A Study On The Mechanism Of Epitaxial Crystallization Of Poly(L-Lactide) On Oriented Polyolefin Substrates

In recent years,with the rapid development of society,lack of resources and environmental pollution have become more serious,and then biodegradable and renewable polymers have become the focus of scientists'attention.As a degradable and renewable thermoplastic polyester which has been studied and applied widely,the modification of poly(lactic acid)(PLA)become a popular research.It is well known that materials' properties depend on the materials' structure,among all the modified method,epitaxial crystallization is an important one which can regulate and control the structure of materials,and oriented polyolefins are one of the most common substrates for epitaxial crystallization.In the previous study,we found that poly(L-lactic acid)(PLLA)on the highly oriented polyethylene(PE)substrate can occur epitaxial crystallization,the molecular chains of PLLA were perpendicular to the molecular chains of PE.And we also knew that in the PLLA/PE bilayers,PLLA had already existed some pre-oriented molecular chain in the amorphous state.However,the previous research mainly studied PLLA/PE bilayers,we didn't realize the effects of the PE substrate during the cold-crystallization,and we can not completely sure the epitaxial orientation was induced by those pre-oriented molecular chains.In order to solve this problem,we used three different methods to prepare highly oriented PE substrates,and then we removed the PLLA films from PE substrates before annealing,we studied the orientation of PLLA films before and after annealing.At the beginning of the experiment,we also studied the effect of temperature on the epitaxial cold-crystallization of PLLA/PE system.The results show that PLLA has heterogeneous and homogeneous epitaxy phenomenon on the oriented PE substrate at the same time,and generate the cross-hatched structure.The higher temperature is more favorable to heterogeneous epitaxy.Moreover,through the analysis,we believe the highly oriented PE film has a strong nuclear ability to the cold crystallization of PLLA and this effect induce pre-oriented molecular chains of PLLA in the amorphous state when PLLA contact with the PE surface.During the cold-crystallization,these pre-orientation molecular chains induce other molecular chains to orient,and the PE substrate have an effect on fixing orientation of the PLLA molecular chains during the cold-crystallization.In addition,we have studied the mechanism of epitaxial crystallization of PLLA/iPP system.In this paper,the highly oriented iPP substrate was prepared by melt-draw method.After the research,it was found that PLLA which is on PLLA/PE bilayers existed pre-oriented molecular chains in amorphous state before the cold-crystallization,during the cold-crystallization,these pre-oriented molecular chains induced other molecular chains to orient,and the oriented iPP substrate also play a role on fixing pre-orientation of PLLA molecular chains during cold crystallization.In this paper,we study the epitaxial crystallization of PLLA on polyethylene and isotactic polypropylene substrates,it can provide some datas for the study of the mechanism of polymer epitaxy and it's helpful to explain the mechanism of polymer epitaxy better.And in this study,we removed PLLA film from the oriented substrates through using different methods to prepare the oriented substrates,it can be used to provide a new idea for the preparation of oriented polymer films without oriented substrates using epitaxial crystallization.