Free-volume study of ethylene - vinyl alcohol copolymer evaluated through positronium lifetime measurement

The free-volume, of size ranging from 0.2 to 0.4 nm in radius, in an ethylene-vinyl alcohol copolymer was estimated using positronium lifetime measurement to elucidate the dependence of oxygen permeability on the free-volume size and fraction, on the ethylene content and on the crystallinity. The permeability and the free-volume fraction with varying the ethylene content were well related and the relation was interpreted based on the free-volume theory near below and above the glass transition temperature. On the other hand, the crystallinity significantly influenced the fraction of the amorphous region, where the free-volume hole exists, along with a slight change of the free-volume size. The variation of the permeability with the crystalline degree cannot be explained from the averaged free-volume fraction estimated by the whole volume of the polymer, but the permeability correlated with the free-volume size apparently.     

Direct measurement of free-volume hole distributions in polymers by using a positronium probe

We have directly measured the free-volume hole distributions in semicrystalline polypropylene by positron lifetime annihilation spectroscopy. A Laplace inversion technique was engaged to analyze the positron lifetime spectra measured under quasi-isotropic external pressures of 0, 4.2, and 14.7 kbar into continuous lifetime distributions. The hole radii distributions as determined from the ortho-positronium lifetime distributions are found to be between 4.0 and 0.5 Å and to have maxima at 3.0, 1.9, and 1.1 Å under the external pressures of 0, 4.2, and 14.7 kbar, respectively. © 1992 John Wiley & Sons, Inc.     

Raman spectral study of solid and dissolved poly(vinyl alcohol) and ethylene-vinyl alcohol copolymer

We have measured the Raman spectra of ethylene-vinyl alcohol copolymer (EVOH) and poly(vinyl alcohol) (PVOH). Spectra of 88% hydrolyzed PVOH were examined from the partially crystalline solid, from PVOH dissolved in both H₂O and D₂O, and from films precipitated from these solutions. The spectrum in H₂O differs from that of the starting material by disappearance of sharp bands having Raman shift values of 1146 and 1093 cm^?1, strengthening of a band near 915 cm^?1, decrease in frequency of bands at 480, 1356, and 1441 cm^?1, and increase in frequency of bands at 369, 413, 1023, 1371, and 2910 cm^?1. The spectrum of the film shows partial reversal of these trends. With D₂O as the solvent, the band shifts are slightly different from those listed above and new bands appear. These changes are indicative of loss of crystallinity, change in stereochemistry, and partial deuteration of hydroxyl during dissolution of this PVOH sample at room temperature. © 1994 John Wiley & Sons, Inc.     

Characterization of extruded ethylene-vinyl alcohol copolymer based barrier blends with interest in food packaging applications

Based on previous work a number of optimum extruded blends with high contents of a high barrier ethylene-vinyl alcohol copolymer were selected and characterized in terms of phase morphology, water sorption and barrier properties. Blend components were an ethylene vinyl-alcohol copolymer (EVOH with 32 mol% ethylene), an amorphous polyamide (aPA) and a nylon-containing ionomer. A fine two phase structure was found for these blends in all cases. However, Raman spectroscopy results indicated a poor interface interaction between the blend components in the case of the EVOH/aPA blends. Higher interface interaction had been previously found in the dry EVOH/ionomer blends. Equilibrium moisture solubility and diffusion were found to be higher than expected from simple additivity. However, the oxygen transmission rate was found to be clearly lower than expected from the rule of mixtures, particularly under dry conditions, fitting closely a simple Maxwell model.     

Measurement of free-volume hole size distribution in Nafion-117 using positron annihilation spectroscopy

We report a new result of free-volume hole size distribution in water and ethanol-swollen Nafion-117 polymer. With the increase in water content, free-volume hole size decreases, but overall the volume fraction increases. The hole size distribution in dry polymer is seen to be distinctly different from hydrated membranes. The narrow and symmetric distribution in hydrated membrane as compared to dry membrane is believed to be a consequence of crosslinking due to cluster formation. In alcohol-swollen membranes, on the other hand, not only are the free-volume size and fraction seen to be higher, the hole size distribution is seen to be broader compared to dry or hydrated membranes, indicating the effect of penetration of alcohol into the hydrophobic backbone region. We have also examined our results vis-a-vis reported gas diffusion studies in Nafion in the framework of existing free-volume model. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 983–989, 1998     

Diffusion and solubility of mineral oils through ethylene-vinyl acetate copolymer

This paper reports a study of mineral oil diffusion through a filled ethylene-vinyl acetate crosslinked polymer, together with some comparisons with aliphatic linear hydrocarbons. Permeation was monitored by classical gravimetric measurements leading to values of solubility and diffusion coefficient at several temperatures ranging from 20 to 120 °C. Diffusion coefficients display a change in activation energy at at ca. 70 °C for mineral oils but not for simple hydrocarbons. The values obtained were discussed regarding available structure-diffusivity relationships and diffusion models derived from free volume theory. A relationship between penetrant evaporation temperature and its diffusivity was observed and tentatively justified.     

Solubility of alcohols in ethylene-vinyl alcohol copolymers by inverse gas chromatography

Sorption isotherms of alcohols in EVOH copolymers were determined by inverse gas chromatography below and above the glass transition temperature. The sorbates studied were: methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, s-butanol, and t-butanol. From the sorption isotherms, Henry's law solubility coefficients (S) were evaluated and correlated to size, condensability, and (SINGLE BOND)OH position in the molecule as well as on temperature. Molecular dimension was an important factor, affecting the solubility via two contrary effects: molecular size and condensability. When comparing alcohols of the same size, primary alcohols were sorbed more than secondary alcohols and these more than tertiary alcohols at every temperature in the study. At low temperatures, the larger the size the greater the solubility, contrary to what was observed at high temperatures. Temperature effect on solubility was also tested applying Van't Hoff's equation. Since S behaved differently below and above T_g for every sorbate two values of the enthalpy of solution were obtained, positive above T_g and negative below T_g. © 1996 John Wiley & Sons, Inc.     

Equation-of-state properties and surface tension of ethylene-vinyl alcohol random copolymers

Pressure-volume-temperature (PVT) data were studied for ethylene-vinyl alcohol random copolymers (EVOH) and served to determine bulk reduction parameters for the equation-of-state by Flory-Orwoll-Vrij. The compressibility factor as a function of copolymer composition displays positive deviation from additivity. Sessile drop method yielded surface tensions of EVOH melts at different temperatures. To a good approximation linear variation of surface tension with respect to copolymer composition was observed. It indicates that conformational restrictions prevent surface excess of the low-energy units. Results on surface tensions are discussed in terms of different theoretical approaches. It turns out that the parachor approach is in excellent agreement with experimental data. The corresponding state approximation revealed decreasing surface energy with ascending ethylene content whereas the surface entropy remains approximately constant as long as vinyl alcohol units are in excess.     

Thermal variation of free-volumes size distribution in polypropylenes. Probed by positron annihilation lifetime technique

Positron annihilation lifetime measurement was applied to the study of free-volume properties in three kinds of polypropylene as a function of temperature in the range of 25–180°C at thermal equilibrium. Positron lifetime data for polypropylenes were analyzed with a Laplace inversion technique in order to obtain continuous positron annihilation lifetime (PAL) distributions. At each temperature, four distinct PAL distributions were recognized. The distribution of the longest lived component was associated with a pick-off annihilation of ortho-positronium (o-Ps) trapped in free-volume of amorphous region, which grew bigger as the temperature increased. The hole radius distributions of free-volumes were estimated from the results of o-Ps lifetime distributions. A detailed analysis showed a mean radius of free volumes was 0.34 nm at room temperature and that was 0.42 nm near the melting point for each specimen. The distributions of hole radii of free volumes were found to be broader after thermal treatments. The relaxation of free volumes was attributed to the thermal equilibrium and the evacuation of included molecules in free volumes. © 1995 John Wiley & Sons, Inc.     

Temperature dependence of the inhibition of positronium by chlorine-substituted hydrocarbons in non-polar liquids

Positron annihilation lifetime spectra were measured for solutions of 1,2,3,5-C₆H₂Cl₄ in hexane, toluene, m-xylene and mesitylene, CCl₄ in hexane and toluene, and C₂HCl₃ in n-hexane for concentrations below 1 M and at various temperatures between ?30°C and 67°C. The Ps inhibition by C₆H₂Cl₄ was roughly 14 times stronger at ?30°C than at 67°C in toluene, m-xylene, and mesitylene, while that of CCl₄ displayed hardly any temperature dependence in hexane and toluene. Previously, the Ps inhibition by C₆H₂Cl₄ in various liquids at 20°C was explained by a competition between electron pick off by the positron from C₆H₂Cl₄^? formed by electron scavenging in the positron spur, and dehalogenation of the same anion at picosecond times after the spur formation. Three effects might reasonably explain the strong temperature dependence of the Ps inhibition of C₆H₂Cl₄: (1) The dechlorination time of C₆H₂Cl₄^? might be much shorter at low than at high temperatures. (2) The positron mobility might decrease very strongly with decreasing temperature. (3) Positrons possibly trapped on C₆H₂Cl₄ might thermally detach at high but not at low temperatures.     

Crystallinity in PEEK and PEK after mechanical testing and its dependence on strain rate and temperature

This work extends the results of previous workers on the influence of drawing on the crystallinity of PEEK and PEK to test temperatures well above T_g and to strain rates up to 10³ s⁻¹. The study thus includes measurement of crystallinity in samples tested in the strain rate regime where large increases in flow stress have previously been noted in these and other polymers. The results are in reasonable agreement with other workers on PEEK and are representative of the behavior of both PEEK and PEK at temperatures up to 200°C and strain rates up to 10² s⁻¹. However at a strain rate of 10³ s⁻¹ a dramatic increase in crystallinity and reduction in d-spacing is observed. It is speculated that a change in crystalline morphology induced by the high rate testing may account for the observed changes in spacing, crystallinity, and flow stress. © 1996 John Wiley & Sons, Inc.     

Effect of crystal form and particle size of titanium dioxide on the photodegradation behaviour of ethylene-vinyl acetate copolymer/low density polyethylene composite

The photodegradation behaviour of ethylene-vinyl acetate copolymer (EVA)/low density polyethylene (LDPE) composite containing four different types of titanium dioxide (TiO₂) was investigated through colour difference, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and mechanical tests. The results showed that the performance losses of composites were qualitatively correlated with the degradation degree. The vinyl acetate (VA) groups in EVA were sensitive to UV light and the photodegradation mainly occurred in the amorphous region. The chain scission and annealing effect facilitated the secondary crystallization of composites. The heterogeneous nucleation effect of TiO₂ on the crystallization of composites was related to the particle size of TiO₂. The micro rutile TiO₂, micro anatase TiO₂ and their mixture (rutile/anatase = 13/87) exhibited a photo-stabilising effect, while the nano mixed crystals TiO₂ (rutile/anatase = 20/80) had an opposite effect.     

Comprehensive study of positronium formation in polymer blends between polyethylene and ethylene-vinylacetate copolymer

The intensity I₃ of ortho-positronium (o-Ps) in a polymer blend system consisting of polyethylene (PE) and ethylene-vinylacetate (EVA, random copolymer with a vinylacetate content of about 14%) was measured as functions of EVA weight content (Φ=0–100%), electric field (E=0–60 kV/cm ), positron irradiation time (t=0–200 h) and temperature (T=100–300 K). It was found that the addition of small amounts of EVA to PE significantly alters the electric field, positron irradiation time and temperature dependence of I₃. Positron trapping on polar EVA is suggested to be responsible for the sensitive effects of EVA.     

Thermal lifetime of cellulose insulation material evaluated by an activation energy based method

A rapid method, based on a logarithmic degradation model of insulation material, is proposed to reduce the test duration in lifetime assessment of cellulose paper insulating materials. This method proposes the determination of the activation energy from a non-isothermal measurement made by differential scanning calorimetry or another thermal analysis technique and an aging test at a single elevated temperature. The use of the onset temperature of the exothermal peak at ca. 300 °C is proposed for evaluation of the activation energy of degradation. For comparison, the thermal aging of Kraft cellulose paper for power transformer insulation was performed according to the general standard IEC 60216-1/2001 at three different temperatures: 155, 135 and 115 °C, and subsequently, the lifetimes at different service temperatures were estimated. The experimental data proved to have good agreement between the applied methods, the differences being <10 % in terms of the estimated lifetime across the range of service temperatures. The novel proposed method is effective in terms of both energy and manpower costs as compared to the current method: a factor of around 10 in the case of reducing the aging time, a factor of 3 for the time needed for measurements, and a factor of 10 for the reduction of power intake.     

An NMR investigation of compatibility in blends of poly vinyl chloride and ethylene-vinyl acetate copolymer

A series of technical composite suspension polymerized poly vinyl chloride products with additions of ethylene-vinyl acetate copolymer (Levapren 450M. 45 wt % VAc) have been investigated with NMR. Partial compatibility is found in these samples, and the phase conditions have been shown to be very much dependent upon the methods of forming of the materials. Complete phase separation is observed after prolonged heating.     

A DSC investigation of compatibility in blends of poly vinyl chloride and ethylene-vinyl acetate copolymer

In DSC-thermograms from blends of suspension-polymerized vinyl chloride and ethylenevinyl acetate copolymer (Levapren 450M, 45 wt % VAc), two glass transitions are observed. The transition temperatures are shifted towards each other. In precipitated samples, intermediate phases seem to be formed. The PVC-EVA (45 wt % VAc) system can be regarded as semi-compatible.