Non-intuitive Trends in Flory-Huggins Interaction Parameters in Polyether-Based Polymers

Recently, a variety of "click"or other additive chemistries have been introduced to functionalize polymers after polymerization to target specific applications, for example, membranes, catalysis, or drug delivery systems. It is generally assumed that the inclusion of these "click"linking groups has minimal impact on the thermodynamics of the polymer as a whole. In this study, we demonstrate that the introduction of these click-derived units has a profound impact on the Flory-Huggins parameter of polyether derivatives. Using random phase approximation fits for small-angle X-ray scattering data from block copolymer pairs to estimate the Flory-Huggins interaction parameter (χ), we determined that poly(ethylene oxide) (PEO) and poly(allyl glycidyl ether) (PAGE), which differ only by the inclusion of an allyl sidechain, have a χ of 0.030 (at T = 34 °C). While PEO is miscible with poly(lactide) (PLA) at nearly all temperatures, the PLA/PAGE χ determined experimentally is 0.015 (at T = 30 °C). Atomistic molecular dynamics simulations of PEO/PAGE oligomer blends show that upon blending, PEO chains contract and move closer together, while PAGE chains stretch and spread apart, indicating an enthalpic contribution to the χ parameter due to changes in polymer coordination resulting from the conformational asymmetry of PAGE and PEO. These studies demonstrate the large impact that functionalization and side-chain units have on the χ parameter of polymer pairs.