Multiscale Modeling Of Solid Propellants And Polymer Blends: From The Atomistic To The Mesoscopic And Macroscopic Scales

With the improvement of computational power, computer simulations have played anincreasingly important role in materials modeling and subsequent technology development, asthey can reveal the microscopic pictures of underlying mechanisms that are otherwiseexperimentally inaccessible or difficult to obtain. Polymersarecharacterizedby theirabroadrangeof lengthandtime scales, which, up till now, cannot be encompassed by anysinglemodel or simulation algorithm currently available. In this paper, the moleculardynamics(MD), dissipative particle dynamics (DPD), mesoscopic dynamics (MesoDyn)simulation and the finite element (FEM) methodare employed toinvestigate the relationshipbetween structures and properties in several polymer systems includingpolymerbinder/plasticizer blends, binary polymer/polymer blends and polymer-clay nanocompositessystem.(1) It has been found that the mechanical properties of solid propellants and plasticbonded explosives (PBX) are largely decided by the compatibility of the polymer binder andplasticizer used in theirs formulation. To predict this compatibility, MD and MesoDynsimulations are conducted in this paper to calculate the density (ρ), cohesive energy density(CED), solubility parameters (δ), the Flory–Huggins interaction parameters,χ, bindingenergies, the glass transition temperature (T_g) and the mechanical properties ofhydroxyl-terminated Polybutadiene (HTPB)/Dioctyl sebacate (DOS) blend and HTPB/nitroglycerine (NG) blend in the COMPASS force field. Then by comparing differentsolubility parameters value (Δδ), the radial distribution functions g(r), or the change in density,it can be all found that HTPB/DOS is a miscibility system while HTPB and NG are not. Inaddition, by analyzing the volume–temperature curve, this paper determines the T_gof HTPB,HTPB/DOS and HTPB/NG, which are 197.54, 176.30 and 200.03 K respectively. The factthat HTPB/DOS has only one glass transition indicates it is a miscible system. Meanwhile,MD simulations can also be used to predict the mechanicalproperties of blends such as tensilemodulus (E), bulk modulus (K) and shear modulus (G) of blends, which will greatly decreasewith the adding of DOS.In the following process,Δδcalculated through MD simulation serves as the inputparameters of MesoDyn, which is then used to simulate the mesoscale morphologies of blendsand the dynamic evolution process of the system. The obtained isosurface of the density fields,order parameters and free energy density can be further used to predict the compatibility ofthe blend system. The results of both simulations prove that HTPB/DOS is miscibleHTPB/NG is not, which is consistent with the existing experimental observations.(2) The miscibility, mesoscopic structures and their mechanical properties ofPolypropylene (PP)/Polyamide-11 (PA11) and Polylactide (PLA)/Polyethylene terephthalate(PET) blends are investigated by MD, MesoDyn simulation and FEM method. Five PP/PA11and PLA/PET blends (with the weight ratio at 10/90, 30/70, 50/50, 70/30 and 90/10) areexamined. Theδvalues of pure polymer obtained by using the MD simulation are in goodagreement with the reference data. By comparing the calculatedχwith the critical valuesχcand comparing the different heights of g(r) of the inter-molecular atomic pairs, it can be foundthat PP/PA11 blend is miscible only when its composition is 90/10, while PLA is completelymiscible with PET over the entire composition range. For each PP/PA11 blend system, two T_gcan be detected, each of which indicates the frozen temperature of one component, while onlyone T_gis observed for PLA/PET blends. This also can indicate the miscibility/immiscibility ofstudied blend system.In order to further study the mesoscopic structures of PP/PA11 and PLA/PET blends, MesoDyn is applied to simulate the phase separation dynamics of the blends at themesoscopic level.χparameters calculated by MD simulation between polymer–polymer areconverted into the interaction MesoDyn parameters. The miscibility/immiscibility is provedby the mesoscopic morphologies of polymer blends.The distribution of the grid-based concentration density of thetwo phases in the periodicsimulation cell,which are calculated from MesoDyn simulations, are converted into theinputmorphology of FEM method, which is then used to obtain the mechanical propertiesofoverall modulus and local stress distribution.The result indicates that all blends have overallisotropic behavior; in the case of PP/PA11 blends, the increase of PP will lead to the lineardecrease of the modulus (E, G and K), while in the case PLA/PET blends, the increase of PLAwill result in the rise of E and G and the fall of K. which is in consistent with the existingexperimental conclusion.(3) The atomistic structures, mesoscopic structures and mechanical properties ofPA11/Quat/Montmorillonite (MMT)nanocomposites system are investigated by MD,DPDsimulationand FEM method. The equilibrium conformationsobtained by MD show that theQuat chains are flattenonto the MMT, covering almost the entire surface, and the polymermolecules collapse onto the Quat themselves rather than directly on the MMT surface. Strongfavorableinteractions between MMT and the surfactants exist; the polar groups of the Quat aremaintainedin the proximity of the MMT layer by virtue of a strongelectrostaticattraction.While the apolar groups of the Quat positively interact, mainlyvia vdW forces, withthe polymer chain, and the bindingenergy between PA11 and the Quat is favorable.The DPDis adopted as the mesoscopic simulation technique,and the interactionparameters of the mesoscopic model areestimated by mapping the corresponding nonbondedinteraction energy values obtainedfrom MD simulations. The predicted structure of polymerblend obtained by DPD is in excellent agreement with MD simulation results. The output ofDPD serves as the input morphology for FEM simulations, which are used to predict theirmechanical properties basedon the simulated morphology.The FEM simulationresults showthe blend has an anisotropic behavior, themodulus along z direction show a good agreement with those of the existing experiments and is lower thanthose perpendiculars to it (xand y).