Relaxational transitions in cis-polyisoprene from structural relaxation data

PolymerScienceU.S.S.R. Vol. 29, No. 3, pp. 556-561, 1987 Printed in Poland

RELAXATIONAL FROM

0032-3950/87 Sl0.00+.00 O 1988PergamonPress plc

TRANSITIONS

STRUCTURAL

IN cis-POLYISOPRENE

RELAXATION

DATA*

G. M. BARTENEV and M. V. KARASEV

Institute of Physical Chemistry, U.S.S.R. Academyof Sciences

(Received 23 July 1985) The temperatures of the relaxational transitions in cis-l,4-polyisoprene measured by the methods of structural (heat capacity)and mechanicalrelaxation clearlymatch and may be calculated from the formulae of relaxational spectrometry. ONE way to study the structural relaxation in polymers is to investigate the temperature dependence of the heat capacity at a set rate of heating. In [1] in the curve of heat capacity of polybutadiene methylstyrene above Tg a series of weak relaxational 2transitions was observed also characteristic of the spectrum of internal friction and the spectrum of the relaxation times obtained by the method of mechanical relaxation. With the methods of relaxational spectrometry it is, in principle, possible to calculate using data of mechanical relaxation the temperatures of the relaxational transitions observed during structural relaxation. Recently the method of stress relaxation [2, 3] (Table) for SKI above the glass transition point (~-transition) yielded a discrete spectrum r~ of the relaxation times (i= 1, 2,..., n) characterizing the various physical and chemical processes of relaxation and the corresponding relaxation constants Us and B~ in the Boltzmarm-Arrhenius equation. We investigated weakly crosslinked SKI (natural rubber) with 3 ~, sulphur [2, 3] in which (Table) a number of relaxational transitions above the glass transition point ( - 72°C) was observed: the It relaxational transition associated with the breakdown of the local physical nodes formed by the interaction of the methyl side groups, the ns-transition associated with the local intermolecular dipole-dipole interactions of intramolecularly attached sulphur, the group of ,~-transitions asSociated with the breakdown of the microvolumetric physical nodes of the molecular network and the t~-group of the chemical processes of relaxation. The transitions It and ;is were first detected in [4] and a classification of the other relaxational transitions is presented in line with [5].

The relaxational transitions are clearly visible in the internal friction spectrum (Fig. 1). Below the ~ maximum (vitrification) two small-scale p-transitions are observed and above it at -25°C the 2, maximum associated with the process of crystallization at the temperature of the maximum rate of crystallization of SKI [2]. This maximum does not depend on the frequency. Above the temperature of the 2c maximum a group of relaxational transitions is observed indicated in the Table. They were identified from the temperatures of transitions calculated from the formula * Vysokomol. soyed. A29; NO. 3, 498-502, 1987.

Relaxational transitions in

cis-polyisoprene

557

V¢¢t AND THE TEMPERATURESOF THE RELAXATIONALTRANSITIONS T= OF S K I CALCULATED FROM THE RELAXATIONALCONSTANTS AT W--~3"3 X 10-2 deg/sec

FREQUENCIES

Relaxational transition

U, k J / m o l e 49 60 29 29 34 34 34 34 23 119 126 153

P 7~S

2' 21 22 22

24 61 ~sx Os2 Oc

Bt, see

p

1-7x 1"15 x 4"4 x 2"5 x 2"1 x 1"1 x 5x 2"3 x 2"5 6.7x 6"7 x 3"2 x

{ T~t, K [ (ri = 10 ~ see)

10 - x ° 10 - t l 10 - s 10-* 10 - 4 10 - a 10 - a 10 - 2 10-14 10- ~4 10- t4

221 245 246 277 307 345 400 476 746 415 435 515

Co, K

v=, x 104, H z

8.3 8"3 17 22 23 29 39 55 201 12"0 12-5 14"4

6"3 6.3 30 24 23 18 13 9"5 2"6 4"4 4"2 3"6

Ti, K:

I

214/-238/24

i !

219/-241/24 266/26 291/29 318/32 355/36 357/36 400/39 417/41 493/--

! i

~t

* N u m e r a t o r - calculated values; d e n o m i n a t o r - experimental data.

£i

(J)

T~= ( U J2. 3k ) /log2~vB i

where Ut is the activation energy of the given relaxational process; Bt is a coefficient (pre-exponent in the Boltzman-Arrhenius equation) obtained from analysis of the continuous and discrete relaxation time spectra; k is the Boltzmann constant; v is frequency; q is a dimensionless constant. For small-scale processes (/1, ns, ~sl, 5s2 and 5c)c~ ~- 1.

[o9 "a,lO) z 2 -

o(.

V=L/N

V=2Hz

o 2

A

7-

-200

I

1

I

0

200

qO0

T°

FIG. 1. Total spectrum of internal friction of crosslinked NR-tempcratur¢ dependence of logarithmic decay decrement at the frequency 4 (below) and 2 (above the glass transition point) Hz of samples 1 (l) and 2 (2) taken from same plate. Measurements on passing from low ( - 150°) to high (460 °) temperatures.

G. M. BARTEIqEVand M. V.

558

For the A-transitions and the 51-transition c~"-~10. As may be seen (Fig. 1) in it are realized all the transitions indicated in the Table apart from Aa, A4 and 81 since their temperatures T~ at the frequency v=2 Hz are above 460°C. Thus, the data obtained from the relaxation time and internal friction spectra well agree. Schmieder and Wolf [6] for NR. with 1"5 Yo sulphur observed in the internal friction spectrum (v= 1-2 Hz) above the glass transition point four maxima at 25, 65, 175 and 220°C. Although these measurements date from 1953 until recently they remained enigmatic and in [6] their origin was not explained at all. Now, analysis of the data of the Table and calculation of the temperatures of the transitions from formula (1) show that the first two maxima at 25 and 65°C (at v= 1-2 Hz) exactly correspond to the/z and rCs transitions (Table).

-750

-50

50

tSO

7"*

Fio. 2. Temperature dependence of heat capacity C~ of crosslinked NR at the heating rate w=2 deg/min.

The temperature dependence of the heat capacity C,, was measured by the method of Godovskii-Barskii [7] with continuous rise in temperature at the rate 3"3 × 10 -2 deg/sec. To identitify the fine structure in the curve C~-Tthe heat capacity was measured through 2-3 ° with an accuracy +0.01 kJ/kg.deg and over a wide temperature interval ( - 175- + 175°C) (Fig. 2). In the view of Wunderlich and Bauer [8] one may consider the best data on the heat capacity of NR the results of Bekkedahl and Matheson [9] and Wood and Bzkkedalai [10]. In these studies attention was focussed on the jump in heat capacity at the glass transition point and obtaining the mathematical dependences of the smoothed temperature course of Ca below and above Ts. These findings (Fig. 3) were obtained in a wide interval - 2 5 8 - + 147° but their distinguishing feature is that the thermal capacity was measured through 20°C (Fig. 3). This prevents one from detecting the fine

Relaxational transitions in cis-polyisoprene

559

~tructural transitions occurring in the temperature intervals below 20 ° unlike our findings (Fig. 2) where the heat capacity was measured through 2-3°C. Now let us look at the results obtained from measuring the heat capacity (Fig. 2). The data on mechanical relaxation were obtained by us for relatively small deformations in the region of linear visco-elasticity. It is assumed that the relaxational transitions excited by thermal motion in absence and under the influence of external forces do not change the structure of the polymer. Therefore, the relaxational transitions observed by the methods of mechanical (Fig. 1, Table) and structural (Fig. 2) relaxation are determined by the same relaxational times ~ . Therefore, the temperatures of the relaxational transitions T~ during structural relaxation (Fig. 2) may be calculated from the formula (1) from the data on mechanical relaxation if one knows the equivalent frequency Ci vcq= 2rcc0 w (2) where w is the heating rate in the experiments with measurement of the temperature dependence of the heat capacity (in our case w=3.3 × 10 -2 deg/sec); c~ is a dimensionless constant figuring in the condition for observing the loss maximum (2ztvr~=c,); Co is a constant with the dimensionality of temperature determined by the known Vol'kenshtein-Ptitsyn relation

co-

kr, 2

(3)

tr,

where T~= Tr t is the temperature of the ith transition in standard experimental conditions (rl = 10 +2 sec). The Table gives the calculated values of Co for each relaxational transition and the corresponding equivalent frequencies v,q. From the formula (1) we calculated Tl using

cp, J/9"e

ao

~['

0.6/ 2OO

~K O00

Fro. 3. Temperature dependence of heat capacity Cv of NR from the data of [9, 10].

566

~. M. BARTENi~Vand M. V. ~,~ARA~EV

which one may analyse the data in Fig. 2. The Table also gives TS/t calculated from the Boltzman-Arrhenius equation for the values U~ and B~. In the heat capacity curve (Fig. 2) each relaxational transition is characterized by a certain heat capacity jump. Thus, at low temperatures fl~transitions and the ~-transitions (vitrification) are clearly observed and the fl-transition less clearly. Here T , = - 7 0 ° at w=3.3x.10 -2 deg/sec (r~=0.5× 10 2 see)while the standard temperature of the structural vitrification of SKI Ts = - 72°. The thermal capacity jump at T, (Figs. 2 and 3) has the same value of A Co = 0.47 kJ/kg" deg. Above the vitrification region a number of weaker but clearly noticeable relaxational transitions are observed (accuracy of measurement _0.01 kJ/kg.deg). To identify them let us calculate from formula (1) the temperatures of the transitions indicated in the Table at the equivalent frequencies v = v,q. In Fig. 2 the arrows indicate the positions of the calculated temperatures Ti. As may be seen t h e / t and 2'-transitions at the equivalent froquency are shifted to lower temperatures and adjoin the vitrification region. Therefore, in the heat capacity curve it is hard to detect them. The transitions ns and 2" practically match forming the region of an appreciable heat capacity jump. Then at higher temperatures the transitions 2~, 22, 2~ and 24 correspond to small heat capacity jumps. The temperature of the 24 transition almost coincides with that of the J~-transition (ultraslow process of physical relaxation of the crosslinked elastomer). As Fig. 3 shows some of these transitions were also observed by Bzkkedahl. The calculated temperatures of the #sx and Js2 transitions corresponding to the breakdown of the polysulphide and monosulphide chemical transverse bonds are in the region where the chemical structure of crosslinked SKI breaks down and the thermal effects of the chemical reactions appear. From the results it follows that study of the structural relaxation of SKI leads to the same relaxational transitions observed by the methods of mechanical relaxation. The temperatures of the structural and mechanical relaxational transitions agree with satisfactory precision and may be calulated from the formulae of relaxational spectrometry. The authors wish to thank M. V. Lazarenko for assistance in measuring the heat capacity of the SKI studied. Translated by A. CRoz't REFERENCES

1. G. M. BARTENEV,V. P. DUSHCHENKO, N. I. SHUT and M. V. LAZAZRENKO,Vysokotool. soyed. A27: 405, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 2, 453, 1985) 2. G. M. BARTENEV and M. V. KARASEV, Ibid. A27: 582, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 3,651, 1985) 3. Idem., Ibid. A27: 2217, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 10, 2492, 1985) 4. Idem., Ibid. A27: 1782, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 8, 2004, 1985) 5. BARTENEV, G. M., Struktura i relaksatsionnyesvoistva elastomerov (Structure and~Relaxational Properties of Elastomers). 288 pp., Moscow, 1979 ~' 6. K. SCHMIEDER and K. WOLF, Kolloid. Z. B. 134: 149, 1953 7. Yu. K. GODOVSKII and Yu. P. BARSKII, Plast. massy, 7, 12, 1965

Thermo-oxidative degradation of polymers

561

8. B. WUNDERLICH and G. BAUER, Teployemkost' lineinykh polimerov (Heat Capacity of Linear Polymers). 238 pp., Moscow, 1972 9. N. BEKKEDAHL and H. MATHESON, J. Res. Nat. Bur. Stand. 15: 503, 1935 10. L. A. WOOD and N. BEKKEDAHL, Polymer Lett. 5: 169, 1967

Polymer Science U.S.S.R. Vol. 29, No. 3, pp. 561-565, 1987 Printed in Poland

0032--3950/87 $10.00+.00 1988 PergamonPress ple

THERMO-OXIDATIVE DEGRADATION OF P O L Y M E R S IN PRESENCE OF O R G A N O C A D M I U M C O M P O U N D S * Y c . D. SEMCrlIKOV, N. L. KHVATOVA, V. G. EL'SON a n d R. F. GALIULLINA Chemistry Research Institute at the Lobachevskii State University, Gorkii (Received 24 July 1985)

The use of the method of "non-chain" inhibition for stabilization of PMMA, PS and butyl rubber by organocadmium compounds has been explored. It is shown that the latter are more effective for non-chain inhibition of thermo-oxidative degradation as compared with a dispersion of metallic cadmium. METHODS o f n o b - c h a i n inhibition o f d e g r a d a t i o n o f t h e l m a l l y s t a b l e p o l y m e r s devised by G l a d y s h e v et al. [1-4] a r e n o w widely k n o w n . T h e extension o f this m e t h o d o f stabiliz a t i o n t o a wide class o f p o l y m e r s f o r m e d f r o m vinyl m o n o m e r s was h e l d b a c k by t h e a b s e n c e o f c o m p o u n d s b r e a k i n g d o w n w i t h release o f t h e m e t a l at t e m p e r a t u r e s well b e l o w t h e t e m p e r a t u r e s o f the b r e a k d o w n o f the p o l y m e r s . It, t h e r e f o r e , a p p e a r e d p r o m i s i n g t o s t u d y t h e possibility o f using t h e o r g a n o m e t a l l i c c a d m i u m c o m p o u n d s ( O M C ) - b i s - ( t r i e t h y l g e m a n y l ) c a d m i u m ( B G C d ) [5] a n d d i m e t h y l - , diethyl- a n d d i i s o a m y l c a d m i u m ( D M C d , D E C d a n d D A M C d ) [6]. The synthesis of PM MA and PS in presence of BGCd and investigation of the polymers obtained were desribed by us in reference [7]. DMCd, DECd and DAMCd were introduced in vacuo into the monomer with initiator and then polymerized. Metal-filled butyl rubber (BR) of grade BR-2045 was obtained from a 10% solution in benzene containing the rubber and the OMC. The OMC in the rubber solution was broken down at 433 and 353 K for 1 hr and then the benzene removed by vacuum treatment. The size and shape of the Cd particles in the polymers were determined with the Tesla 242 E electron microscope. In determining M of polymethylmethacrylate they were freed of Cd by repeated centrifugation of the polymer solutions in acetone. The kinetics of the thermo-oxidative degradation was evaluated by the TGA method. The light transmission of the metal-filled samples was studied with the SF-26 spectrophotometer at ).= 380 nm. Hydrolysis of P M M A containing DMCd was with 0.1 N H2SO4 solution in chloroform at 293 K in vacuo. * Vysokomol. soyed. A29: No. 3, 503-506, 1987.