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Triplet-state ESR spectra of aromatic molecules oriented in stretched polyethylene films
Volume 132, number 3
CHEMICAL PHYSICS LETTERS
12 December 1986
TRIPLET-STATE ESR SPECTRA OF AROMATIC MOLECULES ORIENTED INSTRETCHED POLYETHYLENE FILMS*
I.M. BROWN McDonnell Douglas Research Laboratones, St. Louis, MO 63166, USA
P.J. PHILLIPS and D. PARIKH Department of MatertalScience
and Engineering,
University of Tennessee, Knoxville, TN 37994, USA
Received 28 August 1986; in final form 16 September 1986
Triplet-state ESR spectra show that when the aromatic molecules anthracene, anthracene-d,,,, terphenyl, and acridine are dissolved m stretched, low-density polyethylene films, a fraction of these solute molecules become oriented with their X axis along the stretch direction but with their XY planes oriented randomly to the stretch direction. The fraction of molecules involved in the orientation process is largest for terphenyl.
2. Experimental 1. Introduction
Oriented polymers containing aromatic solute molecules have been studied with several techniques such as visible and infrared spectroscopy [ l-31, thermal differential scanning calorimetry [ 41, dielectric relaxation [ 51, and magnetic resonance [ 61. All of these studies show that any orientation of the aromatic solute molecules by oriented polymer chains can significantly affect the resulting mechanical, optical, dielectric and magnetic properties of the polyethylene films. In this Letter we report some additional information on the identity of the potential sites for the location of aromatic solute molecules that have been oriented in stretched polyethylene. The excited triplet states of the aromatic hydrocarbon solute molecules were studied by electron spin resonance (ESR). The triplet-state ESR spectra are particularly informative about the nature and extent of the molecular orientation, because the zero-field splittings which determine these lineshapes are highly anisotropic.
* This work was supported by the McDonnell Douglas Independent Research and Development Program.
The polyethylene films were produced by the techniques described earlier [ 51. The aromatic solutes were introduced into the polyethylene by soaking the stretched films in chloroform solutions of the aromatic solutes. All the ESR experiments were performed by use of an X-band spectrometer (IBM model 200D). The UV irradiation was performed with a 100 W high-pressure Hg lamp, a 20 cm water filter to remove the long infrared wavelengths, and a Corning # 9863 UV filter.
3. Results and discussion Low-density polyethylene film samples containing up to 0.5 wt”h anthracene-dlO, terphenyl, or acridine were examined. The triplet-state ESR spectra of the aromatic solutes in stretched and unstretched film were studied at 77 K. In the unstretched film samples the ESR spectra of the triplet states associated with anthracene, anthracene-d,,-, and terphenyl correspond to that expected from a collection of randomly oriented individual molecules whose energy levels are given by the following spin Hamiltonian in the principal coordinate system (X, Y, Z) :
0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
273
CHEMICAL PHYSICS LETTERS
Volume 132, number 3
Magnetic
Magnetic
field (mT)
Fig. 1. Triplet-state ESR spectra observed from anthracene-d,, in polyethylene: (a) before stretching, i.e. randomly oriented molecules; (b) after stretching and with the magnetic field direction parallel to the stretch direction; (c) after stretching and with the magnetic field direction perpendicular to the stretch direction.
H=/?H-g*S+D(Sf
- f)+E(S;-S;)
)
(1)
where D and E are the zero-field splitting parameters that are determined primarily by the dipolar spin-spin interaction between the unpaired electrons. In the example shown in fig. 1, the spectrum for the triplet state of anthracene, the designated X (Y, 2) lines correspond to the two transitions associated with molecules oriented with their X (Y, Z) molecular axes parallel to the magnetic field direction. Moreover, from anthracene single-crystal studies [ 7,8 1, the principal axis system for the zero-field tensor in the spin Hamiltonian given by eq. (1) was found to be as shown in fig. 2a. The triplet-state ESR spectra of stretched polyethylene film samples containing the four aromatic hydrocarbon solutes were recorded with the magnetic field oriented both parallel and perpendicular to the film’s stretch direction. Figs. lb and lc show the triplet spectra for anthracene in films in which the magnetic field is oriented parallel and perpendicular to the stretch direction, respectively. As can be seen in comparing fig. 1b with fig. 1a, when the magnetic field is parallel to the stretch direction, the X lines are greatly enhanced, the Ylines are just detectable, and the Z lines are absent. On the other hand, for the spectrum taken with the magnetic field per274
12 December 1986
field (mT)
Frg. 2. (a) Principal axes system for the zero-field tensor in anthracene. Triplet-state ESR spectra observed from acridine in polyethylene (b) before stretching, i.e. randomly oriented molecules, and (c) after stretching and with the magnetic field direction parallel to the stretch direction.
pendicular to the stretch direction (see fig. lc) the Y and Z lines are intense whereas the X lines are weak. In all instances where the X, Y, and Z lines are observed in the stretched films, the lines always occur at the same field values observed in the unstretched films. These results suggest that two types of anthracene molecules exist after stretching: (1) those that are unoriented and (2) those oriented with their X axis along the stretch direction, but with their XY plane oriented randomly with respect to the stretch direction. Results of dielectric relaxation studies of l-bromonaphthalene in polyethylene films [ 51 indicate that the latter type of anthracene molecules are epitaxially located on the surfaces of the polyethylene crystallites ‘whereas the former type of molecules are in the amorphous regions. Furthermore, the melting characteristics determined by differential scanning calorimetry for acridine in polyethylene [ 41 suggest that the aromatic solute molecules can conglomerate to form microcrystals which are oriented epitaxially on the surfaces of the polyethylene crystallites. However, the ESR results in the stretched films can be interpreted unequivocally in terms of triplet states The polyethylene crystallites are thought to consist of folded chain arrangements with the chain axis along the stretch direction.
Volume 132, number
3
CHEMICAL
PHYSICS
associated with isolated individual aromatic molecules. For example, in the case of anthracene-d,, in the stretched and unstretched films, the D and E values are given by 1D 1lhc= 0.0725 f 0.002 cm- ‘, and 1E 1lhc= 0.0084 k 0.002 cm- ‘, which are in excellent agreement with the values reported [ 7,8] for single molecules of anthracene (Dlhc=0.0688 f 0.004 cm-‘, E/he= -0.0081 f0.002 cm-‘). In the spectra of the excited triplet state for terphenyl, only the two X lines were observed when the magnetic field direction was along the stretch direction and only the Y and 2 lines were detected when the magnetic field was perpendicular to the stretch direction. These results indicate that following stretching there is only one type of terphenyl solute molecule, namely that oriented with its X axis along the stretch direction and its XY plane oriented randomly with respect to the stretch direction. This randomness may be attributed to the randomness of the crystallites around the stretch direction and/or to the possible random orientation of the aromatic XY plane of the solute molecule with respect to the surface of a polyethylene crystallite. At this stage it is not possible to separate these contributions, but work is in progress to study the ESR triplet lineshapes as a function of aromatic solute concentration. Any change in orientation of the aromatic plane of the solute molecule with respect to the polymer crystallite surface which occurs with increasing coverage of the polymer surface (e.g. from a “flat-on” position to a “slotted-in” position [4]) should be apparent from a comparison of the experimental and computer-simulated triplet-state ESR lineshapes. The lack of any detectable second type of terphenyl molecule in the ESR spectrum indicates (a) the absence of randomly oriented molecules in the amorphous phase and also (b) the uniform, high alignment of the polymer crystallites along the stretch direction.
LETTERS
I2 December
1986
Any triplet excitons generated in the aromatic microcrystals were not detected from their ESR spectra. We therefore conclude that either (1) no microcrystals are formed in our samples (containing 0.5 wt% solute) and all the solute is present as single, isolated molecules or (2) microcrystals are present along with the isolated molecules but the remaining randomness around the stretch direction leads to enough inhomogeneous broadening of the ESR triplet exciton spectrum to render it undetectable. Thus, the absence of triplet exciton spectra in these samples need not be in conflict with the previously reported melting characteristics [ 41 that indicate the presence of a&dine microcrystals. Even under the conditions of continuous light irradiation, the spectra of several of the stretched film samples exhibited spin-polarization effects, which have already been observed in transient experiments with a pulsed laser source [ 61. Thus, as shown in fig. 2c, in the spectrum for acridine with the magnetic field direction along the stretch direction, the intensity of the low-field X line is 35O/bless than that for the high-field X line.
References [ I ] E.W. Thulstrup and J. Michl, J. Phys. Chem. 84 (1970) 82. [2] J.G. Radziszewski and J. Michl, J. Phys. Chem. 85 (1981) 2934. [ 31 M.B. Mitchell, W.A. Guillory, J. Michl and J. Radziszewski, Chem. Phys. Letters 96 (1983) 413. [ 41 D. Parikh and P.J. Phillips, J. Chem. Phys. 83 (1985) 1948. [5] Y.T. Jang, P.J. Phillips and E.W. Thulstrup, Chem. Phys. Letters 93 (1982) 66. [6] S.S. Rim and S.I. Weissman, J. Am. Chem. Sot. 101 (1979) 5864. [7] D. Haarer, D. Schmid and H.C. Wolf, Phys. Stat. Sol. 23a (1967) 633. [ 81 D. Haarer and H.C. Wolf, Mol. Cryst. Liquid Cryst. 10 (1970) 359.
275
CHEMICAL PHYSICS LETTERS
12 December 1986
TRIPLET-STATE ESR SPECTRA OF AROMATIC MOLECULES ORIENTED INSTRETCHED POLYETHYLENE FILMS*
I.M. BROWN McDonnell Douglas Research Laboratones, St. Louis, MO 63166, USA
P.J. PHILLIPS and D. PARIKH Department of MatertalScience
and Engineering,
University of Tennessee, Knoxville, TN 37994, USA
Received 28 August 1986; in final form 16 September 1986
Triplet-state ESR spectra show that when the aromatic molecules anthracene, anthracene-d,,,, terphenyl, and acridine are dissolved m stretched, low-density polyethylene films, a fraction of these solute molecules become oriented with their X axis along the stretch direction but with their XY planes oriented randomly to the stretch direction. The fraction of molecules involved in the orientation process is largest for terphenyl.
2. Experimental 1. Introduction
Oriented polymers containing aromatic solute molecules have been studied with several techniques such as visible and infrared spectroscopy [ l-31, thermal differential scanning calorimetry [ 41, dielectric relaxation [ 51, and magnetic resonance [ 61. All of these studies show that any orientation of the aromatic solute molecules by oriented polymer chains can significantly affect the resulting mechanical, optical, dielectric and magnetic properties of the polyethylene films. In this Letter we report some additional information on the identity of the potential sites for the location of aromatic solute molecules that have been oriented in stretched polyethylene. The excited triplet states of the aromatic hydrocarbon solute molecules were studied by electron spin resonance (ESR). The triplet-state ESR spectra are particularly informative about the nature and extent of the molecular orientation, because the zero-field splittings which determine these lineshapes are highly anisotropic.
* This work was supported by the McDonnell Douglas Independent Research and Development Program.
The polyethylene films were produced by the techniques described earlier [ 51. The aromatic solutes were introduced into the polyethylene by soaking the stretched films in chloroform solutions of the aromatic solutes. All the ESR experiments were performed by use of an X-band spectrometer (IBM model 200D). The UV irradiation was performed with a 100 W high-pressure Hg lamp, a 20 cm water filter to remove the long infrared wavelengths, and a Corning # 9863 UV filter.
3. Results and discussion Low-density polyethylene film samples containing up to 0.5 wt”h anthracene-dlO, terphenyl, or acridine were examined. The triplet-state ESR spectra of the aromatic solutes in stretched and unstretched film were studied at 77 K. In the unstretched film samples the ESR spectra of the triplet states associated with anthracene, anthracene-d,,-, and terphenyl correspond to that expected from a collection of randomly oriented individual molecules whose energy levels are given by the following spin Hamiltonian in the principal coordinate system (X, Y, Z) :
0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
273
CHEMICAL PHYSICS LETTERS
Volume 132, number 3
Magnetic
Magnetic
field (mT)
Fig. 1. Triplet-state ESR spectra observed from anthracene-d,, in polyethylene: (a) before stretching, i.e. randomly oriented molecules; (b) after stretching and with the magnetic field direction parallel to the stretch direction; (c) after stretching and with the magnetic field direction perpendicular to the stretch direction.
H=/?H-g*S+D(Sf
- f)+E(S;-S;)
)
(1)
where D and E are the zero-field splitting parameters that are determined primarily by the dipolar spin-spin interaction between the unpaired electrons. In the example shown in fig. 1, the spectrum for the triplet state of anthracene, the designated X (Y, 2) lines correspond to the two transitions associated with molecules oriented with their X (Y, Z) molecular axes parallel to the magnetic field direction. Moreover, from anthracene single-crystal studies [ 7,8 1, the principal axis system for the zero-field tensor in the spin Hamiltonian given by eq. (1) was found to be as shown in fig. 2a. The triplet-state ESR spectra of stretched polyethylene film samples containing the four aromatic hydrocarbon solutes were recorded with the magnetic field oriented both parallel and perpendicular to the film’s stretch direction. Figs. lb and lc show the triplet spectra for anthracene in films in which the magnetic field is oriented parallel and perpendicular to the stretch direction, respectively. As can be seen in comparing fig. 1b with fig. 1a, when the magnetic field is parallel to the stretch direction, the X lines are greatly enhanced, the Ylines are just detectable, and the Z lines are absent. On the other hand, for the spectrum taken with the magnetic field per274
12 December 1986
field (mT)
Frg. 2. (a) Principal axes system for the zero-field tensor in anthracene. Triplet-state ESR spectra observed from acridine in polyethylene (b) before stretching, i.e. randomly oriented molecules, and (c) after stretching and with the magnetic field direction parallel to the stretch direction.
pendicular to the stretch direction (see fig. lc) the Y and Z lines are intense whereas the X lines are weak. In all instances where the X, Y, and Z lines are observed in the stretched films, the lines always occur at the same field values observed in the unstretched films. These results suggest that two types of anthracene molecules exist after stretching: (1) those that are unoriented and (2) those oriented with their X axis along the stretch direction, but with their XY plane oriented randomly with respect to the stretch direction. Results of dielectric relaxation studies of l-bromonaphthalene in polyethylene films [ 51 indicate that the latter type of anthracene molecules are epitaxially located on the surfaces of the polyethylene crystallites ‘whereas the former type of molecules are in the amorphous regions. Furthermore, the melting characteristics determined by differential scanning calorimetry for acridine in polyethylene [ 41 suggest that the aromatic solute molecules can conglomerate to form microcrystals which are oriented epitaxially on the surfaces of the polyethylene crystallites. However, the ESR results in the stretched films can be interpreted unequivocally in terms of triplet states The polyethylene crystallites are thought to consist of folded chain arrangements with the chain axis along the stretch direction.
Volume 132, number
3
CHEMICAL
PHYSICS
associated with isolated individual aromatic molecules. For example, in the case of anthracene-d,, in the stretched and unstretched films, the D and E values are given by 1D 1lhc= 0.0725 f 0.002 cm- ‘, and 1E 1lhc= 0.0084 k 0.002 cm- ‘, which are in excellent agreement with the values reported [ 7,8] for single molecules of anthracene (Dlhc=0.0688 f 0.004 cm-‘, E/he= -0.0081 f0.002 cm-‘). In the spectra of the excited triplet state for terphenyl, only the two X lines were observed when the magnetic field direction was along the stretch direction and only the Y and 2 lines were detected when the magnetic field was perpendicular to the stretch direction. These results indicate that following stretching there is only one type of terphenyl solute molecule, namely that oriented with its X axis along the stretch direction and its XY plane oriented randomly with respect to the stretch direction. This randomness may be attributed to the randomness of the crystallites around the stretch direction and/or to the possible random orientation of the aromatic XY plane of the solute molecule with respect to the surface of a polyethylene crystallite. At this stage it is not possible to separate these contributions, but work is in progress to study the ESR triplet lineshapes as a function of aromatic solute concentration. Any change in orientation of the aromatic plane of the solute molecule with respect to the polymer crystallite surface which occurs with increasing coverage of the polymer surface (e.g. from a “flat-on” position to a “slotted-in” position [4]) should be apparent from a comparison of the experimental and computer-simulated triplet-state ESR lineshapes. The lack of any detectable second type of terphenyl molecule in the ESR spectrum indicates (a) the absence of randomly oriented molecules in the amorphous phase and also (b) the uniform, high alignment of the polymer crystallites along the stretch direction.
LETTERS
I2 December
1986
Any triplet excitons generated in the aromatic microcrystals were not detected from their ESR spectra. We therefore conclude that either (1) no microcrystals are formed in our samples (containing 0.5 wt% solute) and all the solute is present as single, isolated molecules or (2) microcrystals are present along with the isolated molecules but the remaining randomness around the stretch direction leads to enough inhomogeneous broadening of the ESR triplet exciton spectrum to render it undetectable. Thus, the absence of triplet exciton spectra in these samples need not be in conflict with the previously reported melting characteristics [ 41 that indicate the presence of a&dine microcrystals. Even under the conditions of continuous light irradiation, the spectra of several of the stretched film samples exhibited spin-polarization effects, which have already been observed in transient experiments with a pulsed laser source [ 61. Thus, as shown in fig. 2c, in the spectrum for acridine with the magnetic field direction along the stretch direction, the intensity of the low-field X line is 35O/bless than that for the high-field X line.
References [ I ] E.W. Thulstrup and J. Michl, J. Phys. Chem. 84 (1970) 82. [2] J.G. Radziszewski and J. Michl, J. Phys. Chem. 85 (1981) 2934. [ 31 M.B. Mitchell, W.A. Guillory, J. Michl and J. Radziszewski, Chem. Phys. Letters 96 (1983) 413. [ 41 D. Parikh and P.J. Phillips, J. Chem. Phys. 83 (1985) 1948. [5] Y.T. Jang, P.J. Phillips and E.W. Thulstrup, Chem. Phys. Letters 93 (1982) 66. [6] S.S. Rim and S.I. Weissman, J. Am. Chem. Sot. 101 (1979) 5864. [7] D. Haarer, D. Schmid and H.C. Wolf, Phys. Stat. Sol. 23a (1967) 633. [ 81 D. Haarer and H.C. Wolf, Mol. Cryst. Liquid Cryst. 10 (1970) 359.
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