Structures and interactions in collapsed hydrogels of thermoresponsive interpenetrating polymer networks

Abstract

Temperature-induced collapse of hydrogels of interpenetrating polymer networks (IPNs) poly(Nvi nyl cap ro lac t am) / po ly( N -i sop ro pyl ac ryla mide ) (PVCL/PNIPAm) and poly(N-isopropylmethacrylamide) (PNIPMAm)/PNIPAm, where both components are thermoresponsive, was studied by combination of 1 H nuclear magnetic resonance (NMR) spectroscopy, small-angle neutron scattering (SANS), differential scanning calorimetry (DSC), and dynamic mechanical measurements. Behavior of studied hydrogels (one or two transitions) was found to depend on the ratio of both IPN components. For hydrogels of IPNs containing around 50 mol% of PNIPAm monomer units, separate transitions were revealed for both components. From SANS curves, it follows that compact three-dimensional multi-chain globules are formed in PNIPMAm/PNIPAm and PVCL/PNIPAm IPN hydrogels at temperatures above the phase transition, with a gyration radius of 14-28 nm. A certain portion of spatially restricted bound water (HDO) was established for all the studied IPNs at temperature above the volume phase transition from measurements of 1 H NMR spectra, spin-spin relaxation times (T 2 ), and diffusion coefficients (D) of HDO. Slow exchange regime between bound and free water was revealed. Spin-spin relaxation times (T 2 ) and diffusion coefficients (D) as obtained for the bound HDO are up to 2 orders of magnitude smaller in comparison with "free" HDO. Higher content of bound water as found for collapsed hydrogels of IPN PVCL/PNIPAm in comparison with PNIPMAm/PNIPAm hydrogels is in accordance with swelling experiments and lower values of the shear mechanical modulus; this shows the decisive role of bound water in this respect.

Poly(N-vinylcaprolactam), a comprehensive review on a thermoresponsive polymer becoming popular

“…The preparation of double thermosensitive IPNs was attempted by sequential thermal and photochemical crosslinling polymerizations. PNVCL (thermal) and PNIPAAm (photochemical) IPN´s were obtained using ethylidene-bis-3(N-vinyl-2-pyrrolidone) (EBVP) and N,N´-methylene-bisacrylamide (BIS) as crosslinkers respectively in each step[151]. However, results showed that by DSC only one transition temperature was observed for these IPNs, allegedly because the LCST of the single chains of PNIPAAm and PNVCL are very similar.…”

Hydrogels based on N-isopropylmethacrylamide and N-isopropylacrylamide

“…IPN poly(NIPMAM)-co-poly(NIPAM) was examined by Hanyková et al IPN network was synthesized through swelling of poly(NIPMAM) network in the ethanol solution of NIPAM monomer, in the presence of photoinitiator Darocur 1173 and BIS cross-linker, in the refrigerator without the presence of light and air. On the basis of the research findings, it was concluded that the behavior of IPN depends on the ratio of both components [51,83]. Double temperature sensitive core-shell microgels based on poly(NIPMAM) and poly(NIPAM) were synthesized through emulsion polymerization.…”

“…LCST of poly(NIPMAM-co-NIPAM) copolymer in water was examined by the combination of DSC method and the measurement of turbidity of the solution [80]. IPN poly(NIPMAM)-co-poly(NIPAM) were characterized by the 1 H NMR method, DSC method, dynamic mechanical spectroscopy [51,83], optical microscopy [83], SANS [51] in order to determine the IPN contraction inducing temperature [51,83]. In order to take advantage of specific properties of hydrogel due to their application in various fields, it is essential that we know their structure.…”

“…Abrupt macromolecular transition in poly(NIPAM) from hydrophilic to hydrophobic structure occurs as a consequence of LCST [47]. Poly(NIPAM) aqueous solutions have LCST at approximately 32 °C [49][50][51][52][53]. The chemical structure of poly(NIPAM) is shown in Figure 2.…”

Phase Transition in Hydrogels of Thermoresponsive Interpenetrating Polymer Networks

“…There are virtually no studies of IPNs where both chemically different networks are thermoresponsive, but each at different temperature. Recently we studied collapse phase transition in hydrogels of IPNs poly( N‐ isopropylmethacrylamide) (PNIPMAm)/PNIPAm and poly( N ‐vinylcaprolactam) (PVCL)/PNIPAm, i.e., IPNs where both components are thermoresponsive, by combination of NMR, DSC, small‐angle neutron scattering (SANS), dynamic mechanical measurements and swelling measurements . For neat PNIPAm and PNIPMAm homopolymers LCST ≅307 K and 316 K, respectively; for neat PVCL homopolymer LCST ≈303–311 K depending on the concentration and molecular weight .…”