Polymer Brushes1) Definition (Brittain, Minko Wiley review)

a) Tethered polymer with strong interactions between densely grafted polymer chains

b) Sufficient proximity so that unperturbed solution dimensions are altered in good solvent

2) Advantagesa) Compared to SAMs

i) SAMs self-assembled so difficult to get large-area defect-free monolayers

ii) Mechanically chemically fragile

b) Stimuli-responsive behavior

c) Free chain energy F = statistical segments Fint + E difference btw stretched and unstretched polymer chains Fel (Elastic free E caused by chain entropy loss)

i) Types of stimuli(1) Solvent quality

(a) Solvent quality determines size of isolated chain

(i) Flory-Huggins interaction parameter

(ii) temperature(b) Theta solvent ideal coil 1/2~N1/2 (indep of [polym])

(c) Good solvent/high [polym]

(i) Semi-dilute chain sz dec (1/23 volume fraction)

(d) Poor solvent

(i) Phase separation

1. Pure solvent

2. Concentrated polym w/ overlapping Gaussian coils

(e) Assembled polymer brushes expand in good solvents and collapse in poor solvents (drastically different than free chains in soln)

(i) Chains stretch in good solvents due to excluded volume effect compensated by polymer coil elastic energy (stretching entropy)3) Critical parameters

a) Segment density profile ((z)) of surface-attached chains and/or brush h (graft density )

b) Molecular weight/degree of polymerization of surface-attached chains

c) Solvent quality of contacting medium4) Mechanismsa) Physisorption

i) One segment attracted to surface (e.g., PEO on mica)

ii) Other block preferentially solvated

iii) Unstable under solvent and T conditions, displaced by other adsorbents

b) Grafting-to

i) Increased [polym] ( increased grafting densitiesii) Grafting density is usually low ( can regulate density with grafting time

c) Grafting-from

i) In situ polym or initiator

5) First time TBAG used for large polymers