Effects of Porogen on Structure and Properties of Poly Lactic AcidHydroxyapatite Nanocomposites (PLAHAp)

15, 1–10, 2015 www.aspbs.com/jnn Effects of Porogen on Structure and Properties of Poly Lactic Acid/Hydroxyapatite Nanocomposites PLA/HAp Dinh Thi Mai Thanh1 ∗, Pham Thi Thu Trang1, Ngu

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Printed in the United States of America

Article Journal of Nanoscience and Nanotechnology

Vol 15, 1–10, 2015

www.aspbs.com/jnn

Effects of Porogen on Structure and Properties of Poly Lactic Acid/Hydroxyapatite Nanocomposites (PLA/HAp) Dinh Thi Mai Thanh1
∗, Pham Thi Thu Trang1, Nguyen Thi Thom1, Nguyen Thu Phuong1,

Pham Thi Nam1, Nguyen Thi Thu Trang1, Jun Seo-Park2, and Thai Hoang1

1Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay, Hanoi, Vietnam

2Department of Chemical Engineering Hankyong National University, 327 Jungang-ro, Anseong-si, Gyeonggi-do, 456-749, Korea

PLA/md-HAp/PEO porous nanocomposites for applications in bone engineering from poly lactic

acid (PLA) incorporated with different NH4HCO3porogen content were prepared by solvent casting

method The porosity, morphology and mechanical properties of the nanocomposites were

deter-mined The obtained results showed that the porosity of the nanocomposites increases from 10 to

49% with the increase of NH4HCO3porogen content from 0–30 wt% However, their Young’s

mod-ulus decreased 78% in comparison with the nanocomposite without using NH4HCO3porogen The

bioactivity of the nanocomposite with 20 wt% NH4HCO3porogen was evaluated by examining the

formation of hydroxyapatite (HAp) on its surface when being immersed in simulated body fluids

(SBF) solution The in vitro degradation behavior of the nanocomposites immersed in the SBF

solu-tion at 37C was systematically monitored at different time periods of 1, 3, 7, 14, 21 and 28 days

SEM images showed the formation of hydroxyapatite on the surface of the nanocomposite after

1 immersion day in the SBF solution The measurements of weight loss, pH solution, and XRD

of the samples indicated that PLA/md-HAp/PEO nanocomposite without NH4HCO3 porogen was

degraded more slowly than the PLA/md-HAp/PEO nanocomposite with 20 wt% NH4HCO3porogen

Keywords: Hydroxyapatite (HAp), Modified Doped Hydroxyapatite (md-HAp), Poly Lactic Acid

(PLA), Porogen, PLA/HAp Nanocomposite, Solvent Casting Method, Simulated Body Fluids (SBF)

1 INTRODUCTION

Hydroxyapatite (Ca10(PO4
6(OH)2, HAp) has been

recog-nized as a promising bone substitute thanks to its

chemi-cal and biologichemi-cal similarities to the mineral phase of the

native bones This bioceramic has been used for several

years for medical applications.1 2 However, HAp being

synthesized artificially did not have mechanical properties

which are necessary for applying in bone implants One

of the solutions to solve the above problem is to develop

biocomposites such as HAp/metal, HAp/polymer3–5which

have been widely used in medicine and stomatology for

the repair of bone tissue HAp/polymer composite has

more advantages than original HAp or neat polymer.6

Polymer phase is able to have the same chemical

compo-sition as the polymer in bone tissue (collagen) but it could

be synthesized as well.2 5–10So far, special attentions have

∗ Author to whom correspondence should be addressed.

been paid to biodegradable polymer applied in surgery and bio-medicine in general

Poly(-hydroxyesters) such as poly(lactic acid) (PLA),

poly(glycolic acid) (PGA) and their copolymers have been widely used to fabricate different kinds of scaffolds in tissue engineering because of their good biodegradability, bio-compatibility and feasibility.11–17 However, there are few problems when using these polymers for tissue engi-neering in practice One of the limitations of these poly-mers is the lack of bioactivity so that the new bone tissue cannot bond to the polymer surface tightly when they are applied for the bone tissue engineering.4 Another prob-lem is their high hydrophobicity.18A previous study had shown that the adhesion rate of human endothelial cells on PLA is much lower than on the polystyrene The reason is the contact angle of PLA (71) is higher than that of the polystyrene (35).19

Recently, nanocomposite of nano HAp and PLA (PLA/HAp) has attracted much attention from researchers

because of their ability in replacing the metal and alloy

implants Compared with HAp in the micron range,

the nano-HAp has a larger surface area which exhibits

enhanced mechanical properties due to the strong

hydro-gen bonding interactions between the nano-HAp and

PLA.20 21 The dispersion of HAp in the PLA matrix is

one of critical factors determining the properties of the

PLA/HAp nanocomposite There are many methods to

fabricate this composite such as emulsion method, melt

mixing, high pressure processing, electrospinning,

sol-vent casting method which have their own advantages

and disadvantages.22–24 The solvent casting method is the

facility of preparation and operation without any

special-ized equipment Fabrication of PLA/HAp

nanocompos-ite by the solvent casting method has been developed

by many researchers.25–27 In order to be applied in bone

implant, PLA/HAp nanocomposite needs to have

compat-ibly mechanical stability, mechanical strength and highly

open porous structure which are necessary to develop

tis-sue fluids; the size and distribution of pore should be

suitable for cell in-growth.28–30 Several techniques have

been developed to fabricate porosity materials, including

porogen leaching,31–35 gas expansion,36 emulsion

freeze-drying,37thermally induced phase separation38–41 and

3D-printing,42 43 etc Compared with other techniques, the

porogen leaching technique controls pore structure easily

and has been well established in the preparation of porous

nanocomposite Xu et al fabricated composite scaffolds

for application in bone engineering from poly(D,L-lactide)

(PDLLA) incorporate with different proportional

bioac-tive wollastonite powders through a salt-leaching method,

using NH4HCO3 as porogen.44 In vitro bioactivity of

PLA/HAp nanocomposites can be evaluated by immersing

the material in saline,45phosphate buffered saline (PBS)46

and the simulated body fluids (SBF).34 47 48

In this study, the porous PLA/HAp nanocomposites

with different contents of NH4HCO3 porogen were

pre-pared by the solvent casting method The

characteriza-tion, properties including IR spectra, water contact angle,

tensile property, porosity morphology and phase structure

of the nanocomposites were investigated The formation

of HAp on the surface of the nanocomposites immersed

in the SBF solution and their weight change were also

discussed

2 MATERIALS AND METHODS

2.1 Materials

Poly lactic acid (PLA) was provided by Nature

Works-USA (weight-average molecular weight Mw =

250105 g/mol, density d = 124 g/cm3) Poly(ethylene

oxide) (PEO) was provided by Sigma Aldrich

(aver-age molecular weight Mw= 105 g/mol) Calcium nitrate

tetrahydrate (Ca(NO3
2· 4H2O, M= 23615 g/mol, 99%

pure), magnesium nitrate hexahydrate (Mg(NO3
2· 6H2O,

M = 25641 g/mol, 99% pure), zinc nitrate

hex-ahydrate (Zn(NO3
2· 6H2O, M = 29749 g/mol, 99%

pure), diammonium hydrogen phosphate ((NH4
2HPO4,

M= 13206 g/mol, 99% pure), ammonium bicarbonate

(NH4HCO3, M = 7906 g/mol), tetrahydrofuran (THF,

C4H8O, M = 7412 g/mol, 95.5% pure), lactic acid

(C3H6O3, M = 9008 g/mol, 85.5–90% pure), xylene

(C8H10, M= 10617 g/mol, 99% pure) were purity

mate-rials of China

2.2 Preparation of Doped HAp

The nano-spherical HAp powder doped with magnesium

and zinc (d-HAp: 13–22 nm) was synthesized by the

chemical precipitation method at room temperature The (NH4
2HPO4 aqueous solution was added drop by drop into [0.4 M Ca(NO3
2· 4H2O, 0.05 M Mg(NO3
2· 6H2O, 0.05 M Zn(NO3
2· 6H2O] aqueous solution (the ratio Ca/Mg/Zn of 9/0.5/0.5) at a rate of 1 ml·min−1 during 2 h under strong stirring (750 rpm) The M/P ratio was 1.67 (M= Ca, Mg, Zn) The pH of the mixture solution was adjusted to 10 by adding NH4OH solution The process was performed within 2 h by stirring, then within 24 h without stirring at room temperature The precipitate was washed for several times with distilled water to pH 7 The

obtained doped-HAp powder (d-HAp) was dried at 80C for 48 h

2.3 Preparation of Modified Doped HAp

The reaction system was prepared as following: 20 g of

d-HAp powder was dispersed in 70 ml THF via

stir-ring, heating to 65 C Lactic acid (LA) was added drop by drop into the above reaction mixture system for

30 minutes (d-HAp/LA = 1/2 wt/wt) and then 180 ml of

xylene was added The resulted suspension was heated to

150C and stirred for 8 h Then, the modified doped HAp

(note md-HAp) was obtained through filtering and being

washed with ethylene ether for several times to remove the

adsorbed solvent on md-HAp.

2.4 Fabrication of PLA/md-HAp/PEO

Nanocomposites

The PLA/md-HAp/PEO nanocomposites were made by the solvent casting method The md-HAp, PEO and NH4HCO3 powders were dispersed in 30 ml dichloromethane (DCM)

by stirring in 30 minutes Ammonium bicarbonate salt (NH4HCO3) was used as a porogen at different contents (0, 3, 7, 10, 20 and 30 wt%) The PLA was dissolved

in 70 ml DCM in 30 minutes And then, combining two above mixtures together by stirring (110 rpm) during 2 h,

to form a gel paste mixture The gel paste mixture was then put into a die (4× 5 cm) and compressed at a pres-sure of 10 MPa for 2 minutes at room temperature After that, the above die was put into vacuum and dried at room temperature for 24 h, and then continuously dried at 80C within 24 h to remove the porogen

4000 3500 3000 2500 2000 1500 1000 500

Wave number (cm –1 ) PLA/HAp/PEO

OH –C=O –CH

OH

PLA

(70/30/5 wt/wt/wt) nanocomposite.

2.5 Porosity of PLA/md-HAp/PEO Nanocomposite

Porosity of the porous material was determined by the

Archimedes’ method with an absolute ethanol as the

immersion medium The specimens were dried at 80 C

within 2 h before being tested The dried sample was

weighed as m1 All the air in specimens were removed

by a vaccum pump After that, the specimens were totally

submerged in the absolute ethanol The liquid saturated

Figure 2. SEM images of nanocomposites with the different PLA/md-HAp ratios: (a) 80/20, (b) 70/30, (c) 60/40 and (d) 50/50 (wt/wt).

specimen was weighed as m2 A pycnometer filled with ethanol was weighed as m3 Then, the liquid-saturated sample was put in filled pycnometer, m4 is the weight of the liquid-saturated sample after taken out of the liquid The open porosity obtained by:

%
= m2− m1
/m3+ m2− m4
 · 100

2.6 TestIn Vitro

The in vitro degradation properties of the samples were

evaluated in the simulated body fluids (SBF) In order

to prepare 1 litre of the SBF solution, 8 g NaCl; 0.35 g NaHCO3; 0.4 g KCl; 0.48 g Na2HPO4· 2H2O; 0.1 g MgCl2·6H2O; 0.18 g CaCl2·2H2O; 0.06 g KH2PO4; 0.1 g MgSO4· 7H2O and 1 g glucoza were dissolved in distilled water The pH of the SBF solution is 7.4 (this value is in the pH range of the human body fluids pH= 7.35–7.45).49–51 The samples of PLA/md-HAp with and

without NH4HCO3 were immersed in the cell containing

40 ml SBF, and kept at 37C, during different immersion times: 1, 3, 7, 14, 21 and 28 days These samples were gently rinsed with distilled water before being dried within

24 h at 80C The measurement of weight loss, pH and SEM images of these samples were determined

The mass of PLA/md-HAp/PEO nanocomposites with

and without porogen were determined by Precisa XR 205 SM-DR analysis balance The pH value of the SBF solu-tion was measured by using pH3110 Meter

E Modulus / MPa

0

200

400

600

800

1000

1200

1400

1600

1800

2000

PLA

80/20 70/30

60/40 50/50 (a)

0 5 10 15 20 25 30 35 40 45 50 55

PLA

80/20 70/30

60/40 50/50 (b)

Figure 3. The mechanical properties: (a) Young’s modulus and (b) tensile strength of PLA and PLA/md-HAp/PEO nanocomposites with the different ratios of PLA/md-HAp.

2.7 FT-IR

FT-IR spectra analysis for PLA, md-HAp and

PLA/md-HAp/PEO nanocomposite is used to determine

character-istic groups of their molecules The FTIR spectra of the

samples were recorded by using Nicolet/Nexus 670

Spec-trometer (USA) at room temperature by averaging 16 scans

with a resolution of 4 cm−1 in transmission mode by using

KBr pellet method The FT-IR spectra were recorded in

the wave numbers range from 400 to 4000 cm−1

2.8 Scanning Electron Microscopy (SEM)

The surface of PLA/md-HAp/PEO nanocomposites was

examined by using Hitachi S-4800 Scanning Electron

Microscope (SEM)

2.9 X-ray Diffraction

The phase structure of PLA/md-HAp/PEO with and

with-out NH4HCO3 porogen after 7 immersion days in the

SBF solution were analyzed by X-ray Diffraction (XRD)

(Siemens D5000 Diffractometer, CuK radiation ( =

154056 Å) with step angle of 0.030, scanning rate of

2.10 Mechanical Properties

The mechanical properties (Young’s modulus and

ten-sile strength) of PLA, PLA/md-HAp/PEO

nanocompos-ites with and without porogen were measured by using a

Zwick-Tensile Tester at room temperature with crosshead

speed of 100 mm/min, the dumbbell shaped specimens and

the measurements were carried out according to ASTM

D638

2.11 Hydrophilicity or Hydrophobicity

Determination

The hydrophilicity or hydrophobicity of PLA and

PLA/md-HAp/PEO nanocomposites with and without

NH4HCO3 porogen were evaluated through the

measure-ment of water contact angles Each determination was

obtained by averaging the results of five measurements

Water contact angle measurements were performed by using a SEO Phoenix 150 Contact Angle Analyzer

3 RESULTS AND DISCUSSION

3.1 Influence ofmd-HAp Content on the

Morphology and Mechanical Properties of

The Figure 1 presented the FT-IR spectra of PLA,

md-HAp and PLA/md-HAp/PEO nanocomposite (70/30/5 wt/wt/wt) All characteristic peaks of md-HAp (PO3−4 ,

OH−, CO2 −

3 ) and PLA (C O) were appeared in

PLA/md-HAp/PEO nanocomposite:

(i) characteristic peaks of md-HAp (PO3 −

4 ) at 560, 607,

1061 cm−1 moved back to 563, 610, 1095 cm−1 in the nanocomposite The –CH vibration peaks in PLA (1457 cm−1) and in the nanocomposite (1465 cm−1) also

shifted It indicates the molecular interaction between

md-HAp and PLA in the nanocomposite

(ii) In the nanocomposite, the vibration of the liaison –C O of neat PLA at 1761 cm−1 shifted to 1767 cm−1 This movement may be attributed to the formation of

hydrogen bonding between the –OH of md-HAp and

–C O of PLA

Scanning electron microscopy (SEM) was used to

observe the surface morphology of PLA/md-HAp/PEO

nanocomposites with using 5 wt% of PEO and the

differ-ent ratios of PLA/md-HAp: 80/20, 70/30, 60/40 and 50/50

(wt/wt) (Fig 2) The content of HAp plays an important

Table I. The variation of porosity of PLA/md-HAp/PEO

nanocompos-ites versus NH4HCO3porogen content.

Porogen content (wt%) Porosity (%)

role in controlling the morphology of PLA/md-HAp/PEO

nanocomposites With 20 wt% and 30 wt% of md-HAp,

md-HAp powder was dispersed more regularly in PLA

matrix Higher amounts of md-HAp (40 and 50 wt%)

might cause the aggregation of md-HAp particles in PLA.

However, in order to apply in bone implants, the large

content of HAp is good for biocompatibility, therefore,

30 wt% of md-HAp has been chosen for following studies.

The Young’s modulus of PLA/md-HAp/PEO

nanocom-posites decreased with the increase of md-HAp content

(Fig 3) The Young’s modulus was 1806± 51 MPa with

neat PLA sample; while the Young’s modulus of the

nanocomposite dropped to the value of 593±52 MPa with

Figure 4. SEM images of PLA/md-HAp/PEO nanocomposites with NH4HCO3different porogen content: (a) 0 wt%, (b) 3 wt%, (c) 7 wt%, (d) 10 wt%, (e) 20 wt% and (f) 30 wt%.

20 wt% md-HAp added (down more than 67%) When the md-HAp content is 50%, the Young’s modulus of the

nanocomposite was only 115± 426 MPa (a decrease of

over 93%) The tensile strength of the nanocomposites was deduced similarly to the Young’s modulus

3.2 Influence of Porogen Content on the Porosity, Morphology and Mechanical Properties of

The content of the porogen (NH4HCO3) influenced on

the porosity of the PLA/md-HAp/PEO nanocomposites.

As seen in Table I, the open porosity of the nanocom-posites increased with the increase of NH4HCO3 porogen

100

200

300

400

500

600

700

800

(a)

30 %

20 %

10 %

7 %

3 %

PLA/HAp/PEO/NH 4 HCO 3 70/30/5/x wt/wt

0 %

0 2 4 6 8 10 12 14 16 18 20 22 24 (b)

0 % 3 %

7 %

10 %

20 %

30 %

PLA/HAp/PEO/NH 4 HCO 3 70/30/5/x wt/wt

Figure 5. The (a) Young’s modulus and (b) tensile strength of PLA/md-HAp/PEO nanocomposites without and with 3, 7, 10, 20 and 30 wt%

NH4HCO3porogen content.

content The open porosity was only 12% when the

porogen content was 3 wt%, while it reached 49% at

30 wt% of porogen content Without using NH4HCO3

in the nanocomposites, the porosity of the

nanocom-posite was 10% because md-HAp nano powder itself

also has the ability to increase the porosity of the

nanocomposites.53During the fabrication of

nanocompos-ite, NH4HCO3molecules were uniformly distributed in the

samples At 80 C, NH4HCO3 was degraded to form air

pores with small size (Fig 3) When drying at 80C within

24 h, NH4HCO3 in the nanocomposite was decomposed

to form CO2 and NH3 gas (Fig 3) With high amounts

of the porogen (20, 30 wt%), a part of generated gas was

compressed inside of the nanocomposite and a rest

gener-ated gas was able to release out the surface to form high

porosity of the nanocomposite However, the high porosity

of the nanocomposite was able to destroy the structure in

size and reduced tensile properties of the nanocomposites

In the case of low porogen content (3, 7%), the generated

gas still exist mainly in the nanocomposite by compressing

and only a little generated gas was able to release out

The SEM images of the nanocomposites with different

contents of NH4HCO3 porogen were shown in Figure 4

In absence of NH4HCO3porogen, the PLA/md-HAp/PEO

nanocomposites still have porous structure (Fig 4(a)) The

porosity of this nanocomposite was nearly constant at

low content of NH4HCO3 porogen (3 or 7 wt%) but it

increased significantly when the NH4HCO3 porogen

con-tent was up to 10, 20, 30 wt% In the nanocomposite, HAp

interacts with PLA by hydrogen bonds and NH4HCO3

porogen with low and high content was dispersed in the

nanocomposite When drying the nanocomposite at 80C

within 24 h, NH4HCO3was decomposed to form CO2and

NH3 and pore size of the nanocomposite changed from

small to high depending on NH4HCO3 porogen content as

above explained

The effect of NH4HCO3porogen content on mechanical

properties of the nanocomposites was also studied As seen

in Figure 5, Young’s modulus and tensile strength of the

nanocomposite decreased when porogen content increased

For the samples without and with low porogen content (3 or 7 wt%), the Young’s modulus and tensile strength changed not much, in agreement with determination results

of the porosity of the nanocomposites The Young’s mod-ulus of the nanocomposites decreased from 549± 54 MPa (sample without porogen) to 421± 49 and 400 ± 50 MPa for the sample having the porogen content of 10 and

20 wt%, respectively Specially, with the nanocomposite using 30 wt% porogen content, the Young’s modulus of the nanocomposites was only 120± 39 MPa, which decreased

about 78% compared with PLA/md-HAp/PEO

nanocom-posite without porogen Therefore, component ratio of

PLA/md-HAp/PEO= 70/30/5 with 20 wt% NH4HCO3

porogen content was chosen to test in vitro bioactivity

of the nanocomposite in the simulated body fluids (SBF) solution

The hydrophilicity or hydrophobicity of PLA and

PLA/md-HAp/PEO nanocomposites with and without

porogen were evaluated by measuring the water contact angle (Table II)

Table II demonstrated the measurement results of water

contact angle of surfaces of neat PLA and

PLA/md-HAp/PEO nanocomposites with and without 20 wt%

NH4HCO3 porogen The water contact angle of neat PLA is 83.1± 2.9,3 and its high value shows that PLA

is a hydrophobic polymer PLA/md-HAp/PEO (70/30/5)

nanocomposite has water contact angle of 63.7 which

is lower than that of neat PLA because md-HAp

pow-der is hydrophilic and it also increased the porosity

of the nanocomposite.9 In the presence of 20 wt%

Table II. Water contact angle of PLA, PLA/md-HAp/PEO and PLA/md-HAp/PEO nanocomposites with 20 wt% NH4HCO3porogen Samples Water contact angles (degrees)

PLA/md-HAp/PEO (70/30/5) 637 ± 19

PLA/md-HAp/PEO (70/30/5) 506 ± 19

with 20 wt% NH4HCO3

(1) (2) (3)

Figure 6. Water contact angle images of (1) PLA, PLA/md-HAp/PEO nanocomposites (2) without and (3) with 20 wt% NH4HCO3porogen.

NH4HCO3 porogen, water contact angle of the

nanocom-posite decreased to 50.6 compared to nanocomposite

without porogen (63.7) due to the increase of the porosity

of the nanocomposite (Fig 6) This result indicated that

the incorporation of HAp and NH4HCO3 into

hydropho-bic polymers is a feasible approach to improve the

hydrophilicity of the hydrophobic polymer

3.3 In Vitro Bioactivity of PLA/md-HAp/PEO

Nanocomposites With and Without 20 wt%

NH4HCO3Porogen in Simulated Body Fluids

(SBF) Solution

The in vitro degradation of PLA as well as the formation of

HAp on/in PLA/md-HAp/PEO nanocomposites with and

without 20 wt% NH4HCO3 porogen into the SBF solution

were evaluated by the variation of the pH of the SBF

solu-tion When nanocomposites were immersed into the SBF

solution, there are two processes occurring simultaneously:

the first process is hydrolysis of PLA expressed by two

Eqs (1) and (2) to generate acid lactic, and release H+ion;

the second process is the formation of HAp, which

con-sumes OH−ion Both of processes reduced pH of the SBF

solution The formation of HAp can be explained as

fol-lowing: the hydrolysis of PLA released H+ion, leading to

the dissolution of HAp The calcium ions dissolved from

the HAp increased the calcium ion concentration in the

surrounding SBF, which was already supersaturated with

respect to apatite; and the nancomposite surfaces provided

favorable sites for apatite nucleation As a result of SEM,

a large number of apatite nuclei formed on nanocomposite

surfaces, grew spontaneously, and consumed the calcium

and phosphate ions from the surrounding fluid.54

RCOOH−→ RCOOKa −+ H+ (1)

(2) 10Ca2++ 6HPO2−

4 + 8OH−

−→ Ca10PO4
6OH
2+ 6H2O (3)

Figure 7 showed the pH values of the SBF solution

when immersing nanocomposites at different immersion

time, at 37 C The pH value of the solution before soaking nanocomposites is 7.4 During the immersion time, the pH of the SBF solution

contain-ing PLA/md-HAp/PEO nanocomposites with and

with-out NH4HCO3 porogen decreased but the pH of the

SBF solution containing PLA/md-HAp/PEO

nanocompos-ite with 20 wt% NH4HCO3 porogen decreased more

strongly (39%) because PLA/md-HAp/PEO

nanocompos-ite with NH4HCO3 porogen (39%) has higher porosity

than PLA/md-HAp/PEO nanocomposite without porogen

(10%) Therefore, water molecules easily permeate into

PLA/md-HAp/PEO nanocomposite with NH4HCO3 poro-gen and the contact surface area of the nanocomposite with the SBF solution become higher

The variation of weight of PLA/md-HAp/PEO

nanocomposites with and without NH4HCO3 porogen during immersion time was displayed in Figure 8 The weight of the above nanocomposites decreased strongly after 7 and 3 immersion days It indicated that the decom-position of PLA in the nanocomposites happened strongly than the formation of HAp crystals And then, the weight

of the nanocomposites increased continuously with 28 immersion days It is clear that the formation of HAp crystals on/in the nanocomposites increased significantly This can be explained by the formation HAp crystals

0 3 6 9 12 15 18 21 24 27 30 6.0

6.2 6.4 6.6 6.8 7.0 7.2 7.4

2 1

Time (day)

Figure 7 The pH variation of SBF solution according to immersion

time of PLA/md-HAp/PEO nanocomposites (1) with and (2) without

20 wt% NH4HCO3porogen.

0 3 6 9 12 15 18 21 24 27 30

–0.0025

–0.0020

–0.0015

–0.0010

–0.0005

0.0000

0.0005

0.0010

2 1

Time (day)

Figure 8. The variation of weight of PLA/md-HAp/PEO

nanocompos-ites (1) with and (2) without NH4HCO3porogen according to immersion

time in SBF solution.

on/in the pore that will prevent hydrolysis process of PLA

in the SBF solution

Figure 9 displayed images of PLA/md-HAp/PEO

nanocomposites with 20% NH4HCO3 which was

immersed in the SBF solution during 0, 1, 3, 7, 14, 21

(g)

Figure 9. SEM images of PLA/md-HAp/PEO (70/30/5) nanocomposites with 20 wt% NH4HCO3porogen at the different immersion times in SBF solution: (a) 0, (b) 1, (c) 3, (d) 7, (e) 14, (f) 21 and (g) 28 immersion days.

and 28 days The sample after 1 immersion day appeared HAp nucleation crystals After 3 or 7 immersion days, HAp crystals grew with higher density The surface of the nanocomposites nearly covered fully with HAp crystals after 14, 21 or 28 immersion days in the SBF solution Specially, with the sample immersed during 28 days

in the SBF solution, HAp crystals grew up to form a thicker block and it showed the degradation of PLA in the nanocomposite

Figure 10 performed the XRD patterns of

PLA/md-HAp/PEO nanocomposites before being immersed in

the SBF solution; PLA/md-HAp/PEO without and with

NH4HCO3 porogen after 7 immersion days in the

SBF solution The XRD pattern of PLA/md-HAp/PEO

nanocomposite before being immersed in the SBF solution expressed that PLA in the nanocomposite is a semicrys-talline polymer (Fig 10(1)) Besides that, in the XRD pat-degree= 25,84and 31,93 The diameter of HAp crystals

in PLA/HAp nanocomposite based on the Scherrer

equa-is 19.87 nm.

The XRD patterns of PLA/md-HAp/PEO

nanocompos-ites with and without NH4HCO3 porogen after 7 immer-sion days (Fig 10(2) and 10(3)) performed the appearance

10 20 30 40 50 60

20

60

100

140

180

3

2

1

2 θ (degree)

0

100

200

300

400

500

600

0

100

200

300

400

500

600

before immersing, PLA/md-HAp/PEO (2) without and (3) with

NH4HCO3porogen after 7 immersion days in SBF solution.

of 2 characteristic peaks for crystal structure of PLA at

 and 18,9.55 After 7 immer-sion days in the SBF solution, PLA amorphous part in

the nanocomposites was hydrolysed and PLA crystal part

remained And two characteristic peaks of HAp at about

 and 31,94 were also shown in these patterns However the intensity of the characteristic peaks

of PLA crystal in PLA/md-HAp/PEO nanocomposite with

NH4HCO3porogen was higher than that in the

nanocom-posite without NH4HCO3 porogen This was able to be

explained as following: PLA/md-HAp/PEO

nanocompos-ite with 20 wt% NH4HCO3 porogen was more porous

than PLA/md-HAp/PEO nanocomposite (Fig 4), so

amor-phous PLA part was hydrolysed more strongly,

crys-tal PLA dominated and the formation of HAp became

easily The formation of HAp after being immersed in

the SBF was exhibited by the intensity of the

charac-teristic peaks of HAp in the nanocomposite which was

arranged as following order: PLA/md-HAp/PEO before

being immersed < PLA/md-HAp/PEO without NH4HCO3

porogen after 7 immersion days < PLA/md-HAp/PEO with

20 wt% NH4HCO3porogen after 7 immersion days

4 CONCLUSION

PLA/md-HAp/PEO porous nanocomposites using

NH4HCO3 porogen was prepared by the solvent

casting method The incorporation of md-HAp and

NH4HCO3porogen greatly improved the porosity and the

hydrophilicity of the nanocomposites The porosity of the

nanocomposites increased and their mechanical properties

decreased with the increase of NH4HCO3porogen content

The results of characterisations, properties, morphology

and the degradation of PLA/md-HAp/PEO

nanocompos-ites with and without NH4HCO3 porogen in the SBF solution showed the formation of the HAp on the surface

of the nanocomposites and the hydrolysis process of PLA after being immersed in the SBF solution These porous nanocomposites are promising potential applications for bone implant

Acknowledgments: The authors gratefully acknowl-edge the Ministry of Science and Technology of Vietnam for financial support through the Bilateral Project Vietnam—Korea number 49/2012/HD-NDT

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Received: 27 February 2015 Acceptance: 10 June 2015