- Email: [email protected]
Relaxation transitions in polyvinylcyclohexane
484
V. A. TA.Lx,xov et al.
4. V. N. TSVETKOV, L. N. ANDREYEVA, Ye. V. KORNEYEVA, P. N. LAVRENKO,
N. A. PLATE, V. P. SHTRAYEVand B. S. PETRUKHIN, Vysokomol. soyed. A15: 2226, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 10, 1971) 5. V. N. TSVETKOV,. I. N. SHTENNIKOVA, Ye. I. RYUMTSEV, G. F. KOLBINA, L I. KONSTANTINOV, Yu. B. AMERIK and B. A. KRENTSEL, Vysokomol. soyed. Al1:
2528, 1969 (Translated in Polymer SOL U.S.S.R. 11: 11, 2874, 1969) 6. V. N. TSVETKOV, V. Ye. ESKIN and S. Y&. FRENIgEL', Struktura makromolekul v rastvorakh (Macromolecular Structure in Solutions). Izd. "Nauka", 1964 7. J. M. G. COWIE, S. BYWATER, Polymer 6: 197, 1965 8. W. H. STOCKMAYER and M. FIXMAN, .J. Polymer Sci. Cl: 137, 1963 9. L. MANDELKERN and P. FLORY, J. Chem. Phys. 20: 212, 1952 10. V. N. TSVETKOV, Uspekhi khlmii 38: 1674, 1969 11. W. KUHN and H. KUHN, Heir. Chim. Acta 26: 1394, 1943
R E L A X A T I O N TRANSITIONS IN P O L Y V I N Y L C Y C L O H E X A N E * V. A. TALYKOV,V. I. KLEINER and L. L. STOTSKAYA A. V. Topchiev Institute of Petroohemical Synthesis, U.S.S.R. Academy of Sciences V. I. Lenin All-Union Electrotechnieal Institute
(Received 16 July 1970) ISOTACTIO polyvinylcyclohexane (PVCH) is a polymer of high heat stability and satisfactory electrical insulating properties in the temperature range of 180220 °. A combination of these valuable properties with the possibility of processing I~VCH suggests t h a t it is a promising material for electrical insulating coatings, films and fibres [1-3]. This s tu d y examined b y dielectric methods the thermal molecular motion in PVCH and therefore investigated relaxation processes in iso- and atactic PVCH in comparison with iso- and atactic polystyrene (PS) and the electrical conductivity ~ of these polymers. To explain the t y p e of molecular motion of P VC H and PS in the range of negative temperatures, a st udy was made of the temperature frequency dependences of the t angent of dielectric loss angle t a n J and dielectric constant e of vinylcyclohexane, ethylcyclohexane and styrene. The materials examined can be used as models to establish a relation between structure, heat motion and dielectric properties. EXPERIMENTAL
Relaxation properties of PVCH and PS were examined in stereospceifie and atactio polymer Alm~ 40 to 50/~ thick. The films were prepared by evaporation of eyclohexane or benzene solutions of polymers at room temperature. Before the measurements the samples * Vysokomol. soyed. AI4: No. 2, 432-439, 1972.
Relaxation transitions in polyvinylcyclohexane
485
wore dried to constant weight /n vacuo (10 -s ram) at 80°. The synthesis of storeospeoific PVCH and PS is similar and has boon previously dosoribod [4]. Atactic PVCH was prepared by catalytic hydrogenation of ataetio PS or by fraetionation of PVGH synthesized on oomplex organo-motallio catalysts (other-soluble fraction). Methods of measuring tan 6, 8 and 7 and the apparatus used were described in former reports [5, 6]. Silver, vacuum sprayed on to the surface of films was the electrode material for isotaotie PVGH and PS specimens. Ataotio PVCH and PS specimens and liquid low mol.wt, compounds were tested in special stainless stool cells [7]. Tan 6 and 8 wore measured under conditions of regular dooreaso in temperature of specimens at a rate of N 1 degree/rain in an inert atmosphere. ~ was measured in a constant field of 2 × 108 V/em and regular decrease of temperature of preheated samples. RESULTS
I t follows f r o m Fig. 1 a n d Table 1 t h a t t w o different f o r m s of molecular h e a t m o t i o n occur in P V C H over t h e t e m p e r a t u r e r a n g e o f - - 1 8 0 - 2 1 5 °, each of which is o f r e l a x a t i o n t y p e a n d m a y be characterized b y effective a c t i v a t i o n energies o f dielectric r e l a x a t i o n W a n d " u n f r e e z i n g " or glass-transition t e m p e r a tures Tg. 2.5 [-
a
0.005" F
-sso
-too
-5o
o
too
5o
c
ISo
2o0 r, oc
b
2.2
~ 3
tan~
0.005 -
- 15o
w~
i
+.+.~v~. -ZoO
2 34 x'-'×'x
-t-
-50
0
50
lO0
150
200 "IT,°C
FzG. 1. Temperature dependence of tan 6 and 8 of isotactic (a) and atactio (5) PVOH. Frequencies: 1--10=; 2--108; 3--5 × 104; 4--1.5 × 10e c/s. F o r high t e m p e r a t u r e processes W values are 43 a n d 60 kcal/mole, for isot a c t i c a n d a t a e t i c polymers, respectively, a n d 9 a n d 10.5 kcal/mole for low t e m p e r a t u r e processes (Table 1). The order o f W values is t y p i c a l of a c t i v a t i o n
V.A. TALYKOVe$ al.
486
energies of dielectric relaxation of dipole-segmental motion (or a-process) in the first case and dipole-group motion (or fl-process), in the second [8]. The T~ value for isotactic P V C H determined b y a dielectric method from the temperature-frequency dependence of tan 5 was 113 ° which is close to the Tg values determined dilatometrically and thermomechanically [2]. For atactic PVCH dipole-segmental motion begins considerably earlier (T~-97°). This process is characterized b y higher absolute values of tan ~ in the maxima w i t h a less regular shape of curves (Fig. 1). Obviously, the lower parameter of distribution of relaxation times a in atactic PVCH reflects an increased freedom of movement of macromolecules taking part in the a-process of large sections, as compared with isotactic PVCH. TABLE 1. PROPERTIES OF POLYMERS
Polymer
PVCH PVCH PS PS
Chain structure
Isotactic Atactic Isotactic Atactic
~ o
113 97 91 88
60-0 62.0 66.0 - - ~ 0
9-0 10.5 10.0
108 90 105 87
31.0 34.0 39-0 48-0
160 168 162 146
46.0 27.0 34.0 37.0
18o 170 170 140
Dipole-group motion in isotactic PVCH also encounters more considerable difficulties than in atactic PVCH. This is confirmed b y the higher T~ value of this motion (Table 1) and lower Wp value. The fl-process in isotactic PVCH, like the a-process, is characterized b y a very regular shape of curves tan 5 : f (T) in the maxima; the temperature range of dipole-group motion, compared with atactic motion, considerably widens (Fig. 1). This process of low temperature molecular motion, apparently, also accounts for the mechanical loss maxima observed dynamically in atactic PVCH in the low temperature range [9]. As with PVCH, in iso and atactic PS at temperatures higher than 90 ° dipolesegmental relaxation takes place which reflects the segmental mobility of macromolecules (Fig. 2, Table 1). This process, however, in PS is characterized b y lower T~ values and higher absolute values of tan 5ma~ than in PVCH. The increased ordering of isotactic PS structure, as with PVCH, is accompanied b y a reduction of dipole-segmental dielectric losses (Fig. 2), which enables us to relate the a-process to amorphous polymer regions. The increased hindrance to segmental mobility in PVCIt is evidently due to the larger dimensions of the cyclohexane ring, compared with the benzene ring (the natural volume of the monomer unit in P V C H is 74.05, compared with 66.4 cm3/mole in PS [10]). The higher macromolecular rigidity of PVCH is determined b y the type of interaction of
Relaxation transitior~_ in polyvinylcyclohexane
487
eyclohexane rings, which are distinguished from benzene rings by shape and flexibility. A comparison of temperature-frequency dependences of tan J and 8 of PVCH and PS of different structures and chemical composition (Figs. 1 and 2) suggests
_- x
_. - x~ ~ x : ~~ xx
-~--x- -- -~---.-~~ x
~--x-×~ -
-
.
-
-
-
04! rand
--x^
2
/
/\ ./;
0-005
x~-w.
-/50
-~0~
-50
0
5g
1oo
15o
T,°g
Fie. 2. Temperature dependence of tan J and e of isotactic (1) and atactie (2) PS; frequency i0 ~ e/s.
that the higher ordering in polymer structure due to stereo-specific properties considerably reduces macromolecular mobility, but cannot fully eliminat~ dipole-segmental heat motion; the replacement of the benzene ring by a cyclohexane ring has a marked effect on macromolecular rigidity. In contrast to PVCH an increase in stereospecific properties of PS causes a degeneration in dipole-group motion, although this process takes place in atactic PS (Figs. 1, 2 and Table 1). However, the kinetic units, which are responsible for low temperature relaxation in PVCH and atactic PS are different. Thus, for atactie PS this process is apparently due to vibratory motions o f - - C H 2 - - C H ~ groups of the main chain contained in the macromolecules by the anomalous addition of " h e a d - h e a d " type monomer units [11] and having a dipole moment as a consequence of structural asymmetry of adjacent macromoleeular sections. Stereo-specific PS does not contain such groups, which results in the disappearance of the low temperature range of dielectric relaxation. For PVCH the --CH2--CH 2groups are present both in the atactic (anomalous addition, cyclohexane ring) and isotactie (cyclohexane ring) polymers therefore, a dipole-group process occurs in both types of PVCH. I n contrast to benzene rings, the cyclohexane rings are not only capable of complete participation in complex motion, but also undergo internal conformational transitions [12]; the stereo-specific properties of PVCH do not, of course, result in the degeneration of these transitions. An increase in molecular mobility on replacing the benzene ring by a eyelohexane ring is also confirmed by the example of vinylcyclohexane and styrene. I t follows from Fig. 3 (curves 1 and 1') and Table 2 that in the temperature
V. A. T~Yxov et aL
488
range of --180-160 ° ~he motion of vinylcyclohexane molecules undergoes relaxation, which is maintained at temperatures considerably lower than the melting point of the monomer (--126.7 ° [I3]). On transition to the solid state no sudden 2-8
2
\z.e
%
2.0 ~ao~
i ~i'
-150
0.005
-100
-50
O
T,°C
50
I~'~G. 3. Tempe~sture dependence of tan ~ and a: 1, / ' - - v i n y l c y c l o h e x a n e ; 2, 2'---et,yrene~
3, 3'---ethyleycIohexane. Frequencies: I-3--10 =, 1"3'--10 =e]s. change in dielectric constant is observed as only the centres of molecules are fixed while the mobility of individual groups of atoms in vinyleyclohexane is mainatained up to T~=176 ° (Table 2). TABLE ~. PROPERTIES OF LOW MOLECULAR WEIGHT COMPOUNDS
Compound Vinylcyclohoxane St,yrene Ethylcyclohexano
~melt
-- 126.7 --30.6 --111.3
T~, °C
WB
kcal/mole
--176
II'0
/--178
11.0
I n contrast to vinyloyelohexane molecules, for styrene near the melting point (--30.6 °) [13] the displacement of both molecular centres and individual atom groups is no longer possible. The change-over of styrene molecules in the region of --30 ° and the variation of mobility is not of relaxation type (Fig. 3, curves 2 and 2'). Absolute maxima tan ~ decrease in proportion to the increase in the frequency of the field applied without a temperature displacement in the positions of these maxima.
Relaxation transitions in poly~nylcyclohexane
489
Over the temperature range examined below melting point relaxation processes in styrene were not detected dielectrically, due to an increase in packing density of styrene molecules in the crystalline sta~e. Crystallization evidently prevents rotation of benzene rings around single C-C bonds and as noted previously, conformation transitions in these rings are impossible.
-11
-/3
\ \
-/7
2.0
2.5
iO~T,°K-1
3JO
I~G. 4. Temperature dependence of the electrical conductivity of polymers. ]--isotactio and 2--atactic PVCH, 3--iso~actic and 4--atactic PS, ~--isotactic PVCH irradiated by X-rays for 30 rain. Dipole relaxation losses are also observed in ethylcyclohexane, from which unlike vinylcyclohexane, double bonds capable of being readily polarized during conformation transitions of the cyclohexane ring (Fig. 3, curves 3 and 3'), are absent. For ethylcyclohexane there are dielectric loss maxima, although these
490
V.A.
TALYKOV et al.
maxima and v a l u e s of dielectric c o n s t a n t s are much lower at all frequencies than for vinylcyclohexane (Fig. 3 curves 1 and 1'). The proximity of T~ and W~ values for vinylcyclohexane and ethylcyclohexane (Table 2) suggests that the s a m e kinetic units are responsible for the appearance of molecular mobility. I t can therefore be concluded that the molecular heat motion in iso- and atactic PVCH is closely related to the specific mobility of lateral cyclohexyl substituents. I t is known [5, 14] that the electrical conductivity of polymers is also closely related to chemical structure and the type of molecular heat motion. It was therefore interesting to study the actual type of this relation and examine the temperature dependence of electrical conductivity for iso- and atactic PVCH in comparison with iso- and atactic PS (Fig. 4). Figure 4 shows that in the temperature range of 20-220 ° all the polymers studied have two inflexions on the curves of the dependence of log y ~ f (I/T). The average temperature of low temperature inflexion is denoted by T 1 and is close to T~ values derived from the temperature-frequency dependences of tan g (Table 1). The average temperature of high temperature inflexions is 50-70 ° higher than T1, is denoted by T~ and corresponds to Tmin (Table 1), at which tan g at a frequency of 100 c/s passes through a minimum and then begins to increase as a result of through conductance (Figs. 1, 2). The linear parts of dependences log ~ = f (l/T) enabled us to calculate the effective activation energies of electrical conductivity W1 in the temperature range of T1-T2 and W~ in the ranges from T 2 to temperatures several degrees higher than T~ (Table 1, Fig. 4). In the temperature range below T~ activation energies of electrical conductivity were 10-12 kcal/mole. A variation in electrical conductivity near the glass transition temperature, which is accompanied by a sudden increase in the activation energy of this process, is observed in m a n y polymers and is due to a considerable increase in molecular mobility on transition from the glassy to the high elastic state [14]. Temperature T~ should also be regarded as one of the characteristics reflecting processes of internal change-over in the polymer. It is known that at temperatures which are 50-70 ° higher than T~ a variation is observed in the activation energies of m a n y physical processes--viscous flow [15], dielectric relaxation [16] and electrical conductance [6]. These change-over processes are, apparently, based on the disruption of the co-operative motion of segments due to the varying initial structural ordering of the polymer and the subsequent reduction or increase of this ordering under the effect of high temperatures. It is assumed by some authors [15] that at temperatures 50-70 ° higher than Tg liquid-liquid type transitions occur in atactic polymers, while others [8] assume a variation in supermolecular structural formations. I t follows from Fig. 4 and Table 1 that stereo-specific properties of PVCH and PS result in a considerable reduction in absolute values of 7, a displacement ot curves log ? = f (I/T) in the direction of higher temperature and a certain reduc-
Relaxation transitions in polyvinylcyclohexane
491
tion in W1 values. Lowest W1 values, as also W~ values, are observed for PVCH which has the highest chain rigidity and retarded segmental motion of macromolecules. At temperatures higher than T~ the variation in electrical conductivity for isotactie PVCH differs from t h a t observed for other polymers. While with atactie PVCH and PS W2 noticeably decreases compared with W1, with isotactic PVCH W2 increases. A reduction in the activation energy of electrical conductivity at temperatures higher t h a n T~, compared with W1 values, is typical of m a n y polymers [5]. The causes of anomalous variation of Ws in isotactic PVCH near T~ require detailed study as t h e y are determined by complex structural changes in the polymer. I t is possible that this effect, like the abnormally high melting point of crystalline isotactic PVCH, is due to a transition of the polymer from one crystalline modification to another. This transition is confirmed b y X-ray data of PVCH [17], according to which the initial rule of variation of the interplanar distance according to temperature does not hold good in the temperature range of 170-200 ° and a marked change is observed in the coefficient of thermal expansion, which corresponds to a change in the "order-disorder" type structure [4]. In approximately the same region (180-200 °) an inflexion is observed in the dependence on temperature [18] of optical density of a band at 830 cm -1, which characterizes the helical conformation of PVCH macromolecules. In this temperature range transition is observed in the tetragonal structure of PVCH 247 in a polymer oriented at temperatures lower than 200 ° to structure 41, which is characterized by higher stability at high temperatures [19]. W, kcal/rno/e
50
1,o/~14, 5g /
3g 20 - fO
~ . . . . . 0
# I,j lg
IogA
FIG. 5. Dependence o£ activation energy of electrical conductivity W on the logarithm of pro-exponential factor A for polymers (see explanations of curves in Fig. 4); 1 - 5 correspond to the dependence of Wl-log A t , l ' - 5 " - - t o W,-log A,. It is well known [5, 14] that for m a n y polymers a so-called "compensation effect" is observed, whereby the dependence of W-log A, where A - - t h e pre-exponential factor in the expression 7 = A e -~vmT , is linear. For polymers with Tg>60 ° previously studied [5] and differing in structure, chain rigidity and degree macromolecular crosslinking, the dependence W1,2----27-2+1-78 log A1, 2 is valid. A similar relation appeared to hold good for atactic PVCH and iso- and atactic PS (Fig. 5).
~g2
V. A. TALXXOVet a/.
For initial isotactic PVCH and PVCH irradiated by y-rays points 1' and 5' of the dependence Wz-log A n deviate from the general relation (Fig. 5), which also proves that this polymer behaves abnormally. The polymers were irradiated by soft X-rays with a beam diameter of 60 mm (tube voltage 20 kV, current 200 mA), the specimens being retained for 15-180 rain. Irradiation for 30 rain did not alter the general dependence of log r=f (l/T) (Fig. 4, curve 5), reduced the absolute values of 7 with a slight change in IV1 and IVy. However, a further increase in the irradiation dose increased ttrl from 30 to 40 kcal/mole with a slight reduction in 1¥~ and an increase in 7. The data confirm that two different processes take place during irradiation-crossllnklng and simultaneous degradation of polymer macromolecules. A small dose of irradiation promotes crossllnking, structure formation in the linear polymer, reduction in macromolecular mobility and consequently reduces the mobility of ionic charge carriers and the value of 7. Further increase in the irradiation dose with an exposure of over 30 min results in degradation, higher concentration of ionic charge carriers as a result of products of decomposition during oxidation and degradation. As a result absolute values of electrical conductivity increase. Exposure of PVCH specimens for a period of more than 180 min produced increased brittleness and decomposition of films. It should be noted that an increase in the crystallinity of isotactic PVCH during annealing causes a reduction of 7, similar to the effect caused by irradiation for a brief period (curve 5, Fig. 4). CONCLUSIONS
(1) Relaxation processes in iso- and atactie polyvinylcyclohexane (PVCH) and polystyrene (PS), styrene, ethyl- and vinylcyclohexane were examined by a dielectric method. Temperature dependences of electrical conductivity of PVCH and PS were investigated. (2) It was shown that in isotactic PVCH, as in atactic PVCH and PS, there are two types of molecular heat motion: dipole-group and dipole-segmental motion. The stereospecific structure of PVCH macromolecules reduces molecular mobility without causing degeneration of the dipole-group motion, as observed in isotactic PS. (3) Dipole-group relaxation in atactic PVCH is due to the motion o f - - C H ~ - --CHt-groups of the main chain (irregular addition of monomer units) or the cyclohexane ring, while in isotactic PVCH it is caused by a change in the position of these groups in the cyclohexane rings alone as a result of the rotary, vibratory or conformation motion of the latter. (4) It was shown that for isotactic PVCH the activation energy of electrical conductivity increased abnormally in the temperature range T 2 which is 50-70 ° above the glass transition temperature T~ and a deviation was observed from the
Relaxation transitions in polyvinylcyclohexane
493
linear d e p e n d e n c e W - l o g A, which is t y p i c a l o f t h e o t h e r p o l y m e r s studied. I t is a s s u m e d t h a t t h e a b n o r m a l b e h a v i o u r o f isotactic P V C H is due to t h e presence of t h e " c r y s t a l - c r y s t a l " transition. Translated by E. S~MEm~ REFERENCES
1. V. I. KLEINER, B. A. KRENTSEL' and L. L. STOTSKAYA, Plast. massy, No. 4, 3, 1967 2. V. L KLEINER, S. G. BARSAMYAN, A. S. APRESYAN and L. L. STOTSKAYA, Plast. massy, ~o. 3, 13, 1968 3. Ye. F. ZININ, M. S. AKUTIN et aL, Plast. massy, No. 1, 32, 1970 4. V. I. KT.EINER, Dissertation, 1969 5. V. A. TALYKOV, Dissertation, 1969 6. V. A. TALYKOV, G. E. GOLUBKOV, A. I. IKONNIKOVA and A. P. BEI~YAYEVA, Vysokomol. soyed. A l l : 1303, 1969 (Translated in Polymer Sci. U.S.S.R. l l : 6, 1480, 1969) 7. G. Ye. GOLUBKOV, Dissertation, 1959 8. G. P. MIKHAILOV, ZhVKhO im. D. I. Mendeleyeva 6: 404, 1961 9. K. A. WOLF, Z. Elektrochem. 65: 604, 1961 10. G. L. SLONIMSKII, A. A. ASKADSKH and A. I. KITAIGORODSKII, Vysokomol. soyed. A12: 494, 1970 11. M. BACCAREDA, E. BUTTA, V. FROSINI and P. MAGAGNINI, J. Polymer Sci. 4, A-2: 789, 1966 12. E. ILYEL, N. ~tLLINZHER, S. ENIZHIAL and G. MORRISON, Konformatsionnyi ~naliz (Conformational Analysis). Izd. "Mir", 1969 13. V. M. TATEVSKII, Fiziko-khlmicheskie svoistva individual'nykh uglevodorodov (Physical and Chemical Properties of Individual Hydrocarbons). Gostoptetrhi~.dat, 1960 14. B. I. SAZHIN, Elektroprovodnost' polimerov (Electrical Conductivity of Polymers). Izd. "Khlmiya", 1965 15. R. BOIER (Ed.), Perekhody i relaksatsionnye yavleniya v polimerakh (Transitions and Relaxation Effects in Polymers). Izd. "Mir", 1961 16. A. M. LOBANOV, G. P. MEgHAILOV and V. A. SHEVELEV, Vys0komol. soyed. 8: 794, 1961 (l~ot translated in Polymer Sci. U.S.S.R.) 17. R. A. RAFF and K. V. DOK (Ed.), Kristallieheskie poliolefiny (Crystalline Polyolefms). Izd. "Khimiya", 2, 1970 18. O. A. N ~ I T I N A , Dissertation, 1969 19. N. D. NOETHER, J. Polymer Sci. CI6: 725, 1967
V. A. TA.Lx,xov et al.
4. V. N. TSVETKOV, L. N. ANDREYEVA, Ye. V. KORNEYEVA, P. N. LAVRENKO,
N. A. PLATE, V. P. SHTRAYEVand B. S. PETRUKHIN, Vysokomol. soyed. A15: 2226, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 10, 1971) 5. V. N. TSVETKOV,. I. N. SHTENNIKOVA, Ye. I. RYUMTSEV, G. F. KOLBINA, L I. KONSTANTINOV, Yu. B. AMERIK and B. A. KRENTSEL, Vysokomol. soyed. Al1:
2528, 1969 (Translated in Polymer SOL U.S.S.R. 11: 11, 2874, 1969) 6. V. N. TSVETKOV, V. Ye. ESKIN and S. Y&. FRENIgEL', Struktura makromolekul v rastvorakh (Macromolecular Structure in Solutions). Izd. "Nauka", 1964 7. J. M. G. COWIE, S. BYWATER, Polymer 6: 197, 1965 8. W. H. STOCKMAYER and M. FIXMAN, .J. Polymer Sci. Cl: 137, 1963 9. L. MANDELKERN and P. FLORY, J. Chem. Phys. 20: 212, 1952 10. V. N. TSVETKOV, Uspekhi khlmii 38: 1674, 1969 11. W. KUHN and H. KUHN, Heir. Chim. Acta 26: 1394, 1943
R E L A X A T I O N TRANSITIONS IN P O L Y V I N Y L C Y C L O H E X A N E * V. A. TALYKOV,V. I. KLEINER and L. L. STOTSKAYA A. V. Topchiev Institute of Petroohemical Synthesis, U.S.S.R. Academy of Sciences V. I. Lenin All-Union Electrotechnieal Institute
(Received 16 July 1970) ISOTACTIO polyvinylcyclohexane (PVCH) is a polymer of high heat stability and satisfactory electrical insulating properties in the temperature range of 180220 °. A combination of these valuable properties with the possibility of processing I~VCH suggests t h a t it is a promising material for electrical insulating coatings, films and fibres [1-3]. This s tu d y examined b y dielectric methods the thermal molecular motion in PVCH and therefore investigated relaxation processes in iso- and atactic PVCH in comparison with iso- and atactic polystyrene (PS) and the electrical conductivity ~ of these polymers. To explain the t y p e of molecular motion of P VC H and PS in the range of negative temperatures, a st udy was made of the temperature frequency dependences of the t angent of dielectric loss angle t a n J and dielectric constant e of vinylcyclohexane, ethylcyclohexane and styrene. The materials examined can be used as models to establish a relation between structure, heat motion and dielectric properties. EXPERIMENTAL
Relaxation properties of PVCH and PS were examined in stereospceifie and atactio polymer Alm~ 40 to 50/~ thick. The films were prepared by evaporation of eyclohexane or benzene solutions of polymers at room temperature. Before the measurements the samples * Vysokomol. soyed. AI4: No. 2, 432-439, 1972.
Relaxation transitions in polyvinylcyclohexane
485
wore dried to constant weight /n vacuo (10 -s ram) at 80°. The synthesis of storeospeoific PVCH and PS is similar and has boon previously dosoribod [4]. Atactic PVCH was prepared by catalytic hydrogenation of ataetio PS or by fraetionation of PVGH synthesized on oomplex organo-motallio catalysts (other-soluble fraction). Methods of measuring tan 6, 8 and 7 and the apparatus used were described in former reports [5, 6]. Silver, vacuum sprayed on to the surface of films was the electrode material for isotaotie PVGH and PS specimens. Ataotio PVCH and PS specimens and liquid low mol.wt, compounds were tested in special stainless stool cells [7]. Tan 6 and 8 wore measured under conditions of regular dooreaso in temperature of specimens at a rate of N 1 degree/rain in an inert atmosphere. ~ was measured in a constant field of 2 × 108 V/em and regular decrease of temperature of preheated samples. RESULTS
I t follows f r o m Fig. 1 a n d Table 1 t h a t t w o different f o r m s of molecular h e a t m o t i o n occur in P V C H over t h e t e m p e r a t u r e r a n g e o f - - 1 8 0 - 2 1 5 °, each of which is o f r e l a x a t i o n t y p e a n d m a y be characterized b y effective a c t i v a t i o n energies o f dielectric r e l a x a t i o n W a n d " u n f r e e z i n g " or glass-transition t e m p e r a tures Tg. 2.5 [-
a
0.005" F
-sso
-too
-5o
o
too
5o
c
ISo
2o0 r, oc
b
2.2
~ 3
tan~
0.005 -
- 15o
w~
i
+.+.~v~. -ZoO
2 34 x'-'×'x
-t-
-50
0
50
lO0
150
200 "IT,°C
FzG. 1. Temperature dependence of tan 6 and 8 of isotactic (a) and atactio (5) PVOH. Frequencies: 1--10=; 2--108; 3--5 × 104; 4--1.5 × 10e c/s. F o r high t e m p e r a t u r e processes W values are 43 a n d 60 kcal/mole, for isot a c t i c a n d a t a e t i c polymers, respectively, a n d 9 a n d 10.5 kcal/mole for low t e m p e r a t u r e processes (Table 1). The order o f W values is t y p i c a l of a c t i v a t i o n
V.A. TALYKOVe$ al.
486
energies of dielectric relaxation of dipole-segmental motion (or a-process) in the first case and dipole-group motion (or fl-process), in the second [8]. The T~ value for isotactic P V C H determined b y a dielectric method from the temperature-frequency dependence of tan 5 was 113 ° which is close to the Tg values determined dilatometrically and thermomechanically [2]. For atactic PVCH dipole-segmental motion begins considerably earlier (T~-97°). This process is characterized b y higher absolute values of tan ~ in the maxima w i t h a less regular shape of curves (Fig. 1). Obviously, the lower parameter of distribution of relaxation times a in atactic PVCH reflects an increased freedom of movement of macromolecules taking part in the a-process of large sections, as compared with isotactic PVCH. TABLE 1. PROPERTIES OF POLYMERS
Polymer
PVCH PVCH PS PS
Chain structure
Isotactic Atactic Isotactic Atactic
~ o
113 97 91 88
60-0 62.0 66.0 - - ~ 0
9-0 10.5 10.0
108 90 105 87
31.0 34.0 39-0 48-0
160 168 162 146
46.0 27.0 34.0 37.0
18o 170 170 140
Dipole-group motion in isotactic PVCH also encounters more considerable difficulties than in atactic PVCH. This is confirmed b y the higher T~ value of this motion (Table 1) and lower Wp value. The fl-process in isotactic PVCH, like the a-process, is characterized b y a very regular shape of curves tan 5 : f (T) in the maxima; the temperature range of dipole-group motion, compared with atactic motion, considerably widens (Fig. 1). This process of low temperature molecular motion, apparently, also accounts for the mechanical loss maxima observed dynamically in atactic PVCH in the low temperature range [9]. As with PVCH, in iso and atactic PS at temperatures higher than 90 ° dipolesegmental relaxation takes place which reflects the segmental mobility of macromolecules (Fig. 2, Table 1). This process, however, in PS is characterized b y lower T~ values and higher absolute values of tan 5ma~ than in PVCH. The increased ordering of isotactic PS structure, as with PVCH, is accompanied b y a reduction of dipole-segmental dielectric losses (Fig. 2), which enables us to relate the a-process to amorphous polymer regions. The increased hindrance to segmental mobility in PVCIt is evidently due to the larger dimensions of the cyclohexane ring, compared with the benzene ring (the natural volume of the monomer unit in P V C H is 74.05, compared with 66.4 cm3/mole in PS [10]). The higher macromolecular rigidity of PVCH is determined b y the type of interaction of
Relaxation transitior~_ in polyvinylcyclohexane
487
eyclohexane rings, which are distinguished from benzene rings by shape and flexibility. A comparison of temperature-frequency dependences of tan J and 8 of PVCH and PS of different structures and chemical composition (Figs. 1 and 2) suggests
_- x
_. - x~ ~ x : ~~ xx
-~--x- -- -~---.-~~ x
~--x-×~ -
-
.
-
-
-
04! rand
--x^
2
/
/\ ./;
0-005
x~-w.
-/50
-~0~
-50
0
5g
1oo
15o
T,°g
Fie. 2. Temperature dependence of tan J and e of isotactic (1) and atactie (2) PS; frequency i0 ~ e/s.
that the higher ordering in polymer structure due to stereo-specific properties considerably reduces macromolecular mobility, but cannot fully eliminat~ dipole-segmental heat motion; the replacement of the benzene ring by a cyclohexane ring has a marked effect on macromolecular rigidity. In contrast to PVCH an increase in stereospecific properties of PS causes a degeneration in dipole-group motion, although this process takes place in atactic PS (Figs. 1, 2 and Table 1). However, the kinetic units, which are responsible for low temperature relaxation in PVCH and atactic PS are different. Thus, for atactie PS this process is apparently due to vibratory motions o f - - C H 2 - - C H ~ groups of the main chain contained in the macromolecules by the anomalous addition of " h e a d - h e a d " type monomer units [11] and having a dipole moment as a consequence of structural asymmetry of adjacent macromoleeular sections. Stereo-specific PS does not contain such groups, which results in the disappearance of the low temperature range of dielectric relaxation. For PVCH the --CH2--CH 2groups are present both in the atactic (anomalous addition, cyclohexane ring) and isotactie (cyclohexane ring) polymers therefore, a dipole-group process occurs in both types of PVCH. I n contrast to benzene rings, the cyclohexane rings are not only capable of complete participation in complex motion, but also undergo internal conformational transitions [12]; the stereo-specific properties of PVCH do not, of course, result in the degeneration of these transitions. An increase in molecular mobility on replacing the benzene ring by a eyelohexane ring is also confirmed by the example of vinylcyclohexane and styrene. I t follows from Fig. 3 (curves 1 and 1') and Table 2 that in the temperature
V. A. T~Yxov et aL
488
range of --180-160 ° ~he motion of vinylcyclohexane molecules undergoes relaxation, which is maintained at temperatures considerably lower than the melting point of the monomer (--126.7 ° [I3]). On transition to the solid state no sudden 2-8
2
\z.e
%
2.0 ~ao~
i ~i'
-150
0.005
-100
-50
O
T,°C
50
I~'~G. 3. Tempe~sture dependence of tan ~ and a: 1, / ' - - v i n y l c y c l o h e x a n e ; 2, 2'---et,yrene~
3, 3'---ethyleycIohexane. Frequencies: I-3--10 =, 1"3'--10 =e]s. change in dielectric constant is observed as only the centres of molecules are fixed while the mobility of individual groups of atoms in vinyleyclohexane is mainatained up to T~=176 ° (Table 2). TABLE ~. PROPERTIES OF LOW MOLECULAR WEIGHT COMPOUNDS
Compound Vinylcyclohoxane St,yrene Ethylcyclohexano
~melt
-- 126.7 --30.6 --111.3
T~, °C
WB
kcal/mole
--176
II'0
/--178
11.0
I n contrast to vinyloyelohexane molecules, for styrene near the melting point (--30.6 °) [13] the displacement of both molecular centres and individual atom groups is no longer possible. The change-over of styrene molecules in the region of --30 ° and the variation of mobility is not of relaxation type (Fig. 3, curves 2 and 2'). Absolute maxima tan ~ decrease in proportion to the increase in the frequency of the field applied without a temperature displacement in the positions of these maxima.
Relaxation transitions in poly~nylcyclohexane
489
Over the temperature range examined below melting point relaxation processes in styrene were not detected dielectrically, due to an increase in packing density of styrene molecules in the crystalline sta~e. Crystallization evidently prevents rotation of benzene rings around single C-C bonds and as noted previously, conformation transitions in these rings are impossible.
-11
-/3
\ \
-/7
2.0
2.5
iO~T,°K-1
3JO
I~G. 4. Temperature dependence of the electrical conductivity of polymers. ]--isotactio and 2--atactic PVCH, 3--iso~actic and 4--atactic PS, ~--isotactic PVCH irradiated by X-rays for 30 rain. Dipole relaxation losses are also observed in ethylcyclohexane, from which unlike vinylcyclohexane, double bonds capable of being readily polarized during conformation transitions of the cyclohexane ring (Fig. 3, curves 3 and 3'), are absent. For ethylcyclohexane there are dielectric loss maxima, although these
490
V.A.
TALYKOV et al.
maxima and v a l u e s of dielectric c o n s t a n t s are much lower at all frequencies than for vinylcyclohexane (Fig. 3 curves 1 and 1'). The proximity of T~ and W~ values for vinylcyclohexane and ethylcyclohexane (Table 2) suggests that the s a m e kinetic units are responsible for the appearance of molecular mobility. I t can therefore be concluded that the molecular heat motion in iso- and atactic PVCH is closely related to the specific mobility of lateral cyclohexyl substituents. I t is known [5, 14] that the electrical conductivity of polymers is also closely related to chemical structure and the type of molecular heat motion. It was therefore interesting to study the actual type of this relation and examine the temperature dependence of electrical conductivity for iso- and atactic PVCH in comparison with iso- and atactic PS (Fig. 4). Figure 4 shows that in the temperature range of 20-220 ° all the polymers studied have two inflexions on the curves of the dependence of log y ~ f (I/T). The average temperature of low temperature inflexion is denoted by T 1 and is close to T~ values derived from the temperature-frequency dependences of tan g (Table 1). The average temperature of high temperature inflexions is 50-70 ° higher than T1, is denoted by T~ and corresponds to Tmin (Table 1), at which tan g at a frequency of 100 c/s passes through a minimum and then begins to increase as a result of through conductance (Figs. 1, 2). The linear parts of dependences log ~ = f (l/T) enabled us to calculate the effective activation energies of electrical conductivity W1 in the temperature range of T1-T2 and W~ in the ranges from T 2 to temperatures several degrees higher than T~ (Table 1, Fig. 4). In the temperature range below T~ activation energies of electrical conductivity were 10-12 kcal/mole. A variation in electrical conductivity near the glass transition temperature, which is accompanied by a sudden increase in the activation energy of this process, is observed in m a n y polymers and is due to a considerable increase in molecular mobility on transition from the glassy to the high elastic state [14]. Temperature T~ should also be regarded as one of the characteristics reflecting processes of internal change-over in the polymer. It is known that at temperatures which are 50-70 ° higher than T~ a variation is observed in the activation energies of m a n y physical processes--viscous flow [15], dielectric relaxation [16] and electrical conductance [6]. These change-over processes are, apparently, based on the disruption of the co-operative motion of segments due to the varying initial structural ordering of the polymer and the subsequent reduction or increase of this ordering under the effect of high temperatures. It is assumed by some authors [15] that at temperatures 50-70 ° higher than Tg liquid-liquid type transitions occur in atactic polymers, while others [8] assume a variation in supermolecular structural formations. I t follows from Fig. 4 and Table 1 that stereo-specific properties of PVCH and PS result in a considerable reduction in absolute values of 7, a displacement ot curves log ? = f (I/T) in the direction of higher temperature and a certain reduc-
Relaxation transitions in polyvinylcyclohexane
491
tion in W1 values. Lowest W1 values, as also W~ values, are observed for PVCH which has the highest chain rigidity and retarded segmental motion of macromolecules. At temperatures higher than T~ the variation in electrical conductivity for isotactie PVCH differs from t h a t observed for other polymers. While with atactie PVCH and PS W2 noticeably decreases compared with W1, with isotactic PVCH W2 increases. A reduction in the activation energy of electrical conductivity at temperatures higher t h a n T~, compared with W1 values, is typical of m a n y polymers [5]. The causes of anomalous variation of Ws in isotactic PVCH near T~ require detailed study as t h e y are determined by complex structural changes in the polymer. I t is possible that this effect, like the abnormally high melting point of crystalline isotactic PVCH, is due to a transition of the polymer from one crystalline modification to another. This transition is confirmed b y X-ray data of PVCH [17], according to which the initial rule of variation of the interplanar distance according to temperature does not hold good in the temperature range of 170-200 ° and a marked change is observed in the coefficient of thermal expansion, which corresponds to a change in the "order-disorder" type structure [4]. In approximately the same region (180-200 °) an inflexion is observed in the dependence on temperature [18] of optical density of a band at 830 cm -1, which characterizes the helical conformation of PVCH macromolecules. In this temperature range transition is observed in the tetragonal structure of PVCH 247 in a polymer oriented at temperatures lower than 200 ° to structure 41, which is characterized by higher stability at high temperatures [19]. W, kcal/rno/e
50
1,o/~14, 5g /
3g 20 - fO
~ . . . . . 0
# I,j lg
IogA
FIG. 5. Dependence o£ activation energy of electrical conductivity W on the logarithm of pro-exponential factor A for polymers (see explanations of curves in Fig. 4); 1 - 5 correspond to the dependence of Wl-log A t , l ' - 5 " - - t o W,-log A,. It is well known [5, 14] that for m a n y polymers a so-called "compensation effect" is observed, whereby the dependence of W-log A, where A - - t h e pre-exponential factor in the expression 7 = A e -~vmT , is linear. For polymers with Tg>60 ° previously studied [5] and differing in structure, chain rigidity and degree macromolecular crosslinking, the dependence W1,2----27-2+1-78 log A1, 2 is valid. A similar relation appeared to hold good for atactic PVCH and iso- and atactic PS (Fig. 5).
~g2
V. A. TALXXOVet a/.
For initial isotactic PVCH and PVCH irradiated by y-rays points 1' and 5' of the dependence Wz-log A n deviate from the general relation (Fig. 5), which also proves that this polymer behaves abnormally. The polymers were irradiated by soft X-rays with a beam diameter of 60 mm (tube voltage 20 kV, current 200 mA), the specimens being retained for 15-180 rain. Irradiation for 30 rain did not alter the general dependence of log r=f (l/T) (Fig. 4, curve 5), reduced the absolute values of 7 with a slight change in IV1 and IVy. However, a further increase in the irradiation dose increased ttrl from 30 to 40 kcal/mole with a slight reduction in 1¥~ and an increase in 7. The data confirm that two different processes take place during irradiation-crossllnklng and simultaneous degradation of polymer macromolecules. A small dose of irradiation promotes crossllnking, structure formation in the linear polymer, reduction in macromolecular mobility and consequently reduces the mobility of ionic charge carriers and the value of 7. Further increase in the irradiation dose with an exposure of over 30 min results in degradation, higher concentration of ionic charge carriers as a result of products of decomposition during oxidation and degradation. As a result absolute values of electrical conductivity increase. Exposure of PVCH specimens for a period of more than 180 min produced increased brittleness and decomposition of films. It should be noted that an increase in the crystallinity of isotactic PVCH during annealing causes a reduction of 7, similar to the effect caused by irradiation for a brief period (curve 5, Fig. 4). CONCLUSIONS
(1) Relaxation processes in iso- and atactie polyvinylcyclohexane (PVCH) and polystyrene (PS), styrene, ethyl- and vinylcyclohexane were examined by a dielectric method. Temperature dependences of electrical conductivity of PVCH and PS were investigated. (2) It was shown that in isotactic PVCH, as in atactic PVCH and PS, there are two types of molecular heat motion: dipole-group and dipole-segmental motion. The stereospecific structure of PVCH macromolecules reduces molecular mobility without causing degeneration of the dipole-group motion, as observed in isotactic PS. (3) Dipole-group relaxation in atactic PVCH is due to the motion o f - - C H ~ - --CHt-groups of the main chain (irregular addition of monomer units) or the cyclohexane ring, while in isotactic PVCH it is caused by a change in the position of these groups in the cyclohexane rings alone as a result of the rotary, vibratory or conformation motion of the latter. (4) It was shown that for isotactic PVCH the activation energy of electrical conductivity increased abnormally in the temperature range T 2 which is 50-70 ° above the glass transition temperature T~ and a deviation was observed from the
Relaxation transitions in polyvinylcyclohexane
493
linear d e p e n d e n c e W - l o g A, which is t y p i c a l o f t h e o t h e r p o l y m e r s studied. I t is a s s u m e d t h a t t h e a b n o r m a l b e h a v i o u r o f isotactic P V C H is due to t h e presence of t h e " c r y s t a l - c r y s t a l " transition. Translated by E. S~MEm~ REFERENCES
1. V. I. KLEINER, B. A. KRENTSEL' and L. L. STOTSKAYA, Plast. massy, No. 4, 3, 1967 2. V. L KLEINER, S. G. BARSAMYAN, A. S. APRESYAN and L. L. STOTSKAYA, Plast. massy, ~o. 3, 13, 1968 3. Ye. F. ZININ, M. S. AKUTIN et aL, Plast. massy, No. 1, 32, 1970 4. V. I. KT.EINER, Dissertation, 1969 5. V. A. TALYKOV, Dissertation, 1969 6. V. A. TALYKOV, G. E. GOLUBKOV, A. I. IKONNIKOVA and A. P. BEI~YAYEVA, Vysokomol. soyed. A l l : 1303, 1969 (Translated in Polymer Sci. U.S.S.R. l l : 6, 1480, 1969) 7. G. Ye. GOLUBKOV, Dissertation, 1959 8. G. P. MIKHAILOV, ZhVKhO im. D. I. Mendeleyeva 6: 404, 1961 9. K. A. WOLF, Z. Elektrochem. 65: 604, 1961 10. G. L. SLONIMSKII, A. A. ASKADSKH and A. I. KITAIGORODSKII, Vysokomol. soyed. A12: 494, 1970 11. M. BACCAREDA, E. BUTTA, V. FROSINI and P. MAGAGNINI, J. Polymer Sci. 4, A-2: 789, 1966 12. E. ILYEL, N. ~tLLINZHER, S. ENIZHIAL and G. MORRISON, Konformatsionnyi ~naliz (Conformational Analysis). Izd. "Mir", 1969 13. V. M. TATEVSKII, Fiziko-khlmicheskie svoistva individual'nykh uglevodorodov (Physical and Chemical Properties of Individual Hydrocarbons). Gostoptetrhi~.dat, 1960 14. B. I. SAZHIN, Elektroprovodnost' polimerov (Electrical Conductivity of Polymers). Izd. "Khlmiya", 1965 15. R. BOIER (Ed.), Perekhody i relaksatsionnye yavleniya v polimerakh (Transitions and Relaxation Effects in Polymers). Izd. "Mir", 1961 16. A. M. LOBANOV, G. P. MEgHAILOV and V. A. SHEVELEV, Vys0komol. soyed. 8: 794, 1961 (l~ot translated in Polymer Sci. U.S.S.R.) 17. R. A. RAFF and K. V. DOK (Ed.), Kristallieheskie poliolefiny (Crystalline Polyolefms). Izd. "Khimiya", 2, 1970 18. O. A. N ~ I T I N A , Dissertation, 1969 19. N. D. NOETHER, J. Polymer Sci. CI6: 725, 1967