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

Molecular Dynamics Study of Charged Dendrimers in Salt-Free Solution: Effect of Counterions

Polyamidoamine (PAMAM) dendrimers, being protonated under physiological conditions, represent a promising class of nonviral, nano-sized vectors for drug and gene delivery. We performed extensive molecular dynamics simulations of a generic model dendrimer in a salt-free solution with dendrimer's terminal beads positively charged. Solvent molecules as well as counterions were explicitly included as interacting beads. We find that the size of the charged dendrimer depends non-monotonically on the strength of electrostatic interactions demonstrating a maximum when the Bjerrum length equals the diameter of a bead. Many other structural and dynamic characteristics of charged dendrimers are also found to follow this pattern. We address such a behavior to the interplay between repulsive interactions of the charged terminal beads and their attractive interactions with oppositely charged counterions. The former favors swelling at small Bjerrum lengths and the latter promotes counterion condensation. Thus, counterions can have a dramatic effect on the structure and dynamics of charged dendrimers and, under certain conditions, cannot be treated implicitly

Charge inversion of dendrimers in complexes with linear polyelectrolytes in the solutions with low pH

Complexes of fully ionized third-generation dendrimers with oppositely charged linear polyelectrolyte chains are studied by the Brownian dynamics method. A freely jointed model of a dendrimer and a linear chain is used. Electrostatic interactions are considered within the Debye-Hückel approximation with the Debye radius exceeding the dimensions of a dendrimer. In these systems, the phenomenon of charge inversion is observed, and the degree of overcharging is higher as compared with that taking place in analogous complexes formed by dendrimers in which only terminal groups are charged. The dependence of the amount of chain units adsorbed on a dendrimer on the polyelectrolyte chain length is nonmonotnic and agrees qualitatively with the predictions of the theory proposed by Nguyen and Shklovskii for a complex composed of a spherical macroion with an oppositely charged linear chain. This nonmonotonic character also manifests itself for certain other structural characteristics of the complexes. Upon the formation of a complex, a chain is shown to penetrate deeply into a dendrimer

Linker formation in an overcharged complex of two dendrimers and linear polyelectrolyte

The complexes formed by two dendrimers with charged terminal groups and oppositely charged long linear polyelectrolyte (LPE) have been studied using Brownian dynamics simulations. The structural properties of the complexes and their dependence on the LPE chain length were investigated. It was observed that dendrimers in the considered complexes are sufficiently overcharged; i.e., the number of adsorbed LPE monomers is larger than required for the neutralization. The degree of overcharging increases with the increase of the LPE length and is accompanied by the linker appearance until saturation in overcharging is reached. Nonmonotonic dependence of the linker size on the LPE length was observed. To describe the structural properties of the complexes formed by two macroions and a polyelectrolyte chain, the correlation theory has been developed

Coarse-grained molecular-dynamics simulations of nanoparticle diffusion in polymer nanocomposites

\u3cp\u3eMolecular-dynamics simulations have emerged as an effective tool to characterize polymer systems. Molecular level effects (even on microsecond time scales) are nowadays well reproduced by atomistically detailed models. Beyond this, further insights into the properties of the polymer system at a mesoscopic level can be gained by resorting to simulations based on appropriate coarse-grained models. However, reducing the number of degrees of freedom during the coarse-graining procedure may have a significant impact on atomistic level effects. A common example is the overall enhancement of the diffusive motion of polymer chains in coarse-grained simulations, which arises from the reduced friction of the coarse-grained beads. In the present work we investigate this well-known effect and study how the diffusive properties of the nanoparticle are affected by the coarse-graining procedure. To this end, we apply iterative Boltzmann inversion to develop two coarse-grained models of a nanocomposite based on the thermoplastic polyimide R-BAPB, containing a single fullerene C\u3csub\u3e60\u3c/sub\u3e nanoparticle. By changing the size and, correspondingly, the total number of coarse-grained beads in each polymer chain, we can control the effect of chemical detalization on various phenomena. We exploit this idea to study the influence of the degree of detalization of polymer chains on their structural properties as well as on the diffusive properties of the fullerene nanoparticle, whose detalization is kept fixed. Although the structural properties of the coarse-grained systems are in good agreement with those of the fully atomistic system, the nanoparticle diffusion is significantly affected by the local chain structure. In particular, we find that the coarse-graining of the polymer chains on the length scale of the nanoparticle size leads to a full suppression of the subdiffusive regime observed in the fully atomistic system.\u3c/p\u3

Structural effects in overcharging in complexes of hyperbranched polymers with linear polyelectrolytes

New insight is provided by a combined theoretical and simulational approach regarding the effects of structural characteristics of the constituents, on the overcharging phenomena in complexes formed by hyperbranched polymers with linear polyelectrolytes

Structural ordering in SWCNT-polyimide nanocomposites and its influence on their mechanical properties

Using fully-atomistic models, tens-microseconds-long molecular-dynamic modelling was carried out for the first time to simulate the kinetics of polyimides ordering induced by the presence of single-walled carbon nanotube (SWCNT) nanofillers. Three polyimides (PI) were considered with different dianhydride fragments, namely 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 2,3',3,4'-biphenyltetracarboxylic dianhydride (aBPDA), and 3,3',4,4'-oxidiphthalic dianhydride (ODPA) and same diamine 1,4-bis[4-(4-aminophenoxy)phenoxy]benzene (diamine P3). Both crystallizable PI BPDA-P3 and two amorphous polyimides ODPA-P3 and aBPDA-P3 reinforced by SWCNTs were studied. The structural properties of the nanocomposites at temperature close to the bulk polymer melting point were studied. The mechanical properties were determined for the nanocomposites cooled down to the glassy state. It was found that the SWCNT nanofiller initiates' structural ordering not only in the crystallizable BPDA-P3 but also in the amorphous ODPA-P3 samples were in agreement with previously obtained experimental results. Two stages of the structural ordering were detected in the presence of SWCNTs, namely the orientation of the planar moieties followed by the elongation of whole polymer chains. The first type of local ordering was observed on the microsecond time scale and did not lead to the change of the mechanical properties of a polymer binder in considered nanocomposites. At the end of the second stage, both BPDA-P3 and ODPA-P3 PI chains extended completely along the SWCNT surface, which in turn led to enhanced mechanical characteristics in their glassy state

Structural ordering in SWCNT-polyimide nanocomposites and its influence on their mechanical properties

\u3cp\u3eUsing fully-atomistic models, tens-microseconds-long molecular-dynamic modelling was carried out for the first time to simulate the kinetics of polyimides ordering induced by the presence of single-walled carbon nanotube (SWCNT) nanofillers. Three polyimides (PI) were considered with different dianhydride fragments, namely 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 2,3',3,4'-biphenyltetracarboxylic dianhydride (aBPDA), and 3,3',4,4'-oxidiphthalic dianhydride (ODPA) and same diamine 1,4-bis[4-(4-aminophenoxy)phenoxy]benzene (diamine P3). Both crystallizable PI BPDA-P3 and two amorphous polyimides ODPA-P3 and aBPDA-P3 reinforced by SWCNTs were studied. The structural properties of the nanocomposites at temperature close to the bulk polymer melting point were studied. The mechanical properties were determined for the nanocomposites cooled down to the glassy state. It was found that the SWCNT nanofiller initiates' structural ordering not only in the crystallizable BPDA-P3 but also in the amorphous ODPA-P3 samples were in agreement with previously obtained experimental results. Two stages of the structural ordering were detected in the presence of SWCNTs, namely the orientation of the planar moieties followed by the elongation of whole polymer chains. The first type of local ordering was observed on the microsecond time scale and did not lead to the change of the mechanical properties of a polymer binder in considered nanocomposites. At the end of the second stage, both BPDA-P3 and ODPA-P3 PI chains extended completely along the SWCNT surface, which in turn led to enhanced mechanical characteristics in their glassy state.\u3c/p\u3