Synergistic Effect Of Carbon Nanotube Grafted Polymer On The Crystallization Behavior Of Polylactic Acid And Its Stereocomplex

Since the 21st century,with the shortage of resources and the increasingly serious problem of environmental pollution,it has become a hot topic for people to search for environmentally-friendly green materials in recent years.Among them,polylactic acid?PLA?has good reproducibility,degradability,biocompatibility and excellent processability,mechanical strength.However,its shortcomings such as poor crystallization ability and brittleness limit its application.In this paper,PLA was modified by blending with modified carbon nanotubes?MWCNTs?,and PLA stereocomplex?SC-PLA?.The synergistic effect of MWCNTs and MWCNTs grafted polymers on the crystallization behavior of poly?L-lactide??PLLA?and SC-PLA was investigated.Firstly,the effect of hydroxylated multi-walled carbon nanotubes?MWCNTs-OH?as nucleating agent on the crystallization behavior of PLLA was investigated.The results show that after the addition of MWCNTs-OH,the spherulite density of PLLA increases,the spherulite size decreases,the cold crystallization temperature(T_cc)decreases by about 20?,and the melting enthalpy??H_m?increases due to heterogeneous nucleation of MWCNTs-OH.However,with the increase of MWCNTs-OH content,?H_m gradually decreases,indicating that the addition of MWCNTs-OH improves the crystallization of PLLA.However,when the content of MWCNTs-OH is too high,MWCNTs-OH disperses unevenly or even aggregates in the PLLA matrix,which hinders the movement of the PLLA molecular chain and makes the crystallization of high content blends worse than that of low content blends.Secondly,MWCNTs-OH grafted poly?L-lactide??MWCNTs-g-PLLA?,MWCNTs-OH grafted polycaprolactone?MWCNTs-g-PCL?and MWCNTs-OH grafted polyethylene glycol?MWCNTs-g-PEG?were synthesized,and the synergistic effect of nucleating agent and plasticizers on the crystallization of PLLA was studied.Through DSC characterization,the results show that after the addition of MWCNTs-OH graft polymers,T_cc decreases greatly due to their heterogeneous nucleation.The?H_m of the blends increases,and the maximum value of?H_m appeares when the content of MWCNTs-g-PEG is 2wt%.And the cold crystallization peak of PLLA gradually disappeares with the increase content of MWCNTs-g-PEG.These results show that MWCNTs-g-PEG has the most obvious promotion effect on PLLA crystallization.At the same time,the crystallization rate of all blends increases,and the Avrami index is around 3,indicating that the crystallization of PLLA is improved without changing the crystallization mode.Based on the POM observation,it can be found that the spherulite density and the spherulite growth rate of the blends increase,indicating that when MWCNTs-OH grafted different polymers as additives,they play the roles of nucleation and plasticization during the PLLA crystallization process.Through XRD test,it is found that when the MWCNTs-g-PEG content is 1.5wt%,the intensity of the diffraction peak is largest.It is shown again that MWCNTs-g-PEG has the best effect in improving PLLA crystallization.Thirdly,the blends of PLLA and PDLA with different proportions were prepared to investigate the competition mechanism of homogeneous crystals?HC?and stereo crystals?SC?in PLLA/PDLA blends.Through DSC characterization,the results show that with the mass ratio approaches,the crystallinity of SC in the blends increases,the melting peak of SC increases,and the melting peak of HC almost disappears.Based on the POM observation,it can be found that when the mass ratio is quite different,the spherulite structure is clearly blurry,and the SC in the blends crystallizes first,then SC as nucleating agent,the homopolymer molecular chains diffuse and entangle on its surface to form HC.The XRD results show that when the mass ratio of PLLA and PDLA is the same,the blends show weak HC diffraction peak,and the characteristic diffraction peak of SC is very obvious.Finally,the synergistic effect of nucleating agent?MWCNTs-OH?and plasticizers?PLLA,PCL,and PEG?on the crystallization of the PLLA/PDLA blend with the same mass ratio was studied.The DSC results show that as the MWCNTs-OH content increases,SC crystallization is inhibited and HC crystallization is promoted;as the MWCNTs-OH grafted polymers content increases,HC crystallization is inhibited and SC crystallization is promoted.When the MWCNTs-g-PEG content is 1.5wt%,the maximum value of?H_m?SC?appears,indicating that MWCNTs-g-PEG can most effectively promote SC crystallization.Based on the POM observation,it can be found that compared with the PLLA/PDLA blend,the spherulite density of the ternary blends increases and the size decreases,indicating the nucleation effect of the additives.With the increase of MWCNTs-OH content,the spherulite growth rate of SC decreases significantly,indicating that the high content of MWCNTs-OH inhibites SC crystallization.However,the growth rate of the spherulite of the ternary blends increases after adding MWCNTs-OH grafted polymers,indicating that PLLA,PCL and PEG as plasticizers can improve the mobility of molecular chains during the crystallization process of ternary blends.Based on the POM observation,it can be found that the additives hardly change the HC crystallization in the PLLA/PDLA blend.For SC,when the MWCNTs-OH content is 1wt%and the MWCNTs-OH grafted polymers content is 1.5wt%,the SC diffraction peak intensity is the largest.It is shown that when MWCNTs-OH grafted polymers is used as additive,due to the presence of soft segments,its promotion effect on SC crystallization is greater than that of MWCNTs-OH on SC crystallization.