In this thesis, we present a thorough investigation into the rigidity of a polymer melt above and below its glass transition (GT) at a temperature TG = 0.465 (in reduced Lennard-Jones units). We use an isothermal-compression method to enter the glassy phase and NPT ensemble is realized through molecular dynamics simulations. We monitor such quantities as the mean-square displacement, the heat capacity CP, the volume and time-dependent shear modulus G(t). Whenever possible, these quantities are monitored below the GT as well. We also compute the shear modulus mu via external deformations and, in the zero-shear limit, find reasonably good agreement with G(t → infinity). The rigidity transition (RT) in the system is found to occur slightly below the GT at a temperature TR = 0.44. The results are explained in terms of sufficient free volume above TR allowing collective motion and local stress relaxation. This is seen through dynamics which are not only heterogeneous, but also spatially correlated. We appeal to notions such as "jamming" within the system and the presence of floppy modes (which allow for deformations without energy cost) to interpret the RT phenomenon. We also characterize the response to external deformation: small and large deformation regimes can be identified, the latter type causing a non-negligible reconfiguration, an over-stretching of the chains and a move to a more shallow potential energy "well." Furthermore, we analyze the "aging" phenomenon as a series of intermittent collective rearrangements and show that the two types of instantaneous shear deformations both induce "overaging," but in two completely different manners.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/26794 |
| Date | January 2004 |
| Creators | Wallace, Matthew L |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 86 p. |
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