Supramolecular Structure for Large Strain Dissipation and Outstanding Impact Resistance in Polyvinylbutyral

Water Clustering in Polyvinyl Butyral (PVB): Evidenced by Diffusion and Sorption Experiments

Polyvinyl butyral (PVB) is a transparent amorphous polymer often used to protect fragile surfaces such as glass or photovoltaic panels. The polymer is then packaged in the form of adhesive sheets and bonded to the surfaces. The transport and retention of water in PVB are crucial properties to understand as they modulate the polymer's adhesion properties. In this work, we propose a detailed experimental study of water diffusion and sorption in PVB over a wide range of temperatures and humidity levels in the surrounding atmosphere. Using spectroscopic and gravimetric measurements, our study elucidates how the diffusion coefficient varies with temperature or vapor concentration and provides the activation energy for this process. In addition, dynamic vapor sorption experiments reveal (i) a strong dependence of sorption on hydroxyl group (-OH) concentration and (ii) that the solubility of vapor in PVB decreases with temperature. This enables us to trace the heat of the solution of water in PVB. A comparison of the thermodynamic data obtained with those for water in volume and with the engaged species induced clustering model supports the microscopic view of water organization in PVB in the form of clusters induced by hydrogen bonding.

Multiscale Relaxation Behavior of Amorphous Plasticized Poly(vinyl butyral)

As a key component in laminated glass, plasticized polyvinyl butyral (PVB) interlayer is a kind of impact-resistant polymer material with high toughness. Recently, by using ultrasmall angle X-ray scattering (USAXS) technique, Stretch-induced phase-separated structure on the scale of hundreds of nanometers formed in plasticized PVB for the first time is reported. In this work, the multiscale relaxation behavior of plasticized PVB is further investigated. The relaxation behavior of deformed plasticized PVB is studied from macroscopic stress, mesoscopic phase-separated structure, and microscopic chain segment by combining USAXS, and birefringence with in situ stretching device. The contributions of chain segments and hydrogen bonding clusters for the multiscale relaxation behavior are discussed.

Study on Bone-like Microstructure Design of Carbon Nanofibers/Polyurethane Composites with Excellent Impact Resistance

It is a challenge to develop cost-effective strategy and design specific microstructures for fabricating polymer-based impact-resistance materials. Human shin bones require impact resistance and energy absorption mechanisms in the case of rapid movement. The shin bones are exciting biological materials that contain concentric circle structures called Haversian structures, which are made up of nanofibrils and collagen. The "soft and hard" structures are beneficial for dynamic impact resistance. Inspired by the excellent impact resistance of human shin bones, we prepared a sort of polyurethane elastomers (PUE) composites incorporated with rigid carbon nanofibers (CNFs) modified by elastic mussel adhesion proteins. CNFs and mussel adhesion proteins formed bone-like microstructures, where the rigid CNFs are served as the bone fibrils, and the flexible mussel adhesion proteins are regarded as collagen. The special structures, which are combined of hard and soft, have a positive dispersion and compatibility in PUE matrix, which can prevent cracks propagation by bridging effect or inducing the crack deflection. These PUE composites showed up to 112.26% higher impact absorbed energy and 198.43% greater dynamic impact strength when compared with the neat PUE. These findings have great implications for the design of composite parts for aerospace, army vehicles, and human protection.

Multi-instrumented dynamic loading experiments on laminated glass

Describing quantitatively the response of laminated glass to low-velocity (~5 m/s) impacts by rigid bodies is an important issue because of its significance in terms of structural degradation and integrity, key parameters for people safety and anti-intrusion performances. This study aims to address the formation of cracks during graveling and steel ball drop tests, so, two well-instrumented experimental set-ups are proposed to study cracking in reproducible conditions. The first device can be seen as a mini-Hopkinson bar system, which from two strain gauges, allows to estimate force and velocity at a sharp indent tip. The second device, reproducing a blunt impact, exploits stereo-Digital Image Correlation (D.I.C.) measurements of the laminated glass surface.

Time dependent fracture of soft materials: linear versus nonlinear viscoelasticity

Toughness of soft materials such as elastomers and gels depends on their ability to dissipate energy and to reduce stress concentration at the crack tip. The primary energy dissipation mechanism is viscoelasticity. Most analyses and models of fracture are based on linear viscoelastic theory (LVT) where strains are assumed to be small and the relaxation mechanisms are independent of stress or strain history. A well-known paradox is that the size of the dissipative zone predicted by LVT is unrealistically small. Here we use a physically based nonlinear viscoelastic model to illustrate why the linear theory breaks down. Using this nonlinear model and analogs of crack problems, we give a plausible resolution to this paradox. In our model, viscoelasticity arises from the breaking and healing of physical cross-links in the polymer network. When the deformation is small, the kinetics of bond breaking and healing are independent of the strain/stress history and the model reduces to the standard linear theory. For large deformations, localized bond breaking damages the material near the crack tip, reducing stress concentration and dissipating energy at the same time. The damage zone size is a new length scale which depends on the strain required to accelerate bond breaking kinetics. These effects are illustrated by considering two cases with stress concentrations: the evolution of spherical damage in a viscoelastic body subjected to internal pressure, and a zero degree peel test.