Blend formation

Materials. Supercritical fluids offer many opportunities in materials processing, such as crystallization, recrystallization, comminution, fiber formation, blend formation, and microceUular (foam) formation. 

Because of the aqueous solubiUty of polyelectrolyte precursor polymers, another method of polymer blend formation is possible. The precursor polymer is co-dissolved with a water-soluble matrix polymer, and films of the blend are cast. With heating, the fully conjugated conducting polymer is generated to form the composite film. This technique has been used for poly(arylene vinylenes) with a variety of water-soluble matrix polymers, including polyacrjiamide, poly(ethylene oxide), polyvinylpyrroHdinone, methylceUulose, and hydroxypropylceUulose (139—141). These blends generally exhibit phase-separated morphologies. 

Randomisation in miscible polymer blends (formation of random copolymers). 

For nanocomposites produced via melt blending, formation of mixed type composite occurs already at 5 wt% of layered silicate in polyethyleneterephthalate. X-ray diffraction analysis patterns are shown on Figure3. 

The introduction of reactive components during blend formation further increases the complexity of the process and affects results. Polypropylene functional-... 

CAR Cardelli, C., Conti, G., Gianni, P., and Porta, R., Blend formation between homo-and copolymers at 298.15 K. PMMA-SAN blends, J. Therm. Anal. Calorim., 71, 353,2003. 

Fig. 2.27 CPMAS NMR spectra of PMAA top line), PVAc bottom line), and several PMAA/PVAc blends, (a) carboxyl regions of PMAA and carbonyl regions of PVAc (b) aliphatic regions. The weighted sums (of the pure PMAA and pure PVAc NMR spectra) are also depicted on the right of the corresponding observed spectra left columns). The blend formation results in strong qualitative changes in the OC=0 carbon, but not so much in the carbons of the aliphatic region (Data from Asano et al. 2002)...

In most cases, some mode of sample preparation has to be used after the blend formation, viz., staining, swelling, fracturing, or etching. These are very appropriate for and have been extensively used to characterize morphology of immiscible blends, but they have obvious severe shortcomings in miscible or partially miscible... 

Creating compatibilized polymer blends allows polymer compounders to supply potential product fabricators with a very broad spectrum of required properties. Over the last 50 or so years, progress in successful blends formation has rested on experimental studies. This has been achieved, primarily, by the international... 

Olefin copolymerization and reactor blend formation are important processes to tailor polyolefins. Copolymer properties depend upon the sequence distribution of the comonomers, which is controlled by means of catalyst as well as process technology. Today most copolymers are produced either in solution processes or in solvent-free gas phase polymerization. Recent breakthroughs in catalyst development are stimulating production of a novel range of copolymers, especially of ethylene copolymers. In the past, special catalysts were designed to produce three classes of ethylene copolymers with different comonomer content ... 

As diffusion phenomena are extremely important in miscible blend formation and phase separation of polymer blends, these phenomena are discussed in detail in this chapter. 

In blends of ABC with AB block copolymers a mixing at the molecular level of both blocks of the diblock copolymer with the corresponding blocks of the triblock terpolymer leads to a centrosymmetric superstructure. However, in the case of a blend of ABC and AC block copolymers a noncentrosymmetric structure is obtained when all diblock and triblock terpolymer molecules prefer to form common domains with the corresponding block of the other species. Before discussing this situation in some detail, other general possibilities for blend formations need to be considered. Besides the trivial case of macrophase separation and the random sequence of ABC and AC block copolymers due to equal energetic situations between the different A blocks and C blocks, two different centrosymmetric double-layered structures are also possible. 

The technique is straightforward but neglects the thermodynamics and kinetics of morphology formation. Another technique involves using a modified Cahn-Hilliard equation to describe blend formation [17]. This approach takes better account of the physical processes of blend formation, with the additional benefit of being able to include further processes, such as differing surface energies [17] or solvent evaporation [26] as appropriate, as shown in Fig. 2. 

Fig. 2 Simulated blend structures using a modified Cahn—Hilliard equatirai to desraibe blend formation, illustrating the ability to control the formation of surface wetting laytas (prestait in (a) but not in (b)). Reproduced with pmnission from [17]. American Chemical Society...

Processing with an environmentally benign supercritical fluid, such as carbon dioxide, is an attractive alternative to conventional processing of electrically conducting polymer blends. The advantages of supercritical carbon dioxide as a solvent for polymerization and blend formation have been outlined by many investigators (30,31). Carbon dioxide is inexpensive, nonflammable, offers high mass transport rates, and allows in situ removal of unreacted monomer and other impurities. It is also known to swell host polymers (32), which facilitates blend formation. 

In this chapter, we review processes to obtain blends of conducting polymers and host substrates, with particular emphasis on processes that employ carbon dioxide as the solvent. The effect of the solvent on the synthesis of the conducting polymer, on blend formation, and on doping of the conducting polymer is reviewed. Furthermore, morphologies that lead to high electrical conductivity are identified. 

Supercritical carbon dioxide is generally a poor solvent for polymers (91). However, it does have the capacity to swell many polymers (92,93), and this can be of considerable advantage in blend formation. Watkins and McCarthy (32) found that the solubility of carbon dioxide in PCTFE reaches a maximum at a temperature of 313 K and a pressure of 10.4 MPa. This maximum represents a mass gain of 4.5% carbon dioxide in the host substrate PCTFE. Wissinger and Paulaitis (94) found that PMMA swells by about 20 vol % after reaching equilibrium in carbon dioxide at a temperature of 305.7 K. However, it is important to note that PMMA undergoes foaming with supercritical carbon dioxide treatment, as observed by Shieh et al. (95,96). 

VIII. RECENT STUDIES INVOLVING IN SITU BLEND FORMATION USING SUPERCRITICAL CARBON DIOXIDE... 

We have smdied the polymerization of pyrrole and 3-undecylbithiophene and blend formation with several host polymers using supercritical carbon dioxide... 

P Rajagopalan, TJ McCarthy. Two-step modification of chemically resistant polymers blend formation and subsequent chemistry. Macromolecules 1998 31 4791-4797. 

Similar activation energy values have been reported for pure polyamides such as PA-6,6, PA-6,10, PA-6 and poly(ether ester amide)s 502570. The decrease in the activation energy values of polyamides, due to the addition of a component or in a blend formation, was observed for mixtures of PA-6,10, PA-6,6, PA-11 and PA-12 with a fire-retardant ammonium polyphosphate and in blends of PA-6 with functionalised or non-functionalised polypropylene 502570. The presence of EPDM-g-MA in the blend decreased the activation energy and significantly affected the absorption bands of solid residues in the FTIR spectra. This behaviour suggested a decrease in the thermal stability of polyamide due to the presence of functionalised EPDM. 

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