Low Levels Of Cross-linking Research Articles

Challenges in predicting and controlling particle morphology in latices of commercial complexity

AbstractIndependent, quantitative models for free energy associated with crosslinked seed latices, and monomer and surfactant at the polymer/water interface have been integrated into a predictive model for latex particle morphology. It has been found that very low levels of crosslinking are predicted to influence the particle morphology in a dramatic fashion. Free energy calculation also highlight the often critical role played by surfactant. Highly active surfactants tend to equalize the two polymer/water interfacial tensions and allow the polymer/water interfacial energy to significantly influence the particle morphology.

Rheological and Mechanical Properties of Filled Rubber: Silica-Silicone

Abstract This work focuses on the relationship between the morphology of filled rubbers and their behavior at small and large deformations. Composites were made from vinyl terminated polydimethylsiloxane (PDMS) and different fumed silicas. Filler concentration range was kept between 0 to 40 phr (parts of silica per hundred parts of PDMS). Small deformation tests done in oscillatory shear showed that there is coupling of the filler agglomeration and crosslinking effects at high crosslink density while the coupling is negligible at low crosslinking levels. Since crosslinking (hydrosilation) proceeds through the end vinyl groups of the PDMS, lower molecular weight polymers produce rubbers with higher elastic modulus than do high molecular weight silicones. However, the rubbers formed from low molecular weight PDMS break at lower extensions. As in the case of uncured composites, the moduli of the samples increase with silica concentration and amount of silanols on the silica. A silica treated with divinyltetramethyldisilazane gave rubbers of high modulus similar to those obtained from untreated silicas, highlighting the importance of good adhesion between filler and polymer.

Cross-linking of Escherichia coli RNA polymerase subunits: identification of beta' as the binding site of omega.

The omega protein is a peptide found in near-stoichiometric levels in highly purified Escherichia coli RNA polymerase. In order to determine the binding site of omega to RNA polymerase, we cross-linked omega to RNA polymerase with the hetero-bifunctional cross-linker N-hydroxysuccinimidyl 4-azidobenzoate and analyzed for cross-linked partners using antibodies raised against each of the subunits. Our analysis indicates that omega cross-links predominantly with the beta' subunit, while a very low level of cross-linking was detected to the alpha subunit. We did not detect cross-linking to either the sigma 70 or the beta subunits. This report demonstrates the utility of combining cross-linking and immunological techniques to determine interactions between RNA polymerase subunits.

Gene-specific DNA repair of interstrand cross-links induced by chemotherapeutic agents can be preferential.

The gene-specific formation and repair of interstrand cross-links (ICL) were measured in the dihydrofolate reductase (DHFR) gene in hamster cells. Cells were treated with two different chemotherapeutic agents, nitrogen mustard and cisplatin, and the frequency of cross-links was quantified in the active gene and in a downstream, inactive region. About 5% of total lesions induced by these agents were ICL. Whereas the frequencies of cross-links formed were similar in the gene and in the noncoding region after cisplatin treatment, there were more nitrogen mustard-induced cross-links in the inactive region than in the active gene. At low levels of cross-linking, we found preferential DNA repair in the active gene as compared to the inactive region. At higher levels of cross-linking, there was no difference in repair rates between the gene and the noncoding region due to an increase in the repair efficiency in the inactive DNA. Implications of fine structural organization of cross-link repair are discussed.

Radiation Crosslinking of HDPE/LLDPE Blends in the Presence of Dicumyl Peroxide

Blends of HDPE and LLDPE have been modified by gamma radia tion in the presence of dicumyl peroxide. The modification is found to introduce a low level of crosslinking in the matrix without seriously affecting the crystal linity. Properties, such as yield stress, tensile strength, and abrasion resistance improve with the irradiation.

Ribosomal localization of the mRNA in the 30S initiation complex as revealed by UV crosslinking

Translation initiation complexes consisting of 30S ribosomal subunits, 32P-labelled mRNA (002 mRNA), fMet-tRNA and the three initiation factors were subjected to UV-crosslinking to determine the protein and rRNA neighbors of the bound mRNA by immunochemical methods and by nucleic acid hybridization techniques. The mRNA was found to be crosslinked to a specific region of the 16S rRNA spanning from nucleotide 418 to 615 and to ribosomal proteins S1 and S21 (the main targets), S3, S10, S12 and S14; a low level of crosslinking was also detected with S2, S7, S13, S18 and S19.

Interpretation of non-uniform oxide thickness measurements using electron spectroscopy for chemical analysis (ESCA) : A. Unal, (Imperial College, London, England), Powder Metallurgy, Vol 34, No 2, 1991, 135–138

In this study, a novel technique combining surface imprinting and polymer crosslinking was applied to synthesize the Ag+-imprinted thiourea/glutaraldehyde grafted O-carboxymethyl chitosan (ITG-OCMC) beads which demonstrate extremely good selective adsorption of Ag+ from a bimetallic aqueous solution. Results indicated that high degree of carboxymethylation and low level of crosslinking would help to achieve higher uptake capacity of Ag+. The maximum uptake of Ag+ was found to be 156.32 mg g−1 at 40 °C with an initial Ag+ concentration of 160.5 mg L−1 and the biosorbent dosage of 1.0 g L−1. Langmuir isotherm and Lagergren’s pseudo-second-order kinetic can be used to describe the sorption process of Ag+. Analyses from FT-IR and XPS confirmed that selective adsorption of Ag+ took place on the surfaces of ITG-OCMC beads by chelation through >CS, amine, carboxyl and hydroxyl groups.

Small Angle Neutron Scattering Studies of Single Phase IPNs

ABSTRACTInterpenetrating polymer networks (IPNs) have been synthesized from polymers that form miscible polymer blends. Full, semi-I and semi-II IPNs made from polystyrene-d8 and poly(vinylmethylether) can be made to phase separate by incorporating low levels of crosslinking. However, blends of these polymers have a negative Flory-Huggins interaction parameter, making them highly miscible. This indicates that formation of IPNs favors phase separation relative to blends.IPNs made from polystyrene-d8 and polystyrene-h8 show that increased crosslink density also destabilizes the mixture as shown by small angle neutron scattering.IPNs have also been made by crosslinking end functionalized polydimethylsiloxanes in the presence of nonfunctionalized, deuterated siloxanes. These IPNs are also destabilized by increasing crosslink density, suggesting that the destabilization is due to the network, and not to the particular type of network forming reaction.

Influence of cross-linking on the hysteresis behavior of poly(urethane-diacetylene) segmented copolymers

The influence of diacetylene polymerization within the hard domains on the hysteresis behavior of segmented copolymers with different hard-segment-structures and different levels of phase separation and hard-domain ordering is studied. For elastomers with weakly associated hard segments, it is found that low levels of crosslinking signifcantly reduce the amount of hysteresis that they exhibit at low strains. For elastomers with crystalline hard domains and highly interlocked microstructures, crosslinking increases the level of hysteresis regardless of the level of crosslinking

Vitrification and curing studies of a photopolymerizable semi‐interpenetrating polymer network. II

AbstractThe photopolymerizable semi‐interpenetrating polymer network‐II being studied consists of a linear copolyester, a crosslinked network of multifunctional acrylate monomers and a very efficient photoinitiator system. Using both Raman and dynamic mechanical spectroscopy (DMS), it has been determined that the polymerizing network vitrifies after a very low level of crosslinking is attained. This results in a sample that has a Tg which is substantially lower than would be expected from knowledge of the values of Tg for the individual components of the blend. The extent of reaction and, therefore, the glass transition temperature can be increased by heating the sample during photopolymerization. In addition, the glass transition temperature of the network can be increased by thermally curing the sample after photopolymerization. It has been found that once the network has been fully cured, the glass transition temperature is much higher than would be predicted. This has been interpreted in terms of synergistic effects; conceivably, the acrylates form a tight matrix around the linear copolyester and effectively reduce the molecular mobility of the components. The results also show that the acrylates and the copolyester are not inherently miscible. However, if a sufficient concentration of acrylates is present, it is possible to prevent the copolyester from phase separating. Dynamic mechanical spectroscopy data do indicate that these semi‐inter‐penetrating polymer networks‐II (semi‐IPN‐II) samples are somewhat heterogeneous; however, there does not seem to be any gross phase separation. Transmission, electron microscopy (TEM) results show that the most fully cured sample has domains on the order of 50–250 Å associated with the copolyester phase.

Magnesium-ADP-induced changes in the structure of myosin S1 near the ATPase-related thiol SH1 probed by cross-linking

The structural consequences of MgADP binding at the vicinity of the ATPase-related thiol SH1 (Cys-707) have been examined by subjecting myosin subfragment 1, premodified at SH2 (Cys-697) with N-ethylmaleimide (NEM), to reaction with the bifunctional reagent p-phenylenedimaleimide (pPDM) in the presence and absence of MgADP. By monitoring the changes in the Ca2(+)-ATPase activity as a function of reaction time, it appears that the reagent rapidly modifies SH1 irrespective of whether MgADP is present or not. In the absence of nucleotide, only extremely low levels of cross-linking to the 50-kDa middle segment of S1 can be detected, while in the presence of MgADP substantial cross-linking to this segment is observed. A similar cross-link is also formed if MgADP is added subsequent to the reaction of the SH2-NEM-pre-modified S1 with pPDM in the absence of nucleotide. Isolation of the labeled tryptic peptide from the cross-linked adduct formed with [14C]pPDM, and subsequent partial sequence analyses, indicates that the cross-link is made from SH1 to Cys-522. Moreover, it appears that this cross-link results in the trapping of MgADP in this S1 species. These data suggest that the binding of MgADP results in a change in the structure of S1 in the vicinity of the SH1 thiol relative to the 50-kDa "domain" which enables Cys-522 to adopt the appropriate configuration to enable it to be cross-linked to SH1 by pPDM.

Cross-linking of α2-plasmin inhibitor and fibronectin to fibrin by fibrin-stabilizing factor

Two plasma proteins, α 2-plasmin inhibitor and plasma fibronectin, are cross-linked to fibrin by plasma transglutaminase (R-glutaminyl-peptide : amine γ-glutamyl-yltransferase, EC 2.3.2.13, fibrin stabilizing factor) when blood coagulation takes place. The cross-linking reactions of these proteins were analyzed by polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) using these radioactively labeled proteins. Both proteins were cross-linked exclusively to the α-chain of fibrin, and each of these cross-linking reactions proceeded independently without being influenced by the other cross-linking reaction. The cross-linking of fibronectin to the α-chain proceeded steadily at a rate similar to that of the cross-linked polymerization of the α-chain. In contrast, the cross-linking reaction of α 2-plasmin inhibitor to fibrin proceeded markedly faster than that of fibrin polymerization but did not proceed further after reaching a certain relatively low level of cross-linking. Most of the cross-linked α 2-plasmin inhibitor molecules at this stage of the fibrin cross-linking process were in the form of complex with the α-chain monomer. The complex with the α-chain monomer was gradually transformed to a complex with the α-chain polymer as the cross-linking polymerization of the α-chain proceeded. The rate of the transformation was the same as that for the disappearance of the α-chain monomer, indicating that whether the α-chain was cross-linked to α 2-plasmin inhibitor or not, the α-chain underwent cross-linking polymerization at the same rate.

An improved method for measuring cross-links in the DNA of mammalian cells: The effect of nitrogen mustard

This paper describes a technique for quantitatively measuring both the number of interstrand cross-links and the number of strand breaks introduced into the DNA of cells by a drug. The method has been illustrated by studying the effect of nitrogen mustard on the DNA of a murine lymphoma cell (L5178Y). The results have been compared with measurements of the molecular weight of DNA in alkaline sucrose gradients. It was found that treatment of L5178Y cells for one hour at 37 °C with 1 μM nitrogen mustard introduced two cross-links and one strand-break per 10 9 daltons of DNA. The method is also suitable for observing low levels of cross-linking. It is estimated that one cross-link in 2 · 10 9 daltons of DNA can be detected.

Hydrophobic interaction in poly(2-hydroxyethyl methacrylate) homogeneous hydrogel.

AbstractHomogeneous poly(2‐hydroxyethyl methacrylate) (PHEMA) hydrogel exhibits a narrow range of swelling at equilibrium in water (% H2O, 41.09 ± 0.15 standard error of the mean of 24 samples), regardless of the dilution of the monomer solution and relatively low level of crosslinking. It is postulated that PHEMA hydrogel has, in addition to its covalently linked network structure, a secondary structure stabilized by hydrophobic bonding. The addition of microsolutes to the hydrogel seems to confirm this hypothesis. The hydrogel swells beyond its swelling equilibrium in water in presence of urea and its methyl derivatives. Swelling is also induced by organic solvents like alcohol and acetone, and by anions like iodide, acetate, trichloroacetate, and thiocyanate. Chlorides and sulfates produce a less swollen hydrogel than pure water, while bromides and cetylpyridinium chloride, in the concentrations tested, induce only a slight deswelling of the gel. When PHEMA gel prepared in organic solvent–water solutions is placed in water, the gel passes through an opaque state before becoming transparent again. This phenomenon is interpreted as being caused by the inability of water to solvate the hydrophilic ends of the unorganized polymer segments. Homogeneity returns to the gel after a rearrangement of the chains, directed by the interaction of the hydrophobic portions of the polymer segments, exposing to the solvent–water most of the hydrophilic sites in the network.