Molar mass dependent spatial distribution of form II to I transition inside spherulites of disentangled isotactic Polybutene revealed by scanning Confocal Raman microscopy

Form II to I transition in isotactic polybutene-1 (PB-1) is known to depend on the molar mass due to complicated interplays between crystalline phase and the amorphous inter-crystalline links. In general, higher molar mass favored the transition at the early stage as more inter-crystalline links result in more form I nuclei. At the late stage, it was observed that the low molar mass samples possess higher transition rate because of an easy relaxation of the unfavored stress generated during the transition. In this work, form II to I transition at room temperature in spherulites with few inter-crystalline links between adjacent lamellae, which were obtained by isothermally crystallizing two commercial PB-1 of different molar mass at sufficiently high temperature, were investigated using confocal Raman microscope scanning along the radii. Surprisingly, the spherulite of the higher molar mass PB-1 showed a higher gross transition rate at late stage than that of the lower molar mass one. Considering the impacts from the thermal stress, exterior stress, the neighbor effect of form I as well as the coordination of thermal fluctuation driven transition and retarding effect by nearest form I, a detailed analysis of the spatial distribution of the transition within the spherulites revealed that the lamellae inside the higher molar mass sample led to a higher form II to I transition in the late stage through its preponderance of abundant long-chain ends, numerous folding parts and more effective lateral connectivity along the radial arms.