Hydrophobically Modified Ethoxylated Urethane Research Articles

Tailoring Waterborne Coating Rheology with Hydrophobically Modified Ethoxylated Urethanes (HEURs): Molecular Architecture Insights Supported by CG-MD Simulations.

A novel investigation of the effects of the hydrophilic and hydrophobic segments of hydrophobically modified ethoxylated urethanes (HEURs) on the rheological properties of their aqueous solutions, latex-based emulsions, and waterborne paints is demonstrated. Different HEUR thickeners were produced by varying the poly(ethylene glycol) (PEG) molecular weight and terminal hydrophobic size. Results reveal that the strength of hydrophobic associations and, consequently, the rheological properties of HEUR formulations can be effectively controlled by modifying the structure of the hydrophobic segment, specifically, the combination of diisocyanate and monoalcohol. This allows for the on-demand attainment of diverse rheological behaviors ranging from predominantly Newtonian profiles exhibiting lower viscosities to markedly pseudoplastic behaviors with significantly higher viscosities. The length of the hydrophilic group appears to affect viscosity only marginally up to a molecular weight of 23,000 g/mol, with more notable effects at 33,000 g/mol. Additionally, it was indicated that the rheological responses observed in water solutions provide a reliable forecast of their behavior in latex-based emulsions and waterborne paints. Coarse-grained molecular dynamics (CG-MD) simulations were also applied to gain insight into HEUR micelle dynamics in aqueous solutions. Guided by the DBSCAN algorithm, the simulations successfully captured the concentration-dependent behavior and the impact of hydrophilic chain length, aligning with the experimental viscosity trends. Various metrics were employed to provide a comprehensive analysis of the micellization process, including the hydrophobic cluster volume, the total micellar volume, the aggregation number, and the number of chains interconnecting with other micelles.

Understanding associative polymer self-assembly with shrinking gate fluorescence correlation spectroscopy.

The self-assembly of polymers is integral to their role in liquid formulations. In this study, we combine a dye whose lifetime is sensitive to the nanoviscosity of its local environment with shrinking gate fluorescence correlation spectroscopy (sgFCS) to study the self-assembly of a model telechelic polymer, hydrophobically modified ethoxylated urethane (HEUR). Fluorescence lifetime measurements show a monotonic increase in average lifetime with increasing HEUR concentration driven by a small fraction of dye (<1%) with long lifetimes strongly bound to HEUR. Despite this small fraction, sgFCS isolates the diffusional dynamics of the bound fraction with no a priori assumptions as to the distribution of lifetimes. Sensitivity is greatly enhanced compared to standard FCS, revealing micellar aggregates forming between 0.2 and 1 wt% followed by formation of a percolated network. This sgFCS approach, which we apply for the first time to polymers in this work, is readily extendable to any dye that changes lifetime on binding.

Sustainable associative thickeners based on hydrophobically modified ethoxylated poly(hydroxy-urethane)s end-capped by long alkyl chains

Hydrophobically modified ethoxylated urethanes (HEURs) are associative polymers applied as thickening additives in various waterborne systems, such as emulsions, coatings and printing inks. Their conventional synthesis, based on diisocyanates, poses environmental and health threats during both manufacture and application. Hence, in this paper we present an approach for the next generation of sustainable and CO2-rich polyurethane rheological modifiers – isocyanate-free HEURs (IFHEURs) obtained through poly(hydroxy-urethane) route. The hydrophilic core of the IFHEURs was prepared by step-growth polyaddition between diamine and poly(ethylene glycol) bis(cyclic carbonate) – synthesized using CO2 as a green source of the carbonate bonds. Such obtained prepolymer with terminal carbonate groups was subsequently modified with hydrophobic amines using reactive extrusion method, which enabled efficient homogenisation of the viscous reaction systems and reaching high end-capping functionality after only 2 h. In contrast to previously reported studies, the synthetic procedure was both solvent- and catalyst-free and allowed to incorporate up to 11 wt% of CO2 into the backbone of the final products. The structural analysis of the obtained IFHEURs using NMR and FT-IR spectroscopy and MALDI-ToF mass spectrometry confirmed obtaining the desired telechelic architecture resembling standard HEUR materials. The associative behavior of the IFHEURs in aqueous solutions was addressed in a rheological study, which revealed typical thickening mechanisms exhibited by rheological additives.

Intensification of solventless production of hydrophobically-modified ethoxylated urethanes (HEURs) by microwave heating

Hydrophobically modified ethoxylated urethanes (HEURs) are among the most widely studied class of waterborne polyurethanes. HEURs are amphiphilic polymers that are usually composed of a polyethylene glycol (PEG) backbone, typically end-capped with aliphatic alkyl chains. HEUR synthesis in the industry is a slow and energy-intensive step-growth polymerization process, due to heat and mass transfer limitations inherent in such highly viscous systems. We investigate for the first time the effect of microwave heating on both the solvent-free HEUR synthesis step and the initial pretreatment step of the reagents prior to the HEUR synthesis. We show that microwaves can drastically reduce the overall processing time, and thereby reduce total energy consumption by: A) faster melting of solid PEG and B) faster completion of the polymerization reaction, when operating with a novel rapidly rising transient temperature profile that cannot be reproduced by conventional heating. However, it was found that PEG pretreatment/dehydration by microwaves leads to faster degradation of PEG, compared to dehydration by conventional heating at the same bulk temperature, and should therefore be avoided.

Insight Into Solventless Production of Hydrophobically Modified Ethoxylated Urethanes (HEURs): The Role of Moisture Concentration, Reaction Temperature, and Mixing Efficiency.

In this work, we report for the first time on the influence of the quality of reactants and reaction conditions on the production of hydrophobically modified ethoxylated urethanes (HEURs) and selected prepolymers without the use of solvents. We show that the polyol water concentration is detrimental to the progress of the main urethane forming reaction, confirming the necessity of carefully drying the reactants below 1000 ppm to suppress the consumption of diisocyanate toward urea during HEUR synthesis. Increasing the mixing speed (≈30 to 750 rpm), reaction temperature (80–110 °C), and catalyst concentration (0.035–2.1 wt % bismuth carboxylate) can significantly increase the rate of molecular weight buildup, but their effect decreases with time as the bulk viscosity increases and mixing limitations eventually take over, leading to the Weissenberg effect and chain growth termination. Consequently, for the selected formulation, the maximum product molecular weight attained lies in the range of ≈20 000–22 000 g/mol, irrespective of the specific process conditions applied.

Reactive Extrusion Synthesis of Biobased Isocyanate-Free Hydrophobically Modified Ethoxylated Urethanes with Pendant Hydrophobic Groups.

Development of hydrophobically modified ethoxylated urethane (HEUR) rheology modifiers enabled the widespread application of waterborne paints and coatings, replacing their environmentally burdening solvent-based predecessors. However, the diisocyanates, required for the conventional synthesis of HEURs, pose severe eco-sustainability threats. In this paper, we demonstrate an innovative approach to avoiding toxic components in the preparation of rheology modifiers by obtaining a new class of water-soluble isocyanate-free hydrophobically modified ethoxylated poly(hydroxy-urethane)s (IFHEURs). The first step in the synthetic pathway was the preparation of CO2-based five-membered poly(ethylene glycol) bis(cyclic carbonate) and its subsequent aminolysis using 4,7,10-trioxa-1,13-tridecanediamine, yielding poly(hydroxy-urethane) (PHU) prepolymers terminated with cyclic carbonate groups. The PHU prepolymers were further extended in a reactive extrusion (REX) synthesis using biobased hydrophobic diamine PRIAMINE 1075. The REX technique made it possible to overcome the typical limitations of the aminolysis reaction and to reach the desired conversion within a moderate reaction time. IFHEURs have been structurally elucidated using FT-IR and NMR spectroscopy techniques, MALDI-ToF mass spectrometry, and SEC analysis and applied as rheology modifiers. The study of their associative behavior in aqueous solutions confirmed that the architectural flexibility of the obtained IFHEURs, containing terminal and pendant hydrophobic groups, opens a perspective for tuneable thickening performance.

Orientational Dynamics of Magnetic Iron Oxide Nanoparticles in a Hydrogel: Observation by Magnetic Linear Dichroism under Oscillating Field.

For the success of biomedical applications of magnetic iron oxide nanoparticles (MION), such as magnetic hyperthermia and magnetic particle imaging, it is essential to understand the orientational dynamics of MION in a complex fluid under an alternating field. Here, using the magnetic linear dichroism (MLD) measurement, we directly observed the orientational behavior of MION in a hydrogel under a damped oscillating magnetic field (DOMF) of 33 kHz in frequency. Hydrophobically modified ethoxylated urethane (HEUR) is examined as the network polymer because the mesh size of the network is controllable with its concentration. We used two MIONs: a bare MION (MION1) and a MION coated with an amphiphilic polymer (MION2). Where the mesh size of the gel network is larger than the particle's hydrodynamic diameter, MION1 in the hydrogel rotates in the same manner in a simple solution, although the macroscopic rheological property of the medium is quite different. Meanwhile, the orientational behavior of MION2 is dramatically changed by the addition of HEUR molecules even below the minimum gelation concentration, indicating that MION2 is associated with the flower micelles of HEUR. By analyzing the MLD waveform, the orientational behavior of MION1 in the HEUR gel under a DOMF can be explained with single-mode relaxation, whereas that of MION2 is complicated; a rapid partial rotation near the particle and a whole slow rotation of the particle-flower micelle associate are superimposed. It is hard to distinguish this difference in orientational behaviors from the dynamic magnetization curve because the dominant magnetization reversal process is Néel rotation, the rotation of the magnetic moment in the particle. The MLD measurement is a potential tool for optimizing biomedical techniques utilizing MIONs and for nanorheology or colloid science in a complex matrix such as a hydrogel or cytoplasmic matrix.

Synthesis of hydrophobically modified ethoxylated non-isocyanate urethanes (HENIURs) and their use as rheology additives

Telechelic associative polymers such as Hydrophobically modified ethoxylated urethanes (HEURs) are an important class of rheology additives used in many industrial fields allowing a precise control of the rheology. To avoid the use of toxic isocyanates, more eco-friendly HEURs were synthesized based on the cyclo-carbonate/amine addition reaction. Hydrophobically modified ethoxylated hydroxyurethanes (HEHURs) were thus obtained by the aminolysis of telechelic 5-membered cyclic carbonates functionalized PEG in the presence of hydrophobic amines of 14, 16 and 18 carbons. The rheological properties (G’, G’’ and η*) of these amphiphilic structures were investigated, as well as the influence of different parameters such as: temperature (5–25 °C), concentration (10–20 wt%) and hydrophobic chain length. Their rheological properties were also compared to isocyanate-based equivalent structures (HEURs). Finally, HEHURs were used in latex solution to illustrate their potential use as rheology additives and their thickening efficiency at 0.25 wt%. Mainly Newtonian to fully pseudoplastic profiles were obtained depending on the variation of the hydrophobic chain length.

Preparation of silicon modified HEUR thickener with good rheological and solvent resistance properties

Hydrophobically modified ethoxylated urethanes (HEUR) has often been used in paint fabrication to adjust rheology, prevent cracks, and improve resistance to sag. However, in water-based coatings, organic solvents are often used as film-forming aids and co-solvents. These solvents will greatly weaken the thickening performance of HEURs. Herein, we adopt one-step polymerization to prepare silicon modified HEUR with good solvent stability and anti-organic-solvent performance. FTIR spectroscopy and H-NMR were used to evaluate the chemical structure of HEUR. Aggregation behavior, rheological behavior, thickening performance and anti-solvent performance were conducted to assess the performance of HUER. It was found that HEUR modified with 1,3-bis(3-aminopropyl) tetramethyldisiloxane displays an excellent behavior to resist organic solvents.

Influence of prepolymer molecular weight on the rheology and kinetics of HEUR-thickened Latex Suspensions

Hydrophobically Modified Ethoxylated Urethanes (HEURs) were synthesized by a step growth polymerization of polyethylene glycol (PEG, Mn = 4000, 6000, 8000) with isophorone diisocyanate (IPDI). The thickeners were produced with different sizes of the hydrophilic section by changing the molar ratios of reactants. The hydrophobic ends, 1-tetradecanol (C14), was constant for all prepared samples. The structure, molecular weight and polymer dispersity index (PDI) of HEUR were characterized by 1H NMR, IR and GPC respectively. The rheological behavior of the HEUR-thickened latex (Latex/HEUR) was determined by oscillatory and flow experiments. Dramatic increases in viscosity of Latex/HEUR are obtained. Under flow conditions the Latex/HEUR fits the Cross model, showing two regions, a Newtonian and a shear thinning one. Broad thixotropy are observed in hysteresis tests and three interval thixotropy test (3ITT) with decreasing molecular weight of the prepolymer, which was analyzed using a second-order kinetic equation. The results of atomic force microscopy (AFM) and the flow-levelling performance tests for Latex/HEUR films show smoother surface and better levelling with the increase of the hydrophilic component size.

Polymerizable Hydrophobically Modified Ethoxylated Urethane Acrylate Polymer: Synthesis and Viscoelastic Behavior in Aqueous Systems

Hydrophobically modified ethoxylated urethanes (HEURs) comprising hydrophilic poly(ethylene glycol) backbone and hydrophobic end groups are widely applied as rheological additives to regulate the viscoelastic behavior of waterborne systems such as coatings and inks. However, the conventional HEURs are usually nonreactive and remain free in final films, which may bring some defects and damages to the final cured films during the service time. An appealing way to improve filming properties may be to design reactive HEURs, which can participate in the post-crosslinking reactions with other components in formula during the film curing. In this work, a polymerizable acrylic-type HEUR (acrylic-HEUR) end-functionalized by the reactive methacrylate group and hydrophobic octadecyl tail is reported. The typical free radical polymerization feature of acrylic-HEUR based on the highly reactive methacrylate group is carefully clarified by dynamic light scattering, in situ nuclear magnetic resonance (1H NMR), and rheological measurements. The excellent thickening performance of acrylic-HEUR in waterborne UV emulsions and its effective copolymerization reactivity with the UV-curable matrix resin are demonstrated by in situ rheological measurements under UV irradiation in detail. Compared to conventional nonreactive HEURs, the copolymerization reactivity of acrylic-HEUR may make it especially suitable for applications in waterborne UV coatings to avoid its migration from final UV-cured films. This work may not only open a new perspective for the fabrication of HEUR polymers with high chemical reactivity but also promise the potential of polymerizable HEURs as superior associative thickeners in industry.

From Architectures to Cutting-Edge Properties, the Blooming World of Hydrophobically Modified Ethoxylated Urethanes (HEURs)

Hydrophobically modified ethoxylated urethanes (HEURs) are associative polymers that, compare to others, are far less sensitive to solution parameters, especially pH and ionic strength. Therefore, they are widely used as rheology modifiers in many different waterborne systems such as inks, coatings, emulsions, etc. In solutions, HEURs form transient networks through molecular associations between their hydrophobic groups. The transient network formation and its characteristics are responsible for the rheological properties and depend on the HEUR chains architecture. Many parameters such as molecular weight, size and nature of hydrophobic groups, polymer concentration, etc must be controlled to tune the several physicochemical properties of HEUR solutions. This article aims to give an overview of the HEUR studies reported in the literature in order to highlight the structure/rheological properties relationship. The article also draws attention to new trends in HEURs with innovative architectures and stimuli-responsive properties.

Influence of the first normal stress differences on model hydrophobically modified ethoxylated urethane-thickened waterborne paints brush drag

Commercial paints are complex formulations and their application properties are typically explained based on their steady shear viscosity profiles. Hydrophobically modified ethoxylated urethane (HEUR) polymers are widely used as rheology modifiers (RM) in waterborne paints. In this work we formulated a series of HEUR-based model paints with matching high-shear viscosity (0.12 Pa-s at 10,000 s−1). Compositionally these paints were similar, differing only in the HEUR end-hydrophobe length (Cn, n = 10–18) and the amount used to obtain the targeted high-shear viscosity. Their linear viscoelastic response was dominated by a transient network of HEUR-bridged latex particles as revealed from oscillatory shear measurements. The low (steady) shear viscosities were related to the HEUR end-hydrophobe length while the high-shear viscosities tend to approach the targeted value. The first normal stress difference (N1) at high shear changed sign from negative for low Cn (n = 10, 12) to positive for high Cn (n = 16, 18) paints. Further, brush drag was measured in real time where the typical application shear rate was ≈ 10,000 s−1. Paint brush drag was found to increase monotonically with HEUR Cn and this variation/trend could not be explained through their high-shear viscosity. It is proposed that the observed trend can be explained based on the paint N1 sign, where a negative N1 provides a compressive force pulling the brush towards the surface and facilitates paint transfer at relatively lower drag. In contrast, a positive N1 exerts a tensile force tending to pull the brush out from the surface and extra force is required to overcome this effect. This work emphasizes the importance of normal stress (along with viscosity) to understand paint properties, especially for high-shear applications.

A reactive hydrophobically modified ethoxylated urethane (HEUR) associative polymer bearing benzophenone terminal groups: Synthesis, thickening and photo-initiating reactivity

Hydrophobically modified ethoxylated urethanes (HEURs), as a representative class of associative polymers, are widely used as rheology modifiers in waterborne systems such as coatings, emulsions, inks etc. due to their unique associative network structure and finite relaxation behavior in solution. However, conventional HEURs are non-reactive and generally stay free in final cured films, which may induce some damages to the performance of the coatings. To solve the problem, a new concept of reactive HEUR has been proposed. That is, a reactive HEUR should bear reactive hydrophobic end-groups which can chemically crosslink with matrix resins during the curing process of waterborne coatings. In this work, with the help of the hydrogen abstraction photo-initiating reactivity of benzophenone (BP) group, a reactive HEUR (BPC11HEUR) end-functionalized by a photo-initiator BP moiety has been prepared. In aqueous solution, as demonstrated by steady shear measurement, an exponential increment of solution viscosity along with the increasing of polymer concentration (C) was found when C surpassed its critical percolation concentration (Cp ≈ 23.6 g/L). Besides, ultraviolet (UV) light induced photolysis in aqueous solution manifested a typical photochemical reaction behavior of BPC11HEUR polymer with a final photolysis degree (D) of approximate 78% after 300 s of irradiation. In situ 1H NMR and rheological measurements collectively revealed the efficient photo-initiating reactivity of BPC11HEUR for waterborne UV monomers or resins. Further, rheological measurements demonstrated that the emulsion zero/low shear viscosity can be greatly enhanced by ca. 50 folds for a commercially available waterborne UV polyurethane acrylate emulsion when the addition amount of BPC11HEUR varied from 0 to 1.0 wt%. These results shown that the reactive BPC11HEUR polymer can not only be used to efficiently modify the rheological properties of solutions or emulsions but also can chemically crosslink with waterborne UV acrylic monomers or resins by light irradiation. The work may be of interest for development of reactive HEURs used in waterborne UV coatings, inks, and adhesives, etc.

Surfactant modulated interactions of hydrophobically modified ethoxylated urethane (HEUR) polymers with penetrable surfaces

HypothesisAdsorption of hydrophobically modified ethoxylated urethane polymers (HEURs) at the soft colloid interfaces of emulsion droplets will stabilise oil-in-water emulsions (a) via steric stabilisation induced by adsorption of the polymer at the droplet surfaces through the hydrophobic groups, and (b) via continuous phase viscosity enhancement through polymer self-association. Both of these mechanisms will be modulated by the presence of the surfactant, sodium dodecylsulfate (SDS). ExperimentsDodecane-in-water emulsions stabilised by three HEUR polymers with different structural composition were examined in the absence and presence of SDS by NMR spectroscopy and small-angle neutron scattering (SANS). The effect of adsorption of the polymer to the dodecane droplet surfaces, and the conformation of the self-associating polymer in the aqueous solution were quantified. FindingsAll emulsions were stable for days-weeks. Diffusion data showed the formation of oil droplets of hundreds of nm in size in the presence of all three HEURs, here denoted C6-L-(EO100-L)9-C6, C10-L-(EO200-L)4-C10, and C18-L-(EO200-L)7-C18, where EOx represents a block of ethylene oxide of x monomers, L denotes the linker group, and Cn the length of the hydrophobic end-group. No significant changes in droplet size across this series of polymers was observed. Collectively, the results point to adsorption of the polymer to the droplet surfaces, which results in a small decrease in the effective polymer solution concentration, thereby driving to significant changes in the structure and dynamics of the system. Evident in the SANS data in particular, is a subtle balance between the characteristic features reflecting polymer self-association, and those associated with polymer structures commensurate with a larger length-scale, dependent on the system composition. Surprisingly, the polymer and polymer/SDS complex in the presence of oil show slightly greater diffusive rates relative to the analogous systems in the absence of the oil.Finally, the partitioning of the three polymers in phase-separated samples was studied by 1H NMR, and it was shown that the C18-L-(EO200-L)7-C18 exhibited a greater partitioning in the oil phase compared with C6-L-(EO100-L)9-C6 and C10-L-(EO200-L)4-C10, an observation that may be understood in terms of the structural composition of the HEURs. The SDS showed a positive correlation between its partitioning in the two layers with the polymer partitioning, evidence of a strong interaction between the surfactant and the polymer, consistent with the behaviour observed in the oil-free system.

Surfactant modulated interaction of hydrophobically modified ethoxylated urethane (HEUR) polymers with impenetrable surfaces

HypothesisThe presence of surfactant modulates the surface-chemistry-specific interaction of hard colloidal particles (latex) with HEUR polymers, principally through introducing a preferential solution interaction rather than a competitive surface interaction; addition of surfactant leads to a preponderance of polymer/surfactant solution complexes rather than surface-bound complexes. ExperimentsA range of model formulations comprising a hexyl end-capped urethane polymer (C6-L-(EO100-L)9-C6), sodium dodecylsulfate (SDS) and a series of polystyrene-butylacrylate latices (PS-BA-L) have been characterised in terms of rheology, particle surface area (solvent relaxation NMR), polymer conformation (small-angle neutron scattering) and solution composition to build up a detailed picture of the distribution of the HEUR in the presence of both surfactant and latex. FindingsThere is very weak adsorption of C6-L-(EO100-L)9-C6 to only the most hydrophobic latex surface studied, an adsorption that is further weakened by the addition of low levels of surfactant. Macroscopic changes in the hydrophobic latex system may be interpreted in terms of bridging flocculation at low polymer concentrations. No adsorption of C6-L-(EO100-L)9-C6 is observed in the case of hydrophilic surfaces. In most cases, the observed behaviour of the ternary system (polymer/surfactant/particle) is highly reminiscent of the binary (polymer/surfactant) system at the appropriate composition, suggesting that the polymer/surfactant solution interaction is the dominant one.

Oscillatory and Steady Shear Rheology of Model Hydrophobically Modified Ethoxylated Urethane-Thickened Waterborne Paints.

Hydrophobically modified ethoxylated urethane (HEUR) thickeners are widely used as rheology modifiers for waterborne paints. Although the rheology of HEUR solutions in water is fairly well-understood, their impact on the rheology of waterborne latex/pigment suspensions (formulated paints) is more complicated. We study the shear rheology of model HEUR/latex/TiO2 suspensions in water and investigate the dependence of both oscillatory and steady shear behaviors on the strength of the HEUR hydrophobes. We observe that in both oscillatory and steady shear experiments, rheological curves could be shifted onto a single master curve, demonstrating a "time-hydrophobe superposition". We also note that the oscillatory shear behavior exhibits a power-law spectrum of relaxation times, unlike the single-Maxwellian behavior of pure HEUR solutions. On the basis of these results and earlier experimental and theoretical findings, we propose that the rheology of the HEUR-thickened latex/TiO2 suspensions is mainly determined by the transient network of HEUR-bridged latex particles, with a broad distribution of the characteristic lifetimes of the bridge. The model is found to be in good qualitative and semiquantitative agreement with the experiments for both steady shear and oscillatory shear.

Studying the interaction of hydrophobically modified ethoxylated urethane (HEUR) polymers with sodium dodecylsulfate (SDS) in concentrated polymer solutions

HypothesisHydrophobically modified ethoxylated urethane polymers (HEURs) are widely used to control the rheological profile of formulated particulate dispersions through associative network formation, the properties of which are perturbed by the presence of surfactants. At high polymer concentrations and in the presence of surfactants, it is hypothesised that the dominant factors in determining the rheological profile are the number and composition of the mixed hydrophobic aggregates, these being defined by the number and distribution of the hydrophobic linkers along the polymer backbone, rather than the end-group hydrophobe characteristics per se that dominate the low polymer concentration behaviour. ExperimentsThree different HEUR polymers with formulae (C6-L-(EO100-L)9-C6, C10-L-(EO200-L)4-C10 and C18-L-(EO200-L)7-C18 (where L = urethane linker, Cn = hydrophobic end-group chain length, and EO = ethylene oxide block) have been studied in the absence and presence of SDS employing techniques that quantify (a) the bulk characteristics of the polymer surfactant blend, (b) the structure and composition of the hydrophobic domains, (c) the dynamics of the polymer and surfactant, and (d) the polymer conformation. Collectively, these experiments demonstrate how molecular-level interactions between the HEURs and sodium dodecylsulfate (SDS) define the macroscopic behaviour of the polymer/surfactant mixture. FindingsBinding of the SDS to the polymer via two mechanisms – monomeric anti-cooperative and micellar cooperative – leads to surfactant-concentration-specific macroscopic changes in the viscosity. Binding of the surfactant to the polymer drives a conformational rearrangement, and an associated redistribution of the polymer end-groups and linker associations throughout the hydrophobic domains. The composition and size of these domains are sensitive to the polymer architecture. Therefore, there is a complex balance between polymer molecular weight, ethylene oxide block size, and number of urethane linkers, coupled with the size of the hydrophobic end-groups. In particular, the urethane linkers are shown to play a hitherto largely neglected but important role in driving the polymer association.

Influence of substituted structure of Percec-type mini-dendritic end groups on aggregation and rheology of hydrophobically modified ethoxylated urethanes (HEURs) in aqueous solution

Percec-type substituted benzyl alcohol mini-dendrons as new hydrophobic end-cappers can effectively improve the thickening performance of hydrophobically modified ethoxylated urethanes (HEURs) in aqueous solution. To demonstrate the effects of substituted structure of Percec-type mini-dendritic end groups on aggregation and rheological properties of HEURs in aqueous solution and latex, three model HEURs end-functionalized by mono-, di- and tri-substituted benzyl alcohol mini-dendrons were synthesized, respectively. For a reasonable comparison, the total carbon number of substituted alkyl tails is set to be 24 for the three mini-dendritic end groups: HOCH2-Ph-C24H49 (C24), HOCH2-Ph-(C12H25)2 (2C12) and HOCH2-Ph-(C8H17)3 (3C8), which respectively correspond to single-, two- and three-tail according to their substituted degrees. Interestingly, rheological properties of these HEURs in aqueous solution and latex significantly depend on the substituted degree of mini-dendritic hydrophobes. The high-substituted mini-dendritic hydrophobes help HEURs to form a strong physical network with high thickening performance and fast relaxation in both aqueous solution and latex, because shorter alkyl tails in high-substituted dendritic hydrophobes will lead to a smaller aggregation number and a lower rheological activation energy. This work not only provides new insights into the connection of networks in solutions of mini-dendron functionalized HEURs but also offers a new approach to preparation of high performance thickeners used in water-borne systems.

An End-Bifunctionalized Hydrophobically Modified Ethoxylated Urethane Model Polymer: Multiple Stimuli-Responsive Aggregation and Rheology in Aqueous Solution

Hydrophobically modified ethoxylated urethanes (HEURs) belong to an important class of telechelic associative polymers for improving solution rheological properties in many industrial fields. In this work, we describe the synthesis and solution behavior of a novel hydrophobically modified ethoxylated urethane model polymer (AzoFc-HEUR) end-bifunctionalized by Percec-type mini-dendron 3-(6-ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl alcohol. The telechelic polymer containing both azobenzene and ferrocene moieties in aqueous solution performs multiple stimuli-responsive aggregation and rheological properties dependent on the magnitude of amphiphicity change of functional end groups upon exposure to different external stimuli such as light irradiation and/or redox reaction. It has been demonstrated that the stimuli-responsive hydrophobicity change of end groups induces a reversible micellar transition or rearrangement of micellar aggregates in dilute AzoFc-HEUR aqueous solution and a rearrangement of ...