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Mechanical properties of a hybrid-oxide glass
EUROGEL '91 S. Vilminot, R. Nass & H. Schmidt (editors) © 1992 Elsevier Science Publishers B.V. All rights reserved.
399
Mechanical properties of a hybrid-oxide glass C. Maï, J.F. Cornu, S. Bouras, R. Vassoille, J. Perez Groupe d'Etudes de Métallurgie Physique et de Physique des Matériaux Institut National des Sciences Appliquées de Lyon, U R A CNRS 341 20 Avenue Albert Einstein, 69621 Villeurbanne, France
Abstract A sol-gel process has been successfully utilized to produced large bulk hybrid-oxide glasses from tetraethoxysilane (TEOS) or tetramethoxy silane ( T M O S ) incorporating polymeric components of 3-(trimethoxysilyl) propyl methacrylate and methylmethacrylate ( M M A ) . A l l samples are transparent and without porosity. The morphology of hybrid-oxide glass looks like an oriented fibers composite materials. Mechanical tests show that hybrid-oxide glasses have intermediary behaviors between brittle and plastic materials. Particularly, the plasticity at room temperature of hybrid-oxide glasses has been observed.
1. I N T R O D U C T I O N Oxide glasses are brittle materials below glass transition temperature ( T g ) . It means that they exhibit no region of ductility or plasticity, particularly in tensile stress. The brittleness is due to the nature of the chemical bonds, i.e chemical composition. Studies of 3 oxynitride glasses have shown that the substitution of 0 2 + by N + modifies the physical, chemical and mechanical properties of these glasses. The observed variation in properties are due to the formation of a more highly crosslinked structure [ 1 - 3 ] . The chemical bonds in inorganic oxide glass can also be modified by introducing organic groups in glass composition. Recent studies about inorganic-organic glasses [46 ] , termed hybrid materials, have shown that properties of glasses could be modified by properly choosing polymeric species. However, as far as we know, few studies have been undertaken in the mechanical behaviors of hybrid glasses.
400
C. Maï et al.
The objective of this study was to investigate how the presence of organic units could modify mechanical properties of oxide glasses obtained by sol-gel techniques. The sol-gel process for preparing oxide glasses has been extensively studied and developments in the techniques for preparing oxide glasses from alkoxides have been reviewed [7-11]. Due to the nature of this chemistry (hydrolysis and condensation at room temperature), it was shown possible to incorporate some polymeric or oligomeric species into the glass network if these components have appropriate functional groups to undergo condensation. The present paper reports results of a comparative study of some mechanical characteristics, i.e. Young modulus, hardness, fracture toughness etc...between inorganic silica-based glass, P M M A and hybridoxide glass.
2. E X P E R I M E N T A L 2.1.
Materials-Samples preparation The materials and supplier were listed in table I. A l l materials were used without further purification. Table 1 Name tetraethoxysilane
tetramethoxysilane
Formula (TEOS)
(TMOS)
S i ( O C 2H 5) 4 S i ( O C H 3) 4
3-(trimethoxysilyl)propyl H 2 C=C(Π3 X:02C3H 6 Si(OCH3)3 methacrylate methylmethacrylate ( M M A ) H 2 C = C ( C T 3 ) C O O - Π3 benzoyl peroxide (C 6 H 5 CO)202
Mw 208.1 152.2 248.3
Supplier
Aldrich
100.1 242.2
At first, TEOS (or T M O S ) were mixed with 3-(trimethoxysilyl)propyl methacrylate. Then an appropriate amount of distilled water and hydro-chloric acid were added. Separately a solution of monomer: methyl methacrylate and benzoyl peroxide as catalyst had been prepared. The two mixtures were blended and at last the suitable solvents were added. This sol was poured into a Teflon petri disk and stirred for 4 hours at 50°C. Then it was covered with a parafilm (not tightly to permit evaporation of the solvents) and let at 50°C for 48 hours. The solution formed a leatherlike transparent gel after this delay. Bulk hybrid-oxide glasses were obtained after 4 days.
Mechanical Properties of Hybrid-Oxide Glass
401
2.2.
Microstructure observations The chemical analysis of hybrid oxide-glasses by Infra-Red and Raman spectroscopy will be reported elsewhere [12]. Microstructure and morphology of samples were studied by Differential Scanning Calorimetry ( D S C ) , Dynamic Mechanical Spectroscopy ( D M S ) and Wide Angle X-ray Scattering ( W A X S ) . Both optical and Scanning Electron Microscopy ( S E M ) methods were utilized for investigating the fracture and optical characteristics. 2.3.
Mechanicals tests Microhardness was measured by V i c k e r s Indentation ( H v ) technique. Young Modulus ( E ) was determined using a resonance method. Fracture toughness ( K c ) , Plane Strain Compression and Hertzian Indentation techniques have been done to determine mechanical characteristics of hybrid-oxide glasses, silica-based glass and P M M A .
3. R E S U L T S A N D DISCUSSION 3.1.
Microstructure and Morphology of hybrid-oxide glass Large and transparent samples without porosity of hybrid-oxide glass are obtained. Most samples display a certain flexibility. The higher flexibility is occured for the lower atomic concentration of silicon in hybrid-oxide glass. This result suggests that the rubbery siloxane and the M M A components have been incorporated into the network. Since the hybrid-oxide glasses are transparent, there is most likely no phase separation in the dimension of the wawelength of visible light. SAXS have been done on hybrid-oxide glasses. N o phase separation was observed [12]. However, at higher temperature (T=150°C) a phase separation phenomenon occurs as shown in a DSC curve (Figure 1). The glass transition temperature is missing in DSC measurement, while W A X S (Figure 2) shows only a wide scattering peak characteristic of amorphous material. The morphology of hybrid-oxide glass, observed on the fracture surface by SEM is quite different from polymeric P M M A and inorganic glass. A likely composite material with a homogeous matrix associated with oriented fibers is observed.(Figure 3 ) . The measurements of micromechanical properties are performed with a special DMS operating in the low frequency range(lHz to 1 0 ' ^ H z ) under low applied stress and described elsewhere [13]. A typical isochronal dynamic mechanical spectrum at 1Hz is given in Figure 4. The general behavior of the storage modulus and the internal friction, Q~l, are similar for all samples. Three relaxation peaks can be observed.
402
C. Mai et al.
Figure 1. DSC curve of a hybridoxide glass
Figure 3. a: b: c:
Figure 2. W A X S curves- a:PMMA b:Hybrid-oxide glass ; ciSilica glass
SEM observation on the fracture surface, A general view of the fracture surface, Higher magnification of a fiber, Some fibers are pull out from the matrix.
Mechanical Properties of Hybrid-Oxide Glass
403
However, only the higher temperature relaxation peak at 362K is associated with a decrease of the storage modulus. This indicates that there is a transition from glassy to the rubbery state. T o confirm this result isothermal spectra measured at different frequencies have been done and the apparent activation energy ( U a ) for the higher temperature relaxation peak (T=362K ; U a = 3 e V ) and the second relaxation peak (T=265K; U a = 0 . 4 e V ) are calculated. These results suggest that the two relaxations correspond respectively to the α and β processes. In addition, the α relaxation temperature of hybrid-oxide glass is higher than the one of PMMA.(with small molecular weigth). This indicates there is probably incorporation of M M A in the network.
Figure 4. Isochronal dynamic mechanical spectrum of a hybrid oxide glass at 1Hz. 3.2.
Figure 5. Plane strain compression curves at room temperature.
Mechanical properties Mechanical characteristics have been measured in a silica-based glass, P M M A and hybrid-oxide glasses. Results of Young modulus ( E ) , Vickers hardness ( H v ) are listed in Table 2. T w o characteristics of hybrid-oxide glass can be pointed out:
404
C. Mai et al
- elastic behavior of hybrid-oxide glass is close to a polymeric material. - in the contrary, the hardness is close to the inorganic glass. Table 2 Material
E(GPa)
PMMA hybrid TEOS 53 hybrid TEOS 71 hybrid T M O S 66 silica-based glass
1.9 4.2 4.2 4.2 70
Hv(GPa) 20 320 330 375 467
1 / 2
Kc(MPa m ) 1,6 [14] 1.1 1.0 1.1 0.8
In addition, hybrid-oxide glass obtained with T M O S seems to have higher hardness than those obtained with TEOS. Length of crack associated with Vickers indentation prints are measured and the stress intensity factor ( K c ) are calculated (Table 2) utilizing the relation [15]: ( c / 1 8 a ) - L 5 1 H al / 2
( c / a )
KC= α Φ LΓ ^ ]J EO
0 4
where: Φ = 3 , E=Young Modulus (GPa) ; H v =Vickers Hardness (GPa) a= half indentation print (μπι) ; c= lenght of indentation crack ( μ π ι ) 4
α=14[1-8(4ν-0.5)/1+ν)) ] v= Poisson coefficient The stress intensity factor of hybrid-oxide glass is 25% to 35% higher than those of inorganic oxide glass. These results strongly suggest that hybrid-oxide glass should have a plastic deformation behavior. T o confirm this hypothesis, plane strain compression and Hertzian indentation techniques have been done. Figure 5 shows the stress-strain curves. Results show that hybrid-oxide glass has intermediary behavior between polymeric material and inorganic glass. More evidence of the plasticity behavior of hybrid-oxide glass is obtained by Hertzian indentation as shown in Figure 6. For P M M A (figure 6a) the penetration of the indentor is important without cracks' formation. In contrary, the formation of typical cracks occurs rapidely in silica-based glass (figure 6c). As shown in figure 6b, hybrid-oxide glass displays a real plasticity. However, the plastic deformation in hybrid-oxide glass is less important than in P M M A .
Mechanical Properties of Hybrid-Oxide Glass
405
Figure 6. Hertzian indentation at room temperature: a- P M M A b- Hybrid-oxide glass c- Silica-based glass
4. C O N C L U S I O N Hybrid-oxide glasses, with new mechanical properties, based on the incorporation of organic functionalized polymer into a sol-gel reaction with inorganic alkoxide have been successfully synthesized. Three characteristics of hybrid-oxide glasses can be pointed out: -The morphology of hybrid-oxide glasses looks like an oriented fibers composite material. -Mechanical behaviors of hybrid-oxide glasses are intermediary between polymeric material and inorganic glass. For instance, Young modulus of hybrid-oxide glass is close to those of polymeric materials, while their hardness is close to the inorganic glasses. -In introducing organic bonds in inorganic oxide glass the fracture toughness Kc increases and correlatively the hybrid-oxide glass display a real plasticity. The present paper gives first results of hybrid-oxide glass. Work in this regard is continuing and will be reported at later date.
406
C. Mai et al.
5 ACKNOWLEDGEMENTS W e wish to acknowledge financial supports from the "Programmes Pluriannuels de Recherche"-Région R H O N E - A L P E S and from the "Direction de Recherche-Développement" of BSN - Emballage company.
6 REFERENCES 1 R. Pastuzak and P. Verdier, J. of Non Cry st. Solids, 56 (1983) 141. 2 S. Sakka, K. Kamiya and T. Yoko, Preparation, Properties and Structure of special glasses, North Holland, Amsterdam, 1983. 3 J. Homeny and, D.L. Mc Garry, Comm. of Amer. Cer. S o c , (1984) C.225. 4 H. Dislish, J. of Non Cryst. Solids, 57 (1983) 371. 5 B.E. Yoldas, J. of Non Cryst. Solids, 12 (1977) 1203. 6 S. Sakka and K. Kamiya, J. of Non Cryst. Solids, 42 (1980) 403. 7 C. Friedel and A . Ladenberg, C R. Acad. Sei. Paris, 818 (1968) 66. 8 H.H. Huang, B. Orler and G.L. Wilkes, Polymer Bull., 14 (1985) 557. 9 H. Schmidt, J. Non Cryst. Solids, 73 (1985) 681. 10 H. Schmidt and B. Seiperling, Mat. Res. Soc. (Symp. P r o c ) , 73 (1988) 739. 1 1 C. Sanchez, J. Livage, M . Henry and F. Babonneau, J. Non Cryst. Solids, 100 (1980) 403. 1 2 C. Mai, J.F. Cornu, J. Perez (to be published) 1 3 S.Etienne, J.Y.Cavaillé, J.Perez, R.Point and M.Salvia, Rev. Sei. Instr. 53 (1982) 1261. 1 4 Ph.Béguelin and H.H.Kausch Deformation Yield and Fracture of Polymers, The Plastic and Rubber Institute, London, 1991. 1 5 K. Liang, Thesis, INS A Lyon (1990).
399
Mechanical properties of a hybrid-oxide glass C. Maï, J.F. Cornu, S. Bouras, R. Vassoille, J. Perez Groupe d'Etudes de Métallurgie Physique et de Physique des Matériaux Institut National des Sciences Appliquées de Lyon, U R A CNRS 341 20 Avenue Albert Einstein, 69621 Villeurbanne, France
Abstract A sol-gel process has been successfully utilized to produced large bulk hybrid-oxide glasses from tetraethoxysilane (TEOS) or tetramethoxy silane ( T M O S ) incorporating polymeric components of 3-(trimethoxysilyl) propyl methacrylate and methylmethacrylate ( M M A ) . A l l samples are transparent and without porosity. The morphology of hybrid-oxide glass looks like an oriented fibers composite materials. Mechanical tests show that hybrid-oxide glasses have intermediary behaviors between brittle and plastic materials. Particularly, the plasticity at room temperature of hybrid-oxide glasses has been observed.
1. I N T R O D U C T I O N Oxide glasses are brittle materials below glass transition temperature ( T g ) . It means that they exhibit no region of ductility or plasticity, particularly in tensile stress. The brittleness is due to the nature of the chemical bonds, i.e chemical composition. Studies of 3 oxynitride glasses have shown that the substitution of 0 2 + by N + modifies the physical, chemical and mechanical properties of these glasses. The observed variation in properties are due to the formation of a more highly crosslinked structure [ 1 - 3 ] . The chemical bonds in inorganic oxide glass can also be modified by introducing organic groups in glass composition. Recent studies about inorganic-organic glasses [46 ] , termed hybrid materials, have shown that properties of glasses could be modified by properly choosing polymeric species. However, as far as we know, few studies have been undertaken in the mechanical behaviors of hybrid glasses.
400
C. Maï et al.
The objective of this study was to investigate how the presence of organic units could modify mechanical properties of oxide glasses obtained by sol-gel techniques. The sol-gel process for preparing oxide glasses has been extensively studied and developments in the techniques for preparing oxide glasses from alkoxides have been reviewed [7-11]. Due to the nature of this chemistry (hydrolysis and condensation at room temperature), it was shown possible to incorporate some polymeric or oligomeric species into the glass network if these components have appropriate functional groups to undergo condensation. The present paper reports results of a comparative study of some mechanical characteristics, i.e. Young modulus, hardness, fracture toughness etc...between inorganic silica-based glass, P M M A and hybridoxide glass.
2. E X P E R I M E N T A L 2.1.
Materials-Samples preparation The materials and supplier were listed in table I. A l l materials were used without further purification. Table 1 Name tetraethoxysilane
tetramethoxysilane
Formula (TEOS)
(TMOS)
S i ( O C 2H 5) 4 S i ( O C H 3) 4
3-(trimethoxysilyl)propyl H 2 C=C(Π3 X:02C3H 6 Si(OCH3)3 methacrylate methylmethacrylate ( M M A ) H 2 C = C ( C T 3 ) C O O - Π3 benzoyl peroxide (C 6 H 5 CO)202
Mw 208.1 152.2 248.3
Supplier
Aldrich
100.1 242.2
At first, TEOS (or T M O S ) were mixed with 3-(trimethoxysilyl)propyl methacrylate. Then an appropriate amount of distilled water and hydro-chloric acid were added. Separately a solution of monomer: methyl methacrylate and benzoyl peroxide as catalyst had been prepared. The two mixtures were blended and at last the suitable solvents were added. This sol was poured into a Teflon petri disk and stirred for 4 hours at 50°C. Then it was covered with a parafilm (not tightly to permit evaporation of the solvents) and let at 50°C for 48 hours. The solution formed a leatherlike transparent gel after this delay. Bulk hybrid-oxide glasses were obtained after 4 days.
Mechanical Properties of Hybrid-Oxide Glass
401
2.2.
Microstructure observations The chemical analysis of hybrid oxide-glasses by Infra-Red and Raman spectroscopy will be reported elsewhere [12]. Microstructure and morphology of samples were studied by Differential Scanning Calorimetry ( D S C ) , Dynamic Mechanical Spectroscopy ( D M S ) and Wide Angle X-ray Scattering ( W A X S ) . Both optical and Scanning Electron Microscopy ( S E M ) methods were utilized for investigating the fracture and optical characteristics. 2.3.
Mechanicals tests Microhardness was measured by V i c k e r s Indentation ( H v ) technique. Young Modulus ( E ) was determined using a resonance method. Fracture toughness ( K c ) , Plane Strain Compression and Hertzian Indentation techniques have been done to determine mechanical characteristics of hybrid-oxide glasses, silica-based glass and P M M A .
3. R E S U L T S A N D DISCUSSION 3.1.
Microstructure and Morphology of hybrid-oxide glass Large and transparent samples without porosity of hybrid-oxide glass are obtained. Most samples display a certain flexibility. The higher flexibility is occured for the lower atomic concentration of silicon in hybrid-oxide glass. This result suggests that the rubbery siloxane and the M M A components have been incorporated into the network. Since the hybrid-oxide glasses are transparent, there is most likely no phase separation in the dimension of the wawelength of visible light. SAXS have been done on hybrid-oxide glasses. N o phase separation was observed [12]. However, at higher temperature (T=150°C) a phase separation phenomenon occurs as shown in a DSC curve (Figure 1). The glass transition temperature is missing in DSC measurement, while W A X S (Figure 2) shows only a wide scattering peak characteristic of amorphous material. The morphology of hybrid-oxide glass, observed on the fracture surface by SEM is quite different from polymeric P M M A and inorganic glass. A likely composite material with a homogeous matrix associated with oriented fibers is observed.(Figure 3 ) . The measurements of micromechanical properties are performed with a special DMS operating in the low frequency range(lHz to 1 0 ' ^ H z ) under low applied stress and described elsewhere [13]. A typical isochronal dynamic mechanical spectrum at 1Hz is given in Figure 4. The general behavior of the storage modulus and the internal friction, Q~l, are similar for all samples. Three relaxation peaks can be observed.
402
C. Mai et al.
Figure 1. DSC curve of a hybridoxide glass
Figure 3. a: b: c:
Figure 2. W A X S curves- a:PMMA b:Hybrid-oxide glass ; ciSilica glass
SEM observation on the fracture surface, A general view of the fracture surface, Higher magnification of a fiber, Some fibers are pull out from the matrix.
Mechanical Properties of Hybrid-Oxide Glass
403
However, only the higher temperature relaxation peak at 362K is associated with a decrease of the storage modulus. This indicates that there is a transition from glassy to the rubbery state. T o confirm this result isothermal spectra measured at different frequencies have been done and the apparent activation energy ( U a ) for the higher temperature relaxation peak (T=362K ; U a = 3 e V ) and the second relaxation peak (T=265K; U a = 0 . 4 e V ) are calculated. These results suggest that the two relaxations correspond respectively to the α and β processes. In addition, the α relaxation temperature of hybrid-oxide glass is higher than the one of PMMA.(with small molecular weigth). This indicates there is probably incorporation of M M A in the network.
Figure 4. Isochronal dynamic mechanical spectrum of a hybrid oxide glass at 1Hz. 3.2.
Figure 5. Plane strain compression curves at room temperature.
Mechanical properties Mechanical characteristics have been measured in a silica-based glass, P M M A and hybrid-oxide glasses. Results of Young modulus ( E ) , Vickers hardness ( H v ) are listed in Table 2. T w o characteristics of hybrid-oxide glass can be pointed out:
404
C. Mai et al
- elastic behavior of hybrid-oxide glass is close to a polymeric material. - in the contrary, the hardness is close to the inorganic glass. Table 2 Material
E(GPa)
PMMA hybrid TEOS 53 hybrid TEOS 71 hybrid T M O S 66 silica-based glass
1.9 4.2 4.2 4.2 70
Hv(GPa) 20 320 330 375 467
1 / 2
Kc(MPa m ) 1,6 [14] 1.1 1.0 1.1 0.8
In addition, hybrid-oxide glass obtained with T M O S seems to have higher hardness than those obtained with TEOS. Length of crack associated with Vickers indentation prints are measured and the stress intensity factor ( K c ) are calculated (Table 2) utilizing the relation [15]: ( c / 1 8 a ) - L 5 1 H al / 2
( c / a )
KC= α Φ LΓ ^ ]J EO
0 4
where: Φ = 3 , E=Young Modulus (GPa) ; H v =Vickers Hardness (GPa) a= half indentation print (μπι) ; c= lenght of indentation crack ( μ π ι ) 4
α=14[1-8(4ν-0.5)/1+ν)) ] v= Poisson coefficient The stress intensity factor of hybrid-oxide glass is 25% to 35% higher than those of inorganic oxide glass. These results strongly suggest that hybrid-oxide glass should have a plastic deformation behavior. T o confirm this hypothesis, plane strain compression and Hertzian indentation techniques have been done. Figure 5 shows the stress-strain curves. Results show that hybrid-oxide glass has intermediary behavior between polymeric material and inorganic glass. More evidence of the plasticity behavior of hybrid-oxide glass is obtained by Hertzian indentation as shown in Figure 6. For P M M A (figure 6a) the penetration of the indentor is important without cracks' formation. In contrary, the formation of typical cracks occurs rapidely in silica-based glass (figure 6c). As shown in figure 6b, hybrid-oxide glass displays a real plasticity. However, the plastic deformation in hybrid-oxide glass is less important than in P M M A .
Mechanical Properties of Hybrid-Oxide Glass
405
Figure 6. Hertzian indentation at room temperature: a- P M M A b- Hybrid-oxide glass c- Silica-based glass
4. C O N C L U S I O N Hybrid-oxide glasses, with new mechanical properties, based on the incorporation of organic functionalized polymer into a sol-gel reaction with inorganic alkoxide have been successfully synthesized. Three characteristics of hybrid-oxide glasses can be pointed out: -The morphology of hybrid-oxide glasses looks like an oriented fibers composite material. -Mechanical behaviors of hybrid-oxide glasses are intermediary between polymeric material and inorganic glass. For instance, Young modulus of hybrid-oxide glass is close to those of polymeric materials, while their hardness is close to the inorganic glasses. -In introducing organic bonds in inorganic oxide glass the fracture toughness Kc increases and correlatively the hybrid-oxide glass display a real plasticity. The present paper gives first results of hybrid-oxide glass. Work in this regard is continuing and will be reported at later date.
406
C. Mai et al.
5 ACKNOWLEDGEMENTS W e wish to acknowledge financial supports from the "Programmes Pluriannuels de Recherche"-Région R H O N E - A L P E S and from the "Direction de Recherche-Développement" of BSN - Emballage company.
6 REFERENCES 1 R. Pastuzak and P. Verdier, J. of Non Cry st. Solids, 56 (1983) 141. 2 S. Sakka, K. Kamiya and T. Yoko, Preparation, Properties and Structure of special glasses, North Holland, Amsterdam, 1983. 3 J. Homeny and, D.L. Mc Garry, Comm. of Amer. Cer. S o c , (1984) C.225. 4 H. Dislish, J. of Non Cryst. Solids, 57 (1983) 371. 5 B.E. Yoldas, J. of Non Cryst. Solids, 12 (1977) 1203. 6 S. Sakka and K. Kamiya, J. of Non Cryst. Solids, 42 (1980) 403. 7 C. Friedel and A . Ladenberg, C R. Acad. Sei. Paris, 818 (1968) 66. 8 H.H. Huang, B. Orler and G.L. Wilkes, Polymer Bull., 14 (1985) 557. 9 H. Schmidt, J. Non Cryst. Solids, 73 (1985) 681. 10 H. Schmidt and B. Seiperling, Mat. Res. Soc. (Symp. P r o c ) , 73 (1988) 739. 1 1 C. Sanchez, J. Livage, M . Henry and F. Babonneau, J. Non Cryst. Solids, 100 (1980) 403. 1 2 C. Mai, J.F. Cornu, J. Perez (to be published) 1 3 S.Etienne, J.Y.Cavaillé, J.Perez, R.Point and M.Salvia, Rev. Sei. Instr. 53 (1982) 1261. 1 4 Ph.Béguelin and H.H.Kausch Deformation Yield and Fracture of Polymers, The Plastic and Rubber Institute, London, 1991. 1 5 K. Liang, Thesis, INS A Lyon (1990).