Structural Analysis of PEO−PBO Copolymer Monolayers at the Air−Water Interface (original) (raw)
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Langmuir, 2008
Surface films of two copolymers of ethylene oxide (E) and butylene oxide (B), namely E 23 B 8 and E 87 B 18 , have been examined by Brewster angle microscopy (BAM) and atomic force microscopy (AFM). Isotherms taken on unsupported films of these copolymers at the air-water interface showed a clear gas to liquid phase transition for E 87 B 18 and a barely discernible phase transition for E 23 B 8 . The BAM studies showed a gradual brightening of the films as the surface pressure was increased, which was associated with a film thickening and/or a film densification. Several bright spots were also observed within the films, with the number of spots increasing gradually as the film surface pressure was increased. AFM studies of these films did not show any localized ordering, which fits in with the results from our previous X-ray study of these copolymers [Hodges, C. S.; Neville, F.; Konovalov, O.; Gidalevitz, D.; Hamley, I. W.; Langmuir 2006, 22 (21), 8821-8825], where no long-range ordering was observed. AFM imaging showed two sizes of particulates that were irregularly spaced across the film. The larger particulates were associated with silica contaminants from the copolymer synthesis, whereas the smaller particulates were assumed to be aggregated copolymer. An analysis of the semidilute region of the isotherm showed that while both copolymers had intermixed ethylene oxide and butylene oxide units, the lower molecular weight E 23 B 8 copolymer manifested significantly more intermixing than E 87 B 18 .
Langmuir, 2006
The dynamic dilational elasticity of adsorbed and spread films of PEO-PPO-PEO triblock copolymers at the air-water interface was measured as a function of surface pressure, surface age, and frequency. At low surface pressures (<10 mN/m), the surface viscoelasticity is identical to that of PEO homopolymer films. The results at higher surface pressures can be explained by the desorption of PPO segments from the interface and then mixing with PEO segments in water. Unlike some recent results, the spread and adsorbed films are not identical. Spread films exhibit a maximum real part of the dynamic surface elasticity of about 20 mN/m and probably begin to dissolve in water at surface pressures above 19 mN/m. However, the surface elasticity of the adsorbed films decreases beyond the maximum, indicating the formation of a loose surface structure.
Journal of colloid and interface science, 2015
The behavior of a series of amphiphilic triblock copolymers of poly(ethylene oxide) (PEO) and poly(isobutylene) (PIB); including both symmetric (same degree of polymerization (DP) of the terminal PEO blocks) PEOm-b-PIBn-b-PEOm and non-symmetric (different DP of the terminal PEO blocks) PEOm-b-PIBn-b-PEOz, is investigated at the air/water interface by measuring surface pressure vs mean molecular area isotherms (π vs mmA), Langmuir-Blodgett (LB) technique, and infrared reflection-absorption spectroscopy (IRRAS). The block copolymer (PEO32-b-PIB160-b-PEO32) with longer PEO segments forms a stable monolayer and the isotherm reveals a pseudo-plateau starting at π∼5.7 mN/m, also observed in the IRRAS, which is assigned to the pancake-to-brush transition related to the PEO dissolution into the subphase and subsequent PEO brush dehydration. Another plateau is observed at π∼40 mN/m, which is attributed to the film collapse due to multilayer formation. The pancake-to-brush transition could no...
Langmuir, 1996
Spread monolayers of poly(styrene)-poly(ethylene oxide) diblock copolymers (PSm-PEOn, m ) 38, n ) 90, 148, 250, and 445) have been studied at the air-water interface by measuring the surface pressurearea (π-A) isotherms at several temperatures. The π-A isotherms exhibit several regions which can be ascribed to different conformations of the polymer chains: a pancake structure at low surface pressures and high areas when the isolated chains are adsorbed by both the PS globule and the PEO segments at the interface; an intermediate structure, quasi-brush, when the PEO segments are solubilized in the subphase; and finally a brush developed at low surface areas when the PEO chains are obliged to stretch away from the interface to avoid overlapping. At surface pressures near 10 mN/m there is a transition between a high-density pancake and the quasi-brush regime. The compression and the subsequent expansion curves superpose at the transition and quasi-brush regions but not at the brush and pancake stages. This points to a high cohesion in the brush structure after compression and to some irreversible entanglement and hydration of the PEO chains when immersed in the subphase. These two local hystereses depend differently on the PEO chain length and temperature. The hysteresis observed at high surface pressures (brush conformation) decreases with the PEO length and temperature, whereas the low surface pressure hysteresis (pancake) increases with PEO chain length, decreases with temperature in the range 283-298 K, and increases in the range 298-315 K. A negative mean transition entropy change was obtained from the temperature dependence of the quasi-SSAL-quasi-brush transition. The results indicate that the extensive properties of the present diblock copolymers at the interface, such as the pancake limiting area and the mean transition entropy, when expressed by PEOmer, are independent of the PEO length.
Monolayer formation of PBLG–PEO block copolymers at the air–water interface
Journal of Colloid and Interface Science, 2005
Physicochemical properties of PBLG (poly(γ -benzyl-L-glutamate))-PEO (poly(ethylene oxide)) diblock copolymers composed of PBLG as the hydrophobic rod component and PEO as the hydrophilic component were investigated at the air-water interface. Surface pressure-area isotherms obtained by the Wilhelmy plate method provide several variables such as molecular size, compressibility of PEO, and the free energy change of the PBLG-PEO block copolymer. GE-1 (M w of PBLG:PEO = 103,700:12,000), with a relatively longer rod, has negative temperature effects and GE-3 (M w of PBLG:PEO = 8400:12,000), with a relatively shorter rod, shows a positive temperature effect because of the large entropy loss. These competitions were based on the block size of PBLG and PEO and were affected by various microstructures of the PBLG-PEO diblock copolymer. Monolayer aggregations transferred onto mica from the air-water interface were analyzed with AFM. AFM images of GE-1 monolayers show cylindrical micelles, but the self-assembled structure has many large domains. The monolayer of GE-2 (M w of PBLG:PEO = 39,800:12,000), which has a medium size rod, forms a spherical structure at the air-water interface. Monolayers of GE-3, with a short rod length, form bilayer structures. These results demonstrate that the microstructures of PBLG-PEO diblock copolymers are related to free energy changes between rod and coil blocks. 2004 Elsevier Inc. All rights reserved.
Langmuir, 2005
The dilatational rheological properties of monolayers of poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide)-type block copolymers at the air-water interface have been investigated by employing an oscillating ring trough method. The properties of adsorbed monolayers were compared to spread layers over a range of surface concentrations. The studied polymers were PEO26-PPO39-PEO26 (P85), PEO103-PPO40-PEO103 (F88), and PEO99-PPO65-PEO99 (F127). Thus, two of the polymers have similar PPO block size and two of them have similar PEO block size, which allows us to draw conclusions about the relationship between molecular structure and surface dilatational rheology. The dilatational properties of adsorbed monolayers were investigated as a function of time and bulk solution concentration. The time dependence was found to be rather complex, reflecting structural changes in the layer. When the dilatational modulus measured at different concentrations was replotted as a function of surface pressure, one unique master curve was obtained for each polymer. It was found that the dilatational behavior of spread (Langmuir) and adsorbed (Gibbs) monolayers of the same polymer is close to identical up to surface concentrations of ≈0.7 mg/m 2. At higher coverage, the properties are qualitatively alike with respect to dilatational modulus, although some differences are noticeable. Relaxation processes take place mainly within the interfacial layers by a redistribution of polymer segments. Several conformational transitions were shown to occur as the area per molecule decreased. PEO desorbs significantly from the interface at segmental areas below 20 Å 2 , while at higher surface coverage, we propose that segments of PPO are forced to leave the interface to form a mixed sublayer in the aqueous region.
Monolayers of diblock copolymer at the air-water interface: the attractive monomer-surface case
European Physical Journal B, 1998
We have studied both experimentally and theoretically the surface pressure isotherms of copolymers of polystyrene-polyethyleneoxide (PS-PEO) at the air-water interface. The SCMF (single chain meanfield) theory provides a very good agreement with the experiments for the entire range of surface densities and is consistent with the experiments if an adsorption energy per PEO monomer at the air-water interface of about one kBT is taken. In addition, the chain density profile has been calculated for a variety of surface densities, from the dilute to the very dense ones. The SCMF approach has been complemented by a mean-field approach in the low density regime, where the PEO chains act as a two-dimensional layer. Both theoretical calculations agree with the experiments in this region.
Monolayers of some ABA block-copolymers at the airwater interface
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1996
Monolayers of non-ionic ABA block-copolymers at the air-water interface have been investigated by means of the Langmuir surface balance technique. The A-and B-blocks of the polymer are biodegradable polyester and poly(ethylene oxide) (PEO) chains, respectively. The interfacial properties such as surface pressure and area per molecule have been measured for polymers with variable A-blocks and a constant B-block. The results showed that there is a dependence between the critical and extrapolated areas per molecule and the length of the A-blocks. The results were interpreted by a modified equation of state for gases in two dimensions which suggested that the final polymer chain conformation was adopted relatively early during the compression and that the B-block was displaced away from the interface only with the two B-block monomers that are bound to the A-blocks anchored at the interface. The parameter, i, characterising the interaction between the molecules at the interface, was an approximately linear function of the molecular weight and the size of the A-blocks, and when extrapolated to the molecular weight corresponding to a pure B homopolymer a value close to the value for a B homopolymer monolayer was obtained. Interpretation of the results by an equation of state developed by Gaines indicated different monolayer compositions in the high and low compressibility regions, showing that the B-block is displaced away from the interface above a surface pressure of approximately 9 mN m-I which is the collapse surface pressure of a pure PEO monolayer. A comparison between this model and a modified Semenov model showed only a small contribution to the surface pressure from the polymer chain conformational changes, in line with the suggestion that the polymer chain adopts the final conformation at relatively high areas.
Langmuir, 2005
A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (MW) 141k, 11.4 wt% PEO) diblock copolymer in the hydrophobic regime was spread from chloroform solutions of various concentrations at the air-water interface, and the resultant monolayers were transferred to glass substrates and imaged using atomic force microscopy. Monolayers prepared under identical conditions were also characterized at the airwater interface via Langmuir compression isotherms. The effects of spreading solution concentration on surface features, compressibility, and limiting mean molecular area were determined, revealing several interesting trends that have not been reported for other systems of PS-b-PEO. Spreading solutions g0.50 mg/mL resulted almost exclusively in dot and spaghetti morphologies, with no observed continent features, which have been commonly found in more hydrophobic systems. For lower spreading solutions, e0.25 mg/mL, we observed a large predominance of two novel surface morphologies, nanoscale rings and chains. The surface pressure (π)-area (A) isotherms also exhibited a unique dependence on the spreading solution concentration, with limiting mean molecular areas and isothermal compressibilities of PS-b-PEO monolayers increasing below a critical concentration of spreading solution, suggesting a greater contribution from the PEO blocks. These results suggest that PS chain entanglement prior to solvent evaporation plays an important kinetic role in the extent of PEO adsorption at the air-water interface and in the morphologies of the resulting self-assembled surface aggregates.
Monolayers of Symmetric Triblock Copolymers at the Air−Water Interface. 1. Equilibrium Properties
Langmuir, 2000
Surface pressure isotherms and ellipsometric measurements of monolayers of two triblock symmetric copolymers, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), at the air-water interface have been carried out. These copolymers are water-soluble, and the difference in hydrophobicity between the blocks is small. This represents a different scenario for brush formation than for most of the hydrophobic-hydrophilic block copolymers reported so far. The surface pressure curves show two different phase transitions. The ellipsometric measurements indicate a thickness transition when the monolayer saturates, which supports the hypothesis for brush formation. The experimental data have been analyzed in terms of the scaling theory of adsorption of polymer brushes. Despite the possibility of diffusion from the interface, the PPO block acts as an efficient anchoring element in the formation of an adsorbed brush, once the adsorption sites at the interface are fully occupied. This is analogous to what has been reported for diblock copolymers with a much larger difference in the hydrophobicity of the blocks.