Fully biobased robust biocomposites of PLA with assisted nucleation by monodispersed stereocomplexed polylactide particles

Abstract

Fully biodegradable biocomposites are a desirable choice among the synthetic plastics due to their increasing adverse ecological issues. Stereocomplexed polylactide submicronparticles (s-PLA SP), which provide dense nucleating sites to PLLA in the crystallization process, were used to prepare homogenous PLLA biocomposites to enhance the thermal and mechanical properties of PLLA. Herein, we report the synthesis of monodispersed s-PLA SP and preparation of biocomposites of PLLA incorporating s-PLA SP. The synthetic system for s-PLA SP is comprised of two steps. The first step involves solvating PLLA and PDLA in respective reactors. The second step is the stereocomplexation of the dissolved polymer in the mixing reactor. The polymer solvation conditions greatly affect the s-PLA particle size and distribution. Therefore, we studied the variation of s-PLA particle size and polydispersity index (PDI) due to the effect of the polymer solvation parameters, such as homopolymer concentration, solvation time, temperature, pressure and molecular weight, as the first step. The optimum conditions established were 50 °C, 300 b, with 5% homopolymer solvation for 12 h. In the second step, s-PLA SP formation was carried out at the previously established reaction conditions of 70 °C, 250 b for 2 h. The PLA homopolymers (M_n ∼ 57 kg mol⁻¹) produced s-PLA SP smaller than 963 nm with a PDI of 0.12, as analyzed by DLS and SEM at the optimum reaction conditions. We cast completely biobased homocomposites (BC) for high performance by homogeneously dispersing s-PLA SP in the high molecular weight PLLA matrix (M_n ∼ 200 kg mol⁻¹) in chloroform, followed by annealing. The solution casting DSC results show the enhanced crystallization of PLLA for various compositions of BC ranging from 0.5% to 5%. An increase in the concentration of s-PLA SP increases the thermal and mechanical properties of the PLA BC. Effective nucleation with decreased spherulite size was noticed for the 5% BC with 963 nm s-PLA SP incorporated into the casting solution, as inferred by SEM analysis. The tensile strength and Young's modulus increased from 29 MPa to 73 MPa and 1.3 GPa to 3.2 GPa, respectively, whereas the X_cc increased from 20% to 35% and T_cc increased from 98 °C to 113 °C with the 5% composition involving 963 nm s-PLA SP.

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Introduction

PLA has wide applications in commercial and biomedical fields since it is an excellent biocompatible candidate among the synthetic polyesters. However, its slow crystallization rate and low mechanical properties largely limit its processing and implementation.¹

Stereocomplexation of PLA can improve its thermomechanical properties. However, the innate poor mechanical and thermal tolerance of the PLLA homopolymer remains challenging. To date, several investigations have addressed the improvement of the innate brittleness of PLA via various methods such as co-polymerization, blending with various organic and inorganic additives, cross-linking and nanocomposite fabrication.^2–4 Polymer composites differ from traditional mixtures of polymers in terms of the resulting new material with advanced properties. The combined properties of the composite are not merely the result of the addition of two materials and their properties.⁵ To increase the crystallization rate of PLA, usually a nucleating agent is added to the PLA matrix.⁶ Fine crystalline particles with a melting temperature higher than the crystallization temperature of the matrix are said to be the nucleating agent.⁷ The accelerated crystallization of the polymer is a direct consequence of increased nuclei density and reduced spherulite size under the effect of a nucleating agent.⁸ Not only small molecule additives, but also polymers and stereocomplexes can act as nucleating agents.^9,10

Much effort has been made to investigate organic and inorganic nucleating agents for PLA, e.g. poly(butylenes adipate-co-terephthalate) (PBAT), PBS, poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG), to act as nucleating agents for PLA.^11–14 Some inorganic additives, e.g. talc, CaCO₃, magnesium silicate and SiO₂, have also been reported to reinforce PLA.¹⁵ Reducing the crystallization induction time by adding specific nucleating agents is an important industrial implementation of this concept. Crystallization temperature, crystallinity and crystallization rate are boosted with a nucleating agent.¹⁶ Nucleating agents reduce polymer spherulite size and provide surface area for nuclei sites.^17–20 The large surface area to volume ratio of nucleating agents gives rise to more nucleating sites.²¹ Nanoparticle size fillers offer larger surface areas with better crystallization behavior than microparticle fillers.^21–23 Another challenge is the phase difference of the filler and the matrix of the composites. Heterogeneous composites are limited in processibility due to the inherent phase difference of the materials combined. The existing commercial composites have nondegradable contents, which are purposed for long lasting applications.²⁴ The good dispersion of filler particles in the polymer matrix is the basis of the improvement in thermomechanical properties. Therefore, homogenous and biodegradable nanocomposites with improved physical properties have a bright scope for implementation in biomedical fields such as bone implants, cardiovascular grafts and bone tissue regeneration.^25–27

Homogenous BCs of the biodegradable and biocompatible PLA with tuned physical properties are necessary due to arising ecological issues. The s-PLA SP are a perfect match for the nucleation of PLLA since they are in the same class of biocompatible polymers. These submicronparticles can provide a larger surface to area ratio for seeding a higher number of nuclei and faster crystal growth. Herein, we present a completely biodegradable, homogenous composite system of polymers for biocompatible and commercial implementation. We assume that narrowly dispersed s-PLA nanoparticles can generate a uniform platform of evenly distributed nuclei centers to boost the crystallization of PLLA. In this study, we synthesized monodispersed s-PLA SP, and incorporated them as nucleating agents to prepare completely homogenous PLLA biocomposites to enhance the thermal and mechanical properties of PLLA. We observed the effects of different concentrations and particle size of s-PLA SP on the thermal and mechanical properties of high molecular weight PLLA.

Experimental

Materials and methods

D-Lactide and L-lactide (M_w = 144.3 g mol⁻¹) were purchased from Purac America Inc. and used as received. Tin(II) 2-ethylhexanoate (Sn(Oct)₂), 1-dodecanol (purity 92.5–100%) and toluene (purity 99.9%) were purchased from Sigma-Aldrich. Tin(II) 2-ethylhexanoate and 1-dodecanol were dissolved in toluene prior to use. Chloroform (Sigma-Aldrich, purity 99.9%) and N₂ (Shin Yang Oxygen Industry, minimum purity 99.9%) were used as received.

Synthesis of stereocomplex polylactide nanoparticles

Submicron s-PLA particles were prepared via a solution feed system using the supercritical fluid technology, as shown in Fig. 1. PLLA and PDLA were enclosed in high pressure autoclaves R1 and R2, respectively, equipped with magnetic stirrers. DME was inserted in R1 and R2 via high pressure needle valves V1 and V2, respectively. The homopolymers were allowed to dissolve with continuous vigorous stirring. Once they dissolved at the desired solvation conditions, the polymer solutions were shifted through another pair of high pressure valves, V3 and V4, respectively, into a stereocomplexation autoclave R3 where stereocomplexation of the polymers proceeded at previously established reaction conditions.²⁵ At the end of the reaction the stereocomplex PLA–sc-DME fluid was sprayed into the air at high pressure to obtain s-PLA nanoparticle powder.

Fig. 1 Schematic of the solution feed system. Abbreviations used are: P = pressure gauge, V = high pressure valves, T = temperature controller, and s = magnetic stirrer. Black arrows represent the flow of DME. The numerals 1, 2, …5 represent the order of the equipment.

Preparation of bionanocomposites of PLLA

PLA biocomposites (BC) were prepared by dispersing s-PLA submicron particles (s-PLA SP) of various concentrations ranging from 0.5% to 5% in the PLLA matrix (M_n = 2.0 × 10⁵ g mol⁻¹). After the evaporation of chloroform at room temperature, the PLA BC were annealed in a vacuum oven.

Characterization

The molecular weight of the polymers was determined by gel permeation chromatography (GPC) (Viscotek model 302 TDA). The cold crystallization temperature T_cc (°C), melting temperature T_m (°C), glass transition temperature T_g (°C), and degree of crystallinity X_cc (%) were measured on a modulated differential scanning calorimeter (modulated DSC 2910, TA Instrument). The crystallization of PLLA films containing different contents of s-PLA SP was observed by non-isothermal DSC scanning. The films were heated and cooled at a rate of 10 °C min⁻¹. Herein, a holding temperature of 200 °C was selected to melt the PLLA crystallites, but not to melt the stereocomplex crystallites. X-ray diffraction spectra were recorded using a Rigaku D/Max 2500 X-ray diffractometer with a Cu KR (λ = 1.54056 Å, 30 kV, 100 mA) source, a quartz monochromator, and a goniometric plate. Thermogravimetric analysis (TGA) was conducted on a Hi-Res TGA 2950 (TA Instrument) under N₂ flow. Mechanical properties were measured on an Instron apparatus. For the tensile tests, dumbbell shaped strips with a specimen size of 10 × 3 mm and 100 μm thick were cut from PLLA BC films. The gauge length was 10 mm, and the extension rate was 1 mm min⁻¹. Ten specimens were tested. SEM images were obtained for these film segments using a FEI-Nova Nano SEM 200. Particle size was analyzed via dynamic laser scattering (DLS) on an EL-Zeta potential particle analyzer. The morphology of the films was observed using a Carl Zeiss Axio Imager A2m (Germany) polarized optical microscope equipped with a camera. Cryofractured film surfaces were formed with liquid N₂ at room temperature.

Results and discussion

In this report, we generate monodispersed s-PLA SP and incorporate them into PLLA to study their nucleating effect. The s-PLA SP initiated nucleation and crystal growth of PLLA synergistically, thus providing ultimate mechanical strength and thermal tolerance to the BC. In order to observe the effect of particle size of s-PLA, we generated different submicronparticle sizes by tuning various physical parameters such as concentration, time, temperature, pressure and molecular weight of the homopolymer.

We developed a solution feed system employing supercritical fluid technology for the synthesis of monodispersed s-PLA submicronparticles. PLLA and PDLA homopolymers with variable molecular weights were prepared by ring opening polymerization using Sn(Oct)₂ and dodecanol as the initiator and co-initiator, respectively. The homopolymers were inserted in respective high pressure autoclaves equipped with magnetic stirrers and wrapped with bent heaters. DME was inserted to dissolve the homopolymers with continuous stirring, as aforementioned (Fig. 1).

In solution feed system processing, homopolymer solutions have variable solvating conditions for various sets of experiments. However the stereocomplexation of PDLA and PLLA was performed at predetermined reaction conditions (specifically, 250 b, 70 °C for 2 h)²⁵ for all sets of experiments, which will be described throughout the discussion as predetermined stereocomplexation reaction conditions (PSC). The particle size results for s-PLA obtained with variable parameters are plotted in Fig. 2(a)–(e).

Fig. 2 Effect of (a) concentration (b) solvating time, (c) pressure, (d) temperature, and (e) molecular weight on the particle size and PDI of s-PLA nanoparticles and (f) DSC scan of s-PLA SP. For the synthesis of s-PLA submicron particles we used PLLA with M_n = 57 kg mol⁻¹ and PDLA with M_n = 53 kg mol⁻¹.

To study the effect of concentration we varied the polymer to DME total feed concentration from 2% to 10% with the solvating conditions fixed as 40 °C, 300 b for 12 h and PSC. For the total homopolymer (PLLA + PDLA) feed concentration (T.H.P.) from 2% to 5%, the s-PLA particle size decreased from 1021 nm to 995 nm with a PDI of 0.18 to 0.14, respectively, as shown in Fig. 2(a). However, the increase in total homopolymer feed concentration to 10% resulted in an increased s-PLA particle size up to 1669 nm with a comparatively broad PDI of 0.19. For 2% T.H.P., we assume that the density of polymer–SCF is below the cloud point of the sc-DME, hence precipitation could not occur to generate monodispersed nanoparticles s-PLA. At higher T.H.P. (e.g., 10%), the s-PLA particles remain intact and coagulate rather than disperse fairly in the sc-DME fluid to generate homogenous tiny particle sizes when sprayed into air.

However, the feed concentration matches the density values to give uniform monodispersed small particle sizes of s-PLA powder at 5% T.H.P. The higher DME concentration contributes a strong dissolution power to the polymers and great pressure for good dispersion to give monodispersed small size s-PLA dry powder sprayed into air.

The homopolymer solutions had a variable solvating time from 3 h to 24 h, while maintained at 40 °C, 300 b, 5% T.H.P. and PSC. For short durations, e.g. 3–6 h, the s-PLA particle size and PDI were 2081–1885 nm and 0.23–0.19, respectively. Solvation for 12 h generated the smallest nanoparticles s-PLA (992 nm) with a PDI of 0.12, as shown in Fig. 2(b). However, the particle size and PDI of s-PLA were not much different for 24 h polymer solvation time as compared to that for 12 h. Less time for the solvating homopolymers could not provide complete dissolution and a good consistency for monodispersity or small s-PLA particle sizes. However, dissolution of the homopolymers for a night produced completely dissolved, freely mobile homopolymer chains ready for uniform distribution to generate small size monodispersed s-PLA particles.

Homopolymer dissolution experiments were conducted at variable pressures with 5% T.H.P. from 100–350 b at 40 °C for 12 h to evaluate the immensely important phenomenon of the effect of pressure on the particle size of s-PLA. As evident from Fig. 2(c), lower solvating pressures for the homopolymers from 100–200 b resulted in larger s-PLA particle sizes with a broad PDI. This could be the result of the polymer–sc-DME fluid density scarcity for sufficient solubility. This yielded a solution above the cloud point and the precipitates remained aggregated and coagulated, which resulted in the formation of comparatively bigger particles. Increasing the pressure to 300 b resulted in uniformly distributed monodispersed s-PLA particles 997 nm in size and PDI of 0.13. Higher pressures yield a completely dissolved and freely mobile polymer chain system.

Solvating temperature experiments were conducted with 5% T.H.P. from 40–70 °C at 300 b for 12 h and PSC. As shown in Fig. 2(d), increasing the temperature up to 50 °C reduced the particle size to 963 mm with PDI 0.12. Higher values of polymer dissolution temperature like 60–70 °C produced larger s-PLA particles from 1072–2563 nm with PDI from 0.18–1.4 respectively. This is due to the fact that the elevated temperature decreases the viscosity and density of polymer–SCF system required for good dissolution of homopolymers.

We observed that high molecular weight homopolymers generate larger s-PLA particle sizes, as shown in Fig. 2(e). Varying the molecular weight of the homopolymers from M_n = 57–137 kg mol⁻¹ resulted in the particle size of s-PLA increasing from 963–2297 nm with a PDI from 0.12–1.3. These experiments were conducted with 5% T.H.P. at 50 °C, 300 b and 12 h solvation.

Several reports indicate that it is difficult for larger PLA chains to settle as stereocomplex-crystallites, which is much easier for low molecular weight PLA chains.^29,30 The s-PLA particle shape, size and number distribution are shown in Fig. 3. The spherical s-PLA SP can be seen in the SEM image with their PDI shown in the DLS number distribution histogram. These s-PLA SP have a melting temperature of 229.6 °C, as shown in Fig. 2(f), which is higher than the melting point of PLLA. Hence, the s-PLA SP are suitable nucleating agents for PLLA. We obtained a dry fine powder of s-PLA SP from these experiments for further applications. The optimum conditions established for the first step as 50 °C, 300 b, with 5% homopolymers solvating for 12 h. Whereas, in the second step the s-PLA SP formation carried out at previously established reaction conditions as 70 °C, 250 b for 2 h using the PLA homopolymers with molecular weight of M_n ∼ 57 kg mol⁻¹).

Fig. 3 SEM image (left) and DLS histogram (right) of the s-PLA submicronparticles. For the synthesis of the s-PLA submicron particles we used PLLA with M_n = 57 kg mol⁻¹ and PDLA with M_n = 53 kg mol⁻¹ with the optimum reaction conditions of 5% T.H.P. at 50 °C, 300 b and 12 h solvation.

After obtaining different size monodispersed particles of s-PLA, we incorporated them into PLLA biocomposites (BC). The thermal and mechanical properties of the PLLA BC were investigated under the effect of s-PLA SP. Solution castings with a concentration of 50 g L⁻¹ were prepared by dispersing s-PLA SP into the high molecular weight PLLA matrix (M_n = 2.3 × 10⁵ g mol⁻¹) in chloroform, followed by annealing. The s-PLA SP were well dispersed before adding PLLA into the solution in all castings. In order to optimize the compositions and annealing conditions in our experiments, we selected the s-PLA particle size of 963 nm. The 0.5% s-PLA compositions were cast for a variable time duration at 70 °C in one experiment set, as shown in Table 1. For comparison, neat PLLA was also cast as non-annealed and annealed samples at room temperature and at 70 °C, respectively. Increasing the annealing time of the 0.5% BC from 6 to 24 h resulted in an enhancement in the mechanical properties and X_cc (%) of the samples, as shown in Table 1. Annealing of the 0.5% BC at 70 °C for 6 h gives the tensile strength of 39.2 MPa with 1.5 GPa Young's modulus. However, increasing the annealing duration up to 24 h, increases the tensile strength up to 54 MPa with an increased Young's modulus up to 1.8 GPa. The corresponding X_cc increased from 21% to 25% with the T_cc increment from 98.9 °C to 113.9 °C, respectively. This enhancement in the 0.5% composition is appreciable than the neat annealed sample of PLLA. The neat PLLA film annealed at 70 °C for 24 h has a tensile strength of 29 MPa with a 1.3 GPa Young's modulus. The X_cc observed by DSC for the neat, annealed PLLA film was 20% with a T_cc of 98 °C. This comparison shows the validity of s-PLA SP as nucleating agents for PLLA. The increment in the mechanical properties in the 0.5% BC as compared to the neat PLLA sample infers the role of s-PLA nanoparticles in boosting crystallinity.

Table 1 The effect of various parameters on the thermal and mechanical properties of BC

Annealing conditions				DSC analysis		Mechanical properties
Time (h)	Temp. (°C)	Conc.^a (%)	Particle Size (nm)	Tcc (°C)	Xcc (%)	Tensile strength (MPa)	Young's modulus (GPa)
a Concentration total homopolymer/total DME (w/w%).
24	70	Control	—	98	20	29	1.3
6	70	0.5	963	98.9	21	39.2	1.5
12	70	0.5	963	99.7	24	43.9	1.67
24	70	0.5	963	113.9	25	54	1.8
48	70	0.5	963	99.96	16	48.7	1.7
24	50	0.5	963	99.1	18	35.9	1.2
24	60	0.5	963	100.2	19.5	40.4	1.3
24	70	0.5	963	113.9	25	54	1.8
24	80	0.5	963	103.2	23	55	1.83
24	70	0.5	963	113.9	25	54	1.8
24	70	1	963	113.6	24	54	1.9
24	70	2	963	110.5	26	57	2.3
24	70	3	963	107.9	28	62	2.5
24	70	5	963	106.4	35	73	3.2
24	70	0.5	668	115.6	32	69	3.0
24	70	0.5	2016	104.02	23	43	2.9
24	70	0.5	3190	100.6	21	35	2.5

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The annealing temperature variation experiment conducted for 24 h from 50 °C to 70 °C with the 0.5% s-PLA compositions showed an increase in tensile strength from 35.9 MPa at 50 °C to 54 MPa at 70 °C with an increase in Young's modulus from 1.2 GPa to 1.8 GPa. The corresponding increase in the X_cc and T_cc was from 18% to 25% and 99.1 °C to 113.9 °C, respectively. However the tensile strength and Young's modulus of these composites were not much different from the samples annealed at 80 °C, as shown in Table 1.

Crystallization is a process that takes place in two distinct steps, nucleation and growth.^31,32 In the nucleation step, the density of nuclei depends on the temperature of crystallization. The nucleation of the polymer cast occurs below the melting temperature of the nucleating agent. The randomly oriented molecules in solution align and form small ordered regions, which are called nuclei.³² The successive increase in the annealing temperature from 50 °C to 70 °C results in the melting of the amorphous regions and development of crystalline sites in the polymer matrix, which provides favorable geometric dimensions to the polymer chains to adhere to a more ordered state for PLLA crystal growth. The added s-PLA SP functions like seedlings. The PLA chains get entangled and align to initiate and promote crystallization. Therefore, the s-PLA SP provide a platform for crystallization.

We optimized the annealing conditions for our solution castings to be 70 °C for 24 h. We expect that the addition of a greater quantity of s-PLA SP will accelerate the nucleation and crystal growth process in PLLA nanocomposites.³³ To observe the effect of concentration of s-PLA SP on the crystallization of the PLLA BC, we prepared composites with varying s-PLA concentrations between 0.5% and 5% at the optimized annealing conditions. The results of these experiments are given in Table 1 and shown in Fig. 4. There is a direct relationship between the s-PLA SP concentration and thermo-mechanical properties of PLLA, as shown in Fig. 4(a). The tensile strength of the BC increased from 54 MPa with the Young's modulus of 1.9 GPa to 73 MPa with the Young's modulus of 3.2 GPa for 1 to 5% of s-PLA SP dispersed in PLLA respectively, as given in Table 1 (Fig. 4(a)). The degree of crystallinity, X_c, increased from 25–35% with the increase in concentration of s-PLA SP from 0.5–5%. The DSC cooling exotherms of the BC with varying concentrations and sizes of s-PLA SP are shown in Fig. 5. It can be seen that the T_cc of the 0.5% BC increased compared to that of the neat annealed PLLA solution casting from 98 °C to 113.9 °C. The increased exothermic peak with an increase in concentration of s-PLA SP suggests the increase in crystallinity of the BC. The T_cc decreased with the increase in concentration of the same s-PLA SP from 113.9 °C to 106.4 °C, as given in Table 1. According to Nelley et al.,²⁸ the addition of the low molecular weight PDLA into the PLLA matrix increases the T_cc of the solution casting initially by up to 1%. However, when the concentration of the low molecular weight PDLA exceeds 1%, the T_cc of the composites decrease. The drop in T_cc for the compositions with high concentrations of PDLA can be attributed to the low molecular weight of the homopolymers from which the s-PLA SP were generated. SEM images of the BC with increasing concentrations are shown in Fig. 6. It is evident that the successive increase in concentration of s-PLA SP enhances the density of spherulites, but reduces the spherulite size. The addition of more nucleating agent tends to provide more nuclei centers for crystallization. To study the effect of particle size, we conducted experiments with monodispersed spherical particles with variable sizes of s-PLA SP at the concentration of 0.5% and annealed at optimum conditions. The s-PLA SP with larger sizes cannot improve the thermal and mechanical properties of the PLLA BC. The addition of 0.5% s-PLA of small size particles, such as 668 nm, enhances the tensile strength up to 69 MPa with 3.0 GPa Young's modulus, as shown in Fig. 4(b). Similarly, the X_cc of the 668 nm containing BC increased to 32% with an increase in T_cc to 115.6 °C, as shown in Fig. 5(b). The smaller the s-PLA particle, the larger the surface area available for crystal seedlings to sow, proliferate and grow.

Fig. 4 Effect of s-PLA SP (a) concentration and (b) particle size on the tensile strength of the BC.

Fig. 5 DSC cooling curves at a cooling rate of 10 °C min⁻¹ from 200 °C for neat PLLA and the BC. (a) Effect of s-PLA SP concentration and (b) effect of different particle sizes on the cold crystallization temperature (T_cc).

Fig. 6 SEM images of annealed solution castings of neat and s-PLA SP with concentrations between 0.5% and 5% on the scale bar of 1, 100, 50 and 50 μm, respectively.

An enhanced number of nuclei give smaller size spherulites dispersed evenly throughout the cast film, as shown in Fig. 8. The availability of a large number of nuclei results in the growth of smaller radii spherulites. At high concentrations, it is difficult for the polymer chain to get enough mobility to settle in the required geometric dimensions. In these circumstances, there may be a large number of nuclei sites due to available s-PLA SP, however crystal growth is retarded, which results in the development of small sized tightly packed spherulites. We can see the extremely tight entanglement of the spherulites for the 5% BC in Fig. 6 and 10. The cryofractured surface morphology of the samples (neat, 3% and 5%) was analyzed by SEM and is shown in Fig. 7. It is clearly shown by the SEM images of the neat PLLA solution casting that the fractured surface was left mostly complete spherulites with few ruptured spherulites. In comparison, the cryofractured surface of the 5% BC (Fig. 7) shows all ruptured spherulites, whereas the composite with 3% s-PLA showed mostly ruptured spherulites along with a few complete spheres.

Fig. 7 SEM images of the cryofractured surface morphology of neat and BC solution castings with compositions from 0.5 to 5% on the scale bar of 100, 100, 50 and 30 μm, respectively.

Stereocomplexation is preferred over homopolymer crystallization, even at low concentrations of PDLA in the PLLA matrix. The enhanced crystallization is induced by the nucleation effect of the s-PLA particles. The high molar mass PDLA chains in the PLLA matrix induce the formation of s-PLA crystallites faster under a slow increase in temperature for crystallization. Racemic crystallites are dominant with the favourable enantiomeric geometry of homopolymers. These crystallites act as nucleation sites and increase the number of PLLA spherulites and thus enhance the overall crystal growth initially.

The effect of stereocomplex PLA crystallites on the mechanical properties of blended and non-blended solution castings composed of PLLA and PDLA was studied by Tsuiji. For a non-annealed 1 : 1 blended solution castings with a homopolymer molecular weight M_w of 1 × 10⁵ to 1.3 × 10⁶ Da, a tensile strength of 45–49 MPa with a Young's modulus of 1.7–1.8 GPa was determined.³⁴ In another report the effect of different annealing conditions was observed, where direct annealing of PLLA with a M_w of 1.3 × 10⁵ g mol⁻¹ at 25–160 °C resulted in a tensile strength of 50–59 MPa with a Young's modulus of 1.8–2.9 GPa.³⁵ Another study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization. In that article, Tsuji determined that for a non-isothermal annealing system containing PLLA with M_n 1.2 × 105 g mol⁻¹ and PDLA with an M_n of 5.2 × 104 g mol⁻¹, the addition of PDLA into the PLLA matrix between 0.1% and 3% can enhance the tensile strength from 50.7–53.4 MPa with a small decrease in Young's modulus from 1.29 to 0.96 GPa.³⁶

The literature proves that the strong intermolecular binding forces of attraction between molecules of a material rupture the molecules in cryofractured surfaces, which indicates a strong material. However if the fractured surface morphology gives almost complete particles detached from each other, this indicates weak intermolecular bonding and hence a weak material.³⁷ The higher compositions such as 3% and 5% have strong intermolecular forces, which make it difficult to separate or break the spherulites. This effect is more pronounced in the 5% composite, which is a measure of high strength and greater Young's modulus. Therefore, this contributes towards the material being stronger and tougher. The s-PLA SP are evenly distributed throughout the PLLA film, as shown in the comparison of the neat and 0.5% composition in Fig. 8.

Fig. 8 Dispersion of s-PLA SP in the BC with TEM analysis of the neat and 0.5% BC solution castings.

The existence of s-PLA SP after annealing is indicated by the XRD patterns and DSC comparative scans in Fig. 9(a) and (b). The s-PLA SP promote crystallization of PLLA by providing intense nuclei centers and smaller spherulites, as shown in Fig. 10. However, due to restricted polymer chain mobility, crystal growth is sluggish. A comparison of the PLLA spherulite nuclei centers and radii can clearly be seen in the POM micrographs shown in Fig. 10. The BC can be used to accommodate bacteria or load drugs in their interstices. This requires further research to explore the structure and functional relationship of this spacious BC.

Fig. 9 Analysis of the existence s-PLA SP in the BC by (a) XRD and (b) DSC.

Fig. 10 PLLA spherulite growing density and reducing radii. Polarized optical micrographs (scale bar 200 μm) of neat, 3% and 5% annealed BC solution castings.

Conclusion

Biodegradable polymeric biocomposites serve as green materials for commercial, biomedical and pharmaceutical fields such as robust, transparent, commercial and household biodegradable packaging; biomimicking tissue regenerating scaffolds and bone implants and sustained release carriers for drugs, agrochemicals, medical imaging agents and personal care ingredients.

The s-PLA SP function as seedlings that can adhere and align the randomly tangled polymeric chains in the correct dimensions to grow as a crystal. This mechanism paves new ways to generate mechanically strong green homocomposites. The thermomechanical properties of these green homocomposites can be tuned by the control of the s-PLA SP concentration. The s-PLA SP can enhance the thermomechanical properties of PLLA with a concentration as low as 0.5%. SEM images show small interstices in the low concentration BC. This type of biocomposite can be used to accommodate bio-organic fillers, drugs or microbes.

This study proves the synthesis of s-PLA submicron particles via a solution feed system using DME as a supercritical fluid. The solution feed strategy provides the possible extreme of homopolymer dissolution to obtain a homogenous environment. The dissolved solutions then result in stereocomplexed PLA submicronparticles with a possible uniformed distribution. We availed the chance to spray the nanoparticles into the air at room temperature to evaporate DME quickly, thus leaving the dry dust of s-PLA behind. The SCF assisted solution feed system coupled with the air spray mechanism makes this process swift, economic and feasible.

The DME/polymer feed ratio is a strong tool to control the s-PLA particle size. In our studies, the optimum conditions for the synthesis of monodispersed s-PLA SP are 5% total homopolymer (PLLA + PDLA) dissolved at 50 °C and 300 b for 12 h, while stereocomplexation proceeded using previously established reaction conditions. We obtained 963 nm s-PLA particles with a 0.12 size distribution as inferred by DLS and SEM analyses. The BC were optimized as 5% s-PLA SP dispersed in PLLA solution castings, annealed at 70 °C for 24 h with the improved tensile strength of 73 MPa and Young's modulus of 3.2 GPa.

We believe that the future prospects of dispersing s-PLA particles into biopolymeric degradable systems can generate mechanically strong compositions that are promising for use as bone alternatives, cartilage tissue regeneration, bacterial residencies or drug delivery systems.

Acknowledgements

This work was supported by a National Research Foundation of Korea grant funded by the Korean Government (MEST) NRF-2010-C1AAA001-2010-0028939.

Notes and references

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