New Design of Shape Memory Polymers: Mixtures of an Elastomeric Ionomer and Low Molar Mass Fatty Acids and Their Salts

ReceiVed January 24, 2008 ReVised Manuscript ReceiVed March 24, 2008 Introduction. Shape memory polymers (SMP) are materials that can change shape when exposed to an external stimulus, such as temperature. SMPs have a permanent shape that is provided by a cross-linked network, but they can be deformed above a critical temperature (Tc) of a second, reversible network and fixed into a temporary shape when cooled under stress to below Tc. When reheated above Tc without stress, the material assumes its permanent shape. SMPs have applications as medical devices (e.g., stents and sutures), actuators, sensors, artificial muscles, switches, smart textiles, and self-deployable structures. All previous work on SMPs has involved synthesizing new polymers or modifying existing polymers, though many applications include fillers and additives to adjust the modulus and/or strength of the material. The permanent networks are either covalently cross-links or physical cross-links with sufficiently long relaxation times that, mechanically, the “bonds” behave as permanent above Tc. The temporary networks typically rely upon vitrification, melting, or some other physical interaction such as nanophase separation that is reversible with temperature (or another external stimulus, such as photosensitivity). There are currently four classes of SMPs: (1) covalently cross-linked glassy polymers, (2) covalently cross-linked semicrystalline elastomers, (3) physically cross-linked glassy polymers, and (4) physically cross-linked semicrystalline block copolymers. Ionomers possess a strong physically cross-linked network due to nanophase separation of ion-rich domains that can persist to relatively high temperatures. Ionomers have been used to design SMPs, but either the shape memory properties were poor or the ionomer was used primarily to vary the stiffness of the hard segments or melting point of a polyurethane. This paper describes a new type of SMP based on blends of an elastomeric ionomer and low molar mass fatty acids or their salts. Nanophase separation of the ionomer was used to develop the permanent network, and fatty acids (salts), FA, were used to produce a secondary network where Tc was related to the melting point of the FA, Tm. Experimental Details. Sulfonated EPDM (SEPDM) is a thermoplastic elastomer that was developed by Exxon in the mid-1970s. It possesses properties characteristic of a crosslinked rubber but exhibits viscous flow at temperatures >150 °C. Poly{ethylene-r-propylene-r-(5-ethylidene-2-norbornene [ENB])}, EPDM (Royalene 521: Mooney viscosity ) 40 (ML (1 + 4)/100 °C) and composition of 49% ethylene, 46% propylene, and 5% ENB), was obtained from Crompton Chemical Co. The zinc salt of sulfonated EPDM (ionomer) with a zinc sulfonate concentration of 0.03 mequiv/g was prepared by sulfonating EPDM with acetyl sulfate (see Scheme 1) and neutralizing the product with zinc acetate. Although SEPDM can flow at elevated temperatures, the melt viscosities are too high to be practical for any conventional melt-processing operation, e.g., extrusion or injection molding. This problem can be resolved by adding a polar plasticizer such as zinc stearate (ZnSt), which not only lowers the melt viscosity but when the compounds are cooled to below 100 °C, the ZnSt crystallizes and acts as a reinforcing filler. Zinc stearate (ZnSt)/ionomer composites were prepared by dispersing the ZnSt in a solution of the ionomer, flashing off the solvent with steam, and drying. Film samples were compression-molded at 200 °C. Shape memory cycles to assess fixation and recovery of the SMPs were measured with a TA Instruments dynamic mechanical thermal analyzer (DMTA) 2980 using the tension mode and a frequency of 1 Hz.