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.