The study of glass formation is largely framed by semiempirical models that emphasize the importance of progressively growing cooperative motion accompanying the drop in fluid configurational entropy, emergent elasticity, or the vanishing of accessible free volume available for molecular motion in cooled liquids. We investigate the extent to which these descriptions are related through computations on a model coarse-grained polymer melt, with and without nanoparticle additives, and for supported polymer films with smooth or rough surfaces, allowing for substantial variation of the glass transition temperature and the fragility of glass formation. We find quantitative relations between emergent elasticity, the average local volume accessible for particle motion, and the growth of collective motion in cooled liquids. Surprisingly, we find that each of these models of glass formation can equally well describe the relaxation data for all of the systems that we simulate. In this way, we uncover some unity in our understanding of glass-forming materials from perspectives formerly considered as distinct.glass formation | elasticity | cooperativity | free volume | strings T here are numerous theoretical approaches aiming to describe the universal liquid dynamics approaching the glass transition. One class of theories emphasizes the importance of the congested nature of the local atomic environment in cooled liquids, focusing on the amount of "free volume" available to facilitate molecular rearrangement (1). This free-volume approach is also linked to the more modern jamming model of glass formation (2). Older treatments of glass formation based on this perspective can be traced back to Batchinski (3), Doolittle (4), and Hildebrand (5) for small liquids, and to Williams and coworkers (6) and Duda and Vrentas (7, 8) for polymer materials. There is also more recent work based on the free-volume perspective, for example, positron lifetime measurements (9) that probe the cavity structure of glass-forming (GF) liquids. DebyeWaller measurements (9, 10), based on neutron, X-ray, or other scattering measurements, emphasize another type of free volume that is associated with the volume explored by particles as they rattle about their mean positions in a condensed material. This type of free-volume modeling has also been refined to take into account the shape of these "rattle" volumes (11,12).Another family of glass-formation models emphasizes the emergent elasticity in glassy materials (13). These approaches build on the idea that the solid-like nature of glasses is one of their most conspicuous, and perhaps defining, properties. Dyre (13) and Nemilov (14) have argued that the activation energy for transport should grow in proportion to the shear modulus. The models of Hall and Wolynes (15) and Leporini and coworkers (10, 16) can also be included in this class if the Debye-Waller factor is taken as a measure of local material stiffness.Approaches emphasizing the underlying complex potential energy surface have also found consider...

Thin polymer films are ubiquitous in manufacturing and medical applications, and there has been intense interest in how film thickness and substrate interactions influence film dynamics. It is appreciated that a polymer-air interfacial layer with enhanced mobility plays an important role in the observed changes and recent studies suggest that the length scale x of this interfacial layer is related to film relaxation. In the context of the Adam-Gibbs and random first-order transition models of glass formation, these results provide indirect evidence for a relation between x and the scale of collective molecular motion. Here we report direct evidence for a proportionality between x and the average length L of string-like particle displacements in simulations of polymer films supported on substrates with variable interaction strength and rigidity. This relation explicitly links x to the theoretical scale of cooperatively rearranging regions, offering a promising route to experimentally determine this scale of cooperative motion.

Changes in the dynamics of supported polymer films in comparison to bulk materials involve a complex convolution of effects, such as substrate interactions, roughness, and compliance, in addition to film thickness. We consider molecular dynamics simulations of substrate-supported, coarse-grained polymer films where these parameters are tuned separately to determine how each of these variables influence the molecular dynamics of thin polymer films. We find that all these variables significantly influence the film dynamics, leading to a seemingly intractable degree of complexity in describing these changes. However, by considering how these constraining variables influence string-like collective motion within the film, we show that all our observations can be understood in a unified and quantitative way. More specifically, the string model for glass-forming liquids implies that the changes in the structural relaxation of these films are governed by the changes in the average length of string-like cooperative motions and this model is confirmed under all conditions considered in our simulations. Ultimately, these changes are parameterized in terms of just the activation enthalpy and entropy for molecular organization, which have predictable dependences on substrate properties and film thickness, offering a promising approach for the rational design of film properties. C 2015 AIP Publishing LLC. [http://dx

Despite extensive efforts, a definitive picture of the glass transition of ultra-thin polymer films has yet to emerge. The effect of film thickness h on the glass transition temperature T(g) has been widely examined, but this characterization does not account for the fragility of glass-formation, which quantifies how rapidly relaxation times vary with temperature T. Accordingly, we simulate supported polymer films of a bead-spring model and determine both T(g) and fragility, both as a function of h and film depth. We contrast changes in the relaxation dynamics with density ρ and demonstrate the limitations of the commonly invoked free-volume layer model. As opposed to bulk polymer materials, we find that the fragility and T(g) do not generally vary proportionately. Consequently, the determination of the fragility profile--both locally and for the film as a whole--is essential for the characterization of changes in film dynamics with confinement.

Group-IV monochalcogenides are a family of two-dimensional puckered materials with an orthorhombic structure that is comprised of polar layers. In this article, we use first principles calculations to show the multistability of monolayer SnS and GeSe, two prototype materials where the direction of the puckering can be switched by application of tensile stress or electric field. Furthermore, the two inequivalent valleys in momentum space, which dictated by the puckering orientation, can be excited selectively using linearly polarized light, and this provides additional tool to identify the polarization direction. Our findings suggest that SnS and GeSe monolayers may have observable ferroelectricity and multistability, with potential applications in information storage. The discovery of 2D materials that can be isolated into single layers through exfoliation and exhibit novel properties has established new paradigms for ultrathin devices based on atomically sharp interfaces [1,2]. In particular, transition metal dichalcogenides (TMDs) have been studied extensively and have shown potential for many technological applications ranging from photovoltaics to valleytronic devices [3][4][5][6][7][8][9]. The family of monolayer 2D materials has recently grown to include other 2D semiconductors, such as phosphorene and related materials.However, one of the features thus far lacking for applications both in 2D electronics and in valleytronics is non-volatile memory. Ferromagnetism, an essential element in spintronic memories, is believed to be achievable in graphene and other 2D materials but so far remains difficult to realize and control [10]. Ferroelectric memories, in which the information is stored in the orientation of the electric dipole rather than in the magnetization are a possible option. Single-layer graphene (SLG) ferroelectric field-effect transistors (FFET) with symmetrical bit writing have been demonstrated [11], but the prototypes rely on bulk or thin film ferroelectric substrates [11] or ferroelectric polymers [12], rather than on crystalline atomically thin ferroelectric materials. An altogether different approach to information storage relies on phase change materials, where the bit value corresponds to a distinct structural phase of the material. Researchers have recently optimized the phase switching energy by using superlattice structures where the movement of the atoms is confined to only one dimension [13].In this article, we analyze the stability of group-IV monochalcogenide MX (M=Ge or Sn, and X=S or Se) monolayers, paying particular interest to their potential as memory functional materials. As prototypes, we use SnS and GeSe. In ambient conditions, bulk SnS and GeSe crystallize in the orthorhombic structure of the P nma space group. At 878 K, SnS goes through a secondorder displacive phase transition into the β-SnS phase with Cmcm symmetry [14,15], which is also a layered phase that can be viewed as a distorted rocksalt structure. For bulk GeSe, such a phase transition has not been observed. Ins...