Langmuir 1991,7, 1158-1160
1158
Photopolymerization of Thin Films of Triazinedithiols Kunio Mori' and Seichi Sai Department of Applied Chemistry, Faculty of Engineering, Iwate University, Ueda, Morioka 020, Japan
Tokuji Miyashita Department of Biochemistry and Engineering, Tohoku University, Aoba, Aramaki, Sendai 980, Japan
Minoru Matsuda Chemical Research Institute of Nonaqueous Solutions, Tohoku University, Katahira 2-1 -1, Sendai 980, Japan Received March 13,1990. In Final Form: December 3, 1990 Three kinds of 6-(stearylamino)-1,3,5-triazine-2,4-dithiol (ST)and 6-(cis-9-octadecenylamino)-1,3,5triazine-2,4-dithiol(OL)films, a cast film, an immersionfilm, and a LB film,were prepared and polymerized under UV light irradiation. The polymerization rate was influenced by film structures and obeyed the rate equation of pseudo-first-order reaction for ST films and paraboric law for OL films. 1. Introduction
Recently, the photopolymerization of various organic thin films has attracted considerable attention for possible applications such as microphotographs,' photoresists,z semiconductors,3 electrical devices,' and optical device^.^ At present, these monomer thin films can be prepared readily by Langmuir-Blodgett! vacuum vapor deposition,7 immersion,B-lO and spin coating techniques. However, little is known about the effects of structure and properties of various films" on the rate of photopolymerization. This paper describes the photopolymerization of three kinds of triazinethiol monomer films prepared by LB, spin coating, and immersion techniques. 2. Experimental Section
The monomers (triazinedithiols) used in this work, 6-(stearylamino)-1,3,5-triazine-2,4-dithiol (ST) and its 1,3,5monosodium salt (STN),6-(cis-9-octadeceny1amino)triazine-2,4-dithiol (OL) and its monosodium salt (OLN), were prepared as described previou~ly.~>~ Cast Films.12 The 0.1 M solutions of ST and OL in tetrahydrofuran (THF) were poured onto a quartz plate rotated at 3000 rpm at 25 "Cto yield spin-coated ST and OL thin films (STand OL cast films). The cast films thus obtained were dried by rotating in air for 30 min at 25 "C and then in a vacuum desiccator, for 24 h at 20 "C. The cast films, about 100 nm thick, were kept over blue silica in a desiccator before use. (1)Barraud, A.;Rosilio, C.; Teixier, A. R. Thin Solid Films 1980,68, 91. (2)Barraud, A. Thin Solid F i l m 1982,70,317. (3)M a n , B.; Kuhn, H. J.Appl. Phys. 1971,42,4398. (4)Agawal, V. K. Thin Solid Film 1978,50,3. ( 5 )Vincentt, P.5.;Roberta,G. G. Thin Solid Films 1980,68,135. (6)Blodgett, K. B. J. Am. Chem. SOC.1931,57,1007. (7)Sasabe, Y.Kogyo Zairyo 1983,3I,31. (8)Mori, K.; Muroi, Y. Kobunshi Ronbunshu 1986,43,617. (9)Mori, K.;Saito, M.; Nakamura, Y.Nippon Kagaku Kaishi 1978, 727. (10) Mori, K.; Muroi, Y. J. Polym. Sci., Part A: Polym. Chem. 1987, 25,2983. (11)Kawaguchi, T. Nippon Kagaku Kaishi 1987,2148. (12)Mori, K.; Sai, S.; Miyaahita, T.; Matauda, M. Kobonshi Ronbunshu 1989,46,819.
Immersion F i l m ~ . ~Quartz J~ plates (30 X 60 X 0.1 mm) were coated copper, using a vapor-depositing apparatus, and were kept in a vacuum desiccator to prevent corrosion. The copper-coated quartz plates were passed through UV light. The quartz plates were immersed in M solution of either STN or OLN in methanol a t a 60 "C for 30 min to yield ST or OL immersion thin films on the surface. Film thickness was calculated a t about 100 f 15 nm, from weight difference before and after immersion. LB Films.l3 The LB films of ST were prepared on quartz slides (10X 30 X 0.1mm) by using an automatically working Langmuir-trough (Kyowa Kaimen Kagaku HBMAP using a Wilhelmy type film balance) as follows: Quartz slides on which LB multilayer had been deposited were cleaned in boiling HzSOd-HNOs (2:l) solution and their surfaces were made hydrophobic with dimethyldichlorosilane. The monolayer of STwas spread from the chloroform solution (10-3 mol/L, 0.2 mL) onto distilled, deionized water at 10 "C.After standing for 15 min, the monolayers were compressed at a surface pressure of 15 or 30 mN/m. The condensed ST monolayers were transferred to the quartz slides in successivelydownward and upward strokes at a transfer ratio of 1.0f 0.1 up to 50 layers (about 100 nm), giving Z-type LB uniforms. OL LB monomer film on quartz plates could not be prepared since the OL molecules polymerized so easily, even during spreading of OL on water in air in the dark. 3. Results and Discussion Generally, the structures and properties of thin films depend on the mode of preparation. A cast film is an amorphous assembly with low density; its molecules are arranged in random directions. Immersion film was previously found to be considerably packed and ordered as in the case of the Y-type film in the LB meth~d.~JO LB films are cross-packed and ordered assemblies, as a result of compressing ST monolayers on the water surface at a surface pressure of 15 to 30 mN/m. To confirm whether the three kinds of ST or OL films had the same general structure and properties as described (13)Blodgett, K.B.;Langmuir, I. Phys. Rev. 1937,5,964.
0743-7463I91 /24Q7-1158%Q2.5010 .~ , 0 1991 American Chemical Society I
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Langmuir, Vol. 7, No. 6, 1991 1159
Photopolymerization of Triazinedithiol Films Table I. Contact Angles of Various ST and OL Films contact angle, deg films ST OL
-
~
~~~~~~
cast immersion LB tablet
I9 103 102 80
75 101
Table 11. Rate Constant in the Photopolymerization of ST and OL Films ST films OL films t = -k In (1 - Y) P = kt k, min-1 k, min112 cast film immersion film LB film p = 3.6 mol/nm2 p = 4.0 mol/nm2
-
76
. film 0.8
0.0069 0.016
0.012 0.013
0.021 0.038
-
I
ST film
1 .o
0.8 Q,
0.6 01
0
4
C
m
P
Li 0
m
9
0.6
f?
9
0.4
0.4
0.2
0.2 n 200
I
I
0 200
300 400 Wave Length("
1 500
Figure 1. UV spectra of various ST thin films on quartz plates: - - -,ST cast film; -, ST immersion film;-, ST LB film.
300
400
300
200
Wave Length(m)
400
500
Wave Length(-)
Figure 2. Effect of oxygen on the polymerization of ST and OL cast films under UV light radiation: - - -, monomers; -, polymerized for 60 min in air; -, polymerized of 60 min in argon. - e
- a
above, contact angles and UV spectra were measured. Table I shows the contact angles of water to ST and OL films prepared by the three above techniques and tablets prepared by the press mold of ST and OL. The contact angle of water to hydrohobic-treated quartz glass was about 63O. The contact angles of water to ST and OL cast films agreed roughly with those of the tablets, since these top surface layers consisted of polar and nonpolar parts. ST or OL molecules thus could possibly be arranged randomly and packed loosely. ST or OL immersion film as well as LB films had considerably higher contact angles than cast films because the top surface layer were compressed only of nonpolar parts, namely alkyl chains. Thus, the molecules are ordered in a regular fashion and oriented. Figure 1shows the UV spectra of three kinds of ST thin films. The cast film showed two peaks at 230 and 298 nm and had a shoulder at 272 nm. The peak at 230 nm was assigned to a C=N bond of the thiol type in ST, based on a 7r- 7r * transition, and the peak at 298 nm, to a S=C-N group of the thione type, based on n H * transition. The shoulder at 272 nm may be assigned to a -CS- r N H + group, being an intermediate of thiol and thione types in ST. This intermediate indicates the cast film could possibly be loosely packed at low density. The ST-LB film on quartz had peaks at 229 and 320 nm, which were assigned based on the C=N bond and S=C-N< group. The peak at 320 nm in the ST LB film corresponded to that at 298 nm in the ST cast film. The red shift in the LB ST film was due to the orientation of ST molecules and formation of a stable J-aggregateI4J5in the triazine ring of the thione type. The immersed ST film on the copper-coated quartz slide gave two peaks at about 230
-
-
~~
_______
~
(14) Murrell, J. N. The theory of the Electronic Spectra of Organic
Molecules; Methuen: London, 1963; p 133. (15) Becker, R.S.TheoryandInterpretationofFZuorensene andPhorescence; Wiley-Interscience: New York, 1969; p 234.
and 300 nm as did also the cast film, although the peaks were weak since the quartz slides had been coated by copper. In conclusion,the above UV spectra data indicated the films to possibly consist of the thione type of ST or
OL. The monomer films were polymerized under UV irradiation with a Wacom R & D cord xenon light KXL-500F. The UV light irradiation system produced a wavelength of 250 to 400 p m by color filters and the irradiation temperature was maintained at 20 "C by a water filter. The conversion of monomer (Y)could be determined from changes in absorbance peaks near 300 nm or at 320 nm. With irradiation time ( t ) , the peaks based on the S=C-N< group decreased and the peak based on the C=N bond increased. Thus, the S-S or C-S bonds may have been formed by irradiation. The rate constant (k) of polymerization was determined from eq 1 for ST monomer and eq 2 for OL monomer
t = -k In (1- Y)
(1)
= kt (2) Table I1 lists the effects of the kinds of triazinedithiol films on the rate constant and rate equation. These films were all polymerized under UV irradiation, obeying eq 1. The rate increased with the packing density of ST molecules, in the order cast film < immersion film C LB film. The two kinds of OL film were also polymerized under UV light, obeying eq 2. In OL films, the rate of polymerization for cast and immersion films was the same. Such differences in photopolymerization behavior and rate equation of ST and OL films indicate the possibility of two different polymerization mechanisms. To make the difference in the mechanism more clear, the effect of polymerization atmosphere was investigated and the results are shown in Figure 2. UV irradiation of ST and OL cast in air gave a polymer. In argon, the OL
Mori et al.
1160 Langmuir, Vol. 7, No. 6, 1991
Scheme I
ST polymer NHCS~HI, ~
N
~
,Ay ICs
NHCisHir
H
NAN Hs
NHClaH,,
h ?/O'
_[,dtS-+
( 1)
HI 0
h Xs H
H
ST
ST p o l y m e r
monomer
NHC. H l b C H = C H C a H,,
-
NANH ,Ay J,.
N H C s H l s C H = C H C a H,r
hv
NAN H& HJ S
H
I
1 I Soluble OL polymer
OL m o n o m e r
I SH
N
~
S-
N
C a HI, h v / O *
3
A . 4 i h H C a H l s C H 2 CI H I THF
1
1
105
lo4
1G3 Molecular Weight
1 102
Figure 3. GPC elution patterns of ST polymer and soluble OL polymer before and after NaSH treatment: -, before treatment; - - -,after treatment.
monomer was polymerized under UV irradiation, but the ST monomer was hardly polymerized at all. That is, the ST monomer was polymerized under UV irradiation in the presence of oxygen in air while the OL monomer was polymerized in the absence of oxygen. This shows clearly ST and OL films each possess a different polymerization mechanism. ST film after UV irradiation for 60 min was easily soluble in benzene and THF, but OL film after UV irradiation produced an insoluble polymer. This is because the ST polymer is linear and the OL polymer contains a network formed of the C-S bonds. For greater clarification of the polymer structure, gel permeation chromatography (GPC) of ST and OL polymer films before and after a NaSH treatment was conducted as shown in Figure 3. The OL polymer after irradiation was found to contain a linear polymer soluble in THF. A T H F solution of ST and OL polymers was treated with a methanol solution of NaSH a t 30 "Cfor 60 min. By NaSH
+PN&HC. THF
soluble
OL
1
NaN
Ca HI, HllcHt dH+
(2)
insoluble
polymer
treatment, the presence of S-S or C-S bonds in the polymers could be determined. The ST polymer gave only ST monomer after NaSH treatment since it had been produced by SS bonds during irradiation. The molecular weight distribution of the soluble OL polymer shifted to the low molecular wight side after NaSH treatment, although the polymer was never converted to the monomer. This shows the OL polymer to have many C-S bonds and a small number of S-S bonds. From the above results, the polymerization of ST and OL films under UV light irradiation may be expressed as shown in Scheme I. ST and OL molecules are tautomers as shown in these reactions. First, thiol-type ST molecules are oxidized by photoactivated oxygen1%in air to form S-S bonds. Equilibrium in the ST tautomer shifts to the right with decrease in the concentration of the thiol by oxidation. The formation of the ST polymer can thus be observed from UV spectra data as a decrease in concentration of the thione type. For OL films as shown in reaction 2 of Scheme I, photoaddition occurs mainly between thiol groups and double bonds in the films to C-S bonds and produce a network polymer. This reaction easily occurs only under UV irradiation even in the absence oxygen. Actually, the oxidation of thiol groups in OL film as well as STfilm also occurs slightly under UV irradiation. Registry No. ST,29556-81-8; ST (homopolymer), 12592573-7; OL, 105658-72-8; OL (homopolymer), 111623-84-8. (16)Steer, R. S.;Kmight, A. R.J. Phys. Chem. 1968, 72, 2145.
