Carmen Albano¹, Gema González², Jordana Palacios³, Arquímedes Karam³, Reina Verónica Castillo³, María Covis³

1 Universidad Central de Venezuela, Facultad de Ingeniería, Caracas, Venezuela. *e-mail: carmen.albano@ucv.ve

2 Instituto Venezolano de Investigaciones Científicas, Departamento de Ingeniería de Materiales y Nanotecnología, Caracas, Venezuela.

3 Instituto Venezolano de Investigaciones Científicas, Centro de Química, Laboratorio de Polímeros, Caracas, Venezuela.

ABSTRACT

The chemical, thermal, thermo-mechanical and morphology properties of poly (L-lactide) (PLLA) composite with 30% hydroxyapatite (HA) were evaluated. The composite was prepared employing the solvent casting technique. Hydroxyapatite was synthesized by chemical precipitation method. The degradation kinetic parameters were calculated using the Coats – Redfern integral method to obtain the reaction order and the E₂ function methodology to calculate the activation energy (Eα). The addition of HA to PLLA matrix increased its glass transition temperature. This was confirmed by DSC and DMTA analysis. Also, the presence of HA increased the crystallization temperature of PLLA, implying a nucleation effect. The PLLA-HA composite exhibited better thermal stability than the neat polymer; additionally higher decomposition temperature and activation energy for the decomposition process were obtained. HA nanoparticles have a rod shape morphology that might improve the interfacial interactions, increasing the thermal stability of the composite. The storage modulus (E’) of this composite was enhanced mainly at temperatures above the glass transition.

Keywords: PLLA composites, Hydroxyapatite, Thermal degradation, DMTA.

Caracterización del compuesto poli (L-Lactida) / Hidroxiapatita: Propiedades químicas, térmicas y termomecánicas.

RESUMEN

Palabras clave: Compuestos de PLLA, Hidroxiapatita, Degradación térmica, DMTA.

Recibido: julio 2012 Recibido en forma final revisado: enero 2013

INTRODUCTION

The addition of specific nanoparticles to a polymeric matrix enhances the properties and broadens the field of application of polymers. For these reasons, over the past few decades, researchers have been focused on the development of better and improved polymer composites evaluating different nanofillers like organoclays (Pavlidoua & Papaspyridesb, 2008; Bordes et al. 2009), carbon nanotubes (CNT) (Sahoo et al. 2010; Spitalsky et al. 2010) and hydroxyapatite (HA) (Rezwan et al. 2006; Roeder et al. 2008).

Hydroxyapatite has been intensively investigated as bone repair material due to its chemical composition (Ca₁₀(PO₄)₆(OH)₂) (Rezwan et al. 2006) and structure, which is similar to the main inorganic constituent of natural bone, and also to their bioactive, biocompatible, and osteoconductive properties (Kokubo et al. 2003; Kumta et al. 2005; Rezwan et al. 2006; Zhou et al. 2007; Roeder et al. 2008). However, their low mechanical fracture performance and high brittleness restrict their application in bone regeneration at high load-bearing sites (Kokubo et al. 2003).

The combination of HA properties with those of the polymeric matrices seem to be promising for bone tissue engineering purposes, due to a good combination of flexural strength and high toughness [8]. To accomplish this improvement, HA particles should exhibit an acicular structure with nanometric size (Kokubo et al. 2003; Kothapalli et al. 2005; Li & Wang, 2005; Zhou et al. 2007); this morphology promotes a better interaction and load transfer between the filler and the matrix.

Different biodegradable polymers (Nair & Laurencin, 2007) have been used as biomaterials for bone tissue engineering applications, some poly(α-esters)/HA composites have attracted much interest due to their good biocompatibility, specific biodegradability and good mechanical properties. Among them, the most employed composites with HA are Poly(lactide) (PLA) and its stereoisomeric forms, poly(glycolide) (PGA) and poly(lactide-co-glycolide) (PLGA) copolymer (Shikinami & Okuno,. 1999; Kim et al. 2006; Petricca et al. 2006; Lee et al. 2008; Cui et al. 2009; Chłopek et al. 2009; Aboudzadeh et al. 2010; Zhang et al. 2010). These polymers are FDA approved and therefore exhibit biocompatible properties. In addition, they are easily processed and their physical and mechanical properties and degradation features can be adjusted by copolymerization (Shikinami & Okuno, 1999; Kim et al. 2006; Petricca et al. 2006; Lee et al. 2008; Cui et al. 2009; Chłopek et al. 2009; Aboudzadeh et al. 2010; Zhang et al. 2010).

PLA-HA composites have been synthesized through several methods, focusing in most of the cases on the final application. Solution mixing and in-situ precipitation were employed in order to characterize and evaluate the properties of the composite material. Zhang et al. (2010) used a modified in situ precipitation method to prepare PLLA/nano-HA composites, obtaining a well distributed HA and improvement on Young’s modulus and compressive strength. Scaffolds based on PLA-HA composites have been obtained using many different methods: thermally induced phase separation (Nejati et al. 2008), porogen-leaching (Li et al. 2010), rapid prototyping technique (Li et al. 2010), electrospinning (Peng et al. 2011), and electrospun techniques (Seyedjafari et al. 2010). In general, these works deal with the bioactive performance of the composite device for a bone tissue engineering application and less emphasis is made on the physico- chemical, thermal and mechanical properties of the composite.

In the present work PLLA with 30 wt. % HA composite was prepared to elucidate the effect of HA on the chemical, mechanical, thermal and thermal degradation properties of PLLA. The filler content (30wt. % HA) was chosen based on previous studies conducted (Albano et al. 2009; Albano et al. 2010; Albano et al. 2010). The kinetics of the thermal degradation of the composites using the Coats – Redfern integral method (Coats & Redfern, 1964) was evaluated to obtain the reaction order, and the E2 function method (Chen et al. 2004) was used to calculate the activation energy.

Materials

A Poly(L-lactide) (PLLA) homopolymer from Sigm-Aldrich, was used as polymer matrix (Lactel® BP-0600, Mw: 66.000-107.000). For the HA synthesis, calcium hydroxide (Ca(OH)₂) 98,6% and ammonium hydrogen phosphate ((NH₄)₂HPO₄) 99%, (Fisher Chemicals), were used as starting materials. Chloroform 99.8% supplied by Merck was employed as the solvent for composite preparation.

Synthesis of hydroxyapatite

The synthesis of HA was carried out according to ref (Spadavecchia & González, 2007). Stoichiometric deionized water solutions of Ca(OH)₂ and (NH₄)₂HPO₄ were prepared by magnetic stirring for 30min. Then, (NH₄)₂HPO₄ solution was transferred to an addition funnel and dropped into Ca(OH)₂ solution under continuous stirring. After that, the reaction was allowed to continue for 30min. Then, the mixture was decanted for 3 days. The solid obtained was centrifuged and washed with de-ionized water repeatedly until neutral pH was achieved. The HA was vacuum-dried at 60ºC for 62h. Finally, it was grinded and sieved until a fine powder was obtained.

Composite preparation

The composite (PLLA-HA) was prepared by solvent casting technique using chloroform as solvent. Specific quantities of PLLA and HA were separately dissolved and suspended, respectively, in chloroform for 4 hours. After that, the PLLA solution was slowly added to the HA suspension under continuous magnetic stirring for 4 hours. Then, the mixture was poured into a petri dish and allowed to dry at room temperature to remove the excess of solvent and later in a vacuum oven for a week.

Characterization

Crystallographic analysis of the synthesized HA was performed in a SIMENS D5005 diffractometer. The XRD data were recorded using CuKα radiation (λ = 0.15406nm) and the diffraction patterns were scanned between 10 and 80º (2θ) with step size of 0.02º and 0.52s counting time per step.

Fourier infrared spectroscopy was carried out for the pure components and composite in a Thermo Scientific Nicolet iS10 spectrometer with a resolution of 4cm^-1. HA powdered sample was prepared with KBr and the spectrum was recorded in transmission mode between 4000 and 400cm^-1 (32 scans). The composite film was evaluated by Attenuated Total Reflection (ATR) technique between 4000 and 650 cm-1 (32 scans, 4cm-1 resolution).

The thermal stability was studied by thermogravimetric analysis (sample mass 10mg), employing a Mettler–Toledo TGA/SDTA 851 thermal analyzer under nitrogen atmosphere. The equipment was calibrated with indium and aluminum standards. The non-isothermal tests were carried out from 25 to 700ºC at 10°C/min. The thermograms obtained were analyzed employing the Coats-Redfern kinetic models (Coats & Redfern, 1964) and the E₂ function method (Chen et al. 2004) to determine the global activation energy value (Ea).

Equation (1) was used to determine the reaction order n according to Coats-Redfern method:

Using the previous expression and the G(α) functions (Mamleev et al. 2000; Bourbigot et al. 2001; Budrugeac et al. 2001), the reaction order n was determinate from the best fit of equation (1) suggesting the reaction mechanism.

With n known, the E₂ function method (Chen et al. 2004) was used to determine Ea and A values, using equations (2) and (3):

The activation Energy Ea is calculated using three conversion values (α1, α2 and α3) and three corresponding decomposition temperatures (T₁, T₂ and T₃).

The mechanical properties were evaluated by dynamic – mechanical – thermal analysis (DMTA) using a Rheometrics Solids Analyzer RSA II in tension mode, film geometry. Rectangular film samples (22 mm long, 8 mm width, 0.1 mm thickness) were employed. A dynamic temperature ramp was recorded from 25 to 100ºC at 1ºC/min, with a frequency of 1 Hz and 0.015% strain. This strain value was previously determined using a dynamic strain sweep test at 1Hz, using the value located in the linear viscoelastic zone. The storage modulus (E’) and the loss factor (tan δ) of PLLA and the composite were measured.

RESULTS AND DISCUSSION

Characterization of the synthesized HA

The FT-IR spectrum of the synthesized HA is presented in Figure 1. The small signal around 3567 cm^-1 belongs to the stretching mode of the hydroxyl group (OH) present in HA. A broad absorption band located at 3423 cm^-1 and a band at 1638 cm^-1 associated to the bending (ν2) mode, corresponding to adsorbed water are present. The four vibrational modes of the phosphate ions of HA are located at 470 cm^-1 for the double degenerated bending (ν2) mode of the O−P−O bonds; at 565 and 603 cm^-1 for the triply degenerated bending (ν4) mode of the O−P−O bonds (well-defined bands); at 962cm^-1 (very weak signal) for the non-degenerated symmetric stretching (ν1) mode of the P−O bond; and at 1033 and 1096 for the triply degenerated asymmetric stretching (ν3) mode of the P−O bond of the phosphate group (Zhou et al. 2007). Also, the vibration bands located between 1500 and 1400 cm^-1 and at 875cm-1 are indicative of the carbonate groups. These groups show three vibrational modes: the bending modes (ν3 or ν4) signals situated at 1453 and 875 cm^-1, and the stretching mode (ν3) located at 1413cm^-1 (Zhou et al. 2007). These signals are characteristic of the presence of carbonates in substitution of some of the hydroxyl groups, consequence of the interaction between HA and ambient CO2 during the synthesis (Wang et al. 2007).

The crystallographic structure of the HA was also confirmed by x-ray diffraction analysis (Figure 2), and the needle shape morphology of average dimensions of 74 ± 21 nm long and 22 ± 7 nm width, was observed by TEM (Figure 3). The high aspect ratio of HA nanocrystals results in a high interfacial area, this is an important feature to improve the contact area between these nanoparticles and the matrix in polymer composites, improving the mechanical reinforcement.

The thermal analysis of HA is shown in Figure 4. The total mass loss of this nanoceramic is 11%. Three differentiated regions can be observed, the first region from 25 to 250°C, is associated to the loss of the physically adsorbed water molecules. The second stage between 250-550 °C could be attributed to the slow loss of carbonate ions on the HA , the last region is associated to the slow dehydroxylation of HA (Ivanova et al. 2001).

The FT-IR spectra of Poly (L-lactide), PLLA-HA and HA is shown in Figure 5. The spectrum of PLLA presents a signal located at 1748 cm^-1 associated to the stretching mode of the carbonyl group and the vibrations at 1180 and 1080 cm^-1 corresponding to the symmetric and asymmetric C-O-C stretching modes of the ester groups. The spectrum of PLLA-HA composite shows a shift in the position of the band associated to the phosphate group of HA, from 1033 cm^-1 to 1020 cm^-1, overlapping the νC-CH3 stretching of PLLA (Table 1). Also, the band at 1638 cm^-1, associated to the bending (ν2) mode of adsorbed water molecules, is shifted to 1605 cm-1 in the composite. A slightly displacement of the carbonyl signal from 1748 cm-1 to 1755 cm-1 is also observed in the composite spectrum. This band splits in two peaks (inset Figure 5), attributed to interaction of the hydrogen bond of the carbonyl group of PLLA and HA (Zhou et al. 2007). Therefore, the different displacements observed suggest possible interactions between the functional groups of the HA and the polymer matrix.

The distribution of HA on the polymer matrix was analyzed by TEM (Figure 6). It can be observed that the nanoparticles are well distributed in the polymer matrix, preserving the rod shape morphology; although some agglomeration was obtained. It is possible that the solvent casting methodology employed to prepare the PLLA-HA composites is not entirely effective to promote a high dispersion of the filler, this could be due to the high content of HA. Similar results have been reported previously in HDPE-HA composites obtained by solution method (Albano et al. 2006).

Thermal behavior study by DSC of the PLLA-HA composite

The thermograms of PLLA and PLLA-HA are shown in Figure 7 and the results are presented in Table 2. For PLLA, the exothermal peak at 94.5 ºC corresponds to the crystallization temperature (T_mc) (see Fig. 7 (a)). During the heating scan, PLLA presents its glass transition temperature (T_g) at 52.5ºC, and a cold crystallization exothermal peak (T_cc) at 102.9 ºC and an endothermal peak (T_m) at 172.2 ºC corresponding to the melting transition (see 7 (b)), these values are within the range reported in the literature (Velde & Kiekens,. 2002; Nair et al. 2007; Wilberforce et al. 2011). The presence of a cold crystallization process is a consequence of the slow crystallization kinetics of PLLA during cooling.

Besides the cold crystallization and the melting transition, an exothermal peak located just before the melting temperature of PLLA at 152. 3 ºC was observed (identified as Tr in Table 2). This behavior could be explained by the melt-recrystallization model (Yasuniwa et al. 2004; Aboudzadeh et al. 2010), which suggests that smaller and less perfect crystallites would change into more stable forms, as the temperature increases. So the melting and recrystallization process are competitive during heating and this exothermic peak would appear when the rate of recrystallization overcomes that of the melting process (Yasuniwa et al. 2004).

Once HA is incorporated into PLLA matrix, it is clear that these nanoparticles would act as nucleating agents [35] and promote the crystallization of the polymer matrix during cooling. This is evident by the narrow peak observed in PLLA-HA composite thermogram (Figure 7(a)). Additionally, the presence of HA suppresses the cold crystallization and melt-recrystallization process observed in neat PLLA. Deplaine et al. ( 2010) reported a decrease in the cold crystallization temperature in PLLA-HA membranes, with a filler content up to 15 %wt. HA. In our case, the high HA content could induce the nuclei and crystal growth during the cooling, so more stable crystals are formed and no reorder or additional recrystallization occurs during heating.

The composite PLLA-HA showed an increase of 6 ºC on the transition temperature, respect to PLLA (Table 2). The HA nanoparticles reduce the chain segment mobility of the polymeric amorphous phase around them, so the glass transition will occur at higher temperatures (Pham et al. 2003; Grady et al. 2009). Also, the increase on Tg has been attributed to the existence of some interfacial interactions or non-covalent bonding between HA and the polymer matrix, that restricts the movements of the polymer chains, consistent with FT-IR analysis. An increase in the crystallization temperature of 10 ºC was observed, while no major changes on its melting temperature and crystallinity degree were present. The increase on the crystallization temperature indicates a nucleation effect of the HA nanoparticles, with no detriment on the crystallinity of the polymer matrix. This behavior has been previously observed with different fillers into polymer matrices (Reyes-de-Vaaben et al. 2008).

Thermal degradation behavior of the PLLA composite

Figure 8(a) shows the thermograms obtained by TGA for PLLA, PLLA-HA and HA. The PLLA thermogram shows a single step around 360 °C, however, also a shoulder around 340ºC is observed in the derivative graph (Figure 8(b)). It is known that these polyesters exhibit a degradation mechanism that could involve a random chain scission at the beginning of the decomposition and specific chain scission at the end. At lower temperatures, the main degradation mechanism for neat poly(lactide) involves a non-radical, backbiting ester interchange reaction involving OH chain ends, leading mostly to cyclic oligomers rather than linear, or acetaldehyde plus carbon dioxide. As the temperature increases the release of carbon monoxide, methylketene and ketene, as main degradation products are produced as a consequence of a radical chain scission mechanism (McNeill & Leiper, 1985).

The addition of 30% HA to PLLA matrix improves the thermal stability of the polymer, as can be seen in Figure 8. A rise on the initial decomposition temperature (Tonset) and on the degradation peak temperature (Tpeak) was observed for PLLA, in the composite (Table 3). On the other hand, an increase on the degradation rate of the polymer matrix was observed with the presence of the HA (Figure 8 (b)). Also, an increase in the activation energy of the polymer in PLLA-HA composite was obtained. Therefore, HA improves the thermal stability of PLLA, but once the degradation process is initiated, the decomposition rate is high.

Dynamic mechanical thermal analysis of PLLA-HA composite

The addition of 30% HA to PLLA confers some reinforcement effect to the matrix at temperatures approaching the glass transition (T_g). Above this transition, the stiffness effect of HA particles maintain the structural mechanical stability of the composite at higher temperatures. Also, a possible interface interaction between the polymer matrix and HA, due to the high interfacial effect of the HA nanorods could lead to a more effective load transfer between them and therefore an improvement on the storage module was obtained. Similar results were reported in PLLA-HA composites with low nanofiller content (Wilberforce et al. 2011).

On the other hand, the addition of HA to PLLA increased the glass transition temperature (Figure 10). The loss factor curves for PLLA and PLLA-HA composite confirm the results obtained by DSC. The peak of the curve corresponds to the glass transition relaxation. The increase in temperature associated to this relaxation is mostly due to the restriction of the cooperative motions of chain segments imposed by the presence of the rigid HA nanoparticle. As a consequence, a better mechanical stability of the composite with temperature is obtained.

CONCLUSIONS

A composite based on PLLA -HA was prepared by the solvent casting technique and characterized in terms of its chemical, thermal and thermo-mechanical properties. The activation energy of the composite was obtained from the thermal degradation study. An interaction between the polymer matrix and HA through the carbonyl and phosphate groups was obtained by FTIR The several thermal transitions of PLLA were evaluated by DSC: the glass transition, crystallization, cold crystallization, melt-recrystallization and melting. The addition of HA to PLLA matrix increases its glass transition temperature, this was confirmed by DSC and DMTA analysis. Also, the presence of HA increases the crystallization temperature of PLLA, implying a nucleation effect. On the other hand, the cold crystallization phenomenon was suppressed by HA presence and no major changes on the melting temperature and crystallinity were observed. The PLLA-HA composite showed better thermal stability than the neat polymer. The introduction of nano-HA particles increased the decomposition temperature and the activation energy retarding the decomposition process of PLLA. The rod shape morphology of nano-HA implies a possible interfacial interaction that increases the thermal stability of the composite. Additionally, an improvement of the mechanical stability of the composite with temperature is obtained.

ACKNOWLEDGEMENTS

The authors are grateful to Prof. Alejandro Müller from the Polymer Group at Simón Bolivar University for providing the facilities for the dynamic – mechanical – thermal analysis of the samples.

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