This review article is an effort to combine the recent developments, concerns and prospective applications of environmentally friendly nano- with micro-structured polymeric materials such as chitin, starch, polycaprolactone and nanocellulose.
Trang 1Review Article
Recent progress in selected bio-nanomaterials and their engineering
applications: An overview
Raghvendra Kumar Mishraa, Sung Kyu Hab,**, Kartikey Vermac, Santosh K Tiwarib,d,*
a International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India
b Department of Mechanical Engineering, Hanyang University, South Korea
c Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India
d Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad, Jharkhand, India
© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
The prevailing development in polymeric composites
applica-tions is gaining momentum around the world [1e3] The
extraordinary features of innovative polymeric composites and
their establishment with existing environmentally friendly
analytical techniques adduce considerable triumph to improve the
era of environmental research[2,3] The food packaging materials
obtained from petroleum-derived polymers are widely employed
in several different functions due to their lesser density, more
affordable cost, and extraordinary mechanical as well as barrier
properties[1e3] Even though, various forms of petroleum-based
product packaging polymer materials are generally recoverable
and reused, huge ranges of these types of materials completelyturn out in the form of landfill[2,3] Consequently, progressivebiosphere issues parallelled the growth and development of ver-satile barrier bio-based product packaging materials as a reason-able alternative when dealing with these materials[2] Together,the lack of fossil fuel energy sources and the raised costs of crudeoil have heightened the worldwide interest in bio-based materials[1e3] It turned out by analysers that the petroleum sourceswould appear to be inadequate in successive 60 years [2e4].Controlling the forthcoming concerns because of the plasticwastes as well as petroleum resources triggers the production ofleading-edge and more environmentally friendly materials in themodern era Severe efforts are undertaken for growth anddevelopment of bio-based composites composed of renewablesources to replace petroleum-based polymers by obtaining eco-friendly materials[1e4] Edible coatings andfilms involved with
an appreciable unique class of packaging materials that offer anadditional strategy over the traditional packaging materials likelydue to their outstanding biodegradable, biocompatible as well as
* Corresponding author Department of Applied Chemistry, Indian Institute of
Technology (ISM), Dhanbad, Jharkhand, India.
** Corresponding author.
E-mail addresses: sungha@hanyang.ac.kr (S.K Ha), ismgraphene@gmail.com
(S.K Tiwari).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirectJournal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2018.05.003
2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Journal of Science: Advanced Materials and Devices 3 (2018) 263e288
Trang 2edibility characteristics[2e5] Although a sizeable number of bio
extracted materials are analysed for their precise practical
appli-cations, scientists and experts have anticipated a fair number of
persistent issues that confine their wider industrial applications
[3] For example, some bio-degradable polymer materials in
general exhibit poor mechanical properties in comparison with a
lot of petroleum-based polymers This is exactly due to the
intrinsic low stiffness as well as strength characteristics of
biodegradable polymer materials Enhancing the properties of
these kinds of biopolymers can often lead to innovative materials
Low production levels, competition with food crops and high costs
are the important aspects that can further reduce the broader
technical applications of biopolymer packaging materials Thus,
scientists are attempting to enhance the mechanical and barrier
characteristics of bio-based films There are several options
available to enhance the barrier and mechanical properties of
packaging [4] Nowadays, studies regarding bio-polymer
nano-composites have witnessed a considerable improvement in the
targeted properties in both industrial and academic laboratories
According to the recent studies, the worldwide foods and
beverage trade are going to display progression rates of near to 8%
CAGR over the expected duration [4,5] The total food and
beverage business enterprise in 2005 were approximately around
USD 8 trillion and achieved around USD 15 trillion in 2015; a
development related to, among other activities, an increasing
middle class of the community with an amplified customer
investing potential in the Asia Pacific and Latin America The huge
research and development on this material around the globe
(Scopus data 2017,Fig 1a and b), have produced extensive items
for the various industries, that would eventually raise the total
biopolymer sell capacity by 20,246, as shown inFig 1 Therefore,
this review article is a boon for the research outputs regarding the
bio-polymer inspired micro and nanomaterials
2 Green materials and their derivatives
2.1 Bio-polymer
Biopolymers are usually polymeric biomolecules consisting
monomeric units, which are generally covalently joined to
fabri-cate larger sized molecules[1,2] The term‘bio’ implies that these
are in fact naturally degradable materials derived from formal
living microorganisms[1] A group of materials generally
manu-factured from organic natural resources just to illustrate microbes,
crops, or even plants are defined by means of the expression
“biopolymer” Materials based synthetic routes derived from the
biological resources for instance vegetable oils; sucrose, fats,
resins, proteins, amino acids, etc are also referred to as biopolymer
(because of the natural compositions)[2e4] In comparison with
artificial polymers that contain a less complicated and additional
random configuration, biopolymers are complicated molecular
assemblies that employ accurate, described 3D patterns and
architectural structures [5] This is certainly one vital character
rendering biopolymers active molecules in-vivo Their specified
shapes together with structure are key elements of their
perfor-mance As an illustration, haemoglobin is unable to have oxygen in
the blood when it was not folded inside a quaternary architecture
Biopolymers are categorized in various ways according to distinct
scales[5,6] According to their degradability, biopolymers are split
into two wide categories, specifically biodegradable in addition to
non-biodegradable, and alternatively, into bio-based as well as
non-bio-based biopolymers[6] On the structure of their polymer
main chain, biopolymers are undoubtedly grouped generally into
the following categories: polyesters, polysaccharides,
poly-carbonates, polyamides, as well as vinyl polymers These types of
classes are further defined directly into a bunch of subgroups inagreement with their source [3,4] Biopolymers are grouped,dependent upon the type of the repeating unit which is composed
of three classes: (i) polysaccharides which are produced of sugars,(ii) proteins that come from different amino acids and (iii) nucleicacids that are composed of nucleotides Relevant to the applica-tion, biopolymers are known for their role of bioplastics, bio-surfactant, bio detergent, bio-adhesive, bioflocculant, etc[6,7].2.2 Biopolymer source and preparation
Biopolymers are usually a variety of plastics produced fromenvironmentally friendly biomass sources, for example, cornstarch, pea starch, vegetable oil, and so on[6,7] Combined withbio-inspired polymeric materials (also many biopolymers) whichare artificially extracted from particular polymers for preferredapplications [6] The existence of unique natural polymers incrops, vegetation and plants grant a bio-renewable opportunityfor their preparation It is noticeable that, almost all regular man-made polymers, are created in bulk after which they are mouldedfor the purpose of scientific research works Various types ofmicrobes performed a key part in creating a variety of bio-polymers, for example, polyesters, polysaccharides, as well aspolyamides within the range of viscous solutions to plastics(Table 1)[7,8] Their physical characteristics are influenced by theconstitution, design of repeated units as well as the molecularweight of the polymer [8] The physical, as well as chemicalcharacteristics of a variety of biopolymers synthesized by the help
of microbes, may be tailored to the consistent treatment of croorganisms which makes it suitable for healthcare applications,for example, drug delivery and tissue mechanism[9] Biopolymerswhich are generated by making use of microorganismsrequire specific nutrition as well as maintained surroundingenvironment These are commercially developed by means ofdirect fermentation or even by chemical polymerization by usingrepeating units that are consequently prepared by means offermentation procedure Generally, most of the biopolymers arebiocompatible as well as biodegradable without negative impact
mi-on biological systems [9] The functional mechanism ofmanufacturing of biopolymers from the microorganism origin iswidely seen either as a result of their particular defence mecha-nism or as the storage material[9] It is well recognized that thesetypes of materials tend to get degraded by natural processes asthat of microorganisms and enzymes to ensure that it may lastly
be reabsorbed in the environment[9,10] By focussing our centration, a bit more into the biopolymers, the modification offossil materials as well reducing of CO2pollutants are achieved,therefore promoting environmental friendly development [10].Among the variety of microorganisms, algae work as anoutstanding feedstock for the plastic generation because of theirhigh output along with its potential to cultivate in a variety ofconditions[9,10] The application of algae reveals the chance formaking use of carbon as well as neutralizing greenhouse gasemissions from all sorts of industrial facilities Algae-based plas-tics have been a newly released inclination in the period of bio-plastics in comparison with conventional strategies of employingfeedstocks of corn and potatoes as plastics[1,10] Although algae-based plastics are in their infancy, once they are into commer-cialization they likely find applications in a wide range of in-dustries At present, microbial plastics are viewed in the form of acrucial root of polymeric material which has an incredible op-portunity for commercialization in the next generation They cantailor the movement abilities offluids, be encapsulating materials,flocculate particles, create emulsions as well as stabilize suspen-sions[10]
con-R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 264
Trang 3Fig 1 (a) Scopus database (09/12/2017) for the research output in form of research articles from the top ten countries and (b) applications of bio-polymer inspired micro and nanomaterials (Scopus database 09/12/2017).
Table 1
Different sources and methods of preparation for bio-polymer and related materials [1]
Hybrid plastics Introducing denatured algae biomass to petroleum-based
plastics exactly like polyurethane and polyethene
in the role of fillers.
Filamentous green algae, Cladophores
Cellulose-based plastics Biopolymer of glucose 25e30% of the biomass created after extraction of
algal oil is known to comprise cellulose Poly-lactic acid (PLA) Polymerization of lactic acid lactic acid Microorganism fermentation of algal biomass
Bio-polyethylene Ethylene manufactured from ethanol, by a chemical
reaction called cracking Ethanol derived from natural gas/petroleum sources
Microorganism fermentation of algal biomass
Polyhydroxybutyrate (PHA) Obtained as a carbon and energy storage polymer
under nutrient limiting growth environments
Microorganism such as Alcaligenes eutrophus Polycaprolactone (PCL) Ring-opening polymerization using dibutyl
zinc-triisobutylaluminum as a catalyst
ε-caprolactone Chitosan Alkali NaOH treatment Treating shrimp and other crustacean shells such as crabs and krills Gelatin Partial hydrolysis of collagen Collagen from white connective tissue, animal bones and skin Alginic acid Treatment Brown algae in aqueous alkali solutions Brown algae, including Laminaria hyperborea, Laminaria digitata,
Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 265
Trang 43 Structure- property relationship of green polymeric
materials
Bio-polymers are adaptable in structure as well as properties and
hence regarded as the most preferred biomaterials The demand for
polymeric biomaterials in healthcare enterprise is about 60% of the
global market, which has been expanding exponentially in recent
years [9] This is due to the structure and surface properties of
polymers, which can be tuned according to the specific desires of
the biomaterials for various applications[9,10] The polymer-based
materials derived from renewable resources support the
develop-ment of sustainable composites via economically feasible and
environmentally friendly technology[1,9] If a polymer or a
poly-meric blend or a polymer composite material with biodegradation
property is obtained solely from a sustainable source, then it is
called as a green polymeric product Nature presents a wide range of
polymers with biodegradability property and exhibits the potential
aspects to replace conventional fossil fuel-based polymers[1e3]
The common examples for naturally derived polymers include
proteins, starch, chitin and cellulose[1e3] In addition to this,
nat-ural rubber latex (NR), polylactic acid (PLA) derived from corn and
polyhydroxyalkanoates (PHA) produced from bacteria serve as
ex-amples for other green polymers Polymers such as poly(a-hydroxy
acid)s, poly(ε-caprolactone), poly(glycolic acid), poly(methyl
methacrylate), poly(dimethylsiloxane), PU, cellulose, silk etc are
utilized as biomaterials for various applications like contact lenses,
bone cement, wound dressings, artificial organs, tissue scaffolds,
cardio-vascular apparatus, breast implants, catheters, drug delivery,
sutures and so forth[1e3]
Green composites include a group of materials, which aregathered from renewable resources and undergo complete degra-dation through microorganisms [1e3] These materials act as apotential substitute for conventional petroleum-based polymericmaterials for which recycling seems to be unpractical or uneco-nomical The biodegradability aspect of green polymers associatedwith their biocompatibility makes them an efficient advancedbiomaterial[10,11] However, lack of adequate mechanical strength,
as well as bioactivity characteristics, insufficient control withrespect to degradation rate and poor biomimetic structural orcompositional features, limit their practical applications,Table 2shows a list of frequently used polymers and their applications[10,11]
4 CelluloseCellulose is an abundantly available natural biopolymer and can
be readily obtained from sustainable sources[2,10] The examples
offibrous form of cellulose include cotton, wool and hemp As sugarconstitutes the monomeric units of cellulose, it falls under thecategory of the polysaccharide[11] The molecular formula for anorganic cellulose is (C6H10O5)n This denotes polysaccharide, whichconsists of hundreds or thousands of 1e4 linkedD-glucose unitsthat are linked together in a linear fashion[10e13] It is found thatcertain bacterial species also secrete cellulose to promote the for-mation of biofilms In general, plants consist of 33% of cellulosecontent on an average[12,13] The plant sources rich in celluloseinclude cotton and wood The cotton consists of ~90% cellulose,whereas the cellulose content present in wood corresponds to ~50%
Table 2
Most frequently used polymers for different applications with remarks on their pros and cons [10]
Poly(methyl
methacrylate)
Contact lenses, bone cements, dentures etc Comparable elastic modulus to bone, bio-stable or bio-inert, brittle, low
tolerance to the organic solvents, inability to modify with biomolecules etc Polyurethane(PU) Wound dressings, artificial organs, tissue scaffolds,
cardio-vascular devices, etc.
Tuneable properties, blood-compatibility, biodegradable with no significant
pH change, Toxic degradation products, lack of bio-stability for permanent implants etc.
Poly(dimethylsiloxane) Contact lenses, breast implants etc Skin protectant, bio-durable, immunogenic activation of anti-silicon
antigens etc.
Polyethylene Orthopaedic joint implants, components of catheters etc Good toughness, resistance to fats and oil, cannot withstand sterilization
temperature etc.
Poly(ethylene glycol) Wound dressings, fillers etc Hydrophilicity, biocompatibility, low immunogenic, insufficient strength,
high degradation rate etc.
Polycaprolactone Drug delivery, sutures and scaffolds etc Good ductility, biocompatible, low tensile strength, slow degradation rate
Gelatin Tissue engineering, wound dressing, gene transfection,
drug delivery, weight loss as well as for treating osteoarthritis, rheumatoid
arthritis, for foods, cosmetics, and medicines etc.
Haemostatic, non-immunogenic, pro-angiogenic, biodegradable as well as biocompatible, cross-linked to form hydrogels etc.
Starch Paper, textiles as well as adhesives, pharmaceutical tablets,
pesticides, cosmetics, detergents, oil-drilling fluids etc.
Cheap as well as degradable, sensitivity to moisture and poor mechanical properties etc.
Cellulose Biomedical field, Textile applications etc Natural biological polymer, environment-friendly, biodegradable,
remarkable strength etc.
Chitin Water treatment, biomedical applications such as Fibroblast
migration and proliferation wound dressing, raw materials for chitosan etc.
It is insoluble in most of the solvents, as applicability, oxygen permeability, water sorptivity, blood coagulating property etc.
Chitosan Drug carrier, coating agent, gel former etc The excellent film forming property etc.
Polylactic acid Medical implants, wound management, drugs delivery etc Biodegradable thermoplastic, aliphatic polyester, soluble in many organic
solvents, higher transparency compare with other biodegradable polymers, superior in weather resistance etc.
Poly-vinyl-Alcohol Thickener in glues, paper-making, a sizing agent in textiles,
water-soluble films useful for packing, coatings, optical, pharmaceutical, medical applications
Colourless, white, and odourless, water-soluble synthetic polymer etc.
Polyvinyl acetate Biomedical, synthesis of metal nanoparticles,
sensing activity etc.
Polyvinyl ester family, Thermoplastic resin etc.
Collagen Medical such as healing and repairing of the body's
tissues and skin-deep application.
Damage the production of collagen includes sunlight, smoking, and high sugar consumption.
R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 266
Trang 5or more[12,13] Apart from cellulose, several natural components
namely lignin, hemicellulose and pectin are also present in plant
fibres As cellulose is an abundantly available raw material in nature
and has attractive features, it is considered to meet the growing
demand for eco-friendly products with ensured biocompatibility
[12,13] It is well known that cellulose is insoluble in many solvents,
which brings a limitation to its reactivity and processability for
utilization[11e13] In thermoplastic-based polymer matrices,
cel-lulose fibres are recognized as an efficient reinforcement in the
recent years This is mainly due to the significant features of
cel-lulose, which include low density, reduced wear problems while
processing and readily active surface for functionalization,
cost-effective nature and easy availability[13] The cellulose reinforced
polymer composites can be easily combusted during a recycling
process as compared to polymer matrices filled with inorganic
fillers[13,14] Despite all these advantages, the utilization of
cel-lulosefibres in the industrial scale is limited The reason behind this
problem is the inability in attaining satisfactory dispersion levels in
polymer matrices[14]
4.1 The basic structure of cellulose
It is well known that cellulose is a polysaccharide with glucose
as its monomeric units Structural isomers of glucose include
fructose and galactose [14,15] Throughout the cellulose chain,
glucose molecules are linked via glycosidic bridges which are
formed by loss of hydrogen atom from one monomer and hydroxyl
groups to another monomeric unit[15] This leads to the formation
of microfibrils[14] These microfibrils are arranged together by the
intermolecular hydrogen bonding to form biggerfibrils It can be
observed that a cellulose homopolymer consists of D
-anhy-droglucopyranose units which are then linked via (1e4) glycosidic
bonds As glucose exists as a six-membered ring in the cellulose
structure, it is named as pyranose[15] The oxygen linkages
pro-vide a connection between the pyranose monomeric units Native
cellulose refers to the cellulose produced by plants, which exists in
two types of crystalline structure namely cellulose I and cellulose II
Type II cellulose is found to exist in marine algae[15,16] Cellulose
II is usually crystalline in nature and can be produced when
cel-lulose I is subjected to undergo treatment with aqueous phase
sodium hydroxide Apart from type I and type II cellulose, other
forms such as cellulose III and cellulose IV do exist Cellulose I is
recognized to be less stable as compared to other polymorphs,
whereas cellulose II is known to present a highly durable structure
among all the types[16]
4.2 Polymorphisms of cellulose
Polymorphism is the property by virtue, a compound can
appear in more than one form Cellulose consists of many hydroxyl
groups contributing to more intramolecular and intermolecular
hydrogen bonds which lead to separate ordered arrangements[17]
In one cellulose bonding unit, there prevails six hydroxyl groups
and three oxygen atoms Hence, there are many possibilities of
crystal packaging, different cellulose units, and change in chain
polarity The polymorphs of cellulose are classified generally into
four heads[17]:
4.2.1 Cellulose I
Cellulose I is a native cellulose, and it is abundantly found in the
environment[2,3] In 1984, Atalla and Vander Hart discovered that
the native cellulose is present in two forms, namely Iaand Ib They
used13C CP/MAS NMR spectra to characterize native cellulose into
two distinct allomorphs[17,18] Cellulose Iahas a triclinic unit with
one hydrogen bonding chain per unit cell, where the cellulose
chains are stacked parallel to each other by Van Der Waal's action[17,18] Cellulose Ibhas a monoclinic unit with two hydrogenbonding chains per unit cell These two forms of cellulose I mutu-ally co-exist and their percentage varies with varying sources ofcellulose [18] Cellulose obtained from primitive organisms (bac-teria, algae) is rich in cellulose Ia while cellulose obtained fromdeveloped higher woody plants are rich in cellulose Ib Cellulose Ia
inter-can be transformed into more stable Ibby annealing at around
2600C to 2800C in some special solvents[18].4.2.2 Cellulose II
Cellulose II is much more thermodynamically stable than lulose I It was in 1844 that John Mercerfirst invented the technique
Cel-of mercerization for converting cellulose I to Cellulose II[17e19] Inthe process of mercerization cellulose is treated with alkali with aconcentration of about 17%e20% w/v[17e19] This leads to theswelling of cellulosefibres when Naþ ions penetrate the spacesbetween the cellulose molecules without causing any dissolution.During the process, the parallel chain arrangement of the cellulosemolecules gets reversed into antiparallel chains In 2015, B J C.Duchemin was able to convert cellulose I into cellulose II by usingonly 1% w/v NaOH solution at the temperature below 0C withoutchanging the crystallinity of the cellulose microstructure This wasindeed a great advancement in the mercerization technique.Another process of dissolution and regeneration can also convertcellulose I into cellulose II[18,19] As the name suggests, this pro-cess involves the complete dissolution of cellulose followed byregeneration of cellulosefibres Regeneration of cellulose can bedone by various processes like copper ammonium and N-methylmorpholine N-oxide (NMMO) processes[19] The cellulose I to IIconversion was reported by regeneration using phosphoric acid.The alteration of cellulose I to cellulose II is an irreversible processindicating cellulose II is much more thermodynamically stable[19].4.2.3 Cellulose III
Interestingly, Cellulose III exists in two forms: Cellulose IIIIandCellulose IIIII Cellulose IIIIis obtained by the ammonia merceriza-tion of Cellulose I and Cellulose IIIIIis obtained by the ammoniamercerization of Cellulose II In 1986, Yatsu did the stable trans-formation of cellulose I to cellulose III by immersing cellulose inammonia solution followed by degassing[20] The conversion toCellulose III is reversible in nature where chain orientation is notchanged However, fragmentation of crystal takes place during thetransformation of Cellulose I to cellulose III During the reversetransformation, distortion of the morphological structure is notrestored[20]
4.2.4 Cellulose IVCellulose IV is obtained from Cellulose III by treating it withglycerol at a higher temperature In 1946, Hermans and Weidingerwere able to convert mercenaries' ramie cellulosefibres into cel-lulose IV by treating it at high temperature in the presence ofglycerol[21]
4.3 NanocelluloseNanocellulose is one of the dominant biodegradable and sus-tainable nanomaterials found in nature In simpler terms nano-cellulose corresponds to cellulose in the nanometer scale[20e22].Till 1970, humans have been making cellulose from plant materialslike wood, plants, waste materials and algae[22,23] In 1970, Tur-bak, Snyder and Sandberg could successfully synthesize micro-fibrillated cellulose by homogenization at high temperature andhigh pressure accompanied by an impact ejection at a hard surface[22,23] Generally, cellulose is slightly crystalline and amorphous inR.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 267
Trang 6nature[23] It is extremely difficult to break the crystalline part of
cellulose because of its remarkably strong hydrogen bonds [23]
Hence, cellulose has to go through a sequence of chemical and
mechanical treatments in order to extract crystalline nanocellulose
and nanocellulosefibres[22] Cellulose nanofibrils (CNF) include
extremely thin (nearly 5e20 nm) and in length (up to severalmm)
fibrils with significantly high surface area (aspect ratio) Every
microfibril is usually defined as a chain of cellulose crystals
con-nected along the microfibril axis disordered amorphous domains
[22,23] In small quantities, it makes a translucent gel-like material
that can be used for developing biodegradable combined with
eco-friendly safe, uniform, as well as densefilms for several purposes,
specifically in the biomedical area[23] Extraction of CNF is being
discussed from a variety of resources such as coir, banana, sugar
beet, hemp, softwood, in addition to wood pulps [23] After
employing a range of plasticizers, range of thermal, mechanical,
barrier, and also physical features of the cellulose are enhanced in
order that it has been employed in a variety of market applications
[23] Plasticizing enhances the features such as oil and grease
protection, and significant barrier against oxygen transfer
particu-larly at a dry environment[23,24] Almost all cellulose-synthesizing
microorganisms consisting microbes, algae, tunicates, as well as
considerably higher plants possess cellulose syntheses proteins,
which catalyse the polymerization reaction of glucan chains[24]
At present, cellulose is generally obtained from a vast variety of
crops, vegetation, plants, animals, as well as bacteria[17,23] The
origin is crucial as it influences the dimensions as well as
charac-teristics of the obtained cellulose An array of the crop, vegetable
and plant materials are being analysed in relation to the extraction
of cellulose as well as nanocellulose, which includes timber, rice
husk, sisal, hemp,flax, kenaf, and in the coconut husk Cotton fibres
have likewise been applied in form of an efficient source material,
benefiting from their low non-cellulosic constituent content than
wood Wood is an elegant initiating material for cellulose as well as
nanocellulose isolation, due to its terrific quantity[23] It is a
nat-ural composite material with a hierarchical structure comprised of
cellulose, hemicelluloses, as well as lignin[23,24] Wood includes a
porous anisotropic configuration, which displays an extraordinary
combination of excellent strength, rigidity, resilience, as well as
lesser density [25] The preparation of nanocellulose from wood
involves a multistage operation concerning vigorous chemical and/
or mechanical treatments Tunicates are aquatic invertebrate
ani-mals, particularly, members of the subphylum Tunicata Most of the
study in this field includes a highlighted category of tunicates
which are typically referred to as sea squirts (Ascidiacea), that is a
breed of aquatic invertebrate filter feeders Experts are working
over 2300 varieties of Ascidiacea and cellulose microfibrils to
un-derstand above mentioned phenomenon For the same reason,
scientists are also working on many dissimilar species like
Hal-ocynthia roretzi, HalHal-ocynthia papillosa and Metandroxarpa uedai
[23,26,27] The tunicates create cellulose in the external tissue,
referred to as tunic, from which a refined cellulose portion referred
to as tunicin is obtained Tunicate cellulose consists of nearly fresh
cellulose of CIballomorph form with significant crystallinity[26]
The nano/microfibrils of tunicate cellulose contain a huge aspect
ratio as well as the excellent specific surface area Algae of a variety
of breeds, green, red as well as brown, are also defined as cellulose
as well as nanocellulose sources For example, Valonia, Micrasterias
denticulate, Micrasterias rotate, Cladophora, Boergesenia, as well as
other kinds of algae are being employed[26,27] Cellulose
micro-fibrils with a huge aspect ratio (>40) are generally obtained from an
algae cell wall by way of acid hydrolysis or mechanical refining[27]
The architectural structures of CMFs separated from various kinds
of algae change To illustrate, Valonia microfibrils hold square
cross-sectional area (nm2) as they are mainly of Iacrystalline form
[26] However, M denticulate microfibrils possess rectangularcross-sectional area (nm2) because they are mainly of the CIb
crystalline form Bacterial cellulose (BC) is a result of the majormetabolic operations of a particular kind of bacteria[26,27] Themost well-known BC-producing bacterial breeds are Gluconaceto-bacter xylinus [23,26,27] Under specific culturing environment,these types of bacteria create a dense gel which is made up ofcellulose microfibrils together with 97e99% water [23,26,27].Bacteria cellulose crystallites are predominantly of the CIacrys-talline form along with the degree of polymerization (DP) of bac-terial cellulose, which is commonly between two thousand and sixthousand The benefit of bacterial cellulose is the fact that it is easy
to adapt the culturing environment to modify the microfibrilconfiguration as well as crystallization [23] The supplementaryvital ability of bacterial cellulose is its superior chemical purity,which distinguishes it from the kinds of plant cellulose that aregenerally related to hemicelluloses as well as lignin In spite of this,both celluloses synthesized by bacteria or cellulose obtained from anumber of plants contain identical molecular arrangements[15,23,26]
The different kinds of nanocellulose are generally categorizedinto various subcategories according to their shape, dimension,functionality, as well as generation strategy, which predominantlyrely on the cellulosic resource together with processing environ-ment [23,26] Various terminologies can be employed for thedifferent types of nanocellulose [26] A serious problem thatneeds to be eliminated for effective commercialization of cellu-lose nano-fibrils is the excessive energy consumption needed forthe mechanical disintegration of the preliminary cellulosemicrofibers into nano-fibres, may possibly consist of many passesthrough the disintegration machine To deal with, it looks like themode of cellulose feedstock performs a noteworthy role in theenergy consumption; despite this, it seems to have merely anyeffect on the resultant cellulose nano-fibrils features It should bementioned that cellulose nano-fibrils experience some specificunfavourable characteristics, which reduce their application inseveral sectors, for illustration, in papermaking due to sluggishdewatering or even as polymer composites because of inadequatecompatibility of hydrophilic reinforces with hydrophobic poly-mers A possible method for fixing this issue is the chemicalmodification of cellulose nanofibrils to decrease the quantity ofhydrophilic hydroxy active groups [28] Cellulose nanocrystals(CNCs) show an elongated rod-like appearance and provide verylimitedflexibility in comparison to cellulose nanofibrils, due to itsconsiderably higher crystallinity[28,29] Cellulose nanocrystalsare usually called as nanocrystalline cellulose, nanowhiskers,nanorods, or rod-like cellulose crystals[29] The nanocrystallineparticles structure are produced through the splitting of amor-phous domains, along with the splitting of localized crystallinecontacts between nanofibrils, by means of hydrolysis withconcentrated acids (6e8 M) This chemical type method isaccompanied by high-power mechanical or sometimes ultrasonictreatment methods[29] A significant feature of cellulose nano-crystals made from sulfuric acid is the negative particle charge,because of the generation of sulphate ester active groups, whichimproves the phase durability of the nanocrystalline particles in
an aqueous environment [28,29] The geometrical shapes andsizes of cellulose nanocrystals may vary extensively, with adiameter ranging from 5 to 50 nm as well as a length within therange of 100e500 nm The dimensions, as well as the crystallinity
of a cellulose nanocrystal, are controlled by the cellulose resourceand consequently extraction circumstances Researchers havedescribed that nanocrystalline particles obtained from tunicatesand bacteria cellulose are likely to be bigger in comparison tocellulose nanocrystals received from wood or cotton This isR.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288
268
Trang 7because tunicates or bacteria cellulose are extremely crystalline
and possess extended nanocrystallites Cellulose nanocrystals
derived from pure cellulose materials display greater crystallinity
Nanocrystalline cellulose particles manifest outstanding
me-chanical characteristics[29,30] The theoretical Young's modulus
of a cellulose nanocrystal along the cellulose chain axis is
ex-pected to be 167.5 GPa, which is certainly just like the modulus of
Kevlar or maybe greater than the modulus of steel [30] The
experimental Young's modulus of cotton cellulose nanocrystals is
105 GPa, in addition the modulus of tunicate cellulose
nano-crystals is 143 GPa Amorphous Nanocellulose is generally
extracted by using acid hydrolysis of regenerated cellulose with
succeeding ultrasound disintegration Amorphous nanocellulose
particles commonly contain an elliptical shape with typical
di-ameters of 50e200 nm [30,31] Due to its amorphous con
fig-uration and arrangement, amorphous nanocellulose has
extraordinary capabilities, including a much higher functional
group content, a significant availability, an improved sorption, as
well as an enriched thickening potential[3,17,22] Even though,
amorphous nanocellulose particles possess inadequate
mechani-cal characteristics because they are inappropriate to be used in the
form of reinforcing nanofillers[3,17] Thereby, the main
applica-tions of amorphous nanocellulose are in the role of carriers for
bioactive ingredients, thickening agents in a variety of aqueoussystems, etc [30e34] Cellulose Nanoyarn is another form ofnanocellulose, it is produced by electrospinning a solutioncomposed of cellulose or cellulose derivatives However, it has notbeen widely studied till date [34e37] A transmission electronmicroscope (TEM) image of nanocellulose forms, produced fromdifferent sources is shown as an example inFig 2
4.4 Preparation of nanocelluloseWhen plant cell wall is exposed to powerful mechanical disin-tegration, the initial structure of cellulose fibre is transformed,therefore thefibres transform into nanofibrils (CNF) or even theirmicrofibrils bundles (CMF) with diameters which range from 10 to
100 nm based on the disintegration power [33e37] Many chanical methods are often used to obtain cellulose nanofibrils orcellulose microfibrils from a variety of feedstocks, which includehomogenization, microfluidization, grinding, cryocrushing, as well
me-as ultrme-asonication, me-as shown in Fig 3 [36] The preparation ofnanocellulose consists of many steps which includes mechanicaland chemical treatments Such treatments are assigned towardsrestructuring the specific coherent organization of microfibrilspresent in a natural cellulose[23,36] The nature and properties of
Fig 2 TEM micrographs of (a) Microcrystalline Cellulose from fodder grass [32] , (b) Cellulose microfibril from sugar beet [33] , (c) Cellulose nanofibril from banana peel [34] , (d)
fiber
R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 269
Trang 8the ultimate nanocellulose product that we receive depend upon
the individual steps involved as well as the individual length of
step The aspect ratio of thefinal nanocellulose depends upon the
length of de-structuring that the raw material undergoes[38]
4.4.1 Methods of nanocellulose preparation
There are several different extraction techniques to collect
nanofibrils (Fig 3) They are often carried out by mechanical
stra-tegies, for example, grinding, cryocrushing in the presence of liquid
nitrogen, high-pressure homogenization, and so forth Similarly,
various chemical alkali, as well as enzymatic hydrolyses, are
applied before mechanical techniques to be able to increase the
gain access of hydroxyl active groups, which improve the inner
surface, modify the crystallinity, split cellulose hydrogen bonds,
thereby enhancing the reactivity of thefibres[37,38]
4.4.2 Pre-treatment
Two main issues frequently take place during thefibrillation
stage, and most importantly throughout the mechanicalfibrillation
of cellulose are: (i)fibril aggregation, whenever slurry is pumped
by means of the disintegration equipment in addition to (ii)excessive energy intake involved with fiber delamination, maypossibly engage multiple feeds into the disintegration equipmentuntil effective delamination of cell walls is attained[36e39] Theexcessive energy contribution is essential with the intention toproduce the nanofibers In order to reduce the interfibrillarhydrogen bonding, based on earlier scientific studies, an effectivepre-treatment can reduce the energy intake The selection of pre-treatment methodology is based on the cellulose source[39] It isworthwhile stating that a suitable pre-treatment of cellulosefibressupports reliability, improves the inner surface, adjusts crystal-linity, decreases the energy demand and promotes the process ofnanocellulose generation[40] As an example, the pre-treatment ofvegetable, crop, fruit, plant materials enhances the total or evenlimited elimination of noncellulose constituents hemicellulose,lignin as well as the isolation of specific fibres Pre-treatment oftunicate entails the elimination of the proteins matrix, isolation ofthe mantel, along with the isolation of specific cellulose fibrilsR.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288
270
Trang 9[36e40] Pre-treatment of algae usually helps in the elimination of
the matrix material of algae cell walls, while pre-treatment of
bacterial NC is targeted on the elimination of microbes as well as
other contaminants from the slurry [39] Pre-treatment is an
extremely crucial stage since it can adjust the structural foundation,
crystallinity, as well as polymorphism of the cellulose, in addition
to numerous characteristics of the treated feedstock The
pre-treatment method is employed to assist the cell wall
delamina-tion together which produces nano-sizedfibrils The well-known
pretreatment approach includes pulping techniques, bleaching,
alkaline-acid-alkaline treatment, enzymatic treatment, ionic
liq-uids, oxidation, and steam explosion[1,38e40]
4.4.3 High-pressure homogenization
High-pressure homogenization refers to a technique for the
large-scale fabrication of CNF, along with laboratory-scale
prepa-ration of nanofibrils[41,42] This approach includes pushing the
solution by using an extremely trim channel or alternatively an
orifice taking advantage of a piston, under an elevated pressure of
50e2000 MPa [41,42] The width of the homogenization gap is
dependent upon the viscosity of the solution along with the exerted
pressure[42] The consequential high solution streaming velocity
leads to an intensification of the dynamic pressure as well as a
lowering the static pressure underneath the vapour pressure of the
aqueous phase This contributes to the production of gas bubbles,
which breakdown instantly if the liquid departs from the
homog-enization gap, getting once again under a standard air pressure
[41,42] The gas bubble production, as well as implosion incident,
can cause the creation of shockwaves and cavitations, which trigger
disturbances of thefibrillar configuration of the cellulose Cellulose
fibre size drop is accomplished by means of a significant pressure
drop, excessive shear forces, turbulentflow, as well as interparticle
collisions The level of the cellulosefibrillation is determined by the
range of homogenization cycles as well as on the exerted pressure
[42]
4.4.4 Microfluidization
A microfluidizer is an additional technique which can be
employed for cellulose nanofibrils or even cellulose microfibrils
production In contrast to the homogenizer, which works at the
steady pressure, the microfluidizer performs at a consistent shear
rate Thefluid slurry is pumped via a z-shaped chamber, which
attains an elevated shear force[42,43] The pressure can achieve
ranges up to 40,000 psi, which is about 276 MPa[42] Purposely
designed predetermined-geometry microchannels are placed
in-side the chamber, by which the slurry speeds up to higher
veloc-ities The preferred shear, as well as impact forces, are built
whenever the slurry stream strikes on wear-resistant surfaces
Several check valves enable recirculation of the slurry Upon leaving
the interaction section, the product is sent in a heat exchanger,
recirculated in the system for additional operation, or perhaps sent
from the outside to the subsequent step in the operation It is
required to perform repeatedly the process many times to adapt to
different sized chambers to be able to enhance the level of
fibril-lation Authors inspected the influence of the number of passes of
microcrystalline cellulose MCC slurry in a microfluidizer on the
morphology of the extracted cellulose nanofibrils[44] They
iden-tified that the aspect ratio of the nanofibrillar bundles enhanced
after 10e15 transferring cycles, while extra flows resulted in
agglomeration of the CNFs because of the expanded surface area as
well as a greater level of the surface hydroxyl group[45]
4.4.5 Grinding
An additional method for isolating cellulosefibres into
nano-sizedfibrils is grinding process[46] Throughout grinding process, a
fibre fibrillation operation is performed by transferring the lose slurry between static as well as rotating grindstones rotating atabout 1500 rpm, which exert a shearing stress to thefibres[46] Thefibrillation procedure in the grinder makes use of shear forces todecompose the cell wall configuration and arrangement as well asseparate the nanoscalefibrils[45,46] The level offibrillation reliesupon the distance between the disks, the actual morphology of thedisk tunnels, along with the number of feeds into the grinder.Regarding a homogenizer, numerous passes have been instructed
cellu-to produce thefibrillated cellulose The requirement for disk stoneroutine maintenance as well as a replacement may be a downside
of this method since wood pulpfibres are able to deteriorate thegrooves as well as grit[46,47] But, an important benefit of grinderoperation is the fact that extra mechanical pre-treatments areusually not required[47]
4.4.6 CryocrushingCryocrushing is a mechanicalfibrillation way to cellulose in arefrigerated condition This process generatesfibrils with reason-ably big diameters, varying within 0.1 to 1mm Within this method,water-swollen cellulosefibres are usually refrigerated in liquid ni-trogen and after that progressively crushed [46,48] The use ofsubstantial collision forces to the frozen cellulosicfibres contributes
to breaking of the cell walls because of the pressure implemented
by the help of the ice crystals This draws out the nanofibers Thecryo-crushed fibres can subsequently be distributed as well asdispersed uniformly in water with the help of a routine disinte-grator[49] This technique is relevant to numerous cellulose ma-terials which enable it to be considered as afibre pre-treatmentprocedure before homogenization Authors developed nanofibersfrom soybean stock by means of cryocrushing together with suc-ceeding high-pressurefibrillation TEM confirmed that the nano-fiber diameters have been found in the 50e100 nm range[48] Thenanofibers produced manifested outstanding dispersion capability
in the acrylic emulsion in comparison to water In spite of this, thecryocrushing technique offers a low efficiency and is not costworthy, due to its high energy expenses[49]
4.4.7 High-intensity ultrasonicationHigh-intensity ultrasonication is a very common laboratorymechanical treatment method employed for cell disturbances inaqueous conditions [50,51] This technique produces effectivecavitation which includes the growth, extension, as well ascollapsing of microscopic gas bubbles once the water moleculesintercept ultrasonic energy The effect of the hydrodynamic forces
of the ultrasound on the pulp contributes to the defibrillation of thecellulosefibres[50] Number of researchers have investigated theuse of high-intensity ultrasonication (HIUS) to the separation ofnanofibers from a variety of cellulosic resources, for example, plaincellulose, microcrystalline cellulose, pulp, culinary banana peel,rice waste, as well as microfibrillated cellulose[3,17,50,51] The testresults confirm that a mixture of microscale as well as nanoscalefibrils can be accomplished directly after ultrasonication of thecellulose samples; the diameters of the extracted fibrils areextensively found in the range from 20 nm to several microns,suggesting that a few nanofibrils are extracted from the fibres,although a few stay on thefibre surface[50] As a result, this pro-cess provides aggregatedfibrils with a vast width distribution It isadditionally noticed that the crystalline structure of certain cellu-losefibres is modified by means of ultrasonic treatment method[50] These types of alterations change for individual cellulose re-sources, for instance, the crystallinity after remedy enhanced for100% pure cellulose, diminished for microcrystalline cellulose,although it continued to be consistent for pulp fibre Authorsexamined the consequences of temperature, concentration, power,R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 271
Trang 10dimensions, duration, as well as distance from the probe tip on the
level offibrillation of plenty of cellulose fibres with the help of HIUS
treatment[51] They claimed that outstandingfibrillation was due
to a higher power as well as temperature, even though lengthier
fibres were much less defibrillated Greater pulp fraction and larger
sized distance from the probe to beaker were not beneficial for the
fibrillation[51] These researchers noticed that a conjunction of
HIUS together with HPH enhances the fibrillation together with
uniformity of the nanofibers, in comparison to high-intensity
ultrasonication (HIUS) alone The nanofibrils cellulose output is
additionally improved if TEMPO-oxidized pulp is employed for
HIUS treatment method[50,51]
4.4.8 Acid hydrolysis
To extract cellulose nanocrystals, acid hydrolysis of purified
cellulosic material can be carried out by intense mineral acids
(6e8 M) under-governed environment, time, agitation, and acid/
cellulose ratio conditions[52] Various mineral acids are employed
for this specific purpose, for example, sulfuric acid, hydrochloric,
phosphoric, maleic, hydrobromic, nitric, as well as formic acids
[3,17,52] A combination made up of hydrochloric together with
organic acids (acetic or butyric) has been discussed in the previous
sections Sulfuric is regarded as the most widely applied acid for
cellulose nanocrystals production Throughout hydrolysis,
disor-dered amorphous regions, as well as interfibrillar contacts of
cel-lulose, are selectively hydrolysed; on the other hand, consistent
crystallites stay unchanged which enable it to be separated as
rod-like nanocrystalline materials [52] The cellulose nanocrystals
dispersion in an intense acid is antiquated by using water and
cleaned via consecutive centrifugations Neutralization or even
dialysis by using distilled water is conducted to take away
remaining acid from the dispersion Supplementary steps for
example filtration, centrifugation, or perhaps ultracentrifugation,
in addition to mechanical or possibly ultrasound disintegration,
have likewise been discussed[53,54] In case cellulose nanocrystals
are produced taking advantage of cellulose hydrolysis together
with hydrochloric acid, the uncharged nanocrystalline particles are
likely toflocculate in the aqueous medium[53,54] The other
hy-drolysis techniques are hyhy-drolysis with solid acids, hyhy-drolysis with
gaseous acids, hydrolysis with metal Salt catalyst (Novo et al., 2015)
The main benefits of hydrolysis in the presence of the solid acid areeasy restore of the solid acid, relative safe, lesser corrosion rate ofthe equipment [52e54] Hydrolysis with gaseous acids methodcould provide numerous environmentally harmful as well as time-consuming steps which are used for traditional acid hydrolysis isruled out Without any doubt, less quantity of water is required, theacid reusing is much easier, and in fact the dialysis stage is left out.The cellulose nanocrystals output is considerably more, due tolesser cellulose feedstock damage throughout the gaseous hydro-lysis operation [53] Hydrolysis with metal salt catalyst is per-formed using a transition metal-dependent catalyst A transitionmetal-dependent catalyst offers a satisfactory, preferential, aswell as feasible hydrolysis operation with minor acidity Thevalence condition of the metal ion is the paramount aspect toinduce the hydrolysis performance, in which an acidic solution (Hþ)produces during the period of polarization between metal ions aswell as water molecules[55,56] A greater valence state producesmuch more Hþ ions, which behave efficiently in the co-catalysedacid hydrolysis reaction in the existence of metal ions[1e3].4.5 Chitin
Chitin is a polysaccharide which is highly basic in nature Chitinfalls under the category of natural polymer usually found in shells
of crabs After cellulose, chitin is recognised as the most abundantlyavailable polymer in nature Both chitin and cellulose belong topolysaccharide category Chitin differs from cellulose by the pres-ence of acetamide group rather than hydroxyl group It is estimatedthat the crab and shrimp shell waste generated fromfishing in-dustry contains 8e33% of chitin polymer Chitin consists of b-1, 4-N-acetyl-D-glucosamine monomer units arranged in a linearfashion, as shown inFig 4 [57,58]
The isolation of chitin starts with the choice of shells Preferably,shells of the identical size, as well as kinds, are selected For shrimpshells, the relatively slim walls render restoration of chitin moreconvenient[57,58] The washing, as well as drying of the shellspursued by extensive crushing, is the subsequent step in themethod The small shell fragments are dealt with dilute hydro-chloric acid to take away calcium carbonate Proteins along withother organic contaminants are eliminated by an alkali treatmentR.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288
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Trang 11method (20% sodium hydroxide) The pigments, predominantly
carotenoids are taken away by extraction with ethanol or even
acetone after the demineralization method[58] Chitin exhibits
biodegradability and anti-microbial properties Chitosan is not
water soluble and it exhibits resistance towards alkalis, acids and
other solvents[55e58] Chitin is being widely used in several
ap-plications such as biosensors and medicalfield In the case of a
medicalfield, chitin finds utilization in the form of wound dressing
material and drug delivery vehicles Chitin exhibits biological
ac-tivity apart from being biocompatible and biodegradable in nature
Chitin exhibits optical properties in addition to the ability to form
polyoxy salt,films and chelation of metal ions[56,57]
Chitin does not represent block or random orientation The
properties of chitin and cellulose are entirely different Like
chi-tosan, chitin is also not a water-soluble polymer and it readily
dissolves in organic solvents The solvents in which chitin is
sol-uble includes hexafluoroacetone, hexafluoro isopropanol and
several other chloro alcohols in combination with mineral acids
[57e59] Chitin is also known to exhibit reactivity towards certain
solvents Chitin begins to degrade even prior subjecting to melting
due to the existence of hydrogen bonding Chitin contains a
pro-tein matrix in which microfibrillar structure is embedded The
diameter of microfibrils is found to be in the range of 2.5e2.8 nm
In crab shell, chitin prevails in the thixotropic state as well as
liquid crystalline form The crystalline regime of chitin is isolated
from squid pens and crab shells by various authors[58,59] For
isolation of crystalline regions of chitin, acid hydrolysis is
per-formed using hydrochloric acid It is documented that the aspect
ratio of chitin determines its reinforcing ability in polymer
matrices The crystal structure of chitin and chitosan is
investi-gated in several published articles[59,60] Three kinds of
crystal-line forms are found in the chitin, these area-,b-, andg-chitins,
these crystalline forms are dependent upon the configuration and
arrangement of the polymeric chains[60] For example, authors in
their investigation of the crystal structure found that a-chitin and
b-chitin derived from shrimp shell and squid pen displayed similar
diffractograms when subjected to X-ray diffractometer (XRD)
analysis[61] Additional evidence on the crystalline structure of
a-and b-chitin is acquired by analysis of XRD patterns of highly
crystalline specimens[61] However, the crystallographic
param-eters differed for both the forms of chitin Each unit cell of a-chitin
is found to contain two anti-parallel molecules[61] In contrast,
b-chitin is reported to contain parallel construction per unit cell In
spite of exhibiting such difference in terms of crystallographic
parameters, both the forms of chitin demonstrate independent
crystallographic unit for N-acetyl glycosyl functional group[61]
For the examination of physicochemical properties of chitin and
chitosan, a variety of methods can be found from the previous
publications for the main purpose The comprehensive
informa-tion is explained inTable 3
The XRD patterns for a-chitin derived from different sourcesnamely Lobster and Sagitta exhibited difference[61,62] In the case
of a-chitin derived from lobster, the diffraction peak at 001 crystallattices is found whereas the similar peak position is found to beabsent for a-chitin derived from Sagitta[60] The a-chitin obtainedfrom Sagitta is found to be highly crystalline as compared to a-chitin derived from the lobster[60] Hence, much attention needs
to be given to investigating the crystal structure of a-chitin in ordersolve the ambiguities[60] However, well-defined crystal structurefor b-chitin is well documented other than certain issues related tounit cell parameters The crystallinity indices of commercial chitin,and chitin extracted from cocktail protease treatment method,were 97.9 and 81.0%, respectively (the baseline at 2q¼ 16) On the
whole, it had been noticed that the use of protease cocktaildecreased the crystallinity of chitin from 97.9% in commercial chitin
to 88.0% in the enzymatic treatment method (Table 4)[62].4.6 Lignin
Lignin, the 2nd most abundant biopolymer next to cellulose onearth planet, has been considered as an auspicious alternative forexisting fossil fuel resources, exhibits some advanced propertieslike antimicrobial activity, antioxidant properties, low density, goodstiffness, high carbon content, and ultraviolet (UV) radiation pro-tection [60e63] With such properties, wide research leads toexplore the possibility of converting lignin into value-added in-dustrial products Lignocellulosic materials chiefly composed ofcellulose, lignin and hemicellulose along with small fractions ofwaxes, and several water-soluble mixtures Lignocellulosic materiallike naturalfibres, strength to the materials is provided by cellulose,moisture absorption thermal, degradation and biodegradation byhemicellulose Besides possessing high thermal stability, Lignin isresponsible for the ultraviolet (UV) biodegradation of the materials[63] Being hydrophobic in nature, it makes the cell wall imper-meable to water and ensures a well-organized water and nutritiontransport in the cells Lignin is known as a cross-linked macro-molecular material based on a phenylpropanoid monomer struc-ture[62,63] Molecular masses of lignin have been reported to be inthe range of 1000e20,000 g/mole, isolated from different re-sources Lignin consists of several types of substructures, whichrepeat in an apparently haphazard manner, and it is found invari-ably fragmented during extraction, so measurement of the degree
of polymerization is very difficult i.e varying functionality and highcross-linking The process of extraction and the source from whichthe lignin is extracted like hardwood/softwood/grass determinesthe overall physicochemical properties of the lignin[63] Differentproperties for some selected lignins are summarized inTable 5.Watkins et al represented that the processing methods of thelignin have a strong impact on the adhesive properties of thesynthesized phenol-formaldehyde adhesive [63,64] In theirobservation, it is found that the properties of kraft lignin-derivedphenol-formaldehyde resin were more superior to the steam-exploded lignin-based phenol-formaldehyde resin [63] This
Table 3
Physicochemical characteristics of chitin and chitosan and the determination
methods [61]
Physical characteristics Determination methods
Degree of deacetylation Differential scanning calorimetry, Conductometric
titration, Nuclear magnetic resonance spectroscopy (1HNMR) and (13CNMR), Infrared spectroscopy, First derivative UV-spectrophotometry, Potentiometric titration
Ash contents Gravimetric analysis
Average Mw and/or
Mw distribution
Light scattering, Viscosimetry, Gel Permeation Chromatography
Moisture contents Gravimetric analysis
Crystallinity X-ray Diffraction
Table 4 The degree of acetylation and crystallinity index of commercial chitin, chitin ob- tained from crude protease, commercial protease, alkali and acid treatment [62]
of acetylation (%)
Crystallinity index (%) Chitin obtain from acid treatment 55.25 ± 0.06 e Chitin obtain from commercial protease 80.34 ± 0.11 e Chitin obtain from cocktail
protease treatment
82.25 ± 0.07 88.0 Chitin obtain from alkaline treatment 84.46 ± 0.14 e
R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 273
Trang 12superiority of kraft lignin is due to the high-density network like
hydrogen bonding between the constituents of kraft lignin and
therefore Glass transition temperature (Tg) is another imperative
property that affects the properties of thefinal product[63] The
hydrogen bonding between stilbene and amylose of lignin is shown
inFig 5 Tgof different Lignin Processed by Using Different
tech-niques, as displayed inTable 6
Leskinen et al have reported in their widespread research that
Tgdepends upon the amount of polysaccharides and water in the
lignin, along with the molecular weight and functionalities present
in the lignins[64,65] Different chemical procedures are being used
to process Lignin from different resources and each process has
some advantage and disadvantage and most of the procedures
follow either an acid or base-catalysed mechanism [65] Thus
Lignin is broken down into low molecular weight fractions while
handling through these processes, and its properties got affected
greatly[65] In the same line, many researchers have developed a
new technique for the processing of lignin, in which sulfite, kraft,
and soda, are three major processes along with many others
[64e66] The sulfite process is an acid-catalysed process,
conven-tionally used in pulping technology, involves the cleavage of thea
-ether linkages and b-ether linkages of lignin [65,66] In this
method, a chemical reaction between free sulphurous acid and
lignin leads to the formation of lignosulfonic acid, soluble
ligno-sulfonates formation with cations and lignoligno-sulfonates
fragmenta-tion along with the producfragmenta-tion of carbohydrates take place[65,66]
Kraft lignin is a product of kraft pulping process, which exhibit a
dark colour and is insoluble in solvents including water[66] The
kraft lignin comprises the highest quantity of the phenoliceOH in
contrast to other of lignins[65,66] With a decrease in the
molec-ular mass of lignin, an increment in the number ofeOH is reported
Because of cleavage of the bonds, ionic strength, temperature, and
pH of the solution also influence the solvency of kraft lignin in anaqueous medium [66] Commonly, diluted NaOH is used as thecooking chemical to produce wood pulp in soda pulping process.Soda lignin usually exhibits similar properties such as high phenolichydroxyl content, relatively low glass transition temperature (Tg)and low molecular weight Gupta et al used spray-dried lignin-coated cellulose a bio-basedfiller in PLA host matrix, to modify therheological and thermo-mechanical properties of poly(lactic acid)(PLA) composites The lignin coating on CNCs improves thedispersion of CNCs as well as improved their interfacial interactionwith the PLA matrix, resulting in a substantial enhancement inthermo-mechanical and rheological properties, which make them asuitable candidate for the end user applications [67] The com-posites show significantly higher storage modulus (G0) than the
neat PLA at all loading of L-CNCs in both glassy and rubbery region
in the Dynamic Mechanical Analysis[67] In the presence of L-CNCs,crystallization behaviour of the PLA matrix was also found toimprove significantly[67]
Kai et al synthesized a series of the composite with differentloading of lignin in PLA via the ring-open polymerization of lactideonto selectively alkylated lignin [68] First, this copolymer wasblended with poly(L-Lactide) (PLLA) and then the electrospunprocess is taken place to fabricate uniform nanofibres withcontrolledfibre diameters To examine the biocompatibility of PLA-lignin composites, three different cell types ePC12, humanmesenchymal stem cells (MSCs) and human dermal fibroblasts(HDFs) were cultured In mechanical properties study, it was foundthat elongation at break and toughness of PLA/lignin composites arefive times higher than neat pure PLA Antioxidant activities wereevaluated by DPPH assay for PLA-lignin copolymers and lignin-based nanofibres It is observed that Neat PLLA nanofibers showlow antioxidant activities[68] Even after 72 h, it attains only 15.5±6.2% free radical inhibitions, which is much lower than lignin-containing nanofibers Such type of lignin-based nanofibers used
as biomaterials may reduce oxidative stress-related tissue damages
or functional disorders The biocompatibility of PLLA/PLA-ligninwas studied Due to the oxidative stress induced by the polyesteritself, all three cell types show low metabolic activities on neat PLLAnanofibers[67,68] Higher cell proliferation values were found forall lignin-containing nanofibers which indicate that the antioxidantactivities may enhance the viability of the cells In locally attenu-ating cellular oxidative stress, such materials could be used as tissueengineering scaffolds In the same line, Kai D et al [68], againevaluated the antioxidant activities of lignin-PCLLA copolymers andtheir electrospun nanofibres by DPPH assay They proved thathigher lignin loaded copolymer shows the higher antioxidantproperty Even all PLLA/lignin-PCLLA nanofibers exhibit antioxidantactivity more than 70% which enables their application for bio-materials and food packaging to address issues of oxidative stress[67,68]
4.7 Unvulcanised natural rubber particlesNatural rubber (NR) is the main biopolymer and it is commonlyutilized in manyfields such as medical, tyre and glove due to its
Table 5
Constituents of different lignins [64]
Lignin type M n (g mol1) COOH
(%)
OH phenolic (%)
Methoxy (%)
Milled wood lignin steam explosion lignin 113e139
R.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 274
Trang 13good physical properties[69] Natural rubber (NR) is a bio-based
polymer which can be derived from renewable resources namely
Hevea brasiliensis The overall production of natural rubber in 2003
was 8 million tonnes with the major producers being Thailand (2.8
million tonnes), Indonesia (1.8 million tonnes), Malaysia (0.9
million tonnes) and India (0.6 million tonnes)[69] In India, 90% of
the rubber production is accounted from Kerala NR has excellent
extrudability and launderability and has a high rate of cure Apart
from its application in the tyre industry, NR is employed in the
production of thin-walled soft products with high strength This is
because; NR can crystallize easily upon stretching[69] The physical
properties of NR are enhanced by the addition offiller, chemical
alteration and blending with other polymers such as polyethene
(PE), propylene (PP) and nitrile rubber (NBR) The greenfibre and
‘natural fibre’ covers a broad range of vegetable fibres such as wood
fibre and plant-based bast, leaf, seed, stem fibre and animal fibres
such as collagen, keratin andfibroin[69,70] Recently, they are used
as reinforcements in polymer composites to improve the
me-chanical properties The naturalfibres were also blended with NR to
enhance its modulus and biodegradability properties[70] In the
past work, the raw materials were picked up from many natural
sources such as sugar beet pulp as well as eucalyptus kraft pulp
because H2SO4, HCl, HClO4, NaOH are capable to react with
cellu-losefibre[69,70] The problem with NR composite is poor adhesion
between NR and cellulosefibre The factors affecting the properties
of the NR-based polymer composite arefibre type, fibre amount,
and chemical treatment, the shape of the fibre, adhesion and
arrangement offibres in the polymer matrix[69,70]
4.8 Properties of starch, separation and applications: an overview
Starch is a sustainable and cheap biodegradable polymer[71]
After cellulose, starch is recognised as the widely available polymer
[3,71] The main sources from which starch is obtained include rice,
wheat, potato as well as corn However, the certain features of the
plants namely shape as well as size; morphology along with the
composition of different plant sources used for starch production
varies from each other[71] The United States is reported to be as
the World's top producers of starch The European countries
contribute next to the United States in starch production in the
World Both of these nations play significant contribution towards
half of the World's starch production [71e73] Native starch is
naturally composed of nano-sized blocklets which have
semi-crystalline arrangements of starch chains It is now well established
that the crystalline regime is made up of thin lamellar domains by
the intertwining of amylopectin side chains to result in the
for-mation of double helices[71e73] Such double helices are tightly
packed such that they tend to lead to the basis of crystalline
do-mains In the amorphous regime, the molecules of starch are
ar-ranged in a single-chain conformation whereas the crystalline
regime is so ordered such that the molecules of starch exist in the
double-helix state Both the amorphous as well as crystalline
re-gions get arranged resulting in the formation of the ring structure
which in turn encompasses the initiation point for the granule
[71e73] The presence of ring structure which initiates the
forma-tion of the granule is visible via optical and electron microscopy
The morphological view of starch represents granules of spherical
shape The diameter of the spherical starch granules ranges from 2
to 100mm, which depends on the botanical source from where the
starch is produced Irrespective of the source used for the
produc-tion of starch, the density is found to be consistent and reported to
be 1.5 kg/m3[72,73]
Those components include amylose, an essentially linear or
slightly branched (1e4)-a-D-glucan, which exhibits molecular
mass as high as 106 g mol1, and amylopectin, with the molecular
weight between 106 and 107 g mol1[72,73] The amylopectin is abranched polymer with a short length of (1e4)-a-D-glucan unitslinked througha-(1e6) bonds In most of the starch varieties, theamylose content varies in the range of 72e82%, whereas theamylopectin content varies from 18 to 28%[73,74] Morphologi-cally, the branched amylopectin component consists of crystallineareas and the linear amylose is mostly composed of amorphous orsemi-crystalline Amylose is therefore soluble in hot water whereasamylopectin is insoluble In Industries, the steps used for extraction
of starch from plant sources include wet grinding, washing, servingandfinally drying The white powder which resembles like flourafter its extraction from plant sources is termed as“native starch” Ifthe white powder obtained is subjected to undergo chemicaltreatment to meet significant characteristics, then it is termed as
“modified starch” [71,73] Native starch is classified into threedifferent classes namely class A, class B and Class C The XRDanalysis suggests a long-range order for the starch granules irre-spective of three different classes The chain length of amylopectinwas known to influence the crystallinity of starch biopolymer[73]
To figure out the difference between class A and class B type ofstarch, authors proposed a model which represents double-helixpacking It is found that the transition occurs from class A to class
B and vice-versa via rearrangement phenomenon of double-helixstructure It is found that the A-type adopts a closely packedarrangement in which the water molecules exist in between everydouble-helix structure[73,74] However, in the case of class B, thepacking is more open such that the water molecules exist in thecentral cavity created by six double helices Class c is reported toexhibit the combination of both the class A and class B types, which
is confirmed from the diffractogram obtained from XRD analysis[74] The class C starch is found to be present in bean starch It hasbeen reported by authors that the class B and C type starch granulesare larger in diameter than that of class A[74] The diameter of class
B and C types are found to be in the range of 400e500 nm whereasclass A type demonstrated only 25e100 nm Several reports areavailable on the investigation of structural characteristics of class Cstarch derived from pea seeds[74] The reports suggest that theclass C starch exhibit polymorphism of both the class A and class B
It has been found that the class B is present at the central part of thestarch granule whereas class A exists in the surrounding or pe-ripheral region It is found that the starch crystals belonging to class
A and C show much resistance towards acid hydrolysis as compared
to class B [73e75] It has also been reported that amylose andbranched amylopectin contribute to the amorphous regime andamylopectin with the short form of branched chains correspond tothe crystalline regime for the starch [74,75] However, it is notestablished whether the presence of side chains of amylopectinwith clustered form leads to the partial crystallinity of the starch[75] The ratio of amylose to amylopectin is dependent on the type
of plant source used for extraction In fact, the ratio also depends onthe steps involved in the extraction process In recent decades, eventhough considerable efforts are made on the investigation of themolecular structure of starch, significant information at the mo-lecular level remains unclear Starch demonstrates highly complexstructure that could be better understood when classified intoseveral levels of the organization as shown inFig 6 [74e76] Starchgranules can be gelatinized in water at lower temperatures and inalkaline solutions The starch in its paste form can be used forglueing or stiffening agent The application of starch in industriallevel is limited due to its functional difficulties
These natural limitations can be substantially solved using avariety of modifications, including physical, chemical and enzy-matic techniques [74e76] The physical methods include heattreatment to remove moisture, an annealing process, pre-gelatinization, treatment under high pressure, radiation processR.K Mishra et al / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 275