Chapter 15
Structure-Property Relationships for Poly(dimethylsiloxane) Networks In Situ Filled Using Titanium 2Ethylhexoxide and Zirconium n-Butoxide 1
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Downloaded by FUDAN UNIV on February 21, 2017 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch015
S. Murugesan , J. E. Mark , and G. Beaucage 1
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Departments of Chemistry and Materials Science, University of Cincinnati, Cincinnati, O H 45221
Abstract In situ filling of elastomeric networks has been used as an alternative to the conventional ex situ filling of such materials. The in situ process has advantages over the ex situ process, for example, it is a low-temperature process, giving uniform distributions of particles compared to the energy demanding ex situ process. The sizes and shapes of the particles can be controlled by adjusting the reaction parameters during the synthesis. In this present work titania and zirconia filler particles were synthesized in situ in poly(dimethylsiloxane) networks from titanium 2-ethylhexoxide and zirconium n-butoxide. The advantage of the former is its long backbone chain (compared to that of titanium n-butoxide). Also the ethyl side group attached to the back bone is expected to reduce the initial growth of the titania particles compared to those from titanium n-butoxide. The sizes of the particles were obtained from SAXS (small angle X-ray scattering), and the mechanical properties were determined from stress-strain measurements.
© 2003 American Chemical Society
Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Introduction For the past decade or so, an alternative to the energy demanding exsitu filling of elastomers has been in situfilling.The in situ technique is a solvent-free synthesis at room temperature that can be used to produce (in a continuous process) significant quantities of colloidal-size particles (7-5). These particles can be produced from various alkoxides using either base or acid catalysts. There are many advantages to this in situ process, for example particle sizes and distributions (6,7) can be controlled by reaction conditions, low temperatures are involved, and solvent is unnecessary. SAXS (small angle x-ray scattering) is one of the important tools used to characterize suchfillerparticles (8-10) with their different shapes and sizes. Using Porod's law and Guinier's law it is possible from the SAXS data to obtain values of the R (radius of gyration), and the slope of the Porod regime, gives an indication of the shapes of the particle. In this study an effort has been made to synthesize in situ zirconia and titania particles in poly(dimethylsiloxane) (PDMS) networks. Zirconium n-butoxide and titanium 2-ethylhexoide (TEH) were used in this study to generate the particles. The advantages of using T E H are that it has a longer chain and a side group, ethyl, which will reduce the speed at which these particles are formed (compared to those in an earlier study with titanium n-propoxide (8)). g
Experimental Procedures PDMS used in this study was obtained from Gelest and has a molecular weight (Mn) of 18000 gms/mol. Equimolar quantities of the PDMS and the crosslinking agent tetraethoxysilane (TEOS) were mixed well with a small quantity (-1%) of the catalyst stannous oleate, and left undisturbed for 3 days. Once a film was formed, it was removed and extracted first with toluene and then with a mixture of toluene and methanol with decreasing portions of toluene until it was 100% methanol. The film was then dried for 24 hrs under vacuum, and then soaked in the alkoxides for up to 24 hours. In order to monitor the speed at which these particles were formed, several strips were cut and then soaked in the alkoxide for various times. The soaked strips were then immersed in a dilute base solution of 2% w/v diethyl amine in distilled water for 24 hours. The base acted as a catalyst for the hydrolysis and condensation of the alkoxides absorbed into the PDMS network.
Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
165 Some mechanical properties of the filled samples were studied using an Instron machine. Micrographs were obtained from a Hitachi scanning electron microscope, and SAXS data were obtained using pinhole 2D SAXS apparatus.
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Results and Discussion The total amount of filler in situ precipitated into the PDMS networks was 15wt.% for titania and 17wt.% for zirconia. Micrographs revealed that the zirconia particles occurred as strands with good interlocking (Figure 1). In the case of titania there were no fibrillar particles. Also, the reinforcement of the PDMS networks by the zirconia filled was larger than thatfromthe titania. The SAXS data (Figure 2) showed that particle size increased up to 10-12 hours of reaction time, and after that the particle size decreased (Table I). This may be attributed to the fact that zirconia has strong tendency to form coordination compounds with the alcohol byproduct (n-butanol). This could make the particles bend around and thus show a decrease in size after 12 hours. Also, the particles have a tendency to form aggregates. Titania samples also showed a similar trend, specifically a decrease in size after 10-12 hours. This may be due to fact that the 2-ethyl hexanoic acid, which is increased in size by its side group, may find it difficult to diffuse out of the PDMS network, thus enabling the particles to bend and become smaller. The mechanical properties showed good improvement (Figure 3), especially in toughness (Table Π), with zirconia giving the larger improvements. The sizes along with the stranded nature of the zirconia particles could explain these differences.
Table I Values of Rg of thefillerparticles Hours in alkoxides
Rg of particles (À) Zirconia Titania
1
50
60
6
65
79
11
76
105
24
68
75
Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Figure 1. Micrographs of PDMS networksfilledwith a) zirconia b) titania.
Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Figure 2. Small angle X-ray scattering of a) Titania b) Zirconia filled PDMS networks
Clarson et al.; Synthesis and Properties of Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Downloaded by FUDAN UNIV on February 21, 2017 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch015
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