Jianna 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.

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.

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.

Crystallization behavior and crystalline structural changes of poly(glycolic acid) investigated via temperature-variable WAXD and FTIR analysis

Poly(glycolic acid) (PGA) is a unique biodegradable, aliphatic polyester having a higher melting point (Tm) and better mechanical properties and chemical resistance than its analogs. The unusual physical properties of PGA can be attributed to its crystalline structure, chain packing and interactions. Herein, we systematically investigated the crystalline structure, spherulitic morphology, and structural evolution of PGA in the crystallization and melting processes by time-resolved wide-angle X-ray diffraction (WAXD) and FTIR spectrometry. PGA exhibits fast crystallization and it completely crystallizes in the fast cooling process. The Tm, degree of crystallinity, and long period of PGA increase as the crystallization temperature increases. PGA forms unique hedrites but not normal spherulites during crystallization. Because of the thermal expansion of the crystal lattice, the a and b axes of PGA crystal cells increase linearly with temperature in the heating process. The band splitting in the FTIR spectrum of PGA is observed in the ν(CO) and δ(CH2) regions and the spectral splitting intensifies with cooling, indicating the existence of intermolecular dipolar interactions in PGA crystals. It is speculated that the C–H⋯OC H-bonds are formed in the crystalline phase of PGA, as indicated by the significant red shifts of the ν(CO) band during crystallization.

Polymorphic Crystallization and Crystalline Reorganization of Poly(l-lactic acid)/Poly(d-lactic acid) Racemic Mixture Influenced by Blending with Poly(vinylidene fluoride)

The effects of poly(vinylidene fluoride) (PVDF) on the crystallization kinetics, competing formations of homocrystallites (HCs) and stereocomplexes (SCs), polymorphic crystalline structure, and HC-to-SC crystalline reorganization of the poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic mixture were investigated. Even though the PLLA/PDLA/PVDF blends are immiscible, blending with PVDF enhances the crystallization rate and SC formation of PLLA/PDLA components at different temperatures that are higher or lower than the melting temperature of the PVDF component; it also facilitates the HC-to-SC melt reorganization upon heating. The crystallization rate and degree of SC crystallinity (Xc,SC) of PLLA/PDLA components in nonisothermal crystallization increase after immiscible blending with PVDF. At different isothermal crystallization temperatures, the crystallization half-time of PLLA/PDLA components decreases; its spherulitic growth rate and Xc,SC increase as the mass fraction of PVDF increases from 0 to 0.5 in the presence of either a solidified or a molten PVDF phase. The HCs formed in primary crystallization of PLLA/PDLA components melt and recrystallize into SCs upon heating; the HC-to-SC melt reorganization is promoted after blending with PVDF. We proposed that the PVDF-promoted crystallization, SC formation, and HC-to-SC melt reorganization of PLLA/PDLA components in PLLA/PDLA/PVDF blends stem from the enhanced diffusion ability of PLLA and PDLA chains.

Competing Stereocomplexation and Homocrystallization of Poly(l-lactic acid)/Poly(d-lactic acid) Racemic Mixture: Effects of Miscible Blending with Other Polymers

Promoting the stereocomplexation ability of high-molecular-weight poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) is an efficient way to improve the thermal resistance of the resulting materials. Herein, we studied the competing crystallization kinetics, polymorphic crystalline structure, and lamellae structure of the PLLA/PDLA component in its miscible blends with poly(vinyl acetate) (PVAc) and proposed a method to improve the stereocomplexation ability of PLLA and PDLA through miscible blending with the other polymer. Crystallization of the PLLA/PDLA component is suppressed after the addition of PVAc, due to the dilution effect. The stereocomplexation ability of PLLA and PDLA is enhanced by blending with PVAc; this becomes more obvious at a high PVAc content (≥50 wt %) but less significant with the further increase of PLLA, PDLA molecular weights. Almost exclusive formation of SCs is achieved for PLLA and PDLA after blending with a large proportion of PVAc (e.g., 75 wt %). Incorporation of PVAc also facilitates the HC-to-SC structural reorganization upon heating. The increased chain mobility, decreased equilibrium melting point, and enhanced intermolecular interactions may account for the preferential stereocomplexation in PLLA/PDLA/PVAc blends.