Studies on the aggregation behaviour of a side-chain liquid crystalline polymer in Langmuir-Blodgett films

ELSEVIER

Thin SolidFilms286 (1996) 232-236

Stu es on the aggregation behaviour of a side-chain liquid crystalline polymer in Langmuir-Blodgett films Xiao Chen ~, Qing-Bin Xue ~, Kong-Zhang Yang ~'*, Qi-Zhen Zhang b • I~dtute of Colloid and Interface Chemistry, Shandong University, Jinan 250100. People "sRepublic of China b Deparunent of Chemistry. $handong University. Yinan 250100. People's Republic of China

Received23 August 1995;accepted7 December1995

Almraet The aggregation behaviour of a newly synthesized ferroelectric side-chain liquid-crystalline polysiloxane (denoted as PSLC) in LangmuirBlodgett (LB) films has been investigated. The surface pressure(fr)--area (A) isotherm and hysteresis experiments for arachidic acid (AA) / I~LC mixed monolayer confirm existence of aggregation or phase-separation during compression. From the UV-Vis spectra, we can clearly see H-aggregate formation. To study the aggregate structure and molecular packing of PSLC, cyclic voltammetry combined with transmission degtmn migrescopy and scanning electron microscopy morphology observations was used. The exciton model previously advanced by Kasha end coworkers is successfully introduced for a qualitative explanation of the spectra observed in the LB films. g#ywords: l.mqgmuir-Blodgettfilms; Monolayen; Polymers;Electrochemistry

1, Introduction

Recently, much attention has been given to fertoelectric side-chain liquid-crystalline polymers (FSLCP) [!-4] because of their scientific and technical potentials, such as appli~tions to non-linear optics, flat panel displays, polarintion switching, electro-optic devices. These kind of polymen combine the ferroelectrio properties of the monomeric side.chain mesogen with the structural features of the polymer. Therefore, FSLCP not only have self-ordering capabilities, but also show additional advantages over low molecular weight compounds such as greater mechanical and thermal stabilities. It is known that the smectic C* phase of FSLCP, which shows fermelectrieity, is highly influenced by an interface with a dissimilar material [5 ]. Therefore, a more detailed knowledge on molecular ordering in Sc, materials near the interface is of fundamental interest and practical importance. This kind of study is possible by using the Langmuir-Blodgett (i,B) technique. Sc, material LB films in carefully designed experiments may allow us to study the interfaces, since they give us the models of the stepwise development from a two-dimensional monolayer deposited on a solid substrate surface to a three-dimensional bulk-layered system of * Ccxt~pondln8rasher, 0040-60901961515.00O 1996ElsevierScienceS.A. All rightsreserved

S&D!0040-6090(95 )08519-X

the liquidcrystal.The LB multilayersmay also he usefulas a model to understand the effectsof molecular orientation and aggregationon ferrcelectricpolarization. Based on this consideration,monolayer hehaviours of some FSLCPs at the air-water interface have been studied [6--8]. These monolayers can also be transferred to suitably prepared substrates to form LB films [9-11 ]. Such multilayers have been shown to exhibit ferroelectric polarization similar to that of the bulk [ 12]. However, there has been no systematically investigation on the aggregation effects of FSLCP in monolayers and LB films. Often, the formation of aggregates makes it complicated to evaluate the properties of such polymer monolayer and multilayer films. The changes of molecular stacking in the aggregate result in a stronger dipole-dipole interaction which may enhance the spontaneous dipole moments, and therefore should affect polarization properties. Thus, it is interesting to study the physical processes of agsregetion during the film formation and the structural characteristics in the LB films, because the better understanding of the phenomena will enable us to design and fabricate monolayer and multilayer assemblies with controlled properties. In this paper, we report the first investigation of the aggregation and spectroscopic behaviour of a newly synthesized ferroelectric side-chain liquid-crystalline polysiloxane

X, (:hen et al. I Thin Solid Films 286 (1996) 232-236 O,Hs

(.3c)ssio÷~.~-o~,sl(CH~)3

CH8

glass as°C-~s e 97°C --S^ 15§0C~ Isolmplc (S: Smectic) Fig. l, Structure and bulk thermal properties of PSLC.

(denoted as PSLC, shown in Fig. 1) in mixed monolayers and LB films. The chiral center is in the head group and can induce smectic C* phase in the bulk which exhibits ferroelectricity. Besides usual measurements, such as UV-Vis spectra, X-ray diffraction, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) observations, cycfic voltammetry was first introduced to study the organized structure and the aggregation effect in the films. The spectral behaviour of the aggregate is discussed using molecular exciton model.

2. Experimental details The chemical structure of PSLC and the bulk thermal phase behaviour are shown in Fig. 1. Its synthesis and characterization are discussed elsewhere [ 13]. The experiments for monolayer spreading and LB film deposition were performed on a commercially available Langmuir trough NIMA 2000 (NIMA Tech. England) with computerized control. Because the material in our research is not typical amphiphile and forms only a rigid monolayer which is difficult to transfer for LB film preparation, we incorporate PSLC molecules into matrices of arachidic acid (AA) to fabricate a stable and easy transferable film. A mixed monolayer was obtained by spreading chloroform solution of AA/PSLC blend (molar ratio - 1:1 ) at a concentration of about 0.4 mg ml- ~on the redistilled water surface. LB films were built up on glass or quartz plates by the vertical dipping method with a typical dipping rate of 10 mm min-I. All the measurements were carried out at room temperature (25 + 1 0(2). The details of the performance are given in Ref. [ 14]. Small-angle X-ray diffraction patterns were obtained using a D/MAX-'yB X-ray diffractometer with Cu Ko: radiation (Affi0.154 nm). A Schimadzu recording spectrometer (model UV-3000) was used to take UV-Vis absorption spectra. Fused silica plates whose surfaces were made hydrophobic were used for measurements of LB films. Morphological characterizations were performed using a JEM- 100CXII analytical electron microscope and a Hitachi S-520 scanning electron microscope. The mixed monolayer was deposited onto Formvar-coated Cu grid for TEM and onto slide glass for SEM observation. Electrochemicalcyclic voltammetry experimentswere carried out using a PAR M 173 potentiostat equipped with a PAR M175 function generator and a RE-0091 X-Y recorder. The ITO (indium tin oxide) electrode covered by LB films was used as working electrode. Pt plates were used as counter electrodes. The reference electrode was a saturated calomel

233

electrode (SCE). Potentials reported here are all referred to SCE. All the measurements were made at 27 °(2.

3. Results and discussion

3.1. Surface pressure (It)--area (A) isotherm Fig. 2 shows the ~r-A isotherm for an AA/PSLC mixed monolayer (molar ratioffi 1:1) at the air-water interface. It exhibits a distinct inflection region at about 30 mN m - ~.The apparent areas for the mesogen repeat unit (Ao) extrapolated from the linear high-pressure regions before and after the inflection are 63 and 52 A2, respectively. The area obtained before the inflection is larger than the sum of the cross-sectional ,area for AA (20.5 A2) and the side chain ( ~ 25 A2) calculated based on the molecular CPK model [ 14], indicating the side chains do not reach the closest packing in AA matrices. A smaller value of Ao after the inflection, however, suggests that the monolayer undergoes a phase transition at the inflection region which corresponds to the initiation process of aggregation or crystallite formation as will be shown later. Fig. 3 shows hysteresis curves with the compression direction reversed before and after the inflection respectively. Little hysteresis and good reproducibility are observed in the compression-expansion cycle below the inflection, but large hysteresis and irreversibility appear for compression to pressure higher than the inflection point, which fact may be due to the irreversible formation of aggregates. 6O I

5O 40 30 2O 10 0

I

50 1O0 150 200 A (A :/repeat unit) Fig. 2. Surface pressure-area isotherm for an AA/PSLC mixed monolayer with molar ratio = 1: I. 0

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~ 10 0

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so

100

150

A (A2/repeat unit) Fig. 3. Hysteresis curves of an AA/PSLC mixed monolayer (molar ratio= I: I) at revelse pressures above the inflection region (. • • ) and below the inflection region ( ).

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X, Chert el ai, / Thin Solid Films 286 (1996) 232-236

3.2. UV-Vis spectra and X.ray d~fraction

0,6

,

,

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Absorption spectral characterizations are useful for examining chmmophore orientations in the aggregates. Fig. 4 shows the absorption spectra for PSLC and its blend with AA in CHCI~ solution and LB film. In solution PSLC is present as a single chain species with a lower energy band at 334 nm, corresponding to the n ~ ~rr*transition ofbenzalaniline chromophore and a band at 257 nm arising from the ~'--* ~r* transition of the phenyi moieties. A blue-shifted band at 245 nm is clearly seen in LB film and is assigned as an absorption for the H-aggregate of PSLC, where the transition moments orient parallel to each other, on the basis of exciton energyshift model. This absorption band moves to shorter wavelength with increasing deposition pressure (Fig. 5), which reflects possible structural changes of the monolayer corresponding to a transition from a fluid-like condensed phase to the crystalline packing upon compression. These spectroscopic observations are consistent with the results obtained from the Ir-A isotherm and X-ray diffraction. X-ray diffraction measurements can give layer spacings in the direction normal to the film. Corresponding to surface pressures of 25, 35, and 45 mN m - t, layer spacings are 51.9, $2.5, and 54.5 ,~,, respectively. Since the cha:.n length of AA is 26.9 ,~ and the calculated length of the side chain group including the siloxane backbone is 31.8 ,~, the data indicate that AA molecules dominate the layer spacing of Y-type films with their chains tilted with respect to the layer plane. Increased layer spacings suggests that the molecular chains become more closely packed and more perpendicular to the film plane with increase of ~r, which fact agrees well with the results of UV absorgtion measurements (Fig, 5), 3,3. Cyclic voltammetry measurements Electrochemical cyclic voltammetry is a very useful and sensitive method to detect defects such as pin-bolas in LB films. In this technique, a redox couple which exhibits a fast, reversible, heterogeneous electron transfer step under applied fields is used to probe the LB film coated electrodes. The current and potential characteristics which monitor electron transfer events between the probe and working electrode in

0,4. ~ 0,3 ,'~ .< 0.2 0.1 0.0

200

250

300

350

400

Wavelength ( n m ) Fig. 5. UV-Vis absorption spectra for an AA/PSLC blend ( 1:1) in 26-layer LB films deposited at different surface pressures: (a) 45 mN m - t (b) 3 5 m N m - I , ( c ) 2 5 m N m - l , ( d ) 15mNm -I.

an electrochemical cell directly reflect the structural defects of the thin film. Fig. 6 shows typical cyclic voltammograms for a Fe(CN)~-/4- redox reaction on ITO electrodes coated with AA/PSLC LB films deposited at different surface pressures. It is clear that these two films show different redox blocking effect for the Fe(CN) 3-14- probes. Usually the denser and the thicker the film is, the stronger the blocking effect is. Thus, from the ~'-A isotherm and X-ray diffraction we should consider the film deposited at 35 mN m - t shows larger blocking effect than that deposited at 25 mN m - i because of its smaller surface area per repeat unit and greater layer spacing. From Fig. 6, however, we see a reversed blocking sequence. With the same number of layers and film com. position, the film deposited before the inflection shows a larger blocking effect than that deposited after the inflection. Such an unusual result gives us additional evidence of aggregation at higher Tr. At 35 mN m-~, the PSLC molecules are more closely packed and the aggregation may proceed to such an extent that phase-separation of PSLC from AA matrices occurs, resulting in larger pin-holes or cavities in the film, and hence lower blocking capability. Such a supposed film structure change is consistent with the results described below.

I t.s 1.4 1.2 1.0 0.8 ~0 O.S 0.4. 0.2 0.0 200

\./ ~ 250

, 300

350

400

Wavelength ( n m ) Fig, 4, UV-Vis absorption spectra of PSLC ( ) and its blend with AA ( . . -) in chloroform solution and in a 26-layer LB film (. • • ).

.e.z g (V • SCr) Fig, 6. Cyclic voltammograms for ITO electrodes coated with a 5-layer AA/ PSLC LB film deposited at 25 mN m - i (dashed line) and 35 mN m - i (solid line), Scan rate--100 mV s - ' . Electrolyte solution is 0.1 M KCI + l,O× lO-3M K3Fe(CN)6.

X. Chen et al. / Thin Solid Films 286 (1996) 232-236

235

Fig. 7. El) patterns for one-layer AA/PSLC LB films deposited at (a) 25 mN m- t and (b) 45 mN m - ~.

3.4. TEM and SEM morphology observation

3.5. The structure model of the AA/PSLC LB film

TEM and SEM observations give additional information about aggregation structure in the films, TEM images at surface pressures lower or near the inflection region show a homogeneous surface, and the electron diffraction (ED) pattern of the monolayer exhibits an amorphous halo (see Fig. 7(a)), indicating the monolayer is amorphous and contains no detectable aggregation. With increasing ~rto 45 raN m-~, crystalline patches are observed and the ED pattern shows crystalline hexagonal spots (Fig. 7(b)), indicating that crystalline domains are formed or recrystaUized at the interfacial lateral surfaces owing to aggregation or phaseseparation behaviour caused by higher ~r. The surface morphology viewed by SEM supplies further support to the TEM results interpretation. Fig. 8 shows SEM photographs of the LB films deposited at two differentsurface pressures on the slide glass. A five-layer mixed LB film deposited at 35 mN m-J (after the inflection) (Fig. 8(b) ) shows homogeneouslydistributed pin-holes or islands, which we consider as the results of aggregation and phase-separation of PSLC from the mixed matrices as described in Section 3.3. The film deposited at 25 mN m - t (before the inflection), however, shows only a dense and uniform appearance. This obvious aggregation behaviour at ~rhigher than the inflection point also illustrates the blocking effect difference between these two films in cyclic voltammetry measurements.

To better understand the aggregate structure, the molecular exciton model is useful because it helps us interpret obtained results, in particular the specific spectroscopic excited-state traits. According to this model [ 15,16], transition moments of the chromophores in H-aggregate are oriented parallel to each other and nearly perpendicular to the film plane, that is, a card-pack arrangement. This leads to marked splitting of the excited state into N bands, where N is the number of monomers in the aggregate. However, the ground state should not split because these wave functions are not coupled with a dipole Hamiltonian. Because of the regular transition moment alignments, we can predict what transitions will be allowed or forbidden. The card-pack aggregate formally has one dipole allowed transition, that being to the topmost energy level of the exci~on band. Transition to or from any lower level are forbidden. Thus we could expect that the absorption of benzalaniline in PSLC's side-chains should be blue-shifted. The extent of the band shift can also be semi-quantitatively estimated. The spectral shift in wavenumber (A v) from the monomer peak produced by aggregation is given by

Fig. 8. SBM photographs for S-layerAA/PSLCLB filmson slide glass depositedat (a) 25 mNm-~ and (b) 35 InN m-~.

A z, ffi2 ( N - l )/z2( 1 - 3 cos2 0) INhcr~ where h is Planck's constant, c is the light velocity, N is the number of monomers in the aggregate,/z is the transition dipole moment, r is the distance between molecular centers, and 0 is the tilt angle between the line of molecular centers and the direction of the transition moment. Clearly the calculated A,~, value according to the above equation varies with the change of the mutual orientation (0) and the aggregation number of the chromophores (N). The larger the N or the more normal the chromophore, the larger shift can be expected. So the surface pressure induced changes of UV spectra (Fig. 4) can be easily understood. From the above results and discussions we suggest that the PSLC exists as an ordered fluid phase in the mixed monolayer below the inflection region. The siloxane backbone in the matrices of AA can extend to some extent. Therefore, the mesogens cannot get close enough to each other to be densely packed and may have a tilted confm marion. Above the inflection pressure, the mixed film is a highly incompressible solid film. At this point we may assume that the mesogenic side

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X. Cken et aL /Thin Solid Films 286 (1996) 232-236

magnetic fields. Further studies on these effects are in progress.

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Acknowledgements The authors would like to thank the National Natural Science Foundation of China and State Major Basic Research Project for financial support.

.

~mpma~m I m l ~ ~

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lUig. 9. The slructm~ changes of an AA/PSLC mixed monolayer upon compmssion.

chains are more perpendicular to the water surface and get close together from two sides of main chain to ensure denser packing of mesogens. Upon occurrence of aggregation the PSLC molecules may be separated from the AA matrices. This structure changes is shown in Fig. 9.

4. Coadudon

The LB technique is an excellent method for systematically manipulating the layer-by-layer stacking of a liquid-crystalline polymer whose manipulation is impossible using the bulk technique. Results in our experiments have given clear evi-

dencethat high H-like aggregationof PSLC moleculesoccurs in the monolayer and LB film with increasing surface pressure. The dipole-dipole interaction in aggregates results in an exciton energy splitting which can be observed in the UVVi~ spectra. The morphology change caused by aggregation

or phase.separation could be effectively revealed by cyclic voltammetry combined with SEM examinations.Sincedipole interaction is important in the overall bulk properties for ferroelectric I,C materials, difference in the packing order will affect the overall polarization. Therefore, by using LB method it may also be possible to optimize the spontaneous polarization of a ferroelectric film without using electric or

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