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To better understand the processing and final properties of multilayer films obtained by layer forced assembly in coextrusion as well as their structure morphology, the present study focuses on model films with LDPE as the base polymer. More specifically, the layer confinement of LDPE by a glassy amorphous polymer (here, PC or PS) was investigated. First, the viscosity and elasticity ratios of the different neat polymers were measured by rheological tests to simulate the processing conditions in the feedblock and multipliers during coextrusion. These results together with the observation of film transparency at the macroscopic scale and the layer breakup phenomena between layers at the microscopic scale enabled us to build a comprehensive stability map rationalizing the conditions required for a well-controlled multinanolayer architecture. Second, the morphology of the coextruded films was analyzed by SEM and TEM. The onset of the layer breakup in the LDPE/PS system was determined at 2048 layers with a layer thickness of 95 nm, while in the LDPE/PC system, it was at 256 layers with a layer thickness of 980 nm. The layer breakup happened at a fewer number of layers for the LDPE/PC system due to the viscoelastic mismatched properties between the base polymers. Interestingly, we have demonstrated that it is nonetheless possible to prepare some nanolayer structures with 16,380 layers of the PS/LDPE system with some defects but still maintain an overall property improvement despite their high mismatched viscoelastic properties. Finally, the orientation and crystalline structure of the coextruded films were characterized by 2D-WAXS and DSC, and the ultimate properties of the films were determined through tensile testing. The geometrical confinement of the LDPE nanolayer did not affect the thermal crystalline properties of LDPE chains, but it affected the crystalline morphologies as well as the final mechanical response of the obtained multilayer films.

Tough hydrogels of many chemical compositions have been developed in recent years, but their fatigue fracture has not been studied. The lack of study hinders further development of hydrogels for applications that require long lifetimes under cyclic loads. Examples include tissue engineering, soft robots, and stretchable electronics. Here we study the fatigue fracture of a polyacrylamide–alginate tough hydrogel. We find that the stress–stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles. The threshold for fatigue fracture is about 53 J/m2, much below the fracture energy (∼10,000 J/m2) measured under monotonic load. Nonetheless, the extension of crack per cycle in the polyacrylamide–alginate tough hydrogel is much smaller than that in a single-network polyacrylamide hydrogel.

Liquid propylene-glycol (PG) has long been used as an anti-icing substance, for example, by spraying on an airplane parked in an airport. In applications, large quantities of PG flow away, which is costly and raises environmental concerns. Here we report propylene-glycol materials, including PG-gels and PG-gel/cotton composites. A PG-gel consists of PG molecules as a solvent and a polymer network. PG evaporates slowly, and the polymer network retains the PG molecules so long as the gel is not in contact with running water. Water and PG form a eutectic system with an eutectic temperature of −60 °C. When ice falls on the surface of the gel, the ice and the PG molecules compete for water molecules, and thermodynamics dictates that the ice should lose water molecules to the PG molecules, so that ice melts and water molecules dissolve in the gel. A liquid-like layer exists on the ice/gel interface, the adhesion energy between the gel and ice is low, and ice readily slides on the gel. We peel a PG-gel from ice, and measure a low adhesion energy of ∼3 Jm−2at temperatures about −35 °C. We further demonstrate PG-gel/cotton composites as tough, anti-icing blankets. The blankets are reusable if one removes water by dehydration, and replenish PG by submerging the blanket in liquid PG.

Abstract The deformation behavior of polymer materials below their glass transition temperature involves a complex intermingling of elasticity, of viscoplastic yielding with softening and large strain hardening, and of significant rate, temperature and pressure dependency. During unloading, the response exhibits large non-linearity as well, combining forward and backward creep. Even though the nature of the atomistic underlying mechanisms is relatively well understood, a full picture of the interactions between the elementary distortion mechanisms and their collective organization capturing the complexity of the macroscopic behavior is still lacking. Here, we propose a mesoscale approach based on the heterogeneous activation of molecular level shear transformation zones and we show that it is rich enough to unravel the physical origin of most aspects of the macroscopic viscoplastic response. The study is based on a wide micro- and macro-mechanical test campaign on a thermoset material. In addition to providing a full characterization of all macroscopic properties, evidence of a complex rate reversal mechanism upon unloading is given. Other experiments provide insights about the characteristic length scales at play. Compared to existing continuum models involving typically more than 25 parameters, the present formalism is based on only 7 physical parameters while allowing quantitative predictions of all the experimental data. The present approach opens new avenues for the prediction of the mechanical response of a variety of polymer applications in confined state such as in composites or in adhesive joints. It also offers a new line of thought to address the mechanics of polymers at small and intermediate length scales. [ABSTRACT FROM AUTHOR]