Victor M. Nazarychev

Molecular-dynamics simulation of polyimide matrix pre-crystallization near the surface of a single-walled carbon nanotube

Polyimide-based composite materials with a single-walled carbon nanotube as filler were studied by means of extensive fully-atomistic molecular-dynamics simulations. Polyimides (PI) were considered based on 1,3-bis-(3′,4-dicarboxyphenoxy)-benzene (dianhydride R) and various types of diamines: 4,4′-bis-(4′′-aminophenoxy)-diphenylsulfone (diamine BAPS) and 4,4′-bis-(4′′-aminophenoxy)-diphenyl (diamine BAPB). The influence of the chemical structure of the polyimides on the microstructure of the composite matrix near the filler surface and away from it was investigated. The formation of subsurface layers close to the nanotube surface was found for all composites considered. In the case of R–BAPB-based composites, the formation of an organized structure was shown that could be the initial stage of the matrix crystallization process observed experimentally. Similar structural features were not observed in the R–BAPS composites. Carbon nanotubes induce the elongation of R–BAPB chains in composites whereas R–BAPS chains become more compact similar to what is observed for EXTEM™ polyimide. It was shown that electrostatic interactions do not influence the microstructure of composites but slow down significantly the dynamics of PI chains in composites.

Evaluation of the characteristic equilibration times of bulk polyimides via full-atomic computer simulation

The full-atomic computer simulation of bulk plastic polyimides based on dianhydride 1,3-bis(3′,4-dicarboxyphenoxy)benzene and two types of diamines, 4,4′-bis(4″-aminophenoxy)diphenyl sulfone and 4,4′-bis(aminophenoxy)diphenyl oxide, is performed on the microsecond scale via the moleculardynamics method. For the investigated molecules, which consist of eight repeating units, the limiting values of the characteristic sizes of individual polymer chains are established. The limiting sizes obtained via computer simulation are in good agreement with theoretical values calculated in terms of virtual-bond formalism. It is found that the time of sample equilibration for the full-atomic computer simulation of bulk plastic polyimides is ∼1 μs, which agrees in order of magnitude with the displacement time of the center of mass of an individual molecule by a distance equal to its own size.

Influence of the carbon nanotube surface modification on the microstructure of thermoplastic binders

The structural properties of polymer nanocomposites based on thermoplastic polyimides filled with surface-modified carbon nanotubes (CNT) have been studied by means of fully-atomistic molecular-dynamics simulations. The influence of the distribution of functional carboxyl groups over the CNT surface on the polymer-matrix density distribution, and the orientational ordering of polymer chains have been investigated. It was shown that the polymer shifts far away from the nanoparticle surface with increase of the CNT modification degree. The orientational ordering of PI chains was not observed in the case of nanocomposites filled with modified CNTs where carboxyl groups are distributed uniformly on the surface. However, in case of the edge-modified CNTs the polymer can interact with the CNT surface; such edge-modified nanoparticles induce orientational ordering of crystallisable polyimide chains which can be considered as an initial stage of the polymer matrix crystallization.

Multiscale computer simulation of polymer nanocomposites based on thermoplastics

A literature review is presented on a multiscale approach to the simulation of nanocomposites based on thermoplastic polymers that includes calculations using quantum-chemical methods and molecular dynamics simulations with the use of full-atomic and mesoscopic models. Common problems arising during the multiscale simulation of thermoplastic nanocomposites and the ways to solve them are discussed. The results of studies of the structural, thermal, and mechanical properties of thermoplastic nanocomposites obtained via the simulation with consideration for the detailed chemical structures of components are given.

Computer simulation of the heat-resistant polyimides ULTEM™ and EXTEM™ with the use of GROMOS53a6 and AMBER99 force fields

An atomistic computer simulation was performed for the polyimides ULTEM™ and EXTEM™ via the molecular-dynamics method with the use of Gromos53a6 and Amber99 force fields. For parameterization of electrostatic interactions, the partial atomic charges were calculated through quantum-chemical methods. The temperature dependence of density and the thermal-expansion coefficients for the polyimides were obtained. The calculated density values of the polyimides at room temperature and their coefficients of thermal expansion in the glassy state are in agreement with available experimental data. It is shown that inclusion of electrostatic interactions is necessary for simulation of the thermophysical characteristics of the considered polyimides.

Computational Modeling of Polylactide and Its Cellulose-Reinforced Nanocomposites

Computer simulations based on all-atom models can provide significant assistance to researchers in understanding molecular mechanisms that lead to the appearance of new properties in composites different than those of the individual components. Recently, this simulation of polymer nanocomposites has become more popular, since it provides answers that cannot be obtained experimentally. To date, there have been only a few publications devoted to the computer simulations of polymer nanocomposites based on cellulose, even though these materials are very promising and attract great attention because of their renewable character. One of the main biodegradable polymers, obtained from natural sources and used for developing such biocomposites, is polylactide (PLA). This chapter focuses on computer simulations of PLA and nanocellulose, as well as on composite materials based on these materials.

Atomistic Molecular Dynamics Simulations of the Initial Crystallization Stage in an SWCNT-Polyetherimide Nanocomposite

Crystallization of all-aromatic heterocyclic polymers typically results in an improvement of their thermo-mechanical properties. Nucleation agents may be used to promote crystallization, and it is well known that the incorporation of nanoparticles, and in particular carbon-based nanofillers, may induce or accelerate crystallization through nucleation. The present study addresses the structural properties of polyetherimide-based nanocomposites and the initial stages of polyetherimide crystallization as a result of single-walled carbon nanotube (SWCNT) incorporation. We selected two amorphous thermoplastic polyetherimides ODPA-P3 and aBPDA-P3 based on 3,3′,4,4′-oxydiphthalic dianhydride (ODPA), 2,3′,3,4′-biphenyltetracarboxylic dianhydride (aBPDA) and diamine 1,4-bis[4-(4-aminophenoxy)phenoxy]benzene (P3) and simulated the onset of crystallization in the presence of SWCNTs using atomistic molecular dynamics. For ODPA-P3, we found that the planar phthalimide and phenylene moieties show pronounced ordering near the CNT (carbon nanotube) surface, which can be regarded as the initial stage of crystallization. We will discuss two possible mechanisms for ODPA-P3 crystallization in the presence of SWCNTs: the spatial confinement caused by the CNTs and π–π interactions at the CNT-polymer matrix interface. Based on our simulation results, we propose that ODPA-P3 crystallization is most likely initiated by favorable π–π interactions between the carbon nanofiller surface and the planar ODPA-P3 phthalimide and phenylene moieties.

Linear Viscoelasticity of Polymers and Polymer Nanocomposites: Molecular-Dynamics Large Amplitude Oscillatory Shear and Probe Rheology Simulations

In this chapter, we discuss coarse-grained and atomistic molecular-dynamics simulation studies of the rheological properties of bulk polymer systems and polymer nanocomposites. Both systems contain monodispersed and non-crosslinked chain molecules. A multiscale strategy is applied to characterize the rheological behavior on different length scales of the systems structural organization. Fully atomistic simulations provide insights in rheological properties on smaller length scales than those accessible through coarse-grained simulations. Different approaches are utilized to obtain rheological moduli at these different length scales. At both levels of description, cyclic shear deformation is performed to characterize macroscopic properties of the systems before and after filler insertion. In the fully atomistic simulations of polyimide R-BAPB, passive microrheology approach is employed in addition to active rheology. To this end, a probe particle is immersed into the atomistic polymer matrix. Then, local rheological properties on the length scales at and beyond the Kuhn length are estimated. Results are compared with macroscopic rheological properties obtained by shear deformation. Additionally, the influence of the strain amplitude on the resulting rheological properties is examined. The reported coarse-grained simulations show a strong decrease of the nanocomposites storage modulus with increasing strain amplitude, which is accompanied by a maximum in the loss modulus (the so-called Payne effect); the onset of the softening is observed in the linear regime of deformation at strain amplitude of about 0.01. Moreover, the dependence of the storage modulus on the instantaneous strain exhibits both softening and hardening regimes, in agreement with recently reported [22] Large Amplitude Oscillatory Shear (LAOS) experiments. The simulations suggest that the observed hardening is caused by the shear-induced decrease of the non-affine diffusion of the polymer segments due to filler particles acting as effective crosslinks between polymeric chains and, hence, hindering diffusion. Moreover, the formation of “glassy” immobile layers at the nanoparticle interface strongly increases the storage modulus at low strain amplitudes. The strain softening with increasing strain amplitude is connected to the mobilization of these glassy layers and an increase in the dynamic heterogeneity of the polymer matrix. A breakup of the network structure plays a role as well.