Study On Cold Crystallization Behavior Of Poly (Lactic Acid)

The cold crystallization behaviors of amorphous poly?lactic acid??PLA?have been investigated by synchrotron wide-angle and small-angle X-ray scattering?SR-WAXS and SR-SAXS?,differential scanning calorimetry?DSC?and Fourier transforminfraredspectroscopy?FTIR?,focusing onthequiescent cold-crystallization of amorphous PLA and poly?L-lactic acid?/poly?D-lactic acid??PLLA/PDLA,1/1?blends and strain-induced cold-crystallization of amorphous PLA.The crystalline structure and thermal properties of cold-crystallized PLA samples are influenced by crystallization conditions and sample preparation method.It shows that different crystal forms???and/or?form?can form when amorphous PLA samples cold-crystallize at different temperatures,which lead to the non-monotonic variations with crystallization temperatures of lamellar thickness and melting temperatures.During isothermal cold-crystallization at different temperatures,the degree of structural ordering of crystal keeps almost unchanged with crystallization time.More local ordered structures with higher order can form in amorphous PLA samples obtained by higher quenching temperatures,which is in favor of the formations of??form with higher order and lamellae with smaller thickness.Different ordered structures can be induced by strain when amorphous PLA samples are stretched at different temperatures.Upon stretching within 50-60°C?below T_g?,the disordered mesocrystal forms,of which the onset corresponds to the certain orientation degree of amorphous chains.Drawn within 70-90°C?above T_g?,the mesocrystal and crystal of PLA appears successively at 70°C,while only the crystal forms and leads to the formation of analogous‘shish-kebab'morphology at 80and 90°C.All of the crystals induced by strain are disordered??form but with a slightly more compact chain packing than that of the cold crystallization.When stretched at 100-150°C,the more ordered??and?form could be induced by strain.Specially,??form could be transformed partly into?form when PLA is stretched at100°C,which leads to the decrease in the thickness of lamellae.The cold-crystallization temperature(T_cc)has a significant influence on crystal composition,crystalline structure and thermal properties of PLLA/PDLA blends.When T_cc is below 100°C,only homopolymer crystal?hc?could be detected in PLLA/PDLA blends.The hc and stereocomplex crystal?sc?coexist in PLLA/PDLA blends cold-crystallized within 100-180°C.And only sc appears in PLLA/PDLA blends when T_ccc is above 180°C.When T_ccc is below 180 ^oC,the crystal form and structural ordering of hc in PLLA/PDLA are same with that of PLLA at the same T_cc.The lamellar thickness of PLLA/PDLA is slightly smaller than that of PLLA due to the existence of sc,while has the similar variation with T_ccc to that of PLLA.The lamellar thickness increases rapidly with T_ccc higher than 180 ^oC.The melting temperature of hc increases gradually while that of sc keeps almost constant with T_ccc blow 180 ^oC.When T_ccc is above 180 ^oC,with T_cc increasing,there firstly appears a double melting peak of sc,which then gradually transforms into the single melting peak at higher temperature.