Blends demixing

The kinetics of polymer/polymer demixing are many orders of magnitude slower than those for polymer/solvent demixing. In a typical instrument used to study blend demixing a polymer film supported on a thin glass cover slip is placed in a ther-mostatted pressure cell (Fig. 7.18c). The sample is annealed for some hours well... 

Although the scientific principles behind this simple example of practical technology are easily understood, it illustrates the benefits that can be realized by considering the blending process as a dispersion operalion that may be followed, if necessary, by an operation to retard the rate at which the ingredients of the blend demix. In special cases, of course, the latter operalion may be rendered unnecessary by the selection of blend ingredients that are miscible in the first instance. 

Mixing of two amorphous polymers can produce either a homogeneous mixture at the molecular level or a heterogeneous phase-separated blend. Demixing of polymer chains produces two totally separated phases, and hence leads to macrophase separation in polymer blends. Some specific types of organized structures may be formed in block copolymers due to microphase separation of block chains within one block copolymer molecule. 

The mechanism of formation of morphology structures in iPP-E-plastomers blends via shear-dependent mixing and demixing was investigated by optical microscopy and electron microscopy. A single-phase stmcture is formed under high shear condition in injection machine after injection, namely under zero-shear environments, spinodal decomposition proceeds and leads to the formation of a bicontinuous phase stmcture. The velocity of spinodal decomposition and the phase separation depend on the molecular stmcture of iPP and E-plastomer components. 

Fig. 8 Theoretical liquid-liquid demixing curve (solid line) and the bulk melting temperature (dashed line) of a flexible-polymer blend with one component crystallizable and with athermal mixing. The chain lengths are uniform and are 128 units, the linear size of the cubic box is 64, and the occupation density is 0.9375 [86]...

One practical example of demixing that might be attributed to a difference in crystallizability is the incompatibility in blends of polymers with different stereochemical compositions. The stereochemical isomers contain both chemical and geometrical similarities, but differ in the tendency of close packing. In this case, both the mixing energy B and the additional mixing entropy due to structural asymmetry between two kinds of monomers are small. However, the stereochemical differences between two polymers will result in a difference in the value of Ep. Under this consideration, most experimental observations on the compatibility of polymer blends with different stereochemical compositions [89-99] are tractable. For more details, we refer the reader to Ref. [86]. 

Due to light scattering, crystalline polymers mostly yield turbid films.Their blends with other polymers are always demixed because polymers are not able to form mixed crystals. Consequently, crystallizable polymers only yield homogeneous blends above their melting point. As soon as crystallization sets in, the components will separate. 

In practice, blends are often prepared by mechanical mixing in the melt. As a rule the products are demixed, but dispersed so finely that the phase structure can only be detected with a transmission electron microscope (TEM). 

The advantages of spray-dried flavours are the high flavour load and the fast release. The process is very economical. A disadvantages of the flavour powder is the physical demixing in dry blends with sugar, tea, cereals or granulates. 

Compacted flavours are granules of size between 0.5 and 5 mm. The main application of the compacted flavour granules in the food industry is tea leaf flavouring for tea bags. Powdered flavours are not suitable because of demixing of the leaves and the powder during the blending process. 

It is well-known that one can reduce the droplet size of a demixed polymer blend by adding a well-chosen block-copolymer at given p and T, and achieve interesting improvements of the blend s mechanical properties. 

EOS models were derived for polymer blends that gave the first evidence of the severe pressure - dependence of the phase behaviour of such blends [41,42], First, experimental data under pressure were presented for the mixture of poly(ethyl acetate) and polyfvinylidene fluoride) [9], and later for in several other systems [27,43,44,45], However, the direction of the shift in cloud-point temperature with pressure proved to be system-dependent. In addition, the phase behaviour of mixtures containing random copolymers strongly depends on the exact chemical composition of both copolymers. In the production of reactor blends or copolymers a small variation of the reactor feed or process variables, such as temperature and pressure, may lead to demixing of the copolymer solution (or the blend) in the reactor. Fig. 9.7-1 shows some data collected in a laser-light-scattering autoclave on the blend PMMA/SAN [46],... 

Macrophase separation after microphase separation has been observed in an AB block copolymer/homopolymer C blend (Hashimoto et al 1995). Blends of a PS-PB starblock copolymer (75wt% PS) and PVME homopolymer were prepared by solvent casting. Binary blends of PS and PVME exhibit a lower critical solution temperature (LCST), i.e. they demix at high temperatures. The initial structure of a 50% mixture of a PS-PB diblock and PVME shown in Fig. 6.20(a) consists of worm-like micelles. Heating led to macrophase separation as evident... 

Until recently, very little quantitative information was available on blends of block copolymers. The literature is summarized in Table 6.3. Hoffman et al. (1970) reported microscopic demixing of blends of PS-PB diblocks, with two maxima in the domain size distribution, but with no evidence tor macrophase separation. These findings must be treated with caution in the light of more recent results. Hadziioannou and Skoulios (1982) used SAXS and SANS to investigate the morphology of binary blends of PS-PI diblocks, and binary PS-PI/PS-PI-PS or PS-PI/PI-PS-PI blends or blends of the two types of triblock. They found that the blends were microphase separated, and that the sharpness of the interface was not reduced in blends compared to neat copolymers. The transition between a lamellar and a cylindrical structure was shown to depend primarily on blend composition. In contrast, the transition from a lamellar to a disordered phase at... 

Does extensive blending cause demixing and segregation of components This is important, especially if particle size/density of the powder/granula-tion vary widely. 

Compared to bulk polymer mixtures, the interfacial behaviour of polymer blends is essentially different [341]. The demixing process in thin films is strongly affected by the thin film confinement and the interfacial interactions of the blend components with the confining phases (e.g., substrate and air). Even in the one-phase region of the phase diagram, preferential segregation of the components at one of the interfaces leads to a certain composition profile as a function of the distance from the free surfaces and the substrate plane [342,343]. In the... 

The following problems have been studied experimentally using different SFM techniques (i) the surface topography and its dependence on the preparation conditions, (ii) the chemical composition of the surface, and (iii) the role of the substrate structure in polymer demixing. Here, one has to distinguish between asymmetric blends, where one of the components concentrates at the surface and the other at the substrate interface, and symmetric blends where one of the components segregate at both interfaces. 

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