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

Effect of Temperature on Flow-Induced Crystallization of Isotactic Polypropylene:A Molecular-Dynamics Study

Molecular-dynamics simulations are employed in order to study the flow-induced crystallization (FIC) of isotactic polypropylene from a supercooled state at different temperatures. The study found that FIC displayed the highest rate at a temperature range of Tmax = 330-360 K. By applying the mean first passage time method, the pre-nucleation, nucleation, and growth stages were successfully identified. The pre-nucleation stage was thoroughly examined, and multiple phenomena were observed, including unexpected strain hardening in the vicinity of Tmax and the formation of high ordering areas that acted as nuclei precursors with limited motion along the tensile direction. Additionally, a non-uniformly slowed segmental relaxation was noted, which suggested the existence of cooperative rearranging regions, the percolation of which could potentially explain the strain hardening effect. Furthermore, the size of the critical clusters at the nucleation point was independent of temperature. Finally, stable clusters grew and merged, resulting in the formation of a shish network.</p

Self-assembly of block-copolymer chains to promote the dispersion of nanoparticles in polymer nanocomposites

In this paper we adopt molecular-dynamics simulations to study the amphiphilic AB block-copolymer (BCP) mediated nanoparticles (NPs) dispersion in polymer nanocomposites (PNCs), with the A-block being compatible with the NPs and the B-block being miscible with the polymer matrix. The effects of the number and components of BCP, as well as the interaction strength between A-block and NPs on the spatial organization of NPs are explored. We find the increase of the fraction of the A-block brings different dispersion effect to NPs than that of B-block. We also find that the best dispersion state of the NPs occurs in the case of a moderate interaction strength between the A-block and the NPs. Meanwhile, the stress-strain behaviour is probed. Our simulation results verify that adopting BCP is an effective way to adjust the dispersion of NPs in the polymer matrix, further to manipulate the mechanical properties.</p

Non-Gaussian nature of glassy dynamics by cage to cage motion

A model based on a single Brownian particle moving in a periodic effective field is used to understand the non-Gaussian dynamics in glassy systems of cage escape and subsequent recaging, often thought to be caused by a heterogeneous glass structure. The results are compared to molecular-dynamics simulations of systems with varying complexity: quasi-two-dimensional colloidlike particles, atactic polystyrene, and a dendritic glass. The model nicely describes generic features of all three topologically different systems, in particular around the maximum of the non-Gaussian parameter. This maximum is a measure for the average distance between cages

Direct Atomistic Modelling of Deformed Polymer Glasses

We use molecular-dynamics computer simulations to explore the influence of thermal and mechanical history of typical glassy polymers, atactic polystyrene (PS) and (bis)phenol A polycarbonate (PC), on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the experimentally observed mechanical rejuvenation and overaging of polymers. Energy partitioning reveals essential differences between mechanical and thermal rejuvenation. All results are qualitatively interpreted by considering the ratio's of relevant timescales: for cooling down, for deformation, and for intrinsic segmental relaxation

Deforming glassy polystyrene: Influence of pressure, thermal history, and deformation mode on yielding and hardening

The toughness of a polymer glass is determined by the interplay of yielding, strain softening, and strain hardening. Molecular-dynamics simulations of a typical polymer glass, atactic polystyrene, under the influence of active deformation have been carried out to enlighten these processes. It is observed that the dominant interaction for the yield peak is of interchain nature and for the strain hardening of intrachain nature. A connection is made with the microscopic cage-to-cage motion. It is found that the deformation does not lead to complete erasure of the thermal history but that differences persist at large length scales. Also we find that the strain-hardening modulus increases with increasing external pressure. This new observation cannot be explained by current theories such as the one based on the entanglement picture and the inclusion of this effect will lead to an improvement in constitutive modeling