Study of the crystallization of elongated polyorganosiloxane elastomers

STUDY OF THE CRYSTALLIZATION OF ELONGATED POLYORGANOSILOXANE ELASTOMERS* V. Yu. L ~ w ~ , G. L. SLOlVII~ISKII,K. A. AI~DRIANOV,A. A. ZHDAI~OV, Y r . K . GODOVSKII, V. S. PApKOV a n d YE. A. LYUBAVSKAYA Institute of Hetero-organic Compounds, U.S.S.R. Academy of Sciences (Received 16 June 1971)

ORG:AI~O-SILICO~ p o l y m e r s p r e p a r e d f r o m linear p o l y d i m e t h y l siloxane m a y crystallize a t low t e m p e r a t u r e s which n a t u r a l l y restricts t h e t e m p e r a t u r e r a n g e o f

use [1]. It is known that stress has a marked effect on crystallization. Information in the literature is mMnly concerned with organic polymers [2-4]. Earlier we examined in detail the crystallization of an extensive class of organo-silicon polymers in the non-deformed state and found that these polymers behave in a specific manner [5, 6]. In view of the foregoing the investigation of crystallization of organo-silicon polymers under stress is of fundamental interest. OBJECTS OF INVESTIGATION AND METHODS

P o l y d i m e t h y l siloxane a n d p o l y d i m e t h y l m e t h y l v i n y l siloxane (containing 0 . 3 m o l e % v i n y l groups) r u b b e r s w i t h M = 5 0 0 , 0 0 0 were e x a m i n e d . R u b b e r s were v u l c a n i z e d b y a d d i n g v a r i o u s a m o u n t s o f c u m y l peroxide. Aerosfl-300 a n d U-333 c a r b o n b l a c k were used for p r e p a r i n g filled elastomers. T h e n u m b e r of n e t w o r k units was controlled b y v a r y i n g t h e a m o u n t of c u m y l p e r o x i d e a n d filler a n d calculated f r o m swelling in benzene using t h e F l o r y - R e h n e r r e l a t i o n s h i p [7]. T h e objects studied are t a b u l a t e d . The variation of tensile stress required for maintaining a given deformation during crystallization of elongated polyorgano-siloxane elastomers was studied in a specially constructed apparatus (Fig. 1). This is a thermostatically controlled unit, in which the elongated specimen was placed, joined by shafts to the tcnsometric system. The apparatus made it possible to carry out experiments both under isothermal conditions at ± 0.5 temperature and under conditions of linear cooling. Experimental procedure included: initial specimen (length 4, width 0-3 and thickness 0-05 cm) was elongated at room temperature to a certain a value (~ is the ratio of the final length to the initial length) and fixed in clamps by means of a special appliance. The specimen was then kept at the same temperature for a certain time required to establish equilibrium stress which is determined in special experiments from stress relaxation. I t was then placed in a unit kept at experimental tern* Vysokomol. soyed A15: No. 1, 224-233, 1973. 256

Crystallization of elongated polyorganosiloxane elastomers

257

LIsT OF I~.LASTOM~ERSSTUDIED

@

Name of elastomer

Polydimethyl vinyl siloxane (0"3~/o vinyl groups Same Polydimethylsiloxane

Same

Polydimethyl methyl vinyl sfloxane

Crosslinking Filler, Type of crosslinking part by agent, agent and filler wt. parts by wt.

Cumyl peroxide Same Cumyl peroxide, U-333 silica filler Cumyl peroxide, aerosil-300 Same Cumyl peroxide, aerosfl- 175

Mc*

X

× 103

E,? kg/em ~

0.05 0.1

0 0

85 25

3"00

20

40

5

3"00 3"0

15 35

20 8

9 32

0"5

35

4

40

M c - molecular weight of the chain segment between the network units. t E - - m o d e l of high elasticity with a deformation of 100 %. *

perature (above the glass transition point) and joined by rods to the tensometric system. After disconnecting the fixing pin the stress corresponding to a certain elongation was measured. When the experiments were carried out under non-isothermal conditions the elongated specimen was placed in the unit at room temperature, oonnected with tensometric data units and after establishing equilibrium stress, the temperature of the unit was reduced by linear proportions using special cooling equipment.

/7 z8 8<

CoolinE agent FIG. 1. Layout of apparatus used for measuring tensile stress during crystallization: / - - s t r a i n gauges, 2, 3--shaft system, 4--pin; 5--lid, 6--thermocouple, 7--specimen, 8--thermostatically controlled unit, 9--clamps.

V. Yu. LEVIN et al.

258

The relation between stress and deformation and curves of stress relaxation were plotted using a Polanyi apparatus. Investigations involving heat of melting and crystallization were carried out in micro-calorimetric apparatus, designed in our laboratory [8]. The specimen previously elongated and relaxed to equilibrium stress was placed in the cell of a thermostatically controlled micro-calorimeter and retained there at constant temperature during the period required for crystallization. Experimental conditions (temperature and retention time) were determined from earlier data concerning crystallization kinetics of undeformed, vulcanized and filled polyorganosiloxane elastomers. The crystallized specimen was then transferred to another micro-calorimeter, cooled to --150 ° which was heated in a linear manner at a rate of 2 deg/min. The dependence of specific heat C~ on temperature was examined by methods previously described [9]. Melting point was determined from the position of peak corresponding to the specific heat anomaly in the melting range. RESULTS AND DISCUSSION

The effect of crystallization on the state of stress in polymers. I n a p a p e r b y W a r w i c k et al. it was s h o w n t h a t c r y s t a l l i z a t i o n of e l o n g a t e d p o l y o r g a n o s i l o x a n e e l a s t o m e r s u n d e r n o n - i s o t h e r m a l conditions is a c c o m p a n i e d b y a m a r k e d increase in tensile stress, which is o b s e r v e d a t t h e t e m p e r a t u r e of crystallization [10]. A n increase in stress, a c c o r d i n g t o t h e a u t h o r s of this s t u d y , is t h e result of r e d u c t i o n in s p e c i m e n length due to crystallization. X - r a y analysis shows t h a t d u r i n g c r y s t a l l i z a t i o n of e l o n g a t e d p o l y o r g a n o s i l o x a n e e l a s t o m e r s t h e crystallites f o r m e d are a r r a n g e d a t a n y angle to t h e direction of elongation. A certain prop o r t i o n of crystallites is p e r p e n d i c u l a r to t h e axis of elongation. I t is precisely these crystallites [10] which are responsible for r e d u c t i o n of s p e c i m e n length, i.e. a n increase o f tensile stress. I t is well k n o w n , h o w e v e r , t h a t during crystallization o f e l o n g a t e d organic elastomers, e.g. v u l c a n i z e d n a t u r a l r u b b e r , t h e length of t h e s p e c i m e n increases ~,kg/cm e ~, k#/cm z 50-

[00

-

80-

3

25 20 [O 0

20

60 T/me, rain FIG. 2

100

10 -/g -30 -50 -70 r,°C FIG. 3

FIe. 2. Relationship between a and time at --40 ° for polyorganosfloxane specimen 6 (1'-3') and vulcanized natural rubber (1-4) when a = 4 (1'); 3.5 (2'); 3 (3'); 0 (1); --10 (2); --20 (3) and --30 ° (4). FIG. 3. Relationship between a of deformed polyorganosiloxane elastomer and temperature (linear cooling) when ~----1.5 (•); 3 (2); 5 (3) and 7 (d).

Crystallization of elongated polyorganosiloxane elastomers

259

resulting in a reduction of stress to zero. After complete stress relaxation several authors observed a spontaneous elongation of the specimen, which was a sign of continuing orientation crystallization. This elongation is about 4% of the initial length, independent of the degree of preliminary elongation [11, 12]. I n every study dealing with the crystallization of elongated organic elastomers, a reduction in tensile stress during crystallization due to elongation of the specimen was explained by the orientation of crystallites along the axis of elongation. I t should be emphasized that all studies concerning organic elastomers were carried out under isothermal conditions, in contrast to experimental studies of organosiloxanes. We found it inappropriate to compare results of the effect of crystallization on the state of stress of organic and organo-silicon elastomers [10] since these results were obtained under different temperature conditions. In order to compare the properties of organic and organo-silicon elastomers, a study was made of the effect of crystallization on the state of stress of these elastomers under. identical conditions.

60

ep, c~l/g.deg

0

-#0

-60

Fie. 4

T,°O

-lgo

0

[00

T, °C

Fie. 5

Fie. 4. Relationship between a of vulcanized natural (1) and butyl rubbers (2) and temperature (linear cooling). Fie. 5. Temperature dependence of specific heat O~ of a n undeformed polyorganosiloxano elastomer specimen.

Experiments were carried out at temperatures which ensured crystallization of elongated elastomers. This range was determined by X-ray [10] and confirmed by our investigations. Within the temperature range of crystallization experiments were carried out every 1° with elongations ~ ranging from 2 to 5, and in some cases to 8. Figure 2 shows the typical relation between tensile stress and time at c o n s t a n t temperature for some polyorganosiloxanes. Similar relationships were derived for all polyorganosiloxanes studied in a wide temperature range. A study of this relationship shows that, in contrast to natural rubber, isothermal crystallization of elongated polyorganosiloxane elastomers is not accompanied by stress variation maintaining a given deformation. Experiments carried out under quite:

260

V. Yu. LEvr~ e$ al.

similar conditions with natural vulcanized rubber (Fig. 2), as could be expected, demonstrated the stress reduction during crystallization described in the literature. The higher the elongation, the higher the rate of reduction, i.e. the rate o f crystallization. I t may be concluded from this information that an increase in stress during crystallization of polyorganosiloxane elastomers is only observed under non-isothermal experimental conditions (Fig. 3). To confirm this, we

~/

-'=/-¢0~ I

v2

I

2

I

3

FIG. 6

A4

I

#

J

5~ FIG. 7

~ o . 6. Relationship between Tm~l~ of polyorganosiloxane elastomers and the degree of elongation for specimens 2 (•); 3 (2); 4 (3) and 5 (4). ~G. 7. Relationship between A T and T for different degrees of elongation of a polyorganosiloxane elastomer when a-~ 1 (1) 2; (2); and 3 (3). investigated stress variation in non-isothermal crystallization of natural and butyl rubbers (Fig. 4). A study of these relationships indicates that during non-isoChermal crystallization stress increases abnormally even for these polymers. This is caused, in our opinion, b y the following: during cooling of the system which incorporates the elongated specimen studied, the length of fibres decreases which should inevitably result in an increase of stress. However, up to the pore, where the elongated specimen is in the high-elastic state, stress does not increase :since the modulus of high-elasticity of both polyorganosiloxane elastomers and natural and butyl rubbers is low and their slight elongation due to a reduction o f fibres does not markedly alter stress. Therefore, the elongated elastomer in the high-elastic state functions as a soft spring in a rigid system of clamps and rods offsetting reduction. As soon as crystallization or glass transition occurs, the elasticity modulus markedly increases and the entire system becomes rigid. A slight reduction of fibres during cooling causes a sudden increase in stress, which is recorded in experiments on the study of crystallization under nonisothermal conditions. We note that if some glassy or crystalline polymer is placed between the clamps, stress increases immediately after the inception o f cooling. Results [10] should therefore be attributed to a methodical error d u e to the absence of heat compensation. A comparison of results suggests that during isothermal crystallization o f extended polyorganosiloxane elastomers, in contrast to organic polymers, no change of stress occurs, which would be necessary to maintain a given deformation. This is due to the fact [13] that during the crystallization of elongated polyorganosiloxane elastomers the crystallites formed are arranged with ap-

Crystallization of elongated polyorganosfloxane elastomers

261

proximately equal probability at angles in relation to the axis of elongation. This, in our view, is due to the specific properties of crystallization of polyorganosiloxanes noted previously [5] and is explained by the high segmental mobility of polysiloxane macromolecules.

Effect of the degree of elongation on the qnelting l~oin~ of polyorganosiloxane elastomers crystallized in the elongated state. I t is well known that during melting elastomers crystallized in the elongated state, melting point increases with an increase in the degree of elongation [10]. An increase in melting point should be accompanied b y a displacement of the maximum peak on the curve showing the temperature dependence of specific heat. Figure 5 illustrates the temperature dependence of specific heat C~ of an undeformed polyorganosfloxane elastomer specimen which was crystallized isothermally and then placed in a pre-cooled (to --150 °) microcalorimeter. 5

6

Tmee~t

#

v

-20

.

.

.

I

2

.

.

X

I

8

3

-

-

X

i

#

-

-

- -

-

[

5o~

FI~. 8. Relationship between Teelt and a for polyorganosfloxane elastomers. Here and in Fig. 9 the numbers of curves correspond to the numbers of specimens in the Table. It can be seen that at --126 ° specific heat increases more suddenly, which corresponds to the transition of the amorphous part of this specimen from the glass-like to the high-elastic state. Then to a temperature of --70 ° specific h e a t increases in a practically linear manner and the peak, corresponding to melting of the specimen, is observed in the temperature range of - - 7 0 - - - 3 5 ° . The maximum peak on the curve showing specific heat corresponds to a temperature which should be regarded as the melting point of the crystallized polymer. When studying the temperature dependence of C~ of elongated polyorganosiloxanes it appeared that with an increase in the degree of elongation, the maximum peak of specific heat is displaced slightly reaching 5 ° when ~ = 2 f o r all the polyorganosiloxane elastomers studied. With a further increase in the degree of elongation the temperature corresponding to the maximum of the melting peak on the curve showing specific heat (T~eal~), reamins constant, approximately --35 ° (Fig. 6). However as shown b y Fig. 7 on increasing the degree of elongation, the shape of the peak corresponding to melting of an elongated specimen, changes (the position of the maximum remains unchanged, but the t e m p e r a t u r e

262

V. Yu. L~.w~ et al.

which restricts the end of the peak (T°melt),increases). X-ray investigation showed t h a t this temperature corresponds to the disappearance of the last traces of erystallinity. It is natural to assume t h a t the final temperature of the peak corresponds to the melting of erystallites situated in parallel to the axis of elongation. An analysis of curves showing the temperature dependence C~ of elongated polyorganosiloxane elastomers suggests that the value of Teelt shows a linear o', A ,,/cm2 300 -

/G

200 -

/ / /5

a ~,,kg/cm z

////

6

SO-

~

8

I

3

5oc

Ol

P'~'~"4

~" 3

'7! E oc

F i e . 9. Stress-deformation curves (a) and the same curves calculated on the i n i t i a l stress (b) o f i~he po]yorganosi]oxane e]astomers. The broken lines represent equilibrium stresses.

increase with an increase of the degree of elongation (Fig. 8). For each elastomer Che t y p e of increase of Temel~varies according to the degree of elongation. A comparison of the properties of the elastomers studied indicates that the cause o f this difference is the varying modulus of high-elasticity. With an increase in the value of the modulus, T~elt depends increasingly on the degree of elongation. For an elastomer with a modulus of N 4 kg/em ~, the value of T~elt is practically independent of the degree of elongation, whereas for an elastomer with a modulus of 40 kg/cm ~, when a = 2 , the value of T~elt increases by 15 °. For a more detailed analysis of the dependence of T~elt on the mechanical properties of polyorganosiloxane elastomers, curves were plotted to show stress-deformation for all the elastomers studied (Fig. 9). Since it is interesting to compare equilibrium parameters, these curves were reconstructed using relationships of stress relaxation (Fig, 10). A comparison Of relationships of T~olt and equilibrium stress aa°ct (in terms of actual cross see-

Crystallization of elongated polyorganosiloxane elastomers

263

tional area) on ~ enable us to obtain the dependence of T~elt on aaeet (Fig. 11), which for all the polyorganosiloxane elastomers studied, independent of whether they are erosslinked or filled, is described b y a straight line (comparison was made in a region which is restricted b y the value of aaeot 100 kg/cm ~, since high stress values in experiments on determining melting points were not obtained owing to difficulties). Proceeding from the fact that the properties of polyorganosiloxane elastomers studied (Table) vary considerably, b u t that t h e y crystallize

-/0

GT

I

-20 I

I

0

5

7-z.rne , re~z; Fro. 10

.2 3

14~ /0

0

I

I

50

100

~ hq/cm z Fro. 11

FIG. 10. Curves of stress relaxation of polyorganosiloxane elastomers with deformation of 100 (•); 200 (2); 300 (3) and 400~o (4). FIG. 11. Relationship between Temptand initial equilibrium stress. to form the same crystalline lattice, it m a y be assumed that for all polyorganosiloxane elastomers formed of linear polydimethylsiloxane the dependence of T~nelt on the equilibrium tensile stress is described b y a simple analytical expression Temelt= 246~-0"25a, (1) where 246 is the temperature at which traces of crystallinity of an undeformed specimen disappear, °K; a is the equilibrium stress required for maintaining a given deformation, kg/cm ~. This relationship enables ut so predict the temperature range over which stressed polyorganosiloxane elastomers can exist in the high elastic state. It is natural to assume that similar expressions can be derived for various classes of elastomers crystallizing to form identical crystalline lattices. This approach is, in our view, valuable since it enables a relation to be derived between the thermodynamically significant value of aT~elt/aa and the actual morphological properties of crystalline elastomers.

V. Yu.

264

LEw~

e~ al.

For a more detailed analysis of the results it is advisable to compare t h e m in the light of present theories. Flory established the following relationship [14]: 1

1

Tmelt

/~

F(6~'/"

Tmelt AH-meltL\~m/ a

a~

1 ],

2m

a-m

(2)

where Tmelt is the equilibrium temperature of an undeformed network; Trael t is the melting point of the deformed network; AHmeit--the heat of melting per mole of statistical segments; m - - t h e number of statistical segments between the network units; a - - t h e degree of uniaxial elongation. On deriving equation (2) it was assumed t h a t crystallites only grew in the direction of elongation and it was shown t h a t it can hardly be expected that this equation will give the correct equilibrium melting points for slight elongations and, even less so, for no elongation. Values of Tmeit calculated from the Flory equation would be too high for slight elongations. A similar relationship describing the relation between deformation and temperature was proposed by Krigbaum and Roe [15] 1

1

f

R

\1/~

2

Using these relations an analysis was made of experimental results. Bearing in mind t h a t equations (2) and (3) were derived with the prerequisite t h a t crystallites are mainly oriented in the direction of the axis of elongation, the value of T~ett was substituted in place of Tm¢lt which, in our opinion, corresponds to melting of these crystallites in oriented polyorganosiloxanes. For elastomers formed from linear polydimethylsiloxane (specimens 3-5, Table) the value of m can be determined from the well known value of Mo and from data given in another paper [16], according to which the statistical segment of the polydimethylsiloxane chain is 4.2 of the recurrent unit. The m value thus determined for specimen 4 is 65, for specimen 5-26 (Table). The heat of melting of isothermally crystallized elastomers was previously determined calorimetrically and crystallin i t y by X-rays. A comparison of these data enable the heat of melting to be calculated for elastomers with 100% crystallinity per mole of statistical segments (AHmelt). This value is about 6.2 kcal/mole of statistical segments for both elastomers. From the parameters thus obtained the theoretical dependence of the difference of 1/Tmelt-1/Tmelt on the a value was calculated using equations (2) and (3). Figure 12 shows t h a t the experimental dependence of 1/Tmelt-- 1/Treea on the a value is satisfactorily described by the Flory relationship; as might be expected, in the range of low elongation the theoretical values of Tmel~ are too high. Relationship (3) gives too low values of Tmelt. Therefore, although this equation enables us to obtain Tmelt values for unstretched specimens, it is not very suitable for the description of the variation of melting point with elongation. We note t h a t several authors [17] made similar evaluation of equation (3).

Crystallization of elongated polyorganosiloxane elastomers

265

Comparing these results it m a y be concluded t h a t crystallization of elongated polyorganosiloxane elastomers takes place somewhat differently from crystallization of elongated organic elastomers. The main cause of this difference, as in the case of crystallization of undeformed polyorganosiloxanes, is the high segmental mobility of macromolecules, which determines the globular supermolecular e

/

l

1 x ,0a ¢)

113 / ~/,I 4..¢

/ ol,

2

3

0o~

FIG. 12. Relationship between the difference of 1 / T m e l t - 1 / T m e l t and the extent of elongation for specimens 4 (3) and 5 (4): 1, 2--theoretical curves according to Flory; 5, ~6-according to Krigbaum-Roe. k

structure of these elastomers and results in numerous specific properties. It follows from these results that high segmental mobility is also maintained in the elongated state, creating conditions for the equally likely distribution of crystallites in relation to the axis of elongation. This determines the constancy of stress required for maintaining a given deformation in crystallization. The absence of significant displacement from the maximum peak of melting oll increasing the degree of elongation is also evidence t h a t the position of crys~allites during crystallization both in the unstretched and the elongated condition is practically the same. The temperature at which the last traces of crystallinity disappear and which represents the end of the melting peak of elongated elastomers, corresponds to melting of the erystallites parallel to the axis of elongation and shows a linear increase with the increase of degree of elongation. This temperature is related simply to the modulus of high-elasticity of elongated elastomers. The proportion of crystallites parallel to the axis of elongation is low, but is significant when increasing the overall rate of crystallization. CONCLUSIONS

(1) During crystallization of polyorganosiloxane elastomers extended to a constant degree of elongation, stress remains unchanged. (2) During melting polyorganosiloxane elastomers erystalhzed in the elongated

266

V. Yu. LEw~ e~ al.

state, w i t h a n increase in elongation, a v a r i a t i o n is o b s e r v e d in t h e s h a p e o f t h e p e a k corresponding to m e l t i n g on the c u r v e showing t h e t e m p e r a t u r e d e p e n d e n c e of specific h e a t (the m a x i m u m p e a k is n o t displaced in practice). (3) T h e t e m p e r a t u r e which corresponds to c o m p l e t e m e l t i n g of p o l y o r g a n o siloxane e l a s t o m e r s crystallized in t h e e l o n g a t e d state, is d e t e r m i n e d b y t h e m o d u l u s of high elasticity a n d shows a linear d e p e n d e n c e on equilibrium stress required for m a i n t a i n i n g a g i v e n d e f o r m a t i o n . (4) E l o n g a t i o n considerably increases the r a t e of crystallization of p o l y o r g a n o siloxane elastomers, displacing the t e m p e r a t u r e r a n g e of crystallization to higher temperatures. (5) The relationships d e r i v e d which distinguish p o l y o r g a n o s i l o x a n e elastom e r s f r o m o t h e r classes of e l a s t o m e r s involve a n equally p r o b a b l e d i s t r i b u t i o n of the position of crystallites in respect of t h e direction of elongation, which is due to t h e high flexibility of m a c r o m o l e e u l e s also r e t a i n e d in t h e e l o n g a t e d state. Translated by E. SEM:ERE

REFERENCES 1. G. D.4.M.ASIIIYN,Plaste und Kautsehuk 10: 68, 1963 2. L. i~I&N'DEL'KERN, Kristallizatsiya polimerov (Crystallization of Polymers). Izd. "Khimiya", 1967 3. P. J. FLORY, J. Amer. Chem. Soe. 78: 5222, 1956 4. J. F. OTH and P. J. FLORY, J. Amer. Chem. Soe. 80: 1297, 1958 5. V. Yu. LEVIN, Dissertation, 1967 6. Yu. K. GODOVSKII, V. Yu. LEVIN, G. L. SLONIMSKII, A. A. ZH]DANOV and K. A. ANDRIANOV, Vysokomol. soyed. A l l : 444, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 11, 2778, 1969) 7. P. J. FLORY and J. REHNER, J. Chem. Phys. 2: 521, 1943; 18: 108, 1950 8. Yu. K. GODOVSKII, Dissertation, 1966 9. Yu. K. GODOVSKII and Yu. P. BARSKII, Plast. massy, 1~o. 7, 57, 1965 10. E. L. WARRICK, J. Polymer Sei. 27: 19, 1958 11. A. N. GENT, Trans. Faraday Soc. 50: 521, 1954 12. W. H. SMITH and S. P. SAYLOR, J. Res. Nat. Bur. Standards 21: 257, 1938 13. S. M. OHLBERG, L. E. ALEXANDER and E. L. WARRICK, J. Polymer Sei. 27: 1, 1958 14. P. J. FLORY, J. Chem. Phys. 15: 397, 1947 15. W. R. KRIGBAUM and R. J. ROE, J. Polymer Sci. A2: 4391, 1964 16. G. F. KARTASHEVA, Ye. G. EREN'BURG and I. Ya. PODDUBNYI, Vysokomol. soyed. B l l : 693, 1969 (Not translated in Polymer Sci. U.S.S.R.) 17. HUO-GUN KIM and L. MANDELKERN, J. Polymer Sci. 6: 181, 1968