Langmuir 1991, 7, 2438-2442
2438
The Interface between Two Strongly Incompatible Polymers: Interfacial Broadening and Roughening near Tg S. Huttenbach,? M. Stamm,*vfG. Reiter,j and M. foster^^^ Institut fur Festkorperforschung, KFA Jiilich, Postfach 1913, 5170 Jiilich, FRG, and Mar-Planck-Institut fur Polymerforschung, Postfach 3148, 6500 Mainz, FRG Received July 5,1990. I n Final Form: December 10, 1990 The technique of X-ray reflectometryis used to investigatethe interface between two strongly incompatible polymers, polystyrene (PSI and poly(p-bromostyrene) (PBrS). Two thin polymer films are deposited on a glass substrate and various film parameters are determined during annealing. The interfacial width between the polymers is determined with subnanometer resolution. It increases with annealing temperature. A maximum thickness of 1.3 nm is reached at approximately the glass transition temperature T,of PBrS where both components become mobile. The increase of the interfacial width with annealing temperature is explained on the basis of mobility and interdiffusion. The Flory-Huggins interaction parameter obtained from the experiment is in good agreement with mean field theoretical approximations. By phase interference microscopy a drastic increase in surface roughness is observed during annealing above T,of PBrS. It is explained with respect to a minimization of the interfacial contact area between the components giving rise to a lateral reorganization on a micrometer scale.
Introduction Most polymers are incompatible with each other resulting in a phase-separated structure after blending. The extent of phase separation is different for the various materials and depends for instance on temperature treatment of the samples. To obtain compatibility, the chemical composition of the components usually has to be similar or a specific interaction between monomers is necessary. In the system polystyrene (PS)blended with poly@-bromostyrene) (PBrS) the degree of compatibility can be varied over a wide range, changing the degree of bromination of the PBrS. PS and PBrS are strongly incompatible at a high degree of bromination and are compatible at a lower degree of bromination where the two components become more similar with respect to their chemical composition. Blends are largely used in applications to combine preferential properties of the materials and to compensate for bad properties of one component. In incompatible systems it is crucial for good mechanical properties to have still a good contact between the microscopic phases formed in the blend and to achieve a reasonable intermixing at the interface between the components. This interfacial region is not easy to determine because it might be as small as some nanometers, depending on the Flory-Huggins interaction parameter x between the components. We use X-ray reflectometry, a technique which has only recently been applied to the investigation of polymer surfaces and interfaces and which offers the resolution to measure the polymer intermixingat an interface with subnanometer precision. Neutron reflectometry has on the other hand been used to investigate thin polystyrene films’ as well as the interface formation between PS and PMMA homopolymersa2 With suitable contrast between the components, it is possible to perform similar experiments with X-ray reflectrometry, and data on the interdiffusion +
Institut for Festkorperforschung.
* Max-Planck-Institut for Polymerforschung.
Present address: Department of Polymer Science, University of Akron, Akron, OH 44325-3909. (1)S t a ” , M.; Majknak, C. F. Polym. Prepr. 1987, 28 (2), 18. (2) Fernandez, M. L.; Higgins, J. S.; Penfold, J.; Ward, R. C.; Shackleton, C.; Walsh, D. J. Polymer 1988,29, 1923.
of PMMA and PVC have been reported? The contrast between PS and PBrS is sufficient to resolve accurately the interfacial region by X-ray reflectivity experiments. The resolution is much better as compared to other techniques like forward recoil spectroscopy, Rutherford backscattering, or secondary ion mass spectrometry. We first briefly describe the application of this technique to the interface problem of incompatible polymers. Reflectivity experiments are performed at different temperatures starting above T,of PS. The interfacial broadening is discussed with respect to mobility and chain interaction. Finally measurements of the surface roughness obtained from a phase interference microscope are presented where a significant roughening is observed at temperatures higher than T,of PBrS.
Experimental Section Sample Preparation. For our investigations we used polystyrene (PS) obtained from Polymer Lab, Ltd., and poly(bromostyrene) (PBrS)from Aldrich. The degree of bromination is almost 100%. Molecular weights and polydispersities are determined by GPC and viscosimetry: Mw(PS)= 117 OOO; Mw(PBrS) = 47 OOO. The degrees of polymerization are thus N(PS) = 1125 and N(PBrS) = 260. The polydispersity of PS is better than MwJM,= 1.05,while PBrS has a broad distribution, MI/ M. = 3.1. Each polymer is dissolved in toluene at a concentration of approximately 10 mg of polymer in 1 mL of solvent and then spread by spin coating at 2000 rpm onto high-quality flat float glass (7X 3 cm2) to form a smooth, uniform layer. To obtain a bilayer sample, one of the films is floated off onto distilled water and picked up by the other film, which is still on the substrate. The samples are then dried under vacuum at 60 O C for several hours. In this work we will discuss two bilayer samples, one with PS and one with PBrS as the top layer. The glass substrates were fist checked by X-rayreflectivity and usually showed a roughness of less than 1 nm. Similarly the films are characterized prior to the bilayer preparation and directly afterward. The results are summarized in Table I. Roughness and interface profile are approximatedby an error function profile‘ with width parameter 0. The indices are counted from air, thus e.g., 0 1 is the surface roughness and dl is the thickness of the top layer. For (3) S t a ” , M.; Reiter, G.; Foster, M.; Motachmann, H.; Hllttenbach, S.; Toprakcioglu, C. Sei. Eng. ACSIPMSE 1990, 62, 843. (4) Nevot, L.; Croce, P. Rev. Phys. Appl. 1980, 15, 761.
0743-746319112407-2438$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 11, 1991 2439
The Interface between Incompatible Polymers Table I. Characteristic Data on Thickness d, Surface Roughness q,and Interface Width 02 of Single Films and Bilayer Samples As Determined by X-ray Reflectometry. SB-bilayers PBrS on top of PS PS on top of PBrS BS-bilayer PS film sample SB1 SB2 SB3 SB4 BS5 a
dl
01
PBrS film dl
61
53.1 1.0 34.2 39.3 33.4 36.5 28.4
0.9 1.0 0.6 0.8
41.4 38.9 45.8 29.7
bilayer dl
1.0 34.2 1.0 41.0 1.0 37.2 0.6 45.0 0.8 28.9
dp
54.9 39.7 34.4 37.8 30.6
up(RT) ~ ~ ( 1 O3C 5) 1.2 1.8 1.0 2.0 1.0 1.8 1.3 2.2 1.1 1.7
All values are in nanometers. X-RAY REFLECTOMETER
Monochro-
mator
Anode
F i g u r e 1. Schematic diagram of the X-ray reflectometer setup at Mainz.
the investigations with X-ray reflectometry and phase interference microscopy the films were successively annealed in a vacuum furnace at various temperatures and quenched to room temperature (RT) between the experiments. Thus a whole series of experiments is performed with one sample. X-ray Reflectometer. The small incident angles needed for reflectivity experiments make it necessary to work with an excellent collimation. Therefore a special reflectometer has been built at the MPI fur Polymerforschung in M a i d using a Rigaku 18-kW rotating anode with a Cu target as an X-ray source and a graphite monochromator set for X = 0.154 nm. A schematic diagram of the reflectometer setup is shown in Figure 1. The slit collimating system provides a beam divergence of O.0lo and a beam size of 50 pm X 10 mm at sample position. The incident angle can be varied in steps of 0.002O. Incident angles range from 0.lo to 2O. The intensity is typically ZO = 5 X lo6 counts/s with a total background $0.4 counts/s. Reflectivities as low as 10-7Z0can be measured. The experiment is completely controlled by a PC and typical run times for a reflection curve range from 1 to 12 h depending on sample and angular range. Standard sample size is 7 x 3 cm2. Depending on the incident angle, different areas on the sample are illuminated. Results of the experiment like roughness, density profile, or film thickness are laterally averaged within this area and the sample has to be extremely flat and homogeneous to obtain optimum resolution. Radiation damage of the sample is negligibly small at experimental times, fluxes, and X-ray energies used during the experiments. Phase Interference Microscope. The phase interference microscope purchased from LOT/ZYGO is used to optically characterize the sample surface with a lateral resolution of approximately 1 pm and a height resolution 20.6 nm. The interference pattern of monochromatic light reflected from a flat reference surface and the sample under investigation is recorded in an area detector while the reference plane is moved with a piezoelectric drive. The principle of operation is described elsewhere.6 We use a magnification of 100 times. The lateral resolution is given by the detector pixel size or the wavelength of the used light whichever is larger and is of the order of 1pm. Because of the excellent reference surface and piezo-drive, a resolution perpendicular to the test surface of the order of 0.6 nm can be achieved. The root mean square roughness calculated from the two-dimensional height distribution is averaged over a defined area (typically ca. 50 X 50 pm2). An example of the surface of a PBrS film spin coated onto float glass is shown in Figure 2a. This method is especially used to visualize surface
5 vm
Figure 2. (a) Phase interference microscopy picture of the surface of a single PBrS film spun cast on float glass. The shown area in the picture is 42 X 42 pump. The root mean square roughness is 0.6 nm and the peak to valley distance 5.6 nm. (b) Picture of the surface of bilayer sample SB4 (PBrS on PS) after annealing a t 144 OC for 30 min. The root mean square roughness increased to 37 nm and the peak to valley distance to 252 nm because of deep holes in the layer structure. The holes indicate the start of a three-dimensional decomposition. While the shown area is 55 x 55 pmz, the scale in the .z direction has been changed with respect to (a) by a factor of 50. waviness, holes, bumps etc. in the micrometer range which might occur during sample preparation and which cannot be resolved with X-ray reflectrometry since our equipment does not have this lateral resolution. It is in particular helpful to investigate changes in surface roughness on a micrometer scale during annealing/interdiffusion,as will be discussed below.
Theory X-ray reflectometry has proven to be a powerful tool for the investigation of thin films with respect to thickness, roughness, and density with a subnanometer res~lution.~ The reflectivity is determined by the refractive index n of the sample, which f9r X-rays is given by7
io;
n = 1- 6 - 6 = ( ~ ~ / 2 7 ) r , p , (1) where X is the wavelength and re the classical electron radius. The electron density pe determines largely the values of n. The imaginary part 0 is responsible for the absorption of the X-rays and contains the absorption coefficient of the material for X-rays. For X-ray reflectivity studies on polymers, it is necessary to have a good scattering contrast, larger than usually needed for, e.g., X-ray small angle scattering. Therefore we chose the couple polystyrene (PS) and polybromostyrene (PBrS). Due to the heavy Br atom the contrast between the two polymers is good enough for our investigations,if the degree of bromination of PBrS is high. For our samples we get from eq 1
Htittenbach, S. Vuccum 1990,
n(PS) = 1- (3.6X lo*) - i(2 X lo*) n(PBrS) = 1- (5.0 X lo*) - i(2 X 10") (la) n(g1ass) = 1- (8.0 X lo*) - i(l X Values of n are very close to 1and are always smaller than 1for X-rays. The reflectivity, R, of X-rays incident at a glancing angle Bi on an ideal interface between medium 1
( 6 ) Biegen, J. F.; Smythe,R. A. SPIE O/E LASE '88 Symposium,Los Angeles, CA, Jan 1988.
(7) James, R. W. The Optical Principles of Diffraction of X-rays; Cornel1 University Press: Ithaca, NY, 1967.
( 5 ) Foster, M.; Stamm, M.; Reiter, G.; 41, 1441.
Hiittenbach et al.
2440 Langmuir, Vol. 7, No. 11, 1991
.-. -‘.
“3
\
Figure 3. Schematic drawing of a reflection experiment of a thin film. Symbols are explained in the text.
croroughness averaged over the lateral coherence length of the X-rays. PS and statistical styrene-bromostyrene copolymers are known to phase separate a t low molecular weights and low degree of bromination with an upper critical solution temperat~re.~ At 100% bromination and the molecular weights used, the two polymers are strongly incompatible. Compatibility is expressed by the Flory-Huggins interaction parameter xl0. For x larger than a critical value xCthe polymers are incompatible. xc can be calculated from the degree of polymerization N, and Nb of the two polymers10
and 2 is given by the Fresnel equation8
(A’,”’
k, sin Bi R = ;1-1 ki = 2n ni kl + k , x
xc
k1-
For angles below the critical angle 01, the reflectivity is nearly unity. For 81 > elcthe reflectivity drops off sharply going with k4for 81 >> elc. 81, can be calculated from Snell’s law n, cos O1 = n2 cos 8,
(3)
which for nl = 1 yields
e,,
= (26,)”2
(4)
For polymers and A = 0.154 nm, 81, is typically below 0.2”. In the case of a thin polymer film on a substrate (Figure 3) the incident beam will be reflected from the different interfaces and those reflected beams will interfere with each other. One obtains a more or less strongly modulated curve characteristic for the sample. For a single film on a substrate the reflectivity R is given by
R=
r:
+ r< + 2rtrbcos (2d k2,,)
+
1 r:rt
(5)
+ 2rtrbcos (2d kZ,J
where rt and rb are the reflected amplitudes at the top and the bottom interface, respectively, d is the thickness of the layer, and k2,2is the component of the wave vector k perpendicular to the interface within the layer. The cosine is responsible for the characteristic modulation. By use of a matrix formalism8 reflectivities of multilayer systems can be calculated. Any profile can be approximated by a sequence of small homogeneous layers. Comparing experiment and calculated reflectivity curves gives information on model electron density profiles. The deviations between calculated and experimental curves are minimized and emphasis is given to larger angles corresponding to small characteristic distances. Thickness, roughnesses, and interface widths of the individual layers with an accuracy up to 0.1 nm can be obtained. The lateral coherence length of X-rays is of the order of 1 pm. Since the beam, on the other hand, covers different areas F on the sample depending on the angle of incidence (F a sin-’ 81) X-ray reflectrometry averages (a) coherently over a lateral dimension of the order of micrometers (average of reflected amplitudes) and (b) incoherently over an area of square centimeters to square millimeters (intensity average). The latter effect, which resembles the waviness of the sample, might also be detected via broadening of the reflected beam. It smears out the reflectivity curve independent of the angle of incidence. The influence of the first effect on the reflectivity curve on the other hand is angle dependent. The reflectivity curve thus contains information on the mi~~~
(8) Lekner,J. Theory o f l i e f l e d o n ;MartinusNijhoff Publishers: Amsterdam, 1987.
=
+ N:/2)2 2NaNb
For our polymers one obtains xc = 4.2 X x can be roughly estimated by extrapolation of investigations with mixtures of PS and styrene-bromostyrene copolymers of low degree of bromination (> xc. While the interdiffusion of compatible polymers will progress steadily with time, interfacial mixing will come to a standstill after the formation of a small interfacial region for incompatible polymers. The width of this interfacial region will depend on the x parameter between the materials and is a measure of the “degree of incompatibility” of the materials. It will be small for strongly incompatible materials, which however still intermix on a segmental scale. For the equilibrium state a profile of the form 4(z)
-
tanh
(f); 1 = a21J2(x-l)-l/z 3X,’J2
(7)
xc
is predicted by theory.13-17 a is the mean polymer segment length and z the coordinate perpendicular to the sample surface. For strong incompatibility, i.e. x >> xcthis reduces to
and 1 becomes independent on the molecular weight of the polymers.
Results and Discussion X-ray reflectivity curves have been measured as a function of annealing time and annealing temperature until an equilibrium interface width was obtained for a given temperature. Each sample was annealed at a fixed temperature for a specific time and quenched to room temperature to perform the reflection experiments. This procedure was repeated until no significant changes in the reflectivity curves of two succeeding measurements were evident, thus indicating that the sample has reached equilibrium for this temperature. Thereafter the annealing temperature was successivelyincreased to investigate the temperature dependence of the equilibrium interface (9)Strobl, G . R.; Bendler, J. T.; Kambour, R. P.; Schulz, A. R. Macromolecules 1986,19,2683. Bruder, F.; Brenn, R.; Stiihn, B.; Strobl, G.R. Macromolecules 1989,22,4434. (10) de Gennes, P. G. Scaling Concepts in Polymer Physics; Comell University Press: Ithaca, NY, 1979. (11) Meier, H. Thesis,Univ. Mainz, FRG, 1986. (12) Urban, G . Thesis, Univ. Mainz, FRG, 1987. (13) Helfand, E.; Tagami, Y. J. Chem. Phys. 1972,56,3592. (14) Helfand, E.; Sapee, A. M. J. Chem. Phys. 1975,62,999. (15) de Gennes,P. G.J. Chem. Phys. 1980, 72,4756. (16) Binder, K. J. Chem. Phys. 1983, 79, 6387. (17) Cheturvedi, U. K.; Stainer, U.; Zak, 0.;Krausch, G.; Klein, J. Phys. Rev. Lett. 1989, 63, 616.
The Interface between Incompatible Polymers X
Langmuir, Vol. 7, No. 11,1991 2441
reflectivity curve
PS/PBrS
r.1
1
0.6
Figure 4. Measured X-ray reflectivity curve (single points) of the unannealed bilayer sample SB4 (PBrS on PS) and the best fit (dashed line). The obtained parameters are given in Table I and are explained in the text. width. The X-ray reflectivity curve of the unannealed PS-PBrS double layer sample SB4 (see Table I) is shown in Figure 4. At small angles one observes the total reflection region with first the cutoff from the critical angles of PS and PBrS. In this region the beam penetrates the polymer layers but is still totally reflected at the pol--merglass interface. At larger angles the beam also penetrates the glass substrate and the reflected intensity drops significantly. The fringes are due to interferences of the beams reflected at the various interfaces of the sample. The total bilayer thickness can be obtained very accurately from the periodicity of the fringes. The thicknesses of the top and the bottom layer, respectively, as well as the width of the interface between the polymer films are responsible for the characteristic intensity modulation of the maxima. The best fit for the unannealed bilayer sample SB4 is also shown in Figure 4 (dotted line). We obtain the film thicknesses of dl = 45.0 f 0.6 nm and d2 = 37.8 f 0.6 nm for the upper PBrS and lower PS films, respectively. The mean electron densities correspond to the usual bulk values of the polymers according to eq 1. The surface and interface roughnesses are u1= 0.6 f 0.1 nm, 62 = 1.3 f 0.2 nm, and u3 = 0.4 f 0.1 nm. Those values correspond to the width in the error function profiles used for the broadening of the electron density profile at the corresponding interface. The quality of the fit might still be improved by using more complicated models (e.g. asymmetric profiles) which was not tried in this study. During annealing the reflectivity curve changes as indicated in Figure 5. Table I1 shows the results of the reflectivity measurements performed at different annealing temperatures with two samples. Listed are the annealing temperature T, the annealing time t, the interface width between PS and PBrS films u2(T), and a reduced width 1. 1 is calculated from the measured interface width u2(T )and the roughness 4 R T ) before annealing the sample
l ( T )= 0.844(u?(T) - CT,~(RT))"~ (9) This assumes two independent processes which are convoluted to yield the value of the measured interface width. 1 is supposed to express the increase of the interfacial width due to interdiffusion alone. The factor 0.844 takes the different density distribution functions for the interface into account. One underlying assumption in eq 9 is the constancy of the roughness distribution u2(RT) during the whole temperature treatment. This assumption proves to be correct since the surface roughness of the samples does not change during the annealing procedure listed in Table 11. Furthermore it was checked by additional experiments where only single films of PS
' ' s
I
* '*
0.8
4
&A-,
9 [degl
1.0
Figure 5. Change of experimental reflectivity curves of bilayer sample SB4 (PBrS on PS; see Table 11) with temperature. The interface width of the unannealed sample (solid line) is 1.3 nm and changes during annealing at 130 O C to 2.0 nm (dashed line). This corresponds to an intermixing of PS and PBrS of 1 = 1.3 nm at the interface. Table 11. X-ray Reflectometry Results for Two Samples Annealed at Different Temperatures. sample SB4 sample BS5 time, u2, 1, temp, time, us, I, temp, "C h nm nm "C h nm nm RT 1.3 0 RT 1.1 0 1 114 2 1.3 0 114 1 1.1 0 2 114 6 1.5 0.6 114 3 1.3 0.6 3 114 10 1.5 0.6 114 14 1.3 0.6 120 2 1.3 0.6 4 120 2 1.6 0.8 5 120 14 1.6 0.8 120 11 1.3 0.6 6 125 2 1.7 0.9 125 2 1.4 0.7 7 125 10 1.8 1.1 125 11 1.4 0.7 8 125 12 1.8 1.1 125 15 1.4 0.7 9 130 2 1.8 1.1 130 2 1.6 1.0 10 130 11 2.0 1.3 130 6 1.6 1.0 11 135 8 2.2 1.5 135 2 1.7 1.1 12 135 18 2.2 1.5 135 7 1.7 1.1 13 139 10 2.2 1.5 139 12 1.7 1.1 14 139 15 1.7 1.1 15 139 21 1.7 1.1 16 144 2 1.7 1.1 17 144 14 1.7 1.1 a Interface width u2 and broadening 1 as a function of annealing conditions are indicated.
and PBrS on glass were annealed a t 160 "C for several hours. Within experimental accuracy we did not observe any changes in the surface roughness of the films and thus can conclude that roughness does not increase during the annealing procedure. The observed changes in the interfacial width are therefore an effect of interdiffusion of PS and PBrS. For a further interpretation of the data listed in Table I1one has to take into account that most of the experimenta were performed above the glass transition temperature of PS (T,(PS) = 103 "C)but below T,(PBrS) = 142 0C.18 Thus below T,(PBrS) the PBrS chains are frozen in and cannot move. Nevertheless interfacial mixing is observed after annealing the samples. Especially between 100 and 135 O C the interface width increases until a final constant value of 1 is reached (Figure 6). The small differences between the two samples are within error bars and are possibly an effect of sample preparation. This initial increase of 1(T)with temperature is believed to occur due to a limited mobility of PBrS segments at the interface. At temperatures below T,(PS) both compounds are immobile. Starting above T,(PS) one component can (18)Wilhelm,T.;Hofmann,R.;Fuhrmann,J.Makromol. Chem.,Rapid Commun. 1983,4,81.
Hlittenbach et al. 1.5
1.0
-
PS/PBrS
0
0
1
0 T
I
110
120
0
-__---
o
I
1
I
130
1LO
TITI
Figure 6. Variation of the interfacial broadening 1 due to intermixing as a function of annealing temperature. The values of 1 are the equilibrium values given in Table I1 which are constant with time. (0) Sample SB4 (PBrS on PS); ( 0 )Sample BS5 (PS on PBrS).
diffuse into the other, thus reducing the common Tgof the mixture according to the concentration. According to the dependence of Tgon the concentration of the components, one would expect at temperatures between T (PS) = 103 O C and Tg(PBrS) = 142 O C that chains in a blend up to a certain concentration would still be mobile, while for higher concentrations Tgof the blend will not be reached. Those mixtures will thus not be formed at this temperature leading to a highly asymmetric density profile at the interface. The temperature dependence of the interface width is thussolely an effect of the different glasstransition temperatures of PS and PBrS. Due to the strong incompatibility of the materials, the interface width is generally very small and we cannot resolve the details of the profile. It should be emphasized that all the observed values of 1 in Table I1 and Figure 6 at T < Tg(PBrS)are equilibriumvalues and do not change anymore with time. The initial time dependence at small times cannot be resolved due to the small interface width. Taking the limiting value at T 1 135 "C of 1 = 1.3 f 0.2 nm as the interface width which should be compared with theory from eq 8 we obtain an experimental x parameter for the investigated system of x = 0.1 f 0.03. We have assumed a mean segment length of a = 0.82 nm according to Meier.11 Thus we obtain reasonableagreement between the theoretical estimate and the experimentally determined Flory-Huggins parameter x taking into account the large uncertainties in the extrapolation of the parameters used. An intriguingeffect arises when the samples are annealed at temperatures above Tg(PBrS). While SB-type bilayer samples with PBrS on top roughen after annealing at 144 O C for only a few minutes, the BS-type samples with a PS top layer remain perfectly smooth even after annealing for a day. Only at 153 "C and annealing for several hours, these samples get very rough, too. Further investigations by X-ray reflectrometry for both types of samples are then impossible, because no interference fringes occur anymore in the reflectivity curves and thus no information about the polymer layers can be deduced. We therefore investigated the sample surfaces by phase interferencemicroscopy. Figure 2a shows a typical surface of a single PBrS film on glass. The sketched area has lateral dimensions 42 X 42 pm*. The root mean square roughness within this area is 0.6 nm. This value compares well with corresponding X-ray fits. The surface of a single polymer film of either PS or PBrS on glass remains unchanged after annealing the sample at 160OC for several hours which again is in full agreement with the X-ray reflectometry results. When a bilayer sample with PBrS on top is annealed at 144 O C for 30 min, the sample surface however looks like the area sketched in Figure 2b.
Spherical holes with steep rims emerge. The bottom level is about 50 nm below the surrounding mean level. The overall thickness of the bilayer as measured by X-ray reflectrometry is 82 nm, while for the top layer 46 nm is found. This indicates that only the upper layer begins to break up and the bottom film seems to remain intact. The mean roughness of the area shown in Figure 2b is 37 nm. After the same sample was annealed at 144 OC for a longer period, the whole sample becomes opaque as aconsequence of the scattering of light at the rough surface. A possible explanation for this behavior might be that the upper PBrS film breaks up due to the strong incompatibility in order to minimize the common contact area with the PS film below (dewetting). That this effect is indeed a consequence of the strong incompatibility can be seen from the fact that a single layer of PBrS on glass remains smooth even after annealing at higher temperatures. The low molecular weight of the PBrS is supposed to support the roughening, since the chains can diffuse faster and are less entangled. Rough bilayers with PS on top show pictures similar to those with a PBrS top layer. Besides the different molecular weights of PS and PBrS used, also different surface tensions may explain that bilayers with PS on top roughen at higher temperatures as compared to those with PBrS on top.
Conclusion The interface between PS and PBrS has been accurately investigated as a function of annealing temperature by X-ray reflectrometry. The technique proves to be extremely sensitive on the interface width. Due to the surface roughness of the individual films an initial broadening of approximately 1.0 nm is observed when two films are brought into contact. Intermixing starta at annealing temperatures above the glass transition temperature of PS where PS is the mobile component and swellsthe immobile PBrS. After some hours equilibrium is reached and the intermixing comes to a standstill due to the strong incompatibility of the materials. A maximum of 1.3 nm for the interface broadening is reached at 135 "C. The corresponding interaction parameter agrees nicely with theory while the detailed density profile at the interface cannot be resolved. To get this profile, which is expected to be discontinuous below Tg(PBrS), one would have to perform the experiments with other polymers which form a wider interface. The choice of polymers is, however, largely limited since a strong electron density difference between the two components is necessary for X-ray reflectivity experiments. The application of neutrons offers in that respect the advantage that a contrast can easily be achieved by deuteration of one component. Quite unexpectedly we also observe a significant roughening of the film surface when the layer system is heated above T&PBrS). Since this effect depends on the nature of the top layer, PS or PBrS, we believe that the surface tensions of the materials as well as their mobility play a significant role. The formation of lateral structures is nicely seen in a phase interference microscope which is a perfect supplementarytechnique to reflectrometry because of its lateral resolution. The application of both techniques gives a much more complete picture on the interfacial mixing of incompatible polymers in the direction both perpendicular and parallel to the surface and helps in the understanding of the interface formation. Acknowledgment. We gratefully acknowledge technical help of M. Bach and financial support by BMF'T. RegietryNo. PS (homopolymer),9003-53-6;PBrS (homopolymer), 24936-50-3.
