Fabrications Of Networks In PLLA And Improvements Of Shape Memory Properties

Shape memory materials become the focus of smart materials with the capacity to be fixed as a desired shape and recover to permanent shape upon certain stimulus.Shape memory polymers(SMP)exhibit potential applications in many fields due to their diverse external stimuli,highly flexible programming,various structural designs and the tunable properties.Poly(L-lactic acid)(PLLA),a semi-crystalline polymer with environmental friendliness and biocompatibility,has the advantages of easymanufacture and good mechanical properties,making it an excellent candidate for shape memory polymers.PLLA shows poor shape memory performance including the absence of shape-fixed phase,lower modulus at high temperature and the lower recovery ratio,which does limit its wide applications.In this work,therefore,novel strategies have been developed to introduce networks including tiny crystal,inorganic filler and cross-linking points to improve the shape memory performance of PLLA.Furthermore,the enhanced modulus as well as recovery ratio were obtained based on the interplay between network and crystallization behaviors.(1)Fabrication of tiny crystal network and crystallization condition dependence of PLLA shape memory performances: The crystalline network of PLLA is prepared by means of cold crystallization and melt crystallization following quench from melts to room temperature.The crystallinity and crystal structures are under the control of crystallization time and temperature.During shape memory cycles,the tiny crystal and amorphous part of PLLA can act as the shape-fixed phase and shaperecovery phase respectively.The shape fixity of PLLA is nearly 100% since glass transition temperature of it plays the role of switch temperature.The recovery ratio and modulus at high temperature,however,depend crucially on the crystallinity of PLLA(x).In the case of x<6.5 %,both of them exhibit higher magnitude;when the crystallinity is higher than 6.5%,the recovery ratio suffers from the irreversible deformation of crystal structures although modules has been enhanced.(2)Improvement of PLLA shape memory properties based on double roles of polyvinyl acetate grafted graphene oxide: The polyvinyl acetate grafted graphene oxide(GO-g-PVAc)was fabricated by ?-ray irradiation-induced graft polymerization at a high graft level of 31.9 %.The grafted PVAc chains make the graphene nanoplates well dispersed in PLLA matrix based on the good miscibility between PLLA and PVAc.The physical entanglement between them produces better interfacial bonding.Furthermore,the modified GO plays double roles in shape memory performance.On one hand,storage modulus of the composite at high temperature has been improved upon the incorporation of GO accompanied by the excellent dispersion of it in the PLLA matrix,resulting in higher recovery stress;on the other hand,it can act as the new shape fixed phase to prevent the irreversible sliding of polymer chains during deformation,accounting for the enhanced shape recovery ratio(a max of 96.8 % at angle method).(3)Improvement of PLLA shape memory properties based on tiny crystal and cross-linking networks: TAIC(Triallyl isocyanurate),the cross-linking agent,shows excellent miscibility with PLLA.Upon irradiation by ?-ray,shape memory PLLA with both tiny crystal network and cross-linking network was fabricated successfully.It is facile to tailor the cross-linking degree by means of TAIC content and absorbed dose.In the SMP(30kGy),the cross-link network improves the modulus of PLLA(80 °C)from 1.03 MPa to 2.03 MPa without the loss of elongation(above 400 %).With the aid of both tiny crystal network and cross-link network,the shape recovery ratio exhibits not only high magnitude of 99.5 % but also the excellent cycle stability.The crosslinking network(with 1% TAIC)endows PLLA with the triple shape memory performance,in which glass transition temperature and melting temperature of PLLA act as the switch temperatures.(4)Improvement of PLLA shape memory properties based on the interplay between cross-linking network and PLLA crystallization behaviors: The interplay between cross-linking network and PLLA crystallization behaviors has been discussed in detail.Our results indicate that they produce significant influence each other.On one hand,the cross-linking network shows the depression effect on PLLA crystalline,leading to slower crystallization velocity,lower crystallinity and thinner crystal lamella;on the other hand,TAIC is expelled out to amorphous regions upon cold or melting crystallization.As a result of selective distribution of TAIC,?-ray irradiation produces heterogeneous cross-linking networks.These networks correspond to weaker depression effect on crystallization.This is the reason for the enhanced shape fixity relative to the homogenous cross-linking network prepared by ?-ray irradiation without cold or melting crystallization of PLLA.