Evelyne Van Ruymbeke

Determination of the molecular weight distribution of entangled linear polymers from linear viscoelasticity data

In a companion paper [Macromolecules 35 (2002) 2689], we have addressed the direct problem of predicting the linear viscoelastic response of entangled linear polymers from their molecular weight distribution, using a suitable model inspired by reptation theory. By comparing the theoretical results to experimental data for a variety of samples of different nature (polystyrene, polycarbonate, high-density polyethylene) and distribution (monomodal and multimodal), we found that the time-dependent diffusion reptation model proposed by des Cloizeaux, suitably modified to treat short chains and to include Rouse processes, is capable of quantitative predictions. In the present paper, we use the modified des Cloizeaux model to address the inverse problem of predicting molecular weight distribution from dynamic moduli. A parametric approach is implemented to deal with the ill-posedness of the problem. Results are given for most of the samples studied in [Macromolecules 35 (2002) 2689]. They are in quantitative agreement with size-exclusion chromatography data. In particular, the proposed methodology is able to resolve small amounts of short chains in bimodal blends containing large amounts of long chains

Decoding the viscoelastic response of polydisperse star/linear polymer blends

We probed the viscoelastic behavior of polydisperse star/linear polymer blends with varying composition and average arm length. The linear viscoelastic behavior has been investigated over a wide range of frequencies using both oscillatory and long creep/recovery experiments. A method was developed for splitting the molar mass distributions of the blends into the contributions of different architectures. These results were used to implement a time-marching algorithm based on a parameter-free tube model for predicting the linear viscoelastic behavior. The model predictions were successfully validated with the experimental data, opening the route for designing blends with desired rheological properties.

Molecular rheology of branched polymers: decoding and exploring the role of architectural dispersity through a synergy of anionic synthesis, interaction chromatography, rheometry and modeling

An emerging challenge in polymer physics is the quantitative understanding of the influence of a macromolecular architecture (i.e., branching) on the rheological response of entangled complex polymers. Recent investigations of the rheology of well-defined architecturally complex polymers have determined the composition in the molecular structure and identified the role of side-products in the measured samples. The combination of different characterization techniques, experimental and/or theoretical, represents the current state-of-the-art. Here we review this interdisciplinary approach to molecular rheology of complex polymers, and show the importance of confronting these different tools for ensuring an accurate characterization of a given polymeric sample. We use statistical tools in order to relate the information available from the synthesis protocols of a sample and its experimental molar mass distribution (typically obtained from size exclusion chromatography), and hence obtain precise information about its structural composition, i.e. enhance the existing sensitivity limit. We critically discuss the use of linear rheology as a reliable quantitative characterization tool, along with the recently developed temperature gradient interaction chromatography. The latter, which has emerged as an indispensable characterization tool for branched architectures, offers unprecedented sensitivity in detecting the presence of different molecular structures in a sample. Combining these techniques is imperative in order to quantify the molecular composition of a polymer and its consequences on the macroscopic properties. We validate this approach by means of a new model asymmetric comb polymer which was synthesized anionically. It was thoroughly characterized and its rheology was carefully analyzed. The main result is that the rheological signal reveals fine molecular details, which must be taken into account to fully elucidate the viscoelastic response of entangled branched polymers. It is important to appreciate that, even optimal model systems, i.e., those synthesized with high-vacuum anionic methods, need thorough characterization via a combination of techniques. Besides helping to improve synthetic techniques, this methodology will be significant in fine-tuning mesoscopic tube-based models and addressing outstanding issues such as the quantitative description of the constraint release mechanism.

Entanglement relaxation time of polyethylene melts from high-frequency rheometry in the mega-hertz rangea)

The determination of relevant rheological properties and parameters in a very broad frequency range can be achieved for a number of thermoplastic polymers, for example, polystyrene, by applying the time-temperature-superposition principle. In contrast, polyethylene can only be explored rheologically in a limited frequency range, due to its fast crystallization below the crystallization temperature and its weak viscosity temperature-dependence. In this paper, various commercially available polydisperse and narrowly distributed linear and branched polyethylenes and ethylene-vinylacetate-copolymers were characterized. A piezoelectric- and a new quartz (crystal resonator) rheometer (QR) with an extended frequency range were utilized for the characterization. Introduction of high frequency rheological techniques and implementation of these new measurement methods are shown. For the first time, the entanglement relaxation time in the higher MHz frequency range was determined by applying the QR-technique and com...

Decoding the linear viscoelastic properties of model telechelic metallo-supramolecular polymers

This work focuses on the linear viscoelastic properties of entangled telechelic bulk metallo-supramolecular polymers. The latter are based on linear and star poly(n-butyl acrylate)s functionalized with a terpyridine ligand at each chain extremity, in the presence of transition metal ion of varying nature. The systems are investigated both experimentally and theoretically using small amplitude oscillatory shear and a modified version of the tube-based time marching algorithm (TMA), respectively. The experimental data reveal that sample relaxation depends on both disentanglement and association dynamics, with the respective importance of these two processes depending on the nature of the metal ion and on the temperature. A good description of the data is achieved using the modified TMA model, provided that dissociation events of metal-ligand complexes occur via ligand exchange. The model contains two fitting parameters, i.e., the fraction of unassociated stickers and the longest time needed in order to ensu...

Linear Viscoelasticity of Weakly Hydrogen-Bonded Polymers near and below the Sol–Gel Transition

Supramolecular polymers bearing weak hydrogen bonds (sticker) can express outstanding dynamic properties due to their labile association. Studying the linear viscoelasticity (LVE) of this type of polymer can provide us with sufficient knowledge to design polymeric materials for applications that need dynamic properties such as self-healing. Using different compositions of flexible weak stickers, LVE analysis showed scalings corresponding to a transition from a linear precursor to a cluster. By introducing one sticker per repeating unit of the precursor polymer, the effect of sticker distribution along the chain as well as phase separation is excluded. However, even a fully functionalized polymer could not show any network formation, whereas surprisingly, a stable cluster was formed. This proves that weakly associated networks do not dissociate rapidly and can relax as a cluster at extended time before the dissociation of stickers can lead to the relaxation of linear analogous (slow kinetics similar to strong physical or even chemical bonds.) On the other hand, the absence of a gel even in fully sticker-functionalized polymers shows that the weakness of these polymers can be described as their weakness in complete association (thermodynamically not favored).