PVCL microgels were prepared by surfactant-free precipitation polymerization using Cross-PAOS as a crosslinker. At polymerization temperature (70°C) water-soluble azo- initiator decomposed producing cationic free radicals and initiating the polymerization of VCL with vinyl group of PEG-modified Cross-PAOS. It is believed that the polymerization process in the present system follows the classical mechanism of precipitation polymerization proposed for the aqueous microgels. After formation of the primary radicals and fast chain growth process, polymer chains reaching a critical chain length precipitated to become precursor particles. These precursor particles created nuclei for PVCL growing chains which collapsed on the nuclei surface at the polymerization temperature of 70 °C which is far above their lower critical solution temperature (LCST) and became additionally crosslinked by silica nanoparticle (Si-O-Si bonds) obtained via condensation of Cross-PAOS, where the PEG chains were cleaved. The PEG chains were cleaved by hydrolysis/condensation of siloxane groups at reaction temperature and later on removed during the dialysis step. It is well known that Si-O-C bond is hydrolytically unstable, and can be cleaved by water to form silanol and alcohol. This reaction can be facilitated by increasing water solubility of the alkoxysilanes as indicated by Husing et al.19 To address the extent of PEG hydrolysis FT-IR measurements were performed on the silica samples obtained by the hydrolysis of Cross-PAOS samples under the conditions of microgel synthesis, i.e. stirring at 70 °C for 5h in water and subsequent dialysis. Afterwards, the sample was dried in vacuum oven for 3 hrs and then IR was measured. Figure 3.1.2 shows the IR spectra of pure silica, CP-V15-PEG40 and CP-V15- PEG40-10mol%-hyd (where “hyd” stands for the product obtained after hydrolysis). The comparison of all the spectra show that the hydrolysis of Cross-PAOS is almost done and the amount of remaining PEG groups is almost negligible. It is clear that nearly all the PEG
groups are cleaved during the synthesis and dialysis. Therefore, it was concluded that the PEG groups influence the microgel size and morphology only by controlling the aggregation of the crosslinker molecules in water during particle formation at the early stages of polymerization process, but not by modifying the hydrophilic-hydrophobic balance of the microgels due to the incorporation in polymer network. Similar results have also been reported by Gill et al.20 The growing PVCL microgel particles were stabilized by steric mechanism and partly by electrostatic repulsion provided by charged groups of the cationic initiator. Control experiments were performed to synthesize microgels in absence of crosslinking agent or in presence of PEG-PAOS without vinyl groups. In both cases no microgel formation was observed.
Figure 3.1.2. FT-IR spectra of pure silica, CP-V15-PEG40, and CP-V15-PEG40-10mol%-
hyd.
To monitor the chemical structure of the microgels XPS analysis was done. The overlay XPS spectra of various samples along with PVCL-BIS are shown in Figure 3.1.3. The XPS spectra show the presence of silicon atoms in microgel samples prepared with Cross- PAOS. Contrary, no Si-signal can be observed in the XPS spectrum of PVCL-BIS microgel. It is evident from the high resolution spectrum of C 1s (Figure 3.1.4a) that three peaks were fitted and the main peak at 284.78 eV was assigned to the C-C binding energy; the other two peaks were assigned to C-N at 285.6 eV and C=O at 287.0 eV in PVCL, respectively.
Figure 3.1.3. Overlay XPS spectra of CP-V15-PEG20-5mol% microgel (A), CP-V20-
PEG20-15mol% microgel (B), CP-V30-PEG20-15mol% microgel (C) and PVCL-BIS microgel (D).
Figure 3.1.4. High-resolution XPS spectra of C 1s (a), N 1s (b) and Si 2p (c) of CP-V20-
The spectrum of N 1s with a peak at 399.3 eV was symmetrical because there is only one type of nitrogen atom in the microgels. The existence of Si in the microgels was observable in Figure 3.1.4c because a peak appeared at 102.3 eV, which belongs to the Si 2p. The intensity was very low, so it is not possible to fit and distinguish the Si-O and Si-C species. By taking the XPS results into consideration, it was concluded that cross-PAOS was successfully incorporated into the microgels.
The incorporation of Cross-PAOS into the microgels has also been proved by FT-IR spectroscopy. The FT-IR spectra of Cross-PAOS (CP-V15-PEG20), microgels (CP-V20- PEG20-15 mol%) and the PVCL-BIS microgels are shown in Figure 3.1.5. The typical amide (1640 cm-1) peak was evident in both microgel spectra (PVCL-BIS and CP-V20-PEG20-15 mol%), which is due to the C=O stretch in PVCL chains. The typical siloxane peak at around 1104 cm-1 was visible in IR spectrum of both Cross-PAOS and microgel sample crosslinked by Cross-PAOS, which is assigned to Si-O-Si group. This peak was obviously absent in PVCL microgels crosslinked by BIS. The presence of this peak can confirm the existence of the crosslinker in microgel although this peak was very weak and appeared as a shoulder peak due to very low content of the crosslinker in the microgel network.
Figure 3.1.5. FT-IR spectra of PVCL-BIS microgels, CP-V20-PEG20-15 mol% microgels
Electron energy loss spectroscopy (EELS) and solid state 29Si NMR were also used to confirm the crosslinker incorporation. However, due to very low concentration of crosslinker inside the microgels quantitative determination was not possible.
Figure 3.1.6. AFM images of PVCL-BIS microgel particles before H2 plasma (a), CP-V15- PEG20-5mol% microgel particles (b); CP-V20-PEG20-10 mol% microgel particles (c), after H2 plasma PVCL-BIS microgel (d); CP-V15-PEG20-5mol% microgel (e); CP-V20-PEG20- 10 mol% microgel particles (f). Inset shows the images of the particles at higher resolution with all the scale bars 200 nm.
To further confirm the crosslinker incorporation, AFM measurements were done with and without H2 plasma treatment of PVCL microgels crosslinked by BIS and Cross-PAOS.
Figure 3.1.6a shows the AFM images of PVCL/BIS microgel particle before H2 plasma treatment. The height of these well-defined particles is approximately 100 nm. After treating the sample with H2 plasma, only some residues are left which can be seen in Figure 3.1.6d. CP-V15-PEG20-5mol% (Figure 3.1.6b) and CP-V20-PEG20-10mol% (Figure 3.1.6c) microgel particles are about 50 nm and 60 nm in height respectively before H2 plasma treatment. A well-defined inorganic network skeleton based on ultrasmall silica particles with height ≈5 nm is found after H2 plasma treatment of CP-V15-PEG20-5mol% microgels giving almost 90% reduction in height because of the removal of organic species (Figure 3.1.6e). Similarly, CP-V20-PEG20-10mol% microgels are 10 nm in height showing 84% reduction (Figure 3.1.6f). The size of the skeleton is in agreement with the size of microgel, which supports the information obtained from IR and XPS about crosslinker incorporation inside the microgels.
Finally, elemental analysis of the two selected microgel samples was done to determine the element composition (Table 3.1.6). The data of CP-V20-PEG20-15mol% and CP-V30-PEG20-15mol% confirm the presence of silicon inside the microgels and are in good agreement with the theoretical values calculated from the feed ratio.
Table 3.1.6. Elemental analysis results of two microgel samples synthesized using Cross-
PAOS Sample Elements C H N Si CP-V20-PEG20-15mol% Calculated 61.92 % 8.18 % 8.75 % 6.8 % Measured 63.91 % 9.85 % 8.68 % 5.79 % CP-V30-PEG20-15mol% Calculated 63.24 % 8.59 % 8.85 % 5.19 % Measured 63.94 % 9.51 % 8.68 % 4.34 %