Structural differences exhibited by networks prepared by chemical or photochemical reactions. I stress-strain properties

Stress-strain properties and swelling measurements were used to characterise networks obtained either by peroxide curing or by photocrosslinking. It was shown that the value of the average molecular weight of a network strand (M _c) is dependent on its methods of determination (m _cf from modulus, M _cv2m from swelling ratios) and that the value of the ratio M _cp/M_cv2m is a characteristic of each crosslinking process. Moreover, the results have shown that the homogeneity of distribution of the crosslinks is dependent on the crosslinking process used.     

Dependence of properties of swollen and dry polymer networks on the conditions of their formation in solution

A marked difference between the properties of networks first crosslinked and then swollen, and those synthesized in a solvent medium, has been shown. The present investigation was carried out using two different types of regular networks, polysiloxane and polyisocyanurate networks, which had different chain flexibilities, crosslinking densities, and intermolecular interactions. The concentration at which the network is formed v₀ and the quality of the solvent were varied over a wide range. It was found that with a decrease of v₀ from 1-0.05, the elastic modulus of both gels and dry networks obtained from these gels after removing the solvent decreased by a factor of 2–4 and equilibrium swelling decreased a few times.     

Swelling behavior of anionic acrylamide-based hydrogels in aqueous salt solutions: Comparison of experiment with theory

A series of hydrogels were prepared from acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) monomers with 0–80 mol % AMPS and using N,N′-methylenebis(acrylamide) as the crosslinker. The swelling capacities of hydrogels were measured in water and in aqueous NaCl solutions. The volume swelling ratio q_v of hydrogels in water increases sharply when the mole fraction f_c of AMPS increases from 0 to 0.06. At higher values of f_c from 0.06 up to 0.18, no change in the swelling capacities of hydrogels was observed; in this range of f_c, q_v becomes nearly constant at 750. However, as f_c further increases, q_v starts to increase again monotonically over the entire range of f_c. At a fixed value of f_c, the swelling ratio of hydrogels decreases with increasing salt concentration in the external solution. The results of the swelling measurements in aqueous salt solutions were compared with the predictions of the Flory–Rehner theory of swelling equilibrium. It was shown that the theory correctly predicts the swelling behavior of hydrogels up to 80 mol % charge densities. The method of estimation of the network parameters was found to be unimportant in the prediction of the experimental swelling data. The network parameters used in the simulation only correct the deficiency of the swelling theory. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 567–575, 1998     

Semi- and fully interpenetrating polymer networks based on polyurethane–polyacrylate systems. I. The polyurethane networks

The polyurethane networks based on commerical prepolymer, Adiprene L-100, and trimethylol propane (system 1) and on toluene diisocyanate, polypropylene gylcol, and trimethylol propane (system 2) were prepared and characterized in a number of ways. The materials constitute the first formed networks in a series of interpenetrating polymer networks and semi-interpenetrating polymer networks to be reported in subsequent papers in this series. System 1 networks were characterized by swelling tests which showed the M _c values to be sensitive to the amount of polyurethane present in the polymerization solvent. Stress–strain, stress–relaxation, and dynamic mechanical analyses wer also conducted. For system 2, M _c was measured, by both the swelling and the Mooney–Rivlin techniques, for materials in which the diol-to-triol ratios had been altered. the latter showed C₁ increasing as M _c decreased while C₂ was small and changed onlyy slightly indicating approximately ideal behavior. These M _c values were about 13 % larger than predicted by swelling.     

Phase transition in swollen gels: 7. Effect of charge concentration on the temperature collapse of poly(N,N-diethylacrylamide) networks in water

The swelling and mechanical behaviour of gels of the copolymer of diethylacrylamide (DEAAm) with a small quantity of sodium methacrylate (mole fraction x_MNa = 0–0.067) swollen in water was investigated in the temperature range 1–80°C. For networks in the range x_MNa > 0.0095 phase transition was observed; both the critical transition temperature T_c,c and the extent of the collapse Δ_c increase with increasing x_MNa. The formation of associations in the collapsed state contributes to the overall extent of the transition; these structures give rise to stable turbid gels at elevated temperatures. Evidence for the formation of ‘associated’ structures is also supported by the observed independence of cloud temperature of the concentration of DEAAm, c, in the polymerization DEAAm-water mixture without the crosslinking agent in the range c = 0.5–80 vol%. While PDEAAm solutions are formed in the range c < 4.5–6 vol%, physical gels arise at higher concentrations.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—III. The stoichiometric number

The stoichiometric number of the hydrogen electrode reaction on Rh, Ni, Pt, Au, Ag, and Ir was determined by means of deuterium tracer at the reversible potential. It is shown that, along with the change of the rate-determining step in the reaction route, H₂ ? 2H(a) followed by H(a) + B ? H⁺B + e, where B = H₂O or OH^?, the stoichiometric number v? experimentally observed should change by the relation, v? = (m + 2)/(m + 1), where m = v_d/2v_c and v_c and v_d are the exchange rates of these steps. The experimental v?vs m relation observed on these metals qualitatively satisfied this relation. Disagreement which existed were quantitatively explained by taking the isotope effect into account. The isotope effect of the individual steps, which were needed to interpret the disagreements, were found to be of reasonable magnitude.     

Rubber elasticity of poly(n-butyl acrylate) networks formed with multifunctional crosslinkers

The rubber elasticity characteristics of poly(n-butyl acrylate) networks crosslinked with tetrafunctional (EGDM and TEGDM), hexafunctional (TMPTM), and octafunctional (PETMA) vinyl crosslinkers were investigated. Both gel—sol analyses and crosslinking efficiency theories were used to evaluate the chemical crosslink contribution v_c and the entanglement contribution v_p to the elastically effective network chains v_e, and the effect of the crosslink junction functionality f on the front factor g. The front factors obtained were in the range of 0.50–0.92, depending upon the network system and the counting method for v_c. The relationship of g = [(f ? 2)/f]/〈r²〉/〈r²〉_o seems reasonable in the case of the tetrafunctional and hexafunctional networks, but deviates in the case of the octafunctional network. It is also evident that the functionality scheme for the front factor could only be valid under the postulate of a high v_p, which increases with increasing v_c, especially in the high v_c region near the Gaussian limit. The average energetic contribution to the retractive force of the present systems, expressed as F_e/F, is ?0.30 ± 0.1.     

Preparation and characterization of pH-sensitive,interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid)

Hydrogels with varying cross-linking ratio and ionic content were prepared from interpenetrating networks of poly(vinyl alcohol) and poly(acrylic acid). Equilibrium swelling studies were conducted and the average molecular weight between cross-links, M _c, and mesh size were determined. Hydrogels with large M _c, values were found to swell to a greater extent than those with small M _c values. It was also observed that an increase in M _c yielded faster swelling and deswelling rates, as the rates for membranes with M _c = 18,000 were about twice as fast as were the rates for membranes with M _c = 34,000. Oscillatory swelling behavior was investigated in response to changes in the pH and ionic strength of the swelling medium. A change in pH from 3 to 6 caused an ionization of the hydrogels and an increase in the weight swelling ratio, with a greater increase exhibited by IPNs with a higher ionic content. Increase in pH also caused an increase in the average mesh size. © 1995 John Wiley & Sons, Inc.     

Mechanical and swelling behaviour of well characterized polybutadiene networks

Endlinking of hydroxyl-terminated polybutadiene with the appropriate isocyanate has been used to prepare well characterized networks. Two networks have been studied with molecular weights of the prepolymers being 6100 and 2400 g/mole by g.p.c. Cylindrical specimens were prepared and the derivatives of the stored energy function with respect to the stretch invariants were determined by torque and normal force measurements in torsion. From these data the Valanis-Landel stored energy function derivatives w′(λ) were determined for both networks. The stored energy function for the junction constraint model of Flory, which is a special form of the Valanis-Landel function, has been fitted to that determined from the experiments. The contributions, ΔA_ph and ΔA_c, to the stored energy function from the phantom network and from the junction constraints respectively do not agree with predictions from the topologies of the networks. In spite of this the form of w′(λ) for the junction constraint model gives an excellent ‘curve fit’ to the data. Comparison is also made with equilibrium swelling.     

Characterization of crosslinking of random polymer network by rheological and equilibrium swelling data

The transesterification reaction in the molten state of ester groups of ethylene vinyl acetate (EVA) copolymers and ethylene acrylic ester (EMA) copolymers has been used to crosslink the chains of this polymeric system. The relative EVA copolymers (or EMA copolymers) concentration dependence of the network formation by co-crosslinking of EVA/EMA miscible blends has been assessed. EVA/EMA networks were characterized by swelling experiments, rheological measurements, and determination of the extent of the reaction through a chromatographic technique. All results can be compared in a master curve. The influence of the polydispersity and the microstructure of EVA and EMA samples was put in evidence. On the other hand, a scaling law (v2～M_c^?3/5) was observed in agreement with predictions of the Flory-Rhener expression. © 1994 John Wiley & Sons, Inc.     

Fast transient fluorescence method for measuring swelling and drying activation energies of a polystyrene gel

The time-resolved, fast transient fluorescence (TRF) technique which uses a strobe master system (SMS), was employed for studying swelling and drying of disc-shaped polystyrene gels. Disc-shaped gels were prepared by free-radical crosslinking copolymerization of styrene and ethylene glycol dimethacrylate. Pyrene was introduced as a fluorescence probe during polymerization and lifetimes, τ of pyrene were measured during in situ swelling and drying processes. Chloroform was used as an organic vapor agent to induce gel swelling at various temperatures. It was observed that τ values decreased as swelling is proceeded. Li-Tanaka equation was used to determine the swelling time constant, τ_c and cooperative diffusion coefficients, D_c for the swelling processes. It is observed that lifetimes, τ of pyrene increased during drying process and an empirical equation was introduced to determine the desorption coefficient, D for drying at various temperatures. The activation energies, ΔE were measured for the swelling and drying processes and found to be 80.0 and 33.5 kJ mol⁻¹, respectively.     

Effect of electrolytes in aqueous solutions on oxygen transfer in gas–liquid bubble columns

The effects of inorganic electrolytes (NaCl, MgCl₂, CaCl₂) in aqueous solutions on oxygen transfer in a bubble column were studied. Electrolyte concentrations (c) below and above the critical concentrations for bubble coalescence (c_tc), and six superficial gas velocities (v_sg), were evaluated. The volumetric mass transfer (k_La) and the mass transfer (k_L) coefficients were experimentally determined. It was found that the concentration of electrolytes reduced the k_L, but the interfacial area (a) increased enough to result in a net increase of k_La. Using as independent variable a normalizing variable (c_r = c/c_tc), and maintaining fixed v_sg, similar values of k_La were observed regardless the kind of electrolyte; the same happened for k_L. This suggests that c_r quantifies the structural effects that these solutes exert on mass transfer. Also, once c_r = 1 was reached, no significant variations were found in k_La and k_L for constant v_sg. It is concluded that the gradual inhibition of bubble coalescence (c_r < 1) governs the significant changes in hydrodynamics and mass transfer via the reduction of bubble size and the consequent increment of a and gas holdup (?_g). Finally, regarding the effects of v_sg on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed.     

The effects of the synthesis parameters on the xerogels structures and on the swelling parameters of the poly(methacrylic acid) hydrogels

Poly(methacrylic acid) hydrogels were synthesized. The effects of the synthesis parameters: the neutralization degree of methacrylic acid and the concentrations of monomer, crosslinker and initiator on the xerogels structural properties: the xerogel density (ρ_xg), the number average molar mass between the network crosslinks (M_c), the crosslink degree (ρ_c), the number of elastically effective chains totally induced in a perfect network per unit volume (V_e), the distance between the macromolecular chains (ξ) and the equilibrium swelling degree (SD_eq) and the swelling kinetics was investigated. As the concentrations of crosslinker, monomer and initiator increase, the value of ρ_xg, ρ_c and V_e increases and decreases the value of M_c, ξ and SD_eq. With the increase in the neutralization degree of methacrylic acid, the values of ρ_xg, M_c, ξ, SD_eq increase, while the ρ_c and V_e decrease. The xerogels structural properties, SD_eq and swelling kinetic parameters are mainly in power law form functional relationships with the synthesis parameters as well as with the xerogels crosslinking degree.     

Characterization of the network structure of hydroxyl terminated poly(butadiene) elastomers prepared by different reactive systems

Three types of hydroxyl terminated poly(butadiene) (HTPB) networks having different NCO/OH reactive group ratios were prepared using three different reactive systems (i.e., Desmodur N-100 pluriisocyanate, isophorone diisocyanate (IPDI)/trimethylol propane (TMP), and hexamethylene diisocyanate (HMDI)/TMP). Desmodur N-100 and TMP were used as polyfunctional reactants. The polymer–solvent interaction parameters (χ₁) for HTPB–tetrahydrofuran (THF) and HTPB–chloroform systems at 45°C were determined, using a vapor pressure osmometer (VPO), as 0.31 and 0.24, respectively. Equilibrium swelling values of the networks in THF and chloroform and the data obtained by osmometry were further used for the calculation of the mean number-average molecular weight between junction points (M _c) of HTPB networks through the Flory-Rehner equation. The M _c values of each network, which were calculated from the swelling data obtained in THF and chloroform using the corresponding χ₁ parameters for polymer–solvent systems, were found to be nearly the same. The changes in the mechanical, swelling, and the solubility properties of the networks prepared by three different reactive systems were followed. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1129–1135, 1998     

CFD Modeling of Active Volume Creation in a Non‐Newtonian Fluid Agitated by Submerged Recirculating Jets

Computational fluid dynamics (CFD) models were employed to investigate flow conditions inside a model reactor in which yield stress non‐Newtonian liquid is mobilized using submerged recirculating jets. The simulation results agree well with the experimental results of active volume in the reactor obtained using flow visualization by the authors in a previous study. The models developed are capable of predicting a critical jet velocity (v_c) that determines the extent of active volume obtained due to jet mixing. The v_c values are influenced both by the rheological properties of the liquid and the nozzle orientation. The liquid with higher effective viscosity leads to higher v_c for a downward facing injection nozzle. However, an upward facing injection nozzle along with a downward facing suction nozzle generates enhanced complementary flow fields which overcome the rheological constraints of the liquid and lead to lower v_c.     

Effect of chain length distribution on thermal characteristics of model polytetrahydrofuran (PTHF) networks

A series of model polytetrahydrofuran (PTHF) networks were synthesized via end-linking reactions of α, ω-allyl PTHF oligomers with a stoichiometric tetrafunctional crosslinker. The telechelic PTHF oligomers were synthesized by living cationic ring-opening polymerization of tetrahydrofuran followed by a termination reaction with allyl alcohol. Networks thus prepared have well-controlled architecture in terms of the inter-crosslink chain length (M_c) and chain length distribution: resulting in unimodal, bimodal and clustered structures. Unimodal network was prepared by using polymer chains of same molecular weight, bimodal networks were synthesized by using two groups of polymer chains with different average molecular weights, and the clusters are prepared by incorporating clusters of networks with small molecular weight chains in a network matrix made of longer chains. Thermal characteristics of these model networks were investigated as a function of crosslink density, as well as inhomogeneities of crosslink distribution using DSC. We demonstrate that glass transition temperature (T_g) and crystallization behavior (melting temperature and crystallinity) of the networks are both strongly influenced by crosslink density (M_c). By comparing the unimodal, bimodal and clustered networks with similar average M_c, the effects of inhomogeneities in the crosslink distribution on the thermal properties were also investigated. Results show that inhomogeneities have trivial influence on T_g, but strongly affects the crystallization behavior. Moreover, the effects of the content ratio and length ratio between long and short chains, and the effects of cluster size and size distribution on the thermal characteristics were also studied.     

Influence of poly(acrylic acid) xerogel structure on swelling kinetics in distilled water

Summary The basic structural properties of xerogels of crosslinked poly(acrylic acid) were defined and determined: xerogel density (ρ_xg), xerogel volume fraction in the equilibrium-swollen state (v₂), the number average molar mass between network crosslinks , the crosslink density (ρ_c) and the distance between macromolecular chains (d). A crosslinking ratio (X) increase leads to a linear increase in the values for ρ_xg and ρ_c, while the values and d decrease. The isothermal swelling kinetic curves of four samples of structurally different poly(acrylic acid) xerogels in bidistilled water at different temperatures ranging from 25 to 45 °C were determined. It is shown that isothermal kinetic swelling curves could not be described with the model of first-order reaction kinetics in entire. It was found that these curves could be described by the Johanson-Mampel-Avrami (JMA) equation. For all of the investigated xerogel samples, the initial swelling rate (v_in), effective reaction rate constant (k) and equilibrium swelling degree increased with swelling temperature increase. Based on the determined values of the v_in and k, the activation energy (E_a) and pre-exponential factor (lnA) were determined. It was concluded that the activation energy linearly increased with increasing distance between macromolecular chains (d) and molar mass between the network crosslinks . The relationship between the activation energy changes with pre-exponential factor (compensation effect) caused by xerogel structural properties was established. Isothermal swelling kinetics could be completely described by the kinetics of phase transition of the xerogel transformation from glassy to rubbery state, i.e. with the JMA kinetic equation.     

Swelling behavior of polystyrene networks in some aliphatic esters and linear polystyrene solutions

Summary Polystyrene networks were synthesized by free radical polymerization of styrene in the presence of dibenzoyl peroxide as an initiator, divinylbenzene as a crosslinker at 50 °C in toluene under nitrogen atmosphere. The swelling behavior of the synthesized polystyrene networks were studied at different temperatures in methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, tert-butyl acetate, and isopentyl acetate. Equations were given for the variation of the polymer-solvent interaction parameter, χ₁₂, as a function of reciprocal absolute temperature in the temperature range from 25 to 75 °C. Polystyrene networks were swollen in linear polystyrene solutions in toluene of different concentrations. Linear polystyrene samples were almost monodisperse and of high molecular weight. The equilibrium swelling degrees were evaluated using the Flory-Rehner theory. The relation of volume swelling ratio, q_v, to the volume fraction of the linear polymer in solution, φ, was evaluated and characterized by interaction parameter of crosslinked polystyrene with linear polystyrene, χ₂₃. The parameter χ₂₃ was found around zero as expected from mixtures of similar chemical structures by extrapolating the values of χ₂₃ at tested concentrations to zero solvent concentration.     

Swelling and mechanical behavior of charged poly(<Emphasis Type="Italic">N</Emphasis>-isopropylmethacrylamide) and poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) networks in water/ethanol mixtures. Cononsolvency effect

Summary Swelling and mechanical behavior of negatively (ionic comonomer-sodium methacrylate) and positively (ionic comonomer-(2-acrylamidoethyl)trimethyl ammonium chloride) charged networks of poly(N-isopropylmethacrylamide) (PIPMAm) and poly(N-isopropylacrylamide) (PIPAAm) was investigated in water/ethanol mixtures at room temperature. Strong cononsolvency effect was observed for uncharged and negatively charged gels of both systems; while for neat solvents high degree of swelling Q was observed, for solvent mixtures pronounced minima in swelling were found. Swelling minima are connected with the coil-to-globule transition of network chains and their characteristic parameters-concentration of ethanol at the minimum, c_eth,m, and corresponding swelling degree, Q_m, were determined. With increasing amount of negatively charged ionic comonomer the minimum in swelling shifts to higher c_eth,m values and Q_m increase. On the other hand, positively charged networks of both systems exhibit roughly constant Q values practically independent of solvent mixtures compositions. For all four systems it was found that mechanical behavior is predominantly determined by the degree of swelling regardless of charge concentration.