The behavior of ultrathin polymer films below the glass temperature is not well understood despite its importance in micro- and nano-scale engineering of polymeric structures. Research is to be performed to understand the physical aging response of ultrathin polymer films as a function of temperature and of film thickness for several different polymeric materials. The approach includes making measurements using a novel bubble inflation method of measurement developed in the labs of Texas Tech University (TTU) to perform equi-biaxial and nonequibiaxial deformation geometries. These will be the first physical aging measurements on freely standing ultrathin polymer films. In addition, the work adapts a unique dewetting method in order to perform physical aging experiments below but near to the glass transition for polystyrene films. These dewetting experiments provide completely different film constraint than do the nanobubbles and comparison of the results between the two methods provides a strong challenge to both techniques that are nominally similar, but in the literature have given different and, to-date, unreconciled results. The work also directly probes the dynamics of confined polymers and addresses unreconciled mesoscale vs. nanoscale aging responses that have been reported in the literature. Finally, material failure at the nanometer size scale has been little investigated and the TTU bubble inflation technique will be exploited to examine the rupture behavior of ultrathin polymer films.
NON-TECHNICAL SUMMARY: Polymers are widely used in micro-, now becoming nano-, electronics applications. Because it is observed that polymer material properties change when feature sizes are smaller than 100 nm, this makes design and prediction of the polymer behavior at the nanoscale difficult for the electronics developer and designer. Similar effects occur in nanocomposites and other futuristic applications of polymers at the nanoscale. The present work is designed to put two different methods of measurement of nanoscale properties into direct confrontation in order to reconcile differences of nanoscale properties that have been reported in the literature. One method is the TTU nanobubble inflation test developed by the principal investigator in Lubbock, TX and the other is a nanofilm dewetting method developed by researchers at the E.S.P.C.I. in Paris, FR. The outcome of the work will provide highly important insights into the reasons for similarities and differences in reported material behaviors at the nanometer size scale. In addition to the technical work the research is to be carried out by graduate students in the Chemical Engineering Laboratories at TTU and will provide opportunities for two students to be trained and to achieve the bulk the research education to obtain their Ph.D. degrees. Finally, the results of the research will be widely disseminated through journal publication and presentations by the PI and students at national meetings.
We have made important contributions to the physics underlying the viscoelasticity of ultrathin polymer films and we developed a novel experimental method in the field. The broader impacts included training of graduate students in a way that impacted materials physics outside of discipline. There were also significant outreach activities during the project in which students were engaged in K-12 STEM related activities. Each of these aspects is described below. Viscoelasticity in ultrathin polymer films Two major discoveries were made in the field of the behavior of ultrathin polymer films. First, we made measurements of the viscoelastic behavior of polycarbonate ultrathin films using a nanobubble inflation method we had previously developed. Measurements were made on films as thin as 3 nm and we discovered that the glass transition temperature Tg of polycarbonate at this thickness was reduced by 122 oC relative to the bulk material (Figure 1). This is a "record" in terms of the reduction of Tg in nano-confined materials and has important implications for the use of polymers at the nano-scale because the glass transition temperature Tg determines the use temperature. The second discovery was in investigations of the dewetting of ultrathin films from liquid glycerol and from the surface of an ionic liquid. While we were not the inventors of the technique (credit goes to researchers at the E.S.P.C.I. in Paris, FR), we did make measurements on films thinner than previously reported with the technique, i.e., to 4 nm in thickness. These measurements indicate that the surface energy is extremely important in determining the magnitude of the Tg reduction in "supported" thin films. The ionic liquid showed a stronger effect than did the glycerol (Figure 1). This opens a new research area that is fundamental to the understanding of mobility and confinement effects on the glass transition behavior of polymers. Novel experimental technique The liquid dewetting method was modified in our labs by using a step-wise temperature profile to investigate the ultrathin films. Because the glass transition of the ultrathin films depends on thickness, the dewetting behavior changes as the film thickens. When the film thickens enough that it "revitrifies" it quits dewetting. The plateau in dewetting thickness vs. time determines the Tg at the specific thickness and temperature at which dewetting ceases. Consequently, a single experiment determines the glass transition as a function of film thickness over a range of thicknesses and a range of glass transition temperatures. Figure 2 gives results for a polystyrene polymer dewetting from either glycerol or an ionic liquid. The steps occur when temperature increases and the plateaus occur when the film revitrifies as it thickens. (Also Figure 1). Impact outside specific discipline At Texas Tech we admit students to the Ph.D. program through a qualifying examination in which they resolve an open ended question. If they do very well at this, they frequently publish a paper describing their findings. The work sometimes leads to novel outcomes beyond the original intent of the question. In the case of the present grant, one student had a very successful qualifying exam in which she was asked to address the question of diverging time-scales in glasses below their glass transition temperature. This is a fundamental question that has plagued researchers for many decades. However, the "geologic age" problem in glasses makes measurement of the dynamics far below Tg virtually impossible and the limitations of conventional aging experiments makes it obvious that another method than laboratory aging is necessary to address the issue of time-scale divergence very far below the Tg. To address this problem we chose a 20 million year old (Cenozoic) amber and were able to show that as much as 42.6 oC below the glass transition temperature, there is no divergence in the dynamics. This result challenges the classical paradigms of an "ideal" glass transition and the often received wisdom that time-scales in glass-forming materials diverge at temperatures above absolute zero. Such a finding, if further substantiated, has important implications for the field of glass physics. Outreach The graduate students, in addition to their research, participated actively in the Texas Tech STEM outreach program referred to as Science-It’s a Girl Thing (SIGT). SIGT is aimed at girls from the 5th to the 11th grades and is a 3 night and 4 day residence camp with the goals to 1) Provide girls with strong role models, 2) Spark interest in science, 3) Dispel myths and misconceptions about science and careers in science and 4) Introduce under-represented girls to a collegiate experience. The students are given hands-on experiments under the graduate student and faculty guidance. Figure 3 shows an SIGT class.
