Time-Resolved ATR of a Photopolymerization Reaction

Abstract

The photopolymerization curing process of a UV-curable polymer was investigated using time-resolved Attenuated Total Reflection (ATR) spectroscopy. Spectral band shifts and absorbance changes resulting from of UV lamp exposure were analysed to describe the chemical changes occurring during this reaction.

Introduction

ATR spectroscopy is a powerful tool for understanding the chemical structure of samples. Although ATR is most frequently used to analyze equilibrium chemical systems, it can also be used to analyse non-equilibrium systems, such as chemical reactions. Here we use the IRIS Monolithic Diamond ATR to study the spectral band evolution of a common UV-curing adhesive.

A typical reaction leading to photopolymerization is the radicalization of a thiol via absorption of an ultraviolet photon. The unstable radical then rapidly forms a bond with the terminal alkene group of a conjugated monomer, which forms another radical, thus propagating the polymerization reaction.

Experimental Methods

The photopolymer investigated was a popular UV-curing epoxy, Norland Optical Adhesive-61 (NOA-61). To observe the spectral changes during the photopolymerization reaction, time-resolved ATR spectra were collected at 1.67 s intervals. Spectra were collected on a commercial FTIR spectrometer, equipped with a DTGS detector and IRIS diamond ATR (PIKE Technologies, Madison, WI). The spectral resolution was 4 cm-1 and 1 scan per time interval was used to ensure maximum possible time resolution.

An ultraviolet lamp emitting 365 nm light was used to initiate the photopolymerization reaction, 38 seconds after the start of the time-resolved ATR measurement.

Results

The ATR spectrum of the monomer, prior to exposure with the UV lamp is shown in Figure 2. The spectral band evolution of the adhesive before and during photocuring is shown as a time, wavenumber, ΔAbsorbance contour plot in Figure 3, emphasising the multidimensionality of the time-resolved dataset and the temporal progression of the photopolymerization reaction.

A series of spectral band shape changes occur across the 600-1200 cm-1 spectral region, as shown in Figure 3. Notably a negative ΔAbsorbance feature appears near 924 cm-1, consistent with a reduction in the out-of-plane bending of the =CH2 groups common in photoinitiators prior to curing.

Conclusion

The IRIS Monolithic Diamond ATR accessory provides high optical throughput from over the spectral region of critical absorbing species present in photopolymers. By performing a time-resolved ATR investigation of a photopolymerization reaction, the spectral lineshape changes of a photopolymer before during and after the photocuring process can be quantitated. Given the high optical performance of the IRIS, combined with robust and inert physical properties of its monolithic diamond ATR crystal, the IRIS is a highly capable ATR accessory for performing analyses of equilibrium and non-equilibrium chemical species.