Molecular weight characterization of soluble high performance polyimides. 1. Polymer-solvent-stationary phase interactions in size exclusion chromatography

Soluble, fully cyclized m-amino phenyl acetylene terminated polyimides based on several anhydride/diamine monomers were prepared in N-methylpyrrolidine (NMP) and cyclized by solution imidization to controlled molecular weight. The polyimides and a polyamic acid precursor were successfully analyzed by size exclusion chromatography (SEC) utilizing online parallel coupled refractive index and differential viscometer detectors. The calculated M _nvalues were varied from 3,000 to 20,000 daltons. N-methylpyrrolidone (NMP), tetrahydrofuran (THF), and chloroform served as mobile phases for the cross-linked polystyrene gel packings. Normal retention behavior of the polyimides was observed in chloroform, THF, and NMP containing LiBr, or in NMP stirred over P₂O₅ before use. Values of Mark-Houwink-Sakurada exponents for narrow distribution linear polystyrene indicate that pure NMP and NMP with 0.06 M LiBr are good solvents for polystyrene standards at 60°C. In contrast, SEC behavior of polyimides in pure NMP leads to splitting of the peaks with the major portion observed to pass through the columns at the exclusion limit. In contrast to strong polymeric chain expansion of the polyamic acid in dilute solution, presumably due to a polyelectrolyte effect, no increase of intrinsic viscosity of polyimide samples in pure NMP was observed. This exclusion effect of polyimides analyzed in NMP is discussed in terms of possible ion-exclusion from pores of the stationary phase. Differences in polystyrene calibration in NMP with or without additives and the temperature dependence of calibration curves in these mobile phases is discussed as well. ©1995 John Wiley & Sons, Inc.     

Control of macromolecular architecture of polyamides by poly-functional agents. 2. Use of oligomerization in polycondensation study

In the first paper of the series, a statistical model for star-branched polycondenzation of AB type monomers in the presence of a polyfunctional agent RA_f was completely developed. The analytical expressions obtained for the number-average (D̄P̄_n̄) and weight-average (D̄P̄_w̄) degree of polymerization, and the dispersion index (D) for whole polymer species, linear and star macromolecular chains, are now derived as function of the feed and of end-group analysis. Also the important molecular parameter, mole fraction of star-branched polymer, can be evaluated. Some numerical examples are presented. It is illustrated that the molecular weight properties of the linear and star-branched polymers in the mixture of the products, very important factors for the application of this kind of polymeric materials, can be determined starting from the feed and terminal group analysis. Polymerization and oligomerization of 6-aminocaproic acid were carried out in the presence of trimesic (T3) acid and 2,2,6,6-tetra(β-carboxyethyl)cyclohexanone (T4) and EDTA as tri- and terra-functional agents. The molecular weights calculated are in good agreement with those obtained by Size Exclusion Chromatography (SEC), end group analysis and NMR spectra.     

On-line measurement of the RMS radius of gyration and molecular weight of polyimide precursor fractions eluting from a size-exclusion chromatograph

The recent introduction of multiangle light-scattering detectors for size-exclusion chromatography has made possible the measurement of the root mean square radius of gyration (R_g) and molecular weight (M) of polymer fractions eluting from a size-exclusion chromatography column. The characterization of the dimensions of a polymer may be accomplished with only a few milligrams. The dimensions of a polyimide precursor prepared by the condensation of the meta-diethyl ester of pyromellitic dianhydride with para-phenylene diamine have been measured with this technique. The dependence of R_g on M across the distribution is compared with that predicted for a freely rotating chain, and with other similar polymers measured with hydrodynamic techniques.     

Rapid determination of molecular parameters of synthetic polymers by precipitation/redissolution high‐performance liquid chromatography using “molded” monolithic column

Rapid high‐performance liquid chromatography (HPLC) of polystyrenes, poly(methyl methacrylates), poly(vinyl acetates), and polybutadienes using a monolithic 50 × 4.6 mm i.d. poly(styrene‐co‐divinylbenzene) column have been carried out. The separation process involves precipitation of the macromolecules on the macroporous monolithic column followed by progressive elution utilizing a gradient of the mobile phase. Depending on the character of the separated polymer, solvent gradients were composed of a poor solvent such as water, methanol, or hexane and increasing amounts of a good solvent such as THF or dichloromethane. Monolithic columns are ideally suited for this technique because convection through the large pores of the monolith enhances the mass transport of large polymer molecules and accelerates the separation process. Separation conditions including the selection of a specific pair of solvent and precipitant, flow rate, and gradient steepness were optimized for the rapid HPLC separations of various polymers that differed broadly in their molecular weights. Excellent separations were obtained demonstrating that the precipitation‐redissolution technique is a suitable alternative to size‐exclusion chromatography (SEC). The molecular weight parameters calculated from the HPLC data match well those obtained by SEC. However, compared to SEC, the determination of molecular parameters using gradient elution could be achieved at comparable flow rates in a much shorter period of time, typically in about 1 min. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2767–2778, 2000     

Oxidation of ultra high molecular weight polyethylene (UHMWPE) part 3: Decomposition of hydroperoxides with SO2 and its effect on the chemiluminescence signal

Samples of slightly oxidised UHMWPE were treated with SO₂ to decompose hydroperoxides prior to chemiluminescence measurements. It was found that the effect was quite substantial, but it was not just due to the elimination of hydroperoxides; a stabilising effect was also manifested. This effect increased with an increasing concentration of hydroperoxides in the samples. The stabilising effect could not be seen in fresh samples or when the hydroperoxides were decomposed by heating the sample in nitrogen. In addition, a stabilising effect was also observed for samples in which the hydroperoxides were reduced by DMS. It was concluded that the stabilisation was due to the formation of H₂SO₄ in the case of SO₂ and DMSO in the case of DMS. Both these substances can act as stabilisers against oxidative degradation of polymers and were evenly distributed in the sample films after the reaction with the hydroperoxides.     

Estimating molecular masses of petroleum-derived fractions: High mass (>2000 u) materials in maltenes and asphaltenes from Maya crude oil

Molecular mass ranges and average masses of fractions from a heavy Mexican crude oil (Maya) have been studied, using mainly size exclusion chromatography (SEC) and laser desorption-mass spectrometry (LD-MS). Method development focused on the use of planar chromatography and size exclusion chromatography (SEC), to isolate narrow bands of material from solubility-separated fractions of the crude oil. The procedure provides a planar chromatography based method for studying mass ranges in complex hydrocarbon mixtures. It allows the calculation of ‘best estimate’ values for number and mass-averages. These can then be used in average structural parameter (ASP) calculations, for studying structural features of the samples. The method is applicable to both coal and petroleum-derived samples. The molecular mass estimates arrived at in this work for petroleum-derived samples are considerably higher than those reported by other workers for similar samples. The results presented here provide strong evidence for the presence of ions approaching m/z 10,000 in the Maya asphaltene. The maltene fraction was found to contain a small amount of ions with mass (m/z) in excess of 2000.