PLLA Segments Research Articles

Electrospinning and melt electrowriting of a tunable triblock-copolymer composed of poly(ε-caprolactone) and poly(L-lactic acid) for biomedical applications

Both electrospinning (ES) and melt electrowriting (MEW) garner emerging interest within the field of biomedical applications. Poly(ε-caprolactone) (PCL) is still the gold standard for both techniques, demonstrating excellent processability due to its solubility, melt flow properties and thermal stability. However, when evaluating biomaterials for soft tissue applications, PCL exhibits limitations such as low elasticity and extended biodegradation times. Additionally, the limited availability of materials that are processable by MEW constrains their broader clinical translation. To address this limitation, a tunable triblock-copolymer of PCL and poly(L-lactic acid) (PLLA) is synthesized and processed using ES and MEW. This synthesis employs a PCL-diol backbone with a molar mass of 30 000 g mol-1, to which various lengths of PLLA end-blocks are added on both termini (PLLA-b-PCL-b-PLLA). The successful incorporation of these tunable end-blocks is verified through spectroscopic analysis. The ES of PLLA-b-PCL-b-PLLA reveals excellent fiber formation, with fiber diameters ranging between 2–4 µm as the PLLA molar mass increases. The tunable nature of the polymer allows for MEW at 90 °C, producing fibers with diameters ranging between 40–50 µm. Additionally, the materials exhibit adjustable mechanical properties and biodegradation rates by varying the PLLA segment lengths. Finally, in vitro cell testing using MC3T3-E1 cells demonstrates the polymer's excellent biocompatibility.

Biodegradable thermoplastic elastomers synthesized from C7–C10 aliphatic dicarboxylic acids, 2-methyl-1,3-propanediol, and L-lactide

Elastic materials with high biodegradability should replace those with low biodegradability in some applications. We previously reported biodegradable thermoplastic elastomers (TPEs) composed of poly(l-lactide) (PLLA) as a hard segment and aliphatic polyesters from 2-methyl-1,3-propanediol (MP) and short aliphatic dicarboxylic acids as soft segments. In this study, we synthesized the biodegradable thermoplastic elastomers using longer aliphatic dicarboxylic acids to develop the TPEs with lower glass-transition temperature (Tg) and high biodegradability. A series of aliphatic polyesters were prepared by polycondensation of MP and aliphatic dicarboxylic acids bearing 7–10 carbons. Among them, poly(2-methyl-1,3-propylene azelate) (PMP9) was found to have lower Tg, amorphous nature, and high biodegradability in seawater. The ring-opening polymerization of l-lactide (LLA) using PMP9 as a macroinitiator afforded triblock copolymers, PLLA-b-PMP9-b-PLLA (TPE9). The successive addition of LLA to in-situ generated PMP9 resulted in the successful one-pot synthesis of high molecular weight TPE9. The TPE9s showed both low Tg of the PMP9 segment and high melting temperature of the PLLA segment. The TPE9s exhibited elastic behavior showing elongation at break of up to 2500 % in tensile tests and also high biodegradability in seawater. Thus, we developed potentially practical TPE with tunable physical properties and high biodegradability obtainable from easily available and renewable starting materials.

Effect of poly(L-lactic acid)/poly(ethylene glycol)-poly(L-lactic acid) block copolymer blend film on preservation of Chinese winter jujube (Ziziphus Jujuba Mill. cv. Dongzao)

The effect of poly(L-lactic acid)/poly(ethylene glycol)-poly(L-lactic acid) block film on preservation of Chinese winter jujube (Ziziphus Jujuba Mill.cv.Dongzao) was investigated. Eight arm poly(ethylene glycol)-poly(L-lactic acid) block copolymer (8-PEG/PLLA) and net structure 8-PEG/PLLA (NET-PEL) were successfully synthetized by ring-opening polymerization, and different percentages (5, 10, and 20 %) of them were blended with PLLA to prepared blends films. Mechanical properties, modulated different scanning calorimetry (MDSC), gas and water vapor permeability results showed that 8-PEG/PLLA and NET-PEL block copolymer greatly increased the toughness of blend films, could be decrease PLLA segment glass transition temperature (Tg)from 59.5 °C to 41.6–46.7 °C and cold crystallization temperature(Tcc)from 89.4 °C to 73.5–77.7 °C, and increased the oxygen (O2), carbon dioxide (CO2), and water vapor transmission rate. The an appropriate gas concentration [O2 (2.56–3.51 %), CO2 (5.05–5.56 %)] was created inside the PLLA/NET-PEL20% (NEPEL20)group, which could restrain increase of total soluble solids (TSS), malonaldehyde content. The firmness, color luminosity (L*), total phenols, and ascorbic acid were maintained at higher level，and kept its commercial value after 40 days of storage. The present data indicated that treating post-harvest winter jujubes with NEPEL20 MAP packaging was an effective method for preservation of postharvest winter jujube.

Poly(L‐lactide)‐b‐poly(ε‐caprolactone)‐b‐poly(D,L‐lactide) copolymers with enhanced toughness and strength by regulating crystallization and phase separation

AbstractCopolymerizing poly(lactide) with other materials to obtain better comprehensive performance is a effective way to expand its application range. In this work, the precursors of hydroxyl terminated (poly(L‐lactide) [PLLA], poly(ε‐caprolactone) [PCL], poly(D,L‐lactide) [PDLLA]) were prepared, and PLLA‐PCL‐PDLLA copolymer were synthesized by chain extension. The effects of the proportion and molecular weight of each component and the amount of chain extender on crystallization, phase structures, mechanical properties and thermal stabilities of PLLA‐PCL‐PDLLA copolymer were studied in detail. Based on small‐angle X‐ray scattering results, the competition between crystallization and microphase separation was regulated by the composition and chain length of prepolymers. As the ratio of PLLA/PDLLA was 1:1, crystallization was prevailing and no obvious peak was observed in SAXS pattern. The tensile test results showed that as the ratio of PLLA/PDLLA increased from 1:1 to 1:5, the elongation at break of the copolymer changed from 1.8% to 343%. By using shorter length of PCL and PLLA segments in chain extension, improvement in strength and flexibility were obtained due to moderate degree of crystallization and microphase separation. This work used biodegradable materials to prepare extraordinary toughness copolymers without losing the biocompatibility, which may provide a feasible method for obtaining high toughness and biodegradable PLA‐based materials.

Fabrication and evaluation of 3D printed poly(l-lactide) copolymer scaffolds for bone tissue engineering

The application of poly(L-lactic acid) (PLLA) in tissue engineering is limited due to its brittleness and uncontrollable degradation rate. In this study, the flexible p-dioxanone (PDO) and highly reactive glycolide (GA) units were introduced into PLLA segments by chemical modification to prepare poly(l-lactide-ran-p-dioxanone-ran-glycolide) (PLPG) copolymers. The copolymers were then processed into the PLPG scaffold by a 3D printing technology. The physicochemical properties of the PLPG copolymers were studied by NMR, DSC, XRD, GPC, and SEM. Furthermore, the mechanical properties, degradation properties, and biocompatibility of the PLPG scaffolds were also studied. The results showed that introducing PDO and GA units disrupted the regularity of PLLA, decreasing the crystallinity of the PLPG copolymers. However, introducing PDO and GA units could effectively improve the mechanical and degradation properties of the PLLA scaffolds. In vitro cell culture experiments indicated that the PLPG scaffolds supported proliferation, growth, and differentiation of MC3T3-E1 cells. The PLPG scaffolds reported herein, with controllable degradation rates and mechanical performance, may find applications in bone tissue engineering.

Super-Tough and Biodegradable Poly(lactide-co-glycolide) (PLGA) Transparent Thin Films Toughened by Star-Shaped PCL-b-PDLA Plasticizers.

To obtain fully degradable and super-tough poly(lactide-co-glycolide) (PLGA) blends, biodegradable star-shaped PCL-b-PDLA plasticizers were synthesized using natural originated xylitol as initiator. These plasticizers were blended with PLGA to prepare transparent thin films. Effects of added star-shaped PCL-b-PDLA plasticizers on mechanical, morphological, and thermodynamic properties of PLGA/star-shaped PCL-b-PDLA blends were investigated. The stereocomplexation strong cross-linked network between PLLA segment and PDLA segment effectively enhanced interfacial adhesion between star-shaped PCL-b-PDLA plasticizers and PLGA matrix. With only 0.5 wt% addition of star-shaped PCL-b-PDLA (Mn = 5000 g/mol), elongation at break of the PLGA blend reached approximately 248%, without any considerable sacrifice over excellent mechanical strength and modulus of PLGA.

Polylactide/poly[(R)‐3‐hydroxybutyrate] (PHB) blend fibers with superior heat‐resistance: Effect ofPHBon crystallization

AbstractBlending low content of poly[(R)‐3‐hydroxybutyrate‐co‐4‐hydroxybutyrate] (P34HB ≤8 wt %) with poly(L‐lactide) (PLLA) has proven effective in enhancing heat resistance of PLLA fibers. Unfortunately, the enhancement is relatively limited with boiling water shrinkage of PLLA/P34HB blend fibers reduced to 9.9% at most. In this study, a series of PLLA/poly[(R)‐3‐hydroxybutyrate] (PHB) blend fibers with low PHB content (≤8 wt %) was prepared with sound spinnability. Surprisingly, the PLLA/PHB blend fibers showed much lower boiling water shrinkage than the PLLA/P34HB blend fibers with the same blend ratio. In particular, the boiling water shrinkage was dropped from ca. 80% for neat PLLA fibers to 9.9% for the PLA/P34HB blend fibers and further to 3.8% for the PLA/PHB blend fibers with 8 wt % P34HB or PHB added. Such an exceptionally improved heat resistance of the PLLA/PHB blend fibers could be closely related to an increase in crystallinity (Xc,PLLA) of the PLLA phase. Specifically, the mobility of the PLLA segments was promoted in the presence of PHB, along with notably improved orientation of the PLLA phase in the PLLA/PHB blend fibers. More importantly, investigation on crystallization behaviors revealed that the PHB phase served as an effective nucleating agent to accelerate the overall crystallization of PLLA. By contrast, the P34HB phase only acted as spherulite growth‐accelerating and/or nucleation‐assisting agents. These findings help provide a facile and effective strategy to improve heat resistance of PLLA fibers more significantly.

Biodegradable membrane of poly(l-lactide acid-dioxanone-glycolide) and stereocomplex poly(lactide) with enhanced crystallization and biocompatibility.

The membranes of poly(l-lactide acid-p-dioxanone-glycolide) (PLPG) with stereocomplex poly(lactic acid) (sc-PLA) were prepared by the solution blending way. It was observed that sc-PLA significantly heightened the crystallizing behavior of PLLA segments of the PLPG matrix. The crystallizing behavior displayed that the temperature of crystallization shifted to a higher range than that of PLPG. Moreover, the half-time of crystallization sharply decreased in value as the sc-PLA content increased in value on account of the pre-eminent nucleation ability of sc-PLA. TGA results revealed the thermal stability of the samples with the increase of sc-PLA compared to PLPG. Meanwhile, enzymatic degradation results indicated that the mass loss rate of the membrane decreased with the introduction of sc-PLA, but the overall degradation ability was still greater than that of PLLA. In the meantime, the biological experiment indicated that the membrane possessed low cytotoxicity.

Effects on the crystallization behavior and biocompatibility of poly(LLA-ran-PDO-ran-GA) with poly(d-lactide) as nucleating agents.

The blends of poly(l-lactide acid-p-dioxanone-glycolide) (PLPG) with poly(d-lactide) (PDLA) (PLPG/PDLA) were prepared by a solution-casting method. The effects of PDLA on the properties of the PLPG were studied. DSC and WAXD results confirmed that PLA stereocomplex (sc-PLA) crystals were formed by blending PLLA segments in PLPG with PDLA, and the melting endotherm for both PLLA and sc-PLA relied on PDLA content. The non-isothermal crystallization results indicated that the crystallization process was remarkably accelerated by the addition of PDLA. Meanwhile, the results of isothermal crystallization indicated that the half-time of crystallization decreased with the increase of PDLA content. Besides, the enzymatic degradation behavior of the samples showed that with the increase of PDLA content, the mass loss gradually decreased. Furthermore, TGA and DTG results indicated that the thermal degradation of the samples was a complex process. Moreover, the biocompatibility of the samples was tested by cell culture and using CCK-8 and live/dead staining. Results showed that the samples possessed lower cytotoxicity. Therefore, the PLPG/PDLA blends are promising candidate materials in biomedical applications.

Synthesis and properties of biodegradable thermoplastic elastomers using 2-Methyl-1,3-propanediol, succinic acid and lactide

Triblock copolymers, poly(L-lactide)-b-poly(2-methyl-1,3-propanediyl succinate)-b-poly(L-lactide) (PLLA-b-PMPS-b-PLLA), were synthesized as new biodegradable thermoplastic elastomers by two-step polymerization. In the first step, hydroxy-telechelic poly(2-methyl-1,3-propanediyl succinate) (PMPS) was synthesized from 2-methyl-1,3-propanediol (MP, small excess) and succinic acid (SA) using two-stage reaction of esterification and simple polycondensation processes. In the second step, plant derived L-lactide (LLA) was polymerized using the bifunctional PMPS as a macroinitiator to synthesize PLLA-b-PMPS-b-PLLA triblock copolymers. The structures of the triblock copolymers as well as the PMPS macroinitiator were characterized by NMR and GPC analysis. PMPS is an amorphous polymer with Tg at lower than −24 °C. The triblock copolymers also exhibited low Tg for the PMPS segment at lower than −20 °C as well as Tm at around 150 °C for the PLLA segment. The tensile tests of the triblock copolymers demonstrated their low modulus and high elongation. The biodegradation tests of these polymers were also performed using enzymes and seawater to reveal their biodegradability.

Polyester urethane functionalizable through maleimide side-chains and cross-linkable by polylactide stereocomplexes

Sustainable multifunctional alternatives to fossil-derived materials, which can be functionalized and are degradable, can be envisioned by combining naturally derived starting materials with an established polymer design concept. Modularity and chemical flexibility of polyester urethanes (PEU) enable the combination of segments bearing functionalizable moieties and the tailoring of the mechanical and thermal properties. In this work, a PEU multiblock structure was synthesized from naturally derived L-lysine diisocyanate ethyl ester (LDI), poly(L-lactide) diol (PLLA) and N-(2,3-dihydroxypropyl)-maleimide (MID) in a one-step reaction. A maleimide side-chain (MID) provided a reactive site for the catalyst-free coupling of thiols shown for L-cysteine with a yield of 94%. Physical cross-links were generated by blending the PEU with poly(D-lactide) (PDLA), upon which the PLLA segments of the PEU and the PDLA formed stereocomplexes. Stereocomplexation occurred spontaneously during solution casting and was investigated with WAXS and DSC. Stereocomplex crystallites were observed in the blends, while isotactic PLA crystallization was not observed. The presented material platform with tailorable mechanical properties by blending is of specific interest for engineering biointerfaces of implants or carrier systems for bioactive molecules.

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.

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

AbstractThe confined crystallization of poly(ϵ‐caprolactone) (PCL) block in poly(ϵ‐caprolactone)–poly(l‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry

Crystallization behavior of plasticized poly(lactide) film by poly(l-lactic acid)-poly(ethylene glycol)-poly(l-lactic acid) triblock copolymer

Poly(l-lactic acid)-poly(ethylene glycol)-poly(l-lactic acid) triblock copolymer (PLLA-PEG-PLLA) prepared by simple esterification of l-lactic acid (l-LA) and diol-polyethylene glycol (diol-PEG) is proposed as an effective plasticizer for poly(lactide) film (PLA). Differential scanning calorimetry and X-ray diffraction analyses reveal improvement in PLA/PLLA-PEG-PLLA blend miscibility without PEG segment recrystallization in the PLA matrix. A comparative study on the performance of PLLA-PEG-PLLA with two different PLLA chain lengths clarifies that long PLLA-PEG-PLLA with balanced block lengths of PLLA and PEG segments performs a semicrystalline structure of PLA crystal without PEG-crystal formation, resulting in blend miscibility. This microstructure clearly induces the perfect α-form of the PLA crystal with increased degree of crystallinity to 50%. Additionally, the sufficiently long PLLA segment in triblock copolymer can get entangled with PLA chains, allowing to enhance melt strength of the PLA/long PLLA-PEG-PLLA blend. In contrast, short PLLA-PEG-PLLA is amorphous, obstructing the formation of the highly ordered PLA packing structure in the blend, and contributing to poor melt strength and mechanical weakness. Furthermore, the PLA/long PLLA-PEG-PLLA blend improves toughness with increased elongation at break to 80% while maintaining the relatively high tensile strength, as compared to that of PLA/PEG or PLA/short PLLA-PEG-PLLA films at the same blending weight ratio.

Enhancing the recrystallization ability of bio-based polylactide stereocomplex by in situ construction of multi-block branched conformation

To enhance the formation and regeneration ability and the toughness of stereocomplex polylactide (sc-PLA), a high molecular weight stereocomplex branched multi-block polylactide (sb-PCLDA) was synthesized from d-lactide, l-lactide and poly(e-caprolactone) diol (PCL). High molecular weight sc-PLA has attracted much attention in bioengineering because of its good mechanical and thermal properties, but its brittleness and the poor regeneration ability of the sc crystallites hindered the practical processability, that is, the mixed homochiral and sc crystallites would form in the sc-PLA products after the complete melting and recrystallization processing. The high molecular weight sc-PLA which is easy to form complete stereocomplex and suitable for melt processing needs further research. Herein, sb-PCLDA having PCL, PLLA and PDLA segments on one molecular chain has been synthesized by polymerization of l-lactied and d-lactide with PCL and in situ hydrolysis condensation with silane coupling agents. Differential scanning calorimetry and wide angle X-ray diffraction indicated that sb-PCLDA formed complete sc with high crystallinity and exhibited good sc reformulation ability after multiple melting, suggesting much improved processability. Meanwhile, the sb-PCLDA maintained good thermal properties and hydrolysis resistance. The polymer exhibited significant tensile toughness with an elongation at break of 185%. This facile method could simultaneously improve the mechanical property and recrystallization ability of sc-PLA.

Synthesis and properties of partially biodegradable fluorinated polyacrylate: Poly(l‑lactide)‑co‑poly(hexafluorobutyl acrylate) copolymer

In order to alleviate the environmental pollution from fluoropolymer, the biodegradable poly(l‑lactide) (PLLA) segments were introduced into fluorinated polyacrylate. The PLLA macromonomers with vinyl terminal group and different molecular weight, i.e. 2500, 7400 and 14,700, were synthesized by ring opening polymerization of l‑lactide (LLA) first, then copolymerized with hexafluorobutyl acrylate (HFBA) through free radical copolymerization. By changing the macromonomers and feeding ratio of the macromonomer and HFBA, the poly(l‑lactide)‑co‑poly(hexafluorobutyl acrylate) (PLLA-co-PHFBA) copolymers with different structures were obtained. Structures of the PLLA macromonomers and the copolymers were confirmed by FTIR, 1H NMR and GPC. The crystallization, thermal decomposition resistance, surface hydrophobicity and hydrolytic degradation were also investigated. The results show that the PLLA segments in the copolymers still have the crystallization capacity and the highest crystallinity reaches 36%. PLLA-co-PHFBA-4 with the longest PLLA segments has the highest initial decomposing temperature up to 355 °C. The hydrophobicity of the copolymers is mainly related to the content of PHFBA segments. The length of PLLA segments also have some effect on the hydrophobicity. An obvious mass loss of the copolymers was observed in 35 days under the hydrolytic degradation and it was related to PHFBA content and PLLA segment's length, exhibiting partially biodegradable behavior.

Precise Synthesis of PS-PLA Janus Star-Like Copolymer.

In this work, an efficient strategy is designed for the precise synthesis of novel Janus star-like copolymer (polystyrene)8 -b-(poly(l-lactide))8 , (PLLA)8 -b-(PS)8 , consisting of two types of chemically distinct polymer arms, PS and PLLA, in an asymmetric structure. During the synthesis, PLLA hemisphere carrying protected hydroxyl groups at the focal point was first synthesized via a combined reactions of esterification, light-induced "Click" chemistry, and ring opening polymerization (ROP) using a specially designed dendron as initiator. After removing the protecting moiety, the terminal hydroxyl groups on the dendron segment is increased fourfold and further modified into bromopropionate-based macroinitiator through a two-step end group transformation reaction, followed by atom transfer radical polymerization (ATRP) of styrene to obtain the desired (PLLA)8 -b-(PS)8 Janus star-like copolymer. The versatility and efficiency of the designed synthetic strategy are demonstrated by the well-defined molecular characteristics and high yields of the targeted product. In addition to the controlled degradation behavior of the PLLA segments, the remaining bromide groups located at the distal end of PS arms could allow for further fabrication of diverse building blocks through consecutive ATRP of various monomers. This work signifies the first time for facile and precise synthesis of Janus star-like copolymer with unique biphasic structure and function control.

Stereocomplexed and Homochiral Polyurethane Elastomers with Tunable Crystallizability and Multishape Memory Effects.

Design of the polymer networks with tunable mechanical properties and multishape memory effects (multi-SMEs) is highly desired in the engineering applications. Herein, we report on the stereocomplexed and homochiral polyurethane (PU) elastomers with tunable multi-SMEs by cross-linking the triblock prepolymers bearing the poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) enantiomeric segments. The homochiral PU is nearly amorphous, yet the stereocomplexed PU becomes highly crystalline due to the stereocomplexation of enantiomeric segments. Moreover, the two distinct thermal (glass, melting) transitions of PLLA (or PDLA) segments in PUs are integrated to realize the thermally induced triple- and quadruple-SMEs. Control over the enantiomeric segmental ratios allows the feasible manipulation of crystallizability, mechanical and thermal properties, and multi-SMEs of PUs.

Crystallization and Stereocomplexation of PLA-mb-PBS Multi-Block Copolymers.

The crystallization and morphology of PLA-mb-PBS copolymers and their corresponding stereocomplexes were studied. The effect of flexible blocks (i.e., polybutylene succinate, PBS) on the crystallization of the copolymers and stereocomplex formation were investigated using polarized light optical microscopy (PLOM), differential scanning calorimetry (DSC), infrared spectroscopy (FTIR), and carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR). The PLA and PBS multiple blocks were miscible in the melt and in the glassy state. When the PLA-mb-PBS copolymers are cooled from the melt, the PLA component crystallizes first creating superstructures, such as spherulites or axialites, which constitute a template within which the PBS component has to crystallize when the sample is further cooled down. The Avrami theory was able to fit the overall crystallization kinetics of both semi-crystalline components, and the n values for both blocks in all the samples had a correspondence with the superstructural morphology observed by PLOM. Solution mixtures of PLLA-mb-PBS and PLDA-mb-PBS copolymers were prepared, as well as copolymer/homopolymer blends with the aim to study the stereocomplexation of PLLA and PDLA chain segments. A lower amount of stereocomplex formation was observed in copolymer mixtures as compared to neat L100/D100 stereocomplexes. The results show that PBS chain segments perturb the formation of stereocomplexes and this perturbation increases with the amount of PBS in the samples. However, when relatively low amounts of PBS in the copolymer blends are present, the rate of stereocomplex formation is enhanced. This effect dissappears when higher amounts of PBS are present. The stereocomplexation was confirmed by FTIR and solid state 13C-NMR analyses.

Poly(l-lactic acid)-polyethylene glycol-poly(l-lactic acid) triblock copolymer: A novel macromolecular plasticizer to enhance the crystallization of poly(l-lactic acid)

Different from traditional small molecular plasticizers, poly(l-lactic acid) (PLLA)-polyethylene glycol (PEG)-PLLA triblock copolymers were synthesized by ring-opening polymerization of L-lactide on the end hydroxyls of PEG and blended with PLLA, to enhance the crystallization of PLLA. Non-isothermal and isothermal crystallization studies show that the triblock copolymer enhances the crystallization of PLLA due to the nucleation effect from PLLA segments and the plasticization effect from PEG segments, which strongly depends on the structure of copolymers. With shorter PLLA segment and longer PEG segment of the triblock copolymer, the glass transition temperature and cold crystallization temperature of the blends decrease, and the degree of crystallinity and spherulite growth rate increase owing to the higher content of PEG. However, triblock copolymers with relatively longer PLLA segment and shorter PEG segment show unique nucleation effects for PLLA, leading to the elevation of crystallization temperature of PLLA. The results confirm that the crystallization acceleration of PLLA can be ascribed to the nucleation and plasticization effect, and the plasticization effect is more significant. What’s more, the introduction of triblock copolymers improves the compatibility and durability due to the good interfacial interaction with PLLA matrix even at a high loading.