Temperature‐Sensitive Simultaneous Interpenetrating Polymeric Networks With Improved Mechanical Properties and Shrinking Kinetics

Abstract

Hydrogels are hydrophilic, three-dimensional polymeric networks, which can absorb and retain a large amount of water, but do not dissolve in it due to the existence of crosslinking points. [1,2] In the past three decades, stimuli-responsive hydrogels, which exhibit drastic volume phase transitions in response to environmental changes, have been considerably investigated. [1][2][3][4] Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels undergo sharp swelling/deswelling behavior at around 32-34 8C, which is called lower critical solution temperature (LCST). Due to the unique property, PNIPAAm-related hydrogels have been developed to design different biodevices, such as drug release systems, biosensors, matrix for tissue engineering, etc. [5][6][7][8][9][10] Conventional PNIPAAm hydrogels have certain limitations, such as a slow deswelling rate when changing the temperature from below to above LCST due to the formation of a thick, dense layer on the surface region, which prevents the freed water molecules from diffusing out. In some applications, such as controlled release systems and sensors, fast responsive properties are critically important. Many approaches, such as the phase separation technique, [11][12][13][14] and template polymerization [15,16] have been explored to prepare fast responsive PNIPAAm hydrogels with porous microstructures. These porous structures are able to destroy the dense surface layer and allow water molecules to diffuse out freely, so these PNIPAAm hydrogels exhibited fast responsive properties. However, due to the porous and heterogeneous structures, the polymer fractions of these networks are very low and the water swelling ratios (SRs) are high, leading to poor mechanical strength, which made it difficult to handle these materials. For example, when the hydrogels are used as an implant drug delivery device, the materials need to be implanted as well as removed once the release is finished. If these materials were too soft and easy to break during handling, it would be difficult to be completely removed through traditional surgical procedures. [5] There exists a need to improve the mechanical properties of the hydrogels.Much attention has been dedicated to the enhancement of the mechanical properties of PNIPAAm hydrogels. Introducing another polymeric network into PNIPAAm hydrogel to form interpenetrating polymeric networks (IPN) is also a promising and exciting choice to obtain improved properties as compared to the conventional PNIPAAm hydrogel. [17][18][19][20][21][22] According to the synthesis process, there are two different types: sequential IPN and simultaneous IPN. [18] For sequential IPN, one network is firstly obtained and then swollen with a second cross-linking system consisting of monomer(s), cross-linker and initiator, which is subsequently polymerized within the first one. In the case of simultaneous IPN, precursors or monomers for both kinds of polymeric networks are mixed together with their specific cross-linking agents and initiators, and then the resulting mixture is c...

Reaction-Induced Self-Assembly of Gel Structure: A New Insight into Chemical Gelation Process of N-Isopropylacrylamide as Studied by Two-Dimensional Infrared Correlation Spectroscopy

“…Finally, the local environments of the monomers in the vicinity of copolymer chains change as follows. (4) The dehydration NH band of monomers changes to (5) hydration NH band of the monomers, which causes the sequence of time change in the peak intensities given by 1556 → 1589 → 1572 cm −1 .…”

“…This suggests in turn the following four fundamental events to occur, which are designated as events 5−8 in the order of the increasing time, continuously to events 1−4 occurring in the first-stage reaction process. (5) The intensity change of the band at 1629 cm −1 reveals formation of the cross-linked, dehydrated network chains (Table 1). However, it would be very important to note that the cross-linking occurs within the beginning of the first-stage reaction, while they are selected in the beginning of the second-stage reaction.…”

“…Thus, the collision-controlled reactivity in the former may be much higher than that in the latter, which may account for the difference of the monomer selectivity for the reaction. This event (5) creates microgels that are expected to coexist with the hydrated droplets newly created as well as newly created globules with their concentration smaller than the critical concentration, as shown in Scheme 1e.…”

“…Poly­( N -isopropylacrylamide) (PNiPAAm) is a temperature-sensitive polymer in aqueous solutions, and hence, it forms a thermo-responsive hydrogel, when the monomer (NiPAAm) reacts with N,N ′-methylenebis­(acrylamide) (BIS) in an aqueous reaction medium. − The PNiPAAm hydrogel is hydrophilic and swollen in water below the lower critical solution temperature (LCST, T LCST ) of aqueous PNiPAAm solution. By contrast, above the T LCST , the PNiPAAm hydrogel becomes hydrophobic and shrunken via the volume phase transition. − The PNiPAAm hydrogel is one of the most promising materials in biotechnology as well as in pharmaceutical and industrial applications because of its reversible volume phase transition triggered by external stimuli such as temperature, pH, solvent composition, magnetic field, and so on. − Moreover, the characteristics of the hydrogels, such as turbidity, elasticity, the amount of the volume change upon the volume phase transition, and so on, dramatically change, when the preparation conditions of gels slightly change, e.g., when the preparation temperature, T p , varies only a few degree Celsius around T LSCT .…”