type-A Polymer Research Articles

Chain Dimension and Dynamics of Polymers in Well-Defined Non-sticky Nanocomposites of Molecular Nanoparticle Polyhedral Oligomeric Silsesquioxane/Poly(butylene oxide)

The chain dimension and dynamics of type-A polymer, poly(1, 2-butylene oxide) (PBO), in the nanocomposites comprising polyhedral oligomeric silsesquioxane (POSS) nanoparticles have been investigated in broad frequency and temperature windows by utilizing broadband dielectric spectroscopy and rheology. Non-crystalline and dielectric “invisible” molecular nanoparticle POSS was specially designed to ensure its miscibility with the PBO matrix, which was verified by quantitative estimation of solubility parameters and subsequent X-ray scattering experiments. Dispersions of POSS in the PBO matrix up to 10 vol % have been investigated. Dielectric experiments revealed a negligible change of the dielectric relaxation strength of the normal mode and thus the statistical chain dimension of PBO, regardless of the POSS loadings and the polymer chain lengths spanning from the unentangled to the entangled region. The introduction of nanoparticles is found to influence both the chain and segment dynamics when the molecular weight of the polymer is relatively low. However, by renormalized with the segmental relaxation, the dielectric chain relaxation shows identical characteristic relaxation time with that of the neat polymer, independent of the volume fractions of POSS, which is consistent with the terminal relaxation time determined in the rheology experiments. The slowing down of segmental relaxation has been attributed to the influence of nanoparticles on the chain ends associated with excess free volume. The possible confinement, solvent, and filler effects in this type of a non-sticky nanocomposite model system have been further discussed.

ENTANGLEMENT-LOOSENING DYNAMICS RESOLVED THROUGH COMPARISON OF DIELECTRIC AND VISCOELASTIC DATA OF TYPE-A POLYMERS: A REVIEW

ABSTRACTFor so-called type-A polymer chains having electrical dipoles aligned parallel along their backbone, the large-scale chain motion over the end-to-end distance results in not only viscoelastic but also dielectric relaxation. These two relaxation processes detect the same motion but with different averaging moments, which enables us to experimentally resolve some details of the chain dynamics through comparison of viscoelastic and dielectric data of type-A polymers. For a typical type-A polymer, high-cis polyisoprene (PI), results of such an experimental approach are summarized to discuss characteristic features of an entanglement-loosening process (constraint release and/or dynamic tube dilation process) resolved from the data comparison.

Molecular Dynamics of Polyfarnesene

This paper represents the first comprehensive report of the molecular weight dependence of the dynamics of polyfarnesene (PF), utilizing oscillatory shear rheology and broadband dielectric spectroscopy. Extended PF chain conformations arising from tightly packed C11/C13 pendant groups reduce the probability of chain entanglements and lead to Rouse-like melt dynamics up to a critical molecular weight ∼105 g/mol. At higher molecular weights, PF behaves as an entangled polymer melt. Dielectric spectroscopy measurements establish PF as a type-A polymer, whose normal mode relaxation is strongly dependent on molecular weight, providing a compliment to melt rheology for the exploration of PF global chain dynamics.

Measurement of temperature profiles in turbulent pipe flow of polymer and surfactant drag-reducing solutions

A device was built to measure temperature profiles of turbulent pipe flows of various drag-reducing fluids. It is easy to use and reliable. We measured temperature profiles over a range of conditions leading to accurate measurements down to y+≈10, for tests carried over Reynolds numbers (Re) between 10 000 and 90 000. The effects of high heat fluxes and buoyancy, in particular, were quantified to ascertain the parameter range for accurate measurements. Temperature profiles measured for type-A polymer solution and for cationic surfactant solutions allowed us to see strong similarity between velocity and temperature profiles for drag-reducing surfactant solutions. A comparison between the slopes of the thermal and velocity buffer layers resulted in calculated turbulent Prandtl numbers between 6 and 9 for those drag-reducing solutions. We also used this tool to investigate drag reduction for a nonionic surfactant solution, which showed a significantly different fan-type profile, and also for a type-B drag-reducing polymer solution (Xanthan gum).

Time Autocorrelation Function of Intramolecular Dipole Moments for Type-A Polymer Glass

We compute the high-frequency spectra of the intermolecular dipole moment correlation for a model polymer near the glass transition by molecular dynamics simulation without charge distribution. The dielectric properties are calculated by introducing the inherent dipoles which are virtually placed along the chains as forming the type-A polymer. The high-frequency loss peak corresponding to the local motions of molecules is observed at around 10 THz. The spectra are strongly deviated from a Lorentian distribution, which causes a large so-called tangential delta near the peak frequency as well.

Temperature and Density Effects on the Local Segmental and Global Chain Dynamics of Poly(oxybutylene)

Dielectric spectroscopy measurements over a broad range of temperature and pressure were carried out on poly(oxybutylene) (POB), a type-A polymer (dielectrically-active normal mode). There are three dynamic processes appearing at lower frequency, the normal and segmental relaxation modes, and a conductivity arising from ionic impurities. In combination with pressure-volume-temperature measurements, the dielectric data were used to assess the respective roles of thermal energy and density in controlling the relaxation times and their variation with T and P. We find that the local segmental and the global relaxation times are both a single function of the product of the temperature times the specific volume, with the latter raised to the power of 2.65. The fact that this scaling exponent is the same for both modes indicates they are governed by the same local friction coefficient, an idea common to most models of polymer dynamics. Nevertheless, near Tg, their temperature dependences diverge. The magnitude of the scaling exponent reflects the relatively weak effect of density on the relaxation times. This is usual for polymers, as the intramolecular bonding, and thus interactions between directly bonded segments, are only weakly sensitive to pressure. This insensitivity also means that the chain end-to-end distance is invariant to P, conferring a near pressure-independence of the (density-normalized) normal mode dielectric strength.

Molecular dynamics in thin grafted and spin-coated polymer layers.

Broadband Dielectric Spectroscopy is employed to study the molecular dynamics in thin layers of grafted and spin-coated polydimethylsiloxane (PDMS) and of the type-A polymer cis-1,4-polyisoprene (PI). Strong resemblance with low-molecular-weight systems is found but additionally the conformation of the chain and its extension becomes important. For PI a confinement-induced relaxation is found. It is observable if the layer thickness is comparable to the end-to-end vector of the chain and it depends strongly on the separation between interfaces. In contrast, the dynamic (and hence the calorimetric) glass transition is not affected by the geometrical confinement.

Local and global dynamics of polylactides.: A dielectric spectroscopy study

Polylactides (PLAs) are a family of degradable plastics having a component of the dipole moment both perpendicular and parallel to the polymer backbone (i.e. is a type-A polymer). We have studied the sub-glass, segmental and global chain dynamics in a series of fully amorphous samples having an L: D ratio of 80:20, with molecular weights in the range: 3.8×10 3< M n<4.7×10 4. Finite size effects have been observed for both the segmental and the sub-glass process. The segmental mode was found to have a steeper temperature and pressure dependence compared to the longest normal mode in accord with findings from other type-A polymers. The molecular weight dependence of the longest normal mode relaxation times ( τ∼ M n 3.1) reflects on the intermediate molecular weight regime ( M e∼6×10 3) from Rouse to entangled chains. These findings are compared with the viscoelastic results on the same system. Agreement in the c 2 value of the WLF equation is found but the scaling of the normalized longest relaxation time derived from the rheological measurements shows a stronger than expected dependence on the molecular weight.

Effect of pressure on the segmental and chain dynamics of polyisoprene. Molecular weight dependence

We report on the effect of pressure on the relaxation modes of the type-A polymer polyisoprene as a function of the chain length for M/Me ratio’s (Me is the entanglement molecular weight): 15, 12, 23, 2, and 5. We find that the spectral shape of the segmental and normal modes is invariant under temperature (T) and pressure (P) variations, but the time–pressure superposition fails as a consequence of the stronger P dependence of the segmental mode. The segmental mode activation volume for the different molecular weights was found to scale with the temperature difference from the respective glass transition temperature. Pressure is found to slow down both modes in a quantitatively similar way, irrespective of the chain length, implying that Me is largely independent of pressure.

Pressure dependence of the local and global dynamics of polyisoprene

We report on the pressure dependence of the local segmental and normal modes of a type-A polymer (polyisoprene) using dielectric spectroscopy. We find that external pressure exerts a stronger influence on the dynamics of the segmental mode as compared to the longest normal mode which results in the crossing of the two modes at higher pressures. The predictions of the Rouse model are not adequate to explain the different shift factors even at elevated temperatures.