Controlling Swelling Behavior of Upper Critical Solution Temperature Micelle Containing Layer-by-Layer Films

We explore the role of binding partner strength on temperature-responsive swelling profiles of hydrogen-bonded layer-by-layer (LbL) films containing upper critical solution temperature micelles (UCSTM) composed of poly(acrylamide-co-acrylonitrile)-block-polyvinylpyrrolidone. LbL films were constructed at acidic pH, at temperatures below the UCST, using poly(methacrylic acid) (PMAA) and the PVPON corona of the UCSTM. By using isothermal microcalorimetry, the interaction between PMAA and PVPON in solution was found to be pH dependent and entropically driven at pH 4. The internal structure of UCSTM LbL films deposited at various pHs was monitored with neutron reflectometry using deuterated PMAA to highlight film stratification, while growth and temperature-responsive swelling profiles of multilayers were studied with spectroscopic ellipsometry. Films assembled at higher pH, featuring more ionized PMAA chains, were less well stratified and displayed larger amplitude temperature-triggered swelling changes than those assembled at lower pH. Using pH to control film stratification and functionality can potentially be used in other hydrogen-bonding polymer assemblies.