A clear trend in the nominal relaxation times ( τ *) is observed, following the order: 1-a < 1-b < 1-c < 1-d ( Table 1 ). τ * ( τ KWW ) is defined as the characteristic relaxation time obtained from fitting the stress relaxation curves with a Kohlrausch–Williams–Watts (KWW) stretched exponential function G ( t )
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