Biodegradable synthetic and semisynthetic polymers

As a part of our continuing interest in the preparation of multifunctional polymeric materials and evaluation of their potential in pharmaceutical, medical and environmental fields, a survey on new polymeric systems of synthetic and semisynthetic origin, susceptible to bioerosion and/or biodegradation attack is presented. In particular, attention has been focused on the design, preparation and characterization of the following classes of hydrophilic and/or water-soluble polymeric materials: hydroxyl-containing poly(ester)s, hydroxyl-containing poly(amide)s, hydrophilic polymers containing carboxylic groups, perhydroxylated oligo- and poly(saccharide)s, oxidized poly(saccharide)s, and hybrid polymeric materials based on blends of carboxyl-containing synthetic polymers and proteins. The rationale of the research activity undertaken over the years is illustrated by focusing on some specific achievements of practical significance.     

Stereoelective polymerization of racemic α-olefins: Influence of conversion and of nature of catalyst components

The dependence of stereoelectivity on conversion has been investigated by polymerizing racemic 4-methyl-1-hexene (I) and 3,7-dimethyl-1-octene (II) in the presence of catalysts prepared from TiCl₄ and bis[(S)-2-methylbutyl]-zinc(III) or tris [(S)-2-methylbutyl]aluminum (IV). The influence of the nature of transition metal halide has also been examined by polymerization of racemic I and II with catalysts obtained from III and transition metal halides as well as from TiCl₄ and optically active organometallic compounds containing phenyl groups. Stereoelectivity remains practically constant or increases slightly with increasing conversion, but it is affected by the nature of transition metal halide. Moreover a detectable variation of stereoelectivity was observed by changing the chemical structure of the alkyl group in the optically active organometallic component of the catalyst. In any case, however, stereoelectivity is not higher than 5–10%. Evidence in favor of asymmetric induction by the catalytic complex is reported and discussed.     

ɛ-nets and simplex range queries

We demonstrate the existence of data structures for half-space and simplex range queries on finite point sets ind-dimensional space,d≥2, with linear storage andO(n ^α ) query time, $$\alpha = \frac{{d(d - 1)}}{{d(d - 1) + 1}} + \gamma for all \gamma > 0$$ . These bounds are better than those previously published for alld≥2. Based on ideas due to Vapnik and Chervonenkis, we introduce the concept of an ?-net of a set of points for an abstract set of ranges and give sufficient conditions that a random sample is an ?-net with any desired probability. Using these results, we demonstrate how random samples can be used to build a partition-tree structure that achieves the above query time.     

Molecular Interactions and Inclusion Phenomena in Substituted β-Cyclodextrins. Simple Inclusion Probes: H2O, C, CH4, C6H6, NH4 +, HCOO−

Using a simple molecular mechanics approach interaction energy profiles of simple probes (C, CH₄, C₆H₆, H₂O, NH₄ ⁺, and HCOO^-) passing through the center of the -CD ring cavity along the main molecular symmetry axis were first evaluated. Molecular Electrostatic Potential (MEP) values along the same path were also evaluated. The effect of the flexibility of the host -CD molecule together with solute-solvent (H₂O) interactions have been represented by averaging structures of MD calculations for -CD alone and -CD surrounded by 133 H₂O molecules. The effect of various substitutions of -CD has also been evaluated. Small symmetric hydrophobic probes (such as C, CH₄, C₆H₆) are predicted to form stable inclusion complexes with non-substituted and substituted -CDs, the probe position typically being near the cavity center. The stability of the inclusion complexes will increase with increasing size and aliphatic character of the probe. Small polar and charged probes (such as H₂O, NH₄ ⁺, HCOO^-) are predicted to prefer the interaction with the solvent (water) in the bulk phase rather than the formation of inclusion complexes with non-substituted and substituted -CDs. Guest–host interactions in the stable inclusion complexes with hydrophobic probes are almost entirely dominated by dispersion interactions. The MEP reaches magnitudes close to zero in the center of the non-substituted -CD ring cavity and in most of the studied substituted -CDs and shows maximum positive or negative values outside of the cavity, near the ring faces. Substitution of -CD by neutral substituents leads to enhanced binding of hydrophobic probes and significant changes in the MEP profile along the -CD symmetry axis.     

Discrepancy and approximations for bounded VC-dimension

Let (X, ) be a set system on ann-point setX. For a two-coloring onX, itsdiscrepancy is defined as the maximum number by which the occurrences of the two colors differ in any set in . We show that if for anym-point subset the number of distinct subsets induced by onY is bounded byO(m ^d) for a fixed integerd, then there is a coloring with discrepancy bounded byO(n ^1/2–1/2d(logn)^1+1/2d ). Also if any subcollection ofm sets of partitions the points into at mostO(m ^d) classes, then there is a coloring with discrepancy at mostO(n ^1/2–1/2dlogn). These bounds imply improved upper bounds on the size of -approximations for (X, ). All the bounds are tight up to polylogarithmic factors in the worst case. Our results allow to generalize several results of Beck bounding the discrepancy in certain geometric settings to the case when the discrepancy is taken relative to an arbitrary measure.Work of J.M. and E.W. was partially supported by the ESPRIT II Basic Research Actions Program of the EC under contract no. 3075 (project ALCOM). L.W. acknowledges support from the Deutsche Forschungsgemeinschaft under grant We 1265/1-3, Schwerpunktprogramm Datenstrukturen und effiziente Algorithmen.     

Application of EU Water Framework Directive for monitoring of small water catchment areas in Hungary, II. Preliminary study for establishment of surveillance monitoring system for moderately loaded (rural) and heavily loaded (urban) catchment areas

For the elaboration of a monitoring guidance corresponding to the EU Water Framework Directives, a comprehensive monitoring network on small water catchment areas was developed on three investigated areas in Hungary. In the present study the experience of monitoring of the Galga and Rákos streams are presented, which represent a moderately loaded rural and a heavily impacted urbanic catchment area, respectively. Hydrological, biological and chemical parameters were monitored during one year. The influence of human impact on the spatial and temporal variability of parameters and the appropriate frequency of sampling were compared. Necessary actions to improve the ecological status can be suggested on the basis of the above monitoring programme.     

Visibility graphs and obstacle-avoiding shortest paths

Summary Two closely related problems in Computational Geometry are determining visibility graphs and shortest paths in a two- or three-dimensional environment containing obstacles. Applications are within Computer Graphics and Robotics. We give a survey on recent research done on efficient algorithms for these problems.

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Zusammenfassung Zwei eng miteinander verwandte Probleme in der algorithmischen Geometrie sind die Bestimmung von Sichtbarkeitsgraphen und kürzesten Wegen in einer zwei- oder dreidimensionalen Umgebung mit Hindernissen. Anwendungen finden sich insbesondere auf den Gebieten Computergrafik und Robotik. Diese Arbeit gibt einen Überblick über kürzlich erschienene Arbeiten zum Entwurf effizienter Algorithmen für diese Probleme.

     

Cutting dense point sets in half

A halving hyperplane of a set S of n points in R ^d contains d affinely independent points of S so that equally many of the points off the hyperplane lie in each of the two half-spaces. We prove bounds on the number of halving hyperplanes under the condition that the ratio of largest over smallest distance between any two points is at most , δ some constant. Such a set S is called dense. In d = 2 dimensions the number of halving lines for a dense set can be as much as , and it cannot exceed . The upper bound improves over the current best bound of which holds more generally without any density assumption. In d = 3 dimensions we show that is an upper bound on the number of halving planes for a dense set. The proof is based on a metric argument that can be extended to d≥ 4 dimensions, where it leads to as an upper bound for the number of halving hyperplanes. Received March 22, 1995, and in revised form January 15, 1996.     

Liquid Crystalline Side Chain Polysiloxanes with 4-Cyano- and 4-Alkoxy-Stilbene Mesogenic Units

Abstract

Side chain homo- and copolysiloxanes with 4-cyano- and 4-alkoxy-4′-stilbene mesogens, spaced apart from the backbone by oligomethylene segments of variable lengths, were synthesized via a hydrosilylation coupling reaction of five stilbene-containing α-olefins with four commercial poly(methylhydrosiloxane)s and poly(methyl-hydro-∞-dimethylsiloxane)s. Broad smectic phases were observed for the polysiloxanes with cyanostilbene mesogens, whereas the homologous with alkoxy-terminated stilbenes displayed only narrow mesophases in the high temperature range. Preliminary room tem-perature X-ray diffraction studies on mechanically oriented samples evidenced the occurrence of side chain crystallization, microphase separation and indicated the interdigitated smectic A nature of the mesophases.     

Nanostructured films of amphiphilic fluorinated block copolymers for fouling release application

New amphiphilic block copolymers S nSz m consisting of blocks with varied degrees of polymerization, n and m, of polystyrene, S, and polystyrene carrying an amphiphilic polyoxyethylene-polytetrafluoroethylene chain side-group, Sz, were prepared by controlled atom transfer radical polymerization (ATRP). The block copolymers, either alone or in a blend with commercial SEBS (10 wt% SEBS), were spin-coated in thinner films (200-400 nm) on glass and spray-coated in thicker films ( approximately 500 nm) on a SEBS underlayer (150-200 microm). Angle-resolved X-ray photoelectron spectroscopy (XPS) measurements proved that at any photoemission angle, varphi, the atomic ratio F/C was larger than that expected from the known stoichiometry. Consistent with the enrichment of the outer film surface (3-10 nm) in F content, the measured contact angles, theta, with water (theta w > or = 107 degrees ) and n-hexadecane (theta h > or = 64 degrees ) pointed to the simultaneous hydrophobic and lipophobic character of the films. The film surface tension gamma S calculated from the theta values was in the range 13-15 mN/m. However, the XPS measurements on the "wet" films after immersion in water demonstrated that the film surface underwent reconstruction owing to its amphiphilic nature, thereby giving rise to a more chemically heterogeneous structure. The atomic force microscopy (AFM) images (tapping mode/AC mode) revealed well-defined morphological features of the nanostructured films. Depending on the chemical composition of the block copolymers, spherical (ca. 20 nm diameter) and lying cylindrical (24-29 nm periodicity) nanodomains of the S discrete phase were segregated from the Sz continuous matrix (root-mean-square, rms, roughness approximately 1 nm). After immersion in water, the underwater AFM patterns evidenced a transformation to a mixed surface structure, in which the nanoscale heterogeneity and topography (rms = 1-6 nm) were increased. The coatings were subjected to laboratory bioassays to explore their intrinsic ability to resist the settlement and reduce the adhesion strength of two marine algae, viz., the macroalga (seaweed) Ulva linza and the unicellular diatom Navicula perminuta. The amphiphilic nature of the copolymer coatings resulted in distinctly different performances against these two organisms. Ulva adhered less strongly to the coatings richer in the amphiphilic polystyrene component, percentage removal being maximal at intermediate weight contents. In contrast, Navicula cells adhered less strongly to coatings with a lower weight percentage of the amphiphilic side chains. The results are discussed in terms of the changes in surface structure caused by immersion and the effects such changes may have on the adhesion of the test organisms.