Architectural Dispersity in Model Branched Polymers: Analysis and Rheological Consequences

Abstract

We combine state-of-the-art synthetic, chromatographic, rheological, and modeling techniques in order to address the role of architectural polydispersity in the rheology of model branched polymers. This synergy is shown to be imperative in the field and leads to several important results. Even the best available synthesis is prone to “contamination” by side-products. The exact targeted macromolecular structure can be analyzed experimentally and statistically and eventually fractionated. Temperature-gradient interaction chromatography proves to be an indispensible tool in this process. All techniques are sensitive to the various macromolecular structures, but in different ways. In particular, the presence of side-products may or may not influence the linear rheology, due to competing contributions of the different relaxation processes involved, reflecting different structures at different fractions. Hence, combination of all these techniques is the key for fully decoding the architectural composition of branched polymers and its influence on rheology.

Synthesis and temperature gradient interaction chromatography of model asymmetric star polymers by the “macromonomer” approach

“…The efficiency of the two coupling methods are compared and the 'purified' star polymers characterised by both SEC and Temperature Gradient Interaction Chromatography (TGIC). Whilst SEC has been the characterisation method of choice for many decades, for the analysis of molecular weight and molecular weight distribution of polymers, in recent years TGIC has emerged as a technique capable of significantly enhanced resolution compared to SEC, especially in the characterisation of branched polymers [12,14,[68][69][70][71]] -a subject recently reviewed in detail [72]. In the current work TGIC revealed low levels of heterogeneity in the purified (fractionated) stars -heterogeneity which could not be detected by SEC.…”

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.…”