Transactions of the Materials Research Society of Japan 35[2] 303-306 (2010)

Lamella and Fine Crystallite Arrangement in “Crystalline”, Flexible, Heat-Resistant, and Transparent Films of Fluorinated Copolymers

Kazuya Numakura and Atsuhiro Fujimori* Graduate School of Science and Engineering, Yamagata University, Yonazawa, Yamagata, 992-8510, Japan Fax: +81-238-26-3073, e-mail: [email protected]

In recent times, a “crystalline” and flexible optical waveguide candidate with excellent heat-resistance and dimensional stability are developed. For the practical use of this crystalline optical film in the near future, an accurate control of the solid-state structure is indispensable because of the necessity of reducing light refraction at the crystalline/amorphous interface. In the present study, changes in the fine structure and lamella arrangement upon drawing poly[tetrafluoroethylene-co-(perfluoroethylvinylether)] (abbrev. EFA) transparent crystalline films are investigated by using wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) methods. The EFA is crystallized as a lamella crystal in the films and forms a thicker lamella. Upon the drawing of the EFA films, four-point SAXS diagrams develop in the photograph at through direction to the film, which implied that a particular type of layer structure, an alternately tilted lamella arrangement known as the herringbone, is formed. From the result of WAXD and SAXS measurements at edge direction to the film, it is found that formation of isotropic disordered lamella arrangement. Therefore, it is indicated that three dimensional lamella arrangement in this fluorinated transparent film forms uniaxially cylindrical symmetry.

Key words: Fluorinated copolymer, Lamella Arrangement, Optical Waveguide, Wide-angle X-ray diffraction, Small-angle X-ray scattering

1. INTRODUCTION (PTFE; –(CF2-CF2)n–) and its copolymers easily form General “polymer crystals” essentially have both rigid helices in order to yield extended-chain crystals. crystalline and amorphous regions. It is well-known It seems difficult for PTFE to form a lamellae structure that crystalline polymers construct hierarchical because of its rigid molecular chain.2 In addition, since structures ranging from crystal structure and crystallite tetrafluoroethylene copolymers obtained by the arrangement on the angstrom scale to spherulite on the incorporation of several comonomers exhibit extremely mesoscopic scale via lamella on the nanometer scale.1 fast crystallization rates,3 their spherulites generally These crystals in these crystalline polymers are cannot be observed until they are sufficiently large. generally formed by the folding of the main-chain. In Therefore, PTFE exhibits a high degree of crystallinity many cases, since these folded parts and interspherulite of over 90%.4 chains form the amorphous region, crystalline polymers Poly[tetrafluoroethylene-co-(perfluoroalkylvinyl are essentially intermingled states of the crystalline and ether)] (abbrev. EFA (alkyl = ethyl))5 has a the amorphous regions. Therefore, crystalline unique role in the plastics industry due to its polymers are not a suitable candidate for use in several inertness, heat-resistance, and low coefficient of light conductive materials such as film-type optical friction in a wide temperature range. Generally, waveguides (FOWs) and plastic optical fibers (POFs), fluorinated compounds and fluoropolymers have because of the occurrence of remarkable light refraction excellent chemical resistance, oil resistance, and at the crystalline/amorphous interface. Consequently, oil- and water-shedding resistance.6 They have amorphous FOWs lack heat-resistance and dimensional been used as rubbers at high temperatures and in stability. several lubricating fluorine manufactured However, if the construction of extremely products. homogeneous crystalline FOWs made by low-priced However, in the field of fundamental science, plastics is realized, “crystalline” FOWs with excellent structural studies on fluorinated polymers have heat-resistance and dimensional stability can be progressed slowly since the time these polymers developed. The heat-resistant FOWs will efficiently were first reported by Bunn and Howells in demonstrate their optical ability in a circuit exposed to a 1954.7 We could find very few reports on the high temperature; so far there have been no products of systematic structural studies on PTFE or heat-resistant FOWs that can sustain temperatures. If tetrafluoroethylene-based fluorinated copolymer the heat-resistant FOWs are realized, light wiring will because this compound is difficult to synthesize also be spread under the several high temperature due to the emission of poisonous gases.2 environment; the heat-resistant FOWs will not only Further, although EFA is a crystalline polymer, connect the AV equipment or personal computers but processed EFA samples with a high crystallinity also connect the control system around a terminal unit of are as transparent as amorphous polymers such as mobile phone. This future technology is based mainly polymethymethacrylate (PMMA)8 and on “crystalline fluorinated polymers” having a high poly(acrylic acid) (PAA), as shown Fig. 1. This crystallinity. Generally, polytetrafluoroethylene experimental fact is not well-known. Probably,

303 Lamella and Fine Crystallite Arrangement in “Crystalline”, Flexible, 304 Heat-Resistant, and Transparent Films of Fluorinated Copolymers

–(CF2-CF(OCF2CF3))n–. The amount of comonomers of these materials is about a few wt%. The molecular weight of the EFA processed to a crystalline film form is about 600,000. This molecular weight is examined by a computer simulation on the basis of the Figure 1 Photograph of crystalline, transparent, viscoelasticity of the film in a molten state and flexible film made by fluorinated because it is difficult to dissolve these polymers copolymer, and their SAXS and in an organic solvent. WAXD patterns. 2.1.2 Drawing of EFA films EFA transparent films are produced by the hot since the transparency of organic materials press method using a uniaxial press (Toyoseiki, depends on the existence of differences in Co. Ltd, Mini-Test Press-10) at 330 °С under 15 electron density between the crystalline and the MPa pressure. These EFA transparent films are amorphous regions, it is considered that a high drawn uniaxially by using a hand-drawing crystallinity of EFA relates closely to the ° occurrence of transparency. It is obvious that apparatus in an air oven at 200 C. The surface the enhancement of these unique properties of the of the film specimen is marked at intervals of 2 processed EFA films is a result of the changes in mm in order to measure the draw ratios. The the crystal structure and crystalline morphology drawing speed is fixed at 20 mm/min, and, in of EFA films that take place during the drawing order to estimate the fixed annealing effect, the ° process. Further, fluorinated polymers do not film was fixed and annealed at 280 C for 60 min absorb infrared light because of a lack of C-H after drawing. Using these methods, we bonds.9 Hence, a “crystalline” FOWs made by obtained crystalline flexible films with excellent fluorinated polymers transports not only visible transparency (Fig. 1). light but also infrared light. 2.2 Experimental methods Recently, we proposed heat-resistant 2. 2. 1 Small-angle X-ray scattering (SAXS) “crystalline” fluorinated POFs candidates The crystalline morphology of the drawn EFA constructed by uniaxial drawing, and their crystal copolymers is characterized with a SAXS structure in this fiber was investigated in detail.10 instrument (M18XHF, MAC Science Co.) In the view of industrial study, if heat-resistant consisting of an 18-kW rotating-anode X-ray generator with a Cu target (wavelength, λ = 0.154 “crystalline” FOWs are also realized, an 11 application range of light conductive materials is nm) operated at 50 kV and 300 mA. This still expanding. On the other hand, in the view instrument comprised a pyrographite of fundamental science, structural estimation of monochromator, pinhole collimation system (φ = film-shaped polymer processing materials by 0.3, 0.3, and 1.1 mm), vacuum chamber for the uniaxial drawing have great possibility to bring scattered beam path, and a two-dimensional out the detailed information related to imaging plate detector (DIP-220). stereo-structure because incident radiation from a 2. 2. 2 Wide-angle X-ray diffraction (WAXD) various direction (ex. edge direction, though In order to obtain the WAXD data for the drawn direction, and so on) to the film sample is films, an R-axis diffractometer (Rigaku Co.) is possible in analytical experiments using X-ray. operated at 45 kV and 200 mA to generate CuKα In this study, the changes in the fine structure and radiation (λ = 0.1542 nm). WAXD photographs lamella arrangement of the two-dimensional of the samples are taken at room temperature by fluorinated films formed by tetrafluoroethylene using a graphite monochromator and a 0.3-mm copolymers upon drawing are investigated by pinhole collimator. Diffraction data are using wide-angle X-ray diffraction (WAXD) and recorded on a cylindrical imaging plate detector small-angle X-ray scattering (SAXS) methods. equipped with an interface to a computer system. We have found very few reports on the studies on The camera length is 127.4 mm, and the exposure the structural changes in fluorinated polymers time is 600 s. upon drawing, whereas there are many reports of studies on hydrogenated polymers. Therefore, 3. RESULTS AND DISCUSSION this study may also be valuable as fundamental Figure 2 shows the SAXS patterns and schematic research in the field of polymer physics. In illustrations of lamella arrangement of the through addition, we have discussed the origin of direction to the films for the drawn and undrawn occurrence of transparency based on a decrease in transparent crystalline EFA films. In the case of difference between density of crystal and undrawn film, a ring-shaped SAXS pattern is observed, amorphous region. which indicated the formation of an isotropic random lamella texture. In the case of PTFE homopolymer, the 2. EXPERIMENTAL SAXS pattern is obscure, and the corresponding profile 2.1 Materials exhibited extremely low intensity because this polymer 2.1.1 Fluorinated Copolymer almost forms an extended chain and not a lamella EFA is a random copolymer obtained from the structure.12 On the contrary, it is found that the copolymerization of tetrafluoroethylene tetrafluoroethylene copolymer forms lamella structures –(CF2-CF2)n– and perfluoroethylvinylether since the undrawn EFA film used in this study exhibited K. Numakura et al. Transactions of the Materials Research Society of Japan 35[2] 303-306 (2010) 305

Figure 3 SAXS and WAXD patterns of drawn EFA film (fixed annealing at 280 °C after drawing at 200 °C) with through, side, and edge direction.

Figure 2 Changes in SAXS patterns and occurred, forming the small kink bands in the lamella. corresponding lamella arrangements In accordance with the changes in lamella, the grain of EFA ”crystalline” transparent boundaries or amorphous parts between two neighboring films with drawing: (a) undrawn (b) lamella are also distributed regularly towards the draw drawing at 200 °C (c) fixed annealing direction, and they thus result in a periodic change in at 280 °C after drawing at 200 °C. density in the direction normal to them, which account

for the four-point diffraction pattern. That is, with an isotropic SAXS pattern and value of long period. In increase in the elongation of the EFA sample, a the case of drawn films at several drawn ratios and particular kind of layer structure, an alternately tilted 200 °C, characteristic changes in SAXS patterns are lamella arrangement known as the herring-bone, is systematically confirmed. An elliptical pattern is formed inside the films. In comparison of Fig. 2(b) shown in photographs of twice drawn sample. system, it is suggested that formation of stacked lamella Generally, these like pattern mean the formation of arrangement and existence of crystalline/amorphous assembling organization of cylindrical symmetric interface become quite clear by fixed annealing at crystallite. An elliptical SAXS pattern is often 280 °C. It is interesting information associated with observed in the middle of drawing process for crystalline structural control at lamella level although clearly polymers. Therefore, it is suggested that lamella existence of crystalline/amorphous interface brings arrangement at drawn ratio (DR)=2 of EFA film is the about decreasing of light conductivity and transparency state on the way of structural transition. In the case of of the FOWs. DR=3, the obscure two-point pattern, which mean It is suggested that volume of amorphous parts is formation of an arrangement of lamella parallel to the decreased by pseudo- crystallization brought about the draw direction, is confirmed in photograph. The SAXS uniaxial drawing. The parallel arrangement to the pattern disappear by further elongation at DR=4. This drawing direction is attained by three times drawing at phenomenon means disappearance of lamella interface 200 °C or fixed annealing at 280 °C after twice drawing formed by crystalline/amorphous boundary. That is to at 200 °C. Further, “herring-bone” arrangement of say, it is supposed either disordering of lamella lamella forms by fixed annealing at 280 °C after three arrangement or extremely reduction of difference times drawing at 200 °C. between electron densities of crystalline and amorphous In order to estimate three-dimensional structural region by increase of density for amorphous part. formation, SAXS and WAXD measurements from the Judging form enhancement of transparency of this EFA several incident direction of piled up crystalline EFA film, there is high possibility of increase of amorphous transparent films are carried out by using annealed density. Reduction of differences in electron densities DR=3 sample. Figure 3 shows SAXS and WAXD at crystalline/amorphous interface directly connects patterns of EFA film at through, side, and edge direction. advancement of light conductivity in FOWs because it At the side-direction, obscure four-point SAXS pattern suppresses conductive loss by light refraction at the with void scattering and WAXD fiber pattern are interface. These SAXS results reflect the important confirmed. WAXD pattern in side-direction rotates on characteristics on functionality of FOWs. 90 degree to the though one according to incident Figure 2(c) shows the fixed annealing effect at 280 °C direction. An appearance of arc-type pattern in after drawing of the EFA film. In the SAXS patterns of equatorial line indicates existence of orientated this system, two- or four-point patterns are observed for crystalline. The void scattering in SAXS pattern means the DR2 or DR3 and 4 film samples, respectively. The the formation micro-defect into lamella. Obscure appearance of the two-point SAXS patterns implied the SAXS pattern correspond to a decrease in difference formation of an arrangement of lamella parallel to the between density of crystal and amorphous regions. It is draw direction. As the film is drawn further, the suggested that density of amorphous region in interlamellar and/or interlamellar slips probably side-direction become high by twisted elongation in this Lamella and Fine Crystallite Arrangement in “Crystalline”, Flexible, 306 Heat-Resistant, and Transparent Films of Fluorinated Copolymers

4. CONCLUSION The changes in fine structure upon drawing transparent crystalline EFA films are investigated by SAXS and WAXD measurements. EFA is crystallized as a lamella crystal in the films although the polytetrafluoroethylene homopolymer itself usually forms extended-chain crystals. In this type of crystalline fluorinated copolymers, we considered the formation of a switchboard-type lamellae model. With an increase in the drawing of the films with fixed annealing at 280 °C, four-point SAXS diagrams develop in the photograph of EFA transparent films, which imply that a particular type of layer structure, the alternately tilted lamella arrangement known as the herring-bone, is formed. From the result of WAXD and SAXS measurements at edge-direction to the film, it is Figure 4 Illustration of stacked lamellar in EFA found that formation of isotropic disordered film fixed annealing at 280 °C after lamella arrangement. Therefore, it is indicated drawing at 200 °C (insert figure, that three-dimensional lamella arrangement in switch board type lamella). this fluorinated transparent film forms uniaxially direction. In the case of edge-direction, SAXS patterns cylindrical symmetry. show only void scattering, and WAXD indicate isotropic Debye ring. This result indicates lack of periodic REFERENCES arrangement of lamella and randomly existed crystallite [1] A. Keller, Philos. Mag., 2, 1171-172 (1957) in this direction. From the results of these [2] J. Burdon, and J. C. Tatlow, Advances in measurements, schematic illustration of Fluorine Chemistry, Vol.1, eds. M. Stacey, J. three-dimensional lamella arrangement is shown in Fig. C. Tatlow, A. G. Sharp, Academic Press, New 4. In this case, according to our previous work10, 12 York, (1960), pp.129–65 “switch-board” type lamella13) is adopted as structural [3] T. Ozawa, Bull. Chem. Soc. Jpn., 57, 639-43 units. From the view of through and side direction, (1984) two-dimensional stacked lamella arrangement forms the [4] C. Marega, A. Marigo, V. Causin, V. “herring-bone” arrangement. This result is almost Kapeliouchko, E. D. Nicoló, A. Sanguineti, similar tendency to the previous reported fiber sample. Macromolecules, 37, 5630-637 (2004) That is to say, in the case of an application of uniaxial [5] J. C. Lee, S. Namura,S. Kondo, A. Abe, drawing, crystalline fluorinated copolymers containing Polymer, 42, 5453-461 (2001) bulky amorphous region in processed materials transit to [6] R. M. Overney, E. Meyer, J. Frommer, D. almost same arrangement whether macroscopic sample Brodbeck, R. Luthi, L. Howald, H. J. shapes are film or fiber. In other words, lamella Güntherodt, M. Fujihira, H. Takano, Y. Gotoh, arrangement and their transition tendency in microscopic Nature, 359 133-35 (1992) scale along the uniaxial direction are independent on the [7] C. W. Burn, E. R. Howells, Nature, 18, macroscopic processed form of materials under the 549-51 (1954) drawing. However, in the case of film form, randomly [8] Y. Koike, Polymer, 32, 1737-745 (1991) isotropic structure is observed from edge direction. [9] T. Ishigure, M. Kano, Y. Koike, J. Lightw. That is to say, lamella in the drawn EFA films formed Technol., 18, 178-84 (2000) uniaxially cylindrical symmetric arrangement.11 In this [10] A. Fujimori, Y. Hayasaka, Macromolecules, case, lamella arrangement in the side-direction is not 41, 7606-615 (2008) layered structure analyzed by two-point SAXS pattern [11] A. Fujimori, K. Numakura, Y. Hayasaka, but “herring-bone” arrangement obtained by four Poly. Eng. Sci., 50, (2010) in press. point-SAXS pattern. That is to say, this-type top down [12] A. Fujimori, M. Hasegawa, T. Masuko, technique related to drawing of polymer film is not Polym. Int., 56, 1281-287 (2007) enough to obtain completely orientated structure of [13] P. J. Flory, J. Am. Chem. Soc, 84, 2857-867 crystalline fluorinated copolymers. In this film, (1962) herring-bone arranged lamella wound into cylindrical rod along the drawing direction, almost same structure (Received December 3, 2009; Accepted April 5, 2010) in both though and side direction is observed from probing area of WAXD and SAXS. In the case of using this type EFA film as FOWs, it supposes that anisotropy of light conductivity direction occur. Along the through and side direction, visible and infrared light will be efficiently conducted, while edge direction will impede the transmission of lights.