Interchain crosslinking

Flow Behavior of Polymer Blends as Affected by Interchain Crosslinking... 

The treatment of blends as a two phase system opened up an interesting field of modifying the composite properties by the use of a (third component within the interface boundaries, which is termed as compatibilizers [1]. Such modifications are still being extended to the formation of microgel out of the interaction between the two blend partners having a reactive for functionalities. This type of interchain crosslinking does not require any compatibilizer to enhance the blend properties and also allows the blends to be reprocessed by further addition of a curative to achieve still further improved properties [3,4]. Such interchain crosslinking is believed to reduce the viscoelastic mismatch between the blend partners and, thus, facilitates smooth extrusion [5,6]. 

The non-Newtonian index is plotted against the blend ratio in Fig. 2. There are three distinct stages of the change of value with %NBR in the preblend. First, a decrease of up to 40% of NBR, a rapid rise of up to 60% of NBR, and beyond this ratio a further decrease are observed. Heating of blends shows the only minimum at 50 50 ratio. It is obvious that 60-50% of Hypalon in the NBR-Hypalon blend is an optimum range where maximum extent of interchain crosslinking reaction is expected, and this blend is supposed to be more pseudoplastic. 

Earlier studies [14,15] clearly reveal that there is a reaction between two polymers and that the extent of reaction depends on the blend ratio. As 50 50 ratio has been found to the optimum (from rheological and infrared studies) ratio for interchain crosslinking, the higher heat of reaction for the NBR-rich blend may be attributed to the cyclization of NBR at higher temperatures. There is an inflection point at 50 50 ratio where maximum interchain crosslinking is expected. Higher viscosity, relaxation time, and stored elastic energy are observed in the preheated blends. A maximum 50-60% of Hypalon in NBR is supposed to be an optimum ratio so far as processibility is concerned. 

The non-Newtonian index is plotted against the blend ratio in Fig. 8. We see that for both the preblends and the preheated blends, the w value increases with NBR content up to about 50% of NBR. Beyond this while the preblend shows a continued increase in n value at a slower rate, the preheated blends seem to show a saturation in n value. The low values of may be attributed to the interchain crosslinking at higher levels of NBR in the blends. 

The product of the reaction (5.5.29) is, however, unstable against subsequent interchain crosslinking and insertion of supporting electrolyte cations. The electrochemical carbonization of fluoropolymers was recently reviewed by L. Kavan. 

Early in the development of liposomes, it was recognized that their plasma instability could be a serious detriment in certain applications. Consequently, there were efforts to Lrst incorporate polymerizable lipids into the liposomal bilayers, and then initiate polymerization by, for example, photolysis, to form interchain crosslinks to stabilize the bilayer. The most commonly used polymerizable lipids have been PCs-containing diacetylene or butadiene moieties in the tailgroups... 

Another laboratory invoked the role of water in bread staling involving a zipper mechanism that results in the formation of interchain crosslinks 492 Moisture redistribution from crumb to crust played a significant role in crumb firming and amylopectin recrystallization for breads stored for more than seven days 493 According to Piazza and Masi494 to inhibit staling it is more important to slow the dehydration phenomena than it is to increase the initial moisture content in the bread. 

Unfortunately, reactions (4.11a and 4.11b) do not stop at the stage of pure polyyne, -(C=C) -, but there are two other pathways of product transformation. Polyyne is unstable against interchain crosslinking, which leads to sp2 carbon structures [33] ... 

In the following sections we describe the preparation of polymers which incorporated DEC chromophores either as components of a polymer backbone which is stabilized by interchain crosslinking or as a pendant to mainchain poljmers where the DEC chromophore is acting as the interchain crosslinking agent. The objective of this research, which has been realized, is the retention of significant optical nonlinearity at elevated temperatures (e.g., 125 °C) for extended periods of time. This is accomplished with the realization of an acceptable magnitude of optical nonlinearity (x(2) = 100-300 pnVV). 

Recently, Prigodin and Efetov have developed a theoretical model for interchain interaction at the M-I transition in a random network of coupled metallic chains [94]. In this model, the interchain disorder due to intrinsic defects and the randomness in the distribution of interchain contacts induce localisation. The M-I transition in such a system is determined by the critical concentration of interchain crosslinks, which in turn depends on the localisation lengths and interchain coupling. Moreover, a metallic state can exist in such a random network of coupled metallic chains only if the concentration of interfibril contacts is large enough to overcome the percolation threshold. 

FIGURE 8.37 Schematic diagram of an interchain crosslink between neighboring PPy chains induced by radical coupling. (From Song, K.T., Synthesis of electrically conducting soluble polypyrrole and its characterization. Ph.D. thesis, 2000. With permission.)... 

To obtain water-insoluble sorbitol copolyesters, 1,8-octanediol was used in place of a fraction of sorbitol in the monomer feed. Adipic acid, 1,8-octanediol, and sorbitol were copolymerized in the molar ratio 50 35 15 (Table 1, entry 2). The methanol-insoluble product had an Mw of 1.17x10. The solubility in water and THF of entry 1 and 2 products, respectively, is direct proof that they have few interchain crosslinks. Furthermore, 0.6- 1.0% w/v solutions of the products passed through 0.45 Um filters with complete recovery of the products which demonstrates the absence of microgels. This was shown by the fact that 99.9%wt of polymer was recovered when a known concentration of polymer solution was passed through the filter and the solvent was evirated. 

Pendent sulfur groups are the supposed precursors prior to an interchain crosslink formation. The synthesis of model pendent groups containing benzothiazolyl functions [132,133] has enabled their thermal behavior to be studied directly. Again, the experiments evidenced that the zinc complexes play an important active role in the desulfuration reaction of the pendent polysulfidic groups. 

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