Structure and mechanical properties of transparent ZnO/PBDMA nanocomposites

The structural, optical and mechanical characterization of ZnO/PBDMA (poly(butanediolmonoacrylate)) nanocomposites is presented. ZnO nanoparticles are homogenously dispersed in the polymer matrix in transmission electron microscopy (TEM) images of ultramicrotomed sections. The size of the ZnO nanoparticles can be controlled during synthesis in a range of 6-10 nm as determined by dynamic light scattering (DLS). TEM and small-angle X-ray scattering (SAXS) investigations show a homogeneous dispersion for the 6 nm sized particles in the resulting nanocomposites. Due to the low scattering of small, well dispersed particles, the transparency for visible light of the nanocomposites is very high (transmittance > 91% for lambda = 600 nm), while the haze is below 1%. ZnO nanoparticles act as a strong UV-absorber, causing a transmittance below 0.05% for wavelengths smaller than 350 nm in the nanocomposites. For the composite containing 6 nm sized particles, a green luminescence band, centered at 538 nm, is observed using fluorescence spectroscopy, while the excitation of the fluorescence has a maximum at 357 nm. Both, excitation and emission maxima, depend on the size of the particles and are shifted to higher wavelength when larger particles were used. Furthermore, the nanoparticles strongly influence the mechanical properties and the glass transition temperature of the nanocomposites. The addition of 4.5 wt% ZnO to PBDMA leads to an increase in modulus from 70.8 MPa to 139.1 MPa (increase 10 nearly 200%) and in tensile strength from 5.2 MPa to 9.5 MPa (increase to 180%) retaining the elongation at break nearly unchanged (decrease from 13.4% to 10.1%). The sample is much stiffer and exhibits a higher work of fracture due to the nanofiller addition. As compared to the unmodified materials, the glass transition temperature is enhanced by 5 K in the case of the nanocomposite sample.