3.2.4. Dynamic mechanical properties of UHMWPE fibers/NR composites
The dynamic mechanical properties of NR and UHMWPE fibers/ NR composites were also performed. The variation of storage modulus (E0) and tangent delta (tan d) as a function of temperature are reported in Figs. 8 and 9. The dynamic properties reflect the amount of the energy in the composite stored as elastic energy and the amount of energy dissipated during the strain process. These properties are highly dependent on the existence of fillers:dispersion within the matrix, volume fraction, geometrical characteristics,and load transfer from the filler to the matrix [24]. As illustrated in Fig. 8, at low temperatures, NR was in the glassy state and its storage modulus remained roughly constant at 1.5 GPa. This was due to the fact that in the glassy state, the molecular motions were largely restricted to vibration and short-range otational
motions. The glassy modulus are 1.9 and 0.5 GPa with the addition of as-received and treated UHMWPE fibers, respectively. It is remarkable that the as-received UHMWPE fibers/NR composites gives the highest energy dissipation in the temperature region over the glass transition temperature. This can be attributed to the very weak fiber/polymer bond in the composite [25]. As-received UHMWPE fibers/NR composites give an increase in E0 in both the rubbery region and glassy region, treated UHMWPE fibers/NR composites only in the rubbery region. For instance, at 40 C the storage modulus of the NR and the composites with as-received and treated UHMWPE fibers are 3.3, 20.7 and 16.1 MPa,
respectively.The evolution of the tangent of the loss angle as a function of temperature for NR and UHMWPE fibers/NR composites is presented in Fig. 9. The dynamic glass transition temperature was for all the materials close to 55 C. An insignificant decrease of the maximum tan d values was observed when adding the UHMWPE fibers. The tan d peak height is 2.47, 1.35 and 1.22 respectively for NR, as-received and treated UHMWPE fibers/NR composites. The
decrease of tan d peak reflects that UHMWPE fibers have effectively immobilized NR chains near the polymer-fiber interface due to favorable interfacial interactions with the NR matrix [26]. Meanwhile,the maximum tan d value of the composites with treated UHMWPE fibers are lower than that of as-received UHMWPE fibers/ NR composites. This result indicates that there is a stronger interaction between treated UHMWPE fibers and NR matrix [27]