In Situ Characterization of the Reaction-Diffusion Behavior during the Gradient Interphase Formation of Polyetherimide with a High-Temperature Epoxy System

Abstract

This study presents two novel methods for in situ characterization of the reaction-diffusion process during the co-curing of a polyetherimide thermoplastic interlayer with an epoxy-amine thermoset. The first method was based on hot stage experiments using a computer vision point tracker algorithm to detect and trace diffusion fronts, and the second method used space- and time-resolved Raman spectroscopy. Both approaches provided essential information, e.g., type of transport phenomena and diffusion rate. They can also be combined and serve to elucidate phenomena occurring during diffusion up to phase separation of the gradient interphase between the epoxy system and the thermoplastic. Accordingly, it was possible to distinguish reaction-diffusion mechanisms, describe the diffusivity of the present system and evaluate the usability of the above-mentioned methods.

Keywords: Raman spectroscopy; characterization; interphase formation; optical properties/techniques; reaction-diffusion; thermoplastic; thermoset resin.

Figure 1

Adapted measurement stack of the in situ Raman spectroscopy coupled with hot stage microscopy including a thermoplastic film with an epoxy gap.

Figure 2

Identification of robust features and extraction of reliable tracking points: (a) initial frame at the beginning of diffusion from optical microscopy without post-processing; (b) binarized initial frame with allocated region of interest (ROI) in MATLAB; (c) magnified initial frame at diffusion step t₀ including strongest feature points detected by Shi–Tomasi corner detector algorithm; and (d) magnified frame at diffusion stept₀ + ∆t, excluding eliminated points due to exceedance of the forward–backward error threshold.

Figure 3

Reference spectra for normalization of PEI, epoxy precursor and epoxy system signals.

Figure 4

Monocomponent diffusion experiments of chosen precursors with PEI. From top to bottom: diffusion of amine precursor at 190 °C; point tracker image details and epoxy precursors at 160 °C, where the left image shows the starting time, and the right shows the diffusion after a defined time; diffusion of epoxy precursors at 160 °C.

Figure 5

Diffusivity of monocomponent diffusion experiment on optical hot stage setup derived by point tracker algorithm: (a) epoxy precursor at various temperatures; (b) amine precursor at various temperatures.

Figure 6

Reactive multicomponent diffusion experiment with the epoxy system at 160 °C: (a) start of partial dissolution; (b) end of diffusion; (c) initiation of reaction-induced phase separation; (d) end of phase separation with visible decomposed microstructure.

Figure 7

Diffusivity of reactive multicomponent diffusion experiments on an optical hot stage setup derived by the point tracker algorithm. Values in the positive direction indicate diffusion from the epoxy system into PEI, whereas values in the negative direction indicate diffusion from PEI into the epoxy system.

Figure 8

Temperature-dependent concentration curves normalized to Raman intensity amplitude 1819@988 cm⁻¹ at coordinate x = 40 μm (positive) for diffusion of the (monocomponent) epoxy precursor into PEI.

Figure 9

Temperature-dependent concentration curves normalized to Raman intensity amplitude 2437@1003 cm⁻¹ at coordinate x = −80 μm (negative) for diffusion of the PEI into (monocomponent) epoxy precursor.

Figure 10

Temperature-dependent concentration curves normalized to Raman intensity amplitude 4504@988 cm⁻¹ at coordinate x = 10 μm (positive) for diffusion of the (reactive multicomponent) epoxy system into PEI.

Figure 11

Temperature-dependent concentration curves normalized to Raman intensity amplitude 2437@1003 cm⁻¹ at coordinate x = −20 μm (negative) for diffusion of the PEI into (reactive multicomponent) epoxy system.

Figure 12

In situ Raman spectra featuring the reactive and reference peaks during curing at 180 °C.

Figure 13

Comparison of in situ Raman diffusion measurements with optical hot stage experiments with the epoxy precursor.

Figure 14

Comparison of the concentration onset error at 180 °C, 160 °C and 140 °C at different positions.