Question

High density polyethylene (HDPE) has a glass transition temperature below 0 C. However, HDPE appears as plastic rather than a rubbery material at room temperature. Why? (Max 100 words) (5 Points) Enter your answer What is the glass state? What is the glass transition temperature? (Max 100 words) (5 Points) ) Enter your answer Why polymers do not want to mix? (Consider the entropy and enthalpy effects, and compare to small molecular compounds.) (Max 100 words) (5 Points) Enter your answer What is the basic principle of size exclusion chromatography? (Regarding to how does it separate molecules with different sizes and how do you calculate the average molecular weight and molecular weight distribution.) (Max 100 words) (5 Points)

Answer 1

Key points:

1. Glass Transition is a property of the amorphous region while melting is the property of crystalline region.
2. Below glass transition temperature (Tg), there exists disordered amorphous solid where chain motion is frozen and molecules start wiggling around above Tg. The more immobile the chain, the higher the value of Tg.
3. While below melting temperature (Tm), it is an ordered crystalline solid which becomes disordered melt above Tm

Solution to question 21:

HDPE is made out of PE. Tg of PE is low because of its high flexibility and the absence of side groups. Additionally PE is semicrystalline due to its realtively simple and symmetrical structure, therefore it remains tough and exhibits plastic behavior between Tg and Tm.

Further, HDPE is plastic based on consideration of its thermal behavior. That is it softens when heated and returns to initial state when it is cooled .