Polystyrene-block-Poly(ethylene oxide

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Polystyrene-block-Poly(ethylene oxide) Stars as Surface Films at the Air/Water Interface Jennifer L. Logan,† Pascal Masse,† Yves Gnanou,# Daniel Taton,# and Randolph S. Duran*,† The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, and Laboratoire de Chimie des Polyme` res Organiques, ENSCPB, Universite´ Bordeaux I, 16 Avenue Pey-Berland, 33607 Pessac Cedex, France Received March 24, 2005. In Final Form: May 19, 2005 Star diblock copolymers containing polystyrene (PS) and poly(ethylene oxide) (PEO) were investigated as surface films at the air/water interface. Both classic and dendritic-like stars were prepared containing either a PS core and PEO corona or the reverse. The investigated polymers, consisting of systematic variations in architectures and compositions, were spread at the air/water interface, generating reproducible surface pressure-area isotherms. All of the films could be compressed to higher pressures than would be possible for pure PEO. For stars containing 20% or more PEO, three distinct regions appeared. At higher areas, the PEO absorbs in pancakelike structures at the interface with PS globules sitting atop. Upon compression, a pseudoplateau transition region appeared. Both regions strongly depended on PEO composition. The pancake area and the pseudoplateau width and pressure increased in a linear fashion with an increasing amount of PEO. In addition, minimum limits of PEO chain length and mass percentage were determined for observing a pseudoplateau. At small areas, the film proved less compressible, producing a rigid film in which PS dominated. Here, the film area increased with both molecular weight and the amount of PS. Comparison with pure linear PS showed the stars spread more, occupying greater areas. Among the stars, the PEO-core stars were more compact while the PS-core stars spread more. The influence of architecture in terms of the core/corona polymers and branching were also examined. The effects of architecture were subtle, proving less important than PEO chain length or mass percentage.

Introduction Diblock copolymers represent an interesting class of compounds due to the combination of two polymer blocks containing different, unique properties. A myriad of proposed applications includes information storage, drug delivery, and photonic crystals.1 Amphiphilic diblock copolymers, in which one block is hydrophobic and the other hydrophilic, prove especially intriguing in terms of biological membrane applications. These polymers can be self-assembled into a variety of morphologies, depending on the relative amounts of both blocks. In choosing an amphiphilic diblock copolymer, a popular choice is poly(ethylene oxide)-b-polystyrene (PEO-b-PS). PEO is biocompatible, while PS represents an inexpensive polymer that can be easily prepared. In addition, this diblock copolymer is appropriate for surface pressure studies involving Langmuir troughs. This technique provides a simple means for controlling the surface density of a self-assembled monolayer. When applied to the air/ water interface, the PEO is attracted to the water while the hydrophobic PS serves as an anchor, maintaining the PEO at the air/water interface. Linear PEO-b-PS has been shown to form stable, condensed surface films.2-12 * To whom correspondence should be addressed. Phone: 352392-2011. Fax: 352-392-9741. E-mail: [email protected]. † University of Florida. # Universite ´ Bordeaux. (1) Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; John Wiley and Sons: Hoboken, NJ, 2003. (2) Bijsterbosch, H. D.; de Haan, V. O.; de Graaf, A. W.; Mellema, M.; Leermakers, F. A. M.; Cohen Stuart, M. A.; van Well, A. A. Langmuir 1995, 11, 4467. (3) Gonç alves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547.

Upon spreading linear PEO-b-PS at the air/water interface, numerous studies have observed different molecular arrangements dependent upon surface pressure. At large areas, the PEO exists in what has been described as a pancakelike structure. The hydrophobic PS sits atop these pancakes in the form of compact globules. When compressed, the macromolecules crowd together, their interaction resulting in an increase in surface pressure. The PEO is believed to desorb at a pressure equivalent to homopolymer PEO collapse (10 mN‚m-1),13 forming a plateaulike region. Due to the anchoring effect of the PS, the PEO remains attached to the interface, stretching into the water to form a brush structure. Several studies support this interpretation. Bijsterbosch et al.2 and Gonç alves da Silva et al.,3,4 for example, both examined the same series of PS-b-PEO diblock copolymers containing a constant PS length and varying amounts of PEO. The copolymers were postulated to undergo a pseudofirst-order transition from a pancakelike structure to that (4) Gonç alves da Silva, A. M.; Simoes Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998, 14, 5327. (5) Faure´, M. C.; Bassereau, P.; Carignano, M. A.; Szleifer, I.; Gallot, Y.; Andelman, D. Eur. Phys. J. B 1998, 3, 365. (6) Faure´, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32, 8538. (7) Cox, J. K.; Yu, K.; Constantino, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15, 7714. (8) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417. (9) Richards, R. W.; Rochford, B. R.; Webster, J. R. P. Polymer 1997, 38, 1169. (10) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 1921. (11) Gragson, D. E.; Jenson, J. M.; Baker, S. M. Langmuir 1999, 15, 6127. (12) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432. (13) Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 786.

10.1021/la050787c CCC: $30.25 © 2005 American Chemical Society Published on Web 06/29/2005

Surface Films at the Air/Water Interface

of a brush upon compression. Faure´ et al.5,6 investigated a similar series of PS-b-PEO copolymers and also found that the PEO underwent a change from an adsorbed layer to a brush. Brewster angle microscopy (BAM) studies indicated this to be a first-order phase transition. While this brush model is discussed the most often in the literature, Cox et al.7,8 provided a second interpretation. They agreed with Richards et al.9 in contending that brush formation was not possible due to PEO’s low surface energy. The PEO instead forms a surface excess layer; a very high surface concentration would be required to reduce the surface tension of water below that of PEO, thereby pushing the PEO into a brush. This pressure would be so high that film collapse would occur before the necessary conditions are achieved. In the expanded region, Cox et al. envisioned a model similar to that of Gonç alves da Silva et al.3,4 in which the PEO exists as a coherent film covering the water surface with PS globules sitting on top. They disagreed, however, with the popular interpretation that the isotherm pseudoplateau represented a pancake-brush transition. They instead proposed that this transition reflects dehydration of the PEO followed by a subsequent change in conformation. In addition to linear PS-b-PEO systems, star architectures have also been prepared. These include classic stars in which one branching point exists and dendritic stars.14,15 The latter involves a core of one block in which each arm has two branches of the second block forming the corona. At the air/water interface such stars exist, in essence, as unimolecular micelles. This assembled structure can thus occur on an even smaller scale than micelles formed from numerous linear chains. Preliminary isotherm studies were presented for the synthesis of three- and four-arm PS-core stars.14 We found the three regions apparent in linear PS-b-PEO to be present in star systems as well. In addition, we have investigated aggregation and surface morphology of PS-b-PEO stars through atomic force microscopy (AFM).16,17 Both works showed spontaneous aggregation among domains, similar to behavior described in linear systems.7 Peleshanko et al.18,19 have also investigated alternative architectures with heteroarm PS-b-PEO stars. While they found their stars to behave similarly to linear chains, the heteroarm stars proved to be more compact, forming more stable circular domains when seen through AFM. The existence of branching points would presumably lead to new interfacial properties due to a difference in chain density. Comparing such structures to linear homologues could lead to better behavioral control in terms of architecture and composition. In this study, we continue the investigation into architecture and how composition affects star behavior at the air/water interface. A series of structures, including both PEO- and PS-core stars shown in Figure 1, was investigated. The PEO-core structures consist of classic and dendritic-like stars. These were prepared according to a previously reported procedure.15 The PS-core stars consist mainly of classic stars, with one dendritic-like one. (14) Francis, R.; Taton, D.; Logan, J. L.; Masse, P.; Gnanou, Y.; Duran, R. S. Macromolecules 2003, 36, 8253. (15) Angot, S.; Murthy, K. S.; Taton, D.; Gnanou, Y. Macromolecules. 2000, 33, 5418. (16) Francis, R.; Skolnik, A. M.; Carino, S. R.; Logan, J. L.; Underhill, R. S.; Angot, S.; Taton, D.; Gnanou, Y.; Duran, R. S. Macromolecules 2002, 35, 6483. (17) Logan, J. L.; Masse, P.; Dorvel, B.; Skolnik, A. M.; Sheiko, S. S.; Francis, R.; Taton, D.; Gnanou, Y.; Duran, R. S. Langmuir 2005, 21, 3424. (18) Peleshanko, S.; Gunawidjaja; R.; Jeong, J.; Shevchenko, V. V.; Tsukruk, V. V. Langmuir 2004, 20, 9423. (19) Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V. Macromolecules 2004, 37, 6511.

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Figure 1. Model of the various PS-b-PEO star architectures. Both PS- and PEO-core structures exist, as well as classic and dendritic-like stars. The samples consist of varying PS/PEO compositions.

The synthesis of these samples can also be found in the literature.14 Both synthetic techniques involved the anionic polymerization of ethylene oxide and the atom transfer radical polymerization (ATRP) of styrene. The resulting macromolecules were well defined and of controlled molar mass, providing novel architectures suitable for characterization at the air/water interface. Study of these stars will allow the influence of both architecture and PS/PEO chain lengths to be investigated in terms of surface film behavior and flow properties at the interface. Experimental Section Materials. Stars containing a PS core and a PEO corona (PSnb-PEOy, in which n and y refer to the number of PS and PEO arms, respectively) were synthesized by R. Francis according to a previously reported procedure.14 Using a benzyl halide core, PS stars containing either three or four arms were prepared through the ATRP of styrene (St). The branch ends were then modified, allowing for the growth of PEO chains through the anionic polymerization of ethylene oxide (EO). In addition to PS core-PEO corona stars, the reverse structure was also prepared (PEOy-b-PSn) by S. Angot.15 Using a method detailed in a prior publication, a PEO star was produced through the core-first method based on a hydroxyl multifunctional initiator. The end groups were then modified followed by the growth of PS through ATRP. The resulting architectures are represented in Figure 1, while Table 1 lists the characteristics of each star. The number-average molecular weight (Mn) was calculated through 1H NMR, while SEC determined the polydispersity index (PDI). Linear chains of PS were obtained from Polysciences, Inc. Langmuir Films. The PS-b-PEO stars were characterized as surface films at the air/water interface. Each star was dissolved in chloroform at a concentration of 1 mg‚mL-1. Using a Hamilton syringe, the solution was then spread dropwise across a layer of Millipore filtered water (Ω g 18.2 MΩ‚cm-1) in a Teflon Langmuir trough system (KSV Ltd., Finland), equipped with two moving barriers and a Wilhelmy plate microbalance system. After waiting 30 min to allow for complete evaporation of the chloroform, the surface film was compressed at (10 mm‚min-1 with a linear compression rate of 0.5 mN‚m-1‚min-1 at 25 °C. Compressing the film generates an isotherm of surface pressure (π) vs mean molecular area (MMA). The latter represents the average area each molecule occupies at the air/water interface. Isobaric measurements, in which a constant π was maintained, were recorded over a period of 10 h at a compression rate of (10 mm‚min-1. The change in area following this 10 h period was then reported.

Results Pressure-Area Isotherms. The PS-b-PEO stars exemplify a wide range of amphiphilicity. The PEO-core

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Table 1. The Composition, Including the Amounts of PS and PEO, of the Classic and Dendritic Star Copolymers Depicted in Figure 1

a Architecture represents the star structure where m and x are the number of St and EO units/branch, respectively, while n and y indicate the number of PS and PEO branches (in that order). The polymer listed first represents the core, while the second one is the corona. For example, (PS70)3-(PEO36)3 is a three-arm star containing a PS core of 70 styrene units/branch with a PEO corona of 36 units/branch.b Total molecular weight (Mn) was calculated using 1H NMR. c Polydispersity indices (Mw/Mn) were determined using size exclusion chromatography (SEC).

stars are predominantly hydrophobic with typically 15% mass PEO ((PEO45)3-(PS77)3 and (PEO35)4-(PS4)8 being the most hydrophilic). In contrast, the PS-core stars are more hydrophilic (>20% mass PEO) (exceptions being (PS70)3-(PEO36)3 and (PS104)4-(PEO10)4). As a result, the two groups demonstrate very different behavior. Nonetheless, some direct comparisons are possible and overall conclusions can be drawn. Another interesting note is the presence of homologous series. Within the PEO-core stars, (PEO34)2-(PS50)4, (PEO36)3-(PS48)6, and (PEO35)4-(PS42)8 represent a series in which each star contains equivalent chain lengths of PEO and PS. The difference occurs in the number of branches. For the PS-core stars, a series appears with (PS111)3-(PEO107)3, (PS111)3-(PEO195)3, (PS111)3-(PEO237)3, and (PS111)3-(PEO415)3. These three-arm stars all consist of the same PS core but different amounts of PEO corona. All of the star structures, however, proved suitable for characterization at the air/water interface. The surface pressure-area (π-A) isotherms of several star copolymers are shown in Figure 2a and b, with a log scale on the x axis for convenient visualization. (PEO36)3(PS48)6 and (PEO35)4-(PS4)8 represent the PEO-core stars, while (PS83)3-(PEO2109)3, (PS70)3-(PEO36)3, and (PS104)4(PEO247)4 are PS-core stars. Though not displayed, the other architectures showed similar π-A isotherms. The surface films were reproducible and could be compressed up to surface pressures as high as 60 mN‚m-1. As expected, stars of higher molar mass initially occupied larger areas.

One note of interest is that linear compression was necessary in order to consistently reproduce star isotherms. Faster compression rates overcompress the surface films, with the resulting isotherm being masked by viscoelastic character. This same phenomenon was remarked upon by Kato in his comment that a constant strain rate was necessary in measuring relaxation processes as its reciprocal represents a “time of observation”.20,21 Adams et al.22 also illustrated the necessity of a constant strain rate in describing a collapse process as a viscoelastic relaxation with different relaxation times. Pancake Region (I). A notable feature in some of the isotherms is the presence of three distinct regions. In Figure 2a, these areas are defined as I, II, and III and can be seen for (PS83)3-(PEO2109)3, (PS104)4-(PEO247)4, and to a smaller extent, (PEO35)4-(PS4)8. Initially, the surface film is expanded at large molecular areas (I). Here, the PEO is believed to attain a pancakelike shape when absorbed at the air/water interface. The area of this pancake region can be quantified by extrapolating the linear portion of this region’s π-A curve to a pressure of zero, yielding Apancake (Figure 2b, Table 2). Within this region, compression was reversible, yielding the same isotherm upon expansion. (20) Kato, T. Langmuir 1991, 7, 2208. (21) Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1990, 6, 870. (22) Adams, J.; Buske, A.; Duran, R. S. Macromolecules 1993, 26, 2871.


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For those stars with measurable pancake areas (Table 2), a graph of Apancake vs the number of EO units demonstrates a linear dependence (R2 ) 0.9984) with a trendline of y ) 0.3315x - 16.06. In contrast, no such Apancake dependence was found for PS. The negative y intercept of this trendline indicates that not all PEO contributes to Apancake. Pseudoplateau Region (II). Upon further compression, the surface pressure first increases and then plateaus at pressures of 8-10 mN‚m-1 (II). Though relatively constant, the pressure still slightly increases, producing a pseudoplateau. Its width (∆Apseudoplateau) is defined as the change in area between ATransition1 and ATransition2 (Table 2). These transition areas are determined as the intersections of the extrapolated lines shown in Figure 2b. For PS-core stars exhibiting a pseudoplateau, a graph of ∆Apseudoplateau vs the number of EO units yielded a linear dependence (R2 ) 0.9996) (Figure 3).23 In contrast, the amount of PS has no observable effect. Our data indicate that along the pseudoplateau, a monomer of EO occupies 14.7 Å2. In addition, an x intercept of 209 EO units shows that each PEO branch in a three-arm PS-core star must contain at least 69 EO units in order for a pseudoplateau to be observed. For a four-arm PS-core star, 52 EO units are needed. Both calculations are confirmed by (PS70)3(PEO36)3 (36 EO units/branch) and (PS104)4-(PEO10)4 (10 EO units/branch), the only PS-core stars to not exhibit a pseudoplateau. When compressed, these two block copolymers instead undergo a minimal increase in pressure prior to the steep pressure rise seen at small molecular areas.

PEO also plays a role in the pressure at which the pseudoplateau occurs. As the amount of PEO present in a star increases, so does the pseudoplateau pressure (Figure 4). Eventually, this pressure levels out at 10 mN‚m-1, as illustrated by (PS83)3-(PEO2109)3 (not shown in Figure 4). Our finding confirms that observed by Kuzmenka and Granick24 where the collapse pressure of homopolymer PEO increased with molecular weight until attaining a constant value of ca. 10 mN‚m-1. Condensed Region (III). The third region (III) occurs beyond the pseudoplateau, where the surface pressure sharply increases at low areas. In pure homopolymer PEO, such a region does not exist as the PEO is completely submerged into the aqueous phase at ca. 10 mN‚m-1. The presence of PS, however, anchors the PEO, allowing for further compression. For this region, extrapolation of the linear portion of the isotherm to π ) 0 mN‚m-1 yields Ao, the theoretical area that the most compact surface film would occupy at zero pressure. For the sake of comparison, Table 2 shows Ao normalized with respect to the number of styrene units and EO units, for those stars demonstrating some dependence. Within the condensed film area, Ao increases with increasing molecular mass. This dependence, shown in Figure 5, is expected since larger molecules would occupy larger space. One interesting note, however, is that copolymer PS samples occupy much more area than homopolymer analogues. The PS-b-PEO stars consistently occupy areas larger than would be expected in a pure PS sample of similar molecular weight. For example, a PEOcore star of smaller molar mass, (PEO35)4-(PS4)8 (Mn ) 9600 g‚mol-1), has a larger Ao than the linear PS of Mn ) 18 600 g‚mol-1. In fact, the pure PS samples yielded an average area of only 1.3 Å2/St unit and hence do not form Langmuir surface films in the usual sense. This small area indicates that the PS forms a many-multilayered 3D structure at the air/water interface, stretching into the air in order to minimize interaction with the water subphase. In addition to molecular weight, Ao also depends on the amount of PS present (Figure 6). While less linear than the pancake and pseudoplateau trendlines, the PS-core stars show that Ao increases with increasing PS (R2 ) 0.5726).25 A similar trend was also noted for the PEO-core stars. Both structures demonstrate slopes of ca. 0.05, meaning Ao depends on PS in an equivalent fashion for both structures. PEO-core stars, however, generally had lower Ao values than the PS-core ones. The average Ao/St was 8.9 and 6.2 Å2/St unit for PS- and PEO-core stars, respectively (Table 2). Another interesting note in Figure 6 is that the Ao vs PS graphs for both PS-core and PEO-core stars revealed positive y intercepts. These values mean that in a theoretical star with no PS units, some interfacial area would still be occupied in Region III. For star pairs containing the same PS core, those with longer PEO chains generally possessed larger Ao values. When comparing the PS-core and PEO-core stars, the same type of observation can be made. The shorter PEO chains in the PEO-core stars lead to lower Ao values than those seen in the PS-core stars. One exception, (PEO51)4-(PS167)4, has a higher Ao value due to longer PEO chains. Normalization with Respect to PS and PEO. The π-A isotherms could also be normalized with respect to either St units or EO ones. In some cases, this process

(23) PEO-core stars containing pseudoplateau were not included in Figure 3, as insufficient data exists to accurately characterize this architecture’s dependence of ∆Apseudoplateau on PEO.

(24) Kuzmenka, D. J.; Granick, S. Macromolecules 1988, 21, 779. (25) The trendline for Ao vs St was obtained without (PS83)3(PEO2109)3, considered an outlier due to significantly longer PEO chains.

Figure 2. (a) Several representative isotherms are shown, depicting the dependence of surface pressure (π) on mean molecular area. Three main regions appear in most of the star isotherms. (b) An isotherm of (PS83)3-(PEO2109)3 depicting how measurements for the three principal regions are obtained.

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Table 2. Measurements Obtained from Isotherm Experiments of the Stars. Values Were Extrapolated Using the Procedure Shown in Figure 2b and Then normalized on the Basis of the Total Number of Either Styrene (St) or EO Units Ao/Sta (Å2)

Ao/EO (Å2)b

∆Apseudoplateau (nm2)

Apancake (nm2)c

Apancake/EO (Å2)

17.9 22.4 25.4 53.5 30.3 30.0 30.9 28.8 25.8 33.3 27.4 -

8.5 10.7 10.2 21.5 9.1 9.0 9.3 8.6 6.2 8.0 9.4 8.9

-

112.4 113.5 903.3 15.7 55.0 81.5 139.6 105.8 81.2 -

280.1 302.7 2080 99.4 164.1 213.6 373.1 371.3 215.5 -

30.6 32.4 32.9 31.0 28.0 30.0 30.0 37.6 28.5 31.2

13% 14% 7% 15% 63% 20% 12% -

10.6 17.2 28.1 23.4 4.33 17.7 44.5 -

5.4 6.0 4.4 7.0 12.9 7.7 6.7 6.2

15.6 16.1 26.3 16.9 3.1 13.0 22.0 16.1

12.7 38.5 -

9.1 28.3 18.7

0% 0% 0% -

0.472 2.12 27.4 -

1.3 1.2 1.4 1.3

-

-

-

polymer

mass % PEO

(PS70)3-(PEO36)3 (PS70)3-(PEO305)3 (PS83)3-(PEO311)3 (PS83)3-(PEO2109)3 (PS111)3-(PEO107)3 (PS111)3-(PEO195)3 (PS111)3-(PEO237)3 (PS111)3-(PEO415)3 (PS104)4-(PEO10)4 (PS104)4-(PEO247)4 (PS97)3-(PEO126)6 average

18% 65% 61% 91% 29% 42% 47% 61% 4% 50% 52% -

(PEO34)2-(PS50)4 (PEO36)3-(PS48)6 (PEO36)3-(PS107)6 (PEO35)4-(PS42)8 (PEO35)4-(PS4)8 (PEO45)3-(PS77)3 (PEO51)4-(PS167)4 average PS36 PS179 PS1896 average

Ao (nm2)

3.44 10.2 -

a The PS-core star, (PS ) -(PEO 83 3 2109)3, and the PEO-core star, (PEO35)4-(PS4)8, were not included in the average calculation due to either significantly longer PEO chains or shorter PS chains, respectively. b Ao/EO was not calculated for those stars demonstrating no dependence on EO for the condensed film area. c Apancake values were not obtained for those stars lacking such a region.

Figure 3. The range of the pseudoplateau region depends linearly on the PEO chain length. (The inset provides an expanded view of the graph for smaller amounts of PEO.) The PS-core stars indicate that as the total number of EO units increases, the width of the pseudoplateau does too.

produced overlapping isotherms, indicating similar behavior for those stars. Figure 7 displays the normalized isotherms that result for a series of PEO- and PS-core stars. In Figure 7a and b, the series of (PEO34)2-(PS50)4, (PEO36)3-(PS48)6, and (PEO35)4-(PS42)8 (three stars containing equivalent PEO and PS compositions, differing only in the number of branches) is shown. When normalized with respect to EO, the star isotherms overlap, producing a single curve (Figure 7a). Differences are seen only in the more-chaotic collapse regime. The Ao obtained from the normalized graph indicates that each EO unit occupies 16 Å2 at the air/water interface. In contrast, normalizing the isotherms with respect to styrene produces curves that, while similar, remain separated (Figure

Figure 4. The pressure at which the pseudoplateau begins increases with the molecular mass (Mn) of the PEO. (PS83)3(PEO2109)3 (not shown), with significantly longer PEO chains, has a pseudoplateau pressure of 10 mN‚m-1, indicating that the increase in pressure eventually reaches a limit corresponding to the collapse pressure observed in homopolymer PEO.24

7b). Here, the two-arm star, (PEO34)2-(PS50)4, occupies the smallest area while the four-arm star, (PEO35)4(PS42)8, lies at a larger one. PS-core stars were also normalized, in particular, a series involving the same PS-core and differing PEO corona ((PS111)3-(PEO107)3, (PS111)3-(PEO195)3, (PS111)3(PEO237)3, and (PS111)3-(PEO415)3). Figure 7c shows that for EO-based normalization, the stars yield overlapping curves within the pancake region. Together, the stars show that PEO occupies 30 Å2/unit within the pancake region. This number is within experimental error of the 33 Å2/EO unit found for all of the stars. In contrast, the condensed region produces no overlap upon PEO normalization.


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Figure 5. A graph of Ao vs Mn shows a general dependence in which the area a perfectly ordered star would occupy at the air/water interface increases with molecular weight.

Figure 6. The extrapolated area, Ao, generally increases with increasing amounts of PS. While a linear fit of the data is less than perfect, both the PEO-core and PS-core stars demonstrate a similar slope of ca. 0.05 nm2/St unit with the PS-core stars typically at higher areas than the PEO-core ones.

Instead, styrene normalization is necessary in order to obtain similarity in the condensed isotherm region, as shown in Figure 7d. Here, all four stars show that the same PS core leads to the same condensed behavior, emphasizing the role of PS within this region. The average Ao is 9 Å2/St unit. Not all normalization leads to the same type of behavior, however. The other pairs of PS-core stars ((PS83)3(PEO2109)3/(PS83)3-(PEO311)3, (PS70)3-(PEO36)3/(PS70)3(PEO305)3, and (PS104)4-(PEO10)4/(PS104)4-(PEO247)4) result in different curves when normalized with respect to styrene. A key difference, however, is that in each of these pairs, one star contains a significantly greater amount of PEO (by a factor of ca. 10) than the other. (The (PS111)3core series, however, contains PEO lengths of the same magnitude (107-415 units/branch)). In contrast, EO-based normalization yields similar pancake regions, regardless of PS content. This observation even proves true for the pair (PS83)3-(PEO2109)3 and (PS83)3-(PEO311)3. Despite the significantly longer chains of (PS83)3-(PEO2109)3 (2109 PEO/branch), (PS83)3(PEO311)3 still gives a similar pancake area of 32 Å2/EO unit. All of the PS-core stars with pancake regions normalize to approximately the same curve in Figure 7c, producing an average Apancake of 30 Å2/EO unit. Differences between PS-core three- and four-arm stars are seen upon normalizing the isotherms with respect to

Figure 7. Isotherms of the PEO- and PS-core stars were normalized with respect to the total number of St units and EO units. Part (a) shows that for the PEO-core series, normalization based on the number of EO units yields the same isotherm. In contrast (b), normalizing with respect to St produces different curves. Parts (c) and (d) demonstrate a series of PS-core stars. Here, normalization with respect to EO (c) results in similar pancake region behavior while St (d) leads to the same condensed-area isotherm.

the number of styrene (St) units. The three-arm stars occupy ca. 8.5-10.7 Å2/St, while the four-arm stars ((PS104)4-(PEO10)4 and (PS104)4-(PEO247)4) are smaller (6.2 and 8.0 Å2/St, respectively) (Table 2). (The exception is (PS83)3-(PEO2109)3 whose significantly longer PEO chains affect molecular area, even in a highly compressed state.) Surface Compressibility Modulus. The isothermal compressional modulus (Ks) of a surface film is defined by the following equation:26

Ks ) -∂π/∂(ln A) We determined Ks by measuring the slope for both the condensed-film and pancake regions of π vs ln(A) where A refers to the mean molecular area of the surface film. The higher the Ks, the more rigid the film. The stars demonstrate compressional moduli of similar magnitude within the brush region. The PS-core stars prove to be more compressible with an average Ks(condensed) of 110 ( 90 mN‚m-1. In contrast, the PEO-core stars are less compressible with an average Ks(condensed) of 140 ( 20 mN‚m-1. A series of linear homopolymer PS chains were the most rigid (Ks(condensed) of 160 ( 70 mN‚m-1), indicating that increased compressibility most likely results from (26) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966.

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Figure 8. When compressing and then maintaining a surface film at a certain pressure (in this case, π ) 20 mN‚m-1), a dependence of A/Ao on time is observed. This ratio denotes the area of the surface film at a certain time (A) with respect to the original area (Ao) and represents isobaric creep.

PEO. While these Ks(condensed) values lie within the margin of error of each other, a general trend can nonetheless be considered. Stars containing a large amount of PEO are expected to be the most compressible. The PEO-core stars, being more hydrophobic, are more rigid, while the relatively hydrophilic PS-core stars are more compressible. For those stars exhibiting a pancake area, a Ks value for this region could also be obtained. Within the pancake area, the films were more compressible than in the condensed region. Here, the PEO- and PS-core stars demonstrate similar pancake compressibility with Ks(pancake) values of 9 ( 2 and 9.6 ( 0.7 mN‚m-1, respectively. Overall, the compressional moduli within the pancake region compare to the range expected for a liquid-expanded state.27 In addition, Ks values in the condensed region relate to those reported within a liquid-condensed state.27 Isobaric Creep. We also examined the degree of isobaric creep in the stars. Surface films were compressed to a specified pressure that was then maintained for a period of 10 h. At the end of the experiment, the area of the surface film (A) was compared to the initial area obtained at time zero (Ao). The ratio A/Ao was used to determine the extent of creep in which a value of 1 indicates no creep. Pressures of π ) 5, 10, and 20 mN‚m-1 were chosen as they represent the pancake, pseudoplateau, and condensed regions, respectively. A representative graph of the dependence of creep (A/ Ao) on time can be seen in Figure 8. Here, an initial decrease in area levels out to a relatively constant value within the first 3 h of compression (A/Ao is within 5% of the final ratio). This type of behavior was observed within all of the stars. The creep data indicate that at a low pressure (5 mN‚m-1), the surface films are relatively stable (with values generally greater than 0.80). At an intermediate pressure (10 mN‚m-1), a large change in area occurs for each of the stars. Once the surface pressure reaches 20 mN‚m-1, less area change occurs. Figure 9 demonstrates the pattern of change in the A/Ao ratios at different pressures. The graph is presented such that the most hydrophobic sample (linear PS) is listed first with the most hydrophilic star last (based on mass % PEO). The most hydrophobic sample (PS) demonstrates minimal creep that remains constant across all three pressures. In contrast, (PS83)3-(PEO2109)3, the most hydrophilic star, exhibits a significant increase in creep when progressing from 5 to 10 mN‚m-1. Another means for representing this change in A/Ao is illustrated in Figure 10 in which the dependence of creep difference (∆A/Ao) on the mass percentage of PEO is shown. Specifically, this graph involves the difference in creep between the pressures of 5 and 10 mN‚m-1. In general, the degree of isobaric creep increases with increasing PEO. (27) Davies, J. T. Adv. Catal. 1954, 6, 1.

Figure 9. The creep ratio (A/Ao), representing the degree of creep observed in a surface film, depends on surface pressure. The legend is ordered such that the most-hydrophobic sample (linear PS) is listed first while the last entry is the least hydrophobic.

Figure 10. The dependence of creep difference on the mass percentage of PEO is illustrated. Creep difference is defined as the change in A/Ao between two pressures, in this case, 5 and 10 mN‚m-1.

Those stars lacking a pseudoplateau region typically exhibit the lowest degree of creep. Discussion Characterization of this wide array of stars at the air/ water interface indicates that film behavior depends on a complex combination of chain length, structure, and branching. Due to the variety of stars, determining the role that each parameter plays requires minute attention to detail throughout the isothermal analysis. Quantifying the regions seen in star isotherms (Figure 2) revealed the more obvious result that PEO plays a large role at higher areas while PS dominates at lower ones. The effects of the core/corona architecture and branching, however, are not as clear-cut. Effect of PEO and PS. The effects of PEO and PS occur through both chain length and relative amounts (as expressed through mass percentage). While chain length is relevant throughout the isotherm, mass percentage is of particular importance within the pseudoplateau region. Pancake Region (I). At higher areas, the expanded region of the stars (I) compares to that seen in linear PSb-PEO, which has been described as a film of PEO at the surface with small globules of PS atop it.3,4,7,8 Considering the affinity of PEO for the air/water interface28 and the hydrophobicity of PS, the same interpretation appears (28) Glass, J. E. J. Phys. Chem. 1968, 72, 4459.


likely for stars, regardless of architecture. Due to the linear dependence of Apancake on EO units for our stars, such a model supports our findings. The area per PEO monomer obtained from the slope of this graph (0.33 nm2) compares to the average obtained from the Apancake/EO calculated individually for each star (0.31 nm2, Table 2). Both are lower than the value found in the literature for pure PEO homopolymer (0.40-0.48 nm2).13 Instead, our Apancake/EO is equivalent to that determined for linear PS-b-PEO block copolymers by Gonç alves da Silva et al. (0.27 or 0.31 nm2)3,4 and Bijsterbosch et al. (0.31 nm2).29 For heteroarm PS-b-PEO samples, Peleshanko et al.18,19 calculated a lower surface area of 0.23-0.33 nm2/EO. These comparisons suggest that the pancake structure of our star block copolymers resembles expanded linear chains, both being more compact than PEO homopolymer but less compact than heteroarm stars. Our value is also similar to 0.28 nm2, the estimated value of an EO monomer hydrogen-bonded to 1-3 water molecules.8 It may be that the PS segments, forced to be in local proximity to EO units, increase the effective hydrophobic character of the interfacial region and thereby densify EO units compared to the packing they would adopt as a homopolymer at the air/water interface. The negative y intercept of Apancake vs EO indicates that not all of the PEO occupies the interface. (Ideally, zero EO units should yield an Apancake of zero.) While the PS contribution to the pancake structure is negligible, the unaccounted PEO may exist entangled within the PS globules and is certainly a second contribution to the morecompact EO packing noted in the previous paragraph. Pseudoplateau Region (II). The importance of PEO also appears in region II, as demonstrated in Figures 3 and 4. Our finding that a minimal length of 209 PEO units is necessary for a pseudoplateau to occur is confirmed by several literature studies. Devereaux and Baker10 observed that a large PS block (93% mass) with only 82 EO units results in no plateau. In addition, two heteroarm PEO-b-PS stars containing only one PEO branch of either 159 or 170 units gave no plateau either.19 Seeming contradictions within our own data and the literature also exist. Examples in our own stars are (PEO35)4-(PS4)8 and (PEO45)3-(PS77)3. Lacking the necessary 52 and 69 EO units/branch, respectively, we would predict that neither star has a pseudoplateau. Surprisingly, both do. Another discrepancy appears in two PEOb-PS heteroarm stars with 161 and 170 PEO units/ branch.19 Though less than the necessary 209 PEO units/ branch, these stars also demonstrated a pseudoplateau. Linear PS-b-PEO samples of 90 and 148 EO units also yielded pseudoplateau.2,3 While all of these samples represent architectures different than the PS-core stars from which the minimal PEO chain lengths were obtained (Figure 3), architecture does not adequately explain the surprising presence of a pseudoplateau. Instead, the more interesting connection is that all of the PS-b-PEO samples contain more than 20% PEO. While the pseudoplateau width does not linearly depend on the mass percentage of PEO, this relative amount of PEO does influence whether a pseudoplateau appears in the isotherm. Present in both linear and star diblock systems, the interpretation of the pseudoplateau remains under discussion. Gonç alves da Silva et al.,3,4 for example, consider the region as PEO dissolving into the water subphase. Since homopolymer PEO monolayers collapse at 8.5-10.2 mN‚m-1,13 they interpret the subsequent, sharp increase (29) Slope determined by data presented in ref 2.

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in pressure for the PS-b-PEO as the formation of a brushlike conformation, with PEO chains stretching into the water subphase. Such a model appears in several other studies as well.2,5,6,10-12 Lennox and co-workers, however, propose an alternative view in which the PEO undergoes dehydration and conformational change.8 While the exact nature of the surface film in this region remains a matter of debate, the pseudoplateau clearly reflects PEO behavior. Our observation of width dependence on PEO was also noted by Gonç alves da Silva et al.3 and Faure´ et al.5 in linear PS-b-PEO systems. Condensed Region (III). The effect of PEO also appears within the condensed region (III). Figure 6 reveals that for star pairs containing the same amount of PS but different PEO lengths, stars containing significantly more PEO appear at greater condensed areas, even when normalized with respect to styrene (Ao/St, Table 2). Also, the positive y intercepts and the imperfect linear dependence of Ao on PS in Figure 6 implies that some PEO remains at the interface. These findings confirm those of the pancake region in which PS-PEO interentanglement was also suggested. Within the condensed region, the stars occupy significantly more area per St unit than a PS homopolymer (Table 2). The magnitude of these surface areas, however, is significantly smaller than the cross-sectional area of a St repeat, indicating that the surface films are not perfectly flat monolayers in the same sense as low-molar-mass amphiphiles. Instead, both the stars and PS homopolymer most likely form three-dimensional coils at high compression. With larger areas, the stars are presumably films of lower vertical height than the PS homopolymer. We suppose that the presence of PEO in the stars constrains the extent of possible PS displacement, therefore reducing the PS height. Anchored to the surface-active PEO, the PS spreads more laterally than observed in the homopolymer. In a similar vein, Peleshanko et al.18,19 found that multiple PS chains connected to a single joint are more tightly packed than in a linear copolymer. When in close proximity, PS chains thus prefer to aggregate unless forced to spread by adjoining PEO chains. Comparison between Isotherm Regions. The effect of PEO can be seen in all three isotherm regions. While dominant within the pancake and pseudoplateau regions, PEO also affects the surface film area in the condensed region. Another example of PEO’s influence appears in the creep studies where stars containing more PEO exhibit the largest degree of creep. This trend is expected. The large degree of change in molecular area indicates significant compaction or possibly a loss of chains at the air/water interface over the course of an extended period of time. Since 10 mN‚m-1 represents a pressure at which PEO submerges into the water subphase, the greatest change in creep would presumably occur within the pseudoplateau region and be due to submersion of PEO segments. Once the condensed region is attained (20 mN‚m-1), the films again stabilize, showing higher A/Ao ratios than those seen in the pseudoplateau area. Nevertheless, creep in the condensed region is still greater than the pancake area, implying that the surface film is the most stable at lower pressures, and most likely that the copolymers have viscoelastic character at higher surface pressures. Effect of Core/Corona Architecture. Comparison of the stars with heteroarm stars and linear chains implies that architecture has the effect depicted in Figure 11. For PEO-core stars, the PS corona aggregates within the star as a result of hydrophobic repulsion with PEO and the water subphase. The PEO, submerged, is generally too

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Figure 11. The model depicts the principle effect of architecture to be the location of PEO, whether in the core or the corona. For either structure, the hydrophobic PS aggregates. When PEO is the core, the PS tends to dominate, leading to greater intra- and intermolecular PS aggregation. When PEO resides in the corona, more spreading occurs. PS has a less-dominating effect, leading to a greater probability of observed pancake and pseudoplateau regions.

short to form significant pancake structures at the air/ water interface. Only the more-hydrophilic stars, (PEO35)4(PS4)8 and (PEO45)3-(PS77)3, have sufficient PEO to overcome the dominating PS hydrophobicity to reveal some pancake and pseudoplateau behavior. This model is supported by Figure 6 in which PEO-core stars are generally more compact than PS-core ones. When PEO is at the periphery, the PS chains stretch horizontally more than they do in the PEO-core stars (where PS is, instead, the corona). More evidence appears in the creep studies. While hydrophilic stars generally had greater creep than hydrophobic ones (Figure 10), an exception occurs with (PEO35)4-(PS4)8. This PEO-core star, containing 63% PEO, has less creep than (PS111)3-(PEO107)3, with only 29% PEO. This discrepancy shows that, here, a PEO core is less susceptible to creep than when in the corona. The predominant effect of the PEO-core also appears in normalization studies. For PEO-core stars, isotherms normalized with styrene result in equivalent St units in which differences are due to PEO. Shown in Figure 7b, the normalized curves differ slightly as a result of PEOcore branching. In contrast, the normalized curves in Figure 7a overlap, indicating that PS-corona branching has no effect. For the PEO-core stars, PS in the corona behaves in a similar fashion regardless of architecture. Unlike the PEO-core stars, the PS-core ones show less of a PS-aggregation effect. While the PS still experiences the same type of repulsion-driven aggregation seen in the PEO-core stars, the PEO-corona, being at the perimeter of the star, can better spread at the air/water interface. As a result, most of these stars demonstrate distinct regions in the isotherms. The only exceptions are the overly hydrophobic (PS70)3-(PEO36)3 and (PS104)4-(PEO10)4 (18 and 4% PEO, respectively). The extent of core/corona architecture influence can be seen in comparing isothermal compressional moduli for the pancake and condensed regions. Within the pancake region, most of the PEO remains at the air/water interface. Whether the PEO resides in the core or corona has little effect. The PS- and PEO-core stars thus have comparable Ks(pancake) values. Within the brush region, most of the PEO presumably resides in the water subphase. The effect of lingering PEO would most likely depend on its location,

Logan et al.

whether in the core or the corona. In the core, the PEO is surrounded by PS and has minimal effect on compressibility, as evidenced by the higher Ks values of the PEOcore stars. In contrast, PEO within the corona is less affected by PS and could thus have a larger effect on film rigidity. As a result, the PS-core stars are more compressible within the condensed region. These subtle effects of core/corona architecture are not noticeable in significantly hydrophobic stars. In the creep studies, (PEO36)3-(PS107)6 (93% PS), (PS70)3-(PEO36)3 (82% PS), and (PS104)4-(PEO10)4 (95% PEO) all showed constant creep at π ) 5, 10, and 20 mN‚m-1, respectively. While (PEO36)3-(PS107)6 contains a PEO core, (PS70)3(PEO36)3 and (PS104)4-(PEO10)4 have PEO coronas, indicating that architecture does not play a significant role for predominantly hydrophobic stars. This contrasts the more-hydrophilic stars where architecture is relevant. Effect of Branching. While the choice of polymer within the core or corona affects star film properties, another parameter to consider is branching. This effect can be seen through isotherm normalization with respect to styrene (Figure 7b). Here, the PEO-core star area depends on the number of PEO branches present. With four PEO branches at the core, (PEO35)4-(PS42)8 spreads more than the two-arm star, (PEO34)2-(PS50)4. In contrast, lower numbers of PS branches spread more than higher ones. This effect appears in normalization with respect to styrene (Ao/St, Table 2). With a range of 8.5-10.7 Å2/St, the stars with three PS arms occupy greater area than the four-arm PS stars (6.2 and 8.0 Å2/St). This finding illustrates the high tendency of PS to aggregatesmore PS branches result in greater aggregation and smaller area at the air/water interface. When comparing the effect of branching versus PEO chain length, the latter proves more important in determining surface film area. This finding results from comparing Ao values of (PS97)3-(PEO126)6 and (PS111)3(PEO195)3 (Table 2). Though (PS97)3-(PEO126)6 has the higher molecular mass, this dendritic star occupies less space than (PS111)3-(PEO195)3, a three-arm linear star of similar molecular weight and PS chain length. While the more-compact (PS97)3-(PEO126)6 contains twice the number of PEO arms found in (PS111)3-(PEO195)3, these chains are shorter. Such results indicate that PEO chain length plays a greater role than the number of branches in star spreading. The dominance of PEO chain length over PEO branching also appears in creep studies. While more-hydrophilic stars tend to have greater creep, an exception is again the dendritic PS core star, (PS97)3-(PEO126)6. While (PS97)3(PEO126)6 and (PS111)3-(PEO237)3 both consist of ca. 50% PEO, the former has twice as many PEO chains. As a result, the PEO chains in (PS97)3-(PEO126)6 are half the length of those in (PS111)3-(PEO237)3. Since (PS111)3(PEO237)3 demonstrates almost three times the creep that (PS97)3-(PEO126)6 does, longer PEO chain length thus affects film stability more than higher PEO branching. Architecture vs Composition. On the basis of normalization, general comments can be made regarding architecture and composition. For the same architecture, maintaining a constant chain length of one polymer and varying amounts of the other lead to normalizeable behavior. This observation is seen in the PS-core star series (Figure 7c and d). Our finding is also confirmed by (PS83)3(PEO311)3, (PS70)3-(PEO305)3, and (PS111)3-(PEO415)3, three-arm stars that all contain ca. 62% mass PEO. When normalized, these isotherms overlap. In contrast, different architectures require the same composition of PEO or PS in each branch in order to achieve


the same isotherm upon normalization. The PEO-core series demonstrates this (Figures 7a and 7b) as do (PS104)4(PEO247)4 and (PS111)3-(PEO237)3. The latter two are three- and four-arm PS-core stars containing similar chain lengths of PEO and PS. They, too, normalize to the same isotherm, demonstrating that different architectures can still exhibit the same behavior depending on composition. Conclusion The ability of the stars to form stable, reproducible surface films was thus demonstrated. For stars containing 20% or more PEO, three distinct regions appear in the isotherm. Typically, such stars were the PS-core ones. The PEO absorbs at the air/water interface, forming pancakelike structures greatly dependent on PEO. With compression, the PEO is forced into the water within the pseudoplateau. The width and pressure of this region increase with PEO. While a certain PEO chain length is necessary, a predominantly hydrophobic star does not demonstrate this region due to PS masking PEO behavior. All of the stars examined were compressible to surface pressures as high as 60 mN‚m-1. At these smaller areas, PS dominated with Ao increasing with both molecular weight and PS. In addition, this Ao was greater than that expected for pure PS, indicating that PEO helps spread the PS. The dependence of the distinct isotherm regions on either PEO or PS indicates that molecular arrangement can be controlled through varying the relative amounts

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of these polymers. In addition, the architecture, as reflected through the placement of PEO in either the core or corona and the number of branches, also affects the surface film. For example, the amount of PS played a more dominant role when PS resides in the corona. Also, a larger number of branches in the core leads to greater spreading for PEO but higher compactness for PS. When comparing PEO branching to PEO chain length, the latter proved more important in determining surface film behavior. Our analysis of star behavior at the air/water interface was conducted in the hope of providing a foundation for predicting the behavior of new PS-b-PEO architectures and compositions. On the basis of our findings, new stars could be tailor-made with specific properties. Future work involves transferring these stars as Langmuir-Blodgett films and examining them through AFM. Such a study could provide insight into the morphology of the isotherm regions and the role that both architecture and PS/PEO composition play. Acknowledgment. Much appreciation is given to R. Francis and S. Angot for preparing and providing the stars. Also, support from DOE-BES and NSF/CNRS Grant No. DE-FG02-01ER45933 and NSF Grant No. INT-9816175 is acknowledged. J.L. received support from the NSF Graduate Fellowship Program, while P.M. participated in the NSF REU Program at the University of Florida. LA050787C

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