How do the properties change when T < Tg? When T > Tg? g
Plasticizers • Small organic molecules that act as lubricants • Lower Tg, make material flexible • Dialkyl phthalates are commonly used as plasticizers • Due to safety and health questions they have been banned in children’s toys Fibers • Thin threads produced by extruding molten polymer through small holes in a die, or spinneret • Fibers are cooled and drawn out orienting the crystallite regions along the axis of the fiber • Contain considerable tensile strength • Extend – gets stronger = more ordered Elastomers • Amorphous polymers that stretch out and spring back to original shapes • Entropy driving force for its return. • Low T g values and small amount of cross-linking Molecular weight: M Mass of a mole chains
Not all chains in a polymer are the same length because there is a distribution of molecular weights. Degree of Polymerization : average number of repeat unit per chains M= the molecular weight of the repeating units. Number Average Molecular weight: the sum of the products of the molar mass of each fraction multiplied by its mole fraction. Where x i is the mole fraction of molecules of length i , the ratio of the number of molecules of length i , n i , to the total number of molecules, n . m is the mass of each species: M w ≥ M n Molecular weight Distribution curve: Synthetic polymers have molecules that vary in molecular weight: Natural products such as proteins and peptides have a single defined molecular weight and are called monodisperse 1 1 2 2 3 3 1 1 2 3 1 . . . . . . n i i i n n i i n M n M n M n M M n n n n = = + + + = = + + + å å
The relative viscosity rev : the ration of the viscosity of the solution , to the velocity of the pure solvent o .
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- Spring '17
- Polymer, Erythromycin