Time Effects In Thermoplastics

Question 1

Background to Polymers

Answer 1

• Polymers are light materials with densities varying between 0.9-1.5 times that of water.
• They are corrosion resistant relative to metals and have low thermal and electrical conductivity.
• One disadvantage is their low melting temperatures, though this doesn’t mean that low temperature processes can shape them.
• Another problem when designing load-bearing components is that their strength and modulus are very low, and properties are very temperature and time dependent.
• They will creep at room temperature when subjected to constant load or stress, or if a constant strain is applied they will exhibit stress relaxation.
• They will also show different stress versus strain curves if tested at different strain rates.
• The engineering designer, therefore, needs to take these time effects into account.

Question 2

Thermo-softening Polymer Properties: Bonding, Structure & Properties

Answer 2

• Thermosoftening polymers (or Thermoplastics) melt or soften in the temperature range 60oC to 350 oC to form a molecular liquid.
• This low temperature is because the bonds holding the molecules together are weak secondary hydrogen bonds with a strength almost 100-th of the carbon-carbon bonds found along the carbon backbone.
• The long chain molecules are made up of monomers or repeat units, the simplest being polythene with repeated CH2 units.
• If one or more of the hydrogen atoms is replaced with larger atoms or side groups, a range of thermoplastics can be produced.
• As the size of the side group increases the strength increases and the ductility (% elongation) decreases. This is because the mechanism of plastic flow in thermoplastics is one of molecular uncoiling and inter-molecular sliding, and this becomes more difficult with greater interlocking of the larger side groups.

Question 3

Glass Transition Temperature

Answer 3

• This increases with larger side group size.
• The glass transition temperature is the temperature above which thermal energy allows inter-molecular movement to occur.
• Above this temperature the properties are therefore increasingly more ductile, and polymer shaping processes can be applied, while below this temperature the polymer is brittle or glassy.

Question 4

Branched Polymers

Answer 4

• The configuration of a polymer refers to the way each chain is constructed from its covalently bonded atoms, and different types of configuration are possible.
• Branched polymers differ from linear chain polymers in that side chains are attached to the main chain e.g. High Density Polyethene (HDPE) versus Low Density (LDPE).

Question 5

Copolymers

Answer 5

• Copolymers have two or more monomers, which maybe random, alternating or block copolymers.

Question 6

Homopolymers

Answer 6

• These have identical repeat units while cross-linked polymers have covalent linking chains to give a three dimensional molecules.
• Chains of different length have different molecular mass which also increases properties like modulus as mass increases.

Question 7

Stereoisomers

Answer 7

• These have different spatial arrangements of the same atoms around a double bond, cis, and trans being two important types.

Question 8

Crystallinity in polymers

Answer 8

• The more complex the structure, the more difficult it is to crystallise it, since the time required to orientate molecules into a regular pattern.
• Polythene can be crystallised by slow cooling from the molten state, but with faster cooling, the proportion of amorphous material in the structure increases
• When the close packing of crystallisation occurs there is a decrease in free volume in the structure, and therefore a decrease in the specific volume.

Question 9

Specific Volume vs. temperature for a thermoplastic

Answer 9

• The melting temperature Tm is not as sharp as in metals because the segments of molecular chain bonded together can have variable length.
• If no crystallisation occurs due to rapid cooling from melt, there is a smaller decrease in free volume as the average interatomic spacing decrease, but the structure remains amorphous or random.
• The material becomes more viscous and then solid, though still ductile, until intermolecular movement stops below Tg when the material becomes brittle or glassy.

Question 10

Temperature & Time dependence of polymer properties

Answer 10

• The melting temperature is not as sharp in metals because the segments of molecular chain bonded together can have variable length.
• If no crystallisation occurs due to rapid cooling from the melt, there is a smaller decrease in free volume as the average interatomic spacing decreases, but the structure remains amorphous or random. The material becomes more viscous and then solid, though still ductile, until intermolecular movement stops below Tg when the material becomes brittle or glassy.
• When a simple molecule like polyethene crystallises, parallel sections of the chain align in a zigzag way, to accommodate the C-C bond angles. On a larger scale the segments fold to produce lamellae about 10nm thick & these grow from a centre, twisting as they do so and forming a spherulite, which is the equivalent of a grain in a metal.

Question 11

Temperature & time dependence of polymer properties:

Answer 11

• Perspex’s tensile properties varies with temperature. It can be seen that the material is brittle below it’s glass transition temperature and ductile above it.
• It can be seen that, like metals, necking occurs and causes a fall in the engineering stress, but that unlike metals the phenomenon of drawing can occur. When stress is applied to a semi-crystalline polymer the crystallites reorientate themselves.

Question 12

Strain Crystallisation

Answer 12

• The degree of crystallisation after drawing is higher, and this is called strain crystallisation.
• Because the molecules are aligned with the strong covalent bonds now parallel to the stressing direction, the material in the drawn neck is very much stronger. So despite the higher stress in the neck, the neck becomes stable and the draw font moves towards each end of the specimen.
• When the draw front meets the thicker ends of the specimen, a higher force is required to continue drawing and at some point the fracture stress is exceeded.

Question 13

Creep Modulus

Answer 13

-This applies where there is continuing deformation under constant load

Question 14

Relaxation Modulus

Answer 14

• This applies where there is a decrease in stress for a sample held at constant deformation.

Question 15

Tacticity

Answer 15

• This refers to different ways the side groups can be arranged along the polymer chain.
• Isotactic with side groups on one side. This is the most symmetrical and easiest to crystallise.
• Atactic with random placing of side groups. This is the least symmetrical and hardest to crystallise.

Question 16

Crazing in polymers

Answer 16

• When amorphous thermopllastics like polystyrene or polycarbonate are stressed at an appropriate rate, crazes may be seen in the material, especially close to the fracture surface. They are the result of local yielding on planes perpendicular to the tensile stress and are very thin.