2901
J. Phys. Chem. 1983, 87,2901-2911
Alignment of Solutes in Stretched Polyethylene. Determination of the Five Second and Fourth Moments of the Orientation Distribution of 2-Fluoropyrene from Polarized Fluorescence. Additional Evidence for the Twisting of Weak Transition Moments by the Solvent Environment Frans W. Langkllde, Markus Glsln, Erik W. Thulstrup,' and Josef Michl' Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112 (Received: January 6, 1983)
Measurements of linear dichroism and of polarized fluorescence of 2-fluoropyrene (2-F-1) in stretched linear low-density polyethylene (LLDPE) at 77 K have been used to evaluate the two independent second moments, (cos2z) and (cos2y ) , as well as the three independent fourth moments, (cos4t),(cos4y ) , and ( cos4x ) , of the orientation distribution function. The results are used to discuss two previously proposed detailed models for the mechanism of the orientation of aromatics in stretched polyethylene. For pyrene (1) and 2-methylpyrene (2-Me-l), four of the five moments were obtained. In these two molecules the direction of the Lb transition moment does not coincide with the molecular short in-plane y axis, but shows a distribution of orientations within the molecular framework, revealed by a site-selection experiment. On the average, it is inclined approximately 40' and 20' away from the y axis in the two compounds, respectively. This behavior is ascribed to a symmetry-lowering perturbation by the environment, related to the Ham effect.
Introduction The characterization of the partial alignment of solutes in uniaxial anisotropic media such a stretched polymers, nematic liquid crystals, or lipid bilayers is of considerable interest in a variety of fieldsS2v3Most optical studies to date have relied on one-photon processes, in particular polarized absorption (linear dichroism). From these, only the second moments of the orientation distribution function can be obtained. A few studies used the two-photon processes, Raman4 or fluorescence5spectroscopy, to obtain one of the fourth moments for various molecules. In a series of papers, Dekkers et a1.6 have used polarized fluorescence of solutes in stretched polyethylene to obtain spectroscopic information about the solutes without attempting to separate the effects of photoselection and of polymer-induced orientation. Such a separation is easy in principle using the equations of ref 7, particularly for molecules of high symmetry such as C2"or DZh. In the present paper, we report the first determination of all five independent moments of the orientation distribution function of a symmetrical solute in a uniaxial solvent which are accessible from one- and two-photon measurements. These are the two independent second moments and the three independent fourth moments of 2-fluoropyrene (2-F-l ) embedded in stretched linear low-density polyethylene (LLDPE) at 77 K. They (1) Permanent address: Department of Chemistry, Royal Danish School of Educational Studies, Emdrupvej 115B, Copenhagen NV, Denmark. (2) I. M. Ward, Ed., "Structure and Properties of Oriented Polymers", Applied Science, Ltd.,London, 1975. (3) H. Kelker and R. Hatz, "Handbook of Liquid Crystals", Verlag Chemie, Weinheim, West Germany, 1980. (4) S.Jen, N.A. Clark, P. S. Pershan, and E. B. Priestley, J. Chem. Phys., 66,4635 (1977);S.K. Satija and C. H. Wang, ibid., 69,2739 (1978); J. Maxfield, R. S. Stein, and M. C. Chen, J . Polym. Sci., Polym. Phys. Ed., 16, 37 (1978). (5) Y.Nishijima, Ber. Bunsenges. Phys. Chem., 74,778 (1970);J. H. Nobbs. D. I. Bower. I. M. Ward, and D. Patterson. Polymer, 15, 287 (1974);s. Hibi, M. Maeda, H. Kubbta, and T. Miura, ibid., i 8 , 143 (1977); J. Fuhrmann and M. Hennecke, Makromol. Chem., 181, 685 (1980). (6) J. J. Dekkers, W. P. Cofino, G. Ph. Hoornweg, C. Maclean, and N. H. Velthorst, Chem. Phys., 47, 369 (1980), and references therein. (7) J. Michl and E. W. Thulstrup, J . Chem. Phys., 7 2 , 3999 (1980).
I
1
I
2-F-1
I
2-Me-1
were determined from the measurement of the polarization of two differently polarized fluorescence peaks upon excitation into two differently polarized absorption peaks. We also report four of the five moments for pyrene (1) and 2-methylpyrene (2-Me-1) in this environment. Similar methods should be applicable to the determination of the second and fourth moments of other fluorescent probes in stretched polymers and other anisotropic media. The acquisition of such results for a suitably selected series of molecules should provide an improved picture of the relation of molecular shape, polarizability, and other properties to the nature of the solute orientation distribution in stretched polymers, and thus contribute to the understanding of the underlying orientation mechanism and of the microscopic structure of stretched polymers. The reason for our failure to obtain all three independent fourth moments for the orientation distribution of 1 and 2-Me-1 is the absence of purely y-polarized features in the fluorescence spectrum (and the weakness of the phosphorescence). In analogy to the results obtained on 1 and 2-Me-1 in glassy 3-methylpentane: we find that the traditional assignment of the origin of the Lb band in absorption and in emission as purely y polarized, while undoubtedly correct in an isolated molecule, is not valid for molecules in the solvent environment. The evidence for this solvent-induced solute symmetry lowering obtained in the stretched polyethylene samples is very direct and (8) F.W. Langkilde, E. W. Thulstrup, and J. Michl, J. Chem. Phys., 78, 3372 (1983).
0022-3654/83/2087-2901$01.50/0@ 1983 American Chemical Society
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Langkilde et ai.
The Journal of Physical Chemistry, Vol. 87, No. 15, 1983
leaves little doubt that the effect is real.
Experimental Part Samples. "Gold Label" pyrene was purchased from Aldrich, and 2-methylpyrene and 2-fluoropyrene were obtained as a gift from Professor A. Berg (Aarhus University). They were chromatographed on silica gel, sublimed, and finally gradient sublimed. Chemically pure LLDPE pellets (melt index 2) were obtained from Dow Chemical Co. They were hot-pressed into -0.2 mm thick sheets at 156 'C with pressures gradually increasing to 3600 psi, quenched in cold water, cut into 1 X 4 in. strips, extracted with spectral-grade chloroform for 24 h, air-dried, and stored. Immediately before use, they were washed with chloroform for 15 min and dried. The sheets were stretched, soaked in a chloroform soluton of pyrene or its derivative, and air-dried. Their surface was then washed with spectral-grade methanol to remove all traces of crystalline material. In the UV measurements, pyrene concentrations were -1 X M (absorption) and -5 X M (abM (emission) for the Lb band and -1 X sorption) and 1 X lo-* M (emission) for the other transitions, as judged by UV absorption spectroscopy assuming extinction coefficients comparable to those in ordinary hydrocarbon solvents. These were the smallest concentrations for which acceptable signal-to-noise ratios were obtainable. A variation of the concentration by a factor of severalfold had no observable effect on the measured polarization ratios and we assume that concentration depolarization is negligible. For 2-F-1 IR dichroism was measured on one of the sheets used to obtain the UV dichroism of the Lb transition. The stretching was performed on a homemade stretcher and reduced the sheet thickness by a factor of 3 and the sheet width by a factor of 2. In the process, the length of the specimen increased about sixfold. The stretched sheets were clear and showed no evidence of depolarization when viewed between crossed polarizers. While still in the stretcher and under tension, the center of the sheets was clamped to an aluminum sheet holder over a circular hole for beam passage and then cut off on both sides from the rest. For UV absorption and emission measurements, the holder was mounted on a nylon rod and immersed in a quartz Dewar filled with filtered liquid nitrogen so that the stretching direction 2 was vertical in the measurements of fluorescence and a t 45' from the vertical in the measurements of polarized absorption. The Dewar was equipped with Suprasil optical windows. Besides acting as a coolant, the liquid nitrogen improves the optical contact on the surface of the sheet, which is then actually hard to see. For the IR measurements, the sheet was mounted between two CsI plates attached to the cold end of an Air Products closed-cyclehelium cryostat kept at 77 K. The UV measurement on the Lb transition was also repeated with this setup. The UV polarizers were calcite UV prism polarizers (Karl Lambrecht Corp.); the IR polarizers were aluminum grid polarizers (Cambridge Physical Sciences, Ltd.). One set of meaurements of polarized absorption and fluorescence of 1 was performed on an oxygen-free sample as follows. The stretched polymer sheet was prepared in torr, and the usual manner, degassed for 6 h at transferred in a drybox under argon atmosphere into the optical Dewar. The Dewar was then filled with liquid nitrogen and an absorption base line was recorded. Pyrene was then soaked into the sheet in the usual manner, the degassing procedure was repeated, and absorption and fluorescence spectra were taken. Measurements. Polarized absorption spectra were
measured on five independent samples by using a Cary 17 spectrometer and taking care that the polyethylene sheet and the Dewar windows were perpendicular to the optical beam (directed along X)and that all of the beam passed through the sample. Polarizers were placed in both beams of the instrument and rotated f45O relative to the vertical for the measurement. The spectra E Z ( t )and E*(?) were recorded by rotating the electric vector direction of the polarizer parallel with the stretching direction (2) or perpendicular to it (Y), respectively. Base lines were recorded separately by using similarly prepared sheets without pyrene. Fluorescence spectra were measured on a homemade f/7 spectrofluorimeter described e l ~ e w h e r e . ~The , ~ polarizer position was rotated without changing the orientation of the sample. The spectra are not corrected for the wavelength dependence of the instrumental response. The exciting beam was focused on the sheet and defines the X direction. It passed at normal incidence through the Dewar windows and the sheet. The optical density due to the pyrene in the sample was kept below 0.25 at the exciting wavelength. The stretching direction Z was vertical. The emitted light was collected in the front-surface excitation arrangement at an angle of 14.5'. The polarization bias due to this nonnormal passage through the surface of the sheet and through the two Suprasil windows was calculated and found to be negligible. The observed emission intensity is Iuv(ij1,t2),where U = Y or Z represents the direction of the electric vector of the exciting light (Y, horizontal; 2, vertical), whose wavenumber is tl,and V = Y'or 2 represents the direction of the electric vector of the observed light, whose wavenumber is t2.The direction Y' deviates from Y by 14.5': Y' = Y cos 14.5' X sin 14.5', where U is a unit vector along laboratory axis U. A series of measurements was also performed with the stretching direction 2 of the polymer oriented horizontally (then, Z' = Z cos 14.5' X sin 14.5'). All the results were compatible with those obtained on sheets mounted with their stretching direction vertical. In the limit Y 9 Y', or Z E Z', these arrangements would permit the measurement of four of the five independent polarized fluorescence intensities obtainable in principle with polarization vectors directed only along the unique axis of the sample, 2, or perpendicular to it, in which passage through the birefringent sample does not affect the state of polarization of the light. A measurement of the fifth intensity, I x y , would require that light propagate along 2 and this was not feasible due to the thinness of the sheet. Some attempts were made to use thicker sheets (1 mm) but these were found to depolarize light excessively. In reality, Y and Y' differ by 14.5'. The appropriate correction to be made to convert I y y into Iw is I w = ( I w - I y x sin2 14.5')/cos2 14.5'. The intensity I y x could not be measured directly but it can be expressed through the same factors K and L as the other intensities, so that the correction can be introduced conveniently in the evaluation of the spectra. The effect of this correction was only a few percent and thus smaller than experimental uncertainties. Note that I Z r = I z y All measurements were repeated at least 5 times on independent samples. The results were highly reproducible; the results shown are averages or typical spectra. Measurement of the polarization of fluorescence of a fluid solution of pyrene in chloroform at room temperature,
+
+
(9) H. J. Dewey, H. Deger, W. Frolich, B. Dick, K. A. Kingensmith, G. Hohlneicher, E. Vogel, and J. Michl, J. Am. Chem. Soc., 102, 6412 (1980).
Alignment of Solutes in Stretched Polyethylene
assumed to be completely depolarized, revealed a bias of 1.05/1.00 in the detecting part of the spectrofluorimeter in favor of vertically polarized light. Measurement of unpolarized fluorescence intensity from this sample, and also from a room-temperature solution of Rhodamine 6G in alcohol, revealed a bias of 1.15/1.00 in the exciting part of the spectrofluorimeter in favor of vertically polarized light for the region of the Lb and La transitions and 1.28/1.00 in the region of the Bb transition. All results shown have been corrected for these factors: e.g., the measured intensities Izz, Izy, Iyz, and IW were divided by 1.24,1.16, 1.05, and 1.00, respectively, when the excitation was at Lb or La. For Lb and Bb excitation, the signal intensity is much lower than for La excitation for different reasons: weak absorbance and low lamp intensity, respectively. In these two cases, the correction of the bias in the exciting beam is less reproducible and reliable than that for the detected beam. Since the rotation of the polarizers caused small beam displacements in spite of our best attempts a t their alignment, and since the slit widths used were 1 mm or less, it was found necessary to optimize the position of the collecting spherical mirror in front of the analyzing monochromator to maximum signal intensity each time a polarizer was rotated. The wavelength chosen for the optimization was the same in any one series of experiments. These adjustments were small and reproducible, but their effect was important. Our efforts to measure the polarization of the weak phosphorescence did not produce data of sufficient accuracy to warrant their use for the present purposes. Results Effect of Oxygen. In principle, it is conceivable that oxygen could be preferentially associated with molecules imbedded in a particular kind of environment in the stretched polymer, aligned differently from others, quenching their fluorescence. Such a situation would mean that the molecular ensembles and orientation distributions observed in absorption measurements (all molecules of 1) and in emission measurements (those molecules of 1 which do not have an O2molecule as an immediate neighbor) were not the same. In such a case, orientation factors derived from absorption and from fluorescence measurements would not be comparable. We have found that all results obtained on an oxygenfree sample of 1 in stretched polyethylene were identical within experimental error with those obtained on ordinary samples saturated with atmospheric oxygen. This finding eliminates the possible complication. Effect of Site Selection. Molecules located in different environments have slightly different excitation energies and produce inhomogeneously broadened lines in disordered solvents. In an anisotropic solvent such as stretched polyethylene, it is conceivable that molecules located in different types of environments are aligned to different degrees or have their transition moments twisted by the environment to different degrees. This should be observable, for instance, as a variation of the dichroic ratio in absorption across the inhomogeneously broadened line. Very weak variations of this kind and slight but reproducible differences in the positions of the absorption peaks in the two dichroic spectra of aromatics in stretched polyethylene have been observed repeatedly in earlier work.'O While there may be other causes for these observations, we believe that many of the "wiggles" in the (10) E.g., for anthracene: (a) L. Margulies and A. Yogev, Chem. Phys., 34, 253 (1978); (b) J. Michl, E. W. Thulstrup, and J. H. Eggers, Ber. Bunsenges. Phys. Chem., 78, 575 (1974).
The Journal of Physical Chemistry, Vol. 87, No. 75, 1983 2903
I t
t Flgure 1. Site selection in fluorescence of pyrene in stretched linear low-density polyethylene at 77 K for four excitation energies within the 0-0 absorption band of the 'L, transition. Stretching direction: horizontal. Excitation bandwidth: 70 cm-I. The portion of the fluorescence spectrum near the 0-0 band is distorted by scattered light and is not shown.
reported reduced spectra can be assigned to this phenomenon. When the orientation distribution of the total molecular assembly rather than those of selected subassemblies are to be characterized, it is necessary to average over the inhomogeneously broadened line and the simplest procedure is to use its integrated areas in the two dichroic spectra. In photoluminescence experiments, the site-selection effects are particularly pronounced and need to be considered before the orientation factors obtained from absorption and those obtained from luminescence can be combined. When narrow bandwidth excitation is used, only molecules in a particular set of environments are excited and these are the only ones observed in fluorescence. This provides a means of subdividing the total molecular assembly into subassemblies whose orientation distribution functions can be studied separately. At the relatively high temperatures used in our work, zero-phonon lines cannot be expected to stand out separately and phonon wings should dominate the spectra. Figure 1 shows the results of the site-selection experiment on 1 contained in stretched polyethylene. The narrowest excitation bandwidth compatible with a reasonable signal-to-noise ratio was used to excite the inhomogeneously broadened Lb origin at four different wavelengths. Four differently displaced emission spectra resulted, and the fluorescence polarization ratios are very different for each one (Figure 2). We believe that this is the first experimental demonstration that at least a part of the variation of the dichroic ratio across the absorption line observed for some molecules in stretched polymer sheets is indeed due to the existence of different environments for molecules characterized by different excitation energies. While the possibilities which this offers for a more detailed characterization of the molecular environment are obvious, in the present paper we concentrate on the orientation averages taken over the whole molecular assembly, permitting a simple comparison of the results obtained in linear dichroism and in polarized emission. We have therefore used larger bandwidths, in effect integrating over the inhomogeneously broadened lines, while paying attention to properly restricting the excitation and ob-
2904
The Journal of Physical Chemistry, Vol. 87, No. 15, 1983 25
26
Langkilde et al.
25
2 7
30
35
40
45
t ~ ' " ' ' " " ' ~ " " ' ' ' ~
I
I1
i-
D '
I r L
0A
,
l
,
,
,
,
,
,
,
,
,
l
,
,
c
i; (103cni'J
Figure 3. Pyrene in stretched LLDPE at 77 K. Top: base-line-corrected polarized absorption spectra and the dichroic ratio D = ,EZ/€,. Bottom: reduced absorption spectra. The absorbance scale is in arbitrary units, different for the two sections of the spectrum (left and right).
,
. .. 1
25
1
26
1
I
I
I
27
i , IlO'cm")
Flgure 2. Site-selected polarized fluorescence of pyrene in stretched LLDPE at 77 K. Stretching direction: horizontal. Top: excitation at the high-energy edge of the 0-0 absorption band of the 'b transition. Bottom: excitation at the low-energy edge of the 0-0 absorption band of the 'L, transition.
servation to one line at a time. It is possible that some of the scatter in the results for 1 and 2-Me-1 derived below is due to the incomplete removal of these site-selection effects. When the same type of measurement was performed on 2-F-1, similar regularly displaced fluorescence spectra again resulted, but this time there was no difference in the fluorescence polarization ratios as a function of the location of the excitation wavenumber within the origin of the Lb band. We shall discuss the significance of this interesting difference below. Linear Dichroism. Second Moments of the Distribution and "perpendicular" ( E y ) Function. The "parallel" (EZ) absorption spectra of 1, 2-F-1, and 2-Me-1 in LLDPE at 77 K are shown in Figures 3-5, respectively. They permit of the three the determination of the orientation molecular axes K , (u = x , y , z ) , defined by K , = (cos2 u ) where cos u is the direction cosine of the polymer stretching direction Z with respect to the molecular axis (11)E. W.Thulstrup, J. Michl, and J. H. Eggers, J.Phys. Chem., 74, 3868 (1970);J. Michl, E. W.Thulstrup, and J. H. Eggers, ibid., 74, 3878 11970). - -,I--
(12)E. (13)E. (1982).
W.Thulstrup and J. Michl, J . Phys. Chem., 84, 82 (1980). W.Thulstrup and J. Michl, J. Am. Chem. Soc., 104, 5594
c (1O7cm 'I
Figure 4. 2-Fluoropyrene in stretched LLDPE at 77 K. See caption to Figure 3.
u and the brackets indicate averaging over the molecular
ensemble. The observed spectra also permit the determination of the purely y - and purely z-polarized "reduced" spectra A, and A,, assuming that the absorption in the observable region has a negligible component in the out-of-plane direction x (i.e., that only T T * transitions need to be considered). The z-polarized reduced spectra A , = EZ d,,Eywith dy = 1.0 for 1,O.a for 2-F-1,and 0.76 for 2-Me-1,
Alignment of Solutes in Stretched Polyethylene
The Journal of Physical Chemistry, Vol. 87, No. 15, 1983 2905
TABLE I: Orientation Factors of Pyrenes in Stretched Linear Low-Density Polyethylene (LLDPE)at 7 7 K from Linear Dichroism"
1 lit. (UV)c
lit. (IR)C 2-F-1, UV
IR
* 0.02 * 0.02 0.33 i- 0.01 0.30 * 0.02
0.33 0.34
0.58 f 0.02 0.58 * 0.03 0.56 c 0.02 0.60 c 0.02 0.63 c 0.015 0.62 c 0.02
0.29 i- 0.01 0.28 0.02
* 0.04 * 0.05 0.10 +. 0.01 0.10 * 0.04 0.08 * 0.005 0.09
0.42
* 0.02
0.29
* 0.02
0.08
37 c 1 0 Ot
15d
20* 5
5* 5
2-Me-1 * 0.10 i- 0.04 0.33 f 0.02 2 3 c 10 2 5 i- 5 K , = (cosz u ) is the orientation factor of the u-th molecular axis, KLb is that of the Lb (0-0) transition moment, e is the "average" angle between the latter and the y axis (see text). The errors shown are maximum deviations. In 3-methylpentane glass.8 Previously reported values from measurements of UV and IR d i ~ h r 0 i s m . l ~ Using the more accurate K values from IR measurements. a
25
30
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4
35 1
,
1
/
1
40 1
+
,
,
,
,
45 1
,
B'l
I
7 4
Bh
25
30
35
40
45
i (1o3Cm")
Figure 5. 2-Methylpyrene in stretched LLDPE at 77 K. See captlon to Figure 3.
and the y-polarized reduced spectra A, = [(2 + d,)/(2 + d,)](d,E,- ICz),with d, = 2.8 for 1, 3.0 for 2-F-1, and 3.3 for 2-Me-1, are also shown. They were obtained by the stepwise reduction pr~cedure'l-'~using the already established fact'^'^ that the origin of the Bb band is purely y polarized and that the origin of the La band is purely z polarized. Thus, the curve A, was chosen as that linear combination of EZ and EY in which the origin of the Bb transition is absent, and A, as that linear combination in which the origin of the La transition is absent. The orientation factors K , of the molecular axes in the three molecules in stretched polyethylene were obtained from the d, values of the La and Bb origins using the relation~"-'~K , = d,/(d, + 2) and C,K, = 1and are collected in Table I. The error limits have been estimated from the incomplete reproducibility in a series of measurements. The orientation factors for 1 are compared with the previously published'* values obtained from IR and UV dichroism at 77 K. Those for 2-F-1 have been obtained in two ways: from dichroism in the UV region as described above, and from dichroism in the IR region. As indicated by the estimated error limits, the latter measurement is more accurate. This is due to the absence of spectral overlap and the negligible (14)J. G. Radziszewski and J. Michl, J.Phys. Chem., 85,2934 (1981).
role of site effects, to the large number of observable transitions, and to the presence of transitions of all three possible orientations, which permits the use of the relation C,K, = 1 as a check on the accuracy rather than as the only means of obtaining K , as in the UV measurement. Reductions on the Lb origin lead to results different from those on the Bb origin. In the case of 2-F-1the difference is very small. The reduction factor optimized on the Bb transition is 0.86, yielding K, = 0.30, while a value of about 0.83 would be more appropriate for the origin of the Lb transition, and this yields K, = 0.29. We do not assign much physical significance to this small difference and believe that the Lb and Bbtransition moments in 2-F-1 are parallel. This is supported by the near coincidence with the dichroic ratio of 0.83 measured for the strongest ypolarized transition in the IR spectrum using the same sheet without dismounting the sample from the cryostat or changing its temperature. The value 0.86 is considered less reliable since at the shorter wavelenghs there is more uncertainty in the position of the base line. In the case of 1 and 2-Me-1 the reduction factors for the Lb origin are 0.42 f 0.02, and 0.33 f 0.02, respectively, quite different from those of the Bb origin. This is in accordance with a previous report15of a difference between the dichroic ratios of the Lb and Bb origins of 1 imbedded in stretched poly(viny1 alcohol) and with the information obtained from photoselection experiments on 1 and 2-Me-1 in 3-methylpentane glass.' In all of these cases, the Lb origin is clearly of mixed polarization. At first we wondered whether the higher dichroic ratio of the Lb origin is not due to the higher solute concentration necessary for the measurement of this weak transition. However, even higher concentrations are used in the measurement of IR dichroism14and yet for 1 and for 2-F-1 it yields the same dichroic ratios as the purely polarized UV transitions La and Bb (the IR measurement was not performed on 2-Me-1 since too little sample was available). We have therefore dismissed this possibility. The "average" deviation 10avlof the Lb transition moment from the y axis can be obtained if it is assumed that it lies in the molecular plane, by using the re1ati0nll-l~ ( Cot2 0 ) =
( K , - KLJ/ (KL,- K,)
The resulting values leav1for the various media are listed in Table I. The fact that lO,l vanishes within experimental error for 2-F-1 in stretched polyethylene also agrees with the result obtained in 3-methylpentane glass in ref 8 and attributed there to the much larger intrinsic electronic transition moment of the Lb origin of 2-F-1 relative to 1 and 2-Me-1. (15) T. Yoshinaga, H. Hiratsuka, and Y. Tanizaki, Bull. Chem. SOC. Jpn., 50, 3096 (1977). (16) B. E. Read and R. S. Stein, Macromolecules, 1, 116 (1968).
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The Journal of Physical Chemistty, Vol. 87,No. 15, 1983
TABLE 11: Relative Intensities of Polarized Fluorescence of 2-Fluoropyrene in Stretched LLDPE a t 77 K
Langkilde et at. 25000
The reduced spectra of all three compounds look alike, except for the fine structure of the Lb band and, particularly, the increased intensity of its origin, which can be attributed to a perturbation of the alternant symmetry of pyrene by the fluoro substituent.8 Polarized Fluorescence. Fourth Moments of the Solute Orientation Distribution Function. The four polarized fluorescence spectra Izz, Izy, Iyz, and I y y were recorded with excitation at the Lb, La, and Bb origins, with the stretching direction vertical. 2-Fluoropyrene. Since the absorption of this compound is purely polarized in all three absorption regions, the calculation of its fourth moments L,, = (cos2 u cos2 u ) becomes quite simple and will be discussed first. The top two lines of Table I1 collect the relative intensities IUVof the purely y-polarized8 origin of the fluorescence for excitation at the purely z-polarized La origin and the purely y-polarized Bb origin (scattered light made the use of excitation at the Lb origin impractical). Figure 6 shows the four polarized fluorescence curves Iuv for excitation at the Lb and La origins. The fluorescence origin is labeled a. For such purely polarized absorption (along u ) and emission (along u ) the relative intensities of the polarized fluorescence Iuv are proportional to the tensor elements Suv(uu) defined in ref 7 . It is known8 that the weak fluorescence features labeled 0 and y in Figure 6 are polarized along the z direction. Since they overlap with y-polarized peaks, a stepwise reduction is necessary to reveal their true z-polarized contributions to the four spectra, as outlined in ref 7 . The ratio of the tensor elements Suv(uu) and S s T ( u ~where ), S, T, U , and V c a n have the values X , Y , or 2, is obtained by fiiding those linear combination of I w and IST in which the uu-polarized spectral features just disappear: Iuv [ s ~ V ( u ~ ) / s s T ( u u )Proper ] ~ ~ p care has been taken to convert Y' to Y as shown below. The correction is smaller than the experimental error. The disappearance of a spectral feature is recognized by comparison with the already known reduced spectra measured in glassy 3methylpentane.8 Figure 7 shows two examples of such reductions for the important case u = u = z (excitation at La): Izz against I Z y and 1, against Iyr. The spectral features (3 and y disappear in the linear combination I z y - 0.25122, yielding Szu(zz)/SZ&) = 0.25 and in the linear
~
~
#
1
1
27000 ,
,
,
,
1
,
~, 1 > -
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L,
i
11 z (La) y (e) 0.54 * 0.01 1.46 t 0.2 0.93 * 0.04 9 y (Bb) y ( 0 ) 1.14 t 0.04 1.45 f 0.4 0.44 * 0.03 4.0 t 1.0 1.8 i. 0 . 2 11 z (La) z ( 0 , ~ )4.0 t 1.0 0.50 t 0 . 2 3.3 f 0.7 11 y (Lb) z (0,~)2.2 t 0.3 0.70 t 0.2 3.3 f 0.7 9 y (Bb) Z ( c ) , ~ ) 2 . 2 i 0.3 a Number of independent measurements of the four polarized spectra, taken o n a total of eight different polymer sheets. Absorbing transition (in parentheses) Transition moment and its transition moment direction. direction of t h e emitting Lb transition (the observed For t h e first t w o fluorescence feature in parentheses). rows, S u v ( u u )= I,, is t h e observed polarized intensity and the error shown is t h e standard deviation. For the third t o fifth rows, S u v ( u u ) / S s T ( u u ) was obtained by t h e reduction procedure described in the text and t h e errors shown are maximum deviations. Those in t h e second, fourth, and fifth rows of t h e second column also take into account t h e uncertainty in the correction for the polarization bias due t o t h e rotation of the exciting polarizer at t h e wavenumbers of t h e Lb and Bb excitations, where the signals are very weak. e S y y , ( u u ) = S y y ( u u ) cos' 11.5" + S x y ( u u ) sin2 14.5".
26000 1
.!
ZY
25000
27000
26000 ij2 Icm'l
Figure 6. Polarized fluorescence of 2-fluoropyrene in stretched LLDPE at 77 K, excited at the origins of the L, transition (top)and La transition (bottom). The vertical scale is arbitrary.
combination Iw - 0.551yz, yielding [Syy(zz).cos214.5' + Sxy(zz).sin214.5']/Sy&) = 0.55. Similar reductions have been performed for other combinations of the Iuv)s for different choices of excitation wavelengths; both the ypolarized reduced fluorescence spectrum and the z-polarized reduced spectrum are independent of which UV combinations they come from and, moreover, they are virtually identical with the reduced spectra previously determined in glassy 3-meth~lpentane.~ The results are collected in the bottom three lines of Table 11. The ratios of the tensor elements Suv(uu) are related to the orientation factors K , and L,, as follows:
SZZ(UU):S~Z(UU):SZ~(UU):[S~~(UU) cos2 14.5' + Sxy(uu)sin2 14.5'1 = L,,:(K, - L,,)/2:(K, - L,,)/2
+ 3L,,] cos2 14.5' + [(3 26,,)(1 - K , - K,) + L,,] sin2 14.5')/8
: { [ (+l 26,,)(1 - K , - K,)
The K , values are now known, and we use those obtained in the IR measurement as being the most accurate. Then, every line of Table I1 permits the evaluation of L,, in three independent ways. The top two lines (excitation at La and Bb, emission at a ) yield three independent values for L,, and L, which are most reliable since no reduction is necessary. Lines four and five (excitation at L b and Bb, respectively, and observation at P,y) each yield three additional independent checks for Lyz. Finally, line three permits three independent determinations of Lzz. The values obtained in this fashion are collected in Table 111. The agreement among the numerous independent determinations is very satisfactory. From the three directly determined values of L,, L,,, and Ly2)it is possible to obtain all six L values by using the relations' 2L,, = ( K , - L,)
+ ( K , - L,)
-
K , = Lux + L,, + L,,
( K , - L,)
Alignment of Solutes in Stretched Polyethylene
The Journal of Physical Chemistry, Vol. 87, No. 15, 1983 2907
0
25000
c,: La
I
i , , , ,
,
,
,
i , , , ,
,
,
,
25 000
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
26 000
,
i
,
1
27000
c2 (cm") Flgure 7. Stepwlse reduction of the z-polarized spectral features fi and y in the fluorescence of 2-fluoropyrene in stretched LLDPE. The steps for dare 0.1, for the top curve in each set, d = 0. The curves which most closely resemble the reduced y-polarized spectrum' of 2-F-1 in glassy 3-methylpentaw shown on top, are dashed.
TABLE 111: Orientation Factors L,, of 2-Fluoropyrene in Stretched LLDPE at 7 7 K O LU"
U
z y z y y
(La) (Bb)
U
y (CY) y (CY)
(ZZ/ZY)
0.13 + 0.02 0.11 + 0.02
(ZZiYZ) 0.12 + 0.02 0.12 + 0.02
(YZ/YY) 0.16 f 0.02 0.12 t 0.02 0.42 t 0.04
(La) .z (0,~) 0.42 + 0.04 0.42 f 0.04 (Lb) Z ( 0 , y ) 0.15 * 0.02 0.13 f 0.04 0.16 t 0.04 (Bb) 2 (0,~) 0.15 f 0.02 0.16 t 0.04 0.16 f 0.04 Each entry represents an independent determination of L,, , using the excitation and emission frequencies specified in parentheses o n the left and the polarizer arrangements shown on the top.
The results are presented in Table IV. Pyrene and BMethylpyrene. For these two compounds, only the La and Bb origins are purely polarized and we are not able to derive all three independent fourth moments. Figure 8 shows the four polarized fluorescence spectra of 1, Iuv,excited at the origins of the Lb, La, and Bb bands. The spectra of 2-Me-1 are similar and are not shown. Since the Lb origin is of mixed polarization in the absorption, there is no reason to expect it to be purely polarized in emission, and it is necessary to rely exclusively
2908
The Journal of Physical Chemistry, Vol. 87, No. 75, 1983
TABLE V:
Langkiide et al.
Relative Intensities of Polarized Fluorescence of Pyrene and 2-Methylpyrene in Stretched LLDPE a t 7 7 K
Suv(U~)lSu'v'(~~)" UVI U'V' : Ub
uc, d
z (La) y (%) z (La) Y (%I
CY)^ CY)^ z (0,~)
z (La) y (Bb) (La) Y (Bb)
"y'' ''y"
ZZIZY
ZZI Y Z
(1.00 i 0.04)d (2.26 t O . l l ) d 4.0 t 1.0
(1.79 i 0.13)d (1.43 i 0.08)d 3.6 i 0.8 0.65 i 0.1
(037)
CY)^ CY)^ (P,r)
''y" ''y"
2 (P,T)
(0.78 i 0.05)d (1.13 i 0.07)d 5.0 i 1 . 5 2.5 fr 0.3
ZZIYY' Pyrene" (3.19 i 0.25)d (1.11 i 0.06)d
2-Methylpyrenea (2.23 i 0.32)d (1.85 fr 0.28)d (1.15 i 0.09)d (0.66 i 0.05)d 4.0 i 1.0 6.7 f 2.0 2.2 t 0.3
ZYIYZ
(1.82 (0.64
(2.86
i f
i
O.lO)d 0.02)d
0.45)d
(1.01f 0.08)d 0.27
i
ZYIYY'
YZIYY'
(3.20 i 0.21)d (0.50 i 0.02)d 1.1 i 0.2 1.5 i 0.3
(1.77 t 0.05)d (0.77 t 0.03)d
(2.45 t 0.44)d (0.57 i 0 . 0 3 ) d
(0.83 (0.57
0.05
3.3
i
1.8 i 3.3
i
0.7 0.06)d 0.03)d 0.2 0.7
i i
For the first t w o rows, Suv(uu) = I u v is the observed polarized intensity and the error shown is t h e standard deviation For the third and fourth rows, S u v ( u u ) / S s T ( u u was ) obtained by t h e reduction procedure described in the text, and the errors shown are maximum deviations. The measurement of t h e four polarized spectra was repeated 6 times (on six different sheets). The stepwise reductions were carried o u t independently for all U V / U ' V ' combinations shown. Only Absorbing transition (in parentheses) and its three of the combinations are independent and the rest are redundant. Transition moment direction of the emitting Lb transition (the observed feature in transition moment direction. Ratios calculated under the assumption that the Lb origin, CY, is polarized along y. This is actually not parentheses), correct; see text.
on the spectral features labeled p and y in the fluorescence spectra of ref 8, which have been shown there to be z polarized. Since these features overlap to some degree with y-polarized fluorescence features, it is not possible to rely on the dichroic ratios and a stepwise reduction procedure is called for. Again, a comparison with the reduced fluorescence spectra obtained in 3-methyl~entane~ helps us to determine in which linear combination of Iuvand IST the z-polarized features just disappear. Similarly as for 2-F-1, these linear combinations are Iu, - [Suv(uz)/ ssT(uz)]lsTwhere u = z for La excitation and u = y for Bb excitation. Figure 9 shows two examples of such reductions for 1: Izz against Izyfor u = z and Izy against Iyyt for u = y . In the first of these two examples, the spectral features and y disappear in the linear combination Izy- 0.25lZz, yielding S z y ( z z ) / S & z ) = 0.25. In the second, they disappear in the linear combination I y y - 0.651zy,yielding SwCyz)/Szy(yz) = 0.65. The reduction factors obtained in this fashion for 1 and 2-Me-1 for both excitation wavelengths (La and Bb) are listed in Table V. Procedures already outlined for the case of 2-F-1 permit the determination of the orientation factors L,, and L,, in three indepdendent ways. The K, values used for 1were those obtained in the IR measurement, and those for 2Me-1 in the UV measurement. The results are listed in Table VI. The absence of a purely y-polarized feature in the emission makes it impossible to obtain L,, in this manner. Our efforts to obtain the missing third independent L,, value from measurements of phosphorescence polarization were unsuccessful due to a combination of its excessive weakness and mixed nature.8 The best values for the orientation factors K and L for d three compounds are collected in Table IV. Discussion TransitionMoment Twisting. We believe that sufficient evidence was given in ref 8 to establish the fact that the transition moment of the Lb origin of 1 and 2-Me-1 imbedded in glassy 3-methylpentane deviates significantly from the molecular y axis, while that of 2-F-1in the same environment does not. It was proposed there that this is due to site effects which effectively lower the D 2 h 0.r CZu symmetry of the chromophore, and whose effect will be felt most strongly when the transition moment in question is very small, i.e., in 1 and 2-Me-1 but not 2-Fe-1 (cf. the Ham effect1'). Since most solvents will interact with
I
25000
I "
26000
27000
F~ i c m ' i
Figure 8. Polarized fluorescence of pyrene in stretched LLDPE at 77 K, excited at the origins of the L,, transition (top), La transition (center), and Bb transition (bottom). The vertical scale is arbitrary.
solutes at least as strongly as 3-methylpentane, the phenomenon should be generally observable. The results presented here for stretched linear low-density polyethylene as solvent provide further strong support for the proposal that the transition moment of the Lb origin de(17) J. s. Ham, J. Chem. Phys., 21, 756 (1953); J. R. Platt, J. Mol. Spectrosc., 9, 288 (1962).
The Journal of Physical Chemistry, Vol. 87, No. 15, 1983 2909
Alignment of Solutes in Stretched Polyethylene
TABLE VI: Orientation Factors L,, of Pyrene and 2-Methylpyrene in Stretched LLDPE at 77 K as Obtained from Various Types of Measurements' L,, Ub
z (La) y (Bb)
(Bb)
z
z (La) y
0.37
(P,r)
z
t
*
0.04
*
(P,r)
(h) z (P,r)
(0.15 i
(0.10 0.41
(ZYiYZ)
Pyrenea
(0.16 O.O1)d (0.14 t O.O1)d 0.36 t 0.04 0.14 i 0.02
O.O1)d
(0.17 0.02)' (0.10 t O.O1)d 0.44 t 0.05 0.16 t 0.02
" y " (a)' " y " (a)'
y (Bb) z (La)
i:
(0.18t
(P,r)
(La) Y
(0.19
" y " (a)' " y " (a)'
(22/ YY' )
(ZZ/ YZ)
(ZZlZY)
UC
t i
(-0.31 i 0.35)' (0.23 i 0.04)d
2-Methylpyrene 0.02)d (0.08 * 0.10)' 0.01)' (0.28 i: 0.33)' 0.05
0.41 i: 0.22 0.16 i 0.32
(0.05
(0.10
0.15
*
t
t
0.09)'
0.11)' 0.07
(ZYiYY') (0.14 (0.15 0.37 0.14
t
0.02)'
t
0.03
(0.20 (0.07
t
0.05)' 0.03)'
* 0.02)' * 0.05
t
(YZl YY' ) (0.12 (0.09
t t
0.01)' 0.02)d
0.14
t
0.04
(0.16 t 0.02)' (0.07 i: 0.03)' 0.40 t 0.04 0.15 i 0.04
' Each entry represents an independent determination of L,,
using the excitation and emission frequencies specified in parentheses on the left and the polarizer arrangements shown on the top, Absorbing transition (in parentheses) and its transition moment direction. Transition moment direction of the emitting Lb transition (the observed fluorescence feature in parentheses). L determined under the assumption that the Lb origin is polarized along y . This is actually not correct: see text.
'
25 000 I
1
27000
26000 ,
,
,
l
I
I
!~ l
~,
,
I
l
,
,
,
,
l
/
dichroic ratio of the Lb origin in the polarized absorption spectra of 1 and 2-Me-1 differs from that of the Bb origin; this was also observed in stretched poly(viny1 alcohol) but was interpreted differently at the time.I5 Moreover, it is very clear that the dichroic ratio of the Lb absorption origin of 1 differs from that common to all y-polarized vibrations observed in its IR spectra,14which in turn is the same as that of the Bb absorption origin. The polarized absorption measurements are particularly important in that it is not possible to attribute the anomalies to rotational depolarization; this argument has been proposed to explain the fluorescence polarization data in glassy solvents6 and has been already questioned in ref 8. The conclusion that the transition moment of the Lb origin is not parallel to y clearly follows. The polarized fluorescence results are also in full agreement with this conclusion. Thus, the reduction factors for both the nominally y-polarized and the z-polarized features in the fluorescence of 1 and 2-Me-1 obtained upon excitation into the nominally y-polarized Lb origin differ from those obtained upon excitation into the actually y-polarized Bb origin. If it is assumed that the Lb origin is purely y polarized and an attempt is made to calculate L,, and L, from the polarized fluorescence intensity ratios, contradictory values arise as shown in Table VI (values in parentheses). Also, the equality Iyz = Izu which should hold whenever the absorbing and emitting transition moments are parallel, and which holds reasonably well for 2-F-1 and even 2-Me-1, considering the above-mentioned difficulties with corrections for polarization bias when exciting at Bb, does not hold at all for 1. Once again, 2-F-1 behaves normally and its transition moments all lie in molecular symmetry axes. The agreement of the results observed for this molecule with those expected for such a normal case reassures us that the anomalies observed for 1 and 2-Me-1 are not due to experimental artifacts or shortcomings in data evaluation. Although the conclusion that in the stretched polyethylene environment the Lb origin in absorption and in emission is not polarized purely along y in 1 and 2-Me-1, in contrast to 2-F-1, is inescapable, it is difficult to analyze the results quantitatively in order to derive information about the orientation distribution of the Lb transition moment direction relative to the y axis. Progress appears possible only if it is assumed that this orientation distribution is independent of the molecular orientation distribution within the sample. This appears unlikely, since there probably is some correlation between the degree of asymmetry of the molecular environment in stretched polyethylene which determines 0 and the degree of align-
n
25 ooo
26000
27000
i;,(em")
Figure 9. Stepwise reduction of the r-polarized spectral features /3 and y in the fluorescence of pyrene in stretched LLDPE. The steps for dare 0.1, for the top curve in each set, d = 0. The curves which most closely resemble the reduced y-polarized spectrum of 1 in glassy 3-methylpentane, shown on top, are dashed.
viates from the symmetry axis y, so that this origin is only nominally y polarized. The measurements of linear dichroism are particularly direct and convincing. There simply is no doubt that the
2910
The Journal of Physical Chemistry, Vol. 87, No. 75, 1983
ment relative to the 2 axis which the environment imparts to the molecule. It is of interest to note that the average deviation 10avl of the transition moment direction of the Lb origin from the y axis induced in stretched polyethylene is clearly larger in 1 than in 2-Me-1 while glassy 3-methylpentane induces a comparable value of 10avl in both. This shows that the bSgcomponent of the solvent-induced perturbation, believeda responsible for the twisting of the Lb transition moment by admixing La character into the excited state, is larger in the former solvent. Following the argument of ref 8, for 1, we estimate that IOav[ = 40° corof the La wave function in responds to a weight 4 X the perturbed Lb wave function in stretched polyethylene while the value was only 1.2 X lo-* in glassy 3-methylpentane. The effective matrix elements needed to introduce this degree of ?&--La mixing are of the order of 50 cm-' in stretched polyethylene and 30 cm-' in glassy 3methylpentane. These numbers can be compared with the 50 cm-' estimated for 2-Me-1, which shows no significant difference between the two solvents. The difference in the results of site-selection experiments for 1 and for 2-F-1 is also readily interpreted by using the concept of environment-induced transition moment twisting. As we shall see in the following, there is very little scatter in the orientation of the molecules of either 1 or 2-F-1 around their average orientation. This is certainly in agreement with the observation that site (energy) selection does not cause orientation selection in the case of 2-F-1, and is hardly compatible with the notion that it does in 1. The orientation factors of the two solutes appear to be very similar (Table IV), providing an additional argument in this direction. If we admit that site selection causes no orientation selection for either 1 or 2-F-1, it follows from the observed results that it causes a selection of the Lb transition moment direction within the molecular framework of 1 but not 2-F-1. The degree of Lb-L, mixing may well be a similarly sensitive function of the environment in both 1 and 2-F-1 and should be affected by the same properties of the environment which determine the 0-0 energy. As noted above, a given degree of solvent-induced Lb-L, mixing will produce a far greater deviation of the Lb transition moment from the y axis in 1 than in 2-F-1. Orientation Distribution. The known orientation factors for 1,2-F-1, and 2-Me-1 are collected in Table IV.The data are most complete for 2-F-1 but are very similar for all three compounds. To our knowledge, this is the first time that all five independent second and fourth moments of the orientation distribution have become available for a solute (2-F-1)in a stretched polymer (or, for that matter, for a solution in any anisotropic solvent). The most striking property of the L , values is their closeness to the K,2 values. The limit L , = KU2is reached when the angle u which the u-th molecular axis makes with the stretching direction 2 is identical for all of the solute molecules. This situation occurs for the out-of-plane axis x within our experimental accuracy, and the limit is nearly reached for the other two axes as well, indicating that the fraction of molecules whose orientation deviates substantially from the average must be quite small. While the knowledge of the five orientation factors permits us to reject those orientation distribution functions that do not fit, it does not permit us to determine what the correct orientation distribution function actually is, since an infinite number of acceptable functions exist. If we were to identify one as a fair representative of those which are acceptable, it would be simplest to use the fact
Langkilde et al.
that the limit L , = K,* is nearly reached and to approximate the orientation distribution by one in which all molecules are oriented alike, with the angles 37.5O, 57.5O, and 73.5O, respectively, between the z, y, and x molecular axis and the stretching direction. That this cannot be strictly correct is clear from the small but real differences between K U 2and L , for u = y and z. In itself, even the complete knowledge of the orientation distribution function does not represent an answer on the ultimate level of inquiry. The really interesting questions deal with the nature of the microscopic environment of the solute molecules and of the forces which cause them to be oriented (i.e., with the detailed orientation model). In the following, we shall consider briefly the implications of our results for two of the detailed orientation models which have been proposed in the literature. Margulies and Yogevloahave proposed that the orientation of aromatics in stretched polyethylene is bimodal: a fraction f , presumably located inside the crystalline domains of polyethylene, is oriented so that their molecular planes are exactly aligned with the stretching direction (K, = 0), and a fraction 1 - f , presumably located within the amorphous domains, is distributed isotropically. Our results are not in particularly good agreement with this model. If we label the orientation factors of the former fraction K,' and L,,' and those of the latter fraction K," and L,,", we can write for the observed orientation factors K, and L,, K, = fK,' (1 - f)K,'so
+
+ (1 - f)L,,'"" where Kuiso= 1/3, LuuLSo = (1 + 26,,)/15, and K,' L,, = fL,,'
= L,' =
Ls,' = L,,' = 0, so that
f = 1 - 3K, K,'
( K , - K,) / (1 - 3K,)
In our case, the K, values for 2-F-1 yield f = 0.76, Ky' = 0.28, and K,' = 0.72; thus, 24% of the molecules would have to be oriented randomly if the above bimodal hypothesis were correct. It is seen immediately that it is difficult to reconcile the measured L,, values with this model. The model demands L , = 0.05 and L,, = L,, = 0.02, and this disagrees with the experimental results (Table IV): L , = 0.00 f 0.04, L,, = 0.06 f 0.04, L,, = 0.02 f 0.02, where the error limits represent maximum deviations. A t least for 2-F-1 in stretched LLDPE, we are inclined therefore to consider this particular proposed bimodal model as unlikely. It was originally proposed to fit the measurements of polarized fluorescence of anthracene in stretched polyethyleneloa and we cannot exclude the possibility that it is correct there. Konwerska-Hrabowska and Kryszewskil8 have proposed that the pyrene molecules absorbed in polyethylene are located at the surface of the microscopic crystallites. This proposal has been refined more recentlylg to state that a relatively well-oriented fraction of the solute molecules is located at the surface of the crystallites while a relatively poorly oriented remainder is located within the amorphous phase. This modification thus reintroduces the notion of a bimodal distribution, with the relative weight of the two components dictated by the nature of the solute. (18) J. Konwerska-Hrabowska and M. Kryszewski, Bull. Acad. Pol. Sci., Ser. Sci. Math., Astron. Phys., 21, 673, 771 (1973). (19) Y. T. Jang, P. J. Phillips, and E. W. Thulstrup, Chem. Phys. Lett., 93,66 (1982).
J. Phys. Chem. 1983, 87, 2911-2914
Our results are compatible with this more general model but place some reasonable restrictions on it. It needs to be postulated that (i) pyrene is one of those solutes which strongly prefer to be located on the surface of the crystallites, as proposed in ref 18, (ii) the polyethylene crystallites are all aligned almost alike, and (iii) the pyrene molecules prefer a particular orientation on the crystallite surface to the near exclusion of others. The postulates ii and iii are required in order to account for the near equality of L, and KU2. More detailed comparison with both models will require the orientation distribution factors for a few more solutes and for a range of stretching ratios on the one hand, and additional information on the structure of the stretched
2911
polymer, such as the orientation distribution of the crystallites from X-ray diffraction patterns, on the other hand. It is likely that considerable further effort will be required before the detailed solute orientation mechanism is elucidated with anything like final validity. Acknowledgment. This work was supported by the U.S. Army Research Office under contract DAAG29-82-K-0024. F.W.L. acknowledges a travel grant from the Danish Natural Science Research Council, and M.G. a grant from the Swiss National Science Foundation. We are grateful to Prof. A. Berg for a generous gift of samples. Registry No. 2-Fluoropyrene, 1714-25-6; polyethylene, 9002-88-4.
Conformational Properties of Perfluoroalkane Chains. 7. Carbon-I 3, Fluorine, and Proton Spin-Lattice Relaxation of Poly(decamethylene perfluorosebacate) in Solution and the Local Mobility of the Fluorocarbon Chain Kelro Matsuot and Walter H. Stockmayer' Department of Chemistry, Dartmouth College, Hanover, New Hampshlre 03755 (Received:January 13, 1983)
Chloroform-d solutions of poly(decamethy1ene perfluorosebacate) were studied by I3C, 19F, and 'H NMR. Proton-decoupled or fluorine-decoupled 13C spin-lattice relaxation times were measured over the temperature range 5-75 "C. Mean orientational correlation times for C-F and C-H bonds are found to differ by a factor of only about 2.3 (e.g., 104 and 45 ps at 25 "C), and have equal activation energies of about 3 kcal mol-'. The 19Frelaxation data are consistent with the 13C results.
Introduction The various physical properties of poly(tetrafluor0ethylene),'J PTFE, particularly its very high melt viscosity and supposedly3" low entropy of fusion, have customarilp but not alwaysgbeen regarded as indicative of considerable chain stiffness, both equilibrium and dynamic, as compared to hydrocarbon polymers including poly(ethy1ene). To some extent this view, though not in an extreme form, has been regarded as consistent also with properties of nonpolymeric fluorocarbons and their derivatives,lOJ1but there is some r e a ~ o n ~to, ~suppose J~ that the difference in conformational behavior between alkane and perfluoroalkane chains is after all rather modest. Molecular motions in both crystalline and amorphous phases of bulk PTFE have been elegantly probed with multiple-pulse NMR methods by Vega and Eng1i~h.l~ Information on the dynamic flexibility of single perfluoroalkane chains in mobile liquids or solutions is perhaps more elusive. The insolubility of PTFE and related copolymers in all solvents at ordinary temperatures is a formidable obstacle; and for liquid or dissolved non'Present address: Technisch-Chemisches Laboratorium, Swiss Federal Institute of Technoloev. ETH-Zentrum. CH-8092 Zurich, Switzerland. L"
polymeric fluorocarbons or derivatives the internal motions of the chain are greatly if not totally obscured by the (1) Sperati, C. A. In "Polymer Handbook"; Brandrup, J.; Immergut, E. H., Ed.; Wiley-Interscience: New York, 1975; 2nd ed, V-29.
(2) Wall, L. A., Ed. "Fluoropolymers"; Wiley-Interscience: New York, 1972. (3) Until recently the accepted value of the entropy of fusion of PTFE was 1.14 cal K-' (mol CF#, as compared to a figure of 2.37 for polyethylene. See, for example, Sperati, C. A.; Starkweather, H. W., Jr., Adu. Polym. Sci. 1961,2,465. New experiments on PTFE have produced an upward revision (see following reference)' to 1.84 cal K-I (mol CF2)-I, which is still somewhat lower than the polyethylene figure. The main reason for the difference between the two polymers is the existence of disorder in crystalline PTFE.5 Indeed, the entropy difference between the most ordered crystalline phase of PTFE (below 19 "C) and the melt is greater than 2.5 cal K-l (mol CF2)-l. (4) Starkweather, H. W. Jr.; Zoller, P.; Jones, G. A.; Vega, A. J. J. Polym. Sci., Polymn. Phys. Ed. 1982, 20, 751. (5)Bates, T. W.; Stockmayer, W. H. Macromolecules 1968, 1 , 17. (6) Nishioka, A.; Watanabe, M. J. Polym. Sci. 1957, 24, 298. (7) Billmeyer, F. W., Jr. "Textbook of Polymer Science"; Wiley-Interscience: New York, 1971; 2nd ed, pp 224, 424. (8) Starkweather, H. W.; Boyd, R. H. J . Phys. Chem. 1960, 64, 410. (9) Flory, P. J. "Statistical Mechanics of Chain Molecules"; Wiley-Interscience: New York, 1969; pp 153, 157. (10) (a) Bates, T. W. Trans. Faraday SOC.1967, 63, 1825. (b) Ibid. 1968, 64, 3180. (c) Bates, T. W.; Stockmayer, W. H. Macromolecules 1968, 1 , 13. (11) Lyerla, J. R., Jr., VanderHart, D. L. J. Am. Chem. SOC.1976,98, 1697.
0022-3654/83/2087-2911$01.50/00 1983 American Chemical Society
