Alexander Xenopoulos

Mesophases of Alkylammonium Salts. I. First-Order Transitions

Abstract Thermal properties of symmetric tetra-n-alkylammonium salts are interpreted based on the concept of conformational motion and disorder (condis crystals). A special effort is made to separate the well-known plastic crystalline states for the lower homologs of this series from the condis state. New transition parameters for (C2H5)4NBr, (C3H7)4NBr, (C8H17)4NBr, (C10H21)4NBr, (C12H25)4NBr, (C12H25)4NI, (C16H33)4NBr, and (C18H37)4NBr are added to data in the literature. Three groups of compounds displaying distinct behavior are found: A) Short-chain tetra-n-alkylammonium salts do not melt nor disorder conformationally, they transform to plastic crystals at a single disordering transition, followed by sublimation or decomposition. B) Long-chain tetraalkylammonium salts show conformational disorder and motion before fusion and do not exhibit a plastic crystalline phase. C) In the intermediate alkyl-chain lengths both plastic and condis crystals may exist, not all phases are, however, equilibrium phases.

Heat capacities of solid polyamides

Abstract Heat capacities of 10 polyamides were analysed by using an approximate group vibration spectrum and fitting the skeletal heat capacity to a Tarasov function. New data were measured between 230 and 320 K for nylon 11, nylon 12, nylon 6.9, nylon 6.10 and nylon 6.12 by differential scanning calorimetry. A new method to account for the difference between C p and C v was tested. For all synthetic nylons good agreement could be obtained between experiment and calculation (nylon 6, nylon 11, nylon 12, nylon 6.6, nylon 6.9, nylon 6.10, nylon 6.12 and polymethacrylamide). Two poly(amino acids), (polyglycine and polyalanine) were also analysed and found to have larger discrepancies at higher temperatures, perhaps due to experimental error. The θ temperatures are similar for most polyamides and extrapolate to polyethylene for larger numbers of CH 2 groups.

Mesophases of Alkylammonium Salts. VI. The Crystal Structures of Tetra-n-butylammonium Bromide and Iodide

Abstract The structures of tetra-n-butylammonium bromide,N(C4H9)4Br (abbreviated: 4Br) and tetra-n-butylam-monium iodide, N(C4H9)4I (abbreviated:4I) have been determined. For 4Br: Mw = 322.4 Da, c-centered monoclinic space group C2/c, a = 14.409(5) A, b = 13.901(2)A, c = 20.345(9)A, β= 110.18(3)A, V =3729.4A3, Z =8, Dc = 1.148 g/cm3, F(000)= 1392, R =0.098 for 1311 reflections. For 41: Mw = 369.4 Da, c-centered monoclinic space group C2, α = 14.482(4)A, b = 14.318(4)A, c = 19.975(7)A, β = 110.86(2)°, V =3870A3, Z = 8, Dc = 1.230 g/cm3, F(000) = 1536, R = 0.142 for 1899 reflections. The structure analysis shows that the n-butyl chains are distorted to different degrees from their all-trans conformations and are connected through the quaternary nitrogen with one or two gauche- bonds. The plastic crystalline mesophase for 41 seems to be possible because of a lower packing fraction that produces larger deviations of the bond-rotation angles from the intramolecular energy-minimum, not found for 4Br. The condis...

Thermodynamic Characterization of the Plastic Crystal and Non-Plastic Crystal Phases of C70

Abstract Heat capacity measurements were performed on chromatographically purified C70 from 120 to 560 K by DSC. The experimental values were linked to the vibrational heat capacity, calculated based on normal-mode vibration frequencies available from the literature. Good agreement is found, as prior for C60 Major orientational disorder is introduced in two closely spaced transitions, reflecting the change to the phase that displays anisotropic rotation of the rugby-ball shaped molecules. The total entropy of the disordering transitions is 22.4 J/(Kmol), slightly lower than that of C60, but additional entropy is gradually acquired between 120 K and the transition temperature, making the total entropy gain higher than that of C60, as expected for a less symmetric molecule. The Debye temperature for the six lattice vibrations of C70 was calculated to be 45 K, compared to 53 K for C60 No melting temperature could be detected for C70 up to 950 K, but the entropy of fusion is estimated to be 12 J/(K mol). Enth...

Mesophases of Alkylammonium Salts. V. Heat Capacities of Symmetric Halides with Alkyl Chains Longer than Ethyl

Abstract The heat capacities of tetra-n-alkylammonium bromides and iodides in the solid state have been measured from 130 K through their respective transitions and fitted at low temperatures to two θ temperatures, using group vibrations computed from the vibrational spectra and the Tarasov analysis, as developed for linear macromolecules. The compounds analyzed were (C3H7)4bNBr, (C4H9)4NBr, (C5H11)4NBr, (C6H13)4NBr, (C7H15)4NBr, (C8H17)4NBr, (C10H21)4NBr, (C12H25)4NBr, (C16H33)4NBr, (C18H37)4NBr, (C3H7)4NI, (C4H9)4NI, (C5H11)4NI, (C6H13)4NI, (C7H15)4NI, and (C12H25)4NI. The total vibrational contribution to the heat capacity is thus derived and compared to the experimentally measured heat capacity over the whole temperature range. Positive deviations of the measured heat capacity from the calculation occur already below room temperature, implying the existence of large-amplitude, nonvibrational motion in the solid state, as suggested before by solid-state 13C NMR analysis. Quantitative information on the...

Mesophases of Alkylammonium Salts III. Disorder and Motion in the Intermediate Homologs: [CH3(CH2) n-1]4NX, n = 4, 5, 6, 7 and X = Br and I

Abstract Disorder and motion in crystals and mesophases of tetra-n-alkylammonium halides of intermediate chain length alkyl groups, [CH3(CH2) n-1]4NX, where n = 4, 5, 6, 7 and X = Br and I, have been analyzed based on entropy changes of phase transitions known from differential scanning calorimetry (DSC) and new measurements of solid state 13C nuclear magnetic resonance spectra (NMR). Some mesophase transitions from the crystal state involve only conformational motion and disordering to lead to condis crystals. The conformational motion and disordering in the alkyl chains may also be introduced gradually (transitionless change to a condis crystals). Further heating of the samples increases the degree of conformational disorder (entropy) to reach different, more disordered condis states, or causes orientational motion and disorder of the cation as a whole to form plastic crystals, which in this intermediate alkyl chain-length group of salts contain always conformational disorder. Finally, isotropization oc...

A possible glass transition for poly(l-methionine) and poly(l-serine)

Abstract The possibility of a low-temperature glass transition is proposed for two poly(amino acid)s: poly( l -methionine) and poly( l -serine). The suggestion is based on deviations of the experimental heat capacity from values calculated from the vibrational spectrum. The discrepancy consists in an increase of the heat capacity over a wide temperature range (260–400K) to values more typical of liquids. Qualitative experiments are performed that show the rubbery nature of the samples above room temperature and brittleness below. The broadness of the proposed glass transition may be caused by the presence of some low level of crystallinity. It is speculated that the motion begins in the side groups around room temperature and causes a gradual devitrification of the whole molecule at higher temperatures.

Mesophases of Alkylammonium Salts II. Disorder and Motion in the Low Homologs: [CH3(CH2) n-1]NX, n = 1, 2, 3 and X = Br and I Analyzed by Solid State 13C NMR

Abstract Disorder and motion in crystals of tetra-n-alkylammonium halides of short chain length alkyl groups, [CH3(CH2) n-1]4NX, where n = 1, 2, 3 and X = Br and I, have been analyzed based on solid state NMR. The solid-mesophase phase transitions involve orientational motion and disorder of the cation as a whole, so that the mesophases must be identified as plastic crystals. Except for the methyl rotation, the conformational motion, i.e., the rotation about single-bonds, is limited to small-angle librations that do not reach other conformational isomers. The analyzed salts show, thus, a plastic-crystal me-sophase without conformational disorder. The information on the disorder and motion, obtained initially by thermal analysis of the transition, agrees well with the more detailed NMR results.

Investigations of the transitions of some tetraalkylammonium halides by DSC, NMR, and X-ray diffraction

Abstract The transitions of several symmetric tetraalkylammonium halides were studied by differential scanning calorimetry (DSC), solid-state nuclear magnetic resonance (NMR) and X-ray diffraction. The reversibility of the transitions of Bu 4 NI is investigated by cycling of the samples in the DSC. A plastic-crystalline phase is found and characterized. A glass transition is observed for Bu 4 NCl at 230 K. Solid-state CP-MAS 13 C NMR is performed on Hex 4 NI in order to reveal the large-amplitude molecular motion. The DSC transitions are confirmed by changes in the chemical shifts, but additional, more continuous changes are seen. Condis-crystalline phases are probable for the latter two salts. Room-temperature X-ray diffraction on various iodides shows that the appropriate d -spacings increase linearly with size of the alkyl groups.

Defect generation and motion in polyethylene-like crystals, analyzed by simulation with supercomputers

Defects in polymers were observed by high resolution electron microscopy and inferred from their mechanical and dielectrical behavior. The details of their generation was not known, however, in the past. During the last few years we have been able to extend the molecular dynamics simulation of polyethylene to crystals containing up to 6100 atoms and to times as long as 100 ps. The major observation was that single bond rotations of more than 90{degree} become possible already more than 100 K below the melting temperature. These defects have lifetimes of only a few ps. By coupling to kinks (2g1) they can extend their lifetime considerably. Addition of a thermal, mechanical or dielectric free energy gradient to the thermally created defects seems to be able to account for the microscopic motion needed to explain the macroscopically observed annealing, deformations and relaxation effects. Key to the mechanical and dielectric properties is thus the existence of conformational disorder (condis crystal). 47 refs., 9 figs.