Abstract
The present work provides a comparative study on the photopatterning of epoxy-based thermosets as a function of network structure and network mobility. Local switching of solubility properties by light of a defined wavelength is achieved by exploiting versatile o-nitrobenzyl ester (o-NBE) chemistry. o-NBE derivatives with terminal epoxy groups are synthetized and thermally cured with different types of cycloaliphatic anhydrides via nucleophilic ring opening reaction. By varying the structure of the anhydride, glass transition temperature (Tg) and surface hardness are adjusted over a broad range. Once the network has been formed, the photolysis of the o-NBE groups enables a well-defined degradation of the 3D network. Fourier transform infrared (FT-IR) spectroscopy studies demonstrate that cleavage rate and cleavage yield increase with rising mobility of the network, which is either facilitated by inherent network properties (Tg below room temperature) or a simultaneous heating of the thermosets above their Tg. The formation of soluble species is evidenced by sol-gel analysis, revealing that low-Tg networks are prone to secondary photoreactions at higher exposure doses, which lead to a re-crosslinking of the cleaved polymer chains. The change in solubility properties is exploited to inscribe positive tone micropatterns within the thermosets by photolithographic techniques. Contrast curves show that the resist performance of rigid networks is superior to flexible ones, with a contrast of 1.17 and a resolution of 8 µm.
Highlights
Due to their permanent network structure, classic epoxy-based thermosets are characterized by superior mechanical properties, high resistance to creep, and high thermal stability
The results revealed that the differences in network mobility significantly affect the efficiency of the cleavage reaction, the formation of soluble species, and, the contrast curve of respective positive tone photoresists
Once formed, the alkoxide esters react with further mechanism of the accelerated curing follows a ring opening reaction theable anhydride byepoxy the amine anhydride groups yielding carboxylate anion functional esters
Summary
Due to their permanent network structure, classic epoxy-based thermosets are characterized by superior mechanical properties, high resistance to creep, and high thermal stability. These salient features make them ideal candidates for structural applications [1] and indispensable materials in functional coatings [2], electrical [3], and electronic devices [4]. The network formation typically relies on thermal curing routes with amines, amides, anhydrides, phenols, thiols, or imidazoles as crosslinkers [5]. The material properties of epoxy-based thermosets are governed by the structure, the curing parameters and the stoichiometric ratio both of monomer and crosslinker [6]. Advancing from epoxy-based thermosets with permanent covalent crosslinks, recent research is geared towards the introduction of exchangeable chemical bonds and dynamic crosslinks, which offers a promising strategy for reshaping [8], self-healing [9,10], reprocessing [11,12], or recycling [13]
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