Poly (vinylidene fluoride) the Great Electroactive Polymer

2022-04-29

By: Asst. Prof. Dr. Raid Khider Salman

PVDF-based gel electrolytes play a major role in manufacturing essential applications such as rechargeable batteries, security tags, LCD displays,  ...etc.

The best employment for this great polymer within gel making come with lithium rechargeable batteries.

Polymer gels can be obtained either by cross-linking agent chemically or by dissolving polymer in a solvent physically. A physical gel is a polymeric network containing solvent. In this criterion, the gel is made either by forming a polymeric network in a solvent in which the chains are relaxed, rubber elasticity theory may be work on interpreting such gel; or by a previously cross-linked polymer subsequently swollen in a solvent (swollen gel), and such gel follows Flory-Rehner equation. Poly (vinylidene fluoride) PVDF is a semi-crystalline fluoropolymer with almost 70% crystalline structure, that can form thermoreversible gels when it dissolved in a suitable solven. Due to its wide range advantages such as high mechanical, thermal and chemical stabilities as well as superior conductive performance, PVDF was chosen as a polymer host for many polymer gels and polymer gel electrolytes.

Poly (vinylidene fluoride) (PVDF) distinctly crystallizes with five crystalline phases, which can be affected by crystallization conditions and processing of the polymer. These include the nonpolar α-phase with TGTG’ conformation, the polar β-phase or all Trans phase TTTT and γ-phase or the semipolar phase with TTTGTTTG’, δ- and ε-phase, and the different crystal phases are associated with varying properties.

Among these crystal polymorphs, the α- and β-phases are the two most important crystalline modifications. The key applications for PVDF are in the piezoelectric, pyroelectric and ferroelectric fields due to the β-phase, which has an all-trans conformation (TTTT) and is the strongest polar phase of all other crystals. In contrast, the PVDF chains in the α crystalline phase with respective polarizations in alternating directions are nonpolar. The all-trans mode (β-form), in which all C–F bonds are normal to the backbone chain and vibrate in one side, gives a highly polar unit cell with dipole moment around 7.0 × 10−30 C m. This is the main reason for the prominent electrical properties of PVDF.

References

- Voice, A.M., et al., Thermoreversible polymer gel electrolytes. Polymer, 1994. 35(16): p. 3363-3372.

- A. M. Voice, G.R.D., I. M. Ward, Structure of poly (vinylidene fluoride) gel electrolytes. Polymer Gels and Networks, 1997. 5: p. 123- 144.

- Southall, J.P., et al., Ionic conductivity and viscosity correlations in liquid electrolytes for incorporation into PVDF gel electrolytes. Solid State Ionics, 1996. 85(1-4): p. 51-60.