Biomedical subjects

Wim H de Jeu

Publications and source records attributed to Wim H de Jeu.

10 recordsLinked to original sources

PubMed2007

Morphology of a highly asymmetric double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) block copolymer.

The morphology of a highly asymmetric double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) (PCL-b-PEO) block copolymer has been studied with in situ simultaneously small and wide-angle x-ray scattering as well as atomic force microscopy. The molecular masses Mn of the PCL and PEO blocks are 24,000 and 5800, respectively. X-ray scattering and rheological measurements indicate that no microphase separation occurs in the melt. Decreasing the temperature simultaneously triggers off a crystallization of PCL and microphase separation between the PCL and PEO blocks. Coupling and competition between microphase separation and crystallization results in a morphology of PEO spheres surrounded by PCL partially crystallized in lamella. Further decreasing temperature induces the crystallization of PEO spheres, which have a preferred orientation due to the confinements from hard PCL crystalline lamella and from soft amorphous PCL segments in different sides. The final morphology of this highly asymmetric block copolymer is similar to the granular morphology reported for syndiotactic polypropylene and other (co-) polymers. This implies a similar underlying mechanism of coupling and competition of various phase transitions, which is worth further exploration.

Biocompatible Materials↗

PubMed2006

Dynamic heterogeneity in hydrogen-bonded polymers.

We report on neutron spin echo experiments on hydrogen-bonded polymers and compare the experimentally found dynamical structure factor with theoretical predictions. Surprisingly, we find that in the melt phase the expected scaling of the Rouse dynamics is not satisfied. We propose an explanation based upon the large spatial volume occupied by the connecting groups. When the effects of these bulky groups on the local friction are taken into account, the usual scaling behavior is restored.

Journal Article↗

PubMed2005

Dynamics of fluctuations in smectic membranes.

We present a comprehensive account of the dynamics of layer-displacement fluctuations in smectic liquid-crystal membranes as studied by x-ray photon correlation spectroscopy (XPCS) and neutron-spin echo (NSE). Combining these two techniques at fast relaxation times, three distinct relaxation regimes can be distinguished. For thin membranes, at the specular Bragg position oscillatory relaxation occurs, which transforms for thicker samples into exponential decay. Above a critical off-specular angle, in XPCS exponential relaxation is observed that does not depend on the scattering angle. This indicates relaxation times that are independent of the wavelength of the fluctuations. In this regime the relaxation of the fluctuations is dominated by the surface tension. Using NSE larger off-specular angles can be reached than by XPCS, for which the relaxation time decreases with the scattering angle. This regime is dominated by the bulk elasticity of the smectic membrane. The results are compared with theoretical models for the fluctuation behavior of smectic membranes, in which effects of the mosaic distribution and of the center of mass movement of the smectic membranes must be incorporated.

Journal Article↗

PubMed2005

Self-assembly of the perfluoroalkyl-alkane F14H20 in ultrathin films.

Scanning force microscopy on monomolecular films of eicosylperfluorotetradecane, F(CF(2))(14)(CH(2))(20)H, on mica, silicon oxide, or water revealed spontaneous organization to well-defined nanoscopic ribbon and spiral or toroidal superstructures. Whether ribbons or nanospirals were formed depended on the solvent from which the molecular monofilm was cast. Ribbons were observed when a hydrocarbon or a perfluorocarbon solvent was used, e.g., decalin or perfluorodecalin. When the compound, however, was deposited from nonselective hexafluoroxylene, the molecules assembled into spirals of defined size. The spirals/toroids transformed to ribbons when exposed either to decalin or perfluorodecalin vapor, and the ribbons transformed to toroids when exposed to hexafluoroxylene vapor. These changes could be observed in situ. Scanning force microscopy yielded an identical height and width for the bands forming the spirals and for the parallel flat ribbons. X-ray reflectivity yielded a height of 3.61 +/- 0.05 nm, again identical for both morphologies. Yet, the length of the extended F(CF(2))(14)(CH(2))(20)H molecule, i.e., 4.65 nm, exceeds the layer thickness obtained from X-ray reflectometry. It is, however, consistent with an arrangement where the fluorinated chains are oriented normal to the surface layer and where the alkyl segments are tilted with a 122 degrees angle between the two segments. Within the plane defined by the tilt, this angle allows a dense packing of the alkyl segments compensating for the larger cross-section of the fluorocarbon segment. The tilt plane defines an "easy" direction along which the monolayer structure can preserve order. In the plane perpendicular to this axis, long-range ordered dense packing of the alkyl chains is not possible. Incommensurable packing can in principle explain the finite and regular width of the ribbons and the stepwise turn in the spirals.

Journal Article↗

PubMed2005

Shear-induced smectic ordering and crystallisation of isotactic polypropylene.

Shear-induced smectic ordering and crystallisation of isotactic polypropylene (iPP) has been studied with in-situ small- and wide-angle X-ray scattering. Shear-induced smectic bundles with a periodicity of about 4 nm have been observed at temperatures below as well as above the melting point. This applies to iPP of different molecular weight and from different sources. An increase in the average molecular weight leads to a larger periodicity of the smectic layers. The smectic layers assemble in a fibrillar morphology with a length and a width up to 200 and 10 microm, respectively. After crystallisation, the smectic bundles show upon heating higher melting temperatures than their crystalline counterparts. In agreement with this behaviour, the correlation length along the smectic layer normal is of the order of tens of nanometers, much larger than the crystal thickness. We present an anisotropic drop model of smectic domains forming a conserved system in which the smectic layers can rotate. On the basis of this model we can explain the relative orientation of the smectic layers, the crystalline lamellae and the long axis of the drop, as well as the reversibility of the smectic periodicity during cooling and heating. In the supercooled melt, the smectic ordering is followed by crystallisation; during this process crystals grow epitaxially on the surface of the smectic bundles. This leads to a new picture of the shish-kebab structure in which smectic bundles rather than extended-chain crystals play the role of the shish. The crystallisation process of the smectic regions themselves indicates that the 'mesophase' previously reported in fast-quenched iPP, is a metastable state formed during the transition from the high-temperature smectic phase to a crystal. Moreover smectic domains rather than alpha crystallites form the nuclei for crystallisation of the beta phase. The high-temperature smectic phase presents an ideal model system to study the coupling between density and conformational ordering under shear flow. Our results support a nucleation and growth process for polymer crystallisation, in which smectic bundles or other mesophases are the primary nuclei.

Journal Article↗

PubMed2004

Disorder by random crosslinking in smectic elastomers.

We present a high-resolution x-ray study of the effects of disorder due to random crosslinking on the one-dimensional translational ordering in smectic elastomers. At a small crosslink density of about 5%, the elastomer network stabilizes the smectic structure against layer-displacement fluctuations, and the algebraically decaying layer ordering extends up to several micrometers. With increasing concentration of crosslinks, the finite size of these domains is strongly reduced, indicating that disordering takes over. Finally, at a crosslink concentration of 20%, the structure factor can be described by a Lorentzian, which signals extended short-range correlations. The findings are discussed in terms of recent theories of randomly quenched disorder.

Journal Article↗

PubMed2004

Hydrophilic elastomers for microcontact printing of polar inks.

A moderately hydrophilic, thermoplastic elastomer (poly(ether-ester)) was investigated as a stamp material for microcontact printing of a polar ink: pentaerythritol-tetrakis-(3-mercaptopropionate). Stamps with a relief structure were produced from this polymer by hot embossing, and a comparison was made with conventional poly(dimethylsiloxane) (PDMS) and oxygen-plasma-treated PDMS. It is shown that the hydrophilic stamps can be used for the repetitive printing (without re-inking) of at least 10 consecutive patterns, which preserve their etch resistance, and this in rather sharp contrast to conventional and oxygen plasma-treated PDMS stamps. It is argued that these enhanced printing characteristics of the hydrophilic stamps originate from an improved wetting and solubility of polar inks in the hydrophilic stamp.

Journal Article↗

PubMed2004

Shear-induced smectic ordering in the melt of isotactic polypropylene.

We report an unusual shear-induced smectic phase of high-molecular-mass isotactic polypropylene at temperatures well above its melting point. The necessary stiffness in the polymer chain is provided by shear-induced coil-helix transitions. Coupling between the conformational ordering and the density generates the smectic layering. The periodicity of about 4 nm is built up from a rigid helical sequence of at least 11 monomers and a random-coil part of about 1.4 nm. These results also provide new clues to the question of preordering before polymer crystallization.

Biomimetic Materials↗

PubMed2003

Two-stage surface freezing of a single top layer in a smectic-A membrane.

The crystallization of a single liquid top layer of smectic membranes of the compound 4O.8 has been studied with grazing-incidence x-ray diffraction. As this process takes place in two steps, involving an intermediate hexatic smectic-B layer before the final crystalline-B surface structure is reached, it provides a model for melting in two dimensions. The positional order has been investigated quantitatively by measuring the scattering profiles and the associated correlation lengths. The surface liquid-hexatic phase transition is found to be continuous, while the hexatic-crystal transition is weakly first order with an abrupt change of the in-plane positional correlations. The surface phase transitions do not modify the liquid in-plane structure of the interior layers.

Journal Article↗

PubMed2002

Smectic membranes in motion: approaching the fast limits of x-ray photon correlation spectroscopy.

The dynamics of the layer-displacement fluctuations in smectic membranes have been studied by x-ray photon correlation spectroscopy (XPCS). We report transitions from an oscillatory damping regime to simple exponential decay of the fluctuations, both as a function of membrane thickness and upon changing from specular to off-specular scattering. This behavior is in agreement with recent theories. Employing avalanche photodiode detectors and the uniform filling mode of the synchrotron storage ring, the fast limits of XPCS have been explored down to 50 ns.

Journal Article↗