Fluorinated hydroxytelechelic polybutadiene as additive in cationic photopolymerization of an epoxy...

Fluorinated Hydroxytelechelic Polybutadiene as Additive inCationic Photopolymerization of an Epoxy Resin

B. AMEDURI,1 R. BONGIOVANNI,2 M. SANGERMANO,2 A. PRIOLA2

1Ingenierie et Architectures Macromoleculaires; Institut Charles Gerhardt, UMR (CNRS) 5253, Ecole Nationale Superieurede Chimie de Montpellier, 8, rue Ecole Normale F-34296 Montpellier Cedex, France

2Politecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, C.so Duca degli Abruzzi 24,10129 Torino, Italy

Received 15 January 2009; accepted 26 February 2009DOI: 10.1002/pola.23376Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Fluorinated hydroxytelechelic polybutadiene (PBF), which is synthesizedfrom the radical addition of a fluorinated mercaptan onto a hydroxytelechelic polybu-tadiene was used as an efficient surface and mechanical modifier in original formula-tions to enable the UV cationic polymerization of a telechelic diepoxy cycloaliphaticoligomer, leading to crosslinked polymers. Various amounts of PBF were used (rang-ing between 0 and 15 wt %). The kinetics of photocrosslinking revealed that the die-poxyde conversion reached 70% only in the absence of PBF, whereas it was about90% in the presence of 10 wt % of PBF. When the PBF content increased, thedynamic mechanical measurements showed that: (i) the glass transition temperaturevalues decreased and (ii) the resilience increased. The hydrophobicity of these poly-mers was investigated from the water contact angle (WCA) values showing a surfacemodification of the epoxy-based system. Indeed, great modifications were noted onthe extreme surface (i.e., air side) of the polymers (with WCA values as high as 120�)in contrast to those observed on the substrate side. VVC 2009 Wiley Periodicals, Inc. J Polym

Sci Part A: Polym Chem 47: 2835–2842, 2009

Keywords: crosslinking; fluoropolymers; morphology; photopolymerization;polybutadiene; surface modification

INTRODUCTION

UV-induced cationic photopolymerization of epoxymonomers is a well-known process and is of greatinterest as a result of its high number of indus-trial applications.1 The UV-polymerizable formu-lations are solvent free, the production rates arehigh, and the energy required can be less com-pared with thermal curing.2 Moreover, the cati-

onic photopolymerization of epoxy monomerspossesses some distinct advantages such as lackof inhibition by oxygen, a low shrinkage, goodmechanical properties of the UV-cured materials,and good adhesion properties of UV-cured filmsto various substrates.3 Finally, the monomersemployed are generally less toxic and less irri-tant4 than acrylates and methacrylates exten-sively used in the radical mechanism. Therefore,extensive work has been carried out to investigatethe photocuring of epoxides, focusing mainly onthe reaction kinetics and the properties of thecured materials.5–13 Recently, a study by Fernan-dez-Francos et al.14 discussed the influence of the

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 2835–2842 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: B. Ameduri (E-mail: [email protected])

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photocuring temperature and the amount of theinitiator on the properties of fully cured epoxy ma-terial. Other interesting reports showed that cer-tain epoxide monomers display high reactivity inphotoinitiated cationic polymerization and aresuitable for industrial applications, whereasothers undergo apparently slow reactions and aretherefore not attractive. Some epoxy monomersthat possess basic groups as part of their struc-tures are not at all polymerizable under cationicconditions.15–17 For this reason, the number ofavailable epoxide monomers that are useful inapplications involving photoinitiated cationic po-lymerization is still limited. Accordingly, most ofpresent and past research emphasis has beenplaced on developing and optimizing only thosemonomer-photoinitiator systems that displaythe highest possible reactivity. One of this mono-mer is 3,4-epoxycyclohexylmethyl-30,40-epoxycy-clohexyl carboxylate, which leads to cured poly-mers characterized by high mechanical propertiesand solvent resistance, but having a rigid struc-ture. They exhibit rather poor elongation and flex-ibility characteristics. It is well-known that, inmany coatings applications, there is a need fortoughness and flexibility at the same time ensur-ing considerable elongation at break. For this rea-son, it is desirable to find out ways to obtain goodmechanical characteristics of UV cured epoxycoatings, together with high flexibility. In the lit-erature, the introduction of reactive plasticizingagents in the UV curable formulations is reportedto increase the resilience behavior5,18,19: the pres-ence of a plasticizing agent linked to the epoxynetwork increases the flexibility and the impactresistance of the cured product giving rise to agood balance of the mechanical properties.Hydroxyl-containing additives were investigatedwith the aim of introducing flexibilization on UVcured networks,20 because the use of alcohols asthe chain transfer agents in cationic polymeriza-tion was first indicated by Penczek et al.,21 thenconfirmed for the UV curing processes by Crivelloet al.22

It was shown that in a network forming poly-merization, the growing chain end becomes fixedto the immobile three-dimensional network. If thenetwork is glassy, the polymerization is stopped,and a large number of oligomers remain trappedinto the glassy polymer network. When the chaintransfer occurs, new chains carrying reactive spe-cies are formed: they are not linked to the net-work and are, therefore, highly mobile. Higher po-lymerization rates result from the removal of the

restriction to diffusion as well as from a decreasein the average crosslinking density. Also, epoxy-hydroxyl functionalized polybutadiene can beincorporated into an epoxy polymeric network,during cationic UV curing of an epoxy formula-tion, both via a copolymerization reaction of theepoxy groups and via a chain transfer reactioninvolving hydroxyl groups.23 The investigation ofthe properties of the cured films showed that theirTg decreased sharply in the presence of thehydroxylated additives with an important gain intoughness.

Pursuing these investigations, a fluorinatedhydroxytelechelic polybutadiene was synthesizedand used as an original additive in cationic UVcuring of an epoxy-based system. The use of fluo-rinated monomers or fluorinated alcohols to mod-ify epoxy networks was previously described24–26:peculiar characteristics, connected to the presenceof fluorine atoms at the surface, were imparted tothe coatings, in particular, hydrophobicity andoleophobicity.

The aim of this study is to induce not only sur-face modification of the epoxy-based system byintroducing the fluorinated hydroxyl additive butalso to achieve a bulk modification by choosing arubbery oligomer as a modifier, so that coatingswith hydrophobic surface properties and im-proved network flexibility and resilience can beobtained.

EXPERIMENTAL

Materials

A fluorinated hydroxytelechelic polybutadiene(PBF) was synthesized by radical (various initia-tors were used such as AIBN, di-tert butyl perox-ide, and t-butyl peroxypivalate, or photochemicallyinduced) addition of 1H,1H,2H,2H-perfluorooctyl-1-mercaptan onto hydroxytelechelic polybutadiene(HTPBd, kindly supplied by Atofina) of averagemolecular weights of 1000 g mol�1, polydispersityindex of 2.5, and functionality of 2.3. The reactionis represented by Scheme 2.

A typical recipe is as follows: in a three-neckedround-bottomed flask equipped with a condenserand a device for a nitrogen flow, was magneticallystirred, under nitrogen, 0.046 g (0.192 mmol) ofter-butyl peroxypivalate, 0.923 g (2.43 mmol) of1H,1H,2H,2H-perfluorooctyl-1-mercaptan, 10.051g (9.61 mmol) of HTPBd, and 50 mL of THF. Afterstirring for 7 h at 75 �C, the mixture was cooled toroom temperature and the solvent evaporated.

2836 AMEDURI ET AL.

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Then, the modified HTPBd (PBF) was precipi-tated from methanol twice to allow the unreactedmercaptan to be eliminated (this was checked bythe absence of the characteristic triplet centeredat 1.50 ppm in the 1H NMR spectrum, assigned tothe SH end-group of the mercaptan) to lead to anorange viscous oil. As previously reported,27 1HNMR spectroscopy and elemental analysesenabled to assess the grafting percentage (11%),while the 19F NMR spectrum confirmed the pres-ence of the perfluorohexyl pendant chain. The flu-orine content in the resulting grafted PBF was28.2%.

3,4-Epoxycyclohexylmethyl-30,40-epoxycyclohexylcarboxylate (CE), UVACURE1500 from CYTECCompany, was employed as a reference epoxyresin. The photoinitiator was a commerciallyavailable mixture of antimonate sulfonium salts(UVI 6976) supplied by the Dow Company: it wasgiven as a solution in propylene carbonate (50%w/w), and it was added into the curable mixturesat a concentration equal to 2% w/w. The struc-tures of the monomer and of PBF additive arereported in Scheme 1.

Preparation of the Samples

The photocurable formulations were prepared bymixing different amounts (ranging between 5 and15 wt %) of the fluorinated hydroxytelechelic poly-butadiene (PBF) additive to the epoxy resin, andadding 2 wt % of the cationic photoinitiator. Thelist of formulations is reported in Table 1. The for-mulations were coated onto a glass slide using awire-wound applicator. The films were exposed toUV irradiation by using a Fusion lamp (Hg bulb),with radiation intensity on the surface of the sam-ple of 280 mW/cm2 and a belt speed of 6 m/min.Fully-cured films of about 100-lm thick wereobtained.

Techniques of Characterization

The photopolymerization was monitored by real-time FTIR spectroscopy, using a Thermo-Nicolet5700 instrument. The formulations were coatedonto a silicon wafer. The sample was exposedsimultaneously to the UV beam, which inducedthe polymerization and to the IR beam, which an-alyzed in situ the extent of the reaction. Becausethe IR absorbance is proportional to the monomerconcentration, conversion versus irradiation timeprofiles can be obtained. Epoxy group conversionwas monitored by the decrease of the absorbanceof the epoxy groups in the region 760–780 cm�1:the absorbance was normalized using the esterband at 1670 cm�1 as an internal reference. Amedium pressure mercury lamp equipped with anoptical waveguide was used to induce the photo-polymerization (light intensity on the surface ofthe sample of about 30 mW/cm2).

The gel content was determined on the curedfilms by measuring the weight loss after 24 hextraction with chloroform at room temperature,according to the standard test method ASTMD2765-84.

Dynamic mechanical thermal analyses (DMA)were carried out with a Rheometric ScientificMKIII (UK) instrument, at a frequency of 1 Hz inthe tensile configuration.

Impact resistance measurements were per-formed with an ATS FAAR Charpy pendulum,according to ASTM D256 method. The fracturesurfaces of the samples employed for the impactresistance tests were analyzed by scanning elec-tron microscopy (SEM) microscopy.

The morphology and the energy dispersiveanalysis of X-rays of the materials were investi-gated by SEM analysis using LEO (ex LEICA, exCAMBRIDGE) S260 equipped with a system ofmicroanalysis-X. Accelerating voltage of 300 V to3 kV (accelerating rate of 100 V) and 4 to 30 kV(accelerating rate of 1 kV) was applied. Sampleswere put into liquid nitrogen to be cryofractured,

Scheme 1. Chemical structure of the 3,4-epoxycy-clohexylmethyl-30,40-epoxycyclohexyl carboxylate (CE)monomer and fluorinated hydroxytelechelic polybuta-diene (PBF) additive.

Table 1. Composition of the Formulations forVarious Coatings

Coating Type Composition

CE Pure CEPBF-5 CE/PBF 95:5 wt/wtPBF-10 CE/PBF 90:10 wt/wtPBF-15 CE/PBF 85:15 wt/wt

FLUOROPOLYBUTADIENE IN PHOTOPOLYMERIZATION 2837


the cross section was analyzed. Samples weremetallized with carbon under vacuum.

Contact angles were measured with a KrussDSA10 instrument, equipped with a video cam-era. Analyses were made at room temperature bymeans of the sessile drop technique. Five meas-urements were performed on every sample, andthe values were averaged. The measuring liquidwas double distilled water (c ¼ 72.1 mN/m).

RESULTS AND DISCUSSION

Photopolymerization of the Epoxide in thePresence of the Fluorinated Polybutadiene

The fluorinated polybutadiene (PBF) synthesizedby the radical addition of a fluorinated mercaptanonto the double bonds of a hydroxytelechelic poly-butadiene (Scheme 2) was employed, as additive,in the cationic UV curing of 3,4-epoxycyclohexyl-methyl-30,40-epoxycyclohexyl carboxylate (CE). Itwas added to the epoxy monomer in differentamounts as summarized in Table 1. The compati-bility of PBF in the CE resin was assessed by vis-ual inspection; the highest concentration insuringhomogeneous formulations was about 15 wt %.

The effect of the presence of PBF on thekinetics of the CE photopolymerization was inves-tigated by means of real-time FTIR. The telechelicdiepoxycycloaliphatic resin is able to polymerizevia a cationic photoactivation leading to a cross-linked network, while a chain transfer reactioncan take place involving the hydroxyl function ofPBF. The conversion curves as a function of irra-diation time for the neat CE resin and in the pres-ence of increasing amounts of PBF, are reportedin Figure 1.

While the slope of the curves gives an indica-tion of the rate of polymerization, the plateau

value gives the final epoxy group conversion. Inagreement with the literature,15 the CE resinpolymerization proceeded quite rapidly at the be-ginning and then slowed down markedly whenthe glassy network was formed (Tg ¼ 190 �C). Asa result, a large amount of epoxy groups remainedtrapped in the polymer network and could not dif-fuse further so that the propagation reactionstopped. It was noted that the final conversion isaround 70%. When the PBF was added, the initialreaction rate did not change, while an increase ofthe epoxy group conversion was achieved. In thepresence of 10 wt % of PBF, the epoxy groupconversion rose to almost 90%.

This behavior is explained on the basis of achain transfer mechanism involving the hydroxylgroups of PBF, as reported in Scheme 3: the carbo-cationic growing chain is terminated by reactionwith hydroxyl group hence inducing consequentdecreases of the chain length and of the crosslinkdensity. The introduction of ether free-danglingends makes the network more flexible; therefore,the propagation reaction proceeds increasing theconversion. Simultaneously, a proton is producedthat can initiate a new polymeric chain. As a con-sequence of this reaction, the network structure istherefore characterized by a greater flexibility.Hence, the lowering of the glass transition tem-perature of the cured networks is expected, aswill be discussed further.

Figure 1. Real-time FTIR kinetic curves of pure CEresin in the presence of increasing amounts of PBF asthe additive. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com.]

Scheme 2. Synthetic route for the fluorinatedhydroxytelechelic polybutadiene (PBF) by radical (orphotochemical) addition of 1H,1H,2H,2H-perfluoro-octyl-1-mercaptan onto hydroxytelechelic polybutadiene.

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The occurrence of the chain transfer reaction,and thus, the covalent incorporation of PBF in thenetwork structure, is confirmed by the high valueof the gel content also in the presence of a highamount of the additive (always above 96%, seeTable 2). These results indicate that the additiveis tightly crosslinked to the polymer network andtherefore takes part to the growing polymericchain. In fact, if polybutadiene would not havebeen part of the crosslinked network, it wouldhave been extracted giving rise to definitely lowergel content which, on the contrary, is alwayshigher than 96%.

Bulk Properties

Dynamic mechanical analyses (DMA) were per-formed on cured films to evaluate the glass transi-tion temperatures (Tg), defined as the maximumpeak of tan d curves. The values are reported inTable 2. A decrease of Tg values is observed whenincreasing the amount of PBF in the photocurableformulation, arising from the chain transfer reac-tion previously explained.

Scheme 3. Chain transfer reaction of 3,4-epoxycyclohexylmethyl-30,40-epoxycyclo-hexyl carboxylate (CE) resin by a generic hydroxyl containing additive.

Table 2. Epoxy Group Conversion After 2 min ofIrradiation and Bulk Properties of the Cured Films

Sample

EpoxyGroup

Conv. (%)

GelContent

(%)Tg (�C)DMTA

Resilience(J/cm2)

CE 71 97 190 7.53PBF-5 80 96 184 8.66PBF-10 88 96 180 9.09PBF-15 – 95 176 9.55

Figure 2. Scanning electron microscopy analysis offracture surface of PBF-15 UV cured sample.



The resilience behavior of the cured sampleshas been investigated by Charpy measurements.The data, reported in Table 2, show that the resil-ience increases from 7.53 J/cm2 for the pure CEcured resin to 9.55 J/cm2 for the cured formula-tion containing 15 wt % of PBF. This increase intoughness can be attributed both to the flexibiliza-tion effect induced by the chain transfer reaction,as indicated by the decrease of the Tg values, aswell as to the introduction of polybutadiene in theepoxy matrix. As already observed in a pastwork,11 polybutadiene can segregate and give riseto phase separation. SEM inspections of the cryo-fracture of the sample containing 15 wt % PBFshowed the formation of polybutadiene domainswith an average size of about 50–80 lm (Fig. 2).

Energy dispersive X-ray (EDX) analysis (Fig.3) on this area (clear zones) exhibited theexpected peaks of carbon and oxygen atoms butalso revealed the presence of fluorine atoms ofPBF, in contrast to the dark zones that did notshow any trace of fluorine.

Examining the picture, a good interaction ofthe polybutadiene with the surrounding epoxypolymer matrix appears. The interaction betweenthe polybutadiene domains, and the polymer net-work could be the result of the chemical linkagebetween both the components due to the chaintransfer reaction.

Surface Properties

As reported in the experimental section, the for-mulations having different contents of PBF wereused to coat glass slides. Glass slides were coatedwith mixtures having different contents of PBF.The homogeneous films obtained were peeled offthe substrate, and the contact angles versus

water on both sides of the films have been mea-sured. On the surfaces exposed to air, that is, theair side of the films, an interesting surface modifi-cation as a function of the fluorinated additiveconcentration was noted. On the contrary, theglass side showed no modification of the surfaceproperties, whatever the amount of the additive.The plot in Figure 4 clearly shows that PBF iseffective only on the air side of the film, where thecontact angle versus water increases and over-comes 90�, meaning that the wettability isreduced and the surface becomes hydrophobic.

On the glass side, the same wettability of thepure CE resin, around 70�, has been observed.This is an important result because it is possible,in this way, to obtain a hydrophobic surface with-out affecting the good adhesion properties ofepoxy coatings onto polar substrates.

The contact angle for the film containing 15wt % of PBF is high (120�) and similar to thatexhibited by a fully fluorinated polymer, whichare always highly hydrophobic.28 This high

Figure 3. Energy dispersive X-ray analysis of PBF-15 sample taken from the bulk,showing carbon and oxygen atoms and only traces of fluorine.

Figure 4. Advancing contact angles for cured filmscontaining various amounts of PFB (on glass (n) andon air (^) sides).

2840 AMEDURI ET AL.


hydrophobicity suggests a surface segregation ofthe fluorinated groups: due to its low surfaceenergy, PBF can migrate towards the air interfaceand concentrate at the outermost layer of thecured film. A similar behavior was previouslyobserved for different photocured systems in thepresence of fluorinated additives.29

This surface segregation was confirmed by EDXmeasurements taken both from the air surface ofthe cured samples. EDX analysis of the surface(Fig. 5) exhibited a strong peak due to fluorineatoms whose intensity is higher than the peaksassigned to carbon and oxygen. A negligibleamount of fluorine was found on the other surface.

The surface modification was stable and irre-versible. The wettability was checked many weeksafter film preparation, and the contact angle valueswere unchanged. These interesting results arisefrom the fact that the additive is covalently linkedto the network as it takes part in the polymeriza-tion process through a chain transfer reaction.

CONCLUSIONS

A fluorinated hydroxytelechelic polybutadiene(PBF) was synthesized and used as an additive inthe cationic UV curing of a dicycloaliphatic epoxyresin (CE). An increase of epoxy group conversionwas achieved by increasing PBF content in thephotocurable formulations. This result wasexplained on the basis of a chain transfer reac-tion, involving OH groups, which induces thedecrease of the chain-length and crosslinking den-sity, achieving a delay on ‘‘glassification’’ process.As a consequence, the bulk properties of the filmswere modified; the glass transition temperaturewas decreased and the toughness enhanced. A

surface modification was also observed in thepresence of PBF; selectively, the air sides of thefilms were made hydrophobic. In fact, a surfacesegregation of the low energy component, thefluorinated polybutadiene, was found by EDXanalysis.

In conclusion, the fluorinated hydroxytelechelicpolybutadiene showed interesting potentiality asthe additive with the possibility to tailor the finalproperties of the cured films. Highly hydrophobicepoxy coatings endowed with good adhesion andmechanical properties, and improved toughnesscould thus be achieved.

The authors thank Nathalie Morcelet for conductingSEM/EDX analyses and Atofina for supplying free sam-ples of hydroxyl telechelic polybutadiene and fluori-nated thiol.

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Figure 5. Energy dispersive X-ray analysis of PBF-15 cured sample taken from thesurface, showing different elements (C, O, and F). [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]



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