Polymer Dynamics in Melts

Abstract

The dynamics of linear polymer chains in the melt depends strongly on the chain length: for short, unentangled chains, the dynamics is determined by a balance of viscous and entropic forces; for long chains, topological constraints are dominant. In this chapter, the experimental exploration of chain dynamics is introduced and discussed in detail. The focus is on neutron spin-echo (NSE) spectroscopy, which is one of the most powerful tools to explore the different dynamic regimes in polymer melts on a microscopic scale. It allows direct observation of the transition from a regime of free relaxation at short times, which can be described in terms of the Rouse model, to constrained motion at longer times. The constrained motion is caused by the entanglements that emerge in long-chain polymer systems. The tube concept models these topological confinements by assuming the chain to be confined in a virtual tube formed by adjacent chains. This concept of chains reptating in a tube is strongly supported by experiments in the limit of long chains. However, there is also strong experimental evidence that the tube model starts to fail if the polymer chains become shorter. In this regime of intermediate chain length, neither the Rouse model nor the pure reptation concept are applicable. A close comparison of linear rheology data with the predictions of the reptation model indicates the existence of additional degrees of freedom that release the topological confinement. Fluctuating chain ends that destroy the tube confinement starting from both ends were proposed as one candidate. This process, called contour length fluctuations (CLF), indeed accounts for the observed behavior of the mechanical relaxation function. In this chapter, we present a systematic study of this mechanism on a microscopic scale.