Pieter Leezenberg | Stanford University (original) (raw)

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Papers by Pieter Leezenberg

Pure Appl Chem, 1996

Naphthalene and dansyl chromophores are covalently attached to trifunctional cross-link junctions... more Naphthalene and dansyl chromophores are covalently attached to trifunctional cross-link junctions in poly(dimethy1 siloxane) elastomers. Photophysical measurements are subsequently used to monitor the junction dynamics in unfilled elastomers and the local junction environment in nanocomposites prepared by sol-gel in-sifu precipitation techniques. Transient fluorescence anisotropy measurements of the naphthalene chromophore are interpreted in terms of a fast, partial reorientation within a restricted cone and a slow, complete reorientation driven by cooperative motions of larger chain segments connected to the cross-link. Steady state fluorescence measurements of the dansyl chromophore in unfilled elastomers and in silica-containing nanocomposites suggest the existence of two populations of chromophores: one that is tightly immobilized through interactions with inorganic domains and a second that is free to move within the elastomeric domain.

The Journal of Physical Chemistry, 1992

Dynamics in poly(dimethylsi1oxane) (PDMS) melts from Tg + 75 to T, + 175 K have been measured by ... more Dynamics in poly(dimethylsi1oxane) (PDMS) melts from Tg + 75 to T, + 175 K have been measured by the fluorescence anisotropy decay of a probe chromophore. The reorientational dynamics of the probe chromophore, 5-(dimethylamino)-1-naphthalenesulfonamide (dansylamide), attached to a trifunctional silane, are characterized in a small molecule solvent, cyclohexanol, and compared to its reorientation in the polymer system. In cyclohexanol, the orientational dynamics obey the Debye-Stokes-Einstein equation with a thermal activation energy equal to that of the cyclohexanol viscosity. In contrast, the rate of reorientation of the probe dispersed in PDMS polymer melts does not reflect the bulk properties of the samples. The local dynamics are exponentially activated, with activation energies that are higher than that of the viscosity of the bulk material. This result is different than conclusions of analogous studies made on carbon-based polymers. Two possible explanations are given based on the unique characteristics of the silicon-oxygen bonds in PDMS.

The Journal of Physical Chemistry, 1996

A series of end-linked poly(dimethylsiloxane) (PDMS) networks were prepared with different cross-... more A series of end-linked poly(dimethylsiloxane) (PDMS) networks were prepared with different cross-link functionalities and molecular weights. This was achieved by simultaneous end-linking and self-condensation of a trifunctional silane cross-link precursor. These networks had a nonpolar naphthalene chromophore covalently attached to a fraction of the cross-link junctions. We probe the time-dependent reorientation of the naphthalene, and infer reorientation of the cross-links, by determining the time-dependence of the fluorescence depolarization in the picosecond time domain. A two-step relaxation model describes the orientational dynamics. Fast, partial depolarization in a restricted geometry is superimposed on a slower relaxation that completely depolarizes the fluorescence. We determine the two rotational diffusion constants at temperatures varying from 235 to 298 K, while we vary network parameters such as cross-link density, molecular weight, and macroscopic strain. These diffusion constants have an Arrhenius activation energy of 11.4 ( 0.8 kJ/mol. The fast relaxation is driven by motions of a few chain segments; this process is dominated by the density of the network polymer around the labeled cross-links. The slower, complete reorientation is driven by cooperative motions of a larger number of chain segments connected to the cross-link that are insensitive to steric constraints in the immediate vicinity of the cross-links. fluorescence anisotropy from nonpolar 2-naphthalene that is directly bound to the cross-link junctions. We incorporate this probe into the network by having naphthyltriethoxysilane (NTES) participate as a cross-linking precursor in the networkforming reaction. The aim of this study is to correlate the structure of the network to the local rotational motions, as probed by timedependent fluorescence depolarization. In particular, we monitor the change of probe rotational dynamics as a function of the network chain length, M c , and cross-link functionality, φ avg . We also determine the influence of macroscopic strain on the crosslink rotational dynamics by monitoring the fluorescence depolarization when the network is swollen in a low viscosity oil. For networks with these varying parameters, we determine r(t), the time-dependent fluorescence anisotropy. Subsequently using the fitting parameters in a model that describes rotational diffusion, we determine the rotational diffusion constants.

InSAR time series analyses can be fine-tuned for specific applications, yielding a potential incr... more InSAR time series analyses can be fine-tuned for specific applications, yielding a potential increase in benchmark density, precision and reliability. Here we demonstrate the algorithms developed for gas pipeline monitoring, enabling operators to precisely pinpoint unstable locations. This helps asset management in planning, prioritizing and focusing in-situ inspections, thus reducing maintenance costs. In unconsolidated Quaternary soils, ground settlement contributes to possible failure of brittle cast iron gas pipes and their connections to houses. Other risk factors include the age and material of the pipe. The soil dynamics have led to a catastrophic explosion in the city of Amsterdam, which triggered an increased awareness for the significance of this problem. As the extent of the networks can be very wide, InSAR is shown to be a valuable source of information for identifying the hazard regions. We monitor subsidence affecting an urban gas transportation network in the Netherlands using both medium and high resolution SAR data. Results for the 2003-2010 period provide clear insights on the differential subsidence rates in the area. This enables characterization of underground motion that affects the integrity of the pipeline. High resolution SAR data add extra detail of door-to-door pipeline connections, which are vulnerable due to different settlements between house connections and main pipelines. The rates which we measure represent important input in planning of maintenance works. Managers can decide the priority and timing for inspecting the pipelines. The service helps manage the risk and reduce operational cost in gas transportation networks.

Pure and Applied Chemistry, 2000

Macromolecules, 1995

Poly(dimethylsi1oxane) (PDMS) networks were fluorescently labeled with the 1-(dimethylamino)-5-na... more Poly(dimethylsi1oxane) (PDMS) networks were fluorescently labeled with the 1-(dimethylamino)-5-naphthalenesulfonyl (dansyl) chromophore. The networks were prepared by end-linking silanolterminated PDMS with a mixture of methyltriethoxysilane (MTES) and N-((triethoxysily1)propyl)dansylamide (DTES). The selective sorption behavior of these networks swollen in binary mixtures of 1,Cdioxane and a series of water and linear alcohols was studied. Refractometry and equilibrium swelling measurements were used to determine the amounts of PDMS and solvent components inside the swollen networks. We find that the cosolvency behavior of the solvent mixture depends on the alkyl chain length of the amphiprotic solvent. Steady-state fluorescence spectra of the dansyl compound dissolved in binary solvent mixtures are compared to the fluorescence of dansyl-labeled networks swollen with the same solvent mixture. Differences in emission energy are related to differences in the polarity of the solvation shells surrounding the dansyl moieties. The composition around the labeled cross-link junctions is compared with the bulk composition to determine spatial heterogeneity in the distribution of solvent and polymer molecules. We find that while dioxane is preferentially sorbed, the alcohols preferentially solvate the fluorescent moieties. Water has a tendency to cluster inside the networks that is stronger than its tendency to preferentially solvate dansyl.

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1999

Journal of Applied Physics, 2001

Methods to chemically passivate the surfaces of amorphous-carbon films (a-C) produced by dc magne... more Methods to chemically passivate the surfaces of amorphous-carbon films (a-C) produced by dc magnetron sputtering were studied. The chemical composition of carbon surfaces produced via sputtering are dependent upon the environment to which the carbon is exposed immediately following deposition. When the sputtered film is vented to ambient conditions, free radicals produced at the surface during the deposition process are

IEEE Transactions on Magnetics, 1999

Surface energy is an important surface property of a magnetic recording disk There are three pred... more Surface energy is an important surface property of a magnetic recording disk There are three predominant factors that influence the surface energy of a disk: (1) lubricant, (2) carbon overcoat and (3) interaction between the lubricant and carbon overcoat. In this work we show how the lubricant thickness and carbon overcoat composition influence the surface energy of the disk Furthermore,

Tribology Letters, 1998

Perfluoropolyethers (PFPE) are low surface tension liquids that are commonly employed in magnetic... more Perfluoropolyethers (PFPE) are low surface tension liquids that are commonly employed in magnetic recording devices (hard-disk drives) as disk lubricants. In current drives, a single monolayer (or less) of a PFPE is applied to the amorphous carbon overcoat of the hard disk to ...

Chemistry of Materials, 1995

... Muter. 1993, 5, 422. ... (34) Shea, KJ; Stoddard, GJ Macromolecules 1991, 24, 1207. (35) Shea... more ... Muter. 1993, 5, 422. ... (34) Shea, KJ; Stoddard, GJ Macromolecules 1991, 24, 1207. (35) Shea, KJ; Stoddard, GJ; Shavelle, DM; Wakui, F.; Choate, (36) Seo, T.; Take, S.; Miwa, K.; Hamada, K.; Iijima, T. Macromol-(37) Holmes-Farley, SR; Whitesides, GM Langmuir 1986,2, 266. ...