Network structure of spontaneously forming physically cross-link hydrogel composed of two-water soluble phospholipid polymers

Hydrogels containing 2-methacryloyloxyethyl phosphorylcholine (MPC) moieties were prepared from aqueous solutions with water-soluble poly[MPC-co-methacrylic acid (MA)] (PMA) and poly[MPC-co-n-butyl methacrylate (BMA)] (PMB). We had found that the hydrogel would swell under acidic pH and dissociate under neutral and alkaline conditions. Investigating these properties together with the complex modulus and the calculating cross-link molecular weight, we tried to determine how the network is constructed. The hydrogel showed a low swelling and a low elastic modulus E as the PMA feed ratio increases. However, a slow dissociation behavior was shown at the same time. This is due to the formation of the intramolecular cross-links caused by hydrogen bonds provided by carboxyl groups inside the hydrogel, which does not contribute to the swelling or elastic property. However, they had affected the ionization of the carboxyl groups. The average cross-link molecular weight decreased with the higher PMA feed ratio, indicating that the network is actually denser. The calculated cross-link molecular weight for the high PMA feed ratio is low, but the number of cross-links is also low, indicating that there are many intramolecular cross-link networks, but fewer valid entanglements for high mechanical strength. Therefore, denser networks are formed for higher the PMA feed ratio, but they do not function as cross-link junctions. This could be confirmed by Fourier transform infrared spectroscopy (FTIR). When the release behavior of the loaded drugs was tested, it showed that the drugs with molecular weights higher than the cross-link molecular weight would be slower than that of drugs with lower molecular weights.     

The effect of ionic cross-links on the deformation behavior of homoblends made of poly(styrene-co-styrenesulfonic acid) and poly(styrene-co-4-vinylpyridine)

Deformation behavior of stoichiometric blends made from poly(styrene-co-styrenesulfonic acid) (SPS) and poly(styrene-co-4-vinylpyridine) (SVP) was investigated by TEM observation of strained thin films. An FTIR investigation revealed that ionic cross-links were formed between the component polymers upon blending due to intermolecular ion-ion interactions, which arose from proton transfer from sulfonic acid groups to pyridine groups. TEM observations indicate that the deformation mode of the blends changed from crazing only to crazing plus shear deformation, with the shear contribution becoming larger, as the ion content in the blends increased. Such changes in deformation mode can be understood as arising from an increase in the ‘effective’ strand density due to the formation of ionic cross-links upon blending. It was also found that the ionic cross-links via pyridinium cation/sulfonate anion ion pairs were more effective in inducing the transition of deformation mode than ionic cross-links via -SO₃⁻/Na⁺ or -SO₃⁻/Ca²⁺ ion pairs.     

New polyisobutylene-based model ionomers

Mechanical properties of tri-arm polyisobutylene based ionomers have been investigated. Number average molecular weights (¯M_n's) of the polymers were varied from 8,300 to 34,000. The ionomer of lowest ¯M_n (8,300) exhibits very low extension at break (～ 150%) while the others with ¯M_n's of 11,000, 14,000 and 34,000 show very high extensions, often exceeding 1,000%, and display relatively low permanent set and low hysteresis behavior. Since the ionic bonding is located exclusively at the chain ends, end-linked pseudo networks are formed due to coulombic attractions.At higher temperatures the coulombic interactions are weakened and the networks can be compression molded (～150°C).     

Ionic binding in pressure-sensitive adhesive polymers

Ionic binding is affected in pressure-sensitive acrylate-based copolymers containing carboxylic groups by introduction of Na ions. The tack and peel force of pressuresensitive polymers decreased and the resistance to creep increased with increasing degree of ionic binding. An infrared spectroscopic method for the assay of sodium content has been developed based on the ratio of absorbances at 5.75 and 6.28 μ due to ester carboxyl and ionized carboxyl, respectively.     

Disorder effects on the strain response of model polymer networks

Molecular dynamics simulations are used to investigate polymer networks made by either end-linking or randomly crosslinking a melt of linear precursor chains. The resulting network structures are very different, since end-linking leads to nearly ideal monodisperse networks, while random crosslinking leads to polydisperse networks, characterized by an exponential strand length distribution. Networks with average strand length 20 and 100 were generated. These networks were used to study the effects of disorder in the network connectivity on observables averaged either over the entire network or selected sub-structures. Heterogeneities in the randomly crosslinked networks cause significant differences in the localization of monomers, however, neither the localization of crosslinks nor the microscopic strain response are significantly affected. Compared to end-linked networks, randomly crosslinked networks have a slightly increased tube diameter, and as a result a slightly decreased shear modulus, but otherwise identical stress-strain behavior. For the investigated systems, we conclude that the microscopic strain response, tube diameter, and stress-strain relation are all insensitive to the heterogeneities due to the linking process by which the network were made.     

Humidity dependence of visible absorption spectrum of gelatin films containing cobalt chloride

Gelatin films containing cobalt chloride were investigated to obtain information on the humidity dependence of optical absorbency in view of the coordinate state of cobalt ion and the polymer structure. The visible absorption spectra of the films exhibited that, with increasing relative humidity (RH), the absorption bands between about 550 and 750 nm decreased, accompained by sharpening of the 693 nm peak. Further, the decrease of cobalt chloride in gelatin resulted in decreased absorption intensity of the 693 and 668 nm peaks, more than the overall lowering in intensity. The infrared spectral data indicated that most of the carboxyl groups of gelatin are linked to the complex even in the strongly hydrated state. In addition, the water content of the films containing cobalt chloride is a little less below 42% RH, but is much greater above 62% RH than that of the original gelatin film. These results indicated that there are cross-linked networks, composed of the hydrogen bonding and the coordination bond via the complex in the dry film; as water molecules are adsorbed by the film, they gradually destroy these bonds and swell the polymer. © 1993 John Wiley & Sons, Inc.     

Elastomeric properties of polysiloxane networks: birefringence measurements on bimodal elastomers that are presumed to be spatially inhomogeneous

In-situ measurements of birefringence during simple extension were performed on a series of end-linked bimodal poly(dimethylsiloxane) elastomers that were templated to possess high cross-link density nanostructures. The strain birefringence showed marked deviations from predictions of linearized Gaussian rubber-like elasticity theory for a bimodal elastomer with well-developed structure (nanoscale to microscale). However, the stress-birefringence relation for this same network seemed to be described by Gaussian rubber-like elasticity theory. Reinforced bimodal networks that were macroscopically homogenous (although potentially possessing nanoscale structure) were observed to have strain birefringence scaling similar to theoretical predictions, although the magnitude was below the Gaussian prediction. Analysis of the strain birefringence shows that a non-linear, non-Gaussian effect (likely limited chain extensibility) can occur. However, in all cases the birefringence was found to have a significant linear regime over a large range of strains which suggests that limited chain extensibility may not be the sole contributor towards the well known “bimodal” reinforcement.     

Polyisobutylene model elastomers prepared using demonstrably complete end-linking reactions

Summary Linear polyisobutylene (PIB) molecules with either hydroxyl or isopropenyl groups at the chain ends were prepared using cationic polymerizations with bifunctional initiator-chain transfer agents. Extensive spectroscopic analyses confirmed the essentially perfect difunctionality of the two types of polymers. The former polymers were end-linked using an aromatic triisocyanate, and the latter by means of a tetrafunctional silane. The resulting trifunctional and tetrafunctional model PIB networks were found to have absolutely negligible sol fractions, which demonstrates that the end-linking reactions used to prepare them were essentially complete. The networks were studied with regard to their equilibrium stress-strain isotherms in uniaxial extension at 25 °C. The results thus obtained are in satisfactory agreement with theory and yield no evidence whatever for significant elastic contributions from inter-chain entanglements.     

Hydrothermally treated chitosan spontaneously forms water-soluble spherical particles stable at a wide pH range

The authors report the spontaneous formation of water-soluble chitosan-tartaric acid (CS-TA) spherical particles. Particles are formed by heating chitosan in the presence of tartaric acid under hydrothermal conditions. Tartaric acid serves as an ionic cross-linker, a depolymerizing agent, and a particle stabilizer in aqueous phase. The CS-TA particles exhibit superior colloidal stability at a wide pH range due to their surface charge tunability, which is due to the colocalization of surface hydroxyl, amine, and carboxyl groups. At physiological pH condition, particles have zwitterionic structure as determined by the zeta potential measurements. Still, CS-TA maintains colloidal stability at neutral pH due to the abundance of surface hydroxyl groups. As a proof-of-concept study, the CS-TA particles were labeled with a model insoluble cargo (fluorescein isothiocyanate [FITC]) to demonstrate their capacity for solubilizing hydrophobic drugs. The CS-TA/FITC conjugates were found to remain well dispersed at neutral pH, while maintaining FITC fluorescence properties.     

Preparation of amphoteric latex by modification of styrene–acrylamide copolymer latex

Styrene–acrylamide copolymer latex prepared from the polymerization in an emulsifier-free aqueous medium was treated with hypochlorite and sodium hydroxide. The resulting latex indicated amphoteric property due to amino and carboxyl groups formed by the Hofmann reaction and competitive hydrolysis of amide groups, respectively. Amphoteric latices with the same particle size but different charge density (1.0–3.0 ionic groups/100 Ų) and different isoelectric points (6.4–9.2) have been prepared by changing the reaction conditions.     

Dynamic mechanical properties of weakly clustered poly(styrene-co-sodium citraconate) ionomers

Summary The properties of poly(styrene-co-citraconate) ionomers were investigated dynamic mechanically. It was found that the loss tangent peak for the matrix glass transition decreased with increasing content of ionic units, while the loss tangent peak for the cluster glass transition was barely detectable. However, the ionic modulus values of the citraconate ionomers as a function of the amount of ionic units were comparable to those of well-clustered methacrylate ionomers. Thus, it was postulated that the ionic modulus was affected by both clustering due to the overlapping of regions of restricted mobility and strong physical cross-links originated from the ionic association of ca. 4 ion pairs. Received: 16 March 2001/Revised version: 25 April 2001/Accepted: 27 April 2001     

Studies on preparations and properties of drug-eluting embolization microspheres made from oxidated alginate and carboxymethyl chitosan

Embolization microspheres were prepared through two steps. The microspheres demonstrated regular sphericity with a main size distribution in the range of 100?µm?～?270?µm. An anticancer drug doxorubicin could be loaded into the microspheres with high loading rates from 21% to 32% due to physical and ionic interaction of carboxyl groups in the microspheres and amino groups in doxorubicin. The drug could be controlled released at different pH conditions. The blank microspheres were biodegradable, non-cytotoxic, and the drug-eluting embolization microspheres could be potentially applied as embolic agents in interventional therapy.     

Conductimetric titration of polyelectrolytes having sulfate and carboxyl groups

Summary Conductimetric titration of polyelectrolyte solutions was carried out to determine the contents of sulfate and carboxyl groups. In the mixtures of poly(acrylic acid) and poly(vinyl sulfate), and of alginic acid and dextran sulfate, the contents of sulfate and carboxyl groups observed by the conductimetric titration were in agreement with those derived from the concentration of the original homopolymers. This method was applied to a viscous polysaccharide from red algae, funoran, which contains both the sulfate and carboxyl groups. These results suggested that this method is applicable to either the samples consisting of the mixture of homopolymers or those of copolymers containing both ionic species.     

Morphology and deformation behavior of polymer blends made of poly(styrene-co-styrenesulfonic acid) and poly(methyl methacrylate-co-4-vinylpyridine)

The effect of a small amount of ionic groups (interactions) on the morphology and deformation behavior of stoichiometric blends made of poly(styrene-co-styrenesulfonic acid) (SPS) and poly(methyl methacrylate-co-4-vinylpyridine) (MVP) was investigated by Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). FTIR data revealed that intermolecular ion-ion interactions were formed between SPS and MVP polymers, arising from proton transfer from sulfonic acid groups to pyridine groups upon blending. TEM observations show that the morphology of the blends changes from macroscopic phase separation to microscopic phase separation, and to miscibility, with increasing ion content of the blends from 0 to 6 mol%. Correspondingly, deformation behavior of the blends changes from crazing only, to curved and branched crazing, and to crazing plus shear deformation. Such changes in deformation mode can be understood as arising from the morphological changes and the increase of ‘effective’ strand density due to the formation of ionic cross-links in the blends.     

Effect of filler amount on thermoelastic properties of poly(dimethylsiloxane) networks

End-linked poly(dimethylsiloxane) (PDMS) networks were prepared in the presence of fumed silica particles with hydroxyl groups at their surfaces. The silica particles were introduced into the polymer solution prior to end-linking. Hydroxyl ended PDMS chains were end-linked via the tetra functional crosslinker, tetraethoxysilane. The filler content varied in the range 0-5 wt%. Atomic Force Microscopy was used to image and characterize the silica particles. Swelling, stress-strain and thermoelasticity experiments were performed. The temperature coefficient and the energetic part of the force in uniaxial extension are found to increase with increasing silica amount. This observation is ascribed to effects contributed possibly by the adsorption layer around the silica particles.     

Thermal oxidation of gallium arsenide with transition metal nanolayers on the surface

The physicochemical features of the interaction between nanofilms of iron triad metals and the gallium arsenide surface during thermal oxidation are analyzed. The role of oxides formed upon oxidation of iron, cobalt, and nickel metal films in the course of thermal oxidation of gallium arsenide is demonstrated, and the influence of the metal deposition method on the kinetics of oxidation, the composition, and the properties of the prepared samples is determined. Schemes are proposed for the development of the oxidation processes under investigation with due regard for the chemical specific features of the iron triad metals and their compounds.     

Telechelic ionomeric poly(butylene terephthalate): Synthesis,characterization and comparison with random ionomers

Telechelic poly(butylene terephthalate) (PBT) ionomers have been prepared by melt synthesis using a new polycondensation process that involves a pre-reaction of sulfobenzoic acid sodium salt with butanediol. No side reaction occurs and the incorporation of the ionic groups is quantitative. The addition of a buffer agent, such as Na₃PO₄, to the catalyst reduces the THF formation and improves the polycondensation rate. The comparison of the thermo-mechanical and physical properties between random and telechelic ionomers is also reported. The ionic groups act as chain-extension reversible electrostatic links for telechelic ionomers while act as cross-links in random ionomers. Therefore, random ionomers present a consistently higher melt viscosity compared to telechelic ionomers and to PBT and for this reason high molecular weight random ionomers cannot be obtained by melt polycondensation. Thermal and hydrolytic stabilities of telechelic ionomers are comparable with those of commercial PBT and consistently higher respect to those of random ionomers. The presence of ionic groups decreases the translation mobility of the polymer chains thus lowering the crystallization rate.     

Facile fabrication of polystyrene/carbon nanotube composite nanospheres with core-shell structure via self-assembly

In this paper, a novel method for fabrication of core-shell nanospheres with polystyrene (PS) as the core and multi-walled carbon nanotubes (MWNTs) as the shell via hydrogen-bonding self-assembly is introduced. The PS nanospheres with carboxyl acid groups on the surface (PS-COOH nanospheres) were prepared by typical soap-free emulsion copolymerization with acrylic acid as comonomer. The MWNTs were grafted with poly(vinyl pyrrolidone) (PVP), in which the carbonyl oxygen can act as proton acceptors to form hydrogen bonds with the carboxyl acid groups. The results show that the functionalized MWNTs can self-assemble onto the surface of PS-COOH nanospheres rapidly via hydrogen bonding interaction, and the process is reversible and can be well controlled by adjusting pH value of the system. These core-shell nanospheres have the potential to be used as conductive and synergistic reinforcement fillers in fabricating high-performance and functional nanocomposites.     

Polymeric corrosion inhibitors for aluminium pigment

In aqueous alkaline media (e.g. water-borne metallic paints) aluminium pigments react by the evolution of hydrogen. This corrosion reaction can be inhibited by addition of different water-soluble polymers with carboxyl groups like polyacrylic acids, styrene–maleic acid or styrene–acrylate copolymers. As a rough empirical rule can be stated that the corrosion-inhibiting effect of polymers with carboxyl groups increases with decreasing molecular mass and decreasing acid number. Moreover, the isoelectric point (IEP) of aluminium oxide (pH≈9) seems to be an important factor controlling corrosion inhibition (and adsorption) of polymers with carboxyl groups. Thermosetting phenolic resins (resoles) inhibit the corrosion reaction of aluminium pigment excellently at pH 8 but less effectively at pH 10. The corrosion-inhibiting functional group of resoles seems to be the chelating ortho-hydroxybenzyl alcohol structural part. In contrast, the nonionic water-soluble polymer polyvinyl alcohol does not inhibit the corrosion reaction. So, one may assume that an ionic interaction between aluminium pigment surface and polymer is necessary (but not sufficient) for corrosion inhibition.     

Interaction of iron with humic acid extracted from lignite

The role of different functional groups in humic acid was investigated for complex formation during the interaction of Fe⁺² and Fe⁺³ ions with humic acid, oxidized humic acid, acetylated humic acid and decarboxylated humic acid. The formation of two types of complexes, ionic and non-ionic, was observed. The results indicate that the proton releasing groups, e.g. carboxyl and phenolic hydroxyl groups, are responsible for the formation of a ionic complex, and the formation of a non-ionic complex involves the carboxyl and carbonyl (keto) groups. The percentage of the ionic iron in the complex increases and that of the non-ionic iron decreases with increase of pH.