**6. Poled PVDF films and asymmetric reflection experiments**

#### **6.1 Background on the PVDF material ((Pallathadka, 2006), (Zhang et al., 2002)) and its preparations**

Polyvinylidene difluoride, or PVDF (molecular formula: -(CH2CF2)n-), is a highly nonreactive, pure thermoplastic and low-melting-point (170C) fluoropolymer. As a specialty plastic material in the fluoropolymer family, it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, and bases. With a glass transition temperature (Tg) of about -35oC, it is typically 50-60% crystalline at room temperature. However, when stretched into thin film, it is known to manifest a large piezoelectric coefficient of about 6-7 pCN-1, about 10 times larger than those of most other polymers. To enable the material with piezoelectric properties, it is mechanically stretched and then poled with externally applied electric field so as to align the majority of molecular chains ((Pallathadka, 2006) , (Zhang et al., 2002)). These polarized PVDFs fall in 3 categories in general, i.e., alpha (TGTG'), beta (TTTT), and gamma (TTTGTTTG') phases, differentiated by how the chain conformations, of trans (T) and gauche (G), are grouped. FTIR (Fourier transform IR) measurements are normally employed for such differentiation purposes ((Pallathadka, 2006) , (Zhang et al., 2002)). With variable electric dipole contents (or, polarization densities) these PVDF films become ferroelectric polymers, exhibiting efficient piezoelectric and pyroelectric properties, making them versatile materials for sensor and battery applications.

In our experiments, PVDF films of Polysciences (of PA, USA) are subjected to non-uniform mechanical and electric polings to generate -PVDF films of distributed dipolar regions. By applying infrared light beams on these poled -PVDF films, the evidences of enhanced asymmetric refraction at varying incident angles as well as its inverse dependence on the incident power are sought for.
