Formation and Collapse of Single-Monomer-Thick Monolayers of Poly(n-butyl acrylate) at the Air−Water Interface

Abstract

The behavior of poly(n-butyl acrylate) (PnBA) spread at the air-water interface has been studied for a full range of surface coverages and several molecular weights. At low and intermediate surface coverages, the surface pressure-area isotherm behavior of the polymer is found to follow the expected scaling laws. In the dilute regime the pressure is an increasing function of surface coverage and a decreasing function of molecular weight. In the semidilute regime the surface pressure becomes independent of molecular weight, and a Flory exponent for the twodimensional radius of gyration is found to be ν = 0.57 ( 0.02. Beginning in the high coverage concentrated regime, at a surface pressure of around 15 mN/m, and through the full coverage regime (where the water in the subphase is fully covered and not exposed to air), X-ray reflectivity (XR) measurements show the formation of a continuous waterfree monolayer (i.e., one monomer thick) film of the polymer. At surface concentrations above the transition point to the full coverage regime (alternatively called the "collapsed" regime hereafter for the reason that will become apparent below), Brewster angle microscopy (BAM) shows that the excess polymer material does not distribute uniformly in the polymer film layer but instead leads to formation of micrometer-scale isolated globular domains of roughly uniform size. Further, it was observed that the number of such domains increases as the surface polymer concentration is increased, whereas the size of the globular domains is largely unaffected by the concentration variation. X-ray grazing incidence diffraction (GID) indicates that these domains are regions of bulklike (amorphous) polymer. These and other observations, including the invariant nature of the monolayer throughout the compression (confirmed by XR), the plateau nature of surface pressure-area isotherm throughout the collapsed regime, and the reversible nature of the domain formation (evidenced by BAM), suggest that the globular domains formed at high surface concentrations of PnBA are in a type of coexistence with the uniform monolayer. A simple thermodynamic model considering the entropic penalty of confining the polymer chains to monolayer, the translational entropy of the domains, and the surface energy of the interface is made in order to understand the behavior of the polymer as it becomes excluded from the monolayer. This argument suggests that the excess polymer should form a single large domain in order to minimize the large surface energy at the water-polymer interface. The presence of many small domains suggests the domains are kinetically trapped in a local, rather than global, equilibrium.

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

“…Figure 1(A) displays the results from these measurements in plots of monolayer surface pressure (Π) vs. area per hydrophobic (i.e., PnBA) monomer (α). 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.…”

“…Specifically, a laterally-mobile mixed PEO and PDMAEMA brush was prepared by spreading a mixture of two diblock copolymers, poly(ethylene oxide)-poly( n -butyl acrylate) (PEO-PnBA) and poly(2-(dimethylamino)ethyl methacrylate)-poly( n -butyl acrylate) (PDMAEMA-PnBA), onto the air-water interface. Here, PnBA was used as the common hydrophobic block for these copolymers, because the PnBA polymer forms a flat, single monomer-thick, continuous film (free of bare areas) at the air-water interface, 56 and also the grafted PEO and PDMAEMA chains do not have any tendency to adsorb to the surface of the anchoring domain formed by PnBA, 57, 58 which makes the PEO-PnBA and PDMAEMA-PnBA block copolymers uniquely suited for preparation of model non-adsorbing PEO/PDMAEMA brush systems. A laterally-mobile mixed PEO and PDMAEMA brush was prepared from a mixture of PEO 113 -PnBA 100 and PDMAEMA 118 -PnBA 100 diblock copolymers, where the subscripts refer to the number-average degrees of polymerization of the individual blocks; note that, as will be analyzed, in this case the heights of the PEO and PDMAEMA brushes are estimated to be (slightly) mismatched (that is, they differ by a factor less than about two).…”

Water Is a Poor Solvent for Densely Grafted Poly(ethylene oxide) Chains: A Conclusion Drawn from a Self-Consistent Field Theory-Based Analysis of Neutron Reflectivity and Surface Pressure–Area Isotherm Data

“…21 The relevant findings of the study can be summarized as follows. As demonstrated in Figure 1A, a typical surface pressure (Π) vs area per chain (α) isotherm of a PnBA polymer exhibits a sharp transition upon compression from a compressible fluid state to a plateau of surface pressure (for instance, in the case demonstrated in the figure (DP n = 102), the transition occurs at an area per chain value of α o = 2120 Å 2 /chain).…”

“…As mentioned in the previous section, in a monolayer formed by a PnBA homopolymer, this monolayer collapse leads to formation of micrometer-scale aggregates ("domains"). 21 To test whether the same happens in the PEO−PnBA monolayer, the lateral uniformity of the monolayer film was examined at an area per chain condition of α = 2000 Å 2 /chain by atomic force microscopy (AFM). The resultant image is presented in Figure S1 of the Supporting Information (SI).…”

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

“…Thus, during monolayer compression, this chain collapsed regime starts at the surface polymer concentration corresponding to the full surface coverage condition. 4 In 1946, Crisp reported extensive data comparing surface pressure-area isotherm data for a wide range of different polymers;…”