Simulation of End-Coupling Reactions at a Polymer−Polymer Interface: The Mechanism of Interfacial Roughness Development

A Simple Stochastic Reaction Model for Heterogeneous Polymerizations

The stochastic reaction model (SRM) treats polymerization as a pure probability‐based issue, which is widely applied to simulate various polymerization processes. However, in many studies, active centers were assumed to react with the same probability, which cannot reflect the heterogeneous reaction microenvironment in heterogeneous polymerizations. Recently, we have proposed a simple SRM, in which the reaction probability of an active center is directly determined by the local reaction microenvironment. In this paper, we compared this simple SRM with other SRMs by examining living polymerizations with randomly dispersed and spatially localized initiators. The results confirmed that the reaction microenvironment plays an important role in heterogeneous polymerizations. This simple SRM provides a good choice to simulate various polymerizations.

Interfacial reaction-induced roughening in polymer thin films

Reactive blending of immiscible polymers is an important process for synthesizing polymer blends with superior properties. We use a phase-field model to understand reaction dynamics and morphology evolution by diffusive transport in layered films of incompatible, end-reactive polymers. We thoroughly investigate this phenomenon over a large parameter space of interface shapes, layer thicknesses, reaction rates specified by a Damkohler number (Da_f), and Flory-Huggins interaction parameter (χ), under static conditions with no external fields. For films of the same thickness, the dynamics of the system is not significantly influenced by the length of the film or the initial shape of the interface. The interface between the polymers is observed to roughen, leading to the formation of a spontaneous emulsion. The reaction progresses slower and the interface roughens later for thicker films, and systems with higher χ. Increasing Da_f increases the reaction rate and hastens the onset of roughening. The quasi-static interfacial tension decreases with the extent of reaction, but does not become vanishingly small or negative at the onset of roughening. Simulations with reversible reactions and systems where only a fraction of the homopolymers have reactive end groups show that a critical diblock (reaction product) concentration exists, below which interfacial roughening and spontaneous emulsification is not observed. We also demonstrate that thermal fluctuations accelerate the onset of interfacial roughening, and help sustain the system in an emulsified state.

Reaction Kinetics at PDMS-E Emulsion Droplet-Gelatin Interface

In this work, interfacial reaction kinetics between α-[3-(2,3-epoxypropoxy)propyl]-ω-butyl-polydimethylsiloxane emulsion droplets with different sizes and gelatin was studied. The results of amino conversion rate determination show that the reaction proceeded in two steps. Fluorescence spectra analysis indicates that step 1 (0-2 h) should be the adsorption of gelatin on droplet surface. In step 2 (2-13 h), amino conversion rate increased rapidly. The reaction rate in step 2 ( k₂) was obtained by using the 2nd-order approach to model the grafting reaction kinetics. The quantitative relationships among amino conversion rate, droplet size, the concentration of surfactant, reaction temperature, and time were described by linear regression models in given ranges of conditions in step 2. Thermodynamic analysis indicates that the interfacial reaction is an endothermic reaction. After 13 h, the reaction was almost stopped.

Thermoset Polymer Matrix Structure and Properties: Coarse-Grained Simulations

The formation of a thermoset polymer network is a complex process with great variability. In this study, we used dissipative particle dynamics and graph theory tools to investigate the curing process and network topology of a phthalonitrile thermoset to reveal the influence of initiator and plasticizer concentration on its properties. We also propose a novel way to characterize the network topology on the basis of two independent characteristics: simple cycle length (which is mainly affected by the initiator amount) and the number of simple cycles passing through a single covalent bond (which is determined primarily by plasticizer concentration). These values can be treated in the more familiar terms of network "mesh size" and "sponginess", correspondingly. The combination of these two topological parameters allows one to characterize any given network in an implicit but precise way and predict the resulting network properties, including the mechanical modulus. We believe that the same approach could be useful for other polymer networks as well, including rubbers and gels.

Dynamic and Static Mechanical Properties of Crosslinked Polymer Matrices: Multiscale Simulations and Experiments

We studied the static and dynamic mechanical properties of crosslinked polymer matrices using multiscale simulations and experiments. We continued to develop the multiscale methodology for generating atomistic polymer networks, and applied it to the case of phthalonitrile resin. The mechanical properties of the resulting networks were analyzed using atomistic molecular dynamics (MD) and dissipative particle dynamics (DPD). The Young’s and storage moduli increased with conversion, due both to the appearance of a network of covalent bonds, and to freezing of degrees of freedom and lowering of the glass transition temperature during crosslinking. The simulations’ data showed good quantitative agreement with experimental dynamic mechanical analysis measurements at temperatures below the glass transition. The data obtained in MD and DPD simulations at elevated temperatures were conformable. This makes it possible to use the suggested approach for the prediction of mechanical properties of a broad range of polymer matrices, including ones with high structural heterogeneity.

Copolymerization on Selective Substrates: Experimental Test and Computer Simulations

We explore the influence of a selective substrate on the composition and sequence statistics during the free radical copolymerization. In particular, we study the radical copolymerization of styrene and acrylic acid in bulk and in silica pores of different sizes. We show that the substrate affects both polymer composition and sequence statistics. We use dissipative particle dynamics simulations to study the polymerization process in detail, trying to pinpoint the parameters responsible for the observed differences in the polymer chain composition and sequences. The magnitude of the observed effect depends on the fraction of adsorbed monomer units, which cannot be described in the framework of the copolymerization theories based on the terminal unit model.

Rheological and dynamic insights into an in situ reactive interphase with graft copolymers in multilayered polymer systems

We provide rheological and dynamic insights into the role of an in situ reactive interphase with graft copolymers in multilayered polymer systems, using a polyamide-6 (PA6)/maleic anhydride grafted poly(vinylidene fluoride) (PVDF-g-MAH) bilayer as a model. Firstly, the influence of the reactive interphase on macroscopic melt flow behavior was studied. The in situ generated interphase from coupling reactions in bilayers significantly contributed to overall viscoelastic responses in both linear and nonlinear regimes. Specifically, under fast extensional flows, the reactively healed bilayer showed enhanced strain hardening mainly due to the formed graft copolymers in the interphase. Secondly, the evolution of a reactive interphase and its effects on microscopic dynamics and structural properties were further probed using dielectric relaxation spectroscopy (DRS). Interestingly, the reactive interphase drastically altered the dielectric responses of the bilayer upon healing, manifesting in the distinct interfacial relaxation/polarization. The relaxation strength of the interfacial polarization increased linearly as a function of reaction time, and was further improved by increasing the number of layers. In agreement with the rheology, DRS also demonstrated the retarded microscopic dynamics of a reactive interphase in healed bilayers. Using the dielectric molecular relaxation spectrum as a probe for the structure, the effects of the reactive interphase on charge dynamics and the resulting structural properties of bilayers were further evaluated. These findings are aimed at providing a better understanding of the effects of the reactive interphase on rheology, dynamics and dielectric properties, towards controlling the interface/interphase in multi micro-/nano-layered polymer structures and for further applications.

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

Nucleation and Growth of Ordered Copolymer Structures at Reactive Interfaces between PA6 and MA-g-HDPE

We have studied the effect of the interfacial chemical reaction between PA6 and MA-g-HDPE in static conditions at a macroscopically flat interface. Interface destabilization and the growth of instabilities, somehow similar to myelin figures observed in surfactants put in the presence of water, are observed. For the first time in this system, it is shown that ordered microphase-separated copolymer domains, whose morphologies depend on the architecture of the copolymer, namely, essentially on the relative length of the blocks on each side of the interface, may nucleate and grow at a static interface between reactive polymers. We discuss the stability of the plane interface in the case of nonsymmetrical formed graft copolymers. The density of copolymers in the interface (coverage) can be estimated accurately from the long period of the formed structures. We confirm the predictions of Berezkin et al. This observation is very important since it confirms that nanometric domains are certainly generated during reactive extrusion, in addition to micrometric domains formed by rheological processes.