Novel way to control PDMS cross-linking by gamma-irradiation

Available online at www.sciencedirect.com NIM B Beam Interactions with Materials & Atoms Nuclear Instruments and Methods in Physics Research B 266 (2008) 1105–1108 www.elsevier.com/locate/nimb Novel way to control PDMS cross-linking by gamma-irradiation S.R. Gomes a,*, F.M.A. Margacßa a,*, D. Faria Silva a, L.M. Ferreira a, I.M. Miranda Salvado b, A.N. Falcão a b a Department of Physics, Nuclear and Technological Institute, E.N. 10, 2686-953 Sacavém, Portugal Department of Glass and Ceramics Engineering, CICECO, Aveiro University, 3810-193 Aveiro, Portugal Received 8 December 2007; received in revised form 27 February 2008 Available online 18 March 2008 Abstract Using a 60Co gamma source polydimethylsiloxane-based materials have been prepared by gamma-irradiation of a mixture of polydimethylsiloxane, PDMS, silanol-terminated and the tetraethylortosilicate, TEOS, with no other chemicals. Samples were prepared with varying TEOS concentrations. The obtained materials are monolithic, flexible and transparent. X-ray measurements have shown them to be amorphous. The thermal behaviour has also been investigated by using differential scanning calorimetry and thermal gravimetric analysis. The results suggest that TEOS have a tailoring effect on the conformation of the polymer chains, affecting the network formation, as shown by the swelling behaviour of the material. Ó 2008 Elsevier B.V. All rights reserved. PACS: 81.05.t; 81.05.Lg; 61.25.Hq; 61.82.Pv; 68.60.Dv Keywords: PDMS network; Gamma-irradiation; Thermal analysis; Swelling degree 1. Introduction Polydimethylsiloxane (PDMS) or silicone rubber is the most widely used silicon-based organic polymer. It is optically clear and is generally considered to be inert, non-toxic and non-flamable. PDMS can be cross-linked into networks. It has many different uses. In particular, it is used as a sealant elastomer in different applications. PDMS is also used as a dense separative layer for volatile organic components extraction by selective permeation through membranes [1]. The elastomer has some special features which make it an interesting membrane material for organic extraction: high hydrophobicity, high permeability, good thermal and mechanical resistance and easy manufacture [2]. Membrane transport properties depend very much on the micro-structure of the polymer network and there* Corresponding authors. E-mail addresses: susanag@itn.pt (S.R. Gomes), fmargaca@itn.pt (F.M.A. Margacßa). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.046 fore on its preparation conditions. In this paper, we focus our attention on the thermal and swelling properties of PDMS materials cross-linked in different conditions. PDMS has also been widely used to prepare hybrid inorganic-organic materials with applications ranging from waveguides and anti-corrosion coatings to biomaterials. Most of the obtained hybrids were prepared via the sol– gel method [3]. Recently the authors have prepared hybrid materials through gamma-irradiation of the precursors PDMS and silicium and zirconium alkoxides [4–6]. The cross-linking of polymer chains can be promoted by many different ways, one of them being gamma-ray irradiation [7]. Previous work has been done in this area of research by different authors [4,5,8,9]. As to the mechanism of PDMS cross-linking it is believed that the irradiation fractures Si–C, C–H and Si–O bonds, since they all have similar bond energies. However because no oxygen was detected in the evolved gases during irradiation the fracture of the Si– O bond must be immediately followed by recombination [6,10]. 1106 S.R. Gomes et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1105–1108 2.1. Sample preparation Tetraethylortosilicate (TEOS), purity 99%, from Riedelde Haën and polydimethylsiloxane (PDMS), silanol terminated, p.a. reagent, from ABCR – Speciality Chemicals for Research, Development and Production, with average molecular weight, Mw = 43 500 g/mol, referred as S33, were used as raw materials to prepare samples with compositions xPDMS (100-x)TEOS, x in wt%. All samples were prepared with 1.00 g of mass. The sample irradiation was carried out at the 60Co irradiation facility, Radiation Technology Unit, UTR [11]. The description of the preparation method is given elsewhere [5]. All samples were irradiated with a dose of ca. 450 ± 20 kGy, slightly above the minimum dose necessary to reach gelation of these samples [5]. 2.2. Sample characterization A Sartorius electronic analytical balance, with error ±0.01 g, was used to determine the mass of precursors in the preparation of samples and the mass of the obtained materials. The atomic structure of samples was characterized by X-ray diffraction using Cu Ka = 1.5418 Å in a Rigaku, Geigerflex instrument at the University of Aveiro. The thermal properties of the materials prepared by gamma-irradiation were analysed by thermogravimetry (TGA) and differential scanning calorimetry (DSC) techniques. The measurements were carried out in a nitrogen atmosphere, ranging from 25 °C to 700 °C, at a rate of 10 °C/min, using equipment from TA Instruments (TGAmodel 951; DSC-model 910), at the Polymer Characterisation Laboratory of Nuclear and Technological Institute, ITN. Swelling experiments were performed using toluene, purity 99.5%, p.a. reagent, from Merk, that acts as a good solvent for PDMS. Samples were immersed in the solvent during 24 h at a temperature of 24 °C. The sample mass values, before (m0) and after swelling (m), were measured and used to calculate the swelling degree, Q [12]: Q¼ m  m0 ; m0 q ð1Þ where q is the density of the solvent (q = 0.87 g cm3). In this way the swelling degree which represents physically 3. Results and discussion The samples, as prepared and after drying in air at room temperature, Tamb, are monolithic, transparent and flexible materials. Fig. 1 represents the mass of PDMS and PDMS + TEOS samples during the drying stage. It is observed that the mass of these samples, after the drying stage, approaches the mass of the polymer used in their preparation. Thus, most of TEOS evaporated and only a very small part was retained in the dried sample. X-ray diffraction was carried out in dried samples and some of the results are shown in Fig. 2. All samples are amorphous with two broad peaks at small 2h in both PDMS and PDMS + TEOS systems, as indicated by arrows in the figure. These peaks, revealing short-range order, are characteristic of amorphous structures, whose elementary units are randomly positioned for long distances but ordered locally, at nearest neighbours. PDMS 1.0 1.0 80% PDMS + 20% TEOS 0.8 M dried (g) 2. Experimental the volume occupied by the solvent in the network is determined. 0.8 0.6 0.6 50% PDMS + 50% TEOS 0.4 0.4 20% PDMS + 80% TEOS 0.2 0.2 0 10 20 30 40 50 60 70 Drying time (day) Fig. 1. Sample mass as a function of drying time for xPDMS(1-x)TEOS, gamma-irradiated (450 kGy). 1000 1 2 3 Intensity (a.u.) In the present work this method was used to prepare materials from a mixture of polymer and silicium alkoxide, with no addition of any other chemical compound. The precursors that have been used are PDMS, silanol terminated and tetraethylortosilicate, TEOS. This paper reports the investigation of the thermal behaviour and the swelling degree of the prepared materials. PDMS 50% PDMS+50% TEOS 20% PDMS+80% TEOS 500 1 2 3 0 0 10 20 30 40 50 60 70 80 90 2 theta (°) Fig. 2. X-ray diffractograms from PDMS and PDMS + TEOS samples. 1107 S.R. Gomes et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1105–1108 Heat Flow (W/g) -0.2 PDMS before irradiation PDMS irradiated 80%PDMS+20%TEOS 20%PDMS+80%TEOS -0.4 -0.6 -0.8 0 100 200 300 400 500 Temperature (°C) Fig. 3. DSC plots from PDMS and PDMS + TEOS samples. To investigate the prepared materials as to structural transitions as well as their weight loss as a function of temperature, all samples were studied by DSC and TGA. Fig. 3 shows DSC curves obtained from PDMS before irradiation, PDMS gamma-irradiated and 80%PDMS + 20%TEOS and 20%PDMS + 80%TEOS to represent the system PDMS/TEOS. The DSC curves present one exothermic peak at Texo 6 300 °C for all samples and show different behaviour in the temperature range T P 400 °C, for which the materials degrade producing no stable product. The exothermic peak area is proportional to the liberated energy, its shape being narrower or broader, depending on the freedom and flexibility of the PDMS chains for each structural reorganization. In case the system is very rigid (e.g. for very organized materials or high reactional mixtures) the peak is narrow around a certain temperature. On the other hand if the polymer chains have freedom enough to re-organise then the peak is broad, the reorganization progresses without the need to overcome large energy barriers. The former seems to be the case of sample with 80%PDMS + 20%TEOS whereas the latter is that of 20% PDMS + 80%TEOS. For the same materials the TGA plots have been obtained. Some of the collected curves are shown in Fig. 4. The DSC and TGA curves (not shown) for interme- diate % PDMS samples presented a similar behaviour to the other studied samples of the system PDMS/TEOS. The TGA curves are similar for all samples at the respective Texo values, showing no weight loss. Therefore, the DSC exothermic peak is attributed to structural changes involving spatial ordering of the polymer chains although not extending to the whole volume of the sample as shown by XRD. The temperature associated to the crossover that shows the beginning of the accelerated weight loss, here designated by thermal rupture temperature, is related with the polymer degradation. This temperature is observed to increase with the TEOS content. The TGA residual mass is always smaller than 15%. Fig. 5 shows both the variation of the temperature of thermal rupture and the variation of the position of the exothermic peak temperature, Texo, as a function of the TEOS/PDMS ratio. The temperature necessary to promote the thermal rupture increases, by 50 °C, when the samples are prepared with TEOS in comparison with that of irradiated PDMS. However this temperature remains approximately constant with the TEOS content. Since in all samples, regardless of composition, TEOS evaporates almost completely during the drying stage, the increase in the temperature of thermal rupture seems to be related to the increasing strength of the network of cross-linked polymer. Therefore, the development of this network is conditioned by the presence of TEOS during the irradiation processing of the materials. The variation of the Texo, that is associated to PDMS chain ordering, occurs at T < 300 °C and its value decreases as the TEOS content increases. This shows that TEOS facilitates the spatial ordering of the polymer chains in the sample volume. As the system is only composed by TEOS and PDMS, increasing TEOS means that a smaller quantity of PDMS chains is present in the sample volume. This fact allows more freedom for the motion of the polymer chains leading to an easier rearrangement. Since the irradiation occurs while TEOS is present in the sample, it is expected 480 Temperature (°C) 100 Weight (%) 80 60 40 440 400 280 260 240 20 PDMS before irradiation PDMS irradiated 80%PDMS+20%TEOS 20%PDMS+80%TEOS 0 0 100 200 220 300 400 500 600 700 Temperature (°C) Fig. 4. Thermograms from PDMS and PDMS + TEOS samples. Thermal rupture Exothermic peak 0.0 0.1 1 TEOS/PDMS Fig. 5. Variation of the exothermic peak position and the temperature of thermal rupture for different sample compositions. 1108 S.R. Gomes et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1105–1108 4. Conclusions 6.0 Swelling Degree 5.5 5.0 4.5 4.0 3.5 3.0 0 1 2 3 4 TEOS/PDMS Fig. 6. Variation of the swelling degree with the TEOS/PDMS ratio. that even after TEOS evaporation (which occurs during the sample drying), the system keeps memory of its earlier conformation in the presence of TEOS. To further investigate these results, experiments of swelling were performed in the samples. The polymer swelling behaviour in a good solvent is indicative of the crosslinking density of the polymer network, the swelling increasing with the decrease of the cross-linking. Fig. 6 shows the results obtained for the present samples. The swelling degree shows a critical point at TEOS/ PDMS 6 1/4. Bellow this point, in the case of low TEOS content in the sample, the polymer swelling decreases with the addition of TEOS. This means that TEOS contributes to the cross-linking of the polymer chains. The role of cross-linking agent of TEOS in PDMS hybrids prepared by the sol–gel route is well known from the literature, e.g. [3]. Above the critical point, the network swelling increases with TEOS. In this case, for larger TEOS/PDMS ratios, TEOS has a dilution effect that surpasses its crosslinking ability. These results show that the addition of TEOS–PDMS provides a new effective way to control PDMS cross-linking in samples prepared by gamma-irradiation. This has the advantage that after irradiation almost all TEOS evaporates leaving the cross-linked PDMS network as the only component in the prepared material. Materials of the system TEOS/PDMS prepared with different relative compositions using gamma-irradiation have been investigated as to mass loss in the drying stage, X-ray Diffraction, thermal properties and swelling. It was found that, although the majority of TEOS evaporates after irradiation, nevertheless its presence has a major impact in thermal properties as well as in the swelling properties of the material prepared by this method. The latter showed that the polymer cross-linked network strongly depends on the TEOS content present during the irradiation stage. This provides a new approach to control PDMS cross-linking by gamma-irradiation. Acknowledgements The authors wish to acknowledge support from the Portuguese Foundation for Science and Technology, FCT, in the framework of project POCTI/CTM/44150/ 2002. References [1] P. Côté, C. Lipski, in: R. 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