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The application of nuclear magnetic resonance to the study of rubber-like fluorine-containing polymers
THE APPLICATION OF NUCLEAR MAGNETIC RESONANCE TO THE STUDY OF RUBBER-LIKE FLUORINE-CONTAINING POLYMERS* A. N. LYUBIMOV, A. S. NOVIKOV, F. A. GALIL-OGLY, A. V. GRIBACISEVA a n d A. F. V A R E N I K Scientific-Research Institute of the Rubber Industry (Received 17 November 1960)
I N RECENT y e a r s t h e nuclear m a g n e t i c r e s o n a n c e ( N M R ) m e t h o d has ])ecome m o r e widely used for t h e s t u d y o f the s t r u c t u r e a n d p r o p e r t i e s o f p o l y m e r s
[1, 2, 3]. This m e t h o d enables t h e r e l a x a t i o n p r o p e r t i e s of t h e molecules to be studied in r e l a t i o n to t h e c o m p o s i t i o n a n d m e t h o d o f p r e p a r a t i o n o f the polyn'mr, a n d to t e m p e r a t u r e , degree of swelling, v u l c a n i z a t i o n etc. [4, 5, 6]. I t has also b e e n used for t h e s t u d y o f the effect of t h e c o n f o r m a t i o n a n d chemical s t r u c t u r e of t h e p o l y m e r chains on t h e i r m o b i l i t y in t h e glassy a n d viscous-flow states, a n d f o r d e t e r m i n a t i o n o f the c r y s t a l l i n i t y o f t h e p o l y m e r . I n v e s t i g a t i o n s b y this m e t h o d o f t h e effect o f t h e degree o f fluorination o f p o l y e t h y l e n e on the m o b i l i t y o f t h e molecules a t t e m p e r a t u r e s f r o m - - 2 0 0 ° to -b150 ° [7, 8] should also be m e n tioned. I n spite of the f a c t t h a t a h n o s t all the w o r k so f a r carried o u t on t h e a p p l i c a t i o n o f N M R to t h e s t u d y o f p o l y m e r s has b e e n of a n e x p l o r a t o r y n a t u r e t h e r e is e v e r y r e a s o n to believe t h a t t h e s y s t e m a t i c s t u d y o f p o l y m e r s b y t h e NMI~ m e t h o d will be v e r y profitable for o b t a i n i n g i n f o r m a t i o n on t h e m e c h a n i s m o f m o v e m e n t of p o l y m e r molecules, p a r t i c u l a r l y if this m e t h o d is used in c o n j u n c t i o n w i t h o t h e r p h y s i c o - c h e m i e a l a n d p h y s i c a l m e t h o d s of investigation. T h e p u r p o s e of t h e p r e s e n t w o r k was to s t u d y t h e t e m p e r a t u r e d e p e n d e n c e o f t h e w i d t h of the n u c l e a r r e s o n a n c e line a n d t h e second m o m e n t for a n u m b e r of fluorine-containing, r u b b e r - l i k e p o l y m e r s differing in t h e i r elastic properties. The materials studied were copolymers of the following monomers: (I) trifluorochloroethylene and vinylidene fluoride; (II) hexafiuoropropylene and vinylidene fluoride; (III) trifluoroehloroethylene, vinylidene fluoride and p3rfluoromethoxyperfluoropropyl acrylate; (IV) peri~luoromethoxyperfluoropropyl acrylate homopols~ner and (V) polyhexafluoropentamethyleneadipate. The NMR spectrometer used for recording the nuclear resvnance lines was the ordinary type, of our own design, with linear sweeping, sinusoidal modulation of the polarizing field and an autodyne indicator. The polarizing field (1500 G) of the spectrometer was produced by a high-resistance electromagnet with a supply stabilized with respect to bott~ voltage and current, giving a field stability of about 10=5. * Vysokomol. soedin. 3: No, 10, 1511-1515, 1961. 66*
1005
1006
A. lq. LYUB~MOV et al.
T h e h o m o g e n e i t y o f t h e field m e a s u r e d b y a n a l y s i s o f t h e c h e m i c a l shifts o f t h e F 19 r e s o n a n c e w a s 10 -5 i n a v o l u m e o f 0.5 c m a. For the linear sweeping an electronic integrator system was used which ensured linearity w i t h a p r e c i s i o n o f a b o u t 1 ~o. H e a t i n g a n d c o o l i n g o f t h e s p e c i m e n s o v e r t h e t e m p e r a t u r e r a n g e f r o m - - 110 ° t o - ~ 120 ° was achieved by the contact method and the temperature was measured by means of a thermoeouple. A t a n a m p l i t u d e o f m o d u l a t i o n c o n s i d e r a b l y less t h a n t h e line w i d t h a n d w i t h sinep h a s e d e t e c t i o n t h i s s p e c t r o m e t e r r e c o r d s a p p r o x i m a t e l y t h e first d e r i v a t i v e s o f t h e a b s o r p t i o n lines a l o n g t h e p o l a r i z i n g field. I t is k n o w n t h a t t h e n u c l e a r r e s o n a n c e line w i d t h is a p a r a m e t e r c h a r a c t e r i z i n g i n t r a m o l e c u l a r m o v e m e n t s i n liquids, b u t i n solids a n d in s t r u c t u r e s o f low m o b i l i t y t h e s e c o n d m o m e n t is s u c h a p a r a m e t e r . H e n c e t h e d e r i v a t i v e s o f t h e p r o t o n a n d f l u o r i n e r e s o n a n c e lines w e r e r e c o r d e d a t v a r i o u s t e m p e r a t u r e s b e t w e e n - - 1 5 0 ° a n d + 1 2 0 ° for all t h e polymers studied. I n o r d e r t o t e s t t h e a c c u r a c y o f t h e c a l i b r a t i o n o f t h e s p e c t r o m e t e r t h e F 19 r e s o n a n c e s i g n a l for p o l y t e t r a f l u o r o e t h y l e n e w a s fa-st r e c o r d e d . A c c o r d i n g t o S l i c h t e r [8] t h e v a l u e s o f t h e s e c o n d m o m e n t for t h i s p o l y m e r a t - - 7 0 ° lie b e t w e e n 11 a n d 12 G 2 for s p e c i m e n s differing in m o l e c u l a r w e i g h t a n d degree o f c r y s t a l l i n i t y . O u r v a l u e w a s 12 G 2, w h i c h is i n agreement with Slichter's results and confirms the accuracy of the calibration of the apparatus. T h e s e c o n d m o m e n t s were f o u n d b y t h e g r a p h i c a l i n t e g r a t i o n m e t h o d a n d t h e t e m p e r a t u r e d e p e n d e n c e o f t h e s e c o n d m o m e n t s was p l o t t e d . T h e n a r r o w i n g o f t h e r e s o n a n c e lines w i t h i n c r e a s i n g t e m p e r a t u r e i n d i c a t e s i n c r e a s e d molecular movement in the polymer and at temperatures above +90 ° the width of the p r o t o n a n d f l u o r i n e lines is d e t e r m i n e d o n l y b y t h e i n h o m o g e n e i t y o f t h e p o l a r i z i n g field a n d t h e m o b i l i t y o f t h e c h a i n s is e q u i v a l e n t t o t h e m o b i l i t y o f m o l e c u l e s in a liquid. I n o n e o f t h e c o p o ] y m e r s ( h e x a f l u o r o p r o p y l e n e a n d v i n y l i d e n e fluoride) fine s t r u c t u r e i n t h e fluorine r e s o n a n c e was o b s e r v e d a t + 90 °, p r o b a b l y d u e t o c h e m i c a l s h i f t o f t h e r e s o n a n c e in t h e CF s a n d C F a g r o u p s . AH' (G') b
/2
8
g7
I
I
-130
t
i
-~L~
i
J
-5~
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i
-lO ÷I0
I-..*--
p3L7
~,~--
---.~--
-I10
"~0
-50
"10 +l~
+5~
Temperature (°C) V a r i a t i o n o f s e c o n d m o m e n t for f l u o r i n e (a) a n d h y d r o g e n (b) w i t h t e m p e r a t u r e for c o p o l y m e r s of: (1) v i n y l i d e n e fluoride a n d t r i f l u o r o c h l o r o e t h y l e n e ; (2) v i n y l i d e n e fluoride a n d h e x a f l u o r o p r o p y l e n e ; (3) h o m o p o l y m e r o f p e r f l u o r o m e t h o x y p e r f l u o r o p r o p y l a c r y l a t e ; (4) v i n y l i d e n e fluoride, t r i f l u o r o c h l o r o e t h y l e n e a n d p e r f l u o r o m e t h o x y p e r f i u o r o p r o p y l acryl a t e ; (5) p o l y h e x a f l u o r o p e n t a m e t h y l e n e a d i p a t e .
Nuclear magnetic resonance for studying fluorine-containing polymers
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I t should be noted t h a t for the vinylidene fluoride-hexafluoropropylene and vinylidene fluoride-trifluorochloroethylene copolymers complete narrowing of both the proton and fluorine lines occurred at higher temperatures (+700--[-90 ° ) than for the other polymers (-~10°-~-30°). This difference carl evidently be explained by strong intermolecular interaction in these polymers. Figures (la) and (b) show the observed temperature dependence of the sccon(} moments of the proton and fluorine resonance lines. I t is seen from these curves t h a t strictly speaking a rigid structure is not shown by some of the polymers at the lowest temperature (--150°). Whereas the second moment of the proton resonance remains practically constant at temperatures below --110 ° for all the polymers the second moment of the fluorine resonance continues to increase with decreasing temperature, indicating slight movement of the groups containing fluorine. It m a y therefore be considered t h a t the observed values of the second moments of the proton resonance in all the polymers at temperatures below - - l l 0 ° correspond to the moments of rigid structures. In this temperature region the second moment for protons lies between 16 G 2 and 19.5 G 2. The vinylidene fluoride-hexafluoropropylene and vinylidene fluoride-trifluorochloroethylene copolymers show the highest values of the second moment. For these polymers the second moments of the proton resonance are the same at 19.5 G ~-. I t is interesting to note t h a t this value is very close to t h a t for proto~ls in fluoro-derivatives of ethylene of the type (CH2--CF2),~ found experimentally by Slichter [S]. For the trifluorochloroethylene-vinylidene fluoride copo]ymer the second moment was calculated theoretically from the Van Vleck formula. For the calculation it was assumed t h a t the polymer chains are immobile, t h a t the C--C bonds form a planar, zigzag structure with tetrahedral angles, t h a t the C - - H and C - - F bonds lie in planes perpendicular to the plane of the C--C bonds aad form tetrahedral angles between themselves and with the C--C bonds, and t h a t polymerization is exclusively of the "head-to-tail" type with alternating vinylidene fluoride and trifluorochloroethylene units in the chain. The length of the C--C bond was taken as 1.42 A, of the C - - H bond 1.094 A and of the C - - F bond 1.35 =~. When the magnetic interaction of the fluorine and hydrogen nuclei in one such chain is considered, without taking into account the effect of neighbouri~lg chains the second moment for proton r e s o n a n c e , with the above assumptio~ls, is 15.7 G 2. The difference of 3.8 G 2 between this and the experimental figure can be explained by the effect of intermolecular interaction, particularly since this almost exactly coincides with the correction for intcrmolec~dar interaction in (CH 2- CF2) ~ chains calculated by Slichter. The nature of the variation of the second moment with time is the same for all the polymers with the exception of (V).
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A.N. LYUBIMOVet al.
All the curves can he divided into three sections: firstly the second moments are constant, secondly they decrease relatively slowly and thirdly there is a fairly rapid decrease. The end of the first and beginning of the second practically coincides for all the polymers at about --110 °. The end of the second section and beginning of the third for the copolymers of vinylidene chloride with trifluorochloroethylene and hexafluoropropylene occurs at about --20 °, for the homopolymer (IV) and the ternary copolymer (III) at about --40 ° and for polyhexafluoropentamethylene-adipate (V) at about --60 °, though in the last case this transition is not very clearly defined. The temperatures -- 20 °, -- 40 ° and -- 60 ° for these copolymers correspond to their glass temperatures obtained by measurement of the deformation characteristics on a Kargin dynamometric balance. Hence above these temperatures movement of segments of the chain molecules sets in, which also explains the increase in slope of the curves of the variation of the second moment in the third sections above these temperatures. The decrease in second moment in the second sections is evidently explained b y reorientation of the methylene groups present in all the polymers. The absence of a well defined second section for pol:~mer (V), obtained by condensation of the fluorinated diol with adipyl chloride, is possibly associated with the fact t h a t the chains are more mobile as a result of the presence of ester "hinge" groups (OCO) and thermal movement in these begins ahnost simultaneously with reorientation of the CH~ groups. These results therefore lead to the conclusion t h a t some thermal movement of chain segments occurs in these polymers in addition to rotational movement of individual groups. I t is interesting to note the almQst complete coincidence of the proton resonance curves in Figure (1) b for the copolymers of vinylidene fluoride with trifluorochloroethylene and hexafluoropropylene. This coincidence can be explained by the fact t h a t the second moment for hydrogen in the two polymers is created mainly by interaction of protons with methylene groups, which are distributed almost identically in the two. The contributions to this moment by the fluorine nuclei in the CF 2 groups of vinylidene fluoride are also identical. On the other hand the contributions to this moment of the CF~ groups of hexafluoropropylene or trifluorochloroethylene can differ depending on how these monomers are joined to the CH2CF 2 unit. However th6 latter are small in comparison with the first. The general nature of the variation of the fluorine second moment with temperature is similar to t h a t for hydrogen. Here again three sections can be seen--sections corresponding to constant, relatively slow and relatively rapid variation of the second moment. However, as mentioned above, the fluorinecontaining groups of these polymers are more mobile t h a n the methylene groups. For the copolymers of vinylidene fluoride with hexafluoropropylene and trifluorochloroethylene the fluorine second moments practically coincide at tern-
Nuclear magnetic resonance for studying fluorine-containing polymers
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p e r a t u r e s a b o v e - - 1 3 0 °. A t t e m p e r a t u r e s below - - 1 3 0 ° the second m o m e n t for the first p o l y m e r remains c o n s t a n t and for the s o : . , l d p o l y m e r it continues to increase, lit is difficult to find a simple ~.,planation tbr this b u t it m a y be supposed t h a t the " t h a w i n g " o f i n t e r a m o l e c u l a r m o v e m e n t in the first p o l y m e r occurs at higher t e m p e r a t u r e s as a result of the presence of CF a groups d i s t r i b u t e d along the chain in the f o r m o f side groups. T h e OCF a groups in polymers (III) and (IV) e v i d e n t l y give a similar effect. The i n t r o d u c t i o n o f p e r f l u o r o m e t h o x y d i h y d r o p e r f l u o r v p r o p y l a c r y l a t e groups into the c o p o l y m e r o f vinylidene fluoride and trifluorochloroethylene leads firstly to a decrease in the fluorine second m o m e n t and secondly to a decrease in tile t e m p e r a t u r e at which t h e r m a l m o v e m e n t of segments of the p o l y m e r chain begins. The first effect is a result of the breaking up of the - - C F 2 CFC1CH2CF e chain a n d a decrease in intermolecular i n t e r a c t i o n due to the presence on the chain of fairly large p e n d a n t groups. The second is p r o b a b l y the result of an increase in the mobility of the chain s~gments due to the presence of the OCO " h i n g e " groups a l r e a d y m e n t i o n e d . The h y d r o g e n and fluorine second m o m e n t s for the fluorinated d i o l - a d i p y l chloride c o p o l y m e r (V) and the vinylidene f l u o r i d e - h e x a f l u o r o p r o p y l e n e copolym e t (II) in the t e m p e r a t u r e region from - - 9 0 ° to --150 ° are close to one another, b u t when the t e m p e r a t u r e is increased a b o v e - - 7 0 ° the decrease in the second m o m e n t is considerably m o r e rapid in the case of the first p o l y m e r because of the presence of OCO groups in the molecule. A l t h o u g h the fluorine second m o m e n t of the rigid s t r u c t u r e for the vinylidene fluoride-trifluoroehloroethylene c o p o l y m e r c a n n o t strictly speaking be measured, because " f r e e z i n g " e v i d e n t l y occurs below - - 1 5 0 ° the general course of the relationship AH~,~=f(T °) indicates t h a t it is unlikely to be greater t h a n 12.5 G 2. C o n s e q u e n t l y for this p o l y m e r the fluorine second m o m e n t was also calculated theoretically. I n this case the w a y in which the m o n o m e r s are joined tog e t h e r in the c o p o l y m e r chain is not u n i m p o r t a n t , i.e. w h e t h e r the s t r u c t u r e s CF2CFC1CH2CF 2 - ( 1) or -- CF2CFC1CF2CH 2 - (2) are present. I n the calculation o f t h e h y d r o g e n second m o m e n t practically the same result is o b t a i n e d for b o t h s t r u c t u r e s because, as shown above, the m a i n contributions to this m o m e n t are the same for the two structures. T h e theoretical fluorine second m o m e n t for the first s t r u c t u r e is 7-2 G 2 a n d for the second, 8.6 G 2. Comparison of these values with the e x p e r i m e n t a l value (12.5 G 2) shows t h a t the difference o f 3.9 G 2 for the second s t r u c t u r e lies w i t h i n the limits of Slichter's corrections (3.1--4.[ G 2) for intermolecular i n t e r a c t i o n b e t w e e n the chains (CHe--CHF),~, (CH 2 - CF2),, and (CF2--CFH), c F o r the first s t r u c t u r e this difference is 5.3 G 2. This of course does not provide a simple solution to the problem of the actual s t r u c t u r e of the copolymers b u t it suggests t h a t the second s t r u c t u r e is more probable.
1010
A.N. LYUBIMOVet aL
I t m a y be considered t h a t in o t h e r cases, a n d also a f t e r t h e precision of the calculations a n d t h e s e n s i t i v i t y o f t h e a p p a r a t u s has b e e n i m p r o v e d , t h e N M R m e t h o d can be of v a l u e for t h e identification o f t h e s t r u c t u r e o f p o l y m e r s . T h e a u t h o r s express t h e i r g r a t i t u d e to A. I. K i t a i g o r o d s k i i for v a l u a b l e a d v i c e d u r i n g discussion of the results o f this work. CONCLUSIONS
(1) T h e t e m p e r a t u r e d e p e n d e n c e of t h e second m o m e n t s a n d w i d t h s of t h e p r o t o n a n d fluorine m a g n e t i c r e s o n a n c e lines has b e e n studied for a n u m b e r o f fluorine-containing c o p o l y m e r s o v e r the t e m p e r a t u r e r a n g e f r o m - - 1 5 0 to + 9 0 °. I t is s h o w n t h a t t h e p a r a m e t e r s of nuclear m a g n e t i c r e s o n a n c e decrease w i t h increasing t e m p e r a t u r e . (2) T h e existence o f two t y p e s of t h e r m a l m o v e m e n t in the c o p o l y m e r chains has b e e n c o n f i r m e d - - r o t a t i o n a l m o v e m e n t of i n d i v i d u a l f u n c t i o n a l groups a n d t h e m o v e m e n t of whole s e g m e n t s of t h e chain. (3) I t is s h o w n t h a t t h e m o b i l i t y of p o l y m e r chains is increased b y the effect o f OCO groups. (4) B y c o m p a r i s o n of t h e e x p e r i m e n t a l values of t h e h y d r o g e n a n d fluorine second m o m e n t s o f the v i n y l i d e n e f l u o r i d e - t r i f l u o r o c h l o r o e t h y l e n e c o p o l y m e r w i t h t h o s e calculated theoretically, w i t h t w o possible m e t h o d s o f a d d i t i o n of t h e - - C F 2 - - C F C 1 - - a n d - - C H 2 - - C F 2 - m o n o m e r units, it is s h o w n t h a t t h e --CF2CFCICF2CH 2 - s t r u c t u r e is m o r e p r o b a b l e t h a n --CF2CFC1CH2CF2--. (5) Chemical shift o f t h e fluorine r e s o n a n c e of t h e CF 2 a n d CF 3 g r o u p s in t h e v i n y l i d e n e f l u o r i d e - h e x a f l u o r o p r o p y l e n e c o p o l y m e r was detected a t -+-90 °. Translated by E. O. PHILLIPS.
REFERENCES
1. C. H. HOLROYD, R. S. CODRINGTON, B. A. MROWCA and E. GUTH, J. Appl. Phys. 22: 696, 1951 2. W. P. SLICHTER, J. Appl. Phys. 26: 1099, 1955 3. N. M. B A Z H E N O V , M. V. V O L ' K E N S H T E I N , A. I. KOL'TSOV a n d A. C. KHACHATUROV, Vysokomol. soedin. 1: 1048, 1959 4. V. R. HONNOLD, F. McCAFFREY and B. A. MROWCA, J. Appl. Phys. 25: 1219, 1954 5. A. ODAJIMA and T. NAGAY, Chem. High Polymers (Japan) 150: 512, 1957 6. H. S. GUTOWSKY and L. H. MEYER, J. Chem. Phys. 12: 2122, 1953 7. C. W. WILSON and G. E. PAKE, J. Polymer Sci. 10: 503, 1953 8. W. P. SLICHTER, J. Polymer Sci. 24: 178, 1957
I N RECENT y e a r s t h e nuclear m a g n e t i c r e s o n a n c e ( N M R ) m e t h o d has ])ecome m o r e widely used for t h e s t u d y o f the s t r u c t u r e a n d p r o p e r t i e s o f p o l y m e r s
[1, 2, 3]. This m e t h o d enables t h e r e l a x a t i o n p r o p e r t i e s of t h e molecules to be studied in r e l a t i o n to t h e c o m p o s i t i o n a n d m e t h o d o f p r e p a r a t i o n o f the polyn'mr, a n d to t e m p e r a t u r e , degree of swelling, v u l c a n i z a t i o n etc. [4, 5, 6]. I t has also b e e n used for t h e s t u d y o f the effect of t h e c o n f o r m a t i o n a n d chemical s t r u c t u r e of t h e p o l y m e r chains on t h e i r m o b i l i t y in t h e glassy a n d viscous-flow states, a n d f o r d e t e r m i n a t i o n o f the c r y s t a l l i n i t y o f t h e p o l y m e r . I n v e s t i g a t i o n s b y this m e t h o d o f t h e effect o f t h e degree o f fluorination o f p o l y e t h y l e n e on the m o b i l i t y o f t h e molecules a t t e m p e r a t u r e s f r o m - - 2 0 0 ° to -b150 ° [7, 8] should also be m e n tioned. I n spite of the f a c t t h a t a h n o s t all the w o r k so f a r carried o u t on t h e a p p l i c a t i o n o f N M R to t h e s t u d y o f p o l y m e r s has b e e n of a n e x p l o r a t o r y n a t u r e t h e r e is e v e r y r e a s o n to believe t h a t t h e s y s t e m a t i c s t u d y o f p o l y m e r s b y t h e NMI~ m e t h o d will be v e r y profitable for o b t a i n i n g i n f o r m a t i o n on t h e m e c h a n i s m o f m o v e m e n t of p o l y m e r molecules, p a r t i c u l a r l y if this m e t h o d is used in c o n j u n c t i o n w i t h o t h e r p h y s i c o - c h e m i e a l a n d p h y s i c a l m e t h o d s of investigation. T h e p u r p o s e of t h e p r e s e n t w o r k was to s t u d y t h e t e m p e r a t u r e d e p e n d e n c e o f t h e w i d t h of the n u c l e a r r e s o n a n c e line a n d t h e second m o m e n t for a n u m b e r of fluorine-containing, r u b b e r - l i k e p o l y m e r s differing in t h e i r elastic properties. The materials studied were copolymers of the following monomers: (I) trifluorochloroethylene and vinylidene fluoride; (II) hexafiuoropropylene and vinylidene fluoride; (III) trifluoroehloroethylene, vinylidene fluoride and p3rfluoromethoxyperfluoropropyl acrylate; (IV) peri~luoromethoxyperfluoropropyl acrylate homopols~ner and (V) polyhexafluoropentamethyleneadipate. The NMR spectrometer used for recording the nuclear resvnance lines was the ordinary type, of our own design, with linear sweeping, sinusoidal modulation of the polarizing field and an autodyne indicator. The polarizing field (1500 G) of the spectrometer was produced by a high-resistance electromagnet with a supply stabilized with respect to bott~ voltage and current, giving a field stability of about 10=5. * Vysokomol. soedin. 3: No, 10, 1511-1515, 1961. 66*
1005
1006
A. lq. LYUB~MOV et al.
T h e h o m o g e n e i t y o f t h e field m e a s u r e d b y a n a l y s i s o f t h e c h e m i c a l shifts o f t h e F 19 r e s o n a n c e w a s 10 -5 i n a v o l u m e o f 0.5 c m a. For the linear sweeping an electronic integrator system was used which ensured linearity w i t h a p r e c i s i o n o f a b o u t 1 ~o. H e a t i n g a n d c o o l i n g o f t h e s p e c i m e n s o v e r t h e t e m p e r a t u r e r a n g e f r o m - - 110 ° t o - ~ 120 ° was achieved by the contact method and the temperature was measured by means of a thermoeouple. A t a n a m p l i t u d e o f m o d u l a t i o n c o n s i d e r a b l y less t h a n t h e line w i d t h a n d w i t h sinep h a s e d e t e c t i o n t h i s s p e c t r o m e t e r r e c o r d s a p p r o x i m a t e l y t h e first d e r i v a t i v e s o f t h e a b s o r p t i o n lines a l o n g t h e p o l a r i z i n g field. I t is k n o w n t h a t t h e n u c l e a r r e s o n a n c e line w i d t h is a p a r a m e t e r c h a r a c t e r i z i n g i n t r a m o l e c u l a r m o v e m e n t s i n liquids, b u t i n solids a n d in s t r u c t u r e s o f low m o b i l i t y t h e s e c o n d m o m e n t is s u c h a p a r a m e t e r . H e n c e t h e d e r i v a t i v e s o f t h e p r o t o n a n d f l u o r i n e r e s o n a n c e lines w e r e r e c o r d e d a t v a r i o u s t e m p e r a t u r e s b e t w e e n - - 1 5 0 ° a n d + 1 2 0 ° for all t h e polymers studied. I n o r d e r t o t e s t t h e a c c u r a c y o f t h e c a l i b r a t i o n o f t h e s p e c t r o m e t e r t h e F 19 r e s o n a n c e s i g n a l for p o l y t e t r a f l u o r o e t h y l e n e w a s fa-st r e c o r d e d . A c c o r d i n g t o S l i c h t e r [8] t h e v a l u e s o f t h e s e c o n d m o m e n t for t h i s p o l y m e r a t - - 7 0 ° lie b e t w e e n 11 a n d 12 G 2 for s p e c i m e n s differing in m o l e c u l a r w e i g h t a n d degree o f c r y s t a l l i n i t y . O u r v a l u e w a s 12 G 2, w h i c h is i n agreement with Slichter's results and confirms the accuracy of the calibration of the apparatus. T h e s e c o n d m o m e n t s were f o u n d b y t h e g r a p h i c a l i n t e g r a t i o n m e t h o d a n d t h e t e m p e r a t u r e d e p e n d e n c e o f t h e s e c o n d m o m e n t s was p l o t t e d . T h e n a r r o w i n g o f t h e r e s o n a n c e lines w i t h i n c r e a s i n g t e m p e r a t u r e i n d i c a t e s i n c r e a s e d molecular movement in the polymer and at temperatures above +90 ° the width of the p r o t o n a n d f l u o r i n e lines is d e t e r m i n e d o n l y b y t h e i n h o m o g e n e i t y o f t h e p o l a r i z i n g field a n d t h e m o b i l i t y o f t h e c h a i n s is e q u i v a l e n t t o t h e m o b i l i t y o f m o l e c u l e s in a liquid. I n o n e o f t h e c o p o ] y m e r s ( h e x a f l u o r o p r o p y l e n e a n d v i n y l i d e n e fluoride) fine s t r u c t u r e i n t h e fluorine r e s o n a n c e was o b s e r v e d a t + 90 °, p r o b a b l y d u e t o c h e m i c a l s h i f t o f t h e r e s o n a n c e in t h e CF s a n d C F a g r o u p s . AH' (G') b
/2
8
g7
I
I
-130
t
i
-~L~
i
J
-5~
~
i
-lO ÷I0
I-..*--
p3L7
~,~--
---.~--
-I10
"~0
-50
"10 +l~
+5~
Temperature (°C) V a r i a t i o n o f s e c o n d m o m e n t for f l u o r i n e (a) a n d h y d r o g e n (b) w i t h t e m p e r a t u r e for c o p o l y m e r s of: (1) v i n y l i d e n e fluoride a n d t r i f l u o r o c h l o r o e t h y l e n e ; (2) v i n y l i d e n e fluoride a n d h e x a f l u o r o p r o p y l e n e ; (3) h o m o p o l y m e r o f p e r f l u o r o m e t h o x y p e r f l u o r o p r o p y l a c r y l a t e ; (4) v i n y l i d e n e fluoride, t r i f l u o r o c h l o r o e t h y l e n e a n d p e r f l u o r o m e t h o x y p e r f i u o r o p r o p y l acryl a t e ; (5) p o l y h e x a f l u o r o p e n t a m e t h y l e n e a d i p a t e .
Nuclear magnetic resonance for studying fluorine-containing polymers
1007
I t should be noted t h a t for the vinylidene fluoride-hexafluoropropylene and vinylidene fluoride-trifluorochloroethylene copolymers complete narrowing of both the proton and fluorine lines occurred at higher temperatures (+700--[-90 ° ) than for the other polymers (-~10°-~-30°). This difference carl evidently be explained by strong intermolecular interaction in these polymers. Figures (la) and (b) show the observed temperature dependence of the sccon(} moments of the proton and fluorine resonance lines. I t is seen from these curves t h a t strictly speaking a rigid structure is not shown by some of the polymers at the lowest temperature (--150°). Whereas the second moment of the proton resonance remains practically constant at temperatures below --110 ° for all the polymers the second moment of the fluorine resonance continues to increase with decreasing temperature, indicating slight movement of the groups containing fluorine. It m a y therefore be considered t h a t the observed values of the second moments of the proton resonance in all the polymers at temperatures below - - l l 0 ° correspond to the moments of rigid structures. In this temperature region the second moment for protons lies between 16 G 2 and 19.5 G 2. The vinylidene fluoride-hexafluoropropylene and vinylidene fluoride-trifluorochloroethylene copolymers show the highest values of the second moment. For these polymers the second moments of the proton resonance are the same at 19.5 G ~-. I t is interesting to note t h a t this value is very close to t h a t for proto~ls in fluoro-derivatives of ethylene of the type (CH2--CF2),~ found experimentally by Slichter [S]. For the trifluorochloroethylene-vinylidene fluoride copo]ymer the second moment was calculated theoretically from the Van Vleck formula. For the calculation it was assumed t h a t the polymer chains are immobile, t h a t the C--C bonds form a planar, zigzag structure with tetrahedral angles, t h a t the C - - H and C - - F bonds lie in planes perpendicular to the plane of the C--C bonds aad form tetrahedral angles between themselves and with the C--C bonds, and t h a t polymerization is exclusively of the "head-to-tail" type with alternating vinylidene fluoride and trifluorochloroethylene units in the chain. The length of the C--C bond was taken as 1.42 A, of the C - - H bond 1.094 A and of the C - - F bond 1.35 =~. When the magnetic interaction of the fluorine and hydrogen nuclei in one such chain is considered, without taking into account the effect of neighbouri~lg chains the second moment for proton r e s o n a n c e , with the above assumptio~ls, is 15.7 G 2. The difference of 3.8 G 2 between this and the experimental figure can be explained by the effect of intermolecular interaction, particularly since this almost exactly coincides with the correction for intcrmolec~dar interaction in (CH 2- CF2) ~ chains calculated by Slichter. The nature of the variation of the second moment with time is the same for all the polymers with the exception of (V).
1008
A.N. LYUBIMOVet al.
All the curves can he divided into three sections: firstly the second moments are constant, secondly they decrease relatively slowly and thirdly there is a fairly rapid decrease. The end of the first and beginning of the second practically coincides for all the polymers at about --110 °. The end of the second section and beginning of the third for the copolymers of vinylidene chloride with trifluorochloroethylene and hexafluoropropylene occurs at about --20 °, for the homopolymer (IV) and the ternary copolymer (III) at about --40 ° and for polyhexafluoropentamethylene-adipate (V) at about --60 °, though in the last case this transition is not very clearly defined. The temperatures -- 20 °, -- 40 ° and -- 60 ° for these copolymers correspond to their glass temperatures obtained by measurement of the deformation characteristics on a Kargin dynamometric balance. Hence above these temperatures movement of segments of the chain molecules sets in, which also explains the increase in slope of the curves of the variation of the second moment in the third sections above these temperatures. The decrease in second moment in the second sections is evidently explained b y reorientation of the methylene groups present in all the polymers. The absence of a well defined second section for pol:~mer (V), obtained by condensation of the fluorinated diol with adipyl chloride, is possibly associated with the fact t h a t the chains are more mobile as a result of the presence of ester "hinge" groups (OCO) and thermal movement in these begins ahnost simultaneously with reorientation of the CH~ groups. These results therefore lead to the conclusion t h a t some thermal movement of chain segments occurs in these polymers in addition to rotational movement of individual groups. I t is interesting to note the almQst complete coincidence of the proton resonance curves in Figure (1) b for the copolymers of vinylidene fluoride with trifluorochloroethylene and hexafluoropropylene. This coincidence can be explained by the fact t h a t the second moment for hydrogen in the two polymers is created mainly by interaction of protons with methylene groups, which are distributed almost identically in the two. The contributions to this moment by the fluorine nuclei in the CF 2 groups of vinylidene fluoride are also identical. On the other hand the contributions to this moment of the CF~ groups of hexafluoropropylene or trifluorochloroethylene can differ depending on how these monomers are joined to the CH2CF 2 unit. However th6 latter are small in comparison with the first. The general nature of the variation of the fluorine second moment with temperature is similar to t h a t for hydrogen. Here again three sections can be seen--sections corresponding to constant, relatively slow and relatively rapid variation of the second moment. However, as mentioned above, the fluorinecontaining groups of these polymers are more mobile t h a n the methylene groups. For the copolymers of vinylidene fluoride with hexafluoropropylene and trifluorochloroethylene the fluorine second moments practically coincide at tern-
Nuclear magnetic resonance for studying fluorine-containing polymers
1009
p e r a t u r e s a b o v e - - 1 3 0 °. A t t e m p e r a t u r e s below - - 1 3 0 ° the second m o m e n t for the first p o l y m e r remains c o n s t a n t and for the s o : . , l d p o l y m e r it continues to increase, lit is difficult to find a simple ~.,planation tbr this b u t it m a y be supposed t h a t the " t h a w i n g " o f i n t e r a m o l e c u l a r m o v e m e n t in the first p o l y m e r occurs at higher t e m p e r a t u r e s as a result of the presence of CF a groups d i s t r i b u t e d along the chain in the f o r m o f side groups. T h e OCF a groups in polymers (III) and (IV) e v i d e n t l y give a similar effect. The i n t r o d u c t i o n o f p e r f l u o r o m e t h o x y d i h y d r o p e r f l u o r v p r o p y l a c r y l a t e groups into the c o p o l y m e r o f vinylidene fluoride and trifluorochloroethylene leads firstly to a decrease in the fluorine second m o m e n t and secondly to a decrease in tile t e m p e r a t u r e at which t h e r m a l m o v e m e n t of segments of the p o l y m e r chain begins. The first effect is a result of the breaking up of the - - C F 2 CFC1CH2CF e chain a n d a decrease in intermolecular i n t e r a c t i o n due to the presence on the chain of fairly large p e n d a n t groups. The second is p r o b a b l y the result of an increase in the mobility of the chain s~gments due to the presence of the OCO " h i n g e " groups a l r e a d y m e n t i o n e d . The h y d r o g e n and fluorine second m o m e n t s for the fluorinated d i o l - a d i p y l chloride c o p o l y m e r (V) and the vinylidene f l u o r i d e - h e x a f l u o r o p r o p y l e n e copolym e t (II) in the t e m p e r a t u r e region from - - 9 0 ° to --150 ° are close to one another, b u t when the t e m p e r a t u r e is increased a b o v e - - 7 0 ° the decrease in the second m o m e n t is considerably m o r e rapid in the case of the first p o l y m e r because of the presence of OCO groups in the molecule. A l t h o u g h the fluorine second m o m e n t of the rigid s t r u c t u r e for the vinylidene fluoride-trifluoroehloroethylene c o p o l y m e r c a n n o t strictly speaking be measured, because " f r e e z i n g " e v i d e n t l y occurs below - - 1 5 0 ° the general course of the relationship AH~,~=f(T °) indicates t h a t it is unlikely to be greater t h a n 12.5 G 2. C o n s e q u e n t l y for this p o l y m e r the fluorine second m o m e n t was also calculated theoretically. I n this case the w a y in which the m o n o m e r s are joined tog e t h e r in the c o p o l y m e r chain is not u n i m p o r t a n t , i.e. w h e t h e r the s t r u c t u r e s CF2CFC1CH2CF 2 - ( 1) or -- CF2CFC1CF2CH 2 - (2) are present. I n the calculation o f t h e h y d r o g e n second m o m e n t practically the same result is o b t a i n e d for b o t h s t r u c t u r e s because, as shown above, the m a i n contributions to this m o m e n t are the same for the two structures. T h e theoretical fluorine second m o m e n t for the first s t r u c t u r e is 7-2 G 2 a n d for the second, 8.6 G 2. Comparison of these values with the e x p e r i m e n t a l value (12.5 G 2) shows t h a t the difference o f 3.9 G 2 for the second s t r u c t u r e lies w i t h i n the limits of Slichter's corrections (3.1--4.[ G 2) for intermolecular i n t e r a c t i o n b e t w e e n the chains (CHe--CHF),~, (CH 2 - CF2),, and (CF2--CFH), c F o r the first s t r u c t u r e this difference is 5.3 G 2. This of course does not provide a simple solution to the problem of the actual s t r u c t u r e of the copolymers b u t it suggests t h a t the second s t r u c t u r e is more probable.
1010
A.N. LYUBIMOVet aL
I t m a y be considered t h a t in o t h e r cases, a n d also a f t e r t h e precision of the calculations a n d t h e s e n s i t i v i t y o f t h e a p p a r a t u s has b e e n i m p r o v e d , t h e N M R m e t h o d can be of v a l u e for t h e identification o f t h e s t r u c t u r e o f p o l y m e r s . T h e a u t h o r s express t h e i r g r a t i t u d e to A. I. K i t a i g o r o d s k i i for v a l u a b l e a d v i c e d u r i n g discussion of the results o f this work. CONCLUSIONS
(1) T h e t e m p e r a t u r e d e p e n d e n c e of t h e second m o m e n t s a n d w i d t h s of t h e p r o t o n a n d fluorine m a g n e t i c r e s o n a n c e lines has b e e n studied for a n u m b e r o f fluorine-containing c o p o l y m e r s o v e r the t e m p e r a t u r e r a n g e f r o m - - 1 5 0 to + 9 0 °. I t is s h o w n t h a t t h e p a r a m e t e r s of nuclear m a g n e t i c r e s o n a n c e decrease w i t h increasing t e m p e r a t u r e . (2) T h e existence o f two t y p e s of t h e r m a l m o v e m e n t in the c o p o l y m e r chains has b e e n c o n f i r m e d - - r o t a t i o n a l m o v e m e n t of i n d i v i d u a l f u n c t i o n a l groups a n d t h e m o v e m e n t of whole s e g m e n t s of t h e chain. (3) I t is s h o w n t h a t t h e m o b i l i t y of p o l y m e r chains is increased b y the effect o f OCO groups. (4) B y c o m p a r i s o n of t h e e x p e r i m e n t a l values of t h e h y d r o g e n a n d fluorine second m o m e n t s o f the v i n y l i d e n e f l u o r i d e - t r i f l u o r o c h l o r o e t h y l e n e c o p o l y m e r w i t h t h o s e calculated theoretically, w i t h t w o possible m e t h o d s o f a d d i t i o n of t h e - - C F 2 - - C F C 1 - - a n d - - C H 2 - - C F 2 - m o n o m e r units, it is s h o w n t h a t t h e --CF2CFCICF2CH 2 - s t r u c t u r e is m o r e p r o b a b l e t h a n --CF2CFC1CH2CF2--. (5) Chemical shift o f t h e fluorine r e s o n a n c e of t h e CF 2 a n d CF 3 g r o u p s in t h e v i n y l i d e n e f l u o r i d e - h e x a f l u o r o p r o p y l e n e c o p o l y m e r was detected a t -+-90 °. Translated by E. O. PHILLIPS.
REFERENCES
1. C. H. HOLROYD, R. S. CODRINGTON, B. A. MROWCA and E. GUTH, J. Appl. Phys. 22: 696, 1951 2. W. P. SLICHTER, J. Appl. Phys. 26: 1099, 1955 3. N. M. B A Z H E N O V , M. V. V O L ' K E N S H T E I N , A. I. KOL'TSOV a n d A. C. KHACHATUROV, Vysokomol. soedin. 1: 1048, 1959 4. V. R. HONNOLD, F. McCAFFREY and B. A. MROWCA, J. Appl. Phys. 25: 1219, 1954 5. A. ODAJIMA and T. NAGAY, Chem. High Polymers (Japan) 150: 512, 1957 6. H. S. GUTOWSKY and L. H. MEYER, J. Chem. Phys. 12: 2122, 1953 7. C. W. WILSON and G. E. PAKE, J. Polymer Sci. 10: 503, 1953 8. W. P. SLICHTER, J. Polymer Sci. 24: 178, 1957