CC BY
13
ФИЗИКА МОЛЕКУЛ
DOI: 10.5862/JPM.213.15 UDC 537.226:544.163.2
T.P. Stepanova 1, G.I. Nosova 1, N.A. Solovskaya 1, V.M. Kapralova 2, A.V. Yakimansky '
' Institute of Macromolecular Compounds RAS 2 St. Petersburg Polytechnic University
the constant electric field effect on the dipole moment of a comb-like polymer
with chromophore groups in side chains
The study ofconformational properties and tendency to association for chromophore-containing comb-like copolymer of p-(3,4-dicyanophenylazobenzenethyazole) meth-acrylate (A) and amylmethacrylate (B) (1:1) has been carried out. The copolymer AB is of particular interest because of nonlinear optical properties of its films. Dielectric permittivity and dipole moment temperature dependences in dilute cyclohexanone solutions in the temperature range from 20 to 70 °C, in the electric field E < 104 V/cm were investigated by means of static dielectric polarization. It was shown that temperature and concentration dependences of dielectric permittivity for the solvent, copolymer AB, monomer A and polymer B were linear indicating low molecular interactions at temperatures and fields used. The invariable stoichiometry of components in solution for concentration lower than 10-3 mol/mol was proved. The values of dielectric permittivity were extrapolated to infinite dilution and increments a = (As12/Ax2)x2=0 were calculated. The solvent dipole moments were calculated in terms of the Onsager theory whereas dipole moments of AB, A and B were calculated in terms of the Backingham statistical theory of dielectric polarization. Intramacro-molecular conformational transition was found to be at -40 oC. Dipole moment of A was shown to increase with both temperature and electric field strength. Copolymer side chains trans-location takes place due to intramacromolecular association resulting in the compensation of dipole moments and Kirkwood factor g « 0.6. The association of A units increases in the electric field reducing the dipole moment per monomer unit significantly and g values approximately twice.
comb-like polymer, dielectric polarization, dipole moment, association.
1. Introduction
Nowadays, the investigation of the correla-
tion between comb-like polymers properties
and molecular structure is of great interest for polymer physics. Various functional groups like imine, chromophore, mesogenic ones with specific molecular interactions can be included into the side-chains of macromolecules, so it is possible to synthesize new polymers with liquid-crystalline, nonlinear optical, bioactive properties, etc. for the up-to-date engineering
and medicine [1— 3].
The task of the paper was to investigate the solution of a chromophore-containing comblike polymer by means of static dielectric polarization method in order to find correlations between the conformation as well as association tendency in a single macromolecule coil and nonlinear optical properties of the polymer material. External orienting electric field was used to gain additional information about orientation dielectric polarization of the polymer.
(A)
(B)
Scheme 1. Structural formulae of the objects
The comb-like copolymer of (dicyanophenyl) azobenzenethyazolemethacrylate (A) and amyl-methacrylate (B) 50 : 50 (see Scheme 1) was studied.
Films of this copolymer polarized in the corona discharge demonstrate the second order nonlinear optical properties [3].
The comparative study of temperature dependences of dielectric permittivity and dipole moments for copolymer AB, polymer B and monomer A under the electrical field = 104 kV/cm and Е = 0 in the cyclohexanone dilute solutions was carried out.
2. Experimental
2.1. Copolymer AB synthesis technique
Chromophore-containing monomer A was synthesized in two steps:
(a) synthesis of 4-{4-[methyl(2-hydroxy-eth-yl)amino] -phenylazo}-phthalonitrile (MAPN) from N-methyl-N- (2-hydroxyethyl)aniline and 3-aminophthalonitrile using the technique described in Ref. [4]. Some properties of this compound are as follows: melting temperature Tm = 169 — 171 °C; wavelength of the absor-bance spectra maximum 1 = 483 nm;
r max 7
(b) synthesis of {{4-{4-[methyl(2-hydroxy-ethyl)amino]-phenylazo}-phthalonitrile}}meth-acrylate from MAPN and methacrylic acid chloride in dimethylacetamide at 0 °C. Reaction yield was as high as 83 %.
Copolymer AB was synthesized by radical polymerization of chromophore-containing monomer A and amylmethacrylate B with the ratio of 50 : 50 in the 30 mass. % dimethylacet-
amide solution using dinitrileazoisobutyric acid as initiator (1 mass.%) at 70 °C. Water precipitation was used. After drying the polymer was purified by the precipitation from the cy-clohexanone solution into the water-methanol 1 : 3 mixture. The copolymer intrinsic viscosity [n] in the cyclohexanone was 0.72 cm3/g. Structure and composition of monomer A and copolymer AB were verified by means of NMR spectroscopy.
The technique of measuring solution dielectric permittivity in the external field as well as specific volume evaluation is described in Refs. [5-8].
2.2. The equations used
The dipole moment of the solvent (cyclohexanone) was calculated in terms of Onsager theory of dielectric polarization [9]:
2 _ 9kT M (2s + n2)(2s - n2) ^ _ 4%Na ' p ' s(n2 + 2)2 . ()
Copolymer AB was considered regular. Both monomers contain polar side groups joined with the same methacrylate chain. In this case, the summarized dipole moment of the macromolecule (AB) is
fp _ Z (+H )>
(2)
and the mean square dipole moment in the first approximation (the first term of the series) is given by
_ (^A + M"B ),
(3)
i _1
n
where ^A is the mean square of the monomer A dipole moment and ^B is the mean square of the dipole moment of monomer unit B included into the copolymer chain.
The mean square dipole moments of copolymer AB and homopolymer B macromolecules per monomer unit and that of monomer A molecule were calculated in terms of Frochlich — Buckingham theory of statistical polarization [10—13] for a two-component system (dissolved substance — solvent) in the case of infinite dilution:
lim 4 = kT l(8 - + «
X2
4nN.
( dv л
M
V dX2 J
- Mlvl + M2v2
X2 =0 2
+V
2e2 +1 ( dЛ
1 s2
- 6V1
(n2 -1)
V dX2 Jx2 =0 2 ( d 8^
(4)
V dX2 J
(281 + n2)2 3(281 +1)(n2 -1)
281 + n2
3(281 + 1)(n22 -1) 281 + n2
V -
V
In Eqs. (1) and (4) and below the following
symbols are used: e is a dielectric permittivity;
n is a refraction index; V is a molar volume
(V = Mv , v is a specific volume; Mis a mov sp ' sp A 7
ds,.
lecular mass);
dXr,
and dv12sp
dx^
are in-
crements of the dielectric permittivity and the specific volume in the case of infinite dilution; x is a mole fraction; Т is a temperature; NA is the Avogadro number; k is the Boltzmann constant; ^ is a dipole moment; lower indices 1 and 2 refer to the solvent and the dissolved substance respectively.
Refraction indices were determined from the molar refraction R by the Lorentz — Lor-ence equation:
"2 -1 M
Rd =
n2 + 2p p
(5)
The molar refraction RD for the molecules of cyclohexanone and monomer A as well as monomeric units A and B were calculated as a sum of bond increments and atomic refractions [10].
3. Results and discussion
3.1. The dipole moment of cyclohexanone molecule
Temperature dependences of experimental values for the cyclohexanone dielectric constant, the density, the dipole moment and the Kirkwood factor are presented in Table 1. Dielectric permittivity values are in the close agreement with those given in Ref. [12].
The correlation factor in the orientation of dipolar cyclohexanone molecules
g = (KJ2/(VgJ2
was calculated using the tabulated value of the dipole moment ^gas of gaseous cyclohexanone molecules; ^gas = 2.8 D [11].
As indicated in Table 1, the cyclohexanone dipole moment increased from 2.87 to 2.89 D
Table 1
Temperature dependence of cyclohexanone physical properties
Т оС £ p, g/ cm3 v, cm3/g n2 щ D g
20 15.102 0.94654 1.0521 2.072710 2.87 1.050
25 14.811 0.94209 1.0615 - 2.88 1.060
30 14.520 0.93764 1.0614 2.059060 2.88 1.060
35 14.229 0.93319 1.0716 - 2.89 1.060
40 13.939 0.92874 1.0708 2.045500 2.89 1.065
50 13.357 0.91984 1.0804 2.032039 2.89 1.066
60 12.776 0.91094 1.0902 2.018663 2.89 1.060
70 12.195 0.90204 1.1002 2.005375 2.88 1.060
X =0
X =0
Fig. 1. Plots of the electrical capacitance, C, of the measuring cell filled with cyclohexanone, vs temperature, T, at 500 (1), 750 (2), and 1000 (3) V
with increase in temperature from 25 to 60 °C. This change can be caused by the variation of statistical weights of the "boat" and more polar "chair" conformers. The g value being of the order 1.1 shows a certain correlation in the orientation of dipole moments of cyclohexanone molecules which causes a high value of the dielectric permittivity e = 15.1 at the temperature of 20 °C.
Electrical capacitance C of the measuring cell filled with cyclohexanone under the external orienting constant electric field at the voltage of U = 500, 750 and 1000 V in the temperature range from 25 to 60 °C is shown in Fig. 1. It can be seen that the field mentioned above does not affect C values and, therefore, does not change the orientation of the dipole moment vector for the >C = O group in the cyclohexanone molecule and also does not influence on the ratio of the "boat" and "chair" conformers statistical weights in the chosen temperature and voltage range.
3.2. The dipole moment of monomer A.
External electric field influence
Temperature and concentration dependences for the electrical capacitance of the measuring cell filled with the solutions of monomer A in cyclohexanone at Е = 104 V/cm are shown in Fig. 2. The identical dependences measured at Е = 0 are similar. The linearity of these de-
pendences confirms the invariable stoichiom-etry of the solution composition at the constant temperature, making possible to extrapolate the data to infinite dissolution and to evaluate increments of dielectric permittivity (Ae)/ (A*2)L=0 . Here Ae = (AC)/C0; C = eC0 + C,a,; C0 is a self-capacitance of the cell determined by the interelectrode gap geometry; Cpar is a wire capacitance. C0 and Cpar values were calculated from the data of capacitance measurements for the cell filled with standard liquids with the tabulated dielectric permittivity.
Fig. 2. Plots of the electrical capacitance, C, of the measuring cell filled with cyclohexanone solution of monomer A, at E = 104 V/cm, vs temperature (a) and concentration (b); the concentration x2 = 0 (1); 1 (2); 2 (3); 3 (4) mol/mol (a); the temperature T = 20 (1); 30 (2); 40(3); 50 (4); 60 (5); 70 (6) °C (b)
Fig. 3. Plots of the dielectric permittivity increments for cyclohexanone solution of monomer A vs temperature at 0 (1), 750 (2), 1000 (3) V
6 s
,-T 4 3
• 1
1.0 1.5 2.0 2.5 3,0
x210-3, mol/mol
Fig. 4. Plots of the differential static dielectric permittivity of cyclohexanone solution of monomer A at E = 104 V/cm, vs solution concentration, x2 , at 20 (1), 30 (2), 40(3), 50 (4), 60 (5), and 70 (6) °C
Temperature dependences of dielectric permittivity increments for monomer A at different E values are shown in Fig. 3. It is well-known that the dielectric permittivity increment is in direct proportion to squared dipole moment. Obviously (see Fig. 3), the dipole moment of monomer A increases with temperature at E = 0. The tendency remains the same at nonzero field. Increment values also rise when E is increased.
The molecule A structure is rather complicated; it contains several polar groups and axes
of internal rotation. Previous investigations [14 -16] of low molecular liquid crystals with the same polar cyanophenyl end groups both in solution and in condensed state by means of dielectric polarization method showed the formation of associates with antiparallel laying of dipole moments (about 4 D) of cyanophenyl groups.
The increasing temperature gives rise to the disordering the antiparallel orientation of the polar groups. The external electric field has the same effect forcing the orientation of the end cyanophenyl groups along the field. Also, one can suppose that the external electric field forces the appearance of the induced dipole
Fig. 5. Plots of the electrical capacitance, C, of the measuring cell filled with cyclohexanone solution of copolymer AB at E = 0 (a) and
E = 104 V/cm (b) vs temperature, T; concentration x2 = 0 (1), 0.00649 (2), 0.00359 (3), 0.00253 (4) mol/mol
a)
C. pF 17sr
170 165 160 165 160 145 140 135
* 4
x210 3, mol/mol
6) C. pF
176 r
170 166 160 1s6 1so 145 140 135
___* ■>
.4- -- " -
" ■< 6
x210 3, mol/mol
Fig. 6. Plots of the electrical capacitance, C, of the measuring cell filled with cyclohexanone solution of copolymer AB at E = 0 (a) and E = 104 V/cm (b) vs concentration, x2. Temperature T = 20(7), 30 (2), 40(J), 50 (4), (20 ( 5), and 70 (6) °C
moment from the chromophore groups which makes the whole dipole moment value of the molecule or associate larger.
Concentration dependences of the static polarization for the monomer A at E = 104 V/cm are presented in Fig. 4. One can see that the monomer A polarization increases with temperature nonlinearly and tends to have a limit as the monomer concentration in solution rises. This seems to be coupled with the association and partial dipole moments compensation caused by it.
3.3. The effect of the external electric field on the dielectric polarization of the copolymer AB
Temperature and concentration dependences for the electrical capacitance of the measuring cell filled with copolymer AB solutions in cyclohexanone at E = 0 and E = 104 V/cm are shown in Figs. 5 and 6. The dependences are linear, and this makes it possible to calculate the dielectric permittivity increment (Ae)/(Ax2)|x2=0 for the copolymer AB solutions in cyclohexanone.
The temperature dependences of (Ae)/ (Ax2)|x2=0 for infinitely diluted solutions of the copolymer AB without external orienting electric field and in the field are presented in Fig. 7. One can see that temperature behavior of the dielectric permittivity increment for the copo-lymer is quite different from that of the monomer A. The values of (Ae)/(Ax2)|x2=0 decline in the temperature range of 20—40 °C and then increase with temperature. The change of the temperature rate of the (Ae)/(Ax2)|x2=0 dependence at 40 °C is caused by the intramacromo-lecular conformational transition and reflects the change of the intramolecular structure as well as the ratio of statistical weights of con-formers with different dipole moments. This effect is typical for previously investigated comblike homopolymethacrylates, side chains of
Fig. 7. Plots of the dielectric permittivity increments of the cyclohexanone solution of the copolymer AB at E= 0 (1), and 104 V/cm (2) vs temperature
-о.ю
-0.11 -1-1-1-1-'
2 3 4 5 6
i. lO mol/mol
Fig. 8. Plots of the static dielectric permittivity of the cyclohexanone solution of the copolymer AB at E = 104 V/cm, vs solution concentration
which contain fragments of the same structure as low-molecular liquid crystals do [17—23].
When external electric field is applied, the temperature dependence of (Ae)/(Ax2)|x2=0 is the same. However, the values of (Ae)/(Ax2)|x2=0 are less in the whole temperature range. It can be caused by the fact that in electric field the value of projection of the monomer unit effective dipole moment on the field direction decreases. At the same time the ratio of the conformer statistical weights in copolymer is not affected by the applied external electric field.
As it was shown in Refs. [19, 22], side chains intramacromolecular association in comb-like polymers was rather typical because of molecular groups specific interactions inside the polymer coil. Moreover, the intramolecular conformational transitions in comb-like polymers determine further structure formation in dilute solutions on the intermolecular level and lead to the gel formation when the thermody-namic parameters of the solution are changed [24, 25].
Table 2
Т, оС v2, cm3/g n 2 П2 а = (Ae/Ax2)x2=0 -ß = (Av/Ax2)x2=0 ц2, D
E = 0 104 V/cm E = 0 104 V/cm
Copolymer AB
20 1.1000 1.979101 92.70 7S.11 0.0435 S.54 7.92
30 1.1104 1.967664 S9.S4 75.96 0.043S S.61 7.99
40 1.20S0 1.955S24 SS.90 74.95 0.0440 S.77 S.12
50 1.1312 1.944271 S9.77 75.19 0.0440 9.00 S.20
60 1.4160 1.932993 90.46 75.S9 0.043S 9.26 S.55
70 1.5200 1.9219S2 91.94 77.37 0.0434 9.51 S.79
Polymer B
20 0.9493 2.20727 26.06 42.74 0.107 3.54 4.S3
30 0.9500 2.20602 26.1S 42.94 0.116 3.74 4.96
40 0.9507 2.2047S 26.30 43.14 0.126 3.74 5.10
50 0.9515 2.20336 26.43 43.33 0.136 3.SS 5.25
60 0.9522 2.20212 26.54 43.53 0.146 3.9S 5.40
70 0.9529 2.200SS 26.66 43.72 0.156 4.10 5.55
Monomer A
20 1.0052 2.132S9 134.409 194.705 0.0439 10.27 12.05
30 1.0096 2.11SS9 157.973 236.949 0.042S 11.25 13.4S
40 1.0192 2.104S9 1S1.552 279.193 0.0516 12.23 14.SS
50 1.02SS 2.090S9 205.139 321.436 0.0516 13.22 16.27
60 1.03S4 2.076S9 22S.751 363.707 0.051S 14.22 17.67
70 1.04S0 2.062S9 252.345 405.951 0.0522 15.22 19.06
The temperature dependence of the properties of the investigated compounds in cyclohexanone solutions
Fig. 9. Plots of the dipole moment of the monomer A (1, 2), and of the partial dipole moment of monomer unit A included into the copolymer AB (3, 4), measured in the cyclohexanone solution E = 0 (1, 3), and 104 V/cm (2, 4) vs temperature, T.
The intramolecular association is reflected also in the nonlinear effect of dielectric polarization. The concentration dependence of the eE and eE=0 difference is depicted in Fig. 8. The abrupt decrease of the difference of the dielectric permittivity in the field and without the field as the polymer concentration increases and the plateau at x2 > 0.004 mol/mol is due to the association of the monomeric units A with the compensation of the tangential partial dipole moments on the intramolecular level [6, 22, 23].
3.4. The dipole moment and the internal rotation of monomer units A in the copolymer macromolecule
It is well known [10, 14, 26] that the orientation of the polar monomer units in polymers depends on the internal rotation hindering in the chain and is determined by the macromolecule conformation properties. As a result, the dipole moment per monomer unit is different from the dipole moment of a free monomer molecule.
As it was mentioned above, the presence of monomer units A in the copolymer macromolecules AB gives nonlinear optical properties of the copolymer in the condensed state. Therefore, it is important to know the conditions of monomer units A internal rotation in the copolymer chain, which gives information about structure formation in the single molecular coil and order of the copolymer in the condensed state.
According to Eq. (3), it is possible to calculate molar orientation polarization of the monomer unit A in the copolymer macromolecule by subtracting molar orientation polarization of polymer units B from the copolymer molar orientation polarization (see Table 2). Calculated values of orientation dielectric polarization (dipole moment squared) of monomer unit A in copolymer AB are also given in Table 2.
Dipole moments of isolated monomer A and monomer unit A in the copolymer AB are compared in Fig. 9 in the absence and in the presence of the electric field. The Kirkwood factor
Fig. 10. Plots of the Kirkwood factor of the monomer unit A included into the copolymer AB measured in the cyclohexanone solutions at E = 0 (1) and 104 V/cm (2) vs temperature
g (^(A)coi) /(^(A)iso)
values are presented in Fig. 10 for the same two cases. It can be seen that the correlation factor values g of polar monomer unit A are typical for the polymethacrylate homologous series being equal to approximately 0.6 at T = 20 °C [6, 21-23]. When electric field is applied, the g value decreases to about 0.35 reflecting the larger degree of association in the molecular coil. The received value g < 1 characterizes the tendency to antiparallel orientation of the monomer A side chains and the increase of the units A association when the solution is in the electric field.
4. Conclusions
Thus, the analysis of experimental data on comparative study of the dielectric polarization of the copolymer AB, the polymer B and the monomer A leads to the following conclusions.
(1) There are associates of constant stoichiometry at given temperature due to intermolecular interactions in the monomer A solutions, and their dipole moments increase both with temperature and with external electric field.
(2) In the copolymer solution, intra-macromolecular association of chromophore-containing side chains in trans conformation takes place leading to dipole moment compensation. The Kirkwood factor is less than 1. When external electric field is applied, intramolecular association increases: g values decrease almost twice.
(3) The dipole moment of the copolymer molecule per monomer unit AB decreases under the external electric field.
(4) The intramacromolecular conforma-tional transition takes place at the temperature of about 40 °C.
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THE AuTHORS
STEPANOVA Tamara P.
Institute of Macromolecular Compounds RAS
31 Bolshoy Ave. V.O., St. Petersburg, 199004, Russian Federation
t_stepanova2005@mail.ru
NOSOVA Galina I.
Institute of Macromolecular Compounds RAS
31 Bolshoy Ave. V.O., St. Petersburg, 199004, Russian Federation
nosova@physicist.net
SOLOVSKAYA Nina A.
Institute of Macromolecular Compounds RAS
31 Bolshoy Ave. V.O., St. Petersburg, 199004, Russian Federation
KAPRALOVA Victoriia M.
St. Petersburg Polytechnic University
29 Politekhnicheskaya St., St. Petersburg, 195251, Russian Federation kapralova2006@yandex.ru
YAKIMANSKY Alexander V.
Institute of Macromolecular Compounds PAS
31 Bolshoy Ave. V.O., St. Petersburg, 199004, Russian Federation
yak@hq.macro.ru
Степанова Т.П., Носова Г.и, Соловская Н.А., Капралова В.М., Якиманский А.В. ВЛИЯНИЕ ПОСТОЯННОГО ЭЛЕКТРИЧЕСКОГО ПОЛЯ НА ДИПОЛЬНЫЙ МОМЕНТ ПОЛИМЕРА ГРЕБНЕОБРАЗНОГО СТРОЕНИЯ С ХРОМОФОРНЫМИ ГРУППАМИ В БОКОВЫХ ЦЕПЯХ.
Работа посвящена исследованию конформационных свойств и тенденции к ассоциации макромолекул хромофорсодержащего гребнеобразного сополимера дицианфенилазобензтиазолметакрилата (А) и амилметакрилата (В) в соотношении 1 : 1, пленки которого обладают нелинейно-оптическими свойствами. Методом статической диэлектрической поляризации изучены температурные зависимости диэлектрической проницаемости и дипольных моментов сополимера АВ, полимера В и мономера А в разбавленных растворах в циклогексаноне, в том числе во внешнем постоянном ориентирующем электрическом поле. Показано, что в сополимере происходит внутримолекулярная ассоциация хромофорсодержащих фрагментов с компенсацией их дипольных моментов, которая усиливается при наложении электрического поля. Дипольный момент макромолекулы в расчете на мономерное звено уменьшается под действием поля. При температуре около 40 оС имеет место внутримакромолекулярный конформационный переход.
гребнеобразный полимер, диэлектрическая поляризация, дипольный момент, ассоциация.
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СВЕДЕНИЯ ОБ АВТОРАх
СТЕПАНОВА Тамара Павловна — кандидат физико-математических наук, старший научный сотрудник лаборатории люминесценции, релаксационных и электрических свойств полимерных систем Института высокомолекулярных соединений РАН.
199004, Российская Федерация, г. Санкт-Петербург, Большой пр. В.О., 31 t_stepanova2005@mail.ru
НОСОВА Галина Ивановна — кандидат физико-математических наук, старший научный сотрудник лаборатории люминесценции, релаксационных и электрических свойств полимерных систем Института высокомолекулярных соединений РАН.
199004, Российская Федерация, г. Санкт-Петербург, Большой пр. В.О., 31 nosova@physicist.net
СОЛОВСКАЯ Нина Аркадьевна — кандидат химических наук, старший научный сотрудник лаборатории синтеза полимерных наноматериалов и композиций для оптических сред Института высокомолекулярных соединений РАН.
199004, Российская Федерация, г. Санкт-Петербург, Большой пр. В.О., 31
КАПРАЛОВА Виктория Маратовна — кандидат физико-математических наук, доцент кафедры интегральной электроники Санкт-Петербургского государственного политехнического университета. 195251, Российская Федерация, г. Санкт-Петербург, Политехническая ул., 29 kapralova2006@yandex.ru
ЯКИМАНСКИЙ Александр Вадимович — заведующий лабораторией синтеза полимерных наноматериалов и композиций для оптических сред Института высокомолекулярных соединений РАН. 199004, Российская Федерация, г. Санкт-Петербург, Большой пр. В.О., 31 yak@hq.macro.ru
© Санкт-Петербургский государственный политехнический университет, 2015
