Real-time temperature and photon transmission measurements for monitoring phase separation during the formation of poly(N-isopropylacrylamide) gels (original) (raw)
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The Effect of Preparation Temperature on Phase Transitions of N -Isopropylacrylamide Gel
Phase Transitions, 2003
Temperature sensitive N-isopropylacrylamide (NIPA) hydrogels were prepared by free-radical crosslinking polymerization of NIPA in the presence of various N,N -methylenebisacrylamide (Bis) as a crosslinker in aqueous solution at 22 • C. Photon transmission technique at 550 nm wavelength was used to study the spinodal decomposition of these NIPA gels prepared with various molar concentrations of crosslinker (Bis). During the phase transition experiments temperatures of the gel samples increased from 20 to 40 • C with 0.4 • C min −1 rate. It was observed that the increase in turbidity is much faster in the gel prepared at lower Bis content than higher Bis content sample which indicated that NIPA-water system reaches the spinodal decomposition much faster for the gel prepared with low Bis content. It is understood that high Bis content NIPA gel posses more heterogeneities which are gained during gelation, present low spinodal temperature, T s . However, NIPA gels prepared with lower Bis content go to spinodal decomposition at higher T s values.
Phase transitions of N-isopropylacrylamide gels prepared with various crosslinker contents
Materials Chemistry and Physics, 2003
Temperature sensitive N-isopropylacrylamide (NIPA) hydrogels were prepared by free-radical crosslinking polymerization of NIPA in the presence of various N,N -methylenebisacrylamide (Bis) as a crosslinker in aqueous solution at 22 • C. Photon transmission technique at 550 nm wavelength was used to study the spinodal decomposition of these NIPA gels prepared with various molar concentrations of crosslinker (Bis). During the phase transition experiments temperatures of the gel samples increased from 20 to 40 • C with 0.4 • C min −1 rate. It was observed that the increase in turbidity is much faster in the gel prepared at lower Bis content than higher Bis content sample which indicated that NIPA-water system reaches the spinodal decomposition much faster for the gel prepared with low Bis content. It is understood that high Bis content NIPA gel posses more heterogeneities which are gained during gelation, present low spinodal temperature, T s . However, NIPA gels prepared with lower Bis content go to spinodal decomposition at higher T s values.
2013
The first part of the thesis aims at a careful inspection of the influence of temperature on the microstructure of PNIPA gels synthesized by free radical cross-linking copolymerization, FRC, in the range 10-27.5 °C, i.e. below LCST by means of static and dynamic light scattering methods. The total scattering intensity was divided into two parts: thermal scattering due to Brownian motion and static scattering due to topological or spatial inhomogeneity. Different methods of data evaluation, i.e. the non-ergodic and the partial heterodyne approaches, were carefully compared. Consistent results were obtained clearly demonstrating that both parts of the scattering rise markedly upon increasing the observation temperature from 10 to 27.5 °C thus approaching the lower critical solution temperature. While the temperature dependence of the thermal scattering component is well understood, the rise of the static component was attributed to the establishment of local swelling equilibrium in a ...
Journal of Polymer Science Part B: Polymer Physics, 2000
Linear and crosslinked polymers based on N-isopropylacrylamide (NIPAAm) exhibit unusual thermal properties. Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAAm) phase-separate upon heating above a lower critical solution temperature (LCST), whereas related hydrogels undergo a swelling-shrinking transition at an LCST. A linear copolymer made of NIPAAm/acryloxysuccinimide (98/2 mol/mol) and two hydrogels with different hydrophilicities were prepared. Fourier transform infrared (FTIR) spectroscopy was employed to determine the transition temperature and provide insights into the molecular details of the transition via probing of characteristic bands as a function of temperature. The FTIR spectroscopy method described here allowed the determination of the transition temperature for both the linear and crosslinked polymers. The transition temperatures for PNIPAAm and the gel resulting from the crosslinking with polylysine or N,NЈ-methylenebisacrylamide (MBA) were in the same range, 30-35°C. For the gels, the transition temperature increased with the hydrophilicity of the polymer matrix. The spectral changes observed at the LCST were similar for the free chains and the hydrogels, implying a similar molecular reorganization during the transition. The COH stretching region suggests that the N-isopropyl groups and the backbone both underwent conformational changes and became more ordered upon heating above the LCST. An analysis of the amide I band suggests that the amide groups of the linear polymer were mainly involved in hydrogen bonding with water molecules below the LCST, the chain being flexible and disordered in a water solution. During the transition, around 20% of these intermolecular hydrogen bonds between the polymer and water were broken and replaced by intramolecular hydrogen bonds. Similar changes were also observed at the LCST of a gel crosslinked with MBA.
Phase Separation Induced Mechanical Transition of Poly(N-isopropylacrylamide)/Water Isochore Gels
Macromolecules, 1994
Poly(N4sopropylacrylamide) (NIPAAm) gels immersed in water (isobar gels) are known to undergo a volume phase transition at T, 2 34 O C. This transition, due to the release of hydrophobic hydration, leads to dramatic changes in physical properties, such as mechanical, optical, and thermal properties. We dealt with NIPAAm isochore gels, where the gels were isolated from water to keep the gel concentration, and investigated the mechanical and thermal properties of the gels as functions of temperature, cross-link density, and polymer concentration. The dynamic storage modulus, E', increased suddenly at T , by a factor of 10 or more and became frequency dependent with increasing temperature. The enthalpy change related to the dissociation of the hydrophobic hydration was obtained as a function of the gel concentration. It was found that about 13 water molecules were released to dehydrate one NIPAAm molecule. Attenuated total reflection (ATR) infrared absorption spectroscopy was also conducted to elucidate the local environment of the NIPAAm gel network.
Journal of Applied Polymer Science, 2001
Photon transmission experiments were performed using a UV±visible (UVV) spectrometer during the swelling of polyacrylamide (PAAm) gels. These gels were prepared from acrylamide (AAm) with various N,N 0 -methylenebisacrylamide (Bis) contents by free-radical crosslinking copolymerization (FCC) in water and dried before use for swelling experiments. Transmitted light intensity, I tr increased at very early times when PAAm gels are immersed in water, then decreased continuously as swelling time is increased. Decrease in I tr was attributed to the increase in the scattered light intensity, I sc which may originate from the contrast between`frozen blob clusters' and holes in the swelling gel. Decrease in I tr was modelled using the Li±Tanaka equation from which time constants, t 1 , and collective diffusion coef®cients, D o were determined for various Bis content PAAm gels. t 1 and D o were found to be strongly correlated with the wavelength, l, of I tr and the Bis content of the gel samples. The correlation between l and t 1 and D o predicts the existence of`frozen blob clusters' in PAAm gels. q
Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study
Macromolecular Symposia, 2009
Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems, depending on monomer concentration. In addition to pure hydrogels, the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. N,N 0 -methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine (trisodium 8-hydroxypyrene-1,3,6-trisulfonate acid, HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization, thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phase transition without disturbing the system mechanically and to test the universality of the sol-gel transition as a function of polymer concentration ratios. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent b obeyed the percolation result.
Phase separation of poly(N-isopropylacrylamide) solutions and gels using a near infrared fiber laser
Journal of Applied Polymer Science, 2007
We used a tunable near infrared fiber laser in the wavelength range of 1533-1573 nm to induce photothermal phase changes in poly(N-isopropylacrylamide) (PNIPAM) aqueous solutions and hydrogels. The laser induces the hydrophilic to hydrophobic transition of the polymer by heating the surrounding water. We report our observations of the phase changes based on using a novel fiber backreflectance method coupled with visual cloud point measurements. At 1533 nm the phase transition was induced at the end of a single mode optical fiber with 9 mW of power at an ambient temperature of 248C. We found that the power required to reach the lower critical solution temperature was inversely proportional to the absorption spectrum of water. In addition, phase changes in both solutions and gels occurred very rapidly (<1 s) as the laser was turned on and off.
Dependence of shrinking kinetics of poly(N-isopropylacrylamide) gels on preparation temperature
Polymer, 2002
Preparation temperature dependence of equilibrium swelling degree and shrinking kinetics of poly(N-isopropylacrylamide) gel has been investigated by optical microscopic measurements. The degree of swelling, d/d 0 , at 20 8C was found to be strongly dependent on the preparation temperature, T prep , where d and d 0 are the diameter of gel during observation and preparation, respectively. The value of d/d 0 was about 1.2 for T prep 20 8C, but steeply increased by approaching the phase separation temperature < 32.0 8C. Above 32.0 8C, d/d 0 decreases stepwise to 1.46. This upturn in d/d 0 was correlated with spatial inhomogeneities in gels. That is, the gel became opaque by increasing T prep. Though the shrinking half-time, t 1/2 , of gel was on the order of 500 min for T prep # 20 8C, t 1/2 decreased to 2 min for T prep $ 26 8C. Hence, a rapid shrinking was attained by simply increasing T prep. The physical implication of this rapid shrinking in gels was discussed in conjunction with the gel inhomogeneities and a thermodynamic theory of swelling equilibrium.