Synthesis and characterization of styrene-b-dimethylsiloxane diblock and styrene-b-isoprene-b-dimethylsiloxane triblock copolymers

Eur. Pol.vm. J. Vol. 22, No. 5. pp. 391 -397. 1986 Printed in Great Brntain. All rights reserved

0014-30577.86 $3.0tl ÷ 0.00 Copyright .~ 1986 Pergamon Press I_td

SYNTHESIS A N D CHARACTERIZATION OF STYRENE-b-DIMETHYLSILOXANE DIBLOCK A N D STYRENE-b-ISOPRENE-b-DIM ETHYLSI LOXANE TRIBLOCK COPOLYMERS S. L. MAI.HOTRA. T. L. BI.UIIM and Y. DESLANDt{S Xerox Research (7entre of Canada. 2660 Speakman Drive. Mississauga. Ontario, Canada L5K 2I.I ( Received 22 August 1985)

A~traet--Poly(styrene-h-dimethylsiloxane) (PS-PDMS) diblock and poly(styrene-h-isoprene-hdimethylsiloxane) (PS-PI-PDMS) triblock copolymers have been synthesized by living anionic sequential addition polymerization in tetrahydrofuran as solvent with n-BuLl as initiator. These copolymers were characterized by gel permeation chromatography (GPC), membrane osmometry, viscometry, nuclear magnetic resonance (NMR) spectroscopy, small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The universal calibration curve of [q].~, vs GPC peak elution volume in methyl ethyl ketone for polystyrene (PS) and poly(dimethylsiloxane) (PDMS) standards was not unique suggesting that this method cannot be used for the calculation of true molecular weights of these diblock copolymers. Number-average molecular weights (Mo) of the diblock copolymers were computed from their uncorrected GPC chromatograms using log M vs elution volume calibration curves of PS and PDMS in combination with the following equation: log '~bl,,~k= Wps log 'Ql's + WmMs log AtPnMSwhere Wps and WpnMs are the weight fractions of PS and PDMS in the block copolymer (Wr.s + 14"pi)Ms= 1). For /13.<4 x 104. the calculated values were close to those measured experimentally. Micelles composed of PS-PDMS diblock copolymer formed in heptane were studied using SAXS. Radii of gyration of these micelles varied as the I/3 power of the block copolymer molecular weight. SAXS measurement of the toluene cast films of the PS-PI-PDMS triblock copolymer yielded lamellar thickness of 54 nm. This value was largcr than that (18.5 35 nm) obtained with TEM.

INTROI)UCTION Block copolymers comprised of polystyrene and poly(dimethylsiloxanc) segments have been synthesized by .sequential polymerization [I-13] as well as by coupling reactions [14] of dianionic or terminally reactive polymers. Molecular weights a n d molecular weight distributions of these block copolymers have been studied by gel p e r m e a t i o n c h r o m a t o g r a p h y in a good solvent, toluene, whose refractive index (n D = 1.494) lies between those of the two c o m p o n e n t s , viz. P D M S (nt>= 1.43) and PS (nt~ = 1.59), of the block copolymer. In the present studies, G P C molecular weight data have been gathered in methyl ethyl ketone ( M E K ) which is a p o o r solvent for P D M S and good solvent for PS, but whose n o (I.377) is lower than those of the two c o m p o n e n t s of the block copolymer. Weight-average (M,,) and n u m b e r - a v e r a g e ( , ~ ) molecular weights of the block copolymers can be c o m p u t e d from the uncorrected G P C c h r o m a t o g r a m s using the summ a t i o n m e t h o d [15] in c o m b i n a t i o n with the R u n y o n [16] and T u n g [17] equation: log '~hl,,~ = Wps log :'Ops + H'PDMSlog 'QPD',ts ( I ) where Wps and 14"m~,~s are the weight fractions o f the two c o m p o n e n t s of the block c o p o l y m e r (Wes + WpDs~s = 1). These can also be calculated using the universal calibration curve [18, 19]. Both a p p r o a c h e s have been considered. In an earlier publication [20] micelles c o m p o s e d of poly(styrene-b-isoprene) (PS-PI) formed in h e p t a n e

were studied using small angle X-ray scattering (SAXS). It was shown that these radii of gyration varied as the I. 2 power of the molecular weight of the copolymer. Similar studies o n micellc f o r m a t i o n in h e p t a n e for P S - P D M S yielded different results and these are presented in this c o m m u n i c a t i o n . SAXS m e a s u r e m e n t s were also m a d e on the toluene cast films of the triblock copolymer P S - P I - P D M S . Values of lamellar thickness were calculated and c o m p a r e d with data obtained using transmission electron microscopy (TEM).

EXPERIMENTAl. ,i tateria/~

Styrene IFisher Chemical Co.). he×ammhyl cyclotrisiloxane (Petrarch Systems Inc.) and isoprene (Aldrich Chemical Co.) were stored over Call, and degassed on a vacuum line for 1 week. These were then distilled under vacuum, the head and tail fractions being discarded. Tetrahydrofuran ITHF: BDH Omnisol) was kept over CaFt, and degassed for I week and subsequently distilled over a Na mirror prior to use. n-BuLi solution in hexane (I .6 M, Aldrich Chemical Co.) v,as used as receivcd. Polystyrene standards of molecular weights between 9iN and 6.0 x 10' were purchased from Prcssure Chcmical Co. Poly(dimethylsiloxane) standards of .~.;1~= 1.03 × 10~ and ,'~, = 4.39 x 10"~ (Aid-2) and .~;~,= 7.18 x 10"~, .'ff~= 3.2 x I1)4 (Aid-3) were purchased fron'l Aldrich Chemical Co. Poly(dimethylsiloxane) samples X-471, X-474. X-475 having polydispersity ratios 1.39, 1.14 and 1.75 respectively were also synthesized by polymerizing 391

S . L . MALHO'rRA et al.

392

hexamethyl cyclotrisiloxane at 25 in T H F with n-BuLi as initiator.

Pol.l'(st t'rene-b-dirneth)'lsilo.x'ane ) Block copolymers were synthesized via anionic sequential polymerization in a simple reusable apparatus employed for styrene-h-isoprene copolymers and described elsewhere [21]. Initiation and polymerization of styrene were carried out at -100 with n-BuLi in T H F as solvent. After 30min of rcaction, a solution of hexamethyl cyclotrisiloxane in THF at - I00 was transferred on to living polystyrene. A colour change from red to greyish white was observed. This mixture was polymerized at 25 ~ for 3 hr and quantitatively removed from the reactor into a beaker containing methanol. The precipitated polymer was filtered through a weighted sintered glass filter, thoroughly washed with methanol and dried in a vacuum oven at 60: to constant weight. 40-50 g of the block copolymers were synthesized using this apparatus.

Poly(styrene-b-isoprene-b-dirnethylsiloxane)(PS-PI-PDMS) Triblock copolymer was also synthesized via anionic sequential polymerization in T H F as solvent and n-BuLl as the initiator. Initiation and polymerization o f styrene were achieved at - 1 0 0 ' . On the addition of pure isoprene monomer, a colour change from red to yellow was observed. This solution was polymerized at 25 ':. Once this reaction was complete, hexamethyl cyclotrisiloxane solution in T H F was added on top of living poly(styrene-b-isoprene). A change from yellow to greyish white confirmed that cross-initiation was complete. This mixture was allowed to polymerize for 4 hr. The triblock was terminated with methanol and the precipitated polymer was dried as described earlier. Prior to each cross-initiation reaction, small samples of the polymers were removed from the reactor for GPC analysis. Some of the block copolymers synthesized have high polydispersity suggesting that they may not be pure and are contaminated with low molecular weight deactivated polystyrene formed in the early stages of the reaction.

system consisted o f 5~-Styragel columns connected in series, each packed with cross-linked polystyrene gel having (by the Waters method) pore sizes of 100, 500, I x 103, I × 104 and 1 x 10~,~,, respectively. The flow of solvent MEK was maintained at 1 ml/min while the concentration of polymer solution was limited to 0.2% in order to render negligible the "concentration effects" on the peak position in the chromatograms. Calibration of the instrument was performed with PS standards as well as with PDMS samples of predetermined molecular weights. Weight-average (/Q,) and number-average (.,~,) molecular weights of the block copolymers were computed from the uncorrected GPC chromatograms using the summation method [15]. The molecular weights of the copolymer were assumed to be weighted averages of the log molecular weights of the homopolymers of the constituent comonomers [16, 17]. X-ray. Small-angle X-ray scattering curves were obtained using a Kratky camera with a 40p entrance slit, a 75p receiving slit and CuK~, radiation (2 = 1.542 A). Scattered intensity was measured by means of a proportional counter equipped with a pulse height analyzer. Micelle solutions (concentration < 20 mg/ml) were placed in beryllium-glass capillaries (2.0 mm diameter) and the data were collected at room temperature. The intensity scattered by a solvent blank in the same capillary was subtracted in each case. Guinier plots were constructed using the slit smeared intensity data. SAXS measurements were also made on a toluene cast film of PS-PI-PDMS triblock copolymer. Transmission electron microscopy. Toluene cast films were sectioned at cryogenic temperature using a Reicbert-Jung ultramicrotome, model Ultracut-E, to obtain sections of about 80 nm thick, in the direction perpendicular to the film surface. When stained sections were needed, they were exposed to vapour of a 2% aqueous solution of OsO4 for 10 min. The sections were examined with a Philips transmission electron microscope, model 400, operated at an accelerating voltage of 100 kV. The size of the different domains were measured on the micrographs. These spacings represent an average obtained by measuring at least ten different micrographs.

Polymer characterization Nuclear magnetic resonance (N M R ). Proton N M R spectra of the block copolymers were measured at 80 and 250 MHz in CDCI 3 using Bruker WP80/54 and WM250 Instruments. Osmometr)' (OSM). Number-average molecular weights ~ , of the block copolymers were determined at 37 ~ in MEK using osmometry. A Knauer vapour-pressure osmometer (type 11.00) was used for samples with 11~,< 2 x 104 g/mol. The instrument was calibrated with ultrapure benzil solution as well as with a polystyrene standard (]Q = 9 x I0~). Molecular weights higher than 2 × I0"*g-mol were obtained with a Knauer membrane osmometer (type 01.00) using gel cellophane membrane (type 0-8). Viscometry. Measurements of the intrinsic viscosity of PDMS homopolymers and poly(styrene-b-siloxane) block copolymers were made in MEK at 25 ~ making use of the flow time measurement data obtained with a Ubbelohde viscometer. Values of the Huggins coefficient kH for the block copolymers were computed from the relation: rhpc= [r/] + k,[~]2C.

RESULTS AND DISCUSSION

Polk' (styrene-b-dirnethylsiloxane ) In o r d e r to use G P C for m o l e c u l a r weight determ i n a t i o n s o f p o l y ( s t y r e n e - b - d i m e t h y l s i l o x a n e ) block c o p o l y m e r s , a c a l i b r a t i o n curve o f [r/]h.~',~. vs e l u t i o n v o l u m e , vc, was p r e p a r e d with p o l y s t y r e n e s t a n d a r d s as well as P D M S s a m p l e s o f p r e d e t e r m i n e d m o l e c u l a r weights (Tables 1 a n d 2, Figs I a n d 2). O n e n o t e s that, in M E K as solvent, PS a n d P D M S d o not follow a u n i q u e curve o f [q]M,~ vs elution volume. Alternatively, o n e m a y use the a p p r o a c h suggested by R u n y o n [16] a n d T u n g [17] w h e r e the m o l e c u l a r w e i g h t s o f the c o p o l y m e r are a s s u m e d to be the w e i g h t e d a v e r a g e s o f the log m o l e c u l a r weights o f

Table I. Solution properties of polystyrene

(2)

Molecular weights for PDMS samples were calculated from viscosity data in toluene at 25 ~ using literature [22] values for the Mark Houwink constants. Calculated values of [r/} for PS standards in MEK were also obtained with literature values of K and ~ [23-25, 30]. Gelpermeation chromatography (GPC). Molecular weight distributions of polymers were measured using a Waters Associates GPC equipped with a high pressure solvent delivery system (Model 6000A), and a differential refractometer (Model R401) operated at 25. The separating

Sample PS PS PS PS PS PS PS PS PS

~x

Elution [r/], volume 10 ~ MEK at 25 C (ml) [~]x ~ x l 0

4.0 9.0 17.5 35.0 50.0 100.0 235.0 390.0 600.0

0.041 0.068 0.102 0.156 0.194 0.298 0.505 0.691 0902

47.5 45.0 42.5 40.5 39.0 36.5 35.0 33.0 31.25

0.16 0.6 I 1.78 5.46 9.70 29.80 118.67 269.50 541.20

~

Styrene (DMS) diblock and styrene (I-DMS) triblock copolymers

393

Table 2. Solution properties of poly(dimethylsiloxane) It/] (dl!g)(25) Sample No. X-471 X-474 X-475 ADL-3 AI.D-2

Toluene 0.085 0.140 OI90 0-275 0.410

MEK 0.079 0.122 0.143 0.182 0.240

GPC GPC data peak . . . . . . evaluation [71 x .9, M* x 10 ~ ,Q, x 10 ~ ,'14,x 10 } M,,'M, volume (ml) xlO ~ 11.0 23.4 37.1 65.2 119.5

12.2 31.3 45.2 74.0 107.0

0.88 27.5 25.5 31.1 65.4

1.39 1.14 175 2.38 1.64

41.5 39.0 38.0 36.5 355

IO0 3.80 6.46 13.2 25.7

M* calculated from [,7] in toluene at 25 using :( ~ 0.658, k = 187 x 10 "dlg. the homopolymers of the constituent comonomers. This requires knowledge of calibration curves, M vs elution volume, for both homopolymers. As PS standards of narrow disperity are readily available, calibration of G P C for this polymer in M E K is straightforward (Fig. I). Commercially available standard samples of PDMS, on the other hand, have broad distributions and although their precise molecular weights are known, their elution volumes are not precise (Fig. 2). In order to overcome this problem, PDMS samples of lower polydispersity were synthesized and characterized by viscometry and GPC (Fig. 2). Due to the polydispose nature of all PDMS samples, the GPC calibration of M vs F< was prepared indirectly using the basic definitions (equations 3 and 4) of weight-(h,~'~) and number-(5,7,) average molecular weights: /

= E w,M,i E w,

X-471

32

34

36

38

40

42

Elution volume l

l

21x106

Fig. 2. GPC

44

46

(ml)

I

l

84x104 80x103 Molecular weight

chromatograms

I 50

48

10x103

of poly(dimethy]siloxane)

samples.

(3)

where W, is the weight of species i of molecular weight M, and can be obtained from the G P C chromatogram. Assuming that the variation of M vs V~ is linear, one can optimize a calibration curve

10 5

which used in combination with equations (3) and (4) yields h7n values similar to those obtained by the classical method of osmometry. One such curve is shown in Fig. 1. One notes that, for identical molecular weights of PS and PDMS, M vs elution volume curves for the two homopolymers are quite different. Use of R u n y o n [16] and T u n g [17] equations requires the GPC chromatograms of the block copolymers (Fig. 3) and the weight fractions of the two components of the block copolymer. These weight fractions can be computed from the N M R spectra of the block copolymers. One such spectra is shown in Fig. 4 for PS-PDMS sample X-483. Aromatic protons of PS appearing between 6.25 and 7.25 ppm, and dimethylsiloxane protons between 0 and 0.25 ppm

10 5

X-484 I I~

105

I 36

1 42

X - (181 / ' ~

¢t

A/\

\\l X Ik I // \7 I\11 \

o 103

~-

102

X-466

L

104

×-4o/i

\/\

/

\/

\

\

102

48

54

Elufion volume (ml)

Fig. I. Plots of log M and log {q]h3.~ vs elution volume for polystyrene and poly(dimethylsiloxane),

32

34

36

38 40

42

44

46

48

50

52

.54

Elution volume (ml)

Fig. 3. GPC chromatograms of poly(styrene-b-dimethylsiloxane) samples.

S . L . MALHOI"RA et al.

394

X-483

I "t

I 6

F

II

I 2

I 1

1 0

ppm Fig. 4. 250 M H z proton N M R spectrum of poly(styrene-b-dimcthylsiloxane) sample X-483.

w e r e t a k e n f o r t h e c o m p u t a t i o n o f Wps a n d WpDMS respectively. T a b l e 3 s h o w s t h e c o m p u t e d G P C d a t a o f M,,, M , f o r t h e b l o c k c o p o l y m e r s . A l s o listed for c o m p a r i s o n a r e t h e ~ . v a l u e s o b t a i n e d via o s m o m e t r y . O n e n o t e s t h a t f o r low m o l e c u l a r w e i g h t s ( < 4 x 104 g / m o l ) t h e t w o sets o f v a l u e s a g r e e well w i t h d a t a o b t a i n e d u s i n g Tung's method. However, for molecular weights g r e a t e r t h a n 4.0 x 104 g / m o l , t h e ,t70 ( O s m ) is c l o s e r to t h e d a t a o b t a i n e d w i t h PS c u r v e a l o n e . T h e disagreement between the calculated and the experim e n t a l ~ , m a y arise f r o m t h e p o o r r e s o l u t i o n in GPC because of MEK being a poor solvent for the block copolymer.

Poly(styrene-b-dimethylsiloxane ) micelles. W h e n a b l o c k c o p o l y m e r is a d d e d to a s o l v e n t w h i c h is selective f o r o n e o f t h e c o m p o n e n t s , t h e f o r m a t i o n o f s p h e r i c a l micelles results. S u c h micelles c o n s i s t o f a core made up of the insoluble component and a h i g h l y s w o l l e n shell i n v o l v i n g t h e s o l u b l e c o m p o n e n t [26]; P S - P D M S in h e p t a n e , a selective s o l v e n t for t h e P D M S b l o c k , is o n e s u c h s y s t e m w h i c h e x h i b i t s micelle f o r m a t i o n . T h e sizes o f t h e s e micelles c a n be d e t e r m i n e d u s i n g S A X S m e a s u r e m e n t [20, 27]. G u i n i e r ' s a p p r o x i m a t i o n o f t h e s c a t t e r i n g c u r v e at very s m a l l a n g l e s c a n be w r i t t e n as: l(h) = I(0) e x p ( - h : R 2 s / 3 )

Table 3. Solution properties of (styrene-b-dimethylsiloxane) diblock copolymers GPC molecular weights GPC molecular weights Tung's method [17] polystyrene calibration Sample % PS [t/] (dl/g), Huggins ,i~, x 10 J . . . . . . . . . . . . . . . . No. NMR MEK at 25" coefficient, k H (Osm) h,7w × 10 ~ h.~',x 10 ~ a4"w/At. ~ x 10 3 /~, x 10 ~ A4,/'/~n X-484" 45 X-481" 13 X-456 62 X-464" 47 X-485 29 X-483" 27 X-466 38 X.-468" 12 X-467 60 X--486 27 X-470" 15 X..469 50 *Block copolymer

0.041 --0.073 -6.9 0. 100 -6.7 0.110 0.47 -0.146 0.83 33.3 0.158 1.08 33.6 0.180 -40.8 0.166 0.87 53.5 0.186 0.37 44. I 0.215 0.73 65.7 0.225 0.40 77.7 0.270 0.34 88.0 samples used in micelle studies.

5.5 13.5 18.0 20.5 44.0 48.0 54.0 50.4 68.0 71.6 80.0 100.0

2.75 5.4 9.6 15.4 30.0 31.0 37.0 33.5 50.0 45.0 50.0 68.6

2.0 2.5 1.9 1.3 1.5 1.5 1.5 1.5 1.4 1.6 1.6 1.3

9.07 20.5 25.0 32.7 69.6 76.0 80.5 87.0 87.0 106.0 123.5 128.0

7.0 14.8 15.0 26.5 53.7 56.7 59.0 67.0 67.9 78.2 89.0 96.6

1.3 1.4 1.7 i.2 1.3 1.3 1.4 1.3 1.3 1.4 1.4 1.3

Styrene (DMS) diblock and styrene (I-DMS) triblock copotymers where:

395

PS-PI

h = 4zr0 ,~;.,

v,

0 is one-half the scattering angle and 2 is the wavelength of the incident radiation. The radius of gyration, Rs, is obtained from the slope of a plot of In I (tl) vs h-'. It should be noted here that this treatment according to Guinier's Law is valid only for h:R28 < I. In a previous report [20] it was shown that the radius of gyration of micelles measured in this way is not the true total radius of the micelle. Due to interference from the swollen micelle shell, the measured radius of gyration lies between that of the micelle core and the total micelle (core plus shell). The relationship between the total micelle radius and the measured radius depends on the volume fraction of polymer in the highly swollen shell, which in turn depends on the X parameter between the polymer in the shell and the solvent, in this case heptane. From a log--log plot of total copolymer numberaverage molecular weight, 3,3",, vs the radii of gyration, Rx+ the power law relating 317, to micelle radius of gyration was found to have the power equal to ~1/3 (Fig. 7). The scatter in the data may be attributed to the fact that the copolymers vary somewhat in composition and polydispersity (Table 3). This power law dependence differs from that found previously for poly(styrene-b-isoprene) micelles [20] where it was ~ I/2. The smaller value of 1/3 for the poly(styrene-b-dimethylsiloxane) micelles indicates that the micelle sizes are not increasing as rapidly with molecular weight as in the case of poly(styreneb-isoprene) micelles in heptane. We may speculate that the lower power in this case may be connected with the fact that the shell polymer block makes up a smaller proportion (12-47%) of the copolymer than in the previously studied case of poly(styrene-b-

PS-PI-PDMS

,-,'5

.c_ u

F'S

32

J 34

56

42

44

E l u t i o n volume

58

40

(ml)

46

isoprene) where the isoprene blocks comprised 33-69% of the copolymer. However, it must be emphasized that the difference in the power law for the two systems is not. at present, fully understood.

PoO, (styrene -b-isoprene -b-dimethylsiloxane ) Figure 5 shows the GPC molecular weight distributions of the PS-PI-PDMS triblock copolymer as well as the individual distribution of the PS-PI diblock and PS homopolymer segments of the triblock. NMR analyses of the PS-PI-PDMS block copolymer (Fig. 6) showed it to be made up of 23% PS, 20% PI and 57% PDMS. Solution properties of the homopolymer PS. diblock PS-PI and the triblock PS-PIPDMS are presented in Table 4. One notes that as ; ~ of the living polymer increases from 2.52 × 104 (PS

X -482

/ I

L

1

I

6

5

4

3

I

I

1

2

I

0

nDm

Fig. 6. 250 MHz proton spectrum of pol)(styrene-h-isoprene-h-dimethytsiloxane) triblock. P J 22 ~ [)

50

Fig. 5. GPC chromatograms of poly(styrene-h-isoprene-hdimethylsiloxane) as well as those of poly(styrene-hisoprene) and homopolymer polystyrene segments of the triblock copolymer.

F

7

48

S.L. MALHOTRA et al.

396

Table 4. Solution properties orpolystyrene, poly(styrene-isoprene) and poly(slyreneisoprene-dimethylsiloxane) in M E K as solvent at 25 G P C molecular weights •" h.~'~ x 10 3 .Q. x 10 -3 ~tw;M n

Polymer PS PS-PI PS-PI-PDMS

25.2 33.3 65.6

12.4 24.0 42.7

2.0 1.4 1.5

homopolymer) to 3.33× 104 (PS-PI block) to 6.56 x 104 (PS-PI-PDMS triblock), the intrinsic viscosity increases from 0.134 to 0.154 to 0.184, whereas the Huggins coefficient did not change greatly and was very low. This result suggests that MEK at 25: is very close to being a solvent for the diblock PS-PI as well as the triblock PS-PI-PDMS copolymcr. JQ,, and AT, shown in Table 4 were computed using M vs elution volume cure for PS. M, of 4.27 x 104 of the triblock is very close to that of 4.66 x 10 4 measured by osmometry. TEM analyses o f P S - P I - P D M S triblock. The morphology of both the PS-PI-PDMS triblock and the PS-PI diblock (which was actually used as precursor to the triblock) were examined using transmission electron microscopy (TEM). Figure 8 shows two electron micrographs of the PS-PI-PDMS triblock before (A) and after (B) staining with OsO4. The morphology adopted by the copolymer is lamellar. In the case of the unstained section, the dark phase corresponds to the PDMS phase which, because of the higher absorbing power of the Si atom, provides good scattering contrast without staining. In the case of the stained section, the dark layer contains both PDMS and PI phases, while the bright area corresponds to the PS phase. The lamellar spacing (sce Fig. 8) measured from thc micrographs was found to be 25.5 + 4.5 nm. No significant differences were observed between the spacings of both the stained and the unstained sections. The average thickness of each individual layer is 10 + 2 nm for PS. 7 + 1.5 nm for PI and 8.5 + 1.5 nm for PDMS. The morphology of the corresponding PS-PI diblock is also lamellar. The spacing is 15.5 __+2rim. The average thickness of the PS layer was found to be 9 + 1.5 nm while the thickness of the PI layer was 6.5 + 1.5 nm thick. These values compare wcll with those obtained for the corresponding PS-PI segment in the triblock. It is an indication that the presence of

[r/}, M E K at 2 5 (dig)

Huggins coefficient k u

0.134 0.154 0.184

0.15 0. I 0 0.10

the PDMS did not affect significantly the morphology of the PS-PI segment. There is always a large error associated with the measurement of the spacing from electron micrographs which may explain why the domain sizes reported hcre seem to bc always smaller than the values (54 nm) obtained by SAXS. In the case of the unstained section of the triblock, spacings measured on different micrographs ranged from 18.5 to 35 nm. As suggestcd by Handlin and Thomas [28], variations in the angle of the plane of the film to the microtome knife and the tilt angle of the interface to the electron beam are partly responsiblc for the differences. Com-

i

/"

.1'

j/

A

.

,,

\,,

,\

3.0 PS-PI i" I ~

i~ ~ 2.5

PS-PDMS

2' 20

I.~

5.0

I

L

I

I

3.5

4..0

4.5,

5.0

I .5.5

logic M N

Fig. 7. Log-log plot of the radius of gyration ,RM of the micelles and the experimentally determined (by Osm) ~ , of the copolymer.

B

Fig. 8. Transmission electron micrographs of PS-PI-PDMS triblock copolymer before (A) and after (B) staining with OsO4.

Styrene (DMS) diblock and styrene (I-DMS) triblock copolymers pression of the layers d u r i n g m i c r o t o m i n g can result in the o b s e r v a t i o n of lamellae that are actually thinner than the " t r u e " lamellae as d e m o n s t r a t e d by Odell et al. [29]. Interestingly, m e a s u r e m e n t of the spacing of the triblock before a n d after staining with OsOa seem to indicate that no real change occurred in the phase dimensions due to staining. This is in agreement with H a n d l i n and T h o m a s [28] who performed such a c o m p a r i s o n on PS-PI-PS triblock using phase contrast imaging.

CONCLUSIONS The principal conclusions to be d r a w n from this study may be s u m m e d up as follows: I. 40-50 g of poly(styrene-b-dimethylsiloxane) with polydispersities ranging between 1.5 and 2.0 can be synthesized with the reusable polymerization apparatus [20]. 2. Gel p e r m e a t i o n c h r o m a t o g r a p h y analyses of poly(styrene-h-dimethylsiloxane) in MEK on t~-styragel columns c a n n o t employ the universal calibration curve for c o m p u t i n g .,~ or M, as the two c o m p o n e n t s of the diblock c o p o l y m e r do not satisfy this curve. 3. W h e n M vs elution volume curves are k n o w n for b o t h h o m o p o l y m e r s , c o m p u t a t i o n o f ~,, a n d JQn for the block copolymers with low molecular weights may be o b t a i n e d from the G P C c h r o m a t o g r a m s using the R u n y o n [16] and T u n g [17] equation. 4. Variations in the radii of gyration for micelles formed in h e p t a n e by P S - P D M S and PS-PI block copolymers of equivalent JQ, may be attributed to the differences in the c o m p o s i t i o n of the shell materials in the two systems. 5. In the T E M m e a s u r e m e n t s of the triblock copolymer, OsO4 staining did not affect the lamellar Ihickness. 6. Smaller lamellar thickness in T E M analyses of the triblock as c o m p a r e d to that by SAXS may partly be attributed to m i c r o t o m e sectioning used in the ['ormer technique [29]. X-ray and T E M studies of triblock copolymers v,ith various c o m p o s i t i o n of PS, PI and P D M S segments are planned for better u n d e r s t a n d i n g of the system. .Icknowh,dgemcnts The authors are grateful to M. Breton and Carol Low for the osmometry data. They thank G. Ilamer for running the NMR spectra on polymers.

3'-)7

REFERENCES

1. J. C. Saam, D. J. Gordon and S. l.indsey..'da(.romolecules 3, 1 (19701. 2. D. S. Brown. K. U. Fulcher and R. E. Wetton. P,;lvm. l,ett. 8, 659 (1970). 3. J. W. Dean, Polvm. Lett. 8, 677 (19701. 4. D. G. Lcgrand. J. Polym. Sci. B 8 , 195 (1970). 5. A. Skoulios. Macromolecules 4, 268 ( 1971 ). 6. W. G. Davies and D. P. Jones, Intl. Engne ('hem. Prod. Res. Del'l 10, 168 (10711. 7. H. G. Kim. Macromoh'cule.s 5, 594 (197"). 8. C. Price, A. (i. Watson and M. 1. Chow. Polvm,:'r 13, 833 (1972). 9. F. R. Jones. Eur. Polvm. J. 10, 249 (1974). 10. M. Morton. Y. Kestcn and L J. Fetters, .4ppl. t'olvm. Syrup. 26, 113 (19751. 1I. A. Marsiat and Y. Gallot. Makromoh,k. Chem. 176, 164 (1975). 12. P. Bajaj. S. K. Varshney and A. Misra, J. Polvm. Sol.. Polym. C/wm. Ed. 18, 295 (19801. 13. S. Krausc. M. lskandar and M. lqbal, Macromoh'cuh'.s IS, 105 (1982). 14. G. Zilliox. J. E. Roovers and S. By~ater, .~lacromolecules 8, 573 (1975). 15. Waters Associates, Gel Permeation Chromatograph Instruction Manual, Bull. No. 2, p. 2064 (1966). 16. J. R. Runyon, D. E. Barnes, J. F. Rudd and L. H. Tung, J. appl. Polym. Sci. 13, 2359 (1969). 17. L. H. Tung, J. appl. Polym. Sci. 24, 953 (1979). 18. A. Dondos, P. Rempp and |I. Benoit. Makromoh,k. Chern 130, 233 (1969). 19. N. Ho-Duc and J. Prud'homme, Macromoh, cuh's 6, 472 (1973). 20. T. L. Bluhm and S. L. Malhotra, Eur. Polvm. J. 22, 249 (1986). 21. S. L. Malhotra, M. Breton and J. I)utL J. :~tacromol. Sci. Chem. A20, 733 (1983). 22. J. Brzezinski, Z. Czlonkowska-Kohutnicka, B. Czarnecka and A. Kornas-Calka. Eur. Polvm. J. 9, 733 (1973). 23. C. E H Bawn, C. Freeman and A. Kamaliddin, Fran.~. Faradtzv Soc. 46, 1107 (1950). 24. P. Outer, C. I. Carr and B. H. Zimm. J. chem. Phys. 18, 830 (1950). 25. J. Oth and V. Desreux, Bull. Sin.. chim Belg. 63, 285 (1954). 26. Ch. Sadron and B. Gallot, Makrmmdek. ('hem. 164, 3 [0 (1973). 27. J. Plestil and J. Baldwin. Makromoh'k. ('hem. 176, 1009 (1975). 28. D. L. Handlin and E. I.. Thomas, Macr,moh'cz,h'.~ 16, 1514 (1983). 29. J. A. Odell, .I. Dlugosz and A. Keller. J. l'olvm. S¢;.. Pol~m. Phys. Ed. 14, 861 11976} 30. J. Prudhomme, J. E. L. Roovers and S. By~,atcr, Eur. Polvm. J. 8, 901 [1972).