Banerjee Handbook of Specialty Fluorinated Polymers

Preparation, Properties, and Applications
1. Auflage 2015
ISBN: 978-0-323-36996-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 340 Seiten

Reihe: Plastics Design Library

Fluoropolymers are used in applications demanding service at enhanced temperature while maintaining their structural integrity and have excellent combination of chemical, physical and mechanical properties. Advancements in materials and processing technology mean that a huge amount of research is currently taking place into new, high performance applications for specialty fluorinated polymers. This book is a complete review of the current research in synthesizing new fluorinated high performance polymers and their application in the field of low dielectric constant materials, membrane based separation (gas and liquid) and proton exchange membranes. Special emphasis is given to the preparation of soluble high performance polymers by incorporating fluorine and different structural elements so as to use these classes of polymers in different membrane based applications, including low dielectric constant materials, gas separation, pervaporation, proton exchange membranes in fuel cells, and more. The coverage of processing properties and commercial aspects - as well as a practical assessment of the advantages and disadvantages of specialty fluoropolymers compared to other materials - enables engineers and product designers to apply the latest scientific developments in this area in a practical setting. - Thorough coverage of modern applications for specialty fluorinated polymers, including membranes and coatings - giving insight into recent research and the future direction of this technology - Brings researchers and engineers up to date with the latest developments in specialty fluoropolymers, to assist in future materials research and part design - Includes detailed assessment of the advantages and shortcomings of specialty fluorinated polymers, for ease of comparison with alternative materials

Autoren/Hrsg.

Banerjee, Susanta

Leseproben

6.2. Fluorosilicone Polymers
Traditional silicones consist of a backbone that contains alternating Si and O atoms joined by single bonds; this configuration is termed a siloxane or silicone. Because silicon is a tetravalent atom, there are two extra positions in which functional groups are subsequently attached. By definition, fluorosilicones must contain a fluorinated component in the silicone backbone. Based on this definition, the simplest fluorosilicone consists of a siloxane backbone with a fluorine atom attached directly to the silicon. However, the reactivity of this combination makes these compounds useful only as intermediates [12]. In addition, the formation of SiF is possible through thermal rearrangement. To overcome this shortcoming, instead of introducing a simple fluorine atom, it is useful to attach a fluorinated methyl group to the silicon atom. To prevent rearrangement, a hydrocarbon spacer with a minimum length of two carbons must be placed between the fluorinated group and the siloxane backbone if a polymer with high thermal stability is desired; this results in the use of a 3,3,3-trifluoropropyl (TFP) group (–CH2CH2CF3). The first fluorosilicone, PTFPMS, synthesized by Pierce et al. [13], contains this requisite structure. It was produced through ring-opening polymerization of a tricyclosiloxane containing one TFP group and one methyl group on each silicon with current ring-opening polymerization methods using anionic initiation as presented in Scheme 6.2 [13,14]. Condensation chemistry has also been examined as an alternative to ring-opening polymerization for PTFPMS [15] (Scheme 6.3). However, this method of synthesis is not typically used for short-chain fluorinated groups because ring-opening polymerization, particularly with the advent of living systems, provides more control over the final molar mass without the issues of reactant purity associated with condensation polymerization. However, using condensation polymerization to form fluorosilicones still has some utility. As mentioned, if the desired pendant group is bulky, a fluorosilicone can be synthesized through the condensation of siloxane oligomers. Most often, the condensed polymers have silanol or chlorosilane termini and the method of condensation is the same as for non-fluorinated siloxane polymers [14,15] (Scheme 6.3). Hydrosilylation has also been used as a linking reaction [16]. The Tg of the PTFPMS ranges from -75 to -65 °C and, unlike PDMS, it does not exhibit low-temperature crystallization at -40 °C. This is because of the inability of the polymeric chain to pack into a crystalline lattice [3]. It can be cross-linkable from various classical processes of the chemistry of silicones such as in the presence of peroxides or from the SiH/Si–CH-CH2 systems [12]. However, the relatively low strength limits the use of fluorosilicones to static applications such as seals for fuel lines [3]. Various approaches have been taken to improve the properties of fluorosilicones. In this connection, Kobayashi et al. [17] synthesized several methyl-3,3,3-trifluoropropylsiloxane (F)-dimethylsiloxane (D) random and block copolymers. The random copolymers were prepared by equilibrium copolymerization starting from a mixture of cyclic F and D siloxanes with potassium silanolate as the catalyst. The F–D block copolymer was prepared by sequential anionic living polymerization of strained cyclotrisiloxanes using a much weaker catalyst, lithium silanolate. The prepared copolymers were soluble in both tetrahydrofuran (THF) and 1,l,2-trichloro-l,2,2-trifluoroethane. The polydispersity indexes (PDIs) of the polymers were recorded as ~1.7–2.0. The differential scanning calorimetry (DSC) data suggested low-temperature flexibility of the F–D copolymers prepared by equilibrium polymerization. The copolymers showed no transition peaks other than Tg. Interestingly, the Tg of the copolymers appeared in a lower-temperature region than that of PTFPMS homopolymer, indicating its better low-temperature flexibility.
Scheme 6.2 Anionic ring-opening polymerization of PTFPMS. R1 = methyl; R2 = trifluoropropyl. Taken from Ref. [14].
Scheme 6.3 Condensation of dihalosilanes to high-molecular-weight linear siloxanes. Taken from Ref. [14]. In a further study, Kobayashi [18] measured the surface tension of fluorosilicones based on the pendant drop method. The polymers used for the study were PTFPMS, poly(3,3,4,4,5,5,6,6,6-nonafluorohexylmethylsiloxane) (PNFHMS), poly(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylmethylsiloxane) (PTDFOMS), and poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,l0-heptadecafluorodecylmethylsiloxane) (PHDFDMS), with PDMS as the standard silicone polymer fluid. As the length of the fluorinated side chain in the polymers increased, a decrease in refractive index and an increase in density were observed. The surface tension of PNFHMS, PTDFOMS, and PHDFDMS was in the range of 18.4–19.1 mN/m. Kobayashi was curious to estimate the service temperature range of these low-surface-tension fluorosilicone polymer fluids. The PTFPMS and PNFHMS polymers had a Tg at a similar temperature range, -71 and -75 °C, respectively. The Tg of PTDFOMS was observed at -58 °C. The PHDFDMS polymer had no Tg but two endotherm peaks were recorded at -0.3 and 12.6 °C, which the author attributed to a melting, crystallization, and remelting pattern. The most common standard polysiloxane, PDMS, had a Tg at -123 °C, as expected. After that, Kobayashi [19] prepared fluoroalkylsilsesquioxane polymers, namely, poly(3,3,3-trifluoropropylsilsesquioxane) (Prf-T) and poly(3,3,4,4,5,5,6,6,6-nonafluoro hexylsilsesquioxane) (Hxf-T), via hydrolysis of trichloro-3,3,3-trifluoropropylsilane (CF3CH2CH2SiCl3) and trichloro-3,3,4,4,5,5,6,6,6-nonafluorohexylsilane (C4F9C2H4SiCl3), respectively. The silanol functional groups present in the silsesquioxane polymers could be used to cross-link the polymers. Contact angle measurements were carried out after the polymers were cross-linked by condensation. Water and methylene diiodide contact angle data were used to determine the solid surface tension of the polymer coatings. The solid surface tensions decreased in the order of PDMS, PTFPMS, and poly[methyl-(3,3,4,4,5,5,6,6,6-nonafluorohexyl)siloxane] (PMNFHS) with an increasing degree of fluorination. The contact angle for Prf-T was 49° and that of Hxf-T was 85°. The surface tension of Prf-T was recorded at 18.9 mN/m and that of Hxf-T at 7.5 mN/m. Fluorosilicone with 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl groups (C8F17–CH2CH2–fluorosilicone) were also synthesized by Kobayashi et al. [20] using condensation polymerization of C8F17CH2CH2–(CH3)Cl2, and its surface properties were reported. The cross-linked films produced from poly(1-,1-,2H,2H-heptadecafluorodecyl)-methylsiloxyne (PHDFDMS) possessed the lowest surface tension value at 7.0 mN/m. In addition, the 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoro-1-undecene was successfully reacted with poly(hydromethylsiloxane) (PHMS) through a hydrosilylation reaction. Furukawa et al. [21] examined a new synthetic method for fluorosilicones with a high content of fluoroalkyl side chains based on the hydrosilylation of fluorinated olefins with PHMS. They introduced perfluoro-octyl groups into silicone polymers in one step without using the cracking process usually used in the conventional method. Employing the same methodology, Furukawa et al. [22] prepareda series of fluorosilicone homopolymers with 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecylmethylsiloxane (–C8F17CH2CH2CH2(CH3)SiO– [HDFUSiO]) and copolymers based on dimethylsiloxane (–[CH3]2SiO–) by the hydrosilylation of 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoro-1-undecene (C8F17CH2CHaCH2) with poly(hydromethylsiloxane)s (Scheme 6.4). The thermal stability of C8F17CH2CH2CH2–FLS was evaluated by thermogravimetric analysis (TGA) under air atmosphere, and the decomposition of fluoroalkyl side chains occurred at about 245 °C. With increasing HDFUSiO content, the refractive index decreased, whereas the dielectric constant increased. The authors theorized that the –CH2CF2– electric dipole was responsible for the increase in the dielectric constant. The liquid surface tension of the FLS containing 10 mol% HDFUSiO was as low as that of the highly fluorinated FLSs. The incorporation of longer C8F17CH2CH2CH2– groups into the polymers had no significant effect on their liquid surface tension, as shown by comparison with that of C4F9CH2CH2–FLSs. The authors thought that the C8F17CH2CH2CH2– groups were not fully packed in the surface with the CF3– groups (with the lowest surface tension) outermost in the liquid state for flexible siloxane backbones. Furukawa’s group further synthesized C8F17CH2CH2CH2–FLSs with trichlorosilylethyl side chains (Cl3SiCH2CH2–; C8F17CH2CH2CH2–Cl3Si–FLSs) to evaluate their surface free energy by coating them onto glass plates....