Network architecture and shape memory behavior of cold-worked epoxies

Abstract

The shape memory behavior of polymers derives from a combination of their molecular architecture and thermomechanical history. In this study, several epoxies with various network architectures were prepared using mixtures of a diepoxide resin, a monoepoxide resin, and an aliphatic diamine hardener. A nonconventional cold-working programming, carried out below T-g, was employed to set the materials in a temporary configuration and allowed to fix considerable amounts of the applied strain. The shape memory behavior was evaluated through transient heating and isothermal recovery tests. All the resins are capable of complete recovery, which occurs as a sequence of an early process taking place below T-g and a major one close to T-g, which acted as the switching temperature (T-switch). The proximity of the deformation temperature to T-g influenced the amount of strain recovered within each process. It was shown that resins with different structures, although presenting similar T-switch, may have different recovery kinetics, and the roles of the network density and the chain stiffness on the recovery rate were evidenced

Effect of the Network Structure and Programming Temperature on the Shape-Memory Response of Thiol-Epoxy “Click” Systems

“…The excellent mechanical properties found between T g E 1 and T g + 20 broaden the applicability of our materials, since T prog not only modifies the mechanical response, but also affects the SMR [41]. In this sense, it is possible to obtain materials with specific SMR maintaining and excellent mechanical response.…”

“…Furthermore, when programming at Tg, the σb (over 23 MPa) is still higher than those reported by the mentioned authors [10,14,15] and εb is considerably high (up to 82%). The excellent mechanical properties found between Tg E′ and Tg + 20 broaden the applicability of our materials, since Tprog not only modifies the mechanical response, but also affects the SMR [41]. In this sense, it is possible to obtain materials with specific SMR maintaining and excellent mechanical response.…”

“…Furthermore, the differences in Rr between the different formulations when they are programmed at Tg E′ are higher than when they are programmed at Tg + 20. According to Pandini et al [41], loading at a temperature below the Tg involves more energetic changes (elastic and hardening processes) while loading at a temperature above the Tg involves mostly entropic changes. The energy lost during the loading process is higher when the loading involves more energetic molecular changes (at Tg E′ ) [42] because of molecular friction.…”

Recent advances in shape memory epoxy resins and composites

“…It is noteworthy that the R f data for the "cold worked" EPs were somewhat below those of the traditionally deformed (i.e., T trans > T g ) counterparts. A further important conclusion of Pandini et al [19] was that the SM recovery behavior is governed by the cross-linking density and the stiffness of the segments in between the net points plays a minor role. There are two possibilities to produce multi-shape SMEPs using neat EP resins: (i) exploitation of the "width" of the T g range and (ii) preparation of layered systems composed of two or more EPs with different T g s. The former approach assumes that the T g range can be split into several sections in which a fraction of the segments is in the rubbery while the others in the glassy state.…”

“…Understanding the coupling between cooling (shape fixing) and heating rates (shape recovery) and the evolution of the recovery response is a key issue for many applications. This aspect was addressed in a recent in-depth study by Pandini et al [19]. The cited authors studied the effects of network architecture of the SM behavior of SMEPs under both dynamic and isothermal recovery conditions.…”