Sensing Polymer Chain Dynamics through Ring Topology: A Neutron Spin Echo Study

Using neutron spin echo spectroscopy, we show that the segmental dynamics of polymer rings immersed in linear chains is completely controlled by the host. This transforms rings into ideal probes for studying the entanglement dynamics of the embedding matrix. As a consequence of the unique ring topology, in long chain matrices the entanglement spacing is directly revealed, unaffected by local reptation of the host molecules beyond this distance. In shorter entangled matrices, where in the time frame of the experiment secondary effects such as contour length fluctuations or constraint release could play a role, the ring motion reveals that the contour length fluctuation is weaker than assumed in state-of-the-art rheology and that the constraint release is negligible. We expect that rings, as topological probes, will also grant direct access to molecular aspects of polymer motion which have been inaccessible until now within chains adhering to more complex architectures.

Figure 1

SANS data for the 20k ring ($\varphi =0.01$) in a deuterated linear 20k matrix (full symbols) as compared to a pure ring polymer melt of identical molecular weight (empty symbols, data from [17]) in the Kratky representation. The lines represent fits to the data considering a Gaussian ring and a compact ring, respectively.

Figure 2

NSE spectra for the 20k ring ($\varphi =0.1$) in a deuterated linear 2k matrix for $Q$ values (from the top down) 0.05, 0.08, 0.1, 0.13, and $0.2{\AA }^{-1}$ at $T=413\mathrm{K}$. Upper inset: NSE data at a shorter time scale. The dashed black lines represent fits to the data (for details see text). Lower inset: PFG NMR data of a 20k ring (red circles) and a linear 20k chain (black triangles) in a deuterated 2k matrix at $T=413\mathrm{K}$.

Figure 3

NSE spectra for the 20k ring ($\varphi =0.1$) in a deuterated linear 80k matrix for $Q$ values (from the top down) 0.05, 0.08, 0.1, 0.13, and $0.2{\AA }^{-1}$ at $T=413\mathrm{K}$. Full and empty symbols refer to two different wavelength setups. Dashed black lines: initial Rouse decay. Solid lines: plateaus with $d_e=42\pm 1\AA$. Dashed colored lines: internal dynamics of pure 20k ring melt (data from Ref. [17]).

Figure 4

NSE spectra for the 20k ring ($\varphi =0.1$) in a deuterated linear 20k matrix (empty symbols) for $Q$ values (from the top down) 0.05, 0.08, 0.1, 0.13, and $0.2{\AA }^{-1}$ at $T=413\mathrm{K}$. Full symbols: 20k ring in a linear 80k matrix. Dashed lines: fit to the data considering CLF with $\alpha =1.1$. Thin solid lines: CLF contribution with $\alpha =1.5$.