Diffusion-controlled transitions in high polymers

Ohne Zusammenfassung The online version of the original article can be found at     

Features of diffusion-controlled bimolecular reaction of fluorescence quenching in sol-gel-xerogel transitions

Reaction kinetics have been investigated in the diffusion-controlled quenching of pyrene fluorescence by N,N-dimethylaniline in a medium that gradually changes from homogeneous to heterogeneous as a result of solution-sol-gel-xerogel transitions. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 2, pp. 122–125, March–April, 1998.     

Glass transitions in ionic polymers

The glass transitions of an ionic polymer in bulk have been studied as a function of the counterion for various homo- and copolymers. It is shown that the glass transition temperature can vary by as much as 530°C., from ?10°C. for the nonionic material to +520 for Ca²⁺ or Zn²⁺ substituted polymer. From simple electrostatic considerations, it is shown that a linear correlation should exist beween the glass transition and the ratio of the cation charge, q, to the internuclear distance, a, between cation and chain anion; for this particular material (the polyphosphate chain) the relation is T_g = 625(q/a) ?12 where q is in units of one electron and a is in A. If the data are interpreted in the light of the Gibbs-DiMarzio theory, it is seen that both the chain stiffness and the intermolecular energy increase in an approximately parallel manner as q/a increases.     

Photon-echo experiments on singlet-triplet transitions in organic mixed crystals

The homogeneous optical linewidth of a spin-forbidden S_o → T₁ transition in a chemically mixed organic crystal was directly measured in a two-pulse photon-echo experiment. The experimental values of T₂^Opt for the system dibromobenzophenone in dibromodiphenyl ether ranged from 1.4(3) to 3.8(2) μs, depending on the nuclear spins of the host. A temperature-dependent contribution to (T₂^Opt)^?1 revealed an activation energy ΔE of 13.8(5) cm^?1, which is tentatively attributed to a librational mode frequency.     

Mechanism of the diffusion-controlled stage of bimolecular reactions in crystalline and vitreous polymers

Polymerization processes in solid and highly viscous media are considered. It is supposed that many processes in solids are ratelimited by rotational diffusion related to conformational transitions of structure elements containing reaction centers. This supposition agrees well with data on radical combination kinetics and the overall kinetics of polymerization in solids.     

Cooperative relaxations/transitions in ferroelectric polymers

Dielectric relaxation and thermal transitions in β-PVDF have been investigated by Thermo-Stimulated Current spectroscopy and Differential Scanning Calorimetry respectively. A comparative study of spectra and thermograms has been performed. The relaxation mode associated with the glass transition of the true amorphous phase is characterized by relaxation times obeying a compensation law due to cooperative molecular movements. A conformationally disordered structure is proposed for β-PVDF to explain thermal events occurring around 60°C. Ageing of ferroelectric properties of β-PVDF has been associated with cooperative molecular movements liberated largely below the melting point.     

Carbon-13 NMR relaxation and transitions in polymers

High-resolution proton-decoupled carbon-13 nuclear magnetic resonance relaxation parameters have been obtained as a function of temperature for a set of completely amorphous polymers, semicrystalline polymers, and a series of ethylene–vinyl acetate copolymers. With these samples the nature of the glass temperature, other postulated amorphous transitions, and the β transition were investigated. For the completely amorphous polymers, the average correlation times depend on temperature according to the Williams–Landel–Ferry relation. Spectral collapse occurs at temperatures whose ratio to T_g is in the range 1.2–1.4 and corresponds to a correlation time of about 10^?7s. The loss of resolvable spectra is demonstrated to be a consequence of experimental methods and is not due to the occurrence of another amorphous transition. Both the methylene and methine carbons can be resolved for the ethylenevinyl acetate copolymers. Although the correlation time for the methylene carbon is continuous and resolvable through the β transition region, the methine branch-point resonance is lost. The implication of these results to the molecular nature of the β transition is discussed.     

Effect of phase transitions on the rheology of polymers

Polymeric materials near phase transition conditions exhibit some common features in their linear viscoelastic relaxation patterns. This has been shown by comparing several recent experimental studies of our laboratory. A universal pattern seems to evolve in the case of liquid-to-solid transitions such as chemical gelation, crystallization of semi-crystalline polymers, and nematic to smectic transition in thermotropic liquid crystalline polymers. They all exhibit the self-similar CW-spectrum at low frequencies (for ω→0) or long times. Phase separating di-block copolymers and polymer blends also show this same type of relaxation time spectrum, however, it most likely does not extend to ω→0 and it does not seem to be restricted to the close vicinity of the transition.     

Conformational transitions in ultrahigh-molecular-mass polymers and their manifestation in chromatography on monolithic columns

For a monolithic capillary column based on polydivinylbenzene, a change in the elution profile of polystyrene standards with variation in the eluent-flow rate is studied. It is shown that, for polymers with molecular masses up to 3 × 10⁶, the elution profile does not depend on the flow rate. For higher molecular mass polymers at low flow rates, there is a single almost Gaussian peak that splits into two peaks that move to different sides from the initial peak with an increase in the flow rate. A peak with a smaller retention time (peak I) rapidly attains the limiting elution time, and later on, its retention is independent of the eluent-flow rate. In contrast, the retention time of the other peak (peak II) continuously increases with an increase in the flow rate of the mobile phase, so that, at high flow rates, this peak is retained for a longer time than the low-molecular-mass marker. The intensity of peak I decreases, while the intensity of peak II increases with an increase in the eluent-flow rate; the ratio of their intensities tends toward a certain limiting value (≤1). The observed profile of elution of ultrahigh-molecular-mass polymers assumes the presence of a dynamic equilibrium similar to that existing in the case of first-order reversible reactions. For the assumed equilibrium, the rate constants of direct and back reactions are determined. It is found that these constants are close to the inverse maximum relaxation time of a polymer molecule. The character of the transformations of the polymer molecule in the chromatographic column is discussed.     

Relaxation spectroscopy on amorphous high polymers

Summary Measurements of the dynamic Youngs modulus and shear modulus of Polymethylmethacrylate (PMMA) in the frequency range from 10^–3 cps to 10^–1 cps and in the temperature range from –20°C to 100°C up to stress amplitudes of 4 kN/cm² are presented. The measured nonlinear viscoelastic behavior is discussed with regard to nonlinear elasticity and nonlinear effects caused by the shift of relaxation times due to the reaction rate theory.

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Zusammenfassung Es wird über Messungen des dynamischen Elastizitätsmoduls und Schubmodells von Polymethacrylsäuremethylester (PMMA) im Frequenzbereich von 10^–3 Hz bis 10^–1 Hz und im Temperaturbereich von –20°C bis +100°C bei Spannungsamplituden bis zu 4 kN/cm² berichtet. Das gemessene nichtlineare viskoelastische Verhalten wird hinsichtlich einer nichtlinearen Elastizität und hinsichtlich nichtlinearer Effekte, die durch eine Verschiebung der Relaxationszeiten nach der Platzwechseltheorie verursacht sind, diskutiert.

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With 21 figures     

Pressure-driven phase transitions in NaBH4: theory and experiments

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.     

Further experiments on pyrolysis of 2,2-dimethylpropane behind shock waves

Pyrolysis of a dilute mixture of neopentane (2,2-dimethylpropane) has been studied behind incident shock waves at an average pressure of 0.35 atm; the reaction was followed by absorption spectroscopy for H atoms. In the temperature range 1230–1455 K, the rate constant for dissociation of neopentane to t-butyl and methyl radicals is 1.1 E 13 exp(?62 kcal/RT) s^?1. These data and some of the literature results, between 1000 and 1450 K, can be fitted by an RRKM model of the hindered Gorin type, with five active internal rotors in the complex. To match our data with other literature data down to 800 K, a vibrational model was more satisfactory, but this did not fit very low pressure pyrolysis data in the 1000–1100 K range. Apparently, the VLPP data are too high because of heterogeneous processes or chain reactions.     

Microscopically-viewed structural changes in solvent-induced phase transitions of synthetic polymers

Structural changes occurring in the solvent-induced phase transition have been investigated by carrying out the time-resolved measurements of X-ray diffraction, infrared spectra and Raman spectra. First example is about the solvent-induced crystallization of syndiotactic polystyrene glass. By comparing the time evolution of the various infrared and Raman bands and the X-ray reflections, the process of nucleation, growth, and aggregation of regular helical sequences in the crystalline lattice could be traced concretely. It was also found experimentally that the amorphous chain segments started an active motion immediately after absorbing solvent molecules and became a trigger to induce the local regularization of random coils into regular helical segments. The second example is the reversible solid-state phase transition of poly(ethylene imine) between the anhydrate of doubly-stranded helices and the hydrates of planar zigzag chains. By carrying out the time-resolved infrared spectral measurements in water vapor atmosphere (H₂O or D₂O), the characteristic bands could be identified for these crystalline phases and the structural transformation in the hydration process could be clarified in detail.     

Hydroformylation of high polymers

A new class of polymeric materials has been prepared and subjected to preliminary evaluation. These materials were obtained by hydroformylation of diene-based polymers. The olefinic bonds were saturated and aldehyde groups attached. The extent of reaction may be varied at will up to 100% double bond conversion. The properties of these polymers are dominated by the active aldehyde groups. One of the chief characteristics of simple aldehydes is their ability to polymerize forming mainly aldehyde trimers and tetramers. This same phenomenon has been shown to occur with polymeric aldehydes with consequent gel formation. The gelling tendency has been closely related to the aldehyde content per unit volume which may be conveniently varied by (1) the concentration of the initial polymer, (2) the degree of unsaturation in the polymer, and (3) the extent of double bond conversion. Any combination which gives a free aldehyde concentration in excess of 1 × 10^?4 mole/ml. is very prone to gel. The loose gels can be readily reversed by conditions which depolymerize aldehydes. The propensity of the aldehyde polymers to crosslink upon evaporation of the solvent was used to form films. These films were not discolored by ultraviolet light but under such treatment became quite brittle. The products may be stabilized by conversion of the aldehyde groups to more stable derivative, in particular the acetal. The extent of acetal formation has been shown to be controlled by steric factors. Complete polyacetal formation was possible only when the aldehyde groups were well dispersed along the polymer chain. Certain aldehyde derivatives (acetals, oximes) are thermoplastic. They soften at lower temperature than commercial plastics.     

Further polymers derived from bisazlactones and tetraamino compounds

New polymers containing benzimidazole in the polymeric backbone were obtained from the reaction of bis(oxazolones) with known and with a novel aromatic tetraamine. The polymers have inherent viscosities in the range of 0.1-0.4 dL/g (in DMF 30°C).