Applied Polymer ARTICLE would lead to an even stronger reduction in the matrix mobil ity, which is expressed by a stronger shift in Tg. The different behaviors of th es provided fur the influence of the chemical functionalization of the surface on the interfacial adhesion between the nanotubes and the epoxy Tensile Properties The interface adhesion between the reinforcing fiber and the matrix markedly influenced the mechanical performance of the composites. This region is the site synergy in composite materials, as the stress redistribution from the matrix to the fibers takes place through their bond/interphase. Therefore, although the interface region appeared to have an insignificant hatrix and its composites (NFRUP volume fraction, its influence on the overall material properties and EGMRUP) in the second was significant. Indeed, it has been observed that the exis tence of this interface/into terphase brings about significant that hybrid composites showed significant improvement in changes in the relaxation behavior of Te as discussed earlier. 42. 3 fatigue crack propagation resistance, and this could support the Figure 4 illustrates the traction curves of the NERUP and NFRUP hybrid composite, which contained two natural fibers EGMRUP composites. Both curves showed a plateau at the be lfa and wool)and the polymeric fibers(PET-PE). a closer ginning for a low strength attributed to the slide of the speci- look at the E-glass-fiber cross-sectional aspects depicted in Fig- men in the grips of the dynamometer. After this plateau,the ure 2(d) demonstrated a similar close contact in the interfacial curves exhibited a typical mechanical behavior in each compos- region between the fibers and the matrix compared with that of ite. On the NERUP composite traction curve, a depression was the nyrup composite bserved, which could be explained by the stretching of some fibers. Indeed, the tensile properties of the alfa fibers were lower The UP resin matrix and its NERUP and EGMRUP composites than those of the E-glass fibers,and this explained the absence were subjected to DSC to evaluate their thermal properties. The of such a depression on the EGMRUP composite traction curve thermographs of all of the samples are shown in Figure 3. The Accordingly, Figure 5(a-c)shows the Young's modulus, strengt T value of each sample was determined from the midpoint and stress at break of the NeRUP and EGMRUP composites value of the jump in heat flow after the second heating run. From these histograms, we observed that the tensile properties Also, the line construction was done by the Pyris software, and of the NFRUP composite were slightly better than those of he obtained values were about 68, 78, and 54C for the resin EGMRUP with regard to the obtained values of the young's and the NFRUP and the EGMRUP composites, respectively. modulus and tensile strength at break values. This showed, These results indicate that To which was attributed to the tran- according to the Young's modulus, a good cohesion of the sition from a glasslike form to a rubbery and flexible for materials to transfer stress from the matrix to the fiber How increased with the addition of the natural fibers to the matrix. ever, the stress at break of the EGMRUP composite was superior However, this temperature decreased with the addition of to that of the NFRUP composite; this could be attributed to the E-glass fibers to the matrix. Changes in the glass-transition tem- greater strength of the E-glass fibers compared to that of the perature(ATg) can indicate altered polymer chain mobility. natural fibers, as mentioned previously. When specific proper- Indeed, depending on the strength of the interaction between ties were compared, the differences in the tensile performance the polymer and the filler, this region can have a higher or became more marked. In fact, the NFRUP composite exhibited lower mobility than the bulk material, and this can result in decrease or an increase in Tx So ATg is less than 0 if there is a depletion of segments at the boundaries, and AT is greater than 0 if the segment-filler interactions are strong in compari- on with the segment-segment interactions. Furthermore, it EGMRUP was argued by extension that if the system is asymmetric 2 (bounded by a free surface and a substrate), AT is greater than a e monomer-filler interactions are sufficiently strong to dominate the depletion of chain segments near the free surface; otherwise, ATg is less than 0. Moreover, a comparative study carried out of the thermal properties between a functionalized carbon nanotube(CNT)-epoxy composite and a nonfunctional ized CNT-epoxy one revealed a stronger shift in Tg in the case of the composite containing functionalized CNTs. According to this study, it was assumed that covalent bonds between the amino functions on the surface of the CNT and the epoxy Figure 4. Traction curves of the composites NFRUP and EGMRUP Www. MaterialSviewS. cOm WILEYONLINELIBRARY. COM/APP J APPL POLYM. SCL 2013, DOl: 10.1002/APP. 38499 491that hybrid composites showed significant improvement in fatigue crack propagation resistance, and this could support the NFRUP hybrid composite, which contained two natural fibers (alfa and wool) and the polymeric fibers (PET–PE). A closer look at the E-glass-fiber cross-sectional aspects depicted in Fig￾ure 2(d) demonstrated a similar close contact in the interfacial region between the fibers and the matrix compared with that of the NFRUP composite. DSC The UP resin matrix and its NFRUP and EGMRUP composites were subjected to DSC to evaluate their thermal properties. The thermographs of all of the samples are shown in Figure 3. The Tg value of each sample was determined from the midpoint value of the jump in heat flow after the second heating run. Also, the line construction was done by the Pyris software, and the obtained values were about 68, 78, and 54
C for the resin and the NFRUP and the EGMRUP composites, respectively. These results indicate that Tg, which was attributed to the tran￾sition from a glasslike form to a rubbery and flexible form, increased with the addition of the natural fibers to the matrix. However, this temperature decreased with the addition of E-glass fibers to the matrix. Changes in the glass-transition tem￾perature (DTg) can indicate altered polymer chain mobility. Indeed, depending on the strength of the interaction between the polymer and the filler, this region can have a higher or lower mobility than the bulk material, and this can result in a decrease28 or an increase29 in Tg. 30 So DTg is less than 0 if there is a depletion of segments at the boundaries, and DTg is greater than 0 if the segment–filler interactions are strong in compari￾son with the segment–segment interactions.31–34 Furthermore, it was argued by extension35 that if the system is asymmetric (bounded by a free surface and a substrate), DTg is greater than 0 if the monomer–filler interactions are sufficiently strong to dominate the depletion of chain segments near the free surface; otherwise, DTg is less than 0. Moreover, a comparative study carried out of the thermal properties between a functionalized carbon nanotube (CNT)–epoxy composite and a nonfunctional￾ized CNT–epoxy one revealed a stronger shift in Tg in the case of the composite containing functionalized CNTs. According to this study, it was assumed that covalent bonds between the amino functions on the surface of the CNT and the epoxy would lead to an even stronger reduction in the matrix mobil￾ity, which is expressed by a stronger shift in Tg. 36 The different behaviors of the two sample series provided further evidence of the influence of the chemical functionalization of the surface on the interfacial adhesion between the nanotubes and the epoxy resin. Tensile Properties The interface adhesion between the reinforcing fiber and the matrix markedly influenced the mechanical performance of the composites.37,38 This region is the site synergy in composite materials, as the stress redistribution from the matrix to the fibers takes place through their bond/interphase.39 Therefore, although the interface region appeared to have an insignificant volume fraction, its influence on the overall material properties was significant.40,41 Indeed, it has been observed that the exis￾tence of this interface/interphase brings about significant changes in the relaxation behavior of Tg, as discussed earlier.42,43 Figure 4 illustrates the traction curves of the NFRUP and EGMRUP composites. Both curves showed a plateau at the be￾ginning for a low strength attributed to the slide of the speci￾men in the grips of the dynamometer. After this plateau, the curves exhibited a typical mechanical behavior in each compos￾ite. On the NFRUP composite traction curve, a depression was observed, which could be explained by the stretching of some fibers. Indeed, the tensile properties of the alfa fibers were lower than those of the E-glass fibers,44 and this explained the absence of such a depression on the EGMRUP composite traction curve. Accordingly, Figure 5(a–c) shows the Young’s modulus, strength, and stress at break of the NFRUP and EGMRUP composites. From these histograms, we observed that the tensile properties of the NFRUP composite were slightly better than those of EGMRUP with regard to the obtained values of the Young’s modulus and tensile strength at break values. This showed, according to the Young’s modulus, a good cohesion of the materials to transfer stress from the matrix to the fiber.45 How￾ever, the stress at break of the EGMRUP composite was superior to that of the NFRUP composite; this could be attributed to the greater strength of the E-glass fibers compared to that of the natural fibers, as mentioned previously. When specific proper￾ties were compared, the differences in the tensile performance became more marked. In fact, the NFRUP composite exhibited Figure 3. DSC thermograms of the matrix and its composites (NFRUP and EGMRUP) in the second heating run. Figure 4. Traction curves of the composites NFRUP and EGMRUP. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38499 491 ARTICLE