Nonequilibrium States of Symmetric Tetraalkylammonium Halides
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53 Mat. Res. Soc. Symp. Proc. Vol. 321. 01994 Materials Research Society
purities were approximately 99%, except for 4C1 (92%). The abbreviated nomenclature used by us gives the number of carbon atoms in the alkyl group and designates the anion, so that 4C1 stands for tetra-n-alkylammonium chloride. For the DSC experiments a PerkinElmer DSC-7 was used, operating with a heating rate of 10 K/min and a constant flow of dry N2. Temperature and heat-flow calibrations were carried out as before.4 Measurements were made from 220 K to decomposition on 5-25 mg samples. A typical sequence of runs for the detection of nonequilibrium effects included an initial heating to the melt, followed by cooling, a second heating to the melt, quenching at nominally 500 K/min, and a final heating to the melt. Measurements were made during all, but the quenching step. The results from measurements on close to equililbrium samples were described earlier;1"4 the nonequilibrium behavior is discussed here. SALT Supercooling (K) Any weight loss during this sequence of thermal analyses (due to decomposition or Bromides evaporation) was kept to less than 3%. C2H5 20, (PC-.PC), 21 (PC-C) For polarizing microscopy, an Olympus zoom stereo-microscope with up to 40x C3117 4 (PC-PC), 46 (PC-C) magnification was used. Temperature was CAH, 50 (M-CC) controlled with a Mettler FP-82 hot stage, governed by a FP-80 processor. MicroscoC5Hj1 19 (M-C) py was needed mainly to identify crystals C,H,3 49 (M-CC) and their transitions. Plastic crystals usually lack birefringence because of their cubic C7H,5 24 (M-CC), 18 (CC-C?) symmetry, while condis crystals often have C,1117 17 (M-CC), 10 (CC-C?) higher birefringence than rigid crystals. The melt and plastic crystalline states, both ClOH 21 14 (M-CC), 8 (CC-C) without birefringence, can be distinguished CnH25 11 (M-CC), 31 (CC-C?) by their appearance (droplets or crystals). The results of the cooling experiC1,Hu 15 (M-CC), 12 (CC-C) ments at 10 K/min are summarized in the CjgH37 12 (M-C) Table by listing of the supercooling from the equilibrium transition temperatures Iodides established earlier."14 Typical examples of C2H5 23 (PC-C) heating and cooling sequences are shown in Figs. 1 and 4, below, heat capacity analyC3H7 26 (PC-C) ses of conformational disordering can be C4H, 24 (M-PC), 28 (PC-C) seen in Figs. 2 and 3. CsHjj 17 (M-C) C6H 13
10 (M-CC), 4 (CC-CC), 7-17
DISCUSSION
(CC-C)
A liquid sample can solidify either by ordering sufficiently so that the largeC12H2, 11 (M-CC), 3 (CC-CC), 13 (CC-amplitude positional, orientational, and _ C?) conformational motions are frozen (crystallization), or its large-amplitude motions Table Supercooling for transitions of tetra-n- can freeze at a glass transition without alkylammonium halides (10 K/inin cooling change in order. In case the ordering on rate). The phases are identified as C (crystal), crystallization is incomplete (mesophase PC (plasticcrystal), and CC (condis crystal). C7H1 s
11 (M-CC), 17 (CC-C?)
54
formation), final solidification occurs at a lo
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