Lower critical solution temperatures of poly(decyl methacrylate) in hydrocarbons

Eur. Polym. J. Vol. 19, No. 10/11, pp. 963 965, 1983 Printed in Great Britain. All rights reserved

0014-3057/83 $3.00 + 0 . 0 0 Copyright @, 1983 Pergamon Press Ltd

LOWER CRITICAL SOLUTION TEMPERATURES OF POLY(DECYL METHACRYLATE) IN HYDROCARBONS E. MADEREK,G. V. SCHULZ a n d B. A. WOLF Institut fiir Physikalische Chemie der Universit/it Mainz, Jakob-Welder-Weg 15, D-6500 Mainz, FRG Abstract--The high temperature demixing of solutions of poly(decyl methacrylate) (PDMA) was measured as a function of pressure for the following solvents: n-pentane, cyclo-pentane, n-hexane, n-heptane, iso-octane and toluene. In all cases the application of pressure increases the miscibility in the order of 0.4-1.0 K/bar. The lower critical solution temperatures extrapolated to atmospheric conditions range from 155 (n-pentane) to 261°C (toluene); no phase separation could be observed with the base oil AF 1 of Schindler up to 370°C. Noticeable thermal degradation of the polymer sets in when the solutions are heated above c a . 300°C.

~00

INTRODUCTION

Poly(decyl methacrylate) ( P D M A ) is often a d d e d to m o t o r oils in order to improve their rheological properties, i.e. their lubricating behaviour. Since there exists a close correlation of t h e r m o d y n a m i c a n d of flow properties in the case o f polymer solutions [1], the present m e a s u r e m e n t s were p e r f o r m e d in order to o b t a i n i n f o r m a t i o n o n the solubility of P D M A in a n u m b e r of selected h y d r o c a r b o n s a n d in a base oil o f practical application. F u r t h e r m o r e , it was tested w h e t h e r some general features concerning the high t e m p e r a t u r e demixing o f solutions of o t h e r polymers [2] are also observed with P D M A .

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EXPERIMENTAL

The polymer sample was supplied by R6hm GmbH (Darmstadt); its weight average tool. wt and molecular non-uniformity ( ~ / , ] ~ / ~ - 1 ) were [3] 470,000 and 0.20 respectively. All solvents were products of E. Merck (Darmstadt), and were of the following grades:--n-pentane, cyclo-pentane: for spectroscopy; n-hexane, n-heptane, isooctane and toluene: p.a. They were employed without further purification. The base oil AF 1 of Schindler was also supplied by R6hm GmbH (Darmstadt). In all cases the polymer concentration was chosen to be 15 wt~o; this value, estimated for the critical concentration of the present polymer sample dissolved in iso-octane, is likely to be a good average for the various solvents. The exact individual values have not been determined because of the small influence of composition on the demixing conditions in the vicinity of the critical concentration [2]. Determination of the cloud points was performed using a small quartz cell, inserted into a pressure apparatus [4]; a detailed description of the experimental procedure has already been given [2]. The lowest pressure for which high temperature demixing can still be observed is the equilibrium vapour pressure of the solution. If p is reduced below that value by isothermal expansion, the solvent will evaporate until the equilibrium pressure is either reattained or the volatile component is used up. From such characteristic points in decompression experiments, the vapour pressure curve was determined for the pure base oil and for the solution of PDMA in it. Dedicated to Professor Oto Wichterle in honour of his 70th birthday.

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Cyclo-pentane n-Hexane

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Fig. 1. Demixing pressure as a function of temperature for solutions of 15 wt% poly(decyl methacrylate) (~/,. = 470,000) in the indicated hydrocarbons. The full symbols denote the critical points of the pure solvents. For measurements at temperatures above 275°C, the experiments were repeated with the same solution in order to find out, whether the polymer has been degraded thermally to a measurable extent. RESULTS AND DISCUSSION Figure 1 gives the directly m e a s u r e d T-dependence of the demixing pressure for the solutions o f P D M A u n d e r investigation. The two phase region is in all cases situated below the individual lines. This m e a n s that the miscibility o f the c o m p o n e n t s is raised by compression. According to some general considerations following from the c o r r e s p o n d i n g states principle, it was recently concluded [2] t h a t the high t e m p e r a t u r e demixing of polymer solutions u n d e r their equilibrium v a p o u r pressure should n o r m a l l y be

963

964

E. MADEREKet al. Table 1. Lower critical solution temperatures Tc and their pressure dependence (both extrapolated to 1 bar) for poly(decyl methacrylate) in the given solvents (1 bar) Solvent

T~ (°C)

d Tc/dp, (K/bar)

Tb (°C)

Tc/Tb (K/K)

n-Pentane Cyclo-pentane n-Hexane n-Heptane Iso-octane Toluene

155 196 189 218 219 261

0.54 0.50 0.68 0.96 0.96 0.36

36.1 49.3 68.9 98.4 99.2 110.6

1.38 1.45 1.35 1.32 1.32 1.39

Tb: boiling point of the pure liquids. observed within the temperature interval between Tb (K, 1 bar), the boiling point of the pure solvent, and 1.5 Tb. As can be seen from Table 1, this conclusion is corroborated by the present results: The characteristic Tc/Tb values scatter only slightly around an average of 1.37. The general tendency is therefore confirmed that the lower critical solution temperatures increase as the boiling point of the solvent becomes higher. A further conclusion of the cited paper [2] concerning an increase of the pressure influences on Tc associated with an increase of Tc/Tb for the majority of systems could not be corroborated, however; the situation seems to be more complicated. Obviously the purely chemical contributions can still modify the free volume effects in both directions, i.e. towards increased miscibility as well as toward reduced miscibility. An example of these special chemical influences, still active at high temperatures, can be seen from Table 1 on comparing the results for iso-octane and toluene. Although the boiling points are very similar, the pressure influences on Tc differ markedly. Noteworthy also are the almost identical results for n-heptane and for iso-octane. A more detailed picture of the demixing of PDMA in the homologous alkanes n-pentane, n-hexane and n-heptane is given in Fig. 2 together with the corresponding vapour pressure curves of the pure solvents taken from the literature [5]. The T-dependence of the vapour pressure is also given for the base oil under investigation. For the n-alkanes the critical lines meet the corresponding pressure curves at approx. 20 bar; this limiting pressure decreases as the chain length of the solvent is raised. No demixing can be observed for the PDMA solution in the base oil up to 370°C. This 50

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finding is consistent with the corresponding states arguments which can be directly read from Fig. 2 in the present case. The fact that practically the same vapour pressure is measured for the pure solvent and for the solution was expected in view of the high molecular weight of the solute. For iso-octane, the aliphatic solvent with the highest LCST, measurements were performed up to 360°C. Figure 3 shows the results for various polymer concentrations. As long as the solutions are not heated above 300°C, the experimental data can be reproduced when repeated runs are performed with the same solution. However, if 300°C is exceeded, thermal degradation begins to play an important role, as can be seen directly from the appearance of reddish reaction products. This degradation is demonstrated in Fig. 3 for the solution containing 0.85 Wt~o PDMA by the difference between the data for the first and the second runs; the total residence time at T > 300°C during the first run was approx. 1 hr. The considerable lowering of the demixing pressure after that treatment reflects the reduction in chain length via the corresponding increase in solu-

I

I 350

T (=C)

Fig. 2. Vapour pressure curves (solid lines) of the pure solvents [5] n-pentane, n-hexane and n-heptane, of the pure base oil (Schindler) (open symbols) and of a solution of 15 Wt~o PDMA in it (closed symbols). The broken lines represent the corresponding demixing curves (cf. Fig. 1).

225

250

300

I 350

T (*C)

Fig. 3. High temperature measurements of the demixing of solutions of PDMA in iso-octane of the indicated polymer concentration for the purpose of testing the occurrence and the influence of polymer degradation (cf. text). The dotted line represents the vapour pressure curve of the pure isooctane.

Temperatures of poly(decyl methacrylate) in hydrocarbons bility. This effect can also be seen from the flattening of the demixing curves at the higher temperatures for 0.5 and 0.85wt%. Concentrated solutions behave differently. Although they also become coloured at high temperatures, the demixing conditions are either almost unchanged or the solubility of the polymer is even decreased by the polymer degradation as can be seen from the upturn of the curve for 10Wt~o. The explanation of the above findings probably lies in the fact that the molecular weight of the polymer plays an increasingly minor role when its concentration is raised and in the likelihood that the increasing portion of degradation products, like water, change the properties of the solvent to a measurable extent. Figure 3 also demonstrated the already mentioned comparatively low influence of polymer concentration on the conditions of phase separation. For fixed temp., the demixing pressures differ only by ca. 5 bar for 15 and 0.5 wt% PDMA respectively. CONCLUSION As long as the average molecular weight of the base oil exceeds ca. 300 g/tool, the occurrence of a high temperature demixing of solutions of PDMA in it seems very unlikely at temperatures < 300°C on the

965

basis of the present experimental results. Furthermore, it can be concluded that the thermal degradation of PDMA should remain negligible under the normal working conditions of engines. Acknowledgements--The authors thank the AIF (Ar-

beitsgemeinschaft Industrieller Forschungsvereinigungen e. V.) for financial support and R6hm GmbH (Darmstadt) for the supply of the polymer and the base oil. REFERENCES

1. J. R. Schmidt and B. A. Wolf, Makromolek. Chem. 180, 517 0979); Coll. Polym. Sci. 257, 1188 (1979); H. J. Geerissen, J. R. Schmidt and B. A. Wolf, J. appl. Polym. Sci. 27, 1277 (1982); M. Ballauff, H. Kr~imerand B. A. Wolf, J. Polym. Sci. Phys. (in press); B. A. Wolf, Polymer Yearbook and Almanach 1981 of the Michigan Molecular Institute. 2. E. Maderek, G. V. Schulz and B. A. Wolf. Makromolek. Chem. (in press). 3. F. Herold, G. V. Schulz and B. A. Wolf, Mater. Chem. Phys. 8, 243 (1983). 4. B. A. Wolf and G. Blaum, Makromolek. Chem. 177, 1073 (1976). 5. J. Timmermans, Physico-Chemical Constants of Pure Organic Compounds. Elsevier, New York (1950).