PHASE TRANSI TION

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

One recent trend is the use of DSC in combination with other experimental techniques, which include X-ray crystallography and various spectroscopic methods, such as IR, UV-Vis, circular dichroism, and and C NMR spectroscopy (presented in Chapters 7, 9, and 10). Simultaneous analysis of complexation behavior using different methods obviously has a lot of advantages. Indeed, with the help of DSC one can obtain direct information about the thermal stability/phase transi-tion(s) of CyD complexes (including those formed with drug molecules). Furthermore, DSC in combination with crystallographic and spectroscopic methods provides us with definitive experimental evidence for the formation of CyD-drug complexes and their stoichiometry and structure in the solution and solid states. 

Figure 3 Effect of heating (open points)-coolmg (filled points) cycle on ultrasonic velocity in a 20% hexadecane-in-water emulsion (adapted from Ref. 23). The speed of sound in the emulsion decreases with temperature and there is an abrupt change corresponding to the phase transi tion in the droplet oil. Supercooling of the liquid oil is responsible for the hysteresis loop observed.

Straumanis, ME., Vora, P.M., Khan, A. A. ACA Meeting "Mechanism of Phase Transi-tions" Columbia South Carolina USA, Jan. 31-Feb. 4, 1971, p. 36 (Abstract). 

PHASE TRANSI TIONS OF POl.YMFR SOFI A IONS... 

Figure 15. He and Ar concentrations are shown as a function of He/ Ar in the Palfris Marl (after BaUentine et al. 1994). Near-constant Ar concen-trations (all variation in [ Ar J is within 2a of the mean dashed line, top figure) are found in fluids trapped in vugs within the Palfris marl and in free borehole fluids from the same horizon. Stable isotopes and a constant Ar show that these differ-ently sited fluids are derived from the same source. In contrast, He concen-trations are lOx higher in the free fluid than that trapped in the inclusions (lower figure) are likely due to addi-tion of excess " He to the free fluid. This is explained by He, but not Ar, diffusive loss into the fracture gas phase from the host marl. The transi-tion to high He/ Ar ratios has occur-red subsequent to the fluid formation and entrapment in the inclusions, at below 250°C and 6- to...

Nam] Nam, H.D., Kim, E.C., Han, J.S., Mossbauer Study of Iron Sulfides Doped with 3d-Transi-tion Metals , Solid State Commun., 135(5), 327-329 (2005) (Experimental, Crys. Structure, Electronic Stracture, Phase Relations, 8)... 

An interesting variation on the pure block copolymer is the tapered block copolymer, which is formed by copolymerizing two monomers which enter the polymer chain at very different rates. If A enters the chain faster than B, the first part of the chain will be almost polyA. As A is consumed, and the B/A ratio rises, B will enter the chain occasionally, and with increasing frequency, forming random copolymer of decreasing A/B ratio. Toward the end of the reaction, with very little A left, the last part of the chain will be almost polyB. While this does not fit the theoretical model nearly as well, experimental results sometimes suggest that such tapered block copolymers may actually be even more effective compatibilizers than pure block copolymers [9]. The explanation may lie in their ability to form a broader interphase, with a gradual transi-tion/modulation between the two pure phases. 

---