Showing papers in "Macromolecular Chemistry and Physics in 2005"

On the Interpretation of X-Ray Diffraction Powder Patterns in Terms of the Nanostructure of Cellulose I Fibres

Abstract: Summary: The average lateral dimensions of cellulose microfibrils in paper sheets can be determined from the width of X-ray Bragg peaks at half the maximum height. The method assumes it is the finite size of the crystallite which is the main contributor to peak broadening in the directions orthogonal to the long axis of the crystallite. This assumption is examined in the context of samples from different botanical sources, of varying crystallinity and cellulose content and the various instrumental sources of broadening X-ray powder patterns. Excellent fits to the data from paper samples can be obtained by using five Voigt functions to the powder diffraction patterns, for the 101, , 021, 002 and 040 reflections. Of these reflections, the 020 is the clearest, and the best able to determine the lateral dimension of the crystallite. Results indicate that the size of microfibrils is dependent on the botanical source of the fibres.

Synthesis, Characterization and Behavior in Aqueous Solution of Star-Shaped Poly(acrylic acid)

Abstract: We report the synthesis of star-shaped poly(acrylic acid) (PAA), with 5, 8, and 21 arms, by atom transfer radical polymerization of tert-butyl acrylate. We employ the core-first approach using glucose-, saccharose- and cyclodextrin-based initiators. Subsequent acidic treatment of poly(tert-butyl acrylate) (PtBA) leads to star-shaped poly(acrylic acid) (PAA). Alkaline cleavage of the arms enabled us to determine the initiation site efficiency. The PAA stars and arms were esterified to poly(methyl acrylate) (PMA). Molecular weight determination by means of GPC/viscosity, MALDI-TOF MS and NMR end-group determination showed that the initiation site efficiency is close to unity. Results from potentiometric titration of PAA arms and stars show that the apparent pK a values increase with increasing arm number, which is a direct result of increasing segment density. Osmometry measurements of aqueous solutions of the PAA stars result in osmotic coefficients between 0.05 and 0.38, indicating that most of the counterions are confined within the star. The confinement increases with arm number.

Preparation of Fluorescent Carboxyl and Amino Functionalized Polystyrene Particles by Miniemulsion Polymerization as Markers for Cells

Abstract: Summary: A series of fluorescent polystyrene latex particles with carboxyl and amino functionalities on their surface were synthesized by the miniemulsion technique. The fluorescent dye N-(2,6-diisopropylphenyl)perylene-3,4-dicarboximide (PMI) was incorporated into the copolymer nanoparticles formulated from styrene and acrylic acid or styrene and aminoethyl methacrylate hydrochloride. The resulting latexes were stable and showed a monodisperse size distribution. The particle size depended on the amount and nature of the functional comonomer and was in the range 100–175 nm. All latexes were characterized by transmission electron microscopy (TEM), dynamic light scattering, UV-Vis spectroscopy and zeta potential measurements. The amount of surface functional groups was determined by electrolyte titration. Furthermore, the functionalized fluorescent particles were utilized as markers for HeLa cells and cell uptake was visualized using fluorescence microscopy. The correlation of the uptake of nanoparticles with the surface charge was determined by FACS measurements. Confocal fluorescent microscopy of HeLa cells after the uptake of amino functionalized particles (green).

Synthesis of Novel Polycations Using the Chemistry of Ionic Liquids

Abstract: In this work, we report the synthesis of a great variety of polycations with varying counter-anions. These new polymers were obtained by a simple anion exchange reaction facilitated by the phase separation of the resulting products. This strategy has been successfully applied to three different polycations, poly(1-vinyl-3-ethylimidazolium bromide) poly(ViEtIm+Br-), poly(1-ethyl-4-vinylpyridinium bromide) poly(ViEtPy + Br - ), and poly(methacryloyloxyethyltrimethylammonium chloride) poly(EMTMA + Cl - ), with seven counter-anions such as PF - 6 , CF 3 SO - 3 , (CF 3 SO 2 ) 2 N - , (CF 3 CF 2 SO 2 ) 2 N - , dodecylbenzenesulfonate, toluene-4-sulfonate, and bis(2-ethylhexyl) hydrogen phosphate. The solubility range of the new polymeric ionic liquids becomes very broad, including apolar organic solvents and ionic liquids, depending on the nature of the counter-anion. Thermogravimmetric experiments showed that the thermalstability of the PILs also depends on the nature of the counter-anion improving in the order CF 3 SO - 3 > (CF 3 CF 2 SO 2 ) 2 N - > C 12 H 25 C 6 H 4 SO - 3 > PF - 6 > Br - > C 16 H 34 PO - 4 .

Formation of Thermoresponsive Gold Nanoparticle/PNIPAAm Hybrids by Surface-Initiated, Atom Transfer Radical Polymerization in Aqueous Media

Abstract: We investigated the formation of thermoresponsive gold nanoparticle/poly(N-isopropylacrylamde) (AuNP/ PNIPAAm) core/shell hybrid structures by surface-initiated, atom transfer radical polymerization (SI-ATRP) in aqueous media and the effect of cross-linking on the thermoresponsiveness of the AuNP/PNIPAAm hybrids. The disulfide containing an ATRP initiator was attached onto AuNPs and the monomer, NIPAAm, was polymerized from the surface of AuNPs in the absence or presence of a cross-linker, ethylene diacrylate, in aqueous media at room temperature. The resulting brush-type and cross-linked AuNP/PNIPAAm hybrids were characterized by Fourier-transform infrared spectroscopy, transmission electron microscopy, and variable temperature dynamic light scattering.

Surface Functionalized ZnO Particles Designed for the Use in Transparent Nanocomposites

Abstract: Summary: We present a new method for preparing a nanocomposite containing highly dispersed ZnO nanoparticles. The essence of this approach is the formation of surface modified inorganic particles and their homogeneous incorporation into a polymeric matrix. Therefore, an inverse emulsion of an inorganic salt, for example, zinc acetate stabilized by amphiphilic statistical copolymers, is prepared. By hydrolysis under basic conditions ZnO particles of a size below 100 nm are formed within the micelles. As the surfactant remains adsorbed on the surface of the particles, a surface functionalization with hydrophobic chains is performed. These chains can be considered as compatibilizers for blending of the ZnO nanoparticles in PMMA affording transparent nanocomposite films avoiding particle aggregated. The present approach is in principle applicable to all nanoparticles prepared in a sol-gel process. TEM image of redispersed ZnO particles.

Multicompartment Micelles: Has the Long‐Standing Dream Become a Reality?

Abstract: Multicompartment micelles are complex nanosized systems that possess a hydrosoluble shell and a hydrophobic core, which is characterized by segregated incompatible subdomains. With roots starting about ten years ago, the field of multi compartment micelles has evolved slowly, until recently when significant achievements have been made. The present article reviews strategies for building such micellar assemblies as well as morphological studies, highlights the future challenges, and discusses possible applications, which exploit the coexistence of differentiated nano- domains. Formation of multi compartment micelles using miktoarm stars mu-(polyethylethylene)(poly(ethylene oxide))(poly(perfluoropropylene oxide)) and a cryo-TEM image visualizing the process

Study of the Degradation of an Epoxy/Amine Resin, 1

Abstract: Summary: The degradation of an epoxy/amine resin was studied under photolytic conditions in absence and in presence of oxygen and under thermooxidative conditions. The resin is obtained after curing the diglycidyl ether of bisphenol-A (DGEBA) with a poly(propylene glycol) which is diamine terminated. The mechanisms of degradation of the two separate components before curing were also investigated. Analysis by IR and UV-Vis spectroscopies revealed that the oxidation photoproducts detected during the degradation of the epoxy/amine result from the oxidation of the DGEBA part, of the amine crosslinks and, to a lower extent, of the poly(oxypropylene) units. Their mechanism of formation essentially involves chain scission reactions. During irradiation, the yellowing observed is mainly due to the formation of a quinone methide structure. Thermooxidation generates high discoloration with some products only generated in that case. For all experiments, a detailed analysis of the IR spectra is given with the chemical mechanisms of formation of the degradation products. ATR-IR spectra of an epoxy/amine sample in the carbonyl region during photooxidation.

(Plasticized) polylactide/(organo-)clay nanocomposites by in situ intercalative polymerization

Abstract: Both intercalated and exfoliated poly(L,L-lactide) (P(L,L-LA)/organomodified montmorillonite nanocomposites were synthesized by in situ ring-opening polymerization of L,L-lactide, in bulk, directly in the presence of the nano-filler. Intercalation of polyester chains was found to appear even for natural unmodified montmorillonite-Na + , while exfoliation occurred when the aluminosilicate layers were modified by ammonium cations bearing primary hydroxyl groups. Clay delamination was effectively triggered by the grafting reaction of the growing PLA chains onto the hydroxyl groups. Aluminium triisopropoxide, triethylaluminium, and stannous octoate, as initiating or co-initiating species, were compared in terms of polymerization control. The influence of nanoclay content (from 1 to 10 wt.-% in inorganics) on morphology and thermal behavior was also studied. In parallel, a highly filled nanocomposite (called master-batch), prepared by in situ polymerization, was dispersed into a (plasticized) preformed polylactide matrix in the molten state, to reach a better clay delamination than that obtained by direct melt blending. Finally, L,L-lactide and α,ω-dihydroxylated poly(ethylene glycol) (PEG 1000) were copolymerized in presence of clay in order to study the behavior of the resulting triblocks towards nanocomposite formation.

Vacuum ultraviolet photolysis of hydrocarbon polymers

Abstract: The wavelength-dependent vacuum ultraviolet (VUV) photolysis of several polymers, low density polyethylene (LDPE), biaxially oriented poly(propylene) (BOPP), atactic polystyrene (PS), and poly(methyl methacrylate) (PMMA), was studied by irradiation in vacuum with the well- characterized emissions from four different resonant or excimer VUV sources. These lamps comprise radiofrequency (r.f.) discharges in different noble gases, such as krypton, xenon (at low pressures, producing near-monochromatic resonant line radiations), xenon excimer (at high pressure), and a deuterium/argon mixture (producing a broad-band emission). VUV-induced mass loss (ablation or etching) was monitored in situ by quartz crystal microbalance measurements. Following irradiation, samples were analysed by ATR-FTIR and XPS, to evaluate near-surface structural changes (e.g., creation of unsaturation, cross-linking) resulting from the VUV-initiated bond scissions and radical-creation reactions. PMMA was the most readily ablatable polymer, whereas the mass loss of BOPP was higher than that of LDPE, regardless of the irradiation wavelength, λ. All polymers were found to form double bonds, with the exception of PS, which is rather stable, probably due to energy dissipation by fluorescence.

Photodeformable Polymers: A New Kind of Promising Smart Material for Micro- and Nano-Applications

Abstract: Starting in the 1960s, several kinds of photodeformable polymers have been developed, such as monolayers, polymer gels, solid films and liquid-crystalline elastomers with different photodeformation mechanisms. This field evolved slowly until recently when significant achievements have been made. Most recently, Lendlein and co-workers have put forward another new concept - using photo-crosslinking to prepare deformable polymers with various pre-determined shapes (Nature 2005, 434, 879). This highlight gives a general introduction into photodeformable polymers and brings forth future challenges.

Photoreactive Functionalization of Poly(ethylene terephthalate) Track‐Etched Pore Surfaces with “Smart” Polymer Systems

Abstract: Poly(ethylene terephthalate) (PET) track-etched membranes with a pore diameter of ca. 700 nm, optionally surface-functionalized to create a carboxyl or amino surface, were used for heterogeneous graft copolymerizations. Grafted poly(acrylic acid) acted as a pH responsive, smart polymer. To evaluate the surface-specific initiation of graft copolymerizations unmodified and primary functionalized membranes were systematically combined with three differently charged benzophenone derivatives as photoinitiators, which were adsorbed on the PET surface prior to the reaction. The functionalized membranes thus obtained were characterized for chemical structure and permeability as a function of pH. The pore structure with a narrow size distribution and the resulting high sensitivity of the membrane permeability to the grafting functionalizations made the track-etched PET membranes a very useful tool for analyzing structure and function of responsive grafted polymer layers. The primary functionalization of the base polymer with a molecular layer of a multifunctional alkylamine (amino surface) enhanced the efficiency of the subsequent graft copolymerization via photoinitiated hydrogen abstraction considerably because a high density of well accessible reactive groups had been introduced. The preadsorption of the photoinitiator on the base polymer surface was significantly improved by ionic interactions between the respective functional groups of the surface and the photoinitiator. Such a photoinitiator preadsorption, especially when combined with a reactive layer from the primary functionalization, enabled a very efficient and surface selective functionalization because a better control of grafting density and a reduction of photoinitiated side reactions along with a more efficient use of the photoinitiator were possible.

Accessing Chain Length Dependent Termination Rate Coefficients of Methyl Methacrylate (MMA) via the Reversible Addition Fragmentation Chain Transfer (RAFT) Process

Abstract: The RAFT-CLD-T methodology is demonstrated to be not only applicable to 1-substituted monomers such as styrene and acrylates, but also to 1,1-disubstituted monomers such as MMA. The chain length of the terminating macromolecules is controlled by CPDB in MMA bulk free radical polymerization at 80 degrees C. The evolution of the chain length dependent termination rate coefficient, k(t)(i,i), was constructed in a step-wise fashion, since the MMA/CPDB system displays hybrid behavior (between conventional and living free radical polymerization) resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The obtained CLD of k(t) in MMA polymerizations is compatible with the composite model for chain length dependent termination. For the initial chain-length regime, up to a degree of polymerization of 100, k(t) decreases with alpha (in the expression k(t)(i,i) = k(t)(0) . i(-alpha)) being close to 0.65 at 80 degrees C. At chain lengths exceeding 100, the decrease is less pronounced (affording an alpha of 0.15 at 80 degrees C). However, the data are best represented by a continuously decreasing nonlinear functionality implying a chain length dependent alpha.

Synthesis and Properties of Syndiotactic Poly(propylene)/Carbon Nanofiber and Nanotube Composites Prepared by in situ Polymerization with Metallocene/MAO Catalysts

Abstract: The preparation of syndiotactic poly(propylene) (sPP) nanocomposites with multi-walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), and carbon black (CB) as fillers was accomplished by the in situ polymerization of propylene with a metallocene/methylalumoxane (MAO) catalyst. Different pre-treatments were applied to achieve a homogeneous dispersion of the fillers in the matrix. The resulting nanocomposites were investigated with respect to their properties, which were then compared to those of the pure polymer and among each other. The thermal stability of the nanocomposites was slightly enhanced compared to the pure polymer. In addition, the yield strength of the nanocomposites could be slightly raised in comparison to the neat sPP. The most significant influence of the nanofillers was detected on the crystallization behavior. The crystallization temperature was increased with rising filler content in all cases. Moreover, the half-time of crystallization was significantly reduced in the nanocomposites. The rate constant of crystallization was also greatly enhanced for all nanocomposites as compared to the neat sPP. The nanofillers acted, therefore, as nucleating agents. This effect was most pronounced in the case of MWNTs as fillers.

Effect of the Polyimide Structure and ZnO Concentration on the Morphology and Characteristics of Polyimide/ZnO Nanohybrid Films

Abstract: A series of polyimide/ZnO nanohybrid films with different ZnO content were prepared from a rigid pyromellitic dianhydride-4,4'-diaminodiphenyl ether (PMDA-ODA) polyimide (PI) and a flexible 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride-4,4'-diaminodiphenyl ether (BTDA-ODA) PI with ZnO nanoparticles (3-4 nm). Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) depict that the ZnO nanoparticles function as a physical cross-linking agent with PI through hydrogen bonding between the OH on the ZnO nanoparticles and the C=O of the imide groups. ZnO nanoparticles in the rigid PMDA-ODA matrix cause a larger percentage decrease in the coefficient of linear thermal expansion (CTE) than in the flexible BTDA-ODA matrix. The BTDA-ODA/ZnO hybrid films have two transition peaks in dynamic mechanical tan δ curves, but PMDA-ODAIZnO hybrid films only have one transition peak. Thermogravimetric analysis reveals that ZnO decreases the thermal degradation temperature (T d ) in both hybrid films, but less so in PMDA-ODA/ZnO films. Transmission electron microscopy (TEM) images reveal that the rigid matrix induces larger particle size (30-40 nm) compared to the flexible matrix (10-15 nm).

Reactive compatibilization of melt mixed PA6/SWNT composites: Mechanical properties and morphology

Abstract: Styrene maleic anhydride copolymer (SMA) encapsulated single wall carbon nanotubes (SWNT) were melt mixed with polyamide 6 (PA6) utilizing an interfacial reaction in order to disperse SWNT homogeneously and to enhance interfacial adhesion. The intended reactive coupling between PA6 and SMA was evident from IR spectroscopy. Raman spectroscopy was used to characterize the SWNT and the nanocomposites. Studies on crystallization behavior showed the nucleating behavior of SWNT in the composites, as found using DSC and WAXD. The encapsulation of SWNT by SMA copolymer leads to increased elongation at break in the composites, which can be attributed to the enhanced interfacial adhesion between PA6 and SMA modified SWNT as observed from the SEM investigation of tensile fractured surfaces of PA6/SWNT+SMA composites. The glass transition temperature of PA6 increased significantly in the composites.

Photoinduced Polymerization of Thiophene Using Iodonium Salt

Abstract: Summary: Thiophene was polymerized by means of UV irradiation using diphenyliodonium hexafluorophosphate as a photoinitiator. An initiation mechanism involving electron transfer from photochemically generated onium radical cations to thiophene was proposed. N-Ethoxy-2-methylpyridinium hexafluorophosphate and triphenylsulfonium hexafluorophosphate were also found to be effective in facilitating the polymerization of thiophene. Polymerization is accompanied with the film formation on the surface of the reaction tube. The photochemically obtained polymers were characterized by FT-IR, DSC, and SEM analyses and compared with those obtained by oxidative and electrochemical means. Electrical conductivities of the samples were measured by four-probe technique. Photograph of the polymerization mixture containing thiophene and iodonium salt in CH2Cl2 before (a), after 5 min of irradiation (b), and dried film (c).

Metathetic Selective Degradation of Polyisoprene: Low‐Molecular‐Weight Telechelic Oligomer Obtained from Both Synthetic and Natural Rubber

Abstract: Degradation studies of cis-1,4-polyisoprene were carried out using first and second generation Grubbs catalysts to achieve end-functionalized acetoxy oligomers in both an organic solvent and a latex phase at room temperature. Well-defined acetoxy telechelic polyisoprene structures were obtained in a selective manner with arange of M n from 10 000 to 30 000, with a polydispersity index of around 2.5.

Influence of Addition of Organoclays on Morphologies in Nylon 6/LLDPE Blends

Abstract: Polymer blend nanocomposites containing linear low density polyethylene (LLDPE), nylon 6 and organoclays were prepared by melt mixing, and their morphologies and structures were examined with a field emission scanning electron microscope (FE-SEM) and an X-ray diffractometer (XRD). The size of phase-separated domains decreased considerably with increasing content of organoclay. The d-spacing of organoclay in the nanocomposites was increased from about 18.6 to over 28 A. This effect was highly dependent on nylon 6 contents because nylon 6 is more polar and shows higher affinity to the organoclays compared to LLDPE.

Modification of Natural Rubber by Grafting with Hydrophilic Vinyl Monomers

Abstract: The grafting of two hydrophilic vinyl monomers, dimethylaminoethyl acrylate (DMAEA) and dimethylaminoethyl methacrylate (DMAEMA), onto natural rubber latex was carried out by emulsion polymerization using redox initiation. The effects of the redox initiator concentration and type (cumene hydroperoxide/tetraethylenepentamine, CHP/TEPA; tert-butyl hydroperoxide/tetraethylenepentamine, t-BHP/TEPA; potassium persulfate/potassium metabisulfite, K2S2O8/K2S2O5), the monomer concentration and the reaction temperature on the conversion, the grafting efficiency, the water absorption and the contact angles of the grafted copolymers films, and the colloidal stability of the latexes at low pH, were investigated. Infrared spectroscopic analysis confirmed that DMAEA and DMAEMA were grafted onto the natural rubber particles. The hairy layer structure of NR-g-poly(DMAEMA) latex particles was investigated by transmission electron microscopy using positive and negative stainings with OsO4 and phosphotungstic acid, respectively.

Temperature Dependence of Free Volume in Pure and Silica‐Filled Poly(dimethyl siloxane) from Positron Lifetime and PVT Experiments

Abstract: The microstructure of the free volume and its temperature dependence in two poly(dimethyl siloxane)s (PDMS), one in the pure state and the other filled with 35 wt.-% of an in situ hydrophobized fumed silica with a specific surface area of 200-300 m2 g -1 , were studied by pressure-volume-temperature experiments (PVT, T=22-156°C, P= 10-200 MPa) and positron annihilation lifetime spectroscopy (PALS, T=-173-100°C, P=10 -5 MPa). The Simha-Somcynsky equation of state was used to estimate the hole free volume fraction, h, and free and occupied volumes, V f = hV and V occ = (1 - h)V from the specific total volume, V. The PALS spectra were analyzed with the routine LT9.0, which allowed for a dispersion, σ i , in all three of the lifetimes: the para-positronium (p-Ps, τ 1 ), positron (e + , τ 2 ), and orthopositronium (o-Ps) lifetime (τ 3 ). This kind of analysis delivered correct p-Ps lifetime parameters, τ 1 , σ 1 , and I 1 . It was speculated that e + , like o-Ps, undergoes Anderson localization at empty sites of the, static or dynamic, disordered structure. The hole size distribution, its mean value, (v h ), and dispersion, σ h , were calculated from the o-Ps lifetimes. A comparison of (v h ) with V f was used to estimate the specific hole number, N ' h . During melting of the semicrystalline samples at-38°C (T m ), (v h ) increased abruptly, and σ h suddenly decreased. Both effects are explained by the disappearance of the rigid-amorphous fraction (RAF) and, thus, a reduction in the dynamic heterogeneity. The following leveling-off in (v h ) and the low value of σ h are attributed to the fast segmental relaxation in the PDMS melts which leads to a smearing of the molecule density distribution around a hole during the o-Ps lifetime.

Simple Synthesis of Hierarchically Structured Silicas by Poly(ethyleneimine) Aggregates Pre-Organized by Media Modulation

Abstract: In this paper, we describe the multiple morphogenesis of poly(ethyleneimine) (PEI)-directed silicas through the facile tuning of the media in which the PEIs pre-organize The silica shapes and structures ranged from nanofiber-based asters, fans, particles, plates, and/or curved films to large aggregates composed of unit nanoribbon structures and networks that contained nanospheres or particles, depending on the media compositions or media modulations coupled with the polymer concentrations and chain architectures We found that the media modulations not only controlled the formation of the silica unit structures but also manipulated the hierarchical structures based on the unit silicas Furthermore, methanol modulation successfully allows us to elucidate the dependence of silica deposition ability on the polymer chain architectures that cause subtle differences in their aggregates The approach of methanol modulation to obtain shaped silicas, which involves both PEI aggregation and silica deposition at room temperature, is of particular interest to applications for the immobilization of bioactive components

Energetics of Cooperative Transitions of N-Vinylcaprolactam Polymers in Aqueous Solutions

Abstract: Phase and conformational behavior of poly(N-vinylcaprolactam) was studied as a function of polymer concentration, concentration of NaCI, and concentration of sodium dodecyl sulfate (SDS). Precise measurements of the partial heat capacity of the polymer were first carried out within the temperature range 10-125 °C. Thermodynamic parameters of the phase separation transition (transition temperature, enthalpy, and heat capacity increment) do not change with variations in polymer concentration. In the presence of salt the transition temperature decreases, the enthalpy increases, and the heat capacity increment does not vary significantly. Beginning with the SDS concentration of 0.005 M, the phase separation is suppressed and the heat capacity peak splits into two peaks. Upon further increase in surfactant concentration the amplitude of the low-temperature peak increases while that of the high-temperature peak vanishes. Phase separation transition in aqueous solutions of N-vinylcaprolactam-methacrylic acid copolymers of different compositions were studied at different polymer concentrations and pH (4-12). The transition parameters do not depend on polymer concentration and pH.

Cylindrical Polypeptide Brushes

Abstract: Cylindrical brushes with poly(L-lysine) and poly(L-glutamate) side chains were prepared by grafting through and grafting from techniques. Grafting from is shown to be more successful for the synthesis of cylindrical brushes with high molar mass main and side chains.

Syndiotactic polystyrene containing ortho-dichlorobenzene : Crystal structure of the clathrate form

Abstract: The crystal structure of the clathrate form of syndiotactic polystyrene containing ortho-dichlorobenze-ne has been determined, and compared with those of the other crystalline clathrate structures of s-Ps already known. This guest molecule, characterized by a quite different shape and volume from the others already described, gives rise to the only example of clathrate form of s-PS presenting a statistical orthorhombic symmetry (cell constants a = 17.5 A, b = 22.9 A, c = 7.8 A and space group Ccmb). This symmetry is generated by the random coexistence in the crystals of domains, with the same monocliulc P21/a symmetry found for the other clathrate forms, that can be realized by two different but energetically equivalent, packings of the chains. 0 The unit cell constants are a = 17.5 A, b = 14.4 A, c = 7.8 A and γ = 127.4°. The shape of the guest molecule produces a significant modification of the cavities (if compared to δ form) realized mainly through a shift of the ac layers of chains delimiting the cavities along the a axis direction that turns out in the highest y value found for these kinds of structures.

C-1-symmetric heterocyclic zirconocenes as catalysts for propylene polymerization, 2 - ansa-Zirconocenes with linked dithienocyclopentadienyl-substituted indenyl ligands

Abstract: The synthesis of C 1 symmetric zirconocene complexes bearing the 2,5-dimethyl-7H-cyclopenta]1,2-b:4,3-b']dithiophene ligand (S 2 -3) linked to substituted cyclopentadienes is described. Different syntheses of S 2 -3, the common intermediate for the preparation of these complexes, are discussed. Many of these complexes have been found to be highly active in propylene polymerization, to require very low amounts of methylalumoxane to be actived, and to produce poly(propylene)s of low isotacticity and melting points? 13 C NMR analysis shows that the poly(propylene)s are fully regioregular and that the stereoerrors are randomly distributed, as shown by the enantiomorphic-site triad test E ≃ 1. The experimental pentad distribution was fitted using a two-site model with different probability parameters for each site. The probability of chain back-skip was also taken into account. The molecular weight and crystallinity of the poly(propylene)s are dependent upon the type of substituents on the cyclopentadienyl ring, and the correlation between mmmm and melting point of the PP confirms the random distribution of stereoerrors.

Unique effects of cyclodextrins on the formation and colorimetric transition of polydiacetylene vesicles

Abstract: Summary: The effects of cyclodextrins (CDs) on the formation and color changes of polymerized 10,12-pentacosadiynoic acid (PCDA) vesicles have been investigated. The strategy used in this study focuses on three different stages in which the CDs can play a role. Among the cyclodextrins investigated, α-CD had most prominent effect on the perturbation of the ordered structures of the PCDA vesicles. Solid precipitates, formed by the PCDA vesicles in the presence of α-CD were shown to be PCDA - α-CD inclusion complexes by using DSC, FTIR and 1H NMR analysis. The results of studies using 4-nitrophenol as an inhibitor and the consideration of inner and outer diameters of cyclodextrins have led to the conclusion that the color induced by the cyclodextrins is due to the formation of inclusion complexes with the polydiacetylene. Photograph of the polydiacetylene liposomes after 1 h in the presence of 10 mM of the three different cyclodextrins in a HEPES buffer solution.

Metallocenic Copolymers of Isotactic Propylene and 1‐Octadecene: Crystalline Structure and Mechanical Behavior

Abstract: A set of copolymers of isotactic propylene and 1-octadecene (CiPOD) and the corresponding isotactic poly(propylene) homopolymer (iPP) have been synthesized using a metallocene catalyst. The influence of the incorporation of this long chain comonomer on the structure and final properties exhibited by iPP has been examined. Wide-angle X ray diffraction and optical microscopy have been used to analyze the effect of introducing 1-octadecene comonomer within the iPP backbone. The initial monoclinic crystal lattice with a well-developed lamellar morphology is transformed into a mesomorphic modification showing very small entities. All these structural changes significantly influence the mechanical behavior of these copolymers. Thus, stiffness and microhardness are diminished and the brittle-ductile transition can be observed by simply varying composition when deformation takes place at room temperature. On the other hand, the intensity of the process associated with cooperative movements within the amorphous phase (β relaxation) significantly goes up with the incorporation of 1-octadecene and its location is shifted to lower temperature because of the decrease in the crystallinity content.

New Materials for Rapid Prototyping Applications

Abstract: Rapid prototyping (RP) is a widely used manufacturing tool in the product development cycle. This highlight article gives a brief overview of the currently available RP technique with special emphasis on three-dimensional (3D) printing. The advantages and drawbacks of various RP processes regarding material quality, feature resolution, and surface quality are pointed out. New developments in the field of material development allow the use of polymer ionomers for 3D printing. Using polymer ionomers some of the drawbacks of 3D printing can be eliminated. In particular, the mechanical strength can be increased compared to traditional powder systems used for 3D printing. This article describes the chemical background of polymer ionomers and the relevance of these materials for future developments in RP.

Preparation of Functionalized Core-Shell Alumina/Polystyrene Composite Nanoparticles, 1

Abstract: A convenient method for the preparation of well defined core-shell alumina/polystyrene composite nanoparticles (50 nm) through emulsion polymerization from needle-shaped alumina is reported. The pretreatment of alumina with 3-methacryloxypropyltrimethoxysilane (MPTMS) is essential for successful encapsulation and for creating strong interfacial interaction between alumina and polystyrene (PS). High yield and binding efficiency (both higher than 90%) are achieved by a proper selection of the reaction conditions. As revealed by transmission electron microscopy (TEM) and atomic force microscopy (AFM), an ideal core-shell structure is obtained with one alumina needle as the core and PS as the shell when the amount of alumina is appropriate. However, when more alumina is used, the PS latexes contain an agglomerate of a few alumina needles arranging in a parallel fashion without any precipitation of alumina. The analysis of the grafted and free PS has shown that the grafted PS has bigger molecular weight (MW) and larger molecular weight distribution (MWD) presumably due to some condensation between the methoxyl groups of the silane moiety. The polymerization kinetics and the encapsulation mechanism have also been discussed.