Changes In Weight Distribution Research Articles

Molecular weight distribution and mass changes during polyamide hydrolysis

Samples of polyamide 6, 11 and 12 have been hydrolysed at various temperatures ranging from 80 to 120°C in water baths of pH ranging from 0 to 4. The polymer degradation was monitored by steric exclusion chromatography (SEC), viscometry and gravimetry. SEC data indicate that the polydispersity of the polyamides tested does not increase during ageing. This can be considered as proof of the homogeneity of the reaction. Average molecular weight data from SEC or viscometry appear to be consistent with a random chain scission process, however gravimetry (at least in the case of relatively soft conditions, for instance pH ≥ 4) indicates a high monomer yield. These apparently contradictory results are tentatively explained by the equilibrated character of hydrolysis. Acid—amine pairs resulting from hydrolysis events would recombine easily when both belong to macromolecular fragments. When one fragment is the monomer or a low oligomer, it can escape easily from the cage and the corresponding recombination yield is decreased. The monomer extraction by water complicates the gravimetric behaviour, especially in the most severe hydrolysis conditions.

Obesity and reproduction.

Obesity produces a variety of alterations in the reproductive system and, similarly, manipulations of the hypothalamic-pituitary-gonadal axis produce changes in food intake, body weight and fat distribution. In men, the primary effects of obesity are a weight related reduction in testosterone and, with massive overweight, a reduction in free testosterone. In females, the weight-related development of menarche leads to earlier menarche in obese girls than in normal weight girls. One explanation for the relationship of fatness to menarche may be the ob protein (leptin) which is defective in the obese (ob/ob) mouse. Leptin is secreted by adipose tissue in proportion to the quantity of fat and may serve as a signal to the hypothalamus that fat stores are adequate to nourish a conceptus to term. In women, parity affects obesity and obesity in turn affects the regularity of the menstrual cycle. In many experimental animals with obesity, particularly the genetic forms of obesity, there is complete infertility in the females and marked impairment of reproductive function in the males. In animals with hypothalamic lesions, there is a gender effect on the magnitude of weight gain associated with the sexually dimorphic regions in the medial preoptic area. Castration with removal of oestrogen is followed by obesity in female animals and this can be prevented, as can most forms of obesity, by adrenalectomy. The inhibitory effects of oestrogen on food intake may result from suppression of neuropeptide-Y or galanin peptidergic systems in the arcuate nucleus or medial preoptic area.

The treatment and prevention of obesity: a systematic review of the literature.

To determine the effectiveness of interventions designed to prevent and treat obesity, and maintain weight loss. A systematic review of randomised controlled trials. Overweight and obese adults and children. Post-intervention changes in weight, fat content and fat distribution, measured relative to baseline. For obese children, family therapy and lifestyle modification appear to be effective in prevention and treatment, respectively. The effectiveness of interventions to prevent and treat obesity in adults remains unclear, although behavioural therapy and multicomponent strategies may be useful. Continued therapist contact appears to be useful for maintaining weight loss. Pharmacological interventions appear to be effective for up to 9 months, after which regain occurs. Surgery appears to be effective for the morbidly obese and gastric bypass is more effective than gastroplasty. In general, the methodological quality of studies was poor. Due to problems with methodological quality, it is recommended that research findings indicative of promising interventions are replicated.

Fetal death rate in the united states, 1979–1990: Trend and racial disparity

To examine the impact of changes in birth weight distribution in individual groups and in birth weight-specific fetal death rates on the decline in the crude fetal death rate in the United States. Data on live births and fetal deaths in the U.S. for the period 1979-1990 were examined by birth weight group and race using Kitagawa's method for analysis of the crude fetal death rate. In the period 1979-1990, all racial groups had a decrease in the crude fetal death rate, more so in whites and others (about 22%) than in blacks (10%). In the white population, 73.4% of the total reduction in the crude fetal death rate was attributable to the improvement in birth weight-specific fetal death rates, and the remaining portion of the reduction was due to a favorable change in birth weight distribution. In the black population, the reduction in the crude fetal death rate was entirely attributable to the improvement in the birth weight-specific fetal death rates. However, in other groups, a favorable change in the birth weight distribution was the major determinant. Although black births represented 16.5% of all births in the U.S., they accounted for 26-29% of the crude fetal death rate. Disparity in the crude fetal death rates for blacks and whites is explained almost entirely by differences in birth weight distribution. A further decrease in the crude fetal death rate in the U.S. requires a decrease in low birth weights, particularly in blacks.

Rationale for the use of dietary control in toxicity studies--B6C3F1 mouse.

Significant variability in critical study parameters such as tumor incidences and survival, increasing tumor incidence and decreasing survival in common toxicity test models, and agent-induced changes in body weight (BW) and BW distribution all generate concern about the reproducibility, consistency, and equity of chronic toxicity tests used in regulation. These concerns have led to suggestions to control BW in chronic tests by the modulation of dietary intake without inducing malnutrition [dietary control (DC)] thereby minimizing tumor and survival variability both between and within studies. Evaluating the reports of the best controlled set of chronic experiments, the National Toxicology Program bioassay series, from studies initiated from 1981 to 1990, there is an increase in tumor incidence, especially liver tumors, with a consistent increase in BW. The studies are classified as to whether normal or aberrant BW growth curves occur. When the studies with normal growth curves are considered, the variance in the BW at 12 mo on test (BW12) can account for over 50% of the variance in liver tumor incidence. Additional stratification by study type, which alter tumor prevalences, as well as appreciation of housing effects [group housing decreases survival (in male mice) and induces tumors in males and females when compared to individual housing], further increase the strength of the correlations, accounting for up to 90% of the variance seen in tumor incidences. These updated analyses further support the hypothesis that it is the BW variation that is resulting in much of the variability seen in tumor incidences and refine the suggestions for the BW curves used as the desired targets for DC.

Study of polyethylene photodegradation in formulations with a system of interacting photostabilizers and antioxidants

The accelerated UV photodegradation process of low-density polyethylene films, formulated with two photostabilizers and two antioxidants, was studied to evaluate the effect of different combinations of UV stabilizers and antioxidants on the overall photodegradation process. An experimental design, consisting of 33 formulations with different additive ratios and a blank, was used to evaluate the performance of the four mixed additives. From each formulation, 200 micron-thick films were produced by the extrusion-blowing process. Samples from these films were submitted to accelerated UV aging, and the polymer degradation was measured by carbonyl group evolution, molecular weight distribution changes, and maximum elongation loss. The effect of the additive combination on the different degradation reactions is discussed qualitatively by using “relative variables” and a triangular diagram. © 1996 John Wiley & Sons, Inc.

Study of polyethylene photodegradation in formulations with a system of interacting photostabilizers and antioxidants

The accelerated UV photodegradation process of low-density polyethylene films, formulated with two photostabilizers and two antioxidants, was studied to evaluate the effect of different combinations of UV stabilizers and antioxidants on the overall photodegradation process. An experimental design, consisting of 33 formulations with different additive ratios and a blank, was used to evaluate the performance of the four mixed additives. From each formulation, 200 micron-thick films were produced by the extrusion-blowing process. Samples from these films were submitted to accelerated UV aging, and the polymer degradation was measured by carbonyl group evolution, molecular weight distribution changes, and maximum elongation loss. The effect of the additive combination on the different degradation reactions is discussed qualitatively by using “relative variables” and a triangular diagram. © 1996 John Wiley & Sons, Inc.

Effect of posture on hip joint moment during pregnancy, while performing a standing task

In this paper the combined effect on reactive hip joint moment due to changes in (1) segment mass, (2) trunk centre of mass, and (3) working posture during pregnancy was estimated, and the relative contributions of these three changes to the change in the moment were assessed. The situation studied concerned standing work at a table. Sixteen women were studied monthly during pregnancy. The working posture was assessed by two-dimensional photographic posture recording and description. Body dimensions were measured to assess segment characteristics. A two-dimensional static model was used to estimate the reactive hip joint moment at 10, 20, 30 and 40 weeks of pregnancy. Between 10 and 40 weeks of pregnancy the moment increase was 52 Nm, i.e. the load at 40 weeks of pregnancy is 2.8 times the load at 10 weeks of pregnancy. On average half of this increase was due to postural changes. Changes in segment mass, trunk centre of mass, and the interaction between the three changes accounted for rougly 10, 20 and 15% of the load increase respectively. The increase in reactive hip joint moment may be minimized by preventing adverse postural changes, and optimizing the posture to reduce the contribution of changes in body weight (distribution).

Random Degradation of Branched Polymers. 1. Star Polymers

A random sampling technique in which polymer molecules are sampled from an infinite number of polymer molecules in the reaction mixture is used to predict the molecular weight distribution (MWD) change during random scission reactions of star-shaped polymers. For simpler initial MWDs, the analytical solutions for the full MWD change, as well as the average molecular weights, can be derived in a straightforward manner. For any initial MWD, the concept of the random sampling technique provides a unique Monte Carlo simulation algorithm that enables one to estimate the change in the statistical properties during random degradation quite effectively. Compared with the random degradation of linear polymer molecules, the decrease in the weight-average molecular weight is slower; however, such differences are made smaller as the initial MWD becomes broader. In the course of random degradation, linear and branched polymer fractions are formed. As the degradation reaction proceeds, the polydispersity index of the l...

Random Degradation of Branched Polymers. 2. Multiple Branches

The random sampling technique discussed in part 1 of this series1 is used to investigate the molecular weight distribution (MWD) change during random scission reactions of the polymers with multiple branches. By application of the concept of the random sampling technique to the Monte Carlo method, one obtains a very powerful simulation method that can be applied to the random degradation of virtually any type of branched polymer. Suppose branched polymers are formed first by randomly combining linear polymer molecules, and then these branched polymer molecules are used for the random degradation reactions (branching → degradation). If the processes of branching and degradation are interchangeable, then the same MWD can be obtained from the sequence of processes, degradation → branching. In contrast with random cross-linking and degradation, the processes are not interchangeable except when all of the primary chains possess the most probable distribution with randomly distributed branch points. The MWDs given by the random branching of primary chains with the most probable distribution provide the limiting distributions to which polymers with multiple branches approach as the extent of the scission reaction increases, irrespective of the initial MWD including nonrandomly branched polymers. An analytical expression for such a limiting MWD is derived by the application of the random sampling technique.

Uphill locomotion in mole rats: A possible advantage of backward locomotion

Mole rats were videotaped while locomoting in a Plexiglas tube to evaluate the effects of inclination on locomotion. Animals moving uphill preferred to go backward using plantigrade foot postures, presumably to prevent sliding. Uphill backward locomotion also allowed animals to cope with changes in weight distribution between the hind and forelimbs without modifying footfall pattern relative to the direction of progression. When the animals did use uphill forward strides, they switched to asymmetrical gaits, which are associated with increased propulsive forces. These and prior results suggest that weight distribution and direction of progression can modify the natural pattern of stepping in mammalian quadrupeds.

Structural degradation of polyurethane-based elastomeric modules

Degradative characteristics were studied using specimens of three polyurethane-based elastomeric modules that were treated in solutions of varying acidity, oxygen content and temperature. After periods of 10 and 100 days, molecular weight distribution changes, mechanical property changes and elemental chemistry changes were studied using gel permeation chromatography (GPC), stress-relaxation testing and X-ray photoelectron spectroscopy (XPS), respectively. As determined through analyses of the molecular, mechanical, and elemental changes after treatment, the degradative mechanism was influenced by whether the polyurethane was polyester-based or polyether-based, and the degree of structural degradation was influenced by the type and duration of conditioning treatment. Degradation of the polyester-based materials after treatment was dominated by a chain scission mechanism; conversely, the degradation of the polyether-based material was dominated by a crosslinking mechanism. Among the conditioning treatments, the combined effects of an increase in acidity, oxygen content and temperature most influenced the degree of degradation with time, with the increase in temperature having the greatest effect of any of the single variables investigated.

Computer modelling of the degradation of linear polymers

Mechanisms of degradation of linear polymers are considered with the help of a computer program, which uses a pseudo-random number generator to carry out a Monte Carlo-like simulation of the processes. Randomly chosen bonds are degraded either at random positions along a molecule or by a number of specific systematic processes, such as breaking the molecule in half or slicing off a fixed number or percentage of units. The resulting molecular weight distribution (MWD) changes are compared to experimental measurements reported in the literature and generated in the authors' own laboratory. Totally random processes do not explain observed movements of the MWD to lower molecular weight or the formation of new peaks. Both processes require specific systematic scission of the polymer chain, such that, for instance, the length of one of the degradation products falls within the range of the original MWD. New peaks require the splitting off of fragments of a systematic length and/or recombination of fragments with each other or with other molecules in the distribution. Peak movement requires the preferential scission of the longest molecules. Random processes tend to follow standard first-order kinetic plots of reciprocal DP vs time, provided all the products of degradation are included in the DP calculation. Systematic scission processes tend to introduce curvature.

Weight at age 30 and breast cancer risk

Obesity has been associated with the incidence of breast, endometrium and ovarian cancers. Significant changes in weight and body fat distribution can occur in women from puberty, through childbearing years and adulthood. The objective of the study was to observe any pattern in weight gain between ages 16 to adulthood in breast cancer patients and controls and, consequently, to assess if there was a significant decade in a woman's life contributing to breast cancer risk. Anthropometric, medical and hormonal histories were obtained from 218 consecutively recruited, newly diagnosed breast cancer patients admitted to the H. Lee Moffitt Cancer Center and Research Institute, Tampa, Florida and 436 community-based controls, matched in a 2:1 ratio for age and menopausal status. A weight gain of 15 pounds or over was observed in 63.8% of cases as compared to 49.3% controls (P = 0.0006) from age 30 to current age. Similarly, over 48% of cases gained over 15 pound from ages 16 to 30 as compared to 37% (P = 0.01) of the control population. Although weight gain from age 16 to adulthood was significantly higher in breast cancer patients at each decade when compared to controls, a significant and independent association between weight at age 30 (P = <0.0001) and risk of breast cancer was noted. Women who progressively gain weight from puberty to age 30 and specifically in the third decade of life, should be considered at a higher risk for developing breast cancer.

Standing working posture compared in pregnant and non-pregnant conditions

During pregnancy, an increase in body weight occurs together with changes in body weight distribution and in fit between body dimensions and workplace layout. These changes may cause alterations in working posture which may, in turn, have adverse consequences for the biomechanical load on the musculoskeletal system and so increase the risk of musculoskeletal disorders. Using photographic posture registration, the standing working posture was studied in 27 women during the last stage of pregnancy and after delivery (the experimental group). The women performed an assembly task while standing at various workplace layouts. The postural differences between the pregnant condition and the non-pregnant condition were studied and the effect of the various workplace layouts assessed. Ten non-pregnant controls were also studied twice to establish the effect of the time interval between the measuring occasions. We found that the women of the experimental group stood further from the work surface in the pregnant condition compared to the non-pregnant condition, the hips were positioned more backwards, and, in order to reach the tesk, they increased the flexion of the trunk, increased the anteflexion of the upper arms, and extended the arms more. At the workplace layout in which the work surface height was self-selected, the postural differences due to pregnancy were smallest or even absent, compared to the postural differences in the other workplace layouts studied. Ergonomists and workers in occupational health services should be alert to the consequences for the biomechanical load on the musculoskeletal system and the risk of development of health complaints caused by postural changes due to pregnancy. An adjustable workplace layout may prevent some problems.

Effect of Changes in Maternal Age, Parity, and Birth Weight Distribution on Primary Cesarean Delivery Rates

Objective. —To examine the effect of maternal age on cesarean delivery risk and to quantify the impact of demographic changes since 1970 on primary cesarean delivery rates. Design. —A cohort study. Setting. —Nonfederal short-stay hospitals in Washington State. Participants. —All women who delivered live singletons with linked birth certificate and hospital discharge data from 1987 through 1990. Main Outcome Measures. —Maternal age—, birth weight—, and parity-specific primary cesarean delivery rates, Mantel-Haenszel relative risk estimates for primary cesarean delivery by 5-year age category stratified by parity, and direct standardization of 1987 through 1990 primary cesarean rates to 1970 Washington State maternal age, birth weight, and parity distribution. Results. —Primary cesarean rates ranged from 3.2% for multiparous teenage women who delivered infants weighing 3500 g through 3999 g to 58.9% for primiparous women 40 years of age or older who delivered infants weighing 4000 g or more. After adjustment, the risk of cesarean delivery increased with each 5-year age increment among women 20 years of age or older. We estimated that if the maternal age, parity, and birth weight distribution from 1987 through 1990 were identical to what existed in 1970, Washington State's primary cesarean rate from 1987 through 1990 would have been 12.2%, compared with the observed rate of 14.8%. Conclusion. —The lower adjusted primary cesarean rate reflects the demographic changes in the childbearing population, which may be responsible for 18% of the 1987 through 1990 cesarean delivery rates. These findings suggest the importance of using maternal age—, birth weight—, and parity-specific primary cesarean delivery rates to compare populations and study temporal trends. ( JAMA . 1994;271:443-447)

Effect of Changes in Maternal Age, Parity, and Birth Weight Distribution on Primary Cesarean Delivery Rates

To examine the effect of maternal age on cesarean delivery risk and to quantify the impact of demographic changes since 1970 on primary cesarean delivery rates. A cohort study. Nonfederal short-stay hospitals in Washington State. All women who delivered live singletons with linked birth certificate and hospital discharge data from 1987 through 1990. Maternal age-, birth weight-, and parity-specific primary cesarean delivery rates, Mantel-Haenszel relative risk estimates for primary cesarean delivery by 5-year age category stratified by parity, and direct standardization of 1987 through 1990 primary cesarean rates to 1970 Washington State maternal age, birth weight, and parity distribution. Primary cesarean rates ranged from 3.2% for multiparous teenage women who delivered infants weighing 3500 g through 3999 g to 58.9% for primiparous women 40 years of age or older who delivered infants weighing 4000 g or more. After adjustment, the risk of cesarean delivery increased with each 5-year age increment among women 20 years of age or older. We estimated that if the maternal age, parity, and birth weight distribution from 1987 through 1990 were identical to what existed in 1970, Washington State's primary cesarean rate from 1987 through 1990 would have been 12.2%, compared with the observed rate of 14.8%. The lower adjusted primary cesarean rate reflects the demographic changes in the childbearing population, which may be responsible for 18% of the 1987 through 1990 cesarean delivery rates. These findings suggest the importance of using maternal age-, birth weight-, and parity-specific primary cesarean delivery rates to compare populations and study temporal trends.

Effect of changes in maternal age, parity, and birth weight distribution on primary cesarean delivery rates

Effect of changes in maternal age, parity, and birth weight distribution on primary cesarean delivery rates

Radiation degradation of poly(methyl methacrylate) in the soft x‐ray region

AbstractThe molecular weight distribution change has been measured for the photoresist poly(methyl methacrylate) [PMMA] after in‐vacuo exposure to monochromatic soft x‐rays from the Canadian Synchrotron Radiation Facility [CSRF]. The experimental changes in the mo‐lecular weight distribations derived from gel permeation chromatography [GPC], were compared to a simple Monte Carlo simulation model that assumes random main chain scission. Using this model a scission radiation chemical yield of G(S) = 1.28± 0.10 at room temperature was found to give the best fit at a photon energy of 621 eV. This value is similar to values reported previously in the literature using electron beam and γ‐ray sources, but significantly larger than those reported for fast neutrons, α‐particles, or energetically charged particles. It was found that in this soft x‐ray energy regime, that degradation of PMMA involves primarily a random scission process of the main chain. The results of a least‐squares fit of this soft x‐ray G(S) data and all available literature values from other radiation sources, to the linear energy transfer [LET] dE/dx are discussed. © 1993 John Wiley &amp; Sons, Inc.

Regulation of lateral position of body centre of mass in standing balance

On average, normal adults stand with a nearly symmetric distribution of body weight between the legs, although there is a wide range of individual differences. In the static case, the symmetric distribution of weight implies a position for the centre of body mass (CM) that would maximise stability in the face of random, laterally directed, disequilibrating forces. It therefore seems reasonable to hypothesise that people regulate CM position.In a new experiment with 204 normal adult subjects, we examined the change in weight distribution on raising an arm out to the side. Compared to standing symmetrically, arms by the side, holding one arm out resulted in a statistically reliable change in weight distribution. However, the change was significantly less than would be predicted from the uncompensated postural effect on CM of one arm being raised. This finding supports the hypothesis of CM position regulation. We suggest that in the training of standing balance, the traditional emphasis on an even distributi...