Grafting Density Research Articles

Compositional asymmetry in a crystalline-amorphous block copolymer influences the phase and crystallization behaviors of its blend with an amorphous block copolymer.

This study determined the phase and crystallization behaviors of blends composed of asymmetric polystyrene-block-poly(ethylene oxide) (PS-PEO) and symmetric polystyrene-block-poly(methyl methacrylate) (PS-PMMA). The PS blocks in the various binary block copolymers exhibited nearly identical molecular weights, whereas the molecular weight ratios of PEO and PMMA varied. The compatibility of the PEO and PMMA chains aided the binary block copolymers in co-ordering in a lamellar microdomain morphology, with the PEO and PMMA blocks sharing a common microdomain. Adding short tethered PMMA chains to long tethered PEO chains led to a decrease in the common microdomain spacing and an increase in the grafting density. These behaviors increased PEO chain stretching, causing macrophase separation. The mismatch in PEO and PMMA block lengths divided the common PEO/PMMA microdomain into two sections: the coexisting PEO/PMMA section close to the microdomain interface and the neat PEO section far away from it. The high-glass-transition-temperature PMMA reduced PEO chain mobility, inhibiting PEO crystallization in the coexisting PEO/PMMA section but not in the neat PEO section. When the block length ratio of PEO to PMMA decreased, the neat PEO section narrowed. The increase in the extent of PEO confinement led to a reduction in PEO crystallizability.

Amphiphilic titania Janus nanoparticles containing ionic groups prepared in oil-water Pickering emulsion.

Titania nanoparticles with a diameter of 8 nm underwent an anisotropic modification using apolar 6-bromohexylphosphonic acid and cationic polar N,N,N-trimethyl-6-phosphonohexan-1-aminium bromide. The Janus modification was achieved through a straightforward one-step Pickering emulsion approach using toluene-water mixtures. The resulting Janus particles were compared with isotropically and statistically modified titania particles, where either a single coupling agent is attached to the surface or both coupling agents are assembled over the surface randomly, respectively. The covalent binding of the phosphonic acids to the titania surface was confirmed by FTIR and 31P solid-state CP-MAS NMR analyses. The grafting density was assessed using TGA, elemental analysis, and ICP-MS, revealing grafting densities of 0.1 mmol g-1 to 0.5 mmol g-1 for the cationic coupling agent and 1.2 mmol g-1 to 1.5 mmol g-1 for the apolar coupling agent, respectively. ζ-Potential titration measurements of both pristine and modified particles revealed isoelectric points at pH 4.5 to 9.3, depending on the type of modification. The ability of the particles to stabilize Pickering emulsions was tested under various conditions, with statistically and Janus-modified particles demonstrating a significant increase in stabilization compared to their isotropically modified counterparts. Furthermore, Janus particles were deposited onto glass substrates by a simple layer-by-layer approach. Through the self-assembly of these Janus particles, the glass substrate's properties could be tailored from hydrophilic to hydrophobic to hydrophilic, depending on the dipping cycle.

Delineating the role of surface grafting density of organic coatings on the colloidal stability, transport, and sorbent behavior of engineered nanoparticles

Aqueous stability and sorption affinity (towards target environmental contaminants) of engineered nanoparticles, composed of inorganic nanoparticles and surface stabilizers, are dependent on surface grafting density.

Solvent effects on surface-grafted and solution-born poly[N-(2-hydroxypropyl)methacrylamide] during surface-initiated RAFT polymerization

Solvent effects influence grafting density and cause discrepancy between molar mass of surface-grafted and solution-born poly[N-(2-hydroxypropyl) methacrylamide] during surface-initiated RAFT polymerization.

Grafting of cationic molecules to hyaluronic acid improves adsorption and cartilage lubrication.

Synovial fluid lubricates articular joints by forming a hydrated layer between the cartilage surfaces. In degenerative joint diseases like osteoarthritis (OA), the synovial fluid is compromised, which leads to less effective innate lubrication and exacerbated cartilage degeneration. Studies over the years have led to the development of partially or fully synthetic biolubricants to reducethe coefficient of friction with cartilage in knee joints. Cartilage-adhering, hydrated lubricants are particularly important to provide cartilage lubrication and chondroprotection under high normal load and slow speed. Here, we report the development of a hyaluronic acid (HA)-based lubricant functionalized with cationic branched poly-L-lysine (BPL) molecules that bind to cartilageviaelectrostatic interactions. We surmised that the electrostatic interactions between the BPL-modified HA molecules (HA-BPL) and the cartilage facilitate localization of the HA molecules to the cartilage surface. The number of BPL molecules on the HA backbone was varied to determine the optimal grafting density for cartilage binding and HA localization. Collectively, our results show that our HA-BPL molecules adhered readily to cartilage and were effective as a lubricant in cartilage-on-cartilage shear measurements where the modified HA molecules significantly reduce the coefficient of friction compared to phosphate-buffered saline or HA alone. This proof-of-concept study shows how the incorporation of cartilage adhering moieties, such as cationic molecules, can be used to enhance cartilage binding and lubrication properties of HA.

Glycocalyx-inspired dynamic antifouling surfaces for temporary intravascular devices

Protein and cell adhesion on temporary intravascular devices can lead to thrombosis and tissue embedment, significantly increasing complications and device retrieval difficulties. Here, we propose an endothelial glycocalyx-inspired dynamic antifouling surface strategy for indwelling catheters and retrievable vascular filters to prevent thrombosis and suppress intimal embedment. This strategy is realized on the surfaces of substrates by the intensely dense grafting of hydrolyzable endothelial polysaccharide hyaluronic acid (HA), assisted by an amine-rich phenol-polyamine universal platform. The resultant super-hydrophilic surface exhibits potent antifouling property against proteins and cells. Additionally, the HA hydrolysis induces continuous degradation of the coating, enabling removal of inevitable biofouling on the surface. Moreover, the dense grafting of HA also ensures the medium-term effectiveness of this dynamic antifouling surface. The coated catheters maintain a superior anti-thrombosis capacity in ex vivo blood circulation after 30 days immersion. In the abdominal veins of rats, the coated implants show inhibitory effects on intimal embedment up to 2 months. Overall, we envision that this glycocalyx-inspired dynamic antifouling surface strategy could be a promising surface engineering technology for temporary intravascular devices.

Sliding dynamics of ring chain on graft polymer in rotaxane

Rotaxane is a mechanically interlocked molecule in which rings are threaded on the axial linear polymer chain, and a characteristic feature of rotaxane is that ring can slide along the axial chain. In this study, molecular dynamics simulations are performed to investigate the sliding dynamics of ring on a regular/random graft polymer with graft density f and side chain length Ns. The mean-square displacement g3(t) of ring exhibits a novel sub-diffusion behavior in the intermediate time range due to the interaction between the branches and ring, and the sub-diffusive regime occurs at g3(t)∼((Nc−1)*b2)2, qualitatively, where Nc is the number of monomers between two adjacent side chains and b is bond length. For ring chain sliding along either a regular graft polymer or a random one, the sliding dynamic of ring relies strongly on the side chain length Ns、graft density f、ring size as well as the distribution of branches in the graft polymer. The diffusion coefficient D of ring decreases with the increase of Ns or f. However, ring slides faster along the regular graft polymer than that random one, because the dense side chains in the random graft polymer greatly hinder the ring's mobility. The effect of regular distribution on the improved diffusion gets most obvious when the side-chain length is middle. These results may provide a fundamental view for the understanding the effect of topological structure of axial graft polymer chain on the sliding dynamics of ring in rotaxane.

Adsorption of zwitterionic oligomer-grafted silica nanoparticles on rock surface in enhanced oil recovery studied by molecular dynamics simulations

The application of surface-functionalized nanoparticles (NPs) in enhanced oil recovery (EOR) has been rapidly increasing. Adsorption of NPs on the rock surface is the key step. In this study, we systematically investigated the adsorption and desorption mechanisms of various brush-like zwitterionic oligomers functionalized silica NPs on the hydrophilic rock surface in an aqueous solution using molecular dynamics simulations. Our simulations show that the adsorption is primarily influenced by the oligomer chains, while the neutral silica NP core contributes very weakly only when it has direct contact with the rock surface at the grafting density of 5 or below. The interactions from SO3⁻ groups located at the tail of zwitterionic side chains account for more than 72% of the non-bonded interaction energy Enb between the NPs and the rock. The adsorption becomes stronger for NPs with shorter (4 monomer units) oligomer chains but slightly weaker for NPs with longer (9 monomer units) oligomer chains when increasing the grafting density, which is well explained by the change in the number of adsorbed SO3⁻ groups. Desorption simulations with the umbrella sampling method indicate that the free energy of desorption decreases with increasing the grafting density. The silica NPs grafted with longer oligomer chains (9 monomer units) require higher free energies of desorption than the ones with shorter oligomer chains. The adsorption of NPs on the rock surface enhances surface hydrophilicity, and the effect of longer oligomer chains is more profound than that of short oligomer chains. This research provides valuable insights into the adsorption and desorption mechanisms of surface-modified silica NPs on the rock surface at the molecular level and thus gives hints to the design of nanofluids.

The effects of poly(ethylene glycol) grafting density on the extensional solution flows of high molecular weight polyacrylamide

AbstractControlling the extensional rheological properties of solutions is of great interest for a number of applications, such as printing and coatings, as polymeric additives can modify a fluid's droplet size, formation, and breakup. We have measured the extensional flows of polyacrylamide graft poly(ethylene glycol) amine (PAM‐g‐PEGa) solutions as a function of grafting density. PAM‐g‐PEGa is synthesized with grafting densities of 2.5%, 5.5%, 8%, and 30% and extensional flow was analyzed with drop‐on‐substrate rheology. Flow time scales and profiles are then compared to understand the effects of backbone extension from the addition of side chains. At low grafting densities, the viscous friction from the added side chains is comparable to, and in some cases, counteracts the losses in extensibility to the graft polymer's backbone, leading to increases in the relaxation time of the polymer. At higher grafting densities, the extension of the backbone reduces the relaxation time. By characterizing the effects of the graft density architecture on extensional flows, we are able to fine‐tune and control the breakup dynamics of elastic solutions.

Characterization of well‐defined PHEMA‐based bottlebrushes with controlled chain length and grafting density

AbstractBottlebrushes with controlled horizontal‐ and vertical‐axis scales, which have a high density of graft chains on their main chains, are useful as a nanomaterial. The properties of bottlebrushes depend on the polymer‐type, ‐chain length, and grafting density. We fabricate well‐defined bottlebrushes based on poly(2‐hydroxyethyl methacrylate) (PHEMA) using the grafting from method. Macroinitiators with different compositions of an atom transfer radical polymerization (ATRP) initiator group are prepared, and two bottlebrushes are consequently synthesized by ATRP of HEMA with the macroinitiator: a concentrated bottlebrush with graft chains on the repeating units and a semi‐dilute bottlebrush with graft chains on approximately 10%. The hydrodynamic and gyration radii of the concentrated‐ and semi‐dilute bottlebrushes are Rh = 9.2 nm (Rg = 24.9 nm) and Rh = 69.1 nm (Rg = 130 nm), respectively. The shape factors (Rg/Rh) of these bottlebrushes indicate both solid‐rods. The virial coefficient of semi‐dilute bottlebrush, calculated from static light scattering measurements, is higher than that of concentrated bottlebrush, owing to the flexibility and solvation structure. The grafting density play a more prominent role than the graft‐chain length in the excluded volume effect of the bottlebrush. These results provide useful insights for the construction of polymethacrylate‐based bottlebrushes with specific hydrodynamic properties.

The In Vivo Fate of Polycatecholamine Coated Nanoparticles Is Determined by a Fibrinogen Enriched Protein Corona.

Polycatecholamine coatings have attracted significant attention in the past 10 years owing to their ability to functionalize a wide range of materials. Here we apply the use of such coatings to drug nanocrystals, made from a poorly soluble drug compound, to postfunctionalize the nanocrystal surface with the aim of providing steric stabilization and extending their circulation time after intravenous injection. We show that both polydopamine and polynorepinephrine can be used to successfully modify drug nanocrystals and subsequently incorporate end-functionalized PEG to the surface. Even though high grafting densities of PEG were achieved, we observed rapid clearance and increased liver uptake for polycatecholamine functionalized drug nanocrystals. Using both surface sensitive model systems and protein corona profiling, we determine that the rapid clearance was correlated with an increase in adsorption of proteins involved in coagulation to the polycatecholamine surface, with fibrinogen being the most abundant. Further analysis of the most abundant proteins revealed a significant increase in thiol-rich proteins on polycatecholamine coated surfaces. The observed interaction with coagulation proteins highlights one of the current challenges using polycatecholamines for drug delivery but might also provide insights to the growing use of these materials in hemostatic applications.

Ligand‐Mediated Interaction of Nanoparticles with Lipid Membranes

AbstractWhile many studies are performed on the effect of ligands on the adhesion and endocytosis of NPs, the effects of ligand length and surface density on the NPs' interaction with lipid membranes are poorly investigated. Here, a computational investigation is presented, based on molecular dynamics of a coarse‐grained implicit‐solvent model, of the interaction between ligand‐decorated spherical NPs and lipid membranes. Specifically,the case is considered where the ligands interact attractively with lipid membranes only through their ends. In particular, the effects of ligand grafting density, ligand length, and strength of ligand‐lipid interaction is investigated on the degree of wrapping of the NP by the membrane and on the morphology of the membrane close to the NP. Whereas the degree of wrapping is found to increase with increasing the grafting density for a given interaction strength and ligand length, it decreases with ligand length for a given grafting density and interaction strength. For moderate values of the adhesion strength and long ligands, it is found that the end ligands form long linear clusters, which lead to an anisotropic conformation of the membrane around the NP. For short ligands, the wrapping of the membrane around the NP is almost complete, with the wrapped NP showing a regular faceted structure for high adhesion strength.

Spherical Polyelectrolyte Brushes as Flocculants and Retention Aids in Wet-End Papermaking.

As the criteria of energy conservation, emission reduction, and environmental protection become more important, and with the development of wet-end papermaking, developing excellent retention aids is of great significance. Spherical polyelectrolyte brushes (SPBs) bearing polyelectrolyte chains grafted densely to the surface of core particle have the potential to be novel retention aids in wet-end papermaking not only because of their spherical structure, but also due to controllable grafting density and molecular weight. Such characteristics are crucial in order to design multi-functional retention aids in sophisticated papermaking systems. This review presents some important recent advances with respect to retention aids, including single-component system and dual-component systems. Then, basic theory in papermaking is also briefly reviewed. Based on these advances, it emphatically describes spherical polyelectrolyte brushes, focused on their preparation methods, characterization, conformation, and applications in papermaking. This work is expected to contribute to improve a comprehensive understanding on the composition, properties, and function mechanisms of retention aids, which helps in the further investigation on the design of novel retention aids with excellent performance.

Competitive Specific Anchorage of Molecules onto Surfaces: Quantitative Control of Grafting Densities and Contamination by Free Anchors.

The formation of surfaces decorated with biomacromolecules such as proteins, glycans, or nucleic acids with well-controlled orientations and densities is of critical importance for the design of in vitro models, e.g., synthetic cell membranes and interaction assays. To this effect, ligand molecules are often functionalized with an anchor that specifically binds to a surface with a high density of binding sites, providing control over the presentation of the molecules. Here, we present a method to robustly and quantitatively control the surface density of one or several types of anchor-bearing molecules by tuning the relative concentrations of target molecules and free anchors in the incubation solution. We provide a theoretical background that relates incubation concentrations to the final surface density of the molecules of interest and present effective guidelines toward optimizing incubation conditions for the quantitative control of surface densities. Focusing on the biotin anchor, a commonly used anchor for interaction studies, as a salient example, we experimentally demonstrate surface density control over a wide range of densities and target molecule sizes. Conversely, we show how the method can be adapted to quality control the purity of end-grafted biopolymers such as biotinylated glycosaminoglycans by quantifying the amount of residual free biotin reactant in the sample solution.

Architectural control over morphologies of bottlebrush block copolymer superstructures

The morphology of superstructures formed by bottlebrush block copolymers in the melt can be tuned by changing the side chain length or/and their grafting density at constant volume fractions of the blocks. This feature enables fabrication of microphase separated bulk structures and mesoporous materials thereof with spherical or cylindrical domains (precursors of the mesopores), with high porosity unattainable for materials produced from conventional linear block copolymers. These paradigms are proven by DPD simulations that allow constructing morphological phase diagrams of the melt of block copolymers comprising one linear and one bottlebrush block and comparing the simulation results to the predictions of the mean field analytical theory. While the binodal lines separating the stability regions of spherical and cylindrical domains predicted by the theory perfectly match the simulation results, the simulation indicates appearance of a gyroid phase around the theoretical binodal separating the stability ranges of cylinders and lamellae. The results of our work provide guidelines for macromolecular design of novel composite and mesoporous materials with a wide spectrum of potential applications.

Marriage of Organic and Grubbs Catalysts for Tandem Synthesis of Bottlebrush Polyesters.

Bottlebrush polymers (BBPs) have gained wide attention for their special characters, such as rigid main/side chains, stemming from the exceedingly high graft density. This study aims to provide a simple synthetic approach to BBPs with polyester side chains by merging ring-opening alternating copolymerization (ROAP) and ring-opening metathesis polymerization (ROMP). A simple phosphazene base (tBuP1) is employed for the ROAP of phthalic anhydride and epoxide, after which Grubbs third-generation catalyst (G3) is added to in situ switch on ROMP of the macromonomer, i.e., norbornenyl-ended alternating polyester. The compatibility of tBuP1 with G3 and well-controlled ROMP is evidenced by DOSY-NMR of mixed catalysts, characterization of BBPs, and side-chain degradation. The method can also be extended to BBPs with one-step synthesized block copolyesters side chains. These results highlight the strength of the non-nucleophilic organobase catalyst for convenient construction of complex (degradable) polymers with compositional diversity.

Converting Nanoflower-like Layered Double Hydroxides into Solvent-Free Nanofluids for CO2 Capture.

Due to the flexibility and versatility of the layered crystal structure of layered double hydroxides (LDHs), they have shown great potential in various fields. However, LDH nanosheets (LDH-NSs) are easy to agglomerate, leading to the problem of accumulation, which hinders their further application. Accordingly, once LDHs are combined with solvent-free nanofluids (SFNs), the advantages of LDHs and SFNs could be combined to achieve an extraordinary performance. However, the stacked structure of traditional LDHs is not conducive to the exposure of hydroxyl functional groups, and hydroxyl sites are key to the conversion of LDHs to SFNs. Therefore, in this work, nanoflower-like LDHs (NFLs) with abundant exposed hydroxyl groups were prepared and combined with organic oligomers to achieve a solid-to-liquid transition. The formation mechanism of NFLs and the grafting mechanism of OS-PEA on their surface were identified. The prepared NFL-F3 still has good fluidity and dispersion stability in different solvents after storage for 100 days. The high-saturated grafting density on the surface of NFLs increased the steric hindrance effect of the nanoparticles, thereby improving the dispersion stability and reducing the viscosity of NFL-F3. Notably, the CO2 sorption performance of NFL-F3 is significantly improved, which is attributed to the voids between polymers, physical sorption, and good fluidity caused by high-saturation grafting on the surface of NFL-F3. Finally, by combining the sorption behavior and model fitting, it was confirmed that the physical effect was dominant in CO2 sorption by the NFL-F, which saved energy for the sorption-desorption process of its industrial application. Moreover, NFL-F3 has a good CO2/N2 separation performance and cycle stability. We envision that this general strategy will open up new insights into the construction of innovative low-viscosity LDH-based SFNs with high CO2 capacity and facilitate CO2/N2 selectivity and offer new directions for LDH utilizations.

Changes in Volume and Bone Density of Calvarial Grafts After Two Years of Orbital Reconstruction.

One of the greatest challenges in orbital reconstruction is to obtain adequate orbital volume and globe projection after traumatic injury. Autologous bone graft has been considered the gold standard for orbital floor reconstruction, but the harvesting yields many possible complications, such as donor site morbidity. Bone resorption is a frequent problem that can lead to insufficient volume after graft placement. There is a theoretically lower resorption rate of skull bone grafts, even though the exact amount is not known yet. This study is the first to evaluate bone volume and density changes of calvarial split bone grafts after orbital floor reconstruction with a 2-year follow-up. Bone volume and density of calvarial split grafts were determined using computed tomography scans and the software program RadiAnt Dicom Viewer in a total of 10 patients with orbital floor fracture reconstruction. Computed tomography scans taken immediately after surgery (T0) and after a postoperative period of 2 years (T1) were evaluated. The authors found a mean bone volume reduction of 34.25% after 2 years. Bone density was still high after 2 years (over 850 HU), with a mean reduction of 8%. Based on these findings, calvarial split bone grafts represent the ideal reconstruction material for orbital floor fractures.

Ligand concentration determines antiviral efficacy of silica multivalent nanoparticles

We have learned from the recent COVID-19 pandemic that the emergence of a new virus can quickly become a global health burden and kill millions of lives. Antiviral drugs are essential in our fight against viral diseases, but most of them are virus-specific and are prone to viral mutations. We have developed broad-spectrum antivirals based on multivalent nanoparticles grafted with ligands that mimic the target of viral attachment ligands (VALs). We have shown that when the ligand has a sufficiently long hydrophobic tail, the inhibition mechanism switches from reversible (virustatic) to irreversible (virucidal). Here, we investigate further how ligand density and particle size affect antiviral efficacy, both in terms of half-inhibitory concentration (IC50) and of reversible vs irreversible mechanism. We designed antiviral silica nanoparticles modified with 11-mercaptoundecane-1-sulfonic acid (MUS), a ligand that mimics heparan sulfate proteoglycans (HSPG) and we showed that these nanoparticles can be synthesized with different sizes (4–200 nm) and ligand grafting densities (0.59–10.70 /nm2). By testing these particles against herpes simplex virus type 2 (HSV-2), we show that within the size and density ranges studied, the antiviral IC50 is determined solely by equivalent ligand concentration. The nanoparticles are found to be virucidal at all sizes and densities studied.

Surface-Initiated PET-RAFT via the Z-Group Approach.

Surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) is a user-friendly and versatile approach for polymer brush engineering. For SI-RAFT, synthetic strategies follow either surface-anchoring of radical initiators (e.g., azo compounds) or anchoring RAFT chain transfer agents (CTAs) onto a substrate. The latter can be performed via the R-group or Z-group of the CTA, with the previous scientific focus in literature skewed heavily toward work on the R-group approach. This contribution investigates the alternative: a Z-group approach toward light-mediated SI photoinduced electron transfer RAFT (SI-PET-RAFT) polymerization. An appropriate RAFT CTA is synthesized, immobilized onto SiO2, and its ability to control the growth (and chain extension) of polymer brushes in both organic and aqueous environments is investigated with different acrylamide and methacrylate monomers. O2 tolerance allows Z-group SI-PET-RAFT to be performed under ambient conditions, and patterning surfaces through photolithography is illustrated. Polymer brushes are characterized via X-ray photoelectron spectroscopy (XPS), ellipsometry, and water contact angle measurements. An examination of polymer brush grafting density showed variation from 0.01 to 0.16 chains nm-2. Notably, in contrast to the R-group SI-RAFT approach, this chemical approach allows the growth of intermittent layers of polymer brushes underneath the top layer without changing the properties of the outermost surface.