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The effect of layered silicates on the crosslinking reaction of Silanol-terminated polysiloxane.

INTRODUCTION

Poly(dimethylsiloxane) (PDMS) is probably one of the most important elastomers due to its high thermal stability, low surface tension, and outstanding dielectric properties. The above elastomers are produced by vulcanization, i.e., a cross-linking reaction, converting linear elastomeric chains to a three-dimensional network, characterized by the so-called "rubber elasticity." Most applications require that PDMS be reinforced with solid fillers, with fumed silica and titania being the most widely used materials for this reason. Alternatively, fillers having large specific area and aspect ratio, such as clay nanoplatelets, could give better mechanical properties at much lower filler content, keeping the material transparent and lightweight.

An overview of polymer-clay hybrid nanocomposites was published by LeBaron and Pinnavaia [1], with emphasis on the use of alkylammonium exchanged smectite clays as reinforcement for selected polymer matrices. In the case of mixtures of poly(dimethyl siloxane) and organoclays improvement in tensile properties, thermal stability and resistance to solvent swelling was reported.

Siloxane modified montmorillonite nanoclays as well as two commercially available organo-modifed nanoclays were incorpo rated to liquid silicone rubber (LSR) to form composite materials with superior barrier properties [2], Up to a 20% reduction in water vapor permeability was achieved, as well as a 24% improvement in tear strength and a 40% improvement in the compression set.

Kaneko et al. [3] used masterbatches of poly(dimethylsiloxane-co-3-ethylenepropylmethyl oxide siloxane) mixtures containing natural and organomodified clay, in order to prepare high molar mass PDMS rubber composites. Their results showed that compounding by using masterbatch of organomodified montmorillonite (O-MT) clay enhanced its dispersion into the PDMS matrix, resulting in a silicone rubber reinforced with clay nanolayers. Because of this morphology a great improvement of tensile strength and elongation at break was achieved at a low filler content (5 phr), compared to the properties obtained with similar, higher loaded (30 phr) particulate filled composites, obtained by direct O-MT addition.

Studies of PDMS/fluoroectorite systems by LeBaron and Pinnavaia [4], revealed the effect of parameters, such as the polarity match among the linear polymer, the gallery surface and the gallery cation of nanoclay, on the intercalation process of PDMS molecules.

A few recent studies were concerned with the characteristics of the elastomeric matrix and related mechanical properties. For example, Takeuchi and Cohen [5] published on systems of organomontmorillonite and PDMS networks, prepared from hydroxylor vinyl-terminated precursors. No improvement in the Young's modulus of the networks deriving from vinyl-terminated precursors was obtained. The above authors concluded that enhancement of the modulus was obtained only for nonoptimal networks, formed with hydroxyl-terminated precursor chains, in contrast with vinyl-terminated chains. Their results indicate that reinforcing effect of these elastomers can be attributed to an anchorage of hydroxyl end group to the silicate filler, which dramatically reduces the soluble fraction and binds the pendent chain ends.

The vulcanization of elastomeric materials is a complex process, which has a high impact on the properties of the final product. Therefore, it is important to monitor and control this cross-linking reaction. Several articles on the kinetics of vulcanization of various rubbers are available in the literature and particularly for silicone rubbers, which is the material under investigation in this work [6-10]. However, there are only a few reports on vulcanization kinetics of silicone rubber nanocomposites, including the changes of rheological and thermal behavior during vulcanization, due to the addition of nanofillers. On the other hand, there is a debate regarding the difference in the cure rate of organomodified elastomers compared with the cure rate of the pure elastomer. It was reported that the rate of cure increases [11-14], remains unchanged [15, 16] or even decreases [17, 18].

Monsanto measurements on natural rubber (NR) reinforced with octadecylamine-modified bentonite have shown that the organoclay accelerates vulcanization reaction and gives rise to a marked increase of torque, indicating that the elastomer becomes more densely crosslinked [19]. These results are in agreement with those obtained from swelling experiments and from the study of vulcanization reaction with thermal analysis, where an appreciable increase of the involved heat from curing reactions has been observed. Moreover, thermodynamic parameters, conformational entropy [DELTA]S and elastic Gibbs free energy ([DELTA]G), have shown an increase in the structural order of the nanocomposite [19].

The effect of the incorporation of bentonite on the vulcanization kinetics of NR was investigated by means of both, curemeter and differential scanning calorimetry (DSC), under dynamic and isothermal conditions (11). A marked decrease in the induction time and optimum cure time of the elastomer was observed in the presence of organoclay. Although the octadecylamine itself accelerated the vulcanization process, the octadecylamine-modified clay gave rise to a further noticeable increase of the vulcanization rate, due to a synergistic effect between filler and amine. Moreover, in the presence of the organoclay, a dramatic increase in torque value was obtained because of the formation of higher number of crosslinks, which could be attributed to the confinement of the elastomer chains within the silicate galleries and consequently, to better interactions between filler particles and rubber.

Carretero-Gonzalez et al. [20] studied the influence of nanoclay on the morphological and microstructural changes of NR network, based on the results from broadband dielectric spectroscopy and in situ synchrotron wide-angle X-ray diffraction (XRD). It was found that the presence of nanoclay introduces a dual crystallization mechanism due to the alignment of nanoparticles during stretching. Moreover, the presence of strong interfacial adhesion between nanoparticle-rubber matrix can induce an early promotion and enhancement of overall crystallization of NR chains under uniaxial stretching. The above author reported that organoclay does not affect the extent of curing and the observed increase in modulus was attributed to: (a) the hydrodynamic effect of clay and (b) the formation of physical crosslinks due to the presence of clay.

Cataldo [12] observed that the addition of organo-modified montmorillonite in a natural rubber/SBR-based elastomer causes an increase in the cure rate and this effect was explained by the high concentration of ammonium cations present in the exfoliated clay.

The vulcanization kinetics of fluoroelastomer (FKM) filled with unmodified and organo-modified clay was studied by Kader and Nah [13], with both, oscillating disc rheometer and DSC under isothermal and dynamic conditions. The organo-modified clay enhanced curing through the accelerating effect of quarternary ammonium salt used in the clay modification, while the unmodified clay showed cure retardation due to absorption of curative for in situ clay modification. The kinetic analysis showed the suitability of autocatalytic model for cure characterization. The determined kinetic parameters were in good agreement with the experimental values. The result indicated that the organoclay was efficient in reducing the energy requirements for curing.

Decrease in gel fraction (GF) with increasing clay content was observed by Kong et al. [21], in silicone rubber nanocomposites reinforced with Fe-MMT and Na-MMT, modified with cetylthimethyl ammonium bromide. The decrease in GF values was due to the additives of clay having the action of blocking free radicals during the formation of the network structure.

Schmidt and Giannelis [22], in their study on silicate dispersion and mechanical reinforcement in polysiloxane/layered silicate nanocomposites, reported that increased nanofiller content leads to lower swelling, given that compatible nanofillers should increase the (physical) crosslink density. The effectiveness of nanofillers in increasing physical crosslink density is dependent on the effective interfacial area and interfacial interaction strength.

In this work the effect of different types of MMT on the vulcanization reaction of hydroxy-terminated PDMS was studied through viscosity measurements and infrared spectroscopy analysis. To elucidate a rather complex interaction, additional analytical test methods were performed, such as Temperature Modulated DSC for investigation of the conversion versus time, based on the changes of enthalpy of cold crystallization and heat capacity of the system, during the crosslinking reaction. The cure mechanism was further explored by experiments using Brookfield viscosimetry, infrared spectroscopy (attenuated total reflectance Fourier transform infrared spectroscopy [ATR-FT1R]), XRD and swelling which allowed us to speculate on the structural changes that occur upon incorporation of the clay nanoparticles to PDMS. The main scope of this work was to investigate the structural changes taking place during vulcanization of PDMS in the presence of a nanofiller. The conclusions drawn from this study might be of big importance for clinical practice, when designing applications with biomaterials based on PDMS.

EXPERIMENTAL

Materials

Silanol-terminated PDMS. grade DMS-S35 (Gelest) of molecular weight 49,000 g [mol.sup.-1], was the silicone base elastomer used in this work. The vulcanization reaction system consisted of 10 phr tetrapropoxysilane (TPOS, Aldrich) as a crosslinkcr and 0.1 phr dibutyl tin dilaurate (Aldrich) as catalyst.

Commercial MMT clays, under the trade name Nanofil1 116 (Nani 16), Cloisite[R] 30B (C130B) and Cloisite[R] 20A (C120A) supplied by Rockwood Clay Additives GmbH, were used as the reinforcing nanofiller. The main characteristics of organoclays are presented in Table 1.

Preparation of Nanocomposites

Homogeneous dispersions of the nanoparticles in PDMS were obtained by sonicating the mixtures with an ultrasound probe for 6 min at room temperature. The crosslinking agents were then added and dispersed into the mixture. The samples were cast into molds and cured at room temperature for 12 h.

[FIGURE 1 OMITTED]

X-ray Diffractometer

XRD analysis of the clays and nanocomposite samples was performed in order to detect the evolution of the clay [d.sub.001] reflection. A Siemens 5000 apparatus (35 kV, 25 mA) was employed, using Cu K[alpha] X-ray radiation with a wavelength of [lambda] = 0.154 nm. The diffractograms were scanned in the 20 range from 2[degrees] to 10[degrees], at a rate of 2[degrees] [min.sup.-1].

Brookfield Viscosimetry

A Brookfield viscosimeter, model Brookfield DV-1I+ Pro, was used for the study of rheological behavior of PDMS systems during vulcanization. Measurements were conducted using spindle RV6 at a rotation speed of 15 rpm and ambient temperature.

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

For the determination of changes due to vulcanization reactions in the clay/PDMS system, FT1R-ATR measurements were performed, using a Nicolet FT1R spectrometer (model Magna 1R 750; DTGS detector; Nichrome source; beamsplitter; KBr). A total of 100 scans were applied with a resolution up to 4 [cm.sup.-1]. Spectra were obtained at the attenuated total reflectance (ATR) mode using a standard ZnSe 45[degrees] flat plate Contact Sampler (12 reflections; Spectra-Tech, USA) on which samples of PDMS were placed (100 [micro]l). The obtained spectroscopic data were treated using the standard software (OMNIC 3.1, Nicolet).

All spectra were smoothed using the "automatic smooth" function of the above software, which uses the Savitsky--Golay algorithm (five-point moving second-degree polynomial). After the above procedure, the baseline was corrected using the "automatic baseline correct" function.

Differential Scanning Calorimetry

DSC measurements were run in a DSC I model Mettler Toledo differential scanning calorimeter. Temperature modulated differential scanning calorimetry (TMDSC) method, also called Alternating DSC (ADSC), in which temperature modulation is superimposed on the constant heating rate of a conventional DSC, was used for the study of PDMS vulcanization. Measurements were performed at a heating rate 5[degrees]C [min.sup.-1], the temperature amplitude was [+ or -] 0.4[degrees]C, with time period of 48 s and temperature range of the experiments -150 to 20[degrees]C. Samples of ~10 mg were accurately weighed in an analytical balance and encapsulated in aluminum pans. Samples from the vulcanizing mixture were placed in the DSC cell, which was cooled to - 150[degrees]C, and then heated. All runs were conducted under nitrogen flow of 20 [cm.sup.3] [min.sup.-1] to limit thermo-oxidative degradation. Pure PDMS and its nanocomposites loaded with 2 and 5 phr Cloisite 20A and Cloisite 30B were investigated.

The obtained data were analyzed to determine the total, reversing and nonreversing heat flows, as well as in terms of complex, in-phase and out-of-phase specific heat capacity.

Swelling Experiments

The solvent uptake of immersed PDMS nanocomposite samples was also measured at 25[degrees]C. Preweighed samples were immersed in toluene (Chemical pure, Merck) and at different time intervals, the swollen samples were removed, blotted and rapidly reweighed to minimize evaporation of toluene. Reweighing continued for a few days until reaching equilibrium swelling.

RESULTS AND DISCUSSION

X-ray Diffraction

A series of XRD patterns of PDMS composites, containing various clay loadings, are presented in Fig. 1. The XRD patterns of Nanofil 116, Cloisite 20A and Cloisite 30B show a diffraction peak at 20 values of 7.48[degrees], 3.93[degrees], and 4.88[degrees], respectively. As expected, the unmodified MMT (Nanofil 116), does not undergo any significant changes in the d-spacing after incorporation into PDMS, as shown in Fig. 1a. This suggests the formation of a conventional composite, where the polymer is not intercalated into the clay galleries. In contrast, d-spacing of both types of organically modified montmorillonites used in this work, increases after incorporation into silicone elastomer. The featureless patterns of PDMS hybrids for loadings of Cloisite 20A (Fig. 1b) ranging from 1 to 5 phr and for those of Cloisite 30B (Fig. 1c) in the range 1-3.5 phr, indicate that mixed exfoliated/intercalated structures were formed. In Fig. 1b it can also be observed that the scattering peak was significantly broadened in comparison with that of neat Cloisite 20A, suggesting that dispersion of the particles became random. In contrast, sharper peaks are obtained for Cloisite 30B/PDMS nanocomposite, which is an evidence of good ordering.

Generally, the driving force of the PDMS to penetrate the clay galleries results from the enthalphic contribution related to the establishment of many favorable polar polymer-surface interactions [23]. It would be reasonable that --OH-terminated groups of polysiloxane matrix interact with hydroxy groups present on the nanoclay surface, thus facilitating filler exfoliation. In addition, hydrophilicity of the organic surfactant could also be essential for a good dispersion and exfoliation of organoclay reinforcement. In the case of Cloisite 30B, the hydroxy and other polar groups of the organic surfactant are expected to interact with the hydroxy end groups of PDMS and promote intercalation of clay platelets by the elastomer molecules. On the other hand, Cloisite 20A is characterized by relatively high content of organic modifier and high intergallery spacing, as can be seen in Table 1, which is also helpful to the intercalation process by PDMS molecules. The longer vulcanization time required for Cloisite 20A/PDMS hybrids, as observed in Table 2, allows the silicone molecules to penetrate into the clay galleries, giving enough time for intercalation of OMMT, before the increase of viscosity and gelation of the silicone elastomer being on the process of crosslinking.

Brookfield Viscosimetry

Dispersion of high aspect ratio/elongated nanoplatelets in a fluid is known to significantly increase its viscosity and, therefore, the viscosity of uncured materials is reasonable to increase with increasing the efficiency of clay dispersion [24].

The Effect of Clay Content

The changes of viscosity with time for clay/PDMS hybrids indicate faster curing (Fig. 2) of these systems in comparison with pure PDMS. The acceleration of curing, assessed by the increase of viscosity, is proportional to the content of nanoclay. Probably, some interactions between clay system and polysiloxane matrix are responsible for the significant changes of vulcanization rate of reinforced specimens as compared with pure PDMS. The increase of viscosity during vulcanization of nanocomposites can be attributed to higher cross-links density, which might be the result of the confinement of the elastomer chains within the silicate galleries. This mechanism involves increased possibility for interactions, of physical or chemical nature, between filler and rubber and, therefore, the nanoclay particles act as additional crosslinking sites. In fact, condensation reactions between hydroxy groups of PDMS and those hydroxy groups existing on MMT layer surface could be possible [25]. Further to the above, it was reported that bonding between PDMS and silicates is likely to occur mainly at chain ends and not along the polymer chain backbone [26]. This might be further supported by the FT1R data presented in Fig. 3.

Inorganic nanoparticles are characterized by high aspect ratio, so the efficient dispersion of even a small amount of nanoparticles is enough to ensure interactions with PDMS. Furthermore, these nanoparticles promote physical absorption with the elastomer, which leads to the formation of highly ordered and entangled structures, where the mobility of rubber chains is restricted by the presence of neighboring chains [27].

In agreement with the above, the increase in torque values using oscillating disc rheometer, during vulcanization of a fluoroelastomer-clay nanocomposite was attributed by Kader and Nah [13] to the formation of confined elastomer network within the silicate galleries. An accelerating effect of quaternary ammonium salt modified clay was also reported.

The fact that unmodified MMT gave higher viscosity rate increase than the organic modified nanoclays hybrids with the same loading, can lead to the conclusion that organic modification does not essentially participate in the chemical interactions with silicone elastomers, or at least at the same extent with pure MMT nanoparlicles.

[FIGURE 2 OMITTED]

The Effect of Clay Type

Similar curing reaction rates were Found for nanocomposites of both organoclay types at concentrations of 3.5 and 5 phr. At higher loadings, Cloisite 20A gave higher rate of viscosity increase, in comparison with hybrids of Cloisite 30B, probably due to the better intercalation of layered silicates leading to increased reinforcement/matrix interfacial area, which efficiently promotes the interactions between filler and elastomer, leading to higher viscosity values.

An exponential equation was fitted to the viscosity-time data with very good accuracy for all the examined systems:

y = [y.sub.0] + A x exp (n x t) (1)

Figure 2 shows plots of the experimental data points together with the calculated data from the above equation (dashed black lines). Because [R.sup.2] values for those data are in the range o 0.997 to 0.999 a very close fit can be observed.

From the above fitting the exponent n, which is related to the viscosity increase rate, was calculated and the obtained results are presented in Table 3. From this table the higher rate of viscosity increase for Nanofil 116/PDMS systems was confirmed. Both types of organically modified clays showed almost similar n values for loadings up to 5 phr, but at higher filler concentrations nanocomposite hybrids with Cloisite 20A shifted to higher values.

[FIGURE 3 OMITTED]

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

The extent of vulcanization reaction was followed by infrared spectroscopy. Hydroxy-terminated silicones can be crosslinked by orthosilicate esters in the presence of organotin catalysts that are known to accelerate the cure process at room temperature. The reaction scheme involves hydrolysis of the ester group (with the liberation of by-products, usually ethyl alcohol) and then catalyzed condensation of hydroxy group and formation of Si-O-Si linkages. Because orthosilicate esters are tetrafunctional, cross-linking among the silicone chains takes place. The stretching vibration band of Si-O--Si crosslinks is observed between 900 and 1200 [cm.sup.-1].

Monitoring of vulcanization reaction using ATR-FTIR spectroscopy reveals an increase of the area of peaks corresponding to stretching of Si-O--Si bonds (900-1200 [cm.sup.-1]), especially after the first 40 min of reaction for pure PDMS and Cloisite 20A/PDMS nanocomposites (Fig. 3).

By integration the area of these peaks the extent of reaction for the examined PDMS systems was calculated (Fig. 4), which is related with the number of the Si--O--Si bonds.

However, in the case of Cloisite 30B/PDMS hybrids the contribution of hydroxy groups of the intercalation agent must be taken into consideration. The hydrophilic nature of hydroxy groups present in the organic modification of this type of MMT, possibly promotes the reaction with alkoxysilane catalyst, as a by-process of the crosslinking reaction during vulcanization (Fig. 5), and may participate in the network structure of the elastomer.

Wang et al. [28] reported that, in room temperature vulcanized silicon rubber (RTV-SR)/OMMT composites, the intensity of absorption peaks appears to be weak between 1070-1090 [cm.sup.-1] and 800 [cm.sup.-1], in comparison with pure RTV-SR. The above authors interpreted this behavior by chemical interactions of the alkoxyl silicone catalyst with hydroxyls present in OMMT as well as with the hydroxyl-containing base rubber.

Differential Scanning Calorimetry

Temperature modulated DSC (TMDSC) was used for monitoring the vulcanization reaction of hydroxyl-terminated PDMS/ organoclay nanocomposites, in comparison with pure PDMS systems. This technique allows the accurate monitoring of curing reaction of PDMS and elucidates the different mechanisms occurred during vulcanization process because of the presence of clay nanoparticles in the reaction system.

From Fig. 6, it can be observed that the heat flow of cold crystallization ([DELTA][H.sub.c]) decreases because of crystallization taking place during the vulcanization reaction, which is consistent with the results presented by Tang and Tsiang [29]. The decrease of the heat flow of cold crystallization prompted us to quantitatively correlate the degree of completion for vulcanization reaction, as follows:

Degree of completion for vulcanization = [DELTA][H.sub.0] - [DELTA][H.sub.t]/[DELTA][H.sub.0]

where [DELTA][H.sub.0] was the heat flow at the beginning of the vulcanization.

[FIGURE 4 OMITTED]

Another method for the determination of the degree of completion of vulcanization was through the change of heat capacity of the reaction mixture during vulcanization [29]. The heat capacity decreases with reaction time and levels off as the vulcanization reaction tends to complete. After this point, the structure of vulcanized elastomer does not essentially change and, therefore, the heat capacity remains constant. The degree of completion of vulcanization was thus calculated as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] were the heat capacities at the beginning and end of the vulcanization, respectively.

The data obtained from these two methods, presented in Figs. 7 and 8, are in excellent agreement with each other and confirm the suitability of the above approach.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Shorter vulcanization time was observed by increasing the clay content in the examined hydroxy-terminated PDMS hybrids. It was also found that for pure PDMS, the reaction proceeds at a relatively low rate for the first 200 min and then a significant acceleration takes place. A more even progress of the vulcanization reaction with time was observed with both types of the examined organoclay/PDMS composites, which might be due to restriction of the elastomer chains mobility, thus preventing acceleration.

[FIGURE 8 OMITTED]

A similar effect of nanoclay on the rubber microstructure was observed by Carretero-Gonzalez et al. [20] who have investigated natural rubber (NR) systems and reported that the inclusion of highly anisotropic nanoparticles leads to microscale segmental variations, giving rise to the formation of a relative homogeneous rubber network structure. In addition they claim that the presence of strong interfacial adhesion between nanoparticles and rubber can induce an early promotion and enhancement of overall crystallization of NR chains under uniaxial stretching. This behavior can also explain the higher viscosity values observed during vulcanization of clay/PDMS hybrids in comparison with unfiled PDMS. Using this technique, no significant difference in the vulcanization rate of the elastomer was observed by changing the type of clay reinforcement.

Equilibrium Swelling in Toluene of Clay/PDMS Hybrids

The interactions between polymer and filler play a critical role on the performance of silicone rubber/organoclay nanocomposites.

As can be seen in Table 4, swelling of reinforced silicone specimens show decreased solvent uptake as compared with those of unfilled PDMS. The main mechanism for the solvent uptake is the diffusion, which is primarily controlled by the crosslink density of the network. The clay nanoparticles can contribute to this effect as virtual crosslinks and therefore, restrict solvent absorption. However, each type of OMMT has a different effect on the swelling properties of polysiloxane matrix.

As already stated, for the nanocomposite systems studied in this work, the increased surface area of nanoparticles might promote some interactions with the silicone matrix and, therefore, it may lead to the formation of additional crosslinks, thus extending the crosslinked network.

From the result of Table 4, it can be observed that hybrids of Cloisite 20A gave lower toluene uptake especially at high clay loading. This effect is in agreement with that recorded by XRD analysis, where Cloisite 20A was found to give better exfoliated structures with PDMS matrix, and leads to an increase of the tortuosity path which inhibits diffusion of toluene through the bulk elastomer.

CONCLUSIONS

The incorporation of MMT nanoparticles results in a faster increase of the viscosity of PDMS during vulcanization and seems to affect the reaction parameters, due to physicochemical interactions among the crosslinker, hydroxy groups and clay nanoparticles.

Increase of the area of peaks corresponding to the formation of Si--O--Si bonds (900-1200 cm ') was recorded by FT1R spectroscopy for pure and reinforced PDMS. Modulated DSC was found suitable for accurate monitoring of the curing reaction of organoclay/condensation cured PDMS hybrids, through the enthalpy of cold crystallization and the heat capacity measurements. The incorporation of clay nanoparticles in polysiloxane results in a gradual increase of the curing rate whereas in case of unfilled elastomer an "incubation period" followed by a sharp increase in the reaction rate was recorded.

ACKNOWLEDGMENTS

Special thanks go to Dr. Dimitrios Korres for his assistance in the experimental set-up of the temperature modulated DSC experiments.

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Sozon P. Vasilakos, Marianna I. Triantou, Petroula A. Tarantili

Polymer Technology Laboratory, School of Chemical Engineering, National Technical University of Athens, Zographou, Athens 15780, Greece

Correspondence to: Petroula Tarantili; e-mail: taran@chemeng.ntua.gr

DOI 10.1002/pen.23965

Published online in Wiley Online Library (wileyonlinelibrary.com).
TABLE 1. Main characteristics of the organoclays used in this work.

                               Cloisite 30B          Cloisite 20A

Organic modifier             Methyl, tallow,          Dimethyl,
                           bis-2-hydroxylethyl,     dihydrogenated
                           quaternary ammonium    tallow, quaternary
                                                       ammonium

Modifier concentration      90 meq/100 g clay     95 meq/100 g clay

Weight loss in ignition            30%                   38%

[d.sub.001] ([Angstrom])           18.5                  24.2

where HT is hydrogenated tallow (~65% Cl8, ~30% Cl6, ~5% C14).
T is tallow (~65% C18, ~30% C16, ~5% C14).

TABLE 2. Vulcanization time for the examined clay/PDMS hybrids.

                            Vulcanization time (min)
Clay content in
PDMS matrix (phr)   Nanofil 116   Cloisite 30B   Cloisite 20A

0                                     180
1                       90            105            150
2                       70             90            135
3.5                     75             85            110
5                       60             80             75
8                       50             55             60
10                      40             45             55

TABLE 3. Parameters of the exponential equation fitted to viscosity
versus time data of clay/PDMS systems, during vulcanization reaction.

                                       n

MMT content in PDMS (phr)   Nanl 16   C130B   C120A

0                            0.046
1                            0.141    0.073   0.061
2                            0.203    0.098   0.081
3.5                          0.260    0.124   0.110
5                            0.307    0.182   0.137
8                            0.436    0.158   0.258
10                           0.405    0.150   0.323

                                   [y.sup.0]

MMT content in PDMS (phr)   Nanl 16   C130B    C120A

0                           3287.2
1                           3043.6    2041.0   2562.2
2                           3381.3    3164.9   3708.5
3.5                         3728.8    4206.7   3438.8
5                           4092.6    3985.9   3958.0
8                           4845.5    2359.8   4862.2
10                          5582.2    3146.5   5775.0

                                       A

MMT content in PDMS (phr)   Nanl 16   C130B    C120A

0                           1328.1
1                            778.8    1772.2   1485.2
2                            495.6     866.8    904.9
3.5                          470.5     585.9   1127.4
5                            317.8     412.6   1136.9
8                            197.9    1312.7    746.5
10                           394.6    1583.6   1015.6

TABLE 4. Swelling at equilibrium by immersion in toluene of clay/PDMS
nanocomposites.

                      Equilibrium swelling in toluene (%)
Clay content in
PDMS (phr)            Nanofil 116            Cloisite 30B

0                 282.41 [+ or -] 0.99
2                 264.90 [+ or -] 2.44   269.12 [+ or -] 2.45
5                 260.30 [+ or -] 1.54   252.73 [+ or -] 1.74
8                 230.40 [+ or -] 3.01   245.80 [+ or -] 1.00

                  Equilibrium swelling in toluene (%)
Clay content in
PDMS (phr)                    Cloisite 20A

0
2                         249.13 [+ or -] 1.96
5                         233.00 [+ or -] 1.83
8                         229.45 [+ or -] 1.70
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Author:Vasilakos, Sozon P.; Triantou, Marianna I.; Tarantili, Petroula A.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Apr 1, 2015
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