Local Glass Transition Temperature Tg(z) Within Polystyrene Is Strongly Impacted by the Modulus of the Neighboring PDMS Domain

Local dynamics and failure of inhomogeneous polymer networks

Inhomogeneous crosslinked polymers are powerful platforms for materials design, because they can be synthesized from materials that provide complimentary properties to the resulting gel. For example, a membrane with both glassy and rubbery domains will be mechanically robust while enabling transport. The dynamics, and mechanical and failure properties of rubbery/glassy conetworks are only beginning to be studied, and there is likely to be strong heterogeneities in the dynamics and mechanical response. In this study, we use coarse-grained molecular dynamics simulations to generate microphase separated rubbery/glassy polymer networks with a bicontinuous morphology via in silico crosslinking. We study the effect of phase boundary on the local mobility gradient, and our simulation results reveal an asymmetric shift in the local mobility gradient across the interface that extends deeper into the phase with a lower Tg when the system temperature is between the glass transition temperatures of the two phases. Moreover, by employing a model that allows bond breaking, we examine the microscopic mechanism for failure in these networks as a function of the molecular weight of polymer strands between crosslinks and the number fraction of the glassy domain. Under uniaxial extension, we find that the stress is initially larger in the glassy domain. As the deformation proceeds, the segmental dynamics of the two phases homogenize, and subsequently bond breaking begins.

Modulus alteration of thin polystyrene films by their neighboring PDMS: Soft and hard confinement

It is highly demanded to understand the confinement effect on nanoconfined polymers. Recent studies reported a strong perturbation of local dynamics and substantial alteration of glass transition temperature Tg at nanoscale. However, how confinement affects the mechanical properties of polymers is not fully understood. Here, we show that the modulus of thin polymer films could be remarkedly altered through a polymer-polymer interface. The modulus of a thin polystyrene (PS) film next to a polydimethylsiloxane (PDMS) was determined from the PS-PDMS bilayer bulging test. A series of experiments show that the modulus of PS can be increased up to 37%, when the modulus of the neighboring PDMS varies from 1.04 to 4.88 MPa. The results demonstrate a strong sensitivity of mechanical properties of thin polymers to the hard/soft environment, which we attribute to the change of high-mobility layer by the polymer-polymer interface.

High-density stable glasses formed on soft substrates

Enabled by surface-mediated equilibration, physical vapour deposition can create high-density stable glasses comparable with liquid-quenched glasses aged for millions of years. Deposition is often performed at various rates and temperatures on rigid substrates to control the glass properties. Here we demonstrate that on soft, rubbery substrates, surface-mediated equilibration is enhanced up to 170 nm away from the interface, forming stable glasses with densities up to 2.5% higher than liquid-quenched glasses within 2.5 h of deposition. Gaining similar properties on rigid substrates would require 10 million times slower deposition, taking ~3,000 years. Controlling the modulus of the rubbery substrate provides control over the glass structure and density at constant deposition conditions. These results underscore the significance of substrate elasticity in manipulating the properties of the mobile surface layer and thus the glass structure and properties, allowing access to deeper states of the energy landscape without prohibitively slow deposition rates.

Surface Glass Transition Temperature Region of Diarylethene Films Determined by Nano-Marangoni Effect

For the last two decades, research has addressed whether the glass transition temperature and the molecular motions on the surface of organic films are significantly different from those inside the bulk glasses. It is reported that the surface of the photochromic diarylethene film prepared by vacuum deposition has fluidity and the vacuum deposition of small amount of rubrene molecules induces surface tension fluctuations, generating dents due to the Marangoni flow in nanoscale. The depth of the dents increases in proportion to these radii for the colorless diarylethene film with a bulk glass transition temperature (Tg) close to room temperature. On the other hand, in the colored diarylethene obtained by UV irradiation to the colorless film, the depth becomes constant at a certain level. The Tg distribution in the depth direction is clarified based on an analysis of the dent depth. By approximating the obtained Tg depth distribution with an exponential function, the outermost surface Tg is about 100 K lower than the bulk Tg in the case of photoisomerized diarylethene.

Effect of the nature of the solid substrate on spatially heterogeneous activated dynamics in glass forming supported films

We extend the force-level elastically collective nonlinear Langevin equation theory to treat the spatial gradients of the alpha relaxation time and glass transition temperature, and the corresponding film-averaged quantities, to the geometrically asymmetric case of finite thickness supported films with variable fluid-substrate coupling. The latter typically nonuniversally slows down motion near the solid-liquid interface as modeled via modification of the surface dynamic free energy caging constraints that are spatially transferred into the film and which compete with the accelerated relaxation gradient induced by the vapor interface. Quantitative applications to the foundational hard sphere fluid and a polymer melt are presented. The strength of the effective fluid-substrate coupling has very large consequences for the dynamical gradients and film-averaged quantities in a film thickness and thermodynamic state dependent manner. The interference of the dynamical gradients of opposite nature emanating from the vapor and solid interfaces is determined, including the conditions for the disappearance of a bulk-like region in the film center. The relative importance of surface-induced modification of local caging vs the generic truncation of the long range collective elastic component of the activation barrier is studied. The conditions for the accuracy and failure of a simple superposition approximation for dynamical gradients in thin films are also determined. The emergence of near substrate dead layers, large gradient effects on film-averaged response functions, and a weak non-monotonic evolution of dynamic gradients in thick and cold films are briefly discussed. The connection of our theoretical results to simulations and experiments is briefly discussed, as is the extension to treat more complex glass-forming systems under nanoconfinement.

Development of broad modulus profile upon polymer-polymer interface formation between immiscible glassy-rubbery domains

Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ([Formula: see text]) where previous work has demonstrated a long-range (∼200 nm) profile in local [Formula: see text] is established between immiscible glassy and rubbery polymer domains when the polymer-polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus [Formula: see text] is established when the polymer-polymer interface ([Formula: see text]5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad [Formula: see text] profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus [Formula: see text] spanning [Formula: see text]180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in [Formula: see text] and [Formula: see text] arise from a coupling of the spectrum of vibrational modes across the polymer-polymer interface as a result of acoustic impedance matching of sound waves with [Formula: see text] nm during interface broadening that can then trigger density fluctuations in the neighboring domain.

Directly Using Paraffin as the Toughening Agent of Epoxy Composites: An Experimental and Molecular Dynamics Simulation Study

It is still a challenge in studying the toughening mechanism by well combining the experimental and atomistic molecular dynamics (MD) simulation study. This article directly introduced eicosane (C20, model compound of paraffin) into the epoxy matrix (DGEBA) by using a special epoxy resin with alkyl side chains (D12) as a compatibilizer, which was synthesized through thiol-ene click chemistry. The toughening mechanism of the ternary DGEBA/D12/C20 (EPDA-X) systems was systematically investigated by experimental and MD simulation methods. Though C20 can be well dispersed in the curing mixture, the huge polarity difference between C20 and DGEBA can be the driving force for C20 to stay away from DGEBA, demonstrating the self-assembly effect of C20 around the alkyl side chains of D12 because of the good compatibility of D12 and C20. The soft alkyl chains of D12 and C20 as well as the self-assembly effect of C20 around the D12 molecules can simultaneously improve the strength, modulus, and toughness of the EPDA-2.5 system. This article not only provides a brand new toughening strategy by directly using nonfunctional alkyl derivatives as the toughening agent of epoxy composites with superior mechanical properties but also provides a systematic MD simulation method to evaluate whether there is the interaction or not and the strength of interaction between different molecular chains so as to provide a theoretical basis for the cause of the microphase separation structure and related toughening mechanism in cross-linking networks on the atomic and molecular levels.

Polymers under nanoconfinement: where are we now in understanding local property changes?

Polymers are increasingly being used in applications with nanostructured morphologies where almost all polymer molecules are within a few tens to hundreds of nanometers from some interface. From nearly three decades of study on polymers in simplified nanoconfined systems such as thin films, we have come to understand property changes in these systems as arising from interfacial effects where local dynamical perturbations are propagated deeper into the material. This review provides a summary of local glass transition temperature T_g changes near interfaces, comparing across different types of interfaces: free surface, substrate, liquid, and polymer-polymer. Local versus film-average properties in thin films are discussed, making connections to other related property changes, while highlighting several historically important studies. By experimental necessity, most studies are on high enough molecule weight chains to be well entangled, although aspects that connect to lower molecule weight materials are described. Emphasis is made to identify observations and open questions that have yet to be fully understood such as the evidence of long-ranged interfacial effects, finite domain size, interfacial breadth, and chain connectivity.