Effects of Carbon Atom Parity and Alkyl Side Chain Length on the Crystallization and Morphology of Biscarbamates, A Set of Model Compounds for Polyurethanes

Abstract

Solid state morphology and crystallization behavior of a homologous series of biscarbamate molecules having varying alkyl side chain lengths with different carbon atom parity were investigated. These are model compounds for polyurethanes. We synthesized a set of biscarbamates with double hydrogen bonding motifs separated by a (CH(2))(6) spacer and with alkyl side chains of various lengths ranging from C(3) to C(18) at the ends. Thermal analysis showed an odd-even alternation in their melting temperatures and heats of fusion, with the odd number of carbon atoms in the side chain having higher melting temperatures and heats of fusion than the even numbered ones, in contrast to the case of n-alkanes. The effect of carbon atom parity in the alkyl side chains on the spherulite size, spherulite growth rate, and isothermal crystallization kinetics was studied. Although the spherulite size increases with the alkyl side chain length, the maximum is seen at an intermediate length and not with a short or long alkyl chain for both the odd and even series. Along this series of molecules, a maximum in spherulite size, spherulite growth rate, and rate of crystallization is seen for C(7)C(6) (odd series) and C(8)C(6) (even series) biscarbamates. There is a significant difference in spherulite size with respect to carbon atom parity in the alkyl side chains as well as sample preparation protocol. Hence the length of the alkyl side chain, carbon atom parity in the alkyl side chains, and the sample preparation protocol (i.e., quenching versus slow cooling) play an important role in the morphology of these molecules. We rationalize this behavior with the relative contributions of hydrogen bonding and van der Waals forces as discerned from IR spectroscopy. While the van der Waals interaction increases with the alkyl side chain length in this series, the hydrogen bond contribution remains invariant. The rate of crystallization follows the trend seen with the spherulitic growth. The Avrami exponent is significantly smaller than 3, varying from 1.6 to 2.5 depending on the side chain length, which indicates that the crystallization is not truly three-dimensional. This study on the influence of alkyl side chains shows that while hydrogen bonding enables self-assembly the van der Waals interactions play a significant role in the crystallization and morphology of such self-assembled structures.