Phase separation, polymer mixtures temperature

Such phase-separated polymer mixtures serve as an excellent model system where molecular-level interactions between components are small. The system studied here is a film made of an immiscible binary mixture of atactic polystyrene and low-density polyethylene [2], The dynamic IR measurement was carried out as before by mechanically stimulating the system at room temperature with a 23-Hz dynamic tensile strain with an amplitude of 0.r/>. The time-dependent fluctuations of IR absorbance induced by the strain were recorded at a spectral resolution of 4 cm. ... 

In this paper the discussion was concentrated mainly on the phase separation in polymer solutions due to the change of the composition of the mixture. The basic relations are also valid for phase separations induced by temperature changes, that is thermal gelation and can be applied to glass and metal alloys as well as to polymers. 

Similar to the fluorous biphasic concept, a system was developed that formed a homogenous phase at elevated temperature, but phase-separated at room temperature [49c]. It was found that a mixture of functionalized PNIPAM polymer, ethanol, heptane and water exhibited these properties. The hydrogenation of 1-octadecene and 1-dodecene using a phosphine functionalized PNIPAM with a rhodium precursor were taken as test reactions and the high activity was foimd was similar to that of RhCl(PPh3)3. At room temperature the mixture phase separated and the catalysis stopped since the catalyst is completely insoluble in heptane. The substrate is dissolved in the heptane allowing a facile catalyst/product separation without the loss of activity. The concept is obviously limited to substrates that show... 

Equation (5.1) includes only the ideal, combinatorial entropy of mixing and the simplest conceivable regular solution type estimate of the enthalpy of mixing based on completely random mixing of monomers mm ( ) = 1 in the liquid state language i referred to as the bare chi parameter since it ignores all aspects of polymer architecture and Interchain nonrandom correlations. For these reasons, the model blend for which Eq. (5.1) is thought to be most appropriate for is an interaction and structurally symmetric polymer mixture. The latter is defined such that the only difference between A and B chains is a v (r) tall potential, which favors phase separation at low temperatures. The closest real system to this idealized mixture is an isotopic blend, where the A and B... 

Recently, polymer-dispersed liquid-crystal materials have played an important role for many practical processes such as the electro-optical displays [1, 2]. Since the miscibility or phase separation of mixtures of polymers and liquid crystals controls the performance of the materials, the phase behavior and the phase separation kinetics have been of experimental and theoretical interest. One of the main problem is to examine the location of various phases, such as isotropic and nematic phases, depending on temperature and concentration. 

Phase Equilibrium (PE) Binary mixtures of a polymer in a single solvent phase-separate at various temperatures, Tsep, depending on the volmne fi-action (/12 of the polymer. The maximmn of the 7 sep=/(< 2) fiuiction is called the critical solution temperature Test-The experiment is repeated for a series of dilute solutions of polymers of the same constitution and configmation but of different molar mass. The relation between the eritieal solution temperature and the molar mass of the polymer is based on the Flory-Huggins lattice theory which predicts that... 

Some polymer mixtures de-mix (phase separate) as the temperature is increased contrary to the results of the Flory-Huggins model that predicts phase separation with decrease in temperature only. These mixtures are known to have a lower critical solution temperature (LCST). Figure 3.5 shows experimental data for poly(Af-isopropylacrylamide) (PIPPAm) in water with an LCST around 33°C below which the solution remains homogeneous and above which it phase separates. Several models have been developed to explain the existence of an LCST. 

Figure A2.5.27. The effective coexistence curve exponent P jj = d In v/d In i for a simple mixture N= 1) as a fimction of the temperature parameter i = t / (1 - t) calculated from crossover theory and compared with the corresponding curve from mean-field theory (i.e. from figure A2.5.15). Reproduced from [30], Povodyrev A A, Anisimov M A and Sengers J V 1999 Crossover Flory model for phase separation in polymer solutions Physica A 264 358, figure 3, by pennission of Elsevier Science.

The critical point (Ij of the two-phase region encountered at reduced temperatures is called an upper critical solution temperature (UCST), and that of the two-phase region found at elevated temperatures is called, perversely, a lower critical solution temperature (LCST). Figure 2 is drawn assuming that the polymer in solution is monodisperse. However, if the polymer in solution is polydisperse, generally similar, but more vaguely defined, regions of phase separation occur. These are known as "cloud-point" curves. The term "cloud point" results from the visual observation of phase separation - a cloudiness in the mixture. 

Berger [340] has examined the use of pSFC in polymer/additive analysis. As many polymer additives are moderately polar and nonvolatile SFC is an appropriate separation technique at temperatures well below those at which additives decompose [300,341,342], SFC is also a method of choice for additives which hydrolyse easily. Consequently, Raynor et al. [343] and others [284,344] consider that SFC (especially in combination with SFE) is the method of choice for analysing polymer additives as a relatively fast and efficient sample preparation method. Characterisation of product mixtures of nonpolar to moderately polar components encompassing a wide range of molecular masses can be accomplished by cSFC-FID. Unknown polymer additives may be identified quite adequately by means of cSFC-FID by comparison with retention times of standards [343], However, identification by this method tends to be time-consuming and requires that all the candidate compounds are on hand. SFC-FID of some low-to-medium polarity additives on reversed-phase packed columns... 

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