Dynamic mechanical properties of polymer melts: The dependence of viscoelastic properties on molecular weight distribution

Abstract

An investigation of the dynamic mechanical properties of several molten polymers was performed using the Maxwell Orthogonal Rheometer. Relaxation spectra derived from experimental data for the terminal region of viscoelastic response indicate that as molecular weight distribution broadens, terminal relaxation phenomena associated with molecular disentanglement and translation extend over a corresponding wider frequency range. The same data indicate that a true maximum relaxation time beyond which no elastic response is observed exists for the materials studied. Moreover, the maximum relaxation time corresponds to the reciprocal of the frequency where the dynamic viscosity deviates from its zero‐shear value. Thus an estimate of the time necessary for complete elastic recovery in polymer melts is readily obtained experimentally.

Viscoelastic properties of decrosslinked irradiation‐crosslinked polyethylenes in supercritical methanol

“…Generally, in a liquid‐like low‐frequency region known as a terminal region, the power law linear viscoelastic slopes of a monodisperse flexible homopolymer can be expressed as G ′ ∝ ω 2 and G ″ ∝ ω (and η* ∝ ω 0 ) 13. These slopes are very sensitive to changes in the molecular weight,14, 15 molecular weight distribution,16, 17 chain branching,18, 19 crosslinking,20 mesostructure of the polymers,21, 22 and filler incorporation 23, 24. The slope change of G ′ is more sensitive to the structures than that of G ″.…”

“…LDPE with a high molecular weight ( M w = 349,000 g/mol) can form tremendous entanglements, which can serve as physical crosslinking points 14, 15. The high molecular weight distribution ( M w / M n = 7.0) causes a broadening of the relaxation distribution of the polymer chains in the terminal region, thus enhancing the nonterminal elastic behavior 16, 17. However, the LDPE exhibited more liquid‐like properties than the decrosslinked XLPEs that contained a considerable residual network gel structure.…”

Morphological Studies of Lightly Sulfonated Polystyrene Using 23Na NMR: Effects of Polydispersity in Molecular Weight

“…Polydispersity has been shown to affect shear moduli ( G‘ , G‘‘ ), ,,, stress relaxation moduli, compliance, ,, dielectric spectra, melt viscosity, − ,,, tensile properties, ,, and self-diffusion. , Researchers have also investigated the effects of molecular weight and dispersity of polymer chains between cross-links in network systems. − Above the critical entanglement molecular weight, narrow-distribution polystyrenes show two distinct relaxations in shear experiments, while polydisperse materials show only a broad distribution of relaxation times. , Materials with very broad molecular weight distributions exhibit non-Newtonian flow at lower shear rates than the corresponding narrow-distribution polymers. ,, In terms of bulk properties, the tensile strengths and elongations of monodisperse polystyrenes are higher than those of polydisperse materials at the same weight-average molecular weights ( M w ); the reverse behavior is seen with respect to M n . , …”

“…Polydispersity has been shown to affect shear moduli (G′, G′′), 24,26,38,39 stress relaxation moduli, 28 compliance, 24,36,40 dielectric spectra, 41 melt viscosity, [24][25][26][27]30,33,37 tensile properties, 32,33,38 and self-diffusion. 42,43 Researchers have also investigated the effects of molecular weight and dispersity of polymer chains between crosslinks in network systems.…”

“…23,24 Materials with very broad molecular weight distributions exhibit non-Newtonian flow at lower shear rates than the corresponding narrow-distribution polymers. 27,30,39 In terms of bulk properties, the tensile strengths and elongations of monodisperse polystyrenes are higher than those of polydisperse materials at the same weightaverage molecular weights (M w ); the reverse behavior is seen with respect to M n . 32,33 Influences of molecular weight in telechelic ionomers have also been studied.…”