Monoclinic crystal form

Transition to monoclinic crystal form. Approximately 200,000 molecular weight. d 4-h inhalation LC50 > 5 mg/L. 

Molecular Weight Dependence of Phase Structure. Similar line shape analysis was performed for samples with molecular weight over a very wide range that had been crystallized from the melt. In some samples, an additional crystalline line appears at 34.4 ppm which can be assigned to trans-trans methylene sequences in a monoclinic crystal form. Therefore the spectrum was analyzed in terms of four Lorentzian functions with different peak positions and line widths i.e. for two crystalline and two noncrystalline lines. Reasonable curve fitting was also obtained in these cases. The results are plotted by solid circles on the data of the broad-line H NMR in Fig. 3. The mass fractions of the crystalline, amorphous phases and the crystalline-amorphous interphase are in good accord with those of the broad, narrow, and intermediate components from the broad-line NMR analysis. 

In examining a crystalline structure as revealed by diffraction experiments it is all too easy to view the crystal as a static entity and focus on what may be broadly termed attractive intermolecular interactions (dipole-dipole, hydrogen bonds, van der Waals etc., as detailed in Section 1.8) and neglect the actual mechanism by which a crystal is formed, i.e. the mechanism by which these interactions act to assemble the crystal from a non-equilibrium state in a super-saturated solution. However, it is very often nucleation phenomena that are ultimately responsible for the observed crystal structure and hence we were careful to draw a distinction between solution self-assembly and crystallisation at the beginning of this chapter. For example paracetamol, when crystallised from acetone solution gives the stable monoclinic crystal form I, but crystallisation from a molten sample in the absence of solvent... 

Below — 20°C the dimeric form crystallizes from the solution, exhibiting an orthorhombic and a monoclinic crystal form for the planar diphosphetane ring, with trans orientations of the aryl substituents. PC single bond lengths within the ring vary between 179.6 and 183.8 pm, therewith being close to the lowest values measured for endocyclic PC bonds of comparable phosphetanes (Fig. 18) [see also Refs. (55b-57,102, 122,137,141)]. 

With the use of f and as in A the implication is that both the compound in question and its constituent elements arc in standard states and that the elements, moreover, are in their reference states for any given temperature the reference states of the elements will normally be those that are stable at the chosen standard-state pressure and at that temperature. A resulting feature of tabulations and 4 f G as functions of temperature for compounds is that discontinuous changes are sometimes to be seen these correspond to changes in the stable reference states of the elements, as phase-transition temperatures are passed. Thus, values of AfH" (S02Cl2,g) would show discontinuous changes at three temperatures corresponding to the transitions S(cr,I)->S(cr,II), S(cr,II) S(/), and S(/) l/2 82(g), where I refers to rhombic and II to monoclinic crystal forms. 

Drikos G, Dietrich H and Riippel H (1988) The polarized UV-absorption spectra and the crystal structure of two different monoclinic crystal forms of the retinal homologue yj-8 -apocarotenal. Eur. Biophys J 16 193-205 Erank HA and Cogdell RJ (1993) The photochemsitry and function of carotenoids in photosynthesis. In Young A and Britton G (eds) Carotenoids in Photosynthesis, pp 252-326. Chapman and Hall, London... 

All isotactic vinyl polymers can be arranged in four ways in the usual crystals. Polypropylene has as stable conformation a 2 3/1 helix that consists of successive tg(—) or tg rotational isomeric states (left-handed or right-handed helices). In addition, it was possible for the helices to have the —CHj groups point up or down , relative to the crystallographic c-axis. In the ideal, monoclinic crystal form I (P2j /c) all four types of helices occur only at specific, symmetry-related positions The equilibrium melting temperature and entropy are 460.7 K and 15.1 J/(K mol), respectively. 

It is quite common for a solid to change from one structure to another at a temperature below its melting point. For example, sulfur undergoes a phase change from the rhombic crystal structure to the monoclinic crystal form at temperatures above 95°C. 

In all of their air-quenched filaments, only the monoclinic crystal form was produced. Crystallization was observed to occur on the spin line, and during crystallization the birefringence rose rapidly. Henson and Spruiell [193], who participated in this study, plotted temperature versus distance from the spinneret for the H-0660 sample (Figure 3.23). Crystallization begins to show on the x-ray at the knee in the plot. 

Sheehan and Cole [185] prepared monofilaments from polypropylene by extruding them into cold water. This rapid quenching gave a pseudohexagonal or a smectic structure instead of the usual monoclinic crystal form. When these fibers were drawn slowly in an oven at 130 135°C, very strong fibers were attained. The properties of the fibers are shown in Table 3.32. 

Sulfur exists as (a) cyclic molecules, Sg, in liquid sulfur below about 160°C and in the rhombic and monoclinic crystal forms, (b) long chains, in plastic sulfur and the liquid form S, and (c) short chains of various lengths, Sg, S4, and Sg, in the gaseous state. 

Dautant A, Meyer JB, Yariv J, Precigoux G, Sweet RM, Kalb AJ, Frolow F. 1998. Sttucture of a monoclinic crystal form of cytochrome hi (bacterioferritin) from E. coli. Acta Crystallogr Sect D Biol Crystallogr 54 16-24. 

The monoclinic crystal form of polyethylene (also referred to as the triclinic form) is a metastable phase formed under conditions of elongation [18,19]. It may be present to a small extent in commercial samples that have undergone cold working after initial molding. Temperatures in excess of 60-70°C cause it to revert to the orthorhombic form [20]. The monoclinic phase is sometimes present in nascent granules of ultrahigh molecular weight polyethylene due to... 

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