Effects of Polymer Block Length Asymmetry and Temperature on the Nanoscale Morphology of Thermoresponsive Double Hydrophilic Block Copolymers in Aqueous Solutions

Abstract

By complementing small-angle neutron scattering (SANS) with Fourier transform infrared (FTIR) spectroscopy measurements, the influence of temperature as an external stimulus and block length asymmetry is studied on nanoscale assemblies of thermoresponsive double hydrophilic block copolymers in aqueous solutions. Morphologies and molecular hydration characteristics of two poly(N-isopropylacrylamide)-block-poly(oligo ethylene glycol methyl ether acrylate) (PNIPAM-b-POEGA) diblock copolymers, which differ in the length of the PNIPAM block, are compared. SANS provides insights into the nanostructure of the assemblies and FTIR probes vibrations of selected functional groups at the molecular level. Upon temperature increase, the solutions undergo transformations from hierarchical assemblies to more well-defined spherical morphologies. Variations in the block lengths lead to distinct morphological transformations. The variable morphological transformations originate from differences in the strength and/or amount of hydrogen bonding and hydrophobic interactions, as well as the chain density in the clusters formed and hydration. We identify transformations from hierarchical structures to fractal core–shell morphologies in the asymmetric block copolymer with the long PNIPAM block, while spherical and core–shell spherical morphologies are formed for the symmetric diblock copolymer with the short PNIPAM block. In these assemblies of PNIPAM-b-POEGA with the short PNIPAM block, the methyl side group hydration sensitively depends on the temperature increase at temperatures even beyond the nominal volume phase transition temperature. For both blocks, the evolution of the amide I band reflects that solvent–polymer interactions are still favorable even at the highest temperatures. Morphological variations after cooling to ambient temperature are reflected in both SANS and FTIR, pointing to remnant hydrogen bond interactions.