Time and Temperature Dependent Recovery of Epoxy-Based Shape Memory Polymers

Abstract

Shape memory polymers (SMPs) are a group of adaptive polymers that can recover the permanent shape from a temporary shape by external stimuli on demand. Among a variety of external stimuli for polymer actuation, temperature is the most extensively used. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external deformation constraints, or free recovery, and the amount of the recoverable strain. This paper investigates the amount of the recoverable strain and the recovery rate of an epoxy-based SMP (Veriflex® E, VFE1-62 (CRG, Dayton, OH)) under different thermal conditions. In particular, the free recovery behaviors of the SMPs under two experimental protocols, isothermal and shape memory (SM) cycle, are studied. It is found that free recovery in isothermal experiments is much faster than that in a SM cycle at the same recovering temperature and the material is fully recoverable at the temperature above differential scanning calorimetry Tg. Furthermore, for the recovery in SM cycle experiments, reshaping the sample at a low temperature and recovering from the deformation at a high temperature yield the fastest recovery rate, while reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this work’s experimental time frame. The possible mechanism for these observations is discussed.

Recent progress in shape memory polymer: New behavior, enabling materials, and mechanistic understanding

“…The advantage of this solution is that all the parameters in the model can be measured by a few simple rheological experiments, such as DMA tests and stress relaxation tests. However, this model cannot consider the effects of programming temperature on the recovery, that can significantly affect the recovery behavior[229][230][231]309,310].More recently,Yu et …”

Mechanisms of the Shape Memory Effect in Polymeric Materials

“…Hereinafter, we ignore this effect and other minor factors, such as thermal expansion, in the discussion. Of course, time-related and rate-dependent effects, such as relaxation, creeping and strain rate effect, which are commonly observed in polymeric materials, can be included for a more precise investigation [76][77][78][79][80][81][82][83][84][85][86].…”

Reduced time as a unified parameter determining fixity and free recovery of shape memory polymers

“…Previous experimental work revealed complicated dependency of the SM performance on thermo-temporal conditions in an SM cycle. For example, the recovery of SMPs programmed under the same conditions strongly depends on the recovery temperature and heating rate in the recovery step 43,[46][47][48][49][50][51][52] ; in addition, the programming temperature has been shown to affect significantly the free recovery behaviour for SMPs under the same thermo-temporal recovery conditions 31,[53][54][55][56][57] . To date, because the recovery behaviour of an SMP depends on the aforementioned multiple thermo-temporal input parameters, there is no clear understanding how these input parameters can affect the SM performance individually or collectively.…”

“…The programming step includes thermo-temporal conditions such as programming (loading) temperature, loading rate, holding time after loading, cooling rates and shape fixing temperature; the recovery step includes heating rates, recovery temperature and holding time at the recovery temperature. The performance of a thermally triggered SMPs is typically quantified by shape fixity, shape recovery rate and ratio, shape recovery stiffness and stress, and so on [40][41][42][43][44][45][46][47][48][49][50] . Previous experimental work revealed complicated dependency of the SM performance on thermo-temporal conditions in an SM cycle.…”