Considering topology among all polymer architectures polymer rings are unique, as they are the simplest closed structures without ends. In this review we present recent experimental advances addressing the structure and dynamics of rings. We focus mainly on neutron scattering results that reveal experimental insight on a molecular scale. We first briefly reflect on the progress in ring chemistry that made the experimental access possible. Structural investigations characterizing rings as compact objects in the melts are put into theoretical context. In contrast to the plateau regime common for all other high molecular weight polymer systems, the dynamic modulus of pure ring systems is characterized by a power law decay, while the viscosity displays a much weaker molecular weight dependence as a corresponding linear melt. The dynamics of ring melts is uniquely addressed by neutron spin-echo spectroscopy. The sub-diffusive center of mass motion at short times agrees well with simulation as well as theoretical concepts. In the internal dynamics the basic length scale of the ring molecule, the loop size, manifests itself clearly. The experiments reveal strong evidence for loop motions and call for further theoretical work describing them. Finally, small fractions of ring molecules in linear melts turn out to be very sensitive probes in order to scrutinize the dynamics of the host with the potential to reveal fundamental aspects of the dynamics of branched polymer systems.
Random crystalline-amorphous copolymers containing ethylene and butene segments, yielded from dilute-solution, and self-assembled to one-dimensional, needle-shaped aggregates, can modify wax crystal structures through the cocrystallization of the copolymer and wax molecules into hairy platelets. These copolymers show selectivity in their wax crystal modification capacities that depends on the ethylene content of the backbone. Thus, it has been qualitatively established that a copolymer containing larger crystallizable polyethylene sections [poly(ethylene butene) with 7.5 ethyl branches per 100 backbone carbons (PEB-7.5)] is very efficient for longer wax molecules (C 36 ), whereas for shorter waxes (C 24 ), its efficacy diminishes. Here we present a quantitative evaluation of the small-angle neutron scattering results obtained in a complex study of the self-assembling behavior of PEB-7.5 and paraffin waxes (C 24 and C 36 ) in decane and of cocrystallization for different polymer-paraffin combinations and solution conditions. The richness of the morphologies was evaluated with a contrast variation technique and the application of structural models.
The dynamics of water between highly oriented multilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutron scattering (QENS) and deuterium-nuclear magnetic resonance (NMR) longitudinal (T1) relaxation were employed to investigate both the high-frequency motions of water (10−9–10−11 s time scale) and their anisotropy, while 2H-NMR transverse (T2) relaxation was used for obtaining information on low frequency dynamical processes (microsecond time scale). Our results show that high frequency dynamics (picosecond-time scale) at low hydration (three to four water molecules per lipid) can be understood solely as a uniaxial rotation of the water molecules tightly bound to DPPC head groups with a correlation time τrot≊62 ps at 55 °C and a rotational radius of 1±0.1 Å, but with no detectable translational degrees of freedom. The 2H-NMR T1 data (nanosecond-time scale) can be explained satisfactorily on the basis of fast rotations with the correlation time above and a slower reorientation of the rotational axis (correlation time τ1≊6 ns).
Both QENS and 2H-NMR T1 measurements provide an apparent activation energy of Ea=32±1.0 kJ/mol for this process. Increasing the hydration level of the multilayers leaves the rotational motion essentially unchanged, but enables additional translational motion which can be considered as a jump diffusion process (diffusion coefficient D=16±1×10−10 m2/s at 44 °C and a mean residence time of τ0=2.0±0.5 ps) of nonbound water. It is interesting to note that this diffusion is completely isotropic on the characteristic length scale of this QENS experiment (≤10 Å). Temperature variation shows that the phase state of the lipids has no significant effect on the high frequency dynamics of the water molecules. Measurements of the 2H-NMR quadrupolar splitting of water (D2O) at temperatures around the phase transition temperature Tm of the oriented DPPC multilayers clearly show a coexistence of the crystalline Lβ′ phase and of the fluid Lα phase over a range of up to 4 °C at both sides of Tm. The intermediate Pβ′ (‘‘ripple’’) phase is suppressed as we worked at hydration levels below saturation. In the coexistence range, exchange of water takes place between crystalline and fluid lipid domains due to water diffusion. This exchange causes a pronounced minimum of the 2H-NMR transverse relaxation time T2 at Tm since this low frequency process satisfies approximately a critical damping condition for a two-site chemical exchange process.
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