– Prof. Leigh Palmer, in a letter to Feynman While knowledge is most readily assimilated by the prepared mind, that same mind can be refractory to penetration if it is "prepared" to believe that it cannot be taught. – Prof. Leigh Palmer, in a letter to Feynman
Polymers are different
Normal crystalline materials Polymers are different Normal crystalline materials Either crystalline (~100 %, neglecting defects ) or amorphous at a particular temperature Melt at a sharp, well-defined temperature Crystallizable polymers Never 100% Crystalline Melt over a Range of Temperatures
Model for crystalline regions in polymers
Chain folding - polyethylene
So crystalline region looks like this Or maybe this
In many polymers, crystalline regions grow to form spherulites Image of spherulites in polarized light microscope
Pull thin polymer rod in tension Get alignment of crystalline regions
Polymer fibers have aligned crystalline regions One way to make fibers - extrude viscous melt through tiny holes in "spinneret"
Polymer fibers have aligned crystalline regions - alignment gives greater strength to fiber Kevlar is highly aligned
Breaking strength of polymer fibers (tenacity) measure denier (wt. in grams of 9000 meters of fiber) run tensile test
Tenacity also increases w/ chain length - fewer crystal defects
Glass transition temperature (Tg) Molecular wt.
Glass transition temperature (Tg) Chemical structure
Glass transition temperature (Tg) Chain stiffness
Glass transition temperature (Tg) Chain stiffness
Glass transition temperature (Tg) Bulky side groups
Assignment: Review today's classnotes a. crystallinity in polymers b. chain folding c. spherulites d. tenacity e. glass transition temperature