Challenges and limits of upconversion nanoparticles for cationic photopolymerization with UV initiators excited at 980 nm

Abstract

Upconversion nanoparticles (UCNPs) with various modifications around the NaYF₄:Tm/Yb core served as a source for generating UV light upon excitation with laser light at 980 nm. The core was modified with a silica shell containing azido groups. This facilitates reactions with moieties containing an alkyne group, resulting in a triazole structure, via the CuAAC reaction. The alkyne group connects with a UV initiator; that is, a thioxanthone (TX) derivative. The synthesized particles were characterized via TEM and dynamic light scattering. Thus, nanoparticles were obtained, which contain covalently-bound thioxanthone on the UCNP surface, resulting in the generation of the excited state of TX after the absorption of upconverted UV light. This system was compared with one based on a non-covalently-bound UV initiator; that is, i-(propyl) thioxanthone (ITX). The photoexcited thioxanthone moiety served as a source to generate radicals and the conjugate acid upon electron transfer involving an iodonium cation. Here, the system based on covalently-bound thioxanthone generated 10-fold more acid compared to that with non-covalently-bound thioxanthone. Application to cationic polymerization showed no differences whether the matrix exhibited high or low viscosity. Systems with non-covalently-bound TX showed higher reactivity in the case where the sensitizer concentration was high, and when the viscosity of the epoxide used remained at a moderate level. On the other hand, an epoxide with significantly higher viscosity resulted, in general, in lower reactivity.