Toughening of cyanate ester resin by carboxyl terminated nitrile rubber

The carboxyl terminated butadiene‐acrylonitrile (CTBN) rubber was used to improve the toughness of the cyanate ester (CE) resin. The toughness of the modified blends depended on the CTBN content. The addition of 10 phr (g/100gCE) CTBN in CE resin led to a 200% increase in the impact strength with a loss of storage modulus. The transmission electron microscopy result showed the existence of rubber particles, inferring that phase separation had occured after curing. The thermogravimetric analysis curve of CTBN indicated the presence of cavities which also can be observed on the fractured surface in the scanning electron microscopy pictures using high magnification. Thus, phase‐separation and cavities toughening mechanisms function together to improve the toughness. Copyright © 2004 John Wiley & Sons, Ltd.     

Probing the microstructure of Nafion-117 using positron annihilation spectroscopy

We report a new result on positron annihilation studies in acid- and cation-neutralized (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, UO₂²⁺, Ni²⁺) Nafion membranes using positron lifetime and Doppler-broadened annihilation radiation (DBAR) measurements. The free-volume structure is characterized using a simple quantum mechanical model of positronium (Ps) in a spherical well. Our studies indicate that formation and expansion of clusters is always associated with a change in free-volume structure resulting in smaller free-volume holes. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 771–776, 1997     

Free‐volume change during the volume phase transition of polyacrylamide gel studied by positron annihilation: Solvent‐composition dependence

Changes in the free‐volume parameters of polyacrylamide (PAAm) gels during the volume phase transition (VPT) were studied with the positron annihilation lifetime technique. The VPT was induced through the variation of the solvent composition in a mixture of acetone and water. The PAAm gels containing 0 and 4 mol % carboxyl groups in their polymer chains were adapted to compare the effect of the presence of ionic groups on the microscopic environment. The change of the free‐volume property is discussed on a nanoscopic scale, with attention paid to the interactions between the polymer chains and the solvent molecules. It is proven that the variations of the free‐volume parameters correlate significantly with the VPT phenomenon. The results of the free volume for both gels are well‐explained when an interaction parameter, ε_g, is assumed. The interpretation suggests that the state of the interactions among the components (the polymer chain, acetone, and water molecules) plays an important role in the change of the free volume of PAAm gels during the VPT. An increase of the dispersion of the free‐volume size near the VPT point was observed for the ionized PAAm gel. The broadened size distribution of the free volume of the ionized PAAm gel around the VPT point lay between those of pure water and the corresponding mixed solvent, suggesting that a local minimum of the average free‐volume size at the VPT point is caused by the increase of a specific interaction, hydrogen bonding. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 922–933, 2000     

Molecular weight-dependence of free volume in polystyrene studied by positron annihilation measurements

Positron annihilation lifetime measurements are reported for four monodisperse polystyrenes with molar mass M = 4,000, 9,200, 25,000, and 400,000. The temperature dependences of orthopositronium (o-Ps) lifetime (τ₃) and intensity (I₃) were measured from 5°C to T_g + 30°C for each sample. From these data, the free volume hole size, 〈v_f(τ₃)〉, and fractional free volume h_ps=CI₃〈v_f(τ₃)〉 were calculated. The temperature dependences of τ₃, 〈v_f(τ₃)〉 and h_ps show a discrete change in slope at an effective glass transition temperature, T_g,ps, which is measurably below the conventional bulk T_g. This suggests that τ₃ is sensitive to large holes which retain their liquid-like mobility in the glassy state. Good agreement was found for T > h_g,ps between h_ps and the theoretical free volume fraction h_th deduced from experimental P-V-T data for polystyrene using the statistical mechanical theory of Simha and Somcynsky. Below T_g,ps, deviations between h_ps and h_th are observed, h_ps falling increasingly below h_th as temperature decreases. Whereas h_ps and h_th depend strongly on M in the melt, each essentially independent of M in the glass. A free volume quantity, computed from the bulk volume, which is in good numerical agreement with the Simha-Somcynsky h-function in the melt, gives improved agreement with h_ps in the glassy state. © 1994 John Wiley & Sons, Inc.     

Free‐volume change in volume phase transition of polyacrylamide gel as studied by positron annihilation: Salt dependence

Variation of free‐volume parameters—average radius size, number concentration, and size distribution—of a polyacrylamide (PAAm) gel containing 4 mol % carboxylate anions is studied during a volume phase transition (VPT) caused by a change of sodium chloride (NaCl) concentration. A positron annihilation lifetime technique is used for the determination of the free‐volume characteristics. The measurement is performed in an acetone–water 3 : 2 (v/v) [0.27 : 0.73 (mol/mol)] mixed solvent at 20°C, and the free‐volume parameters deduced from the analysis of a positron annihilation curve are utilized. An average free‐volume size of the swollen PAAm gel, ∼ 0.32 nm in radius, almost agrees with that of the mixed solvent for a corresponding salt concentration, while the size of the collapsed gel, which is ∼ 0.28 nm in radius, is smaller than that of the mixed solvent. The results for the collapsed gel indicate that the hydrogen bond plays a significant role in the nanoscopic environment. The radius of the free‐volume of the swollen PAAm gel seems to be influenced by the composition between acetone and water. An inhomogeneity of the nanoscopic structure inside the PAAm gels is discussed in terms of a dispersion of a size distribution of the free‐volume. It is concluded that a change of the nanoscopic environment of the PAAm gel during the VPT can be monitored through the free‐volume parameters obtained by the positron annihilation lifetime technique. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2634–2641, 1999     

Effect of CO2 exposure on free volumes in polystyrene studied by positron annihilation spectroscopy

Free‐volume properties, size and distribution, in amorphous polystyrene exposed to CO₂ gases have been measured as a function of pressure to 800 psi (5.5 MPa), of time, and of temperature using positron annihilation lifetime spectroscopy. The free volume increases significantly and its distribution broadens as a function of pressure. The free volume relaxes as a function of time with a characteristic time of 15 h, and 5.7 h for 400, and 800 psi, respectively, after depressurizing under vacuum. A portion of free volume created by CO₂ exposure remains permanently in the polymer after CO₂ exposure. The glass transition temperature decreases significantly as a function of CO₂ pressure from the free‐volume data and is compared with the differential scanning calorimeter results. The observed free‐volume variations as a function of pressure, time, and temperature are discussed in terms of hole expansion, creation, free‐volume relaxation, plasticization, and hole filling in amorphous polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 388–405, 2008     

Investigation of correlation between microstructure and dielectric properties of cyanate ester/silicate tube hybrids by positron annihilation lifetime spectroscopy

The correlation between microstructure and dielectric properties of cyanate ester (CE)/hollow silicate tube (HST) hybrids was investigated by positron annihilation lifetime spectroscopy, coincidence Doppler‐broadening spectroscopy, Fourier transform infrared spectra, and dynamic mechanical analyses. The addition of HST to CE resin brings a multi‐aspect influence (cross‐linked structure and density, free volume, and interfacial action) on the structure of the cross‐linked network and thus results in significantly varied dielectric properties. There is an optimum content of HST in hybrids to get the lowest dielectric constant and loss. When the content of HST is smaller than the percolation threshold, the hybrid has decreased dielectric constant and loss; this mainly results in the reduced size of free volume and orthopositronium intensity (I₃), although when the content of HST is larger than the threshold, the increased I₃ and the size of free volume as well as the interfacial polarization are responsible for the significantly enlarged dielectric constant and loss. Copyright © 2011 John Wiley & Sons, Ltd.     

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     

Subnanometer hole properties of CO2-exposed polysulfone studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime (PAL) spectroscopy has been employed to study subnanometer hole properties in polysulfone (PSF). In this study, hole properties of size, fraction, and distribution of PSF exposed to CO₂ are reported. In the PSF/CO₂ system, the hole size and fraction significantly increase and the free-volume distribution broadens as a function of CO₂ pressure in the range of 0–1000 psi. Hysteresis in hole properties is observed during CO₂ sorption/desorption cycle. The high sensitivity of PAL results due to CO₂ exposure in PSF is explained in terms of the microstructural changes in the polymer matrix, i.e., filling penetrant and plasticization, gas hydrostatic pressure effect, and creation of free volumes and holes. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3049–3056, 1998     

Effect of thermal history on the free volume properties of semi‐crystalline poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) membranes by positron annihilation lifetime spectroscopy

Free volume properties of a series of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) membranes, which were produced by various nonisothermal crystallization processes (rapid‐, step‐, and slow‐cooling processes), were investigated using positron annihilation lifetime (PAL) spectroscopy over a temperature range of 25–90 °C. From the annihilation lifetime parameters, the temperature dependence of free volume size, amount, size distribution, and fractional free volume and thermal expansion properties of free volume were discussed. A model which assumed that amorphous phase was subdivided into mobile and rigid amorphous fractions (MAF and RAF) in the semicrystalline polymer was considered to interpret the temperature dependence of those free volume properties. Morphological observation of the semicrystalline polymer by small‐angle X‐ray scattering (SAXS) indicated that the rapid‐cooled (cold‐crystallized) membranes showed a much thinner thickness of the repeating lamellar/amorphous layers and most likely higher amount of RAF, which restrained the chain motion, than the step‐ and slow‐cooled (melt‐crystallized) membranes. The difference of free volume properties among various PHBV membranes was created according to the crystalline structure of the polymer from different thermal history. The polymer crystallized with slower cooling rate induced higher crystallinity and resulted in less free volume amount and lower fractional free volume. In addition, the thermal expansion coefficients of free volume size were affected by the crystallization rate of PHBV polymer. Larger distribution of the free volume size of melt‐crystallized membranes was observed as a result of the bimodal distribution of the lamellar periodicity and less amount of RAF than that of the cold‐crystallized membranes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 855–865, 2009     

A study on free volume of acrylonitrile‐styrene copolymer and methyl methacrylate‐styrene copolymer by using positron annihilation

The o‐Ps lifetime τ₃ and the intensity I₃ of ST‐AN copolymers and ST‐MMA copolymers have been determined by using the positron annihilation technique. The average free volume hole radius R is estimated according to Tao's and Eldrup's model. The result shows that the average free volume hole size mainly attributes to lateral group volume and polarity of macromolecular chain as well as polymerizing temperature, and the o‐Ps intensity I₃ to the effect of the lateral group volume and the polarity. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 465–472, 1999     

Phosphorus‐free boron nitride/cerium oxide hybrid: A synergistic flame retardant and smoke suppressant for thermally resistant cyanate ester resin

Establishing a phosphorus‐free strategy to fabricate high‐performance thermosetting resins owning outstanding thermal resistance, good flame retardancy, and smoke suppression is important for sustainable development. Herein, a unique phosphorus‐free hybrid (BN@CeO₂) was synthesized through chemically grafting cerium oxide (CeO₂) on surface of exfoliated boron nitride (BN) nanosheet with the aids of γ‐aminopropyltriethoxysilane and polydopamine coating, which was then embedded into bisphenol A cyanate ester (BCy) resin to fabricate new BN@CeO₂/BCy composites with high thermal resistance. Compared with BCy resin, the BN@CeO₂/BCy composite with 4 wt% BN@CeO₂ not only has delayed initial ignition time by 23 seconds but also severally shows 58.1%, 23.1%, and 44.4% lower smoke produce rate, total heat release, and peak heat release rate. The study on mechanism behind outstanding flame retardancy reveals that the improved heat resistance and flame retardancy of BN@CeO₂/BCy composite are attributed to multiply effects induced by BN@CeO₂ and its interaction with BCy resin; specifically, these effects come from BN (physical barrier) and CeO₂ (free radical trapping effect and catalytic char layer formation) as well as those from the synergistic effect of BN and CeO₂. These excellent comprehensive properties of BN@CeO₂/BCy composites demonstrate that BN@CeO₂ is an environment‐friendly and synergistic modifier for developing heat‐resisting thermosetting resins with outstanding flame retardancy and smoke suppression.     

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.     

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     

Damage‐depth profiling of ion‐irradiated polyimide films with a variable‐energy positron beam

Various polyimide layers [2.2–2.6 μm of hexafluoroisopropylidene bis(phthalic anhydride‐oxydianiline), pyromellitic dianhydride‐oxydianiline, and 3,3′‐4,4′‐biphenyltetracarboxylic dianhydride‐p‐phenylenediamine] spin‐coated on silicon substrates were studied with a variable‐energy positron beam in combination with a Doppler‐broadened annihilation radiation technique. From the experiments, the thickness of the layers was estimated with the VEPFIT routine. These values corresponded well to the values determined from interferometry and ellipsometry. Irradiation of the polyimides with 1 × 10¹⁵ boron ions/cm² at an energy of 180 keV led to a strong chemical modification of the irradiated top layer. This caused the inhibition of positronium formation in the irradiated layer, which was observed as a lowering of the annihilation line S parameter. The thickness of the modified layer was estimated to be 700–800 nm. This value did not agree with the ellipsometric measurements but corresponded to the maximum implantation depth of boron ions calculated with TRIM (Transport of Ions in Matter) code. The positron results appeared somewhat larger than the TRIM estimates. Reasons for these relations are discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3062–3069, 2000     

Free volume changes on optical switching in azobenzene‐polymethylmethacrylate blends studied by a pulsed low‐energy positron beam

Polymers including chromophores, which can be switched by light, have been studied extensively during the last years due to a host of potential applications which arise from the marked changes in physical properties on switching. Even though there is clear evidence that the free volume has a significant influence on the isomerization kinetics, the question of free volume changes on switching was only addressed recently. Using a pulsed low‐energy positron beam the ortho‐positronium lifetime τ₃ was taken as a very sensitive free volume probe, and no change in free volume was detected on isomerization in an azobenzene‐polymethylmethacrylate (PMMA) copolymer containing about 8 wt % of the azobenzene moiety. Here, we report for the first time on free volume changes in an azobenzene‐PMMA blend with an azobenzene moiety concentration as high as 40 wt %. Using the same pulsed low‐energy positron beam, small but significant changes of τ₃ were observed between the structurally relaxed dark and the UV‐illuminated states suggesting a decrease in free volume of the order of 10%. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Study of relaxation processes in polyethylene and polystyrene by positron annihilation

Relaxation processes in polyethylene (PE) and polystyrene (PS) were studied by positron annihilation technique. For PE, above the glass transition temperature, T_g, the size of free volumes and its concentration were increased by the micro-Brownian motion of molecules. For PS, local motions of molecules in backbone chains were found to start above 260 K. However, these local motions were suppressed by an interphenyl correlation. For both PE and PS, below 250–260 K, the formation probability of positronium atoms increased with decreasing temperature. This fact was assigned to the freezing in of the local motions of molecules. For PS, an onset of the local motions of molecules was observed above 100 K. These motions were expected to be associated with liberation of phenyl groups. © 1996 John Wiley & Sons, Inc.     

Analysis of swelling process of protein by positron annihilation lifetime spectroscopy and differential scanning calorimetry

The change in the nanoscopic structure and bound state of water in the protein gel were investigated using positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). Gelatin was used as a protein. To examine the bound state of water in gelatin gel, the amount of freezing and nonfreezing water in gelatin gels were evaluated by fusion enthalpy of DSC curves. Below water content of 40% (w/w), the whole amount of water was nonfreezing water, whereas above water content of 40% (w/w), the amount of freezing water increased according to increase in water content. To investigate the nanoscopic spatial structure under coexistence of polymer and water, positron annihilation lifetime measurement was performed. The lifetime of o‐Ps or the pore size increased according to increase in water content, particularly below the water content of 40% (w/w). When the water penetrates into the gelatin network, the water molecules form hydrogen bonds with hydrophilic groups inside the helical structure in gelatin gel. The water molecules inside the helical structure expand the structure outward, leading to increase in pore size. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2031–2037, 2007.     

Deterioration of a polyurethane coating studied by positron annihilation spectroscopy: Correlation with surface properties

Deterioration of a polyurethane coating by Florida natural environments as a function of time up to 16 weeks was studied by positron annihilation spectroscopy. Doppler broadening energy spectroscopy (DBES) of annihilation irradiation and positron annihilation lifetime (PAL) were measured as a function of incident positron energy (0–30 keV). A significant decrease in the S‐defect parameter from DBES and the intensity of orthopositronium from PAL was observed as a function of weathering time. This is interpreted as a loss of free volume and holes as a result of the weathering process. The gloss and surface morphology in the same system were measured by glossimetry and atomic force microscopy (AFM), respectively. The gloss decreased and surface roughness increased as a function of weathering time. The AFM images showed a new feature of a spherically coagulated microstructure on the surface after weathering. Direct correlations between the decrease in gloss and the increase in roughness as well as the decrease in the S‐defect parameter from the DBES data and in the free volume from the PAL data were observed. These results were used to discuss the weathering process in terms of chemical and physical changes as a result of photodegradation in protective polymeric systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2290–2301, 2001