Formation of Multiple Distinct Lamellar Stacks in Semicrystalline Homogeneous Random Copolymers

Abstract

Semicrystalline polymers are often idealized as forming lamellar stacks with an average long period (d-spacing) varying little throughout the specimen. In this work, two distinct types of lamellar stack, each with a different d-spacing and melting temperature, were observed in hydrogenated poly(norbornene-r-hexylnorbornene) random copolymers (hPNrH) following slow, continuous cooling from the melt. Such a structure is surprising, considering that these copolymers possess a statistically homogeneous comonomer distribution. Upon heating, the small-angle X-ray scattering (SAXS) peak corresponding to the secondary stacks disappears prior to complete melting. At that same temperature, there is a prominent shoulder in the melting endotherm of the slow-cooled films. The prominence of the secondary stacks in the SAXS pattern was tied to molecular weight, and thermal fractionation experiments revealed that lower molecular weight specimens crystallize into a broader distribution of crystal thicknesses. Finally, the maximum crystallizable sequence length (MCSL) distributions were modeled for ensembles of polymer chains with a set length, revealing a shift in the MCSL distribution to lower median values as chain length was shortened. The following crystallization scheme thus emerges: upon slow cooling from the melt, chains with low MCSL values are unable to crystallize into the lamellae formed during primary crystallization by chains possessing a higher MCSL. At low molecular weights, enough of these low-MCSL chains are present that they coalesce into molten “pockets” before ultimately crystallizing into a distinct type of lamellar stack at a substantially lower temperature.