Superfast responsive ionic hydrogels with controllable pore size

A series of strong polyelectrolyte hydrogels was prepared from the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as the monomer and N,N′-methylene(bis)acrylamide (BAAm) as a crosslinker in aqueous solutions. The gel preparation temperature (T_prep) was varied between −22 and 25 °C. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at T_prep=−8 °C. The hydrogels prepared below −8 °C exhibit a discontinuous morphology consisting of polyhedral pores of sizes 30-50 μm, while those formed at higher temperatures have a non-porous structure. The pore size of the networks increased by decreasing the charge density of the hydrogels, while addition of low molecular weight salts into the gelation system reduced the size of the pores. Calculations based on the equilibrium between the ice and unfrozen gel phases in the reaction system at low temperatures explain the results of observations. It was also shown that the hydrogels formed below −8 °C exhibit superfast swelling properties as well as reversible swelling-deswelling cycles in water and acetone.     

Temperature sensitive poly(N-t-butylacrylamide-co-acrylamide) hydrogels: synthesis and swelling behavior

A series of temperature sensitive hydrogels was prepared by free-radical crosslinking copolymerization of N-t-butylacrylamide (TBA) and acrylamide in methanol. N,N′-methylenebis(acrylamide) was used as the crosslinker. It was shown that the swelling behavior of the hydrogels can be controlled by changing the amount of TBA units in the network chains. Hydrogels immersed in dimethylsulfoxide (DMSO)-water mixtures exhibited reentrant swelling behavior, in which the gels first deswell then reswell if the DMSO content of the solvent mixture is continuously increased. In water over the temperature range of 2-64 °C, hydrogels with less than 40[percnt] TBA by mole were in a swollen state while those with TBA contents higher than 60[percnt] were in a collapsed state. Hydrogels with 40-60[percnt] TBA exhibited swelling-deswelling transition in water depending on the temperature. The temperature interval for the deswelling transition of 60[percnt] TBA gel was found to be in the range from 10 to 28 °C, while for the 40[percnt] TBA gel, the deswelling started at about 20 °C and continued until the onset of the hydrolysis of the network chains at around 64 °C. It was shown that the Flory-Rehner theory of swelling equilibrium provides a satisfactory agreement to the experimental swelling data of the hydrogels, provided that the sensitive dependence of the χ parameter on both temperature and polymer concentration is taken into account.     

Super porous organic-inorganic poly(N-isopropylacrylamide)-based hydrogel with a very fast temperature response

Porous organic-inorganic (O-I) hydrogels showing a very fast temperature response, including very fast reswelling were prepared: only 6 s are needed for 72% deswelling (gel collapse) as well as for 72% reswelling. Both deswelling and reswelling are practically complete in 14 s. The gels were prepared from N-isopropylacrylamide (NIPA), N,N′-methylenebisacrylamide (BAA) and tetramethoxysilane (TMOS) by simultaneous radical polymerization and hydrolytic polycondensation of TMOS. The syntheses were carried out at temperatures below the lower critical solution temperature (LCST) of poly(NIPA) in two steps: during the first stage the temperature was held at T = +15 °C and during the second the temperature was lowered below the freezing point of the reaction mixture, T = −18 °C. The ice crystals, which grew during the second stage, served as the pore-forming agent. The best samples were obtained if the second stage was started shortly before the gel point of the reaction mixture. The introduction of the inorganic phase (silica) is necessary for the ability of fast reswelling and also results in a strong improvement of the hydrogels' mechanical properties, while the maximum swelling degree remains nearly unaffected.     

Preparation and structure characterization of hairy nanoparticles consisting of hydrophobic core and thermosensitive hairs

Structural characterization of hairy nanoparticles consisting of poly(styrene-co-glycidyl methacrylate) (St/GMA) core and poly(NIPA-co-vinylimidazole) (NIPA/VIm) hair has been carried out by dynamic light scattering. The hairy molecules were introduced by surface graft-polymerization of a mixture of NIPA and VIm monomers to the St/GMA core particles with the hydrodynamic radius R_H of 135±10 nm. The R_H of St/GMA-core-NIPA/VIm-hair particles was R_H=360±20 nm at 20 °C, which gradually decreased to 285±10 nm by heating to 33.0 °C, and then underwent a sharp decrease to 175±10 nm by further heating to 33.8 °C. The final value went to 159±10 nm at 36 °C. This decrease in R_H is due to the shrinking transition of NIPA/VIm chain by hydrophobic association. The degree of shrinking of the hairy particles is compared with that of bulk NIPA gels from the viewpoint of geometrical constraints.     

Physicochemical characterization of poly(tert-butyl acrylate-b-methyl methacrylate) prepared with atom transfer radical polymerization by inverse gas chromatography

Poly(tert-butyl acrylate) (PtBuA) was synthesized by atom transfer radical polymerization (ATRP) using methyl-2-bromo propionate (MBP) as an initiator in bulk at 80 °C. The successive ATRP of methyl methacrylate in diphenyl ether at 80 °C using previously obtained PtBuA as a macroinitiator led to formation of poly(tert-butyl acrylate-b-methyl methacrylate) (poly(tBuA-b-MMA)). The synthesized macroinitiator and block copolymer have controlled molecular weight and low polydispersity (M_w/M_n<1.2). The block copolymer was characterized by gel permeation chromatography (GPC) and ¹H NMR. The retention diagrams of poly(tBuA-b-MMA) for some aliphatic esters and aromatic hydrocarbons were obtained using inverse gas chromatography (IGC) technique. The glass transition temperatures, T_gs of poly(tBuA-b-MMA) were determined by both differential scanning calorimeter (DSC) and IGC. It was observed that the block copolymer represents three T_gs at 50, 75 and 100 °C by IGC although it represents only one T_g at 71 °C by DSC. After the column was quenched from 180 to 0 °C, the T_g at 100 °C shifted to 105 °C however others did not change. Specific retention volumes, and the thermodynamical polymer-solvent interaction parameters such as Flory-Huggins, , equation-of-state, and effective exchange energy, X_eff were found for all studied solvents. Partial molar heat of sorption, , partial molar heat of mixing, and molar heat of vaporization, ΔH_v, were determined. In addition, the solubility parameter of the corresponding block copolymer, δ₂ was determined as 11.0 (cal/cm³)^1/2 at 25 °C.     

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.     

Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with a hydrophobic comonomer

Hydrogels were prepared by free radical polymerisation in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with di-n-propylacrylamide (DPAM), di-n-octylacrylamide (DOAM) or di-dodecylacrylamide (DDAM) as hydrophobic comonomer. N,N-methylene bisacrylamide (BIS) and glyoxal bis(diallyacetal) (GLY) were used as crosslinkers. A series of copolymers with three different comonomer contents was synthesised and for some polymers three different crosslinker concentrations were employed. The swelling equilibrium of these hydrogels was studied as a function of temperature, hydrophobic comonomer species and content in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS). In pure water the gels showed a discontinuous volume phase transition at 33 and 30 °C for PNIPA and hydrophobically modified PNIPA copolymeric hydrogels, respectively. The swelling ratio r and the transition temperature (LCST) increased at low temperatures with the addition of SDS, this is ascribed to the conversion of non-ionic PNIPA gels into polyelectrolyte gels through the binding of SDS. At SDS concentration below 0.5 wt%, gels exhibited a single discontinuous volume transition at 36-38 °C. However, for SDS concentration above 0.5 wt%, two discontinuous volume transitions at 36-40 and 70 °C were observed. Additionally, the replacement of BIS by the novel octafunctional crosslinker glyoxal bis(diallylacetal) (GLY) yielded an increase in the swelling ratio.     

Synthesis, characterization and optical properties of cross-linkable poly(phthalazinone ether ketone sulfone)

Cross-linkable poly(phthalazinone ether ketone sulfone) bearing tetrafluorostyrene groups (PPEKS-FSt) has been prepared by copolycondensation reaction for optical waveguide applications. The resulting amorphous polymer exhibits good solubility in some common polar organic solvents (e.g., N,N′-dimethylacetamide, N-methyl-2-pyrrolidinone, chloroform) at room temperature, and can be easily spin-coated into thin films with good optical quality. The glass transition temperature (T_g) and the temperature of 1% weight loss (1% T_d) are 261 °C and 494 °C, respectively, which could be further increased by 31 °C and 14 °C upon thermal cross-linking. The cross-linked polymer thin films exhibit high refractive index (∼1.65, TE mode), high thermo-optic coefficient value (dn/dT) (−1.455 × 10⁻⁴/°C, TE mode), low optical loss (less than 0.24 dB/cm at 1310 nm) and relatively low birefringence (∼0.007).     

Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (T_c)≥125 °C), the induction period for PLLA spherulite formation (t_i) (T_c≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤T_c≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (t_c(S), t_c(1/2), and t_c(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the t_c(S), t_c(1/2), and t_c(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long t_i and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the t_c(S), t_c(1/2), and t_c(E) were shorter than or similar to those at PDLA content of 0 wt% for the T_c range below 95 °C and above 125 °C, whereas this inclination was reversed for the T_c range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.     

The influence of axial pressure on relaxor properties of BaBi2Nb2O9 ceramics

On the basis of our studies it results that dielectric properties of BaBi₂Nb₂O₉ ceramics are sensitive to axial pressure applied. The pressure causes an increase of dispersion in the real part of dielectric permittivity ?′(T,f) and a rise in the temperature T_m at which the maximum in ?′(T,f) dependence occurs. The applied pressure induces in the ?′(T) dependence an additional step-like anomaly, which appears at the temperature T_A < T_m. The applied pressure shifts both T_m and T_A at the same rate, i.e. dT_A/dX = dT_m/dX = +14 °C/kbar at high axial pressure range, above the threshold pressure X_thresh. The Vogel–Fulcher relationship is employed to determine the axial pressure influence on relaxor properties of BBN ceramics. The simulated order parameter q takes non-zero values below Burn‘s temperature T_B, where the polar clusters appear on cooling. For pressures higher than 0.8 kbar, the T_B changes at the rate dT_B/dX = −200 °C/kbar. The decrease in the difference between Burn's T_B and the freezing T_f temperatures induced by the applied axial pressure is observed. This could be ascribed to the narrowing of temperature range of relaxor behavior.     

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.     

Melting behavior of poly(l-lactic acid): Effects of crystallization temperature and time

Effects of crystallization temperature and time on the melting behavior of poly(l-lactic acid) were studied with differential scanning calorimetry (DSC). The isothermal crystallization was performed at various temperatures (T_cs), and DSC melting curves for the isothermally crystallized samples were obtained at 10 K min⁻¹. When T_c was lower than T_d (∼135 °C), the double melting peaks appeared. The melting behavior, especially T_c dependence of the melting temperature (T_m), discretely changed at T_b (=113 °C), in accordance with the discrete change of the crystallization behavior at T_b, which was previously reported. When T_c was higher than T_d, a single melting peak appeared. In addition, T_c dependence of dT_m/dT_c discretely changed at T_d. That is, the melting behavior, especially T_c dependence of T_m and dT_m/dT_c, are different in three temperature regions of T_c divided by T_b and T_d: Regions I (T_c ≤ T_b), II (T_b ≤ T_c ≤ T_d), and III (T_d ≤ T_c). The effect of crystallization time on the melting behavior, melting temperature and heat of fusion in each temperature region of T_c is also discussed.     

Isothermal calorimetry investigation of Li1+xMn2−yAlzO4 spinel

The heat generation of LiMn₂O₄, Li_1.156Mn_1.844O₄, and Li_1.06Mn_1.89Al_0.05O₄ spinel cathode materials in a half-cell system was investigated by isothermal micro-calorimetry (IMC). The heat variations of the Li/LiMn₂O₄ cell during charging were attributed to the LiMn₂O₄ phase transition and order/disorder changes. This heat variation was largely suppressed when the stoichiometric spinel was doped with excess lithium or lithium and aluminum. The calculated entropy change (dE/dT) from the IMC confirmed that the order/disorder change of LiMn₂O₄, which occurs in the middle of the charge, was largely suppressed with lithium or lithium and aluminum doping. The dE/dT values obtained did not agree between the charge and the discharge at room temperature (25 °C), which was attributed to cell self-discharge. This discrepancy was not observed at low temperature (10 °C). Differential scanning calorimeter (DSC) results showed that the fully charged spinel with lithium doping has better thermal stability.     

Recrystallization studies on isotropic cold-crystallized PET: Influence of heating rate

The nanostructural changes associated to the multiple melting behaviour of isotropic cold-crystallized poly(ethylene terephthalate) (PET) have been investigated by means of simultaneous wide- and small-angle X-ray scattering, using a synchrotron radiation source. Variations in the degree of crystallinity, coherent lateral crystal size and long period values, as a function of temperature, for two different heating rates are reported for cold-crystallized samples in the 100-190 °C range. The Interface Distribution Function analysis is also employed to provide the crystalline and amorphous layer thickness values at various temperatures of interest. Results suggest that samples crystallized at both low (T_a = 100-120 °C) and high (T_a = 160-190 °C) temperatures are subjected to a nearly continuous nanostructural reorganization process upon heating, starting immediately above T_g (≈80 °C) and giving rise to complete melting at ≈260 °C. For all the T_a investigated, a melting-recrystallization mechanism seems to take place once T_a is exceeded, concurrently to the low-temperature endotherm observed in the DSC scans. For low-T_a and slow heating rates (2 °C/min), a conspicuous recrystallization process is predominant within T_a + 30 °C ≤ T ≤ 200 °C. In contrast, for high-T_a, an increasingly strong melting process is observed. For both, high- and low-T_a, an extensive structural reorganization takes place above 200 °C, involving the appearance of new lamellar stacks simultaneously to the final melting process. The two mechanisms should contribute to the high-temperature endotherm in the DSC scan. Finally, the use of a high heating rate is found to hinder the material's overall recrystallization process during the heating run and suggests that the high-temperature endotherm is ascribed to the melting of lamellae generated or thickened during the heating run.     

The phase structure and electric properties of low-temperature sintered (K,Na)NbO3-based piezoceramics modified by CuO

0.25 wt% CuO-doped (Li,K,Na)(Nb,Ta)O₃–AgSbO₃ lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000 °C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100 °C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal–orthorhombic phase transition point (T_T−O) but a lower Curie point (T_c), compared to undoped ones. The KNN-based piezoceramics became “hard” as CuO was added, supported by an increase of Q_m. Fairly good electrical properties of d₃₃^*=383 pm/V, ε_r=860, Q_m=188 and T_c=215 °C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970 °C, demonstrating promising potential in practical applications.     

Radiochemical ageing of an amine cured epoxy network. Part I: change of physical properties

An aromatic rich, amine cured epoxy network (initial glass transition temperature 250 °C), was irradiated in air (pressure 0.22 MPa), at 30 and 120 °C, by gamma rays with two dose rates 2 and 20 kGy/h, for doses upto 70 MGy. The following characteristics were recorded, thickness of oxidised layer (TOL) from IR microspectrophotometry, flexural strength σ_R, toughness K_IC and glass transition temperature T_g. σ_R decreases from 120 MPa to about 40 MPa in the most degraded samples. This decrease is sharply linked to TOL showing the key role of the oxidised layer in crack initiation. K_IC decreases from 0.7 to 0.55 MPa m^1/2. Data are too much scattered to allow a kinetic study but it appears that, in the early period of exposure, K_IC decreases more rapidly at 120 °C than at 30 °C. T_g decreases from 250 to 140 °C in the most degraded samples, and the decrease is faster at 30 °C than at 120 °C. The decrease of T_g is attributed to a predominant chain scission process. The decrease of K_IC can be attributed to a combination of chain scission and physical ageing or chain scission and crosslinking. A relationship between T_g and the number of chain scissions, derived from the Di Marzio's theory, is proposed.     

Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (T_cs) by quenching from different T_maxs (maximum temperature above T_m for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from T_max lower than 110 °C leads to higher fractions of α-crystal, but crystallization from T_max higher than 140 °C leads to higher fractions of β-crystal. In addition to T_max, polymorphism in PHepT is also influenced by crystallization temperature (T_c = 25-75 °C). When PHepT is melt-crystallized from a high T_max = 150 °C (completely isotropic melt), it shows solely β crystal for higher T_c, and solely the α-crystal for T_c < 25 °C; in-between T_c = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower T_max = 110 °C, it shows α-crystal only at all T_cs, high or low.     

Immobilization of polyvinylacetate macromolecules on hydroxyapatite nanoparticles

Concentration dependence of the storage modulus, E′, was investigated for polyvinylacetate (PVAc) filled with hydroxyapatite (HAP) nanoparticles. The filler volume fraction, v_f, varied from 0 to 0.05 and the E′ and loss tangent, tan δ, were measured below neat matrix T_g at −40 °C and above neat matrix T_g at +50 °C at 1 Hz. The T_g determined as the position of the maximum on the temperature dependence of tan δ increased by 14 °C compared to the neat PVAc (39 °C) by adding 5 vol.% of HAP. At −40 °C, the observed small increase of E′ with v_f was in agreement with the prediction based on the simple Kerner equation. At +50 °C, the increase of E′ with v_f observed was an order of magnitude greater than that predicted using the simple continuum mechanics model. An attempt was made to explain the observed deviation employing the hypothesis of immobilized entanglements.     

Determination of reactivity ratios and swelling characteristics of ‘stimuli’ responsive copolymers of N-acryloyl-N′-ethyl piperazine and MMA

‘Stimuli’ responsive copolymers of N-acryloyl-N′-ethyl piperazine (AcrNEP) and methyl methacrylate (MMA) were synthesized by free radical solution polymerization. The copolymers were analyzed as thin films by FTIR spectroscopy. The monomer reactivity ratios were determined by linearization methods of Fineman-Ross (F-R) and Kelen-Tüdös (K-T) giving the results r₁ (AcrNEP)=0.58 and r₂ (MMA)=0.91 by the F-R method and r₁=0.72 and r₂=1.08 by the K-T method. The latter r values in turn yielded Q=0.59 and e=−0.12 for AcrNEP. Crosslinked copolymer hydrogels of AcrNEP and MMA with various compositions were prepared in bulk and solution by photo-initiated free-radical polymerization. The gels were dual responsive to pH and temperature. The response to pH was reversible with a response time of 100 min with good reversibility and with no loss in swelling capacity. Water sorption of the gels was investigated gravimetrically and the collective diffusion coefficients were determined at 10, 25, and 50 °C. The water sorption of the gels in water was Fickian. The temperature dependence of the equilibrium water content was studied by the Gibbs-Helmholtz equation. The enthalpy of mixing decreased with an increase in the hydrophilic content (AcrNEP) of the gel. Other parameters such as type and amount of crosslinker, preparative conditions, nature of buffers, and salts were found to influence the swelling behavior.     

The structure of a cyanobiphenyl side chain liquid crystalline poly(silylenemethylene)

The structure of a side chain liquid crystalline poly(silylenemethylene) (-(SiCH₃R-CH₂)-: R=O(CH₂)₁₁O-Ph-Ph-CN, Ph=phenyl) (CN-11) has been studied by X-ray diffraction and differential scanning calorimetry (DSC). The DSC results showed that CN-11 has transitions at ∼92 °C (T₂) and ∼147 °C (T₁) during both cooling and immediate heating. A third transition occurred at ∼50 °C (T₃) during heating after annealing at room temperature. The X-ray fiber pattern of the CN-11 annealed at room temperature showed several wide and small angle reflections which were indexed by a monoclinic unit cell with parameters a=16.8 Å, b=7.42 Å, c=43.6 Å and β=102.1° (b: fiber direction), representing a crystal structure with layer thickness of ∼43 Å. Upon heating at T₃, the crystal structure became less ordered (but somewhat more ordered than smectic A (S_A) and smectic C (S_C)). This was followed by S_A (or S_C) phase at T₂, and ultimately an isotropic state (I) at T₁. The observed layer thickness (∼43 Å) is about ∼1.5 times the most extended side chain length, indicating a double-layer structure with tilted or interdigitated side chains. The X-ray fiber pattern had a four-point pattern at d=4.52 Å, suggesting that the side chains in the crystal are likely to be tilted by 56° from the polymer fiber axis.