Rheology and crystallization behavior of asymmetric PLLA/PDLA blends based on linear PLLA and PDLA with different structures

In this study, several asymmetric poly(L‐lactide)/poly(D‐lactide) (PLLA/PDLA) blends were prepared by adding small amounts of PDLA with different structures into linear PLLA matrix. The effect of PDLA on rheological behavior, crystallization behavior, nucleation efficiency and spherulite growth of PLLA was investigated. Rheological results indicated that PLLA/PDLA blends showed solid‐like viscoelastic behavior at low temperature (<200°C), and the cross‐linking density of PLLA/PDLA melt at 180°C followed the order: PLLA/6PDLA > PLLA/L‐PDLA > PLLA/3PDLA > PLLA/4PDLA. No‐isotherm and isotherm crystallization results indicated that the crystallization capacity of PLLA/PDLA blends was strongly related to the PDLA structure, crystallization temperature and thermal treatment temperature. Furthermore, the dimension of crystal growth during isotherm crystallization presented the obvious dependent on the PDLA structure. The nucleation efficiency of sc‐crystallites in the blends and spherulite density during isothermal crystallization were also studied. Nucleation efficiency of sc‐crystallites in the PLLA/S‐PDLA blends showed the obvious dependent on thermal treatment temperature with respect to PLLA/L‐PDLA, and nucleation efficiency sc‐crystallites in the PLLA/S‐PDLA blends first decreased and then increased as the thermal treatment temperature increased. Spherulite density of PLLA/PDLA blends was also related to thermal treatment temperature and the PDLA structure. This study has discussed the temperature dependence of the stereocomplex networks between PLLA and PDLA with different structure, and then its consequential influence on rheology and crystallization capacity of PLLA, which would provide the theoretical direction for PLA processing. Copyright © 2016 John Wiley & Sons, Ltd.     

Influence of graphene nanosheets on stereocomplex crystallization behaviors of star‐shaped poly (D(L)‐lactide) stereoblock copolymer

The nanocompsites of star‐shaped poly(D‐lactide)‐co‐poly(L‐lactide) stereoblock copolymers (s‐PDLA‐PLLA) with two‐dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X‐ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s‐PDLA‐PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0°C formed in isothermal crystallization for all samples. The crystallization temperatures of s‐PDLA‐PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3 wt% GNS was about 128.2°C, ie, 15°C higher than pure s‐PDLA‐PLLA. At isothermal crystallization processes, the halftime of crystallization (t_0.5) of the sample with 3 wt% GNS decreased to 6.4 minutes from 12.9 minutes of pure s‐PDLA‐PLLA at 160°C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three‐dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s‐PDLA‐PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s‐PDLA‐PLLA with good nucleation ability in the s‐PDLA‐PLLA material.     

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     

Effect of Star-shaped chain architectures on the polylactide stereocomplex crystallization behaviors

Linear and star-shaped polylactides (PLA) with similar molecular weights of each arm are synthesized via ring-opening polymerization of LA with 3-butyn-l-ol and pentaerythritol as initiators,respectively.By solution blending of equivalent mass of poly(L-lactic acid)s (PLLAs) and poly(D-lactic acid)s (PDLAs),perfect PLA stereocomplexes (scPLAs) are prepared and confirmed by WAXD and FTIR analysis.Effect of chain architectures on stereocomplex crystallization is investigated by studying the non-isothermal and isothermal crystallization of linear and star-shaped polylactide stereocomplexes.In dynamic DSC and POM test,star-shaped PLLA (4sPLLA)/PDLA and PLLA/star-shaped PDLA (4sPDLA) stereocomplexes reach rapid crystallization and higher crystallinity due to larger spherulite density of star-shaped chain and excellent chain mobility of linear chain.In isothermal crystallization test,much faster crystallization and less crystallization half-time is obtained with the increase of star-shaped chain.Meanwhile,4sPLLA/PDLA and PLLA/4sPDLA are found to have the highest crystallinity,suggesting limitation of too much star-shaped chain for 4sPLLA/4sPDLA and restriction of linear chain in nucleation capacity for PLLA/PDLA.The results reveal that star-shaped chain has an important influence on the crystallization of scPLAs.     

Enhancing crystallization behavior for optimized performances of poly(TMC‐b‐(LLA‐ran‐GA)) by PDLA/PLLA stereocomplex crystallization

The blends of poly(1,3‐trimethylene carbonate‐b‐(l ‐lactide‐ran‐glycolide)) (PTLG) with poly(d ‐lactide) (PDLA) were prepared via solution‐casting method using CH₂Cl₂ as solvent. The poly(l ‐lactide) (PLLA) segments of PTLG with PDLA chain constructed as stereocomplex structures and growth stereocomplex crystals of PLA (sc‐PLA). The effects of sc‐PLA crystals on thermal behavior, mechanical properties, thermal decomposition of the PTLG/PDLA blends were investigated, respectively. The differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) results showed that the total crystallinity of the PTLG/PDLA blends was increased with the PDLA content increasing. Heterogeneous nucleation of sc‐PLA crystals induced crystallization of the PLLA segments in PTLG. The crystallization temperature of samples shifted to 107.5°C for the PTLG/PDLA‐20 blends compared with that of the PTLG matrix, and decreased the half‐time of crystallization. The mechanical measurement results indicated that the tensile strength of the PTLG/PDLA blends was improved from 21.1 MPa of the PTLG matrix to 39.5 MPa of PTLG/PDLA‐20 blends. The results of kinetics of thermal decomposition of the PTLG/PDLA blends by TGA showed that the apparent activation energy of the PTLG/PDLA blends was increased from 59.1 to 72.1 kJ/mol with the increasing of the PDLA content from 3 wt% to 20 wt%, which indicated the enhancement of thermal stability of the PTLG/PDLA blends by addition of PDLA. Furthermore, the biocompatibility of the PTLG/PDLA blends cultured with human adipose‐derived stem cells was evaluated by CCK‐8 and live/dead staining. The experiment results proved the PTLG/PDLA blends were a kind of biomaterial with excellent physical performances with very low cytotoxicity.     

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.     

Synthesis,stereocomplex crystallization and properties of poly(l‐lactide)/four‐armed star poly(d‐lactide) functionalized carbon nanotubes nanocomposites

Functionalized carbon nanotubes (F‐CNTs) were synthesized through the nucleophilic substitution reaction between four‐armed star poly(d ‐lactide) (4PDLA) and acryl chloride of carbon nanotubes and were characterized using Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy and thermogravimetric analysis. The results indicated that the 4PDLA was successfully grafted onto carbon nanotubes, and it contained 45.5 wt% of 4PDLA. Poly(l ‐lactide) (PLLA) nanocomposites with different F‐CNTs content were prepared by solution casting. Optical microscopy and scanning electron microscopy results showed that F‐CNTs were uniformly dispersed in the nanocomposites. Crystallization behavior and crystal structure of PLLA nanocomposites were investigated using differential scanning calorimetry, polarizing microscope and X‐ray diffraction. The results found that poly(lactide) stereocomplex crystal could be formed between PLLA and F‐CNTs. F‐CNTs played different roles in the process of solution casting and melting crystallization. Polarizing microscope also revealed that crystallization temperature had a significant effect on the nucleation and spherulites growth of PLLA. Thermal stability and mechanical properties of the nanocomposites were also investigated by thermogravimetric analysis, dynamic mechanical analysis and tensile testing. These results demonstrated that the addition of F‐CNTs obviously improved thermal stability and tensile strength of PLLA. The results showed that PLLA/F‐CNTs would have potential values in engineering fields. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA) is incorporated into a 1:1 polymer blend system of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) that has a high molecular weight to study its addition effect on the stereocomplex (sc) formation of PLLA and PDLA. The ternary polymer blend films are first prepared by casting polymer solutions of sb-PLA, PLLA, and PDLA with different compositions. Upon increasing the content of sb-PLA in the blend films the sc crystallization is driven to a higher degree, while the formation of homo-chiral (hc) crystals is decreased. Lowering the molecular weight of the incorporated sb-PLA effectively increases the sc formation. Consequently, it is revealed that sb-PLA can work as a compatibilizer to improve the poor sc formation in the polymer blend of PLLA and PDLA.     

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     

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.     

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.     

Different enantioselectivity of two types of poly(lactic acid) depolymerases toward poly(l-lactic acid) and poly(d-lactic acid)

Poly(lactic acid) (PLA) depolymerases are categorized into protease-type and lipase-type. Protease-types can hydrolyze poly(l-lactic acid) (PLLA) but not poly(d-lactic acid) (PDLA). Lipase-types, including cutinase-like enzyme (CLE) from Cryptococcus sp. strain S-2 preferentially hydrolyze PDLA. Both enzymes degraded not only PLA emulsion but also PLA film, in which amorphous region is preferentially attacked, but crystalline region can be also attacked. Stereocomplex PLA (sc-PLA) formed by 50:50 blending of PLLA and PDLA included no homo crystals, but a tiny homo crystallization peak appeared and crystallinity increased by 5% when attacked by CLE, although no significant change of molecular weight and crystalline size was found. Enantioselective degradation must occur in amorphous region of PLLA/PDLA film and preferentially hydrolyzed PDLA, resulting in a slightly excess amount of PLLA remained, which must be crystallized.     

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.     

Characteristics of the biodegradability and physical properties of stereocomplexes between poly(L‐lactide) and poly(D‐lactide) copolymers

Random and block copolymerizations of L ‐ or D ‐lactide with ε‐caprolactone (CL) were performed with a novel anionic initiator, (C₅Me₅)₂SmMe(THF), and they resulted in partial epimerization, generating D ,L ‐ or meso‐lactide polymers with enhanced biodegradability. A blend of PLLA‐r‐PCL [82/18; PLLA = poly(L ‐LA) and PCL = poly(ε‐caprolactone)] and PDLA‐r‐PCL [79/21; PDLA = poly(D ‐LA)] prepared by the solution‐casting method generated a stereocomplex, the melting temperature of which was about 40 °C higher than that of the nonblended copolymers. A blend of PLLA‐b‐PCL (85/15) and PDLA‐b‐PCL (82/18) showed a lower elongation at break and a remarkably higher tensile modulus than stereocomplexes of PLLA‐r‐PCL/PDLA‐r‐PCL and PLLA/PDLA. The biodegradability of a blend of PLLA‐r‐PCL (65/35) and PDLA‐r‐PCL (66/34) with proteinase K was higher than that of PLLA‐b‐PCL (47/53) and PDLA‐b‐PCL (45/55), the degradability of which was higher than that of a PLLA/PDLA blend. A blend film of PLLA‐r‐PDLLA (69/31)/PDLA‐r‐PDLLA (68/32) exhibited higher degradability than a film of PLLA/PDLLA [PDLLA = poly(D ,L ‐LA)]. A stereocomplex of PLLA‐r‐PCL‐r‐PDMO [80/18/2; PDMO = poly(L ‐3,D ,L ‐6‐dimethyl‐2,5‐morpholinedion)] with PDLA‐r‐PCL‐r‐PDMO (81/17/2) showed higher degradability than PLLA‐r‐PDMO (98/2)/PDLA‐r‐PDMO (98/2) and PLLA‐r‐PCL (82/18)/PDLA‐r‐PCL (79/21) blends. The tensile modulus of a blend of PLLA‐r‐PCL‐r‐PDMO and PDLA‐r‐PCL‐r‐PDMO was much higher than that of a blend of PLLA‐r‐PDMO and PDLA‐r‐PDMO. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 438–454, 2005     

Stereoselective Interaction between Isotactic and Optically Active Poly(lactic acid) and Phenyl‐Substituted Poly(lactic acid)

Isotactic and optically active poly(D ‐lactic acid) (PDLA) and phenyl‐substituted poly(lactic acid)s (Ph‐PLAs), i.e., poly(D ‐phenyllactic acid) (Ph‐PDLA) and poly(L ‐phenyllactic acid) (Ph‐PLLA), were synthesized and stereospecific interactions between the synthesized polymers were investigated by their thermal properties and crystallization behavior using differential scanning calorimetry (DSC). The DSC measurements indicated that PDLA is miscible with Ph‐PLAs and that the attractive interaction between PDLA and L ‐configured Ph‐PLA is higher than that between PDLA and D ‐configured Ph‐PDLA. In other words, the latter result means that poly(lactic acid) (PLA) has a higher stereoselective attractive interaction with Ph‐PLA with the reverse configuration than with Ph‐PLA of the same configuration. These results strongly suggest that PLA‐based materials with a wide variety of physical properties and biodegradability can be fabricated by blending them with substituted PLAs with the reverse and same configurations.

     

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.     

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     

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.     

Preparation of porous biodegradable PLLA with tunable pore size through simple variation of PEG/PLLA-blend crystallization conditions

In this study, porous poly(L-lactic acid) (PLLA) films are prepared via a facile and low-cost approach using poly(ethylene glycol) (PEG) and solution casting. In contrast to most studies, the PEG/PLLA samples are further processed under different crystallization conditions (i.e., different PLLA crystallization temperatures) before PEG removal. As the PEG is extracted via solvent at higher PLLA crystallization temperatures, the resultant PLLA samples have larger pores. Interconnected fibrillar-shaped pores are found in all systems, and the fibrillar-porous structure width is ~150 nm–1.2 μm, as observed via scanning electron microscopy. These pore sizes can be tuned by adjusting the blend composition and crystallization temperature. In addition, PEG/PLLA blends are subjected to hydrolytic degradation analysis according to their crystallization conditions. Higher PLLA crystallization temperature yields higher PLLA crystallinity and larger pores, as well as reduced surface interaction with water. Therefore, the PLLA degradation rate is decreased. The developed PLLA films have potential applications in drug delivery and tissue engineering.     

Effects of compatibility of poly(l‐lactic‐acid) and thermoplastic polyurethane on mechanical property of blend fiber

Blending poly(l ‐lactic‐acid) (PLLA) and thermoplastic polyurethane (TPU) has been performed in an effort to toughen PLLA without compromising its biodegradability and biocompatibility. The mixing enthalpy calculation of PLLA and TPU predicted that the blend was a thermodynamic miscible system. The viscoelastic properties and phase morphologies of PLLA/TPU blends were investigated further by dynamic mechanical analysis and scanning electron microscopy. It was found that the blend was a partially miscible system. The dynamic mechanical analysis showed that T_g of PLLA and TPU shifted toward with TPU content increasing. Scanning electron microscopy photos showed that the morphologies of the blends changed from a sea island structure to a bicontinuous structure as an increment in TPU content, which suggested that the miscibility of PLLA and TPU was enhanced when the TPU increased. PLLA/TPU blend fibers were fabricated. With the TPU content increasing from 0 wt% to 30 wt%, the tensile strength and initial modulus of blend fibers decreased first then increased, while elongation at break and fracture work gradually increased. The change of tensile properties indicated the toughening effects of TPU on PLLA fibers, also suggested that the formation of blend fibers was influenced by the blend rheological behavior other than the compatibility. Copyright © 2014 John Wiley & Sons, Ltd.