Cross-linking dilatancy

Faergemand, M., Murray, B.S. (1998). Interfacial dilatational properties of milk proteins cross-linked by transglutaminase. Journal of Agricultural and Food Chemistry, 46, 884-890. 

Similar reversible contraction/dilation experiments under constant load were recently performed by Eisenbach on stretched poly(ethyl acrylate) networks, cross-linked however with 4,4 -dimethacryloylaminoazobenzene (0.02 mol-%). 

It is interesting that, upon rubber modification, the CET resin matrix can no longer form dilatation bands (18). Only rubber-particle cavitation and matrix shear yielding are detected. This observation implies that a dilatational stress component is required to trigger the formation of dilatation bands. In other words, upon rubber-particle cavitation, the dilatational stress component in the matrix is reduced. This suppresses the formation of dilatation bands. This conjecture finds support in the work of Glad (27), who investigated thin-film deformation of epoxy resins with various cross-link densities and could not find any signs of dilatation bands in his study. 

Importance of the Findings. The present work indicates that the formation of crazing and dilatation bands in moderately cross-linked engineering thermosets can be quite common. Therefore, it is important to determine whether crazing and dilatation bands are effective in toughening thermosets. It is equally important to find out how to promote these types of fracture mechanisms, either via modification of network architecture or incorporation of toughening agents. 

There is experimental evidence, for many rubber-toughened polymers, that the rubber particles cavitate early in the deformation. The degree of cross-linking is kept relatively low in the polybutadiene phase of ABS to aid cavitation, and sometimes silicone oil is added for the same reason. Figure 4.12 shows both the conventional stress-strain curve and the volumetric strain versus tensile strain for rubber-modified polystyrene. When the polystyrene yields, the volume strain increases at a higher rate. Majority of the dilatational strain is due to cavitation in the rubber phase. 

Abraham G, Purushothaman E. 1998. Synthesis and photostimulated dilation changes of polymers with azobenzene cross links. Indian J Chem Technol 5(4) 213 216. 

On cooling a polymer mixture of a hard and a pliant component, the hard phase will try to desegregate from the pliant phase because of the different expansion coefficients of the demixing components. The two components are bound to each other, however, when the hard component is grafted onto the pliant component. But a non-cross-linked pliant phase further shrinks on cooling, which leads to stresses being set up in the pliant phase. Formation of holes within the pliant phase releases the stresses. If the pliant phase is cross-linked, it must dilate on cooling since it is under... 

Phosphoric acid functions both as an acid catalyst to promote bond cleavage and the formation of cross-links via cyclization and condensation reactions, and to combine with organic species to form phosphate and polyphosphate bridges that connect and cross-link biopolymer fragments. The addition (or insertion) of phosphate groups drives a process of dilation that,... 

RIB Ribeiro, C.P. and Freeman, B.D., Sorption, dilation, and partial molar volumes of carbon dioxide and ethane in cross-linked poly(ethylene oxide). Macromolecules, 41, 9458, 2008. 

Rubbers swell if a solvent penetrates. This isotropic dilatation becomes particularly large if cross-linked polyelectrolytes are swollen by water. The equilibrium state here is controlled by both the osmotic pressure of the mobile counter-ions and the non-linear network properties. 

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