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The study of rotational mobility of stable nitroxyl radicals in polyvinylacetate
European PolymerJournal, Vol. 12. pp. 691 to 695. Pergamon Press 1976.Printed in Great Britain.
THE STUDY OF ROTATIONAL MOBILITY OF STABLE N I T R O X Y L R A D I C A L S IN P O L Y V I N Y L A C E T A T E A. M. WASSERMAN,T. A. ALEXANDROVAand A. L. BUCHACHENKO Institute of Chemical Physics, Academy of Sciences of the U.S.S.R., Moscow 117334, U.S.S.R. (Received 2 December 1975)
Abstrac~Analyses of the rotational diffusion characters of free and bonded nitroxyt radicals in polyvinylacetate were carried out. The radical rotational character essentially depends on the molecular sizes of the radicals. The movement of the "small" radical is matched by the arbitrary jump tumbling model. The rotation of the "large" radical (both probe and label) occurs by the Brownian rotational diffusion mechanism. The correlation times in the slow-motion region are calculated by taking into account the radical rotation mechanism. Comparison of the r, values for the free and bonded radicals with those obtained by the NMR technique shows that movements of the spin probes and labels depend not only on the short polymer segments but on other factors also.
Study of rotational diffusion of stable nitroxyl radicals has been successfully used for investigation of molecular motions in polymers and of the structures of polymers in the solid and in solutions (see, for example,[l-4]). The processes of crystallization, orientation and degradation of polymers can be examined by the spin probe technique [1]. It has been shown that, for a number of solid polymers, the radical-probe rotation is connected with the polymer segment m o v e m e n t s [ l ] . In this paper the character of rotational diffusion of nitroxyl spin labels and probes and its dependence on the molecular size of radicals are discussed. The comparison between the molecular motions of spin labels and probes is particularly emphasized as well as their relation to molecular motions in polyvinylacetate.
One label was incorporated for ~2000 monomer units. The spin labelled polymer was carefully washed with water and dried in high vacuum. ESR spectra were recorded on a Varian E-4 spectrometer.
EXPERIMENTAL
Samples of polyvinylacetate (PVA) with ~I~ = 105, and PVA containing 15 tool % of OH groups were employed. As spin probes the stable nitroxyl radicals 2,2,6,6-tetramethylpiperidine-l-oxyl (I) and 2,2,6,6-tetramethyl-4-amino dichlorotriazine -piperidin-l-oxyl (II) were used.
/•CH3 H3C"
'1: CH3 0
N.~,~/N Ct
(I)
H3c/~CH3
('IT)
Radical [1) was incorporated into the polymer from the gas phase; radical (II) was introduced from ether solution followed by solvent evaporation. The concentration of stable free radicals in polymers was approximately 1.1017 spin/g. The spin labelling of PVA, containing OH-groups, by radical (II) was performed by the following reaction [5].
RESULTS AND DISCUSSION Figure 1 shows the dependence on temperature of the ESR spectra of the probe radicals (I) and (II) and the covalently bonded radical (II). One can see that the spectra of probe radicals (I) and (II) differ noticeably but for probe and label (II) they differ significantly only at high temperatures. In order to analyze the spectra in the region of slow motion and to calculate the rotational correlation times it is necessary to know the character of the rotational diffusion. The shift of the derivative absorption extremum in the high field AH (see Fig. 1) with respect to the rigid limit was found to be sensitive to the character of molecular motion, whereas the corresponding shift of the low field extremum AH+ does not depend on it [6]. Figure 2 illustrates the theoretical dependences of the parameter R = AH_/'AH + on the low field shift AH+ for Brownian rotational diffusion and arbitrary jump tumbling models [6]. Measuring the parameters AH+ and AH_ and comparing the experimental value of R with the theoretical dependences (shown in Fig. 2) leads to choice of the rotational model. Figure 3 shows the experimental dependences of AH+ and AH for radical-probes (I) and (II) as well as for covalently bonded label (II) in PVA at various temperatures. The temperature changes of the spectra occur only above - 1 5 0 J for radical (I) and - 1 1 0 ° for radical (II) (probe or label) therefore the spectra below these temperatures correspond to the rigid limits.
H3C\ CH3 H! _ OH÷Ct~C~.C_NH_K~_£~
I )
IL
N%C~I [ CL
\
60 -70°
2 H_C"~C H
"3
H~.~CH3
PVA
Dimethylfo~mamide
3
691
Spin labelled
CL
A.M. WASSERMAN,T. A. ALEXANDROVAand A. L. BUCHACHENKO
692
The experimental values R calculated from these data are shown in Fig. 2. For radical-probe (I), all the experimental R values are between 0.7 and 0.9 and closely correspond to the theoretical values for an arbitrary jump tumbling model. In the case of radical (II) both probe and label, R lies in the range 1.5-2.3, approximately corresponding to the theoreti(a)
Ho
Fig. I. ESR spectra (a) radical-probe (I) (b)--radical-probe (II) and (c) radical-label (II) in polyvinyl-acetate at various temperatures. cal values of the rotational diffusion model. These data indicate that the theoretical approach [6] for radical rotation in liquids can be used also for investigation of molecular motion in polymers. It is also evident that increase of the probe size (compare formulae of the radicals (I) and (II)) leads to change of the character of rotational motion of
I
0
I
I
I
I
2
AH+,
3
I
4
13
Fig. 2. The dependence of the parameter R on AH+ for PVA 1---corresponding to rotational diffusion [6]; 2-corresponding to arbitrary jump tumbling [6]; O Experimental values for spin-labeled PVA; t for radical-probe (II) in PVA; a for radical-probe (I) in PVA.
Study of nitroxyl radicals in polyvinylacetate
693
//
/°/
o .
AH, A
_
.,~*'
1
+
o,,~'
L~H_
T,
o
/ 5 *
H÷
o .e/,
°C
Fig. 3. The dependence of z~H+ on temperature for radical-probe (I) (&); radical-probe (II) (O); and radical-label (II) (l). radicals in PVA. The molecular models of radicals (I) and (II) were constructed and volumes of these radicals were found to differ by a factor of approximately two. Such a difference is enough to change the rotational motion character of the probe in PVA. It is remarkable that the bonding of radical (II) to polymer chain through the C - O - C bond does not change greatly tile values of R and the character of radical rotation. The bonding of radical to the polymer chain might lead to anisotropy of radical rotation but if the anistropy is not very large this factor was shown [7] to have practically negligible influence on the R values. For that reason, we discuss our results in terms of isotropic rotational models. The Arrhenius plots of the rotational correlation times are given in Fig. 4. In the region of "rapid" rotation (5.10 -11 < "cc < 1.10-gsec), the correlation times were calculated by equation: (see review tl]):
where 3H+ is the width (in Gauss) of the low field line (ran = + 1) and I(+), I ( - ) are the intensities of the low and high field ESR lines. It should be noted that it is necessary to take into consideration the contribution of inhomogenous line broadening induced by unresolved hyperfine proton structure to calculate correctly the correlation times [4, 8]. However in air, the additional oxygeninduced line broadening was shown in paper [9] to allow neglect of inhomogenous line broadening and calculate the correlation time from Eqn. (1). In the region of "slow" movements, the correlation times can be calculated in different ways. At r~ > 1.10-gsec it can be found by use of the parameter . ~ [10]:
curve at the correlation times re, at the "free" (re < 10 -11 sec) and "rigid" (zc > 10 -7 sec) limits respectively. The theoretical dependence of ~ on the correlation time has been given [10]. From previous results [6, 10], it can be concluded that the theoretical value ~ is the same for the arbitrary jump tumbling and Brownian rotational diffusion models. For this reason, the correlation times were calculated by this method for radical (I) as well as radical (II) (probe and label) (see Fig. 4). At ~c > 7.10 -9 sec, the correlation time can also be calculated from the formula E11]: zc = a(1 - S) b
where S = A=/A*=, a and b are parameters dependent upon the rotational model, 2A= and 2A*= are the separations shown in Fig. 1. For radical (I), the arbitrary jump tumbling parameters a = 2.55.10 -1° and b = -0.615 were used when formula (3) was employed. For the molecular
/ ~'
2.4
tI
/ I
I
2.8
I
I
3.2
I I l 140110 80 50
t
I 20
I
.3!6 I 4 0
(2)
Here H(r), H(r---* 0) and H(r--~ oc) are the magnetic field values corresponding to low field hyperfine extrema in the first derivative of the absorption
I
I
4.4
1103/T,I ° K -I0 -40 T,
H(r) - H(r ~ 0) ' ~ = H(r---~ ~ ) - H(r----~0)"
(3)
I
I
4.8
i
t -70
I
5.2
t
I
5.6
L
-I100 I
°C
Fig. 4. Arrhenius plots of the rotational correlation time calculated by various formulae. 1--radical-probe (I): • by formula (l); © by (2); A by (3) with parameters of arbitrary jump tumbling. 2 radical ( I I ~ t h e probe and the label. [] probe, by formula (1); • label © probe, by (2); • label, [] probe, by (3) with parameters of rotational diffusion.
694
A.M. WASSERMAN,T. A. ALEXANDROVAand A, L. BUCHACHENKO
motion description of radical (II), both probe and label, the Brownian rotational diffusion parameters a = 5"4.10 - l ° and b = - 1 . 3 6 were used. ESR spectra over the wide temperature interval were analysed in terms of Eqns. 1-3. zc values calculated by different ways are in good agreement (Fig. 4). The correlation times of probe (I) are smaller than those of probe (II). This discrepancy arises from the differences in molecular sizes of the stable radicals. The values zc of radical (II) both spin-probe and spin-label are practically identical up to 80 °. One can conclude that labelling of PVA through a mobile ordinary bond is not enough to influence the rotational motion of the radical in the solid polymer. In both cases the main barrier to radical rotation is determined by the mobility of macromolecules surrounding the radical. However at temperatures above 80 ° the zc values of the label becomes larger than that of the probe. This difference is unambigiously caused by radical bonding to polymer chain. The rotational mobility of the spin label was studied up to 125 ° since at high temperatures degradation of spin-labelled polymer might occur. The study of rotational mobility of probes (I) and (II) in PVA was carried out up to 155 ° (Fig. 4). In contrast to solid polymer, the mobility of probes and labels in solution differ considerably. The ESR spectra of 5wt% PVA in methanol solutions with probe and label (II) are shown in Fig. 5. T~ values of probe and label calculated from these spectra differ by a factor of approximately four. It should be noted that the Tc values of the radical probe rotation in solid PVA and in PVA containing 15 mol% OH-groups are practically the same. One can conclude that the molecular mobilities in these polymers are the same. These results agree with the Ho
P,
11
,oG
(a)
(b)
Fig. 5. ESR spectra of 5 wt% PVA in methanol solutions at 0 °. (a)--spin-probe (II), rc = 1.5.10-1o sec; (b)--spinlabel (II), zc = 6.3.10-lO sec.
9oL ,~. 8.6
. ..oI_
*1
z2t'
I 22
I 24
[ ~1\ I 26 28 3.0 103/T, °K
I
I 3.2 I
i
T,
1
I
"C
Fig. 6. Arrhenius plots of the rotation correlation times • chain section by NMR [10]; © spin-label (II); A--spinprobe (II). well-known fact that the value Tg of PVA is quite close to those for PVA containing small quantities of hydroxyl-groups [12]. Arrhenius plots of the correlation times are not linear, consisting of two almost linear plots (see Fig. 4); the activation energy at temperatures above Tg exceeds that for temperatures below Tg. The activation energy (Ea¢0 of radical (I) rotation above Tg is 9.3 kcal/mole, the pre-exponent is To = 3.10-13 sec; below Tg, E, ct = 1.2kcal/mole, z0 = 7 . 9 " 1 0 -9 sec. For radical-probe (II) E,,¢, = 7.0kcal/mole, To = 4.8-10-13 sec (above Tg); E~,~, = 1.2 kcal/mole, To = 4.3" 10- 9 sec (below Tg). The activation energy below Tg does not depend on the radical-probe size and depends only slightly upon it at temperatures above
Tg. The differences between Eact above and below Tg are explained by the freezing of segmental motions at Tg. Analogous differences have been observed for the translation diffusion of various molecules in polymers [ 13]. It is interesting to compare the results of our investigation with those from a study of molecular motions in PVA by NMR. Study [14] of the spin-lattice relaxation in PVA showed that, at temperatures considerably above Tg ( + 9 0 - + 160°), the N M R relaxation was caused by the movement of short section (1-2 links) of the main polymer chain [14]. In Fig. 6, the correlation times of the short polymer sections and spin label and probe movements are compared. The activation energy of polymer section movements (13.5 kcal/mole,[14]) is considerably more than for spin-label and probe rotation. This distinction indicates that the movement of the radical probes and labels in PVA depends not only on short sections of polymer chain but on other factors also.
Study of nitroxyl radicals in polyvinylacetate REFERENCES
1. A. L. Buchachenko, A. L. Kovarskii and A. M. Wasserman, in Advances in Polymer Science, (Edited by Z. A. Rogovin), p. 33, Haelsted Press/Wiley, New York (1974). 2. P. T6rmS_la and S. Tulikonra, Polymer 15, 248 (1974). 3. A. T. Bullock, G. G. Cameron and J. M. Elson, Polymer 15, 74 (1974). 4. A. T. Bullock, G. G. Cameron and P. M. Smith, Europ. Polym. J. 11, 617 (1975). 5. U. B. Grebentschikov, V. I. Irdzak, A. I. Koozoob, P. P. Koons and N. S. Enikolopov, Dokl. Acad Nauk. SSSR 210, 1124 (1973). 6. A. N. Kuznetsov and B. Ebert, Chem. Phys. Lett. 25, 342 (1974).
695
7. V. A. Livshits, d. phys. Chem., in Russian 60, 808 (1976). 8. G. Poggi and C. P. Johnson, J. mag. Res. 3, 436 (1970). 9. A. N. Kuznetsov, A. Y. Volkov, V. A. Livshits and A. T. Mirsoian, Chem. Phys. Lett. 26, 369 (1974). 10. A. N. Kuznetsov, A. M. Wasserman, A. U. Volkov and N. N. Korst, Chem. Phys. Lett. 12, 103 (1971). 11. S. A. Goldman, G. V. Bruno and J. H. Freed, J. phys. Chem. 76, 1858 (1972). 12. S. H. Ushakov, Polivinilovy spirt i ego proizvodnie, p. 569, Akad. Nauk SSSR, (1960). 13. B. I. Sadzin, Electricheskie svoistva polymerov, p. 34, Chimiya, 1972. 14. G. P. Michailiv and B. A. Shevelev, Vysokomolek. Soedin. 9a. 1542 (1966).
THE STUDY OF ROTATIONAL MOBILITY OF STABLE N I T R O X Y L R A D I C A L S IN P O L Y V I N Y L A C E T A T E A. M. WASSERMAN,T. A. ALEXANDROVAand A. L. BUCHACHENKO Institute of Chemical Physics, Academy of Sciences of the U.S.S.R., Moscow 117334, U.S.S.R. (Received 2 December 1975)
Abstrac~Analyses of the rotational diffusion characters of free and bonded nitroxyt radicals in polyvinylacetate were carried out. The radical rotational character essentially depends on the molecular sizes of the radicals. The movement of the "small" radical is matched by the arbitrary jump tumbling model. The rotation of the "large" radical (both probe and label) occurs by the Brownian rotational diffusion mechanism. The correlation times in the slow-motion region are calculated by taking into account the radical rotation mechanism. Comparison of the r, values for the free and bonded radicals with those obtained by the NMR technique shows that movements of the spin probes and labels depend not only on the short polymer segments but on other factors also.
Study of rotational diffusion of stable nitroxyl radicals has been successfully used for investigation of molecular motions in polymers and of the structures of polymers in the solid and in solutions (see, for example,[l-4]). The processes of crystallization, orientation and degradation of polymers can be examined by the spin probe technique [1]. It has been shown that, for a number of solid polymers, the radical-probe rotation is connected with the polymer segment m o v e m e n t s [ l ] . In this paper the character of rotational diffusion of nitroxyl spin labels and probes and its dependence on the molecular size of radicals are discussed. The comparison between the molecular motions of spin labels and probes is particularly emphasized as well as their relation to molecular motions in polyvinylacetate.
One label was incorporated for ~2000 monomer units. The spin labelled polymer was carefully washed with water and dried in high vacuum. ESR spectra were recorded on a Varian E-4 spectrometer.
EXPERIMENTAL
Samples of polyvinylacetate (PVA) with ~I~ = 105, and PVA containing 15 tool % of OH groups were employed. As spin probes the stable nitroxyl radicals 2,2,6,6-tetramethylpiperidine-l-oxyl (I) and 2,2,6,6-tetramethyl-4-amino dichlorotriazine -piperidin-l-oxyl (II) were used.
/•CH3 H3C"
'1: CH3 0
N.~,~/N Ct
(I)
H3c/~CH3
('IT)
Radical [1) was incorporated into the polymer from the gas phase; radical (II) was introduced from ether solution followed by solvent evaporation. The concentration of stable free radicals in polymers was approximately 1.1017 spin/g. The spin labelling of PVA, containing OH-groups, by radical (II) was performed by the following reaction [5].
RESULTS AND DISCUSSION Figure 1 shows the dependence on temperature of the ESR spectra of the probe radicals (I) and (II) and the covalently bonded radical (II). One can see that the spectra of probe radicals (I) and (II) differ noticeably but for probe and label (II) they differ significantly only at high temperatures. In order to analyze the spectra in the region of slow motion and to calculate the rotational correlation times it is necessary to know the character of the rotational diffusion. The shift of the derivative absorption extremum in the high field AH (see Fig. 1) with respect to the rigid limit was found to be sensitive to the character of molecular motion, whereas the corresponding shift of the low field extremum AH+ does not depend on it [6]. Figure 2 illustrates the theoretical dependences of the parameter R = AH_/'AH + on the low field shift AH+ for Brownian rotational diffusion and arbitrary jump tumbling models [6]. Measuring the parameters AH+ and AH_ and comparing the experimental value of R with the theoretical dependences (shown in Fig. 2) leads to choice of the rotational model. Figure 3 shows the experimental dependences of AH+ and AH for radical-probes (I) and (II) as well as for covalently bonded label (II) in PVA at various temperatures. The temperature changes of the spectra occur only above - 1 5 0 J for radical (I) and - 1 1 0 ° for radical (II) (probe or label) therefore the spectra below these temperatures correspond to the rigid limits.
H3C\ CH3 H! _ OH÷Ct~C~.C_NH_K~_£~
I )
IL
N%C~I [ CL
\
60 -70°
2 H_C"~C H
"3
H~.~CH3
PVA
Dimethylfo~mamide
3
691
Spin labelled
CL
A.M. WASSERMAN,T. A. ALEXANDROVAand A. L. BUCHACHENKO
692
The experimental values R calculated from these data are shown in Fig. 2. For radical-probe (I), all the experimental R values are between 0.7 and 0.9 and closely correspond to the theoretical values for an arbitrary jump tumbling model. In the case of radical (II) both probe and label, R lies in the range 1.5-2.3, approximately corresponding to the theoreti(a)
Ho
Fig. I. ESR spectra (a) radical-probe (I) (b)--radical-probe (II) and (c) radical-label (II) in polyvinyl-acetate at various temperatures. cal values of the rotational diffusion model. These data indicate that the theoretical approach [6] for radical rotation in liquids can be used also for investigation of molecular motion in polymers. It is also evident that increase of the probe size (compare formulae of the radicals (I) and (II)) leads to change of the character of rotational motion of
I
0
I
I
I
I
2
AH+,
3
I
4
13
Fig. 2. The dependence of the parameter R on AH+ for PVA 1---corresponding to rotational diffusion [6]; 2-corresponding to arbitrary jump tumbling [6]; O Experimental values for spin-labeled PVA; t for radical-probe (II) in PVA; a for radical-probe (I) in PVA.
Study of nitroxyl radicals in polyvinylacetate
693
//
/°/
o .
AH, A
_
.,~*'
1
+
o,,~'
L~H_
T,
o
/ 5 *
H÷
o .e/,
°C
Fig. 3. The dependence of z~H+ on temperature for radical-probe (I) (&); radical-probe (II) (O); and radical-label (II) (l). radicals in PVA. The molecular models of radicals (I) and (II) were constructed and volumes of these radicals were found to differ by a factor of approximately two. Such a difference is enough to change the rotational motion character of the probe in PVA. It is remarkable that the bonding of radical (II) to polymer chain through the C - O - C bond does not change greatly tile values of R and the character of radical rotation. The bonding of radical to the polymer chain might lead to anisotropy of radical rotation but if the anistropy is not very large this factor was shown [7] to have practically negligible influence on the R values. For that reason, we discuss our results in terms of isotropic rotational models. The Arrhenius plots of the rotational correlation times are given in Fig. 4. In the region of "rapid" rotation (5.10 -11 < "cc < 1.10-gsec), the correlation times were calculated by equation: (see review tl]):
where 3H+ is the width (in Gauss) of the low field line (ran = + 1) and I(+), I ( - ) are the intensities of the low and high field ESR lines. It should be noted that it is necessary to take into consideration the contribution of inhomogenous line broadening induced by unresolved hyperfine proton structure to calculate correctly the correlation times [4, 8]. However in air, the additional oxygeninduced line broadening was shown in paper [9] to allow neglect of inhomogenous line broadening and calculate the correlation time from Eqn. (1). In the region of "slow" movements, the correlation times can be calculated in different ways. At r~ > 1.10-gsec it can be found by use of the parameter . ~ [10]:
curve at the correlation times re, at the "free" (re < 10 -11 sec) and "rigid" (zc > 10 -7 sec) limits respectively. The theoretical dependence of ~ on the correlation time has been given [10]. From previous results [6, 10], it can be concluded that the theoretical value ~ is the same for the arbitrary jump tumbling and Brownian rotational diffusion models. For this reason, the correlation times were calculated by this method for radical (I) as well as radical (II) (probe and label) (see Fig. 4). At ~c > 7.10 -9 sec, the correlation time can also be calculated from the formula E11]: zc = a(1 - S) b
where S = A=/A*=, a and b are parameters dependent upon the rotational model, 2A= and 2A*= are the separations shown in Fig. 1. For radical (I), the arbitrary jump tumbling parameters a = 2.55.10 -1° and b = -0.615 were used when formula (3) was employed. For the molecular
/ ~'
2.4
tI
/ I
I
2.8
I
I
3.2
I I l 140110 80 50
t
I 20
I
.3!6 I 4 0
(2)
Here H(r), H(r---* 0) and H(r--~ oc) are the magnetic field values corresponding to low field hyperfine extrema in the first derivative of the absorption
I
I
4.4
1103/T,I ° K -I0 -40 T,
H(r) - H(r ~ 0) ' ~ = H(r---~ ~ ) - H(r----~0)"
(3)
I
I
4.8
i
t -70
I
5.2
t
I
5.6
L
-I100 I
°C
Fig. 4. Arrhenius plots of the rotational correlation time calculated by various formulae. 1--radical-probe (I): • by formula (l); © by (2); A by (3) with parameters of arbitrary jump tumbling. 2 radical ( I I ~ t h e probe and the label. [] probe, by formula (1); • label © probe, by (2); • label, [] probe, by (3) with parameters of rotational diffusion.
694
A.M. WASSERMAN,T. A. ALEXANDROVAand A, L. BUCHACHENKO
motion description of radical (II), both probe and label, the Brownian rotational diffusion parameters a = 5"4.10 - l ° and b = - 1 . 3 6 were used. ESR spectra over the wide temperature interval were analysed in terms of Eqns. 1-3. zc values calculated by different ways are in good agreement (Fig. 4). The correlation times of probe (I) are smaller than those of probe (II). This discrepancy arises from the differences in molecular sizes of the stable radicals. The values zc of radical (II) both spin-probe and spin-label are practically identical up to 80 °. One can conclude that labelling of PVA through a mobile ordinary bond is not enough to influence the rotational motion of the radical in the solid polymer. In both cases the main barrier to radical rotation is determined by the mobility of macromolecules surrounding the radical. However at temperatures above 80 ° the zc values of the label becomes larger than that of the probe. This difference is unambigiously caused by radical bonding to polymer chain. The rotational mobility of the spin label was studied up to 125 ° since at high temperatures degradation of spin-labelled polymer might occur. The study of rotational mobility of probes (I) and (II) in PVA was carried out up to 155 ° (Fig. 4). In contrast to solid polymer, the mobility of probes and labels in solution differ considerably. The ESR spectra of 5wt% PVA in methanol solutions with probe and label (II) are shown in Fig. 5. T~ values of probe and label calculated from these spectra differ by a factor of approximately four. It should be noted that the Tc values of the radical probe rotation in solid PVA and in PVA containing 15 mol% OH-groups are practically the same. One can conclude that the molecular mobilities in these polymers are the same. These results agree with the Ho
P,
11
,oG
(a)
(b)
Fig. 5. ESR spectra of 5 wt% PVA in methanol solutions at 0 °. (a)--spin-probe (II), rc = 1.5.10-1o sec; (b)--spinlabel (II), zc = 6.3.10-lO sec.
9oL ,~. 8.6
. ..oI_
*1
z2t'
I 22
I 24
[ ~1\ I 26 28 3.0 103/T, °K
I
I 3.2 I
i
T,
1
I
"C
Fig. 6. Arrhenius plots of the rotation correlation times • chain section by NMR [10]; © spin-label (II); A--spinprobe (II). well-known fact that the value Tg of PVA is quite close to those for PVA containing small quantities of hydroxyl-groups [12]. Arrhenius plots of the correlation times are not linear, consisting of two almost linear plots (see Fig. 4); the activation energy at temperatures above Tg exceeds that for temperatures below Tg. The activation energy (Ea¢0 of radical (I) rotation above Tg is 9.3 kcal/mole, the pre-exponent is To = 3.10-13 sec; below Tg, E, ct = 1.2kcal/mole, z0 = 7 . 9 " 1 0 -9 sec. For radical-probe (II) E,,¢, = 7.0kcal/mole, To = 4.8-10-13 sec (above Tg); E~,~, = 1.2 kcal/mole, To = 4.3" 10- 9 sec (below Tg). The activation energy below Tg does not depend on the radical-probe size and depends only slightly upon it at temperatures above
Tg. The differences between Eact above and below Tg are explained by the freezing of segmental motions at Tg. Analogous differences have been observed for the translation diffusion of various molecules in polymers [ 13]. It is interesting to compare the results of our investigation with those from a study of molecular motions in PVA by NMR. Study [14] of the spin-lattice relaxation in PVA showed that, at temperatures considerably above Tg ( + 9 0 - + 160°), the N M R relaxation was caused by the movement of short section (1-2 links) of the main polymer chain [14]. In Fig. 6, the correlation times of the short polymer sections and spin label and probe movements are compared. The activation energy of polymer section movements (13.5 kcal/mole,[14]) is considerably more than for spin-label and probe rotation. This distinction indicates that the movement of the radical probes and labels in PVA depends not only on short sections of polymer chain but on other factors also.
Study of nitroxyl radicals in polyvinylacetate REFERENCES
1. A. L. Buchachenko, A. L. Kovarskii and A. M. Wasserman, in Advances in Polymer Science, (Edited by Z. A. Rogovin), p. 33, Haelsted Press/Wiley, New York (1974). 2. P. T6rmS_la and S. Tulikonra, Polymer 15, 248 (1974). 3. A. T. Bullock, G. G. Cameron and J. M. Elson, Polymer 15, 74 (1974). 4. A. T. Bullock, G. G. Cameron and P. M. Smith, Europ. Polym. J. 11, 617 (1975). 5. U. B. Grebentschikov, V. I. Irdzak, A. I. Koozoob, P. P. Koons and N. S. Enikolopov, Dokl. Acad Nauk. SSSR 210, 1124 (1973). 6. A. N. Kuznetsov and B. Ebert, Chem. Phys. Lett. 25, 342 (1974).
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