Excluded chain dimensions and network size in g.p.c.

Attempts have been made to correlate the average network size of polystyrene, poly(methyl methacrylate), poly(methyl acrylate), poly(n-butyl acrylate) and poly(vinyl acetate) gels with the maximum size of the polymer molecule which can permeate the gels. The average size of the network structure (r_c) was estimated from the average molecular weight between crosslinkages (M_c) derived from the glass transition temperatures of the gels. The maximum molecular size which could permeate into the network was expressed as the root-mean-square end-to-end distance [$\text{(}\text{r}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$] of the permeable substance with the molecular weight at exclusion limit (M_lim) obtained by gel permeation chromatography. Both r_c and $\text{(}\text{r}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreased with increasing total dose of γ-irradiation or increasing amount of the crosslinking agent. Linear relations were also obtained between r_c and $\text{(}\text{r}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ covering a wide range of total dose and amount of the crosslinking agent.     

Development of techniques to compare mechanical properties of reversible hydrogels with spherical, square columnar and ocular lens geometry

We have developed a new experimental technique for determining the elastic modulus of the ocular lens by using hydrogel phantoms. We successfully prepared disulfide-reversible polyacrylamide hydrogels in the forms of a square column, a sphere, and a lens. The M_n (number average molecular weight) of the reduced copolymer ranged from 696,800 to 870,900 Da. The physical and swelling properties of the gels were independent of shape. The M_c (molecular weight between cross-links) ranged from 1776 to 1887 compared with the theoretical M_c of 1638 Da. The gels exhibited non-linear rubber elasticity, but at low strains the elastic moduli (E) were 4680±150, 5010±280, and 4870±220 Pa for the square column, sphere, and lens, respectively. The shear modulus (G) was 1531±70 Pa with an E/G ratio of approximately 3:1, indicating an incompressible gel at low strains. At high strains (∼15%) the Mooney-Rivlin plot was linear and the magnitude of 2C₁ was 1515 Pa, which was comparable to the shear modulus of the gels. Finally, the Tatara mechanical model for large deformation of rubber spheres was successfully applied to extract the elastic modulus of the lens. The modulus of the lens obtained with this technique was consistent with the moduli for the square column and sphere. The new technique will be used to determine the mechanical properties of the ocular lens.     

Polybenzimidazole gel membrane for the use in fuel cell

Thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA) is investigated by studying the gel morphology, thermodynamics of the gelation, and gelation kinetics utilizing test tube tilting and UV-Vis spectroscopy techniques. Gelation kinetics studies reveal that both the gelation rate and critical gelation concentration (C^∗_t=∞) are function of gelation temperature (T_gel) and the molecular weight of PBI. Highly dense fibrillar network morphology with large number of longer and thinner fibrils is obtained for higher gel concentration and higher molecular weight PBI. Both the gel melting (T_gm) and gelation (T_gel) temperature depend upon the gelation concentration and molecular weight of PBI. The presence of self-assembled chains of PA molecules, which help to produce the PBI crystallites, is observed from the thermodynamical study. I.R. and Raman studies prove the presence of strong hydrogen bonding interaction between the PBI and the PA molecules, and the free PA molecules in the gel network. The gelation occurs in two-step processes which include a slow rate determining conformational transition from coil to rod and followed by aggregation of rod via crystallization. The PA loading of PBI membrane obtained from the PBI-PA gel is significantly high compared to the conventional imbibing process membrane. The PBI gel membrane displays very high thermal and mechanical stabilities. The high acid loading and superb thermo-mechanical stability are due to the gel network structure of the membrane. The proton conductivity of the membrane at 160 °C and 0% relative humidity (RH) is ∼0.1 S cm⁻¹, which is higher than the reported values in the literature for the PBI. The activation energy of the proton conduction is 14-15 kJ/mol indicating faster proton transfer by hopping process inside the gel network.     

Gelation mechanism and mechanical properties of Tetra-PEG gel

Tetra-PEG gel has drawn much attention as a polymer gel with extremely suppressed heterogeneity, which is known as the intrinsic characteristics of polymer networks, and is difficult to control. Tetra-PEG gel is formed by a novel fabrication method, i.e., AB-type crosslink coupling, which is polycondensation between mutually reactive multi-armed polymers forming a polymer network. AB-type crosslink coupling has advantages on suppressing the heterogeneity over conventional polycondensation. In this review, we focused on the gelation reaction and the relationship between the mechanical properties and structure. The reaction proceeded as a simple second order reaction of A and B end-groups from the initiation to the end (approx. 90%), suggesting the homogeneous mixing of two prepolymers. The model predicting the elastic modulus of Tetra-PEG gel shifts from the phantom to affine network models with an increase in polymer concentration. The fracture energy is well predicted by the Lake–Thomas model. These experimental studies also suggest that Tetra-PEG gel has extremely homogeneous network structure and is a candidate material for evaluating the basic physical properties of polymer gels.     

Dynamic viscoelastic behavior of triple helical Lentinan in water: Effects of concentration and molecular weight

The dynamic viscoelasitc behavior of Lentinan, one triple helical β-(1 → 3)-d-glucan from the fruiting body of Lentinus edodes, in water was investigated as a function of concentration and molecular weight at 25 °C by using dynamic rheology. It was revealed that the shear storage moduli (G′), viscous loss moduli (G″), and the dynamic complex viscosity (η^∗) exhibited strong dependence on concentration and molecular weight. At low concentrations, the Lentinan/water systems displayed liquid-like behavior with G′ lower than G″ at low frequencies and crossing-over at high frequencies. With increasing concentration, the elastic response of the Lentinan/water system was stronger than the viscous response, leading to the conclusion that the Lentinan/water systems displayed a predominantly solid-like behavior. The gel point (c_gel) was determined from Winter-Chambon method (frequency-independence of tan δ). The most important point is that the c_gel was much lower than some synthesized polymers and other flexible polysaccharides, which may be attributable to the high stiffness of triple helical Lentinan and strong intra- and intermolecular interactions among polysaccharide chains. Furthermore, a decrease in molecular weight leads to a sharp increase of c_gel. The dynamic strain sweep measurements proved that the gelation of Lentinan in water is induced by the extremely entangled and stiff triple helices forming continuous network, and the Lentinan gel is structurally more like a solution that is unable to flow within a timescale of usual observation.     

Swelling and elasticity of ionic poly(N-isopropylacrylamide) gels immersed in the melt of poly(ethylene glycol) chains

The equilibrium network concentration ν₂ and the elastic modulus G of a series of poly(N-isopropylacrylamide) (PNIPA) gels immersed in the melt of poly(ethylene glycol) (PEG) chains were investigated. 2-Acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) was used as the ionic comonomer in the gel synthesis. The molecular weight of PEG was varied between 200 and 1000 g/mol. It was found that the decrease in ν₂ with increasing charge density of the gels is considerable weaker than expected with the assumption of Gaussian statistics. This deviation is interpreted as the ion pair and multiplet formation from the mobile counter ions of AMPS units of the gel in the presence of PEG chains. It was shown both theoretically and experimentally that the PNIPA gel undergoes a swelling-deswelling transition in PEG, if the chain length of PEG exceeds the critical value y_cr=N^1/2, where N is the network chain length calculated from the elastic moduli of gels immersed in PEG melt.     

Swelling behavior of amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate

The amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate with different 1-bromoalkanes (1-C_nH_2n+1Br, n = 2, 4, 6, 8, 12) were synthesized by radiation-induced polymerization and crosslinking. The length of alkyl side chains had significant influence on the swelling behavior of the resulting gels. The swelling degree of the gels decreased with the increase of side chain length, and the gel hardly swelled when n = 12. The effect of temperature and ionic strength on the swelling behavior of the resulting gels revealed that (1) the gels with longer side chains (n ≥ 8) had upper critical solution temperature, while other gels were not thermo-sensitive. (2) Antipolyelectrolyte effect was observed when immersing the gels (n ≥ 8) in NaCl solutions in certain concentration range. The dramatic difference in swelling behavior was attributed to the different gel structures. The gels with short side chains (n ≤ 6) had cellular structure of normal polyelectrolyte gels. The gels (n ≥ 8) had an aggregation gel structure caused by the hydrophobic interaction among alkyl groups and the formation of ion-cluster between tetra-alkyl ammonium cation and Br⁻, which had been analyzed with the aid of SEM, Br⁻-selective electrode and fluorescence molecular probe.     

Drying of agar gels using supercritical carbon dioxide

The use of supercritical carbon dioxide (scCO₂) for the removal of water from agar gels has been investigated and compared to air and freeze drying. Experiments were conducted to evaluate how gel formulation (with and without sucrose) and drying conditions (with and without ethanol as a co-solvent, flow rate and depressurisation rate) affected the microstructure of the gels dried using scCO₂. X-ray micro-computed tomography (X-ray micro-CT) was used to determine the voidage (% open pore space) of the dried structures, which can be used to indicate the extent of drying-induced structural collapse (in general, the lower the voidage, the greater the collapse). For formulations containing sucrose, which displayed the best structural retention, voidage was found to increase in the order: air drying (4% voidage) < supercritical drying with pure CO₂ (48%) < supercritical drying with ethanol-modified CO₂ (68%) < freeze drying (76%). The relatively high voidage of samples dried in the presence of ethanol, was due in part to foaming of the gels, hypothesised to result from an interaction between the agar and ethanol, rather than an effect of the supercritical fluid. CO₂ flow rate (1 vs. 3 l/min) during supercritical drying and depressurisation rate (0.4 vs. 1.6 MPa/min) had no effect on the dried microstructure.     

Influence of molecular weight of polymer matrix on the structure and rheological properties of graphene oxide/polydimethylsiloxane composites

The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (C_cr). Moreover, at GO concentration just above C_cr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (M_c). However, positive ΔN is found in samples with PDMS molecular weight above M_c. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.     

Glass transition temperature and critical properties

In a preceding paper it was shown that for high molecular weight polymers, the volume of the repeating unit at T_g is proportional to the total free space defined as V_c?V₀ where V_c is the volume of the repeating unit at the critical temperature and V₀ is the volume of the repeating unit at absolute zero. Here it will be demonstrated that this proportionality is also true for a host of low molecular weight liquids. In addition, an empirical method of estimating T_c for a high molecular weight polymer is proposed which provides values of T_c which are consistent with those for low molecular weight liquids.     

Influence of solvent size on the mechanical properties and rheology of polydimethylsiloxane-based polymeric gels

Soft polymeric gels have utility in a broad range of medical, industrial, and military applications, which has led to an extensive research investment over the past several decades. While most gel research exploits a cross-linked polymer network swollen with small molecule solvents, this article systematically investigates the impact of the solvent molecular weight on the resulting gel mechanical properties. The model polymer gel was composed of a chemically cross-linked polydimethylsiloxane (PDMS) network loaded with a non-reactive PDMS solvent. In addition to investigating the impact of solvent loading, the solvent molecular weight was varied from 423,000 g/mol to 1250 g/mol, broadly spanning the molecular weight of entanglement for PDMS (MW_ENT ∼29,000 g/mol). The gels exhibited a strong frequency dependent mechanical response when the solvent molecular weight >MW_ENT. In addition, scaling factors of shear storage modulus versus solvent loading displayed a distinct decrease from the theoretical value for networks formed in a theta solvent of 2.3 with increasing measurement frequency and solvent molecular weight. The frequency dependent shear storage modulus could be shifted by the ratio of solvent molecular weights to the 3.4 power to form a master curve at a particular solvent loading indicating that mobility of entangled solvent plays a critical role for the mechanical response. In addition, the incorporation of entangled solvent can increase the toughness of the PDMS gels.     

Characteristics of zwitterionic sulfobetaine acrylamide polymer and the hydrogels prepared by free-radical polymerization and effects of physical and chemical crosslinks on the UCST

A zwitterionic sulfobetaine polymer, poly(N,N-dimethyl(acrylamidopropyl) ammonium propane sulfonate) (poly(DMAAPS)), and the hydrogels of this polymer were synthesized by free-radical polymerization in an aqueous redox system using a wide range of monomer concentrations (C_m). The resulting polymers were characterized in terms of polymer yield, intrinsic viscosity, molecular weight, gel fraction, and thermoresponsive phase-transition behavior. Parameters in the Mark–Houwink–Sakurada equation, including the molecular-weight exponent α, were determined for poly(DMAAPS) in 0.1 M NaCl aqueous solution. The physical state and transparency of the poly(DMAAPS) samples were strongly dependent on C_m and temperature. At higher values of C_m (i.e. above a critical molecular weight), poly(DMAAPS) became a gel comprising a physically crosslinked network consisting of entangled polymer chains and interchain associations of the zwitterionic groups. The poly(DMAAPS) solutions or gels exhibited a thermoresponsive phase transition with an upper critical solution temperature (UCST). The gels obtained were completely soluble in aqueous NaCl solution at ambient temperature as well as in water at temperatures above UCST. The effects of molecular weight, chemical crosslink density and copolymerization on the UCST were also elucidated.     

Gelation behaviors of cellulose solution dissolved in aqueous NaOH/thiourea at low temperature

Cellulose was dissolved rapidly in 9.5 wt% NaOH/4.5 wt% thiourea aqueous solution pre-cooled to −5 °C, as a result of the formation of an inclusion complex (IC) associated with cellulose, NaOH and thiourea, which could bring cellulose to the aqueous system. To clarify the rheological behaviors of the system dissolved at low temperature, this cellulose solution was investigated by dynamic viscoelastic measurement. The shear storage modulus (G′) and loss modulus (G″) as a function of the angular frequency (ω), concentration (c), temperature (T) and weight-average molecular weight (M_w) were analyzed and discussed. The results revealed that gels could form in the cellulose solution at either high temperature or low temperature, or for longer time. Interestingly, 4 wt% cellulose solution having cellulose M_w of 12.0 × 10⁴ remained at liquid state for longer time (12 days) at the temperature ranging from 0 to 5 °C. The gels already formed at elevated temperature were irreversible, i.e., after cooling to lower temperature including the temperature of cellulose dissolution (−5 °C), they could not be dissolved to become liquid. The Arrhenius analysis of the temperature dependence of viscosity in the cellulose solution indicated that a high apparent activation energy (E_a) occurred at 0 to −5 °C, suggesting the relatively stable IC structure. However, the viscosity of the cellulose solution increased slowly with an increase in the temperature at 0-40 °C, leading to the negative E_a values. The results suggested that the cellulose solution in NaOH/thiourea system is complex to differ from normal polymer systems.     

Composition dependence of crystallized lamellar morphology formed in crystalline-crystalline diblock copolymers

We have investigated the crystallized morphology formed at each temperature T_c (20 °C ≤ T_c ≤ 45 °C) in double crystalline poly(?-caprolactone)-block-polyethylene (PCL-b-PE) copolymers as a function of composition (or volume fraction of PE blocks ?_PE) by employing small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. When PCL-b-PE with ?_PE ≤ 0.58 was quenched from a microphase-separated melt into T_c, the crystallization of PE blocks occurred first to yield an alternating structure consisting of thin PE crystals and amorphous PE + PCL layers (PE lamellar morphology) followed by the crystallization of PCL blocks, where we can expect a competition between the stability of the PE lamellar morphology (depending on ?_PE) and PCL crystallization (on T_c). Two different morphologies were formed in the system judging from a long period. That is, the PCL block crystallized within the existing PE lamellar morphology at lower T_c (<30 °C) to yield a double crystallized alternating structure while it crystallized by deforming or partially destroying the PE lamellar morphology at higher T_c (>35 °C) to result in a significant increase of the long period. However, the temperature at which the morphology changed was almost independent of ?_PE. For PCL-b-PE with ?_PE ≥ 0.73, on the other hand, the morphology after the crystallization of PE blocks was preserved at every T_c investigated.     

Burning of liquid fuels: effect of burner diameter on burning rate and measurements of quenching diameter

Laminar pool flames of four straight-chain alcohols were studied in uncooled glass and copper burners and water-cooled glass burners. The mass burning rate $\text{m}\text{?}$ increased with the burner diameter d decreasing as $\text{m}\text{?}$ ∝ d^?n, where n ≈ 1.2 for uncooled glass burners. The values of $\text{m}\text{?}$ in copper burners were lower by a factor of 1.5–2 than those in glass burners of the same diameter. Vigorous boiling of the subsurface layer of liquid occurred in sufficiently narrow uncooled glass burners, and numerous burning droplets were observed to traverse the flame. The quenching diameter d_c was measured. For uncooled copper burners d_c increased with increasing molecular weight M_r of the alcohol, from ≈ 2 mm for propanol to ≈ 20 mm for decanol. For uncooled glass burners d_c was much lower (1–2 mm) and less dependent on molecular weight. The product of d_c and the critical mass burning rate is assumed to have a characteristic value for the stability of diffusion burning, in accordance with the Zel'dovich criterion (Pe) = (Pe)_c = const. for flame stability in gas mixtures. However, the stability of diffusion burning increased with increasing heat losses from the burner wall. Two possible interpretations of this paradoxical effect are suggested.     

Temperature dependence of density of polymer gels: Effects of ionizable groups in copoly(N-isopropylacrylamide/acrylic acid or sodium acrylate)-water systems

Effects of ionizable groups in hydrogels of copolymer networks on the volumetric contraction-expansion process were investigated. Polymer networks used were: copoly[N-isopropylacrylamide (NIPA)(1 − x)/acrylic acid (HAc) or sodium acrylate (NaAc)(x)] with mole fraction of minor component (x) assuming 0.0114 and 0.0457. From the temperature (T) dependence of total volume of gels, densities of the polymer and solvent (water) components, and stoichiometry, we evaluated (1) the volume of gels occupied by a single mean polymeric residue and associated water molecules (expressed in units of nm³), mean v_sp(gel), and (2) number of water molecules per single mean polymeric residue, mean N_s(gel), from near 273 K to 323 K. These quantities (1) and (2) listed above showed how acid and salt forms affect differently on volumetric changes of gels over 50 K. We developed an approach to evaluate volumetric changes of gels solely caused by a single polymeric residue of a minor component (x < 0.05) plus associated water by applying thermodynamic first-order perturbation theory. They are specific v_sp(gel)(T) for a single HAc or NaAc polymeric residue plus associated water and the corresponding specific N_s(gel)(T). Specific v_sp(gel)(HAc or NaAc)(T) and the corresponding specific N_s(gel(T)) revealed specific characteristics in thermal behavior near their respective transition temperatures from the swollen to shrunken states. We found these thermal changes shown at the nano-scale match very well with specific changes in the molality(T) of both ionizable groups. In fact, these are directly triggered by varying contents of water in gels. Based on the understanding of dissociative equilibrium attained by ionizable groups, we successfully replaced Na⁺ in hydrogels of copoly[NIPA(1 − x)/NaAc(x)] (x = 0.0457) by hydrogen ions. Absence of Na⁺ in treated hydrogels was experimentally verified by ²³Na NMR and Na atomic absorption flame photometry. Discontinuity in the volumetric contraction-expansion process from the swollen to shrunken states and vice versa was not observed in contradiction to the previous reports [Hirotsu S, Hirokawa Y, Tanaka T. J Chem Phys 1987;87:1392-5. Matsuo SE, Tanaka T. J Chem Phys 1988;89:1695-703.] obtained by the conventional swelling experiments.     

AN EXPERIMENTAL STUDY OF POLYMER DIFFUSION IN CONCENTRATED SOLUTION: IMPLICATIONS FOR DIFFUSION IN POLYMERIZATION

The Trommsdorf or gel effect in free radical polymerization is due to the fact that the termination reaction becomes strongly diffusion controlled above a critical concentration associated with the onset of molecular entanglements. Therefore, an understanding of polymer self-diffusion in entangled systems becomes essential to understanding the Trommsdorf effect. Our group has previously proposed a molecular model for the gel effect which uses a specific theory for polymer diffusion (reptution). The present work represents an experimental attack on the same problem Experimental studies of polymer self-diffusion in entangled systems are scarce. Quasielastic light scattering from ternary systems composed of solvent(l)-polymer(2)-polymer(3), in which species (3) is isorefractive to the solvent (i.e. 0n/0c ₃ = 0), offers an attractive way to study the tracer diffusion coefficient of species (2) in a binary mixture of composition c ₃. In regimes of low momentum transfer (qR _G < 1,) where q is the scattered wave vector and R _G is the polymer radius of gyration, we have shown that the correlation function of the scattered electric field should decay with a single exponential decay time, given by (D₂₂ q ²)^?1 where D ₂₂(c ₂, c ₃) is the main ternary diffusion coefficient of component (2). Extrapolation to zero concentration of 2) at fixed concentration of (3) yields the tracer diffusion coefficient of (2) in the binary mixture of (1) and (3). The systems toluene(l)-polystyrene(2)-polymethylmethacrylate(3) (0n/0c ₃ ≈ 0.007 at 25°C) and toluene(l)-polystyrene(2)-polyvinylmethylether (3) (0n/0c ₃ ≈ 0.012 at 60°C) very nearly satisfy the above criteria. In both systems, we have found that the tracer diffusion coefficient of the polystyrene decreases with increasing concentration of the isorefractive polymer. Further studies have focused upon the dependence of the tracer diffusion coefficient upon polystyrene molecular weight, and upon the effect of incomplete index matching.     

Low frequency ac conduction and dielectric relaxation behavior of solution grown and uniaxially stretched poly(vinylidene fluoride) films

The measurements of ac conductivity [σ_m(ω)], dielectric constant [?′(ω)] and loss [?″(ω)] have been performed on solution grown (thickness ∼85 μm) and uniaxially stretched (thickness ∼25, 45 and 80 μm) films of poly(vinylidene fluoride) (PVDF) in the frequency range 0.1 kHz-10 MHz and in the temperature range 77-400 K. The σ_m(ω) can be described by the relation σ(ω) = Aω^s, where s is close to unity and decreases with increase in temperature. Three relaxations, observed in the present investigation, have been designated as the α_c-, the α_a- and the β-relaxations appearing from high temperature side to the low temperature side. The α_c-relaxation could not be observed in the case of uniaxially stretched poly(vinylidene fluoride) films. The α_c- and α_a-relaxations are associated with the molecular motions in the crystalline regions and micro-Brownian motion in the amorphous regions of the main polymer chain, respectively, whereas the β-relaxation is attributed to the rotation of side group dipoles or to the local oscillations of the frozen main polymer chain.     

Development and evaluation of pH-dependent interpenetrating network of acrylic acid/polyvinyl alcohol

The objective of the present work was to synthesize interpenetrating networks (IPNs) of acrylic acid/polyvinyl alcohol (AA/PVA) by free radical polymerization using N,N-methylenebisacrylamide (MBAAm) and glutaraldehyde as cross-linkers. The IPNs were evaluated for swelling, diffusion coefficient and network parameters by using Flory–Huggins theory, i.e., the average molecular weight between cross-links (M _c), polymer volume fraction in swollen state (V _2,s), number of repeating units between cross-links (M _r) and cross-linking density (N). It was found that the degree of swelling of AA/PVA interpenetrating network increases greatly within the pH range 5–7 depending on composition. The gel fraction and porosity increased by increasing the concentration of AA or PVA, while by increasing the degree of cross-linking, porosity decreased and gel fraction increased. Selected samples were loaded with chlorpheniramine maleate as a model drug. Drug release was studied in USP, hydrochloric acid buffer solution of pH 1.2 and phosphate buffer solutions of pH 5.5 and 7.5. Drug release data were fitted into various kinetics models, e.g., zero-order, first-order, Higuchi and Peppas models. The results of the kinetics investigation showed that the drug release from IPNs followed non-Fickian diffusion. Fourier transform infrared spectra confirmed the formation of cross-linked IPNs as there was a shifting to lower frequency of 1,713–1,718 cm^?1 with reduced intensity, while scanning electron microscopy revealed uniform distribution of drug in IPNs.     

Rheological characterization of a charged cationic hydrogel network across the gelation boundary

The in situ rheological behavior across the gelation threshold has been investigated for an affine network of a completely charged cationic monomer (3-acrylamidopropyl)-trimethylammonium chloride (APTMAC1) when it is crosslinked with a neutral crosslinker (N,N′-methylenebisacrylamide) to form a fully charged novel cationic hydrogel. The elastic moduli (G′) near the gel point (during the crosslinking or ‘curing’ process) show a power law dependence of the form G′(t)=ε^z, where ε=((t−t_c)/t_c) is the distance from the gel point (t_c). The critical exponent, z, for the hydrogel series investigated is estimated to be 1.5, slightly lower than the predictions based on percolation theory (z∼1.7-1.9). From the equilibrium (after the curing process) rheological measurements of a series of samples, it is inferred that there is a critical crosslinker mole percent (X_c) with respect to the monomer concentration, required to form a well-defined three-dimensional network with a solid-like behavior. The value of this X_c is found to be between 0.5 and 1%. The theoretically predicted value of X_c using the percolation theory (for the percolation of crosslinks, G₀(X)∝[|X−X_c|/X_c]^z) and the exponent estimated from the in situ measurements (z=1.5), is X_c∼0.6, which is in good agreement with the experiments. The results may have applicability in translating from bulk systems to the nanoscale in hydrogel design.