NONINVASIVE MONITORING OF BULK-POLYMERIZATION USING SHORT-WAVELENGTH NEAR-INFRARED SPECTROSCOPY

Short-wavelength near-infrared spectroscopy is evaluated as a method f or monitoring the percent conversion of methyl methacrylate to poly(me thyl methacrylate) in situ in a mold. To reduce lensing effects of the mold and Schlieren patterns caused as the reaction progressed, the il lumination beam was modified to pass through the sample twice, using a retroreflecting array. The major spectral change observed involved th e conversion of the vinyl group of the methyl methacrylate monomer to the methylene groups of the polymer. Observation of this change was en hanced by taking the second-derivative spectra of the sample and subtr acting the initial monomer spectrum from it. With this preprocessing s tep, the disappearance of the third overtone stretch of the CH of the vinyl group at 868 nm could be observed together with the concomitant appearance of the CH of the methylene group at 890 nm. The spectral da ta at these two wavelengths showed an excellent correlation to percent conversion as determined by two independent reference methods.