Effect of Post-Curing and Solvent Type on Swelling and Residual Dipolar Couplings in Star-Based Model Homo- and Conetworks

Abstract

Solution-synthesized amphiphilic polymer co-networks (APCNs) are known to undergo post-crosslinking when dried. This decreases macroscopic swelling degree and the SAXS-detected correlation length. Using disulfide-crosslinked tetra PEG as model end-linked network, proton multiple-quantum (MQ) NMR is used to probe the residual dipolar couplings ($D_{\text{res}}$) of chain segments in networks swollen to $Q=V/V_0=5$ in water, toluene, THF, acetonitrile and DMSO, after being subjected to different post-curing conditions and de/re-swelling histories. Post-curing, i.e., the reaction of about 5% previously unreacted ends increases the $D_{\text{res}}$ more than twofold. Such effects can be minimized with precursor stoichiometry control and passivation of unreacted arms. Despite different post-curing conditions, PEG chains are found to show a consistent relationship between the solvent-dependent equilibrium degree of swelling and $D_{\text{res}}$, proving the latter to reliably reflect chain conformational statistics and thus the thermodynamic state. Microstructural variation during swelling in different solvents hamper the detailed study of additional influencing factors such as solvent effects on sub-segmental conformational averaging. A possible mechanism of the increase of $D_{\text{res}}$ and the decrease in the equilibrium swelling degree due to trapped entanglements formed by post-crosslinked strands is discussed.