Near-infrared NIR analysis of polymer reactions

In condensation polymerization, the use of NIR to follow the reactant concentrations at elevated temperatures (200-300 °C) has been applied to the synthesis of aromatic and aliphatic polyesters (Dallin, 1997). This provides an accurate alternative to the routine measurement of acid values, hydroxyl number and viscosity, with the added advantage of providing continuous data throughout the polyesterification, which allows optimum conversion. Related examples of the use of NIR include studies of esterification of low-molar-mass analogues (Blanco and Serrano, 2000) or transesterification of an existing polymer or copolymer (Sasic et al, 2000). The NIR method allowed quantitative determination of rate constants, end points and yield and equilibrium constants as well as mechanistic information, which allowed the esterification to be optimized through the use of higher temperatures and an excess of acetic acid (Blanco and Serrano, 2000). 

The assignment of the bands in the spectrum to combination and overtone bands of the fundamental vibmtions assigned earlier for both TGDDM and DDS (Tables 3.2 and 3.3) is shown in Table 3.4 for both the resin and the amine. 

Peak position (cm ) Band assignment (and fundamental bands) 

Primary-amine combination (N-H symmetric stretch + C-N stretch) Primary-amine combination (N-H symmetric stretch + deformation) Primary-amine combination (N-H asymmetric stretch + deformation) Methylene first overtones (C-H stretch) 

Primary-amine first overtone (N-H symmetric stretch) 

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