Reaction Kinetics in Polymer Melts

TL;DR

This paper develops a theoretical framework for understanding reaction kinetics of end-functionalized polymers in melts, revealing regimes from mean field to diffusion-controlled, with specific power-law behaviors depending on reactivity and density.

Contribution

It generalizes previous dilute-limit models to arbitrary initial densities and reactivities, deriving a closed-form rate constant equation for polymer melt reactions.

Findings

Mean field kinetics dominate at short times with rate proportional to reactivity Q.

High reactivity leads to diffusion-controlled kinetics with a t^{-1} dependence.

Different regimes exhibit distinct power-law decay of reactive group density over time.

Abstract

We study the reaction kinetics of end-functionalized polymer chains dispersed in an unreactive polymer melt. Starting from an infinite hierarchy of coupled equations for many-chain correlation functions, a closed equation is derived for the 2nd order rate constant $k$ after postulating simple physical bounds. Our results generalize previous 2-chain treatments (valid in dilute reactants limit) by Doi, de Gennes, and Friedman and O'Shaughnessy, to arbitrary initial reactive group density $n_0$ and local chemical reactivity $Q$. Simple mean field (MF) kinetics apply at short times, $k \sim Q$. For high $Q$, a transition occurs to diffusion-controlled (DC) kinetics with $k \approx x_t^3/t$ (where $x_t$ is rms monomer displacement in time $t$) leading to a density decay $n_t \approx n_0 - n_0^2 x_t^3$. If $n_0$ exceeds the chain overlap threshold, this behavior is followed by a regime where $n_t \approx 1/x_t^3$ during which $k$ has the same power law dependence in time, $k \approx x_t^3/t$, but possibly different numerical coefficient. For unentangled melts this gives $n_t \sim t^{-3/4}$ while for entangled cases one or more of the successive regimes $n_t \sim t^{-3/4}$, $t^{-3/8}$ and $t^{-3/4}$ may be realized depending on the magnitudes of $Q$ and $n_0$. Kinetics at times longer than the longest polymer relaxation time $\tau$ are always MF. If a DC regime has developed before $\tau$ then the long time rate constant is $k \approx R^3/\tau$ where $R$ is the coil radius. We propose measuring the above kinetics in a model experiment where radical end groups are generated by photolysis.