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The chain dynamics and viscoelastic properties of poly(ethylene oxide) (PEO) were studied covering a wide range of molecular weights and temperatures. Two experimental techniques were used: rheology, in order to study the large scale viscoelastic properties, and neutron spin−echo (NSE) spectroscopy, to investigate the chain dynamics at the molecular level. We aimed to explore the characteristic dynamical parameters of the pure homopolymer system and describe its dependence on the polymer molecular weight and temperature. We will show that, after accounting for the molecular weigth dependence of the glass transition temperature, the dynamics observed for the different molecular weight samples can be consistently described by the Vogel−Tammann−Fulcher (VTF) temperature dependence.

We have investigated the molecular dynamics of syndiotactic poly(methyl methacrylate) well above the glass transition by combining quasielastic neutron scattering and fully atomistic computer simulations. The incoherent scattering measured by backscattering on a sample with deuterated ester methyl groups has revealed the single-particle motions of hydrogens in the main chain and in the α-methyl groups. Moreover, with neutron spin-echo experiments on the fully deuterated sample we have accessed the collective motions at the two first maxima of the structure factor. The simulated cell, which has been previously validated regarding the structural properties [Genix, A.-C.; et al. Macromolecules2006, 39, 3947], shows a dynamical behavior that, allowing a shift in temperature, reproduces very accurately all the experimental results. The combined analysis of both sets of data has shown that: (i) The segmental relaxation involving backbone atoms deviates from Gaussian behavior. (ii) The dynamics is extremely heterogeneous: in addition to the subdiffusion associated with the α-process and the methyl group rotations, we have found indications of a rotational motion of the ester side group around the main chain. (iii) At a given momentum transfer and depending on the molecular groups considered, the time scales for collective motion are spread over about 1 order of magnitude, the correlations involving the main chain decaying much more slowly than those relating side groups. (iv) At the length scale characteristic for the overall periodicity of the system (that corresponding to the first structure factor peak), the experimentally observed collective dynamics relates to the backbone motions and is of interchain character; there, coherency effects are observed for all correlations, though side groups display weaker collectivity. (v) At the second structure factor peak, coherency remains only for correlations involving the main chains., This research project has been supported by the European Commission NoE SoftComp, Contract NMP3-CT-2004-502235. A.A., F.A., and J.C. acknowledge support from the projects MAT2004-01017 and 9/UPV00206.215-13568/2001. “Donostia International Physics Center” is also acknowledged. The European Commission, NMI3 Contract RII3-CT-2003-505925, supported the experiments at the Forschungsreaktor in Jülich.

We have investigated the short-range order in 1,4-polybutadiene by combining fully atomistic MD simulations and neutron diffraction with polarization analysis on isotopically labeled samples. Thereby the structure factor corresponding to the fully deuterated sample and the partial structure factor of the fully protonated polymer have been obtained. Both functions show a first main peak centered at about 1.4 Å-1 and a second one at around 3 Å-1. With varying temperature, we observe opposite tendencies for the intensity of the first diffraction peak in each sample. The direct comparison between simulation and experimental data has allowed validation of our simulated cell. Exploiting the information on the simulations, we have found that CC correlations dominate the first diffraction peak for both the deuterated and the protonated samples, while the origin of the second maximum depends on the labeling. Finally, the temperature behavior of the first peaks has also been rationalized. To a large extent, density changes are driving the evolution of the peaks, and the way they affect mainly the CC correlations and the mixed correlations involving C and H determines the behavior observed in the diffraction patterns., This research project has been supported by the European Commission, NMI3 Contract RII3-CT-2003-505925, and the NoE SoftComp, Contract NMP3-CT-2004-502235. J.C., F.A., and A.A. acknowledge support from the Projects MAT2004-01017 and 9/ UPV00206.215-13568/2001, and A.N. acknowledges the FPI grant of the Spanish Ministery of Science and Technology.

We present scattering data from a melt of H-shaped polybutadiene polymers, in which the central crossbar of the polymers is wholly labeled with deuterium. The melt was uniaxially stretched by a factor of 2, and using careful temperature control of the sample, the neutron scattering pattern was measured after several different times of relaxation. The scattering data are compared against the predictions of a new theory, based upon the tube model but which includes the additional effect of elastic inhomogeneities in the entanglement network. We find that with this new theory one can obtain a good representation of the data, using parameters that are consistent with relaxation times obtained from a tube model description of the linear rheological data. The most important aspect of these data is an increasing anisotropy of the correlation hole peak, and this appears to be directly related to the elastic inhomogeneities. One further aspect of the comparison to theory is that the best fits are obtained using a Warner-Edwards tube diameter that deforms in a manner consistent with recent predictions for polymer networks.

Liquid-crystalline polyfluorene (PF) homopolymers substituted with chiral alkyl side chains were synthesized, and their chiroptical properties in the solid state were investigated by means of circular dichroism (CD), circularly polarized photoluminescence (CPPL), and circularly polarized electroluminescence (CPEL) measurements. Polarization-selective scattering of light is shown to cause artifacts in the circularly polarized absorption and emission spectra in the wavelength range near or above the absorption edge, and a measurement scheme to avoid these is presented. For all derivatives, significant chiroptical effects appeared only after the solid layers have been annealed at elevated temperatures, preferably into the liquid-crystalline state of the polymer. The largest anisotropy factors were measured for a polyfluorene substituted with chiral (R)-2-ethylhexyl side chains, yielding absolute values of up to 0.28 for CPPL and up to 0.25 for CPEL. These are among the highest ever reported for a chiral conj...

We have made detailed comparison of the local and chain dynamics of a melt of 1,4-polybutadiene (PBD) as determined from experiment and molecular dynamics simulation at 353 K. The PBD was found to have a random microstructure consisting of 40% cis, 50% trans, and 10% 1,2-vinyl units with a number-average degree of polymerization 〈Xn〉 = 25.4. Local (conformational) dynamics were studied via measurements of the 13C NMR spin−lattice relaxation time T1 and the nuclear Overhauser enhancement (NOE) at a proton resonance of 300 MHz for 12 distinguishable nuclei. Chain dynamics were studied on time scales up to 22 ns via neutron spin−echo (NSE) spectroscopy with momentum transfers ranging from q = 0.05 to 0.30 A-1. Molecular dynamics simulations of a 100 carbon (Xn = 25) PBD random copolymer of 50% trans and 50% cis units employing a quantum chemistry-based united atom potential function were performed at 353 K. The T1 and NOE values obtained from simulation, as well as the center of mass diffusion coefficient an...

Despite the great experimental effort carried out over the past years by means of different techniques, the segmental dynamics in the model blend system poly(vinylethylene)/polyisoprene (PVE/PI) is still a topic of strong debate. One of the most controversial questions is the possibility of a bimodal distribution of segmental relaxation times for polyisoprene in the blend. In this work, we have combined a microscopic technique, quasielastic neutron scattering (QENS), with molecular isotopic labeling and broad-band dielectric spectroscopy. The measurements have been carried out on the same samples and at the same temperature (270 K). By means of this combination, we have shown that all the PI in the miscible blend sample PVE/PI (50 wt % PVE−50 wt % PI) dynamically behaves very close to pure PI. In contrast, the α-relaxation of PVE component in the blend is drastically “plasticized”. In addition, it has been shown for the first time by QENS that the secondary β-like processes seem not to be modified by blending, at least for the blend component displaying the higher Tg. These experimental facts contradict many of the results previously published about the dynamics of the PI/PVE system, and as far as our knowledge, they cannot be explained in the frame of the models presently available for polymer blends dynamics., A. Arbe, A. Alegría, and J. Colmenero acknowledge support from the following projects: DGICYT, PB97-0638; GV, EX 1998-23; UPV/EHU, 206.215-G20/98. Support from “Donostia International Physics Center” is also acknowledged by them and D. Richter.

In this work we have investigated how the dynamics of poly(vinyl methyl ether), PVME, changes by blending with deuterated polystyrene. The experimental techniques used were dielectric spectroscopy, quasielastic neutron scattering, and 13C nuclear magnetic resonance. By means of these techniques, the dynamics of the poly(vinyl methyl ether) units in the blends can be selectively investigated in a huge time range (101−10-11 s). Two different blend compositions have been investigated. The main relaxation processes observed in this range are the secondary β-process and the segmental α-relaxation. It turns out that the β-relaxation is not affected by blending. The data analysis procedure followed by us in the case of the α-process is based on the assumption that the dynamics of the PVME segments in the blends is a superposition of dynamical processes with the same shape as that in pure PVME, but with the relaxation times distributed due to the presence of concentration fluctuations. From this analysis we found...

Recently by dielectric spectroscopy a new secondary relaxation process in polyisobutylene (PIB) was detected showing all the signatures of a Johari-Goldstein relaxation [Richter, D; Arbe, A.; Colmenero, J.; Monkenbusch, M.; Farago, B.; Faust, R. Macromolecules 1998, 31, 1133]. Using high resolution neutron backscattering, we investigated the quasielastic neutron spectra from protonated PIB which are related to this process as a function of momentum transfer and temperature. In addition we studied the elastically scattered intensity over a wide temperature range. After multiple scattering corrections all results can be described consistently in terms of a local jump process with the distribution parameters from the dielectric beta-relaxation. The spatial extent of the associated protonic motion was determined to be d = 2.7 Å. A comparison with the existing body of data for PIB leads to the conclusion that the dielectric beta-process and the earlier found delta-process must be identical, thereby revising the assignment of the delta-process as due to methyl group rotation. Finally, we remark on aspects of the relation between quasielastic coherent and incoherent scattering and address the seemingly contradictory result of different length scales revealed for the same process with the two techniques.

We present neutron spin−echo and dielectric results on the local dynamics of polyisobutylene. The dielectric spectra reveal the existence of a so far unknown secondary relaxation process being distinctly different to previous theoretical predictions. Neutron spectra have been taken over a large range in momentum transfer Q and temperature. At the Q value of the first structure factor maximum the dynamic pair correlation function is selective for interchain motions. There the neutron spectra display the same temperature dependence and shape as the classical rheological data taken in the terminal zone and do not follow the temperature laws based on spectroscopic results. A quantitative evaluation combining the information content of the dielectric and neutron results reveals a small stepwidth of 0.5−0.9 A involved in the secondary process. The α process is diffusive and follows the Gaussian approximation resulting in a sublinear time development of associated mean squared displacements.

Employing neutron spin-echo spectroscopy, we have studied the dynamic structure factors for the relaxation of a single chain in polymer melts. We have varied the molecular weights through the transition region from unrestricted Rouse dynamics to entanglement controlled behavior. Investigating the dependence of the dynamic structure factor on the momentum transfer Q, it is possible to access the different relaxation modes separately. We found that, depending on their spatial extension in relation to the entanglement distance, larger scale relaxations are successively slowed down compared to Rouse relaxation. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency

The form factor of a labeled chain randomly cross-linked into a network in a tubelike approach is presented. Within a harmonic approximation for the constraining potential, the action of cross-links and entanglements is modeled by an effective tube, which is deformation dependent. The only parameters of the model are the reduced microscopic deformation, following a power law in the strain, and the diameter of the tube, which is inversely proportional to the strength of the constraints. First experimental results on polybutadiene networks with varying cross-link density are discussed. The conclusions drawn confirm a less than affine deformation at the molecular level while segmental fluctuations are found to be unperturbed despite the network topology and of the same size as obtained from rheology

The dynamic structure factors of ethylene-propylene alternating copolymer (PEP) and saturated polybutadiene (PEB-2) was studied. To investigate different models of entanglement formation, the polymer volume fraction (PEB-2) and temperature (PEB-2 and PEP) were varied, thereby the two length scales considered to be important for the buildup of entanglements, namely, the contour length density and the random walk step length, were systematically changed. In the framework of the scaling ansatz, the result is in the range of the binary contact models and excludes the packing model

The experimental small angle neutron scattering results were compared with a RIS calculation using the statistical bond parameters of Mark. Experimental and theoretical results show good agreement as far as low and intermediate temperatures are concerned. The temperature coefficient for chain dimensions of (-1.16±0.03)x10 -3 K -1 extracted from the data in the range of 25-180°c is in good agreement with the value of (-1±0.2)x10 -3 K -1 obtained from measurements in θ solutions over a smaller temperature range.