Article Reactive Molecular Dynamics Study of the Thermal Decomposition of Phenolic Resins Marcus Purse 1, Grace Edmund 1, Stephen Hall 1, Brendan Howlin 1,* , Ian Hamerton 2 and Stephen Till 3 1 Department of Chemistry, Faculty of Engineering and Physical Sciences, University of Surrey, Guilford, Surrey GU2 7XH, UK; [email protected] (M.P.); [email protected] (G.E.); [email protected] (S.H.) 2 Bristol Composites Institute (ACCIS), Department of Aerospace Engineering, School of Civil, Aerospace, and Mechanical Engineering, University of Bristol, Bristol BS8 1TR, UK; [email protected] 3 Defence Science and Technology Laboratory, Porton Down, Salisbury SP4 0JQ, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-1483-686-248 Received: 6 March 2019; Accepted: 23 March 2019; Published: 28 March 2019 Abstract: The thermal decomposition of polyphenolic resins was studied by reactive molecular dynamics (RMD) simulation at elevated temperatures. Atomistic models of the polyphenolic resins to be used in the RMD were constructed using an automatic method which calls routines from the software package Materials Studio. In order to validate the models, simulated densities and heat capacities were compared with experimental values. The most suitable combination of force field and thermostat for this system was the Forcite force field with the Nosé–Hoover thermostat, which gave values of heat capacity closest to those of the experimental values. Simulated densities approached a final density of 1.05–1.08 g/cm3 which compared favorably with the experimental values of 1.16–1.21 g/cm3 for phenol-formaldehyde resins. The RMD calculations were run using LAMMPS software at temperatures of 1250 K and 3000 K using the ReaxFF force field and employing an in-house routine for removal of products of condensation. The species produced during RMD correlated with those found experimentally for polyphenolic systems and rearrangements to form cyclopropane moieties were observed. At the end of the RMD simulations a glassy carbon char resulted. Keywords: molecular simulation; reactive molecular dynamics; polyphenolics 1. Introduction The field of reactive molecular dynamics of polymers has progressed considerably since the review by Lyon et al. in 2003 that studied models consisting of 20 monomer chains of the polymers under investigation [1]. There have been several papers on reactive molecular dynamics (RMD) of bituminous coal in particular. Bhoi, Banerjee, and Mohanty [2] studied the combustion and pyrolysis of brown coal by RMD, and they concluded that hydrogen was abstracted to form water and that the levels of simulated formaldehyde agreed with the experimental literature. Zan et al. [3] studied the initial reaction of sub-bituminous coal using RMD and Illinois no. 6 coal was simulated in a study by van Duin et al. [4]. Most relevant to this work, Liu at al. [5] studied the pyrolysis of high-density polyethylene using RMD and a model of 7000 atoms and they found that the reaction time for 90% of the loss of structure agreed with the experimental values. This study used a new graphical interface to help interpret the results [6]. An interesting study [7] used RMD to probe the oxidation resistance in hydrogen peroxide of ultra-high molecular weight polyethylene and polyoxymethylene. Two recent studies looked at energetic materials [8,9] and composites featured in a study of carbon nanofiber supported particles [10], graphene sheets [11], and lignite [12]. Tetrabutylphosphonium glycinate/CO2 J. Compos. Sci. 2019, 3, 32; doi:10.3390/jcs3020032 www.mdpi.com/journal/jcs J. Compos. Sci. 2019, 3, 32 2 of 12 J. Compos. Sci. 2018, 2, x 2 of 12 glycinate/mixturesCO were2 m alsoixtures investigated were also byinvestigated van Duin by [13 van]. A Duin new force[13]. A field new was force derived field was for Pt-Oderived systems for Ptby-O Fantauzzisystems by et al.Fantauzzi [14], and et an al. organic [14], and mechanistic an organic study mechanistic was carried study out onwas polymerization carried out on by polymeriSchoenfelderzation [15by]. Schoenfelder Adaptive accelerated [15]. Adaptive RMD onaccelerated hydrogen RMD was on published hydrogen by was Goddard published et al. [by16 ], Goddardand an implementation et al. [16], and an of RMDimplementation on GPU’s was of RMD developed on GPU by’ Kylasas was developed et al. [17]. Theby Kylasa second et generation al. [17]. Theof RMDsecond methods generation were of reviewed RMD methods by Farah were et al. inreviewed 2012 [18 by], which Farah used et al. empirical in 2012 reactive[18], which force use fieldsd empiricalto generate reactive the new force species. fields to Recently, generate a the transferable new species reactive. Recently, force a field transferable was developed reactive by force Xiao, field Shi, wasHao, developed Liao, Zhang, by Xiao, and Chen Shi, Hao, for phosphorous Liao, Zhang and, and hydrogen Chen for which phosphorous showed aand distinct hydrogen improvement which showedin predicting a distinct the improvement mechanical properties in predicting of pristine- the mechanical and defect-laden properties black of pristine phosphorous- and defect [19].-laden In this blackstudy phosphorous we concentrate [19 on]. In phenol-formaldehyde this study we concentrate resins (PFRs), on phenol also- knownformaldehyde as phenolic resins resins, (PFRs), which also are knowna subclass as phenolic of synthetically resins, which derived are thermosetting a subclass of polymers synthetically obtained derived from thermosetting a reaction of phenol polymers with obtainedformaldehyde from a (Figurereaction1). of phenol with formaldehyde (Figure 1). Figure 1. Scheme for the formation of polyphenolic resins. Figure 1. Scheme for the formation of polyphenolic resins. AlthoughAlthough substituted substituted derivativesderivatives of of either either may may also also be used be [used20], under [20], aqueousunder aqueous acidic conditions acidic conditionsformaldehyde formaldehyde reacts with react waters with to formwater methylene to form methylene glycol, a difunctionalglycol, a difunctional monomer monomer that is the that key isto the linking key to the linking phenol th units.e phenol A 0.8:1units. mixture A 0.8:1 of mixture phenol of and phenol methylene and methylene glycol produces glycol anproduce alternatings an alternatingcopolymer, copolymer known as, aknown novolac, as a with novolac, only awith small only amount a small of amount branching. of branching. The addition The of addition a hardener, of a hexamethylenetetramine,hardener, hexamethylenetetramine crosslinks, thecrosslink novolacs the prepolymers, novolac prepolymers, forming the forming thermoset the [thermoset20]. Under [20aqueous]. Under basic aqueous conditions, basic conditions, phenol is converted phenol is toconverted the phenoxide to the phenoxide ion which ision substituted which is substituted in the ortho- inor the para-positions ortho- or para by-positions a formaldehyde by a formaldehyde molecule. A 1.5:1molecule. mixture A 1.5:1 of formaldehyde mixture of formaldehyde and phenol is and used phenolalong withis used water along and with a suitable water catalyst.and a suitable These reactantscatalyst. areThese heated reactants to produce are heated a prepolymer to produce which a prepolymeris rich in hydroxymethylphenols which is rich in hydroxymethylphenols [20]. Further heating [20]. Further of this prepolymerheating of this causes prepolymer reactions cause of thes reactionhydroxymethyls of the hydroxymethyl groups with other groups phenols with to other form phenols large cross-linked to form large structures, cross-linked known structures as resoles,, knownwithout as theresole needs, without for a hardener the need [ 20for]. a PFRs hardener have [ found20]. PFRs a wide have variety found a of wide uses variety in the modernof uses in world the modernranging world from workranging surfaces from work to circuit surfaces boards to tocircuit billiard boards balls. to They billiard have balls. been They utilized have by been the aerospaceutilized byindustry the aerospace to construct industry ablative to construct heat shields ablative for spaceheat shields vehicles for which space work vehicles by charring which work and sublimingby charring on andre-entry. subliming The expulsionon re-entry. of The gaseous expulsion pyrolysis of gaseous products pyrolysis creates aproducts comparatively creates cool a comparatively cushion of gas cool that cushionprotects of the gas vehicle that protects from the the hot vehicle shock gasfrom layer. the Thesehot shock PFR-based gas layer. heat These shields PFR have-based been heat used shields by both havethe Nationalbeen used Aeronautics by both the and National Space AdministrationAeronautics and (NASA) Space andAdministration the Soviet Union (NASA [21)]. and When the possible,Soviet Unionit is important [21]. When that possible, a model it is reproduces important experimentalthat a model reproduces parameters, experimental and in particular parameters as in this, and case, in particularwhen the as model in this is case going, when to be the used model to study is going decomposition. to be used to study There decomposition. is a wide range There of experimental is a wide range of experimental comparisons that can be used, e.g., density, elastic moduli, and glass transition temperature, many of which we have previously reported [22]. Because we were interested in J. Compos. Sci. 2019, 3, 32 3 of 12 comparisons that can be used, e.g., density,