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1.
This article describes dielectric properties of complex Zn(II) salts of ethylene-methacrylic acid copolymer (5.4 mol% methacrylic acid) with n-hexylamine. In all samples, the β′ relaxation near 340 K and γ relaxation near 170 K are observed. These are assigned, respectively, to micro-Brownian molecular motion of long segments above Tg and to local molecular motion of short segments below Tg. The dielectric results indicate that ionic clusters are not formed in these systems.  相似文献   

2.
Dielectric properties of four methacrylate polymers (methyl, ethyl, n-butyl and n-octyl) were studied in the frequency range 0.0001 cps–300 kcps at temperatures above and below the glass transition temperature and at various pressures up to 2500 atm. At temperatures well above Tg a single relaxation peak (α′ peak) was observed in the case of the higher n-alkyl methacrylates. However, this peak was split into two peaks, α and β, with decrease in temperature or increase in pressure. The molecular motions corresponding to the α and the β relaxation processes are the micro-Brownian motions of amorphous main chains and of flexible side chains, respectively. From the temperature and the pressure dependence of the average dielectric relaxation time of these polymers the single relaxation process (the α′ process) was attributed to the micro-Brownian motion of the main chain coupled with that of the side chain. The effects of temperature and pressure on the d.c. conductivity of these polymers were also studied.  相似文献   

3.
Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ3, and its intensity, I3, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, Tg. For PE, above Tg (140 K), the value of τ3 increased, but the temperature coefficient of I3 was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (Tg = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1601–1609, 1997  相似文献   

4.
For photo-charge generation of polymeric photorefractive (PR) composites, temperature is important factor with electric field, but no study for temperature dependence of photo-charge generation efficiency (φ) at the glass transition temperature (Tg) region has been done yet. In this work, we investigate the temperature dependency of φ of a low Tg PR composite. With increasing temperature the photo-charge generation efficiencies increased below Tg but decreased above Tg. This behavior could be attributed to temperature dependence of heat capacity and the dielectric constant of PR composite. The space charge field was strongly dependent on temperature and increased with increasing φ.  相似文献   

5.
Broadband dielectric spectroscopy was used to study the segmental (α) and secondary (β) relaxations in hydrogen‐bonded poly(4‐vinylphenol)/poly(methyl methacrylate) (PVPh/PMMA) blends with PVPh concentrations of 20–80% and at temperatures from ?30 to approximately glass‐transition temperature (Tg) + 80 °C. Miscible blends were obtained by solution casting from methyl ethyl ketone solution, as confirmed by single differential scanning calorimetry Tg and single segmental relaxation process for each blend. The β relaxation of PMMA maintains similar characteristics in blends with PVPh, compared with neat PMMA. Its relaxation time and activation energy are nearly the same in all blends. Furthermore, the dielectric relaxation strength of PMMA β process in the blends is proportional to the concentration of PMMA, suggesting that blending and intermolecular hydrogen bonding do not modify the local intramolecular motion. The α process, however, represents the segmental motions of both components and becomes slower with increasing PVPh concentration because of the higher Tg. This leads to well‐defined α and β relaxations in the blends above the corresponding Tg, which cannot be reliably resolved in neat PMMA without ambiguous curve deconvolution. The PMMA β process still follows an Arrhenius temperature dependence above Tg, but with an activation energy larger than that observed below Tg because of increased relaxation amplitude. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3405–3415, 2004  相似文献   

6.
We report dielectric relaxation and Rayleigh-Brillouin spectroscopic measurements on the side chain polymer poly(n-hexylmethacrylate), PHMA (Tg = 268 K), exhibiting a broad glass transition region. The dielectric loss curves can be represented by single Havriliak-Negami functions in the temperature range of 260–450 K. The width of the distribution relaxation function is a decreasing function of temperature up to T = 333 K ≊ 1.24 × Tg and remains virtually constant above that temperature. This is interpreted as marking the merging of the α-process with a slow β-relaxation in agreement with the value of the cooperativity length associated with the α-mode. Hence above that temperature, the relaxation times confirm well to an Arrhenius temperature dependence. The hypersonic dispersion deduced from the Brillouin spectra (210–550 K) surprisingly peaks at temperatures near Tg which bears no relation to the main α-relaxation. This structural relaxation is rather associated with the side hexyl group motion showing striking resemblance with the hypersonic dispersion in molecular liquids. It is conceivable that the observed damping in PHMA is dynamically related to the internal plasticization effect of the hexyl group. © 1996 John Wiley & Sons, Inc.  相似文献   

7.
The effect of hydrostatic pressure up to 1,361 atms on the dielectric properties of a segmented polyurethane elastomer (Dow 2103‐80AE) is studied at temperatures from 0°C to 80°C. The experimental results show that the relaxation time for both the I–process, associated with the molecular motions in the hard segments, and the α–process, associated with the glass transition, increases with pressure, and this shift is more pronounced for the I–process. Besides the glass transition, it is found that the I–process can be described by the Vogel–Fulcher (V–F) and Williams–Landel–Ferry (WLF) relations. At atmospheric pressure, Tg and T0 for the I–process are 235.9 K and 4.2 × 103 K, respectively. Based on the V–F and WLF relations and experimental results, it is found that a parameter, C1, in the WLF relation is independent of the pressure. Thus, a method is introduced to determine the values of both the characteristic transition temperature (Tg) and activation energy (T0) for the processes at different pressures. As the pressure increases from atmospheric to 1,361 atms, the increase of Tg for the I–process is about 30°C. The results also show that, for both the I– and the α–processes, T0 decreases with increasing pressure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 983–990, 1999  相似文献   

8.
Three reactive epoxy–amine systems based on diglycidyl ether of bisphenol A (DGEBA) with 4,4′-diaminodiphenylsulfone (DDS), 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA), and 4,4′-methylenebis [2,6-diethylaniline] (MDEA), were studied during isothermal curings at 140 and 160°C. The simultaneous kinetic and dielectric studies allow to express conductivity, σ, in terms of conversion, x, and of glass transition temperature, Tg. The conductivity, σ0, of the initial monomer mixture and, σ of the fully cured network are measured. It is found that:
  • The glass transition temperature, Tg, versus conversion, x, curves follows the equation of Di Benedetto modified by Pascault and Williams
  • There exists a linear relation between log σ/log σ0 and Tg.
So, it is possible to predict both kinetic and dielectric behaviors of these epoxy-amine systems by the knowledge of Tg0, ΔCp0, and σ0, respectively, glass transition temperature, heat capacity, and conductivity of initial monomer mixture, Tg and ΔCp, and σ, respectively, glass transition temperature and heat capacity and conductivity of fully cured network. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2911–2921, 1998  相似文献   

9.
We used neutron reflectivity to measure the interfacial width in the immiscible system polystyrene/poly(n‐butyl methacrylate) (PS/PnBMA). Measurements were made on the same samples at temperatures ranging from below the glass‐transition temperature (Tg) of PS to slightly above. We observed significant broadening of the interface at temperatures below the Tg of PS, indicating chain mobility below the bulk Tg value. The interfacial width exhibited a plateau at a value of 20 Å in the temperature range of 365 K < T < 377 K. A control experiment involving hydrogenated and deuterated PS films (hPS/dPS) showed no such broadening over the same temperature region. The results are consistent with a reduction of the Tg of PS in the interfacial region of ~20 K. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2664–2670, 2001  相似文献   

10.
Macromolecular dynamics of sulfonated poly(styrene-b-ethylene-ran-butylene-b-styrene) (sSEBS) triblock copolymers were investigated using broadband dielectric spectroscopy (BDS). Two main relaxations corresponding to the glass transitions in the EB and S block phases were identified and their temperature dependences were VFT-like. Tg for the S block phase shifted to higher temperature due to restrictions on chain mobility caused by hydrogen bonded SO3H groups. While the EB block phase Tg appeared to remain constant with degree of sulfonation in DMA experiments, it shifted somewhat upward in BDS spectra. A low temperature relaxation beneath the glass transition of the EB block phase was attributed to short range chain motions. The Kramers–Krönig integral transformation was used to calculate conductivity-free loss permittivity spectra from real permittivity spectra to enhance true relaxation peaks. A loss permittivity peak tentatively assigned to relaxation of internal S-EB interfacial polarization was seen at temperatures above the S block phase glass transition, and the temperature dependence of this relaxation was VFT-like. The fragilities of the EB and S block domains in sulfonated SEBS decreased after sulfonation. The temperature dependence of the dc conduction contribution to sSEBS loss spectra also followed VFT-like behavior and S block segmental relaxation time correlated well with conductivity according to the fractional Debye–Stokes–Einstein equation.  相似文献   

11.
The miscibility behavior of ternary blends made by the addition of di(ethyl-2 hexyl) phthalate (DOP) to a mixture of chlorinated polymers was investigated by differential scanning calorimetry. Two chlorinated polymer mixtures were selected: polyvinyl chloride (PVC) with a chlorinated polyethylene containing 48 wt% Cl (CPE48), and PVC with a chlorinated PVC containing 67 wt% Cl (CPVC67). Each binary DOP/chlorinated polymer pair is miscible whereas PVC/CPE48 and PVC/CPVC67 blends are immiscible. DOP/CPE48/PVC and DOP/PVC/CPVC67 ternary blends containing, respectively, more than 55 and 20% DOP exhibit a single glass transition temperature (Tg). The spinodal between the one-Tg zone and the two-Tg zone is symmetrical in the two cases. At high DOP concentrations, a quantitative analysis of the results leads to the conclusion of the presence of a true ternary phase. At low DOP concentrations where two Tgs are observed, the DOP is distributed equally between the two chlorinated polymers forming, in the DOP/CPE48/PVC case for instance, two binary DOP/CPE48 and DOP/PVC phases. The broad immiscibility zone observed in the DOP/CPE48/PVC ternary blend as compared to the DOP/PVC/CPVC67 blend appears to be mainly caused by the high molecular weight of CPE48, as compared with PVC and CPVC67. © 1994 John Wiley & Sons. Inc.  相似文献   

12.
Summary Volume and enthalpy relaxation in polycarbonate subjected to double temperature jumps in the Tg region has been analysed. It concerns both initial Tdown-jump from equilibrium above Tg to consolidation temperature below Tg and fina1 Tup-jump to relaxation temperature, also below Tg. The measured H and V data after Tup-jump were compared with respect to aging time calculating (dH/dV) ratio denoted as aging bulk modulus, Ka. According this new methodology H and V relaxation response after Tup-jump demonstrates differences in relaxation responses.  相似文献   

13.
Relaxation processes in polyethylene (PE) and polystyrene (PS) were studied by positron annihilation technique. For PE, above the glass transition temperature, Tg, 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.  相似文献   

14.
13C- and 31P-NMR spin lattice relaxation in the rotating frame have been measured on a series of networks prepared from monodisperse and deliberately bimodal poly(propylene glycols) (PPG) crosslinked with tris(4-isocyanatophenyl) thiophosphate. The T1pC minima correspond to loss maxima in the DMTA (Dynamic Mechanical Testing) measured at 10Hz. The T1pP minima fall at higher temperatures than those of T1pC for the same network indicating that these crosslinks lag the segments in frequency of motion at a given temperature. The carbon relaxation is biphasic below Tg of the segments indicating two relaxation domains which we assign to bulklike PPG segments and PPG segments proximal to he crosslink. Lineshape analysis by a diffusional model indicates crosslink reorientation is not isotropic until well above Tg. Relaxation and lineshapes for the bimodal networks indicate that junctions are not uniformly plasticized by the segments.  相似文献   

15.
The physical aging of polycarbonate was investigated with dynamic viscoelastic measurements. Physical aging was observed for samples aged at 110 °C (QA) and room temperature (QP) after being quenched from the molten state. The shapes of the temperature dispersion curves of the dynamic viscoelastic functions (E′, E″, and tan δ) of the QA and QP samples changed with aging time in a temperature range below the glass‐transition temperature (Tg). However, at temperatures close to but below Tg, the curves for the aged samples merged into the curve of the quenched sample at a temperature denoted TH. TH increased with aging time. The experimental results suggest that the aged sample has a memory of having been quenched and that as the sample approaches the equilibrium state, this memory is lost. Differential scanning calorimetry thermograms showed an endothermic peak below Tg for the QA samples. The peak temperature (Tp) also increased with aging time. TH and Tp of the QA samples were approximately the same. The increase of both TH and Tp with aging time indicates that the structure of the polymeric chain in the glassy state relaxes over larger segment scale lengths because the scale of the movable segments is related to temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 337–341, 2001  相似文献   

16.
The conductivity of poly(p-phenylene sulfide) (PPS) amorphous samples sandwiched between metallic electrodes has been studied as a function of applied voltage, temperature, and electrode material. The voltage (U) dependence of the currents for electric fields within the range 103–106 V/cm exhibits exp βU1/2 behavior with β = βSchottky below the glass transition temperature (Tg ≊ 90°C), and β = βPoole-Frenkel above Tg. Coordinated temperature measurements of dc currents with different metallic contacts and thermally stimulated currents (TSC) indicate, however, that the conductivity at T < Tg is consistent with the so-called “anomalous” Poole-Frenkel effect rather than the Schottky effect. Consequently, the p-type conductivity in amorphous PPS is proposed to be a bulk-limited process due to ionization of two different types of acceptor centers in the presence of neutral hole traps. © 1996 John Wiley & Sons, Inc.  相似文献   

17.
Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume of 4-(diethylamino)-4′-nitrotolane (DEANT) in poly(methyl methacrylate) (PMMA). The decay of the SHG signal from films of DEANT/PMMA was recorded at hydrostatic pressures up to 3060 atm and at different temperatures between 25°C below the glass transition temperature to 35°C above it. The activation volume, ΔV*αβ associated with the long range α-type motion of the polymer remained constant at 213 ± 10 Å3 between Tg − 25°C and Tg + 10°C. At higher temperatures, ΔV*αβ decreased linearly with increasing temperature. The activation volume, ΔV*αβ, associated with short range secondary relaxations was constant over the entire temperature range with a value of 77 ± 10 Å3. The data suggest that above Tg chromophore reorientation is coupled to both the long range and local motions of the polymer; whereas, well below Tg chromophore reorientation is closely coupled to the local relaxations of the polymer. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 901–911, 1998  相似文献   

18.
The dielectric permittivity and loss of Bisphenol-A-polycarbonate (PC) was measured over the frequency range 100 Hz to 200 kHz and temperature range 77–383 K. One sub-Tg relaxation peak is observed which rapidly broadens with a decrease in temperature. This is attributed to a progressive separation of the γ and β peaks, which at high temperatures are merged to form one peak of high strength. The strength of the sub-Tg relaxations decreases on physical aging of PC but is increased if the sample is quenched from a temperature above its Tg. Slowly cooled PC has a lower strength of its sub-Tg relaxation than a quenched specimen. The thermal history of PC affects the magnitude of its sub-Tg relaxation.  相似文献   

19.
Radiation-induced copolymerization of acrylonitrile with methyl acrylate was carried out in aqueous medium at room temperature. Different compositions of the copolymer were prepared and characterized by IR, 1H-NMR, thermal, and dielectric studies. NMR spectroscopy was used to determine the composition and stereochemistry of the copolymer. Glass transition temperature values (Tg ) were determined by DSC. Dielectric studies were carried out to understand the segmental motions and the effect of composition on dielectric loss.  相似文献   

20.
The effects of diluent on molecular motions and glass transition in the polystyrene–toluene system was studied by means of dielectric, thermal, and NMR measurements. Three dielectric relaxations were observed between 80 and 400°K. On the basis of NMR measurements on solutions in toluene and in deuterated toluene, relaxation processes were assigned to segmental motions of polystyrene, rotations of toluene, and the local motions of polystyrene and toluene in order of appearance from the high-temperature side. The concentration dependence of the relaxation strength and of the activation energy for the primary relaxation (that at the highest temperature) show a step increment at about 50% by weight. The activation plots for the primary process were expressed by the Vogel–Tamman equation. With this equation, the temperatures at which the mean dielectric relaxation time becomes 100 sec is determined. This agrees well with the glass-transition temperature Tg and hence Tg in concentrated solution is expressed by in terms of the parameters A, B, and T0 of the Vogel–Tamman equation. The values of A and B are, respectively, about 12 and 0.65 and independent of the concentration. The physical meaning of these parameters is discussed.  相似文献   

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