Comparison of chain-growth polymerization in solution versus on surface using reactive coarse-grained simulations

Molecular Dynamics Simulations of Ideal Living Polymerization: Terminal Model and Kinetic Aspects

Living polymerization is an important synthetic approach to achieving precise control of synthesized polymers, which is crucial for their applications. The molecular weight distribution (MWD) prescribes the macroscopic properties of polymers and hence is a key feature to characterize polymerization. In this work, we present a systematic molecular dynamics simulation study of ideal living polymerization in bulk and surface-initiated systems based on a terminal stochastic reaction model. The evolution of polymer dispersity and MWD along with the polymerization process is examined. We demonstrate that MWD is generally well captured by the Schulz-Zimm distribution for bulk and surface-initiated systems with low grafting densities. However, as the grafting density in the surface-initiated case increases, heterogeneity in chain growth emerges due to the kinetic trapping of reactive sites, which causes the starving of short chains and the thriving of minority long chains such that a shoulder region shows up in MWD. This effect can be enhanced by kinetic compressing induced by polymerization. In addition, the interplay of bonding reaction kinetics and other kinetic properties (e.g., mass transfer and polymer relaxation) is further explored, alongside the influences of bonding probability and reactant concentration. We expect that this investigation will aid in our understanding of typical kinetic aspects of living polymerization.

Molecular Dynamics and Structure of Poly(Methyl Methacrylate) Chains Grafted from Barium Titanate Nanoparticles

Core−shell nanocomposites comprising barium titanate, BaTiO₃ (BTO), and poly(methyl methacrylate) (PMMA) chains grafted from its surface with varied grafting densities were prepared. BTO nanocrystals are high-k inorganic materials, and the obtained nanocomposites exhibit enhanced dielectric permittivity, as compared to neat PMMA, and a relatively low level of loss tangent in a wide range of frequencies. The impact of the molecular dynamics, structure, and interactions of the BTO surface on the polymer chains was investigated. The nanocomposites were characterized by broadband dielectric and vibrational spectroscopies (IR and Raman), transmission electron microscopy, differential scanning calorimetry, and nuclear magnetic resonance. The presence of ceramic nanoparticles in core–shell composites slowed down the segmental dynamic of PMMA chains, increased glass transition temperature, and concurrently increased the thermal stability of the organic part. It was also evidenced that, in addition to segmental dynamics, local β relaxation was affected. The grafting density influenced the self-organization and interactions within the PMMA phase, affecting the organization on a smaller size scale of polymeric chains. This was explained by the interaction of the exposed surface of nanoparticles with polymer chains.

Polymerization and Structure of Opposing Polymer Brushes Studied by Computer Simulations

A model of the polymerization process during the formation of a pair of polymer brushes was designed and investigated. The obtained system consisted of two impenetrable parallel surfaces with the same number of chains grafted on both surfaces. Coarse-grained chains embedded in nodes of a face-centered cubic lattice with excluded volume interactions were obtained by a ‘grafted from’ procedure. The structure of synthesized macromolecular systems was also studied. Monte Carlo simulations using the dynamic lattice liquid model were employed using dedicated parallel machine ARUZ in a large size and time scale. The parameters of the polymerization process were found to be crucial for the proper structure of the brush. It was found that for high grafting densities, chains were increasingly compressed, and there is surprisingly little interpenetration of chains from opposite surfaces. It was predicted and confirmed that in a polydisperse sample, the longer chains have unique configurations consisting of a stretched stem and a coiled crown.

The structure of polymer brushes: the transition from dilute to dense systems: a computer simulation study

Monodisperse polymer brushes were studied by means of Monte Carlo simulations. A coarse-grained model of a polymer brush was designed and the Cooperative Motion Algorithm was employed to model the polymerization process 'grafted from' and to study the structure of a brush immersed in a good solvent. The structure of brushes was determined as a function of the chain length and the grafting density. The influence of these parameters on the scaling properties of the brush was presented and discussed. A thorough analysis of the distribution of concentrations of the polymer segments and the distribution of chain free ends was also carried out. The analysis of the depth of penetration of the low molecular weight solvent into the brush area showed that the main factor determining the penetration is the grafting density. Good agreement between the simulation results and theoretical predictions is observed, especially for longer chains and higher grafting density. The origin of small quantitative differences between the simulation and theoretical results is discussed.

Dynamics of Opposing Polymer Brushes: A Computer Simulation Study

Opposing polymer brush systems were synthesized and investigated by molecular modeling. Chains were restricted to a face-centered cubic lattice with the excluded volume interactions only. The system was confined between two parallel impenetrable walls, with the same number of chains grafted to each surface. The dynamic properties of such systems were studied by Monte Carlo simulations based on the dynamic lattice liquid model and using a highly efficient parallel machine ARUZ, which enabled the study of large systems and long timescales. The influence of the surface density and mean polymer length on the system dynamic was discussed. The self-diffusion coefficient of the solvent depended strongly on the degree of polymerization and on the polymer concentration. It was also shown that it is possible to capture changes in solvent mobility that can be attributed to the regions of different polymer densities.

Impact of PEGDA photopolymerization in micro-stereolithography on 3D printed hydrogel structure and swelling

3D printing complex architectures of responsive-hydratable polymers are enabled by stereolithography via photopolymerization. Yet, insufficient crosslinking leads to compromised structural integrity of the photopolymerized samples, which affects the functionality and reliability of hydrogel devices significantly. Here we investigate how curing parameters and ink formulation affect 3D printed PEGDA samples by using a combination of microfabrication, structural characterization, and reactive coarse-grained molecular dynamics simulation. Our findings show that the degree of curing exhibits a graded profile from confocal Raman spectroscopy and submicron pores from atomic force microscopy, both of which are also observed in our molecular simulations. Moreover, with environmental scanning electron microscopy, we probe the microscopic swelling and bending dynamics of 3D printed hydratable PEGDA structures as well as their structural integrity. Our in-depth characterization results reveal how hydrogel elasticity and irreversible densification due to pore formation highly depends on the exposure time, light intensity and the associated degree of crosslinking. This work provides new molecular insights into processing-structure relation in stereolithography 3D printing.

Simultaneous polymer chain growth with the coexistence of bulk and surface initiators: insight from computer simulations

By Brownian dynamics simulations we study the simultaneous polymer chain growth process with the coexistence of bulk and surface initiators. We find that when the surface initiator density is low enough, the practical experimental way to estimate the dispersity (Đ) of surface-initiated chains on the basis of the dispersity of bulk-initiated chains remains valid as long as the conformations of grafted chains remain within the mushroom regime (i.e., the grafted chains are sparsely distributed). On the other hand, although the average chain lengths of surface and bulk polymers could be equivalent when certain conditions are met, their mass distributions are still different. We also find that increasing the fraction of surface initiators leads to an enlarged disparity in Đ and average length between surface and bulk chains, which is inconsistent with previous studies. This study helps in better understanding the cooperative competition and suppressing effect of bulk chains on surface grown chains, as well as the cause of the dispersity of the surface grown chains as compared to their bulk counterparts with the coexistence of bulk and surface initiators.