Morphological Structure Regulation And Crystallization Behavior Of The Polylactide Enantiomeric Blends

Polylactide(PLA)has been widely studied owing to its good biocompatibility,biodegradability and mechanical strength,as well as relatively high modulus of elasticity and thermoformability.The PLA-based products have been successfully applied in various fields,such us orthopaedics,drug carriers,surgical sutures,and packaging materials.However,there are many deficiencies of PLA(e.g.,low crystallization ability,toughness and thermal resistance),which greatly limit its large-scale production and industrial application.In order to broaden the development of PLA,we have explored the relationship of crystalline form,structural transformation,morphology and properties,and have made the targeted improvements,which will improve their comprehensive performance and promote the practical application.In this paper,the formation of stereocomplex crystallite network in the asymmetric PLLA/PDLA blends and its effect on matrix crystallization were investigated.The asymmetric PLLA/PDLA blends were prepared by electrospinning,and the orientation memory effect of electrospun fibers was studied by annealing.The influences of molecular weight and annealing temperature on the formation of polylactide stereocomplex mesophase in PLLA/PDLA blends were also studied.The conclusions are as follows:(1)A honeycomb stereocomplex crystallite network was clearly observed by the scanning electron microscopy in the blends with high-molecular-weight PDLA as a minor component via selective dissolution.In asymmetric PLLA/PDLA blends,the polylactide stereocomplex crystals were bridged with low molecular weight and high molecular weight PDLAs respectively to form networks.For the PDLA with different molecular weight,the structure of network was completely different.The high molecular weight PDLA could span and connected stereocomplex crystals to produce honeycomb networks that survive during selective dissolution.The network generated by the low molecular weight PDLA was disintegrated.Moreover,the stereocomplex crystallite network had obvious enhancement of nucleation effect,and the honeycomb network accelerated the crystallization of PLLA matrix remarkably.The crystallization temperature of PLLA matrix in PLLA/PDLA blends increased by about 40 °C with only 5 wt% PDLA content.(2)The asymmetric polylactide enantiomer blend fiber films were prepared by electrospinning.The PLLA/PDLA blend samples with the same proportion were prepared by solution precipitation method.After annealing at 190 °C,electrospinning fiber films had the memory effect due to the orientation of molecular chains produced by the strong electric field traction.And the homogeneous thermal crystallization temperature of PLLA matrix was significantly increased.At the same time,the growth of crystals was accelerated remarkably.When the content of dextral polylactic acid reached 1 wt%,the orientation memory effect of molecular chains in electrospun fiber membranes reached a saturated state.(3)The formation of polylactide stereocomplex mesophase in three PLLA/PDLA blends with different molecular weight was studied.The formation of mesophase in PLLA/PDLA blends showed a significant dependence on molecular weight.Similar to the polylactide stereocomplex crystals,the polylactide stereocomplex mesophase was just higher than Tg was related to the intermolecular interaction between PLLA and PDLA chains.The decrease of molecular weight was beneficial to the formation of the polylactide stereocomplex mesophase and inhibits the homogeneous crystallization.At 62 °C,owing to the remarkable molecular permeation between PLLA and PDLA chains,a unique constitutive composite mesophase was formed in the blends with low molecular weight.What's more,the formation of the stereocomplex mesophase in PLLA/PDLA blends was controlled by the thermodynamics.Because of the low thermal stability,the formation of the polylactide stereocomplex mesophase could only be formed at low temperature because of the driving force.The melting of the polylactide stereocomplex mesophase will cause the lag phenomenon of subsequent cold crystallization,especially in the low molecular weight PLLA/PDLA blends,the effect of delaying cold crystallization was more remarkble.