Crosslinking microvoids

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. 

Relationships between microvoid heterogeneity and physical properties in crosslinked elastomers, poly-(isobutylene-/7-methylstyrene-p-bromomethylstyrene) (PIB-PMS/BrPMS) terpolymers, were identified by a 3D-NMR imaging study. Three-dimensional reconstruction of the sample images reveals that the voids are spherically shaped. The experimental results indicate that high microvoid density in cured elastomers leads to crack initiation and accelerated crack growth, thereby resulting in premature mechanism failure of the materials. 

The fluctuation free volume in crosslinking epoxy polymers has a fractal structure and the microvoids formed in the matrix are simulated by Df dimensional spheres. The size of a microvoid is considered as the volume that is necessary for its formation and is a consequence of the accumulation of thermal fluctuations. 

Thns, the flnctnation free volume in a crosslinking epoxy polymer has a fractal structure and the microvoids, forming are simulated by Df dimensional sphere. The volume that is necessary for accnmulation of the thermal fluctuation energy, sufficient for its formation controls the size of a microvoid. 

There is evidence to suggest that exposure to humid environments, at temperatures above about 60°C can produce permanent damage in epoxy adhesives.Apicella and coworkers have explained this in terms of the formation of microcavities produced by clusters of water molecules. This mechanism of damage is expected to be evident in thermoset resins in which the rigid crosslinked structure does not allow the matrix to relax after microvoid formation. 

Let us consider the physical significance of the model and the determined dimension As is well known, both the length of a macromolecule part between the crosslinking nodes, having molecular mass [160], and the sizes of the free volume microvoids [84] for crosslinked polymers possess a certain special distribution. For the studied... 

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