Photo-Oxidation of High Performance Epoxy Networks: Correlation between the Molecular Mechanisms of Degradation and the Viscoelastic and Mechanical Response

Abstract

The photo-oxidative degradation of a highly cross-linked epoxide/diamine network has been investigated by FTIR spectroscopy, dynamic-mechanical thermal analysis (DMTA) and compressive mechanical tests. The FTIR measurements allowed us to monitor the degradation process and to obtain reliable kinetic data. Two dimensional infrared (2D IR) correlation spectroscopy, was employed to analyze the spectra collected as a function of time. This approach effectively enhanced the spectral resolution and revealed details about the reaction mechanism hardly detectable in the one-dimensional, frequency domain. In particular, the sites of initiation of the auto-oxidative sequence were identified, and the main pathways through which the degradation reaction proceeds were proposed. Dynamic-mechanical measurements performed in a multifrequency mode were used to reveal the effect of the photo-oxidative degradation on the relaxation processes of the epoxy network. It was found that the T g decreases with increasing the exposure time. An analogous trend was found for the storage modulus, E′, at temperatures above the glass transition. The overall cross-linking density was found to decrease as a consequence of the chain scission reactions. A marked reduction of the mechanical performances of the material was evidenced at high exposure times, especially in terms of yield-strength and strain.

Spectroscopic, Mechanical and Dynamic‐Mechanical Studies on the Photo‐Aging of a Tetrafunctional Epoxy Resin

“…[6] It was found that for all profiles the best fit was achieved with a first order model (n ¼ 1), which yielded a very satisfactory description of the experimental data. The curves obtained by Equations 3 or 4 through a non-linear least-square regression of the experimental data are reported, as continuous lines, in Figures 2-4 that the methylene groups are the more susceptible to photo-oxidation and, possibly, the site of initiation of the whole process.…”

“…The first step of the curing schedule was carried out at 140 8C for 16 h, followed by a post-curing step at 200 8C for 4 h. Further details about the sample preparation are reported elsewhere. [6] The photoaging experiments were carried out at 50 8C and 50% of relative humidity in an environmental chamber equipped with a UV bulb lamp mainly emitting in the spectral range of 320-600 nm (simulating the solar spectrum). Transmission FTIR spectra were obtained using a PerkinElmer System 2000 spectrometer.…”

“…On the other hand, the two carbonyl bands centred at 1718 and 1665 cm À1 increase substantially with exposure time. Among the various carbonyl species formed by photo-oxidation the two dimensional FTIR spectroscopy [6] identified essentially amide groups at 1675 cm À1 , aldehyde/ketone functionalities at 1728 cm…”

“…[11,12] Thus, the rubber elasticity theory along with the experimental storage modulus in the rubbery region (E 0 r ) were used to estimate the cross-link density (n) of the neat and photooxidized epoxy networks. [6,11,13] The values of n are reported in Table 2. It is found that for the two photo-oxidized samples n decreases by 22.2% and 48.1% with respect to the original network.…”

“…This mechanism, however, cannot be responsi- ble for the whole production of amide groups in the system, since the spectroscopic data demonstrate that the carboxylic acid concentration is very limited with respect to the other carbonyl species, whereas the amide group concentration is strongly predominant. In a previous paper [6] it was found that the principal route for amide formation starts from the peroxy radical III of Scheme 1, with a chain-scission occurring at the carboncarbon bond, rather than at the carbonnitrogen bond (see Scheme 3). In summary, the photo-oxidative degradation of the TGDDM/DDS network is a complex process involving several mechanisms which ultimately bring about chain-scission mainly localized on the aliphatic segment of the network.…”