polymers 3

Question 1

what is glass transition temperature (Tg)?

Answer 1

above Tg segments of the polymer chain are free to move from thermal agitation. vobrations become coordinated. below Tg, the material is brittle, above Tg it becomes tougher

Question 2

what are the effects of changing the strain rate and temperature?

Answer 2

behaviour is highly dependent on temperature. increasing temperature: decreases elastic modulus, reduces tensile strength and increases ductility. opposite for increasing strain rate.

Question 3

what are the four stages of deformation (tensile)?

Answer 3

1) amorphous regions extended, crystalline lamellae ‘shear’ past each other
2) lamellae orientate to the load direction
3) lamellae segments separate from the bulk connected by tie chains
4) blocks and tie chains align with the loading direction

leads to highly orientated structure, high strength and stiffness

Question 4

structural effects of drawing into fibres?

Answer 4

any cross linking will increase stiffness, strength, reduce ductility. in crystalline regions there is extensive secondary bonding, with an increase in stiffness, strength and reduced ductility. (strength also increased with molecular weight up to a point)

Question 5

what is the melting point of polymers?

Answer 5

point where there is a sudden change in specific volume (regular array of molecular chains goes into random arrangement) (rigid solid goes into rubbery solid/viscous liquid depending on molecular weight due to thermal vibrations breaking secondary bonds)

Question 6

melting points for amorphous vs crystalline polymers?

Answer 6

totally amorphous - no melting point observed, rather glass transition temp observed by change in rate of change of specific volume.
partially crystalline - both Tm and Tg will be detected (melting point of crystalline or partially crystalline polymer depends on factors to do with 2ndary bonding)

Question 7

what can cause melting point to increase?

Answer 7

reduced branching, increased molecular weight (ends are more mobile), crystallite size, larger/stiffer backbone units and side groups.

Question 8

what can cause glass transition temperature to increase?

Answer 8

things that reduce chain flexibility - large side groups, branching, cross-linking, increased molecular weight

Question 9

what is the influence of plasticisers on glass transition temperature?

Answer 9

small molecules that fit between larger molecules, increasing spacing b/w chains and reducing secondary bonds, essentially lubricate and allow movement. lowers Tg, as rearrangements can occur easier. (increase flexibility, ductility, toughness of otherwise brittle polymers)

Question 10

what are requirements for a good plasticiser?

Answer 10

high boiling point, low vapour pressure, ideally non-soluble in liquids that could come in contact with plastic, compatible with polymer so segregation doesn’t occur. involves extensive testing

Question 11

what is viscoelasticity?

Answer 11

behaviour commonly related to the amorphous component of polymer above glass transition temperature. (visco - behaviour like viscous liquid. elastic - behaviour like elastic solid)

Question 12

how can the elastic component of viscoelasticity be modelled?

Answer 12

a spring with a spring constant equal to youngs modulus, instantaneous response to application or removal of load, complete recovery once load is removed

Question 13

how can the viscous component of viscoelasticity be modelled?

Answer 13

‘dashpot’ (piston kinda) with time-dependent response to the application of load, no tendency to undergo recovery (stays deformed when load removed) (think loose piston filled with something like honey)

Question 14

what behaviour does a spring and dashpot in series model?

Answer 14

‘rubbery’ viscoelastic behaviour - rapid partial recovery and secondary creep (not total recovery)

Question 15

what behaviour does a spring and dashpot in parallel model?

Answer 15

‘leathery’ viscoelastic behaviour - primary creep, time delayed complete recovery

Question 16

difference between leathery and rubbery state?

Answer 16

leathery - small instantaneous elastic strain followed by much larger time dependent elastic strain (primary creep) along with small amount of viscous flow (secondary creep) (primary creep reaches some equilibrium value with time while secondary creep continues)
rubbery - more instantaneous elastic strain, negligible primary creep, increased viscous flow

Question 17

look at slide 30 polymer 3

Answer 17

go on

Question 19

what is the viscoelastic relaxation modulus?

Answer 19

basically a time dependent elastic modulus, affected by temperature

Question 20

what are the regions on relaxation modulus vs temperature graph?

Answer 20

glassy (elastic behaviour with frozen chains), leathery (deformation is time dependent, molecules have coordinated motion), rubbery plateau (deformation easy due to low viscoelastic modulus), rubbery flow (soft rubbery state), viscous flow (just viscous)

Question 21

how does structure affect relaxation modulus?

Answer 21

for small amount of cross-linking, rubbery plateau extends til decomposition.
more cross-linking, value of relaxation modulus at plateau increases
highly crystalline, small drop in relaxation modulus at glass transition temperature as T increases, then relaxation modulus maintained at high value until melting point reached