Water-soluble copolymers and their hydrogels with pH-tunable diverse thermoresponsive behaviors enabled by hydrogen bonding

Abstract

Water-soluble copolymers of poly(acrylic acid-co-N-vinylcaprolactam) (PAN) and poly(acrylic acid-co-N-vinylcaprolactam-co-dimethyl acrylamide) (PAND) were synthesized and found to exhibit opposite, and pH-tunable, UCST (upper critical solution temperature) and LCST (lower critical solution...

Modulation of the Lower Critical Solution Temperature of Thermoresponsive Poly(N-vinylcaprolactam) Utilizing Hydrophilic and Hydrophobic Monomers

“…Poly- N -vinylcaprolactam (PNVCL) is a synthetic hydrogel-forming polymer. It is a non-ionic, water-soluble, and thermoresponsive polymer with the capability of transitioning from a liquid state to a gel state when exposed to temperatures above its lower critical solution temperature (LCST), which is commonly reported as being close to the physiological temperature range 34–37 °C [ 14 , 15 , 16 , 17 , 18 , 19 ]. Overall, PNVCL is an inexpensive and highly used polymer due to its low cytotoxicity, which enables the polymer to be employed in the biomedical field for applications, such as wound healing, drug delivery, and tissue engineering [ 20 ].…”

Color-Shifting Fluorescent System Based on an AIE Polymer with an Upper Critical Solution Temperature

“…Enhancing polymer–polymer hydrogen-bonding (H-bonding) is not only an effective strategy to design thermoresponsive polymers with a UCST property − but also a facile method to construct AIE polymers. , Take poly­(acryl amide) (PAAm) as an example. The chain unit of PAAm has a carbonyl group as a hydrogen-donor and an amide group as a hydrogen-acceptor.…”

Hydrogen Bonding Enables Polymer Hydrogels with pH‐Induced Reversible Dynamic Responsive Behaviors

“…The hydrogel treated in 3.0 M HCL solution displays the highest tensile stress and elongation at break, reaching about 223 kPa and 333 %, respectively, which is consistent with the lowest water content (Figure 3a) and the most compact structure (Figure 3b) of this hydrogel. It is likely that with fully protonated COOH groups, the hydrogen bonds involving AA-AA and AA-NVCL [17] enhance the cohesion between polymer chains and make the hydrogel mechanically strong and tough. By further increasing the HCl concentration above 3.0 M, the fast opaquetransparent transformation (relatively short-lived transient heterogeneous phase) somehow retain more water molecules in the hydrogel, resulting in reduced mechanical properties.…”

“…When moved to an acid bath (HCl, 5.0 M, 50 mL), the transparent hydrogel turns opaque quickly (less than 30 s), which is mainly due to the hydrogen bonds between AA-AA and AA-NVCL being enhanced with the protonation of the COOH groups at pH < pKa and the reducing water solubility of the polymer. [17] Then, the opaque hydrogel gradually regains its transparent state over a period of 16 minutes. The PA 85 N 15 -2 % hydrogel's transparency changes as a result of pH decrease, and it undergoes a time-dependent transparency return from the turbid state.…”

“…Due to the dense hydrogen bonds formed between acrylic acid (AA) and Nvinylcaprolactam (NVCL) comonomer units (Figure 1a), it is difficult to generate a hydrogel with homogeneous network structure in deionized (DI) water. [17] To address this issue, the preparation of the hydrogels consists in two steps: 1) DMSO was used to break the hydrogen bonds in order to reduce the heterogeneities during free radical polymerization with AIBN as the initiator and N,N'-methylenebis(acrylamide) (MBA) as the chemical crosslinker; 2) the organogels were then immersed into DI water to reconstruct the multi hydrogen bond interactions between active protons (COOH) and electro-negative carbonyl groups (C=O from NVCL) or AA-AA dimers (Figure 1a). Thus, highly transparent poly(AA-co-NVCL) (PAN) hydrogels under water equilibration were obtained and used to explore the pH-induced transformation from transparent to turbid and return to transparent state.…”