Main-chain chiral poly(2-oxazoline)s:Influence of alkyl side-chain on secondary structure formation in the solid state

Abstract

\u3cp\u3eThe influence of the side-chains of main-chain chiral poly(2-oxazoline) s on their thermal properties was investigated using differential scanning calorimetry (DSC) and the nature of the obtained melting endotherms was further investigated by thermal annealing of the polymers. Poly(R-2-ethyl-4-ethyl-2-oxazoline) (p-R-EtEtOx) was found to be amorphous, while polymers with longer side-chains are semicrystalline. Previously we reported that the chirally ordered crystals of poly(R-2-butyl-4-ethyl-2-oxazoline) (p-R-BuEtOx) have a high melting temperature of more than 200°C. in this work we demonstrate that elongation of the side-chains frombutyl to octyl results in a decrease in the crystallization rate and melting temperature suggesting that the chirally ordered crystals of p-R-BuEtOx are based on close packing of the mainchain enhancing diploar interactions between the tertiary amide moieties. Crystallizaiton of chiral polymers with longer side-chains resultsmay then be driven by close packing of the side-chains. This is supported by the observation that further elongation of the sidechain length increases the crystallization rate. Moerover, an additional melting endothermappears for thesepolymers at a lower temperature upon annealing ascribed to a dual crystal size population. Circular dichroism (CD) measurements of the semicrystalline main-chain chiral polymer films revealed the presence of chirally ordered crystals while X-ray diffraction (XRD) patterns revealed a closer packing of the chiral poly(2-alkyl-2-oxazoline) s compared to the non-chiral polymers, suggested to result form the chiral ordering in the crystals. Grazing incidence wide angle x-ray scattering (GIWAXS) patterns indicated that the chiral crystals of p-R-BuEtOx do not form a helical structure, however, the substrate does influence the type of structure formed.\u3c/p\u3