Yongzhong Bao

Competitive Stereocomplexation, Homocrystallization, and Polymorphic Crystalline Transition in Poly(l-lactic acid)/Poly(d-lactic acid) Racemic Blends: Molecular Weight Effects

Competitive crystallization kinetics, polymorphic crystalline structure, and transition of poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends with a wide range of molecular weights (MWs) were symmetrically investigated. Stereocomplex (sc) crystallites are exclusively formed in the low-MW racemic blends. However, stereocomplexation is remarkably depressed, and homocrystallization becomes prevailing with increasing MWs of PLLA and PDLA. Suppressed stereocomplexation in high-MW (HMW) racemic blends is proposed to be due to the low chain diffusion ability and restricted intermolecular crystal nucleation/growth. Equilibrium melting point of sc crystallites first increases and then decreases as MW increases. Crystallinity and relative fraction of sc crystallites in racemic blends enhance with crystallization temperature (Tc), and the sc crystallites are merely formed at Tc > 170 °C because of their higher thermodynamic stability. In situ wide-angle X-ray diffraction (WAXD) analysis reveals that the stereocomplexation and homocrystallization are successive rather than completely simultaneous, and the stereocomplexation is preceding homocrystallization in isothermal crystallization of HMW racemic blends. Both initial crystalline structure of homocrystallites (hc) and MW influence the heating-induced hc-to-sc transition of HMW racemic blend drastically; the hc-to-sc transition becomes easier with decreasing Tc and MW. After crystallization at the same temperature, sc crystallites show smaller long period than their hc counterparts.

Core-shell structure, biodegradation, and drug release behavior of poly(lactic acid)/poly(ethylene glycol) block copolymer micelles tuned by macromolecular stereostructure.

Poly(ethylene glycol)-b-poly(L-lactic acid)-b-poly(D-lactic acid) (PEG-b-PLLA-b-PDLA) stereoblock copolymers were synthesized by sequential ring-opening polymerization. Their micelle formation, precise micelle structure, biodegradation, and drug release behavior were systematically investigated and compared with the PEG-b-poly(lactic acid) (PEG-b-PLA) diblock copolymers with various PLA stereostructures and PEG-b-PLLA/PEG-b-PDLA enantiomeric mixture. Stereoblock copolymers having comparable PLLA and PDLA block lengths and enantiomerically-mixed copolymers assemble into the stereocomplexed core-shell micelles, while the isotactic and atactic PEG-b-PLA copolymers formed the homocrystalline and amorphous micelles, respectively. The PLA segments in stereoblock copolymer micelles show smaller crystallinity than those in the isotactic and enantiomerically-mixed ones, attributed to the short block length and presence of covalent junction between PLLA and PDLA blocks. As indicated by the synchrotron radiation small-angle X-ray scattering results, the stereoblock copolymer micelles have larger size, micellar aggregation number, core radius, smaller core density, and looser packing of core-forming segments than the isotactic and enantiomerically-mixed copolymer micelles. These unique structural characteristics cause the stereoblock copolymer micelles to possess higher drug loading content, slower degradation, and drug release rates.

Exclusive Stereocomplex Crystallization of Linear and Multiarm Star-Shaped High-Molecular-Weight Stereo Diblock Poly(lactic acid)s.

Linear, 3-arm, and 6-arm star-shaped stereo diblock copolymers of l- and d-lactic acid (PLLA-b-PDLA) with high molecular weights (MWs) were synthesized via two-step ring-opening polymerization (ROP) with 1-dodechanol, glycerol, and d-sorbitol as the initiators, respectively. The chemical structure, nonisothermal and isothermal crystallization kinetics, crystalline structure, lamellar morphology, and mechanical thermal properties of PLLA-b-PDLAs with different macromolecular topologies were investigated. Compared to the high-molecular-weight (MW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends, PLLA-b-PDLAs exhibit faster crystallization upon cooling and isothermal melt crystallization; they crystallize exclusively in stereocomplex (sc) crystallites under all of the conditions investigated. This is attributable to the enhanced interactions between enantiomeric blocks linked covalently. Macromolecular topology influences the crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly. The crystallization temperature upon cooling, melting temperature, degree of crystallinity, spherulitic growth rate, crystallite size, long period, and crystalline layer thickness of PLLA-b-PDLA decrease with increasing branching number because of the retarding effect of branching on the crystallization rate and crystallizability. Because of the formation of high-melting-point sc crystallites, both the linear and star-shaped PLLA-b-PDLAs exhibit better thermal resistance and higher storage moduli at high temperature than does homocrystalline PLLA.

In situ formation and gelation mechanism of thermoresponsive stereocomplexed hydrogels upon mixing diblock and triblock poly(lactic acid)/poly(ethylene glycol) copolymers.

A novel in situ formed gel system with potential biodegradability and biocompatibility is developed by mixing the diblock and triblock poly(lactic acid)/poly(ethylene glycol) (PLA/PEG) copolymers with opposite configurations of PLA blocks. In situ gelation of such system is extremely fast, which happens within 10 s after mixing. In situ gelation, gel-to-sol transition, crystalline structure, microstructures, and mechanical properties of PLA-PEG/PLA-PEG-PLA enantiomerically mixed gels are significantly influenced by the mixing ratio, degree of polymerization for PEG block in triblock (DPPEG,tri) and diblock copolymers (DPPEG,di). It is found that in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixture just happen at relatively smaller PLA-PEG/PLA-PEG-PLA mass ratio and larger DPPEG,tri. Hydrodynamic diameters of PLA-PEG and PLA-PEG-PLA copolymers in dilute solution increase remarkably upon mixing, indicating the formation of bridging networks. Stereocomplexed crystallites are formed for the PLA hydrophobic domains in PLA-PEG/PLA-PEG-PLA enantiomeric mixtures. As indicated by synchrotron-radiation SAXS analysis, the enantiomeric mixture changes from a compactly to loosely aggregated structure and the intermicellar distance enhances with increasing DPPEG,tri, DPPEG,di, or PLA-PEG-PLA fraction. Gelation mechanism of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is proposed, in which part of PLA-PEG-PLA chains act as the connecting bridges between star and flower-like micelles and the stereocomplexed crystallites in micelle cores act as physically cross-linked points.

Alternating poly(lactic acid)/poly(ethylene-co-butylene) supramolecular multiblock copolymers with tunable shape memory and self-healing properties

Alternating supramolecular multiblock copolymers with hard poly(lactic acid) (PLA) and soft poly(ethylene-co-butylene) (PEB) segments were prepared by terminal functionalization of PLA–PEB–PLA triblock oligomers with the 2-ureido-4[1H]-pyrimidinone (UPy) self-complementary quadruple hydrogen bonding units. Such supramolecular copolymers (SMPs) exhibit the characteristic properties of thermoplastic elastomers. The thermal, morphological, mechanical, shape memory, and self-healing properties of SMPs can be readily modulated by varying the composition, stereostructure, and crystallizability of PLA blocks. The prepared SMPs are shown as transparent and elastic films, while their PLA–PEB–PLA precursors are viscous or brittle solids. Crystallization of isotactic PLA blocks, i.e. poly(L-lactic acid) (PLLA), in SMPs is significantly impeded by the end-caped UPy motifs. The prepared SMPs show a well-defined microphase-separated structure, which varies from cylindrical to lamellar morphology with the increasing fraction of PLA blocks. Compared to the PLA–PEB–PLA precursors, SMPs exhibit improved mechanical strengths, modulus, elongation-at-break, good thermally-induced shape memory and light-triggered self-healing properties. The recovery ratios of SMPs containing atactic poly(D,L-lactic acid) (PDLLA) blocks are nearly 100%. The shape memory and self-healing properties of SMPs can be modulated by the stereostructure of PLA segments and they become worse when the isotactic, crystallizable PLLA segments are presented.

Preferential Stereocomplex Crystallization in Enantiomeric Blends of Cellulose Acetate-g-Poly(lactic acid)s with Comblike Topology

Although stereocomplex (sc) crystallization is highly effective for improving the thermal resistance of poly(lactic acid) (PLA), it is much less predominant than homocrystallization in high-molecular-weight (HMW) poly(l-lactic acid)/ poly(d-lactic acid) (PLLA/PDLA) racemic blends. In this contribution, the sc crystallization of HMW PLLA/PDLA racemic blends was facilitated by using comblike PLAs with cellulose acetate as the backbone. Competing crystallization kinetics, polymorphic crystalline structure, and structural transition of comblike PLLA/PDLA blends with a wide range of MWs were investigated and compared with the corresponding linear/comblike and linear blends. The HMW comblike blend is preferentially crystallized in sc polymorphs and exhibits a faster crystallization rate than does the corresponding linear blend. The sc crystallites are predominantly formed in nonisothermal cold crystallization and isothermal crystallization at temperatures above 120 °C for the comblike blends. Except for the facilitated sc formation in primary crystallization, synchrotron radiation WAXD analysis indicates that the presence of a comblike component also facilitates the transition or recrystallization from homocrystallite (hc) to sc crystallite upon heating. Preferential sc formation of comblike blends is probably attributable to the favorable interdigitation between enantiomeric branches and the increased mobility of polymer segments. After crystallization under the same temperature, the comblike blends, which mainly contain sc crystallites, show smaller long periods and thinner crystalline lamellae than do the corresponding PLLA with homocrystalline structures.

Polylactide-b-poly(ethylene-co-butylene)-b-polylactide thermoplastic elastomers: role of polylactide crystallization and stereocomplexation on microphase separation, mechanical and shape memory properties

Polylactide-b-poly(ethylene-co-butylene)-b-polylactide (PLA–PEB–PLA) triblock copolymers containing PLA segments with different stereo-regularities such as poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(D,L-lactide) (PDLLA) were prepared via the ring-opening polymerization of various lactides using α,ω-dihydroxy PEB as the macromolecular initiator. Molecular weight and chemical composition of copolymers were adjusted by changing the monomer-to-initiator ratio. Morphological, thermal, mechanical, and shape memory behaviors of PLA–PEB–PLA were explored. As confirmed by small angle X-ray scattering (SAXS) and transmission electrical microscopy (TEM), PLA–PEB–PLA adopted ordered microphase-separated morphology, depending on the copolymer composition and crystallizability of PLA segments. Spherical, hexagonally packed cylindrical, and lamellar structures were observed in PLA–PEB–PLA upon increasing the volume fraction of PLA. However, the morphological order was diminished in PLLA–PEB–PLLA/PDLA–PEB–PDLA enantiomeric blends, due to the preferential stereocomplexation of PLLA and PDLA segments before microphase separation. PLA–PEB–PLA showed the properties of thermoplastic elastomers. Their Youngs modulus and tensile strength increased while the strain at break decreased upon increasing the fraction of PLA hard segments or with the crystallization or stereocomplexation of PLA domains. Interestingly, PLA–PEB–PLA elastomers showed shape memory behavior, which could be controlled by the crystallizability of PLA hard segments.

Synthesis of end-functionalized hydrogen-bonding poly(lactic acid)s and preferential stereocomplex crystallization of their enantiomeric blends

The solvent-free ring-opening polymerization (ROP) of lactide using the self-complementary quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidinone (UPy)-functionalized alcohol as the initiator was achieved to attain UPy mono-functionalized poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) capable of undergoing supramolecular self-assembly. This ROP polymerization exhibits good controllability and the synthesized polymers have controlled molecular weights and well-defined terminal structures. The specific viscosities of UPy-functionalized PLLA and PDLA in dilute solution show strong concentration dependence, demonstrating the formation of a supramolecular structure by UPy dimerization. The crystallization kinetics, polymorphic crystalline structure, and crystalline structural organization of UPy-functionalized PLLA/PDLA blends were investigated and compared to the corresponding non-functionalized blends. The UPy end functionalization not only accelerates the crystallization but also facilitates the formation of high-melting-point stereocomplexes (SCs) in the PLLA/PDLA blends. The stereocomplexation ability of UPy-functionalized PLLA/PDLA blends further enhances on decreasing the molecular weights of PLLA and PDLA, and increasing the content of UPy end functionality. The incorporation of UPy end functionality also promotes the melt recrystallization of homocrystallites (HCs) to SCs upon heating. It is proposed that the promoted SC formation of the UPy-functionalized PLLA/PDLA blend originates from the enhanced interactions between enantiomeric chains.

Click chemistry synthesis, stereocomplex formation, and enhanced thermal properties of well-defined poly(L-lactic acid)-b-poly(D-lactic acid) stereo diblock copolymers

Stereoblock copolymerization of lactide enantiomers has been a feasible method to prepare stereocomplexed poly(lactic acid) (PLA) with highly improved thermal resistance. However, synthesis of high-molecular-weight (HMW) poly(L-lactic acid)-b-poly(D-lactic acid) (PLLA-b-PDLA) stereoblock copolymers with controlled stereoblock length and composition is still challenging. Herein we synthesized well-defined PLLA-b-PDLA stereo diblock copolymers with different molecular weights (MWs, 14–110 kDa) and PLLA and PDLA block lengths by a combination of ring-opening polymerization and azide/alkyne click chemistry. The crystallization kinetics, polymorphic crystalline structure, lamellar morphology, and thermomechanical properties of the PLLA-b-PDLAs were systematically investigated. All the PLLA-b-PDLAs exhibit fast crystallization and predominantly form stereocomplexes (SCs) during the cooling and heating processes. Symmetric PLLA-b-PDLAs with similar PLLA and PDLA block lengths exclusively crystallize in the SCs at all the investigated crystallization temperatures (Tcs) in melt crystallization; but asymmetric PLLA-b-PDLAs with very different PLLA and PDLA block lengths crystallize in both SCs and homocrystallites (HCs) at a low Tc (<160 °C). Because of the formation of high-melting-point SCs, HMW PLLA-b-PDLAs exhibit better thermal resistance and higher storage moduli at a high temperature range (170–200 °C) than the homocrystalline PLLA.

Thermosensitive Poly(N-isopropylacrylamide-co-acrylonitrile) Hydrogels with Rapid Response

Abstract Acrylonitrile (AN) was copolymerized with N -isopropylacrylamide (NIPA) to synthesize thermosensitive hydrogels, and the on-off switch behavior of poly(NIPA-co-AN) hydrogels with different fraction of hydrophobic component (AN) was investigated. It is found that the lower critical solution temperature (LCST), the swelling ratio at certain temperature and the reswelling rate of poly(NIPA-co-AN) hydrogels decreased as AN unit fraction in copolymers increased. In order to improve the responsive rate of poly(NIPA-co-AN) hydrogels, they were further treated by surface crosslinking using N , N′-methylene bisacrylamide (BIS) as a crosslinking agent. The swelling and deswelling behaviors of these copolymers were compared with those of the untreated hydrogels. The results indicated that the responsive rate of poly(NIPA-co-AN) hydrogel was improved by surface crosslinking. The resulting hydrogels bearing cyano groups with fast response have potential applications in the field of drug-controlled release and immobilization of biomolecules.