Guorong Shan

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.

Preparation of Mesoporous Submicrometer Silica Capsules via an Interfacial Sol–Gel Process in Inverse Miniemulsion

Mesoporous silica capsules with submicrometer sizes were successfully prepared via the interfacial hydrolysis and condensation reactions of tetraethoxysilane (TEOS) in inverse miniemulsion by using hydrophilic liquid droplets as template. The inverse miniemulsions containing pH-controlled hydrophilic droplets were first prepared via sonication by using poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/B)-PEO) or SPAN 80 as surfactant. TEOS was directly introduced to the continuous phase of an inverse miniemulsion. The silica shell was formed by the deposition of silica on the surface of droplets. The formation of capsule morphology was confirmed by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The mesoporous structure was verified by nitrogen sorption measurements. The specific surface area could be tuned by the variation of the amount of cetyltrimethylammonium bromide (CTAB) and TEOS, and the pore size by the amount of CTAB. The influences of synthetic parameters on the particle size and morphology were investigated in terms of the amount of CTAB, pH value in the droplets, TEOS amount, surfactant amount, and type of solvent with low polarity. A formation mechanism of silica capsules was proposed.

Free radical copolymerization and kinetic treatment of styrene with N‐phenylmaleimide

The copolymerization of styrene (M1) with N-phenylmaleimide (M2) in chloroform with 2,2′-azobis(isobutyronitrile) as an initiator was investigated. The kinetic parameters, such as reactivity ratios, overall activity energy, and the effect of molar fraction of monomers on the initial copolymerization rate, were determined. The bimolecular termination of the copolymerization was proved. The treatment method proposed by Yoshimura and colleagues was used to estimate quantitatively the contribution of the charge-transfer complex (CTC) and the free monomers in the copolymerization process. The propagation reactivity ratios of CTC and free monomers were calculated by a new method.

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.

Temperature and pH-dependent swelling and copper(II) adsorption of poly(N-isopropylacrylamide) copolymer hydrogel

Poly(N-isopropylacrylamide-co-acrylamide-co-maleic acid) (P(NIPAM-AM-MA)) hydrogel has been synthesized by free radical polymerization. The incorporation of functional monomer in the hydrogel was confirmed by Fourier transform infrared spectrometer (FTIR). Swelling measurements and differential scanning calorimeter (DSC) were employed to investigate the volume phase transition of P(NIPAM-AM-MA) hydrogel. P(NIPAM-AM-MA) shows higher swelling ratio and LCST than poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-AM)). The adsorption behavior of copper(II) (Cu2+) ions on P(NIPAM-AM-MA) hydrogel is temperature and pH-dependent. The adsorption isotherm is well fitted by the Freundlich model and the adsorption kinetics can be described by the pseudo-second order equation. In 20 mL of CuSO4 solution containing 4 mg Cu2+, the adsorption capacity reaches 24.4 mg g−1 dry hydrogel at 30 °C and pH = 5. After the volume phase transition, the Cu2+-loaded P(NIPAM-AM-MA) hydrogel can release almost 90% of absorbed water containing few heavy metal ion. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to study the effect of temperature and Cu2+ ions on the microstructure of P(NIPAM-AM-MA) hydrogel. The occurrence of volume phase transition increases the size of cross-linked domains and mass fractal dimension, while the presence of Cu2+ ions has an opposite effect. The adsorbed hydrogel can be easily regenerated by hydrochloric acid and reused in the following adsorption process. This pH and temperature sensitive hydrogel may be used for water purification and enrichment of heavy metal ions.

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.