Stretching-Induced Phase Transition Of The 1-Butene/Ethylene Random Copolymer

Poly(1-butene)is the typical polymorphic polymer and the crystal modification formed determines properties of the final products.Therefore,control of crystal structure is the key point to optimize product performance.For poly(1-butene),stretching can induce occurrence of phase transition from tetragonal form II into hexagonal form I and consequently improve the material strength.Although the stretching-induced phase transition has been recognized,the fundamental mechanism is not clear yet.In this thesis,we combined mechanical testing with the in-situ wide angle X-ray diffraction(WAXD)to study the phase transition of 1-butene/ethylene random copolymer with 1.5 mol% ethylene co-units.We first studied the phase transition process under stretching with various stretching speeds and temperatures.Three transition pathways were distinguished,including the phase transition accomplishing within off-axis oriented crystallites(pathway 1),simultaneous occurrence of phase transition with fibrillation of transformed form I(pathway 2),and phase transition happening within fibrillar crystallites(pathway 3).Transition kinetics was correlated with the macroscopic mechanical response,which was found strongly dependent on pathways.In pathway 1,triggering of phase transition corresponds to the mechanical yielding,which is attributed to the generation of new tie chains.Moreover,the kinetics of entire transition exhibits the identical dependence with stress.However,when fibrillation occurs in pathways 2 and 3,transition kinetics is substantially altered,which should be associated with the the stress bearing by fibrillar network.During stretching,fraction of form I transformed and the corresponding mechanical property both evolve continuously,which are coupled with each other.To understand the relationship between form I fraction with mechanical property and phase transition,we utilized quiescent phase transition to prepare mixture systems containing various fractions of form I and studied their stretching-induced phase transition.It was found that at 30 °C,different mixture systems still start in the yielding region and follow the same stress-determined kinetics.The established correspondence between form I fraction and stretching stress can be used to predict the yielding stress of mixture systems.Furthermore,using the cyclic stretching protocol,it was found that phase transition can proceed even during stress recovery process and the secondary stretching.Only when stretching strength exceeds the stress of initial yielding,phase transition kinetics can be significantly accelerated.Moreover,the kinetics of phase transition still follows the stress dependence of pathway 1.