Atomistic picture of isothermal volume relaxation behavior of atactic polystyrene glass provided by a molecular dynamics simulation

Abstract

Using an atomistic molecular dynamics (MD) simulation, we study the volume relaxation behavior of atactic polystyrene and attempt to correlate this macroscopic behavior with certain microscopic aspects. To this end, the gyration radius, the dimensionless relative shape anisotropy, the mean-squared displacement, and the non-Gaussian parameter are examined simultaneously. Our result shows that the structures characterized at different length scales change in a self-similar way and these changes are intimately correlated to the translational mobility of atoms. The initial incubation of structural changes at the different scales originates from the restriction of mobility due to the cage effect. The applicability of the MD simulation to the investigation of the bulk properties is discussed.

Molecular simulation of structural relaxation in ultrathin polymer films

“…polymer glass, 40,41 the impact of nanoparticles on physical aging of polymer nanocomposite, 42 and the isothermal volume relaxation behavior of atactic polystyrene. 43 Some interesting MD simulations concerning the stringlike cooperative motions of supercooled liquid 44 have been employed to study the relaxation and physical aging behavior of coarse-grained polymer glass. 45,46 Other simulations based on constitutive equations, 47 commercial software such as Materials Studio, 48 and a generalized fractional Maxwell model 49 were also employed to simulate the physical aging of polymers.…”

“…6,17,25,26 Previous MD simulations could also generate the reduction of specific volume during the relaxation process. 43 In these off-lattice models, there indeed exists implicitly some vacancy diffusion processes. However, it is not easy to clarify exactly how the vacancies diffuse within films and to detail the relation between vacancy diffusion and chain architectures.…”

“…In the past two decades, some simulation progress in structural relaxation and physical aging behaviors has been achieved in bulk polymer systems. [38][39][40][41][42][43][47][48][49] For example, Andrejew et al used the Monte Carlo simulation to investigate the physical aging of glassy polymers during the cooling process. 38 The bondfluctuation model on a simple lattice with an energy associated with long bonds was applied to study the glassy state.…”

Determination of Glass Transition Temperatures in Bulk and Micellar Nanoconfined Polymers Using Fluorescent Molecular Rotors as Probes for Changes in Free Volume

“…In the literature, it is known that the intrinsic timescale of rotor rotation is of the order of a picosecond (or less); the non-radiative decay of DCVJ, which has the same julolidine headgroup as FCVJ (Figure a), for instance, was measured to occur within a timescale of ∼1 ps in low-viscosity media. , Therefore, it is reasonable to expect that the local vibration of the monomers does not actually cause much friction against the rotation of FCVJ (because the FCVJ rotation is almost as fast as the monomer vibration) and, therefore, the relevant free volume quantity that is being probed by FCVJ is the total free volume ( V f,tot = V f,exc + V f,vib ), instead of the excess free volume ( V f,exc ) (Figure b). Note, on the other hand, the monomer relaxation (“Rouse”) time, defined as the time it takes for a monomer to travel a distance comparable to its size (⟨ r 2 ⟩ 1/2 ≈ b ≅ 0.7 nm for a PS monomer unit), was much greater than the simulation time (∼0.5 ns) even at 460 K (Figure c), and it further increased rapidly with decreasing T , as also shown in other previous studies. , At temperatures close to the glass transition, the monomer relaxation times of polymers are known to be on the order of micro- to milliseconds (at the minimum) . Therefore, the monomer relaxation must be controlled by the friction from the vibrational volume ( V vib ) (not by the friction from the hard-core volume ( V hc )); in this case, the relevant free volume is the conventional, excess free volume ( V f,exc ).…”

“…Note, on the other hand, the monomer relaxation ("Rouse") time, defined as the time it takes for a monomer to travel a distance comparable to its size (⟨r 2 ⟩ 1/2 ≈ b ≅ 0.7 nm for a PS monomer unit), was much greater than the simulation time (∼0.5 ns) even at 460 K (Figure 4c), and it further increased rapidly with decreasing T, as also shown in other previous studies. 41,42 At temperatures close to the glass transition, the monomer relaxation times of polymers are known to be on the order of micro-to milliseconds (at the minimum). 43 Therefore, the monomer relaxation must be controlled by the friction from the vibrational volume (V vib ) (not by the friction from the hard-core volume (V hc )); in this case, the relevant free volume is the conventional, excess free volume (V f,exc ).…”