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Langmuir 1995,11, 1068-1071
Self-AssembledAlternating Multilayers Built-up from Diacetylene Bolaamphiphiles and Poly(allylamine hydrochloride): Polymerization Properties, Structure, and Morphology Farnaz Saremi, Ellen Maassen, and Bernd Tieke* Institut f i r Physikalische Chemie der Universitat zu Koln, Luxemburger Strasse 11 6,D-50939 Koln, Germany
Guntram Jordan and Werner Rammensee Mineralogisch-PetrographischesInstitut der Universitat zu Koln, Ziilpicher Strasse 49b, 0-50674 Koln, Germany Received November 28, 1994. In Final Form: February 13, 1995@ A study of the polymerization properties and the morphology of a new type of ultrathin diacetylene multilayers is presented. The multilayers were built-up on charged surfaces by alternating self-assembly of dipolar amphiphilic diacetylenes (2,4-hexadiyne 1,g-disulfate (HDDS), 5,7-dodecadiyne 1,12-disulfate (DDDS), 10,12-docosadiyne 1,22-disulfate (DCDS))and a cationic polyelectrolyte (polyallylamine hydrochloride) from aqueous solution. Upon W- and y-irradiation, the multilayers containing DCDS dianions polymerized. X-ray studies indicate a layered structure with a layer thickness of 2.7 f 0.1 nm. The DCDS dianions are tilted by 38 f 4" with regard to the layer plane. The morphological study by scanning force microscopy indicates that the multilayers do not represent a homogeneous film but consist of a multitude of small, separate domains of up to 300 nm in diameter covering the substrate in a way that the macroscopic appearance is fairly regular and homogeneous. 1. Introduction Purpose of our paper is to report on polymerization properties and morphology of a new type of ultrathin polydiacetylene films prepared by a simple self-assembly technique. As recently reported by Decher e t al.,1-3 ultrathin organic films can be easily built-up on charged surfaces by consecutive adsorption of anionic and cationic bipolar amphiphiles (bolaamphiphiles) and/or polyelectrolytes from aqueous solution. By use of this simple dipping technique, multilayer assemblies are obtained which are alternatingly composed of cationic and anionic organic layers in a regular fashion. Recent studies have shown t h a t a p-phenylene bis(acry1ic acid) amphiphile4 and a diacetylene amphiphile5 with polar headgroups in the aand o-position can be W polymerized within these layers. Since the two amphiphiles only polymerize in latticecontrolled reactions,6s7the monomers must attain a highly ordered arrangement in the multilayer assemblies. In the present paper, the polymerization properties of a series of diacetylene bolaamphiphiles in the selfassembled multilayers will be reported and the morphology of the polymerized multilayers will be described in more detail. The general formula of the diacetylene bolaamphiphiles is Na' -O,S-O-(CH,),-C~C-C~C-(CH,),-O-SO,-
Na' with n being 1,4, and 9. The compounds are the disodium salts of 2,4-hexadiyne 1,6-disulfate (HDDS), 5,7-dodeca-
* Author to whom correspondence should be addressed.
Abstract published inAdvanceACSAbstracts,March 15,1995. (11 Decher, G.; Hong, J. Makromol. Chem. Macromol. Symp. 1991 46,321. (2) Decher, G.; Hong, J. Ber. Bunsenges. Phys. Chem. 1991 95,1430. (3)Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993,9, 481. (4)Mao, G.;Tsao,Y.; Tirrell, M.; Davies, H. T.; Hessel, V.; Ringsdorf, H . Langmuir 1993,9,3461. (5)Saremi, F.;Tieke, B. Adv. Mater., in press. @
diyne 1,la-disulfate (DDDS), and 10,12-docosadiyne 1,22-disulfate (DCDS). Polymerization properties were studied by W/visible spectroscopy and structure and morphology were investigated using X-ray diffraction and scanning force microscopy (SFM).
2. Experimental Section Materials. The disodium salts ofthe alkadiyne a,w-disulfates (ADDS) were prepared by sulfation of the corresponding diols using the pyridine-SO3 complex (Fluka). 2,4-Hexadiyne-1,6were prepared according to diol and 10,12-docosadiyne-l,22-diol the literature,8-1°while 5,7-dodecadiyne-l,l2-diol was purchased from Polyscience, Ltd. Preparation of the Disodium Salt of2,4-Hexadiym 1,6-Disulfate (HDDS). 2,4-Hexadiyne-l,&diol(3.0 g, 0.027 mol)was dissolved
under a nitrogen atmosphere in anhydrous dimethylformamide (DMF)(40 mL) in a three-necked flask fitted with stirrer, reflux condenser, and dropping funnel. A solution of pyridine-SO3 complex (8.6 g, 0.054 mol) in anhydrous DMF (40 mL) was put into the dropping funnel and added dropwise with stirring. Subsequently,the mixture was stirred at 40 "C for 7 h and kept at room temperature overnight. Then the mixture was worked up by addition of aqueous NaOH (5% by weight) until the pH was between 8 and 9. Upon neutralization, a pale brown precipitate appeared. Finally, DMF was evaporated and the residual solid product dried in vacuum. A 4.0 g portion (48.2%) of a yellowish polycrystalline powder was obtained, which was twice recrystallizedfrom water. Thermalbehavior: endothermic phase transition at 117 "C, exothermicdecomposition at T > 300 "C, no melting point. IR (KBr): 1360 cm-I (vas (S=O)), 1230 cm-l (vs (S=O)); W-NMR (DMsO-ds, 200 MHz, ppm) 6 75.93 (C=C-CHz), 68.56 (C=C-CHz), 53.70 (CHz);W (water) 258.4 nm (6 = 71.1 m2 mol-'), 244.4 nm (88.8), 231.6 nm (91.1). (6)Hasegawa, M. Chem. Rev. 1983,83,507. (7) See, for example, Polydiacetylenes, Cantow, H.-J., Ed.; Springer Verlag: Berlin, 1984. Polydiacetylenes. Advances in Polymer Science Bloor, D., Chance, R. R., Eds.; Martinus Nijhoff: Boston, MA, 1985; Vol. 63. (8) Reppe, W. Liebigs Ann. Chem. 1966,596,70. (9) Bader, H.; Ringsdorf, H. J.Polym. Sci., Polym. Chem. Ed. 1982, 20,1623. (10)Plachetta, C.; Rau, N. 0.; Hauck, A.; Schulz, R. C. Makromol. Chem., Rapid Commun. 1982,3,249.
0743-746319512411-1068$09.00/00 1995 American Chemical Society
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Table 1. List of Dipolar Diacetylene Amphiphiles and Their Photoreactivity in the Bulk State and in Self-Assembled Multilayers W- and y-polymerization in amphiphile bulk phasea multilayer HDDS DDDS DCDS + + ~~
a
As disodium salt.
O"O
1
O 0.00 450
'
500
O
550 2
600 0
Wavelength [nm]
Figure 1. Optical absorption spectra of self-assembled DCDS/ PAH multilayers monitored after 8 min of UV irradiation. Samples differ in the number, n, of dipping cycles applied. Disodium Salt of 5,7-DodecadiynelJ2-Disulfate (DDDS). The same procedure as described for HDDS was applied except that 5,7-dodecadiyne-1,12-diol(2.0 g, 0.01 mol) was dissolved in anhydrous DMF (30 mL) and reacted with the pyridine-SO3 complex (3.2g, 0.02mol) in anhydrous DMF (30mL). Yield: 1.83 g (46.3%) of white polycrystalline powder. Thermal behavior: endothermic transition at 47.9 "C and exothermic decomposition at T > 200 "C, no melting point. IR (KBr): 1440 cm-l (vas (S=O)), 1230 cm-I (vs (S=O)), 820 cm-I (v(C-0-S);
13C-NMR(DMSO-d6,200 MHz, ppm) 6 77.97(CW-CHd, 65.48 (CEC-CH~), 64.92(CHz-O), 35.86-18.68((CH2)3);TJV (water) 252.4 nm ( e = 37.6m2 mol-'), 239 nm (51.4). Disodium Salt of 10,12-Docosadiyne 1,22-Disulfate (DCDS). The same procedure as described for HDDS was applied except g,0.012mol) was dissolved that lO,l2-docosadiyne-l,22-di01(4.0 in anhydrous DMF (70mL) and reacted with the pyridine-SOs complex (3.8g, 0.024mol) in anhydrous DMF (40 mL). Yield: 5.1 g (75%)of polycrystalline powder which was twice recrystallized from water. Thermal behavior: endothermic phase transitions at 87 and 120 "C, exothermic decomposition at 177 "C, no melting point. IR (KBr) 1370 cm-l (v,(S=O)), 1230 cm-l (vs(S-O)), 830 cm-' (4C-0-s)); 13C-NMR(DMso-d~j,200 m z , ppm) 6 77.49 (CmC-CHg), 65.05(CHz-O), 64.83 (CSC-CH~), 28.56-17.79((CH2)8),UV (water) 282 nm ( E = 26.1 m2 mol-'), 266 nm (32.0),253 nm (46.3),239 nm (61.1). Poly(styrenesulfonate) (sodium salt, M = 100 000)(PSS)was obtained from Janssen and poly(ally1amine) (hydrochloride,M = 50000-65000) (PAH)from Aldrich. All polyelectrolyteswere used without further purification. Milli-Q (Millipore GmbH) water was used for all cleaning steps and as the solvent for the adsorption. Methods. The multilayers were built up on quartz slides (30 x 12 x 1 mm) which were purified and silanized with (3-aminopropy1)trimethoxysilane (Fluka) according to the literature.'-3 A typical multilayer preparation was as follows: The silanized substrate was immersed (a) in a 0.01monomolar PSS solution in 0.01 M aqueous HC1 (monomolar refers to the molar concentration of monomer residues), (b) in a 0.01monomolar PAH solution in 0.01M aqueous HC1. Steps a and b were repeated twice. Then the substrate was immersed (c) in a 0.2 mM solution of the disodium salt of DCDS in Milli-Q water and (d) in the PAH solution described under (b). Steps c and d were repeated up to 15 times. Immersion times were 1 h for the first layer (described under (a))and 30 min for all subsequent layers. Between two immersion steps, the substrate was always washed with Milli-Q water for 1min and subsequently dried in air for 10 min. For UV polymerization, the samples were irradiated with a 40-WUV lamp (K. Benda, Wiesloch, I = 254 nm). The distance between sample and lamp was 3 cm. No efforts were made to cool the substrate or t o exclude oxygen from the irradiation process. Already after a few seconds of U V irradiation, a red coloration of the samples became visible.
Figure 2. Scheme of U V polymerization of DCDS in the self-assembled multilayer. The tilt angles of the dianions are arbitrary.
1070 Langmuir, Vol. 11, No. 4, 1995
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looa,
100
50
I
0
I I
1000
, 2000 I
I I
3000
I
I
4000
nm
0
nm
I
loo0
I
2ooo
-I.
3ooo
I
4000
Figure 3. SFM surface views of a polymeric DCDSPAH multilayer (12 dipping cycles) on quartz in air with a scan area of (a) 10 x 10 pm2 (top view image) and (b) 4 x 4 pm2 (perspective image). (c) Cross section along the line indicated in image (d) which is a top view of (b). The section shows the maximum height differenceto be about 40 nm which corresponds with the total thickness of the multilayer. The morphologywas studied with a NanoscopeI1SFM (Digital Instruments) working in contact mode. Self-assembled multilayers of 30-40 nm thickness were built-up on polished quartz substrates and investigated in air at room temperature. All SFM images presented here were taken in equiforcemode. The force used for each sample was so low that even after prolonged scan times no changes of the surface structure were detectable. Commercially available Si/N cantilevers with integrated tips were used. Scan sizes were 4 x 4pm2and 11.2 x 11.2 pm2. X-ray data were recorded using a Philips powder diffractometer in the reflection mode using Ni-filtered Cu K a radiation.
3. Results and Discussion The disodium salts of the alkadiyne disulfates (ADDS) were prepared from the corresponding alkadiyne diols by sulfation with the pyridine-SO3 complex in dimethylformamide and subsequent treatment with sodium hydroxide. At 25 "C, the monomeric salts of HDDS and DDDS were excellently soluble in water, while the DCDS salt formed a clear solution in water only when the concentration was lower than 0.6 mmoVL. The aqueous solutions were stable against W polymerization. The polymerization behavior of the bulk materials is compiled in Table 1. The disodium salts of HDDS and DDDS were inactive upon W and y-irradiation, while the DCDS salt rapidly turned deep red. The different reactivities can be explained by a different packing of the dianions. The packing of the short chain HDDS and DDDS dianions is most likely determined by the bulky sulfate groups; i.e., the intermolecular distances are probably too large for polymerization, while the long chain DCDS dianions form a dense packing of the alkyl chains which is known to be
very suitable for the solid-state polymerization. None of the compounds could be polymerized upon thermal treatment. Self-assembly multilayers were prepared by alternate dipping of a quartz substrate silanized with (3-aminopropy1)trimethoxysilane into aqueous solutions of the disodium salts of the corresponding ADDS and acidified PAH. In order to adsorb the disulfate as homogeneously as possible, it was advantageous to precoat the substrate with three polystyrenesulfonate (PSS)and PAH layers in alternate sequence. The precoating provided a more homogeneous charge distribution on the substrate. After the precoating, the substrate was dipped into the ADDS solution and the first, negatively charged diacetylene disulfate layer was adsorbed. In the next step, the substrate was dipped into the PAH solution and a positively charged PAH layer was adsorbed. Following this procedure, up to 16 alternating ADDSPAH layers were built up on each side of the substrate. For UV polymerization, the multilayers were irradiated with a 40-Wlow-pressure mercury lamp (A = 254 nm). The polymerization behavior is compiled in Table 1. Only multilayers containing DCDS were photoreactive. They rapidly turned red upon W or y irradiation, while samples containing HDDS or DDDS remained colorless. Obviously the sandwiched monolayers have the same packing as in their normal crystal structure (see also the X-ray studies below). Absorption spectra of DCDS multilayers are shown in Figure 1. The formation of the conjugated polymer is indicated by the broad absorption in the visible region with a maximum at 536 nm and a shoulder at 498 nm. After an irradiation time of 8 min the optical
Letters absorbance did not further increase. The polymerization process is schematically shown in Figure 2. X-ray measurements of polymerized PAWDCDS multilayers indicate two reflections a t 2 8 values of 13.4 and 16.5" from which a layer spacing of 2.7 f 0.1 nm could be calculated. Subtraction of 0.7 nm for the PAH layer3gives a thickness of about 2 nm for a single DCDS layer. Since the maximum length of the DCDS dianions can be calculated to about 3.3 f0.3 nm, the molecules must attain a highly tilted arrangement in the individual layers. The tilt angle is about 38 f 4" with regard to the layer plane. For comparison, bulk crystalline samples of the DCDS disodium salt in the polymerized state were also investigated. X-ray measurements indicate a layer spacing of 2.51 f 0.05 nm. Subtraction of 0.4-0.5 nm for the disodium interlayer (diameter of the sodium ion: 0.2 nm) gives a thickness of2.0-2.1 nm for the DCDS layer. Apparently, the packing of the DCDS dianions is the same as in the self-assembled multilayers (and therefore the polymerization behavior is also the same as discussed above). The morphology of the polymerized multilayers was studied using SFM. In parts a and b of Figure 3, SFM images of a polymerized DCDSPAH multilayer consisting of 12 alternatingDCDS/PAHlayersare shown. In general, the multilayer is fairly uniform. It consists of a multitude of small, isolated domains. Domain sizes vary between 100 and 300 nm; the average distance between individual domains lies between 100 and 200 nm. The cavities between the domains are 20 to 40 nm deep. This can be concluded from the cross section (Figure 3c) along the line indicated in the SFM image (Figure 3d). Since the layer spacing is about 2.7 nm, the multilayer of 12 alternating DCDSPAH layers should have a total thickness of approximately 32 nm. This agrees with the depth of the deepest cavities; i.e., some ofthe cavities are actually holes in the multilayer. The SFM images also show a different height of the individual domains. This probably arises from the adsorption of bi- and multilayer aggregates of the DCDS dianions and/or the PAH polyelectrolyte chains. The aggregate formation becomes especially pronounced after some dipping cycles, as can be concluded from the nonlinear increase of the polymer absorbance a t 536 nm with the number of dipping cycles applied (Figure l).5 The polymerized multilayers are fairly heat resistant. This can be concluded from absorption spectra of the polymer monitored after annealing of the sample a t
Langmuir, Vol. 11, No. 4, 1995 1071
A,
T = 20 "C
, 135'C
0.00 ,
1
I
500 550 600 Wavelength [nm] Figure 4. Temperature effects on the polymer absorption of a DCDSPAH multilayer (14 dipping cycles). Spectra were monitored a h r annealing of the multilayer at temperature T for 10 min and subsequent recooling. 450
different temperatures and subsequent recooling to room temperature (Figure 4). A significant decrease of the polymer absorption only occurs a t temperatures above 150 "C. The polymer turns yellow indicating a reduction of the backbone planarityll and the onset of degradation. Our studies show that ultrathin diacetylene multilayers can be built-up on charged surfaces by alternating selfassembly of dipolar amphiphilic diacetylenes and a polyelectrolyte from aqueous solution. DCDS dianions form a condensed, photoreactive 2D structure and can therefore be UV polymerized on the substrate. The morphological study by SFM indicates that the multilayers consist of domains, which do not exceed 300 nm in diameter and thus are considerably smaller than in LB films.12 Domains are usually not in contact with each other, so that the multilayers do not represent a homogeneous film but a fairly regular coverage of the substrate with small islands oflayered structure. Further studies are necessary to better control the formation of the multilayers. L4940937B (11)Patel, G.N.; Chance, R. R.; Witt, J. D.J. Chem. Phys. 1979,70, 4387. (12)Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films l9S3,99,95.
