Supramolecular Elastomers. Particulate β-Sheet Nanocrystal-Reinforced Synthetic Elastic Networks

Expanding the "Magic Triangle" of Reinforced Rubber Using a Supramolecular Filler Strategy

A strategy for optimizing the rolling resistance, wet skid and cut resistance of reinforced rubber simultaneously using a supramolecular filler is demonstrated. A β-alanine trimer-grafted Styrene Butadiene Rubber (A₃-SBR) pristine polymer was designed and mechanically mixed with commercially available styrene butadiene rubber to help the dispersion of a β-alanine trimer (A₃) supramolecular filler in the rubber matrix. To increase the miscibility of A₃-SBR with other rubber components during mechanical mixing, the pristine polymer was saturated with ethanol before mixing. The mixture was vulcanized using a conventional rubber processing method. The morphology of the assembles of the A₃ supramolecular filler in the rubber matrix was studied by Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM). The Differential Scanning Calorimetry study showed that the melting temperature of β-sheet crystals in the vulcanizates was around 179 °C and was broad. The melting temperature was similar to that of the pristine polymer, and the broad melting peak likely suggests that the size of the crystals is not uniform. The Transmission Electron Microscopy study revealed that after mixing the pristine polymer with SBR, some β-sheet crystals were rod-like with several tens of nanometers and some β-sheet crystals were particulate with low aspect ratios. Tensile testing with pre-cut specimens showed that the vulcanizate containing A₃-SBR was more cut-resistant than the one that did not contain A₃-SBR, especially at a large cut size. The rolling resistance and wet skid were predicted by dynamic mechanical analysis (DMA). DMA tests showed that the vulcanizates containing A₃-SBR were significantly less hysteretic at 60 °C and more hysteretic at 0 °C based on loss factor. Overall, the "magic triangle" was expanded by optimizing the rolling resistance, wet-skid, and cut resistance simultaneously using a β-alanine trimer supramolecular filler. The Payne effect also became less severe after introducing the β-alanine trimer supramolecular filler into the system.

Design of Interfacial Crowding for Elastomeric Reinforcement with Nanocrystals

Design of the crowding of chains tethered at the faces of β-sheet nanocrystals self-assembled from β-alanine trimers grafted on polyisobutylene (PIB) rubber tailors nanocrystal size and thus the elastic matrix morphology, thereby altering the material's macroscopic elastic properties. Results from transmission electron microscopy, small-angle X-ray scattering, and small-angle neutron scattering characterizations of the morphology demonstrate that increasing the density of chain tethering at the crystalline nanodomain/matrix interface can sharply limit the nanodomain growth in the direction of hydrogen bonding in the crystals. The nanocrystal size, in turn, impacts the gradient in chain stretching away from the crystal surface and the macroscopic volume fraction of unperturbed chains. Nanocomposite mechanical and dynamic mechanical properties at low degrees of deformation are related to the structural hierarchy resulting from the control of interfacial tethering density.

REINFORCEMENT OF RUBBER USING REACTIVE OLIGO(β-ALANINE) SUPRAMOLECULAR FILLERS

Oligo(β-alanine)–based supramolecular fillers that possess an 11-mercaptoundecanoyl oleophilic moiety and an oligo(β-alanine) oleophobic moiety of varied lengths (SA1, SA2, and SA3, having a β-alanine unimer, dimer, and trimer moiety, respectively) have been systematically investigated for reinforcement of SBR. An analog of SA2 with 11-mercapto-undecanoyl being replaced by 3-mercaptopropanoyl, SA2′, was also studied for comparison. Fourier transform infrared evidence has confirmed that all supramolecular fillers exist exclusively as hydrogen-bonded β-sheets in the rubber composites. Differential scanning calorimetry studies have revealed that SA2, SA2′, and SA3 form crystalline domains in the SBR phase at all filler loadings, while SA1 only forms crystalline domains at >15 phr, indicating that microphase separation from the SBR phase is weaker for SA1 than for its higher congeners. Transmission electron microscopy investigations have shown that the filler crystalline domains are dispersed uniformly as short fibers in a continuous SBR phase. The widths of the fibers are below 10 nm, and the lengths range from a few tens to a couple of hundreds of nanometers. The oligo(β-alanine)–based supramolecular fillers exhibit diverse reinforcing characteristics among themselves and in comparison with carbon black. SA1 gives comparatively high extensibility and low tensile strength, while SA2 and SA3 give relatively low extensibility, high stiffness, and high strength. SA2′ behaves similarly to SA1 at low filler loadings but significantly compromises the extensibility at relatively high loadings. Cyclic tensile testing shows that SA1 gives high hysteresis and high set, while SA2 and SA3 give low hysteresis at low elongation and high hysteresis at high elongation in comparison with carbon black. Dynamic mechanical study at 2% strain shows that SA1, SA2, and SA3 result in markedly lower tan δ at 60 °C than carbon black. In addition, abrasion study shows that SA1 and SA2 result in lower weight loss than carbon black at 30 phr of filler loading.

Direct Observation of Single-Molecule Stick-Slip Motion in Polyamide Single Crystals

Stick-slip is a ubiquitous motion in the hydrogen bonding network, which confers the corresponding materials with excellent toughness and strength. The experimental study of the stick-slip mechanism remains challenging because of the complexity of stress accumulation and release. An ideal system for study of this motion should comprise a defined molecular structure and chain arrangement and strong intermolecular interactions. In this study, we detected the stick-slip motion at the single-molecule level in the hydrogen bonding network of polyamide (PA) single crystals through atomic force microscopy (AFM)-based single-molecule force spectroscopy. Our results show that a stiffer force-loading device can enhance the stick capacity by increasing the fracture force and facilitating stress release. We confirm that the chain rotates while slipping and the slip distance is dependent on the unit structure of the hydrogen bonding network.

Facile Preparation of Highly Stretchable and Recovery Peptide-Polyurethane/Ureas

In this work, a new class of highly stretchable peptide-polyurethane/ureas (PUUs) were synthesized containing short β-sheet forming peptide blocks of poly(γ-benzyl-l-glutamate)-b-poly(propylene glycol)-b-poly(γ-benzyl-l-glutamate) (PBLG-b-PPG-b-PBLG), isophorone diisocyanate as the hard segment, and polytetramethylene ether glycol as the soft phase. PBLG-b-PPG-b-PBLG with short peptide segment length (<10 residues) was synthesized by amine-initiated ring opening polymerization of γ-benzyl-l-glutamate-N-carboxyanhydrides (BLG-NCA), which shows mixed α-helix and β-sheet conformation, where the percent of β-sheet structure was above 48%. Morphological studies indicate that the obtained PUUs show β-sheet crystal and nanofibrous structure. Mechanical tests reveal the PUUs display medium tensile strength (0.25⁻4.6 MPa), high stretchability (>1600%), human-tissue-compatible Young's modulus (226⁻513 KPa). Furthermore, the shape recovery ratio could reach above 85% during successive cycles at high strain (500%). In this study, we report a facile synthetic method to obtain highly stretchable and recovery peptide-polyurethane/urea materials, which will have various potential applications such as wearable and implantable electronics, and biomedical devices.

Wearable and Washable Conductors for Active Textiles

The emergence of stretchable electronics and its potential integration with textiles have highlighted a challenge: Textiles are wearable and washable, but electronic devices are not. Many stretchable conductors have been developed to enable wearable active textiles, but little has been done to make them washable. Here we demonstrate a new class of stretchable conductors that can endure wearing and washing conditions commonly associated with textiles. Such a conductor consists of a hydrogel, a dissolved hygroscopic salt, and a butyl rubber coating. The hygroscopic salt enables ionic conduction and matches the relative humidity of the hydrogel to the average ambient relative humidity. The butyl rubber coating prevents the loss and gain of water due to the daily fluctuation of ambient relative humidity. We develop the chemistry of dip-coating the butyl rubber onto the hydrogel, using silanes to achieve both the cross-link of the butyl rubber and the adhesion between the butyl rubber and the hydrogel. We test the endurance of the conductor by soaking it in detergent while stretching it cyclically and by machine-washing it. The loss of water and salt is minimal. It is hoped that these conductors open applications in healthcare, entertainment, and fashion.