Fig. 4.1 shows the cross-sectional microstructure of the SiO2/pHEMA composites with different amounts of H2O and SiO2 concentrations during polymerization. It was observed that, in Fig. 4.1(a), sample 35H showed a considerably dense and uniform microstructure with no visually observable voids, which is similar to that of pure pHEMA membrane (0H) (not shown in Fig. 4.1), i.e., without the use of water. However, on increasing the amount of water during the synthesis of pHEMA, i.e., 50H, the resultant samples presented a loose microstructure, as illustrated in Fig. 4.1(b). This is because although water is a good solvent for the HEMA monomer, it become a poor solvent upon polymerization, i.e., for the HEMA polymer or pHEMA. Phase segregation between water and pHEMA phases may accordingly cause a change in the resultant microstructure. In the presence of water, a transparent, homogeneous pHEMA membrane can be obtained only when the concentration of water is below a certain critical value (between 40% and 60%) [141]. When the water concentration exceeds the critical value, phase separation occurs due to the thermodynamic interaction between water and the polymer network, resulting in an opaque, heterogeneous pHEMA hydrogel.
With the incorporation of the silica nanoparticles together with a sufficient amount of water in the starting solution, the resulting composite membranes of 4Si35H and 4Si50H, shown in Fig. 4.1(c) and (d), exhibited a wrinkled structure. In contrast to the sample without SiO2
particles, 4Si35H and 4Si50H presented a rougher surface morphology. With a further increase in the content of silica nanoparticles, i.e., sample 9Si35H (Fig. 1(e)) showed a nanoporous structure developed in the composite. Similarly, for sample 9Si50H, with higher water and silica nanoparticles contents, a submicroporous structure was developed, as illustrated in Fig. 1(f), indicating that the addition of SiO2 nanoparticles induced the formation of nano-to-micro pores. The variation in microstructural evolution of the composites suggests that the presence of the SiO2 nanoparticles does play an important role
in the evolution of nanoporosity during synthesis.
Fig. 4.1 SEM micrographs of cross-section image of various SiO2/pHEMA composites
The development of a nanoporous structure in the presence of the nanoparticles suggests that the nanoparticles may act as a heterodomain that induce phase separation, thus forming nanoporosity in the SiO2/pHEMA composite. As shown in Fig. 4.1(e), the nanopores developed during UV polymerization were randomly distributed in the composite. These
(e) 9Si35H
500 nm 500 nm
(f) 9Si50H 500 nm (b) 50H
500 nm
(d) 4Si50H
500 nm (a) 35H
500 nm
(c) 4Si35H
4.1(f)).
Radical polymerization has been known as the reaction responsible for the synthesis of polymeric HEMA; however, this reaction is expected to take place without interference in the regions at a distance away from the site of the SiO2 nanoparticles, which is termed as a “bulk region.” Nevertheless, in the region (termed “domain region”) that is close to the vicinity of the nanoparticles, the polymerization rate of the HEMA monomers is catalytically accelerated over the “bulk region.” Thus, the monomers near the silica nanoparticles can be cured at a faster rate under UV irradiation than those of the bulk regions [142-144]. Hence, the difference in the rate of polymerization between the “domain regions” and “bulk regions”
induced the formation of heterodomains. It is believed that a phase separation should take place once the heterodomains are fully developed; this would result in a nanoporous structure in the final composite. Furthermore, the diffraction efficiency of the UV light can be enhanced by the presence of nanoparticles [145, 146] so that the presence of SiO2
nanoparticles is expected to accelerate the polymerization rate and accordingly, the regions near the nanoparticles polymerize faster than other regions that are far from the nanoparticles is expected. By increasing the content of SiO2 nanoparticles, it was found that the formation of porous structure of the membranes was enhanced for the samples of 4Si50H and 9Si50H, as shown in Fig. 4.1(d) and (f). In other words, it indicates that the acceleration of polymerization rate could be enhanced by SiO2 nanoparticles, inducing the phase separation of polymer and the nanoporous structure.
Figure 4.2 shows that the tensile strength is decreased with increasing water concentration for the samples without silica nanoparticles. This has been explained to be a result of a lower degree of crosslinking during polymerization. However, a considerable improvement in the tensile strength can be detected for the SiO2/pHEMA composites. It was noted that the tensile strength of the 35H samples increased with the addition of silica
nanoparticles up to 4 wt%. This mechanical reinforcement effect of the silica nanoparticles can be referred to an earlier argument where the silica nanoparticles enhanced the degree of polymerization and possibly interacted with the pHEMA matrix although numerous nanopores were formed in the matrix polymer [142]. Those nanopores appear to have little effect on the deterioration of the tensile strength of the 9Si35H composites, and it is believed that the silica nanoparticles do provide an advantage in mechanical reinforcement.
The same scenario is prevalent for other compositions except for the sample 9Si50H for which the tensile strength was decreased compared with that of the sample 4Si90H. The explanation for this exception in the sample 9Si50H is due to the water content exceeding a critical value (40%) during the synthesis of SiO2/pHEMA where a considerable phase separation may have occurred, resulting in a composite with a higher porosity, as evidenced in Fig. 4.1(f).
Fig. 4.2 Mechanical properties of pHEMA and SiO2/pHEMA composites
A further characterization of the composite materials SiO2/pHEMA, in comparison to
pHEMA were placed in the distilled water and the water uptake was recorded by gravimetric measurement at different time durations. The swelling properties of the composites and pHEMA are shown in Fig. 4.3. Both SiO2/pHEMA composites of ySi35H and ySi50H series showed higher equilibrium swelling than pHEMA. The water absorption of SiO2/pHEMA composites increased corresponding to the content of silica nanoparticles and the effect is enhanced by increasing the amount of water in the starting reaction solution as illustrated in the ySi50H curves. The swelling ratio of the hydrogel was observed to be dependent on its free volume, the degree of chain flexibility, cross-linking density, and hydrophilicity [147]. The change in the swelling ratio might be attributed to the presence of hydrophilic nanosilica in the composite and an enhanced ability of water storage as a result of the nanoporous structure developed in the composites. Thus, an increased swelling ratio can be expected [148].
0 2 4 6 8 10
25 30 35 40 45 50 55 60 65
ySi50H
ySi35H pHEMA
ySi50H ySi35H pHEMA
Swelling ratio (%)
SiO
2concentration wt%
Fig. 4.3 Amount of water absorption of pHEMA and SiO2/pHEMA composites
0 1000 2000 3000 4000 5000 6000 7000 8000