718
A. ODAJIMB, J. A. SAUEjR, AND A. E. J$O ' ODTYARD
species as the w-value of 1.64 makes the ionization potential lower. This value is 3.45 and yields the affinity values (in the same order) of -1.4, -4.2, - 1.0, and $0.4. These are quite close to the corresponding values reported previously. We may only conclude from these remlts that the inclusion of overlap does not provide significantly better agreement with the electron affinities of Pople than is obtained from the Hiickel MO's. The ionization potentials, it would seem from this analysis, also are not significantly different when overlap is included. Summary Detailed comparisons of the w-technique and Pople closed form solutions for the ionization potentials of the cyclic polyenes unambiguously indicate these methods to be fundamentally different. With the parameters presently used for both methods, poor agreement for the potentials of very large molecules is to be expected. Whether either method will yield the correct results for such molecules is a question which must await further experimental work. I n the meantime, the correlational usefulness of the w-technique for the smaller organic molecules of common interest so
Vol. 66
broadly demonstrated by Streitwieser should not be overlooked. Extrapolation methods introduced to hasten convergence or circumvent divergence in the cycling procedure are shown to be useful in some calculations. The question of whether a particular case of divergence should be subjected to extrapolation is raised. If the cause is due to the initial choice of parameters or Huckel orbitals and a continuous functional dependence of the property of interest (e.g., energy) upon the latter exists, it would seem that extrapolation is justified. The possibility of intrinsic limitations of the w-technique preventing the correct charge density from being self-consistent must be considered, however. An erroneous result may thereby be obtained upon extrapolation. Until this distinction is more clearly made, justification can only be a posteriori; if the result agrees with experiment, extrapolation seems justified. The extension of the technique t o overlap included ill0 methods also was investigated in terms of some of the closed form cases. For calculations of ionization potentials and electron affinities at least, little if any improvement is to be found over the overlap-included results.
PROTON AIBGNETIC RESONAKCE OF SOME NORMAL PARAFFINS AND POLYETHYLENE1 BY A. ODAJIMA, J. A. SAUER,AXD A. E. WOODTARD Department of Physics, Pennsylvania State University, University Park, Pennsylvania Received October 12. 1961
Proton magnetic resonance spectra have been obtained in the temperature region from 77°K. to the melting point for eleven normal paraffins including d&H190 and for two preparations of polyethylene crystals grown from solution having fold periods of 112 and 120 A. At, or a few degrees below, the crystal-crystal transition point, the n.m.r. line narrows markedly in a 1-2" range for n-C1QH40,nG1H44, n-CZ8H58, n-C32H66, and n-C35HT2ftnd at, or just below, the melting point a narrow line characteristic of that found with liquids is obtained. The line-narrowing rocess for n-CSBHT4 takes lace in two steps due to the presence of two crystal-crystal transitions. These transitions are b e h r e d to be accompanied Ey the onset of long-axis rotation of the paraffin molecule. The results indicate that considerable methyl group rotation occurs in the n-paraffins at 77-100°K. The s ectra at temperatures 20-30" below the transition oints for n-C3&j, n-C35Hn! and nC36H74 are found to be markedly &pendent on thermal history and purity. For n-Cap&o and n-C94Hm, line narrowing, presumably due to torsional oscillation takes place about 25 and 50' below the melting point, respectively. The appearance of a component 0.1 to 2 gauss in width at temperatures of ~ 2 0 0 ° K for . CISto -300°K. for was found to be dependent on sample history and purity, One polyethylene crystal preparation (120 d. fold period) showed a narrow and broad component at 300"K., both of which underwent further narrowing a t higher temperatures. Heating the polyethylene crystals at tern eratures from 390°K. to the melting point shifted the fine line-narrowing process to lower temperatures and the broad {ne-narrowing to higher temperatures causing these processes to be coincident with those for melt-formed polymer. crystals showed no narrow line at 300°K. but one The polyethylene crystal sample with 112 A, fold period and the n-C~dHi~o appeared after subsequent heat treatments. This component found for C Mand polyethylene is attributed to motion of chain segments in crystal defects.
Introduction Phase changes and other transformations taking place in normal paraffin compounds in the solid state have been investigated by various physical methods, including: X-ray c r y s t a l l ~ g r a p h ypro,~~~ ton magnetic resonance4 (n.m,r.), cooling curves13 specific volume,6 and calorimetry.6 Using n.m.r., (1) Supported by the U. 8. Atomic Energy Commission under Contract AT(30-1) 1868. (2) A. Mliller, Proc. Roy. Sac. (London), Al20, 437 (1928); 8137, 417 (1930); A l W , 514 (1932). (3) J. D. Hoffman and B. F. Decker, J . Phys. Chem., 67, 620 (1963). (4) E. R. Sndrew, J . Chsm. Phys., 18, 670 (1950).
Andrew4 investigated three normal paraffins with an even number of carbon atoms: n-octadecane (GI*), n-octacosane (Cz8), and n-dotriacontane (C,,), and reported the presence of considerable motion in all three compounds a t temperatures well below their respective melting points. For Cl8 a t 95OK., the lowest temperature employed, Andrew reported an experimental second moment well below the calculated rigid-lattice value. On (5) P. R. Templin, Ind. Eng. Chem., 48, 164 (1956). (6) A. A. Schaerer, C. J. Busso, A. E. Smith, and L. B. Skinner, J. Am. Chem. Sac., 7 7 , 2017 (1965).
April, 1962
P R O T O N h!!AGNETIC
RESONAXCE O F %-PARAFFIN!
the other hand, and C32were believed to be in a rigid-lattice state a t 95OK., but did undergo motional narrowing about 100O below their respective melting points. In order t o obtain further information about motion in the solid state, proton magnetic resonance spectra have been obtained for eleven normal paraffins and one branched paraffin a t various temperatures in the range from 77OK. t o the respective melting point. The materials studied include a consecutive series of four compounds: CIS, n-nonadecane (CH), n-eicosane (Cm), and nheiieicosane (C,,), as well as eight others : hexadecane (@le), 2-methylheptadecane (2MHD), CB, C32, n-pentatriacontane (Cs5), n-hexatriacontane (c86),n-tetratetracontane (c,,), and n-tetranonacontane (Cg4). Some effects of thermal history and impurities d s o have been investigated. The results of these measurements will be presented and discussed herein. Proton magnetic resonance spectra also have been obtained over a wide temperature range on two samples of polyethylene single crystals grown from solution. It is known' that such crystals have a structure a t room temperature identical with that for crystals of normal paraffins with the c h a i p being packed normal to layers of thickness 90 A. or greater. However, for polyethylene this type of packing requires that the chains be folded back on themselves. N.m.r. studies of solutiongrown crystals of polyethylene first were given by Herring and Smiths and marked differences found between such a sample and melt-formed specimens. Results also have been published by ~ Slichter.lo I n this paper, Peterlin, et c ~ l . , and the n.m.r. data for the two samples of polyethylene crystals with fold periods of 112 and 120 A. are compared with those obtained for various normal paraffins. Thermal treatment of polyethylene crvstals is known to affect segmental mobility. For example, Slichterla found for single-crystal preparations, which had been heated between 100' and the melting point, that the segmental mobility became irreversibly greater as the annealing temperature was increased. This type of treatment also is found11J2 t o increase irreversibly the long spacing of the crystal lamellae as a consequence of chain refolding. I n the present investigation, the effect of thermal treatment and annealing on the occurrence of segmeiital motion in polyethylene crystals has been studied further by examining two crystal preparations with different fold periods which had received prior heat treatments. Experimental The apparatus arid procedures used in this study have been described previ0us1y.l~ For samples exhibiting the (7) A. Keller and A. O'Connor, Natwe, 180, 1289 (1957). (8) M. J. Herring and J. A. S. Smith, J . Chem. Soc., 273 (1960). (9) (a) A. Peterlin, 3%. KraSoveo, E. Pirkmajer, and I. Levstek, Mucromol. Chem., 37, 231 (1960); (b) A. Peterlin and E. Plrkmajer, J . Polymer Sei., 46, 185 (1960). (10) (a) W. P. Slichter, J . A p p l . Phys., S I , 1865 (1960); (b) 32, 2339 (1961). (11) A. Keller and A. O'Connor, Discussions Faraday Soc., 25, 114 (1958). (12) W. 0. Statton and P. H. Geil, J . A p p l . Polymer Sci.,'s, 357 (1960).
AND POLYETHYLENE
719
narrow n.m.r. lines of width < 0.5 gauss care was taken to avoid modulation broadening by using modulation widths of 0.25-0.04 gauss and a slow scanning rate of 0.2-0.1 gauss/min. To avoid saturation broadenin a t low temperatures a radio-frequency intensity below t i a t where such effects were noticed was used. The experimental second moments were calculated from the central derivative absorption peak, the limits being where d[f(H)/dH] goes to zero, as determined experimental1 . The samples of C18(m.p. as"), sample no. 1 (m.p. 56"), and Css sample no. 1 (m.p. 68.5") were purchased from Humphrey-Wilkinson, Inc. A comparison of the melting points given here for C Zand ~ C ~ with Z those in the l i t e r a t ~ r e "indicate ~ that these samples contain some impurities, the amounts of which are not known. Upon redistillation and recrystallization a sample of the CIS was obtained with a 98.5% purity as determined from the boiling point. Other paraffin samples, including Cle(rn 18.0°), BMHD, Cls(m.p. 32.O0), Czo(m.p. 36.6"), sample no. 2 (m.p. 61.1"), C32 sample no. 2 (m.p. 69.3'), C35(m 73.4-73.8"), Cae(m.p. 75.9"), Cer(m.p. 86'), and (m .p. 114.1-114.5°) were high-purity samples supplied by the American Petroleum Institute Project NO.42 a t the Pennsylvania State University. The sample of Czl(m.p. 39.8")1* was furnished by Professor E. R. Fitzgerald of the Pennsylvania State University. Except for the sample of Cg4, most of the n.m.r. measurements were made on solid samples of the above paraffins obtained by cooling from the liquid state; samples with elevated meltin points were removed from the oven and allowed to coo? in the laboratory atmosphere. As noted on the figures to follow, some samples of the same hydrocarbon were stored at room temperature for 7-21 days prior to measurement and others were measured immediately. N.m.r. measurements also have been made on crystal aggregates of CZ?,C3j, and '294. These crystals were prepared from acetone in the case of CZIand from n-pentane in the case of C35 and Cs4and were vacuum-dried for 48-72 hr. a t 300°K. Following the n.m.r. measurements on this C S ~ sample, crystals again were grown using the same material and these were dried a t 350°K. under vacuum for 48 hr. I n some cases, solid mixtures of paraffins were made up by mixing the components in the liquid state. One of the polyethylene crystal preparations was supplied by Dr. A. Keller of the University of Bristol, who reported a 120 d. fold period, as obtained by low angle X-ray diffractioq. The second sample, with a reported fold period of 112 A., was supplied by Dr. F. E. Karasz of the National Physical Laboratory, England; this sample was prepared from a 0.1% xylene solution and was dried under vacuum for 150 hr. a t 53" prior to use. Subsequent heat treatments of specimens from both preparations were carried out in a vacuum oven at the temperatures indicated on Fig. 7 and 8.
28
2;
8s;
Results N.m.r. spectra have been obtained for meltformed samples of CI8,C19,Czo,and Czl a t various temperatures from 77'K. to the respective melting points. The width of the broad component was found to be 15.5 rt 0.3 gauss at 77OK. This width decreased slowly with increasing temperature, reaching values of 14 gauss at 270'K. At the melting point for GIs, the line narrows from a value of 13 gauss t o that observable for liquids (
