CONICET | Buscador de Institutos y Recursos Humanos

Thermo-responsive shape memory polymers (SMPs) are polymeric materials which have the capability of changing their shapes from a temporary shape to a permanent shape upon application of an external thermal stimulus (1). They attracted great attention of scientists and engineers due to the novel capacity to be manipulated into one desired shape and then recover to another desired shape when temperature varies from below to above the transition temperature. Shape memory provides great potential for applications in sensors, actuators, packaging, medical materials, etc. In this work, we study new SMPs based on an epoxy resin, crosslinked with a mixture of a bi-functional and tetra-functional amine, such that the glass transition temperature (Tg) can be tuned by modifying the composition. The polymer was obtained by polymerizing a di-epoxy monomer based on diglycidyl ether of bisphenol A (DGEBA, DER 332, Dow), with a mass per mol of epoxy groups equal to 174.25 g/mol, and a mixture of dodecilamine (DA, Fulka) and Meta-xylylene-diamine (MXDA, Aldrich), such that stoichiometry between epoxy and amine groups is respected. As the DA becomes easily carbonatated when stored, it was heated to 130ºC for 30 minutes immediately before use (in this conditions carbonatation is reverted). Polymerization kinetics was analyzed by differential scanning calorimetry (DSC, Perkin Elmer Pyris 1). The curing reaction of DGEBA with DA is faster than with MXDA. From the analysis of the kinetics, the heating cycle selected to synthesize the materials was: 60 minutes at 60ºC, then 120 minutes a 100ºC. The first step at low temperature was used to avoid evaporation of DA, the second step was used to react completely the MXDA, in a reasonable time. Table 1. Variation of Tg for materials with different amounts of DA. DA fraction 0 0.5 0.667 1 Tg (ºC) 120 60 34 15 The variation of Tg with the ratio DA:MXDA was analyzed by DSC. If we consider potential applications of this material as a thermo-responsive SMP, the glass transition determines the range of temperature in which in can be used. Tg varied from 15ºC for the material synthesized with pure DA, to 120ºC for the material with pure MXDA. Table 1 shows the Tg for different amounts of DA (defined as the ratio equivalents of DA/equivalents of total amine).  The original sample was subject to the following cycle. Heating at 100ºC, manual deformation, cooling at room temperature, heating at 100ºC, without external deformation, for 1 minute, cooling at room temperature . It can be seen that the original shape is recovered, and the recovery is fast. This cycle was repeated several times, obtaining the same results.