The Effect Of Poly(ethylene Oxide) On The Crystalline Structure Of Poly(lactic Acid)

Poly(lactic acid)(PLA)is an important biodegradable crystalline polymer,derived from renewable resources.Neveretheless,PLA is brittle,limiting its scope of application.Blending PLA with another polymer which has lower glass transition temperature(Tg)is an important approach to improve its toughness.Poly(ethylene oxide)(PEO)is an interesting low Tg polymer,as it possesses low toxicity and good biocompatibility.Therefore,PLA/PEO blends have received increasing attention.This thesis focuses on the effect of the addition of PEO on the polycrystalline structure of PLA.The main results of this thesis are as follows:(1)The effect of PEO on the crystalline structure of PLA crystallized from the melt.PLA/PEO blends,with various compositions including 100/0,90/10 and 80/20,were prepared by melt blending.The effect of PEO on the crystal structure of PLA crystallized isothermally from the melt was investigated by wide angle X-ray diffraction(WAXD).It was found that,for neat PLA,?' and ? crystals were formed at crystallization temperature Tc ? 110 oC and Tc > 110 oC,respectively.For the PLA/PEO blends,the(110/200)and(203)diffractions of the WAXD profiles didn't shift with Tc,and no obvious ?' crystal was observed,indicating that the presence of PEO promoted the formation of ? crystals in PLA.The effect of PEO on the crystallization behavior of PLA was studied by differential scanning calorimetry(DSC).The crystallization kinetics was analyzed by the Avrami model.The results showed that the addition of PEO did not change the Avrami exponent of PLA,indicating that PEO did not change the crystallization mechanism of PLA.On the other hand,the incorporation of PEO significantly improved the crystallization rate of PLA.This could be ascribed to the decrease of Tg and equilibrium melting point of PLA,which promoted the movement of PLA molecular chain.(2)The effect of PEO on the crystalline structure of PLA crystallized from the glassy state.PLA/PEO blends,with various compositions including 100/0,99/1,98/2,95/5,90/10 and 80/20,were prepared by melt blending.The effect of PEO on the structure of PLA isothermal cold crystallization was studied by WAXD.The results showed for neat PLA,?' and ? crystals were formed at crystallization temperature Tc < 110 oC and Tc ? 110 oC,respectively.With the increase of PEO content in the blend,the critical Tc for forming ?' crystal and ? crystal of PLA was not significantly changed,but the difference of the 2? value between ?' and ? crystals was gradually reduced.As PEO content increased to 10% and above,there were no apparent evidence of the presence of ?' crystal.In addition,the evolution of the crystalline structure during heating process for PLA and its blends crystallized at Tc = 90 oC was studied by WAXD.The results showed that an obvious ?' to ? crystal transitation occured for neat PLA,and this trend was also observed for PLA/PEO(99/1),(98/2),(95/5)blends.