Systematic study of glass transition in low-molecular phthalonitriles: Insight from computer simulations

Abstract

Phthalonitrile compounds with Si bridges were recently suggested for producing thermosetting polymer composites with reduced T and thus expanded processing range. The detailed experimental investigation of this class of phthalonitriles is still difficult due to development time and costs limitations and the need to take into account the structural changes during the crosslinking. In this paper, we try to overcome these limitations using computer simulations. We performed full-atomistic molecular dynamics simulations of various phthalonitrile compounds to understand the influence of molecular structure on the bulk glass temperature T. Two molecular properties affect T of the resulting bulk compound: the size of the residue and the length of the Si bridge. The larger residues lead to higher Ts, while compounds with longer Si bridges have lower Ts. We have also studied relaxation mechanisms involved in the classification of the samples. Two different factors influence the relaxation mechanisms: energetic, which is provided by the rigidity of molecules, and entropic, connected with the available volume of the conformational space of the monomer.

Crosslinking mechanisms, structure and glass transition in phthalonitrile resins: Insight from computer multiscale simulations and experiments

“…The ratio between the repulsion parameter a , bond strength b , and angle strength c was set to 1:1:0.02 to keep the distribution of ABC and BAB angles (see Fig. ) in monomers close to the one in MD simulations of monomer melts . Equilibrium bond length was set to 0.5, equilibrium angles between bonds – to π.…”

“…All equilibration and production runs were performed using pcff force field for interatomic interactions. The same force‐field was used in our recent papers (for the simulation of polyisoprene–silica composites and phthalonitrile bulks) . The non‐bonded atoms interacted through LJ 9‐6 potential with 1 nm cut‐off.…”

“…The temperature and pressure were fixed using Berendsen thermostat and barostat, respectively. The same parameters for thermo‐ and barostat and for the Ewald summation were used in the recent papers . The trajectory was saved every 5 or 10 ps, depending on the system.…”

“…Computer simulations provide a tool of direct examining how chemistry of constituent monomers and reaction kinetics influence macroscopic material properties. In our recent papers, a systematic study of a set of industrially important phthalonitrile monomers was carried out using molecular dynamics simulations. We estimated their T g s and found a good agreement with experiments.…”

Thermoset Polymer Matrix Structure and Properties: Coarse-Grained Simulations

“…Harmonic bond (equilibrium bond length lb=0.5, bond strength Kb=90) and angle potentials (equilibrium angle γa=180∘, angle strength Ka=1.8) were applied, which required using a time step of δt=5×10−3 for the stable performance of the integration scheme. The initial simulation box had a size of 30×30×30 DPD units, which is approximately 23×23×23 nm3 at room temperature, according to known density data [22]. We prepared eight systems with different ratios of the initial compounds.…”