The pH effect on microsphere formation in the dispersion copolymerization of methyl methacrylate with vinylbenzyl-terminated poly(acry1ic acid) (PAA) macromonomers is studied. The diameter of poly(methy1 methacrylate) (PMMA) microspheres is minimum around pH = 8. Dynamic light scattering data indicate that the shell thickness of PAAgrafted chains shows a maximum around this pH. The degree of expansion of PAA chains drastically affects the particle size of PMMA microspheres during the dispersion copolymerization.In previous work'), we prepared polymer microspheres by dispersion copolymerization of methyl methacrylate (MMA) with poly(methacry1ic acid) (PMA) macromonomers in aqueous ethanol solution. The particle size (submicrometre to micrometre range) and size distribution are controlled by selecting the solvent composition and the feed amount and molecular weight of the PMA macromonomers. However, the particle diameter does not depend strongly on the pH of the solvent medium.More recently, we synthesized polymer microspheres by dispersion copolymerization of MMA with poly(acry1ic acid) (PAA) macromonomers in aqueous ethanol solution*). The particle diameter of the polymer microspheres decreases as the water fraction in the mixed solvent, macromonomer concentration, and macromonomer molecular weight increased. For this dispersion copolymerization system it is expected that the pH in an aqueous solution drastically affects the particle size of the microspheres.In this short communication, we studied the pH effect in aqueous solution upon microsphere formation in the dispersion copolymerization of MMA with PAA macromonomers.
Experimental partThe preparation of vinylbenzyl-terminated poly(acry1ic acid) (PAA) macromonomer has been described in detail earlier'). Copolymerizations were carried out at 60 "C under a nitrogen atmosphere in a glass vessel. A mixture of the PAA macromonomer (numberaverage molecular weight fin = 1.5 x lo4, ratio of weight-to number-average molecular weights adfin = 1.35), methyl methacrylate (MMA), and 2,2'-azodiisobutyronitrile (AIBN) were dissolved in water. The mixture was kept in a thermostated bath at 60°C with shaking (150 shakes per min) for 7 h. The resulting products were dialyzed through 0 1997, Hiithig &
We have performed open cell transmission electron microscopy experiments through pure water vapor in the saturation pressure regime (>0.6 kPa), in a modern microscope capable of sub-Å resolution. We have systematically studied achievable pressure levels, stability and gas purity, effective thickness of the water vapor column and associated electron scattering processes, and the effect of gas pressure on electron optical resolution and image contrast. For example, for 1.3 kPa pure water vapor and 300kV electrons, we report pressure stability of ± 20 Pa over tens of minutes, effective thickness of 0.57 inelastic mean free paths, lattice resolution of 0.14 nm on a reference Au specimen, and no significant degradation in contrast or stability of a biological specimen (M13 virus, with 6 nm body diameter). We have also done some brief experiments to confirm feasibility of loading specimens into an in situ water vapor ambient without exposure to intermediate desiccating conditions. Finally, we have also checked if water experiments had any discernible impact on the microscope performance, and report pertinent vacuum and electron optical data, for reference purposes.
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