Form II to Form I Transition in Butene-1/Ethylene Copolymer at Isothermal Crystallization Temperatures

Abstract By means of differential scanning calorimetry and wide-angle X-ray diffraction techniques, the form II to form I polymorphic transition in a butene-1/ethylene random copolymer (9.88 mol % ethylene co-units) under isothermal crystallization conditions was discovered. The results indicated that this peculiar phase transition at the isothermal crystallization temperature was driven by the local internal stress due to the formation of less-stable form I′ in amorphous regions between existing form II lamellae via a confined crystallization process. Such internal stress was then transmitted to the form II crystals by intercrystalline links of tie molecules and entangled loops, effectively overcoming the nucleation barrier and accelerating the phase transition from form II to form I. A sufficient amount of the less-stable form I′ generated after the sample had completed the form II isothermal crystallization was necessary to accelerate this phase transition. Consequently, a higher proportion of form II was observed to undergo phase transition to form I in samples that started from a melt at a lower temperature or crystallized at a lower temperature.