Some thermal characteristics of branched and crosslinked polyphenylenes

Branched and crosshnked polyphenylenes

2465

13. L. BELLAMY, Infrakrasnye spektry molekul (Infrared Spectra of Complex Molecules). pp. 16, 170, 314, Foreign Literature Publishing House. 1951 (Russian translation) 14. T. CURT/US and K. THUN, J. prakt. Chem. 44: 175, 1891 15. D. J. LYMAN, Rev. Makromolek. Chem. 1: 191. 1966 16. Yu. S. LIPATOV, Struktura , svoistva polinretanov (Structure and Propertms of Poly11rethanes). p. 44, "Naukova dumka", 1970

SOME THERMAL CHARACTERISTICS OF BRANCHED AND CROSSLINKED POLYPHENYLENES* V. V KORSHAK, V. A. SERGEYEV, V. G. DANILOV and V. K. SHITIKOV Institute of Hetero-orgamc Compounds, U.S.S.R. Academy of Scmnces (Received 23 June 1971)

The thermal degradatmn oi polymers of the polyphenylene type, obtained by polycyclotrlmerlzatlon of dmthynylbenzene, has been stuched by thermogravlmetrm and gas-chromatographm analyms. It m shown that polyphenylenes from dmthynylbenzene lose yery httle weight when they are heated to 900 ° (about 10-15~) and the loss becomes less as the molecular weight of the ohgomers m reduced. Benzene, toluene, methane, hydrogen, ethane, ethylene and CO2 are found among the products of thermal degradatmn. The apparent energs" of activation for thermal degradation of the polyphenylenes has been found by Kofstad's method. A mechamsm of hardening and degradation of the polymers ,s suggested. INSOLUBLE a n d soluble p o l y m e r s ofthe p o l y p h e n y l e n e t y p e , o f different molecular weight, h a v e been o b t a i n e d previously b y p o l y c y c l o t r i m e r i z a t i o n o f d i e t h y n y l benzene, with trialkyl p h o s p h i t e complexes o f cobalt as catalysts [1]. I n the general f o r m p o l y c y c l o t r i m e r i z a t i o n of d i e t h y n y l c o m p o u n d s can be r e p r e s e n t e d as follows

CH--C--Ar--C_----CH~ cat:-~HC----C--Ar--[ ~ "

Ar--I--C--CII Ar--C

~_ CH

-Jn

The p r e s e n t p a p e r r e p o r t s a s t u d y of some t h e r m a l characteristics o f polyphenylenes prepared by polycyclotrimerization of diethynylbenzene, diethynyld i p h e n y l oxide and d i e t h y n y l d i p h e n y l e t h a n e in the presence of a trialkyl phosp h i t e - c o b a l t c o m p l e x as catalyst. Some relationships in t h e d e g r a d a t i o n of p o l y m e r s o f this t y p e and o f linear p o l y - p - p h e n y l e n e were r e p o r t e d in reference [2]. The t h e r m a l characteristics and p h y s i c o m e c h a n i e a l properties of linear p o l y - p - p h e n y l e n e p r e p a r e d b y oxidative d e h y d r o p o l y c o n d e n s a t i o n o f benzene are discussed in reference [3]. * Vysokomol. soyed. AI5: No. 10, 2180-2184, 1973.

2466

v.V. KORSHAKet

al.

It is seen from Fig. 1 that introduction of oxygen atoms or ethylene groups between the aromatic fragments of polyphenylene lowers the thermal stability of the polymers considerably. Decomposition of the polyphenylene from diethynylbenzenc begins earlier than decomposition of poly-p-phenylene and the region of vigorous decomposition of the former occurs at lower temperatures. It should be noted ~hat both polymers are infusible and are insoluble in ordinary organic solvents. Under high moulding pressures however poly-p-phenylene forms monolithic articles [3] and it dissolves in chlorosulphonic acid. This shows that the polyphenylene from benzene is not crosslinked. On the other hand the insoluble polyphenylene from diethynylbenzene does not form monolithic articles even at very high moulding temperatures and pressures, and it does not dissolve in chlorosulphonic acid, which shows that it has a crosslinked structure. foo

-

I

800

~

FIG. 1

700

foo

3OO

/

5OO ~ °c

FIG. 2

Fro. 1. Dynamm TGA curves of polyphenylenes m helmm: 1--crosshnked polyphenylene from diethynylbenzene; 2--the same from dmthynyldlphenyl oxide; 3--the same from dlethynyldlphenylethane; 4--poly-p-phenylene from benzene. l~m. 2. Thermomeehanical curves of soluble polymers with M : 1200 (1) and 1540 (2) and of a crosshnked polymer (3) from dmthynylbenzene. The lower temperature of the beginning of decomposition of the polyphenylene from diethynylbenzene, in comparison with poly-p-phenylene, can be explained on the basis of the nature of three-dimensional polycyelotrimerization. I t is well known that in three-dimensional polycondensation or polymerization the degree of conversion at which gel formation begins is not high and is dependent on the functionality of the monomers. In our case the centre of branching is the benzene ring formed b y cyclotrimerization and according to Flory's theory [4] the degree of reaction at the onset of structure-formation should be in the region of 60%. Determination b y infrared spectroscopic analysis of the concentration of triple bonds in the insolublo polyphenylene from diethynylbenzene, showed that the degree of reaction is in fact 60%.

Branched and crosshnked polyphenylenes

2467

As a result of this the action of heat on the polyphenylene from diethynylbenzene gives rise primarily to processes involving the triple bonds, with formation of structures with lower thermal stability than the aromatic nucleus. The amorphous structure of the polymer and the fact that it contains trisubstituted benzene rings must also play an important part in the lowering of the decomposition temperature of the polyphenylene from diethynylbenzene, in comparison with poly-p-phenylene. Later the large number of polymerizable triple bonds present lead to formation of a highly crosslinked polymer. Consequences of this are that the rate of loss ill weight in the region of vigorous decomposition is lower and the total loss of weight at 900 ° is less in the polyphenylene from diethynylbenzene than in poly-p-phenylene. In addition to infusible and insoluble polymers, polyphenylenes that are soluble in solvents such as toluene and dioxan and that soften at relatively low temperatures (Fig. 2) can be obtained by polycyclotrimerization of diethynylbenzene. The deformation at 100-150 ° (Fig. 2) is due to softening of the original polymer and the cessation of deformation is due to crosslinking of the polymer by interaction of terminal triple bonds, in the following way for example + ~ C H =CH--C---- C ~ ~C----- CH +HC------ C~

CH2 ....

_~C--CH----C--C--

C~

+HC=-C~

Increase in the molecular weight of the soluble polymers brings about an increase in softening point and a decrease in deformation, resulting from increase in the rate of crosslinking [1]. A soluble polyphenylene of molecular weight 2500

+ I £08

400

6~

~00

F1G. 3. Kmetms of gas evolution in thermal degradation of a polyphenylene from dmthynylbenzene: / - - m e t h a n e , 2--hydrogen, 3--ethylene-~-C02, 4--ethane, 5--benzene, 6--toluene.

does not show appreciable deformation at 100-200 ° m thermomechanical tests, because the polymer becomes crosslinked at temperatures below its softening point. The thermomechanical curve of this polymer almost coincides with cor-

2468

V . V . KORSHAK e~ al.

responding curves for insoluble polyphenylenes crosslinked during the course of preparation or hardened subsequently. The deformation of the polyphenylenes at ~ 600 ° is due to their decomposition in the presence of air [1]. Analysis of the gaseous products of thermal degradation of polyphenylenes from diethynylbenzene, obtained during thermogravimetric analysis (Fig. 3) showed that the spectrum of these products is considerably broader than the spectrum of the products from poly-p-phenylene [3]. In addition to benzene,

80~

"~

I

I

JO0

~00 :FIG. 4

600

8OO

I000

FIG. 5

FIG. 4. Dynamm TGA curves (1-3) and curves of the evolution of benzene (1'-3') and toluene (1"'-3") for polydicthynylbenzene (1) and of polyphenylene from diethynylbenzene with M~-640 (2) and 1200 (3). FIG. 5. Differential curves of loss m weight of poly-p-phenylene (1), a crosslinked polyphe~ylene from diethynylbenzene (2), soluble polyphenylenes from diethynylbenzene with M = 1200 (3) and 640 (d), and polydiethynylbenzene (5). m e t h a n e a n d h y d r o g e n t h e s p e c t r u m also shows ethane, ethylene, toluene a n d small a m o u n t s o f aliphatic a n d a l k y l a r o m a t i c p r o d u c t s . * The evolution o f benzene is due to r u p t u r e of t h e b o n d b e t w e e n a r o m a t i c nuclei, a n d of m e t h a n e a n d * Carbon dioxide is also evolved during degradation of polyphcnylcne from diethynylbenzene, but COs and ethylene are not separated in a column filled with sihca gel, therefore Fig. 3 shows the total quantity of these two gases evolved. Carbon dioxide is obviously produced in breakdown of products of oxidation of the triple bond.

Branched and crosslinked polyphenylenes

2469

h y d r o g e n (at 700-850 °) to b r e a k d o w n o f t h e a r o m a t i c s y s t e m and to carbonizat i o n o f t h e polymer. T h e a p p e a r a n c e a m o n g t h e d e g r a d a t i o n p r o d u c t s o f ethane, e t h y l e n e and toluene, a n d t h e m u c h earlier a p p e a r a n c e of m e t h a n e can be e x p l a i n e d b y reactions o f t h e triple bond. W h e n the p o l y p h e n y l e n e f r o m d i e t h y n y l b e n z e n e is h e a t e d the strength of t h e E P R signal increases, b u t its m a x i m a l value is given b y the residue h e a t e d t o 600 ° and n o t to 700 °, which was f o u n d for p o l y - p - p h e n y l e n e f r o m benzene [3]. T h u s t h e change f r o m a linear p o l y p h e n y l e n e s t r u c t u r e to a b r a n c h e d a n d crosslinked s t r u c t u r e containing triple bonds and polyene f r a g m e n t s results in lowering of t h e decomposition t e m p e r a t u r e , as d e t e r m i n e d b y loss in weight, gas evolution a n d t h e v a r i a t i o n in t h e E P R s p e c t r u m . I t is seen f r o m Fig. 4 t h a t as the molecular weight of t h e soluble p o l y p h e n y l enes is decreased t h e r a t e o f decomposition and t h e loss in weight o f t h e p o l y m e r decrease. This indicates the high t e n d e n c y of these polymers to crosslink and f o r m condensed products. T h e m a x i m a l r a t e evolution o f benzene a n d toluene f r o m all the p o l y m e r s coincides with t h e most rapid loss of weight in t h e T G A curves (Figs. 4 a n d 5) (taking a c c o u n t o f the t i m e lag b e t w e e n t h e c h r o m a t o g r a p h and t h e r m a l balance readings). The fact t h a t evolution of benzene and toluene f r o m p o l y e t h y n y l benzene and p o l y p h e n y l e n e s occurs in the same region of t e m p e r a t u r e s (500-700 °) shows t h a t it is in this region t h a t d e g r a d a t i o n o f t h e main p o l y p h e n y l e n e chain occurs. I t should be n o t e d t h a t in t h e p o l y m e r s t h a t we are discussing t h e evolution o f benzene a n d the loss in weight are generally m u c h less t h a n in p o l y p h e n y l a cetylene [5], because these polymers do n o t contain m o n o s u b s t i t u t e d benzene rings. We attempted to calculate the energy of activation for thermal degradation from the TGA results for these polymers by Kofstad's method [6, 7]. The following equatmn was used for calculating the kinetm parameters: log W-- log ( -- d W/clt)

E 1 2.303R T

{log A -- (1 -- n) log W0},

where W m the weight of the sample m mg and d W / d t is the rate of loss in weight m mg/min. Thin equation permits determination of the energy of activation for degradation (E) and the pre-exponential factor A, whmh is dependent on the value of the order of reaction (~) between 0 and 2 or above. In our case the loss m weight m very small, the term n log W is small and it varies little during the course of degradation. Therefore the effect of n on the potation of the points on the graph and on the straightening out of the Arrhemus curve is small and it m not possible to select n. Moreover the rate of degradation is very low and It varies very slowly, especially m the first stage and it m therefore difficult to determine d W / d t graphically. For thin reason we were restricted to making only an estimate of the energy of activation for degradation.

2470

V.V. KORSH~LKet al.

For the polyphenylenes from diethynylbenzene and for polyethynylbenzene up to 2-3% loss in weight E : 8-9 kcal/mole. In the region of vigorous decomposition of polyphenylenes from diethynylbenzene E rises to 12-13 kcal/mole, whereas it remains at the original level for polyethynylbenzene. Calculation of ~he energy of activation for degradation of poly-p-phenylene showed t h a t in the early stage of degradation (~ 4%) it is 4 kcal/mole, but in the region of vigorous decomposition E rises steeply, reaching 26-27 kcal/mole. Such low values of E are evidently attributable to the fact t h a t alongside degradation very extensive crosslinking of the polymer occurs, which reduces the rate of loss in weight considerably. Furthermore, because of the high density of the polymer network only a small fraction of the acts of bond rupture result in loss in weight, because the radicals can recombine or remain in the free state. This is indicated by increase in the E P R signal when the sample is heated to 600 °. Since it is very difficult to separate the degradation and crosslinking processes the apparent energy of activation for degradation is of a formal nature and cannot serve as a measure of the thermal stability of polymers of this type. EXPERIMENTAL

Thermal degradation was studied with a DAM (France) B-60 electronic thermal balance in an atmosphere of helium at a rate of heating of 5 deg/min, with a sample weight of 20 rag. Chromatographic analysis of the degradation products was carried out during the course of thermogravimetric analysis in a "Chromdam" chromatograph, in three columns, filled with zeolite CaA, silica gel and 5% of silicone E-301 on Celite 545. Thermomeehanical analysis was carried out in the apparatus of reference [8] at a rate of heating of 2 deg/min and under a specific pressure of 0-8 kg/cm 2 Translated by E. O. P~ILLIPS REFERENCES

1. V. V. KORSHAK, V. A. SERGEYEV, V. K. SHITIKOV and V. G. DANILOV, Vysokomol.

soyed. AI5: 27, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 1, 28, 1973) 2. V. G. DANILOV, V. A. SERGEYEV and V. K. SHITIKOV, XVII konferentsia po vysokomolekulyarnym soyedineniyam (17th Conference on Macromolecular Compounds). p. 67, Moscow, 1969 3. V. V. KORSHAK, V. A. SERGEYEV, V. G. DANILOV, G. L. SLONIMSKII, A. A. ASKADSKII and A. I. MZHEL'SKII, Izv. Akad. Nauk SSSR, ser. khim., 744, 1971

4. P. J. FLORY, J. Amer. Chem. Soc. 63: 3083, 1941 5. P.P. KISLINA,M. I. CHERKASHINand A. A. BERLIN, Izv. Akad. Nauk SSSR, ser. khim., 2453, 1967 6. P. KOFSTAD, Nature 179: 1362, 1957 7. T. ASAHARA, M. SENUI and M. FUKUI, Kogyo kagaku zasshi 72: 1387, 1969 8. V. L. TSETLIN, V. I. GAVRILOV, N. A. VELIKOVSKAYAand V. V. KOCHKIN, Zavod. lab. 22: 352, 1956