Impact of Changing PDMS Modulus on the Local Glass Transition Temperature Tg(z) of Neighboring Polystyrene

Our group recently characterized the local glass transition temperature profile Tg(z) of polystyrene (PS) vs. distance from the interface of PS with a higher Tg polymer (PSF and PMMA) or a lower Tg polymer (PnBMA and PiBMA) [J Chem Phys 2017, 146, 203307]. For PS next to an interface with a lower Tg material ("soft confinement") a distance of z ≈ 225-250 nm away from the interface was needed for PS to recover its bulk Tg, while z ≈ 100-125 nm for an interface with a higher Tg material ("hard confinement"). This difference between hard and soft interfaces, along with recent theoretical efforts in the field, motivated us to investigate a system where the modulus could be varied without also changing chemical structure. Here we present measurements of Tg(z) in PS next to cross-linked polydimethylsiloxane (PDMS). By varying the ratio of cross-linker to base monomer, we varied the cross-link density and hence modulus of PDMS. We find that the Tg(z) in PS at a distance of z = 50 nm away from the PS/PDMS interface can vary by more than 40 K when the PDMS modulus varies from ~1 to 3 MPa. Interestingly, the length scale z ≈ 70-90 nm at which bulk Tg(z) of PS is recovered is significantly shorter than what would be expected for this soft neighboring domain based on our previous results.