Article pubs.acs.org/Langmuir
Polystyrene−Poly(ethylene oxide) Diblock Copolymer: The Effect of Polystyrene and Spreading Concentration at the Air/Water Interface Cameron P. Glagola,† Lia M. Miceli,† Marissa A. Milchak,† Emily H. Halle, and Jennifer L. Logan* Department of Chemistry, Washington & Jefferson College, 60 S. Lincoln St., Washington, Pennsylvania 15301, United States S Supporting Information *
ABSTRACT: Polystyrene-block-poly(ethylene oxide) (PS-PEO) is an amphiphilic diblock copolymer that undergoes microphase separation when spread at the air/water interface, forming nanosized domains. In this study, we investigate the impact of PS by examining a series of PS-PEO samples containing constant PEO (∼17 000 g·mol−1) and variable PS (from 3600 to 200 000 g·mol−1) through isothermal characterization and atomic force microscopy (AFM). The polymers separated into two categories: predominantly hydrophobic and predominantly hydrophilic with a weight percent of PEO of ∼20% providing the boundary between the two. AFM results indicated that predominantly hydrophilic PS-PEO forms dots while more hydrophobic samples yield a mixture of dots and spaghetti with continent-like structures appearing at ∼7% PEO or less. These structures reflect a blend of polymer spreading, entanglement, and vitrification as the solvent evaporates. Changing the spreading concentration provides insight into this process with higher concentrations representing earlier kinetic stages and lower concentrations demonstrating later ones. Comparison of isothermal results and AFM analysis shows how polymer behavior at the air/water interface correlates with the observed nanostructures. Understanding the impact of polymer composition and spreading concentration is significant in leading to greater control over the nanostructures obtained through PS-PEO self-assembly and their eventual application as polymer templates.
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hydrophilic PEO resides at the interface,22 anchored by the PS. The surface density of this polymer monolayer is readily controlled through the use of compressible barriers. Upon compression of the monolayer, a π−area isotherm depicting the dependence of surface pressure (π) upon the mean molecular area can be generated. (Surface pressure is defined as the change in surface tension, or π = γ0 − γ, where γ0 and γ are the surface tension of pure water and the monolayer-covered water, respectively.) A typical PS-PEO isotherm contains three well-defined regions (Figure 1, inset). At larger areas, the molecules exist in an expanded state (pancake region) where the surface-active PEO resembles a “pancake” with the hydrophobic PS globule sitting on top.16−19,21,23−25 Upon compression, the molecules crowd together, resulting in an increase in surface pressure. Eventually, a plateau region is observed at a pressure of ca. 10 mN/m. Further compression leads to a steep increase in pressure, denoting a condensed film region where additional compression would lead to collapse as the PS-PEO film breaks into multilayers. The behavior behind the plateau and condensed regions has been described as PEO being pushed into the water subphase and subsequently stretched into a polymer brush.6,21,23−25 Several studies, however, disagree with
INTRODUCTION A primary goal in nanoscience is to create structures controlled by size, shape, population, and function, ranging in size from 5 to 200 nm.1 One popular strategy is self-assembly, in which molecules spontaneously organize into a structure of predictable degree and ordering.2 An intriguing class of compounds that self-assemble is block copolymers, macromolecules composed of two or more polymer chains. Chemical differences in these blocks can result in phase separation that, on a surface, can lead to the creation of a predictable pattern. The resulting polymer template can be used to direct chemical reagents to an interface based on selective solubilization with the polymer blocks.3 Polystyrene-b-poly(ethylene oxide) (PS-PEO) is an amphiphilic diblock copolymer whose ability to self-assemble permits its use for nanoparticle arrays,4−9 nanoelectrode ensembles,10 and scaffolds for particle growth.11,12 In addition, the development of a method for selective removal of PEO extends the possible applications of PS-PEO templates.13 When applied to the air/water interface, linear PS-PEO has been observed to form two-dimensional, nanosized structures ranging from continent-like planar features to spaghetti and dot domains.5,7,14−21 Additional shapes like rings and chains20 have also been reported. PS-PEO nanostructures are easily formed using a Langmuir trough. When a solution of PS-PEO is spread at the air/water interface, the hydrophobic PS remains in the air while the more © 2012 American Chemical Society
Received: October 19, 2011 Revised: January 6, 2012 Published: February 16, 2012 5048
dx.doi.org/10.1021/la204100d | Langmuir 2012, 28, 5048−5058
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its resulting aggregation and further entanglement to reduce such interaction. The polymer block that ultimately dominates is a function of the polymer composition as well as the spreading solvent, since its evaporation would dictate the mobility of the system. To better understand the role of PS in nanostructure formation, we propose to examine a series of polymers containing a constant amount of PEO (average Mn of 17 000 g·mol−1) and variable blocks of PS (Table 1). This
this interpretation. Cox and co-workers argue that the changes in pressure result from reorganization of surface aggregates rather than individual molecules.14,15 This model is supported by studies that suggest that PEO remains at the air/water interface, regardless of surface pressure.26,27 Instead, compression causes dehydration and conformational changes in the PEO rather than expulsion from the air/water interface. In addition, Richards et al determined through neutron reflectometry that the surface concentration required to sufficiently reduce the surface tension of the interface below that of PEO, thereby forming a polymer brush, is too high; film collapse would occur first.27 The areas of these regions have been observed to depend on PEO at surface pressures lower than 10 mN/m and both PS and PEO at higher surface pressures.16,23,28 More hydrophobic PS-PEO (generally 20% PEO) generated isotherms displaying the three regions (pancake, plateau, and condensed) that represent conformational changes in PS-PEO (Figure 1).14,16,21,23 The 49k, 27.5k, and 20.2k (not shown) clearly exhibit these three regions as the film compresses from larger to smaller areas. The more hydrophobic 78k, 148k, and 216k polymers (≤20% PEO) show only the steep increase in pressure representative of the condensed film region. These samples lacked the PEO necessary for a plateau and pancake region, similar to other samples containing ∼12 wt % or less PEO17,20,36 as well as pure PS.36,37 A less well-defined 2D−3D transition (pseudoplateau) has been seen in polymers with ∼16 wt % PEO,16 representing a boundary between the polymers whose interfacial behavior illustrates either PS- or PEO-dominant contributions. The 78k (20.5% PEO) falls within this category, showing a gradual increase in pressure upon compression that is not as dramatic as the steep increase in more hydrophobic polymers. The boundary between those polymers which exhibit pancake and plateau regions and those that do not is not well-defined. Two PS-PEO samples (185k and 18k), each containing 19 wt % 5050
dx.doi.org/10.1021/la204100d | Langmuir 2012, 28, 5048−5058
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and 20.2k, the 49k has a notably smaller plateau (9 Å2/EO), suggesting that its higher amount of PS leads to quicker compression of the PEO pancakes within the plateau region. Once the PEO pancakes come into closer contact and reach the plateau region, the longer PS chains more readily entangle, inhibiting PEO overlap to a greater degree than seen in the shorter PS chains of the 27.5k and 20.2k samples. The pressure at which the plateau begins (πplateau) was 7.7, 9.0, and 8.2 mN/m for the 49k, 27.5k, and 20.2k, respectively (Table 2). Though the amount of PEO is relatively constant in our series, the 27.5k does have the highest amount (18k) while the 20.2k has the lowest (16.6k). Comparison of these two polymers suggests that higher Mn,PEO leads to higher plateau pressures. This relationship is supported by previous work where the transition pressure increased with increasing PEO for a series of constant PS and variable PEO.29 Other reports have noted a similar dependence of plateau pressure on PEO.23,38 Kuzmenka and Granick found that the collapse pressure of homopolymer PEO increased with molecular weight until attaining a constant value of ca. 10 mN/m.40 Along these lines, the 49k (with 17k PEO) should have a slightly higher πplateau than the 20.2k (with 16.6k PEO) but does not. The 49k plateau, in fact, is less well-defined than the 27.5k and 20.2k, implying that the higher amount of PS impacts the plateau region. In their own study of PS-PEO, Fauré et al. noted that as PS increases, the height and slope of the plateau decreases.41 The culmination of these results suggests that the plateau pressure reflects primarily PEO but PS plays a role. Increasing the amount of PS (and thereby decreasing % PEO) reduces the pressure at which the 2D-3D transition occurs. The condensed region of the isotherm is characterized by the limiting area, A0. Figure 2a shows that as the percent of PEO
increases (and therefore the Mn decreases), the A0 decreases. This result is expected, since it indicates that smaller molecules occupy smaller areas at the air/water interface. What is interesting, though, is that this dependence is not linear across the series. A0 exhibits a remarkably different dependence on percent of PEO depending on whether the polymer has more or less than ∼20% PEO. In the brush model, this region represents PEO stretched into the water subphase with A0 reflecting the area occupied by the PS at the air/water interface. If this assumption was correct, the normalized area (A0/St) would be constant. However, Figure 2b shows how A0/St changes with percent of PEO. For more hydrophobic polymers (≤20% PEO), the A0/St is relatively constant (4.2 ± 0.6 Å2/ St). Kumaki found that for dilute solutions of PS homopolymers, A0 = 0.04M where M is the molecular weight of the PS.37 Simple conversion equates this relationship to 4.2 Å2/St. Most of our polymers (with 34% PEO and lower) yield the same A0/St value, indicating that within this condensed region the PS-PEO does indeed behave as if the interface consisted entirely of PS. A dramatic increase in A0/St occurs, though, for the polymers containing higher % PEO (>65% in our study). Based on the other samples in our series as well as Kumaki’s results, the 27.5k and 20.2k polymers occupy more area at the air/water interface than would be expected in macromolecules of such small size. This discrepancy implies that for polymers containing a higher % PEO, the PEO does in fact influence film packing within the condensed region. This observation suggests that the PS-PEO does not form a brush and that PEO remains at the air/water interface. It is interesting to remember that each of these polymers contains ∼17 000 g·mol−1 PEO (or 390 EO units). The observation that PEO impacts the A0 of the >65% PEO polymers but not the more hydrophobic ones suggests that % PEO is the most important factor in determining PS-PEO behavior. AFM Analysis. The polymer domains were transferred as LB films using mica as a substrate. Since mica is hydrophilic, the PEO adsorbs onto the substrate, resulting in a film containing a bottom layer of PEO with PS sitting on top. As a result, the brighter (taller) domains in the topographical AFM images (for example, Figure 3) represent PS while the darker background reflects either bare mica or PEO. Each LB film was obtained at a pressure of 2 mN/m. We chose this pressure as it occurs at larger mean molecular areas (when PS-PEO with ∼20% or more PEO are still within the pancake region). Enough spacing exists between the molecules to reflect the domains formed upon spreading as opposed to aggregation that may result from significant compression. Figure 3 demonstrates the typical features observed for our series of PS-PEO (all obtained at 1.0 mg/mL in chloroform). Circular shapes were identified as dots while spaghetti were the longer, stringlike features with an aspect ratio of at least 10:1. Continents describe the large, relatively flat structures. The 216k (7.4% PEO) displays a random mixture of continents, spaghetti, and dots, depending on where the film is imaged. Figure 3a, for example, shows large continents bordered by spaghetti. These features are similar to those observed in other PS-PEO containing 1 which has been reported to lead to complex 2D morphologies while NPS/NPEO
< 1 (the 49k, 27.5k, and 20.2k) results in dots (Supporting Information).19 The shape and size of the nanosized aggregates have been described as reflecting entanglement within the PS chains.16−18,20,21 The question of whether entanglement occurs in the solution prior to spreading at the air/water interface was considered by Cox et al.14 Through dynamic light scattering, they showed that no micelles formed in toluene (a highly selective solvent for PS) at
