J Sol-Gel Sci Technol (2010) 56:310–319 DOI 10.1007/s10971-010-2307-5
ORIGINAL PAPER
Polydimethylsiloxane/silica/titania composites prepared by solvent-free sol–gel technique Mihaela Alexandru • Maria Cazacu • Alexandra Nistor Valentina E. Musteata • Iuliana Stoica • Cristian Grigoras • Bogdan C. Simionescu
•
Received: 7 May 2010 / Accepted: 10 August 2010 / Published online: 21 August 2010 Springer Science+Business Media, LLC 2010
Abstract Composites based on polydimethylsiloxane incorporating silica and titania were prepared by mixing polydimethylsiloxane with proper oxides precursors, tetraethyl-orthosilicate and tetrabutyl-orthotitanate. In the presence of environmental humidity and in acid catalysis, hydrolysis/condensation processes take place with formation of oxides and concomitantly polymer crosslinking. Partial replacement of SiO2 in a polydimethylsiloxane/ silica composite with titania (both generated in situ by sol–gel process) affects surface hydrophilicity (evaluated by dynamic contact angle), water vapor sorption ability (determined by dynamic vapor sorption) and thermal stability. The dielectric properties are also controlled by composition. Keywords Composites Sol–gel technique Polysiloxanes Titania Silica
1 Introduction The obtaining of composite materials by sol–gel technique has been widely investigated [1–5]. As compared to other techniques, the sol–gel approach has several advantages: low costs, low temperature of heat treatment, unique ability to achieve molecular level uniformity in the synthesis of M. Alexandru (&) M. Cazacu A. Nistor V. E. Musteata I. Stoica C. Grigoras B. C. Simionescu ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania e-mail:
[email protected] B. C. Simionescu Department of Natural and Synthetic Polymers, ‘‘Gh. Asachi’’ Technical University of Iasi, Iasi 700050, Romania
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organic–inorganic composites, and strong adhesion of the coating to the substrate [6, 7]. Polydimethylsiloxane/SiO2 composites prepared by the reaction of polydimethylsiloxane (PDMS) and tetraethylorthosilicate (TEOS) have also been extensively studied [8–16]. These materials can be considered as ‘‘ceramic rubbers’’ depending on the TEOS/PDMS molar ratio. When PDMS concentration is increased, the final material presents rubber-like properties. For high TEOS concentrations, hard composites are obtained. The rubbery properties are dependent on various reaction parameters, such as temperature, acid concentration, reaction time, etc. The incorporation of different inorganic components, instead of or aside from TEOS, into the hybrid structure is usually carried out in order to improve the mechanical, thermal, and optical properties or to obtain new properties derived from the hybrid nature of the material [17–22]. Due to their interesting properties such as elasticity, insulating ability and easy processing, silicones are used in microelectromechanical systems (MEMS) where they play a structural role as protective layers, encapsulating elements, valves and diaphragms. However, by using active fillers, the dielectric properties of silicones can be modified [23, 24]. Titania has a high dielectric constant (e * 89) being of real interest for this purpose. In addition, it is an important inorganic functional material, with good physical properties, which make it suitable for thin film applications. Films containing TiO2 have been often used in microelectronic devices, e.g. in capacitors, or as a dielectric gate in metal-dielectric-semiconductor devices. Titania occupies also an important place as a photocatalyst, due to its high photocatalytic activity, excellent functionality, high chemical and thermal stability and non-toxicity [25]. The preparation of PDMS/SiO2/TiO2 composites by the sol–gel method has already been reported in literature
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[19, 26, 27] and phase-separated materials in which SiO2 and TiO2 particles are dispersed in the silicone matrix were generally obtained [28, 29]. As an alternative to literature approaches, the present paper deals with the obtaining of PDMS/SiO2/TiO2 composites in the absence of solvents. Solvent-free systems present environmental advantages and, on the other hand, this method diminishes the problem created by the solvent removal at the end of the reaction, which determines porosity in the material. In addition, this would be a preferred alternative when a high molecular mass polydimethylsiloxane is used as polymer matrix, taking into account that this is insoluble in solvents commonly used in sol–gel technique. A series of composites based on PDMS filled with in situ generated SiO2 and TiO2 was prepared. The influence of TiO2 content on the dielectric properties was investigated. Other properties of interest in dielectric applications, namely surface (dynamic contact angle, dynamic vapor sorption) and thermal properties were also studied.
2 Experimental 2.1 Materials Polydimethylsiloxane-a,x-diol (Mv ¼ 48; 000) was prepared according to a previously described procedure [8, 30]. Tetraethyl-orthosilicate (TEOS), purchased from Fluka (purity [ 98%, b.p. = 163–167 C, d20 4 = 0.933) was used as received. Tetrabutyl-orthotitanate (TBT) (d = 0.966 g/cm3, b.p. = 310–314 C, m.p. = -55 C, d20 4 = 1.486). Dibuthyltin dilaurate (DBTDL, d20 = 1.055) was received 4 from Merck-Schuchardt, and was used as received. 2.2 Equipments Fourier transform infrared (FTIR) spectra were obtained on a Bruker Vertex 70 FTIR analyzer. The analyses were performed in transmission mode, in the 400–4,100 cm-1 range, at room temperature with 2 cm-1 resolution and accumulation of 32 scans. The ground samples were incorporated in dry KBr and processed as pellets. Water vapors sorption capacity of the film samples was measured by using the fully automated gravimetric analyzer IGAsorp supplied by Hiden Analytical, Warrington (UK). An ultrasensitive microbalance measures the weight change as the humidity is modified in the sample chamber at a constant regulated temperature. The measurement system is controlled by a user-friendly software package. Dynamic contact angles (DCA) and contact angle hysteresis were measured on films by using a KSV Sigma 700 tensiometer system—a modular high-performance, computer controlled surface tension/contact angle meter. The
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measurement parameters were: advancing–receding speed, 5 mm/min; start depth, 0 mm; immersion depth, 8 mm; number of cycles, 3; the average values were taken into consideration. Thermogravimetric measurements (TGA) were performed in the 0–750 C temperature range at a heating rate of 10 C/min in air using a Q-1500D System. Glass transition and melting processes of PDMS based composites were followed using a Pyris Diamond DSC (Perkin Elmer USA) instrument. The samples were cooled from room temperature to -150 C, held at this temperature 2 min, and then heated up to 50 C at a heating rate of 20 C/min. Helium gas was purged through the cells at 20 ml/min to assure an inert atmosphere and good thermal conductivity. Before measurements, the DSC instrument was calibrated for temperature and energy scale using nhexane and pure water as recommended standards for LN2 range of DSC analysis. The glass transition temperature was calculated as a midpoint of the heat capacity of the sample. Novoncontrol setup (Broadband dielectric spectrometer Concept 40, Germany), integrating an ALPHA frequency response analyzer and a Quatro temperature control system, was used to investigate the dielectric properties of the polymer composites over a broad frequencies window, 100–106 Hz, in the -140 7 30 C temperature range. The bias voltage applied across the sample was 1.0 V. Samples having uniform thickness in the 0.2–0.9 mm range were placed between gold plated round electrodes, the upper electrode having a 20 mm diameter. The AFM measurements were made on a Scanning Probe Microscope Solver Pro-M platform (NT-MDT, Russia), in air, in semi-contact mode, using a rectangular NSG10/Au cantilever with a nominal elasticity constant KN = 11.5 N m-1 and a 10 nm radius of curvature of the tip. A 257.8 kHz oscillation frequency was used. The scan area was 20 lm 9 20 lm, 256 9 256 scan point size images being thus obtained. The AFM image processing and the calculation of the surface texture parameters were realized by the Nova Software (NT-MDT, Russia).
2.3 Composites preparation The synthesis of a polydimethylsiloxane/silica/titania composite was carried out as follows. 1.000 g (0.020 mmol) of PDMS with Mv ¼ 48; 000 was introduced in a Teflon dish and mixed with 0.500 g (2.400 mmol) TEOS and 0.038 g (0.111 mmol) TBT. After about 10 min of stirring, 0.003 g (0.005 mmol) of DBTDL was added and the stirring was continued for another 10 min. The mixture was poured on a Teflon foil and vacuumed for 10 min to eliminate the incorporated air. This sample was labeled as T5. The other
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Si–OH PDMS end groups, a crosslinking of PDMS occured. Obviously, TEOS and TBT also act as crosslinking agents (Scheme 1). The method used for the samples preparation in this study is based on the same principle as the room temperature vulcanization of the silicone rubber (RTV), in which the crosslinking of the polydimethylsiloxane-a,x-diol occurs by polycondensation reactions in the presence of environmental humidity, and needs a few days for curing. Water must diffuse deep into the film, while low molecular condensation compounds (i.e. alcohol) migrate outwards [31]. Therefore, the films were kept in the laboratory environment for about 2 months before investigations, when the weighting demonstrated the mass stabilization. In our case there was a relative humidity of 40–60% in the laboratory. The obtained white opaque films (of about 0.2–0.9 mm thickness) were easily peeled off from the substrate. The inorganic part of these composites is considered to be the sum of SiO2 and TiO2, while the organic one is represented by PDMS. The SiO2/TiO2 ratio was varied, but their cumulated amounts remained constant. While silica is introduced as a reinforcing material, the expected effect of titania is to modify PDMS dielectric properties [32].
Table 1 The recipes used to prepare PDMS/SiO2/TiO2 composites Sample
Inorganic
Organic
Catalyst
TEOS (wt. %)
TBT (wt. %)
PDMS (wt. %)
DBTDL (wt. %)
T0
35
0
65
0.165
T5
32.5
2.5
65
0.196
T7.5 T10
31 30
4 5
65 65
0.245 0.222
T15
28
7
65
0.132
T20
26
9
65
0.166
Tm
63
5
32
0.174
composites were obtained following the procedure described above but using amounts of reactants according to Table 1.
3 Result and discussions A polydimethylsiloxane-a,x-diol was used as matrix for preparing composite materials with SiO2 and TiO2 by adapting a solvent-free sol–gel procedure. We chose this way because as is known, PDMSs of high molecular mass (e.g., 48,000) have a low solubility in solvents that are common for sol–gel technique, such as alcohols, ketones and dimethylsulfoxide. Instead, they are soluble in nonpolar solvents like hydrocarbons and their halogenated derivatives, many of them being toxic. For this purpose, oxide precursors (TEOS and TBT) and a condensation catalyst (DBTDL) were added in preestablished amounts to the polymer. After energic stirring and gas removing, the mixtures were processed as films. The hydrolysis of the corresponding precursors took place under the influence of the atmospheric humidity followed by catalytic condensation with the formation of silicon and titanium oxides networks [26]. In addition, as a result of the reactions of the alkoxydes and their hydrolysates with Scheme 1 Proposed networking within composites
3.1 Fourier transform infrared spectroscopy Figure 1 shows the FT-IR spectra of PDMS based composites obtained by using different TEOS/TBT mass ratios in the spectral range between 4,100 and 400 cm-1. The composites show absorption bands around 2,900 and 2,964 cm-1, assigned to stretching vibrations of C–H in methyl groups. The absorption band around 1,262 cm-1 is the main characteristic band of methyl groups bonded to silicon and assigned to the symmetric deformation of C–H. The absorption bands around 1,020–1,097 cm-1 are assigned to Si–O–Si stretching vibrations in siloxane network [33, 34]. The broad peaks of OH bond are observed at 3,000–3,500 cm-1 for all samples [29]. TBT
OC2H5 C2H5
O
Si
CH3
OC2H5
+ HO
OC2H5
Si
CH3 O
Si
CH3
OC4H9
CH3 O
n
CH3
Si
OH + C4H9
O
Ti
OC4H9
OC4H9
CH3
room DBTDL temp. Si
O
O Si
Si O
O
Si O O Si
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Si CH3
CH3 Si CH3
O
Si CH3
CH3 O
n
Si CH3
O
O O
Si
O
O O
O Si
Si
Si
O
O
O O
Si
Ti
O
Si
O
Ti
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substitution in the titanium dioxide-enriched composite is difficult, probably due to the limited coordination ability [38]. A strong band is developed at 478 cm-1 assigned to both Si–O–Si and Ti–O–Ti overlaped bonds [39]. 3.2 Vapor sorption capacity
Fig. 1 IR spectra of the PDMS/SiO2/TiO2 composites
self-condensation reaction gives Ti–O–Ti bonds, while its copolymerization with TEOS or PDMS gives Ti–O–Si bonds [27]. Some reports suggest that the Ti–O–Si bonds formed as a result of the reactions between TBT, TEOS and PDMS are unstable, disappearing during the aging process [35]. However, a certain amount of Ti–O–Si bonds can be found in the final material. To verify the formation of the Si–O–Ti bond, not visible in our spectra due to the higher content in PDMS as compared with the inorganic part, a model sample was prepared based on an inversed mass ratio between the organic and inorganic components (sample Tm, Table 1). One can observe, in the FTIR spectrum of the Tm sample, a shoulder in the 910–960 cm-1 region, shoulder to be assigned to the Si–O–Ti bond [36]. Due to the fact that in this region Si–OH bond could be also present [37], the sample was calcinated at 900 C (Tm-c), the band centered around 951 cm-1 and corresponding to Si–O–Ti vibration thus becoming well defined. According to literature [38], the intermolecular interaction between titania and silica in composites with predominant SiO2 content occurs by replacement of silicon atoms in the SiO-4 4 tetrahedra with titanium atoms, while maintaining the tetrahedral coordination of titanium with respect to oxygen. The reverse
Water vapors uptaking capacity for the samples at 25 C in the 0–90% relative humidity range (RH) was investigated by using the IGAsorp equipment. The vapors pressure was increased in 10% humidity steps, every having a preestablished equilibrium time between 30 and 40 min (minimum time and time out, respectively). At each step, the weight gained was measured by electromagnetic compensation between tare and sample when equilibrium was reached. An anti-condensation system was available for vapor pressure very close to saturation. The cycle was ended by decreasing the vapor pressure in steps to obtain also the desorption isotherms. The drying of the samples before sorption measurements was carried out at 25 C in flowing nitrogen (250 ml/min) until the weight of the sample was in equilibrium at RH \ 1%. The sorption/ desorption isotherms registered in these conditions are presented in Fig. 2. The partial replacement of SiO2 in a PDMS-silica composite with TiO2 (both generated in situ by the sol–gel process) yielded an increase of water vapor sorption capacity from 0.4 (T0) up to 1.0–1.1 (T5–T20). According to IUPAC classification, the sorption–desorption curves can be associated to type III curves. These types of isotherms describe sorption on hydrophobic/low hydrophilic material with weak sorbent–water interactions [40]. GAB and BET kinetic models applied to the obtained data gave the values presented in Table 2.
Fig. 2 Rapid water vapors sorption isotherms for composites
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Table 2 Maximum water vapors sorption values for the composites
Sample
The average pore size (nm)
Weight (% d.b.)
T0
1.37
0.4552
6.622
0.001886
T5
1.27
1.0891
17.107
0.004873
20.866
0.005943
T7.5
1.39
1.0947
15.773
0.004492
18.806
0.005356
T10
1.59
1.0282
12.996
0.003702
15.207
0.004331
T15
1.25
1.0414
16.675
0.004749
20.703
0.005897
T20
1.44
1.1290
15.679
0.004466
18.422
0.005247
As one can see, the surface area values increase with TiO2 content although the average pore size remains almost constant (1.2–1.6 nm). The incompatibility between the oxide and the polymer, leading to the increasing pore number, can explain this behavior [41]. 3.3 Dynamic contact angle Film surface wettability was analyzed by measuring the dynamic contact angle on composites films by using the tensiometric method (Wilhelmy plate technique). Water was used as measurement liquid. The DCA runs were performed on rectangular films. The obtained contact angle values are given in Table 3. The water contact angle depends on the polarity of the surface, i.e. by increasing the polarity, the hydrophilicity increases. This happened by incorporating TiO2 in the studied composites. The presence of Ti–O–Ti bonds contributes to the hydrophilicity. A decrease of advancing contact angles was observed with increasing titania content in the samples. The differences between the maximum advancing and minimum receding contact angle values, known as contact angle hysteresis—a measure of surface heterogeneity and roughness [42]—were calculated. The decreasing hysteresis value is determined by the surface smoothing caused by the increase of TiO2 content in the sample.
BET analysis (5–35%) Area (m2/g)
Monolayer (g/g)
GAB analysis (5–80%) Area (m2/g) –
Monolayer (g/g) –
3.4 Thermogravimetric measurements Results of thermogravimetric analysis of the prepared composites in the 0–750 C temperature range in air are shown in Fig. 3. Two weight loss stages are observed, below 350 C and between 350 and 550 C, the latter one being a significant weight loss stage. The weight loss below 350 C is attributed to the evaporation of free water, the volatilization and the thermal decomposition of the remnant organic solvents. Between 350 and 550 C, the weight losses could be probably ascribed to the further combustion of organic moieties [43, 44]. The samples containing TiO2 yield a larger amount of residue during the thermo-oxidative decomposition than sample T0 that contains only SiO2. It was previously shown [31, 45, 46] that traces of organometallic catalyst favors depolymerisation with formation of volatile compounds and lowers the residue amount. According to the
Table 3 The main parameters of the water contact angle measurements Sample
T0 T5 T7.5
%TiO2
0 5
A
R
H
109.89 97.13
77.82 74.63
32.07 22.5
83.74
75.06
8.68
T10
10
94.42
81.6
12.82
T15
15
84.46
73.70
10.76
T20
20
76.39
77.32
123
7.5
H2O
–
Fig. 3 The TGA curves of PDMS/SiO2/TiO2 composites
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here reported data, in the presence of TiO2 this process is hindered.
315 Table 4 The main parameters of the DSC curves registered for the composites Sample
Tg (C)
DCp J/g(C)
Tm (C)
DHm (J/g)
T0
-118.93
0.09
-44.60
24.425
T5
-123.27
0.229
-40.61
19.145
T7.5
-123.52
0.073
-41.89
18.38
T10
-123.32
0.269
-41.23
19.713
T15
-123.36
0.089
-40.84
17.634
T20
-123.66
0.059
-40.58
17.65
3.5 Differential scanning calorimetry PDMS is well known as a polymer having very low glass transition and melting temperatures down to 0 C. Glass transition processes in PDMS/SiO2/TiO2 systems resulted from DSC scans are shown in Fig. 4a for all samples, including the T0 sample. It is noticeable that the T0 sample has a higher glass transition temperature (-118 C) as compared to the composites with TiO2 (T5….T20) (Table 4) which show glass transition processes around -123 C for all compositions (with small, insignificant differences up to 1 C). It is reasonable to consider that, by filling with SiO2 and crosslinking of PDMS, a reduction of chain mobility occurs, which leads to the increase of Tg. The incorporation of TiO2 in the PDMS/SiO2 system in various fractions generally has a contrary effect; the increasing of the free volume and the increased chain mobility allows a lower glass transition temperature. The
amorphous phase of the material is affected by both the chemical linking of SiO2 and by the addition of TiO2 (no matter the amounts added). The melting endotherms of PDMS/SiO2/TiO2 composites are shown in Fig. 4b. Sample T0 clearly shows a melting process around -45 C. This sharp endothermic peak suggests that the PDMS/SiO2 system has a crystalline morphology consisting of well-defined crystallites that need a high thermal energy to melt (24.42 J/g). All composites containing TiO2 present lower-energy melting endotherms with enthalpy values decreasing from T5 to T20, as shown in Table 4. The shape of melting endotherms for samples which contain TiO2 shows a duality of their crystalline morphology, which comes from the development of two major types of crystallites. The first one is formed by the PDMS/SiO2 composite and melts at -47 C, while the second type is developed by the PDMS/SiO2/ TiO2 composite and has a melting interval around -43 C. This duality of morphology is not very clearly separated because the two types of crystallites have almost the same thermodynamic stability, and, in addition, at least one polymer chain that forms a crystallite type may pass through the other crystallite type. According to Fig. 4b, for higher contents of TiO2 (higher than 15%) in the PDMS/ SiO2 system the dual morphology becomes well defined. The morphology developed only by PDMS/SiO2 fraction seems to be preserved. 3.6 Dielectric measurements
Fig. 4 Glass transition (a) and melting endotherms (b) in PDMS/ SiO2/TiO2 composites
Measurements of the complex dielectric permittivity e* = e0 - ie00 (where e’ is the storage component and e00 is the loss component) were carried out at seven fixed frequencies, by sweeping the temperature from -140 C up to 30 C with 5 C/min heating rate. The real and imaginary parts of complex dielectric permittivity have a direct physical interpretation. e0 is related to the reversible energy stored in the material by polarization, whereas e00 is proportional to the energy which is dissipated per cycle, divided into relaxation and conductivity contribution (energy required to align dipoles and move ions).
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For the T5 sample the data are represented as the real part of complex dielectric permittivity (e0 ) and the imaginary part or dielectric loss (e00 ) as a function of temperature and frequency in Fig. 5a and 5b, respectively. The temperature dependence of dielectric parameters, e0 and e00 , exhibits three regions which are associated to the mobility of the polymer chains. Initially, e0 presents an increasing step and e00 a peak which corresponds to the segmental a relaxation associated with the glass transition of amorphous PDMS. The shifting of these peaks to higher temperatures by increasing frequency is a characteristic of the dielectric relaxation. Due to the frequency of the dielectric measurement, the loss peak appears at a temperature significantly higher (-100 C) than the calorimetric Tg (-123 C). A a relaxation peak follows, this one being partly superposed by a smaller peak at higher temperatures. The last one could result from crystallization of
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the sample during the heating scan. The idea of crystallization is supported by e0 behavior, which displayed a decrease at the same temperature; at this temperature the mobility of the dipoles is reduced due to the immobilization and/or constraint of some fractions of the responding dipoles by increasing bulk crystallinity. At temperatures higher than -40 C, an increase in the permittivity of composites by increasing temperature is observed, especially at lower frequencies, due to the
Fig. 6 Dielectric permittivity, dielectric loss versus temperature at 1 kHz
Fig. 5 Dielectric permittivity e0 (a) and dielectric loss e00 (b) as a function of temperature and frequency for T5 sample
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Fig. 7 Dielectric permittivity versus TiO2 percent at 25 C
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Fig. 8 AFM images of the composites
interfacial polarization at electrode/sample or amorphous/ crystalline interface. This process appears at lower frequencies, when the mobile charges have enough time to migrate between boundaries.
From dielectric measurements, it appears that e00 increases with temperature, especially at low frequencies, this behavior being attributed to the increased conductivity; the mobility of the charge carriers increase by increasing
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Table 5 Surface particles characteristics and roughness parameters of T0–T10 films, corresponding to the 2D AFM images AFM scanned area (lm2)
Particle characteristics
T0
20 9 20
10
T5
20 9 20
10
T 7.5 T 10
20 9 20 20 9 20
10 10
922 1427
Sample
Number of particles measured
Surface roughness parameters Average roughness, Saa (nm)
Root mean square roughness, Sbq (nm)
812
13.56
23.37
1169
10.13
13.15
4.10 6.03
5.48 10.40
Average particle length (nm)
a
The average roughness parameter, Sa, is the most used surface roughness parameter. It is the arithmetic mean or average of the absolute distances of the surface points from the mean plane. The digital equation that represents this algorithm is displayed below, where M is the number of columns in the surface and N is the number of rows in the surface: N P M P 1 Sa ¼ MN ð1Þ jzj xi ; yj j1 i1
b
The root mean square (RMS) roughness parameter, Sq, is the root mean square of the surface departures from the mean plane within the sampling area: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N P M P 1 Sq ¼ MN z2 xi ; yj ð2Þ j1 i1
temperature. These effects on e0 and e00 values are more pronounced in the composite samples containing TiO2. No significant variation of the a relaxation temperature with composition is observed, this demonstrating that the glass transition of polymer matrix is not considerably influenced by the introduction of TiO2, as also emphasized by DSC (Fig. 6). As shown by the vapor sorption study, the samples with higher TiO2 content have an increased hydrophilicity, suggesting that it’s possible for those samples to have a higher content of absorbed atmospheric water. It is known that the dielectric constant of a material increases by uptaking water [47] because the concentration of mobile dipoles becomes higher, leading to the increase of the permittivity [48, 49]. Therefore, the increase of the dielectric constant by rising the TiO2 content could have two causes: the polarity of TiO2 (e = 89) and the polarity of water (e = 78.5 at 25 C) [50]. Taking into account that the amount of absorbed water is smaller (*1.1%) than the TiO2 content (5–20%), it is presumed that the increase in dielectric constant is mainly due to the titania amount. However, the increase in dielectric loss at positive temperatures for the samples containing TiO2 could be due to the absorbed water molecules. On the other hand, the slight increasing of the dielectric constant values of the samples with TiO2 against those without (Fig. 7) implies that the TiO2 particles are uniformly dispersed in the PDMS matrix. Otherwise, the porosities associated with the agglomerated TiO2 particles would significantly degrade the dielectric constant.
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3.7 Atomic force microscopy Figure 8 shows AFM images of the T0–T10 composites. The films have relatively high surface roughness (the root mean square roughness is about 10 nm). Table 5 presents the surface particles characteristics and roughness parameters of T0–T10 films, with 20 9 20 lm2 scanned areas, corresponding to the 2D AFM images. Although the T0 particles are smaller (the average of 10 measurements is 812 nm), roughness parameters have higher values (Sa is 13.56 nm and Sq is 23.37 nm), and the particles tend to agglomerate. In the case of T10 sample, the particles have a larger average diameter, about 1427 nm, but are not crowded, thus determining a lower film roughness (Sa is 6.03 nm and Sq is 10.40 nm). The introduction of TiO2 in PDMS/SiO2 composites leads to an increase of particle size, but one can not differentiate between TiO2 and SiO2 particles.
4 Conclusions A series of PDMS/SiO2/TiO2 composites have been prepared by solvent-free sol–gel technique. The amounts of TiO2 were quite small and didn’t induce spectacular effects, but higher amounts of TiO2 yield low quality films and structuration processes. The partial replacement of SiO2 in a PDMS-silica composite with TiO2 (both generated in situ by the sol–gel process) increases the
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hydrophilicity, porosity, and thermal stability of the composites. While the Tg value is only slightly influenced by TiO2 addition, the crystallization and melting temperatures values change as compared to those of the blank sample. Both e0 and e00 values increase throughout the studied range of temperatures and frequencies following the addition of TiO2. The temperature dependence of both curves show inflections at Tg, Tm, and Tc values close to those detected by DSC. The possibility to control the dielectric parameters recommends these materials as elastic dielectric—active elements for nano-actuation devices. Acknowledgments This research was financially supported by European Regional Development Fund, Sectoral Operational Programme ‘‘Increase of Economic Competitiveness’’, Priority Axis 2 (SOP IEC-A2-O2.1.2-2009-2, ID 570, COD SMIS-CSNR: 12473, Contract 129/2010-POLISILMET).
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