Recent Gelation Studies on Irreversible and Reversible Systems with Dynamic Light Scattering and Rheology ‐ A Concise Summary

“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.

     

Mimic of Protein: A Highly pH‐Sensitive and Thermoresponsive Polyampholyte

As a mimic of protein, a polyampholyte‐based material should be able to duplicate the properties and functions of protein. A polyampholyte that is highly sensitive to both pH and temperature under physiological conditions is obtained when 80% butylamide‐terminated poly(amidoamine) dentron is grafted to the backbone of styrene and maleic anhydride. The phase separation occurs at 33.7 °C at pH = 6.20, while the transition point increases to 43.9 °C at pH = 6.30. The superior performance is the consequence of the cooperative interactions among basic, acidic, and thermoresponsive groups. The polyampholyte with a suitable isoelectric point provides a platform for the development of multifunctional materials for biomedical applications.     

Physical Gelation of Amphiphilic Poly(N‐isopropylacrylamide): Influence of the Hydrophobic Groups

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.

     

Clusters of Optimum Size Formed by Hydrophobically Associating Polyelectrolyte in Homogeneous Solutions and in Supernatant Phase in Equilibrium with Macroscopic Physical Gel

Summary: The formation of finite‐sized microgel particles was revealed by photon correlation spectroscopy in aqueous solutions of hydrophobically associative polyelectrolyte polystyrene‐block‐poly(sodium methacrylate)‐block‐polystyrene. Starting from the lowest polymer concentration (1 × 10^?6 g · L^?1) up to 1 g · L^?1 a single relaxation process dominates the spectrum of relaxation times of the polymer solutions. This dominant mode was attributed to the diffusive motion of supramolecular microgel particles with a size of about 100 nm. Above a polyelectrolyte concentration of 1.0 g · L^?1, the size of the microgel grows up to ca. 300 nm. At a polymer concentration 20 g · L^?1, macroscopic phase separation occurs: the liquid phase of the microgel‐cluster solution coexists with the phase of the macroscopic gel.

Schematic representation of the formation of supramolecular structures in aqueous solutions of hydrophobically associative polyelectrolyte.     

Thermo‐Responsive Copolymers Based on Poly(N‐isopropylacrylamide) and Poly[2‐(methacryloyloxy)ethyl phosphorylcholine]: Light Scattering and Microscopy Experiments

Aqueous self‐assembly of thermosensitive triblock copolymers based on poly(N‐isopropylacrylamide) and poly[2‐(methacryloyloxy)ethyl phosphorylcholine] (PNIPAM_m–PMPC_n–PNIPAM_m and PNIPAM_m–PMPC_n–S–S–PMPC_n–PNIPAM_m) 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.

     

Molecular Characteristics and Gelling Properties of the Carrageenan Family, 1. Preparation of Novel Carrageenans and their Dilute Solution Properties

The carrageenans are a family of sulfated polysaccharides extracted from red seaweed, and are conventionally classified into three major groups as κ‐, ι‐ and λ‐carrageenan according to the maximum number (1, 2 and 3, respectively) of sulfate groups per disaccharide repeating unit in corresponding ideal structures. β‐ and θ‐carrageenan (not available in nature) possessing none and two sulfate groups per repeating unit, respectively, were prepared from κ‐ and λ‐carrageenan by desulfonation. Here the aqueous solutions of κ‐, ι‐, β‐ and θ‐carrageenans undergo the thermoreversible gelation by lowering temperature, whereas the aqueous solution of λ‐carrageenan forms no gel.Light scattering and small‐angle X‐ray scattering were observed from the aqueous solution of κ‐, ι‐, λ‐, β‐ and θ‐carrageenans, and the results were analyzed conventionally and by adapting the molecular model in order to elucidate the effect of sulfate groups and anhydrous residue on the conformational characteristics of carrageenans.     

Swelling,Elasticity and Spatial Inhomogeneity of Poly(N,N‐dimethylacrylamide) Hydrogels Formed at Various Polymer Concentrations

Summary: The equilibrium swelling degree, modulus of elasticity and the spatial inhomogeneity of poly(N,N‐dimethylacrylamide) (PDMAAm) hydrogels were investigated over the entire range of the initial monomer concentration. The degree of dilution of the networks after their preparation was denoted by ν, the volume fraction of crosslinked polymer after the gel preparation. The linear swelling ratio of the gels increased linearly with increasing ν. Depending on the value of ν, three different gel regimes were observed: (1) For ν < 0.3, increasing ν decreases the extent of cyclization during crosslinking so that the effective crosslink density of gels increases with rising ν. (2) For 0.3 < ν < 0.7, increasing ν reduces the accessibility of the pendant vinyl groups during crosslinking due to steric hindrance at high polymer concentrations. As a result, the effective crosslink density of gels decreases with increasing ν. (3) For ν > 0.7, the modulus of elasticity increases sharply with increasing ν due to the increasing extent of chain entanglements in this high concentration regime. Static light scattering measurements on the gels show that the degree of spatial gel inhomogeneity in PDMAAm gels attains a maximum value at ν = 0.06. The appearance of a maximum as well as the ν‐dependence of scattered light intensities from gels was successfully reproduced by the theory proposed by Panyukov and Rabin.

Effective crosslink density ν_e of the hydrogels shown as a function of ν.     

Phase Separation of Hydrophilic Monomer/Surfactant Aqueous Solution Studied by a Combined Light Scattering and Interference Approach

Summary : Polymerization‐induced phase separation behavior of 2‐acryllamido‐2‐methyl‐1‐propanesulfonic acid (AMPS)/sodium dodecylsulfonate (SDS) aqueous solution was investigated by using a combined small angle light scattering (SALS) and laser light interference (LI) system. Compared with the conventional SALS technique that has been widely used in studying the phase separation kinetics and mechanism of multicomponent polymer blends or solution, the present combined SALS and LI system can additionally provide valuable information on polymerization kinetics prior to the onset of phase separation. It was found that the 1 mol · L^?1 AMPS aqueous solution in the presence of 0.2 mol · L^?1 SDS underwent apparent phase separation when polymerization conversion approached 82%. The phase separation rate is so rapid that it takes only 90 s for scattering intensity to reach the maximum. The early stage phase separation follows the spinodal decomposition (SD) mechanism that can be described by the classic linear theory sponsored by Cahn and Hilliard. At late stage of phase separation, however, an observable shift of characteristic vector q_m to larger values take place, which may result from the secondary phase separation in the monomer‐rich phase that leads to a deceased average domain size in sample solution. On the other hand, the integral scattering intensity decreased apparently at late stage of phase separation due to the decreased refractive index difference between the monomer‐rich and polymer‐rich phases as a result of the continued polymerization in the monomer‐rich phase.

Illustration of the combined SALS and LI system.     

Static and Dynamic Light Scattering of Polyelectrolyte/Surfactant Solutions: the Na‐Hyaluronate/(C10TAB) System

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 R_g 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 L_t, assuming a worm‐like chain at high ionic strength, is found about 58 Å in the case of surfactant and L_t = 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.     

Preparation and Characterization of Acetoacetoxyethyl Methacrylate‐Based Gels

Novel polymeric gels have been prepared by radical copolymerization of acetoacetoxyethyl methacrylate (AAEM) and hydroxyethyl methacrylate (HEMA) in water‐ethanol medium. The influence of the HEMA:AAEM ratio and crosslinker concentration on properties of gels was studied. Independently on gel composition the maximum swelling was detected in chloroform. It was found that PAAEM gels possess phase transition temperature or upper critical solution temperature (UCST) in alcohol‐water solutions. UCST decreases in linear order from 75 to 10 °C when HEMA content in gel structure increases. The minimal UCST of AAEM/HEMA gels in binary alcohol‐water mixtures is shifted toward lower temperatures and lower alcohol concentrations when the alkyl chain of alcohol increases.

AAEM/HEMA gels prepared at different BIS concentrations (1:1 mol‐%, 2:2.5 mol‐%, 3:5 mol‐%).     

Asymmetric Behavior of Temperature‐Responsive Poly(N‐isopropylacrylamide) Ultrathin Layers Observed by Atomic Force Microscopy

During characterization of a temperature‐responsive poly(N‐isopropylacrylamide) (PIPAAm) layer grafted onto a Si(100) substrate, atomic force microscopy (AFM) is able to probe the interactions between the microscope tip and the polymer. The modification of the AFM tip surface with octadecyltrichlorosilane (OTS) changes the interaction between the PIPAAm surface and the tip. Although a repulsive interaction is observed between a commercially available Si tip and the PIPAAm surface, a strong attractive interaction between the OTS‐modified Si tip and the surface is observed. Adhesion‐force analysis shows changes in the hydrophilic/hydrophobic character of ultrathin PIPAAm surfaces immediately after a change in temperature. The PIPAAm surface becomes hydrophobic less than 30 min after temperature increase, but requires 120 min to become hydrophilic after temperature reduction.

     

Characterization of Polyelectrolyte Complexes Based on Maleic Anhydride Alternating Copolymers by Static and Dynamic Light Scattering

The influence of the hydrophobicity of oppositely charged polyelectrolytes based on maleic anhydride alternating copolymers on the complex formation has been investigated by means of DLS and SLS. The change of hydrophobicity of the stock polyions was provided by variation of the comonomer (ethylene, isobutylene and styrene) in the polymer backbone. The particle size and the polydispersity at certain mixing ratios increase with increasing hydrophobicity of the used polyelectrolytes. Below the isoelectric point (1:1 interaction stoichiometry) an increase in the mixing ratio n^?/n⁺ leads to an increasing molar mass and density of the PECs within separate series while the polydispersity is decreasing. The radius of gyration and the hydrodynamic radius are less affected by the mixing ratio in this range. Above the isoelectric point, size and molar mass of the PECs increase strongly and depend on the amount and stabilizing efficiency of the added stabilizing polyelectrolyte. All studied PECs are found to be of spherical shape with different internal structure depending on the used polymers and the mixing ratio.

     

Tracer Diffusion in Heterogeneous Polymer Networks

The diffusion of linear polymer chains in swollen polymer networks is studied with different extents of spatial heterogeneity of their crosslinking density. Structural heterogeneity is imparted to these gels by photo‐crosslinking of polymer precursor chains in semidilute solution, which comprise either just one batch of precursors, yielding gels with just a little spatial heterogeneity, or mixtures of two different batches of precursors, one with high and one with low degree of functionalization with crosslinkable moieties, yielding gels with more pronounced heterogeneity. In addition, heterogeneous gels are also prepared from crosslinkable precursor chains and defined fractions of pre‐crosslinked microgel particles that serve as doped‐in nanometer‐scale domains of high local crosslinking density. The resulting gels are probed by static light scattering and shear rheology, and the self‐diffusion of fluorescent linear tracer chains within the gels is studied by fluorescence recovery after photobleaching. The outcome of these investigations is that spatial heterogeneity of a given crosslinking density in swollen polymer networks decreases their ability to store mechanical deformation energy, but has no marked impact on the above‐micrometer‐scale diffusivity of linear tracer chains.

     

pH‐Sensitive Micelles Formed by Interchain Hydrogen Bonding of Poly(methacrylic acid)‐block‐Poly(ethylene oxide) Copolymers

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 R_h 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 ΔR_h/R_h ≤ 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.

     

Influence of Short‐Chain Branching of Polyethylenes on the Temperature Dependence of Rheological Properties in Shear

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.