Cavitation in crosslinked polymers: Molecular dynamics simulations of network formation
M Zee and AJ Feickert and DM Kroll and SG Croll, PROGRESS IN ORGANIC COATINGS, 83, 55-63 (2015).
Crosslinked 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. (C) 2015 Elsevier B.V. All rights reserved.
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