Densely crosslinked polymer compositions based on various types of dimethacrylates

Polymer ScienceU.S.S.]I. V,]. 23, 1~'o.10, pp. 2438-2445, 1 9 8 1 Printed in Poland

0032-3950/81/102488-08507.50/0 © 1982 Pergamon Press Ltd.

DENSELY CROSSLINKED POLYMER COMPOSITIONS BASED ON VARIOUS TYPES OF DIMETHACRYLATES* ZH. D. CHERNOVA, T. D. GLUMOVA, M. F. LEBEDEVA, YE. V. KRUCHINI~A, L. V. KRASN~R, L. S. ANDRIA_~'OVA, S. K. Z ~ o v , T. I. BomsovA and G. P. B~ONOVSKAYA High Polymers Institute, U.S.S.R. Academy of Sciences

(Received 13 June 1980) Mixt~tres of various dimethacrylates with polar vinyl monomers were conv e t t e d by radical polymerization to densely erosslinked polymer compositions; their thermal and deformation properties were studied. Dielectric relaxation was used t~ study the structure of the compositions based on triethylene glycol dimethacrylate with m e t h y l acrylate.

Trot combination of a rigid polymer network with linear or slightly crosslinked rubber-like polymers is known to yield compositions possessing a complex of valuable properties [1]. Where thermodynamically miscible erosslinked and linear polymers are combined, or immiscible ones are forced to combine, one can produce optically translucent polymeric materials. This was confirmed on the synthesis of glasses in the combined polymerization of diisoeyanates (M1) with a number of polar monomers (M2) in the presence of tertiary amines [2]. The specifics of producing these densely crosslinked compositions are cyelotrimerization of the diisocyanate (DIC) in M2 as solvent to a network followed, after the almost, complete exhaustion of the 5T(30-groups, by initiation of polymerization of the M2 fixed in the network and orientated as a result of polar reaction. The above eon,~ecutive processes gave rise to peculiar mutually penetrating cvosslinked (MPC) systems [3]; the most interesting were the properties of glasses ill which the rigid polyisocyanate network was combined with a linear rubber-like polymer at a M1 : 312 ratios of 1 : 2 to 1 : 4 (e.g. of a toluylene DIG system with propylene sulphide or methyl acrylate). These materials had a relatively high softening temperature (520 ° ) and satisfactory deformation-strength properties [2]. The main disadvantage of the compositions was their yellow eolour which varied in intensity a~d was due to the presence of the diisoeyanurate rings. The colour intensified on heating thus limiting the practical uses of the glass, but it did not exclude other uses of this peculiar material. We studied the possibility of producing this type of material by synthesizing densely crosslinking polymer compositions from monomers forming a network * Vysokomol. soyed. A28: No. 10, 2244-2250, 1981.

2438

Polymer compositi.:)ns based on various types of diraethacrylates

2439

(Mx) using various d i m c t h a c r y l a t e s , a m o n g s t t h e m the oligo-ether a c r y l a t e s (OEA). The M2 were m o n o - f u n c t i o n a l polar v i n y l m o n o m e r s a n d t h e Mx:M~ molar ratios were 1 : 1-1 : 4. The M1-M ~ m i x t u r e s were p o l y m e r i z e d with radicalt y p e initiators. W h e r e there are large relative r e a c t i v i t y differences (rl>>r~) t h e p r o d u c t i o n of t h e p o l y m e r composition o u g h t to be similar to some e x t e n t t o t h e i b r m a t i o n o f glasses based on DIC. T h e 1~ 1 p o l y m e r i z a t i o n will be d o m i n a n t a t t h e s t a r t a n d give a dense network; this will be followed b y the M 2 h o m o p o l y m e r i z a t i o n a n d the result will be a composition similar in s t r u c t u r e an interlaced l~etwork t y p e of M:PC [2, 3]. T h e homo- a n d c o p o l y m e r i z a t i o n s of p o l y f u n c t i o n a l m o n o m e r s capable of crosslinking, especially of the O E A , has been studied in some detail [4]; nevertheless, there was little i n f o r m a t i o n a b o u t the relative reactivities o f d i m e t h a c r y l a t e s in their radical copolymerizations w i t h various m o n o m e r s . W e s y n t h e s i z e d densely crosslinked compositions in radical p o l y m e r i z a t i o n s o f d i m e t h a c r y l a t e m i x t u r e s w i t h various polar m o n o m e r s h a v i n g k n o w n r~ a n d r 2, a n d also those pairs for w h i c h this i n f o r m a t i o n was u n k n o w n . EXPERIMENTAL

The following compotmds were used: for Mx were the dimethaerylates of diphenylolp~.x)paue (DPPDM), of ethylene glycol (EGDM), of triethylene glycol (TGM-3), and of tetraethylene glycol (TGM-4). As M2 we used methyl acrylate (MA), methyl metham*ylate (IM:MA), ethyl acrylate (EA), butyl acrylate (BA), ethoxyethyl acrylate (EEA), vinyl acetate (VA), tuld N-vinyl pyrrolidone (VP). The DPPDM and the TGM-3 were synthesized as described m the Russian Authors' Cert. No. 327162, 1972, and by Sivergin and co-workers [6]; tech.~ical TGM-3 and TGM-4 were also used and were a mixture of the dimethacrylates of mono-, di-, tri-, tetra- and penta-ethylem- glycols respectively, having wt. % contents of: TGM-3 (first batch) TGM-3 (second b~ltc]~) TGM-4

7-8 18 2

15 23 12

48 42 16

23-25 24 47

6-7 -

-

10

The EGDM was 99.9O/o pure, n~°=1. 4545, acid number 0.0026. The monofunctiolml n~,~nolner.~, i.e. MA, MMA, BA, EEA, VA, VP were purified by the known methods. A glass ampoule wa.s filled with the initiator and the M~ and M~ metered by syringe ~ d e r ~z counter-current of inert g~s. The ampoule was sealed and shaken 10 hr at rooru v,,mperature. Th(~ contents wer~ ~hen transferred by syringe into a 60 × 60 × 4 mm mould filled with inert gas and sealed. Tht, mould was placed iu an oven and kept at tim required ~,.,.~mperature, which depended ~n the initiator type, tultil the whole volume had gelled. This was followed by cooling to room temperature and then placing back in the oven for 48 hr. This caused the samples to set and they were the~ subjected to stepwise heating t~, 373-413°K. The heating conditions varied according to the type and ratio of M1 : Mr, and ~,l~e type and concentration of the initiator. Any disturbance of the moulding conditions, ,.~specially at the start, led to sy~(,resis and cracking of the samples. Tlm compositions produced in optimal conditions were solid or flexible plastics; this depended on the MI:M~ ratio and on their chemical nature. The deformation-strength properties (strength fs, elongation at break sb and elasticity modulus E) were determined on the "Instron" universal fracturing machine at room temperature and at a 8 × 10 - I ram/see stretching rate. The thermomcctmnical curves were got by the penetration method on a n automatic

instrument

[7].

Z~r. D. C m ~ n o v A et ~ .

2440

RESULTS We established on the DPPDM-MA compositions, by an indirect method f r o m t h e a m o u n t o f u n p o l y m e r i z e d M A a t v a r i o u s c o n v e r s i o n s t a g e s , t h a t 1~I~ w a s m u c h m o r e r e a c t i v e t h a n M~. T h e c o m p o s i t i o n o f t h e p r o d u c t a s a f u n c t i o n o f c o n v e r s i o n s is s h o w n f o r M~ : M ~ 1 : 2 i n F i g . 1. O n e g a t h e r s f r o m t h i s r e s p o n s e line that the DPPDIK polymerization was dominant in early stages under the s e l e c t e d e x p e r i m e n t a l c o n d i t i o n s a n d t h a t : > 5 0 % o f D P P D M , a n d a b o u t 13~/~ of MA were present in the composition at 40% conversion. These findings indic a t e d t h e c o m p o s i t i o n s t o h a v e a s t r u c t u r e close t o a M P C o f t h e i n t e r l a c e d network type. The network produced by the DPPDM was however much denser and the composition containing it therefore had unsatisfactory deformations t r e n g t h c h a r a c t e r i s t i c s (see T a b l e ; c o m p o s i t i o n s 1 a n d 2). B r i t t l e m a t e r i a l s w e r e I)~.F0~MATION-STRENGTIC[ P R O P E R T I E S

OF C O M P O S I T I O N S

BASED

ON

VARIOUS

DI~TH-

ACRYLATES

Composition, No.

M1

i M1 : M=,

M2

mole

Max. heating temp., oK

Thennomechanical transition temp., °K T1

1 2

3 4

5 6 7 8

9 10 11 12 13 14 15 16 17 18

i9 20 21 22 23

DPPDM DPPDM DPPDM EGDM EGDM T G M -3 TGM-3 TGM- 3* TGM-3 TGM-3 TGM-3

TGM-3 t TGM-3 TOM-3 TGM-3 TGM-3 TGM-3 TGM-3 TGM-3 TGM-3 TGM-4 TGM-4 TGM-4

MA MA EEA MA EEA MA EA MA MA M.A~-MMA MA-~ MMA

1:3 1:4 1:2 1:2 1:2 1:2 1:1 1.2 1:2 1 : 0.4 : 1.6 1:0"4:1.6

MA MA MA MA VA VP EEA

1:2 1:2 1:3 1:4 1:2 1:2 1:2

MA V'P

1:2 ] 1:2

373 373 373 373 373 373 373 388 413 393 413 373 413 373 413 373 373 373 413 373 393 373 393

310 353 368 382 399

349 394 363 387 367 400 307 35O 335

fs$~

11 O /

E*.

MPa

8=,/o

GPa

31.4 34"3 25-5 24.5 17.2 51.0 33.4 49-1 64.7 5~0 59.8 42.7 3@9 41.2 6~5 50.0 58.4 62.3 69.7 18.6 43.2 48.1 48.6

1.2 2.0 2-0

2.3(

[ T2 608 610 575 651 534 615 593 611 613 594 632 527 551 487 497 495 464 538 564 533 539 573 608

1.0 3-5

4.5 4.9 3.6 4.7 3.8 4-0 2-3 1.6 2.2 3.0 2.4 2.9 3.6 3.7 6.1 3.3 3.6 4.2

1.8t 1.44 2.41 @7"

1.9' 1.2]

1.8( 2.6( 1.6(

2.6(, 2.2" 2.2~

2.71 2-6( 3.0~ 2.9( 2-94 2.1~ 0.64 1.9( 2.4~ 1.8(

* The fa, ¢~ and E-values were measured a t room temperature. They changed greatly at 423°K and were 9.3, 4.5~ 0'31, v~ad15-7, 8-(}and 0.55 respectively for compositions !5 and 19. ? TOM-3 of industrial batch 1. $ TGM-3 of industrial batch 2.

Polymer compositions based on various types of dimethacrylates

2441

also got when EGDM was used as M1 (composition 4). The compositions based on I)PPD]K and EGDM had unsatisfactory physico-meehanical properties when E E A was used as M2 (compositions 3, 5). I t appears t h a t EEA, although having a low glass temperature (Tg----213°K) cannot compensate for the brittleness of dense networks. Al,~m

6

22

240

~7 760

80-

~.~0.2 It. I 0

L

I

20

i

;

qO ~.,%

A /2 350

Fro. 1

450

/

-j

550

7],K

Fro. 2

Fro. 1. The composition as a function of conversion; DPPDM DPPDM:MA molar ratio

=1 : 2 at the start. FIG. 2. Typical thermomechanical curves for polymer compositions based on EA and monofimction~flmonomers obtained by the penetration method. The specific load on the indentation tool was 9. 8 MPa. The numbers against the curves are the same as those given in the Table for the compositions. Compositions having a similar MPC structure formed from a pair of monomers known to hwve strongly differing copolymerization constants (e.g. TGM-3~-VP [8] or TGM-3~-VA [4]) and a M 1 produce a more elastic network owing to the chains between network joints being longer and more flexible. In such a ease the compositions had better physico-mechanical characteristics (compositions 18, 19). Fairly satisfactory the~nal and deformation-strength properties were got when OEA mixtures with MA@ere polymerized (compositions 6, 8, 9, 14-17, 22). Although we did not determine the copolymerization constants of these monomers, one would expect them not to differ greatly. Nevertheless, the properties of these compositions were similar to those based on EA with VP or VA, as the Table shows. These materials combine high melting temperatures resulting from their network structure, with good physico-mechanical properties which are due to t h e elasticity of the PM_A. For example, a number of compositions based on TGM-3 and M~, produced under identical conditions, showed an improvement in strength and deformation when the MA content was increased. The reason is most probably

2442

Z~. D. CtC~TOVA

e~ af.

that given by Korol'ev [9] according to which the structure of the compositions, liable to gel during early conversion stages, will depend on the chemistry of the comonomers and also the physical conditions of moulding. We stated earlier on that the heating conditions strongly affect the properties of the materials. Raising the upper temperature limit greatly improved the deformation-strength properties of the compositions (8-11, 14, 15). Please note that such a condition reduced these properties bl the case of the TGM-3 homopolymer (compositions 12, 13). The results given in the Table (compositions 5-9~ 14-20, 22, 23) and Fig. 2 show that most of the compositions based on TGM-3 and TGM-4 have two transition temperatures; the first in the case of TGM-3 and MA is T l = 3 4 9 399°K, the second T~=613°K where pure TGM-3 (composition 6) or its industrial batch 1 (compositions 8, 9) were used. T~ was at 493°C where TGM-3 industrial batch 2 was used (compositions 14-16). Note that the samples having the low temperature transition normally possessed improved strength properties at room temperature, especially a better elongation at break. The systems without such a transition were very brittle, i.e. eb ~<2 % (with the exception of ternary systems 10, 11). An upper temperature limit increase displaced T~ into the higher temperature range (compositions 8, 9, 14, 15). Some conclusions about the molecular structure and the morphology of the compositions based on TGM-3 (batch 2) with MA were obtained by the molecular mobility study of chains by the dielectric relaxation method. These tests were carried out in the 0.1-100 kHz range and the temperature dependence of the dielectric loss, tan ~, was examined on the pure polymer components as well as on the compositions having various component ratios. Figure 3 contains the respective curves produced at 1 kHz. Two dielectric relaxation ranges had been detected earlier [10] on linear PMA, namely at 179 and 313°K. The low temperature of the tan $ peak is attributed to the ~oupdipole relaxation which is due to the local movements of the ester groups in the branches (fl-process). The high-temperature peak is linked with the polymer transition to the highly-elastic state (~-process). The crosslinked polymer based on TGY[-3 has 3 relaxation ranges of dipolar polarization and therefore 3 types of dielectrically active intra-chain mobilities (~ 173, ~ 370 and 440°K at 1 kHz; Fig. 3), The process at 173°K has an activation energy (46 kJ/mole) which is typical of the group-type polarization and can be ascribed to the contributions by the oxyethylene group and unreacted methacrylate group mobilities. The polarization relaxation parameters (relaxation time v, activation energy U) were similar at 360-370°K although above those of the fl-process for PMMA (for which U~- 103 kJ/mole). One can therefore assume the origin of this relaxation range to be the same in both cases, i.e. the local mobility of the ester groups. The oxyethylene groups adjacent to the latter can be regarded as lateral bridges between the PIMMA molecules which produce an

Pols~mer compositions based on various types of dimcthacrylates

2443

additional obstacle to the COO-group movements (compared with linear PMMA) and cause the larger ~- and U-values. We established t h a t v and U of the process are independent in the TGM-3 polymer of the crosslink density which was increased by the annealiug of the samples at a high temperature. This confirmed the correlation of the examined phenomenon with the local movement type au(| the group-dipole mechanism of polarization. ~an d'. 10z

8 q

220

260

300

3qo

~ K

3. The dielectric loss as a function of the temperature: /--hardened TGM-3. 2-4--TGM-3-MA eompositiol~s at 1 : 2, 1 : 3, and 1 : 4 molar ratios respectively,

FIG.

The third process occurred in the crosslinked TGM-3 polymer ~t 440°K. Typical for this relaxation is the dependence of the temperature-frequency coordinates on the thermal history of the sample. The higher the temperature and the longer its action, the larger will be the crosslink density of the dimethacrylate and the t a n Smax shift towards higher temperatures. From this one can conclude t h a t the process is linked with the movements of the larger fragments of the molecules including the network joints. I t is known t h a t amorphous, incompatible mixtures of polymers with a large enough mol.wt., i.e. those outside the "oligomeric range" have the components segregated and each has its own relaxation transition. The z- and U-changes of the components present in the blend or the appearance of an additional range of dipole relaxation will indicate the presence of a molecular interaction between the components, i.e. a smaller or lesser degree of miscibility and a formation of transition layers. The segmental dipole relaxation processes can serve as a "pointer" to the compatibility of the macro-chains when multi-component

2444

Zm D. ~ N O V A

et al.

polymer systems are studied b y dielectrical spectroscopy; these processes depend dominantly on intrermolecular reactions. The group-dipole processes normally depend on molecular interactions in very small volumes and depend only slightly on the presence of other types of macromolecules in the sample volume. The group-dipole relaxations take place in the same temperature range for TGM-3 and PMA and have similar activation energies. The compositions will therefore show an overlap of the tan Jmax peaks for the low-temperature processes; the tan Jmax was found to be lower for the compositions than for each of the components separately. This can be explained as due to the fact that partial copolymcrization takes place when the monomers mixture is polymerized and that ¢~statistical TGM-3-MA copolymer is produced. This copolymerization suppresses the dielectric losses at ~173°K because the group dipole relaxation of such a copolymer shifts to another temperature-frequency range. The examined range of compositions will show 2 or 3 incompletely resolved dipole loss ranges in the 290-390°K range. At 313°K, the temperature of tan ~max for the pure PMA, there will be a ridge on the tan 5 - T curves for most of the compositions, but it normally disappears after heating to 430-450°K (Fig. 3). This indicates the presence of 20-30% PMA in a segregated state in the samples; this amount diminishes as a result of heating to the mentioned temperatures. There is a twinned tan J peak at 350-370°K which becomes more intense with increase in the MA content. It can be assumed that one of the processes causing the twinning is associated with the segmental dipole polarization of the COO-groups in PMA contained in the molecular network of the set TGM-3. Furthermore, one can say after consideration of the molecular mobility results on MA-MMA copolymers [11] that the COO-group polarizations contribute to this loss range and also to the group dipole process in the set TGM-3 described above. The existence of the dielectric relaxation in the 350-370°K range correlates with the molecular mobility showing as a thermomechanical change at T1 (see Table and Fig. 2). The high-temperature tan Jmax range (440°K) for T G ~ - 3 in its compositions will shift to low temperatures with increasing MA content. The fall of the a-transition temperature (and therefore of the Tg) in the TGM-3 containing compositions can be regarded as being the consequence of the formation of interlacing transition layers in the network with the linear PMA, i.e. as a specific "plasticizing" action of the PMA. The results of the dielectric relaxation studies thus show the structure of polymer compositions based on TGM-3 and MA to be a network of the statistical TGM-3 copolymer with MA which is topologically combined with a moderate content (20-30%) of linear, separate PMA. The union of the components present in the system can be improved b y heating the samples at high temperature; this will have a favourable effect on the thermal and deformation-strength properties of the glasses, as pointed out earlier. The whole collection of results makes it clear that radical copolymerization

Polymer compositions based on various types of dimethacrylates

2445

of dimethacrylates with mono-functional polar monomers can be used to synthesize densely crosslinked polymer compositions of the MPC type in which a crosslinked network is interlaced with a linear (or slightly crosslinked) polymer. In conclusion the authors thank B. A. Dolgoplosk for his criticism of the work and V. A. Frolov for the supply of EA and advice of how to use it. Translated by K. A. A L L E ~ REFERENCES 1. P. G. BABAYEVSKII, Termoplasty konstruktsionnogo naznachoniya (Thermoplastics as Constructional Material), p. 151, " K h i m i y a " , 1976 2. G. P. BELONOVSKAYA, J. D. CHERNOVA, L. A. KOROTNEVA, L. S. ANDRIANOVA et al., Europ. Polymer J. 12: 817, 1976 3. L. H. SPERLING, Macromol. Roy. 12: 141, 1977 4. A. A. BERLIN, T. Ya. K E F E L I mad G. V. KOROLEV, Poliefirakrilaty (Polyacrylates). " N a u k a " , 1967 5. Russian Authors' Cert. No. 327163; Byull. Izobret., No. 5, 1972 6. Yu. M. SIVERGIN, N. P. MIRENSKAYA, V. T. SHASH.KOVA and A. A. BERLIN, Vysokomol. soyed. A l l : 1919, 1969 (Translated in Polymer Sei. U.S.S.R. 11: 9, 2186, 1969) 7. S. K. ZAKHAROV and Ye. V. KUVSHINSKII, Zavod. Labor. 30: 1399, 1964 8. V. V. KORSHAK, L. B. ZUBAKOVA and L. Ya. NIKIFOROVA, Vysokomol. soyed. B15: 419, 1973 (Not translated in Polymer Sci. U.S.S.R.) 9. G. V. KOROLEV, Dokl. I. Vsesoyuz. Konf. po khimii i fizikokhimii polimerizatsionnosposobnykh oligomorov (Reports I. Chemistry and Physical Chemistry Conf. on Oligomers capable of Polymerization), p. 144, Chernogolovka, 1977 10. G. P. ~ O V and L. V. YJtASNER, Vysokomol. soyed. 4: 1071, 1962 (Not translated in Polymer Sci. U.S.S.R.) 11. G.P. MIKHAILOV, T. I. BORISOVA, N. N. IVANOV and A. 8. NIGMANKHODZHAYEV, Vysokomol. soyed. A9: 20, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 1, 20, 1967)