Linear Viscoelastic Characterization of Polymer Melts with Long Relaxation Times

Abstract

Melts of linear polyethylenes (HDPE) have broad retardation and relaxation spectra that can be determined by combining oscillatory and long duration creep/creep recovery tests. For a commercial HDPE the necessary sample preparation (stabilization), testing (at 170 °C), and data conversion are described. For long time testing incomplete creep/creep recovery is preferred because the strain is kept within the linear range and reaches a final, constant value. The measured recoverable compliance, (t), is converted into a discrete retardation spectrum {L i , τ i ‘}. For the short time range the dynamic moduli are measured and converted into the dynamic compliances and finally the retardation spectrum L(τ‘). Both spectra overlap and form a combined spectrum over 7.5 decades of retardation time τ‘. From this the dynamic compliances are recalculated and converted into the dynamic moduli. These are used for computing the weighted relaxation spectrum, τH(τ), ranging from τ = 0.01−2.5 × 105 s. τH(τ) has a maximum at τ = 104 s, explaining the difficulties in characterizing HDPE melts. The viscosity functions η(t) and the first normal stress functions Ψ1(t) are measured at shear rates between 10-3 and 10 s-1 and compared with their limits for zero shear rate, η0(t) and (t), respectively, predicted from H(τ). The measured results are close to these predictions, but only at short times, such that the equilibrium values for t → ∞, η0 and Ψ1,0, are never reached in such tests. In contrast, creep/creep recovery reveals η0 = 113.5 kPas, the (high!) linear equilibrium compliance J e = 0.05 Pa-1, and 2η0 2 J e = Ψ1,0 = 1.3 × 109 Pa s2. The latter value coincides well with the one calculated from the second moment of H(τ).

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…In this context, incomplete creep tests are performed by the method of Meissner. [85,86] After a series of mathematic transformations of the raw data, the rheological functions such as creep compliance J(t), recoverable compliance J r (t r ) and steady-state elastic compliance (J 0 e ) can be obtained. [87] The most common rheological methods of elongation are stressing experiments and creep-recovery experiments.…”

Towards a rheological classification of flow induced crystallization experiments of polymer melts

“…1992, Aguliar et al 2003, Vega et al 2003 and theory (Pattamaprom and Larson 2001) can only be related to the HMW chains provided the storage, G¢, and loss modulus, G 00 , are measured in the terminal regime, where G 0 / x 2 and G 00 / x: However, the terminal regime can not always be reached for iPP (Swartjes 2001;Elmoumni et al 2003) and PE melts (Kraft et al 1999) containing long chains. (This problem can be resolved by performing creep experiments; Kraft et al 1999;Gabriel and Mu¨ndstedt 2002. ) Second, the contribution of the HMW tail may be screened by the remaining part of the melt (Eder et al 1989;Suneel et al 2003;Hepperle 2002).…”

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

“…The temperature chosen was 145°C to avoid degradation, since some of the materials are characterised by broad relaxation and retardation spectra and long duration experiments are needed. Incomplete creep tests were performed according to the method of Meissner [166,167]. Preliminary incomplete creep tests were performed by varying shear stress until the same value as the viscosity was achieved, indicating the condition of the linear viscoelastic response.…”