Glass Transition Behavior of PS Films on Grafted PS Substrates

Dynamics of Polymer Films on Polymer-Grafted Substrates: A Molecular Dynamics Simulation

For substrate-supported polymer films, the tails of adsorbed chains are generally assumed to play important roles in the propagation of the substrate's effect inside polymer films. The effects of the grafting density and the rigidity of substrate-grafted polymers, the simplest model for the adsorbed tails, on the diffusivity of film polymers are investigated by performing molecular dynamics simulations. An optimal grafting density σo, around the critical grafting density for the transition from "mushroom" to "brush", is found with the most pronounced suppression of diffusivity on the film polymers; i.e., the penetration of the film polymers into the grafting layer reaches the maximum. However, at high grafting density, the crowded and vertically stretched brush excludes the coil-like film polymers, and the suppression is thus reduced. At σo, with an increase in the rigidity of the grafted polymers, the suppression is increased quickly at low rigidity but slowly at high rigidity. The dynamic suppression is attributed to the combination of the conformation change from stretching at low rigidity to tilted orientation at high rigidity and decelerated mobility induced by the rigidity. The stretching conformation enhances, whereas the tilted conformation weakens the interpenetration between the grafted polymers and the film polymers. Our results reflect the importance of both conformational variation and interchain interaction in the interface region.

Exploring the Molecular Origin for the Long-Range Propagation of the Substrate Effect in Unentangled Poly(methyl methacrylate) Films

The polymer/substrate interface plays a significant role in the dynamics of nanoconfined polymers because of its suppression on polymer mobility and its long-range propagation feature, while the molecular origin of the long-range substrate effect in unentangled polymer material is still ambiguous. Herein, we investigated the propagation distances of the substrate effect (h*) by a fluorinated tracer-labeled method of two unentangled polymer films supported on silicon substrates: linear and ring poly(methyl methacrylate) films with relatively low molecular weights. The results indicate that the value of h* has a molecular weight dependence of h*∝N (N is the degree of polymerization) in the unentangled polymer films, while h*∝N1/2 was presented as previously reported in the entangled films. A theoretical model, depending on the polymer/polymer intermolecular interaction, was proposed to describe the above long-range propagation behavior of the substrate effect and agrees with our experiment results very well. From the model, it revealed that the intermolecular friction determines the long-range propagation of the substrate effect in the unentangled system, but the intermolecular entanglement is the dominant role in entangled system. These results give us a deeper understanding of the long-range substrate effect.

Improving the Stability of Non-fullerene-Based Organic Photovoltaics through Sequential Deposition and Utilization of a Quasi-orthogonal Solvent

The development of organic photovoltaic (OPV) devices based on non-fullerene acceptors (NFAs) has led to a rapid improvement in their efficiency. Despite these improvements, significant performance degradation in the early stages of operation, known as burn-in, remains a challenge for NFA-based OPVs. To address this challenge, this study demonstrates a stable NFA-based OPV fabricated using sequential deposition (SqD) and a quasi-orthogonal solvent. The quasi-orthogonal solvent, which is prepared by incorporating 1-chloronaphthalene (1-CN) into dichloromethane (DCM), reduces the vapor pressure of the solvent and allows for the efficient dissolution and penetration of the Y6 (one of efficient NFAs) into a PM6 polymer-donor layer without damaging the latter. The resulting bulk heterojunction (BHJ) is characterized by a higher degree of crystallinity in the PM6 domains than that prepared using a conventional single-step deposition (SD) process. The OPV fabricated using the SqD process exhibits a PCE of 14.1% and demonstrates superior thermal stability to the SD-processed OPV. This study conclusively reveals that the formation of a thermally stable interface between the photoactive layer and the electron-transport layer (ETL) is the primary factor contributing to the high thermal stability observed in the SqD-processed OPV.

End-Tethered Chains Increase the Local Glass Transition Temperature of Matrix Chains by 45 K Next to Solid Substrates Independent of Chain Length

The local glass transition temperature T_g of pyrene-labeled polystyrene (PS) chains intermixed with end-tethered PS chains grafted to a neutral silica substrate was measured by fluorescence spectroscopy. To isolate the impact of the grafted chains, the films were capped with bulk neat PS layers eliminating competing effects of the free surface. Results demonstrate that end-grafted chains strongly increase the local T_g of matrix chains by ≈45 K relative to bulk T_g, independent of grafted chain molecular weight from M_n = 8.6 to 212 kg/mol and chemical end-group, over a wide range of grafting densities σ = 0.003 to 0.33 chains/nm² spanning the mushroom-to-brush transition regime. The tens-of-degree increase in local T_g resulting from immobilization of the chain ends by covalent bonding in this athermal system suggests a mechanism that substantially increases the local activation energy required for cooperative rearrangements.

Spectroscopic ellipsometry as a route to thermodynamic characterization

Strategies for synthesizing molecularly designed materials are expanding, but methods for their thermodynamic characterization are not. This shortfall presents a challenge to the goal of connecting local molecular structure with material properties and response. Fundamental thermodynamic quantities, including the thermal expansion coefficient, α, can serve as powerful inputs to models, yielding insight and predictive power for phenomena ranging from miscibility to dynamic relaxation. However, the usual routes for thermodynamic characterization often require a significant sample size (e.g. one gram), or challenging experimental set-ups (e.g. mercury as a confining fluid), or both. Here, we apply spectroscopic ellipsometry, which is an optical technique for thin film analysis, to obtain thermodynamic data. We clarify issues in the scientific literature concerning the connection between ellipsometric and volumetric thermal expansion coefficients for substances in both the glass and melt states. We analyze temperature-dependent data derived using both ellipsometry and macro-scale dilatometric techniques for ten different polymers. We find superb correlation between the α values obtained via the two techniques, after considering the effects of mechanical confinement by the substrate for a glassy thin film. We show how the ellipsometric α can serve as input to the locally correlated lattice theory to yield predictions for the percent free volume in each polymer as a function of temperature. We find that the ellipsometric α at the glass transition temperature, T_g, is not only material dependent, but it is linearly correlated with T_g itself. Spectroscopic ellipsometry, which requires only very small quantities of sample and is straightforward to perform, will significantly expand the range of systems for which thermodynamic properties can be characterized. It will thus advance our ability to use theory and modeling to predict the miscibility and dynamic relaxation of new materials. As such, ellipsometry will be able to underpin materials synthesis and property design.

Immobilization of Conjugated Polymer Domains for Highly Stable Non-Fullerene-Based Organic Solar Cells

To commercialize organic solar cells (OSCs), changes in the optimized morphology of the photoactive layer caused by external stimuli that cause degradation must be addressed. This work improves OSC stability by utilizing the cross-linking additive 1,8-dibromooctane (DBO) and a sequential deposition process (XSqD) to fabricate the photoactive layer. The cross-linking additive in the donor polymer (PTB7-Th) improves polymer crystallinity and immobilizes the crystalline morphology by partial photo-cross-linking. Ellipsometry experiments confirm the increase in the glass transition temperature of cross-linked PTB7-Th. The polymer crystallinity is further improved after removal of non-cross-linked polymer and residual additive by chlorobenzene. The cross-linked polymer layer forms an efficient and stable heterojunction with a nonfullerene acceptor (IEICO-4F) layer via an XSqD process. The OSC based on the immobilized PTB7-Th exhibits excellent stability against light soaking and thermal aging.

Interphase in Polymer Nanocomposites

The lightweight and high-strength functional nanocomposites are important in many practical applications. Natural biomaterials with excellent mechanical properties provide inspiration for improving the performance of composite materials. Previous studies have usually focused on the bionic design of the material's microstructure, sometimes overlooking the importance of the interphase in the nanocomposite system. In this Perspective, we will focus on the construction and control of the interphase in confined space and the connection between the interphase and the macroscopic properties of the materials. We shall survey the current understanding of the critical size of the interphase and discuss the general rules of interphase formation. We hope to raise awareness of the interphase concept and encourage more experimental and simulation studies on this subject, with the aim of an optimal design and controllable preparation of polymer nanocomposite materials.

Artificial Neural Network Modeling of Glass Transition Temperatures for Some Homopolymers with Saturated Carbon Chain Backbone

The glass transition temperature (Tg) is an important decision parameter when synthesizing polymeric compounds or when selecting their applicability domain. In this work, the glass transition temperature of more than 100 homopolymers with saturated backbones was predicted using a neuro-evolutive technique combining Artificial Neural Networks with a modified Bacterial Foraging Optimization Algorithm. In most cases, the selected polymers have a vinyl-type backbone substituted with various groups. A few samples with an oxygen atom in a linear non-vinyl hydrocarbon main chain were also considered. Eight structural, thermophysical, and entanglement properties estimated by the quantitative structure-property relationship (QSPR) method, along with other molecular descriptors reflecting polymer composition, were considered as input data for Artificial Neural Networks. The Tg's neural model has a 7.30% average absolute error for the training data and 12.89% for the testing one. From the sensitivity analysis, it was found that cohesive energy, from all independent parameters, has the highest influence on the modeled output.

Temperature-Dependent Spectroscopic Ellipsometry of Thin Polymer Films

Thin polymer films have found many important applications in organic electronics, such as active layers, protective layers, or antistatic layers. Among the various experimental methods suitable for studying the thermo-optical properties of thin polymer films, temperature-dependent spectroscopic ellipsometry plays a special role as a nondestructive and very sensitive optical technique. In this Review Article, issues related to the physical origin of the dependence of ellipsometric angles on temperature are surveyed. In addition, the Review Article discusses the use of temperature-dependent spectroscopic ellipsometry for studying phase transitions in thin polymer films. The benefits of studying thermal transitions using different cooling/heating speeds are also discussed. Furthermore, it is shown how the analysis and modeling of raw ellipsometric data can be used to determine the thermal properties of thin polymer films.

Optimizing the Grafting Density of Tethered Chains to Alter the Local Glass Transition Temperature of Polystyrene near Silica Substrates: The Advantage of Mushrooms over Brushes

We measured the local glass transition temperature T_g(z) of polystyrene (PS) as a function of distance z from a silica substrate with end-grafted chains using fluorescence, where competing effects from the free surface have been avoided to focus only on the influence of the tethered interface. The local T_g(z) increase next to the chain-grafted substrate is found to exhibit a maximum increase of 49 ± 2 K relative to bulk at an optimum grafting density that corresponds to the mushroom-to-brush transition regime. This perturbation to the local T_g(z) dynamics of the matrix is observed to persist out to a distance z ≈ 100-125 nm for this optimum grafting density before bulk T_g is recovered, a distance comparable to that previously observed by Baglay and Roth [J. Chem. Phys. 2017, 146, 203307] for PS next to the higher-T_g polymer polysulfone.

Effect of Entrapped Solvent on the Evolution of Lateral Order in Self-Assembled P(S-r-MMA)/PS-b-PMMA Systems with Different Thicknesses

Block copolymers (BCPs) are emerging as a cost-effective nanofabrication tool to complement conventional optical lithography because they self-assemble in highly ordered polymeric templates with well-defined sub-20-nm periodic features. In this context, cylinder-forming polystyrene-block-poly(methyl methacrylate) BCPs are revealed as an interesting material of choice because the orientation of the nanostructures with respect to the underlying substrate can be effectively controlled by a poly(styrene-random-methyl methacrylate) random copolymer (RCP) brush layer grafted to the substrate prior to BCP deposition. In this work, we investigate the self-assembly process and lateral order evolution in RCP + BCP systems consisting of cylinder-forming PS-b-PMMA (67 kg mol^-1, PS fraction of ∼70%) films with thicknesses of 30, 70, 100, and 130 nm deposited on RCP brush layers having thicknesses ranging from 2 to 20 nm. The self-assembly process is promoted by a rapid thermal processing machine operating at 250 °C for 300 s. The level of lateral order is determined by measuring the correlation length (ξ) in the self-assembled BCP films. Moreover, the amount of solvent (Φ) retained in the RCP + BCP systems is measured as a function of the thicknesses of the RCP and BCP layers, respectively. In the 30-nm-thick BCP films, an increase in Φ as a function of the thickness of the RCP brush layer significantly affects the self-assembly kinetics and the final extent of the lateral order in the BCP films. Conversely, no significant variations of ξ are observed in the 70-, 100-, and 130-nm-thick BCP films with increasing Φ.

Influence of grafting on the glass transition temperature of PS thin films

We present an investigation of the effect of the interaction between a thin polystyrene film and its supporting substrate on its glass transition temperature ([Formula: see text]). We modulate this interaction by depositing the film on end-tethered polystyrene grafted layers of controlled molecular parameters. By comparing [Formula: see text] measurements versus film thickness for films deposited on different grafted layers and films deposited directly on a silicon substrate, we can conclude that there is no important effect of the film-subtrate interaction. Our interpretation of these results is that local orientation and dynamic effects substantial enough to influence [Formula: see text] cannot readily be obtained by grafting prepolymerized chains to a surface, due to intrinsic limitation of the surface grafting density.

Unexpected segmental dynamics in polystyrene-grafted silica nanocomposites

Establishing the relationship between interfacial layer chain packing and dynamics remains a continuing challenge in polymer nanocomposites (PNCs). This issue is expected to be significant in our understanding of the mechanism of the dynamic response of such materials and the manner in which these parameters affect the macroscopic properties of PNCs. In this study, we report the dynamics of free polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrix chains, as well as those of polymer chains surrounding the spherical silica nanoparticles (NPs) where silica NPs are either bare or PS grafted, to discriminate the role of grafted chains and interfacial interactions between grafted NPs and the matrix. The α-relaxation dynamics of the PS matrix is unaffected by silica NP loadings, it slows down in PMMA nanocomposites because of polymer-NP interfacial interactions and steric hindrance. More interestingly, we probe the enhanced mobility of the interfacial layer (α'-relaxation) in PNCs filled with grafted NPs, and this phenomenon is further corroborated by the accelerated Maxwell-Wagner-Sillars polarization process in the presence of grafted silica NPs. Moreover, the α'-relaxation time in the vicinity of glass transition temperature of the polymer matrix unexpectedly increases with increasing temperature. Such an anomalous temperature-dependent behavior can be attributed to the influence exerted by slow α-relaxation dynamics. Considering these phenomena and the mechanical properties, we propose a three-layer model to explain the observed behavior of grafted silica NP-filled nanocomposites. These findings provide new insight into the mechanisms responsible for mechanical reinforcement and therefore provide guidance in designing PNCs with tunable macroscopic properties.

The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films

To elucidate the mechanism underlying the effect of polymer/solid interfacial interactions on the dynamics of thin polymer films, the glass transition of thin end-functionalized polystyrene films supported on SiO₂-Si, such as proton-terminated PS (PS-H), α,ω-dicarboxy-terminated PS (PS-COOH), and α,ω-dihydroxyl-terminated PS (PS-OH), was investigated. All the PS films exhibited a substantial depression in T_g with decreasing film thickness, while the extent of such depression was strongly dependent on the chemical structure of the end groups and molecular weights. It was found that T - T of the various PS films increased linearly with increasing h_ads/R_g, in which h_ads is the thickness of the interfacial adsorbed layer and R_g is the radius of gyration of PS. The h_ads/R_g is a direct reflection of the macromolecular chain conformation within the adsorbed layer which was affected by its end groups and molecular weights. These findings are in line with the work of Napolitano, and present direct experimental evidence.

Densification and depression in glass transition temperature in polystyrene thin films

Ellipsometry and X-ray reflectivity were used to characterize the mass density and the glass transition temperature of supported polystyrene (PS) thin films as a function of their thickness. By measuring the critical wave vector (qc) on the plateau of total external reflection, we evidence that PS films get denser in a confined state when the film thickness is below 50 nm. Refractive indices (n) and electron density profiles measurements confirm this statement. The density of a 6 nm (0.4 gyration radius, Rg) thick film is 30% greater than that of a 150 nm (10Rg) film. A depression of 25 °C in glass transition temperature (Tg) was revealed as the film thickness is reduced. In the context of the free volume theory, this result seems to be in apparent contradiction with the fact that thinner films are denser. However, as the thermal expansion of thinner films is found to be greater than the one of thicker films, the increase in free volume is larger for thin films when temperature is raised. Therefore, the free volume reaches a critical value at a lower Tg for thinner films. This critical value corresponds to the onset of large cooperative movements of polymer chains. The link between the densification of ultrathin films and the drop in their Tg is thus reconciled. We finally show that at their respective Tg(h) all films exhibit a critical mass density of about 1.05 g/cm(3) whatever their thickness. The thickness dependent thermal expansion related to the free volume is consequently a key factor to understand the drop in the Tg of ultrathin films.

Mechanical and dielectric properties of microcellular polycarbonate foams with unimodal or bimodal cell-size distributions

This article reports on the compressive, dynamic mechanical and dielectric properties of microcellular polycarbonate foams with unimodal or bimodal cell-size distributions fabricated using the environment-friendly supercritical carbon dioxide. The effects of cell morphologies such as relative density, cell-size distribution and porosity on the compressive strength, Young’s modulus, storage modulus, loss modulus, dielectric constant and loss tangent of the microcellular polycarbonate foams are investigated quantitatively. Experimental values of the compressive strength and Young’s modulus fit very well with the theoretical values calculated from the Gibson–Ashby model at low relative densities. The higher relative density leads to higher storage modulus and loss modulus. The bimodal foams significantly improve the compressive and dynamic mechanical properties compared to the unimodal foams with the same relative density. The dielectric properties of microcellular foams depend only on the total porosity, but not on the cell-size distribution or microstructure of the foams. With increasing porosity, the dielectric constant of the microcellular foams gradually decreases, and agrees very well with the curve calculated from the Maxwell-Garnett-spheres model.

Solvent-assisted directed self-assembly of spherical microdomain block copolymers to high areal density arrays

The fabrication process for 5 Tb/in(2) bit patterns using solvent-assisted directed self-assembly is investigated. The N-methyl-2-pyrrolidone solvent vapor-annealing method was used to achieve good long-range lateral ordering of low-molecular-weight polystyrene-block-polydimethylsiloxane with a lattice spacing of 11 nm on flat Si substrates, PS modified substrates and lithographically patterned substrates, respectively.