Crosslink distribution

Other imperfections are developed in the process of crosslinking network defects (unreacted functionalities, intramolecular loops, chain entanglements), inhomogeneity in crosslink distribution, or heterogeneity of the network due to phase separation. These four types of network imperfections are interdependent and a sharp borderline between them does not exist. 

The latter result shows that to interpret the mechanical properties of networks we do not need to take into account the spatial inhomogeneities of crosslink distribution in a sample, at least in the rubbery state. The analysis of epoxy networks performed under the framework of a tree-like model and experiments 7,10-26) brought the... 

Sakamoto, H., Kumazawa, Y, Kawajiri, H., and Motoki, M. (1995). s-(y-glutamyl)-lysine crosslink distribution in foods as determined by improved method. J. Food Set. 60, 416-419. Stryer, L. (1995). Biochemistry. 4th edition. WH Freeman Company, New York, pp. 252-258. Toldra, F. (1992). The enzymology of dry-curing of meat products. In F. Smulders, F. Toldra, J. Flores, and M. Prieto (eds.). New Technologies for Meat and Meat Products, Audet, Nijmegen, pp. 209-231. 

An example system is PVC and polyethylene wire and cabling irradiated to improve stresscracking resistance, abrasion resistance, high-temperature properties and flame retardance, via controlled electron-beam crosslinking (Loan, 1977). Additionally electron-beam crosslinking is utilized to impart memory into a polymer system, such as crosslinked PE materials for heat-shrinkable films and pipe applications (Baird, 1977). The control of electron-beam processing has advanced the quality of cell size and shape of PE foams via control of crosslink distribution (Paterson, 1984). 

The copolymerization diagrams plotted for the mixtures with the molar ratios of p- and m-DVB ranging from 2 1 to 1 4 show that when the p-DVB content in the initial monomer mixture increases, the nonuniformity in crosslinks distribution in the resulting network is also increased (Fig. 1.2). 

Trying to completely avoid the technically unpleasant process of chloromethylation, Negre et al. [48, 49] prepared a linear styrene copolymer with p-vinylbenzyl chloride and then subjected the product to self-crosslinking. Alternatively to the earlier-mentioned crosslinking of linear polystyrene with MCDE, this procedure results in local inhomogeneity of crosslinks distribution, because of the uneven distribution of the two comonomers along the initial chain (the monomer reactivity ratios of vinylbenzyl chloride and styrene are 1.41 and 0.71, respectively). Nevertheless, vinylbenzyl chloride became a popular comonomer for styrene and DVB in the preparation of beaded hypercrosslinked products [50-52]. 

Graessley [17,18] has developed a statistical mechanical calculating device based on small network regions, the micronet, for estimating the change in chain conformation as a consequence of crosslinking. He found that the front factor depended only on the functionality (/) and not on the details of network topology, formation or crosslink distribution. His final equation is... 

NMR of swollen rubber has been used to determine crosslink distributions in blends [233,234]. Swelling enhances chain mobility, and the isotropic motion of nuclei averages local fields, thereby narrowing the spectral lines.This allows individual resonances to be characterized. Crosslinking constrains motion and makes it more anisotropic, and thereby the incoherent averaging is less complete, broadening line widths (Fig. 21). Empirical correlations with crosslink density are required for quantitative results. 

Boochathum, P, Prajudtake, W. (2001). Vulcanization of cis-and transpolyisoprene and their blends cure characteristics and crosslink distribution, Eur. Polym. J. 37, 417 27. 

Apart from the elfect of the overall crosslink density of NR/NBR on the tensile strength of the blend, the crosslink distribution in separate phases is of great importance. Lewan" showed that for a 50/50 blend the higher tensile strength was evident for a NR/NBR crosslink density ratio of 1.0-1.2. Further tensile strength enhancement in a NR/NBR vulcanizate was obtained when methyl methacrylate grafted NR was added to the blend instead of NR." ... 

Without the use of functional side-groups, the network formatiOTi can also be realized by y-irradiation as demonstrated by Vazilets and co-workers. A benefit of this method is that good control of the crosslink distribution can be achieved and that the crosslinking works with non-functionalized and thus thermally stable polymers that are not light sensitive. However, for this method an irradiatimi source is required that is not usually readily available [20, 21]. 

Most experiments on LC elastomers have so far used single crystal elastomers made via the two-step crosslinking process, which involves stretching in the LC state. There is increasing evidence that this situation represents a special thermodynamic state - smectic elastomers made in such a way are well aligned but their layer positions are frustrated due to the random crosslink distribution. Evidently, there is room for experiments on nematic and smectic elastomer samples oriented in different ways, for example by photo-crosslinking. In such a way, any memory of the aligning procedure imprinted in the samples will be avoided (at least partially) and new features of phases and phase transitions could be revealed. 

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