Radiation chemistry of aqueous poly(ethylene oxide) solutions. II. Highly purified samples

The radiation chemistry of highly purified, rigorously degassed aqueous solutions of poly(ethylene oxide) is markedly different from solutions not so treated. Gelation doses are lower in the purified samples by almost two orders of magnitude. Degradation is unimportant in dilute solutions. The very broad molecular weight distribution as well as the very high molecular weight of these polymers makes the interpretation of the results difficult. Neither gel dose measurements nor solubility measurements give valid crosslinking yields. The crosslinking yield is highly concentration-dependent.     

Isothermal crosslinking studies on polyquinoxalines by dynamic mechanical methods

Torsional braid analysis was used to investigate the crosslinking behavior of linear quinoxaline polymers with and without reactive side groups. The kinetic parameter followed was the glass transition temperature during isothermal exposure in an inert atmosphere. With high molecular weight polyamide-quinoxaline copolymers (PPAQ), an initial decrease in T_g was observed during heat exposure which was followed by a subsequent increase in T_g. This was attributed to simultaneous chain scission and crosslinking reactions. Since the effect of random chain scission on the initial change in T_g of the highest molecular weight polymer samples is much stronger than on low molecular weight analogues, a T_g minimum was observed only on the highest molecular weight polymers. Because of the complexity of the reactions occurring one must consider the activation energies obtained from the Arrhenius plots as “apparent” activation energies. No attempt was made to elucidate the mechanisms of these reactions. It has been shown that isothermal heat exposure of high-temperature aromatic polymers in an inert atmosphere leads to crosslinking. In general, however, linear polymers that have reactive side groups such as methyl or carboxyphenyl groups along the polymer chain crosslink more rapidly than the analogs without these groups.     

Polystyrene degradation induced by stresses resulting from the freezing of its solutions

Solutions of polystyrene in p-xylene were frozen in liquid nitrogen. No changes in molecular weight and distribution were caused by freezing solutions for a series of narrow distribution polystyrenes with molecular weights of near 2 × 10⁶ and lower. Likewise a commercial polystyrene of M?_w = 234,000 showed no change, even after 45 cycles of freezing and thawing. However, an ultrahigh molecular weight polystyrene (M?_w = 7.3 × 10⁶) showed appreciable degradation even after a few freezing cycles of its solutions. The changes in molecular weight and distribution were analyzed by gel-permeation chromatography. The results depended very much on the choice of solvent, cooling rate, and concentration. The extent of degradation was found to depend on polymer concentration in two distinct ways. Indeed, two different degradation mechanisms have been distinguished at low and at high concentrations. The change between mechanisms took place between 1.0 and 2.5 g/l. for polystyrene in p-xylene. This appears to provide a rare measure of polymer-polymer interactions (entanglements) in dilute solutions. Degradation in the entanglement region proceeded via a random chain-scission mechanism as tested by the Scott method. In contrast, at low concentrations degradation was characterized by the formation of appreciable amounts of low molecular weight polystyrene. The presence of an antioxidant (Ionol) during freezing did not change the extent of degradation significantly.     

Polymerization of Dioxolane by Triethyloxonium Hexafluorophosphate

Abstract

The polymerization of dioxolane by triethyloxonium hexafluorophosphate in methylene chloride has been studied with a view to determine the nature of the active center. NMR studies of solutions of the initiator with low ratios of monomer led to little reaction of the initiator over long times. Analysis of normal reaction mixtures showed that only a small amount of the initiator was consumed during the reaction. The polymer was studied by GPC, UV spectroscopy, and NMR. The polymer appears to consist largely of high molecular weight material with M_w/M_n less than 2, and also low molecular weight material perhaps formed by a different mechanism. The high molecular weight material appears not to have been formed by a simple linear trialkyloxonium ion, from end-group studies, and it is suggested the active center is a secondary oxonium ion on a large cyclic polymer.     

Polyurethane network structure from tensile tests

The mechanical behavior of polyurethane networks based on polyether (PE-PU) and polyester (PES-PU) diols is studies in light of recent molecular theories of rubber elasticity. The relationship between reduced stress (or modulus), extension ratio α, and network structure is discussed. In the range of low extensions, the deformation behavior of PESPU appears to be more affinelike than PE-PU networks. When crosslinking agents with higher functionality are used, behavior closer to the affine limit is observed. Data in the low deformation range (α → 1) allow the estimation of network molecular weights predicted by the limiting phantom and affine networks. However, an exact determination of the true chain length cannot be obtained. The uncertainty in the molecular weight is due to the uncertainty in the theoretical structural parameter A'_?, which can assume values 1–2/? ≤ A'_? ≤ 1. The range of possible network chain molecular weights can be narrowed when the real network, which always presents defects, is formed starting from building blocks of known molecular weight M_s, as in our case. In these real cases the molecular weight is between the value M_a Predicted by the affine model and the stoichiometric value M_s.     

THE REACTION OF SINGLET OXYGEN WITH PROTEINS, WITH SPECIAL REFERENCE TO CRYSTALLINS

Photosensitized oxidation of the eye lens proteins, the crystallins, is thought to lead to protein crosslinks and high molecular weight aggregates. Such protein modifications may be important factors in the formation of lens opacities or cataracts. We focus attention here on type 2 photo-oxidation involving the reaction of singlet oxygen (1O2) with crystallins and some "control" proteins. We find that: (1) trp residues are oxidized to N-formyl kynurenine and related products, but this in itself does not lead to the production of high molecular weight protein aggregates of the protein; (2) tyr residues react with 1O2 but we do not detect dihydroxyphenylalanine or bityrosine nor are protein crosslinks formed as a result; (3) oxidation of his residues appears necessary for high molecular weight protein covalent aggregates to form. Proteins devoid of his, e.g. melittin or bovine pancreatic trypsin inhibitor, do not form high molecular weight products upon reaction with 1O2. Prior reaction and blocking of his inhibits the crosslinking reactions. (4) The oxidized protein is seen to be more acidic than the parent and has an altered tertiary structure. (5) Among the crystallins, reactivity towards 1O2 varies in the order gamma greater than beta greater than alpha and also gamma A/E greater than gamma D greater than gamma B crystallin.     

Liquid crystalline,highly luminescent poly(2,5-dialkoxy-p-phenyleneethynylene)

A high-molecular-weight poly(2,5-dialkoxy-p-phenyleneethynylene) derivative has been prepared by the Heck reaction of 1,4-bis(2-ethylhexyloxy)-2,5-diiodobenzene and 1,4-diethynyl-2,5-dioctyloxybenzene. The highly luminescent polymer exhibits excellent solubility and can readily be processed into high-optical-quality films. The weight-average molecular weight M̄_w was 240000 g · mol⁻¹, with a polydispersity index of 2.9. Thermal analysis revealed a glass transition around 90°C, and an onset of chemical crosslinking at 130°C. The high M̄_w and the remarkable solubility enabled the preparation of liquid crystalline solutions of the new PPE.     

Viscometric properties of dilute polystyrene/dioctyl phthalate solutions

We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 10⁵ ≤ M_w ≤ 3.0 × 10⁶ Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (M_w ≥ 2.0 × 10⁶ Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006     

Calculation of molecular weight distributions for polymers undergoing random crosslinking and scission

A simple, practical calculation procedure has been developed for predicting the changes in molecular weight distribution of a polymer undergoing random crosslinking and/or degradation. Simulations of the random crosslinking and degradation of narrow and broad Poisson-type distributions have been made. The results agree with those calculated from Kimura's analytical solutions to Saito's general equations after a correction has been made for a mathematical error in Kimura's solution. This method can be applied to determining the probabilities of crosslinking and scission for any arbitrary molecular weight distribution expressed in tabular form. The importance of using narrow distribution samples to estimate crosslinking from changes in molecular weight distribution is graphically demonstrated.     

The Fractionation and Properties of Gluten Proteins

Abstract

Gluten protein may be separated into fractions of differing molecular weight distributions by a successive extraction procedure using urea solutions and dilute sodium hydroxide solution. Fractions containing mainly low molecular weight protein decrease dough strength and mixing stability but increase dough plasticity. Fractions rich in high molecular weight protein have the opposite effect. Doughs or glutens behave as uncross-linked systems in which entanglement coupling appears to play an important role.     

Influence of crosslinking conditions on the phase behavior of a polyacrylate-based liquid-crystalline elastomer

The influence of crosslinking conditions (γ-radiation-induced and chemical crosslinking above and below the clearing point) on the phase behavior of a liquid-crystalline polyacrylate-based elastomer is shown and discussed. The network formation in samples γ-irradiated in the nematic phase results in an increase of the clearing point T_n-i which does not depend on the crosslinking density expressed as the ratio of weight-average molecular weight over number-average molecular weight between crosslinks M _w/M _c within a wide M _w/M _c range. The distorting action of the crosslinking agent at the same temperature leads to the opposite tendency in the change of T_n-i. Crosslinking in the isotropic phase shows a strong decrease in T_n-i.     

Unusual behaviour of crosslinked and linear forms of a zwitterionic polymer in aqueous alkali

Novel findings are reported on hydrogels and aqueous solutions of the zwitterionic polymer, poly[1‐(3‐sulfopropyl)‐2‐vinylpyridinium‐betaine]. In aqueous media of high pH: (a) the chemically crosslinked polymer develops a reddish‐brown coloration followed by physical disintegration and dissolution, and (b) the corresponding linear polymer exhibits the same coloration and undergoes a reduction in molecular weight, solution viscosity and T_g. These finding lie in sharp contrast to normal behaviour exhibited in neutral and acidic aqueous media. Although an unequivocal definitive mechanism is not known, a possible explanation consistent with the experimental observations is suggested.     

Free-radical polymerization of a reducing vinyl sugar ester in dimethylformamide and water

To develop a new synthetic polymer containing sugar branches, radical polymerization of the reducing vinyl sugar ester 6-O-vinyladipoyl-D -glucose ( 1 ) was performed in an organic solvent or in water. The polymers obtained with several azoinitiators in dimethylformamide (DMF) showed comparatively low average molecular weight (M̄_n ≈ 4500). In contrast, the use of a redox initiator (FeSO₄ and H₂O₂) in water gave polymers of higher average molecular weight (M̄_n ≈ 33000) in higher yield (90%), followed by crosslinking at high conversions.     

Polymer coupling and theory of on-random crosslinking: an analytical solution

Gel formation is an important feature in free-radical polymer coupling. Due to the different possible combination reactivities of each polymer backbone radical, polymer chains are crosslinked in a non-random manner. Equations of the moments have been derived to predict the pregel molecular weight development and the crosslink density at gel point. This work provides an analytical solution for the differential equations. The model agrees with the Flory-Stockmayer gelation theory under the condition of random crosslinking. The magnitude of deviations from the classical theory for non-random crosslinking depends on the product of the radical termination reactivity ratios (r₁r₂), the ratio of the rate constants of backbone radical generation (k), the ratio of the weight-average chain lengths of primary polymers (y), and the polymer weight fractions (w₂).     

Rheological aspect on electrospinning of polyamide 6 solutions

Polyamide 6 (PA6) solutions in formic acid (FA) and deionized water cosolvent may behave as polyelectrolyte or neutral solutions depending on the cosolvent composition. In this study, both polyelectrolyte and neutral PA6 solutions were prepared for electrospinning, and their spinnability was correlated with their rheological properties. In addition, the effects of PA6 average molecular weight and carbon nanocapsule (CNC) nanoparticle addition on solution rheology and electrospinnability were investigated. Microstructure and thermal properties of the as-spun fibers were identified by wide-angle X-ray diffraction, polarized Fourier infrared spectroscopy, and differential scanning calorimetry (DSC). Due to the chain expansion, polyelectrolyte solutions with 99 vol.% FA solvent possess much lower entanglement concentration (?_e, ∼1 wt.%) than neutral solutions (∼7 wt.%) prepared by 90 and 85 vol.% FA solvent. Compared with the neutral solution, the polyelectrolyte solution is more advantageous because a lower concentration is sufficient to obtain bead-free PA6 fibers. However, at a concentrated regime of 15 wt.% solution, the obtained fibers exhibit a larger diameter due to the higher entanglement density. For the crystalline structure, the content and orientation of α-form crystals are higher in the PA6 fibers obtained from the polyelectrolyte than from the neutral solution. When PA6 with a lower molecular weight is used, a higher concentration is required to develop the entangled chains to produce bead-free fibers. Homogeneous PA6 solutions filled with CNCs exhibit more elastic behavior than unfilled solutions due to the presence of the CNC–CNC network, aside from the entangled network of PA6 chains. Electrospinning of the CNC-filled solutions yields PA6 fibers with CNC aggregates protruding from the fiber surface. The inclusion of CNC in the PA6/FA solution produces fibers possessing enhanced α-form crystals with reduced orientation. In all cases, DSC heating traces of the as-spun fibers identify a high melting temperature (HMT) phase of PA6. The amount of HMT phase decreases, provided that more water or CNCs are added into the PA6/FA solution for electrospinning.     

Crosslinking Index,Molecular Weight Distribution and Rubber Equilibrium Shear Modulus During Polyfunctional Crosslinking of Existing Polymer, 6. Primary Polymer with a Schulz–Zimm Distribution of Molecular Weights

A network model for the crosslinking of already existing polymer molecules with a so‐called Schulz–Zimm distribution of their molecular weights is presented. It is an extension of previously developed statistical network models applied to the crosslinking of primary polymers with several other molecular weight distributions and with crosslinks of any functionality. The model results in the possibility to obtain more insight into the structure of polymers, especially those with narrow distributions of the molecular weight. In more detail, the model can give a perspective on structural network parameters such as the weight fractions of ideal network, of dangling polymer ends, and of those molecules not connected to the network, i. e., the sol fraction, the number of crosslinks in which a polymer molecule is bound, the functionality of the crosslinks, or the average molar mass of the polymer molecules in between the crosslinks M̄_c. Results of calculations are shown for a hypothetical crosslinking process of polymers with various molecular weight distributions. Moreover, the dependency of the network parameters on the polydispersity index and the type of molecular weight distribution is shown. Finally the increase of the functionality of the crosslinks during the ageing process of a 9.9% poly(vinyl chloride) gel as a function of the polydispersity index of the molecular weight distribution is presented.     

Modified graessley theory for non-newtonian viscosities of polydimethylsiloxanes and their solutions

A deviation from Graessley's theory of entanglement viscosity appears at very high shear rates when the flow of polydimethylsiloxanes of various molecular weights and their solutions with various concentrations is measured by the capillary method. In order to explain this deviation, a modified Graessley theory is proposed according to the previously reported suggestion that frictional viscosity appears not to be negligible at high shear rates. A reducing procedure taking a frictional viscosity parameter into account was performed. All of the reduced data are combined to give a master curve in spite of a wide range of molecular weight, concentration, and shear rate (from the lower Newtonian to very highest non-Newtonian flow region). The findings from the reducing procedure completely explain the mechanism of non-Newtonian flow for the bulk polymers with various molecular weights, including those below the critical molecular weight for entanglement, and for polymer solutions at any concentration. The viscosity of the linear polymer system consists of the shear-dependent entanglement term η_ent proposed by Graessley and the shear-independent frictional term η_fric. The non-Newtonian behavior depends on the ratio of η_ent/η_fric at the shear rate of measurement. The ratio of zero-shear entanglement viscosity η_ent,0 to η_fric and the critical shear rate for onset of the non-Newtonian flow may be used as a measure of the non-Newtonian behavior of the system and a measure of capability for its rising, respectively. The Graessley theory is to be included in the present modified theory and is applicable to the case of η_entη_fric ? 1.     

Thermomechanical behavior of a polyphenylimide-quinoxaline

Linear polyphenylimide-quinoxalines (PPIQ) can be crosslinked by isothermal heat treatment in an inert atmosphere. To show this, three polyphenylimide-quinoxalines were prepared which differed only in molecular weight and polymer chain endings. Apparent activation energies of thermal crosslinking were then obtained from the rates of change of T_g as a function of time and temperature. The values (60 kcal/mole) were essentially the same as those for the thermal degradation of the same polymer in vacuum. Differences in polymer molecular weight had a distinct effect on the rates of change of T_g but the polymer chain ends seemed to have a lesser effect than previously observed on polyphenylquinoxalines (PPQ). Nevertheless, the rate of change in T_g is greater for PPIQ than for PPQ of a similar molecular weight. This indicates that the imide portion of the polymer chain leads to faster crosslinking under isothermal conditions.     

Postgel properties in the statistical crosslinking of heterochains. I. Systems with N types of chains

The heterochain crosslinking model describes nonrandom crosslinking of polymer chains and is an extension of the classical Flory/Stockmayer gelation theory. We consider the postgelation relationship for the system consisting of N types of polymer chains, in which the probability that a crosslink point on an i‐type chain is connected to a j‐type chain is explicitly given by p_ij. The analytical solutions for the weight fraction of the sol, the number‐average and weight‐average molecular weights within the sol fraction, and the crosslinking density within the sol and gel fractions are derived for the systems, with each type of chain conforming to the Schulz–Zimm distribution. Illustrative calculations are shown for the systems consisting of two and three types of chains, and the obtained results agree with those from the Monte Carlo method. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2333–2341, 2000