Cavitation in crosslinked polymers: Molecular dynamics simulations of network formation

Abstract

a b s t r a c tCrosslinked organic polymers are used in a wide variety of coatings and composites to distribute stress, increase toughness and protect the substrate by limiting the passage of aggressive chemicals. Enhancing performance of crosslinked polymers requires understanding how precursor chemistry and geometry, as well as crosslinking protocol, determine the structure and performance of the resulting network. Previous molecular dynamics studies have indicated that cavitation produces pores in simulated liquids, even metals (and the resultant solids), when there is only a single type of force, usually van der Waals, between particles. Here we show that nano-sized cavitation voids also occur in a system bound by van der Waals (Lennard-Jones) forces that is additionally crosslinked with strong covalent (FENE) bonds to form 3 or 6 functional solid networks. Cavitation was observed in both systems. These voids are not a consolidation of "free volume", nor due to a loss of volatiles, but happen as the solidification/cooling stresses exceed the local tensile strength of the material. At temperatures well above the glass transition temperature, "free volume" is distributed evenly throughout the sample in very small pores. As the system cools through its rubbery phase, a few larger voids form via cavitation. Although the loci of these larger voids is associated with crosslinked nodes, cavitation involves the rupturing of weak van der Waals (Lennard-Jones) bonds between molecular chains in regions not constrained by the strong intramolecular bonds. Voids were observed to form during rapid quenches, as well as during much slower cooling at fixed volume, which emulates adhesion of the network to a more rigid body. The voids are large compared to the dimensions of aggressive ionic species and water molecules, and may potentially reduce the barrier properties of a crosslinked coating or composite. Such pore formation, via cavitation, during network formation and curing is not incorporated in current theories of the crosslinking process.

Influence of crosslinking functionality, temperature and conversion on heterogeneities in polymer networks

“…Simple coarse-grained networks studied here are generated from precursors consisting of 'chain extenders' composed of two beads (dimers) and a crosslinker of functionality, f = 3 or 6. These, and similar models, have been used by Stevens et al [15 -17] to study interfacial fracture in highly cross-linked polymer networks and by us to determine the influence of crosslink density on the structural and thermo-mechanical properties [18] and void formation [19] in crosslinked networks. As in these earlier investigations, the 6-functional network is formed with one chain extender molecule already bonded to the 6-functional crosslink node (bead) prior to starting the dynamics.…”

“…As in these earlier investigations, the 6-functional network is formed with one chain extender molecule already bonded to the 6-functional crosslink node (bead) prior to starting the dynamics. It has been shown that this pre-bonding has no noticeable effect on the resulting network connectivity [19]. In the 3-functional model the crosslinker consists of a central 3functional bead connected to three beads, each having a single reactive functionality.…”

“…This bond potential is the sum of the purely repulsive LJ interaction with a cutoff at 2 1/6 σ (the minimum of the LJ potential) and a finite-extensible nonlinear elastic (FENE) attractive potential. (6) where k = 30/ 2 and R 0 = 1.5σ as used previously [18,19]. The MD simulations are performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [20].…”

“…The upper bound on the sum of the frequency values at a particular temperature will be the number of crosslinkers originally present in the precursor mixture, i.e. 1632 [19].…”

Optimizing coating performance for diffusion under cyclic moisture exposure

“…Transport of moisture, and especially the time scales at which it occurs, has broad effects on the thermal and mechanical properties of coating systems over time [3], as well as on the adhesion of a coating to other layers or a substrate [4]. Moisture permeation has also been linked to the presence of pores [5] and pathways [6] within a coating that are often the result of the statistics and chemistry of the crosslinking process [7] both before and after gelation [8,9], and of cavitation [10].…”