Gelation and Cross-Linking in Multifunctional Thiol and Multifunctional Acrylate Systems Involving an in Situ Comonomer Catalyst

Abstract

Dynamic rheology in combination with Fourier transform infrared spectroscopy (FTIR) is used to examine the gelation kinetics, mechanism, and gel point of novel thiol−acrylate systems containing varying concentrations of an in situ catalyst. Gelation, as evidenced from the gel time determined using the Winter− Chambon criterion, is found to occur more quickly with increasing catalyst concentration up until a critical catalyst concentration of 22 mol %, whereupon the gel time lengthens. Such a minimum in gel time may be attributed to changes in the number of available reaction sites and percentage conversion required for gelation. Chemical conversions at the gel point measured for representative samples are consistent with theoretical values calculated using Flory−Stockmayer's statistical approach, confirming our hypothesis. Relaxation exponents of 0.97 and fractal dimensions of 1.3 are calculated for all samples, consistent with coarse-grained discontinuous molecular dynamics (DMD) simulations. The elevated value of n may be due to the low molecular weight prepolymer. The relaxation exponent and fractal dimensions are invariable over all systems studied, suggesting the cross-linking mechanism remains unaffected by changes in catalyst concentration, allowing the gel time to be tailored by simply modulating the catalyst concentration.

Mechanistic Kinetic Modeling of Thiol–Michael Addition Photopolymerizations via Photocaged “Superbase” Generators: An Analytical Approach

“…Predicted gel times can be confirmed experimentally by comparison with those obtained from dynamic rheometry when applying the Winter–Chambon criterion and/or from conversion measurements via real-time infrared (FT-IR) spectroscopy using X crit predicted by eq 33 and by subsequently checking their agreement with the theoretical value. 46–48 It should be noted that the gel times are not solely dependent on the critical conversion achieved for a given comonomer number-average functionality and reaction system stoichiometry but are also related to the reaction kinetics, including conditions of initiation and intrinsic reactivity of thiol–vinyl groups.…”

Gelation studies on acrylic acid‐based hydrogels via in situ photo‐crosslinking and rheology

“…In FTMS experiments, multiwave (multi‐frequency) time sweep experiments are employed to monitor frequency independence of tan δ at a certain time/point during the crosslinking reaction . Combining small‐amplitude oscillatory dynamic rheology with the FTMS provides a means to monitor the evolution of G ′ and G ″ over time for several frequencies within a single experiment . It is important to note that the sum of the stress amplitude, or the strain amplitude, must be within the linear viscoelastic region of the material…”

“…In addition to the network relaxation characteristics, there are other properties that define the critical gel, at the gel point. These include properties such as stiffness ( S ) and fractal dimensions ( D f ) . S defines the gel strength and is dependent on the starting reactant species .…”

Allyl sulfides and α‐substituted acrylates as addition–fragmentation chain transfer agents for methacrylate polymer networks

“…S3), by comparison, is fairly linear (on a logarithmic scale) up to conversions of 60% for all of the polymers. This type of behavior is common for the polymerization of initially low molecular weight monomers …”

“…S4). It is important to point out that using the intercept of storage and loss modulus to define the gel point is only valid when the relaxation exponent n of the gel is equal to 0.5 . While this approximation is reasonable for the difunctional methacrylate system under investigation, gelation may actually be occurring slightly earlier (also at lower conversions) than the given values.…”