Langmuir 1993,9, 1971-1973
1971
Orientational Disordering Transition in Langmuir-Blodgett Films of Fluorinated Polymers H. Hsiung,* R. Beckerbauer, and J. M. Rodriguez-Parada Central Research and Development, DuPont Company, P.O.Box 80356, Wilmington, Delaware 19880-0356 Received May 3,1993. In Final Form: June 18,1993
A thermally induced disordering transition of molecular orientation in polymeric Langmuir-Blodgett (LB) films is observed with a nonlinear optical probe-second-harmonic generation (SHG). The LB films consist of alternate monolayers of a SHG-active polyacrylate and a buffering polyoxazoline. The observed disordering transition appears to be dominated by intralayer thermal relaxation of the active polymer, rather than the interlayer interactions between the active polymer and the buffer polymer. Furthermore, we find that the transition kinetics can be described with an Arrhenius rate equation, with a high apparent activation energy indicative of a collective relaxation process.
Introduction Thermal stability of Langmuir-Blodgett (LB) films is an issue of both fundamental and practical importance, but one that we have achieved only a limited understanding.' While previous research activities in this area have primarily dealt with LB films of simple monomeric compounds, particularly fatty acids,l several thermal studies of LB films of polymers have been reported,24 the latter being partly motivated by the expectation that polymeric LB films should hold more promise for practical use. Unlike monomeric compounds, many polymers exhibit little crystallinity in the bulk phase, and their thermal properties are best characterized by glass transitions. Therefore, fadors that determine thermal stability of polymeric LB films may not be the same as that for monomeric LB films. Among the experimental methods used for studying thermal stability of polymeric LB films, small-angle X-ray scattering213 and interlayer energy transfers are used to probe the stability of the periodic, layered structure, and polarized infrared spe~troscopy4~~ is used to probe the orientational stability of the polymer side chains. These methods tend to be qualitative, and from which certain dynamic parameters-such as the activation energy and the transition rate of the thermal disordering-are difficult to extract. In the present study, we use optical second-harmonic generation (SHG)7 to investigate thermal relaxations in LB films of several polymers containing fluorinated side chains (Figure 1). The LB films, prepared by alternate Y-type deposition,l contain alternating monolayers of an SHG-active polymer (A) and a buffer polymer (B1 or B2) that exhibits negligible SHG a c t i ~ i t y The . ~ ~SHG ~ activity of polymer A arises from the p(perfluoroalkylsulfony1)phenylamino group (denoted "probe" in Figure 1)in the (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett t o Self-Assembly;Academic Press: San Diego, CA, 1991; Part 2. (2) Rmgsdorf, H.; Schmidt, G.; Schneider,J. Thin Solid Films 1987, -152. - -, 207. -- . . (3) Biddle, M. B.; Lando, J. B.; Ringsdorf, H.; Schmidt, G.; Schneider, J. Colloid Polym. Sci. 1988,266, 806. (4) Schneider, J.; Ringedorf, H.; Rabolt, J. F. Macromolecules; 1989, 22, 206. (5) Schneider, J.; Erdelen, C.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989, 22, 3475. (6) Ueno, T.; Ito, S.;Ohmori, S.;Onogi, Y.; Yamamoto,M. Macromolecules 1992, 25, 7150. (7) Shen, Y. R. Nature 1989,337,519, and references therein. (8)Hsiung,H.; Rodriquez-Parada,J. M.; Beckerbauer,R. Chem.Phys. Lett. 1991, 182, 88. (9) Beckerbauer,R.;Hsiung,H.;Rodriguez-Parada,J. M.Po1yn. Prep. 1992,33, 1204.
Polvmer A
Polvmer B1
Polvmer 82
-CFs
W '
f=O
+N$
I
+T
Figure 1. Chemicalstructuresof the polymers used in this study.
side chain. Within the multilayer (ABAB ...) structure, the neighboring active polymer and buffer polymer assume a backbone-to-backbone, side-chain-tu-side-chain conformation, resulting in a net polar alignment for the "probe" and consequently an SHG signal-the size of the SHG signal is determined by the degree of the polar alignmenL8 We report in this Letter the observation of an irreversible disordering transition in the side-chain orientation, as indicated by a sharp drop in the SHG signal when the temperature of an LB film is increased at a constant rate. The thermal behavior is quantitatively analyzed using an Arrhenius rate equation, yielding a high activation energy and a large transition rate resembling that of glass transitions in bulk polymers. Experimental Section The synthesis of poly(N-@-heneicosafluorodecylsulfonylphenyl)-L-prolinolacrylate) (polymer A) and poly(2-@-(l-oxy1-(trifluoromethyl)-2,2-bis~eptafluoro~propyl)e~ylene)phenyl)-
2-oxazoline) (polymer B1) has been described in previous For the synthesis of poly(2-@-(tridecafluoropublications.~~~ hexyl)phenyl)-2-oxazoline)(polymer B2),the oxazoline monomer was prepared from methylp-tridecafluorohexylbenzoate(kindly provided by D. P. Higley) by common methods described in the literature.1° The monomer was then polymerized in bulk at 120 O C using methyl trifluoromethanesulfonate as the initiator. The molecular weight of polymer A was kept low, with a degree of polymerization less than 20, typically44;both polymers B1 and B2 had a degree of polymerization -20. Thermal properties of the polymers in the bulk phase were determined by differential scanning calorimetry at a heating rate of 20 OC/min. Polymer A and polymer B1 are amorphous with glass transition temperatures of 81 O C and 120 O C , respectively, while polymer B2 is highly crystalline with a melting point of 270 "Cand no detectable glass transitions. (10)Kobayeshi, S.;Saegusa, T. In Ring-Opening Polymerization; Ivin, K. J.; Saeguea, T.; Eds.; Elsevier Applied Science: New York, 1984; Vol. 2, Chapter 11.
0743-7463/93/2409-1971$04.00/00 1993 American Chemical Society
1972 Langmuir, Vol. 9,No. 8,1993
Letters
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$
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-icn? c B -
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c
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'? v)
0.2
0.1
0.2 0.0
0.0 20
30
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70
SO
0
90
5
10
Figure 2. Thermal scan of SHG intensity for a 25-bilayer LB
min or (b) 5.0 OC/min. The dotted curves are theoretical fits using eq 6, yielding (a) E, = 59.1 kcal mol-1, v = 1.3 X 1P8-1 and (b) E, = 59.9 kcal mol-', v = 3.5 X 1Ps-l. The preparation of alternate Y-type films of polymer A and polymer B1, or that of polymer A and polymer B2, is described in ref 8. The transfer of the polymer monolayers from the airwater interface to a glass substrate was carried out at a surface pressure of 35 mN/m for all three polymers, which corresponds to an average surface area of 0.41,0.45, and 0.36 nm2per repeat unit of polymers A, B1, and B2, respectively. The bulky and rigid side chains of the polymers do not allow a tight molecular packing in the monolayers.ll We have shown previously that, for the polymeric LB films comprising at least up to 80 monolayers, the ordering of the side-chain orientation is maintained throughout the multilayer structure; the orientational order also exhibits an excellent long-term stability at room temperature.8 LB film samples used in the present study include some prepared over 2 years ago. For the SHG measurements,the fundamental light was derived from a diode-pumped, Q-switched NdYLiF, laser operated at 1.047pm. The laser beam consisted of 12-11s pulses at a repetition rate of 1 kHz, with the energy per pulse typically set at 15 rJ. At the sample, the laser beam was focused to a diameter of -20 pm. The fundamental light was directed onto the LB f i b at an incident angle of 60°, and the second-harmonicphotons at 523.5 nm were detected in the transmission direction. We used either p-polarized or s-polarized fundamental light to generate p-polarized second-harmonic light in the L B - f i i samples? Both polarization combinationsyielded similar results, and hence only the data obtained with p-polarized fundamental light are presented in the following discussion. The LB-film sample was situated in a thermally insulated copper cell with its temperature controllable to within f0.1 "C. Experimentally, two different procedures were used to study the thermal disordering process: (1)While the LB-fib sample was heated at a constant rate (0.65 OC/min), the SHG intensity was measured as a function of temperature. (2) The sample was f i t heated and maintained at a constant temperature within the transition region of the thermal disordering, then its SHG signal was monitored as a function of time.
Results and Discussion As illustrated in Figure 2, upon heating at a constant rate, the SHG signal of an LB film exhibited a relatively sharp, irreversible drop at around 70 "C, indicating a substantial loss of the orientational order in polymer A. Figure 3 shows the temporal behavior of the orientational disordering at a fixed temperature within the transition region. In both cases, there remained a small residual SHG signal after the completion of the disordering transition. (11) Hsiung, H.; Beckerbauer, R. Chem. Phys. Lett. 1992, 193, 123.
20
25
30
35
40
Tlme ( min.)
Temperature ( "C )
f i i of polymer A and polymer B1, at a heating rate of (a) 0.5OC/
15
Figure 3. Time dependence of SHG intensity for a 25-bilayer i b of polymer A and polymer B1 at 68.8 "C. The dotted LB f curve is a theoretical fit using eq 5, yielding 7 = 10.7 min and fLO/fHO = 0.13.
In the following, we present a simplifiedtwo-statemodel to analyze the experimental results. For simplicity,let us assume that the "probe" (or the side chain of polymer A) in the LB film only occupies one of two orientational states: a low-temperaturestate of high orientational order and a high-temperature state of low orientational order. Here, a "state" represents an orientational distribution of the "probe". We further assume that the transition from one state to the other can be described statistically by an Arrhenius rate equation where NHo and NLo represent the populations of the highorder and the low-order states, R is the gas constant, and E, and u are respectively the apparent activation energy and the apparent preexponential factor. From eq 1,NHO decays exponentially at a constant temperature ( T ) as
.r = u-'
exp[Ea/(RT)l
On the other hand, if the sample temperature is increased at a constant rate of r = dT/dt, eq 1 can be integrated to yield where
v(T) = ur-' J TTO exp[-EJ(RIV)l dT' and TOis the initial (room) temperature. Since the total population must remain constant, we have NHO+ NLO= NHO(TO). Furthermore, the SHG intensity can be expressed as (4)
1%Q: WH&o + NLdLoI'
where f H 0 and fLo are coefficients reflecting the degree of the polar alignment8 It is expected from our experimental observation that fH0 >> fL0. Combining eq 2 or eq 3 with eq 4$we obtain I% OC WHO(t=O)(f,O -fLO) exp(-t/7) + NHO(TO>f~O12
(constant T ) ( 5 ) or
Langmuir, Vol. 9,No. 8, 1993 1973
Letters
1%
a (NHO(TO)[VHO
-tLo) ~xP(-v(T)) + /Lo112
(constant r) (6) respectively, for the two experimental conditions. As shown in Figures 2 and 3, in spite of the substantial simplifications in the above analysis, eq 5 and eq 6 agree satisfactorily with our experimental observations. In Figure 2, the SHG intensity for two samples (both containing 25 bilayers of polymers A and B1) under very different heating rates is plotted as a function of temperature; a theoretical fit using eq 6 (but ignoring the small factor f ~ ohas ) yielded nearly identical activation energies but different preexponential factors. Indeed, we find that the deduced values of Ea (in the range of 58-60 kcal mol-') vary only slightly from sample to sample, but that of v (in the range of lOL1036 s-l) can differ substantially. Due to the highly nonlinear nature of eq 3, more reproducible values of v may be difficult to obtain with the present technique. The consistency of our analysis is further verified in Figure 3, where the temporal behavior of the orientational relaxation is shown to follow eq 5 approximately. The time constant ( 7 = 10.7 min) deduced from Figure 3 is in fair agreement with the calculated values using eq 2 and the values of Ea and v obtained from Figure 2: (a) 8.1 min and (b) 9.2 min. The high apparent activation energy and the large preexponential factor are indications of a collective relaxation process in the polymeric LB fiis. Such large values are not uncommon for characterizing glass transitions in the bulk phase of polymers.12 However, these quantities are phenomenological parameters not bearing the same physical meanings as that in the usual Arrhenius rate equations for a chemical reaction. Furthermore, despite the substantial structural difference between the two buffer polymers (B1 and B2), the A-B1 and the A-B2 multilayer films exhibited very similar thermal behavior. This is shown in Figure 4, where LB films of the two different structures were studied under the same heating rate. Our observation suggests that the disordering transition may be dominated by intrinsic relaxations of polymer A, rather than the interlayer interactions between polymer A and the buffer polymers. This is consistent with the fact that the observed transition range is close to the range of polymer A's bulk glass transition (74-90 OC). (12) Kauzmann, W.Rev. Mod. Phye. 1942,14, 12.
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Figure 4. Thermal scan of SHG intensity for an LB film made of polymers A and B1 and that made of polymers A and B2. Both sampleswere 20 bilayers thick and were heated at the same rate of 1.0 OC/min. The dotted curves are theoretical fits using eq 6, yielding E, = 59.0 kcal mol-' and Y = 4.4 X l W 5-l for the A-Bl sample, and E, = 58.1 kcal mol-', Y = 1.3 X 1W s-1 for the A-B2 sample.
After being heated to well above the temperature range of the disordering transition, the LB films displayed no observable macroscopic damage. We were also unable to detect any change in the film thickness as measured by ellipsometry (with an uncertainty of - 5 % ) . Whether the orientational disordering is accompanied by a disordering in the periodic, layered structure is an important question demanding further investigations. Multilayer LB films of polymers represent a unique model system for experimental investigations of polymer monolayers and polymer mixtures. This is because monolayers of different polymers can be incorporated in an LB film in a variety of designed sequences, allowing the manipulation of the microscopic environment surrounding a particular polymer. Through our study, we have demonstrated a novel approach for investigating interlayer interactions in LB films, as well as polymer glass transitions at the monolayer level. Acknowledgment. The authors are very grateful to F. C. Zumsteg for his assistance during the early stage of this experiment and to B. B. Sauer for valuable discussions.
