A novel approach to determine the cross-linking level of epoxy resins
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The cross-linking density of epoxy resins is a crucial parameter that determines its mechanical properties and characteristic data such as the glass transition temperature. Typically, the glass transition temperature increases with increasing cross-linking level of the epoxy system. A direct measurement of glass transition temperature is possible with DSC, but the cross-linking level has to be calculated from the remaining enthalpy compared to a completely cured sample. Raman spectroscopy is often used for the determination of the cross-linking density of epoxy resins. The decrease of the epoxide vibration peak is compared to the vibration peak of the phenyl ring peak, which is not affected by the curing of the epoxy system. Our approach is to determine the glass transition temperature of the resin with various crosslinking levels using an AFM equipped with a heatable cantilever. Performing local thermal analysis is based on a temperature calibration with known polymer standards to correlate the tip temperature with the applied voltage on the tip. AFM has a high spatial resolution which allows the determination of local differences in the glass transition temperature. The local thermal analysis with AFM is compared with conventional methods like Raman spectroscopy and DSC.