Temperature polymer blend phase separation

Most polymer blends phase-separate with LCST thus the miscibility region stretches from the melting point or Tg up to the binodal while the phase-separated region exists above the spinodal temperature, Ts [1, 212]. Within the region between binodal and spinodal the system is metastable, characterized by strong interrelation between the rheology and thermodynamics [211, 213-215]. 

The miscibility limit between thermotropic liquid crystal polymers and flexible chain polymers can be predicted by calculations corresponding to the spinodal curve at constant temperature. This miscibility is increased with the increase of the degree of disorder (y/jc,) of the liquid crystal polymer and with the decrease of the degree of polymerization. Two quantitative parameters from the Hory s lattice theory can be used to estimate the phase behavior of this kind of blend at melt processing temperature the polymer-polymer interaction parameter and the degree of disorder (y/Xj). In binary polymer blends phase separation may occur for any value of degree of disorder (y/Xj)... 

In such systems, spatial temperature modulations may cause, via the thermodiffusion effect, concentration modulations in the composition of the polymer blend already above the critical temperature. If the mean temperature crosses the critical temperature from above, phase separation sets in. However, a spatially periodic temperature modulation changes the phase separation process in polymer blends considerably. 

PMMA is typical of many polymer pairs, for which the parameter is positive and of order 0.01, making only low molar mass polymers form miscible blends. PVME/PS, PS/PPO, and PS/TMPC have a strongly negative x parameter over a wide range of temperatures (of order — 0.01) but since >0 and Bblends phase separate on heating. PEO/ PMMA, PP/hhPP and PlB/hhPP, all represent blends with very weak interactions between components (x = 0). 

The formulation of Scott (44) does not present the range of phenomena occurring in polymer blends. Various binary blends exhibit lower critical solution temperatures (LCST) where phase separations occur at lower temperature. Other blends exhibit upper critical solution temperatures (UCST) where miscible blends exhibit phase separations at higher temperatures (45). It was shown by McMaster (46) that volume changes occurred in mixing. 

The polymer/solvent blend is extruded or cast at elevated temperatures. As the temperamre approaches an ambient temperature, a polymer-ricb phase separates from the solvent. The solvent is subsequently removed... 

Polymer blends may be characterized in terms of the temperature dependence of the Flury-Huggins interaction parameter (j)- In the case of an upper critical solution temperature (UCST) blend, / decreases with temperature, and the blend remains miscible. For phase separation to occur in a UCST blend, the temperature must be lower than the critical solution temperature. In the case of a lower critical solution temperature (LCST) blend, x increases with temperature, and thus phase separation occurs above the critical solution temperature. The ability of CO2 to mimic heat means that miscibility is enhanced in the case of UCST blends, and for the case of LCST blends the miscibihty is depressed. Ramachandrarao et al. [132] explained this phenomenon by postulating a dilation disparity occurring at higher CO2 concentration as a result of the preferential affinity of CO2 to one of the components of the blend, inducing free-volume and packing disparity. 

This approach was further explored by Hakemi (2000) who prepared blends that contain both a wholly aromatic and an aromatic-aliphatic LCP that are miscible with each other. The ultimate goal of this approach was to develop multi-component blends that have components of thermoplastics. The miscible LCP blends could be useful as reinforcing agents for the thermoplastic matrix polymer and, due to the fact that the LCP s contain some of the components of the thermoplastic polymer, there is expected to be improved adhesion between the LCP portion and the matrix portion of the mixture. This is another example of an attempt to balance the phase separation that is inherent in high temperature polymer blends due to molecular conformation differences by strengthening the enthalpic interaction between the two polymers. 

Vlassopoulos, D., Koumoutsakos, A., Anastasiadis, S.H., Hatzikiriakos, S.G., and Englezos, P. (1997) Rheology and phase separation in a model upper critical solution temperature polymer blend. 

Odian, G., 1991. Principles of Polymerization, John WUey and Sons, New York Oh, J., and Rey, A. D., 2002. Computer simulation of functional polymeric materials formation via polymerization-induced phase separation imder a temperature gradient , in Phase Separation in Polymer Solutions and Blends, R K. Chan (Ed.), Research Signpost, Trivandrum, India. Panayiotou, C., and Vera, J. H., 1982. Polym. J., 14,681-694. 

The thermal history has a profound influence on the DSC curves of polymer blends containing at least one crystalline component. In order to obtain by DSC experiments, the samples are usually first heated to a temperature between the phase separation temperature and the melting point of the crystalline component and held for several minutes to remove the thermal history. 

When a homogeneous mixture solution is cooled, phase separation is induced at a certain temperature. This critical phase separation temperature is termed the upper critical solution temperature (UCST). It is a convex upward curve in the plot of composition versus temperature (C-T plot) and its maximum point shifts to a higher temperature with increasing relative molecular mass of the polymer. However, for many polymer-solvent and polymer-polymer blend systems, a decrease in mutual solubility is also observed as the temperature increases. The critical phase separation temperature is called the lower critical solution temperature (LCST). It is a convex downward curve in the C-T plot and the minimum point shifts to a lower temperature with increasing relative molecular mass of the blend components. LCST occurs at a higher temperature than UCST. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. Phase morphology with a single characteristic length scale can be synthesized by quenching a partially miscible polymer blend below the critical temperature of demixing. 

Sample. Three PS samples with different molecular weights and glass transition temperatures were investigated (Table 1). A blend of PMMA and PS was used to obtain adhesive force images. These polymers show phase separation due to their chemical difference. A PMMA with a molecular weight of 100000 g/mol and a glass transition temperature Tg of 109 was chosen as second component for the blend. Tg has been estimate by DSC measurements. 

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