The network density as tailoring parameter for the two-way shape memory response of crosslinked poly(ε-caprolactone)

Abstract

The "two-way" shape memory response of semicrystalline networks was studied on poly(epsilon-caprolactone)-based systems, crosslinked by thermal curing of methacrylic end-capped linear chains. By changing the methacrylation degree of the precursors, it was possible to vary the network density over one order of magnitude, without any remarkable change in their transition temperatures and crystallinity content. When subjected to a constant stress and to a cooling-heating cycle from above T-m to below T-c, the materials display reversible two-way shape memory capabilities, consisting in a cyclic elongation-contraction effect, which involves significant variations of strain. Two different cooling induced elongational processes are evidenced, one due to entropy elasticity and the other to a crystallization driven effect. The amount of elongation that may be achieved depends on the network density and on the applied stress, and it is maximized for systems with a crosslink density that allows to exploit both the entropy-and the crystallization-induced effect

The two-way shape memory behaviour of crosslinked poly(ε-caprolactone) systems with largely varied network density

“…The elongation taking place within the cooling stage displays different increasing rates depending on the temperature region: a less steep strain increase is found on the rubbery region, whereas a steeper increase in strain, evidenced as a sigmoidal increase, is displayed at temperature below 40°C, in proximity to the crystallization region. This suggests that the whole elongation originates from two distinguished mechanisms, which, on the basis of previous observation on similar PCL-based networks (Pandini et al, 2012, 2013, 2014a, 2014b), are interpreted as follows. The less steep elongation process may be ascribed to an entropy elasticity effect: in fact, since the stiffness of a rubbery material decreases with temperature, the application of a fixed stress leads to an increase in strain as the cooling proceeds within the rubber-like behaviour region.…”

“…This suggests that the whole elongation originates from two distinguished mechanisms, which, on the basis of previous observation on similar PCL-based networks (Pandini et al, , 2013(Pandini et al, , 2014a(Pandini et al, , 2014b, are interpreted as follows. The less steep elongation process may be ascribed to an entropy elasticity effect: in fact, since the stiffness of a rubbery material decreases with temperature, the application of a fixed stress leads to an increase in strain as the cooling proceeds within the rubber-like behaviour region.…”

“…Furthermore, the significant changes in the crystallinity content and the transition temperatures, which accompanied the variation of the molecular architecture, prohibit a straightforward comparison of the two-way shape memory response of the systems investigated and a clear understanding of the driving force of this cyclic process. To provide a larger variation in the crosslinking text, a strategy successfully proposed by Li et al (2011), consisting in reducing the gel fraction content, was employed to prepare novel materials which preliminary results have revealed capable of shape memory response (Pandini et al, 2014a). In this work, the two-way shape memory behaviour of these PCL-based networks is discussed, at the light of their mechanical properties and structural evolution, and the enhancement of their elongation capabilities is highlighted.…”

A polycaprolactone‐based syntactic foam with bidirectional reversible actuation

“…Polycaprolactone is known to display 2W-SME. [50][51][52][53][54][55][56][57][58][59] It is interesting to know the 2W-SME of the cPCL syntactic foam because of the better thermal stability and lower cost of the composite. The two-way shape memory performance of the cPCL syntactic foam was studied using DMA by tuning temperature ranges, external loads and tension programming levels.…”

“…Among these polymers, PCL was widely studied for its 2W-SMEs due to its potential bio-applications. [50][51][52][53][54][55][56][57][58][59] Mather et al…”