Timothy C. Schunk

Chemical composition separation of synthetic polymers by reversed-phase liquid chromatography

Abstract Synthetic polymers present many unique separations challenges because, unlike small organic molecules, they consist of a distribution of structurally different chains. Each macromolecule can differ in chain length and end groups; stereochemical and structural isomers are possible along with unique branching architectures. Copolymers are further complicated by the combination of these potential variables with varying ratios and sequence distributions of their comonomer units. Liquid chromatography of individual synthetic polymers is limited by the solution properties of macromolecules to a narrow selection of solvents. In particular the thermodynamic considerations of polymer dissolution are strongly influenced by entropic factors. The application of reversed-phase liquid chromatography to the separation of synthetic polymers is characterized by the delicate balance between strong solvation interactions and weak adsorption interactions and is often further complicated by polymer solubility limitations in gradient elution separations.

Bonded phase conformation and salvation effects on the stationary phase structure in reversed-phase liquid chromatography

Abstract The stationary phase in reversed-phase liquid chromatography on chemically modified silica bonded with alkyl modeties is understood to be a dynamic multicomponent mixture. Through the work of many researchers, an understanding of this near-surface region has been developed as a salvation layer mixture in terms of the length of the bonded hydrocarbon moieties, the surface bonding density, the hydrogen-bonding water adsorption at the silica surface, and the enhanced concentration of organic solvent from the mobile phase. However, the structure and chemical interactions in this near-surface region are not well understood; in fact, some researchers have even considered the stationary phase to be a chemically passive participant in chromatographic separations. Detailed investigation of the thermodynamic and kinetic quantities associated with a temperature-induced conformational change in bonded octadecyl moieties yields information on this interfacial region. The solvation dependence of this surface change as probed by a variety of solutes on different bonding-density octadecyl-bonded silica materials provides a model elucidating the three-dimensional structure and retention interactions of the stationary phase. The general applicability of this model is shown to provide consistent description of solute retention interactions with four different organic modifiers on five octadecyl bonded silicas differing in bonding chemistry, bonding density, and base silica.

Quantitative polymer composition characterization with a liquid chromatography-Fourier transform infrared spectrometry-solvent-evaporation interface

Abstract Factors that are important to the quantitative analysis of polymer composition distribution by high-performance liquid chromatography (HPLC)-Fourier transform infrared (FT-IR) using a solvent-evaporation interface are investigated. These factors include the effects of the location and distribution of the deposited polymer films, as well as the morphology of the deposit. Consideration is given to the influence of these factors on the chromatographic resolution and FT-IR spectral quality. Size-exclusion separations of polystyrene and poly(methyl methacrylate) in tetrahydrofuran are used to demonstrate the impact of these effects on the quantitative use of the resulting FT-IR spectra. Results indicate that all absorbance bands are not uniformly affected by spectral distortions, but that compositional information can be obtained on simple blends. The effectiveness of a post-sample-collection solvent-annealing procedure is also considered.

Quantitative FTIR detection in size-exclusion chromatography

With the increasing popularity of evaporative interfaces, detection using Fourier Transform Infrared (FTIR) spectrometry in the mid-infrared region is becoming more important in size-exclusion chromatography (SEC). FTIR spectrometry is a powerful, and potentially very widely applicable, method for obtaining chemical functional group information for each molecular size fraction. Quantitative evaluation of polymer composition across the SEC chromatogram can provide more accurate characterization of heterogeneous polymer samples for problem solving and for material specification. The evaporative interface removes the SEC mobile phase at the exit of the column and deposits the eluting polymer as a continuous film stripe or as a series of discrete films on infrared transparent substrates. Initially this detection approach was used only for qualitative analysis. More recently, it is being used quantitatively. Previously we demonstrated that the quality of the film generated by the evaporative interface was critical to determining the suitability of the resulting FTIR spectra for quantitative analysis. In a continuation of this work, the objective of this paper is to develop a procedure for obtaining valid quantitative results for polymer blends with the interface. Experimental topics include improving the quality of polymer films by post-SEC treatments, off-line FTIR calibrating using other means to obtain high quality polymer films, and utilizing in-line SEC detectors in calibration. Interpretation aspects focus upon peak fitting of FTIR spectra, linear regression, partial least squares, and data pre-processing. PLS prediction with internal calibration using the second derivative of solvent-annealed film spectra was found to provide the best compromise between processing time, accuracy and precision.

Methyl cyclohexane as a new eluting solvent for the size-exclusion chromatography of polyethylene and polypropylene at 90°C

Abstract Size-exclusion chromatography using halogenated aromatic solvents such as trichlorobenzene at 145°C is widely used for industrial polyolefins. Such high temperature operation requires special instrument design and causes many operational problems. In this work, a new eluting solvent, methyl cyclohexane, has been successfully used at 90°C for the size-exclusion chromatography of industrial (high-molecular-mass) grades of polypropylene and polyethylene. Sample preparation involves dissolution in decalin at temperatures as high as 140°C, followed by dilution in methyl cyclohexane at 90°C previous to injection into the SEC column running with methyl cyclohexane as the mobile phase. In addition to permitting operation at 90°C, methyl cyclohexane is less toxic than the usual solvents and has a differential refractive index sensitivity advantage as well. It also provides new opportunities for ultraviolet, fluorescence, and infrared detection for functionalized polyolefins. However, one disadvantage is that polystyrene adsorbs from this solvent on styrene-based packings. Thus, narrow fractions of polyisobutylene (PIB) were used in place of polystyrene for universal calibration. Another disadvantage is that methyl cyclohexane is more flammable than halogenated aromatic solvents. A quantitative assessment of the new solvent system is in progress. Initial results are promising and are presented in this paper.

Compositional distribution characterization of poly(methyl methacrylate)-graft-polydimethylsiloxane copolymers

Abstract Graft copolymers prepared by radical polymerization of a low-molecular-mass monomer with a macromonomer display heterogeneity in both molecular mass and chemical composition. The characterization of these joint distributions by a single technique [e.g., size-exclusion chromatography (SEC)] is hindered by the effects of both variables on the separation mechanism. Separation emphasizing chemical composition heterogeneity can be efficiently performed by gradient elution high-performance liquid chromatography (HPLC) combining precipitation and adsorption retention. Comparison of Fourier transform IR and evaporative light-scattering detection indicated decreasing polydimethylsiloxane (PDMS) macromonomer incorporation corresponding to increasing retention time for a poly(methyl methacrylate) (PMMA)-graft-PDMS copolymer. More detailed information was obtained by multidetector SEC of composition fractions from gradient elution HPLC. SEC separation in an isorefractive solvent for PDMS (tetrahydrofuran) with low-angle laser-light scattering, differential viscometry, and differential refractive index detection allowed determination of the molecular mass of both the whole copolymer and that of the PMMA backbone for each HPLC fraction. Comparison with an independent SEC determination of the PDMS macromonomer molecular mass allowed estimation of the number of pendant PDMS chains per graft copolymer molecule across the HPLC chromatogram. Results indicated a relatively constant incorporation of the number of PDMS side chains with increasing PMMA backbone molecular mass, leading to a relative decrease in weight fraction PDMS incorporation with increasing molecular mass of the whole graft copolymer molecule.

Composition distribution separation of methyl methacrylate-methacrylic acid copolymers by normal-phase gradient elution high-performance liquid chromatography

Abstract The characterization of copolymers requires the determination of chemical composition distribution in addition to molecular mass (M) distribution. Techniques commonly applied to copolymer analysis provide either average bulk composition or a convolution of M and composition distributions. The application of gradient elution adsorption chromatography, under appropriate conditions, allows the separation of copolymers based solely upon their chemical composition in a M-independent mode. Separation independent of M requires the elimination of size-exclusion effects with the porous adsorbent and the minimization of precipitation contributions. For separation over a wide copolymer composition range, the relative contributions of adsorption and precipitation retention mechanisms change with solvent composition. Under gradient elution conditions where adsorption retention dominates, composition fractionation can be combined with subsequent size-exclusion chromatography to provide a complete composition/M map of the copolymer.

Size Exclusion Chromatography: Practical Methods for Quantitative Results

ABSTRACTSize exclusion chromatography (SEC) is now a common polymer analysis method employed in polymer reaction engineering studies. However, this once simple technique is no longer very simple. In this overview we selectively review the literature relevant to the practical use of SEC to obtain quantitative results. Fractionation, detection, calibration, resolution correction and system development are examined in turn. It becomes evident that, at this stage in SEC development, each of these areas is advancing very rapidly. This means that valuable additional quantitative information on polymer molecular properties is now obtainable.However, this additional information is accompanied by significant additional sources of error and uncertainty. The original SEC with only a differential refractive index detector is currently the most reliable and precise system, as well as being the most limited in what information it provides. New methods of linking results from such simple systems with multi-detector “res...

Detecting “Perfect Resolution” Local Polydispersity in Size Exclusion Chromatography

Abstract The objective of this work is to obtain a simple method for detecting local polydispersity. Local polydispersity is the presence of a variety of different types of molecules at the same retention volume in SEC. One source of local polydispersity is axial dispersion. However, the topic of this paper is the detection of local polydispersity which is independent of axial dispersion effects This “perfect resolution” local polydispersity can occur because SEC separates on the basis of molecular size in solution and thus for complex polymer molecules, such as copolymers or branched polymers, a variety of combinations of molecular weight and composition can produce the same molecular size. In conventional SEC interpretation, it is assumed that with high resolution columns local polydispersity is absent. Highly misleading analyses can result if this assumption is invalid. Two very simple methods were developed in this work. The first method enabled polystyrene-poly(dimethyl siloxane) blends to be examine...

Chapter 22 Synthetic polymers

Publisher Summary This chapter describes synthetic polymers and the use of chromatographic techniques to analyze them. Synthetic polymers present unique separations challenges because, unlike many small organic molecules and biopolymers, they always consist of a distribution of structurally different chains. For example, each macromolecule differs in length and may have different end groups. Vinyl monomers can combine in head-to-head, head-to-tail, or tail-to-tail configurations, as well as in stereochemically different conformations to give a distribution of homopolymer structural isomers. Homopolymers may also be randomly or non-randomly branched or possess unique architectures, such as combs, stars, ladders, or macrocycles. Synthetic polymers are described by single, unique values, such as average molecular weight, average chemical composition, average length among branch sites, average comonomer sequence of triads or pentads, or average block length. For many purposes, these averages suffice. However, it is often a shape or characteristic of a structural distribution that is not easily described by an average value that dictates polymer properties. Size-exclusion chromatography (SEC) remains the principal chromatographic method for determining molecular weight distributions of synthetic polymers. The development and commercialization of low-angle laser light scattering (LALLS), and more recently, multi-angle laser light scattering have provided absolute molecular weight distributions directly from size-exclusion experiments. The characterization of branching distribution in synthetic polymers usually involves the measurement of both molecular weight and polymer dimensions in solution. Long-chain branching in poly(viny1 alcohol) and poly(viny1 acetate) have also been studied quantitatively by SEC after establishing a relationship between molecular weight and intrinsic viscosity of isolated fractions.