Synthesis of poly(N-isopropylacrylamide)-based star-block and dendritic copolymers and their thermal and pH dual-responsive behaviors
Date of Issue2013
School of Materials Science and Engineering
Thermal responsive polymers have aroused extensive research interest due to their potential applications in areas such as controlled drug release and gene delivery, molecular switch and separation membranes. Poly (N-isopropylacrylamide) (PNIPAm) is probably the most widely investigated thermal responsive polymer. PNIPAm can be dissolved in aqueous solution at low temperature and exhibit an inverse soluble-insoluble phase transition at ca. 34 ˚C, well-known as the lower critical solution temperature (LCST) or cloud point (Tc). At the transition the PNIPAm chains change from extended random coils to collapsed globules, expelling water and small molecules. The transition temperature of PNIPAm and its copolymers is of great importance for scientific research and technical applications and is affected by many factors such as solution’s ionic strength, molecular weight, polymer architecture, hydrophilicity of the copolymer and the end groups, etc. This work is focused on synthesis of PNIPAm-based star-block and dendritic copolymers and study of the effects of molecular architecture and chain length on the solution phase transition behaviors of the copolymers. Star-shaped PNIPAm with short arms was successfully synthesized by “core first” atom transfer radical polymerization (ATRP) using a multi-functional polyhedral oligomeric silsesquioxane (POSS) initiator. The star architecture of POSS-PNIPAm was verified by molecular weight analysis of the star polymers and the corresponding linear arms produced by hydrolysis. POSS-PNIPAm exhibits Tc which is found to be much lower than that of the linear counterpart having similar chain length. It could be attributed to the high local chain density in the near-core region that significantly increased intra-molecular interaction among the neighbour arms. On the other hand, such interactions are inhibited in solutions of low molecular weight linear PNIPAm that adopts a rigid rod-like conformation when their lengths are close to the persistent length. To reveal the influence of the high local chain density in the near-core region on temperature and pH dual-response behavior of PNIPAm-based copolymers, short blocks of pH sensitive poly(acrylic acid) (PAAc) are inserted between the POSS core and the PNIPAm block via sequential steps of ATRP. By changing the length of PAAc block and pH of the solution, the hydrophilicity of the molecules and intra-molecular interaction can be manipulated. It is found that when PAAc block is very short, Tc of the star-block copolymers varies in a wide temperature range while when PAAc block is relatively long the phase transition is broad and the Tc had almost no response to pH. The results suggest that with high local chain density in the near-core region, the intra-molecular interaction is significantly enhanced between PAAc and PNIPAm blocks to lower the Tc, whereas the synergic effect is weakened when the PAAc block is longer. To improve biocompatibility of temperature and pH dual-responsive polymers, dendritic POSS-PNIPAm with a peripheral poly(2-hydroxyethyl methacrylate) (PHEMA) layer are successfully synthesized via the combination of ATRP and click reaction. The dendritic architecture is constructed by clicking between POSS-PNIPAm and Y-shape PNIPAm or PHEMA precursors. The dendrimers with a peripheral layer of short PHEMA chains (DN2H) is found to be temperature and pH sensitive. Its Tc decreases with decreasing pH from 10.0 to 5.0, whereas exhibits a small increase from pH 5.0 to 4.0 due to better solvation of PHEMA at highly acidic condition. Dynamic light scattering studies reveal that DN2H forms large aggregations at high pH values, while at pH 4.0 it has a mono-dispersed distribution with Rh of about 5 nm. The controlled release experiments at different temperature and pH demonstrate that dendritic PNIPAm-PHEMA is a promising candidate for thermal and pH dual responsive nanocarrier.