08 Polymers Free Online FlashCards

What is a polymer?

Polymers are macromolecules built from repeating chemical units called monomers.

What defines a cross-linked polymer structure?

Covalent bridges connect neighboring chains. Light cross-linking can produce flexible elastomers, while dense networks are typical of thermosets.

What are typical effects of greater polymer crystallinity?

Greater crystallinity generally increases stiffness and strength and can improve heat and chemical resistance, but may reduce clarity and toughness.

How is number-average molecular weight, $M_n$, calculated?

The number-average molecular weight weights each chain equally: $M_n = \frac{\sum_i N_i M_i}{\sum_i N_i}$.

What does it mean for a polymer to be viscoelastic?

Their response combines elastic, spring-like behavior with time-dependent deformation. It depends on temperature, loading rate, and how long a load is applied.

What changes at a polymer’s glass-transition temperature, $T_g$?

The glass-transition temperature marks a change in the mobility of amorphous regions: below $T_g$ they are relatively stiff and glassy; above it they become more flexible and rubbery.

How do thermoplastics respond to heating and cooling?

Heating increases chain mobility so they soften and can flow; cooling solidifies them again. This allows reshaping, though repeated heating can eventually cause degradation.

How does injection molding form a thermoplastic part?

Softened polymer is injected into a closed mold, cooled, and ejected. The method is widely used to make complex parts in large quantities.

How can branching affect a polymer?

Side chains interfere with close packing and can change a polymer’s density and flexibility.

How is weight-average molecular weight, $M_w$, calculated?

The weight-average molecular weight gives heavier chains more influence: $M_w = \frac{\sum_i N_i M_i^2}{\sum_i N_i M_i}$.

What does dispersity measure?

Dispersity measures the breadth of the molecular-weight distribution: $Đ = M_w/M_n$. It is normally at least $1$, since $M_w$ is usually at least as large as $M_n$.

What happens at a semicrystalline polymer’s melting temperature, $T_m$?

At the melting temperature, $T_m$, crystalline regions in a semicrystalline polymer lose their order.