A. Arbe

Neutron Spin Echo in Polymer Systems

Neutron spin echo spectroscopy (NSE) provides the unique opportunity to unravel the molecular dynamics of polymer chains in space and time, covering most of the relevant length and time scales. This article reviews in a comprehensive form recent advances in the application of NSE to problems in polymer physics and describes in terms of examples expected future trends. The review commences with a description of NSE covering both the generic longitudinal field set-up as well as the resonance technique. Then, NSE results for homopolymers chains are presented, covering all length scales from the very local secondary β-relaxation to large scale reptation. This overview is the core of the review. Thereafter the dynamics of more complex systems is addressed. Starting from polymer blends, diblock copolymers, gels, micelles, stars and dendrimers, rubbery electrolytes and biological macromolecules are discussed. Wherever possible the review relates the NSE findings to the results of other techniques, in particular emphasizing computer simulations.

Study of the dynamics of poly(ethylene oxide) by combining molecular dynamic simulations and neutron scattering experiments.

We performed quasielastic neutron scattering experiments and atomistic molecular dynamics simulations on a poly(ethylene oxide) (PEO) homopolymer system above the melting point. The excellent agreement found between both sets of data, together with a successful comparison with literature diffraction results, validates the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field used to produce our dynamic runs and gives support to their further analysis. This provided direct information on magnitudes which are not accessible from experiments such as the radial probability distribution functions of specific atoms at different times and their moments. The results of our simulations on the H-motions and different experiments indicate that in the high-temperature range investigated the dynamics is Rouse-like for Q-values below approximately 0.6 A(-1). We then addressed the single chain dynamic structure factor with the simulations. A mode analysis, not possible directly experimentally, reveals the limits of applicability of the Rouse model to PEO. We discuss the possible origins for the observed deviations.

Design and Preparation of Single-Chain Nanocarriers Mimicking Disordered Proteins for Combined Delivery of Dermal Bioactive Cargos

Inspired by the multifunctionality of vitamin D-binding protein and the multiple transient-binding behavior of some intrinsically disordered proteins (IDPs), a polymeric platform is designed, prepared, and characterized for combined delivery of dermal protective and anticancer bioactive cargos on the basis of artificial single-chain nano-objects mimicking IDPs. For the first time ever, simultaneous delivery of folic acid or vitamin B9 , and hinokitiol, a relevant natural bioactive compound that exhibits anticancer activity against human malignant melanoma cells, from these multidirectionally self-assembled unimolecular nanocarriers is illustrated.

Neutron scattering study of the dynamics of a polymer melt under nanoscopic confinement

Poly(ethylene oxide) confined in an anodic aluminum oxide solid matrix has been studied by different neutron scattering techniques in the momentum transfer (Q) range 0.2<or=Q=/Q/<or=1.9 A(-1). The cylindrical pores of the matrix present a diameter (40 nm) much smaller than their length (150 microm) and are parallel and hexagonally ordered. In particular, we investigated the neutron intensity scattered for two orientations of the sample with respect to the incident beam, for which the Q direction was either parallel or perpendicular to the pores for a scattering angle of 90 degrees . Diffuse neutron scattering at room temperature has shown that the aluminum oxide has amorphous structure and the polymer in the nanoporous matrix is partially crystallized. Concerning the dynamical behavior, for Q<1 A(-1), the spectra show Rouse-like motions indistinguishable from those in the bulk within the uncertainties. In the high-Q limit we observe a slowing down of the dynamics with respect to the bulk behavior that evidences an effect of confinement. This effect is more pronounced for molecular displacements perpendicular to the pore axis than for parallel displacements. Our results clearly rule out the strong corset effect proposed for this polymer from nuclear magnetic resonance (NMR) studies and can be rationalized by assuming that the interactions with the pore walls affect one to two adjacent monomer monolayers.

From Rouse dynamics to local relaxation: A neutron spin echo study on polyisobutylene melts

We investigated the single chain motions of monodisperse polyisobutylene chains in the melt by neutron spin echo spectroscopy. Thereby a wide range in momentum space over a large dynamic range was covered. Motional processes from the center of mass diffusion, the Rouse dynamics to the more local relaxation processes which limit the validity of the standard Rouse model, were elucidated. The observed dynamic structure factors were analyzed in terms of relevant theoretical approaches addressing the limiting factors of the Rouse model. We found that other than claimed in the literature effects of local chain stiffness—they were treated in terms of the all rotational states model and a bending force model—cannot account for the experimental observations. It appears that additional damping effects related to an internal viscosity of the chain have to be involved, in order to explain the experimental results.

The dynamics of the α- and β-relaxations in glass-forming polymers studied by quasielastic neutron scattering and dielectric spectroscopy

Abstract The dynamics of the α-relaxation in three different polymeric systems, poly(vinyl methyl ether) (PVME), poly (vinyl chloride) (PVC) and poly (bisphenol A, 2-hydroxypropylether) (PH), have been studied by means of relaxation techniques and quasielastic neutron scattering. By using these techniques, we have covered a wide timescale ranging from mesoscopic to macroscopic times (10−10 – 101 s). In this wide timescale, the dynamics of the α-relaxation show a clear non-Debye behaviour which can be described by assuming a Kohlrausch-Williams-Watts functional form for the relaxation function or the intermediate scattering function. The shape of the relaxation function is found to be similar for the different techniques used and independent of temperature and momentum transfer, Q. Moreover, the characteristic relaxation times deduced from the fitting of the experimental data can also be described using only one Vogel-Fulcher functional form. Besides we found that the Q-dependence of the relaxation times obtained by quasielectric neutron scattering is given by a power law, τ(Q) ∝ Q−n (n > 2), n being dependent on the system, and that the Q-behaviour and the non-Debye behaviour are directly correlated. In the case of PVC, the dynamics are also investigated in a shorter timescale (10−11 – 10−13 s). In this range, we have observed the short-time part of the α-relaxation together with a fast process (timescale ∼ 10−12 s). It appears that both dynamical processes are separated by a critical time which hardly depends on Q and T. The results also indicate that the fast process observed is not related to the dielectric β-relaxation. All of these results are reviewed and discussed in the framework of the different theoretical approaches to the problem of the glass transition.

Efficient Synthesis of Single-Chain Globules Mimicking the Morphology and Polymerase Activity of Metalloenzymes.

Endowing unimolecular soft nanoobjects with biomimetic functions is attracting significant interest in the emerging field of single-chain technology. Inspired by the compartmentalized structure and polymerase activity of metalloenzymes, copper-containing compact nanoglobules have been designed, synthesized, and characterized endowed with metalloenzyme mimicking characteristics toward controlled synthesis of water-soluble polymers and thermoresponsive hydrogels. When compared to metalloenzymes, artificial nanoobjects endowed with metalloenzyme mimicking characteristics offer increased stability against thermal changes and reduced degradability by hydrolytic enzymes.

On the origin of the non-exponential behaviour of the -relaxation in glass-forming polymers: incoherent neutron scattering and dielectric relaxation results

The question of whether the primary -relaxation in glass-forming systems is homogeneous or heterogeneous in nature is investigated by incoherent neutron scattering techniques for glass-forming polymers. The results obtained for more than ten different polymers show that the incoherent intermediate-scattering function is well described by a Kohlrausch-Williams-Watts function with a constant shape parameter. Moreover, the momentum-transfer dependence of this function approximately follows the Gaussian approximation. These results do not agree with those expected from the heterogeneous picture usually invoked as the origin of the non-exponential behaviour of the -process. It seems that the apparent stretching of the -relaxation function in glass-forming polymers relates dominantly to sub-linear diffusion and is not a result of a superposition of single-exponential processes, at least in the mesoscopic timescale covered by neutron scattering techniques. Deviations from the Gaussian behaviour are evaluated for the different polymers investigated and interpreted in terms of distributions of stretched relaxation functions. The validity of the extension of these results to the low-temperature range close to the glass transition temperature, where neutron scattering measurements are not feasible, is discussed in the light of new dielectric results for this range.

Anomalous relaxation of self-assembled alkyl nanodomains in high-order poly(n-alkyl methacrylates)

We have exploited the selectivity of neutron scattering combined with isotopic substitution to study the structure and dynamics of poly(n-alkyl methacrylates). Our diffraction data strongly support the suggested nanosegregation of main-chains and side groups. Moreover, we have been able to separately follow the dynamics of both subsystems at a molecular scale. While the structural relaxation observed at the main-chain level is standard, for high-order members the correlations involving side groups within the alkyl nanodomains relax through an exotic logarithmic decay. In these polymers, a strong dynamic asymmetry also develops. We discuss possible theoretical frameworks for the anomalous relaxation observed.

Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions

(2)H-nuclear magnetic resonance (NMR) and neutron scattering (NS) on isotopically labelled samples have been combined to investigate the structure and dynamics of polyvinylpyrrolidone (PVP) aqueous solutions (4 water molecules/monomeric unit). Neutron diffraction evidences the nanosegregation of polymer main-chains and water molecules leading to the presence of water clusters. NMR reveals the same characteristic times and spectral shape as those of the slower process observed by broadband dielectric spectroscopy in this system [S. Cerveny et al., J. Chem. Phys. 128, 044901 (2008)]. The temperature dependence of such relaxation time crosses over from a cooperative-like behavior at high temperatures to an Arrhenius behavior at lower temperatures. Below the crossover, NMR features the spectral shape as due to a symmetric distribution of relaxation times and the underlying motions as isotropic. NS results on the structural relaxation of both components-isolated via H/D labeling-show (i) anomalously stretched and non-Gaussian functional forms of the intermediate scattering functions and (ii) a strong dynamic asymmetry between the components that increases with decreasing temperature. Strong heterogeneities associated to the nanosegregated structure and the dynamic asymmetry are invoked to explain the observed anomalies. On the other hand, at short times the atomic displacements are strongly coupled for PVP and water, presumably due to H-bond formation and densification of the sample upon hydration.