Structural origin of the cryogenic relaxations in poly(ethylene terephthalate)

The effect of molecular organization (crystallinity, orientation) on the internal friction of poly(ethylene terephthalate) was studied by means of dynamic mechanical measurements at temperatures from 300 to 4.2°K, with a free-oscillating torsion pendulum at 1 Hz. It was found that crystallinity decreases the intensity of the composite γ relaxation at 210°K and gives rise to an additional loss maximum ε at 26°K. Uniaxial orientation broadens the γ relaxation and gives rise to an additional loss peak δ, at 46°K. The δ and ε losses are dependent on molecular organization, occurring only in samples containing aligned, taut chain segments and crystalline structures, respectively. They have a common activation energy of 4 kcal/mole. All three low-temperature relaxations in oriented specimens show pronounced directional anisotropy, which, in the γ loss, may be due to the preferred orientation of noncrystalline chain segments, while in the δ and ε losses, may be associated with the direction of defect structures. On the basis of the observed behavior of the δ and ε relaxations it is suggested that they may involve motions of defect structures and may thus participate in stress-transfer mechanisms at large deformations.     

Molecular motion and spin relaxation in polydiethylsiloxane

Quantitative comparison of previously published NMR spin-relaxation data for polydiethylsiloxane with theoretical predictions for a variety of motional processes allowed both the nature and time scale of molecular motions to be identified. At the lowest temperatures, methyl reorientation produced a T₁ minimum and was found to proceed with an activation energy of 2.4 kcal/mole in both amorphous and crystalline phases. Reorientation of the ethyl groups in the amorphous phase was observed at a higher temperature with an activation energy of 9.3 kcal/mole. Relaxation in the melting region was influenced by flexing and stretching of the helical polymer chain. The maximum angular displacement of the chain was estimated to be 24°, with an activation energy for this process of 2.6 kcal/mole.     

Kinetics and mechanism of the transformation in antimony trioxide from orthorhombic valentinite to cubic senarmontite

The kinetics of the polymorphic transformation in antimony trioxide from metastable orthorhombic valentinite to cubic senarmontite has been studied in polycrystalline material between 490 and 530°C. Quantitative analysis of the mixtures was done using infrared spectrophotometry. The kinetic data was analyzed and the activation energy for the process was obtained: (i) On the basis of Avrami's equation, which is derived on the basis of a nucleation and growth mechanism; and (ii) from the time required for a constant fraction of the transformation to take place. The values obtained were 50.8 and 46.0 kcal/mole. Observations have also been made on partly transformed single crystals of valentinite using a polarizing microscope. The latter studies and the value of the activation energy suggest that a better understanding of the transformation could be obtained on the basis of a vapor phase mechanism.     

Radiation copolymerization of vinyl acetate with the diethyl esters of maleic and fumaric acids

The radiation-induced copolymerization of vinyl acetate with diethyl maleate and with diethyl fumarate was investigated in the temperature range from ?40 to 90°C over a wide range of comonomer compositions. Both the rates of copolymerization and the molecular weights of the resulting copolymers were found to depend strongly on the initial comonomer compositions. The apparent activation energy was found to change at 13°C with an increase in temperature from a value of 1.76 kcal/mole to a value of 4.31 kcal/mole in the copolymerization with diethyl maleate, while in the case of the copolymerization with diethyl fumarate the apparent activation energy changed at 21°C from a value of 1.76 kcal/mole to a value of 5.98 kcal/mole. Scavenger studies indicate that a free-radical mechanism prevails over the entire temperature range investigated in the case of both copolymerizations.     

The Electrocyclic Cyclopropyl-Allyl Rearrangement. Preliminary communication

MINDO-2 SCF calculations indicate that ring-opening of cyclopropyl radical (I) to allyl radical (II) is more favourable via a disrotatory reaction path, the calculated activation energy being ～30 kcal/mole. (For conrotatory opening the activation energy was found to be ～44 kcal/mole.) The two critical motions of the nuclei during the transformation are found to be strongly decoupled, i.e. rupture of the CH₂βCH₂ bond precedes rotation of the CH₂ groups. As predicted by qualitative theories both ring-opening modes are unfavourable since they involve a change in electronic ground-state symmetry between I and II. The preferred ring-opening mode is discussed qualitatively in terms of Evans' principle.     

Proton NMR study of molecular motion in polydimethylsiloxane

Proton spin–lattice relaxation times T₁ were measured for two samples of polydimethylsiloxane (PDMS), one with weight-average molecular weight M_w = 77,400 and the other with M_w = 609,000. Two T₁ minima and a T₁ discontinuity were observed for each compound. The high-temperature T₁ minima were attributed to a stretching and flexing motion of the PDMS chain. Quantitative comparison of the relaxation data with a theoretical model developed for this motion allowed the activation energy, 2.3 kcal/mole, and the maximum angular displacement of the methyl group symmetry axis to be determined. The latter was found to be 31°, independent of sample molecular weight. The low-temperature minima were ascribed to methyl reorientation with an activation energy of 1.6 kcal/mole. The T₁ discontinuities were attributed to melting and allowed the degree of crystallinity to be estimated.     

A differential scanning calorimeter study of the crystal forms and polymerization kinetics of 2,4-hexadiynyl-1,6-bis(p-toluenesulfonate)

The thermal properties of 2,4-hexadiynyl-1,6-bis(p-toluenesulfonate) have been explored by program temperature and isothermal differential calorimetry. The heat of fusion for the rapidly heated pure solid was 8254 cal/mole (34,540 J/mole) at 367.1°K (93.8°C). This amounts to an entropy change of 22.5 cal/mole °K (94.1 J/mole °K). The energy of activation for the thermal polymerizations was 18.97 kcal/mole (79.37 kJ/mole). The thermal polymerization appears to follow a solid–solid phase transition which proceeds by random homogeneous nucleation throughout the process. The kinetics were simple first order over 70% of the reaction. Programmed temperature studies indicate that during the first 10% of the polymerization a new high temperature (mp 375.4°K) solid phase is formed which acts as the monomer form during the bulk of the reaction.     

Effect of ammonia on the thermal decomposition of orthorhombic and cubic ammonium perchlorate

The inhibiting effect of ammonia vapours on the kinetics of the thermal decomposition of ammonium perchlorate(AP) in the temperature range 215–270°C has been investigated. An initial ammonia pressure of about 200 Torr is necessary for the practically full suppression of the decomposition of the orthorhombic crystals at temperatures close to the point of AP polymorphic transformation (240°C). With the cubic crystals, 0.5 Torr is the corresponding pressure required. In the case of complete inhibition of the decomposition in the presence of ammonia, AP crystals become yellowish. The activation energy of decomposition of the orthorhombic modification is 29 ± 0.6 kcal mole^?1 in the absence of ammonia, and 38 ± 1.1 kcal mole^?1 under ammonia vapour pressure of 6.5 Torr. A kinetic analysis of the traditional proton model of AP decomposition has been made showing that the increase of the activation energy in the presence of ammonia may be derived from this model.     

Self-consistent-field potential energy surface in three dimensions for the Cl + H2 → ClH + H chemical reaction

Non-empirical self-consistent-field calculations have been carried out for 38 points on the potential surface for the Cl + H₂ → ClH + H chemical reaction. A basis set of seven s, five p, and one d functions on chlorine and three s and one p on each hydrogen atom was used. The least energy path occurs for the linear Cl---H---H arrangement. A much higher barrier is found for the approach of Cl along the H---H perpendicular bisector. The linear barrier height is predicted to be 26.2 kcal/mole and the saddle point occurs for R(Cl---H) ≈ 1.46 Å, R(H---H) ≈ 0.94 Å. The experimental activation energy is 5.5 kcal/mole. It seems likely that a general feature of the Hartree-Fock approximation is an overestimation of barrier heights. The exothermicity is calculated to be −6.7 kcal/mole, compared to the near Hartree-Fock result −2.3 kcal/mole and experiment −3.0 kcal/mole.     

Trimethylenemethane: Activation energy for ring-closure of the diradical

The energy of activation for ring-closure of ground state triplet trimethylenemethane (I) to methylenecyclopropane has been measured by following the rate of dissappearance of the electron spin resonance spectrum over the temperature range –155° to –140°, in a series of frozen solid matrices. The experiments described make use of 3-methylenecyclobutanone and methylenecyclopropane as precursors to trimethylene-methane.Kinetic results obtained starting from methylenecyclopropane were most satisfactory and lead to an energy of activation for ring-closure of 7 kcal/mole. This value is significantly smaller than the aprpox. 20 kcal/mole barrier estimated on the basis of theoretical models. Truncation of the barrier by a tunnelling mechanism is made unlikely by the finding that trimethylenemethane-d₆(I-d₆) undergoes ring-closure with the same 7 kcal/mole energy of activation as the parent I.     

Kinetic study of dissolution of poly(vinyl chloride) in cyclohexanone

The kinetics of dissolution of five fractions of commercial poly(vinyl chloride) in cyclohexanone was studied at temperatures from 20 to 70°C. Good agreement was observed between the experimental results and equations expressing the dependence of the induction periods and the rates of dissolution on temperature and molecular weight. It was found that the apparent activation energy for the swelling process lies in the range 9–14 kcal/mole and the apparent activation energy for the dissolution diffusion process in the range 8–12 kcal/mole. The apparent dependence of activation energies on number-average molecular weight indicates that the chain ends are more important in determining the dissolution rate than the centers of the polymer chains.     

Dielectric relaxation in styrene-related polymers at low temperatures

The complex dielectric constants ?^* = ?′ ? j?″ of each of several members of a system of copolymers of 4-chlorostyrene and 4-methylstyrene have been measured from 1.6°K to 300°K and from 0.1 kHz to 20 kHz. The principal experimental findings are: the strength of the relaxation process which occurs near 50°K at 1 kHz varies linearly with changing copolymer composition; both the apparent activation energy (H = 2.7 ± 0.7 kcal/mole) and the shape of the relaxation curve are independent of the composition variable and of the temperature (or frequency) within the ranges studied; and the ratio of the relaxation strength of poly-4-methylstyrene to that of poly-4-chlorostyrene in the 50°K process is about 25 times the corresponding ratio for the primary relaxation process that occurs in the neighborhood of the glass-transition temperature. These findings suggest that in the 50°K process the phenyl groups relax independently of one another; that the apparent activation energy and the shape of the relaxation spectrum are determined primarily by the nature of the intrachain forces; and that the strength of the relaxation process depends primarily on effects of intermolecular forces that are governed by the molar “free volume” of the copolymers.     

Secondary processes of poly(methyl methacrylate) and their activation energies as determined by shear and tensile creep compliance measurements

The results of measurement of the shear and tensile creep compliance of poly(methyl methacrylate) between ?150 and 75°C. are presented. The master curves show the creep behavior from essentially elastic response to the onset of the α-process. The logarithmic retardation spectra and shear loss compliance reveal two molecular processes, one process being partially obscured by the α-process and therefore not as well defined as the other. These processes manifest themselves as breaks in an Arrhenius plot of the shift factors at about ?35 and +25°C. The activation energies in the ?35 to 25°C. and 25 to 80°C. ranges are 17.8 kcal./mole and 42.2 kcal./mole, respectively. The former temperature range and activation energy corresponds to the well-known ß-process in poly(methyl methacrylate), the latter to a process which is apparently detectable using various long-time experimental techniques but whose molecular interpretation is at present obscure. The activation energy in the ?150 to ?35°C. range is about 8.7 kcal./mole.     

Radiation-induced ionic polymerization of isobutyl vinyl ether in bulk

The radiation-induced ionic polymerization of isobutyl vinyl ether was investigated under conditions where the monomer was dried with molecular sieves. The investigation covered the temperature range from ?16°C to 90°C, and the dose-rate range from 10¹⁵ to 10²⁰ eV/g-sec, using both γ-rays and electrons. A very high overall activation energy of 15.9 kcal/mole was found for the process below 30°C. Above 30°C, however, the value of the overall activation energy dropped to 4.9 kcal/mole, a phenomenon which is ascribed to the solvation of the propagating carbonium ion below 30°C. The dose-rate dependence of the rate of polymerization was found to be 0.58 over the entire dose-rate range investigated. The molecular weight of the polymer was found to be far less sensitive to trace amounts of water than the rate of polymerization. The molecular weight of the polymer depended strongly on the irradiation temperature, reaching a maximum value of about 120,000 at 35°C. It is shown that at temperatures above 20°C regenerative chain transfer processes play an important role in determining the molecular weight of the polymer.     

Internal motions in an alternating copolymer of ethylene and tetrafluoroethylene

Internal motions in an alternating copolymer of ethylene and tetrafluoroethylene were investigated by dynamic mechanical and dielectric measurements and by nuclear magnetic resonance. At 1 Hz the α, β, and γ relaxations were observed at 110, ?25, and ?120°C in a quenched sample. The activation energy was 76 kcal/mole for the α relaxation and 10.6 kcal/mole for the γ relaxation. These relaxations are attributed to the motion of long and short segments in the amorphous regions, respectively. The β relaxation, which was observed only in the dynamic mechanical experiments, appears to occur in the crystalline regions. The copolymer is isomeric with poly(vinylidene fluoride), but it has a higher melting point and a much lower dielectric loss.     

Photolysis of poly(tert-butyl acrylate) in the region of the glass transition temperature

The thin film (1 × 10^?5 cm.) photolysis (2537 A.) of poly(tert-butyl acrylate) under 1 atm. helium pressure has been investigated in detail. Isobutene was the only significant volatile product in the temperature range 20–110°C. The reaction was demonstrated to be initially a first-order decomposition with an energy of activation of 3.3 kcal./mole in the glassy state. Above the glass transition temperature a value of 1.8 kcal./mole was found. The rate of isobutene formation is autoaccelerated when a minimum of one acrylic acid unit is generated per chain. The initial quantum yield for the formation of isobutene varies from 0.083 to 0.17 over the temperature range studied. There was no dependence of quantum yield on the exciting wavelength using sources of 1849, 2537, and 3660 A. The intensity exponent was found to be unity, consistent with first-order decomposition kinetics.     

Mechanical and dielectric absorption of poly(vinyl chloride) and some related polymers

The mechanical and dielectric low temperature absorptions of poly(vinyl chloride) (PVC) and several modified PVC's have been studied over the temperature range from ?60 to +60°C. with some tests extending to ?150°C. and others to +170°C. The results indicate that the low-temperature absorption near ?50°C (β₂ absorption) decreases in intensity with chlorination, while the absorption at a higher temperature near 0°C (β₁ absorption) decreases in intensity with hydrogenation. The apparent activation energies of the β₁ and β₂ absorptions were calculated to be 16 kcal/mole and 10.7 kcal/mole, respectively. Besides, the β₂ absorption markedly decreases in intensity with addition of plasticizer, while the intensity of β₁ absorption is not much affected by increasing plasticizer content. From these results, the β₁ and β₂ processes are concluded to be the results of molecular motion in crystalline and amorphous region in PVC, respectively. For samples of reduced Cl content, another low-temperature absorption was located near ?120°C (γ absorption) and attributed to the presence of short sequences of ethylene units. It has also been observed that the temperature location of the high temperature absorption near 100°C (α absorption) shifts linearly to higher temperature with increasing chlorine content and to lower temperature with increasing hydrogen content.     

Ab initio calculations of the rotational potential functions for propanol and ethyl methyl ether

Calculations at the STO-3G and 4–31G levels have been carried out on propanol and ethyl methyl ether, with geometries obtained from molecular mechanics calcualations. Full relaxation was allowed in all degrees of freedom except for the torsion about the central bond, which was varied at 30° increments. A butane-type potential was found, the maximum and minimum values of energy are from 0° to 180° 5.55, 0.00, 4.02, and 0.00 kcal/mole for propanol, and 8.35, 2.74, 3.31, and 0.00 kcal/mole for ethyl methyl ether (4–31G).     

NMR and ESR studies of the solid-state polymerization of vinyl monomers. II. Acrylic acid

The post-polymerization of acrylic acid, γ-irradiated at 77°K, has been studied by broad-line NMR and ESR between 240°K and 286°K (mp). The changes in the structure of NMR spectra during the polymerization has been related to the conversion yield checked by gravimetry of the polymer formed. The very fast reaction occurring above ?270°K has been followed simultaneously by NMR and thermal analysis. This last method indicates an activation energy of chain propagation of 18.6 ± 2 kcal/mole in satisfactory agreement with the value given by the initial slope of the conversion—time curves. ESR experiments show that, while the average radical concentration decays by a factor of about ten between 77°K and 240°K, the local concentration of radicals persisting after prolonged annealing at T > 240°K, remains the same as at 77°K, i.e., 1.4 × 10¹⁹ spins/g. A kinetic scheme, assuming an exponential decay of propagation and termination rate constants with chain length, has been proposed to explain the shape of conversion—time curves as well as the almost constancy of the local concentration of growing polymer chains.     

The pyrolysis of trimethylene sulfide. A unimolecular decomposition

The kinetics of decomposition of trimethylene sulfide to ethylene and thioformaldehyde was investigated in a single-pulse shock tube using the «relative rate» technique. The extent of reaction was measured in the reflected shock regime from 860° to 1170°K, but experimental difficulties limited the useful data to the temperature range of 980°–1040°K. The first-order rate constant was found to be k = 10^13.0 exp (?48,200/RT) sec^?1. This result sets an upper limit of 50 kcal/mole for the standard enthalpy of formation of CH₂S, with 35 kcal/mole as a more likely value. The isomerization of cyclopropane to propene was used for the reference reaction; in turn, this was checked, in a relative rate experiment, against the pyrolysis of cyclohexene.