Francis W. Starr, PhD, MA, BS, Wesleyan University
Ultra-thin polymer films have ubiquitous technological applications, ranging from the electronic devices to artificial tissues. Changes in the dynamics of polymer films as compared to the bulk have a profound effect on potential uses. Solidification by the formation of a glass below the glass transition temperature Tg is one of the most important material properties for the processing polymeric systems. Consequently, our research has focused on developing a fundamental understanding of factors that control polymer dynamics and Tg, and how these factors are altered in ultra-thin polymer films.
Impact of the Research
To systematically tackle the problem, we have taken a multi-faceted approach. First, we have examined glass formation of a bulk polymer melt where we can test theoretical descriptions in comparatively controlled environment. Simultaneously, we have examined ultra-thin polymer films of varying thickness, surface interactions, and surface roughness to provide a broad characterization of the factors that alter dynamics. Initial studies in these directions have already been published, and we are preparing follow-up results for publication. Finally, since it has been argued that there is a close correspondence between the dynamical changes in thin polymer films and polymer-nanoparticle composites [Starr et al., Phys. Rev. E 64, 021802 (2001); A. Bansal at al., Nature Materials 4, 693 (2005)], we have examined to what degree the same framework used to describe the bulk and ultra-thin films can be applied to a model polymer-nanoparticle composite. We briefly describe the key findings of these publications below.
In [J. Chem. Phys. 138, 12A541 (2013)], we examined the geometrical structure of clusters and string-like cooperative motions in a model glass-forming polymer melt. We showed that these clusters conform to statistical geometry of equilibrium branched polymers. Most importantly, we examined the question of whether these heterogeneity scales can be identified with the scales anticipated by the Adam-Gibbs and random first-order transition descriptions of glass formation. We showed the strings apparently provide the most consistent description of the cooperative scale described by these theories.
The similarity of these cooperative string-like motions to the properties of 'living polymerization' has led us to consider the implications of the hypothesis that these strings can be quantitatively described as dynamic polymers that form and disintegrate in equilibrium. This hypothesis has proven highly successful, and we are preparing this work for publication. Our identification of the living polymerization model with the coherent string-like displacements of molecules implies that glass formation can be interpreted as a kind order-disorder transition, in which particles reversibly associate into string-like clusters at equilibrium, but where the 'polymerized' ordered state does not correspond to a particle configuration.
In [J. Chem. Phys. 137, 244901 (2012)], we focused on how confinement perturbs the dynamics of ultra-thin polymer films. We systematically characterized the effects of film thickness of supported polymer films on Tg and fragility, both for the entire film and locally. We showed that there is a substantial and non-monotonic variation of Tg with thickness, both for the film as a whole, as well as locally (see fig. 1); Tg alone provides an incomplete picture of the effects of film confinement, a fact not well appreciated in much literature. We further examined to what degree a free-volume layer picture can be applied, and demonstrated the limitations of this model. Moreover, the dynamics of the middle of a film can differ dramatically from the bulk, even though the density is bulk-like. We concluded that there are significant non-local effects of confinement on dynamics, so the film cannot be treated as a slice of the bulk with only local surface effects. Evidently, a successful model must incorporate both fragility variations and non-local changes to the film relaxation dynamics.
Figure 1: The effect of film thickness on the local variation of Tg and fragility: Tg (top) and fragility (bottom) as a function of distance z from the wall.
Following up on these findings, we have are preparing a new publication in which we rationalize all of the changes to the film dynamics by corresponding changes to the scale of cooperative string-like motion – linking back to our studies of pure polymer systems. This will be an important contribution, since it provides a unifying framework to explain the seemingly disconnected affects of surface interaction strength, roughness, and film thickness on the dynamics.
Lastly, in [Soft Matter 9, 241 (2013)], we examined the validity of these ideas for polymer-nanoparticle composite materials. In that work, we showed that the changes in Tg and fragility are reflected in the overall scale of the string-like collective motion. By comparing dynamical changes with those of density, we showed that free-volume based approaches are not adequate to describe dynamical changes. These findings appear to validate the qualitative, and to a lesser degree, quantitative relation between the dynamical changes in polymer nanocomposites and ultra-thin films.
Impact on Students and PI
Students have been the driving force behind the research performed and published during the reporting period.
Beatriz Pazmio Betancourt is a Latina graduate student and primary author of Soft Matter 9, 241 (2013). She is presently writing up results for two further publications, and has a tentative Ph.D. defense date in October this year. Support from this grant has to finish her Ph.D. much sooner than anticipated. More importantly, the impact of this work has earned her a post-doctoral position supported by the National Institute of Standards and Technology (NIST).
Paul Hanakata is an undergraduate and the primary author of J. Chem. Phys. 137, 244901 (2012). This grant has supported his research during both summer and winter holiday periods. This funding has been especially valuable to Paul, since he is from Indonesia and is financially challenged. He is presently preparing his follow-up work for publication, as well as a senior honors thesis.
For the PI, this support has enabled me to expand my research on the polymer dynamics to an important new direction, tackling the practically important problem of confinement effects. I am looking to leverage this new expertise to obtain further support from the National Science Foundation.