Microscopic observation of ordered colloids in sedimentation equilibrium and the important role of Debye screening length. 3. Heavy and monodisperse polystyrene type spheres in aqueous solution of neutral polymers

The ordered structures are observed directly by the metallurgical microscope for the heavy (specific gravity=1.50) and monodisperse polystyrene type spheres in sedimentation equilibrium and in the presence of neutral polymers, polyvinylalcohol (PVA), polyethyleneglycol (PEG), polyvinyl-pyrrolidone (PVP), hydroxypropylcellulose (HPC), and polyacrylamide (PAA_m). The intersphere distances in the ordered lattices decrease by the addition of neutral polymer in the order, PVA < PEG < PVP < HPC < PAA_m. These effects of neutral polymers are reasonably well-explained by the intersphere repulsion from the electrical double layer interaction, which is influenced by the polymer adsorption on the latex surface by the hydrophobic and/or dipole-dipole interactions.     

Monte Carlo simulations of the exchange kinetics of polymers adsorbed on a solid surface

The exchange kinetics of polymers adsorbing on a solid surface is extensively studied by dynamic Monte Carlo simulations. A model employed simulates a semidilute polymer solution placed in contact with a solid surface that attracts polymer segments by the adsorption interaction (χ_s). The exchange process of polymer chains, between the solution and the adsorbed polymer layer, is examined under various conditions. The exchange kinetics shows two characteristic regimes with increasing chain length. One is the diffusion‐controlled regime found with a small χ_s , and the other the detachment‐controlled regime with a large χ_s . These two regimes are well described by a kinetic theory. Various dynamic quantities show that the diffusion‐controlled regime is not due to sluggish dynamics near the surface, but rather to bulk diffusion of chains. The diffusion‐controlled regime found in this study is considered to appear at the high temperature limit.     

Adsorption Mechanism of Comb Polymer Dispersants at the Cement/Water Interface

Comb polymers are commonly used as dispersants to stabilize highly concentrated cement suspensions. The effectiveness of such polymeric additives to stabilize these suspensions is determined to a large extent by the amount adsorbed. In this study we investigated the adsorption characteristics of various comb dispersant containing different graft densities on surfaces of cement particle. The effect of inorganic salts on their adsorption was also examined in order to elucidate their adsorption mechanism. The results show that the adsorption of comb polymer dispersants on cement surface conforms approximately to Langmuir's adsorption isotherm and the characteristic plateau A _s and adsorption free energy ΔG_ads are largely dependent on the anionic group content of the comb polymers. The A _s and ΔG_ads increase with increasing anionic group content. This information suggests that the adsorption of comb polymers on cement surfaces is dominated by electrostatic interaction between COO-groups on the comb polymers and the positive surface of the cement. This conclusion is supported by effects of inorganics such as calcium and sulfate ions, and diffuse reflectance FTIR spectroscopy. The implication of results for tuning polymers for the required performance in cement manufacture should be noted.     

Biodegradable thermo‐ and pH‐responsive hydrogels for oral drug delivery

In this article, novel smart hydrogels based on biodegradable pH sensitive poly(L ‐glutamic acid‐g‐2‐hydroxylethyl methacrylate) (PGH) chains and temperature‐sensitive hydroxypropylcellulose‐g‐acrylic acid (HPC‐g‐AA) segments were designed and synthesized. The influence of pH and temperature on the equilibrium swelling ratios of the hydrogels was discussed. The optical transmittance of the hydrogels was also changed as a function of temperature, which reflecting that the HPC‐g‐AA part of the hydrogels became hydrophobic at the temperature above the lower critical solution temperature (LCST). At the same time, the LCST of the hydrogels had a visible pH‐dependent behavior. Scanning electron microscopic analysis revealed the morphology of the hydrogels before and after enzymatic degradation. The biodegradation rate of the hydrogels was directly related to the PGH content and the pH value. The in vitro release of bovine serum albumin from the hydrogels were investigated. The release profiles indicated that both the HPC‐g‐AA and PGH contents played important roles in the drug release behaviors. These results show that the smart hydrogels seem to be of great promise in pH–temperature oral drug delivery systems. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Sol-Gel Synthesis and Phase Evolution Behavior of Sterically Stabilized Nanocrystalline Zirconia

Nanocrystalline as well as submicron sized, non-agglomerated, spherical ZrO₂ particles have been successfully synthesized using the sol-gel technique utilizing hydroxypropyl cellulose (HPC) as a polymeric steric stabilizer. The effect of various parameters such as the ratio of molar concentration of water and alkoxide (R), the molar concentration [HPC] and the molecular weight (MW_HPC) of HPC polymer as well as the calcination temperature on ZrO₂ nanocrystallites size and their phase evolution behavior is systematically studied. The phase evolution behavior of nanocrystalline ZrO₂ is explained and correlated with the adsorption behavior of HPC polymer on ZrO₂ nanoparticles surface, which is observed to be a function of R, [HPC], MW_HPC and the calcination temperature. Optimum synthesis parameters for obtaining 100% tetragonal phase in nanocrystalline ZrO₂ are identified for the present sol-gel method of synthesizing nanoparticles.     

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (VB _1s ).     

Determination of surface area of copolymer latex particles by surface tension measurements

The surface tension γ of four copolymer latices, of their respective serums, and of aqueous solutions of dispersant alone were measured at various dilutions. By extrapolating the surface excess of dispersant (calculated by the Gibbs adsorption equation) both at the aqueous solution surface and at the serum surfaces to 1/c = 0 (c being the bulk concentration of dispersant) the same limiting site area A_lim per adsorbed molecule was determined. Amounts of dispersant adsorbed by copolymer particles at various dilutions were determined from differences between the known total concentrations of the dispersant in latex and in serum at the same γ. These values were then extrapolated to the maximum adsorption at 1/c = 0 in latex. The surface area of copolymer particles was determined therefrom by using A_lim. The average particle radius calculated this way agrees reasonably well with electron microscope measurements. Thus it appears that the method for determining latex particle surface area by surfactant titration may be calibrated by means of the Gibbs adsorption equation, provided one uses A_lim and not the site area at the critical micelle concentration of dispersant.     

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (V?B _1s ).     

Interfacial profiles of miscible poly(4‐trimethylsilylstyrene)/polyisoprene bilayer films

Compositional profiles of bilayer films in the direction normal to the interfaces have been investigated by neutron reflectivity measurements and analyzed with mean field theory. The bilayer films were prepared with poly(4‐trimethylsilylstyrene) (PTMSS) and polyisoprene (PI), which constitute a miscible polymer pair and whose blends show phase separation at the lower critical solution temperature (LCST) by heating. Because we can accurately control the degree of polymerization of component polymers and can adjust the Flory–Huggins interaction parameter, χ, with the temperature, T, according to the relationship χ = 0.027–9.5/T, the phase behavior and the interfacial structure of PTMSS and PI are predictable by mean field theory. When the bilayer films of PTMSS and PI were set at 90 °C, which is a temperature below the LCST, diffusion at the interface was observed, and the original interface disappeared in several hours; this supports the idea that the polymer pair is miscible. No clear interfaces were identified below the LCST, whereas broad interfaces, compared with that of the strong segregation pairs, were observed above the LCST. The compositions of each layer are consistent with that of the coexisting phase in the polymer blends, and the interfacial widths agree well with the theoretical prediction considering the effect of capillary waves. In addition, all annealed films have a thin surface layer of PTMSS corresponding to surface segregation induced by the lower surface energy of PTMSS (with respect to that of PI). Thus, the interfacial profiles of PTMSS/PI bilayer films have been totally prospected in the framework of mean field theory. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1486–1494, 2005     

Surfactant effect on the lower critical solution temperature of poly(organophosphazenes) with methoxy-poly(ethylene glycol) and amino acid esters as side groups

 The surfactant effect on the lower critical solution temperature (LCST) of thermosensitive poly(organophosphazenes) with methoxy-poly(ethylene glycol) and amino acid esters as side groups was examined in terms of molecular interactions between the polyphosphazenes and surfactants including various anionic, cationic, and nonionic surfactants in aqueous solution. Most of the anionic and cationic surfactants increased the LCST of the polymers: the LCST increased more sharply with increasing length and hydrophobicity of the hydrophobic part of the surfactant molecule. The ΔLCSTs (T _0.03M− T _0M), the change in the LCST by addition of 0 and 0.03 M sodium dodecyl sulfate (SDS), were found to be 7.0 and 14.5 °C for the polymers bearing ethyl esters of glycine and aspartic acid, respectively. The LCST increase of poly(organophosphazene) having a more hydrophobic aspartic acid ethyl ester was 2 times larger compared with that of the polymer having glycine ethyl ester as a side group. The binding behavior of SDS to the polymer bearing glycine ethyl ester as a hydrophobic group was explained from the results of titration of the polymer solutions containing SDS with tetrapropylammonium bromide. Graphic models for the molecular interactions of polymer/surfactant and polymer/surfactant/salt in aqueous solutions were proposed. Received: 17 February 2000/Accepted: 25 April 2000     

Adsorption of cationic polyelectrolyte onto a model carboxylic latex and the influence of adsorbed polycation on the charge regulation at the latex surface

 Adsorption of a well-characterized cationic polyacrylamide (CPAM) onto the surface of a model colloid (monodisperse polystyrene latex with carboxylic functional groups) was studied over a wide range of pH (4–9) and KCl concentration (c _s =10^-3–0.3 M). The surface charge density of the latex particles with and without adsorbed CPAM was also measured over the same range of electrolyte compositions. The adsorbed amount of CPAM increases with increase in c _s and pH. The polyelectrolyte adsorption alters substantially the surface charge density of the latex particles as compared to the polymer-free case. A large overcompensation of the surface charge by the adsorbed polyelectrolyte is established at high c _s and low pH. A qualitative explanation of the observed features is put forward. Received: 3 December 1996 Accepted: 20 January 1997     

A Review of Temperature Influence on Adsorption Mechanism and Conformation of Water Soluble Polymers on the Solid Surface

The temperature effect on adsorption behavior of water soluble polymer and structure of its adsorption layer formed on the solid surface is presented. The main reasons for such problem explanation are wide application possibilities of stabilization-flocculation properties of polymers in many technological and ecological processes and very poor knowledge about temperature dependences of high-molecular compounds adsorption on the solid surface. To systematize the information about the effect of temperature on polymer chain conformation on the adsorbent surface the following aspects were considered: thermodynamic and physicochemical characteristics of polymer solutions, stabilization-flocculation properties of polymers in the dispersed systems, and presentation of respective experimental results. The theoretical and experimental evidence of the temperature effect on polymer macromolecule conformation on the solid surface presented in the article can contribute to better knowledge and understanding of the adsorption process at the metal oxide-polymer solution interface in the temperature function.     

Adsorption of poly(N-isopropylacrylamide) on glass substrata

The adsorption of poly(N-isopropylacrylamide) (PNIPAAM), a well known thermosensitive polymer, on glass was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The polymer was dissolved in water at low (0.02 g/L) and high (2 g/L) concentration and the tested temperatures were below (25 degrees C) and above (50 degrees C) the lower critical solubility temperature (LCST). Whatever the conditions, a smooth layer of adsorbed molecules spread along the surface was observed. The thickness was about twice higher for high concentration compared to low concentration. The cohesion in the adsorbed layer, as revealed by scraping tests performed by AFM, was higher above the LCST than below the LCST. On top of this adsorbed layer, single-chain coils, globules, or aggregates were present, depending on concentration and temperature. The observation of these additional adsorbed entities was poorly reproducible, presumably due to the lack of shear control upon rinsing. These results emphasize the importance of the characterization of surface morphology to interpret amounts of adsorbed polymers.     

Dissolution equation of polymer

The dissolution of the polar polymer, polyacrylamide (PAM), in water is studied kinetically. The rate of dissolution, ?, of PAM is determined by means of measuring the conductivity of water as dissolution of the polymer proceeds. The thickness of the swollen surface layer, δ, and the rate of swelling, ?, are determined with the aid of penetration and followed surface track by conductometric electrode. A kinetic equation reflecting PAM dissolution in water is obtained directly as 2? + ? = D?_s/δ, where D?_s is the average diffusion coefficient of solvent in polymer and is shown to be true in cases of nonpolar as well as polar polymers. © 1996 John Wiley & Sons, Inc.     

Mesophases in a Gel from Hydroxypropyl Cellulose/Polyacrylamide

The objective of the present work is to analyze the different mesophases observed in a gel synthesized from hydroxypropyl cellulose (HPC) and polyacrylamide (PAAm) and its phase behavior in water. Hydroxypropyl cellulose is a material derived from cellulose it is non toxic and degradable. HPC is widely used in pharmaceutics, food additives, stabilizer, thickener, etc., HPC is approved by the FDA for use in the food industry. HPC has the characteristic that it forms liquid crystals, depending on the solvent and on the concentration of the polymer. In this work a lower critical solution temperature (LCST) and an upper critical solution temperature (UCST) were observed, the resulting gel showed anisotropic, nematic and cholesteric phases in water, the phase formed depended on the concentration of the polymer.     

Investigation of the swelling behavior of hydrogels derived from high-molecular-weight poly(2-ethyl-2-oxazoline)

Thermoresponsive hydrogels are of great importance as smart materials. They are usually composed of cross-linked polymers with a lower critical solution temperature (LCST). Although much is known about networks of poly(N-isopropylacrylamide), all other polymers are somewhat neglected. In this work, the temperature-dependent swelling behavior of differently cross-linked thermoresponsive poly(2-ethyl-2-oxazoline) (PEtOx) hydrogels were investigated with regard to varying parameters of the network composition. It was found that the degrees of swelling of the hydrogels converge for a certain polymer/solvent system at a distinct temperature independent of its degree of cross-linking. Furthermore, this temperature correlates with the LCST of the respective starting PEtOx. Its net chain molecular weight M_c only affects the maximum degree of swelling and thus, the swelling–deswelling rate of the hydrogel. The fundamental structure/property relations found in this study could be useful to predict the behavior of other thermoresponsive hydrogels.     

Fabrication of thermoresponsive micelles by functionalizing hydroxypropyl cellulose via azide-alkyne click reaction

In this work, we functionalized hydroxypropyl cellulose (HPC) by attaching tetraphenylethylene (TPE) via copper-catalyzed azide-alkyne cycloaddition (CuAAC). The obtained HPC-TPE samples displayed water-solubility, biocompatibility, fluorescence and thermoresponsive properties. The degree of substitution (DS_TPE) of HPC-TPE1 ~ 4 was determined to be 0.002, 0.006, 0.025, and 0.053, respectively. HPC-TPE could self-assemble into micelles in water with the hydrodynamic radius (R_h) ranging from 164 to 190 nm. Under different DS_TPE, HPC-TPE samples showed different lower critical solution temperature (LCST) behaviors in light transmittance, R_h and fluorescence. The critical transition temperatures in light transmittance for HPC-TPE1 ~ 4 solutions were 55–49 °C during the heating process, and were 44–40 °C during the cooling process, respectively. Moreover, HPC-TPE demonstrated a rapid and sensitive response to Fe³⁺ with ignoring interferences in the presence of other common metal ions, and could also be used to image 4T1 cells. Therefore, this work offered a general approach for the synthesis of functionalized polymers with promising applications in sensing and bioimaging.     

Thermoresponsive Polymer Brush Modulation on the Direction of Motion of Phoretically Driven Janus Micromotors

We report a thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) brush functionalized Janus Au–Pt bimetallic micromotor capable of modulating the direction of motion with the change of the ambient temperature. The PNIPAM@Au–Pt micromotor moved along the Au–Pt direction with a speed of 8.5 μm s^?1 in 1.5 % H₂O₂ at 25 °C (below the lower critical solution temperature (LCST) of PNIPAM), whereas it changed the direction of motion (i.e., along the Pt–Au direction) and the speed decreased to 2.3 μm s^?1 at 35 °C (above LCST). Below LCST, PNIPAM brushes grafted on the Au side were hydrophilic and swelled, which permitted the electron transfer and proton diffusion on the Au side, and thus the motion is regarded as a self‐electrophoretic mechanism. However, PNIPAM brushes above LCST became hydrophobic and collapsed, and thus the driving mechanism switched to the self‐diffusiophoresis like that of Pt‐modified Janus silica motors. These motors could reversibly change the direction of motion with the transition of the hydrophobic and hydrophilic states of the grafted PNIPAM brushes. Such a thermoresponsive polymer brush functionalization method provides a new strategy for engineering the kinematic behavior of phoretically driven micro/nanomotors.     

Preparation of thermo- and redox-responsive branched polymers composed of three-armed oligo(ethylene glycol)

We herein report the preparation of thermo- and redox-responsive branched polymers by the condensation reaction of three-armed oligo(ethylene glycol) (trisOEG) and cystamine (CA). The prepared branched polymers exhibited a soluble–insoluble transition at a lower critical solution temperature (LCST) and formed coacervate droplets through a liquid–liquid phase separation process. We then demonstrated control of the LCSTs of the branched polymers by varying the feed ratio of CA and the surrounding salt concentration close to body temperature. In addition, the trisOEG-cys _x polymer formed coacervate droplets above the LCST, in which hydrophobic molecules were condensed. The redox response of the branched polymers was also investigated. Interestingly, the branched polymers degraded to low-molecular-weight materials (i.e., trisOEG) in the presence of dithiothereitol as a reducing agent through cleavage of the disulfide bond of CA. This facile preparation of branched polymers is expected to be valuable in the context of functional biomedical materials and modifiers for materials surfaces, such as the bases for drug delivery carriers and separation materials. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2623–2629     

Converting Poly(Methyl Methacrylate) into a Triple-Responsive Polymer

Multiresponsive polymers that can respond to several external stimuli are promising materials for a manifold of applications. Herein, a facile method for the synthesis of triple-responsive (pH, temperature, CO₂) poly(N,N-diethylaminoethyl methacrylamide) by a post-polymerization amidation of poly(methyl methacrylate) (PMMA) is presented. Combined with trivalent counterions ([Fe(CN)₆]³⁻) both an upper and lower critical solution temperature (UCST/LCST)-type phase behavior can be realized at pH 8 and 9. PMMA and PMMA-based block copolymers are readily accessible by living anionic and controlled radical polymerization techniques, which opens access to various responsive polymer architectures based on the developed functionalization method. This method can also be applied on melt-processed bulk PMMA samples to introduce functional, responsive moieties at the PMMA surface.