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Reports: ND753456-ND7: Supramolecular Gels and Nanocomposites with Multiple Stimuli Responsive Behavior

Christopher J. Ellison, PhD, University of Texas at Austin

              Stimuli-responsive polymers are materials that can reversibly respond to one or more stimuli inducing large changes in their properties [1]. Due to their response to a variety of stimuli, these polymers have been proven useful in many applications including sensors, actuators [2], drug delivery systems [3], and re-healable materials [4]. The origin of most examples of stimuli-responsiveness can be attributed to specific interactions inherent to the material that enable it to dynamically change in a fast and reversible way. Example interactions that enable such extraordinary behavior include dynamic covalent bonds and various non-covalent bonds, such as hydrogen bonding, ionic interactions, and π- π interactions [5]. A major goal of our research is to synthesize a pyrene end-labeled telechelic polymer that has dual stimuli-responsiveness (both temperature and light stimuli). Such materials could be used as heat sensors or light repositionable adhesives. We hope to illustrate how significant changes can be obtained by nanoscale confinement of pyrene chain ends in a polydimethylsiloxane (PDMS) matrix; these pyrene chain ends consist of less than 1 mol % of the total polymer repeat units in the system. While the nanoconfined crystallization properties of similar materials have been studied, details of the responsiveness of their flow properties to heating/cooling cycles or different light exposure protocols, and how this is connected to their molecular architecture, is unknown. In this research, the end-groups of amino-propyl terminated PDMS starting material were fully functionalized with pyrene by straightforward chemistries. Figure 1 shows how the material is prepared in detail. All of the reactions 95%+ conversion as confirmed by proton nuclear magnetic resonance; the final product was also fully characterized with size exclusion chromatography, differential scanning calorimetry, and rheometry.   Figure 1. Reaction scheme for synthesizing γ-oxo-1-pyrenebutyric acid N-hydroxysuccinimide ester (top) and pyrene-end-labeled PDMS (bottom).   As shown in Figure 2, after developing adequate washing and degassing procedures, we were able to obtain a transparent stimuli-responsive pyrene end-labeled PDMS material that acts as a solid-gel at room temperature. This behavior is strongly dependent on the molecular weight of the material. For example, if the material has a lower molecular weight, the higher concentration of pyrene ends in the system allow it to have stronger and robust interactions producing a gel that does not flow.  In contrast, the lower concentration of pyrene ends associated with higher molecular weight polymer produces a material that is a viscous liquid due to fewer pyrene end group interactions. Figure1

Figure 2. Photographs of telechelic PDMS for comparison: a) Mw=5,000 g/mol (left) and Mw=25,000 g/mol (right) both show good transparency at room temperature and, b) Mw=5,000 g/mol (top) is a gel at room temperature, while Mw=25,000 g/mol (bottom) flows like a liquid.

 

  This difference becomes more obvious when comparing the rheological properties of materials with different molecular weights, as illustrated in Figure 3. Material with low molecular weight has a broad and distinctive phase transition in storage modulus over a wide temperature range from 20 °C to 60 °C, while the high molecular weight material shows no significant changes in storage modulus. This result is in good agreement with DSC results (not shown here) and it shows the significance of π-π interactions of pyrene ends in controlling the properties of the material. As illustrated in Figure 3, by substituting 1 mol% of PDMS end groups with pyrene moieties give rise to a tremendous reversible change in storage modulus with temperature, by more than 6 orders of magnitude. Such a result cannot typically be achieved by crystallization of polymer main chains, which makes this finding more unique and valuable.

Figure 3. Heating (unfilled) and cooling (filled) curves for storage modulus as a function of temperature. Heating curves start from the left and continue to the right, while cooling curves follow the opposite of the heating curves. Heat/cool rate: 5 °C/min, angular frequency: 10 rad/s.   In order to explore the stimuli-responsiveness, we modulated temperature while simultaneously measuring the rheology of the materials. Figure 4 shows an example of the stimuli-responsive behavior from a predesigned temperature program. By modifying the temperature program, one can obtain a significant amount of information, such as the importance of pyrene crystallization kinetics, the role of π-π interactions of pyrene ends, the effect of molecular weight, etc. We plan to submit an abstract to present these findings at the 31st International Conference of The Polymer Processing Society held in Jeju Island, Korea in June 2015. The immediate future research will focus on these types of experiments to develop a complete physical picture of the responsiveness to temperature. Then we will develop the understanding and capabilities for modulation by light. Figure 4. Stimuli-responsive behavior (bottom) for designated temperature program (top): a) Mw=5,000 g/mol and b) Mw=25,000 g/mol, where red line represents the storage modulus of the material while blue line represents the loss modulus. Angular frequency: 10 rad/s.   The American Chemical Society – Petroleum Research Fund has primarily supported one full time graduate student who is mainly working on this study for his doctoral degree. This has provided him an ability to deepen his understanding of polymer synthesis and characterization, and hone his skills with rheometry and other scientific instrumentation. He was also recently awarded the Graduate Dean's Prestigious Fellowship Supplement for his work. This PRF grant has also partially supported a postdoc and two other graduate students that have made important contributions. Finally, this research has spawned several completely new projects in the group involving π-π interactions that are focused on composite applications.     References [1] Tenhaeff, W. E.; Gleason, K. K. Chem. Mat. 2009, 21, 4323. [2] Palacios, M. A.; Nishiyabu, R.; Marquez, M.; Anzenbacher, P. Journal of the American Chemical Society 2007, 129, 7538. [3] Li, J.; Li, X.; Ni, X.; Wang, X.; Li, H.; Leong, K. W. Biomaterials 2006, 27, 4132. [4] Montarnal, D.; Tournilhac, F. o.; Hidalgo, M.; Couturier, J.-L.; Leibler, L. Journal of the American Chemical Society 2009, 131, 7966. [5] Wojtecki, R. J.; Meador, M. A.; Rowan, S. J. Nat. Mater. 2011, 10, 14.