Dependence of critical tie molecule content on crystallinity for semicrystalline polymers

Tie molecules - individual polymer chains that quite literally tie together adjacent polymer crystallites - are key to providing structural integrity to semicrystalline polymers. Without such intercrystallite stress transmitters, a semicrystalline polymer will be brittle and unsuitable for most applications. In order to probe the brittle-ductile transition (BDT), we systematically vary the molecular weight, crystallinity, and domain spacing of hydrogenated polynorbornene homopolymers, hPN, and hydrogenated norbornene-hexylnorbornene random copolymers, hP(N-r-H). How these factors affect tie molecule fraction (as calculated by the Huang and Brown model) is also investigated and correlated with brittle/ductile fracture. We find crystallinity to be a major factor in predicting critical tie molecule content (the fraction of tie molecules needed to reach the BDT); a factor of 2 increase in crystallinity reduces the fraction of tie molecules at the BDT by an order of magnitude. Comparing these data with those from linear polyethylene homopolymers (LPE), we find similar trends in both cases, but for identical crystallinities, the critical tie molecule content is lower by nearly an order of magnitude for hPN than LPE.