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Thiourea-formaldehyde condensate: Synthesis, characterization and electrical resistivity
Mat. R e s . B u l l . , Vol. 18, p p . 897-902, 1983. P r i n t e d i n t h e USA. 0025-5408/83 $3.00 + .00 C o p y r i g h t (e) 1983 P e r g a m o n P r e s s L t d .
THIOUREA-FORMALDEHYDE CONDENSATE: SYNTHESIS, CHARACTERIZATION AND ELECTRICAL RESISTIVITY Hari S i n g h Nalwa a n d Padma V a s u d e v a n D e p a r t m e n t of C h e m i s t r y and C e n t r e f o r R u r a l D e v e l o p m e n t & Appropriate Technology I n d i a n I n s t i t u t e of T e c h n o l o g y New Delhi l l 0 016, I n d i a ( R e c e i v e d May 26, 1983; C o m m u n i c a t e d b y S. A m e l i n c k x )
ABSTRACT A t h i o u r e a - f o r m a l d e h y d e c o n d e n s a t e was s y n t h e s i z e d b y c o n d e n s i n g t h i o u r e a with f o r m a l d e h y d e i n a l k a l i n e medium u s i n g sodium h y d r o x ide. T h e c o n d e n s a t e was c h a r a c t e r i z e d b y e l e m e n t a l a n a l y s i s and infrared spectroscopy. D e r i v a t i v e t h e r m o g r a v i m e t r y (DTG) a n d t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) t e c h n i q u e s were c a r r i e d o u t to e x a m i n e t h e t h e r m a l s t a b i l i t y of t h e material. D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) was also u s e d for i n d e n t i f y i n g t h e p h y s i c a l and chemical t r a n s i t i o n s o c c u r r e d i n t h e m a t r i x . E l e c t r i c a l r e s i s t i v i t y of t h e m a t e r i a l was m e a s u r e d in t h e t e m p e r a t u r e r a n g e of 30 to 180°C. T h e e l e c t r i c a l a n d t h e r m a l d a t a were c o r r e l a t e d .
Introduction T h i o u r e a is a w e l l - k n o w n f e r r o e l e c t r i c (1). T h i o u r e a c r y s t a l r e p r e s e n t s a form of f e r r o e l e c t r i c i t y i n which t h e p o l a r i z a t i o n a r i s e s from t h e motion of t h e whole molecule in t h e l a t t i c e (2}. S e v e r a l i n v e s t i g a t i o n s on e l e c t r i c a l p r o p e r t i e s (3), s u c h as v a r i a t i o n of d i e l e c t r i c c o n s t a n t with t e m p e r a t u r e , p y r o e l e c t r i c , p i e z o e l e c t r i c and f e r r o e l e c t r i c e f f e c t s , and s p o n t a n e o u s p o l a r i z a t i o n p h e n o m e n o n h a v e p r o v e d t h i o u r e a to b e a n i n t e r e s t i n g o r g a n i c c o m p o u n d . Hence it was of i n t e r e s t to e x a m i n e t h e e l e c t r i c a l b e h a v i o u r of s u c h a f e r r o e l e c t r i c s u b s t a n c e w h e n i n c o r p o r a t e d as a p a r t of a p o l y m e r m a t r i x . K e e p i n g t h i s i n v i e w , work has b e e n d o n e i n o u r l a b o r a t o r y on t h i o u r e a - f o r m a l d e h y d e (TUF) c o n d e n s a t e s . Two t y p e s of TUF c o n d e n s a t e s , one in acidic medium u s i n g acetic acid and one i n b a s i c medium u s i n g sodium h y d r o x i d e , were p r e p a r e d a n d t h e i r e l e c t r i c a l p r o p e r t i e s were s t u d i e d ( 4 , 5 ) . T h e p r e s e n t a r t i c l e r e p o r t s t h e s y n t h e s i s of TUF c o n d e n s a t e i n b a s i c medium, its c h a r a c t e r i z a t i o n and e l e c t r i c a l r e s i s t i v i t y .
Experimental The TUF condensate was prepared analogously to urea-formaldehyde resins by the reaction of thiourea with formaldehyde in sodium hydroxide. Weighed 897
898
H.S.
NALWA, e t al.
Vol. 18, No. 7
a m o u n t s of t h i o u r e a and 37% formaline were t a k e n i n t h e molar ratio of 1 : 1 . 7 and c h a r g e d i n t o a r e a c t i o n k e t t l e e q u i p p e d with a m e c h a n i c a l s t i r r e r and r e flux c o n d e n s e r . T h e pH of the m i x t u r e was a d j u s t e d to 7-8 with 2N sodium h y d r o x i d e s o l u t i o n . T h e m i x t u r e was r e f l u x e d for f o u r h o u r s and cooled; t h e p r o d u c t was t h e n w a s h e d s e v e r a l times with distilled w a t e r and d r i e d u n d e r v a c u u m at room t e m p e r a t u r e . T h e c l e a r pale yellow TUF c o n d e n s a t e was t h u s obtained. C a r b o n a n d h y d r o g e n were d e t e r m i n e d b y C a r b o n - H y d r o g e n A n a l y z e r (Coleman Model 33, P e r k i n - E l m e r ) . N i t r o g e n was d e t e r m i n e d b y Kjeldahl m e t h o d . S u l f u r was c o n v e r t e d i n t o s u f a t e s b y u s i n g o x i d i z i n g a g e n t s and e s timated as b a r i u m s u l f a t e . T h e i n f r a r e d s p e c t r u m was r e c o r d e d as KBr pellet o v e r t h e r a n g e of 400-4000 cm -1 b y IR S p e c t r o m e t e r (SP 1200, Unicam I n s t r u m e n t s L t d . , E n g l a n d ) . DTG, TGA a n d DSC s c a n s were r e c o r d e d s i m u l t a n e o u s l y at a h e a t i n g r a t e of 5°C/rain. i n a i r , u s i n g D e r i v a t o g r a p h (MOM, B u d a p e s t , Hungary). T h e e l e c t r i c a l r e s i s t i v i t y of t h e sample was m e a s u r e d i n t h e form of p e l l e t s p r e s s e d at a load of 9 t o n s at room t e m p e r a t u r e u n d e r v a c u u m . T h e p e l l e t s were s a n d w i c h e d b e t w e e n two aluminium e l e c t r o d e s o r coated b y aluminium u n d e r v a c u u m for making e l e c t r i c a l c o n t a c t s . A t w o - p r o b e method was employed for m e a s u r i n g e l e c t r i c a l r e s i s t i v i t y u s i n g a s t a b i l i z e d power s u p p l y a n d a d . c . m i c r o v o l t m e t e r . T h e e l e c t r i c a l r e s i s t i v i t y was d e t e r m i n e d b e t w e e n th~ t e m p e r a t u r e r a n g e of 30 to 180°C u n d e r the applied e l e c t r i c field of 2.50x10 ° V/cm. Results and Discussion The elemental analysis obtained for several samples of T U F condensate from different batches show reproducibility with the average values of C = 29.41%, H = 4.93%, N = 27.45%, S = 31.37% and O = 7.84%. The empirical formula corresponding to this elemental analysis is C5HION4S20. The chemical formula obtained could not easily be matched with any one of the k n o w n compounds (6). No literature data is ava/lable on the formation of condensates in the presence of sodium hydroxide; condensates obtained in barium hydroxide have been reported to be monomethylol or dlmethylo] thloureas and their higher condensates. The sulfur to nitrogen ratio is 1:2, as to be expected since both of them are obtained from thiourea having this ratio. However, carbon besides being present in thlourea segments will also be present in bridges like -CH2, C H 2 - O - C H 2 - , and at the side group such as - N H - C H 2 - O H , etc. Looking into the literature, it is seen that very few compounds have been reported having carbon content as high as 30%. Only in the condensate of 2 tool of formaldehyde with 1 tool of thlourea, formation of such compositions have been reported (6). The melting point of the compound is 198°C. It is insoluble in organic solvents except in dimethyl formamide and caprolactum. The molecular weight determined by the depression of freezing point in caprolactrum gives a value of 206. From the elemental and molecular weight data, the following chemical structures are proposed:
Vol. 18, No. 7
THIOUREA-FORMALDEHYDE
899
S
(i)
ii
/c \
S ii
HN
N - C H 2 - NH - C - NH 2
O
(it)
S
S
||
i!
C H 2 = N - C- NH-CH2-O-CH2-NH-C-NH 2
The p o s s i b i l i t y of t h e formation of e t h e r l i n k a g e i n TUF c o n d e n s a t e is likely b e c a u s e t h e c o n d e n s a t i o n r e a c t i o n b e t w e e n t h i o u r e a a n d f o r m a l d e h y d e was c a r r i e d o u t at h i g h e r t e m p e r a t u r e for f o u r h o u r s , w h e r e t h e elimination of w a t e r may t a k e place from t h e two methylol g r o u p s o r from methylol and amino e n d g r o u p s . T h e p o s s i b l e r e a c t i o n mechanism of m e t h y l o l a t i o n and c o n d e n s a t i o n p r o c e s s e s i n a l k a l i n e medium may be i l l u s t r a t e d as follows: S
S
H 2 N - C - N H 2 + O--H--~H2N - ~ - N H +
S
H20
S
H 2 N - C - N H + C H 2 0 - - - ~ H 2 N - C - NH - C H 2 0 S
S
H 2N-C - NH-CH20
+ H 2 0 - - + H 2 N - C -"N H - C H 2 O H
+ OH
monom e t h y l o l - t h i o u r e a
-NH-CH2OH
+ HOCH2-NH-
-
~-NH-CH2-O-CH2-NH-+H20
ether l i n k a g e T h e p r i n c i p a l IR a b s o r p t i o n b a n d s o b s e r v e d a n d t h e i r a s s i g n m e n t s h a v e b e e n l i s t e d i n T a b l e 1 and c o m p a r e d with t h e l i t e r a t u r e calues (7). T h e r e is a good a g r e e m e n t b e t w e e n t h e s p e c t r a o b t a i n e d and r e p o r t e d . T h e IR a b s o r p tion b a n d s o b s e r v e d a r e : 678, 955, 1010, 1130, 1300, 1350, 1460, 1520, 1570, 1630, 2060 a n d 3330 cm -1. T h e IR s p e c t r a of some TUF c o n d e n s a t e s s u c h as monomethylol t h i o u r e a , dimethylol t h i o u r e a , m e t h y l e n e b i s t h i o u r e a , d i m e t h y l e n e t r i s t h i o u r e a a n d o t h e r s u b s t i t u t e d d e r i v a t i v e s h a v e b e e n r e p o r t e d in t h e literature. T h e IR s p e c t r u m of t h e a b o v e p r o d u c t c o r r e s p o n d s e x t e n s i v e l y to an o v e r l a p p i n g from t h e s p e c t r a of mono a n d d i m e t h y l o l - m e t h y l e n e b i s t h i o u r e a . I n c o m p a r i s o n with t h e IR s p e c t r u m of t h e s a m p l e , t h e p r e s e n c e of -NH, CH2,
900
H . S . NALWA, et al.
Vol. 18, No. 7
TABLE 1 IR S p e c t r a l Data of TUF C o n d e n s a t e IR A b s o r p t i o n b a n d s observed
IR A b s o r p t i o n b a n d s reported in the Htera t u r e (7)
Band Assignments
(era-I)
(cm -I)
678
675
-SCN
955
955
N-CH2-N, - N - C H 2 - O
1010
1012
-CH2OH
1130
1130
-N-CH2-O ,
1300
1295
-NH
1350
1351
-NH
1460
1468
-CH 2
1520
1525
-CN 2
1570
1563
-NH 2
1630
1622
-NH 2
2060
2060
-SCN
3330
3330
-NH 2, -OH
-N-CH2-O-CH2-N-
- N - C H 2 0 , - N - C H 2 - O - C H 2 - N - , -NH 2 and SCN b a n d s a r e c l e a r l y i n d i c a t e d . T h e g e n e r a I f e a t u r e s of IR s p e c t r u m seem to confirm t h e p r o p o s e d chemical s t r u c t u r e of TUF c o n d e n s a t e i n which t h i o u r e a s e g m e n t s are l i n k e d t h r o u g h e t h e r badges. F o r e x a m i n i n g t h e r m a l decomposition a n d p h y s i c a l as well as chemical t r a n s i t i o n s o c c u r r i n g i n t h e sample, t h e t h e r m a l methods s u c h as DTG, TGA a n d DSC were s t u d i e d , as r e p r e s e n t e d i n Fig. 1. T h e c u m u l a t i v e w e i g h t loss is less t h a n 2% up to 150°C, a n d an i n c r e a s e of u p to 75% is r e c o r d e d at 200°C. T h e m e l t i n g p o i n t of t h e compound as s h o w n b y DTG is 198°C. I n t h e DSC s c a n , two t r a n s i t i o n s at 140 and 156°C were r e c o r d e d . The data obtained from t h e r m a l a n a l y s i s h a v e b e e n c o r r e l a t e d s u b s e q u e n t l y with t h e e l e c t r i c a l b e h a v i o u r of t h e c o m p o u n d . F i g u r e 2 shows t h e plot of log s p e c i f i c r e s i s t i v i t y (p) v e r s u s 1/T h a v i n g t h r e e b r e a k s at 80, 120 a n d 140°C d i v i d i n g t h e line i n t o f o u r r e g i o n s . At room t e m p e r a t u r e t h e p was of t h e o r d e r of 8.9x1012 ohm cm, i n d i c a t i n g t h e i n s u l a t l n g p r o p e r t y of t h e material, a n d d e c r e a s e s with t h e i n c r e a s e of t e m p e r a t u r e . T h e a c t i v a t i o n e n e r g y (Ea) was c a l c u l a t e d from t h e A r r h e n i u s e q u a t i o n P = P0 e x p
Ea
and Ea v a l u e s were f o u n d : 0.45, 1.10, 1.96 and 5.00 eV for t h e t e m p e r a t u r e r e g i o n s of 30 to 80, 80 to 120, 120 to 140 a n d 140 to 180°C, r e s p e c t i v e l y . T h e DSC s c a n of t h e sample (Fig. 1) showed an e x o t h e r m i c peak at 140°C a n d log p v e r s u s 1/T c u r v e also e x h i b i t e d a b r e a k at t h i s p a r t i c u l a r t e m p e r a t u r e ; h e n c e t h e r e is a t r a n s i t i o n at 140°C. A d i e l e c t r i c t r a n s i t i o n n e a r 140°C was also o b -
Vo]. 18, No. 7
THIOUREA-FORMALDEHYDE
901
lOO[
70 ~so 8O
so
~ 3o
~2o 10 I
0
100
1
200 Temperoture ~ °C
FIG. DTG,
TGA
and D S C
I
I
300
400
1
scans of T U F
13.0
condensate. 0.45 eV
Eo12.0
E e-
o,11.o
lO.O.oo/ T 9,Q.
~I 2.3
I 2.5
I 22
I 2.9
I
31
1/T x 10-3,( K-1 ) FIG. Log p v e r s u s
2
1/T plot of TU F c o n d e n s a t e .
902
H.S.
NALWA, e t al,
Vol. 18, No. 7
s e r v e d in t h e s a m p l e while s t u d y i n g t h e d i e l e c t r i c c o n s t a n t a n d loss t a n g e n t as a function of temperature (8). Moreover, the thermally stimulated depolarizat i o n (TSD) c u r r e n t s t u d i e s (9) of p o l a r i z e s as well as u n p o l a r i z e d TUF s a m p l e s e x h i b i t e d TSD p e a k at 140 to 145°C. T h u s , t h e e l e c t r i c a l d a t a along with t h e DSC t e c h n i q u e i n d i c a t e t h e p r e s e n c e of a d i s t i n c t t r a n s i t i o n in t h i o u r e a - f o r m a l d e h y d e c o n d e n s a t e in t h e t e m p e r a t u r e r a n g e of 140 to 145°C. I t is p r o b a b l y d u e to t h e p h y s l c o - c h e m i c a l c h a n g e s o c c u r r i n g in t h e m a t e r i a l . Acknowledgments T h e a u t h o r s e x p r e s s t h e i r t h a n k s to t h e I n d i a n S p a c e R e s e a r c h O r g a n i z a t i o n (ISRO) a n d t h e C o u n c i l of S c i e n t i f i c and I n d u s t r i a l R e s e a r c h (CSIR) f o r financial assistance.
References 1.
G. 7. G o l d s m i t h a n d :[. C. White, Chem. P h y s .
2.
N . E . Hill, W. E. V a u g h a n , A. H. P r i c e a n d M. D a v i e s , D i e l e c t r i c P r o p e r t i e s and Molecular B e h a v i o u r , R e i n h o l d , L o n d o n (1969).
3.
I.S. Z h e l u d e v , P h y s i c s of C r y s t a l l i n e D i e l e c t r i c s , Plenum P r e s s , Y o r k (1971).
Nalwa and P. Vasudevan,
3..1_1, 1175 (1959).
New
4.
H.S.
5.
H . S . Nalwa, Ph.D. Thesis: Electrical Properties of Macromolecules: Studies on Some Thlourea and Phthalocyanine Systems, lIT, Delhi (1982).
Eur. Polym. J. 17, 145 (1981).
6.
H. Staudlnger and K. Wagner, Makromol. Chem. 12, 168 (1954).
7.
H. [[. Becher and F. Grlffel, Chem.
8.
P. Vasudevan, H. S. Nalwa, K. L. TaneJa and U. S. Tewarl, [[. App]. Phys. 50, 4324 (1979).
Ber. 91, 691, 2025, 2032 (1958).
9.
H . S . Nalwa, P. S. Vlswanathan and P. Vasudevan, Chem. 82, 39 (1979).
Angew.
Makromo].
THIOUREA-FORMALDEHYDE CONDENSATE: SYNTHESIS, CHARACTERIZATION AND ELECTRICAL RESISTIVITY Hari S i n g h Nalwa a n d Padma V a s u d e v a n D e p a r t m e n t of C h e m i s t r y and C e n t r e f o r R u r a l D e v e l o p m e n t & Appropriate Technology I n d i a n I n s t i t u t e of T e c h n o l o g y New Delhi l l 0 016, I n d i a ( R e c e i v e d May 26, 1983; C o m m u n i c a t e d b y S. A m e l i n c k x )
ABSTRACT A t h i o u r e a - f o r m a l d e h y d e c o n d e n s a t e was s y n t h e s i z e d b y c o n d e n s i n g t h i o u r e a with f o r m a l d e h y d e i n a l k a l i n e medium u s i n g sodium h y d r o x ide. T h e c o n d e n s a t e was c h a r a c t e r i z e d b y e l e m e n t a l a n a l y s i s and infrared spectroscopy. D e r i v a t i v e t h e r m o g r a v i m e t r y (DTG) a n d t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) t e c h n i q u e s were c a r r i e d o u t to e x a m i n e t h e t h e r m a l s t a b i l i t y of t h e material. D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) was also u s e d for i n d e n t i f y i n g t h e p h y s i c a l and chemical t r a n s i t i o n s o c c u r r e d i n t h e m a t r i x . E l e c t r i c a l r e s i s t i v i t y of t h e m a t e r i a l was m e a s u r e d in t h e t e m p e r a t u r e r a n g e of 30 to 180°C. T h e e l e c t r i c a l a n d t h e r m a l d a t a were c o r r e l a t e d .
Introduction T h i o u r e a is a w e l l - k n o w n f e r r o e l e c t r i c (1). T h i o u r e a c r y s t a l r e p r e s e n t s a form of f e r r o e l e c t r i c i t y i n which t h e p o l a r i z a t i o n a r i s e s from t h e motion of t h e whole molecule in t h e l a t t i c e (2}. S e v e r a l i n v e s t i g a t i o n s on e l e c t r i c a l p r o p e r t i e s (3), s u c h as v a r i a t i o n of d i e l e c t r i c c o n s t a n t with t e m p e r a t u r e , p y r o e l e c t r i c , p i e z o e l e c t r i c and f e r r o e l e c t r i c e f f e c t s , and s p o n t a n e o u s p o l a r i z a t i o n p h e n o m e n o n h a v e p r o v e d t h i o u r e a to b e a n i n t e r e s t i n g o r g a n i c c o m p o u n d . Hence it was of i n t e r e s t to e x a m i n e t h e e l e c t r i c a l b e h a v i o u r of s u c h a f e r r o e l e c t r i c s u b s t a n c e w h e n i n c o r p o r a t e d as a p a r t of a p o l y m e r m a t r i x . K e e p i n g t h i s i n v i e w , work has b e e n d o n e i n o u r l a b o r a t o r y on t h i o u r e a - f o r m a l d e h y d e (TUF) c o n d e n s a t e s . Two t y p e s of TUF c o n d e n s a t e s , one in acidic medium u s i n g acetic acid and one i n b a s i c medium u s i n g sodium h y d r o x i d e , were p r e p a r e d a n d t h e i r e l e c t r i c a l p r o p e r t i e s were s t u d i e d ( 4 , 5 ) . T h e p r e s e n t a r t i c l e r e p o r t s t h e s y n t h e s i s of TUF c o n d e n s a t e i n b a s i c medium, its c h a r a c t e r i z a t i o n and e l e c t r i c a l r e s i s t i v i t y .
Experimental The TUF condensate was prepared analogously to urea-formaldehyde resins by the reaction of thiourea with formaldehyde in sodium hydroxide. Weighed 897
898
H.S.
NALWA, e t al.
Vol. 18, No. 7
a m o u n t s of t h i o u r e a and 37% formaline were t a k e n i n t h e molar ratio of 1 : 1 . 7 and c h a r g e d i n t o a r e a c t i o n k e t t l e e q u i p p e d with a m e c h a n i c a l s t i r r e r and r e flux c o n d e n s e r . T h e pH of the m i x t u r e was a d j u s t e d to 7-8 with 2N sodium h y d r o x i d e s o l u t i o n . T h e m i x t u r e was r e f l u x e d for f o u r h o u r s and cooled; t h e p r o d u c t was t h e n w a s h e d s e v e r a l times with distilled w a t e r and d r i e d u n d e r v a c u u m at room t e m p e r a t u r e . T h e c l e a r pale yellow TUF c o n d e n s a t e was t h u s obtained. C a r b o n a n d h y d r o g e n were d e t e r m i n e d b y C a r b o n - H y d r o g e n A n a l y z e r (Coleman Model 33, P e r k i n - E l m e r ) . N i t r o g e n was d e t e r m i n e d b y Kjeldahl m e t h o d . S u l f u r was c o n v e r t e d i n t o s u f a t e s b y u s i n g o x i d i z i n g a g e n t s and e s timated as b a r i u m s u l f a t e . T h e i n f r a r e d s p e c t r u m was r e c o r d e d as KBr pellet o v e r t h e r a n g e of 400-4000 cm -1 b y IR S p e c t r o m e t e r (SP 1200, Unicam I n s t r u m e n t s L t d . , E n g l a n d ) . DTG, TGA a n d DSC s c a n s were r e c o r d e d s i m u l t a n e o u s l y at a h e a t i n g r a t e of 5°C/rain. i n a i r , u s i n g D e r i v a t o g r a p h (MOM, B u d a p e s t , Hungary). T h e e l e c t r i c a l r e s i s t i v i t y of t h e sample was m e a s u r e d i n t h e form of p e l l e t s p r e s s e d at a load of 9 t o n s at room t e m p e r a t u r e u n d e r v a c u u m . T h e p e l l e t s were s a n d w i c h e d b e t w e e n two aluminium e l e c t r o d e s o r coated b y aluminium u n d e r v a c u u m for making e l e c t r i c a l c o n t a c t s . A t w o - p r o b e method was employed for m e a s u r i n g e l e c t r i c a l r e s i s t i v i t y u s i n g a s t a b i l i z e d power s u p p l y a n d a d . c . m i c r o v o l t m e t e r . T h e e l e c t r i c a l r e s i s t i v i t y was d e t e r m i n e d b e t w e e n th~ t e m p e r a t u r e r a n g e of 30 to 180°C u n d e r the applied e l e c t r i c field of 2.50x10 ° V/cm. Results and Discussion The elemental analysis obtained for several samples of T U F condensate from different batches show reproducibility with the average values of C = 29.41%, H = 4.93%, N = 27.45%, S = 31.37% and O = 7.84%. The empirical formula corresponding to this elemental analysis is C5HION4S20. The chemical formula obtained could not easily be matched with any one of the k n o w n compounds (6). No literature data is ava/lable on the formation of condensates in the presence of sodium hydroxide; condensates obtained in barium hydroxide have been reported to be monomethylol or dlmethylo] thloureas and their higher condensates. The sulfur to nitrogen ratio is 1:2, as to be expected since both of them are obtained from thiourea having this ratio. However, carbon besides being present in thlourea segments will also be present in bridges like -CH2, C H 2 - O - C H 2 - , and at the side group such as - N H - C H 2 - O H , etc. Looking into the literature, it is seen that very few compounds have been reported having carbon content as high as 30%. Only in the condensate of 2 tool of formaldehyde with 1 tool of thlourea, formation of such compositions have been reported (6). The melting point of the compound is 198°C. It is insoluble in organic solvents except in dimethyl formamide and caprolactum. The molecular weight determined by the depression of freezing point in caprolactrum gives a value of 206. From the elemental and molecular weight data, the following chemical structures are proposed:
Vol. 18, No. 7
THIOUREA-FORMALDEHYDE
899
S
(i)
ii
/c \
S ii
HN
N - C H 2 - NH - C - NH 2
O
(it)
S
S
||
i!
C H 2 = N - C- NH-CH2-O-CH2-NH-C-NH 2
The p o s s i b i l i t y of t h e formation of e t h e r l i n k a g e i n TUF c o n d e n s a t e is likely b e c a u s e t h e c o n d e n s a t i o n r e a c t i o n b e t w e e n t h i o u r e a a n d f o r m a l d e h y d e was c a r r i e d o u t at h i g h e r t e m p e r a t u r e for f o u r h o u r s , w h e r e t h e elimination of w a t e r may t a k e place from t h e two methylol g r o u p s o r from methylol and amino e n d g r o u p s . T h e p o s s i b l e r e a c t i o n mechanism of m e t h y l o l a t i o n and c o n d e n s a t i o n p r o c e s s e s i n a l k a l i n e medium may be i l l u s t r a t e d as follows: S
S
H 2 N - C - N H 2 + O--H--~H2N - ~ - N H +
S
H20
S
H 2 N - C - N H + C H 2 0 - - - ~ H 2 N - C - NH - C H 2 0 S
S
H 2N-C - NH-CH20
+ H 2 0 - - + H 2 N - C -"N H - C H 2 O H
+ OH
monom e t h y l o l - t h i o u r e a
-NH-CH2OH
+ HOCH2-NH-
-
~-NH-CH2-O-CH2-NH-+H20
ether l i n k a g e T h e p r i n c i p a l IR a b s o r p t i o n b a n d s o b s e r v e d a n d t h e i r a s s i g n m e n t s h a v e b e e n l i s t e d i n T a b l e 1 and c o m p a r e d with t h e l i t e r a t u r e calues (7). T h e r e is a good a g r e e m e n t b e t w e e n t h e s p e c t r a o b t a i n e d and r e p o r t e d . T h e IR a b s o r p tion b a n d s o b s e r v e d a r e : 678, 955, 1010, 1130, 1300, 1350, 1460, 1520, 1570, 1630, 2060 a n d 3330 cm -1. T h e IR s p e c t r a of some TUF c o n d e n s a t e s s u c h as monomethylol t h i o u r e a , dimethylol t h i o u r e a , m e t h y l e n e b i s t h i o u r e a , d i m e t h y l e n e t r i s t h i o u r e a a n d o t h e r s u b s t i t u t e d d e r i v a t i v e s h a v e b e e n r e p o r t e d in t h e literature. T h e IR s p e c t r u m of t h e a b o v e p r o d u c t c o r r e s p o n d s e x t e n s i v e l y to an o v e r l a p p i n g from t h e s p e c t r a of mono a n d d i m e t h y l o l - m e t h y l e n e b i s t h i o u r e a . I n c o m p a r i s o n with t h e IR s p e c t r u m of t h e s a m p l e , t h e p r e s e n c e of -NH, CH2,
900
H . S . NALWA, et al.
Vol. 18, No. 7
TABLE 1 IR S p e c t r a l Data of TUF C o n d e n s a t e IR A b s o r p t i o n b a n d s observed
IR A b s o r p t i o n b a n d s reported in the Htera t u r e (7)
Band Assignments
(era-I)
(cm -I)
678
675
-SCN
955
955
N-CH2-N, - N - C H 2 - O
1010
1012
-CH2OH
1130
1130
-N-CH2-O ,
1300
1295
-NH
1350
1351
-NH
1460
1468
-CH 2
1520
1525
-CN 2
1570
1563
-NH 2
1630
1622
-NH 2
2060
2060
-SCN
3330
3330
-NH 2, -OH
-N-CH2-O-CH2-N-
- N - C H 2 0 , - N - C H 2 - O - C H 2 - N - , -NH 2 and SCN b a n d s a r e c l e a r l y i n d i c a t e d . T h e g e n e r a I f e a t u r e s of IR s p e c t r u m seem to confirm t h e p r o p o s e d chemical s t r u c t u r e of TUF c o n d e n s a t e i n which t h i o u r e a s e g m e n t s are l i n k e d t h r o u g h e t h e r badges. F o r e x a m i n i n g t h e r m a l decomposition a n d p h y s i c a l as well as chemical t r a n s i t i o n s o c c u r r i n g i n t h e sample, t h e t h e r m a l methods s u c h as DTG, TGA a n d DSC were s t u d i e d , as r e p r e s e n t e d i n Fig. 1. T h e c u m u l a t i v e w e i g h t loss is less t h a n 2% up to 150°C, a n d an i n c r e a s e of u p to 75% is r e c o r d e d at 200°C. T h e m e l t i n g p o i n t of t h e compound as s h o w n b y DTG is 198°C. I n t h e DSC s c a n , two t r a n s i t i o n s at 140 and 156°C were r e c o r d e d . The data obtained from t h e r m a l a n a l y s i s h a v e b e e n c o r r e l a t e d s u b s e q u e n t l y with t h e e l e c t r i c a l b e h a v i o u r of t h e c o m p o u n d . F i g u r e 2 shows t h e plot of log s p e c i f i c r e s i s t i v i t y (p) v e r s u s 1/T h a v i n g t h r e e b r e a k s at 80, 120 a n d 140°C d i v i d i n g t h e line i n t o f o u r r e g i o n s . At room t e m p e r a t u r e t h e p was of t h e o r d e r of 8.9x1012 ohm cm, i n d i c a t i n g t h e i n s u l a t l n g p r o p e r t y of t h e material, a n d d e c r e a s e s with t h e i n c r e a s e of t e m p e r a t u r e . T h e a c t i v a t i o n e n e r g y (Ea) was c a l c u l a t e d from t h e A r r h e n i u s e q u a t i o n P = P0 e x p
Ea
and Ea v a l u e s were f o u n d : 0.45, 1.10, 1.96 and 5.00 eV for t h e t e m p e r a t u r e r e g i o n s of 30 to 80, 80 to 120, 120 to 140 a n d 140 to 180°C, r e s p e c t i v e l y . T h e DSC s c a n of t h e sample (Fig. 1) showed an e x o t h e r m i c peak at 140°C a n d log p v e r s u s 1/T c u r v e also e x h i b i t e d a b r e a k at t h i s p a r t i c u l a r t e m p e r a t u r e ; h e n c e t h e r e is a t r a n s i t i o n at 140°C. A d i e l e c t r i c t r a n s i t i o n n e a r 140°C was also o b -
Vo]. 18, No. 7
THIOUREA-FORMALDEHYDE
901
lOO[
70 ~so 8O
so
~ 3o
~2o 10 I
0
100
1
200 Temperoture ~ °C
FIG. DTG,
TGA
and D S C
I
I
300
400
1
scans of T U F
13.0
condensate. 0.45 eV
Eo12.0
E e-
o,11.o
lO.O.oo/ T 9,Q.
~I 2.3
I 2.5
I 22
I 2.9
I
31
1/T x 10-3,( K-1 ) FIG. Log p v e r s u s
2
1/T plot of TU F c o n d e n s a t e .
902
H.S.
NALWA, e t al,
Vol. 18, No. 7
s e r v e d in t h e s a m p l e while s t u d y i n g t h e d i e l e c t r i c c o n s t a n t a n d loss t a n g e n t as a function of temperature (8). Moreover, the thermally stimulated depolarizat i o n (TSD) c u r r e n t s t u d i e s (9) of p o l a r i z e s as well as u n p o l a r i z e d TUF s a m p l e s e x h i b i t e d TSD p e a k at 140 to 145°C. T h u s , t h e e l e c t r i c a l d a t a along with t h e DSC t e c h n i q u e i n d i c a t e t h e p r e s e n c e of a d i s t i n c t t r a n s i t i o n in t h i o u r e a - f o r m a l d e h y d e c o n d e n s a t e in t h e t e m p e r a t u r e r a n g e of 140 to 145°C. I t is p r o b a b l y d u e to t h e p h y s l c o - c h e m i c a l c h a n g e s o c c u r r i n g in t h e m a t e r i a l . Acknowledgments T h e a u t h o r s e x p r e s s t h e i r t h a n k s to t h e I n d i a n S p a c e R e s e a r c h O r g a n i z a t i o n (ISRO) a n d t h e C o u n c i l of S c i e n t i f i c and I n d u s t r i a l R e s e a r c h (CSIR) f o r financial assistance.
References 1.
G. 7. G o l d s m i t h a n d :[. C. White, Chem. P h y s .
2.
N . E . Hill, W. E. V a u g h a n , A. H. P r i c e a n d M. D a v i e s , D i e l e c t r i c P r o p e r t i e s and Molecular B e h a v i o u r , R e i n h o l d , L o n d o n (1969).
3.
I.S. Z h e l u d e v , P h y s i c s of C r y s t a l l i n e D i e l e c t r i c s , Plenum P r e s s , Y o r k (1971).
Nalwa and P. Vasudevan,
3..1_1, 1175 (1959).
New
4.
H.S.
5.
H . S . Nalwa, Ph.D. Thesis: Electrical Properties of Macromolecules: Studies on Some Thlourea and Phthalocyanine Systems, lIT, Delhi (1982).
Eur. Polym. J. 17, 145 (1981).
6.
H. Staudlnger and K. Wagner, Makromol. Chem. 12, 168 (1954).
7.
H. [[. Becher and F. Grlffel, Chem.
8.
P. Vasudevan, H. S. Nalwa, K. L. TaneJa and U. S. Tewarl, [[. App]. Phys. 50, 4324 (1979).
Ber. 91, 691, 2025, 2032 (1958).
9.
H . S . Nalwa, P. S. Vlswanathan and P. Vasudevan, Chem. 82, 39 (1979).
Angew.
Makromo].