Paul Steeman

The structure of a styrene-acrylonitrile/butadiene polymer studied by 2D-i.r. spectroscopy

2D-i.r. spectroscopy involving mechanical excitation is employed to study the mechanical relaxation phenomena in a styrene-acrylonitrile/butadiene (ABS) sample, revealing detailed information regarding the molecular segments involved in macroscopic mechanical relaxation. From the temperature dependence of the dynamic signals it is corroborated that in the ABS sample the polybutadiene (PB) rubber relaxes entirely independently from the styrene-acrylonitrile (SAN) matrix, and thus is present as a separate phase in the material. The rapid-scan variation developed to perform 2D-i.r. experiments offers a significantly reduced measurement time in comparison with the step-scan mode of operation. In particular, in relation to research in the polymer field, in which case stress relaxation is expected to occur during the 2D-i.r. experiment, this presents a major advantage.

Linear and nonlinear viscoelasticity of polymer/silica nanocomposites: an understanding from modulus decomposition

We investigate the linear and nonlinear viscoelasticity of a model polymer nanocomposite of fumed silica nanoparticles in poly-(ethylene-co-α-butene) (PEB). Above a critical filler fraction, a space-filling network builds up as a result of cluster agglomeration and causes the material to change from liquid-like to solid-like states. Using the Winter-Chambon criterion, the percolation threshold from the measured dynamic moduli is found to depend strongly on the matrix viscoelasticity even when the particle dispersion is identical. Such phenomenon comes from the implicit assumption in the method that the contribution from particles (or particle agglomerates) should dominate the dynamic moduli, which is usually inapplicable when polymer has a high viscosity (or elasticity). A new method is suggested to decompose the moduli of composites into the hydrodynamic part and the part from particle agglomerates by taking into consideration the different effects of particles on the strain rate and stress. The two portions are shown to depend on oscillatory frequency, which are adopted to extrapolate the dynamic moduli to lower frequency. The percolation threshold from these extrapolated data becomes independent of the matrix viscoelasticity. Furthermore, it is found that the appearance of nonlinear behavior in oscillatory shear is related to the portion of particle agglomerates in storage modulus and the strain rate is the key factor to destroy the structures.

Rapid-Scan Two-Dimensional FT-IR: An Efficient Approach to Dynamic Infrared Spectroscopy

It is shown that it is possible to use a standard rapid-scan Fourier transform infrared FT-IR spectrometer for dynamic infrared measurements. Spectrum acquisition time is significantly reduced in comparison with two-dimensional infrared (2D IR) spectrometers employing the step-scan technique. In the case of polymers, for which stress relaxation often occurs over time, the reduction of the total measurement time proves to be an essential feature. Experimental and mathematical details are discussed. Application of the rapid-scan technique is illustrated with some results for a styrene–acrylonitrile/butadiene copolymer.

On Mooney and Mooney stress relaxation. II. A nonlinear viscoelastic description: Experimental and numerical results

It has been investigated whether the stress build-up and the stress relaxation involved in a Mooney test, with subsequent Mooney stress relaxation, can be described by nonlinear viscoelastic theory, more particularly the Wagner constitutive model. For this purpose, the viscoelastic behavior of three nonvulcanized EPDM materials, with similar Mooney viscosity but varying elasticity, has been studied. Relaxation time spectra were obtained from dynamic mechanical experiments, from which the step-strain stress-relaxation modulus was calculated. Stress build-up experiments were performed with a cone and plate system in order to obtain the so-called damping function (a measure for the deformation sensitivity) of the materials. Using these material functions, the Mooney test was successfully described with the Wagner constitutive model. Experimental and theoretical Mooney stress-relaxation rates are in close agreement. The predicted Mooney viscosity is up to 25% lower than the measured value. This may be due to nonideal conditions during the Mooney test, such as inhomogeneous heating and secondary flows, and to inaccuracy of the damping function. The model calculations confirm the strong experimental dependence of Mooney measurements on small variations in instrumental conditions such as geometry, rotation speed, and so forth.

Analysis as a Function of Temperature of the Dynamic Linear Infrared Dichroism Spectra of Isotropic and Cold-Drawn High-Density Polyethylene

A dynamic linear infrared dichroism (DLIRD) study was carried out as a function of temperature on isotropic and cold-drawn high-density polyethylene (HDPE) with the use of a rapid-scan setup. The dynamic in-phase dichroic spectrum and its components (parallel and perpendicular polarized spectra) were analyzed. From the results, it turned out that, in addition to molecular orientation effects, other effects such as frequency shifts are clearly identified in the dynamic signals, which furthermore were in agreement with previously reported results on polyethylene under stress from the use of conventional infrared spectroscopy. Moreover, the frequency shifts were more apparent at the lowest temperatures and in the cold-drawn material, i.e., with increasing sample moduli. A revision of the interpretation of the results published earlier, in which the dynamic signals were solely attributed to molecular orientations, is also carried out in view of our results. From our results it seems that indeed an orientation of the crystallographic b axis parallel to the applied strain is suggested from the –CH2–bending and rocking ranges of the isotropic sample. However, an orientation of the crystallographic a axis of the unit cell perpendicular to the strain direction as suggested from previous studies is questioned, as well as the assignment of a certain dynamic signal to a reorientation of chains in the amorphous phase. Besides these results, the behavior of the –CH2– wagging at 1176 cm−1, assigned to the crystalline phase, suggests an orientation of the crystallographic c axis of the unit cell parallel to the strain direction. As a consequence, two conclusions are made: (1) a more complex orientation motion occurs in the crystals than has been reported so far, and (2) a more thorough study of the DLIRD signals has to be carried out in which other factors such as thickness variations and instrumental artifacts need to be addressed.

Dynamic Infrared Spectroscopy Using the Rapid-Scan Technique

A technique using rapid-scan Fourier transform infrared (FT-IR) spectroscopy to observe reversible dynamic changes of systems under an externally applied perturbation is introduced. The technique does not require a lock-in amplifier or synchronization between the applied perturbation and the spectral acquisition. With the use of a carefully designed sampling rate with respect to the perturbation period and reconstruction of the observed spectral intensities, dynamic spectral intensities of the system are obtained. Due to the repeated behavior, the reconstructed spectral intensities can be used to represent the dynamic characteristic of the system. The experimentation time of the rapid-scan technique is significantly shorter than that of the step-scan technique. The influences of noise and experimental parameters on the observed spectral intensities are verified. Applications of the technique to systems with sinusoidal dynamic changes and exponentially decaying dynamic changes are demonstrated.

A Non-Linear Viscoelastic Description of the Mooney and the Mooney-Relaxation Test

In the rubber industry the standardised Mooney test is used on a routine basis for quantifying the flow behaviour and the Mooney-relaxation test on a non-routine basis for characterizing the elasticity of non-vulcanised rubbery materials. These tests involve the measurement of the shear stress, during start-up and after the cessation of steady shear flow, using a visco-simeter equipped with a closed chamber measurement cell which includes a rotor embedded in the material. Because of the very large deformations applied during the Mooney test, typically the deformation amounts to several hundred shear units, the results are highly nonlinear viscoelastic. Therefore, the relation between the Mooney results and the linear viscoelastic material functions, known to be a sensitive indicator for details of the molecular structure, is not straightforward.

Synthesis, characterization and mechanical properties of networks prepared from poly(propylene glycol) di(ethylmaleate) and a trifunctional thiol

Abstract One option to understand and possibly improve the properties of thermosets is to investigate well-defined polymer networks. Various novel, α,ω-ethylmaleate functionalized poly(propylene glycol)s were endlinked with a trifunctional thiol, 2-ethyl-2-(hydroxymethyl)-1,3-propanediol tris(3-mercaptopropionate) (EHPT). Model reactions of EHPT and diethyl maleate indicated that a substitution effect in the thiol is absent. Characterization of the model networks revealed that the molecular weight between junction points, M c , calculated from the sol fraction (branching theory) agrees with the M c calculated from the tensile storage rubber modulus E ′. The glass transition temperature ( T g ) decreases linearly with increasing M c . The K 1c (critical stress intensity factor) seems to be independent of the crosslink density.