Reaction thermal analysis of organopolysiloxanes

REACTION

JAROSLAV

Research (Ilcccivccl

THERMAL

FRANC

Imslilulc April

ASD

of Organic 25th.

ANALYSIS

JAROSJAV

Syuflrcscs.

OF

OlCGRNOPOLYSILOSANISS

POUR

Pardtcbicr-Rybilvi

(Czechoslovdin)

1gCq)

Thermal analysis is a comparatively recent analytical method, but has already lxcn modified and estencled in various different ways, e .g, therrnogravimctry (TGA) , differential thermal analysis (DTA), gas evolution analysis (GEA), thermogravimctry connected with mass spectrometry (TGA-MS), etc. Thermal analysis has very many uses, as can lx seen from several recent review articlesI-“. Generally, only changes caused by increasing temperatures are investigated. In some cases, the changes caused by the reaction between the test substance and the ambient atmosphere” have been investigated; recently the reactions during which new compounds are formed, have also been studied”, this method being usecl to identify certain substances. Some papers have dealt with the identification of cleavage products created by thermal clecomposition. The identification is carried out either by mass spectronietryo-0 or by gas chromatography lo,*1 by accumulating the gaseous cleavage products and feeding them regularly to the mass spectrograph or gas chromatograph. Th
Chi?rz. Ada,

48 (1969)

129-137

J. FRASC,

130

J. POUR

The reaction thermal analysis (RTA) apparatus is shown schematically in Fig. I. It consists of am pressure vessel (I) which is the source of the carrier gas (nitrogen). The carrier gas passes through a manostat (2), a needle valve (3) and a manometer (4), into an oven (5) in which on a thermo-couple (6) (iron-constantan), a platinum vessel (7) is fixed with the sample and the reagent. The oven is designed so that the

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temperature can be programmed. The oven is followed by an absorption device (8) which is designed to capture any volatile portions of the reagent. This absorption device however, lets through the gaseous cleavage products, which are then fed into a combustion oven (9) to be burnt to carbon dioxide and hydrogen: after the carbon dioxide has been removed in an absorber (LO) the amount of hydrogen is measured by a thermal conductivity sensor (II) 13. The carrier gas then passes through a flow meter (12) to the atmosphere. The sensor and the thermo-couple are connected to a recorder (13) which alternately records the signals from the sensor and the thermocouple. The switching is carried out automatically by a switch (14). In some cases it is more convenient to record the temperature curve separately. In order to be able to determine which functional group splits out in the temperature interval investigated, a zo-ml injection syringe (15) is inserted between the oven and the absorption device. The syringe has an injection needle of very small diameter (0.4 mm) and is inserted into a rubber connection between the oven and the absorption device. A small portion of the carrier gas, together with the cleavage products, raises the piston of the injection syringe during the measurement. At suitable intervals, 5-10 ml of these products are fed to the gas chromatograph and the individual components are identified. Methane and benzene were identified by normal gas-chromatographic techniques. Procedure The sample is weighed into a small platinum vessel (Fig. 2). In order to guarantee that the sample is perfectly distributed and that it comes into the greatest possible contact with the reagent used, first about 20 mg of glass beads (diameter 0.1 mm) or asbestos are put into the vessel, followed by about 2.5 mg of the sample. This is

REACTION

THERhIAL

ANALYSIS

OF

ORGANOPOLYSILOSANES

131

covered by a layer of about rg mg of liquid reagent. It is very important that the optimal ratio of sample to reagent be found, in order to prevent a large excess or insufficient amount of reagent. The vessel is placed on the thermo-couple and inserted into the oven. The carrier-gas input is opened so that the flow rate is 15 ml/min. The programmed oven heating is then switched on at a rate of zg”/min. Simultaneously, the recorder is switched on (o-x mV range) and the thermogram is recorded.

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For cleavage of the organopolysiloxanes examined, concentrated sulphuric acid was used; this was,pre-heated until white fumes evolved. Soda asbestos of grain size cu. 2 mm, was used as the absorbing agent in the absorption device. After each measurement, it is necessary to clean not only the vessel in whicli the reaction takes place, but also the whole oven, in which the vessel is located. It is, therefore, best to design the equipment so that a quartz tube (diameter 4 mm, length 9 cm) can be inserted into the oven orifice; this is then easy to remove and clean. The thermo-couple is located in a stainless steel pipe of diameter 2 mm, sealed at the end. The positioning of the thermo-couple and the feeding of the carrier gas have been described previouslylz. The cleavage reaction involves splitting out of methane and benzene, and the residue consists mainly of silicic acid with, possibly, some unclecomposed residue. High-molecular-weight organopolysiloxanes may result from the reaction with sulphuric acid, which acts as a polymerisation catalyst; however, their cleavage reactions appeared to be the same as those of lower-molecular-weight organopolysiloxanes. RESULTS

AND

DISCUSSION

Wgure 3 shows curves which were obtained by reaction thermal analysis during the splitting of organopolysiloxanes by concentrated Sulphuric acid in the temperature range zo-300’. These curves show that if the molecule has the same structure, the temperatures at which the splitting takes place, are identical within a small interval. An attempt was, therefore, made to attribute the individual maxima of the thermogram to certain functional groups and their arrangement in the molecule. This is reviewed in Table I. The Table was also assembled with the help of gas chromatography, which made it possible to determine to which functional group the maximum of the appropriate thermogram belonged. The Table indicates the following facts: in cleavage of organopolysiloxanes by sulphuric acid, the phenyl groups split first, the ease of removal of the groups being in the order PhSi E > PhzSi = > Ph&i - . The second group is formed by methyl groups bound to silicon. It is interesting to note that if the molecule contains the organopolysiloxane group -Si(CH3)3, a total of three temperature values is obtained, at which the methyl groups separate in the form of methane. It can thus be concluded Anal.

Chint.

Acta,

48 (x+9)

129-137

J.

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that at a tcinperature of x55-IS5” the first methyl group separates, at about 215O the second and, finally, at 275” the third. In the course of the separation of the first methyl group a set of two temperatures can be observed: about 155” and about 185”. In some cases both may be observed, although one of them is less well defined. This may be explained by the fact that if a methyl group and a phenyl group are bound to silicon simultaneously, their spatial distribution is different, and in the case of steritally more accessible positions, the separation may take place at a lower temperature. If the molecule contains only a =Si(CHa)z group, the latter behaves like the last two methyl -Si(CHa) groups, i.e. the separation takes place at temperatures of about 215 and 275”. If only a zSi(CH3) group is present, the splitting temperature is 255”, i.e. a little lower than the 275” corresponding to the last methyl group of -Si(CH& or =Si(CHa)a. The accuracy of the temperature record corresponding to the individual maxima of the thermogram, is determined by the instrumentation; in the arrangement used, the accuracy of the temperature maxima was f 3-4”. As can be seen from the EXPERIMENTAL, there is a certain delay between the time when the appropriate temperature is recorded and the signal of the detector, because the velocity of the carrier gas is 15 ml/min. Experimentally, it was found that this delay with the arrangement used amounted to 24 set, which corresponds to a And.

Chinr. Acfn,

48 (1gGg)

129-137

REACTION THERMAL ANALYSIS OF ORGANOPOLYSILOXANES

135

TABLE I S~MMARY OF TEMPERATURES (~) SILOXANES BY Sb'LPIIUR1C ACID

NO,

C o m p o u n d n,n C or L':

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OF

THE

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a F o r the sake of sHnphcttv ,tbbre~ rations for organopols~tloxant~ u~cd m ~pecmh2ed literature .ire .q~phed in t h e Table M denotes (CH~)aStO t D (CHa)o.SIO ~/~t (CHa)2(C~Hs)StO i M" (CH3) (C~H~)zSIO Dt (CH@ (C6H~)StO D" (C~H~)2bIO M"' (C~Hs)~S10 j Numerical reflex lndicatiug t h e number oi thc~e formatton~ m ,~ melee ule C means cyclic and L h n e a r arrangement of the molecule

d i f f e r e n c e of 6 °, c o n s i d e r i n g t h e t e m p e r a t u r e m t r e a s e of i 5 ° / m m , w h w h m u ~ t be %ubtracted from the recorded temperature m order to obtain the correct value R e a c t i o n t h e r m a l a n a l y s i s c a n b e e x p l o i t e d m a n a l y t i c a l c h e m i s t r y in v a r i o u s ways. Firstly, substances can be characterized, because only very similar substances possess the same thermograms S e c o n d l y , a c e r t a i n t y p e o f s t r u c t u r a l a n a l y s i s is p o s s i b l e , w h i c h is e s p e c i a l l y c o n v e n i e n t , e g . , f o r o r g a n o p o l y s l l o x a n e s , a s a s u p p l e m e n t t o r e a c t i o n g a s c h r o m a t o g r a p h y o f t h e s e ~ u b s t a n c e s 12 T h e a u t h o r s a r e of t h e o p i n i o n t h a t t h i s m e t h o d c o u l d b e u s e d in i d e n t i f y i n g v e r y v a n e d o r g a m c s u b s t a n c e s c o n t a i n e d i n a c e r t a i n s e t F u r t h e r r e s e a r c h i n t o t h i s f i e l d s h o u l d p r o v i d e f u r t h e r p o s s l b f l x t l e s of structural analysis O t h e r a d v a n t a g e s of t h i s m e t h o d a r e f o u n d m t h e c o m p a r a t i v e l y s i m p l e e q m p ment required and in the fact that the whole analysis can be carried out with a small a m o u n t of t h e s u b s t a n c e , o n l y m i l l i g r a m o r e v e n s m a l l e r a m o u n t s a r e r e q u i r e d , d e A n a l Chum A a a , 48 (~969) I29--x37

136

J.

FRANC,

J.

POUR

pending on the type of splitting reaction and on the sensitivity of the thermalconductivity detector available. In some cases it should even be possible to operate with ,ug-amounts. The splitting of a comparatively small sample, which is suitably distributed (glass beads), is not subject to the errors found with larger amounts owing to imperfect heat transfer. Moreover, the gas-chromatographic detectors usually provide a differential record. This is another advantage, because with most thermoanalytical methods, special equipment is necessary to obtain a differential record. Further differentiation can be achieved by suitable changes of the reaction media which cause the splitting (e.g., at present the splitting of organopolysiloxanes by concentrated soclium hydroxide solutions is being studied), In summarizing the above results, it can be stated that the reaction thermal analysis method could become another useful extension of the existing range of tl~crn~oannlytical techniques. SUhIhIARY

A new method of thermal analysis based on the reaction of the test substance with a suitable reagent under programmed tcmpcraturc conditions is described. The gaseous reaction products are detected by gas chromatography. The splitting of low-molecular-weight organopolysiloxanes by sulphuric acid is used as an example to illustrate the relationship between the structure of the test substance and the temperature at which the splitting of the individual functional groups occurs. The method seems applicable for identifying chemical groups and their location in certain types of molecules. ICXkXlMI::

On ddcrit une nouvelle methode d’analyse thermique basee sur la reaction de la substance a tester avec un reactif approprie dans des conditions de temperature programmees. Les procluits de reaction gazeux sont decel& par chromatographie gazeuse. On a choisi comme esemple des organopolysiloxanes de faible poids moleculnire avec acicle sulfurique. Cette mCthode semblc applicable pour l’identification de groupes chimiques et pour leur localization dans certains types de molecules. ZUSAMhIENFASSUNG

Es wird eine neue Methode zur thermischen Analyse von organischem Polysilosan beschrieben, die auf der Reaktion mit geeignetem Reagenz bei programmierten Temperaturbeclingungen beruht. Die dabei auftretenden gasfijrmigen Reaktionsprodukte werden mit einem Gaschromatographen nachgewiesen. Die Aufspaltung niedrigmolekularer Polysiloxane durch Schwefelsaure client als Beispiel, urn die Beziehung zwischen der Struktur der gepriiften Substanz und der Temperatur, bei der die Aufspaltung einzelner funktionaler Gruppen erfol& zu zeigen. Die Methode scheint geeignet zu sein, chemische Gruppen zu identifizieren und bei gewissen Typen von Molekiilen zu lokalisieren. Anal.

Chitn.

Acta, 453 (IgGg)

x29-137

REACTLOX

I 2 3 4 5 6 7 8 g IO II IZ 13

THER?.lAL

ASALYSIS

OF OI~Ghh’OPOLYSIl~OSh?SES

137

A. V. COATS AND J. P. REDFERN, A~~nlysf, 88 (1953) 306. c. 13. i%IURPlIY, /lllcl~.c/rO,a., 38 (1966) 443 I<. I<. I~ERI~o~' AND A. MAT~IIEU, Cltinr. Ambl. (Puris), dC, (I#.+) 293. D. I