Enrique Saiz

Combination of SEC/MALS experimental procedures and theoretical analysis for studying the solution properties of macromolecules.

Size-exclusion chromatography (SEC) with dual detection, i.e., employing refractive index (RI) and multiangle light-scattering (MALS) detectors, has been applied to study the solution properties of two very different polymer-solvent systems at 25 degrees C: poly(N-vinylcarbazole) (PVCz) in an organic solvent THF that is a very good solvent and a system under theta conditions that is formed by polyvinylpyrrolidone (PVP) in water containing a 0.1 M concentration of NaNO(3). In both cases, the analysis of a single highly polydisperse sample obtained by free radical polymerization is enough for obtaining molecular weight and radius of gyration calibration curves, molecular weight distributions (MWD) (and thus, molecular weight averages), molecular dimensions, scaling laws coefficients and unperturbed dimensions. Extrapolation to theta conditions produces values of the characteristic ratio of the unperturbed dimensions C(n)=(o)/nl(2)=15.9 and 14, respectively, for PVCz and PVP. Unperturbed dimensions are also theoretically calculated with different models such as Kuhn equivalent chain, worm-like chain and rotational isomeric states model. Conformational parameters required for this last model were taken from the literature in the case of PVCz; however, they are calculated by molecular dynamics simulations in the case of PVP. Theoretical values obtained with the RIS model are in good agreement with the experimental results.

Analysis of copolymers of styrene and methyl methacrylate using size exclusion chromatography with multiple detection

Size exclusion chromatography (SEC) with differential refractive index (RI), multiangle light scattering (MALS) and UV detectors has been used to obtain information about copolymers of styrene (S) and methyl methacrylate (MMA). Homopolymers PS and PMMA having broad molecular weight distributions, mixtures of PS and PMMA homopolymers, and copolymers containing different weight fractions of S and MMA units were studied. Molecular weight distributions, molecular dimensions and scaling laws are reported for these systems. The behavior of the random copolymers is very different from that exhibited by the heterogeneous mixtures of homopolymers. Thus, the results obtained for the copolymers can be explained by assuming that these molecules have homogeneous distributions of S and MMA units along their chains. On the contrary, the mixtures produce the results that would be expected from heterogeneous combinations of different molecules. The SEC technique offers the possibility of determining the composition of random copolymers and discriminating among random copolymers (having homogeneous distributions of units) and block copolymers.

A simple method to determine unperturbed dimensions of polymers using size exclusion chromatography and multiangle light scattering

A very simple method to obtain unperturbed dimensions from measurements of just one sample of a polydisperse polymer, using differential refractive index and multiangle laser light scattering detectors in size exclusion chromatography is presented. Poly(methyl methacrylate)s of broad molecular weight distribution have been measured, in a good solvent, in order to obtain the scaling law between the radius of gyration and the molecular weight. The unperturbed dimensions have been calculated using three different mathematical procedures for the extrapolation and compared with data found in the literature.

Experimental and simulation studies on the transport of gaseous diatomic molecules in polycarbonate membranes

Measurements on the pure transport of oxygen, nitrogen, and carbon monoxide in membranes prepared from poly[bisphenol A carbonate-co-4,4′-(3,3,5-trimethylcyclohexylidene) diphenol carbonate] are reported. The isotherms representing the variation of the permeability coefficient with the upstream pressure can be described by assuming that the diffusants are retained in the matrix by Henry and Langmuir mode sorptions. The values of the permeability coefficient of oxygen, nitrogen and carbon monoxide at 35 °C and p0=1 atm are 8.75, 1.87, and 2.91 barrers, respectively. The values of the diffusion coefficient in 10−8 cm2/s, measured in the same conditions, are 8.26, 2.81, and 3.08, respectively. These membranes exhibit higher permeability than polycarbonate membranes with the two methyl groups of bisphenol A replaced by other different molecular groups, without a substantial diminution of permselectivity. The permeability and the diffusion coefficients obey Arrhenius behavior. The application of the transition...

Direct estimation of the true Mark-Houwink-Sakurada parameters from gel permeation chromatography, intrinsic viscosity and one average of molecular weight

A method is described for the simultaneous determination of the true constants in the Mark-Houwink viscosity equation and the absolute calibration of gel permeation chromatography data. In this method, polydisperse samples can be used. The experimental data required on each sample are its gel permeation chromatogram, its intrinsic viscosity and one average of molecular weight, Mw or Mn. The method does not require any calibration curve for another polymer (e.g. polystyrene monodisperse standards). The method is applied to the system poly(N-vinyl-3,6-dibromocarbazole) in tetrahydrofuran at 25°C. The procedure may be especially useful in those cases in which, due to the existence of secondary exclusion mechanisms resulting from polymer-stationary phase or polymer-solvent interactions, the hydrodynamic volume universal calibration concept is not valid.

The use of molecular dynamics for the study of solution properties of guar gum

Size exclusion chromatography with dual detection, i.e., employing a refractive index, concentration sensitive, detector together with a multiangle light scattering detector which is sensitive to molecular size, has been applied to study the solution properties of guar gum in water with different concentrations of K2SO4 at 25 °C. The analysis of a single highly polydisperse sample is enough for obtaining calibration curves for molecular weight and radius of gyration and the scaling law coefficients. The influence of the ionic strength on the conformational properties of the polymer can also be analyzed. Moreover, unperturbed dimensions can be obtained by extrapolation of the values measured in a good solvent. The value of the characteristic ratio of the unperturbed dimensions thus obtained is Cn=〈r2〉0/nl2≈19±1. A theoretical analysis is also included. Thus, molecular dynamics procedures were employed to analyze the conformational properties of an oligomer of guar gum under different conditions; namely, st...

Molecular dynamics of model compounds of polymers with chlorocyclohexyl groups in their structure

The conformational mobility of 2‐chlorocyclohexyl isobutyrate (CCHI), a model compound for the repeating unit of vinyl polymers containing chlorocyclohexane residues as side groups, is analyzed employing molecular dynamics (MD) procedures. Close to room temperature (ca. 300 K), the interconversion between axial (i.e., both chlorine atom and ester group in axial positions) and equatorial (both substituents in equatorial orientations) is not observed within the total time of 5 ns allowed to the MD trajectories. The analysis was then performed at temperatures in the range 1000 to 1500 K and the results extrapolated to lower temperatures. These extrapolations give energetic barriers of 5.72 and 8.15 kcal/mol, respectively for axial→equatorial and equatorial→axial transformations, with life times of τax≊9.6 and τeq≊46.3 ns for these two conformations at 300 K. The same procedure applied to unsubstituted cyclohexane gives an energetic barrier of 10.6 kcal/mol for the chair to chair interconversion, in excellent...

Simulations of diffusive and sorption processes of gases in polyimide membranes: Comparison with experiments

The trajectories of carbon dioxide, oxygen and nitrogen in membranes prepared from poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine) were simulated by assuming that the molecules of gas migrate through the glassy polymer in a sequence of hops between local minima of the potential energy. The values of the diffusion coefficients in units of 10−9 cm2 s−1 were 1.8, 7.3 and 7.8, in fair agreement with the experimental results, 1.3, 5.4 and 1.6, respectively, in the same units. The solubility coefficient was simulated by using Monte Carlo techniques in which the probabilities of inserting/removing a molecule of diffusant in a matrix containing n molecules were evaluated as the product of three factors depending, respectively, on: (a) the pressure p and temperature T, (b) the interaction energy between the diffusant particle and the polymer host matrix, and (c) the volume of the diffusant. The simulated pressure dependence of the concentration of carbon dioxide in the membrane displays the same pattern as that experimentally found. The simulated values of the solubility coefficients of CO2, O2 and N2, at 30°C and 76 cmm Hg of pressure, are in rather good agreement with the experimental results obtained from both permeation and sorption experiments.

Intramolecular excimers and energy migration in polyesters with 2,6-naphthalene dicarboxylic acid units separated by spacers of 1–4 ethylene oxide units

Abstract The fluorescence has been measured in dilute solution and in glassy poly(methyl methacrylate) for polyesters of 2,6-naphthalene dicarboxylate and HO(CH2CH2O)mH, m = 1−4. The model compounds in which the glycol connects two molecules of 2-naphthoate were also studied. Intramolecular energy migration is implied by the anisotropy of the fluorescence in rigid media. The results are consistent with Forster radii of about 12 A in the model compounds and 14 A in the polymers, as found previously for a similar series based on HO(CH2)mOH instead of HO(CH2CH2O)mH.

Intramolecular excited state complexes in trichromophoric model compounds for polyesters derived from 2,6‐naphthalenedicarboxylic acid and aliphatic glycols: experiment, rotational isomeric state model, and molecular dynamics

Steady-state fluorescence was used to measure the ratio of emission intensities, denoted ID/IM, for excited state complexes and excited monomers of five trichromophoric compounds, 2-naphthyl-COO-(CH2)m-OOC-2,6-dinaphthyl-COO-(CH2)m-OOC-2-naphthyl, m = 2–6. The linear aliphatic alcohols H(CH2)nOH, n = 1–7, as well as mixtures of ethylene glycol and methanol, were used to change the viscosity of the medium, η. The values of ID/IM depend on η and m. A Rotational Isomeric State model and Molecular Dynamics simulations were used for interpretation of the experimental results.