Partially miscible blends

In this section the crystallization kinetics of blends whose components are miscible in some proportion in the melt will be discussed. In such partially miscible systems, liquid-liquid phase separation could intervene and influence the crystallization process. Therefore, in order to properly analyze the crystallization kinetics in such systems it is necessary that the phase diagram be established. These systems represent a classical example where the melt structure plays an important role in governing the ensuing crystallization. The reason for this requirement will become clear as some typical phase diagrams, involving a crystallizable polymer, are examined. 

A unique situation can be observed with a UCST type phase diagram. In this particular case the blend is miscible above Tn, but displays a miscibility gap be-low.(69,70) There is no equilibrium basis for liquid-liquid phase separation to take place below the melting temperature-composition boundary. However, it is possible for the melt to be sufficiently supercooled below Tn that liquid-liquid phase separation does in fact occur. Therefore, in a practical sense crystallization occurs in competition with liquid-liquid phase separation. 

An excellent example of the role played by liquid-liquid phase separation in the ensuing crystallization is found in blends with syndiotactic poly(styrene).(77) Measurements of the glass temperature in mixtures with poly(2,6-dimethyl-l,4-diphenylene oxide) (PPO) indicate that the components are miscible in all proportions in the melt. However, mixtures of syndiotactic poly(styrene) with poly(vinyl methyl ether) represent partially miscible blends. When the poly(vinyl methyl ether) content exceeds 20% by weight, the melt separates into two liquid phases, one rich in syndiotactic poly(styrene), the other in poly(vinyl methyl ether). Thus, the two blends have a common crystallizing component. However, in one the crystallization takes place from a homogeneous melt in the other from one that is phase separated. The different melt structures profoundly affect the crystaflization kinetics. This can be seen when a comparison is made between the crystallization kinetics of syndiotactic poly(styrene) from a homogeneous or phase separated melt. 

The dependence of the overall crystallization rate on the composition of these two blends differs from those of the growth rates.(77) The half-time for syndiotactic poly(styrene) crystallization is plotted against the crystallization temperature for both type blends in Fig. 11.33. The half-times for the pure syndiotactic polymer are represented again by the sohd circles. For this type of measurement the halftimes increase with the addition of either poly(vinyl methyl ether) or PPO. The addition of poly(vinyl methyl ether) causes a larger increase in the half-time and thus a greater reduction in the crystallization rate. The overall crystallization rate is a reflection of both initiation and growth of crystalUzation. Both of these processes 

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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, ( ),... 

The most complex types of interfaces are those between two amorphous polymer phases, as in immiscible or partially miscible blends and block copolymers. Such interfaces differ in a... 

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]. 

Polymer blending is to combine two or more components and has superior mechanical, optical, or thermal properties than these individual polymers. From the practical and economical points of view, polymer blending from existing polymers is the most effective and convenient route to create new and useful materials with greater versatility and flexibility than the development of new polymers. Basically, three different types of blends can be distinguished completely miscible, immiscible, and partially miscible blends [1,2] as shown in Figure 2.1. 

L. T. Yan, J. Sheng, Analysis of phase morphology and dynamics of immiscible PP/PAlOlO blends and its partial-miscible blends during melt mixing from SEM patterns. Polymer 2006, 47,2894. 

Miscibility between the individual polymers is the most important factor to determine the performance characteristics of a polymer blend. Mutual solubility of the phases, the thickness and properties of the interphase formed during blending and the structure of the blend are mainly dependent on the miscibility of individual polymers within a polymer. As a result, a quantitative estimation of interactions is very much important for the prediction of blend properties. Comparison of solubility parameters of individual polymers is an effective method to predict the extent of miscibility within a blend. According to the Hildebrand solubility theory, a large difference in solubility parameters (6p) of individual matrices results in immiscibility between them in the absence of any interfacial compatibil-izer [222]. Jandas et al. have reported that PLA and PHB have Hildebrand solubility parameters (6p) of 23.5 J /cm and 19.8 J Vcm which can turn out to be partially miscibile blends in between them [35]. In case of partially miscible blends, the miscibility can be controlled by compatibili-zation using proper interactables. 

D. G. Baird, S. S. Bafna, J. P. DeSouza, and T. Sun, Mechanical properties of in situ composites based on partially miscible blends of polyetherimide and liquid crystalline polymers, Polymer Composites, vol. 14, No. 3, 214-223, June (1993). 

A number of miscible polymer blends are only completely miscible and form one-phase systems over a limited concentration, temperature, and pressure range. Under certain conditions of temperature, pressure, and composition, miscible binary blends may phase separate into two liquid phases with different compositions, called partially miscible blends. Important characteristics of this type of blends are the overall blend composition, the morphology, and the composition of the different phases as well as the nature of the interface between the phases. 

C.H. Stephens, A. Hlltner, E. Baer, Phase behavior of partially miscible blends of linear and branched polyethylenes. Macromolecules 36(8), 2733-2741 (2003)... 

Often the TgS of miscible (or partially miscible) blends are compared with values calculated from the Kelley-Bueche equation (Eq. 21) [55]. which is based on the additivity of free and occupied volumes of the individual components, the Gordon-Taylor equation (Eq. 22) [56] or the Fox equation (Eq. 23) [57] for a binary mixture of A and B is the glass-transition temperature of the mixture, (l>A> g,A> g,B volume and weight fractions and glass-tran-... 

The compatibility of two components in a partially miscible blend is a function of the extent to which the blend properties are altered as compared to a completely immiscible pair. The compatibility can be characterized by a fine dispersion of one phase within the other, adhesion of the dispersed phase to the matrix, shifting of glass transition temperatures relative to the pure components, reduced interfacial energy between the phases, and molecular interchange [/]. 

Free energy, spinodal, and binodal Partially miscible blends—AA5 ... 

Partially miscible blend—a polymer blend that is at a phase-separated state of mixing at a molecular level with the composition of the separated phases impure or not identical to the pure components prior to blending. 

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