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Effect of the concentration, distribution and degree of neutralization of the carboxyl groups, and of the nature of the cation, on the thermomechanical properties of ionomers
EFFECT OF THE CONCENTRATION, DISTRIBUTION AND DEGREE OF NEUTRALIZATION OF THE CARBOXYL GROUPS, AND OF THE NATURE OF THE CATION, ON THE THERMOMECHANICAL PROPERTIES OF IONOMERS* S. :I~. P~AFIKOV, YU. B. MO:NAKOV, I . A. Io~ovA, G. P . GLADYSHEV, A. A. ANDRUSENKO, O. A. ~OI~OMAREV,A. I. VOROB'EVA, A. A. BERG, L. F. ANTONOVA, ~ . I. ABLYAKIMOV, ~¢[. F . SISI:N a n d A. A. SMORODI~ I n s t i t u t e of Chemistry, Bashkir Branch of the U.S.S.R. Academy of Sciences (Received 13 December 1971) A study has been made of the effect of the concentration of carboxyl groups a n d their distribution along the copolymer chain on the thermomechanical and physicomechanical properties of ionomers. I t is shown t h a t when there is a random distribution a n d a low concentration of carboxyl groups in the polymer the degree of neutralization and the nature of ~he cation has little effect on the transition temperatures of ionomers. When the concentration of carboxyl groups is high the thermomechanical characteristics of neutralized samples are dependent on the concentration and nature of the cation.
COPOLYMERS containing carboxyl groups are used extensively. The introduction of certain quantities of carboxyl and other acidic groups, and partial ionization of these with various mono- and divalent cations disturbs the supermolecular structure of the polymers and at the same time increases intermolecular interaction between the polymer chains, which has a marked effect on their physicomechanical and physicochemical properties. In these polymers in addition to Van der Waals forces and hydrogen bonding ionic interaction, which is thermally reversible, occurs. Consequently in the solid state ionomers are reminiscent of crosslinked polymers, but in the molten state they behave as typical thermoplasts [1]. The formation of this network of ionic bonds provides ionomers with a number of unique properties, such as high strength, rigidity, transparency, high resistance to solvents and good adhesion [2]. There is however no information in the literature on ionic interaction in polymers in the solid state and the effect of this interaction on the thermomechanical properties of ionomers, apart from a paper by Nikolayev and Gal'perin [3] on the heat-distortion characteristics of ionomers, and one by Eisenberg [4], in which problems associated with the glass temperature of ionic polymers are discussed. The present paper discusses the effect of the nature and concentration of the * Vysokomol. soyed. A15: No. 9, 1974-1981, 1973. 2225
2226
S . R . RAFIKOV et al.
cations on the thermomechanical properties of ionomers based on random copolymers of ethylene with acrylic and methacrylic acids, a graft copolymcr of ethylene and maleic acid, a random and a block copolymer of styrene and mcthacrylic acid, and an alternating copolymer of 1-hexene with maleic acid.
EXPERIMENTAL The random copolymers of ethylene with acrylic and mettlacrylic acid were prepared in an autoclave a t 120 ° and a pressure of 1400 arm, with tert. b u t y l peroxide as initiator, a t low degrees of conversion. The copolymers were purified b y two reprecipitations in ethanol from hot, filtered solutions in xylene. The intrinsic viscosities of the original copolymers were 0.35-0.40 dl/g a t 70 ° in o-xylene, and the concentration of the acids in the copolymers was 3-4 mole Yo. Graft copolymers of ethylene and maleic anhydride were prepared in the presence of benzoyl peroxide a t 110 ° in solution in o-xylene b y the method of reference [5]. To obtain the pure copolymer the reaction product was twice-reprecipitated in ethanol from hot solutions in xylene, washed several times on the filter and dried i n v a c u o at 60 °. The polyethylene used for this purpose had a molecular weight of 3 × 104 and the ethylene-maleic anhydride graft copolymers contained 11.0, 5.4 a n d .1-4 mole ~o. of maleic anhydride. The random styrer~e-methacrylic acid copolymer (8 : 2) was prepared b y polymerization in bulk at 60 °, with 0.01 ~ b y weight of azobisisobutyronitrile (AZBN} as initiator. W h e n a degree of conversion of 10yo was reached the reaction mixture was diluted with dioxan and precipitated in petroleum ether. The copolymer thus precipitated was washed several times with flesh portions of petroleum ether a n d dried at 60 ° i n v a c u o . The molecular weight of the copolymer, determined b y the light-scattering method in dioxan, was 8-9 × 105. F o r preparation of a block copolymer of styrene and methacrylic acid a polystyrene of molecular weight 3 × 10 ~ was used. The concentration of the acidic monomer in the block copolymer was 3 0 ~ b y weight. 1-Hexene was copolymerized with maleic anhydride in previously evacuated ampoules in solution in acetone a t 70 °. The concentration of initiator (AZBlq) was 5 × 10 -~ mole/1. a n d the total concentration of monomers was 2 mole/1. The 1 : 1 copolymer was precipitated b y ethyl ether from solution in acetone. T o ' o b t a i n a copolymer of maleic acid the anhydride copolymer was hydrolyzed with HC1 for 5 hr a n d the product was washed with water to neutral reaction, dried and purified b y two reprecipitations in ethyl ether from solution in acetone. The molecular weight of the copolymer, determined b y light scattering in dimethylformamide a t an angle of 90 °, was 28,000. The structure of all the copolymers was confirmed b y infrared spectroscopic analysis in a UR-20 spectrometer, and the acid content of the copolymers was found b y elementary ana]ysls for carbon and b y potentiometric titration. The styrene-methacrylic acid copolymer was neutralized b y treating 1 ~/~ solutions of the copolymer in dioxan with the calculated quantities of methanolic solutions of N a O H or R b O H , or with acidified solutions of calcium or zinc acetate at 40 °. The carboxyl groups in copolymers of ethylene with acrylic a n d methacrylic acids were converted b y addition of solutions of the hydroxides or acetates of various metaIs in alcohol or dimethylformamide to hot solutions of t h e copolymers in xylene. I n this w a y products containing Na, Rb, K, Zn, Cd and Co ions were obtained. Salts of a copolymer of ethylene a n d maleic acid were obtained b y neutralizing the ethylene-maleic anhydride graft copolymer b y heating the l a t t e r under reflux in solution in xylene, with the calculated q u a n t i t y of I~aOH dissolved in a 9 : 1 mixture of alcohol and water. Copolymers of 1-hexene and maleic acid neutralized to different extents with sodium
Concentration, distribution and degree of neutralization of carboxyl groups
2227
hydroxide were obtained by hydrolysis of the anhydride copolymer by refluxing with the calculated quantities of NaOH for 3 hr, followed by two reprecipitations from aqueous solution in a 1 : 3 mixture of dioxan and ether and drying in vacuo at 40 °. The hydrogen of the carboxyl groups was replaced by cadmium by addition to an acetone solution of the original copolymer of a 9 : 1 aqueous acetone solution of cadmium acetate. The product was washed with water and twice reprecipitated from solution in acetone by ethyl ether, and dried in vacuo. The presence of neutralized carboxyl groups was confirmed by the occurrence of a band in the region of 1570 cm -1 in the infrared spectrum. The quantity of metal introduced was determined by flame photometry and the degree of neutralization of the carboxyl groups was calculated from the analytical results. The thermomechanieal tests were made with powdered samples compressed to form pellets of diameter 6 mm and thickness 2 mm. The pressure was 1850 kg/cm 2 and the time in the press was 5 min. In order to release the internal stresses brought about by themoulding process the pellets were annealed at 100° for 20 min. The two methods of constant and periodic load application were used in the thermomechanical tests. In both methods the load was the same at 8.8 g/mm ~. The temperature was raised uniformly at the rate of ~ 2.5 deg/min. The transition temperatures (softening point T s and flow temperature Tt) were determined from the points of intersection of the tangents to the initial parts of the inflexions in the thermomechanical curves, the results of two or three determinations being averaged. The effect of the experimental conditions (annealing of the pellets, the loading routine and the magnitude of the load) on the transition temperatures was tested with the styrenemethacrylic acid copolymer (Table 1). It is seen from Table 1 that the methods of loading and of preparation of the test specimen do not have any important effect on the transition temperatures of the copolymer Any difference in the transition temperatures is explained by difference in the test methods. RESULTS AND DISCUSSION I o n o m e r s based on ethylene copolymers. I t is seen from Fig. 1 t h a t Tf of the neutralized copolymers is n o t d e p e n d e n t on the n a t u r e of the cation or its c o n c e n t r a tion, or on the n a t u r e of the acid. I t is the same for all the samples a n d is the same as Tf of t h e original copolymer. This can be a t t r i b u t e d t o the low c o n c e n t r a tion of acid in t h e eopoly~ner (3-4 mole %), as a result of which t h e ionic b o n d s f o r m e d are so widely spaced o u t t h a t t h e y do n o t affect the t h e r m o m e e h a n i c a l characteristics of t h e ionomers. The ionic bonds nevertheless do impair the physieomechanical characteristics of t h e ionomers, as is seen f r o m Table 2. I t is seen f r o m Table 2 t h a t neutralization of t h e c o p o l y m e r increases the breaking s t r e n g t h considerably, Co(CH3CO0)2 affecting the s t r e n g t h to a g r e a t e r extent than NaOH. I n c r e a s e in t h e molecular weight o f the e t h y l e n e - m e t h a c r y l i c acid c o p o l y m e r t o 30,000 did n o t bring a b o u t a n y appreciable difference in the t h e r m o m e e h a n i c a l characteristics of the u n t r e a t e d c o p o l y m e r or of neutralized samples (Fig. 1, curves 6 a n d 9). I n this instance h o w e v e r t h e p h y s i c o m e c h a n i c a l characteristics o f t h e ionomers are impaired, t h o u g h it is true these changes a t t h e high molecular w e i g h t are less. T h u s molecular weight does n o t exert an i m p o r t a n t effect on the thermomech.-
2228
S. R, RAFI~rOVet al.
anical characteristics o f ionomers based on r a n d o m copolymers of ethylene and m e t h a c r y l i c acid. The situation is different in the case of graft c o p o l y m e r s of ethylene. I t is seen f r o m Fig. 2 t h a t t h e i n t r o d u c t i o n o f 1-4% a n d 5.4% of an acid does not alter t h e I00
-8,Z8 1,2,~'.~
0
I
I
I
2g
~g
8g
ag
-a,w
log r,o¢
~FIG. 1. Thermomeehanical curves of ionomers based on random copolymers of ethylene, neutralized with various ions: /--polyethylene; 2--copolymer of ethylene and methacrylic acid of low molecular weight; 3--copolymers neutralized to the extent of 30yo; 4 - - 6 0 ~ a n d 5 - - 9 0 ~ with Na ions; 6--eopolymer 90~o neutralized with zinc ions; 7--90yo with Co ions; 8--ethylene-methacrylic acid copolymer of high molecular weight; 9--copolymer 9 0 ~ neutralized with Zn ions; 10--ethylene-acrylic acid copolymer; //-acrylic acid copolymer 90 ~ neutralized with Zn ions. t r a n s i t i o n t e m p e r a t u r e of the copolymer, which are e v i d e n t l y associated with melting of the p o l y e t h y l e n e , a n d only in the case of a graft c o p o l y m e r containing 11% o f acid does T~ fall b y ~ 1 0 °. W h e n t h e graft copolymers are neutralized additional transition t e m p e r a t u r e s appear, due p r o b a b l y to f o r m a t i o n of ionic crosslinkages, the t e m p e r a t u r e s being d e p e n d e n t on the acid c o n t e n t a n d on the degree o f neutralization. TABLE 1.
EFFECT
OF T H E M E T H O D
OF L O A D I N G A ~ D
ON T H E T R A N S I T I O N T E M P E R A T U R E S
Method of preparation of specimen treatment Without annealing Annealing Hot compression Hot compression * Without annealing
load, I °C P' g/mm~ ] T, 8-8 8.8 8.8 8-8 21
100 110 110
* W i t h f a r t h e r a n n e a l i n g a t 120 ° for 20 m i n .
OF P R E P A R A T I O N
OF T H E
SPECIMENS
OF A S T Y R E N E - - M E T H A C R Y L I C A C I D C O P O L Y M E R
Constant load
Periodic loading
time, rain
%
T~
T~
T~
20 5 5
118 120 120 119 113
,182 183 182 181 177
113 113 112 113 110
180 180 179 180 175
Concentration, distribution and degree of neutralization of carboxyl groups
2229
•Whereas neutralization of the copolymer containing 11% of maleic anhydride to the extent of 30% and 60% results in the appearance of a second transition in the region of 154 ° and 170 ° respectively (Fig. 2a, curves 2 and 3), after which the samples are crosslinked, this second transition temperature does not appear after 90% neutralization and the sample crosslinks immediately when heated to 120 ° (Fig. 2a, curve 4). These crosslinkages are so strong that they are not broken when the polymer is heated to 420 ° . T A B L E 2. P H Y S I e O M E C H A N I C A L C H A R A C T E R I S T I C S OF ETHYLENE--M_ETHACRYLIC ACID COPOLYMERS : N E U T R A L I Z E D "WITH S O D I U M A N D COBALT I O N S
Neu-
Degree of neutraliing agent zation,
traliz-
0
NaOH
30 60 90
Tensile strength at break,
kg/cm2 53"5* 72"3 99'8 108"6
Elongation at break, % 47"0 37"5 28-0 24-2
Neutralizing agent
Co(CHaCO0)~
Degree Tensile Elongaof neu- strength tion at tralizaat break, break, kg/em 2 % tion, 30 60 90
91"2 140-7 150'1
39'5 44"0 63"7
* The low tensile strength is due to the low molecular weight ( ~ 8000) of the origi~al copolymer.
When the graft copolymer containing 5.4% of maleic anhydride in the side chains is neutralized to the extent of 30% and 60% the second transition temperatures are moved to 185 ° and 221 ° respectively, and as in the first instance when the degree of neutralization is 90% this transition temperature is not seen (Fig. 2b). In contrast to the copolymer containing 11°/0 when the temperature is raised to 400 ° the network is broken down and the polymer acquires flow. I t is interesting to note the non-reproducibility of the second transition temperature after repeated heating of the sample to this temperature and subsequent cooling to room temperature (Fig. 2b, curve 3), evidently because of irreversible scission of some of the intermolecular bonds. When the q u a n t i t y of acid in the copolymer is still less (1-4%) neutralization has a smaller effect on the transition temperatures. For example samples neutralized to the extent of 30%, 60% and 90% with Na ions (Fig. 2c, curves 1-~) have the same second transition temperature, in the region of 190 °, and not until the degree of neutralization is reduced to 15% does this temperature fall to 175 °. The samples neutralized to 15% and 30% do not crosslink however but continue to exhibit flow. I t seemed of interest to follow the course of the thermomechanical curves when the conditions of preparation of the pellets and the method of loading in the tests are varied. Figure 2 (curves 6 and 7) shows thermomechanical curves, recorded with periodic loading, of a graft copolymer containing 1.4% of maleic anhydride, neutralized to the extent of 60%. Curve 6 relates to a sample moulded under a pressure of 1870 kg/cm ~ and annealed at
2230
S. let. RAFIKOVet al.
100 ° for 20 min, and curve 7 to a sample hot-compression moulded at 100 ° under the same pressure. Comparison of the curves shows t h a t in the case of crystallizable polymers the method of preparation of the test specimen has a considerable effect on its thermomechanical characteristics. In the second instance there is no second transition temperature in the curve. The form of the curves in both instances is characteristic of crosslinkable polymers. In the first instance (curve 6) crossli~king begins during the time of the thermomechanical test [6], but in the second we are dealing with a sample t h a t became crosslinked during preparation of the pellet.
~,°C
260 Ot''-~l
I
~
m
-
I
2
220
2"
!
zl
Oi
J
I
/80
I
I001 f
c
6 0 ~ 1 0
I
,,,,2
Ioo
I
200
FIG. 2
"LI I
300
I qoo T,'C
30 80 90 D~eeeof neuf~I/zot/on , % FxG. 3
Fxo. 2. Thermomechanical curves of ionomers based on graft copolymers of ethylene with 11~ (a) 5.4~ (b) and 1.4~ (c) of maleie anhydride: /--original copolymers; 2--samples neutralized to the extent of 30yo; 3--60~ and d--90~; 3--repeated heat cycle; 5--sample 15yo neutralized; 6--sample 60~ neutralized, prepared by crompression moulding with subsequent annealing; 7--the same, but pellet prepared by hot compression moulding. FIG. 3. Dependence of Ts (1, 1') and Tt (2, 2') on the degree of neutralization with Na ions (1, 2) and Rb ions (1', 2') of random eopolymers of ethylene and methaerylic acid. Neutralization of the carboxylic acid copolymer of styrene and methacrylic acid (8 : 2) with Na and Rb ions alters the thermomechanical characteristics of the samples, the degree of change being dependent on the nature and q u a n t i t y of the cation. I t is seen from Fig. 3 t h a t at the same degree of neutralization the values of Ts and Tf of the ionomers containing Na ions are higher t h a n for the copolymer neutralized with Rb ions.
Concentration, distribution and degree of neutralization of carboxyl groups
2231
According to reference [7] crystalline and amorphous regions are present in the supermolecular organization of ionomers, and the amorphous regions contain chain segments with ionic groups disposed along the chain. These segments form so-called "ionic pockets" in which there are intra- and intermolecular associations containing carboxyl, carbonyl and carboxylate groups, and cations.
As a result of such cooperation it is possible that ionized carboxyl groups could be arranged opposite one another, so that electrostatic interaction will occur between them. In addition the polarizing effect of the metal ion on the oxygen of the carbonyl group will be superimposed. Therefore in addition to /00
J
0 ~ 120
I
I
I
[/iO
IGO
180
I 200 ~ °g
FTG. 4. Thermomechanieal curve of a random copolymer of styrene and methaerylie acid, treated with calcium acetate (1-3) and zinc acetate (4, 5) to degrees of neutralization of 30 ~g (1), 60~o (2, 4) and 90~ (3, 5). hydrogen bonding an important part is played in ionomers b y electrostatic forces between the macro-anions and the metal cations. Because interaction between ions is inversely proportional to the distance between them, which is dependent on the ionic radius, "ionic crosslinking" with the Na+ cation, which has a smaller ionic radius than the R b + ion, will be stronger. Hence the ionomers containing Na + will have higher transition temperatures than those containing R b +. As the degree of neutralization is increased the number of ionic crosslinkages increases also, and therefore the transition temperatures rise.
2232
S.R. RAFIKOVe~ al.
The situation is different when the carboxyl groups of the copolymer are neutralized with acetates of divalent metals. I t is seen from Fig. 4 that neutralization with Zn and Ca ions has practically no effect on the thermomeehanical characteristics of the ionomers. This can evidently be explained b y the fact t h a t 100
/ O log
I,TO
i 200
~ ..... i 250 300
1 36"0
I T,°C
Fro. 5. Thermomechanical curves of a block copolymer of styrene and methacrylic acid (1) and of this copolymer 90~ ncutralized~with Zn ions (2).
oj
in this instance the neutralizing agents are not divalent metal ions (M2+), b u t monovalent ions of the type M(CH3.CO0) +, which are formed predomin a n t l y when metal acetates dissociate in solution M(CHsCO0)~ ~ M{CH3. COO) + +CHACO0-.
I° i
i
I
0 i
I~O FIG. 6
I
i"
200
250
i
i
I
Fro. 7
FIo. 6. Thermomeehanical curves of a copolymer of 1-hexane and maleic anhydride (•); of the hydrolyzcdeopolymer 2 and of the latter neutralized to the extent of 20~o (3); 30~ (d); 50~/o (5); 70~/o (6) and 100~/o (7) with l~a ions. FIG. 7. Thermomechanical curves of samples neutralized to the extent of 5 ~ (•); 10~/~a(2); 20yo {3); 50~ (4, 7); 70~ (5, 8) ~nd 100~/o (6) with Cd, recorded with loads of 8.8 g/ram2 (1-6) and 21 g/mm~ (7, 8). Because of their large size the acetate groups of these monovalent ions screen the ionic charges and thus reduce the strength of the ionic erosslinkages, i.e. they exert a special plasticizing effect. This is confirmed b y the shape of the thermomechanical curves, which display all three states characteristic of thermoplasts, and also by the fact t h a t ionomers with a 30% degree of substitution with Ca and Zn ions are soluble in dioxan and dimethylformamide.
Concentration, distribution and degreo of neutralization of carboxyl groups
2233
Increase in the quantity of methacrylic acid and introducing it into the polystyrene chain in the form of a block alters the thermomechanical characteristics of the samples. It is seen from Fig. 5 that neutralization with Zn ions in these circumstances brings about a considerable shift in Tf and crosslinking of the polymer at temperatures ~above Tf. It seemed of interest to follow the change in the thermomechanical properties of a copolymer of 1-hexene with maleic anhydride, its carboxylic acid derivative and their sodium and cadmium salts. Figure 6 shows that replacement of the anhydride groups by carboxyl groups brings about additional intermolecular interaction by hydrogen bonding and consequently Tf of the carboxylic acid copolymer is 12° higher than Tf of the original copolymer. Neutralization of the copolymcr containing carboxyl groups with sodium ions (Fig. 6, curves 3-7) considerably changes the nature of the temperature dependence of the deformation of the samples. For example T~ of the 100% neutralized sample is almost 150 ° above Tf of the copolymer containing free carboxyl groups. When the copolymer is neutralized to the extent of 20~o, 30% and 50% (curves 3-5) regions of high-elastic deformation appear in the thermomechanical curves, T s varying between 155 ° and 255 °, depending on the degree of neutralization. When the degree of neutralization is further increased (70~o and above) again on]y one transition temperature, namely Tf, appears in the thermomechanical curves at a given load, but now in the region of 290 ° . These results can again be explained by formation of ionic bonds. As a result of neutralization of copolymers containing carboxyl groups intermolecular ionic bonds are formed, which in contrast to covalent bonds are thermally reversible. Therefore increase in temperature causes breakdown of these ionic crosslinkages and segments of the polymer chains acquire mobility, which appears on the thermomechanical curves as the softening point. When the temperature is further increased the copolymers flow like ordinary, uncrosslinked polymers. As the degree of neutralization is increased the frequency of ionic crosslinking increases, Tf and T s shift to higher temperatures in curves 4 and 5 (Fig. 6) and approach one another, and finally at 70-100°/o neutralization (curves 6 and 7) the frequency of ionic crosslinking is so high that the rigidity of the chain is increased considerably and as a result appreciab]e softening of the polymer does not occur under the loads used. This is indicated by decrease in deformation of the ionomers in the region of their softening points when the load is increased. In the thermomcchanical curve of the sample neutralized to the extent of 70% (curve 6) a high-elastic region again appears, at 320 °, which can evidently be explained by crosslinking by formation of covalent bonds (of the ketone type, by decomposition of salts, for example) [8]. This sparse network of covalent bonds breaks down at temperatures above 320 ° and the sample again flows. As would be expected, when the degree of neutralization is increased (curve 7) the crosslinking density increases. As a result of this the temperature
2234
S . R . RA~IKOV et al.
range of the high-elastic state broadens and when the samples are heated above 300 ° carbonization occurs. This is confirmed by visual observation. For example samples neutralized to the ext ent of 70% are dark-brown in colour and flow after having been heated at 330 °, while those neutralized to 100% are black and do not flow when t h e y are heat ed to 400 ° . Comparison of Figs. 6 and 7 shows t h a t neutralization with cadmium ions brings about an improvement in the thermomechanical characteristics of the ionomers at the same degree of neutralization. Merging of T s and Tf occurs at lower degrees of neutralization and also covalent crosslinkages are formed a t a lower percentage substitution. The fact t h a t repeated heating of the ionomers to temperatures 10-30 ° above Ts, with intermediate cooling to room temperature, causes practically no change in the thermomeehanical curves or the value of T s provides confirmation of the thermal reversibility of the ionic crosslinking. W h en the load is increased to 21 g/mm 2, instead of the usual lowering of the thermomechanical characteristics t h a t occurs at low degrees of neutralization, Tf is found to increase for ionomers with a high percentage substitution of t h e carboxyl groups. The greater deformation indicates t h a t as the load is increased the covalent bonds break, t he samples exhibit flow at higher temperatures and th ey do not crosslink. Translated by E. O. PHILLIPS REFERENCES
1. 2. 3. 4. 5. 6.
R. W. REES and D. VAUGHAN, Polymer Preprints 6: 287, 296, 1965 A. S. SEMENOVA and A. F. NIKOLAYEV, Plast. massy, 1~o. 10, 67, 1967 A. F. NIKOLAYEV and V. M. GAL'PERIN, Plast. massy, No. 8, 16, 1969 A. EISENBERG, Macromolecules 4: 125, 1971 S. POREI~KO, W. GABARU and J. KULESZA, J. Polymer Sci. A - I , 5: 1563, 1967 V. A. KARGIN, V. A. KABANOV and I. Yu. MARCHENKO, Vysokomol. soyed. 1: 94, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 1, 41, 1959) 7. J. S. WARD and A. V. TOBOLSKY, J. Appl. Polymer Sei. 11: 2403, 1967 8. G. L. SLONIMSKIT~V. A. KARGIN and L. I. GOLUBENKOVA, Zh. fiz. khim. 30: 2435, 2656, 1956; 31: 28, 1957
COPOLYMERS containing carboxyl groups are used extensively. The introduction of certain quantities of carboxyl and other acidic groups, and partial ionization of these with various mono- and divalent cations disturbs the supermolecular structure of the polymers and at the same time increases intermolecular interaction between the polymer chains, which has a marked effect on their physicomechanical and physicochemical properties. In these polymers in addition to Van der Waals forces and hydrogen bonding ionic interaction, which is thermally reversible, occurs. Consequently in the solid state ionomers are reminiscent of crosslinked polymers, but in the molten state they behave as typical thermoplasts [1]. The formation of this network of ionic bonds provides ionomers with a number of unique properties, such as high strength, rigidity, transparency, high resistance to solvents and good adhesion [2]. There is however no information in the literature on ionic interaction in polymers in the solid state and the effect of this interaction on the thermomechanical properties of ionomers, apart from a paper by Nikolayev and Gal'perin [3] on the heat-distortion characteristics of ionomers, and one by Eisenberg [4], in which problems associated with the glass temperature of ionic polymers are discussed. The present paper discusses the effect of the nature and concentration of the * Vysokomol. soyed. A15: No. 9, 1974-1981, 1973. 2225
2226
S . R . RAFIKOV et al.
cations on the thermomechanical properties of ionomers based on random copolymers of ethylene with acrylic and methacrylic acids, a graft copolymcr of ethylene and maleic acid, a random and a block copolymer of styrene and mcthacrylic acid, and an alternating copolymer of 1-hexene with maleic acid.
EXPERIMENTAL The random copolymers of ethylene with acrylic and mettlacrylic acid were prepared in an autoclave a t 120 ° and a pressure of 1400 arm, with tert. b u t y l peroxide as initiator, a t low degrees of conversion. The copolymers were purified b y two reprecipitations in ethanol from hot, filtered solutions in xylene. The intrinsic viscosities of the original copolymers were 0.35-0.40 dl/g a t 70 ° in o-xylene, and the concentration of the acids in the copolymers was 3-4 mole Yo. Graft copolymers of ethylene and maleic anhydride were prepared in the presence of benzoyl peroxide a t 110 ° in solution in o-xylene b y the method of reference [5]. To obtain the pure copolymer the reaction product was twice-reprecipitated in ethanol from hot solutions in xylene, washed several times on the filter and dried i n v a c u o at 60 °. The polyethylene used for this purpose had a molecular weight of 3 × 104 and the ethylene-maleic anhydride graft copolymers contained 11.0, 5.4 a n d .1-4 mole ~o. of maleic anhydride. The random styrer~e-methacrylic acid copolymer (8 : 2) was prepared b y polymerization in bulk at 60 °, with 0.01 ~ b y weight of azobisisobutyronitrile (AZBN} as initiator. W h e n a degree of conversion of 10yo was reached the reaction mixture was diluted with dioxan and precipitated in petroleum ether. The copolymer thus precipitated was washed several times with flesh portions of petroleum ether a n d dried at 60 ° i n v a c u o . The molecular weight of the copolymer, determined b y the light-scattering method in dioxan, was 8-9 × 105. F o r preparation of a block copolymer of styrene and methacrylic acid a polystyrene of molecular weight 3 × 10 ~ was used. The concentration of the acidic monomer in the block copolymer was 3 0 ~ b y weight. 1-Hexene was copolymerized with maleic anhydride in previously evacuated ampoules in solution in acetone a t 70 °. The concentration of initiator (AZBlq) was 5 × 10 -~ mole/1. a n d the total concentration of monomers was 2 mole/1. The 1 : 1 copolymer was precipitated b y ethyl ether from solution in acetone. T o ' o b t a i n a copolymer of maleic acid the anhydride copolymer was hydrolyzed with HC1 for 5 hr a n d the product was washed with water to neutral reaction, dried and purified b y two reprecipitations in ethyl ether from solution in acetone. The molecular weight of the copolymer, determined b y light scattering in dimethylformamide a t an angle of 90 °, was 28,000. The structure of all the copolymers was confirmed b y infrared spectroscopic analysis in a UR-20 spectrometer, and the acid content of the copolymers was found b y elementary ana]ysls for carbon and b y potentiometric titration. The styrene-methacrylic acid copolymer was neutralized b y treating 1 ~/~ solutions of the copolymer in dioxan with the calculated quantities of methanolic solutions of N a O H or R b O H , or with acidified solutions of calcium or zinc acetate at 40 °. The carboxyl groups in copolymers of ethylene with acrylic a n d methacrylic acids were converted b y addition of solutions of the hydroxides or acetates of various metaIs in alcohol or dimethylformamide to hot solutions of t h e copolymers in xylene. I n this w a y products containing Na, Rb, K, Zn, Cd and Co ions were obtained. Salts of a copolymer of ethylene a n d maleic acid were obtained b y neutralizing the ethylene-maleic anhydride graft copolymer b y heating the l a t t e r under reflux in solution in xylene, with the calculated q u a n t i t y of I~aOH dissolved in a 9 : 1 mixture of alcohol and water. Copolymers of 1-hexene and maleic acid neutralized to different extents with sodium
Concentration, distribution and degree of neutralization of carboxyl groups
2227
hydroxide were obtained by hydrolysis of the anhydride copolymer by refluxing with the calculated quantities of NaOH for 3 hr, followed by two reprecipitations from aqueous solution in a 1 : 3 mixture of dioxan and ether and drying in vacuo at 40 °. The hydrogen of the carboxyl groups was replaced by cadmium by addition to an acetone solution of the original copolymer of a 9 : 1 aqueous acetone solution of cadmium acetate. The product was washed with water and twice reprecipitated from solution in acetone by ethyl ether, and dried in vacuo. The presence of neutralized carboxyl groups was confirmed by the occurrence of a band in the region of 1570 cm -1 in the infrared spectrum. The quantity of metal introduced was determined by flame photometry and the degree of neutralization of the carboxyl groups was calculated from the analytical results. The thermomechanieal tests were made with powdered samples compressed to form pellets of diameter 6 mm and thickness 2 mm. The pressure was 1850 kg/cm 2 and the time in the press was 5 min. In order to release the internal stresses brought about by themoulding process the pellets were annealed at 100° for 20 min. The two methods of constant and periodic load application were used in the thermomechanical tests. In both methods the load was the same at 8.8 g/mm ~. The temperature was raised uniformly at the rate of ~ 2.5 deg/min. The transition temperatures (softening point T s and flow temperature Tt) were determined from the points of intersection of the tangents to the initial parts of the inflexions in the thermomechanical curves, the results of two or three determinations being averaged. The effect of the experimental conditions (annealing of the pellets, the loading routine and the magnitude of the load) on the transition temperatures was tested with the styrenemethacrylic acid copolymer (Table 1). It is seen from Table 1 that the methods of loading and of preparation of the test specimen do not have any important effect on the transition temperatures of the copolymer Any difference in the transition temperatures is explained by difference in the test methods. RESULTS AND DISCUSSION I o n o m e r s based on ethylene copolymers. I t is seen from Fig. 1 t h a t Tf of the neutralized copolymers is n o t d e p e n d e n t on the n a t u r e of the cation or its c o n c e n t r a tion, or on the n a t u r e of the acid. I t is the same for all the samples a n d is the same as Tf of t h e original copolymer. This can be a t t r i b u t e d t o the low c o n c e n t r a tion of acid in t h e eopoly~ner (3-4 mole %), as a result of which t h e ionic b o n d s f o r m e d are so widely spaced o u t t h a t t h e y do n o t affect the t h e r m o m e e h a n i c a l characteristics of t h e ionomers. The ionic bonds nevertheless do impair the physieomechanical characteristics of t h e ionomers, as is seen f r o m Table 2. I t is seen f r o m Table 2 t h a t neutralization of t h e c o p o l y m e r increases the breaking s t r e n g t h considerably, Co(CH3CO0)2 affecting the s t r e n g t h to a g r e a t e r extent than NaOH. I n c r e a s e in t h e molecular weight o f the e t h y l e n e - m e t h a c r y l i c acid c o p o l y m e r t o 30,000 did n o t bring a b o u t a n y appreciable difference in the t h e r m o m e e h a n i c a l characteristics of the u n t r e a t e d c o p o l y m e r or of neutralized samples (Fig. 1, curves 6 a n d 9). I n this instance h o w e v e r t h e p h y s i c o m e c h a n i c a l characteristics o f t h e ionomers are impaired, t h o u g h it is true these changes a t t h e high molecular w e i g h t are less. T h u s molecular weight does n o t exert an i m p o r t a n t effect on the thermomech.-
2228
S. R, RAFI~rOVet al.
anical characteristics o f ionomers based on r a n d o m copolymers of ethylene and m e t h a c r y l i c acid. The situation is different in the case of graft c o p o l y m e r s of ethylene. I t is seen f r o m Fig. 2 t h a t t h e i n t r o d u c t i o n o f 1-4% a n d 5.4% of an acid does not alter t h e I00
-8,Z8 1,2,~'.~
0
I
I
I
2g
~g
8g
ag
-a,w
log r,o¢
~FIG. 1. Thermomeehanical curves of ionomers based on random copolymers of ethylene, neutralized with various ions: /--polyethylene; 2--copolymer of ethylene and methacrylic acid of low molecular weight; 3--copolymers neutralized to the extent of 30yo; 4 - - 6 0 ~ a n d 5 - - 9 0 ~ with Na ions; 6--eopolymer 90~o neutralized with zinc ions; 7--90yo with Co ions; 8--ethylene-methacrylic acid copolymer of high molecular weight; 9--copolymer 9 0 ~ neutralized with Zn ions; 10--ethylene-acrylic acid copolymer; //-acrylic acid copolymer 90 ~ neutralized with Zn ions. t r a n s i t i o n t e m p e r a t u r e of the copolymer, which are e v i d e n t l y associated with melting of the p o l y e t h y l e n e , a n d only in the case of a graft c o p o l y m e r containing 11% o f acid does T~ fall b y ~ 1 0 °. W h e n t h e graft copolymers are neutralized additional transition t e m p e r a t u r e s appear, due p r o b a b l y to f o r m a t i o n of ionic crosslinkages, the t e m p e r a t u r e s being d e p e n d e n t on the acid c o n t e n t a n d on the degree o f neutralization. TABLE 1.
EFFECT
OF T H E M E T H O D
OF L O A D I N G A ~ D
ON T H E T R A N S I T I O N T E M P E R A T U R E S
Method of preparation of specimen treatment Without annealing Annealing Hot compression Hot compression * Without annealing
load, I °C P' g/mm~ ] T, 8-8 8.8 8.8 8-8 21
100 110 110
* W i t h f a r t h e r a n n e a l i n g a t 120 ° for 20 m i n .
OF P R E P A R A T I O N
OF T H E
SPECIMENS
OF A S T Y R E N E - - M E T H A C R Y L I C A C I D C O P O L Y M E R
Constant load
Periodic loading
time, rain
%
T~
T~
T~
20 5 5
118 120 120 119 113
,182 183 182 181 177
113 113 112 113 110
180 180 179 180 175
Concentration, distribution and degree of neutralization of carboxyl groups
2229
•Whereas neutralization of the copolymer containing 11% of maleic anhydride to the extent of 30% and 60% results in the appearance of a second transition in the region of 154 ° and 170 ° respectively (Fig. 2a, curves 2 and 3), after which the samples are crosslinked, this second transition temperature does not appear after 90% neutralization and the sample crosslinks immediately when heated to 120 ° (Fig. 2a, curve 4). These crosslinkages are so strong that they are not broken when the polymer is heated to 420 ° . T A B L E 2. P H Y S I e O M E C H A N I C A L C H A R A C T E R I S T I C S OF ETHYLENE--M_ETHACRYLIC ACID COPOLYMERS : N E U T R A L I Z E D "WITH S O D I U M A N D COBALT I O N S
Neu-
Degree of neutraliing agent zation,
traliz-
0
NaOH
30 60 90
Tensile strength at break,
kg/cm2 53"5* 72"3 99'8 108"6
Elongation at break, % 47"0 37"5 28-0 24-2
Neutralizing agent
Co(CHaCO0)~
Degree Tensile Elongaof neu- strength tion at tralizaat break, break, kg/em 2 % tion, 30 60 90
91"2 140-7 150'1
39'5 44"0 63"7
* The low tensile strength is due to the low molecular weight ( ~ 8000) of the origi~al copolymer.
When the graft copolymer containing 5.4% of maleic anhydride in the side chains is neutralized to the extent of 30% and 60% the second transition temperatures are moved to 185 ° and 221 ° respectively, and as in the first instance when the degree of neutralization is 90% this transition temperature is not seen (Fig. 2b). In contrast to the copolymer containing 11°/0 when the temperature is raised to 400 ° the network is broken down and the polymer acquires flow. I t is interesting to note the non-reproducibility of the second transition temperature after repeated heating of the sample to this temperature and subsequent cooling to room temperature (Fig. 2b, curve 3), evidently because of irreversible scission of some of the intermolecular bonds. When the q u a n t i t y of acid in the copolymer is still less (1-4%) neutralization has a smaller effect on the transition temperatures. For example samples neutralized to the extent of 30%, 60% and 90% with Na ions (Fig. 2c, curves 1-~) have the same second transition temperature, in the region of 190 °, and not until the degree of neutralization is reduced to 15% does this temperature fall to 175 °. The samples neutralized to 15% and 30% do not crosslink however but continue to exhibit flow. I t seemed of interest to follow the course of the thermomechanical curves when the conditions of preparation of the pellets and the method of loading in the tests are varied. Figure 2 (curves 6 and 7) shows thermomechanical curves, recorded with periodic loading, of a graft copolymer containing 1.4% of maleic anhydride, neutralized to the extent of 60%. Curve 6 relates to a sample moulded under a pressure of 1870 kg/cm ~ and annealed at
2230
S. let. RAFIKOVet al.
100 ° for 20 min, and curve 7 to a sample hot-compression moulded at 100 ° under the same pressure. Comparison of the curves shows t h a t in the case of crystallizable polymers the method of preparation of the test specimen has a considerable effect on its thermomechanical characteristics. In the second instance there is no second transition temperature in the curve. The form of the curves in both instances is characteristic of crosslinkable polymers. In the first instance (curve 6) crossli~king begins during the time of the thermomechanical test [6], but in the second we are dealing with a sample t h a t became crosslinked during preparation of the pellet.
~,°C
260 Ot''-~l
I
~
m
-
I
2
220
2"
!
zl
Oi
J
I
/80
I
I001 f
c
6 0 ~ 1 0
I
,,,,2
Ioo
I
200
FIG. 2
"LI I
300
I qoo T,'C
30 80 90 D~eeeof neuf~I/zot/on , % FxG. 3
Fxo. 2. Thermomechanical curves of ionomers based on graft copolymers of ethylene with 11~ (a) 5.4~ (b) and 1.4~ (c) of maleie anhydride: /--original copolymers; 2--samples neutralized to the extent of 30yo; 3--60~ and d--90~; 3--repeated heat cycle; 5--sample 15yo neutralized; 6--sample 60~ neutralized, prepared by crompression moulding with subsequent annealing; 7--the same, but pellet prepared by hot compression moulding. FIG. 3. Dependence of Ts (1, 1') and Tt (2, 2') on the degree of neutralization with Na ions (1, 2) and Rb ions (1', 2') of random eopolymers of ethylene and methaerylic acid. Neutralization of the carboxylic acid copolymer of styrene and methacrylic acid (8 : 2) with Na and Rb ions alters the thermomechanical characteristics of the samples, the degree of change being dependent on the nature and q u a n t i t y of the cation. I t is seen from Fig. 3 t h a t at the same degree of neutralization the values of Ts and Tf of the ionomers containing Na ions are higher t h a n for the copolymer neutralized with Rb ions.
Concentration, distribution and degree of neutralization of carboxyl groups
2231
According to reference [7] crystalline and amorphous regions are present in the supermolecular organization of ionomers, and the amorphous regions contain chain segments with ionic groups disposed along the chain. These segments form so-called "ionic pockets" in which there are intra- and intermolecular associations containing carboxyl, carbonyl and carboxylate groups, and cations.
As a result of such cooperation it is possible that ionized carboxyl groups could be arranged opposite one another, so that electrostatic interaction will occur between them. In addition the polarizing effect of the metal ion on the oxygen of the carbonyl group will be superimposed. Therefore in addition to /00
J
0 ~ 120
I
I
I
[/iO
IGO
180
I 200 ~ °g
FTG. 4. Thermomechanieal curve of a random copolymer of styrene and methaerylie acid, treated with calcium acetate (1-3) and zinc acetate (4, 5) to degrees of neutralization of 30 ~g (1), 60~o (2, 4) and 90~ (3, 5). hydrogen bonding an important part is played in ionomers b y electrostatic forces between the macro-anions and the metal cations. Because interaction between ions is inversely proportional to the distance between them, which is dependent on the ionic radius, "ionic crosslinking" with the Na+ cation, which has a smaller ionic radius than the R b + ion, will be stronger. Hence the ionomers containing Na + will have higher transition temperatures than those containing R b +. As the degree of neutralization is increased the number of ionic crosslinkages increases also, and therefore the transition temperatures rise.
2232
S.R. RAFIKOVe~ al.
The situation is different when the carboxyl groups of the copolymer are neutralized with acetates of divalent metals. I t is seen from Fig. 4 that neutralization with Zn and Ca ions has practically no effect on the thermomeehanical characteristics of the ionomers. This can evidently be explained b y the fact t h a t 100
/ O log
I,TO
i 200
~ ..... i 250 300
1 36"0
I T,°C
Fro. 5. Thermomechanical curves of a block copolymer of styrene and methacrylic acid (1) and of this copolymer 90~ ncutralized~with Zn ions (2).
oj
in this instance the neutralizing agents are not divalent metal ions (M2+), b u t monovalent ions of the type M(CH3.CO0) +, which are formed predomin a n t l y when metal acetates dissociate in solution M(CHsCO0)~ ~ M{CH3. COO) + +CHACO0-.
I° i
i
I
0 i
I~O FIG. 6
I
i"
200
250
i
i
I
Fro. 7
FIo. 6. Thermomeehanical curves of a copolymer of 1-hexane and maleic anhydride (•); of the hydrolyzcdeopolymer 2 and of the latter neutralized to the extent of 20~o (3); 30~ (d); 50~/o (5); 70~/o (6) and 100~/o (7) with l~a ions. FIG. 7. Thermomechanical curves of samples neutralized to the extent of 5 ~ (•); 10~/~a(2); 20yo {3); 50~ (4, 7); 70~ (5, 8) ~nd 100~/o (6) with Cd, recorded with loads of 8.8 g/ram2 (1-6) and 21 g/mm~ (7, 8). Because of their large size the acetate groups of these monovalent ions screen the ionic charges and thus reduce the strength of the ionic erosslinkages, i.e. they exert a special plasticizing effect. This is confirmed b y the shape of the thermomechanical curves, which display all three states characteristic of thermoplasts, and also by the fact t h a t ionomers with a 30% degree of substitution with Ca and Zn ions are soluble in dioxan and dimethylformamide.
Concentration, distribution and degreo of neutralization of carboxyl groups
2233
Increase in the quantity of methacrylic acid and introducing it into the polystyrene chain in the form of a block alters the thermomechanical characteristics of the samples. It is seen from Fig. 5 that neutralization with Zn ions in these circumstances brings about a considerable shift in Tf and crosslinking of the polymer at temperatures ~above Tf. It seemed of interest to follow the change in the thermomechanical properties of a copolymer of 1-hexene with maleic anhydride, its carboxylic acid derivative and their sodium and cadmium salts. Figure 6 shows that replacement of the anhydride groups by carboxyl groups brings about additional intermolecular interaction by hydrogen bonding and consequently Tf of the carboxylic acid copolymer is 12° higher than Tf of the original copolymer. Neutralization of the copolymcr containing carboxyl groups with sodium ions (Fig. 6, curves 3-7) considerably changes the nature of the temperature dependence of the deformation of the samples. For example T~ of the 100% neutralized sample is almost 150 ° above Tf of the copolymer containing free carboxyl groups. When the copolymer is neutralized to the extent of 20~o, 30% and 50% (curves 3-5) regions of high-elastic deformation appear in the thermomechanical curves, T s varying between 155 ° and 255 °, depending on the degree of neutralization. When the degree of neutralization is further increased (70~o and above) again on]y one transition temperature, namely Tf, appears in the thermomechanical curves at a given load, but now in the region of 290 ° . These results can again be explained by formation of ionic bonds. As a result of neutralization of copolymers containing carboxyl groups intermolecular ionic bonds are formed, which in contrast to covalent bonds are thermally reversible. Therefore increase in temperature causes breakdown of these ionic crosslinkages and segments of the polymer chains acquire mobility, which appears on the thermomechanical curves as the softening point. When the temperature is further increased the copolymers flow like ordinary, uncrosslinked polymers. As the degree of neutralization is increased the frequency of ionic crosslinking increases, Tf and T s shift to higher temperatures in curves 4 and 5 (Fig. 6) and approach one another, and finally at 70-100°/o neutralization (curves 6 and 7) the frequency of ionic crosslinking is so high that the rigidity of the chain is increased considerably and as a result appreciab]e softening of the polymer does not occur under the loads used. This is indicated by decrease in deformation of the ionomers in the region of their softening points when the load is increased. In the thermomcchanical curve of the sample neutralized to the extent of 70% (curve 6) a high-elastic region again appears, at 320 °, which can evidently be explained by crosslinking by formation of covalent bonds (of the ketone type, by decomposition of salts, for example) [8]. This sparse network of covalent bonds breaks down at temperatures above 320 ° and the sample again flows. As would be expected, when the degree of neutralization is increased (curve 7) the crosslinking density increases. As a result of this the temperature
2234
S . R . RA~IKOV et al.
range of the high-elastic state broadens and when the samples are heated above 300 ° carbonization occurs. This is confirmed by visual observation. For example samples neutralized to the ext ent of 70% are dark-brown in colour and flow after having been heated at 330 °, while those neutralized to 100% are black and do not flow when t h e y are heat ed to 400 ° . Comparison of Figs. 6 and 7 shows t h a t neutralization with cadmium ions brings about an improvement in the thermomechanical characteristics of the ionomers at the same degree of neutralization. Merging of T s and Tf occurs at lower degrees of neutralization and also covalent crosslinkages are formed a t a lower percentage substitution. The fact t h a t repeated heating of the ionomers to temperatures 10-30 ° above Ts, with intermediate cooling to room temperature, causes practically no change in the thermomeehanical curves or the value of T s provides confirmation of the thermal reversibility of the ionic crosslinking. W h en the load is increased to 21 g/mm 2, instead of the usual lowering of the thermomechanical characteristics t h a t occurs at low degrees of neutralization, Tf is found to increase for ionomers with a high percentage substitution of t h e carboxyl groups. The greater deformation indicates t h a t as the load is increased the covalent bonds break, t he samples exhibit flow at higher temperatures and th ey do not crosslink. Translated by E. O. PHILLIPS REFERENCES
1. 2. 3. 4. 5. 6.
R. W. REES and D. VAUGHAN, Polymer Preprints 6: 287, 296, 1965 A. S. SEMENOVA and A. F. NIKOLAYEV, Plast. massy, 1~o. 10, 67, 1967 A. F. NIKOLAYEV and V. M. GAL'PERIN, Plast. massy, No. 8, 16, 1969 A. EISENBERG, Macromolecules 4: 125, 1971 S. POREI~KO, W. GABARU and J. KULESZA, J. Polymer Sci. A - I , 5: 1563, 1967 V. A. KARGIN, V. A. KABANOV and I. Yu. MARCHENKO, Vysokomol. soyed. 1: 94, 1959 (Translated in Polymer Sci. U.S.S.R. 1: 1, 41, 1959) 7. J. S. WARD and A. V. TOBOLSKY, J. Appl. Polymer Sei. 11: 2403, 1967 8. G. L. SLONIMSKIT~V. A. KARGIN and L. I. GOLUBENKOVA, Zh. fiz. khim. 30: 2435, 2656, 1956; 31: 28, 1957