This Materials World Network brings together world-expertise on the experimental characterizations of polymer films (Boston University, USA and Universität Freiburg, Freiburg, Germany on the film stress and viscoelasticity, respectively) and computer simulations on the dynamics and structure of confined polymers (Institut Charles Sadron, Strasbourg, France) to seek for the possibility of a unified picture that can account for all the unusual properties observed of polymer films to date (namely the dynamics, residual stress and thermal expansivity), and at the same time attempt to relate these properties to the (out-of-equilibrium) polymer structures therein as well as the fabrication and/or post-fabrication processes producing them. By establishing such structure-property and processing-property relations, the Network aims to ultimately institute a strategy that can predict, design and tune the properties of polymer films. Because the structures of the polymer films that allegedly cause the unusual properties are metastable and change with time, a cross-linkable copolymer of polystyrene are used to freeze-in the structures and whereby their properties. This research is largely guided by several published experiments and a model conjectured by Reiter (the PI from Germany) and de Gennes portraying how a non-equilibrated polymer network structure engendered by rapid solvent evaporation could explain negative thermal expansivity. The results of this program are expected to substantiate this model, and additionally expand its relevance to the dynamical behaviors of the films. The resultant, more encompassing model will provide a pivotal platform upon which the structure-property and processing-property relations can be portrayed and/or formulated.
Applications of polymer films are widespread. Success of this program leads to a simple strategy to design and fabricate polymer films with premeditated thermal expansion and/or dynamical properties. Having such ability significantly enhances our leverage to adapt these materials in a cost effective manner to the ever-changing demands on their properties by the applications employing them. Six PhD students are trained in this program, providing them an excellent training ground in the academically challenging and technologically relevant problem concerning the unusual properties of spin-coated polymer films. Members of the Network benefit from the international setting of the program. In particular, there are regular meetings and physical exchange between the PhD students besides frequent video conferences. This program actively engages undergraduate and high-school student participants.
This award is jointly funded by the DMR Polymers Program and the DMR Office of Special Programs.
A. Intellectual outcome: Polymer films have widespread applications. For a long time, there have been numerous observations showing that when the film thickness is decreased below 10 nanometers, the films may exhibit unconventional properties such as negative thermal expansion and unusually long structural relaxation times. One finding of this Materials World Network (MWN) award shows that polymer films can adsorb irreversibly to the substrate surface even there is no obvious interactions between the polymer and the substrate, which may explain the unusually long relaxation time. This finding has stimulated further investigations on the properties of the adsorbed layers by several other groups, as evident from the literature. Another prominent result is the finding that the properties of a polymer film can be adjusted by either spin-coating – a prevalent method of making thin polymer films – or swelling it with a solvent that has different qualities. The basic idea is that these processes can bring the films to different out-of-equilibrium states. Possibly because of irreversible adsorption, these states are long-lived and their diversified properties can be seen in the films. Our result thus points to the use of solvent quality to simply and economically modify the properties of polymer films. This MWN award has generated twelve refereed journal articles, one handbook entry and one PhD thesis. Below is a full list of these publications: Fujii, Y; Yang, ZH; Leach, J; Atarashi, H; Tanaka, K; Tsui, OKC, "Affinity of Polystyrene Films to Hydrogen-Passivated Silicon and Its Relevance to the Tg of the Films", Macromolecules, p. 7418, vol. 42, (2009). Tsui, OKC; Epps, TH, "Introduction", Journal of Polymer Science Part B: Polymer Physics, p. 2531, vol. 48, (2010). Clough, A; Peng, DD; Yang, ZH; Tsui, OKC, "Glass Transition Temperature of Polymer Films That Slip", Macromolecules, p. 1649, vol. 44, (2011). Raegen, A; Chowdhury, M; Calers, C; Schmatulla, A; Steiner, U; Reiter, G, "Aging of Thin Polymer Films Cast from a Near-Theta Solvent", Physical Review Letters, p. 227801, vol. 105, (2010). Fujii, Y; Yang, ZH; Clough, A; Tsui, OKC, "Shear Modulus of a Polymer Brush", Macromolecules, p. 4310, vol. 43, (2010). Yang, Z; Peng, D; Clough, A; Lam, CH; Tsui, OKC, "Is the dynamics of polystyrene films consistent with their glass transition temperature?", European Physical Journal Special topics, p. 155, vol. 189, (2010). Thomas, KR; Chenneviere, A; Reiter, G; Steiner, U, "Nonequilibrium behavior of thin polymer films", Physical Review E, p. 021804, vol. 83, (2011). Reiter, G; Napolitano, S, "Possible Origin of Thickness-Dependent Deviations from Bulk Properties of Thin Polymer Films", Journal of Polymer Science B: Polymer Physics, p. 2544, vol. 48, (2010). Li, RN; Clough, A; Yang, ZH; Tsui, OKC, "Equilibration of Polymer Films Cast from Solutions with Different Solvent Qualities", Macromolecules, p. 1085, vol. 45, (2012). Botiz, I; Schlaad, H; Reiter, G, "Processes of Ordered Structure Formation in Polypeptide Thin Film Solutions", in SELF ORGANIZED NANOSTRUCTURES OF AMPHIPHILIC BLOCK COPOLYMERS II, p. 117, vol. 242, (2011). Chowdhury, Mithun, "Thin Polymer Films Out of Thermodynamic Equilibrium", (2012). Thesis, Inaugural-Dissertation zur Erlangung des Doktorgrades der Fakultät für Mathematik und Physik der Albert-Ludwigs-Universität Freiburg Reiter, G. "Spreading of Liquids on Substrates", (2011). Book, Published, Editor(s): Lucas Filipe Martins da Silva, Andreas Ochsner, Robert Adams, in Handbook of Adhesion Technology, Springer (2011). Reiter, G., "Probing Properties of Polymers in Thin Films Via Dewetting", Advances in Polymer Science (2012). Clough, A.; Chowdhury, M.; Jahanshahi, K.; Reiter, G.; Tsui, OKC, "Swelling with a Near-Theta Solvent as a Means to Modify the Properties of Polymer Thin Films", Macromolecules, p. 6196, vol. 45, (2012). B. Broader impact outcome: The new knowledge generated in this MWN award opens up new possibilities to adjust the physical properties of polymer films by simple and economic means. In the education front, this MWN award has enabled the training of seven graduate students, two postdocs and two undergraduate students in the emergent area of polymer nanometer films. Since leaving the group, the two postdocs have become university faculties. One of the graduate students completed his PhD this August and was offered a postdoctoral position in Dublin, Ireland. Besides university personnel, this project had also involved three high school research interns from around the country, one in each summer during the life of this MWN award. Almost all the participants of this award had given presentations on their work at professional meetings. Among eight other lectures, the PI had given a four 1.5-hour lecture series in March 2012 to the students of Kyushu University, Japan on the Stability and Wetting Properties of Polymer Films. She also served as a co-editor for the Special Issue for the Division of Polymer of the 2010 American Physical Society March Meeting, published in the December 2010 issue of the Journal of Polymer Science B: Polymer Physics.
