Melt miscible blends

Crystallization behavior in miscible blends containing crystallizable components has been extensively studied [174-180]. Generally, when a crystallizable component is mixed with an amorphous component its melting temperature goes down and its crystallinity lowers. The same trend has been reported for blends with intercomponent hydrogen bonding such as PCL/STVPh [181], PCL/poly(hydroxyl ether of bisphenol A) [182] and phenoxy resin/PEO [183]. 

Correlation between blend morphology, melt-elongation, and foaming behavior — despite the significant influence only addressed for neat polymers [38], miscible blends [4, 5], and some filled systems [39,40]... 

Breakdown of the stress-optic rule can also occur in multiphase or multicomponent mixtures, as well as in melts with crystalline domains. However, as in glassy polymers, for miscible blends, a revised stress-optic law can sometimes be recovered by breaking the stress and birefringence tensors into two components, one for each component in the blend (Zawada et al. 1994 Kannan and Komfield 1994). 

Melt flow, however, also affects the phase separation, usually enhancing the miscibility for partially miscible blends that show LCST behavior. From Lyngaae-Jorgensen work (46) one may derive the following relation between the shear stress and the change in the spinodal temperature, T, ( ),... 

PEEK/PEI Sample melt mixed at 400°C, quenched in ice water into 0.2-0.4 mm sheets. DSC (10 mg) at 20°C/min. Miscible blends. Goodwin and Simon, 1996... 

The thermodynamic requirement for crystallization in a miscible blend is that the blend exhibits a free energy on crystallization that is more negative than the free energy of the liquid-liquid mixture. A liquid-solid phase separation can occur when the miscible melt is cooled to a temperature between the glass-transition of the blend and the equilibrium melting point of the crystallizable component(s) (section 3.3.1). 

When dealing with miscible blends containing two crystalline components, several modes of crystallization are possible separate crystallization, concurrent crystallization, co-crystallization, etc. Only those blends in which both components are miscible in the melt are considered here (Table 3.3). PET/PBT blends were reported to be an example of separate crystallization [Escala and Stein, 1979 Stein et al., 1981]. A spherulitic crystallization was observed for the neat components as well as for blends with small amounts of one component, and the crystals of the minor component were included within the spherulites of the major component, which results in a coarsening of the spherulitic texture. Transesterihcation is, however, the reason for the homogenous amorphous phase. 

The Equilibrium Melting Temperature in Miscible Blends - Hoffinan-Weeks Plot... 

Hoffman-Weeks plots have also been drawn for several other amorphous/crystalhne miscible blends, such as PVDF/PEMA [Eshuis et al., 1982], PEG/PMMA [Martuscelh, 1984], PCL/SARAN [Zhang and Prud homme, 1987], as well as for some miscible blends containing two semicrystalline components, PCL/PC [Jonza and Porter, 1986] and PCL/Penton [Guo, 1990]. Table 3.10 represents equilibrium melting points derived from... 

Figure 3.20. Schematic diagram of the free energy of a crystalline phase (G ) and the free energy of a melt phase of a homopolymer (G ) and a miscible blend as function...

Table 3.10. Equilibrium melting points derived from Hoffman-Weeks plots for several crystallizable miscible blends...

Melting point depression data are often used to determine the Huggins-Flory interaction parameter, X12 Table 3.11), that is a measure for the miscibility of the blend, i.e., X12 is negative for a miscible blend. A lack of melting point depression means that is zero. Eq 3.39 is only valid for systems in which the crystalline morphology is not affected by the composition. 

For some miscible blends, a melting point elevation has been reported with respect to that of the neat crystallizable component, both crystallized at the same temperature [Eshuis et al, 1982 Rim and Runt, 1983 1984]. These observations may originate from recrystallization, enhanced crystal perfection, and increased crystal size. 

Crystallization may take place only within the temperature region limited on the upper side by the melting point, T, and on the lower side by the glass transition temperature, T. The crystallization behavior very much depends on the state of miscibility, as well as on the nature of other blend s components. In miscible blends Tg is a monotonic function of the components T s. When the second component is amorphous, its presence can either decrease or increase the tendency of the first resin to crystallize. The process depends on the blending effects on T and on the chain mobility (the free-volume effect). When T of the amorphous component is lower... 

A miscible blend of amorphous and crystalline polymers usually means a single phase in the melt and a neat crystalline phase with a mixed amorphous region in the sohd. Because of chain folding during crystallization, the crystal lamellae are formed. Their radical growth usually lead to the formation of spheniUtes [Nadkami and Jog, 1991]. 

It has been known for more than a century that impurities reduce the melting point. This observation has been used to determine the molecular weight of the contaminant by Raoult in 1882-5. Nearly a hundred years later, this concept was used to calculate the thermodynamic binary interaction parameter, from the melting point depression of a crystalline polymer in miscible blend with low concentration of another polymer. The relation is usually used in the simplified form for very high molecular weight components [Nishi and Wang, 1975, 1977] ... 

Polyarylate/PET blends prepared by solution or melt blending under short residence times at T < 280°C with or without an added ester interchange inhibitor such as triphenylphosphite, are essentially phase-separated, exhibiting two glass transition temperatures, one each for a PET phase and a polyarylate-rich phase. From the observed glass transition temperamres, one can conclude that it is a partially miscible blend in which more PET dissolves in the polyarylate phase than polyarylate does in PET. The interaction parameter has been estimated to be slightly positive (Xj = 0-1) [Chung and Akkapeddi, 1993]. 

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