Shear Effects On Crystallization Structures And Behaviors Of PLA And PLA/Graphene Composites

Nanocomposites are materials with excellent properties, in which nanoparticlesare put into polymer materials by melt, solution or in-situ polymerization blendingmethods. Meanwhile, polymers are usually subjected to flow filed when they areprocessed in industry, and flow has an effect on the physical structures of polymermaterials, and, therefore, will influence their properties and application. Thus, it isvery important to investigate the crystalline structure and crystallization behavior ofnanocomposites under shear flow.In this thesis, we mainly investigated the effects of shear on crystalline structuresand crystallization behavior of poly（lactide） and poly（lactide）/graphene composites.First, effects of shear on crystalline structure and crystallization behavior ofpoly（lactide） were investigated. Under different shear conditions, there are onlyα’-form crystal in samples crystallized at86℃and96℃, α-form crystal at126℃and136℃; But there exist α’-and α-form crystal at106℃and116℃. Meanwhile,long period, along the meridian, of pure poly（lactide） increased with shear rate, whilethat along the equator almost stayed constant. The degree of lamellae orientation alsoincreased with shear rate. But with the increase of shear temperature, long periods andthe degree of orientation decreased gradually. With the same strain, long periods alongthe meridian and degree of orientation both increased with shear rate. In addition,shear could accelerate the crystallization kinetics and increase the spherulites’ growthrate.Then, crystalline structures and crystallization behavior of poly（lactide）/graphenecomposites were investigated. WAXD results show that crystal forms ofpoly（lactide）/graphene composites are the same with that of poly（lactide）. However,the lamellae’s long period of poly（lactide）/graphene composites is smaller than that ofpure poly（lactide）. Meanwhile, POM results show that poly（lactide）/graphenecomposites have the same crystal morphology, but different spherulites’ growth rateswith poly（lactide）.Finally, effects of shear on poly（lactide）/graphene composites were investigated.Shear has no influence on crystal forms, but can change the crystal structure ofpoly（lactide）/graphene composites. Under shear flow, the long period of lamellae firstincreased with shear rate when shear rate was smaller than5s^-1, and did not changewith shear rate when shear rate was larger than5s^-1. The reason leaded to this special phenomenon is, in our opinion, the interaction of shear and graphene. Meanwhile,under all conditions, the long period along the equator, and the thickness ofamorphous layer changed little with shear rate. Under shear, the crystallization time ofpoly（lactide） decreased from100min （under quiet condition） to20min; Forpoly（lactide）/graphene composites, the application of shear make crystallization timedecreased from34min to5min. Meanwhile, cylinderic morphology appears in thesamples after application of relative strong shear.