Electroless metal plating has been employed to insert nickel inside the pores of the PVDF matrix. The affinity between the plating solution and substrate surface is an important factor for successful metal deposition. Hence, considering the hydrophobic nature of PVDF (contact angle on water is 82°),[60] both sensitization and activation steps of the plating procedure were performed in MeOH/H2O mixtures in order to completely wet the surface of the pores.
TEM images of the electroless plated films (Figure 4.11) demonstrate the successful deposition of metal inside the nanoporous PVDF. Nickel penetrated the pores in the polymer matrix, resulting in a PVDF/Ni nanocomposite. The images were obtained without sample staining due to the significant contrast from the strong scattering of the nickel phase. The lamellar morphology of the porous template (Figure 4.9), originating from the block copolymer phase separation (Figure 4.7), is clearly preserved. The electroless deposition of nickel on the PVDF matrix results in an intimate contact between the polymer and metal phase. The PVDF/Ni composite is therefore a promising nanomaterial, since strain-coupled multiferroic composites require such an intimate contact between the piezoelectric and magnetostrictive phase.[61] Furthermore, the self-assembled morphology may greatly enhance the magnetoelectric response, since the lamellar nanostructure represents the optimal laminated configuration.[62] The WAXS pattern of the PVDF/Ni nanocomposite (Figure 4.8b4) is comparable to the pattern of the block copolymer and porous template, demonstrating the presence of the β-polymorph of PVDF. This suggests that the crystalline phase in the block copolymer film is preserved both during acid etching and electroless nickel deposition.
To study the chemical composition of the PVDF/Ni composite, the plated films were cleaved and investigated by energy-dispersive X-ray spectroscopy. Figure 4.12 displays the EDX spectrum of a cross section. Both carbon (0.28 keV) and fluorine (0.68 keV) peaks represent poly(vinylidene fluoride) within the composite, while the signals at 0.85 and 7.48 keV correspond to nickel. The presence of oxygen in the spectrum indicates some oxidation of nickel when the composite is stored in air. Additionally, the complete WAXS pattern of the nanocomposite (Figure 4.13) confirms the presence of nickel, given the observed diffractions at higher q values corresponding to the (110) and (111) Ni crystal planes.
Nanocomposites from PS-b-PVDF-b-PS precursors
Figure 4.12 EDX spectrum of PVDF/Ni nanocomposite, obtained after nickel plating of the nanoporous template.
Figure 4.13 Complete WAXS pattern of PVDF/Ni nanocomposite, obtained after nickel plating of the nanoporous template.
Chapter 4
4.4 Conclusion
PS-b-PVDF-b-PS block copolymers have been used as precursors for the fabrication of nanoporous PVDF and PVDF/Ni nanohybrids. First, the triblock copolymers were successfully synthesized via a two-step synthesis method, involving functional benzoyl peroxide initiated polymerization of VDF, followed by ATRP of styrene from the resulting macroinitiator. Kinetic analysis demonstrated the controlled behavior of the radical polymerization.
The morphology of the semicrystalline block copolymers has been investigated, and revealed an alternating crystalline-amorphous lamellar nanostructure inside a spherulitic superstructure for a range of block copolymer compositions (fPS = 0.29-0.58), confirming the dominant role of crystallization during structure formation.
In addition, the β-polymorph has been detected within the block copolymer crystal structure, supposedly due to the PS domains that stimulate the nucleation of the all-trans chain conformation of PVDF.
The amorphous PS block has been removed selectively by applying a facile chemical etching method with fuming nitric acid, leading to a nanoporous PVDF matrix. The use of semicrystalline block copolymers as precursors for these materials will enable us to tune the porosity by altering the copolymer composition. Subsequently, a PVDF/Ni nanocomposite has been successfully prepared through electroless nickel plating. The lamellar nanostructure and β-crystalline phase, both originating from the block copolymer phase separation, are conserved within the porous template and nanohybrid. Considering the ferroelectric properties of PVDF and the ferromagnetic behavior of nickel, both the nanoporous PVDF network and the PVDF/Ni nanocomposite are promising materials for nanotechnological applications.
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