Utilizing 3-methyl-1-butene co-units to tailor phase transition behavior in butene-1 copolymers

Abstract

Copolymerization with extra co-units is often utilized to tailor the structure of crystalline polymers. In this work, a unique type of 3-methyl-1-butene (3M1B) co-monomer was chosen to introduce branched co-units into the polybutene-1 main chain. For this series of butene-1/3-methyl-1-butene random copolymers synthesized, the stretching-induced phase transition from the kinetically favored tetragonal form II into the thermodynamically stable trigonal form I was investigated using a combination of tensile tests and in situ wide-angle X-ray diffraction. The results revealed that for the II–I phase transition, not only the kinetics but also the orientation pathway shows strong dependence on the concentration of 3M1B co-units. For copolymer 3M1B1.27 with a low co-unit concentration of 1.27 mol%, form II transformed into form I as crystallites reached the off-axis orientation. With a high co-unit concentration of 3.12 mol%, form II of copolymer 3M1B3.12 could develop into a highly oriented state with c-axes parallel to the stretching direction, which subsequently transformed into highly oriented form I. More importantly, the stretching-induced II–I phase transition was suppressed by increasing 3M1B co-units of copolymers. Therefore, as the 3M1B co-unit concentration was increased in copolymers, the resistance to external deformation was decreased, resulting in decreased mechanical strength.