Dr. F. Snijkers

UMR 5268 - CNRS/Solvay, 69192 Saint-Fons, France

Effects of interchain interactions on the viscoelasticity of polyamides



The viscoelasticity of linear polymers is well understood. The relaxation of short chains was elucidated in the 50s by Rouse and, for long chains, the breakthrough came with the tube idea from Edwards, de Gennes and Doi in the 70s. Many very diverse polymers, such as PS, PE, PP, PBD, PI, and PMMA, displaying an incredibly wide range of solid state properties, were shown to obey the theoretical predictions with amazing accuracy, although issues remain when polymers are polydisperse and branched. An important common feature of the quoted polymers is their purely entropic nature. A whole different class of commercial “engineering plastics”, including PA, PEEK, and PFU, however exists. Their properties are typically largely determined by the existence of high densities of enthalpic interactions between chains.

We studied the viscoelasticity of unentangled and mildly entangled polyamides (PAs) obtained by regular polycondensation reactions.  PAs are typically semi-crystalline and their molar mass evolves at elevated temperatures.  To be able to readily obtain rheological data over the full range of temperatures, we selected a special class of semi-aromatic, amorphous PAs with blocked chain-ends.  They display strong variations in interchain interaction strengths and densities, while having an identical main-chain structure.  We studied their glass transition temperatures, viscoelasticity, and dielectric properties with differential scanning calorimetry, parallel plate rheometry, and broadband dielectric spectrometry, respectively.  We found strong and systematic variations in the glass transition temperatures and alpha relaxations with varying bond strengths and densities. The variation of the viscoelasticity with temperature obeyed the WLF-equation, for all studied PAs, but the dependency was relatively mild, and they are relatively strong glass formers compared to most other polymers.  Surprisingly, the dynamics in the molten state appeared unaffected by the bonds and could be well described with regular polydisperse Rouse models using the molar mass distributions as obtained by size-exclusion chromatography as input (with a single fit parameter remaining).

Date: 14/1/2016
Time:12:00 (coffee & cookies will be served at 00:00)
Place:FORTH Seminar Room 1