Network Formation Kinetics of Poly(dimethylsiloxane) Based on Step-Growth Polymerization

Abstract

Kinetic study of the step-growth polymerization of hydroxyl-terminated poly(dimethylsiloxane) (PDMS) and triisocyanate has been done in this work. Kinetic Monte Carlo simulations are performed to understand the experimental investigation of the cross-linked network formation. This approach enables us to afford a visualization of the entire network formation in detail and evaluate its characteristic properties. A comparison between the current simulation results and that from classical theories (e.g., Flory and Macosko−Miller theoretical calculations) is done. In addition, the Cole−Poehlein diffusion model based on the free volume theory, involving a parallel summation of the chemical and diffusional rate coefficients, is used to describe the effect of diffusion-controlled mechanism on polymerization. Moreover, the effect of experimental conditions, such as the ratio of [NCO]/[OH] and the molar mass of PDMS, on network formation kinetics is also investigated. The simulation results demonstrate that the [NCO]/[OH] ratios within the range of 1−1.2 and PDMS with lower molar mass are preferred to prepare a polymer network with a high cross-linking density. It is believed that the current kinetic modeling provides a way for better understanding the kinetics and optimizing the experimental conditions.

New mechanism for autoxidation of polyolefins: kinetic Monte Carlo modelling of the role of short-chain branches, molecular oxygen and unsaturated moieties

“…Hence, their rate law (and reaction probability) is not proportional to the number of involved molecules but rather to the number of involved reaction centres in a single molecule. 38 This second case is special because larger macromolecules have a larger probability to undergo hydrogen abstraction reactions compared to smaller molecules. Hence, the rate laws for this second type of reactions are treated using so-called "massweighted" binary trees (or mass binary trees) (see section S2 in ESI † for an example).…”

Kinetic modeling of simultaneous polycondensation and free radical polymerization for polyurethane/poly(methyl methacrylate) interpenetrating polymer network

“…Figure 10A shows the simulated results of conversion vs. time for different [NCO]/[OH] ratios, where the arrows indicate the location of the gel point. In this simulation, the reduced mass‐average degree of polymerization (RDP) excluding the largest molecule is employed to determine the gel point, 43 as shown in Supporting Information. It can be seen that with the increase of [NCO]/[OH] ratio, the formation rate of the PU network increases.…”

“…The kinetic scheme of polycondensation of diols with triisocyanates was reported in detail in our previous work, 43 and a brief summary is presented in Scheme 1. All the reactions accounted for are sketched below using the following notation: A 2 is the poly(propylene glycol) (PPG, M n = 2000 g/mol), B 3 is the polyisocyanate (adduct of trimethylolpropane and toluene diisocyanate), and S O , N is the polyurethane chain, where the subscripts O and N denote the number of reactive hydroxyl and isocyanate groups, respectively.…”

“…Corresponding to Flory's theory, 44 the assumption that all unreacted functional groups have equal reactivity is adopted in the polycondensation. Intramolecular reactions are taken into account in the simulation, because these reactions have a great influence on the molecular structure and can cause an increase in the cross‐linking density 34,35,43,45 . The reaction between hydroxyl and isocyanate occurs not only between the two polyurethane chains but also within the same polyurethane chain to form a loop.…”

Synergy of Advanced Experimental and Modeling Tools to Underpin the Synthesis of Static Step-Growth-Based Networks Involving Polymeric Precursor Building Blocks

“…Kinetic modeling techniques are often restricted to basic reaction schemes, an approximate description of diffusional limitations, the calculation of only average molecular characteristics or at most two-dimensional (2D) cross-linking patterns, and low synthesis times during which no gel formation has taken place yet. − Most emphasis has been on the description of the gel point, − with an overview on the experimental and theoretical characterization methodologies presented in previous works. , Specifically, molecular dynamic (MD) simulations aim at 3D network representations although under the assumption that ideal (theoretical) high-yield equilibrated network structures can be formed . Such structures are in practice unlikely to be obtained, as the interplay of chemistry and diffusional limitations, due to viscosity, increases, thereby continuously altering the actual network buildup.…”