Fractal dimensions of silica gels generated using reactive molecular dynamics simulations

Polymerization kinetics of a multi-functional silica precursor studied using a novel Monte Carlo simulation technique

Silica polymerization has been extensively used to synthesize various fascinating materials for industrial and technological applications. The polymerization protocol is modified by altering several parameters (such as the concentration of the precursor, temperature, pH) heuristically to obtain the desired end product. To properly understand the effect of such parameters, knowledge of molecular events occurring during the process of polymerization is essential. In this work, we developed algorithms to capture molecular events such as translation, rotation, and reactions using the reaction ensemble Monte Carlo (REMC) technique. Our algorithms simulate molecular events in accordance with physical time by correctly scaling the movements of a cluster with the monomer, thereby capturing the kinetics of the process. We studied the polymerization of the four coordinated silica (f₄) precursor using our algorithm and observed excellent agreement between simulation results and experimental data. The algorithm was also used to study the polymerization of the three coordinated silica (f₃) precursor and it was found that our simulations capture experimental kinetics well, thereby confirming that the developed algorithms are robust. We studied the effect of the functionality of the precursor on polymerization kinetics and the resulting structure by simulating silica systems having a mixture of two, three and four functional (f₂, f₃, and f₄) silica precursors. We observed that network formation and cluster size decrease with the increase in the concentration of the f₂ precursor. The radius of gyration (R_g) of the system initially increases due to network formation and decreases later due to the collapse of a large cluster. The R_g is directly correlated with the total number of primitive rings present in the system. The molecular level understanding obtained will be useful in the design of tailored silica nanoparticles.

Elastic and thermal expansion asymmetry in dense molecular materials

The elastic modulus and coefficient of thermal expansion are fundamental properties of elastically stiff molecular materials and are assumed to be the same (symmetric) under both tension and compression loading. We show that molecular materials can have a marked asymmetric elastic modulus and coefficient of thermal expansion that are inherently related to terminal chemical groups that limit molecular network connectivity. In compression, terminal groups sterically interact to stiffen the network, whereas in tension they interact less and disconnect the network. The existence of asymmetric elastic and thermal expansion behaviour has fundamental implications for computational approaches to molecular materials modelling and practical implications on the thermomechanical strains and associated elastic stresses. We develop a design space to control the degree of elastic asymmetry in molecular materials, a vital step towards understanding their integration into device technologies.