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Molecular microscopy of polydiacetylene monolayers
Ultramicroscopy 9 (1982) 225-230 North-Holland Publishing Company
225
MOLECULAR MICROSCOPY OF POLYDIACETYLENE MONOLAYERS L.T. G E R M I N A R I O * and P.C. G I L L E T T E Department of Macromolecular Science, Case Institute of Technology, Case Western Reserve University, Cleveland, Ohio 44106, USA
Received 14 June 1982 (presented at Workshop January 1982)
High resolution electron microscopy, selected-area electron diffraction (SAD), and digital recording of electron diffraction intensities have been used to study the structure properties and electron beam sensitivity of pristine and osmium-tetroxide-treated monolayer (Langmuir-Blodgett) films of the polydiacetylene, 10,12-nonacosadiynoicacid. Electron diffraction and computed b-c projections of the Patterson function of self-supporting polydiacetylene monolayers indicated that the packing symmetry is unaffected by the osmate ester reaction. These results are combined with published data and used to support the contention that the deleterious secondary processes which limit attainable resolution of organic polymers can be minimized by heavy-metal "staining".
1. Introduction The polydiacetylenes are an important new class of organic polymers owing to their strongly anisotropic behavior. This anisotropy reflects the underlying structure of the materials: they are made up of m a n y parallel chains with unusual optical and electrical properties [1]. The planar and fully conjugated polymer b a c k b o n e is best represented by the following resonant structures: R~)C_~C_ ~ \ R2
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The chemical and physical properties of polydiacetylenes undergo dramatic changes on exposure to selected impurities or dopants such as A s F 5 [ 1]. These changes are believed to be due to the formation of charge-transfer complexes between the polydiacetylene and the doping species or diacetylene side groups (R~, R2). The polydiacetylene monomer, about which this study is concerned, is 10,12-nonacosadiynoic acid, * Inquiries should be addressed to Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, Tennessee 37662, USA.
where R I - - - ( C H 2 ) I 6 - H and R 2 - - - ( C H 2 ) 8C O O H . Its molecular and crystal structure [2] is shown in fig. 1. It is surface active and thus forms two-dimensional arrays of m o n o m e r units (Langm u i r - B l o d g e t t films) at a g a s - w a t e r interface. The m o n o m e r crystal can be polymerized by either ionizing or ultraviolet (UV) radiation to form long conjugated polymer chains which are rigid, selfsupporting and can form large two-dimensional crystalline domains [3]. The structure and properties of the polydiacetylenes and the charge-transfer complex have been investigated by optical, U V as well as by
l o
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Fig. 1. a - c projection of polymer monolayer, a =34.5 ,~ and c =4.89 .~.
0 3 0 4 - 3 9 9 1 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
226
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Fig. 2. Untreated monolayerfilms, dose= 1 C/cm2. Crystalline, amorphous and holey regions are evident.
photoelectron, Raman, electron spin resonance, X-ray, spectroscopies, and electron diffraction [14]. The underlying theme of all these studies is to gain basic knowledge of the effect of microstructure on the properties of the polymer. These techniques can, however, only provide indirect information on the molecular structure. This work takes advantage of the unique electrical properties of the polydiacetylenes, as well as the high spatial resolution capabilities of state-of-
O
Fig. 3. Second exposureof fig. 2 to a dose of about 1.5 C//cm2.
the-art electron microscopes, for directly observing molecular structures.
2. Experimental Monolayers were spread and polymerized in a Teflon trough, as previously described [3]. Polymer monolayer samples were deposited on grids by lifting the grid support from the water through the
L.T. Germinario, P.C. Gillette / Molecular microscopy of polvdiacet~lene monolayers
227
Fig. 4. Osmium-treated polymer monolayer. Only crystalline and amorphous regions are evident. Dose = 20 C / c m 2.
gas interface. Monolayer images and diffraction patterns were recorded on Kodak electron image plates with a JEOL JEM 100C electron microscope at 100 kV. A JEOL JEM 100B was modified for computer control of collection of selected electron diffraction intensities. The hardware is built around a Digital Minc 11 microcomputer which is controlled from a VT 105 data terminal with an alphanumeric keyboard. A dual floppy RX O 2 is used for recording intensity data and for loading the system operating programs. Patterson maps were computed using a Digital VAX 11/780. Small angle laser scattering patterns (SALS) were recorded from exposed electron image plates on an optical bench equipped with a helium-neon laser. Grids containing monolayers were exposed to osmium tetroxide vapors for 15 min before examination in the electron microscope.
3. Results and discussion
The a - c projection of the polymer monolayer is shown in fig. 1 [3], where a = 34.5 A and x = 4.89 A. The polymer backbone is the c axis and consists of conjugated double and triple bonds. The b - c
projection is along the electron beam axis and is shown in the insert of fig. 2, where b--8.11 A. This polymer thus consists of two polyethylenetype R groups, where R~ consists of a chain containing 16 carbon atoms, with 8 atoms being superimposed on the b - c projection, while R 2 consists of an 8-carbon-atom chain with 4 atoms being superimposed on the b - c projection [3]. In analyzing untreated monolayer films in the electron microscope (fig. 2) at a magnification of 2 × 105 and a dose of about 1 C / c m 2 (625 e/A2), crystalline, as well as amorphous and holey regions, were evident. A second exposure of the same region to a dose of about 1.5 C / c m 2 (937 e / A 2) reveals (fig. 3) further degradation of the monolayer film. The high sensitivity of these films to the electron beam precluded optimization of astigmatism correction. The coexistence of crystalline, amorphous and holey regions in the monolayer films attests to the highly localized nature of the deleterious inelastic scattering events [5]. Examination of polymer monolayers which were exposured to osmium tetroxide vapors revealed the absence of holey regions, even after exposures of 20 C / c m 2 (1.25 × l04 e / A 2) (fig. 4). The use of thin crystalline films provides several
L. T. Germinario, P. C. Gillette / Molecular microscopy of polydiacetylene monolayers
228
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advantages. It provides molecular packaging to minimize thermal vibration, as well as providing stacked rows of molecules and atoms to increase the contrast of the projected image. The thickness of the monomer (34.5 A) is such that the specimen acts' like a phase object and produces its best image at the Scherzer value of underfocus. The effect of osmium atoms on electron scattering in the b-c projected polymer structures is shown to be negligible by the similarity in the Patterson maps of treated and untreated polymer films (fig. 5).
Finally, radiation damage was monitored by measuring fading of the diffraction spots (002), (020), and (011) of both pristine and doped polymer films. The data are shown in fig. 6. These reflections correspond to the spacings of 4.89 ~, (c), 8.11 .~ (b), and 8.38 .~ (a), respectively. In each case the dose rate is 3.1 × 10 - 4 C / c m 2- S (0.2 e / A . s) and data were taken for a period of 60 s (12 e/cm2). This method of monitoring radiation damage shows the osmium-treated specimens to be slightly more resistant to radiation damage. However, examination of the molecular images pro-
L. 7". Germinario, P.C. Gillette / Molecular microscopy of polydiacetylene monolayers
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vides a m o r e realistic m e c h a n i s m for the fading of the Bragg reflections, i.e., the intensity loss in o s m i u m - t r e a t e d specimens a p p e a r s to b e due p r i m arily to m o l e c u l a r disorderir~g. Therefore, the presence of o s m i u m in the p o l y m e r significantly increases its resistivity to r a d i a t i o n damage.
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H i g h r e s o l u t i o n electron m i c r o s c o p y can thus offer a direct m e a n s of observing m o l e c u l a r structures a n d when used in c o n j u n c t i o n with o t h e r i n d i r e c t m e t h o d s for structure analysis can p r o v i d e a m o r e c o m p r e h e n s i v e m e a n s of s t u d y i n g the chemical a n d p h y s i c a l p r o p e r t i e s of this i m p o r t a n t new class of o r g a n i c polymers•
References
[1] G. Wegner, in: Chemistry and Physics of One-Dimensional Metals, NATO Advanced Studies, Set. B, Physics, Ed. H.J. Keller (Plenum, New York, 1977) p. 297. [2] D.R. Day and J.B. Lando, Macromolecules 13 (1980) 1483. [3] D.R. Day and J.B. Lando, Macromolecules 13 (1980) 1478. [4] W. Deits, P. Cukor and M. Rubner, in: Conductive Polymers, Ed. R.B. Seymour (Plenum, New York, 1981) p. 171. [5] M.S. Isaacson, in: Principles and Techniques of Electron Microscopy, Vol. 7, Ed. M.A. Hayat (Van Nostrand-Reinhold, New York, 1976) p. 1.
225
MOLECULAR MICROSCOPY OF POLYDIACETYLENE MONOLAYERS L.T. G E R M I N A R I O * and P.C. G I L L E T T E Department of Macromolecular Science, Case Institute of Technology, Case Western Reserve University, Cleveland, Ohio 44106, USA
Received 14 June 1982 (presented at Workshop January 1982)
High resolution electron microscopy, selected-area electron diffraction (SAD), and digital recording of electron diffraction intensities have been used to study the structure properties and electron beam sensitivity of pristine and osmium-tetroxide-treated monolayer (Langmuir-Blodgett) films of the polydiacetylene, 10,12-nonacosadiynoicacid. Electron diffraction and computed b-c projections of the Patterson function of self-supporting polydiacetylene monolayers indicated that the packing symmetry is unaffected by the osmate ester reaction. These results are combined with published data and used to support the contention that the deleterious secondary processes which limit attainable resolution of organic polymers can be minimized by heavy-metal "staining".
1. Introduction The polydiacetylenes are an important new class of organic polymers owing to their strongly anisotropic behavior. This anisotropy reflects the underlying structure of the materials: they are made up of m a n y parallel chains with unusual optical and electrical properties [1]. The planar and fully conjugated polymer b a c k b o n e is best represented by the following resonant structures: R~)C_~C_ ~ \ R2
l-
.
.
.
.
.
.
.
.
.
.
.
.
7 RI)C"C"C< R2
The chemical and physical properties of polydiacetylenes undergo dramatic changes on exposure to selected impurities or dopants such as A s F 5 [ 1]. These changes are believed to be due to the formation of charge-transfer complexes between the polydiacetylene and the doping species or diacetylene side groups (R~, R2). The polydiacetylene monomer, about which this study is concerned, is 10,12-nonacosadiynoic acid, * Inquiries should be addressed to Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, Tennessee 37662, USA.
where R I - - - ( C H 2 ) I 6 - H and R 2 - - - ( C H 2 ) 8C O O H . Its molecular and crystal structure [2] is shown in fig. 1. It is surface active and thus forms two-dimensional arrays of m o n o m e r units (Langm u i r - B l o d g e t t films) at a g a s - w a t e r interface. The m o n o m e r crystal can be polymerized by either ionizing or ultraviolet (UV) radiation to form long conjugated polymer chains which are rigid, selfsupporting and can form large two-dimensional crystalline domains [3]. The structure and properties of the polydiacetylenes and the charge-transfer complex have been investigated by optical, U V as well as by
l o
~-Cq
Fig. 1. a - c projection of polymer monolayer, a =34.5 ,~ and c =4.89 .~.
0 3 0 4 - 3 9 9 1 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
226
H [ ~ j
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Fig. 2. Untreated monolayerfilms, dose= 1 C/cm2. Crystalline, amorphous and holey regions are evident.
photoelectron, Raman, electron spin resonance, X-ray, spectroscopies, and electron diffraction [14]. The underlying theme of all these studies is to gain basic knowledge of the effect of microstructure on the properties of the polymer. These techniques can, however, only provide indirect information on the molecular structure. This work takes advantage of the unique electrical properties of the polydiacetylenes, as well as the high spatial resolution capabilities of state-of-
O
Fig. 3. Second exposureof fig. 2 to a dose of about 1.5 C//cm2.
the-art electron microscopes, for directly observing molecular structures.
2. Experimental Monolayers were spread and polymerized in a Teflon trough, as previously described [3]. Polymer monolayer samples were deposited on grids by lifting the grid support from the water through the
L.T. Germinario, P.C. Gillette / Molecular microscopy of polvdiacet~lene monolayers
227
Fig. 4. Osmium-treated polymer monolayer. Only crystalline and amorphous regions are evident. Dose = 20 C / c m 2.
gas interface. Monolayer images and diffraction patterns were recorded on Kodak electron image plates with a JEOL JEM 100C electron microscope at 100 kV. A JEOL JEM 100B was modified for computer control of collection of selected electron diffraction intensities. The hardware is built around a Digital Minc 11 microcomputer which is controlled from a VT 105 data terminal with an alphanumeric keyboard. A dual floppy RX O 2 is used for recording intensity data and for loading the system operating programs. Patterson maps were computed using a Digital VAX 11/780. Small angle laser scattering patterns (SALS) were recorded from exposed electron image plates on an optical bench equipped with a helium-neon laser. Grids containing monolayers were exposed to osmium tetroxide vapors for 15 min before examination in the electron microscope.
3. Results and discussion
The a - c projection of the polymer monolayer is shown in fig. 1 [3], where a = 34.5 A and x = 4.89 A. The polymer backbone is the c axis and consists of conjugated double and triple bonds. The b - c
projection is along the electron beam axis and is shown in the insert of fig. 2, where b--8.11 A. This polymer thus consists of two polyethylenetype R groups, where R~ consists of a chain containing 16 carbon atoms, with 8 atoms being superimposed on the b - c projection, while R 2 consists of an 8-carbon-atom chain with 4 atoms being superimposed on the b - c projection [3]. In analyzing untreated monolayer films in the electron microscope (fig. 2) at a magnification of 2 × 105 and a dose of about 1 C / c m 2 (625 e/A2), crystalline, as well as amorphous and holey regions, were evident. A second exposure of the same region to a dose of about 1.5 C / c m 2 (937 e / A 2) reveals (fig. 3) further degradation of the monolayer film. The high sensitivity of these films to the electron beam precluded optimization of astigmatism correction. The coexistence of crystalline, amorphous and holey regions in the monolayer films attests to the highly localized nature of the deleterious inelastic scattering events [5]. Examination of polymer monolayers which were exposured to osmium tetroxide vapors revealed the absence of holey regions, even after exposures of 20 C / c m 2 (1.25 × l04 e / A 2) (fig. 4). The use of thin crystalline films provides several
L. T. Germinario, P. C. Gillette / Molecular microscopy of polydiacetylene monolayers
228
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Fig. 5. Patterson maps of treated (OsDCT) and untreated (DCT) polymer.
advantages. It provides molecular packaging to minimize thermal vibration, as well as providing stacked rows of molecules and atoms to increase the contrast of the projected image. The thickness of the monomer (34.5 A) is such that the specimen acts' like a phase object and produces its best image at the Scherzer value of underfocus. The effect of osmium atoms on electron scattering in the b-c projected polymer structures is shown to be negligible by the similarity in the Patterson maps of treated and untreated polymer films (fig. 5).
Finally, radiation damage was monitored by measuring fading of the diffraction spots (002), (020), and (011) of both pristine and doped polymer films. The data are shown in fig. 6. These reflections correspond to the spacings of 4.89 ~, (c), 8.11 .~ (b), and 8.38 .~ (a), respectively. In each case the dose rate is 3.1 × 10 - 4 C / c m 2- S (0.2 e / A . s) and data were taken for a period of 60 s (12 e/cm2). This method of monitoring radiation damage shows the osmium-treated specimens to be slightly more resistant to radiation damage. However, examination of the molecular images pro-
L. 7". Germinario, P.C. Gillette / Molecular microscopy of polydiacetylene monolayers
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H i g h r e s o l u t i o n electron m i c r o s c o p y can thus offer a direct m e a n s of observing m o l e c u l a r structures a n d when used in c o n j u n c t i o n with o t h e r i n d i r e c t m e t h o d s for structure analysis can p r o v i d e a m o r e c o m p r e h e n s i v e m e a n s of s t u d y i n g the chemical a n d p h y s i c a l p r o p e r t i e s of this i m p o r t a n t new class of o r g a n i c polymers•
References
[1] G. Wegner, in: Chemistry and Physics of One-Dimensional Metals, NATO Advanced Studies, Set. B, Physics, Ed. H.J. Keller (Plenum, New York, 1977) p. 297. [2] D.R. Day and J.B. Lando, Macromolecules 13 (1980) 1483. [3] D.R. Day and J.B. Lando, Macromolecules 13 (1980) 1478. [4] W. Deits, P. Cukor and M. Rubner, in: Conductive Polymers, Ed. R.B. Seymour (Plenum, New York, 1981) p. 171. [5] M.S. Isaacson, in: Principles and Techniques of Electron Microscopy, Vol. 7, Ed. M.A. Hayat (Van Nostrand-Reinhold, New York, 1976) p. 1.