Microscopic structure and dynamics of glass transition under phase separation in a model linear polymer melt (PhD)

Abstract

Glass transition is a subject of extensive study due to its relation to the important outstanding problems in physics of complex systems. In this thesis, we present results obtained from molecular dynamics simulations of linear polymer melt system. This thesis mainly focuses on the study of structural and dynamical properties of the system near glass transition temperature at two densities where system phase separate at low temperatures. Dynamics of these systems are compared to a system at a higher density where it undergoes glass transition without in homogeneity in the density distribution. Cavities are forming in both lower density systems and pressure becomes negative at low temperatures. Higher density system also shows negative pressure at low temperatures. The glassy dynamics of the polymer melt systems under negative pressure and phase separation is rarely studied computationally in literature. We computed various density distribution functions such as radial distribution functions (intra and inter parts) and the density of monomers from the center of mass of a molecule to the monomers of the same chain and other surrounding chains. These distribution functions show that both lower density systems (ρ = 0.7 and 0.85) show phase separation due to the formation of cavities, where pressure becomes negative, while the higher density system (ρ = 1.0) shows homogeneous density distributions. We see that the average local density is comparable at low temperatures (T = 0.5 to 0.4) for all three densities. The potential of mean force between molecules shows a pronounced long-ranged attractive tail along with the Gaussian potential at shorter distances, showing the importance of attractive interaction during phase separation for both lower density systems, whereas the higher density system shows a short-ranged attractive tail. Finally, the shape and anisotropy of molecules are examined, which shows that the molecular asymmetry is more in both lower density systems than the higher density system. We also find that molecular asymmetry varies in both lower density systems at low temperatures, however, it remains unchanged in the higher density system. The role of local structure and local density in the arrested dynamics of the supercooled linear polymer melt are compared at three different densities mentioned above. We find that local density at the first coordination shell obtained from the radial distribution function is related to the α−relaxation time, Debye-Waller factor, and the dynamic correlation length. We proposed an exponential relation between the local density,ρloc andα−relaxation time, τα ∝exp(ρ0/(ρ0−ρloc)) (ρ0 is a critical local density). It found that the local density and Debye-Waller factor⟨u2⟩have a fractional power-law relation (⟨u2⟩∝(ρ0−ρloc)α). These relations state that if local density approaches ρ0, the α−relaxation time diverges and the Debye-Waller factor becomes zero. Additionally, the relation between dynamical quantities e.g., α−relaxation time, Debye-Waller factor, and dynamic correlation length are examined at three different densities. Violations of Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) relations are presented in the supercooled state of the linear polymer melt. The role of multiple lengths and time scales in the violation of SE and DSE relations is revealed from the various time correlation functions in the supercooled state of the system at three number densities. The incoherent scattering function examines the non-exponential relaxation dynamics, which attributes to the caged monomers and molecules in the system. The NGP shows a pronounced peak at low temperatures of lower density system compared to the higher density system. This pronounced peak appears due to the large disparity in the mean squared displacements of the monomers and molecules in both lower density systems. The self-part of von-Hove correlation functions for monomers and molecules show a major peak at a smaller distance and a hump at a distance equal to the diameter of the monomers and molecules. This shows that particles with two type of mobilities are present in the supercooled state of the linear polymer melt system. The SE relation violates in the supercooled regime for all three densities. However, the pronounced violations are observed in both lower density systems. The end-to-end vector and reorientational correlation functions, and the trajectories of the orientational unit vector of a randomly selected polymer chain show that DSE relation is obeyed for higher density system, while it violates for both lower density systems. This violation of DSE relation is attributed to the step-like orientational relaxations and jump like motion of the orientational unit vector. Finally, the violations of SE and DSE relations are attributed to both the mobile and caged particles, which agrees with the existing literature. Local order, shear stress correlations, and its fluctuations. The computed local bond orientational order parameters show that this system of linear polymer chains exhibits a mixture of incomplete local face-centered cubic and body-centered cubic crystalline structures,which are similar for both densities (ρ = 1.0, 0.85) and could be a reason for the slow down of the dynamics. Variousstresscorrelationsshowthatitsoscillationsaredecayingatthevibrational time scale, which are localized at low temperatures because the amplitude becomes smaller. The trajectories of shear stress show that the stress is more localized in the lower density system compare to the higher density system at low temperatures.