The Effect Of Rapid Compression On The Solidification Behavior Of Poly(Lactic Acid) And Poly(lactic Acid)/poly(ethylene Oxide) Blends,and Study The Cold Crystallization Behavior Of The Products

Poly?lactic acid??PLA?is an important biodegradable and biocompatible crystalline polymer,derived from renewable resources.Nevertheless,owing to its slow crystallization rate,PLA products are usually amorphous,and the products are easy aging and have poor toughness.Therefore,the cold crystallization behavior,aging behavior and toughening have received more and more attention.Different from the common method?quenching cooling?of preparing amorphous materials,the rapid compression method is adopted in this paper for preparing the amorphous PLA.The influences of rapid compression on the solidification,cold crystallization and aging behavior of PLA was investigated.In addition,poly?ethylene oxide??PEO?was added in PLA.The detail is that,PLA/PEO blendswith various mass ratios of 100:0,95:5,90:10,85:15 and 80:20were prepared by melt blending,and then treated by rapid compression.The effect of rapid compression on the solidification behavior of the blends were studied.Based on this solidification method,the influence of PEO on the crystalline behavior of PLA was studied.The main results are as follows:?1?Two kinds of amorphous PLA samples were prepared by rapid compression?RC?and quenching cooling?QC?from melts,respectively.Firstly,the initial structure of the two kinds of amorphous samples were investigated by wide angle X-ray diffraction?WAXD?and fourier transform spectroscopy?FTIR?.The results show that two kinds of samples are amorphous,but not exactly the same in microstructure.In other words,the 2??corresponding to the 2?of maximum intensity of the 1-D WAXD curve?of RC and QC samples are 15.9oand 15.6o,respectively.It is speculated that the reason for this difference is that RC sample has some local ordered structure on the scale of molecular chain,which leads to the reduction of the average inter-chain distance.Secondly,the nonisothermal cold crystallization of the RC and QC samples were investigated by differential scanning calorimeter?DSC?,and the Avrami model was used to describe the crystallization kinetics.The results show that the cold crystallization temperature?T_c?of RC and QC samples are 92.8?and 104.6?,and the half crystallization time(t_1/2)are 9.2min and 12.3 min,respectively.The results indicate that the RC sample has a higher crystallization rate than that of the QC sample.Finally,using polarized optical microscopy?POM?to observe the isothermal cold crystallization morphology of the two kinds of amorphous samples at 140?.The results show that the nucleation density of RC sample is remarkably higher than that of the QC sample,which can be ascribed to the considerable ordered structure in the initial structure of the RC sample.The presence of the local ordered structure increases the number of nucleation sites and improves the nucleation capability.?2?The amorphous samples of RC and QC were aged at 50?for 0 h,2 h,4 h and 8 h,respectively.The results show that aging can increase the crystallization rate of RC sample,significantly,but has little influence on the crystallization rate of QC sample,the reason for this difference may be due to the different thermal stability of the ordered structure foe the two aged samples.?3?The initial structure of the PLA/PEO blends treated and untreated by rapid compression was compared,WAXD and FTIR results show that rapid compression inhibits crystallization.The initial structures of neat PLA and PLA/PEO blends treated by rapid compressing were also analyzed by WAXD and FTIR.The results show that the 100:0 sample is glassy state,but has some local ordered structure;the mesophase phase appeared in the 95:5 and 90:10 samples,and the content of mesophase increases as increasing PEO content;the?'crystal appeared in the 85:15and 80:20 samples,and the degree of crystal perfection increases as increasing PEO content.The nonisothermal cold crystallization of the neat PLA and PLA/PEO blends treated by rapid compression were investigated by DSC and in-situ WAXD.The results show that the?'to?phase transition of 100:0,95:5 and 90:10 samples occur at ca.150?,but the phase transition temperature of 85:15 and 80:20 samples occur at ca.90?.