Size Exclusion Chromatography of Meiamine-, Urea-, and Phenolformaldehyde Resins Using N,N-Dimethylformamide as Eluent

Abstract

Resins, hardly soluble in THF or chloroform, in the oligomer region were analyzed using DMF as solvent and a polystyrene column for oligomers. This column, 25 cm × 8 mm i.d., is the exclusive use of DMF and has the number of theoretical plates of 6000 per 25 cm. Polynuclear methylol melamines were resolved into individual polynuclear species and peaks from mono- to penta nuclear methylol melamines were identified. The thermal stability of methylol melamines can be monitored by this system. Molecular weight distributions of urea-formaldehyde resins and phenol-formaldehyde resole resins were observed. This PSt gel - DMF system will be useful for the process control or the quality control of these resins.     

Mass spectrometric monitoring of change of resole structures by compounding and curing of EPDM compound

Change of resole structures in a rubber compound by compounding and curing processes were investigated using ethylene-propylene-diene terpolymer (EPDM) compound and atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). Resole structures were divided into three groups of reactive, pendent, and unreactive species to examine in detail the structural changes. Reasonable resole species of the major resole species having two possible structures were determined using variations of the relative abundances by compounding and curing. Of the major resole species, there was no resole species having both terminals of methylol. It was found that resole was not simply mixed in a rubber matrix and reactive resole species were diversified and activated through the compounding process. Almost all reactive resole species participated in crosslinking reactions, but some reactive resole species still remained in the vulcanizate.     

Structural characteristics of p‐t‐Octylphenol formaldehyde resole resins using molecular simulation

p‐t‐Octylphenol formaldehyde resole resins have two linkage types of methylene‐ and dimethylene ether‐linkages and have three terminal types of hydrogen, methylol, and o‐methylene quinone. Variation of structural characteristics of the resins due to different types of linkages and terminals were studied using molecular dynamics and molecular mechanics. The structural characteristics of the methylene‐bridged resins were intramolecular hydrogen bonds between hydroxyl groups of the adjacent p‐t‐octylphenols. In the dimethylene ether‐bridged resin, the intramolecular hydrogen bonds between oxygen atoms of the dimethylene ether‐linkages and hydroxyl groups of the neighboring phenolic units were found. For the resins with both methylol terminals, one of both terminals of the resins was hidden at the center of the molecule when the resin size is large. The number of hydrogen bonds in the resins with the methylol terminal was larger than for the resins with the o‐methylene quinone terminal. Variation of the structural characteristics of the resins by dehydration of the terminal methylol was discussed. Using the calculated results, dissociation of the dimethylene ether linkage and crosslinking reaction of rubber chains by the resin were explained. Copyright © 2002 John Wiley & Sons, Ltd.     

Determination of molecular weight distribution for polypropylene by column fractionation and gel-permeation chromatography

Investigations on the molecular weight distribution of cyrstalline polypropylene were carried out by column fractionation and gel-permeation chromatography for the purpose of comparing the determination methods. Average molecular weight, standard deviation, skewness, and kurtosis were calculated as statistical parameters from the distribution curve of each, and the number-average and weight-average molecular weights were determined by osmometry and light scattering. The molecular weight distribution curves obtained from column fractionation were found to be narrower than those from gel-permeation chromatography, and it was confirmed that the molecular weight distribution curve obtained from GPC was more accurate and reliable than that from column fractionation by the fact that the average molecular weight from GPC agreed fairly well with that from the absolute methods. On the other hand, no clear difference between these determination methods was observed with respect to skewness and kurtosis of the data, while the standard deviation from GPC seemed to be greater than that from column fractionation.     

Comparison of number–average molecular weights calculated by GPC and membrane osmometry

Data are presented which show that when a polymer contains an appreciable amount of low molecular weight species below the diffusion limit of the osmometer membrane, the osmotic molecular weight, M?_n, is generally higher than the M?_n calculated from gel-permeation chromatography (GPC). Experiments were performed on samples of poly(vinyl chloride) (PVC) and high-cis polybutadiene polymers. Osmotic data were obtained in the usual manner, while GPC data were obtained using the universal calibration approach. It was found that when all polymer species below approximately 10,000 molecular weight were excluded from the calculation of M?_n by GPC, agreement in M?_n was obtained between membrane osmometry and GPC. The data obtained suggest that the choice of M?_n as measured by membrane osmometry in the calibration of the GPC should not be done casually, as the measured M?_n may not reflect the “true” value of that sample, especially when the polymer sample contains an appreciable amount of low molecular weight material.     

Gel-permeation chromatography of polydimethylsiloxanes

The molecular weight distributions of three polydimethylsiloxanes (PDMS) have been determined by gel-permeation chromatography (GPC), by gas–liquid chromatography (GLC), and by precipitation fractionation. The GPC data are greatly improved by correction for band spreading. For a high molecular weight polymer the corrected distribution agrees closely with the theoretical molecular weight distribution calculated from polymerization kinetics.     

Formaldehyde Condensation Products of Model Phenols for Conifer Bark Tannins

Abstract

Gel permeation chromatography of the condensation products of phenols and formaldehyde proved effective in understanding the reactions of condensed tannins with formaldehyde. Rates of condensation of phloroglucinols, resorcinols, catechols, (+)catechin, and (-)epicatechin were examined to determine if methylol-tannins from southern pine bark could be prepared as resin intermediates. The phloroglucinols (models for the A-ring of pine bark tannins) were so much more reactive than were the resorcinols (models for the A-ring of wattle tannins), that preparation of methylol-tannins from pine bark tannins seems unlikely even though this approach has been applied successfully to synthesis of resins from wattle tannins. The methylol content of catechin-formaldehyde condensation products was very low. The catechol B-ring was unreactive under conditions in which condensations of pine bark tannins could be controlled. Although there is much less steric hindrance of the A-ring of (-)epicatechin than of (+)catechin, differences in the rates of condensation of these isomers were not detected. GPC and H-NMR spectra of (+)catechin condensation products showed that a dimer formed rapidly and that further condensation occurred more slowly.     

Gel-permeation chromatography and cellulose. I. Effect of degree of nitration of cellulose on molecular weight distribution data

Consideration is given to the effect on gel-permeation chromatographic (GPC) data of the extent of substitution in nitrated cellulose. GPC parameters for samples containing 13.55–13.81% nitrogen (14.14% corresponds to complete substitution, DS = 3) were hardly affected by this variation in substitution. Variations that were observed are considered to arise within the samples themselves. Experiments with low molecular weight organic iodides, nitrates, and hydroxyl compounds indicate longer chain lengths than actual; this is attributed to extensive solvation of the substituent groups. The very long chain lengths obtained for cellulose nitrate by the present GPC procedure may arise from such an affect.     

Resolution of GPC columns

A resolution index for gel-permeation chromatograph (GPC) columns is defined by the relation log (RI) = αW, where RI is the resolution index, α is the slope of the calibration curve of logarithm of molecular weight against peak position for narrow polymer fractions, and W is the width of the GPC curves for narrow fractions. This resolution index is calculated approximately for several GPC columns individually and in series combinations by using data obtained on narrow fractions of polystyrene. The index increases with number of columns and with flow rate over a limited range of flow rate, does not vary much with solvent, and does not correlate well with plate count obtained with low molecular weight molecules. It is believed that the resolution index provides an improved indication of the performance of GPC columns.     

Molecular weight determinations by gel-permeation chromatography and viscometry

The hydrodynamic volume concept can be used effectively with gel-permeation chromatographic (GPC) and viscosity data to estimate the molecular weight of a variety of polymers. Agreement is within ±5–10% of the absolute values and thus is satisfactory for many purposes. An iterative computer technique and a method developed by Funt and Hornof for analyzing GPC–viscosity data were found to be equivalent with respect to estimating the molecular weights for the five cases studied. The latter is easily employed but restricted to the case where the sample of interest and the GPC calibration standards have approximately equal Mark-Houwink parameters. Since GPC measurements are commonly performed in thermodynamically good solvents, the general applicability of the method is not impaired. Using the unperturbed dimensions of the polymer chain to estimate the molecular weight of a variety of polymers was not as satisfactory as the above techniques. This approach generally gave biased molecular weight values (consistently low or consistently high). Agreement with the absolute values ranged from 10 to 30%. We therefore believe that either of the techniques based on the hydrodynamic volume concept can be used more effectively to estimate the molecular weight of a series of polymers than the treatment based on the unperturbed dimension.     

Study of radiation-induced polymerization of vinyl monomers adsorbed on inorganic substances. V. Effects of p-benzoquinone and dose rate on methyl methacrylate–silica gel system

In order to elucidate the mechanism of radiation-induced polymerization of methyl methacrylate adsorbed on silica gel, the effects of p-benzoquinone addition and dose rate were studied in detail. Most of the polymerization is inhibited by p-benzoquinone at levels above 10^-2 mole/l. The GPC spectra of both graft polymers and homopolymers show two peaks. The high molecular weight material appears to have been formed by polymerization by a radical mechanism, because these peaks decrease as p-benzoquinone concentration increases; on the other hand, their low molecular weight polymers seem to be products of an ionic polymerization mechanism because those peaks are almost not affected by p-benzoquinone. The four GPC peaks differ in dose rate dependences of their polymerization rate. The dose-rate exponents of polymerization rate were obtained for the four GPC peaks. The behavior of the low molecular weight peaks of graft polymers and homopolymers were quite different, suggesting that the polymers differ considerably in formation mechanism.     

Simulation model for urea-formaldehyde reactors

A comprehensive simulation model for urea/formaldehyde (UF)-resins has been developed. The model utilizes kinetic results reported in the literature for low molecular weight systems and extends these to higher molecular weight systems by using a functional group approach. The model consists of - 9 functional groups, 5 urea groups and 4 formaldehyde groups. - 9 reactions, 5 methylolation reactions and 4 condensation reactions. In all these reactions both the formation and the decomposition reactions are included. All the kinetic constants are pH and temperature dependent. Two batch runs with different U/F ratios were used to adjust a couple of model parameters. Samples from these batch runs were analyzed with HPLC for low molecular weight compounds, and GPC for the average molecular weight. A run with continuous IR measurement showed excellent fit for the formaldehyde concentration. The model also estimates the viscosity from the functional groups.     

Determination of the width of a narrow molecular weight distribution by gel-permeation chromatography

A recycle gel-permeation chromatography (GPC) method for extrapolating to infinite resolution is proposed. From the GPC calibration curve, the extrapolated elution peak width volume can be converted into peak width in decades of molecular weight. For an essentially Gaussian distribution, the weight-average/number-average molecular weight ratio can be determined from a calculated conversion table.     

Low-density polyethylene: Variation of branching frequency with molecular weight and its influence on molecular weight as determined by gel-permeation chromatography

A method for determining long-chain branching frequency and molecular weight averages for unfractionated low-density polyethylene (LDPE) by the combined use of gel-permeation chromatography (GPC) and intrinsic viscosity data has been reported (GPC–IV method). The method assumes that the number of long branches λ per unit molecular weight is a constant independent of molecular weight. Recent data reported on λ as a function of molecular weight M in commercial LDPE indicate that this assumption is not generally valid, and concern has been expressed as to the size of the errors in molecular weights calculated using this assumption. The errors associated with assuming that λ is constant were evaluated in this study by first determining the way in which λ varies with M for two typical commerical LDPE resins by fractionation and application of the GPC–IV method to representative fractions. The experimentally determined relations between λ and M were then employed in the calculation of molecular weight and molecular size averages from GPC–IV data on the original unfractonated samples. Although it was found that λ increases with molecular weight for both samples, the results indicate that the error involved in assuming that λ is a constant is no greater than the precision with which molecular weight averages can be evaluated by GPC.     

Some Aspects of the Reaction between Chitosan and Formaldehyde

Reactions of chitosan (degree of deacetylation 67%, weight-average molecular weight 850,000) with formaldehyde were performed in a dilute aqueous acetic acid solution at a molar ratio of amino groups of the polymer to HCHO of 0.06:O.1 mol/L and at different temperatures (45, 60, and 75°C). In each case the pH of the reaction mixtures was 3.4 during the experiments. The process was investigated by measuring the kinetic changes of the free formaldehyde concentration and the sum of free formaldehyde and methylol groups produced. No gelation of the reaction product was observed. The experimental results led to the conclusion that the methylol groups formed by the addition of HCHO to chitosan functions take part in some intramolecular reactions with amino and/or hydroxyl groups of chitosan. Initially, the concentration of the resultant methylene bridges grows rapidly and then drops again until an equilibrium is established. This rather unusual decomposition of the methylene links once formed without changing the reaction conditions is accompanied by a substantial growth of the amount of methylol derivative whose concentration at the beginning of the process increased rather slowly.     

Studies of the formation of thermosetting resins. XII. Computer simulation of the reactions of phenols with formaldehyde

The mechanism of the reaction of phenols with formaldehyde was studied by computer simulation. At first starting molecules were stored in a computer and the hypothetical reaction yielded conditions like the reactivity ratio of hydroxymethyl group to formaldehyde and that of orthohydrogen to parahydrogen. The molecular weight distribution of the hypothetical product in the computer was compared with that of the prepared resin determined from GPC measurement. Reaction mechanisms were discussed. We also confirmed, by computer simulation, that the rate of methylenation is larger than that of hydroxymethylation in an acid-catalyzed system and that the reactivity ratio of hydroxymethyl group to formaldehyde is 5–12 for the reaction of phenols such as o-cresol, p-cresol, and phenol with formaldehyde. The opposite results were obtained in a base-catalyzed system. It also became apparent that information regarding molecular structures, such as the number of branches, the number of phenolic nuclei in the longest chain, and the number of o,o′-,o,p′- and p,p′-methylene linkages, can be obtained by computer simulation. The most probable values of these factors for 10 mer of a phenol–formaldehyde condensation molecule are 2, 7, 2, 5, and 2.     

Universal calibration in GPC

The M[η]-elution volume calibration curve for gel-permeation chromatography (GPC) is based on the implicit assumption that the hydrodynamic volume of a solvated polymer species in the GPC columns is that which pertains at infinite dilution. This is not true of highly solvated high molecular weight fractions and results in apparent failure of this calibration in some instances. A model is presented to estimate hydrodynamic volumes of polymers at finite concentrations. The parameters required are polymer concentration, molecular weight, amorphous density, and the Mark-Houwink constants for the particular polymer–solvent combination. The calculated log (hydrodynamic volume)–elution volume relation provides a universal GPC calibration. The model accounts for the occasional shortcomings of the infinite dilution calibration and is essentially equivalent to it in noncritical cases. The use of the proposed calibration method is illustrated.     

Study of radiation-induced polymerization of vinyl monomers adsorbed on inorganic substances. IX. Preirradiation polymerization of styrene–silica gel system

Preirradiation polymerization of the styrene–silica gel system was studied in detail. Both graft polymers and homopolymers have bimodal GPC spectra. High molecular weight peaks were formed in a radical mechanism and the low molecular weight peaks were formed in a cationic mechanism as same as those in the simultaneous irradiation polymerization. The rate of formation of the low molecular weight peaks was very high compared with that of the high molecular weight peaks. Monomer conversion and percent grafting leveled off at about 1–2 Mrad. Radiation dose dependence of the four peaks were different from each other. Monomer conversion and percent grafting decreased as the preheating temperature of silica gel increased. The amount of the low molecular weight peaks of graft polymers depended on the number of silanol groups, as in the case of the simultaneous irradiation polymerization. A reaction mechanism for the preirradiation polymerization is proposed based on the results obtained.     

Polymer characterization by coupling gel-permeation chromatography and automatic viscometry

A method is proposed for use of the universal calibration curve, i.e., the product of molecular weight and intrinsic viscosity versus retention volume, in calculating the molecular weight distribution of a polymer from gel-permeation chromatography (GPC) when the Mark-Houwink relation of the polymer in the solvent used for the GPC is unknown. This is achieved by measuring the viscosity of each fraction with an automatic capillary tube viscometer. Application of this technique to poly(vinyl chloride) and poly(vinyl acetate) proved to be successful.     

Methylol‐functional benzoxazines as precursors for high‐performance thermoset polymers: Unique simultaneous addition and condensation polymerization behavior

A new class of high‐performance resins of combined molecular structure of both traditional phenolics and benzoxazines has been developed. The monomers termed as methylol‐functional benzoxazines were synthesized through Mannich condensation reaction of methylol‐functional phenols and aromatic amines, including methylenedianiline (4,4′‐diaminodiphenylmethane) and oxydianiline (4,4′‐diaminodiphenyl ether), in the presence of paraformaldehyde. For comparison, other series of benzoxazine monomers were prepared from phenol, corresponding aromatic amines, and paraformaldehyde. The as‐synthesized monomers are characterized by their high purity as judged from ¹H NMR and Fourier transform infrared spectra. Differential scanning calorimetric thermograms of the novel monomers show two exothermic peaks associated with condensation reaction of methylol groups and ring‐opening polymerization of benzoxazines. The position of methylol group relative to benzoxazine structure plays a significant role in accelerating polymerization. Viscoelastic and thermogravimetric analyses of the crosslinked polymers reveal high T_g (274–343 °C) and excellent thermal stability when compared with the traditional polybenzoxazines. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012