PLLA Matrix Research Articles

The Influence of Cu3(BTC)2 metal organic framework on the permeability and perm‐selectivity of PLLA‐MOF mixed matrix membranes

ABSTRACTPoly(l‐lactic acid) (PLLA) ‐ 20% (w/w) and Cu3(BTC)2 metal organic framework (MOF) based mixed matrix membranes (MMMs) were fabricated by a vertical corotating twin screw microcompounder followed by an injection molding process. Water vapor, CO2, O2, and selected aroma mass transfer properties of PLLA and PLLA MMMs were evaluated. The CO2/O2 perm‐selectivity of PLLA (αCO2/O2) MMMs increased from 7.6 to 10.3 with the incorporation of 20% Cu3(BTC)2 MOF. Gravimetric permeability studies of trans‐2‐hexenal performed at 23°C and 50% RH indicated that permeability coefficient of PLLA MMMs increased by around 60% as compared to regular PLLA film. However, no changes in mass transfer rates were observed for acetaldehyde. Furthermore, the thermal processing parameters as well as the presence of MOF did not show any significant effect on the molecular weight of the PLLA matrix nor on the crystalline structure of the Cu3(BTC)2 MOF, which was confirmed by both gel permeation chromatography and X‐ray diffraction studies. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42764.

Phase morphology, crystallization behavior and mechanical properties of poly(L-lactide) toughened with biodegradable polyurethane: Effect of composition and hard segment ratio

Polyester-based biodegradable polyurethane (PU) with different hard segment ratios was selected to modify the impact toughness of poly(L-lactide) (PLLA). The influence of blending composition and hard segment ratio of PU on the phase morphology, crystallization behavior and mechanical properties of PLLA/PU blends has been investigated systematically. The results showed that the PU particles were uniformly dispersed in PLLA matrix at a scale from submicrons to several microns. The glass transition temperature of PU within these blends decreased compared to that of neat PU, but rose slightly with its content and hard segment ratio. The presence of PU retarded the crystallization ability of PLLA, whereas enhanced its elongation at break and impact resistance effectively. As the PU content reaches up to 30 wt%, the phenomenon of brittle-ductile transition occurred, resulting in a rougher fracture surface with the formation of fibril-like structure. Moreover, under the same concentrations, the elongation at break and impact strength of PLLA blends decreased slightly with the increase of hard segment ratio of PU.

Pyrene-end-functionalized poly(L-lactide) as an efficient carbon nanotube dispersing agent in poly(L-lactide): mechanical performance and biocompatibility study

In order to improve the mechanical properties of poly(L-lactide) (PLLA) based implants, a study was made of how far well dispersed multi-walled carbon nanotubes (MWCNTs) within a PLLA matrix were able to positively affect these properties. To this end, pyrene-end-functionalized poly(L-lactide) (py-end-PLLA) was evaluated as a dispersing agent. Transmission electron microscopy (TEM) analyses and mechanical tests of MWCNTs-based materials demonstrated an enhancement of MWCNT dispersion in the PLLA matrix and improved Young’s modulus (E) when 4 wt% of py-end-PLLA was used as the dispersing agent. Subsequently, the bioacceptance of PLLA/py-end-PLLA/MWCNTs nanocomposites was evaluated using human bone marrow stromal cells (HBMC) in vitro. The inclusion of py-end-PLLA and MWCNTs supported HBMC adhesion and proliferation. The expression levels of the bone-specific markers indicated that the cells kept their potential to undergo osteogenic differentiation. The results of this study indicate that the addition of MWCNT combined with py-end-PLLA in PLLA/py-end-PLLA/MWCNTs nanocomposites may widen the range of applications of PLLA within the field of bone tissue engineering thanks to their mechanical strength and cytocompatibility.

Toughening of poly(l‐lactide) modified by a small amount of acrylonitrile−butadiene−styrene core‐shell copolymer

ABSTRACTA small amount of acrylonitrile‐butadiene‐styrene (ABS) core shell copolymer particles are used to improve the toughness of poly(l‐lactide) (PLLA) matrix. The incorporation of ABS copolymer dramatically increased the elongation yield at break of PLLA. For PLLA blend with 6.0 wt % ABS copolymer particles, the elongation yield at break increased by 28 times and the notched impact strength improved by 100% comparing with those of neat PLLA. Fourier transformed infrared (FTIR) and dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM) measurement results indicated that the special polarity interaction between ester group of PLLA matrix and nitrile group of PSAN shell phase enhanced the interfacial adhesion between PB rubber phase and PLLA matrix and promoted the fine dispersion of ABS particles in PLLA matrix. Meanwhile, ABS core shell particles also showed a certain extent of effects on the crystallinity behavior of PLLA. A small amount of ABS particles became the nucleating sites, and then the degree of crystallinity of PLLA/ABS blends increased. However, the notched impact of PLLA blends decreased because of the aggregation of more ABS particles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42554.

Role of PEG Segment in Stereocomplex Crystallization for PLLA/PDLA-b-PEG-b-PDLA Blends

The isothermal crystallization behavior and morphology of neat poly(l-lactic acid) (PLLA) and its blends with poly(d-lactic acid) (PDLA) and PDLA-b-poly(ethylene glycol) (PEG)-b-PDLA have been investigated. The glass transition temperature of the PLLA/PDLA blend, and especially the PLLA/PDLA-b-PEG-b-PDLA blend, is much lower than that of neat PLLA, indicating the plasticization effect of both low molecular weight PDLA and flexible PEG block. Both the PLLA/PDLA and PLLA/PDLA-b-PEG-b-PDLA blends can form stereocomplex (SC) crystals. However, the PDLA in the PLLA/PDLA blend postponed the crystallization rate of the blend, whereas the PEG chains in PLLA/PDLA-b-PEG-b-PDLA not only promoted the formation of SC crystals but also facilitated the crystallization of the PLLA matrix. The evolution of the crystal structure and morphologies of PLLA and its blends during the isothermal crystallization process have revealed the significant role of flexible PEG chains in the stereocomplex crystallization.

Superior Performance of Fully Biobased Poly(lactide) via Stereocomplexation-Induced Phase Separation: Structure versus Property

Superior properties such as high strength, toughness and transparency of fully biobased poly(lactide) (PLA) were achieved simultaneously without any external modifiers. The improvement in properties is well explained by a structural/morphological study. Stereocomplex PLA (SC-PLA) was obtained by melt compounding asymmetric poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blends at 200 °C and confirmed by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) analyses. The SC-PLA domains (d = 950–1200 nm) lead to a physical cross-link network in the PLLA matrix. Rheology and Molau experiment reveal two different microstructures as a function of SC-PLA content, i.e., the connection of SC-PLA domains varied from chain entanglement to direct molecular bridging when the SC-PLA content increased from 10% to 23%. The SC-PLA crystals and the cross-link network reinforced the PLLA matrix, resulting in increases in melt viscosity, modulus and yield strength. Surprisingly, the elongation at break of the PLLA/PDLA blends was increased concomitantly from 11% to 200% with the SC-PLA content up to 10%. The brittle-to-ductile transition is ascribed to the cross-link network and easy deformation/cavitation of the SC-PLA domains. In addition, the asymmetric PLLA/PDLA blends exhibit an average visible light transmittance as high as 70% and the blends showed excellent heat-resistance after a short annealing at 100 °C.

Improved mechanical properties of polylactide nanocomposites-reinforced with cellulose nanofibrils through interfacial engineering via amine-functionalization

One of the main factors responsible for the mechanical and physical properties of nanocomposites is the effectiveness of the interfacial region to transfer loads and mechanical vibrations between the nano-reinforcements and the matrix. Surface functionalization has been the preferred approach to engineer the interfaces in polymer nanocomposites in order to maximize their potential in structural and functional applications. In this study, amine-functionalized cellulose nanofibrils (mCNF-G1) were synthesized via silylation of the hydroxyl groups on the CNF surface using 3-aminopropyltrimethoxysilane (APTMS). To further increase the amine density (mCNF-G2), dendritic polyamidoamine (PAMAM) was grafted onto mCNF-G1 by the Michael addition of methacrylate onto mCNF-G1, followed by the transamidation of the ester groups of methacrylate using ethylenediamine. Compared to native CNF-reinforced, poly(l-lactide) (PLLA) nanocomposites, amine-functionalized CNF exhibited significantly improved dispersion and interfacial properties within the PLLA matrix due to the grafting of PLLA chains via aminolysis. It is also a more effective nucleating agent, with 15% mCNF-G1 leading to a crystallinity of 32.5%, compared to 0.1 and 8.7% for neat PLLA and native CNF-reinforced composites. We have demonstrated that APTMS-functionalized CNF (mCNF-G1) significantly improved the tensile strength compared to native CNF, with 10% mCNF-G1 being the most effective (i.e., >100% increase in tensile strength). However, we also found that excessive amines on the CNF surface (i.e., mCNF-G2) resulted in decreased tensile strength and modulus due to PLLA degradation via aminolysis. These results demonstrate the potential of optimized amine-functionalized CNF for future renewable material applications.

Confinement crystallization of poly(l-lactide) induced by multiwalled carbon nanotubes and graphene nanosheets

Two sets of multiwalled carbon nanotubes (MWCNTs)/PLLA nanocomposites and graphene nanosheets (GNSs)/PLLA nanocomposites with various MWCNTs and GNSs loadings prepared by a melt blending approach were investigated in terms of both nonisothermal and isothermal crystallization behaviors. The results indicated that MWCNTs and GNSs not only acted as heterogeneous nucleating agents for PLLA crystallization but also restricted the mobility and diffusion of PLLA chains. At low MWCNTs and GNSs concentrations, the nucleation effect of MWCNTs and GNSs was dominant to achieve accelerated overall crystallization kinetics. As the MWCNTs and GNSs concentration rose up to 2.0 and 2.5 mass%, respectively, the MWCNTs and GNSs network structures were formed in the PLLA matrix, which were proved by solid-like rheological behavior at low frequencies in rheological measurement. With further increasing concentration of MWCNTs above the critical concentration, an enhanced nucleation density but an almost unchanged overall crystallization rate for the MWCNTs/PLLA nanocomposites indicated that the expected increase in the crystallization promoting effect from more MWCNTs was offset by some confining effect. However, for GNSs, the formed network structure provided a more severely confined space for PLLA crystal nucleation and growth in contrast to MWCNTs, resulting in the decreased nucleation density and retarded crystallization rate.

Toughening effect of poly(d-lactide)-b-poly(butylene succinate)-b-poly(d-lactide) copolymers on poly(l-lactic acid) by solution casting method

The block copolymers of poly(d-lactide)-b-poly(butylene succinate)-b-poly(d-lactide) (PDLA-b-PBS-b-PDLA) were added into PLLA matrix to enhance the toughness of PLLA by solution casting method. The properties of the films were investigated by wide angle X-ray diffractometry (WAXD), differential scanning calorimetry (DSC) and tensile testing. The WAXD and DSC results showed that the stereocomplex was the major crystalline when the amount of copolymer addition exceeded 30wt%. The blending samples with high mechanical strength and toughness could be obtained only when certain amount of PDLA-b-PBS-b-PDLA copolymers were added.

Nucleating effect and crystal morphology controlling based on binary phase behavior between organic nucleating agent and poly(l-lactic acid)

A new kind of nucleating agent for poly(l-lactic acid) (PLLA), N,N'-bis(benzoyl) hexanedioic acid dihydrazide (BHAD), was synthesized. Differential scanning calorimetry (DSC), polarization optical microscopy (POM), wide-angle X-ray diffraction (WAXD) and rheometer were used to study the crystallization behavior and crystal morphology of PLLA nucleated by BHAD. In order to ensure the same thermal history of the samples characterized by DSC, POM and XRD, these samples were all thermally treated by DSC. In nonisothermal crystallization, BHAD showed excellent nucleating ability to PLLA. However, it was different from the traditional heterogeneous nucleation mechanism. POM observation showed that morphologies of PLLA crystals varied with the concentration of BHAD and can be classified into three types. Through the systematic investigation of DSC, POM and rheological analysis, it was clearly demonstrated that BHAD had solubility in PLLA matrix, depending on its concentration and processing temperature. Consequently, a highly schematic binary phase diagram of the PLLA/BHAD system was presented, accompanied with a reasonable mechanism of how the PLLA crystal morphologies controlled by BHAD.

Poly(l -lactide)/four-armed star poly(l -lactide)-grafted multiwalled carbon nanotubes nanocomposites: Preparation, rheology, crystallization, and mechanical properties

Four-armed star poly(l-lactide) grafted multiwalled carbon nanotubes (CNTs-g-4PLLA) were synthesized through the nucleophilic substitution reaction between 4PLLA and acryl chloride of CNTs and then characterized by transmission electron microscope, X-ray photoelectron spectroscopy, thermal gravimetric analysis (TGA), and ultraviolet visible spectrophotometer. The results indicated that 4PLLA was successfully grafted onto CNTs, and CNTs-g-4PLLA contained 37.7 wt% of 4PLLA. PLLA/CNTs-g-4PLLA nanocomposites were prepared by solution casting with different CNTs-g-4PLLA content. Rheological behavior of PLLA/CNTs-g-4PLLA nanocomposites was measured using a rheometer. The result showed that CNTs-g-4PLLA formed a network structure at percolation concentration, which improves obviously rheological properties of PLLA in the molten state. The crystallization behavior and crystal structure of the nanocomposites were comprehensive evaluated through differential scanning calorimetry, X-ray diffraction, and polarizing optical microscope. The results found that CNTs-g-4PLLA played two roles in PLLA crystallization. The addition of CNTs-g-4PLLA acted as nucleating agent and obviously accelerated the spherulites growth under percolation concentration, while it inhibited the movement of PLLA chains at above percolation concentration, resulting in the decrease of crystallinity. Thermal stability and mechanical properties of the nanocomposites were also investigated using TGA, dynamic mechanical analysis, and tensile test. These results indicated that the incorporation of CNT-g-4PLLA into the PLLA matrix improved the thermal stability, storage modulus, and tensile strength of the nanocomposites. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

Amphiphilic multiblock copolymers of PLLA, PEO and PPO blocks: Synthesis, properties and cell affinity

Poly(l-lactide) (PLLA) is a biodegradable and bioabsorbable polyester recognized for its potential uses in many fields, including biomedical applications. However, its hydrophobic and highly crystalline nature can be limitations for tissue engineering applications; these limitations can be overcome by PLLA copolymerization with more hydrophilic and amorphous blocks. Therefore, PLLA blocks were attached to block copolymers based on poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) – PEO-b-PPO-b-PEO and PPO-b-PEO-b-PPO – and to random PEO-ran-PPO through ring-opening polymerization of l-lactide (LLA) to obtain a series of multiblock copolymers containing amorphous/crystalline blocks with variable mass ratios of hydrophilic/hydrophobic blocks. PLLA-b-P(EO-PO)-b-PLLA copolymers were evaluated for chemical composition and molar mass (hydrogen nuclear magnetic resonance – 1H NMR and gel permeation chromatography – GPC). Phase behavior was investigated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), which showed the suppression of the polyether phase’s crystallinity and the complexity of the relaxation spectra of the copolymers, respectively. The combination of AFM and DMA results revealed polyether domains that were segregated from the PLLA matrix morphology. A more hydrophilic surface compared with neat PLLA was evidenced by water contact angle measurements. All copolymers are able to allow cell adhesion and to support a suitable osteoblast cell proliferation rate, as revealed by PicoGreen® assays. These amphiphilic block copolymers are potentially biocompatible candidates for tissue engineering devices.

Enhancing Thermomechanical Properties and Heat Distortion Resistance of Poly(l-lactide) with High Crystallinity under High Cooling Rate

In this work, a novel, effective and simple approach to largely improve the thermomechanical properties and heat distortion resistance of biodegradable poly(l-lactide) (PLLA) by using a new nucleating agent (NA), i.e., itself high-melting-point homocrystallites (hPLLA crystallites) is reported. Specially, hPLLA crystallites with a melting temperature (Tm) of 187 °C were introduced into the PLLA matrix with a lower Tm, i.e., 168 °C via simply melt blending at 170 °C which is between the Tms of the two PLLAs. Nonisothermal and isothermal crystallization results reveal that hPLLA crystallite is an efficient nucleating agent for PLLA. Also, hPLLA crystallites show much more prominently promoting effect on the crystallization rate of PLLA in comparison with two widely reported NAs for PLLA, talc and stereocomplex crystallites. Most importantly, this promoting effect is still efficient at very high cooling rate, leading to a crystallinity of 39.1% at a cooling rate of 100 °C/min, which can help to obtain high-crystallinity PLLA products in conventional manufacturing processes. The optical microscopic observation reveals that the remarkable crystallization promotion can be attributed to the outstanding heterogeneous nucleation effect, as a result of both identical chemical constitution and lattice constitution between hPLLA crystallites and PLLA matrix. Further characterizations indicate that the enhancement of PLLA crystallinity by using such a new efficient NA can enhance the thermomechanical properties and heat distortion resistance of PLLA remarkably. For instance, at 80 °C (above Tg of PLLA), the elastic modulus increases by 60 times from 8 to 477 MPa with the incorporation of 5 wt % hPLLA.

Rheology and crystallization behavior of PLLA/TiO2‐g‐PDLA composites

Poly(L‐lactide) (PLLA) composites with TiO2‐g‐poly(D‐lactide) (PDLA), which was synthesized by surface‐initiated opening ring polymerization with TiO2 as initiator and Sn(Oct)2 as catalyst, were prepared by solution casting. The synthesized TiO2‐g‐PDLA was characterized by transmission electron microscope (TEM) and dynamic laser scattering (DLS), showing larger size corresponding to that of TiO2. Fourier transform infrared spectroscopy (FT‐IR) and X‐ray photoelectron spectroscopy (XPS) measurements were further carried out and indicated that PDLA was grafted onto TiO2 through covalent bond. For PLLA/TiO2‐g‐PDLA composites, the stereocomplex crystallites were formed between PDLA grafted on the surface of TiO2 and the PLLA matrix, which was determined by FT‐IR, differential scanning calorimetry (DSC), and X‐ray diffractometer (XRD). The influence of stereocomplex crystallites on the rheological behavior of PLLA/TiO2‐g‐PDLA was investigated by rheometer, which showed greater improvement of rheological properties compared to that of PLLA/TiO2 composites especially with a percolation content of TiO2‐g‐PDLA between 3 wt%–5 wt%. The crystallization and melting behavior of PLLA/TiO2‐g‐PDLA composites were studied by DSC under different thermal treatment conditions. The formed PLA stereocomplex network acted as nucleating agents and a special interphase on the functional surface of TiO2, which resulted in imperfect PLLA crystal with lower melting temperature. When the thermal treatment was close to the melting temperature of PLA stereocomplex, the crystallinity approached to the maximum. The isothermal crystallization study by polarizing microscope (POM) indicated that stereocomplex network presented stronger nucleation capacity than TiO2‐g‐PDLA. Copyright © 2015 John Wiley & Sons, Ltd.

Synthesis of Novel Hyper-Branched Polymers From Trimethoxysilyl-Terminated Polylactides and Their Utilization for Modification of Poly(l-Lactide) Materials

Silyl-terminated oligomers of poly(l-lactide) (mSi-PLLA) and poly(d-lactide) (mSi-PDLA) were synthesized by hydrosilylation of allyl-terminated derivatives. They were readily hydrolyzed into hyper-branched polymers having particulate morphology. When a mSi-PDLA oligomer was blended with a PLLA sample (52.5 kDa) in 1–10 wt.-% by solution casting, small cross-linked particles with stereocomplex form were generated and distributed homogenously in the PLLA matrix, and by which the homo-chiral crystallization of PLLA was efficiently suppressed. When another mSi-PLLA oligomer was blended with PLLA, the cross-linked particles formed from mSi-PLLA were amorphous and distributed inhomogeneously. It was, therefore, indicated that the hydrolytic polycondensation of mSi-PDLA and mSi-PLLA was induced during the film casting, changing the inner structure and properties of the blend films with PLLA.

Uniformly Dispersed Poly(ethylene glycol) Functionalized Graphene Oxide/Poly(L-lactic acid) Nanocomposites with Balanced Mechanical Properties

Balanced mechanical properties are always the goal of high-performance general plastics for engineering purposes. In order to develop uniformly dispersed graphene oxide/poly(L-lactic acid) nanocomposites with balanced mechanical properties, a poly(ethylene glycol) (PEG) grafted graphene oxide was introduced into poly(L-lactic acid) (PLLA) matrix. The PEG grafted graphene oxide (GEG) was confirmed by the results of fourier transform infrared (FTIR) spectra and thermal gravimetric analysis (TGA). Then the mechanical properties of the nanohybrids were measured. The results showed that in contrast to the aggregation of GO, GEG could uniformly disperse in PLLA matrix. An intriguing phenomenon is that thanks to the strong interfacial interaction between GEG and PLLA, it exhibits a substantial enhancement of the elongation at break (EB) as well as a simultaneous improvement of the tensile strength (TS), while the addition of GO decreases the EB of PLLA matrix. This work provides a potential industrialized technique for high-performance PLLA nanocomposites

Effect of nanoparticle SiO2 grafted poly (methyl methacrylate) on poly(l-lactic) acid crystallization

For practical applications of poly(l-lactic acid) (PLLA), PLLA crystallization with keeping transparency is preferable. As one method to keep transparency without inhibiting crystallization, we prepared SiO2 nanoparticles which were modified with poly (methyl methacrylate) (PMMA-g-SiO2). FT-IR and TG–DTA measurement indicated that PMMA was grafted on SiO2 surface successfully. TG–DTA and GPC measurement of free PMMA cleaved from PMMA-g-SiO2 determined graft density of PMMA as 0.039 chains/nm2. The effect of PMMA-g-SiO2 on PLLA crystallization was characterized by DSC measurement and POM observation. Although PMMA-g-SiO2 could not promote PLLA crystallization significantly like commercial crystal nuclear agents, it did not inhibit PLLA crystallization. By UV measurement and Haze meter, it was indicated that crystallized PLLA with PMMA-g-SiO2 generated 3.5–5.3 times higher transparency than crystallized PLLA with/without commercial nucleating agent. TEM observation indicated that the aggregation size of PMMA-g-SiO2 was 21.7 ± 13.9 nm which was smaller than visible wavelength. Transparency improvement likely resulted from size-reduced aggregation of PMMA-g-SiO2 in PLLA matrix. On the other hand, PMMA-g-SiO2 made size of generated spherulite smaller but increased its number in PLLA matrix, which inhibited transparency improvement.

Synthesis of Reactive Comb Polymers and Their Applications as a Highly Efficient Compatibilizer in Immiscible Polymer Blends

Reactive comb (RC) polymers were synthesized by copolymerizing methyl methacrylate (MMA) with glycidyl methacrylate (GMA) and a kind of MMA macromer. The obtained molecules had both epoxide groups and long PMMA side chains distributed randomly along the PMMA backbone. The synthesized RC polymers were applied as compatibilizers in an immiscible poly(l-lactic acid)/poly(vinylidene fluoride) (PLLA/PVDF) system. Compared to their linear counterparts, only 1 wt % of RC polymers with appropriate length of side chains significantly reduced the size and enhanced the uniformity of the PVDF dispersed phase in PLLA matrix and the RC-compatibilized blends exhibited a significant improvement in fracture strain. The higher compatibilization efficiency of RC polymers was ascribed to the existence of PMMA side chains, which improved the stability of RC polymers at the interface of immiscible blend, and this was demonstrated by transmission electron microscopy.

Prediction of stress-strain curves for TCP/PLLA composites: effect of hydrolysis and strain rate

In this study, effect of hydrolysis in simulated body environment on mechanical behavior of tricalcium phosphate (TCP)/poly(L-lactic acid) (PLLA) composites was analytically characterized. In order to predict stress–strain behavior after hydrolysis, damage of micromechanical analysis proposed by the authors was utilized. In this model, nonlinear behavior in stress–strain relationship was simulated considering interfacial debonding between TCP particle and PLLA matrix. For the purpose of deciding the interfacial strength, such as critical energy release rate, curve fitting was conducted on the result of the composites with 15wt% TCP content at three types of strain rates. Theoretical results on 5 and 10wt% composites using the interfacial strength obtained were in good agreement with experimental results. These results indicated that interfacial strength was dependent only on strain rate and was independent from TCP fraction.

Enantiomeric poly(d-lactide) with a higher melting point served as a significant nucleating agent for poly(l-lactide)

In the present work, a new approach to significantly accelerate the crystallization of poly(L-lactide) (PLLA) by utilizing poly(D-lactide) (PDLA) crystallites is presented. In particular, a PDLA with a high melting point (25 °C higher than that of PLLA) and high crystallinity (hPDLA) was introduced into the PLLA matrix via melt blending at a temperature between the melting points of PLLA and hPDLA. It was proved that the hPDLA crystallites were not melted and were instead reserved in the PLLA matrix. Results showed that a small amount of hPDLA crystallites can largely elevate the crystallization temperature, reduce the crystallization half-time and promote the crystallization rate of PLLA. In situ polarizing microscope observations showed that the great improvement of the crystallization rate was caused by the excellent nucleation effect of the hPDLA crystallites. Wide-angle X-ray diffraction results showed that the crystalline structures of the PLLA crystallites and hPDLA crystallites showed very good lattice match, which results in a largely reduced energy barrier for nucleation and increased crystallization kinetics.