We present a facile two-stage UV/UV activation method for the polymerization of off-stoichiometry thiol-eneepoxy, OSTE1, networks. We show that the handling and processing of these epoxy-based resins is made easier by introducing a material with a controlled curing technique based on two steps, where the first step offers excellent processing capabilities, and the second step yields a polymer with suitable endproperties. We investigate the sequential thiol-ene and thiolepoxy reactions during these steps by studying the mechanical properties, functional group conversion, water absorption, hydrolytic stability, and thermal stability in several different thiol-ene-epoxy formulations. Finally, we conclude that the curing stages can be separated for up to 24 h, which is promising for the usefulness of this technique in industrial applications. INTRODUCTIONThe huge impact of thermoset polymers in applications ranging from biological implants to airplane building materials stems from the ability to precisely control their curing behavior while simultaneously allowing for tuned final properties. Epoxy resins belong to a particular versatile family of polymers that are widely used in industrial applications exhibiting good mechanical end-use properties, excellent bonding to a variety of substrates, and good chemical resistance. Considerable effort has been devoted to develop a plethora of different epoxies with end-properties optimized for many different applications.
In this article, we present OSTE+RIM, a novel reaction injection molding (RIM) process that combines the merits of off-stoichiometric thiol–ene epoxy (OSTE+) thermosetting polymers with the fabrication of high quality microstructured parts. The process relies on the dual polymerization reactions of OSTE+ polymers, where the first curing step is used in OSTE+RIM for molding intermediately polymerized parts with well-defined shapes and reactive surface chemistries. In the facile back-end processing, the replicated parts are directly and covalently bonded and become fully polymerized using the second curing step, generating complete microfluidic devices. To achieve unprecedented rapid processing, high replication fidelity and low residual stress, OSTE+RIM uniquely incorporates temperature stabilization and shrinkage compensation of the OSTE+ polymerization during molding. Two different OSTE+ formulations were characterized and used for the OSTE+RIM fabrication of optically transparent, warp-free and natively hydrophilic microscopy glass slide format microfluidic demonstrator devices, featuring a storage modulus of 2.3 GPa and tolerating pressures of at least 4 bars.
Electron beam lithography (EBL) is of major importance for ultraminiaturized biohybrid system fabrication, as it allows combining biomolecular patterning and mechanical structure definition on the nanoscale. Existing methods are limited by multistep biomolecule immobilization procedures, harsh processing conditions that are harmful to sensitive biomolecules, or the structural properties of the resulting protein monolayers or hydrogel-based resists. This work introduces a thiol-ene EBL resist with chemically reactive thiol groups on its native surface that allow the direct and selective "click" immobilization of biomolecules under benign processing conditions. We constructed EBL structured features of size down to 20 nm, and direct functionalized the nanostructures with a sandwich of biotin and streptavidin. The facile combination of polymer nanostructuring with biomolecule immobilization enables mechanically robust biohybrid components of interest for nanoscale biomedical, electronic, photonic, and robotic applications.
Thiol–enes are a group of alternating copolymers with highly ordered networks and are used in a wide range of applications. Here, “click” chemistry photostructuring in off-stoichiometric thiol–enes is shown to induce microscale polymeric compositional gradients due to species diffusion between non-illuminated and illuminated regions, creating two narrow zones with distinct compositions on either side of the photomask feature boundary: a densely cross-linked zone in the illuminated region and a zone with an unpolymerized highly off-stoichiometric monomer composition in the non-illuminated region. Using confocal Raman microscopy, it is here explained how species diffusion causes such intricate compositional gradients in the polymer and how off-stoichiometry results in improved image transfer accuracy in thiol–ene photostructuring. Furthermore, increasing the functional group off-stoichiometry and decreasing the photomask feature size is shown to amplify the induced gradients, which potentially leads to a new methodology for microstructuring.
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