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coating on the performance of polydimethylsiloxane-based silicone elastomers for outdoor applications
Accepted Manuscript Effect of TiO2 addition/coating on the performance of polydimethylsiloxane-based silicone elastomers for outdoor applications D.T. Vaimakis-Tsogkas, D.G. Bekas, T. Giannakopoulou, N. Todorova, A.S. Paipetis, N.-M. Barkoula PII:
S0254-0584(18)30969-6
DOI:
https://doi.org/10.1016/j.matchemphys.2018.11.011
Reference:
MAC 21095
To appear in:
Materials Chemistry and Physics
Received Date: 3 September 2018 Revised Date:
19 October 2018
Accepted Date: 9 November 2018
Please cite this article as: D.T. Vaimakis-Tsogkas, D.G. Bekas, T. Giannakopoulou, N. Todorova, A.S. Paipetis, N.-M. Barkoula, Effect of TiO2 addition/coating on the performance of polydimethylsiloxanebased silicone elastomers for outdoor applications, Materials Chemistry and Physics (2018), doi: https:// doi.org/10.1016/j.matchemphys.2018.11.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Effect of TiO2 addition/coating on the performance of polydimethylsiloxanebased silicone elastomers for outdoor applications D. T. Vaimakis-Tsogkas1, D. G. Bekas1,2, T. Giannakopoulou3, N. Todorova3, A. S. Paipetis1, N.-M. Barkoula1* Materials Science & Engineering Department, University of Ioannina, Ioannina
45110, Greece 2
Department of Aeronautics, Imperial College London, South Kensington Campus,
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Exhibition Road, SW7 2AZ, London, UK 3
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1
Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Agia Paraskevi,
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15341, Greece
*corresponding author: [email protected]
Abstract
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In the current study we investigated the effect of TiO2 addition on the performance of silicone elastomer films with focus on their response under UV radiation. TiO2 nanoparticles (0.1 – 5 wt. %) were dispersed to a poly-dimethylsiloxane-based
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silicone and the behavior of the films was assessed as a function of TiO2 content. TiO2
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nanoparticles (0.1 wt. %) were also deposited as a coating on silicone elasomer films. The dispersion of TiO2 nanoparticles into the silicone matrix resulted in an overall increase of strength (up to app. 32%) and strain at break (up to app. 44%) as well as in reduced Young’s Modulus (up to app. 30%). Application of TiO2 as coating resulted in lower stiffness (app. 60%) and strength (app. 43%), and almost identical strain at break compared to the unmodified silicone matrix. An increase in the thermal stability (up to app. 5 °C) and dielectric permittivity (up to app. 10%) was observed with increased TiO2 content. Furthermore addition of TiO2 resulted in higher stability
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ACCEPTED MANUSCRIPT against UV radiation and contributed to photocatalytic NO oxidation. Best performance was found in coated films which presented limited deterioration in their strength (residual values 65% vs. 23% for unmodified ones) and enhanced NO conversion. Overall it was shown that addition of small amounts of TiO2, especially in
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the form of coating, significantly improved the performance of silicone elastomer films for outdoor applications.
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Keywords: TiO2, silicone, PDMS, photocatalysis, UV resistance
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1. Introduction
Polydimethylsiloxane (PDMS) is by far the most widely used silicone elastomer in industries as diverse as textiles, automobiles, aerospace, construction, electronics, and biomedical materials thanks to its soft and flexible nature, transparency to ultraviolet
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and visible light, permeability to oxygen, high dielectric constant, low permeability to water, excellent thermal and chemical stability, resistance to UV radiation and weathering, nontoxicity and low cost [1-7]. PDMS has recently drawn the attention of
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engineers who aim at introducing dielectric elastomers in architectural applications in
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order to mimic nature and create dynamic structural elements and full-scale kinetic surfaces [8].
TiO2 has attracted significant amount of interest as filler in PDMS-based composites, due to its relatively high permittivity, chemical inertness, non-toxicity and ease of dispersion [9-13]. Relevant studies concluded that the addition of an appropriate amount of TiO2 may result in increased dielectric permittivity and reduced modulus of elasticity. Apart from its high permittivity, TiO2 has been very frequently used as a photo-catalyst due to its strong oxidative ability [14]. TiO2 has been shown to
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ACCEPTED MANUSCRIPT effectively degrade air pollutants (e.g. NO, NO2) in indoor and outdoor environments [14-16]. At the same time, TiO2 has been widely used as a UV-blocking additive due to its UV-absorption capacity, in addition to its reflecting and/or scattering capability, minimizing photodegradation [17-19]. The anatase polymorph of TiO2 is considered
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to be more active photo-catalyst, while the rutile polymorph has been used as UV absorber due to its high refractive index and hiding power, as well as good chemical stability and UV light screening effects [17]. Similar to the liquid and gaseous TiO2
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photocatalytic reactions, the free radicals created during UV radiation may however
matrix in the solid phase [20-25].
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attack the polymeric chains and initiate photocatalytic degradation of the polymer
Based on the above, it is clear that the addition of TiO2 in PDMS-based elastomers may significantly affect their performance, especially in outdoor applications. Degradation of air pollutants is of particular interest for architectural applications.
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Facades and kinetic surfaces made out of TiO2 modified PDMS-based elastomers may function as adaptive structural elements and air purification systems simultaneously. On the other hand, durability of the PDMS-based elastomers after TiO2 modification,
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and maintenance of their long-term structural integrity is of key importance for their
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successful application. Very few studies however exist on the effect of TiO2 addition on the photodegradation and photocatalytic performance of PDMS-based elastomer films [26-29]. It has been documented that TiO2 promote the photodegradation of salicylic acid in aqueous solution and show good photocatalytic performance for the decontamination of water and decomposition of organic dyes. However, no information is available on the durability of TiO2 modified PDMS-based elastomers. Thus, the scope of the current study is to offer an understanding on the effect of TiO2 modification on the overall performance of PDMS-based elastomers, including their
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ACCEPTED MANUSCRIPT response under UV radiation. TiO2 nanoparticles (80/20 anatase/rutile) were added at 0.1 – 5 wt. % loadings in the silicone matrix and their effect on mechanical, thermal, thermomechanical, dielectric and photo-catalytic properties of the composite were investigated. For comparison reasons, PDMS was also coated with TiO2
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nanoparticles. The amount of TiO2 in the form of coating was kept low (0.1 wt. %) to ensure stable and reproducible deposition as well as minimize the potential degradation due to extensive UV absorption. To the best of our knowledge the effect
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of UV radiation on the residual mechanical properties and NO oxidation process of
part of this study.
2. Materials and Methods
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TiO2 modified PDMS-based silicone elastomers is been assessed for the first time as
2.1 Materials and specimens preparation
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A commercial polydimethylsiloxane (PDMS)-based silicone (TC-5005 A/B-C, BJB Enterprises Inc., USA.) and titanium dioxide (TiO2) (AEROXIDE TiO2 P25, Evonik Industries, Germany) in the form of fine powder were used as matrix and filler,
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respectively. AEROXIDE TiO2 P25 consists of aggregated primary particles with size
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of several hundred nm and mean diameter of app. 21 nm. The weight ratio of anatase and rutile is app. 80/20 which provides high photocatalytic activity (due to the high amount of the anatase polymorph in the powder). According to the manufacturer specifications, PDMS was processed by adding curing agent B at a ratio of 10:100 w/w of prepolymer A. Components A and B were hand mixed for app. 5 minutes. Component C (plasticizer) was added at 50:100 w/w of the mixture A/B in order to increase the softness of the final elastomer. TiO2 previously dried at 100°C for 24h in vacuum oven (Thermo Scientific – Heaaeus Vacutherm), was dispersed in component
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ACCEPTED MANUSCRIPT C using a sonication bath (Branson 3510 Ultrasonics Corp) for 30 s, with pulse frequency set at 10 Hz and pulse width at 50%. At the end of the dispersion process, appropriate amounts of C/TiO2 and A/B were hand mixed for app. 5 min in a laboratory beaker following the manufacturer’s specifications, to obtain mixtures with
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0.5-5 wt. % TiO2. Τhe different mixtures were placed in aluminum molds and allowed to crosslink at room temperature for 24 hours which resulted in silicone-based films with an average thickness of app. 1 mm. Instead of dispersion, one series of samples
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was created by coating TiO2 onto the silicone matrix. For this purpose, neat silicone films were placed on the aluminum mold and TiO2 powder was deposited on the films
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using a 200 mm diameter laboratory sieve. The amounts of the different components were adjusted in order to reach a silicone film coated with 0.1 wt. % TiO2. Table 1 provides an overview of the prepared films and their designation.
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Table 1: Amounts (wt. %) of prepolymer A, curing agent B, plasticizer C and TiO2 and designation of the prepared PDMS-based nanocomposite films (*c denotes that TiO2 has been applied as coating on the films) A
B
C
TiO2
wt. (%)
wt. (%)
wt. (%)
wt. (%)
60.61
6.06
33.33
0
0.1
60.55
6.05
33.30
0.1
0.5
60.31
6.03
33.16
0.5
1
60.00
6.00
33.00
1
5
57.58
5.76
31.66
5
0.1c*
60.55
6.05
33.30
0.1
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0
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code name
2.2 Characterization Mechanical properties 5
ACCEPTED MANUSCRIPT To evaluate the effect of TiO2 addition on the mechanical properties dump-bell specimens were cut from the elastomer films using an ISO 527 type 4 die. Specimens were subjected to uniaxial tensile loading using an Instron universal testing machine equipped with a ±30kN load cell. Tests were performed at ambient temperature, at an
samples from each formulation were tested sequentially. Thermogravimetric Analysis (TGA)
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extension rate of 100 mm/min, as suggested by ASTM D412-06a. At least five
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The thermal stability of the elastomer films was monitored with a STA 449C analyzer (Netzsch-Gerätebau GmbH Germany) at a heating rate of 10oC/min over temperatures
atmosphere purge of 40 mL/min.
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ranging from 25oC to 1000oC on an average 20 mg samples, under a nitrogen
Differential Scanning Calorimetry (DSC)
Measurements were carried out using a STA 449C analyzer (Netzsch-Gerätebau
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GmbH Germany) at a constant heating and cooling rate of 20oC/min on 20 mg samples, in a flowing nitrogen atmosphere (N2). Samples were initially heated up from room temperature up to 100oC and held at that temperature for 5 min to
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eliminate previous residual thermal history. Afterwards, specimens were cooled down
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to -120oC and heated up to 0oC. Dynamic Mechanical Analysis (DMA) A DMA 242C apparatus (Netzsch-Gerätebau GmbH Germany) was used to evaluate the thermomechanical response of the elastomer films under tensile mode at a frequency of 1 Hz, and temperatures ranging from −125 to 25oC (rate 5°C/min). The amplitude of the deformation was 200 µm. Dynamic Dielectric Analysis
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ACCEPTED MANUSCRIPT The dielectric properties were determined using a high-resolution dielectric spectrometer/frequency spectrum analyzer (DETA SCOPE L1 provided by Advice Ltd.) Samples were mounted on an interdigital dielectric sensor which was placed in a purposely-made dielectric cell. In order to ensure an effective contact between the
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material and the sensor, samples were slightly compressed during the dielectric measurements. The complex dielectric function of each sample was measured by performing consecutive isothermal frequency scans over a frequency range of 10 to
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105 Hz while the effective voltage amplitude was selected at 5 V. It must be pointed out that the thickness of all specimens was the same and the recorded differences in
of the PDMS. Photocatalytic oxidation reactions
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terms of the complex dielectric functions can be solely attributed to the modification
The photocatalytic oxidation of nitric oxide (NO) was monitored using a standard
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procedure [30, 31] based on ISO/DIS 22197-1. The samples were placed in a flowtype photocatalytic reactor and their photocatalytic activity was evaluated under UVA light illumination with intensity 10 W/m2 for 30 min. The samples were exposed to
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model air containing 1 ppm NO with flow rate 3 L/min. The starting concentration of
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NO2 was ∼0 ppm and the NOx concentration was calculated as the sum of NO and NO2 concentrations. The photocatalytic tests were performed at room temperature. Photodegradation Tests Photodegradation tests were performed using a SUNTEST XLS+ (Atlas MTS) sunlight simulator. Selected specimens were placed in the Weatherometer, having a 2200 W air cooled xenon light source that produce an energy spectrum similar to natural sunlight. Specimens were exposed at four different time periods: 8, 24, 72 and 216 h. The mechanical properties of the films after radiation were determined as
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ACCEPTED MANUSCRIPT described above. The fractured cross-sections of 216h irradiated samples were sputtered with gold (Sc760 sputter coater Polaron) and inspected using Scanning Electron Microscope (SEM) (JEOL JSM 6510LV SEM/ Oxford Instruments X- Act
3. Results and Discussion Mechanical, thermal and thermo-mechanical properties
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EDX).
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In order to evaluate the effect of TiO2 addition on the mechanical behavior representative stress-strain curves of the silicone-based elastomer films are presented
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in Fig. 1 while key mechanical properties, comprising of Young’s Modulus (E), tensile strength (σts) and strain at break (εb), are included in Table 2. All tested films presented a rubber-like response with an initial linear region (up to app. 300% strain) followed by stress-hardening at higher deformations, indicative of molecular chain
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orientation. The initial linear part of the stress-strain curve was used for the estimation of Young’s Modulus of the tested films. The dispersion of TiO2 nanoparticles into the silicone matrix resulted in an overall increase in strength (up to app. 32%) and strain
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at break (up to app. 44%) and reduced Young’s Modulus (up to app. 30 %). However,
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no straight relationship between mechanical properties and TiO2 content was found, as observed in Table 2. Stiffness and strength increased steadily up to 0.5 wt. % TiO2 content while non monotonic evolution was found in stain at break with the TiO2 content. The addition of 0.1 and 1 wt. % TiO2 nanoparticles led into highest stain at break and lowest stiffness compared with unmodified matrix while the highest reinforcement was obtained at 0.5 wt. % loading. The application of TiO2 as coating resulted in lower stiffness (app. 60%) and strength (app. 43%), and almost identical strain at break compared to the unmodified silicone matrix. Although one would
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ACCEPTED MANUSCRIPT expect that the dispersion of nano-TiO2 would lead to reinforcement of the silicone matrix manifested as enhanced stiffness and strength and hence lower strain at break values, this is not the case here. This phenomenon has been documented previously and has been attributed to different antagonistic mechanisms [5, 32]. It has been
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suggested that the nanometer sized TiO2 particles interfere with the crosslinking process of the silicone elastomer resulting in softening [5, 12]. This is counterbalanced to some extend by the hardening effects due to the intrinsic high
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elastic modulus of TiO2. At the same time, the filler agglomeration at high TiO2 contents weakens the interface and results in slippage of polymer chains on the filler
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surface during deformation [12, 32]. Finally, due to the direct dispersion of TiO2 in the plasticizer part of the elastomer, it can be also claimed that the nanoparticles act as plasticizer distributors enhancing the strain at break of the elastomer films, as
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observed in the case of clay modified polymer films [33, 34].
Fig. 1: Representative stress-strain curves of silicone-based elastomers as a function of TiO2 content
Table 2: Effect of TiO2 addition on the mechanical, thermal and thermo-mechanical properties of silicone-based elastomers
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σTS
εb
Tpeak
mr
Tg
Tm
(kPa)
(MPa)
(%)
(°C)
(%)
(°C)
(°C)
0
38±4
1.02±0.20
1403±120
567.7
15.86
-115.8
-28.9
0.1
27±5
1.05±0.20
1861±181
570.5
16.67
-113.2
-23.6
0.5
38±4
1.35±0.02
1535±27
570.8
15.06
-116.7
-25.7
1
30±10
1.15±0.21
2014±151
571.4
16.37
-115.9
-25.9
5
32±8
1.33±0.19
1796±180
589.2
22.30
-115.7
-24.6
0.1c
25±5
0.58±0.07
1455±128
567.1
16.18
-115.9
-29.1
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specimen
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Fig. 2 shows the thermogravimetric weight loss curve (TG) and the weight loss
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derivative curve (DTG) of the silicone elastomer films before and after the addition of TiO2 nanoparticles. In agreement with previous observations degradation was found in temperatures ranging between 400 and 650 °C [37]. Although the starting decomposition temperature was almost the same for all tested films (Fig. 2a), the incorporation of TiO2 nanoparticles influenced the degradation peak temperature
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(Tpeak) (Fig. 2b) and the remaining silicone mass (mr) after decomposition (see Table 2). Overall an increase in both Tpeak and mr was observed with increased TiO2 content, which was more evident at 5 wt. % (up to 22 °C and 7%, respectively). This can be
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linked with the presence of the TiO2 nanoparticles, which act as barriers against heat
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and volatiles diffusion [28, 35, 36]. Coating of silicone elastomer with TiO2 did not contribute to the thermal stability as expected, since the nanoparticles interacted only with the surface of silicone and not with the volume of the material.
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Fig. 2: a) TG and b) DTG curves of silicone-based elastomers as a function of TiO2
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content
Fig. 3 shows the DSC thermograms of the unmodified and TiO2-modified silicone elastomer films. All curves presented an abrupt exothermic change in the heat flow at app. -110 °C and a main endothermic peak at app. -30 °C. This behavior is in agreement with previous observations on PDMS and silica-reinforced PDMS
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nanocomposites [37-39]. The first change can be associated with the glass transition temperature (Tg) of the amorphous part of the silicone-based elastomers, while the
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second peak is related to the melting of the crystalline fraction of the materials (Tm) (see Table 2). Intermediate peaks between Tg and Tm can be associated with
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crystallization and lamellar thickening of the silicone-based elastomers upon heating [38]. Tg did not show any significant variation with the addition of TiO2 nanoparticles. The addition of TiO2 resulted however to an up-to 5 °C increase in Tm. Similar trends were observed in silica modified PDMS composites, where nanoclusters did not modify the relaxation behavior of the amorphous phase and the glass transition kinetics, they however promoted crystallization of the nanocomposites [38].
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Fig. 3: DSC thermograms of silicone-based elastomers as a function of TiO2 content.
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The storage modulus (Eˊ), loss modulus (E˝) and loss factor (tanδ) response of the silicone-based elastomers is being illustrated in Fig. 4. Unmodified and TiO2 modified samples behaved similarly presenting a small steady decrease in Eˊ and E˝ as the temperature increased, followed by a steep drop (2-3 orders of magnitude) of moduli
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at app. -60 °C and a plateau at higher temperatures. At the same temperature range, tanδ curves presented a step increase followed by a very broad peak. Normally one would ascribe such change to the glass transition region [28], however as discussed
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previously, the peak absence in the E˝ curve suggests that this behavior is due to the melting process of the crystalline regions of the semi-crystalline silicone elastomer [7,
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38, 39]. TiO2 addition resulted in a small shift of the transition towards higher temperatures, in agreement with the DSC results. Furthermore an increase was observed in Eˊ and E˝ after the addition of TiO2 nanoparticles, which was more pronounced in the rubbery region. The stiffness increase was higher than the losses in that region, resulting in smaller damping of the TiO2 modified samples. The increment of the Eˊ in the -95° to -65°C region has been attributed to secondary crystallization and/or lamellar thickening while at higher temperatures this
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ACCEPTED MANUSCRIPT phenomenon has been linked with higher crosslink density of the nanomodified
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silicone elastomers [38].
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Fig. 4: a) Storage Modulus (Eˊ), b) Loss Modulus (E˝) and c) Loss factor (tanδ) of
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silicone-based elastomers modified with various amounts of TiO2
Dielectric properties
The complex dielectric permittivity ε* is a very important electrical property of a dielectric elastomer that greatly affects the force required for its actuation [40]. The complex dielectric permittivity (ε*= εˊ-iε˝), can be expressed by a real part, which is
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the dielectric permittivity (εˊ), an imaginary part called dielectric loss (ε˝) and their ratio, which is the dissipation factor (tanδe = ε˝/εˊ), in analogy with the dynamic
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mechanical response of the materials.
Fig. 5 illustrates the effect of the TiO2 addition on the dielectric permittivity (εˊ), the
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dielectric loss (ε˝) and the dissipation factor (tanδe) of the tested elastomers. As observed in Fig. 5a, independent to the TiO2 content, the dielectric permittivity of the silicone elastomers decreased with frequency which denotes that the material dipoles were not able to efficiently follow the external field changes at high frequencies [41]. Even though the permittivity values of TiO2 is several orders of magnitude higher than that of the silicone elastomer, the addition of small contents of TiO2 nanoparticles did not significantly affect the permittivity of the system. After the addition of 1 wt. % TiO2 a small increase in permittivity throughout the entire 13
ACCEPTED MANUSCRIPT frequency spectrum was observed which become more evident at 5 wt. % TiO2 content. The improvement in permittivity (app. 10%) is in line with previous observations and is quite significant considering the very low TiO2 content [5, 9]. The addition of small amounts of TiO2 (<0.5 wt. %) resulted in small decrease in the
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dielectric loss, while increased TiO2 contents led to higher losses at the low and high end of the frequency spectrum (Fig. 5b). As aforementioned, the dielectric loss greatly affects the electrical performance since its increase results in higher conductivity of
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the elastomer [11]. At frequencies between 102 and 104 Hz, the dielectric losses of all TiO2 modified elastomers were very close to those of the unmodified system. Thus,
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the increase in dielectric loss is not expected to elevate the material temperature and the dissipation of the elastomer will not be considerably increased at intermediate
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frequencies as observed in Fig. 5c.
Fig. 5: a) Dielectric permittivity (εˊ), b) Dielectric Loss (ε˝) and c) Dissipation Factor
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(tanδe) of silicone-based elastomers modified with various amounts of TiO2
Photocatalytic oxidation reactions The results from the photocatalytic response of the prepared materials are presented in Fig. 6. As illustrated on the example of the coated material (0.1c sample), NO adsorption/desorption equilibrium in dark was initially reached over the material that is visualized as a peak in the curves before the illumination point (Fig 6a). After the light was turned on, a decrease of NO concentration and a simultaneous increase of 14
ACCEPTED MANUSCRIPT NO2 concentrations occurred followed by stabilization of the gas concentrations. The variation of the NO, NO2 and NOx concentration recorded for rest of the samples is not presented due to similarity. The photocatalytic activity is related to the intensity of the processes: NO to NO2
following parameters: decrease of NO (1ppm-NO and removal of NOx (1ppm – NOx
meas).
measured),
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oxidation and NO2 to NO3- oxidation. The processes can be demonstrated by the NO2 formation (NO2 meas)
The parameters’ values for the neat and
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composite samples taken after the gas stabilization are given in Fig. 6b. The neat sample showed no photocatalytic activity, as expected. In general, the TiO2 containing
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composites exhibited very low activity that can be ascribed to the absence of a sufficient amount of TiO2 on the surface of the silicone. A small increase in the NO oxidation and NOx removal was recorded with the increase of the TiO2 loading in the composites. The sample with the highest photocatalytic activity was 0.1c. This was
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attributed to the enrichment of the silicone surface with TiO2 photocatalyst. The NO concentration remained constant till the end of the illumination evidencing the
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absence of catalyst saturation by reaction products.
Fig. 6: a) Concentration profiles of NO, NO2 and NOx gases over the coated sample (0.1c) under UV irradiation and b) NO conversion by silicone-based elastomers modified with various amounts of TiO2 15
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Photodegradation Tests As discussed previously the addition of TiO2 is expected to alter the mechanical response of the silicone elastomer films under UV radiation. In order to verify this
TiO2-modified silicone elastomers after UV radiation.
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effect, Fig. 7 illustrates the residual mechanical properties of unmodified and selected
As seen in Fig. 7a, unmodified and films with dispersed TiO2 nanoparticles presented
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a reduction in Young’s modulus after 8h of irradiation followed by a respective increase with irradiation time. At the same time, the results revealed an abrupt
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reduction in mechanical strength and strain at break within the first 24h of irradiation, while further exposure resulted in limited degradation (Fig. 7b-c). Dispersion of small amounts of TiO2 within the silicone elastomer matrix led to higher stability against UV radiation, since residual strength increased from 23 to 30% at steady state and
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respective residual strain from 38 to 46%. Application of TiO2 nanoparticles as coating resulted in steady increase of the residual Young’s modulus and limited degradation of the strength, with residual values as high as 65% compared to app.
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23% found in unmodified films.
Fig. 7: Residual mechanical properties of silicone-based elastomers as a function of Ti02 content and UV irradiation duration: a) E/E0, b) σ/σ0, and c) ε/ε0
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ACCEPTED MANUSCRIPT The observed reduction in strength and ductility after UV radiation can be explained on the basis of oxidative degradation of the chains of the silicone elastomer which resulted in reduction of their molecular weight and this in turn led to deterioration of their mechanical properties. The variation in mechanical properties with composition
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and exposure time indicates that TiO2 nanoparticles interfere with the crosslinking process and/or UV absorbance process in the elastomer films. As discussed in the past irradiation results in intermolecular crosslinking of silicone rubbers as well as
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increased apparent crosslink density due to chemical links at the polymer-particle interface of nanomodified elastomers [42, 43]. Thus, the increase in Young’s modulus
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with exposure time implies an increase in the degree of crosslinking due to UV exposure, which compensates the reduction in stiffness due to chain scission [7]. Increase in the photostability of the modified silicone elastomer films especially after coating can be associated with the ability of TiO2 to absorb the UV radiation and
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transform it into thermal energy inline with the photocatalysis results. Overall modification with TiO2 had a positive influence on the photostability of silicone elastomer in agreement with previous results on nano-rutile TiO2 natural rubber
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composites [18].
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Images of the fractured cross-sections after UV radiation provide further insight into the effect of TiO2 addition on the photostability of the elastomeric films. Unmodified films (Fig. 8a) and films with 0.1 wt. % TiO2 (Fig. 8b) presented parting lines which were almost eliminated after the dispersion of 1 wt. % TiO2 (Fig. 8c). On the other hand absence of cracks and evidence of plastic deformation was seen after the application of TiO2 as coating (Fig. 8d). These observations support the fact that dispersion of higher amount of TiO2 nanoparticles or application as coating increases
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ACCEPTED MANUSCRIPT the photostability of the modified silicone elastomer films due to shielding and/or
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absorption of the UV radiation and transforming into heat.
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Fig. 8: Fractured surfaces of silicone-based elastomers after 216h of UV irradiation as a function of TiO2 content: a) 0 wt. %, b) 0.1 wt. %, c) 1 wt. % and d) 0.1c wt. %
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4. Conclusions
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In this study the performance of TiO2-modified PDMS-based silicone elastomers as a function of TiO2 content and application method (dispersion vs. coating) was evaluated. Focus was given on the performance of the films after UV irradiation. Based on the mechanical results it can be concluded that the addition of TiO2 was beneficial to the performance of the PDMS-based silicone elastomers since it lowered the Young’s Modulus and enhanced the strain at break of the obtained films. Furthermore the thermal stability was improved after the addition of higher amounts of TiO2 (5 wt. %). A positive impact was also found on the thermomechanical
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ACCEPTED MANUSCRIPT properties with TiO2 modified films presenting smaller damping compared to unmodified ones. Addition of TiO2 improved the photostability of the silicone elastomer films, with coated films showing the best performance against deterioration of mechanical
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properties after UV radiation. Coated films presented also the highest rate of NO oxidation. Dielectric permittivity increased with higher TiO2 contents, however dielectric losses also increased. Overall it can be concluded that addition of small
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amounts of TiO2, especially in the form of coating, significantly improve the performance of PDMS-based silicone elastomer films for outdoor applications since
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they inhibit the mechanical deterioration due to UV irradiation of the films and at the same time can contribute to the air purification by photocatalytic NO oxidation.
Acknowledgements
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The authors wish to thank the Ring of the Laboratory Units and Centers and the MSSNDE Laboratory of the University of Ioannina for providing access to the thermal analysis unit, scanning electron microscopy unit and mechanical testing facilities,
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respectively. Special thanks to Prof. T. Vaimakis for the support during the execution
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of the DSC and TGA experiments and to Prof. Konstaninou for providing access to the Suntest apparatus.
Conflicts of interest
The authors wish to declare that there is no conflict of interest in this research work.
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3. 4.
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5.
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Fa, W., et al., Enhanced photodegradation efficiency of polyethylene-TiO2 nanocomposite film with oxidized polyethylene wax. Journal of Applied Polymer Science, 2010. 118(1): p. 378-384. Nawi, M.A., et al., Photocatalytic-oxidation of solid state chitosan by immobilized bilayer assembly of TiO2–chitosan under a compact household fluorescent lamp irradiation. Carbohydrate Polymers, 2011. 83(3): p. 1146-1152. Jawad, A.H. and M.A. Nawi, Characterizations of the Photocatalytically-Oxidized Cross-Linked Chitosan-Glutaraldehyde and its Application as a Sub-Layer in the TiO2/CS-GLA Bilayer Photocatalyst System. Journal of Polymers and the Environment, 2012. 20(3): p. 817-829. Jawad, A.H. and M.A. Nawi, Oxidation of crosslinked chitosan-epichlorohydrine film and its application with TiO2 for phenol removal. Carbohydrate Polymers, 2012. 90(1): p. 87-94. Tavares, M.T.S., et al., TiO2/PDMS nanocomposites for use on self-cleaning surfaces. Surface and Coatings Technology, 2014. 239: p. 16-19. Lamberti, A., Microfluidic photocatalytic device exploiting PDMS/TiO2 nanocomposite. Applied Surface Science, 2015. 335: p. 50-54. Silva, V.P., et al., Silicone rubbers filled with TiO2: Characterization and photocatalytic activity. Materials Chemistry and Physics, 2009. 113(1): p. 395-400. Zhu, Y.F., et al., Preparation and evaluation of photocatalytic activity of poly(dimethylsiloxane)–titanium dioxide composites. Plastics, Rubber and Composites, 2007. 36(7-8): p. 360-364. Giannakopoulou, T., et al., Composite hydroxyapatite/TiO2 materials for photocatalytic oxidation of NOx. Materials Science and Engineering: B, 2012. 177(13): p. 1046-1052. Mitsionis, A., et al., Hydroxyapatite/titanium dioxide nanocomposites for controlled photocatalytic NO oxidation. Applied Catalysis B: Environmental, 2011. 106(3): p. 398-404. Razzaghi Kashani, M., S. Javadi, and N. Gharavi, Dielectric properties of silicone rubber-titanium dioxide composites prepared by dielectrophoretic assembly of filler particles. Smart Materials and Structures, 2010. 19(3). Giannakas, A., et al., Preparation, characterization, mechanical and barrier properties investigation of chitosan–clay nanocomposites. Carbohydrate Polymers, 2014. 108: p. 103-111. Vlacha, M., et al., On the efficiency of oleic acid as plasticizer of chitosan/clay nanocomposites and its role on thermo-mechanical, barrier and antimicrobial properties – Comparison with glycerol. Food Hydrocolloids, 2016. 57: p. 10-19. Buzarovska, A., et al., Poly(hydroxybutyrate-co-hydroxyvalerate)/titanium dioxide nanocomposites: A degradation study. Journal of Applied Polymer Science, 2009. 114(5): p. 3118-3124. Huang, P., et al., High performance surface-modified TiO2/silicone nanocomposite. Scientific Reports, 2017. 7(1). Carpi, F., et al. Electroactive polymers: New materials for spacecraft structures. in European Space Agency, (Special Publication) ESA SP. 2005. Bosq, N., et al., Melt and glass crystallization of PDMS and PDMS silica nanocomposites. Physical Chemistry Chemical Physics, 2014. 16(17): p. 7830-7840. Fragiadakis, D. and P. Pissis, Glass transition and segmental dynamics in poly(dimethylsiloxane)/silica nanocomposites studied by various techniques. Journal of Non-Crystalline Solids, 2007. 353(47): p. 4344-4352.
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Carpi, F., et al., Chapter 6 - ENHANCING THE DIELECTRIC PERMITTIVITY OF ELASTOMERS, in Dielectric Elastomers as Electromechanical Transducers. 2008, Elsevier: Amsterdam. p. 51-68. Sheng, J., et al., Thermal, Mechanical, and Dielectric Properties of a Dielectric Elastomer for Actuator Applications. Journal of Macromolecular Science, Part B, 2012. 51(10): p. 2093-2104. Stevenson, I., et al., Influence of SiO2 fillers on the irradiation ageing of silicone rubbers. Polymer, 2001. 42(22): p. 9287-9292. McCarthy, D.W. and J.E. Mark, Poly(Dimethylsiloxane) Elastomers from Aqueous Emulsions: III. Effects of Blended Silica Fillers and γ-Radiation-Induced Crosslinking. Rubber Chemistry and Technology, 1998. 71(5): p. 941-948.
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ACCEPTED MANUSCRIPT Table captions Table 1: Amounts (wt. %) of prepolymer A, curing agent B, plasticizer C and TiO2 and designation of the prepared PDMS-based nanocomposite films (*c denotes that TiO2 has been applied as coating on the films)
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properties of silicone-based elastomers
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Table 2: Effect of TiO2 addition on the mechanical, thermal and thermo-mechanical
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ACCEPTED MANUSCRIPT Figure captions Fig. 1: Representative stress-strain curves of silicone-based elastomers as a function of TiO2 content Fig. 2: a) TG and b) DTG curves of silicone-based elastomers as a function of TiO2
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content
Fig. 3: DSC thermograms of silicone-based elastomers as a function of TiO2 content. Fig. 4: a) Storage Modulus (Eˊ), b) Loss Modulus (E˝) and c) Loss factor (tanδ) of
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silicone-based elastomers modified with various amounts of TiO2
Fig. 5: a) Dielectric permittivity (εˊ), b) Dielectric Loss (ε˝) and c) Dissipation Factor
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(tanδe) of silicone-based elastomers modified with various amounts of TiO2 Fig. 6: a) Concentration profiles of NO, NO2 and NOx gases over the coated sample (0.1c) under UV irradiation and b) NO conversion by silicone-based elastomers modified with various amounts of TiO2
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Fig. 7: Residual mechanical properties of silicone-based elastomers as a function of Ti02 content and UV irradiation duration: a) E/E0, b) σ/σ0, and c) ε/ε0 Fig. 8: Fractured surfaces of silicone-based elastomers after 216h of UV irradiation as
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a function of TiO2 content: a) 0 wt. %, b) 0.1 wt. %, c) 1 wt. % and d) 0.1c wt. %
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ACCEPTED MANUSCRIPT Highlights -
Increased strength and strain at break due to dispersion of TiO2 in PDMS matrix
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Higher thermal stability and dielectric permittivity in TiO2 modified PDMS
-
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films Improved performance of PDMS for outdoor applications after the addition of small amounts of TiO2 -
Coating with TiO2 is more effective than addition through dispersion in PDMS
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matrix
S0254-0584(18)30969-6
DOI:
https://doi.org/10.1016/j.matchemphys.2018.11.011
Reference:
MAC 21095
To appear in:
Materials Chemistry and Physics
Received Date: 3 September 2018 Revised Date:
19 October 2018
Accepted Date: 9 November 2018
Please cite this article as: D.T. Vaimakis-Tsogkas, D.G. Bekas, T. Giannakopoulou, N. Todorova, A.S. Paipetis, N.-M. Barkoula, Effect of TiO2 addition/coating on the performance of polydimethylsiloxanebased silicone elastomers for outdoor applications, Materials Chemistry and Physics (2018), doi: https:// doi.org/10.1016/j.matchemphys.2018.11.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Effect of TiO2 addition/coating on the performance of polydimethylsiloxanebased silicone elastomers for outdoor applications D. T. Vaimakis-Tsogkas1, D. G. Bekas1,2, T. Giannakopoulou3, N. Todorova3, A. S. Paipetis1, N.-M. Barkoula1* Materials Science & Engineering Department, University of Ioannina, Ioannina
45110, Greece 2
Department of Aeronautics, Imperial College London, South Kensington Campus,
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Exhibition Road, SW7 2AZ, London, UK 3
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1
Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Agia Paraskevi,
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15341, Greece
*corresponding author: [email protected]
Abstract
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In the current study we investigated the effect of TiO2 addition on the performance of silicone elastomer films with focus on their response under UV radiation. TiO2 nanoparticles (0.1 – 5 wt. %) were dispersed to a poly-dimethylsiloxane-based
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silicone and the behavior of the films was assessed as a function of TiO2 content. TiO2
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nanoparticles (0.1 wt. %) were also deposited as a coating on silicone elasomer films. The dispersion of TiO2 nanoparticles into the silicone matrix resulted in an overall increase of strength (up to app. 32%) and strain at break (up to app. 44%) as well as in reduced Young’s Modulus (up to app. 30%). Application of TiO2 as coating resulted in lower stiffness (app. 60%) and strength (app. 43%), and almost identical strain at break compared to the unmodified silicone matrix. An increase in the thermal stability (up to app. 5 °C) and dielectric permittivity (up to app. 10%) was observed with increased TiO2 content. Furthermore addition of TiO2 resulted in higher stability
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ACCEPTED MANUSCRIPT against UV radiation and contributed to photocatalytic NO oxidation. Best performance was found in coated films which presented limited deterioration in their strength (residual values 65% vs. 23% for unmodified ones) and enhanced NO conversion. Overall it was shown that addition of small amounts of TiO2, especially in
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the form of coating, significantly improved the performance of silicone elastomer films for outdoor applications.
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Keywords: TiO2, silicone, PDMS, photocatalysis, UV resistance
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1. Introduction
Polydimethylsiloxane (PDMS) is by far the most widely used silicone elastomer in industries as diverse as textiles, automobiles, aerospace, construction, electronics, and biomedical materials thanks to its soft and flexible nature, transparency to ultraviolet
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and visible light, permeability to oxygen, high dielectric constant, low permeability to water, excellent thermal and chemical stability, resistance to UV radiation and weathering, nontoxicity and low cost [1-7]. PDMS has recently drawn the attention of
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engineers who aim at introducing dielectric elastomers in architectural applications in
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order to mimic nature and create dynamic structural elements and full-scale kinetic surfaces [8].
TiO2 has attracted significant amount of interest as filler in PDMS-based composites, due to its relatively high permittivity, chemical inertness, non-toxicity and ease of dispersion [9-13]. Relevant studies concluded that the addition of an appropriate amount of TiO2 may result in increased dielectric permittivity and reduced modulus of elasticity. Apart from its high permittivity, TiO2 has been very frequently used as a photo-catalyst due to its strong oxidative ability [14]. TiO2 has been shown to
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ACCEPTED MANUSCRIPT effectively degrade air pollutants (e.g. NO, NO2) in indoor and outdoor environments [14-16]. At the same time, TiO2 has been widely used as a UV-blocking additive due to its UV-absorption capacity, in addition to its reflecting and/or scattering capability, minimizing photodegradation [17-19]. The anatase polymorph of TiO2 is considered
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to be more active photo-catalyst, while the rutile polymorph has been used as UV absorber due to its high refractive index and hiding power, as well as good chemical stability and UV light screening effects [17]. Similar to the liquid and gaseous TiO2
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photocatalytic reactions, the free radicals created during UV radiation may however
matrix in the solid phase [20-25].
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attack the polymeric chains and initiate photocatalytic degradation of the polymer
Based on the above, it is clear that the addition of TiO2 in PDMS-based elastomers may significantly affect their performance, especially in outdoor applications. Degradation of air pollutants is of particular interest for architectural applications.
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Facades and kinetic surfaces made out of TiO2 modified PDMS-based elastomers may function as adaptive structural elements and air purification systems simultaneously. On the other hand, durability of the PDMS-based elastomers after TiO2 modification,
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and maintenance of their long-term structural integrity is of key importance for their
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successful application. Very few studies however exist on the effect of TiO2 addition on the photodegradation and photocatalytic performance of PDMS-based elastomer films [26-29]. It has been documented that TiO2 promote the photodegradation of salicylic acid in aqueous solution and show good photocatalytic performance for the decontamination of water and decomposition of organic dyes. However, no information is available on the durability of TiO2 modified PDMS-based elastomers. Thus, the scope of the current study is to offer an understanding on the effect of TiO2 modification on the overall performance of PDMS-based elastomers, including their
3
ACCEPTED MANUSCRIPT response under UV radiation. TiO2 nanoparticles (80/20 anatase/rutile) were added at 0.1 – 5 wt. % loadings in the silicone matrix and their effect on mechanical, thermal, thermomechanical, dielectric and photo-catalytic properties of the composite were investigated. For comparison reasons, PDMS was also coated with TiO2
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nanoparticles. The amount of TiO2 in the form of coating was kept low (0.1 wt. %) to ensure stable and reproducible deposition as well as minimize the potential degradation due to extensive UV absorption. To the best of our knowledge the effect
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of UV radiation on the residual mechanical properties and NO oxidation process of
part of this study.
2. Materials and Methods
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TiO2 modified PDMS-based silicone elastomers is been assessed for the first time as
2.1 Materials and specimens preparation
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A commercial polydimethylsiloxane (PDMS)-based silicone (TC-5005 A/B-C, BJB Enterprises Inc., USA.) and titanium dioxide (TiO2) (AEROXIDE TiO2 P25, Evonik Industries, Germany) in the form of fine powder were used as matrix and filler,
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respectively. AEROXIDE TiO2 P25 consists of aggregated primary particles with size
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of several hundred nm and mean diameter of app. 21 nm. The weight ratio of anatase and rutile is app. 80/20 which provides high photocatalytic activity (due to the high amount of the anatase polymorph in the powder). According to the manufacturer specifications, PDMS was processed by adding curing agent B at a ratio of 10:100 w/w of prepolymer A. Components A and B were hand mixed for app. 5 minutes. Component C (plasticizer) was added at 50:100 w/w of the mixture A/B in order to increase the softness of the final elastomer. TiO2 previously dried at 100°C for 24h in vacuum oven (Thermo Scientific – Heaaeus Vacutherm), was dispersed in component
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ACCEPTED MANUSCRIPT C using a sonication bath (Branson 3510 Ultrasonics Corp) for 30 s, with pulse frequency set at 10 Hz and pulse width at 50%. At the end of the dispersion process, appropriate amounts of C/TiO2 and A/B were hand mixed for app. 5 min in a laboratory beaker following the manufacturer’s specifications, to obtain mixtures with
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0.5-5 wt. % TiO2. Τhe different mixtures were placed in aluminum molds and allowed to crosslink at room temperature for 24 hours which resulted in silicone-based films with an average thickness of app. 1 mm. Instead of dispersion, one series of samples
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was created by coating TiO2 onto the silicone matrix. For this purpose, neat silicone films were placed on the aluminum mold and TiO2 powder was deposited on the films
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using a 200 mm diameter laboratory sieve. The amounts of the different components were adjusted in order to reach a silicone film coated with 0.1 wt. % TiO2. Table 1 provides an overview of the prepared films and their designation.
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Table 1: Amounts (wt. %) of prepolymer A, curing agent B, plasticizer C and TiO2 and designation of the prepared PDMS-based nanocomposite films (*c denotes that TiO2 has been applied as coating on the films) A
B
C
TiO2
wt. (%)
wt. (%)
wt. (%)
wt. (%)
60.61
6.06
33.33
0
0.1
60.55
6.05
33.30
0.1
0.5
60.31
6.03
33.16
0.5
1
60.00
6.00
33.00
1
5
57.58
5.76
31.66
5
0.1c*
60.55
6.05
33.30
0.1
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0
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code name
2.2 Characterization Mechanical properties 5
ACCEPTED MANUSCRIPT To evaluate the effect of TiO2 addition on the mechanical properties dump-bell specimens were cut from the elastomer films using an ISO 527 type 4 die. Specimens were subjected to uniaxial tensile loading using an Instron universal testing machine equipped with a ±30kN load cell. Tests were performed at ambient temperature, at an
samples from each formulation were tested sequentially. Thermogravimetric Analysis (TGA)
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extension rate of 100 mm/min, as suggested by ASTM D412-06a. At least five
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The thermal stability of the elastomer films was monitored with a STA 449C analyzer (Netzsch-Gerätebau GmbH Germany) at a heating rate of 10oC/min over temperatures
atmosphere purge of 40 mL/min.
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ranging from 25oC to 1000oC on an average 20 mg samples, under a nitrogen
Differential Scanning Calorimetry (DSC)
Measurements were carried out using a STA 449C analyzer (Netzsch-Gerätebau
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GmbH Germany) at a constant heating and cooling rate of 20oC/min on 20 mg samples, in a flowing nitrogen atmosphere (N2). Samples were initially heated up from room temperature up to 100oC and held at that temperature for 5 min to
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eliminate previous residual thermal history. Afterwards, specimens were cooled down
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to -120oC and heated up to 0oC. Dynamic Mechanical Analysis (DMA) A DMA 242C apparatus (Netzsch-Gerätebau GmbH Germany) was used to evaluate the thermomechanical response of the elastomer films under tensile mode at a frequency of 1 Hz, and temperatures ranging from −125 to 25oC (rate 5°C/min). The amplitude of the deformation was 200 µm. Dynamic Dielectric Analysis
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ACCEPTED MANUSCRIPT The dielectric properties were determined using a high-resolution dielectric spectrometer/frequency spectrum analyzer (DETA SCOPE L1 provided by Advice Ltd.) Samples were mounted on an interdigital dielectric sensor which was placed in a purposely-made dielectric cell. In order to ensure an effective contact between the
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material and the sensor, samples were slightly compressed during the dielectric measurements. The complex dielectric function of each sample was measured by performing consecutive isothermal frequency scans over a frequency range of 10 to
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105 Hz while the effective voltage amplitude was selected at 5 V. It must be pointed out that the thickness of all specimens was the same and the recorded differences in
of the PDMS. Photocatalytic oxidation reactions
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terms of the complex dielectric functions can be solely attributed to the modification
The photocatalytic oxidation of nitric oxide (NO) was monitored using a standard
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procedure [30, 31] based on ISO/DIS 22197-1. The samples were placed in a flowtype photocatalytic reactor and their photocatalytic activity was evaluated under UVA light illumination with intensity 10 W/m2 for 30 min. The samples were exposed to
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model air containing 1 ppm NO with flow rate 3 L/min. The starting concentration of
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NO2 was ∼0 ppm and the NOx concentration was calculated as the sum of NO and NO2 concentrations. The photocatalytic tests were performed at room temperature. Photodegradation Tests Photodegradation tests were performed using a SUNTEST XLS+ (Atlas MTS) sunlight simulator. Selected specimens were placed in the Weatherometer, having a 2200 W air cooled xenon light source that produce an energy spectrum similar to natural sunlight. Specimens were exposed at four different time periods: 8, 24, 72 and 216 h. The mechanical properties of the films after radiation were determined as
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ACCEPTED MANUSCRIPT described above. The fractured cross-sections of 216h irradiated samples were sputtered with gold (Sc760 sputter coater Polaron) and inspected using Scanning Electron Microscope (SEM) (JEOL JSM 6510LV SEM/ Oxford Instruments X- Act
3. Results and Discussion Mechanical, thermal and thermo-mechanical properties
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EDX).
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In order to evaluate the effect of TiO2 addition on the mechanical behavior representative stress-strain curves of the silicone-based elastomer films are presented
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in Fig. 1 while key mechanical properties, comprising of Young’s Modulus (E), tensile strength (σts) and strain at break (εb), are included in Table 2. All tested films presented a rubber-like response with an initial linear region (up to app. 300% strain) followed by stress-hardening at higher deformations, indicative of molecular chain
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orientation. The initial linear part of the stress-strain curve was used for the estimation of Young’s Modulus of the tested films. The dispersion of TiO2 nanoparticles into the silicone matrix resulted in an overall increase in strength (up to app. 32%) and strain
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at break (up to app. 44%) and reduced Young’s Modulus (up to app. 30 %). However,
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no straight relationship between mechanical properties and TiO2 content was found, as observed in Table 2. Stiffness and strength increased steadily up to 0.5 wt. % TiO2 content while non monotonic evolution was found in stain at break with the TiO2 content. The addition of 0.1 and 1 wt. % TiO2 nanoparticles led into highest stain at break and lowest stiffness compared with unmodified matrix while the highest reinforcement was obtained at 0.5 wt. % loading. The application of TiO2 as coating resulted in lower stiffness (app. 60%) and strength (app. 43%), and almost identical strain at break compared to the unmodified silicone matrix. Although one would
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ACCEPTED MANUSCRIPT expect that the dispersion of nano-TiO2 would lead to reinforcement of the silicone matrix manifested as enhanced stiffness and strength and hence lower strain at break values, this is not the case here. This phenomenon has been documented previously and has been attributed to different antagonistic mechanisms [5, 32]. It has been
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suggested that the nanometer sized TiO2 particles interfere with the crosslinking process of the silicone elastomer resulting in softening [5, 12]. This is counterbalanced to some extend by the hardening effects due to the intrinsic high
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elastic modulus of TiO2. At the same time, the filler agglomeration at high TiO2 contents weakens the interface and results in slippage of polymer chains on the filler
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surface during deformation [12, 32]. Finally, due to the direct dispersion of TiO2 in the plasticizer part of the elastomer, it can be also claimed that the nanoparticles act as plasticizer distributors enhancing the strain at break of the elastomer films, as
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observed in the case of clay modified polymer films [33, 34].
Fig. 1: Representative stress-strain curves of silicone-based elastomers as a function of TiO2 content
Table 2: Effect of TiO2 addition on the mechanical, thermal and thermo-mechanical properties of silicone-based elastomers
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σTS
εb
Tpeak
mr
Tg
Tm
(kPa)
(MPa)
(%)
(°C)
(%)
(°C)
(°C)
0
38±4
1.02±0.20
1403±120
567.7
15.86
-115.8
-28.9
0.1
27±5
1.05±0.20
1861±181
570.5
16.67
-113.2
-23.6
0.5
38±4
1.35±0.02
1535±27
570.8
15.06
-116.7
-25.7
1
30±10
1.15±0.21
2014±151
571.4
16.37
-115.9
-25.9
5
32±8
1.33±0.19
1796±180
589.2
22.30
-115.7
-24.6
0.1c
25±5
0.58±0.07
1455±128
567.1
16.18
-115.9
-29.1
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specimen
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Fig. 2 shows the thermogravimetric weight loss curve (TG) and the weight loss
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derivative curve (DTG) of the silicone elastomer films before and after the addition of TiO2 nanoparticles. In agreement with previous observations degradation was found in temperatures ranging between 400 and 650 °C [37]. Although the starting decomposition temperature was almost the same for all tested films (Fig. 2a), the incorporation of TiO2 nanoparticles influenced the degradation peak temperature
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(Tpeak) (Fig. 2b) and the remaining silicone mass (mr) after decomposition (see Table 2). Overall an increase in both Tpeak and mr was observed with increased TiO2 content, which was more evident at 5 wt. % (up to 22 °C and 7%, respectively). This can be
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linked with the presence of the TiO2 nanoparticles, which act as barriers against heat
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and volatiles diffusion [28, 35, 36]. Coating of silicone elastomer with TiO2 did not contribute to the thermal stability as expected, since the nanoparticles interacted only with the surface of silicone and not with the volume of the material.
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Fig. 2: a) TG and b) DTG curves of silicone-based elastomers as a function of TiO2
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content
Fig. 3 shows the DSC thermograms of the unmodified and TiO2-modified silicone elastomer films. All curves presented an abrupt exothermic change in the heat flow at app. -110 °C and a main endothermic peak at app. -30 °C. This behavior is in agreement with previous observations on PDMS and silica-reinforced PDMS
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nanocomposites [37-39]. The first change can be associated with the glass transition temperature (Tg) of the amorphous part of the silicone-based elastomers, while the
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second peak is related to the melting of the crystalline fraction of the materials (Tm) (see Table 2). Intermediate peaks between Tg and Tm can be associated with
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crystallization and lamellar thickening of the silicone-based elastomers upon heating [38]. Tg did not show any significant variation with the addition of TiO2 nanoparticles. The addition of TiO2 resulted however to an up-to 5 °C increase in Tm. Similar trends were observed in silica modified PDMS composites, where nanoclusters did not modify the relaxation behavior of the amorphous phase and the glass transition kinetics, they however promoted crystallization of the nanocomposites [38].
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Fig. 3: DSC thermograms of silicone-based elastomers as a function of TiO2 content.
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The storage modulus (Eˊ), loss modulus (E˝) and loss factor (tanδ) response of the silicone-based elastomers is being illustrated in Fig. 4. Unmodified and TiO2 modified samples behaved similarly presenting a small steady decrease in Eˊ and E˝ as the temperature increased, followed by a steep drop (2-3 orders of magnitude) of moduli
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at app. -60 °C and a plateau at higher temperatures. At the same temperature range, tanδ curves presented a step increase followed by a very broad peak. Normally one would ascribe such change to the glass transition region [28], however as discussed
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previously, the peak absence in the E˝ curve suggests that this behavior is due to the melting process of the crystalline regions of the semi-crystalline silicone elastomer [7,
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38, 39]. TiO2 addition resulted in a small shift of the transition towards higher temperatures, in agreement with the DSC results. Furthermore an increase was observed in Eˊ and E˝ after the addition of TiO2 nanoparticles, which was more pronounced in the rubbery region. The stiffness increase was higher than the losses in that region, resulting in smaller damping of the TiO2 modified samples. The increment of the Eˊ in the -95° to -65°C region has been attributed to secondary crystallization and/or lamellar thickening while at higher temperatures this
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silicone elastomers [38].
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Fig. 4: a) Storage Modulus (Eˊ), b) Loss Modulus (E˝) and c) Loss factor (tanδ) of
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silicone-based elastomers modified with various amounts of TiO2
Dielectric properties
The complex dielectric permittivity ε* is a very important electrical property of a dielectric elastomer that greatly affects the force required for its actuation [40]. The complex dielectric permittivity (ε*= εˊ-iε˝), can be expressed by a real part, which is
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the dielectric permittivity (εˊ), an imaginary part called dielectric loss (ε˝) and their ratio, which is the dissipation factor (tanδe = ε˝/εˊ), in analogy with the dynamic
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mechanical response of the materials.
Fig. 5 illustrates the effect of the TiO2 addition on the dielectric permittivity (εˊ), the
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dielectric loss (ε˝) and the dissipation factor (tanδe) of the tested elastomers. As observed in Fig. 5a, independent to the TiO2 content, the dielectric permittivity of the silicone elastomers decreased with frequency which denotes that the material dipoles were not able to efficiently follow the external field changes at high frequencies [41]. Even though the permittivity values of TiO2 is several orders of magnitude higher than that of the silicone elastomer, the addition of small contents of TiO2 nanoparticles did not significantly affect the permittivity of the system. After the addition of 1 wt. % TiO2 a small increase in permittivity throughout the entire 13
ACCEPTED MANUSCRIPT frequency spectrum was observed which become more evident at 5 wt. % TiO2 content. The improvement in permittivity (app. 10%) is in line with previous observations and is quite significant considering the very low TiO2 content [5, 9]. The addition of small amounts of TiO2 (<0.5 wt. %) resulted in small decrease in the
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dielectric loss, while increased TiO2 contents led to higher losses at the low and high end of the frequency spectrum (Fig. 5b). As aforementioned, the dielectric loss greatly affects the electrical performance since its increase results in higher conductivity of
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the elastomer [11]. At frequencies between 102 and 104 Hz, the dielectric losses of all TiO2 modified elastomers were very close to those of the unmodified system. Thus,
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the increase in dielectric loss is not expected to elevate the material temperature and the dissipation of the elastomer will not be considerably increased at intermediate
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frequencies as observed in Fig. 5c.
Fig. 5: a) Dielectric permittivity (εˊ), b) Dielectric Loss (ε˝) and c) Dissipation Factor
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(tanδe) of silicone-based elastomers modified with various amounts of TiO2
Photocatalytic oxidation reactions The results from the photocatalytic response of the prepared materials are presented in Fig. 6. As illustrated on the example of the coated material (0.1c sample), NO adsorption/desorption equilibrium in dark was initially reached over the material that is visualized as a peak in the curves before the illumination point (Fig 6a). After the light was turned on, a decrease of NO concentration and a simultaneous increase of 14
ACCEPTED MANUSCRIPT NO2 concentrations occurred followed by stabilization of the gas concentrations. The variation of the NO, NO2 and NOx concentration recorded for rest of the samples is not presented due to similarity. The photocatalytic activity is related to the intensity of the processes: NO to NO2
following parameters: decrease of NO (1ppm-NO and removal of NOx (1ppm – NOx
meas).
measured),
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oxidation and NO2 to NO3- oxidation. The processes can be demonstrated by the NO2 formation (NO2 meas)
The parameters’ values for the neat and
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composite samples taken after the gas stabilization are given in Fig. 6b. The neat sample showed no photocatalytic activity, as expected. In general, the TiO2 containing
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composites exhibited very low activity that can be ascribed to the absence of a sufficient amount of TiO2 on the surface of the silicone. A small increase in the NO oxidation and NOx removal was recorded with the increase of the TiO2 loading in the composites. The sample with the highest photocatalytic activity was 0.1c. This was
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attributed to the enrichment of the silicone surface with TiO2 photocatalyst. The NO concentration remained constant till the end of the illumination evidencing the
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absence of catalyst saturation by reaction products.
Fig. 6: a) Concentration profiles of NO, NO2 and NOx gases over the coated sample (0.1c) under UV irradiation and b) NO conversion by silicone-based elastomers modified with various amounts of TiO2 15
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Photodegradation Tests As discussed previously the addition of TiO2 is expected to alter the mechanical response of the silicone elastomer films under UV radiation. In order to verify this
TiO2-modified silicone elastomers after UV radiation.
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effect, Fig. 7 illustrates the residual mechanical properties of unmodified and selected
As seen in Fig. 7a, unmodified and films with dispersed TiO2 nanoparticles presented
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a reduction in Young’s modulus after 8h of irradiation followed by a respective increase with irradiation time. At the same time, the results revealed an abrupt
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reduction in mechanical strength and strain at break within the first 24h of irradiation, while further exposure resulted in limited degradation (Fig. 7b-c). Dispersion of small amounts of TiO2 within the silicone elastomer matrix led to higher stability against UV radiation, since residual strength increased from 23 to 30% at steady state and
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respective residual strain from 38 to 46%. Application of TiO2 nanoparticles as coating resulted in steady increase of the residual Young’s modulus and limited degradation of the strength, with residual values as high as 65% compared to app.
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23% found in unmodified films.
Fig. 7: Residual mechanical properties of silicone-based elastomers as a function of Ti02 content and UV irradiation duration: a) E/E0, b) σ/σ0, and c) ε/ε0
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ACCEPTED MANUSCRIPT The observed reduction in strength and ductility after UV radiation can be explained on the basis of oxidative degradation of the chains of the silicone elastomer which resulted in reduction of their molecular weight and this in turn led to deterioration of their mechanical properties. The variation in mechanical properties with composition
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and exposure time indicates that TiO2 nanoparticles interfere with the crosslinking process and/or UV absorbance process in the elastomer films. As discussed in the past irradiation results in intermolecular crosslinking of silicone rubbers as well as
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increased apparent crosslink density due to chemical links at the polymer-particle interface of nanomodified elastomers [42, 43]. Thus, the increase in Young’s modulus
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with exposure time implies an increase in the degree of crosslinking due to UV exposure, which compensates the reduction in stiffness due to chain scission [7]. Increase in the photostability of the modified silicone elastomer films especially after coating can be associated with the ability of TiO2 to absorb the UV radiation and
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transform it into thermal energy inline with the photocatalysis results. Overall modification with TiO2 had a positive influence on the photostability of silicone elastomer in agreement with previous results on nano-rutile TiO2 natural rubber
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composites [18].
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Images of the fractured cross-sections after UV radiation provide further insight into the effect of TiO2 addition on the photostability of the elastomeric films. Unmodified films (Fig. 8a) and films with 0.1 wt. % TiO2 (Fig. 8b) presented parting lines which were almost eliminated after the dispersion of 1 wt. % TiO2 (Fig. 8c). On the other hand absence of cracks and evidence of plastic deformation was seen after the application of TiO2 as coating (Fig. 8d). These observations support the fact that dispersion of higher amount of TiO2 nanoparticles or application as coating increases
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ACCEPTED MANUSCRIPT the photostability of the modified silicone elastomer films due to shielding and/or
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absorption of the UV radiation and transforming into heat.
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Fig. 8: Fractured surfaces of silicone-based elastomers after 216h of UV irradiation as a function of TiO2 content: a) 0 wt. %, b) 0.1 wt. %, c) 1 wt. % and d) 0.1c wt. %
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4. Conclusions
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In this study the performance of TiO2-modified PDMS-based silicone elastomers as a function of TiO2 content and application method (dispersion vs. coating) was evaluated. Focus was given on the performance of the films after UV irradiation. Based on the mechanical results it can be concluded that the addition of TiO2 was beneficial to the performance of the PDMS-based silicone elastomers since it lowered the Young’s Modulus and enhanced the strain at break of the obtained films. Furthermore the thermal stability was improved after the addition of higher amounts of TiO2 (5 wt. %). A positive impact was also found on the thermomechanical
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ACCEPTED MANUSCRIPT properties with TiO2 modified films presenting smaller damping compared to unmodified ones. Addition of TiO2 improved the photostability of the silicone elastomer films, with coated films showing the best performance against deterioration of mechanical
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properties after UV radiation. Coated films presented also the highest rate of NO oxidation. Dielectric permittivity increased with higher TiO2 contents, however dielectric losses also increased. Overall it can be concluded that addition of small
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amounts of TiO2, especially in the form of coating, significantly improve the performance of PDMS-based silicone elastomer films for outdoor applications since
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they inhibit the mechanical deterioration due to UV irradiation of the films and at the same time can contribute to the air purification by photocatalytic NO oxidation.
Acknowledgements
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The authors wish to thank the Ring of the Laboratory Units and Centers and the MSSNDE Laboratory of the University of Ioannina for providing access to the thermal analysis unit, scanning electron microscopy unit and mechanical testing facilities,
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respectively. Special thanks to Prof. T. Vaimakis for the support during the execution
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of the DSC and TGA experiments and to Prof. Konstaninou for providing access to the Suntest apparatus.
Conflicts of interest
The authors wish to declare that there is no conflict of interest in this research work.
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ACCEPTED MANUSCRIPT Table captions Table 1: Amounts (wt. %) of prepolymer A, curing agent B, plasticizer C and TiO2 and designation of the prepared PDMS-based nanocomposite films (*c denotes that TiO2 has been applied as coating on the films)
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properties of silicone-based elastomers
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Table 2: Effect of TiO2 addition on the mechanical, thermal and thermo-mechanical
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ACCEPTED MANUSCRIPT Figure captions Fig. 1: Representative stress-strain curves of silicone-based elastomers as a function of TiO2 content Fig. 2: a) TG and b) DTG curves of silicone-based elastomers as a function of TiO2
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content
Fig. 3: DSC thermograms of silicone-based elastomers as a function of TiO2 content. Fig. 4: a) Storage Modulus (Eˊ), b) Loss Modulus (E˝) and c) Loss factor (tanδ) of
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silicone-based elastomers modified with various amounts of TiO2
Fig. 5: a) Dielectric permittivity (εˊ), b) Dielectric Loss (ε˝) and c) Dissipation Factor
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(tanδe) of silicone-based elastomers modified with various amounts of TiO2 Fig. 6: a) Concentration profiles of NO, NO2 and NOx gases over the coated sample (0.1c) under UV irradiation and b) NO conversion by silicone-based elastomers modified with various amounts of TiO2
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Fig. 7: Residual mechanical properties of silicone-based elastomers as a function of Ti02 content and UV irradiation duration: a) E/E0, b) σ/σ0, and c) ε/ε0 Fig. 8: Fractured surfaces of silicone-based elastomers after 216h of UV irradiation as
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a function of TiO2 content: a) 0 wt. %, b) 0.1 wt. %, c) 1 wt. % and d) 0.1c wt. %
1
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Increased strength and strain at break due to dispersion of TiO2 in PDMS matrix
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Higher thermal stability and dielectric permittivity in TiO2 modified PDMS
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films Improved performance of PDMS for outdoor applications after the addition of small amounts of TiO2 -
Coating with TiO2 is more effective than addition through dispersion in PDMS
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matrix