Network formation of catanionic vesicles and oppositely charged polyelectrolytes. Effect of polymer charge density and hydrophobic modification

In nonequimolar solutions of a cationic and an anionic surfactant, vesicles bearing a net charge can be spontaneously formed and apparently exist as thermodynamically stable aggregates. These vesicles can associate strongly with polymers in solution by means of hydrophobic and/or electrostatic interactions. In the current work, we have investigated the rheological and microstructural properties of mixtures of cationic polyelectrolytes and net anionic sodium dodecyl sulfate/didodecyldimethylammonium bromide vesicles. The polyelectrolytes consist of two cationic cellulose derivatives with different charge densities; the lowest charge density polymer contains also hydrophobic grafts, with the number of charges equal to the number of grafts. For both systems, polymer-vesicle association leads to a major increase in viscosity and to gel-like behavior, but the viscosity effects are more pronounced for the less charged, hydrophobically modified polymer. Evaluation of the frequency dependence of the storage and loss moduli for the two systems shows further differences in behavior: while the more long-lived cross-links occur for the more highly charged hydrophilic polymer, the number of cross-links is higher for the hydrophobically modified polymer. Microstructure studies by cryogenic transmission electron microscopy indicate that the two polymers affect the vesicle stability in different ways. With the hydrophobically modified polymer, the aggregates remain largely in the form of globular vesicles and faceted vesicles (polygon-shaped vesicles with largely planar regions). For the hydrophilic polycation, on the other hand, the surfactant aggregate structure is more extensively modified: first, the vesicles change from a globular to a faceted shape; second, there is opening of the bilayers leading to holey vesicles and ultimately to considerable vesicle disruption leading to planar bilayer, disklike aggregates. The faceted shape is tentatively attributed to a crystallization of the surfactant film in the vesicles. It is inferred that a hydrophobically modified polyion with relatively low charge density can better stabilize vesicles due to formation of molecularly mixed aggregates, while a hydrophilic polyion with relatively high charge density associates so strongly to the surfactant films, due to strong electrostatic interactions, that the vesicles are more perturbed and even disrupted.     

Temperature-dependent vesicle formation of aqueous solutions of mixed cationic and anionic surfactants

The phase behavior of aqueous solutions of mixed cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) was examined at different temperatures (20, 30, 40, and 50 degrees C). While stable vesicles were formed in a narrow composition range on the SOS-rich side at 20 degrees C, the range widened remarkably when the temperature was raised to 30 degrees C. Thus, the vesicle region extended to cover almost the entire composition range, CTAB:SOS = 0.5:9.5-5.0:5.0, at the total surfactant concentrations of 50-70 mM on the SOS-rich side. To analyze the temperature dependence of this phase behavior of the mixed surfactant system, DSC and fluorescence polarization measurements were performed on the system. The experimental findings obtained revealed that pseudo-double-tailed CTAB/SOS complex, the major component of the bimolecular membrane formed by the surfactant mixture, undergoes a gel (Lbeta)-liquid crystal (Lalpha) phase transition at about 26 degrees C. This phenomenon was interpreted as showing that the bimolecular membrane has no curvature and is rigid and easy to precipitate at temperatures below the phase transition point, whereas it has a curvature and is loose enough to disperse in the solution as vesicles at temperatures above the phase transition point. Vesicles formed by the anionic/cationic surfactant complex were then stable at temperatures above the phase transition temperature of the complex.     

Characterization and Aggregation Behaviors of Mixed DDAB/SDS Solution With and Without Poly(4-styrenesulfonic Acid-Co-Maleic Acid) Sodium

Synthetic vesicles are formed by cationic and anionic surfactants, didodecyldimethylammonium bromide (DDAB), and sodium dodecylsulfate (SDS). The morphology, size, and aqueous properties of cationic/anionic mixtures are investigated at various molar ratios between cationic and anionic surfactants. The charged vesicular dispersions made of DDAB/SDS are contacted or mixed with negatively charged polyelectrolyte, poly(4-styrenesulfonic acid-co-maleic acid) sodium (PSSAMA), to form complexes. Depending on DDAB/SDS molar ratio or PSSAMA/vesicle charge ratio, complexes flocculation or precipitation occur. Characterization of the cationic/anionic vesicles or complexes formed by the catanionic vesicles and polyelectrolytes is performed by transmission electron microscope (TEM), dynamic light scattering (DLS), conductivity, turbidity, and zeta potential measurements. The size, stability, and the surface charge on the mixed cationic/anionic vesicles or complexes are determined.     

Aqueous phase separation in giant vesicles

We report the synthesis and initial characterization of approximately 10 mum diameter lipid vesicles that contain two distinct aqueous phases. The aqueous two-phase system is a dextran/poly(ethylene glycol) solution that exhibits temperature-dependent phase behavior. Vesicles were prepared above the phase transition temperature of the polymer solution. Upon cooling to room temperature, the polymer solution phase separated both within the vesicles and in the bulk solution. The location of poly(ethylene glycol)-rich and dextran-rich phases was determined by fluorescence microscopy. These structures are exciting in that they enable for the first time the interior volume of liposomes to be structured.     

Correlation of surfactant/polymer phase behavior with adsorption on target surfaces

In this contribution, the phase behavior of a surfactant/polymer mixed system is related to the adsorption of a complex derived from the mixture onto a target surface. The phase map for the system sodium dodecyl sulfate (SDS, a model anionic surfactant)/pDMDAAC (poly(dimethyl diallyl ammonium chloride), a cationic polymer) shows behavior very typical of surfactant/oppositely charged polyelectrolyte mixtures. The predominant feature is a broad, two-phase region in the phase map which lies asymmetrically around the 1:1 stoichiometry of surfactant charge groups to polymer charge units. The overall controlling principle driving the phase separation is charge compensation. Excess of polymer yields an isotropic solution, as does a great excess of surfactant (termed resolubilization). The phase separating in the SDS/pDMDAAC system is characterized by a positive zeta-potential when the polymer is in excess and a negative zeta-potential when the surfactant is in excess. The surface charge properties of the precipitated phases are essentially identical to those of target particles (ground borosilicate glass) dispersed at the same approximate position in the phase map, suggesting that the surfactant/polymer complex at the precipitation boundary is the same as that adsorbing onto the pigment particle. This conclusion is confirmed by depletion studies which allow the polymer adsorption density to be determined. For polymer-rich systems, essentially all of the surfactant adsorbs along with the polymer via a high-affinity isotherm with a plateau coverage of about 0.8 mg polymer/m (2). Surfactant-rich systems adsorb with a similar affinity, despite the mismatch of the complex charge matching that of the particle surface. Once adsorbed, these complexes are not readily removed by rinsing, though complexes adsorbed from SDS-rich systems will lose excess surfactant upon extreme dilution. Over a wide range of surfactant-rich compositions, from 1:1 stoichiometry out toward the resolubilization zone, a chemical analysis reveals that the surfactant/polymer precipitate species consists of a 1:1 charge complex with the addition of about 0.25 mol of surfactant/mol of complex. Resolubilization of these sparingly soluble species is achieved simply by dilution to below their solubility limit.     

DNA encapsulation by biocompatible catanionic vesicles

The encapsulation of DNA by catanionic vesicles has been investigated; the vesicles are composed of one cationic surfactant, in excess, and one anionic. Since cationic systems are often toxic, we introduced a novel divalent cationic amino-acid-based amphiphile, which may enhance transfection and appears to be nontoxic, in our catanionic vesicle mixtures. The cationic amphiphile is arginine-N-lauroyl amide dihydrochloride (ALA), while the anionic one is sodium cetylsulfate (SCS). Vesicles formed spontaneously in aqueous mixtures of the two surfactants and were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM); the vesicles are markedly polydisperse. The results are compared with a study of an analogous system based on a short-chained anionic surfactant, sodium octylsulfate (SOS). Addition of DNA to catanionic vesicles resulted in associative phase separation at very low DNA concentrations; there is a separation into a precipitate and a supernatant solution; the latter is first bluish but becomes clearer as more DNA is added. From studies using cryo-TEM and small angle X-ray scattering (SAXS) it is demonstrated that there is a lamellar structure with DNA arranged between the amphiphile bilayers. Comparing the SOS containing DNA-vesicle complexes with the SCS ones, an increase in the repeat distance is perceived for SCS. Regarding the phase-separating DNA-amphiphile particles, cryo-TEM demonstrates a large and nonmonotonic variation of particle size as the DNA-amphiphile ratio is varied, with the largest particles obtained in the vicinity of overall charge neutrality. No major differences in phase behavior were noticed for the systems here presented as compared with those based on classical cationic surfactants. However, the prospect of using these systems in real biological applications offers a great advantage.     

Investigations on the mechanisms of ionic conductivity in PEO-PU/PAN semi-interpenetrating polymer network-salt complex polymer electrolytes: an impedance spectroscopy study

This paper is a first comprehensive study on the correlated ion transport mechanisms contributing to the overall conductivity behavior in a new class of poly(ethylene oxide)-polyurethane/polyacrylonitrile (PEO-PU/PAN) semi-interpenetrating polymer networks (semi-IPNs)-salt complex polymer electrolytes. A simultaneous investigation of the electrical response on PEO-PU/PAN/LiClO(4) and PEO-PU/PAN/LiCF(3)SO(3) semi-IPNs with varying EO/Li mole ratios (100, 60, 30, 20, 15, 10) has been carried out by impedance spectroscopy. Analysis of the complex plane and spectroscopic plots indicated the presence of two microscopic phases corresponding to the PEO-PU and PAN domains, which leads to space charge polarization in these systems. A suitably modified approach based on the universal power law (UPL) considering the independent contribution from the individual microphases of semi-IPNs facilitates a complete interpretation of the spectroscopic profiles for the real component of conductivity (sigma'(omega)). The sigma'(omega) spectroscopic profiles were fitted with two power law equations, where the frequency region up to approximately 300 kHz is the conductivity profile associated with the PAN phase and beyond this is the superimposed contribution of the PEO-PU phase. Simulated fits using the UPL equation revealed two relaxation times (tau(PEO)(-)(PU), tau(PAN)) related to ionic hopping in the PEO-PU and PAN phases in addition to the conductivity relaxation time (tau(peak)) determined from the Debye peaks. The respective power law exponents (n(PEO)(-)(PU) approximately 0.5-0.8, n(PAN) approximately 1.0-1.6) indicate that though cationic hopping in the softer PEO-PU phase is favored, anionic hopping in the PAN phase contributes significantly to the charge transport processes in these semi-IPNs. Correlation of the experimental results with the simulated fits enable us to distinguish the effects of semi-IPN composition, temperature, morphology, ion-ion, and ion-polymer interactions, which influence the microscopic molecular events, involved in the charge transport in these complex semi-IPN polymer electrolytes.     

Swelling of Lamellar Gel from Catanionic Hydro- and Perfluoro-Carbon Surfactant Mixtures by Refractive-Index Matching

Bilayer swelling behavior of cationic and anionic surfactant mixtures in solution induced by adding glycerin was investigated. The measurements were performed a system, cationic tetradecyltrimetylammonium bromide (TTABr), and anionic sodium perfluorodecanoate (C₉F₁₉CO₂Na) surfactant mixtures with their stoichiometric mole ratio being exactly 1 in aqueous solution. The non-precipitated phase of cationic and anionic hydro- and perfluoro-carbon surfactant mixtures being the mole ratio of 1:1 could be identified to be lamellar gel phase, which was characterized by freeze-fracture transmission electron microscopy (FF-TEM) and x-ray diffraction (XRD) measurements. Deuterium nuclear magnetic resonance (²H NMR) and rheology were used to characterize the phase transition from the lamellar gel to smaller vesicles. Phase transition from lamellar gel to smaller vesicles can be induced by adding glycerin to replace water. The addition of glycerin lowers the turbidity of the dispersion and swells the interlamellar distance between bilayers, which could be explained by matching of refractive index between solvent and bilayers.     

Surfactant vesicles for high-efficiency capture and separation of charged organic solutes

We demonstrate the unique ability of catanionic vesicles, formed by mixing single-tailed cationic and anionic surfactants, to capture ionic solutes with remarkable efficiency. In an initial study (Wang, X.; Danoff, E. J.; Sinkov, N. A.; Lee, J.-H.; Raghavan, S. R.; English, D. S. Langmuir 2006, 22, 6461) with vesicles formed from cetyl trimethylammonium tosylate (CTAT) and sodium dodecylbenzenesulfonate (SDBS), we showed that CTAT-rich (cationic) vesicles could capture the anionic solute carboxyfluorescein with high efficiency (22%) and that the solute was retained by the vesicles for very long times (t1/2 = 84 days). Here we expand on these findings by investigating the interactions of both anionic and cationic solutes, including the chemotherapeutic agent doxorubicin, with both CTAT-rich and SDBS-rich vesicles. The ability of these vesicles to capture and hold dyes is extremely efficient (>20%) when the excess charge of the vesicle bilayer is opposite that of the solute (i.e., for anionic solutes in CTAT-rich vesicles and for cationic solutes in SDBS-rich vesicles). This charge-dependent effect is strong enough to enable the use of vesicles to selectively capture and separate an oppositely charged solute from a mixture of solutes. Our results suggest that catanionic surfactant vesicles could be useful for a variety of separation and drug delivery applications because of their unique properties and long-term stability.     

Preparation of an ion-exchangeable polymer bead wrapped with bilayer membrane structures for high performance liquid chromatography

We synthesized a chromatographic packing material that has a non-covalently attached dihexadecyl phosphate (DHP) bilayer membrane structure on a CA08S, a nonporous-type cationic polymer bead with a diameter ranging from 11 to 14 μm. Confocal fluorescence microscopic and differential scanning calorimetric analyses of the DHP-CA08S complex revealed that the DHP bilayer membrane structures were formed on the surface of the CA08S polymer beads. When the functionality of the DHP-CA08S complex was evaluated in the ion-exchange HPLC of proteins, the retention behavior of the proteins on the DHP-CA08S complex column totally mirrored the anionic property of the DHP bilayer membrane surface, not the cationic property of the CA08S bead. Methylene blue (MB) was eluted from the DHP-CA08S complex column in the isocratic elution mode, but not at all from a CK08S column, a styrene-divinylbenzene based cation-exchange polymer. When the column temperature was elevated from 50 to 60 °C, the peak shape of MB on the DHP-CA08S complex column became fairly sharp without a change in its peak area, which mirrored the characteristic phase transition of the DHP bilayer membrane formed on the DHP-CA08S complex.     

Gelation of "catanionic" vesicles by hydrophobically modified polyelectrolytes

The gelation of mixed cationic/anionic surfactant vesicles of sodium dodecyl sulfate/didodecyldimethylammonium bromide and sodium dodecylbenzenesulfonate/cetyltrimethylammonium tosylate by hydrophobically modified sodium polyacrylate is studied rheologically. When the vesicles are cationically charged, mixtures with this anionic polyelectrolyte form precipitates. When the vesicles are anionically charged, however, these mixtures display a progression from a Maxwell fluid to a critical gel to a solidlike gel with increasing vesicle and/or polyelectrolyte concentration. Consideration of the viscous behavior with increasing vesicle and polymer volume fraction indicates that the gel network is formed by the bridging of the hydrophobically modified polymer between vesicles. The similarity between the gelation results for the two anionic systems suggests the results can be generalized to other similarly charged mixtures.     

Temperature-responsive phase transition of polymer vesicles: real-time morphology observation and molecular mechanism

Novel thermosensitive polymer vesicles with controlled temperature-responsive phase transition at the lower critical solution temperature (LCST) varying from 8 to 81 degrees C were prepared via self-assembly of amphiphilic hyperbranched star copolymers having a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many hydrophilic polyethylene oxide (PEO) arms. Real-time optical microscopic observation revealed that the polymer vesicles have undergone sequential morphology changes including enrichment, aggregation, fusion, and vesicle-to-membrane transformation near the LCST. Molecular-level investigation indicates that the LCST transition results from the decreasing water solubility of the polymer vesicles with increasing temperature based on the partial dehydration of the PEO vesicle corona. On the basis of these results, a LCST transition mechanism, in view of the molecular configuration, balance of hydrophilic and hydrophobic moieties, and the vesicle morphology transformations, was proposed. As far as we know, the work presented here is the first demonstration of thermosensitive vesicles based on PEO, and the finding may be useful to design the thermosensitive core-shell structures by introducing the PEO segments.     

Sol-gel reaction in acrylic polymer emulsions: the effect of particle surface charge

Acrylic polymer-silica hybrid emulsions were synthesized from both anionic and cationic polymer emulsions by simple post-addition of tetraethoxysilane as a silica precursor. Solvent resistance of the films from the hybrid emulsions and the zeta-potential of the hybrid emulsions suggested the different forms of silica components in each hybrid emulsion. Thermal gravimetric analysis, 29Si NMR measurements, and transmission electron microscope observations revealed that the hybrid emulsion from the anionic polymer emulsion was a mixture of anionic polymer particles and homogeneously dissolved silicate oligomer-polymer. On the contrary, the hybrid emulsion from cationic polymer emulsion consisted of polymer core-silica shell particles. The electrostatic interaction between the cationic polymer particle surface and the silicate would be responsible for the accumulation of the silicate onto the particle surface, leading to the silica shell layer formation. The sol-gel condensation reaction of silicate in the acidic emulsion phase was revealed to be controllable by the surface charge of the coexisting particles.     

Fluorescence delineation of the surfactant microstructures in the CTAB-sOS-H2O catanionic system

A key feature of amphiphilic molecules is their ability to undergo self-assembly, a process in which a complex hierarchical structure is established without external intervention. Ternary systems consisting of aqueous mixtures of cationic and anionic surfactants exhibit a rich array of self-assembled microstructures such as spherical and rodlike micelles, unilamellar and multilamellar vesicles, planar bilayers, and bicontinuous structures. In general, multiple complementary techniques are required to explore the phase behavior and morphology of aqueous systems of oppositely charged surfactants. As a novel and effective alternative approach, we use fluorescence spectroscopic measurements to examine the microstructures of aqueous cationic/anionic surfactant systems in the dilute surfactant region. In particular, we demonstrate that the polarity-sensitive fluorophore prodan can be used to demarcate the surfactant microstructures of the ternary system of cetyltrimethylammonium bromide, sodium octyl sulfate, and water. As the fluorescence signature of this probe is dependent on the nature of the surfactant aggregates present, our method is a promising new approach to effectively map complex surfactant phase diagrams.     

Synthesis and Characterization of Piperazinium Polyelectrolytes Carrying Hydrophobic Functionalities: Effect of Counter Ion and Charge Density on Flocculation

Following our recent synthesis and characterization of three new cationic polyelectrolytes with subtle hydrophobic variability, this paper reports their physical and chemical properties in aqueous media in relation to their chemical structure. Aryl substituted cationic polyelectrolytes varying with their charge density are reported for the first time. Viscosity studies show that these polymers display typical polyelectrolytic behavior. The flocculation efficiency of the polyelectrolytes was investigated with different counter ions. The zeta potential of the polyelectrolytes indicates the charge of the mono and diquaternary ammonium salts which is supported by chloride analysis. The morphology of polymer before and after flocculation was investigated. The introduction of methylene group and quaternary nitrogen play an important role in the flocculation process. It was shown that increasing the hydrophobicity and charge density of the aryl substituted polymer affects the flocculation in the industrial tannery effluent and bentonite suspension.     

Two routes to vesicle formation: metal-ligand complexation and ionic interactions

Two routes to vesicle formation were designed to prepare uni- and multilamellar vesicles in salt-free aqueous solutions of surfactants. The formation of a surfactant complex between a double-chain anionic surfactant with a divalent-metal ion as the counterion and a single-chain zwitterionic surfactant with the polar group of amine-oxide group is described for the first time as a powerful driving force for vesicle-phases constructed from salt-free mixtures of aqueous surfactant solutions. As a typical example, a Zn(2+)-induced charged complex fluid, vesicle-phase has been studied in aqueous mixtures of tetradecyldimethylamine oxide (C(14)DMAO) and zinc 2,2-dihydroperfluorooctanoate [Zn(OOCCH(2)C(6)F(13))(2)]. This ionically charged vesicle-phase formed due to surfactant complexation has interesting rheological properties and is not shielded by excess salts because there are no counterions in the solution. Such a vesicle-phase of surfactant complex is important for many applications; for example, the vesicle-phase was further used to produce in situ the vesicle-phase of the salt-free cationic/anionic (catanionic) surfactants, C(14)DMAOH(+)-(-)OOCCH(2)C(6)F(13). The salt-free catanionic vesicle-phase could be produced through injecting H(2)S gas into the C(14)DMAO/Zn(OOCCH(2)C(6)F(13))(2) vesicle-phase, because the zwitterionic surfactant C(14)DMAO can be charged by the H(+) released from H(2)S to become a cationic surfactant and Zn(2+) was precipitated as ZnS. After the ZnS precipitates were removed from C(14)DMAO/Zn(OOCCH(2)C(6)F(13))(2) solutions, the final mixed solution does not contain excess salts as do other cationic/anionic surfactant systems. Both the C(14)DMAO-Zn(OOCCH(2)C(6)F(13))(2) complex and the resulting catanionic C(14)DMAOH(+)-(-)OOCCH(2)C(6)F(13) solution are birefringent Lalpha-phase solutions that consist of uni- and multilamellar vesicles. Ring-shaped semiconductor ZnS materials with encapsulated ZnS precipitates and regular spherical ZnS particles were prepared, which resulted in a transition from vesicles composed of metal-ligand complexes to vesicles held together by ionic interactions in the salt-free aqueous systems. This strategy should provide a new method to prepare inorganic materials. The present routes to form vesicles solve a problem: how to prepare nanomaterials using surfactant self-assembly, with structure controlled not by the growing material, but by the phase behavior of the surfactants.     

Dispersions of polymer ionomers: I

The principal subject discussed in the current paper is the effect of ionic functional groups in polymers on the formation of nontraditional polymer materials, polymer blends or polymer dispersions. Ionomers are polymers that have a small amount of ionic groups distributed along a nonionic hydrocarbon chain. Specific interactions between components in a polymer blend can induce miscibility of two or more otherwise immiscible polymers. Such interactions include hydrogen bonding, ion-dipole interactions, acid-base interactions or transition metal complexation. Ion-containing polymers provide a means of modifying properties of polymer dispersions by controlling molecular structure through the utilization of ionic interactions. Ionomers having a relatively small number of ionic groups distributed usually along nonionic organic backbone chains can agglomerate into the following structures: (1) multiplets, consisting of a small number of tightly packed ion pairs; and (2) ionic clusters, larger aggregates than multiplets. Ionomers exhibit unique solid-state properties as a result of strong associations among ionic groups attached to the polymer chains. An important potential application of ionomers is in the area of thermoplastic elastomers, where the associations constitute thermally reversible cross-links. The ionic (anionic, cationic or polar) groups are spaced more or less randomly along the polymer chain. Because in this type of ionomer an anionic group falls along the interior of the chain, it trails two hydrocarbon chain segments, and these must be accommodated sterically within any domain structure into which the ionic group enters. The primary effects of ionic functionalization of a polymer are to increase the glass transition temperature, the melt viscosity and the characteristic relaxation times. The polymer microstructure is also affected, and it is generally agreed that in most ionomers, microphase-separated, ion-rich aggregates form as a result of strong ion-dipole attractions. As a consequence of this new phase, additional relaxation processes are often observed in the viscoelastic behavior of ionomers. Light functionalization of polymers can increase the glass transition temperature and gives rise to two new features in viscoelastic behavior: (1) a rubbery plateau above T(g) and (2) a second loss process at elevated temperatures. The rubbery plateau was due to the formation of a physical network. The major effect of the ionic aggregate was to increase the longer time relaxation processes. This in turn increases the melt viscosity and is responsible for the network-like behavior of ionomers above the glass transition temperature. Ionomers rich in polar groups can fulfill the criteria for the self-assembly formation. The reported phenomenon of surface micelle formation has been found to be very general for these materials.     

Semi‐interpenetrating polymer networks composed of poly(N‐isopropyl acrylamide) and polyacrylamide hydrogels

Three series of semi‐interpenetrating polymer networks, based on crosslinked poly(N‐isopropyl acrylamide) (PNIPA) and 1 wt % nonionic or ionic (cationic and anionic) linear polyacrylamide (PAAm), were synthesized to improve the mechanical properties of PNIPA gels. The effect of the incorporation of linear polymers into responsive networks on the temperature‐induced transition, swelling behavior, and mechanical properties was studied. Polymer networks with four different crosslinking densities were prepared with various molar ratios (25:1 to 100:1) of the monomer (N‐isopropyl acrylamide) to the crosslinker (methylenebisacrylamide). The hydrogels were characterized by the determination of the equilibrium degree of swelling at 25 °C, the compression modulus, and the effective crosslinking density, as well as the ultimate hydrogel properties, such as the tensile strength and elongation at break. The introduction of cationic and anionic linear hydrophilic PAAm into PNIPA networks increased the rate of swelling, whereas the presence of nonionic PAAm diminished it. Transition temperatures were significantly affected by both the crosslinking density and the presence of linear PAAm in the hydrogel networks. Although anionic PAAm had the greatest influence on increasing the transition temperature, the presence of nonionic PAAm caused the highest dimensional change. Semi‐interpenetrating polymer networks reinforced with cationic and nonionic PAAm exhibited higher tensile strengths and elongations at break than PNIPA hydrogels, whereas the presence of anionic PAAm caused a reduction in the mechanical properties. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3987–3999, 2004     

Effects of thermosensitivity of poly (N-isopropylacrylamide) hydrogel upon the duration of a lag phase at the beginning of drug release from the hydrogel

The release rates of three kinds of drugs, with different charges, from poly (N-isopropylacrylamide) hydrogels were studied. The release rate was observed to be temperature dependent for the types of drug. When the temperature was lower than the phase transition temperature, the release rate was higher at lower temperatures and increased as the temperature rose. The amount of drugs released from a poly (N-isopropylacrylamide) hydrogel disk was plotted against the square root of time. It was found that the amount of drugs released was proportional to the square root of time over a certain time interval. A lag phase was observed before the amount of drug released became proportional to the square root of time. The longest time lag was observed at the phase transition temperature of poly (N-isopropylacrylamide); LCST (33°C). This suggests that the penetration rate of water into the hydrogels is lowest at the phase transition temperature and drastically changes around it. The release rates of drugs was also affected by the charges of the drug molecules. This may be caused by the interaction of drug molecules with polymer chains. When anionic drugs are released, the electrostatic repulsion seems to act between polymer chains and drug molecules. Therefore, the lag phase observed at the beginning of the release of anionic drugs was shorter, as compared with other kinds of drugs at any temperatures between 25 and 40°C. On the other hand, when cationic drugs are released, the time lag was longer at temperatures higher than 33°C as compared with the time lag at lower temperatures. At temperatures higher than 33°C, drugs are released from the surface skin layer of the hydrogel where water molecules are less mobile than those in bulk distilled water. The drug release thus shows a long lag phase.     

Electric Conductivity of Sandwich Structures with Films of Poly-N-epoxypropylcarbazole Doped with Organic Dyes of Various Ionicity

The conducting characteristics of samples of a sandwich structure with films based on the photoconducting polymer poly-N-epoxypropylcarbazole doped with cationic, anionic, cationic–anionic, neutral, and bipolar organic dyes were investigated. It was shown that the conductivity of the samples is caused by ion drift, thermofield generation of holes from uncontrollable impurity centers, thermofield generation of electrons and holes from the dopant molecules, and thermofield injection of electrons and holes from the electric contacts. The contribution from the injection currents of the charge carriers increases in the transition from a cationic dye to anionic, cationic–anionic, and intraionic dyes.