Radiation Effects on Polymers - American Chemical Society

Chapter 6

Downloaded by STANFORD UNIV GREEN LIBR on August 10, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch006

Radiation Effects on the Structure and Properties of Poly(vinylidene fluoride) and Its Ferroelectric Copolymers Andrew J. Lovinger AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

Electron- and γ-kradiation of copolymers of vinylidene fluoride with tri- or tetra-fluoroethylene has been shown to induce solid-state ferroelectric-to-paraelectric transitions. Their Curie temperatures decrease with increasing radiation dose, while at the higher doses full amorphization is obtained as a result of cross-linking. Poly(vinylidenefluoride)homopolymer shows only amorphization without an intervening Curie transition. A structural model is presented to explain this behavior, and the thermal, dielectric, and dynamic mechanical properties of the irradiated polymers are discussed.

The radiation chemistry of polymeric materials is a rapidly expandingfieldbecause of the extensive modification of properties which it renders possible. This may be a result of chemical changes (e.g., cross-linking or chain scission) as well as structural effects (e.g., on crystallinity, molecular conformation, or interchain packing). When crystalline polymers are irradiated by X-rays or electrons, they typically undergo cross-linking through formation and subsequent recombination of free radicals, as typified by polyethylene. Rarely (e.g., in polyoxymethylene) chain scission is the dominant effect, leading to progressive reduction in molecular weight and eventual degradation to nonpolymeric materials. These aspects of ionizing radiation on crystalline polymers have been reviewed thoroughly (see, e.g. réf. 1 and 2). Among such crystalline polymers, poly(vinykdene fluoride), -(CH2-CF ) - [abbrev. PVF ], and its copolymers continue to attract high interest because of their exceptional electroactive properties. These include piezo-, pyro-, and ferro-electricity, and are leading to important applications in areas such as electromechanical transducers, infrared-, motion-, and impact detectors, microphones, hydrophones, speakers, and medical devices. Copolymers with 2

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0097-6156/91/0475-0084S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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Structure & Properties ofPVF & Copolymers

trifluoroethylene, - ( C H F - C F 2 ) - [abbrev. F3E], are particularly promising because they have higher crystallinities, piezoelectricities, and electromechanical coupling coefficients. Another reason is that they can be crystallized directly into their polar phases without requiring mechanically induced transformations as does PVF2. The structures, properties, processing, and applications of PVF2 and its copolymers have been summarized recently in a textbook (3) and in a number of reviews (e.g., 4-7). The effects of irradiation on PVF2 and its copolymers have been investigated only recently, and interest in them is growing because of the extraordinary findings obtained. Among these are major structural changes, including ferroelectric-to-paraelectric transformations at room temperature controllable by radiation dose. These effects on structure and properties are summarized in this report. Irradiation of PVF

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Poly(vinylidenefluoride)was found (8) to be quite sensitive to radiation-induced degradation. Typical changes in the electron-diffraction pattern of the electrically active polar β-form of PVF2 are seen in Figure 1 as a function of irradiation dose. Here and in Figures 2 and 3 the actual electron doses were measured in C/m . The absorbed dose is a function of the stopping power of the particular material, which depends upon chemical composition, density, and film thickness. For common carbon-based polymers, 1 C/m is equivalent to ca. 25-40 Mrad (7); for PVF and its copolymers discussed here, Macchi et al (9) found an equivalence of ca. 33-38 Mrad. As is seen in Figure 1, the crystallographic reflections become progressively weaker and more diffuse during irradiation, until their eventual replacement by amorphous haloes. This is indicative of a cross-linking process but occurs at doses much smaller than is typical for polymers (e.g., ca. one-third of that for polyethylene). No substantial differences were found (8) in the rates or doses of this radiation-induced cross-linking among the α, β, and γ polymorphs of PVF2, Remarkable phenomena were observed for this polymer at much lower doses. Wang (10) showed that for γ-ray doses of up to 50 Mrad there was an improvement in thermal stability of polar β - Ρ \ Φ , as well as in its piezoelectric properties. Specifically, a slower piezoelectric decay that rendered thefilmsusable to higher temperatures was associated with the slight formation of cross-links, which was not sufficient to cause amorphization but only to impede the molecular relaxation of the dipole-containing groups. For similar reasons, an enhanced dimensional stability was also observed at high temperatures (10). Pae and co workers (77) have performed a similar study using 175-keV electronsfroma Van de Graaf accelerator at doses up to 50 Mrad, this time on the anti-polar α-phase of PVF . They found that the limited introduction of cross-links leads to a lowering in melting point as expected, but also detected a significant increase in crystallinity. They ascribed the latter to chain scission, which could have increased the mobility of molecular chains, thus leading to further recrystallization (77). 2

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Irradiation of PVF Copolymers 2

The effects of γ - and electron irradiation have also been explored for copolymers of PVF with trifluoroethylene (F3E) and tetrafluoroethylene (F E). For the 2

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RADIATION EFFECTS ON POLYMERS

Figure 1. Changes in the electron-diffraction pattern of the piezoelectric βphase of PVF that had been uniaxially oriented (fiber axis vertical) and subjected to 100 keV electrons at 25 C. (a) initial exposure, (b) 9 C/m , (c) 20 C/m , (d) 25 C/m , (e) 32 C/m , and (f) 38 C/m . Reproduced from ref. 8. Copyright 1985 American Chemical Society. 2

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In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991. 2

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Figure 2. Effects of electron irradiation on the selected-area electrondiffraction pattern from a single crystal of a 73/27 mol % V F / F E copolymer, grown at 135C and examined at 25°C at 100 keV. (a) initial exposure, (b) after 3 C/m , (c) 4 C/m , (d) 6 C/m , (e) 9 C/m , (f) 14 C/m . Reproduced from ref. 8. Copyright 1985 American Chemical Society.




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Figure 3. Consecutive electron-diffraction patterns (recorded at 25 C and 100 keV) from edge-on crystals of a 73/27 VF2 / F 3 E copolymer that had been grown epitaxially on KBr as seen in part d. Reproduced from ref. 8. Copyright 1985 American Chemical Society.


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Structure & Properties of PVF & Copolymers

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copolymers, compositions containing 50-80 mol % VF2 have been studied extensively. These have the advantage that they generally crystallize directly into the ferroelectric β-phase of PVF2, since the trifluoroethylene units promote adoption of the trans conformation (12, 7). Among tetrafluoroethylene copolymers, an 81/19 mol % VF /F4E composition has been studied in some detail, even though others also adopt the ferroelectric β-structure (13,14). Contrary to PVF homopolymer, both of these copolymers were found to undergo major and highly unusual crystallographic transformations upon irradiation. These are summarized below in terms of structure and properties. VF2/F3E

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Structural Transformations in PVF* Copolymers. The discovery of electroninduced structural transformations in VF2 /F3E copolymers was reported in ref. 8. It was shown that single crystals of electron copolymers undergo solid-state transformations during electron irradiation at room temperature in the manner depicted in Figure 2. At extremely low doses (Fig. 2b and c) new reflections appear at a higher interplanar spacing but with the same azimuthal disposition as the original ferroelectric-phase reflections. At a slightly higher dose, the new reflections increase dramatically in both intensity and sharpness, as the original reflections disappear (Fig. 2d). With further irradiation, the new reflections begin to fade, leading to an amorphous, cross-linked structure (Fig. 2e and f). It was concluded (8) that the ferroelectric lattice is replaced by a paraelectric one since the spacings of the new reflections corresponded to those of the paraelectric lattice that had previously been obtained only at high temperatures (12,15). The much higher intensity and reduced arcing of these paraelectric-phase reflections (Fig. 2d) compared to their ferroelectric counterparts (Fig. 2a) is exactly opposite to the usual degradative effects of electron irradiation. The greater perfection of this radiation-induced paraelectric lattice was also demonstrated by dark-field electron microscopy (8). However, the true paraelectric nature of the individual macromolecules could not be ascertained from this evidence, since all the reflections in Fig. 2 were of the hkO type, i.e. wtermolecular. For this reason, the conformational changes accompanying electron irradiation were probed after crystallizing the copolymer lamellae on edge, i.e. with the chains parallel to the substrate. These are seen in Figure 3d, together with the corresponding consecutive changes in the electrondiffraction pattern during irradiation (Fig. 3a-c). Comparison of Figures 3a and 3c shows that the sharp reflectionfromthe intramolecular all-trans repeat (at 2.57 Â) is replaced by a broad and diffuse reflection at 2.30 Â, which had been shown earlier (12,15) to typify the disordered conformation ofthe paraelectric phase; this conformation consists of irregular sequences of TG, TG, and TT groups. In this manner, this unique electron-induced ferroelectric-to-paraelectric transformation at room temperature was established both intra- and inter-molecularly (8). It is also noteworthy that this transformation occurs at extremely low doses, which are ca. 20-25 times smaller than those needed to impart the usual electron-induced changes (i.e., amorphization) to typical polymers (e.g., polyethylene). Similar changes were also observed for other vinylidene-fluoride copolymers using different irradiation sources and different detection techniques. Specifically, Odajima and co-workers (16) utilized a Co source to expose thicker (20-40 μπι)filmsof a 65/35 mol % V F / F E copolymer to γ-rays. As seen in 60

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Figure 4, irradiation at 120 Mrad caused the major intermolecular reflection of the ferroelectric phase (i.e., the combined {200, 110} at ca. 19.50 2Θ) to disappear in favor of the equivalent paraelectric peak (at ca. 18.2), in excellent agreement with the electron-microscopic results from the same copolymer (8). For a lower dose of 60 Mrad (Fig. 4b), only an incipient transformation was found by irradiation at ambient temperature; however, exposure at 120*C (i.e., with the polymer in the paraelectric state) was more effective in inducing this structural conversion. This conclusion was apparently also supported by IR spectroscopy, which was reported (16) to show an increasing absorption of a new band at 1710 cm (attributed (16) to - C F = C H - bonds) with temperature of irradiation. For lower VF2 contents, the behavior is more complex because the roomtemperature interchain peak consists of two components that have been ascribed (17,18) to a coexistence of ferroelectric and paraelectric phases (an orthorhombic packing of the ferroelectric phase has also been proposed (19). As was shown by Odajima, et al. (20) (see Figure 5a), the higher-angle one of these two peaks (consistent with the ferroelectric lattice (17,18)) continuously decreases in intensity as both peaks shift to a lower 2Θ angle (characteristic of the paraelectric lattice) with increasing γ-ray dose. The partial transformation to the paraelectric phase at intermediate doses (e.g., 80 Mrad) could be fully completed by subsequent heating (see Figure 5b). Such transformations have also been documented for VF2 / F 3 E copolymers richer in vinylidenefluoride(e.g. 78 mol %) (820), as well as for VF2 copolymers with ferrofluoroethylene (8). The central question following thesefindingsis, of course, why all of these copolymers undergo this extraordinary radiation-induced polymorphic transformation, while the homopolymer itself does not This may seem surprising particularly in terms of the very strong chemical and structural similarities between the two materials. As seen in Figure 6, the chains have the same (trans) conformation in their ferroelectric phases, as well as the same chain packing. The major difference lies in the expansion of the a and b axes of the unit cell by the presence of the randomly added tri- (or tetra-)fluoroethyleneunits. An explanation was proposed in ref. 8 based upon this lattice expansion (see Figure 7). In P-PVF2 the chains are held at their closest possible contacts along the bdirection through their strong dipolar alignment, litis renders rotation away from the trans conformation energetically unfavorable. On the contrary, the closest contacts between the copolymer chains will be at the randomly located -CHF or - C F groups, providing increased separation to the vinylidenefluorideunits (see Figure 7). This should allow expansion of the effective cross-sectional area of the chains during the early stages of electron irradiation by internal rotations to the G and G conformations (as occurs commonly through introduction of defects or stresses), leading to adoption of the paraelectric packing. This expansion was confirmed by the X-ray studies of Odajima et al. (16), and the above model (8) was supported by the analysis of Daudin and co-workers (21). β

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Properties of Irradiated PVF Copolymers. Thermal, mechanical, and electrical properties of irradiated PVF copolymers have been the subject of very recent, but increasing, interest. Of these, the dielectric properties werefirstinvestigated by Odajima and co-workers (1620) for samples irradiated with Co γ-ray beams or 2 MeV electron beams. Typical results are shown in Figure 8. The unirradiated 2

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6. LOVINGER

Structure & Properties ofPVF

Radiation Effects on Polymers - American Chemical Society

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