Ion specific effects in polymer conformation

Sammanfattning: It is well known that ions affect polymers in specific ways not solely based on electric charge, usually referred to as the Hoffmeister effect or ion specific effects. Poly(N-isopropylacrylamid) (PNIPAM) is a thermosensitive polymer with a LCST at 32℃. PNIPAM is a well studied polymer due to its similarities with denaturation of proteins in aqueous solutions. Utilizing diffusion NMR this report studied the effect different Hoffmeister anion concentrations have on the configuration of pre-transitional PNIPAM. A fractionation process was developed for PNIPAM, yielding a product of about 87 000 g/mol, used for diffusion measurements. Diffusion coefficients for PNIPAM in saline solutions ranging from 0 to 800 mM were measured for NaCl, NaClO4, NaSCN and NaI. Diffusion coefficients for PNIPAM were also measured at some concentrations of NaF, Na2SO4 and Na2CO3. Hydrodynamic radius was calculated from the diffusion coefficients. The report found a pre-transitional chain collapse of PNIPAM which increased with ionic concentrations of NaCl, NaClO4, NaSCN, NaF and Na2CO3, but not for NaI and Na2SO4. At 800 mM the hydrodynamic radius decreased with 9% for NaCl, 13% for NaClO4 and 5% for NaSCN. The hydrodynamic radius decreased with 19% at 300 mM Na2CO3 and with 10% at 400 mM NaF. There was a significant decrease in hydrodynamic radius for high concentrations of NaCl and NaClO4 but exact decrease needs to be replicated to validate the findings due to an unexpected large decrease in hydrodynamic radius already at 50 mM. Values for NaF and Na2CO3 should be replicated with internal standard to accomodate for possible precipitation of the longer polymer chains within the fraction.