Enhanced mechanical and thermal properties of polystyrene nanocomposites prepared using organo-functionalized Ni-Al layered double hydroxide via melt intercalation technique

In comparison with pristine PS, the tensile modulus for all PS nanocomposites exhibit signi ficant enhancement with the highest tensile enhancement for PSNL 1 sample (23% higher value).. [r]

Original Article

Enhanced mechanical and thermal properties of polystyrene

double hydroxide via melt intercalation technique

Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

a r t i c l e i n f o

Article history:

Received 22 March 2017

Received in revised form

7 May 2017

Accepted 10 May 2017

Available online 17 May 2017

Keywords:

Ni-Al LDH

Polystyrene

Nanocomposites

Tensile test

Thermal degradation kinetics

a b s t r a c t

The article reports upon the preparation and characterization of organo-functionalized NieAl layered double hydroxide (LDH)-polystyrene (PS) nanocomposites Initially, pristine NieAl LDH was synthesized via the co-precipitation technique and was subsequently treated using sodium dodecyl sulfate to obtain organo-functionalized NieAl LDH (ONieAl LDH) PS nanocomposites were fabricated by melt interca-lation using a twin screw extruder in presence of ONieAl LDH nanofiller (1, 3, 5, and 7 wt.%) The PS nanocomposites were characterized for their structural, thermal and mechanical properties The dispersion and morphology of the obtained PS nanocomposites were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) Mechanical and thermal properties of the PS nanocomposites as a function of LDH content were examined by tensile tests, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) The XRD and TEM results revealed the formation of an exfoliated structure of the PS nanocomposite with 1 wt.% ONieAl LDH loading The maximum im-provements of the mechanical and thermal properties of the nanocomposites with ONieAl LDH loading over pristine PS included tensile strength ¼ 34.5% (1 wt.%), thermal decomposition temperatures (T15%)¼ 27.4C (7 wt.%), and glass transition temperature (Tg)¼ 4.3C (7 wt.%) The PS nanocomposites possessed higher mechanical strength and thermal degradation resistance compared to the pristine PS The activation energy (Ea) and reaction mechanism with respect to thermal degradation of the pristine PS and its nanocomposites were evaluated by the Coats-Redfern and Criado model, respectively

© 2017 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

Layered double hydroxides (LDHs) constitute an important class

of hydrotalcite materials Typically, LDH are expressed as [M2þ1x

M3þx(OH)2]xþ(Am)x/myH2O, with M3þ, M2þand Amdesignated

for trivalent cations (e.g Mn3þ, Al3þ, Cr3þ, Ga3þ), divalent cations

(e.g Ni2þ, Mg2þ, Co2þ, Cu2þ) and interlayer anion (e.g NO3, CO3,

Cl, OH) Being inexpensive to fabricate and environmentally

friendly, such materials are of serious interest Hence, several

au-thors targeted their applications such as catalysts and catalyst

precursors [1,2], adsorbents and ion exchangers [3,4], electrode

modifiers[5], optical materials[6], precursors for preparing CO2

adsorbents [7], fire retardant additives [8], host materials to

facilitate drug delivery[9], additives in cement[10]and polymer/ LDH nanocomposites[11] Very recently, their potential as nano-fillers for preparing polymer nanocomposites has received considerable attention due to their significantly enhanced proper-ties such as thermal stability, mechanical properproper-ties,flame retar-dency and impermeability to gas [8,12]compared with polymer matrix Usually, nanocomposites are fabricated using in-situ poly-merization, solution intercalation and melt intercalation methods [13] Compared to all other mentioned methods, melt intercalation

is promising due to high productivity, low cost, environmentally benign (elimination of organic solvents) and ease to adapt con-ventional polymer processing techniques (e.g.: extrusion and in-jection moulding)

A brief outline of fabrication aspects associated to polymer nanocomposites can be presented as follows Paul et al.[14]and Yeh

et al.[15]prepared clay based PS nanocomposites using solution intercalation technique and in-situ thermal polymerization method, respectively The authors inferred that the nanocomposites

* Corresponding author Fax: þ91 361 2582291.

E-mail address: pugal@iitg.ernet.in (G Pugazhenthi).

Peer review under responsibility of Vietnam National University, Hanoi.

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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

http://dx.doi.org/10.1016/j.jsamd.2017.05.003

2468-2179/© 2017 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

possessed enhanced thermal stability due to nanofiller

incorpora-tion in the polymer matrix Adopting melt intercalaincorpora-tion method,

Limpanart et al.[16]prepared clay based conventional and

inter-calated PS nanocomposites Solvent blending coupled with

soni-cation was adopted by Morgan et al.[17]for similar nanocomposite

fabrication The authors inferred that in the solvent blending

pro-cess, sonication contributed significantly towards exfoliation of the

nanocomposite structure Adopting emulsion polymerization

method, Noh and Lee [18]prepared the PS/clay nanocomposites

(30 wt.% MMT) and observed that glass transition temperatures (Tg)

of the nanocomposites is 5C higher than that of the pristine PS PS/

ZneAl LDH nancomposites were prepared by He et al.[19]using

solvent blending technique For the nanocomposites with 3 wt%

ZneAl LDH loading, crystallization temperature was 2 C higher

than that of the polymer For similar nanocomposites prepared by

Qiu et al.[20]using solution intercalation method, considering 50%

weight loss as a reference point, an enhancement of 17C was

re-ported for thermal stability of 20 wt.% inorganic loaded

nano-composites in comparison with the pristine PS Du et al [21]

obtained nylon 6/MgeAl LDH nanocomposites via melt

intercala-tion of nylon 6 into part of organomodified MgeAl LDH interlayers

Their XRD results demonstrated that too high LDH loading makes it

difficult for LDH layers to undergo exfoliation Chen and Wang[22]

prepared polypropylene (PP) composites by melt blending method

and studied the thermal decomposition kinetics of the

nano-composites They utilized Coats-Redfern and Criado model to

mea-sure the activation energy and determine the reaction mechanism of

PP nanocomposites, respectively

Given significant fabrication research emphasis in the

inorganic-polymer nanocomposites, it can be observed that the

synthesis of PS/ONieAl LDH nanocomposites using melt

interca-lation has not been conducted till date The work primarily focuses

upon sodium dodecyl sulfate based modification of pristine NieAl

LDH for the enhancement of its compatibility with the polymer

matrix The subsequent section addresses the experimental

methods adopted towards the synthesis of said PS nanocomposite

for targeted enhancement in thermal and mechanical properties

2 Experimental

2.1 Materials

PS polymer was procured from National Chemicals Ltd., Gujarat

(India) Nickel nitrate (Ni(NO3)2$6H2O), sodium nitrate (NaNO3),

aluminum nitrate (Al(NO3)3$9H2O), sodium hydroxide (NaOH),

sodium dodecyl sulfate (SDS), methanol (CH3OH) were purchased

from Merck India Ltd

2.2 Preparation of ONieAl LDH

The pristine NieAl LDH was prepared with co-precipitation

technique The solution mixture containing nickel, aluminum and

sodium nitrate was synthesized with the molar ratio of 2:1:2,

respectively Thereby, 2 M NaOH solution was added to the mixture

under constant stirring condition to enhance the solution pH to 10

Eventually, stirring was continued for 16 h at ambient temperature

condition After this step,filtration was carried out and the retained

precipitate was washed thoroughly using Millipore water (Model:

Elix 3, Milli-Q; Make: M/s Millipore, USA) up to achieving neutral

pH condition of the washed solution After 16 h of room

tempera-ture stabilization, the precipitate was collected in a petridish

fol-lowed by drying for 16 h at 60C For further use, dried pristine LDH

was thoroughly grinded to result as a powder

The prepared pristine NieAl LDH was organo-functionalized

through regeneration method to obtain ONieAl LDH To do so,

thefirst step involved calcination of 2.5 g of pristine NieAl LDH powder at 500C (using Box furnace) with 10C/min heating rate and atmospheric pressure The calcined gray colored powder was dissolved in 120 mL of 2.5 g SDS containing solution and was

refluxed for 12 h at 80C After this step, the residue was separated using a centrifuge and washed thoroughly using Millipore water to remove adhering SDS Finally, after drying at 70C, the ONieAl LDH sample was grounded as powder to serve as a starting material for

PS nanocomposite fabrication

2.3 Preparation of PS/ONieAl LDH nanocomposites

A co-rotating twin-screw extruder was used to prepare PS/ ONieAl LDH nanocomposites using melt compounding method Initially, the moisture from pristine PS and ONieAl LDH was removed by drying the grounded samples for 16 h at 60C and

70C, respectively Further, the ONieAl LDH (1, 3, 5 and 7 wt.% with respect to pristine PS) was dispersed in 100 mL methanol (see Table 1) The solution was sprayed on a precise quantity of pristine

PS pellet for ensuring uniform distribution of ONieAl LDH on the pristine PS matrix Subsequent drying step for 24 h facilitated methanol evaporation from the samples The coated PS pellets were fed to the extruder (Model: ZV-20 HI-Torque; Make: Specifiq En-gineering and Automats, India) that was operated at temperature values of 185, 195, 210 and 200C, for feed, metering, compression and adaptor zones, respectively After operation, extrudate was thoroughly quenched with water at ambient condition and was cut into pellets and dried using hot air oven at 60C for 24 h There-after, dried pellets were fed to injection molding equipment (Model: 180 High Pressure; Make: JSW, Japan) that was operated at

195e210 C This step was followed to achieve samples for me-chanical characterization studies As controls and reference set, a clean PS sample was also prepared using similar procedure without ONieAl LDH In summary, the experimental investigations allowed fabrication of samples designated as NieAl LDH, PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7 to represent nickel-aluminum LDH, polystyrene, PS/ONieAl LDH 1 (wt.%), PS/ONieAl LDH 3 (wt.%), PS/ONieAl LDH 5 (wt.%), and PS/ONieAl LDH 7 (wt.%), respectively

2.4 Characterization X-ray diffractometer (Model no: D8 Advance; Make: Bruker, Germany) facilitated with Cu-Karadiation (0.15406 nm wavelength) and Nifilter at room temperature was used to obtain XRD profiles of ONieAl LDH, pristine PS and PS nanocomposites Further, the structural morphology of the PS nanocomposites was evaluated with transmission electron microscopy (TEM) analysis, using TEM instrument (Model: JEM 2100; Make: JEOL, Japan) operated at

200 kV accelerating voltage Fourier transform infrared (FTIR) ana-lyser (Model: IRAffinity1; Make: Shimadzu, Japan) was used to re-cord FTIR spectra at room temperature in the range 400e4000 cm1. Mechanical characterization studies involved evaluation of impact strength, tensile, and flexural properties of the nanocomposites Specimens having dimensions of 168 13  3 mm3of pristine PS and the nanocomposites were evaluated for tensile strength and

Table 1 Preparation chart for PS/ONieAl LDH nanocomposites.

K Suresh et al / Journal of Science: Advanced Materials and Devices 2 (2017) 245e254 246

modulus using ASTM D 638 method and INSTRON (M 3382, UK)

universal testing machine The nanocomposites along with pristine

PS specimens of dimension of 126 13  3 mm3were taken for

flexural tests were carried out using universal testing machine

INSTRON (M 3382, UK) and ASTM D790 test method Impactometer

(M/s Tinius Olsen, USA) was used to conduct impact strength

mea-surements (specimens having dimension of 62 13  3 mm3) For

each case, six specimens were tested and average value has been

reported Thermal stability of the prepared samples was determined

under nitrogen atmosphere with aflow rate of 60 mL/min using

high temperature thermo-gravimetric (TG) analyser (Model no: TGA

851e/LF/1100; Make: Mettler Toledo, Switzerland) at 10 C/min

heating rate and 30e700C temperature range DSC instrument

(Model no: 1; Make: Mettle Toledo, Switzerland) operated at 5C/

min heating rate under nitrogen environment (and 40 mL/min) was

used to determine glass transition temperature of pristine PS and

other nanocomposite samples

3 Results and discussion

3.1 XRD analysis

XRD analysis facilitates the evaluation of the degree of

interca-lation and exfoliation in LDHs based nanocomposites In such

ma-terials, these features are dependent upon several factors such as

LDH composition, organic modifier's chemical nature and method

of fabrication For intercalated nanocomposites, d-spacing (defined

as d003or 003 peak is shifted to lower angle) is higher than that of

the original LDH The exfoliated nanocomposites exhibit well

separated inorganic layer from one another with no peaks

corre-sponding to basal plan (003) and good distribution of nanofillers in

the polymer matrix The intercalation extent of LDH in PS is based

on the LDH interlayer spacing evaluated using Bragg's law (gallery

height or d003spacing determined using expression nl¼ 2dsinq,

where n¼ 1 andlcorrespond to X-ray wave length (1.5406 Å))

[23]

Fig 1depicts XRD diffractograms of ONieAl LDH, pristine PS and

PS/ONieAl LDH nanocomposites The XRD patterns convey that for

ONieAl LDH, d003 peak exists at 6.54 with a basal spacing of

1.35 nm (see Fig 1(a)) The amorphous nature of pristine PS is

characteristically reflected in two small halos that are centered at

2qvalues of 10 and 20[24] No peaks corresponding to d003 at

lower angles can be observed for all PS nanocomposite samples

(Fig 1(cef)), thereby conveying that the LDH layers might be

exfoliated or delaminated in the PS matrix However, to further confirm such ambiguity, electron microscopy based analysis is to be considered for the nanocomposite morphology

3.2 TEM analysis For the PS nanocomposites, the extent of dispersion and dis-tribution of ONieAl LDH fillers in the polymer matrix has been investigated using TEM microscopy.Fig 2depicts the LDH layers microstructure in the PS matrix.Fig 2(a) depicts TEM image for PSNL 1 sample and the pertinent dark lines and bright region signify LDH galleries and PS matrix, respectively Black arrows have been presented in thefigure to indicate LDH layers Based on the image analysis, it can be inferred that LDH layers got totally exfo-liated in the matrix and do not exhibit ordered stack structure The TEM image of PSNL 3 sample depicted inFig 2(b) involves arrows and circle mark to signify exfoliated and intercalated structure, respectively Thus, the TEM image of the sample conveys that the sample refers to the partially exfoliated structure Similar morphology with mixed exfoliation characteristics was reported by Alansi et al.[25]for PS/LDH nanocomposites with 4% loading of inorganicfiller (MgeAl LDH) The TEM image of PSNL 5 and PSNL 7 nanocomposites depicted inFig 2(c) and (d), respectively consti-tute circles indicating intercalated regions and stacked LDH layers This conveys that the intercalated structure is attained at higher doping of the nanofiller Similar intercalated structures have been reported by Qiu et al.[20]for PS/ZneAl LDH nanocomposites for higher values of inorganicfiller loading

3.3 FTIR analysis The FTIR analysis involved comparative assessment of FTIR spectra of PS nanocomposites and pristine PS Fig 3depicts the spectra obtained from FTIR analysis for ONieAl LDH, pristine PS and

PS nanocomposites For ONieAl LDH (Fig 3(a)), lattice vibration bands have been observed at 400-800 cm1(NieO, AleO, OeAleO modes) [26] The ONieAl LDH has a characteristic peak at

1063 cm1(asymmetric vibration (vo S ]O) of sulfate from dodecyl sulfate anion), 1218 cm1 (symmetric vibration (vS ]O) of sulfate from dodecyl sulfate anion)[27], 2850 cm1(stretching vibration of

CH3found in the modifier SDS), 2920 cm1(stretching vibration of

CH2found in the modifier SDS), 3528 cm1(OeH stretching vi-bration of metal hydroxide layer and interlayer water molecules) [25].Fig 3(b) affirmed upon the existence of several absorption bands in pristine PS These refer to the wavelengths of 698 cm1 (monosubstituted benzene), 1368, 1453 cm1(vibrational mode of

CH2 bending), 1496, 1504 cm1 (C]C bending vibration),

2930 cm1(aliphatic CeH stretching vibration), 3070 cm1 (aro-matic CeH stretching vibration) Compared with ONieAl LDH (Fig 3(a)) and pristine PS (Fig 3(b)), the PS nanocomposite (PSNL 1, PSNL 3, PSNL 5 and PSNL 7) samples (Fig 3(cef)) exhibit fewer new absorption peaks at 1218 cm1(symmetric vibration of sulfate from dodecyl sulfate anion), 3528 cm1(OeH stretching vibrations) and

400e800 cm1 (lattice vibration bands (NieO, AleO, OeAleO modes)) The FTIR analysis of various samples and their compara-tive assessment indicates that LDH layers are well dispersed in the

PS polymer matrix Similar trends have been reported by Wang

et al.[28]for MMT dispersed in PS polymer matrix

3.4 Tensile properties

A primary goal of LDH nanofiller reinforcement into the PS matrix is to enhance mechanical properties of the nanocomposites

in conjunction with the pristine PS As LDH loading increases, tensile strength and modulus of nanocomposites increases This is Fig 1 XRD patterns of (a) ONieAl LDH, (b) pristine PS, (c) PSNL 1, (d) PSNL 3, (e) PSNL

due to the incorporation of LDH into the composite system which

enhances stiffness and rigidity of polymer nanocomposites A sharp

enhancement in tensile modulus for very small LDH loading

(1 wt.%) can be observed in thefigure However, the subsequent

enhancement in tensile modulus was not significant upto 7 wt.% LDH loading In this regard, it can be observed that Genity et al.[29] conveyed that thefiller modulus, loading and aspect ratio are three main factors to enhance the composite modulus A composite with high stiffness needsfiller particles possessing higher combinations

of modulus and aspect ratio at higherfiller loading In line with this hypothesis, in the carried out work, with minimal LDH loading (1 wt.%), significant enhancement in tensile modulus of the PS/ ONieAl LDH nanocomposites has been possible This is possibly due

to higher combinations of aspect ratio and LDH modulus.Fig 4 depicts the tensile strength and modulus profiles of PS and PS/ LDH nanocomposites Compared to pristine PS and all other nanocomposites, PSNL 1 sample exhibited highest tensile strength

of 39.01 MPa (36.29 MPa for PSNL 3, 32.30 MPa for PSNL 5, 30.22 MPa for PSNL 7 and 29.01 MPa for PS samples) Stronger interfacial interactions (between ONieAl LDH and PS) facilitated due to maximum exfoliation in the structure are responsible for maximum tensile strength values at lower LDH loading (1 wt.%) Similar trends have been reported by Li et al.[30]for ZnO nano-particles embedded polyurethane polymer composites The mar-ginal reduction in tensile strength for higher loading of inorganic nanofiller (3e7 wt.%) is possibly due to non-uniform distribution of LDH layers in the polymer along with agglomeration and segre-gation to thereby allow formation of weak spots in the nano-composites Similar behavior has been reported by Uthirakumar

Fig 2 TEM images of (a) PSNL 1, (b) PSNL 3, (c) PSNL 5 and (d) PSNL 7 nanocomposites.

Fig 3 FTIR spectra of (a) ONieAl LDH, (b) pristine PS, (c) PSNL 1, (d) PSNL 3, (e) PSNL 5

and (f) PSNL 7 nanocomposites.

K Suresh et al / Journal of Science: Advanced Materials and Devices 2 (2017) 245e254 248

et al.[31] for clay loaded PS polymer nanocomposites Further,

Tanniru et al.[32]elaborated that clayfiller aggregation in

poly-ethylene nanocomposites facilitates concentration of stressfield at

the aggregates thereby triggering easy crack propagation and rapid

premature failure

The tensile modulus profiles of pristine PS and its

nano-composites have been demonstrated inFig 4 In comparison with

pristine PS, the tensile modulus for all PS nanocomposites exhibit

significant enhancement with the highest tensile enhancement for

PSNL 1 sample (23% higher value) PSNL samples with higher

inorganicfiller loading (3e7 wt.%) exhibit marginally lower tensile

strength than PSNL 1 sample Reasons for the same are similar to

those presented for the tensile strength profiles

3.5 Flexural properties

Fig 5depicts trends inflexural strength and modulus of pristine

PS and PS nanocomposites (1e7 wt.% ONieAl LDH) The trends

confirm that PS/ONieAl LDH nanocomposites exhibit improved

tensile strength in comparison with pristine PS While pristine PS

possessed 54.08 MPaflexural strength, PSNL 1 possessed highest

flexural strength of 68.95 MPa followed by PSNL 3 (64.46 MPa),

PSNL 5 (59 MPa) and PSNL 7 (55.59 MPa) samples This is due to

homogeneous and uniform distribution of LDH in the polymer

matrix that resulted in maximum exfoliation in the nanocomposite

structure at 1 wt.% loading of the inorganic nanofiller At higher

loading, LDH agglomeration might have reduced the flexural

strength Also, several factors such as extent of intercalation,

orientation and distribution of LDH platelets in stress/load

direc-tion have been reasoned to influence the flexural properties of

polymer nanocomposite materials Chow et al.[33]also inferred upon the enhancement in flexural strength for nanocomposites compared with pristine polymer

Fig 5depicts trends in flexural modulus of PS and PS nano-composite Similar trends have been obtained forflexural modulus, given the fact that flexural strength of both pristine PS and PS nanocomposites underwent similar variations with variant con-centration of nanofiller loading The PSNL 1 nanocomposite possessed 12% higherflexural modulus value than that of the PS polymer PSNL 3, 5 and 7 samples with higher ONieAl LDH loading did not indicate even significant enhancement Reasons for such trends are similar to those presented for theflexural strength 3.6 Impact strength

Fig 6shows impact strength profiles for PS and PS/ONieAl LDH nanocomposites Compared to pristine PS, the impact strength for all PS nanocomposites enhanced significantly The impact strength for PS/ONieAl LDH sample with 1 wt.% LDH loading is 22% higher in comparison with pristine PS Yuan et al [34] reported 50% enhancement in impact strength for 0.32 wt.% nanofiller loaded PS-m-MWCNT nanocomposites Similarly, Yilmazer and Ozden [35] opined that 1.6 wt.% organoclay loaded PS nanocomposites possessed 4% higher impact strength than that of pure PS For PSNL

1 sample uniform and homogenous distribution of LDH in the polymer matrix is reasoned to be crucial for measured enhance-ment in the impact strength For higher loading of ONieAl LDH cases (3, 5 and 7 wt.%), impact strength did not enhance signi fi-cantly compared to PS samples and was lower than that of the PSNL

1 sample Non-uniform distribution of inorganic filler and agglomeration are possible reasons for the reduction in impact energy absorption for the samples with higher inorganic loading 3.7 DSC analysis

The DSC analysis was conducted to evaluate the effect of nanofiller on the molecular mobility of PS chains in the polymer nanocomposite This is typically reflected in the glass transition temperature (Tg) of the sample For various samples including pristine PS and PS composites, results obtained from DSC analysis have been presented inFig 7and the Tghas been evaluated as the value corresponding to inflection point of onset and end-set tem-perature profiles For PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7, the Tg values are 68.1, 70.5, 71.2, 72.1 and 72.4C, respectively This in-dicates that ONieAl LDH addition into the PS matrix enhanced Tg Fig 4 Tensile properties of pristine PS and PS nanocomposites.

gradually with increasing inorganicfiller content For the PSNL 7

sample, the Tgvalue is 4.3C higher that of the pristine PS sample

This is probably due to the restriction or hindrance of polymer

chain motion in the nanocomposite that has been brought forward

due to inclusion of the inorganic nanofiller Similar insights have

been presented by Zidelkheir et al [36] who inferred that the

10 wt.% clay-PS nanocomposites possessed 4C higher Tgvalues

than the pristine PS polymer

3.8 TGA analysis

TGA analysis was conducted to evaluate the comparative

ther-mal degradation/stability of PS/ONieAl LDH nanocomposites in

comparison with pristine PS Fig 8 (a) depicts TGA profiles of

pristine PS, ONieAl LDH and all nanocomposite samples For the

ONieAl LDH sample, significant weight loss from room

tempera-ture up to 200C is due to evaporation of physically adsorbed and

interlayer water molecules (Fig 8(1)) Subsequent weight loss from

200 to 350C is due to decomposition of interlayer dodecyl sulfate

Above 350 C, weight loss occurred due to LDH degradation to

NieAl oxides Considering 15% mass loss as reference point, the

thermal decomposition temperature of ONieAl LDH is 639 C,

which is in agreement with the value reported in the literature

[37,38]

For pristine PS sample (Fig 8(2)), thermal degradation occurs at

345e445C and residue did not exist above 480C. Fig 8(3e6)

depicts TGA curves of PSNL 1, PSNL 3, PSNL 5 and PSNL 7

nanocomposite samples The PS nanocomposite TGA profiles involved simpler variations in comparison to the pristine PS TGA profiles This is due to PS nanocomposite degradation in two stages namelyfirst stage decomposition (135e330C) and second stage decomposition (330e460 C) The first stage decomposition occurred due to evaporation of physically adsorbed and interlay-ered water molecules and thermal degradation of sodium dodecyl sulfate molecules The second stage involved thermal decomposi-tion of polymer molecules to form charred (black) residues The TGA curves also exhibit thermal degradation rate of PSNL 1, 3, 5 and

7 samples and convey that these are slower than that of the pristine

PS sample This is due to significant interactions between pristine

PS and LDH layers that contribute towards higher diffusional resistance for oxygen and volatile compounds[8] Beyond 500C, the TGA curves for all samples areflat thus indicating that only inorganic constituents are left behind in the sample

Quantitative information with respect to mass loss at various temperature values has been presented inTable 2for pristine PS and all polymer nanocomposite samples It could be inferred from Table 2that the degradation temperature (Td) corresponding to 15% mass loss (T15%) of PS nanocomposites samples depicts significant enhancement with respect to pristine PS, thereby indicating increased thermal stability Moreover, the enhancement can be observed to be proportional to ONieAl LDH content in the PS polymer The T15% for pristine PS is 358 C, which got further enhanced by 11, 20.1, 25.3 and 27.4C for PSNL 1, 3, 5 and 7 samples respectively The observed enhancement in higher degradation temperature is due to the incorporation of the inorganic nanofiller

in PS polymer matrix [39] Among all nanocomposites, PSNL 7 exhibited highest thermal stability (Table 2) Such behavior is reasoned due to the presence of barrier effect of LDH lamellar layers which limit emission of produced degradation gases and heat transmission and eventually result in improving thermal stability of nanocomposite materials In the nanocomposite, volatile products have to take a long and tortous path around impermeable clay platelets that are distributed in PS matrix as compared to pristine

PS, where the diffusion of volatile products is much easier Fig 7 DSC analysis of pristine PS and PS nanocomposites.

Fig 8 (a) TGA curves of (1) ONieAl LDH, (2) pristine PS, (3) PSNL 1, (4) PSNL 3, (5) PSNL 5 and (6) PSNL 7 nanocomposites (Inset shows the TGA profiles between 335 and 435  C).

Table 2 Thermal degradation temperatures of pristine PS and PS nanocomposites.

Sample Temperature at 15% weight loss (T 15 )  C DT 15% (  C) T max (  C)

K Suresh et al / Journal of Science: Advanced Materials and Devices 2 (2017) 245e254 250

Fig 8(b) illustrates DTG curves of PS and PS nanocomposites.

The peaks inFig 8(b) convey the maximum degradation

temper-ature (Tmax) of the prepared samples It could be inferred from

Fig 8(b) that Tmaxpeaks of the DTG curves of PS nanocomposites

exhibit shift towards higher temperature as compared to pristine

PS, thus indicating enhancement in thermal stability The Tmaxfor

pristine PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7 nanocomposites

have been evaluated to be 415.7, 418.1, 420.5, 421.7, and 422.5C,

respectively For the PSNL 7 nanocomposite, Tmaxis 6.8C higher

than that of pristine PS (seeTable 2) Thus, both TGA and DTG

analysis based trends essentially confirm upon enhanced thermal

stability of the polymer due to the incorporation of LDH layers in the polymer matrix

3.9 Kinetics analysis 3.9.1 Coats-Redfern method for kinetics analysis The kinetic analysis of thermal degradation process for pristine

PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7 nanocomposites has been conducted to understand degradation behavior of composites in comparison with pristine PS Coats-Redfern method [40] was deployed for the kinetic analysis Compared to other kinetic

Fig 9 Determination of kinetic parameters by plots of the left part in Equation (1) against 1/T using Coats-Redfern method.

models, the method requires only single but not multiple heating

data Relevant model expressions in the method are presented as

follows:

lnð1 aÞ

T2



¼ ln

 AR

bEa

Ea



Ea

ð1  nÞT2

!

¼ ln

 AR

bEa

Ea



Ea

where, Ea, n, A, R and T refer to apparent activation energy, order of reaction, pre-exponential factor, gas constant and absolute tem-perature, respectively

Using the Coats-Redfern method, for each TG curve corre-sponding to a precise heating rate, kinetic parameters (Ea, n and A) can be obtained To do so, the left side expressions in Equations(1a) and (1b)have been plotted with respect to1for visualization as a straight line plot with slope and intercept as Activation Energy and pre-exponential factor, respectively In this regard, it is important to note that the intercept is determined by considering the expression

12RT

E a



as 1 To determine the value of n, different values of n

Table 3

Thermal degradation kinetics of pristine PS and PS nanocomposites obtained from

Coats-Redfern method.

Fig 10 Determination of the thermal degradation mechanism by plotting Z(a) versusausing Criado model: (a) pristine PS, (b) PSNL 1, (c) PSNL 3, (d) PSNL 5 and (e) PSNL 7 nanocomposites.

K Suresh et al / Journal of Science: Advanced Materials and Devices 2 (2017) 245e254 252

have been assumed and regression coefficient of plots

corre-sponding to various n values have been compared and the n value

for which best regression coefficient value was obtained has been

inferred as the n value for the measured data

Fig 9depicts linearfitness plots of Coats-Redfern model for PS,

PSNL 1, PSNL 3, PSNL 5 and PSNL 7 nanocomposites and different n

values The obtained kinetic parameter values including A, Eaand

n for the prepared samples are enlisted inTable 3 The evaluated

Eavalues for PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7 samples are

88.1, 108.5, 125.4, 132.2, and 134.8 kJ/mol Thus, the activation

energy for nanocomposite degradation enhanced by 20.4e46.7 kJ/

mol in comparison with the value obtained for pristine PS This is

due to the enhanced thermal stability of the nanocomposite

samples

3.9.2 Criado method for reaction mechanism determination

Criado method[41]involves reaction mechanism determination

using Ea, A and n evaluated from Coats-Redfern method Relevant

expressions for the solid-phase reactions are as follows:

ZðaÞ ¼b

AgðaÞda

dte

Ea

ZðaÞ ¼da

dt

Ea

Re

Ea

where, PðxÞ ¼exx x4þ 20xx3þ 18x3þ 120x2þ 86x þ 962þ 240x þ 120

In the above equations, while Equation (2)is used to obtain

master Z(a) versusacurves[42], Equation(3)is used to represent

experimental Z(a) versusacurve Thereby, comparative assessment

of master Z(a) versusacurve with experimental Z(a) versusacurve

to predict pertinent reaction mechanism of thermal degradation

process Analysis was conducted for pristine and all nanocomposite

samples

Figs 10(aee) illustrate master and experimental Z(a) versusa

curves for PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7 samples For

pristine PS (Fig 10(a)), the experimental curve is in good agreement

with the master curve of Z(F1), thus indicating that the pristine PS

thermal degradation process follows F1 reaction mechanism

(random nucleation of one nucleus on individual particle) For the

nanocomposite samples (Figs 10(bee)), the initial phase involved

thermal decomposition as per F1 reaction mechanism (a < 0.4)

followed with gradual transition to A4 mechanism (nucleation and

growth for a ¼ 0.7e0.9) Thus, it is apparent that the thermal

degradation mechanism involved a shift at higher temperature

3.10 Integral procedural decomposition temperature

Integral procedure decomposition temperature (IPDT) method

[43]was used for thermal stability evaluation of prepared pristine

PS and PSNL 1, PSNL 3, PSNL 5 and PSNL 7 nanocomposite samples

Relevant expressed as presented as follows:

where, N¼ ðR1 þR 2 Þ

ðR 1 þR 2 þR 3 Þ

B¼ðR1þ R2Þ

ðR1Þ ;

where N, Tfand Tirefer to area ratio of total experimental curve

specified by the total TGA thermogram, the final and initial

experimental temperature Fig 11 depicts a graphical

representation of typical TGA thermogram in terms of three areas (R1- R3) Using parameters of these regions, the IPDT values for all samples were determined using Equation(4) The IPDT values for

PS, PSNL 1, PSNL 3, PSNL 5 and PSNL 7 samples are 379.1, 387.1, 401.1, 408.9 and 413.2C Thus, PS composites possess higher IPDT values in comparison with pristine PS and hence comparatively improved thermal stability Among all samples, PSNL 7 possessed highest IPDT values (Table 3)

4 Conclusion Melt intercalation facilitated in twin screw extruder equipment was followed in this work to fabrication PS nanocomposites with variant constitution of ONieAl LDH (1, 3, 5 and 7 wt.%) XRD and TEM analyses affirmed good dispersion of ONieAl LDH layers in the

PS matrix TGA analysis affirmed enhanced thermal decomposition temperature for samples with enhanced inorganic nanofiller con-tent Compared to pristine PS, enhancement in tensile strength, flexural strength and impact strength is 34.5%, 27.5% and 22%, respectively With 15% mass loss as reference point, thermal decomposition temperature of PS nanocomposites has been determined to be 11.0e27.4C higher than that of the pristine PS polymer The DSC graphs revealed marginal improvement in the glass transition temperature (2.4e4.3C) for the nanocomposites. The Coats-Redfern method based activation energy values have been estimated to be 20.4e46.7 kJ/mol higher than that of the pristine PS Reaction mechanism of nanocomposites using Criado method indicated F1 reaction mechanism at lower degradation temperature followed with A4 reaction mechanism at higher temperature The IPDT value of PS nanocomposites increases with

an increase in the ONieAl LDH concentration confirmed by TGA data In summary, PS/ONieAl LDH nanocomposites possessed su-perior mechanical strength and thermal stability parameters in comparison with pristine PS

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