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Impact behavior of sugarcane bagasse waste–EVA composites
Polymer Testing 20 (2001) 869–872 www.elsevier.com/locate/polytest
Material Behaviour
Impact behavior of sugarcane bagasse waste–EVA composites G.C. Stael a, M.I.B. Tavares a, J.R.M. d’Almeida a
b,*
Instituto de Macromole´culas, Universidade Federal do Rio de Janeiro, PO Box 68525, 21945-970, Rio de Janeiro, RJ, Brazil b Materials Science and Metallurgy Department, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225-22453-900, Rio de Janeiro, RJ, Brazil Received 8 November 2000; accepted 5 February 2001
Abstract The impact performance of chopped bagasse–EVA matrix composites is evaluated and compared with the behavior of bagasse filled PP and PE matrix composites and wood-based materials. The volume fraction and size of the chopped bagasse used as filler was varied. The experimental results show that the incorporation of bagasse strongly reduces the deformation capacity of EVA polymer. The reduction of the deformation capacity of the composites was also inferred by solid-state NMR relaxation analysis. The impact strength was independent of the bagasse size, but varied with the volume fraction. As a function of the volume fraction it was shown that the mechanical performance of bagasse–EVA composites could be tailored to reproduce the behavior of wood-based particle boards. 2001 Elsevier Science Ltd. All rights reserved. Keywords: EVA matrix composites; Chopped sugar cane bagasse; Impact behavior; NMR analysis
1. Introduction Nowadays, composite materials are largely used in many industrial applications that range from offshore structures used by the petroleum industry to common household goods [1,2]. The great majority of these composites are resin matrix-based materials. In fact, the large number of polymers that have a good performance as matrix materials provides a vast range of properties and, therefore, the versatility of resin matrix composite materials [3]. Poly(ethylene-co-vinyl acetate), EVA, polymer could be a feasible possibility as resin matrix, although its low mechanical properties could restrict the use of EVAbased composites [4–6]. Nevertheless, due to the high flexibility presented by EVA polymer, EVA-based com-
* Corresponding author. E-mail address: [email protected] d’Almeida).
(J.R.M.
posites could present some advantages under impact conditions, where toughness plays an important role. In this work, the impact performance of chopped bagasse–EVA matrix composites were evaluated. The Izod impact strength of the composites was measured as a function of the volume fraction and size of the bagasse and the values obtained were compared to those of bagasse filled-polyethylene, PE, and polypropylene, PP, matrix composites and common wood-based materials. Solid-state NMR analysis was performed to provide information on the matrix–bagasse interaction.
2. Experimental methods and materials The bagasse used as filler was directly obtained from sugar cane mills, after being processed to extract sugar and liquor. This “as received” material was dried at 80°C for 48 h and then was chopped and sieved. Bagasse pieces with lengths, l⬍3 mm, 3⬍ l ⬍5 mm and 20⬍ l ⬍30 mm were separated and used in this work. Herein-
0142-9418/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 0 1 4 - 9
870
G.C. Stael et al. / Polymer Testing 20 (2001) 869–872
after these chopped materials will be referred to as the small length (SL), medium length (ML) and long length (LL) bagasse pieces. The chopped bagasse was dried again at 80°C for 48 h before being incorporated into the EVA matrix. The composites were fabricated by mixing the proper quantities of EVA and chopped bagasse in a Haake Rheocord 9000 plastograph using the rollermix rotor at 60 rpm for 20 min. The processing temperature used was 200°C. The mixed material was then pressed at 3.5 MPa for 15 min at 200°C to obtain plates of the composites. These processing parameters were shown to give the best fabrication performance for these EVA–bagasse mixtures [6]. Composites with volume fraction, Vf, of bagasse of 0.13, 0.30, 0.48 and 0.59, respectively, corresponding to a matrix to bagasse ratio of 80/20, 60/40, 40/60 and 30/70 in weight, were fabricated. The specimens for the impact tests were machined from the fabricated plates and were 62 mm long, 12.7 mm wide and 4 mm thick. The notch to depth ratio was 0.20 and the radius of the notch tip was 0.25 mm. The included angle of the notch tip was 45°, as specified by the ASTM standard D-256. The tests were conducted at room temperature, 23±3°C, in a non-instrumented equipment with a maximum pendulum capacity of 2.7 J. At least 10 specimens were tested per composite. Using the same fabrication procedure bagasse–PP and bagasse–PE composites were also fabricated. These composites have a volume fraction of 0.48 and the chopped bagasse used was the SL material. Test specimens from these materials and from three commercial wood-based materials, were also machined and tested using the same experimental parameters as for the bagasse–EVA composites. Only the thickness of the wood-based materials were different, because these materials were tested with their commercial thickness, without any machining. The three wood-based materials tested were a 4.4 mm thick low density particle board (LDPB); a high density particle board (HDPB), 3 mm thick, composed of pressed cellulosic material; and a soft plywood, SP, 3.7 mm thick plate. The fracture surface of the tested specimens were analyzed by scanning electron microscopy (SEM), on gold– palladium coated specimens. The SEM observation was performed with secondary electrons imaging and acceleration of the electron beam between 15 to 20 kV. The solid state NMR spectra were obtained on a VARIAN INOVA 300 spectrometer operating at 299.9 and 75.4 MHz for 1H and 13C, respectively. All experiments were performed at probe ambient temperature using gated high power decoupling. A zirconium oxide rotor with a diameter of 7 mm was used to acquire the NMR spectra at rates of 6 kHz. 13C spectra are referenced to the chemical shift of the carbon atoms of the methyl group of hexamethyl benzene (17.3 ppm). The 13 C measurements were carried out in the cross-polariz-
Table 1 Impact strength (in kJ/m2), of the chopped bagasse–EVA composite as a function of the volume fraction and bagasse length Volume
Bagasse length
fraction
SL
ML
LL
0.13 0.30 0.48 0.59
– 8.4±1.1 3.7±0.5 2.7±0.5
20.0±2.2 7.8±0.8 3.9±0.4 2.9±0.4
20.1±0.6 8.8±0.6 4.2±0.4 3.6±0.3
ation mode with magic-angle spinning (CPMAS) with 2 s of delay. A variable contact-time experiment was also recorded and the range of contact-time established range from 200 to 8000 µs. The T1Hr values were determined from the intensity decay of carbon-13 peaks with increasing contact-time [7–9].
3. Experimental results and discussion The experimental results obtained are shown in Tables 1 and 2 for the chopped bagasse–EVA composites and for the materials used for comparison, respectively. From Table 1, one can see that the impact behavior of EVAbased composites is almost independent of the bagasse length. Only for the higher volume fraction, viz. 0.59, is there a slight tendency for the energy absorbed to increase with increase of the bagasse length. This same trend was also observed for the tensile behavior of these composites [10] and, in practice, it implies that for chopped bagasse with sizes smaller than 30 mm there is no need for sieving the bagasse pieces. On the other hand, the variation of volume fraction causes a strong effect on the impact strength of the composites. As one can see from the data in Table 1, the incorporation of chopped bagasse produces a strong decrease in the impact strength. The brittle behavior associated with the composites with higher volume fractions could be attributed to the inhibition of the deformation capacity of EVA matrix due to the presence of the much more stiff Table 2 Impact strength of the chopped bagasse–PP and PE composites and commercial wood based materials Material
Impact strength (kJ/m2)
48% ML/PP 48% ML/PE LDPB HDPB SP
17.1±2.1 19.7±1.8 3.9±1.1 7.4±0.7 28.2±2.3
G.C. Stael et al. / Polymer Testing 20 (2001) 869–872
Fig. 1. Common aspects observed at the bagasse–EVA interface, showing stretched polymer attached to a bagasse piece. ML bagasse–EVA composite with Vf=0.13.
bagasse material. In fact, the Young modulus of bagasse is on the order of 1800 MPa [11] against the low value of only 28 MPa for the bare and flexible EVA polymer [10]. Besides that, the topographic aspects observed at the fracture surfaces show that there was a good interaction between EVA matrix and chopped bagasse. Hence, not only the difference between the stiffness of the resin matrix and bagasse contributed to decrease the deformation capacity of the composites, but the matrix deformation was also constrained by the chopped bagasse material. Fig. 1 shows the common aspects observed at the bagasse–EVA interface. One can see fibrils stretched from the matrix and adhered to the bagasse material. This is a clear evidence that a good bagasse–EVA interaction occurred. Fig. 2 shows another evidence that a
Fig. 2. Cohesive fracture of bagasse. ML bagasse–EVA composite with Vf=0.30.
871
good EVA–bagasse interface was developed. One can see a bagasse piece that was split in two parts, i.e. the crack run inside the bagasse and not at the interface. Cohesive fractures of this nature are usually found in systems showing good adhesion [12,13]. The good adherence developed between bagasse and EVA is also shown in Fig. 3, where one can see a piece of bagasse that was torn apart from the bulk bagasse pieces but is still adhered to the EVA resin matrix. The results of the solid-state NMR relaxation time measurements that were also carried out in these composites corroborated the fractographic analysis. From the values obtained for the proton spin–lattice relaxation time in the rotating frame, T1Hr, that was measured through the variable contact-time experiment, it was characterized that EVA–bagasse composites have a better physical interaction between bagasse and the polymeric matrix at molecular level, due to the decrease in the T1Hr values in relation to those shown by the bare bagasse. This interaction can come from the polar groups randomly distributed in the EVA macromolecular chains. This good interaction between bagasse–EVA could be interpreted as a sign that bagasse is really acting as a reinforcement. From the above results, one can highlight the variation of the impact strength as a function of volume fractions as the one with the greatest practical interest. In fact, although showing a lower performance than the PP and PE composites, as shown in Tables 1 and 2, EVA composites could be tailored to have the same performance of wood-based particle board materials. From the experimental results obtained, one can see that for composites with a volume fraction of 0.30, which corresponds to 40% in weight of bagasse, the impact strength is comparable to that of HDPB. With the higher volume fractions, what means more material usually disregarded as waste
Fig. 3. Broken bagasse with attached polymer. SL bagasse– EVA composite with Vf=0.48.
872
G.C. Stael et al. / Polymer Testing 20 (2001) 869–872
being used on an useful way, the impact strength of the chopped bagasse–EVA composites are comparable to the value shown by LDPB, largely used in low cost furniture. The soft plywood has a much higher impact strength than EVA composites and, therefore, cannot be considered as substitutable by these low strength composites.
Acknowledgements The authors would like to acknowledge the National Council for Scientific and Technological Development (CNPq).
References 4. Conclusions From the experimental results obtained one can conclude that: 1. The incorporation of chopped bagasse reduces the deformation capacity of EVA polymer; 2. A good bagasse/EVA interface was developed, as revealed by the topographic features observed at the fracture surface; 3. The impact strength varies with the volume fraction of bagasse, which enables tailoring the mechanical performance of these composites to reproduce the mechanical behavior of wood-based particle boards. This last conclusion points to a feasible application for these bagasse–EVA composites. The environmental problems posed today by bagasse wastes could be partially reduced if new low cost applications could be envisaged for these wastes to replace wood products.
[1] I.E. Winkle, M.J. Cowling, S.A. Hashim, E.M. Smith, J. Ship Prod. 7 (1991) 137. [2] R.F. Gibson, Principles of Composite Materials Mechanics, McGraw-Hill, New York, 1994, pp. 1–33 (Chapter 1). [3] K.K. Chawla, Composite Materials. Science and Engineering, Springer, New York, 1987, pp. 3–5 (Chapter 1). [4] G.C. Stael, M.I.B. Tavares, J.R.M. d’Almeida, Polym.Plast. Technol. Engng. (in press). [5] S. Schwartz, S.H. Goodman, Plastics Materials and Processes, Van Nostrand, New York, 1982, pp. 93–105. [6] G.C. Stael, PhD Thesis, Science and Technology Center, Universidade Estadual do Norte Fluminense, Campos dos Goytacazes, Rio de Janeiro, 1997 (in Portuguese). [7] F.A. Bovey, P.A. Mirau, NMR of Polymers, Academic Press, New York, 1996. [8] M.I.B. Tavares, Polym. Testing 16 (1997) 271. [9] N.M. Silva, M.I.B. Tavares, E.O. Stejskal, Macromolecules 31 (2000) 115. [10] G.C. Stael, M.I.B. Tavares, J.R.M. d’Almeida, Polym. Polym. Compos. (in press). [11] E.C. McLaughin, J. Mater. Sci. 15 (1980) 886. [12] S.M. Spearing, AIAA J. 35 (1997) 1638. [13] Z. Abdo, H. Aglan, J. Mater. Sci. Lett. 15 (1996) 469.
Material Behaviour
Impact behavior of sugarcane bagasse waste–EVA composites G.C. Stael a, M.I.B. Tavares a, J.R.M. d’Almeida a
b,*
Instituto de Macromole´culas, Universidade Federal do Rio de Janeiro, PO Box 68525, 21945-970, Rio de Janeiro, RJ, Brazil b Materials Science and Metallurgy Department, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225-22453-900, Rio de Janeiro, RJ, Brazil Received 8 November 2000; accepted 5 February 2001
Abstract The impact performance of chopped bagasse–EVA matrix composites is evaluated and compared with the behavior of bagasse filled PP and PE matrix composites and wood-based materials. The volume fraction and size of the chopped bagasse used as filler was varied. The experimental results show that the incorporation of bagasse strongly reduces the deformation capacity of EVA polymer. The reduction of the deformation capacity of the composites was also inferred by solid-state NMR relaxation analysis. The impact strength was independent of the bagasse size, but varied with the volume fraction. As a function of the volume fraction it was shown that the mechanical performance of bagasse–EVA composites could be tailored to reproduce the behavior of wood-based particle boards. 2001 Elsevier Science Ltd. All rights reserved. Keywords: EVA matrix composites; Chopped sugar cane bagasse; Impact behavior; NMR analysis
1. Introduction Nowadays, composite materials are largely used in many industrial applications that range from offshore structures used by the petroleum industry to common household goods [1,2]. The great majority of these composites are resin matrix-based materials. In fact, the large number of polymers that have a good performance as matrix materials provides a vast range of properties and, therefore, the versatility of resin matrix composite materials [3]. Poly(ethylene-co-vinyl acetate), EVA, polymer could be a feasible possibility as resin matrix, although its low mechanical properties could restrict the use of EVAbased composites [4–6]. Nevertheless, due to the high flexibility presented by EVA polymer, EVA-based com-
* Corresponding author. E-mail address: [email protected] d’Almeida).
(J.R.M.
posites could present some advantages under impact conditions, where toughness plays an important role. In this work, the impact performance of chopped bagasse–EVA matrix composites were evaluated. The Izod impact strength of the composites was measured as a function of the volume fraction and size of the bagasse and the values obtained were compared to those of bagasse filled-polyethylene, PE, and polypropylene, PP, matrix composites and common wood-based materials. Solid-state NMR analysis was performed to provide information on the matrix–bagasse interaction.
2. Experimental methods and materials The bagasse used as filler was directly obtained from sugar cane mills, after being processed to extract sugar and liquor. This “as received” material was dried at 80°C for 48 h and then was chopped and sieved. Bagasse pieces with lengths, l⬍3 mm, 3⬍ l ⬍5 mm and 20⬍ l ⬍30 mm were separated and used in this work. Herein-
0142-9418/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 0 1 4 - 9
870
G.C. Stael et al. / Polymer Testing 20 (2001) 869–872
after these chopped materials will be referred to as the small length (SL), medium length (ML) and long length (LL) bagasse pieces. The chopped bagasse was dried again at 80°C for 48 h before being incorporated into the EVA matrix. The composites were fabricated by mixing the proper quantities of EVA and chopped bagasse in a Haake Rheocord 9000 plastograph using the rollermix rotor at 60 rpm for 20 min. The processing temperature used was 200°C. The mixed material was then pressed at 3.5 MPa for 15 min at 200°C to obtain plates of the composites. These processing parameters were shown to give the best fabrication performance for these EVA–bagasse mixtures [6]. Composites with volume fraction, Vf, of bagasse of 0.13, 0.30, 0.48 and 0.59, respectively, corresponding to a matrix to bagasse ratio of 80/20, 60/40, 40/60 and 30/70 in weight, were fabricated. The specimens for the impact tests were machined from the fabricated plates and were 62 mm long, 12.7 mm wide and 4 mm thick. The notch to depth ratio was 0.20 and the radius of the notch tip was 0.25 mm. The included angle of the notch tip was 45°, as specified by the ASTM standard D-256. The tests were conducted at room temperature, 23±3°C, in a non-instrumented equipment with a maximum pendulum capacity of 2.7 J. At least 10 specimens were tested per composite. Using the same fabrication procedure bagasse–PP and bagasse–PE composites were also fabricated. These composites have a volume fraction of 0.48 and the chopped bagasse used was the SL material. Test specimens from these materials and from three commercial wood-based materials, were also machined and tested using the same experimental parameters as for the bagasse–EVA composites. Only the thickness of the wood-based materials were different, because these materials were tested with their commercial thickness, without any machining. The three wood-based materials tested were a 4.4 mm thick low density particle board (LDPB); a high density particle board (HDPB), 3 mm thick, composed of pressed cellulosic material; and a soft plywood, SP, 3.7 mm thick plate. The fracture surface of the tested specimens were analyzed by scanning electron microscopy (SEM), on gold– palladium coated specimens. The SEM observation was performed with secondary electrons imaging and acceleration of the electron beam between 15 to 20 kV. The solid state NMR spectra were obtained on a VARIAN INOVA 300 spectrometer operating at 299.9 and 75.4 MHz for 1H and 13C, respectively. All experiments were performed at probe ambient temperature using gated high power decoupling. A zirconium oxide rotor with a diameter of 7 mm was used to acquire the NMR spectra at rates of 6 kHz. 13C spectra are referenced to the chemical shift of the carbon atoms of the methyl group of hexamethyl benzene (17.3 ppm). The 13 C measurements were carried out in the cross-polariz-
Table 1 Impact strength (in kJ/m2), of the chopped bagasse–EVA composite as a function of the volume fraction and bagasse length Volume
Bagasse length
fraction
SL
ML
LL
0.13 0.30 0.48 0.59
– 8.4±1.1 3.7±0.5 2.7±0.5
20.0±2.2 7.8±0.8 3.9±0.4 2.9±0.4
20.1±0.6 8.8±0.6 4.2±0.4 3.6±0.3
ation mode with magic-angle spinning (CPMAS) with 2 s of delay. A variable contact-time experiment was also recorded and the range of contact-time established range from 200 to 8000 µs. The T1Hr values were determined from the intensity decay of carbon-13 peaks with increasing contact-time [7–9].
3. Experimental results and discussion The experimental results obtained are shown in Tables 1 and 2 for the chopped bagasse–EVA composites and for the materials used for comparison, respectively. From Table 1, one can see that the impact behavior of EVAbased composites is almost independent of the bagasse length. Only for the higher volume fraction, viz. 0.59, is there a slight tendency for the energy absorbed to increase with increase of the bagasse length. This same trend was also observed for the tensile behavior of these composites [10] and, in practice, it implies that for chopped bagasse with sizes smaller than 30 mm there is no need for sieving the bagasse pieces. On the other hand, the variation of volume fraction causes a strong effect on the impact strength of the composites. As one can see from the data in Table 1, the incorporation of chopped bagasse produces a strong decrease in the impact strength. The brittle behavior associated with the composites with higher volume fractions could be attributed to the inhibition of the deformation capacity of EVA matrix due to the presence of the much more stiff Table 2 Impact strength of the chopped bagasse–PP and PE composites and commercial wood based materials Material
Impact strength (kJ/m2)
48% ML/PP 48% ML/PE LDPB HDPB SP
17.1±2.1 19.7±1.8 3.9±1.1 7.4±0.7 28.2±2.3
G.C. Stael et al. / Polymer Testing 20 (2001) 869–872
Fig. 1. Common aspects observed at the bagasse–EVA interface, showing stretched polymer attached to a bagasse piece. ML bagasse–EVA composite with Vf=0.13.
bagasse material. In fact, the Young modulus of bagasse is on the order of 1800 MPa [11] against the low value of only 28 MPa for the bare and flexible EVA polymer [10]. Besides that, the topographic aspects observed at the fracture surfaces show that there was a good interaction between EVA matrix and chopped bagasse. Hence, not only the difference between the stiffness of the resin matrix and bagasse contributed to decrease the deformation capacity of the composites, but the matrix deformation was also constrained by the chopped bagasse material. Fig. 1 shows the common aspects observed at the bagasse–EVA interface. One can see fibrils stretched from the matrix and adhered to the bagasse material. This is a clear evidence that a good bagasse–EVA interaction occurred. Fig. 2 shows another evidence that a
Fig. 2. Cohesive fracture of bagasse. ML bagasse–EVA composite with Vf=0.30.
871
good EVA–bagasse interface was developed. One can see a bagasse piece that was split in two parts, i.e. the crack run inside the bagasse and not at the interface. Cohesive fractures of this nature are usually found in systems showing good adhesion [12,13]. The good adherence developed between bagasse and EVA is also shown in Fig. 3, where one can see a piece of bagasse that was torn apart from the bulk bagasse pieces but is still adhered to the EVA resin matrix. The results of the solid-state NMR relaxation time measurements that were also carried out in these composites corroborated the fractographic analysis. From the values obtained for the proton spin–lattice relaxation time in the rotating frame, T1Hr, that was measured through the variable contact-time experiment, it was characterized that EVA–bagasse composites have a better physical interaction between bagasse and the polymeric matrix at molecular level, due to the decrease in the T1Hr values in relation to those shown by the bare bagasse. This interaction can come from the polar groups randomly distributed in the EVA macromolecular chains. This good interaction between bagasse–EVA could be interpreted as a sign that bagasse is really acting as a reinforcement. From the above results, one can highlight the variation of the impact strength as a function of volume fractions as the one with the greatest practical interest. In fact, although showing a lower performance than the PP and PE composites, as shown in Tables 1 and 2, EVA composites could be tailored to have the same performance of wood-based particle board materials. From the experimental results obtained, one can see that for composites with a volume fraction of 0.30, which corresponds to 40% in weight of bagasse, the impact strength is comparable to that of HDPB. With the higher volume fractions, what means more material usually disregarded as waste
Fig. 3. Broken bagasse with attached polymer. SL bagasse– EVA composite with Vf=0.48.
872
G.C. Stael et al. / Polymer Testing 20 (2001) 869–872
being used on an useful way, the impact strength of the chopped bagasse–EVA composites are comparable to the value shown by LDPB, largely used in low cost furniture. The soft plywood has a much higher impact strength than EVA composites and, therefore, cannot be considered as substitutable by these low strength composites.
Acknowledgements The authors would like to acknowledge the National Council for Scientific and Technological Development (CNPq).
References 4. Conclusions From the experimental results obtained one can conclude that: 1. The incorporation of chopped bagasse reduces the deformation capacity of EVA polymer; 2. A good bagasse/EVA interface was developed, as revealed by the topographic features observed at the fracture surface; 3. The impact strength varies with the volume fraction of bagasse, which enables tailoring the mechanical performance of these composites to reproduce the mechanical behavior of wood-based particle boards. This last conclusion points to a feasible application for these bagasse–EVA composites. The environmental problems posed today by bagasse wastes could be partially reduced if new low cost applications could be envisaged for these wastes to replace wood products.
[1] I.E. Winkle, M.J. Cowling, S.A. Hashim, E.M. Smith, J. Ship Prod. 7 (1991) 137. [2] R.F. Gibson, Principles of Composite Materials Mechanics, McGraw-Hill, New York, 1994, pp. 1–33 (Chapter 1). [3] K.K. Chawla, Composite Materials. Science and Engineering, Springer, New York, 1987, pp. 3–5 (Chapter 1). [4] G.C. Stael, M.I.B. Tavares, J.R.M. d’Almeida, Polym.Plast. Technol. Engng. (in press). [5] S. Schwartz, S.H. Goodman, Plastics Materials and Processes, Van Nostrand, New York, 1982, pp. 93–105. [6] G.C. Stael, PhD Thesis, Science and Technology Center, Universidade Estadual do Norte Fluminense, Campos dos Goytacazes, Rio de Janeiro, 1997 (in Portuguese). [7] F.A. Bovey, P.A. Mirau, NMR of Polymers, Academic Press, New York, 1996. [8] M.I.B. Tavares, Polym. Testing 16 (1997) 271. [9] N.M. Silva, M.I.B. Tavares, E.O. Stejskal, Macromolecules 31 (2000) 115. [10] G.C. Stael, M.I.B. Tavares, J.R.M. d’Almeida, Polym. Polym. Compos. (in press). [11] E.C. McLaughin, J. Mater. Sci. 15 (1980) 886. [12] S.M. Spearing, AIAA J. 35 (1997) 1638. [13] Z. Abdo, H. Aglan, J. Mater. Sci. Lett. 15 (1996) 469.