Dispersed melt phase

The crystaUization behavior of a dispersed melt-phase, for example discrete melt droplets, in an amorphous matrix can be dramatically affected... 

The crystallization behavior of a dispersed melt phase, for example, discrete melt droplets, in an amorphous matrix can be dramatically affected compared to that of the bulk polymer. It has been reported by several authors that crystaUizable dispersed droplets can exhibit the phenomenon of fractionated crystallization originating from the primary nucleation of isolated melt particles by species with different nucleating activities (heterogeneities, local chain ordering)... 

The crystallization behavior of a dispersed melt phase in an amorphous or semi-crystalline matrix phase has generated a lot of interest in recent years. In polymer blends in which the crystallizable phase is dispersed into tine droplets in the matrix, crystallization upon cooling from the melt can occur in several temperature intervals that are initiated at different undercoolings, often ending up with a crystallization at the homogeneous crystallization temperature T(.,hom- This phenomenon is often called fractionated crystallization [73, 74]. The phenomenon of delayed crystallization was directly related to the... 

More recently KuHchikhin [83] has found that dispersed melt phases (droplets) organize themselves during shear flow into a series of parallel linear arrangements. This would seem to be the mechanism by which the filaments of the dispersed phases [71 to 76] of the previous paragraph are formed. 

Most elastomers that are used for nylon modification contain a small amount of maleic anhydride (0.3 to 2%). In the melt blending process, these elastomers react with the primary amine end groups in nylon, giving rise to nylon grafted elastomers. These grafts reduce the interfacial tension between the phases and provide steric stabili2ation for the dispersed mbber phase. Typically, thermally stable, saturated mbbers such as EPR, EPDM, and styrene—ethylene/butylene—styrene (SEBS) are used. 

In preliminary tests, melt mixed blends of PP and LCP were processed at six different temperatures (Tcyi 230, 240, 250, 260, 270, and 280°C) with a Brabender Plasti-Corder PLE 651 laboratory single-screw extruder. The measured melt temperatures were about 10°C higher than the cylinder temperatures (Tcyi). The objective was to study the influence of temperature on the size and shape of the dispersed LCP phase. Two different polypropylenes were used to ascertain the effect of the viscosity of the matrix on the final morphology. Different draw ratios were obtained by varying the speed of the take-up machine. 

The reactive extrusion of polypropylene-natural rubber blends in the presence of a peroxide (1,3-bis(/-butyl per-oxy benzene) and a coagent (trimethylol propane triacrylate) was reported by Yoon et al. [64]. The effect of the concentration of the peroxide and the coagent was evaiuated in terms of thermal, morphological, melt, and mechanical properties. The low shear viscosity of the blends increased with the increase in peroxide content initially, and beyond 0.02 phr the viscosity decreased with peroxide content (Fig. 9). The melt viscosity increased with coagent concentration at a fixed peroxide content. The morphology of the samples indicated a decrease in domain size of the dispersed NR phase with a lower content of the peroxide, while at a higher content the domain size increases. The reduction in domain size... 

Another result shown in Table 3.15 is the slight shift of the main phase transition towards lower temperatures. Similar results have been found for water ethanol dispersions of DPPC [445,453,454]. The strong influence of ethanol on the enthalpy of the main phase transition of DMPC water-ethanol dispersions shown in Table 3.15 is similar to the substantial increase in this enthalpy in the case of water-ethanol dispersions of DPPC [445,453]. Thus, a correspondence is found for the temperature of the chain-melting phase transitions in the cases of foam bilayers and the fully hydrated water-ethanol dispersions of DMPC. 

Hence, the interaction between lipid molecules is very similar in these foam bilayers and it can be supposed that the AF foam bilayers are in the liquid crystalline state within the temperature range studied. This assumption is in agreement with the fact that amniotic fluid contains substantial amount of unsaturated phospholipids, which as known [45], lower considerably the temperature of the chain-melting phase transition. Bearing in mind the similarity of the phase behaviour of a phosphatidylcholine aqueous dispersion and foam bilayers [38-40], it can be supposed that at the temperatures which are important for in vivo systems, the foam bilayers are in the liquid crystalline state. This assumption allows to determine the critical concentration of phosphatidylcholines in amniotic fluid, necessary for formation of a foam bilayer by extrapolation of the Arrhenius dependence of C, for AF foam bilayers to 37°C. Thus, at 37°C C, = 19.9 jxg cm 3 and d, = 1.47. This value of C, at 37°C corresponds to the lower limit (found by other methods [46,47]) of phosphatidylcholine concentration which permits to classify as mature a sample of amniotic fluid. The above value... 

Another interesting phenomenon - the fractionated crystaUisatiOTi of PTFE -could be observed by DSC. PTFE melts near 325°C and crystallises near 310°C. The crystallisation behaviour of the dispersed PTFE phase in a PA matrix is influenced by the melt viscosity of the polyamide matrix, the processing conditions in twin-screw extruders and the irradiation dose of PTFE. With increasing... 

Provided Mn was greater than 3000 g/mol, both atactic and isotactic PP-g-MA enhanced the dispersion of PA6 in the continuous PP matrix. Because of the miscibility of atactic and isotactic PP in the melt phase, stereoregularity did not influence PA6 dispersion, which was controlled primarily by the PP-g-... 

In immiscible blends, the phases are separated in the molten state, before crystallization of the matrix starts. The dispersed amorphous phase is assumed to be homogeneously distributed in the melt in droplet-like domains. 

The behavior of binary blends with only one crystaUizable component has been studied by several authors, who have investigated different systems. The crystals of the crystaUizable matrix have grown in equilibrium with their own melt phase. The presence of separate domains of non-crystallizable component, dispersed in the molten matrix during the crystallization process, (owing to the kinetic and morphological effects), may cause a depression of the observed melting tem-... 

Shingankuli et al. [1988] studied the crystallization behavior of dispersed PET droplets in a PPS matrix. A serious increase of the crystallization temperature of the dispersed PET phase (by about 20°C) during cooling experiments from the melt, was explained as a result of... 

R. E. Jacobs, B. Hudson, and H. C. Andersen, A Theory of the Chain Melting Phase Transition of Aqueous Phospholipid Dispersions, Proc. Natl. Acad. Sci. USA 11, 3993-3997 (1975). 

During the last few years, most attention has been paid to the blending of PLCs with less expensive thermoplastic engineering polymers (EPS). Addition of PLCs to such polymers not only enhances mechanical properties (strength and stiffness) of the resulting composites obtained due to the orientation of the PLC phase, but also improves their processing properties. Even relatively small amounts of a PLC may induce a reduction in the melt viscosity and thus improve the processability. In most cases, under appropriate processing conditions the dispersed PLC phase can be deformed into a fibrillar one. The... 

Because of these reasons, it would be desirable to find an approach in which the reinforcing elements are not present before processing but are formed during the extrusion or injection molding process. To produce these in situ composites, a thermotropic PLC is first blended with a thermoplastic in the melt. During the subsequent extrusion or injection molding, the dispersed PLC phase is deformed into the fibrillar... 

A further variation was proposed by the Finnish company Neste Oy and is at the stage of being tested in various ESD applications by Panipol, which attempted to prepare a melt processable polyaniline [61 ]. It still remains a matter of debate what the melt behavior they observed resulted from, but it was evident that the resulting blend again is a two-phase system with nanosize network structures formed by the dispersed PAni phase. 

Nanocomposite technology using small amounts of silicate layers can lead to improved properties of thermoplastic elastomers with or without conventional fillers such as carbon black, talc, etc. Mallick et al. [305] investigated the effect of EPR-g-M A, nanoclay and a combination of the two on phase morphology and the properties of (70/30w/w) nylon 6/EPR blends prepared by the melt-processing technique. They found that the number average domain diameter (Dn) of the dispersed EPR phase in the blend decreased in the presence of EPR-g-MA and clay. This observation indicated that nanoclay could be used as an effective compatibilizer in nylon 6/EPR blend. X-ray diffraction study and TEM analysis of the blend/clay nanocomposites revealed the delaminated clay morphology and preferential location of the exfoliated clay platelets in nylon 6 phase. 

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