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Polymer Research in the Roth Lab

Polymers are long chain molecules that often exhibit unique properties because of their macromolecular nature.

Polymers are ubiquitous in life.  Synthetic polymers make up much of the objects we use in daily life (nylon, polyester, styrofoam, plexiglass), while natural polymers make up much of who we are (DNA, proteins, polysaccharides) and much of our natural world (cellulose, cotton, wood, paper).

The large, entangled nature of polymer molecules leads to slow dynamics, which means that polymer can easily become trapped in nonequilibrium states during processing and fabrication of materials.

We want to understand the effects of forces, interfaces, and other perturbing influences on the properties of polymer materials. Our lab uses a number of optical techniques such as fluorescence and ellipsometry.

 

Polymer blends are mixtures of two or more different polymers with different chemical compositions.  The extent to which two different polymers will mix together depends on their interaction energies, how long the molecules are, and the temperature.

⇒ How does the presence of electric fields affect the miscibility of polymer blends?
 

Most polymer pairs are not miscibile and are thus forced together by various mixing and processing strategies to form phase separated materials with various morphologies.  In general, the best properties and performance of the material are found when the domain sizes can be made as small as possible (on the order of 100 nm).  These nanostructured blends have so many intertwined domains that their properties are often dominated by the polymer-polymer interfaces between the domains.

⇒ How do polymer-polymer interfaces affect the local properties of materials?
 

Polymers are also frequently used at the nanoscale in thin films (photoresist layers in microelectronics, anti-reflection coatings, barrier layers, gas separation membranes).  The properties of polymers in thin films (on the order of 100 nm or less in thickness) are often significantly different from what it would be in a bulk (large scale) material.

Again the perturbing influences of the interfaces seem to dominate the behavior of the material.  It is believed that similar mechanisms are behind the success of polymer nanocomposites.

⇒ What causes different material properties when the polymer is confined to nanoscale dimensions?
 

The slow dynamics of polymers also implies that polymers readily form glasses on cooling.  Glasses are nonequilibrium materials formed on cooling when packing frustration exceeds the available thermal energy for molecular rearrangments.  To a good approximation, a glass can be considered to be a liquid that has just frozen into place.  Structurally, we have yet to identify how a glass is different from a liquid, other than it simply slowed down to the point where it is no longer really moving.  There is still a big mystery surrounding how and why exactly this massive (super-Arrhenius) slowing down in dynamics occurs leading to glass formation.

⇒ Our lab is interested in understanding how stress and other forces present during glass formation alters the resulting state of the glassy polymer.

Physical aging is the long term evolution of the nonequilibrium glassy state often characterized by a decrease in free volume of the system over logarithmic time scales.  We have been able to link anomalous physical aging behavior in glassy polymer films as arising from differences in stress inadvertently applied to polymer films on cooling due to thermal expansion mismatches between the film and support.

⇒ Understanding anomalous physical aging behavior in polymer films.