Maria P. Tarazona

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.

Dilute solution characterization of polyphosphazenes

Abstract A survey of the data reported in the literature on the dilute solution characterization of polyphosphazenes is presented. There are relatively few studies dealing with this kind of characterization, at least in comparison with the enormous amount of work devoted to the synthesis, solid-state properties and possible practical applications of these materials. Furthermore the analyses published to date give surprising and often contradictory results which indicate that the solution properties of these polymers are difficult to measure and not yet well understood. High polydispersity of the samples, formation of intermolecular aggregations and branching are the problems more frequently invoked to explain the experimental results. Some conclusions are presented that could facilitate future characterizations of these polymers.

Solution properties of poly[bis(piperidino)phosphazene] by size exclusion chromatography and multiangle light scattering

The synthesis, fractionation and characterization of poly[bis(piperidino)phosphazene] are described. Dilute solution properties of the polymer in THF at 25°C have been studied by size exclusion chromatography, using simultaneously multiangle light scattering and differential refractive index detectors. The anomalous elution behavior and the dependence of the dimensions, i.e. radius of gyration, of the polymer on the molecular weight are discussed. Coefficients for the scaling law have also been calculated.

Solution properties of poly(diphenoxyphosphazene) below the θ temperature obtained by SEC/MALLS

Abstract Dilute solution properties of poly(diphenoxyphosphazene), PDPP, in THF at 25°C have been studied by size exclusion chromatography, using simultaneously multiangle light scattering and differential refractive index detectors. The anomalous elution behavior and the dependence of the dimensions, i.e. radius of gyration, of the polymer on the molecular weight are discussed. Coefficients for the scaling law and unperturbed dimensions have also been calculated. Molecular Dynamics simulations of some model compounds were performed in order to analyze the conformational characteristics of the P–N–P and N–P–N pairs of skeletal bonds. The conformational model thus obtained indicates that the PDPP chain is mainly formed by sequences of about four skeletal bonds in the trans conformation separated by one bond in cis, with negligible incidence of cis–cis orientations. The chain thus generated is very rigid so that the characteristic ratio of the unperturbed dimensions CN increases with the number of repeat units of the chain x, up to values as high as x≈800, reaching an asymptotic value of ca 30 when x→∞, in very good agreement with the experimental result.

Kerr constants of random coil polypeptides

SummaryCalculations of average molar Kerr constants of polypeptides of the twenty natural α-aminoacids in the random coil state are presented. The computation was carried out according to the Rotational Isomeric States model with the values of energies, dipole moments and optical anisotropies of the repeating units reported elsewhere. In the case of homopolypeptides, the ratios /x extrapolated to x→∞ range approximately from −6000 to +5000 in units of 10−27V−2m5mol−1. Results obtained for ten actual proteins and three enzymes in the random coil state are also reported; their values of /x are very sensitive to the kind of aminoacid residues and to the sequence of those residues.

Characterization of poly(diethoxyphosphazene) by size exclusion chromatography and viscometry

Two samples of poly(diethoxyphosphazene) (PDEP) having very different molecular weights have been studied by viscometry and size exclusion chromatography in THF solution. The results obtained, together with light scattering data of these samples, allow the calculation of the Mark-Houwink constants a=0.65 and K=2.5 10-4 in THF at 25°C. The method of calculation employed takes into account the great polydispersity of the samples. The characteristic ratio of the unperturbed dimensions was also calculated giving Cn = 〈r2〉o/nℓ2 ≈ 18, a value slightly higher than those previously reported for poly(dihexoxyphosphazene), Cn≈13 and poly (dichlorophosphazene), Cn≈13.5.

Kerr constant of poly (methyl phenyl siloxane)

SummaryThe molar Kerr constant at infinite dilution of atactic (Wm=0.5) poly (methyl phenyl siloxane) (PMPS) with ¯Mv=8 105, was determined from experimental measurements of electrical birefringence on its solutions in p-dioxane at 21±1 °C. The experimental result is /x=(1.9±0.5) 10−25 m5V −2mol−1; the large estimated error of this result is due to the surprisingly low value of /x of the PMPS. The theoretical value of this magnitude, calculated according to the RIS model is /x=1.6 10−25 m5V −2mol−1 wich is in good agreement with the experimental result. Moreover, theoreticical analysis indicates that the reason for the low value of this magnitude, is that the dipole contribution to /x is negative although not large enough to overcome the positive contribituion of polarizability.

Determination of the viscometric parameters for the system poly(dihexoxyphosphazene)-benzene at 25°C

Abstract A viscometric analysis of four samples of poly(dihexoxyphosphazene), [NP(R2)−]x with R = O(CH2)5CH3, having very different molecular weights has been carried out using benzene as solvent. The results for intrinsic viscosity obtained at 25°C were [η] = 1.61, 1.09, 0.41 and 0.057 dl g−1, respectively, for samples having weight-average molecular weights, determined by light scattering, of 10 −5 M w = 22.8, 9.17, 2.45 and 0.26 g mol−1. Although the samples are polydisperse, it is possible to determine the true values of the K and a coefficients appearing in the Mark-Houwink equation [η] = KMa, by using the results obtained in a previous analysis performed on a different solvent. The values for K and a for benzene at 25°C are 2.01 × 10−5 and 0.79, respectively. The extrapolation to unperturbed conditions allows the evaluation of the characteristic ratio of dimensions as C n = 〈r 2 〉 0 nl 2 = 12.9 , in good agreement with the value of 13.2 previously obtained. This agreement validates the hypothesis used for the determination of the K and a coefficients.

Use of a Reliable Homemade Dilatometer To Study the Kinetics of the Radical Chain Polymerization of PMMA: An Undergraduate Polymer Chemistry Laboratory Kinetics Experiment

In this laboratory experiment, a simple, reliable homemade dilatometer was used to study the kinetics of the radical chain polymerization of PMMA. The reaction was carried out in toluene with benzoyl peroxide as the initiator at a temperature of 80 °C. Each student studied the kinetics at a different initiator concentration constant. Pseudo-first-order plots permit students to obtain kapp and to demonstrate order 1 with respect to the monomer concentration. Finally, a log-log plot of kapp versus the initiator concentration from the data collected by each student demonstrates order 0.5 with respect to the initiator concentration. Results also agree with the rate constants of the process implicated in this type of polymerization.

Scaling law for the radius of gyration of poly[bis(2-naphthoxyphosphazene)] (PBNP)

AbstractA numerical procedure that allows for the evaluation of the scaling law between radius of gyration and molecular weight, i1/2=Q Miq, of ideally monodisperse fractions of any polymer is proposed. The method is similar to that previously used to determine viscometric constants and chromatographic calibration functions and is based on the combination of experimental results obtained by Static Light Scattering and Size Exclusion Chromatography on polydisperse samples. The method is used to study poly[bis(2-naphthoxyphosphazene)] in THF solution at 25°C for which the relationship i1/2=0.557 Mi0.29 is obtained. The comparison between the dimensions computed with this equation and those obtained by viscometric measurements, performed under the same experimental conditions, suggest the formation of intermolecular aggregates in the solutions of this polymer.