Conformations and flexibility
Units of a chain can rotate around single bonds, like the joints of a snake. So the chain constantly changes shape and is usually curled into a coil. The easier the rotation, the more flexible the chain.
Conformations are shapes of a macromolecule arising from rotation around single bonds without breaking them. Chain flexibility depends on rotation barriers and is characterised by the Kuhn (statistical) segment length: the longer it is, the stiffer the chain. Polyethylene has a flexible chain; cellulose a stiff one because of its pyranose rings and intramolecular hydrogen bonds.
Amorphous and crystalline
If chains are regular, they can line up in rows like pencils in a box: these are crystalline regions. Where order breaks down, chains are tangled: these are amorphous regions. Cellulose has both.
Supramolecular structure comprises ordered crystalline and disordered amorphous regions; the degree of crystallinity depends on chain regularity and processing conditions. Crystallising polymers form lamellae, fibrils and spherulites. Orientation of macromolecules on drawing sharply raises fibre strength along the axis; in wood the oriented fibres are the cellulose microfibrils.
Three physical states
Heat a piece of plastic under a weight. At first it is hard and barely bends. Then it softens and becomes rubbery: it stretches but springs back. Heat it further and it starts to flow.
On heating, an amorphous linear polymer passes from the glassy to the high-elastic (rubbery) state at the glass transition temperature Tg, and from the rubbery to the viscous flow state at the flow temperature Tf. Deformation plotted against temperature is the thermomechanical curve. The rubbery plateau widens as chains get longer; a crosslinked polymer shows no viscous flow.
Glass transition and plasticisation
Some substances slip between chains and make a polymer softer: these are plasticisers. For wood the main plasticiser is water. That is why steamed wood can be bent and, once dry, keeps its shape.
Plasticisers lower the glass transition temperature by increasing segment mobility. Water strongly plasticises the amorphous wood polymers: when moist, the softening temperature of hemicelluloses falls below room temperature, and that of lignin drops into a range reachable by steaming. This underlies bending of steamed wood and thermomechanical processing (refining chips to make mechanical pulp).
Polymers in solution
A polymer dissolves slowly: first it swells by soaking up solvent, and only then does it come apart. A crosslinked polymer never gets past swelling.
Dissolution of polymers begins with swelling; unlimited swelling turns into dissolution, while limited swelling is typical of network polymers. In solution macromolecules form coils whose size depends on solvent quality. Polymer solutions are highly viscous even at low concentrations.
Key facts
- flexibility
- Kuhn segment
- cellulose
- stiff chain
- states
- glassy, rubbery, flow
- Tg
- glass transition
- wood plasticiser
- water
- crosslinked polymer
- does not flow
Sources for this chapter
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.
- Odian G. Principles of Polymerization. 4th ed. Hoboken, NJ: Wiley-Interscience, 2004.
- Salmén L. Viscoelastic properties of in situ lignin under water-saturated conditions. Journal of Materials Science 19, 3090-3096 (1984).