Effect of crosslinking agent content, monomer functionality, and repeat unit chemistry on properties of unfilled resins

Seven mechanical/physical properties were used to evaluate 10 unfilled resins: eight aromatic dimethacrylates and two urethane dimethacrylates. Physical property tests included compressive strength, Young's modulus in compression, uniaxial tensile strength, intrinsic yield point, toothbrush abrasion, Knoop hardness, and water sorption. Controlled changes were made in the following four material parameters: amount of crosslinking diluent present in the uncured monomer, functionality of the monomer, repeat unit chemistry of the monomer (urethane vs. aromatic structure) and mode of activation (chemical vs. visible light). Polymers containing a high concentration of crosslinking agent (50 wt%) were found to be tougher and to possess lower hardness than materials containing lesser amounts of crosslinking agent. This was attributed to the flexible nature of the triethylene glycol dimethacrylate crosslinking molecule. Exposure to water plasticized the highly crosslinked materials to the degree that the yield point and elastic modulus were effectively lowered. Most of the tested properties were unaffected by differences in functionality except resistance to toothbrush abrasion, which was enhanced for polymers derived from high functionality monomers. The urethane-based polymers sorbed substantially more water than the aromatic-based materials, which presumably resulted in lower values for surface hardness. However, the urethane resins were very tough, and excellent resistance to toothbrush abrasion was observed. Property differences caused by differences in activation mode were small, although the visible light materials did sorb more water.     

The effect of particle distribution on damage formation in particulate reinforced metal matrix composites deformed in compression

Image analysis results are reported on the generation of damage in particulate reinforced metal matrix composites during compressive deformation. The technique allows the automated collection of data on the incidence of particle fracture and void formation in the matrix as a function of important microstructural parameters such as local particle volume fraction and particle size. There is a strong relationship between damage and the local volume fraction of the reinforcement proving that damage formation is accentuated in regions of particle clustering. With the SiC reinforced materials examined, there was observed to be a change in dominance of damage mechanism from particle fracture at low local volume fractions to void formation in the matrix within strongly clustered regions. The results are compared with finite element (FE) modelling of the compressive deformation of clustered particles using a simple cluster of equi-spaced particles. The FE results suggest that plastic flow is generally inhibited in clustered regions. In certain highly clustered configurations shielding is such that flow does not occur in the heart of the cluster even at high levels of average plastic strain. The modelling suggests that the change in dominance of damage mechanism is related to the dramatic increase in tensile hydrostatic stresses in the matrix with higher levels of particle clustering.     

Making darkness visible: the discovery of X-ray and its introduction to dentistry

Wilhelm Conrad Roentgen's discovery of the X-ray one century ago has been ranked in importance with the discovery and development of anesthesia and the discovery of microorganisms and their role in disease. Roentgen's scientific devotion and the persistence of C. Edmund Kells, who took Roentgen's discovery and applied it to dentistry, had a great impact on how dentists practice today.     

Stochastic complexity measures for physiological signal analysis

Traditional feature extraction methods describe signals in terms of amplitude and frequency. This paper takes a paradigm shift and investigates four stochastic-complexity features. Their advantages are demonstrated on synthetic and physiological signals; the latter recorded during periods of Cheyne-Stokes respiration, anesthesia, sleep, and motor-cortex investigation     

Reaction time during cocaine versus alcohol withdrawal: longitudinal measures of visual and auditory suppression

Visual and auditory stimulus discrimination tasks, analogous to those used in the Reitan-Klove Sensory Perceptual Examination, were performed by 12 cocaine-dependent and 5 alcohol-dependent patients after 1 week, 3 weeks, and 3 months of verified abstinence. Sixteen control subjects, who were not substance-dependent, performed the same tasks after comparable intervals. During each task, either visual or auditory stimuli were presented in the left, in the right, or in both sensory fields. A simple key press was made to discriminate these conditions. Cocaine-dependent patients responded more slowly than control subjects during both tasks. The reaction-time slowing persisted across all three sessions, spanning a 3-month period of abstinence. There were no significant differences between the cocaine-dependent and control groups in response accuracy. In the context of other findings, these findings are interpreted as reflecting an enduring effect of prior cocaine dependence on motor as opposed to sensory functioning.     

Outbreak of serogroup C meningococcal disease associated with campus bar patronage

Between February 1991 and April 1992, eight undergraduates at a US residential university and one at a nearby 2-year college contracted serogroup C meningococcal disease. A case-control investigation with 20 controls per case, oropharyngeal carriage surveys, and multilocus enzyme electrophoresis (MEE) of serogroup C isolates were used to identify factors contributing to the outbreak. All eight sterile-site isolates from cases were closely related by MEE and were similar (though not identical) to the strain associated with the 1991-1992 epidemic of meningococcal disease in eastern Canada. Disease was associated with cigarette smoking (p = 0.012), recent patronage of campus-area bars (p = 0.034), estimated amount of time spent in campus-area bars (p = 0.0003), and, especially, recent patronage of one specific bar, bar A (p = 0.0006; odds ratio = 23.1, 95% confidence interval 3.0-571.5). In carriage surveys, 1,528 throat cultures taken from (primarily student) noncases yielded only five (0.3%) strains that were identical by MEE to those from cases. Two of these were found among 22 cultures obtained from bar A employees in spring 1992. Some cases in this outbreak may have followed transmission of the epidemic strain in bar A. Campus bar environments may facilitate the spread of meningococcal disease among teenagers and young adults.     

Information processing deficits associated with developmental coordination disorder: a meta-analysis of research findings

A meta-analysis was conducted to identify information processing factors that characterise children with Developmental Coordination Disorder (DCD). A total of 50 studies yielded 374 effect sizes based on 983 DCD and 987 control children. A mild generalised performance deficit was indicated, since motor-impaired children were inferior on almost all measures of information processing. There were, however, several areas where their deficiencies were more pronounced. The greatest deficiency was in visual-spatial processing. This was evident regardless of whether or not the tasks involved a motor component. Most other deficiencies were in the small-to-moderate range and included kinaesthetic and cross-modal processing. The findings support the notion that perceptual problems, particularly in the visual modality, are associated with difficulties in motor coordination.     

Reproductive senescence predicts cognitive decline in aged female monkeys

The present investigation provide evidences from a non-human primate model that naturally occurring menopause predicts a prominent signature of age-related cognitive decline. Young and aged rhesus monkeys were tested on a delayed response (DR) task known to the sensitive to aging, and reproductive status was evaluated according to menstrual cyclicity and urinary hormone profiles. Peri-/postmenopausal monkeys exhibited significant DR impairments relative to either age-matched premenopausal females, or young control subjects. In addition, markers of endocrine decline in the aged animals were selectively correlated with behavioral performance measures that distinguished premenopausal and peri-/postmenopausal monkeys. These results document that menopause is coupled to cognitive decline in the monkey, and they establish a valuable primate model for defining the effects of endocrine aging on brain and behavioral function.     

Implementing Information Systems: Some Practical Advice and a Richer Model

Recent research suggests that implementing information systems presents considerable difficulties and that many implementations are total or partial failures. This paper argues that what both practitioners and students require are richer and more acceptable models of information systems implementation. Accordingly, case study data concerning the introduction of manufacturing resource planning (MRP II) are used to illustrate five patterns of behaviour (rites) which capture important social actions; and four components of changing psychological orientation. It is argued that appreciation of the rites and psychological developments identified here will support effective change programmes. Guidelines based on our model are provided for practitioners.