A4 Moisture Absorption and Environmental Aging

Topic

When this polymer matrix composite is exposed to the environment, ambient moisture diffuses into the matrix following Fickian diffusion: moisture moves from the composite's surface toward its interior, creating a concentration profile that evolves in space and time much like any diffusion process driven by a concentration gradient. Continued environmental exposure causes moisture to keep diffusing into the matrix.

However, this moisture diffusion into the matrix does not continue indefinitely; over time, the matrix reaches a saturation moisture content defined by Henry's isotherm. A higher ambient relative humidity results in a proportionally higher saturation moisture content—consistent with that isotherm—while a lower relative humidity results in a proportionally lower saturation level.

As the polymer matrix absorbs moisture and approaches saturation, the moisture plasticizes the matrix: water molecules lodge between the polymer chains, increasing the free volume between them and facilitating chain slippage, much like a plasticizer deliberately added to the polymer. This plasticization lowers the matrix's glass transition temperature: the higher the absorbed moisture content, the lower the temperature at which the matrix transitions from behaving as a rigid solid to behaving as a soft material. In addition to plasticizing the matrix, this same moisture absorption generates a hygroscopic stress within the composite: the matrix swells locally as it absorbs moisture—analogous to thermal expansion, yet driven here by moisture concentration rather than a thermal gradient. Because this hygroscopic swelling of the matrix is ​​constrained by the composite's fibers—which neither absorb moisture nor swell in the same way—an internal hygroscopic stress arises, analogous to the thermal stress produced by a temperature gradient rather than a moisture gradient.

Consequently, this model—based on the Fickian diffusion of moisture into the polymer matrix—describes an anomaly unique to polymer-matrix composites: the fact that the matrices of metal-matrix or ceramic-matrix composites do not absorb moisture in the same way as polymer matrices means that matrix plasticization, the reduction of the glass transition temperature, and the hygroscopic stress described by this model do not occur in those other two types of composites.

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