Cure Depth of Photo‐Polymerized Gels

Abstract

Cure depth of photo‐polymerized gels is theoretically analyzed. The critical point in a frontally photo‐polymerized sample is determined by the critical degree of polymerization, which is derived by the concentration ratio of monomer and initiator. While the total polymer depth or gel depth weighed by the monomer conversion depends on both radiation energy and light intensity, the averaged cure depth around the critical point depends on radiation energy only. The profile is consistent with experimental results. This is a practical formulation of the cure depth of photo‐polymerized gels without any empirical parameter.

Modeling the Optimal Conditions for Improved Efficacy and Crosslink Depth of Photo-Initiated Polymerization

“…As previously reviewed by Lin [ 37 ] for corneal crosslinking, the photochemical kinetics have three pathways which are revised for a more general polymer system and are briefly summarized as follows. We will limit the kinetic to the simple case of radical-mediated mechanism, although a more complex, two-step thiol-Michael mechanism, involving anionic centers reactive intermediates may also occur [ 29 ].…”

“…The significance of Equation (5) with the fit A-factor is that it provides the simplest formula to find various optimal conditions, analytically, without involving complex simulations based on the exact solution of coupled Equation (1), which is both temporally and spatially dependent. We note that Equation (1) can be analytically solved only for the special case of b’ = Q = 0, or an assumption of spatial average over the PI concentration, C(z, t) [ 29 ].…”

“…The kinetics of photoinitiated polymerization systems (PPS) have been studied by many researchers for uniform photoinitiator (PI) distribution and for oversimplified cases without photolysis absorption or constant light intensity [ 21 , 22 , 23 , 24 , 25 , 26 ]. Previous models of PPS assumed either a constant light intensity [ 20 ] (for thin polymers), or a conventional Beer–Lambert law (BLL) [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ] for the light intensity. For more realistic systems, one should expect non-uniform PI distribution, and the UV light may still be absorbed by the photolysis product besides the absorption of the monomer.…”

“…Radical photoinitiation systems are broadly divided into two classes associated with their radical generation mechanism following photon absorption. Type I (cleavage type) photoinitiators dissociate into two radicals following photon absorption, while type II initiating systems, in an excited state after photon absorption, abstract a hydrogen atom from a second, coinitiator species [ 20 , 29 ]. Sequential network formation has also been achieved with many different types of polymerization methods, such as thiol–Michael/acrylate hybrid, epoxy/acrylate curable resins, thiol–acrylate/thiol–acetoacetate thermosets, and thiol–ene/epoxy-based polymers [ 20 ].…”

“…Sequential network formation has also been achieved with many different types of polymerization methods, such as thiol–Michael/acrylate hybrid, epoxy/acrylate curable resins, thiol–acrylate/thiol–acetoacetate thermosets, and thiol–ene/epoxy-based polymers [ 20 ]. More recently, wavelength-selective systems were developed to achieve precise temporal and spatial control on polymerization for customizable, highly tunable polymer behavior in optical lithography, photodegradable materials, and drug delivery [ 21 , 22 , 29 ].…”