Solution behaviors and microstructures of PNIPAm-P123-PNIPAm pentablock terpolymers in dilute and concentrated aqueous solutions

Abstract

The solution behaviors and microstructures of poly(N-isopropylacrylamide)_x-poly(ethylene oxide)₂₀-poly(propylene oxide)₇₀-poly(ethylene oxide)₂₀-poly(N-isopropylacrylamide)_x (PNIPAm_x-PEO₂₀-PPO₇₀-PEO₂₀-PNIPAm_x or PNIPAm_x-P123-PNIPAm_x) pentablock terpolymers with various PNIPAm block lengths in dilute and concentrated aqueous solutions were investigated by micro-differential scanning calorimetry (micro-DSC), static and dynamic light scattering (SLS & DLS), and synchrotron small angle X-ray scattering (SAXS). Two lower critical solution temperatures (LCSTs) were observed for PNIPAm_x-P123-PNIPAm_x pentablock terpolymers in dilute solutions, which corresponded to LCSTs of PPO and PNIPAm blocks, respectively. The LCST of PPO block shifted from 24.4 °C to 29 °C when the length x of PNIPAm block increased from 10 to 97. The LCST of PNIPAm is around 34.5 °C–35.3 °C and less dependent on the block length x. The PNIPAm_x-P123-PNIPAm_x pentablock terpolymers formed “associate” structures and micelles with hydrophobic PNIPAm and PPO blocks as cores and soluble PEO blocks as coronas in dilute aqueous solutions at 20 °C and 40 °C, respectively, regardless of the relative lengths of PNIPAm, PPO and PEO blocks. The size of “associate” structures of PNIPAm_x-P123-PNIPAm_x pentablock terpolymers at 20 °C increased with increasing the length of PNIPAm block. The microstructures of PNIPAm_x-P123-PNIPAm_x hydrogels formed in concentrated aqueous solutions (40 wt%) were strongly dependent on the environmental temperatures and relative lengths of PNIPAm, PPO and PEO blocks as revealed by SAXS. Increasing the length of PNIPAm block weakened the order structures of PNIPAm_x-P123-PNIPAm_x hydrogels. The microstructures of PNIPAm_x-P123-PNIPAm_x hydrogels changed from mixed fcc and hex structures for PNIPAm₁₀-P123-PNIPAm₁₀ to isotropic structure for PNIPAm₉₇-P123-PNIPAm₉₇. Increasing temperature led to the transition from mixed hex and fcc structure to pure hex structure for PNIPAm₁₀-P123-PNIPAm₁₀ hydrogel at temperature above the LCSTs.