Obstruction of crystallisation by shorter fold-length or different crystal form: A quantitative study of self-poisoning mechanisms in precisely brominated polyethylene

Recently growth rate minima were observed in a series of polyethylenes, PEBr-m, having a hydrogen replaced by bromine on every m-th carbon (m = 21,19 and 15). It is generally accepted that minima in temperature dependence of crystal growth rate are caused by self-poisoning (SP), whereby the crystal growth face is “poisoned” by frequent deposition of chains with wrong but nearly stable conformation. Whilst until recently in all examples of SP the blocking chains had too short a fold-lengths, it was suggested instead that in PEBr they adopt a wrong unstable crystal form. Here we construct two different kinetic models of SP of PEBr, one “conventional”, based on a quantised jump in fold length, the other based on competing polymorphs. By fitting the experimental data, we have qualitatively compared the two models and tested their validity. The results strongly favour the first model, based on quantised fold lengths. In order to fit the experiments by the polymorphic poisoning model, highly unrealistic assumptions would have to be made. Recently, however, polymorphic self-poisoning has indeed been confirmed in some other polymers.