Profiles in local glass transition temperature near and across polymer interfaces in nanostructured blends
November 10-14, 2019
Small domain sizes and adhesion between interfaces are the cornerstones of creating high performance multicomponent materials. Combined with an optimized morphology, the strategic use of interfacial interactions to perturb local properties is a promising avenue for creating designer materials where the desired global properties are obtained from an amalgam of local property changes. To achieve this grand goal, a detailed understanding of how interfaces perturb local properties are needed, along with knowledge of how the global macroscopic characteristics result from these local effects. This presentation will discuss our efforts to understand how various interfaces perturb local material properties and how different experimental techniques contribute to this picture.
Our group has employed a localized fluorescence method to measure the profile in local glass transition temperature Tg(z) across a glassy-rubbery polymer interface between polystyrene (PS) and poly(n-butyl methacrylate) (PnBMA) . For a single interface between two semi-infinite domains, the local Tg(z) profile was found to be extremely broad and asymmetric, spanning 350-400 nm in extent as the local Tg(z) value transitioned the 80 K difference in bulk Tgs far from the interface from one side to another. Subsequent studies found this profile to be common to a range of weakly immiscible systems where the stiffer polymer domain with higher bulk Tg consistently showed a longer-ranged Tg(z) perturbation extending further from the interface . The observed Tg(z) profiles were found to be strongly dependent on the interface formed between the two polymer domains during thermal annealing  and on the finite size of domains . This difference between hard vs. soft interfaces, along with theoretical studies in the literature, has led us to investigate the Tg(z) profile in PS next to polydimethylsiloxane (PDMS) with varying crosslink density to systematically change the modulus of the neighboring domain without also changing the chemistry of the interface. We find the local Tg(z) in PS at a distance of z = 50 nm away from the PS/PDMS interface can vary by 45 K when the PDMS modulus changes from ~1 to 3 MPa.
These broad Tg(z) profiles demonstrating strong coupling of dynamics across the interface appear to only occur when the dissimilar polymer-polymer interface is well formed by annealing . This suggests that some aspect during polymer interface formation (broadening of interface, chain interpenetration, or interfacial roughening) may be significant in controlling the observed behavior. Efforts to separate these different factors have led us to investigate silica substrates with rough interfaces and with end-tethered chains. We have observed similar long range Tg(z) profiles near PS end-grafted silica substrates, with the largest Tg(z) increase of 50 K next to the substrate corresponding to a low grafting density coinciding with the “mushroom-to-brush” crossover regime . The Tg(z) profile is comparable to that observed near a polymer-polymer interface for a lower Tg polymer next to one with a much higher Tg, suggesting that chain connectivity across the interface is key to long range coupling of dynamics.
1. R.R. Baglay C.B. Roth, J Chem. Phys. 2015, 143, 111101. Communication: Experimentally determined profile of local glass transition temperature across a glassy-rubbery polymer interface with a Tg difference of 80 K.
2. R.R. Baglay C.B. Roth, J Chem. Phys. 2017, 146, 203307. Local glass transition temperature Tg(z) of polystyrene next to different polymers: Hard vs. soft Confinement.
3. R.R. Baglay C.B. Roth, ACS Macro Letters 2017, 6, 887-891. Experimental study of the influence of periodic boundary conditions: effects of finite size and faster cooling rates on dissimilar polymer-polymer interfaces.
4. X. Huang X, C.B. Roth, ACS Macro Letters 2018, 7, 269-274. Optimizing the grafting density of tethered chains to alter the local glass transition temperature of polystyrene near silica substrates: The advantage of mushrooms over brushes.
Connie B. Roth, "Profiles in local glass transition temperature near and across polymer interfaces in nanostructured blends" in "Composites at Lake Louise 2019", John Kieffer, University of Michigan, USA Erik Spoeke, Sandia National Laboratories, USA Meisha Shofner, Georgia Institution of Technology, USA Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/composites_all_2019/44