Viscoelastic behavior of semicrystalline polymers at elevated temperatures on the basis of a two-process model for stress relaxation

Cyclic thermo-viscoplasticity of high density polyethylene

“…As modeling of a semicrystalline polymer as a two-phase composite leads to a noticeable growth in the number of material constants, observations in additional tests are to be employed to ensure an acceptable accuracy in their determination. An attempt to apply experimental data in relaxation tests for this purpose was undertaken by Djokovic et al (2000). However, their treatment of observations (matching of relaxation curves by a sum of two exponential functions that are presumed to describe the time-dependent response of amorphous and crystalline regions separately) may be questioned.…”

“…The apparent activation energies H 1 and H 2 adopt similar values (H 2 exceeds H 1 by 26%, but some deviations of experimental data for E 1 from the theoretical dependence should be mentioned). Closeness of H 1 and H 2 is in accord with the results reported by Djokovic et al (2000) based on treatment of observations in relaxation tests. The activation energies in the range 21-26 kJ/mol are similar to those provided by Zubova et al (2007) for a-relaxation in HDPE (between 29 and 36 kJ/mol) and Bin Wadud and Baird (2000) for relaxation in polyethylene melts (between 27 and 32 kJ/mol).…”

Structure and properties of phase change materials based on HDPE, soft Fischer‐Tropsch paraffin wax, and wood flour

“…Linear polyethylenes (LPE) usually do not show a β‐relaxation, which normally occurs in branched polyethylenes (BPE). This transition is the result of the motion in the interfacial regions (amorphous portion between crystallites) of the semicrystalline material, and depends on the degree of branching 34, 35. Djoković et al35 reported that the minimal interfacial content, which can produce a visible β‐peak in the DMA curve of polyethylene, is about 10% of semicrystalline material.…”

Thermo-viscoelastic and viscoplastic behavior of high-density polyethylene

“…Fig. 7 demonstrates that the apparent activation energy of HDPE far below the melting temperature is about 78 kJ/mol, which is lower than the values provided in some studies grounded on the standard method of superposition of observations in dynamic tests: E a ¼ 110-140 (Zamfirova et al, 2002), E a ¼ 137 (Pegoretti et al, 2000), E a ¼ 150-170 (Mano et al, 2001), E a ¼ 174 (Djokovic et al, 2000), and E a ¼ 207 kJ/mol (Tajvidi et al, 2005), but agrees well with the activation energies E a ¼ 50-140 (Stadler et al, 2005), E a ¼ 65-80 (Boiko et al, 1995), E a ¼ 79-106 (Matsuo et al, 1988), and E a ¼ 89 kJ/mol (Ohta and Yasuda, 1994). The fact that the apparent activation energy of HDPE in the solid state exceeds that of HDPE melt by twice [in the latter case, the activation energy belongs to the interval between 27 and 32 kJ/mol (Bin Wadud and Baird, 2000)] appears to be natural, whereas rather high values of the activation energies (close to the activation energies for thermal degradation of this polymer [E a ¼ 210-270 (Sinfronio et al, 2005) and E a ¼ 260-290 kJ/mol (Marazzato et al, 2007)] ''shows that the physical significance of E a .…”

“…In the past decade, the time-and rate-dependent responses of polyethylenes at ambient conditions, as well as at elevated temperatures have been studied in a number of publications, see Zhang and Moore (1997), Nitta and Suzuki (1999), Beijer and Spoormaker (2000), Djokovic et al (2000), Pegoretti et al (2000), Mano et al (2001), van Dommelen et al (2003), Dasari and Misra (2003), Bergstrom et al (2004), Hong et al (2004), Remond (2005), Mrabet et al (2005), Nikolov et al (2006), Colak and Dusunceli (2006), Elleuch and Taktak (2006), Christiansen (2007a,b), andBen Hadj Hamouda et al (2007), to mention a few. Although several variants of constitutive equations have been proposed for the viscoelastic and viscoplastic behavior of semi-crystalline polymers that reveal an acceptable agreement with observations, these models share common shortcomings: (i) they disregard thermally-induced evolution of microstructure of HDPE in the vicinity of a-relaxation point (in the interval of temperatures between 60 and 80°C), and (ii) their application to fitting observations results in rather high values of the apparent activation energy of solid polyethylene (in the interval between 100 and 200 kJ/mol) which substantially exceed those for polyethylene melts [20-30 kJ/mol (Bin Wadud and Baird, 2000)] and are close to the activation energies for thermal degradation of high-density polyethylene [ranging between 200 and 300 kJ/ mol (Sinfronio et al, 2005;Marazzato et al, 2007)].…”