Deciphering the 3D Microstructures of a Doubly Charged Homopolymer through a Complementary Correlation of Monomer Crystallography and Polymer Powder X-ray Diffraction

This work unravels the bulk microstructure of a doubly charged homopolymer by correlating crystallographic details from single-crystal X-ray diffraction (SCXRD) of the monomer with the crystalline order of the polymer utilizing powder X-ray diffraction (PXRD). The homopolymer, synthesized through reversible addition-fragmentation chain transfer (RAFT) polymerization, features a styrenic backbone with amphiphilic pendant groups containing a doubly charged 1,4-diazabicyclo[2.2.2]octane (DABCO) salt that is attached to a terminal octadecyl (C18) chain. SCXRD of the single crystals grown from the monomers reveals that the amphiphilic monomers prefer to pack into a highly ordered herringbone lamellar structure that facilitates electrostatic interactions between the DABCO salt units and hydrophobic associations of the styrene moieties and pendant C18 chains. Following living radical polymerization from a homogeneous solution, the resulting homopolymer was found to be semicrystalline, despite the expected stereoirregularity (atactic configuration) of the styrenic backbone. Surprisingly, comparisons of the PXRD patterns of the monomer and annealed homopolymer suggest that the homopolymer also crystallizes into a herringbone lamellar structure similar to that of the monomer. Moreover, the V-shaped counterion geometry likely plays an important role in the formation of the herringbone structure for the monomer and the homopolymer to maximize dipole-dipole interactions. Through correlations of the precise crystallographic details of the monomer with the PXRD patterns of the semicrystalline homopolymer, this study highlights a powerful approach in developing a structural model to define the morphology of polymers with complex chemical structures and hierarchical ordering.