Chain Relaxation Processes Research Articles

Electric‐field‐induced chain orientation in poly(L‐lactic acid)

AbstractThe orientational order was studied for melt‐state poly(L‐lactic acid) under an external direct‐current electric field. A birefringence as high as 1.1 × 10−2 was recorded against an external field of 1.0 MV/m at 190 °C. The evidence proved that a field–dipole interaction transferred from a randomly coiled conformation to a uniaxially drawn conformation. The field‐induced birefringence was temporally resolved, and the chain orientation and relaxation processes on the order of 100 s were observed in a real timescale after the field was turned on and off. A mechanism of chain orientation was examined with respect to the orientation polarization and viscoelasticity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4433–4439, 2004

Observations of structure development during crystallisation of oriented poly(ethylene terephthalate)

Two types of SAXS and WAXS experiments have been made using synchrotron radiation to observe the transformation from smectic to crystalline phases in oriented poly(ethylene terephthalate) (PET). In step-anneal experiments, PET was drawn slowly at 30 °C and then observed after annealing at 5 °C steps up to 100 °C. In the other experiments, time-resolved observations were made while drawing at 90 °C at rates up to 10 s−1. Up to 70 °C the WAXS data in the step-anneal experiments showed the smectic meridional reflection reducing in lateral width, indicating an increase in lateral long range order with annealing. Between 70 and 100 °C, there was a reduction in the intensity of the smectic reflection which correlated with an increase in the intensity of crystalline reflections. The SAXS from the step-anneal experiments showed an intense equatorial streak which has a correlation peak around 20 nm and which diminishes with annealing above 70 °C. It is concluded that this feature is a characteristic of the presence of the mesophase in oriented PET and is due to elongated domains of smectic mesophase with a length >75 nm and with an interdomain spacing of around 20 nm. Between 70 and 100 °C the SAXS data showed additional diffuse diffraction which correlated quantitatively with the crystalline phase and evolved from a cross-like appearance to a well resolved four-point pattern. The time-resolved drawing experiments were limited by the time resolution of the SAXS detector. They showed the same development of four-point diffuse SAXS patterns as was observed in the step-anneal experiments and a very weak equatorial streak. Differences in phase transformation kinetics between the two types of experiment are attributed to the different chain relaxation processes available under different conditions.

A molecular-dynamics simulation study of dielectric relaxation in a 1,4-polybutadiene melt

We have carried out atomistic molecular dynamics simulations of a melt of 1,4-poly(butadiene) from temperatures well above the experimentally observed merging of the primary α process and secondary β process down to temperatures approaching the experimentally observed bifurcation temperature. The relaxation strength and maximum loss frequency and its temperature dependence for the combined α-β dielectric relaxation process from simulations were in good agreement with experiment. The maximum loss frequency, melt viscosity, chain normal-mode relaxation times and torsional autocorrelation times were found to exhibit nearly identical non-Arrhenius temperature dependencies well represented by a Vogel–Fulcher fit with parameters in good agreement with experimental values obtained from dielectric and viscosity measurements. The dielectric susceptibility showed increasing intensity at high frequency for the lower temperatures investigated, indicative of a breakdown in time-temperature superposition due to an emerging β process. Comparison of time scales for the chain normal-mode dynamics and dielectric relaxation revealed that the latter is associated with motions on the segmental length scale. The correspondence of time scales and temperature dependence for the dielectric relaxation and the torsional autocorrelation function further confirmed the localized nature of the dielectric relaxation and indicated that the combined α-β dielectric process is fundamentally tied to microscopic conformational dynamics of individual dihedrals. However, the mean conformational transition rates were found to exhibit Arrhenius temperature dependence, leading to a divergence of time scales between the torsional, dielectric, chain and mechanical relaxation processes and the rates of conformational transitions with decreasing temperature. This divergence was associated with the increasingly heterogeneous character of conformational dynamics in the melt with decreasing temperature. Hence, the time scale of the principal (α) relaxation in the melt is fundamentally correlated with the time scale for homogenization of conformational dynamics, and not to the time scale of the conformational transitions themselves.

Physical Ageing in Polymethylmethacrylate — A Dielectric Investigation

Polymethylmethacrylate aged for various times at temperatures below the glass transition temperature, Tg, were studied using dielectric spectroscopy. The study uses dipole relaxation as a probe for the physical ageing process. Isothermal measurements indicate that the dominant changes occurring in the dielectric loss spectrum at temperatures between the Tg and the side chain (β) relaxation process are associated with changes in the dipole relaxation distribution and the ionic charge mobility. Differences between the spectra for aged and unaged samples are used to assess the nature of the changes occurring at a molecular level. The rate of change of dielectric loss was estimated as a function of the ageing temperature and the observed behaviour was found to be similar in form to that observed from creep and structural relaxation.

Orientation and crystallisation mechanisms during fast drawing of poly(ethylene terephthalate)

Synchrotron radiation has been used to record the diffraction patterns from Poly(ethylene terephthalate) for a range of draw rates (0.1 to 10sec−1) and temperatures (90 to 120°C). The patterns were analysed to derive the development of the order parameter and the rates of crystallisation. The effects of temperature and draw rate can be unified with a WLF time-temperature shift factor. Comparison with estimates of chain relaxation processes show that, when the draw rate is faster than the chain retraction process, the onset of crystallisation is delayed until the end of drawing. Crystallisation is very sensitive to both temperature and orientation and has an approximate 4th power dependence on .

Exploring molecular origins of sharkskin, partial slip, and slope change in flow curves of linear low density polyethylene

This paper explores the molecular mechanism for sharkskin formation on extrudate of linear low density polyethylenes (LLDPE) and investigates the rheological origin of a characteristic curvature (i.e., a slope change) in the flow curve of LLDPE. Rheological measurements, performed at various temperatures from 160 to 240 °C with a controlled‐pressure capillary rheometer and a variety of dies, suggest that the slope change in the flow curve, interpreted by many as demonstrating wall slip in the die land, arises from a combination of interfacial slip and cohesive failure due to chain disentanglement, first initiated on the die wall in the exit region. Since the disentanglement state is unstable for the adsorbed chains within a certain stress range below the critical stress for the global stick–slip transition, a partial slip flow cannot sustain itself and occurs only periodically. This time‐dependent molecular entanglement–disentanglement fluctuation produces the sharkskin like extrudate in the regime where the slope change takes place. Sharkskin dynamics are found to precisely correlate with chain relaxation processes. Specifically, the characteristic time scale τ (i.e., the sharkskin periodicity) is found to be of the same magnitude and have the same WLF (Williams–Landel–Ferry) temperature dependence as that of the characteristic molecular relaxation time τ* as determined by oscillatory shear measurements in a parallel‐plate flow cell. The LLDPE resins are also observed to undergo interfacial stick–slip transitions as well as a rarely seen cohesive slip–slip transitions at various temperatures.

Dynamic viscoelasticity and thermal behaviour of pectin—calcium gels

The influence of temperature on the equilibrium shear modulus of low-methoxyl pectin—calcium gels have been investigated in order to determine the standard breakdown enthalpy of the gel crosslinks and the activation energy of the polymer chain relaxation process.

Relaxation spectra and viscoelastic properties of the binary linear flexible polymers

A binary blending law was proposed for a mixture which was composed of two narrowly distributed linear polymer fractions with different molecular weights and the high entanglement density. The relaxation spectrum was used to express the binary blending law and it was derived from the chain relaxation processes based on tube model theories as well as from the earlier observations on the viscoelasticity of linear polymers with bimodal molecular weight distribution. The proposed law showed good agreements with experimental data over a wide range of time or frequency for the viscoelastic properties such as shear stress relaxation modulus and dynamic moduli. It also showed good-fitted results for the compositional dependences of some viscoelastic constants such as zero-shear viscosity and recoverable compliance.