polyisoprene block copolymers

An investigation of the effect of chain geometry on the two-phase morphology of polystyrene/polyisoprene block copolymers C. Price, A. G. Watson and Mei T. Chow Department of Chemistry, University of Manchester, Manchester M13 9PL, UK (Received 14 December 1971) Block copolymers having structures of the type A-B, (A-B-)2X, (A-B-)3Y and (A-B-)4Z, where A is polystyrene, B is polyisoprene, and X, Y and Z are linking agents, have been synthesized by anionic polymerization. Electron microscopy of thin films cast from benzene solutions showed that all the polymers formed ordered arrays of regular domains. The inter-domain distances and domain radii of the two phase structures were not influenced by changes in chain geometry, provided the overall composition and the length of the A blocks remained constant. Some results are reported concerning the compatibility of polystyrene/ polyisoprene and polystyrene/polybutadiene block copolymers. INTRODUCTION Microphase separation in many block copolymers has been shown to give rise to ordered arrays of regular domainsl, L For the case of styrene-isoprene block copolymers [noue et al. 3 showed that for solvent cast films the two-phase structure obtained is governed by the incompatibility between the A and B segments, the solvation of the segments in solution, the casting temperature, the total chain length of the block copolymer and the overall composition. The three types of domains most often observed were cylinders, spheres and lamellae. Supporting evidence can be drawn from the work of Matsuo et al. 4 on styrene-butadiene block copolymers and from a wide range of general studies carried out by Skoulios and his colleagues 5. For the case of A-B block copolymers several theories have been put forward which attempt to establish an interrelationship between the type and size of domains and the important molecular, or thermodynamic, variables3, 6. The relationships obtained have been found to provide a useful semiquantitative understanding of the observed behaviour. The theoretical problem clearly becomes more difficult if we move on to consider A - B - A and other multi-block systems. However, it has been tentatively suggested that simple changes in chain geometry (such as going from A-B to an A - B - A polymer whilst keeping the length of the A blocks and the overall composition constant) may not have a significant effect upon the tertiary structure, i.e. on the type, size and arrangement of the domains 6. This does not mean that changes in chain geometry will not influence the mechanical behaviour of the polymer. Indeed it is well established that for polymers in which B is a long block of a rubbery polymer and A is a shorter block of a glassy polymer, the A - B - A system can have properties similar to a conventionally crosslinked elastomer (due to the formation of a continuous network) whilst the A-B system behaves like an uncrosslinked rubber. In the present study we have prepared and characterized two series of styrene/isoprene block copolymers of the type A-B, (A-B-)2X, (A-B-)aY and (A-B-)4Z, where A is polystyrene, B polyisoprene and X, Y and Z

short linking agents; the overall composition of each of the polymers was ~ 2 5 ~ by weight of polystyrene, and within each series the lengths of the A blocks were the same. The overall aim of the work of which this paper constitutes the first report, is to investigate experimentally the influence of chain geometry on domain morphology and physical properties. In recent times a considerable amount of research has been carried out in synthesizing and characterizing star-shaped homopolymers. Thus, tetra-chain star-type polystyrenes have been synthesized by the reaction of polystyryl lithium with silicon tetrachloride 7 and with 1,2,4,5-tetrachloromethyl benzene and by the reaction of polystyryl potassium with the latter s, 9. Tri-chain polystyrenes have been prepared by the reaction of polystyryi lithium with 1,2,4-trichloromethyl benzene TM and methyl trichlorosilane la. In the present study we used n-butyl lithium as the initiator in preparing an isoprenyl lithium-ended polystyrene-polyisoprene block copolymer and chose the series of silicon compounds, SiCl4, SiClaCHa, SiCI2(CH3)2 as linking agents. are

EXPERIMENTAL AND RESULTS Materials Styrene m o n o m e r . Styrene (BDH, Laboratory Reagent Grade) was first dried over calcium hydride, and then degassed and distilled. The middle fraction (,,~75~o by wt.) was collected and allowed to stand over lithium aluminium hydride for 48 h and then it was distilled in a vacuum system on to a freshly prepared sodium mirror. As soon as polymerization began to take place monomer was distilled off and sealed into ampoules. Isoprene m o n o m e r . Isoprene (BDH, Laboratory Reagent Grade) was also dried over calcium hydride, degassed and distilled, and then the middle fraction allowed to stand over calcium hydride for 48h. The monomer was next introduced into a small vacuum system consisting of a reservoir and a series of ampoules. n-Butyl lithium was added until the characteristic colour of poly(isoprenyl lithium) appeared. The ampoules were cleansed of reactive impurities by washing with the polymerizing mixture and rinsed by repeated distillation

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333

Effect of chain geometry on morphology of block copolymers : C. Price et al. of monomer from the reservoir. Finally known volumes of isoprene were distilled into the ampoules and these sealed off. Linking agents. These were tetrachlorosilane, trichloromethylsilane and dichlorodimethylsilane. They were each distilled several times in a vacuum system before being introduced into ampoules attached to the main apparatus. Benzene. This was dried over calcium hydride and fractionally distilled. After degassing on the vacuum line it was stored ready for use. n-Butyl lithium. This was obtained as a 15~o solution in hexane from the Koch-Light Chemical Co. and was used without further purification. The concentration of the n-butyl lithium solution was estimated using the method of Gilman and Haubein 12.

Polymerizations The technique used was somewhat similar to that developed by Bywater et al. 13. Reactions were carried out in benzene solution at approximately 25°C in a sealed vacuum system. All glassware was first washed with a polystyryl lithium solution and rinsed by repeated distillation of solvent from a reservoir. So as not to employ greased joints extensive use was made of break seals. The all glass reaction vessel is shown in Figure 1. Initially it was attached to the vacuum line at I, and ampoules G and H contained known volumes of styrene and isoprene. 400cm 3 of benzene were distilled into bulb B, and the reaction vessel was sealed off at J. About l cm 3 of styrene was introduced into the apparatus via serum cap K followed by n-butyl lithium until the characteristic red colour of polystyryl lithium persisted. The resulting solution was washed around the vessel and then run back into flask B. Repeated distillation of benzene from flask B to all the extremities of the apparatus served to rinse out any residual polystyryl lithium. The solution was then stirred for some time to ensure all the styrene had polymerized, after which benzene was distilled from flask B to flask A and flask B sealed off at the constriction L. The styrene ampoule G was broken using a glass breaker, and a known quantity of catalyst injected through serum cap F to initiate polymerization. The serum cap was removed by sealing it off at the constriction and the solution, which gradually became orange-red in colour, was stirred for 4h. At the end of this period a small sample was removed at M, and after it had been terminated with methanol, it was analysed by gel permeation chromatography (g.p.c.). The next stage of the polymerization was started by breaking the isoprene ampoule H. The solution, which became immediately pale yellow in colour, was left to stir for 24h. At the end of this period a sample of the styrene-isoprene block copolymer was removed (via ampoule N) terminated with methanol, and analysed by g.p.c. Provided the g.p.c, analysis indicated a sharp molecular weight distribution, the remaining solution was divided as equally as possible between the graduated flasks C, D and E, and the flasks sealed off at the constrictions. To each of the flasks, via a break seal, was added one of the coupling agents SiCI~, SiCI3CHa and SiCI2(CH3)~. In each case it was arranged that the molar ratio isoprenyl lithium to silicone-chlorine bonds was 1.25. The mixtures were stirred for 4 days after which time any residual anions

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were terminated by adding methanol, and the polymers were subjected to g.p.c, analysis.

Fractionation The polymers were fractionated at 25°C using the method of successive liquid-liquid phase separation. Briefly successive quantities of methanol were added to dilute solutions of the polymer in benzene ( ~ 0 . 2 ~ by wt. concentration). The liquid phases so obtained were equilibrated using the usual heating and cooling cycle and then separated by syphoning off the dilute phase. Approximately ten fractions were collected in each fractionation case and these were isolated from solution by freeze drying. The number-average molecular weights of the fractions were determined using a 'Hi-Speed' C5M2 recording osmometer fitted with a Sartorius filter (pore size L 50 A). The measurements were carried out in toluene solutions at 25°C; approximately an hour was required for each solution to reach equilibrium. The fractions (in tetrahydrofuran solution) were also analysed by gel phase chromatography using a Waters instrument. This technique was employed primarily to establish the homogeneity of fractions rather than to assess molecular weight. In Figure 2 a g.p.c, trace for a fraction is compared with that for an unfractionated sample. The fractionation technique outlined was found to provide a sharp fractionation of species on the basis of chain geometry. A typical set of data are given in Table 1 for a polymer prepared using the tetrafractional linking agent. After completing such fractionation and molecular weight analysis the fractions corresponding to the dominant species were combined together, whilst the rest of the fractions were discarded. The number-average molecular weight of each of the polymer samples isolated by this procedure are recorded in Table 2.

i ~,

N

(

H

( G

t

rL

Figure I Apparatus used in the synthesis of the block copolymers

Effect of chain geometry on morphology of block copolymers : C. Price et al. Table 2 Molecular weight and electron microscopy (EM) results for the two series of styrene/isoprene block copolymers Series 1

Sample Type

1P1

1P2 1P3 1P4

AB (AB)2X

% Volume polystyrene D dint M n x l 0 -4 (nm) (nm) from EM assuming from chain h.p.c, structure 4.81 9.8

( A B ) 3 Y 14.2 ( A B ) 4 Z 18.8

18.7 17.0

36.0 33.7

24.5 23.1

23'5 23'0

18.1 33.7 26.2 18.6 34.1 27.0

23.9 24-0

26'9 26'0 26"2 26"0

22'9 23"8 24"1 24"2

Mn of the A block=12 500 Series 2 25

2P1 2P2 2P3 2P4

AB (AB)2X (AB)3Y (AB)4Z

9'6 18"5 27"5 36" 5

48'1 28'4 49'0 25"5 48"6 26"4 50'0 24'5

Mn of the A block=24 400 D=domaindiameter;dint=interdomain gonally packed array of cylinders

Electron microscopy Electron micrographs of ultra-thin films were obtained using an AE[ EM6G electron microscope. The instrument was operated at an accelerating voltage of 100 kV under which conditions the stated resolution was better than Into. Ultra-thin films were prepared by carefully evaporating solutions of the polymers in benzene. The concentration of the solutions used was calculated to give a film thickness of 50nm. The films were stained using osmium tetroxide, which selectively combines with the olefinic bonds of the polyisoprene chains. On using a fairly slow rate of evaporation and casting from mercury, each polymer gave regular hexagonal arrays of 'circular' domains of the type shown in Figure 3. At larger fields of view, grain structures were clearly visible. On varying the method of casting more irregular arrays of the type described previously by one of us 14 were observed, but in the present study they were not analysed. For the regular arrays the measured domain radii and inter-domain distances (i.e. nearest distance between centres) are given in Table 2.

<3

y

20

Ill

distance; h,p.c,=hexa-

I E[ution volume

Figure 2 G.p.c. traces for sample 2P4 before (upper) and after

DISCUSSION

(lower) fractionation

Table 1 Fractionation from benzene-methanol mixtures at 25°C of a copolymer synthesized by coupling an AB polymer with SiCI4

Fraction no.

Vol. methanol Weight %

Vol. benzene

g.p.c. suggests

0' 452 0.465

(A B)4Z (A B)4Z

0.475

4 5

39" 04 9.13 29.17 9.50 0.89

6 7 8

1.34 5.49 5.41

(A B)4Z (AB)3Y (AB)3Y (AB)3Y AB, (AB)3Y A, AB, (AB)3Y

1 2

3

0.482 0.491

0.523 0. 576 Residue

Mnx10 -4 18.8 18.6 18.8 14.2

The AB polymer isolated before the addition of SiCI4 was found to have Mn=48 100 Fractions 1 and 2 were combined together and designated sample 1P4.

Let us first consider the evidence which will enable us to make assignments to the two-phase structures of the polymers we have studied. If we assume there is complete phase separation of the two components, the volume fraction of polystyrene may be calculated by two independent methods. Firstly we can calculate it from a knowledge of the chain geometry and block lengths, and secondly from measured area fractions assuming a particular domain structure (spheres, cylinders or lamellae). From such a comparison we conclude that the two-phase structures of all 8 polymers (listed in Table 2) are consistent with a model in which cylinders of polystyrene are hexagonally packed in a polyisoprene matrix. The data are given in Table 2. For reasons we have discussed in detail previously the use of ultra-thin solvent cast films to investigate morphological features is not completely satisfactory t4,1'~'.

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Effect of chain geometry on morphology of block copolymers : C. Price et aL

Figure 3 Electron micrograph of solvent cast film of sample 2P4

Figure 4 Electron micrograph of solvent cast film 50/50% by weight mixture of sample 1P4 and an SBS polymer

Nevertheless, provided a suitable solvent is chosen, information obtained by this method in the past has been found, in the light of more detailed studies involving low-angle scattering 15 and electron microscopy of ultramicrotomed sections 1~, to provide a very useful guide to the actual behaviour of the bulk polymer. For the purpose of the present investigation the main advantage of using solvent cast film is that the technique is simple to apply, and very reproducible. Hence any small differences in the two-phase behaviour of the samples could readily have been detected. Examination of Table 2 shows that within experimental error the cylindrical radii and inter-domain distances are independent of the changes in chain geometry we have made provided the polystyrene end-block lengths and the overall composition remain constant. This can be interpreted to mean that the configurational entropy of the matrix plays only a secondary role in determining the two-phase morphology. Each (A-B-)4Z polymer molecule has the possibility of occupying by way of its four A end-blocks, from one to a maximum of four separate domains. In practice one would expect to find a statistical distribution. Each A-B polymer molecule on the other hand can only occupy one domain. Thus moving along the series from A-B to (A-B-)4Z the degree of constraint on the rubbery chains is progressively increased. This increase can be expected to be quite marked in view of the fact that the large cylindrical domains will have a very much lower mobility than that normally associated with the low functionality chemical crosslink encountered in the case of rubbery vuicanizates. In spite of this very little change in structural behaviour is observed moving along a series. Comparison with styrene/butadiene block copolymers Listed in Table 3 for comparison are results we

obtained in a previous study for a (polystyrene)(polybutadiene)(polystyrene) block copolymer 15, where Mn fort'his polymer is 84000 and for the polystyrene blocks l l 000. Perhaps not surprising in view of the similar nature of polyisoprene and polybutadiene the domains are seen to compare closely with those observed for sample 1P2. In view of this similarity we decided to make up a 50% by wt. mixture of the two polymers, and then reinvestigate the structure. Because of the incompatibility problem we expected to observe the usual

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POLYMER, 1972, Vol 13, July

Table 3

Electron microscopy

results Sample

D (nm)

dint (nm)

1P2 SBS 1P2/SBS

17"0 It0-0 18"2

33"7 43"0 34"5

macroscopic separation of the two polymers. However, under the conditions of our solvent casting technique the two polymers appear to blend together to form a single two-phase structure. If anything the ordering of the cylindrical domains was even more regular than for the case of the individual block copolymers (see Figure 4). The characteristic dimensions for the mixed polymer structure are given in Table 3. To what extent the polybutadiene and polyisoprene chains are blended together within the matrix has not yet been established by us, and is currently being subjected to a more detailed investigation. ACKNOWLEDGEMENTS We wish to acknowledge the assistance of the departmental glassblowers, Mr P. Le Pinnet and Mr D. Greenhalgh. During the course of investigation A.G.W. was supported by a grant from Dunlop Research Ltd and M.T.C. by SRC. REFERENCES 1 2 3 4 5 6 7

Matsuo, M. Japan Plastics 1968, 19, 6 Lewis, P. R. and Price, C. Nature 1969, 223, 494 Inoue, T., et. al. J. Polym. Sci. (,4-2) 1969, 7, 1283 Matsuo, M., .Sagal, S. and Asai, H. Polymer 1969, 1O, 79 Skoulios, A. Adv. Colloid Interface Sci. 1967, 1, 3 Meier, D. J. J. Polym. ScL (C) 1969, 26, 81 Morton, M., Helminiak, T. E., Gadkary, S. D. and Beuche, F. Conf. High Temperature Polymer Fluid Res. 1962, 1, 165 8 Altares, T., et. al. J. Polym. Sci. (A) 1965, 3, 4131 9 Yen, S.-P. S. Polym. Preprints 1963, 4, 332 10 Wenger, F. and Yen, S.-P. S. Polym. Preprints 1962, 3, 162 I 1 Zelinski, R. P. and Wofford, C. F. J. Polym. Sci. (A) 1965, 3, 93 12 Gilman. H. and Haubein, A. H. J. Am. Chem. Soc. 1944, 66, 1515 13 Worsfold. D. J. and Bywater, S. Can. J. Chem. 1960, 38, 1891 14 Lewis, P. R. and Price. C. Polymer 1972, 13, 20 15 Lewis, P. R. and Price, C. Polymer 1971, 12, 258 16 Dlugosz, J. et. al. Kolloid-Z. 1970,242, 1125