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

[Display omitted] •Mesophase formation of random butene-1/propylene copolymers was quantified.•Form II mesophase formation of butene-1 chains is unaffected by propylene co-units.•Form I′ crystals develop, if propylene content is higher than 10mol%.•Direct Form I′ crystallization from melt was quantified as function of temperature. 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 343K, 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 10mol%. For a random butene-1/propylene copolymer with 11.4mol% propylene it was found that Form I′ crystals only form below 310K, 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 273K. 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.