Volume Transition Temperature Research Articles

The effects of interaction energy on the volume phase transition of N-isopropylacrylamide-co-N-isopropylmethacrylamide nano-sized gel particles: Applicability of molecular simulation technique

Abstract We investigated the volume phase transition temperatures of N -isopropylmethacrylamide (NIPMA)-co- N -isopropylacrylamide (NIPA) gels with various mole ratios. Nanometer-sized NIPA-co-NIPMA gel particles were prepared by precipitation polymerization and their swelling behaviors were measured using photon correlation spectroscopy (PCS). After applying the interaction energies found using the molecular simulation technique, we predicted the swelling equilibria for the hydrogels in a water system. For studying cross-linked hydrogel particles in water, we have combined the modified double-lattice theory with Flory and Erman's theory of elasticity. To correct for the deviation of the volume transition temperature, we considered the additional energy parameter ( ɛ h ) between the cross-linker and solvent molecules. We used the corrected model to describe the swelling behavior of the hydrogel and the volume transition temperature. The corrected model was agreed well with their experimental data with no fitting parameters.

Study of optical band gap and FTIR spectroscopy of Li 2O·Bi 2O 3·P 2O 5 glasses

Glasses having composition (25 + x)Li 2O·(65 − x)P 2O 5·10Bi 2O 3 (0 ≤ x ≤ 25 mol%) were prepared by normal melt quenching technique. The optical absorption spectra of the polished samples were recorded in spectral range from 200 to 3300 nm at room temperature. The optical absorption edge ( λ cutoff), optical band gap ( E g) and Urbach energy (Δ E) were determined from the absorption spectra. Variation in these optical parameters has been associated with the structural changes occurring in these glasses with increase in Li 2O:P 2O 5 ratio. Variation in density ( D), molar volume ( V M) and glass transition temperature ( T g) has also been correlated with structural changes in the glass samples. The FTIR spectra have been recorded in the spectral range from 400 to 1400 cm −1. The FTIR spectra show the rapid depolymerization of phosphate chains with increase in Li 2O content and P–O–Bi bonds are formed.

Effect of Li 2O on structure and optical properties of lithium bismosilicate glasses

Bismuth–silicate glasses containing lithium oxide having composition xLi 2O·(85 − x)Bi 2O 3·15SiO 2 (5 ≤ x ≤ 45 mol%) were prepared by melt quench technique. Density, molar volume and glass transition temperature for all the glass samples were measured. IR spectroscopy was used for structural studies of these glasses in the range from 400 to 1400 cm −1. The increase of Li 2O content in glass matrix results in the decrease of the Si–O–Si bond angle and increase in the covalence nature of Bi–O bond. IR spectra suggest the presence of distorted [BiO 6] octahedral units and the degree of distortion increases with the addition of Li 2O in these glasses. The optical transmission spectra in the wavelength range from 200 to 3300 nm were recorded and optical band gap ( E g) was calculated. The values of E g lie in between 2.81 and 2.98 eV. The values of average electronic oxide polarizability as well as optical basicity in these glasses were found to be dependent directly on Bi 2O 3/Li 2O ratio.

Effects of sulfone/ketone in poly(phthalazinone ether sulfone ketone) on the gas permeation of their derived carbon membranes

A series of copolymer, poly(phthalazinone ether sulfone ketone)s (PPESKs) with the sulfone over ketone unit (S/K) ratio varying from 20/80, 50/50 to 80/20, were used as precursors to prepare carbon membranes. The effects of chemical structure as S/K ratio of PPESKs on the microstructure and gas separation performance of their derived carbon membranes were mainly investigated. The properties of PPESKs were detected in terms of density, fractional free volume, char yield, interlayer distance and glass transition temperature. During the formation process of carbon membranes (i.e., stabilization and pyrolysis), the changes in functional groups, microstructural parameters and gas permeation were monitored by FTIR, X-ray diffraction, TEM and single gas permeation techniques. The results have shown that the microstructure and gas permeation of obtained carbon membranes are significantly affected by the S/K ratio in precursor PPESKs. Carbon membranes exhibit higher selectivity and lower permeability when prepared at low pyrolytic temperature (i.e., 650 °C and 800 °C) and from PPESKs with S/K ratio equaling 50/50, followed with 20/80 and 80/20. As for carbon membranes prepared at high pyrolytic temperature (i.e., 950 °C), the selectivity order of them is well in accordance with S/K mole ratio in precursor PPESKs: 20/80 > 50/50 > 80/20, and vice versa for permeability.

Influence of chemical structure of the repeating unit on physical properties of aromatic polymers containing phenylquinoxaline rings

Conformational parameters and free, occupied, accessible, and Van der Waals volume values for a group of polymers containing phenylquinoxaline rings have been calculated using an original method and have been correlated with the data regarding glass transition temperature and transport parameters given in the literature. Thus, it was shown that conformational rigidity determines the packing of macromolecules in solid state, and therefore the free volume and glass transition temperature. The introduction of phenylquinoxaline rings in polyimides leads to a decrease of selectivity to all gases due to the increase of free volume in comparison with initial polyimides. But it leads to the increase of selectivity of the gas pair H2/G, where G is any simple gas because of an increase of the number of small microcavities. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1868–1877, 2008

A modified Vegard’s law for multisite occupancy of Ca in BaTiO3–CaTiO3 solid solutions

The site occupancy of Ca in the CaTiO3–BaTiO3 solid solution has been investigated through an analysis of the lattice parameters and ferroelectric phase transition temperature variation. The lattice volume was found to vary in a nonlinear manner with different amounts of Ca2+ ions occupying the Ba and Ti sites near the pure BaTiO3 end member. To model the lattice volume trends in this complex case, we modify the classical Vegard’s law and incorporate a lattice volume correction to account for the ionic compensation of the acceptor charge CaTi″, with oxygen vacancies VO••. The paraelectric-ferroelectric phase transition temperature shows further evidence of a non-linear variation with composition. This is also consistent with site occupancy changes. From lattice volume and phase transition temperature variations on the Ba-site occupancy compositions, the maximum concentration of Ti-site occupancy (y) was determined and found to be approximately 0.007±0.002 and then found to remain constant for higher concentrations of Ca, specifically (0.03<x+y<0.1,Ba1−xCax+yTi1−yO3−y).

Optical, electrical and thermal studies on (As 2Se 3) 3− x(As 2Te 3) x glasses

Optical properties and conductivity of glassy (As 2Se 3) 3− x (As 2Te 3) x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.

Effect of filler networking on the response of thermosensitive composite hydrogels

Several composite hydrogels of poly( N-isopropylacrylamide) (pNIPAAm) with sodium montmorillonite (NaMM) have been synthesized using a fixed polymer/NaMM ratio (4:1 wt./wt.), but various monomer concentrations, in order to obtain hydrogels with different degrees of swelling, and thus different clay contents in the swollen state. For comparison, unfilled pNIPAAm gels have been also prepared at the same concentrations. The equilibrium swelling behaviour of the gels has been studied both in the swollen and in the shrunk state. In the swollen state, the polymer volume fraction increases with the initial monomer concentration C 0. In the shrunk state, the polymer fraction in pNIPAAm hydrogels is dependent on the specimen size and on C 0, whereas in the composite gels a constant polymer content is observed. When subjected to stepwise heating from 25 to 45 °C, unfilled gels undergo only poor deswelling. By contrast, complete deswelling takes place in composite gels. The latter show half-shrinking times varying over two orders of magnitude, depending on the monomer concentration and on the procedure followed to disperse NaMM, which determine the overall dispersion state of the filler, as evidenced by transmission electron microscopy (TEM). In particular, TEM observations show clay networking above a percolation threshold near 2.5 wt.% of NaMM. The effect of the incorporation of clay on the response to thermal stimuli is discussed in terms of the ability of NaMM to hinder the hydrophobic association of pNIPAAm segments and in terms of its dispersion state. It is suggested that, above the percolation threshold, NaMM forms a hydrophilic, physical network, through which water can flow also above the volume transition temperature, where pNIPAAm acquires a hydrophobic character.

Influencia del proceso de curado y del contenido de carga sobre la Tg y el volumen libre en compuestos particulados de matriz epoxi

Se presenta un estudio acerca de la influencia del proceso de curado sobre la temperatura de transición vítrea (Tg) y volumen libre en compuestos particulados de matriz epoxi. Como carga de la resina se utilizaron partículas de aluminio con tamaños típicos de ~100 mym. La información experimental se obtuvo usando calorimetría diferencial de barrido y espectroscopia temporal de aniquilación de positrones. Los resultados obtenidos estudiando el proceso de curado del polímero blanco permitieron encontrar que existe una muy buena correlación entre el volumen libre y la temperatura de transición vítrea. Al estudiar los compuestos particulados se encontró que el volumen libre promedio se modifica en forma no monótona con el incremento de la fracción de carga. Se analiza dicho comportamiento junto con la variación detectada en la Tg para cada muestra estudiada y se discuten los resultados considerando los cambios en la reticulación del material que se lleva a cabo durante el curado y los efectos de interacción en la interfase matriz polimérica-carga.

Swelling Kinetics of Poly(N-isopropylacrylamide) Minigels

We synthesize poly(N-isopropylacrylamide) (PNIPAM) gels with different sizes in the micrometer scale by a slight variation of a recent emulsion polymerization method (ref 1). The procedure is different than that typically used for obtaining macroscopic PNIPAM hydrogels. The resultant minigel suspension is polydisperse thus allowing the swelling kinetics for different gel sizes to be studied; we do so at temperatures below the volume-transition temperature by wetting with water previously dried particles. The resultant swelling is followed by optical video microscopy. We find that the characteristic swelling time scales with the inverse of the particle dimension squared, in agreement with theoretical predictions (ref 2). The proportionality constant is the network diffusion coefficient D, which for the minigels under consideration appears to be in between that of PNIPAM macrogels and the self-diffusion coefficient of water.

Effects of organophilic montmorillonite on hydrogen bonding, free volume and glass transition temperature of epoxy resin/polyurethane interpenetrating polymer networks

Epoxy resin nanocomposites containing organophilic montmorillonite (oM) and polyurethane were prepared by adding oM to interpenetrating polymer networks (IPNs) of epoxy resin and polyurethane (EP/PU). The dispersion degree of oM in EP/PU matrix was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fourier transform infrared spectrometry (FT–IR) showed that strong interactions existed between oM and EP/PU matrix, and oM had some effect on hydrogen bonding of these EP/PU IPNs nanocomposites. Positron annihilation spectroscopy (PALS) and differential scanning calorimetry (DSC) measurements were used to investigate the effect of oM and PU contents on free volume and glass transition temperature ( T g) of these nanocomposites. The PALS and DSC results clearly showed that the presence of oM led to a decrease in the total fractional free volume, which was consistent with increasing T g upon addition of oM, ascribed to increasing hydrogen bonding in interfacial regions of oM and EP/PU matrix and enhancing the miscibility between EP phase and PU phase. In addition, with increasing PU content, the total fractional free volume increased, corresponding to decreasing T g.

Preparation of Polystyrene‐Poly(N‐isopropylacrylamide) (PS‐PNIPA) Core‐Shell Particles by Photoemulsion Polymerization

AbstractSummary: Monodisperse thermosensitive PS‐NIPA core‐shell particles composed of a PS core and a cross‐linked PNIPA shell can be successfully synthesized by a novel method: photoemulsion polymerization. Cryo‐TEM images indicate clearly the core‐shell morphology of the PS‐NIPA particles: A homogeneous regular PNIPA shell has been affixed on the spherical PS core. DLS measurements indicate that the obtained PS‐NIPA latex particles are thermosensitive. The shell of PNIPA networks with different cross‐linking densities can shrink and re‐swell with temperature and the volume transition temperature is around 32 °C in all cases.Cryo‐TEM image of PS‐NIPA core‐shell particles.magnified imageCryo‐TEM image of PS‐NIPA core‐shell particles.

Body temperature-responsive gels derived from hydroxypropylcellulose bearing lignin II: adsorption and release behavior

The lower critical solution temperature (LCST) of hydroxypropylcellulose bearing lignin (HPC-L) prepared from unbleached pulp depends on the amount of residual lignin. An HPC-L gel having thermal properties reflective of original HPC-L was prepared using ethyleneglycol diglycidylether as a crosslinker, as previously reported [Uraki et al. (2004) Carbohydr. Polym. 58:123–130], and the volume transition temperature was detected as an endothermic peak by differential scanning calorimetry (DSC). The adsorption and release behavior of the guest molecules to/from this gel was then examined. When the adsorption of cationic and anionic guests was compared, cationic methylene blue (MB) was adsorbed in larger amounts than anionic methyl orange (MO). In addition, MB adsorption into the HPC-L gel was greater than MB adsorption into the HPC gel prepared from commercially available HPC. This suggests that residual lignin affects the adsorption of organic dyes. Significant differences were not observed with respect to the release of MB from HPC-L at 38 °C and lower temperatures. In the adsorption of surfactants, marked adsorption at around the critical micelle concentration of the ionic surfactants and gel swelling were observed. Such swelling did not occur in the aqueous nonionic surfactant solution, although the nonionic surfactant was adsorbed into the gel. Gel swelling may have been caused by the electrostatic repulsion of the ionic surfactants adsorbed onto the polymer chains within the gel structure.

Influence of osmotic pre-treatment and microwave application on properties of air dried strawberry related to structural changes

Strawberry halves, with and without osmotic pre-treatment, were dried at 40 °C air temperature, with and without microwave application (0.2 W/g). Changes in volume, pectic fractions, mechanical behaviour and glass transition temperature due to dehydration treatments were evaluated. Microwave application resulted in an increased water soluble pectic fraction as a consequence of both protopectin and oxalate soluble pectic fraction reductions. This solubilisation leads us to obtain dried samples with a more rigid, firmer structure which, indeed, showed greater glass transition temperature values. Nevertheless, these samples are softer when rehydrated, thus indicating a greater structural damage. The sugar presence induced by an osmotic dehydration pre-treatment seems to reinforce the solid cellular matrix, which is evident from the mechanical behaviour of rehydrated samples. So, the greatest structural damage caused by microwaves may be reduced by an osmotic dehydration pre-treatment.

Magnetic entropy change and magnetic phase transition of LaFe 11.4 Al 1.6 C x ( x =0–0.8) compounds

The unit cell volume and phase transition temperature of LaFe11.4Al1.6Cx compounds have been studied. The magnetic entropy change, refrigerant capacity and the type of magnetic phase transition are investigated in detail for LaFe11.4Al1.6Cx with x=0.1. All the LaFe11.4Al1.6Cx (x=0-0.8) compounds have the cubic NaZn13-type structure. The addition of carbon atoms brings about a considerable increase in the lattice parameter. The bulk expansion results in the change of phase transition temperature (T.). T increases from 187 K to 269 K with x varying from 0.1 to 0.8. Meanwhile an increase in the lattice parameter can also cause a change of the magnetic ground state from antiferromagnetic to ferromagnetic. Large magnetic entropy change vertical bar Delta S vertical bar is found over a large temperature range around T and the refrigerant capacity is about 322J/kg for LaFe11.4Al1.6C0.1. The magnetic phase transition belongs in weakly first-order one for x=0.1.

Volume transition and internal structures of small poly(N‐isopropylacrylamide) microgels

AbstractMonodispersed poly(N‐isopropylacrylamide) (PNIPAM) nanoparticles, with hydrodynamic radius less than 50 nm at room temperature, have been synthesized in the presence of a large amount of emulsifiers. These microgel particles undergo a swollen–collapsed volume transition in an aqueous solution when the temperature is raised to around 34 °C. The volume transition and structure changes of the microgel particles as a function of temperature are probed using laser light scattering and small angle neutron scattering (SANS) with the objective of determining the small particle internal structure and particle–particle interactions. Apart from random fluctuations in the crosslinker density below the transition temperature, we find that, within the resolution of the experiments, these particles have a uniform radial crosslinker density on either side of the transition temperature. This result is in contrast to previous reports on the heterogeneous structures of larger PNIPAM microgel particles, but in good agreement with recent reports based on computer simulations of smaller microgels. The particle interactions change across the transition temperature. At temperatures below the transition, the interactions are described by a repulsive interaction far larger than that expected for a hard sphere contact potential. Above the volume transition temperature, the potential is best described by a small, attractive interaction. Comparison of the osmotic second virial coefficient from static laser light scattering at low concentrations with similar values determined from SANS at 250‐time greater concentration suggests a strong concentration dependence of the interaction potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 849–860, 2005

Body temperature-responsive gels derived from hydroxypropylcellulose bearing lignin

Hydroxypropylcellulose bearing lignin (HPC-L) prepared from softwood by the atmospheric acetic acid pulping followed by derivatization showed the lower critical solution temperature (LCST) of approximate 38 °C in 2% aqueous solution, which were 5° lower than that of pure HPC with a similar molecular mass to HPC-L. Two types, urethane and epoxy, of chemical gels were prepared from HPC-L and HPC using hexamethylene diisocyanate and polyethylene glycol diglycidyl ether as crosslinkers, respectively. The urethane-type gels from both HPC's have undergone shrinking at less than 20 °C. This thermoresponse of the gels did not reflect LCST of the sources. Such low temperature shrinking was brought about by the hydrophobicity of crosslinker. On the other hand, the epoxy-type gels showed a very similar volume transition temperature (VTT) upon shrinking to LCST; the epoxy-type HPC-L gel showed VTT at 38 °C, while the corresponding HPC gel had VTT at 46 °C. This difference in VTT as well as LCST was attributable to hydrophobicity of lignin dangled to HPC. Thus, the body temperature-responsive gel can be prepared from HPC-L with the water-soluble crosslinker.

Contributions to the thermodynamics of polymer hydrogel systems

In this work, an extended version of a quasichemical thermodynamic model is presented. The swelling behavior of crosslinked acrylamide polymer gels and N-substituted derivatives, such as N-isopropylacrylamide and N- tert-butylacrylamide has been compared to predictions from such model which takes into account the specific hydrogen bonding interactions encountered in these systems. The calculated volume transition temperature of the poly( N-isopropylacrylamide) gel is 0.8 °C lower than the experimental value and the predicted solvent volume fraction in the collapsed and swollen gel states are about 2% larger than the corresponding experimental data measured at the transition point. Applying the same energy parameters obtained from regressing poly( N-isopropylacrylamide) gel swelling pressure data, the model has also been capable to correctly represent the major features found in the swelling behavior of linear poly( N- tert-butylacrylamide) and poly( N- tert-butylacrylamide) gels, after the model parameters that characterize the molecular structure were changed in accord to each polymer repetitive unit.

Fluorescence Study on the Swelling Behavior of Comb-Type Grafted Poly(N-isopropylacrylamide) Hydrogels

The shrinking mechanism of comb-type grafted poly(N-isopropylacrylamide) gel due to temperature jump across its volume transition temperature has been investigated. Grafted chains or gel networks were labeled by a dansyl probe, and the temporal change in microenvironment of the dansyl-labeled gel was investigated by means of fluorescence spectroscopy. The comb-type grafted poly(N-isopropylacrylamide) gel exhibited a rapid shrinking compared to normal-type NIPA gel, and the change in its microenvironment was found to become hydrophobic more than 10 times faster than normal-type poly(N-isopropylacrylamide) gel by observation of temporal change in the maximum emission wavelength, λem, of the dansyl group. The freely mobile characteristics of grafted chains are expected to show the rapid dehydration to make tightly packed globules with temperature, followed by the subsequent hydrophobic intermolecular aggregation of dehydrated graft chains. The dehydrated grafted chains created the hydrophobic cores, which en...

Structural and optical properties of lithium bismuthate glasses

Structural and optical properties of ion - conducting lithium bismuthate glasses are reported here. The structure of these glasses has been explored from the compositional variation of the density, molar volume, and glass transition temperature. The optical study in the visible and infrared region indicates a large transmission window for these glasses. The BiO6 octahedra were identified as the main structural unit from the Raman spectra of these bismuthate glasses.