time effects in thermoplastics

Question 1

What are polymers

Answer 1

-light materials with densities varying between 0.9-1.5* that of water

Question 2

disadvantages of polymers

Answer 2

• low melting temperatures, and therefore easily shaped by low temp processes
• strength and modulus values are low, and properties are strain rate defendant, creeping when subjected to constant load or stress, or exhibiting stress relaxation.
• they show different stress/strain curves if tested at different strain rates. Time effects must be considered by designer.

Question 3

Properties of thermosoftening polymers

Answer 3

• melt in temp range 60-350oC to form a molecular liquid
• low temp because bonds holding molecules together are weak secondary hydrogen bonds
• long chain molecules are made up of mers (repeat units)

Question 4

Glass transition temperature

Answer 4

• Temperature above which thermal energy allows intermolecular movement to occur.
• increases with larger side group size
• above temp the properties are more ductile, and polymer shaping processes can be applied
• below temp the polymer is brittle or glassy

Question 5

Types of polymers

Answer 5

-Branched polymers
Side chains attached to main chain
-Copolymers
Two or more mers, maybe random, alternating or block copolymers
-Homopolymers
Identical repeat unites 
-Crosslinked polymers
Covalent bonds linking chains to give 3d molecules

Question 6

Crystallinity in polymers

Answer 6

• Complexity of structure makes it more difficult to crystallize
• this is due to to the time taken to orientate molecules into a regular pattern
• Polythene can be crystallized by slow cooling from molten state.
• Fast cooling means a rise in the amount of amorphous material in structure

Question 7

Melting temperature in polymers

Answer 7

not as sharp as in metals due to the segments of molecular chain bonded together having variable length

Question 8

Temp and time dependance of polymer properties

Answer 8

• like metals, necking occurs and causes a fall in engineering stress, but unlike metals, drawing can occur.
• Degree of crystallization after drawing is higher, and this is called strain crystallization.
• Due to molecules being aligned with bonds , material in drawn neck is much stronger.

Question 9

What is drawing?

Answer 9

-As stress increases, lamellae are broken down, form fibrils and these break down polymer chains unfolding, and flow into alignment

Question 10

definitions for modulus of polymer

Answer 10

Creep Modulus
-continuing deformation under constant load
Stress relaxation modulus
-Decrease in stress for a sample held at constant deformation

Question 11

Tacticity

Answer 11

-Different ways thatside groups can be arranged along polymer chains

Question 12

Types of tacticity

Answer 12

Isotactic
-side groups all on one side, most symmetrical and easiest to crystallize
Syndiotactic
-Side groups alternating
Atactic
-Random placing of side groups, least symmetrical and hardest to crystallize

Question 13

Crazing in polymers

Answer 13

• happens when Amorphous thermoplastics are stressed at an appropriate rate
• may be seen in material especially close to fracture surface
• a result of local yielding on planes perpendicular to tensile stress and are very thin.
• look like cracks, but still load bearing since they are bridged by polymer fibrils
• Density is about half that of a solid polymer
• reduce craze by adding small rubbery particles to toughen polystyrene.