Epoxy-nanocomposites containing exfoliated zirconium phosphate: Preparation via cationic photopolymerisation and physicochemical characterisation

European Polymer Journal 45 (2009) 2487–2493

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Epoxy-nanocomposites containing exfoliated zirconium phosphate: Preparation via cationic photopolymerisation and physicochemical characterisation R. Bongiovanni a, M. Casciola b,*, A. Di Gianni a, A. Donnadio b, G. Malucelli a b

Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino c.Duca degli Abruzzi 24, 10129 Torino, Italy Dipartimento di Chimica, Università di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy

a r t i c l e

i n f o

Article history: Received 18 March 2009 Received in revised form 19 June 2009 Accepted 28 June 2009 Available online 2 July 2009 Keywords: Photochemical processing Dynamic mechanical analysis Thermogravimetric analysis Epoxy nanocomposite Zirconium phosphate

a b s t r a c t Gels of exfoliated a-zirconium phosphate in 3,4-epoxycyclohexylmethyl-30 ,40 -epoxycyclohexane carboxylate (hereafter BCDE) were polymerised by a UV-induced process and the resulting nanocomposites were characterised by TEM, X-ray diffraction, thermogravimetric analysis and dynamic-mechanical thermal analysis. Real-time FT-IR spectroscopy was used to investigate the photopolymerisation kinetics. The polymerisation rate of the gels is faster than that of neat BCDE, especially when the filler is functionalised with aminoalcohols (HO(CH2)nNH2, with n = 3, 4, 5). The composites exhibit lower Tg values in comparison with neat polymerised BCDE. Because of the barrier effect towards oxygen diffusion due to the high aspect ratio of the filler particles, all composites also exhibit enhanced thermal stability. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, polymer nanocomposites have attracted great interest because of their excellent performances compared to conventional composites. They usually consist of nanosized mineral particles dispersed into a polymeric matrix: among them polymer–clay nanocomposites are within the most explored, since the early works on polyamide 6–montmorillonite hybrids by the Toyota researchers [1]. The polymer–clay nanocomposites consist of clay nanolayers dispersed in a polymeric matrix. For their preparation several strategies have been developed: the filler can be directly dispersed in the polymer melt or in a polymer solution, alternatively it is swollen within the liquid monomer and the polymer formation can occur in between the intercalated or exfoliated sheets [2,3]. The latter method is called in situ intercalative polymerisation. The UV-curing technique, which is widely used to prepare thermoset coatings, adhesives, inks, can be success* Corresponding author. Fax: +39 075 585 5566. E-mail address: [email protected] (M. Casciola). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.06.022

fully employed to perform this kind of nanocomposite preparation [4]. After dispersion of the clay in the liquid monomer and upon irradiation in the presence of a suitable photoinitiator, polymeric thermoset matrices can be built through a process which is fast, does not use solvents and does not require any heating. In the literature some examples of UV-cured nanocomposites based on modified montmorillonites and different polymeric matrices (mainly acrylates) are described [5]. Two main types of structures are obtained when the layered clays are present within the crosslinked polymers: the intercalated structure, in which the polymer chains are intercalated between the silicate layers and form an ordered multilayer morphology with alternating polymeric and inorganic layers; the exfoliated structure, in which the silicate layers are completely and uniformly dispersed as single platelets in a continuous polymer matrix. In the last years, in the field of nanocomposites, increasing attention has been directed towards the use of synthetic fillers, at least for research purpose if not as an alternative to clays. Among the many layered fillers, a-zirconium phosphate (Zr(HPO4)2, hereafter ZrP) is particu-

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a

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larly interesting. It was first synthesized by Clearfield [6] and since then extensive research has shown that tailormade ZrP can be prepared: not only size and shape can be controlled by altering the reaction conditions, but also its surface can be easily modified to make it compatible to different polymers. Up to now Nafion, sulfonated polyetherketones, polyethyleneoxide, polyacrylamide and polyvinylidene fluoride has been used as polymer matrices (see [7–14] and references therein). Also epoxy nanocomposites were recently prepared [15–20], using a conventional process: both the ZrP and the polymer precursor (i.e., the epoxy monomer) were first dispersed in a solvent, then after its removal the curing agent was added and the mixture was cured in a oven for few hours. In the present work the preparation of epoxy-nanocomposites containing ZrP is made using the UV-curing process, which is much faster than thermal curing; in addition the novelty of this process is based on the few reports that investigate the in situ polymerisation of epoxides by UV light in the presence of either natural lamellar silicates or organophilic montmorillonites [21–24], none of them concerning UV-cured materials containing synthetic ZrP. In order to prepare UV-cured epoxy composites, ZrP was properly modified with different aminoalcohols having different molecular weight. Besides assuring a better compatibility with the epoxy matrix, the presence of hydroxy terminated modifier is interesting. In fact upon irradiation, in the presence of a suitable photoinitiator, epoxides polymerise following a cationic mechanism: in the presence of hydroxyl groups a chain transfer reaction takes place, which changes the reaction kinetics [25–27]. The paper therefore considers at first the effect of ZrP, in particular its organic modifier, on the kinetics of the polymerisation process of a bis-cycloaliphatic diepoxy resin, (3,4-epoxycyclohexylmethyl-30 ,40 -epoxycyclohexane carboxylate), then reports the characterisation of the resulting composites by means of X-ray diffraction, transmission electron microscopy (TEM), thermogravimetric analysis and dynamic-mechanical thermal analysis.

2. Experimental 2.1. Chemicals The bis-cycloaliphatic diepoxy monomer, 3,4-epoxcarboxylate ycyclohexylmethyl-30 ,40 -epoxycyclohexane (BCDE) was supplied from Cytec Inc. (USA). A sulfonium salt was used as photoinitiator: Cyracure UVI-6974, which is a mixture of triarylsulfonium hexafluoro antimonate salts in 50% solution of propylene carbonate, was kindly given by Dow Chemicals. It was used as the photoinitiator at 2 wt%. All other reagents were supplied by Aldrich and were used as received without further purification. The structures of BCDE and of the photoinitiator are shown in Fig. 1. 2.2. Preparation of ZrP, its modification by propylamine and aminoalcohols, dispersion in the epoxy monomer Crystalline ZrP was prepared by the direct precipitation method in the presence of hydrofluoric acid [28]. A colloidal dispersion of a-ZrP intercalated with propylamine (ZrPPrN, where PrN = C3H7NH2) in water was prepared according to Ref. [29] by titrating a suspension of 0.5 g ZrP in 33 mL water with 16.6 mL of 0.1 M propylamine. The dispersion was treated with 6 mL of 1 M HCl (final pH < 2) so as to regenerate the hydrogen form of zirconium phosphate. The solid was separated from the solution and washed with water under vigorous stirring. A gelatinous precipitate settled by centrifugation at 3000 rpm. Washing was repeated up to elimination of chloride ions. The gel thus obtained contained 1.5–2.5 wt% anhydrous ZrP. Functionalization of ZrP with aminoalcohols (NH2(CH2)n OH, with n = 3, 4, 5) was carried out by slowly adding a suitable volume of 0.1 M aqueous solutions of aminoalcohol to the ZrP/H2O gel so that the NH2(CH2)nOH/Zr molar ratio was 0.6. Aminoalcohol functionalised ZrP will be hereafter indicated as ZrPAn, where n is the number of carbon atoms of the aminoalcohol.

Fig. 1. Structure of BCDE and of the photoinitiator.

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To obtain gels of ZrP or ZrPAn in tetrahydrofuran (THF), an amount of the corresponding gel in water, containing x mL water, was washed three times with 4x mL of THF. After each washing, the gel was separated from the solution by centrifuging at 3000 rpm. Gels of ZrP or ZrPAn in BCDE were prepared by mixing suitable volumes of THF gels and BCDE and subsequent THF evaporation at 40 °C in a rotavapor. The volumes of THF gels and BCDE were chosen so that the filler content was 3 wt% after THF evaporation.

copper grid. The glass transition temperature (Tg) of the samples was determined by dynamic-mechanical thermal analysis (DMTA) on a MK III Rheometrics Scientific Instrument at 1.0 Hz frequency in the tensile configuration and heating rate of 5.0 °C min1. The size of the specimen was about 40  5  2 mm. The storage modulus, E0 , and the loss factor, tan d, were measured as a function of temperature in the range 20–250 °C.

2.3. Film preparation

3.1. Preparation of nanocomposites and kinetics measurements

2.4. Kinetic investigations The kinetics of the photopolymerisation was determined by real-time FT-IR spectroscopy, employing a Thermo - Nicolet 5700 apparatus (USA). The formulations were coated onto a silicon wafer in a very thin layer (around 10 lm). The samples were exposed simultaneously to the UV beam, which induces the polymerisation and to the IR beam, which analyses in situ the extent of the reaction. Because the IR absorbance is proportional to the monomer concentration, conversion versus irradiation time profiles can be obtained. Epoxy group conversion was followed, by monitoring the decrease in the absorbance due to epoxy groups in the region 760–780 cm1 and normalized it to the ester band at 1670 cm1. A medium pressure mercury lamp equipped with an optical waveguide was used to induce the photopolymerisation (light intensity on the surface of the sample of about 10 mW/cm2). All the polymerisation reactions were performed at room temperature at constant humidity (RH = 25–30%). 2.5. Techniques The samples, after irradiation, were stored for one night at 50 °C before characterising them. The gel content of the cured products was determined by measuring the weight loss after 24 h of extraction at room temperature with chloroform according to the standard test method ASTM D2765-84. X-ray powder patterns were collected with a Philips XPert PW3710 powder diffractometer using a Cu Ka radiation source. Thermogravimetric determinations were carried out in an air flow by a NETZSCH STA 449 Jupiter thermal analyser connected to a NETZSCH TASC 414/3 A controller at a heating rate of 10 °C min1. TEM analysis was carried out by an HRTEM JEOL, JEM2010 high resolution transmission electron microscope with an operating voltage of 200 kV. Ultrathin sections of about 100 nm were cut by a Power TOME X microtome equipped with a diamond knife and placed on a 200-mesh

Gels of exfoliated ZrP in water can be prepared through intercalation–deintercalation of propylamine in aqueous solution; water can then be replaced by a large variety of organic solvents without significant alteration of the degree of exfoliation of ZrP [7]. This allows to obtain easily ZrP gels in organic solvents which are miscible with water. If one wants to prepare ZrP gels in solvents that are immiscible with water, it is necessary to prepare an intermediate gel in a solvent which is miscible with both water and the final solvent. The gel solvent can also be a liquid monomer. In this case the gel can be polymerised and form nanocomposites containing ZrP, if the original degree of exfoliation is retained upon polymerisation. We followed this strategy in the present work. First ZrP gels in water were prepared as described in the experimental section; functionalised ZrP was also prepared, by addition of aminoalcohols NH2(CH2)nOH, with n = 3, 4, 5 (ZrPAn). Gels of 3 wt% ZrP or ZrPAn in BCDE (which is immiscible with water) were then prepared by replacing water with THF in the starting gel and THF with BCDE in the intermediate gel. The gels thus obtained could be cured in the presence of a photoinitiator by UV irradiation. BCDE is a reactive monomer that can homopolymerise via a cationic mechanism. This mechanism is widely discussed in the literature [30]: the initiation depends on Brönsted acids whose generation from a photoinitiator (as the one used in this study) involves complex reactions; the propagation is the addition of the epoxy monomer onto the initial protonated species. Chain transfer reactions occur when hydroxyl groups are present in the form of alcohols, polyols, water or humidity [25,26]. In a chain transfer reaction the hydroxyl attacks a carbon atom bonded to a positively charged epoxy function, binds to the polymeric chain thus blocking its growth and releases a proton which can catalyze the growth of a new chain (Fig. 2). As a consequence, shorter polymer chains are formed, the resulting network is more flexible and the photopolymerisation kinetics faster. The photopolymerisation of BCDE and its gels containing ZrP or ZrPAn was followed by infrared spectroscopy, monitoring in real time, during the irradiation, the decrease of the epoxy group concentration. The experimental curves of the conversion as a function of irradiation time are plotted in Fig. 3. The curve concerning the neat BCDE shows that the rate of photopolymerisation is very high at the beginning of the process, then it diminishes and the conversion reaches a nearly constant value of 40% in

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The BCDE gels were added of 2% of photoinitiator; then they were coated onto a PET substrate using a 2 mm wirewound applicator and exposed to UV irradiation. The curing reaction was performed under air with a medium pressure Hg lamp (Italquartz, Milano, Italy), having a light intensity I = 25 mW/cm2 at the sample level. The mixtures were exposed for 2 min to UV light.

3. Results and discussion

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Fig. 2. Mechanism of the chain transfer reaction for a generic RC2H3O epoxide; R0 is the growing polymer chain.

2 min. Notwithstanding the limited conversion, the polymer obtained is not tacky and is highly crosslinked as its insoluble fraction is quite high, around 88%. These results are in agreement with the literature: differential scanning calorimetry (DSC) and real time infrared spectroscopy data show that the conversion of BCDE is uncomplete and generally is less than 50% [31]. As a cycloaliphatic epoxide, BCDE is more reactive in cationic polymerisation than open-chain based epoxides, which in turn are more reactive than glycidyl ethers and glycidyl esters [32,33]. The stop of the conversion was explained by Scott, in his work on cationic polymerisation of vinyl esters [34]: he stated that the photocuring process proceeds until vitrification, when the material reaches a glass transition temperature close to the photocuring temperature. Examining the conversion curves of BCDE when either neat or functionalised ZrP are present (Fig. 3), it is evident that the photopolymerisation of BCDE is faster and the conversion of the gels is about 10–20% higher compared to the conversion of neat BCDE. The photopolymerisation kinetics is therefore affected by the filler and by the type of organic modifier. The reasons can be many. First of all the acidic groups of the filler surface can enhance the ring opening of the epoxide. Moreover, on the basis of past results on the photopolymerisation of epoxides in the presence of other inorganic fillers such as sodium montmoril-

lonite [23,35] and silica [36], we can suggest that also in this case the filler takes part of chain transfer reactions through its alcoholic or acidic OH groups. From Fig. 3 one can also observe that the initial photopolymerisation rate of the BCDE/ZrPAn gels changes according to the sequence: A5 > A4 > A3. Interestingly, the same sequence, but with higher conversions, was found for mixtures of BCDE and n-alkanols (CnH2n+1OH, with n from 3 to 5) where the alkanol amount, in moles, was the same as the aminoalcohol amount in the corresponding BCDE/ZrPAn gel (Fig. 4). The similar kinetic behaviour exhibited by the BCDE/ZrPAn gels and by the BCDE–n-alkanol mixtures indicate that also in the gels the alcoholic OH groups contribute significantly to the chain transfer reactions. As to the slower photopolymerisation rate of the gels compared to the alcohol–BCDE mixtures, an obvious explanation lays in the different viscosity of the systems and different mobility of the reactive species. Besides that, it can be pointed out that aminoalcohols are basic enough to deprotonate the monohydrogen phosphate group of ZrP [37]: the surface of the filler particles bears basic „PO groups whose negative charge is balanced by the protonated aminoalcohols. Therefore, in the presence of the aminoalcohol functionalised ZrP, protons which catalyze the polymerisation reaction may bind to the „PO groups of the filler surface, thus breaking the ionic bond between „PO and ANH3+. On the basis of these considerations, the aminoalcohol functionalised ZrP appears to have a twofold role: if on one hand it contributes to the formation of shorter polymer chains through the chain transfer mechanism, on the other hand the protonation of the surface sites decreases the catalyst concentration thus making the reaction kinetics of the modified gels slower in comparison with the mixtures of BCDE and free alcohols. 3.2. Nanocomposite characterisation

Fig. 3. Photopolymerisation kinetic curves for neat BCDE and for BCDE/ ZrP and BCDE/ ZrPAn gels.

In order to characterise the composites obtained upon irradiation, 100 lm thick samples were prepared. After a 2 min irradiation, they were subjected to a 24 h treatment at 50 °C: at the end of this annealing the insoluble fraction was found equal to 99–100%. This treatment was necessary as the BCDE photopolymerisation is essentially non-terminating and the active centers formed are long living, therefore there is a significant post-curing: Scranton also

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showed that these long-living species can cause ‘shadow curing’, i.e., they can lead to the polymerisation of BCDE monomers that were never exposed to light [38]. Both post-curing and shadow curing can be enhanced by increasing the temperature, but it is proved that these processes take place also at 30–40 °C [38,39] or at temperatures close to the photocuring temperature. The composite samples obtained were homogeneous and transparent. Fig. 5 shows TEM pictures of a ZrP based composite. The ZrP particles are randomly dispersed in the photopolymerised BCDE matrix and have thickness of few nanometres with an extremely high aspect ratio. As the thickness of a single ZrP layer does not exceed 1 nm, the ZrP particles consist of stacks of few layers. Therefore the nanocomposite can be defined as partially exfoliated. Further information on the filler exfoliation is obtained by comparing the X-ray powder patterns of a sample (A) prepared from a BCDE/ZrP gel with the pattern of a sample (B) obtained from a physical mixture of BCDE and the same amount of ground microcrystalline ZrP (3 wt%). In Fig. 6 the pattern of sample B shows sharp characteristic reflections of ZrP at 2h = 11.6° and 25.3° [40,41] besides the broad signal centred around 2h = 17.5°, which is due to the polymeric matrix. In particular, the reflection at 2h = 11.6° arises from the stacking of the a-layers with an interlayer distance, d, of 0.76 nm. This reflection is missing in the pattern of sample A, obtained from the gel: here a weak peak appears at 2h = 8.9° (d = 0.98 nm) which may be ascribed to BCDE intercalated ZrP. On the basis of the width at half peak height, the thickness of the ZrP particles is estimated by using the Debye–Scherrer formula to be around 9 nm, in qualitative agreement with the TEM picture. On the other hand, the X-ray powder pattern of the ZrPA5 composite (Fig. 6) do not show any filler reflection thus indicating that the aminoalcohol favours the filler dispersion in the organic solvents used for the gel preparation. The thermal behaviour the cured BCDE oligomer and its ZrP based composites was investigated by DMTA analysis and thermogravimetric determinations. DMTA experiments allowed to determine Tg values from the position

Fig. 5. TEM pictures for a sample of cured BCDE containing 3 wt% exfoliated ZrP.

Fig. 6. X-ray powder patterns of composites prepared from gels of BCDE/ ZrP (A), BCDE/ZrPA5 (C) and from a physical mixture of BCDE and 3 wt% ZrP (B).

of the maximum of the tan d vs. temperature curves. The spectra describing the cured BCDE and its homologue containing ZrPA3 are reported in Fig. 7, while all the Tg values

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Fig. 4. Photopolymerisation kinetic curves for mixtures of BCDE and the indicated n-alkanols.

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are collected in Table 1. The presence of the filler decreases the Tg from 212 °C for neat BCDE to 187 °C for BCDE/ZrP, while the composites containing functionalised ZrP have Tg values in the range 170–181 °C. These values are close to those of samples prepared from the BCDE–n-alkanol mixtures: the flexibilisation effect due to the functionalised filler is thus comparable with that arising from alkanols. This is a further confirmation that the filler interacts with the growing cationic species through the alcoholic

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Fig. 9. Temperature shift (T) of the weight loss curves of cured BCDE/ ZrPand BCDE/ZrPAn gels referred to the curve of cured BCDE as a function of the number of carbon atoms of the aminoalcohols. DT is the shift corresponding to 20% weight loss.

Fig. 7. Log E0 and tan d as a function of temperature for cured BCDE and for the cured BCDE/ZrPA3 gel.

Table 1 Glass transition temperatures for photopolymerised BCDE, for its composites with ZrP and its mixtures with n-alkanols. Sample

Tg [°C]

BCDE BCDE/ZrP BCDE/ZrPA3 BCDE/ZrPA4 BCDE/ZrPA5 BCDE + C3H7OH BCDE + C4H9OH BCDE + C5H10OH

212 187 170 171 184 178 192 182

function and acts as a chain transfer agent, reducing the chain length and enhancing the epoxy conversion. The presence of the nanosized filler resulted in all cases in the shift towards higher temperatures of the thermal decomposition of the polymer. As an example Fig. 8 shows the weight loss in air of composites containing ZrP and ZrPA5. The shift to high temperature of the thermogravimetric curves of the composites referred to the curve of cured BCDE is maximum, in all cases, for a weight loss around 20%. Fig. 9 shows that the temperature shift corresponding to 20% weight loss increases with the number of carbon atoms of the aminoalcohol chain and reaches a maximum of about 40 °C for NH2(CH2)5OH. It can be pointed out that no shift was observed for the samples obtained from BCDE and alcohol mixtures where the amount of alcohol is equal to the amount of aminoalcohol used in the composites to functionalise ZrP. Thus the enhanced thermal stability of the composites must be ascribed to the barrier effect towards oxygen diffusion determined by the presence of high aspect ratio filler particles. Moreover, the dependence of the temperature shift on the number of carbon atoms of the aminoalcohols appears to be a consequence of the fact that the increasing length of the alkyl chain favours the filler dispersion in the organic solvents used for gel preparation and its retainment upon polymerisation. 4. Conclusion

Fig. 8. Weight loss curves for cured BCDE and for cured BCDE/ZrP and BCDE/ZrPA5 gels.

UV-curing of gels of exfoliated ZrP in BCDE allowed the preparation of polymerised-BCDE/ZrP nanocomposites. In comparison with the neat polymer, the presence of the exfoliated filler makes the photopolymerisation kinetics faster, enhances the thermal stability and lowers the glass transition temperature of the composites. These changes are even greater when ZrP is functionalised with aminoalcohols. The filler influence on the photopolymerisation kinetics and on the physical properties of polymerised BCDE is consistent with the flexibilisation of the polymer network induced by the occurrence of chain transfer reactions.

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