Dramatic Enhancements in Toughness of Polyvinylidene Fluoride Nanocomposites via Nanoclay‐Directed Crystal Structure and Morphology

Abstract

Addition of nanoclays or other nanoparticles into various polymers to produce nanocomposites has been extensively utilized in an attempt to enhance the mechanical, physical, and thermal properties of polymers. While some interesting properties have been demonstrated, the resulting nanocomposites have yet to realize their full potential. Nanoparticles in general, and nanoclays in particular, with their nanometer size, high surface area, and the associated predominance of interfaces in the nanocomposites, can function as structure and morphology directors, for example stabilizing a metastable or conventionally inaccessible polymer phase, or introduce new energy dissipation mechanisms. Thus, what distinguishes nanoparticles from conventional micrometer-size rigid reinforcements is that their role might not be limited to only adding stiffness to the polymer, but also to directing morphology, as well as introducing new energy-dissipation mechanisms leading to enhanced toughness in the nanocomposites. Herein we demonstrate this potential by reporting a remarkable (order of magnitude) increase in toughness with a concurrent increase in stiffness in a poly(vinylidene fluoride) (PVDF) nanocomposite.The kinetics of crystallite growth and the details of crystallite morphology of semicrystalline polymers can be affected by the presence of layered silicates. [1,2] Although some changes in morphology have been described in polymer/nanoparticle hybrids, [3±7] near-total stabilization and control of a crystalline phase, coupled with dramatic enhancements in materials properties, has not yet been reported. PVDF is an important engineering plastic. It is used extensively in the pulp and paper industry due to its resistance to halogens and acids, in nuclear-waste processing for radiation-, and hot-acid applications, and in the chemical processing industry for chemical and high-temperature applications. It is also used in various device applications, due to its unique piezoelectric [8±10] and pyroelectric [11] properties. There are five known crystalline forms or polymorphs of PVDF: a, b, c, d, and e.[12] The a phase is the most common in melt crystallization, and remains the dominant crystalline form versus the b, and c phases. The c phase does not form except at high temperatures and pressures. Earlier reports have shown that the a phase (chain conformationÐtrans-gauche trans-gauche, tgis inactive with respect to piezo-and pyroelectric properties, while the b form (all trans) exhibits the most activity, and is thus the focus for electromechanical and electroacoustic transducer applications. Thus, the b form has great technological utility and there have been numerous attempts to stabilize this phase. For example, the b form of the PVDF has been obtained by careful crystallization from solution, [13] by melt crystallization at high pressure, by application of a strong electric field, [14] by molecular epitaxy, [15] and by preparing a carbon-coated, highly oriented ultrathin film.[16] Earlier reports have indicated the possibility...

Clay nanocomposites based on poly(vinylidene fluoride-co-hexafluoropropylene): Structure and properties

“…Alternatively, a mechanism in analogy to that developed for the epitaxial crystallization of β modification on KBr substrate has been proposed [31]. The mechanism highlights the geometrical similarities between the crystal lattice of the inorganic phase (KBr, clay) and the β phase of PVDF [17,32]. It should be noted that the β modification was also found to be favored in the presence of Ag nanoparticles [33], functionalized multiwall carbon nanotubes [34] and functionalized graphene sheets [35].…”

“…Dramatic enhancements in toughness have been reported for a limited number of clay based nanocomposite systems including PVDF [17] and PVDF based blends [39]. In general, nanofillers homogeneously dispersed in a polymer matrix, in addition to inducing changes in crystallinity (both degree and type of phase) can give rise to a percolated network that has a protective role against the onset of catastrophic cracking; the load transfer efficiency largely depends upon the interfacial nanofiller/polymer shear stress [40].…”

“…However, martensiticlike transformations of the crystalline phase have been observed only in selected polymer-clay combinations. The most well explored examples of this type are the development of γ phase in Nylon 6 at the expense of the α phase in the neat polymer [12,13] and the evolution of β phase instead of the α form in PVDF [14][15][16][17][18][19][20][21][22][23][24]. The latter has a considerable impact in applications where pyroelectric, piezoelectric, ferroelectric or magnetostrictive response is desirable [25], given that the all-trans configuration of β phase imparts a high dipole moment (7.0 10 -30 Cm /repeat unit) due to the alignment of the polar C-F bonds perpendicular to the polymer axis chain.…”

A facile method to enhance ferroelectric properties in PVDF nanocomposites

“…Lately, it has been reported that addition of clay platelets into PVDF polymer matrix induces polar crystalline phase 12,[18][19][20][21][22][23][24] . Priya and Jog [18][19][20] reported the effectiveness of organically modified montmorillonite for the induction of β polymorph formation in PVDF films.…”

“…Various methods have been proposed and implemented [11][12][13][14] to increase the β phase content of PVDF films; however high temperature mechanical stretching and high voltage poling using are proved to be efficient and practical [15][16][17] .…”