PhD Thesis
Smart organic-inorganic nanohybrids of functionalized silsesquioxane nanoparticles
Manuela Schumacher (07/2004-11/2008)
Support: Axel H. E. Müller
Summary
The formation and characterization of smart organic-inorganic nanohybrids was studied. The inorganic part was formed by N,N-di(2,3-dihydroxypropyl)3-aminopropylfunctional silsesquioxane nanoparticles being highly functionalized with ca. 14 tertiary amino groups per particles, each amino group bearing four hydroxyl groups. Two different polymer systems were used for the organic side: amphiphilic block copolymer micelles of poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBAx-b-PAAy with x = 90, 100 and y = 100, 150, 300) and star-shaped poly(acrylic acid)s (PAA100)21 and (PAA200)24, the latter serving as a model system for frozen micelles. In all cases the mixing of aqueous solutions of anionic block copolymer micelles or the anionic stars with solutions of the silsesquioxane nanoparticles led to the easy and straightforward formation of organic-inorganic nanohybrids. The structure of the complex nanohybrids depends on pH and salinity. The amount of incorporated silsesquioxane nanoparticles within the micelles or the stars under varying external stimuli was determined using a large number of methods.
Complexation preserved the original size of the micelles - consisting of a PnBA core and a PAA corona - according to dynamic light scattering (DLS) and static light scattering (SLS) as well as light scattering titration measurements and asymmetric flow field-flow fractionation (AFFFF) experiments. Fourier-transform infrared spectroscopy (FT-IR) and dialysis measurements with fluorescently labelled silsesquioxane nanoparticles confirmed the nanohybrid formation over a relatively wide range in pH. Cryogenic transmission electron microscopy (cryo-TEM) micrographs indicated a core-shell structure of the nanohybrids with gradual decreasing density of silsesquioxane nanoparticles.Complexation preserved the original size of the micelles - consisting of a PnBA core and a PAA corona - according to dynamic light scattering (DLS) and static light scattering (SLS) as well as light scattering titration measurements and asymmetric flow field-flow fractionation (AFFFF) experiments. Fourier-transform infrared spectroscopy (FT-IR) and dialysis measurements with fluorescently labelled silsesquioxane nanoparticles confirmed the nanohybrid formation over a relatively wide range in pH. Cryogenic transmission electron microscopy (cryo-TEM) micrographs indicated a core-shell structure of the nanohybrids with gradual decreasing density of silsesquioxane nanoparticles.
LS titrations gave an insight in the postulated interaction mechanism. Complexation in acidic media is driven by hydrogen-bonding and ionic interaction; in alkaline media nanohybrids are mainly formed due to ionic interaction. Depending on ionic strength, attractive Coulomb interactions may be (i) either sufficient to promote complexation even at high pH, where hydrogen-bonding is absent (low ionic strength), or are (ii) screened (high ionic strength), resulting in less favourable interactions between micelles and silsesquioxane nanoparticles. The reason for the size conservation is most probably due to the kinetically frozen micellar core and the compensation of (i) increased steric repulsion due to complexation and (ii) attractive interactions between the silsesquioxane nanoparticle and the charged PAA. The maximum of the interaction at 0.1 M could be deduced to be at 3.5 < pH < 7.5 NaCl. At low salinity (0.01 M NaCl) more nanoparticles were incorporated within the micelles. Nanohybrids exist even up to very basic conditions (pH < 9.5). The responsiveness of the system on pH and salinity as external stimuli was demonstrated by LS titration, dialysis and FT-IR measurements, thermogravimetric analysis (TGA) and AFFFF measurements.
Quantifying the amount of nanoparticles incorporated in the micelles turns out to be a arduous task. SLS of dialysed and undialysed samples and AFFFF of undialysed samples clearly showed increased molecular weights of the formed nanohybrids. TGA - requiring an exhaustive dialysis procedure prior to the measurements - provided information about the amount of incorporated silsesquioxane nanoparticles within the micelles. Isothermal titration calorimetry (ITC) provided the possibility to investigate the complexation mechanism in greater detail. Small angle neutron scattering (SANS) experiments, conducted at basic conditions, provided information on the inner structure of the nanohybrids. A newly developed fitting model enabled the determination of the radial profiles of the organic-inorganic nanohybrids. Additionally, it enabled the quantification of the amount of interacting nanoparticles under these conditions.
All methods to determine the amount of nanoparticles incorporated within the micelles sustained the formation of the organic-inorganic nanohybrids. The absolute number of nanoparticles per micelle is quite high (in the range from 160 to 4300, depending on the used method and conditions), however, the calculated numbers of nanoparticles per acrylic acid unit are quite low (in the range from 0.002 to 0.053).
The PAA-stars (PAA100)21 and (PAA200)24 showed comparable behaviour to the micelles. According to DLS and SANS experiments their size was preserved during complexation. SANS and LS titration measurements demonstrated the increased mass of the nanohybrid stars compared to the net stars. Cryo-TEM micrographs confirmed the formation of organic-inorganic nanohybrid stars, indicating a morphology with gradual decreasing density of nanoparticles. An appropriate fitting model for the SANS data for this challenging system was developed that proved the interaction between the silsesquioxane nanoparticles and the PAA and enabled the calculation of the amount of entrapped silsesquioxane nanoparticles within one star. The determined values were comparable to the ones calculated for the micellar nanohybrids.