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what are the pendant groups in polystyrene and polypropylene
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Network polymers, characterized by their highly crosslinked, three-dimensional structures, tend to exhibit brittle rather than ductile behavior. This is due to the rigid and extensive crosslinking that restricts the mobility of polymer chains, making it difficult for the material to deform plastically under stress. Polymers like Bakelite and epoxy resins are classic examples of network polymers and are known for their hardness and high dimensional stability, but not for ductility. Unlike linear or lightly branched polymers, which can exhibit significant plastic deformation through chain sliding mechanisms, the crosslinks in network polymers effectively lock the structure in place, leading to a brittle fracture when a critical stress is exceeded. However, modifications can be made to the polymer's composition to alter its properties, but fundamentally, network polymers are not naturally ductile.
Wetting agents, or surfactants, play a crucial role in pharmaceutical formulations by enhancing the wetting of solid particles. This improves the dissolution rate of poorly soluble drugs, ensuring quicker and more consistent bioavailability. Wetting agents achieve this by reducing the surface tension between a liquid and a solid, facilitating the spreading of liquid over the solid particles. This is particularly important in tablet manufacturing, where even wetting ensures uniform compaction and dissolution, and in topical preparations, where it aids in the dispersion of active ingredients across the skin or mucous membranes. Additionally, wetting agents can improve the flow properties of powders, aiding in capsule filling and tablet compression processes. By optimizing drug solubility and absorption, wetting agents ensure effective drug delivery and enhance patient compliance.
Determining the ceiling temperature of a polymer, also known as its maximum service temperature, involves assessing the temperature at which the material begins to degrade or lose its mechanical properties significantly. This is crucial for applications requiring thermal stability. One common method is Differential Scanning Calorimetry (DSC), which measures the heat flow into or out of the material as it is heated. Thermogravimetric Analysis (TGA) is another technique that evaluates weight loss under controlled temperature conditions, providing insights into decomposition temperatures. Additionally, Dynamic Mechanical Analysis (DMA) can be used to observe changes in modulus or other mechanical properties with temperature increases. For more accurate results, these tests should be conducted in an inert atmosphere to prevent oxidation. It's essential to consider the specific application and select a polymer accordingly, ensuring it can withstand the expected thermal stresses without significant degradation.
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