Influence of chain stiffness on the dynamical heterogeneity and fragility of polymer melts

Abstract

It is well accepted that stiffer polymers have higher glass transition temperatures. However, the influence of chain stiffness on the slow dynamics and dynamical heterogeneity when approaching the glass transition point is still not well understood. In this work, we investigate the influence of chain stiffness on the dynamic heterogeneity and fragility of supercooled polymer melts by using molecular dynamics simulation. The chain stiffness is tuned by varying the bending strength, and the diffusion and relaxation of polymer segments are studied. We find that the power law relation between the rescaled diffusion coefficient and the structural relaxation time does not change with changing chain stiffness, indicating similarities of glass-forming behavior of polymer melts with different chain stiffness. The dynamical heterogeneities are characterized by the non-Gaussian parameter and dynamic susceptibility, and the string-like cooperative motion is analyzed by the string-length. It is found that the non-Gaussian parameter and dynamic susceptibility characterize a different aspect of dynamical heterogeneities. Though both decreasing temperature and increasing bending strength lead to slower dynamics and growing dynamical heterogeneities, there is no simple superposition between temperature and bending strength. Our work may shed new light on the glass transition behavior of polymers with different chain stiffness.

Role of Cohesive Energy in Glass Formation of Polymers with and without Bending Constraints

“…These trends are supported by our simulation results, as shown in Figure b. Regarding the influence of chain stiffness on fragility, Figure indicates that fragility is generally larger in polymer melts with bending constraints than without bending constraints, but the difference between these two classes of models diminishes as the cohesive interaction strength increases, indicating that cohesive energy and chain stiffness parameters are highly coupled as in previous studies on molecular binding of polymers. , The trend that fragility increases with chain stiffness was predicted even earlier by the GET. , This general trend has also been identified in a number of simulation studies. ,,,,, …”

“…58,59 This general trend has also been identified in a number of simulation studies. 34,36,37,41,46,48 Therefore, the combined simulation and GET results in this section, along with those in Section 3.2, reveal both similarities and differences between polymer melts with and without bending constraints regarding the role of cohesive interaction strength in determining their thermodynamic and dynamic properties. We attribute these phenomena to the coupling effect of cohesive energy and chain stiffness on polymer glass formation.…”

“…As another example, cohesive interaction strength is a highly relevant variable in ionic and polar polymeric materials − despite the complexity of the intermolecular interactions involved in these materials. A number of experimental − and computational studies − aim to elucidate the role of cohesive energy and chain stiffness in polymer glass formation, and in particular, we mention the comprehensive experimental observations of Rössler and co-workers − on molecular and polymer fluids, along with a serious attempt to arrive at an improved parametric description of the functional form of the structural relaxation time τ α governing this extensive body of data. Despite progress in both experimental and modeling fronts, the development of a quantitative molecular theory capable of accurately predicting thermodynamic and dynamic properties of GF polymers in terms of basic molecular parameters and varying thermodynamic conditions remains a challenge.…”

Non-Gaussian and Cooperative Dynamics of Entanglement Strands in Polymer Melts

“…The decaging process relates to the α-relaxation and exhibits a maximum near τ α . Both extensive simulations − and experimental studies combined with simulations , were reported. The characteristic time τ α 2 for α 2 ( t ) was found to relate nonlinearly to τ α : τ α 2 ∼ τ α ≅0.7 . − …”

“…For such chains, Pan and Sun have studied NG behavior by MD simulations on coarse-grained melts including chain stiffness effects. The evaluated α 2 ( t ) display two maxima, a stronger one around the monomer decaging process and a second one in the Rouse-like subdiffusive regime that increases in strength with increasing chain stiffness.…”