Supersoft and Hyperelastic Polymer Networks with Brushlike Strands

Using a combination of the scaling analysis and molecular dynamics simulations, we study relationship between mechanical properties of networks of graft polymers and their molecular architecture. The elastic response of such networks can be described by replacing the brushlike strands with wormlike strands characterized by the effective Kuhn length which is controlled by the degree of polymerization of the side chains <i>n</i>_sc and their grafting density 1/<i>n</i>_g. In the framework of this approach we have established relationships between the network structural shear modulus <i>G</i>, strands extension ratio β, and architectural triplet [<i>n</i>_sc, <i>n</i>_g, <i>n</i>_x], where <i>n</i>_x is the degree of polymerization of the backbone strand between cross-links. Analysis of the simulation data shows that <i>G</i> could increase with β (<i>G</i> ∝ β), which reflects the “golden rule” of elastomers: softer materials are more deformable. However, networks of graft polymers can also break this rule and demonstrate an increase of the modulus <i>G</i> with decreasing extension ratio β such as <i>G</i> ∝ β^–2. This can be achieved by changing the grafting density of the side chains 1/<i>n</i>_g and keeping <i>n</i>_x and <i>n</i>_sc constant. This peculiar mechanical response of graft polymer networks is in agreement with experimental studies of poly­(dimethylsiloxane) graft polymer elastomers

Synthesis of Molecular Bottlebrushes by Atom Transfer Radical Polymerization with ppm Amounts of Cu Catalyst

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(<i>n</i>-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%

Grafting Poly(OEGMA) Brushes from a Shape Memory Elastomer and Subsequent Wrinkling Behavior

An azide-functionalized shape memory elastomer, poly­(octylene diazoadipate-<i>co</i>-octylene adipate), has been grafted with poly­(oligoethylene glycol) methacrylate (poly­(OEGMA)) brushes via aqueous ARGET (activators regenerated by electron transfer) ATRP. Sequential swelling of the substrate followed by a grafting-from reaction yielded an incompressible brush layer on the shape-memory substrate. Upon heating the substrate above the <i>T</i>_m to return to the primary shape, uniaxial wrinkles perpendicular to the direction of strain with sizes of 27–33 μm appear in addition to micrometer-sized features formed on the temporary shape after grafting. Swelling equilibration time (<i>t</i>₁) and grafting reaction time (<i>t</i>₂) were varied to control wrinkle formation and size. In this manner, we were able to create unique, anisotropic hierarchical surface structures with different length scales and patterns

Switchable Micropatterned Surface Topographies Mediated by Reversible Shape Memory

Reversibly switching topography on micrometer length scales greatly expands the functionality of stimuli-responsive substrates. Here we report the first usage of reversible shape memory for the actuation of two-way transitions between microscopically patterned substrates, resulting in corresponding modulations of the wetting properties. Reversible switching of the surface topography is achieved through partial melting and recrystallization of a semi-crystalline polyester embossed with microscopic features. This behavior is monitored with atomic force microscopy (AFM) and contact angle measurements. We demonstrate that the magnitude of the contact angle variations depends on the embossment pattern

How To Measure Work of Adhesion and Surface Tension of Soft Polymeric Materials

Knowledge of the work of adhesion and surface tension directs the design of new materials for coatings, adhesives, and lubricants. We develop an approach to determine both properties from analysis of equilibrium indentations of rigid particles in contact with soft polymeric materials. In accord with coarse-grained molecular dynamics simulations, the indentation depth is described by the crossover expression combining together the adhesion and wetting models, which takes into account both the elastic energy of the contact and full surface free energy change outside and inside the contact area. The crossover expression is applied to obtain the work of adhesion and substrate surface tension for polystyrene (PS), carboxyl-modified polystyrene (PS-COOH), and poly­(methyl methacrylate) (PMMA) particles in contact with poly­(dimethylsiloxane) (PDMS) networks made of brush-like and linear chains. This analysis results in the work of adhesion <i>W</i> = 48.0 ± 2.9 mN/m for PS/PDMS, <i>W</i> = 268.4 ± 27.0 mN/m for PS-COOH/PDMS, and <i>W</i> = 56.2 ± 2.4 mN/m for PMMA/PDMS and the surface tension of the PDMS substrate to be γ_s = 23.6 ± 2.1 mN/m

Combs and Bottlebrushes in a Melt

We use a combination of the coarse-grained molecular dynamics simulations and scaling analysis to study conformations of bottlebrush and comb-like polymers in a melt. Our analysis shows that a crossover between comb and bottlebrush regimes is controlled by the crowding parameter, Φ, describing overlap between neighboring macromolecules. In comb-like systems characterized by a sparse grafting of side chains (Φ < 1), the side chains and backbones belonging to neighboring macromolecules interpenetrate. However, in bottlebrushes with densely grafted side chains (Φ ≥ 1), the interpenetration between macromolecules is suppressed by steric repulsion between side chains. In this regime, bottlebrush macromolecules can be viewed as filaments with diameter proportional to size of the side chains. For flexible side chains, the crowding parameter is given by Φ ≈ [<i>v</i>/(<i>lb</i>)^3/2]­[(<i>n</i>_sc/<i>n</i>_g + 1)/<i>n</i>_sc^1/2], which depends on both the architectural parameters (degree of polymerization of the side chains, <i>n</i>_sc, and number of backbone bonds between side chains, <i>n</i>_g) and chemical structure of monomers (bond length <i>l</i>, monomer excluded volume <i>v</i>, and Kuhn length, <i>b</i>). Molecular dynamics simulations corroborate this classification of graft polymers and show that the effective macromolecule Kuhn length, <i>b</i>_K, and the mean-square end-to-end distance of the backbone, ⟨<i>R</i>_e,bb²⟩, are universal functions of the crowding parameter Φ for all studied systems

Advancing Reversible Shape Memory by Tuning the Polymer Network Architecture

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

Tuning Multiphase Amphiphilic Rods to Direct Self-Assembly

New methods to direct the self-assembly of particles are highly sought after for multiple applications, including photonics, electronics, and drug delivery. Most techniques, however, are limited to chemical patterning on spherical particles, limiting the range of possible structures. We developed a lithographic technique for fabrication of chemically anisotropic rod-like particles in which we can specify both the size and shape of particles and implement multiple diverse materials to control interfacial interactions. Multiphase rod-like particles, including amphiphilic diblock, triblock, and multiblock were fabricated in the same template mold having a tunable hydrophilic/hydrophobic ratio. Self-assembly of diblock or triblock rods at a water/oil interface led to the formation of bilayer or ribbon-like structures

Submicrometer-Encapsulation of NaBH₄ by Dopamine End-Functionalized Polystyrene: Gas Generation at Oil–Water Interfaces

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

Dynamics of Bottlebrush Networks

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 (<i>b</i>_K), and (iii) relaxation of the bottlebrush network strands between cross-links. The relaxation of side chains having a degree of polymerization (DP), <i>n</i>_sc, dominates the network dynamics on the time scales τ₀ < <i>t</i> ≤ τ_sc, where τ₀ and τ_sc ≈ τ₀(<i>n</i>_sc + 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 <i>G</i>₀^BB(<i>t</i>) ∼ <i>t</i>^–1/2. On time scales <i>t</i> > τ_sc, bottlebrush elastomers behave as networks of filaments with a shear modulus <i>G</i>₀^BB(<i>t</i>) ∼ (<i>n</i>_sc + 1)^−1/4<i>t</i>^–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, τ_BB ≈ τ₀<i>N</i>²(<i>n</i>_sc + 1)^3/2, where <i>N</i> is DP of the bottlebrush backbone between cross-links. In this time interval, the network shear modulus depends on the network molecular parameters as <i>G</i>₀^BB(<i>t</i>) ∼ (<i>n</i>_sc + 1)⁻¹<i>N</i>^–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 (<i>n</i>_sc = 14 and <i>N</i> = 50, 70, 100, 200) measured experimentally