Perturbing effects of bulky comonomers on the chain conformation of poly(vinylidene fluoride) Yuning Yang a , Suriyakala Ramalingam a , Guolin Wu a , Shaw Ling Hsu a, * , Lothar W. Kleiner b , Fuh-Wei Tang b , Nadine Ding b , Syed Hossainy b a Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003, United States b Abbott Vascular Systems, Santa Clara, CA 95054, United States Received 3 January 2008; received in revised form 6 February 2008; accepted 8 February 2008 Available online 14 February 2008 Abstract The comonomer effect on the structures of poly(vinylidene fluorideehexafluoropropylene) P(VDFeHFP) copolymers was analyzed by Raman spectroscopy. The HFP content of these copolymers varies from 5% to 15%. Because of steric interactions involving the bulky HFP comonomers, the predominant chain conformation has extensively more gauche conformers in comparison to the neat PVDF. Based on both experimental and simulation studies, specific spectroscopic features in the 400e900 cm 1 region have been identified that are characteristic of irregular chain conformations elucidating the perturbing effect of HFP on the equilibrium chain statistics of PVDF in the amorphous phase. In addition, these spectroscopic features were revealed to be extremely sensitive to the relative placements of the CF 3 units with respect to other fluorine atoms along the chain. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Amorphous; Conformation; Fluorinated 1. Introduction Fluorinated polymers have a broad spectrum of applica- tions. A well known polymer in this family is poly(vinylidene fluoride) (PVDF). This polymer, known for its high piezoelec- tric constant, is used in numerous commercial electromechan- ical devices such as speakers and microphones [1]. When mechanical performance is valued, this polymer also possesses toughness. Although fluorineefluorine interactions are signif- icant, the PVDF chain has the usual rotational isomeric states designated as trans or gauche . The relative distribution of these states associated with the multitude of crystalline forms can differ substantially depending on the processing condi- tions and application of an electric field [2]. PVDF homopolymer can crystallize in at least three distinct crystalline phases consisting of different chain conformations. For the a phase, the chain possesses the regular sequence of tgtg 0 conformation. The all-trans (ttt) conformation is generally referred to as the b phase, and t 3 gt 3 g 0 is known as the g phase [3e5]. These polymorphic structures have clearly been de- scribed in previous studies [2]. Due to its specific chain confor- mation in the crystalline unit cell, the polar b phase has the highest remnant polarization, thus attracting considerable atten- tion in the polymer community [6,7]. The polar b phase can be produced by uniaxial mechanical deformation with the simulta- neous application of a strong electric field [8e10]. The struc- tural rearrangement accomplished under different processing conditions has been discussed extensively. Effects of an electric field [11e14], thermal annealing [15,16] and ultra-quenching of molten PVDF [17] have been discussed previously. It is also known that the copolymers of PVDF with a num- ber of other monomers can alter morphological features and create interesting structures and associated physical properties [3,4,18e20]. The focus of the present study is to elucidate the effects of hexafluoropropylene (HFP). The usual commoners used previously are generally smaller than the CF 2 units thus * Corresponding author. Tel.: þ1 413 577 1125. E-mail address: slhsu@polysci.umass.edu (S.L. Hsu). 0032-3861/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.02.015 Available online at www.sciencedirect.com Polymer 49 (2008) 1926e1933 www.elsevier.com/locate/polymer