Dispersion characteristics of nanocomposites based on functionalized block copolymers

The dispersion characteristics of organoclay nanocomposites based on functionalized block copolymers have been investigated. For the investigation, polystyrene-block-polybutadiene (SB diblock) copolymers synthesized via anionic polymerization were first hydroxylated via hydroboration/oxidation to obtain polystyrene-block-hydroxylated polybutadiene (SBOH diblock) copolymers. Then, the SBOH diblock copolymer was attached with pyridine, pyrimidine, terpyridine, or terpyridine-Ruthenium (Ru) complex functional groups to obtain SB-pyridine, SB-pyrimidine, SB-terpyridine (SB-Terpy), and SB-Terpy-Ru complex diblock copolymers. Subsequently, each of these functionalized block copolymers was used to prepare, via solution blending, organoclay nanocomposites, for which natural clay (montmorillonite, MMT) and two commercial organoclays (Cloisite 30B and Cloisite 15A) were employed. The dispersion characteristics of the organoclay nanocomposites were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and oscillatory rheometry (OR). We have made the following observations. The SBOH/Cloisite 30B nanocomposite had a very high degree of dispersion of Cloisite 30B aggregates, whereas the SBOH/Cloisite 15A and SBOH/MMT nanocomposites had a very low degree of dispersion of the aggregates of Cloisite 15A or MMT. In situ Fourier transform infrared (FTIR) spectroscopy has revealed that hydrogen bonds were formed between the hydroxyl groups in the SBOH diblock copolymer and the surfactant residing at the surface of Cloisite 30B in the former nanocomposite, yielding a very high degree of dispersion of Cloisite 30B aggregates, while no hydrogen bonds were formed in the latter two nanocomposites. The (SB-pyridine)/Cloisite 30B nanocomposite had intercalation of aggregates of Cloisite 30B, while the (SB-pyridine)/Cloisite 15A and (SB-pyridine)/MMT nanocomposites had a very low degree of dispersion of the aggregates of Cloisite 15A or MMT in the SB-pyridine matrix. The aggregates of Cloisite 30B or Cloisite 15A were well dispersed in the nanocomposites based on the SB-pyrimidine diblock copolymer, whereas the aggregates of MMT were poorly dispersed in the SB-pyrimidine matrix. The difference in the dispersion characteristics of the two nanocomposites is explained via FTIR spectroscopy in terms of the presence of ion-dipole interactions between the polar groups in pyrimidine and the positively charged N+ in the surfactant residing at the surface of an organoclay employed (Cloisite 30 or Cloisite 15A). The (SB-Terpy)/Cloisite 30B nanocomposite had a very high degree of dispersion of Cloisite 30B aggregates, while the (SB-Terpy)/Cloisite 15A and (SB-Terpy)/MMT nanocomposites had a very low degree of dispersion of the aggregates of Cloisite 15A or MMT in the SB-Terpy matrix. FTIR spectroscopy has revealed that hydrogen bonds were formed between the hydroxyl groups in the SB-Terpy diblock copolymer and the surfactant residing at the surface of Cloisite 30B, yielding a very high degree of dispersion of Cloisite 30B aggregates. The very high degree of the dispersion characteristics of the nanocomposites based on the SB-Terpy-Ru complex diblock copolymer is interpreted in terms of the Coulombic interaction present in the respective nanocomposites.