Tailoring the rigid amorphous fraction of isotactic polybutene-1 by ethylene chain defects

•The rigid amorphous fraction (RAF) is tailored by incorporation of random co-units.•Exclusion of ethylene co-units from crystallization leads to an increase of the RAF.•Increase of the RAF with ethylene content is due to lower perfection of fold-surfaces.

The effect of different amounts of ethylene co-units in the butene-1 chain, on the fold-surface structure of crystals of isotactic polybutene-1, has been probed by analysis of the rigid amorphous fraction (RAF). The exclusion of ethylene co-units from crystallization in random butene-1/ethylene copolymers and their accumulation at the crystal basal planes leads to a distinct increase of the RAF with increasing concentration of co-units. A specific RAF was determined by normalization of the RAF to the crystal fraction. While in the butene-1 homopolymer a specific RAF of 20-30% is detected, it increases to more than 100% in copolymers with 5-10 mol% of ethylene co-units, being in accordance with the previously observed increase of the free energy of the crystal fold-surface due to copolymerization. It has also been shown that the specific RAF increases with decreasing temperature of crystallization, due to formation of a fold-surface of lower perfection, containing an increased number of chain segments traversing the crystalline-amorphous interface.

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