Journal of Elastomers and Plastics

Journal of Elastomers and Plastics http://jep.sagepub.com/

Characterization of poly(lactic acid)/hydroxyapatite prepared by a solvent-blending technique: Viscoelasticity and in vitro hydrolytic degradation Mujtahid Kaavessina, Achmad Chafidz, Ilias Ali and Saeed M. Al-Zahrani Journal of Elastomers and Plastics published online 12 November 2014 DOI: 10.1177/0095244314557973 The online version of this article can be found at: http://jep.sagepub.com/content/early/2014/11/12/0095244314557973

Published by: http://www.sagepublications.com

Additional services and information for Journal of Elastomers and Plastics can be found at: Email Alerts: http://jep.sagepub.com/cgi/alerts Subscriptions: http://jep.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav Citations: http://jep.sagepub.com/content/early/2014/11/12/0095244314557973.refs.html

>> OnlineFirst Version of Record - Nov 12, 2014 What is This?

Downloaded from jep.sagepub.com by guest on November 13, 2014

Article

Characterization of poly(lactic acid)/ hydroxyapatite prepared by a solventblending technique: Viscoelasticity and in vitro hydrolytic degradation

Journal of Elastomers & Plastics 1–16 ª The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0095244314557973 jep.sagepub.com

Mujtahid Kaavessina1, Achmad Chafidz2,3, Ilias Ali3 and Saeed M. Al-Zahrani2,3

Abstract Poly(lactic acid) (PLA) was solvent blended in a chloroform solution using multiple weight fractions of hydroxyapatite (HAp) (5, 10, and 20 wt%). A miniature laboratory mixing extruder equipped with a ribbon die was used to produce thin ribbons of PLA/ HAp biocomposites. The dynamic mechanical parameters (storage modulus (G0 ), loss modulus (G00 ), complex viscosity (*), and degree of crystallinity) increased with increasing HAp loading. In vitro hydrolytic degradation of the PLA biocomposites was conducted in a 0.01-M phosphate-buffered saline solution at 37 C. The presence of HAp tended to increase both the hydrolytic degradability of the PLA and the crystallinity, possibly resulting from the hydrophilicity of the HAp. The thermal stability of the PLA was slightly higher in the composites with HAp. Following hydrolytic degradation, several microholes and cracks appeared on the surface of these biocomposites, as observed by scanning electron microscopy. The Coats–Redfern method was used to evaluate the thermal degradation kinetics of the biocomposites with support from a chemical reaction model. From this evaluation, the activation energies of the biocomposites were found to exceed that of the neat PLA. These energies were observed to decrease after the hydrolytic degradation process. 1

Department of Chemical Engineering, Sebelas Maret University, Surakarta, Indonesia Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia 3 SABIC Polymer Research Center (SPRC), King Saud University, Riyadh, Saudi Arabia 2

Corresponding author: Saeed M. Al-Zahrani, Department of Chemical Engineering, King Saud University, Riyadh 11421, Saudi Arabia. Email: [email protected]


2

Journal of Elastomers & Plastics

Keywords Poly(lactic acid), hydroxyapatite, hydrolytic degradation, viscoelasticity, biocomposite

Introduction Recently, researchers from both academia and industry have devoted increased attention to the development of biomaterials (including bio-based polymers) for various medical applications. Aliphatic polyesters are promising biopolymers with hydrolyzable and flexible ester bonds that can be naturally degraded into nontoxic matter in a humid environment or in solutions with various pH values,1 suggesting that these polymers may be useful in clinical applications. Owing to their outstanding biocompatibility and controllable degradability, poly(lactic acid) (PLA), polycaprolactone, polyhydroxybutyrate, and polyglycolide, as well as their copolymers/blends,2 have been widely used as drug carriers,3 in medical devices/implants4 and as scaffolds in tissue engineering applications.5,6 Among the aliphatic polyesters, PLA has been recognized as having excellent mechanical properties that are comparable with those of conventional polymers, such as polystyrene, polyethylene, and polyethylene terephthalate.7 Owing to its favorable mechanical properties, PLA has been used in various biomedical applications, including bone screws, scaffolding materials, and sutures.8–10 Several technological advances have been made to tailor the different critical properties of PLA to meet the stringent criteria for multiple biomedical applications, such as their controllable compressive and tensile strength, biodegradation rate, and bioactivity. Several authors have reported on various efforts to prepare PLA composites with multiple fillers11–15 to enhance the mechanical properties and the degree of crystallinity (Xc) of the PLA composites. The reported fillers have various effects on the hydrolytic degradation of the PLA composites, and several reports have concluded that both montmorillonites12 and hydroxyapatites (HAps)2 can act as catalysts in the hydrolytic degradation of the composites because of their hydrophilic characteristics. In contrast, other studies have demonstrated that nanoclays can act as inhibitors during the degradation process because of the barrier properties of clay,1,16 although permeability data are not available. Tsuji and Ikada17 and Zhou and Xanthos14 reported that increased crystallinity is accompanied by improvements in the hydrolytic degradation kinetics. Another report has suggested the opposite relationship, indicating that hydrolytic degradation occurs mainly in the amorphous phase.18 The degradation kinetics are influenced by the chemical structure, molecular weight and distribution, filler type, and processing conditions (temperature and pH). HAp is used in the medical field as an artificial bone material to replace amputated bone and has been observed to stimulate bone growth. As a filler in a PLA composite, HAp particles increase the rate of cell proliferation, suggesting that both bioactivity and biocompatibility of PLA composites increase with increasing amounts of HAp.2 In fiber form, HAp enhances the modulus of elasticity for PLA with a negative effect on the bending strength.13


Kaavessina et al.

3

The aims of this work were to prepare PLA/HAp biocomposites through solvent blending and to determine the viscoelasticity and in vitro hydrolytic degradation characteristics of the resultant composites. The HAp used in this investigation was in particle form. Viscoelasticity tests were performed through dynamic mechanical analysis (DMA). In vitro hydrolytic degradation experiments were performed by placing the samples in a 0.01-M phosphate-buffered saline (PBS) solution at 37 C. The morphology and thermal properties of the neat PLA and the composites (before and after hydrolytic degradation) were investigated by scanning electron microscopy (SEM) and dynamic scanning calorimetry (DSC), respectively. Thermal degradation evaluations of the biocomposites were performed under nitrogen atmosphere. The kinetic parameters were calculated using the Coats–Redfern method with support from a chemical reaction model.

Experimental Sample preparation Extrusion grade PLA with the commercial name ‘‘PLE001’’ was kindly supplied by NatureplastTM (France). PLE001 is a transparent resin, with a density and melt flow index of 1.25 g mL1 and 2–8 g 10 min1 (190 C, 2.16 kg1), respectively, as eported by the supplier. HAp particles were purchased from Sigma-Aldrich (no. 55496, St Louis, Missouri, USA). The particle size was predominantly 40 m, as reported by the supplier. PLA/HAp composites were prepared using a solvent-blending procedure to produce uniform blends. Prior to blending, the PLA and HAp, 50 g of PLA was dissolved in 250 mL chloroform at 30 C. Various amounts of HAp particles were added to the solutions. A mechanical stirrer was used to mix the suspensions at 50 r min1 at room temperature for 30 min. Chloroform was removed from the mixtures by air-drying in an exhaust chamber at room temperature for 24 h, followed by drying in a vacuum oven at 60 C until a constant weight of PLA/HAp biocomposite was obtained. The composites were extruded in a miniature lab mixing extruder (Dynisco, Franklin, Massachusetts, USA) equipped with a ribbon die to obtain ribbon composites. The extrusion experiments were performed at a barrel and ribbon die temperature of 170 C and a rotor speed of 60 r min1. Four composite samples were prepared with 0, 5, 10, and 20 wt% HAp and were designated PLA, PLA5, PLA10, and PLA20, respectively.

Hydrolytic degradation Hydrolytic degradation studies were performed by immersing approximately 40 mg ribbons of PLA or composite (Winitial) in 20 mL of a 0.01 M PBS solution. The incubation temperature was adjusted to 37 C. At each degradation time point, samples were obtained from the PBS solution (without calcium (Ca2þ) or magnesium (Mg2þ)) and washed intensively with distilled water at room temperature prior to drying in an oven at 50 C to a final constant weight (Wfinal). The weight loss of the composites and the pH values of the PBS solutions were determined for each time point, and the data were analyzed to determine the thermal properties, thermal stability, and morphology.


4


The PBS solution contained sodium chloride, potassium chloride, anhydrous disodium hydrogen phosphate and potassium dihydrogen phosphate at concentrations of 8.0, 0.2, 1.56, and 0.2 g L1, respectively. The pH values of the initial PBS solutions were adjusted to 7.41 by adding a hydrochloric acid (HCl) solution (0.1 M HCl). The pH was measured using a PH211 meter (Hanna instrument, Romania)

Characterizations Dynamic mechanical properties were determined using an AR-G2 instrument (TA analysis, TA Instruments, New Castle, Delaware, USA). Viscoelastic tests were performed on extruded ribbons using a temperature sweep program. Ribbons were placed in a dual cantilever with a controlled strain of 1% at a frequency of 1 rad s1 to determine the temperature dependence of the storage modulus (G0 ) of the solid samples. The temperature ranged from 25 C to 90 C in incremental steps of 5 C. Melt rheological tests were performed on extruded ribbons using a frequency sweep program. The melt ribbons were placed in a parallel plate geometry with a diameter of 25 mm and a gap of 1 mm at 170 C. The frequency sweep was performed within the range of 0.1–628 rad s1 at a controlled strain of 1% to elucidate the frequency dependence of * and G0 of the melts. The thermal properties of the materials were analyzed using a DSC60 apparatus (Shimadzu, Japan). Dynamic scanning was used to discard the anterior thermal history. The first heating profile used a thermal scanning procedure from 25 C to 185 C with a ramp of 10 C min1, followed by a hold for 5 min at 185 C. The samples were then cooled with a ramp of 10 C min1 to 25 C. Finally, the second heating process employed a scanning procedure that increased the temperature from 25 C to 300 C with a ramp of 10 C min1. A thermogravimetric analysis (TGA) Q600 analyzer (TA Analysis) was used to determine the thermal degradation of the biocomposites at a heating rate of 10 C min1 under nitrogen atmosphere (100 mL min1). The morphologies of the biocomposites before and after immersion in the PBS solution were observed by SEM (JEOL JSM6360A Japan, at 15 kV).

Results and discussion Viscoelasticity As reported by several authors, the addition of fillers to form PLA composites can increase several mechanical properties (tensile strength, modulus, etc.) and the crystallinity of the samples.2,19 The temperature resistance of the PLA samples should increase with increasing crystallinity of the biocomposites. To characterize the samples, solid ribbons of the biocomposites were subjected to DMA. As shown in Figure 1, the values of G0 and tan were determined for the PLA biocomposites in the temperature range of 25–90 C. Compared with the neat PLA in this temperature range, the G0 of the biocomposites gradually increased with increasing percentage of HAp content. The increasing crystallinity resulting from the presence of HAp may have restricted the chain


Kaavessina et al.

5

Figure 1. Dynamic storage modulus and tan d spectra of neat PLA and its biocomposites as a function of temperature. PLA: poly(lactic acid).

mobility of the PLA. The storage modulus behaviors of the biocomposites were obtained, as previously described. For the ribbon samples, the G0 values were similar, with a plateau at a specific temperature (approximately 55–65 C), followed by a significant decrease prior to converging to a new relatively constant value. The plateau length of the G0 increased with temperature, indicating that the material samples had increased temperature resistance/thermal stability. As shown in Figure 1, the thermal stabilities of the biocomposites exceeded the stability of the neat PLA, although these differences were not statistically significant. For the neat PLA, the G0 tended to gradually decrease at temperatures of approximately 45 C, decreasing rapidly approximately above 55 C. HAp had little effect on the thermal stability at amounts of 5 wt% in the biocomposites. For 10 and 20 wt% HAp in the biocomposites, significant effects were observed. A rapid decrease in the G0 was observed at temperatures above approximately 55 C. This phenomenon was also studied by TGA, as reported in this article. By plotting tan versus temperature, the significant drop in the G0 observed at approximately 55–65 C was found to correspond with the a-relaxation of the amorphous structure, indicating that the neat PLA and the biocomposites were semicrystalline. The peak of tan can be used to determine the reversible change in the molecular mobility transition between the rubbery and glassy states.11 The peak of tan at approximately 60 C indicates the glass transition temperature (Tg). Compared with the neat PLA, the peak of tan was slightly lower, as also reported in the literature.20,21 The quantifications of the Xc and Tg were confirmed by the DSC results. Theoretically, Tg is a function of molecular rotational freedom. Cowie and Arrighi22 suggested that the introduction of stiff chemical groups (such as benzene rings) that interfere with molecular movement will result in an increase in Tg. In this


6


Figure 2. Z of PLA and its biocomposites as a function of angular frequency in melt. PLA: poly(lactic acid); Z: complex viscosity.

regard, the presence of HAp may limit the molecular rotation and increase the Tg value of the PLA composite. In contrast with this theory, the Tg value was observed to decrease slightly, as previously discussed. The presence of HAp of a certain size may also have acted as a nucleating agent, leading to an increase in the Xc of the PLA composites. Suryanegara et al.19 suggested that an increase in the crystallinity of PLA may correspond to a decrease in Tg. The combination of the restriction of molecular rotation and the action as a nucleating agent may have produced the Tg values observed in this investigation. The * of a polymer provides a measure of the overall resistance to flow in the melt phase. This parameter is associated with both the free volume and the chain mobility of the polymer, with a high degree of sensitivity to the macromolecular chain structure and impurities in the polymer.23 As shown in Figure 2, the values of * of PLA and the biocomposites as a function of angular frequency at 170 C exhibit similar trends and can be divided into two virtual regions. The first region is below 10 rad s1 and exhibits typical * behavior, which suggests Newtonian behavior that is relatively independent of frequency. The second region is characterized by values above 10 rad s1 and displays a shear thinning response that exhibits decreasing * with increasing frequencies. * measurements were performed on ribbons of neat PLA and the biocomposites. Inducing changes to the chain mobility of the polymers, the presence of HAp as an impurity affected the *. The molecular chain mobility of PLA was restricted in the presence of HAp, increasing the * of the melt PLA. At 0.1 rad s1, the addition of HAp amounts ranging from 0 to 5, 10 and 20 wt% increased the * from 1.84 kPas to 2.106, 2.463 and 3.695 kPas, respectively.


Kaavessina et al.

7

Figure 3. G0 and G00 of PLA and its biocomposites as a function of angular frequency in melt phase at 170 C phase at 170 C. G0 : storage modulus; G00 : loss modulus; PLA: poly(lactic acid).

As shown in Figure 3, the storage modulus (G0 ) and the loss modulus (G00 ) of PLA and the biocomposites as a function of angular frequency at 170 C demonstrate that the moduli of these materials tended to increase with increasing angular frequency, suggesting that the molecular chain mobility decreases at higher frequencies. The G0 of PLA was enhanced from 3 Pa to 40,000 Pa as the frequency increased from 0.1 rad s1 to 100 rad s1. These decreases in mobility grew proportionally with the percentage of HAp in the biocomposites, resulting in an increased modulus. Considering the G0 and G00 values of PLA and the biocomposites, a phenomenon was observed for the angular frequency range of 0.1 to 628.3 rad s1. All samples measured at values approximately below ! ¼ 125 rad s1 exhibited lower G0 values in comparison with the G00 values, displaying a liquid-like behavior in the melt samples. For each sample, the curves of G0 and G00 crossed approximately at ! ¼ 125 rad s1. At values above this point, the values of G0 exceeded the values of G00 , exhibiting pseudo solid-like behavior. Similar phenomena have also been reported for other fillers.24,25

In vitro hydrolytic degradation As discussed in the introduction, various fillers in the PLA composites may act as either an inhibitor or a catalyst in the hydrolytic degradation of aliphatic polyesters. Lin et al.2 investigated the hydrolytic degradability of medical grade poly(DL-lactic acid) (PDLLA) and PDLLA/HAp in PBS solution, reporting that the exposed surface area of the PLA in PBS solution increased with increasing HAp loading, which in turn increased the rate of hydrolytic degradation. As shown in Figure 4, the hydrolytic degradation in PBS solution without Ca2þ or Mg2þ was determined by measuring the weight loss and pH. The weight loss represents


8


Figure 4. Weight loss and pH value of PLA and its biocomposites during the hydrolytic degradation process in PBS solution (pH ¼ 7.41). PLA: poly(lactic acid); PBS: phosphate-buffered saline.

hydrolytic chain cleavage accompanied by the formation and release of water-soluble monomer/oligomers from the samples.26 The pH values indicate the type of materials that were degraded in the experimental conditions.2,27 During the period of hydrolysis (0–6 weeks), the degradation of PLA and the biocomposites occurred, but the HAp fillers may not be degraded, as measured by the release of Ca2þ. With regard to pH, HAp will degrade at pH < 4.2.2,27 For each experiment, the weight loss was calculated by the following equation: Winitial Wfinal Weight loss ð%Þ ¼ 100%; ð1Þ Winitial where Winitial and Wfinal are the weight of the samples before and after immersion in the PBS solution at a given time, respectively. The degradation of PLA in aqueous media18 occurs in two stages; the process is initiated by water diffusion into the amorphous regions and then degradation occurs in these regions. After degradation occurs in the amorphous regions, the degradation process continues to the crystalline parts. With the degradation mainly occurring in the amorphous regions, high degradation rates suggest decreased crystallinity of the PLA samples. This expectation was confirmed by measuring the crystallinity and the thermal properties of PLA both before and after the hydrolytic process in PBS solution, as shown in Figure 5 and Table 1. Figure 5 shows that the Tgs decreased slightly with increasing HAp content. The melting temperatures (Tm) were essentially unchanged, indicating that HAp may facilitate the arrangement of the PLA chains in a crystalline structure. The value of Xc was calculated by the following formula and is shown in Table 1:28–30


Kaavessina et al.

9

Figure 5. DSC thermogram of PLA and its biocomposites before hydrolytic degradation in PBS solution at a heating rate of 10 C min1. DCS: dynamic scanning calorimetry; PLA: poly(lactic acid); PBS: phosphate buffered saline. Table 1. Thermal properties PLA and its biocomposites before and after hydrolytic process in PBS solution. Sample PLA

PLA5

PLA10

PLA20

t (weeks)

Tg ( C)

Tm ( C)

DHcc (J g1)

DHm (J g1)

Xc (%)

0 2 4 0 2 4 0 2 4 0 2 4

59.9 59.4 60.0 58.6 59.1 58.8 58.0 57.6 57.7 57.1 56.8 56.7

150.7 150.2 150.5 150.1 150.2 150.5 150.5 150.9 150.2 150.9 151.3 150.7

5.06 7.13 9.21 10.12 11.69 12.98 10.49 13.13 13.94 11.14 12.63 14.10

5.40 7.61 9.83 11.37 13.13 14.58 12.44 15.57 16.53 14.86 16.85 18.81

PLA: poly(lactic acid); PBS: phosphate-buffered saline; Tg: glass transition temperature; Tm: melting temperature; DHcc: cold crystallization enthalpy; DHm: melting enthalpy; Xc: degree of crystallinity; t: immersion time in PBS solution.

Xc ð%Þ ¼

Hm Hcc 100%; X Hmo


ð2Þ

10


where Hm and Hcc are the experimental melting and cold crystallization enthalpies, respectively. X is the weight fraction of the PLA in the blends. The value of Hmo ¼ 93:7 J g1 was used in the calculations, based on the reported Hm of PLA crystal of infinite size.29 The presence of HAp significantly increased the crystallinity of the PLA. The inclusion of HAp at 0, 5, 10, and 20 wt% produced crystallinity values of 5.4, 11.37, 12.44, and 14.86%, respectively. As shown in Figure 4 and Table 1, increases in the degradation rates and in the Xc were obtained upon the addition of HAp. The fillers tended to congregate in the amorphous regions, facilitating the penetration of their irregular structures into the polymer matrix. HAp particles could act as nucleation agents, increasing the Xc of the PLA. The enhanced barrier properties that resulted from the increased crystallinity were not significantly affected by the hydrophilicity of the PLA. The hydrolytic degradation rates were observed to actually increase with increasing Xc. These results are consistent with previous reports.2,21 The presence of HAp may have increased the hydrophilicity of the PLA,21 thus increasing the permeation of water and facilitating degradation in the amorphous regions. As also shown in Table 1, the the values of Xc of PLA immersed in PBS solution increased over time, with the crystallinity of the neat PLA measured at 5.4, 7.61, and 9.83% after immersion in the solution for 0, 2, and 4 weeks, respectively. Similar phenomena have also been reported by other authors.14,18,26 These results support the finding that the degradation occurred in the amorphous regions. With a decrease in the amount of amorphous phase, the bulk crystallinity of the PLA would increase. These phenomena were also observed in the biocomposites. For each sample, the the values of Tg and Tm temperatures were unaffected by the degradation process. However, the Hm increased, suggesting that the degradation had not continued into the crystalline parts during the hydrolytic degradation process. The presence of carboxylic acid in the degradation medium arose from chain scission of the lactic acid oligomer after formation within the amorphous regions. Similar trends were reported by Fukushima et al.11 The hydrolysis of PLA and PLA/Cloisite 30B nanocomposites exhibited a significant increase in Hm, with relatively unchanged values for Tg and Tm in PBS solution after 6 weeks at 37 C. Significant changes in the values for Tg and Tm were observed after 8 weeks at 37 C. Figure 6 shows surface images of PLA20 before and after immersion in the PBS solution at 37 C for 4 weeks. The image of PLA20 before hydrolytic degradation exhibits a smooth surface (Figure 6(a)). The HAp tended to congregate in microclumps spread within the PLA matrix. A possible explanation of these microclumps is discussed previously. With improved penetration, greater amounts of HAp were located in the amorphous regions in comparison with the crystalline regions. This image further highlights the two regions in the biocomposites (dense phase and sparse phase in the virtual investigation). The dense phase is lighter than the sparse phase. The image of PLA20 after hydrolytic degradation shows several microholes and cracks (Figure 6(b)). The microholes may have resulted from the removal of PLA chains from the amorphous regions. The degradation of PLA occurred in the PBS solution at 37 C for 4 weeks, influencing the brittleness of the PLA. In visual observations, several


Kaavessina et al.

11

Figure 6. SEM image of the surface morphology of PLA20: (a) before and (b) after hydrolytic degradation for 4 weeks. SEM: scanning electron microscopy; PLA: poly(lactic acid).

cracks were found, indicating an increase in both brittleness and the Xc. Similar phenomena have also been reported by other authors.11,14 As shown in Figure 7, TGA curves of the PLA and the biocomposites before hydrolytic degradation in PBS solution were measured as a function of temperature at a heating rate of 10 C min1. The decomposition of PLA and the biocomposites began at approximately 300 C. Based on data provided by Aizawa et al.,31 HAp discharges H2O from the OH group at temperatures of 800 C or higher with further decomposition at


12


Figure 7. TGA curves of PLA and its biocomposites before hydrolytic degradation process in PBS solution. TGA: thermogravimetric analysis; PLA: poly(lactic acid); PBS: phosphate buffered saline.

temperatures exceeding 1050 C. The residues of the nonvolatile matter in samples at 600 C were identified as HAp. The content of the residues from PLA, PLA5, PLA10, and PLA20 were approximately 0, 5, 10, and 20 wt%, respectively, confirming that the solvent-blending process ensured good mixing of the PLA with the HAp. The Coats–Redfern method was selected to determine the kinetic parameters from the TGA data, as presented in Table 2. This powerful method has been commonly used to examine the thermal degradation kinetics of solid substances.32 By omitting the 2RT/Ea term, the Coats–Redfern method simplifies to the following equation:20,32 gðÞ AR Ea ; ð3Þ ln 2 ¼ ln T Ea RT where R is the universal gas constant (8.314 J mol1 K1), b is the heating rate (10 C min1), T is temperature (K), and g(a) is dependent on the kinetic model, which is used to describe the degradation. The constants Ea and A refer to the activation energy and a pre-exponential factor (not a function of temperature), respectively. In this investigation, a chemical reaction model was selected to determine the kinetic parameters for the thermal degradation. In this model, interparticle diffusion phenomena and intraparticle heat transfer were neglected. Thus, equation (3) is reduced as follows:20,32 lnð1 xÞ AR Ea ; for n ¼ 1 ð4Þ ln ¼ ln T2 Ea RT or


Kaavessina et al.

13

Table 2. Thermal stability and kinetic parameter of PLA and its biocomposites before and after immersion in PBS solution for certain times. Sample PLA

PLA5

PLA10

PLA20

t (weeks)

Tr ( C)

0 2 4 0 2 4 0 2 4 0 2 4

298–392 315–384 301–388 310–396 300–395 311–392 309–395 310–391 307–395 325–391 326–386 327–389

T1 ( C) T50 ( C) Ea (kJ mol1) 312.1 302.3 296.1 305.1 301.9 291.3 299.4 296.1 295.3 285.2 276.6 257.3

361.4 361.3 363.5 362.4 362.5 363.5 362.8 363.5 363.7 363.7 363.9 364.5

234.09 223.86 221.85 237.53 227.56 225.65 240.52 231.38 226.26 242.58 233.09 227.73

A (s1)

N 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.95 0.9 0.9

4.3124 1.2102 8.2597 1.2257 1.7918 1.4330 2.1054 4.1284 1.4822 4.3257 6.8704 2.4942

1018 1018 1017 1019 1018 1018 1019 1018 1018 1019 1018 1018

R2 0.997 0.990 0.996 0.991 0.987 0.989 0.989 0.991 0.989 0.995 0.994 0.995

PLA: poly(lactic acid); PBS: phosphate buffered saline; Tr: range of evaluated temperature; T1: temperature at weight loss fraction of 1% (assumed as initial thermal degradation); T50: temperature at weight loss fraction of 50%; Ea: activation energy; N: order of reaction; A: pre-exponential factor; R2: correlation coefficient of the linear regression; Y: immersion time in PBS solution.

"

# lnð1 xÞ1n AR Ea ; ln ¼ ln Ea ð1 nÞT 2 RT

for n 6¼ 1;

ð5Þ

where x and n are the fractions of mass loss and the order of reaction, respectively. The procedures employed to calculate the Ea in defined temperature ranges are to (a) estimate the n value; (b) fit the experimental TGA data to equations (4) or (5) and (c) verify the correlation coefficient of the linear regression (R2). Optimal values of R2 close to 1 indicate that the n value is suitable for the experimental data. Finally, the Ea and the A can be obtained from the slope and the intercept of a straight line, respectively. As shown in Table 2, the Ea of PLA tended to decrease with time after immersion in the PBS solution. The Ea of PLA before immersion was 234.0864 kJ mol1. After immersion for 2 and 4 weeks, the Ea values decreased to 223.8596 and 221.8475 kJ mol1, respectively. As explained in the literature,28,33 the Ea is the minimum energy required to transport molecular fragments to a desired state, suggesting that the hydrolytic degradation of PLA promoted further thermal decomposition of the chains. These phenomena were also observed in the biocomposites. With increasing amounts of HAp, the values of 50% weight loss temperature (T50%) slightly increased, with values of 361.4, 362.4, 362.8, and 363.7 C for the PLA composites loaded with 0, 5, 10, and 20 wt% HAp, respectively. The presence of HAp did not significantly improve the thermal stability of PLA. Balakrishnan et al.34 suggested that a significant improvement in the thermal stability may be associated with the properties of the filler and the surface interaction between the filler particles and the PLA. Additionally, the quantity of the fillers in the matrix can also affect the


14


thermal stability,15 suggesting that the surface interactions of HAp particles with the PLA were weak. An alternative explanation is also available for these observations. After immersion in the PBS solution, the PLA crystallinity increased, the thermal stability increased slightly, and the Ea decreased. With the presence of microholes and cracks from the degradation process, an enlargement of the surface area of the samples may have occurred, facilitating the heat transfer in the bulk of the samples. With the crystallinity and thermal stability only slightly increased, these effects could be neglected in the initial thermal degradation. These observations would suggest that the T1 value of the samples increased after the hydrolytic degradation (see Table 2). These trends were also observed in the biocomposites. With the increased Ea in the presence of HAp, the T1 values shifted to lower temperatures in comparison with the neat PLA. The incorporation of HAp may have increased the bulk heat conductivity of the biocomposites. Improved conductivities facilitate heat transfer in bulk samples. With the irregular structure of the amorphous regions in the biocomposites, this region could more easily undergo thermal decomposition. The initial thermal degradation (T1) of the samples increased with increasing HAp content in the samples. As shown in Figure 7, the T1 value of PLA20 curve occurred at 285 C, with values gradually decreasing up to 320 C, and finally, decomposition occurred at temperatures above 320 C. Compared with the PLA curve, the T1 value was observed at 312 C, followed by decomposition.

Conclusions PLA /HAp biocomposites were prepared using a solvent-blending method. The presence of HAp enhanced the dynamic mechanical properties of the PLA, that is, the G0 , G00 , and *. The HAp nanoparticles may have acted as nucleation agents to increase the Xc of the PLA chains. The enhanced barrier properties resulting from the increased crystallinity did not affect the hydrophilicity of the PLA, suggesting that the hydrolytic degradability of the PLA was enhanced, even with the increased crystallinity. Additionally, the thermal stability was slightly enhanced in the presence of HAp, with a tendency for the initial temperature to shift toward lower values. The surface interactions of the HAp particles with the PLA were weak. This investigation provides beneficial data for use in optimizing the composition of PLA biocomposites for various medical applications. Acknowledgments The authors are very grateful to Deanship of Scientific Research (DSR) and Research Center – College of Engineering at King Saud University for the financial supports.

Funding This investigation was supported by the Deanship of Scientific Research (DSR), Research Center – College of Engineering, King Saud University.


Kaavessina et al.

15

References 1. Huang LP, Jin B, Lant P, et al. Simultaneous saccharification and fermentation of potato starch wastewater to lactic acid by Rhizopus oryzae and Rhizopus arrhizus. Biochem Eng J 2005: 23: 265–276. 2. Lin P-L, Fang H-W, Tseng T, et al. Effects of hydroxyapatite dosage on mechanical and biological behaviors of polylactic acid composite materials. Mater Lett 2007; 61: 3009–3013. 3. Rancan F, Papakostas D, Hadam S, et al. Investigation of polylactic acid (PLA) nanoparticles as drug delivery systems for local dermatotherapy. Pharmaceut Res 2009; 26: 2027–2036. 4. Wee Y-J, Yun J-S, Park D-H, et al. Biotechnological production of L(þ)-lactic acid from wood hydrolyzate by batch fermentation of enterococcus faecalis. Biotechnol Lett 2004; 26: 71–74. 5. Yang J, Wan Y, Tu C, et al. Enhancing the cell affinity of macroporous poly(L-lactide) cell scaffold by a convenient surface modification method. Polym Int 2003; 52: 1892–1899. 6. Zhou Z, Liu X, Liu L, et al. Cytocompatibility of poly(L-lactide-co-glycolide) porous scaffold materials for tissue engineering. Int J Polym Mater Polym Biomater 2008; 57: 1026–1035. 7. Auras R, Harte B and Selke S. An overview of polylactides as packaging materials. Macromol Biosci 2004; 4: 835–864. 8. Zhang D, Zhang L, Xiong Z, et al. Preparation and characterization of biodegradable poly(D,L-lactide) and surface-modified bioactive glass composites as bone repair materials. J Mater Sci: Mater Med 2009; 20: 1971–1978. 9. Chang JH, An YU and Sur GS. Poly(lactic acid) nanocomposites with various organoclays. I. Thermomechanical properties, morphology, and gas permeability. J Polym Sci B: Polym Phys 2003; 41: 94–103. 10. Hou J-Z, Sun X-P, Zhang W-X, et al. Preparation and characterization of electrospun fibers based on poly(L-lactic acid)/cellulose acetate. Chin J Polym Sci 2012; 30: 916–922. 11. Fukushima K, Tabuani D, Dottori M, et al. Effect of temperature and nanoparticle type on hydrolytic degradation of poly(lactic acid) nanocomposites. Polym Degrad Stabil, 2011; 96: 2120–2129. 12. Paul MA, Delcourt C, Alexandre M, et al. Polylactide/montmorillonite nanocomposites: study of the hydrolytic degradation. Polym Degrad Stabil, 2005; 87: 535–542. 13. Kasuga T, Ota Y, Nogami M, et al. Preparation and mechanical properties of polylactic acid composites containing hydroxyapatite fibers. Biomaterials 2000; 22: 19–23. 14. Zhou Q and Xanthos M. Nanoclay and crystallinity effects on the hydrolytic degradation of polylactides. Polym Degrad Stabil 2008; 93: 1450–1459. 15. Solarski S, Mahjoubi F, Ferreira M, et al. (Plasticized) Polylactide/clay nanocomposite textile: thermal, mechanical, shrinkage and fire properties. J Mater Sci 2007; 42: 5105–5117. 16. John R, Nampoothiri K and Pandey A. Fermentative production of lactic acid from biomass: an overview on process developments and future perspectives. Appl Microbiol Biotechnol 2007; 74: 524–534. 17. Tsuji H and Ikada Y. Properties and morphology of poly(L-lactide) 4. Effects of structural parameters on long-term hydrolysis of poly(L-lactide) in phosphate-buffered solution. Polym Degrad Stabil 2000; 67: 179–189. 18. Hakkarainen M. Aliphatic polyesters: abiotic and biotic degradation and degradation products. Adv Polym Sci 2002; 157: 113–138. 19. Suryanegara L, Nakagaito AN and Yano H. The effect of crystallization of PLA on the thermal and mechanical properties of microfibrillated cellulose-reinforced PLA composites. Compos Sci Technol 2009; 69: 1187–1192.


16


20. Li J, Zheng W, Li L, et al. Thermal degradation kinetics of g-HA/PLA composite. Thermochimica Acta 2009; 493: 90–95. 21. Calandrelli L, Immirzi B, Malinconico M, et al. Preparation and characterisation of composites based on biodegradable polymers for ‘‘in vivo’’ application. Polymer 2000; 41: 8027–8033. 22. Cowie JMG and Arrighi V. Polymers: Chemistry and Physics of Modern Materials. 3rd ed. New York: CRC Press, 2007. 23. Yu L, Pei X, Lei T, et al. Genome shuffling enhanced L-lactic acid production by improving glucose tolerance of Lactobacillus rhamnosus. J Biotechnol 2008; 134: 154–159. 24. Pluta M, Jeszka JK and Boiteux G. Polylactide/montmorillonite nanocomposites: structure, dielectric, viscoelastic and thermal properties. Eur Polym J 2007; 43: 2819–2835. 25. Sinha Ray S, Yamada K, Okamoto M, et al. New polylactide-layered silicate nanocomposites. 2. Concurrent improvements of material properties, biodegradability and melt rheology. Polymer 2003; 44: 857–866. 26. Tsuji H, Shimizu K and Sato Y. Hydrolytic degradation of poly(L-lactic acid): combined effects of UV treatment and crystallization. J Appl Polym Sci 2012; 125: 2394–2406. 27. Dias MOS, Modesto M, Ensinas AV, et al. Improving bioethanol production from sugarcane: evaluation of distillation, thermal integration and cogeneration systems. Energy 2011; 36: 3691–3703. 28. Kaavessina M, Ali I, Elleithy R, et al. Crystallization behavior of poly(lactic acid)/elastomer blends. J Polym Res 2012; 19: 9818. 29. Garlotta D. A literature review of poly(lactic acid). J Polym Environ 2001; 9: 63–84. 30. Kaavessina M, Ali I, Elleithy R, et al. Preparation and characterization of poly(lactic acid)/ elastomer blends prepared by melt blending technique. J Elastomers and Plast 2014; 46: 253–268. 31. Aizawa M, Hanazawa T, Itatani K, Howel FS and Kishioka A. Characterization of hydroxyapatite powders prepared by ultrasonic spray-pyrolysis technique. J Mater Sci 1999; 34: 2865–2873. 32. Vlaev LT, Markovska IG and Lyubchev LA. Non-isothermal kinetics of pyrolysis of rice husk. Thermochimica Acta 2003; 406: 1–7. 33. Huang J-W, Chang Hung Y, Wen Y-L, et al. Polylactide/nano- and microscale silica composite films. II. Melting behavior and cold crystallization. J Appl Polym Sci 2009; 112: 3149–3156. 34. Balakrishnan H, Hassan A, Wahit MU, et al. Novel toughened polylactic acid nanocomposite: mechanical, thermal and morphological properties. Mater Des 2010; 31: 3289–3298.
