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X‐ray diffraction patterns of linear and branched polyethylenes typically show two sharp reflections and an amorphous halo. The position of the halo depends on branch content and temperature. A single curve describes the position of the halo maximum (2θhalo) for a range of liquid hydrocarbons and polyethylenes in the 20–140°C range. At temperatures well below their melting point, branched polymers give 2θhalo values which differ significantly from those observed for the liquid Linear polymers show a greater divergence, indicating that some of the material giving rise to the halo is much better packed than in the liquid.Parallel 13 C NMR spin‐lattice relaxation studies suggest that this relatively ordered material has a trans conformation but a low average T1c value. © 1993 John Wiley & Sons, Inc.

Can. J. Chem. 53,878 (1975).Phase relationships in the copper/selenium system in the composition range 30-70 atomic % selenium have been studied a t temperatures from 298 to 850 K and at pressures to 5 0 kbar. A revised atmospheric pressure phase diagram is given, as well as an outline of the phase diagram a t 20 kbar. aCu,Se is monoclinic at 298 K with a = 14.087, b = 20.481, c = 4.145 A, P = 90" 23'. The a -, p (f.c.c.) transformation is complex, and occurs over a 30 K interval centered o n 396 K.The overall enthalpy change is 6.4 f 2 kJ mol-'. The maximum in the DTA signal for this transition decreases slowly with increasing pressure a t < 1 K kbar-'. The signal disappears above 42 kbar, presumably due to the formation of a high pressure modification.Variable composition in CuZ-,Se (berzelianite) extends from Cu,.,,Se to Cu1.,,Se in the range 402-523 K. C U , .~, S~, with a = 5.765 at 298 K, is stable throughout the temperature and pressure ranges investigated.Cu3SeZ (umangite) is stable a t 298 K to a t least 35 kbar. Dimensions of the tetragonal cell at 298 are a = 6.385, c = 4.217 A. At atmospheric pressure it disproportionates to Cu,-,Se and pCuSe a t 386 K with an enthalpy change of 10.0 f 5 kJ mol-'. The reverse reaction is very slow. Above ca. 5 kbar it disproportionates to CuZ-,Se and CuSe,II at a temperature less than 413 K. The reverse reaction is fast. clCuSe (klockmannite) is hexagonal with a = 3.934, c = 17.217 a t 298 K. It transforms to pCuSe a t 323 K under atmospheric pressure with an enthalpy change of 0.84 f 5 k J mol-'. This modification is C end-centered orthorhombic with a = 3.948, b = 6.958, c = 17.239 at 324 K. With increasing temperature the orthorhombic a/b ratio increases rapidly until, at 393 K, the transition to yCuSe is complete. This modification is hexagonal with a = 3.984, c = 17.288 a t 430 K. CuSe is unstable above 5 kbar at 298 K, decomposing to form CuaSez and CuSe211.Marcasite-type CuSe,, with a = 5.0046, b = 6.182,, c = 3.739, A at 298 K, disproportionate~ to CuSe and Seat 605 K under atmospheric pressure with a n enthalpy change of 9.6 + 4 kJ mol-'. It transforms a t less than 5 kbar a t 298 K to the cubic pyrite modification CuSeZII, Du CuSez du type marcasite avec a = 5.0046, b = 6.182,, c = 3.739, A a 298 K, se disproportionne en CuSe et S e a 605 K a pression atrnosphkrique avec un changement d'enthalpie de 9.6 f 4 kJ mol-'. I1 se transforme a moins de 5 kbar a 298 K pour conduire la modification cubique de pyrite CuSe'II avec a = 6.1 16

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The low temperature modifications of the normal paraffins n-CnH2n+2crystallize in three groups (Broadhurst, 1962). The structure is triclinic for n even, 6 < n < 26 (Muller and Lonsdale, 1948; Nyburg and Luth, 1972); orthorhombic for n odd, 11 < n < 39 (Smith, 1953; Teare, 1959); and monoclinic for n even, 28 < n < 36 (Shearer and Vand, 1956). In all of these structures the hydrocarbon chains are linear and in trans configuration. The chains are parallel to one another, the terminal methyl groups forming the surfaces of lamella which are more or less perpendicular to the chain axis. For n < ca.36, it is apparently the interlamellar interaction between end methyl groups which dictates the symmetry. For longer chains the structure is usually orthorhombic and comparable to the structure of highly crystalline polyethylenes. Chains do not fold (as they undoubtedly do in polyethylenes) unless n is greater than 102 (Bidd and Whiting, 1985; Ungar and Keller, 1986).The several crystal forms differ in the manner in which the nearest neighbor chains are related to one another. In the triclinic lattices the packing is such that a triclinic sublattice containing one methylene group is evident. In the other modifications the sublattice is orthorhombic and contains four methylene groups. If the overall symmetry is orthorhombic the long chain axes are perpendicular to the interlamellar surface; the x and y translations, perpendicular to the long axis, are common to both cells. If the nearest neighbor chains are displaced by two or four methylene groups along the chain axis, overall monoclinic symmetry results (Sullivan and Weeks (1970)).

The phase structure of random copolymers of ethylene and ethylene‐d4 with 1‐octadecene and other 1‐alkenes has been investigated. CPMAS 13C NMR spectra show that a fraction of the central sections of C16H33 side chains in ethylene‐d4 copolymers are in ordered environments at 298 K. They give rise to resonances from 32.9 ppm to 33.8 ppm, which show that they are in trans conformations; T1C values for this group of resonances range from 1 s to 7 s. The remaining side chains are in an amorphous environment, the internal methylenes having a chemical shift of 30.8 ppm and a T1C close to 0.4 s. A Raman band at 1062 cm−1 in the spectrum of an ethylene‐d4‐1‐octadecene copolymer is consistent with partial crystallization of side chains. Some side‐chain crystallization also occurs in a 1‐tetradecene copolymer. X‐ray diffraction studies suggest that smaller side chains do not crystallize to a significant extent at 298 K. © 1996 John Wiley & Sons, Inc.

Random copolymers of ethylene with 1‐butene, 1‐octene, and 1‐octadecene have been prepared using a homogeneous vanadium‐based catalyst system. Comonomer contents determined by 13C‐NMR analysis of polymer solutions are in the range 1–10 mol%. Crystallinities were estimated by means of density measurements, x‐ray diffraction, differential scanning calorimetry, laser Raman spectroscopy, and CPMAS 13C‐NMR spectroscopy. The results are compared with those obtained for heterogeneous copolymers of ethylene containing 1–4 mol% 1‐butene. As the comonomer content is increased, the crystallinity decreases. The dimension perpendicular to the 110 plane in orthorhombic crystallites decreases linearly with crystallinity. This decrease in crystallite size is accompanied by an increase in the size of the orthorhombic unit cell. For copolymers containing large amounts of 1‐octene and 1‐octadecene, a second crystalline form appears. Differences in estimates of crystallinity are discussed in terms of looser packing in highly branched copolymers and the extent to which the second crystalline form participates in the phase structure.