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.     

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.     

Experimental evidence for trapped chain entanglements: their influence on macroscopic behaviour of networks

The swelling and elastic properties of poly(dimethyl siloxane) networks prepared by end-linking and subsequently swollen in heptane and toluene have been investigated as a function of the volume fraction, v_c, at which networks are generated. Increases in both swelling degree and shear modulus with v_c demonstrate the increase in number of trapped entanglements with v_c. The results may be described by a simple scaling law approach.     

Floating Ricobendazole Delivery Systems: A 3D Printing Method by Co-Extrusion of Sodium Alginate and Calcium Chloride

At present, the use of benzimidazole drugs in veterinary medicine is strongly limited by both pharmacokinetics and formulative issues. In this research, the possibility of applying an innovative semi-solid extrusion 3D printing process in a co-axial configuration was speculated, with the aim of producing a new gastro-retentive dosage form loaded with ricobendazole. To obtain the drug delivery system (DDS), the ionotropic gelation of alginate in combination with a divalent cation during the extrusion was exploited. Two feeds were optimized in accordance with the printing requirements and the drug chemical properties: the crosslinking ink, i.e., a water ethanol mixture containing CaCl₂ at two different ratios 0.05 M and 0.1 M, hydroxyethyl cellulose 2% w/v, Tween 85 0.1% v/v and Ricobendazole 5% w/v; and alginate ink, i.e., a sodium alginate solution at 6% w/v. The characterization of the dried DDS obtained from the extrusion of gels containing different amounts of calcium chloride showed a limited effect on the ink extrudability of the crosslinking agent, which on the contrary strongly influenced the final properties of the DDS, with a difference in the polymeric matrix toughness and resulting effects on floating time and drug release.     

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.     

Analysis of crystal habits bounded by asymmetrically curved faces: Polyethylene oligomers and poly(vinylidene fluoride)

Curved lateral faces of lamellar polymer crystals have previously been described mathematically using a model of initiation and spreading of molecular steps on the growth surface. Previously it has been assumed that the steps spread with equal velocity v in both directions, and hence the model only applied to faces that have a two-fold axis or are bisected by a mirror plane normal to the lamella. Many lateral faces in polymer crystals do not have such symmetry. We recently solved the growth equation and reconstructed the growth profile for the case where the velocities in the left and right directions (v_l and v_r) are different, using a square lattice model. Here we adapt the model to oblique lattices suitable for {110} growth faces of polyethylene oligomers (ultralong alkanes) and PVDF. Very good fits are obtained with the observed unusual habits of crystals with curved {110} faces. It is shown that the shape of an asymmetric lateral crystal face is defined by two kinetic parameters, v_r/v_l and il₀²/(v_r + v_l), where i is the nucleation rate and l₀ the interchain distance in the growth plane. The value of v_r/v_l is found to be furthest from 1 at the smallest supercooling, which is consistent with the general principles of crystallization kinetics. The observed ten-fold difference between right- and left-moving steps on α-crystals of PVDF is attributed to the difference between edge-on and flat-on chain attachment at these steps, respectively. The high value of il₀²/(v_r + v_l), the cause of high curvature, gives a picture of a crystallization process that is only marginally nucleation controlled. Specific combinations of the above two parameters are predicted to produce exotic single crystals with re-entrant corners, so far unobserved.     

Molar volumes of multicharged electrolytes in water and aqueous sulphuric acid

The apparent molar volumes, φ_v, of aluminium ammonium sulphate and potassium aluminium sulphate in water and aqueous sulphuric acid (0.01, 0.05, 0.10 and 0.15 N) at different concentrations and temperatures have been estimated from the densities of the solutions measured by hydrostatic balance. The φ_v values vary linearly with square root of concentrations. The limiting apparent molar volume, φ⁰_v, and the experimental slope S_v, have been interpreted in terms of solute-solvent and solute-solute interactions, respectively. The φ⁰_v values vary with temperature and can be represented in the power series of temperature. Structure making/breaking capacity of the electrolyte is inferred from the sign of [?φ⁰_v/?T²] values. Both the electrolytes behave as structure breakers in water but structure makers in aqueous sulphuric acid. The molar expansibility, at infinite dilution have been found to be the same for both electrolytes at 290.5 K.     

Effect of swelling on spatial inhomogeneity in poly(acrylamide) gels formed at various monomer concentrations

The effect of swelling on the spatial inhomogeneity in poly(acrylamide) (PAAm) gels has been investigated with the static light scattering measurements. Four sets of gels were prepared using N,N′-methylenebis(acrylamide) (BAAm) as a crosslinker at various initial monomer concentrations. The crosslinker ratio X (the mole ratio of BAAm to the monomer acrylamide) was fixed at 1/50, 1/61.5, 1/66, and 1/100 in each set of gels. The gels, both at the state of preparation and at the equilibrium swollen state in water, exhibit a maximum degree of spatial gel inhomogeneity at a critical monomer concentration (ν_2,cr⁰). ν_2,cr⁰ shifts toward smaller concentrations as X is decreased or, as the gel swells beyond its swelling degree after preparation. Swelling enhances the extent of spatial inhomogeneity in PAAm gels and, this enhancement mainly occurs at low crosslinker ratios. The theoretical prediction of the Panyukov-Rabin theory was found to be in qualitative agreement with the experimental findings. It was also shown that three different effects, namely crosslinker, concentration, and swelling effects determine the extent of inhomogeneities in gels formed at various monomer concentrations.     

The measurement of the solubility of electrolytes in water—miscible organic solvent mixture by using the “solventing out” process

Twenty water—MOS-electrolyte systems were studied. An analytical expression which describes the relative solubility S/S^w as a function of the MOS amount, v, was found. Expressions for predicting the solubility constants λ and v₀/1 + v₀ from precipitation constants were found. These constants are applicable to the entire range, in most cases, and applicable for most of the range in other cases.     

Fe-Zn film stripping voltammetry: Theoretical and experimental study

The voltammetric dissolution of an iron-zinc alloy film, deposited potentiostatically from −0.2 to −1.4 V, was studied. Dissolution curves of the film at different sweep rates showed only one anodic peak, whose current density (j_p) was higher than the divacancy current density, implying that Fe and Zn in the film dissolved simultaneously. Equations relating j_p to sweep rate (v) and peak potential (E_p) to v, for the dissolution process of thick alloy film, were developed on the basis of Brainina's theory of pure metal layer dissolution. These equations were applied to the dissolution of Fe-Zn alloy film. The results obtained showed that the j_p × v and E_p × v plots were linear, corroborating the equations developed based on Brainina's theory and indicating that this alloy dissolves as one metal.     

Role of network nematicity in swelling and phase equilibria of polymer networks in nematic solvents

Swelling and phase equilibria of polymer networks in a low molecular mass liquid crystal (LC) have been investigated as a function of network nematicity. LC networks with varying nematicity were prepared by copolymerization of mixtures of mesogenic monomers and non-mesomorphic (styrene) monomers with various compositions. Molar fraction of mesogenic monomer (x) in copolymer network strongly influences the swelling behavior as well as the nematic-isotropic (N-I) transitions in both dry and swollen states. The swollen networks of sufficiently high x with strong nematicity exhibit a sharp N-I transition and simultaneously undergo a discontinuous change in gel volume, i.e. volume phase transition. Meanwhile, the swollen networks of x≤0.8 with less nematicity show a broad N-I transition, and the resulting volume change proceeds continuously over a finite temperature range. When x decreases further down to less than 0.5, the nematicity of the dry networks vanishes. The nematic ordering in the swollen copolymer networks of x<0.5 occurs at the same temperature as the N-I transition temperature of the pure nematic solvent (T_NI^S), which yields the inflection in the swelling-temperature curve. A mean field theory considering network nematicity as a variable describes the effects of x on volume phase transition such as a shift of T_NI^G and a change in the magnitude of volume transition, apart from the emergence of continuous volume transition due to the broad N-I transition. The purely isotropic network of x=0 appreciably swells in the nematic solvent. The solvent inside the gel forms the nematic phase at the temperatures below which conflicts with the classical theoretical prediction that nematic ordering of LC solvent in fully swollen isotropic network never occurs.     

Propagation rate coefficient of acrylic acid: theoretical investigation of the solvent effect

The free-radical polymerization propagation rate coefficient (k_p) of acrylic acid (AA) was calculated using transition state theory and ab initio quantum chemistry in order to shed light on the very strong solvent effects observed experimentally. Calculations were performed using a gas-phase reaction simulation, and the contribution of solvent then taken into account using the Polarizable Continuum Model for two solvents—water and toluene. The frequency factor (A) for AA propagation was insensitive to both varying the level of quantum theory applied as well as introducing a solvent field into the calculations. The activation energy (E_a) decreased substantially (∼10 kJ mol⁻¹) upon introduction of the water solvent field relative to the gas phase; however, the toluene solvent field had little impact on E_a. From analyzing transition state (TS) bond lengths and angles, as well as electronic density, this variation in E_a was attributed to greater resonance stabilization of the TS in the polar solvent, as well as more substantial mixing of reactant molecular orbitals, assisting in the transfer of electrons from the alkene to the radical. This provides a partial explanation to the marked variation in aqueous-phase k_p of water-soluble monomers seen experimentally with changes in monomer and electrolyte concentrations, which alters the hydrogen bonding patterns in solution.     

Solvated structure of C60 nanowhiskers

This work characterizes the structure of C₆₀ nanowhiskers prepared by the liquid-liquid interfacial precipitation method in the C₆₀-saturated m-xylene and isopropyl alcohol system. Transmission electron microscopy and X-ray diffraction measurement show that the C₆₀ nanowhiskers had a hexagonal structure with cell dimensions a = 2.407 nm and c = 1.018 nm which is different from pristine C₆₀. The structure of the C₆₀ nanowhiskers in solution is different from that of the solvated structure reported for the C₆₀ nanotubes but similar to that reported for the C₆₀ bulk crystal solvated with m-xylene. X-ray diffraction analysis also showed a shift to fcc structure after solvent evaporation. The C₆₀ nanowhiskers prepared using toluene as solvent also showed a similar solvated structure, and a more rapid structural change into fcc upon drying was again observed.     

Diffusion regimes at nanoelectrode ensembles in different ionic liquids

The electrochemical and diffusion behaviour of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles (NEEs) in comparison with millimetre sized gold (Au-macro) and glassy carbon (GC) disk electrodes. The redox probes are neutral ferrocene (Fc), the ferrocenylmethyltrimetylammonium cation (FA⁺) and the ferrocenylmonocarboxylate anion (FcCOO⁻). The ILs are the dicyanamide, [N(CN)₂] or bis(trifluoromethylsulfonyl)amide), [N(Tf)₂] salts of the following cations: 1-butyl-3-methylimidazolium, [BMIm], 1-butyl-3-methylpyrrolidonium, [BMPy], or tris(n-hexyl)tetradecylphosphonium [P_14,666]. These ILs are characterized by different viscosities, ranging from 32 to 277 cP. The cyclic voltammetric behaviour of the redox probes is reversible and diffusion controlled at GC electrodes. Diffusion coefficients (D) calculated by the Randles-Sevcik equation scales inversely with the IL viscosity, ranging from 2 × 10⁻⁸ to 3 × 10⁻⁷ cm² s⁻¹. Ionic solutes, namely FA⁺ and FcCOO⁻, present slightly lower D values than neutral Fc. At the Au-macro the electrochemical behaviour of the redox probes is diffusion controlled in the ILs containing the [N(Tf)₂] anion, while it involves relevant adsorption processes in the [N(CN)₂] containing electrolyte. For this reason the diffusion at gold NEEs is studied only in the former ILs.The CVs of the redox probes at the NEEs are peak shaped at low scan rate (v), while they are sigmoidally shaped at high v, but with some shift between forward and backward patterns. This is indicative of the occurrence of a total overlap (TO) diffusion condition when v is low which becomes a mixed diffusion layers (MDL) regime, with only a partial overlapping of individual diffusion layers, at high v values. In the most viscous IL, namely [P_14,666] [N(Tf)₂], at v higher than 0.8 V s⁻¹, a plateau current independent on the scan rate is achieved, indicating the tendency to reach the pure radial regime in this IL. The v values at which the transition between TO and MDL is observed scales directly with D and inversely with the IL viscosity. This behaviour is interpreted on the basis of the dependence of individual diffusion layers at each nanoelectrode on redox probe/IL interaction which fits with existing theoretical models very recently developed for nanoelectrode arrays.     

A review of the effects of covapors on adsorption rate coefficients of organic vapors adsorbed onto activated carbon from flowing gases

Published breakthrough time, adsorption rate, and capacity data for components of organic vapor mixtures adsorbed from flows through fixed activated carbon beds have been analyzed. Capacities (as stoichiometric centers of constant pattern breakthrough curves) yielded stoichiometric times τ, which are useful for determining elution orders of mixture components. Where authors did not report calculated adsorption rate coefficients k_v of the Wheeler (or, more general, Reaction Kinetic) breakthrough curve equation, we calculated them from breakthrough times and τ. Ninety-five k_v (in mixture)/k_v (single vapor) ratios at similar vapor concentrations were calculated and averaged for elution order categories. For 43 first-eluting vapors the average ratio (1.07) was statistically no different (standard deviation 0.21) than unity, so that we recommend using the single-vapor k_v for such. Forty-seven second-eluting vapor ratios averaged 0.85 (standard deviation 0.24), also not significantly different from unity; however, other evidence and considerations lead us to recommend using k_v (in mixture)=0.85k_v (single vapor). Five third- and fourth-eluting vapors gave an average of 0.56 (standard deviation 0.16) for a recommended k_v (in mixture)=0.56k_v (single vapor) for such.     

Acetone absorption in UV‐irradiated polycarbonate

Isothermal acetone absorption in UV‐irradiated polycarbonate (PC) was investigated at temperatures from ?23 to +25°C. The crystalline fraction of PC increased with UV dose. The anomalous transport data were analyzed based on Harmon's model which includes contributions from Case I transport characterized by the diffusion coefficient, D, and Case II transport characterized the velocity, v. Diffusion coefficients were monitored by both mass uptake using the model (D and v), and movement of the sharp front (D_f). Plots of log of D, v, and equilibrium swelling ratio (ESR) versus 1/T exhibited similar features. That is, plots were linear and superimposable for all doses at the higher temperatures, and exhibited a transition temperature and then a linear region that varied with dose. The values of D, D_f, and v increased with increasing dose, whereas the ESR decreased with dose. The direction of Case II transport was opposite to that of Case I diffusion during which time, sorbed solvent was expelled from the samples. Results are discussed in terms of a solvent‐induced change in lattice structure at the lower temperatures and in terms of UV radiation effects on the PC matrix that enhance solvent crystallization. POLYM. ENG. SCI., 58:1174–1183, 2018. © 2017 Society of Plastics Engineers     

Synergistic gelation of solutions of isotactic polypropylene and bis-(3,4-dimethyl benzylidene) sorbitol and its use in gel-processing

We report the synergistic, rapid gelation of solutions of isotactic polypropylene (i-PP) and the nucleating agent 1,3:2,4-bis-(3,4-dimethyl benzylidene) sorbitol (DMDBS) in decalin. Cooling to room temperature of a solution comprising, for instance, 3.0 wt% of a moderately high molecular weight i-PP (M_v=1.3×10⁶ g/mol) and as little as 0.0075 wt% DMDBS (0.25 wt% based on the polymer) resulted in the fast formation of highly ductile gels. In reference experiments without DMDBS, often ‘mud-cracked’, brittle polymer films were obtained, and decaline solutions of DMDBS alone at the above concentration did not form macroscopically coherent gels. In the present work we employed this useful occurrence for gel-processing/drawing of i-PP, yielding material with Young's moduli of 35 GPa, tensile strengths of approximately 1 GPa and melting temperatures measured for constrained samples as high as 228 °C.     

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.     

Crystallization kinetics and morphological behavior of reactively processed PBT/epoxy blends

Polybutylene terephthalate (PBT), a versatile engineering thermoplastic, has been processed using epoxy resin as a reactive solvent. Following processing of this blend, the epoxy was cured using a bi-functional amine curing agent, resulting in phase separation and phase inversion thus producing a different morphology. Change in crystallization kinetics of PBT in the presence of the epoxy monomer and cured epoxy resin has been studied using differential scanning calorimetry. Half time of crystallization (t_1/2) of PBT decreased in the presence of epoxy monomer while it remained constant in the presence of cured epoxy resin. The value of Avrami exponent varied between 1 and 2 in pure PBT as well as for uncured and cured blends, indicating mixed type of spherulitic growth. Morphology of the uncured and cured blends was studied using small angle light scattering (SALS) and polarizing microscopy for samples crystallized at different temperatures at all levels of the epoxy resin. Scattering pattern in H_v and V_v mode of SALS provided information about the type of spherulites as well as volume filling nature of the spherulites. In general, typical unusual type of spherulitic pattern for PBT, in which scattering lobes lie along the polar axis, changed to usual type of pattern for PBT/epoxy blends, in which scattering lobes lie at 45° to the polar axis.