Søren Hvidt

Towards a phenomenological definition of the term ‘gel’

Abstract The term ‘gel’ is used so indiscriminately that it has become ambiguous. Existing definitions are reviewed, examples of unfortunate uses of the term are discussed, and important phenomenological characteristics of gels are identified. We propose that the term ‘gel’ should be limited to systems which fulfil the following phenomenological characteristics: (a) they consist of two or more components one of which is a liquid, present in substantial quantity and (b) they are soft, solid, or solid-like materials. We further propose a definition of the solid-like characteristics of gels in terms of the dynamic mechanical properties, viz. a storage modulus, G′(ω), which exhibits a pronounced plateau extending to times at least of the order of seconds and a loss modulus, G″(ω), which is considerably smaller thatn the storage modulus in the plateau region.

Self-aggregation and phase behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solution

The phase behavior and aggregation properties of block copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronics, poloxamers) in aqueous solution have recently attracted much attention. Both experimental and theoretical studies are reviewed, not comprehensively, but with the focus on studies, partly cooperative, partly independent, performed by groups in Uppsala (light scattering and fluorescence), Roskilde (rheology and calorimetry), Risø (SANS), Graz (x-ray and speed of sound), and Lund (theoretical model calculations).The phase behavior of these copolymers is similar in many respects to that of conventional nonionic surfactants, with the appearance of hexagonal, cubic, and lamellar liquid crystalline phases at high concentrations. In the isotropic solution phase the critical concentration for micelle formation is strongly temperature dependent, and at a given concentration the monomer to micelle transition occurs gradually over a broad temperature range, partly due to the broad size polydispersity of both the PO- and EO-blocks. For some Pluronic copolymers a transition from globular to long rod-like micelles occurs above a transition temperature, resulting in a strong and sudden increase of viscosity and viscoelasticity of the solution.Size and aggregation numbers have been determined for the globular micelles in some cases, and also the rod-like micelles have been characterized. NMR and fluorescence measurements have provided further information on the properties of the micellar core and mantle. In combination, results from different measurements on the same Pluronics material indicate that the aggregation number of the micelles increases with the temperature, whereas the hydrodynmic radius varies much less. The PEO-mantle of the micelles seems to contract with increasing temperature. The core appears to contain appreciable amounts of PEO in addition to PPO (and also some water). The segregation between core and mantle is not as distinct as in normal micelles, a conclusion which is in line with the predictions from the model calculations.

Insulin association in neutral solutions studied by light scattering

Molecular weights and weight distributions of sulfated, Zn-free, and 2Zn insulins have been measured at pH 7.3 as a function of concentration from 0.1 to 2 mg/ml by use of a combination of light scattering, refractometry, and size-exclusion chromatography. Results show that sulfated insulin is monomeric over the studied concentration range. Weight average molecular weights between those of a monomer and a hexamer were found for both zinc-free and 2Zn insulins. Zinc stabilizes the hexamer, and the dimer-hexamer equilibrium constant is approx. 400-times higher in the presence of zinc than in its absence. An average hydrodynamic radius of 5.6 nm, close to the crystallographic size of the insulin hexamer, was determined from dynamic light scattering of 2Zn insulin solutions.

Solubility of nonylphenol and nonylphenol ethoxylates. On the possible role of micelles

The water solubility of nonylphenol (NP) has been estimated to be 4.9 +/- 0.4 mg/l corresponding to (2.22 +/- 0.18) x 10(-5) mol/l at 25 degrees C using shake flask and surface tension techniques. The low solubility in combination with an observed rather slow dissolution process will limit the leachability of NP in the terrestrial environment. Based on indirect evidence, it is suggested that NP, in contrast to nonylphenol ethoxylate (NPEO) with, e.g., 12 ethoxylate moieties, is not subject to micelle formation, and as such does not constitute a potential vehicle for the transport of hydrophobic pollutants in the environment. For NPEOs with a very high number of ethoxy moieties, e.g., 100, the compounds appear water soluble without micelle formation.

Basic rheology for biologists.

Many cellular processes lead to changes in elastic and viscous properties of cells. Rheology is the science that deals with deformation and flow of materials. Fundamental rheologic concepts are explained, and some of the main techniques are discussed. Nonperturbing oscillatory techniques are especially useful for monitoring structure formation including gelation, whereas other techniques such as steady shear flow and creep are useful for determining flow properties. Sample preparation is often a major obstacle, and advantages of different deformation geometries are discussed. Simple biological samples such as purified biopolymers can be investigated with a range of rheologic techniques, and factors affecting gelation of, for example, blood or cytoskeletal proteins can be studied in detail. More complex biological systems such as intact tissues can often only be studied with more qualitative techniques and results. With proper choice of experimental setup, rheologic techniques can give valuable information about cellular systems and dynamics on a timescale that is closely related to biological functions.

Pressure- and temperature-dependent micellization of a poly(ethylene oxide)poly(propylene oxide) triblock copolymer in aqueous solutions

Bulk moduli, densities, and heat capacities of aqueous solutions of a triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer with average composition (EO) 28 (PO) 48 (EO) 28 (where EO is ethylene oxide, PO is propylene oxide) and homopolymers with average compositions (EO) 133 and (PO) 37 have been measured as a function of temperature and concentration. Block copolymers form micelles with increasing temperature. Micellization is shown to be an endothermic transition with a ΔH of 50 ± 4 J g -1 of copolymer and with an increase of the apparent copolymer volume of 0.029±0.003 ml g -1 . The measured storage bulk moduli show that the dissolved unimer structures are nearly incompressible. The modulus decreases during micellization for copolymers or phase separation for poly(propylene oxide) (PPO). A simple two-phase model of micelles with a core of pure PPO and a mantle of water-swollen poly(ethylene oxide) chains describes both the measured bulk moduli and volume properties well. The relaxation time for unimer to micelle dynamics has been estimated from the loss bulk modulus maximum with temperature to be approximately 1 μs at 20° C.

Dynamic Mechanical Properties

Publisher Summary This chapter focuses on various dynamic mechanical properties and viscoelastic functions. Rubber is frequently used for applications in which it undergoes rapid cyclic deformations at a certain frequency or over a range of frequencies. The dynamic mechanical properties are strongly dependent on temperature, frequency, the presence of fillers, and the extent of deformation if it is large. For very small deformations, the properties are found to be independent of the magnitude of the deformation (linear viscoelasticity), and temperature–frequency superposition can be applied in many cases, a feature that greatly facilitates obtaining or predicting the dynamic response over a wide frequency range. It is important to note that the dynamic mechanical properties of different unfilled amorphous elastomers are quite similar when the proper reference state is used. Dynamic mechanical properties refer generally to responses to periodically varying strains or stresses. They are most simply defined for a small sinusoidally varying strain or stress for which the response is a small sinusoidally varying stress or strain, respectively, with the same frequency but generally out of phase.

Characterization and micellization of a poloxamer block copolymer

Abstract Several poloxamers that are symmetrical EPE block copolymers (E and P are ethylene and propylene oxide, respectively) have been characterized by size exclusion chromatography on Superose columns in water. The poloxamers contain between 12 and 26 wt% of smaller-size UV-absorbing impurities. Poloxamer P94 (E28P48E28) forms micelles with increasing temperature, and micellization was investigated by eluent gel permeation chromatography (EGPC). EGPC results demonstrate that P94 impurities are not incorporated into the micelles up to 38°C. The importance of poloxamer heterogeneity for thermodynamic and structural studies is discussed.

Thermally induced chain exchange of frog αβ-tropomyosin

Abstract The thermally induced unfolding of the α-helix of Rana esculenta αα, αβ and ββ tropomyosin and two tryptic fragments approximately corresponding to the N- and C-terminal halves of αβ have been investigated by use of optical rotation, circular dichroism and UV difference spectroscopy. Reversible unfolding transitions of αα and ββ occur around 49°C and 32°C, respectively. The helix unfolding of αβ shows two major transitions at 36°C and 48°C, with only the latter being reversible. The major unfolding transitions of each of the N- and C-terminal αβ peptides roughly correspond to the low and high temperature transitions of intact αβ, respectively. This suggests that the unfolding of αβ could be due to unfolding of two independent domains in αβ. UV difference data, crosslinking and chromatography results show, however, that the unfolding of αβ at 36°C is due to chain exchange with the formation of αα homodimers and largely β monomers, and that the transition at 48°C is due to unfolding of αα dimers.

The molar hydrodynamic volume changes of factor VIIa due to GlycoPEGylation

The effects of GlycoPEGylation on the molar hydrodynamic volume of recombinant human rFVIIa were investigated using rFVIIa and two GlycoPEGylated recombinant human FVIIa derivatives, a linear 10kDa PEG and a branched 40kDa PEG, respectively. Molar hydrodynamic volumes were determined by capillary viscometry and mass spectrometry. The intrinsic viscosities of rFVIIa, its two GlycoPEGylated compounds, and of linear 8kDa, 10kDa, 20kDa and branched 40kDa PEG polymers were determined. The measured intrinsic viscosity of rFVIIa is 6.0mL/g, while the intrinsic viscosities of 10kDa PEG-rFVIIa and 40kDa PEG-rFVIIa are 29.5mL/g and 79.0mL/g, respectively. The intrinsic viscosities of the linear PEG polymers are 20, 22.6 and 41.4mL/g for 8, 10, and 20kDa, respectively, and 61.1mL/g for the branched 40kDa PEG. From the results of the intrinsic viscosity and MALDI-TOF measurements it is evident, that the molar hydrodynamic volume of the conjugated protein is not just an addition of the molar hydrodynamic volume of the PEG and the protein. The molar hydrodynamic volume of the GlycoPEGylated protein is larger than the volume of its composites. These results suggest that both the linear and the branched PEG are not wrapped around the surface of rFVIIa but are chains that are significantly stretched out when attached to the protein.