| Poly(lactic acid)/poly(ethylene oxide)(PLA/PEO)blends present good biocompatibility and hold much potential for applications in biomedical and other fields.PLA/PEO blends are miscible systems.The relevant research in the literature mainly focused on the influence of low melting point component PEO on the crystallization behavior of high melting point component PLA,while the confined crystallization behavior of PEO in PLA crystal was rarely reported.In this work,PLA/PEO blends were prepared by melt blending.The effect of the composition of the blends as well as PLA crystals crystallized under different conditions on the crystallization behavior of PEO was studied systematically.Since the molecular weight of PEO has an important influence on the crystallization behavior of PLA,the PLA/PEO blends containing three different molecular weights(2×106,1×105,6×103 g/mol)PEO were investigated in this work.In particular,atomic force microscopy(AFM)was used to study the crystal morphology of PLA/PEO blends at nanometer scale.The main results are as follows:(1)Confined crystallization behavior of PEO in PLA/(high molecular weight)PEO blends.The experiments were carried out in two groups.In one group,the PLA/PEO blends were first cooled from the melting state at a rate of 2.5,5,10,20?/min to 60 ? to completely crystallize the PLA component.Then the samples were cooled to-50 ? at a rate of 10 0C/min,during which the crystallization of PEO was followed with DSC measurement in order to investigate the effect of PLA crystals on the crystallization of PEO.The differential scanning calorimeter(DSC)results show that two crystallization peaks of PEO,which are lower than 60 ? for PLA/PEO(70/30)blends.It indicates the fractional crystallization of PEO.The cooling rate from blends melting state to 60 ? has an important influence on the temperature of two crystallization peaks of PEO.Different from PLA/PEO(70/30)blends.PLA/PEO(50/50)blends show only one crystallization peak of PEO at high temperature during the cooling process,which indicates that no fractional crystallization occurred,and the initial temperature of crystallization is higher.It indicates that the PEO is more confined in PLA/PEO(70/30)blends.In the other group,PLA/PEO blends were first crystallized at 110,120,130 ? from melting state,then completely crystallized the PEO component at 30?,and the melting behavior of PEO was observed at a heating rate of 10 ?/min.The DSC results show that PLA/PEO(70/30)blends present two melting peaks of PEO,which are not affected by the crystallization temperature of PLA.In PLA/PEO(50/50)blends,only one melting peak of PEO is observed.(2)Confined crystallization behavior of PEO in PLA/(medium and low molecular weight)PEO blends.PLA/PEO blends was first cooled down from the melting state to 60 ? to completely crystallize the PLA component at a different rate of 2.5,5,10,20 ?/min.Then cooled down at 10 ?/min to study the crystallization of PEO.DSC results show that three crystallization peaks of PEO appear in PLA/PEO(70/30)blends.The multiple crystallization peaks indicate the fractional crystallization of PEO in blends.Moreover,the initial temperature of low molecular weight PEO crystal is significantly lower than that of medium molecular weight PEO,and the degree of PEO confinement increases.(3)The crystal morphology of PLA/PEO blends was performed through AFM.In PLA/(high molecular weight)PEO(50/50),PLA spherulites form a continuous phase,in which PEO microdomains are entrapped.With the rise of PLA isothermal crystallization temperature,the size of PEO phase increases.The sizes of PEO microdomains in the vertical orientation direction arrive to micron scale.During the crystallization of PLA,the PEO in melting state is separated into a series of microdomains.In PLA/(medium and low molecular weight)PEO(70/30)blends,the sizes of PEO microdomains in the vertical orientation direction are in nanometer scale,ranging from tens of nanometers to several hundred nanometers. |
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