The colloidal behaviour of kraft lignin

Kraft lignin gels have been found to exhibit both macrosyneresis and hysteresis in swelling. The effects of temperature and prehistory on swelling and on the mechanical properties have been investigated. Thermal treatment of kraft lignin gels in the protonised state induces an irreversible deswelling of the gels. This irreversible deswelling can, however, be released by deprotonization of the carboxyl groups. Deswelling also occurs when partly dried protonised gels are placed in water again. Furthermore, the gels were found to exhibit pH-hysteresis.It is concluded that the above-mentioned effects are closely related to the state of dissociation of the carboxylic groups and to their ability to form intermolecular hydrogen bonds in the network structure.It is suggested that syneresis is due to a structural rearrangement induced by breaking and formation of hydrogen bonds and promoted by the long-range van der Waal's interaction between the colloidal units in the gel. Swelling hysteresis is assumed to be related to repeptization phenomena commonly encountered in lyophobic colloids.     

Swelling behaviour of poly(butadiene) gels in liquid crystal solvents

The swelling behaviour of poly(butadiene) gels in four different nematogenic liquid crystalline (LC) solvents has been investigated as a function of temperature (T). Microscopy with crossed polarizers reveals that the nematic to isotropic phase transition temperature of the LC solvents inside the gels (T_NI ^g) is slightly lower than that of the surrounding pure LC solvents (T_NI ^o), but the degrees of depression in T_NI ^g in each system are comparable regardless of the considerable differences in the degrees of equilibrium swelling (Q) at T_NI ^g between the various systems. In general, Q in the isotropic phase is larger than that in the nematic phase, but a unique swelling behaviour of the gel is found in the vicinity of T_NI due to the phase transition of the LC solvents. Q remains constant in the temperature range of T_NI ^g ≤ T ≤ T_NI ^o in which the phases of the LCs outside or inside the domain of the gels are different, namely, nematic and isotropic phase, respectively. In addition, a finite abrupt (discontinuous-like) change in Q is observed at around T_NI. The gels swollen in the LCs, having an ability to interact with the crosslinking points via hydrogen bonding, show a significant thermal hysteresis for the temperature dependence of Q in the vicinity of T_NI, while no discernible thermal hysteresis is observed for the gels in the LCs incapable of forming hydrogen bonds.     

Effect of fatty and resin acids on properties of the surface of kraft lignin

The effect of oleic and abietic acids on the properties of the surface of alkali lignins and the capability of lignins to stabilize water-oil emulsions was examined with kraft spruce and birch lignins as examples. The kinetics of the breakdown of oleic acid-water and abietic acid-water emulsions stabilized by kraft lignins was studied.     

Swelling of sodium acrylate gels

The swelling process of sodium acrylate gel is experimentally investigated. It is found that sodium acrylate gels weakly crosslinked with N,N-methylene-bis-acrylamide may undergo volume phase transition and that different kinds of mechanical instabilities occur in sequence at the transition. Peculiar wrinkle patterns appear on the free surface of an unstable gel and are changed in geometry as swelling proceeds. Cellular patterns seen at various instances in the late period of swelling are ascertained to be geometrically similar to each other and different only in size. The radii of spherical acrylate gels allowed to swell in water are measured as functions of time. The results are discussed and compared with the kinetic theories of swelling. As a result, these theories are proved to be unsatisfactory to fully describe the experimental facts.     

Swelling and drug releasing properties of poly(N-isopropylacrylamide) thermo-sensitive copolymer gels

A series of N-isopropylacrylamide (NIPAAm) copolymer gels with different hydrophilicities were prepared from NIPAAm, hydrophilic acrylamide (AAm) and hydrophobic butyl methacrylate (BMA). The swelling and thermo-responsive properties of PNIPAAm P (NIPAm-co-BMA) and P(NIPAm-co-AAm) copolymer hydrogels were investigated. The drug loading and releasing behaviors for two kinds of model drug with different hydrophilicities were studied. The result shows that the copolymer gels present negative thermo-sensitivities. The lower critical solution temperature (LCST), equilibrium swelling degree and the initial swelling rate increase as the hydrophilicity of gels increases when the temperature is below the LCST. With increasing gel hydrophilicity the loading ratio for sodium salicylate increases, while for salicylic acid, the reverse is observed. The initial drug releasing rate of sodium salicylate and salicylic acid also increase with increasing gel hydrophilicity. The initial drug releasing rate of sodium salicylate is significantly higher than that of salicylic acid. For salicylic acid which is less hydrophilic, the equilibrium releasing ratio at high temperature is lower than that at low temperature while for sodium salicylate which is more hydrophilic, the equilibrium releasing ratio at high temperature is almost the same as that at low temperature. Equilibrium releasing ratios of the three gels are significantly different from each other for salicylic acid when the temperature is below LCST while the equilibrium releasing ratios of the three gels are all 100% for sodium salicylate. __________ Translated from Journal of Central South University (Science and Technology), 2007, 38(5): 906–911 [译自: 中南大学学报(自然科学版)]     

Phase transition in swollen gels 31. Swelling and mechanical behaviour of interpenetrating networks composed of poly(1-vinyl-2-pyrrolidone) and polyacrylamide in water/acetone mixtures

Swelling and mechanical behaviour of interpenetrating positively charged polymer networks (IPNs), composed of poly(1-vinyl-2-pyrrolidone) (PVP) networks and polyacrylamide (PAAm) networks, was investigated in water/acetone mixtures. The first PVP networks were prepared by radiation polymerization at room temperature; after that the PVP networks were swollen in PAAm aqueous solutions and the networks were prepared by thermal copolymerization at 65 °C. The IPNs were prepared with various amounts of the two charged comonomers (quaternary ammonium salts) in the presence of crosslinkers. Two transition regions, detected in the dependence of swelling ratio X on acetone concentration a, suggest that a two-phase structure was formed. The first transition, located between 44 and 60 vol% of acetone, corresponds to PAAm networks, while the second transition, located at 75 vol% of acetone, corresponds to PVP networks. Depending on the amount of positive charges bound to chains, both transitions exhibit continuous or discontinuous character; this fact indicates that intermolecular interactions between the two components occur with the formation of IPNs (e.g., more polar, charged PVP component increases the extent of hydrogen bonding and makes acetone less effective solvent for IPNs at the PAAm transition). The dependences of log G on log X are roughly the same regardless of charge concentrations; this means that the mechanical behaviour is predominantly determined by the degree of swelling for all gels.     

Aging effect on the mechanical properties of hybrid gels

Sol-gel derived unsupported films and thin rods have been obtained from co-hydrolysis of triethoxysilane and methyldiethoxysilane. The materials are flexible, dense and transparent. Films and rods have been aged for different periods of time in air at room temperature. The elastic modulus has been measured by means of tensile or flexural tests. Measurements showed an increase of elastic modulus with aging time and showed different values for films and rods. The observed evolution of mechanical properties has been related to a corresponding structural modification as highlighted mainly by MAS-NMR studies. Analyses pointed out the crucial role of condensation processes and showed that the stiffness increase arises from the formation of relatively few bonds which link and constrain pre-existing mobile network regions.     

Yielding behavior of thermo-reversible colloidal gels

The breakdown of structure in gelled suspensions due to the application of an external stress results in flow. Here we explore the onset of flow by investigating the onset of nonlinear behavior in the elastic moduli of a widely studied class of thermo-reversible gels over a range of volume fractions. We employ the system composed of octadecyl-coated silica particles (radius = 24 nm) suspended in decalin that displays a transition from a liquid to a gel below a volume-fraction-dependent gel temperature, Tgel. The perturbative yield stress at which the gel modulus drops to 90% of its value in the linear viscoelastic limit is found to increase monotonically with volume fraction and decreasing temperature. The recently proposed activated barrier-hopping theory of Schweizer and co-workers1,2 presents a framework to capture the impact of external forces on the mechanical properties of structurally arrested systems. By characterizing particle interactions with a Yukawa potential and employing the resultant static structure factor as input into the activated barrier-hopping theory, we make predictions for how the elastic modulus evolves with the applied stress. Comparisons of these calculations with experiments reveal that the theory does an excellent job of quantitatively capturing the perturbative yield stresses over the entire range of volume fractions and temperatures explored in the study. The match of predictions with experimental results suggests that the theory not only captures particle localization but also how this localization is modulated in the presence of an external stress.     

Swelling of polyacrylamide gels in polyacrylamide solutions

Swelling behavior of polyacrylamide (PAAm) and polyacrylamide-co-polyacrylic acid (PAAm-co-PAAc) gels was investigated in aqueous solutions of monodisperse PAAms with molecular weights (M_w) ranging from 1.5 × 10³ to 5 × 10⁶ g/mol. The volume of the gels decreases as the PAAm concentration in the external solution increases. This decrease becomes more pronounced as the molecular weight of PAAm increases. The classical Flory–Huggins (FH) theory correctly predicts the swelling behavior of nonionic PAAm gels in PAAm solutions. The polymer–polymer interaction parameter χ₂₃ was found to decrease as the molecular weight of PAAm increases. The swelling behavior of PAAm-co-PAAc gels in PAAm solutions deviates from the predictions of the FH theory. This is probably due to the change of the ionization degree of AAc units depending on the polymer concentration in the external solution. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1313–1320, 1998     

Reactions of lignin in chlorine dioxide bleaching of kraft pulps

The reactions between chlorine dioxide and the residual lignin in oxygen-bleached softwood kraft pulps have been studied. In a first series, isolated lignin samples have been subjected to chlorine dioxide oxidation at different pH values and subsequently analysed by oxidative degradation and elemental analysis. Different analytical techniques have also been employed to follow the gradual chemical changes in lignins isolated from kraft pulps after each of the bleaching stages in the OD(EOP)DD sequence. The results demonstrate that, in order to minimize chlorination of the lignin, the first chlorine dioxide stage should be carried out at a pH around or above three. At this pH level, a high degree of lignin oxidation is also achieved. A certain (mono)-chlorination of the lignin in the first D stage cannot be avoided, but this chlorine is to a large extent removed in the later bleaching stages. The efficient and non-selective oxidation of the various phenolic lignin end groups by chlorine dioxide is clearly illustrated by the analytical data. Moreover,¹³C NMR reveals that reduced lignin structures formed during the kraft cook survive the oxidative bleaching stages to a large extent.     

Swelling behaviour of succinylated fibers

The swelling behaviour of cellulosic fibers bearing various amounts of carboxylic groups introduced by succinylation was studied as a function of pH. Upon an increase of pH, the perimeter of the succinylated fibers expanded as measured with the Wilhelmy plate technique. The fibers pass two pH regimes of increased expansion, pH = 5 and pH = 9. These pH levels correlate with the conductometric titration, which reveals two inflection points in both the pH and conductivity values for the succinylated fibers. Determinations of fiber saturation points (FSP) confirm that the cell wall is largely affected by increased pH. Analysis of the fibers with ESEM (Environmental Scanning Electron Microscope) showed that bundles of fibrils were released from the surface of the succinylated fibers at higher pH. Wilhelmy measurements also showed that the surface roughness was more than doubled in fibers succinylated for 12 h as pH rose from 3.2 to 10. These results indicate that, as the charge of the fibers is increased, the swelling forces reach such levels of magnitude that they overcome the structural network forces holding the fiber wall together. The methodology applied can hence be used to quantify the fundamental gel properties of the fiber wall.     

Filled PMMA: mechanical properties and fracture behaviour

The aim of this work is to study the influence of the filler fraction and that of the filler/matrix interfacial adhesion on the mechanical properties and on the fracture behaviour of a poly(methyl methacrylate) PMMA (for injection moulding). The variation of the tensile and flexural mechanical properties with the filler volume fraction was determined. The changes in the fracture behaviour produced by the fillers were studied by evaluating the K_IC and G_IC parameters of the LEFM (Linear Elastic Fracture Mechanics) by carrying out tests with SENB geometry at room temperature and low strain rates. After fracture surfaces examination by SEM (Scanning Electron Microscopy), it was found that the surface treatment had been rather effective and that the fracture toughening mechanism was multiple crazing.     

Structure of colloidal gels during microchannel flow

We investigate the structure and flow behavior of colloidal gels in microchannels using confocal microscopy. Silica particles are first coated with a cationic polyelectrolyte and then flocculated by the addition of an anionic polyelectrolyte. In the quiescent state, the suspension is an isotropic and homogeneous gel. Under shear flow, the suspension contains dense clusters that yield at intercluster boundaries, resulting in network breakup at high shear rates. These structural changes coincide with a transition from pluglike flow at low pressures to fluidlike flow at high pressures.     

Influence of ozonized kraft lignin on the crystallization of CaCO3

Results are reviewed from a study examining how structural modifications introduced by ozonization enhance the influence of kraft lignin on the crystallization of CaCO(3). Ozone treatment of kraft lignin in an aqueous environment is shown to increase its carboxylic acid and overall oxygen content and reduce its molecular weight. Calcium concentration and temperature were monitored in heated supersaturated solutions containing ozonized kraft lignins to gauge their influence on CaCO(3) crystallization processes. The presence of kraft lignin raises the temperature necessary to induce crystallization. This effect is shown to level off at relatively low lignin concentrations and be dependent on the extent of ozone treatment the kraft lignin has undergone. A linear correlation is found between crystallization temperatures and the carboxylic acid content of ozonized lignin samples indicating the introduction of these functional groups plays an important role in enhancing its inhibitory effect. Scanning electron microscopy images of crystals grown in the presence of kraft lignins show significant morphological modifications. These are consistent with specific or pseudo specific interactions between the lignin and crystal faces of calcite to inhibit growth parallel to its c axis. The influence over crystal morphology demonstrated by modified kraft lignin increases with increasing ozonization. Also presented here are crystallization temperature data for a range of kraft lignin ultrafiltration fractions, which indicate that the optimal (nominal) molecular weight of kraft lignin for inhibiting the crystallization of CaCO(3) lies between 5000 and 10000.     

Third‐generation amphiphilic conetworks. III. Permeabilities and mechanical properties

While two of our earlier papers on poly(dimethyl acryl amide)/polymethylhydrosiloxane/polydimethylsiloxane (PDMAAm/PMHS/PDMS) amphiphilic conetworks concerned synthesis and biological properties, respectively, the present contribution focuses on oxygen and insulin permeabilities, and select mechanical properties. We show that by increasing the PDMAAm content from 20 to 60% (i.e., by decreasing the hydrophobic content from 80 to 40%), oxygen permeabilities decrease from ～240 to ～130 barrers. Evidently, oxygen permeability is a function of the sum of the oxyphilic components, PDMS + PMHS, in the conetworks. In contrast, insulin permeability is a function of the hydrophilic component, and reaches a desirable 1.5 × 10^?7 cm²/s at 61% PDMAAm. We also studied the permeabilities of glucose, dextran, and albumin through a PDMAAm₆₁/PMHS₆/PDMS₃₃ membrane and found, unsurprisingly, that the permeability of these molecules follows their hydrodynamic radii, and we project that the permeability of IgG is infinitesimally low. Tensile strengths and ultimate elongations of water‐swollen membranes are also a function of conetwork composition: by increasing the PDMAAm content from 30 to 60%, strengths decrease from 1.6 to 1.2 MPa, and elongations from ～60 to ～40%. Overall, the permeabilities and the mechanical properties of these membranes are appropriate for implantation and, specifically, for immunoisolation of living tissue. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4276–4283, 2007     

Swelling and deswelling kinetics of poly(N-isopropylacrylamide) gels

Swelling and deswelling kinetics was investigated for three types of cylindrical poly(N-isopropylacrylamide) (PNIPA) gels differing in crosslink density. The temperature dependence curves of the volume of the gel specimens were different from one another. One of the gel specimens was considered as a critical gel showing the continuous volume phase transition. The volume change process of the specimens after a temperature jump was examined. In the deswelling processes with temperature jumps to temperatures higher than 35 degrees C, a phase separation was observed in the gel specimens and the volume change slowed down due to the homogenization after the phase separation. The value of the diffusion constant obtained without the phase separation decreased rapidly as temperature approaches the transition temperature. The rapid decrease for the critical gel indicates the emergence of the critical slowing-down. The value of the critical exponent for the correlation length suggests that the universality class for the volume phase transition of the critical PNIPA gel belongs to the class for the classical theory.     

Swelling of mechanical pulp fines

The objective of the study was to determine the swelling of different types of mechanical pulp fines. The physical and chemical characteristics of the fines were also examined. It was found that the degree of swelling correlates with the proportion of fibrillar material, that is fibrillar content of the fines. The fines with the lowest fibrillar content had a swelling comparable to mechanical pulp fibres (0.69g/g), whereas the fines with a high fibrillar content had a swelling comparable to neverdried kraft pulp fibres (1.41g/g). Hemicellulose content and charge could not explain the differences in swelling of different types of the mechanical pulp fines. While the lignin content appears to be an important factor in the degree of swelling of mechanical pulp fines, the results suggest that structural differences between the particles are also important. The bulk elastic modulus was determined by measuring the change in swelling for a known change in osmotic pressure. All the mechanical pulp fines had a high bulk elastic modulus compared to kraft fines. However, fibrillar fines had a lower bulk elastic modulus than flakelike fines. Mechanical pulp fines, both fibrillar and flakelike varieties, did not hornify appreciably. The swelling of both the fines and the fibre fractions increased slightly with the specific energy consumption in the refining.