Kristoffer Almdal

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.

Fluctuations, conformational asymmetry and block copolymer phase behaviour

Phase behaviour near the order–disorder transition (ODT) of 58 model hydrocarbon diblock copolymers, representing four different systems, is summarized. Six distinct ordered-state microstructures are reported, including hexagonally modulated lamellae (HML), hexagonally perforated layers (HPL) and a bicontinuous cubic morphology with Iatext-decoration:overline3d space group symmetry. Two non-classical parameters, Iµ and text-decoration:overlineN, control the occurrence and distribution of these phases, in addition to the classical variables ƒ and χN, where ƒ, χ and N are the composition, segment–segment interaction parameter and degree of polymerization, respectively. Iµ accounts for differences in the conformational and volume-filling characteristics of each block. Conformational asymmetry, Iµ≠ 1, produces an asymmetric phase diagram around ƒ= 1/2. The importance of fluctuation effects are inversely related to the magnitude of text-decoration:overlineN, a type of Ginzburg parameter that is proportional to N. As text-decoration:overlineN decreases, the bicontinuous Iatext-decoration:overline3d phase appears adjacent to the ODT. Development of this cubic phase can be rationalized based on chain-packing frustration near the lamellar hexagonal state. Apparently the Iatext-decoration:overline3d cubic state is stabilized by fluctuations since it disappears when text-decoration:overlineN becomes large. These findings provide new insights into the origins of phase complexity in condensed soft matter.

Evaluating Nanoparticle Sensor Design for Intracellular pH Measurements

Particle-based nanosensors have over the past decade been designed for optical fluorescent-based ratiometric measurements of pH in living cells. However, quantitative and time-resolved intracellular measurements of pH in endosomes and lysosomes using particle nanosensors are challenging, and there is a need to improve measurement methodology. In the present paper, we have successfully carried out time-resolved pH measurements in endosomes and lyosomes in living cells using nanoparticle sensors and show the importance of sensor choice for successful quantification. We have studied two nanoparticle-based sensor systems that are internalized by endocytosis and elucidated important factors in nanosensor design that should be considered in future development of new sensors. From our experiments it is clear that it is highly important to use sensors that have a broad measurement range, as erroneous quantification of pH is an unfortunate result when measuring pH too close to the limit of the sensitive range of the sensors. Triple-labeled nanosensors with a pH measurement range of 3.2-7.0, which was synthesized by adding two pH-sensitive fluorophores with different pK(a) to each sensor, seem to be a solution to some of the earlier problems found when measuring pH in the endosome-lysosome pathway.

Can a single function for χ account for block copolymer and homopolymer blend phase behavior

Most theoretical treatments of polymer–polymer phase behavior assume that homopolymer mixtures and block copolymer melts are controlled by a common segment–segment interaction parameter knows as χ. This publication describes the results of small-angle neutron scattering (SANS) experiments conducted as a function of temperature and composition from homogeneous mixtures of poly(ethylene) (PE) and poly(ethylenepropylene) (PEP) of equal molecular weight. Analysis of these SANS measurements based on the random phase approximation indicates that χPE/PEP is independent of composition and linear in T−1. The associated symmetric phase diagram calculated with Flory–Huggins theory contains a stability curve that is consistent with the divergence in single phase susceptibility obtained by SANS. This function χPE/PEP(T) is compared with functions for χPE–PEP(T) associated with the homologous PE–PEP diblock copolymers, extracted both from the temperature dependence of the disordered state scattering structure factor, a...

Epitaxial growth and shearing of the body centered cubic phase in diblock copolymer melts

Two poly(ethylenepropylene)–poly(ethylethylene) (PEP‐PEE) diblock copolymer melts, containing 25% and 83% by volume PEP, were investigated using small‐angle neutron scattering (SANS) and rheological measurements. The SANS measurements were performed with the aid of an in situ shearing device operated directly in the neutron beam. Each sample was observed to possess three equilibrium phases: two ordered phases at low temperature and a disordered phase at elevated temperatures. The low and high temperature ordered phases have been evaluated to be hexagonally packed (hex) cylinders and body centered cubic (bcc) spheres, respectively. Application of a large amplitude dynamic shear deformation to the hex phase leads to well‐aligned cylinders, with a specific crystallographic orientation relative to the shear plane. Upon heating through the cylinder‐to‐sphere transition, the bcc phase grows epitaxially, with the [111] direction coincident with the original cylinder axis, leading to a well‐defined twinned micros...

Laboratory‐scale setup for anionic polymerization under inert atmosphere

‘‘Living’’ anionic polymerization offers a readily feasible preparation of very well‐controlled homo‐ or copolymers in a wide range of molar masses and chemical structures. Such samples are vital in experimental polymer physics and the technique presented here offers self‐sufficiency with such samples. The technique of anionic polymerization performed under inert atmosphere is presented here. The components of the setup are described in detail. Purification procedures for glassware and chemicals for specific polymerizations are given. Illustrative examples and results are presented.

Complex Phase Behavior in Solvent-Free Nonionic Surfactants

Unsolvated block copolymers and surfactant solutions are “soft materials” that share a common set of ordered microstructures. A set of polyethyleneoxide-polyethylethylene (PEO-PEE) block copolymers that are chemically similar to the well-known alkane-oxyethylene (CnEOm) nonionic surfactants was synthesized here. The general phase behavior in these materials resembles that of both higher molecular weight block copolymers and lower molecular weight nonionic surfactant solutions. Two of the block copolymers exhibited thermally induced order-order transitions and were studied in detail by small-angle scattering. The fundamental microstructural spacing was determined to be a crucial parameter in these transitions. Transitions from one ordered state to another occur only when the lattice spacing is nearly matched. These materials highlight the importance of epitaxy and molecular conformation in the phase transformations of soft material.

Order, disorder, and fluctuation effects in an asymmetric poly(ethylene‐propylene)‐poly(ethylethylene) diblock copolymer

The thermodynamic and dynamic properties of a partially deuterated asymmetric poly (ethylene-propylene)-poly(ethylethylen) (PEP-PEE) diblock copolymer containing 77% by volume PEP were characterized below and above the order-disorder transition (ODT). Both small-angle neutron schattering (SANS) and rheology experiments provided unambiguous evidence of composition fluctuations between the order-disorder transition temperature (T ODT ) and at least 40°C above T ODT . The strength of the fluctuations increases as the weak first-order ODT is approached

Expanding the dynamic measurement range for polymeric nanoparticle pH sensors

Conventional optical nanoparticle pH sensors that are designed for ratiometric measurements in cells have been based on utilizing one sensor fluorophore and one reference fluorophore in each nanoparticle, which results in a relatively narrow dynamic measurement range. This results in substantial challenges when conducting live cell measurements, which often leads to misleading results. In the present work we provide a simple solution to this problem.

Phase continuity and inversion in polystyrene/poly(methyl methacrylate) blends

Dual-phase continuity and phase inversion of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends processed in a twin-screw extruder was investigated using a selective extraction technique and scanning electron microscopy. Emphasis was placed on investigating the effects of viscosity ratio, blend composition, processing variables (mixing time and annealing) and diblock copolymer addition on the formation of bi-continuous phase structure (BPS) in PS/PMMA blends. The experimental results were compared with the volume fraction of phase inversion calculated with various semi-empirical models. The results showed that the formation of a BPS strongly depends on the blend composition and the viscosity ratio of the constituent components. Furthermore, BPS was found in a wide volume fraction interval. Increasing the mixing time and the addition of diblock copolymer, both led to a narrowing range of volume fraction in which BPS exists. Quiescent annealing coarsened the structure but indicated no qualitative changes. Some model predictions for phase inversion could predict qualitative aspects of the observed windows of co-continuity but none of the models could account quantitatively for the observed data.