D5 FTIR/Raman Spectroscopy

Topic

FTIR spectroscopy—the first of the two experimental methods described by this model—illuminates the polymer with infrared light and records the absorbance of the characteristic bands produced by its functional groups within the spectrum; the Beer-Lambert law governs the absorbance of each characteristic band, such that a higher concentration of the corresponding functional group, or a greater sample thickness traversed by the beam, results in a proportionally higher absorbance for that band. Consequently, a higher concentration of a specific functional group within the polymer causes the band characteristic of that group to exhibit higher absorbance, in accordance with the Beer-Lambert law.

The ratio of absorbances between a specific pair of bands in the spectrum—selected for their sensitivity to the polymer chain conformation—determines the polymer's crystallinity index (a parameter described in greater detail by another model in this set): the more ordered the polymer chains are (i.e., the more they exist in crystalline phases), the more this band ratio shifts toward the band associated with that ordered conformation, resulting in a higher crystallinity index. Conversely, a shift of this conformation-sensitive band ratio toward the band associated with the disordered conformation results in a lower crystallinity index. Thus, this model—utilizing the FTIR spectroscopy and band ratio with which it was initiated—tracks the corresponding degree of conversion via a distinct reactive band within that same spectrum (whether during the polymer's curing or its subsequent degradation). As this reactive band loses intensity—because the functional group producing it is consumed during curing or destroyed during degradation—the degree of conversion advances in direct proportion to the band's attenuation; this process is conceptually equivalent to how another model, applied to a different set of materials, tracks the curing conversion of a fiber-reinforced matrix. Consequently, the further this reactive band attenuates within the time-resolved FTIR spectrum, the further the degree of conversion—whether curing or degradation—advances.

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