The present review focuses on summarizing key advances made on controlling polymer blend morphology to improve electrical conductivity in carbon-based polymer composite materials, including those based on carbon black, carbon nanotubes, and graphene. Fundamentals for controlling polymer morphology and the distribution of conductive fillers in various polymer composite systems and the impact on the electrical, rheological, mechanical, and thermal properties are reviewed. The concept of triple percolation and its beneficial effect on electrical conductivity is then reviewed.

The dendritic and axonal arbors of developing retinal ganglion cells (RGCs) are exposed to two sources of BDNF: RGC dendrites are exposed to BDNF locally within the retina, and RGC axons are exposed to BDNF at the target, the optic tectum. Our previous studies demonstrated that increasing tectal BDNF levels promotes RGC axon terminal arborization, whereas increasing retinal BDNF levels inhibits RGC dendritic arborization. These results suggested that differential neurotrophic action at the axon versus dendrite might be responsible for the opposing effects of BDNF on RGC axonal versus dendritic arborization. To explore this possibility, we examined the effects of altering BDNF levels at the optic tectum on the elaboration of RGC dendritic arbors in the retina. Increasing tectal BDNF levels resulted in a significant increase in dendritic branching, whereas neutralizing endogenous tectal BDNF with function-blocking antibodies significantly decreased dendritic arbor complexity. Thus, RGC dendritic arbors react in opposing manners to retinal- versus tectal-derived BDNF. Alterations in retinal BDNF levels, however, did not affect axon terminal arborization. Thus, RGC dendritic arborization is controlled in a complementary manner by both local and target-derived sources of BDNF, whereas axon arborization is modulated solely by neurotrophic interactions at the target. Together, our results indicate that developing RGCs modulate dendritic arborization by integrating signals from discrete sources of BDNF in the eye and brain. Differential integration of spatially discrete neurotrophin signals within a single neuron may therefore finely tune afferent and efferent neuronal connectivity.

SUMMARYSeven halogen-free flame retardant (FR) compounds were evaluated using pyrolysis combustion flow calorimetry (PCFC) and cone calorimetry. Performance of wires coated with the compounds was evaluated using industry standard flame tests. The results suggest that time to peak heat release rate (PHRR) and total heat released (THR) in cone calorimetry (and THR and temperature at PHRR in PCFC) be given more attention in FR compound evaluation. Results were analyzed using flame spread theory. As predicted, the lateral flame spread velocity was independent of PHRR and heat release capacity. However, no angular dependence of flame spread velocity was observed. Thus, the thermal theory of ignition and flame spread, which assumes that ignition at the flame front occurs at a particular flame and ignition temperature, provides little insight into the performance of the compounds. However, results are consistent with a heat release rate greater than about 66 kW/m 2 during flame propagation for sustained ignition of insulated wires containing mineral fillers, in agreement with a critical heat release rate criterion for burning. Mineral fillers can reduce heat release rate below the threshold value by lowering the flaming combustion efficiency and fuel content. A rapid screening procedure using PCFC is suggested by logistic regression of the binary (burn/no-burn) INTRODUCTIONTesting of flame retardant (FR) materials presents technical and scientific challenges. Various enduse applications have unique fire tests, which in most cases have not been correlated to small-scale methods of flammability assessment. To complicate matters further, these tests are often conducted on finished articles rather than pure polymer compounds. Limiting oxygen index (LOI) and cone calorimetry are among the most commonly cited methods for assessing flammability of FR polymer compounds. Although studies have raised questions about the reliability of LOI in flammability research [1, 2], LOI remains one of the most popular flammability test methods. This is likely due in large part to the fact that measurement of LOI can be done quickly using relatively inexpensive and simple equipment. Despite its popularity, correlation of LOI with performance in industry flame tests is limited, and application to predict burn behavior in actual fires is uncertain [3].Cone calorimetry is another method for flammability assessment of materials, and can be run on raw materials as well as fabricated articles, providing a wealth of data from measurement of a single specimen. While cone calorimetry is steadily gaining in popularity, its use outside of research remains limited due to a number of factors, including the high cost, complexity of cone calorimeters, and the resources required to operate and maintain them. Additionally, there is no widespread agreement in the industry about which parameters are most meaningful. Therefore, more studies correlating burn performance in key industry tests with results of cone calorimetry on compounds are needed.Cone calor...

A review of nucleating agent (NA) types and their effect on crystallization in high-density polyethylene (HDPE) is presented. The focus is on methods to improve the physical properties of HDPE due to its widespread use in commercial applications and high volume use in typical industrial processes including extrusion, injection molding, and blow molding. The crystallization process in HDPE significantly affects its final optical, mechanical, and thermal properties. The addition of NAs affects the physical properties of HDPE by controlling the crystallization from the melt state. Specific NAs improve properties such as clarity, cycle time, and modulus. NAs are more widely developed for polypropylene (PP) than HDPE as its slower crystallization rate allows greater control in achieving property improvements. While certain NAs are effective in improving property characteristics in HDPE, greater control over the crystallization process would achieve further improvements in specific properties. Research has progressed in identifying effective NAs for HDPE, though the magnitude of the effects remains lower than those generally observed in PP. Inorganic and organic NAs are reviewed with an emphasis on the mechanisms by which they function. Fundamentals of polymer crystallization and modeling kinetics during both isothermal and nonisothermal studies provide the necessary framework for characterizing the effects of a NA in HDPE. Finally, the interactions between HDPE, NA, and industrial processing conditions as related to practical applications are discussed. POLYM. ENG. SCI., 56:541-554, 2016.