Crystallization and phase transition of butene/propylene copolymers

Abstract

In this work, a series of butene/propylene random copolymers with a broad incorporation range of 0–26.1 mol% were synthesized using the dimethylpyridine amine hafnium/[Ph₃C][B(C₆F₅)₄] catalytic system. The melt crystallization and solid phase transition behaviors of these butene/propylene copolymers were investigated employing differential scanning calorimetry and in situ Fourier transform infrared spectroscopy. The introduction of propylene co-units could not only decrease the cooling crystallization temperature (T_cooling) but also change the crystallization polymorphism. With 13.7 mol% propylene co-units, copolymer BP13.7 crystallizes into kinetically favored tetragonal form II but quickly transforms into form I even during the dynamic cooling and heating process at 10 °C min⁻¹, leading to improvement of thermal stability. As propylene incorporation was increased to 17.9 mol%, the trigonal phase can crystallize directly from an amorphous melt, which is often referred to as form I′. Interestingly, with 13.7 and 17.9 mol% co-units, copolymers are able to trigger the II–I phase transition at the same temperature with form II crystallization, where there is no cooling to generate the thermal internal stress. Furthermore, the copolymer with 26.1 mol% propylene co-units remained amorphous during cooling at 1 °C min⁻¹, but could crystallize into almost pure form I′ within the isothermal process at 0–50 °C. In summary, the presence of propylene co-units within the polybutene main chain facilitates the generation of the thermodynamically stable trigonal phase by inducing the direction crystallization of form I′ and accelerating the phase transition of form II into form I.