Guilhem P. Baeza

Nonlinear shear and uniaxial extensional rheology of polyether-ester-sulfonate copolymer ionomer melts

We present unique nonlinear shear and extensional rheology data of unentangled amorphous polyester ionomers based on polyethers and sulphonated phthalates with sodium/lithium counterions. Previous linear viscoelastic measurements showed significant elasticity in these ionomers due to the formation of strong ionic aggregates. These ionomer melts exhibit viscoelastic properties similar to well-entangled melts with an extended rubbery plateau. To evaluate the effects of nonlinear deformation, the rheology of these ionomers was investigated using uniaxial extension and shear. The measurements were performed on a filament stretching rheometer and on a strain controlled rotational rheometer equipped with a cone-partitioned-plate setup. In extension, ionomer samples exhibited a decreasing strain hardening trend with increasing extension rates. At the same Weissenberg number, the same strain hardening was observed for different counterions. The presence of high solvating poly(ethylene oxide), PEO, along the backb...

Physical Networks from Multifunctional Telechelic Star Polymers: A Rheological Study by Experiments and Simulations

The equilibrium mechanical properties of a cross-linked gel of telechelic star polymers are studied by rheology and Brownian dynamics simulations. The Brownian dynamics model consists of cores to which Rouse arms are attached. Forces between the cores are obtained from a potential of mean force model developed by Likos and co-workers. Both experimentally and in the simulations, networks were created by attaching sticker groups to the ends of the arms of the polymers, which were next allowed to form bonds among them in a one to one fashion. Simulations were sped up by solving the Rouse dynamics exactly. Moreover, the Rouse model was extended to allow for different frictions on different beads. In order to describe the rheology of the non-cross-linked polymers, it had to be assumed that bead frictions increase with increasing bead number along the arms. This friction model could be transferred to describe the rheology of the network without any adjustments other than an overall increase of the frictions due to the formation of bonds. The slowing down at intermediate times of the network rheology compared to that of the non-cross-linked polymers is well described by the model. The percentage of stickers involved in forming inter-star bonds in the system was determined to be 25%, both from simulations and from an application of the Green–Tobolsky relation to the experimental plateau value of the shear relaxation modulus. Simulations with increasing cross-link percentages revealed that on approaching the gel transition the shear relaxation modulus develops an algebraic tail, which gets frozen at a percentage of maximum cross-linking of about 11%.

Linear shear and nonlinear extensional rheology of unentangled supramolecular side-chain polymers

Supramolecular polymers are important within a wide range of applications including printing, adhesives, coatings, cosmetics, surgery, and nano-fabrication. The possibility to tune polymer properties through the control of supramolecular associations makes these materials both versatile and powerful. Here, we present a systematic investigation of the linear shear rheology for a series of unentangled ethylhexyl acrylate-based polymers for which the concentration of randomly distributed supramolecular side groups is systematically varied. We perform a detailed investigation of the applicability of time temperature superposition (TTS) for our polymers; small amplitude oscillatory shear rheology is combined with stress relaxation experiments to identify the dynamic range over which TTS is a reasonable approximation. Moreover, we find that the “sticky-Rouse” model normally used to interpret the rheological response of supramolecular polymers fits our experimental data well in the terminal regime, but is less successful in the rubbery plateau regime. We propose some modifications to the “sticky-Rouse” model, which includes more realistic assumptions with regard to (i) the random placement of the stickers along the backbone, (ii) the contributions from dangling chain ends, and (iii) the chain motion upon dissociation of a sticker and reassociation with a new co-ordination which involves a finite sized “hop” of the chain. Our model provides an improved description of the plateau region. Finally, we measure the extensional rheological response of one of our supramolecular polymers. For the probed extensional flow rates, which are small compared to the characteristic rates of sticker dynamics, we expect a Rouse-type description to work well. We test this by modeling the observed strain hardening using the upper convected Maxwell model and demonstrate that this simple model can describe the data well, confirming the prediction and supporting our determination of sticker dynamics based on linear shear rheology.Supramolecular polymers are important within a wide range of applications including printing, adhesives, coatings, cosmetics, surgery, and nano-fabrication. The possibility to tune polymer properties through the control of supramolecular associations makes these materials both versatile and powerful. Here, we present a systematic investigation of the linear shear rheology for a series of unentangled ethylhexyl acrylate-based polymers for which the concentration of randomly distributed supramolecular side groups is systematically varied. We perform a detailed investigation of the applicability of time temperature superposition (TTS) for our polymers; small amplitude oscillatory shear rheology is combined with stress relaxation experiments to identify the dynamic range over which TTS is a reasonable approximation. Moreover, we find that the “sticky-Rouse” model normally used to interpret the rheological response of supramolecular polymers fits our experimental data well in the terminal regime, but is less s...