measured and subtracted for each sample to give the total coherent scattering for each sample, I1(Q) and 12(Q). 2 it is clear that the single chain from factors From our earlier results, The S(Q) obtained in this experiment is shown in Figures 1 and 2. The solid points are the measured values while the solid and broken curves are computations using the Debye form factor for gaussian coils at various values of the radius of gyration R. Rg. The scatter in these data is smaller by an order of magnitude than similar data taken earlier 7 on polybutadiene using a low concentration of marked polymer. We can see from Figure 1 that the Debye form factor with an R g of 34 A fits the data reasonably well over the range of the measurements. A Zimm plot of the data in the Guinier range (Q R < 1) gives an of R Rg g 33 ± 4 A, while fitting the data to the Debye form factor with varying values of R gives an R of 33.9 ± 0.8 A. The minimum, unperturbed 8 8 8 value of R for polyisoprene occurs for 1,2-addition products and sydiog tactic placements. This value for the molecular weight here is 28.9 A. The maximum value occurs for trans 1,4-addition products and is 39.8 A. Our results and those of Williams et al., show that one can measure single chain form factors for bulk polymer and concentrated solutions using large ratios of marked to normal polymer. The practical impact of this is that the flux requirements to perform any particular experiment are reduced by at least an order of magnitude. This means Figure 1. S S(Q) in arbitrary units for polyisoprene. The solid curve is the Debye form factor for a gaussian coil with an R of Figure 2. Comparison of the measured S(Q) and gaussian coil calculations for three values of R, 32, 34, and 36 A. 33.9+ 8.8 A. The best fit is that previously impossible experiments, such as the polyelectrolyte solution measured by Williams, et al., can now be done at the highest flux facilities. Other experiments, such as the polyisoprene experiment, described here, can now be done at facilities of more modest flux such as NBS reactor with precision. 1. 2. 3. 4. 5. 6. R. G. Kirste, W. A. Kruse, and J. Schelten, Makromol. Chem. 162, 299 (1973). J. P. Cotton, B. Farnoux, G. Jannink, J. Mons, and C. Picot, H. Benoit, J. P. Cotton, D. Decker, B. Farnoux, J. S. Higgins, G. Jannink, R. Ober, and C. Picot, Nature, Phys. Sci. 245, 13 (1973). J. P. Cotton, B. Farnoux, G. Jannink, C. Picot, and G. C. Summerfield, J. P. Cotton, D. Decker, B. Farnoux, G. Jannink, R. Ober, and C. A. Z. Akcasu, G. C. Summerfield, S. N. Jahshan, C. C. Han, C. Y. 7. Jeffrey A. Hinkley, Charles C. Han, Bernard Mozer, and Hyuk Yu, Macromolecules 4, 836 (1978). 8. C. Williams, et al., J. Polymer Sci. Letters (to be published). 4He CRITICAL EXPONENTS FOR He AT THE GAS-LIQUID PHASE TRANSITION Bernard Mozer and Bernard Le Neindre (Laboratoire des Interactions Moleculaires et des Hautes Pressions Centre Universitaire Paris Nord Villetaneuse, France) and John R. D. Copley 1 Small angle neutron diffraction measurements were obtained from "He in the vicinity of the gas-liquid phase transition. 10-4 Diffraction -1 patterns varying from K(wave vector difference) = 0.01 to 0.125 A were taken for each temperature in the reduced temperature range 0.25 to for a fixed value of the helium density at its critical value. The thermodynamic measurements of density and temperature were made in the gas and liquid phases for calibration of the system and were accurate to better than 1% in density and limited to an accuracy in relative temperature from 0.25 m K fluctuations on the control block. Raw neutron data was corrected for background from the sample container and environment, response of the area detector, and multiple scattering. Normalization of the response of the area detector was determined from the scattering of water. Extensive calculations were made for the correction factors for angular resolution of the incoming beam and of the scattering and detector configuration as well as wave length distribution of the incoming beam. Additional diffraction data was obtained at 8 to 10 K at the critical density and at reduced densities along the isotherm of the critical temperature. Preliminary data analysis of the corrected structure factor enabled us to determine the critical exponents ʼn (deviation from Ornstein-Zernike theory), Y (isothermal compressibility), and V (correlation length). Y and V were determined in the reduced temperature range 0.025 to 0.001 and ʼn determined in the range of reduced temperatures less than 0.001. The following table shows the values of these exponents n = 0.10 +0.05 Y = 1.16 ± 0.05 v = 0.63 ± 0.03 More extensive data analysis is being undertaken to refine the values of these exponents and to seek estimates for other exponents associated with corrections to the scaling hypothesis. 1. Now at McMaster University, Hamilton, Ontario, Canada PHASE TRANSITION IN AMMONIUM NITRATE C. S. Choi and H. J. Prask (Energetic Materials Division, LCWSL, ARRADCOM, Dover, NJ) and (National Bureau of Standards, Washington, DC) and E. Prince Ammonium nitrate (AN) is hygroscopic and crystallizes in at least five different polymorphic forms at atmospheric pressure, the existence of which leads to serious problems for the utilization of AN in certain practical applications. In particular, the phase transition between phase IV (-18°C to 32°C) and phase III (32°C to 84°C) is accompanied by a large change in volume, which causes irreversible growth, break-up and caking of cast or pellitized AN. The occurence of phase III, however, depends on the presence of water (≥ 0.1 wt%) which suggests the possiblity, that dopants might be found which would counteract the effect of water and eliminate phase III. Our recent work on AN has included structural studies of AN with dopants and of pure AN to understand the effect of water or dopants on the bonding. |