Volume Transition Research Articles

Structure, topology and chemical order in Ge–As–Te glasses: a high-energy x-ray diffractionstudy

High-energy x-ray diffraction is employed to study the atomic structure of bulkGexAs2xTe100 − 3x glasses with compositionsin the range 25 ≤ 3x ≤ 70. The coordination environments of Te atoms suggest significant violation of chemical orderin these glasses. Analyses of the nearest-neighbor coordination environments and theparameters for the first sharp diffraction peak indicate that these telluride glasses arestructurally and chemically more disordered as compared with their sulfide or selenideanalogs. The compositional evolution of the structural parameters is shown to be consistentwith the corresponding variation in molar volume and glass transition temperature.

Dynamic light scattering studies of ionic and nonionic polymer gels with continuous and discontinuous volume transitions

AbstractWe have performed dynamic light scattering experiments on poly(acrylamide)‐poly(acrylic acid) copolymer gels with controlled crosslink density and copolymer composition, by varying the temperature, amount and valency of added salt, pH, and solvent quality. Our systematic study provides several insights. The correlation length for the monomer density fluctuations, as inferred from the measured diffusion coefficient, is too small to be identified as the mesh size of the gel. The correlation length in an ionic gel, which is found to be smaller than that for an equivalent gel without ionization. Comparison of swelling ratio with the diffusion coefficient shows that these quantities are not simply geometrically related to each other. When a discontinuous volume phase transition is induced by gradually varying the solvent quality, the diffusion coefficient exhibits a pretransitional reduction by two orders of magnitude even before the gel collapse. These findings provoke a need for new theoretical approaches for describing the elastic modes of polyelectrolyte gels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

ChemInform Abstract: Elastic Properties and Molar Volume of Rare-Earth Aluminosilicate Glasses.

This paper reports on the elastic properties, molar volume, and glass transition temperature (T{sub g}) of rare-earth-containing aluminosilicate glasses that were investigated in the compositions of SiO{sub 2}--LnAlO{sub 3} and SiO{sub 2}--Ln{sub 3/4}Al{sub 5/4}O{sub 3}, where Ln is Y, La, Nd, Eu, or Yb. The molar volume decreased with decreased ionic size of the Ln{sup 3+} ion, and T{sub g} and elastic moduli increased in the same order. The Yb-containing glasses showed the highest Young's modulus among all the oxide glasses, even higher than the highest value ever known fro glass containing Y{sub 2}O{sub 3}, as expected from the smaller ionic radius of Yb{sup 3+} than that of Y{sup 3+}. The bulk modulus was found to be almost proportional to the inverse four-thirds power of the molar volume of glasses in each composition, indicating that Ln{sup 3+} ions can substitute for each other without changing the glass structure except for the size of the local structure around themselves. From the comparison of these properties, the structural role of rate-earth ions in these glasses is discussed.

Solvents Effect on the Structure and Properties of Thermosensitive Polyurethane/Poly(N-Isopropylacrylamide) Semi-Interpenetrating Polymer Networks

poly(N-isopropylacrylamide) (PNIPPAm) is a thermally sensitive polymeric material which exhibits large volume transition at temperatures around its lower critical solution temperature (LCST). To get a thermosensitive semi-Interpenetrating Polymer Networks(semi-IPNs), N- isopropylacrylamide (NIPPAm) was dispersed and synthesized with different crosslinking agent amount in thermoplastic polyurethane(TPU) solution. The resulting films were characterized in terms of swelling ration, surface contact angle, and scanning electron micrograph (SEM). The results showed that filming medium has great affect on the structure and swelling of PU/PNIPPAm composite film and the amount of crosslinking agent influents the miscibility of PU and PNIPPAm. Ethanol as filming medium makes the phase separation behavior of PNIPPAm insignificant in the films.

Thermoresponsive Pore Structure of Biopolymer Microspheres for a Smart Drug Carrier

Triggering changes in surface porosity enabled the controlled release of biomolecules from elastin-like polypeptide (ELP) microspheres. The transition temperature (T(t)) of cross-linked microspheres was determined by differential scanning calorimetry, and T(t) was in agreement with the volume transition observed by changing the external temperature of the incubation media. The thermoresponsive pore structure of ELP microspheres and their surface morphology were examined by field-emission scanning electron microscopy. ELP microspheres were investigated as a smart drug carrier using model drug molecules, bovine serum albumin, and prednisone acetate. The release rate was accelerated by squeezing out the entrapped biomolecules as the temperature was increased above T(t) because of the development of micropores at the surface as well as in the bulk. In addition, the stepwise release confirmed that ELP microspheres could be progammed precisely to control the release of drugs by external stimuli.

Analysis of Optical Gradient Profiles during Temperature- and Salt-Dependent Swelling of Thin Responsive Hydrogel Films

Surface-attached, cross-linked hydrogel films based on thermoresponsive N-isopropylacrylamide with a dry thickness >1 microm were studied with surface plasmon resonance/optical waveguide mode spectroscopy (SPR/OWS) to monitor temperature-dependent and salt-induced changes of their swelling state. In combination with the reversed Wentzel-Kramers-Brillouin and Bruggeman effective medium approximation and by modeling the hydrogel film as a composite of sublayers with individual complex refractive indices, refractive index/volume fraction gradient profiles perpendicular to the surface are accessible simultaneously with information about local inhomogeneities. Specifically, the imaginary refractive index kappa of each sublayer can be interpreted as a measure for static and dynamic inhomogeneities, which were found to be highest at the volume transition collapse temperature in the layer center. These results indicate that the hydrogel collapse originates rather from the film center than from its boundaries. Upon addition of NaCl to a swollen hydrogel below its transition temperature, comparable optical loss characteristics as for the thermal gel collapse are observed with respect to inhomogeneities. Interestingly, in contrast to the thermally induced layer shrinkage and collapse, swelling increases at intermediate salt concentrations.

Cooperative Hydration Induces Discontinuous Volume Phase Transition of Cross-Linked Poly(N-isopropylacrylamide) Gels in Water

The concept of cooperative dehydration, defined as the simultaneous dissociation of the bound water molecules from the polymer chains in correlated sequences, is applied to study the collapse of cross-linked poly(N-isopropylacrylamide) (PNIPAM) gels in aqueous solutions upon heating above the critical temperature. We examined the applicability of this concept in two situations: (i) free swelling of PNIPAM gels in water (in the absence of applied force), (ii) swelling of PNIPAM gels in water subjected to a uniaxial tension. The transition becomes sharper as the cooperativity parameter σ of hydration decreases (increasing cooperativity), and there is a critical value σc ≃ 0.30 of the cooperativity parameter below which the volume transition is discontinuous. Hence, the real phase transition with thermodynamic discontinuity is shown to be possible for the nonionic polymer gels if the hydration is sufficiently cooperative. The calculations of the swelling ratio and the spinodal lines are compared with the experimental data of PNIPAM gels by Hirotsu for point i and by Suzuki for point ii.

Poly(vinyl alcohol) microspheres with pH- and thermosensitive properties as temperature-controlled drug delivery

One of the most important inconveniences of the pH- and temperature-sensitive hydrogels is the loss of thermosensitivity when relatively large amounts of a pH-sensitive monomer are co-polymerized with N-isopropylacrylamide (NIPAAm). In order to overcome this drawback, we propose here a method to prepare thermosensitive poly(vinyl alcohol) (PVA) microspheres with a higher content of carboxylic groups that preserve thermosensitive properties. Moreover, PVA possesses excellent mechanical properties, biocompatibility and non-toxicity. PVA microspheres were obtained by suspension cross-linking of an acidified aqueous solution of the polymer with glutaraldehyde. Poly(N-isopropylacrylamide-co-N-hydroxymethyl acrylamide) (poly(NIPAAm-co-HMAAm)), designed to have a lower critical solution temperature (LCST) corresponding to that of the human body, was grafted onto PVA microspheres in order to confer them with thermosensitivity. Then, the pH-sensitive functional groups (COOH) were introduced by reaction between the un-grafted OH groups of PVA and succinic anhydride. The pH- and temperature-sensitive PVA microspheres display a sharp volume transition under physiological conditions around the LCST of the linear polymer. The microspheres possess good drug-loading capacity without losing their thermosensitive properties. Under simulated physiological conditions, the release of drugs is controlled by temperature.

Volume Transition of PNIPAM in a Nonionic Surfactant Hexagonal Mesophase

We investigate the volume transition of a thermoresponsive polymer, poly(N-isopropylacrylamide), PNIPAM, in the presence of an aqueous solution of nonionic surfactant, C12E9. We combine turbidimetry with optical microscopy, NMR, and SAXS to follow the volume transition of the PNIPAM and the H1-isotropic transition of the surfactant/water system. Nonionic surfactants such as C12E9 are known to interact weakly with PNIPAM. Accordingly, we show that there is only a small change in the volume transition temperature for the PNIPAM in isotropic micellar solutions of C12E9, even for relatively high concentrations of C12E9. Interestingly, once the surfactant forms an H1 phase, there is a dramatic decrease in the coil−globule transition onset temperature. We believe that this behavior results from a competition between C12E9 in the H1 phase, and PNIPAM to associate with water. When PNIPAM in the H1 phase is cooled to low enough temperatures so as to be in the coil state, it locally disturbs the hexagonal phase ordering. Thus, we show that for PNIPAM in a weakly interacting surfactant matrix, it is the phase behavior of the matrix rather than the matrix chemistry that governs the coil−globule transition. Finally, we show that in a PNIPAM copolymer with a higher LCST we observe an interesting sequence of transitions in the surfactant phase: on cooling from a high temperature free-flowing turbid globular state (∼75 °C), we enter a free-flowing translucent coil phase (∼47 °C), then a turbid gel (∼25 °C) where the copolymer is collapsed in the H1 phase, and finally a low-temperature clear gel (∼5 °C) where the copolymer is in the expanded coil state.

Multiphysics modeling of responsive characteristics of ionic-strength-sensitive hydrogel

A multiphysics model is developed in this paper for simulation of the volume transition mechanism of the smart hydrogel in response to the changes in the ionic strength of bathing solution as an important measure of the ionic concentration of that solution, which is termed the multi-effect-coupling ionic-strength-stimulus (MECis) model. In the present works, the ionic strength is treated as a main stimulus and incorporated into both the ionic convection-diffusion system in the Nernst-Planck flux and the fixed charge density equation characterized by Langmuir isotherm theory. Due to the diffusion and convection, the osmotic pressure is produced by the difference in the ionic concentration between the interior hydrogel and exterior solution, which drives the swelling of the smart hydrogel. The deformation of the ionic-strength-sensitive hydrogel is described by the momentum conservation law, in which the osmotic pressure is a main driving source. Apart from osmotic pressure, however, the repulsive force between the fixed charges is also considered in the mechanical equilibrium equation as another driving source for the swelling of the hydrogel. The simulation is conducted for one-dimensional steady-state problem, and then compared with the experimental data and other theories from open literature. The comparisons demonstrate that the MECis model can simulate well the swelling behavior of the ionic-strength-sensitive hydrogel qualitatively and quantitatively. Probably it is able to predict the responsive characteristics of the bathing solution including the distribution of diffusive ionic concentrations and electrical potential.

Regulating Volume Transitions of Highly Responsive Hydrogel Scaffolds by Adjusting the Network Properties of Microgel Building Block Colloids

We present a simple method to control the volume change of thermally responsive hydrogel scaffolds, providing a remarkably fast swelling and deswelling response to temperature changes. These scaffolds have 3-dimensional colloidal-network structures which are made from microgel particles while they are above their deswelling transition temperatures. By tuning the cross-link density of the microgel particles, we achieve controllable changes of the volume of the scaffolds in response to temperature. Their fast response rate is determined by the length scale of the unit microgel particles and is not influenced by the properties of the network. The release profile of a model drug (Rhapontin) loaded within the scaffolds can also be regulated by the cross-linking density of the microgel particles. These results offer a new way of fabricating hydrogel scaffolds with tunable matrix geometry and function by adjusting the properties of the unit microgel colloids, without loss of their fast response to temperature change.

Evanescent wave spectroscopy for studying swelling/de-swelling kinetics of soft matter

Optical fibers connected to a spectrophotometer are shown to be analytical devices for recording the kinetics of structural changes within soft materials deposited on the side surface of the fiber core. A portion of electromagnetic radiation propagating inside an optical fiber penetrates out of its core (evanescent field) and interacts with molecules located within the depth of penetration (∼λ/10). It can be absorbed, scattered or can excite molecules around the fiber core surface. To prove the concept, a cylindrical hydrogel film [poly-N-(isopropylacrylamide), PNIPA] was polymerized by an evanescent wave around the fiber core immerged into the hydrogel forming solution. The integral intensity of light passing through the fiber core covered with the dried PNIPA hydrogel film was recorded with time during its hydration to reveal tiny features of swelling kinetics. The volume transition temperature (∼32 °C) was determined for the PNIPA hydrogel by measuring integral intensity and spectral composition of light passing through the combined gel-on-fiber core system at different temperatures. This value agrees well with the one known from the literature. Above this temperature, the PNIPA hydrogel shrank absorbing/scattering the light in different spectral ranges with a temperature increase. The results are of great importance for practical applications such as the time-resolved spectroscopy of structural transformations within not only synthetic, but also natural soft materials (cells, organelles, microorganisms, etc.).

Molecular simulation of CO 2/CH 4 permeabilities in polyamide–imide isomers

Molecular simulation techniques have been adopted for the first time for understanding the structure–property relationships of three highly fluorinated polyamide–imide (PAI) isomers (6FDA/8p, 6FDA/8m and 6FDA/12p). The PAIs being considered in this work are based on 2,2-bis(3,4-decarboxy-phenyl) hexafluoropropane dianhydride (6FDA) and N,N-bis(amino-phenyl)-perfluoroalkane-α,ω-dicarboxamide diamines. The density, fractional free volume (FFV) and glass transition temperature ( T g) of the PAIs are calculated to investigate the microstructure of the PAIs and the structural dependence of sorption and diffusion properties of CO 2/CH 4 pair. The 6FDA/8p and 6FDA/12p give both higher permeability and permselectivity than the 6FDA/8m because the rigid para-isomer functions as a molecular sieve. With a longer length of oligo(tetrafluoroethene) segment, the 6FDA/12p has permeability and selectivity greater than 6FDA/8p. Hence, this study may provide a guideline for designing PAI membranes to have desirable properties of permeability and permselectivity.

Loop quantum cosmology and spin foams

Loop quantum cosmology (LQC) is used to provide concrete evidence in support of the general paradigm underlying spin foam models (SFMs). Specifically, it is shown that: (i) the physical inner product in the timeless framework equals the transition amplitude in the deparameterized theory; (ii) this quantity admits a vertex expansion a la SFMs in which the M-th term refers just to M volume transitions, without any reference to the time at which the transition takes place; (iii) the exact physical inner product is obtained by summing over just the discrete geometries; no ‘continuum limit’ is involved; and, (iv) the vertex expansion can be interpreted as a perturbative expansion in the spirit of group field theory. This sum over histories reformulation of LQC also addresses certain other issues which are briefly summarized.

Influence of SiO 2 on the structure and optical properties of lithium bismuth silicate glasses

Bismuth silicate glasses containing lithium oxide with composition 20Li 2O·(80 − x)Bi 2O 3· xSiO 2 (5 ⩽ x ⩽ 70 mol%) have been prepared by melt-quench technique. Density ( D), molar volume ( V M) and glass transition temperature ( T g) for all the glass samples have been measured. FTIR spectroscopy has been employed to investigate the structure of these glasses in order to obtain information about the competitive role of Bi 2O 3 and SiO 2 in the formation of glass network. The increase of SiO 2 content in the glass matrix results in increasing the Si–O–Si bond angle and hence the ionicity of Si–O bond increases with decrease in Bi 2O 3/SiO 2 ratio. The optical transmittance spectra of all the glasses have been recorded in the wavelength range 200–3300 nm. The values of optical band gap ( E g) have been determined from the cutoff wavelength of these glasses. The average electronic polarizability of oxide ion ( α o 2 - ) and the optical basicity ( Λ th) have been estimated from the calculated values of the E g and were found to be dependent directly on Bi 2O 3/SiO 2 ratio. The variation in different physical parameters such as D, V M and T g and optical parameters; viz., E g, α o 2 - , Λ th with Bi 2O 3/SiO 2 ratio have been analyzed and discussed in terms of change in the glass structure.

A novel potentiometric method for the determination of real crosslinking ratio of poly(aspartic acid) gels

In order to obtain nontoxic functional polymer gels for biomedical applications, chemically crosslinked poly(aspartic acid) gels have been prepared using 1,4-diaminobutane as crosslinker. The presence of COOH and amino groups on the network chains renders these gels pH sensitive. Due to the specific hydrophobic–hydrophilic balance, these gels show a significant volume transition at a well-defined pH close to the p K value of uncrosslinked poly(aspartic acid). Since the magnitude of volume change critically depends on the degree of crosslinking, it is an important task to determine the topological characteristics of these networks. A novel method based on potentiometric acid–base titration has been developed to assess the crosslinking ratio, excluding physical crosslinks and entanglements. It turned out that only 25% of all crosslinker molecules forms real crosslinks between the poly(aspartic acid) chains; the rest react with one of its functional groups and forms short pendant side chains. At a nominal crosslinking ratio of 0.1, the number average molecular mass between crosslinks is found to be M c = 2300.

Corona-Cross-Linked Polymer Vesicles Displaying a Large and Reversible Temperature-Responsive Volume Transition

Departementdemedecinenucleaireet de radiobiologie and Centre d’imagerie moleculaire deSherbrooke, Universite de Sherbrooke, Sherbrooke, Quebec,Canada J1K 2R1Received August 14, 2009Revised Manuscript Received September 4, 2009Amphiphilic block copolymer (BCP) vesicles (polymersomes)represent an active research area due to their potential forapplications such as controlled drug delivery and nanoreactorsfor controlled chemical reactions.

Surfactant Self-Assembly in Oppositely Charged Polymer Networks. Theory

The interaction of ionic surfactants with polyion networks of opposite charge in an aqueous environment is analyzed theoretically by applying a recent theory of surfactant ion-polyion complex salts (J. Colloid. Int. Sci. 2009, 332, 183). The theory takes into account attractive and repulsive polyion-mediated interactions between the micelles, the deformation of the polymer network, the mixing of micelles, polyion chains, and simple ions with water, and the hydrophobic free energy at the micelle surface. The theory is used to calculate binding isotherms, swelling isotherms, surfactant aggregation numbers, compositions of complexes,and phase structure under various conditions. Factors controlling the gel volume transition and conditions for core/shell phase coexistence are investigated in detail, as well as the influence of salt. In particular, the interplay between electrostatic and elastic interactions is highlighted. Results from theory are compared with experimental data reported in the literature. The agreement is found to be semiquantitative or qualitative. The theory explains both the discrete volume transition observed in systems where the surfactant is in excess over the polyion and the core/shell phase coexistence in systems where the polyion is in excess.

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.