Solid-like rheological response of non-entangled polymers in the molten state

TL;DR

This study reveals that non-entangled polymers exhibit solid-like elastic behavior at macroscopic scales in the molten state, challenging traditional views of polymer melt flow and suggesting collective chain motions.

Contribution

It demonstrates for the first time that non-entangled polymers can display elastic-like responses in bulk molten form under standard rheological conditions.

Findings

Non-entangled polymers show elastic behavior above Tg.

Elasticity observed at millimeter-scale thicknesses.

Bulk properties indicate collective chain motions.

Abstract

We show that non-entangled polymers display an elastic-like behaviour at a macroscopic scale (probed at some 0.100 mm thickness) up to at least hundred degrees above the glass transition temperature. This observation, found under non-slippage conditions, both for side-chain liquid crystalline polymers and ordinary polymers, is in contradiction with the typically found flow behaviour of polymer melt. Our measurements were carried out with a conventional rheometer at thicknesses of several tenths millimetres. Thus, we were probing bulk properties. The observed elasticity supposedly implies that even in the melt the chains experience a cohesive effect of macroscopic distances, involving collective motions over time scales longer than the individual relaxation time of an individual polymer chain. The detection of such a solid-like property of molten non-entangled polymers is of considerable importance for a better understanding of the polymer dynamics.