European Polymer Journal 46 (2010) 397–403
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Influence of nanoparticles on elastic and optical properties of a polymeric matrix: Hypersonic studies on ethylene–vinyl alcohol copolymer–titania nanocomposites
a b c
Instituto de Ciencia de Materiales de Madrid, CSIC, Campus Cantoblanco s/n, E-28049 Madrid, Spain Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049 Madrid, Spain Instituto de Ciencia y Tecnología de Polímeros, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 16 February 2009 Received in revised form 5 November 2009 Accepted 9 November 2009 Available online 14 November 2009 Keywords: Polymer composite materials Elastic properties Refractive index Brillouin spectroscopy Semicrystalline polymers Nanoinclusions
a b s t r a c t High resolution Brillouin spectroscopy (HRBS) backscattering elastic data in nanocomposites of ethylene–vinyl alcohol copolymer (EVOH) and TiO2 nanoparticles present anomalous dependence with concentration, while Young’s modulus and microhardness data show the expected behaviour. When performing HRBS with the 90A scattering geometry to asses the effective elastic constant, the expected behaviour for low concentration of TiO2 nanoparticles is again obtained. This unusual disagreement can be solved assuming that the inclusion of TiO2 nanoparticles induces anomalous refractive index behaviour at the applied laser wavelength for the different EVOH–TiO2 nanocomposites. Comparison with experimental elastic and optical data obtained in isotactic-polypropylene–TiO2 nanocomposites proves that EVOH–TiO2 nanocomposites show an unusual optical behaviour at the laser wavelength, possibly due to a singular bonding between the EVOH polymer and the TiO2 nanoparticles. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Hybrid or nanocomposite organo–inorganic materials that combine attractive qualities of dissimilar components have received immense interest for a wide range of mechanical, electronic, magnetic, biological, and optical properties. The control of nature and properties from surface/interface interactions between components appears of prime importance in the development of advanced materials having novel functionalities out of those characteristic of the components [1]. Modification of food packaging polymeric matrices to prevent growth or reduce adhesion of detrimental microorganisms is a highly desired objective. Hence, there is a significant interest in the development of antimicrobial * Corresponding author. E-mail address:
[email protected] (R.J. Jiménez Riobóo). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.11.009
materials for application in the health and biomedical devices, food and personal hygiene industries [2]. TiO2 is an inert, non-toxic and cheap material with a potential activity against all kind of microbes. Anatase and rutile are the two main crystalline structures of TiO2. A crystallographic research [3] demonstrated that anatase-type TiO2 has {0 1 1} and {1 1 0} crystal faces, and that rutile-type one has {0 0 1} and {0 1 1} faces. It was suggested that the different crystal faces make varied roles as oxidation and reduction sites. Concerning the photocatalytic reactivity, it was reported that anatase-type TiO2 exerts higher activity than rutile-type one in many photocatalytic reactions in air and water [4–7]. Moreover, under light excitation, with energy above the TiO2 band gap (3.2 eV), the formation of energy rich electron–hole pairs is produced. Once at the surface of the material, such charge carriers are able to interact with micro-organisms rendering biocidal properties to the TiO2-containing nanocomposite materials. The
MACROMOLECULAR NANOTECHNOLOGY
R.J. Jiménez Riobóo a,*, A. De Andrés a, A. Kubacka b, M. Fernández-García b, M.L. Cerrada c, C. Serrano c, M. Fernández-García c
MACROMOLECULAR NANOTECHNOLOGY
398
R.J. Jiménez Riobóo et al. / European Polymer Journal 46 (2010) 397–403
control of the TiO2 polymorphism (i.e. to ensure the presence of the anatase phase) and primary particle size of these inorganic nanoparticles, yields to a correct handling of the UV-light matter interaction, ultimately leading to an optimized photo-activity in the elimination of microorganisms [8]. The influence of TiO2 nanoinclusions on the acoustic and optical phonons in ethylene–vinyl alcohol copolymer EVOH–TiO2 nanoparticle composites (from now on nanocomposites) for their application in the food packaging industry, previously studied [9,10], can deliver hints on the interactions between polymeric matrix and inorganic nanoparticles. In this work, a combination of different experimental techniques delivering elastic, optical and mechanical information has been used in order to clarify whether the anomalous optical behaviour already observed in EVOH–TiO2 nanocomposites [10] is either real or only a possible artifact of the applied experimental techniques. Moreover, comparison to isotactic-polypropylene i-PP–TiO2 nanocomposites will be drawn, reinforcing the anomalous optical behaviour of EVOH–TiO2 nanocomposites. 2. Experimental The TiO2 component was prepared using a microemulsion synthetic route by addition of titanium (IV) isopropoxide (Aldrich) to an inverse emulsion containing an aqueous phase dispersed in n-heptane (Panreac), using Triton X-100 (Aldrich) as surfactant and 1-hexanol (Aldrich) as cosurfactant [1]. A commercially available ethylene–vinyl alcohol copolymer (EVOH; Kuraray), containing a nominal 71 mol% vinyl alcohol content, was used as polymeric matrix in the preparation of EVOH–TiO2 nanocomposites with different TiO2 nanoparticle contents: 0.25, 0.5, 1, 2, 5, 10 and 13 wt.%. These novel materials were prepared through melt processing in a shear mixer prototype with a volumetric capacity of 3 cm3 at 195 °C and at 60 rpm for 5 min. Films of the nanocomposites were prepared by compression moulding at a pressure of 1.5 MPa for 5 min in a Collin Press (Model 3912) between hot plates at 210 °C for dynamic mechanical thermal analysis and microhardness measurements. A quench was applied to the different films from the melt to room temperature. The question about the dispersion of the TiO2 nanoparticles in the EVOH matrix has been already discussed [9]. In the case of isotactic-polypropylene–TiO2 nanocomposites, a commercially available metallocene-catalysed isotactic-polypropylene, i-PP (Basell Metocene X50081: melt flow index of 60 g/10 min at 230 °C/2.16 kg, ASTM D1238), meeting FDA requirements for food contact (Federal Regulations, 21 CFR 177.1520), is used as polymeric matrix. A polypropylene wax partially grafted with maleic anhydride, PP-g-MAH, is used as interfacial agent (LicoÒ mont AR 504 fine grain supplied from Clariant). The compatibiliser composition added is of 80 wt.% with respect to the content in TiO2 nanoparticles. These biocidal nanocomposites are prepared at 160 °C and at 60 rpm for 5 min and later moulded at 175 °C. The TiO2 nanoparticle contents studied were: 0.5, 1, 2 and 5 wt.%.
Raman spectroscopy and X-ray diffraction data confirmed that the nanoparticles were exclusively of the anatase-type (a polymorph of Rutile and Brookite, other forms for TiO2 to be found in nature, and showing tetragonal symmetry) and had a primary particle size of 10 nm [9]. Absorption spectra were recorded in transmission with a Cary 4000 UV–vis spectrophotometer from Varian. Photoluminescence measurements were performed at room temperature with different laser lines of an Ar+–Kr+ laser, a Jobin–Yvon HR 460 monochromator and a N2 cooled CCD. The excitation light was focused on the sample with an Olympus microscope (except for UV laser lines) which was also used to collect the scattered light. Spectra are corrected by the instrumental function recorded with a calibrated white source and a CaF2 pellet. High resolution Brillouin spectroscopy (HRBS) is used to study the acoustic phonons’ behaviour. HRBS is a very suitable experimental technique to evaluate elastic properties in polymer systems [11]. The Brillouin scattering (HRBS) experiment is performed using as light source a 2060 Beamlok Spectra Physics Ar+ ion laser (k0 = 514.5 nm). The scattered light was analysed using a Sandercock-type 3 + 3 tandem Fabry–Pérot interferometer [12]. No polarization analysis of the scattered light was made. The inclusion of TiO2 nanoparticles in the EVOH copolymer or i-PP homopolymer matrices results in certain opacity since the samples thickness is ca. 200 lm. In this context the backscattering geometry is the one suitable methodology to obtain information about the acoustic phonons using high resolution Brillouin spectroscopy. The propagation velocity of an acoustic wave (v) can be obtained from the acoustic phonon frequency (f) provided the acoustic wave vector (q) [11]:
v ¼ 2pf =q
ð1Þ
The magnitude and direction of the acoustic wave vector is fixed by the scattering geometry used. In the case of backscattering geometry,
q ¼ 4pn=k0
ð2Þ
where n is the refractive index of the sample and k0 is the laser wavelength in vacuum. In order to avoid the direct reflection entering the spectrometer in the backscattering measurements, the sample was inclined so that the incident beam was tilted some degrees off the surface normal. In the case of transparent samples the 90A scattering geometry is the one usually applied, being
q ¼ 4psinðp=4Þ=k0
ð3Þ
and the wave vector lies within the sample plane [11]. It is very important to notice that the first one is refractive index (n) dependent while the second one is not. One step further is to obtain the value of the related elastic constant:
c ¼ q v2
ð4Þ
where q is the mass density of the sample. If the system studied can be assumed to be elastically isotropic this elastic constant corresponds either to c11 or to c44 depending on the light polarization state. Moreover the combination of the two scattering geometries lead, in the absence of acoustic dispersion, to the determination of the refractive
R.J. Jiménez Riobóo et al. / European Polymer Journal 46 (2010) 397–403
399
index of the sample at the laser wavelength (514.5 nm) [11]:
n ¼ f 180 =f 90A
0:5 ! 1 2
ð5Þ
where f180 and f90A stand for the backscattering and 90A scattering geometries related acoustic phonon frequencies, respectively. The assessment of the mass density of the different samples studied was performed by picnometry using mixtures of ethanol and carbon tetrachloride. For the theoretical density, in the case of EVOH–TiO2 nanocomposites, the relationship applied was:
1=q ¼ w1 =q1 þ w2 =q2
ð6Þ
where wi is the weight fraction of components. In the case of i-PP–TiO2 nanocomposites this relationship reads:
ð7Þ
where w1 and w2 is the weight fraction of i-PP and TiO2, respectively and q3 is the density of the interfacial agent. Viscoelastic properties were measured with a Polymer Laboratories MK II dynamic mechanical thermal analyser working in the tensile mode. The complex (loss and storage) modulus and the loss tangent (tan d) of each sample were determined at 3 Hz over a temperature range from 145 to 150 °C, at a heating rate of 1.5 °C/min. The specimens used were rectangular strips of about 4.5 mm wide, around 0.30 mm thick and over 16 mm long. A Vickers indenter attached to a Leitz microhardness tester was used to perform microindentation measurements undertaken at 23 °C. A contact load of 19.235 N and a contact time of 25 s were employed. Microhardness (MH) values (in MPa) were calculated according to the equation:
2
MH ¼ 2sinð68 ÞP=d
ð8Þ
where P (in N) is the contact load and d (in mm) length of the projected indentation area [13]. It should be noticed that all TiO2 nanoparticle concentrations in the text will be related to wt% if no other indications are explicitly given. 3. Results and discussion Fig. 1a shows the concentration dependence of the mass density for the studied EVOH–TiO2 and i-PP–TiO2 nanocomposites. The measured value of the mass density of the pure EVOH copolymer (1.1962 g/cm3) is similar to other values found in the literature [14–17]; therefore we have used the experimental value for an interpolation (squares in Fig. 1a). The experimental results show a much lower slope in the concentration dependence than the one expected assuming a non linear dependence (Eq. (6)) between the values of pure EVOH and TiO2. This fact can be interpreted in the sense that the surface of the TiO2 nanoparticles interact with the EVOH matrix, modifying the surrounding polymer matrix, thus lowering the average density of the composite. No such behaviour has been observed for the i-PP–TiO2 nanocomposites, that behave as expected (following Eq. (7)).
Fig. 1. (a) Experimental concentration dependence of the mass density (triangles) in EVOH–TiO2 nanocomposites (upper graph) and i-PP–TiO2 nanocomposites (lower graph) and linear interpolation between real EVOH (upper graph) value, real i-PP (lower graph) value and TiO2 literature one (squares). (b) Concentration dependences of the mass density (squares) and refractive index (circles) in EVOH–TiO2 nanocomposite samples. The dashed line is the result of a linear least squares fit to the experimental density data. This result was used to calculate the refractive index values using Eq. (10).
In the case of EVOH–TiO2 nanocomposites, these density values can be well fitted to a straight line that in turn can be used to make an estimation of the concentration dependence of the refractive index. As far as these samples present random distribution of nanoparticles and EVOH crystalline and amorphous phases, the Lorentz–Lorenz equation [18]
q ¼ q0 ðn2 1Þ=ðn2 þ 2Þ
ð9Þ
can be used to calculate the refractive index for each concentration. The value for the pure EVOH sample was obtained using an Abbe refractometer being nD = 1.531 at room temperature. The concentration dependence of the refractive index can be obtained applying the expression:
n ¼ ð1 þ 3q=ðq0 qÞÞ0:5
ð10Þ
derived from Eq. (9), using the fitted values from the experimental density and assuming a linear interpolation for the specific refractivity, q0, between the EVOH and the TiO2 anatase measured values. The concentration dependence of the refractive index together with that of the experimental density is shown in Fig. 1b. There is no indication of TiO2 agglomerates formation, clearly detectable by SEM at TiO2 loading superior than 2% [9] (in the case of iPP–TiO2 nanocomposites no agglomeration of TiO2 nanoparticles are detected in the concentration range studied [19]). In principle, it is possible to asses a refractive index value for the TiO2 nanoparticles that turns out to be much lower than expected (TiO2 anatase value of n = 2.49 [20]),
MACROMOLECULAR NANOTECHNOLOGY
1=q ¼ w1 =q1 þ w2 =q2 þ ð0:8 w2 Þ=q3
MACROMOLECULAR NANOTECHNOLOGY
400
R.J. Jiménez Riobóo et al. / European Polymer Journal 46 (2010) 397–403
but its physical meaning is very questionable. A rather similar behaviour was experimentally observed using the immersion method for poly(vinyl-alcohol)–TiO2 nanocomposites at low TiO2 concentrations [21]. The concentration dependence of the elastic constant (ceff) (see Fig. 2a) of the EVOH–TiO2 nanocomposites can easily be obtained by using Eqs. (1), (2), (4), along with the experimental results for the mass density, the calculated refractive index data and the HRBS backscattering frequency data of Ref. [10]. Even though TiO2 elastic constants are higher than those of the EVOH copolymer, the effect observed is contrary to the expected one. The elastic constant softens when increasing the TiO2 nanoparticles content. There is a clear change in behaviour between 2% and 5% TiO2; this feature can be directly related to the appearance of TiO2 agglomerates as described previously [9], but the fact of a diminishing elastic constant with increasing TiO2 concentration cannot be explained. In a previous work [10], based only on backscattering frequency data, it was discussed the possibility of a competing effect between crystallisation degree of the semicrystalline copolymer and the insertion of TiO2 nanoparticles. The TiO2 nanoparticles limiting the EVOH crystal formation induces a lowering of the elastic constant of the compound until the agglomeration of the TiO2 nanoparticles at concentrations above 2% compensated this effect. In order to test this assumption, the degree of crystallinity and long spacing were also studied [9]. From those results no supporting arguments are found to maintain the above
Fig. 2. (a) Concentration dependence of the effective elastic constant in EVOH–TiO2 nanocomposite samples as obtained from the backscattering acoustic phonons. In this case this elastic constant corresponds with c11. (b) Microhardness experimental data (black circles) and estimated storage modulus data (grey circles) for the EVOH–TiO2 nanocomposite samples at room temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
hypothesis, since the most probable crystal size corresponding to the polymeric matrix, determined from the values of long spacing and crystallinity assuming a twophase model, increases with nanoparticle content. Therefore, other techniques to obtain information about the elastic or mechanical properties of the EVOH–TiO2 nanocomposites were used to compare to the HRBS backscattering data. Microhardness (MH) and storage modulus (E0 ) experimental results are depicted in Fig. 2b. Both techniques show very similar behaviours with the nanoparticle content. However, HRBS shows entirely different concentration behaviour as can be clearly seen from a comparison of Fig. 2a and b. This feature is very unusual because, in principle, all these experimental techniques should deliver similar information. A possible explanation for this peculiarity could be the difference in the measuring frequencies used by the different experimental techniques and the fact of the existence of a glass transition located at temperatures slightly above room temperature. The a-relaxation related to mechanical losses could play a key role. Temperature dependent measurements of the storage modulus for different samples are shown in Fig. 3. At room temperature, the amorphous regions of the polymeric matrix are in the glassy state. Even the losses related to the a-relaxation are rather negligible. Techniques using higher frequencies should find the losses-maximum shifted even to
Fig. 3. (a) Temperature dependence of storage modulus (E0 ) and loss tangent (tan d) for EVOH–TiO2 nanocomposite samples obtained at 3 Hz. The a-relaxation peak clearly lies above room temperature.
higher temperatures. It is then clear that there is no reasonable justification why at room temperature these techniques do not bring similar results even for very different measuring frequencies. Only for very low TiO2 nanoparticles content (below 2%) the samples with thickness about 200 lm are not opaque and the light absorption in the visible frequency range allows light transmission. Therefore, for the samples EVOH, 0.25%, 0.50% and 1% TiO2 amount, the 90A scattering geometry could be used (in the case of samples with 2% and 5%, the absorption plays an important role and the obtained results are not reliable). In this case we skip one difficulty because there is no need of knowing the refractive index of the samples to assess the related elastic constant (see Eqs. (3), (4)). Fig. 4a shows the 90A scattering values for the elastic constant. It is straightforward to distinguish that the 90A values follow the same tendency as the Young’s modulus and microhardness data presented in Fig. 2b. The elastic constant values for low TiO2 nanoparticle concentrations lie between the Reuss (series connection) and the Voigt (parallel connection) average limits. At very low concentrations the behaviour of the effective elastic constant is well described by a Reuss–Voigt–Hill average that is an arithmetical average of Reuss and Voigt averages. Fig. 4b presents the relative change (compare to the poly-
Fig. 4. (a) Concentration dependence of the effective elastic constant EVOH–TiO2 nanocomposite samples with low nanoparticle content obtained from the 90A scattering phonons. This elastic constant corresponds to c11. The lines represent the different calculated averages. (b) Comparison between the concentration dependence of the relative variation of the effective elastic constant and the relative variation of the microhardness. One corresponds to the pure EVOH value (pol). (c) Concentration dependence of the refractive index at the laser wavelength (514.5 nm) for EVOH–TiO2 nanocomposite samples with low nanoparticle contents (squares) and corresponding Lorentz–Lorenz refractive index values from experimental density data.
401
mer value) in effective elastic constant (from Fig. 4a) and microhardness (from Fig. 2b). The coincidence is very satisfactory, indicating the correctness of the 90A scattering geometry elastic data. The only indirectly estimated parameter for the calculations of the HRBS backscattering data (Fig. 2a) is the refractive index of the samples. Nevertheless, in absence of acoustic dispersion, as is the case at room temperature, and for an isotropic sample, as in a random oriented semicrystalline copolymer and nanoparticles, HBRS can deliver the refractive index (at the laser wavelength) of the sample studied. This is possible combining two different scattering geometries as backscattering and 90A scattering geometries [11] (see Eq. (5)). The obtained results can be observed in Fig. 4c. The increment in TiO2 nanoparticles content provokes a lowering of the refractive index of the EVOH–TiO2 nanocomposites, contrary to the Nussbaumer et al. findings [21]. Evidently, these experimental data are in contradiction with the simple assumption of the Lorentz–Lorenz formula validity which associates the mass density and refractive index. Moreover the decrement in n is extremely high (more than 5%). From the 90A scattering data the thermal heating effects do not seem responsible for this anomalous behaviour. In order to understand this unexpected effect, a possible influence of non linear optical effects in the backscattering experiment cannot be excluded. Indeed the diminishment of the refractive index values seems to be overstated. The optical activity of TiO2 nanoparticles, especially at UV wavelengths, is already known but such an anomalous behaviour of the refractive index (at visible laser wavelength) with the concentration is puzzling. The photoluminescence (PL) experiments at different laser excitation wavelengths, could give some insight into this phenomenon throughout the TiO2 concentration series. Preliminary PL experiments were discussed in a previous article [9]. The analysis of the PL emission as well as the dependence of its intensity vs. the TiO2 content for different excitation energies (Fig. 5a and b) reveals that the inclusion of TiO2 nanoparticles in the polymer generates new electronic states at the interface of both materials [9]. This is supported also by the large increase of the absorption in the visible range (see Fig. 5c) [10]. Two mechanisms originate this increase: light scattering due to clusters of TiO2 nanoparticles, which is important above around 2%, and the absorption of the combined TiO2–polymer states. In the visible region and in particular at 514.5 nm, in spite of the almost none absorption and emission of the separated components (polymer and TiO2 nanoparticles), the combination of both creates new electronic states and allows complex energy and charge transfer mechanisms that originate the observed PL and biocide effects [9]. Therefore, the optical properties cannot be understood considering a simple linear combination of those corresponding to the individual materials. The imaginary part of the refractive index, in this frequency range where strong absorption occurs, is large and therefore cannot be neglected. We cannot evaluate the contribution of the scattered light from the measured absorption coefficient; therefore, the spectra are not useful to obtain n(x) from a Kramers–Krönig analysis.
MACROMOLECULAR NANOTECHNOLOGY
R.J. Jiménez Riobóo et al. / European Polymer Journal 46 (2010) 397–403
402
R.J. Jiménez Riobóo et al. / European Polymer Journal 46 (2010) 397–403
MACROMOLECULAR NANOTECHNOLOGY
Fig. 6. High resolution Brillouin scattering spectra for EVOH copolymer and i-PP homopolymer showing backscattering and 90A scattering geometry related peaks.
Fig. 5. (a) Normalized PL intensity under 365 + 333 nm excitation for different TiO2 concentrations. (b) Normalized PL intensity under 514.5 nm excitation for different TiO2 concentrations at 521 and 588 nm. (c) Absorption at 514.5 nm for different TiO2 concentrations.
The PL intensity is very sensitive to the different radiative and non-radiative relaxation processes and its short and long time dependence would give valuable information. Further studies in this direction are planned. At this point we have observed an important decrease of the emission intensity when exciting at 514.5 nm that occurs in the scale of seconds. HRBS experiments have been also performed on isotactic-polypropylene–TiO2 nanocomposites, in order to contrast the hypothesis of anomalous optical behaviour observed in EVOH–TiO2 nanocomposites. Thorough information concerning their characterization has been already published [19]. Backscattering and 90A scattering geometries are also used in this case. Fig. 6 shows a comparison between the EVOH and i-PP spectra. The peaks belonging to the i-PP sample present a clearly larger width and in the case of the backscattering peak it is clearly two component. This is consistent with the fact of the large difference in density (consequently also in refractive index) between the crystalline and amorphous phases of i-PP (0.936 and 0.856 g/cm3, respectively) [22,23]. In our case the degree of crystallinity [22,23] is estimated to be 62.2% (density 0.9041 g/cm3). On the other hand, the peak belonging to the 90A scattering geometry cannot be resolved into two independent peaks, leading to only one effective elastic constant of the polymer. The combination of both scattering geometries allows the assessment of two refractive indices following Eq. (5). The evolution of the effective elastic constant and refractive indices with the TiO2 concentration is shown in Fig. 7a and b. Even though the elastic behaviour of both nanocomposites is very similar, the refractive index variation of both nanocomposites presents a completely diverging behaviour, showing an increasing in the refractive index value with TiO2 nanoparticle concentration in the crystalline as well as in the amorphous
Fig. 7. (a) Concentration dependence of the effective elastic constant of EVOH–TiO2 (down triangles) and i-PP–TiO2 (up triangles) nanocomposites with low nanoparticle content obtained from the 90A scattering phonons. This elastic constant corresponds to c11. (b) Concentration dependence of the refractive indices at the laser wavelength (514.5 nm) of EVOH–TiO2 (down triangles) and i-PP–TiO2 (up triangles) nanocomposites with low nanoparticle contents. The dashed lines are linear least square fits to the refractive index data.
phase of i-PP–TiO2 nanocomposites, contrary to the diminishing tendency observed in EVOH–TiO2 nanocomposites. This fact supports the abnormality of the optical behaviour of the EVOH–TiO2 nanocomposites and the different interaction of the TiO2 nanoparticles with different polymeric matrices. 4. Conclusions Clear discrepancies have been observed in the elastic (HRBS-backscattering) and mechanical behaviour of
R.J. Jiménez Riobóo et al. / European Polymer Journal 46 (2010) 397–403
Acknowledgements Dr. A. Kubacka and Ms. C. Serrano thank, respectively, the CSIC and MEC for I3P postdoctoral and PFU predoctoral grants. This work was supported by the CSIC under the projects PIF200580F0101, PIF200580F0102, PIF2005 60F0103 and PIF200560F0102.
References [1] Gómez-Romero P, Sánchez C. Functional hybrid materials. NY: Wiley; 2004. [2] Appendini P, Hotchkiss JN. Innovative Food Sci Technol 2002;3:113. [3] Ohno T, Sarukawa K, Matsumura M. New J Chem 2002;26:1167. [4] Nishimoto S, Ohtani B, Kaizaki H, Kagiya T. J Chem Soc Faraday Trans 1985;1:61. [5] Mills A, Sawunyama P. J Photochem Photobiol A 1994;84:305. [6] Sclafani A, Herrmann JM. J Phys Chem 1996;100:13655. [7] Lucarelli L, Nadtochenko V, Kiwi J. Langmuir 2000;16:1102. [8] Diebold U. Surf Sci Rep 2003;48:53. [9] Cerrada ML, Serrano C, Sánchez-Chaves M, Fernández-García M, Fernández-Martín F, de Andrés A, et al. Adv Funct Mater 2008;18:1949. [10] Jiménez Riobóo RJ, Serrano-Selva C, Fernández-García M, Cerrada ML, Kubacka A, Fernández-García M, et al. J Lumin 2007;128:851. [11] Krüger JK. In: Bässler A, editor. Brillouin spectroscopy and its application to polymers in optical techniques to characterize polymer systems. Amsterdam: Elsevier; 1989. [12] Sandercock JR. In: Cardona M, Guntherdot G, editors. Light scattering in solids III. Berlin: Springer-Verlag; 1982. p. 173. [13] Baltá-Calleja FJ. Adv Polym Sci 1985;66:117. [14] Mavridis H, Shroff RN. Polym Eng Sci 1992;32:1778. [15] Kamal MR, Garmabi H, Hozhabr S, Arghyris L. Polym Eng Sci 1995;35:41. [16] Faisant JB, Aït-Kadi A, Bousmina M, Deschênes L. Polymer 1998;3:533. [17] Huang CH, Wu JS, Huang CC, Lin LS. J Polym Res Taiwan 2004;11:75. [18] See for instance Ashcroft NW, Mermin ND. Solid state physics sounders. China: Saunders College Publishing I.E.; 1988. p. 542n. [19] Kubacka A, Ferrer M, Cerrada ML, Serrano C, Sánchez-Chaves M, Fernández-García M, et al. Appl Catal B Environ 2009;89:441. [20] Lide DR, editor. CRC Handbook of chemistry and physics. 82nd (2001–2002) ed.; London: CRC Press; 2001. p. 4. [21] Nussbaumer RJ, Caseri WR, Smith P, Tervoort T. Macromol Mater Eng 2003;288:44. [22] Dudi D, Djokovi V, Kostoski D. Polym Test 2004;23:621. [23] Mandelkemn L, Alamo RG. In: Mark JE, editor. Physical properties of polymers handbook. Woodbury, New York: Aip Press; 1996. p. 55.
MACROMOLECULAR NANOTECHNOLOGY
EVOH–TiO2 nanocomposites for low TiO2 concentrations. On the other hand, at low TiO2 nanoparticles concentration (below 2%, thus far from the light absorption) the elastic constant, as obtained by HRBS in 90A scattering geometry (n independent), shows similar concentration behaviour as observed by storage modulus and microhardness measurements, and seems to follow a Voigt–Reuss–Hill behaviour. The results have been analysed and an anomalous refractive index behaviour at the laser wavelength (514.5 nm) induced by the combination of EVOH and TiO2 nanoparticles possibly creating new electronic states has been found. The usual linear interpolations between literature values of EVOH and TiO2 mass density and refractive indices turned out to be inaccurate at the wavelength used. This anomalous behaviour can be induced by the laser wavelength used and can be a signature of the effects in the electronic states observed by photoluminescence. Experimental data (elastic constant and refractive indices) on isotactic-polypropylene–TiO2 nanocomposites do not show such anomalous behaviour in the refractive indices while the elastic constants behave similar to the EVOH–TiO2 nanocomposites one. These results support the statement of a singular bonding between the EVOH polymer and the TiO2 nanoparticles.
403
