Phase transitions of N-isopropylacrylamide gels prepared with various crosslinker contents (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.
Journal of Applied Polymer Science, 2002
Phase separation during the formation of poly(N-isopropylacrylamide) (PNIPA) hydrogels was investigated using real-time photon transmission and temperature measurements. The hydrogels were prepared by freeradical crosslinking polymerization of N-isopropylacrylamide (NIPA) in the presence of N,NЈ-methylenebisacrylamide (BAAm) as a crosslinker in an aqueous solution. The onset reaction temperature T 0 was varied between 20 and 28 0 C. Following an induction period, all the gelation experiments resulted in exothermic reaction profiles. A temperature increase of 6.5 Ϯ 0.6 0 C was observed in the experiments. It was shown that the temperature increase during the formation and growth process of PNIPA gels is accompanied by a simultaneous decrease in the transmitted light intensities I tr . The decrease in I tr at temperatures below the lower critical solution temperature of PNIPA was explained by the concentration fluctuations due to the inhomogeneity in the gel network. At higher temperatures, it was shown that the gel system undergoes a phase transition via a spinodal decomposition process.
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.
Cogent Chemistry, 2015
Temperature-responsive poly(N-isopropylacrylamide) microgels crosslinked with a new hydrophobic chemical crosslinker were prepared by surfactant-mediated precipitation emulsion polymerization. The temperature-responsive property of the microgel and the influence of the crosslinker on the swelling behaviour was studied systematically by light scattering and small-angle X-ray scattering (SAXS). The radius of gyration (R g) and the hydrodynamic radius (R h) of the microgels decreased with increase in temperature due to the volume-phase transition from a swollen to a collapsed state. The ratio of R g /R h below the transition temperature was lower than that of hard-spheres due to the lower crosslinking density of the microgels. The SAXS data were analysed by a model in which the microgels were modelled as core-shell particles with a graded interface. The model at intermediate temperatures included a central core and a more diffuse outer layer describing pending polymer chains with a low crosslinking density. In the fully swollen state, the microgels were modelled with a single component with a broad graded surface. In the collapsed state, they were modelled as homogeneous and relatively compact particles. The polymer volume fraction inside the microgel was also derived based on the model
Kinetics of spinodal decomposition in chemically crosslinked gels
Physica A: Statistical Mechanics and its Applications, 1996
We present time-resolved small angle light scattering measurements of spinodal decomposition in a covalently crosslinked N-isopropylacrylamide gel. We find that for shallow quenches and early times the linear Cahn-Hilliard-Cook (CHC) theory can be used to describe the time evolution of the scattered intensity as well as the turbidity. Surprisingly, we find that the initial length scale on which fluctuations grow is much larger than the concentration correlation length and the diffusion constant characteristic of the phase separating gel is much slower than the collective diffusion constant. It appears that the initial process of phase separation in these gels is driven by fluctuations on micron sized regions, perhaps representing large-scale variations in the degree of swelling. We have also obtained the range of validity of the linear CHC theory, and find that the breakdown occurs earlier at higher wave-numbers and deeper quenches.
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 ...
Hydrogels based on N-isopropylmethacrylamide and N-isopropylacrylamide
Advanced Technologies, 2018
Hydrogels are three-dimensional polymer networks which have the capacity to retain a large quantity of water or biological fluids in the swollen state. Thermosensitive hydrogels have received special attention of reserachers since they represent a parameter which frequently changes in chemical, biological and physiological systems. Thermosensitive hydrogels have the critical solution temperature, i.e. they exhibit a substantial change in volume with the temperature change. Homopolymers poly(N-isopropylmethacrylamide) (poly(NIPMAM)) and poly(N-isopropylacrylamide) (poly(NIPAM)) are thermosensitive materials which have lately become the subject of intensive study. Monomer N-isopropylmethacrylamide, NIPMAM, enters into copolymerization with monomer N-isopropylacrylamide, NIPAM, in order to create a system with a phase transition temperature approximate to the human body temperature. In literature data there is available information on the synthesis and characterization of microgels, nanogels and copolymers based on NIPMAM and NIPAM. These thermosensitive polymer materials are used in controlled drug delivery and protein immobilization.
Colloid and Polymer Science, 2014
Hydrogels based on N -isopropylacrylamide and sodium acrylate as ionic comonomer were synthesized by free r a d i c a l p o l y m e r i z a t i o n i n w a t e r u s i n g N , N ′methylenebisacrylamide as crosslinker and ammonium persulfate as initiator. The glass transition of dried copolymers poly(N -isopropylacrylamide) (PNIPA) and poly(sodium acrylate) (SA) gels and demixing/mixing transition of PNIPA-SA hydrogels swollen with increasing amounts of water were studied using conventional differential scanning calorimetry. In the crosslinked polymers, the glass transition linearly increases, and the transition range becomes broader, with increasing crosslinker content. Increasing content of ionic comonomer also produces an increase of glass transition temperature, which moves to higher temperatures with higher sodium acrylate fraction. The influence of chemical structure of PNIPA-SA hydrogels on the lower critical solution temperature (LCST) of PNIPA-SA/water mixtures during heating and cooling was quantified as function of the content of the crosslinker and the ionic comonomer, as well as water content of the hydrogel in the range from 95 to 70 wt%. At parity of water content, the LCST occurs at higher temperatures for gels containing higher amounts of sodium a c r y l a t e . S i m i l a r l y, t h e i n t r o d u c t i o n o f N , N ′methylenebisacrylamide causes an increase of the LCST, which grows with increasing of crosslinking degree of the hydrogel.
International Journal of Polymer Science
In this work, a series of thermally responsive terpolymers of N-isopropylacrylamide (NIPA) with three different comonomer contents was synthesized, and their swelling behaviour was studied as a function of composition and temperature. Temperature-sensitive, random cross-linked terpolymers of NIPA were prepared from methyl methacrylate (MMA), N-tert-butylacrylamide (NTBA), and acrylic acid (AA) using a free radical polymerization method. In the synthesis of terpolymer hydrogels, N,N-methylene bisacrylamide (BIS) was used as cross-linkers and ammonium persulfate (APS) as initiator. The NIPA content of the monomer feed varied from 80 to 50 mol %, and other comonomer feed varied from 40 to 5 mol %. The swelling equilibrium of these hydrogels was studied as a function of temperature and hydrophobic and hydrophilic comonomer contents. The swelling properties of the polymers were investigated in pure water at temperatures from 10 to 80°C. All of the synthesized gels were found to be sensit...