MARKE. DRYASD RALPHA. BEEBE
1x0
Tul. G4
ADSORPTION STUDIES ON BOKE MINERAL AlVD SYNTHETIC HYDROXYA PATITE1r2 BY MARKE. DRYAND RALPHA. BEEBE Department of Chemistry, Amherst College, Amherst, MassachusetLs Received March $9, 1960
Adsorption isotherms have been determined and calorimetric heats of adsorption have been obtained for the adsorption of both methanol and water vapors on hydroxyapatite dehydrated a t 450’ both in the form of anorganic bone and in the form of synthetic hydroxyapatite. In the case of the methanol, it was found practicable both on the basis of the isotherms and the differential heat data to separate the total adsorption more or less sharply into “chemisorbed” and physisorbed fractions. In the case of water such a separation was less clearly delineated. The adsorption characteristics differed remarkably little whether the adsorbing hydroxyapatite surface was of synthetic origin or derived from bone. The relatively high heats of adsorption in the first monolayer, for both methanol and water, indicate a high energy of binding on the hydroxyapatite and can probably be attributed to hydrogen bond formation involving the exposed oxygen atoms of the surface. Some experiments have been done with nitrogen adsorption a t -195’ on the dehydrated hydroxyapatite and on the surface of this material covered by a chemisorbed layer of methanol or of water. It is found that the chemisorbed layers have a rather profound influence on the subsequent physisorption of nitrogen.
Introduction Because of the importance of the surface chemistry of bone mineral in physiological systems, we have undertaken a series of gas adsorption studies on hydroxyapat~iteboth in the synthetic form and in the form of anorganic bone. The present investigation has been particularly concerned with the adsorption of two polar subst’ances, water and methanol, on dehydrated hydroxyapatite surfaces. Because of the ionic character of the hydroxyapatite, we might expect that the polar adsorbate molecules would be strongly bound to the surface and probably oriented with respect to it. To test this supposition me have determined the adsorption isotherms and have measured the heats of adsorption of these polar adsorbates by means of a calorimet,er. The interpretation of the experimental results has enabled us to draw a number of conclusions. In particular we have been able to sort out the LLchemisorption”16 from the physisorptioii in the total adsorbed film, to compare the states of the adsorbing surfaces of the hydroxyapatites from synthet,ic and from natural sources and to rompnre and contrast the characteristics of adsorbed films of water and of methanol on the same 1iydroxyapat)it’esurface. Previous calorimetric ineasuremeiits have been concerned n-it,h t’he adsorption on solid surfaces of polar molecules from the vapor p h a ~ e . ~ -Re~ ceiitly several iiivestigators have employed preclision liquid immersion calorimetry to study the inter:tction of water and other polar molecules with silicafi-* and otherg surfaces. lion-ever we halx ( 1 1 Presentrd in the Symgosinm on Energetirs of Surfaces a n d Interfares hefore the Division of Colloid Chemistry of the American Ciieniical Society, September 7-12, 1958, in Chicago, Illinois. (2) This research was supported by Grant NS:F-G27G.i from the Chemint,ry Branch of the Division of Physical and Engineering Sciences of the National Science Foundation. The work leading to a previous publication [R.A. Beebe and E. R. Camplin, T H IJOURNAL, ~ 63, 480 (1939)] alao received support from the above grant. Our thanks are due to the Foundation for the support which made these tn.0 invostigations poasihle. ( 8 ) A . 1’. Iiiselev, Kolloidny Zhurn., 20, 339 (1958); A. V. K i d e v , N. V. Kovaleva, V. 8.Sinitsyn and E. V. Khrapova, ibid., 2 0 , 444 (1958). (4) B. Millard, R. A. Beehe and J. Cynarski, TEXIS JOURNA 68, L,
468 (1954). (5) C. H. .4mhers. J . A n . Chem. Soc.. 79, 3980 f1957). (6) M . h l . Egorov. Y. E’. ICiseler. IC. G. Krasil’nikov and V. Y. hlurina, ithad. Nauk S.S.S.R., ( J . Plrys. Chem.), 33, No. 1 (1959).
found no record of such studies on hydroxyapatite. I n order to determine the specific surface areas of the adsorbent materials, we have used the B.E.T. method based on the adsorption of nitrogen a t - 195O.lo I n this phase of the work we have determined the adsorption of nitrogen not only on the dehydrated surfaces but also on these surfaces when they already are covered with chemisorbed monolayers of water or methanol. We have observed that these chemisorbed layers exert a marked effect on the subsequent physisorption of nitrogen. A similar effect has been noted by other investigator~.~~-~~ Experimental Materials.-Thc physical chemistrv of ltotli bone mine1al and synthetic hydroxyapatite has bern reviewed in detail by Seuman and Neuman l 4 It is known from electron microscope studies that both bone minerall6 and synthetir hydroxyapatite have a micro-crystalline structure whirh leads to high specific surface areas which ar(1 of the order of magnitude found by the B.E.T. method. The composition and structure of fluoroapatite has been thoroughlv investigated; due to the similarity of the X-ray patterns of hydrovyapntite to that of fluoroapatitr, the unit re11 of the hvdrovyapatit e is, by analogv, taken t o be Ca10(1’04)6(OH), Whrther hvdroxyapatite cornrs from a nntural or :t synthetic soiirce, it has a rariable romposition, which sernlb i n large part to bc due to the substitution of varisble amonnts of I-13C)+ions for C:t+.+ions in the crystal surfaces. Thus it I S necessary to rYer(isc spcci:tl c v r in the contiol of oiitgassing procedurrs in prep:tr ttion for adsorption runs Wf,have found that a fairly reprodurihlr adsorbing surfaw 1~ ohtainrd by overnight degassing a t 450”. The loss of weight for the hone miner:tl with tempcrnturr follows :L p:tttern qriitc similar to that ohserved hy T i u n 1 1 ~for ~ ~synthetic material. Thus, although the per writ Iosq of wzter rhnnges with temperntiire of outgassing rsperi:tlly in thc region from 100 to 400°,the rate of changr of loss with temperature is smalIer betwvecn 400 and 500”. If the (7) A. C. hlakrides and N. Hackerman, Trris Jorynshr., 63, 594 (1958). ( 8 ) G. J. Young and T. P. Bursh, J . Colloid Sci., in prcss. (9) A. C. Zettlemoyer. C,. J. Young, J. J . Chessick and F. €I. IIealey. THISJOCRNAL, 57, 649 (1953). (10) (a) E. P. Barrett, I.. G. Joyner aad P. P. Ilalenda, I n d . Eng. Ckem., 43, 639 (1951); (h) 44, 1847 (1952). i l l ) F. 5. Stone and P. F. Tileg, ‘Vature, 167, 054 (1951). (12) A. V. Kiselev. N. V. Kovaleva, A. Ta. Rorolev and IC. D. Shcherbskova. Compt. rend. Acad. Sci. U R S S ( D o k l n d y .4 lzademii nauk S S S R ) , 124, 617 (1959). (13) D. S. Mclver and H. H. Tobin, TrrIs . J ~ U R S A L . 64, G83 (1960). (14) W. F. a n d M. W. Neuman, “The Chemical Pvnaiiiics of Bone blineral,” Univ. of Chicago Press, 1858, p. :39: (ref t o Kilnin, p. 43). (15) R. A. Robinsonand I1.A. Cameron. J . P?oph!,o?.BioctLcm. Cytol., 2, 2.53 ( 1 9 X ) .
Sept., 1960
ADSORPTION STUDIES os BOKE_~IIXER.U, A N D SYNTHETIC HYDROXYAPATITE 130 1
heating process is estende:! into the 500 t o 800" range, the, hydroxyapatite, stable below 500°, it; broken down with further loss of water and production of tricalcium phosphate. The anorganic bone mineral was supplied by the courtesy of the Research Division of hrmour and Company, Chicago, Illinois. This material had been freed from collagen and other organic matter by ethylenediamine extraction. Th(: sample used in d l t,lie work reported below was the Ossar Bovine Femur Head lot 34. The loss of weight whirh occurred on evacuation of this material a t 500" was made up of 3 to 4% Hd), 1% CO? and 0.3y0of gas uncondensable a.t liquid air temperatures. The amount of water present i n the initial samples and hence the amount lost on evacuRtion a t high temperature was dependent upon the humidity of the atmosphere to which the sample had been exposed before degassing. Slight charring of the initially whit(% bone mineral was observed to occur a t temperaturcs abovc, 200'. The matcrial used in the: experiments described below was first treated in a stream of oxygen a t 500" and in s u b sequent evacuations the temperature was 450'. The specific surface area of this material after dehydrating at 450" was 100 m.* per g. (For brevity in this paper the adsorbent described above will be designated simply as the bone mineral or Ossar.) The synthetic hydroxyaphtite used in the present work was kindly supplied by Dr. W. F. Xeuman of the University of Rochester Medical School. The sample was taken from the apatite preparation batch "31" described in the book bv Neunian and Xeuman.14 In this material the Ca/P atomic ratio is given as 1.48. The loss in weight (H20) on evacuating a t 450' wa.s found to be 5 to 6% (evolved COn and gas uncondensable :st -195' constituted less than 0.01% of t'hc total weight of the sample). The dehydrated material had a specific surface area of 67 m.Z/g. The outgassing procedure \\.as identical to that described above for the Ossar The methanol was de-aerated by bulb-to-bulb distillation in a vacuum system until the vapor pressure reading became constant for a bulb containing liquid methanol (3.07 em. a t 0"). Increments of methanol for adsorption were measured in a gas buret, due consideration being given to the effect of deviation from the perfect gas laws. Water vapor was drawn off from a weight buret (equipped with a stopcock and the male part of a standard ground glass joint) which contained de-aerated distilled watrr. In order to measure small increments ( I X 10-4 mole) with reasonable accuracy. water vapor from the weight buret was allowed to expand into a system consisting of two bulbs (one large and the other small) which were connected by a stopcock. In this manner a relatively large amount of water vapor could be drawn from the weight buret and hence the effect of an error in the weighing was minimized. Knowing the volumes of the two bulbs, the distribution ratio of the vapor het,ween t,he two was obtained readily. The stopcock connecting the bulbs then mas closed and the aliquot amount of vapor contained in the smaller bulb was introduced into the adsorption system. Apparatus.--The gas handling and vacuum systems were standard equipment. The type of calorimeter used in the measurements, and the method of computation of the heats of adsorption were essentially the same as described elsewhere.5 The only major feature of difference in the construction of the presrnt calorimeter was in the nature of the copper filler. The filler, onto which the heater wire used for calibration purposes was directly wound, was considerably more bulky than those used in previous cases, filling the whole of the inner plat,inum bucket except for six slots between the filler and the bucket. To ensure as far as possible the even distribution of an increment of adsorbing gas over the entire sample of adsorbent which was embedded in the six lots, the filler had a hole drilled down the center with smaller holes directed outward from i t toward the slots. The heat capacity of the adsorbent material was only 3.57'0 of that of the metal parts of the calorimeter and this arrangement yielded satisfactory time-temperature curves from which the heats of adsorption could be computed with confidence. Due to the high specific surface areas of the adsorbents involved, it was possible to tolerate the loss in sensitivity of the calorimeter caused by the high ratio in the weight of the inert metal of the calorimeter to that of the actual adsorbent.
14
0
I
1
I
I
1
I
1
0 1
02
0 3
04
0 5
0 6
07
P/P,. Fig, 1.--hdsorption isotherms for methanol on hydroxy,. 3, bone mineral; A, A, synthetic; apatite a t 0": 0, upper curve after degassing a t 450"; lower curves after degassing methanol covered surface a t 0". 24
I
22
.
20 18
u 0
E I6 \
-0 u
I4
Y
12 IO
8 I
o
I
I
II
I
I
2 4 6 a 1 0 1 2 1 4 M O L E S A D S O R B E D / M' x I O 6
Fig. 2.-Differential heats of adsorption for methanol on hydroxyapatite a t 0": 0, 0 , bone mineral; A , V, A, synthetic; upper curves after degassing a t 450'; lower curve after degassing methanol covered surface a t 0".
Results and Discussion I. The Adsorption of Methanol.-The experimental data for methanol adsorption on the hydroxyapatite samples of both synthetic and natural origin are given in the isotherms of Fig. 1 and the differential heat curves of Fig. 2. The isotherms, measured a t Oo, are plotted on the basis of relative pressure vs. moles of methanol adsorbed per m.2 of hydroxyapatite surface as determined by the B.E.T. nitrogen adsorption method. The data in the upper curve in Fig. 1 were taken after degassing the sample overnight a t 450' and those of the lower curves were obtained
1302
ARK E. DRY.iisI) RALPH BEEBE
after the surfaces, already covered with more than a monolayer of met'hanol. were degassed overnight at 0'. It is noten-orthy that the upper curves for total adsorption per miit area of surface are identical for the synthetic apat'ite and the bone mineral. By a comparison of the upper and lower isotherms of I'ig. I , it is possible to sort out, in a qualitative way at least, the "chemisorption" from the physisorption for the system under study. Thus it is reasonable to assume that, while the upper curve represents total adsorption a t any given value of P:,Po, the lower curves represent physisorpt'ioii (removable by pumping a t 0 ") and t'he difference between the upper and lower curves represents chemisorption (not removable at O o ) . I 6 This method of sort'ing out chemisorption from physisorption was first applied by Emmett and Bruiiauer" to the analysis of the adsorption of carbon monoxide and carbon dioxide on iron catalyst surfaces. It is realized that in the adsorption system being studied this method can hardly be expected to give an entirely clean-cut differentiation between the two types of adsorpt'ion. For instmaneea much longer pumping period might ha1-e removed a few per cent. more of the adsorbed methanol, with a corresponding small decrease in the estimated chemisorbed fraction, The upper and lower isotherms of Fig. 1 are separat'ed by a virtually constant distance over a considerable range of pressure. From this distance we est'imate that the chemisorbed layer contains 8.0 X and 8.3 x mole of adsorbed methanol per m.? of surface for the bone mineral and the synt'hetic adsorbent, respectively. If we assume that the chemisorbed methanol comprises the whole first monolayer, then we find cross-sections per adsorbed methanol molecule of 20.7 and 20.0 respectively, for t,he above two surfaces. There seems to be no universally applicable value for the cross section of adsorbed methanol molecules in different systems. The value obtained from the liquid density of methanol assuming close-packed spheres is 17.7 A ? ; this would represent physisorption. For methanol in the chemisorbed state, cros: $ections have been cited whiGh range from 28 A2,011 a Tilica gel sample t'o 15 A.2 on a nickel surface. Is The heats of adsorption represented in Fig. 2 are plotted on the basis of kcal. per mole against moles of methanol adsorbed per m S 2of surface. I n the upper right are given the results of check runs a t 0" on each of the two types of hydroxyapatite after outgassing at 430'. In the lower left are the calorimetric data on the synthetic material after outgassing a t 0"; this material mould presumably retain a chemisorbed monolayer of methanol after outgassing. A dotted vertical line a t 8.2 X moles,/m.' indicates the average value for the completion of the chemisorbed monolayers of the ( l i i ) For convenience 15-e shall refer t o the adsorbate not removable b y ~ i u i n p i n ga t 0' as being chemisorbed, although the forces of dipole attrartion and hydrogen honding which are probahly a t play here might not, be generally accepted a s being cliemical in nature. (17) P. H. E m m e t t a n d S. Brunauer, J . A m . Chem. doc.. 6 9 , 310
(1938). (18) V. A . Dzisko and 1 '. 1841 (l052),
1-01. 64
two adsorbents as estimated from the data of Fig. 1. It is seen in Fig. 2 that the differential heat values are high throughout the estimated chemisorbed layer exceeding the heat of vaporization of methanol (EL = 9.1 kcal./mole) by about 5 kcal. even at the estimated point of completion of the layer. The heats fall throughout the first layer but the rate of fall is greater near its completion and the values rapidly approach the heat of vaporization as the second layer fills. Because of the relatively small diameter of the calcium ions, the surface mould be largely occupied by oxygen atoms associated either with the phosphate or the hydroxyl ions of the surface. Thus, it seems plausible to postulate the oriented adsorption of methanol on the ionic surface of the hydroxyapatite as the result of dipole attraction plus hydrogen bonding. These hydrogen bonds could involve 0-H-0 bonds in which the hydrogen might be supplied either by the methanol or in part by the hydroxyl groups of the hydroxyapatite. Such an adsorbed layer of methanol would present a layer of methyl radicals a t its surface and would have relatively small attraction for building up a second adsorbed layer of methanol. This model seems consistent with the observed differential heat curve. 11. The Adsorption of Water.-We also have studied the adsorption of water vapor on the hydroxyapatite surfaces. The isotherm for the total adsorption of Ivater a t 23' on the bone mineral sample outgassed a t G O " is given in Fig. 3 . An attempt to split this total adsorption into a physisorption and chemisorption was less successful and more arbitrary than was the caSe for methanol. Thus, unless the outgassing was very carefully timed, it mas difficult to obtain reproducible results for the physisorbed water. For comparison, the isotherm for total adsorption of methanol a t 0" is included in Fig. 3. It is immediately obvious that the water isotherm is steeper beyond the estimated monolayer than is the case Tyith methanol. Some differential heat data for water measured a t 23' on bone mineral are shonm in Fig. 4. The dotted vertical line represents moles of water adsorbed/m.e at the monolayer as estimated by point B of Fig. 3. I t is apparent that the experimental points scatter to some degree. In the firyt calorimeter which we used for water adsorption this scatter was even more pronouiiced and it was traced to a non-equilibrium effect previously noticed in earlier work 011 other adsorption qyqterns. 19,?0 Such effects are especially noticeable at low pressures of the ambient adsorbate gabeq. I n the calorimeter used to obtain the data of I i g . 4 the design was altered t o eliminate this nonequilibrium effect as far as possible, by bringing the incoming gas increment into contact with a broad annular layer of the adsorbent surrounding a perforated central inlet tube rather than a narrow layer at the top of the calorimeter. HoweIrer, this device still would not eliminate the possibility of a non-equilibrium state due to slow diffusion of water molecules initially adsorbed on lower energy 119) R. A Reehe and D 1 D o n d e n J d m C h r m S O C 60, , 2912
N.Krasnopol'nkaya. Zhur. Fiz. Khim..
26,
(1938) 1493 OKGl, (20' \T3 E a Gainer and I?. J. J e a l , J . C h c n ~ en?. .
Sept., 1960
A b S O R P T I O X S T U D I E S O S ROSl3 J I I S K R A L ASI) SYSTHETIC
HYDROXYAPATITE 1303
site., into the body of the crystals where less readily available sites of higher energy might be present. Thus, it is felt that the heat data up to the calculated monolayer may be somewhat erratic depending on the time between admissions of increments. If anything, the experimental points may be too low in this region although we believe that the solid line of Fig. 4 comes close to representing measurements under true equilibrium conditions. In comparing the heat data for methanol and for water as shown in Figs. 2 and 4 it seems apparent that (1) the heats for water at a given coverage run somewhat lower than those for methanol and ( 2 ) the drop in the differential heats is much more 0 1 I , I I gradual for water indicating a less clear-cut separa0 1 2 3 4 5 tion in binding energy between the first and second p/po. adsorbed layers. Fig. 3.-Total adsorption isotherms on bone mineral, after We think these observations can be explained on degassing at 150", 0 water a t 23", e methanol a t 0'. the basis of the hydrogen bonding. Pauling*' has shown that the energy per hydrogen bond in the pure liquids is stronger in methanol (6.2 kcal./mole) than in water (4.5 kcal./mole). The higher boiling point and heat of vaporization of water as compared to methanol is due to the formation of more hydrogen bonds per mole in the former than in the latter. Methanol presumably would form one hydrogen bond per molecule, binding it to the adsorbing hydroxyapatite surface. It may well be that the water molecules likewise are adsorbed by forming on the average little more than one hydrogen bond per molecule and from analogy with the bonding in the bulk phases the water would then be less strongly bound to the adsorbing surface than would methanol. If the water molecules are singly bonded to the surface then the molecules in the first layer would possess residual capacity for hydrogen '0 I bonding to subsequently adsorbed molecules in the I I second layer. This model would seem to explain I I : I I the less well-defined separation between the first 0 and second layers as deduced from the heat data as shown in Figs. 2 and 4. This model is also consistent with the observed steeper slope of the water isotherm as compared with that for methanol in Fig. 3. Neuman and his co-workersZ2have studied the adsorption of water on hydroxyapatite by means of a centrifugation technique. From our present study, we would conclude that the above investigators must have been dealing with water adsorption in the second and subsequent layers, since the firqt layer probably would be too tightly bound to be removed by centrifugation. In discussing their results, Xeuman, et al., suggest that there may be some direct bonding between the hydroxyapatite crystals and the organic phase (collagen) in live bone, and they cite the electron micrographs of Robinson and Watsonz3as supporting evidence On the basis of the present research, we suggest that some less direct bonding may be effected through intervening water molecules which are attached by hydrogen bonds to the crystal sur-
-
(21) L. Pauling, "Nature of the Chemical Bond," Cornel1 Univ. Press, I t h a c a , N. Y., Second Edition, 1948, p. 304. (22) W. F. Neuman, T. Y. Toribara a n d B J. AMulryan, J Am. Chem SOC.,?6, 4239 (1953). (23) R A. Rebinson and XI L. Watson, Anal. Record, 114, 383 (1952).
c. If.HUGGINS,L. E.
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S T . PIERRE 9 S D
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11.11.BUECHE TABLE I
DATA FROV KITROCENADSORPTION ON BVNEMISERAL*’ (1)
Treatment (see above)
(a) (b) (C)
,
1
0
02
0 3
I 0 4
PIP,
Fig. 5.-Adsorption isotherms for nitrogen on bone mineral at -I%’, (a) 0 sample degassed a t 450’; (b) A on water covered surface; (c) V on methanol covered surface.
tion of the adsorbing hydroxyapatite surface. Thus in Fig. 5 are shown the isotherms for the surface of the Ossar after the treatments described: (a) surface degassed overnight a t 450°, (b) surface saturated with mater vapor a t 0’ and then degassed overnight a t O o , (c) surface saturated with methanol vapor a t 0’ and then degassed overnight a t 0’. The data of all three isotherms produce the normally encountered straight line B.E.T. plots and the resulting B.E.T. parameters are given in Table I. I n column 1 of this table the treatment prior to nitrogen adsorption is indicated as described above. In column 2 are given the specific surface areas as calculated from the B.E.T. Vm” values, in column 3 are given the B.E.T. “C” values, and in column 4,the net heats of adsorption (E1 - E L )as calculated from the data of column 3. The data of columns 3 and 4 as well as the isotherms of Fig. 5 indicate a weakening of the adsorption potential for physisorption by previously chemisorbed material. The observed decrease
(2) Spec. surf. m.Z/g.
100 86
75
(3) R.E.T.
(4) Net heat of ads., cal./mole
435 142 25
940
”C” value
770 500
in the specific surface areas of column 2 from (a) to (c) may be attributed either to, (1) blocking of very fine pores by the chemisorbed xater or methanol, or (2) a less condensed packing of the nitrogen molecules on the surfaces of lowered adsorption potential. At the present time, we are unable to decide between these two alternatives. This alteration of the physisorption potential of a surface by previously chemisorbed material has been observed by other investigator^."-'^ In particular Kiselev and his co-workers12have found that the substitution of -O-Si(CH& “umbrellas” for the surface hydroxyl groups of silica surfaces has an effect on the nitrogen adsorption very similar to that of adsorbed methanol on a bone mineral surface as shown in Fig. 5 of this publication. The latter authors question the validity of the B.E.T. method for surface area determination in such systems of weakened adsorption potential. Because of our interest in the above phenomena we have undertaken and partly completed the calorimetric measurement of the differential heats of adsorption of nitrogen on the bone mineral surfaces after treatments (a), (b) and (c) as described above. More detailed adsorption-desorption isotherms a t -195” for nitrogen a t high relative pressures are being measured in order to obtain information about pore size distribution in the bone mineral. It is hoped to publish the results of this work in the near future. (24) I n a set of eimilar experiments with the s.i.nthetic hydroxyapatite, the B.E.T. derived “C” values and net heats of adsorption and the variation of these parameters with treatments ( a ) , (b) and ( c ) described above were essentially identical with those listed for the bone mineral in Table I, although the specific siirfnce area of the synthetic material was lower.
SUCLEAR MAGNETIC RESOXANCE STUDY OF MOLECULAR IIOTION IN POLYDIMETHYLSILOXANES‘ BY C. M. HUGGINS, L. E. ST.PIERRE AND A. M. BUECHE General Electric Research Laboratory, Schenectady, New YorB Received April 8, 1960
The proton magnetic resonance (n.m.r.) spectra of a polydimethylsiloxane were determined over the temperature range The spectra are characterized by an unusually narrow resonance line a t room temperature indicating a low microscopic viscosity in the polymer. At 77’K. the line width broadened to 4.9 gauss peak-to-peak, compatible with a rigid lattice and free rotation of the methyl groups. The line narrowing data indicate a chain rotational process with an activation energy of 8 kcal./mole. The combination of the n.m.r. data with X-ray, melting point and dielectric loss data confirms the presence of a “second-order” transition in the crystalline polymer considerably below the crystal melting point. This transition is thought to be primarily the onset of a chain rotation but evidence also is presented for chain translational motion in the crystalline state.
77-300°K.
Introduction The structures and molecular motions of polymers are being subjected to increasing study by (1) Based on a paper presented a t the ACS Meeting, Boston, hlass.? April, 1959.
nuclear magnetic resonance (n.m.r.) techniques. A comprehensive review of the topic has been given by Slichter.2 It has been shown that n.m.r. line shape studies provide a measurement of the relative (2) W.
P. Slichter. Advances in Polgmer Sci., 1, 35 (1958).
