Phase separation in polymer blends

In Ref. [120] the first time has been reported on flow-induced phase separation in polymer blends. When PS/PVME blends were exposed to shear or extensional flow at lower temperatures, 20 to 30 K below the equilibrium coexistence temperature, phase separation was observed in both flow regimes. As the authors suggest, the stress, rather than the deformation rate, appears to be the most important parameter in flow-induced phase separation. 

Our group was the first to report imaging with a diamond ATR accessory that provided a field of view of ca. 1 mm2 and the spatial resolution of ca. 15 pm without the use of an infrared microscope [18], The demonstration of the applicability of a diamond ATR accessory for FTIR imaging opened up a range of new opportunities in polymer research, from compaction of tablets [21-23] to studying phase separation in polymer blends subjected to supercritical fluids [24], This imaging approach was successfully utilised for the study of dissolution of tablets in aqueous solutions [25], We have also demonstrated macro... 

Several implications can be drawn directly from Eq. (2-39). First, A // is always positive. Thus, the rule like attracts like, inferred from Eq. (2-30) for molecular mixtures, should also hold at the continuum level. Second, when dispersion forces are dominant, the Hamaker constant is small when ha= b—that is when the dispersed phase (A) has an index of refraction close to that of the medium (B), These rules also apply to molecular mixtures. Nevertheless, small molecules with a significant difference in index of refraction often mix because of the large entropy thereby gained. But particles lose too little entropy on coagulation to resist doing so when there is an attractive van der Waals interaction, and so particle-particle clumping is the norm in suspensions, unless countermeasures are taken to stop it (see Section 7.1). Analogous considerations explain the prevalence of phase separation in polymer blends (see Section 2.3.1.2). 

As Eq 2.73 indicates, the light scattering intensity, I(q, t) is proportional to S (q, t). For this reason the plot of I(q, t) vs. q (at constant decomposition time and temperature) already provides evidence of the dynamics of phase separation in polymer blends. 

A more sophisticated DIA was proposed for the studies of sphemlite formation and phase separation in polymer blends [Tanaka, 1986 Tanaka and Nishi, 1987]. With a different computer vision, shape features of phase-separated structure was obtained [Gur et al., 1989]. The digitalization gives the possibility of two-dimensional Fourier transformation. A power spectrum of the two-dimensional Fourier transformation was given for the structure developed by spinodal decomposition [Tanaka et al., 1986]. In the real space, one cannot see the order in the image clearly, whereas the characteristic wavelength and the distribution can be seen in the reciprocal space representation. 

We have also reported a new series of asymmetric and color-tunable PPV derivatives, including copolymers, that were developed in order to overcome the phase separation in polymer-blending systems with MEH-PPV units. In order to improve the performance of PLEDs, the bulky dimethyldode-cylsilylphenyl group was introduced into the mcfa-position of the phenyl substitutent, which inhibits the intermolecular interaction between the... 

The combination of experimental evidence and computational modeling show conclusively that stable, homogeneously blended (bulk-immiscible) mixed-polymer composites can be formed in a single microparticle of variable size. To our knowledge, this represents a new method for suppressing phase-separation in polymer-blend systems without compatibilizers that allows formation of polymer composite micro- and nanoparticles with tunable properties such as dielectric constant. Conditions of rapid solvent evaporation (e.g. small (<10 pm) droplets or high vapor pressure solvents) and low polymer mobility must be satisfied in order to form homogeneous particles. While this work was obviously focused on polymeric systems, it should be pointed out that the... 

Wang, H., Composto, R.J. Wetting and phase separation in polymer blend films Identification of four thickness regimes with distinct morphological pathways. Interface Sci. 11(2), 237-248 (2003)... 

Fig. 6.6 Schemes of two possible mechanisms whereby surface chemistry templates can guide the phase separation in polymer blend films. In both cases, the substrate is predominately one stnface chemistry—light green areas—with a second surface chemistry covering a minority of the substrate—black areas. In (a)-(e), the film vertically phase separates as induced by the majority surface chemistry, and then the minority surface chemistry induces dewetting of the lower polymer. In (f)-(j), the minority surface chemistry nucleates phase separation at precise locations before spontaneous nucleation can occur [6, 14]...

Liqnid-liqnid Phase Separation in Polymer Blends.82... 

LIQUID-LIQUID PHASE SEPARATION IN POLYMER BLENDS... 

Mesoscopic simulations have been applied to understand phase separation in polymer blends and in polymer/nanofiller mixtures. 

Katzen, D., and Reich, S. (1993) Image analysis of phase separation in polymer blends, Europhys. Utt. 21, 55. 

Apart from a very brief consideration of phase behavior in polymer solutions, the discussion focused on phase separation in polymer blends and block copolymer melts. Another important area is the self-assembly of block copolymers in low-molecular-weight solvents. A detailed discussion of this topic is outside the scope of this chapter. In general, the self-assembly in selective solvents is characterized by the formation of micelles (spherical, disklike, rodlike, with internal stmctures (e.g., lanus type)) or of vesicles. For recent reviews, we refer to References 191, 218, and 219. 

An adaptation of the method studies phase separation in polymer blends from measurements of light transmission through films. 

This is an intimate combination of two polymers in network form. At least one of the polymers is polymerized and/or cross-linked in the immediate presence of the other (27). While ideally the polymers should interpenetrate on the molecular level, actual interpenetration may be limited owing to phase separation. (Phase separation in polymer blends, grafts, blocks, and interpenetrating polymer networks is the more usual case and is discussed in Chapters 4 and 13). 

The new theories pointed out that LCST behavior is characteristic of exothermic mixing and negative excess entropy (25). This last is caused by den-sification of the polymers on mixing. The entropy of volume change, which ordinarily is relatively small compared to other quantities, comes to the fore in polymer-polymer blends (24). While an introduction to phase separation in polymer blends is presented here, a more detailed development of polymer blends in general is given in Chapter 13. 

The basis for phase separation in polymer blends must be considered further here. For the Flory-Huggins theory of incompressible polymer mixtures (see Section 3.3), the small entropy of mixing is dominated by the heat of mixing, leading to the conclusion that the heat of mixing must be zero or negative to induce miscibility. When the system is compressible, the reverse may be true. Compressible in the current sense means that the theory includes terms for volume changes. 

One of the main characteristics of phase separation in polymer blends is the cloudy appearance of the materials. This cloudiness is a consequence of light scattering due to the inhomogeneity of the phase-separated blend structure. Hence, a cloud-point determination method, based on the static light-scattering phenomenon, has been employed customarily to locate the position of the... 

One further application of FT-IR imaging spectroscopy is its combination with DSC. As well as allowing the detection of phase separation in polymer blends (as described above), information on the state of order in blend films also becomes... 

Using FRES and others techniques, Wang and Composto investigated wetting and phase separation in polymer blend films composed of deuterated poly(methyl methacrylate) (d-PMMA) and poly(styrene-raw-acrylonitrile) (h-SAN) at the critical concentration [176-180]. This blend is characterized by a lower critical solution temperature (LCST) behavior with Tlcst 160 °C and 0d-PMMA 0.5 [180]. In this... 

In summarizing this subsection, it should be confirmed that FRES is a valuable tool for investigating polymer diffusion, wetting and phase separation in polymer blends, either within thin films or in near-surface regions. However, due to the technique characteristics, the best results can be obtained for polymers, blends, or nanocomposites of polystyrene. 

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