Morphology and surface properties of block copolymers of poly(α-methylstyrene) and poly(2-hydroxyethyl methacrylate)

Morphology and surface properties of block copolymers of poly(α-methylstyrene) and poly(2-hydroxyethyl methacrylate)

Morphology and Surface Properties of Block Copolymers of Poly(o~-methylstyrene) and Poly(2-hydroxyethyl Methacrylate) S E U N G K O O K A N G AND M U ...

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Morphology and Surface Properties of Block Copolymers of Poly(o~-methylstyrene) and Poly(2-hydroxyethyl Methacrylate) S E U N G K O O K A N G AND M U S H I K J H O N 1 Department of Chemistry and Centerfor Molecular Science, Korea Advanced Institute of Science and Technology, P.O. Box 150, Cheong Ryang Ni, Seoul, Korea

Received August 22, 1990; accepted November 29, 1990 In this study, we have used XPS to elucidate the surface topography and copolymer composition of a series of poly(a-methylstyrene) (Pc~-MS)and poly(2-hydroxyethylmethacrylate) (PHEMA) block copolymers films. Contact angle can be presented for block copolymer surface composition of Pa-MS/ PHEMA block copolymersat the air-polymer and the water-polymer interfaces. The results show that the Pa-MS concentration at the air-polymer interface is substantially higher than the known bulk concentration of P~-MS, whereas contact angle measurements at the water-polymer interface show the surface enrichment in copolymerof hydrophilic PHEMA. These resultsindicatethat the Pa-MS/PHEMA block copolymer surfaces are not overlayers. By combining scanning electron spectroscopy(SEM), we see that surfacetopographyof Pa-MS/PHEMA block copolymersmatchesthe bulk morphology.Models are presentedfor the 3-D morphologyof the sampleconsistingof discreteregionsof Pa-MS and PHEMA. © 1991 Academic Press, Inc.

INTRODUCTION Block copolymers have long sequences of one chemical structure type of repeating unit, A, joined at one or both ends to long sequences of another type, B. The dissimilar nature of the two blocks combined with the fact that they are chemically linked to each other manifests in a variety of surface and bulk properties quite differently from those of the corresponding homopolymeric systems (1). The surface properties of block copolymers originate from the difference in the surface free energies of the c o m p o n e n t blocks. In m a n y studies ( 2 - 5 ) it showed that in a blockcomponent system, components of lower surface free energy would tend to enrich the surface of a condensed phase. Thus in a phase-separated block copolymer, segments of lower surface free energy preferentially segregate to the air or vacuum surface. The unique bulk properties of block copolymers also arise from their microphase-separated morphology. Due to incompatibility each block tends to generate 1To whom correspondence should be addressed.

separate domains that are restricted to submicroscopic sizes (5-1000 nm) by the chemical link between the blocks. The morphological configuration depends on the parameters mentioned above; m a n y unique microphase structures are possible, giving rise to a variety of properties. X-ray photoelectron spectroscopy (XPS) made it possible to obtain direct chemical information on the top few molecular layers of polymeric surfaces. Studies on pure block copolymers have shown that the surface m a y differ from the bulk in chemical composition and morphology. O'Malley and T h o m a s used angular-dependent XPS to determine surface compositions and topographies of poly(ethylene oxide) ( P E O ) / polystyrene (PS), diblock (6), and triblock (7) copolymers. F r o m the angular-dependent data, they were able to deduce that the surface morphology consisted of cylindrical PS domains slightly elevated above PEO domains. Recently, Schmitt et al. (8) have shown surface enrichment by poly(dimethylsiloxane) ( P D M S ) in bisphenol A polycarbonate ( B P A C ) / P D M S

390 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

SURFACE PROPERTIES OF BLOCK POLYMERS

block copolymers with varying compositions. Using XPS and ion scattering spectroscopy (ISS), they concluded that the morphology of the top 50 A of the surface consisted of discrete regions of PDMS and PBAC oriented perpendicular to the surface. Several earlier efforts involved measurements of contact angles on solids to explain the chemical nature of surfaces (9-12). The method for measuring the surface energy of solid and for resolving the surface energy into contributions from dispersion and polar forces has been developed ( l 3-15 ). In air measurements, the extended Girifalco-Good-Fowkes equation is applied to the Young-Dupre equation, 3~LV(1 + COS0SL) 2 ( T ~ v ) I/2 / ' y ~ v ' ~ 1/2 =

'/2 +

'/2

,

[1]

where d and p are the dispersion and. polar fractional contributions to the total surface free energy and 3'LV is the surface tension at the interface between liquid and vapor phases. The total surface tension (Ts) of surfaces was obtained from the measured contact angle versus the surface tension for a series of specified (16) liquids. When a straight line is drawn through the plotted data points, using a least-squares fit, the intercept is equal to (3,sd)1/2 and the slope is equal to (y~)1/2. In water measurements, the harmonic mean equation for obtaining fluid-solid interface energy is as follows:

"r v

(

+ "r wv

+ 'r .vJ

= q/LwCOS0 + 2/WV-- TLV 4

[2]

Here, as a special case, we choose the octanewater-solid and air-water-solid systems for simple calculation (17).

391

In the present work, we obtain data from combined XPS and contact angle measurements of the surface of block copolymers of a-MS and HEMA. Quantitative interpretations by contact angle measurements at the air-polymer and the water-polymer interfaces can be obtained by dispersion and polar contributions to the total surface free energy for each copolymer. The results obtained by XPS and contact angle measurements will be used to quantify the near-surface region of block copolymers of Pa-MS/PHEMA. The information proves useful in understanding the surface property and morphology of these materials. Clearly, there is no general agreement about either the topography or separated microdomain structure of block copolymer surfaces. So far, the experimental means used to draw conclusions about these important morphological aspects have some deficiencies. The surface and bulk morphologies of films usually have not been specified adequately. In order for the surface phenomenon in block component, block domain systems to be understood, an important question is how do the surface composition and morphology relate to their counterparts in the bulk? In an attempt to gain further insight, XPS(0) was used to obtain the chemical composition gradients at the surfaces. Scanning electron spectroscopy (SEM) ( 18, 19 ) was used to elucidate the microdomain structural features of the bulk and surface. In this study, representative surface topography from the XPS (0) studies reported herein was examined at SEM, and these results are presented for the 3-D morphology of PaMS/PHEMA block copolymers. Surface properties of a block copolymer by the combination of o~-MS and HEMA monomers have stimulated a great deal of interest since the block copolymer generally leads to heterophase materials in which hydrophobic and hydrophilic domains are separated (20). The block copolymers of a-MS and HEMA are good models for studying the influence of the state of aggregation of each segment on the properties of the block copolymer. The two Journal of Colloid and Interface Science, Vol. 144,No. 2, July 1991

392

KANG AND JHON

components have very different surface energies (41.6 dyn/cm for a-MS and 47.1 dyn/ cm for HEMA) at the air-polymer interface, which should favor surface enrichment of ceMS, whereas the water-polymer interface favors surface enrichment of hydrophilic PHEMA. MATERIALS AND METHODS

a-MS/HEMA Diblack and Triblock Copolymer Samples The block copolymers were prepared by a two-stage anionic polymerization technique and purified to remove unwanted homopolymers by fractional precipitation from selected solvents under controlled conditions. The synthesis and characterization of the Pa-MS/ PHEMA block copolymers have been described in detail elsewhere (21). The bulk compositions and weight average molecular weights are shown in Table I.

erating conditions for the X-ray source were 300 W, 15.0 kV, and 20 mA. The base pressure in the source chamber, ca. 10 -9 T o r t , and a pass energy of 25 eV were employed for all spectra. These conditions give a gold 4 J~/2 peak at 83.8 eV binding energy for calibration and full width at half maximum (FWHM) of 1.1 eV at a count rate of 1,000,000 counts/s. No radiation damage was observed during 1.5 times the data collection time. Charge correction in the binding energy scale was accomplished by setting the CHx peak of the carbon ls envelope to 285.0 eV. Angular-dependent XPS instruments were carried out by rotating the samples relative to the fixed analyzer position by an angle 0, designated as the take-off angle between the sample normal and the entrance slit in the analyzer. All data manipulation was accomplished with the standard MACS (version 6) software of the Model 560.

Contact Angle Measurements XPS Measurements

In air measurements, the equilibrium contact angles of each probe liquid on solid surfaces were directly measured with a goniometer (NRL, Contact Angle Goniometer, Model A100, Rarer-Hart Inc.) using different drop sizes (0.2-0.5 #1) and also were measured at TABLEI both sides of each liquid drop edge for several Bulk Compositions and Weight Average Molecular different parts of the surfaces. Weights for the Pa-MS/HEMA and PHEMA/Pa-MS/ Under water measurements, the equilibPHEMA Block Copolymeps rium contact angles of probe fluids were measured on the same goniometer using the en%Pa-MS a vironmental chamber by the two liquid conCopolymer Mw × 10-* type wt tool (by GPC) a Mw/M~ tact angle techniques. The water-immiscible probe fluid bubbles were introduced by a Diblock 3.60 4 2.40 1.27 microsyringe containing an invented syringe Diblock 19.4 21 1.62 1.45 needle (I80 ° bend, Ram,-Hart, Inc.)which Diblock 25. l 27 3.76 1.30 Triblock 34.8 37 5.32 1.19 was connected to a syringe through a Teflon Triblock 49.6 52 5.29 1.27 tube (i.d. = 0.1 cm) onto a block copolymer Diblock 55.6 58 2.27 1.32 surface immersed early in a distilled water in Diblock 64.8 67 2.83 1.28 order to assure complete hydration. Triblock 77.5 79 2.67 1.40 XPS spectra were recorded on a Physical Electronics Model 560 ESCA/SAM system with Mgg~t,2 exiting radiation. Standard op-

"Composition determined by rH NMR. b To determine the molecular weight of the polymers by GPC measurements in THF, the polymers were esterified with poly(2-benzoyloxy)ethyl methacrylate. Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

Testing Liquids The liquids employed for the experiment were formamide, water, ethylene glycol, glyc-

SURFACEPROPERTIESOF BLOCKPOLYMERS erol, methylene iodide, and n-octane. They were purified by conventional distillation. The 7L for the liquids and 7wL for the interfacial surface tension were obtained with the help of Ref. (16).

mains selectively. These fihns were freezedried overnight. An ISI-SX-30E scanning electron microscope was used. Sample chamber vacuum was ca. 10-5 Torr. All samples were gold coated.

RESULTSAND DISCUSSION

Electron-Microscopic Observation The fihn used in this study was prepared by casting a ca. 10 wt% solution of the block copolymer in methanol (a preferential solvent for HEMA)-chloroform (a preferential solvent for a-MS) solvent systems. The thickness of the films obtained a 0.3-0.5 mm by micrometer measurement. For the lateral side, samples were made by the fracture under liquid nitrogen, in order to prevent deformation. The as-cast films were exposed to a 1% aqueous solution ofuranyl acetate for 30 rain, which stained the hydrophilic PHEMA do-

540

535 OlsB.E.eV PHEMA

285.0(CH2)

~~

/f~

290 285 Cls B.F-..eV PHEMA

In order to understand a-MS/HEMA copolymers, it was first necessary to analyze the homopolymer constituents. Figure 1 shows Cls and O]s spectra collected at normal exit angles from the two homopolymers and each block copolymer. The experimental binding energies and peak intensity ratios are tabulated in Table II. The Pa-MS spectra have a strong peak centered at 285 eV associated with the direct photoionization of C~s core levels and a low-energy satellite peak at 291.6 eV arising from shakeup transitions (Tr-~r*) accompanying core

3.P=- MS/PHEMA- 27 4.PHEMA/P-MS/PHEMA--37 5.PHEMA/P=-MS/PHEMA--52 6.P=-MS/PHEMA-58~-I~ 7 • P - MS/PHEMA - 67

533.0(C0) ~ 534.4 ~, \ ( c H2 0 )

530

~

,290--

393

h 8 A7 / I (I / I /~6// [ /

Ill

~A..~-~"I~I-I.--J'~ ~ / / z I 1/ // \ '

~ - ~1 ~- ~ ~ ,

~

~-

-

285

29~B,E . e.~~5 P , MS

FIG.1.XPSspectrafromtheblockcopolymersascomparedto homopolymers. Journal of Colloid and Interface Science, Vol,144, No.2,July199l

394

KANG AND JHON TABLEII

ExperimentalBindingEnergiesand Peak Area Ratios for the ReferenceHomopolymers,Pc~-MSand PHEMA

Pa-MS PHEMA

Carbon environment

Binding energy (eV), Cis

CH2 7r-Tr* CH2 CH20 CO

285 291.6 285.0 286.8 288.9

Oxygen environment

Binding energy (eV), Ois

Peak area ratio O ~ ( P H E M A ) / C ~ s ( P H E M A ) = 0.957

CO CH20

533.0 534.4

CI,(Tr-Tr*)/CI~(Pa-MS) = 0.079

ionization. The peak ratio of the ~r-Tr* to Cls The variation of surface percentage a-MS with core level is 0.079. These results are in agree- angle shows that the a-MS-rich layer has a ment with those previously published by Clark pronounced compositional gradient within the et al. (22, 23). The spectra from PHEMA XPS sampling depth. At bulk concentrations show three peaks for the Ci~ located at 285 eV between 3 and 37% a-MS indicates an accomdue to C-C; 286.8 eV due to CH20; and 288.9 panying rapid rise in surface a-MS concentraeV due to C ~ O transitions, and two peaks tion. for the O15 core levels at 533.0 eV due to In their previous paper (6) Thomas and C ~ O ; 534.4 eV due to CH20 transitions. O'Malley described the fundamentals and Analysis of the relative peak intensities of C~s demonstrated the utility of angular-dependent in PHEMA shows that the proper stoichi- XPS measurements, XPS(0), profiling in ometry of 3:2:1 and the relative peak inten- depth the compositional variations of the sities of O~ in PHEMA yield stoichiometry of block copolymers near the air-polymer inter1:2. The peak ratio of Cls to Ol~ core levels is face. Figure 3 displays the surface concentra1.045. tion gradient in each block copolymer. The In the copolymers, the peak at 291.6 eV a-MS content in a copolymer always gives a (Tr-~r* shake-up) is proportional to the con- higher surface a-MS composition. The subtribution of the a-MS in the main peak. The surfaces consist of an a-MS-rich layer of deremaining contribution is from the HEMA. creasing a-MS content with depth, but the aThe narrow main C~s peak and the presence MS concentration is at least 20% higher in the of 7r-Tr* shake-up satellites at 291.6 eV indi- surface than in the bulk. The slope of the cate the dominance of a-MS at the surface. This allows the calculation of relative peak areas contributed from each component. The 10090relative peak areas are then converted to mole B0DIBLOCK percentage values by correcting the number of ~xm ~ 70~ ",A,';TRIBLOCK carbons. 60The quantitative results on Pa-MS/ 50;~ 40PHEMA block copolymers in Fig. 2 show the ~"~3//'~" "~ o ~o° molecular percentage surface a-MS as detected by XPS plotted against the known molecular percentage a-MS in the bulk. Spectra were 0 0 1'0 2'0 3'0 4? 5'0 6'0 7'0 B~" 9'0 100 collected at three electron exit angles, and the tool% (x-MS IN THE BULK differences between the curves obtained from FIG. 2. Surface vs bulk composition for the Pa-MS/ each angle signify the extent to which concen- PHEMA diblock and triblock copolymers,taken from tration changes occur in the top 40A or so. XPS. Journal of Colloid and Interface Science, VoL 144, No. 2, July 1991

395

SURFACE PROPERTIES OF BLOCK POLYMERS 1O0 - [ - - - - - - [ ] . 90J & 80m x 70~ >m 600"3 z~ 50I

4030E 2010-

A;58

Z~;37 O;21 0;4

0 . . . . .

oooo

o.:~oo o~oo

0.600 COS 0

0.~]00

1.000

FIG. 3. Angle-dependent surface behavior of Pa-MS/ PHEMA block copolymer.

curves is a measure of the change in compositional heterogeneity versus depth, and provides another level of information. The relatively small gradients for all neat block copolymers show that each has a a-MS-rich layer which is thick with respect to the XPS analysis depth. Curves 3, 4, and 5 have nearly identical gradients, whereas the slopes of curves 1 and 2 are higher. This can be interpreted in terms of the block length and the heterogeneity of the copolymers. The a-MS-rich contents have the shorter HEMA block lengths, whereas the low a-MS contents have a longer HEMA block. This implies that the length of the HEMA block plays a role in controlling the compositional gradient of the surface layer.

We can eliminate a continuous overlayer model, in which Pa-MS would form a continuous surface overlayer on top of the PHEMA component, because we do not observe the predicted exponential dependence to the PaMS relative intensity as 0 is varied from 10 to 80 °. The evidence indicates that the Pa-MS and PHEMA components in the copolymers are both exposed at the surface and that they are organized into domains which are thick compared to the XPS sampling depth. The slight angular dependence we observe as 0 is varied again points to a nonpolar surface topography in which the Pa-MS domains are slightly elevated above the PHEMA domains. Total free surface energy of block copolymers at the air-polymer and the water-polymer interfaces is shown as a plot of HEMA compositions in Fig. 4. For the Pa-MS/ PHEMA block copolymers, at the air-polymer interface, the measured surface tensions are Pa-MS (41.6 dyn/cm) and PHEMA (47.1 dyn/cm). The estimated results have some difference in magnitude but the surface tension increased considerably at about 90% HEMA mole fraction in the block copolymer. This indicates that there is a surface excess of the Pa-MS components at the air-polymer interface. At the water-polymer, the measured surface tensions are Pa-MS (44.5 dyn/cm) and PHEMA (69.1 dyn/cm). These results have

70

48 ~ - -

rY Ld

~d

~: 46

~0

z

%

45.

~

44

z o4 ro 50

£3

c~ 45 w

G9 8£ Ld

42. 414 0

~ 10

T 20

5'0 4'0 5'0 6'0 ?0 tool% P c ~ - M S IN COPOLYMER

T-80

9'0

I140 100

FIG. 4. Relation between wettability and copolymer composition by environment: (a) in air, (O) a-MSHEMA diblock copolymer system ( 0 ) triblock copolymer system (b) in water, ([]) a-MS-HEMA diblock copolymer system ( I ) triblock copolymer system. Journal of Colloid and Interface Science, VoL 144, No. 2, July 1991

396

K A N G AND JHON TABLE 1II Computed Surface Energetic Results in Air Surface energy (dyn/cm)

Bulk composition (tool%, a-MS)

Copolymer type

~sd (-+0.1)

7] (+-0.I)

7s (+-0.2)

P~-MS PHEMA 4 21 27 37 52 58 67 79

Homo Homo Diblock Diblock Diblock Triblock Triblock Diblock Diblock Triblock

41.3 30.2 31.6 36.2 38.7 40.4 41.2 41.2 41.3 41.3

0.3 16.9 12.4 5.7 3.1 1.3 0.4 0.4 0.3 0.3

41.6 47.1 44 41.9 41.8 41.7 41.6 41.6 41,6 41.6

interfacial tensions increased dramatically at about 20% HEMA mol% in the block copolymer. Holley and Refujo (24) suggested that hydrophilic groups of the polymer at the airpolymer interface are preferentially drawn into the bulk of the hydrophobic polymer. Considering free rotation around C-C bonds and long-range mobility of macromolecules, this concept appears reasonable, and it is expected that this phenomenon, i.e., orientation of molecules at an interface, should, in general,

also be applicable to polymers in each environment. At the water-polymer interface, the main driving force for the rotation of the molecules at the surface is the strong interaction between water and hydrophilic groups of the macromolecules. Because of this and the high mobility of polymer segments in the hydrated block copolymers, the rotation of molecules or the appearance of hydrophilic groups in the hydrated bulk of the block copolymer appears to take place in Pa-MS/PHEMA block copolymers. Table III presents the results of dispersion (3' ~) and polar (7 ~) contributions to total surface tension at the air-polymer interface of the block copolymer. These data show clearly that the relative enrichment of a-MS is related to the dispersion and polar contributions to the total surface free energy of block copolyreefs. Because the a-MS is 100% hydrophobic and HEMA is amphiphilic polymer at the airpolymer interface, the appearance of the hydrophobic groups is represented by the changes of the Pa-MS/PHEMA block copolymer compositions. From Table IV, the water content in PaMS/PHEMA block copolymers seems to decrease with increasing a-MS due to the increased hydrophobic interaction. The 3"dsand

TABLE IV The Dispersion and Polar Components Surface Free Energies for the P~-MS/PHEMA and PHEMA/Pc~-MS/PHEMA Block Copolymers in Water Surface free energy (dyn/em) Bulk composition (tool%, a-MS)

Copolymer type

H20

(-+0.1)

(_+0.1)

(+-0.2)

Pc~-MS PHEMA 4 21 27 37 52 58 67 79

Homo Homo Diblock Diblock Diblock Triblock Triblock Diblock Diblock Triblock

0 41 34 18 12 8 5 3 2 1

43.4 22.2 22.4 23.7 24.2 24.4 26.0 27.4 29.2 30.8

1.1 46.9 46.6 43.4 42.6 42.1 40.2 38.7 36.7 31.8

44.5 69.1 69.0 67.1 66.8 66.5 66.2 66.1 65.9 62.6

Journal ~?/'Colloid and Interface Science, Vol. 144, No. 2, July 1991

SURFACE PROPERTIES OF BLOCK POLYMERS

q,~ obtained at the water-polymer interface have similar values with PHEMA. This means that the surface at the water-polymer interface is mobile and easily reoriented to adapt the medium environment to its polar contribution due to dipole-hydrogen bonding interaction between the hydrophilic portion of the polymer surface, and water is a major part of the total surface free energy. Estimations of the compositions by dispersion and polar contributions to the total surface free energy were obtained by second-order simultaneous equations ('ydEMA)X + ( ' y d - M s ) Y = TD

[3]

("}/~tEMA)X + ( T P - M s ) Y

[4]

= TP,

where "YI-IEMA, d d P "/u-MS, "YttEMA, and 7~-MS are the dispersion and polar surface free energy contributions to HEMA and oe-MS homopolymers, 3'D and ye are dispersion and polar surface free energy contributions to copolymers, and x and y are the values for HEMA and a-MS, respectively. The solutions of the equations give the values for x and y. Thus, by using (x/x + y) × 100%, we determine the values of PHEMA mol%. These composition series in Table V show that there is a considerable surface excess of Poe-MS at the air-polymer interface, and at

TABLE V Surface Compositions of the Pa-MS/PHEMA and PHEMA/Pa-MS/PHEMA Block Copolymers Calculated from the Contact Angle Data as a Function of Environment Surface composition (tool% of Po:-MS) Bulk composition (tool%, a-MS)

Copolymcr type

In air

In water

4 21 27 37 52 58 67 79

Diblock Diblock Diblock Triblock Triblock Diblock Diblock Tribloek

24 66 76 94 99 99 100 100

1 7 9 l! 16 2! 28 37

397

the water-polymer interface there is a slight surface excess of PHEMA in block copolymer. This is particularly notable at the water-polymer interface where the composition of the surface excess of hydrophilic HEMA is observed. These data show that the POe-MS/ PHEMA block copolymers are not overlayers and separated microdomains. Therefore, this fact also supports the surface topography observations by XPS(0). Figure 5 shows SEM micrographs of the PoeMS/PHEMA block copolymer for HEMA contents of 79, 63, and 33 mol%, depicting their surface morphology. The surface films exhibited partially striped and globular (Fig. 5a), globular (1-30 #m) (Fig. 5b), and globular ( 100-1000 nm) (Fig. 5c) POe-MSdomains embedded in a continuous PHEMA matrix, which were stained selectively by uranyl acetate. The surface morphologies of the other compositions are similar to these morphologies. According to the results, the surface morphology consisted of globular POe-MSdomains slightly elevated above PHEMA domains. Figure 6 shows the morphology of the near surface and the lateral surface at HEMA contents of 96, 79, and 48 tool%. The striped (Fig. 6a), striped (Fig. 6b), and lamellae (Fig. 6c) domains are perpendicular to the free surface. The lateral surfaces show that the alternating structures of Pa-MS (bright domains) and PHEMA (dark domains) are aligned with their interfaces perpendicular to the free surface of the film from striped and to lamellar through the compositions of HEMA. Here, it is found that the domain structures, i.e., the state of aggregation of hydrophilic and hydrophobic segments, obviously are presented for POe-MS/ PHEMA block copolymers at each composition. Therefore, when the results from XPS(0) and contact angle measurements are interpreted in light of the surface topography found by SEM, the following observations can be made. The POe-MS/PHEMA block copolymers show a surface preference for the lower surface energy POe-MS component at the airpolymer interface, where the surface of the Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

FIG. 5. Electron micrographs of c~-MS/PHEMA block copolymer films cast from methanol-chloroform solvents (a) HEMA contents of 79 mol%, (b) HEMA contents of 63 rnol%, (c) HEMA contents of 33 mol%. 398 Journal of Colloid and Interface Science,

Vol.144,No. 2, July 1991

FIG. 6. Electron micrographs of lateral side o f a - M S / H E M A block copolymer films cast from m e t h a n o l chloroform solvents (a) H E M A contents of 96 tool%, (b) H E M A contents of 79 tool%, (c) H E M A contents of 48 mol%. The arrow indicates the free surface. 399 Journal of Colloid and Interface Science, Vol. 144,No. 2, July 199

400

KANG AND JHON ~Free surface

2. 4- Lateral surface 3. 4. • HEMA [] s - M S FIG. 7. The 3-D morphology of the P~-MS/PHEMA52 block copolymer films cast from methanol-chloroform

solvents.

5. 6. 7. 8.

9.

block copolymers appears to be comprised of peaks and valleys. Here, the block copolymers have a morphology consisting of microdomains of each component oriented perpendicular to the surface, with the a-MS segments as the likely regions protruding above the surface. By combining the data from several techniques (XPS(0), contact angle, and SEM), we see that while the block copolymers match that of the bulk, the morphology of this region is consistent with those found in bulk samples with a concentration equivalent to the surface concentration. According to the results from this technique, Fig. 7 illustrates the proposed 3-D morphology for the Pa-MS/ PHEMA-52 block copolymers. ACKNOWLEDGMENTS

10.

11. 12, 13. 14.

15. 16. 17. 18. 19.

This research was supported by the Korea Science and Engineering Foundation and the Korea Research Center for Theoretical Physics and Chemistry. We gratefully acknowledge helpful discussions with Dr. H. K. Kim from the Sam Sung Advanced Institute of Technology.

20.

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23.

1. (a) Noshay, A., and Mcgrath, J. E., "Block Copolymers: Overview and Critical Survey." Academic Press, New York, 1977; (b) Reiss, G., in "Eney-

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clopedia of Polymer Science and Engineering" (J. I. Kroschwitz, Ed.), Vol. 2, p. 324. Wiley-Interscience, New York, 1988. Patel, N. M.z Dwight~ D. W., Hedrick, J. L., Webster, D. C., and Mcgrath, J. E,, Macromolecu[es 21, 2689 (1988). Gibbs, J. W., "The Scientific Papers of J. Willard Gibbs," Vol, 1, p. 219. Dover, New York, 1961. Bhatia, Q. S., Pan, D. H., and Koberstein, J. T., Macromo!ecules 21, 2166 (1988). Gaines, G- L., Jr., Macromoleeules 14, 208 ( 1981 ). Thomas, H. R.~ and O!Malley, J. J., Macromolecules 12, 323 (1979). O'Malley, J. J., Thomas, H. R., and Lee, G. M., Macromolecules 12, 996 (1979). Schmitt, R. L., Gardella, J. A., Jr., Magill; J. H., Salvati, L., Jr., and Chin, R. L., Macromolecules 18, 2675 (1985). Owen, M. J., and Kendrick, T. C., Macromolecules 3, 458 (1970). Legrand, D. G., and Gaines, G. L., Jr., Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 11, 442 (1970). Gaines, G. L., Jr., and Bender, G. W., Macromolecules 5, 82 (i972). Yamashita, Y. J., J. Macromol. Sci. Chem. A13, 401 (1979). Yuk, S. H., and Jhon, M. S,, J.. Colloid Interface Sci. 110, 252 (1986). Andrade, J. D., King, R. D., Gregonis, D. E., and Coleman, D. L., Z Polym. Sci. Polym. Syrup. 66, 313 (1979). Wu, S., J. Polym. Sci. C 34, 19 ( 1971 ). Matsunaga, T~, and Ikada, Y., J. Colloid Interface Sci. 84, 8 (1981). Andrade, J. D,, Ma, S. M., King, R. N, and Gregonis, D. E., J. Colloid Interface Sci. 72,488 (1979). Hasegawa, H., and Hashimoto, T.~ Macromolecules 18, 589 (1985). Schmitt, R, L., Gardella, J. A., Jr., and Salvati, L., Jr., Macromolecules 19, 648 (1986). Okano, T., Katayama, M., and Shinomura, I., J. Appl. Polym. Sci. 22, 369 (1978). Hirao, A, Kato, H., Yamaguchi, K., and Nakahama, S., Macromolecules 19, 1294 (1986). Clark, D. T., Peeling, J., and O'Malley, J. M., J. Polym. Sci. 14, 543 (1976). Clark, D. T., Adams, D. B., Dilks, A., Peeling, J., and Thomas, H. R., £ Electron Spectrosc. Relat. Phenom. 8, 51 (1976). Holley, F. J., and Refujo, M. F., J. Biomed. Mater. Res. 9, 315 (1975).