Reversibly associating polymers containing strong, multiple hydrogen bonding groups, have been a popular research topic for over a decade. Functional groups, including ureidopyrimidinones, oligoamide duplexes, and nucleobase pairs offer great potential in the areas of thermoplastic elastomers, blend compatibilizers, and supramolecular block copolymers. The majority of these studies have involved end-group and side-group functionalization of linear polymers. Our focus has been to attach reversibly associating groups onto classical network polymers in the rubbery state. Specifically, via free radical copolymerization, slightly crosslinked poly(n-butylacrylate)s containing quadruple H-bonding ureidopyrimidinone (UPy) side-groups were synthesized. Resulting elastomeric networks contain both covalent and dynamic non-covalent crosslinks, and this unique architecture affects viscoelastic behavior and permeation properties.

Several elastomers were synthesized, containing, for example ~2 mol % of UPy side groups, 1.5 mol % of a trifunctional crosslinker, and the balance poly(n-butylacrylate). These materials exhibited clear shape-memory effects. A tensile load was applied to samples at elevated temperatures (~65 °C), and the material behaves elastically. If cooled, while loaded, the deformed state can be stabilized, through hydrogen bonding, until subsequent reheating. This phenomenon is repeatable through several shape-memory cycles. Data analysis shows that about 90% of mechanical deformation can be stabilized and the fraction of shape recovery is nearly 100%. To our knowledge, this is the first example where reversible, hydrogen bonding groups are used to stabilize mechanically deformed states. DSC experiments show no glass transition and no indication of melting, which supports the hypothesis that UPy H-bonding is responsible for mechanical stabilization. Unlike the conventional shape-memory polymers, these UPy-containing elastomers do not possess a well defined shape recovery temperature. Instead, the recovery rate of synthesized network polymers strongly depends on temperature. To further study the dynamics and temperature dependence of mechanical relaxation, isothermal creep experiments and dynamic mechanical analysis were performed. Creep data (elongation versus time), were acquired at several different temperatures, and were fit using a simple viscoelastic model. The fit viscosities exhibited an Arrhenius-like temperature dependence with activation energies ~ 90 kJ per mol, which is comparable to the activation energies of dissocation measured in solution¹.

Solvent swelling UPy-containing elastomer films was studied gravimetrically. Diffusion through swollen films was investigated using a custom-built dye diffusion cell apparatus. This setup contains a water jacketed diffusion cell, a photo-detector, and a temperature controller. Dye is introduced into one chamber, and the rise of dye in the other chamber is monitored against time. We will discuss how diffusion depends on temperature, network architecture, solute size, and the interaction between the solute and the network. Membranes with high temperature-sensitive diffusion properties may be useful in applications such as transdermal drug delivery, microfluidics, or liquid chemical separation processes.

1. Wubbenhorst, M.; van Turnhout, J.; Folmer, B. J. B.; Sijbesma, R. P.; Meijer, E. W. IEEE Trans. Dielectr. Elecktr. Insul. 2001, 8, (3), 365.

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