Photo-reactive nanogels as a means to tune properties during polymer network formation

Abstract

Photo-reactive nanogels with an integrated photoinitiator-based functionality were synthesized via a Reversible Addition-Fragmentation Chain Transfer (RAFT) process. Without additional free initiators, this nanogel is capable of radical generation and initiating polymerization of a secondary monomer (i.e. dimethacrylate) that infiltrates and disperses the nanogel particles. Due to the presence of RAFT functionality and the fact that all initiating sites are initially located within the nanogel structure, gelation can be delayed by sequencing the polymerization from the nanogel to the bulk matrix. During polymerization of a nanogel-filled resin system, a progressive delay of gelation conversion from about 2 % for conventional chain growth polymerization to 18 % for the same monomer containing 20 wt% nanogel additive was achieved. A significant delay of stress development was also observed with much lower final stress achieved with the nanogel-modified systems due to the change of network formation mechanics. Compared with the nanogel-free dimethacrylate control, which contained uniformly distributed free initiator, the flexural modulus and mechanical strength results were maintained for the photopolymers with nanogel contents greater than 10 wt%. There appears to be a critical interparticle spacing of the photo-reactive nanogel that provides effective photopolymerization while providing delayed gelation and substantial stress reduction.

Construction of Monomer-free, Highly Crosslinked, Water-compatible Polymers

“…We have demonstrated previously that ~10-to 20-nm reactive nanogels that are dispersed in and swollen by monomer can be used to significantly modify properties of the resultant hybrid polymer networks as applied to monomer reactivity (Liu et al, 2012(Liu et al, , 2014, shrinkage and stress in resinbased composites (Moraes et al, 2011), and mechanical strength in model dental adhesives (Moraes et al, 2012). Here, we focus on nanogels that are water-compatible or near-water-compatible for potential application in dental adhesives; however, for better appreciation of the relationship between nanogel structure and network properties conveyed by the nanogel component rather than nanogel entwined in a resin matrix, this work emphasizes nanogelderived networks created in inert dispersion media.…”

RAFT-mediated control of nanogel structure and reactivity: Chemical, physical and mechanical properties of monomer-dispersed nanogel compositions

“…Nanogel branching is important to allow nanoparticle swelling with monomer as well as to form physical crosslinking entanglements with the resin matrix. Chemical crosslinking can take place when reactive groups are present in the nanogel structure either as residual pendant groups arising from the divinyl monomer 13 or through the introduction of polymerizable groups 12,20 as the final step of the nanogel synthesis. High branching density within nanogel particles is required to maintain a similar level of flexural strength compared with the matrix polymer control, especially at high nanogel loading level (40 – 50 wt%) relative to low branching density nanogel.…”

“…Incorporation of inorganic fillers has a demonstrated ability of shrinkage reduction determined by volume fraction but depending on the effect of the filler on bulk modulus, higher residual shrinkage stress can result for these composite materials 19 . Nanogel, as an organic filler, was recently developed and showed significant potential for shrinkage stress reduction 12,20 . The utilization of nanogels swollen by monomers, has the advantage of forming well-controlled physically interpenetrated and covalently interconnected polymer networks.…”

Simultaneous measurement of fluorescence, conversion and physical/mechanical properties for monitoring bulk and localized photopolymerization reactions in heterogeneous systems

“…9 In the same way, physical and mechanical properties can be correlated with conversion by coupling FT-IR spectroscopy and dynamic mechanical techniques, such as tensometer, 10 dynamic mechanical analyzer 11 or rheometry. 12 This work represents the first time that we can examine changes and heterogeneities at the sub-microscopic level by augmenting an FT-NIR analytical approach with fluorescence spectroscopy.…”