Kaizheng Zhu

Friction in aqueous media tuned by temperature-responsive polymer layers

An atomic force microscope colloidal probe technique has been employed to probe normal and friction forces between silica surfaces coated with adsorbed layers of a diblock copolymer of the composition poly(N-isopropylacrylamide)48-block-poly(3-acrylamidopropyl)trimethylammonium chloride)20, abbreviated PNIPAAM48-b-PAMPTMA(+)20. The interactions between the PNIPAAM48-b-PAMPTMA(+)20-coated surfaces across a 0.1 mM NaCl (pH 6) solution at 25 °C are purely repulsive, due to a combination of steric and electrostatic double-layer forces. However, when the temperature is increased to 35 °C, and subsequently to 45 °C, an attractive force develops at short separations due to the unfavourable PNIPAAM–water interaction at these temperatures. The temperature-dependent polymer–water interaction has implications for the friction force between the layers. At 25 °C a frictional force that increases linearly with increasing load is observed once the surfaces are brought into close contact. At higher temperatures significantly higher friction forces appear as a consequence of attractive segment–segment interactions. Further, a clearly expressed hysteresis between friction forces encountered on loading and unloading is detected. Our results demonstrate that both normal and friction forces between surfaces can be controlled by temperature changes when temperature-responsive polymers are employed, and friction forces can be adjusted as required from low to high.

Effects of temperature and pH on the contraction and aggregation of microgels in aqueous suspensions.

Chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM with different amounts of acrylic acid groups (PNIPAM-co-PAA) were synthesized and the temperature-induced aggregation behaviors of aqueous suspensions of these microgels were investigated mainly with the aid of dynamic light scattering (DLS) and turbidimetry. The DLS results show that the particles at all conditions shrink at temperatures up to approximately the lower critical solution temperature (LCST), but the relative contraction effect is larger for the microgels without acid groups or for microgels with added anionic surfactant (SDS). A significant depression of the cloud point is found in suspensions of PNIPAM with very low concentrations of SDS. The compression of the microgels cannot be traced from the turbidity results, but rather the values of the turbidity increase in this temperature interval. This phenomenon is discussed in the framework of a theoretical model. At temperatures above LCST, the size of the microgels without attached charged groups in a very dilute suspension is unaffected by temperature, while the charged particles (pH 7 and 11) continue to collapse with increasing temperature over the entire domain. In this temperature range, low-charged particles of higher concentration and particles containing acrylic acid groups at low pH (pH 2) aggregate, and macroscopic phase separation is approached at higher temperatures. This study demonstrates how the stabilization of microgels can be affected by factors such as polymer concentration, addition of ionic surfactant to particles without charged acid groups, amount of charged groups in the polymer, and pH.

Temperature-induced formation and contraction of micelle-like aggregates in aqueous solutions of thermoresponsive short-chain copolymers.

A combination of turbidity, light scattering, and steady shear viscosity experiments has revealed that aqueous solutions of an amphiphilic diblock copolymer or a negatively charged triblock copolymer, both containing poly(N-isopropylacrylamide), can undergo a temperature-induced transition from loose intermicellar clusters to collapsed core-shell nanostructures. Turbidity, light scattering, and viscosity results of these short-chain copolymers disclose transition peaks at intermediate temperatures. At high temperatures, the compact core-shell particles from the diblock copolymer aggregate, whereas no renewed interpolymer association is observed for the triblock copolymer or for the solution of the diblock copolymer with added sodium dodecyl sulfate because the electrostatic repulsive interactions suppress the tendency of forming interpolymer clusters. The temperature-induced building up of intermicellar structures and the formation of large aggregates at high temperature in the solution of the diblock copolymer is significantly reduced under the influence of high shear rates.

Temperature-dependent optical properties of gold nanoparticles coated with a charged diblock copolymer and an uncharged triblock copolymer.

We demonstrate that the optical properties of gold nanoparticles can be used to detect and follow stimuli-induced changes in adsorbed macromolecules. Specifically, we investigate thermal response of anionic diblock and uncharged triblock copolymers based on poly(N-isopropylacrylamide) (PNIPAAM) blocks adsorbed onto gold nanoparticles and planar gold surfaces in a temperature range between 25 and 60 degrees C. By employing a palette of analytical probes, including UV-visible spectroscopy, dynamic light scattering, fluorescence, and quartz crystal microbalance with dissipation monitoring, we establish that while the anionic copolymer forms monolayers at both low and high temperature, the neutral copolymer adsorbs as a monolayer at low temperatures and forms multilayers above the cloud point (T(C)). Raising the temperature above T(C) severely affects the optical properties of the gold particle/polymer composites, expelling associated water and altering the immediate surroundings of the gold nanoparticles. This effect, stronger for the uncharged polymer, is related to the amount of polymer adsorbed on the surface, where a denser shell influences the surface plasmon band to a greater degree. This is corroborated with light scattering experiments, which reveal that flocculation of the neutral polymer-coated particles occurs at high temperatures. The flocculation behavior of the neutral copolymer on planar gold surfaces results in multilayer formation. The observed effects are discussed within the framework of the Mie-Drude theory.

Rheological and structural aspects on association of hydrophobically modified polysaccharides

This review covers recent rheological and structural advances in the interactions between hydrophobically modified polysaccharides (HMP) and surfactants or cyclodextrin compounds in aqueous media. Depending on the surfactant concentration, mixtures of HMPs and a surfactant can form strong associating complexes or disrupted networks at high levels of surfactant addition. By adding cyclodextrin monomers to semidilute solutions of HMPs, the hydrophobic interactions can be deactivated and a looser network is formed. For HMPs in the presence of cyclodextrin polymers, network structures with intriguing morphological and rheological features can be constructed. These complex fluids are of potential interest in the development of systems for drug delivery formulations. These rheology modifiers are also of current interest in many other technological applications.

Temperature-induced intermicellization of "hairy" and "crew-cut" micelles in an aqueous solution of a thermoresponsive copolymer.

Temperature-induced intermicellar structures in aqueous solutions of the thermoresponsive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEGn-b-NIPAAM71) copolymer that exhibit a lower critical solution temperature were studied by means of turbidimetry, dynamic light scattering (DLS), shear viscosity, and rheo small-angle light scattering (rheo-SALS) methods. The length of the hydrophilic chains (MPEG) of the copolymer varies from n=0 to n=114. It is shown that this change has a major impact on the temperature-induced association behavior of the polymer in solution. The turbidity results at quiescent conditions revealed a transition peak in the turbidity curve at intermediate temperatures, and this peak as well as the cloud point is shifted toward higher temperatures with increasing length of the hydrophilic chains of the copolymer. The DLS measurements disclosed a fast and a slow relaxation mode, which both are diffusive. From the fast and slow relaxation times the sizes of unimers/micelles and intermicellar clusters, respectively, can be determined. The temperature-induced aggregation is less pronounced in solutions of copolymers with long hydrophilic chains, and the intermicellar structures exhibit an interesting transition at intermediate temperatures. In the shear viscosity measurements large association complexes are formed at high temperatures and at low shear flow for the polymers with short hydrophilic chains, whereas at high shear rates breakup of interaggregate chains was observed. For the copolymer with the highest number of hydrophilic chains (n=114), a novel transition peak was found in the viscosity data. The rheo-SALS results divulged shear-induced structural changes of the association complexes at elevated temperatures. For copolymers with short hydrophilic chains, shear-induced disruption of association complexes was found at higher temperatures, whereas for hairy micelles augmented shear flow promoted the growth of complexes.

Effects of Temperature and Salt Addition on the Association Behavior of Charged Amphiphilic Diblock Copolymers in Aqueous Solution

Effects of temperature on the association behavior in aqueous solutions of a series of charged thermoresponsive poly(N-isopropylacrylamide)-block-poly((3-acrylamidopropyl) trimethylammonium chloride) (abbreviated as PNIPAAM(n)-b-PAMPTMA(+)(20)) with different lengths of the PNIPAAM block (n = 24, 48, and 65) have been studied with the aid of turbidimetry, zeta sizer, and dynamic light scattering (DLS). The turbidity results show that the transition to high turbidity values is shifted to lower temperatures when the length of the PNIPAAM block increases. It was observed that the value of the cloud point (CP) dropped with increasing polymer concentration, enlarged length of the PNIPAAM block, and augmented salinity. It was found that the decay of the correlation function from DLS is bimodal at temperatures well below CP, where the fast mode represents the motion of the unimers and the slow mode the dynamics of micelles/intermicellar complexes. At higher temperatures, larger particles of the system grow at the expense of the smaller ones in the spirit of Ostwald ripening, and clusters with a narrow size distribution evolve at high temperatures. By adding salt (NaCl), enhanced aggregation occurs at elevated temperatures because of screening of Coulomb repulsions.

Effects of temperature and salt concentration on the structural and dynamical features in aqueous solutions of charged triblock copolymers.

Effects of temperature and salt addition on the association behavior in aqueous solutions of a series of charged thermosensitive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide)-block-poly(4-styrenesulfonic acid sodium) triblock copolymers (MPEG(45)-b-P(NIPAAM)(n)-b-P(SSS)(22)) with different lengths of the PNIPAAM block (n=17, 48, and 66) have been studied with the aid of turbidity, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). Increasing temperature and salinity as well as longer PNIPAAM blocks are all factors that promote the formation of association structures. The SAXS data show that, for the copolymers with n=48 and n=66, increasing temperature and salt concentration induce interchain associations and higher values of the aggregation number, whereas no aggregation was observed for the copolymer with the shortest PNIPAAM chain. However, DLS measurements reveal the presence of larger association clusters. The cloud point is found to decrease with raising salinity and longer PNIPAAM block. The general picture that emerges is the delicate interplay between repulsive electrostatic forces and hydrophobic interactions and that this balance can be tuned by changing the temperature, salinity, and the length of the PNIPAAM block.

Characterization of temperature-induced association in aqueous solutions of charged ABCBA-type pentablock tercopolymers

In this work, two novel linear ABCBA pentablock tercopolymers, which are composed of an anionic poly(4-styrenesulfonic acid sodium) (PSSS) block on both ends and two thermosensitive poly(N-isopropylacrylamide) (PNIPAAM) blocks that are separated by a hydrophilic poly(ethylene glycol) (PEG) block have been synthesized via a simple “one-pot” ATRP procedure. The two studied samples have the following compositions: (P(SSS)12-b-P(NIPAAM)63-b-P(EG)34-b-P(NIPAAM)63-b-P(SSS)12 (C34) and P(SSS)14-b-P(NIPAAM)65-b-P(EG)77-b-P(NIPAAM)65-b-P(SSS)14 (C77). The main difference between the copolymers is the length of the hydrophilic block, which plays a crucial role for the physical properties of the copolymers in aqueous solution. A solution of the C34 sample becomes turbid at elevated temperatures and compact interchain complexes are formed with a behavior that is reminiscent of a suspension of particles. A solution of C77 becomes turbid at intermediate temperatures but it is transformed to a transparent solution at high temperatures. Depending on the polymer concentration, this pentablock copolymer forms a fragmented or interconnected network at elevated temperatures. At sufficiently high polymer concentration, the viscosity measurements indicate that a reversible hydrogel is formed. The interchain complexes or networks are readily broken up under the influence of shear flow. The results from the small angle neutron scattering experiments reveal significant structural differences between the pentablock copolymers at different temperatures.

Temperature-responsive inclusion complex of cationic PNIPAAM diblock copolymer and gamma-cyclodextrin

Aqueous mixtures of γ-cyclodextrin (γ-CD) and the thermosensitive cationic diblock copolymer poly(N-isopropylacrylamide)-b-poly(3-acrylamidopropyl)trimethylammonium chloride (PNIPAAM24-b-PAMPTAM(+)9) or the PNIPAAM homopolymer PNIPAAM47 have been investigated using various experimental methods. Solid γ-CD–polymer inclusion complexes (pseudopolyrotaxanes) form at ambient temperatures in fairly concentrated CD solutions. The NMR measurements showed that the stoichiometry of the inclusion complexes is close to two NIPAAM units per CD molecule. The cationic block of the copolymer is not incorporated into the CD cavity. Synchrotron radiation X-ray diffraction spectra of the solid inclusion complexes indicate a compact columnar structure of CD molecules threaded onto the PNIPAAM chains. In water, square-shaped cyclodextrin aggregates were found to co-exist with single cyclodextrin molecules. In mixed solutions of γ-CD and PNIPAAM24-b-PAMPTAM(+)9 these aggregates disintegrate with time as inclusion complexes are formed and the kinetics was studied using time-resolved static and dynamic light scattering and cryo-TEM. Steady-state fluorescence spectroscopy measurements reveal that the CD molecules dethread from the PNIPAAM chains upon increasing the temperature to 40 °C, which is above the lower critical solution temperature of PNIPAAM.