Off-stoichiometry improves the photostructuring of thiol–enes through diffusion-induced monomer depletion

Mikael Hillmering,Alexander Vastesson,Tommy Haraldsson,Carl Fredrik Carlborg,Omkar Supekar,Birgit D Brandner,Wouter Van Der Wijngaart,Gaspard Pardon

doi:10.1038/micronano.2015.43

Mikael Hillmering, Alexander Vastesson + Show 6 more

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https://doi.org/10.1038/micronano.2015.43

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Abstract

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.

Highlights

Thiol–enes are a group of alternating copolymers with highly ordered networks and are widely used in applications ranging from high-performance coatings to optical, biomedical, sensing, and bioorganic modification applications[1,2,3,4,5,6]
Intentional deviation from stoichiometric thiol–ene formulations has recently garnered interest[11,12,13,14]. Such off-stoichiometric formulations enable the creation of polymers containing a predictable amount of residual reactive groups, in which a thiol or ene excess in the prepolymer formulation remains in the final polymer
The residual active groups enable the tuning of mechanical properties and the development of improved surface modification processes while retaining most of the benefits offered by the underlying thiol–ene click chemistry[2,11,12,13]

Summary

Introduction

Thiol–enes are a group of alternating copolymers with highly ordered networks and are widely used in applications ranging from high-performance coatings to optical, biomedical, sensing, and bioorganic modification applications[1,2,3,4,5,6]. A chain transfer step, whereby a carbon-centred ene radical transfers to the thiol, determines the network formation rate and leads to delayed gelation, which results in very low shrinkage stress. These properties are widely exploited in materials technologies, in information and communications technologies, and in life sciences applications[4,7,8,9,10]. Intentional deviation from stoichiometric thiol–ene formulations has recently garnered interest[11,12,13,14] Such off-stoichiometric formulations enable the creation of polymers containing a predictable amount of residual reactive groups, in which a thiol or ene excess in the prepolymer formulation remains in the final polymer. Enhanced sensitivity for holographic data storage in off-stoichiometric formulations of thiol–enes has been predicted[14]

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