Phase transition acceleration

Structural polymorphism has been already reported as a peculiar solid-solid phase transition with a large spectral shift in the cast film of CgAzoCioN+ Br (chapter 4). The type 1 spectrum was thermally transformed to the type VI spectrum and then backed to the type I by the isothermal moisture treatment. The reversible spectral change between the type I and VI is a good experimental evidence of Okuyama s prediction on the molecular packing. Since the type VI state is assumed to be a metastable state, the isothermal phase transition to the type I state is expected to be induced by some external stimuli. Water molecules adsorbed to cast bilayer films might act as an accelerator of the phase transition. 

The sample used to study the relationship between the volume phase transition and the frictional property is poly( /V-isopropylacrylamide) gel which shows a small discontinuous volume phase transition at 33.6 °C. The sample gel is prepared by free radical polymerization 7.8 g of re-crystallized N-iso-propylacrylamide (main constituent, Kodak), 0.133 g lV,iV -methylenebis-acrylamide (cross-linker, Bio-Rad), 240 ml tetramethylenediamine (accelerator, Bio-Rad), and 40 mg ammonium persulfate (initiator, Mallinckrodt) are dissolved in distilled water (100 ml) at 0°C. The gel mold is immersed in the pre-gel solution and then degassed for 40 min at 0°C. The temperature is raised to 20.0 °C after this treatment to initiate the gelation reaction. The sample gel thus obtained is homogeneous and transparent, at least by visual inspection. 

CuO, and PbO react at temperatures >600-700 °C quite strongly with the grain boundary phase and accelerate the oxidation and degradation. At temperatures below the transition temperature Tg of the glassy phase this interaction can be neglected because of the low ion diffusion into the grain boundary. 

Quartz undergoes a minor, rapid and reversible phase transition to a-quartz at 573°C. It is unstable relative to tridymite at 867-1470°C, and to cristobalite above 1470°C. These changes are slow in the absence of other materials, but many admixed materials accelerate them, and lead to formation not of tridymite, but of cristobalite at temperatures over about lOOO C. 

Experiments for investigating the lamellar-Hn transition kinetics have been performed, for example, on DOPE dispersions. The F,p-phase diagram of DOPE in excess water is depicted in Fig. 21. Figures 22, 23 show the diffraction patterns and lattice parameters at 20 °C after a pressure jump from 300 to 110 bar. Clearly, the (001) reflection of the L phase and the (10) reflection of the developing Hu phase can be identified. In this case, a two-state mechanism is observed. Interestingly, we find that successive pressure jumps lead to an acceleration of the phase transition kinetics. The half transit time decays from 8.5 s for the first pressure jump to... 

In the liquid, thermal decomposition and subsequent reactions, as well as phase transition, take place, generating gas bubbles and forming a two-phase region. The propellant then undergoes a sequence of rapid evaporation at the surface (Fig. 5d). Ignition occurs if the heat flux is sufficiently large to initiate the subsequent self-accelerated exothermic reactions which result in substantial heat release (in the gas phase) and emission of light. A luminous flame is... 

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