Composite materials 8, 24 Desorption rate

Polymer chemists use DSC extensively to study percent crystallinity, crystallization rate, polymerization reaction kinetics, polymer degradation, and the effect of composition on the glass transition temperature, heat capacity determinations, and characterization of polymer blends. Materials scientists, physical chemists, and analytical chemists use DSC to study corrosion, oxidation, reduction, phase changes, catalysts, surface reactions, chemical adsorption and desorption (chemisorption), physical adsorption and desorption (physisorp-tion), fundamental physical properties such as enthalpy, boiling point, and equdibrium vapor pressure. DSC instruments permit the purge gas to be changed automatically, so sample interactions with reactive gas atmospheres can be studied. 

TG is frequently used for analysing the composition of adhesives by quantifying the amount of moisture which is present and the amount of volatiles associated with a reaction. Fast heating rate TG allows detection of very low levels of volatiles in small samples. TG is also used for the quantitative determination of solvents in polymeric additives used as pour-point depressants and flow improvers [220], PET moisture analysis by means of TG can be carried out at ppm level [221]. Thermogravimetry (eventually combined with GC or IR and subambient DSC) is very useful for the determination of residual solvents or for the study of interactions of water with polymers (important for modified release formulations for which swelling or gel formation of polymeric excipients is relevant). TGA has also been employed to measure the continuous desorption of sorbed SCCO2 in polymeric materials [222]. 

Often, water reacts with the polymer matrix and causes irreversible chemical changes and diminishes performance. The process of moisture absorption and desorption on the surface layers takes place almost immediately on contact with the environment, but moisture diffusion into the bulk material is usually a slow process. It takes weeks to months for a substantial amount of moisture to be absorbed by the composite and long time (i.e., 1-2 years) before the material reaches saturation. The rate of moisture uptake by a composite laminate depends on the relative humidity, temperature, exposure time and mechanical load. Elevated temperatures accelerate the rate of moisture uptake and material degradation. Similarly, tensile loads accelerates moisture uptake by opening existing internal cavities or voids, and by micro-crack formation. The effect of moismre on polymer composites is potentially... 

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