Synergistic effects in flows of mixtures of wormlike micelles and hydroxyethyl celluloses with or without hydrophobic modifications

This work presents experimental results on simple shear and porous media flow of aqueous solutions of two hydroxyethyl celluloses (HEC) and two hydrophobically modified hydroxyethyl celluloses (HMHEC) with different molecular weights. Mixtures of these polymers with a cationic surfactant, cetyltrimethylammonium p-toluenesulfonate (CTAT) were also studied. Emphasis was given to the range of surfactant concentrations in which wormlike micelles are formed. The presence of hydrophobic groups, the effect of the molecular weight of the polymers, the surfactant and polymer concentrations, and the effect of the flow field type (simple shear versus porous media flow) were the most important variables studied. The results show that the shear viscosity of HEC/CTAT solutions is higher than the viscosities of surfactant and polymer solutions at the same concentrations, but surface tension measurements indicate that no complex formation occurs between CTAT and HEC. On the other hand, a complex driven by hydrophobic interactions was detected by surface tension measurements between CTAT and HMHEC. In this case, the viscosity of the mixture increases significantly more (up to four orders of magnitude at high CTAT concentrations) in comparison with HEC/CTAT aqueous solutions. Increments in the molecular weight of the polymers increase the interaction with CTAT and the shear viscosity of the solution, but make phase separation more feasible. In porous media flow, the polymer/CTAT mixtures exhibited higher apparent viscosities than in simple shear flows. This result suggests that the extensional component of the flow field in porous media flows leads to a stronger interaction between the polymer and the wormlike micelles, probably as a consequence of change of conformation and growth of the micelles.     

Shear rheology and porous media flow of wormlike micelle solutions formed by mixtures of surfactants of opposite charge

The rheology of solutions of wormlike micelles formed by oppositely charged surfactant mixtures (cationic cetyl trimethylammonium p-toluene sulfonate, CTAT, and anionic sodium dodecyl sulfate, SDS), in the dilute and semi-dilute regimes, were studied under simple shear and porous media flows. Aqueous mixtures of CTAT and SDS formed homogeneous solutions for SDS/CTAT molar ratios below 0.12. Solutions of mixtures exhibited a strong synergistic effect in shear viscosity, especially in the semi-dilute regime with respect to wormlike micelles, reaching a four order of magnitude increase in the zero-shear rate viscosity for solutions with 20 mM CTAT. Oscillatory shear results demonstrated that the microstructure of CTAT wormlike micelles is sensitive to SDS addition. The cross-over relaxation times of wormlike micelles of 20 mM CTAT solutions increased by three orders of magnitude with the addition of up to 2 mM of SDS, and the solutions became increasingly elastic. The shear thickening process observed in shear rheology became more pronounced in porous media flow due to the formation of stronger cooperative structures induced by the extensional component of the flow.     

Influence of polymer structure on the rheological behavior of hydroxypropylmethylcellulose–sodium carboxymethylcellulose dispersions

Aqueous dispersions of mixtures of hydroxypropylmethylcellulose (HPMC) and sodium carboxymethylcellulose (NaCMC) were prepared in accordance with a two-component simplex lattice design, using polymer varieties with different molecular weights and substitution characteristics. The resulting systems were characterized rheologically by capillary viscometry, flow rheometry, and oscillatory shear techniques, for the determination of kinematic viscosity, index of consistency, index of fluidity, elastic modulus, and viscous modulus. The values obtained for these parameters were fitted with appropriate canonical models, which revealed synergistic effects for some polymer proportions. Maximum synergy was observed when polymer proportions were optimal for the establishment of between-polymer interactions. The synergistic effects on viscosity and elasticity are attributable to the establishment of hydrophobic interactions and hydrogen bonds between HPMC and NaCMC chains, as revealed by IR spectroscopy and modifications in the cloud-point temperature. The observed among-mixture differences in the polymer proportions at which maximum synergy occurs, and the degree of this synergy, are explained by differences in molecular weights and substitution characteristics, and indeed the degree of synergy (as measured by interaction parameters from the fitted canonical models) showed strong dependence on these variables. Microviscosity values, derived from theophylline diffusion data for some of the mixtures, show that the crossover and chain expansion of the polymers in the mixtures (i.e. increased viscosity and elasticity) give rise to a three-dimensional network with greater mesh size and a more hydrophilic microenvironment, favoring solute mobility. Received: 17 July 2000 Accepted: 20 November 2000     

Supramolecular self-assembly of dendronized polymers: reversible control of the polymer architectures through acid-base reactions

Acid-base switchable supramolecular dendronized polyacetylenes (DPAs) with increasing steric bulk on going from generation one [G1] to three [G3], were constructed using multiple self-assembly processes between Fréchet-type [G1]-[G3]-dendritic dialkylammonium salts and a dibenzo[24]crown-8-containing polymer. The formation of the supramolecular systems is acid-base switchable to either an ON (rodlike dendronized polymers) or an OFF (flexible polymers) state. Thus, by controlling the superstructures of the supramolecular polymers with the [G1]-[G3] dendrons, it is possible to induce conformational changes within the polymer backbones. The supramolecular dendronized polymers, as well as their threading-dethreading properties, were characterized by (1)H NMR and UV absorption spectroscopies, gel permeation chromatography (GPC) and light scattering (LS). Independent measures of molecular weight (GPC, LS) indicate that DPAs behave as increasingly rigid macromolecules with each generation in solution. Molecular dynamics simulations of each DPA suggest that the lengths of the polymer backbones increase accordingly. Atomic force microscopy of the [G3]-dendronized polystyrene (DPS), as well as the DPAs, reveal surface morphologies indicative of aggregated superstructures.     

树枝化聚合物的合成、结构表征及其应用

本文综述了由树枝状大分子和线形聚合物结合而形成的一类新型树形聚合物--树枝化聚合物的研究进展,包括树枝化聚合物的各种合成方法、结构表征和形态分析等.同时对树枝化聚合物在催化载体、纳米材料、生化和光电功能材料等领域的应用研究进行了详尽的综述.     

Synthesis of branched polystyrene by ATRP exploiting divinylbenzene as branching comonomer

Branched polystyrenes have been synthesized using atom transfer radical polymerization (ATRP) of styrene in the presence of divinyllbenzene (DVB) as branching comonomer. The synthesis was completed via facile one pot approach. Mole ratio of styrene to DVB in range of 5:1-30:1 was employed to obtain soluble polymers. The kinetics of the polymerization and evolution of polymer compositions were revealed by determining the conversions of reactants by gas chromatography (GC). The growth of molecular weight was monitored by GPC and the results indicate that the branched polymers were formed by self-condensing vinyl polymerization (SCVP) of AB^∗ monomer or macromonomers. The branched structure of the resulting polymers was confirmed by the remarkable discrepancies of the weight average molecular weights determined by GPC and multi angle laser light scattering (MALLS). The specific viscosity of the resulting polymer is also much lower compared with that of linear analogues. The influence of dosage of initiator and catalyst on the yield and molecular weights of the resulting polymers was also investigated.     

Shear rheology of anionic and zwitterionic modified polyacrylamides

Two groups of copolymers were synthesized from high molecular weight polyacrylamides. One group of copolymers consisted of sulfonated, anionic copolymers (PAM-S) of acrylamide with the sodium salt of 2-acryloamido-2-methyl-1-propane sulfonic acid, and the other consisted of zwitterionic copolymers (PAM-Z) of acrylamide with a sulfobetaine methacrylate monomer. The shear rheology of aqueous solutions of the copolymers and their mixtures was studied experimentally. Solutions of both copolymers exhibit shear thinning behavior in the range of concentrations explored. Solutions of mixtures of two copolymers (PAM-Z and PAM-S) exhibited a slight viscosity synergy at high relative contents of PAM-S. Addition of a relatively high concentration of an electrolyte (0.3 M NaCl) induces decreases in viscosity due to coil contraction and eliminates the synergy of the mixtures. Mixtures of the zwitterionic copolymer and a cationic surfactant, cetyl trimethylammonium p-toluene sulfonate (CTAT), were also studied. These solutions exhibit a strong synergistic effect at low-shear rates when the surfactant forms wormlike micelles. In addition, oscillatory shear measurements demonstrate that PAM-Z/CTAT mixtures are significantly more elastic than CTAT solutions, which indicates that PAM-Z is effective in promoting micelle entanglements, as reflected by the increase in relaxation time with PAM-Z content.     

Viscometric properties of dilute polystyrene/dioctyl phthalate solutions

We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 10⁵ ≤ M_w ≤ 3.0 × 10⁶ Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (M_w ≥ 2.0 × 10⁶ Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006     

Phase behavior of cationic hydroxyethyl cellulose-sodium dodecyl sulfate mixtures: effects of molecular weight and ethylene oxide side chain length of polymers

Novel cationic hydroxyethyl cellulose (HEC) polymers with different molecular weights (1.1 x 10(5) to 1.7 x 10(6) g/mol) and ethylene oxide (EO) side chain lengths (1.5-2.9 EO units) were mixed with sodium dodecyl sulfate (SDS) in aqueous solutions. The phase diagrams of cationic HEC-SDS complexes were determined in the dilute polymer concentration regime (< 0.5 wt %) with gradual addition of SDS molecules. The viscosity and structures of the complexes during the phase evolution were studied using rheometry and dynamic light scattering. The gradual addition of SDS first induced interchain associations with the bound SDS aggregates serving as cross-linkers to form an open network structure, producing a very broad size distribution and high viscosities of the complex solutions, and then condensed the network and induced a structure reorganization, resulting in globular aggregates with narrow size distributions. The growth of these globular aggregates in size eventually led to macroscopic sedimentation near charge neutralization. Further addition of SDS randomly broke the sedimentary aggregates into small particles and SDS micelles with low solution viscosities. The effects of molecular weight and EO side chain length of polymers on the phase boundary, viscosity, and structure of cationic HEC-SDS complexes were discussed.     

Molecular weight determinations by gel-permeation chromatography and viscometry

The hydrodynamic volume concept can be used effectively with gel-permeation chromatographic (GPC) and viscosity data to estimate the molecular weight of a variety of polymers. Agreement is within ±5–10% of the absolute values and thus is satisfactory for many purposes. An iterative computer technique and a method developed by Funt and Hornof for analyzing GPC–viscosity data were found to be equivalent with respect to estimating the molecular weights for the five cases studied. The latter is easily employed but restricted to the case where the sample of interest and the GPC calibration standards have approximately equal Mark-Houwink parameters. Since GPC measurements are commonly performed in thermodynamically good solvents, the general applicability of the method is not impaired. Using the unperturbed dimensions of the polymer chain to estimate the molecular weight of a variety of polymers was not as satisfactory as the above techniques. This approach generally gave biased molecular weight values (consistently low or consistently high). Agreement with the absolute values ranged from 10 to 30%. We therefore believe that either of the techniques based on the hydrodynamic volume concept can be used more effectively to estimate the molecular weight of a series of polymers than the treatment based on the unperturbed dimension.     

Synthesis of dendronized polymer brushes containing metallo‐supramolecular polymer side chains

A new kind of dendronized polymer brush with metallo‐supramolecular polymer side chains was fabricated by a combination of macromonomer and graft‐to approach. The alternating copolymers of maleic anhydride and styryl macromonomers pendant with Fréchet‐type dendrons of three generations were reported previously. In this article, terpyridine groups were introduced along the backbone of the dendronized polymers through the amidolysis of anhydride groups. The terpyridine functionalized PEO linear chains were then incorporated through the complexation of terpyridine and Ru(II) ion. Thus, dendronized polymer brushes with amphiphilic properties were synthesized. AFM analysis showed worm‐like single molecular morphologies of the polymers of three generations, and ¹H NMR analysis indicated that such molecular brushes had an amphiphilic nature in solution. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3303–3310, 2007     

Synthesis of dendronized polymers through bergman cyclization of enediyne‐containing Frechet‐type dendrimers

In this article, dendronized polymers with rigid backbones were synthesized from enediyne‐containing Frechet‐type dendrimers. Two generations of dendrimers were conically incorporated with 3‐(2‐(2‐(trimethylsilyl)ethynyl)phenyl)prop‐2‐yn‐1‐ol. The trimethylsilyl protection groups of enediyne units were subsequently removed, and two types of brush polymers with rigid conjugated backbone were prepared through Bergman cyclization polymerization at elevated temperature under vacuum. The dendronized polymers were characterized with GPC, IR, UV–vis, and NMR spectroscopy. Furthermore, the morphology of the dendronized polymer was revealed by atomic force microscopy. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis of narrow‐polydispersity degradable dendronized aliphatic polyesters

The divergent dendronization of an ?‐caprolactone‐based polymer has been performed to provide access to dendronized polymers with sufficient biocompatibility and degradability for use as drug‐delivery scaffolds. The synthesis was performed through the tin(II) 2‐ethylhexanoate‐catalyzed polymerization of a γ‐functionalized ?‐caprolactone monomer, followed by the divergent growth of pendant polyester dendrons at each repeat unit. The resulting dendronized polymers were obtained up to the fourth generation with molecular weights as high as 80,000 Da and with polydispersities between 1.11 and 1.22. The fourth‐generation hydroxyl‐terminated dendronized polymer was degradable under a variety of aqueous conditions. A comparison of the dendronization approach with a procedure involving the ring‐opening polymerization of a second‐generation dendritic macromonomer reveals that the former procedure is best suited for the preparation of this family of dendronized polyesters because it requires shorter reaction times and affords materials with higher degrees of polymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3563–3578, 2004     

Solution characterization of the novel organometallic polymer poly(ferrocenyldimethylsilane)

A series of four well‐defined poly(ferrocenyldimethylsilane) (PFS) samples spanning a molecular weight range of approximately 10,000–100,000 g mol⁻¹ was synthesized by the living anionic polymerization of dimethyl[1]silaferrocenophane initiated with n‐BuLi. The polymers possessed narrow polydispersities and were used to characterize the solution behavior of PFS in tetrahydrofuran (THF). The weight‐average molecular weights (M_w ) of the polymers were determined by low‐angle laser light scattering (LALLS), conventional gel permeation chromatography (GPC), and GPC equipped with a triple detector (refractive index, light scattering, and viscosity). The molecular weight calculated by conventional GPC, with polystyrene standards, underestimated the true value in comparison with LALLS and GPC with the triple detection system. The Mark–Houwink parameter a for PFS in THF was 0.62 (k = 2.5 × 10⁻⁴), which is indicative of fairly marginal polymer–solvent interactions. The scaling exponent between the radius of gyration and M_w was 0.54, also consistent with marginal polymer–solvent interactions for PFS in THF. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3032–3041, 2000     

树枝化对聚芴类分子发光行为的影响

运用稳态吸收、稳态荧光光谱及皮秒时间分辨荧光光谱等手段, 研究了树枝化对聚芴分子的光谱动力学行为的影响. 在1×10-5 mol/L四氢呋喃溶液中, 非树枝化聚芴和不同代数的树枝化聚芴的稳态吸收和稳态荧光光谱基本一致, 表明树枝化不影响聚芴基团的本征电子性质; 在薄膜状态下, 紧密排列的聚芴分子表现出明显的链间聚集行为. 不同代数树枝化聚芴的时间分辨荧光动力学研究结果表明, 溶液状态下的发光为单指数衰减行为, 而薄膜状态下的发光为多指数衰减行为, 且其发光寿命随树枝化代数增加而增长, 表明树枝化基团抑制了邻近的聚芴分子之间能量传递所致的激发态能量耗散.     

Synthesis of Dendronized Aromatic Polyamides with Chloromethyl Groups in the Periphery by Macromonomer Approach

The preliminary results for one-pot synthesis of dendronized aromatic polyamides with chloromethyl groups in the periphery by macromonomer approach were reported first time. The GPC analysis of the resulting dendronized polymers has shown typical weight average molecular weight (Mw) of 41554 and a polydispersity of 2.74.     

Molecular Dynamics of Polymer Composites Using Rheology and Combined RheoNMR on the Example of TiO2-Filled Poly(n-Alkyl Methacrylates) and Trans-1,4-Polyisoprene

The determination of the interplay between polymeric matrices and filler particles in composites is of great interest to understand structure-property relationships and develop predictive theories. To study the molecular dynamics of polymers in composites, model systems based on poly(n-alkyl methacrylates), trans-1,4-polyisoprene (gutta percha), and titania (TiO₂) were prepared and characterized using rheometry and a combined RheoNMR technique. Apparent entanglement molecular weights were obtained from small amplitude oscillatory shear (SAOS) experiments, which are related to the increasing physical cross-link density as a function of the filler content. Large amplitude oscillatory shear (LAOS) experiments were performed and analyzed within the FT-rheometry framework. The filler had a strong impact on the scaling behavior of the normalized third harmonic. A combined RheoNMR technique was used to simultaneously study the molecular dynamics via NMR and the corresponding mechanical response via rheometry. A strong correlation between the macroscopic mechanical properties and microscopic molecular dynamics was found, which might lead to a new understanding of polymer melt dynamics.     

A dendronized polymer is a single-molecule glass

The molecular architecture of dendronized polymers can be tuned to obtain nanoscale objects with desired properties. In this paper, we bring together experiments and computer simulations to study the thermodynamic and dynamic properties of a single dendronized polymer chain. We find that, upon changing certain architectural features, dynamic correlations characterizing backbone conformational fluctuations of a dendronized polymer exhibit dynamics akin to glass-forming bulk liquids. Thus, a dendronized polymer chain is a novel macromolecule that is a single-molecule glass. Over a range of conditions that lead to glassy dynamics, there does not appear to be any thermodynamic singularities. We discuss how a dendronized polymer is a molecular system that can directly test different models of glassy dynamics. We also show that defect densities characteristic of typical synthesis conditions do not alter the material properties of dendronized polymers.     

Synthesis of an anionically chargeable, high-molar-mass, second-generation dendronized polymer and the observation of branching by scanning force microscopy

An efficient synthesis of a methacrylate-based, second-generation (G2) dendronized macromonomer and its free radical polymerization to the corresponding high-molar-mass G2 dendronized polymer are described. The molar mass is determined by gel permeation chromatography (GPC), light-scattering, and analytical ultracentrifugation and compared with values estimated from a scanning force microscopy (SFM) contour lengths analysis of individualized polymer strands on mica. The polymer carries terminal tert-butyl-protected carboxyl groups, the degree of deprotection of which with trifluoroacetic acid is quantified by NMR spectroscopy using the highest molar mass sample. SFM imaging of both protected (noncharged) and unprotected (charged) dendronized polymers on solid substrates reveals mostly linear chains but also some with main-chain branches. The nature of these branches is investigated and the degree roughly estimated to which they are formed. Finally, a synthetic model experiment is described which sheds some light on the aspect of whether chain transfer, a process that could lead to covalent branching, is of importance in the synthesis of the present dendronized polymers.     

Water‐soluble polymers from acid‐functionalized norbornenes

Unprotected exo,exo‐5‐norbornene‐2,3‐dicarboxylic acid and exo,exo‐7‐oxa‐5‐norbornene‐2,3‐dicarboxylic acid were polymerized via ring‐opening metathesis polymerization. This reaction yielded polymers with molecular weights (M_n from GPC) ranging from 31 to 242 kg/mol and polydispersity indices between 1.05 and 1.12, using Grubbs' third generation catalyst. The water solubility as a function of pH value of the polymers was investigated by dynamic light scattering (DLS). DLS and acid‐base titration revealed that the oxanorbornene polymer was water soluble over a wider pH range than its norbornene analog. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1266–1273, 2009