Recent Advances in Separation-Based Techniques for Synthetic Polymer Characterization

Catalytic Upcycling of Polyolefins

The large production volumes of commodity polyolefins (specifically, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)), in conjunction with their low unit values and multitude of short-term uses, have resulted in a significant and pressing waste management challenge. Only a small fraction of these polyolefins is currently mechanically recycled, with the rest being incinerated, accumulating in landfills, or leaking into the natural environment. Since polyolefins are energy-rich materials, there is considerable interest in recouping some of their chemical value while simultaneously motivating more responsible end-of-life management. An emerging strategy is catalytic depolymerization, in which a portion of the C-C bonds in the polyolefin backbone is broken with the assistance of a catalyst and, in some cases, additional small molecule reagents. When the products are small molecules or materials with higher value in their own right, or as chemical feedstocks, the process is called upcycling. This review summarizes recent progress for four major catalytic upcycling strategies: hydrogenolysis, (hydro)cracking, tandem processes involving metathesis, and selective oxidation. Key considerations include macromolecular reaction mechanisms relative to small molecule mechanisms, catalyst design for macromolecular transformations, and the effect of process conditions on product selectivity. Metrics for describing polyolefin upcycling are critically evaluated, and an outlook for future advances is described.

Evaluation of multi-solvent size exclusion chromatography columns packed with sub-2 μm particles for the characterization of synthetic polymers

With the recent development of small particle stationary-phases and dedicated instrumentation, the combination of size-exclusion chromatography (SEC) with ultra-high performance liquid chromatography (UHPLC) technology has been realized. It opened up a new polymer analysis technique called UHP-SEC. Although high resolution and fast analysis can be achieved, the multi-solvent suitability for a given column was limited to either organic or aqueous eluents. In this work, the capability of novel SEC columns (AdvanceBio SEC columns) packed with 1.9 μm particles for the characterization of synthetic polymers in organic solvents as well as the multi-solvent compatibility for organic and aqueous eluents have been demonstrated. About six times faster separation for both polystyrene (PS) and polyethylene glycol (PEG) with good peak shape and repeatability were achieved in comparison with standard SEC columns at comparable resolution. Especially for PEG, in contrast to other SEC columns, this column could provide close-to-accurate determination of molecular weights with tetrahydrofuran (THF) as mobile phase. Good reproducibility was obtained after switching several times from water to THF and vice versa with RSD% in retention times less than 0.5 %. Different samples such as polyols, isocyanates and additives can also be analyzed for molecular weight and distribution or composition determination. Volume overload, especially with injection volumes higher than 10 µL needs to be considered. This new column offers a powerful choice for oligomer and polymer analysis with both aqueous and organic mobile phase. Ultimately, hyphenating SEC columns to various detectors can enable more information regarding chemical composition, molecular weight, concentration, and structure.

Sample transformation in online separations: how chemical conversion advances analytical technology

While the advent of modern analytical technology has allowed scientists to determine the complexity of mixtures, it also spurred the demand to understand these sophisticated mixtures better. Chemical transformation can be used to provide insights into properties of complex samples such as degradation pathways or molecular heterogeneity that are otherwise unaccessible. In this article, we explore how sample transformation is exploited across different application fields to empower analytical methods. Transformation mechanisms include molecular-weight reduction, controlled degradation, and derivatization. Both offline and online transformation methods have been explored. The covered studies show that sample transformation facilitates faster reactions (e.g. several hours to minutes), reduces sample complexity, unlocks new sample dimensions (e.g. functional groups), provides correlations between multiple sample dimensions, and improves detectability. The article highlights the state-of-the-art and future prospects, focusing in particular on the characterization of protein and nucleic-acid therapeutics, nanoparticles, synthetic polymers, and small molecules.

Use graphene coated silica core shell particles to provide rapid, precise, and equivalent chemical composition distribution analysis for polyolefin materials by high temperature thermal gradient interaction chromatography

High temperature thermal gradient interaction chromatography (HT-TGIC) has been widely used to measure chemical composition distribution due to its applicability to separate crystalline and non-crystalline amorphous polyolefin materials. The compatibility of HT-TGIC with various detectors (infrared (IR), light scattering (LS), and viscometer) has also allowed a comprehensive analysis of molecular architecture of polyolefin and recycled plastics. The introduction of an easy-to-fabricate graphene coated onto non-porous silica particles as HT-TGIC column in 2020 showed a superior chromatographic performance over the traditional graphite column. A reduction in peak broadness (∼47 %) under identical experimental conditions was demonstrated in that research. This paper similarly uses a graphene column but with the focus on optimization of experimental parameters (concentration, and thermal cooling and heating rates etc.). Equivalent chemical composition distribution (CCD) data to that obtained by the incumbent graphite column over a wide range of polyolefins products was achieved, in addition to a shortened analysis time from 120 min down to 88 min per sample. The materials studied included semicrystalline linear low-density polyethylene (LLDPE), elastomers, terpolymers, model blends to mimic recycled plastics. The results also suggest that the elimination of substrate pores enable a better HT-TGIC separation. Coupling the ease and reproducibility of the graphene column fabrication process enables long term chromatographic robustness. This not only results in equivalent CCD data compared to the traditional graphite column but also a 27 % reduction in analysis time. These results demonstrate a substantial advancement of technology in the high throughput industrial laboratory setting where fast testing turnaround time is critical. In addition, simple fabrication with commercially available silica particles and graphene nanopowder provides a cost-effective approach to make HT-TGIC columns reproducibly.

High-Resolution Tracking of Multiple Distributions in Metallic Nanostructures: Advanced Analysis Was Carried Out with Novel 3D Correlation Thermal Field-Flow Fractionation

Multifunctional metallic nanostructures are essential in the architecture of modern technology. However, their characterization remains challenging due to their hybrid nature. In this study, we present a novel photoreduction-based protocol for augmenting the inherent properties of imidazolium-containing ionic polymers (IIP)s through orthogonal functionalization with gold nanoparticles (Au NPs) to produce IIP_Au NPs, as well as novel and advanced characterization via three-dimensional correlation thermal field-flow fractionation (3DCoThFFF). Coordination chemistry is applied to anchor Au³⁺ onto the nitrogen atom of the imidazolium rings, for subsequent photoreduction to Au NPs using UV irradiation. Thermal field-flow fractionation (ThFFF) and the localized surface plasmon resonance (LSPR) of Au NPs are both dependent on size, shape, and composition, thus synergistically co-opted herein to develop mutual correlation for the advanced analysis of 3D spectral data. With 3DCoThFFF, multiple sizes, shapes, compositions, and their respective distributions are synchronously correlated using time-resolved LSPR, as derived from multiple two-dimensional UV-vis spectra per unit ThFFF retention time. As such, higher resolutions and sensitivities are observed relative to those of regular ThFFF and batch UV-vis. In addition, 3DCoThFFF is shown to be highly suitable for monitoring and evaluating the thermostability and dynamics of the metallic nanostructures through the sequential correlation of UV-vis spectra measured under incremental ThFFF temperature gradients. Comparable sizes are measured for IIP and IIP_Au NPs. However, distinct elution profiles and UV-vis absorbances are recorded, thereby reaffirming the versatility of ThFFF as a robust tool for validating the successful functionalization of IIP with Au to produce IIP_Au NPs.

Effects of Machine Washing on the Chromatography Parameters of Polyester Fiber Gel Permeation

Fiber examination is frequently performed in forensics, and gel permeation chromatography (GPC) is one candidate method for discriminating polyester fibers. Here, the effects of machine washing on weight-average molecular weight (M _w), polydispersity index (PDI), and the percentage peak area of cyclic ethylene terephthalate trimer (PPAL) of commercial polyester shirts and manufactured poly(ethylene terephthalate) (PET) yarns were investigated using GPC. GPC was performed using a 1,1,1,3,3,3-hexafluoro-propan-2-ol polymer solubilizer, styrene-divinylbenzene copolymer GPC columns, a chloroform mobile phase, and a 254 nm absorbance monitor. The statistical change in the polyester fibers during machine washing was evaluated by comparing three GPC parameters of the same fiber samples before and after machine washing. Among the commercial polyester shirts examined, the GPC parameters changed significantly after machine washing with a considerable PPAL decrease. In contrast, the GPC parameters of manufactured PET yarns changed significantly with a moderate increase in M _w. This work elucidates the change on GPC parameters of polyester fibers by machine washing.

Gaining New Insights in Advanced Polymeric Materials Using Comprehensive Two-Dimensional Liquid Chromatography

Two-dimensional liquid chromatography (2D-LC) offers new insights into modern polymeric materials such as biodegradable polymers, polymers made from renewable feedstock, and complex formulated systems. Advances in instrumentation and the development of new modulation techniques enable more combinations of different separation modes. Hyphenation with universal and information-rich detectors further enhances the versatility and flexibility of the analytical strategy. Detailed characterization of copolymer composition heterogeneity and identification of polymeric ingredients in complex consumer products are key highlights of new applications.

Physicochemical and Functional Characterization of Differential CRISPR-Cas9 Ribonucleoprotein Complexes

Advances in gene-editing technology enable efficient, targeted ex vivo engineering of different cell types, which offer a potential therapeutic platform for most challenging disease areas. CRISPR-Cas9 is a widely used gene-editing tool in therapeutic applications. The quality of gene-editing reagents (i.e., Cas9 nuclease, single guide (sg)RNA) is associated with the final cellular product quality as they can impact the gene-editing accuracy and efficiency. To assess the impact of the quality of Cas9 protein and sgRNA in the formation of a Cas9 ribonucleoprotein (RNP) complex, stability, and functional activities, we developed a size exclusion chromatography method that utilizes multiple detectors and an in vitro DNA cleavage assay using anion-exchange chromatography. Using these methods, we characterized the formation and stability of Cas9 RNP complexes associated with Cas9 and sgRNA characteristics as well as their functional activities. Multi-angle light scattering characterization showed different types and levels of aggregates in different source sgRNA materials, which contribute to form different Cas9 RNP complexes. The aggregations irreversibly dissociated at high temperatures. When the Cas9 RNP complexes derived from non-heated and heated sgRNAs were characterized, the data showed that specific RNP peaks were impacted. The Cas9 RNP complexes derived from the heated sgRNA retained their biological function and cleaved the double-strand target DNA at a higher rate. This work provides new tools to characterize the Cas9 RNP complex formation, stability, and functional activity and provides insights into sgRNA properties and handling procedures to better control the Cas9 RNP complex formation.

Development of a robust calibration method for improving the long-term precision of polyethylene comonomer content distribution using crystallization elution fractionation

Comonomer content distribution (CCD), also commonly known as chemical composition distribution (CCD) and short chain branching distribution (SCBD), describes the variation of short chain branching composition between individual polymer chains in polyolefin materials. It is of particular importance for controlling polyolefin performance. Crystallization-based separation methods have evolved over the past four decades aiming at resolution, speed, precision, and accuracy. Two of the commonly used techniques are Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF), where polymer chains are physically separated along the column or on the surface of the support based on their crystallinity, respectively. CEF analysis takes much less time than TREF. There is a critical need for precise temperature calibrations for data repeatability. This report demonstrates a novel CEF methodology using a two-point technique to consistently and conveniently calibrate the comonomer composition and column temperature. This column temperature calibration methodology was adopted in a study by tracking the reproducibility over a period of 8 years, using multiple instruments located in different laboratories and in different geographies. The results exhibited superior repeatability, with less than 0.3% of the relative error calculated from 3000 data points of the eluting peak temperature, thus demonstrating this as a robust method for industrial labs that require good quality controls.

Phase equilibria and conformational behavior of dendrimers in porous media: Towards chromatographic analysis of dendrimers

HYPOTHESIS

The intricate entropy-enthalpy interplay of dendrimers confined in pores affects their conformation and retention in the porous stationary phase. This work aims at providing important insights into its impacts on partitioning and chromatographic separation in both size-exclusion chromatography (SEC) and interaction chromatography (IC) regimes.

SIMULATIONS

Using Monte Carlo (MC) simulations, we investigated the bulk-pore phase equilibria and the conformational behavior of flexible dendrimers differing in generation, in spacer length and in fraction of modified terminal groups interacting differently with pore walls than the majority building units.

FINDINGS

With increasing interaction strength, a distinct transition from a roughly spherical shape caused by simultaneous interactions with two walls to an ellipsoidal (or even disklike) conformation tenaciously adhering to only one wall was observed for moderately confined dendrimers. The strongly deformed dendrimers subjected to severe confinement gain high energy and the samples differing in the degree of modification become chromatographically discernable thanks to large energy differences. Consequently, our results suggest that the column fillings with fairly narrow pores which are ineffective in SEC, are highly efficient separation media for dendrimer studies by IC above the critical adsorption point (CAP). Overall, our simulations reveal useful information for advancing and optimizing experimental liquid chromatography studies of dendrimers.

New Advances and Applications in Field-Flow Fractionation

Field-flow fractionation (FFF) is a family of techniques that was created especially for separating and characterizing macromolecules, nanoparticles, and micrometer-sized analytes. It is coming of age as new nanomaterials, polymers, composites, and biohybrids with remarkable properties are introduced and new analytical challenges arise due to synthesis heterogeneities and the motivation to correlate analyte properties with observed performance. Appreciation of the complexity of biological, pharmaceutical, and food systems and the need to monitor multiple components across many size scales have also contributed to FFF's growth. This review highlights recent advances in FFF capabilities, instrumentation, and applications that feature the unique characteristics of different FFF techniques in determining a variety of information, such as averages and distributions in size, composition, shape, architecture, and microstructure and in investigating transformations and function.

Improving temperature gradient interaction chromatography of polyolefins by simultaneous use of different stationary phases

Temperature gradient interaction chromatography (TGIC) at high temperatures is a powerful method for the chemical composition separation of polyolefins. TGIC is a two-step process where the sample is crystallized on the stationary phase at low temperature followed by the elution of the sample components using a temperature gradient towards high temperatures. For TGIC typically a porous graphitic carbon (PGC) stationary phase is used. The separation mechanism is based on crystallization and adsorption/desorption phenomena and it has been shown that co-crystallization and co-adsorption may affect the separation. The present study reports on the simultaneous use of a non-adsorptive and an adsorptive stationary phase (column) in series to utilize both crystallization and adsorption for improved separation in TGIC. A silica column is used as the non-adsorptive support to allow for the crystallization of the polyolefin sample in the absence of an adsorptive force followed by the typical PGC column for adsorption/desorption. Accordingly, the loci of crystallization and adsorption/desorption are well separated from each other and can be adjusted independently. This novel column setup allows the sample to be introduced slowly onto the second (adsorptive) column eliminating possible co-adsorption and poor selectivity. Low molar mass polyethylene comprising of oligomers with approximately C₃₀C₁₃₀ was used to illustrate the importance of a non-adsorptive column for improved separation. Utilizing a non-adsorptive silica column allows for higher dynamic flow rates during crystallization, which improves separation. Shorter adsorptive columns are found to be more efficient in this experimental protocol as compared to standard TGIC experiments. Smaller PGC column sizes result in reduced longitudinal and Eddy diffusion and, hence, higher resolution of low and high molar mass polyolefins.

Crosslinking in Semi-Batch Seeded Emulsion Polymerization: Effect of Linear and Non-Linear Monomer Feeding Rate Profiles on Gel Formation

Waterborne latex is often called a product-of-process. Here, the effect of semi-batch monomer feed rate on the kinetics and gel formation in seeded emulsion polymerization was investigated for the copolymerization of n-butyl methacrylate (n-BMA) and ethylene glycol dimethacrylate (EGDMA). Strikingly, the gel fraction was observed to be significantly influenced by monomer feed rate, even while most of the experiments were performed under so-called starve-fed conditions. More flooded conditions from faster monomer feed rates, including seeded batch reactions, counterintuitively resulted in significantly higher gel fraction. Chain transfer to polymer was intentionally suppressed here via monomer selection so as to focus mechanistic insights to relate only to the influence of a divinyl monomer, as opposed to being clouded by contributions to topology from long chain branching. Simulations revealed that the dominant influence on this phenomenon was the sensitivity of primary intramolecular cyclization to the instantaneous unreacted monomer concentration, which is directly impacted by monomer feed rate. The rate constant for cyclization for these conditions was determined to be first order and 4000 s-1, approximately 4 times that typically observed for backbiting in acrylates. This concept has been explored previously for bulk and solution polymerizations, but not for emulsified reaction environments and especially for the very low mole fraction divinyl monomer. In addition, while gel fraction could be dramatically manipulated by variations in linear monomer feed rates, it could be markedly enhanced by leveraging non-linear feed profiles built from combination sequences of flooded and starved conditions. For a 2 h total feed time, a fully linear profile resulted in 30% gel while a corresponding non-linear profile with an early fast-feed segment resulted in 80% gel.