Reversible volume transitions

For pH-responsive electrochemical systems, on the other hand, the electrical field indirectly influences the polymer conformation to induce a reversible volume transition. The electricity is used to electrolyze water, generating hydrogen ions at the anode and hydroxyl ions at the cathode. This produces a pH gradient across the solution, which can be used to influence the conformation of pH-responsive polymers. Strategic placement of the polymer in the solution can cause expansion, contraction, or even bending (when placed directly between the electrodes) of a polymer actuator. Switching the direction of the current creates the opposite effect. This second method of actuation... 

Minol II will exhibit a reversible phase transition beginning at 32°, producing volume changes of about 3.8%. These volume changes could causfe microcrystalline cracks and pores which could reduce detonation velocity and mechanical strength (Ref 36)... 

In the Cu(DCNQI)2 radical-anion salt in the neighbourhood of the Peierls phase-transition temperature, it was found that the insulating state can be switched optically on a time scale of less than 20 ps into the conducting state (F. O. Karutz, H. C. Wolf et al, Phys. Rev. Lett. 81,140 (1998)). This optical switching process was termed a reversed Peierls transition . From the current transients, it was found that the switched volume must be at least 100 times larger than the directly photo-... 

More recently we utilized the well-known temperature-induced volume phase transition properties of poly(N-isopropylacrylamide) (PNIPAM) (73-75) to create novel CCA materials with variable sphere size and variable array periodicity (76). In water below 30°C, PNIPAM is hydrated and swollen, but when heated above its lower critical solution temperature ( 32 C) it undergoes a reversible volume phase transition to a collapsed, dehydrated state. The temperature increase causes the polymer to expel water and shrink into a more hydrophobic polymer state. 

Choi SH, Lee JH, Choi SM, Park TG. Thermally reversible pluronic/heparin nanocapsules exhibiting 1000-fold volume transition. Langmuir. 2(X)6 22 1758-62. 

Lee and Hsu reported the swelling behaviour of thermosensitive hydrogel based on iV-isopropyl acrylamide-trimethyl methacryloyloxy ethyl ammonium iodide (TMMAl) copolymers, a nearly continuous volume transition and associated phase transition from low temperature, and a highly swollen gel network to high temperature, a collapsed phase near its critical point between 31 and 35 °C. The gel with little TMMAl content (Table 4.3) showed a good reversibility and, the higher the TMMAl content (2-6%) in the copolymer, the higher the gel transition temperature. 

---