A molecular theory is developed to describe quantitatively the permanent set taking place in thin samples of vulcanized natural and synthetic rubbers held at constant extension at elevated temperatures. Permanent set: is considered to be the result of the formation, through the action of molecular scission and cross-linking reactions, of a dual molecular network in the rubber sample, in which the network chains are of two types: chains which are at equilibrium when the sample is at its unstretched length, and chains which are at equilibrium when the sample is at its stretched length. According to the theory the amount of permanent set in a rubber sample is a function of only two quantities: the relative ratio of the number of chains of the two types, and the elongation at which the sample was held. Experimental data on permanent set for various rubber types and under different conditions are presented and are shown to be in good agreement with the theory.

SignificanceExploiting advanced 3D designs in micro/nanomanufacturing inspires potential applications in various fields including biomedical engineering, metamaterials, electronics, electromechanical components, and many others. The results presented here provide enabling concepts in an area of broad, current interest to the materials community––strategies for forming sophisticated 3D micro/nanostructures and means for using them in guiding the growth of synthetic materials and biological systems. These ideas offer qualitatively differentiated capabilities compared with those available from more traditional methodologies in 3D printing, multiphoton lithography, and stress-induced bending––the result enables access to both active and passive 3D mesostructures in state-of-the-art materials, as freestanding systems or integrated with nearly any type of supporting substrate.

High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 µm, enabling the manipulation of light in the visible and infrared range.

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with large refractive index contrasts and varied structural motifs have been successfully fabricated from a wide range of materials. [7,8] However, top-down (e.g., lithographic) formation of large volumes of photonic crystals is a challenge. Selforganization techniques, such as eutectic solidification, have been shown as a possible path to forming large volumes of photonic crystals. [9][10][11][12][13][14] Among possible motifs provided by eutectic solidification, the regular microstructures of lamellar and rod eutectics have direct resemblance to 1D and 2D photonic crystals, respectively, where the phase-separated components provide the required contrast in the refractive index to exhibit a unique optical response. [6] The components of eutectic materials can be chosen from metals, semiconductors, polymers, organics, ceramics, or salts; thus providing metal, dielectric, or even metallodielectric composites with which to synthesize (or to act as templates for) photonic crystals. [11,13,[15][16][17][18][19][20][21][22][23] Recent examples from literature have demonstrated the formation of photonic crystals and other optically interesting structures (for applications like diffraction gratings, phase-separated scintillators with light guiding, and absorption-induced transparency) in directionally solidified chloride-based molten salt eutectics, such as AgCl-KCl, [16,18,22] NaCl-CsI, [23,24] CuI-KCl, [15] and KCl-LiF. [25] The eutectic solidification-based synthesis route is particularly simple if the eutectics have a low melting temperature and low surface energy, are devoid of any corroding components, like fluorides, and do not require controlled atmospheres during fabrication. However, even without these ideal factors, eutectic solidification is a quite well-established industrial process, and many challenging chemistries can be directionally solidified.The binary salt eutectic AgCl-CsAgCl 2 has the advantageous properties of a eutectic temperature (258 °C) and surface energy (135 mJ m −2 ) at its eutectic temperature lower than most other eutectic salt systems, but it has received only minimal attention. [26][27][28] Here, we show that when directionally solidified, AgCl-CsAgCl 2 has a tendency to form either a rod structure or lamellar structure depending on the directional solidification draw rates. While not unprecedented, as some binary metal eutectics, e.g., Al-Al 4 Ca, [29] Au-Co, [30] Cd-Sn, [31] Ni-W, [32] Ag-Cu, [33] and Al-Cu, [34] have been known to show transitions Directional solidification of a eutectic melt allows control over the resultant eutectic microstructure, which in turn impacts both the mechanical and optical properties of the material. These self-organized phase-separated eutectic materials can be tuned to have periodicities from tens of micrometers down to nanometers. Furthermore, the two phases possess differences in their refractive index leading to interesting optical properties that can be tailored within the visible to infrared wavelength regime. It is found the binary salt eutecti...

Certain materials, including natural and synthetic rubbers, after being deformed for finite periods of time and then released, do not return completely to their original dimensions. This phenomenon is called “permanent set”. There are many different types of measurements, made under a great variety of conditions, all of which are described as measurements of permanent set. The molecular causes of permanent set are not the same under all of these conditions. For this reason we shall restrict ourselves in the present article to a discussion of the permanent set taking place in thin rubber samples which are maintained at constant extension at elevated temperatures. We shall show that the permanent set which occurs under these conditions is due to chemical changes which affect the structure of the rubber network, and that a quantitative molecular description of permanent set can be accomplished by the use of concepts already developed concerning the nature of these deteriorative chemical processes.

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polarizers, some waveguides, and many lasers. [1][2][3] Materials with powerful optical functionalities, including negative-index of refraction and optical chirality, can be realized by appropriate placement of materials with suitable properties in 2D or 3D space. [4][5][6] Light in the visible spectrum can be manipulated, although the tolerance for defects is exceedingly low at visible frequencies, and the number of materials with the appropriate properties is limited. [7,8] Most photonic crystals are fabricated by high-resolution top-down 2D patterning methods such as electron-beam lithography, interference lithography, and focused ion beam milling. [9] However, it is challenging to fabricate large-area bulk materials with these techniques, especially with intricate internal structures. [8] Additionally, many materials with promising optical properties are not compatible with these top-down patterning methods. [4,10] As work on colloidal crystals has shown, controlled self-assembly is an effective route to organizing materials into 3D architectures, which interact strongly with light. [9][10][11][12][13] Colloidal self-assembly, however, only offers a limited set of symmetries (generally those of close packed arrangements), and a spherical basis. [12] For many applications, considerably more complex structures are of interest. Particularly promising approaches for forming materials with complex internal microstructures include eutectic solidification and block copolymer self-assembly, [14][15][16][17] and materials with interesting optical properties have been reported using both approaches. These methods are advantageous due to the wide range of microstructures they form. Here, we focus on the structures formed by eutectic solidification since materials with a broader range of optical properties are available compared to that provided by block copolymers, and because the characteristic lengths of structures accessible through eutectic solidification better match the wavelengths of visible and IR light. Further, forming materials with sufficiently large characteristic dimensions for interaction with visible light by block copolymer assembly is synthetically challenging as it requires high molecular weight polymers. [18] Similarly, self-assembly of other building blocks, e.g., nanoparticles, [19,20] molecules, [21,22] and DNA, [23,24] tend to produce structures with characteristic dimensions too small to provide strong light-matter interactions (via diffractive phenomena), [2,3,25] and are thus not the emphasis of this review.Mesostructured materials can exhibit enhanced light-matter interactions, which can become particularly strong when the characteristic dimensions of the structure are similar to or smaller than the wavelength of light. For controlling visible to near-infrared wavelengths, the small characteristic dimensions of the required structures usually demand fabrication by sophisticated lithographic techniques. However, these fabrication methods are restricted to producing 2D and a limited range of 3D...