Study On The Accelerated Phase Transition Of Butene Copolymer

Polybutene is one class of polyolefin materials with excellent creep and impact resistences.However,as the typical polymorphic polymer,polybutene first crystallizes into the tetragonal form II in melt crystallization,due to the kinetic reseacon,and then transforms into the thermodynamically stable form I.Although such II-I phase transition can occur spontaneously,the slow transition kinetics causes a slow shrinkage and long-term instability of the product,which limit the pratical application of material.It has been found that copolymerization,temperature and flow all are the available methods to accelerate the kinetics of phase transition.In present work,copolymers containing various concentrations of different counits were synthesized,and their phase transition behaviors under temperature and flow were studied.We first synthesized a series of butene/1,5-hexadiene random copolymers,which contain the ringlike methylene-1,3-cyclopentane?MCP?structural counits.Comparing with butene/ethylene copolymers,the influences of counit type and concentration on nucleation and growth steps were investigated.Using the stepwise annealing method,it was found that the optimal nucleation and growth temperatures of homopolymer were-10 and 35°C,respectively.For copolymers with 1.5 and 4.3 mol%ethylene counits,the optimal nucleation was reduced to-15°C,but the optimal growth temperature was elevated into 50°C.Clearly,ethylene co-unit has a larger effect on the optimal growth temperature than on the nucleation temperature.For the copolymers with ringlike MCP counit,the optimal nucleation and growth temperatures exhibit different temperature dependences.For instance,the optimal nucleation temperature was reduced to-13°C for the 0.65 mol%MCP counits,but it was reduced to-15°C for 2.15 mol%MCP.It is also interesting to observe that for MCP copolymers with 0.65-2.15 mol%counit,the optimal growth temperature is the same,which is 55°C.The identified optimal temperatures provide crucial parameters for accelerating II-I phase transition.Moreover,we synthesized butene/pentene random copolymers with 12.27-56.56 mol%counits and investigated their phase transition with flow.For the butene homopolymer,extensional rate of 0.1 s^-1 can only orientate the form II crystallites,but 10 s^-1 is sufficient to enhance phase transition even during cooling.For the butene/pentene copolymers,0.1 s^-1 is actually able to trigger phase transition during cooling,where the copolymer with 56.56 mol%pentene completely transforms into form I.For butene/pentene copolymers,increasing strain rate can more accelerate the kinetics of phase transition and the copolymers with 21.46-56.56 mol%pentene counits all finish the II-I phase transition during cooling.