Nano-donuts from pH-dependent block restructuring ...

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Jin Young Park,a Ming Liu,b Jimmy Mays,b Mark Dadmunb and Rigoberto Advincula*a ..... 4 (a) J. L. Logan, P. Masse, Y. Gnanou, D. Taton and R. S. Duran,.
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Nano-donuts from pH-dependent block restructuring in amphiphilic ABA triblock copolymer vesicles at the air-water interface† Jin Young Park,a Ming Liu,b Jimmy Mays,b Mark Dadmunb and Rigoberto Advincula*a Received 27th June 2008, Accepted 18th November 2008 First published as an Advance Article on the web 15th December 2008 DOI: 10.1039/b810149c

To understand the nature of polymers capable of forming multiply bound polymer chains (MBPCs) on surfaces, the Langmuir-Blodgett (LB) technique and Scanning Probe Microscopy (SPM) have been utilized to investigate surface morphology of the amphiphilic triblock copolymer (P2VP-b-dPS-b-P2VP, Mn ¼ 125,000, dPS:P2VP ¼ 1:1) at the air-water interface. The LB monolayer studies were carried out via p–A isotherms, compression-expansion cycles, kinetic stability, and nanoscopic surface morphological behavior. A strong pH effect was observed on the surface limiting area, stability, and morphology forming various 2-D aggregates. In particular, well-ordered interesting nanostructures (nano-donut formation) of the triblock copolymer aggregates depend on the effect of pH on the hydrophilic P2VP blocks, acting as a ‘‘cushion’’ at the air-water interface. In addition, equilibrium factors between the adsorption of the hydrophilic outer blocks (related to its solubility) and the entanglement of the hydrophobic dPS blocks during the evaporation of the spreading solution may play a critical role. Amphiphilic AB block copolymers capable of self-assembly at the air-water interface have been widely studied for their interesting mesoscopic and nanoscopic structures. They are also important in understanding interfacial structure and properties, such as wettability, chemical functionality, and structural stability in amphiphilic polymer systems.1 For novel block copolymer systems, the Langmuir-Blodgett (LB) technique has been used as an eminent tool to investigate surface properties and morphologies of diblock/triblock copolymers at the air-water interface via surface pressure (tension) control.2 For the AB block copolymers, the predominant surface features obtained from LB films at the interface depend on factors, including the nature and relative lengths of the blocks, surface pressure, pH, concentration of the spreading solution, etc.3 The ability of the hydrophobic block to phase separate from the hydrophilic block results in interesting isolated and aggregated polymeric structures at the air-water interface.3(c),4 The stabilization of these structures is also a function of solution equilibrium conditions prior to spreading and after evaporation of the spreading solvent.

a Department of Chemistry, University of Houston, 136 Fleming Bldg, Houston, TX, 77204, USA. E-mail: radvincula @uh.edu; Fax: +1 713743-1755; Tel: +1 713-743-1760 b Department of Chemistry, University of Tennessee-Knoxville, 552 Buehler Hall, Knoxville, TN, 37996, USA. E-mail: [email protected]; Fax: +1 865-974-3141; Tel: +1 865-974-0747 † Electronic supplementary information (ESI) available: Materials synthesis, stability experiments, and additional AFM data. See DOI: 10.1039/b810149c

This journal is ª The Royal Society of Chemistry 2009

Recently, there has been interest in understanding the ordering and morphology of ABA triblock copolymers. Drop-cast solutions under controlled conditions such as dilute concentration and selective solvents has been reported to form aggregates such as hollow micelles and/or vesicles.5 However, reports of ring or donut like structures in LB films are rare even for high molecular weight ABA triblock copolymers (MW >100,000 g/mol). For a fundamental understanding of amphiphilic ABA-type triblock copolymer properties at interfaces, it is necessary to employ surface 2-D manipulation and microscopy methods to understand the nature of short-range and long-range mesoscopic ordering. In this work, the LB technique and Scanning Probe Microscopy (SPM) have been utilized to investigate the surface morphology of an amphiphilic polyvinylpyridine and polystyrene triblock copolymer (P2VP-b-dPSb-P2VP, Mn ¼ 125,000, dPS:P2VP ¼ 1:1)6 at the air-water interface. The LB monolayer studies were carried out via surface pressure-mean molecular area (p-A) isotherms, compression-expansion cycles, isobaric creep of the LB films, and nanoscopic surface morphological characterization. Well-ordered and interesting nanostructures are described, resulting from the effect of pH on the hydrophilic P2VP blocks of the nano-objects at the air-water interface. Fig. 1 shows the compressional p-A isotherms of the triblock copolymer obtained at pH ¼ 2.67 and 12.9. As the pH is increased, it is evident that the p-A isotherm mean molecular area (MmA) onset shifts to a larger area. The pseudo first-order transition region (9– 16 mN/m) which exists at low pH extends to a wider range of surface pressure (5 < p < 33 mN/m) at high pH. Specifically, the change in limiting area (nm2/molecule) represents the water-solubility of P2VP blocks changing with pH and contributes to the p-A isothermal behavior. This value can be obtained by extrapolating the linear region of 35–50 mN/m to 0 mN/m (54  1 nm2/molecule at pH

Fig. 1 p-A isotherms of the P2VP-b-dPS-b-P2VP triblock copolymer at pH 2.67 and pH 12.9 and chemical structure of the material.

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2.67 and 76  1 nm2/molecule at pH 12.9).7 This implies that at low pH, the P2VP chains are ionized and dissolved in the water subphase. At a high pH, when the pyridine is neutral, the chains are at the water surface and occupy greater surface area. For the LB compression experiments, the reduced surface occupancy at low pH is implied from a lower limiting MmA and narrower transition region. It should be noted that vesicles, consisting of hydrophilic P2VP blocks as cores and hydrophobic PS blocks as coronae in the selective solvent (toluene), can be present at first in solution prior to spreading. However, during spreading and solvent evaporation at the water surface, the aggregation phenomenon is transformed to chain entanglements of the glassy and hydrophobic dPS cores and for the P2VP chains, kinetic mobility in adsorption on the water surface or water-inward diffusion of the ionized blocks.1(a),4(b),8 Also, solvent evaporation rate can play an important role in mesophase stability and features.4c The AFM images (Fig. 2A and 2B) reveal the distinct morphologies on the LB films (transferred at p ¼ 2 mN/m) with pH (2.67 and 12.9). Spherical shaped nano-objects on the LB film (h ¼ 2.4  0.3 nm) were formed at pH 2.67 due to the immersion of ionized P2VP units9 as well as aggregation of inner dPS blocks on the water surface. In addition, these submerged blocks can also have mutually repelling interaction with neighboring objects where the PS core-tocore edge distance is constant (d1 ¼ 35–40 nm), forming pearl necklaced structural arrangements in some cases. This result is also consistent with PS-b-PVP diblock copolymer LB films, although no donut-like objects have been reported.7 On the other hand, as shown in Fig. 2B, the LB film at high pH exhibited an interesting surface morphology, i.e, the nano-donut (or ring) formation (h2 ¼ 1.4  0.3 nm). The outer core-to-core distance (d2 # 10–15 nm) between nano-donuts would be determined by the flexible corona chain length of either P2VP outer blocks in each polymer chain, due to the low applied surface pressure. It also needs to be considered that another corona chain on the molecule can

Fig. 2 AFM images of the P2VP-b-dPS-b-P2VP LB films transferred at 2mN/m at A. pH 2.67 and at B. pH 12.9 and inset of the figure is zoom-in area.(150  300nm2) C shows the schematic aggregation behavior of the LB films at two different pHs (blue for P2VP and red for dPS).

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contribute to the inner diameter (d3,hole ¼ 32–33 nm) in each nanodonut. Thus, the spacing between donuts is less or equivalent to the hole radius of the donut (16–16.5 nm), considering external compression acting on each donut. In principle, each isolated nanodonut can form in the gas-phase (no external force) with a slightly compressed state at the onset point or low surface pressure, homogeneous in size. It should also be noted that the nano-donut formation appears only at low surface pressure (before the phase transition zone) and high pH (surface-active P2VP blocks). To our knowledge, this surface feature, showing highly well-ordered donut formation, is a unique morphological behavior observed only on ABA triblock copolymer LB systems (A: surface-active hydrophilic, and B: hydrophobic). Overall the difference between the diameter of the cores (54.0  5.9 nm) and outer diameter of the donuts (83.5  6.8 nm) formed at two pH values is consistent with the diameter of each inner hole, indicating that the flexible P2VP blocks at pH 12.9 eventually determines the size of the donut. Fig. 2C highlights these possibilities. Based on the MmA and the aggregate size on the AFM image at 2 mN/m (only considering the dPS blocks), the aggregation number was calculated to be 34  8 for the spherical nano-object at pH 2.67 and 47  8 for the nano-donut at pH 12.9. The difference between the two may be due to a limit on calculating from a 2-D orientation, not considering the Z-scale (vertical scale). Fig 3 shows the kinetic trapping and reorganizing process for nano-donut formation: (1) Vesicles in the droplets of the solution spread on large water subphase, and (2),(3) then the dPS blocks of the vesicle, closer to the water surface release in/out and aggregate upward during solvent evaporation due to the minimization of surface energy in the hydrophobic blocks. At the same time, (4) either or both sides of P2VP on each molecule can float in/out on the water surface. (5) Finally, an isolated nano-donut can be formed in air. Furthermore, this unique formation is highly relevant to the ratio of dPS and P2VP blocks. For same ABA copolymers with higher ratios of dPS blocks (dPS:P2VP ¼ 4:1 or 10:1), a cylinder structures or large grain was typically observed at high pH (Fig. S1). Thus, both ionization degree of P2VP outer blocks and composition ratio (in other

Fig. 3 An expected scheme of a kinetic process for nano-donut formation. (A. vesicle formation in solution, B. transformation of a vesicle to a nano-donut, and C. the nano-donut formed at water surface).

This journal is ª The Royal Society of Chemistry 2009

Fig. 4 A. pH dependent surface pressure-area (p-A) isotherms of the triblock copolymer and B. AFM images of the LB film transferred at pH 5.34, showing rod, donuts and spherical aggregates (a, b, and c).

words, the relative length of outer P2VP) might play important roles in forming a particular surface morphology. From compression and expansion curves (Fig. S2) and isobaric creep measurements (Fig. S3), the donuts are shown to be highly stable. From Fig. S2, little hysteresis is observed for the isotherms at low pH even after the first compression. On the other hand, some hysterisis is observed with the first compression at high pH compared to subsequent cycles. Moreover, in isobaric creep measurements, when compressed up to 30 mN/m, it produces a greater area shift (6.5 nm2/molecule) at low pH, compared to the high pH (3.6 nm2/ molecule) indicating greater subphase stability under the given surface pressure. Thus, glassy dPS blocks as well as ionized and/or un-ionized P2VP units, depending on the examined phase region (solid or liquid), contribute to the stability of the formed structure. In particular, the P2VP chains at the surface (surface-active at high pH) can act as a type of ‘‘elastic cushion’’ preventing the nano-objects from forming larger domains. To further understand the pH dependent surface morphology, the LB films (transferred at higher surface pressure; 8mN/m) were investigated under various pH values (0.94, 2.67, 5.34, and 12.9) (Fig. 4A) The limiting area increases and the onset point of MmA per molecule shifts to a higher area with increasing pH, generating a continuum of partially ionized or completely surface-active P2VP outer blocks. From the previous discussion, it has been confirmed that the nano-donuts at pH 12.9 is changed to spherical aggregates at pH 2.67, whilst keeping a constant particle distance of 44.1  5.1 nm (Fig. S4). This value is slightly less than the theoretically calculated value (ca. 46 nm) of a fully expanded P2VP block due to the metastable states of P2VP blocks interacting with the subphase. At pH 5.34, in between the low and high pH, the surface morphology showed polymorphism of objects in which donuts, random rods, and spherical vesicles were observed (Fig. 4B). This is attributed to the various interaction states of P2VP blocks at the water interface in

This journal is ª The Royal Society of Chemistry 2009

transition to a fully spherical or a fully donut-shaped object.10 Thus, solubility and pH control of the P2VP blocks enables one to manipulate the various surface morphologies on Langmuir (LB) monolayers at the air-water interface. In conclusion, the unique nano-donut formation in amphiphilic ABA triblock copolymer is dependent on an equilibrium with adsorption of hydrophilic outer blocks and kinetic solubility in the process of spreading at the interface. This is highlighted by pH control for ionizable blocks as in the case of P2VP. From these surface morphological studies and LB experiments, the nano-objects were found to be highly ordered and very stable at the air-water interface. It is possible that control of the triblock architecture and composition can further reveal new morphologies and insight to this interesting behavior.

Acknowledgements The authors gratefully acknowledge funding from NSF DMR-0602896, CHE 0304807, ACS-PRF #45853. We also acknowledge technical support from Agilent.

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