Nucleation Assisted through the Memory of a Polymer Melt: A Different Polymorph Emerging from the Melt of Another One

Abstract

We present investigations of the intricate crystallization and melting behavior of an alternating copolymer synthesized by photopolymerization of 2,2′-dimercaptodiethyl sulfide with di(ethylene glycol)divinyl ether. Upon increasing temperature, we observed the succession of two distinctly separated melting processes, which we related to the sequential formation and disappearance of two crystalline polymorphs. Due to their well-separated melting temperatures Tm1 and Tm2, we labeled these polymorphs as LOW-Tm-form and HIGH-Tm-form, respectively. X-ray diffraction results confirmed differences in the parameters of the crystal unit cells. However, upon cooling from the isotropic melt, we never obtained the HIGH-Tm-form and could only generate the LOW-Tm-form characterized by spherulitic crystals that melted completely at Tm1. Surprisingly, simultaneously everywhere within these molten spherulites, a large number of needle-like crystals were growing as a function of the time the sample was kept (well) above Tm1. All crystals exhibited an orientation largely following the radial direction of the initial spherulites. This observation suggests that molten polymer chains remembered for some time their previous alignment within the crystalline LOW-Tm-form. This memory assisted the nucleation and thus exclusively enabled the formation of the HIGH-Tm-form. Only when partially crystallizing a sample aboveTm1 in the HIGH-Tm-form and subsequently cooling it below Tm1 allowed to achieve coexistence of both polymorphs. When heating such a sample of coexisting crystalline structures above Tm1, only the LOW-Tm-form melted and the HIGH-Tm-form remained. The generality of our findings has been demonstrated by similar results obtained for complementary alternating copolymers. Our study suggests that the otherwise impossible nucleation of polymorphs with a high melting temperature can be enabled by prior orientation of chains within another easily established polymorph characterized by a lower melting temperature.