Effect of strain on dielectric relaxation in filled rubber

Effect of strain on dielectric relaxation in filled rubber

2655

REFERENCES 1. N. GRAZZIE, K h i m i y a protsessov destruktsii (The Chemistry of Processes of Destruction), p. 59, Inost. lit., Moscow, 1959 2. S. MADORSKII, Termicheskoye razlozheniye organicheskikh polimerov (Thermal Degradation of Organic Polymers). p. 315, "Mir", Moscow, 1967 3. V. V. KORSHAK, Termostoikiye polimery (Heat Stable Polymers). p. 8, "l~aul~a", Moscow, 1969 4. Khr. ALAMINOV a n d M. MIKHAILOV, Khim. i industriya 44: 102, 1972 5. N. G R A Z Z l E and J. GILKS, J. Polymer Sci., Polymer Chem. Ed. 11: 1985, 1973 6. E. G. POMERANTSEVA, Ye. M. PEREPLETCHIKOVA and E. O. KRATS, Plast. massy, No. 4, 39, 1979 7. I. KESSLER, Metody infrakrasnoi spektroskopii v khimicheskom analize (Methods of Infrared Spectroscopy in Chemical Analysis). p. 136, "MAr", Moscow, 1964 8. R. Ya. TE1TEL'BAUM Termomekhanichoskii analiz polimerov (Thermomechanical Analysis of Polymers). p. 234, "Nauk~t", Moscow, 1979 9. B. V. OZERKOVSKII and V. P. ROSHCHUPK1N, Dokl. Akad. Nauk SSSR 248: 657, 1979 10. V. P. ROSHCHUPKIN a n d B. V. OZERKOVSKII, Karbotsepnye polimery (Carbochain Polymers). p. 132, " N a u k a " , Moscow, 1977 11. S. V. PIIS~IUGINA, I. N. RAZINSKAYA and A. I. SLUTSKER, Vysokomol. soyod. B20: 171, 1978 (Not translated in P o l y m e r Sei. U.S.S.R.) 12. T. V. BELOPOL'SKAYA, Vysokomol. soyed. A14: 640, 1972 (Translated in P o l y m e r Sci. U.S.S.R. 14: 3, 715, 1972) 13. V. P. ROSHCHUPKIN, B. V. OZERKOVSKII, Yu. B. KALMYKOV and G. V. KOROLEV, Vysokomol. soyed. A19: 699, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 4, 809, 1977)

PolymerScienceU.S.S.R. Vol. 23, No. 11, pp. 2655-2660, 1981 Printed in Poland

0032-3950/811112655--00507.50]0 1982 Pergamon Prc~ Ltd.

EFFECT OF STRAIN ON DIELECTRIC RELAXATION IN FILLED RUBBER* F. G. F~BVLYAK, Y r . S. I~PATOV and D. S. VOVCHUK I n s t i t u t e of C h e m i s t r y of High Molecular W e i g h t Compomlds, Ukr.S.S.R. Academy of Sciences

(Received 9 Ap~'il 1980) The effect of tensile strain on molecular m o b i l i t y in the surface layers of rubber filled wittl Aerosil has been studied. The results of the investigation show t h a t tensile strain significantly influences molecular m o b i l i t y both in volume and in the surface layers. This change in molecular m o b i l i t y results from increase in the r i g i d i t y of t h e maeroehains and change in the eonformational set of t h e macromolecules. * Vysokomol. soyed. A23: No. 11, 2449-2453, 1981.

2~56

F. G. FABULY~.X et ~ .

MUCH experimental material on dielectric relaxation in filled polymers has now been gathered [1]. I t is known that adsorption interaction with the surface of a solid (filler) and conformational restrictions close to the surface result in a considerable fall in segmental mobility manifest in rise in the temperature maximum tan J. The same factors helping to worsen the packing in the surface layer m a y lead, as shown in references [2-4, 5], to falls in the temperature of the maxima of losses corresponding to dipole-group processes. However, the effect of deformation of the filled polymer on dielectric relaxation in the surface layers has still not been studied. Therefore in the present paper we report some results of study of the effect of tensile strain on molecular mobility in the surface layers of Aerosil-filled rubber. The results obtained are compared with the data for unfilled specimens. Molecular mobility was studied b y the method of dielectric relaxation. The measurements were made on an RT-9701 s.c. bridge over the frequency range 0.100-200 k H z at a temperature from 20 to --150°C. The vulcanizate was produced from methylstyrene rubber with a standard formulation which as well as stabilizers and accelerators contained two parts b y weight of sulphur. Vulcanization was carried out at 155°C for 50 min. The samples were stretched to the corresponding tensions with weights and kept under load for 24 hr, then the load was removed and the relaxation processes in such systems studied. The filled specimens contained 10 w~. ~o of unmodified Aerosil. Before measurement the specimens were vacuum-treated in a measuring flask for 3 hr. The vaccum was 1.33 N/m s. The error of measuring the dielectric losses over the temperature scale was 0-5-1°C. Figure 1 presents the temperature dependence of tan ei for unfilled (curve 1) and filled (curve 2) rubber. As the Figure shows, in the region --45 ° the unfilled specimen shows relaxation due to the mobility of the segments of the chains between the nodes of the three-dimensional mesh of the polymer [5] and also weakly marked dipole-group relaxation of the groups of macromolecules or side branchings in the region -104°C. The same processes of relaxation for the filled specimen are observed respectively at --39 and --93°C. For filled rubber the process of relaxation of the segments of the three-dimensional mesh shifts to high temperatures (the shift amounts to 5-6 °) since the conformational set of the macromolecules diminishes and hence the ratio of the trans and gauche conformations of the chain [3, 6] close to the separation boundary changes. This results from the adsorption interaction of the polymer molecules with the surface and restriction on molecular mobility. Of special interest are the results of study of the effect of tensile strain on the changes in molecular mobility both in the volume of the polymer and in the surface layers. Figure 2 shows that with increase in the deformation stress to 2-47 × 107 N/m 2 for the unfilled rubber specimen the maximum ~" of the dipole-segmental relaxation of the mesh shifts to high temperatures. This indicates that after removal of the deformation stress relaxation does not occur

Effect of strain on dielectric relaxation in filled rubber

2657

a n d the maeromolecules deplete oonformational set with increase in the rigidity of the portions of the chains between the nodes of the mesh. The m a x i m u m g ' o f dipole-group relaxation which is manifest in the region --IO0°C also shifts to high temperature with increased stress ON stretching. The shift amounts to 5-8°C (Fig. 2). %

fO2

o! .2 ×3 aq

fan d~lO2

~

1.0 0"6 O.2

-120 -80

"100

t

-qO

]FIG. 1

0

-60

-20

7-°

T°

Fro. 2

FIO. 1. Temperature dependence of tan J for unfilled rubber specimen (1) and rubber specimen with 10% Aerosil (2). FIG. 2. Temperature dependence of dielectric losses for unfilled specimen without load (1), after stretching with a tension of 0. 63 × l07 (2), 1.29 × 107 (3) and 4.47 × 107 (4) N]m 2. From the results it follows t h a t the laxity of packing of the polymer molecules in the specimens exposed to strain decreases. This is obvious since the shift of the m a x i m u m of the dipole-group process to high temperature (Fig. 2) indicates reduction (restriction) of the mobility of the groups at the expense of the high density of packing of the polymer macromolecules, i.e. the density of packing makes its own contribution to the intramolecular interactions. The results of study of the effect of tensile strain on the rubber specimen containing Aerosil show t h a t in filled rubber external loads lead to further reduction of molecular mobility together with increase in the rigidity of the macrochains at the expense of the surface of the filler. This is a consequence of the fact t h a t the m a x i m u m t a n J of the process of relaxation of the segments of the three-dimensional mesh shifts to high temperatures (amounts to 6-9 ° )

2658

F.O.

FAB~YAX

et al.

and the equilibrium value of tensile stresses in the filled specimen appears at, 1-49 × 107 N / m S while in the unfilled one it appears at 1.29× l 0 T N/m ~. As for the dipole-group relaxation, the m a xi m um ~" of this process shifts to high temperatures. The shift of the m a x i m u m points to increase in the density of packing in the surface layer in presence of a hard surface in the deformed systems.

~an,lO3 20

15

-qo~__~

I0

T°

I

k

0.5

1"5

2.5 O,,N/m~=lO 7

0.I

Fro. 3

~ J I : 1.0 10 fO020~? ~, k H z

Fro. 4

FIc. 3. ]~ffcct of tensile stress on shift of maximum tan J over temperature scale for uzlfilled (1) and filled (2) rubber. Fro. 4. Frequency dependence of dielectric losses at --35°C; /--without load; 2--for =4.47)< 107 N/m% The above-outlined results show t h a t tensile strain increases the rigidity of the portions of the chains between the nodes of the mesh, reduces the looseness of packing of the macromolecules both in volume and the surface layers of the filled polymers. A vivid demonstration of change in the character of molecular mobility as a function of stress is provided by the curves in Fig. 3 showing t h a t the mobility of the portions of the chains between the nodes if the mesh decreases, but after the corresponding values of tensile stresses (1.29)< 107 l~/m z for the unfilled and 1.49× 10' lq/m 2 for the filled specimen) the change in metan~=lO 4

I -120

I -80

I -qO

I 0 T°

Fro. 5. Temperature dependence of tan J for low density PE; /--unmodified specimen; 2--deformed by 400~o 3--by 800~.

Effect of strain on dielectric relaxm*i,m in filled rubber

2659,

lecular mobility assumes a constant value--the processes of relaxation of the segments of the mesh do not shift over the temperature scale and are manifest at one temperature. Figure 3 also shows t h a t the constant value of tensile stress for the surface layer of the polymers appears at higher stress values (AJ=0.24 y. × 107 N/m 2) and at higher temperatures t h a n in the volume. This is evidently due to the fact t h a t in the filled mlbber specimen the rigidity of the chains is increased in presence of a hard surface. With increase in the loads on the specimens of the rubbers studied the mobility of the segments of the macromolecules falls. Above a certain critical stress value the polymer macromolecules are not. able to change their conformational set, which leads to partial destruction of the chemical bonds tbr low loads but with increase in them to total destruction of the polymer. We also determined the activation energies of the process of relaxation of the segme~ts of the three-dimensional mesh ¢,aking into account the relation log f , , = ~

f)

~ :. eA U/RT, for j,,~:=jo

where f~ is the frequency of the dielectric measurements at. different temperatures; fo is a pre-exponential coefficient; A U is the activation energy; /~ is the gas constant.. As the Table shows, the activation energy rises on introducing a filler into the polymer, which is explained by increase in the rigidity of the macromolecules. Therefore the activation energy in the stretched specimens rises. VAL~-ES

OF

ACTIVATI01~ ENERGY S E G M E N T S OF T H E

Tensile stress a × 107, N/m z 0 0.63 1.29 1.49 2.28 2.47 2.63

OF

THE

PROCESS

OF

I~ELA~TIOI~

OF

'rl~t~

MESH OF T H E I { U B B E R S STLrDIED

Activation ,~nergyEa × 10-7, J/kmole rubber with 10% ! pure rnbbt,r Aerosil 9.7 10.4 11.1 11-2

11.3 12.1 12.7 13-4

11.6 14-1

The frequency dependence of the unfilled specimen on maximum stretching and without stretching is indicated in Fig. 4. To confirm t h a t the tensile action of the load reduces the looseness of packing we studied a low density P E specimen with 400 and 800% deformation. Naturally, the mechanisms of relaxation in the crystalline polymer (PE) differ from those in mlbber. However, for mobile kinetic uvits of the " c r a ~ k -

2660

F. O. FABULYAK~ ~ .

:shaft" type [7] relaxation in the polymer studied is observed in the region--140 to --80°C. Figure 5 presents the temperature dependence t a n 8 of the P E specimens at low temperatures. As the Figure shows, the process of relaxation is very sensitive to the value of strain under load. With increase in the tension the maximum tan J shifts to high temperatures and becomes wider. This indicates t h a t with increase in the strain the looseness of packing of the macromolecules decreases and the mobility of the methylene sequence of the "cranks h a f t " t)Te is hindered (falls). From the results of the investigations it follows t h a t the value of the tensile strain significantly influences molecular mobility both in volume and the surface layers. This change in molecular mobility results from increase in the rigidity ,of the macrochains and ctlange in the conformational state of the system and ,also decreases in tt~e looseness of packing of the macromolecules. The presence o f a separation boundary makes an appreciable contribution. REFERENCES

1. Yu. S. LIPATOV, Fizicheskay~ khimiya napolnennykh polimorov (Physical Chemistry of Filled Polymers). p. 303, "Khimiya", Moscow, 1977 2. Yu. S. LIPATOV and F. G. FABULYAK, Vysokomol. soyed. A10: 1605, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 7, 1858, 1968) 3. Yu. S. LIPATOV and F. G. FABULYAK, In: Poverkhnostnye yavleniya v polimerakh (Surface Phenomena in Polymers). p. 7, "l~aukova dumka", Kiov, 1970 4. Yu. S. LIPATOV and F. G.FABULYAK,Vysokomol. soyed. All: 708, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 4, 800, 1969) 5. F. G. FABULYAK, Yu. S. LIPATOV, V. M. KUZNETSOVA and Z. N. DIMEDENKO, In: Khimiya i khimich, tekhnol. (Chemistry and Chemical Technology). No. 32, p. 3, "Vsysh. shkola", Dnepropetrovsk, 1973 6. Yu. 8. LIPATOV, F. G. FABULYAK,8. A. SUSLO and G. M. SEMENOVICH, Dokl. Akad. Nauk Ukr.SSR BI: 39, 1979 7. R. BOYER, Perekhody i relaktsatsionnye yavleniya v polhnerakh (Transitions and Relaxation Phenomena in Polymers). p. 11, "Mir", Moscow, 1968