Miloš Netopilík

Polystyrene-equivalent molecular weight versus true molecular weight in size-exclusion chromatography

Abstract The evaluation of the size-exclusion chromatography (SEC) concentration elution curves by means of a calibration dependence obtained in a given SEC set for a polymer different from the polymer to be analyzed results in an error in the determination of both molecular weight and molecular-weight distribution (MWD). The problem is analyzed assuming the validity of the universal-calibration concept. The differences between the true and apparent values of molecular weight, MWD and Mw/Mn depend on and are expressed in terms of the parameters of the Mark–Houwink–Kuhn–Sakurada equation, describing the molecular-weight dependence of intrinsic viscosity, for the polymer to be analyzed and the polymer used for calibration. The differences in molecular weight and the Mw/Mn ratio are typically tens of percent and, in extreme cases, can amount up to a factor of three for molecular weight and a factor of two for the Mw/Mn ratio.

Estimation of width of narrow molecular-weight distributions by size-exclusion chromatography with concentration and light scattering detection.

A new method for the estimation of the weight-to-number-average molecular-weight ratio, Mw/Mn of polymers with a narrow molecular-weight distribution, approximated by log-normal distribution, is proposed using size-exclusion chromatography (SEC) with concentration and light-scattering detectors. From experimental data, the Mw/Mn ratios are calculated by two procedures: one using the concentration and light-scattering elution curve for the polymer measured, and the other based on the concentration elution curve and calibration line for a wide range of molecular masses. An iteration method has been developed making the two Mw/Mn ratios converge. The method was applied to a series of narrow molecular-weight distribution polystyrene standards.

Effect of local polydispersity in size exclusion chromatography with dual detection

Abstract The effect of local polydispersity combined with the error in the pre-determined value of interdetector volume was theoretically examined for a polymer sample with a molecular weight distribution which obeys the log-normal function. The effect results in rotation (change of the slope) of the uncorrected dependences of molecular weight, experimentally determined as local molecular weight average, vs. elution volume. If the interdetector volume is determined correctly, the centre of the rotation due to the peak broadening of each uncorrected correlation lies in the correct correlation on the point corresponding to the n-, w- or z-average molecular weight. The experimental values of the n- and w-average of the whole polymer remain unchanged in the n- and w-detection even if the experimental correlations of molecular weight vs. elution volume are revolved. In the z-detection, on the other hand, the experimental polydispersity does not depend on the interdetector volume.

Combined effect of interdetector volume and peak spreading in size exclusion chromatography with dual detection

The relation between the error in interdetector volume determination and peak spreading was studied for a polymer sample whose molecular weight distribution obeys the log-normal function. A compensation of these quantities was shown, i.e. their values were found for which the ratio of weight- to number-average molecular weight, Mw/Mn, was constant. The influence of non-uniformity in the molecular weight of the sample on the compensation is demonstrated.

Phase transition parameters of potential thermosensitive drug release systems based on polymers of N-alkylmethacrylamides

The phase separation and its thermohysteresis in dilute aqueous solutions of polymeric components of potential drug release systems (homopolymers and copolymers of N-isopropylacrylamide, N-isopropylmethacrylamide, N-propylmethacrylamide, N-sec-butylmethacrylamide, and N-(2-hydroxypropyl)methacrylamide) was studied, both on heating and cooling. Plots of light transmittance vs temperature were constructed and the parameters characterizing them were correlated with polymer structures. Qualitative information was obtained on the rate of formation of the concentrated phase on heating and its disappearance on cooling. Attention has been drawn to the improper identification of the cloud-point temperature, measured at an arbitrary concentration, with the lower critical solution temperature (LCST) as is frequently found in papers dealing with biomedical applications of thermosensitive polymers.

Relations between the separation coefficient, longitudinal displacement and peak broadening in size exclusion chromatography of macromolecules

The separation of a polymer by size exclusion chromatography is described as a series of interactions, i.e. consecutive establishments of equilibria between polymer fractions in the mobile and stationary phases followed by displacements of mobile phase containing the polymer. The elution curve is derived as the longitudinal concentration profile in the column observed in one position in space during the time of the analysis. The mean value of elution volume of a particular polymer species turns out to be the interstitial volume of the separation system divided by the mean fraction of polymer in the mobile phase. The number of the displacement-equilibrium steps can be estimated from the limiting values of the variance of the spreading function.

Model analysis of the determination of the Mark—Houwink—Kuhn—Sakurada parameters and molecular weight distribution by means of size exclusion chromatography with dual detection

The analysis of a polymer obeying the log-normal molecular weight distribution (MWD) by means of size exclusion chromatography with dual concentration/light) scattering or viscosity detection was theoretically examined. The relations between the theoretical and experimental values of MWD parameters and Mark—Houwink—Kuhn—Sakurada constants calculated by comparing viscometric, light scattering and effective linear calibrations to the universal one were calculated as functions of the instrument parameters (interdetector volume and resolution factor). The linearity of calibrations calculated from the dual trace directly or indirectly, by combining with the universal calibration, is preserved.

Effect of interdetector peak broadening and volume in size-exclusion chromatography with dual viscometric–concentration detection

Abstract For a polymer sample with a molecular mass distribution which obeys the log-normal function, the effect of intra- and extracolumn peak broadening, combined with the effect of the error in the interdetector volume determination, on uncorrected data of size-exclusion chromatography with the dual concentration–viscosity detection was theoretically examined. If the separation is not perfect and peaks recorded by the differential refractometric and viscometric detector are broadened to a different degree and/or an error in the interdetector volume occurs, the dependences of the detected molecular mass on elution volume are rotated and deformed. The error in the determination of uncorrected molecular mass averages for the whole polymer from the dual detection data is demonstrated and the effect of the Mark–Houwink–Kuhn–Sakurada exponent on uncorrected data was demonstrated.

Hydrodynamic Parameters of Linear Aromatic Polyester of 3-Phenylglutaric Acid and Bisphenol A

Abstract A series of samples of linear aromatic polyester of 3-phenylglutaric acid and bisphenol A was prepared with molecular weights ranging from 14 700 to 92 700. The polymers were characterized by size exclusion chromatography with triple light-scattering, viscosity, and concentration detection. Hydrodynamic parameters of the chain were determined using the theory of Yoshizaki, Yamakawa, and Nitta modified by Bohdanecký and Netopilík. The chain parameters are a useful reference in the study of aromatic hyperbranched polyesters.

Problems connected with band-broadening in size-exclusion chromatography with dual detection.

The sources of band-broadening (axial dispersion) in size-exclusion chromatography (SEC) of polymers and its influence on data obtained using dual detection of concentration and molecular weight are reviewed. Special attention is paid to the combination of a multiangle light-scattering photometer and a differential refractometer as detectors. The local polydispersity is discussed in the relation to the band-broadening as well as to heterogeneity of the polymer with respect to molecular weight and hydrodynamic volume.