Poly(vinylidene fluoride) (PVDF), a semi-crystalline polymer with a range of interesting properties, shows potential to be used in a variety of technological applications. Our previous research showed that PVDF can be utilized in the thermoset composite welding (TCW) technology. Viscoelasticity is a very important property for the application of polymers. Therefore, the characterisation of viscoelastic behaviour is necessary for PVDF used in TCW technology. Nanoindentation provides a good evaluation method for this purpose. One advantage of the nanoindentation is that this test can tell the local information of the materials, which can better reflect the intrinsic properties of the materials than the traditional macro-test, which may be affected by the factor like, purity of the materials, sample size and shape etc. The results of the viscoelastic behaviours characterised by nanoindentation showed that the creep would be a concern for PVDF used in TCW technology, and the improvement of the creep resistance would benefit a lot for PVDF used in TCW technology. Detail results and discussions about the viscoelastic behaviour by nanoindentation test are given in Appendix A. Therefore, the reinforcing of PVDF was necessary for PVDF used in TCW technology, especially the improvement of the creep resistance.
The reinforcement effect can be achieved by the introduction of a second phase, and the introduction of nanofillers shows promising future due to the huge surface area, high aspect ratio, and better mechanical properties compared with its peer in micro-scale. Therefore, in this research, the nanofillers were chosen as the candidate to improve the properties of PVDF. To screen the best nanofiller candidate, four different types of nanofillers (layered silicates, carbon nanotubes, microcrystalline cellulose, halloysite nanotubes) were used. At the same time, a 3D network was formed by layered silicates and carbon nanotubes to observe if the synergistic effect exists. The effect of those nanofillers on the structures and properties of PVDF was investigated. The final results showed that carbon nanotubes were the best candidate due to the better performance of the creep resistance, simplicity in composition, and less possible side-effect used in TCW technology. Therefore, carbon nanotubes were chosen as the nanofillers to reinforce PVDF.
To further improve the creep resistance, the improvement of the stress transfer from PVDF matrix to carbon nanotubes is necessary. To achieve this, novel “bud-branched” nanotubes, carbon nanotubes decorated by metal particles, were fabricated. The successful fabrication of the bud-branched nanotubes may not only just help the control of the stress transfer, but also give a chance to clarify some fundamental issues for nanocomposites. Therefore, except the creep behaviour of PVDF nanocomposites with carbon nanotubes and bud-branched nanotubes, the other effects by the bud-branched nanotubes, like effect on the rheological behaviours, crystalline structures, fracture behaviours and final properties were also investigated. Results showed that a distorted strain field existed around the restricted buds on the surface of carbon nanotubes during dynamic rheological test, which showed higher improvement of the storage modulus and normal force compared with carbon nanotubes at the melt state. At the same time, for PVDF/bud-branched nanotubes nanocomposites, a significant improvement in the fracture toughness was observed compared with PVDF carbon nanotubes nanocomposites. This showed a way to overcome the traditional drawback to reinforce soft material by rigid fillers, which showed improvement of the stiffness and strength at the cost of dramatic decrease of fracture toughness.
The decoration of the metal particle on the surface of carbon nanotubes showed better reinforcing effect compared with the virgin carbon nanotubes, which was proven by higher Young’s modulus and storage modulus for bud-branched nanotubes compared with virgin carbon nanotubes. However, for the creep resistance, positive effect by the surface decoration of metal particles existed just at low temperature and low stress level. Burgers’ model and Findley power law were employed to model the creep behaviour, and both were found to agree quite well with the experimental data. The relationship between the structures and properties was analysed based on the parameters of the simulation. Eyring rate model was used to model the creep behaviour too, which did not show the suitability to be used for PVDF and PVDF with 5 wt% carbon nanotubes. However, for PVDF with 5 wt% bud-branched nanotubes, it worked very well. PVDF with 5 wt% carbon nanotubes was chosen as the final modified film used in TCW technology because it showed higher improvement of the creep resistance compared with its peer bud-branched nanotubes at high stress level and high temperature.
The modified PVDF by carbon nanotubes with improved creep resistance showed worse performance in term of tensile shear strength in TCW technology compared with pure PVDF prepared by same condition. This is due to the incompletions of welding surfaces that obtained at its optimal welding condition. This conclusion can be supported by the improved performance in TCW technology with higher welding temperature and longer welding time. The fracture surface analysis also showed similar fracture behaviour for pure PVDF and modified PVDF at the co-curing interface (between thermoplastic film and composites). The co-curing interface was considered to be the key factor for the bonding properties between composites and the thermoplastic film. All those information suggests that the modified PVDF has the potential to perform similar in the TCW technology if the problem of the welding interface is solved.