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Binary polymer blends in which one or both components crystallize have been considered. These may be miscible or immiscible in the amorphous state, and phase separation may occur along with crystallization. The rates of these two processes vary differently with temperature. so the consequent morphology and properties depend upon the temperature history. For miscible blends, the glass transition temperature depends upon composition. This affects diffusion so that crystallization kinetics varies as composition changes. If only one component crystallizes and the other is rejected, this rejected component diffuses away from the growing crystalline regions in a manner dependent upon the interaction between components. These phenomena are studied using a variety of techniques such as calorimetry, x-ray diffraction and scattering, and small-angle light and neutron scattering for a number of polymer systems. INTRODUCTION Binary polymer blends may or may not be miscible in the amorphous state depending on the value of the Flory interaction parameter, X. Phase separation may occur upon increasing or decreasing the temperature, depending upon the temperature dependence of X, so that a lower (LCST) or upper (UCST) critical solution temperature may be observed 1 . If one or both components crystallize, such crystallization may occur in the miscible or immiscible region, and will usually result in the rejection of the noncrystallizing component which must then diffuse into the surrounding amorphous phase 2 . This results in a change in the glass transition temperature, Tg, of this phase, with its consequent effect on the rate of crystallization 3. Thus, the crystallization kinetics and the resulting morphology will depend in a complex way upon these phase relationships, and will depend upon the thermal history of the sample during the phase separation and crystallization processes. CRYSTALLIZATION FROM THE MISCIBLE STATE Let us consider the case of a miscible binary blend in which one component crystallizes. Some examples are poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA), poly-e-caprolactone/poly(vinyl chloride) (PCLIPVC), and isotactic/atactic polystyrene (i-PS/a-PS). The crystallization rate depends upon the crystal melting point, Tm, and the T of the amorphous phase, both of which depend5 upon blend composition. As shown by Sgcott4, Tm is depressed by blending according to

[1 - Tdo/Tmo] = - (B V2/AH 2 u)(I

2

(1)

where Tmo is the melting point of the unblended crystallizable component, Tdo is Tm0 for the blend. The constant B, the interaction energy density, is given by 5 X12RT/Vlu. AH2u is the enthalpy of melting of the crystallizable component. The specific volume is V2 , the

531 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society

volume fraction is 01. The amount of melting point depression depends upon X as can be seen from the above relation, as was shown by Morra and Stein 5 in their study of PVDF/PMMA blends. It is important in such studies to achieve equilibrium melting condit