Crystallization network formation

Dittmer JJ, Lazzaroni R, Leclere P, Moretti P, GranstrOm M, Petritsch K, Marseglia EA, Friend RH, Bredas JL, Rost H, Holmes AB (2000) Crystal network formation in organic solar cells. Sol Energy Mater Sol Cells 61 53... 

The initial drying of currently applied alkyd paints is accomplished by evaporation of solvent (physical drying). Subsequently, the eventual curing of the alkyd paint is completed by the formation of a polymer network, which is mainly formed by chemical crosslinks (oxidative drying) but in some cases also physical interactions between the fatty acid side chains occur, such as crystallization or proton-bridge formation [129]. Efficient network formation is crucial in the formation of dry films with good mechanical properties. Due to the presence of unsaturated units in the investigated LOFA- and TOFA-PHA bin-... 

Figure 8-42 Structure Formation in Particulate Systems. (A) Flocculation of an emulsion. (B) Network formation in crystallized fat.

DeMan and Beers (1987) have reviewed the factors that influence the formation of three-dimensional fat crystal networks. The fat crystal networks in plastic fats (Figure 8-44) are highly thixotropic, and mechanical action on these products will result in a drastic reduction of hardness. 

Polymers with longer side-chains (Ciq and Cjs) exhibit non-Newtonian viscosity at room temperature, and the viscosity decreases with increasing shear rate. Because the side-chain T s for and C s polymers are above room temperature, network formation via intermolecular crystallization of the paraffinic side-chains is believed to be responsible for the unusually high... 

Oil exudation results from a reduction in the volume of the fat crystal network over time and is due to the formation of strong primary or secondary bonds. External pressure also is an influencing factor that is particularly relevant with wrapped products stacked at a low point in a pile in storage. Empirical tests include measuring the oil exuded under controlled pressure on absorption into pre-weighed paper (91). 

In addition, PET can be used to form semi-IPNs with other naturally functionalized triglyceride oils, such as vemonia oU (31). The procedures for PET/vemonia semi-IPNs are essentially the same as those of PET/castor ones, but with important differences. For PET/castor mixtures, the diisocyanate cross-hnker was added at 240°C and the mixture was poured into the molds rapidly before the castor gel point had been reached. In this case, PET/castor polyurethane semi-IPNs were formed, in which crystallization and gelation occurred simultaneously resulting in a single, broad glass transition temperature. For PET/vernonia, the sebacic acid was added at 280°C, which reduced the temperature to about 250°C, where the mixture was held for another 5 min, then poured into a preheated mold and allowed to cool, during which time the PET crystalhzes. In this case, PET/vernonia polyester network was formed, and the PET crystallized prior to network formation because the latter... 

FIGURE 17.23 Various stages in the formation of a fat crystal network. Approximate scale the edge of a frame would be several pm. 

The liquid-solid interface supports the growth of 2D crystals on surfaces too. The liquid phase acts as a reservoir of dissolved species which can diffuse towards the substrate, adsorb, diffuse laterally and desorb. These dynamic processes favor the repair of defects. Under equihbrium conditions, relatively large domains of well-ordered patterns are formed. Large domains grow at the expense of small domains via a process which is called Ostwald ripening. Furthermore, the solvent plays a significant role in the network formation. The choice of solvent affects the mobility of molecules, especially, the adsorption-desorption dynamics via the solvation energy and possibly also via solvent viscosity. 

In MEH-PPV modified with tetrabenzyl perylene-3,4,9,10-tetracarb-oxylate, it has been shown that an annealing process effects the formation of crystal networks within the polymer. This network considerably... 

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