Describe the differences between normal phase and reverse phase chromatography.

Flexible plastics, such as polyethylene, have a low modulus of elasticity and can be stretched to a significant extent before they break. This is represented by a relatively flat stress-strain curve. Rigid plastics, such as polycarbonate, have a higher modulus of elasticity and break at a much lower strain. This is represented by a steep stress-strain curve. Fibers, such as nylon, have a high tensile strength and are generally elongated under tension, resulting in a nearly linear stress-strain curve. Elastomers, such as rubber, can be stretched to a high degree and return to their original shape when the stress is released. They have a nonlinear stress-strain curve that can be described as an S-shape. The chemical structures of each type of material determine their properties. Flexible plastics have a long chain of repeating ethylene monomers, which enables them to bend and stretch without breaking. Rigid plastics have a more complex structure with cross-linking between polymer chains, which contributes to their stiffness. Fibers have a highly aligned structure with hydrogen bonding between chains, which gives them their strength. Elastomers have a more random structure with some cross-links between chains, which allows them to stretch and return to their original shape.