Stretch-Induced Formation And Evolution Of Multiscale Structures In Polymorphic Polymers

Understanding the structural evolution mechanism of the semi-crystalline polymers during stretching is of great significance for guiding the processing of polymer products.In the actual material forming process,polymer melt often suffers from complex flow field,and the crystallization is formed under stress.On the other hand,When the semi-crystalline polymer is stretched,it may undergo multiscale evolution processes such as crystal slip,meltingrecrystallization and cavitiation,etc.Besides,polymer blends are often involved in the practical application of materials.The interaction between the different blend components and the microphase structure will also affect the mechanical properties and deformation mechanism.For polymorphic polymers,the stretch-induced crystallization may accompany by the occurrence of crystal transition.These processes intercoupling with each other such make the mechanism more complex.However,the relationship between structures and properties under different temperature and strain,as well the evolution of multiscale structures has not been clearly understood.In this thesis,we select some typical polymorphic polymers and their blends basing on the above problems to study the multiscale evolution process of crystallization,crystal structure and morphology during stretch process.Finally build a relationship among the temperature,strain and structure for polymorphic polymers.Firstly,in order to investigate the stretch-induced formation of multiscale structrues,we studied the stretch-induced structural evolutions of amorphous poly(trimethylene terephthalate)(PTT)below its Tg.Results reveal the sequence of structural ordering of PTT stretched around 25℃.The smectic C phase firstly generated after yield.Unloading at large strain gives rise to the emergence of triclinic crystalline phase via development of gauche conformer.Unlike other polymers where stress has dual function facilitating both chain orientation and conformation ordering,the effect of these two processes for PTT can be decoupled by load-unload stretching tests,thus the key role of conformation ordering on SIC is highlighted.Secondly,in order to investigate the stretch-induced changes of multiscale structrues,we choose the poly(butylene adipate-ran-terephthalate)(PBAT),a representative commercialavailable and flexible biodegradable polyester,as a model system and report its polymorphic crystal transition and lamellae evolution during stretching.Both the lamellae evolution and crystal transition of PBAT depend strongly on the crystallization and deformation temperatures(Tc,Td),due to the effects of stability of original crystals and the mobility of polymeric chains.The original lamellae of PBAT change into the chain-extended fibrillar lamellae during stretching via the melt-recrystallization mechanism;this process is delayed with increasing Tc because of the improved crystal stability.PBAT undergoes α-β crystal transition during stretching;the crystal transition is favored at low Tc and Td.The crystal transition takes place in the newly-formed chain-extended crystals,rather than the original crystals.The transformedβ crystals can recover to their a counterparts upon annealing at high temperature when the stress is released.Then we choose the PBAT/poly(lactic acid)(PLA)pair as the immiscible semicrystalline blend and investigate the evolutions of crystal orientation,polymorphic crystalline structure,lamellar morphology,and the cavitation behavior of PBAT-rich PBAT/PLA blends under stretching.We find that the strength and modulus of PBAT are considerably improved after the incorporation of PLA.Crystal orientation of PLA is much slower than that of PBAT crystals in their blends under stretching.PBAT undergoes the α-to-β phase transition and melt recrystallization of lamellae upon stretching,which is seldom affected by the addition of PLA.Stretch-induced cavitation is observed during stretching the PBAT/PLA blends;the cavities are mainly formed around the PBAT/PLA interface due to the interfacial debonding and in the PLA phase by the fragmentation of crystal lamellae.The formation and development of cavities in PBAT/PLA blends are significantly influenced by the blend composition,crystallization and stretching temperatures(Tc,Td).Cavitation of PBAT/PLA blends is suppressed as Tc is lowered or Td is enhanced.Finally,we aimed to investigate the structural envolution of preoriented poly(vinylidene fluoride)(PVDF)/poly(methyl methacrylate)(PMMA)blends films during post-stretching.The films were prepared by cast extrusion.The PVDF cast film has morphology of parallelly arranged lamellae with good hard elasticity,which was following stretched to induce the formation of pores by the separation of neighboring lamellae.After yielding,the lamellae were destroyed and recrystallized,accompanied by the gradual orientation of the pores.For the PVDF/PMMA blend cast films,PVDF chains can crystallize into either α or β phase depending on the melt-draw ratio.Due to the decrease of the crystallinity,regular oriented lamellar stacks absent in the blend films,thus losing the hard elasticity.The initial deformation of PVDF/PMMA films occurs mainly in the amorphous region,while slippage and rotation occur on the lamellar stacks with small size,but no pores initiated in their post-stretching.This work will provide help for understanding of the effect of chain structures,blend components and stretching conditions of on the mechanism of stretching polymorphic polymers.It will also further provide guidance for the optimization of processing conditions of related materials.