Entanglements in Glassy Polymer Crazing: Cross-Links or Tubes?

Abstract

Models of the mechanical response of glassy polymers commonly assume that entanglements inherited from the melt act like chemical cross-links. The predictions from these network models and the physical picture they are based on are tested by following the evolution of topological constraints in simulations of model polymer glasses. The same behavior is observed for polymers with entanglement lengths N e that vary by a factor of 3. A prediction for the craze extension ratio Λ based on the network model describes trends with N e , but polymers do not have the taut configurations it assumes. There is also no evidence of the predicted geometrically necessary entanglement loss. While the number of entanglements remains constant, the identity of the chains forming constraints changes. The same relation between the amount of entanglement exchange and nonaffine displacement of monomers is found for crazing and thermal diffusion in end-constrained melts. In both cases, about 1/ 3 of the constraints change when monomers move by the tube radius. The results show that chains in deformed glassy polymers are constrained by their rheological tubes rather than by entanglements that act like discrete cross-links.

Load-bearing entanglements in polymer glasses

“…MD simulations have provided valuable insights into the role of entanglements in the properties of polymer melts and glasses ( 11 , 19 , 20 ). Simulations of even simplified, coarse-grained glasses exhibit behaviors that agree very well with experiments during deformation close to T g , where the response is ductile ( 21 , 22 ).…”

“…The role of entanglements in both crazes and shear deformation zones has been well established through experiments, simulations, and theory (6,(8)(9)(10)(11). Crazes form at low entanglement densities, and shear deformation zones form at high entanglement densities.…”

On the Size Effect of Additives in Amorphous Shape Memory Polymers

“…A first set of results obtained within the present model is shown in Figure 5 , for the cases of a pure sample and a polymer-additive mixture. In the both cases investigated, deformation starts at a temperature slightly above the glass transition point ( and ), where the system is soft enough to prevent it from crazing [ 67 , 68 ]. The plot depicts projections of the chains’ gyration tensor onto the directions parallel and perpendicular to deformation, and .…”

Uncovering the rupture mechanism of carbon nanotube filled cis-1,4-polybutadiene via molecular dynamics simulation

“…Therefore, it can be distinguished that the creation and growth processes of nanovoids are key steps in the study of the rupture mechanism. Recently, intensive experimental 3,[5][6][7][8] and simulation [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] studies have been devoted to address this fundamental issue. For instance, the development of crazes is found to depend on the kinetics of local plastic deformation by examining the fracture toughness of homogeneous glassy polymers.…”