No description available.

No description available.

No description available.

No description available.

Styrene-4-vinylpyridinium ionomers of different architectures, random and ABA blocks, were synthesized and their thermal and dynamic mechanical properties investigated. / The Tg's of the random ionomers were found to increase regularly with increasing ion content, at a rate of about 3.5(DEGREES)C per mole % of ions. In contrast with other ionomeric systems studied before, however, these materials showed only one peak in their tan delta curves, associated with the Tg of the matrix. The absence of a second, high temperature peak suggests the absence of clusters in these vinylpyridinium ionomers, at least at temperatures above their Tg. The storage modulus curves were in agreement with this conclusion. / An elaborate polymerization line was constructed for the production of the block copolymers by living anionic polymerization; a basic design was modified considerably and two new units were completely designed in this laboratory. Only one Tg, associated with the Tg of the polystyrene phase, was observed in DSC measurements for the resulting ionomers. The glass transition of the ionic domains was detected in dynamic mechanical measurements as a shoulder on the low temperature side of the polystyrene transition. This unexpected low value for the Tg of the ionic domains was attributed to plasticization by water.

Access to abstract permanently restricted to Ball State community only. / Access to thesis permanently restricted to Ball State community only. / School of Physical Education, Sport, and Exercise Science

The effects of systematic variations in the size and volume fraction of reinforcing phase on the mechanical properties of and fracture processes in silicon carbide particlereinforced aluminium matrix composites have been studied. Tensile tests to failure have been performed to determine the mechanical properties of the composites. A simple model has been proposed for this behaviour. The micromechanisms of fracture have been investigated by a combination of fractographic and dynamic techniques. Matched fracture halves have been obtained from the composites and the fracture processes elucidated. Fracture proceeded by a ductile void nucleation, growth and coalescence mechanism. Void nucleation occurred at the reinforcing phase, with a change in nucleation mechanism on varying the micrstructural parameters. A simple critical stress criterion has been proposed for the nucleation process. Support for this proposal has been obtained by the study of sections through the failed tensile specimens. In situ scanning electron microscopy fracture studies have been performed. These revealed void nucleation before the onset of macroscopic cracking. Crack propagation has been shown to occur by the concurrent formation of microcracks ahead of the crack tip and failure of the joining matrix ligaments. The magnitude of matrix deformation has been shown to determine the extent of microcracking. Acoustic emissions have been monitored during tensile straining. Void nucleation events have been recorded from the onset of plastic deformation and continuing throughout the plastic régime until final failure. The suppression of void coalescence by the constaint imposed on matrix flow by rigidly-bonded interfaces has been proposed to account for the extended void growth in materials containing fractured particles. The importance of the local values of the microstructural parameters on the far-field strain at nucleation has been shown.

When solid particles of different types are mixed together, a random distribution of the components is rarely produced and deterioration of the mixture can occur on subsequent handling. Among the microscopic processes responsible, one important mechanism for free-flowing materials is thought to be interparticle percolation, the drainage of particles through the interstices between larger ones. If the larger particles are stationary this is called spontaneous percolation, whereas if it is produced by shear strain the term strain-induced percolation is used. Here a quantitative evaluation of both and some consequences are described. A practical application of spontaneous percolation has been the design and construction of a new static mixer or distributor, consisting of rows of angle bars mounted horizontally in a vertical channel. Material fed to the top of a unit bounces off the bars and is distributed across the channel. Two mixers were built; one dispersed material in one lateral direction only and could be used for feeding material onto a belt or distributing seed from a moving vehicle. The other produced a two-dimensional dispersion and would be useful in distributing material flowing into hoppers or whenever a good mixture were required. Optimisation of the design was investigated using a computer program which simulated the motion of a spherical particle as it fell through such a mixer. Design data was deduced from the record of the position of the particle. The mixers were not suitable for use with fine materials. Interpretation of experimental results from this equipment requires suitable statistical indices and two were developed here. One related the variance of sample compositions to the number of particles fed to the mixer by assuming that the distributions of material were ordered. The second, using the correlation coefficient between samples, related the variance to the sample size in those situations where two orthogonal processes are in operation. Both techniques are generally applicable to fields other than that of powder mixing. On the theoretical side, an existing model of spontaneous percolation for inelastic materials has been extended and improved. The original form did not account for the motion of a particle between collisions with bulk particles but this has now been included. An entirely new semi-empirical model for partly elastic materials has also been proposed. Both predict percolation velocities which agree with experimental data. In order to extend earlier experimental studies on strain-induced percolation, a simple shear cell was modified by installing a hydraulic drive which enabled the cell to be driven at a constant speed. Advantages of the use of such a cell include the possibility of detecting a percolating particle on entry to and exit from the bed and the constant strain throughout the material. Reliable and accurate readings of residence times of percolating particles were recorded and percolation velocities and both lateral and axial diffusion coefficients were calculated. These were functions of the relative particle size and density, the material properties of the percolating particle and bed conditions such as strain rate and normal stress Denser and softer particles percolated faster. Decreasing the diameter ratio between percolating and bulk particles from 0.67 to 0.27 caused a twenty-five fold increase in the percolation rate. The dependence of this rate on particle diameter was interpreted using statistical mechanics. The percolation rate has been shown to reach a constant value as the strain rate increases, in contrast to the deductions drawn in earlier work by Scott, whose procedure has been proved to be unsound.

A series of experiments are described which explore the relationship between biomechanical properties and the control of jaw movement in speech. This relationship is documented using kinematic analyses in conjunction with a mathematical model of jaw motion and direct measures of jaw stiffness. / In the first experiment, empirical and modeling studies were carried out to examine whether the nervous system compensates for naturally occurring forces acting on the jaw during speech. As subjects walk or run, loads to the jaw vary with the direction and magnitude of head acceleration. While these loads are large enough to produce a measurable effect on jaw kinematics, variation in jaw position during locomotion is shown to be substantially reduced when locomotion is combined with speech. This reduction in jaw motion is consistent with the idea that in speech, the control of jaw movement is adjusted to offset the effects of head acceleration. Results of simulation studies using a physiologically realistic model of the jaw provide further evidence that subjects compensate for the effects of self-generated loads by adjusting neural control signals. / A second experiment explores the idea that a principle mechanical property of the jaw---its spring-like behavior, or stiffness---might influence patterns of kinematic variation in speech movements. A robotic device was used to deliver mechanical perturbations to the jaw in order to quantify stiffness in the mid-sagittal plane. The observed stiffness patterns were non-uniform, with higher stiffness in the protrusion-retraction direction. Consistent with the idea that kinematic patterns reflect directional asymmetries in stiffness, a detailed relationship between jaw kinematic variability and stiffness was observed---kinematic variability was consistently higher under conditions in which jaw stiffness was low. Modeling studies suggested that the pattern of jaw stiffness is significantly determined by jaw geometrical properties and muscle force generating abilities. / A third experiment examines the extent to which subjects are able to alter the three-dimensional pattern of jaw stiffness in a task-dependent manner. Destabilizing loads were applied to the jaw in order to disrupt the ability of subjects to maintain a static jaw posture. Subjects adapted by increasing jaw stiffness in a manner that depended on the magnitude and, to a more limited extent, direction of the destabilizing load. The results support the idea that stiffness properties can be controlled in the jaw, and thus may play a role in regulating mechanical interactions in the orofacial system.

External loads, such as those due to the orientation of body segments relative to gravity, affect the extent to which control signals result in the achievement of desired goals. The degree to which subjects adjust control signals to compensate for loads provides a measure of what the nervous system knows about forces affecting motion and gives an indication of the complexity of control signals needed for voluntary movement. In the present study, we have explored the hypothesis that subjects take no account of the orientation of the head relative to gravity when making jaw movements during speech. We used a simulation model of the jaw to predict the kinematic effect of using a single set of motor commands (which take no account of the relative direction of the gravitational force) to produce speech-like movements while the body was in three different orientations: upright, prone and supine. The simulations predict a systematic change in jaw pitch angle and horizontal translation resulting from the change in body orientation. Empirical results for five subjects tested under the same conditions as those explored in the simulations are for the most part consistent with the pattern predicted by the model. This suggests that in the case of jaw movements during speech, control signals are not adjusted to account for changes in head and body orientation relative to gravity.