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91.
J. A. Ratto Paul T. Inglefield R. A. Rutowski K.-L. Li Alan Anthony Jones Ajoy K. Roy 《Journal of Polymer Science.Polymer Physics》1987,25(7):1419-1430
Carbon-13 and proton spin-lattice relaxation times were measured at two field strengths on solutions 10% by weight of two polycarbonates in C2D2Cl4 from ?20 to +120°C. The first polycarbonate is an asymmetrically substituted form with one chlorine on one of the two phenylene aromatic rings of the bisphenol unit, whereas the second polycarbonate is symmetrically substituted with two chlorines on each of the two rings. The nuclear spin relaxation data are interpreted in terms of several local motions likely in these polymers. Segmental motion was described by the Hall–Helfand correlation function. Segmental motion in the monosubstituted polycarbonate is somewhat slower than in unsubstituted polycarbonate, whereas segmental motion in the tetrasubstituted polycarbonate is considerably slower. Phenylene ring rotation is observed in unsubstituted polycarbonate and in the monosubstituted polycarbonate above 40°C. Below 40°C in the monosubstituted species, and at all temperatures in the tetrasubstituted species, ring rotation is replaced by ring libration as the predominant motion contributing to spin lattice relaxation. In addition, the rotational motion of the two types of rings in the asymmetric monosubstituted form are very similar although not identical. The substituted ring is slightly less mobile than the rings of unsubstituted polycarbonate. This indicates a strong coupling of ring motion, although the coupling leads to less than synchronous motion. Methyl group rotation is present in both polymers and is little affected by the various structural modifications. 相似文献
92.
Muon irradiation of pure liquid 3‐chloropropene, CH2=CH-CH2Cl, yields a primary radical, \dot\mboxCH2-CHMu-CH2Cl, and a secondary radical, MuCH2-\rm\dot\mboxCH-CH2Cl. 2‐methyl‐3‐chloropropene yields only the tertiary radical, MuCH2-\rm\dot\mboxC(CH3)-CH2Cl. These three chloroalkyl radicals have been characterized by μSR and μLCR, and the hyperfine coupling constants (hfcs)
have been determined over a range of temperatures, either in the pure liquid precursor or in concentrated solution. The temperature
variation of the hfcs has been analyzed to obtain estimates of the barrier to internal rotation about the C_\alpha-C_\beta
axis for various alkyl groups, and also their minimum energy conformations, i.e. their orientations with respect to the axis
of the 2p_z orbital of the unpaired electron. The tertiary radical is particularly interesting because all three methyl‐like
groups, -CH3,-CH2Cl and -CH2Mu, are represented. The results can be compared to electron spin resonance data for analogous radicals, to provide information
on the effects of Mu substitution for H.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
93.
This study reports on the effect of input power to hexamethyldisiloxane (HMDSO) plasmas. The power dependence of the plasma-phase species and of the surface chemistry (of the deposits) has been investigated. Neutral and positive molecular species were detected within the plasma using mass spectrometry (MS). Secondary ion mass spectrometry (SIMS) was used to probe the molecular structure of the deposits. The elemental composition of the surface was determined by XPS and the deposition rate was monitored using a vibrating quartz crystal microbalance. Neutral and cationic molecules of mass greater than HMDSO were detected in the plasma. Their formation through ion-molecule reactions is proposed. Changes in the relative concentration of plasma-phase species follow those seen in molecular species detected at the deposit surface. Thus, we believe that the molecular structure of the deposits can be related to the species present in the plasma. While traditionally the dominant mechanism in deposit formation is assumed to be free radical combinations, we propose other possibilities involving cations with the aim of putting forward a new perspective on plasma polymerization mechanisms and thereby stimulating discussion. 相似文献
94.
Arjan J. H. Louter Philo A. Jones J. David Jorritsma Jolan J. Vreuls Udo A. Th. Brinkman 《Journal of separation science》1997,20(7):363-368
An automated system for derivaatization was coupled on-line with solid-phase extraction-gas-chromatography (SPE-GC). The system was optimized for the determination of phenol and chlorinated phenols in aqueous samples. The test analytes were acetylated with acetic anhydride; proper buffering of the sample was a critical factor. Next, the phenol acetates were enriched on a SPE cartridge and transferred to a GC; two appraoaches were studied. In the first approach, the derivatives were enriched on disposable C18 cartridges (ASPEC type) and desorbed with methylacetate. Aan aliquot of the final eluate was injected on-line the GC by means of a loop-type interface. In the second approach, trace enrichment was performed on 10 × 2 mm i.d. LC-type precolumn packed with polystyrenedivinylbenzene copolymer (PLRP-S) this precolumn was dried with a mitrogen purge and the phenol acetates were desorbed with ethyl acetate which was injectedon-line into the retention gap of the GC under partially concurrent solvent evaporation (PCSE) conditions. The Derivatization-SPE-GC system which was based on the loop-type interface has the advantage of simplicity and easy operation, the main drawback is the impossibility to determine phenol acetates which elute prior to trichlorophenol acetates. With the derivatization-SPE-GC approach using PCSE-based desorption, even the most volatile analyte of the test series, phenol acetate, can be determined successfully. The entire procedure, including the derivatization step, was fully automated and integrated in one set-up. The precision data for the integrated on-line derivatization-SP-FID system were fully satisfactory, with RSD values of 1–12 % at the 1 μg/1 level. When a sample volume of 2.2 ml was analyzed, The detection limits for the chlorinated phenol acetates were in the 0.1–0.3 μg/1 range. 相似文献
95.
Measurements of239+240Pu in human tissues, from nuclear weapons testing, provide an invaluable source of data for verifying the uptake and distribution of radionuclides in the body. Measured concentrations of239+240Pu in lung, tracheobronchial lymph nodes, liver and skeleton have been compared with concentrations calculated using estimated plutonium intakes, the ICRP Publication 66 Respiratory Tract Model and the ICRP Publication 67 biokinetic model for plutonium. Measurement data tend to fall between the concentrations estimated on the basis of Type M and Type S absorption parameters. This indicates that the models represent the movement of plutonium through the body reasonably well. 相似文献
96.
Vernon R. Morris Fida Mohammad Lynnette Valdry William M. Jackson 《Chemical physics letters》1994,220(6):448-454
Time-resolved IR emission spectroscopy (TRIES) has been used to study infrared emission in the 3400–3100 cm−1 region from HCN molecules produced when CN radicals abstract a hydrogen atom from ethane, propane, and chloroform. From these observations the nascent vibrational distributions of the HCN produced were derived. The nascent vibrational population distributions of the product HCN in all of the reactions are non-statistical and inverted in both the pure CH stretch (00p) and CH stretch—bend (0np) series. 相似文献
97.
H.O.U. Fynbo Y. Prezado J. Äystö U.C. Bergmann M.J.G. Borge P. Dendooven W. Huang J. Huikari H. Jeppesen P. Jones B. Jonson M. Meister G. Nyman M. Oinonen K. Riisager O. Tengblad I.S. Vogelius Y. Wang L. Weissman K.W. Rolander 《The European Physical Journal A - Hadrons and Nuclei》2002,15(1-2):135-138
98.
Modeling synchronization and suppression of spontaneous otoacoustic emissions using Van der Pol oscillators: effects of aspirin administration 总被引:1,自引:0,他引:1
Many of the aspects of the interaction of spontaneous otoacoustic emissions with external tones (suppression and synchronization) can be qualitatively simulated by the behavior of a single driven Van der Pol oscillator. Analytical and numerical investigations of a model of spontaneous otoacoustic emissions based on such an oscillator (with appropriate parametric changes in the nonlinear and negative damping components) lead to predictions of the nature of the changes in suppression and synchronization (frequency-locking) tuning curves when the levels of spontaneous otoacoustic emissions are modified. Observations of the suppression and synchronization of spontaneous otoacoustic emissions by external tones of different frequencies and levels were obtained while the levels of spontaneous emissions were altered by aspirin administration. Modeling an emission as a single Van der Pol oscillator qualitatively accounts for: (1) the reduction of the level of an external tone required to suppress the emission by a decibel amount equivalent to the level reduction induced by aspirin administration; (2) the broadening of the frequency-locking tuning curve of an emission whose level is reduced; and (3) the pulling of the emission frequency by an external tone. It does not account for: (1) the observed asymmetry in the slopes of the external-tone suppression curves (more gradual for frequencies of the suppressor tone higher, rather than lower, than that of the emission); and (2) the frequency pushing of the emission by an external tone. 相似文献
99.
E. Todd Brown Karen Karp Joseph M. Petrosko Jane Jones Gloria Beswick Carrie Howe Kathy Zwanzig 《School science and mathematics》2007,107(3):102-116
This study investigated K‐12 teachers' beliefs and reported teaching practices regarding calculator use in their mathematics instruction. A survey was administered to more than 800 elementary, middle and high school teachers in a large metropolitan area to address the following questions: (a) what are the beliefs and practices of mathematics teachers regarding calculator use? and (b) how do these beliefs and practices differ among teachers in three grade bands? Factor analysis of 20 Likert scale items revealed four factors that accounted for 54% of the variance in the ratings. These factors were named Catalyst Beliefs, Teacher Knowledge, Crutch Beliefs, and Teacher Practices. Compared to elementary teachers, high school teachers were significantly higher in their perception of calculator use as a catalyst in mathematics instruction. However, the higher the grade level of the teacher, the higher the mean score on the perception that calculator use may be a way of getting answers without understanding mathematical processes. The mean scores for teachers in all three grade bands indicated agreement that students can learn mathematics through calculator use and using calculators in instruction will lead to better student understanding and make mathematics more interesting. The survey results shed light on teachers' self reported beliefs, knowledge, and practices in regard to consistency with elements of the National Council of Teachers of Mathematics Principles and Standards for School Mathematics (2000) technology principle and the NCTM use of technology position paper (2003). This study extended previous research on teachers' beliefs regarding calculator use in classrooms by examining and comparing the results of teacher surveys across three grade bands. 相似文献
100.
Pawan K. Agarwal Richard T. Garner William W. Graessley 《Journal of Polymer Science.Polymer Physics》1987,25(10):2095-2111
We present an analysis of data on the intrinsic viscosity [η] of sulfo-polystyrene ionomers in several solvents for a variety of sulfonation levels and counterions. For solvents of low dielectric constant, 2 < ε < 18, [η] decreases from the base polymer value [η]0 with increasing substitution level. This behavior was attributed to intramolecular association of ionic dipoles. The ratio [η]/[η]0 was found to depend on a single reduced variable αAαSx, where x is the fractional substitution, αA depends only on the counterion, and αS ∝ ε?1 depends only on the solvent. For solvents of high dielectric constant, 36 < ε < 47, [η] increases approximately as x3, and counterion effects are small. This behavior was attributed to ionic dissociation, giving rise to a polyelectrolyte effect. Implications of the low ε results are discussed in relation to association-induced gelation behavior and possible generalizations of the reduced variables approach. 相似文献