Grafting Density Of Side Chains Research Articles

Molecular brushes with long polyfluorene backbones and polymethacrylic acid side chains: Luminescent properties and self‐organization in solutions

AbstractA series of molecular brushes with a polyfluorene (PF) backbone and polymethacrylic acid side chains of varying lengths were prepared by atom transfer radical polymerization. The structure and composition of the synthesized compounds were confirmed by 1H NMR and IR spectroscopy. Effect of the length of the backbone on spectral and conformational parameters of the macromolecules in solutions was analyzed. The grafting density of side chains was about 90%. Spectral methods have been used to determine the dependence of side chain grafting on the luminescent properties of polymer solutions, including quantum yields. It was shown that an optimal length of polymethacrylic acid side chains provides solubility of the polymer brushes. Solutions of PF‐graft‐polymethacrylic acid complexes with the model substance curcumin were investigated. It was established that the molecular brushes containing curcumin form monomolecular micelles. Molecular brushes with zinc phthalocyanine, potential systems for photodynamic, and photothermal therapy, were studied.

Hairy Gels: A Computational Study.

We present results of MD and MC simulations of the equilibrium properties of swelling gels with comb-like or bottlebrush subchains and compare them to scaling-theory predictions. In accordance with theory, the simulation results demonstrate that swelling coefficient of the gel increases as a function of the polymerization degree of the main chains and exhibits a very weak maximum (or is virtually constant) as a function of the polymerization degree and grafting density of side chains. The bulk osmotic modulus passes through a shallow minimum as the polymerization degree of the side chains increases. This minimum is attributed to the onset of overlap of side chains belonging to different bottlebrush strands in the swollen gel.

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.

Synthesis, Structure, Hydrodynamics and Thermoresponsiveness of Graft Copolymer with Aromatic Polyester Backbone at Poly(2-isopropyl-2-oxazoline) Side Chains.

New thermoresponsive graft copolymers with an aromatic polyester backbone and poly(2-isopropyl-2-oxazoline) (PiPrOx) side chains are synthesized and characterized by NMR and GPC. The grafting density of side chains is 0.49. The molar masses of the graft-copolymer, its backbone, side chains, and the modeling poly-2-isopropyl-2-oxaziline are 74,000, 19,000, 4300, and 16,600 g·mol−1, respectively. Their conformational properties in nitropropane as well as thermoresponsiveness in aqueous solutions are studied and compared with that of free side chains, i.e., linear PiPrOx with a hydrophobic terminal group. In nitropropane, the graft-copolymer adopts conformation of a 13-arm star with a core of a collapsed main chain and a PiPrOx corona. Similarly, a linear PiPrOx chain protects its bulky terminal group by wrapping around it in a selective solvent. In aqueous solutions at low temperatures, graft copolymers form aggregates due to interaction of hydrophobic backbones, which contrasts to molecular solutions of the model linear PiPrOx. The lower critical solution temperature (LCST) for the graft copolymer is around 20 °C. The phase separation temperatures of the copolymer solution were lower than that of the linear chain counterpart, decreasing with concentration for both polymers.

Amphiphilic molecular brushes with regular polydimethylsiloxane backbone and poly-2-isopropyl-2-oxazoline side chains. 1. Synthesis, characterization and conformation in solution

The samples of molecular brushes with a polydimethylsiloxane backbone and side chains of poly-2-isopropyl-2-oxazoline have been synthesized. Regular polydimethylsiloxane with undecene tosylate groups in every fourth siloxane unit was used as a macroinitiator. Two prepared samples are characterized by the same grafting density of side chains, but their different length and copolymer molar mass, 400,000 and 710000 g⋅mol−1. Solution behavior study revealed that macroinitiator and its precursor are flexible-chain polymers and macroinitiator’s thermodynamic rigidity is similar to one of polydimethylsiloxane. It was shown that the conformation of the graft copolymer molecules in a selective solvent resembles a polymer star because of the backbone of molecular brushes is strongly folded and shielded from the solvent by the side chains. The thermoresponsiveness of the synthesized molecular brushes in aqueous solutions has been demonstrated.

Breaking translational symmetry via polymer chain overcrowding in molecular bottlebrush crystallization

One of the fundamental laws in crystallization is translational symmetry, which accounts for the profound shapes observed in natural mineral crystals and snowflakes. Herein, we report on the spontaneous formation of spherical hollow crystals with broken translational symmetry in crystalline molecular bottlebrush (mBB) polymers. The unique structure is named as mBB crystalsome (mBBC), highlighting its similarity to the classical molecular vesicles. Fluorescence resonance energy transfer (FRET) experiments show that the mBBC formation is driven by local chain overcrowding-induced asymmetric lamella bending, which is further confirmed by correlating crystalsome size with crystallization temperature and mBBʼs side chain grafting density. Our study unravels a new principle of spontaneous translational symmetry breaking, providing a general route towards designing versatile nanostructures.

Optical and Morphological Characteristics of Polymer Molecular Brushes with Varied Grafting Density and Binary Bioactive Radachlorine-Containing Nanosystems Based on Them

The optical and morphological characteristics of polymer molecular brushes with the polyimide backbone and polymethacrylic acid side chains, and also of binary nanosystems based on the molecular brushes and Radachlorine®, a photosensitizer of second generation, were studied. The brushes had the same backbone and approximately equal molecular mass of side chains, but strongly differed in the side chain grafting density. The side chain grafting density influences the molecular-conformation and morphological parameters of free molecular brushes and of the corresponding binary nanosystems.

Brush-Like Polymers and Entanglements: From Linear Chains to Filaments.

Dynamics of melts and solutions of high molecular weight polymers and biopolymers is controlled by topological constraints (entanglements) imposing a sliding chain motion along an effective confining tube. For linear chains, the tube size is determined by universal packing number Pe, the number of polymer strands within a confining tube that is required for chains to entangle. Here we show that in melts of brush-like (graft) polymers, consisting of linear chain backbones with grafted side chains, Pe is not a universal number and depends on the molecular architecture. In particular, we use coarse-grained molecular dynamics simulations to demonstrate that the packing number is a nonmonotonic function of the ratio Rnsc/Rng of the size of the side chains Rnsc to that of the backbone spacer between neighboring grafting points Rng. This parameter characterizes the degree of mutual interpenetration between side chains of the same macromolecule. We show that Pe of brush-like polymers first decreases with increasing side chain grafting density in the dilute side chain regime (Rnsc < Rng), then begins to increase in the regime of overlapping side chains (Rnsc > Rng), approaching the value for linear chains in the limit of densely grafted side chains. This dependence of the packing number reflects a crossover from chain-like entanglements in systems with loosely grafted side chains (comb-like polymers) to entanglements between flexible filaments (bottlebrush-like polymers). Our simulation results are in agreement with the experimental data for the dependence of a plateau modulus on the molecular architecture of graft poly(n-butyl acrylates) and poly(norbornene)-graft-poly(lactide) melts.

Single-Chain Conformational Characteristics of Comb-Like Polyelectrolytes: Molecular Dynamics Simulation Study

The single-chain conformation of a model comb polyelectrolyte, in which the electric charges are located on the last monomer of side chains, has been studied using molecular dynamics simulations. The model chain has a hydrophobic backbone and hydrophilic side chains. The influence of parameters such as the length of backbone and side chains, the grafting density of side chains, flexibility of the chain and different strengths of backbone hydrophobicity have been investigated. At high grafting density of side chains, a transition from turn to pearl-necklace and finally to globular conformation is observed by increasing the hydrophobicity. Radius of gyration (Rg) of the backbone depends linearly on the number of backbone monomers at low values of the backbone hydrophobicity, but at high values of the backbone hydrophobicity, because of the appearance of turn-like segments between successive globular sections in the pearl-necklace structure, the scaling exponent of Rg versus N is larger. Also, the chain flexibility affects the backbone conformation significantly. The increase of the persistence length leads to the appearance of the folded and then the extended conformations. In addition to the statistical properties of the chains, the investigation of the relaxation of the chains conformation shows that, the backbone takes almost its final conformation in the early time steps of the simulation for all conditions. It means that the relaxation time of the chain is relatively short (fast relaxation).

Synthesis of bottlebrush polymers with v-shaped side chains

Bottlebrush polymers with v-shaped side chains were fabricated via “grafting onto” method by a combination of reversible addition fragmentation chain transfer polymerization (RAFT), atom transfer radical polymerization (ATRP)/ring opening polymerization (ROP), and copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction. RAFT was used to prepare poly(glycidyl methacrylate) backbones with an azide side group on each repeating unit (PGMA-N3). ATRP/ROP was used to synthesize a diversity of v-shaped side chains with an alkynyl group in the middle of polymer chain including poly(methyl methacrylate) (v(PMMA)2), poly(tert-butyl acrylate) (v(PtBA)2), polystyrene (v(PS)2), and poly(ε-caprolactone)-block-polystyrene (v(PCL-b-PS)). CuAAC reaction was then conducted to graft the alkynyl-functionalized side chains onto azide containing backbones and produce the corresponding bottlebrush polymers with varied v-shaped side chains such as PGMA283-g-v(PtBA22)2, PGMA100-g-v(PtBA22)2, PGMA100-g-v(PtBA35)2, PGMA100-g-v(PS10)2, PGMA100-g-v(PMMA30)2, and PGMA100-g-v(PCL10-b-PS24). For all these bottlebrush polymers with varied v-shaped polymer side chains, a high side chain grafting density was obtained with a value above ca. 80% based on the “grafting onto” method developed in this study.

From Adhesion to Wetting: Contact Mechanics at the Surfaces of Super-Soft Brush-Like Elastomers

Fundamental understanding of rigid particle indentation into soft elastic substrates has been elusive for decades. In conventional heterogeneous and multicomponent systems, the ill-defined interplay between elastic and capillary forces has confounded explanation of the crossover region between the classical wetting and adhesion regimes. Herein, we study the indentation behavior of micrometer-sized silica particles on supersoft, solvent-free PDMS elastomers with brush-like network strands. By varying the side chain grafting density and the cross-linking density of the networks, we control their elastic modulus from ∼1 to 100 kPa without adding solvent. This isostructurally regulated balance between elastic and capillary forces allows for accurate mapping of the entire range of particle–substrate interactions by measuring indentation depth as a function of substrate stiffness and particle radius. A generalized theoretical model, accounting for the collaborative contribution of both forces to the system free...

Effects of concentration and pH on the thermosensitive behavior of a graft copolymer with polyimide backbone and poly(N,N-dimethylamino-2-ethyl methacrylate) side chains

Aqueous solutions of the graft copolymer with a polyimide backbone and poly(N,N-dimethylamino-2-ethyl methacrylate) side chains with a molecular mass of M = 4.7 × 105 and a grafting density of side chains of 0.44 are investigated by light scattering and turbidimetry. Solutions are studied in a wide concentration range of 0.0008–0.0250 g/cm3 at рН values varying from 2 to 12 for each concentration. The temperature dependences of optical transmission, scattered light intensity, and hydrodynamic radii of scattering objects are obtained. It is shown that the copolymer is thermosensitive only at pH > 8.0. A decrease in acidity of the medium at a fixed concentration of the copolymer is accompanied by a decline in temperatures corresponding to the onset and end of phase separation Т 1 and Т 2, leading to the narrowing of this interval. At constant рН values, temperatures Т 1 and Т 2 rise with solution dilution, while the phase transition interval becomes wider.

Conformational Properties of Comb-Like Polyelectrolytes: A Coarse-Grained MD Study.

This article presents a coarse-grained molecular dynamics study of single comb-like polyelectrolyte or ionomer chains in aqueous solution. The model polymer is comprised of a hydrophobic backbone chain with grafted side chains that terminate in anionic headgroups. The comb-polymer is modeled at a coarse-grained level with implicit treatment of the solvent. The computational study rationalizes conformational properties of the backbone chain and localization of counterions as functions of side chain length, grafting density of side chains, backbone stiffness, and counterion valence. The main interplay that determines the ionomer properties unfolds between electrostatic interactions among charged groups, hydrophobic backbone interactions, and steric effects induced by the pendant side chains. Depending on the density of branching sites, we have found two opposing effects of side chain length on the backbone gyration radius and local persistence length. Variation in comb-polyelectrolyte architecture is shown to have nontrivial effects on the localization of mobile counterions. Changes in Bjerrum length and counterion valence are also shown to alter the strength of Coulomb interactions and emphasize the role of excluded-volume effects on controlling the backbone conformational behavior. The results of simulations are in qualitative agreement with existing experimental and theoretical studies. The comprehensive conformational picture provides a framework for future studies of comb-polyelectrolyte systems.

Effects of intra/inter-molecular potential parameters, length and grafting density of side-chains on the self-assembling behavior of poly(3′-alkylthiophene)s in the ordered state

The self-assembling behavior of conjugated regioregular poly(3′-alkylthiophene) (P3AT) polymers with various grafting density and length of alkyl side-chains are systematically examined by the coarse-grained molecular dynamics (CGMD) simulations. The effective CG interaction potentials are determined by taking into account the atomistic structural details from all-atom (AA) MD through the mapping route between the AAMD and CGMD data via the Iterative Boltzmann Inversion (IBI) methods. Except the non-bonded interaction potential between two thiophenes is modified by simply calculating the potential energy curve for them in a trans-orientation. With the dihedral angle constraint, two types of the grafted side-chains, which are located on both sides (anti-) and the same side (syn-) of the backbones, are modeled at GD = 100%, 33% and 50%, 25%, respectively. For densely grafted side-chains, such as when GD = 100% and 50%, the strong π–π attraction between the thiophene rings plays the most dominant role on the self-assembling behavior of P3AT molecules. Thus, we observe the formation of single-walled and double-walled lamellae, which however become more twisted with increasing the length of side-chains at GD = 100% and 50%, respectively. This is not surprising as longer flexible alkyl side-chains cause the backbone thiophene rings to suffer a larger distortion degree. When the grafting density is reduced significantly, not only the π–π attraction between the thiophene rings can induce the ordered packing of main-chains, but also the significant space between the side-chains can facilitate the interdigitation and aggregation of side-chains. Accordingly, for the poly(3′-nonylthiophene), poly(3′-dodecylthiophene), and poly(3′-pentadecylthiophene) molecules with 25% grafting density, we predict the formation of polygonal columns, which are formed by the packing of main-chains parallel to the column axes and the aggregation of the side-chains within the columns. When the side-chain length is increased further, these aggregated long side-chains can no longer be constrained within the polygonal minor-domains. Instead, double-walled lamellae become the most stable phase for P3AT molecules with 25% GD.

Solution behavior of polyimide‐graft‐polystyrene copolymers in selective solvents

ABSTRACTA polyimide‐graft‐polystyrene (PI‐g‐PS) copolymer with a polyimide backbone and polystyrene side chains was synthesized by the “grafting from” method using styrene polymerization on a polyimide multicenter macroinitiator via ATRP mechanism. The side chain grafting density z = 0.86 of PI‐g‐PS is rather high for graft‐copolymers synthesized by the ATRP method. Molecular characteristics and solution behavior of PI‐g‐PS were studied in selective solvents using light scattering and viscometry methods. In all solvents, the backbone tends to avoid contact with a poor solvent. To describe the conformation and hydrodynamic properties of PI‐g‐PS macromolecules in thermodynamically good solvents for side chains and PI‐g‐PS, the wormlike spherocylinder model is used. Macromolecules of the studied graft‐copolymer are characterized by high equilibrium rigidities (Kuhn segment length &gt;20 nm). In Θ‐conditions, PI‐g‐PS macromolecules may be modeled by a rigid prolate ellipsoid of revolution with a low asymmetry form and a collapsed backbone as the ellipsoid core. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1539–1546

Synthesis and Characterization of Cyclic Brush-Like Polymers by N-Heterocyclic Carbene-Mediated Zwitterionic Polymerization of N-Propargyl N-Carboxyanhydride and the Grafting-to Approach

Cyclic brush-like polymers were synthesized by tandem organo-mediated zwitterionic polymerization and a grafting-to approach. The cyclic polymer backbone, consisting of poly(N-propargylglycine) (c-PNPG), was synthesized by an N-heterocyclic carbene (NHC)-mediated zwitterionic ring-opening polymerization of N-propargyl N-carboxyanhydride. The polymerization proceeds in a quasi-living manner, allowing access to c-PNPG of well-defined chain length. The cyclic architecture of the polymers was verified by size exclusion chromatography (SEC) and mass spectroscopy (MS), as well as scanning probe characterization. Poly(ethylene glycol) functionalized with azido end-groups was subsequently grafted onto the c-PNPG by the copper-mediated azide/alkyne cycloaddition reaction (CuAAC). The side chain grafting density was determined by 1H NMR spectroscopy and SEC analysis. The grafting efficiency is low (<19%) when the cyclic backbone is comprised of a c-PNPG homopolymer. The efficiency can be significantly improved (up ...

Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: A Molecular Dynamics Simulation

The scission kinetics of bottle-brush molecules in solution and on an adhesive substrate is modeled by means of Molecular Dynamics simulation with Langevin thermostat. Our macromolecules comprise a long flexible polymer backbone with $L$ segments, consisting of breakable bonds, along with two side chains of length $N$, tethered to each segment of the backbone. In agreement with recent experiments and theoretical predictions, we find that bond cleavage is significantly enhanced on a strongly attractive substrate even though the chemical nature of the bonds remains thereby unchanged. We find that the mean bond life time $<\tau>$ decreases upon adsorption by more than an order of magnitude even for brush molecules with comparatively short side chains $N=1 \div 4$. The distribution of scission probability along the bonds of the backbone is found to be rather sensitive regarding the interplay between length and grafting density of side chains. The life time $<\tau>$ declines with growing contour length $L$ as $<\tau>\propto L^{-0.17}$, and with side chain length as $<\tau>\propto N^{-0.53}$. The probability distribution of fragment lengths at different times agrees well with experimental observations. The variation of the mean length $L(t)$ of the fragments with elapsed time confirms the notion of the thermal degradation process as a first order reaction.

Microstructured poly(2-oxazoline) bottle-brush brushes on nanocrystalline diamond

We report on the preparation of microstructured poly(2-oxazoline) bottle-brush brushes (BBBs) on nanocrystalline diamond (NCD). Structuring of NCD was performed by photolithography and plasma treatment to result in a patterned NCD surface with oxidized and hydrogenated areas. Self-initiated photografting and photopolymerization (SIPGP) of 2-isopropenyl-2-oxazoline (IPOx) resulted in selective grafting of poly(2-isopropenyl-2-oxazoline) (PIPOx) polymer brushes only at the oxidized NCD areas. Structured PIPOx brushes were converted by methyl triflate into the polyelectrolyte brush macroinitiator for the living cationic ring-opening polymerization (LCROP) of 2-oxazolines. The LCROP was performed with 2-ethyl-2-oxazoline (EtOx) as well as 2-(carbazolyl)ethyl-2-oxazoline (CarbOx) as monomers, resulting in structured bottle-brush brushes (BBB) with different pendant side chains and functionalities. FT-IR spectroscopy, fluorescence microscopy, and AFM measurements indicated a high side chain grafting density as well as quantitative and selective reactions. Poly(2-oxazoline) BBBs containing hole conducting carbazole moieties on NCD as electrode material may open the way to advanced amperometric biosensing systems.

Bottle-Brush Brushes: Cylindrical Molecular Brushes of Poly(2-oxazoline) on Glassy Carbon

We report on the synthesis of brushes of bottle-brushes of poly(2-oxazoline)s on polished glassy carbon (GC) substrates. First, homogeneous and stable poly(2-isopropenyl-2-oxazoline) (PIPOx) brush layers with thicknesses up to 160 nm were created directly onto GC by the self-initiated photografting and photopolymerization (SIPGP) of 2-isopropenyl-2-oxazoline (IPOx). Kinetic studies reveal a linear increase in thickness with the polymerization time. In a second reaction, the pendant 2-oxazoline ring of the PIPOx brushes were used for the living cationic ring-opening polymerization (LCROP) with different substituted 2-oxazoline monomers to form the side chains. Also for the second surface-initiated LCROP from the surface-bound macroinitiator brushes, the thickness increase with the polymerization time was found to be linear and reproducible. Characterization of the resulting bottle-brush brushes by FTIR spectroscopy, contact angle, and AFM indicates a high side chain grafting density and quantitative reacti...

Cylindrical Molecular Brushes of Poly(2-oxazoline)s from 2-Isopropenyl-2-oxazoline

We report on the synthesis and characterization of cylindrical molecular brushes based on poly(2-oxazoline)s (POx). The dual-functional monomer, 2-isopropenyl-2-oxazoline (IPOx), was first converted to a poly(2-isopropenyl-2-oxazoline), backbone by free radical (PIPOxR) or living anionic polymerization (PIPOxA). Quantitative reaction with methyl triflate yields a macroinitiator salt (PIPOxOTfR/A) for the preparation of molecular brushes via the grafting from approach by living cationic polymerization of 2-oxazolines (2-methyl-, 2-ethyl-, and 2-isopropyl-2-oxazoline). Characterization of the resulting molecular brushes by NMR and FTIR spectroscopy indicates a very high side chain grafting density and quantitative reactions. Visualization of adsorbed molecular brushes by AFM corroborates this assumption. Furthermore, the lower critical solution temperatures of the POx molecular brushes were determined. The transition temperatures were found to be very defined, reversible, and with no noticeable hysteresis.