Shape-Memory Behavior of a Polyethylene-Based Carboxylate Ionomer and Compounds Containing Zinc Stearate
Shape memory polymers (SMP) are materials that can change shape when exposed to an external stimulus, such as temperature. Thermally-actuated shape memory polymers can be deformed when heated above a critical temperature (Tc) of a reversible network, and then fixed into a temporary shape when cooled down under stress below Tc. When the material is reheated above Tc, the reversible network disappears and material recovers its original, permanent shape. The permanent shape of an SMP can be provided by chemical crosslinks or physical crosslinks that persist above Tc. The reversible, temporary network is created by physical crosslinks or hard domains that vanish above Tc. The unique properties of SMPs can be used in various applications, such as intelligent packaging, reconfigurable tooling, aerospace systems, biomedical devices, artificial muscles and self-deployable devices. Shape memory polymers were prepared from Surlyn® 9520, an poly(ethylene-co-methacrylic acid) ionomer and its blends with Zinc Stearate (ZnSt). Surlyn® 9520 is a semicrystalline ionomer with a broad melting point in the range 60-100 °C, physical cross-links in the ionomer due to interchain ionic interactions provided a “permanent” cross-linked network, while its crystals provided a temporary network. Although the ionic associations within the ionomer can be used as the permanent network, the critical role of the ionomer is to facilitate dispersion of the FAS and to provide a complementary functionality to the matrix polymer that stabilizes the FAS dispersion and develops the strong intermolecular interactions. Broad melting point due to the different size of crystals in the ionomer that melt at different temperature allowed demonstrating the tenability of shape memory effect in this ionomer. A separate route of achieving shape memory properties of the samples by blending the ionomer with ZnSt was shown. The strong dipolar interactions between the ionomer and a dispersed phase of crystalline ZnSt can also provide a temporary network. A temporary shape was achieved and fixed by heating and deforming the sample above the melting point (Tm) of the ionomer (or ZnSt) and then cooling down the material below Tm under stress. The original shape was restored by reheating the sample above the Tm of the ionomer (or ZnSt). Two objectives were pursued: 1) ascertain whether PEMA is a SMP and 2) whether multiple-way shape memory can be achieved with PEMA or with a composite of PEMA and a FAS, specifically ZnSt. We focused on SMP developed from methylene/methacrylic acid ionomer and its blends with ZnSt.