- Email: [email protected]
Polyarylates as nonlinear optics materials
REACTIVE & FUNCTIONAL POLYMERS ELSEVIER
Reactive 8~ Functional Polymers 33 (1997) 343-349
Polyarylates as nonlinear optics materials Konrad Noniewicz, Zbigniew K. Brzozowski
*
Department of Specialty Polymer Technology, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
Received 25 September 1996; revised version received 17 March 1997; accepted 17 March 1997
Abstract Several polyarylates based on UV-sensitive bisbenzylidenoketones for use as potential second-order nonlinear optical (NLO) materials have been obtained by interfacial polycondensation. The polymers investigated can be divided into two groups: guest-host systems and polyarylates with NLO side-chains. The electro-optic (EO) coefficient, ~3, was measured in the Pockels experiment. UV crosslinking was applied to increase NLO stability of the investigated materials. Loss and time relaxation of EO coefficient for crosslinked and noncrosslinked polyarylates were also measured. Keywords:
Polyacrylates; NLO materials; Ultraviolet sensitivity; Bisbenzylidenoketones
exhibit very low glass-rubber transition temperatures, and sometimes are liquid crystalline polymers. However, examples of the use of polyesters, especially polyarylates, in the field of nonlinear optics are very limited. This is probably due to the fact that polyarylate guest-host systems exhibit a high degree of impurities and create multiphase systems with the guest molecules which disqualify them from NLO investigations. In this work we would like to present new NLO polyarylates exhibiting very high optical purity in which photocrosslinking reactions were applied to improve their EO stability.
1. Introduction In this work, appropriate modified polyarylates (guest-host and side-chain systems) as potential NLO materials for application in optoelectronics are presented. It is well known from our previous investigations [l-4] that polyarylates exhibit high thermal resistance and give films with good mechanical properties, and that their electrical properties make them applicable in electronics. These properties give a good prognosis for obtaining materials that satisfy all the requirements of an NLO polymer. Despite their good physical and chemical properties, polyarylates have not been investigated in the field of nonlinear optics. Only a few publications [5-71 mention the possibility of obtaining polyesters with NLO side-chains. These polymers usually *Corresponding author. Tel.: +48 22 6214439, Fax: +48 22 6214439, e-mail: [email protected]
2. Material synthesis 2.1. Monomers 2. I. 1. Bisphenols For interfacial polycondensation reactions, photo-sensitive bisphenol bis(3-ethoxy-4-hydro-
1381-5148/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ S1381-5148(97)00070-9
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xybenzylidene)-4-methylcyclohexanone were used. The synthesis of such a bisphenol was carried out in acidic medium at 0-2°C. The reaction is an aldol condensation between an aldehyde, which has no hydrogen atom in the a! position, and a ketone (Claisen-Schmidt reaction) [2]. In this reaction, the protonated carbonyl group from the aldehyde is attached to the o carbon of the keto-enol form, giving an unstable addition product. In acidic conditions, dehydration quickly occurs, leading to an a&unsaturated compound. Bisphenol structure was confirmed by ‘H-NMR spectra [8]. 2.1.2. NLO side-chain monomers To obtain NLO side-chain polymers, monomers which possess aliphatic -CH2CH2OH groups were used and were able to react during interfacial polycondensation. Such monomers were synthesized in diazotiazation and coupling reactions from p-nitroaniline, 2-amino-6-nitrobenzothiazole with N-phenyldiethanolamine [9]. In comparison to bisphenols, monomers with active aliphatic groups have lower solubility in alkaline aqueous solutions. They have higher solubility than simple alcohols, but much less then phenols. The low dye solubility in aqueous NaOH solution limited the possibility of incorporating NLO side-chains into the polymer chain in a quantity greater than 2 mol%. The structure of the monomers was confirmed by 1H-NMR spectra: (1) 6 = 3.44 - 3.59 ppm (2t, 8H, -CH2CH2 OH); 6 = 6.87 - 6.92 ppm and 6 = 7.79 - 7.83 ppm (2d, 4H, Ar-N(CH&H20H)2); 6 = 7.89 7.93 and 8.35 - 8.37 ppm (2d, 4H, Ar-N02). (2) S = 3.37-3.54ppm(2t, 8H,-CH2CH20H); 6 = 6.52 - 8.66 ppm (7H, benzothiazole + At). 2.2. Guest molecules 2.2.1. Azo dyes (Z) 4-(N,N-Dimethylamino)-4’-nitrodiazobenzene and (II), 4-(N,N - dimethylamino) - 4’-nitrobenzothiazole, were obtained in (a) diazotiazation reaction: [9,10] amine was dissolved in appropri-
& Functional Polymers 33 (1997) 343-349
ate volume of water with 2.5-3 equivalents of hydrochloric acid and heating. The solution was then cooled to 0°C and amine hydrochloride was formed as a precipitate during the reaction. The temperature was kept between 0 and 5°C and an aqueous solution of sodium nitrite was added, Sodium nitrite was added until a positive reaction on nitrous acid is observed. In the next step (b), a coupling reaction involving an electrophilic substitution was applied. During coupling with amines, the active form is a free nonionized amine. To the diazo salt, N-phenyldiethanolamine is added. After 15 min of stirring, sodium acetate is added (3.4 g in 5 ml of water). After 1 h, the dye which had precipitated during the reaction was filtered, washed with water and dried under normal atmospheric pressure at 40°C. The structures of the dyes were confirmed by ‘H-NMR spectra: (I) 6 = 3.13 ppm (s, 6H, -CHs); S = 6.746.77 ppm (d, 2H, Ar-N(CH&); S = 7.90-7.94 ppm (2d, 4H, Ar-N=N-Ar); 6 = 8.31-8.34 ppm (d, 2H, &N-Ar), (II) 6 = 3.08 ppm (s, 6H, -CHs); 6 = 6.56 ppm (d, 2H, Ar-N(CH&); 6 = 7.73-7.81 ppm (d, 2H, =CH-Ar); 6 = 7.53 - 8.78 ppm (dds, 3H, nitobenzothiazol). 2.2.2. Shif’s base (ZZZ) With N,N-dimethylamino and nitro groups located on the two sides of -N=CHgroup. This dye was obtained in the following procedure [9,10]. The p-nitroaniline was heated with 1 mol of p-dimethylaminobenzaldehyde at boiling point for 12 h. Acetic acid was a solvent in this reaction. The dye precipitated during the reaction and was crystallized from ethanol. The dye structure was confirmed by’H-NMR spectra: (III) S = 3.09 ppm (s, 6H, -CHs); 6 = 6.74 - 6.76 and 7.75-7.80 ppm (2d, 4H, Ar-N(CH&); 6 = 7.19 - 7.24 and 8.21-8.26 ppm (2d, 4H, Ar-N02). 0&-JN.N~
“c
(I) /$, = 30.36 x 10m3’esu
K. Noniewicz., Z. K. Brzozowski/Reactive
345
& Functional Polymers 33 (1997) 343-349
w~-N-f-JN<; 3
(II) fiO= 56.15 x 10p30 esu
Cl*N~N=CU-Q-_N=~~ 3
(III) PO= 43.64 x 10e30 esu
y = 62% mol UV-sensitive mer useful in crosslinking reaction C
3. Polymer synthesis Via interfacial polycondensation from the photosensitive bisphenols, NLO bisphenols, aliphatic diols and bisphenol A copolyarylates can be obtained. The elaborate conditions employed are as follows [l-4]: The reaction was carried out in a 250 ml threeneck flask, with a mechanical stirrer, dropping funnel, and thermometer. The aqueous phase: bisphenols in alkali solution and catalyst TEBA’Cl- chloride (triethylbenzylammonium chloride) - 5 mol% relative to the bisphenol concentration. The reaction was carried out at 20 f 2°C for 2-5 h. The total time for addition of the acid chloride mixtures in methylene chloride (organic phase) was 5 min. This mixture of terephthalic and isophthalic acid chlorides was then used in an optimal ratio 2 : 1 [3]. The whole system was acidified and then the organic phase was separated, washed with distilled water, and dripped into acetone while intensive mixing was maintained. The polymer, which precipitated in acetone, was filtered, and dried in a vacuum dryer at 70°C. The structure of the investigated polymers are presented below. Polymer matrix PAr is a random terpolymer consisting of three units: A-B-C. Polymers with NLO side-chains are also random terpolymers consisting of A-B-D units, where D is NLO mer.
--L
0
4
z
z = 5% mol mer decreasing glass transition temperature A-B-D random terpolymers side-chains, where D is NLO mer.
d
/ \ -Q CH2
CM2
‘N’
No2
(1) z = 1.5% mol
o-c
_[
12 O-Y2cd a+2
i
/’ --Q -c
CM2
'N'
3 0 N
(2) z = 2% mol x = 33% mol mer giving high tensile strength of polymer
3
0--CH2cutW2cup2-o-~-Q
I
1:
z
with
NLO
346
K. Noniewicz,2 K. Brzozowski/Reactive& FunctionalPolymers33 (1997) 343-349
3.1. Polymer characterization 3.1. I. Polymer matrix PAr
(1) Intrinsic viscosity [q] = 0.90 [lo0 cm3/g]. (2) Time necessary for obtaining maximum crosslinking t = 2 min (3) Maximum tensile strength after crosslinking d = 47.1 [MPa]. (4) Glass transition temperature (TMA) Tg = 89.6O”C. (5) Initial (beginning) decomposition temperature 7” = 197.5”C, half decomposition temperature Th = 518°C. (6) Refractive index n (633 nm) = 1.43 1. 3.1.2. Polymers with NLO side-chains PAr (1). (1) Intrinsic viscosity [Q] = 1.11 [lo0 cm3/g]. (2) Time necessary for obtaining maximum crosslinking t = 5 min (3) Maximum tensile strength after crosslinking cr = 58.8 [Mpa]. (4) Tg = 35°C. (5) Tb = 231=‘C,Th = 448°C. (6) Refractive index IZ(633 nm) = 1.473. PAr (2). (1) Intrinsic viscosity [q] = 0.84 [lo0
cm3/g]. (2) Time necessary for obtaining maximum crosslinking t = 5 min (3) Maximum tensile strength after crosslinking (T = 54.0 [MPa]. (4) Tg = 39°C. (5) Tb = 250°C Th = 537°C. (6) Refractive index n (633 nm) = 1.533. 3.2. Film preparation Three classes of films were studied: guesthost, functionalized polymers with NLO side chains and crosslinked polymers. For preparing the first class we used 4-(N,N-dimethylamino)-4’nitrodiazobenzene (I), 4-(NJ+dimethylamino)-4’-nitrobenzothiazole (II) and Shiff base with N,N-dimethylamino and nitro group located on the two sides of -N=CH-group (III). These chromophores are characterized by /I0 hyperpolarizability, calculated theoretically using the MOPAC 93 computer program [ 111. /? is a molecular analogue of the macroscopic second-order susceptibility Xc2)and is characterized second-order nonlinear optical properties of organic molecules. NLO chromophores are dissolved in the poly-
mer matrix PAr in the quantity of 8 and 10 weight% in relation to polymer - (solvent 1,1,2,2-tetrachloroethane + methylene chloride 1: 1) giving guest-host systems PAr/(I) 10% - n (633 nm) = 1.6117; PAr/(II) 10% - 12(633 nm) - non transparent film; PAr/(III) 8% - IZ (633 nm) = 1.575. Polymers with side-chains were dissolved in 1,1,2,2-tetrachloroethane. The solution was then purified using a Millipore 45 pm filter. The films were obtained by dropping the solution of polymers onto optical glass plates BK7 (refractive index 12 = 1.518). Afterwards the films were dried in a vacuum dryer for 24 h at 60°C. On the glass surface the transparent IT0 electrode was earlier evaporated. The thickness of the polymer layers was 8-14 pm. Another, top, nontransparent electrode was evaporated on the polymer layer. Such prepared samples were next poled in an external electric field (DC). 3.3. Poling procedure A polymer containing the NLO moiety was heated above the glass-rubber transition temperature, a strong electric field was then applied, which orientates the molecules by coupling to the ground-state dipole moment. The material was then cooled with the field applied, locking it in that orientation. The poling process was carried out for 45 min with the applied field at 120°C for guest-host systems and at 80°C for polymers with side-chains. The parameters of the applied field are: guest-host systems: PAr/(I) lo%-100 V/pm, PAr/(II) - 60 V/pm, PAr/(III) - 88 V/pm, polymers with side-chain: PAr(1) - 79 V/pm, PAr(2) - 90 V/pm. While polymers with side-chains and guesthost systems PAr/(I)lO% and PAr/(III) 8% have a very good, transparent optical quality of the surface, polymer PAr/(II) 10% have many small recrystallized dye points on the surface and the film is not transparent. This is due to sulfur atom present in the dye structure causing loss of lustre. Through looking at &, values, dye (II) should be better than (I) or (III). Difficulties with obtaining high optical quality film made the application
K. Noniewicz,
Z. K. Brzozowski/Reactive
of the guest-host system PAr/(II) in EO devices impossible. 3.4. Determination of electro-optic coeficient The EO effect implies that the impermeability cm1 of a material is changed when it is subjected to an applied electric field. In a series expansion we can write [12,13]: S(E-‘)u = QkEK + put&KEL
+ ...
(2)
On derivation the refractive index change caused by the Pockels effect is related to the susceptibility Xc2)by:
341
Polymers 33 (1997) 343-349
The phase change induced by the electric field AqZ in the Pockels experiment was determined from the Fresnel [8] equation and relations results for polymer system geometry [ 15,161. 2rc z:-. A@ h 0 Y
--
n2 ’ n;
sin2 0,
n2 ?2;- -I sin2 0i k n:
(1)
The first term in Eq. 1 corresponds to the linear EO effect, or Pockels effect. The tensor r is defined as the Pockels coefficient. The linear EO effect is a direct consequence of a material’s second-order NLO properties. For small changes of diagonal components of the impermeability tensor, these changes are linearly proportional to changes of the corresponding refractive indices. Choosing principal axes of the indicatrix, the Pockels effect can be rewritten in terms of the refractive index changes as follows [ 121: 6nr = -O.SnzrukEk
& Functional
2d i[
I’
sin2 0, - n2
+
&zsg
i (5)
where: h, = wavelength in vacuum, d = polymer thickness, n = refractive index, 2Oi = angle between laser light and the beam reflected from the glass plate BK7. From Eq. 4 we know that ny = (2n + n,)/3, and we can determine electro-optic coefficient r33: Wh r33 =
AE
-
n> (6)
..3
The electro-optic coefficient, r-33,as a function of A@ for non-crosslinked polymer guest-host PAr/(I) 10% is presented in Fig. 1. The results of EO coefficients are shown in Table 1. 3.5. Time relaxation
EK
(3)
After poling, polymer films possess optical anisotropy with a significant difference between their ordinary refractive index no = ny (in the film plane) and their extraordinary refractive index n, = n, (perpendicular to the film). In the case of the polymers investigated, ne > no. The typical value of birefringence An = n, - n, in the guest-host and side-chain polymers is about -0.003 [14] (poling field 100 V/pm). Eq. 2 taking into account n, and ne could be given:
an,
n3 an, ne3 z = -r33y (4) aE= 2 Our calculations were simplified by assuming in Eq.4thatn,=n,=n. -q3
One of the crucial problems of poled polymers is the relaxation with time, which lowers the Table 1 EO coefficient ~33and q3 values, second-order susceptibility tensor Xzzz and X,,, values and number density N of chromophores in investigated polymers Polymer PAr/(I) 10% PAr/(II) 10% PAr/(IIl) 8% PAr (1) PAr
(2)
(pfl)
N (le6/m3)
X(2) ZLZ (pmN)
X(2) xx.? (pm)
10.08 a 7.14
3.36 a 2.38
1.114 a 0.894
34.00 a
11.33 a
21.98
b
b
b
b
b
b
b
b
b
b
r33
r13
(pmN)
7.33
0;
a Polymer with poor optical film quality. b r33 is impossible to determine under the present experimental conditions due to the fact that the chromophore density number was too low.
K. Noniewicz, Z. K. BrzozowskiIReactive & Functional Polymers 33 (1997) 343-349
348
A# Iradl 0.25-
04
5
0
10
15
20
25
30
35
I
40
r33[pm/VI
Fig. 1. EO coefficient r33 as a function of A@ for polyarylate guest-host system PAr/(I) 10%. Voltage between electrodes 100 V
value of the EO coefficient. To minimize orientation relaxation we decided to apply crosslinking after the poling process. Afterwards, we checked time relaxation behavior for the PAr/(I) 10% guest-host system. The procedures of preparing samples were as follows: after poling, the top aluminium electrode was dissolved in NaOH,, then W-light for 2 min was applied to crosslink the polymer structure and freeze chromophores inside the polymer matrix. After W exposure, the top aluminium
r 33 [pm/VI I2 11 IO
I
9. 8. 7. 6.
r/(l)
5.
10%
after
10%
before
crosslinking
4. 3.
PAr/(l)
2.
crosslinking
Ii ”
24
48
72
96
120
144
168
I92
t[hoursl
Fig. 2. EO coefficient time relaxation for crosslinked guest-host system PAr/(I) 10% held at room temperature for 200 h.
electrode was evaporated again. EO coefficient was measured for 200 h. Results are shown in Fig. 2. 4. Conclusions We observed that limited quantity of NLO chromophores can be dissolved in the polymer matrix. Apart from this it was possible to obtain very pure polyarylate guest-host systems. The maximum amount for (I) and (III) dyes is 10 wt% and 8 wt%, respectively. Dye (II), which possess a sulfur atom in its structure, is nontransparent and due to this it is not useful in EO devices. Polyarylates were obtained in the interfacial polycondensation. NLO chromophores are monomers dissolved in the aqueous phase. Due to the fact that the monomers contain the aliphatic -CH2CH2OH groups, their solubility in Hz0 + NaOH is limited and it is possible to incorporate about -2 wt% NLO side-chains into the polymer matrix. We obtained two guest-host systems exhibiting r-33of about 10 and 7 pm/V. The relaxation investigation demonstrates that in systems in which UV-crosslinking is applied their stability is improved for about 15%. On Fig. 2 the loss of r33 in percentage terms during 200 h is
K. Noniewicz, 2. K. Brzozowski/Reactive
shown. The fastest relaxation is observed in the first 24 h. After this time, the 33 values are stabilized. It is very difficult to compare EO coefficient r33 values achieved for polyarylates with literature data for other polymers, because many things have an influence on the ~33 coefficient, like for instance: the type of polymer matrix, NLO material concentration, poling process, poling time, poling voltage value and temperature. Leaving aside these factors, in general it could be said that NLO polyarylates exhibit r33 values of about 10 pm/V and they are sometimes several times better than for guest-host systems based on PMMA derivatives described in the literature [17,18]. Values ~33 of about 10 pm/V are high enough to apply such polymers in co-operation with classical electronic devices [12], on conditions that such a material have an appropriate electric properties and good EO stability.
References [l] Z.K. Brzozowski and K. Noniewicz, 5th European Polymer Federation Symposium on Polymeric Materials. Information Technology, Basel, Switzeland, October 9-12, 1994. [2] Z.K., Brzozowski and K. Noniewicz, Macromol. Reports, A32 (1995) 959.
C Functional Polymers 33 (1997) 343-349
349
[3] K. Noniewicz, Z.K. Brzozowski and I. Zadroina, J. Appl. Polym. Sci., 60 (1996) 1071. [4] K. Noniewicz, Z.K. Brzozowski and .I. Hajto, Macromol. Reports (1996) in press. [5] W. Chengjiu, A. Nahata, L. Victor, J. Shan, C. Knapp and J. Yardley, Proc. SPIE-Int-Sot. Opt. Eng., 2285 (1994) 360. [6] A. Nahata, W. Chengjiu, C. Knapp, L. Victor, J. Shan and J. Yardley, Appl. Phys. Lett., 64(25) (1994) 3371. [7] K. Servay, H.J. Winkelhahn, M. Schulze, C. Boeffel, D. Neher and G. Wegner, Phys. Chem., 97(10) (1993) 1272. [8] Z.K. Brzozowski and I. Zadroina, Thirteenth European Experimental ‘HNMR Conference, Paris, May, 1996. [9] AI. Vogel, Preparatyka Organiczna. WNT, Warszawa, 1984. [lo] R.N. Boyd and R.T. Morrison, Organical Chemistry. PWN, Warszawa, 1984. [ 1l] MOPAC ‘93 - Fujitsu Ltd. Chemistry Department. Licence No. 3/l/94, 1994. [12] A. Lawrence Homak, Polymers for Lightwave and Integrated Optics, Technology and Applications. New York Press, 1992. [13] J.A. Chilton and M.T. Goosey, Special Polymers for Electronics and Optoelectronics. Chapman and Hall, London, 1995. [ 141 K.D. Singer and W.R. Holland, et al., Proc. SPIE, Nonlinear Optical Properties of Organic Materials II, 1147 (1989) 233. [15] E Ratajczyk, Optyka Osrodkow Anizotropowych. PWN, Warszawa, 1994. [16] J. Petykiewicz, Podstawy Fizyczne Optyki Scalonej. PWN, Warszawa, 1989. [17] D. Alastair, L. McSmith and H. Coles, Polym. Adv. Technol., 6(4) (1995) 230. [18] K.J. Dorst, VP Rao and A. Jen, J. Chem. Sot., Chem. Commun., 4 (1994) 369.
Reactive 8~ Functional Polymers 33 (1997) 343-349
Polyarylates as nonlinear optics materials Konrad Noniewicz, Zbigniew K. Brzozowski
*
Department of Specialty Polymer Technology, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
Received 25 September 1996; revised version received 17 March 1997; accepted 17 March 1997
Abstract Several polyarylates based on UV-sensitive bisbenzylidenoketones for use as potential second-order nonlinear optical (NLO) materials have been obtained by interfacial polycondensation. The polymers investigated can be divided into two groups: guest-host systems and polyarylates with NLO side-chains. The electro-optic (EO) coefficient, ~3, was measured in the Pockels experiment. UV crosslinking was applied to increase NLO stability of the investigated materials. Loss and time relaxation of EO coefficient for crosslinked and noncrosslinked polyarylates were also measured. Keywords:
Polyacrylates; NLO materials; Ultraviolet sensitivity; Bisbenzylidenoketones
exhibit very low glass-rubber transition temperatures, and sometimes are liquid crystalline polymers. However, examples of the use of polyesters, especially polyarylates, in the field of nonlinear optics are very limited. This is probably due to the fact that polyarylate guest-host systems exhibit a high degree of impurities and create multiphase systems with the guest molecules which disqualify them from NLO investigations. In this work we would like to present new NLO polyarylates exhibiting very high optical purity in which photocrosslinking reactions were applied to improve their EO stability.
1. Introduction In this work, appropriate modified polyarylates (guest-host and side-chain systems) as potential NLO materials for application in optoelectronics are presented. It is well known from our previous investigations [l-4] that polyarylates exhibit high thermal resistance and give films with good mechanical properties, and that their electrical properties make them applicable in electronics. These properties give a good prognosis for obtaining materials that satisfy all the requirements of an NLO polymer. Despite their good physical and chemical properties, polyarylates have not been investigated in the field of nonlinear optics. Only a few publications [5-71 mention the possibility of obtaining polyesters with NLO side-chains. These polymers usually *Corresponding author. Tel.: +48 22 6214439, Fax: +48 22 6214439, e-mail: [email protected]
2. Material synthesis 2.1. Monomers 2. I. 1. Bisphenols For interfacial polycondensation reactions, photo-sensitive bisphenol bis(3-ethoxy-4-hydro-
1381-5148/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ S1381-5148(97)00070-9
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K. Noniewicz, Z. K. Brzozowski/Reactive
xybenzylidene)-4-methylcyclohexanone were used. The synthesis of such a bisphenol was carried out in acidic medium at 0-2°C. The reaction is an aldol condensation between an aldehyde, which has no hydrogen atom in the a! position, and a ketone (Claisen-Schmidt reaction) [2]. In this reaction, the protonated carbonyl group from the aldehyde is attached to the o carbon of the keto-enol form, giving an unstable addition product. In acidic conditions, dehydration quickly occurs, leading to an a&unsaturated compound. Bisphenol structure was confirmed by ‘H-NMR spectra [8]. 2.1.2. NLO side-chain monomers To obtain NLO side-chain polymers, monomers which possess aliphatic -CH2CH2OH groups were used and were able to react during interfacial polycondensation. Such monomers were synthesized in diazotiazation and coupling reactions from p-nitroaniline, 2-amino-6-nitrobenzothiazole with N-phenyldiethanolamine [9]. In comparison to bisphenols, monomers with active aliphatic groups have lower solubility in alkaline aqueous solutions. They have higher solubility than simple alcohols, but much less then phenols. The low dye solubility in aqueous NaOH solution limited the possibility of incorporating NLO side-chains into the polymer chain in a quantity greater than 2 mol%. The structure of the monomers was confirmed by 1H-NMR spectra: (1) 6 = 3.44 - 3.59 ppm (2t, 8H, -CH2CH2 OH); 6 = 6.87 - 6.92 ppm and 6 = 7.79 - 7.83 ppm (2d, 4H, Ar-N(CH&H20H)2); 6 = 7.89 7.93 and 8.35 - 8.37 ppm (2d, 4H, Ar-N02). (2) S = 3.37-3.54ppm(2t, 8H,-CH2CH20H); 6 = 6.52 - 8.66 ppm (7H, benzothiazole + At). 2.2. Guest molecules 2.2.1. Azo dyes (Z) 4-(N,N-Dimethylamino)-4’-nitrodiazobenzene and (II), 4-(N,N - dimethylamino) - 4’-nitrobenzothiazole, were obtained in (a) diazotiazation reaction: [9,10] amine was dissolved in appropri-
& Functional Polymers 33 (1997) 343-349
ate volume of water with 2.5-3 equivalents of hydrochloric acid and heating. The solution was then cooled to 0°C and amine hydrochloride was formed as a precipitate during the reaction. The temperature was kept between 0 and 5°C and an aqueous solution of sodium nitrite was added, Sodium nitrite was added until a positive reaction on nitrous acid is observed. In the next step (b), a coupling reaction involving an electrophilic substitution was applied. During coupling with amines, the active form is a free nonionized amine. To the diazo salt, N-phenyldiethanolamine is added. After 15 min of stirring, sodium acetate is added (3.4 g in 5 ml of water). After 1 h, the dye which had precipitated during the reaction was filtered, washed with water and dried under normal atmospheric pressure at 40°C. The structures of the dyes were confirmed by ‘H-NMR spectra: (I) 6 = 3.13 ppm (s, 6H, -CHs); S = 6.746.77 ppm (d, 2H, Ar-N(CH&); S = 7.90-7.94 ppm (2d, 4H, Ar-N=N-Ar); 6 = 8.31-8.34 ppm (d, 2H, &N-Ar), (II) 6 = 3.08 ppm (s, 6H, -CHs); 6 = 6.56 ppm (d, 2H, Ar-N(CH&); 6 = 7.73-7.81 ppm (d, 2H, =CH-Ar); 6 = 7.53 - 8.78 ppm (dds, 3H, nitobenzothiazol). 2.2.2. Shif’s base (ZZZ) With N,N-dimethylamino and nitro groups located on the two sides of -N=CHgroup. This dye was obtained in the following procedure [9,10]. The p-nitroaniline was heated with 1 mol of p-dimethylaminobenzaldehyde at boiling point for 12 h. Acetic acid was a solvent in this reaction. The dye precipitated during the reaction and was crystallized from ethanol. The dye structure was confirmed by’H-NMR spectra: (III) S = 3.09 ppm (s, 6H, -CHs); 6 = 6.74 - 6.76 and 7.75-7.80 ppm (2d, 4H, Ar-N(CH&); 6 = 7.19 - 7.24 and 8.21-8.26 ppm (2d, 4H, Ar-N02). 0&-JN.N~
“c
(I) /$, = 30.36 x 10m3’esu
K. Noniewicz., Z. K. Brzozowski/Reactive
345
& Functional Polymers 33 (1997) 343-349
w~-N-f-JN<; 3
(II) fiO= 56.15 x 10p30 esu
Cl*N~N=CU-Q-_N=~~ 3
(III) PO= 43.64 x 10e30 esu
y = 62% mol UV-sensitive mer useful in crosslinking reaction C
3. Polymer synthesis Via interfacial polycondensation from the photosensitive bisphenols, NLO bisphenols, aliphatic diols and bisphenol A copolyarylates can be obtained. The elaborate conditions employed are as follows [l-4]: The reaction was carried out in a 250 ml threeneck flask, with a mechanical stirrer, dropping funnel, and thermometer. The aqueous phase: bisphenols in alkali solution and catalyst TEBA’Cl- chloride (triethylbenzylammonium chloride) - 5 mol% relative to the bisphenol concentration. The reaction was carried out at 20 f 2°C for 2-5 h. The total time for addition of the acid chloride mixtures in methylene chloride (organic phase) was 5 min. This mixture of terephthalic and isophthalic acid chlorides was then used in an optimal ratio 2 : 1 [3]. The whole system was acidified and then the organic phase was separated, washed with distilled water, and dripped into acetone while intensive mixing was maintained. The polymer, which precipitated in acetone, was filtered, and dried in a vacuum dryer at 70°C. The structure of the investigated polymers are presented below. Polymer matrix PAr is a random terpolymer consisting of three units: A-B-C. Polymers with NLO side-chains are also random terpolymers consisting of A-B-D units, where D is NLO mer.
--L
0
4
z
z = 5% mol mer decreasing glass transition temperature A-B-D random terpolymers side-chains, where D is NLO mer.
d
/ \ -Q CH2
CM2
‘N’
No2
(1) z = 1.5% mol
o-c
_[
12 O-Y2cd a+2
i
/’ --Q -c
CM2
'N'
3 0 N
(2) z = 2% mol x = 33% mol mer giving high tensile strength of polymer
3
0--CH2cutW2cup2-o-~-Q
I
1:
z
with
NLO
346
K. Noniewicz,2 K. Brzozowski/Reactive& FunctionalPolymers33 (1997) 343-349
3.1. Polymer characterization 3.1. I. Polymer matrix PAr
(1) Intrinsic viscosity [q] = 0.90 [lo0 cm3/g]. (2) Time necessary for obtaining maximum crosslinking t = 2 min (3) Maximum tensile strength after crosslinking d = 47.1 [MPa]. (4) Glass transition temperature (TMA) Tg = 89.6O”C. (5) Initial (beginning) decomposition temperature 7” = 197.5”C, half decomposition temperature Th = 518°C. (6) Refractive index n (633 nm) = 1.43 1. 3.1.2. Polymers with NLO side-chains PAr (1). (1) Intrinsic viscosity [Q] = 1.11 [lo0 cm3/g]. (2) Time necessary for obtaining maximum crosslinking t = 5 min (3) Maximum tensile strength after crosslinking cr = 58.8 [Mpa]. (4) Tg = 35°C. (5) Tb = 231=‘C,Th = 448°C. (6) Refractive index IZ(633 nm) = 1.473. PAr (2). (1) Intrinsic viscosity [q] = 0.84 [lo0
cm3/g]. (2) Time necessary for obtaining maximum crosslinking t = 5 min (3) Maximum tensile strength after crosslinking (T = 54.0 [MPa]. (4) Tg = 39°C. (5) Tb = 250°C Th = 537°C. (6) Refractive index n (633 nm) = 1.533. 3.2. Film preparation Three classes of films were studied: guesthost, functionalized polymers with NLO side chains and crosslinked polymers. For preparing the first class we used 4-(N,N-dimethylamino)-4’nitrodiazobenzene (I), 4-(NJ+dimethylamino)-4’-nitrobenzothiazole (II) and Shiff base with N,N-dimethylamino and nitro group located on the two sides of -N=CH-group (III). These chromophores are characterized by /I0 hyperpolarizability, calculated theoretically using the MOPAC 93 computer program [ 111. /? is a molecular analogue of the macroscopic second-order susceptibility Xc2)and is characterized second-order nonlinear optical properties of organic molecules. NLO chromophores are dissolved in the poly-
mer matrix PAr in the quantity of 8 and 10 weight% in relation to polymer - (solvent 1,1,2,2-tetrachloroethane + methylene chloride 1: 1) giving guest-host systems PAr/(I) 10% - n (633 nm) = 1.6117; PAr/(II) 10% - 12(633 nm) - non transparent film; PAr/(III) 8% - IZ (633 nm) = 1.575. Polymers with side-chains were dissolved in 1,1,2,2-tetrachloroethane. The solution was then purified using a Millipore 45 pm filter. The films were obtained by dropping the solution of polymers onto optical glass plates BK7 (refractive index 12 = 1.518). Afterwards the films were dried in a vacuum dryer for 24 h at 60°C. On the glass surface the transparent IT0 electrode was earlier evaporated. The thickness of the polymer layers was 8-14 pm. Another, top, nontransparent electrode was evaporated on the polymer layer. Such prepared samples were next poled in an external electric field (DC). 3.3. Poling procedure A polymer containing the NLO moiety was heated above the glass-rubber transition temperature, a strong electric field was then applied, which orientates the molecules by coupling to the ground-state dipole moment. The material was then cooled with the field applied, locking it in that orientation. The poling process was carried out for 45 min with the applied field at 120°C for guest-host systems and at 80°C for polymers with side-chains. The parameters of the applied field are: guest-host systems: PAr/(I) lo%-100 V/pm, PAr/(II) - 60 V/pm, PAr/(III) - 88 V/pm, polymers with side-chain: PAr(1) - 79 V/pm, PAr(2) - 90 V/pm. While polymers with side-chains and guesthost systems PAr/(I)lO% and PAr/(III) 8% have a very good, transparent optical quality of the surface, polymer PAr/(II) 10% have many small recrystallized dye points on the surface and the film is not transparent. This is due to sulfur atom present in the dye structure causing loss of lustre. Through looking at &, values, dye (II) should be better than (I) or (III). Difficulties with obtaining high optical quality film made the application
K. Noniewicz,
Z. K. Brzozowski/Reactive
of the guest-host system PAr/(II) in EO devices impossible. 3.4. Determination of electro-optic coeficient The EO effect implies that the impermeability cm1 of a material is changed when it is subjected to an applied electric field. In a series expansion we can write [12,13]: S(E-‘)u = QkEK + put&KEL
+ ...
(2)
On derivation the refractive index change caused by the Pockels effect is related to the susceptibility Xc2)by:
341
Polymers 33 (1997) 343-349
The phase change induced by the electric field AqZ in the Pockels experiment was determined from the Fresnel [8] equation and relations results for polymer system geometry [ 15,161. 2rc z:-. A@ h 0 Y
--
n2 ’ n;
sin2 0,
n2 ?2;- -I sin2 0i k n:
(1)
The first term in Eq. 1 corresponds to the linear EO effect, or Pockels effect. The tensor r is defined as the Pockels coefficient. The linear EO effect is a direct consequence of a material’s second-order NLO properties. For small changes of diagonal components of the impermeability tensor, these changes are linearly proportional to changes of the corresponding refractive indices. Choosing principal axes of the indicatrix, the Pockels effect can be rewritten in terms of the refractive index changes as follows [ 121: 6nr = -O.SnzrukEk
& Functional
2d i[
I’
sin2 0, - n2
+
&zsg
i (5)
where: h, = wavelength in vacuum, d = polymer thickness, n = refractive index, 2Oi = angle between laser light and the beam reflected from the glass plate BK7. From Eq. 4 we know that ny = (2n + n,)/3, and we can determine electro-optic coefficient r33: Wh r33 =
AE
-
n> (6)
..3
The electro-optic coefficient, r-33,as a function of A@ for non-crosslinked polymer guest-host PAr/(I) 10% is presented in Fig. 1. The results of EO coefficients are shown in Table 1. 3.5. Time relaxation
EK
(3)
After poling, polymer films possess optical anisotropy with a significant difference between their ordinary refractive index no = ny (in the film plane) and their extraordinary refractive index n, = n, (perpendicular to the film). In the case of the polymers investigated, ne > no. The typical value of birefringence An = n, - n, in the guest-host and side-chain polymers is about -0.003 [14] (poling field 100 V/pm). Eq. 2 taking into account n, and ne could be given:
an,
n3 an, ne3 z = -r33y (4) aE= 2 Our calculations were simplified by assuming in Eq.4thatn,=n,=n. -q3
One of the crucial problems of poled polymers is the relaxation with time, which lowers the Table 1 EO coefficient ~33and q3 values, second-order susceptibility tensor Xzzz and X,,, values and number density N of chromophores in investigated polymers Polymer PAr/(I) 10% PAr/(II) 10% PAr/(IIl) 8% PAr (1) PAr
(2)
(pfl)
N (le6/m3)
X(2) ZLZ (pmN)
X(2) xx.? (pm)
10.08 a 7.14
3.36 a 2.38
1.114 a 0.894
34.00 a
11.33 a
21.98
b
b
b
b
b
b
b
b
b
b
r33
r13
(pmN)
7.33
0;
a Polymer with poor optical film quality. b r33 is impossible to determine under the present experimental conditions due to the fact that the chromophore density number was too low.
K. Noniewicz, Z. K. BrzozowskiIReactive & Functional Polymers 33 (1997) 343-349
348
A# Iradl 0.25-
04
5
0
10
15
20
25
30
35
I
40
r33[pm/VI
Fig. 1. EO coefficient r33 as a function of A@ for polyarylate guest-host system PAr/(I) 10%. Voltage between electrodes 100 V
value of the EO coefficient. To minimize orientation relaxation we decided to apply crosslinking after the poling process. Afterwards, we checked time relaxation behavior for the PAr/(I) 10% guest-host system. The procedures of preparing samples were as follows: after poling, the top aluminium electrode was dissolved in NaOH,, then W-light for 2 min was applied to crosslink the polymer structure and freeze chromophores inside the polymer matrix. After W exposure, the top aluminium
r 33 [pm/VI I2 11 IO
I
9. 8. 7. 6.
r/(l)
5.
10%
after
10%
before
crosslinking
4. 3.
PAr/(l)
2.
crosslinking
Ii ”
24
48
72
96
120
144
168
I92
t[hoursl
Fig. 2. EO coefficient time relaxation for crosslinked guest-host system PAr/(I) 10% held at room temperature for 200 h.
electrode was evaporated again. EO coefficient was measured for 200 h. Results are shown in Fig. 2. 4. Conclusions We observed that limited quantity of NLO chromophores can be dissolved in the polymer matrix. Apart from this it was possible to obtain very pure polyarylate guest-host systems. The maximum amount for (I) and (III) dyes is 10 wt% and 8 wt%, respectively. Dye (II), which possess a sulfur atom in its structure, is nontransparent and due to this it is not useful in EO devices. Polyarylates were obtained in the interfacial polycondensation. NLO chromophores are monomers dissolved in the aqueous phase. Due to the fact that the monomers contain the aliphatic -CH2CH2OH groups, their solubility in Hz0 + NaOH is limited and it is possible to incorporate about -2 wt% NLO side-chains into the polymer matrix. We obtained two guest-host systems exhibiting r-33of about 10 and 7 pm/V. The relaxation investigation demonstrates that in systems in which UV-crosslinking is applied their stability is improved for about 15%. On Fig. 2 the loss of r33 in percentage terms during 200 h is
K. Noniewicz, 2. K. Brzozowski/Reactive
shown. The fastest relaxation is observed in the first 24 h. After this time, the 33 values are stabilized. It is very difficult to compare EO coefficient r33 values achieved for polyarylates with literature data for other polymers, because many things have an influence on the ~33 coefficient, like for instance: the type of polymer matrix, NLO material concentration, poling process, poling time, poling voltage value and temperature. Leaving aside these factors, in general it could be said that NLO polyarylates exhibit r33 values of about 10 pm/V and they are sometimes several times better than for guest-host systems based on PMMA derivatives described in the literature [17,18]. Values ~33 of about 10 pm/V are high enough to apply such polymers in co-operation with classical electronic devices [12], on conditions that such a material have an appropriate electric properties and good EO stability.
References [l] Z.K. Brzozowski and K. Noniewicz, 5th European Polymer Federation Symposium on Polymeric Materials. Information Technology, Basel, Switzeland, October 9-12, 1994. [2] Z.K., Brzozowski and K. Noniewicz, Macromol. Reports, A32 (1995) 959.
C Functional Polymers 33 (1997) 343-349
349
[3] K. Noniewicz, Z.K. Brzozowski and I. Zadroina, J. Appl. Polym. Sci., 60 (1996) 1071. [4] K. Noniewicz, Z.K. Brzozowski and .I. Hajto, Macromol. Reports (1996) in press. [5] W. Chengjiu, A. Nahata, L. Victor, J. Shan, C. Knapp and J. Yardley, Proc. SPIE-Int-Sot. Opt. Eng., 2285 (1994) 360. [6] A. Nahata, W. Chengjiu, C. Knapp, L. Victor, J. Shan and J. Yardley, Appl. Phys. Lett., 64(25) (1994) 3371. [7] K. Servay, H.J. Winkelhahn, M. Schulze, C. Boeffel, D. Neher and G. Wegner, Phys. Chem., 97(10) (1993) 1272. [8] Z.K. Brzozowski and I. Zadroina, Thirteenth European Experimental ‘HNMR Conference, Paris, May, 1996. [9] AI. Vogel, Preparatyka Organiczna. WNT, Warszawa, 1984. [lo] R.N. Boyd and R.T. Morrison, Organical Chemistry. PWN, Warszawa, 1984. [ 1l] MOPAC ‘93 - Fujitsu Ltd. Chemistry Department. Licence No. 3/l/94, 1994. [12] A. Lawrence Homak, Polymers for Lightwave and Integrated Optics, Technology and Applications. New York Press, 1992. [13] J.A. Chilton and M.T. Goosey, Special Polymers for Electronics and Optoelectronics. Chapman and Hall, London, 1995. [ 141 K.D. Singer and W.R. Holland, et al., Proc. SPIE, Nonlinear Optical Properties of Organic Materials II, 1147 (1989) 233. [15] E Ratajczyk, Optyka Osrodkow Anizotropowych. PWN, Warszawa, 1994. [16] J. Petykiewicz, Podstawy Fizyczne Optyki Scalonej. PWN, Warszawa, 1989. [17] D. Alastair, L. McSmith and H. Coles, Polym. Adv. Technol., 6(4) (1995) 230. [18] K.J. Dorst, VP Rao and A. Jen, J. Chem. Sot., Chem. Commun., 4 (1994) 369.