While knowledge is most readily assimilated by the

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

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) 1. measure denier (wt. in grams of 9000 meters of fiber) 2. 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