Degradation of polymer coating systems studied by positron annihilation spectroscopy. IV. Oxygen effect of UV irradiation

The effect of oxygen on the photodegradation of a polyurethane coating induced by accelerated UV irradiation was studied with two different light sources: 313‐nm UVB and Xe lamps. Doppler‐broadened energy spectra (DBES) and positron annihilation lifetimes were measured as a function of incident positron energy (0–30 keV) and irradiation time (0–100 h). The photodegradation of the coating was characterized in terms of subnanometer defect changes. Significant variations of the S parameter (a defect parameter from DBES) and the ortho‐positronium lifetime and intensity were observed as a function of oxygen concentration during exposure to UV irradiation. These results showed a significant enhancement in photodegradation due to the presence of oxygen, as indicated by a decrease in free volume and holes at the atomic level. Enhanced degradation in the presence of oxygen, as indicated by increased free‐radical production (observed with electron spin resonance spectroscopy), was also observed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2035–2047, 2001     

Degradation of polymer coating systems studied by positron annihilation spectroscopy. II. Correlation with free radical formation

The photo‐degradation of polymer coating systems due to irradiation by UV and Xenon light sources is studied using positron annihilation spectroscopy and electron spin resonance (ESR). Doppler broadened spectra of positron annihilation, as a function of slow positron implantation energy and ESR spectra, are measured in two types of polyurethane which were exposed, ex situ, to UV irradiation for up to 800 h. The UV irradiation systematically decreases the S parameter as a function of exposure duration and increases the ESR signals. Thus, significant S parameter decrease is correlated with the ESR signal increase resulting from photo‐degradation of polymers due to UV irradiation. Parallel in situ positron annihilation and ESR experiments are performed as a function of Xenon light exposure for up to 100 min. These results show that the photo‐degradation of the polyurethane coatings involves initial free‐radical formation, which is correlated with the subnanometer defects detected by positron annihilation spectroscopy. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1289–1305, 1999     

Development of positron annihilation spectroscopy to test accelerated weathering of protective polymer coatings

A variable mono-energetic positron beam with a computer-controlled system has recently been constructed at the University of Missouri–Kansas City for weathering studies of polymeric coatings. The beam is designed to measure the S-parameter from Doppler-broadening energy spectra and the sub-nanometer defect properties from positron annihilation lifetimes (PAL). Significant variations of S-parameter and ortho-positronium intensity in coatings, as obtained from the newly built beam and from the Electrotechnical Laboratory’s beam, respectively, are observed as a function of depth and exposure time due to the Xe-light irradiation. A high sensitivity of positron annihilation signal response to the early stage of degradation is observed. Development of positron annihilation spectroscopy to test accelerated weathering of polymeric coatings is discussed.     

Application of slow positrons to coating degradation

Photodegradation of a polyurethane-based topcoat induced by accelerated UV irradiation is studied using Doppler broadened energy spectra (DBES) and positron annihilation lifetime (PAL) spectroscopies coupled with slow positron technique. Significant and similar variations of S-parameter and ortho-positronium intensity (I₃) in coatings are observed as functions of depth and of exposure time. The decrease of S is interpreted as a result of an increase of crosslink density and a reduction of free-volume and hole fraction during the degradation process.     

Study of the photodegradation of epoxy polymers with slow positron annihilation spectroscopy

The photodegradation of an amine‐cured epoxy coating after exposure to accelerated UV‐340 and UV‐313 irradiation was investigated with an atomic‐level technique, positron annihilation spectroscopy (PAS), which detected and characterized the free volumes and defects as a function of the depth. Significant changes in the subnanometer defect parameters S and W were observed as a function of the exposure time near the surface. This was interpreted as due to a loss of the free volume and hole fraction resulting from photodegradation. A dead layer near the surface, resulting from UV irradiation from the surface up to a thickness of 0.4 μm, at which there was nearly no positronium formation, was observed. Correlations between physical defects from PAS in terms of the free volumes and chemical defects from electron spin resonance spectroscopy in terms of free radicals and chemical structural changes measured by ultraviolet–visible and Fourier transform infrared spectroscopy were established. A high sensitivity of PAS for detecting the early stage of degradation, on the order of hours for UV‐313 and on the order of days for UV‐340 irradiation, was observed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2441–2459, 2004     

Characterization of multilayer nanofiltration membranes using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam was used to characterize in situ the layer structure and depth profile of the cavity size in thin film composite (TFC) polyamide nanofiltration (NF) membranes prepared by the interfacial polymerization method. Two techniques, using PAS coupled with a slow positron beam of Doppler broadening energy spectra (DBES) and positron annihilation lifetime spectroscopy (PALS) designed to reveal the layer structure and the cavity sizes contained in a multilayer thin film composite NF membrane, were assessed. To the best knowledge of the authors, a characterization of the depth profile of cavities in NF membranes using PAS coupled with a slow positron beam has never been reported. The membranes selected have a composite structure containing three layers: a selective polyamide layer, a transition layer, and a porous support prepared by the phase inversion technique. Furthermore, the cavity size distribution in the selective top layer plays an important role in determining the performance of the NF membranes.     

Open spaces in the subsurface region of polyethylene probed by monoenergetic positron beams

Open spaces in the subsurface region (<10 μm) of very low density polyethylene were probed by a monoenergetic positron beam. From measurements of Doppler broadening spectra measurements of the annihilation radiation and the lifetime spectra of positrons as a function of incident positron energy, the size of the open spaces in the region of 0–3 μm was found to be larger than that in the deeper region. This was attributed to the cooperative motion of large segments of molecules which is activated near the surface. After the freezing in of such motions (below 230 K), although the lineshape parameter S in the bulk was almost constant, S in the region of 0–3 μm decreased with decreasing temperature. This discrepancy was associated with the presence of the open spaces with an excess content and the resultant contraction of amorphous structure near the surface. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2597–2605, 1998     

The buildup of the electric field during positron annihilation lifetime spectroscopy measurement

Positron annihilation lifetime spectra were measured on pure polystyrene (PS) and PS samples whose film surfaces were coated with gold, graphite (GR‐PS), or M_oS₂. The results showed the longest lifetime remained constant with measured time in all experimental samples, whereas the corresponding intensity decreased with time at different rates. The experimental phenomena were associated with the buildup of an electric field inside the polymer during extended positron annihilation lifetime spectroscopy (PALS) measurement. The decrease in the rate was attributed to the presence of conductive film causing the neutrality between positive charges and negative charges, thereby reducing the buildup of the electric field. Additionally, we also performed PALS measurement on GR‐PS under different experimental conditions, such as the conductive film being grounded or not grounded or the presence of an external electric field. These results further indicated that the buildup of the electric field was responsible for the decrease in the intensity with time. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 332–336, 2001     

Surface correlation effects on gloss

A general expression for gloss within the scalar Kirchhoff's theory is derived in terms of the detector collecting angle, and two statistical parameters that characterize the surface roughness. Analytical expressions for gloss are derived for an exponential and a Gaussian correlation function, and numerical results for these and other quasi-exponential correlation functions are presented. It is shown that the incoherent contribution to gloss is significant in common polymeric surfaces. The latter implies that surface height correlations cannot be neglected in the evaluation of gloss. It is also shown that for a correlation function with a single characteristic length, gloss scales with the correlation length L_c in the same way as with the detector collecting angle. This fact can be used to determine L_c with a glossmeter, and an experimental method to achieve this is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1321–1334, 1998     

A study on free‐volume properties of styrene‐acrylic ester type copolymers

The positron annihilation lifetime measurements have been performed on a number of amorphous styrene–methyl acrylate copolymers and styrene–butyl methacrylate copolymers. The densities of copolymers were obtained with immersion method by using a capillary pycnometer and the average molecular weights were determined by gel chromatography. The lifetime τ₃ of ortho‐positronium (o‐Ps) pick‐off annihilation have been found to correlate with side group volume and polarity of macromolecular chains in the copolymers, and relative intensity I₃ is attributed mainly to the electron‐attracting groups trapping the spur electrons and positrons. The experimental results have been discussed on the basis of the structural variation of macromolecular chains. In addition, the PALS measurement as a function of time for polystyrene and several styrene–methyl acrylate copolymers has also been performed. The result shows that an electric field is built in polymers during extended positron annihilation spectroscopy measurement, and the field effect is a main factor which causes the decrease in I₃ with time. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2476–2485, 1999     

Characterization of film surface treated with ECR plasma by Doppler broadening

Doppler broadened positron annihilation measurements were carried out using the positron beam technique on plasma treated polyethylene films as a function of incident positron energy. In addition, surface properties of the treated films also have been measured using other conventional techniques such as FT-IR, SEM and AFM. The surface tension of the films was also determined using sessile drop method. The S-parameter is seen to decrease on the surface upon plasma treatment that introduces polar groups such as hydroxyl and carbonyl on the surface. The results are discussed.     

Effect of residual water and free volume on the glass‐transition temperature and heat capacity in polystyrene/poly(vinyl acetate‐co‐butyl acrylate) structured latex films

The dynamic heat capacity and glass‐transition temperature of polystyrene (PS)/poly(vinyl acetate‐co‐butyl acrylate) (VAc–BA) (50:50 w/w) structured latex films as a function of annealing time at 70, 77, and 85 °C were examined with modulated‐temperature differential scanning calorimetry. The PS and poly(vinyl acetate‐co‐n‐butyl acrylate) components were considered to be the cores and shells, respectively, in the structured latex. The dynamic heat capacity decreased with time. The glass‐transition temperatures of the PS and VAc–BA phases shifted to higher values after annealing. The results of thermogravimetry showed that there existed about 1.8% residual water in the films. The mean free volume and relative concentration of holes at room temperature (before and after annealing) and 85 °C, as a function of time, were obtained with positron annihilation lifetime spectroscopy (PALS). The PALS results indicated no significant change in free volume during annealing. It is believed that the loss, by diffusion, of residual water mainly caused a decrease in heat capacity and an increase in the glass‐transition temperatures. As little as 1.8% residual water in the structured latex films had a significant influence on the thermal properties. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1659–1664, 2001     

Life science research using positron annihilation spectroscopy: UV-irradiated mouse skin

Positron annihilation spectroscopy (PAS) is applied to study mouse skin under different UV irradiations as a function of positron incident energy (0–30 keV). Significant variations in the depth profile of S parameter are observed in a period of hours and of days for UVA and UVB exposures, respectively. The high sensitivity of positron annihilation signals responding to UV irradiation shows that PAS may be developed as a new noninvasive technique for the detection of molecular damage in life science research.     

Gamma and positron irradiation effects on polypropylene studied by coincidence Doppler broadening spectroscopy

The coincidence Doppler broadening (CDB) technique was applied to study the positron source irradiation effect on γ-irradiated and unirradiated polypropylene. CDB results showed presence of carbonyl groups (CG) whose concentration increase with the γ-irradiation dose. Positron annihilation lifetime (PAL) spectroscopy was used to monitor the positronium (Ps) formation. The γ-irradiation effect on Ps inhibition was significantly weaker than the positron irradiation effect. The joined examination of CDB and PAL data showed that the annihilation on CGs oxygen was not the main reason for the detected Ps inhibition during the positron-irradiation.     

Microstructure and Defect Characterization of Nanostructured Ni<Subscript>3</Subscript>Al

Summary.  Nanostructured Ni₃Al was produced by the inert gas condensation and in situ compaction technique and characterized by means of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and density measurements. The defect structure was investigated using positron annihilation lifetime spectroscopy (PALS). It is shown that in some samples besides the cubic also the martensitic phase can be present. The defect structure can be divided into three major components: vacancy-like defects in the grain boundaries and nano-voids with a size of 1 nm as seen with PALS, and large pores with sizes up to 8 nm as seen with HRTEM. Furthermore, it is shown that an increasing compaction temperature leads to significantly smaller nano-voids. Received October 5, 2001. Accepted (revised) November 12, 2001     

Investigations of (PVA)0.7(NaBr)0.3(H2SO4)xM solid acid polymer electrolyte using positron annihilation lifetime spectroscopy

The free‐volumes of solid polymer electrolytes (SPE) were characterized using positron annihilation lifetime (PAL) spectroscopy, FTIR, and scanning electron microscope (SEM) techniques. The SPE based on poly(vinyl alcohol) (PVA) and sodium bromide (NaBr) complexed with sulfuric acid (SA) H₂SO₄ at different weight percent ratios were prepared using solution cast technique. The PAL results indicate that a higher SA content (more than 0.87 mol/L) in (PVA)_0.7(NaBr)_0.3 matrix increase the free‐volume hole size from 58 ų to 87 ų. The increase in the SPE free‐volume with higher SA content was associated with a decrease in the SPE crystallinity. It is postulated that the incorporated SA interrupt polymeric chain packing and retard crystallization during electrolyte films formation. The FTIR spectral studies indicate that the SA content higher than 0.87 mol/L induces chemical modifications within the PVA, which results in chain scission. The PAL study shows that the chain scissions within the polymer matrix affect the free volume hole density (I₃) and hence the microstructure. I₃ was found to be decreased from ?11 to ?6 %, resulting in lower fractional free‐volume holes in the SPE films. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2038–2044, 2010     

Characterization, transport and sorption properties of poly(thiol ester amide) thin-film composite pervaporation membranes

The effect of acyl chloride chemical structure on the ethanol aqueous solution dehydration through the poly(thiol ester amide) thin-film composite membrane prepared by reacting 2-aminoethanethiol (AETH) with trimesoyl chloride (TMC) or succinyl chloride (SCC) on the surface of the modified asymmetric polyacrylonitrile (mPAN) membrane was investigated. SEM/EDX, ATR-FTIR and water contact angle were applied to analyze the S element, chemical structure, and hydrophilicity of the poly(thiol ester amide) active layer of the composite membrane. In order to estimate the variation in the free volume of the poly(thiol ester amide) active layer and correlate that with the pervaporation performance, positron annihilation spectroscopy (PAS) experiments were conducted, in which a variable monoenergy slow positron beam was used. Doppler broadening S parameters of annihilation radiation energy spectra showed a significant variation with the acyl chloride chemical structures of the poly(thiol ester amide) active layers. The S parameters of the AETH–TMC/mPAN thin-film composite membrane were found to be lower than those of the AETH–SCC/mPAN thin-film composite membrane. In the ethanol aqueous solution dehydration, the AETH–TMC/mPAN thin-film composite membrane exhibited a lower permeation rate and a higher water concentration in the permeate than the AETH–SCC/mPAN. This is in good agreement with the analysis by positron annihilation spectroscopy. The solution effect dominated the pervaporation separation behavior of the poly(thiol ester amide) thin-film composite membrane with TMC substituting for SCC in the poly(thiol ester amide) active layer. The AETH–TMC/mPAN membrane was found to exhibit superior performance compared with some membranes discussed in the literature.     

Correlations between gas permeation and free-volume hole properties probed by positron annihilation spectroscopy

Free-volume hole sizes, fractions, and distributions in a series of polycarbonate-based polymers are measured by using positron annihilation lifetime (PAL) spectroscopy. Correlations between the obtained free-volume hole properties and gas diffusion coefficients are observed. © 1995 John Wiley & Sons, Inc.     

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.     

Study of electrical,mechanical, and nanoscale free‐volume properties of NBR and EPDM rubber reinforced by bentonite or kaolin

The effect of Na bentonite, Ca bentonite, and kaolin fillers on the macrostructure and microstructure of acrylonitrile butadiene rubber, ethylene propylene diene rubber, and their blend (50/50) was studied through electrical and mechanical measurements, as well as with positron annihilation lifetime spectroscopy. The real part of permittivity (ε′), dielectric loss (ε″), and the crosslinking density were found to increase with increasing filler content. The increase of crosslinking density of the blend with increasing amount of fillers reflects a decrease in the equilibrium swelling up to 21.50 wt % compared with that of the unfilled blends. The mechanical investigation showed pronounced increase in the tensile strength, and in elongation at break with the addition of up to 21.50 wt % of filler. In addition, comparing between different fillers showed that the reinforcing effect of Na bentonite is more effective than Ca bentonite and kaolin but the physico‐mechanical of Ca bentonite is less than that for kaolin. The positron annihilation lifetime measurements revealed that the free‐volume properties were strongly affected by the amount and type of filler, in particular, the free‐volume fraction was dramatically decreased with increasing filler content. Furthermore, correlations were made between the free‐volume parameters and both electrical and mechanical properties. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1825–1838, 2009