Individual Polymer Chains Research Articles

Hierarchical Self‐Assembly of Supramolecular Muscle‐Like Fibers

AbstractAn acid–base switchable [c2]daisy chain rotaxane terminated with two 2,6‐diacetylamino pyridine units has been self‐assembled with a bis(uracil) linker. The complementary hydrogen‐bond recognition patterns, together with lateral van der Waals aggregations, result in the hierarchical formation of unidimensional supramolecular polymers associated in bundles of muscle‐like fibers. Microscopic and scattering techniques reveal that the mesoscopic structure of these bundles depends on the extended or contracted states that the rotaxanes show within individual polymer chains. The observed local dynamics span over several length scales because of a combination of supramolecular and mechanical bonds. This work illustrates the possibility to modify the hierarchical mesoscopic structuring of large polymeric systems by the integrated actuation of individual molecular machines.

How surface interactions freeze polymer molecules at room temperature: a single molecule approach

The planar conformation assumed by polyfluorenes is used as a model system to investigate how solid surfaces interact with an individual polymer chain as well as to access the effects of substrates and interchain interactions on the polymer conformation. We demonstrate that confocal fluorescence microscopy is very sensitive to characterize photophysical processes associated to conformation assumed by a single conjugated unit of poly (9,9 dioctylfluorene) or PFO. Emissions from interacting molecules have been resolved for the first time between the energies of the planar (β phase) and the non-planar (amorphous) phase that were associated to perturbations of the primary chain conformation. But we have found the planar conformation is assumed by single PFO molecules due to strong interactions with the hydrophilic quartz surfaces. This PFO conformation persists frozen even at room temperature, although emission lines are significantly broadened due to interactions with vibrating substrate atoms.

Suppressed thermal conductivity of ultrathin carbon nanotube(2, 1) upon hydrogenation.

Non-equilibrium molecular dynamic simulations reveal that the thermal conductivity of ultrathin carbon nanotube (CNT)(2, 1) is significantly suppressed upon hydrogenation. The addition of hydrogen atoms to two-coordinated carbon atoms lowers the participation ratios of phonon modes, thus indicating that the spatial distribution of phonons becomes localized. Furthermore, the phonon lifetimes are remarkably shortened in hydrogenated CNT(2, 1) (HCNT(2, 1)) compared with those of bare CNT(2, 1). The lowered participation ratios and lifetimes of phonon modes are responsible for the significant reduction of thermal conductivity in HCNT(2, 1). Our study is also helpful for understanding the weakened thermal transport abilities in carbon polymers, namely, the cross links formed between individual polymer chains will hinder the thermal conduction along polymers, even though the single straight carbon polymer has a high and divergent thermal conductivity.

Directly Measuring the Complete Stress–Strain Response of Ultrathin Polymer Films

The inherently fragile nature of ultrathin polymer films presents difficulties to the measurement of their mechanical properties, which are of interest in packaging, electronics, separations, and other manufacturing fields. More fundamentally, the direct measurement of ultrathin film mechanical properties is necessary for understanding changes in intrinsic material properties at reduced size scales, for example, when the film thickness alters the equilibrium configuration of the polymer chains. We introduce a method for ultrathin film tensile testing that stretches a two-dimensionally macroscopic, yet nanoscopically thin, polymer film on the surface of water. For polystyrene films, we observe a precipitous decrease in mechanical properties (Young’s modulus, strain at failure, and nominal stress at failure) for film thicknesses down to 15 nm, less than the characteristic size of an individual polymer chain, yielding new insights into the changes in polymer chain entanglements in confined states.

Controlling Macroscopic Properties by Tailoring Nanoscopic Interfaces in Tapered Copolymers

Systematic variation of comonomer concentration in individual polymer chains is responsible for unique phase behaviors in different types of copolymers. Controlled self-assembly of gradient copolymers into desired morphologies is theoretically understood but practically challenging, and identifying heterogeneous phase partitioning of individual comonomers in those resulting morphological regions can be difficult. Building on previous work where improved methods were used to elucidate heterogeneous comonomer partitioning in styrene–butadiene gradient copolymers [Clough Macromolecules 2014, 47, 2625], the arrangement of the styrene and butadiene monomers in only a fraction of the total chain length is used here to significantly perturb the overall morphology and physical properties of copolymers. Importantly, the chemical composition of all copolymers was held nearly fixed in this study. The resulting tapered and inverse tapered block copolymers contain nanometer length scale interfaces that differ from one another and differ dramatically from that observed in a control block copolymer composed of chains with a discrete interface. Evidence is presented that butadiene can reside in rigid environments, styrene can reside in mobile environments, and their relative amounts can be varied based on the gradient design. The connection between the molecular design of the gradient, the resulting nanometer-length scale interfacial structures, and mechanical properties is demonstrated using a combination of variable temperature solid-state NMR, modulated DSC (differential scanning calorimetry), AFM (atomic force microscopy), and rheology experiments.

Self-oscillating AB diblock copolymer developed by post modification strategy.

We prepared AB diblock copolymer composed of hydrophilic poly(ethylene oxide) segment and self-oscillating polymer segment. In the latter segment, ruthenium tris(2,2'-bipyridine) (Ru(bpy)3), a catalyst of the Belousov-Zhabotinsky reaction, is introduced into the polymer architecture based on N-isopropylacrylamide (NIPAAm). The Ru(bpy)3 was introduced into the polymer segment by two methods; (i) direct random copolymerization (DP) of NIPAAm and Ru(bpy)3 vinyl monomer and (ii) post modification (PM) of Ru(bpy)3 with random copolymer of NIPAAm and N-3-aminopropylmethacrylamide. For both the diblock copolymers, a bistable temperature region (the temperature range; ΔTm), where the block copolymer self-assembles into micelle at reduced Ru(bpy)3(2+) state whereas it breaks-up into individual polymer chain at oxidized Ru(bpy)3(3+) state, monotonically extends as the composition of the Ru(bpy)3 increases. The ΔTm of the block copolymer prepared by PM is larger than that by DP. The difference in ΔTm is rationalized from the statistical analysis of the arrangement of the Ru(bpy)3 moiety along the self-oscillating segments. By using the PM method, the well-defined AB diblock copolymer having ΔTm (ca. 25 °C) large enough to cause stable self-oscillation can be prepared. The periodic structural transition of the diblock copolymer in a dilute solution ([Polymer] = 0.1 wt. %) is closely investigated in terms of the time-resolved dynamic light scattering technique at constant temperature in the bistable region. A macroscopic viscosity oscillation of a concentrated polymer solution (15 wt. %) coupled with the periodic microphase separation is also demonstrated.

Multifunctional biocompatible and biodegradable folic acid conjugated poly(ε-caprolactone)-polypeptide copolymer vesicles with excellent antibacterial activities.

Cancer patients after chemotherapy may also suffer bacterial attack due to badly decreased immunity. Although with high bacterial efficacy, conventional antibiotics are prone to inducement of drug resistance and may be not suitable for some cancer patients. In contrast, antibacterial peptides are highly effective in inhibiting bacteria without inducing resistance in pathogens. Presented in this article is a novel kind of highly effective antibacterial peptide-based biocompatible and biodegradable block copolymer vesicle. The copolymer is poly(ε-caprolactone)-block-poly[phenylalanine-stat-lysine-stat-(lysine-folic acid)] [PCL19-b-poly[Phe12-stat-Lys9-stat-(Lys-FA)6]], which can self-assemble into vesicles in aqueous solution. The biocompatible and biodegradable PCL forms the vesicle membrane, whereas the poly[Phe12-stat-Lys9-stat-(Lys-FA)6] block constitutes the vesicle coronas. Compared to the individual polymer chains, the vesicles showed enhanced antibacterial activities against both Gram-positive and Gram-negative bacteria (16 μg mL(-1)) due to the locally concentrated antibacterial poly[Phe12-stat-Lys9-stat-(Lys-FA)6] coronas, which may avoid the inducement of antibiotic-resistant bacteria and side effects of multidrug interactions. Furthermore, folic acid is introduced into the vesicle coronas for potential further applications such as cancer-targeted drug delivery. Moreover, the amino groups can be further functionalized when necessary. This low cytotoxic, biocompatible, biodegradable, and antibacterial vesicle (without antibiotic resistance) may benefit patients after tumor surgery because it is highly anti-inflammatory, and it is possible to deliver the anticancer drug to tumor cells simultaneously.

Modeling of Branching Distributions in Butyl Acrylate Polymerization Applying Monte Carlo Methods

Monte Carlo (MC) methods were applied to the complex kinetic model of butyl acrylate polymerizations. The MC simulator mcPolymer developed in house allows for handling chain‐length‐dependent termination kinetics. The simulations provide detailed information on the microstructure of each individual polymer chain. For example, the number of short chain branches (SCBs) on each polymer chain and the length of the monomer sequence between two short chain branches are captured. It is shown that the maximum of the branching density distribution is shifted to shorter chain lengths with reaction time. Variation of initiator concentration does not lead to significant changes in the sequence length distributions and branching distributions as long as the same monomer conversion is reached.

Efficient Synthesis of Single‐Chain Polymer Nanoparticles via Amide Formation

Single‐chain technology (SCT) allows the transformation of individual polymer chains to folded/collapsed unimolecular soft nanoparticles. In this work we contribute to the enlargement of the SCT toolbox by demonstrating the efficient synthesis of single‐chain polymer nanoparticles (SCNPs) via intrachain amide formation. In particular, we exploit cross‐linking between active methylene groups and isocyanate moieties as powerful “click” chemistry driving force for SCNP construction. By employing poly(methyl methacrylate)‐ (PMMA‐) based copolymers bearing β‐ketoester units distributed randomly along the copolymer chains and bifunctional isocyanate cross‐linkers, SCNPs were successfully synthesized at r.t. under appropriate reaction conditions. Characterization of the resulting SCNPs was carried out by means of a combination of techniques including size exclusion chromatography (SEC), infrared (IR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, dynamic light scattering (DLS), and elemental analysis (EA).

Microcanonical Analysis of Aggregation Transitions in Flexible Polymer Systems

By means of extensive replica-exchange Monte Carlo simulations of a generic coarse-grained model, we examine systems consisting of up to 11 individual polymer chains with 5 monomers each. The application of microcanonical analysis methods reveals new details about the anatomy of aggregation transitions that were previously inaccessible via conventional canonical analysis. We find evidence for phase separation in the transition region and classify the transition as first order. Finally we show that the aggregation transition consists of a hierarchy of sub-phase transitions and discuss the implications of this finding.

Improving the performance of MEH-PPV based light emitting diode by incorporation of graphene nanosheets

The effect of incorporation of graphene nanosheets on the efficiency of poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) based light emitting diodes (LED) has been examined by varying the graphene concentration from 0 to 0.1wt%. It was observed that graphene doping enhances the photoluminescence (PL) emission from the PPV layer by ~6 times at the blending concentration of 0.005wt%. This is attributed to the isolation of individual polymer chains that quenches the inter-chain relaxations and boosts the intra-chain transitions. The improvement in device luminance is also found to be ~6 times as compared to that with MEH-PPV only LED at 0.005wt% graphene concentration. This is due to the high charge carrier mobility in graphene nanostructure that assists in balancing the charge carrier concentration in the emissive layer. Also due to its low LUMO level, graphene improves electron injection from the cathode. Both these effects lead to enhancement in the device luminescence. Employment of graphene in this manner also leads to lowering of turn-on voltage of the device. This is attributed to the ability of graphene sheets to establish an interconnected conducting network in the polymer matrix that lowers the device resistance. However, at higher graphene concentration, this property short circuits the device structure, which greatly deteriorates its performance. The graphene concentration, therefore, should be kept below the percolation threshold level to develop high efficiency devices.

A simulation approach to study of entropic elasticity properties of polymer chain molecules using atomic scale Monte-Carlo

This paper describes atomic scale Monte–Carlo studies of entropic elasticity properties of individual polymer chain molecules. An efficient numerical Monte –Carlo sampling approach is outlined and used to evaluate the entropic contribution to the total elastic force.Theoretic predictions of mechanical properties of polymer molecules, particularly complex bio-molecules (proteins, lipids, etc.), are difficult due to effects of entropic elasticity. Entropic elastic force can be a significant contributor to the free energy F of the polymer chain and can even exceed interatomic potential energy U under external mechanical load. Monte-Carlo based approach allows to achieve atomic resolution for molecular structure in contrast to analytical methods. Specific load-extension curves are obtained numerically for a group of molecules with degenerate potential energy profiles. Results of the atomistic modeling are compared with the limiting continuum model of the same type of polymers. The extent of the linear and nonlinear elastic regimes and dependence on the molecular weight and geometric parameters of the molecules are discussed. A significant divergence with the continuummodel behavior is observed at smaller bond angles for all elongationsof the molecule. Linearity of the entropic force exists in a wide range ofthe elongations, however, molecules with low gyration radii(densely packedpolymers) are linear mostly in extensionor unfolding, while very sparsely packed molecules are linear mostly in thecontraction mode. The achieved result cannot be reproduced withinthe settings of the continuum model and required an application of atomic scale Monte-Carlo approach developed by our group.

Identification and design of novel polymer-based mechanical transducers: A nano-structural model for thin film indentation

Mechanical characterization is important for understanding small-scale systems and developing devices, particularly at the interface of biology, medicine, and nanotechnology. Yet, monitoring sub-surface forces is challenging with current technologies like atomic force microscopes (AFMs) or optical tweezers due to their probe sizes and sophisticated feedback mechanisms. An alternative transducer design relying on the indentation mechanics of a compressible thin polymer would be an ideal system for more compact and versatile probes, facilitating measurements in situ or in vivo. However, application-specific tuning of a polymer's mechanical properties can be burdensome via experimental optimization. Therefore, efficient transducer design requires a fundamental understanding of how synthetic parameters such as the molecular weight and grafting density influence the bulk material properties that determine the force response. In this work, we apply molecular-level polymer scaling laws to a first order elastic foundation model, relating the conformational state of individual polymer chains to the macroscopic compression of thin film systems. A parameter sweep analysis was conducted to observe predicted model trends under various system conditions and to understand how nano-structural elements influence the material stiffness. We validate the model by comparing predicted force profiles to experimental AFM curves for a real polymer system and show that it has reasonable predictive power for initial estimates of the force response, displaying excellent agreement with experimental force curves. We also present an analysis of the force sensitivity of an example transducer system to demonstrate identification of synthetic protocols based on desired mechanical properties. These results highlight the usefulness of this simple model as an aid for the design of a new class of compact and tunable nanomechanical force transducers.

Thermal Conduction in a Single Polyethylene Chain Using Molecular Dynamics Simulations

Research on the thermal conduction in a single polymer chain is significant for the improvement of the thermal property of bulk polymer materials. We calculate the thermal conductivity of a single polyethylene (PE) chain by using both the Green—Kubo approach and a nonequilibrium molecular dynamics simulation method. The results suggest that the thermal conductivity of an individual polymer chain is very high although bulk PE is a thermal insulator, even divergent in our case. Moreover, the thermal conductivity of PE chains is observed to increase with the chain length.

How Far Are Single-Chain Polymer Nanoparticles in Solution from the Globular State?

Single-chain nanoparticles (SCNPs) are unimolecular soft nano-objects, consisting of individual polymer chains collapsed to a certain degree by means of intramolecular bonding. Many of the potential applications of SCNPs rely on their particular molecular architecture. Even if the ultimate goal is to produce globular protein-like soft nanoparticles, recent small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) results-supported by computer simulations-indicate that SCNPs in solution actually adopt sparse configurations. Herein we compile size data from the literature for a large number of SCNPs in solution, covering from covalent to noncovalent bonded SCNPs, and provide a comparison with the corresponding data for compact or partially swollen globules of the same nature and molar mass. This comparison gives a clear idea of how far from the compact globule limit are current SCNPs. A quantification of the departure from the globular state is provided in terms of size scaling laws. This procedure facilitates a comparison with the size scaling laws observed for folded proteins with globular conformation as well as intrinsically disordered proteins which, on average, exhibit a certain local compaction when compared to chemically denatured proteins. Lastly, the underlying physical mechanism for the noncompact morphology of SCNPs in solution is put forward, and guidelines for the potential synthesis of true SCNP globules in solution are suggested.

Nitrosocarbonyl Hetero-Diels–Alder Cycloaddition: A New Tool for Conjugation

It is demonstrated that nitrosocarbonyl hetero-Diels-Alder chemistry is an efficient and versatile reaction that can be applied in macromolecular synthesis. Polyethylene glycol functionalized with a hydroxamic acid moiety undergoes facile coupling with cyclopentadiene-terminated polystyrene, through a copper-catalyzed as well as thermal hetero-Diels-Alder reaction. The mild and orthogonal methods used to carry out this reaction make it an attractive method for the synthesis of block copolymers. The resulting block copolymers were analyzed and characterized using GPC and NMR. The product materials could be subjected to thermal retro [4 + 2] cycloaddition, allowing for the liberation of the individual polymer chains and subsequent recycling of the diene-terminated polymers.

Confinement-driven increase in ionomer thin-film modulus.

Ion-conductive polymers, or ionomers, are critical materials for a wide range of electrochemical technologies. For optimizing the complex heterogeneous structures in which they occur, there is a need to elucidate the governing structure-property relationships, especially at nanoscale dimensions where interfacial interactions dominate the overall materials response due to confinement effects. It is widely acknowledged that polymer physical behavior can be drastically altered from the bulk when under confinement and the literature is replete with examples thereof. However, there is a deficit in the understanding of ionomers when confined to the nanoscale, although it is apparent from literature that confinement can influence ionomer properties. Herein we show that as one particular ionomer, Nafion, is confined to thin films, there is a drastic increase in the modulus over the bulk value, and we demonstrate that this stiffening can explain previously observed deviations in materials properties such as water transport and uptake upon confinement. Moreover, we provide insight into the underlying confinement-induced stiffening through the application of a simple theoretical framework based on self-consistent micromechanics. This framework can be applied to other polymer systems and assumes that as the polymer is confined the mechanical response becomes dominated by the modulus of individual polymer chains.

Structure of a spin-crossover Fe(II)-1,2,4-triazole polymer complex gel in toluene. Small angle neutron scattering and viscoelastic studies

In this paper we report the study of metallo-organic polymer gels of [Fe(II) (4-octadecyl-1,2,4-triazole)3(ClO4)2]n in toluene. A thorough investigation of the molecular structure and the viscoelastic properties of the metallo-organic polymer gels in toluene has been carried out by small angle neutron scattering and controlled stress oscillatory rheology as a function of temperature and concentration in order to understand the self-assembly process leading to the gel formation. The results obtained point out a side-by-side aggregation of the individual metallo-organic polymer chains, behaving as “living” polymers, into fibres of different cross-sections as responsible for the gelation of these systems.

Effect of Polymer Chain Folding on the Transition from H- to J-Aggregate Behavior in P3HT Nanofibers

A combination of wavelength-, time-, and polarization-resolved photoluminescence imaging on isolated P3HT nanofibers of varying molecular weight (from 10 to 65 kDa) has revealed a transition in dominant exciton coupling from primarily interchain (H-aggregation) for low molecular weight nanofibers, to predominantly intrachain (J-aggregation) coupling for high molecular weight nanofibers. Based on nanofiber width measurement from TEM imaging, the driving force for this transition appears to be folding of individual polymer chains within the lamellae, resulting in enhanced chain planarity and reduced torsional disorder.

Single-chain nanoparticles vs. star, hyperbranched and dendrimeric polymers: effect of the nanoscopic architecture on the flow properties of diluted solutions.

The flow properties of dilute solutions of linear, star, hyperbranched and dendrimeric polymers have been the subject of numerous studies. However, no systematic analysis has been carried out for the case of single-chain nanoparticles (SCNPs) of different nature, which are unimolecular soft nano-objects consisting of individual polymer chains collapsed to a certain degree by means of intramolecular bonding. On the basis of the fractal nature of SCNPs and experimental data of the hydrodynamic radius, a simple predictive power-law between the intrinsic viscosity and molecular weight is proposed. Furthermore, a comparison is made between the intrinsic viscosities of SCNPs and of low-functionality stars, hyperbranched and dendrimeric polymers of the same chemical nature and molecular weight. As a consequence of their complex nanoscopic architecture, the intrinsic viscosities of SCNPs are systematically smaller than those of linear chains and low-functionality stars. When compared with hyperbranched and dendrimeric polymers, a complex behaviour is found, this being highly dependent on the molecular weight and amount of X-linkers of SCNPs.