self-healing polymers

Question 1

benefits of self-healing (SH) polymers?

Answer 1

1) no direct human action for repair
2) increased usage/longevity times
3) helps with the limitations of natural resource usage

Question 2

what are the required properties for self-healing?

Answer 2

• the ability to transform physical energy into a chemical and/or physical response (needs to sense the damaging force)
• requires a rest period for healing to take place

Question 3

what is intrinsic self-healing?

Answer 3

when the polymers utilise an inherent material ability to self-heal when triggered by a damage event or stimulus, molecular diffusion etc

Question 4

what is extrinsic self-healing?

Answer 4

the healing agents have to be pre-embedded into a polymeric matrix, which enables their release during a rupture event - this causes self-healing

Question 5

what are the mechanisms of physical self-healing (welding)?

Answer 5

1) molecular interdiffusion
2) patching
3) swelling

Question 6

what is patching?

Answer 6

covering or replacing damaged material by new externally added material

Question 7

what is swelling?

Answer 7

involves the addition of solvents to increase the mobility of the polymer chains and the healing process

Question 8

how does molecular interdiffusion work?

Answer 8

1) occurs at or above the Tg bc at the Tg the chains are mobile enough to self-heal

2) a polymer-polymer interface forms by bringing two polymers together (either side of a crack)

3) there is molecular diffusion of chains across the polymer-polymer interface, the conc grad of this diffusion becomes constant

4) the polymer-polymer interface gradually disappears as the chains diffuse

5) the mechanical properties of the healed region improve with time (more interdiffusion and randomisation)

6) reptation model

Question 9

what are the stages of self-healing by molecular interdiffusion?

Answer 9

1) surface rearrangement
2) surface approach (chains moved to close proximity)
3) wetting (swelling allows the chains to be closer together)
4) diffusion
5) randomisation

Question 10

how does Tg affect self-healing?

Answer 10

polymer must be at the Tg for the chains to move to repair

higher above the Tg = higher thermal energy = faster healing

Question 11

how does Tg affect self-healing?

Answer 11

polymer must be above the Tg for the chains to move to repair, unless a plasticiser is included (then can be a lower Tg)

higher above the Tg = higher thermal energy = faster healing

Question 12

what is the reptation model?

Answer 12

when a chain is in a tube the molecular diffusion of a polymer chain is proportional to M^-2

the movement of the chains are constrained and they move in a snake-like fashion

short chains = faster interdiffusion, long chains = high strength recovery

Question 13

what are the factors controlling Tg?

Answer 13

1) Mw - high Mw = higher Tg bc more intermolecular interactions between longer chains
2) backbone flexibility - less flexible = higher Tg
3) pendant groups - bulky/rigid groups = higher Tg, flexible groups = lower Tg

Question 14

why do pendant groups affect Tg?

Answer 14

bulky/rigid groups = higher Tg because the groups catch on each other when chains move past

flexible groups = lower Tg bc limit how closely the chains can pack together (same as a plasticiser)

Question 15

what is a plasticiser?

Answer 15

a small molecule that gets inbetween polymer chains and spaces them out from each other - prevents close packing of chains and lowers the Tg

Question 16

what types of plastics can self-heal by molecular interdiffusion? why not the other types?

Answer 16

only occurs in themoplastics

NOT thermosets bc they are irreversibly hardened by curing

Question 17

how does viscocity affect self-healing?

Answer 17

lower viscocity upon heating gives more mobility so faster self-healing

Question 18

what are supramolecular self-healing materials?

Answer 18

rely on non-covalent, transient bonds to generate networks e.g. hydrogen bonding, pi-pi stacking or carbon-carbon

normally require many hydrogen bonds specifically

Question 19

describe what is needed for supramolecular polymers to form

Answer 19

1) the strength of the material is from the strength/stickiness of the supramolecular bonds, not covalent chain entanglement

2) the supramolecular bonds are weaker so there are many needed to have good mechanical properties, therefore chain dynamics need to be controllable

3) the chains need to be able to move for the bonds to come together (material needs low Tg)

Question 20

describe the polymer architecture of supramolecular polymers

Answer 20

1) the extension of shorter chains to longer chains

2) chain entanglements can for networks

3) crosslinking points can be formed from groups associating together

4) crosslinks can form inbetween groups and complimentary groups along the backbone

Question 21

how does supramolecular self-healing work?

Answer 21

1) upon damage the supramolecular bonds are the most likely to break compared to covalent

2) unbound supramolecular bonds are ‘sticky’ at the fractured surface

3) when recombined the supramolecular bonds reform to close the gap and heal

the supramolecular bond formation is reversible

Question 22

what does ‘sticky’/’stickiness’ refer to?

Answer 22

binding strength for supramolecular bonds

Question 23

what is a hydrogen bond donor?

Answer 23

molecule attached to the H has a highly electronegative group (N, O, F)

Question 24

what is a hydrogen bond acceptor?

Answer 24

a molecule has a lone pair of e- on a small highly electronegative atom (N, O, F)