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Connie Roth

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Altered Local Properties Across and Near Polymer-Polymer Interfaces

According to the traditional treatment of polymer blends expounded on in textbooks, individual domains within blends retain their bulk component properties such that to a first approximation the blend's macroscopic properties can be considered to be a weighted average of the individual component properties based on the fraction of each component.  It has been recognized for some time that the morphology of the blend plays a large role in determining the blend's macroscopic properties and material performance (a central premise behind structure-property relations).  However, even in this treatment the material differences are assumed to be localized at the polymer-polymer interfaces between domains with the domains themselves still assumed to exhibit bulk properties.  Any deviation from bulk properties is assumed to follow the local composition, which transitions sharply within a few nanometers at the interface between immiscible polymers.

This traditional view is not consistent with studies on polymers in confined systems such as thin films over the past two decades, which have demonstrated large property differences caused by interfacial perturbations when film thicknesses or domain sizes become comparable to approx. 100 nm (see link here).  Given the prevalence of new synthesis and processing technologies that can create so-called nanostructured blends with domain sizes less than 100 nm, there is a pressing need to understand how local properties change across and near polymer-polymer interfaces.

The glass transition temperature (Tg) is a fundamental property that defines a material's transition on cooling from an equilibrium rubbery melt (typical rubbery modulus ~MPa) to a nonequilibrium glassy solid (typically glassy modulus ~GPa).  We are interested in understanding how the local value of Tg and other related properties change across a polymer-polymer interface from one domain to another.
 

Communication: Experimentally Determined Profile of Local Glass Transition Temperature Across a Glassy-Rubbery Polymer Interface with a Tg Difference of 80 K

      Roman R. Baglay and Connie B. Roth, J. Chem. Phys. 2015, 143, 111101. [Link]

Within a model system, we have measured the dynamical profile of the local glass transition temperature Tg(z) as a function of position z across a polymer-polymer interface using a localized fluorescence method.  A canonical glassy polymer, polystyrene (PS) with nominal Tg = 100 oC, is placed in contact with a canonical rubbery polymer, poly(n-butyl methacrylate) (PnBMA), with nominal Tg = 20 oC.  Both domains are made large enough to avoid other interfacial interactions associated with the sample boundary.  The local Tg(z) of 10-15 nm thick pyrene-labeled layers are then measured at different positions z from the polymer-polymer interface (z = 0) within both domains on either side of the interface.  Clearly the local Tg(z) value will transition from one extreme to the other:  from 100 oC far from the interface on the glassy PS side to 20 oC far from the interface on the rubbery PnBMA side.  What is surprising is that the dynamical Tg(z) profile is very broad, spanning 350-400 nm from one bulk Tg value to another, not at all matching the composition profile as would be expected based on traditional textbook teachings.  In addition, the dynamical Tg(z) profile is highly asymmetric, spanning much further into the glassy PS side than the rubbery PnBMA side.  Theoretical considerations suggest this dynamical length scale should be related to that associated with cooperative motion in glasses.
 

Effect of Adjacent Rubbery Layers on the Physical Aging of Glassy Polymers

      Phillip M. Rauscher, Justin E. Pye, Roman R. Baglay, and Connie B. Roth, Macromolecules 2013, 46, 9806-9817. [Link]

Physical aging is the slow logarithmic evolution of the nonequilibrium glassy state towards a denser equilibrium state.  The rate at which physical aging proceeds is a measure of the stability of the glassy state formed, strongly dependent on the cooling rate and the material's current temperature depth below its Tg.  Other factors such as interfacial interactions (see link here) or stress (see link here) can also alter the physical aging rate of polymer films.  We have used ellipsometry to characterize the physical aging rate of thin glassy polystyrene (PS) layers in contact with rubbery poly(n-butyl methacrylate) (PnBMA) layers.  We find that the aging rate of the glassy PS layers atop rubbery PnBMA is not correlated with the observed large decrease in Tg of the glassy PS layer with decreasing layer thickness suggesting that some additional factor is affecting the structural relaxation near the glassy-rubbery interface.  This is in contrast with previous studies of thin single layer PS films where we were able to correlate the local aging rate with the local Tg value [ Pye et al., Macromolecules 2010, 43, 8296-8303].