-Basic Polymer Chemistry, Part 2

ISE 107 POLYMER CHEMISTRY Part 2 1

BASIC POLYMER CHEMISTRYBASIC POLYMER CHEMISTRY

MOLECULAR WEIGHT (Chain Length)Melting point increasesTensile strength, stiffness & hardness increaseBarrier properties improveSolubility in solvents decreases



MOLECULAR WEIGHT (Chain Length)



BRANCHING OF POLYMER CHAINSEffects density and therefore MOLECULAR WEIGHTLinear vs highly “branched” polymer chainsFor the same m/w, vastly different properties result“Straight” chains (HDPE, 0.94 – 0.96) can pack closely

together giving higher density“Highly branched” chains (LDPE, 0.91 – 0.93) take up more

space resulting in lower densityEffects crystallinity (strength, barrier, melt index)



BRANCHING OF POLYMER CHAINS



BRANCHING OF POLYMER CHAINS



• Molecular Structure & Properties• Amorphous vs. Crystalline• Molecules weight distribution• Viscoelastic properties



• Molecular Structure & Properties• Amorphous vs. Crystalline

• Highly ordered = crysalline (translucent to opaque) example is HDPE

• Random arrangements = amorphous (clear) example is LDPE• Depends partly on polymer chain shape• Depends partly on thermal history (rate of cooling)• As Crystallinity increases, stiffness increases, heat tolerance

increases, barrier properties improve, clarity decreases



• Molecular Structure & Properties• Amorphous vs. Crystalline



• Molecular Structure & Properties• Molecular weight distribution

• A polymer can have a wide range of molecular weights depending on many factors. Cited values are averages.

• Higher M/W polymers are not as stable because properties change when the polymer chains break (due to UV, mechanical or chemical attack, etc.)



• Molecular Structure & Properties• Viscoelastic properties

• A combination of elastic & viscous properties• Thermoplastics tend toward more elastic properties• Thermosets tend toward more viscous properties• Rapid application of load causes bending (or other

deformation) and return to original shape after load removal (like a spring) short term (elastic)

• Long term loads cause more permanent deformation called “creep” or “cold flow” (viscous)



• Thermal Behavior• Phase changes are not well defined

• Glass transition• Melt transition• Orientation• Heat shrink properties• Thermal expansion



• Thermal Behavior• Glass transition; (Tg) the temp where the

polymer goes from a solid to a soft or elastic state

• Melt transition; (Tm) the temp where the molecules easily slip by one another allowing the material to flow easily

• Amorphous polymers have a more prominent Tg while crystalline polymers have more change at the Tm


Amorphous

Crystalline



• Orientation• Polymers subjected to stress (stretching) below their Tg results in molecular orientation that can (greatly) change characteristics.

• Used to enhance tensile strength, barrier properties, and allow thinner wall sections.

• Can be MONOAXIAL or BIAXIAL



• Orientation



• Thermal Behavior• Heat shrink properties; results from the “memory”

of the polymer between the Tg and the Tm

• Can be MONOAXIAL or BIAXIAL



• Thermal Expansion



• Thermoplastic vs. Thermoset Polymers• Thermoplastic (Tp) is essentially a 2 dimensional

chain.• Thermoset (Ts) is essentially a 3 dimensional

chain.• Tp’s are supplied fully polymerized. Ts’s polymerize

during the final molding process.• Tp’s melt when heated, Ts’s char when heated

(cross link bonds resist flow)• Ts’s are very solvent resistant



• Thermoplastic vs. Thermoset Polymers

Documents

-Basic Polymer Chemistry, Part 2