Fluorinated Oligomers and Polymers in Photopolymerization

Review pubs.acs.org/CR

Fluorinated Oligomers and Polymers in Photopolymerization Alessandra Vitale,† Roberta Bongiovanni,‡ and Bruno Ameduri*,§ †

Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy § Engineering and Macromolecular Architectures, Institut Charles Gerhardt UMR (CNRS) 5253, Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue Ecole Normale, 34296 Montpellier Cedex 5, France ‡

9.7. Self-Healing Polymers 10. Conclusions and Outlooks Author Information Corresponding Author Notes Biographies Acknowledgments List of Symbols and Abbreviations References

1. INTRODUCTION Fluorinated macromolecules represent a remarkable niche among specialty (co)polymers.1−7 They exhibit a unique combination of exceptional properties, mainly linked to the low polarizability, the strong electronegativity, and the small van der Waals radius (1.32 Å) of the fluorine atom, and to the strong C−F bond (whose bond energy dissociation is worth 485 kJ mol−1). Hence, fluoropolymers that contain a high fluorine amount in the polymeric backbone display high thermal, chemical, aging, and weather resistances, unique inertness to solvents, hydrocarbons, acids, and alkalis, and exhibit low flammability, low refractive index, low dielectric constants, low moisture absorption, and low surface energy (oil and water repellency), especially in the presence of flexible dangling group. Furthermore, the strong C−F bond strongly improves the resistance to oxidation and hydrolytic decompositions. Fluoropolymers can show a wide variety of structures and morphologies (from semi crystalline to totally amorphous), and therefore they give rise to a wide variety of materials, including thermoplastics, elastomers, thermoplastic elastomers, and thermosets. Most fluorochemicals arise from calcium fluoride (also called fluorspar, fluorite, or fluor minerals),8 whose major resources are located in South Africa, China, and Mexico.8 The addition of sulfuric acid to that mineral leads to acid fluoride (Scheme 1) that further enables fluorination of various (chlorinated) intermediates to get fluorocompounds. Despite their high price (related to the cost of the small scale of production, the purification and production of the monomers,

CONTENTS 1. Introduction 2. Photopolymerization Processes 3. Synthesis of Fluorinated Monomers 3.1. Fluoroacrylates 3.1.1. Monofunctional Acrylates 3.1.2. Telechelic Bisacrylates 3.1.3. Fluorinated Polyacrylates 3.2. Fluoroepoxides 3.2.1. Monofunctional Epoxides 3.2.2. Telechelic Bisepoxides 3.3. Fluorinated Vinyl Ethers and ElectronDonor/-Acceptor Couples 3.4. Fluorooxetanes 3.5. Fluorinated Thiol−Ene Systems 4. Fluorinated Photoinitiators 5. Photocuring Processes Based on Multifunctional Fluorinated Monomers 5.1. Radical Photopolymerization 5.2. Cationic Photopolymerization and Hybrid Systems 6. Photocopolymerization of Fluorinated Monofunctional Monomers for the Surface Modification of Polymeric Networks 7. Dual Curing Processes for Obtaining Fluorinated Organic−Inorganic Networks 8. Photografting 9. Applications 9.1. Coatings and Adhesives 9.2. Optical Fibers and Flexible Displays 9.3. Polymer Exchange Membrane for Fuel Cell and Other Energy Devices 9.4. Microfluidic Devices 9.5. Replica Molding and Patterning 9.6. Fluorogels © XXXX American Chemical Society

W W X X X X Y Y Z

A B D D D F H H H I J J K K K L M

M P R R R S

Scheme 1. Preparation of Acid Fluoride from the Reaction between Calcium Fluoride and Sulfuric Acid

T U V W

Received: February 28, 2015

A

DOI: 10.1021/acs.chemrev.5b00120 Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 2. Mechanism of a Photoinduced Polyadditiona

and the unusual processes of polymerization), these polymers are parts of major developments in modern high-tech fields. Important reviews and books on fluoropolymers have been published.1−7 For all of the exceptional properties mentioned above, these specialty macromolecules1−7 find many applications in the following advanced technology areas: automotive industries (ca. 300 g of fluoropolymers per car, fluids for transmission,9 and in components of fuel cells and lithium-ion batteries), aerospace and aeronautics (use of elastomers as seals, gaskets, O-rings for application at extreme temperatures for tanks of liquid hydrogen, or hydrazine in boosters of space shuttles),5,6 petrochemical (pipes and coatings as liners), microelectronics, chemical engineering (high performance membranes),10 textile treatment, building (paints and coatings resistant to UV and graffiti, and stone protection, especially coatings of old monuments for the Cultural Heritage),11−13 and optics (core and cladding of optical fibers). 14 A peculiar class of fluoropolymers can be obtained by photopolymerization processes, which are particularly attractive as they proceed at room temperature, have fast kinetics, and are carried out in bulk without any solvents. This Review reports the state of the art on the synthesis of fluorinated monomers, oligomers, and other fluorinated products suitable for photopolymerization. After the synthesis, this Review describes the main limitations of the photopolymerization of fluorinated monomers (including acrylates, epoxides, vinyl ethers, and oxetanes): a focus is given on telechelic monomers, which ensure a suitable cross-linking, and on the monofunctional monomers, which are mostly employed as comonomers for the surface tailoring of the obtained polymers. Finally, relevant applications of the resulting fluorinated materials obtained by photoinduced processes are illustrated: coatings and adhesives, devices for optics, energy and microfluidics, replica molding materials, fluorogels, and selfhealing (co)polymers.

“■” symbol stands for an unpaired electron or a positive/negative charge.

a

reaction follows and consumes the liquid monomer. Because of the high initiation rate, the polymerization proceeds fast, keeping the propagating chains very short, and bimolecular termination reactions limit the molecular weight of the product. When multifunctional monomers are involved, the products are cross-linked: Scheme 3 represents the curing process of a difunctional monomer, a diacrylate reacting by a radical mechanism and forming a network. Scheme 3. Scheme of the Photoinduced Curing of a Difunctional Acrylic Monomer via a Radical Mechanism Triggered by a Radical Photoinitiator (PI)

2. PHOTOPOLYMERIZATION PROCESSES Photopolymerization is a polymerization reaction in which initiation is triggered by a radiation. According to IUPAC recommendations, the term “photoinduced polymerization” should be employed, while the term “photopolymerization” applies to polymerization processes that require a photon for the propagation step.15 In fact, it is common use to employ in both cases the term “photopolymerization”: this is also the choice we have made in this Review. Because most monomers do not produce any initiating species with sufficiently high yields under light, a photoinitiator that is a photolabile chemical, which generates either radicals or ions under illumination through different reactive pathways, such as a homolytic photoscission, a hydrogen abstraction, a heterolytic cleavage, or a redox reaction, is usually required. The photochemistry of the photoinitiators is comprehensively described in Crivello and Dietliker’s book.16 Moreover, radical fluorinated photoinitiators are also reported in the literature.17 The quantum yield of the photoinitiation reaction is generally high; the concentration of radicals or ions can thus be significant and guarantee a high initiation rate. In most industrial processes, the radiation is provided by UV light as common commercially available photoinitiators respond to this wavelength range. However, other including visible light can be employed, if the photoinitiating system is selected properly. As shown in Scheme 2, most photoinduced polymerizations are polyadditions: once radicals or ions are formed, a chain

The curing process, displayed in Scheme 3, is the basis of the UV-curing technology, where liquid formulations containing monomers, photoinitiators, and eventually additives are transformed into a solid polymer, in either a glassy or a rubbery state, under a UV light. Originally, the process was limited to the field of coatings, inks, and adhesives because of the limited penetration of light.18,19 Nowadays many other applications can be found, as discussed below. Photopolymerization and UV-curing processes are advantageous when compared to thermally induced polymerization for the following reasons:20 (i) the polymer formation is fast (no more than a few minutes, as compared to hours requested by thermal processes); (ii) the process is in bulk and therefore is solvent free; (iii) it is carried out at room temperature; and (iv) the initiation permits spatial resolution, as it mainly occurs in the illuminated areas. Moreover, the properties of the final polymer can be tailored through a wide range of controlling parameters, for example, photoinitiator type, monomer structure, number B


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and type of reactive functional groups, irradiation rate, and atmosphere in which the polymerization is conducted. UV-curing processes (and photopolymerization in general) are mainly based on free radical polymerization of acrylates and methacrylates.21 The length and chemical structure of the crosslink segments (i.e., R bridges in Scheme 3) enable one to reach well-designed properties. When the cross-linking density is high and R is fluorinated, the photocured polymers can exhibit a high resistance to chemicals, heat, and radiation as it will be described below. The major limits of radical processes are the high shrinkage during the liquid−solid phase change, the irritation effect of the most popular acrylic systems, and the sensitivity to oxygen, which induces an inhibition effect as in thermal processes involving radical species.22 Further information on oxygen inhibition will be discussed in section 4. Other UV-reactive systems are unsaturated polyesters (usually maleic or fumaric structures located along the polyester backbone) that copolymerize by direct addition with vinyl monomers such as styrene, vinyl esters, vinyl carbamates, carbonates, and thiol−enes systems. Allyl ethers are used as well, but their reactivity is low and they are preferably employed in copolymerization. Thiol− enes are proposed as photoinitiator-free systems: even in the absence of a photoinitiator, they can in fact exhibit a good reactivity.23 Light-induced cationic polymerization is becoming more popular and displays advantages such as no oxygen inhibition and lower shrinkage. Moreover, it permits one to use monomers that are inactive toward radical species, in particular vinyl ethers24−29 and heterocyclic monomers such as epoxides (or oxiranes), lactones, and cyclic ethers.30 Unfortunately, only a few of them are commercially available. Cationic photoinitiators are mainly onium salts that form strong acids, HMtXn, upon irradiation. Such photoinitiation reactions were comprehensively described by Fouassier and Lalevée.31 The ability of a given acid to protonate a specific monomer is directly related to its inherent acidity as well as the basicity of the monomer. If the monomer basicity is poor, strong acids are required to shift the equilibrium significantly toward the chain growth propagation, as represented by Scheme 4 for the linear photopolymerization of an epoxide (regarded as a weak base).

Scheme 5. Activated Monomer Mechanism in the RingOpening Polymerization of Oxiranes (or Epoxides) by Alcohols

Interestingly, with a cationic mechanism, the polymerization has a “living” character and self-sustains after the light has been turned off.34 The oxonium species are essentially nonterminating, and the active centers stay alive for hours or even days. This means that a postcure effect occurs permitting to achieve higher conversion and enhanced material properties, and may induce reactions in the nonirradiated areas (the so-called “shadow curing”). Termination can happen either by chain transfers producing inactive species or by reaction with nucleophilic impurities, such as water. In the cationic UV-curing field, only cycloaliphatic epoxides have reached commercial significance: the monomers containing epoxy-cyclohexane rings are in fact widely employed, as they present the fastest polymerization rate (the high level of ring strain favors the opening in the presence of an acid), although by 1 order of magnitude lower than that of diacrylates. Besides epoxides, suitable monomers for cationic photoinduced polymerizations are vinyl ethers, propenyl ethers, and oxetanes, the fluorinated homologues of which have also been synthesized and employed in UV-curing. Reports on photoinduced anionic polymerizations are rare.35 Although carbamates, O-acyloximes, and ammonium salts have been proposed as photochemically generated bases since the 1990s, quite a few photobase-catalyzed polymerizations have been described so far.36 Early examples concern the anionic photopolymerization of alkyl cyanoacrylate through the nucleophilic addition of a Lewis photobase onto the carbon− carbon double bond of the monomer.37 Very often, the photolatent base relies on photolabile blocking groups, producing an amine upon UV irradiation. The amine can act as a true photocatalyst as reported, for example, in the anionic polymerization of oxirane monomers38 or in condensation reactions, for example, acetonate/acrylate.39 Alternatively, the amine can be involved in the reaction being incorporated into the polymer network, as it occurs in the ring-opening addition to the pending epoxide groups of a functional acrylate oligomer where a thermal reaction follows.40 So far, to the best of our knowledge, there are no reports on anionic photopolymerization involving fluorinated reactants. By photoinduced processes, one can also covalently link polymeric chains to a solid substrate: this is referred to as photografting. This technique is usually used for the surface modification of materials and is particularly relevant for the modification of polymers and plastics that are thermally sensitive. Photografting can lead to specific surface properties such as adhesion, biocompatibility, etc., without changing the materials’ bulk properties. Another application deals with the sizing of inorganic particles and filler to be introduced into composites (as will be shortly discussed in section 7): in this context, photoprocesses are competitors of thermal processes because they are more efficient as discussed above. Photografting is usually a radical UV process, because photochemical reactions

Scheme 4. Scheme of the Photoinduced Curing of a Monofunctional Epoxidea

a

HMtXn is the strong acid produced by the photoinitiator, when irradiated.

In addition to the paths above, the so-called activated monomer mechanism (Scheme 5) preferably takes place in the presence of alcohols.32 Chain transfer reactions have been extensively reported in the literature, and the effect of alcohols on the polymerization process has been comprehensively discussed.33 The use of fluorinated alcohols has also been proposed, as described below. C


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Scheme 6. Synthetic Industrial Strategies of Fluorinated (Meth)acrylates Based on Tetrafluoroethylene Telomersa

a

Where RF stands for CnF2n+1 with n = 4,6,8,10,..., while Q represents (CH2)xCH3, with x = 1−7, or a functional group such as OH, CO2R (R = H or alkyl), etc...

Scheme 7. Synthesis of Fluorinated Acrylate That Contains a C6 Spacer51

classes of monomers are treated in separate subsections (except those used in copolymerization such as fluorinated allyl ethers, which are mentioned with the fluoroacrylics, and fluorinated maleimides, which are shortly mentioned with vinyl ethers), eventually dividing the descriptions according to the monomer functionality. Photopolymerization of commercially available fluoroalkenes (e.g., tetrafluoroethylene, TFE,4 vinylidene fluoride, VDF,4,6 chlorotrifluoroethylene, CTFE)4,7 are not mentioned here because most studies have already been summarized in reviews 4,6,7 describing the telomerization or (co)polymerization of such fluoroolefins, or more recent modern photoinitiated controlled radical polymerization by iodine transfer polymerization of VDF in the presence of 1iodoperfluorohexane45−47 or a hypervalent iodide carboxylate.48

most frequently involve radical species obtained by UV light. Up to now, radical grafting methods have exclusively been employed when fluorinated modifiers are used. The curing of preformed polymers such as natural rubbers can also be carried out by UV-induced processes: this chemistry is not treated in this Review as it does not involve fluorinated species, but is described in Decker’s works.41,42

3. SYNTHESIS OF FLUORINATED MONOMERS Photopolymerization processes are often used in industrial applications (e.g., coatings, adhesives, and inks) because of their solvent-free formulations and thus their outstanding properties such as environment friendly, energy saving, efficiency, and rapidity to find out various advantages over conventional thermal processes. The introduction of fluorinated monomers and oligomers in photopolymerizable/photocurable systems can allow the preparation of new polymers with unique properties coming from fluorine atoms.43,44 Such materials can show chemical stability, weathering resistance, hydrophobicity, oleophobicity, anticorrosion, antigraffiti, and antifouling characteristics, as well as meet the requirements of high-performance coatings in advanced technological areas such as aerospace and marine. Moreover, with the recent success of photopolymerization processes in new areas such as electrolytic membranes, optical devices, patterning, and 3D-printing, the need for new photoreactive monomers is increasing. This section reports the state of the art on the synthesis of photopolymerizable fluorinated monomers and oligomers: most

3.1. Fluoroacrylates

3.1.1. Monofunctional Acrylates. The synthesis of fluorinated acrylates or methacrylates is still carried out industrially from the (meth)acrylation of fluorinated alcohol precursors obtained after chemical modification of TFE telomers49 (Scheme 6). This reaction occurs by esterification of fluorinated alcohols with either (meth)acrylic acid, (meth)acrylic anhydride, or (meth)acryloyl chloride. [IF] is generated in situ and reacts with TFE to obtain C2F5I. The radical telomerization of TFE with C2F5I, well detailed in a book,4 leads to telomeric C2F5(C2F4)nI distributions that are subsequently end-capped with ethylene and then hydrolyzed to yield C2F5(C2F4)nC2H4OH with an even number of perfluorinated carbon atoms.4 According to the TFE chain length, macroD


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Scheme 8. Synthesis of Fluorinated Acrylate Containing a VDF Unit51

groups alternating to oxygen atoms, that is, −CF2O−, −CF2CF2O−, −CF2CF(CF3)O−, and −CF2CF2CF2CF2O−. These structures do not contain long chain perfluorinated carboxylic acids, and therefore they are considered safe and not bioaccumulative,71−73 and characterized by high inertness and quite low Tg’s (as low as −100 °C). They have found various applications such as elastomers, lubricants, pump fluids, and heat transfer fluids under severe conditions.74 They can be prepared according to three main target strategies: (i) the anionic polymerization of perfluoroepoxides (especially hexafluoropropylene oxide, HFPO) led to a PFPE under the Krytox trade name (marketed by the DuPont de Nemours Company),74,75 and the ring-opening polymerization of HFPO yielded Aflunox product also produced by Unimatec (and formerly by NOK Corp. or Nippon Mektron Ltd.); (ii) the photooxidation of perfluoroalkenes (such as tetrafluoroethylene,76 hexafluoropropylene (HFP),77 and perfluorobutadiene)78 achieved by Solvay Specialty Polymers (formerly Solvay Solexis and ex-Ausimont) Co., giving functional or nonfunctional (or neutral) Fomblin oligomers whose molecular weights range between 1000 and 4000 g mol−1;79,80 and (iii) the Daikin Co. markets Demnum oligomers, inert or functional PFPEs, from the ring-opening polymerization of their commercially available fluorinated oxetanes followed by a fluorination to yield thermostable oils.81 The PFPE chains produced by Solvay Specialty Polymers have been used to obtain monofunctional monomers. In partnership with this company, Bongiovanni et al.82 reported the synthesis of a series of monomethacrylates having the following chemical structure:

monomers are thus obtained, and the chain length has a dramatic influence on the surface properties reported for the first time by Shafrin and Zisman.50 Actually, 2,2,2-trifluoroethyl (meth)acrylates, regarded as the smallest fluorinated chain length, are also produced by several companies. Among the fluoroacrylates proposed in the literature and produced at industrial scale, most have a CnF2n+1−R−OCO− CHCH2 chemical structure (where R represents a polymethylene group, mainly an ethylene unit). There are also some monomers where R is a longer linear alkyl group: the synthesis of acrylate that bears a hexyl spacer between the ester function and the n-C4F9 end-group is summarized in Scheme 7.51 The hydrogenated chain can also include a thioether group.52 Perfluoroalkyl phosphate acrylates monomers have also been reported.53,54 However, the CnF2n+1 (n > 7) chains are too stable, bioaccumulable, toxic, and persistent.55 In particular, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) have been serious concerns since the 1990s, and a list of chemicals that can degrade to fluorinated carboxylic acids is reported in ref 56. The longer the perfluorinated chain, the greater the persistence in the ground or in water. By binding to proteins, they bioaccumulate in organisms (fluorochemicals are widespread globally occurring in wildlife and in human blood); therefore perfluoroalkyl chains longer than C6F13 are banned in several countries.57−64 PFOS was listed in Annex B of the Stockholm convention as a persistent organic pollutant in 2009.65 Surfactants bearing shorter perfluorinated chain lengths (e.g., C4F966 or (CF3)2CF)67 have also been reported. On the other hand, polymeric and oligomeric fluorochemicals are considered safer by global regulatory agencies: intake via food happens only for molecules having sizes able to pass through the human intestinal barrier, corresponding to a molecular weight up to 3600 g/mol in the presence of a high content of fluorine.68 Thus, new alternatives to fluoroalkyl acrylates are desirable as those having a higher MW and based on short perfluorinated alkyl unit such as vinylidene fluoride (VDF) telomers. Hence, macromonomers based on these fluorinated alkene telomers have been prepared: one patent69 reports the synthesis of H2C CHCO2CH2CH2(VDF)nC2F5, while in 2004 the preparation of fluorinated acrylates H2CCHCO2(CH2)3(VDF)nCF3 based on VDF telomers was published.70 Huang et al.51 revisited these above macromonomers and synthesized monoacrylates containing one or two VDF unit(s): first, the addition of RFCH2CF2I onto ethylene followed by hydrolysis gave CF3(CF2)5CH2CF2CH2CH2OH; these alcohols were subsequently modified into nC6F13CH2CF2CH2CH2OCOCHCH2 and nC4F9CH2CF2CH2CF2CH2CH2OCOCHCH2. The synthesis of the former one is displayed in Scheme 8. Another fluorinated building block alternative to the fluoroalkylic one is the perfluoropolyether (PFPE) chain. The structural base units are short C2 and C3 fluorinated alkoxy

where X designates Cl, H, or F, while n varies from 2 to 5. These monomethacrylates monomers were synthesized by reaction of the corresponding X-terminated alcohol separated from a crude product mixture, made of alcohol, diol, and nonfunctional molecules obtained by the photooxidation process of perfluoroolefins.79 The reaction was carried out in fluorinated solvents, in the presence of a catalytic amount of dibutyltindilaurate, by condensation of the hydroxyl group with an isocyanate bringing a methacrylic end group, 2-isocyanatoethyl methacrylate (IEM). The chlorine-terminated precursors (i.e., X = Cl) were reported by Tonelli et al.:83 the photo-oxidation of HFP in the presence of chlorotrifluoroethylene (CTFE) gives an “addition− fragmentation transfer” process producing a series of monofunctional polyether macromonomers having the following general formula:

where n predominantly varies between 1 and 5. The mixture of acyl fluorides is usually converted into ethyl esters by reaction E


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Scheme 9. Synthesis of Aliphatic Fluorinated Telechelic Diacrylates from Telechelic Diols

Scheme 10. Acrylation of Telechelic Diols Leading to Aliphatic Fluorinated Telechelic Diacrylatesa

a

Where HQ stands for hydroquinone.

transfer copolymerization (ITP)94 of vinylidene fluoride (VDF) with perfluoromethyl vinyl ether (PMVE) in the presence of 1,4diiodoperfluorobutane).93 α,ω-Diiodopoly(VDF-co-PMVE) cooligomers were synthesized from various initial [1,4diiodoperfluorobutane]o/([VDF]o+[PMVE]o) molar ratios. Various functionalizations then were achieved in good yield (>60% yield) to obtain telechelic diols, and then diacrylates95 containing a central poly(VDF-co-PMVE) oligomeric chain of different molecular weights bearing various length spacers (C2 and C3) between the ester functions and the poly(VDF-coPMVE) chain (Scheme 11). As mentioned above, another strategy of (meth)acrylation was successfully achieved by Bongiovanni et al.84,96,97 and also by DeSimone’s team98 from the condensation of 2-isocyanatoethyl methacrylate, IEM, with telechelic bis(hydroxy)perfluoropolyethers (Fomblin Z-Dol, Scheme 12) as precursors of original photo-cross-linkable networks. Different synthetic methods for preparing PFPE derivatives suitable to obtain polyfunctional oligomers are reported by Turri et al.76 More recently, various PFPE urethane acrylates were synthesized. The design of UV-curable difunctional oligomers having the general structure G−Rh−PFPE−Rh−G was also reported99 (where G and Rh stand for the acrylic photoreactive group and a hydrogenated spacer, respectively). The spacer enables one to introduce polarity in the structure improving compatibility with other resins and mechanical properties, although maintaining both viscosity and refractive index at the lowest. To extend HOCH2CF2O(CF2O)q(CF2CF2O)pCF2CH2OH PFPE diol bearing a Rh block, the following diisocyanates were used: trimethyl hexamethylene diisocyanate (TMDI) as a 1:1 mixture of 2,2,4-TMDI and 2,4,4-TMDI isomers, and isophorone diisocyanate (IPDI).99 The synthesis of the PFPE acrylates involved then the blocking of NCO groups of the prepolymers with the acrylic functionality by using hydroxyethyl acrylate. The degree of oligomerization at the end of the prepolymerization reaction (i.e., the condensation of the diol with the diisocyanate) was monitored by gas chromatography, analyzing the free NCO function: its value was 1.41 (with TMDI) and 1.23 (with IPDI). The same study99 also reported the synthesis of a highly fluorinated UV-curable oligomer from the direct reaction of PFPE diol with an excess of methacrylic anhydride. Unfortunately, the reactivity of the dimethacrylic oligomer was found very low, due to the poor solubility of the photoinitiators. Similar chemistries were used to functionalize polyethers obtained by polymerization of fluorinated oxetanes (see section

with ethanol, and the esters are then fractionated through distillation. Each macromonomer with a selected n value is reduced by sodium boron hydride to the corresponding alcohol. The equivalent H-terminated series can be obtained with the same synthetic procedure: the homologue chlorine-terminated alcohol is subjected to extensive UV irradiation in the presence of 2-propanol that acts as a hydrogen donor. IEM functionalization of CF 3 −CF 2 O−(CF 2 O) n − (CF2CF2O)m−CF2−CH2OH alcohol has also been described.84 The mixture was first subjected to a chromatographic separation by using a column filled with silica gel; the monoalcohol fraction was then reacted with IEM. Monoacrylates bearing polyether structures with fluorinated side chains are also available. The polyether structure is obtained by polymerizing fluorinated oxetane monomers, which are described in section 3.4. They have at least one pendant −CH2− O−(CH2)n−Rf group, where Rf is a short fluorinated chain. After ring-opening polymerization of the oxetane bearing the fluorinated side chain, this chain will be repeated many times along the main polyether backbone. Typically, an isocyanate or epoxy functionalized polyfluorooxetane is first formed and then reacted with a hydroxy alkyl acrylate, or methacrylate, as described in a patent by Omnova.85 3.1.2. Telechelic Bisacrylates. Two kinds of fluorinated telechelic diacrylates have been synthesized, aliphatic and aromatic, both described below. Various strategies of aliphatic fluorinated telechelic diacrylates have been proposed to enable a further cross-linking. In most cases, diols are interesting precursors (Scheme 9) as they are also involved (i) in the synthesis of various well-defined polymers86−88 (e.g., polyurethanes, polyesters, polyethers, polyamides, polyimides, epoxy resins, etc.) and (ii) as useful intermediates to obtain diepoxides, di(meth)acrylates, or dicyanates.4,89−91 When (meth)acryloyl chloride is used, the esterification requires a HCl scavenger such as triethyl amine (Scheme 9) or poly(vinylpyrrolidone) (Scheme 10).92 The same recipe can be applied to obtain telechelic fluorinated bis(methacrylate)s as the following:

Longer diols of molecular weights ranging from 1500 to 20 000 g mol−1 with a narrow polydispersity ( 100 °C). The refractive indices at 20 °C were in the range 1.416−1.418.238 Furthermore, the Alcatel Co. developed novel photopolymerized networks for core and claddings of optical fibers from fluorinated telechelic diacrylates.239 In addition, polyacrylates based on CTFE and vinylene carbonate were photopolymerized to yield novel cores or claddings for optical fibers (Schemes 13 and 14).111,112 Shashkova et al.240 developed methods to synthesize hydrocarbon and fluorine-containing monomers and oligomers for the creation of planar polymer waveguides. These authors synthesized more than 20 mono- and bifunctional methacrylates of hydrocarbon and fluorinated alcohols, glycols, and diols, and characterized their physicochemical properties. The resulting polymers, endowed with low absorption in telecommunication spectral regions (i.e., near 800 and 1500 nm) and a wide refractive index (1.34−1.557 range), were obtained by photopolymerization. The authors reported the kinetics of photopolymerization of hydrocarbon and fluorine-containing monomers and oligomers, and the overall kinetics of the photoinitiated polymerization of acryl oligomers with different chemical natures of their oligomeric block, molecular weights, and local and macroscopic viscosities. This reaction hardly depends on the number and structure of fluorinated moieties. At high initiation rates, the oligomer block’s flexibility, governed by the number of groups with a low potential rotational barrier (e.g., carbonate groups), is the main parameter that influences the kinetics of polymerization. However, at low initiation rates, the viscosity of oligomers becomes an essential factor. Furthermore, the same team241 also reported the optimal conditions for molding optical articles from hydrocarbon and fluorinated acryl oligomer composites. Imperial Chemical Industries Co.242 claims the synthesis of original telechelic fluorinated diol oligomers bearing −CF2CFHCF3 dangling groups from the radical addition of S


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Scheme 24. Synthesis of (A) the AMPS-Based Network by Free Radical Copolymerization and (B) Cationic Copolymerization of Telechelic Fluorodiepoxide Photoinitiated with a Triarylsulfonium Triflate92

Scheme 25. IPN Based on Poly(vinylidene fluoride) (PVDF) and Divinyl Benzene That Led to Original PEMFC after Sulfonation of Cross-linked Filmsa

a

Reprinted with permission from ref 252. Copyright 2004 Elsevier.

very smooth surfaces. Adhesion between the substrate and the polymer layer was guaranteed by introducing carbonates and carbamates.245 Very recently, low-loss integrated switch arrays suitable for realizing large-scale photonic integrated circuits were also fabricated with a direct UV-writing technique using fluorinated low-loss photopolymers as the waveguide core.246

HFP onto poly(tetramethylene glycol) oligomers (of molecular weight 650) and their reaction with isophorone diisocyanate in the presence of dibutyltindilaurate as the catalyst. 2-Hydroxyethyl acrylate then was added until no free isocyanate remained. That resulting telechelic bis acrylate was photopolymerized in the presence of a photoinitiator (PI) to lead to an interesting soft coating. That original product has been marketed from ICI under the Neorad trademark. Cladding polymeric structures were also searched to tune the numerical aperture of coated optical fibers with the objective to lower the refractive index (n = 1.3−1.5); one example, developed by Optical Polymer Research,243 consists of synthesizing a new fluorinated (meth)acrylate of structure H2CCRCO2CHXY where X designates H, CF3, and Y stands for CF3, CF(CF3)OCF2CF(CF3)OC4F9, (CF2)nF with n = 1−8. Photopolymerization is then conducted in the presence of PI. Fluorinated prepolymers have been proposed as good candidate materials for flexible display applications; the UVcured films prepared from these prepolymers exhibited mechanical flexibility, thermostability even over 350 °C, and optical transparency, with refractive indices and optical losses around 1.43−1.51 and 1.03−1.06 dB cm−1 at a wavelength of 633 nm, respectively.244 The low refractive index exhibited by PFPE systems makes them suitable for optical applications: low-loss polymer waveguides have been prepared by photocuring methacryloyl perfluoroalkyleneoxy carbonate and then reactive ion etching the polymer to produce high-quality channel waveguides with

9.3. Polymer Exchange Membrane for Fuel Cell and Other Energy Devices

Membrane technologies often employ photopolymerization methods. An overview on photoirradiation for the preparation, modification, and stimulation of membranes is proposed by He et al.247 This Review presents a subsection that focuses on membranes and electrolytes used in fuel cell, batteries, and photovoltaics. The proton exchange membrane (PEM) is a fundamental component of fuel cells,248,249 a promising alternative energy conversion approach for low to medium power applications. Existing commercially available proton exchange membrane fuel cells (PEMFCs) are based predominantly on perfluorosulfonic acid polymers (e.g., Nafion, Aquivion, Flemion, Fumion, 3M Membrane).250,251 Delhorbe et al.92 reported the preparation of original membranes achieved with an interpenetrating polymer network architecture composed of a poly(2-acrylamido-2methyl-1-propanesulfonic acid) (AMPS) network and a fluorinated network present in different compositions (Scheme 24). IPNs can act as a powerful tool in simultaneously carrying T


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polymer embedded a luminescent Europium complex able to ensure a luminescent down-shifting and converting UV photons into visible light. A 70% relative increase in power conversion efficiency was achieved as compared to uncoated control devices thanks to the energy management ability of the composite. Moreover, as expected by a fluorinated coating, long-term (>2000 h) weathering tests in real outdoor conditions revealed an excellent stabilizing effect guaranteeing outdoor stability; the fluorinated coating was highly photostable, oleophobic, and hydrophobic.

out a reduction in the methanol permeability and an enhancement of the mechanical stability of polymer electrolytes. The former network was obtained by radical copolymerization of AMPS with a telechelic fluorinated bis(acrylate), while the latter one was obtained by photoinitiated cationic copolymerization of a telechelic fluorinated diepoxide with trimethylol propane triglycidyl ether. The main functional properties related to their use as PEMFC were achieved: low water vapor sorption, limited liquid water uptake, high proton conductivity, gas permeability, and oxidative and thermal stability. More precisely, the effects of the ionic exchange capacity and the cross-linking density of the conducting phase on the morphology and the properties of IPN membranes were studied in detail. Finally, these IPN membranes were tested as fuel cell membrane, and the authors established a correlation between the ex situ and in situ characterizations. Prakash et al.252 obtained original PEMFC membranes from IPNs made in two steps as depicted in Scheme 25. Interestingly, the membrane reduces the methanol crossover when compared to a Nafion 117 membrane (Figure 6).

9.4. Microfluidic Devices

Hydrophobicity, flexibility, and thermal stability of the fluorinated backbone make UV-cured fluorinated polymers excellent materials for the fabrication of microfluidic devices and microfluidics tools (such as valves and pumps). The main limitation is then adapting the photopolymerization process of fluorinated oligomers to the fabrication technologies available for polymer-based microfluidics. These technologies are classified into replication methods and direct fabrication methods. The former ones are characterized by the use of a master (often referred to as the replication tool, mold, or mold insert), which is the geometrical inverse of the desired structure. UV-curing of fluorinated monomers for molds is described in section 9.5. With direct methods, the microfluidic device is fabricated directly without any need of a master and often involves a photochemical reaction, either a polymerization reaction (i.e., in photolithography and stereolithography, the solid structure is built out of a liquid resin, which is called photoresist) or a degradation reaction (i.e., ablating the photoresist with a laser beam). Many photocurable systems (i.e., noncommercial resists) have been used as structural materials for microfluidic devices, and, recently, PFPE oligomers have been proposed.98,259 An unconventional and direct photolithographic approach was developed to transfer micropatterns into photocurable PFPEs with high control and fidelity. It was possible to transfer features with high density, high aspect ratio, and complex shapes. A simple PFPE microfluidic device (Figure 7) was successfully tested,259 closing the

Figure 6. Methanol crossover rate (1M-solution) through a Nafion 117 membrane and a semi-IPN PSSA/PVDF membrane with the same thickness at 23 °C. Reproduced with permission from ref 252. Copyright 2004 Elsevier.

Photopolymerization is a well-established method to prepare solid or quasi-solid polymeric electrolytes for batteries and photovoltaics.253,254 However, salts are poorly soluble in fluorinated solid matrixes, and, at present, no examples of fluorinated electrolytes have been reported yet. Encouraging results come from Balsara’s and DeSimone’s groups255 who presented a unique nonflammable oligomeric electrolyte composed of low molecular weight PFPEs and bis(trifluoromethane)sulfonimide lithium salt. Interestingly, these electrolytes exhibit thermal stability up to 200 °C and a remarkably high transference number (>0.91, i.e., more than double that of conventional electrolytes). Li/LiNi1/3Co1/3Mn1/3O2 cells generated with this electrolyte displayed good performances in galvanostatic cycling, confirming their potential as rechargeable lithium ion batteries with enhanced safety and longevity. Moreover, the same teams256 demonstrated that blending PEG with PFPE is a valuable strategy to enhance the ionic conductivity of pure PFPE electrolytes, allowing at the same time for one to maintain the nonflammability, reduce the Tg, and prevent the crystallization of PEG. In the case of photovoltaics, fluorinated coatings are interesting for protecting the surfaces of the devices exposed outdoors.257 A UV-cured coating was recently presented for dyesensitized solar cells;258 the new photocurable fluorinated

Figure 7. PFPE microfluidic device before (a) and after (b) filling with colored water-based solution. Reproduced with permission from ref 259. Copyright 2013 American Chemical Society.

microfluidic structures by a bonding procedure based on partial UV-curing. PFPE microfluidic devices demonstrated their promising potential for application in microfluidic organic synthesis. Starting from the success in making this prototype, the use of fluorinated monomers was extended to additive manufacturing (AM). AM is a highly versatile layer-by-layer fabrication process where photopolymerization can enter. Lithography-based AM technologies (e.g., stereolithography, two-photon lithography, and digital light processing) can be an ideal tool for generating very complex three-dimensional computer-aided design (CAD) models based on the required internal and external architectures U


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epoxy-based resins are employed, can decrease the transparency of the replica mold.262 High demand exists for the development of new nanofabrication techniques with sub-100 nm resolution patterning. These techniques include nanoimprint lithography (NIL), stepand-flash imprint lithography (SFIL), and scanning probe lithography (SPL). A major challenge for imprint lithography is to minimize the tendency for the resist polymer to adhere to the template, which leads to imprint defects. Large adhesive forces between the template and the imprinted material can result in cohesive failure of the etch barrier material and distort the alignment for imprint. One approach to overcome this issue is the use of fluorinated resists. The hybrid polymeric network via the thiol−ene “click chemistry” reaction described in section 3.5 is proposed for NIL as it exhibited a variety of characteristics desirable for a high-throughput fabrication of nanodevices:160 low viscosity (16−239 cP), low bulk volumetric shrinkage (4.8− 7.5%), good resistance to oxygen inhibition, high transparency to UV light, and resistance to organic solvents. As a soft mold, the excellent mechanical properties (Young’s modulus: 0.31−1.56 GPa) and low surface energy (14−20 mJ m−2) ensured easy mold release without fracture or deformation of the embossed structures. The thermal stability (Td > 300 °C) rendered them capable of being used for both UV and thermal NIL duplication processes. The resolution patterning capacity was in the range of 200 nm to several micrometers: the impressive fidelity in replica and better durability160 are displayed in Figure 9. An alternative approach to reduce interfacial forces between the resist and the template consists of incorporating a fluorinated comonomer, taking advantage of its surface stratification. This is proposed by Wu et al.265 for the etch barrier liquid of SFIL, as fluorinated self-assembled monolayers deposited on the templates without any covalent link have insufficient durability for the thousands of imprints one is expected to make. The authors investigated the effect of template surface on the segregation of fluorinated vinyl ether comonomer; a template with low surface energy and polarity is beneficial to the adsorption of the surfactant at the template−polymer barrier interface. The model developed for the prediction of the fluorine concentration profile and the surface energy of the etch barrier after imprint as a function of comonomer concentration is in qualitative agreement with experimental results and confirms the finding about surface segregation reported by Bongiovanni.103,129,167,179 Although fluorinated monomers can play a major role in the demolding process in NIL, an excessive amount of them (i.e., 30 mol %) may be detrimental, and the desired physical properties of the UV-cured substrate would be degraded.266

of the part while considering the constraints imposed by system capabilities. For instance, three-dimensional fabricated woodpile structures with submicrometer resolution and different geometries have been described by Turri’s group.260 Having proposed ways to bind fluorinated layers and having demonstrated the high reactivity of fluoromonomers in photopolymerization, many other fluorinated AM products are likely to be prepared and used in advanced applications. 9.5. Replica Molding and Patterning

In recent years, soft lithography involved the use of photocurable monomers. As an alternative to thermal-cured polydimethylsiloxane (PDMS), UV-curable formulations based on epoxysilicone and poxydimethylsiloxanes have been used. Similarly, fluorinated materials have proven interesting due to their low surface energy, solvent resistance, chemical and thermal stability, visible transparency, and tunable modulus. As demonstrated by DeSimone’s group,261,262 PFPE urethane diacrylates are suitable for both the fabrication of molds for replica molding and the preparation of patterned structures with high fidelity. Recently, the PRINT (Particle Replication in Nonwetting Templates) technology263 has become a powerful technique for nanomolding enabling the fabrication of particles with precise control over shape, size, composition, and surface functionality (Figure 8).264

Figure 8. Nanoparticles fabricated by using the PRINT process. Figure from DeSimone Web site.264

However, edge defects of the pattern due to the inherent flaws in oxygen inhabitation, and the heat-induced yellowing when

Figure 9. Quartz mold (left) and the replica F-mold (right). Reproduced with permission from ref 160. Copyright 2012 The Royal Society of Chemistry. V


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hydrophobic and chemically resistant, can be used to manufacture 3D functional microfluidic elements requiring a good chemical resistance, as well as hydrophobic tips for AFM, optimizing the imaging of wet and biological samples.

Surface segregation has been recently rediscussed in view of preparing patterns. As the top layer of a photocurable liquid has a different composition, there is a mismatch in shrinkage between the top and bulk layer. This induces self-wrinkling, and leads to the formation of a wrinkled surface: submicrometer wrinkling patterns with a wavelength between 200 and 800 nm were successfully fabricated by tuning the content of the fluorinated polymer.267 Investigating the effect of the factors ruling the stratification, the photopolymerization kinetics, and the mechanical performance of the photocuring systems on the morphology of the wrinkling, a quantitative relationship between the characteristic wavelength, amplitude, and the thickness of the top layer was obtained. In addition, Turri’s group260 reported the synthesis of new PFPE-based hydrophobic and chemically resistant photoresists employed for the successful fabrication of submicrometer 3D structures by two-photon polymerization (2PP) technology (Figure 10). The perfluoropolyether resist, being intrinsically

9.6. Fluorogels

The same telechelic diacrylate PFPEs were also used as crosslinkers in cophotocuring perfluorinated acrylates.268 The resulting original omniphobic fluorogel elastomers displayed a broad range of optical and mechanical properties by tuning the chemical composition or the temperature that controlled the crystallinity of the resulting polymeric chains. These fluorogels also exhibited excellent resistance to wetting by various liquids, some antibiofouling properties, a shape-memory behavior, and maintained cytocompatibility. These characteristics should enable these materials to be involved in new technologies and to improve performance/efficiency in a range of energy, environmental, and biomedical applications that require longterm operations and/or encounter harsh environmental conditions. 9.7. Self-Healing Polymers

A novel class of self-healing PFPEs has recently been reported by Li et al.,269 who also studied their thermomechanical and rheological properties. Using 2-ureido-4[1H]-pyrimidone end groups on low molecular mass of bis(hydroxy) PFPE (Scheme 26), these authors obtained supramolecular fluoropolymers, and the resulting networks arose from the hydrogen-bonding associations. These novel supramolecular polymeric materials exhibit a combination of enhanced modulus and elasticity, along with self-healing properties. Figure 10. SEM images of a 50 μm × 50 μm × 50 μm woodpile structure of the PFPE-based resin obtained by 2PP: (a) top view, (b) tilted (45°) view. The structure is fabricated at a speed of 1 mm/s with an average laser power of 3 mW. Reproduced with permission from ref 260. Copyright 2013 American Chemical Society.

10. CONCLUSIONS AND OUTLOOKS French Professor Henri Moissan, discoverer of fluorine ca. 130 years ago, used to say that “l’étude des composés fluorés réserve encore bien des surprises...”; that is, fluorine will bring many surprises. The history of fluorine chemistry and fluoropolymers confirms he was absolutely right. Fluorine is a key element for

Scheme 26. Synthesis of Original Self-Healing PFPE Polymers from Bis(hydroxy) PFPE269

W


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progress in many fields: it is the most reactive of the elements, and its incorporation in a molecule imparts interesting, if not dramatic, changes of properties. This is the reason Linus C. Pauling stated “Fluorine is a superhalogen; it is a class by itself.” While life would be different if it were not for fluorine, from an academic point of view, fluorine chemistry is always rich in unexpected reactions. This Review shows that fluorine chemistry has also become relevant in many photoinduced processes leading to innovations in the production of materials and devices. Actually, the synthesis of appropriate fluorinated monomers (monofunctional, telechelic and poly(meth)acrylates, epoxides, vinyl ethers, oxetanes) has been developed from various strategies, has already reached industrial scale production, and has opened fluorine to the world of photopolymerization, although much research on well-defined monomers is still growing. In addition, from the nature of these monomers, various appropriate conventional processes, photoinitiators, and photopolymerizations have been used successfully. Easy and clean photocuring processes have led to high value-added networks with remarkable properties. Moreover, photoinduced processes offer many advantages over traditional methods as mentioned here, and because they were originally upscaled, they have been widely adopted in many industries including automotive, telecommunications, electronics, graphic arts, converting, and metal, glass, and plastic decorating. In addition, bottom-up preparations of photopolymerized nanocomposites by UVcuring of curable monomers for the synthesis of an inorganic phase within fluorinated matrix are made by combining sol−gel processes. They led to original composites that exhibited satisfactory mechanical performances with good adhesion between inorganic counterparts or fibers and fluorinated matrixes. The alkoxysilanes can be chosen among those containing fluorinated chains compatible with the matrix and/ or containing functional groups that can coreact in the photopolymerization process. A powerful combination of cost, quality, and environmental benefits, as well as constant refinement of the technologies behind the processes, has constantly helped them to expand into more market segments. Therefore, with the availability of many reactive fluoromonomers that have already reached industrial scale production, high value-added materials with remarkable properties (chemical inertness, hydrophobicity and thermal, mechanical, and radiation resistance) could have been obtained. Examples described in this Review are antistain papers, protective, anticorrosion and antifouling coatings, adhesives, cores and claddings for optical fibers, microfluidic devices, high performance elastomers, energy storage devices (polymer electrolytes for rechargeable lithium ion batteries and membranes for fuel cells), lithographic patterns, replica molding, and fluorogels. Production and markets are expected to still grow as the requirements of specific products endowed with remarkable properties are stringent. For example, in the field of coatings, photoinduced curing experiences a two digit growth. Exciting perspectives come from 3D printing or additive manufacturing, which is a process of making three-dimensional solid objects from a digital file. Among the different technologies, stereolithography is based on photopolymerization (the technique was invented in 1986 by Charles Hull, who also at the time founded the 3D Systems Company). While the most known applications include design visualization, also coming to reality are prototyping/CAD, architecture, education, tissue engineering, and organ printing. Here, the role of fluorinated materials can be

relevant. Therefore, this will stimulate industrial and academic researchers in the coming decades to synthesize new fluoromonomers and better control photoinduced processes, from curing to grafting for surface modification of materials and devices. Innovation in polymeric materials can come from the design of novel monomers; the state of the art in fluorinated photopolymerizable monomers proposed in this Review shows that there are gaps in the present portfolio of fluorinated photocurable structures, such as difunctional fluorinated vinyl ethers and difunctional fluorinated oxetanes. Moreover, further exploring of PVDF, perfluoropolyethers, and polyfluorinated oxetane building blocks by tuning the structural units and molecular weights can be promising. Establishing sound structure−property relationships is also relevant for innovation, and especially in the area of surface properties and toxicologies of fluorinated polymers, there is still a lack of information. Research aiming to this goal requires model compounds; thus, further synthetic efforts should be planned. Concerning changes in photoprocesses, photoinitiator-free polymerization clearly needs to be further explored at least with the fluorinated monomers available. These challenges will probably bring industrial and academic innovations in the coming decades.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Alessandra Vitale received her M.Sc. degree in Materials Engineering from Politecnico di Torino, Italy, and in Polymers for Advanced Technologies from Université Joseph Fourier in Grenoble, France, in 2010. She obtained her Ph.D. from Politecnico di Torino, Italy, under the supervision of Prof. Roberta Bongiovanni. In her laboratory, Alessandra investigated innovative polymers and technologies for the fabrication of microfluidic devices. She has won the AICIng (Italian Association Chemistry for Engineering) Best Ph.D. Thesis Award. In 2014 she joined Imperial College London where she currently works as research associate in the group of Dr. João Cabral. Her research interests include photopolymerization and methods for microfabrication. X


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Boutevin, Prof. G. Kostov, Dr. M. Hung, and M. Holan, and DuPont Performance Elastomers for funding.

LIST OF SYMBOLS AND ABBREVIATIONS AM additive manufacturing AMPS poly(2-acrylamido-2-methyl-1-propanesulfonic acid) ATR-FTIR attenuated total reflection Fourier transform infrared spectroscopy BCMO bischloromethyl oxetane CAD computer-aided design CRM confocal Raman spectroscopy CSM cured site monomer CTA chain transfer agent CTFE chlorotrifluoroethylene DMF dimethylformamide DMFC direct methanol fuel cell DMTA dynamic mechanical thermal analysis DPn number-average degree of polymerization DSC differential scanning calorimetry E ethylene EA epoxy acrylate EB electron beam FTIR Fourier transformed infrared spectroscopy G spacing group (or spacer) GPC gel permeation chromatography HDGE hexanediol diglycidyl ether HDI 1,6-hexamethylene diisocyanate HFPO hexafluoropropylene oxide IEM 2-isocyanatoethyl methacrylate IPDI isophorone diisocyanate IPN interpenetrated polymer network IR infrared ITP iodine transfer polymerization M monomer (or comonomer) m-CPBA 3-chloroperbenzoic acid MATRIFE 2,2,2-trifluorethyl methacrylate MEMO methacryloyl-oxy-propyl-trimethoxysilane MW molecular weight NIL nanoimprint lithography NMR nuclear magnetic resonance PDI polydispersity index PDMS polydimethylsiloxane PEG poly(ethylene glycol) PEMFC proton exchange membrane fuel cell PEO poly(ethylene oxide) PFOA perfluorooctanoic acid PFOS perfluorooctanesulfonic acid PFPE perfluoropolyether PI photoinitiator PMVE perfluoromethyl vinyl ether POSS silsesquioxane PSA pressure sensitive adhesive PTFE poly(tetrafluoroethylene) PVDF poly(vinylidene fluoride) R alkyl group RF perfluorinated group R FI perfluoroalkyl iodide (or 1-iodoperfluoroalkane) SEC size exclusion chromatography SEM scanning electron microscopy SFIL step-and-flash imprint lithography SPL scanning probe lithography TBAHS tetrabutyl ammonium hydrogenosulfate

Roberta Bongiovanni, after a degree in Chemistry at the University of Turin (Italy), received a M.Sc. in Colloid and Interface Science at the University of Bristol (U.K.) where later she obtained a Ph.D. in Physical Chemistry. After working for several years at ENI Central Laboratories in Milan, she joined the Politecnico di Torino where she is now Full Professor of Chemistry. Her work concerns photopolymerization with a special interest for fluorinated polymers and photoinduced modification of polymers. She is the coauthor of 140 papers on international journals, several book chapters, and three international patents.

Directeur de Recherches at CNRS, Bruno Ameduri leads the “Fluoropolymers and Energy” team at the Laboratory “Engineering and Macromolecular Architectures” of Institute Charles Gerhardt in Montpellier, France. His main interests focus on the synthesis and the characterization of fluorinated monomers (including cure site monomers and telechelics), telomers, and (co)polymers for various applications such as surfactants, elastomers, fuel cell membranes, piezoelectric polymers, and solvents, binders, and polymer electrolytes for lithium ion batteries. Coauthor of one book, more than 30 reviews or chapters of books, and more than 250 peer review publications, and coinventor of more than 70 patents, he is also a member of the American and French Chemical Societies and is a member of the Editorial Boards of the Journal of Fluorine Chemistry, European Polymer Journal, Polymer Bulletin, and Polymer Journal (Japan). Out of research, Bruno enjoys cycling, soccer, tennis, and jogging and is an active member of the “Rire” Association and, dressed as a clown, visits sick children in hospitals of Montpellier.

ACKNOWLEDGMENTS R.B. acknowledges support from Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali INSTM (via Giusti 9, Firenze), while B.A. thanks all researchers of his group involved in the project of photo-cross-linking, especially Prof. B. Y


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thiol−ene coupling transmission electron microscopy tetraethoxysilane tetrafluoroethylene glass transition temperature trimethyl hexamethylenediisocyanate ultraviolet vinylidene fluoride (or 1,1-difluoroethylene) X-ray photoelectron spectroscopy heating two-photon polymerization

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Fluorinated Oligomers and Polymers in Photopolymerization

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