By Stephen Z.D. Cheng
A classical metastable country possesses a neighborhood unfastened power minimal at limitless sizes, yet no longer a world one. this idea is part dimension self sustaining. now we have studied a few experimental effects and proposed a brand new idea that there exists a variety of metastable states in polymers on diverse size scales the place their metastability is seriously made up our minds by way of the part measurement and dimensionality. Metastable states also are saw in section adjustments which are kinetically impeded at the pathway to thermodynamic equilibrium. This used to be illustrated in structural and morphological investigations of crystallization and mesophase transitions, liquid-liquid section separation, vitrification and gel formation, in addition to mixtures of those transformation procedures. The section behaviours in polymers are hence ruled via interlinks of metastable states on various size scales. this idea effectively explains many experimental observations and offers a brand new option to attach diverse points of polymer physics.
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Additional info for Phase Transitions in Polymers
Many of the phase transitions are first-order transitions. Some of the liquid crystalline–liquid crystalline phase transitions are second-order transitions. Detailed theoretical descriptions and experimental overviews of liquid crystalline transitions can be found in the Handbook of Liquid Crystals (Barois, 1998; Demus, 1998). An extensive collection of experimental data on the phase transition properties involving liquid crystals, such as transition temperatures and changes of volume, enthalpy and entropy at the transition temperatures, can be found.
One example is fullerenes, such as C60. The transition entropy of this fullerene from the crystal to the plastic crystal phase is attributed to orientational disorder while the positional order is maintained. 3 J/(K mol) change in entropy at À14 C. This value of entropy change fits Walden’s rule (Wunderlich and Jin, 1993). 6 Schematic illustration of the temperature^concentration phase diagram for cyanohexyl-oxybiphenyl (A) and cyanooctyl-oxybiphenyl (B) mixtures at atmospheric pressure. The slope of the concentration change with respect to temperature is positive for the upper half of the phase stability boundary line, and it is negative in the bottom half of the line [replotted from Guillon et al.
Partitioning the thermodynamic contributions of these two dissimilar parts needs to be taken into consideration. When the flexible tail is constructed from methylene units, this part also contributes to the change of entropy during liquid crystalline Thermodynamics and Kinetics of Phase Transitions 27 phase transitions. , 1996a,b). Therefore, the partitioning of the changes in volume and entropy of the mesogen and the flexible tails depends on the composition of the tails. Efforts to figure out the appropriate partitioning have been made based on density measurements (Guillon and Skoulios, 1976a,b), but systematic investigations have not been completed.