Fluorophilic Sigma(σ)-Lock Self-Healable Copolymers.

Although F-Containing molecules and macromolecules are often used in molecular biology to increase the binding with Lewis acidic groups by introducing favorable C-F dipoles, there is virtually no experimental evidence and limited understanding of the nature of these interactions, especially their role in synthetic polymeric materials. These studies elucidate the molecular origin of inter- and intra-Chain interactions responsible for self-healing of F-Containing copolymers composed of pentafluorostyrene and n-butyl acrylate units (p(PFS/nBA). Guided by dynamic surface oscillating force (SOF) and spectroscopic measurements supported by molecular dynamics (MD) simulations, these studies show that the reformation of σ-σ orbitals in -C-F of PFS and CH3CH2- of nBA units enables the recovery of entropic energy via fluorophilic-σ-lock van der Waals forces when PFS/nBA molar ratios are ~50/50. The strength of these interactions determined experimentally for self-healable PFS/nBA compositions is in the order ~0.3 kcal/mol which primarily comes from fluorophilic-σ-lock (~70 %) contributions. These interactions are significantly diminished for non-self-healable counterparts. Strongly polarized -C-F σ orbitals create lateral dipolar forces enhancing the affinity towards -C-H orbitals, facilitating energetically favorable interactions. Entropic recovery driven by non-Covalent bonding offers a valuable tool in designing materials with unique functionalities, particularly self-healable batteries and energy storage devices.

Dynamic Interfaces in Self-Healable Polymers.

It is well-established that interfaces play critical roles in biological and synthetic processes. Aside from significant practical applications, the most accessible and measurable quantity is interfacial tension, which represents a measure of the energy required to create or rejoin two surfaces. Owing to the fact that interfacial processes are critical in polymeric materials, this review outlines recent advances in dynamic interfacial processes involving physics and chemistry targeting self-healing. Entropic interfacial energies stored during damage participate in the recovery, and self-healing depends upon copolymer composition and monomer sequence, monomer molar ratios, molecular weight, and polymer dispersity. These properties ultimately impact chain flexibility, shape-memory recovery, and interfacial interactions. Self-healing is a localized process with global implications on mechanical and other properties. Selected examples driven by interfacial flow and shape memory effects are discussed in the context of covalent and supramolecular rebonding targeting self-healable materials development.

Ring-and-Lock Interactions in Self-Healable Styrenic Copolymers.

Commodity copolymers offer many useful applications, and their durability is critical in maintaining desired functions and retaining sustainability. These studies show that primarily alternating styrene/n-butyl acrylate [p(Sty/nBA)] copolymers self-heal without external intervention when monomer molar ratios are within the 45:55-53:47 range. This behavior is attributed to the favorable interchain interactions between aliphatic nBA side groups being sandwiched by aromatic rings forming ring-and-lock associations driven by pi-sigma-pi (π-σ-π) interactions. Guided by molecular dynamics (MD) simulations combined with spectroscopic and thermomechanical analysis, the ring-and-lock interchain van der Waals forces between π orbitals of aromatic rings and sigma components of aliphatic side groups are responsible for self-healing. Despite the frequent occurrence of these interactions in biological systems (proteins, nucleic acids, lipids, and polysaccharides), these largely unexplored weak and ubiquitous molecular forces between the soft acid aliphatic and soft base aromatic electrons may be valuable assets in the development of polymeric materials with sustainable properties.

Electrically Accelerated Self-Healable Polyionic Liquid Copolymers.

Electrically accelerated self-healable poly(ionic liquids) copolymers that exhibit resistor-capacitor (RC) circuit properties are developed. At low alternating current (AC) frequencies these materials behave as a resistor (R), whereas at higher frequencies as a capacitor (C). These properties are attributed to a combination of dipolar and electrostatic interactions in (1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethyl-sulfonyl)imide) copolymerized with methyl methacrylate (MMA) monomers to form p(MEBIm-TSFI/MMA)] copolymers. When the monomer molar ratio (MEBIm-TSFI:MMA) is 40/60, these copolymers are capable of undergoing multiple damage-repair cycles and self-healing is accelerated by the application of alternating 1.0-4.0 V electric field (EF). Self-healing in the absence of EFs is facilitated by van der Waals (vdW) interactions, but the application of AC EF induces back and forth movement of charges against the opposing force that result in dithering of electrostatic dipoles giving rise to interchain physical crosslinks. Electrostatic inter- and intrachain interactions facilitated by copolymerization of ionic liquid monomers with typically dielectric acrylic-based monomers result in enhanced cohesive energy densities that accelerate the recovery of vdW forces facilitating self-healing. Incorporating ionic liquids into commodity polymers offers promising uses as green conducting solid polyelectrolytes in self-healable energy storage, energy-harvesting devices, and many other applications.

Self-Healable Fluorinated Copolymers Governed by Dipolar Interactions.

Although dipolar forces between copolymer chains are relatively weak, they result in ubiquitous inter- and/or intramolecular interactions which are particularly critical in achieving the mechanical integrity of polymeric materials. In this study, a route is developed to obtain self-healable properties in thermoplastic copolymers that rely on noncovalent dipolar interactions present in essentially all macromolecules and particularly fluorine-containing copolymers. The combination of dipolar interactions between C─F and C═O bonds as well as CH2 /CH3 entities facilitates self-healing without external intervention. The presence of dipole-dipole, dipole-induced dipole, and induced-dipole induced dipole interactions leads to a viscoelastic response that controls macroscopic autonomous multicycle self-healing of fluorinated copolymers under ambient conditions. Energetically favorable dipolar forces attributed to monomer sequence and monomer molar ratios induces desirable copolymer tacticities, enabling entropic energy recovery stored during mechanical damage. The use of dipolar forces instead of chemical or physical modifications not only eliminates additional alternations enabling multiple damage-repair cycles but also provides further opportunity for designing self-healable commodity thermoplastics. These materials may offer numerous applications, ranging from the use in electronics, ion batteries, H2 fuel dispense hoses to self-healable pet toys, packaging, paints and coatings, and many others.

Recent Advances in Stimuli-Responsive Commodity Polymers.

Known for their adaptability to surroundings, capability of transport control of molecules, or the ability of converting one type of energy to another as a result of external or internal stimuli, responsive polymers play a significant role in advancing scientific discoveries that may lead to an array of diverge applications. This review outlines recent advances in the developments of selected commodity polymers equipped with stimuli-responsiveness to temperature, pH, ionic strength, enzyme or glucose levels, carbon dioxide, water, redox agents, electromagnetic radiation, or electric and magnetic fields. Utilized diverse applications ranging from drug delivery to biosensing, dynamic structural components to color-changing coatings, this review focuses on commodity acrylics, epoxies, esters, carbonates, urethanes, and siloxane-based polymers containing responsive elements built into their architecture. In the context of stimuli-responsive chemistries, current technological advances as well as a critical outline of future opportunities and applications are also tackled.

Water accelerated self-healing of hydrophobic copolymers.

Previous studies have shown that copolymer compositions can significantly impact self-healing properties. This was accomplished by enhancement of van der Waals (vdW) forces which facilitate self-healing in relatively narrow copolymer compositional range. In this work we report the acceleration of self-healing in alternating/random hydrophobic acrylic-based copolymers in the presence of confined water molecules. Under these conditions competing vdW interactions do not allow H2O-diester H-bonding, thus forcing nBA side groups to adapt L-shape conformations, generating stronger dipole-dipole interactions resulting in shorter inter-chain distances compared to 'key-and-lock' associations without water. The perturbation of vdW forces upon mechanical damage in the presence of controllable amount of confined water is energetically unfavorable leading the enhancement of self-healing efficiency of hydrophobic copolymers by a factor of three. The concept may be applicable to other self-healing mechanisms involving reversible covalent bonding, supramolecular chemistry, or polymers with phase-separated morphologies.

Entropy and interfacial energy driven self-healable polymers.

Although significant advances have been achieved in dynamic reversible covalent and non-covalent bonding chemistries for self-healing polymers, an ultimate goal is to create high strength and stiffness commodity materials capable of repair without intervention under ambient conditions. Here we report the development of mechanically robust thermoplastic polyurethane fibers and films capable of autonomous self-healing under ambient conditions. Two mechanisms of self-healing are identified: viscoelastic shape memory (VESM) driven by conformational entropic energy stored during mechanical damage, and surface energy/tension that drives the reduction of newly generated surface areas created upon damage by shallowing and widening wounds until healed. The type of self-healing mechanism is molecular weight dependent. To the best of our knowledge these materials represent the strongest (Sf = 21 mN/tex, or σf ≈ 22 MPa) and stiffest (J = 300 mN/tex, or E ≈ 320 MPa) self-healing polymers able to repair under typical ambient conditions without intervention. Since two autonomous self-healing mechanisms result from viscoelastic behavior not specific to a particular polymer chemistry, they may serve as general approaches to design of other self-repairing commodity polymers.

Mechanistic Insights on Spontaneous Moisture-Driven Healing of Urea-Based Polyurethanes.

Self-healing polymeric materials that can spontaneously repair in a perpetual manner are highly appealing to address safety and restoration issues in different key applications. Usually built from reversible moieties that require to be activated using, for example, temperature, light, or pH changes, most of these self-healing materials rely on energy-demanding processes and/or external interventions to promote self-healing. In this work, we propose to exploit rapid dynamic exchanges between urea-based moieties and moisture as an alternative to promote local and spontaneous healing responses to damage using atmospheric moisture as an external stimulus. Non-hygroscopic urea-based polyurethanes with repetitive moisture-induced healing abilities at different degrees of humidity were thus designed through coupling reactions with non-hygroscopic polypropylene glycol and urea moieties. As supported by density functional theory (DFT) calculations coupled to local FTIR experimental studies, we furthermore established that the healing mechanism is ultimately related to the formation of water-urea clusters. Obviously, this work represents a platform for designing more advanced spontaneous self-healing materials beyond the present study, which hold promise for use in a wide range of technological applications.

Key-and-lock commodity self-healing copolymers.

Self-healing materials are notable for their ability to recover from physical or chemical damage. We report that commodity copolymers, such as poly(methyl methacrylate)/n-butyl acrylate [p(MMA/nBA)] and their derivatives, can self-heal upon mechanical damage. This behavior occurs in a narrow compositional range for copolymer topologies that are preferentially alternating with a random component (alternating/random) and is attributed to favorable interchain van der Waals forces forming key-and-lock interchain junctions. The use of van der Waals forces instead of supramolecular or covalent rebonding or encapsulated reactants eliminates chemical and physical alterations and enables multiple recovery upon mechanical damage without external intervention. Unlike other self-healing approaches, perturbation of ubiquitous van der Waals forces upon mechanical damage is energetically unfavorable for interdigitated alternating/random copolymer motifs that facilitate self-healing under ambient conditions.

Stimuli-Responsive Polymeric Nanoparticles.

There is increasing evidence that stimuli-responsive nanomaterials have become significantly critical components of modern materials design and technological developments. Recent advances in synthesis and fabrication of stimuli-responsive polymeric nanoparticles with built-in stimuli-responsive components (Part A) and surface modifications of functional nanoparticles that facilitate responsiveness (Part B) are outlined here. The synthesis and construction of stimuli-responsive spherical, core-shell, concentric, hollow, Janus, gibbous/inverse gibbous, and cocklebur morphologies are discussed in Part A, with the focus on shape, color, or size changes resulting from external stimuli. Although inorganic/metallic nanoparticles exhibit many useful properties, including thermal or electrical conductivity, catalytic activity, or magnetic properties, their assemblies and formation of higher order constructs are often enhanced by surface modifications. Section B focuses on selected surface reactions that lead to responsiveness achieved by decorating nanoparticles with stimuli-responsive polymers. Although grafting-to and grafting-from dominate these synthetic efforts, there are opportunities for developing novel synthetic approaches facilitating controllable recognition, signaling, or sequential responses. Many nanotechnologies utilize a combination of organic and inorganic phases to produce ceramic or metallic nanoparticles. One can envision the development of new properties by combining inorganic (metals, metal oxides) and organic (polymer) phases into one nanoparticle designated as "ceramers" (inorganics) and "metamers" (metallic).

Quantitative Predictions of Shape-Memory Effects in Polymers.

Unique shape-memory transitions manifested by directional extension and subsequent retraction in polymers are attributed to stored conformational entropy. This behavior is quantified in terms of stored (ΔS S ) entropic energy density, the maximum strain (εmax ), and stress (σSF at εmax ). This concept allows quantitative assessments of the shape-memory effect (SME) and can be utilized in any material that exhibits a glass-transition temperature (T g ) and a rubbery plateau.

Instantaneous Directional Growth of Block Copolymer Nanowires During Heterogeneous Radical Polymerization (HRP).

Polymeric nanowires that consist of ultrahigh molecular weight block copolymers were instantaneously prepared via one-step surfactant-free heterogeneous radical polymerization (HRP). Under heterogeneous reaction and initiator-starvation conditions, the sequential copolymerization of hydrophilic and hydrophobic monomers facilitates the formation of amphiphilic ultrahigh molecular weight block copolymers, which instantaneously assemble to polymeric nanowires. As polymerization progresses, initially formed nanoparticles exhibit the directional growth due to localized repulsive forces of hydrophilic blocks and confinement of the hydrophobic blocks that adopt favorable high aspect ratio nanowire morphologies. Using one-step synthetic approach that requires only four ingredients (water as a solvent, two polymerizable monomers (one hydrophilic and one hydrophobic), and water-soluble initiator), block copolymer nanowires ∼70 nm in diameter and hundreds of microns in length are instantaneously grown. For example, when 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and styrene (St) were copolymerized, high aspect ratio nanowires consist of ultrahigh (>10(6) g/mol) molecular weight pDMAEMA-b-St block copolymers and the presence of temperature responsive pDMAEMA blocks facilitates nanowire diameter changes as a function of temperature. These morphologies may serve as structural components of the higher order biological constructs at micro and larger length scales, ranging from single strand nanowires to engineered biomolecular networks capable of responding to diverse and transient environmental signals, and capable of dimensional changes triggered by external stimuli.

One-Step Synthesis of Amphiphilic Ultrahigh Molecular Weight Block Copolymers by Surfactant-Free Heterogeneous Radical Polymerization.

Ultrahigh molecular weight (>106 g/mol) amphiphilic block copolymers were synthesized using one-step surfactant-free heterogeneous radical polymerization (SFHRP). The polymerization initially involves formation of water-soluble homopolymer blocks, followed by copolymerization of a hydrophobic monomer, resulting in ultrahigh molecular weight block copolymers. Facilitating heterogeneous reaction conditions and continuous supply of an initiator controls the process. Using this synthetic approach, we synthesized amphiphilic block copolymers of poly(2-(N,N-dimethylamino)ethyl methacrylate)-block-poly(n-butyl acrylate) (pDMAEMA-b-pnBA), pDMAEMA-block-p(tert-butyl acrylate) (pDMAEMA-b-tBA), and pDMAEMA-block-polystyrene (pDMAEMA-b-pSt) with molecular weights of 1.98 × 106, 1.18 × 106, and 0.91 × 106 g/mol, respectively. These ultrahigh molecular weight block copolymers can self-assemble in nonpolar solvents to form thermochromic inverse polymeric micelles as well as other shapes and exhibit many potential applications.

Self-repairable polyurethane networks by atmospheric carbon dioxide and water.

Sugar moieties were incorporated into cross-linked polyurethane (PUR) networks in an effort to achieve self-repairing in the presence of atmospheric carbon dioxide (CO2) and water (H2O). When methyl-α-D-glucopyranoside (MGP) molecules are reacted with hexamethylene diisocyanate trimer (HDI) and polyethylene glycol (PEG) to form cross-linked MGP-polyurethane (PUR) networks, these materials are capable of self-repairing in air. This process requires atmospheric amounts of CO2 and H2O, thus resembling plant behavior of carbon fixation during the photosynthesis cycle. Molecular processes responsible for this unique self-repair process involve physical diffusion of cleaved network segments as well as the formation of carbonate and urethane linkages. Unlike plants, MGP-PUR networks require no photo-initiated reactions, and they are thus capable of repair in darkness under atmospheric conditions.

Covalent attachment of multilayers (CAM): a platform for pH switchable antimicrobial and anticoagulant polymeric surfaces.

These studies show covalent attachment of multilayers (CAM) to chemically alter surfaces to achieve pH switchable antimicrobial and anticoagulant properties. Polyethylene (PE), poly(tetrafluoroethylene) (PTFE), and silicon (Si) surfaces were functionalized by tethering pH-responsive "switching" polyelectrolytes consisting of poly(2-vinyl pyridine) (P2VP) and poly(acrylic acid) (PAA) terminated with NH2 and COOH groups, respectively. At pH < 2.3, the P2VP segments are protonated and expended, but at pH > 5.5, they collapse while the PAA segments are expanded. The presence of terminal NH2 or COOH moieties on P2VP and PAA, respectively, facilitated the opportunity for covalently bonding ampicillin (AMP) and heparin (HEP) to both polyelectrolyte chains. Such surfaces, when exposed to S. aureus, inhibit the growth of microbial films (AMP) as well as anticoagulant properties (HEP). Comparison of "dynamic" pH dependent surfaces developed in these studies with "static" surfaces terminated with (AMP) entities shows significant enhancement in the longevity of surface activity against microbial film formation.

Tri-Phasic Size- and Janus Balance-Tunable Colloidal Nanoparticles (JNPs).

These studies show synthesis of triphasic size- and Janus balance (JB)-tunable nanoparticles (JNPs) utilizing a two-step emulsion polymerization of pentafluorostyrene (PFS) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) and n-butyl acrylate (nBA) in the presence of poly(methyl methacrylate (MMA)/nBA) nanoparticle seeds. Each JNP consists of three phase-separated copolymers: p(MMA/nBA) core, temperature, and pH-responsive (p(DMAEMA/nBA)) phase capable of reversible size and shape changes, and shape-adoptable (p(PFS/nBA)) phase. Due to built-in second-order lower critical solution temperature (II-LCST) transition of p(DMAEMA/nBA) copolymer, macromolecular segments collapse when temperature increases from 30 to 45 °C, resulting in size and shape changes. The p(DMAEMA/nBA) and p(MMA/nBA) phases within each JNP assume concave, flat, or convex shapes, forcing p(PFS/nBA) phase to adopt convex, planar, or concave interfacial curvatures, respectively. As a result, the JB can be tuned from 3.78 to 0.72. The presence of pH-responsive DMAEMA component also facilitates the size and JB changes due to protonation of the tertiary amine groups of p(DMAEMA/nBA) backbone. Synthesized in this manner, JNPs are capable of stabilizing oil droplets in water at high pH to form Pickering emulsions, which at lower pH values release oil phase. This process is reversible and can be repeated many times.

Simple click reactions on polymer surfaces leading to antimicrobial behavior.

In this study we developed simple reactions that combined microwave plasma reactions in the presence of maleic anhydride with alkyne click chemistries to achieve a platform for unlimited possibilities for further surface modifications of aliphatic polymer surfaces. Using this approach, we covalently attached ampicillin (AMP) to polyethylene (PE) and polypropylene (PP) substrates. As a result, high efficacy against microbial film formation, manifested by efficient antimicrobial activity against S. aureus with a 97-99.8% decrease of bacterial growth, was achieved. This simple and clean process allows functionalization of any polymeric substrate without adverse effects on bulk polymer properties.

Self-healing polymeric materials.

Inspired by nature, self-healing materials represent the forefront of recent developments in materials chemistry and engineering. This review outlines the recent advances in the field of self-healing polymers. The first part discusses thermodynamic requirements for self-healing networks in the context of conformation changes that contribute to the Gibbs free energy. The chain flexibility significantly contributes to the entropy changes, whereas the heat of reaction and the external energy input are the main contributors to enthalpy changes. The second part focuses on chemical reactions that lead to self-healing, and the primary classes are the covalent bonding, supramolecular assemblies, ionic interactions, chemo-mechanical self-healing, and shape memory polymers. The third part outlines recent advances using encapsulation, remote self-healing and the role of shape memory polymers. Recent developments in the field of self-healing polymers undeniably indicate that the main challenge will be the designing of high glass transition (Tg) functional materials, which also exhibit stimuli-responsive attributes. Build-in controllable hierarchical heterogeneousness at various length scales capable of remote self-healing by physical and chemical responses will be essential in designing future materials of the 21st century.