Interaction And Conformation Of Aqueous Poly(N-Isopropylacrylamide) (Pnipam) Star Polymers Below The Lcst

Small-angle neutron scattering (SANS) is used to measure the conformation and solution thermodynamics of low dispersity, star branched poly(N-isopropylacrylamide) (PNIPAM) in water using a newly developed form factor for star branched polymers with excluded volume, in conjunction with the random phase approximation (RPA). Star PNIPAM is synthesized using both ATRP and RAFT, allowing the terminal group and number of arms to be precisely tuned from f = 3 to 6 arms, with bromine, phenyl, and dodecane terminal moieties. SANS measurements show that both the number of arms (f) and synthetic route (i.e., ATRP or RAFT) play a dominant role in the solution behavior of PNIPAM in relation to the interaction parameters, conformation of the arms of the polymer, and clustering/association of the polymers below the LCST. Dodecaneterminated PNIPAM polymers form small, sub-20 nm globules in solution, whereas phenyl- and bromine-terminated polymers form large, micrometer-scale clusters of nearly-Gaussian polymer chains. Multiangle light scattering (MALS) is used to probe the large clusters, finding that their size increases slightly with temperature but is largely independent of terminal group chemistry.