Molecular dynamics between amorphous and crystalline phases of e-beam irradiated piezoelectric PVDF thin films employing solid-state NMR spectroscopy

Abstract

Piezoelectric bi-axially and uni-axially stretched PVDF films subjected to e-beam irradiation doses of 5 kGy, 500 kGy and 1 MGy and films not irradiated have been studied. It was found that the piezoelectric responses of both untreated and e-beam irradiated PVDF films under strain strongly depends on their initial manufacturing processing. The initial structural differences were mainly based on crystalline phase composition, notably the α to β polymorph ratio. 19F MAS Solid State NMR measurements performed with fast sample spinning (31.25 kHz) allow us to accurately determine the proportion of crystal and amorphous phases. Considering only the crystalline phases, it was concluded that the uni-axial ((β)-PVDF) film was almost composed of 100% of β-phase while the biaxially-oriented ((α-β)-PVDF) was composed of 81% β-phase and 19% of α-phase. It was also shown that the changes of local dynamic processes of PVDF chains in kHz timescale enable to reflect the degradation of both the crystalline and amorphous phases. Presence of small molecular-weight chemical compounds was evidenced by Carr–Purcell–Meiboom–Gill (CPMG) 1H MAS NMR experiment. The degradation process was in agreement with melting peak decrease registered by DSC. 19F/1H T1ρ relaxation times and analysis of cross-polarization (CP) buildup curves are discussed. Significant scatter of relaxation parameters (19F T1ρ and 1H T1ρ) suggested an important degradation of polymer structure. The 1H T1ρ values for PVDF samples irradiated at 1 MGy mainly imply chain scission mechanisms of degradation rather than cross-linking processes. SS NMR spectroscopy data supported assumption that e-beam irradiation brings flexibility to piezoelectric PVDF films which improved their piezoelectric voltage output generated by deformation up to doses of 500kGy. Above this dose range, degradations were very important and affected severely PVDF films mechanical properties.