Fluorinated, Main-Chain Chromophoric Polymer - ACS Symposium

Chapter 8

Fluorinated, Main-Chain Chromophoric Polymer Langmuir Layer and Fourier Transform Infrared Spectroscopic Studies Downloaded by UNIV MASSACHUSETTS AMHERST on October 12, 2012 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0493.ch008

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Pieter Stroeve , Roni Koren , L. B. Coleman , and J . D. Stenger-Smith 1

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Department of Chemical Engineering, Organized Research Program on Polymeric Ultrathin Film Systems, and Department of Physics, University of California, Davis, CA 95616 Research Department, Naval Weapons Center, China Lake, CA 93555-6001 3

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A nonlinear optical polymer, which belongs to a new class of fluorinated, main-chain chromophoric polymers has been investigated by equilibrium isotherm studies of the Langmuir layer and FTIR studies of LangmuirBlodgett films. The polymer arranges in an accordion-like manner on the air/water interface, when compressed to moderate surface pressures (12 mN m ), and can be transferred in this configuration as Y-type L B multilayers. At higher surface pressures (> 12 mN m ) the Langmuir layer forms either bilayers or rearranges structurally. The monolayer collapses irreversibly at 20-25 mN m . Analysis of FTIR studies confirm a accordion-like structure. A chemical change occurs in the polymer after aging for one day at the air/water interface. -1

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For future optoelectronic applications, polymeric chromophores appear to be attractive materials because of the robustness of polymeric films. Much effort has been directed at the synthesis ofpolymeric chromophores composed of chromophoric side-chains from a polymer backbone (1-3). Deposition of non-centrosymmetric multilayer Langmuir-Blodgett (LB) films shows quadratic enhancement in second harmonic generation (SHG) with the number of chromophore layers (4). However, improved polymer materials with larger second order susceptibilities are necessary to make practical devices feasible (J). Recently Lindsay et al. (6) synthesized fluorinated, main-chain chromophoric, optically active polymers for L B deposition. Chromophores were incorporated into the polymer backbone in a head-to-head configuration separated by flexible fluorinated spacer groups. The sequence of spacers and chromophores should allow folding of the polymer main-chain in an accordion-like manner out of the interface to facilitate chromophore orientation, which can lead to effective SHG in L B multilayer structures (7). Hoover et al. (7) observed SHG generation in L B films of the accordion-type polymers. 0097-6156/92/0493-0083$06.00/0 © 1992 American Chemical Society

In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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MACROMOLECULAR ASSEMBLIES IN POLYMERIC SYSTEMS

In this work we have investigated the equilibrium isotherm behavior of an accordion-like polymer and we have performed FTTR studies of L B films of the polymer. The purpose of the work is to assess the structure of the polymer at the air/ water interface and in L B films.

Downloaded by UNIV MASSACHUSETTS AMHERST on October 12, 2012 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0493.ch008

Experimental Details The molecular structure of the polymer is shown in Figure 1. Its synthesis has been described by Hoover et al. (7). The polymer was purified by preparative gel permeation chromatography in chloroform solution. A Joyce-Loebl Langmuir trough (Model IV), housed in a class 100 laminar flow hood, was used for the equilibrium isotherm studies of the Langmuir layer and for the deposition of L B films. For the subphase, filtered and de-ionized water was passed through an activated carbon adsorber and then distilled in an all-glass still. The polymer was spread on the subphase from a chloroform solution at a concentration of approximately 1 mg ml" . The compression-expansion speed of the barrier in all experiments was 0.5 c m sec" . Langmuir-Blodgett layers were deposited on aluminized glass slides and ZnSe wafers. The deposition speed was 5 mm min" . Infrared spectra were obtained with a Nicolet 510 Ρ spectrometer equipped with a room temperature DTGS detector. For infrared reflection-absorption spectroscopy (IRAS) studies a fixed angle (80 ) Spectratech reflectance accessory was used with the L B films deposited on aluminized glass. Spectra were obtained with a resolution of 4 cm' by co-adding 256 interferograms. Transmission studies were conducted with L B films on ZnSe wafers. 1

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Results and Discussion The Langmuir Layer. The equilibrium isotherm for the Langmuir layer of fluorinated accordion-type polymer on a distilled water subphase is shown in Figure 2. The temperature in this experiment was 23± 0.5"C. Several compression and expansion cycles are shown. In the first cycle the Langmuir layer was compressed to

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Figure 1. Structure of the repeat unit of the fluorinated, main-chain chromophoric, optically active polymer.


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Fluorinated, Main-Chain Chromophoric Polymer

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5.0 mN m ' . At this point (a) the area was held constant for 5 minutes while the pressure was monitored. The surface pressure remained constant. Expansion of the monolayer retraced the compression curve exactly. The same procedure for a compression to 10.0 m N m (b) gaveaLangmuirlayerwithastablesurfacepressure for 5 minutes. The expansion curve was identical to the compression curve. The second compression (b) accurately retraced the first compression (a) to 5.0 mN m" . The third compression was to 15.0 mN m" (c). From 12 to 15 m N m" a plateau-like region was observed with the specific area per repeat unit decreasing from 78 to about 48 of,A)2 per repeat unit At point c the surface pressure at a constant specific area decreased from 15.0 to 12.1 mN m ' in 5 minutes. The compression curve retraced compression b, but the expansion curve was different The procedure was repeated for 20.0(d), 25.0 (e).30.0 (f), and 45.0 mNm" . The compression curves were retraceable if 20 mN m" was not exceeded. Up to point d compression e was identical. Compression f did not retrace compression e. The surface pressure of the Langmuir layer was increasingly unstable at constant surface area from point c to g. For example, after compression to point d and then the maintenance of constant surface area, the surface pressure dropped from 20.0 to 14.3 mN m ' in 5 minutes. At point e the surface pressure dropped from 25.0 to 14.8 mN m" and at point f the surface pressure decreased from 30.0 to 15.3 m N m ' in 5 minutes. The expansion curves were very different from the compression curves. From these data we conclude that the compression isotherm is reversible from 0 to 20 mN m ' . After 20 mN m ' , irreversible changes in the Langmuir layer take place. From an analysis of the data, the irreversible collapse pressure occurs between 2025 mN m" . Between 10 and 20 mN m' the surface pressure is not stable (at constant area) and monolayer material may be pushed into the subphase or out into the air phase. However, this is a reversible collapse since, after expansion of the Langmuir layer, full recovery of the compression isotherm is obtained upon recompression. The stability of the surface pressure was investigated above 10.0 mN m" in compression-expansion experiments shown in Figure 3. A drop of surface pressure occurs over a 5 minutes period for initial surface pressure values of 13.0,14.0, and 15.0 m N m ' (b,c,andd). At 12.0 mNm" (a) the surface pressure remained constant for 5 minutes at constant surface area. Thus, reversible collapse takes place above 12.0 mN m . The specific surface area at 10 mN m" is about 78 Â per repeat unit. When the isotherm is extrapolated to zero mN m" , the specific area is 95 A . This is reasonably close to the sum of the specific cross-sectional areas of the backbone if each repeat unit is oriented in four segments sticking in the air-water interface in an accordion-like manner as depicted in Figure 1. The segments are: chromophore, chromophore (reversed), hydro-fluorocarbon, and fluoro-hydrocarbon. The crosssectional area of the two chromophoric parts of the chain is about 25 À each and the fluoro-hydrocarbon portions of the backbone are about 20 Â each. When the Langmuir layer is further compressed, a large reduction of the specific surface area takes place at a modest change of surface pressure. It is significant that the reduction in the specific area is approximately one half. This suggests that either a bilayer is 1


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Successive compression-expansion curves for the Langmuir layer at 23 C from 5 to 45 mN m ι e

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Successive compression-expansion curves for the Langmuir layer 1

from 10 to 15 mN m ' at 23 *C. In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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formed, with the monolayer buckling upon itself, or that the four segments are rearranged into two segments. In the latter case, both the chromophoric parts and the fluoro-hydrocarbon parts could form one segment each, again in an accordionlike structure. Alternately, each of the two segments could have a chromophore and part of the fluoro-hydrocarbon section of the backbone arranged in an accordionlike manner. Whatever the structure is, it is not stable as the surface pressure changes at constant area. Equilibrium isotherms and the expansion curves are given in Figure 4 for 16.5,23 and 29 ± 0.5 "C. At 29 "C the plateau-like region is very broad and occurs at a lower surface pressure than at 23 *C, suggesting that the higher the temperature the less stable the Langmuir layer is to buckling or rearrangement The Aged Langmuir Layer. We have also investigated the aging of the Langmuir layer. The polymer was spread on distilled water, compressed to 12 mN m" and then expanded completely. After 24 hours the equilibrium isotherm was measured. Figure 5 shows compression and expansion curves to 10.0 (a), 25.0 (b), 30.0 (c), 35.0 (d), and 42.0 (e) mN m" . Up to 35 mN m" the compression curves are retraceable and the expansion curves are identical to the compression curves. The shape of equilibrium isotherm is very different than that observed in Figure 2. From separate experiments, irreversible collapse was found to occur at 40 mN m" wheremespecificareaperrepeatunitis20Â . The monotonie increase of surface pressure with specific area, exhibited in Figure 4, suggests that no major phase transitions or monolayer rearrangement takes place. Further, up to 30 mN m" the surface pressure was constant at constant surface area for 5 minutes. These results suggest that a major change has occurred in the polymer molecule. Further discussion will be given in the analysis of the FTIR data on L B films. 1

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Π Ί Κ of Langmuir-Blodgett Films, Deposition on a substrate was started with an upstroke. For a fresh Langmuir layer, deposition at 9.0 mN m" gave Ζ type L B layers with a transfer ratio of 1.07 on the upstroke. Deposition at 12.0 mN m" was Y-type with transfer ratios of 0.98 with the upstroke and 0.85 with the downstroke. Higher surface pressures were not used here because of the instability of surface pressure at constant area and consequently the instability of area at constant surface pressure. When the area is not constant, transfer ratios can not be measured accurately. For the aged Langmuir layer Z-type deposition was also found at 9.0 mN m1 with a transfer ratio of 0.90. Langmuir-Blodgett deposition at 17.0 mN m" gave Y-type L B layers with a transfer ration of 1.04 with the upstroke and 0.85 with the downstroke. Figure 6 shows the FTIR reflection-adsorption (IRAS) spectrum of a cast film of the polymer on an aluminized microscope slide. The cast film was made by dripping a chloroform solution of the polymer on the slide followed by drying. The thickness of the cast film is approximately equivalent to the L B films studied here. Figure 7 gives the IRAS of five, Y-type, L B layers on aluminized glass. Comparisons of the figures show that the C H ^ C H stretch bands are much more intense for the 1

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Compression-expansion curves for an "aged" Langmuir layer at 23 -C.



8. STROEVE ET AL

Figure 6.


Infrared reflection-adsorption spectrum of a cast film of the polymer on an aluminized glass slide.

Figure 7.

Infrared reflection-absorption spectrum of 5 Y-type L B layers of 1

the polymer deposited at 12.0 mN m" on an aluminized glass slide.


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cast film than the L B film. The results indicate that for the cast film the C H and C H dipoles are parallel to the substrate plane while for the L B films the dipoles are not parallel. The hydrocarbon portion of the chain lies flat on the surface in the case of the cast film. Figure 8 shows the infrared transmission spectrum of eleven, Y-type, L B layers on a ZnSe wafer. The assignment of the infrared frequencies are given in Table I for reflection and transmission. These assignments are consistent with those found in the literature (8-11). The CH2/CH3 stretch bands are better defined in the transmission spectrum, although they are not large. Comparison of Figures 7 and 8 suggests that the C H and C H dipoles are neither parallel not perpendicular to the substrate plane, but


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Table I:

Infrared Band Assignment of LB Films in Reflection and Transmission ("fresh" layers deposited at π = 12.0 mNm' ; resolution is 4 cm" ) 1

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2960 (w) 2926(w) 2853 (w) 2216 (w) 1723 (w) 1610(w) 1576 (w) 1522 (w) 1462 (w) 1370 (w) 1334 (w) 1272 (s) 1227 (s) 1191 (s) 1100(s) 1138 (w) 818 (w)

2962 (w) 2928(m) 2857 (w) 2216 (w) 1716 (s) 1613 (s) 1570 (s) 1520 (s) 1468(m) 1371(m) 1331(m) 1277 (s) 1226 (s) 1184(s) 1094 (s) 1138 (s) 819 (m)

v.(CH ) V.ÎCHÎ) v.iCHa) CsN ν ( C = O) ester v ( C = C)ring(«>) v ( C = C)ring(4» ν(φ) 5(CH2> viCFj) V(CF2) φ-Ν C F stretch ν ( C-O-C) ester CT„ stretch CTorC-φ? CH()C-CH-)

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rather at an angle, consistent with an accordion-like structure. A precise analysis of the CH2/CH3 orientation is very difficult because these groups do not only appear in the main chains but also as side-chains in the polymer. The transmission spectrum also shows that the C = C stretches of the phenyl groups and the C-O-C ester stretch are more intense than in reflection-absorption. These results indicate that the phenyl


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groups and the ester groups are oriented at an angle with the substrate, again consistent with an accordion-like structure for the polymer. In Figures 6-9, stretches assigned to C F and C F are relatively high. For Figures 7 and 8, the G F stretch is highest in reflection but the stretch in transmission is also very strong. If the hydrofluorocarbon portions of the chains are oriented out of the surface as indicated in Figure 1, for example, the chain would have to bend over bringing some C F groups parallel to the surface. This orientation would give a strong absorbance in both reflection and in transmission as is seen in the spectra. Figure 9 shows the IRAS spectrum of 10, Z-type, L B layers of the aged polymer. Deposition was carried out after the Langmuir layer was 24 hours old. Comparison of the spectrum to those in Figures 7 and 8 shows that the C s Ν stretch has disappeared which indicates that a chemical reaction has taken place at that location of the polymer molecule. It is possible that the organic nitrile group gets hydrolized into ammonia and organic acid. For L B films the hydrolysis reaction could be caused by water vapor in the air, or bound water in the L B film, or both. The monotonie increase in the equilibrium isotherm could be due to scission of the polymer backbone at the O N attachment, because no phase changes are observed for the aged Langmuir layer. For the fresh polymer, phase changes were observed. For the aged polymer, a minimum specific area per repeat unit of about 20 A is obtained in a monotonie fashion. Since the crossectional area of the backbone of the polymer is from 20-25 A , the data can be explained if the cleaved remnants of the polymer are stacking perpendicular to the interface. In addition, the relative weakness of the C H / C H stretch bands means that the C H and C H portions of the molecule are more perpendicular than parallel to the substrate surface, which is consistent with the hypothesis of perpendicular stacking. The change in the chemistry of the polymer should have negative implications for the use of this polymer in nonlinear optical devices. We have observed that IRAS spectra of L B films of the fresh polymer change within a few weeks and show similar spectra to the one shown in Figure 9. Since the nitrile group in the polymer is necessary for resonance in the chromophore, the chemical change would lead to a reduction of SHG with time in L B films. 2

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Conclusion Hoover et al. (7) have shown that accordion-type polymers give an SHG response when deposited in L B films. Obviously, for these accordion-type polymers to be used in optical devices they must be stable to chemical transformation. Our studies show that the polymer does appear to arrange in an accordion-type structure, but also that the polymer is chemically not stable. These results imply mat the chemistry of the polymer needs to be modified to impart chemical stability. Nevertheless, our studies indicate that equilibrium isotherms of Langmuir layers and FTIR studies of L B films are very useful tools to quickly assess new materials and their applicability in non-linear applications of thin films.


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Infrared transmission spectrum of 11 Y-type L B layers of the 1

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Infrared reflection-absorption spectrum of 10 Z-type L B layers 1

of the aged polymer deposited at 9.0 mN m" on an aluminized glass slide.


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An alternative, but more radical hypothesis, is that the aged polymer is not cleaved but it forms a more complex structure on the air/water interface, for example, helices (12-13). Additional experiments are necessary to deduce the structure. Acknowledgment


Funding for this research was provided by the National Science Division (Grant CBT8720282) and a U C Faculty Research Grant to Pieter Stroeve. Literature Cited 1.

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3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13.

Hall, R.C., Lindsay, G.A., Kowel, S.T., Hayden, L.M., Anderson, B.L., Higgins, B.G., Stroeve, P., and Srinivasan, M.P. Proceedings of the Society of Photo-Optical Instrumentation Engineers SPIE, 1988, 824:121. Hall, R.C., Lindsay, G.A., Anderson, B.L., Kowel, S.T., Higgins, B.G., and Stroeve, P., Proceedings of the Materials Research Society Symposium , 1989, 109:351. Anderson, B.L., Hall, R.C., Higgins, B.G., Lindsay, G.A., Stroeve, P., and Kowel, S.T., Synthetic Metals, 1989, 28:D683. Anderson, B.L., Hoover, J.M., Lindsay, G.Α., Higgins, B.G., Stroeve, P., and S.T. Kowel, Thin Solid Films, 1989, 179: 413. Garito, A.F., Wu, J.W., Lipscomb, G.F., Lytel, R., Proceedings of Materials Research Society, 1990, 173: 467. Lindsay, G.A., Fisher, J.W., Henry, R.A., Hoover, J.M., Kubin, R.F., Seltzer, M.D., Stenger-Smith, J.D., Journal of Polymer Science, Part A, 1991, in press. Hoover, J.M., Henry, R.A., Lindsay, G.A., Nadler, M.P., Nee, S.F., Seltzer, M.D., Stenger-Smith, J.D., ACS Symposium Series, 1991, in press. Schneider, J., Ringsdorf, H., Rabolt, J.F.,Macromolecules,1989, 22: 205. Schneider, J., Erdelen, C., Ringsdorf, H., Rabolt, J.R., Macromolecules, 1989, 22:3475. Stroeve, P., Saperstein, D.D., and Rabolt, J.F. Journal of Chemical Physics, 1990 92:6958. Stroeve, P., Spooner, G.J., Bruinsma, P.J., Coleman, L.B, Erdelen, C.H. and Ringsdorf, Η. , ACS Symposium Series, 1991, 447, 177-191. Brinkhuis, R.H.G, and Schouten, A.J.Macromolecules,1991, 24: 1487. Brinkhuis, R.H.G, and Schouten, A.J.Macromolecules,1991, 24: 1496.

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Fluorinated, Main-Chain Chromophoric Polymer - ACS Symposium

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