Surface free energies of silica fillers and their relation to the adsorption of poly(ethylene terephthalate)

The surface free energy of modified silica as well as of PET oligomers was evaluated through measurements of specific retention volumes of several probe molecules by use of the adsorption and adhesion principles in inverse gas chromatography. The nondispersive component of surface free energy of most silica fillers was larger than the dispersive component and the acidic component was much larger than the basic one, which indicated that the surfaces of most silica fillers were rather acidic. These methods were also applied to PET oligomer and it was found that the surface free energy of PET oligomer, regardless of preparation method, consisted of an almost dispersive component, suggesting that the surface of PET was neutral. The amount of PET oligomer adsorbed for the heat-treated silica fillers in acidic solvent increased linearly with increased acidic component of the surface free energy, which indicates that the acidic component of the surface free energy may be responsible for the adsorption. However, the adsorption amount on modified silica is much smaller than that for the heat-treated silica fillers because of steric hindrance caused by the attached organic chain, suggesting that the adsorption cannot be determined only by the surface free energy.     

Structure and properties of epoxy composites cured with new organosilicon amines

The results of the curing of bisphenol A-based epoxy oligomers (ED-20 and NPEL-128) with silicon-containing amines (SCAs) synthesized by the interaction between bifunctional alcohols with different molecular masses and γ-aminopropyltriethoxysilane (AGM-9) are reported. It is shown that, in the case of using SCAs, epoxy oligomer is cured more completely and a higher degree of elasticity in the polymer matrix is achieved without using modifiers, which is impossible in the case when polyethylene polyamines are used.     

Rediscovering the problem of interpretation of chromatographically determined enthalpy and entropy of adsorption of different adsorbates on carbon materials. Critical appraisal of literature data

Adsorbate-adsorbent and adsorbate-adsorbate interactions having decisive influence on the distribution of adsorbate between gas-solid phases in inverse gas chromatography (IGC) have been thermodynamically explained. Specific retention volumes, second adsorption virial coefficients and Kováts retention indices, likewise their dependencies on column temperature, T, number of carbon atoms, n(C) (or methylene groups CH(2)) and mutual ones have been briefly presented. The results of the molar differential enthalpy and entropy of adsorption obtained for different carbon materials employing inverse gas chromatography have been collected and interpreted. An attempt has been made to elucidate abnormal behaviour of the specific and net retention volumes, the second adsorption virial coefficients and the Kováts retention indices, e.g., the magnitudes on which the values of the afore-mentioned thermodynamic values have been determined and compared. The detailed analysis of the errors associated with the experimental parameters necessary for calculating retention volumes, second adsorption virial coefficients and Kováts retention indices has been presented.     

Monofunctional polymers in liquid adsorption chromatography: determination of the interaction parameter in the range of weak interaction

The main physical characteristics of monofunctionals in adsorption chromatography - the adsorption interaction parameter of the repeat units c and the interaction parameter of specific end group q - are discussed. Both parameters are independent on column dimensions and pore diameter, and depend on mobile phase composition. In a plot of elution volumes V(n) vs. the difference DeltaV=V(n)-V(n-1) in elution volumes of consecutive non-functional or monofunctional oligomers, straight lines with the same slope are obtained for sufficiently high molar masses. The intercept of these lines yield the accessible volumes of functionalized and non-functionalized oligomers. In the range of weak interaction, the interaction parameter of the repeat unit can be determined using monofunctional chains with strongly adsorbing end group. Scope and limitations of this approach are studied using monoalkyl ethers of polypropylene glycol as model polymers.     

Electrospray and matrix-assisted laser desorption/ionization mass spectral characterization of linear single nylon-6 oligomers

Synthetic nylon-6 single molecular mass oligomers were studied by matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectrometry. These oligomers, considered as model compounds for the study of nylon-6 polymers, gave good mass spectrometric results using both MALDI and ESI. In spite of the gentle nature of both techniques, the MALDI and ESI spectra showed evidence of end-group cleavage from the oligomer chains. MALDI-MS was found to give similar fragmentation patterns for all of the oligomer samples. An increase in doubly charged ion signals with increasing oligomer mass was observed in the ESI mass spectra, as was end-group fragmentation. Signals from oligomer clusters were observed in ESI-MS for the dimer, tetramer and hexamer, most likely due to non-covalent bonding among the low-mass oligomer molecules.     

Analytical characterization of PEG polymers by MEKC

Characterization of PEGs with average molecular masses of up to 2000 has been achieved using MEKC with UV detection. A rapid derivatization procedure with phenyl isocyanate using microwave radiation, in order to introduce chromophore groups in PEGs, has been developed involving a reaction time of 60 s. Different optimized conditions in accordance with the molecular weight have been studied to obtain the oligomer separation. The weight‐average molecular mass the number‐average molecular mass and the degree of polydispersity (molecular mass distribution) were calculated for the different PEGs obtaining similar results with those certified for standards. A good precision was obtained for characterizing the different oligomers. Ethylene glycol was used as the internal standard for the analysis of low‐molecular‐weight PEGs. The developed method was satisfactorily applied to the characterization of these polymers in several real samples, such as lubricant eye drops, toothpaste, tap water and eye make‐up remover.     

Computational study on the structural diversity of amyloid Beta Peptide (abeta(10-35)) oligomers

We studied the oligomerization of Alzheimer amyloid beta peptide (Abeta) using a replica exchange molecular dynamics (REMD) simulation. The simulation was performed with Abeta(10-35) dimers, trimers, and tetramers. Extensive REMD simulations illustrated several possible oligomer conformations. As the size of the oligomer increased from a dimer to a tetramer, the number of possible configurations was reduced. We identified all the possible conformations for each oligomer and characterized their temperature dependence. It was found that the detailed structures of the oligomers, which may act as folding intermediates, are highly sensitive to the parameters of the simulation environment such as temperature and concentration. Structural diversities of Abeta oligomers suggest multiple pathways of the aggregation process.     

Evaluation of the quantitativeness of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using an equimolar mixture of uniform poly(ethylene glycol) oligomers

Quantitativeness of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was elucidated using an equimolar mixture of uniform poly(ethylene glycol) (PEG) oligomers with no molecular weight distributions. Uniform PEG oligomers with degrees of polymerization n = 6-40 were separated from commercial PEG samples by preparative super-critical fluid chromatography. MALDI-TOF mass spectra of an equimolar mixture of the uniform PEG oligomers were recorded by adding a mixture of 2,5-dihydroxybenzoic acid as a matrix reagent and four chlorinated salts, i.e. LiCl, NaCl, KCl and RbCl. Remarkable non-quantitative effects were observed in the MALDI-TOF mass spectra in both the lower and higher molecular mass regions. At higher molecular masses greater than about 10(3), PEG oligomers with larger molecular mass yielded lower spectral intensities irrespective of the species of adduct cations and higher laser powers induced larger decreases in mass spectral intensities with the increase in their molecular masses. On the other hand, in the lower molecular mass region, less than about 10(3), the observed non-quantitative effect greatly depends on the species of adduct cations, indicating that the stability of the PEG-cation complex affects the MALDI-TOF mass spectral intensities of uniform PEG oligomers.     

Oligomer–oligomer modification upon the synthesis of epoxy polymers with a “shape-memory” effect

The results of studies of the oligomer–oligomer modification of epoxyamine compositions with amine adducts, oligomers, block oligomers, and oligocyclocarbonates differing in their structure and molecular weight upon hardening under the conditions of a general epoxide–amine polycondensation reaction, and competitive epoxide–amine and cyclocarbonate–amine reactions are given. The parameters of the structure and the elastic-deformation properties required for the preparation of high-quality thermally contracting epoxy polymers with a “shape-memory” and for their reliable application are established.     

Synthesis of novel fluoroalkyl end‐capped oligomers/silica gel polymer hybrids possessing antibacterial activity

New fluoroalkyl end‐capped oligomers/silica gel polymer hybrids‐low‐molecular weight biocide (hibitane) composites were prepared by the reactions of tetraethoxysilane (TEOS) with fluoroalkyl end‐capped N‐(1,1‐dimethyl‐3‐oxobutyl)acrylamide oligomer, N,N‐dimethylacrylamide oligomers, and acrylic acid oligomers in methanol under acidic conditions at room temperature. The presence of hibitane in the composites was clarified by the use of elementary analyses of nitrogen in fluorinated acrylic acid oligomer composite and thermogravimetric analysis (TGA) of these fluorinated composites. Thermal stability of fluorinated composites thus obtained were found to increase significantly compared to those of the parent fluorinated oligomers. Thermal stability of fluorinated N,N‐dimethylacrylamide oligomer, acrylic acid oligomer/silica gel polymers hybrid‐hibitane composites decreased compared to those of the corresponding fluorinated oligomers/silica gel polymer hybrids; however, the thermal stability of fluorinated N‐(1,1‐dimethyl‐3‐oxobutylacryl)amide oligomer/silica gel polymer hybrid‐hibitane composite increased significantly compared to that of the corresponding fluorinated oligomer hybrid. The sol methanol solutions of these fluorinated composites were applied to the surface modification of glass to exhibit not only a strong oleophobicity imparted by end‐capped fluoroalkyl groups in oligomers but also a good hydrophilicity on the glass surface. Fluorinated oligomers/silica gel polymer hybrids‐hibitane composites were found to exhibit high anti‐bacterial activity against Pseudomonas aeruginosa and Staphylococcus aureus. Therefore, these fluorinated hibitane composites are suggested to have high potential for new attractive functional materials through not only their excellent surface active property imparted by fluorine and their thermal stability but also through their anti‐bacterial activity. Copyright © 2005 John Wiley & Sons, Ltd.     

Reaction Mechanisms and Kinetics of the Melt Transesterification of Bisphenol‐A and Diphenyl Carbonate

The catalyzed and uncatalyzed reaction mechanisms of the melt transesterification process of bisphenol‐A and diphenyl carbonate are proposed based on nucleophilic substitution at the carbonyl group of the reactants. The reaction paths and energy barriers of the melt transesterification reaction were predicted and identified via density functional theory (DFT) calculations. The calculations reveal that the different oligomers with only one repeating unit are formed through different thermal processes. The theoretical evaluation further indicates that the basic hydroxide catalysts can reduce the energy barrier for the transesterification reaction, which allows subsequent nucleophilic attack to easily occur. Furthermore, the reaction kinetics of transesterification using tetraethyl ammonium hydroxide as a catalyst were investigated experimentally over a temperature range of 155–175°C. The reaction rate constants and equilibrium constants were determined based on the functional group model, and the equilibrium constants decreased with increasing reaction temperature. A detailed molecular species model with a specific repeating unit (n = 3) was developed and applied to predict the change in the reactants, oligomers, and phenol, and the experimental data and model calculation agree quite well. The standard curves of the oligomer were reversely derived, which provide intuitive insight into the concentration change of each oligomer. Both the DFT calculations and experimental results indicate that the C1 oligomer is first formed, and some of which are then converted to other types or higher molecular weight oligomers.     

Effect of the nature and associate structure of an epoxy oligomer on the rate of its curing with diamine

The supramolecular structure of a number of epoxy oligomers, monofunctional epoxy compounds, their mixtures, an oligoamine curing agent, and curing systems on their basis is studied by dynamic light scattering. In all of the starting oligomers and their mixtures, light scattering centers are detected that may be considered as the associates (aggregates) of oligomer molecules. The structure of associates that is determined by the flexibility of the oligomer chain, its branching degree, and the presence of proton-donor groups capable of hydrogen bonding is suggested. The relationship between the rate of curing of epoxy oligomers and their mixtures with oligoamine and the associate structure of oligomers and the nature of the substituent at the glycidyl group of the oligomer is ascertained.     

Protein Oligomer Engineering: A New Frontier for Studying Protein Structure,Function, and Toxicity

Prevalent in nature, protein oligomers play critical roles both physiologically and pathologically. The multimeric nature and conformational transiency of protein oligomers greatly complicate a more detailed glimpse into the molecular structure as well as function. In this minireview, the oligomers are classified and described on the basis of biological function, toxicity, and application. We also define the bottlenecks in recent oligomer studies and further review numerous frontier methods for engineering protein oligomers. Progress is being made on many fronts for a wide variety of applications, and protein grafting is highlighted as a promising and robust method for oligomer engineering. These advances collectively allow the engineering and design of stabilized oligomers that bring us one step closer to understanding their biological functions, toxicity, and a wide range of applications.     

Complexation thermodynamics of free and graft oligomers with complementary polymers

The dilute solution complexation equilibrium between linear macromolecules and smaller complementary oligomers is considered when: (1) the oligomers are free in solution; and (2) the oligomers are covalently attached at one end to the polymer. A general statistical mechanical framework is developed and is illustrated using a simple random walk model for polymer conformation. The statistical mechanical partition functions are formulated using a generating function technique, allowing thermodynamic averages in the complexed state to be calculated. Loops, trains, and tails of all possible length are allowed in the conformation of a complexed oligomer. Simulation results for the free oligomer case are compared with those obtained for oligomers covalently attached to the polymeric molecular. The model provides a theoretical explanation for the experimentally observed enhancement of complexation of oligomers grafted to the complementary polymers.     

Preparation and applications of a variety of fluoroalkyl end-capped oligomer/hydroxyapatite composites

A variety of fluoroalkyl end-capped oligomers were applied to the preparation of fluorinated oligomer/hydroxyapatite (HAp) composites (particle size: 38-356 nm), which exhibit a good dispersibility in water and traditional organic solvents. These fluoroalkyl end-capped oligomer/HAp composites were easily prepared by the reactions of disodium hydrogen phosphate and calcium chloride in the presence of self-assembled molecular aggregates formed by fluoroalkyl end-capped oligomers in aqueous solutions. In these fluorinated HAp composites thus obtained, fluoroalkyl end-capped acrylic acid oligomers and 2-methacryloyloxyethanesulfonic acid oligomer/HAp nanocomposites afforded transparent colorless solutions toward water; however, fluoroalkyl end-capped N,N-dimethylacrylamide oligomer and acryloylmorpholine oligomer were found to afford transparent colorless solutions with trace amounts of white-colored HAp precipitants under similar conditions. HAp could be encapsulated more effectively into fluorinated 2-methacryloyloxyethanesulfonic acid oligomeric aggregate cores to afford colloidal stable fluorinated oligomer/HAp composites, compared to that of fluorinated acrylic acid oligomers. These fluorinated oligomer/HAp composites were applied to the surface modification of glass and PVA to exhibit a good oleophobicity imparted by fluorine. HAp formation was newly observed on the modified polyethylene terephthalate film surface treated with fluorinated 2-methacryloyloxyethanesulfonic acid oligomers and acrylic acid oligomer/HAp composites by soaking these films into the simulated body fluid.     

Near‐monodisperse,catalytically active,imidazole‐containing homooligomers: Synthesis by group transfer polymerization and solution characterization

Oligomers covering degrees of polymerization up to 23 of 2‐(1‐imidazolyl)ethyl methacrylate (ImEMA) were synthesized by group transfer polymerization (GTP) in propylene carbonate and tetrahydrofuran (THF). While GTP proceeds smoothly in propylene carbonate, polymerization in THF is accompanied by oligomer precipitation. The molecular weights (MWs) and molecular weight distributions (MWDs) of the oligomers were obtained by gel permeation chromatography (GPC) using acidic aqueous sodium nitrate as the eluent. Good control over the MW and small polydispersity indices (PDIs) were measured for the oligomers prepared in propylene carbonate but not in THF. The oligomers were evaluated for their ability to hydrolyze 4‐nitrophenyl acetate in aqueous solution. The hydrolytic activity increased with oligomer MW. The oligomer concentration‐ and substrate concentration‐dependencies of the initial hydrolysis rate were both found to be approximately first order. The hydrolytic activity increased with an increase in pH, manifesting the enhanced nucleophilicity and pronounced hydrophobicity of the unprotonated form of the repeating units. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1501–1512, 1999     

Methacrylate-containing oligoorganosilsesquioxanes

Methacrylate-containing organosilsesquioxanes with M _n = (1.5?5.0) × 10³ were synthesized via hydrolytic polycondensation of γ-methacryloxypropyltrimethoxysilane under different conditions. On the basis of comparison of ²⁹Si NMR data and the molecular masses of the oligomers, it was concluded that oligomer molecules contain 8–12-membered siloxane cycles.     

Determination of molecular mass distribution of silicone oils by supercritical fluid chromatography, matrix-assisted laser desorption ionization time-of-flight mass spectrometry and their off-line combination

Silicone oil samples were characterized by supercritical fluid chromatography (SFC), matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI--TOF MS), and their off-line combination. SFC was used to separate samples of silicone oils on micropacked capillary columns. The fractions for the identification studies were obtained from SFC runs at defined time intervals, when the restrictor was pulled out from the chromatographic flame ionization detector (FID) and inserted into a glass vial with acetone. MALDI--TOF MS was used for the identification of individual oligomers in the fractions separated. The molecular mass distributions determined based on SFC and MALDI--TOF MS measurements were compared. From this comparison, it follows that the results are in good agreement. However, certain differences were observed: MALDI--TOF MS was capable of detecting somewhat larger oligomers than the SFC-FID, but the lower molecular mass oligomers were not present in the MALDI spectra. Differences in the region of lower molecular masses can be explained by evaporation of the more volatile low molecular mass oligomers resulting from heating of the sample during the MALDI--TOF MS measurements as a result of the absorption of the laser shot energy. The fact that no high mass discrimination effects of the MALDI--TOF MS measurements, compared with SFC, were observed is very promising for further applications of MALDI--TOF MS in characterizing synthetic polymers of moderate polydispersity.     

Kinetics of curing of diane and aliphatic epoxy oligomer blends: IR-spectroscopy study

The curing of diane and aliphatic epoxy oligomers and their blends is studied by IR spectroscopy. Methods for the quantitative determination of epoxy groups in diane and aliphatic oligomers and their blends during curing are developed. It is shown that, during the joint curing of epoxy oligomers, the reactivity of a more active (diane) oligomer remains practically the same, whereas the reactivity of a less active (aliphatic) epoxy oligomer increases. The rate constants for the consumption of epoxy groups of oligomers and primary amine groups of the curing agent are determined.     

Characterization of molecular weight distribution of oligomers from autocatalyzed batch hydrolysis of xylan

Oat spelt xylan was treated with water in a batch reactor at temperatures of 180 and 200°C. Ion-moderated partition (IMP) chromatography was then applied to separate oligomers in solution according to their molecular size. Calibration of the IMP measurements based on peak height was found to quantify dissolved monomer and oligomer yields well. Oligomer concentrations in the liquid hydrolysate were also determined from the difference in xylose monomer concentrations measured by high-performance liquid chromatography before and after posthydrolysis of dissolved xylooligosaccharides to xylose. Delayed formation and then rapid disappearance of oligomers from DP10 to DP2 was observed by IMP, and total oligomer yields measured by IMP and posthydrolysis were very similar at these times. However, while IMP detected virtually no oligomers initially, posthydrolysis measurements gave significant amounts of soluble oligomers at these times, indicating that oligomers with chain lengths >10 were in solution but not detectable by the IMP system used.