Sensitivity of Polyvinylidene Fluoride Films to Mechanical Vibration Modes and Impact After Optimizing Stretching Conditions Anjana Jain, 1 Jayanth S. Kumar, 1 S. Srikanth, 1 V.T. Rathod, 2 D. Roy Mahapatra 2 1 Materials Science Division, National Aerospace Laboratories, Bangalore 560017, India 2 Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560012, India The b-phase of polyvinylidene fluoride (PVDF) is well known for its piezoelectric properties. PVDF films have been developed using solvent cast method. The films thus produced are in a-phase. The a-phase is trans- formed to piezoelectric b-phase when the film is hot- stretched with various different stretching factors at various different temperatures. The films are then char- acterized in terms of their mechanical properties and surface morphological changes during the transforma- tion from a- to b-phases by using X-ray diffraction, dif- ferential scanning calorimeter, Raman spectra, Infrared spectra, tensile testing, and scanning electron micros- copy. The films showed increased crystallinity with stretching at temperature up to 808C. The optimum conditions to achieve b-phase have been discussed in detail. The fabricated PVDF sensors have been tested for free vibration and impact on plate structure, and its response is compared with conventional piezoelectric wafer type sensor. The resonant and antiresonant peaks in the frequency response of PVDF sensor match well with that of lead zirconate titanate wafer sensors. Effective piezoelectric properties and the variations in the frequency response spectra due to free vibration and impact loading conditions are repor- ted. POLYM. ENG. SCI., 53:707–715, 2013. ª 2012 Society of Plastics Engineers INTRODUCTION In aerospace applications, composites offer unique advantage due to their high strength-to-weight ratio, and consequently reducing fuel consumption. However, they are vulnerable to flaws during fabrication. These flaws lead to premature failure of the components. The detec- tion and identification of these flaws become an important issue. The difficulty in taking remedial measures arises due to the fact that failure mechanisms of composites are not as well understood as in case of metals. The damage caused by these flaws can be detected by real-time moni- toring techniques. One of these method uses distributed sensors on the structure while in use. For such applica- tions, the polyvinylidene fluoride (PVDF) sensor offers unique advantages over other sensors as they are flexible and therefore can be formed easily on to the curved surfa- ces. Further, PVDF is chemically inert, tough, creep resistant, and has good stability when exposed to sunlight [1–4]. As a piezoelectric material, it has excellent sensi- tivity over a wide frequency range, and consequently it is a good candidate for transducer applications [5]. The acoustic impedance matching with water and human tis- sues opens up other areas such as underwater detection of acoustic signals and biomedical applications [6–8]. PVDF is a semicrystalline polymer, which exists in four different phases a, b, c, and d [3, 9–14]. The com- monly occurring a-phase is monoclinic and has nonpolar structure. b-phase (orthorhombic) has polar structure and exhibits piezoelectric effect with highest dipole moment among the other piezoelectric phases of PVDF. a-Phase (polymer chains in TGTG conformation) PVDF films are uniaxially stretched to achieve the b-phase, which has an all-trans TTTT conformation, comprising fluorine atoms on one side and hydrogen atoms on the other side of the polymer backbone. Extensive studies have been carried out in the past on the basic properties of b-phase PVDF films, characterization of the sensors made out of these films, and applications of these sensors in different field [1, 3, 5, 8, 15, 16]. Although authors in the past have worked on conversion of a- to b-phase films, and some of them have also mentioned about the stretching conditions [17, 18] but the structural changes, which occur during the stretching process have not been mentioned. The aim of this article is to optimize the stretching conditions to achieve b-phase PVDF and study the changes that occur during a- to b-phase conversion with variation in temper- ature and stretching factor in detail. Films are then characterized for structural, mechanical, surface, tensile, Correspondence to: Anjana Jain; e-mail: janjana@nal.res.in Contract grant sponsors: CSIR, NPMASS. DOI 10.1002/pen.23318 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2012 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2013