Zhibin Guan

General Strategies for Nanoparticle Dispersion

Traditionally the dispersion of particles in polymeric materials has proven difficult and frequently results in phase separation and agglomeration. We show that thermodynamically stable dispersion of nanoparticles into a polymeric liquid is enhanced for systems where the radius of gyration of the linear polymer is greater than the radius of the nanoparticle. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles. Even when the dispersed state is thermodynamically stable, it may be inaccessible unless the correct processing strategy is adopted, which is particularly important for the case of fullerene dispersion into linear polymers.

Multiphase design of autonomic self-healing thermoplastic elastomers

The development of polymers that can spontaneously repair themselves after mechanical damage would significantly improve the safety, lifetime, energy efficiency and environmental impact of man-made materials. Most approaches to self-healing materials require the input of external energy, healing agents, solvent or plasticizer. Despite intense research in this area, the synthesis of a stiff material with intrinsic self-healing ability remains a key challenge. Here, we show a design of multiphase supramolecular thermoplastic elastomers that combine high modulus and toughness with spontaneous healing capability. The designed hydrogen-bonding brush polymers self-assemble into a hard-soft microphase-separated system, combining the enhanced stiffness and toughness of nanocomposites with the self-healing capability of dynamic supramolecular assemblies. In contrast to previous self-healing polymers, this new system spontaneously self-heals as a single-component solid material at ambient conditions, without the need for any external stimulus, healing agent, plasticizer or solvent.

Self-Healing Supramolecular Block Copolymers†

Polymer, heal thyself! Supramolecular ABA triblock copolymers formed by dimerization of 2-ureido-4-pyrimidinone (UPy) end-functionalized polystyrene-b-poly(n-butyl acrylate) (PS-b-PBA) AB diblock copolymers have been synthesized, resulting in a self-healing material that combines the advantageous mechanical properties of thermoplastic elastomers and the dynamic self-healing features of supramolecular materials.

A biomimetic modular polymer with tough and adaptive properties.

Natural materials employ many elegant strategies to achieve mechanical properties required for survival under varying environmental conditions. Thus these remarkable biopolymers and nanocomposites often not only have a combination of mechanical properties such as high modulus, toughness, and elasticity, but also exhibit adaptive and stimuli-responsive properties. Inspired by skeletal muscle protein titin, we have synthesized a biomimetic modular polymer that not only closely mimics the modular multidomain structure of titin, but also manifests an exciting combination of mechanical properties, as well as adaptive properties such as self-healing and temperature-responsive shape-memory properties.

Malleable and Self-Healing Covalent Polymer Networks through Tunable Dynamic Boronic Ester Bonds

Despite numerous strategies involving dynamic covalent bond exchange for dynamic and self-healing materials, it remains a challenge to be able to tune the malleability and self-healing properties of bulk materials through simple small molecule perturbations. Here we describe the use of tunable rates of boronic ester transesterification to tune the malleability and self-healing efficiencies of bulk materials. Specifically, we used two telechelic diboronic ester small molecules with variable transesterification kinetics to dynamically cross-link 1,2-diol-containing polymer backbones. The sample cross-linked with fast-exchanging diboronic ester showed enhanced malleability and accelerated healing compared to the slow-exchanging variant under the same conditions. Our report demonstrates the possibility of transferring small molecule kinetics to dynamic properties of bulk solid material and may serve as a guide for the rational design of tunable dynamic materials.

Self-healing multiphase polymers via dynamic metal-ligand interactions.

A new self-healing multiphase polymer is developed in which a pervasive network of dynamic metal-ligand (zinc-imidazole) interactions are programmed in the soft matrix of a hard/soft two-phase brush copolymer system. The mechanical and dynamic properties of the materials can be tuned by varying a number of molecular parameters (e.g., backbone/brush degree of polymerization and brush density) as well as the ligand/metal ratio. Following mechanical damage, these thermoplastic elastomers show excellent self-healing ability under ambient conditions without any intervention.

Control of hierarchical polymer mechanics with bioinspired metal-coordination dynamics

In conventional polymer materials, mechanical performance is traditionally engineered via material structure, using motifs such as polymer molecular weight, polymer branching, or copolymer-block design1. Here, by means of a model system of 4-arm poly(ethylene glycol) hydrogels crosslinked with multiple, kinetically distinct dynamic metal-ligand coordinate complexes, we show that polymer materials with decoupled spatial structure and mechanical performance can be designed. By tuning the relative concentration of two types of metal-ligand crosslinks, we demonstrate control over the material’s mechanical hierarchy of energy-dissipating modes under dynamic mechanical loading, and therefore the ability to engineer a priori the viscoelastic properties of these materials by controlling the types of crosslinks rather than by modifying the polymer itself. This strategy to decouple material mechanics from structure may inform the design of soft materials for use in complex mechanical environments.

Direct Synthesis of Polyamides via Catalytic Dehydrogenation of Diols and Diamines

We report a direct synthesis of polyamides via catalytic dehydrogenation of diols and diamines. A PNN pincer ruthenium complex, the Milstein catalyst, was used for this reaction and polyamides with number average molecular weight from ∼10 to 30 kDa could be obtained from a wide variety of diols and diamines bearing aliphatic or aromatic, linear or cyclic spacers. Because of the high catalytic selectivity of primary amine over secondary amine, polyamines could be conveniently incorporated into linear polyamides without tedious protection/deprotection steps. Compared with conventional condensation method, this catalytic system avoids the requirement of stoichiometric preactivation or in situ activation reagents and provides a much cleaner process with high atomic economy.

Enhancing Mechanical Performance of a Covalent Self-Healing Material by Sacrificial Noncovalent Bonds

Polymers that repair themselves after mechanical damage can significantly improve their durability and safety. A major goal in the field of self-healing materials is to combine robust mechanical and efficient healing properties. Here, we show that incorporation of sacrificial bonds into a self-repairable network dramatically improves the overall mechanical properties. Specifically, we use simple secondary amide side chains to create dynamic energy dissipative hydrogen bonds in a covalently cross-linked polymer network, which can self-heal via olefin cross-metathesis. We envision that this straightforward sacrificial bonding strategy can be employed to improve mechanical properties in a variety of self-healing systems.

Multifunctional Dendronized Peptide Polymer Platform for Safe and Effective siRNA Delivery

In this study, we designed and synthesized a biodegradable dendronized polypeptide (denpol) platform for delivery of small interfering RNA (siRNA). The novel denpol architecture combines the multivalency of dendrimers and conformational flexibility of linear polymers for optimal siRNA binding. Multifunctional amino acids were incorporated onto the dendrons and the structure was tuned both systematically and combinatorially to select optimal vectors. By screening a focused library, we identified several denpols that can effectively deliver siRNA to NIH 3T3 cells in vitro and exhibit minimal toxicity. For comparison, the best-performing denpol showed significantly improved transfection efficiency over Lipofectamine in serum-containing media. Fluorescence intracellular trafficking studies indicated that amphiphilicity is important for cell uptake and that the buffering capacity of histidine facilitates endosomal membrane rupture and therefore enhances the transfection efficiency. The combination of high delivery efficiency in serum and low cytotoxicity suggests the denpol system as a promising new carrier for siRNA delivery.