Sticky Multicolor Mechanochromic Labels

Sticky-colored labels are an efficient way to communicate visual information. However, most labels are static. Here, we propose a new category of dynamic sticky labels that change structural colors when stretched. The sticky mechanochromic labels can be pasted on flexible surfaces such as fabric and rubber or even on brittle materials. To enhance their applicability, we demonstrate a simple method for imprinting structural color patterns that are either always visible or reversibly revealed or concealed upon mechanical deformation. The mechanochromic patterns are imprinted with a photomask during the ultraviolet (UV) cross-linking of acrylate-terminated cholesteric liquid crystal oligomers in a single step at room temperature. The photomask locally controls the cross-linking degree and volumetric response of the cholesteric liquid crystal elastomers (CLCEs). A nonuniform thickness change induced by the Poisson’s ratio contrast between the pattern and the surrounding background might lead to a color-separation effect. Our sticky multicolor mechanochromic labels may be utilized in stress-strain sensing, building environments, smart clothing, security labels, and decoration.</p

Processing and Properties of Melt Processable UHMW-PE Based Fibers Using Low Molecular Weight Linear Polyethylene's

The rheology, solid state drawability, morphology, and mechanical properties of polyethylene blends containing ultrahigh molecular weight polyethylene (UHMW-PE) and linear-low molecular weight polyethylene waxes (PEwax) are explored. Addition of PEwax enables melt processing of UHMW-PE and improves solid state drawability, providing opportunities in recycling of UHMW-PE waste. Small angle X-ray scattering results show that both PEwax and UHMW-PE align fully in the drawing direction, irrespective whether the PEwax has an Mn below or above the critical molecular weight at which entanglements can form (Mc). Tensile moduli of drawn specimen are in accordance to the Irvine–Smith model confirming that both UHMW-PE and PEwax align in the drawing direction and no chain slip occurs toward zero strain and both UHMW-PE and PEwax fractions, irrespective of their molecular weight, contribute fully to the modulus. Tensile strength of the blends scales according to the rule of mixtures where PEwax below Mc scale toward zero and those above Mc do contribute to the tensile strength. Modulus hence can be regarded as insensitive to the molecular weight of the PEwax used, whereas strength does show to be sensitive to the molecular weight of the PEwax

Time-dependent failure in load-bearing polymers. A potential hazard in structural applications of polylactides

Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this chapter we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance

Influence of fiber orientation, temperature and relative humidity on the long-term performance of short glass fiber reinforced polyamide 6

As a result of processing of short fiber reinforced thermoplastics, the fiber orientation varies throughout a product giving rise to a pronounced anisotropic mechanical response. Different flow conditions in a product result in spatial variation in both short- and long-term mechanical properties. In this study, a modeling approach is presented to evaluate the lifetime of short fiber reinforced polyamide 6, both in plasticity- and crack growth controlled regions of failure. In the plasticity-controlled region, a viscoplastic model based on separation of the load angle (by means of Hill's equivalent stress formulation) and time dependence of the yield stress is used in the form of an associative flow rule. The influence of temperature and relative humidity on the magnitude of the plastic flow rate is described by using an apparent temperature approach combined with a Ree-Eyring formulation. The depression of the glass transition temperature in the polyamide 6 matrix with increasing amount of absorbed moisture was used to predict the anisotropic deformation kinetics in a humid environment. Similar to the plasticity controlled failure, in slow crack growth controlled failure region the effect of temperature, relative humidity, and load angle on the lifetime under a fatigue load is investigated. The apparent temperature approach could also be successfully applied to predict the slow crack growth failure, while the load angle dependence is shown to scale similar to the plasticity-controlled failure with the Hill's equivalent stress

Physical background of the endurance limit in poly(ether ether ketone)

In this study, it is demonstrated that the apparent endurance (fatigue) limit for plasticity‐controlled failure in poly(ether ether ketone) is related to an evolution of the yield stress. The increase of the yield stress has two separate causes: (a) stress‐ and temperature‐accelerated physical aging of the amorphous phase and (b) strain hardening as a result of texture development. Yield stress evolution is monitored using thermomechanical treatments during which the material is exposed to temperature and load. The combined contributions of both temperature and applied stress to yield stress evolution (below T g ) can be effectively modeled using an effective time approach employing an Arrhenius temperature‐activation as well as Eyring stress activation. Combination of the yield stress evolution with a previously developed model for plasticity‐controlled failure allows prediction of time‐to‐failure under both static and cyclic load, quantitatively capturing the observed apparent endurance limit

Predicting long-term crack growth dominated static fatigue based on short-term cyclic testing

In the present work, the time to failure of a glass fibre reinforced glassy polymer is studied in cyclic fatigue at various frequencies and stress ratios with the goal to predict long-term crack-growth dominated static fatigue. It is demonstrated that the crack propagation rate can be regarded to consist of two components: a time-dependent creep component, and a frequency dependent cyclic component. In static loading, the time-dependent component prevails, while for cyclic loads with large load amplitudes the frequency dependent, cyclic component dominates. For intermediate load amplitudes, the total propagation rate is shown to be a combination of both. Consequently, the contribution of the cyclic component diminishes with decreasing frequency or load amplitude, revealing the contribution of the static component. As such, extrapolation of the stress ratio dependence of the fatigue life to R=1 allows estimation of the long-term static performance. A phenomenological model is provided that captures all relevant aspects and provides an accurate description of the stress ratio and frequency dependence of the lifetime in fatigue loading and allows prediction of the long-term failure, based on short-term cyclic fatigue experiments only

Lifetime Assessment of Load-Bearing Polymer Glasses: An Analytical Framework for Ductile Failure

The most widespread application of polymers in structural applications is their use as pipe material for e.g., gas distribution systems. Pipes have a design lifetime of typically 50 years, which rules out real-time lifetime assessment methods. Here, an engineering approach is presented, which makes it possible to predict long-term ductile failure of loaded glassy polymers based on short-term tests. The approach is based upon the hypothesis that failure is governed by accumulation of plastic deformation up to a critical strain. A pressure-modified Eyring relation is employed to calculate the accumulation of plastic strain for any simple loading geometry. It is demonstrated that the approach can produce accurate quantitative time-to-failure predictions for loaded PC specimens and uPVC pipe segments

Liquid crystal networks on thermoplastics : reprogrammable photo-responsive actuators

Arbitrary shape (re)programming is appealing for fabricating, untethered shape‐morphing photo actuators with intricate configurations and features. We present re‐programmable light responsive thermoplastic actuators with arbitrary initial shapes through spray‐coating of polyethylene terephthalate (PET) with an azobenzene‐doped light responsive liquid crystal network (LCN). The initial geometry of the actuator is controlled by thermally shaping and fixing the thermoplastic PET, allowing arbitrary shapes, including origami‐like folds and left and right handed helicity within a single sample. The thermally fixed geometries can be reversibly actuated through light exposure, with fast, reversible area‐specific actuation, such as winding, unwinding and unfolding. By shape re‐programming, the same sample can be re‐designed and light actuated again. The strategy presented here demonstrates easy fabrication of mechanically robust, recyclable, photo‐responsive actuators with highly tuneable geometries and actuation modes

Predicting plasticity‐controlled failure of glassy polymers: Influence of stress‐accelerated progressive physical aging

This study focuses on the prediction of long-term failure of glassy polymers under static or cyclic loading conditions, including the role of stress-accelerated progressive aging. Progressive physical aging plays a dominant role in a polymer's performance under prolonged loading conditions, and to obtain accurate predictions of failure, its effect has to be considered. First, the aging kinetics, as influenced by temperature and stress history, are studied extensively. Similar to an elevated temperature, the application of a stress (below the yield stress) activates the aging process, and as a result, the yield stress will evolve faster in time. The activation by stress appears to be limited; at some stress level, the activation stagnates and is followed by rejuvenation. This evolution is captured in a model by introducing a state parameter, which describes the thermodynamic state of the material and is directly linked to the yield stress. With the aging kinetics included in the model, an accurate prediction of the failure time for cyclic loading conditions is obtained. For static loading conditions, however, the effect of physical aging is overestimated because of the stagnation of the activation by stress. It appears that there are marked differences in the stress level where stagnation and subsequent rejuvenation occur for a cyclic or static load