Complex Branched Polymers for Structure–Property Correlation Studies: The Case for Temperature Gradient Interaction Chromatography Analysis

Temperature Gradient Interaction Chromatography: A Perspective

The application of temperature gradient interaction chromatography (TGIC) as an advanced technique for the characterisation of polymers is discussed, in comparison to other liquid chromatography techniques and in particular the ubiquitous size exclusion chromatography. Specifically, the use of reversed-phase TGIC for the interrogation of complex branched polymers and normal-phase TGIC for characterisation of high-molar mass end-functionalised polymers is highlighted.

Synthesis of polymeric topological isomers based on sequential Ugi-4CR and thiol–yne click reactions with sequence-controlled amino-functionalized polymers

Polymeric topological isomers have been designed and synthesized with sequence-controlled amino functionalized polymers.

Absolute molar mass determination in mixed solvents. 1. Solving for the SEC/MALS/DRI "trivial" case

Size-exclusion chromatography (SEC) with on-line static light scattering, specifically multi-angle static light scattering (MALS), and differential refractometry (DRI) detection remains the premier method by which to determine absolute, calibrant-independent molar masses of polymers. The method is restricted to the use of either neat solvents or solvents with a small amount of additive. In mixed solvents, preferential solvation (i.e., the enrichment, within the solvated volume of the polymer in solution, of one solvent over the other as compared to the solvent ratio outside said volume) leads to errors in the areas of the MALS and DRI chromatograms, as the solvent baseline does not accurately represent the solvent contribution to these detectors' peaks. A seemingly trivial way by which to overcome this problem is through the use of an isorefractive solvent pair. This "trivial" solution is complicated by the fact that the solvents in the pair must be miscible with each other in all proportions; the individual solvents as well as the mix must be able to fully dissolve the analyte; the solvents must possess sufficient optical contrast with the solution so as to generate an adequate detector signal; the solvent mix must be compatible with the chromatographic stationary phase, such that enthalpic contributions to the separation are minimal and analyte recovery from the columns is quantitative; and the difference in the Rayleigh factors of the solvents can be ignored. Herein, we present the analysis of narrow dispersity polystyrene (PS) and poly(methyl methacrylate) (PMMA) samples, across a four-fold range in molar mass, using SEC/MALS/DRI in a mix of tetrahydrofuran (THF) and methyl isoamyl ketone (MIAK), solvents which are shown to be isorefractive with each other at the temperature and wavelength of the experiments. Molar mass averages and dispersities are demonstrated to be statistically independent of solvent composition and to correspond well to the values in neat THF. The experiments were augmented by the use of on- and off-line quasi-elastic light scattering and of off-line MALS and DRI, to study the effect of solvent composition on polymer size in solution and on dilute solution thermodynamics. Additionally, ¹H nuclear magnetic resonance spectroscopy was used to study the effect of tacticity on the insolubility of PMMA100 in 100% MIAK. We believe this constitutes the first example of obtaining accurate molar masses of polymers by SEC/MALS/DRI employing mixed solvents. The value of these experiments to other forms of macromolecular liquid chromatographic separations is also noted.

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.

Anionic Polymerization of para-(1-Ethoxy ethoxy)styrene: Rapid Access to Poly(p-hydroxystyrene) Copolymer Architectures

Living anionic polymerization of para-(1-ethoxy ethoxy)styrene (pEES) resulting in molecular weights between 2700 and 69 000 g mol^-1 and polydispersity indices ≤1.09 is introduced. PpEES can be used as a precursor for the synthesis of well-defined poly(p-hydroxystyrene) (PHS) architectures, enabling facile and rapid acidic deprotection at room temperature within a few minutes. In addition, a series of block copolymers containing pEES and 2-vinylpyridine (2VP) have been synthesized by anionic block copolymerization, with varied block ratios (X_2VP) between 0.13 and 0.83. Characterization by ¹H NMR spectroscopy, size exclusion chromatography (SEC), and differential scanning calorimetry (DSC) was carried out, and all polymers have been deprotected, leading to the respective PHS-b-P2VP block copolymers. Furthermore, PHS-b-P2VP has been used as a macroinitiator for the anionic ring-opening polymerization of ethylene oxide (EO) to generate ((PHS-g-PEO₅₁)₁₃-b-P2VP₄₀) graft-block copolymers.

Analytical Rheology of Polymer Melts: State of the Art

The extreme sensitivity of rheology to the microstructure of polymer melts has prompted the development of “analytical rheology,” which seeks inferring the structure and composition of an unknown sample based on rheological measurements. Typically, this involves the inversion of a model, which may be mathematical, computational, or completely empirical. Despite the imperfect state of existing models, analytical rheology remains a practically useful enterprise. I review its successes and failures in inferring the molecular weight distribution of linear polymers and the branching content in branched polymers.