W. J. MACKNIGHT, L. W. MCKENNA,B. E. READ,AND R. S. STEIN
1122
Summary 1. Within the temperature range 0-260" and the frequency range 100 cps-10 Rilcps, the permittivity, E', of ceramic BaTia07and ceramic BaTi40svaries linearly with temperature. 2. The loss tangent of both dielectrics a t any chosen temperature within the range 0-150" remains
essentially constant with increase in frequency up to lo5 CPS. 3. Activation energies from direct current conduction measurements are 1.05 eV for BaTi307and 1.03 eV for BaTi40s. 4. No evidence of ferroelectricity was found for either of these dielectrics within the range 0-425".
Properties of EthyleneMethacrylic Acid Copolymers and Their Sodium Salts: Infrared Studies by W. J. MacKnight,' L. W. McKenna, B. E. Read, and R. S. Stein Department of Chemistry and Polymer Science and Engineering Program, University of Massachusetts, Amherst, Massachusetts 01008 (Received June 20, 1967)
An infrared spectroscopic investigation of ethylene-methacrylic acid copolymers and their sodium salts has been carried out. All the copolymers studied were based on a parent copolymer containing 4.1 mol % of methacrylic acid groups. This was then neutralized to various extents (from 0 to 78%) with sodium hydroxide. The per cent ionization was determined from the integrated absorbance of the 1700 om-' un-ionized carbonyl stretching band. Temperature-dependent infraied studies showed that the behavior of the un-ionized acid groups over the entire range of ionization is quantitatively comparable to that of low molecular weight carboxylic acids in nonpolar solvents. A monomer-dimer equilibrium exists among the acid groups and they are almost completely in the form of hydrogen-bonded dimers at room temperature. The heat of dissociation of the dimers was found to be 11.6 kcal mol-'. Thus each hydrogen-bond has a bond strength of 5.8 kcal mol-'. Infrared dichroism studies established that there is a significant amount of crystallinity even a t the highest degree of ionization, that the hydrogen bonds are intermolecular in nature, and that the ionized carboxylate groups have a preferred orientation out of the plane of the main chain of the copolymer.
Introduction This work represents part of a continuing study of the role of intermolecular forces on the physical and mechanical properties of polymers. The ethylenemethacrylic acid copolymers and their sodium salts provide an interesting system for such investigations. It has been established2that the ionization of these acid copolymers results in a significant eiihancement of their tensile strengths and melt viscosities. These findings were explained on the basis of the introduction of strong interchain ionic links.2 The validity of this interpretation is somewhat doubtful, however, and alternative explanations have recently been proposed. a I n this paper are reported the results of an infrared study on films of ethylene-methacrylic acid copolymers, ionized to various extents with sodium. The per cent ionization was determined by analysis of the infrared spectra. The equilibrium constant for the dissociation of carboxylic acid dimers and their heat of dissociation were obtained from temperature-dependent infrared The Journal of Physical Chemistry
studies. Infrared dichroism indicated that the hydrogen bonds f a ~ ~ - between ~ed un-ionized carboxyl groups are intermolecular. The implications of these findings concerning the mechanical properties of the materials are discussed elsewhere.3 Experimental Section The starting material was a partially ionized copolymer of ethylene and methacrylic acid kindly supplied by the Du pant co. Its structure may be represented schematically as
CHa -(CH&HJ
.-(CH2-C-),
I
l
COOH COO-Na+ (1) TOwhom correspondence should be addressed.
1123
PROPERTIES OF ETHYLENE-METHACRYLIC ACIDCOPOLYMERS The methacrylic acid content of the copolymer was 4.1 mol %, For a random distribution of methacrylic acid groups, this would yield an average value of n of about 25 for m = 1. As pointed out in the Discussion section, however, there may be some tendency for the methacrylic acid units to exist in blocks (m > 1). I n this case the sequence lengths of many of the ethylene units would be much greater than 25. The finely divided starting material was refluxed in tetrahydrofurnn with dilute hydrochloric acid in order to produce the ionized copolymer. This was in turn neutralized to various extents by refluxing in tetrahydrofuran with sodium hydroxide for varying periods of time and a t different base concentrations. After this treatment, the ionized copolymers were precipitated twice in a methanol-water mixture, washed thoroughly several times, and dried in uucuo at 100". Samples were compression molded into thin films a t 180" and 20,000 lb of pressure. Infrared spectra of the films were obtained on a Beckman spectrophotometer, Model IR-10. This instrument was operated on the fast-scan setting with the gain control at position 5. The spectral accuracy of the IR-10 is given as 8 cm" from 4000 to 2000 cm-I and as 4 cm-l from 2000 to 300 c ~ n - ' . ~ Temperaturedependent infrared studies were made using a temperature enclosure constructed for this purpose. Temperatures were measured using a calibrated copper-constantan thermocouple which rested against the polymer film surface.
Results and Discussion Figure 1 shows the infrared spectra of the four samples investigated. The following points are of interest. First, there is strong evidence for hydrogen bonding in all cases as shown by the shoulder appearing a t 2650 cm-'. This shoulder is characteristic of the stretching mode of the hydrogen bonded hydroxyl group. Second, the un-ionized carbonyl stretching frequency appears at 1700 cm-l and remains at the same position throughout the entire range of ionization. Finally, the absorption due to the asymmetric stretching mode of the carboxylate ion occurs a t 1560 cm-1 and, of course, increases in magnitude with increasing ionization. The integrated absorbance per centimeter sample thickness of the 1700-crn-' carbonyl band was used to determine the degree of ionization. The absorbance, A, is defined an log J o / l , where lois the incident and I is the transmitted intensity. The integrated absorbance corresponds to the area beneath plots of A against wave number (cm-l). For the 1700-cm-l band, the ratio X:Avl/,, where X is the spectral slit width and A V I / ~is the ablsorption band halfwidth, was found to vary from a value of about 0.13 for the acid copolymer to a value of about 0.06 for the most highly ionized copolymer, Since the peak absorbances did not ex-
WAVELENGTH IN MICRONS
3
5
7
I
I
I
I
I
9 I
II 14 I I /
I
ri
3500
2 5 0 0 1800 1400 1000 WAVENUMBER C d l
Figure 1. Infrared spectra of ethylene-methacrylic acid copolymer films ionized to different extents with sodium. Thickness: O%, 1.2 x 10-3 cm; 20%, 2.5 x 10-3 cm; SO%, 6.1 X 10-3 cm; 787,, 9.4 X 10-3 cm.
ceed 0.8, the correction for instrument resolving power (about 1 %) was considered negligible.5 The success of the present technique for measuring the degree of ionization depends on the lack of overlap of neighboring bands and on accurate film-thickness measurements', The intensity of the carbonyl absorption for the un-ionized acid copolymer is such that it is necessary to CM thickness use a film of approximately 1.27 X in order to get this absorption on scale. The following procedure was developed to measure the thickness accurately. A band was selected with an absorbance that could be measured from film thicknesses of about 2.54 X cm to thicknesses of less than 1.27 X 10-8 om. An appropriate band for this is the 935-cm-1 band in the case of the un-ionized acid copolymer. This band is assigned to the out-of-plane deformation modes of the hydroxyl groups. I t s absorbance should thus be proportional to the number of acid groups within the beam and should be independent of the thermal history of the sample. A plot was made of peak absorbance (Amax) us. thickness for this band. Thicknesses in the range from 2.54 X to 7.62 X (2) R. W. Rees and D. J. Vaughn, Polym. Preprints, 6, 296 (1966). (3) W. J. MacKnight, L. W. McKenna, and B. E. Read, J . A p p l . Phys., 38,4208 (1967). (4) Beckman Instructions 1383-A (available from Beckman Instru-
ments Inc., Scientific and Process Instruments Division, Fullerton, Calif. 92634). (5) D. P. Ramsay, J. Amer. Chern. Soc., 74, 72 (1962). Volunts 79,Number
4 April
1068
1124
W. J. MACKNIGHT, L. W. MCKENNA, B. E. READ,AND R. S. STEIN
cm could be satisfactorily measured by a micrometer. This plot yielded a straight line which extrapolated through the origin. Thus it was established that Beer's law was obeyed. From this plot, the thickness of the films was determined, thus enabling the evaluation of the integrated absorbance of the 1700-cm-l band/unit sample thickness. In the case of the ionized samples a similar procedure was followed, except that at the higher degrees of ionization it was not necessary to employ very thin films and the thicknesses could be measured directly by the micrometer. The per cent ionization was then calculated using the relationship
% ionization
b-
=
integrated absorbance/cm (ionized) integrated absorbance/cm (un-ionized)
Table I lists the integrated absorbances and the per cent ionization for the four samples investigated.
Table I Integrated absorbance of 1700-om-1 peak/ om aample thickness, om-' X 10-8
18.32 14.71 7.26 4.10
A\ p " o I
\cP i
0
/vJw The dissociation constant, K , is defined by
K =
where [-COOHI is the concentration of monomeric carboxyl groups and [ (-COOH),] is the concentration of dimerized carboxyl groups. K was evaluated from the spectra in Figure 2. This was accomplished by measuring the peak abmrbances of the 1700- and 3540cm-' bands at each temperature (54-99'). At 99" the 1750-cm-' band was sufficieritly well resolved so that its peak absorbance could be measured using a base line determined as indicated in Figure 2. The ratio of this absorbance to that of the 3540 cm-I free hydroxyl band was obtained a t 99". This ratio was assumed indepen-
% ionization
0 19.7 60.4
77.6
WAVELENGTH IN MICRONS
'oor--+ '
The results of the temperature-dependent infrared studies on the un-ionized copolymer are shown in Figure 2. Of interest are the appearance of a band at 3540 cm-1 which increases in magnitude with increasing temperature, and the appearance of a shoulder a t 1750 cm-l on the main 1700-cm-l carbonyl band which also increases with increasing temperature. Such behavior is characteristic of low molecular weight carboxylic acids in nonpolar solvents and is interpreted on the basis of a monomer-dimer Thus the 3540-cm-' band is attributed to the free hydroxyl stretching vibration, the 1750-cm-' band to the monomeric carbonyl stretching vibration, and the 1700-cm-' band to the dimerized carbonyl stretching vibration. Figure 3 shows the peak absorbance film thickness for the 3540-cm-l absorption as a function of temperature. From this plot it is evident that free hydroxyl groups are first detected by infrared at about 30". The above results lead to a structure for the copolymer in which the acid groups are dimerized to form interchain links. The dimerization is essentially complete at room temperature. This structure may be schematically represented as The Journal of Physical Chemistry
[-CO0Hl2 [(-COOH)2]
O
0
M
100
100
w,
50
99OC
4000
3200 2 4 0 0 WAVENUMBER CM"
1600
Figure 2. Temperature dependence of the infrared spectrum of the un-ionized ethylene-methacrylic acid copolymer. The dotted line on the 1750-cm-1 band a t 99' was used to determine the base line for the peak-absorbance measurement of the band. (6) C. N. R. Rao, "Chemical Applications of Infrared Bpectroscopy," Academic Press, Inc., New York, N. Y., 1963, Chapter 111.
1125
PROPERTIES OF ETHYLENE-METHACRYLIC ACID COPOLYMERS
II
8 6 n W
Y 2
52
-
3540 CM-' FREE O H STRETCH
P
(-COOH)$2-COOH AH.114 KCALSlMOLE
-'I-
8-
5-
2-
-7 I 2.5
0
I
I
I
I
20
40
60
80
I
100
I
I
I
I
I
2.9 VT(K)XIO~
I
I
I 33
120
Figure 4. van't Hoff plot (log K ws. 1/T) of the dissociation constant for the acid monomer-dimer equilibrium. The open circles are the experimental points. The cross ( X ) is the literature value for low molecular weight carboxylic acids in nonpolar solvents.
dent of temperature and thus the absorbances of the 1750-cm-' band at the other temperatures could be derived from the measured absorbances of the 3540-cm-' band at these temperatures. I€the peak absorbances are expressed as absorbances per centimeter, the following relationships can reasonably be assumed to apply. A1,max
=
EICI
(3)
A2,max
=
4 ' 2
(4)
E2
=
2E1
(5)
where is the peak absorbance per centimeter of the 1750-cm-' band, el is the molar extinction coefficient of the 1750-cm-' band in square centimeters per mole, CI is the concentration in moles per cubic centimeter of the monomeric carboxyl groups, A 2 , m a x is the peak absorbance per centimeter of the 1700-cm-' band, e2 is the molar extinction coefficient of the 1700-cm-' band in square centimeters per mole, and Cz is the concentration in moles per cubic centimeter of the dimerized carboxyl groups. Equation 5 is justified by the work of Chang on the model system pivalic acid in benzene. It was found that the molar extinction coefficient for the dimer in this system is almost exactly twice the molar extinction coefficient for the monomer.' Using eq 3-5, the expression for the dissociation constant becomes
K (mol/cm3)
=
(A1,Inax) 24
(6) ( A2,rnax) €2 e2 can be determined using eq 4, substituting in the value of the peak absorbance per centimeter of the 1700-cm-1 band a t room temperature and the known concentration of the methacrylic acid groups in the copolymer. It is interesting to note that the value thus obtained, 9.14 X lo5 cm2 mol-', compares quite
favorably with the value of 12.6 X lo5 cm2 mol-' obtained for the pivalic acid-benzene system. Small differences would be expected, of course, between values obtained from different spectrometers owing to differences in resolving power, scanning speed, etc. The logarithms of the dissociation constants are plotted as a function of reciprocal temperature in Figure 4. The open circles are the experimental points while the cross represents the value for the dissociation constant of unconjugated carboxylic acids in carbon tetrachloride measured by Wenograd and The heat of dissociation of 11.6 kcal mol-' obtained from the slope of the van't Hoff plot in Figure 4 also agrees well with literature values €or low molecular weight carboxylic acids in nonpolar solvents. Thus it may be concluded that the association of the acid groups of the copolymer is quantitatively comparable to the dimerization of low molecular weight carboxylic acids in nonpolar solvents. It was further established that the peak absorbance of the 3540 cm-l free hydroxyl band is inversely proportional to the degree of ionization. This means that the monomer-dimer equilibrium of the remaining carboxyl groups is unaffected by ionization of a portion of the carboxyl groups. The results of the infrared dichroism studies are presented in Figure 5. Attention is directed to the 720-730-~m-~region. I n the case of the un-ionized copolymer, the unpolarized band in this region consists of a doublet with absorption maxima at 720 and 730 cm-l. Such behavior is characteristic of polyethylene (7) L. C-Y. Chang, Ph.D. Thesis, Polytechnic Institute of Brooklyn, Brooklyn, N. Y., 1955. (8) J. Wenograd and R. A. Spurr, J. Amer. Chem. SOC.,79, 5844 (1957).
Volume 76,Number 4 April 1968
W. J. MACKNIGHT, L. W. MCKENNA,B. E. READ,AND R. S. STEIN
1126 WAVENUMBER CM-' 3000 2000 1500 1200 l O O O 9 0 0 8 0 0 700
u)
2
1
'
I
I
%-
I
0
50
100
150
'lo EXTENSION WAVELENGTH IN MICRONS
Figure 5. Polarized infrared spectra for the 70% ionized copolymer.
and it has been showngthat the 720-cm-' band is due to both the amorphous and crystalline phases, the crystalline component being polarized along the a crystallographic axis. On the other hand, the 730-cm-' band is due entirely to the crystalline phase and is polarized along the b crystal axis. Both the 720- and 730-cm-l bands thus show perpendicular dichroism when the polymer is extended. Turning to the 70% ionized copolymer, it can be seen that the 730-cm-1 band has entirely disappeared in the parallel polarized spectrum but appears as a shoulder on the main 720-cm-I band in the perpendicularly polarized spectrum. It is thus apparent that even at 70% ionization, a significant amount of crystallinity remains. It can also be seen that the shoulder around 2650 cm-l, characteristic of the hydrogen bonded hydroxyl group, shows perpendicular polarization. This indicates that the transition moment of the hydrogen bonded hydroxyl group is perpendicular to the main chain and is thus evidence for the presence of intermolecular hydrogen bonds in the acid dimers present. I n this connection it is interesting to note that the parallel dichroism of the hydrogen-bonded 3350-cm-' hydroxyl band in isotactic polyvinyl alcohol has been cited as evidence for intramolecular hydrogen bonding in this polymer.IO Figure 6 shows a plot of the dichroic ratio (defined as D = A I, / A Lvs. per cent extension for three bands of the 70% ionized copolymer. The 720-em-' band shows relatively large perpendicular dichroism, again emphasizing the significant crystalline contribution to this band. Both the 1700- and 1560-cm-' bands show relatively small perpendicular dichroism. This is consistent with the presence of the carboxyl groups and
The Journal of Physical Chemistry
Figure 6. Dichroic ratio 218. per cent extension for three bands of the 70% ionized copolymer.
carboxylate ions in the amorphous phase, since they would be expected to be too large to be incorporated into the polyethylene crystal latt>ice. The perpendicular dichroism of the 1560-cm-' band indicates that the carboxylate ions have a preferred orientation out of the plane of the main chain. This can be seen schematically as
/"\
0 OQ
As shown, the transition moment direction for the asymmetric stretching mode would be parallel to the main chain if the carboxylate groups were in the plane of the main chain. The out-of-plane orientation of the carboxylate ions may be due to steric hindrance arising from neighboring carboxylate or carboxyl groups. Such steric hindrance would be expected to occur if the methacrylic acid units have a tendency to be in blocks. Acknowledgments. We are grateful to the National Science Foundation for partial support of this research under Grants G P 5840 and G P 5372X. We are also indebted to Dr. W. F. Brondyke of the Plastics Department of the Du Pont Co. for supplying the starting material. (9) R. S. Stein and G . B. B. M. Sutherland, J . Chem. Phye., 21, 371 (1953); 22, 1993 (1954). (10) 8. Murahashi, H. Yuki, T. Sano, U.Yonemura, H. Tadokoro, and Y . Chatane, J . Polymer Sci., 62, 577 (1962).
