Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.     

Impacts of stereoregularity and stereocomplex formation on physicochemical, protein adsorption and cell adhesion behaviors of star-shaped 8-arms poly(ethylene glycol)-poly(lactide) block copolymer films

Biodegradable stereocomplex film exhibiting soft and stretchy character was prepared by simply blending between enantiomeric 8-arms poly(ethylene glycol)-block-poly(l-lactide) (8-arms PEG-b-PLLA) and 8-arms PEG-b-PDLA copolymers with star-shaped structure. The stereocomplex film exhibited higher T_g and PLA crystallinity than those of original copolymer films. Effects of stereoregularity and stereocomplexation on protein adsorption and L929 cells attachment/proliferation behaviors onto the films were analyzed from the viewpoint to design a new class of implantable soft biomaterial. The stereocomplex film was found to exhibit large amount of protein adsorption than original films. Furthermore, cell attachment efficiency and proliferation rate on the film were significantly enhanced by stereocomplexation. This stereocomplex material is expected to be applicable as degradable temporary scaffold for soft tissue regeneration. Consequently, it was indicated that the stereocomplex formation could be proposed to be a novel method to control the protein- and cell-adhesive properties of biodegradable matrix composed of PEG-PLA copolymer.     

Study on the preparation and characterization of biodegradable polylactide/multi-walled carbon nanotubes nanocomposites

In this study, polylactide/multi-walled carbon nanotubes (PLA/MWNTs) hybrids were prepared by means of a melt blending method. To enhance the compatibility between PLA and MWNTs, the acrylic acid grafted polylactide (PLA-g-AA) and the multihydroxyl-functionalized MWNTs (MWNTs-OH) were used to replace PLA and MWNTs, respectively. The crude MWNTs were chemically oxidized by a mixture of H₂SO₄ and HNO₃ and then reacted with thionyl chloride to functionalize them with chlorocarbonyl groups (MWNTs-COCl). The MWNTs-OH was finally obtained by the reaction of MWNTs-COCl and 1,6-hexanediol. The resulting products have been characterized by FTIR, ¹³C solid-state NMR, TGA, DMA, SEM, TEM, and Instron mechanical tester. Due to the formation of ester groups through the reaction between carboxylic acid groups of PLA-g-AA and hydroxyl groups of MWNTs-OH, results demonstrated dramatic enhancement in thermal and mechanical properties of PLA, for example, 77 °C increase in initial decomposition temperature with the addition of only 1 wt%. Based on the result of thermal and mechanical examinations, it was found that the optimal amount of MWNTs-OH was 1 wt% because excess MWNTs-OH caused separation of the organic and inorganic phases and lowered their compatibility.     

Effect of nucleation and plasticization on the stereocomplex formation between enantiomeric poly(lactic acid)s

The effect of nucleation and plasticization on the stereocomplex formation between poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) was investigated in blends where PDLA is added as a minor phase in a major phase of PLLA. The use of small amounts of PDLA is aimed at creating a high melting point stereocomplex phase that in turn can serve as nucleating agent for the major phase of PLLA. Blends containing 5% PDLA with talc or organic phosphonate as nucleants and polyethylene glycol as plasticizer were prepared via melt-blending. Their crystallization behavior was investigated through Differential Scanning Calorimetry (DSC) using various thermal histories. Two peculiar stereocomplex melting endotherms were found. The peak temperature and enthalpy of these two endotherms were correlated to prior isothermal crystallization temperature. The different endotherms were also associated with two different crystalline morphologies observed by optical microscopy and referred to as Network and Spherulitic morphologies. The influence of plasticization and of heterogeneous nucleation on these morphologies was investigated through optical microscopy and calorimetric observations.     

Modification of polylactide upon physical properties by solution-cast blends from polylactide and polylactide-grafted dextran

Polylactide (PLA)-grafted polysaccharides with various lengths and numbers of graft chains were synthesized using a trimethylsilyl protection method. The properties of the cast films prepared from graft-copolymers were investigated through thermal and dynamic mechanical analyses. The graft-copolymer films exhibited a lower glass transition temperature (T_g), melting temperature, and crystallinity, and higher viscosity properties compared to PLA films. Moreover, the usefulness of graft-copolymer as a plasticizer was investigated with 1:4 blend films prepared from the graft-copolymers and PLA. The blend films showed lower T_g and crystallinity, and higher viscosity properties compared to PLA films.     

Crystallisation and spherulite morphology of polylactide stereocomplex

In this study we investigated the crystallisation behaviours of stereocomplex crystals in poly(l ‐lactic acid)/poly(d ‐lactic acid) ( PLLA/PDLA) blends (LD blends) of various weight ratios. The crystallisation and melting behaviours were studied using DSC, and the crystal structure was analysed through wide‐angle X‐ray diffraction. The morphology of homocrystals and stereocomplex crystals in the blends was examined using a hot‐stage polarising microscope and a scanning electron microscope. The DSC results showed that homocrystals and stereocomplex crystals were present in all LD blends except that with 50 wt% PLLA/50 wt% PDLA; in this blend, only stereocomplex crystals were present. The regime II → III transition temperature of stereocomplex crystals in a Lauritzen–Hoffman plot of the LD blends was determined to be 165 °C. A concentric spherulite consisting of stereocomplex crystals and homocrystals formed under two‐step isothermal crystallisation conditions with three growth stages was observed. The confined spherulitic growth rate in the concentric spherulite and the unrestricted spherulitic growth rate in individual spherulites were also analysed. © 2018 Society of Chemical Industry     

Morphology, rheology and crystallization behavior of polylactide composites prepared through addition of five-armed star polylactide grafted multiwalled carbon nanotubes

Functionalized multiwalled carbon nanotubes (F-MWNTs) were prepared by covalent grafting of five-armed star polylactide (fa-PLA), and were characterized by thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR) spectroscopy and Raman spectroscopy. A series of polylactide (PLA)/F-MWNTs composites was prepared via coagulation method. Several techniques were applied to investigate the effects of F-MWNTs on the morphology, melt rheology, and crystallization and melting behaviors of the PLA composites. The optical microscope (OM), field-emission scanning electron microscope (FE-SEM) and transmission electron microscopy (TEM) observations demonstrated that, in comparison with the case of PLA filled with pristine MWNTs, F-MWNTs case showed improved dispersion and interfacial adhesion. Oscillatory frequency sweep measurements showed that addition of about 2.0 wt% F-MWNTs led to a solidlike response where a percolated network structure formed, and the composites exhibited remarkable improvement of rheological properties in the melt state as compared with that of neat PLA. DSC measurements showed that F-MWNTs acted as a nucleating agent to enhance crystallization when below the percolation concentration, while also acted as a hindrance to retard crystallization above the percolation concentration. The double melting peaks on the DSC curves were attributed to melting of the crystals formed in the cold crystallization stage and the melting-recrystallization-remelting (mrr) event during heating, respectively.     

Structure and mechanical properties of polylactide copolymer microspheres and capsules

Polylactide homopolymers, polylactide and poly(ethylene oxide) diblock and triblock copolymers are used to prepare spherical microparticles by using the single oil-in-water emulsion and solvent evaporation technique. We are able to create both bulk and hollow microspheres by altering the conditions of preparation. The experiments are carried out at two fixed temperatures of 15 and 22 °C. We show, from scanning electron microscopy data, that the microspheres produced from the homopolymers are bulk and homogeneous at both temperatures whereas they are hollow when the triblock copolymers are used. The diblock copolymers yield bulk microspheres at 15 °C and microcapsules at 22 °C. Compression experiments emphasize once more the inner morphology of the spheres. As it is expected, bulk microspheres have higher Young’s modulus than the microcapsules. Nevertheless, comparative compression analysis of both morphologies shows that the microcapsules retain relatively high compressive moduli. These results have implications for the design of rigid and biodegradable microcapsules.     

Spontaneous formation of polylactide stereocomplex microspheres containing metal ions

Linear poly(l ‐lactides) (PLLAs ) and poly(d ‐lactides) (PDLAs ) with M _n in the range 2000 ? 4300 containing a different number and placement of carboxyl groups were obtained via cationic ring‐opening polymerization and post‐polymerization functionalization. PLA stereoisomers (PLLA ‐(COOH )_x and PDLA ‐(COOH )_x , where x = 1 ? 3) were used for the investigation of stereocomplexation in solution performed in the presence of metal cations such as Ca²⁺, Mg²⁺, Zn²⁺, Fe³⁺. Spherical microparticles with a diameter in the range 0.7 ? 3.0 µm were obtained in all cases which was confirmed on the basis of scanning electron microscopy (SEM ) analysis. The microsphere size and homogeneity were analyzed depending on the stereocomplexation conditions and the molecular weight as well as the number of carboxyl end groups in the PLLA and PDLA used for stereocomplexation. The PLA microspheres obtained were analyzed by Fourier transform IR spectroscopy, wide angle X‐ray spectroscopy and energy‐dispersive X‐ray spectroscopy methods which confirmed the presence of metal cations inside. The application of regular microspheres with metal ions as drug delivery systems is considered. © 2016 Society of Chemical Industry     

Binary and ternary blends of polylactide, polycaprolactone and thermoplastic starch

In this study binary and ternary blends of polylactide (PLA), polycaprolactone (PCL) and thermoplastic starch (TPS) are prepared using a one-step extrusion process and the morphology, rheology and physical properties are examined. The morphology and quantitative image analysis of the 50/50 PLA/TPS blend transverse phase size demonstrate a bimodal distribution and the addition of PCL to form a ternary blend results in a substantial number of fine dispersed particles present in the system. Focused ion beam irradiation, followed by atomic force microscopy (AFM) shows that dispersed PCL forms particles with a size of 370 nm in PLA. The TPS phase in the ternary blends shows some low level coalescence after a subsequent shaping operation. Dynamic mechanical analysis indicates that the temperature of the tan δ peak for the PLA is independent of TPS blend composition and that the addition of PCL in the ternary blend has little influence on the blend transitions. Both the α and β transitions for the thermoplastic starch are highly sensitive to glycerol content. When TPS of high glycerol content is blended with PLA, an increase in the ductility of the samples is achieved and this effect increases with increasing volume fraction of TPS. The ternary blend results in an even greater ductility with an elongation at break of 55% as compared to 5% for the pure PLA. A substantial increase in the notched Izod impact energy is also observed with some blends demonstrating three times the impact energy of pure PLA. The mechanical properties for the ternary blend clearly indicate a synergistic effect that exceeds the results obtained for any of the binary pairs. Overall, the ternary blend approach with PLA/TPS/PCL is an interesting technique to expand the property range of PLA materials.     

Surprising shape-memory effect of polylactide resulted from toughening by polyamide elastomer

Melt blending of polylactide (PLA) and biodegradable polyamide elastomer (PAE) has been performed in an effort to toughen the PLA. DMA tests showed good compatibility between PAE and PLA blends, and the PAE were dispersed in PLA matrix uniformly shown in SEM photos. Mechanical properties of blends with different PAE concentrations were observed. With the PAE contents increasing, the elongation at break of blends increased and the brittle break became ductile break. When the PAE content is 10%, the tensile strength of blend is similar to neat PLA, and the elongation increased to 194.6% significantly. Remarkably, the blends showed wonderful shape-memory effect. PAE domains act as stress concentrators in system with the stress release locally and lead to energy-dissipation process. These will prevent PLA matrix from breaking under high deformation, and lead to the PLA molecular orientation. Consequently, the blends occurred to deformation upon tensile load, and heating up the material will reform the shape back to the original shape.     

A new biodegradable blends prepared from polylactide and hyaluronic acid

In this article, the biodegradable polylactide/hyaluronic acid (PLA/HA) composites were prepared by a melt blending method. In addition, the acrylic acid grafted polylactide (PLA-g-AA) was studied as an alternative to PLA. The samples were characterized by a fourier transform infrared (FTIR) spectroscopy, a nuclear magnetic resonance (NMR), a differential scanning calorimetry (DSC), an Instron mechanical tester, and a scanning electron microscopy (SEM). As the result, due to the poor compatibility between PLA and HA, the mechanical properties of PLA/HA composites were worse than PLA. Much better dispersion and homogeneity of HA in the polymer matrix could be obtained when PLA-g-AA was used in place of PLA in the composite. The PLA-g-AA/HA composites could obviously improve the mechanical properties of PLA/HA ones, and the former provided a plateau tensile strength at break when the HA content was up to 20 wt%. Furthermore, the PLA-g-AA/HA was more easily processed than the PLA/HA because the former had lower viscosity than the latter, as they were molten. Biodegradation tests of blends were also studied under the enzymatic environment, and the result showed that the mass of blends reduced by about the HA content within 4 weeks.     

Preparation and rheological characterization of long chain branching polylactide

Long chain branching (LCB) of polylactide (PLA) was successfully prepared by the successive reactions of PLA with pyromellitic dianhydride (PMDA) and 1,4-phenylene-bis-oxazoline (PBOZ) together. The topological structures of the LCB generated from functional group reactions were investigated thoroughly by gel permeation chromatography (GPC) and rheology. Qualitative information about the branching structures could be readily obtained from linear viscoelasticity, nonlinear oscillatory shear experiments and strain hardening in elongational experiments. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict probable compositions and chain topologies of the products, which were reasonably explained by the suggested mechanism of functional group reactions. It was found out that the star-like LCB structure generated in these reactions contributed remarkably to the enhancement of strain hardening under elongational flow.     

Preparation and characterization of polylactide/thermoplastic konjac glucomannan blends

In this article, a new degradable thermoplastic konjac glucomannan (TKGM) was synthesized by graft copolymerization of vinyl acetate and methyl acrylate onto konjac glucomannan (KGM). Melt blending of polylactide (PLA) and TKGM has been performed in an effort to improve the processing and comprehensive mechanical properties of PLA and TKGM without compromising its degradability and biocompatibility. The miscibility, processing rheology, phase morphology, thermal properties, interaction, crystallization and mechanical properties of PLA/TKGM blends were investigated in detail. The thermal processing property of PLA/TKGM blend (60/40) was quite close to low density polyethylene (LDPE). As observed from the tan δ curves in dynamic mechanical analysis, all of the blends exhibit a single glass transition over the entire composition range, indicating that the blends were thermodynamically miscible. The TKGM exhibited a relatively broad endothermic peak at around 120 °C, which was lower than that of KGM. And an obvious glass-transition behavior was obtained around 26.6 °C. Furthermore, the PLA/TKGM blend (60/40) had a very high elongation at break of 234.8%, while the tensile strength remained as high as 36.5 MPa. And the PLA/TKGM blend (20/80) resulted in an even greater ductility with an elongation at break of 520.5% as compared with 14.1% for pure PLA. A substantial increase in the non-notched impact strength was also observed with the PLA/TKGM blend (20/80) demonstrating two times the impact strength of pure PLA.     

Rapid stereocomplex formation of polylactide using supercritical fluid technology

It is generally believed that stereocomplex polylactide is a prospect for rigorous application in the future due to enhancement of the physical properties of its homopolymers. There have been many studies about the formation and characterization of this stereocomplex. However, we are the first to report that supercritical fluid technology can yield a high‐degree stereocomplex polylactide (ca 98%) in a relatively short time (1 min). “This method generated a dry product and feasible for polylactide with weight‐average molecular weight (Mw) up to 70 000–160 000 g mol^?1 or higher by modifying the material feed of the homopolymer.” The “feasible” term is used to explained that our system is suitable for stereocomplexation of the polylactide material with molecular weight 70 000–160 000 or above. We also discovered that modification of the material feed did not affect the properties of the stereocomplex polylactide generated. The process and outcome of this research indicate that supercritical fluid technology is suitable for a continuous stereocomplex polylactide formation process. Copyright © 2012 Society of Chemical Industry     

Long chain branching polylactide: Structures and properties

Long chain branching (LCB) of polylactide (PLA) was successfully prepared by the successive reactions of the end hydroxyl groups of PLA with pyromellitic dianhydride (PMDA) and triglycidyl isocyanurate (TGIC) together. The topological structures of the LCB generated from functional group reactions as well as free radical reactions were investigated thoroughly by gel permeation chromatography (GPC) and rheology. Qualitative information about the branching structures could be readily obtained from linear viscoelasticity, non-linear oscillatory shear experiments and strain hardening in elongational experiments. For quantitative information on chain structure, linear viscoelasticity combined with branch-on-branch (BOB) dynamic model was used to predict exact compositions and chain topologies of the products, which were reasonably explained by the suggested mechanism of functional group reactions. It was found out that the tree-like LCB structure generated in these reactions contributed remarkably to the enhancement of strain hardening under elongational flow, which improves the foaming ability substantially.     

Mesophase formation and its thermal transition in the stretched glassy polylactide revealed by infrared spectroscopy

The structural development in the glassy polylactide during stretching and subsequent heating has been investigated by Fourier transform infrared spectroscopy. It is indicated that only when molecular chains in the amorphous phase approach their finite extensibility beyond a critical strain of about 1, accompanied by remarkable conformational ordering, can cohesive mesophase with certain molecular ordering be brought out to trigger strain-induced crystallization. Upon heating cohesive mesophase endures melting during glass transition region where an endothermic peak is observed, and the extent of melting relies on its initial thermal stability and is in particular affected by the subsequent advent of strain-induced crystallization.     

Flow‐induced crystallization of polylactide stereocomplex under pressure

Although shear and pressure fields always coexist in practical polymer processing, their combined influence on the crystallization behavior of polylactide stereocomplex (SC) is ambiguous due to the limit of experiment device. In that case, a homemade device was employed to prepare samples under the coexistence of shear and pressure and explore the crystallization behavior of SC. Differential scanning calorimetry and synchrotron radiation were used to investigate the combined effect of shear flow and pressure on SC crystallization. The results show that shear flow was helpful for SC formation. Shear flow promoted the phase mixing of the polymer blends and improved the nucleation efficiency of SC. Pressure had a negative effect on SC formation because of the decrease in free volume. Regard to polylactide homogeneous (HC), pressure played a positive role on HC formation. Pressure suppressed the formation of SC network which could impede HC generation. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46378.     

Morphology and crystallization kinetics in a mixture of low-molecular weight aliphatic amide and polylactide

Polylactide (PLA)/N,N-ethylenebis(12-hydroxystearamide) mixture was prepared by using melt extrusion. The detailed crystallization kinetics and morphology of neat PLA and a mixture were studied by using polarized optical microscopy, light scattering, differential scanning calorimetry, and wide-angle X-ray diffraction analyses. The overall crystallization rate and spherulitic texture of PLA were strongly influenced in presence of the organic additive. The overall crystallization rate of matrix PLA increased with addition of WX1. These behaviors indicated that WX1 crystallites, which crystallized at the very early stage of PLA crystallization act as a nucleating agent for PLA crystallization.     

Morphology and properties of polylactide modified by thermal treatment, filling with layered silicates and plasticization

Polylactide (PLA) was modified physically by filling with inorganic additives—organomodified particles of clay, unmodified particles of clay as well as with a plasticizer—poly(ethylene glycol). The PLA-based systems were prepared by melt blending of PLA with other components. Combination of PLA with organomodified clay particles formed nanocomposite with intercalated nanostructure, while introducing microparticles led to microcomposite, respectively. The clay concentration was maintained at 3 wt% in the both systems. Additionally unfilled PLA, plasticized PLA and plasticized nanocomposite were prepared under the same blending conditions. For plasticization, 10 wt% of poly(ethylene glycol) was used. Two groups of the PLA samples and PLA-based systems featured by starting amorphous structure or semicrystalline structure were considered, respectively. The structure, thermal behavior, thermo-optical properties and dynamic mechanical response of the PLA-based systems were investigated and compared with unfilled PLA of the same thermo-mechanical history and with the neat (unprocessed) PLA. The influence of the composition and thermal treatment on the structure and physical properties of the considered samples was determined. The effect of aging on the structure and crystallization ability of the PLA material from its glassy amorphous state was evaluated as well. Temperature modulated differential scanning calorimetry, thermo-optical analysis, X-ray diffraction technique, dynamic mechanical analysis (DMTA) were used.