Effect of grafted polymer species on particle monolayer structure at the air-water interface.

We have studied poly(methyl methacrylate)-grafted(PMMA) particle monolayer systems at the air-water interface. In previous papers, we reported that PMMA chains grafted from particles (silica particle and polystyrene latex) were extended on water surfaces. Through observing deposited particle monolayers on substrates using SEM, we have confirmed that PMMA of large molecular weights were either dispersed or arrayed in structure with long inter-particle distances approximately 500 nm. In contrast, low molecular weight PMMA were observed to aggregate upon deposition. We speculated that the difference in morphology in deposited particle monolayers would be attributed to the affinity between the grafted polymer and the substrate. To examine the effect of this affinity three new polymer-grafted silica particles were synthesized with a fairly high graft density of about 0.14 approximately 0.43 nm(-2). As well as PMMA-grafted silica particles (SiO2-PMMA), poly(2-hydroxyethyl methacrylate) and poly(t-butyl methacrylate)--grafted silica particles (SiO2-PHEMA and SiO2-PtBuMA) were also prepared and subjected to pi-A isotherm measurements and SEM observations. These pi-A isotherms indicated that polymer-grafted silica formed monolayer at the air-water interface, and the onset area of increasing surface pressure suggests that the polymer chains are extended on a water surface. However, the morphology of the deposited monolayer is highly dependent on polymer species: SiO2-PHEMA showed that the dispersed particle monolayer structure was independent of grafted molecular weight while SiO2-tBuMA showed an aggregated structure that was also independent of grafted moleculer weight. SiO2-PMMA showed intermediate tendencies: dispersed structure was observed with high grafted molecular weight and aggregated structure was observed with low grafted molecule weight. The morphology on glass substrate would be explaiened by hydrophilic interaction between grafted polymer and hydrophilic glass substrate.

X-ray reflectometry confirms polymer-grafted silica particle monolayer formation at the air-water interface.

We examined the air-water interface of the poly(methyl methacrylate)-grafted silica particle monolayer by X-ray reflectometry to obtain in situ information on the particle monolayer structure suggested by pi-A isotherm measurement and SEM observation. We confirmed the monolayer formation of polymethylmethacrylate-grafted silica particle at the air-water interface by interesting XR profile. We obtained XR profile with many tooth-shaped fringes for one of the PMMA-grafted silica particle monolayer. By the model fitting of this profile, we confirmed monolayer formation at the air-water interface. Furthermore, we found that the interface roughness became large at the plateau region in the pi-A isotherm, which provides insight into the origin of the plateau region. We confirmed the reproducibility of XR profiles at same surface pressure after surface compression. This indicates the structually-reversible nature of the particle monolayer, which is the unique nature of the polymer-grafted particle monolayer.