Linear and four‐armed poly(l‐lactide)‐block‐poly(d‐lactide) copolymers and their stereocomplexation with poly(lactide)s

Linear and four‐armed poly(l ‐lactide)‐block‐poly(d ‐lactide) (PLLA‐b‐PDLA) block copolymers are synthesized by ring‐opening polymerization of d ‐lactide on the end hydroxyl of linear and four‐armed PLLA prepolymers. DSC results indicate that the melting temperature and melting enthalpies of poly (lactide) stereocomplex in the copolymers are obviously lower than corresponding linear and four‐armed PLLA/PDLA blends. Compared with the four‐armed PLLA‐b‐PDLA copolymer, the similar linear PLLA‐b‐PDLA shows higher melting temperature (212.3 °C) and larger melting enthalpy (70.6 J g^?1). After these copolymers blend with additional neat PLAs, DSC, and WAXD results show that the stereocomplex formation between free PLA molecular chain and enantiomeric PLA block is the major stereocomplex formation. In the linear copolymer/linear PLA blends, the stereocomplex crystallites (sc) as well as homochiral crystallites (hc) form in the copolymer/PLA cast films. However, in the four‐armed copolymer/linear PLA blends, both sc and hc develop in the four‐armed PLLA‐b‐PDLA/PDLA specimen, which means that the stereocomplexation mainly forms between free PDLA molecule and the inside PLLA block, and the outside PDLA block could form some microcrystallites. Although the melting enthalpies of stereocomplexes in the blends are smaller than that of neat copolymers, only two‐thirds of the molecular chains participate in the stereocomplex formation, and the crystallization efficiency strengthens. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1560–1567     

Accelerated hydrolytic degradation of Poly(l-lactide)/Poly(d-lactide) stereocomplex up to late stage

Poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blend specimens containing only stereocomplex as crystalline species, together with those of pure PLLA and PDLA specimens, were prepared by solution crystallization using acetonitrile as the solvent. Their accelerated hydrolytic degradation was carried out in phosphate-buffered solution at elevated temperatures of 70-97 °C up to the late stage. During hydrolytic degradation, the stereocomplex crystalline residues were first traced by gel permeation chromatography. Similar to the hydrolytic degradation of pure PLLA and PDLA specimens, the hydrolytic degradation of stereocomplexed PLLA/PDLA blend specimens slowed down at the late stage when most of the amorphous chains were removed and crystalline resides were formed and degraded. The estimated activation energy for hydrolytic degradation of stereocomplex crystalline residues (97.3 kJ mol⁻¹) is significantly higher than 75.2 kJ mol⁻¹ reported for α-form of PLLA crystalline residues. This indicates that the stereocomplex crystalline residues showed the higher hydrolysis resistance compared to that of α-form of PLLA crystalline residues.     

An efficient solid‐state polycondensation method for synthesizing stereocomplexed poly(lactic acid)s with high molecular weight

Simultaneous solid‐state polycondensation (SSP) of the powdery prepolymers of poly(L ‐lactic acid) (PLLA) and poly(D ‐lactic acid) (PDLA) can produce entire stereocomplexed poly(lactic acid)s (sc‐PLA) with high molecular weight and can be an alternative synthetic route to sc‐PLA. Ordinary melt polycondensations of L ‐ and D ‐lactic acids gave the PLLA and PDLA prepolymers having medium molecular weight which were pulverized for blending in 1:1 ratio. The resultant powder blends were then subjected to SSP at 130–160 °C for 30 h under a reduced pressure of 0.5 Torr. Some of the products thus obtained attained a molecular weight (M_w) as high as 200 kDa, consisting of stereoblock copolymer of PLLA and PDLA. A small amount of the stereocomplex should be formed in the boundaries of the partially melted PLLA and PDLA where the hetero‐chain connection is induced to generate the blocky components. The resultant SSP products showed predominant stereocomplexation after their melt‐processing in the presence of the stereoblock components in spite of containing a small amount of racemic sequences in the homo‐chiral PLLA and PDLA chains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3714–3722, 2008     

Stabilization of pH‐Sensitive mPEG–PH–PLA Nanoparticles by Stereocomplexation Between Enantiomeric Polylactides

Methoxy poly(ethylene glycol)–poly(L ‐histidine)–poly(lactide) (mPEG₄₅–PH₃₀–PLA₈₂) triblock copolymers self‐assemble into nanoparticles by sterocomplexation. The properties of the stereocomplex nanoparticles including morphology, stability, and biocompatibility are investigated. The results reveal that the stereocomplexation between PLLA and PDLA segments could prevent the aggregation of the nanoparticles when the pH value is around 6.8. The mean diameter of the stereocomplex nanoparticles is stabilized at about 100 nm when the pH values are changed from 7.9 to 5.0. The cytotoxicity of the stereocomplex nanoparticles is evaluated, and the results demonstrate that the stereocomplexation could decrease the cytotoxicity of the PDLA segments.     

Adsorption of enantiomeric poly(lactide)s on surface-grafted poly(L-lactide)

We show that resistance of densely grafted polymer layers to adsorption of chemically identical free chains, which is known to be caused by entropic expulsion of free chains from the grafted layer, can be suppressed using the grafted and free chains of opposite stereoconfiguration. Specifically, we study adsorption of poly(L-lactide) (PLLA) and its enantiomer poly(D-lactide) (PDLA) onto layers of surface-grafted PLLA in acetonitrile and chloroform by infrared spectroscopy (IR). The grafted layers with thicknesses ranging from 7 to 35 nm are produced by ring-opening polymerization of L-lactide from hydroxyl end-groups of a self-assembled monolayer on gold. The IR data indicate that adsorption on the bare gold surface is the same for the L- and D-form of the polymer. However, covering the gold with the surface-grafted PLLA produces a significant decline in the adsorption of free PLLA and, by contrast, a strong enhancement in the adsorption of free PDLA. In addition, the IR data indicate that the adsorbed PDLA chains are stereocomplexed with the grafted PLLA chains. Thus, entropic expulsion of free chains from the grafted layer, which is responsible for the resistance of surface-grafted PLLA to adsorption of free PLLA, is suppressed in the case of free PDLA by stereocomplexation between the grafted and free chains.     

Morphology,rheological and crystallization behavior in thermoplastic polyurethane toughed poly(l-lactide) with stereocomplex crystallites

Melt-blending poly(l-lactide) (PLLA) with elastomers has been well demonstrated to improve toughness of PLLA. Here, we show a poly(d-lactide) (PDLA) grafted thermoplastic polyurethane (TPU) (TPU-g-PDLA) toughed PLLA with simultaneous formation of few amount stereocomplex crystallites (SCs) which exhibited higher efficient toughening than that of PLLA with TPU. The TPU-g-PDLA was prepared by the in-situ melt-reaction of TPU and PDLA with 4, 4’-diphenylmethane diisocyanate (MDI). A comparative study on morphology, rheological and crystallization behavior was also carried in PLLA/TPU, PLLA/TPU-g-PDLA and PLLA/TPU/PDLA samples. The PLLA/TPU-g-PDLA samples show the highest crystallization rate, complex viscosity, impact strength and tensile strength among PLLA/TPU, PLLA/TPU-g-PDLA and PLLA/TPU/PDLA samples, indicating that the higher interfacial interaction between TPU-g-PDLA and PLLA. Furthermore, TPU chains in TPU-g-PDLA were thought to break the intermolecular interaction of PLLA and rapid its crystallization and increase crystallinity.     

Plasticization of biodegradable stereocomplex polylactides with poly(propylene glycol)

The plasticization of stereocomplex polylactide (scPLA) with poly(propylene glycol) (PPG) is described. The poly(L-lactide) (PLLA), poly(D-lactide) (PDLA) and PPG were completely blended in chloroform before film casting to prepare scPLA/PPG blend films. The PLLA/PDLA ratio was fixed at 50/50 (w/w). The PPG blending enhanced the stereocomplex formation of the scPLA films. The stereocomplex crystallinities of the scPLA films increased as the PPG blend ratio increased, the PPG molecular weight decreased and the PDLA molecular weight decreased. The PPG blending significantly decreased the T _g and film transparency, and improved the elongation at break of the scPLA films. The results indicated that the PPG blending had an effect on the stereocomplexation and it improved the flexibility of the scPLA films.     

Polylactide stereocomplex crystallites as nucleating agents for isotactic polylactide

A nucleation efficiency scale for isotactic poly(L ‐lactide) (PLLA) was obtained with self‐nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self‐nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D ‐lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25–15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 300–313, 2001     

Influence of chain extender on thermal properties and melt flow index of stereocomplex PLA

The effects of chain extension and melt blending temperature on the stereocomplex formation of 50/50 (w/w) poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blends or stereocomplex polylactides (scPLAs) were investigated. Joncryl^® ADR 4368, a styrene-acrylic multifunctional oligomeric agent, was used as a chain extender. Differential scanning calorimetry and X-ray diffractometry were used to confirm the stereocomplex formation of the PLLA/PDLA blends. Melt flow indices (MFI) of the blends were also determined. The stereocomplex crystallinities gradually decreased with increasing blending temperature and Joncryl^® ADR 4368 ratio. The significant decrease in the MFI of scPLAs is believed to be attributed to chain extension at the blending temperatures of 170 °C and 200 °C. The MFI values of scPLAs decreased as the Joncryl^® ADR 4368 ratio and blending temperature increased. The results indicated that the chain extension has an effect on the stereocomplexation and it improved the melt strength of the scPLAs.     

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.     

Visualization of spontaneous aggregates by diblock poly(styrene)‐b‐poly(L‐lactide)/poly(D‐lactide) pairs in solution with new fluorescent CdSe quantum dot labels

Spontaneous stereocomplex aggregation of diblock poly(styrene)‐b‐poly(L ‐lactide) PS‐b‐PLLA/poly(D ‐lactide) PDLA pairs has been investigated under ambient temperature in tetrahydrofuran solution. First, diblock PS₂₆₀‐b‐PLLA₁₆₅ and PS₂₆₀‐b‐PDLA₁₆₂ bearing similar lengths of respective PLLA and PDLA blocks were synthesized through controlled atom‐transfer radical polymerization of styrene, and a subsequent living ring‐opening polymerization of optically pure lactides, and their structures were further characterized by nuclear magnetic resonance spectroscopy (NMR) and gel‐permeation chromatography (GPC). Subsequently, new enantiomeric poly(D ‐lactide) stabilized core‐shell fluorescent CdSe quantum dots (CdSe/PDLA QD) were designed and prepared as sensitive fluorescence labels to shed new lights on the spontaneous stereocomplex aggregation in THF, which was mediated by stereocomplexation of the PLLA and PDLA chains. Upon simply mixing two individual THF solution of diblock PS₂₆₀‐b‐PLLA₁₆₅ and HO‐PDLA₃₀‐SH, spontaneous stereocomplex aggregation was studied, and the aggregated uniform spherical particles were observed by scanning electronic microscopy (SEM) to exhibit average particle diameters of 2.0 μm. Finally, utilizing the prepared CdSe/PDLA QDs as new fluorescent labels, morphologies of the spontaneous aggregates by new diblock PS₂₆₀‐b‐PLLA₁₆₅/HO‐PDLA₃₀‐SH pair were for the first time directly visualized by a confocal laser scanning fluorescence microscopy (CLSFM). These results might suggest alternative ways to simply prepare functional fluorescent particles with tunable diameter sizes and would be helpful to understand the mechanism of stereocomplex particle aggregation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1393–1405, 2009     

Hydrogen bonding assists stereocomplexation in poly(l‐lactic acid)/poly(d‐lactic acid) racemic blends

In this communication, we reported the sequence variation of stereocomplex crystals (SC) and homocrystals (HC) in poly(l ‐lactic acid)/poly(d ‐lactic acid) (PLLA/PDLA) racemic blends melts. It was evidenced that the emerging sequence of the SC and HC depends on the hydrogen bond formation in the melt, and the hydrogen bond is required for the stereocomplexation in PLLA/PDLA racemic blend. First, by combining a commercial fast‐scan chip‐calorimeter (Flash DSC 1) and micro‐FTIR, we found that hydrogen bonds were formed in the melt during cooling at 2.5 K/s, but not at 3000 K/s. Second, annealing the melt without hydrogen bonds at 100 °C led to HC emerging first, while annealing the melt with hydrogen bonds resulted in SC emerging at first. Third, the crystallization kinetics of the racemic blends after cooling to predefined T_c at 2.5 or 3000 K/s further verified that the hydrogen bonding can be inhibited effectively by cooling the racemic blends isotropic melt at fast enough rate. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 83–88     

Mixing of Racemic Poly(L-lactide)/Poly(D-lactide) Blend with Miscible Poly(D,L-lactide):Toward All Stereocomplex-type Polylactide with Strikingly Enhanced SC Crystallizability

Stereocomplex-type polylactide (SC-PLA) consisting of alternatively arranged poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains has gained a good reputation as a sustainable engineering plastic with outstanding heat resistance and durability,however its practical applications have been considerably hindered by the weak SC crystallizability.Current methods used to enhance the SC crystallizability are generally achieved at the expense of the precious bio-renewability and/or bio-degradability of PLAs.Herein,we demonstrate a feasible method to address these challenges by incorporating small amounts of poly(D,L-lactide) (PDLLA) into linear high-molecular-weight PLLA/PDLA blends.The results show that the incorporation of the atactic PDLLA leads to a significant enhancement in the SC crystallizability because its good miscibility with the isotactic PLAs makes it possible to greatly improve the chain mixing between PLLA and PDLA as an effective compatibilizer.Meanwhile,the melt stability (i.e.,the stability of PLLA/PDLA chain assemblies upon melting) could also be improved substantially.Very intriguingly,SC crystallites are predominantly formed with increasing content and molecular weight of PDLLA.More notably,exclusive SC crystallization can be obtained in the racemic blends with 20 wt％ PDLLA having weight-average molecular weight of above 1 ×10s g/mol,where the chain mixing level and intermolecular interactions between the PLA enantiomers could be strikingly enhanced.Overall,our work could not only open a promising horizon for the development of all SC-PLA-based engineering plastic with exceptional SC crystallizability but also give a fundamental insight into the crucial role of PDLLA in improving the SC crystallizability of PLLA/PDLA blends.     

Hydrogen-bonded monolayers and interdigitated multilayers at the air-water interface

Crystalline monolayers of octadecylsulfonate amphiphiles (C18S) separated by hydrophilic guanidinium (G) spacer molecules were formed at the air-water interface at a surface coverage that was consistent with that expected for a fully condensed monolayer self-assembled by hydrogen bonding between the G ions and the sulfonate groups. The surface pressure-area isotherms reflected reinforcement of this monolayer by hydrogen bonding between the G ions and the sulfonate groups, and grazing incidence X-ray diffraction (GIXD) measurements, performed in-situ at the air-water interface, revealed substantial tilt of the alkyl hydrophobes (t = 49 degrees with respect to the surface normal), which allowed the close packing of the C18 chains needed for a stable crystalline monolayer. This property contrasts with behavior observed previously for monolayers of hexadecylbiphenylsulfonate (C16BPS) and G, which only formed crystallites upon compression, accompanied by ejection of the G ions from the air-water interface. Upon compression to higher surface pressures, GIXD revealed that the highly tilted (G)C18S monolayer crystallites transformed to a self-interdigitated (G)C18S crystalline multilayer accompanied by a new crystalline monolayer phase with slightly tilted alkyl chains and disordered sulfonate headgroups. This transformation was dependent on the rate of compression, suggesting kinetic limitations for the "zipper-like" transformation from the crystalline monolayer to the self-interdigitated (G)C18S crystalline multilayer.     

Monolayer properties of a fuzzy rod polymer: Poly(γ-stearyl α, L-glutamate)

Static and dynamic properties, and surface morphologies of monolayers at the air-water interface of a fuzzy rod polymer, poly(γ-stearyl α, L-glutamate), PSLG, have been examined by the Wilhelmy plate method for surface pressure, electrically induced capillary wave diffraction (ECWD), epi-fluorescence microscopy, and atomic force microscopy (AFM). The monolayers were first formed by spreading polymer solutions at the air-water interface and allowing the solvent to evaporate to obtain polymer films, i.e., spread monolayers. The surface mass density was varied by either successive additions of more solutions on a given surface area or step-wise compression of the surface barrier on a Langmuir trough. Surface pressure isotherms at 23–;60°C were confirmed to be reversible and reproducible, and an abrupt change at approximately 60°C was observed, which is reported as the melting point of crystalline stearyl side chains. By AFM, the monolayer director n by surface alignment was confirmed as perpendicular to the compression direction and certain islands of departure from the monolayer state were visualized upon transferring the monolayers horizontally to silicon wafers. Macroscopic anisotropy in the surface alignment was probed by the electrocapillary waves propagated perpendicular (⟂) and parallel (∥) to the director n; the surface tension anisotropy amount to about 7% difference, σ_⟂/σ_⟂ < 0.07, where σ is the surface tension deduced from the wave propagation characteristics. Multidomain morphologies of the monolayers were imaged by epi-fluorescence microscopy and they were found to differ according to the method of monolayer mass density variation, i.e., the successive addition and step-wise compression. © 1996 John Wiley & Sons, Inc.     

Poly(lactide) stereocomplexes: formation, structure, properties, degradation, and applications

Poly(lactide)s [i.e. poly(lactic acid) (PLA)] and lactide copolymers are biodegradable, compostable, producible from renewable resources, and nontoxic to the human body and the environment. They have been used as biomedical materials for tissue regeneration, matrices for drug delivery systems, and alternatives for commercial polymeric materials to reduce the impact on the environment. Since stereocomplexation or stereocomplex formation between enantiomeric PLA, poly(L-lactide) [i.e. poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e. poly(D-lactic acid) (PDLA)] was reported in 1987, numerous studies have been carried out with respect to the formation, structure, properties, degradation, and applications of the PLA stereocomplexes. Stereocomplexation enhances the mechanical properties, the thermal-resistance, and the hydrolysis-resistance of PLA-based materials. These improvements arise from a peculiarly strong interaction between L-lactyl unit sequences and D-lactyl unit sequences, and stereocomplexation opens a new way for the preparation of biomaterials such as hydrogels and particles for drug delivery systems. It was revealed that the crucial parameters affecting stereocomplexation are the mixing ratio and the molecular weight of L-lactyl and D-lactyl unit sequences. On the other hand, PDLA was found to form a stereocomplex with L-configured polypeptides in 2001. This kind of stereocomplexation is called "hetero-stereocomplexation" and differentiated from "homo-stereocomplexation" between L-lactyl and D-lactyl unit sequences. This paper reviews the methods for tracing PLA stereocomplexation, the methods for inducing PLA stereocompelxation, the parameters affecting PLA stereocomplexation, and the structure, properties, degradation, and applications of a variety of stereocomplexed PLA materials.     

Influence of Interfacial Enantiomeric Grafting on Melt Rheology and Crystallization of Polylactide/Cellulose Nanocrystals Composites

Incorporating the surface-grafted cellulose nanocrystals(CNCs)with enantiomeric polylactide(PLLA or PDLA)is an effective and sustainable way to modify PLLA,but their difference in promoting matrix crystallization is still unrevealed.In this study,the CNCs with identical content and length of PLLA and PDLA(CNC-g-L and CNC-g-D)were prepared and blended with PLLA.The rheological properties of PLLA/CNC-gD are greatly improved,indicating that the stereocomplexation can significantly improve the interfacial strength as compared with the conventional van der Waals force in PLLA/CNC-g-L.Surprisingly,the matrix crystallizes at a higher rate in PLLA/CNC-g-L than PLLA/CNC-g-D.PLLA/CNC-g-L15 reaches its half crystallinity in 8.26 min while a longer period of 13.41 min is required for PLLA/CNC-g-D15.POM observation reveals that the superior crystallization behavior in PLLA/CNC-g-L is originated from its higher nucleation efficiency and faster growth rate.The formation of low content of sc-PLA at the interface can restrict the diffusion of PLLA but contribute less to generate crystalline nuclei,which synergistically leads to the retarded crystallization kinetics in PLLA/CNC-g-D.Revealing the mechanism of different interfacial enantiomeric grafting on the melt rheology and crystallization of PLLA is of great significance for the development of high-performance polylactide materials.     

Novel poly(l‐lactide)/poly(d‐lactide)/poly(tetrahydrofuran) multiblock copolymers with a controlled architecture: Synthesis and characterization

Novel poly(l ‐lactide) (PLLA)/poly(d ‐lactide) (PDLA)/poly(tetrahydrofuran) (PTHF) multiblock copolymers with designed molecular structure were synthesized by a two‐stage procedure. Well‐defined PDLA‐PLLA‐PTHF‐PLLA‐PDLA pentablock copolymers were prepared by sequential ring opening polymerization of l ‐ and d ‐lactides starting from PTHF glycol, with the length of the (equimolar) PLLA and PDLA blocks being varied. Then, these dihydroxyl‐terminated pentamers were transformed into multiblock copolymers by melt chain‐extension with hexamethylene diisocyanate–being the first time that the coupling of pentablock units is reported. The successful formation of macromolecular chains with a multiblock and well‐defined architecture was demonstrated by ¹H NMR spectroscopy. The thermal properties and structuring of the resulting materials were investigated by means of DSC and WAXD measurements and DMA analysis. Stereocomplexation was found to be promoted during solution and melt crystallization. This approach affords materials combining the high rigidity and strength (other than improved thermal resistance) of the hard stereocomplex crystallites with the flexibility imparted by the soft block, whereby their properties can be finely tailored through the composition of the basic pentablock units without limitations on the final molecular weight. The adopted reaction conditions make this process highly appealing in view of the possibility to perform it in extruder. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3269–3282     

Rheological and thermal properties of stereocomplexed polylactide films

Poly(l-lactide) (PLLA) and Poly(d-lactide) (PDLA) blended films (PLLA/PDLA) were prepared (5/95; 25/75; 50/50, and 75/25) by solvent casting method. Blend of PLLA and PDLA of medium molecular mass led to the formation of stereocomplex which was evidenced by differential scanning calorimetry, rheological measurement and Fourier transform infrared spectroscopy. The stereocomplex had a higher melting temperature (T _m) (more than 50 °C) and crystallized at higher temperature (T _c) (more than 25 °C) from the melt compared to neat PLLA and PDLA. The T _m and T _c gradually decreased with increasing the number of thermal scans. The enthalpy of fusion (?H_m) for stereocomplex crystallites in 50/50 blend films was the highest than that of homo-crystallites. Rheological measurement at a temperature of 180–195 °C revealed that the neat PLA was predominantly liquid-like behavior (G″ > G′) which transformed to extreme solid-like behavior by incorporation of PDLA into PLLA. Among blends, 50/50 PDLA/PLLA showed the maximum mechanical strength (G′) followed by 25/75, 75/25, and 5/95 blends. The significant increase in mechanical strength is believed to be attributed by stereocomplex formation by blends. Thermal and rheological data supported higher mechanical strength and an increase in melting and crystallization temperature adequately.     

Enhanced crystallization,thermal properties,and hydrolysis resistance of poly(l-lactic acid) and its stereocomplex by incorporation of graphene nanoplatelets

Poly(d-lactic acid) (PDLA) and graphene nanoplatelets were used as nucleating agents for poly(l-lactic acid) (PLLA). The graphene (1 wt%) shows a more pronounced effect than PDLA in facilitating PLLA crystallization. Graphene effect on crystallization of stereocomplex (SC) polylactide is also demonstrated. Although medium molecular weight PLLA was blended with a limited content (1 wt%) of low molecular weight PDLA in the presence of graphene (0.5 phr), SC melting temperature is slightly increased without the use of high molecular weight polylactide pair. Also, optimal graphene content (0.5 phr–1.5 phr) promotes crystallization of PLLA homocrystals in the three-component system (PLLA/PDLA/graphene). Graphene additionally enhances Young's modulus and barrier property to thermal degradation of both PLLA and SC systems. Furthermore, PLLA/graphene is more resistant to hydrolysis than PLLA. Likewise, PLLA/PDLA/graphene is more stable than PLLA/PDLA during hydrolysis.