08 Polymers

Learn how polymer chain structure, molecular-weight distribution, thermal behavior, and processing methods determine the properties and uses of polymer materials.

Polymer structure and performance

Polymers are materials made of very large molecules, or macromolecules, built from repeating chemical units called monomers. Their long-chain structure gives them properties and processing behavior that differ from metals and ceramics. Polymer performance depends on chain chemistry, arrangement, and how chains move relative to one another.

Chain arrangements and crystallinity

A polymer chain has a covalently bonded backbone. Its may be linear, branched, or cross-linked. Mostly unbranched linear chains can often pack relatively closely. Side chains in branched polymers interfere with close packing and can change density and flexibility. In a cross-linked polymer, covalent bridges connect neighboring chains; light cross-linking may produce a flexible elastomer-like structure, while a dense three-dimensional network is typical of a thermoset.

Chains can also differ in the arrangement of their side groups, called , and in the sequence of different monomers in a copolymer. These details affect how regularly chains pack. Many polymers are : they contain both ordered crystalline regions and disordered amorphous regions. Greater crystallinity generally increases stiffness and strength and can improve resistance to heat and chemicals, while reducing clarity and sometimes toughness.

Molecular weight and distribution

A polymer sample usually contains chains of different lengths, so it does not have one exact molecular weight. Two common averages are:

M_n = \frac{\sum_i N_i M_i}{\sum_i N_i} \qquad\text{and}\qquad M_w = \frac{\sum_i N_i M_i^2}{\sum_i N_i M_i}

Here, $N_i$ is the number of chains with molecular weight $M_i$ . The number-average molecular weight, $M_n$ , weights each chain equally. The , $M_w$ , gives more influence to heavier chains. Their ratio is the :

Đ = \frac{M_w}{M_n}

measures the breadth of the molecular-weight distribution. Because larger chains count more strongly in $M_w$ , it is normally at least as large as $M_n$ . Increasing molecular weight often improves toughness and strength, but it also makes molten polymer more viscous and harder to process.

Thermal and mechanical behavior

Polymer chains can rotate, slide, and uncoil over time, making many polymers : their response combines elastic, spring-like behavior with time-dependent deformation. Properties depend on temperature, loading rate, and how long a load is applied. A polymer may feel stiff under a quick impact but gradually deform under a sustained load.

The , $T_g$ , marks a change in the mobility of the amorphous parts of a polymer. Below $T_g$ , those regions are relatively stiff and glassy; above it, they become more flexible and rubbery. polymers also have a , $T_m$ , at which their crystalline regions lose order.

Actual behavior depends on polymer chemistry, molecular weight, crystallinity, , and additives such as plasticizers.

and

have separate chains with little or no permanent cross-linking. Heating increases chain mobility so the material softens and can flow; cooling makes it solid again. This reversible softening and hardening allows reshaping, although repeated heating can eventually cause degradation. Polyethylene, nylon, and polystyrene are examples.

begin as reactive resins or partially formed materials that are shaped and then cured. creates a permanent network of cross-links. Once cured, a thermoset does not melt and flow like a thermoplastic; further heating may damage or decompose it instead. Epoxy resins and phenolic resins are examples. The cross-linked structure often gives good dimensional stability and heat resistance, but makes them difficult to reshape.

Processing methods

Processing must allow a polymer to flow or conform to a shape without damaging it. Temperature, pressure, flow, and cooling or conditions can affect the dimensions and properties of the finished part.

Common thermoplastic processes include:

: softened polymer is pushed continuously through a shaped die to make products such as pipe, sheet, or film.
: softened polymer is injected into a closed mold, cooled, and ejected. It is widely used for complex parts made in large quantities.
: air expands a softened polymer tube against a mold to form hollow products such as bottles.
: a heated plastic sheet is drawn or pressed over a mold, often using vacuum or pressure.

For , molding commonly combines shaping with . In , a measured amount of resin is placed in a heated mold and pressed into the cavity as it cures. This is useful for parts that need to retain their shape under heat.

Matching materials and processes

Polymer performance reflects chain chemistry and architecture, molecular-weight distribution, and the balance of crystalline and amorphous regions. soften reversibly and can be reshaped, whereas turns into permanent cross-linked networks. Selecting a polymer and processing method requires matching material properties and flow or behavior to the needs of the part.