Up Close: Institute C. Sadron Takes a Comprehensive Approach to Polymer Blends
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MRS BULLETIN/FEBRUARY 1994
180 PS / PVME PHASE DIAGRAM
160•
T
binodal
A
T
spinodal
140T° C
120-
100-
80 0.2 PS
0.4
0.6
weight
fraction
O.I
Figure 1. Phase diagram of a polystyrene (PS)-polyvinyl methylether (PVME) blend depicting a lower critical solution temperature.
ent chemical natures. Because of the size and large conformational disorder of polymer chains in the amorphous (molten) state, the combinatorial entropy of mixing
is very small and can hardly overcome the positive enthalpy of mixing resulting from purely dispersive forces. The free enthalpy of mixing is therefore generally positive, resulting in phase separation of the essentially pure constituents. Only when specific forces between the chemical groups belonging to the two constituents exist (such as the formation of hydrogen bonds, the interaction of a large permanent dipole moment and polarizable groups, or charge transfer interaction between electron donors and acceptors) is the enthalpy of mixing negative and total miscibility obtained. The search for pairs of compatible polymers can be carried out by inverse chromatography where one of the polymers, deposited on silica beads, is the stationary phase, which interacts with a small volatile analog of the second. Polymers compatible with polyvinylidene fluoride have been successfully predicted by this method.1 However, even for compatible blends (sometimes designated as alloys), differences in free volume, thermal expansivity and compressibility, which manifest in particular by a contraction upon mixing, may result in a negative entropy of mixing and a phase separation at higher temperatures. A typical phase diagram with a lower critical solution temperature is depicted in Figure 1. While compatible blends are expected to have properties essentially intermediate between those of the constituents (glass transition temperature, mechanical moduli), incompatible blends are expected to behave as a molecular composite. Not only the properties of the individual components but the size and shape of the separated microphases play a major role, especially in nonlinear mechanical properties such as plastic deformation and impact strength. The simplest example is the blending of polystyrene (PS) with an incompatible elastomer which segregates into spherical domains much smaller than the wavelength of light to obtain a transparent, shock-resistant PS. Understanding the morphology-properties relationship involves, first, the control of the formation and stabilization of the microphases. Control of Microphase Morphology Blends with different morphologies can be prepared several ways. At this stage we are essentially interested in covering a large range of sizes and morphologies, characterizing them, and systematically exploring the corresponding properties. The following describes studies carried out at ICS to control the different procedures.
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Up Close: Institute C. Sadron Takes a Comprehensive Approach to Polymer Blends
Mixing in the Molten State The mixing process can be considered as th
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