Polymer Chain Dynamics in a Random Environment: Heterogeneous Mobilities (original) (raw)
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Physical Review Letters, 2000
In order to investigate the origin of the often invoked nanoheterogeneities in miscible polymer blends, we have performed quasielastic neutron scattering experiments on the component dynamics within the miscible polymer blend polyisoprene͞polyvinyl ether including the pure components as a reference. We find that the apparent local heterogeneities observed by spectroscopic techniques originate from the chain specific crossover properties between entropy driven and local chain dynamics and are, thus, a purely dynamical phenomenon. PACS numbers: 61.41. + e, 61.12.Ex, 61.25.Hq The study of thermodynamic and structural properties of polymer blends is a very active field in polymer physics. Recently also dynamical aspects like the socalled "dynamic miscibility," i.e., the question how the chain dynamics is altered upon blending, came into focus . Mainly on the basis of results from spectroscopic techniques many workers have invoked the existence of nanoheterogeneities in thermodynamically miscible blends and disordered block copolymer systems [2-6]. Among the different materials studied, the miscible blend of polyisoprene and polyvinyl ether (PI͞PVE) has been investigated particularly well and can be considered as a model system . 2D NMR revealed [3] that, though the system is homogeneous (Flory Huggins parameter x , 0), the two polymers exhibit significantly different mobilities. By dielectric spectroscopy several groups have found two separate relaxations in the miscible regime (e.g., ). This finding is supported by recent quasielastic neutron scattering (QENS) studies, where at large momentum transfers distinctly different QENS patterns are observed for the two components . Such findings are not restricted to PI͞PVE system but have been observed also in several other miscible polymer blends (e.g., ). Finally, studies by forward recoil spectroscopy [10] on polystyrene͞poly(xylenyl ether) blends resulted in grossly different component diffusion coefficients, indicating very different friction coefficients for the two components in this particular blend.
Collective Intermolecular Motions Dominate the Picosecond Dynamics of Short Polymer Chains
Physical Review Letters, 2013
Neutron scattering and extensive molecular dynamics simulations of an all atom C 100 H 202 system were performed to address the short-time dynamics leading to center-of-mass self-diffusion. The simulated dynamics are in excellent agreement with resolution resolved time-of-flight quasielastic neutron scattering. The anomalous subdiffusive center-of-mass motion could be modeled by explicitly accounting for viscoelastic hydrodynamic interactions. A model-free analysis of the local reorientations of the molecular backbone revealed three relaxation processes: While two relaxations characterize local bond rotation and global molecular reorientation, the third component on intermediate times could be attributed to transient flowlike motions of atoms on different molecules. The existence of these collective motions, which are clearly visualized in this Letter, strongly contribute to the chain relaxations in molecular liquids.
Physica B-condensed Matter, 1992
The dynamics of the cx-relaxation in three glass-forming polymeric systems, poly(vinyl methyl ether) (PVME), poly(vinyl chloride) (PVC), and poly(bisphenol A, 2-hydroxypropylether) (PH) has been studied by means of quasielastic neutron scattering and compared with the results obtained from relaxation techniques. The results indicate that the dynamics of the ~-relaxation in a wide timescale shows a clear non-Debye behaviour and can be well described by means of the same spectral shape, which is found to be independent of temperature and momentum transfer (Q). Moreover, the Havriliak-Negami characteristic times deduced from the fitting of the experimental data can also be described using only one Vogel-Fulcher functional form. This implies a self-consistent description of the dynamics of the c~-relaxalion obtained by very different probes. Besides, we found that the Q-dependence of the characteristic times obtained by QENS is given by a power law, r(Q)~ Q-" (n >2), n being dependent on the system, and that the Q-behaviour and the non-Debye behaviour are directly correlated. These results have main implications about the physical mechanisms behind the dynamics of the c~-relaxation.
Neutron scattering study of the dynamics of a polymer melt under nanoscopic confinement
Chemical Physics, 2009
Poly͑ethylene oxide͒ confined in an anodic aluminum oxide solid matrix has been studied by different neutron scattering techniques in the momentum transfer ͑Q ជ ͒ range 0.2Յ Q = ͉Q ជ ͉ Յ 1.9 Å −1 . The cylindrical pores of the matrix present a diameter ͑40 nm͒ much smaller than their length ͑150 m͒ 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 ជ direction was either parallel or perpendicular to the pores for a scattering angle of 90°. 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 Ͻ 1 Å −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 ͑NMR͒ studies and can be rationalized by assuming that the interactions with the pore walls affect one to two adjacent monomer monolayers.
Static and dynamic contributions to anomalous chain dynamics in polymer blends
Journal of Physics: Condensed Matter, 2011
By means of computer simulations, we investigate the relaxation of the Rouse modes in a simple bead-spring model for non-entangled polymer blends. Two different models are used for the fast component, namely fully-flexible and semiflexible chains. The latter are semiflexible in the meaning that static intrachain correlations are strongly non-gaussian at all length scales. The dynamic asymmetry in the blend is strongly enhanced by decreasing temperature, inducing confinement effects on the fast component. The dynamics of the Rouse modes show very different trends for the two models of the fast component. For the fully-flexible case, the relaxation times exhibit a progressive deviation from Rouse scaling on increasing the dynamic asymmetry. This anomalous effect has a dynamic origin. It is not related to particular static features of the Rouse modes, which indeed are identical to those of the fully-flexible homopolymer, and are not modified by the dynamic asymmetry in the blend. On the contrary, in the semiflexible case the relaxation times exhibit approximately the same scaling behaviour as the amplitudes of the modes. This suggests that the origin of the anomalous dynamic scaling for semiflexible chains confined in the blend is esentially of static nature. We discuss implications of these observations for the applicability of theoretical approaches to chain dynamics in polymer blends.
The Dynamics of Confined Polycarbonate Chains Probed with Incoherent Neutron Scattering
2001
The mean-square atomic displacement <u 2 > of thin (1015 Å to 75 Å) polycarbonate films supported on Si wafers is measured using incoherent elastic neutron scattering. The value of <u 2 > is determined by fitting the scattering intensity data to the Debye-Waller factor and we find that <u 2 > is diminished with decreasing film thickness. When the film thickness is comparable to the unperturbed dimensions of the macromolecule two characteristic temperatures start to emerge in the <u 2 > data, one above and the other below the bulk Tg. Furthermore, preliminary results with a different polymer suggests that <u 2 > does not depend on the probing direction, i.e. the in-plane and the out-of-plane <u 2 > were found to be identical within the experiment uncertainty.
The dynamics of copolymers in homopolymer matrices
The European Physical Journal Special Topics, 2007
The ability of a polymer chain to relax when it is deformed, and the ease to which this relaxation occurs, dramatically influences the macroscopic properties of the polymeric material. However, the local segmental relaxation processes in multi-component polymer systems are not well understood. The impact of the dynamics of one component on the dynamics of the other is not simply proportional to the relative amounts of each component, as one might expect. This paper discusses recent results using neutron techniques and Monte Carlo simulation that monitor the dynamic properties of a copolymer in a homopolymer matrix. In particular, the results indicate that altering either copolymer sequence distribution or copolymer composition will dramatically impact the dynamics of the copolymer that is surrounded by homopolymers. These results provide important fundamental information on the coupling of the dynamics of two components in a multi-component polymer system. This data also offer insight into the local segmental relaxation processes in multi-component polymer systems, which are not well understood and yet influence the ultimate properties of these mixtures
Collective dynamics of glass-forming polymers at intermediate length scales
EPJ Web of Conferences, 2015
Motivated by the proposition of a new theoretical ansatz [V.N. Novikov, K.S. Schweizer, A.P. Sokolov, J. Chem. Phys. 138, 164508 (2013)], we have revisited the question of the characterization of the collective response of polyisobutylene at intermediate length scales observed by neutron spin echo (NSE) experiments. The model, generalized for sublinear diffusion -as it is the case of glass-forming polymers-has been successfully applied by using the information on the total self-motions available from MD-simulations properly validated by direct comparison with experimental results. From the fits of the coherent NSE data, the collective time at Q → 0 has been extracted that agrees very well with compiled results from different experimental techniques directly accessing such relaxation time. We show that a unique temperature dependence governs both, the Q → 0 and Q → ∞ asymptotic characteristic times. The generalized model also gives account for the modulation of the apparent activation energy of the collective times with the static structure factor. It mainly results from changes of the short-range order at inter-molecular length scales. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
On the non-Gaussianity of chain motion in unentangled polymer melts
The Journal of Chemical Physics, 2001
We have investigated chain dynamics of an unentangled polybutadiene melt via molecular dynamics simulations and neutron spin echo experiments. Good short-time statistics allows for the first experimental confirmation of subdiffusive motion of polymer chains for times less than the Rouse time (R) confirming behavior in this regime observed in simulations. Analysis of simulation trajectories obtained over several Rouse times reveals non-Gaussian segmental displacements for all time and length scales. These results, particularly non-Gaussian displacements on large time-and length scales, demonstrate the importance of intermolecular correlations on chain dynamics. Rouse-type analytical models fail to account for this non-Gaussianity leading to large deviations between the experimental dynamic structure factor and model predictions.
Molecular-dynamics simulation of a glassy polymer melt: Incoherent scattering function
The European Physical Journal B, 1999
We present simulation results for a model polymer melt, consisting of short, nonentangled chains, in the supercooled state. The analysis focuses on the monomer dynamics, which is monitored by the incoherent intermediate scattering function. The scattering function is recorded over six decades in time and for many different wave-vectors which range from the size of a chain to about three times the maximum position of the static structure factor. The lowest temperatures studied are slightly above T c , the critical temperature of mode-coupling theory (MCT), where T c was determined from a quantitative analysis of the β-and α-relaxations. We find evidence for the space-time factorization theorem in the β-relaxation regime, and for the time-temperature superposition principle in the α-regime, if the temperature is not too close to T c. The wave-vector (q-) dependence of the nonergodicity parameter, of the critical amplitude, and the α-relaxation time are in qualitative agreement with calculations for hard spheres. For q larger than the maximum of the structure factor the α-relaxation time τ q already agrees fairly well with the asymptotic MCT-prediction τ q ∼ q −1/b. The behavior of the relaxation time at small q can be rationalized by the validity of the Gaussian approximation and the value of the Kohlrausch stretching exponent, as suggested in neutron-scattering experiments.