Low Grafting Density Research Articles

Influence of polystyrene ligand length on the spatial arrangement of quantum dots within PS-b-PEO micelles

The spatial arrangement of functional inorganic nanoparticles within polymer micelles is essential to the nanocomposite performances. Polystyrene (PS) of different lengths (PS24, PS91 and PS163) are grafted onto the surface of fluorescent CdSe/CdS core/shell quantum dots (QDs) through ligand exchange procedure, and their grafting density decreases from 2.80 to 0.54, 0.18 chains/nm2 with increase of PS ligand length. Under two competing effects, i.e. wettability between QDs and block copolymer PS120-b-PEO318 and the attraction between QDs, the precise location of PS-capped QDs inside the co-assemblies can be regulated by the length of PS ligands. The low grafting density of PS163 on the QD surface cannot overcome the van der Waals and hydrophobic attraction between QDs and cause the local aggregation of QDs within the co-assemblies. On the contrary, short PS24 ligands with high grafting density can avoid QD aggregation, but exhibit poor wettability with copolymer, which confines the QDs in the central portion of the core of co-assemblies. PS91 ligands with medium grafting density have good wettability with block copolymer and facilitate the homogeneous distribution of QDs inside the cores of co-assemblies. Furthermore, the influence of stirring time and water addition rate on the structure of co-assemblies is also investigated.

Electron-Equivalent Valency through Molecularly Well-Defined Multivalent DNA.

Oligonucleotide-functionalized nanoparticles (NPs), also known as "programmable atom equivalents" (PAEs), have emerged as a class of versatile building blocks for generating colloidal crystals with tailorable structures and properties. Recent studies have shown that, at small size and low DNA grafting density, PAEs can also behave as "electron equivalents" (EEs), roaming through and stabilizing a complementary PAE sublattice. However, it has been challenging to obtain a detailed understanding of EE-PAE interactions and the underlying colloidal metallicity because there is inherent polydispersity in the number of DNA strands on the surfaces of these NPs; thus, the structural uniformity and tailorability of NP-based EEs are somewhat limited. Herein, we report a strategy for synthesizing colloidal crystals where the EEs are templated by small molecules, instead of NPs, and functionalized with a precise number of DNA strands. When these molecularly precise EEs are assembled with complementary NP-based PAEs, X-ray scattering and electron microscopy reveal the formation of three distinct "metallic" phases. Importantly, we show that the thermal stability of these crystals is dependent on the number of sticky ends per EE, while lattice symmetry is controlled by the number and orientation of EE sticky ends on the PAEs. Taken together, this work introduces the notion that, unlike conventional electrons, EEs that are molecular in origin can have a defined valency that can be used to influence and guide specific phase formation.

Revealing the Critical Role of Probe Grafting Density in Nanometric Confinement in Ionic Signal via an Experimental and Theoretical Study

The grafting density of probes at sensor interface plays a critical role in the performance of biochemical sensors. However, compared with macroscopic interface, the effects of probe grafting density at nanometric confinement are rarely studied due to the limitation of precise grafting density regulation and characterization at the nanoscale. Here, we investigate the effect from the grafting density of DNA probes on ionic signal for nucleic acid detection in a cylindrical nanochannel array (with diameter of 25 nm) by combing experiments and theories. We set up a theoretical model of charge distribution from close to inner wall of nanochannels at low probe grafting density to spreading in whole space at high probe grafting density. The theoretical results fit well with the experimental results. A reverse of ionic output from signal-off to signal-on occurs with increasing probe grafting density. Low probe grafting density offers a high current change ratio that is further enhanced using long-chain DNA probes or the electrolyte with a low salt concentration. This work develops an approach to enhance performance of nanochannel-based sensors and explore physicochemical properties in nanometric confines.

Morphologies of a spherical bimodal polyelectrolyte brush induced by polydispersity and solvent selectivity* *Project supported by the Fundamental Research Funds for the Central Universities of China (Grant No. 3122020080).

It is commonly realized that polydispersity may significantly affect the surface modification properties of polymer brush systems. In light of this, we systematically study morphologies of bidisperse polyelectrolyte brush grafted onto a spherical nanocolloid in the presence of trivalent counterions using molecular dynamics simulations. Via varying polydispersity, grafting density, and solvent selectivity, the effects of electrostatic correlation and excluded volume are focused, and rich phase behaviors of binary mixed polyelectrolyte brush are predicted, including a variety of pinned-patch morphologies at low grafting density and micelle-like structures at high grafting density. To pinpoint the mechanism of surface structure formation, the shape factor of two species of polyelectrolyte chains and the pair correlation function between monomers from different polyelectrolyte ligands are analyzed carefully. Also, electrostatic correlations, manifested as the bridging through trivalent counterions, are examined by identifying four states of trivalent counterions. Our simulation results may be useful for designing smart stimuli-responsive materials based on mixed polyelectrolyte coated surfaces.

Structure and thermodynamics of grafted silica/polystyrene dilute nanocomposites investigated through self-consistent field theory.

Polymer/matrix nanocomposites (PNCs) are materials with exceptional properties. They offer a plethora of promising applications in key industrial sectors. In most cases, it is preferable to disperse the nanoparticles (NPs) homogeneously across the matrix phase. However, under certain conditions NPs might lump together and lead to a composite material with undesirable properties. A common strategy to stabilize the NPs is to graft on their surface polymer chains of the same chemical constitution as the matrix chains. There are several unresolved issues concerning the optimal molar mass and areal density of grafted chains that would ensure best dispersion, given the nanoparticles and the polymer matrix. We propose a model for the prediction of key structural and thermodynamic properties of PNC and apply it to a single spherical silica (SiO2) nanoparticle or planar surface grafted with polystyrene chains embedded at low concentration in a matrix phase of the same chemical constitution. Our model is based on self-consistent field theory, formulated in terms of the Edwards diffusion equation. The properties of the PNC are explored across a broad parameter space, spanning the mushroom regime (low grafting densities, small NPs and chain lengths), the dense brush regime, and the crowding regime (large grafting densities, NP diameters, and chain lengths). We extract several key quantities regarding the distributions and the configurations of the polymer chains, such as the radial density profiles and their decomposition into contributions of adsorbed and free chains, the chains/area profiles, and the tendency of end segments to segregate at the interfaces. Based on our predictions concerning the brush thickness, we revisit the scaling behaviors proposed in the literature and we compare our findings with experiment, relevant simulations, and analytic models, such as Alexander's model for incompressible brushes.

Stress in a Stimuli-Responsive Polymer Brush

The application of a polymer brush in sensing, actuation, self-folding, among others acutely depends on the tuneable bending of a brush-grafted substrate caused by the stress in the brush. However, the stress in a stimuli-responsive brush has not been investigated. In this work, we study the stress in the stimuli-responsive planar polymer brushes of neutral water-soluble polymers with low to very high graft densities using strong stretching theory (SST). First, SST with the Langevin force-extension relation for a polymer chain is extended to the study of stimuli-responsive brushes. Stress profile and other properties of a Poly(N-isopropylacrylamide) (PNIPAm) brush are then obtained using the extended SST and an empirical Flory-Huggins parameter. The model predicts that the stress in a PNIPAm brush is inhomogeneous and compressive at all temperatures and graft densities. The resultant stress is predicted to increase in magnitude with increasing graft density. Moreover, it decreases in magnitude with an increase in temperature before plateauing in low graft density brushes. In contrast, its magnitude increases weakly with increasing temperature in high density brushes. This contrasting behavior is traced to the minimum in interaction free energy density \emph{vs} polymer volume fraction curve for PNIPAm solution at a large volume fraction, and stiffening of chains due to finite extensibility. Furthermore, our results indicate that the ability to tune the resultant stress by changing temperature diminishes with increasing graft density.

On the Effect of Ultralow Loading of Microwave-Assisted Bifunctionalized Graphene Oxide in Stereolithographic 3D-Printed Nanocomposites.

Surface functionalization of graphene oxide (GO) is one of the best ways to achieve homogeneous dispersions of GO within polymeric matrices and composites. Nonetheless, studies regarding how the level of GO functionalization affects the macroscopic properties of three-dimensional (3D) printed nanocomposites are still few. Furthermore, the bifunctionalization of GO with the NH2/NH3+ groups to obtain improved thermomechanical macroscopic properties at ultralow loads has not been reported. In this paper, fast and straightforward surface bifunctionalization of GO with a controlled ratio of NH2/NH3+ groups at low, medium, and high functionalization levels (AGOL, AGOM, and AGOH) in a one-step microwave-assisted synthesis is reported for the first time. The functionalization mechanism was disclosed, wherein three graft densities (Gφ) were obtained. A plateau of maximum functionalization (Gφ = 4.9 μmol/m2 = 2.9 molecules/nm2) was reached, suggesting that full coverage of the GO surface is achievable. Also, an increase in the exfoliation of functionalized layers was obtained, ranging from d002 = 8.6 Å up to d002 = 15.8 Å. X-ray photoelectron spectroscopy (XPS) reveals the successful functionalization of GO, as well as an atomic relationship NH2/NH3+ of about 50/50% in all functionalized samples. Stereolithographic (SLA) 3D-printed nanocomposites (AGOL/R, AGOM/R, and AGOH/R) were obtained using ultralow loads (0.01 wt %) of each bifunctionalized material. This ultralow amount was sufficient to enhance thermal stability (up to 4 °C) and a significant increase in the glass transition temperature (93 °C ≤ Tg ≤ 120 °C). Interestingly, we found that low and medium grafting density promotes a ductile material (ε > 5%); meanwhile, a high graft density produces brittle materials. Also, we observe that the toughness can be tuned as a function of the graft density (AGOH: 24 MPa, AGOM: 342 MPa, AGOL: 562 MPa) at ultralow loadings. The 3D-printed nanocomposites using GO with low graft density (AGOL) increase their tensile strain by 90% in comparison with the control sample (without filler). Finally, the underlying mechanisms were discussed to explain the findings.

Assembly of Polymer-Grafted Nanoparticles in Polymer Matrices.

Leibler pioneered the idea that long enough matrix polymers of length P will spontaneously dewet a chemically identical polymer layer, comprising chains of length N, densely end-grafted to a flat surface ("brush"). This entropically driven idea is routinely used to explain experiments in which 10-20 nm diameter nanoparticles (NPs) densely grafted with polymer chains are found to phase separate from chemically identical melts for P/N ≳4. At lower grafting densities, these effects are also thought to underpin the self-assembly of grafted NPs into a variety of structures. To explore the validity of this picture, we conducted large-scale molecular dynamics simulations of grafted NPs in a chemically identical polymer melt. For the NPs we consider, in the ≈5 nm diameter range, we find no phase separation even for P/N = 10 in the absence of attractions. Instead, we find behavior that more closely parallels experiments when all of the chain monomers are equally attractive to each other but repel the NPs. Our results thus imply that experimental situations investigated to date are dominated by the surfactancy of the NPs, which is driven by the chemical mismatch between the inorganic core and the organic ligands (the graft and free chains are chemically identical). Entropic effects, that is, the translational entropy of the NPs and the matrix, the entropy of mixing of the grafts and the matrix, and the conformational entropy of the chains appear to thus have a second-order effect even in the context of these model systems.

Effects of graft locations on dispersion behavior of polymer-grafted nanorods: A molecular dynamics simulation study

In this paper, we present the effects of graft locations on the dispersion behavior of polymer-grafted nanorods using molecular dynamics simulations. Four graft locations of chains on nanorods, i.e., end-grafted, quarter-grafted, middle-grafted and uniformly grafted, are specially designed, and the effects of graft locations are examined in both sparsely grafted case and densely grafted case. Simulation results show that the end-grafted nanorods exhibit the poorest dispersion in the matrix, whilst the body-grafted (quarter-grafted and middle-grafted) ones show the best dispersion. At low grafting density in the mushroom regime, the middle-grafted disperse slightly better than the quarter-grafted, whereas at high grafting density in the brush regime, the situation is reversed. By analyzing the conformations of graft chains (mushroom height and brush height) and the behavior of grafted nanorods, it is inferred that the inter-rod cohesive energy provides a great contribution to the dispersion, as reflected by the significant role of the center region of nanorods. In general, a dense and wide coverage of the center/middle region of nanorods by graft chains can effectively improve the nanorod dispersion. This work is expected to provide some guidance on controlling and improving the dispersion of nanorods in polymer nanocomposites.

The effect of grafting density and side chain length on the conformation of PEG grafted bottlebrush polymers

PEG grafted polymers are a kind of promising materials, and widely applied in polymer electrolytes, lubricants, biotechnology and biomedical science. While up to now, systematical researches of the relationship between architecture and conformation of PEG grafted polymers are very rare. Here, we efficiently and successfully prepared a series of PEG grafted polymers with different grafting density, and side chain length by combining ring-opening metathesis polymerization (ROMP) and Copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, and then characterized the conformations of the synthesized bottlebrush polymers by AFM technique. The AFM images showed that all the polymers P(NB-alkyne)400-g-PEG with low grafting density were spherical morphology, while the polymers P(NB-2alkyne)400-g-PEG and P(NB-4alkyne)394-g-PEG presented cylindrical morphology, which indicated that only grafting density could determine the shape of the prepared PNB-g-PEG bottlebrush polymers. While, the side chain length could affect the size of prepared PEG grafted bottlebrush polymers.

Gold Nanoparticle Promoted Formation and Biological Properties of Injectable Hydrogels.

Acceleration of gelation in the biological environment and improvement of overall biological properties of a hydrogel is of enormous importance. Biopolymer stabilized gold (Au) nanoparticles (NPs) exhibit cytocompatibility and therapeutic activity. Hence, in situ gelation and subsequent improvement in the property of a hydrogel by employing Au NPs is an attractive approach. We report that stable Au NPs accelerate the conventional nucleophilic substitution reaction of activated halide-terminated poly(ethylene glycol) and tertiary amine functional macromolecules, leading to the rapid formation of injectable nanocomposite hydrogels in vivo and ex vivo with improved modulus, cell adhesion, cell proliferation, and cytocompatibility than that of a pristine hydrogel. NP surfaces with low chain grafting density and good colloidal stability are crucial requirements for the use of these NPs in the hydrogel formation. Influence of the structure of the amine functional prepolymer, the spacer connecting the halide leaving groups of the substrate, and the structure of the stabilizer on the rate promoting activity of the NPs have been evaluated with model low-molecular-weight substrates and macromolecules by 1H NMR spectroscopy, rheological experiments, and density functional theory. Results indicate a significant effect of the spacer connecting the halide leaving group with the macromolecule. The Au nanocomposite hydrogels show sustained co-release of methotrexate, an anti-rheumatic drug, and the Au NPs. This work provides insights for designing an injectable nanocomposite hydrogel system with multifunctional property. The strategy of the use of cytocompatible Au NPs as a promoter provides new opportunity to obtain an injectable hydrogel system for biological applications.

One-pot solvothermal synthesis of silane-functionalized carbon nanodots as compatibilizers for the immiscible TPU/MVQ blends

Herein, silane-functionalized carbon nanodots (Si-CDs) with different sizes and surface grafting density were successfully fabricated by a facile and cost-effective one-pot solvothermal treatment and incorporated for the first time as novel compatibilizers into the thermoplastic polyurethane/methyl vinyl silicone rubber (TPU/MVQ) blends. As obtained fluorescent Si-CDs with a size of 2 nm and functionalized with maximum amido and silane chains, called Si-CDs-High, displayed better compatibilizing efficiency than other Si-CDs with large size and low grafting density. The average diameter of MVQ domain in the blend reduced sharply from 2.51 ± 1.03 μm to 0.91 ± 0.28 μm at a small content of 1 wt% of Si-CDs-High. As effective compatibilizers, Si-CDs-High were located at the interface and formed a stable interface layer between TPU and MVQ phases and the thickness of interface was quantitatively identified by using Atomic Force Microscope. The superior compatibilizing efficiency of Si-CDs-High can be attributed that such Si-CDs with sufficient chains and groups can not only prevent coalescence of polymer domains by rigid nanoparticle core but also enhance interfacial adhesion due to the molecular entanglements and hydrogen bonding. Furthermore, the polymer blends with Si-CDs-High exhibited enhanced mechanical property, thermal stability and tunable fluorescent properties. This strategy compatibilized by fluorescent Si-CDs paves a new possibility for advanced blend nanocomposites.

Molecular Understanding of Ion Effect on Polyzwitterion Conformation in an Aqueous Environment.

Polyzwitterions (PZs) are promising materials for the antifouling in reverse osmosis and nanofiltration membrane technology for water treatment. Fundamental understanding of the structure and molecular interactions involving zwitterions is crucial to the optimal design of antifouling in membrane separation. Here we employ the umbrella sampling and molecular dynamics simulations to investigate molecular interactions between sulfobetaine/carboxybetaine zwitterions and different metal ions (Na+, K+, and Ca2+) in an aqueous solution. The simulation results show that these ions can form stable or metastable contact ionic/solvent-shared-ionic pairs with zwitterions. Simulations at different grafting densities of PZ brush arrays reveal complex competitive association mechanisms, which are attributed to nonbonded electrostatic and van der Waals interactions among zwitterions, water molecules, and different metal ions in an aqueous environment. While the high-grafting density of the PZ brush array leads to a strong branch association between different zwitterions in water, this association is decreased at intermediate- and low-grafting densities due to strong zwitterion-water interactions. More importantly, adding ions into water at intermediate- and low-grafting densities further breaks down the zwitterion branch association, resulting in a randomly oriented and dispersed branch configuration with significant swelling of the polymers. The degree of swelling depends on the type of ions, which further changes the surface electrostatic potential of PZ coatings.

Polystyrene-attached graphene oxide with different graft densities via reversible addition-fragmentation chain transfer polymerization and grafting through approach

Graphene oxide (GO) modified with double bond was utilized in grafting polystyrene chains to its surface through reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene. For this purpose, 3-(((2-aminoethyl)amino)dimethylsilyl)propyl methacrylate (OD) including double bond and amine groups was prepared and used for modification of GO in different grafting densities by a ring-opening nucleophilic reaction. Then, polystyrene-grafted GO was obtained by “grafting through” RAFT polymerization of styrene. Successful using of RAFT polymerization, efficiency of the grafting reaction, different characteristics of the graphene-attached and free polystyrene chains were investigated. Proton nuclear magnetic resonance spectroscopy confirmed successful synthesis of OD. Its grafting on GO was also confirmed by using X-ray photoelectron spectroscopy. The neat and modified GO layers were also investigated by Fourier transform infrared and Raman spectroscopies. Size exclusion chromatography was used to study molecular weight and polydispersity index of the attached polystyrene chains. Thermogravimetric analysis provides degradation temperature, char content, and grafting ratio of the modifier and polystyrene chains. Grafting ratio of OD was 10.3 and 4.4% for the modified GO layers with high and low grafting densities, respectively. Layers morphology was visually studied by scanning and transmission electron microscopies. Flat and smooth graphite platelets were changed to wrinkled layers after oxidation, and converted to opaque layers after grafting with polystyrene chains. High length of small molecule modifier in this study resulted in highly efficient grafting reaction.

Implications from the grafting density and ionic capacity effects on protein adsorption to poly (N,N-dimethylaminopropyl acrylamide)-grafted sepharose FF

Our previous study reported a novel protein ion-exchanger, N,N-dimethylaminopropyl acrylamide (DMAPAA)-grafted Sepharose FF, of very high protein adsorption capacity and uptake rate. To gain new insight into the polymer grafting effects on protein adsorption and chromatography, 18 DMAPAA-grafted anion exchangers of four grafting densities and various chain lengths (i.e., ionic capacities, ICs) were prepared and named as FF-Brx-pDMAPAAn (x represents grafting density and n stands for IC in mmol/L). It was found that an optimal IC existed at each grafting density to present a maximal capacity. Of the four optimal-IC resins, FF-Br3-pDMAPAA587 showed the highest capacity (377 mg/mL), but the protein uptake rate, defined as the ratio of effective pore diffusivity to free diffusivity (De/D0), was smaller than the resin of a similar ionic capacity but lower grafting density (FF-Br2-pDMAPAA592). Consequently, the dynamic binding capacity (DBC) of FF-Br2-pDMAPAA592 was higher than that of FF-Br3-pDMAPAA587 at flow velocities higher than 450 cm/h. Moreover, FF-Br3-pDMAPAA587 showed higher adsorption capacity than FF-Br2-pDMAPAA592 at 0−150 mmol/L NaCl, leading to its higher DBC values as compared to FF-Br2-pDMAPAA592 in the same salt concentration range. Taken together, DMAPAA-grafted resins of different grafting densities would be useful for purifying feedstocks of different ionic strengths.

Swelling of multi-responsive spherical polyelectrolyte brushes across a wide range of grafting densities

Multi-stimulus responsive weak polyelectrolyte brushes were grafted by surface-initiated atom transfer radical polymerization from spherical silica nanoparticles across a wide range of grafting densities approaching the limit of close packing of grafting sites: a regime not previously explored in the brush swelling literature. Brushes consisted of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) homopolymers or poly(2-(dimethylamino)ethyl methacrylate)-r-poly(4-metharyloyloxyazobenzene) random copolymers P (DMAEMA-r-MOAB) and were subjected to pH-, temperature-, and photoisomerization-induced swelling and deswelling. Homopolymer brushes were prepared at grafting densities ranging from 0.12 to 1.4 nm−2 and number average degrees of polymerization ranging from 300 to 1200. Two copolymer brushes with 8 mol% and 13 mol% of the nonionizable, photoresponsive MOAB units were compared with homopolymer brushes prepared at similar grafting densities and degrees of polymerization. Dynamic light scattering was used to measure the hydrodynamic radius of grafted nanoparticles, and thus brush thickness, in water. Brush swelling in response to increased DMAEMA protonation followed a single scaling behavior across the full range of degrees of polymerization and grafting densities. Incorporating azobenzene-containing MOAB comonomers introduced a modest photoresponsive control of brush swelling and decreased the brush thickness relative to the comparable PDMAEMA homopolymer brush, without affecting the scaling with respect to protonation. PDMAEMA brushes exhibited a pH-dependent lower critical solution temperature behavior, resulting in a thermal collapse of up to 50% upon heating. This work demonstrates that scaling behaviors previously tested at low grafting densities hold for extremely high grafting densities and that they are unaffected by moderate nonionizable comonomer content.

Deformation of Chemically Heterogeneous Interfacial Layers of Polymer Nanocomposites.

Dynamics of entangled interfacial polymer layers around nanoparticles determine the linear rheological properties of polymer nanocomposites. In this study, the nonlinear elastic properties of nanocomposites are examined under large-amplitude oscillatory shear (LAOS) flow to reveal the effect of interfacial chemical heterogeneity on the deformation mechanism of polymer-grafted and polymer-adsorbed nanoparticle composites. Adsorbed-poly(methyl methacrylate) (PMMA) layers presented stronger interfacial stiffening and reinforcement than PMMA-grafted layers. Chemical heterogeneities of interfacial layers, provided by polymer-adsorbed and low graft density particles, deformed at smaller strains than the poly(ethylene oxide) (PEO) matrix. Interfaces of loosely bound PMMA and PEO exhibited stiffening at low strains due to the enhanced chain mixing and entanglements. These results demonstrate that chemical and dynamic heterogeneities in interfacial layers have significant importance in designing adaptive polymer nanocomposites for large shear deformation.

Influence of Polymer Bidispersity on the Effective Particle–Particle Interactions in Polymer Nanocomposites

We investigate the role played by the bidispersity of polymer chains on the local structure and the potential of mean force (PMF) between silica nanoparticles (NPs) in a polystyrene melt. We use the hybrid particle-field molecular dynamics technique which allows to efficiently relax polymer nanocomposites even with high molecular weights.The NPs we investigate are either bare or grafted with polystyrene chains immersed in a melt of free polystyrene chains, whereas the grafted and the free polystyrene chains are either monodisperse or bidisperse. The two-body PMF shows that a bidisperse distribution of free polymer chains increases the strength of attraction between a pair of ungrafted NPs. If the NPs are grafted by polymer chains, the effective interaction crucially depends on bidispersity and grafting density of the polymer chains: for low grafting densities, the bidispersity of both free and grafted chains increases the repulsion between the NPs, whereas for high grafting densities we observe two different effects. An increase of bidispersity in free chains causes the rise of the repulsion between the NPs, while an increase of bidispersity in grafted chains promotes the rise of attraction. Additionally, a proper treatment of multi-body interactions improves the simpler two-body PMF calculations, in both unimodal and bimodal cases. We found that, by properly tuning the bidispersity of both free and grafted chains, we can control the structure of the composite materials, which can be confirmed by experimental observations. As a result, the hybrid particle-field approach is confirmed to be a valid tool for reproducing and predicting microscopic interactions, which determine the stability of the microscopic structure of the composite in a wide range of conditions.

Protein adsorption to poly(ethylenimine)-modified Sepharose FF: VIII: Impacts of surface ion-exchange groups at different polymer grafting densities

Our previous studies on protein adsorption to the anion-exchangers of poly(ethylenimine) (PEI)-grafted Sepharose FF found that both adsorption capacity and uptake rate of bovine serum albumin (BSA) increased greatly when the PEI grafting density reached over a critical ionic capacity (cIC) due to the 3D protein binding and occurrence of “chain delivery” of bound proteins. Moreover, by the investigation on the anion-exchangers of diethylaminoethyl (DEAE)-modified and DEAE-dextran-grafted Sepharose FF, we found the unique role of surface ligand in facilitating protein uptake kinetics on dextran-functionalized anion exchangers as the “transfer station” for the “chain delivery” of bound proteins. However, what would be the contribution of surface ligands on the transport of bound proteins on PEI chains at a wide range across cIC, particularly at the lower IC range where no “chain delivery” was present? We have thus designed this research to answer the question. To this purpose, we fabricated three series of PEI-Sepharose FF resins, one without surface ligand (i.e., FF-PEI) and two with surface DEAE modifications at DEAE coupling densities of 60 and 90 mmol/L (i.e., FF-D60-PEI and FF-D90-PEI), and focused on the role of surface DEAE at different PEI densities in BSA adsorption equilibrium and uptake kinetics in a wide IC range of PEI across cIC (∼600 mmol/L for BSA). It was found that, at low grafting-ligand densities (IC<cIC), both adsorption capacity and uptake rate increased significantly (5–14% and 40–140%, respectively) after adding surface DEAE groups. At IC > cIC, however, both adsorption capacity and uptake rate changed only slightly (<5% and <10%, respectively) by the addition of surface DEAE groups. The results revealed that at IC < cIC, the surface DEAE groups provided the supplement of available binding sites and facilitated the happenings of the “chain delivery” of bound proteins as the “transfer station”, both of which were limited at the low IC range. However, at IC > cIC, the extended flexible PEI chains have already afforded 3D binding space with high accessibility and easy happening of “chain delivery” of bound proteins, so the surface DEAE groups between neighboring PEI chains did not work in assisting the transport of bound proteins. Moreover, when the IC was lower but close to the cIC, the FF-D-PEI resins exhibited higher uptake rates than FF-PEI resins at a similar or even lower IC values. These findings indicate that surface modification of charged groups in PEI-based anion exchangers of low grafting density would be an efficient strategy to fabricate high performance protein ion exchangers.

Surface Forces of Asymmetrically Grown Polyelectrolyte Multilayers: Searching for the Charges.

Surface forces are used to investigate the polymer conformation and the surface charge of polyelectrolyte multilayers. Films are prepared from strong polyelectrolytes with low and high linear charge density at 0.1 M NaCl, namely poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS). The multilayer has two growth regimes: in the beginning, the film can contain as many positive as negative monomers. After about 15 deposited layer pairs, a linear growth regime characterized by an excess of cationic PDADMA monomers occurs. Independent of the film composition, at preparation conditions, the film surface is flat, uncharged and partially hydrophobic. Surface force measurements at decreased ionic strength provide insight. For PSS-terminated films electrostatic forces are found. At the beginning of multilayer formation, the surface charge density is negative. However, in the linear growth regime it is positive and low (one charge per 200-400 nm2). This reversal of surface charge density of PSS-terminated films is attributed to excess PDADMA-monomers within the film. PDADMA terminated films show steric forces, chains protrude into the solution and form a pseudobrush, which scales as a polyelectrolyte brush with a low grafting density (1900 nm2 per chain). We suggest a model of polyelectrolyte multilayer formation: PDADMA with its low linear charge density adsorbs with weakly bound chains. Monovalent anions within the film compensate PDADMA monomer charges. When PSS adsorbs onto a PDADMA-terminated multilayer, PSS monomers replace monovalent anions. While electrostatic bonds are formed and dissolved within the polyelectrolyte multilayer, the surface charge density remains zero.