Combining high concentration of reversible hydrogen bonds with a loosely cross-linked chemical network in poly(N,N-dimethylacrylamide-co-methacrylic acid) hydrogels produces dual-network materials with high modulus and toughness on par with those observed for connective tissues. The dynamic nature of the H-bonded cross-links manifests itself in a strong temperature and strain rate dependence of hydrogel mechanical properties. We have identified several relaxation regimes of a hydrogel by monitoring a time evolution of the time-average Young’s modulus ⟨E(t)⟩ = σ(t)/ε̇t as a function of the strain rate, ε̇, and temperature. At low temperatures (e.g., 3 °C), ⟨E(t)⟩ first displays a Rouse-like relaxation regime (⟨E(t)⟩ ∼ t–0.5), which is followed by a temporary (physical) network regime (⟨E(t)⟩ ∼ t–0.14) at intermediate time scales and then by an associating liquid regime (⟨E(t)⟩ ∼ t–0.93) at the later times. With increasing temperature to 22 °C, the temporary network plateau displays lower modulus values, narrows, and shifts to shorter time scales. Finally, the plateau vanishes at 37 °C. It is shown that the energy dissipation in hydrogels due to strain-induced dissociation of the H-bonded cross-links increases hydrogel toughness. The density of dissipated energy at small deformations scales with strain rate as UT ∼ ε̇0.53. We develop a model describing dynamics of deformation of dual networks. The model predictions are in a good agreement with experimental data. Our analysis of the dual network’s dynamics provides general frameworks for characterization of such materials.

We present a single-step, grafting-to synthetic method for the encapsulation of particulate NaBH4 by dopamine end-functionalized polymer chains. Metal–catechol coordination chemistry is used to produce core–shell capsules, which generate H2 gas exclusively upon adsorption to an oil–water interface. Significantly, the synthetic process enables facile control of core diameter, shell thickness, and the chemistry of both shell and core. The interfacial reactivity of these stimuli-responsive capsules may be engineered for various applications such as medical diagnostics, therapeutics, and subsurface imaging. In addition to their triggered reactivity, the capsules react in a manner independent of pressure and are thus well-suited for high pressure subsurface environments.

Thermally expandable microcapsules (TEMs) with wrinkled shells are prepared by one-step suspension polymerization, allowing for encapsulation and controlled release of cargos. Wrinkling results from concurrent crosslinking of shell copolymers and vaporization of volatile reagents along with density increase upon polymerization. Through control of the vapor pressure of the reagents and systematic variation of the suspension composition, microcapsules with different degrees of wrinkling are prepared, ranging from locally dimpled to highly crumpled morphologies. The corresponding increase of the surface-to-volume ratio results in increasing release rate of encapsulated oil red dye as a model cargo. As such, in addition to shell thickness and radius, the wrinkleness provides an effective control parameter for adjusting the release rate. The wrinkled microcapsules with a large surface-to-volume ratio may find applications in drug delivery, chemicals scavenging, and self-healing materials.

This paper describes the synthesis, swelling behavior, and applications of well-defined narrowly dispersed zwitterionic (ZW) microgels prepared by dispersion polymerization in aqueous media. Microgel stability was achieved through precise control of the dispersant composition, timely addition of a cross-linker after the nucleation stage, and the utilization of ionic initiators. Dispersion polymerization allowed for incorporation of both hydrophilic and hydrophobic comonomers, including acrylamide (AAm) and dopamine methacrylamide (Dopa-MA). The broad variety of compositions created many opportunities for practical applications such as encapsulation of mineral acids and synthesis of metal nanoparticles. The swelling behavior of ZW-co-AAm microgels in 6 M HCl was particularly interesting: whereas ZW moieties remained stable in contact with the strong acid, the amide groups underwent hydrolysis to carboxylic acid, resulting in microgel contraction and acid release. Zw-co-Dopa-MA microgels were employed as particulate microreactors, where the ZW moieties played a role of an osmotic pump delivering Ag ions to the DOPA moieties for conversion to silver nanoparticles uniformly dispersed inside the microgel particles.

Because of counteraction of a chemical network and a crystalline scaffold, semicrystalline polymer networks exhibit a peculiar behavior—reversible shape memory (RSM), which occurs naturally without applying any external force and particular structural design. There are three RSM properties: (i) range of reversible strain, (ii) rate of strain recovery, and (iii) decay of reversibility with time, which can be improved by tuning the architecture of the polymer network. Different types of poly(octylene adipate) networks were synthesized, allowing for control of cross-link density and network topology, including randomly cross-linked network by free-radical polymerization, thiol–ene clicked network with enhanced mesh uniformity, and loose network with deliberately incorporated dangling chains. It is shown that the RSM properties are controlled by average cross-link density and crystal size, whereas topology of a network greatly affects its extensibility. We have achieved 80% maximum reversible range, 15% minimal decrease in reversibility, and fast strain recovery rate up to 0.05 K–1, i.e., ca. 5% per 10 s at a cooling rate of 5 K/min.

The deformation dynamics of bottlebrush networks in a melt state is studied using a combination of theoretical, computational, and experimental techniques. Three main molecular relaxation processes are identified in these systems: (i) relaxation of the side chains, (ii) relaxation of the bottlebrush backbones on length scales shorter than the bottlebrush Kuhn length (bK), and (iii) relaxation of the bottlebrush network strands between cross-links. The relaxation of side chains having a degree of polymerization (DP), nsc, dominates the network dynamics on the time scales τ0 < t ≤ τsc, where τ0 and τsc ≈ τ0(nsc + 1)2 are the characteristic relaxation times of monomeric units and side chains, respectively. In this time interval, the shear modulus at small deformations decays with time as G0BB(t) ∼ t–1/2. On time scales t > τsc, bottlebrush elastomers behave as networks of filaments with a shear modulus G0BB(t) ∼ (nsc + 1)−1/4t–1/2. Finally, the response of the bottlebrush networks becomes time independent at times scales longer than the Rouse time of the bottlebrush network strands, τBB ≈ τ0N2(nsc + 1)3/2, where N is DP of the bottlebrush backbone between cross-links. In this time interval, the network shear modulus depends on the network molecular parameters as G0BB(t) ∼ (nsc + 1)−1N–1. Analysis of the simulation data shows that the stress evolution in the bottlebrush networks during constant strain-rate deformation can be described by a universal function. The developed scaling model is consistent with the dynamic response of a series of poly(dimethylsiloxane) bottlebrush networks (nsc = 14 and N = 50, 70, 100, 200) measured experimentally.

•First observation of bond scission in molecular bottlebrushes upon sonication.•Higher scission rates and smaller products of “hairy” macromolecules.•A stronger drag force is consistent with the Rouse dynamics.Polymer bottlebrushes may be viewed as hairy flexible cylinders with a long backbone and a thick corona of densely grafted polymer chains. The corona not only controls the bottlebrush diameter, but also provides an additional drag force. We report the study of backbone scission by external forces caused by ultrasonication. A series of bottlebrushes with the same backbone and different side-chain lengths was prepared by ATRP. Bond fracture was induced by pulsed ultrasound in a dilute chloroform solution and ex-situ monitored through molecular imaging of reaction products by atomic force microscopy. The scission rate was found to increase with side chain length, while the limiting length of fractured bottlebrushes displayed a decrease. The experiment showed a good agreement with the Rouse model of polymer dynamics, which suggests that solvent drains through the corona of bottlebrush side-chains.

Shape memory polymers (SMPs) have been shown to accurately replicate photonic structures that produce tunable optical responses, but in practice, these responses are limited by the irreversibility of conventional shape memory processes. Here, we report the intensity modulation of a diffraction grating utilizing two-way reversible shape changes. Reversible shifting of the grating height was accomplished through partial melting and recrystallization of semicrystalline poly(octylene adipate). The concurrent variations of the grating shape and diffraction intensity were monitored via atomic force microscopy and first order diffraction measurements, respectively. A maximum reversibility of the diffraction intensity of 36% was repeatable over multiple cycles. To that end, the reversible shape memory process is shown to broaden the functionality of SMP-based optical devices.Keywords: elastomers; photonic structures; responsive surfaces; reversible shape memory; shape memory polymers;

•Programming multiple shapes isothermally at one constant temperature.•Triple shape memory for polymers with narrow crystallization transitions.•Individual shapes secured by distinct crystalline scaffolds.•Demonstration of “one-way reversible shape memory”.•Maximization of reversible shape change with control of fixation time.Here we present a new strategy for enabling triple shape memory (TSM), in which different shapes may be programmed at one constant fixation temperature during isothermal crystallization of a poly(octylene adipate) elastomer. Unlike traditional TSM programming techniques that utilize separate thermal transitions to encode each distinct shape, here we report that multiple shapes may be encoded independently at different time stages of the same isothermal crystallization process, and then recovered sequentially at different temperatures upon heating. As such, the new method extends TSM capabilities to conventional semicrystalline elastomers that exhibit a single and narrow thermal transition. Unexpectedly, shape fixation does not depend on crystallinity acquired prior to sample deformation, which suggests that different shapes are secured by an individual crystalline scaffold with minor contribution of the pre-existing crystals. To that end, the isothermal protocol for TSM programming was utilized to demonstrate a “one-way reversible” shape memory transformation controlled by fixation time.

•A persistent macrocycle of 1803.2 g/mol is equipped with two ATRP initiation sides.•ATRP enables positioning of the macrocycle, central between two molecular brushes.•The brush-cycle-brush ABA macromolecules were obtained in a controlled manner.•Molecular imaging by AFM visualized the ring position as defect in the brush structures.A synthesis route towards conjugates comprised of two macromolecular brushes and a central shape persistent macrocycle utilizing atom transfer radical polymerization techniques is presented. The extended brush conformation could be visualized by atomic force microscopy on mica substrates, which allows for the observation of the position of the macrocycle within individual brush-cycle-brush conjugates. As a result of the architecture and the properties of the combined structure elements a functional macromolecule is obtained that might regulate accessibility to a central shape persistent macrocycle via reversible brush-coil transitions.

The structural details of bottlebrush polymers, specifically the grafting density and molecular weight distribution of side chains, influence their physical properties; however, they are difficult to analyze using conventional techniques. Herein we report the synthesis, characterization, and molecular imaging of bottlebrush macromolecules with both uniform and bimodal length distributions of poly(n-butyl acrylate) (PnBA) side chains. The densely grafted copolymers were prepared via the “grafting from” approach using atom transfer radical polymerization (ATRP). Bottlebrush macromolecules with both shorter and longer grafted chains were prepared by removal of a fraction of the bromine chain ends of the initial densely grafted brush by selective capping with 4-butoxy-TEMPO and subsequent chain extension of remaining active chains forming longer PnBA grafts. This procedure provided bottlebrush macromolecules with two distinct degrees of polymerization of the grafted side chains herein called bimodal grafts. AFM imaging of individual macromolecules confirmed the formation of wormlike structures with a distinct halo of diffuse side chains originating from bottlebrushes with bimodal PnBA grafts. To quantify the grafting density and dispersity of the initial monomodal side chains, the side chains were cleaved from the backbone and independently characterized. Utilizing a combination of AFM molecular imaging and the Langmuir–Blodgett technique, the grafting density of monomodal bottlebrushes was measured. The distance between macromolecules is linearly proportional to the weight-average degree of polymerization of the side chains for both the monomodal and bimodal brushes.

Unique star-like polymeric architectures composed of bottlebrush arms and a molecular spoked wheel (MSW) core were prepared by atom transfer radical polymerization (ATRP). A hexahydroxy-functionalized MSW (MSW6-OH) was synthesized and converted into a six-fold ATRP initiator (MSW6-Br). Linear chain arms were grafted from MSW6-Br and subsequently functionalized with ATRP moieties to form six-arm macroinitiators. Grafting of side chains from the macroinitiators yielded four different star-shaped bottlebrushes with varying lengths of arms and side chains, i.e., (450-g-20)6, (450-g-40)6, (300-g-60)6, and (300-g-150)6. Gel permeation chromatography analysis and molecular imaging by atomic force microscopy confirmed the formation of well-defined macromolecules with narrow molecular weight distributions. Upon adsorption to an aqueous substrate, the bottlebrush arms underwent prompt dissociation from the MSW core, followed by scission of covalent bonds in the bottlebrush backbones. The preferential cleavage of the arms is attributed to strong steric repulsion between bottlebrushes at the MSW branching center. Star-shaped macroinitiators may undergo aggregation which can be prevented by sonication.

Bottlebrush macromolecules can be regarded as molecular tensile machines, where tension is self-generated along the backbone due to steric repulsion between densely grafted side chains. This intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion to the substrate. To investigate the effects of tension on the electronic structure of polythiophene (PT), bottlebrush macromolecules were prepared by grafting poly(n-butyl acrylate) (PBA) side chains from PT macroinitiators by atom transfer radical polymerization (ATRP). The fluorescence spectra of submonolayers of PT bottlebrushes were measured on a Langmuir–Blodgett (LB) trough with the backbone tension adjusted by controlling the side-chain length, surface pressure, and chemical composition of a substrate. The wavelength of maximum emission has initially red-shifted, followed by a blue-shift as the backbone tension increases from 0 to 2.5 nN, which agrees with DFT calculations. The red-shift is ascribed to an increase in the conjugation length due to the extension of the PT backbone at lower force regime (0–1.0 nN), while the blue-shift is attributed to deformations of bond lengths and angles in the backbone at higher force regime (1.0–2.5 nN).

We present a general strategy for enabling reversible shape transformation in semicrystalline shape memory (SM) materials, which integrates three different SM behaviors: conventional one-way SM, two-way reversible SM, and one-way reversible SM. While two-way reversible shape memory (RSM) is observed upon heating and cooling cycles, the one-way RSM occurs upon heating only. Shape reversibility is achieved through partial melting of a crystalline scaffold which secures memory of a temporary shape by leaving a latent template for recrystallization. This behavior is neither mechanically nor structurally constrained, thereby allowing for multiple switching between encoded shapes without applying any external force, which was demonstrated for different shapes including hairpin, coil, origami, and a robotic gripper. Fraction of reversible strain increases with cross-linking density, reaching a maximum of ca. 70%, and then decreases at higher cross-linking densities. This behavior has been shown to correlate with efficiency of securing the temporary shape.

We here report the synthesis and characterization of a complex polymeric architecture based on a block copolymer with a cylindrical brush block and a single-chain polymeric nanoparticle block folded due to strong intramolecular hydrogen-bonds. The self-assembly of these constructs on mica surfaces was studied with atomic force microscopy, corroborating the distinct presence of block copolymer architectures.

Molecular tensile machines are bottlebrush molecules where tension along the backbone is self-generated due to steric repulsion between the densely grafted side chains. Upon adsorption onto a substrate, this intrinsic tension is amplified to the nanonewton range depending on the side chain length, grafting density, and interaction with the substrate. In this paper, bottlebrushes with a disulfide linker in the middle of the backbone were designed to study the effect of force and temperature on the scission of an individual disulfide bond. The scission process was monitored on molecular length scales by atomic force microscopy. The scission rate constant has been shown to increase exponentially with bond tension but decrease with temperature. This anti-Arrhenius behavior is ascribed to the decrease of substrate surface energy upon heating, which overpowers the corresponding effects of thermal energy and temperature dependent pre-exponential factor. Quantitative analysis using the force-modified Arrhenius and transition state theory (TST) equations, respectively, was conducted to determine the dissociation energy, maximum rupture force, and activation barrier of a disulfide bond under tension.

Molecular bottlebrushes were prepared by ICAR (initiators for continuous activator regeneration) atom transfer radical polymerization (ATRP) and supplemental activator and reducing agent (SARA) ATRP in the presence of 50 ppm Cu-based catalyst. Poly(n-butyl acrylate) (PBA) side chains were grafted from a polymethacrylate backbone resulting in well-defined molecular bottlebrushes. Imaging of individual bottlebrush macromolecules by atomic force microscopy corroborated the targeted degrees of polymerization of the backbone and side chains. Initiation efficiency was determined by cleaving the side chains to be around 50%.

Spontaneous degradation of bottlebrush macromolecules on aqueous substrates was monitored by atomic force microscopy. Scission of C─C covalent bonds in the brush backbone occurred due to steric repulsion between the adsorbed side chains, which generated bond tension on the order of several nano-Newtons. Unlike conventional chemical reactions, the rate of bond scission was shown to decrease with temperature. This apparent anti-Arrhenius behavior was caused by a decrease in the surface energy of the underlying substrate upon heating, which results in a corresponding decrease of bond tension in the adsorbed macromolecules. Even though the tension dropped minimally from 2.16 to 1.89 nN, this was sufficient to overpower the increase in the thermal energy (kBT) in the Arrhenius equation. The rate constant of the bond-scission reaction was measured as a function of temperature and surface energy. Fitting the experimental data by a perturbed Morse potential V = V0(1 - e-βx)2 - fx, we determined the depth and width of the potential to be V0 = 141 ± 19 kJ/mol and β-1 = 0.18 ± 0.03 Å, respectively. Whereas the V0 value is in reasonable agreement with the activation energy Ea = 80–220 kJ/mol of mechanical and thermal degradation of organic polymers, it is significantly lower than the dissociation energy of a C─C bond De = 350 kJ/mol. Moreover, the force constant Kx = 2β2V0 = 1.45 ± 0.36 kN/m of a strained bottlebrush along its backbone is markedly larger than the force constant of a C─C bond Kl = 0.44 kN/m, which is attributed to additional stiffness due to deformation of the side chains.

Significant tension on the order of 1 nN is self-generated along the backbone of bottlebrush macromolecules due to steric repulsion between densely grafted side chains. The intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion to the substrate. These molecules were employed as miniature tensile machines to study the effect of mechanical force on the kinetics of disulfide reduction by dithiothreitol (DTT). For this purpose, bottlebrush macromolecules containing a disulfide linker in the middle of the backbone were synthesized by atom transfer radical polymerization (ATRP). The scission reaction was monitored through molecular imaging by atomic force microscopy (AFM). The scission rate constant increases linearly with the concentration of DTT and exponentially with mechanical tension along the disulfide bond. Moreover, the rate constant at zero force is found to be significantly lower than the reduction rate constant in bulk solution, which suggests an acidic composition of the water surface with pH = 3.7. This work demonstrates the ability of branched macromolecules to accelerate chemical reactions at specific covalent bonds without applying an external force.

The backbone of molecular bottle-brushes undergoes spontaneous degradation upon spreading on a solid substrate. The self-generated tension in the brush backbone is ascribed to steric repulsion between the densely grafted side chains. The paper discusses two approaches for controlling the bond-scission process on the molecular and macroscopic scales, respectively. On the molecular scale, the tension linearly increases with the distance from the backbone ends and attains its maximum value in the middle section of the backbone. When the backbone becomes shorter than the side chains, the tension is focused precisely on the central bond resulting in the predominant mid-chain fracture of the brush backbone. On the macroscopic scale, addition of a linear polymer to a melt of molecular bottle-brushes alters the film pressure and thus allows controllable positioning of the fracture zone within a spreading film.

Molecular imaging by AFM provides direct and quantitative information about branching topology including length and distribution of branches, not accessible by other methods. In this paper, we report the analysis of branching in linear acrylate-based macromolecules synthesized via free radical polymerization (FRP) and atom transfer radical polymerization (ATRP). The branched structures are formed from chain transfer reactions, specifically transfer to polymer. Quantitative analysis showed that both methods produced branched species, with FRP having degree of branching of 0.035 ± 0.003% and ATRP having a degree of branching of 0.025 ± 0.002%, when measured per backbone repeat unit. The observed lower branching density in ATRP compared to FRP is consistent with a recent 13C NMR study, which also found that controlled radical methods, such as ATRP give lower degrees of branching than FRP. The absolute fraction of long chain branches measured by AFM is significantly lower than the total amount of branches evaluated by measurement of fraction of quaternary carbons using 13C NMR, indicating a predominant intramolecular chain transfer.

The paper presents scaling analysis of mechanical tension generated in densely branched macromolecules tethered to a solid substrate with a short linker. Steric repulsion between branches results in z-fold amplification of tension in the linker, where z is the number of chain-like arms. At large z ∼ 100–1000, the generated tension may exceed the strength of covalent bonds and sever the linker. Two types of molecular architectures were considered: polymer stars and polymer “bottlebrushes” tethered to a solid substrate. Depending on the grafting density, one distinguishes the so-called mushroom, loose grafting, and dense grafting regimes. In isolated (mushroom) and loosely tethered bottlebrushes, the linker tension is by a factor of smaller than the tension in a tethered star with the same number of arms z. In densely tethered stars, the effect of interchain distance (d) and number of arms (z) on the magnitude of linker tension is given by f ≅ f0z3/2(b/d) for stars in a solvent environment and f ≅ f0z2(b/d)2 for dry stars, where b is the Kuhn length and f0 ≅ kBT/b is intrinsic bond tension. These relations are also valid for tethered bottlebrushes with long side chains. However, unlike molecular stars, bottlebrushes demonstrate variation of tension along the backbone f ≅ f0s z1/2/d as a function of distance s from the free end of the backbone. In dense brushes (d ≅ b) with z ≅ 1000, the backbone tension increases from f ≅ f0 ≅ 1 pN at the free end of the backbone (s ≅ b) to its maximum f ≅ zf0 ≅ 1 nN at the linker to the substrate (s ≅ zb).

Mechanical activation of chemical bonds typically involves the application of external forces, which implies a broad distribution of bond tensions. We demonstrate that controlling the flow profile of a macromolecular fluid generates and delineates mechanical force concentration, enabling a hierarchical activation of chemical bonds on different length scales from the macroscopic to the molecular. Bond tension is spontaneously generated within brushlike macromolecules as they spread on a solid substrate. The molecular architecture creates an uneven distribution of tension in the covalent bonds, leading to spatially controlled bond scission. By controlling the flow rate and the gradient of the film pressure, one can sever the flowing macromolecules with high precision. Specific chemical bonds are activated within distinct macromolecules located in a defined area of a thin film. Furthermore, the flow-controlled loading rate enables quantitative analysis of the bond activation parameters.

Spreading of homogeneous mixtures of bottle-brush and linear macromolecules of poly(n-butylacrylate) on a solid substrate has been monitored on the molecular scale by atomic force microscopy. Despite the nearly identical chemical composition and similar molecular weight, brush-like macromolecules move markedly slower than linear chains. Moreover, smaller bottle-brushes have been shown to flow faster than the larger bottle-brushes, resulting in fractionation of the macromolecules along the spreading direction. This behavior was explained by the difference in sliding friction coefficient between the bottle-brush macromolecules and linear chains with the substrate. A theoretical model of molecular size separation is in a good agreement with experimental data.

A series of three cylindrical molecular brushes with poly(ε-caprolactone)-b-poly(n-butyl acrylate) side chains were investigated with regard to the effect of the intramolecular confinement on the crystallization behavior of the PCL core block. While the length of the PCL block was maintained (nPCL = 50), the degree of polymerization of the PBA corona block, prepared by atom transfer radical polymerization, was varied from nPBA = 0 (PCL brush) to nPBA = 52 and 163. The crystallization behavior of the studied polymers was shown to be different in thin films and bulk samples. In thin films, the brushlike macromolecules were fully extended and remained segregated due to the steric repulsion of the adsorbed PBA corona. Under the constraint of the backbone extension, crystallization of the PCL core led to formation of a characteristic spinelike morphology. In bulk samples, the core−shell confinement did not prohibit breakout crystallization, leading to the formation of a spherulitic morphology similar to linear and brushlike PCL. However, the crystallization process of the PCL-b-PBA brushes was significantly slower as compared to the linear and brushlike counterparts as it evolved through a transition from the molecularly segregated core−shell morphology to a lamellar organization of multiple molecules. Drawing of fibers containing nascent crystallites resulted in the final morphology composed of crystalline lamellae parallel to the fiber axis with the PCL blocks oriented perpendicular to the axis. The lamellae thickness was shown to decrease with the length of the amorphous PBA block.

Brush-like macromolecules are unique polymer molecules whose conformation and physical properties are controlled by steric repulsion of densely grafted side chains. Molecules can be either flexible or stiff, depending on the grafting density and the length of the side chains. Molecules can switch their conformation in response to alterations in the surrounding environment, e.g. changes of temperature, solvent quality, pH, and ionic strength. Furthermore, one can control molecular conformation and related properties using external stimuli such as light and electro-magnetic fields. Molecular brushes are also very informative model systems for experimental studies of polymer properties. Molecules are readily visualized by atomic force microscopy, opening unique opportunities to observe single polymer molecules as they move, order, and react on surfaces. Brush-like molecular architectures are well-known in biology where they are responsible for various functions including mucociliary clearance of lung airways and mechanical performance of articular cartilage. Polymer chemistry is currently making the first steps in controlling molecular architecture and understanding the distinctive properties of molecular brushers. This article reviews the characteristic physical properties of well-defined molecular brushes and the different strategies employed for their preparation, with particular focus on synthesis via controlled radical polymerization techniques.

While the effect of the insoluble block length on micelle properties is well understood, the effect of the soluble block is still controversial. We, therefore, have investigated the effect of the molecular weight of the soluble block on the critical micelle concentration (CMC), aggregation number, and hydrodynamic radius of spherical polymer micelles. Spherical micelles were formed from polystyrene-b-polyisoprene (PS-b-PI) in heptane, which was a good solvent for PI and a poor solvent for PS. Measurements were performed on two series of PS-b-PI with a constant PS block (19 and 39 kDa, respectively) and PI blocks varying from 10 to 100 kDa. For samples with large PI blocks, the experimental data were found to be in agreement with the commonly used star-like model. However, the experimental data for samples with short PI blocks deviated from the crew-cut micelle model. To correctly capture the crossover between the crew-cut and star-like regimes, it was found necessary to use recently developed scaling theory which explicitly considers all contributions to the free energy of the micelle. In agreement with theory, the aggregation number decreased while hydrodynamic radius and CMC increased with the molecular weight of the PI block. An interesting finding of these experiments is that the micelles of the 19 kDa series are in equilibrium at 25 °C, whereas the 39 kDa samples with the longer PS core block are “frozen” at room temperature. This was confirmed by SAXS measurements of core expansion upon heating which revealed a glass transition temperature of the 39 kDa samples at 28 ± 1 °C. The temperature value is consistent with 10% swelling of the PS core with heptane as determined by SAXS and SLS.

Soft lithography based on photocurable perfluoropolyether (PFPE) was used to mold and replicate poly(styrene-b-isoprene) block-copolymer micelles within a broad range of shapes and sizes including spheres, cylinders, and torroids. These physically assembled nanoparticles were first formed in a selective solvent for one block then deposited onto substrates having various surface energies in an effort to minimize the deformation of the micelles due to attractive surface forces. The successful molding of these delicate nanoparticles underscores two advantages of PFPE as a molding material. First, it allows one to minimize particle deformation due to adsorption by using low energy substrates. Second, PFPE is not miscible with the organic micelles and thus prevents their dissociation. For spherical PS-b-PI micelles, a threshold value of the substrate surface energy for the mold to lift-off cleanly, that is, the particles remain adhered to the substrate after mold removal was determined to be around γ ≅ 54 mJ/m2. For substrates with higher surface energies (>54 mJ/m2), the micelles undergo flattening which increase the contact area and thus facilitate molding, although at the expense of particle deformation. The results are consistent with theoretical predictions of a molding range for substrate surface energies, which depends on the size, shape, and mechanical properties of the particles. In a similar fashion, cylindrical PS-b-PI micelles remain on the substrate at surface energies γ ≥ 54 mJ/m2 after a mold removal. However, cylindrical micelles behaved differently at lower surface energies. These micelles ruptured due to their inability to slide on the surfaces during mold lift-off. Thus, the successful molding of extended objects is attainable only when the particle is adsorbed on higher energy substrates where deformation can still be kept at a minimum by using stronger materials such as carbon nanotubes for the master.

The backbone of molecular bottle-brushes undergoes spontaneous degradation upon spreading on a solid substrate. The self-generated tension in the brush backbone is ascribed to steric repulsion between the densely grafted side chains. The paper discusses two approaches for controlling the bond-scission process on the molecular and macroscopic scales, respectively. On the molecular scale, the tension linearly increases with the distance from the backbone ends and attains its maximum value in the middle section of the backbone. When the backbone becomes shorter than the side chains, the tension is focused precisely on the central bond resulting in the predominant mid-chain fracture of the brush backbone. On the macroscopic scale, addition of a linear polymer to a melt of molecular bottle-brushes alters the film pressure and thus allows controllable positioning of the fracture zone within a spreading film.