Synthesis, Solution And Adsorption Behavior Of PNIPAM Block Copolymer

Poly(N-isopropylacrylamide)-poly(ethyleneoxide)-poly(N-isopropylacrylamide) (PNIPAM-PPO-PNIPAM) triblock copolymers with narrow molecular weight distribution were successfully synthesized by one or two steps of polymerization via atom transfer radical polymerization (ATRP), with Br-PPO-Br as macro-initiator.Micro-differential scanning calorimetry (Micro-DSC) was used to determine the thermo-sensitive behavior of PNIPAMx-PPO36-PNIPAMx triblock copolymers and PEO45-PNIPAMx diblock copolymers. It was found that the PNIPAM block with x=15 was too short to maintain its own LCST (lower critical solution temperature), so that only LCST of PPO block was observed, i.e.25℃, for PNIPAM15-PPO36-PNIPAM15 in dilute aqueous solution. For the longest PNIPAM block with x= 117, only LCST of PNIPAM block dominated, i.e.32.3 ℃. With middle length x, PNIPAM33-PPO36-PNIPAM33 and PNIPAM75-PPO36-PNIPAM75 exhibited two LCSTs, which were corresponding to LCSTs of PPO and PNIPAM blocks, respectively. While it comes to PEO-PNIPAM diblock copolymers, as the PEO block is hydrophilic, the LCST of the copolymer is relatively higher than that of PNIPAM homopolymer and the higher content of PEO block is, the higher LCST is. The chain conformations of the copolymers were then studied with static and dynamic light scattering (DLS & SLS). It was observed that "associate" structures formed in dilute aqueous solutions of PNIPAMx-PPO36-PNIPAMx triblock copolymers and PEO45-PNIPAMx diblock copolymers at low temperature (i.e.20℃), at which temperature PPO, PNIPAM and PEO segments are theoretically soluble in water. The formation of "associate" structures at low temperature could be attributed to the incompatibility between the blocks and the hydrogen bond between water and copolymers. As the temperature increased to the LCST of PPO segments, core-shell structure with hydrophobic PPO block as core and hydrophilic PNIPAM block as shell was formed in dilute aqueous solutions of PNIPAMx-PPO36-PNIPAMx triblock copolymers. When the temperature increased further above the LCST of PNIPAM segments, the shell shrink to form a hard sphere structure. It is different for PEC45-PNIPAMx diblock copolymers. A stable core-shell structure can be observed at the temperature above its LCST.The effects of chain conformations and block length x on the adsorption behaviors of PNIPAMx-PPO36-PNIPAMx triblock copolymers and PEO45-PNIPAMX diblock copolymers on dodecanethiol-modified gold surfaces were then studied at various temperatures by using a quartz crystal microbalance (QCM) technique. QCM results indicated that the adsorption time constant decreased with increasing adsorption temperature but tended to increase with increasing the length x of PNIPAM block. A complex adsorption behavior with large adsorption amounts was only observed at the corresponding LCST of PNIPAM block for PNIPAMx-PPO36-PNIPAMx with longer PNIPAM blocks with x=33,75, and 117, and PEO45-PNIPAMx with PNIPAM blocks with x=50, and 87. Comparing to our previous study, that a complex adsorption behavior with large adsorption amounts was also observed at the LCST of PNIPAM block for PNIPAM-PEO-PPO-PEO-PNIPAM pentablock terpolymer, it can be concluded that the presence of PNIPAM block is the key reason accounting for the complex adsorption. Furthermore, such complex adsorption behavior could be only observed when the length of PNIPAM block is long enough to maintain its own LCST.