Dependence of the physical properties of polycyanates on their structures

DEPENDENCE OF THE PHYSICAL PROPERTIES OF POLYCYANATES ON THEIR STRUCTURE* V. V. KORSHAK, V. A. PANKRATOV, A. A. ASKADSKII, A. G. PucHI~ and S. V. VINOGRADOVA

I n s t i t u t e for Elemento-Organie Compounds, U.S.S.R. Academy of Sciences

(Received 24 April 1972) A study has benn made of certain physical properties of densely crosslinked heterochain polymers, namely, polycyanates obtained b y the polycyelotrimerization, in bulk, of aromatic dicyanates. Consideration has been given to the effect of the structure of the initial dicyanate esters, in particular, the characteristics of the substituents at the central carbon atom of the dicyanates, on the physical characteristics of the polymers based on them. The packing coefficients of the amorphous and slightly crystalline crosslinked polymer systems were determined first of all and it was shown t h a t they are packed to the same density as in the linear polymers. THE PRESENT w o r k h a s b e e n d e v o t e d t o a s t u d y o f t h e effect o f t h e s t r u c t u r e o f polycyanates on their physical properties: the polymers were obtained by polycyclotrimerization in bulk of dicyanate esters of bisphenols with various substituents at the central carbon atom. As a result of the cyclotrimerization o f - - - O - - C _ ~ N groups in arylcyauates, esters of c y a n n r i c acid are formed by the scheme

Ar

\

/ 0

N

\I 3Ar--0--C~-~N --,

C I N

Ar

0

\/

C II N

\,/ C

I I

0

Ar I n the polymerization of dieyanates with the formation of crosslinkod polymers, tho cyclotrimerization of the O--C------N groups is found to be highly selective [1, 2]. This forms one of the basic features of the polymerization of O - - C - - N groups of dicyanates, which * Vysokomol. soyed. AI6: No, 5, 981-986, 1974. 1133

1134

V.V. KORSm~Ket al.

distinguishes these groups from other functional groups (--lq~--C----O, --C----CH etc.) that participate in cyclotrimerization, since, in the latter cases numerous side reactions occur in addition to the basic reactions (polymerization into chains, dimerization, etc.). This fact and also the high conversion (98-99%) of the N--C--O groups of the initial dicyanate esters during their polymerization are conducive to the formation of densely crosslinked polymers with a regular chemical structure [1, 2]. In the case of polycyanates obtained by the polycyclotrimerization of aromatic dicyanates, the distance between network nodes is equal to the length of the residue of the initial bisphenol. I n connection with what has been put forward above, it was of interest to follow the change in physical properties of polycyanates as a function of the selected dicyanate and, in particular, as a function of the characteristics of the substituents at the central carbon atom of the initial dicyanate ester. In our view, it is also of interest to compare the results obtained with the rules established previously for linear polymers based on bisphenols with the same structure [3-5]. DISCUSSION OF RESUhTS

Polymers 1-9 (Table 1) were obtained from binuclear aromatic dicyanates differing in the nature of the substituents at the central carbon atom. Attention should first of all be given to the fact t h a t the polymers of dieyanates having small and the same substituents at the central carbon atom (polymers 1-3, Table 1), as distinct from all known crosslinked aromatic polymers [6], have an anomalously high index of specific impact thoughness (23-25 kg.cm/cm2). The presence at the central carbon atom of the same aromatic substituents (polymers 4-6, Table 1) and also the presence of groups of the flat card-like type (polymers 7-9) lead to a marked reduction in this index in the ease of cast polyeyanate specimens. However, in the latter cases the bend strength of the polyeyanates is markedly increased. A similar effect of the structure of the substituents at the central carbon atom of bffunetional derivatives has also been found previously with a series of heterochain linear polymers [3-5, 7-9]. In the absence of an sp3-C atom linked between the phenyl nuclei, as occurs in the case of the polycyanate based on 4,4'-dicyanatodiphenyl (polymer 12, Table 1), a polymer is formed with a comparatively low specific impact toughness (6 kg. em/cm2). A reduction in the spacing between network nodes to a single phenylene radical (polycyanates of 1,3- and 1,4-dicyanatobenzene) leads once more to a considerable increase in the specific impact toughness and bend strength of the cast polymer specimens. It is still not possible at present to give an unambiguous explanation of this enhancement of the physical and mechanical properties in the case of polymers based on mononuclear dicyanates (polymers 10 and 11). According to the data from X-ray structural analysis, the majority of the polycyanates obtained are amorphous polymers (Table 1). However, in certain

NCOCeH4--C(CF3)2--C6H4OCN N C O C e H , - - C ( H ) (CsHs)--C~H4OCN NCOC~H~--C(CH3) (C6Hs)~CeH4OCN NCOC , H , - - C (C eI-Is)~--C eI-I4OCN 3,3-Bis- ( 4 - c y a n a t o p h e n y l ) p h t h a l i d e 9,9-Bis- (4-cyanatophenyl)fluorene 9,9-Bis- ( 4 - c y a n a t o p h e n y l ) a n t h r o n e - 10

m-NCOC6H4OCN

p-NCOC6H~OCN

NCOCeH4--C6H4OCN

3 4 5 6 7 8

10

11

12

* Excluding p o l y m e r 10, all the C6H, radicals are

6

12

18

24 10 17 5 6 5 6

25

23

1.47

1.73

1.92

1.46 1.59 1.85 1.62 >2.02 1.54 1.69

1.40

1.22

Specific Bend impact strength, toughness ~ × 10 -8, kg . c m / c m ~ k g / e m 2

para-substituted.

NCOC6H4--C (CHa)~--C6H4OCN

2

9

NCOC 6I-I~--CH~--C ~It~OCN

Initial d y e y a n a t e ester of b i s p h e n o l * •

I

No.

Polymer

1.319

1.431

1.414

1.528 1.255 1-229 1.220 1-322 1.269 1.288

1.235

1.307

Density at 20°C, g/cm3

708"70

480"41

480.41

1158-80 979"08 1021"16 1207-37 1105-06 1207.37 1285.36

834"95

750"78

Molecular mass of repeating unit

615.13

388.33

388"33

858"79 891.85 943-93 1118.53 934.63 1080.73 1134-90

769.63

655"8i

0.689

0.697

0.688

0.682 0.689 0-684 0.681 0.673 0.688 0.685

0.686

0.688

Volume Coeitiof recient of peating packing unit, /ka

Crystalline, average degree of ordering Amorphous with signs of crystallinity Amorphous ,, ,, ,, ,, ,, Crystalline, poorly o r d e r e d Amorphous with signs of crystaUhfity Crystalline, poorly o r d e r e d Crystalline, a v e r a g e degree of ordering

X-ray structure analysis d a t a

TABLE 1. PEYSIOAL PROFE~I~S OF POLYC.t'A~rAT~S BASED OZ~ DICYAN'A~ ESTERS OF BISPHElgOLS WITH VARIOUS STRUCTURES

¢Ji

~

~"

o

~

o

~.

1136

V.V. KORS~AKet

al.

cases where the same substituents, small in volume (H, CHa), are located at the central carbon atom and also in the case of polycyanates based on the dieyanate esters of resorcinol, hydroquinone and 4,4'-dihydroxydiphenyl, structures with a low or moderate degree of ordering are found to be formed. Thus the existence in these polymers of a rigid three-dimensional network is not an obstacle to the appearance of ordered structures.

/~"0

-",-/ 100

'~, ~//1 Z:O

300

I kc'\\: ZOO

~ ', 30g Y,°C

Regions of workability of: 1--polycarbonate based 2,2-bis-(4-hydroxyphenyl) propane; 2, 3 and 4--polycyanates 2, 8 and 10 respectively; 5--polycyanate based on 1,2-bis-(4-cyanatophenyl)-dicarba-closododecaborane.

A crystalline poorly ordered structure is also found in the case of the polycyanate based on the dicyanate ester of 9,9-bis-(4-hydroxyphenyl)anthrone-10 which has a bulky group of the fiat card-like type at the central carbon atom. It has been shown previously [2] that the temperature at which the polycyanates begin to deform (according to the data from thermomechanical tests) lies in a comparatively narrow temperature range (360-410°C). From the characteristics of the thermomechanical curve of the polycyanates, which have a quite considerable negative strain in the region of 370-450°C, it m a y be suggested that the temperature at which deformation begins is connected with degradation processes occurring in these polymers. This is even more probable since the temperatures at which the polycyanates begin to decompose in air (from the TGA data) lie in the same region. As distinct from linear heterochain polymers, in which the nature of the substituents at the central carbon atom of the bifunctional derivatives has a considerable effect on their heat resistance (according to the softening temperature from the thermomOchanical curves) [3-5, 7-9], no such relationship can be traced in the case of polycyanates. In order to compare in more detail the heat resistance of densely crosslinked

Dependence of physical properties of polycyanates on their structure

1137

polycyanates synthesized from the dicyanate esters of bisphenols with various structures, we determined the regions of workability for these polymers over a wide range of temperatures and mechanical stresses (Figure). The determinations were made by the method described previously [5, 10]. TABLE 2. INC~E~.~TS OF VOLUMErO~ C~TA~N ATOMS Atom

Increment of volume

Atom

Increment of volume

8.33

C 1.45 C* t.a4 F

H

C

1.48

C*

1.48 C

H

F

~! c ,.,s

, ,-,____2_,c

10.76

9.54

H

N* .).-~.'.. c ~,,ll

O 1.3e

2.01

C*

N

5.7s

1-34 N

15.79

C

Attention should be given to the correspondence between the curves for anisothermal stress relaxation (broken curves) and the curves delineating the regions of workability both for densely crosslinked systems~ namely, polycyanates, and for the linear heterochain polymers studied in detail previously [5], of which the polycarbonate shown in the Figure is an example. Replacement of an isopropylene group by a fluorene group leads to an increase in the heat resistance of the polymer over the entire temperature and stress range investigated. A marked shift is also observed in the m a x i m u m temperature of workability at which the stress in the specimen is completely relaxed to zero. A similar effect has also been observed in the case of linear aromatic polyesters and polyamides [5]. I n addition, we investigated the effect of a number of other structural features of the initial dicyanate esters on the region of workability of the polycyanates based on them. Thus the replacement of a para-substituted binuclear dicyanate residue between the polymer's network nodes by a mononuclear meta-r adical leads to a

1138

v.V. Ko~sH~ et al.

reduction of the heat resistance of the polyeyanate both with respect to temperatures and also with respect to stresses. It may be seen from the Figure that the introduction of a 1,2-caxborane group instead of a 9,9-fluorene radical as the connecting bridge between the phenyl radicals of the cyanate also reduces the heat resistance of polycyanates over the entire range of temperatures and stresses studied. It may be seen from what has been put forward above that the structure of the initial arylcyanate has a substantial effect on the properties of the polycyanates formed. It had been shown previously that the coefficients of packing for monolithic specimens of a large number of amorphous and poorly crystallized linear polymers with very diverse structures, obtained both by addition polymerization and also by polycondensation, vary within very narrow limits [11], and their density of packing is, to a first approximation, the same (Kay----0"681). There is not, however, a single example of a spatially structured system amongst the numerous data published in the literature about the packing of various polymers. In connection with this, it was of interest to assess qualitatively, by calculating the packing coefficients, the density of packing of the macromolecules in these densely crosslinked polymers and to establish the effect of the chemical structure of the initial cyanates on the density of packing. In this work we calculated the coefficients of packing for twelve polycyanates with various chemical structures (Table 1). In calculating the packing coefficients of the polyeyanatcs, the data for the volume increments for the atoms and. groups of atoms given in reference [ll] were used. We determined the missing increments by using the data for intermolecular radii and bond lengths given in [11-15], (Table 2). The values of density, molecular weight and volume of the repeated link, and also the calculated packing coefficients of the polycyanates with various structures, are shown in Table 1. It may be seen from the results obtained that the packing coefficients of the polycyanates, which are densely crosslinked polymers with very diverse structures, lie in a narrow range (0.673-0.697) and the mean value of this index for polycyanates (Kay----0"686) is close to the mean value determined previously for linear polymers (0.681) [11]. The fact that the packing coefficients of linear and crosslinked polymers are almost the same indicates that amorphous and poorly crystalline spatially structured, infusible and insoluble polymers are packed to the same density as all linear, fusible and soluble polymers. The results from the packing of polycyanates enables it to be assumed that the principal effect on the physical properties of a number of these polymers differing in the nature of the substituents at the central carbon atom is evidently caused by mobility of the C, p3-C,~ ~ bond between the central carbon atom and the phenyl radicals of the internodal network fragments.

Dependence of physical properties of polycyanates on their struoture

1139

EXPERIMENTAL The dicyanate esters of the bisphenols were obtained by the known methods and had constants similar to those described previously [1]. Bulk polymerization of the dieyanates was carried out under identical conditions in argon in the presence of the catalyst ZnC12 (0"25 mole %). As a preliminary, the ampoules with the dieyanates were evacuated to 1 × 10 -5 m m H g and filled with argon, and then the temperature was gradually raised from the melting point of the corresponding dicyanate to 250°C at the rate of 10 deg C/hr and held at 250°C for 4 hr. The degree of conversion of the N----C--O-- groups of the dicyanate esters in the polymers obtained was 98-99%. The polymers densities were determined by the gradient-tube method [16]. The specific impact toughness and the bend strength ("Dinstat" type test machine, German Democratic Republic) were obtained by averaging the results of five measurements of polycyanate specimens 4 × 8 × 15 m m in size.

Translated by G. F. MODLEN REFERENCES 1. S. V. VINOGRADOVA, V. A. PANKRATOV, A. G. PUCH][N and V. V. KORSHAK, Izv. AN SSSR, seriya khimich. 837, 1971 2. V. V. KORSHAK, S. V. VINOGRADOVA, V. A. PANKRATOV and A. G. PUCHIN, Dokl. AN SSSR 202: 350, 1972 3. V. V. KORSHAK, S. V. VINOGRADOVA and V. A. PANKRATOV, Dokl. AN SSSR 156: 880, 1964 4. V. V. KORSHAK and S. V. VINOGRADOVA, Poliarilaty (Polyarylates). Izd. " N a u k a " , 1964 5. A. A. ASKADSKII, Fiziko-Khimiya poliarilitov (Physical Chemistry of Polyarylates). Izd. "Khimiya", 1968 6. Ye. B. TROSTYANSKAYA and P. G. BABAYEVSKII, Uspekhi khimii 40: 117, 1971 7. P. W. MORGAN, Makromolecules 3: 536, 1970 8. V. V. KORSHAK, G. M. TSEITLII~ a n d A. I. PAVLOV, Izv. AN SSSR, seriya khimich. 1912, 1965 9. T. V. DEVDARIANI and D. F. KUTEPOV, Vysokomol. soyed. B I I : 788, 1969 (Not translated in Polymer Sci. U.S.S.R.) 10. G. L. SLONIM~KII and A. A. ASKADSKII, Mekhanika polimerov, No. 1, 36, 1965 11. G. L. SLONIMSKII, A. A. ASKADSKII and A. I. KITAIGORODSKII, Vysokomol. soyed. AI2: 494, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 3, 556, 1970) 12. M. IWASAKI, S. NAGASE and R. KOJIMA, J. Chem. Phys. 22: 959, 1954 13. Tables of Interatomic Distances and Configuration in Molecules and Ions, Special Publication No. 11, London, 1958 14. J. E. LANCASTER and B. P. STOICHEFF, Canad. J. Phys. 34: 1016, 1956 15. E. W. HUGHES, J. Amer. Chem. Soc. 63: 1737, 1941 16. J. M. MILLS, J. Polymer Sci. 19: 93, 585, 1956