Chung and coworkers (39) reported surprising reversible actuation for semicrystalline and covalently
cross-linked poly(cyclooctene) (38) when thermally cycled about the melting transition
under a constant tensile load. The researchers observed that crystallization of this polymer under
an applied tensile stress of ∼1.0 MPa resulted in an incremental tensile strain above that of the
isotropic elastic strain in the range of 20% to 40%. Upon heating at the same load, this incremental
strain was reversed nearly completely (>98%) as the melting transition was surpassed.
Dispersing the same materials with a fluorescent, chromogenic oligo(p-phenylene vinylene) dye,
Kunzelman et al. (8) revealed reversible color changes during one-way SM cycling about the materials
melting transition. Using large, nonpolar (–C18H37) endgroups for the dye design imparted
it with significant solubility; however, the solubility was restricted to the amorphous phase whose
volume fraction decreased during crystallization. Consequently, the same phenomena responsible
for shape fixing—namely, crystallization—caused dye aggregation and excimer formation so that
the fluorescing color of the polymer under UV illumination dramatically changed from green to
orange. So, transient strain fixing and color change occurred simultaneously. Conversely, heating
such samples above Tm led to strain recovery together with a fluorescent color change back to
green, with seemingly high spatial resolution.