CO2-Induced Phase Separation in Polymer Blends

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. 

The effect of CO2 sorption on LCST blends is to depress the lower critical solution temperatures of the polymer system. Watkins et al. [133] observed phase sep- 

The presence of CO2 in UCST polymer blends has been shown to enhance blend miscibility. Walker et al. [136] observed a depression of the cloud point temperature for low-molecular-weight blends of polystyrene/polyisoprene using a combination of visual inspection, small-angle neutron scattering, and spectrophotometry. In the presence of 13.8 MPa of CO2, a reduction of the cloud point was observed compared to the same system in the absence of SCCO2. This demonstrates the ability of CO2 to promote miscibihty in UCST blends, and consequently an increase in the processing window is available. Walker et al. [137] 

Imaging of Polymeric Materials Subjected to High-Pressure CO2 

This section introduces a novel application of IR spectroscopy, namely IR imaging, and the specific sampling technique of attenuated total reflectance (ATR). FTIR imaging in ATR mode allows one to visualize the spatial distribution of different components in polymeric materials and to study directly the effect of high-pressure CO2 on this distribution. This novel approach should benefit polymer scientists studying polymer blends and their processing with SCCO2. 

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