Poly monolayers, properties

To test the basic idea, some experiments are possible. When the side chains are long and flexible, there will be little difference between the energy of interaction between one molecule and another either of the same type or of the opposite enantiomorphic form, irrespective of the backbone directions. For example the properties of poly(c-benzyloxy-carbonyl-L-lysine) can be compared with a 1 1 mixture with the D-lysine enantiomorph. When side chains are short and inflexible, as in poly(L-alanine), significant differences should exist between its monolayer properties and that of a 1 1 racemic mixture, in this case of poly(L-alanine) and poly(D-alanine). 

Monolayer Properties. For solubility reasons it was necessary to spread poly (L-leucine) from chloroform containing 20% trifluoracetic acid. This solvent causes drops of liquid to creep up the outside of the pipet, and the observed area per residue of 16 may be slightly low. Use of a glass pipet treated with a silicone minimized the problem. Both specimens of poly(L-methionine) behaved in a similar manner. Poly(L-nor-leucine) has been investigated previously (12,16). The results presented here are in general agreement, but the work has been repeated under the same experimental conditions as for the other polymers to enable a precise comparison (Figure 3). 

Incidental to this work is more evidence that the a-helix exists at the air-water interface. While some have appeared reluctant to accept this view, no good theoretical reason exists why it should not be stable. Where the nature of the side chain might provoke other conformations [as in poly(l-valine)] or the molecular weight is low or monolayers are spread from poor solvents miscible with water, other conformations are detectable, and the monolayer properties are significantly different (5). 

Polyimide-clay nanocomposites constitute another example of the synthesis of nanocomposite from polymer solution [70-76]. Polyimide-clay nanocomposite films were produced via polymerization of 4,4 -diaminodiphenyl ether and pyromellitic dianhydride in dimethylacetamide (DMAC) solvent, followed by mixing of the poly(amic acid) solution with organoclay dispersed in DMAC. Synthetic mica and MMT produced primarily exfoliated nanocomposites, while saponite and hectorite led to only monolayer intercalation in the clay galleries [71]. Dramatic improvements in barrier properties, thermal stability, and modulus were observed for these nanocomposites. Polyimide-clay nanocomposites containing only a small fraction of clay exhibited a several-fold reduction in the... 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Specific properties of polysilanes have been linked to the method of synthesis.35 For example, in the case of anionic polymerization of poly[l-(6-methoxy-hexyl)-l,2,3-trimethyldisilanylene] a new type of chromism was induced in the polysilane film by the difference in the surface properties of substrates and was termed a surface-mediated chromism. The polysilane exhibited thermochromism with an absorption maximum at 306 nm at 23°C, but <15°C a band at 328 nm began to appear. A monolayer of the polysilane was transferred onto both a clean hydrophilic quartz plate and a hydrophobic one treated with hexamethyldisilazane by the vertical dipping method. With the hydrophobic plate, a broad UV absorption at 306 nm is obtained, whereas the absorption on a hydrophilic plate shifts to 322 nm. The conformation of the polysilane is preserved by hydrogen bonding between the silica surface and the ether section of the substituent on the hydrophilic plate. The polysilane is attached to the hydrophobic surface only by van der Waals forces, and this weaker interaction would not sustain the thermodynamically unstable conformational state that is attained on the water surface. 

Menzel1811 more recently described the properties and behavior of monolayers prepared from azobenzene-containing poly(L-glutamate)s possessing the structures XIX and XX (n= 2) shown in Scheme 11. These monolayers showed photomechanical effects opposite to those described above for azo-modified poly(L-lysine)s. In fact, they expanded when exposed to UV light (trans—>tis isomerization), and shrank when exposed to visible light (ds— trans isomerization). The expansion was found to be smaller than expected from comparison of the monolayer isotherms obtained from irradiated and nonirradiated solutions. This was attributed to the trans-cis photoconversion of the azo units, which occurs with lower yields in monolayers... 

The properties of poly(D, L-lactic acid) monolayers spread at the air - water interface were also shown to be strongly dependent of the nature of the spreading solvent. In this case, the monolayers spread from acetone and tetrahydrofuran exhibited typical reversible collapse behavior in the compression - expansion cycle with a quasi - plateau at large areas followed by a steep rise in the surface pressure at small areas. Conversely, the monolayers spread from chloroform, dichloromethane,... 

The interfacial properties of an amphiphilic block copolymer have also attracted much attention for potential functions as polymer compatibilizers, adhesives, colloid stabilizers, and so on. However, only a few studies have dealt with the monolayers o well - defined amphiphilic block copolymers formed at the air - water interface. Ikada et al. [124] have studied monolayers of poly(vinyl alcohol)- polystyrene graft and block copolymers at the air - water interface. Bringuier et al. [125] have studied a block copolymer of poly (methyl methacrylate) and poly (vinyl-4-pyridinium bromide) in order to demonstrate the charge effect on the surface monolayer- forming properties. Niwa et al. [126] and Yoshikawa et al. [127] have reported that the poly (styrene-co-oxyethylene) diblock copolymer forms a monolayer at the air - water... 

Other recent applications of AFM-SECM included the study of the iontophoretic transport of [Fe(CN)6]4 across a synthetic track-etched polyethylene terephthalate membrane by Gardner et al. [193]. They made the structure and flux measurements at the single pore level and found that only a fraction of candidate pore sites are active in transport. Demaille et al. used AFM-SECM technique in aqueous solutions to determine both the static and dynamical properties of nanometer-thick monolayers of poly(ethylene glycol) (PEG) chains end-grafted to a gold substrate surface [180]. 

There is a vast body of diblock copolymer studies since block choice can be such that they resemble amphiphilic surfactants. For the sake of brevity, we will skip them. Instead, we present an interesting case of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, commonly known by one of its trade names, Pluronics [117]. They have been used as non-ionic surfactants for a variety of applications such as in emulsification and dispersion stabilization. In aqueous solutions, these copolymers form micelles, and there exists a well-defined critical micelle concentration that is experimentally accessible. Several groups have investigated colloidal suspensions of these polymers [118-122], The surface properties of the adsorbed monolayers of the copolymers have been reported with respect to their structures and static properties [123-126]. 

Finally, we note that in a very recent work Heuberger et al. investigated protein-resistant copolymer monolayers of PEG grafted to poly(L-lysine) (PLL) (PLL-g-PEG) in terms of the role of water in surface grafted PEG layers [159], interaction forces and morphology [160], compressibility, temperature dependence and molecular architecture [161], PEG is often used in biomedical applications in order to create protein-resistant surfaces but the mechanisms responsible for the protein-repelling properties of PEG are not fully understood. 

Structure and Physicochemical Properties of Poly alky Isiloxane Monolayers 195... 

Since ordered structural orientation is believed to be a prerequisite for exhibiting alternation in physical properties in monolayer coverage, I have interpreted the alternation exhibited in the C data for the liquids that comprise Z(CH2)nH series to mean that such moleclules are fixed to the monomer unit of poly(styrene) segments in the liquid-saturated gel-state via the Z substituent, and that these adsorbed molecules are distributed around the adsorption sites (i.e, the monomer units) in a well-defined orientation with respect to each site, such that the established orientation relative to that monomer unit is maintained despite the freedom of rotation and serpentine movement of the polymer segments between crosslink junctions in the liquid-saturated gel domain. 

---