Epoxy resins. I. The stability of the epoxy–trimethoxyboroxine system

The useful life of a material depends on its environmental exposure. The diglycidyl ether of bisphenol A (DGEBA) cured with trimethoxyboroxine (TMB) was evaluated under various aging conditions. For isothermal aging, the main factor controlling weight loss appeared to be related to the diffusion of the degradation products (E_act = 22.1 kcal/mole). Chemical decomposition kinetic parameters were obtained using vacuum thermogravimetric analysis (TGA) on powder samples. The thermal decomposition activation energy and the reaction order of cured DGEBA were 37.5 kcal/mole and 1.05, respectively. The hydrolytic aging of this material was also kinetically analyzed, and it was concluded that the weight change was controlled by both water diffusion into the sample and diffusion of hydrolysis products from the sample. During hydrolytic aging below the glass transition temperature, the specimens gained weight up to 0.05 g based on 1-g unaged cured resin and then leveled off. At higher temperatures, the specimens initially gained weight and then began to lose weight, reaching a constant weight gain. The activation energies for water diffusion into the cured resin are 19.5 kcal/mole at temperatures above T_g and 21.5 kcal/mole at temperatures below T_g. The main hydrolysis product was boric acid from reaction of the boroxine ring with water. The time-temperature superposition principle was used for the weight loss study on isothermal and isothermal hydrolytic aging. The scale factor in this approach was found to be the ratio of the diffusion coefficient at the temperature of interest to that at a reference temperature.     

Dielectric relaxations of shellac/amino resin blends

The dielectric constant (ε′) and loss (ε″) of shellac/melamine resin blends have been determined at temperatures between 20° and 120°C and frequencies between 0.1 and 100 kHz. ε′ decreases with increase in the percentage of melamine resin in the blend. Two relaxations have been observed, of ΔH = 4.99 kcal/mole and 11.1 kcal/mole. The glass transition is observed between 60° and 70°C. The cole-cole parameter increases with temperature and becomes constant above T_g.     

Studies on sublimation of disperse dye out of dyed polymers. I. Rate of sublimation and amorphous transition points of poly(ethylene terephthalate)

Four samples of poly(ethylene terephthalate) film of various crystallinities and orientation were dyed with p-nitroaniline and disperse dyes. When these films were heated under a 2–3 × 10^?3 mm Hg vacuum at a specified temperature T, the dye sublimed out of the dyed specimen. The amount (M_t/M_∞) of sublimed dye is in linear proportion to the square root of the sublimation time, t^½, where M_t and M_∞ are the amounts of dye sublimed for times t and t = ∞. The diffusion coefficient D, calculated from the slope of the above plot, is independent of the dye concentration of the film. When log D is plotted against 1/T°K over the temperature range 320–520°K, the relation is composed of two to four intersecting lines with the slope decreasing with elevation of temperature and with the breaks at about 89°–98°, 122°–135°, 155° and 175°–176°C. These breaks are the amorphous transitions: the first is the glass transition temperature T_g, the second and the fourth are the amorphous transitions corresponding to the crystalline transition points, i.e., the cold crystallization temperature and the smectic–triclinic transition temperature. With some exceptions, these amorphous transitions are found also by dilatometry and electrical conductivity measurements. The apparent activation energy for diffusion decreases from about 100 kcal/mole for the glass state to 22–24 kcal/mole for the region above 180°C. The activation energy for each region changes slightly with the size of dye molecule and the crystallinity and orientation of the film.     

Lactone polymers VII. Activation energies for glass transitions of poly-ϵ-caprolactone and poly-ϵ-thiocaprolactone

The activation energies for segmental motion have been determined for partially crystalline poly-?-caprolactone and poly-?-thiocaprolactone over a temperature range that extends from below the glass transition to near the melt region. Based on the shift factors obtained from the time-temperature superposition of stress relaxation data, the activation energy for the glass transition of poly-?-caprolactone is 160 kcal/mole and of poly-?-thiocaprolactone is 200 kcal/mole. The value obtained for poly-?-caprolactone is in excellent agreement with the reported value obtained from dielectric relaxation studies. A relatively broad secondary maximum in the activation energy spectrum was observed for each polymer in the respective temperature regions between the glass transition and the melt zone.     

Decomposition of sec-butyl acetate on charcoal

The decomposition of sec-butyl acetate on de-ashed 20-to 30-mesh coconut-shell charcoal (1500 m²/g) was studied in a fixed bed reactor in the temperature range 300–375°C, and at partial pressures of 1 atm. and 0.29 atm. The ester decomposed principally to n-butene and acetic acid, and only small amounts of other products were found. The ratio of 1-butene to 2-butene was about 1:1, very close to that observed for thermal decomposition of the ester but far removed from the equilibrium butene ratio. The butene selectivity was independent of conversion. The rate of reaction followed the rate equation r = kA P_E/1 + AP_E This expression corresponds to surface reaction on one site being rate controlling. The activation energy of rate constant k was 32.7 kcal/mole, compared to 46.6 kcal/mole for the gas-phase reaction. The temperature dependence of adsorption constant A showed a heat adsorption of 12 kcal/mole. Gas chromatographic measurements confirmed this value and also showed that ester is the principal adsor-bate on the charcoal.     

Chlorination of polystyrene

Chlorination of a low molecular weight polystyrene in the α position was studied by following changes in the infrared spectrum, glass transition temperature, and molecular weights of the polymer. The logarithm of the absorbance ratio at 2920 and 1500 cm^?1 as a function of mole per cent chlorination was linear. The glass transition temperatures, determined by use of a differential scanning calorimeter (DSC), were found to obey an equation developed by Dyvik for copolymers. The molecular weight of the chlorinated polystyrene decreased as a function of chlorination.     

DTA evidence for physical orientation (crystallinity) in PVC

A difference has been observed in the DTA and DSC curves for the glass transition of both PVC homopolymer and acetate copolymer, depending on the rate of quenching or annealing below the glass transition temperature. The difference has the appearance of an endothermal peak added to the glass transition curve and is attributed to an alignment of stereoregular chain segments of adjacent polymer molecules. The lengths of chain segments are assumed to be so short that no x-ray evidence of crystallinity was obtained. The orientation process involved has an activation energy of 47.8 kcal./mole in the homopolymer and 43.6 kcal./mole in the copolymer. The measured rate of the process agrees with the principle of time–temperature superposition, the rates becoming equal for homopolymer and copolymer at equal temperatures below their glass transition temperatures.     

Some rheological properties of molten polytetrafluoroethylene

Measurements of melt viscosity on samples of polytetrafluoroethylene of different molecular weight were carried out at 360°C by means of tensile creep tests in the linear viscoelasticity range. The apparent activation energy for viscous flow in the range between 330° and 380°C was estimated to be 20 kcal/mole. A value of about 7,500 was also determined for the average molecular weight between entanglement points (M_e), from the equilibrium compliance (D_e). Melt viscosity data were compared with zero strength time (ZST) values and a linear correlation was found on a bilogarithmic scale. The dependence of ZST on the applied stress and temperature was also studied and the results are discussed on the basis of Bueche's theory on the creep at rupture above the glass transition temperature.     

Dynamic mechanical properties of polypentenamers with pendant ionic groups and their hydrogenated derivatives

Dynamic mechanical properties of substituted polypentenamers having thioglycolate side groups and their hydrogenated derivatives have been studied. Methyl esters, acids and salts were investigated at two concentrations, 5.5 mole (%) and 9.2 mole (%). In the un-hydrogenated derivatives one principal relaxation, labelled β, exists in the temperature range from-160 to 100°C. This relaxation arises from microbrownian segmental motion accompanying the glass transition. Its temperature is affected by the concentration of substituent but not the chemical nature of the substituent. A ‘rubbery plateau’ region exists in the acid and salt derivatives and in the latter the salt groups act as reinforcing fillers in this plateau region, consistent with the ionic domain model for their structures. In the hydrogenated derivatives, three relaxations occur in the temperature range from ?160 to 120°C and these are labelled α, β, and γ in order of decreasing temperature. The α relaxation arises from motions associated with the crystal phase except in the case of the 9.2 mole (%) sodium salt derivatives where it arises from motions occurring within ionic domains. The β relaxation arises from microbrownian segmental motions accompanying the glass transition, and the γ relaxation arises from localised motions which may involve both amorphous and crystalline phase components.     

Polymer welding relations investigated by a lap shear joint method

A lap shear joint method was used to study strength development during welding of polystyrene surfaces- The surfaces previously had not been in contact and care was taken to insure rapid wetting of the interface. The shear stress at failure, τ_f was measured at room temperature as a function of contact time, t, at constant welding temperatures up no 20°C above the glass transition temperature, T_g. The time dependence of welding could be well described by τ_fαt^1/4. This result is in agreement with predictions of the reptation molecular dynamics model applied to inter-diffusion at a symmetric amorphous polymer-polymer interface. The activation energy for the thermally activated increase in strength development was determined as E = 96 kcal/mol at T = 113.5°C, which compares with E = 93,2 kcal/mole as predicted by the W-L-F theory using C₁ = 13.7, C₂ = 50 and T_g = 100°C. The polystyrene samples had molecular weights, M_n = 143,000 and M_w = 262.000. The time to achieve complete healing, t_∞ ≈? 256 min at 118°C, was found to be of the same order of magnitude as the viscoelastic relaxation time and also with the time required for a polymer chain to diffuse a distance equal to its root mean square end-to-end vector.     

Corresponding states relationship for electrical resistivity of glassy polymers

Using the glass transition as a corresponding state for polymers, a relationship has been found between reduced electrical resistivity and reduced temperature for a large number of polymers in the glassy state. As a consequence of this relationship and the Arrhenius temperature dependence of the unreduced data, the activation energy for electrical conduction is shown to be directly proportional to the glass transition temperature and linearly related to the activation energy for the glass transition. A molecular model is proposed to explain these results in terms of the number of polymer segments involved in electrical conduction (～10) and the activation energy per segment (～4 kcal/mol).     

Nucleation and Growth of Lead Titanate from a Glass

The temperature dependence of the rate of growth of PbTiO₂ crystals from a PbO-B₂O₃-TiO₂ glass was experimentally determined. The Brown-Ginell equation describing the temperature dependence of the growth rate provided an excellent fit for the data. The activation energy for the growth of PbTiO₃ crystals was 34 kcal per mole. Within the same temperature range the activation energy for viscous flow was 75 kcal per mole. Differential thermal analyses of both powdered and solid samples of glass showed that the exotherm associated with crystal growth occurred at 100° to 150°C below that of the maximum rate of crystal growth and corresponded to a temperature at which the rate of surface nucleation was relatively high although the rate of crystal growth was low.     

Thermomechanical behavior of poly(vinyl chloride) in the process of degradation

The thermomechanical behavior of poly(vinyl chloride) (PVC) was investigated during its thermal degradation by using torsional braid analysis. In thermomechanical behavior as a function of temperature, the relative rigidity G_r decreased initially with increasing temperature, then began to increase passing through a minimum at about 200°C, and finally decreased at 340°C. Increase in G_r from 200°C was caused by formation of a conjugated polyene chain accompanied by dehydrochlorination and by crosslinking reaction, and decrease in G_r at 340°C was related to scission reactions of the crosslinking network by oxidation. In the change in logarithmic decrement Δ, three peaks were observed: at 90°C, coinciding with the glass transition of the polymer; at about 200°C, due to the melting transition of crystallites, and at about 300°C, due to a loss of mechanical energy in the rheological transition of the polymer from a liquid state to a glassy state passing through a viscoelastic region. The thermomechanical properties of PVC with different molecular weights were also measured, and the effect of molecular weight G_r and Δ are discussed. In isothermal measurements of the relative rigidity in air, exponentially increasing curves were observed as a function of time. These curves were analyzed kinetically as a first-order reaction, and an activation energy of 22.7 kcal/mole was obtained.     

Synthesis and characterization of 4,4′‐diaminodiphenyl methane‐based benzoxazines and their polymers

A series of diamine‐based benzoxazine precursors have been prepared using 4,4′‐diaminodiphenyl methane, formaldehyde, and different phenol derivatives including phenol, p‐cresol, and 2‐naphthol. Their chemical structures were identified by FTIR, ¹H NMR, and elemental analysis. The curing reactions of those precursors were monitored by FTIR and DSC. The obtained materials exhibited higher glass transition temperature and char yields than the corresponding bisphenol‐A based polybenzoxazines. The polybenzoxazine prepared from phenol showed the highest char yields of 65% and thermal stability with 5 and 10% weight‐loss temperatures at 346 and 432°C, respectively. The polybenzoxazine prepared from 2‐naphthol exhibited the highest glass transition temperature at 244°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermal decomposition of anionic and emulsion polymerized polystyrenes

The thermogravimetric behavior of anionic and emulsion polymerized polystyrenes was investigated in nitrogen at a heating rate of 18°C/min. Kinetic data were obtained by least squares analyses of experimental points obtained by differentiating primary thermograms. Degradation was generally zero order for about the first 25% of the reaction and first order thereafter. No molecular weight effects were observed for anionic polystyrenes with M ≥ 1 × 10⁵. Anionic and emulsion polystyrenes differed significantly in thermal stability. Degradation of emulsion polymers proceeded more slowly and with higher activation energies in both the zero- and first-order regions. Activation energies for anionic polystyrenes were 28 and 44 kcal/mole in the zero- and first-order domains while the corresponding values for emulsion polymers were 36 and 60.5 kcal/mole, respectively. No tacticity differences were detected in 220 MHz NMR spectra. The differences in thermal stability are attributed to differences in end groups in the two polymer types.     

Broad line proton magnetic resonance investigations on solid L-alanine: Crystal structure and molecular reorientations

Broad line NMR investigations on solid L -alanine have been carried out over the temperature range 77°K to 493°K. A comparison between the observed and theoretical values of the second moment has been made, to provide check on the crystal structure and to study the molecular motions. Investigations reveal the presence of NH₃ group motion at about 230°K. The activation energy for NH₃ group rotation has been computed to be 7.19 ± 1.0 kcal/mole. The crystal lattice remains rigid below 230°K. There is a possibility of torsional oscillation of the CH₃ group at about 352°K.     

Polymerization of methyl methacrylate in water in presence of the metal oxides TiO2 and Cu2O

Rates of sodium bisulfite-initiated polymerization of methyl methacrylate in water were determined in absence and in presence of the metal oxides TiO₂ and Cu₂O at 30°, 40°, 50°, and 60°C. Cuprous oxide and titanium dioxide enhanced the rate of polymerization and reduced the molecular weight as compared with the figures obtained in absence of oxide, the effect of the former being more pronounced than the latter. With TiO₂, the rate was increased from 2.3 to 3.2 × 10^?5, while with Cu₂O, it was increased to 8.6 × 10^?5 mole/l./sec, both at concentrations of 9 g/l. water. The apparent energy of activation for the polymerization of methyl methacrylate between 40°C and 50°C was found to be 15.6 kcal/mole in absence of the metal oxides, and 7.6 kcal/mole and 2.8 kcal/mole in presence of titanium dioxide and cuprous oxide, respectively. The number-average molecular weight was found to decrease slightly with the addition of TiO₂ but to decrease greatly when Cu₂O was added.     

Kinetic study of polymerization/curing of filament-wound composite epoxy resin sytems with aromatic diamines

The kinetics of the polymerization/curing of an 80/20 blend of a diglycidyl ether of bisphenol A (DGEBA) and a diglycidyl ether of 1,4-butanediol (DGEBD) with a commercial mixture of methylene dianiline and m-phenylene diamine (Tonox 60/40) was studied, at amine/epoxy ratios ranging from 1.1 to 4.4. Fourier transform infrared (FT-IR) measurements were used to follow the extent of epoxy conversion at 18–122°C, and bulk viscosity measurements were used to define the working range of the resin. For an amine/epoxy ratio of 1.1, the activation energy for the polymerization/curing reaction (based on time to 50% epoxy conversion in the S-shaped conversion-time plots) was 11.9 kcal/mole by FT-IR. This value compared favorably with the corresponding value of 12.7 kcal/mole obtained by Moroni and co-workers in a complementary differential scanning calorimetry (DSC) study of the same system. The FT-IR conversion-time plots were fitted to a kinetic expression that can be accommodated by an autocatalytic mechanism; the expression contains two rate constants with activation energies of 13.7 and 10.0 kcal/mole, respectively. The viscosity of the curing epoxy resin was found to obey the Williams-Landel-Ferry equation, with a Di Benedetto expression for the glass transition temperature.     

Cure kinetics and thermomechanical properties of thermally stable photopolymerized dimethacrylates

A range of dimethacrylates with varying backbone flexibility were partially photocured to various conversions using p‐xylylene bis‐(N,N‐diethyldithiocarbamate) as a photoiniferter and their glass transition region investigated by dynamic mechanical thermal analysis. For isothermally cured samples, the final degree of conversion was found to increase as the length of the spacer group in the monomer was increased or as the crosslink density in the resin was lowered due to the reduced glass transition temperature which allowed greater mobility and, hence, higher cure. Increasing the curing temperature also resulted in a higher degree of conversion as the network was able to polymerize further before vitrification occurred. For the partially photocured samples, the glass transition temperature was raised as the degree of conversion was increased. Most of the measures of the breadth of the glass transition were found to increase with increased conversion for dimethacrylates with short or stiff backbones (TETDMA and bisGMA) while the transition breadth was independent of conversion for either a more flexible dimethacrylate (NEGDMA) or a dimethacrylate network with a lower crosslink density (50 wt % bisGMA/50 wt % PGEMA). This conclusion was generally confirmed by analysis of the viscoelastic parameters in the frequency domain. It is not clear whether these behaviors resulted from differences in the range of molecular motions available in tight networks or if they were due to spatially heterogeneous regions in the network. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3753–3766, 2003     

Water absorption and its effect on polyarylate

Water absorption and its effect on the tensile and impact properties of polyarylate was studied by immersing polyarylate specimens in water baths, between 23 and 98°C. The diffusivity was calculated to be 11 × 10^?9 cm²/s at 23°C with an activation energy of 9.8 kcal/mole. The aromatic ester in polyarylate is hydrolyzed by water, which was found to cause a decrease in molecular weight and in mechanical properties. In the early stage, the reaction is zero-order and the activation energy of the hydrolytic embrittlement is 22 kcal/mole.