Question

Is there any trend between polymer structure and diffusion of a gas through the polymer? Rationalize your answer with 3 key points/facts/ideas.

Answer 1

Solution:

(1) Diffusion of a gas through polymer depends on nature of polymer like as segental mobility of polymer chain, and free volume vailable within polymer. The segmental mobility of polymer is depend on degree of crosslinking, extent of unsaturation, nature of subtituent and degree of crystallinity. It has been noted that transport properties of polymers got effected by glass transition temperature. Polymers having low glass transition temperatures shows greater segmental mobility of polymer chains and will have higher diffusivity. This can be viewed by given figure of trend of diffusion coefficient D₀ for benzene with the glass transition (Tg) of the diffusion medium: PBD (cis polybutadiene) ;NR (natural rubber); PEA (polyethylacrylate) ;PMA (polymethylacrylate) ;PVA (poly7vinylalco-hol) ;EP.(ethylene propylene copolymer) ;PPA (polypropylacrylate) ;PPr (polypropylene) ;PBMA (polybutyl-methacrylate) ;PIB (polyisobutene).

(2) Permeability: As the cohesive energy of the polymer increases due to increase of polarity or change of functional group the permeability of gas which intract with functional groups connected in a polymer can be tend to decrease. For example, In a vinyl polymer when there is an increase in the polarity of the substituent group, oxygen gas permeability is decreased by almost 50,000 times.

(3) Plasticizer effect: While manufacturing polymers if plasticizers (low solubility coefficient) were added then it increases the gas diffusion by increasing the segmental mobility. For example: By research it has been reported that the diffusion coefficients for the transport of neon and hydrogen through PVC (polyvinyl chloride) after addition of tricresyl phosphate as a plasticiser were increased.

Example: If we take example of diffusion of gas through rubbery polymer then we can see that diffusion decreases with penetrant size and solubility rises with penetrant size.

Similar trends were noted for different polymers such as:

Trends for glassy polymers are different than rubbery (described above) due to dual mode adsorption mechanism.