Specific Ion Effect In Several Typical Polymer Systems

In this thesis, we will present the investigations on the specific ion effect in different polymer systems including netural PNIPAM system, negtively charged PSSNa system, and ziwtterionic PSBMA system. The main results are as follows:(1) We have systematically investigated the ion specific reentrant behavior of PNIPAM in water-methanol mixtures. Turbidity measurements demonstrate that SCN-and ClO4-depress the reentrant transition, whereas other anions enhance the transition. As the anion changes from chaotropic to kosmotropic, the minimum critical phase transition temperature (Tmin) decreases and the corresponding volume fraction of methanol (XM) shifts to a larger value. Our results demonstrate that anion specificity is due to the anionic structure making/breaking effect on water/methanol complexes. Cations are found to have a lesser but still significant effect on the reentrant transition and as Tmin decreases the corresponding XM also shifts to larger values as with the anions. Our studies show that cation specificity is induced by specific interactions between cations and PNIPAM chains. Furthermore, both anion and cation specificities are amplified as XM is increased due to the formation of additional water/methanol complexes. Calorimetry measurements demonstrate that the ion specificity is dominated by changes in entropy.(2) We have studied the salt effects on the conformational behavior of PSSNa brushes in the water-methanol mixtures using a quartz crystal microbalance with dissipation (QCM-D). As the methanol fraction increases, PSSNa brushes become more collapsed, indicating that the addition of methanol leads to a poorer solvent for PSSNa chains. When NaCl is introduced, as the salt concentration increases, the extended PSSNa brushes first shrink into an inhomogeneous structure, followed by the formation of a denser and homogeneous polymer layer with the further increase of salt concentration. When CsCl is introduced, besides abovermentioned conformational change, the PSSNa chains will be recharged again at high salt concentrations accompanied by the stretching of grafted chians. PSSNa brushes in the presence of MgCl2exhibit a similar conformational change to that in the presence of CsCl, but the reentrant behavior of conformational change becomes more obvious with the methanol fraction.(3) By use of QCM-D and surface plasmon resonance (SPR), we have systematically investigated the conformational behavior of poly(sulfobetaine methacrylate)(PSBMA) brushes as a function of ionic strength in the presence of different ions. The frequency change demonstrates that the effectiveness of anions to weaken the inter-/intrachain association and to enhance the hydration of the grafted chains increases from kosmotrope to chaotrope in the low ionic strength regime, but the ordering of anions is almost reversed at the high ionic strengths. The dissipation change indicates that some heterogeneous structures are formed inside the brushes in the presence of chaotropic anions with the increase of ionic strength. In SPR studies, the change of resonance unit (ARU) with ionic strength is determined by the balance between the increase of thickness and the decrease of refractive index of the brushes. ΔRU is insensitive to the coupled water molecules inside the brushes so that no anion specificity is observed in the SPR measurements. For the control of protein adsorption/desorption, our studies show that the brushes can more effectively resist the protein adsorption in the presence of a more chaotropic anion and a more chaotropic anion can also more effectively induce the protein desorption from the surface of the brushes.