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43175-G7
Phase Behavior of Binary (Polymer-Supramolecular Polymer) Blends
The objective of this research is to explore how end-associating polymers can influence the phase behavior of binary (polymer polymer) blends. The scope of this research includes both the kinetics and thermodynamics of the mixing process. Understanding how supramolecular bonding in solution and the melt affects physical properties and the phase behavior of these hybrid systems is the key to reducing the high processing costs of commodity polymers and will promote new technologies such as thermoplastic elastomers and shape-memory polymers.
A mean-field model was developed to predict how polymer-polymer miscibility changes if polymers are functionalized with non-covalent, reversibly binding groups. In short, a Flory-Huggins free energy of mixing expression was coupled with a simple association model to account for equilibrium bonding of polymer end-groups. Model inputs include only the length of polymer chains, the strength of their interaction, and a temperature-dependent equilibrium constant. The model was applied to twenty-one different blend combinations involving both mono-functional and telechelic associating chains. Predicted phase boundaries shift according because donor-acceptor interactions can stabilize single phase regions. The model is a useful tool for understanding the delicate balance between the combinatorial entropy of mixing, repulsive interactions, and enthalpic changes due to end-group association.
Our experimental approach was to systematically blend telechelic supramolecular polymers with traditional linear polymers. The ureidopyrimidone (UPy) functional group was chosen as an end-group because of its high association constant (Ka typically exceeds 106 M-1 in chloroform) and its ease of synthesis.[1] Well-defined, mono- and telechelic- polymers are required to obtain meaningful results, and this has been a major part of our research focus.
We have synthesized a series of monodisperse hydroxy-terminated polystyrenes using copper-mediated atom-transfer radical polymerization (ATRP) followed by atom transfer radical coupling.[2] To ensure complete functionalization of chains, an ATRP initiator, 2-hydroxyethyl 2-bromoisobutyrate, was transformed to a UPy-functionalized initiator using isocyanate chemistry. Each initiated chain bears a single UPy group, and chains can be coupled using atom-transfer coupling. Other telechelic polymers containing ureidopyrimidone (UPy) end-groups have been synthesized including poly(butadiene), poly(ethylene oxide), and poly(dimethyl siloxane).
The presence of UPy-functional end groups clearly affects polymer-polymer miscibility. Polymer-polymer miscibility is studied using a custom-built temperature-controlled off-axis light scattering apparatus. Using this technique, the parent blend system of poly(butadiene)-poly(styrene) (PB-PS), was confirmed to exhibits UCST behavior, as reported in the literature. When either component is functionalized with UPy end-groups, miscibility is strongly reduced. Clearing points are above experimentally accessible temperatures. Similar observations were made for the poly(styrene)-poly(dimethylsiloxane) blends, although these results need to be confirmed. Interestingly, even when both components are functionalized, miscibility appears to decrease. This feature is not predicted by the free energy model.
It is difficult to study polymer-polymer blends that are too immiscible to observe clearing points. To circumvent this, we have begun to study polymer-polymer interactions that occur in solution. Our approach is to systematically change polymer-polymer interaction strength by varying total polymer concentration[3]. Additional experimental techniques to study melt and solution association include atomic force microscopy, vapor phase osmometry, dynamic light scattering.
The ACS-PRF type G award has enabled the PI and Mrs. Michelle Wrue, a PhD candidate, to conduct the above investigation. Funds have also been used to support an undergraduate summer research experience (Mr. James A. Viveros) and part of Mr. Jiahui Li's phase behavior studies. Lastly, funds have been used to acquire equipment and chemicals.
1. Sontgens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. Journal of the American Chemical Society 2000, 122, (31), 7487-7493.
2. Sarbu, T.; Lin, K. Y.; Spanswick, J.; Gil, R. R.; Siegwart, D. J.; Matyjaszewski, K. Macromolecules 2004, 37, (26), 9694-9700.
3. Haraguchi, M.; Inomata, K.; Nose, T. Polymer 1996, 37, (16), 3611-3614.
