“What, then, is truth? A mobile army of metaphors, metonyms, and anthropomorphisms ‐ in short, a sum of human relations which have been enhanced, transposed, and embellished poetically and rhetorically, and which after long use seem firm, canonical, and obligatory to a people: truths are illusions about which one has forgotten that this is what they are; metaphors which are worn out and without sensuous power; coins which have lost their pictures and now matter only as metal, no longer as coins.” [Friedrich Nietzsche: On Truth and Lie in an Extra‐Moral Sens (1873)] This contribution is a short summary of the recent achievements of the author in the field of gelation research on irreversible gelling systems based on N‐vinylcaprolactam and on thermoreversible systems (xanthan gum/locust bean gum, and gelatin) mainly based on dynamic light scattering and oscillatory shear rheology. These investigations are discussed in the framework of studies of other authors. It will be pointed out that both methods are well suited to detect the sol‐gel transition, but some expectations that the two different methods can lead to different results will be given, which may be of importance to other systems. In the main focus is the comparison of dynamical critical exponents yielded from the two methods at the sol‐gel transition.
Phase‐separation dynamics are investigated by SALS in aqueous MC solutions in the presence of 5% NaCl, promoted by a quench temperature. The observed scattering peak indicates that the phase separation occurs by the SD mechanism, leading to a bicontinuous structure. A semilog plot of I(q) against time in the early stage of SD gives a straight line, but the position of qmax varies; also, a Cahn–Hilliard plot indicates that diffusive processes dominate and the data can be described by linear CHC theory. Dapp shows a kinetic dependency on the quench temperature. The spinodal temperature of the sample is 41 °C. In the late stages, S(q,t) collapses into a universally time‐independent curve.
Summary: This work discusses the interactions between the anionic polyelectrolyte sodium hyaluronate (Na‐Hy) and the cationic surfactant decyltrimethylammonium bromide (C10TAB). Static and dynamic light scattering experiments were carried out in the single homogenous phases (region I: [C10TAB] < 0.040 M and region III: [C10TAB] > 0.350 M ) in “salt‐free” and at different added salt concentrations (NaCl) in order to gain more understanding on the chain conformation and on the dynamics of polyelectrolyte/surfactant systems. In region I, the Na‐Hy “salt‐free” system exhibits the expected polyelectrolyte properties, i.e. the scattering intensity increases with the addition of surfactant or salt. As a consequence of this screening of the electrostatic interactions, a decrease in both the radius of gyration Rg and the second virial coefficient A2 is observed. This decrease is found more pronounced in the presence of surfactant C10TAB than in the case of added salt (Na‐Hy/NaCl). Consequently, the calculated apparent persistence length Lt, assuming a worm‐like chain at high ionic strength, is found about 58 Å in the case of surfactant and Lt = 71 Å in the case of added salt. This result demonstrates clearly that the binding of the surfactant to Na‐Hy is stronger than in the case of added salt and is at the origin of the phase separation that occurs at the concentration of C10TAB > 0.040 M . As far as the dynamic properties are concerned, the autocorrelation functions in “salt‐free” solutions measured in region I are better described by two relaxation modes (intermediate and slow) and by three relaxation modes (fast, intermediate and slow) in region III. The three relaxation modes reflect respectively the dynamics of free micelles (fast mode), association‐dissociation equilibrium of micelle‐like clusters (intermediate mode) and polymer‐micelles adsorption (slow mode). At high ionic strength (added salt) in region III, the micelle diameter increases (fast mode), the intermediate mode disappears and the polymer‐micelles adsorption diffusion coefficient increases due to the screening of the electrostatic interactions.
Phase diagram of Na‐Hy polyelectrolyte in C10TAB surfactant and NaCl salt. 相似文献
Aqueous self‐assembly of thermosensitive triblock copolymers based on poly(N‐isopropylacrylamide) and poly[2‐(methacryloyloxy)ethyl phosphorylcholine] (PNIPAMm–PMPCn–PNIPAMm and PNIPAMm–PMPCn–S–S–PMPCn–PNIPAMm) were studied using light scattering (SLS and DLS), TEM and fluorescence experiments. These techniques were used to investigate the morphological transition as a function of the temperature, below and above the LCST of the PNIPAM, at various triblock copolymer concentrations ranging from dilute to semi‐dilute regimes. Below the LCST and at low concentrations, aqueous solutions show micellar behavior, while above the LCST self‐assembly leading to large nanoparticles stabilized with PMPC chains. Such behavior is the onset of a gel‐like phase transition observed at higher concentrations.
Dynamic light scattering as well as rheological technique was used to study the semi‐dilute hydrophobically modified alkali‐soluble emulsion (HASE) solutions. The solution viscosity increases dramatically, when the overlap concentration C* is exceeded. Two diffusional modes are evident from the decay time distributions of light scattering and they are related to the hydrodynamic correlation length and the apparent hydrodynamic radius of the HASE clusters. The hydrodynamic correlation lengths, as determined from the fast diffusion mode decrease in proportion to the polymer concentrations but increase with increasing the hydrophobicity of the hydrophobes of the macromonomers. The apparent hydrodynamic radii of the clusters, as determined from the slow diffusion mode, increase with increasing polymer concentrations. When the carbon number of the hydrophobes is increased from C1 to C8, both the viscosities and diffusion coefficients of the slow mode remain almost unchanged. However, they are strongly dependent on the carbon chain length when the carbon number exceeds C12, especially for the polymer with C20 alkyl chains. The solution viscosities decrease and diffusion coefficients of the slow mode increase with increasing temperatures, however, the parameters of Dη/T remain constant.
Comparison of microstructures of semi‐dilute HASE solutions with (a) short hydrophobic alkyl groups and (b) long hydrophobic alkyl groups was made. For (a) with small hydrophobes, the aggregation number per association junction is smaller and there are more aggregation junctions in the network cluster. The hydrodynamic correlation length, ξh, which is the averaged distance between two aggregation junctions, is shorter. As for (b) with larger hydrophobes, the aggregation number is larger with fewer aggregation junctions in the network cluster. The hydrodynamic correlation length is longer. 相似文献
Numerical simulations are performed to study the diffusion behaviors of rigid rods examined by dynamic light scattering (DLS). It was identified that the apparent diffusion coefficient, Dp, determined by DLS obeys a universal relationship Dp/DG = f(qL) = 1.1804 + 0.1764 × tanh [11.799× tan?1(qL) ‐17.169], where DG, L, and q are the mass‐of‐center diffusion coefficient, length of the rod, and scattering vector, respectively. This relationship delineates the DLS experiments into weak (qL < 4.0), intermediate (4.0 < qL < 20.0), and strong translation–rotation coupling regime (qL > 20.0). In weak (strong) coupling regime, Dp is a measure of DG (D// ‐ longitudinal diffusion coefficient) of the rods. In addition, if DLS is performed in the intermediate coupling regime, the rod length can be determined by solving Dp/DG = f(qL). 相似文献
Summary: The applications of polyelectrolyte complexes range from large‐scale industrial products to special uses in biotechnology and medicine, yet one significant problem is their instability against changes in their environmental conditions, particularly the addition of salts. This concerns the colloidal stability as well as the stability of ionic bindings. Previous work on the effect of sodium chloride revealed that not only additional aggregation, but also complete dissolution may occur, depending on the nature of the polyelectrolyte components used. In these studies, the systems required up to one hour to become stable. Multi‐angle static light scattering was used to investigate the processes taking place after the addition of NaCl to polyelectrolyte complexes (PEC) formed in pure water. PECs were prepared with polymethacrylate as polyanion and poly(diallyldimethylammonium chloride) and a copolymer of this polycation with 53 mol‐% of acrylamide. These systems were chosen because of their interesting behavior: secondary aggregation, swelling, and dissolution at a critical salt concentration. The time‐dependence of these effects was studied in detail by static light scattering.
Aggregation and swelling demonstrated by changes in particle mass, radius, and structure density of the PEC. 相似文献
Summary: The sol‐gel transition of radical chain cross‐linking copolymerization system [N‐vinylcaprolactam (monomer)/3,3′‐(ethane‐1,1‐diyl)bis(1‐vinyl‐2‐pyrrolidone) (cross‐linker)] has been studied using in situ by time‐resolved dynamic light scattering (DLS). A critical dynamical behavior was observed near the sol‐gel transition, which is characterized by the presence of power‐law spectra over three decades in the time‐intensity correlation function g2(t) ? 1 ～ t?μ. It was found, that the critical exponent μ is dependent on the amount of the cross‐linker: higher the amount of the cross‐linker, higher is the value of the exponent μ. The occurrence of speckle patterns in the scattered intensity profile and of a power law behavior in the time‐intensity correlation function appeared at the same time. At the lowest concentration of 0.25% cross‐linker, no power law was observed. The estimated values for μ were in the range of 0.39 ≤ μ ≤ 0.577. It can be concluded, that the amount of branching in the critical gel determines the value of μ.
Poly(2‐isopropyl‐2‐oxazoline)s with varied loading of the spiropyran side‐chain are synthesized, and their photochemical and thermosensitive properties are characterized in aqueous solution. The absorption spectrum of the sample aqueous solution (0.1 wt%) indicates that 7.8 mol% of the pendant groups are in the zwitterionic merocyanine form. The introduction of spiropyran groups causes a marked decrease in the cloud point of the aqueous solution even when the pendant groups contain the zwitterionic merocyanine form. The cloud point of the aqueous solution of the samples in the spiropyran form is slightly (less than 1 °C) lower than that bearing the merocyanine group. Moreover, the photoinduced phase transition is demonstrated at an optimum temperature for the spiropyran‐bearing poly(2‐isopropyl‐2‐oxazoline). This material can be made applicable to a light‐driven drug releasing system by introducing a hydrophobic group to form micelles and increasing the photoinduced change of the cloud point. 相似文献
Association of two chitosans of different degrees of acetylation (DA) is studied in dilute water/acid/salt solutions by combined analysis of static and dynamic light scattering. The influence of sodium salt anions Cl? and CH3COO? on conformation of macromolecules and properties of clusters was studied. At DA = 15 mol%, the macromolecules exist either in rigid‐rod (Rg/Rh = 2.7) or in coil (Rg/Rh = 1.8) conformations in the solvents containing NaCl and CH3COONa, respectively. Chitosans with higher DA = 28% dissolved in the solvent containing CH3COONa are in the collapsed state (Rg/Rh < 0.8), predicted by Dobrynin (Macromolecules, 2004). The clusters are of spherical form (0.8 < Rg/Rh < 1.2). CH3COONa promotes higher fractal dimensionality of clusters (2.2–2.4) than NaCl (1.9). Higher critical aggregation concentration is observed in NaCl. The other hydrodynamic methods (capillary viscometry, velocity sedimentation, and Tsvetkov diffusometry) do not detect the clusters, probably, because of their disruption during flow.
pH‐sensitive micelles formed by interchain hydrogen bonding of poly(methacrylic acid)‐block‐poly(ethylene oxide) copolymers were prepared and investigated at pH < 5. Both and Rh of the micelles increase with decreasing pH of the solution, displaying an asymptotic tendency at low pH values. The observed micelles are well‐defined nanoparticles with narrow size distributions (polydispersity ΔRh/Rh ≤ 0.05) comparable with regular diblock copolymer micelles. The CMCs occur slightly below c = 1 × 10?4 g · mL?1. The micelles are negatively charged and their time stability is lower than that of regular copolymer micelles based purely on hydrophobic interactions.
This contribution describes the influence of short‐chain branching on the temperature dependence of rheological properties of polyethylene (PE) melts in shear. The materials investigated are linear and short‐chain branched, metallocene‐catalyzed PEs of narrow molecular mass distribution. The linear viscoelastic properties are determined by dynamic‐mechanical analysis. Short‐chain branching (SCB) leads to an increase of the flow activation energy. The activation energy was found to increase linearly with rising weight comonomer content.
The self‐assembling properties of hydrophobically modified (N‐isopropylacrylamide) have been investigated by dynamic light scattering and rheological measurements. Size of the globules and transmission of the solutions vary strongly in the same range of temperature. The presence of hydrophobic groups leads to contraction of the globules' size compared to poly(N‐isopropylacrylamide) (PNIPAM). The viscoelastic properties of the samples in aqueous solution have been investigated as a function of copolymer concentration, structure of the hydrophobic group (dodecyl or adamantyl group), substitution level (1–5%) and over a temperature range covering the lower critical solution temperature (LCST). At high concentration and high level of adamantyl substitution, gelation is observed several degrees before the phase transition. A physical network is formed due to the strong hydrophobic interactions, and this physical gel undergoes phase transition without macroscopic phase separation.
Inspired by the well‐known amphiphilic block copolymer platform known as Pluronics or poloxamers, a small library of ABA and BAB triblock copolymers comprising hydrophilic 2‐methyl‐2‐oxazoline (A) and thermoresponsive 2‐n‐propyl‐2‐oxazoline (B) is synthesized. These novel copolymers exhibit temperature‐induced self‐assembly in aqueous solution. The formation and size of aggregates depend on the polymer structure, temperature, and concentration. The BAB copolymers tend to agglomerate in water, with the cloud point temperature depending on the length of poly(2‐n‐propyl‐2‐oxazoline) chain. On the other hand, ABA copolymers form smaller aggregates with hydrodynamic radius from 25 to 150 nm. The dependence of viscosity and viscoelastic properties on the temperature is also studied. While several Pluronic block copolymers are known to form thermoreversible hydrogels in the concentration range 20–30 wt%, thermogelation is not observed for any of the investigated poly(2‐oxazoline)s at the investigated temperature range from 10 to 50 °C.
In this paper, a series of polysiloxanes, presenting different contents of densely simple and well‐defined short‐branched chains (G1–G5), are prepared by hydrosilylation of α,ω‐(dimethylvinylsiloxy)‐poly(dimethyl‐methylvinyl)siloxanes (P1–P5) with 1,1,1,3,5,5,5‐heptamethyltrisiloxane (MDHM) and characterized by 1H and 29Si nuclear magnetic resonance, Fourier transform‐infrared spectroscopy, Abbe refractometry, gel permeation chromatography, and differential scanning calorimetry. The rheological behaviors of the G1–G5 products are investigated to study the influences of the short‐branched chains. The rheological parameters including non‐Newtonian index (n ) and the flow activation energy (ΔEη) are obtained and discussed. It is observed that the G1–G5 polymers belonged to pseudoplastic fluids and the ΔEη values of the G1–G5 products are significantly influenced by the short‐chain branching degree of the polymers.
Nanocomposites with organic–inorganic properties represent a new field of basic research and offer prospects for many novel applications in extremely diverse fields, due to their remarkable emerging new properties and multifunctional nature. However, controllable manipulation of their fluorescent properties in different phases is still challenging, which seriously limits the related applications of nanocomposites. In this work, a convenient protocol to fabricate organic–inorganic nanocomposites composed of stilbene chromophores and CdZnS/ZnS quantum dots (QDs) pairs, with controllable fluorescent properties is presented. It is found that stable white‐light emission can be achieved only in solution phase, with negligible energy transfer or reabsorption between chromophores and QDs pairs. By contrast, when the nanocomposites are deposited as blended films, they cannot give rise to white‐light emission, no matter what donor/acceptor volume ratios are used. However, the blended films can exhibit near‐unity efficiency (94%) of Förster resonance energy transfer from QDs to chromophores. The underlying physical mechanisms are revealed through comprehensive steady‐state and time‐resolved spectroscopic analysis. This work suggests that the CdZnS/ZnS QDs/stilbene nanocomposites can be directly used for fluorescence sensors and probes in biological system as well as fundamental investigation of light‐harvesting, and also sheds light on developing other new materials for artificial photosynthesis and optoelectronics.
The molar masses of acetylated kraft lignin and organosolv lignin are determined using both dynamic light scattering and size‐exclusion chromatography (SEC). Preparative gel fractionation of both lignins is allowed for the further analysis of low‐polydispersity lignin fractions. Fundamental information about the aggregation behavior of different molecular weight lignins in different solvent systems is provided. Two different solvent systems are used for SEC. Universal calibration is used for analysis. Applying the Flory–Fox theory, the obtained molecular weights are allowed for determining the maximum glass transition temperature at theoretical infinite molecular weight, Tg∞, of both acetylated lignins.
Full Paper: The phase behavior of polystyrene (PS) and poly(vinyl methyl ether) (PVME) blend has been investigated rheologically as a function of temperature, composition and oscillating shear rate as well as different heating rates. An LCST (lower critical solution temperature)‐type phase diagram was detected rheologically from the sudden changes in the slopes of the dynamic temperature ramps of G′ at given heating and shear rate values. The rheological cloud points were dependent on the heating rate, , and oscillating shear rate, . The cloud points shifted a few degrees to higher temperatures with increasing and reached an equilibrium value (heating rate independent) at °C/min. The phase diagrams of the blends detected at = 0.1 and 1 rad/s were located in lower temperature ranges than the quiescent phase diagram, i.e., oscillating shear rate induced‐demixing at these two values for the shear rate. On the other hand, at = 10 rad/s, the phase diagram shifted to higher temperatures, higher than the corresponding values found under quiescent conditions, i.e., shear induced‐mixing took place. Based on these two observations, shear induced‐demixing and shear induced‐mixing can be detected rheologically within a single composition at low and high shear rate values, respectively, and this is in good agreement with the previous investigation using simple shear flow techniques. In addition, the William, Landel and Ferry (WLF)‐superposition principle was found to be applicable only in the single‐phase regime; however, the principle broke‐down at a temperature higher than or equal to the cloud point. Furthermore, different spinodal phase diagrams were estimated at different oscillating shear rates based on the theoretical approach of Ajji and Choplin.
Spinodal phase diagrams at different oscillating shear rates. 相似文献
Summary: Novel water‐soluble hydrophobically associative polymers, comprising sodium 2‐acrylamido‐2‐methylpropansulfonate , N,N‐dimethylacrylamide and different amounts of a hydrophobically associative macromonomer, were synthesized in an aqueous medium. Formation of a rotaxane‐like complex of this macromonomer, with randomly methylated β‐cyclodextrin, was studied. In addition, a side chain of polyrotaxane, comprising this complex and methyl methacrylate, was synthesized via ATRP. These polymers were characterized by dynamic light scattering and fluorescent‐probe technique. Effects of methylated β‐cyclodextrin on micelle formation of the polymers were also studied.
The diffusion of a thermal stabilizer, i.e., Irganox 1010, has been studied in three different reactor poly(propylene‐co‐ethylene) blends, namely HecoQ, ExpSS, and Super Soft. The diffusion experiments were carried out by using Roe's method based on a stack of several polymer films where each of them had 60–70 μm thickness. The concentration profile of the stabilizer in every film has been monitored by the absorbance at 282 nm of the recorded UV spectra. Under our experimental conditions, the diffusion of Irganox 1010 for all polymer materials can be correctly interpreted by using Fick's second law resolved under certain boundary conditions. Best‐fitting the experimental data with the equation model also allows calculation of the diffusion coefficient, D, for these polymer materials at 40, 70 and 80 °C. By applying an Arrhenius‐type equation to the calculated D coefficients, an estimation of activation energies of the diffusion process has been achieved. An interpretation, in terms of free volume theory, has been applied to explain the observed variation of D which depends mostly on the amount of amorphous phase of the blends.
Arrhenius plots of ln D versus 1/(RT) for the polymers given in legend. 相似文献
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