Polymorphic Phase Transition And Microscopic Mechanism Of Polyolefins Regulated By The Melt Structure

The polymorphic polymer grows in various polymorphs under different crystallization and processing conditions,with one polymorph being thermodynamically stable and the others being metastable.Considering the difference in thermal stability,the metastable polymorphs of polymers can transform into thermodynamically stable ones by altering the external force field and thermal conditions.Such polymorphic phase transition can take place through either solidsolid or melt-recrystallization pathways.At the molecular chain level,phase transition involves changes in chain conformation and packing mode,which are strongly dependent on molecular chain mobility.Chain entanglement,originating from the interpenetration of adjacent chains,is an intrinsic characteristic of macromolecules with long and flexible chains.Entanglements segregated in the amorphous region can be regarded as physical cross-linkers,and they would restrict the chain mobility,thus altering the polymorphic phase transition kinetics and related mechanisms.Additionally,the intermediate melt is the stage that must be crossed for the meltrecrystallization phase transition.Thus,the ordered structure in the melt is also crucial to the polymorphic phase transition.However,this aspect,namely,melt structure,has not been well understood.Therefore,this work uses commercial polyolefins as model systems to systematically study the role of melt structure in polymorphic phase transition.Firstly,we use isotactic polybutenel(PB-1)as a model polymorphic polymer and present the crucial role of chain entanglement in melt crystallization and solid-solid phase transition.A series of less-entangled PB-1 with different entanglement degrees are successfully prepared by freeze-drying.Compared to the bulk sample,the crystallization kinetics of form Ⅱ is suppressed for the less-entangled one.The decrease in entanglement degree would not change the morphology of spherulites but reduce the nucleation density and spherulitic size.Additionally,chain disentangling promotes the phase transition rate and final transition degree from form Ⅱto form Ⅰ.The disentangling-promoted solid-solid phase transition stems from the reduced nucleation barrier and enhanced chain mobility,which could facilitate the conformational transformation in the phase transition.Secondly,the phase transition behaviors of less-entangled PB-1 from form Ⅱ to form Ⅰ are further investigated.The chain mobility is enhanced after disentangling,which favors the formation of form Ⅱ with various lamellar thicknesses during melt crystallization.Thus,formⅠ with different melting temperatures(Tm)are detected after the solid-solid phase transition.Form Ⅰ with a high Tm is mainly formed by the thick lamellae of form Ⅱ after low-temperature annealing.While form I with a low Tm is generated by the thin lamellae of form Ⅱ after hightemperature annealing.Chain entanglement plays a dual role in the form Ⅱ-to-Ⅰ phase transition.The nucleation ability of form Ⅰ is elevated by the effective stress transfer through chain entanglement,while the growth of form Ⅰ crystals is restricted by the lowered chain mobility.The number of entanglements in form Ⅱ with thin lamellae is greater than that in thick lamellae.Thus,the thin and thick lamellae have nucleation and growth advantages of form Ⅰ,respectively.The nucleation and growth of form Ⅰ crystals have different dependencies on the annealing temperature.Accordingly,form Ⅱ with different lamellar thicknesses transforms successively with the change of the annealing temperature during phase transition.Then,PB-1 with different molecular weights(MW)is applied as a representative polymorphic polymer and studied the effects of MW and chain entanglement on the meltrecrystal lized phase transition.PB-1 mainly crystallizes in form Ⅰ’ during freeze-drying.The freeze-dried PB-1 melts and recrystallizes from the intermediate melt into form Ⅱ during heating or high-temperature annealing.Since the recrystallization time is extended with the decrease of heating rate,more form Ⅰ’ could transform into form Ⅱ upon slow heating.The crystallization of form Ⅱ from the melted form Ⅰ’ is promoted with increasing MW.Intriguingly,the more-entangled PB-1 has a faster Ⅰ’-to-Ⅱ phase transition and forms a higher amount of form Ⅱ upon heating and annealing.More-entangled samples are difficult to be fully melted and thus keep more incompletely melted structures in the intermediate melt,which accelerates the following recrystallization of form Ⅱ due to the memory effect.Finally,we choose polymorphic trans-1,4-polyisoprene(tPI)as a model system to explore the effect of melt structure on melt-recrystallization phase transition.The metastable β crystals of tPI melt first into an intermediate state and then recrystallizes to a new crystal phase upon thermal annealing in the melting temperature range.The intermediate melt shows multilevel structural order depending on the melting temperature.At a low Ta(≤52℃).polymer chains cannot be insufficiently relaxed and the chain segments with more ordered conformation may remain in the intermediate melt.Such conformationally-ordered chain segments can not only reduce the nucleation barrier but also memorize the previous crystal polymorph in the following recrystallization process,leading to the enhanced crystallization kinetics and selective formation of β crystals.As Ta increases within the melting range(Ta～55℃),the molecular chains are more relaxed and lose the conformational characteristics of initial β crystals,but the ordered structure may still remain.Therefore,the crystallization rate is still enhanced but the initial crystal polymorph(β crystals)cannot be memorized in the following recrystallization process.