Meridional maxima

It is worth considering that in [97] an explanation of the origin of the diffraction maxima along the meridian at 0.40 and 0.80 A is provided, consistent with the paracrystalline model proposed by Lyndenmeyer and Hosemann [48] for PAN. The halo at f 0.80 is not necessarily the second-order diffraction of the maximum at f 0.40 A since it is apparent from calculations of Fig. 12 that these two meridional maxima may originate from different contributions. The maximum at 0.40 A arises from the average periodicity of lateral - CN groups alone (Fig. 12C), whereas the maximum at C = 0.8oA- arises from the contribution of only the backbone carbon atoms (Fig. 12D). 

X-ray diffraction patterns of fibers spun from liquid crystalline melts of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation. Several meridional maxima are detected which are aperiodic and also change in position and number with the monomer composition. The positions of these maxima can be predicted by calculating the theoretical scattering of random copolymer chains, in which the residues are represented by points separated by the monomer lengths. Both peak positions and intensities are reproduced when intraresidue interferences are allowed for in an atomic model for the random chains. This procedure also allows determination of the stiff-chain persistence (or correlation) length from the breadth of the maximum at d=2.lA which increases from 9 to 13 residues as the HBA content is increased from 25 to 75%. 

The aim of this work is to investigate the structure of these copolyesters at the molecular level both in terms of the sequence distribution and conformation of the individual molecules and how these pack in three dimensions. This paper describes X-ray diffraction work on these copolymers, and primarily addresses the problem of the chain structure. X-ray diffraction patterns of melt-spun fibers of HBA/HNA copolyesters (fig. 1) show a high degree of molecular orientation parallel to the fiber axis. Meridional maxima can be seen which are aperiodic, i.e. they are not orders of a simple fiber repeat these maxima also change in number and position with the monomer ratio. This is seen clearly in fig. 2 which shows 6/20 meridional X-ray diffractometer scans for five HBA/HNA compositions. We have calculated the theoretical diffraction patterns of random chain copolymers of HBA and HNA, and details of these calculations are given below. 

The most striking feature of the X-ray data is that there are several meridional maxima, and that these are not orders of a single repeat distance but are aperiodic and also vary in position depending on the monomer ratio. At present there is no information on the monomer sequence distribution from NMR analysis. (The polymer has... 

The above results demonstrate that we can reproduce both the positions and the intensities of the meridional maxima using a model for a completely random copolymer sequence. In separate work [12] we have checked the sensitivity of the calculations to non randomness, i.e. blockiness, by calculation of Q(z) for unequal monomer reactivities, and the results show that all but minimal blockiness can be ruled out. Tilting the residues is only one of several possible ways to refine the model so as to improve the intensity match. We are presently investigating a distribution of tilt angles (rather than a single tilt for each residue type) and details of this work will be presented at a later date. A further factor that could affect the meridional intensity is the presence of three-... 

Figure 18. Temperature dependences of peak intensity of meridional maxima in the X-ray pattern of as-spun CPE-2 fibers.

Plastic deformation of fibrous structure slightly deforms the microfibrils by shear displacement of chains in the folded chain crystal blocks thus reducing the sharpness of the meridional maximum of It displaces axially the fibrils and... 

The occurrence of the mesophase in the fiber is confirmed by x-ray diffraction examination. The occurrence of three equatorial reflections 010, 110, and 100, the absence of layer and meridional reflections, and the manifestation of the intensity maximum of diffusively scattered radiation at 20 = 19 in the fiber diffraction pattern are the criterion for the presence of the mesophase. The... 

Meridional is a function g (tp), if its maximum is at 9 = 0. Such distributions describe fiber orientation. 

Figure 10.4. MGZ analysis of an undrawn polyamide 6 fiber, (a) Plot of the peak maximum position as a function of the detector coordinates (x,y). Relations to the parameters of the ellipse and the structure (long periods Lm, Lq) are indicated, (b) Separation of meridional long period, Lm, from the long period L . (courtesy S. MURTHY)...

The tungsten tracer observations and the excess -1(,Pb in the tropical atmosphere are explained by the competing influence of sedimentation, slow mean motions, and eddy mixing. Radioactive aerosols in the lower stratosphere become attached to the natural sulfate particles which are of sufficient size and density to oppose upward transport to levels much above 20-25 km. (30). The coefficient of meridional eddy mixing appears to increase with latitude near the equator (29), with little seasonal change (33). At higher latitudes the rate of poleward transport exhibits large seasonal variations, with a maximum in winter (33). 

We have addressed several aspects of STE of ozone and the impact on tropospheric ozone levels. Using ozone observations in the upper troposphere and lower stratosphere from MOZAIC, we have examined the rdation between ozone and PV in the lower stratosphere. A distinct seasonality in the ratio between ozone and PV is evident, with a maximum in spring and minimum in fall associated with the seasonality of downward transport in the meridional circulation and of the ozone concentrations in the lower stratosphere. The ozone-PV ratio is applied in our tropospheric chemistry-climate model to improve the boundary conditions for ozone above the tropopause, to improve the representativity of simulated ozone distributions near synoptic disturbances and realistically simulate cross-tropopause ozone transports. It is expected that the results will further improve when the model is applied in a finer horizontal and vertical resolution. 

The contrast between these tracers is rooted in the pronounced meridional asymmetry in the tritium pattern (lower-southern-hemispheric values), compared to the more balanced CEC distribution. Another aspect is the northern midlatitude maximum in tritium compared to a poleward enhancement of CEC-11. The latter is a result of the temperature dependence of CEC solubility, but the former arises from the progressive sequestration of tritium in the subtropical gyres, a phenomenon similar to that of radiocarbon (see Broecker et al., 1985),... 

Results of small-angle meridional measurements (Figure 1.5. ) show the absence of small-angle diffraction maximum and this shows the absence of large-period stmcture, that is in this case there is no regular alternation of crystalline and amorphous sections along the axis of the fibre. 

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