Christian Jackson

Instrumental effects in the analysis of polymers of wide polydispersity by MALDI mass spectrometry

Abstract Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (tof) mass spectrometry (MS) has been reported to give low number average and weight average molecular weight values for polymers of wide polydispersity. Spectra of wide polydisperse polymers more closely resemble the distribution observed by size exclusion chromatography when obtained in the reflectron mode. However, the highest mass molecules in the distribution are not observed. These high-mass ions can be observed when the more abundant lower mass ions are deflected from reaching the detector. In addition, a mixture of three narrow distribution polymer standards selected to provide a wide mass distribution do not show discrimination against the high-mass molecules. These results are interpreted to suggest that the difficulty in observing accurate molecular weight distributions by MALDI-TOF-MS is at least in part attributable to instrumental limitations rather than ionization limitations.

Size exclusion chromatography with multiple detectors: Solution properties of linear chains of varying flexibility in tetrahydrofuran

Narrow molecular weight distribution polyisoprenes (PI), polybutadienes (PBD), poly(isobuty1enes) (PIB), poly(methylmethacry1ates) (PMMA), polystyrenes (PS), and poly(octadecy1 methacrylates) (PODMA) have been studied by size exclusion chromatography (SEC) in tetrahydrofuran (THF) with light scattering and viscometric detectors. The molecular weight (M) dependence of the intrinsic viscosity, [η], and radius of gyration, Rg, is reported for THF solutions of these polymers, in many instances for the first time. The availability of these data for a series of chains of varying flexibility allows a test of the universal calibration principle in SEC. Furthermore, an apparent dependence of the hydrodynamic parameter in good solvents, Φ, on chain stiffness is observed. All chains appear to exhibit hydrodynamic draining in THF.

Synthesis and dilute solution properties of divinylbenzene-linked polystyrene stars with mixed arm lengths: Evidence for coupled stars

Sequential anionic polymerization of styrene and divinylbenzene (DVB) is known to lead to the formation of star-shaped polymers. This ‘arms-first’ method has been widely used and studied. It is known that this polymerization forms stars with anionically active cores. This article is concerned with the attempt to make asymmetric-star polymers utilizing these living carbanionic sites present in the core to form a second set of shorter arms growing out from the star core. The presence of remaining unreacted DVB within the core was found to cause the stars to couple to form linked double stars and other larger structures. Results from detailed dilute solution studies of the resulting polymers are reported. It was found that the results obtained from size exclusion chromatography for the double stars were flow rate dependent; only at low flow rates was a true size separation obtained.

Computer simulation study of size-exclusion chromatography with simultaneous viscometry and light scattering measurements

Abstract The integration of an on-line viscosity or light scattering (LS) detector with size-exclusion chromatography (SEC) improves the accuracy with which polymer molecular mass distributions can be measured. The coupling of a viscometer and a light scattering detector in one SEC instrument potentially offers improved precision and dynamic range for SEC polymer conformation studies. However, the increased complexity of these experiments and the subsequent data handling introduce a number of problems not present in conventional SEC. A computer simulation of the multiple detector SEC experiment was developed in order to study these effects in detail. The computer model is described and preliminary data are presented to illustrate some of the additional features of SEC with multiple detectors.

Dilute solution properties of randomly branched poly(methyl methacrylate)

Randomly branched poly(methyl methacrylate) samples were prepared by copolymerization with different amounts of ethylene dimethacrylate. The molecular weight distributions, radius of gyration distributions, and intrinsic viscosity distributions were measured by size exclusion chromatography with refractive index, multiangle light-scattering, and viscosity detectors. The effect of branching on the radius of gyration was compared with the effect on the intrinsic viscosity. It was found that the intrinsic viscosity contraction factor gi scales with the radius of gyration contraction factor g, with the exponent, ϵ, having a value in the range 0.8–1.0.

Evaluation of the “effective volume shift” method for axial dispersion corrections in multi-detector size exclusion chromatography

Abstract Band broadening in size exclusion chromatography (SEC) causes errors in the calculated molar mass distribution even when molar-mass-sensitive detectors are used. These errors can be partially corrected by shifting the elution volume of the molar-mass-sensitive detector chromatogram relative to the concentration detector chromatogram, and then re-calculating the molar mass distribution. In this paper a computer simulation of multi-detector SEC of a polymer with a Flory–Schulz molar mass distribution and Gaussian band broadening is used to study the volume shift method of correcting for band broadening. The results are compared with those obtained by applying a conventional band broadening correction. In the case of SEC with a light scattering detector the results from the two methods are comparable. For SEC-viscometry with universal calibration the conventional method is significantly more accurate than the volume shift method. A method for calculating the volume shift required to correct for a known amount of band broadening is also presented.

Size exclusion chromatography of step-growth polymers with cyclic species: theoretical model and data analysis methods

The measurement of the molecular weight averages and the molecular weight distribution (MWD) of many step-growth polymers is complicated due to the presence of cyclic oligomers formed during polymerization. If size exclusion chromatography (SEC) is used to determine the MWD, the cyclic oligomers are generally only partly resolved from the linear polymer, and hence distort the measured linear MWD. Further, the cyclic oligomers require a different calibration curve from the linear species and hence, in general, their molecular weights are not accurately measured. In order to clarify the effect of cyclic species on the measured MWD, a model of the SEC separation of step-growth polymers with cyclic species was developed. In this article, this model is described and used to illustrate aspects of the characterization of these polymers using both conventional SEC and multi-detector SEC. The results from the model are used to develop methods for estimating the MWD of the linear polymer and to determine the weight fraction of cyclic species. The results of the model are compared with experimental data for nylon 6, nylon 6,6 and poly(ethylene terephthalate).

Computer simulation study of multi-detector size- exclusion chromatography of branched molecular mass distributions

Size-exclusion chromatography (SEC) provides a rapid method for determining molecular mass distributions relative to standard calibration materials. If light-scattering and viscosity detectors are used independent measurements of molecular mass and size can be obtained directly, and these can be used to estimate the distribution of branching across the molecular mass distribution. In order to study the effect of branching on the detector signals and the calculated results a computer simulation of the multiple detector SEC analysis of randomly branched polymers was developed. The model is described and results for different amounts of branching, different extents of reaction, and different models of the hydrodynamic size are discussed.

A kinetic study of the formation of polystyrene stars using 1,2‐bis(trichlorosilyl)ethane

The kinetics of formation of a chlorosilane-linked polystyrene six-arm star is reported. The precursor arm material (Mn = 88,000) was made using anionic polymerization in benzene. Prior to addition to the 1,2-bis(trichlorosilyl) ethane linking agent, the anions were endcapped with about five units of isoprene. Size exclusion chromatography using multiangle laser light scattering and viscosity detectors was utilized for characterization. This technique has allowed the molecular weights, radii of gyration, and intrinsic viscosities to be measured for star components in aliquots taken from the reactor at various times. It was found that four-arm star is formed within 30 min after the addition of the chlorosilane linking agent. There is a linear relationship between the logarithm of molecular weight of the star samples and logarithm of time of the reaction after the formation of the four-arm star.

On the Accuracy of Molecular Weight and Radius of Gyration Measured by Right-Angle Light Scattering with SEC-Viscometry

Abstract The use of a right-angle light scattering detector with a size exclusion chromatograph equipped with an online viscometer provides a convenient method of estimating polymer molecular weight distribution. The 90° scattered intensity provides an initial estimate of the molecular weight at each elution slice. This value is then improved using an estimate of the light-scattering asymmetry based on the measured intrinsic viscosity in order to determine the 0° scattering intensity required to obtain the true molecular weight. In addition, an estimate of the radius of gyration can be obtained. In this paper the accuracy of the method is examined for different linear polymers and also for branched polymers. In most cases molecular weights within 2% of the true values can be obtained up to 1 × 106 g/mol. Errors are about 5% for molecular weights of 2 × 106 g/mol and rapidly increase for higher molecular weights. The errors in the radius of gyration estimated using this method are higher (∼ 10%); however, ...