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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 (X c,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 X c,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. [ABSTRACT FROM AUTHOR]

Linear, 3-arm, and 6-arm star-shapedstereo diblock copolymersof l- and d-lactic acid (PLLA-b-PDLA) with high molecular weights (MWs) were synthesized via two-stepring-opening polymerization (ROP) with 1-dodechanol, glycerol, and d-sorbitol as the initiators, respectively. The chemical structure,nonisothermal and isothermal crystallization kinetics, crystallinestructure, lamellar morphology, and mechanical thermal propertiesof PLLA-b-PDLAs with different macromolecular topologieswere investigated. Compared to the high-molecular-weight (MW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemicblends, PLLA-b-PDLAs exhibit faster crystallizationupon cooling and isothermal melt crystallization; they crystallizeexclusively in stereocomplex (sc) crystallites under all of the conditionsinvestigated. This is attributable to the enhanced interactions betweenenantiomeric blocks linked covalently. Macromolecular topology influencesthe crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly. The crystallization temperature uponcooling, melting temperature, degree of crystallinity, spheruliticgrowth rate, crystallite size, long period, and crystalline layerthickness of PLLA-b-PDLA decrease with increasingbranching number because of the retarding effect of branching on thecrystallization rate and crystallizability. Because of the formationof high-melting-point sc crystallites, both the linear and star-shapedPLLA-b-PDLAs exhibit better thermal resistance andhigher storage moduli at high temperature than does homocrystallinePLLA. [ABSTRACT FROM AUTHOR]

Competitive crystallization kinetics,polymorphic crystalline structure,and transition of poly(l-lactic acid)/poly(d-lacticacid) (PLLA/PDLA) racemic blends with a wide range of molecular weights(MWs) were symmetrically investigated. Stereocomplex (sc) crystallitesare exclusively formed in the low-MW racemic blends. However, stereocomplexationis remarkably depressed, and homocrystallization becomes prevailingwith increasing MWs of PLLA and PDLA. Suppressed stereocomplexationin high-MW (HMW) racemic blends is proposed to be due to the low chaindiffusion ability and restricted intermolecular crystal nucleation/growth.Equilibrium melting point of sc crystallites first increases and thendecreases as MW increases. Crystallinity and relative fraction ofsc crystallites in racemic blends enhance with crystallization temperature(Tc), and the sc crystallites are merelyformed at Tc> 170 °C becauseoftheir higher thermodynamic stability. In situwide-angleX-ray diffraction (WAXD) analysis reveals that the stereocomplexationand homocrystallization are successive rather than completely simultaneous,and the stereocomplexation is preceding homocrystallization in isothermalcrystallization of HMW racemic blends. Both initial crystalline structureof homocrystallites (hc) and MW influence the heating-induced hc-to-sctransition of HMW racemic blend drastically; the hc-to-sc transitionbecomes easier with decreasing TcandMW. After crystallization at the same temperature, sc crystallitesshow smaller long period than their hc counterparts. [ABSTRACT FROM AUTHOR]