Mixed polyurethanes containing substituted difluoromethylene groups in the chain

MIXED P O L Y U R E T H A N E S CONTAINING SUBSTITUTED D I F L U O R O M E T H Y L E N E GROUPS IN THE CHAIN * B. F. MALICHENKO, YE. V. SItELUD'KO, YU. Yu. KEgCHA alld R. L. SAVCRENKO Institute of Chemistry and High Molecular Weight Cornpounds, Ukr.S.S.R. Academy of Sciences (Received 3 November 1969)

IN A previous paper [1] we reported oi1 the properties of polyurethanes synthesized by interracial polycondensation from 2,2,3,3,4,4,5,5-octafluoro-l,6-hexamethylenediamine (OF[-IM_DA) and hexamethylene-l,6-bis-(chloroformate) (HMBC) and found t h a t the thermal stability of these polymers was better than t h a t of their non-fluorinated anMogues. A study of the mechanism of thermal degradation of the polyurethanes referred to above revealed a nmnber of quantitative characteristics of the thermal degradation process [2]. I t was desired to investigate the possibility of improving the thermal stability of non-fluorinated polyurethanes by replacing part of the diamine by its fluorinated analogue during the synthesis. With this in view mixed polyurethanes were synthesized from fluorinated and nonfluorinated 1,6-hexamethylenediamine and HMBC. In the diamine mixtures used in preparation of the polyurethanes the content of OFHMDA was varied fi'om 10 to 90 mole % (rising progressively by 10%). The synthesis of these polyurethanes was carried out under identical conditions. In order to compare the physieochemieal properties under similar conditions we synthesized polyurethanes from pure fluorinated or non-fluorinated 1,6-hexamethylenediamine. I t was also desired to study the effect of the method of synthesis (interfacial polycondensation or bulk polymerization) on the behaviour of the polymers, and to do so we used 2,2,3,3,4,4,5,5-octafluorohexamethylene-l,6-diisocyanate (OFHMDI). The phosgenation of OFHMDA in different solvents proceeds only with difficulty, and therefore the approI)ria.te dimethylurethane was prepared from OFHMDA and methyl chloroformate. By heating the dimethylurethane with phosphorus 1)entoxide we were able to obtain OFHMDI in low yield, and this was used for the bulk polymerization. EXPERIMENTAL

2,2,3,4,4,5,5-O(~taJluorol~examethyle~e-l,6-dimethyl urethanc (OFHMDU). To a soluti~m of 2.3 g of OFHMDA ii~ 25 ml of anhydrous benzene were added dropwise 2 g of methyl chloroformate and 2.24 ml of triethylamine, using two dropping funnels simultaneously,

* Vysokomol. soyed. A13: No. 1, 156 161, 1971.

177

B. F. MALICHENKO et al.

178

while stirring and cooling w i t h iced water. The m i x t u r e was stirred for 2 tLr, after which t h e p r e c i p i t a t e was filtered, washed w i t h w a t e r to eliminate C1 ions, and t h e n dried. Yield 2"76 g (83 Yo), m.p. 134-135 ° (from a q u o e u s methanol). F o u n d ~ : F 40.26; 40.30. C10HI~FsN~O 4. Calculated, Yo: F 40.40. OFHMDI. A m i x t u r e of 2.76 g of OFI-IMDU and 10 g of phosphorus p e n t o x i d e was h e a t e d w i t h a gas b u r n e r and the diisocyanate was distilled off simultaneously. Yield 0.57 g (26~o), b.p. 220 ° or 69-70°/3 ram. The m e t h y l u r e t h a n e (m.p. 134-135 ° does n o t depress t h e m e l t i n g p o i n t w h e n i n v o l v e d in a m i x i n g t e s t w i t h t h e d i m e t h y l u r e t h a n e sample. P r i o r to t h e p r e p a r a t i o n of p o l y u r e t h a n e s t h e OFI-IMDI u n d e r w e n t r e p e a t e d v a c u u m - d i s t i l l a t i o n . Starting products. 1 , 6 - H e x a m e t h y l e n e d i a m i n e (HMDA) was purified b y v a c u u m - d i s t i l l a tion; t h e p r o d u c t w i t h m.p. 40 ° was used. O F H M D A u n d e r w e n t v a c u u m - f r a c t i o n and the p r o d u c t w i t h m.p. 44-45 ° was used, this m.p. being in accordance w i t h p a p e r [3]). D e x a n e diol-1,6 was purified b y v a c u u m - d i s t i l l a t i o n and by crystallization f r o m benzene; t h e p r o d u c t w i t h m.p. 42 ° was used. H M B C was purified b y v a e u u m - f r a e t i o n a t i o n ; the p r o d u c t used h a d b.p. 131-133°/4 ram, which is in accordance w i t h [4]. Interracial polycondensation of polyurethanes. A solution of 0.01 mole of HlV[BC in 25 ml of a n h y d r o u s benzene was a d d e d in a single batch, while stirring rapidly, to a solution of 0.01 mole of the d.iamine (or diamine m i x t u r e ) and 0.8 g of s o d i u m h y d r o x i d e in 25 ml of w a t e r at 5 °. Stirring was c o n t i n u e d for 0.5 hr, the p r e c i p i t a t e was t h e n filtered, w a s h e d w i t h w a t e r ~nd t h e n w i t h alcohol (to r e m o v e the small a m o u n t of low m o l e c u l a r w e i g h t products) and v a c u u m - d r i e d to c o n s t a n t weight. The properties of the p o l y u r e t h a n e s are g i v e n in t h e Table.

PROPERTIES

OF

THE

HOMOPOLYURETHANES

BY INTERFACIAL

Molar ratio 0FHMDI

H_MDA

HMDI

MIXED

AND

[e], dl/g

Yield,

OFttMDA

AND

POLYCONDENSATION

%

BULK

POLYURETHANES POLYMERIZATION

SYNTHESIZED *

W e i g h t loss (Yo) u n d e r the action of heating

10 ~o-i2SO~

10 ~oo-NaOH

3.6 1.6 5.6 1.4 2.4 1.5 3.2

13 2'0 2"2 1'7 5'3 5"5 3'1 6"5 1.4 2'7

2 4'3 3.6 3"1 7"5 8.8 5.2 7.4 2.5 4.6

45 5.1 5.25

15 2'5 3"1

14 3"6 4"5

Interracial polyeondensation 0:100 10:90 20:80 30:70 ' 40:60 50:50 60:40 70:30 80:20 90:10

m

80 84 83 82 83 84 83 83 82 81

0:100 10:90 100:0

78 76 76

167-170 164-166 164-165 167 168 170 174-175 176 178 178

1.75 0.67 0.54 0.50 .0.30 0.30 0.20 0-31 0-17 0-17

41 1.5 1.05

Bulk polymerization 170 165 180-181

0.40t 0.38 0.32

* The structure of all the polyurethanes was confirmed by elemental analysis and by IR analysis. t Determined in a capillary. $ In dlmethylformamide; the others in m-cresol.

Mixed polyurethanes containing substituted diflu~mmtethyle,te groups in chaia

179

Preparation of polyurethanes by bulk polymerization. Into a flask fitted with a stirrer, thermometer, dropping funnel and purified nitrogen feed system was placed 0.01 mole of hexanediol-l,6, and while stirring at 70 80° for 30 rain, 0.01 mole of OFI-LMDI or hexamethylenediisocyalmte (HMDI) (or a mixture of both) was added dropwise, and the temperature was simultaneously raised to 180-190 °. The melt was stirred for 30 nfin at this temperature, and then cooled. The polyurethanes were purified by repreeipitation from dimethylfbrmamide or m-cresol, and vacuum-dried to constant weight. The intrinsic viscosity was determined at 30° m dimethylformamide or n~-cresol. The hydrolytic stability (boiling a weighed portion of polymer with ~ 20-fold mnount of lO';,;, sulphuric acid or 10~ sodium hydroxide for 6 hr) and the weight loss (heating a weighed i)ortio,t of polymer for 3.5 hr at 250 ° and 1 ram) were deter'mined by the weight method. The r(-sults are given in the Tabh~. DISCUSSION OF RESULTS

The Table gives the properties of the mixed p o l y u r e t h a n e s synthesized by interfaeiM p o l y c o n d e n s a t i o n or bulk polymerization. I t is noticeable t h a t the melting points of these polymers rise gradually with increase in the a m o u n t of O F H M D A ia the initial diamilm mixture. At the same time all the p o l y u r e t h a n e s melt within a v e r y n a r r o w t e m p e r a t u r e range. I t is k n o w n t h a t the use of interi~eial polycondensation, for instance in the p r e p a r a t i o n of polyamides from a m i x t u r e of different dicarboxylic acids, results in the f o r m a t i o n of block eopolymers [5]. The d e p e n d e n c e of the melting point of these block eopolymers on composition is expressed b y a curve with a euteetic m i n i m u m [5], and the two e n d o t h e r m i c peaks appearing on the difl~relltial era'yes of the heating t h e r m o grams show the melting of the crystalline regions f o r m e d b y the individuM kinds of block. The presence of two melting peaks on the differential heating curves indicates the f o r m a t i o n of block copolyiuers, and the m e t h o d of interfaeial p o l y c o n d e n s a t i o n is suggested for p r e p a r a t i o n of the latter [5]. I n papers [6, 7] we showed t h a t the i n t r o d u c t i o n of fluorine a t o m s into the diisoeyanate c o m p o n e n t of linear Miphatie p o l y u r e t h a n e s results in a change in the mode of melting of the p o l y u r e t h a n e samples crystallized from the melt; the change is m a r k e d b y spikes topping the melting peak of the heating t h e r m o g r a m . W e a t t r i b u t e d this to the fluorinated polym'ethane having the ability to form a p o l y m o r p h i c crystMline s t r u c t u r e owing to the contbrmatiom~l h e t e r o g e n e i t y of its macro-chain. I t was t h e n desired to s t u d y changes in the melting of mixed p o l y u r e t h a n e s as a f u n c t i o n of t h e i r c o n t e n t of O F H M D A milts. The p r o c e d u r e described in p a p e r [8] was used to record the t h e r m o g r a m s . Figure 1 shows the t h e r m o g r a m s for the heating of p o l y u r e t h a n e samples crystallized from the melt and thus having the same t h e r m M prehistory. I t is seen t h a t the m i x e d p o l y u r e t h a n e s containing from 10 to 20 mole ~o O F H M D A h a v e only a single melting p e a k (171 a n d 172°). The shape of the t h e r m o g r a m s for the p o l y u r e t h a n e s containing 30 mole (}/o or more of O F H M D A is similar to theft for the h o m o p o l y u r e t h a n e based on O F H M D A . The melting peak has two spikes, the temperattu'e w d u c of which is displaced into the higher temper-

180

B. F. MALICttENKO et al.

Another feature of major importance is the fact that all the thermograms for the cooling of melts (not given here) of homo- and mixed polyurethanes have a single crystallization peak. In the case of the block copolymers for systems

/

~7 °

--hE J

/b'O°tSl/°

|I

Ill I

f73°~ 18lI °

tszo/os, Fro. 1. Differential curves of mixed polyurethanes containing different amounts of O F H M D A (mole ~ ) : 1 - - 0 , 2 - - 1 0 , 3--20, 4--30, 5--40, 6--50, 7--60, 8--70, 9--80, 10--90, 11-- 100; 5 ' - mechanical mixture of homopolyurethanes cocrystallized from the melt and containing 40 mole ~ of fluorinated polyurethane

having considerable differences in the structure of the elementary units, for instance ethylene-propylene block copolymers [9] we find that the thermograms have two crystallization peaks. It is known that with statistical copolymers an increase in the content of the component present in a smaller amount is reflected in a reduction in the melting point of the copolymer. This is due to the disordering of the crystalline regions in the latter. In such cases we find there is a "eutectic point" in the region of medium compositions on the curve of composition vs. melting point [5, 10]. Figure 2 shows plots of composition vs. melting point and crystallization temperature for the synthesized mixed polyurethanes.

Mixed polyurethanes containing substituted difluommethylene groups in chain

181

It is seen that the curves in question have no euteetic points. In either case the dependence is a linear one. This shows that the phenomenon of isomorphism is found in the given series of polyurethanes. The presence of isomorphism was noted in a series of statistical copolymers (copolymers of styrene with ortho- and para-fluorostyrene [11]) and also in a series of block eopolymers (mixed polyamides based on diacid chlorides of adipic and terephthalie acids and 1,6-hexamethylenediamine [5]). Further confirmation of polymorphism in the series of mixed polyurethanes was sought and to this end a mechanical mixture of homopolyurethanes of HMDA and OFHMDA was prepared. The mixture contained 40 mole % of the OFHMDA homopolyurethane and was prepared by dissolving the given polyurethanes in m-cresol with subsequent precipitation of the polyurethanes by ether. The experiments showed that after eoerystallization the mechanical mixtures of the homopolyurethanes did not have the properties of each homopolymer taken individually. Only a single crystallization peak or melting peak appears in the thermogram (not given in this paper). The shape of the heating curve for the homopolyurethane mixture crystallized from the melt (see Fig. l, curve 5') is similar to that for the mixed polyurethane containing an equal amount of OFHMDA (Fig. 1, curve 5): in either case the melting peak has two spikes. This means that we have the effect of isomorphism in the case under consideration. With a relatively high OFHMDA content (30% or more) in the mixed polyurethane there is a tendency for polymorphie structures to appear as a result of the conformational heterogeneity of the HMDA and OFHMDA units. In view of the above special features of the polyurethane mixtures under review it is difficult to reach an unambiguous conclusion as to the possible existence of block copolymers in the investigated series of block copolymers. Another interesting feature of the mixed polyurethanes is their enhanced thermal stability. The weight loss for the vacuum heating of the polyurethane based on HMDA is 40-450/0 . However, even with quite small additions (10%) of OFHMDA there is a marked improvement in the high-temperature stability of these polyurethanes. When the latter undergo heating under similar conditions the weight loss is only 1"5~o, and it remains practically constant on further increasing the OFHMDA content. With all the mixed polyurethanes the weight loss is quite small, varying within limits of 1-5.6~. We know that polymers prepared by interfaeial polyeondensation generally have broader molecular weight distribution than amdogous polymers synthesized in bulk. It was to be assumed that the molecular weight distribution would likewise be broader in the case of the mixed polyurethanes. This could result in a certain amount of relatively low molecular weight products with terminal hydroxyl or amino groups being present in the mixed polyurethanes. It is possible that these groups might have a stabilizing effect on the macrochains of the polyurethanes when the latter were heated. To verify this assumption h o m o - a n d mixed polyurethanes were prepared in bulk from HMDI and OFHMDI (or mixtures of the two, containing l0 mole ~o OFHMDI) and hexanediol-l,6.

182

B.F. MALICHElqKOet

al.

Samples of these polyurethanes having close intrinsic viscosity values (0.40 and 0.38) were used to compare the thermal stability of the latter, and it was found that the weight losses for heating in a vacuum under the conditions used for the samples prepared by interfacial polycondensation were of the same order.

0

2o

oo 6o 80 OFHMDA , mole

Fro. 2. Crystallization temperature (1) and melting point (2) of mixed polyurethanes vs. composition. The polyurethane based on HMDI suffers a 45% weight loss, while the mixed polyurethane containing 10% OFHMDI has a weight loss of only 5.1% under the same conditions. It is interesting to observe that the homopolyurethane based on OFHMDI also has a weight loss of 5.25% under the same conditions. The stabilization of the fluorinated polyurethane maeroehains is therefore due not to the effect of the end groups but mainly to the intensification of intermoleeular interactions caused by the presence of difluoromethylene groups. The replacement of some of the methylene groups in the polyurethane chain by polar difluoromethylene groups results in the development of further forces of intermolecular interaction. A similar effect was noted by Buck and Livingstone [12] in determining the cohesion energy density for a series of rubbers. It rose quite considerably on introducing difluoromethylene groups into the polymer chain. It is most probable that in this case hydrogen bonds of the type C - - H . . . F - - C are formed. The latter were detected in [13] in a study of the boiling points of the simplest fluorinated hydrocarbons. The authors [13] found in addition that hydrocarbons having molecules containing different numbers of fluorine and hydrogen atoms had boiling points exceeding the calculated ones. The mixed polyurethanes studied in the present paper have not only the usual N - - H . . . O - - C hydrogen bonds common to polyamides and polyurethanes but in addition they also have H-bonds of the type C--F .... H--C tending to increase the thermal stability of the macroehains of these polymers. When 10% of OFHMDI or OFHMDA units, each having four diflnoromethylene units, are introduced into the polyurethane chain this is already sufficient to give rise to a considerable number of additional H-bonds with the result that the thermal stability of the polyurethanes is improved. The rcilfforcement of intermolecular interactions caused by the factors considered above also increases the hydrolytic stability of the mixed polyurethanes to a certain extent. Irrespective of the method of their preparation the nonfluorinated polyurethanes undergo 13-15% weight loss during hydrolysis in an

Mixed polyurethanes containing substituted difluoromethylclm groups ill chain

183

acid m e d i u m , w h e r e a s this loss for the m i x e d p o l y u r e t h a n e s varies f r o m 2 t o 6%. T h e fluorinated p o l y u r e t h a n e s h a v e slightly b e t t e r s t a b i l i t y in a n acid m e d i u m t h a n in a n alkaline one. Most p r o b a b l y the h y d r o l y s i s in a n acid m e d i u m is i]l this ease d e t e r m i n e d b y facilitated a d d i t i o n of h y d r o x o n i u m ions to t h e uns h a r e d electron p a i r of t h e n i t r o g e ~ a t o m s , as was o b s e r v e d [ 14] in the h y d r o l y s i s of p o l y a m i d e s in a n acid m e d i u m . Owing to the i n d u c t i v e effect of t h e electrona c e e p t o r fluorinated radical the electron d e n s i t y oll the n i t r o g e n a t o m of the u r e t h a n e g r o u p is reduced, a n d this t e n d s to inhibit the a d d i t i o n of the h y d r o x o n i u m ion to the l a t t e r w i t h the result t h a t h y d r o l y t i c splitting of t h e u r e t h a n e ~ r o u p p r o c e e d s less readily. CONCLUSIONS

I t has b e e n f o u n d t h a t the r e p l a c e m e n t some of t h e m e t h y l e n e g r o u p s in t h e chain of linear a l i p h a t i c p o l y u r e t h a n e s b y d i f l u o r o m e t h y l e n e g r o u p s is reflected in a considerable i m p r o v e m e n t in t h e t h e r m a l a n d h y d r o l y t i c s t a b i l i t y of these p o l y m e r s . T h e m e t h o d of d i f f e r e n t i a l - t h e r m a l analysis (DTA) has b e e n used t o s t u d y the m e l t i n g a n d c r y s t a l l i z a t i o n of m i x e d p o l y u r e t h a n e s . Isom o r p h i s m has b e e n p r o v e d in the case of these p o l y m e r s .

Translated by R. J. A. H~NDRY REFERENCES 1. B.F. MALICHENKO and Ye. V. SHELUD'KO, Vysokomol. soyed. BIO: 395, 1968 (Not

translated in Polymer Sci. U.S.S.R.) 2. L. L. CHERVYATSOVA, A. A. KACHAN, G. I. MOTRYUK, Yo. V. SHELUD'KO and B. F.

MALICHENKO, Vysokomol. soyed. A12: 981, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 5, 1109, 1970) 3. U.S. Pat. 2515246, 1950; Chem. Abstrs. 44: 9475, 1950 4. T. HOSHIO and I. ICItIKIZAKI, Chem. High Polymers Japan 2: 328, 1945 5. B. KE and W. SISKO, J. Polymer Sci. 50: 87, 1961 6. B. F. MALICHENKO, Ye. V. SHELUD'KO and Yu. Yu. KERCHA, Vysokomol. soyed. A9: 2482, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 11, 2808, 1967) 7. Yu. Yu. KERCHA, L. I. RYABOKON and B. F. MALICHENKO, Sint. i fizikokhim. polluter. (Synthesis and Physicochemistry of Polyurethanes). p. 198, Izd. "Naukova dumka", 1968 8. B. KE, J. Polymer Sci. 61: 47, 1962 9. G. BIER, Angew. Chemic 73: 186, 1961 10. Yu. Yu. KERCItA, Yu. S. LIPATOV, S. V. LAPTII, N. A. LIPATNIKOV and T. M. GRITSENKO, Sint. i fizikokhim, polim. (Synthesis and Physieochemistry of Polymers). p. 5, Izd. "Naukova dumka", 1970 11. G. NATIN, Makromolek. Chem. 35: 94, 1960 12. F. J. BUCK and R. L. LIVINGSTONE, J. Amer. Chem. Soc. 70: 2817, 1948 13. M. GUDLITSKII, Khim. org. soyed, ftora (Chemistry of Organic Fluorine Compounds). p. 257, Izd. Goskhimizdat, 1961 14. V. A. MWYAGKOVand A. B. PAKSIIVER, Koll. zh. 14: 172, 1952