Langmuir 1994,10,2727-2730
2727
Self-Assembly of an Amphiphilic Graft Polymer Bearing Azobenzene Groups Ruifeng Zhang, Xi Zhang, and Jiacong Shen* Department of Chemistry, Jilin University, ChangChun 130023,People’s Republic of China Received December 2, 1993. I n Final Form: April 12, 1994@ Amphiphilicpolymersbearing azobenzenegroups (HRG-g-(NPH),)were synthesized by means of grafting. The polymers can form the “ReversedDuckweed” type monolayer at the aidwater interface. Their behaviors in monolayers at the aidwater interface were studied. The relationship between the chemical structure of the polymers and their behavior at the aidwater interface was discussed. The order and stability of monolayers were determined by graft degree. The UV-visible spectra of this kind of polymer in solution and in LB films were explained by the molecular exciton theory. The orientation of mesogen groups incorporated in the LB film was studied by means of polarized UV-visible spectra, and the layer structure of LB films was studied by X-ray diffraction. A possible schematic model was proposed to describe the relationship between the structure of the LB film and that of this kind of polymer. Introduction
In the last few years, numerous methods to stabilize model membranes have been developed, mainly by using a polymeric system. The use of oligomeric amphiphiles or the incorporation of spacer groups into the polymeric amphiphile to decouple the disordered polymer chain from the ordered membrane allows the formation of membranes from prepolymerized amphiphile~.l-~ In our previous works, we have studied a new type polymeric LB film called the “Duckweed and “Reversed Duckweed type^.^-^ “Duckweed” means that the hydrophobic microgel cores float on the water surface and the hydrophilic chains are down in the water. As for the “Reversed Duckweed type, the hydrophobic chains which are grafted onto a flexible hydrophilic network can form an ordered structure based on self-adjustment of the configuration of the network at the airlwater interface. We think that the flexible network here might have acted as a special “spacer”group. It is impossible for prepolymeric amphiphiles to form extremely ordered structures at the airlwater interface. But, if we ignore the order of the hydrophilic part and ensure that of the hydrophobic groups, the system will be free from the recrystallization of molecules, which is the main reason for the instability of the LB film fabricated from low-molecular-weight molecule^.^ So both order and mobility are necessary in a self-assembly system. This paper deals with another “Reversed Duckweed type of LB film with azobenzene groups incorporated in the hydrophobic part of the membrane. Herein, the azobenzene groups behave as mesogen groups. By use of this model, the liquid-crystal (LC) properties can be combined with the “Reversed Duckweed” LB film to exhibit amphotropic b e h a v i ~ r In .~,~ Abstract published in Advance A C S Abstracts, June 15,1994. (1)Kunitake, T.; Nakashima, N.; Takarabe, K.; Nagai, M.; Tsuge, A,; Yanagi, H. J . Am. Chem. SOC.1981,103,5945. (2) Elbert,R.; Lashewsky,A.; Ringsdorf, H. J . A m . Chem. SOC.1986, @
107,4134. (3)Lashewsky, A,; Ringsdorf, H. Angew. Makromol. Chem. 1986, I ,
145.
(4) Yin, R.; Cha, X.; Zhang, X.; Shen, J. C. Macromolecules 1990,23, 5185. (5) Shen, J. C.; Zhang, X.; Cha, X.; Yin, R. Makromol. Chem. Macromol. Symp. 1991, 46, 157. (6) Shen, J. C.; Zhang, X.; Zhang, R. F. Thin Solid Films 1992,2101 211, 624. ( 7 )Gaines, G. L. Insoluble monolayers at liquid gas interface; Interscience: New York, 1966. (S)Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (9) Finkelman, H.; Schafheutle, M. A. Colloid Polym. Sci. 1986,264,
786.
0743-7463/94/2410-2727$04.50/0
Scheme 1. Approach for the Synthesis of the Amphiphilic Polymer Gel Core
n
Gel Core
--
I r r r - X
HRG-g-(NPH),
tirnlting
NPH
this system, the order and mobility of the membrane may be controlled or adjusted by the structure of the mesogen groups and network, respectively. Experimental Section Synthesis of Amphiphilic Polymer (1) Synthesis of the Gel Core. According to Scheme 1, 13.6g of maleic anhydride, 10 mL of ethenyl acetate, and 4 mL of trimethylolpropane
triacrylate (TMPTA,as a cross-linking agent) were dissolved in 32 mL of dioxane,and 160 mg ofAIBNwas added as the initiator. The materials were polymerized at 80 “C withultrasonic stirring under an Nz atmosphereover approximately10min. The reaction was terminatedby addingan ethyl acetate solutionof benzenediol. The compound was then precipitated twice by adding a large amount of ether, and the precipitate was dissolved in acetone. The slight red powder obtainedwas called the hydrophilicreactive gel (HRG). FT-IR 1860,1785(C=O, in anhydride),1742(C=O, in ester), 1636 (C=C), 1225 (C-O), 2940 (CH3). M , = 3.26 x lo4 (measured by a membrane osmometer). (2) Synthesis of the Functional Group. p-Nitroaniline (28g)was diazotized and reacted with 19g ofphenol. The product was recrystallized in ethanol and then reacted with 15 g of C1(CH&OH in an ethanol solution of KOH with a small amount of KI for 72 h to produce 6-[4-((4-nitrophenyl)azo)phenoxylhexenol-1 (NPH). Anal. Calc for NPH: C, 66.92;H, 5.76;N, 10.51. Found: C, 67.16;H, 5.89;N, 10.68. FT-IR: 1585,1502
0 1994 American Chemical Society
Zhang et al.
2728 Langmuir, Vol. 10,No.8,1994
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i\
0 20
40
60
80
100
120
140
Area/mesogen( A')
Figure 1. II-A isotherms of the amphiphilicpolymer recorded 20 "C: (a)RD-1; 6) RD-% (c)RD-3;(d) RD4. The compression speed is 10 (A~/memgenymin.
20
60
40
80
100
Timdmin)
160
Figure 2. Stabilizationcurve of the amphiphilic polymer at the airhvater interface at 20 "C and a pressure of 30 mN/m: (a) RD-1;(b)RD-2;(c) RD-3; (d) RD4.
each other. In monolayers c and b, mesogen groups are not as close-pacged as in d. However, monolayer a is a Table 1. Strnctnral Parametera of H l U h ~ 4 " H l . typical expanded liquid-analogous membrane without a sample NPH (wt %) n 10-4~,, steep region is the m e . The stability of the monolayers can be seenh m F&me 2. Monolayers b-d are stablefor RD-1 17.5 21 3.95 RD-2 27.3 37 4.48 a long time, and they can be well-transferred onto solid RD-3 34.5 52 4.98 substrates. As might be already obvious h m the II-A RD-4 41.1 69 5.54 isothem, polymer RD-1 cannot form a stable monolayer at the airhater interface at a temperature of 20 "C(curve (benzene C-C), 1385,1319@ 2920,2851 I&), (CHz),3355 (OH), a). An area of 46Az was found initially for one mesogen, 1257 1-(. and after 100 min this was reduced to 14 Az. The (3)Synthesis ofthe Amphiphilie PolymThe HRG instabilitywascausedbycollapseofthefilmanddissolving and Merent amounts of NPH were dissolved in DMF'. A small intothesubphasebecausepolymerRD-1istoohydrophilic amount of triethylamine was then added to this solution. The and soluble in water. These results suggest that reaction mixture was heated to 100 "C for 1 h and then poured thebehaviorof the monolayersof the polymers arestrongly intodiluted-cacidTheprecipitatewaswashedwithwater influenced by the graft degree. It seemed that in the severaltimesaaddriedinvacuumfor24hatroomtemperature. condensed phase the area occupied by close-packed The contentofNPHinthe polymer was determinedby elementary analysis. The polymer we synthesized canbe deacribed as HRGhydrophobic groups should be close to that of a flat g-(NPH),,. The structural parameters are shown in Table 1. hydrophilic network spread at the airhater interface. In preparati0naudCb"za * tionofLBFilms.The the case of a,b, or c, the area of the hydrophilic network amphipmc polymer was diagahred in DMF/chlorohrm (28 vh) might be much bigger than that occupied by all the inaconcentrationof l.OmglmLbeforebeingspreadatthewater/ hydrophobic groups, which means that many gaps have airinterhce. Theaqueoussubphasewaspurifiedanddeionized been lee in the hydrophobic side of the monolayer. As a by using a Millipore Milli-Q system (15 MQ cm). Surface result, monolayers lack enough stability and stdihess. It pressure-molecular area ieothermaweremeasuFedbyusinga is very important to maintain a balance of hydrophilic computer controlled KSV-5000 ' t. Inatypicalexperiand hydrophobicparts not only in thistypeof amphiphilic ment, 25 p L of solution was spread on the water surface. ARer waiting1Ominforsolventevaporation,com~~wasstarted. polymer but also in many other self-assembly system. Y - t y p e L B f i l m s w e r e d e ~ ~ o n ~ e a y s t a l ~ ~ s l i d e s f o rWe also studied the hysteresis of the monolayer at the X-ray di&action and Quartz platea for UV-visible spectral airhater interface. For example, the monolayer of measurementswhile the surkm?pressure was maintained at a polymer RD-3 was iimt compressed up to 30 mN/m; then " t a u t value (30mN/m), with a dipping speed of 5 m"in the compression was stop for 3 min followed by and as upper delay of 10 miu for drying. UV-visible spectra expansiontoanareaof120 Vmesogen. Whenthesame were " l e d with a UV365 apectrophobmeter. X-ray difmonolayer was compressed again, the molecules started fi-actionwas carried out with aD/Max-rA(Cu Ka)d i f b c h meter. to develop pressure in an area (80&mesogen) much s d e r than that in the Grst run (100 AVmesogen; see a e w l t s a n d'o~ n Figure 3). Furthermore, two phase transitionpoints have been observed in the second run at pressures of 5 and 18 The surface pressure-area isothermsfor the polymers mN/m, which means that the monolayer has become more are shown in Figure 1. As for monolayer d, the limiting ordered than that is the 6rst run. According to the point area of every mesOgen estimated by extrapolating the that the phase transition for the polymer is a process of steepest region of the condensed phase to zero pressure relaxation, it can be understood that the self-assemblyof is %A2, which meansthat the mesagengroups are almost polymeric amphiphiles at the airhater interface also perpendiculartothe water surfaceloandclose-med with needs enough time to reach ita h a l ordered structure. When thethirdrun was carried out, no remarkable change (10)Nabahara, H.;Fukuda, K J . cdloid Znterfbae Sei. I-, 39, was found for the shape of the curve. 530.
e
P
Langmuir, Vol. 10, No. 8, 1994 2729
Self-Assemblyof a Polymer Bearing Azobenzene Groups
Area/mesogen(A ')
Figure 3. Hysteresis of the monolayer of polymer RD-3, at 20 "C, with compression speed 10 (Az/mesogen)/min,pause time 3 min, and maximum pressure 30 mN/m: (A) first run;(B) second run.
1. 0
2. 0
3. 0
5. 0
4. 0
2
~
6. 0
7. 0
)
Figure 6. X-ray diffraction pattern of 25 layers of the Y-type LB film of (a) RD-4, (b) RD-3, and ( c ) RD-2.
Table 2. Structure of the "Reversed Duckweed" LB Film sample AJAs tilt angle (deg) layer space d (A) RD-2 RD-3 RD-4
)
300
400
500
600
-
7
Wavelength(nm)
Figure 4. UV-visible spectraof polymer RD-4 in solution and in the LB film. In order to examine the aggregation of chromophores in multilayer films, we compared the W-visible spectrum of PSHAN-a in a solution of DMF/CHC13 (3/7, v/v) with that in a LB film (Figure 4). The absorption maximum of the dye in solution is a t 410 nm, which can be assigned to the transition moment along the long axis of chromophore. The absorption maximum ofthe dye introduced in the LB film shifts to 357 nm. A shift to shorter wavelengths suggests the formation of H aggregates. According to the exciton theory, strong spectral shifts can appear in the absorption spectra due to a sufficiently strong electronic interaction between molecular subunits.'l Since only a blue shift is observed, it can be argued that the transition moments must be aligned nearly parallel in the aggregate. The order of the multilayer can be seen from the orientation of the hydrophobic groups and the X-ray diffraction pattern. The orientation of the mesogen groups ~~
(11)Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Molecular Spectroscopy. Proceedings ofthe VIIIEuropean Congress on Molecular Spectroscopy; Butterworth: London, 1965; p 371.
1.14 1.24 1.61
45 38 23
48.2 51.8 58.4
was studied by polarized W-visible spectra. The tilt angle is defined as the angle of the transition moment to the z-axis which is perpendicular to the plane of the LB film. The incident angle we selected is 30". A, and A, are defined as the absorbance when the polarized directions are parallel and perpendicular to the incident plane, respectively. According to the value of absorption a t 357 nm, we can obtain an average ofthe tilt angle of azo groups using the following equation:12
AP - (n, cos i -_ A, (n, cos r
+ n2 cos r) cos i cos r + 2n,3n3 sin2i + n2 cos i)
n,4
