Metastable polymer solution

It should be pointed out that the twisted conformation shown in Fig. 6.5 is a metastable state, which should only exist in the presence of a proper solvent. Once the solvent is removed, the polymer chains should spontaneously recover the more conjugated conformations, although such recovery is perhaps limited by the restricted motion of the polymer molecules in the solid state. This prediction is supported by the experimental observation that although the absorption Amax of the polymer solution in THF is significantly smaller than the Xmax in aromatic solvents,32 the films spun with aromatic and nonaromatic solvents have essentially the same kmax value. 

The effect of dissolved CO2 on the miscibility of polymer blends and on phase transitions of block copolymers has been measured with spectroscopy and scattering (40). The shifts in phase diagrams with CO2 pressure can be pronounced. Polymer blends may be trapped kinetically in metastable states before they have time to phase separate. Metastable polymer blends of polycarbonate (PC) and poly(styrene-cn-acrylonitiile) were formed with liquid and supercritical fluid CO2 in the PCA process, without the need for a surfactant. Because of the rapid mass transfer between the CO2 phase and the solution phase, the blends were trapped in a metastable state before they... 

Still, there is the most interesting phenomenon that the cationic polymer poly(iV-hexadecyl-/V,/V-dimethy-N-vinylbenzyl ammonium chloride) 28 exhibits very low reduced viscosities but does not show polyelectrolyte behaviour in aqueous solution [103, 292] the plot of reduced viscosity vs concentration is strictly linear, and is insensitive to added salt (Fig. 20). Importantly, this head type vinyl polymer without main chain spacer is not water-soluble and thus not a true polysoap, but forms only metastable aqueous solutions (see Sect 2.2.4). Similar results were reported for alkylated poly(vinylimidazoles) such as 26 [347], It may be speculated that such solutions represent rigid molecular latexes rather than flexible polymeric micelles , and further studies on such systems would be most interesting. 

For amorphous polymer systems, liquid-liquid-phase separation occurs by nucleation and growth of the polymer lean phase when the him composition enters locally the metastable region between the binodal and the spinodal (unstable with respect to l-l demixing). The unstable polymer solution... 

Fig. 4 Schematic phase diagrams of a polymer solution showing LL phase separation with UCST behavior. Curve s is the spinodal, curve b is the binodal, and curve g is the glass transition temperature as a function of polymer concentration. BP indicates the Berghmans point, (a) LL phase separation is the only thermodynamic transformation of the system [17,25, 36]. (b) Curve c shows the crystallization temperature of a polymer fully miscible in a solvent as a function of concentration in the solution [17, 25], The LL phase coexistence curve (combined with vitrification) is a (classical) metastable process that lies beneath the crystallization curve c. In route 1, a polymer solution is supercooled at ALj, and the only active process is polymer crystallization. In route 2, the initially homogeneous solution is supercooled to a larger undercooling than namely AL2. Crystallization may compete either with LL phase separation when reaching point C, or LL phase separation coupled with vitrification when reaching point D. At C, crystallization may take place in the polymer-rich phase. At D, both LL phase separation and crystallization may become arrested by vitrification...

Recent studies have shown that incorporation of boron element into sihcon-based ceramics increases their thermal stability and retard crystallization [202-204]. For example, the materials of the binary system Si—N start to crystallize at 7 =1,000 °C forming a-Si3N4, while metastable solid solutions of the ternary and quaternary systems Si—C—N and Si—B—C—N withstand crystallization up to 1,450 and 1,700 °C, respectively [205]. In order to form an amorphous uniform phase in the final multinary ceramics, the ceramic elements are preferably distributed homogeneously in the preceramic polymers. The general consensus in the ceramics community is that the quaternary system Si—B—C—N as well as the ternary systems Si—B—N and Si—B—C would be particularly suitable for producing amorphous ceramics that resist the microstructural changes even at top loads. 

Fig. 10.28 Schematic illustration of the metastable (nucleation and growth) and unstable (spinodal decomposition) domains in polymer solutions and consequences of pressure quench at different polymer concentrations (Courtesy Online resources)...

The line on the temperature vs. composition phase diagram for a mixture of two components, which separates the metastable region from the single-phase regions. Hence, it represents the limits of stability in a two-phase system, viz., a polymer solution or polymer blend. 

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