Network structure, influencing

The mechanism how a rubber distributed in a network influences the rupture mechanism is not quite well understood yet. It is known that poly(vinyl chloride) forms shear bands when stress is applied and that parts of the rubber which are located in these shear bands may form crazes.13 It might well be that a network structure is efficient for the delocalization of stress energy only in combination with the formation of shear bands. Experimental work is needed to elucidate this further. 

Looking first at the tan 6 curves. Figures 2 and 3, it should be noted that the breadth of the alpha peak is influenced very little by the presence of the alkyl substituents. Imperfections in the network structure are reported to lead to a broadening of the alpha peak ( ). As mentioned earlier, the peak temperature of the alpha peak, a measure of Tg, follows exactly the trend in Tg s as measured by DSC. 

Several studies have considered the influence of filler type, size, concentration and geometry on shear yielding in highly loaded polymer melts. For example, the dynamic viscosity of polyethylene containing glass spheres, barium sulfate and calcium carbonate of various particle sizes was reported by Kambe and Takano [46]. Viscosity at very low frequencies was found to be sensitive to the network structure formed by the particles, and increased with filler concentration and decreasing particle size. However, the effects observed were dependent on the nature of the filler and its interaction with the polymer melt. 

In the case under consideration different physical structures were realized due to the formation of the polymer network in the surface layers the filler surface, as usually happens in filled systems. As is known79, this induces considerable changes in the structure of the material. It is also possible that in these conditions a more defective network structure is formed. These results show that even the purely physical factors influencing the formation of the polymer network in the interface lead to such changes in the relaxation behavior and fractional free-volume that they cannot be described within the framework of the concept of the iso-free-volume state. It is of great importance that such a model has been devised for a polymer system that is heterogeneous yet chemically identical. 

Sub-glass transitions are generally determined by the molecular (local) scale structure. Their location in the (t, T) space undergoes only a second-order influence of the macromolecular (network) structure through internal antiplasticization effects. By contrast, glass transition is directly under the influence of the network structure (Chapter 10), so that it appears interesting to study the influence of crosslinking on the parameters of the time-temperature relationship (WLF equation) ... 

For a given material, there are generally several possible mechanisms of yielding and fracture, each characterized by the influence of temperature, loading rate, hydrostatic pressure, time (physical aging). A vast literature deals with the influence of network structure on yielding or on fracture properties, but we have to be very careful with the results obtained because of the different types of networks used in these experiments. 

There is a vast amount of literature dealing with the influence of network structure (essentially crosslink density) on yielding. Key structural parameters related to yielding will be considered separately for ideal, nonideal, and inhomogeneous networks. 

Grillet et al. (1991) studied mechanical properties of epoxy networks with various aromatic hardeners. It is possible to compare experimental results obtained for networks exhibiting similar Tg values (this eliminates the influence of the factor Tg — T). For instance, epoxy networks based on flexible BAPP (2-2 - bis 4,4-aminophenoxy phenyl propane) show similar Tg values ( 170°C) to networks based on 3-3 DDS (diamino diphenyl sulfone). However, fracture energies are nine times larger for the former. These results constitute a clear indication that the network structure does affect the proportionality constant between ay and Tg — T. Although no general conclusions may be obtained, it may be expected that the constant is affected by crosslink density, average functionality of crosslinks and chain... 

When fine inorganic materials (e.g., bentonite, oxides) are added to water containing suspended particles, the dispersion becomes thixotropic and the inorganic materials form a three-dimensional network (i.e., gel) structure in the medium. The gel network has sufficient elastic properties. It entraps the particles and prevents settling and cake formation. The network structure is broken down upon shaking, thus facilitating pouring. However, the gel structure is influenced by pH and electrolyte concentration. 

In some cases, network structure is modified by aminolysis reactions25. An example is the polymer formed from diglycidylic ester of o-phthalic acid and diaminodiphenilmethane. Aminolysis makes the chain between crosslinks shorter and influences the properties of the polymer (dynamic shear modulus in a rubbery... 

The network structure does not markedly influence the local sub-Tg molecular motions. This is connected with essential differences in the mobility of aliphatic and aromatic polyfunctional fragments of the network. 

Fig. 17 Schematic representation of the temperature gradient occurring during the exothermic curing reaction. Owing to the comparatively high thermal conductivity of the continuous metallic filler (e.g. wires consisting of copper), exothermic reaction heat Q is conducted away from the interface. By changing locally the thermal conditions of the curing reaction, the temperature gradient T(N) may influence the resulting network structure D (N) which in turn defines the elastic properties of the epoxy, e.g. its elastic modulus E(N). jw denotes the heat current density...

The cytoskeletal network is responsible for the mechanical properties of the cell that modulate functions such as cell shape, locomotion, cytokinesis, and translocation of organelles. Experimental evidence suggests that the cytoskeleton also provides connections between cellular structures and presents a large surface area for interactions of various proteins and signaling molecules. Modulation of the cytoskeletal network may influence cell signaling, ion channels and intracellular calcium levels. Cytoskeleton is thus essential for regulation of cellular functions, cell integrity, and viability. 

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