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

Abstract

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.

Viscoelasticity and nonlinear simple shear flow behavior of an entangled asymmetric exact comb polymer solution

“…1 but may contain a number of side products, as shown in [15,16] for similar exact combs. Here, we discard the possible effects of side products on the viscoelasticity and shear flow behavior as we are aiming to find the main effects of controlled branching on the nonlinear flow behavior and for a study of this type, slight effects of architectural dispersity can be reasonably expected to be small for the exact combs, in agreement with [15,16]. The glass transition temperature T g of the exact comb, as measured by differential scanning calorimetry (PL-DSC, TA-instruments, USA), was found to be À90 C (heating/cooling rate was 5 C/min).…”

“…For this type of exact comb samples, slight architectural dispersity is possible, i.e., the sample may not be 100% the target structure from Fig. 1 but may contain a number of side products, as shown in [15,16] for similar exact combs. Here, we discard the possible effects of side products on the viscoelasticity and shear flow behavior as we are aiming to find the main effects of controlled branching on the nonlinear flow behavior and for a study of this type, slight effects of architectural dispersity can be reasonably expected to be small for the exact combs, in agreement with [15,16].…”

“…It is a tube model specially developed for branched polymers, and it is based on the "hierarchical relaxation principle" [58]. Models based on this principle have been found to successfully describe the relaxation behavior of a large variety of model architecturally complex polymers [15,16,57,[59][60][61][62]. For combs, the idea is that the branches relax at high frequencies while the backbone remains frozen after which (at longer times) the backbone can relax while the branches act as a dynamic solvent and the branch points as points of high friction.…”

“…(ii) The syntheses of model polymers are mainly performed by anionic polymerization high-vacuum techniques resulting in well-defined molecular structures, which however come in very small quantities (typically <1 g) [12][13][14]. When theoretical models are being evaluated, such well-defined architecturally complex structures are indispensable, even though some disparity in molecular structure is unavoidable albeit not always problematic, and advanced characterization is often called for [15,16]. The low sample quantities call for the need to use appropriate (or modify existing) characterization methods, e.g., specifically for rheometry one cannot use a capillary rheometer as a characterization tool.…”

“…1, with each segment having a log-normal molar mass distribution with a chosen PDI of 1.1. Architectural dispersity as in [15,16] is not considered here. The final parameter for the model is the relaxation time of an entanglement segment s e .…”