15+16 Polymers

Backbone Bond Type

Covalent Bods (Typically carbon-carbon), (directional)

Natural Polymers

derived from existing biological process(Cellulose, DNA), unlike plastics which are synthetic, semi-synthetic

Polymer Charactertistics

1. lightweight

2. low elastic modulus

3. low strength

4. low fracture toughness

5. good ductility

6. formable (easy to deform)

Degree of Polymerization

represents the average number of mer units in a chain

Higher DP means

1. higher melting point

2. higher stiffness

3. higher strength

Polymer Characteristics which influence mechanical properties

1. DP (degree of polymerization)

2. Linear vs Branches

3. Cross-linking and networking

4. Co-polymers

5. Side chains

more branching means

• lower tensile strength (less intermolecular, dipole-dipole, forces)

• increased ductility (less intermolecular, dipole-dipole, forces)

high-density polyethylene (HDPE)

low degree of short chain branching

Linear-low density polyethylene (LLDPE)

high degree of short-chain branching

Low-density polyethylene

high degree of short chain + long chain brainching

LLDPE vs LDPE

• higher tensile strength

• higher impact and puncture resistance

• ca be less thick

crosslinked polymers

• adjacent linear chain strongly joined together

• non reversible chemical reaction

network polymers

3-dimensional networks (3 active covalent bonds)

Copolymers

polymer composed of two different per units with different combinations

• random

• block

• alternating

• graft polymer

stereoisomerism

atoms linked together in the same order (head to tail) but in different spatial arrangements (isotactic, syndiotactic, atactic)

isotactic

side groups all on the same side

syndiotactic

side groups on alternating sides

atactic

random distribution for side group locations

geometrical isomers

• molecules that have the same molecular formula, but have a different arrangement of the atoms

• when C=C are present, the chain can no longer rotate freely (locked)

long range order

atoms are arranged in a regular, repeating pattern over long distances

amorphous vs crystalline

Amorphous materials do not have long-range order, while crystalline materials do

polymers form crystalline structures when…

side chain groups form van der waals bonds with other side chain groups in the chain

crystal formation inhibiting properties

1. branches

2. extensive cross linking and networking

3. randomness in copolymers

4. bulky side groups

5. atacticity (random distribution of sidechain locations)

6. chemically complex mer structures

7. speed of formation (rapid cooling is less likely to result in organized crystallization)

increased % crystallinity means higher

• strength

• stiffness

• density

• resistance to softening (higher melting points)

• chemical resistance

polymers are highly sensitive to

• rate of deformation (strain rate)

• temperature

• environment (organic solvents and water)

as temperature increases in polymers

• elastic modulus decreases (less stiff)

• tensile strength decreases

• ductility increases

decreasing the rate of deformation has the same effect as

increasing the temperature

thermoset

• polymer decomposes/oxidizes/burns when heated

• formed by cured viscous/soft liquid

• typically stiffer, harder, and can be used at higher temperatures

thermoplastic

polymer melts when heated (without change in properties)

elastomers

• can withstand large deformations and elastically return to original form

• most are thermoset

• lightly crosslinked

fabrication of thermosetting polymers

1. linear polymer with a low molecular weight is prepared (in liquid or malleable solid form)

2. Cured with addition of heat, catalysts, or pressure (generates irreversible crosslinked polymers or network polymers, preventing melting)

glass transition temperature

• occurs when heating or cooling polymers in the amorphous/semicrystalline state

• lower than glass melting temperature

stress relaxation

specimen is strained rapidly to a predetermined strain level, and the change in stress is measured overtime (under constant strain when measuring change in stress, opposite to viscoelastic testing)

elastomers criteria

• must not crystallize easily (amorphous)

• chain bond rotations are relatively free (easy to uncoil)

• limited amount of crosslinking (onset of plastic deformation is delayed)

• above glass transition temperature

air entrainment

pores and air bubbles trapped inside while the polymers are viscous, then are cured

plasticizers

increased flexibility, ductility, and toughness

polymer additives

plasticizers and stabilizers

stabilizers

counteract deterioration of plastics (UV, oxidation, and heat protection)