Evidence of Lateral Nanoscale Heterogeneities in Weak Polyelectrolyte Brushes

Macroscopic lateral heterogeneity observed in a laterally mobile immiscible mixed polyelectrolyte–neutral polymer brush

“…As has been verified in our previous experiments with PnBA, 57, 62 the surface pressure at the transition point is equivalent to the surface tension change associated with the removal of the intervening polymer monolayer from the air-water interface (i.e., Π o = γ air-water – γ polymer-air – γ polymer-water ). On the basis of this concept, the higher transition pressure observed for the diblock monolayers relative to the transition pressure of PnBA (i.e., Π o,PEO-PnBA = Π o,PDMAEMA-PnBA > Π o,PnBA ) is considered to be due to a decrease in the polymer-water interfacial tension (γ polymer-water ) caused by the presence of the end-grafted hydrophilic PEO/PDMAEMA chains at the PnBA–water interface; hereafter, this effect will be called the “(PnBA-water) interface modification” effect.…”

“…As discussed in detail in our previous publication, 56 the sharp plateau transition in the isotherm of the PnBA homopolymer (black curve) at an area per PnBA monomer of α o ≈ 21.6 Å 2 is due to the formation of a flat (i.e., laterally uniform) single-monomer-thick continuous film of PnBA that is free of voids at that polymer surface concentration. Our previous studies have also demonstrated that, thanks to this property of PnBA, at areas per PnBA monomer less than α o , the monolayer domain formed at the air-water interface by PnBA can be used as end-grafting sites for PEO or PDMAEMA chains for preparation of model non-adsorbing PEO 58, 66 or PDMAEMA 57, 62 brush systems. As shown in Figure 1(A), the monolayers of the block copolymers also exhibit a similar transition in their isotherm curves, all at an essentially identical value of area per PnBA monomer (α o = 21.4 ± 0.5 Å 2 ), which indicates that this transition behavior of PnBA is unaffected, regardless of whether the PnBA segment is present as an isolated chain or in the form of a component block of a diblock or a triblock copolymer.…”

“…On the basis of this concept, the higher transition pressure observed for the diblock monolayers relative to the transition pressure of PnBA (i.e., Π o,PEO-PnBA = Π o,PDMAEMA-PnBA > Π o,PnBA ) is considered to be due to a decrease in the polymer-water interfacial tension (γ polymer-water ) caused by the presence of the end-grafted hydrophilic PEO/PDMAEMA chains at the PnBA–water interface; hereafter, this effect will be called the “(PnBA-water) interface modification” effect. We believe that at the transition point (α = α o ), the lateral osmotic repulsion between the PEO or PDMAEMA brush chains is negligible and does not contribute significantly to the increased surface pressure at the transition point, because at this condition the average interfacial area occupied by a PEO or PDMAEMA brush chain ( A o ≈ 2140 Å 2 ) is estimated to be comparable to the π R G 2 values for the PEO and PDMAEMA segments (that is, the grafted PEO or PDMAEMA chains are non-interacting or at most weakly interacting with one another); here, R G is the radius of gyration of the PEO or PDMAEMA block in the three-dimensional random-walk configuration, and its values are approximately estimated to be 25.7 Å for the PEO segment and 27.3 Å for the PDMAEMA segment using relevant parameter values reported in References 67 and 62 , respectively (in the PDMAEMA case the electrostatic interactions are ignored for simplification). This notion that at near the transition point (i.e., at α ≈ α o ) the brush chains contribute negligibly to the lateral surface pressure of the diblock monolayers is also supported by the fact that at the transition point (α = α o ) the PDMAEMA-PnBA monolayer exhibits the same surface pressure as does the PEO-PnBA monolayer even though the PDMAEMA segments are significantly charged at neutral pH conditions; 68 (as will be discussed in the second next paragraph below) the effect of this electrostatic repulsion between adjacent PDMAEMA chains becomes more clearly manifested in the α < α o region where the PDMAEMA-PnBA monolayer shows increasingly higher surface pressure than the PEO-PnBA monolayer with decreasing α.…”

Humidity-dependent compression-induced glass transition of the air–water interfacial Langmuir films of poly(d,l-lactic acid-ran-glycolic acid) (PLGA)

“…Note that in the full surface coverage regime the plateau surface pressure is equivalent to the difference in the interfacial tension quantities, π ୭ = γ ୟ୧୰ି୵ୟ୲ୣ୰ − ൫γ ୟ୧୰ି୮୭୪୷୫ୣ୰ + γ ୮୭୪୷୫ୣ୰ି୵ୟ୲ୣ୰ ൯. 22,23 Using this concept and the data in Figure 2 Figure S4, even after the relaxation process is completed at 25 ℃ , there typically remains a residual stress, and the final surface pressure is always greater than the surface pressure value measured at 40 ℃; therefore, if low temperature alone could cause the glass transformation of a Langmuir PLGA film, the cooling process should still produce an increase in surface pressure. In fact, the opposite is observed (i.e., the surface pressure decreases as the temperature was lowered from 40 to 25 ℃).…”

Structure and Functionality of Polyelectrolyte Brushes: A Surface Force Perspective

“…It has been shown that single‐molecule spectroscopy with a sharp tip can be used to determine the contour length and the persistence length of molecules that are formed by grafting‐from procedures . In a similar manner to SFA force measurements, normal force measurements with a sharp tip or colloidal probe can be used to distinguish electrostatic double‐layer forces from steric ones and, thus, to investigate the swelling of brushes in different solvents and salt concentrations,,, to detect the collapse and recovery of a layer at different normal forces and solution conditions ,.…”