首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The ortho‐positronium (o‐Ps) lifetime τ3 and its intensity I3 in various fluorinated polyimides were determined by the positron annihilation technique and were studied with the spin–lattice relaxation time T1 and the propylene permeability, solubility, diffusivity, and permselectivity for propylene/propane in them. τ3, I3, and the distribution of τ3 changed when the bulky moieties in the polyimides were changed. The polyimides, having both large τ3 and I3 values, exhibited a short T1 and a high permeability with a low permselectivity. The propylene permeability and diffusivity were exponentially correlated with the product of I3 and the average free‐volume hole size estimated from τ3. In highly plasticized states induced by the sorption of propylene, the permeability increased with the propylene pressure in excellent agreement with the change in the free‐volume hole properties probed by o‐Ps. The large and broad distribution of the free‐volume holes and increased local chain mobility for the 2,2‐bis(3,4‐decarboxyphenyl) hexafluoropropane dianhydride‐based polyimides are thought to be important physical properties for promoting penetrant‐induced plasticization. These results suggest that o‐Ps is a powerful probe of not only the free‐volume holes but also the corresponding permeation mechanism and penetrant‐induced plasticization phenomenon. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 308–318, 2003  相似文献   

2.
High‐quality positron lifetime measurements (70 million total counts) are reported for polyethylenes (PEs) of different crystallinities (Xc = 3–82%). The specific volumes of the crystalline and amorphous phases (Vc and Va, respectively) were estimated from density and wide‐angle X‐ray scattering (WAXS) experiments. Some samples (those with low values of Xc) were branched PEs, and those with high values of Xc were linear PEs for which Xc was varied with changes in the crystallization temperature. Both Vc and Va increase with decreasing Xc in the range 0% ≤ Xc ≤ 56% (the branched PEs) but are constant for Xc ≥ 56% (the linear PEs). The lifetime spectra were analyzed with the MELT and LIFSPECFIT routines. Artifacts that can appear in the spectrum analysis were checked via an analysis of computer‐generated spectra. Four lifetime components appeared in all of the PEs; the two long‐lived ones are attributed to pick‐off annihilation of ortho‐positronium (o‐Ps) in crystalline regions (τ3) and in holes of the amorphous phase (τ4). With increasing Xc, τ3 decreases from about 1.2 to 1 ns, τ4 decreases from 3.0 to 2.5 ns, and the intensity I4 decreases from 29 to 0%. An increase in I3 from 6 to 12% was observed. A comparison with simulations shows that the true I3 value approaches 0 for Xc → 0%. The decrease in I4 is weaker than the increase in Xc; this leads to the conclusion that the apparent specific o‐Ps yield in the amorphous phase I4Xc increases with Xc. Possible reasons for this surprising results are discussed. The fractional free hole volume [h = (Va ? Vocc)/Va, where Vocc is the crystalline occupied volume] was estimated from density and WAXS results. Between Xc = 0 and 56%, h decreases from 0.151 to 0.090, but it does not change further above Xc = 56%. The mean size (v) of the local free volumes (holes) estimated from τ4 decreases from 200 to 150 Å3. The number density of holes (Nh) calculated from these values (Nh = h/v) also decreases from 0.8 to 0.6 nm?3 in the range 0% ≤ Xc ≤ 56%. The values of Va, Vc, h, and Nh increase with an increasing degree of branching but do not vary for linear PEs. The possible influence of a crystalline–amorphous interfacial phase (three‐phase model) on the observed lifetime parameters is also discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 65–81, 2002  相似文献   

3.
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  相似文献   

4.
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  相似文献   

5.
The ortho‐positronium (o‐Ps) annihilation parameters, i.e. the mean o‐Ps lifetime, τ3, and the o‐Ps relative intensity, I3, in cis‐1,4‐polybutadiene (cis‐1,4‐PBD) and polyisobutylene (PIB) over a wide temperature range including the glass‐liquid transition have been measured by means of positron annihilation lifetime Spectroscopy (PALS). From them the free volume microstructural characteristics, i.e. the mean free volume hole size, Vh, and the free volume hole fraction, fh, have been determined via a semiempirical quantum‐mechanical model of o‐Ps in a spherical hole or a phenomenological model of volumetric and free volume hole properties, respectively. Consequently, the literature rheological data for both the above‐mentioned polymers have been related to the free volume hole fractions via the WLF‐Doolittle type equation. It has been found that i) in the case of PIB this equation holds over 130K above the glass transition temperature Tg and ii) in the case of cis‐PBD the WLF‐Doolitle equation is valid in the temperature range over 60K above 1.3Tg, but below 1.3Tg down to Tg the modified WLF‐Doolittle‐Macedo‐Litovitz equation with the additional activation‐energy term describes the shift factor data better.  相似文献   

6.
Positron lifetime measurements, performed in the temperature range 80–300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ3 and τ4, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm3 (PE) and between 0.152 and 0.372 nm3 (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of Tg = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at Tg of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 1027 holes/m3 is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513–1528, 1998  相似文献   

7.
The toughness of cyanate ester (CE) resin matrix improves significantly with the addition of carboxyl‐terminated butadiene‐acrylonitrile rubber (CTBN). The curing behavior of the system was studied by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The results show that carboxyl groups on the CTBN chain have a slight activation effect on the CE curing reaction at the beginning of the curing process. Phase separation was found to be the main toughening mechanism for CE/CTBN composites. The existence of macro‐size pores induced by the decomposition of a small amount of the low weight molecular part of CTBN might be another toughening mechanism. It is confirmed that positron annihilation lifetime spectroscopy (PALS) is still valid in such a system where macropores filled with gas molecules exist. When a high weight percentage of CTBN (>8%) was added to CE, free‐positron annihilation was found to be the dominant annihilation process in the macropores. For CTBN weight percentage higher than 8%, the contribution of ortho‐positronium (o‐Ps) annihilation in the macropores to τ3 and I3 was found to be insignificant. It is effective to use PALS as a probe of free‐volume properties in such systems by determining the changes in the τ3 and I3 of the composite. The compatibility and interfacial adhesion of the composites can be estimated from the changes in the free‐volume properties of the composites. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

8.
The temperature dependence of the mean size of nanoscale free‐volume holes, 〈Vh〉, in polymer blend system consisting of polar and nonpolar polymers has been investigated. The positron lifetime spectra were measured for a series of polymer blends between polyethylene (PE) and nitrile butadiene rubber (NBR) as a function of temperature from 100 to 300 K. The glass transition temperatures (Tg) for blends were determined from the ortho‐positronium (o‐Ps) lifetime τ3 and the mean size of free‐volume holes 〈Vh〉 versus temperature as a function of wt % of NBR. The Tgs estimated from the PALS data agree very well with those estimated from DSC in view of different time scales involved in the two measurements. Both DSC and PALS results for the blends showed two clear Tgs of a two‐phase system. Furthermore, from the variation of thermal expansivity of the nanoscale free‐volume holes, the thermal expansion coefficients of glass and amorphous phases were estimated. Variations of the o‐Ps formation probability I3 versus temperature for pure PE and blends with low wt % of NBR were interpreted on the basis of the spur reaction model of Ps formation with reference to the effects of localized electrons and trapping centers produced by positron irradiation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 227–238, 2009  相似文献   

9.
We present the dynamics of a series of three paramagnetic molecules of different volume, mass, and shape in amorphous glass‐forming polymer poly(isobutylene) (PIB) as investigated by means of electron spin resonance (ESR) technique. The reorientation behavior of spin probes is related to the ortho‐positronium (o‐Ps) annihilation in PIB from positron annihilation lifetime spectroscopy (PALS) and the extracted free volume information. It is also related to the dynamic data of PIB from broadband dielectric spectroscopy (BDS), neutron scattering (NS), and nuclear magnetic resonance (NMR) spectroscopy from literature. In the case of the smallest spin probe, 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), a discontinuous course of the spectral parameter 2Azz versus T dependence was observed and the subsequent phenomenological model‐free analyses of the spectral parameter, 2Azz versus T, as well as of the correlation time, τc, versus 1/T plots provided the characteristic ESR temperatures ( , T50G, ) and (T, T, T). These characteristic ESR temperatures were found to be consistent with the characteristic PALS temperatures: T, T = T from temperature dependences of the mean o‐Ps lifetime, τ3, or the width of o‐Ps lifetime distribution, σ3, respectively. In addition, the relationships between the spin probe size, V, and the free volume hole size distributions gn(Vh) at the characteristic ESR temperatures indicate the significant influence of the free volume fluctuation at the crossover from slow to rapid regime as well as within the rapid motional regime. On the other hand, the two larger spin probes exhibit a rather continuous 2AzzT plots with the respective T50G's lying in the vicinity of T independently of their volume, mass and shape, suggesting the common origin of underlying process controlling this T50G transition. Finally, these mutual PALS and ESR findings were compared with the known dynamic behavior of PIB which suggest that the dynamics of the TEMPO and the larger spin probes are related to free volume fluctuation associated with primary α ‐ and secondary β processes, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1058–1068, 2009  相似文献   

10.
Positron annihilation lifetime spectroscopy (PALS) is used to probe structural changes in glassy polycarbonate in terms of changes in the hole volume and the number density of holes during fatigue (cyclic stress) aging. The ortho-positronium (o-Ps) pickoff annihilation lifetime τ3, as well as the intensity I3, were measured as a function of cyclic stresses and various previous thermophysical aging histories. It is found that τ3, the longest of the three lifetime components resolved in the PALS of glassy polycarbonate, increases when a cyclic stress is applied. These results indicate that there is a structural change during fatigue aging. The “holes” where o-Ps can localize become larger upon fatigue aging. These results also suggest that a significant distinction exists between structural changes induced by thermophysical aging and fatigue aging. The o-Ps annihilation intensity, which is a relative measure of the hole density in a material, showed a continuous decrease upon fatigue aging, indicating the possibility of hole coalescence, which could be a precursor of crazing. The interaction between thermophysical aging and fatigue aging corresponds very well with the enthalpy relaxation behavior as reported previously, viz., a well-aged sample is much more sensitive to cyclic stress. More importantly, it is hypothesized that fatigue failure initiation is probably closely related to hole size and density fluctuation.  相似文献   

11.
Positron annihilation lifetime spectroscopy was used to characterize the reentrant volume‐phase‐transition behavior of poly(N‐isopropyl acrylamide) hydrogel in an ethanol/water mixed solvent. The polymer gel was synthesized with γ irradiation. The ortho‐positronium lifetime (τ3) in the gel slowly increased with an increase in the ethanol content in the mixed solvent. τ3 was not influenced by the volume phase transition. The ortho‐positronium intensity decreased with the collapse of the gel in an approximately 10% ethanol/water mixture. When swelled in pure ethanol, τ3 initially increased with the solvent amount in the gel, showing the destruction of intramolecular hydrogen bonding and the relaxation of polymer chains. The lower critical solution temperature of the gel in the 10% ethanol/water mixture was lower than that in pure water, and τ3 for various solvent contents showed behavior similar to that seen in pure solvent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1028–1036, 2002  相似文献   

12.
Positron lifetime spectroscopy has been applied to study the temperature dependence of free-volume properties in a solvent-free polymer–salt complex polyethylene oxide (PEO) doped with ammonium iodide (NH4I, with NH ≈ 0.076) in the temperature range of 298–353 K. The observed lifetime spectra were resolved into three components and the longest lifetime, τ3, was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by the free volume. The lifetime component, τ3, and its intensity, I3, both showed a significant variation with temperature, which followed a different course in the heating and cooling cycle. Changes in the temperature coefficient of τ3 and I3 were observed at T ≈ 328 K, the melting point of the sample. This behaviour is correlated to the temperature variation of the electrical conductivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 969–976, 1998  相似文献   

13.
Polyblend and nanocomposite films of sodium salt of carboxymethylcellulose (Na‐CMC)/polyacrylamide (PAM) and Na‐CMC/PAM modified with carbon nanotubes (CNT) were synthesized by the solution casting technique. The effect of PAM and CNT loading on the structural, optical, and nanoscale free volume properties of Na‐CMC was studied. X‐ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy exhibited the existence of strong interactions between Na‐CMC and PAM and the non‐destructive effect of CNT on Na‐CMC/PAM structure. The HR‐TEM revealed the multi‐walled structure of CNT with a 7.06‐nm wall thickness and a 6.92‐nm wall inner diameter. Positron annihilation lifetime spectroscopy (PALS) was done, in a vacuum and at 30°C to 200°C, to investigate the nanoscale free volume properties by using a conventional fast‐fast coincidence spectrometer. It was found that the o‐Ps lifetime (τ3 ) and free volume (Vh) increase with increasing CNT percentage in the Na‐CMC/PAM blend. The distribution of the o‐Ps lifetime was broadened with increasing CNT ratios. Furthermore, the glass transition temperature (Tg) increases with increasing loads of CNT. For the first time, a correlation was done between Urbach energy (EU) and Vh. Finally, the results were represented and discussed in the frame of free volume properties. Optical measurements showed that the transmittance T% of Na‐CMC/PAM was 91.12% and decreased to 68.42% and 36.45% after loading with 1.0 and 2.0 wt % CNT. In addition, the blend shows higher insulating properties compared with the individual polymers. The CNT incorporation reduces the band gap significantly and increases the EU in the films.  相似文献   

14.
Early stages of cyclic fatigue‐loaded polystyrene (PS) specimens were investigated by positron annihilation lifetime spectroscopy (PALS) at a maximum stress amplitude of 15 MPa. PALS yields information about the average unoccupied hole volume. A linear increase in the ortho‐positronium (o‐Ps) lifetime was observed in a range from 0 to 50,000 cycles. This increase occurs homogeneously distributed at different positions along a sample of 170 mm. The average unoccupied void volume increases by 1.2%. On the other hand, the o‐Ps intensity shows no systematic change upon cycling. The results suggest a homogeneous and linear increase in free volume prior to craze formation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1991–1995, 2008  相似文献   

15.
Synthesis of ethyl 5‐amino‐4‐cyano‐1‐phenyl‐1H‐pyrazole‐3‐carboxylate 5 has been achieved via abnormal Beckmann rearrangement of o‐chloroaldehyde 1 . Reaction of o‐aminocarbonitrile 5 with concentrated H2SO4 furnished expected o‐aminocarboxamide pyrazole 6 . Key intermediates o‐aminocarbonitrile 5 and o‐aminocarboxamide 6 were successfully utilized for the synthesis of pyrazolopyrimidine derivatives. The replacement of Cl in o‐chlorocarbonitrile 3 with secondary amine furnished new synthon 13 , which was further used for the synthesis of polysubstituted heterocycles. The obtained new products were well characterized by IR, 1H and 13C NMR, and mass spectra.  相似文献   

16.
In 2‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐1,2,4‐triazine‐3,5(2H,4H)‐dione (6‐aza‐2′‐deoxy­uridine), C8H11N3O5, (I), the conformation of the glycosylic bond is between anti and high‐anti [χ = −94.0 (3)°], whereas the derivative 2‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐N4‐(2‐methoxy­benzoyl)‐1,2,4‐triazine‐3,5(2H,4H)‐dione (N3‐anisoyl‐6‐aza‐2′‐deoxy­uridine), C16H17N3O7, (II), displays a high‐anti conformation [χ = −86.4 (3)°]. The furanosyl moiety in (I) adopts the S‐type sugar pucker (2T3), with P = 188.1 (2)° and τm = 40.3 (2)°, while the sugar pucker in (II) is N (3T4), with P = 36.1 (3)° and τm = 33.5 (2)°. The crystal structures of (I) and (II) are stabilized by inter­molecular N—H⋯O and O—H⋯O inter­actions.  相似文献   

17.
The ionization (or basicity) constants (pKb) were determined for many 2‐substituted 4,6‐diamino‐s‐tri‐azines ( I ) by means of the electrometric titration. I includes 2‐alkoxy or aryloxy‐( Ia ), 2‐alkyl‐ or 2‐aryl‐( Ib ), and 2‐alkylamino‐ or 2‐arylamino‐4,6‐diamino‐s‐triazines ( Ic ). For the series with the same alkyl or aryl group, the order of the basicity was found to be Ic < Ib < Ia . A study was made of relationships between the pKb, values of I , and the substituent constants, σp, σm, σp+, σm+, σpO, σmo, σI, σn, and σ*. The Hammett relationships were observed between the pKa values of I, and the substituent constants σm, (or the combination ones, [0.97σm + 0.03σp] as well as another [0.77σI + 0.23σR]). The Taft relationships were also found between the pKa values of Ia , Ib , and Ic and the constants σ*, respectively. Furthermore, in the case of Ic a linear relationship was observed between the pKa values and Σσ8.  相似文献   

18.
In this study phenylselenocyanate and some of its derivatives (o‐Cl, p‐Cl, p‐Br, o‐NO2, p‐NO2, o‐CH3, p‐CH3, o‐COOH, p‐COOH, p‐OCH3 substituted) were synthesized ( 3a–3j ). The synthesized compounds were converted to 5‐aryl‐1H‐tetrazole ( 4a–4j ), by Et3N ċ HCl‐NaN3 in toluene, which are a new series of phenylselanyl‐1H‐tetrazoles. The structure of all the presently synthesized compounds were confirmed using spectroscopic methods (FTIR, 1H NMR, MS). © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:255–258, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20293  相似文献   

19.
Positron lifetime measurements were carried out in a series of poly(ethylene oxide)—PEO—of different average molecular weights (M w): 1000, 1500, 6000, 10,000, 300,000, and 4 M. The mean radius (R ) and the mean free volume size (Vf) values were determined using a semiempirical equation that correlates the ortho‐positronium (o‐Ps) lifetime (τ3) and size of holes existing in the amorphous phases. The hole mean radius values determined at room temperature from lifetime spectra were found to be between 2.68 and 2.97 Å, and the hole volumes between 80 and 110 Å3. Free volume size evolution was studied with temperature variation until the melting temperature of the PEO samples. The degree of crystallinity and the melting temperatures were determined by Differential Scanning Calorimetry. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 219–226, 1999  相似文献   

20.
《中国化学》2018,36(2):112-118
Quinazoline‐2,4(1H,3H)‐diones are core structural subunits frequently found in many biologically important compounds. The reaction of 2‐​aminobenzonitrile and CO2, which was frequently studied, only provided N3‐unsubstituted quinazoline‐2,4(1H,3H)‐dione compounds. Herein we report palladium‐catalyzed cyclization reactions of o‐haloanilines, CO2 and isocyanides to prepare N3‐substituted quinazoline‐2,4(1H,3H)‐diones. Electron‐rich o‐bromoanilines participated in the cyclization reaction using Cs2CO3 at high temperature, and electron‐deficient o‐bromoaniline or o‐iodoaniline substrates conducted the reaction using CsF as base to deliver corresponding quinazoline‐2,4(1H,3H)‐dione products in good yields.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号