What are the most important parameters to consider when selecting a polymer for a specific application?

1. Molecular Structure: Consider the polymer's molecular arrangement for its impact on properties. 2. Branching/Cross-Linking: Evaluate presence and effects on flexibility and strength. 3. Polymer Architecture: Linear, branched, or networked affects processing and strength. 4. Stereochemistry: Assess spatial arrangement's influence on properties. 5. Copolymer Composition: Analyze combinations for desired properties.

While all properties naturally flow from molecular structure, one rarely starts from the molecular structure to choose a polymer for an application. Rather, the properties required help determine the class of molecules which could meet those requirements, then further details of the application allow the more detailed determination of the required molecular weight, linearity. crosslinking, stability, etc...

Mechanical and rheological properties form the basis for the general type of polymer to consider. Should it be stiff, soft, flexible, or elastic. Should it be a liquid and if so at what temperatures? Basic properties for the application are always the best starting point.

What forces are expected to act on a molded part/ composite/coating/adhesive in service? Impact, bending, and thermal shock must be considered. Data quantifying these properties are available for many polymers to act as a starting point in material selection, but final parts must always undergo testing with their full complement of additives, fillers, colorants, etc. before being put into service.

1. Strength: Evaluate the polymer's ability to withstand applied forces without deformation or breakage. 2. Flexibility: Consider the polymer's ability to bend or stretch without fracturing. 3. Toughness: Assess the polymer's resistance to fracture under impact or repeated stress. 4. Elasticity: Determine the polymer's ability to return to its original shape after deformation. 5. Hardness: Measure the polymer's resistance to indentation or scratching.

For most materials thermal properties are extremely important, such as those in the electronics and automotive industries, however, some materials such as clothing and indoor items experience relatively small thermal excursions. It depends completely on the application what thermal properties you value. For example, in aerospace polyimides might be the only acceptable materials while hundreds of polymers would be suitable for a water cup. There are a variety of polymers which excel at low temperature performance such as polyisoprene and polybutadiene. Similarly, there are many polymers that are very useful at high temperature such as Kevlar, bismaleimide resins, etc... but few polymers excel at both high and low temperature extremes.

1. Melting Point: Determine the temperature at which the polymer transitions from solid to liquid state. 2. Glass Transition Temperature (Tg): Assess the temperature at which the polymer transitions from a hard, glassy state to a rubbery state. 3. Thermal Conductivity: Evaluate the polymer's ability to conduct heat, which impacts its suitability for insulation or heat transfer applications. 4. Thermal Expansion: Consider the polymer's tendency to expand or contract with changes in temperature, which affects dimensional stability. 5. Heat Resistance: Assess the polymer's ability to maintain its mechanical and chemical properties at elevated temperatures without degradation.

What temperatures will a molded part/ composite/coating/adhesive experience in service? Is it in an automobile engine or a roof in Antarctica? And is the expected temperature range wide or narrow? A polymer's Tg gives you a starting point to gauge suitability, but items must always undergo testing with their full complement of additives, fillers, colorants, etc. before being put into service.

The chemical properties of a polymer also define compatibility with other polymers and additives as well as interactions with coatings and adhesives.

Some polymers can undergo property changes by chemical treatments to modify functional groups on a surface or to develop higher molecular weight or even cross linked networks. Exposure to plasma or radiation are other methods used that can cause major changes to properties.

Durability. How the Mechanical Properties Change as a function of time. Creep, Stress Relaxation, Oxidation, and Stress Crack Resistance come to mind.

When selecting a polymer, it's crucial to consider its compatibility with the manufacturing process. For instance, thermoplastics are suitable for injection molding, while thermosets are often used in resin transfer molding. Additionally, environmental factors such as UV exposure and moisture should be assessed, as they can significantly affect a polymer's performance over time. Lastly, regulatory compliance, especially in food contact and medical applications, can dictate polymer selection to ensure safety and efficacy.