Form I′ crystal formation in random butene-1/propylene copolymers as revealed by real-time X-ray scattering using synchrotron radiation and fast scanning chip calorimetry

The crystallization behavior of random isotactic butene-1/propylene copolymers has been investigated as a function of the propylene concentration and as a function of the crystallization temperature in a wide temperature range from 273 to 343 K, with emphasis placed on the detection of formation of Form I′ crystals. Form I′ crystals in random butene-1/propylene copolymers develop if the propylene content is larger than about 10 mol%. For a random butene-1/propylene copolymer with 11.4 mol% propylene it was found that Form I′ crystals only form below 310 K, with the relative amount increasing with decreasing crystallization temperature. A maximum Form I′ crystal fraction of about 1/3 of the total crystal fraction has been observed on crystallization at 273 K. Regardless the temperature of its formation, the thermal stability of metastable Form I′ crystals is distinctly lower than the thermal stability of unstable Form II mesophase and stable Form I crystals, if formed at identical temperature.