Relation between glass transition temperatures in polymer nanocomposites and polymer thin films

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.

Approaches and Perspective of Coarse-Grained Modeling and Simulation for Polymer-Nanoparticle Hybrid Systems

Molecular modeling and simulations have emerged as effective and indispensable tools to characterize polymeric systems. They provide fundamental and essential insights to design a product of the required properties and to improve the understanding of a phenomenon at the molecular level for a particular system. The polymer-nanoparticle hybrids are materials with outstanding properties and correspondingly large applications whose study has benefited from this new paradigm. However, despite the significant expansion of modern day computational powers, investigation of the long time and large length scale phenomenon in polymeric and polymer-nanoparticle systems is still a challenging task to complete through all-atom molecular dynamics (AA-MD) simulations. To circumvent this problem, a variety of coarse-grained (CG) models have been proposed, ranging from the generic CG models for qualitative properties predictions to more realistic chemically specific CG models for quantitative properties predictions. These CG models have already delivered some success stories in the study of several spatial and temporal evolutions of many processes. Some of these studies were beyond the feasibility of traditional atomistic resolution models due to either the size or the time constraints. This review captures the different types of popular CG approaches that are utilized in the investigation of the microscopic behavior of polymer-nanoparticle hybrid systems. The rationale of this article is to furnish an overview of the popular CG approaches and their applications, to review several important and most recent developments, and to delineate the perspectives on future directions in the field.

Buried Structure in Block Copolymer Films Revealed by Soft X-ray Reflectivity

Interactions between polymers and surfaces can be used to influence properties including mechanical performance in nanocomposites, the glass transition temperature, and the orientation of thin film block copolymers (BCPs). In this work we investigate how specific interactions between the substrate and BCPs with varying substrate affinity impact the interfacial width between polymer components. The interface width is generally assumed to be a function of the BCP properties and independent of the surface affinity or substrate proximity. Using resonant soft X-ray reflectivity the optical constants of the film can be controlled by changing the incident energy, thereby varying the depth sensitivity of the measurement. Resonant soft X-ray reflectivity measurements were conducted on films of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) and PS-b-poly(methyl methacrylate) (PS-b-PMMA), where the thickness of the film was varied from half the periodicity (L₀) of the BCP to 5.5 L₀. The results of this measurement on the PS-b-P2VP films show a significant expansion of the interface width immediately adjacent to the surface. This is likely caused by the strong adsorption of P2VP to the substrate, which constrains the mobility of the junction points, preventing them from reaching their equilibrium distribution and expanding the observed interface width. The interface width decays toward equilibrium moving away from the substrate, with the decay rate being a function of film thickness below a critical limit. The PMMA block appears to be more mobile, and the BCP interfaces near the substrate match their equilibrium value.

Positive effect of functional side groups on the structure and properties of benzoxazine networks and nanocomposites

Calculated glass transition temperature of the MDP-a resin obtained through tetra-functional and penta-functional polymerization paths.

Do Carbon Nanotubes Improve the Thermomechanical Properties of Benzoxazine Thermosets?

Fillers are widely used to improve the thermomechanical response of polymer matrices, yet often in an unpredictable manner because the relationships between the mechanical properties of the composite material and the primary (chemical) structure of its molecular components have remained elusive so far. Here, we report on a combined theoretical and experimental study of the structural and thermomechanical properties of carbon nanotube (CNT)-reinforced polybenzoxazine resins, as prepared from two monomers that only differ by the presence of two ethyl side groups. Remarkably, while addition of CNT is found to have no impact on the glass-transition temperature ( T_g) of the ethyl-decorated resin, the corresponding ethyl-free composite features a surge by ∼47 °C (50 °C) in T_g, from molecular dynamics simulations (dynamic mechanical analysis measurements), as compared to the neat resin. Through a detailed theoretical analysis, we propose a microscopic picture for the differences in the thermomechanical properties of the resins, which sheds light on the relative importance of network topology, cross-link and hydrogen-bond density, chain mobility, and free volume.

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.

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.

The dynamics of unentangled polymers during capillary rise infiltration into a nanoparticle packing

Although highly packed polymer nanocomposites (PNCs) are important for a wide array of applications, preparing them remains difficult because of the poor dispersion of NPs at high loading fractions. One method to successfully prepare PNCs with high loadings is through capillary rise infiltration, as previously shown by Huang et al., although the mechanism of polymer infiltration remains largely unknown. We use molecular dynamics simulations to directly simulate the process of capillary rise infiltration, and we show that the polymers follow Lucas-Washburn dynamics. We observe a wetting front that precedes bulk infiltration, and chains belonging to this front are highly adsorbed to NPs. We also investigate the viscosity of the model polymers both globally and locally in supported and free-standing films, and we find reduced viscosity near the surface of the films and increased viscosity near the supporting substrate, similar to the results of local relaxation times. The reduction in the viscosity at the free surface for short, oligomeric polymers is smaller than for higher molecular weight polymers, and the ratio of the surface viscosities is most consistent with the predictions of the Lucas-Washburn equation. Our results introduce the mechanism by which polymers infiltrate a highly packed NP film, which may shed light on better ways to prepare these materials for energy storage applications and protective coatings.

Effects of nanoscopic-confinement on polymer dynamics

The static and dynamic behavior of polymers in confinement close to interfaces can be very different from that in the bulk. Among the various geometries, intercalated nanocomposites, in which polymer films of ∼1 nm thickness reside between the parallel inorganic surfaces of layered silicates in a well-ordered multilayer, offer a unique avenue for the investigation of the effects of nanoconfinement on polymer structure and dynamics by utilizing conventional analytical techniques and macroscopic specimens. In this article, we provide a review of research activities mainly in our laboratory on polymer dynamics under severe confinement utilizing different polymer systems: polar and non-polar polymers were mixed with hydrophilic or organophilic silicates, respectively, whereas hyperbranched polymers were studied in an attempt to probe the effect of polymer-surface interactions by altering the number and the kinds of functional groups in the periphery of the branched polymers. The polymer dynamics was probed by quasielastic neutron scattering and dielectric relaxation spectroscopy and was compared with that of the polymers in the bulk. In all cases, very local sub-Tg processes related to the motion of side and/or end groups as well as the segmental α-relaxation were identified with distinct differences recorded between the bulk and the confined systems. Confinement was found not to affect the very local motion in the case of the linear chains whereas it made it easier for hyperbranched polymers due to modifications of the hydrogen bond network. The segmental relaxation in confinement becomes faster than that in the bulk, exhibits Arrhenius temperature dependence and is observed even below the bulk Tg due to reduced cooperativity in the confined systems.

Environmental effects on fibre reinforced polymeric composites: evolving reasons and remarks on interfacial strength and stability

The interface between fibre and matrix of fibrous polymeric composites is most critical and decisive in maintaining sustainability, durability and also reliability of this potential material, but unfortunately a comprehensive conclusion is yet to meet the label of confidence for the engineering viability. Fiber reinforced polymer (FRP) composites are being accepted and also utilized as better and reliable alternative materials for repairing and/or replacing conventional materials, starting from tiny objects to mega structure in various engineering applications. The promise and potential of these materials are sometimes threatened in speedy replacement of conventional materials because of their inhomogeneities and inherent susceptibility to degradation due to moist and thermal environments. Environmental conditioning is traditionally believed to be a physical phenomenon but present literature has revealed that the interdiffusion between fiber and polymer matrix resin comprises of physical, chemical, mechanical, physico-chemical and mechano-chemical phenomena. The failure and fracture behavior at ambient conditions itself is a complex phenomenon till at present. The service conditions which are mostly hygrothermal in nature, along with a variation of applied loads make the mechanical behavior nearly unpredictable, far off from conclusions in evaluating the short term as well as long term durability and reliability of FRPs. It is essential to accurately simulate the initial and subsequent evolution process of this kind of damage phenomena, in order to explore the full potential of the mechanical properties of composite laminates. The present review has emphasized the need of complying scattered as well as limited literature on this front, and has focused on creating the urgency to highlight the importance of judicious uses of these materials with minimum safety factors with an aim to achieving lighter weight in enhancing specific properties.

Two glass transitions of polyurea networks: effect of the segmental molecular weight

Polymer-nanoparticle composites (PNCs) play an increasing role in technology. Inorganic or organic nanoparticles are usually incorporated into a polymer matrix to improve material properties. Polyurea is a spontaneously occurring PNC, exhibiting a phase segregated structure with hard nanodomains embedded in a soft (elastically compliant) matrix. This system shows two glass transitions at Tg1 and Tg2. It has been argued that they are related to the freezing of motion of molecular segments in the soft matrix (usual polymer α-glass transition at Tg1) and to regions of restricted mobility near the hard nanodomains (α'-process) at Tg2, respectively. We present detailed dynamic mechanical analysis (DMA) measurements for polyurea networks with different segmental lengths l(c) (2.5, 12.1, 24.5 nm) of the polymer chains, i.e. different volume fractions ϕ(x) (0.39, 0.12, 0.07) of the hard domains. The two glass transitions show up in two distinct peaks in tan δ at Tα and Tα'. Analysing the data using a Havriliak-Negami term for the α- and α'-relaxation, as well as Vogel-Fulcher dependencies for the corresponding relaxation times, it is found that the α-glass transition at Tg1 increases strongly (up to ΔT = 70 K) with increasing ϕ(x), whereas the α'-transition at Tg2 remains unchanged. At ϕ(x)(c) ≈ 0.19 the two curves intersect, i.e. Tg1 = Tg2. This value of ϕ(x)(c) is very close to the percolation threshold of randomly oriented overlapping ellipsoids of revolution with an aspect ratio of about 1 : 4-1 : 5. We therefore conclude that around 19% of the hard nanodomains polyurea changes from a system of hard nanoparticles embedded in a soft matrix (ϕ(x) ≤ ϕ(x)(c)) to a system of soft domains confined in a network of percolated hard domains at ϕ(x) ≥ ϕ(x)(c).

Effect of carbon nanotube functionalization on mechanical and thermal properties of cross-linked epoxy-carbon nanotube nanocomposites: role of strengthening the interfacial interactions

We have used amido-amine functionalized carbon nanotubes (CNTs) that form covalent bonds with cross-linked epoxy matrices to elucidate the role of the matrix-filler interphase in the enhancement of mechanical and thermal properties in these nanocomposites. For the base case of nanocomposites of cross-linked epoxy and pristine single-walled CNTs, our previous work (Khare, K. S.; Khare, R. J. Phys. Chem. B 2013, 117, 7444-7454) has shown that weak matrix-filler interactions cause the interphase region in the nanocomposite to be more compressible. Furthermore, because of the weak matrix-filler interactions, the nanocomposite containing dispersed pristine CNTs has a glass transition temperature (Tg) that is ∼66 K lower than the neat polymer. In this work, we demonstrate that in spite of the presence of stiff CNTs in the nanocomposite, the Young's modulus of the nanocomposite containing dispersed pristine CNTs is virtually unchanged compared to the neat cross-linked epoxy. This observation suggests that the compressibility of the matrix-filler interphase interferes with the ability of the CNTs to reinforce the matrix. Furthermore, when the compressibility of the interphase is reduced by the use of amido-amine functionalized CNTs, the mechanical reinforcement due to the filler is more effective, resulting in a ∼50% increase in the Young's modulus compared to the neat cross-linked epoxy. Correspondingly, the functionalization of the CNTs also led to a recovery in the Tg making it effectively the same as the neat polymer and also resulted in a ∼12% increase in the thermal conductivity of the nanocomposite containing functionalized CNTs compared to that containing pristine CNTs. These results demonstrate that the functionalization of the CNTs facilitates the transfer of both mechanical load and thermal energy across the matrix-filler interface.

Two simultaneous mechanisms causing glass transition temperature reductions in high molecular weight freestanding polymer films as measured by transmission ellipsometry

We study the glass transition in confined polymer films and present the first experimental evidence indicating that two separate mechanisms can act simultaneously on the film to propagate enhanced mobility from the free surface into the material. Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular-weight (MW), freestanding polystyrene films over an extended temperature range. For two different MWs, we observed two distinct reduced glass transition temperatures (T(g)'s), separated by up to 60 K, within single films with thicknesses h less than 70 nm. The lower transition follows the expected MW dependent, linear T(g)(h) behavior previously seen in high MW freestanding films. We also observe a much stronger upper transition with no MW dependence that exhibits the same T(g)(h) dependence as supported and low MW freestanding polymer films.

Effect of nanoparticle dispersion on glass transition in thin films of polymer nanocomposites

We present spectroscopic ellipsometry measurements on thin films of polymer nanocomposites consisting of gold nanoparticles embedded in poly(styrene). The temperature dependence of thickness variation is used to estimate the glass transition temperature, T(g). In these thin films we find a significant dependence of T(g) on the nature of dispersion of the embedded nanoparticles. Our work thus highlights the crucial role played by the particle polymer interface morphology in determining the glass transition in particular and thermo-mechanical properties of such nanocomposite films.

Studying polymer thin films with hybrid optical microcavities

Organic and inorganic polymeric thin films have numerous applications, including solar cells, biodetection, and nanocomposites. Improving our understanding of the fundamental material behavior is critical to designing polymers with ideal behavior and increased lifetime. However, there are limited nondestructive characterization methods that are able to perform these high-resolution measurements. In this Letter, we demonstrate a method that is able to detect temperature-induced changes in the refractive index of polystyrene polymer thin films as small as 10(-7). This approach is based on optical microcavity resonators. The experimental results agree well with the theoretical simulations.

Glass transition of two-dimensional binary soft-disk mixtures with large size ratios

We simulate binary soft-disk systems in two dimensions and investigate how the dynamics slow as the area fraction is increased toward the glass transition. The "fragility" quantifies how sensitively the relaxation time scale depends on the area fraction, and the fragility strongly depends on the composition of the mixture. We confirm prior results for mixtures of particles with similar sizes, where the ability to form small crystalline regions correlates with fragility. However, for mixtures with particle size ratios above 1.4, we find that the fragility is not correlated with structural ordering, but rather with the spatial distribution of large particles. The large particles have slower motion than the small particles and act as confining "walls" which slow the motion of nearby small particles. The rearrangement of these confining structures governs the lifetime of dynamical heterogeneity, that is, how long local regions exhibit anomalously fast or slow behavior. The strength of the confinement effect is correlated with the fragility and also influences the aging behavior of glassy systems.

The glass transition of thin polymer films in relation to the interfacial dynamics

As opposed to measurements on the glass transition of a polymer in the bulk, measurements of thin polymer layers reflect--due to the alterations of the glassy dynamics at the confining interfaces--several contributions acting together to give the net response of a polymer film. This fundamental difference is exemplified in detail for the particular case of broadband dielectric spectroscopy, an experimental tool extensively employed to investigate the glassy dynamics of polymers under condition of geometrical confinement.

Segmental dynamics in polymers: from cold melts to ageing and stressed glasses

Recent progress in developing statistical mechanical theories of supercooled polymer melts and glasses is reviewed. The focus is on those approaches that are either explicitly formulated for polymers, or are applications of more generic theories to interpret polymeric phenomena. These include two configurational entropy theories, a percolated free volume distribution model, and the activated barrier hopping nonlinear Langevin theory. Both chemically-specific and universal aspects are discussed. After a brief summary of classic phenomenological approaches, a discussion of the relevant length scales and key experimental phenomena in both the supercooled liquid and glassy solid state is presented including ageing and nonlinear mechanical response. The central concepts that underlie the theories in the molten state are then summarized and key predictions discussed, including the glass transition in oriented polymer liquids and deformed rubber networks. Physical ageing occurs in the nonequilibrium glass, and theories for its consequences on the alpha relaxation are discussed. Very recent progress in developing a segment scale theory for the dramatic effects of external stress on polymer glasses, including acceleration of relaxation, yielding, plastic flow and strain hardening, is summarized. The article concludes with a discussion of outstanding theoretical challenges.

Polymer chain dynamics and glass transition in athermal polymer/nanoparticle mixtures

Polymer nanocomposites (PNCs), prepared by incorporating nanoparticles within a polymer host, generally exhibit properties that differ significantly from those of the host, even with small amounts of nanoparticles. A significant challenge is how to tailor the properties of these materials for applications (structural and biomedical to optoelectronic), because PNCs derive their properties from a collective and complex range of entropic and enthalpic interactions. Here, we show that PNCs, prepared from athermal mixtures of polymer-chain-grafted gold nanoparticles and unentangled polymer chains, may exhibit increases or decreases in their relaxation dynamics, and viscosity, by over an order of magnitude through control of nanoparticle concentration, nanoparticle size, grafting density and grafting chain degree of polymerization. In addition, we show how the glass transition may also be tailored by up to 10 degrees with the addition of less than 1.0 wt% nanoparticles to the polymer host.