Influence Of Grafting Density Research Articles

Amphiphilic Molecular Brushes with Regular Polydimethylsiloxane Backbone and Poly-2-isopropyl-2-oxazoline Side Chains. 3. Influence of Grafting Density on Behavior in Organic and Aqueous Solutions

Regular and irregular molecular brushes with polydimethylsiloxane backbone and poly-2-isopropyl-2-oxazoline side chains have been synthesized. Prepared samples differed strongly in the side chain grafting density, namely, in the ratio of the lengths of spacer between the grafting points and the side chains. The hydrodynamic properties and molecular conformation of the synthesized grafted copolymers and their behavior in aqueous solutions on heating were studied by the methods of molecular hydrodynamics and optics. It was found that the regularity and the grafting density do not affect the molecular shape of the studied samples of molecular brushes in the selective solvent. On the contrary, the grafting density is one of the most important factors determining the thermoresponsivity of grafted copolymers. It was shown that in analyzing self-organization and LCST values in aqueous solutions of poly-2-isopropyl-2-oxazolines with complex architecture, many factors should be considered. First is the molar fraction of the hydrophobic fragment and the intramolecular density. It was found that molar mass is not a factor that greatly affects the phase transition temperature of poly-2-isopropyl-2-oxazolines solutions at a passage from one molecular architecture to another.

Influence of Grafting Density on Ultrasound-Induced Backbone and Arm Scission of Graft Copolymers

The influence of grafting density on polymer conformations and ultrasonic degradation of polynorbornene-g-polystyrene (PS) or poly(methyl acrylate) (PMA) graft copolymers was explored. Multi-angle light scattering analysis, atomic force microscopy imaging, and molecular dynamics simulations supported that graft copolymers exhibited increased backbone and arm extension and increased arm–arm interactions at higher grafting density. In regard to backbone scission, faster scission rates were observed at higher grafting density, which was further supported by the generation of master curves that account for the rate enhancement due to the polymer being in an extended conformation. The grafting density also influenced arm scission rates, with PS and PMA arms exhibiting opposite trends. Specifically, arm scission was faster for PS at higher grafting density and faster for PMA at lower grafting density, which was attributed to differences in arm extension and arm–arm interactions between the two arm types.

Influence of Grafting Density and Distribution on Material Properties Using Well-Defined Alkyl Functional Poly(Styrene-co-Maleic Anhydride) Architectures Synthesized by RAFT

Poly(styrene-co-maleic anhydride) copolymers (PSMA) with controlled number and distribution of maleic anhydride (MAnh) units were synthesized by reversible addition–fragmentation chain transfer polymerization using chain-transfer agents (CTA) suitable for industrial scale processes. Linear- and star-shaped alternating PSMA polymers were prepared in a single-step synthesis, while a one-pot sequential chain-extension strategy was utilized to prepare diblock, multiblock, and multisite copolymer architectures. A library of grafted PSMAs with controlled density and distribution of side chains was achieved by the subsequent grafting of long aliphatic alcohol chains (C22) to the MAnh units. The influence of structure, composition, and long alkyl chain addition on PSMAs behavior in solution was studied with triple-detection size exclusion chromatography, while their thermal properties were examined by thermogravimetric analysis and differential scanning calorimetry. Overall, the side chain density and distributio...

N‐Oligo(3‐hydroxybutyrate)‐functionalized polypyrroles: towards bio‐erodible conducting copolymers

AbstractNew copolymer materials have been prepared by chemical grafting of oligomeric 3‐hydroxybutyric acid (OHB) onto polypyrrole (PPy) derivatives. The influence of grafting density and molecular weight of OHB brushes on the physicochemical properties of prepared copolymers was investigated. PPy substrates were prepared by FeCl3‐driven oxidative homopolymerization of N‐(2‐carboxyethyl)pyrrole or its copolymerization with pyrrole. The grafting method employed involved controlled anionic polymerization of β‐butyrolactone on pyrrole‐tethered potassium carboxylate active sites. Obtained PPy‐g‐OHB copolymers of varying grafting density and pendant polyester chain length were characterized and the observed structure–property relationships discussed. The impact of real time exposure to phosphate‐buffered saline environment was investigated and the residue products were characterized. Cross‐correlation of spectroscopic, thermal, electrical and elemental analysis data afforded comprehensive evaluation of the structure of prepared materials and their behaviour in hydrolytic medium. Erosion and degradation pathways have been identified, indicating ways to consciously tailor the physicochemical properties of these new biomimetic materials. © 2016 Society of Chemical Industry

Swelling and Surface Interactions of End-Grafted Poly(2-vinylpyridine) Layers in Acidic Solution: Influence of Grafting Density and Salt Concentration.

In previous studies, the authors found that end-grafted layers of the weak polybase poly(2-vinylpyridine) (P2VP) in aqueous solutions do not only swell and collapse if the pH value and salt concentration are varied but also exhibit a pH- and salinity-dependent adhesion to microsized silica spheres. For a better understanding of these effects, in situ force measurements using the AFM colloidal probe technique were applied to end-grafted P2VP layers of different grafting densities in NaCl solutions at pH 2.5. Although a mushroom-to-brush transition could be seen in the dry state, the layers were in the brush regime in aqueous solutions at all NaCl concentrations and grafting densities. We observed an increase of the brush height with increasing grafting density and a salinity-dependent collapse and reswelling of the brushes. The adhesion between the P2VP layer and a silica sphere depended on both grafting density and salinity. At low salt concentrations, the adhesion reached its highest value at the intermediate grafting density and disappeared with denser brushes. Maximum adhesion was obtained for high NaCl concentrations and the lowest grafting density. From a detailed analysis of the experiments, we gained insight into chain stretching and density profiles under complex ionic conditions and into the mechanism of adhesion of polyelectrolytes to solid surfaces.

Stimuli–responsive track-etched membranes via surface-initiated controlled radical polymerization: Influence of grafting density and pore size

The surface-functionalization of poly(ethylene terephthalate) track-etched membranes of different nominal pore sizes (400, 1000 and 3000 nm) with stimuli–responsive poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) via surface-initiated (SI) atom transfer radical polymerization (ATRP) was performed. Variations of grafting density and grafted chain length were achieved by variation of synthesis conditions. It could be clearly demonstrated that mixtures of reaction solutions containing different ratios of acyl bromides, only one bearing the initiator group necessary for the SI ATRP, led to different initiator group densities on the resulting track-etched membrane surface which had been verified by X-ray photoelectron spectroscopy. Moreover the mass increase as function of reaction time strongly correlated with the amount of initiator bound to the membrane surface indicating that the ATRP reaction was not limited by monomer diffusion into the pores. Scanning electron microscopy images and permporometry measurements indicated an even functionalization on the entire membrane surface which was the basis for further investigations. The stimuli–responsive properties of PDMAEMA grafted track-etched membranes were studied by permeability measurements with citrate and glycine buffers as function of pH (2 and 10) and temperature (25 and 60 °C). By that the barrier properties of the membranes could be effectively changed in two steps. The results agree with the expectation that a change in grafting density and chain length has an effect on the stimuli–responsive properties of the membrane. Results for membranes having similar degrees of grafting clearly showed that the reversible swelling of grafted polymeric layers was more pronounced for lower grafting density.

Influence of Grafting Density of Diblock Copolymers on Interfacial Assembly Behavior and Interfacial Shear Strength of Glass Fiber/Polystyrene Composites

To investigate the influence of the grafting density and the molecular structure of block copolymers on the interfacial assembly behavior and interfacial shear strength, macromolecular coupling agents, hydroxyl-terminated poly(n-butyl acrylate-b-styrene) (HO-P(BA-b-S)) were synthesized by atom transfer radical polymerization, and then chemically anchored on the glass fiber surfaces to form a well-defined monolayer. The phase separation and 'hemispherical' domain morphologies of diblock copolymer brushes at the polystyrene/glass fiber interface were observed. The interfacial assembly morphology differs with changes in the grafting density of diblock copolymers. When the grafting density is greatest, the highest height difference of the hemispherical domain and the largest surface roughness are achieved, as well as the best interface shear strength. It was also found that the copolymer brush with a PBA block of the polymerization degree (Xn) about 77 is the optimal option for the interfacial adhesion of PS/GF composites. Thus, the grafting density and molecular structure of diblock copolymers determines the interfacial assembly behavior of copolymer brushes, and therefore the interfacial shear strength.

Nanomechanical behaviors of microcantilever-based single-stranded DNA chips induced by counterion osmotic effects

Experiments show that deflections of microcantilever-DNA chip can be induced by many factors, such as grafting density, hybridization efficiency, concentration, length and sequence of DNA molecules, buffer salt concentration, time, and temperature variation. However, there are few theoretical works on microcantilever-DNA chips. The present paper is aimed to study the influence of counterion effects of single-stranded DNA (ssDNA) polyelectrolyte solution on the nanomechanical behaviors of microcantilever-based ssDNA chips during packing process. First, the effect of osmotic pressure induced by ingress of counterions into DNA brush structures is studied with Hagan's model for a cylindrical polyelectrolyte brush system on the basis of Poisson-Boltzmann distribution hypothesis. Second, Zhang's two-variable method for a laminated cantilever is used to formulate a four-layer energy model for ssDNA chips with weak interactions. Third, the influence of grafting density, ssDNA chain length, and salt concentration on packing deflection is investigated using the principle of minimum energy. The predictive tendency is qualitatively similar to those observed in some related ssDNA chip experiments. The difference between the four-layer model and the simplified two-layer model for ssDNA chips is also discussed.

Influence of Graft Density on Kinetics of Surface-Initiated ATRP of Polystyrene from Montmorillonite

Here we report the kinetics of the surface-initiated atom transfer radical polymerization (ATRP) of styrene from the surface of functionalized montmorillonite clay as a function of graft density. Compared with analogous ATRP reactions with free initiator, we observe a seven-fold increase in the polymerization rate at the highest graft density, ∼1 chain/nm2, whereas bulk kinetics are recovered as the graft density is reduced. We hypothesize that this phenomenon is a consequence of local concentration heterogeneities that shift the ATRP equilibrium in favor of the active state and present a simple phenomenological kinetic model that accounts for our data. These findings present an important consideration relevant to the design of precisely defined molecular architectures from surfaces via surface-initiated ATRP.

Screening effect of PEG on avidin binding to liposome surface receptors

This study investigates the screening effect of poly(ethylene glycol)-phospholipids (PE-PEG) on the interaction of avidin with PEGylated liposomes containing surface-bound biotin ligands. The influence of grafting density and lipopolymer chain length is examined. A simple fluorescence assay involving a receptor-mediated fluorescence increase of BODIPY-labeled avidin upon binding to biotinylated lipids is employed to study the screening effect of submicellar concentrations of 1,2-dipalmitoyl- sn-glycero-3-phosphatidylethanolamine- N-[poly(ethylene glycol)-2000] (PE-PEG 2000) and 1,2-dipalmitoyl- sn-glycero-3-phosphatidylethanolamine- N-[poly(ethylene glycol)-5000] (PE-PEG 5000) incorporated into 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC) liposomes. The results show that incorporation of lipopolymers into DPPC lipid bilayers reduces binding of avidin to the biotinylated liposomes, and it is found that the screening effect of PE-PEG 5000 is stronger than that for PE-PEG 2000. Thus, the results reveal that both the grafting density and the polymer length of the PE-PEG lipopolymers are of importance for the ability of water-soluble macromolecules to reach the surface of PEG liposomes. Furthermore, it is found that none of the lipopolymers completely prevents avidin from reaching the surface-bound biotin ligands.