Microvoid structure

The parallel array of fibrils and voids may be the more appropriate model for the fiber after orientational drawing, for example. In any event, fibers that tend to have relatively small values of R, whether it be Rr, Rc, or Rs, and large values of can be described as having a fine microvoid structure, and vice versa large R values and small Ns values are characteristic of a coarse microvoid structure. 

Microindentation anisotropy 143, 145 Microspherulitic structure 139 Microvoiding 193 Microvoids 194, 205 Model membranes 49, 53, 55 Modulus 135 Moisture sensitivity 199... 

The inhomogeneous structure of the a-Si H films most likely leads to H2 motion that cannot be described as uniformly diffusive throughout the film. It has been suggested that H2 motion occurs along microvoids, present even in device quality films, that are aligned preferentially perpendicular to the films surface (Vanderheiden et al., 1987). These oriented microvoids can be considered as remnant micro-columnar structure in the films. The fact that the amount of H2 that can be reintroduced can never... 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

The principal physical structural parameters that control the modes of deformation and failure and mechanical response of epoxies are (1) macroscopic inhomogenieties such as microvoids or concentrations of unreacted monomer, (2) the glassy-state free volume and (3) the crosslinked network structure characteristics. 

The flexibility and extensibility of a crosslinked epoxy network are determined by the available glassy-state free volume. If the free volume is insufficient to allow network segmental extensibility via rotational isomeric changes then the brittle mechanical response of the epoxy glass is not controlled by the network structure but rather by macroscopic defects such as microvoids. For epoxies with sufficient free volume that allows plastic network deformation the mechanical response is controlled by the network structure. 

Although structurally Kevlar fibres are highly crystalline or ordered it is interesting to note that appreciable moisture is absorbed at equilibrium. For example in variant T950 the moisture uptake is about 5 % at 22 °C and 55 % relative humidity. As discussed in Sect. 4.1.4.1 it is likely that the water is retained in microvoids distributed close to the surface of the fibres. Certainly in the short term there appears to be little effect of moisture on the tensile properties. 

The choice of dimension D, depends on the value of relation dhldm [9], At dm<0,6dh interaction of diffusant molecules with walls of free volume microvoid is small and transport process is controlled by fractal dimension of structure (structural transport). At dm<0,6dh on transport processes has strong influence interaction of diffusant molecules with walls of free volume microvoid, which are polymeric macromolecules surface with dimension >/(/)/ is the dimension of excess energy localization regions) [10], In this case Dt=Df (molecular transport) [9] is adopted. 

The followers of a phase transformation toughening (PTT) state that the change from a less densely packed crystalline structure (f)) to a more packed one (a) (i) promotes microvoiding in the earliest stages of the deformation and (ii) facilitates plasticity due to its exothermic character [72,78,193]. 

Finally, a number of associated techniques have been used to sited light on the phenomenon of molecular fracture. These techniques do not estimate the inddence of fracture events but rather omcern phenomena or structures whidi bear strongly on the subject. Thus infra-red spectroscopy is employed to demonstrate the state of molecular stress prior to fracture, whilst low angle X-ray studies can be used both to assess the distribution of strain within a semi-crystalline polymer and to detect and measure the microvoids which are assodated with molecular fracture in such materials. 

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