Seeds and the city: a review of municipal home food gardening programs in Canada in response to the COVID-19 pandemic

In the wake of the COVID-19 pandemic in Canada, home food gardening articles have saturated popular media outlets. Home food gardening is more popular than ever, and community gardens and community greenhouses are at capacity with long waiting lists for plots. Several local governments across the country are also participating in the food gardening craze. This study compares 19 municipal urban home food gardening programs that ran in 2020. These municipalities provided program participants with free gardening supplies and instructions on how to grow food at home. This study reveals a complicated relationship among municipalities, food gardening programs and household and community food security. The study also determines that the social and emotional challenges brought about by the COVID-19 pandemic are somewhat alleviated through gardening. Ultimately, municipalities are limited in their policy capacities to adequately move the needle on food insecurity in Canada.

Combinations of peptides synergistically activate the regenerative capacity of skin cells in vitro

Objective

To explore synergistic effects related to skin regeneration, peptides with distinct biological mechanisms of action were evaluated in combination with different skin cell lines in the presence or absence of niacinamide (Nam). Furthermore, the synergistic responses of peptide combinations on global gene expression were compared with the changes that occur with fractional laser resurfacing treatment, a gold standard approach for skin rejuvenation, to further define optimal peptide combinations.

Methods

Microarray profiling was used to characterize the biological responses of peptide combinations (+/− Nam) relative to the individual components in epidermal keratinocyte and dermal fibroblast cell lines. Cellular functional assays were utilized to confirm the synergistic effects of peptide combinations. Bioinformatics approaches were used to link the synergistic effects of peptide combinations on gene expression to the transcriptomics of the skin rejuvenation response from fractional laser treatment.

Results

Microarray analysis of skin cells treated with peptide combinations revealed synergistic changes in gene expression compared with individual peptide controls. Bioinformatic analysis of synergy genes in keratinocytes revealed the activation of NRF2‐mediated oxidative stress responses by a combination of Ac‐PPYL, Pal‐KTTKS and Nam. Additional analysis revealed direct downstream transcriptional targets of NRF2/ARE exhibiting synergistic regulation by this combination of materials, which was corroborated by a cellular reporter assay. NRF2‐mediated oxidative stress response pathways were also found to be activated in the transcriptomics of the early skin rejuvenation response to fractional laser treatment, suggesting the importance of this biology in the early stages of tissue repair. Additionally, the second combination of peptides (pal‐KT and Ac‐PPYL) was found to synergistically restore cellular ATP levels that had been depleted due to the presence of ROS, indicating an additional mechanism, whereby peptide synergies may accelerate skin repair.

Conclusion

Through combinatorial synergy studies, we have identified additional in vitro skin repair mechanisms beyond the previously described functions of individual peptides and correlated these to the transcriptomics of the skin rejuvenation response of fractional laser treatment. These findings suggest that specific peptides can act together, via complementary and synergistic mechanisms, to holistically enhance the regenerative capacity of in vitro skin cells.

Treatment using the Progressive Return to Activity Clinical Recommendation may return individuals with elevated symptomatology to normal levels at an expedited rate

Objective

To determine whether the use of a clinical recommendation (CR) for activity progression to treat concussion will benefit symptom profile months post-injury beyond what is seen with usual care.

Methods

This study included 58 individuals who participated in the Defense and Veterans Brain Injury Center Progressive Return to Activity (PRA) Study. Eligibility included sustaining a concussion within 72 hours of study enrollment and no previous concussion within 12 months of injury. Poisson regression with robust error variance was used to evaluate the relationship between treatment group assignment (usual care versus PRA-CR) and having normal symptomatology (Neurobehavioral Symptom Inventory score below the upper 10th percentile) at the 1-, 3-, and 6-month follow-up. Participants with normal symptomatology within 72 hours post-injury were excluded from analyses.

Results

Among participants with abnormal symptoms within 72 hours post-concussion, those receiving the PRA-CR to treat concussion had a 70% (CI = 1.06, 2.72) greater prevalence of normal symptomatology by 1 month post-injury, and a 39% (CI = 1.04, 1.87) greater prevalence of normal symptomatology by 3 months post-injury compared to those receiving usual care. By 6 months post-injury, there was no difference in the prevalence of having normal symptoms between groups.

Conclusion

The findings of this study show that participants with abnormal symptomatology decreased in symptom level to normal at a faster rate among those receiving the PRA-CR compared to those receiving usual care, providing evidence for the use of the PRA-CR as a means to enhance treatment of patients with concussion.

Pyridone-conjugated monobactam antibiotics with gram-negative activity

Herein we describe the structure-aided design and synthesis of a series of pyridone-conjugated monobactam analogues with in vitro antibacterial activity against clinically relevant Gram-negative species including Pseudomonas aeruginosa , Klebsiella pneumoniae , and Escherichia coli . Rat pharmacokinetic studies with compound 17 demonstrate low clearance and low plasma protein binding. In addition, evidence is provided for a number of analogues suggesting that the siderophore receptors PiuA and PirA play a role in drug uptake in P. aeruginosa strain PAO1.

Novel monobactams utilizing a siderophore uptake mechanism for the treatment of gram-negative infections

Novel siderophore-linked monobactams with in vitro and in vivo anti-microbial activity against MDR Gram-negative pathogens are described.

Pyridone methylsulfone hydroxamate LpxC inhibitors for the treatment of serious gram-negative infections

The synthesis and biological activity of a new series of LpxC inhibitors represented by pyridone methylsulfone hydroxamate 2a is presented. Members of this series have improved solubility and free fraction when compared to compounds in the previously described biphenyl methylsulfone hydroxamate series, and they maintain superior Gram-negative antibacterial activity to comparator agents.

Potent inhibitors of LpxC for the treatment of Gram-negative infections

In this paper, we present the synthesis and SAR as well as selectivity, pharmacokinetic, and infection model data for representative analogues of a novel series of potent antibacterial LpxC inhibitors represented by hydroxamic acid.

Requirement for the heart-type fatty acid binding protein in cardiac fatty acid utilization

Nonenzymatic cytosolic fatty acid binding proteins (FABPs) are abundantly expressed in many animal tissues with high rates of fatty acid metabolism. No physiological role has been demonstrated for any FABP, although these proteins have been implicated in transport of free long-chain fatty acids (LCFAs) and protection against LCFA toxicity. We report here that mice lacking heart-type FABP (H-FABP) exhibit a severe defect of peripheral (nonhepatic, non-fat) LCFA utilization. In these mice, the heart is unable to efficiently take up plasma LCFAs, which are normally its main fuel, and switches to glucose usage. Altered plasma levels of LCFAs, glucose, lactate and beta-hydroxybutyrate are consistent with depressed peripheral LCFA utilization, intensified carbohydrate usage, and increased hepatic LCFA oxidation; these changes are most pronounced under conditions favoring LCFA oxidation. H-FABP deficiency is only incompletely compensated, however, causing acute exercise intolerance and, at old age, a localized cardiac hypertrophy. These data establish a requirement for H-FABP in cardiac intracellular lipid transport and fuel selection and a major role in metabolic homeostasis. This new animal model should be particularly useful for investigating the significance of peripheral LCFA utilization for heart function, insulin sensitivity, and blood pressure.

Decreased fat and nitrogen losses in patients with AIDS receiving medium-chain-triglyceride-enriched formula vs those receiving long-chain-triglyceride-containing formula

OBJECTIVE

The purpose of this study was to compare two enteral formulas, differing only in fat source, for product acceptance, tolerance, and effect on fat malabsorption and nutritional status in subjects with acquired immune deficiency syndrome (AIDS).

DESIGN

The double-blind, randomized 15-day trial was divided into a 3-day period in which solid food was consumed followed by a 12-day experimental period in which liquid formulas were consumed.

SETTING/SUBJECTS

Twenty-three men and one woman with AIDS and fat malabsorption completed the study. The study was conducted in the General Clinical Research Center, University of Alabama Hospital, University of Alabama at Birmingham. Laboratory assays were performed in the Department of Nutrition Sciences.

INTERVENTIONS

After 3 days of consuming a controlled, solid food diet containing 100 g fat per day from mixed sources to document fat malabsorption, subjects were randomly assigned to one of two groups. Each group received a liquid formula containing 35% of energy as fat for 12 days. One group received a formula containing 85% medium-chain triglycerides (MCTs) and the control group received a formula containing 100% long-chain triglycerides.

MAIN OUTCOME MEASURES

Determinations included stool number, consistency, weight, and fat and nitrogen content; urine nitrogen and creatinine levels; and body weight.

STATISTICAL ANALYSIS PERFORMED

Subject demographic and other baseline characteristics were compared using two-sample t tests; stool and urine assessments were compared between groups at the initial experimental period using two-sample t tests; changes from initial to final experimental periods were assessed by means of analysis of covariance; changes in pooled intake, body weight, and the number and consistency of bowel movements were also assessed using analysis of covariance. All statistical tests were two-tailed and considered significant at P < .05.

RESULTS

Within-group comparisons indicated that subjects fed the MCT formula showed significantly decreased stool fat and stool nitrogen content (P = .01 and P = .03, respectively) and increased fat absorption (P = .03), whereas those fed the control formula did not. Differences in stool fat between the groups were not statistically significant. However, the difference in fat absorption from the initial to final formula period was significant (P = .04). Subjects consuming the MCT formula also tended to have a decreased number of bowel movements and abdominal symptoms, whereas subjects fed the control formula showed no improvement. All subjects maintained their body weights.

APPLICATIONS

There may be advantages to using an MCT-based formula in the treatment of AIDS-associated malabsorption.

Coping with general and disease-related stressors by patients with multiple sclerosis: relationships to psychological distress

This study examined the relationships between coping styles and psychological distress in a non-institutionalized sample of individuals with multiple sclerosis (MS). The MS sample completed a measure of psychological distress and identified coping strategies utilized for stressors that were disease-related and general in nature. They also self-reported their perceived efficacy of the coping strategies utilized for both stressors. Individuals with MS exhibited greater levels of depression and their indices of psychological distress than demographically matched controls but the patterns of coping strategies endorsed and the ratings of coping effectiveness were similar for the two groups. When coping with disease-related stressors, individuals with MS utilized coping strategies that were more emotion-focused and dependent but no less effective than when the same patients coped with general stressors. Within the MS group, high levels of psychological distress were positively correlated with the use of emotion-focused coping strategies, but were unrelated to the use of problem-focused strategies. Most individuals with MS individuals with MS appear to be able to modify their coping strategies to adopt to varying sorts of stressors.

Candidate biomarkers of aging: age-sensitive indices of immune and muscle function covary in genetically heterogeneous mice

A longitudinal experiment was designed to test the hypothesis that individual mice differ in their aging rate and to validate candidate biomarkers proposed to measure the rate of aging. Mice were bred as the genetically heterogeneous progeny of a cross between CB6F1 mothers and C3D2F1 fathers. Half of the mice were fed ad libitum (AL group), and the other half were subjected to 60% calorie restriction (CR group). Each mouse was tested at about 9 months of age using age-sensitive tests of immune status, and then again at about 12 months of age using age-sensitive tests of muscle function. The data were then analyzed using the method of partial least squares to determine the combinations of test weights that maximize the covariance of the weighted sum of immune measures with the weighted sum of muscle function measures. Both AL and CR mice exhibited a statistically significant relation between the immune status tests and the muscle function tests. Maximal covariance was obtained with a set of weighting coefficients consistent with our working hypothesis: mice with high levels of CD4 memory T cells (which increase with age) also had relatively low levels of muscle strength and endurance. Low strength was associated with low CD8 cells in the AL mice, with high numbers of CD8 memory cells in the CR mice and with low CD3 cells in both diet groups. The partial least squares method generates composite indices of immune status and muscle function that can be evaluated as biomarkers of aging rate in these mice. Further work will be needed to assess whether these tests predict either longevity or the trajectory of change in other age-sensitive molecular and physiological traits.

Identification of Grf1 on mouse chromosome 9 as an imprinted gene by RLGS-M

Normal mammalian development requires a diploid combination of both haploid parental genomes. Uniparental disomy for certain segments of specific chromosomes results in aberrant development or prenatal lethality, indicating that the parental genomes have undergone modifications during gametogenesis. These modifications result in parent-of-origin specific expression for some genes, a phenomenon called genomic imprinting. Recent work with DNA methyltransferase deficient mice showed that differential methylation is the probable basis of the imprinted character of several genes. Screening for endogenous imprinted loci using restriction landmark genomic scanning with methylation sensitive enzymes (RLGS-M) identified eight imprinted RLGS (Irigs) candidate loci. Molecular analysis of the genomic region of one of the loci (Irigs2) resulted in the discovery of the paternally imprinted U2afbp-rs gene within a previously identified imprinted region on mouse chromosome 11 (refs 5, 7). This paper describes the characterisation of a novel imprinted RLGS-M locus, Irigs3, on mouse chromosome 9 (ref. 6). Within this locus we identified the Grf1 (also called Cdc25Mm) gene, which is homologous to the RAS-specific guanine nucleotide exchange factor gene, CDC25, in Saccharomyces cerevisiae. Grf1 is located about 30 kb downstream of the methylation imprinted site, identified by RLGS-M, and shows paternal allele specific expression in mouse brain, stomach and heart. Our results indicate that imprinting may have a role in regulating mitogenic signal transduction pathways during growth and development.

The surveillance of birth defects: the usefulness of the revised US standard birth certificate

To assess the sensitivity and positive predictive value of birth defects reported on the 1989 revision of the US Standard Birth Certificate, a population of 76,862 Atlanta-area births during 1989 and 1990 was used as the basis for comparing 771 birth certificates that reported birth defects with 2428 live-born infant records in a birth defects registry that uses multiple sources of case ascertainment. Only 14% of birth defects in the registry records were reported on birth certificates. After the analysis was restricted to defects recognizable at birth, the sensitivity and positive predictive value of the birth certificates were 28% and 77%, respectively. Birth certificates underestimate birth defect rates and should be used cautiously for birth defect surveillance and epidemiological studies.

Infertility in a transgenic rat due to impairment of cytoplasmic elimination and sperm release from the Sertoli cells

In line TGR(mRen2)26 transgenic rats (TGR26) bearing a randomly inserted additional renin transgene, the males, but not the females, were found to be infertile. Tissue was obtained from TGR26 males and littermate controls after perfusion fixation, and the morphology of the testes and epididymides was examined. Testis size was normal as was gross morphology, but careful examination revealed that the release of many spermatozoa at stage IX of the spermatogenic cycle was impaired. In addition, the process of cytoplasmic elimination was abnormal, as cytoplasmic fragments of elongate spermatids were present in the epididymis. In TGR26 males, seminiferous tubule lumen size was significantly larger (p < 0.001) than in littermate controls, a difference that was most marked at stages IX-XIV--an effect that could be related to the retention of spermatozoa. In situ hybridization confirmed that expression of renin mRNA could be detected in testes of TGR26 rats but not in normal controls or in a fertile line (TGR27) of rats bearing the same transgene. Immunocytochemistry and in situ and Northern hybridization were used to elucidate the pattern of expression of genes that previous studies have implicated in the process of sperm maturation and/or release. Of the gene products examined (sulphated glycoproteins 1 and 2 [SGP-1, SGP-2], transition proteins 1 and 2 [TP-1, TP-2], urokinase, and cyclic protein 2 [CP-2]), none showed any major change in the pattern of expression compared with that in controls. We postulate that TGR26 transgenic male rats may be infertile because the expression of a gene (or genes) involved in the process of cytoplasmic elimination and/or sperm release has been disrupted by the presence of the transgene close to or within the gene(s). Future planned studies will involve determination of the insertion site(s) and ultrastructural analysis of the final phases of spermiogenesis.

Infertility in a transgenic rat due to impairment of cytoplasmic elimination and sperm release from the Sertoli cells

In line TGR(mRen2)26 transgenic rats (TGR26) bearing a randomly inserted additional renin transgene, the males, but not the females, were found to be infertile. Tissue was obtained from TGR26 males and littermate controls after perfusion fixation, and the morphology of the testes and epididymides was examined. Testis size was normal as was gross morphology, but careful examination revealed that the release of many spermatozoa at stage IX of the spermatogenic cycle was impaired. In addition, the process of cytoplasmic elimination was abnormal, as cytoplasmic fragments of elongate spermatids were present in the epididymis. In TGR26 males, seminiferous tubule lumen size was significantly larger (p < 0.001) than in littermate controls, a difference that was most marked at stages IX-XIV--an effect that could be related to the retention of spermatozoa. In situ hybridization confirmed that expression of renin mRNA could be detected in testes of TGR26 rats but not in normal controls or in a fertile line (TGR27) of rats bearing the same transgene. Immunocytochemistry and in situ and Northern hybridization were used to elucidate the pattern of expression of genes that previous studies have implicated in the process of sperm maturation and/or release. Of the gene products examined (sulphated glycoproteins 1 and 2 [SGP-1, SGP-2], transition proteins 1 and 2 [TP-1, TP-2], urokinase, and cyclic protein 2[CP-2], none showed any major change in the pattern of expression compared with that in controls. We postulate that TGR26 transgenic male rats may be infertile because the expression of a gene (or genes) involved in the process of cytoplasmic elimination and/or sperm release has been disrupted by the presence of the transgene close to or within the gene(s). Future planned studies will involve determination of the insertion site(s) and ultrastructural analysis of the final phases of spermiogenesis.

An examination of demographic, socio-cultural, and health differences between congregate and home diners in a senior nutrition program

Building upon prior research, this study investigates characteristics of participants in a senior nutrition program in Hillsborough County, Florida. The Senior Citizens Nutrition and Activities Program serves approximately 2,600 meals per day to county residents 60 years of age and over. In this study, a sample of 1,071 participants are surveyed for background characteristics, interpersonal relationships and emotional closeness, health and economic condition, emotional condition, and participation in the program. Crosstabular analyses are performed on the collected data with age, sex, and race controlled. The results indicate that the program is reaching its goals of promoting good health among the older population of Hillsborough County by providing nutritious meals and reducing social isolation, which often accompanies aging.

Transgenic approaches to modification of cell and tissue function

The past 18 months have seen rapid advances in the use of transgenic techniques for elucidating cellular mechanisms. The modification of gene, cellular and tissue function has been enhanced by developments in the use of antisense and ribozyme constructs, and by improvements in strategies for cell ablation and homologous recombination.

Health care utilization of Canadian snowbirds: an example of strategic planning

Older English-speaking Canadian seasonal migrants to Florida (N = 2046) were surveyed during the winter of 1986. They are users of two health care systems and are generally in good health. When they do need to use health care resources, however, they show a distinct preference for the Canadian system. There is a sensitivity to health concerns, especially among the older of the migrants. This sensitivity generates considerable precautionary behavior including pretravel visits to their Canadian family doctors, stocking up on drugs before the trip, and taking out health insurance for Canadians traveling abroad. The behavior is strategic in that it is both intentional and oriented to contingencies. Cost of health care may be a motivating factor because such strategic behavior is inversely related to income in this relatively affluent migrating population. The flock of Canadian snowbirds is not likely to place exorbitant demands on the U.S. health care system.

Endogenous inhibition of red blood cell Na,K-ATPase in essential and pregnancy-induced hypertension

Digoxin-like inhibitors of Na,K-ATPase have been implicated in the pathophysiology of essential (EH) and pregnancy-induced hypertension (PIH). A technique that enhances dissociation of digoxin from red blood cells (RBC) was used to displace endogenous digoxin-like substances from RBCs. RBC membranes were preincubated in Na and ATP (Release) or Na,K,Mg and ATP (Retention) prior to measuring ATPase activity. Groups studied were: 39 men with EH and 34 controls plus 10 women with PIH and 17 normotensive controls. All displayed similar increases in Na,K-ATPase activity (24.0 +/- 7.9%) following Release. Plasma digoxin immunoreactivity (DI) was measured in pregnant women, m = 0.25 +/- 0.07 ng/ml. No DI was detected in nonpregnant women, but RBCs from these women demonstrated the same increase in Na,K-ATPase activity after Release. The 24% increase in activity achieved by Na and ATP preincubation can be reversed by adding K and Mg to the Release suspension. However, after RBC-bound digoxin is displaced by Release preincubation, addition of K and Mg cannot promote renewed binding and pump inhibition. Thus, the observed endogenous inhibition is not due to displacement of a digoxin-like substance but probably is related to alteration of the enzyme-membrane interaction. Furthermore, even though pregnant women demonstrate DI, an inhibitory substance with digoxin-like binding could not be recognized using this technique.

Factor analysis of the Children's Depression Inventory

Factor analyses of responses to the Children's Depression Inventory were compared for two groups: 123 children at a psychiatric outpatient clinic and 175 elementary school children who attended Grades 4, 5, and 6. Varimax rotations of principal-axis factors yielded four factors for the clinic children and two factors for the non-clinic student group. The items constituting the factors for the clinic group appeared to be theoretically related. These four factors were considered to reflect cognitive, motivational, social integration, and somatic components. In contrast, for the students there were a general factor with no predominant theme and a second factor characterized by non-compliant behavior. The implications for the relationship between adult and childhood depressive symptomatology are discussed as well as suggestions for studying the inventory as a screening instrument.

Softening of Rubber by Deformation

It has been known for many years that deformation results in softening of rubber and that the initial stress-strain curve determined during the first deformation is unique and cannot be retraced. Further the effect of repeated deformation is to cause rubber asymptotically to approach a steady state with a constant or equilibrium stress-strain curve. Softening in this way occurs in vulcanizates either with or without fillers although the effect appears to be much more pronounced in vulcanizates containing high proportions of reinforcing fillers. After the hardness test the simple extension stress-strain test is the test most widely used by rubber technologists. The conventional stress-strain curve is obtained on samples which have not been previously deformed and for design purpose the unique value of stiffness given by this curve may be of little significance. Thus it appears that the values of stress—strain properties determined after “conditioning” cycles of deformation would be of more practical use than the unique value obtained in the conventional test. In recent years much interest has been shown in the factors responsible for this softening behavior particularly in regard to the implications of the loss of the stiffening action of reinforcing fillers on the mechanism of reinforcement.

Stress Softening in Natural Rubber Vulcanizates. Part II. Stress Softening Effects in Pure Gum and Filler Loaded Rubbers

Considerable stress softening occurs in both gum and filler-loaded vulcanizates and when compared at the same stress the extent of softening is similar in both gum and filled vulcanizates. It thus appears that the softening process is mainly due to the rubber phase alone. Apparent differences between the stress—strain hysteresis loops of gum and filler-loaded vulcanizates are traced to an increase in the effective strain in the rubber phase resulting from the presence of black. This is discussed in Part I of this series. The mechanism of stress softening in gum vulcanizates is not well understood. Possible sources include (1) breaking and remaking of crosslinks during extension, (2) residual local orientation of network chains persisting after recovery, and (3) breaking of network chains.

Stress Softening in Rubber Vulcanizates. Part I. Use of a Strain Amplification Factor to Describe Elastic Behavior of Filler-Reinforced Vulcanized Rubber

Measurements of Young's modulus of vulcanized rubbers containing thermal carbon black show the predicted dependence on the volume concentration given by relationships derived for a suspension of spherical particles, for example, that due to Guth, Simha, and Gold which gives E=E0 (1+2.5c+14.1c2). A simple interpretation of the results is that the strain in the rubber is increased by the presence of filler so that the ratio of the average strain to the measured overall strain is given by the factor X=1+2.5c+14.1c2. This factor was used to analyze simple extension stress-strain data obtained at larger extensions. For this purpose the Mooney—Rivlin relation was used to describe the behavior of the rubber phase. Values of C1 independent of the volume concentration and in close accord with measurements of the equilibrium volume swelling of the rubbers were obtained. Values of λ* were also consistent with those of #5#. Analysis of stress—strain data obtained on rubbers containing smaller particle-sized carbon blacks is more complex. For these materials the relation due to Guth, viz., E=E0 (1+2.5c+14.1c2), was chosen. By the choice of suitable values of f, good agreement with the Mooney—Rivlin stress—strain relation was achieved at volume concentrations less than about 0.15.

Structural Characterization of Vulcanizates. I. Crosslinking Efficiency of Sulfur in Unaccelerated Natural Rubber-Sulfur Systems

This paper deals with the determination of crosslinking efficiencies of sulfur in a series of unaccelerated XR-sulfur vulcanizates produced at 140° C. The method used is that described under (1) above, values of the degree of chemical crosslinking being derived from stress-strain measurements on the unextracted vulcanizates.

Determination of Degree of Crosslinking in Natural Rubber Vulcanizates. Part III

The two previous parts of this series described an attempt to relate the physical properties of natural rubber vulcanizates to their network structure. The first established an empirical relationship between the stress-strain behavior of highly swollen natural rubbers in simple extension, and their equilibrium volume swelling in n-decane. It also examined the effect of changes in initial molecular weight of the unvulcanized masticated rubber on these properties, and an empirical correction making allowance for network flaws due to chain-segments terminated by a crosslink at only one end was obtained. The second part gave measurements of the equilibrium volume swelling and the intrinsic viscosity before vulcanization for each of a range of natural rubber vulcanizates which had been prepared by a method which enabled the number of crosslinks to be determined by chemical analysis. These measurements permitted a comparison to be made between (i) the chemical estimate of the actual number of crosslinks introduced and (ii) the number derived from the data on equilibrium volume swelling, by means of the empirical relations derived in Part I. For this purpose, an assumption was made that the stress-strain behavior of highly swollen rubbers was in accord with the predictions of the statistical theory. Recent studies of both the stress-strain properties of highly swollen rubbers and the method of determining number-averaged molecular weight from intrinsic viscosity measurements have provided an improved basis for the quantitative determination of the degree of crosslinking from measurements of physical properties. The results described in Parts I and II of this series are reinterpreted here to take account of both of these developments ; in addition, the effect of the introduction of a correction for a network defect equivalent to chain entanglements is examined.

Radiation Crosslinking of Rubber. III. Chain Fracture

Exposure of hydrocarbon polymer molecules to high energy radiation results either in a decrease in their molecular weight or else in an increase with the eventual formation of an insoluble network. The primary faetor determining the behavior is the relative incidence of fracture of the main chain of carbon atoms to crosslink formation. The ratio of fractures to crosslinks β has usually been estimated from studies of the solubility of irradiated polymers. Most of this work has been on polyethylenes, but even under comparable experimental conditions widely differing estimates of β have been reported. The use of stress-relaxation measurements to detect the fracture of network chains during irradiation has also been reported. A recent study of the elastic behavior of crosslinked networks has indicated that comparative estimates of the number-average molecular weights of both (1) the chain segments lying between adjacent crosslinks and (2) the molecular chains crosslinked into the network may be obtained from stress-strain measurements alone. In the present work an attempt has been made to apply all three methods in a study of chain fracture occurring during the irradiation of purified natural rubber in vacuo.

Reinforcement of Rubber by Fillers Tear Resistance

The results of experimental studies on 1) the effect of reinforcing fillers on tear strength and on the roughness of the resultant torn surfaces, 2) the relative adhesion of rubber to fillers and 3) the effect of hysteresis on tear strength are critically examined. They show that the maximum reinforcing effect of fillers is only exhibited if sufficient time is allowed for their strengthening role to develop during tearing, and that under these conditions the tip of the growing tear is broader and the torn surface rougher. They also confirm that the surface or immediate environment of filler particles provide a region of relative weakness, and that failure takes place at the interfaces ahead of the advancing tear, the rupture path wandering from one internal flaw to another. In addition they show that the more reinforcing fillers adhere more strongly to the rubber. Further, for gum vulcanizates, it appears that there is a simple relationship between tearing energy and hysteresis and that rubbers with higher hysteresis possess higher tear strength. A synthesis of the results of these three investigations provides a qualitative picture of the role of reinforcing fillers in increasing the strength of rubbers. It appears that a process giving rise to sufficiently rapid relaxation of stress ahead of the tip of a growing tear can in principle result in broadening of the tip of the tear and lead to increased strength. It is considered that this type of process plays an important and sometimes dominant role in the reinforcement of rubbers by fillers or by crystallization, and provides a consistent and coherent mechanism which indicates a source of the roughening of rupture surfaces of filler reinforced rubbers and of their possible branching and knotty tearing.

Mastication. IX. Shear-Dependence of Degradation on Hot Mastication

The rate of degradation of rubber during mastication is minimal at about 115° C, degradation increasing progressively on lowering or raising the temperature as far below or above this temperature as practicable. Designation of the degradation processes with the negative and positive temperature coefficients as “cold” and “hot mastication”, respectively, is supported by differences in their chemical mechanisms. The essential degradation step of cold mastication is rupture of rubber molecules by the imposed deforming forces to radicals which are converted to the degraded molecules after reaction with oxygen or other radical acceptor present in the rubber. Hot mastication is less well understood; scission appears to be by an oxidative reaction, with the implication that mastication serves in the main to expose fresh surfaces for absorption of oxygen. The mechanicodegradation of cold mastication produces an unusually rapid decrease in viscosity with molecular weight, logarithmic plotting of these two quantities giving an apparent α, according to the relation [η]=KMα, of greater than unity. The value of α for fractionated rubber is 0.67. The apparent high α is most readily explicable by a decrease in K on degradation, while α has its normal value of 0.67 for natural rubber. A decrease in K would occur if the molecular weight distribution around the mean became sharper. This distribution change has been considered to be caused by rupture only of molecules above a certain molecular weight, and this in the central sections of the molecules. For the oxidation of thin films and latex or any other chemical process whereby scission conceivably occurs with equal probability at every monomeric unit, the chain-length distribution would tend to a random one. In particular, if the rubber had initially a random distribution, viscosity-molecular weight decrease would yield an α equal to that for fractionated rubber and a K which is Γ (α + 2) times higher. Viscosity-molecular weight data on hot mastication have not been previously published. If a normal chemical degradation takes place, a difference in the viscosity-molecular weight relationship from that on cold mastication should be detectable. Differences in molecular weight distribution should also be reflected in differences in other properties of masticated rubber and, to a lesser extent, of vulcanized products. A comparison of viscosity-molecular weight relationships and other properties of rubbers masticated in air forms a starting point for answering the complex query of the relative merits of cold and hot mastication. Interaction of rubber with fillers and vulcanizing ingredients would then have to be systematically investigated.

Carbon-Black Loaded Rubber Vulcanizates: Volume Changes in Stretching

The changes in volume which occur during the stretching of carbon black reinforced vulcanizates have been measured, and the results used to examine crystallization which accompanies the extension of natural rubber. The effect of previous extension on these changes is interpreted in terms of a simple model previously advanced by the authors to describe the stress-strain properties of reinforced vulcanizates.