Analysis On The Structure And Flame Retardancy Of PLLA Modified By Stereocomplex

Poly(lactic acid)(PLLA)is a kind of biodegradable polymer material,which is synthesized from renewable plant resources and frees people's reliance from petroleum resources.Due to its excellent biocompatibility,mechanical properties and processability,PLLA is considered one of the most promising biodegradable polymer materials.However,PLLA has some disadvantages,such as slow crystallization speed and brittleness,which limit the application of PLLA.Therefore,adding nucleating agent is as commonly used method is used to improve the crystallization and mechanical properties expand the scope of PLLA application.Firstly,Poly(L-lactic acid)(PLLA)and poly(D-lactic acid)(PDLA)with equal weight ratio were mixed in solution.The effects of different mixing methods,mixing time and stirring rate on the structure,thermodynamic properties and mechanical properties of the reaction product stereocomplex(SC-PLA)were studied.Stirring conditions have a significant effect on the crystallinity of the product.Under the stirring rate of 200 rpm,the crystallinity of the product is the largest.Co-mixing method has a higher stereo ratio than the products obtained by separate mixing.The stereocomplex prepared by mixing for 6 h at a stirring rate of 200 rpm has higher purity and crystallinity,and the Young's modulus and tensile strength are higher than the products prepared under other conditions.SC-PLA and PLLA were crystallized under the same supercooling temperature and the number of crystal nuclei and spherulite growth rate of stereocomplex were significantly higher.Secondly,SC-PLA prepared under the optimal conditions was used as an off-line nucleating agent and PDLA was used as an on-line nucleating agent to melt blend with PLLA at different processing temperatures to prepare s/PLLA and d/PLLA composites.The effects of the two kings of nucleating agents on isothermal crystallization kinetics and mechanical properties of PLLA were studied.The two nucleating agents had great influence on the enthalpy value of PLLA and the nucleation of crystals.With the increase of isothermal crystallization temperature,the induction time of crystallization decreased first and then increased.The nucleation rate of d/PLLA sample was faster than that of s/PLLA,but the enthalpy value of s/PLLA sample was higher and reached its peak at 125?.By observing the growth radius of spherulites,it was found that the spherulite growth rate of s/PLLA at 210?was the fastest.With the increase of processing temperature,the nucleation point density of spherulites decreased significantly.After heat treatment at 125?,the enthalpy values of s/PLLA and d/PLLA were both larger than PLLA and s/PLLA at processing temperature of 210? had the largest enthalpy value.The mechanical properties test showed that the storage modulus and maximum tensile strength of s/PLLA samples were larger than d/PLLA,while the storage modulus and maximum tensile strength of samples with lower and higher processing temperatures were slightly larger.Finally,the flame-retardant composites s/PLLA/TPP,s/PLLA/PSQ and s/PLLA/PPSQ were respectively prepared by adding additives triphenyl phosphate(TPP),self-made polysiloxane(PSQ)and phosphite functionalized polysiloxane(PPSQ)additives.The combustion performance,mechanical and crystallization properties were studied.With the addition of 2 wt%TPP,PSQ and PPSQ,the flame retardant properties were 22.5%and 21.1%and 23.3%respectively.The peak value of heat release rate was reduced The comparision resulet of oxygen index value of the composite material is s/PLLA/PPSQ>s/PLLA/TPP>s/PLLA/PSQ>s/PLLA.The carbon residue amount of s/PLLA/PPSQ is the highest and the surface of the carbon layer was the densest with the fewest pores.The addition of TPP,PSQ and PPSQ.did not change the PLLA crystal form,but also increased the crystallinity and crystallization rate,and the crystallinity of s/PLLA/PPSQ increased to 37.3%.PSQ could effectively improve the mechanical properties of the composite.Compared with s/PLLA,the tensile strength and impact strength were increased by 10.47%and 38.58%,and the storage modulus was also increased by 45.68%.