Gelation in solutions of poly(diphenyl)(phenyltolyl)siloxanes

Gelation in solutions of poly(diphenyl) (phenyltolyl)siloxanes

2231

REFERENCES

1. V. V. KORSHAK, A. A. ZHDANOV, L. M. TARTAKOVSKAYA, N. G. VASILENKO, T. M. BABCHINITSER, L. G. KAZARYAN, L. B. JTSIKSON and A. A. FII.IPPOV, Vysokomol. seyed. 1327: 300, 1985 (Not translated in Polymer Sci. U.S.S.R.) 2. Yu. A. YUZHELEVSKII, T. V. KURLOVA, Ye. G. KAGAN and M. A. SUVOROVA, Zh. obshch. khim. 42: 2006, 1972 3. J. NEALE and B. B. MILLEVARD, J. Chem. Soc., 4602, 1963 4. A. A. ZHDANOV, L. M. TARTAKOVSKAYA, V. M. KOPYLOV and N. G. VASILENKO, Vysokomol. soyed..4,27: 2090, 1985 (Translated in Polymer Sci. U.S.S.R. A27: 10, 2346, 1985) 5. C. L. LEE and O. W. MARKO, Polymer Preprints 19: 250, 1978 6. T. N. BARATOVA, V. P. MILESHKEVICH and V. E. GURARI, Vysokomol. soyed. A25: 2497, 1983 (Translated in Polymer Sci. U.S.S.R. A25: 12, 2899, 1983)

Polymer Science U.S,S.R. Vol. 31, No. 10, pp. 2231-2238, 1989 Printed in Poland

0032--3950/89 $10.00+ .00 1990 Pergamon Press pie

GELATION IN SOLUTIONS OF POLY(DIPHENYL)(PHENYLTOLYL)SILOXANES* L. Z. ROGOVINA, M. 1. BUZIN, N. G. VASILENKO, L. M. TARTAKOVSKAYA,V. G. VASIL'EV, A. A. ZHDANOV and and G. L. SLONIMSKII Nesmeyanov Institute of Elmento-Organic Compounds, U.S.S.R. Academy of Sciences

(Received 21 March 1988) The process of gelation of poly(diphenyl) (pnenyl-p-tolyl)siloxane and poly(diphenyl) (phenyl-m-tolyl)siloxane in o-xylol has been studied. The binary phase diagrams of these systems axe characterized by a combination of amorphous and crystalline separation, their invariance being reached after several heating-cooling cycles indicating change in the structure of the macromolecules in solution. Comparison of the time of the induction period of gelation, the melting point, the hysteresis of the temperatures of melting and gelation, the enthalpy of destruction of the gel nodes, the elasticity modulus and the activational energy of viscous flow in the polymeric gels indicates more refined packing of the crystallites in the case of poly(diphenylXphenyl-p-tolyl)siloxane. SILOXANE polymers in many respect possess unique properties which depend on the structure of the chain and nature o f the organic radicals at the silicon atom. Thus, in a series of linear polydiorganosiloxanes PDMS is the most flexible chain polymer characterized by the lowest glass transition temperature and polydiphenylsiloxane is a ther* Vysokomol. soyed. A31: No. 10, 2031-2037, 1989.

2232

• L. Z. ROGOVINAet al.

mally stable, highly crystalline polymer [1] with a high melting point (above 500°C) [2] possessing high thermo-oxidative and radiation stability. However, high crystallinity leads to poor solubility of the latter which makes it harder to process to products and study the properties of a given polymer. The replacement of some phenyl substituents in polydiphenylsiloxane by tolyl substituents gave polymers with good solubility at low temperatures able to form oriented films [3]. The polymers synthesized retain a crystalline structure. Increase in the number of tolyl substituents to a certain limit reduces the crystallization capacity of the polymers. For a ratio of phenyl and tolyl substituents in the starting monomers (cyclotrisiloxanes 5 : 1 readily crystallizable polymers form and their crystallization begins already in solution leading to gelation. Thermally reversible gelation both of amorphous and crystalline polymers has recently attracted growing attention [4-9]. In the case of crystallizable polymers, for example, isotactic PS or poly-4-methyl-pentene-1 the gelation capacity in certain solvents is related to the specific character of crystallization: on formation of folded crystals they drop out of solution forming a suspension while gelation occurs during micellar crystallization when in the macromolecule there are more or less ordered regions. For some polymers only one conformation of the macromolecules and one type of crystallization are realized. This applies, for example, to PVS [10] and PAN [11] for which crystallizable gels formed as a result of the crystallization type of phase separation in solutions have long been obtained and studied [12]. Recently it was shown that for a series of polymers crystallization is possible by different mechanisms determined both by the nature of the solvent and the conditions of crystallization - temperature and rate of cooling. The formation of small seeds ensuring the growtla of micellar crystals requires heavy supercooling. For example, accolding to reference [9] crystallization of PVC through gelation proceeds at low'temperatures (below the 0-temperature) or on mixing. The method of crystallization through gelation a_as, in particular, found important practical application for obtaining high molar mass fibres from ultrahigh molar mass polyolefins [13-15]. In some polymers for example isotactic PS and poly-4-methyl-pentene-1 not only are changes in the size and shape of the crystallites found on gelation but also different types of crystalline structures (helices) in certain solvents which do not form in absence of solvent. One probable reason for such crystallization is the change in the free energy of crystal formation with change in the medium, i.e. the nature of the solvent. In particular, in reference [8] the advent of the Clz helix in isotactic PS in aromatic solvents [16] instead of the usual Ca helix is interpreted as the formation of solvate crystals. However, in other laolymers, for example, polyolefins, specific conformations are not found. In line with reference [17] gelation, i.e. formation of less refined crystallites, is promoted by any disturbance of the regularity of structure of the macromolecules an extreme case of which is copolymerization. In all the cases considered control of gelation is a means of regulating the crystalline structure appearing in such conditions. Therefore, extensi 9n of the range of polymers in which crystallization through gelation is possible is both of scientific and practical interest.

Gelation in solutions of poly(diphenyl) (phenyltolyl)siloxanes

2233

The aim of the present work is to study the features of the gelation of poly(diphenyl) (phenyltolyl)siloxanes and establish the role of the p- or m-tolyl substituent in the macromolecule in the course of this process. The test objects were poly(diphenyl) (phenyl-p-tolyl)siloxane(I) and poly(diphenyl) (phenyl-mtolyl)siloxane (II) obtained by anionic polymerization of 2,2,4,4,6-pentaphenyl-6-p-tolyland 2,2,4,4,6pentaphenyl-6-m-tolyl cyelotrisiloxane respectively with deactivation of the active centres by the introduction of trimethyichlorosilane [3]. To bind the hydrochloric acid evolved pyridine was added to 10 ~o solution of the polymer in toluene. The solution was washed in a separating funnel with distilled water to neural reaction with universal indicator. The polymer solution in toluene was diluted to 59/0 concentration, precipitated into a three-fold excess of ethanol and dried in vacuo at 60°C to constant weight. The polymers studied were characterized by specific viscosity in chloroform: 2. 58 and 2.15 for the polymer I and 2. 74 for polymer II. Such specific viscosity when compared with the data obtained for other siloxane polymers corresponds to M ~ 105. The gelation process was studied in orthoxylol although gels form in toluene starting from an even lower concentration of the polymer but investigations of them are made difficult by the low elasticity of the toluene vapour. In all cases the gels were obtained in sealed ampoules. The phase diagrams were plotted by the Alekseyev method from the cloud points [18]. The melting points of the gels were determined by the method of descent of a ball placed under the gel surface. Heating was stepwise at the rate 2 deg/hr. In a separate case the melting point of the gel was determined by the DSC method. The heat effects of melting and crystallization of the dry films were also measured by the DSC method with the DSM-1 scanning calorimeter at the heating rate 8 deg/thin. The elasticity modulus and the creep of the gels were measured by the method of penetration with a ball indentor (R=2 mm) on modified Kargin-Sogolova balances [19]. The elasticity modulus was determined for values of the applied force F for which creep is absent. In calculating the viscosity from the data on the creep of the gels we took the value of the constantly acting pressure at tlae centre of the area of contact P = 3 F / 2 n r 2 where r is the radius of the contact circle. The temperature dependence of the viscosity of the polymer solutions and also the kinetics of gelation were recorded with the VPN-1 constant stress viscometer [20]. To determine the temperature-concentration region of the existence of the gels we plotted the binary phase diagrams of the polymer-solvent systems. From Fig. 1 it will be seen that characteristics of both polymers is a combination of amorphous and crystalline separation to which correspond the binodal (A) and the liquiduus curve (B) [12]. For the sample I both processes are shifted to the region of considerably lower polymer concentrations. The phase diagrams of the above gelating polymers described in the literature are also of a similar character. 7-o

a

9O

70

i

I

8

18

1

j

8

!

l

1G 24 c, .z' %

FIG. I. Binary phase diagrams and concentration dependence of the melting point of the gels I (a) and II (b) in o-xylol. 1 - Clounding curve; 2, 2 " - clearing curves after first (2) and second (2') heatingcooling cycles; 3-concentration dependence of the melting point of the gels. Explanations in text.

L.Z. ROGOVlNAet al.

2234

It should be noted that the final equilibrium in the system is not established at once. The running of the clouding-clearing cycles with change in temperature of the polymers studied showed that the phase diagrams in the repeat cycles shift to the region of lower temperatures and only after two cycles for polymer I and four to five cycles for polymer II does complete reversibility ensue. This indicates rearrangement of the macromolecules in the solvent medium. Gelation occurs both in the region of the liquid and in the region of crystalline separation but far below the phase boundary and is a lengthy process. From the kinetic gelation curves given in Fig. 2 it will be seen that the induction period of gelation of sample I rises with temperature. Gelation of sample II occurs at a measurable rate only at 25°C and is far slower than for sample I (Fig. 2, curve 4). Characteristic of both samples is hysteresis of the melting and crystallization temperatures, the temperature of the disappearance of the crystalline phase for sample I practically matching the melting pont of the gel determined by the descending ball method and for sample I1 the gel melts with retention of the crystallinity of the polymer. The higher temperatures and the lower concentrations of crystallization on gelation of sample I and also the faster rate of this process as compared with sample II indicate its greater capacity for crystallization evidently due to the possibility of more refined packing of the macromolecules containing the p-tolyl grouping. Also chalacteristic of such a system is the considerable hysteresis of the crystallization and melting tem~Pa.sec

( IO~/T)K -~

25

2"95~~2

15 5

2"85 ~

q~ P-r'f~

2.75

lO 20 30 2z/0250 7"irne,min FIG.2

-O.Z

t

I

OZt FIG.3

1"2 loac

Fxo. 2. Kinetics of gelation of 10~ solutions of I (1-3) and II (4) in o-xylol at 25 (1, 4) 40 (2) and

50 (3)°C.

FIG. 3. Concentration dependence of the melting point of the gels I (1) and II (2) in o-xylol in the coordinates of the Ferry-Eldridge equation. peratures. This is also confirmed by study of the crystalline structure of the polymers obtained from gelating and non-gelating solvents [21]. It should be noted that the gels formed at different concentrations retain this concentration when placed in an excess of solvent without tending to "preswell" to the corresponding equilibrium state. In the case of networks formed through chemical crosslinks the inability to "preswell" in an excess of solvent is peculiar only to heavily cross-

Gelation in solutions of poly(diphenyl) (phenyltolyl)siloxanes

2235

linked systems. The similarity observed in the gels studied with physical nodes maY indicate a very large number of nodes formed in the system studied even when highly diluted. It was of interest to evaluate quantitatively the strength of the structure of the gels formed which evidently must be a consequence of the degree of refinement of the crystals lying in the nodes of the gel network. This structure is commonly characterized by the value of the enthalpy of melting d/'/'melt, i.e. destruction of the gel network, determined from the concentration dependence of the melting point from the Ferry-Eldridge equation [22] and also from study of the visco-elastic properties of the gels. Figure 3 presents the plots in the coordinates of the Ferry-Eldridge equation log c = c o n s t - AHme~t/ /2'303 RT for the systems studied. The melting points of the gels were determined by the descending ball method and in the case of the 10~ gel of polymer I it was found that it exactly corresponds to the melting point measured by the DSC method (85°C). The dHmelt values calculated from the tangent of the slope of the linear portions corresponding to the region of crystalline layering in the phase diagram amount to 215 and 157 kJ/mole for samples I and II respectively. Such high values of the enthalpy of breakdown of the nodes of the network of the gels were previously found for gels of gelatine, pectin and x-carageen characterized by the presence of helical regions of the macromolecules in the network nodes [23, 24] and also for the gels of poly-4-methyl-pentene-1 [7, 8] the crystallization of which in solution also leads to the formation of a helical conformation. In crystalline PVC gels [25], in amorphous gels of atactic PS [26] and also in the gels of the above biopolymers in absence of the helical conformation Zlnmeltis considerably lower and amounts to tens of kJ/mole. The high/IHmclt values in the crystalline gel s of poly(diphenyl) (phenyltolyl)siloxanes are evidently the result of strong intermolecular interaction due to the phenyl and tolyl groups which rises considelably on passing from m- to p-tolyl substituents. Let us look at the results of measurement of the visco-elastic properties of the gels, namely, the elasticity modulus and viscosity calculated from the data on creep as is common irl investigating gels [23, 24]. TABLE 1. EFFECT OF THE NUMBER OF MELTING-CRYSTALLIZATION CYCLES ON THE PROPERTIES OF 10~oo GELS I AND [ I IN O-XYLOL AND DRY FILMS OBTAINED FROM THEM

Polymer !

H

No. of cycles

Melting point of gel, °C 79 85 68 75

Elasticity modulus Heat of melting of of gel, dry films, Cx 10-4, Pa Qmclt, J/g 0-1 4-62 28 8.41 1-2 10.6 6. 72

The measurements made on gels of 10~o concentration in the interval 25-70°C showed that the process of the repeat crystallization-melting cycles (depicted in the phase diagram) raises the rigidity of the gel. In particular, Table 1 shows sharp rise

L.Z. ROGOVINAet aL

2236 C'10-~ j ;'a

ri • 10 ; ?a-:. c 30 15 10

2o

• 30

'I

I 60

FIG. 4 FIG. 5 FIG. 4. Temperature dependence of the elasticity modulus of 10% gels I (1) and II (2) in o-xylol. FIG. 5. Temperature dependence of the viscosity of 10% gels I (1) and II (2) in o-xylol. in the elasticity modulus of sample II after a fifth cycle and of sample I after the second cycle which also corresponds to increase in AHmelt of the dry films obtained from gels measured by the DSC method* presented in the same Table. A similar dependence was found in poly-4-methyl-pentene-1 gels where it was related to the destruction of the conformation appearing in the course of synthesis and the formation of a conformation most favourable for gelation [8]. From all this for the rheological tests we used TABLE 2. ACTIVATIONAL ENTHALPY OF VISCOUS FLOW A n v f OF GELS ][ AND

System 10% gel I in o-xylol 10yo gel II in o-xylol 10% solution of I in o-xylol

Temperature region, T O 25-40 50-70 25-30 40-60 80-110 110-120

10yo solution of II in o-xylol 10Yo solution I in tetrachloroethane

70-100 100-120 25-74 74--120

II

AND SOLUTIONS

,4H,r, k J/mole 57.8 46.1 72. 3 45.2 13.25 22.16 11-4 17.8 12-47 16. 93

the samples obtained after two cycles for the polymer I and four to five cycles for polymer i I when a structure practically unchanged in time is established. The values of the elasticity modulus of the gels are equivalent to those characteristic o f equiconcentrated gelatine gels [23]. Study of the process of creep of gels in the interval 25-70°C and the subsequent elastic after-effect showed that as with most crystalline gels characteristic of the samples studied is insignificant development of highly elastic strain. But in the interval 45-55°C * The measurements were made by I. I. Dubovik to whom the authors wish to express their gratitude.

Gelation in solutions of poly(diphenyl) (phenyltolyl)siloxaaes

2237

for polymer I and 35--45°C for sample II the strain of the samples increased. In this temperature interval as is made clear by Figs. 4 and 5 there is rise in the elasticity modulus and the viscosity of the gels which over all the remaining temperature range decrease with rise in temperature. The nature of this interesting phenomenon undoubtedly connected with structural conversions of the gel calls for further investigations. It is natural that the activational enthalpies of viscous flow of the gels AH, r determined from the temperature dependence of viscosity and presented in Table 2 also have different values in the temperature intervals before and after this transition. The AHvf values are substantially smaller than AHme~t of the gels which is also typical of the previously studied biopolymer gels [24]. This is related to the fact that AH, r characterizes the destruction of the greater part but not all the nodes of the network occurring in response to stress below the melting point while AH~eIt characterizes the destruction of the strongest nodes. Their difference indicates the heterogeneity of the nodes in the gel network. The activational enthalpies of viscous flow of these systems determined after melting of the gel, i.e. in the state of the transparent solution, have as shown in Table 2, considerably lower valu~,~ diff,~ring in the two temperature intervals studied. It is interesting to note that the temperature dependence of the viscosity of polymer I in a non-gelating solvent-tetrachloroethane-is also characterized by change in slope. From Table 2 it follows that AHvf of the solutions both in gelating o-xylol and in non-gelating tetrachloroethane are close in absolute terms, AHvf of the solutions risip.g with temperature unlike the gels where it falls. Change in the activation enthalpy both in gels and solutions may be a pointer to change in the packing of the macromolecules with change in temperature. Thus, the investigation has established that the gelation-crystallization process may proceed starting from very low polymer concentrations (0.1-1 ~). Thermally reversible siloxane gels were not previously known. The considerable difference in the gelation parameters and the properties of the gels depending on the position of the methyl group in the side tolyl substituent (shorter induction period of gelation, higher melting point and elasticity modulus of the gels) and also the enthalpy of destruction of the network nodes for the polymer with p-tolyl substituent indicate the considerable refinement of the packing of the macromolecules on para-substitution and show what significant changes in properties may result from such a slight change in the chemical structure of the macromolecule. Rise in the elasticity modulus and melting point of the gels, the heat of melting of the dry films obtained from them with change in the number of heating-cooling cycles and also rise in the elasticity modulus and viscosity of the gels in certain temperature intervals Foint to the structural rearrangement occurring in the state of the gels. However, as shown in reference [21] the crystalline structure of these polymers does not depend on the solvent from which (gelating or non-gelating) they were obtained. Moreover, the crystallinity of the polymers evaluated from the heats of melting is greater for the polymer obtained directly in synthesis (Qmelt=21.8 J/g) than that obtained from the gel state (Qmelt= 8"41 J/g).

2238

L . Z . ROGOVlNAet aL

I t should be b o r n e irt m i n d t h a t even an identical degree o f c r y s t a l l i n i t y a n d also identity o f the lattices o f the crystallites still d o n o t signify i d e n t i t y o f s t r u c t u r e a n d the c o m p l e x o f p r o p e r t i e s o f the p o l y m e r since the c r y s t a l s m a y differ in size a n d shape. Therefore, the two types o f p o l y m e r s o b t a i n e d in different c o n d i t i o n s p a r t i c u l a r l y with a different degree o f crystallinity m a y significantly differ in their c r y s t a l l i n e structure. T h e crystalline structure a p p e a r i n g in the gel state m a y n o t p e r s i s t on r e m o v a l o f the solvent a n d refinement o f the given structure m a k e s n e c e s s a r y a p p r o p r i a t e investigations d i r e c t l y in the solvent m e d i u m . Translated by A. CRozY

REFERENCES

1. E. E. BOSTIK, Polymer Preprints 10: 877, 1969 2. J. IBEMESI, N. GVOZDIC, M. KEUMIN, M. J. LYNCH and D. J. MEIER, Ibid. 26: 18, 1985 3. A. A. ZHDANOV, L. M. TARTAKOVSKAYA, V. M. KOPYLOV and N. G. VASILENKO, Vysokomol. soyed. B27: 2090, 1985 (Not translated in Polymer Sci. U.S.S.R.) 4. M. GILORAMO, A. KELLER, K. MIYASAKA and hi. OVERBERGH, J. Polymer Sci. A-2: 14: 39, 1976 5. J. M. GUENET, B. LETZ and J. C. WITTMANN, Macromolecules 18: 420, 1985 6. H. M. TAN, A. MOET, A. HILTNER and E. BAER, Ibid. 16: 28, 1983 7. G. CHARLET, H. PHUONG-NGUYEN and G. DELMAS, Ibid. 17: 1200, 1984 8. T. TANIGANI, H. SUZUKI, K. JAMAURA and S. MATSUZANA, Ibid. 18: 2595, 1985 9. S. H. GUERERO and A. KELLER, J. Macromolec. Sci. 1320: 167, 1981 10. S. P. PAPKOV, S. G. YEFIMOVA, M. V. MIKHAILOV and L. F. VYRKOVA, Vysokomol. soyed. 8: 69, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 1, 72, 1966) 11. A. H. BISSHOPS, J. Polymer Sci. 17: 167, 1955 12. S. P. PAPKOV, Ravnovesiye faz v sisteme polimer-rastvoritel' (Phase Equilibrium in a Polymer--Solvent System). 272 pp., Moscow, 1981 13. P. SMITH and P. J. LEMSTRA, J. Mater. Sci. 15: 505, 1980 14. M. MATSUO, G. SAWATARI, M. ]IDA and M. JONEDA, Polymer J. 17: 1197, 1985 15. M. MATSUO and G. SAWATARI, Macromolecules 19: 2028, 1986 16. P. R. SANDARARAJAN, N. J. TYRER and T. L. BLUHM, Ibid. 15: 286, 1982 17. L. MANDELKERN, C. O. EDWARDS, R. C. DOMSZY and M. W. DAVIDSON, Microdomains in Polymer Solutions, p. 121, N. Y., 1985 18. A. A. TAGER, Fizikokhimiya polimerov (Polymer Physical Chemistry), 544 pp., Moscow, 1978 19. L. Z. ROGOVINA, V. G. VASIL'EV and G. L. SLONIMSKII, Vysokomol. soyed. A24: 254, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 2, 264, 1982) 20. V. G. VASIL'EV, O. A. KOZLOV, A. A. KONSTANTINOV, L. Z. ROGOVINA, K. S. KRASHENINNIKOV and G. L. SLONIMSKII, Kolloid. zh. 39: 938, 1977 21. T. M. BABCHINITSER, L. G. KAZARYAN, L. M. TARTACOVSKAY, N. G. VASILENCO, A. A. ZDANOV and V. V. KORSHAK, Polymer 26: 1527, 1985 22. J. E. ELDRIDGE and J. D. FERRY, J. Phys. Chem. 58: 992, 1954 23. L. Z. ROGOVINA and G. L. SLONIMSKII, Usp. khim. 43: i102, 1974 24. E. E. BRAUDO, J. G. PLASHINA and V. B. TOLSTOGUZOV, Carbohydrat. Polymer, No. 4, 23, 1984 25. M. A. H A R R I S O N , P. H. MORGAN and G. S. PARK, Europ. Polymer J. 8: 1361, 1972 26. H. M. TAN, B. H. CHANGE, E. BAER and A. HILNER, Ibid, 19; !031, 1983