DEP - ART http://hdl.handle.net/1822/12002 2013-05-25T17:29:19Z 2013-05-25T17:29:19Z Martins, J. A. http://hdl.handle.net/1822/15019 2012-10-23T15:21:08Z 2011-12-09T00:00:00Z

Title: Toward a physical definition of entanglements Authors: Martins, J. A. Abstract: To examine the role of chain entanglements on polymer melt properties, the interaction potential energy between a loop and chain at its centre is evaluated. The interaction potential energy between binary contacts of two adjacent Kuhn monomers is also evaluated. The elasticity in polymer melts and the evaluation of the different contributions to flow activation energy of linear polymer chains are used to demonstrate that chain loops, or any other binary interactions between polymer chain segments, cannot justify the properties assigned to entanglements. These properties may be understood if interactions between several parallel chain segments are considered instead, which implies the assumption of the existence of short-range local ordered regions in polymer melts. Their existence is demonstrated from abundant literature experiments, molecular dynamics simulation results, and also from a detailed discussion on the interaction potential energy values. Because more than 80% of conformational states in one chain are random sequences of chain segments, the remaining states being in short-range ordered regions, we conclude that the random coil model is not an exact model for the morphology of polymer melts, but it is still a very good description of chain conformations at the molten state.

2011-12-09T00:00:00Z Martins, J. A. Cruz, Vera S. Paiva, M. C. http://hdl.handle.net/1822/14698 2011-12-01T01:18:03Z 2011-11-01T00:00:00Z

Title: Flow activation volume in composites of polystyrene and multiwall carbon nanotubes with and without functionalization Authors: Martins, J. A.; Cruz, Vera S.; Paiva, M. C. Abstract: A new experimental protocol for the evaluation of the flow activation volume is proposed. It is based on stepshear experiments carried out on polymer melts. The flow activation volume evaluated agrees with the volume of a tube confining the chain, and is the same for the polymer melt and its composites with as received and functionalized carbon nanotubes. The functional groups bonded to the carbon nanotubes surface facilitate the polymer melt flow, eliminating the solidlike behavior in the high temperature flow region. A model for the morphology of polymer melts with carbon nanotubes is discussed. Polymer -nanotube interaction energies are discussed and the relaxation time of these interactions is estimated. The link between the solid-like behavior at the flow region and the strong shear thinning observed for the carbon nanotube composites is explained analyzing the different response to shear flow of each of the three networks considered by a model for the morphology of these composites. Description: Em publicação

2011-11-01T00:00:00Z