Langmuir 1994,10, 1865-1870
1865
Temperature-Modulated Environmental Responses on the Surface of Poly(viny1 alcohol)-Polystyrene Graft Copolymers Yasuyuki Tezuka* and Akitoshi Araki Department of Material Science and Technology, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-21,Japan Received August 9,1993. I n Final Form: February 22,1994' Environmental responses of a polystyrene, PS, surface layer formed on a poly(viny1 alcohol), PVA, sublayer were studied. The thin organic surface was prepared by casting the solution of a copolymer consisting of covalently-connectedPVA and PS segments, PVA-PS graft copolymer. X-ray photoelectron spectroscopic inspection of the graft copolymer sample films confirmed the formation of a surface layer of the PS component of a thickness of a few tens of angstroms. By changing the surface contacting medium from air to water at 25 O C , which is significantly below the Tgof the PS segment, an unexpected dynamic surfacerearrangement was observed by means of a contact angle measurement with an air-in-watertechnique, where the contact angle changed from one similar to the PS homopolymer to one close to the PVA homopolymer within a 1-3-h period. At 10 "C, in contrast, the environmental response was strongly suppressed. The graft copolymer film was immersed into water to effect surface rearrangement,recovered, dried at 120 OC, Le., above the Tgof the PS segment,and found to restructure itself reversely to the original surface state, while at 60 O C the reverse process was found to be incomplete.
Introduction
A thermodynamic consideration of a surface formation process by a mixture of two fluid phases implies the complete covering of the free surface by the component fluid having the lower surface energy.l This principle is considered to hold also during the surface formation by casting the solution of two-component polymer systems, i.e., polymer blends and block and graft copolymer systems, where the polymer solution maintains sufficient fluidity until the last moment of the formation of a solid film. And since two-component block and graft copolymers,in which immiscible segments are covalently connected to each other, generally result in a microphase separation structure with the domain size basically relative to the radius of gyration of each component, the thin surface layer (tens to hundreds of angstroms in thickness) of the component with the lower surface energy can be formed over the sublayer of the component with the higher surface energy.2-9 In contrast, immiscible polymer blends generally result in a macroscopic phase separation to form a thicker overlayer of one polymer component.1° This surface coverage of one component in block and graft copolymer systems is particularly distinct in those containing a polysiloxane component possessing significantly lower surface energy than most organic p0lymers,2*~*~ and even in block and graft copolymer systems comprised of ~-
~~~
Abstract published in Advance ACS Abstracts, May 1, 1994. (1) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. f2) Tezuka Y.: Fukushima A.:Mataui. S.: Imai. K. J . ColloidInterface Sci:-1986, 114, 16.
(3) Tezuka, Y.; Okabayashi, A.;Imai,K. J. ColloidInterface Sci. 1991, 141, 586. (4) Tezuka, Y.; Ono, T.; Imai, K. J. Colloid Interface Sci. 1990, 136, 408. (5) Tezuka, Y.; Kazama, H.; Imai, K. J. Chem. SOC.,Faraday Trans. 1991, 87, 147. (6) Tezuka, Y.; Yoshino, M.; Imai,K. Langmuir 1991, 7, 2860. (7) Haaegawa, H.; Hashimoto, T. Polymer 1992,33,475. (8) Jones, R. A.; Kramer, E. J. Polymer 1993,34, 115. (9) Budkowski, A.; Klein, J.;Shiner,U.; Fetters, L. J. Macromolecules 1993,26, 2470. (10) Steiner,U.; Klein,J.;Eiser,E.;Budkowski,A.;Fetters,L. J. Science 1992,258,1126.
0743-7463/94/2410-l865$04.50/0
two immiscible polymer components of minimal differences in surface energy.%g At the surface state,polymer molecules can interact not only with the surrounding identical molecules as in the bulk state but also with another in a different phase within a short distance through medium- and long-range surface forces. A polymer chain conformation at the surface or interface can also be distorted compared with that in the bulk state. Hence, the block and graft copolymer film surface is considered to provide a unique opportunity to examine the characteristics of the surface state of organic materials, which can then be compared to those of the bulk state. Although model surfaces produced by LangmuirBlodgett or by self-assembled monolayer techniques are alternative means for the elucidation of the nature of the surface state and surface interactions,ll it should be pointed out that the surfaces prepared by these methods are rigid in most cases due to the self-association and the eventual self-packing of long alkyl chains to form a pseudocrystallinestate.12 Therefore, they are not directly relevant to common polymer material surfaces, which consist of polymer molecules of random coil conformation with inter- and intramolecular entanglements. In the present study, we report on the surface study of a two-component polymer system, poly(viny1 alcohol) (PVA)-polystyrene (PS)graft copolymers, in which uniform size PS segments are covalently connected to the main-chain PVA segment.13 PVA is hydrophilic but not readily swelled or dissolved in water due to its crystalline property, and PS is, in contrast, hydrophobic and glassy in the bulk state (Tgof 80-90 OC). The surface formation process by casting this graft copolymer is expected to produce a complete coverage by the PS component. And the dynamic surface nature of the PS surface layer formed on the PVA sublayer was studied by changing the surface contacting phase from air to water at various temperatures. (11) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28,506. (12) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M. J. Am. Chem. SOC. 1989,111,321. (13) Tezuka, Y.; Araki, A. Makromol. Chem. 1993,194, 2827.
0 1994 American Chemical Society
Tezuka and Araki
1866 Langmuir, Vol. 10, No. 6, 1994
Chart 1
Scheme 1 Ch;
sample film
air
I
sample film " k% 2
'%Pis
3
Table 1. Poly(viny1 alcohol)-Polystyrene Graft Copolymer Samples
M,,of polystyrene polystyrene contact sample graft" content (wt %) MWb ()(lo4) anglec(deg) 7.9 4.5 88 1 2500 43.2 2.2 d 2 3200 9.6 5.9 96 3 4400 3.8 d 4 4400 21.7 36.7 2.5 88 5 4400 70.3 1.6 86 6 4400 9.8 9.2 86 7 5500 4.4 90 8 5500 29.1 3.8 88 9 5500 44.3 10 5500 62.7 1.6 85 "See also Scheme 1. *By GPC measurements of poly(viny1 acetate)-polystyrene graft copolymers. By a water-in-airtechnique. d Not determined.
Experimental Section 1. Materials. A series of poly(viny1 alcohol) (PVA)polystyrene (PS)graft copolymers used in the present study were prepared according to the method shown in Scheme 1 and are listed in Table 1, where uniform size PS macromonomers having a vinylsilane end group were synthesized and subjected to the radical copolymerization with vinyl acetate, followed by the conventional saponification with methanolic NaOH.I3 The samples were purified by Soxhlet extraction with methanol. The molecular weight of graft copolymers was in the range of 10L105 as estimated by gel permeation chromatographic (GPC) measurements of the precursor poly(viny1acetate)-polystyrene graft copolymers. P S homopolymer was synthesized through a bulk radical polymerization with azobisisobutyronitrile as an initiator for 3 h a t 60 "C. The molecular weight (GPC) of the P S obtained was 1.1 X 105. PVA homopolymer (PVA 117H, D P = 1700) was obtained from Kuraray Co., and purified by reprecipitation from the water/methanol system, followed by Soxhlet extraction with methanol. Other reagents were used after conventional purification procedures. 2. Measurements. X-ray Photoelectron Spectroscopic ( X P S ) Measurements. X P S measurements were carried out with a Shimadzu ESCA 750 equipped with an ESCAPAC 760 data system.6 Typical operating conditions were as follows: Mg K q p radiation with 8 kV and 30 mA. The pressure in the instrumental chamber was kept below 5 x 106 Pa. Angledependent X P S measurements were carried out by using a sample holder adjusted to maintain the photoelectron escape angle a t 45" and 15". Samples of PVA-PS graft copolymers as well as PVA homopolymer for X P S measurements were prepared by casting dimethyl sulfoxide (DMSO) solution (ca. 2 w t %), or DMSO/N,N-dimethylformamide (DMF) ( l / l by volume) solution (ca. 2 wt 5%) in cases where the P S content in the graft copolymer exceeds 40 mol % ,onto a n aluminum sheet, evacuating the solvent for 1day a t room temperature and for 1day a t 40 "C, and finally annealing the sample for 4 days a t an elevated temperature (80120 " C ) in order to achieve an equilibrium surface structure. A
air water
PS homopolymer sample film was prepared by casting 2 w t % T H F solution onto an aluminum sheet, and the solvent was evaporated for 1 day under ambient conditions and evacuated for 4 days at room temperature. T h e samples thus prepared were stored under dry nitrogen until the X P S measurement to avoid any contamination from ambient atmosphere. Contact Angle Measurements. Contact angle measurements were performed with a contact angle meter CA-A manufactured by Kyowa Kagaku Co. A water-in-air and an airin-water technique were employed in the present s t ~ d y A. ~sessile droplet technique was applied in the water-in-air system, where a water droplet was placed on the film surface in an environmental chamber. In the air-in-water system, a film sample was immersed into water (chromatography-grade distilled water from Nacalai Tesque) thermostated a t a prescribed temperature and an air bubble was placed from the bottom using a curved needle a t prescribed time intervals. The contact angle (0) recorded in the water-in-air and in air-in-water methods is indicated in Chart 1. The contact angle of each film sample was measured on a t least five spots (generally within &2"), and these measurements were averaged. PVA-PS graft copolymer samples and the relevant homopolymer samples for contact angle measurements were prepared on a clean glass plate (2.5 x 7.5 cm slide glass pretreated with hot HN03 for more than 15 h) by procedures similar to those described for X P S measurements. Reversibility of the SurfaceRearrangement Process. The reversibility of the surface rearrangement process on PVA-PS graft copolymer films was examined by means of the following two contact angle measurements. In the first procedure, a PVAPS graft copolymer sample was immersed into water for 24 h a t 25 "C, and the sample was recovered from water and was subjected to evacuation for 48 h a t either 60 or 120 "C. The surface of the sample film was examined again by a contact angle measurement with a water-in-air and with a n air-in-water method. The immersion into water and the recovery from water to dry at either 60 or 120 "C were then repeated once again to observe contact angles of the sample. In the second procedure, the sample film, which was immersed into water for 24 h at 25 "C, was recovered and subjected to drying under vacuum for 48 h a t 60 "C, and then immersed into toluene for a specified period a t 25 "C. The sample was recovered and evacuated for 24 h a t 60 "C to perform the contact angle measurement.
Results 1. XPS Measurements. XPS analysis on the d r y surface of a series of poly(viny1 alcohol) (PVA)-polystyrene ( P S ) g r a f t copolymers was carried out t o c o m p a r e w i t h PVA and PS homopolymers. A typical e x a m p l e of fullr a n g e XPS results for PVA-PS g r a f t copolymer s a m p l e s is s h o w n i n Figure 1. Peaks d u e solely to C1,and 01,f r o m PVA and PS s e g m e n t s were observed, indicating that t h e s a m p l e surface w a s free f r o m c o n t a m i n a t i o n e i t h e r by a l u b r i c a n t of t h e joint of the glass apparatus used i n t h e s y n t h e t i c p r o c e d u r e (Si) or b y o t h e r incidental sources. XPS results for t h e C1, and 01, regions on PVA-PS g r a f t copolymers and the relevant homopolymers are collected i n Figure 2, w h e r e the results of angle-dependent XPS m e a s u r e m e n t s are also listed to observe the elemental ratio closer to the surface of the sample film b y changing the escape angle of the photoelectron f r o m 90" to 45" and 15". The relative i n t e n s i t y of the 01, signal f r o m the P V A component i n the graft copolymer s a m p l e film w a s notably lower than that i n t h e P V A homopolymer, a n d diminished
Langmuir, Vol. 10, No. 6,1994 1867
Poly (vinyl a Zcoho I)-Polystyrene Graft Copolymers
Gs
1
Figure 1. Full-range XPS result for a poly(viny1 alcohol)polystyrene graft copolymer (sample 8 in Table 1).
A
2
3 J 290
l
l
L i
288 286 284 282 CIS B. E.(ev)
-
n 538536534532 Ols B.E.(eV)
Figure 2. C1, and 01,region XPS results for (1) poly(viny1 alcohol) homopolymer, (2) polystyrene homopolymer, and (3AC) poly(viny1alcohol)-polystyrene graft copolymer (sample 8 in Table 1). The escape angle of the photoelectron is given in degrees).
further at the lower escape angles of photoelectrons, Le., closer to the surface. And as shown in Figure 2, the XPS profile of the topmost surface (presumably 10-20 A) of the graft copolymer was nearly identical to that of the PS homopolymer. 2. Contact Angle Measurements. Contact angle measurements for the surface of the PVA-PS graft copolymers were performed both in air by means of a waterin-air technique and in water a t various temperatures ranging from 10 to 57 O C by means of an air-in-water technique. In the dry state, the contact angle measurement with a water droplet could distinguish the surface of the PVA homopolymer (0 = 5 6 O ) from that of the PS homopolymer (0 = 87'). And a series of PVA-PS graft copolymers showed almost identical contact angle values with that of the PS homopolymer (Table 1). Since the top few angstromscan be sensed by contact angle measurement,"
4
Figure 3. Views of an air bubble a t the surface of poly(viny1 alcohol)-polystyrene graft copolymer (sample 4 in Table 1) immersed in water a t 25 "C (1)immediately after the immersion and after (2) 10 min, (3) 30 min, and (4) 90 min.
the uppermost surface of the graft copolymer films is considered to be covered totally with the polystyrene component. This also agrees with the observation by XPS described in the previous section.
Tezuka and Araki
1868 Langmuir, Vol. 10, No. 6, 1994
1
i 0;
m T i m e
120
180
I
I
0;
60 T i m e
< m i n >
Figure 4. Time vs contact angle variations on poly(viny1 alcohol)-polystyrene graft copolymer films after immersion into water at 25 "C, measured by the air-in-water technique: (A) poly(viny1alcohol)homopolymer,(A)polystyrenehomopolymer, and poly(viny1alcohol)-polystyrenegraft copolymersof different polystyrene content, (0)sample 7, ( 0 )sample 8, (0)sample 9, and (m) sample 10 in Table 1. Then a series of PVA-PS graft copolymer film samples prepared on a glass plate were immersed in distilled water thermostated at a prescribed temperature. And an air bubble was placed a t the surface of the graft copolymer film in water a t appropriate time intervals. The surface of PVA and that of polystyrene could be distinguished from each other and contact angle values for both homopolymers were scarcely changed during 3-h immersion in water. On the other hand, the contact angle of the graft copolymer surfaces underwent dynamic change within a 1-3-h period from one similar to that of the PS homopolymer to one similar to that of the PVA homopolymer. Typical views of an air bubble attached to the graft copolymer sample film surface immersed in water a t 25 "Care shown in Figure 3. This change of the contact angle is believed to correspond to the surface rearrangement process occurring on PVA-PS graft copolymer films. The dynamics of this surface rearrangement process was examined at 25 "C for a series of PVA-PS graft copolymers with various PS contents and segment lengths. The results are collected in Figures 4 and 5, respectively. As was previously observed in the system of PVA and polyurethane-based graft and block copolymers having low Tgsegments such as polysiloxane and poly(tetramethylene the dynamic change of the contact angles was observed at the surface of PVA-PS graft copolymers. And the higher the P S content in the graft copolymer, the slower the surface responded (Figure 4), and the longer the PS segment length, the marginally faster the surface rearrangement occurred (Figure 5). The results of contact angle measurements for the graft copolymer sample film in water a t different temperatures from 10 to 57 "C are listed in Figure 6. The surface response was observed at 25 "C and at higher temperatures, while the increase of the temperature did not cause noticeable influence on the rate of the surface rearrangement process. At 10 "C, in contrast, the contact angle remained almost unchanged and was close t o that of the PS homopolymer during a 2-h period. The suppression of the environmental response at 10 "C was observed for all PVA-PS graft copolymer samples with different PS segment contents and segment lengths.
1M
180
( m i n l
Figure 5. Time vs contact angle variations on poly(viny1 alcohol)-polystyrene graft copolymerfilms after immersion into water at 25 "C, measured by the air-in-water technique: (A) poly(viny1 alcohol)homopolymer,(A)polystyrenehomopolymer, and poly(viny1alcohol)-polystyrenegraft copolymersof different sample 2 , ( 0 )sample 5 , and (0) polystyrene segment lengths, (0) sample 9 in Table 1.
150 140
130
~
y 120
"
110
2 100
L
00
60
120
100
'1. m (m I "> Figure 6. Time vs contact angle variations on poly(viny1 alcohol)-polystyrene graft copolymer films after immersion into water at different temperatures,measured by the air-in-water technique: (A) poly(viny1alcohol) homopolymer at 57 "C,(A) polystyrene homopolymer at 40 "C, and a poly(viny1alcohol)10 "C, polystyrene graft copolymer (sample 8 in Table 1)at (0) ( 0 )25 "C, (m) 45 "C, and (0) 57 "C. I
(5
Discussion The XPS and contact angle inspection of the dry surface of poly(viny1 alcohol) (PVA)-polystyrene (PSI graft copolymers with reference to that of PVA and PS homopolymers revealed that the PS component covers the surface of cast film samples. The surface accumulation of the segment component having the lower surface energy in the two-component block and graft copolymers was previously observed also for other PVA-based graft copolymers having either poly(dimethylsi1oxane) or poly(tetramethylene oxide) graft segments, which have lower surface energies than PVA.2,3 The bulk structure of PVA-PS graft copolymers constitutes a "sea-island"-type microphase separation morphology as schematically shown in Figure 7 , due to the incompatibility of each component.13 And during the film formation process by casting the solution of the graft
Poly(viny1 alcohol)-Polystyrene Graft Copolymers Aic rPS
Langmuir,
Vol. 10, No.6, 1994 1869
Table 2. Contact Angle Variations on Poly(viny1 alcohol)-Polystyrene Graft Copolymers after Annealing and after Toluene Treatment. contact angle (dea) run treatmentb temp ("C) time (h) air-in-water water-in-air
1 Water
J
m / / P/
P/ /
//f////
Figure 7. Schematic picture of a poly(viny1alcohol)-polystyrene graft copolymer surface in the dry (top) and wet (bottom) states at 25 "C.
copolymer, the PS component is favored to accumulate at the film/air interface due to ita lower surface energy than the PVA component, while the PS component of low concentration forms "island" domains of several tens of angstroms in the bulk. The thickness of the surface PS layer is considered to increase along with the total PS content in PVA-PS graft copolymers, as in the previous polyurethane-polysiloxane block copolymer system.5 By immersion into water at 25 "C, the surface of PVAPS graft copolymer films restructures itself from a hydrophobic to hydrophilic structure within a few hours. The rate of this surface rearrangement is correlated to the structural parameters of the graft copolymer,in particular the total content and the segment length of the PS component, which forms the topmost surface layer in the dry state. The surface of PVA-based graft copolymers having either poly(dimethylsi1oxane) or poly(tetramethylene oxide) segments underwent relevant surface rearrangement much more rapidly (within 10-15 min).2J On the other hand, the polyurethane-polysiloxane block copolymer surfaces responded to the environmental change within a few hours.495 Thus, the mobility of the surface layer is influenced not only by the nature of the graft segment component but also by the nature of the main chain segment component, which forms the sublayer. The occurrence of an environmental response at the PS surface is believed to be of significant interest since this implies high mobility of the PS segment at a significantly lower temperature than its Tg(80 "C). The lower Tgof PS segments at the surface layer than that in the bulk was recently suggested from an AFM study,14J5particularly when the molecular weight of PS is lower than its Me (critical molecular weight for the entanglement). Indeed the Me of PS is 3 X lo4and substantially higher than the molecular weight of the present PS graft segment.16 In addition, the structure of water at the vicinity of the surface is reportedly different from that in the bulk, and this can promote the surface rearrangement to occur, as proposed recently to explain the surface rearrangement behavior of plasma-treated polymer ~ u r f a c e s . ~ ~The J * presence of the specific interaction of phenyl groups in PS segments and water molecules, as detected in benzene and water, may also be ~0nsidered.l~ On the other hand, no surface rearrangement was observed at 10 "C, indicating that the PS surface layer (14) Meyers, G. F.; DeKoven, B. M.; Seitz, J. T. Langmuir 1992, 8, 2330. (15)Annis, B. K.; Schwark, D. W.; Reffner, J. R.; Thomas, E. L.; Wunderlich, B. Makromol. Chem. 1992, 193, 2589. (16) Wool, R. P. Macromolecules 1993,26, 1564. (17) Yasuda,H.;Charlson,E.J.;Charlson,E.M.;Yasuda,T.;Miyama, M.; Okuno, T. Langmuir 1991, 7, 2394. (18) Yasuda, T.; Miyama, M.; Yasuda, H. Langmuir 1992,8, 1425. (19) Suzuki,S.;Green,P. G.;Bumgarner,R. E.; Dasgupta, S.;Goddard 111, W. A.; Blake, G. A. Science 1992,257, 942.
2 3 4
anneal anneal toluene toluene
60 120 25 25
48 48 24 72
131 (136)' 106 (102)' 134 138
78(80)c 90 (88)' 83 80
The sample (no. 8 in Table 1)was immersed into water at 25 O C for 24 h, and subjected to either annealing or toluene treatment. The contact angles by the air-in-water technique (deg) were 147 (run 1) and 145 (run 2), respectively. Anneal: the sample was recovered from water and annealed. Toluene: the sample was recovered from water and first annealed at 60 "C for 48h, then immersed into toluene for a prescribed period, and finally dried at 60 O C for 48 h. e The immersion into water and drying were repeated. The value for the second run is given in parentheses.
*
becomes rigid and glassy in contrast to the flexible nature at 25 "Cor higher temperatures. This result demonstrates that a unique temperature modulation is operating in the present surface response on PVA-PS graft copolymers. A plausible picture of the surface rearrangement process is shown in Figure 7. The rearrangement of the PS surface layer occurs to form a coagulated structure from the spreading one. Alternatively, the PVA domain covers the PS domain to form a practically pure PVA homopolymer surface, although any noticeable swellingwas not observed in contact with water at the range of the measurement temperatures within a 1-3-h period. It will be of interest to elucidate the driving force of this surface rearrangement process to occur. At the immersion of the sample film into water, the water phase is separated from the hydrophilic PVA sublayer by the presence of the top PS layer of several tens of angstroms. Therefore, the formation of the thermodynamically favored PVAIwater interface requires first the penetration of water molecules into the PS layer in a short period. Since simple diffusion of water vapor through a polystyrene layer is probably too slow to cause a drastic structural rearrangement within such a short time, a long-range attractive force is considerable to operate between the two phases. The relevant surface forces between the hydrophobic layers and between the hydrophobic and polar forces are reportedly effective as far as several tens of angstroms.20-23 The reversibility in the present surface rearrangement process was examined by recovering the sample film from water and drying under vacuum for 48 h at 120 "C (Table 2). The contact angles of the dried sample surface were examined by the water-in-air and air-in-water techniques, and they were almost identical to those of the original dry state, thus demonstrating the reversible nature of this rearrangement process when the sample is dried at higher temperature than the Tgof the PS segment (80 "C). In contrast, the drying at 60 "C did not restore completely the original state as indicated by the contact angle values, in particular, by the air-in-water technique. As summarized in Table 2, the rearrangement process from the wet to the dry condition could be repeated at the annealing temperature of 120 "C,but not at 60 "C. This result is unique and interesting from the viewpoint of the surface technology of polymer materials, since this implies that the hydrophilic, high-energy surface can be fixed simply (20) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (21) Claesson, P. M.;Christenson, H. K. J.Phye. Chem. 1988,92,1650. (22) Tsao, Y. H.; Evans, D. F.; Wennerstrom, H. Langmuir 1993, 9, 779. (23) Kurihara, K.; Kunitake, T. J. Am. Chem. SOC. 1992,114,10927.
1870 Langmuir, Vol. 10, No. 6, 1994
Tezuka and Araki
through the immersion of the graft copolymer film into water and drying in vacuum a t room temperature. A different treatment of the PVA-PS graft copolymer film was carried out to examine the reversibility of this surface rearrangement process. Thus, the sample film was first immersed into water at 25 "C to cause the surface rearrangement. The resulting film with a hydrophilic surface was then dried a t 60 "cto maintain the hydrophilic nature a t the surface. The sample film was subsequently immersed into toluene. Since toluene is a good solvent for ps and nonsolvent for PVA, the treatment Of the sample film is expected to lower the TgofthePSSurface domains by the plasticizing effect but to have no influence on the PVA domains. However, no Surface rearrangement was detected by the contact angle measurements after the immersion in toluene even for 72 h. Consequently, the rearrangement process under drying was considered to occur only when the sample film was annealed at higher temperatures than both Te'sof PS and poly(viny1alcohol). The Tgof poly(viny1alcohol) depends on the degree of crystallization of the sample,24and is around 60 OC. In addition, the surface tension of the plasticized PS, which is assumed to be identical to that of PS (39.4 d y n / ~ m ) , is ~ 5marginally higher than the critical surface tension of wetting of the PVA surface (37 dynl
~ m ) This . ~ can ~ prevent the spreading of the PS domain over the surrounding PVA solid surface. In conclusion, the present study revealed an unexpected mobility of the PS surface layer formed on a PVA sublayer a t 25 "C, lower than the bulk Tgof PS. Thus, the surface can respond to the change of the contacting phase from air to water by reorganizing the interfacial structure within a relatively short time. In addition, this rearrangement process was strongly suppressed at oc, to indicate the possibility of the temperature modulation of the environmental responses occurring on the organic surfaces. And the rearrangement process progressed reversely by drying the sample film at higher temperatures than those required for the rearrangement to Occur in water. This indicates that a high-energy state may be fixed a t the polymer surfaces. Hence, the present findings provide new insight into the nature of the surface and interface of organic and polymer materials, and also into the m h c u l a r design of Polymer material Surfaces applied in wide fields from biology to tribology.
Imai, K.; Shiomi,T.; Tezuka, Y.; Itamochi, H.; Miya, M. J.Appl.
(25) Brandrup, J.; Immergut, E. H., Ed. Polymer Handbook, 3rd ed.; Wiley Interscience: New York, 1989.
(24)
Polym. Sci. 1993,48, 1525.
Acknowledgment. We thank Professor Y. Yoshida, Toyo University, Saitama, for kind cooperation in the XPS measurements. Financial support from the Sumitomo Foundation is gratefully acknowledged. This work was also supported partly by a grant from the Ministry of Education, Science and Culture, Japan (04205055).
