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.     

Dynamic mechanical properties of quinazolone–imide block copolymer

Dynamic mechanical properties have been investigated over the temperature range of?150–360°C for the quinazolone–imide copolymers, prepared by condensation of the amine-terminated quinazolone prepolymer with a stoichiometric quantity of pyromellitic dianhydride or 3,3′,4,4′-benzophenone-tetracarboxilic dianhydride. Both copolymers have, respectively, the low-temperature β-relaxation and the β^*-relaxation, as well as the case of poly(4,4′-oxydiphenylene–pyromellite-imide) (Kapton) examined for comparison. These relaxations seem to contribute to toughness of the copolymers. The α-relaxations for both copolymers occurred at much the same temperature of 320°C, which can be assigned to a large scale segmental motion of the quinazolone chain sequence. The α-peak temperatures shifted into higher temperatures by heat aging. This can be explained in terms of crosslinking in the copolymers, supported by swelling test in hot m-cresol and IR spectroscopy.     

Relationship between the mechanical relaxations in the α zone and the calorimetric transitions in low density polyethylene

The α-relaxation spectra of LDPE, irradiated and unirradiated, has been compared with their calorimetric thermograms. There exists a correlation between the two mechanical relaxations that appear in the α zone and the two fusion peaks observed in the calorimetric thermograms, confirming that the parameter that governs the temperature of the relaxations α_I and α_II is the most probable crystallite thickness, whereas the height of the α-relaxation zone, as measured by means of tan δ_max, depends on the total crystalline content of the sample.     

Free volume in poly(n-alkyl methacrylate)s from positron lifetime and PVT experiments and its relation to the structural relaxation

Free volume data from positron annihilation lifetime spectroscopy (PALS) experiments are combined with a Simha-Somcynsky (S-S) equation of state analysis of pressure-volume-temperature (PVT) data to model free volume contributions to structural mobility in a series of poly(n-alkyl methacrylate)s. From the PALS data the glass transition temperature, T_g, decreases (from 382 to 224 ± 5 K) and a given mean free volume is observed at lower temperatures as the side-chain length increases (going from methyl- to hexyl-). This is evidence of an internal plasticization whereby the side-chains reduce effective packing of molecules. By comparing PALS and PVT data, the hole number per mass unit, N_h′, is calculated using different methods; this varies between 0.54 and 0.86 × 10²¹ g⁻¹. It is found that the extrapolated free volume becomes zero at a temperature T₀′ that is smaller than the Vogel temperature T₀ of the α-relaxation. The α-relaxation frequencies can be fitted by the free volume theory of Cohen and Turnbull, but only when the free volume V_f is replaced by (V_f − ΔV) where ΔV( = E_f(T₀ − T₀′), E_f is the thermal expansivity of V_f) varies between 0.060 and 0.027 ± 0.003 cm³/g, decreasing with side-chain length, apart from poly(n-hexyl methacrylate) where ΔV increases to 0.043 ± 0.003 cm³/g. One possible interpretation of this is that the α-relaxation only occurs when, due to statistical reasons, a group of m or more unoccupied S-S cells are located adjacent to one another. m is found to vary between 8 and 2 for poly(methyl methacrylate) and poly(n-butyl methacrylate), respectively. We found that no specific feature in the free volume expansion was consistently in coincidence with the dynamic crossover.     

Glass transition behaviour of poly(ether ether ketone)/poly(aryl ether sulphone) blends: dynamic mechanical and dielectric relaxation studies

Blends of poly(ether ether ketone) (PEEK) and poly(aryl ether sulphone) (PES) have been prepared in the whole composition range. The molecular dynamics and α-relaxation behaviour of these materials have been studied using dynamic mechanical and dielectric relaxation spectroscopy. From dynamic mechanical relaxation studies, two α-relaxation peaks corresponding to the segmental relaxation process of pure components in the blend was observed. Also, it was found that the temperature at which α-process of the homopolymers occurs, shows a slight change with blend composition, corresponding to a PEEK-rich and PES-rich phase. The relaxation intensities of the homopolymers in the blend compared to that in pure state were approximately proportional to their respective content in the blend. From the phase composition of the respective phases obtained using Fox equation, it has been inferred that PEEK dissolves more in PES than vice-versa. The α-relaxation of PES could not be detected from dielectric relaxation spectroscopy because of the possible influence of dc conduction and electrode polarization losses. Otherwise, the α-relaxation behaviour of PEEK-rich phase observed from dielectric relaxation studies agree with those inferred from dynamic mechanical relaxation studies. Furthermore, activation energies for molecular motions (E_a) at the α-relaxation have also been determined using an Arrhenius form of equation and it has been found that E_a for both PEEK-rich and PES-rich phase show variation with composition. Similarly, the relaxation times associated with the mobility of relaxing species in both PEEK and PES are influenced in the blends. It is likely that these observations are related to some interactions and a partial segmental mixing between the blend components, which result in changes in the local molecular environment on blending.     

The effect of flexible chain length on thermal and mechanical properties of poly(m-methylene 2,6-naphthalate)s

The effect of flexible chain length on the thermal and mechanical properties such as melting temperature, glass transition temperature, dynamic mechanical relaxation behavior, and uniaxial tensile deformation for melt-quenched poly(m-methylene 2,6-naphthalate) (PmN) films was investigated using differential scanning calorimeter (DSC), dynamic mechanical thermal analyzer, and universal tensile machine. It was found from DSC thermograms that PmNs with even number of methylene group have higher melting temperatures and faster crystallization rates than PmNs with odd number of methylene group, showing an odd-even fluctuation. The plots of versus temperature show that all PmN samples have three relaxation processes (β, β^∗, and α) regardless of the number of methylene group in their backbone. It was found that both β^∗- and α-relaxations are cooperative processes and that the activation energies of both relaxations as well as the glass transition temperature associated with the α-relaxation show odd-even fluctuations as a function of the number of methylene group. The initial tensile modulus at the low drawing rate of 0.15 cm/min also shows an odd-even fluctuation. In summary, the macroscopic thermal and mechanical properties of PmN such as melting temperature, glass transition temperature, crystallization rate, activation energies of α- and β^∗-relaxations, and initial modulus measured under a slow drawing rate exhibit odd-even fluctuations as the number of methylene group in PmN increases.     

Polyurethane-POSS hybrids: Molecular dynamics studies

A series of hybrid polyurethane-POSS materials have been synthesized on the basis of poly(tetramethylene glycol) (Terathane 1400^®) as soft component, 4,4′-methylenebis(phenylisocyanate) (MDI) as hard component, and 1,4-butanediol as chain extender. POSS particles properly modified have been tethered on the main chain by substitution of the chain extender to weight fractions up to 10%. AFM measurements indicate the formation of POSS crystallites in the PU matrix, extended structures at low POSS content and more regular, smaller structures at higher POSS content. A detailed investigation of molecular mobility by means of Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Currents (TSDC) and, mainly, Broadband Dielectric Relaxation Spectroscopy (DRS) has been conducted in all samples of the series and in addition in neat Terathane, as reference. Four relaxations have been studied in detail: two secondary relaxations γ and β, the segmental α relaxation (dynamic glass transition) and an α′ relaxation slower than α associated with crystallinity in neat Terathane and with the presence of hard microdomains in the polyurethane and the hybrids. Secondary relaxations remain unaffected by POSS. The glass transition temperature rises by a few degrees and, in consistency with that, segmental dynamics slightly slows down with increasing POSS content. In addition, the dielectric strength of the segmental relaxation decreases with increasing POSS content, suggesting that a fraction of polymer is immobilized, making no contribution to the relaxation. These results are discussed in relation to morphology.     

Copoly(1,3,4-oxadiazole-ether)s containing phthalide groups and thin films made therefrom

A series of aromatic copolyethers containing 1,3,4-oxadiazole rings and phthalide groups was prepared by nucleophilic substitution polymerization technique of phenolphthalein, 1, or of an equimolecular amount of 1 and different bisphenols 2, such as: 4,4′-isopropylidenediphenol, 4,4′-(hexafluoroisopropylidene)diphenol, 4,4′-(1,4-phenylene-diisopropylidene)bisphenol, 4,4′-cyclohexylidene-bisphenol and 2,7-dihydroxynaphthalene, with 2,5-bis(p-fluorophenyl)-1,3,4-oxadiazole, 3. The polymers were easily soluble in polar solvents such as N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide and chloroform and can be cast from solutions into thin flexible films. They showed high thermal stability, with decomposition temperature being above 400 °C. The polymers exhibited a glass transition temperature in the range of 220-271 °C, with reasonable interval between glass transition and decomposition temperature. Electrical insulating properties of some polymer films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. The values of the dielectric constant at 10 kHz and 20 °C were in the range of 2.98-3.15.     

Synthesis and characterization of organo-soluble thioether-bridged polyphenylquinoxalines with ultra-high refractive indices and low birefringences

Two aromatic tetraketones, 4,4′-thiobis[(p-phenyleneoxy)benzil] (STK, 1) and 4,4′-thiobis[(p-phenylenesulfanyl)benzil] (3STK, 2) were synthesized by the nitro nucleophilic substituent reactions of 4-nitrobenzil and corresponding diol compounds. The two tetraketones were polymerized with three aromatic tetraamines, including 3,3′-diaminobenzidine (a), 3,3′,4,4′-tetraaminodiphenylether (b) and 3,3′,4,4′-tetraaminodiphenylsulfone (c), respectively to afford six thioether-bridged polyphenylquinoxalines (PPQs) - PPQ-1a-1c and PPQ-2a-2c. The obtained PPQs exhibited good solubility not only in conventional m-cresol and chloroform, but in the aprotic solvent - N-methyl-2-pyrrolidinone (NMP). PPQ-1c and 2c containing sulfone units were even soluble in tetrahydrofuran at room temperature with a solid content of 15 wt%. Flexible and tough PPQ films cast from their NMP solution showed good thermal stabilities, including glass transition temperatures in the range of 215-248 °C and 5% weight loss temperatures exceeding 500 °C in nitrogen. The PPQ films at a thickness of ∼10 μm exhibited moderate optical transparency at 450 nm. The best optical transmittance around 80% was achieved by PPQ-1c and 2c containing electron-withdrawing sulfone moieties. The synergic effects of flexible thioether linkages and highly conjugated quinoxaline rings in the present PPQs endowed them with ultra-high refractive indices up to 1.7953 at 632.8 nm and birefringences close to zero.     

Structure–property relationships in thermoplastic elastomers. IV. Dynamic mechanical relaxations in polydioxolane,polybutyltrioxocane, and poly(propylene oxide) polyurethanes

Dynamic mechanical measurements had been made on polyurethanes of a range of hard segment content (HSC) obtained from α,ω-dihydroxy derivatives of polydioxolane, polybutyltrioxocane, and poly(propylene oxide) of 2000–12,000 MW coupled with MDI and TDI with hydroxyethyl phthalamide chain extenders. N-methylated chain extender was used to investigate the effect of hydrogen bonding. A broad γ transition was observed between −100 and −115°C attributable to the local motion of methylene sequences with a distribution of relaxations. The β-transition between −55 and −72°C is influenced by the degree of hydration; the α-transition occurs at nearly the same temperatures for all the TPEs, independent of HSC and thermal history. The magnitude of the loss tangent peak is nearly proportional to the soft segment content; the peak corresponds to the glass transition. An α-transition at higher temperature is sensitive to the structure of soft segment, HSC, and thermal history. It is probably associated with the melting of some weakly ordered regions in the soft domain. The hard domain counterpart was observed at the next higher temperature as the ϵ-relaxation. The final large loss of storage modulus occurs at temperatures much below the melting transition.     

Viscoelastic and dielectric studies on comb- and brush-shaped poly(n-butyl acrylate)

The dynamic compliances J′(ω) and J″(ω) and the dielectric permittivities ?′(ω) and ?″(ω) are reported over a wide range of frequency ω and temperature for comb-branched and brush-shaped poly(n-butyl acrylate) prepared by atom transfer radical polymerization. The analysis here of the viscosity η for the comb- and brush-shaped polymers indicates the need to account for an increase of the persistence length with increasing density of the side chains. Enhanced values of J_S are attributed to a dilution effect arising from the side chains on the values that would otherwise arise from the backbone chain. The dielectric loss ?″(ω) demonstrates a deviation from frequency-temperature superposition at a certain range of frequency, with the deviation increasing with increasing density of the side chains. The deviation occurs for a frequency range for which J′(ω) and J″(ω) are approaching their terminal response, but no corresponding deviation from frequency-temperature superposition is noted for these functions. The dielectric behavior in this region is attributed to a δ-relaxation at frequencies lower than the principal α-relaxation, similar to behavior reported for certain polymers with mesogenic side chains.     

Preparation and characterization of aliphatic/aromatic copolyesters based on bisphenol-A terephthalate,hexylene terephthalate and lactide mioties

To obtain higher molecular weight processable polyesters which are of potential interest for tissue engineering, a series of biodegradable aliphatic/aromatic copolyesters, poly(4,4′-isopropylidenediphenyl terephthalate)-co-poly(hexylene terephthalate)-co-polylactide (PBHTL), were synthesized via direct polycondensation from terephthaloyl dichloride, bisphenol-A, 1,6-hexanediol, and oligolactide. The resulting copolyesters, PBHTL, were characterized by proton nuclear magnetic resonance (¹H NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetry (TG) and wide-angle X-ray scattering (WAXS), and their glass transition temperature (T_g), and thermal decomposition temperature (T_d) were obtained. The PBHTL shows two-step thermal decomposition, which is corresponding to segment of 4,4′-isopropylidenediphenyl terephthalate (BAT) and segments of hexylene terephthalate (HT) and lactide (LA), respectively. Due to the incorporation of flexible aliphatic unit into the main chains of copolyesters, PBHTL has lower T_g, and shows enhanced hydrolytic degradability under a physiological conditions with increasing lactide and hexylene moieties. Initial experiments indicated excellent biocompatibility based on cell seeding experiments and microscopic evidence.     

Use of the dielectric analysis to complement previous thermoanalytical studies on the system diglycidyl ether of bisphenol A/1,2 diamine cyclohexane

The dielectric analysis technique was used to characterize an epoxy cured system consisting of a diglycidyl ether of bisphenol A (DGEBA, n=0) and 1,2 diaminecyclohexane (DCH). With this purpose, the permittivity ε′, the loss factor ε″, and the dissipation factor were determined experimentally. The transition observed within the temperature range studied was associated to a α-relaxation process, the T_g of which was taken as that corresponding to the maximum of the ε″−T curve recorded.Argand diagrams for the different temperatures were constructed, and owing to the asymmetry showed by these diagrams, the Havriliak-Negami model was used to analyze data. It was observed that, within the frequency range (0.1-300 Hz) used for this study, the relaxation time follows an Arrhenius behaviour, thus allowing calculation of the activation energy E_a and T_g in good agreement with literature values.     

Dielectric analysis of poly(methyl methacrylate) zinc(II) mono-pinacolborane diphenylporphyrin composites

Poly(methyl methacrylate) (PMMA) composites were made from a polar metalloporphyrin [5-(4′,4′,5′,5′-tetramethyl[1′,3′,2′]dioxaborolan-2′-yl)-10,20-diphenylporphyrinato]zinc(II) (Zn(II)Bpin-DPP) in select weight % (wt%). Differential Scanning Calorimetry (DSC) showed that porphyrin acted as an antiplasticizer raising the glass transition (Tg) from 105 °C to 123 °C. Dielectric Analysis (DEA) was performed in the frequency range of 0.3 Hz to 100 kHz between −150 and 270 °C. Permittivity (?′), loss factor (?″) and dielectric response of beta (β), alpha beta (αβ), and conductivity relaxations were studied. Previous DEA data was limited to 190 °C. This study brings analysis to 270 °C which is start point for the first part of PMMA degradation. Thus forwarding DEA can be used to evaluate PMMA degradation. The electric modulus formalism is used to reveal the β and conductivity relaxations. The apparent activation energies (E_a) for the molecular relaxations are presented. AC (σ_AC) and DC (σ_DC) conductivity are also evaluated.     

Dynamic-mechanical behavior of polyethylenes and ethene-/α-olefin-copolymers. Part I. α′-Relaxation

Polyethylenes and polyethylene/α-olefin-copolymers covering a range in crystallinity between 12 and 85% were investigated by means of dynamic-mechanical measurements between −145 °C and their melting point. From the temperature and frequency dependence of the complex modulus α′-, α-, β- and γ-relaxations were analyzed. The α′-relaxation was discovered in all HDPE-, LDPE- and LLDPE-samples but not in plastomer- and elastomer-samples. The activation energies (30-140 kJ/mol) of this relaxation were found to decrease with increasing crystallinity. The α′-transition temperature at a fixed frequency rises with increasing degree of crystallinity and tends to reach the melting point when approaching the fully crystalline state. Thus, it is concluded that the α′-relaxation originates from the interface between crystal lamellae and amorphous interlamellar regions. By extrapolation of the storage modulus to the amorphous state the entanglement molar mass was calculated as 2300 g/mol for a completely amorphous polyethylene/α-olefin-copolymer.     

Structure, phase transition and electric properties of poly(vinylidene fluoride-trifluoroethylene) copolymer studied with density functional theory

The geometry, energy, internal rotation, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level and compared with those of the poly(vinylidene fluoride) (PVDF) homopolymer. The chain length and the trifluoroethylene (TrFE) concentration were examined to discuss the copolymer chain stabilities, chain conformations and electric properties. The asymmetrical internal-rotation potential energy curve shows that the angles for the g and g′ conformations in the α-chain (tg and tg′) models are 53° and −70°, respectively, and the β-chain (ttt) conformation is a slightly distorted all-trans plane with dihedral angle at 177°. The energy differences, E_β − E_α(g) and E_β − E_α(g′), between the β- and the α-conformation are 2.1 and 7.8 kJ/mol, respectively. These values are smaller than that in PVDF (8.4 kJ/mol), suggesting that the β-conformation in the copolymer will be more stable than in PVDF. The energy barriers for β → α(g) and β → α(g′) transitions are 16.2 and 5.8 kJ/mol, respectively. The former is almost twice of the energy barrier in PVDF by 8.2 kJ/mol and the latter is slightly smaller (by 2.4 kJ/mol) than that in PVDF. The respective energy barriers for α(g) → β and α(g′) → β transitions are 18.3 and 13.6 kJ/mol compared with the value 16.3 kJ/mol in PVDF. The asymmetrical energy barriers may be one of the reasons for the copolymers with 0.5-0.6 (mole fraction) VDF exhibiting complicated phase transition behavior. The conformation of α-chain P(VDF-TrFE) exhibits from a helical (containing higher TrFE) to a nearly beeline (containing lower TrFE). This behavior is different from that in the PVDF and the nearly beeline conformation might be responsible for the increasing crystallizability. The helical might also be associated with the complicated phase transition behavior and the larger lattice strain in the P(VDF-TrFE)s with higher TrFE concentration. The energy difference per monomer unit between the β- and α-chain decreases with increasing TrFE content. The ideal β-chain is curved with a radius of about 30 Å, which is similar to that in PVDF. The chain curvature and the TrFE content will affect the dipole moment contribution per monomer. The chain length and TrFE content will not significantly affect the mean polarizability. The calculations indicated that there are some additional characteristic vibrational modes that may be used in identification of the α- or β-phase P(VDF-TrFE)s with different TrFE contents.     

Mechanical properties and fractography of block copolymers based on NR and MDI-based polyurethanes

Five series of block copolymers were synthesized from hydroxyl-terminated liquid natural rubber (HTNR) and polyurethane (PU) oligomers, from various diols and diphenyl methane-4,4′-diisocyanate (MDI). They were characterized by mechanical testing and fracture studies (SEM analysis). The block copolymer characteristics were assessed on the basis of the composition and the type of extender diols. Mechanical properties were found to be strongly dependent on the copolymer composition in all the series. Tensile properties were found to improve with the hard segment content. At low hard segment content samples resemble flexible elastomers whereas at high hard segment content they behave as rigid plastics. Where bisphenol A (BPA) is used as the extender diol sample rigidity was higher compared to the samples with aliphatic diols which is attributed to the presence of aromatic ring system in the former samples. Fracture mechanism was found to vary from ductile fracture to rigid and brittle fracture as the hard segment content increased. Fractography also shows the presence of some beads disposed on the sample surface which could be the uncombined polyurethane homopolymer fractions.     

Dynamic mechanical and dielectric relaxations in poly(di-n-chloroalkylitaconates)

Dielectric and viscoelastic relaxation measurements have been carried out on poly(2-chloroethyl diitaconate) (PDCEI) and poly(3-chloropropyl diitaconate) (PDCPI) between 123 K and temperatures about 293 K above the glass transition temperatures.The two polymers exhibit three peaks, a γ-relaxation in the range from 133 to 193 K (at 1 Hz), a broad β-process, in the range from 193 to 293 K and a third peak observed in mechanical measurements at 323 K (PDCEI) and 293 K (PDCPI) probably corresponding to the α dynamic glass transition phenomena. In dielectric measurements, conductive contributions overlap the α-relaxation precluding the observation of peaks at temperatures above room temperature. The apparent activation energies for the γ-relaxation according to the mechanical and dielectric measurements are close to the values derived from the empirical force field molecular mechanics calculations. A comparison is made between the relaxational data of PDCEI and PDCPI by one hand and poly(di-n-propyl itaconate) (PDPI) and poly(di-n-butyl itaconate) (PDBI) by other. This comparison allows us to conclude the relevant role played by the chlorine atoms not only in the γ relaxations but also in the β relaxations of PDCEI and PDCPI.     

Peculiarities of δ- and α-relaxations in thermotropic side chain liquid crystalline polymers with and without nematic reentrant phase

We have performed a dielectric spectroscopy study of four homologous cyanobiphenyl polyacrylates with long side chains. The α- and δ-relaxation times were found to be sensitive to the sequential transformations between mesophases. The τ^δ in the isotropic phase exhibits the characteristics that obeys VFT relation and depend strongly on spacer length. The relaxation times, τ^δ, for the crossover from short range intermolecular interactions to long range LC ordering, decreases with increasing side chain length, implying that the cooperative motions of mesogenic dipoles arrange long range order at shorter time scales, as the spacer length is increased. In the SmA mesophases of CBPAn compounds with n = 8, 9 and 11 α-relaxation times were found to be nearly temperature independent at high temperatures. Thus, segmental motions take place in the state of diminished dynamical constraints of backbones, which can be attributed to the plasticization effects of polymeric layers in the case of long methylene spacers. In CBPA6 and CBPA8, with even numbers of methylene groups, anomalies of δ-relaxation processes were observed at the nematic reentrant (N_re) transitions. The anomalies of α-relaxation processes in the SmA mesophases were found to be precursors of N_re transitions as temperature decreases. The changes in backbone conformations of the SmA layers with decreasing temperature create the conditions for molecular corrugations of the side chains leading to the formation of the nematic order of N_re phases.     

Water sorption in cross-linked poly(vinyl alcohol) networks

Poly(vinyl alcohol) (PVA) networks of different cross-linking densities were prepared by reaction with hexamethylene diisocyanate in solution and casting. The dynamic-mechanical properties of PVA films have been investigated in the temperature range of −150 to +150 °C. Two relaxations processes labeled α and β in order of decreasing temperature were observed. The α-relaxation shifts to lower temperature and the average molecular weight between cross-links decreases with increasing cross-linking density. Isothermal sorption from vapor and liquid water allowed determination of the Flory-Huggins interaction parameter between water and the polymer chain segments, which decreased with the water activity in the hydrogel and increased with the cross-linking density as a consequence of the hydrophobic character of the cross-linking agent. The water diffusion coefficients, D, in the networks obtained by means of dynamic sorption experiments increased with increasing water activity. This behavior is interpreted in terms of plasticization of the polymer by water molecules.