Porosity, polymer, factors affecting

Current understanding of the erosion of polymer laminates has come chiefly from rain erosion studies of composite aircraft materials. Analogies have been drawn between cavitation and liquid impingement processes on metallic materials both involve extensive structural damage early in the incubation period as a result of short-term shock pulses. Furthermore, the deformed surfaces of materials which are subjected to both processes are very similar, and so are the methods used to improve their performance [99]. Attempts have been made to prioritize the factors affecting erosion of FRP by liquid impingement, with more or less predictable results [100] in respect of resin composition, fiber volume fraction and orientation, porosity and thermal conductivity. All these factors were found to affect performance. 

This chapter discusses crosslinked polymers with permanent porosity and the factors affecting their synthesis, formation mechanism, and properties. A characterization of these polymers can be made based on network formation theory or on experimental results. Because of differences between experimental results and theoretical predictions, individual methods are used to indicate areas of uncertainty that need further exploration. 

The factors, which influence the permeability or mass transport, are the following chemical composition of the polymer matrix and its free volume. In fact, crystallinity, molecular orientation, and physical aging in turn influence the free volume of a polymer matrix. In addition, porosity and voids, like free volume, offer sites into which molecules can absorb and are far less of a barrier to transport than solid polymer. Temperature also affects permeability and diffusion properties of small molecules in polymers. With increased temperature, the mobility of molecular chains (in polymer) increases and thermal expansion leads to reduced density therefore, the free volume in the system will increase. External tensile stress applied is expected to increase free volume and open up internal voids or crazes, providing additional sites into which molecules can absorb. Of course, there may be unquantified internal residual stresses, arising from processing, present in the polymers. It is well established that the properties of materials... 

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