Thermogelling Behaviors of Aqueous Poly(N-Isopropylacrylamide-co-2-Hydroxyethyl Methacrylate) Microgel-Silica Nanoparticle Composite Dispersions

Gelation behaviors of hydrogels have provided an outlook for the development of stimuli-responsive functional materials. Of these materials, the thermogelling behavior of poly(N-isopropylacrylamide) (p(NiPAm))-based microgels exhibits a unique, reverse sol–gel transition by bulk aggregation of microgels at the lower critical solution temperature (LCST). Despite its unique phase transition behaviors, the application of this material has been largely limited to the biomedical field, and the bulk gelation behavior of microgels in the presence of colloidal additives is still open for scrutinization. Here, we provide an in-depth investigation of the unique thermogelling behaviors of p(NiPAm)-based microgels through poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) microgel (p(NiPAm-co-HEMA))–silica nanoparticle composite to expand the application possibilities of the microgel system. Thermogelling behaviors of p(NiPAm-co-HEMA) microgel with different molar ratios of N-isopropylacrylamide (NiPAm) and 2-hydroxyethyl methacrylate (HEMA), their colloidal stability under various microgel concentrations, and the ionic strength of these aqueous solutions were investigated. In addition, sol–gel transition behaviors of various p(NiPAm-co-HEMA) microgel systems were compared by analyzing their rheological properties. Finally, we incorporated silica nanoparticles to the microgel system and investigated the thermogelling behaviors of the microgel–nanoparticle composite system. The composite system exhibited consistent thermogelling behaviors in moderate conditions, which was confirmed by an optical microscope. The composite demonstrated enhanced mechanical strength at gel state, which was confirmed by analyzing rheological properties.

Equilibrium swelling of thermo-responsive copolymer microgels

Thermo-responsive (TR) hydrogels with a lower critical solution temperature swell strongly at temperatures below their volume phase transition temperature T_c and collapse above T_c. Biomedical application of these materials requires tuning the critical temperature in a rather wide interval. A facile method for modulation of T_c is to polymerize the basic monomers with hydrophilic or hydrophobic comonomers. Although the effectiveness of this method has been confirmed by experimental data, molar fractions of comonomers necessary for fine tuning of T_c in macroscopic gels and microgels are unknown. A simple model is developed for the equilibrium swelling of TR copolymer gels. Its adjustable parameters are found by fitting swelling diagrams on several macro- and microgels with N-isopropylacrylamide as a basic monomer. Good agreement is demonstrated between the experimental swelling curves and results of numerical analysis. An explicit expression is derived for the volume phase transition temperature as a function of molar fraction of comonomers. The ability of this relation to predict the critical temperature is confirmed by comparison with observations.

A model is developed for equilibrium swelling of thermo-responsive copolymer gels and is applied to predict the effect of molar fraction of comonomers on the volume phase transition temperature of macroscopic gels and microgels.

Temperature-Induced Mechanomodulation of Interpenetrating Networks of Star Poly(ethylene glycol)-Heparin and Poly(N-isopropylacrylamide)

Thermoresponsive interpenetrating networks (IPNs) were prepared by sequential synthesis of a biohybrid network of star-shaped poly(ethylene glycol) [starPEG] and heparin and a poly(N-isopropylacrylamide)-polymer network. Amide bond formation was used for cross-linking of the starPEG-heparin network and photo-cross-linking with N,N'-methylenebis(acrylamide) was applied for the formation of the second polymer network. Both networks were linked by chain entanglements and hydrogen bonds only. The obtained sequential IPNs (seq-IPNs) showed temperature-dependent network properties as reflected by swelling and elasticity data as well as by the release of glycosaminoglycan-binding growth factors. The elastic modulus of the seq-IPNs was found to be amplified up to 50-fold upon temperature change from 22 to 37 °C compared to the intrinsic elastic moduli of the two combined networks. The heparin concentration (as well as the complexation of growth factors with the hydrogel-contained heparin) was demonstrated to be variably independent from the mechanical properties (elastic moduli) of the hydrogels. Illustrating the usability of the developed seq-IPN platform for cell fate control, the thermo-modulation of the release of vascular endothelial growth factor (VEGF) and bone morphogenetic protein 2 (BMP-2) is shown as well as the osteogenic differentiation of human mesenchymal stem cells exposed to stiff and BMP-2 releasing seq-IPNs.

Exploration of the unusual two-step volume phase transition of the poly(N-vinylcaprolactam-co-hydroxyethyl methacrylate) hydrogel

It is important to investigate the phase transition mechanism of stimuli-sensitive hydrogels due to its great guiding significance for the application of stimuli-sensitive hydrogels in biomedical applications.

The role of unique spatial structure in the volume phase transition behavior of poly(N-isopropylacrylamide)-based interpenetrating polymer network microgels including a thermosensitive poly(ionic liquid)

By comparing with the linear homopolymer mixture, the influence of spatial structure on the phase behavior of thermosensitive interpenetrating polymer network (IPN) microgels was clarified.

Process analytical approaches for the coil-to-globule transition of poly(N-isopropylacrylamide) in a concentrated aqueous suspension

The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM) microgel particles suspended in water has been investigated in situ as a function of heating and cooling rate with four optical process analytical technologies (PAT), sensitive to structural changes of the polymer. Photon Density Wave (PDW) spectroscopy, Focused Beam Reflectance Measurements (FBRM), turbidity measurements, and Particle Vision Microscope (PVM) measurements are found to be powerful tools for the monitoring of the temperature-dependent transition of such thermo-responsive polymers. These in-line technologies allow for monitoring of either the reduced scattering coefficient and the absorption coefficient, the chord length distribution, the reflected intensities, or the relative backscatter index via in-process imaging, respectively. Varying heating and cooling rates result in rate-dependent lower critical solution temperatures (LCST), with different impact of cooling and heating. Particularly, the data obtained by PDW spectroscopy can be used to estimate the thermodynamic transition temperature of PNIPAM for infinitesimal heating or cooling rates. In addition, an inverse hysteresis and a reversible building of micrometer-sized agglomerates are observed for the PNIPAM transition process.

Micro-dynamics mechanism of the phase transition behavior of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) hydrogels revealed by two-dimensional correlation spectroscopy

The micro-dynamics mechanism of the volume phase transition of PNIPAM-co-HEMA hydrogels was established using temperature-dependent FTIR spectroscopy, PCMW2D, and 2DCOS analysis.

Influence of the structure on the collapse of poly(N-isopropylacrylamide)-based microgels: an insight by quantitative dielectric analysis

The collapse of poly(N-isopropylacrylamide)/poly(acrylic acid) semi-interpenetrating polymer network (PNIPAM/PAA SIPN) and poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-co-AA)) microgel suspensions is studied by dielectric spectroscopy in a frequency range from 40 Hz to 110 MHz as a function of temperature. Dielectric measurements show that the structure affects the relaxation behavior of microgels: two relaxations (micro-Brownian motion and interfacial polarization at low frequency and counterion polarization at high frequency) are observed in the SIPN microgel whose charges mainly exist in domains and one relaxation (interfacial polarization) is observed in the copolymer microgel whose charges distribute in the whole network. A dielectric model is proposed to describe the collapsed microgel suspensions, from which some parameters, such as the volume fraction and the permittivity of microgels, were calculated using Hanai's equation. The temperature dependencies of these parameters show that the SIPN microgel has better low-temperature swelling properties and thermal responsiveness. This is caused by different polymer-solvent and electrostatic repulsion interactions in different microgels. Compared with pure PNIPAM, the relationship of volume phase transition temperature (VPTT) is VPTTP(NIPAM-co-AA) > VPTTPNIPAM/PAA SIPN > VPTTPNIPAM, and it is explained from the viewpoint of interaction. Besides, the activation energy data prove that the structure influences the electrical properties of microgels, which is consistent with the results obtained from quantitative dielectric analysis.

Two-dimensional correlation infrared spectroscopy reveals the detailed molecular movements during the crystallization of poly(ethylene-co-vinyl alcohol)

The key role of hydrogen bonding during the crystallization of EVOH was elucidated from the enthalpy and molecular movements.

Volume phase transition mechanism of poly[oligo(ethylene glycol)methacrylate] based thermo-responsive microgels with poly(ionic liquid) cross-linkers

Thermodynamic volume phase transition mechanisms of poly[oligo(ethylene glycol)methacrylate] (POEGMA) microgels with poly(ionic liquid) (PIL) cross-linking moieties were investigated in detail on the basis of Fourier transform infrared (FTIR) spectroscopy.

Hydrogen bond breaking of TPU upon heating: understanding from the viewpoints of molecular movements and enthalpy

Hydrogen bond breaking of TPU based on 4,4′-methylenediphenyl diisocyanate (MDI)/1,4-butanediol (BDO) upon heating was studied and elucidated from molecular movements and enthalpy.