Study on the Curie Transition of P(VDF-TrFE) Copolymer

Abstract

A systematic study was carried out to decipher the mechanism of Curie transition of piezoelectric crystals of poly(vinylidene fluoride trifluoroethylene) P(VDF-TrFE) The unique polarity of P(VDF-TrFE) crystalline phase below curie transition temperature is attributed to the lattice packing of all-trans molecular chains which allocates all the substituted fluorine atoms on one side of molecular chains and hydrogen atoms on the other side Therefore a net dipole moment is created across the lateral packing of molecular chains Nevertheless due to the mutual repulsion among fluorene atoms this all-trans conformation is not stable and ready to change above Curie temperature where thermal kinetic energy is sufficient to cause segmental rotation As being illustrated by in-situ recorded X-ray diffraction and thermal analysis the concerned curie transition is deciphered as a two-step process different from conventional one-step solid-solid transitions The lattice expansion and change of molecular conformation are derived to take place first and appears as a progressive process which is followed by the thermal axial rotation of molecular stems within lattice packing This thermal axial rotation of molecular stems is similar to what has been found in plastic crystals Accompanied with this two-step process during heating the occurrence of lamellar bending is inferred for elucidating the decline of stacking regularity of crystalline lamellae which reversibly recover during subsequent cooling However as the crystalline lamellae of P(VDF-TrFE) are confined in between the stacking of crystalline lamellae of PVDF lamellar bending is restricted accordingly As a result a certain fraction of the piezoelectric crystalline lamellae were found to survive through the Curie transition Thus in addition to the suggestion of a two-step process as a new concept for understanding the Curie transition the relationship between the lamellar stacking and transition of molecular packing is unveiled as well in this research

Date of Award	2016 Aug 26
Original language	English
Supervisor	Jr-Jeng Ruan (Supervisor)