Photopolymerization of methacrylate monomers using polyhedral silsesquioxanes bearing side-chain amines as photoinitiator

Abstract

Organotriethoxysilanes (APS-PGE2) were synthesized by reacting 1 mol of 3-(aminopropyl)triethoxysilane with 2 mol of 1,2-epoxy-3-phenoxypropane (PGE). Polyhedral oligomeric silsesquioxanes functionalized with bulky amino groups (ASSQO) were prepared by hydrolytic condensation of APS-PGE2 catalyzed by formic acid. Methacrylate resins were activated for visible light polymerization by the addition of 1 wt.% CQ in combination with the synthesized ASSQO at loadings between 0 and 30 wt.%. The progress of monomer conversion versus irradiation time showed that the CQ/ASSQO pair is an efficient photoinitiator system because a fast reaction and high conversion result from 60 s irradiation at 600 mW/cm². The lack of methacrylate groups in the ASSQO, able to polymerize with the methacrylate resin, results in the absence of chemical bond between the ASSQO cages and the matrix. Debonding of ASSQO cages from the polymer give rise to nanovoids; which allows the methacrylate matrix to yield and deform plastically. Consequently, the final effect is a decrease in the flexural modulus and compressive strength with increasing amounts of ASSQO. The present study highlights the surface effect on the overall properties in nanostructured materials.

Introduction

Photopolymerization science has assumed in recent years an increasing relevance in many applications, ranging from the production of paper, coatings, in dentistry, imaging science, and printing. This technology is based on the use of photoreactive systems suited to absorb a light radiation of the appropriate wavelength and to produce primary radical species able to convert a multifunctional monomer into a crosslinked network. Photopolymerization of resins upon irradiation with visible light is commonly photoinitiated by the pair camphorquinone (CQ)/amine [1], [2], [3]. Generally, in free radical mediated polymerization, photogenerated radicals and the growing macroradicals are quenched by oxygen, which reduces the rate of polymerization. The presence of amine is useful in suppressing inhibition by the reaction of oxygen with the C-based radicals through a radical chain transfer process which regenerates the initiation reaction [4].

Low-molecular-weight amines have intrinsic disadvantages such as odor, toxicity, and migration in UV-curing technology. An issue of primary concern in packaging systems is that the low molecular weight species may migrate from the coating into a packaged product. Similarly, in light-cured dental restorative resins, research has been directed towards the synthesis of amines with improved biocompatibility [5], [6]. The toxicity of restorative materials containing amine co-initiator is connected to the mobility of the amine molecule if other components are essentially non-toxic. These complications may be avoided by the use of high molecular weight photoinitiators that reduce the tendency to migrate owing to their macromolecular nature. A way to reduce its leaching into tissue is by using polymerizable amines that are incorporated into the polymer chain [6] or to increase the size of the molecule by bulky substituent so that it does not diffuse out of the resin. In this regard, polyhedral oligomeric silsesquioxanes (SSQO) appear as an interesting approach to increase the size of the amine molecules. Polyhedral oligosilsesquioxanes are an interesting class of clusters derived from the hydrolytic condensation of trifunctional organosilicon monomers. SSQO molecules are cage-like organic–inorganic structures, which consist of a Si–O–Si inorganic cage surrounded by an organic substituent [7]. The inorganic cage may be a fully condensed closed structure of formula (RSiO_1.5)_n, (R represents the organic substituent) or partially condensed open structure of generic formula T_n(OH)_m, where $\text{T}={\text{RSiO}}_{1.5-\text{m}/2\text{m}}$. The diameter of these structures ranges from 1 to 3 nm [7], depending on the number of silicon atoms in the central cage and the peripheral substitution groups surrounding this core. The SSQO unit can be viewed as a nanoparticle for both its size and filler function or a co-initiator for its ability to initiate photopolymerization. Interest in SSQO has grown rapidly over the past several years, particularly for polymer-related applications, where several families of SSQO monomers have been developed as precursors to hybrid inorganic–organic polymers [8], [9], [10]. The affinity of polyhedral oligosilsesquioxanes to various polymer materials can be easily controlled by the selection of the structure of the surrounding organic group [11]. Methacrylate [12], [13], epoxy [14], [15], amine [16], and (β-carboxyl) ester [17] are some examples of functional groups that have been incorporated into the organic part of oligosilsesquioxanes in order to optimize the molecular structure of cubic-oligosilsesquioxane according to different objectives [18].

The present study was carried out in order to explore the possibility of using silsesquioxanes bearing pendant tertiary amine groups as co-initiator of CQ. To our knowledge, the use of polyhedral oligomeric silsesquioxanes functionalized with bulky amino groups [ASSQO] as photoinitiators has not been previously reported in the scientific literature. The photopolymerization of methacrylate monomers using CQ in combination with amine functionalized polyhedral oligomeric silsesquioxanes (ASSQO) at loadings between 0 and 30 wt.% was investigated. Characterization of the polymerized materials was carried out by measuring flexural and compressive properties. The morphology of the photopolymerized materials was examined by means of Field Emission Scanning Electron Microscopy.