CO2-Assisted Blending of Polymers

The role of CO2 in assisting the blending of polymers may be viewed in one of the following ways  

In situ polymerization to prepare immiscible blends was pioneered by Watkins and McCarthy [108], stimulating other researchers to apply this methodology to prepare novel polymer blends [109-112], fiber-reinforced composite materials[39], and electrically conducting composites [66, 67, 113-116]. Polymer blends produced in this manner include polystyrene/poly(vinyl chloride) [117, 118], polysty-rene/PET [119], nanometer-dispersed polypropylene/polystyrene interpenetrating networks [120], polypropylene/polystyrene [121] and polyethylene/polystyrene [122]. The resultant polymer blend may have a unique morphology compared to the traditionally prepared counterpart (if it is feasible to prepare such a blend via conventional procedures) and therefore demands a thorough investigation. 

Shieh et al. [130] probed the C02-induced morphological alterations in a poly(-ethylene oxide)/poly(methyl methacrylate) blend using small-angle X-ray scattering (SAXS). The crystal and amorphous layer thickness of PEO was found to increase following exposure to CO2, and the extent of the layer thickening was found to be controlled by the fraction of PMMA in the blend. The increase in thickness of the lamellae was attributed to recrystaUization of the PEO domains. 

The preferential interaction of CO2 with individual phases of polymer blends is dictated by the affinity of CO2 to the individual components. Zhou et al. [131] applied atomic force microscopy (AFM) and phase contrast microscopy to study the effect of SCCO2 on the surface structure and phase morphology of PMMA/ PS (50/50 w/w) thin films. AFM was used to monitor the return to thermodynamic equilibrium of the cast film as a function of temperature, pressure, and exposure time. Initially, PMMA appeared as a dispersed surface phase elevated from a continuous PS matrix. The series of AFM images in Fig. 10.7 indicate that the Tg of PMMA/PS blend at 20 MPa is 60°C, and at a temperature of 70 °C the Tg occurs at a pressure greater than 20 MPa, as seen by the initially elevated PMMA phase falhng into the PS phase. 

Yang and X. Xie, Effect of supercritical CO2 on the surface structure of PMMA/PS blend thin films, pp. 137-145, Copyright (2004), with permission from Elsevier). 

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