On the difference in scattering behavior of cyclic and linear polymers in bulk

Abstract

It has been suggested that, due to topological constraints, rings in the melt may assume a more compact shape than Gaussian chains. In this paper, we exploit the availability of narrow fractions of perdeuterated linear and cyclic polydimethylsiloxane ͑PDMS͒ and, through the analysis of the small angle neutron scattering ͑SANS͒ profiles, demonstrate the difference in scattering properties of linear and cyclic PDMS molecules. As expected for Gaussian chains, for the H/D linear PDMS samples, log-log plots of the scattered intensity versus scattering vector Q display a Q ͑−2͒ dependence. However, for H/D cyclic blends, the scaling exponent is higher than 2, as predicted by computer simulations reported in the literature. We show that cyclic molecules in bulk display the characteristic maximum in plots of scattered intensity versus Q ͑−2͒ that is expected on the basis of Monte Carlo calculations and from the Casassa equation ͓E. F. Casassa, J. Polym. Sci. A 3, 605 ͑1965͔͒. It is also shown that, for rings, the Debye equation ͓P. Debye, J. Appl. Phys. 15, 338 ͑1944͔͒ is no longer appropriate to describe the SANS profiles of H/D cyclic blends, at least up to M w Ϸ 10 000. For these samples, the Casassa form factor gives a better representation of the SANS data and we show that this function which was developed for monodisperse cyclics is still adequate to describe our slightly polydisperse samples. Deviations from all above observations are noted for M w Ͼ 11 000 and are attributed to partial contamination of cyclic samples with linear chains. The failure of both the Debye and the Casassa form factors could be due to contamination of the cyclic fractions by linear polymers or to a real conformational change.

A Study of the Morphology, Dynamics, and Folding Pathways of Ring Polymers with Supramolecular Topological Constraints Using Molecular Simulation and Nonlinear Manifold Learning

“…4 Synthetic ring polymers are useful components in the engineering of artificial micro-devices and molecular machines, such as catenane nanoswitches made up of two interlocked ring polymers. [5][6][7][8][9][10][11] Due to the absence of free ends, the topology of ring polymers gives rise to significant structural and dynamic differences compared to their linear counterparts, and the structure and rheological properties of ring polymers have been subject to extensive theoretical, [12][13][14][15] experimental, [16][17][18] and simulation [19][20][21][22] investigations.…”

Monte Carlo simulation of a single ring among linear chains: Structural and dynamic heterogeneity

“…The diffusion mechanism of rings has been suggested theoretically as amoebalike motion: they thrust and pull on unentangled loops. 1,2 For ring polymers in bulk, in spite of extensive work using experiments, [3][4][5][6][7][8][9][10] theories, [11][12][13][14] and simulations, [15][16][17][18][19][20][21][22][23] many controversies still remain.…”

Why polymer chains in a melt are not random walks

“…An experimental verification -e.g. following the lines of a promising recent study [6] -would be of great fundamental interest; it could also delineate the conditions where the predicted corrections to ideality can be observed in real polymer systems and must be considered in understanding their structure, phase behavior, and dynamics. Such an impact on structure and dynamics is expected theoretically, for the bulk [8,17,18] and also for thin polymer films [7].…”

“…Possible causes for deviations are effects of chain stiffness and finite chain thickness. In special cases, the scattering signals from chain stiffness and thickness may compensate one another, leading fortuitously to an extended Kratky plateau [5,6]. Apparently, 'Kratky plots have to be interpreted with care' [4].…”