Nov.>1963
MOLECULAR REORIER‘TATION IN MCLTIMOLECULAH. FILMS
2413
MECHANICALLY INDUCED MOLECULAR. REORIENTATION IK MULTIMOLECULAR FILMS1*2 BY L. S.BARTELL AND C. L. SUTULA Contribution No. lSi6 f r o m the Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa Received M a y 24, 1965 Multimolecular films of a variety of long-chain hydrocarbons and hydrocarbon derivatives were subjected to the shearing action of rubbing with soft tissue paper. If the effective total chain length (monomers of most substances, dimers of acids) exceeded 25-30 carbon atoms, the molecules were found by electron diffraction to fall over with their horizontal component parallel t o the direction of rubbing. The molecular axes pointed upward against the direction of rubbing, forming an angle of several degrees with the plane of the surface. The angle was larger for orthorhombic packing than monoclinic, larger for short chains than for long. Chains shorter than the critical length were observed t o disorder instead of reorienting regularly. Impurities often, but not always, interfered seriously with reorientation. Commercial paraffin wax, for example, completely resisted reorientation during the strongest shearing actions. The mechanism of the reorientation process is discussed. Measurements also were made of the rate of evaporation of the films as a function of chain length. It was found that molecules shorter than the critical length for reorienting were significantly more volatile than molecules exhibiting reorientation. Accordingly, the possibility is not ruled out that surface layers of molecules below the “critical length” are reoriented by shear but lost by evaporation into the vacuum chamber of the electron diffraction unit. For chains just under the critical length the thickness of such a superficial layer appears to be no greater than a molecular length.
In 1938, Finch3 observed that molecules in multimolecular films of stearic acid could be reoriented by rubbing the films with filter paper. Before rubbing, the molecular chains were stacked together with molecular axes tilted away from the normal to the surface by 37”. The azimuths of independent clumps of molecules were random. After rubbing, the molecular chains were found to have been knocked over and swung around parallel to the direction of rubbing and nearly parallel to the surface. Unlike rubbed bristles in a brush, however, they pointed upward against the direction of rubbing, forming an angle of about 5’ with the plane of the surface. Following Finch’s report, Germer and Storks4 performed a detailed study of the effect of unidirectional rubbing on Langmuir-Blodget multimolecular films of stearic acid and barium stearate deposited upon metal surfaces. r\iIultimolecular films of stearic acid were reoriented by the applied shear, as found by Finch, whereas multimolecular films of barium stearate resisted reorientation and suffered only slight disordering. Brummages has examined the effect of shear on multimolecular films of several long-chain n-hydrocarboils on surfaces of atainless steel and copper. Unidirectionally applied shear was observed to reorient only a small fraction of the molecules in these films. I n the course of investigating the structures of films of various n-hydrocarbon derivatives as described in an earlier paper6 (hereafter referred to as I),it was observed that different derivatives of a given hydrocarbon radical exhibited markedly diff ereiit responses to shearing forces. Therefore it was decided to study multimolecular films of a variety of long-chain derivatives to seek correlations between structure and mechanically induced molecular reorientation. An advantage over previous investigations was the availability of a con(1) Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission, presented at the 135th National Meeting of the American Chemical Society, Boston, Massachusetts. April 6, 1959. (2) Based on a dieisertation by C. L. Sutulrt to the Graduate School, Iona State University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1959. (3) G. I. Finch, J . Chem. ~ o c . ,1137 (1938). (41 L. H. Germer and K. H. Storks, Phys. Rev., 55, 648 (1939). ( 5 ) K. G . Brummrtge, Proc. Roy. SOC.(London), Al88, 414 (1947). ( 6 ) C. L. Sutula and L. S. Bartell, J. P h y s . Chem., 66, 1010 (1962).
venient optical method permitting the optical thickness of the films to be followed routinely as a function of the treatment, to within a few per cent of a molecular length. The optical method also made it possible to study the rate of evaporation of the films, a factor which was found to be significant.
Experimental The films were prepared by evaporating dilute solutions of longchain n-hydrocarbon derivatives on carefully polished platinum and chromium surfaces, or by spreading the fused compounds over the metal substrates, as described in I. After a determination of the optical thickness of a given film, its initial structure and molecular orientation were examined by electron diffraction. The film was then subjected to the shearing of unidirectional polishing, and its thickness and structure were re-examined a t intervals as the film was polished down, For the most part, unrubbed films were of the order of 1000 A.o Rubbed films were usually examined in the range of 300 to 500A., and often at several thinner stages down to the order of 10 A. The shear was applied with controlled pressure transmitted through several layers of soft lens or facial tissue mounted on a flat steel slider. The velocity of sliding was approximately 3 cm./sec., and pressures ranging from about 5-25 X lo4 dynes/ cm.2 were tested. Increasing the pressure fivefold increased the response of the film but it was not clear that the effect was as great as that of increasing the number of shearing strokes fivefold. In general, only a few strokes (1 to 3) were required to reduce a film from 1000 A. to 100 A. in thickness. Films rapidly became increasingly resistant to loss of material as optical thicknesses approached molecular lengths. Considerable variation from compound to compound was observed. There can be no doubt that additives and natural waxes in the polishing tissues are a source of contamination in studies of such exceedingly thin films. Vigorous rubbing of freshly cleaned slides with soft tissyes left a deposit which approached a limiting value of about 20 A. after extensive rubbing. To minimize the introduction of foreign materials into the films during polishing, the tissue paper used was pretreated with a dilute benzene solution of the film material before polishing, and was used for only one rub. When films are polished down to 10 or 20 A., it is uncertain how much contamination has been introduced. I n a parallel study7 with C14-tagged octadecylamine, however, the amine radioactivity remained proportional to the total optical thickness down to submolecular thicknesses. Optical thicknesses were measured with a polarization spectrometer, and structures by means of electron diffraction as described in I. The purity of compounds is discussed in I along with techniques of preparation of the metal surfaces. (7) L. S. Bartell and J. F. Betts, ibid., 64, 1076 (1960).
L. S. BARTELL AND C. L. SUTULA
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Vol. 67
TABLE I OBSERVED ORIENTATIONS AXD CRYSTAL FORM IN FILMS OF HYDROCA CAR BOX DERIVATIVES BEFOREAND AFTERAPPLYING SHEAR Compound
(?I
(?I
90 90
Orthorhombic Orthorhombic Triclinic Orthorhombic Monoclinic Orthorhombic Monoclinic Orthorhombic Monoclinic Orthorhombic Orthorhombic
Random Random Random Random 4-6 12-14 3-4 10-11 3-6 3-10 90 and 3
Monoclinic Monoclinic Monoclinic Orthorhombic
0-6 0-5 Random Random
Orthorhombic
90
n-Octadecyl stearate n-Hexadecyl stearate n-Dodecyl laurate n-Octadecyl alcohol n-Stearamide (impure) Paraffin wax
52 52 60 90 70 90
'
'i
1
...
'
1
. . . ... .
'
1
\ALVOHOLS
a I
I
I
a,deg.
Monoclinic, C Monoclinic, C Possibly triclinic Monoclinic, C Possibly triclinic Possibly triclinic
90 52 90 52 90 32 90 90
'
-
52 51-52 70 51-53 45-50 55
Stearic acid Arachidic acid n-Heptadecanoic acid Palmitic acid Isostearic acid Isopalmitic acid Lauric acid n-Octadecane n-Eicosane n-Docosane n-Tricosane n-Hexacosane n-Heptacosane n-Octacosane n-Xonacosane n-Triacontane n-Hentriacontane n-Dotriacontane (impure)
loot
Unrubbed film Crystal form
a,den.
I
I
20 30 40 10 NO. OF C ATOMS IN MOLECULE.
Fig. 1.-Dependence of evaporation rateoon chain length of compounds in films 40-100 A. thick.
Results Reorientation.-Molecules of many of the substances studied underwent a profound reorientation when subjected to shear. By the time a susceptible film of a pure material had been polished down by several hundred angstrom units its reorientation, in the upper layers a t least, was virtually complete. In the case of some impure natural waxes with very long chain lengths, a few light strokes often induced both disorder and some ordered reorientation. In these cases, perfection of packing in reoriented layers often improved substantially with hard polishing. The diffraction patterns of rubbed films could be classified conveniently into three types. Type I corresponded to films xvhich mere appreciably disoriented by applied shear. Type I1 corresponded to films which were quite thoroughly reoriented by shear, a t least in the superficial layers penetrated by the electron beam.
3-8 2-3 5-6 4-9 2-5 4-7
....
....
Rubbed film Crystol form
Monoclinic, C Monoclinic, C Possibly triclinic Monoclinic, C Possibly triclinic Possibly triclinic Disordered film Disordered film Disordered film Disordered film Disordered film Monoclinic Orthorhombic Monoclinic Orthophombic Monoclinic Orthorhombic Orthorhombic (partially reoriented) Mosoclinic Monoclinic Disordered film Disordered film
.......
Orthorhombic
Pattern type
I1 I1 I1 I1 I1 I1 I I I I I I1 I1 I1 I1
I1 I1 I11 I1 I1 I I 111 111
Type I11 corresponded to films which were reoriented only partially or not a t all bj7 the shearing force. The molecular orientations in multimolecular films before and after applying a unidirectional shearing force were generally readily evident from the layer lines of the diffraction patterns. The angle between the long axis of the molecules and the plane of the substrate surface is designated by a! in the tabulated results. The molecules in the multimolecular films were aggregated into submicroscopic crystallites for which it was often possible to determine lattice parameters. Lattice parameters were compared whenever possible with X-ray and electron diffraction results reported for bulk crysstals of the same or closely related compounds. In this way the molecular packing in the film could be related to the known polymorphic forms of the bulk crystals. The molecular packing and response of the substances to shear are indicated in Table I. Evaporation Rates.-Rates of evaporation of films were measured in a vacuum desiccator at a pressure of about 10-3 torr. This pressure, while a rather poor vacuum, roughly duplicated that enountered in diffraction studies during the greater part of the exposure of the slides to a vacuum in the electron diffraction apparatus. The resuAts for films raizgiag in thickness from about 100 to 40 A. are plotted iiZ Fig. 1 in terms of the thickness evaporated in 30-min. intervals (approximately the time required to pump and operate the diffraction apparatus). Not indicated in Fig. 1 is the observation that the rate of evaporation always dropped markedly when the film approached a thickness of one or two molecular lengths. T T h e scatter of data points was considerable in the runs, owing in part to the great difficulty in preparing the exceedingly thin films required with a sufficient UIIiformity of thickness. Severtheless, fairly definite trends appear to be established, as shown in Fig. 1.
Nov., 1963
A!fOLECULAR R E O R I E N T A T I O N IN
The evaporation rates were two or three orders of magnitude slower than the maximum rates calculated according to kinetic theory. It is judicious to regard them, then, as characteristic of particular experimental conditions and not as absolute rates relevant for any system. Discussion Comparison with Other Results ; Effects of Impurities.-Brummage has obtained diffraction patterns of type I11 from rubbed multiniolecular films of ntriacontane and n-tetracontane. In his work only a small proportion of the molecules reoriented, but those which did were observed to pack in aggregates with chains inclined t o the surface by about 3”, pointed upward against the direction of rubbing. Brummage attributed this orientation to crystallization with 201 planes parallel to the surface. He suggested that rubbing caused local me1ting, followed by crystallization with random azimuths and 201 orientations, preferred for crystallographic reasons. Further rubbing, he reasoned, preferentially removed 201 crystallites unless their azimuths were against the rubbing direction. I n the present investigation the fraction of long-chain hydrocarbon molecules that reoriented was much greater than observed by Brummage, provided the compounds were pure. I n fact, with rubbed films several molecular layers in thickness or thinner, the reorientation was often essentially complete, with no evidence of the 001 orientation remaining. It is probable that Brummage made his observations on films which were impure, as discussed in I, for the molecular packing before shear was not that characteristic of the pure compounds. An incomplete response to rubbiiig similar to that found by Brummage was noted in the present study with films of impure palmitic acid, impure octacosane, and impure dotriacontane. Also, cornmercial paraffin wax which contains a wide distribution of chain lengths and at few branched chains was found to be exceedingly immune to effects of shear. It remained in a well ordered, vertical, orthorhombic packing throughout the most vigorous rubbing treatments, retaining this orientation even when the average film thickness was decreased below one molecular Another fact, casting doubt on Brummage’s iiiterpretation is that the angles of incliiiation of the orthorhombically packed molecules which tipped over in the present study were several degrees larger than the angles implied by a 201 orientation. The present patterns (type I1 instead of type 111) made the determination of the angles much more direct than was possible in Brummage’s work. Nevertheless, despite the close correlation between observed and crystallographic angles reported in paper I for unrubbed films, it must be admitted that the relationship between observed angles of molecular objects and planes of macro objects remains speculative. Perhaps rubbing caused a disproportionate accuniulation of material on the leeward (downhill) slopes of surface asperities, biasing the observed slope. In any event, we are inclined to regard the mechanism (8) The presence of iinpurities or inhomogeneity of chain length does not necessarily prevent ieorientation. Virtually coinylete reoiientatioii xith striking perfection of packing was obseried for several natural waxes consisting of mixtures of many compounds. Nevertheless, in these cases a greater amount of buffing was necessary to achieve complete ordering. The principal components of these films, however, had chains perhaps 20 oarbons longer than those in paraffin wax.
n/rULTTMOLECULAR
FILMS
2415
of the reorientation more as a direct consequence of plastic flow under shear than as a random recrystallization from local melting, followed by preferential wiping away of crystallites. Effect of Chain Length.-Whether a multimolecular film of a particular n-hydrocarbon derivative was found by electron diffraction to be disordered or reoriented orderly by shear depended largely on its chain length. I n the saturated n-hydrocarbon series, films of compounds containing less than 24 carbon atoms were disoriented by shear. Films of n-hexacosane, however, were regularly reoriented by a shearing force. The critical chain length for the reorientation appears to occur in this series a t 25 or 26 carbon atoms at room temperature. It is possible that the critical length depends on temperature, perhaps increasing as the temperature increases. Films of lauric acid, which mere difficult to study by electron diffraction, seemed disordered by shear, whereas films of palmitic acid were reoriented by shear. Because the n-aliphatic acids pack as dimers in their crystals, films of the n-aliphatic acids can be seen to change in their response to shear at an effective chain length only slightly greater than that of n-hydrocarbons. Quite analogously, in films of the long-chain esters, n-dodecyl laurate disordered, whereas films of nhexadecyl stearate and n-octadecyl stearate reoriented readily. It is not implausible that a somewhat shorter length suffices in the reorientation of a uniform chain like a hydrocarbon than for a bumpy derivative like an acid dimer or ester. The data for other types of n-hydrocarbon derivatives are less complete. Films of n-octadecyl alcohol were rapidly disordered by shear. Films of mellisyl alcohol (&), however, have been reoriented by shear.g It is probable that the critical length for alcohols and many a-substituted derivatives where dimerization is weak or absent, is similar to that for n-hydrocarbons. It is tempting to conclude that shear-induced reorientation is a simple mechanical property related to niolecular chain length. While there undoubtedly is a certain validity in this view, it is not definitely established that the electron diffraction “critical length” for reorielitation is directly correlated with this property. The “critical length” for reorientation of molecules in the present study is uncomfortably close to the molecular length at which evaporation rates are 10 to 30 B./hr. Longer molecules have trivial rates insofar as the present experiments are concerned but shorter molecules have rates which might seriously deplete a reoriented surface layer, if one were formed. Consequently, the possibility is not ruled out that surface layers of molecules below the “critical length” are reoriented bj7 shear but lost by evaporation into the vacuum chamber of the electron diffraction unit. Lauric acid, in particular, beyond its firmly adsorbed base layer, is certainly too volatile for its reorientation to be definitively studied in the apparatus of the present work. On the other hand, for molecules just shorter than the critical length, the layer evaporated js appreciably thinner, according to Fig. 1, than the completely reoriented layers in films of longer molecules. Moreover, barium stearate, which was observed to re(9) J. v. Sander8 and D. Tabor, Proc. R o y . SOC.(London), 8204, 525 (1951).
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L. S. BARTELL AND C. L. SUTULA
Vol. 67
sist reorientation in an earlier study,4 is essentially nonvolatile. Therefore, the “critical length” may be more than an artefact of evaporatioii rates. Other significant structural features emerged from t,he study. As a rule the angle a was greater in reoriented films for orthorhonibic packing than for nionoclinic. It was also observed that, for a given type of packing, the longer molecules in a homologous series tended to lie flatter. The orientation of the short axes of the essentially crystalline arrays in the reoriented filins was usually far less definite than that of the long axes. The range of aiigles of the a and b axes with respect to the plane of the substrate seemed to narrow as the chaiii length increased. Preferred orientation of the a axis of some of the shorter chains was scarcely discernible (e.g., CM hydrocarbon). Stearic acid, effectively longer than Ca6,exhibited a strong tendency to orient its a axis parallel to the surface. Nevertheless, reflections observed when the beam was parallel to the chains revealed a distribution of a orientations over a range of 35’. Certain very long chain natural waxes, in films 40 A. thick, packed with astonishingly well defined directions of short axes. Twinning was observed in these cases, however, associated with the pseudohexagonal packing often enountered with long chaiii compounds5 in which the a axis may take o n any of three orientations differing by 120’. Considerations of Mechanism of Reorientation.-It is worthwhile to illustrate the stresses to which the system is subjected with a few rough order-of-magnitude calculations. Let us consider a film of intermediate thickness encountered in this work, 300 A., with a coefficient of friction of, say, -0.1. For a slider 5 em. long with the lightest pressure used (5 x l o 4 dynes/cm.*), the energy dissipated in one rubbing stroke would be 0.1 X 5 X 5 X lo4 = 2500 ergs/cmS2or 70 kcal.jmole of film (800 kcal./mole of surface molecules). The work of COhesion ( 2 ~ ~ of 4 ) a surface of methyl groups is of the order of 50 ergs/cm2. Clearly, the input energy per stroke is sufficient to influence the molecules profoundly if fully utilized. On the other hand, the rate of input of energy is so low that the mean temperature rise calculated from the thermal conductivity is less than deg./sec.lO Therefore, melting could not occur uniformly over the entire surface of the film but only, at most, at points of adhesional contact in the friction process. Even a t these localized spots the energy would be quickly dissipated.11 The temperature rise of such contacts can be computed using various models discussed by Bowden and Tabor.12 It is fouiid that the most extreme choices of models and estimates of asperity sizes lead to calculated temperature rises far too low to cause local melting, if films are assumed to be uniform in thickness. This is due to the relative mildness, as frictional processes go, of the surface treatment in which the low values of the applied
pressure, sliding velocity, coefficient of friction, and yield pressure of the organic materials minimize the heating. It is possible, nevertheless, that appreciable local temperature rises occur in those few spots where the tissue fibers contact the nietal slide directly. Although thermal effects seem small, the mechanical stress is considerable. According to the work of Bowdeli and Taborla 011 friction, plastic deformation occurs at contacts of asperities and continues until the area of real contact reaches the area that would theoretically support the load at the pressure associated with the commencement of plastic flow. In the present problem such a flow presumably occurs in films which are not too thin, although a significant fraction of the load a t low film thicknesses may be borne by tissue fiber deformation 011 thinly covered nietal asperities. The shear rates met in the flow process are enornious despite the low sliding velocity, because the velocity gradient in the highly noli-Newtonian flow must occur over a thickness of not many molecular lengths. The shear rate, assuming a shear across one molecular length, would be over lo7 sec.-l. This would lead to an orientatioii of the anisotropic molecules in the viscous inediuni somewhat aiialgous to, but more complete than that occurring in streaming birefringence experiments. l 4 These foregoing considerations make it simpler to understand the molecular reorientations in terms of an ordered plastic flow rather than Brummage’s melting mechanism. Moreover, the work of Tabor, et aZ.,15 indicates that simple melting and recrystallization do not lead spontaneously to the 201 orientation required by the melting theory. It is also clear that the reorientatioii is more than a simple rotation of microcrystals, since multilayers are sometimes observed to transform during reorientation from one crystal form to another, as A to C in the case of stearic acid.4 The most definitive generalization to be drawn from the present investigation is the dependency of the observed reorientation on the effective length of the chains and on the presence of impurities. A more detailed study of the mechanism of reorientation would be helpful in clarifying the role of evaporation, and might prove fruitful in casting light on other problems, as rheological properties of polymers. Acknowledgments.--We are pleased to acknowledge the assistance of Mr, R . R. Roskos in measurements of rates of evaporation. We are deeply indebted to Dr. A. E. Smith and the Shell Development Co. for samples of a series of extremely pure n-hydrocarbons, and to Dr. 11.Senkus and the R. J. Reynolds Tobacco Co. for a sample of pure n-hentriaconlane. We should also like to acknomledge support from the American Petroleum Institute in the early phases of this investigation.
(10) This assumes all energy is degraded into heat and that the metal slide is a n infinite sink. (11) The calculated half-life of cooling is of the order of 10-10 880. for a disturbed region 100 A. thick. (12) F. P. Bowden and D. Tabor, “The Friction and Luhrioation of Solids,’’ Oxford University Press, London, 1950.
(13) F. P. Bowden and D. Tabor, “Friction and Lubrication,” Methuen and Co., Ltd., London, 1956. (14) J. T. Edsall, “Advances in Colloid Sclence,” Vol. I , Intersclence Publishers, Inc., New York, N. P.,1942, p. 269. (15) J. W. Mentor and D. Tabor, Proc. Roy. SOC.(London), A204, 514 (1961); J. V. Sandersand D. Tabor, zbzd., 8204,525 (1951).
