Glass Transition Temperature of Polymer–Nanoparticle Composites: Effect of Polymer–Particle Interfacial Energy

Acrylonitrile Butadiene Styrene-Based Composites with Permalloy with Tailored Magnetic Response

This work reports on tailoring the magnetic properties of acrylonitrile butadiene styrene (ABS)-based composites for their application in magnetoactive systems, such as magnetic sensors and actuators. The magnetic properties of the composites are provided by the inclusion of varying permalloy (Py-Ni₇₅Fe₂₀Mo₅) nanoparticle content within the ABS matrix. Composites with Py nanoparticle content up to 80 wt% were prepared and their morphological, mechanical, thermal, dielectric and magnetic properties were evaluated. It was found that ABS shows the capability to include high loads of the filler without negatively influencing its thermal and mechanical properties. In fact, the thermal properties of the ABS matrix are basically unaltered with the inclusion of the Py nanoparticles, with the glass transition temperatures of pristine ABS and its composites remaining around 105 °C. The mechanical properties of the composites depend on filler content, with the Young's modulus ranging from 1.16 GPa for the pristine ABS up to 1.98 GPa for the sample with 60 wt% filler content. Regarding the magnetic properties, the saturation magnetization of the composites increased linearly with increasing Py content up to a value of 50.9 emu/g for the samples with 80 wt% of Py content. A numerical model has been developed to support the findings about the magnetic behavior of the NP within the ABS. Overall, the slight improvement in the mechanical properties and the magnetic properties provides the ABS composites new possibilities for applications in magnetoactive systems, including magnetic sensors, actuators and magnetic field shielding.

Bacteriostatic and Cytotoxic Properties of Composite Material Based on ZnO Nanoparticles in PLGA Obtained by Low Temperature Method

A low-temperature technology was developed for producing a nanocomposite based on poly (lactic-co-glycolic acid) and zinc oxide nanoparticles (ZnO-NPs), synthesized by laser ablation. Nanocomposites were created containing 0.001, 0.01, and 0.1% of zinc oxide nanoparticles with rod-like morphology and a size of 40-70 nm. The surface of the films from the obtained nanomaterial was uniform, without significant defects. Clustering of ZnO-NPs in the PLGA matrix was noted, which increased with an increase in the concentration of the dopant in the polymer. The resulting nanomaterial was capable of generating reactive oxygen species (ROS), such as hydrogen peroxide and hydroxyl radicals. The rate of ROS generation increased with an increase in the concentration of the dopant. It was shown that the synthesized nanocomposite promotes the formation of long-lived reactive protein species, and is also the reason for the appearance of a key biomarker of oxidative stress, 8-oxoguanine, in DNA. The intensity of the process increased with an increase in the concentration of nanoparticles in the matrix. It was found that the nanocomposite exhibits significant bacteriostatic properties, the severity of which depends on the concentration of nanoparticles. In particular, on the surface of the PLGA-ZnO-NPs composite film containing 0.001% nanoparticles, the number of bacterial cells was 50% lower than that of pure PLGA. The surface of the composite is non-toxic to eukaryotic cells and does not interfere with their adhesion, growth, and division. Due to its low cytotoxicity and bacteriostatic properties, this nanocomposite can be used as coatings for packaging in the food industry, additives for textiles, and also as a material for biomedicine.

Interfacial Contributions in Nanodiamond-Reinforced Polymeric Fibers

We study the interfacial energy parameters that explain the reinforcement of polymers with nanodiamond (ND) and the development of mechanical strength of electrospun ND-reinforced composites. Thermodynamic parameters such as the wettability ratio, work of spreading and dispersion/aggregation transition are used to derive a criterion to predict the dispersibility of carboxylated ND (cND) in polymeric matrices. Such a criterion for dispersion (D_c) is applied to electrospun cND-containing poly(vinyl alcohol) (PVA), polyacrylonitrile (PAN), and polystyrene (PS) fiber composites. The shifts in glass transition temperature (ΔT_g), used as a measure of polymer/cND interfacial interactions and hence the reinforcement capability of cNDs, reveal a direct correlation with the thermodynamic parameter D_c in the order of PAN < PS < PVA. Contrary to expectation, however, the tensile strength of the electrospun fibers correlates with the D_c and ΔT_g only for semicrystalline polymers (PAN < PVA) while the amorphous PS displays a maximum reinforcement with cND. Such conflicting results reveal a synergy that is not captured by thermodynamic considerations alone but also factor in the contributions of polymer/cND interface stress transfer efficiency. Our findings open the possibility for tailoring the interfacial interactions in polymer-ND fiber composites to achieve maximum mechanical reinforcement.

Introducing a novel low energy gamma ray shield utilizing Polycarbonate Bismuth Oxide composite

The fabrication of different weight percentages of Polycarbonate-Bismuth Oxide composite (PC-Bi2O3), namely 0, 5, 10, 20, 30, 40, and 50 wt%, was done via the mixed-solution method. The dispersion state of the inclusions into the polymeric matrix was studied through XRD and SEM analyses. Also, TGA and DTA analyses were carried out to investigate the thermal properties of the samples. Results showed that increasing the amount of Bi2O3 into the polymer matrix shifted the glass transition temperature of the composites towards the lower temperatures. Then, the amount of mass attenuation coefficients of the samples were measured using a CsI(Tl) detector for different gamma rays of 241Am, 57Co, 99mTc, and 133Ba radioactive sources. It was obtained that increasing the concentration of the Bi2O3 fillers in the polycarbonate matrix resulted in increasing the attenuation coefficients of the composites significantly. The attenuation coefficient was enhanced twenty-three times for 50 wt% composite in 59 keV energy, comparing to the pure polycarbonate.

Introducing a novel low energy gamma ray shield utilizing Polycarbonate Bismuth Oxide composite

The fabrication of different weight percentages of Polycarbonate-Bismuth Oxide composite (PC-Bi₂O₃), namely 0, 5, 10, 20, 30, 40, and 50 wt%, was done via the mixed-solution method. The dispersion state of the inclusions into the polymeric matrix was studied through XRD and SEM analyses. Also, TGA and DTA analyses were carried out to investigate the thermal properties of the samples. Results showed that increasing the amount of Bi₂O₃ into the polymer matrix shifted the glass transition temperature of the composites towards the lower temperatures. Then, the amount of mass attenuation coefficients of the samples were measured using a CsI(Tl) detector for different gamma rays of ²⁴¹Am, ⁵⁷Co, ^99mTc, and ¹³³Ba radioactive sources. It was obtained that increasing the concentration of the Bi₂O₃ fillers in the polycarbonate matrix resulted in increasing the attenuation coefficients of the composites significantly. The attenuation coefficient was enhanced twenty-three times for 50 wt% composite in 59 keV energy, comparing to the pure polycarbonate.

Tunable Crosslinking, Reversible Phase Transition, and 3D Printing of Hyaluronic Acid Hydrogels via Dynamic Coordination of Innate Carboxyl Groups and Metallic Ions

This article reports tunable crosslinking, reversible phase transition, and three-dimensional printing (3DP) of hyaluronic acid (HyA) hydrogels via dynamic coordination of Fe³⁺ ions with their innate carboxyl groups for the first time. The concentrations of Fe³⁺ and H⁺ ions and the reaction time determine the tunable ratios of mono-, bi-, and tridentate coordination, leading to the low-to-high crosslinking densities and reversible solid-liquid phase transition of HyA hydrogels. At the monodentate-dominant coordination, the liquid hydrogels have low crosslinking densities (HyA_L). At the mixed coordination of mono-, bi-, and tridentate bonding, the solid hydrogels have medium crosslinking densities (HyA_M). At the tridentate-dominant coordination, the solid hydrogels have high crosslinking densities (HyA_H). The reversible solid-liquid phase transitions among HyA_L, HyA_M, and HyA_H were achieved via controlling the concentrations of Fe³⁺ and H⁺ ions and reaction time. When the crosslinking densities are between HyA_L and HyA_M, the hydrogels become 3D printable (HyA_P). HyA_P hydrogels were 3D-printed successfully using cold-stage or direct writing methods, and the 3D constructs achieved better structural stability using the latter method. In the direct exposure culture with bone marrow-derived mesenchymal stem cells, the 3D-printed HyA_H (HyA_H_3D) and HyA_H hydrogels showed higher average cell adhesion densities than the HyA_M, HyA_P, and HyA_L hydrogel groups under both direct and indirect contact conditions. For all hydrogel groups, cell adhesion densities under direct contact conditions were statistically lower than the same groups under indirect contact conditions. In this article, we elucidated the mechanisms of dynamic coordination and the relationships among the key parameters in controlling the tunable crosslinking, reversible phase transition, and 3DP of HyA hydrogels without blending with other polymers or adding functional groups. This approach can be potentially adapted to crosslink and 3D print other polymeric hydrogels with carboxyl groups, which is promising for a wide range of applications.

A simulation study on the glass transition behavior and relevant segmental dynamics in free-standing polymer nanocomposite films

In polymer/nanoparticle composite (PNC) thin films, polymer chains experience strong confinement effects not only at the free surface area but also from nanoparticles (NPs). In this work, the influence of NP-polymer interaction and NP distribution on the polymer segmental dynamics and the glass transition behavior of PNC free-standing films are investigated through molecular dynamics simulations. We demonstrate that NPs will migrate to the film surface area and form an NP-concentrated layer when NP-polymer interactions are weak, while NPs are well dispersed in the bulk region when NP-polymer interactions are strong. In both cases, we find increases in the glass transition temperature Tg compared with the pure film without NPs, although with a different degree. The weakly interacting system has the same Tg as the pure bulk system without NPs. The NP layer formed at the surface area reduces both the mobility of the surface polymer beads and the mobility gradient in the film normal direction (MGFND), therefore resulting in an increase in the Tg which highlights the vital role of the mobile surface layer. In contrast, the NPs in the bulk region enlarge the MGFND. NPs have opposite influences on the polymer bead dynamic anisotropy when they interact weakly or strongly with polymers, weakened for the former and enhanced for the latter. These findings offer a clear picture of the segmental dynamics and glass transition behavior in free-standing PNC films with different NP-polymer interaction strengths. We hope these results will be helpful for the property design of related materials.

Optimizing the Coverage Density of Functional Groups over SiO2 Nanoparticles: Toward High-Resistant and Low-Friction Hybrid Powder Coatings

Hybrid powder coatings (HPC) with low friction and high hardness enhance the sliding speed and allow interlocking or meshing products to slide effortlessly within each other, saving energy. In automobiles, they decrease fuel consumption and greenhouse gas emission. In the present work, a new insight of the key role played by the coverage density of triethoxyphenylsilane (TPS) grafted to SiO₂ nanoparticles over the friction coefficient, hardness, elastic modulus, and roughness of HPC is presented for the first time. In all cases, a very low amount (0.1 wt %) of functionalized or unfunctionalized SiO₂ nanoparticles were added to a powder-coating formulation based on polyester resin. HPC formulated with functionalized nanoparticles at a suitable coverage density (HPC-TPS3) exhibited significantly low friction coefficient (μ = 0.12), strong wear resistance (under dry sliding conditions at 1 and 5 N of load), low roughness (R _q = 3.5 nm), and high hardness and elastic modulus on the surface. We demonstrated that it is possible to tune the macroscopic properties by varying only the coverage density of TPS that is chemically attached to SiO₂ nanoparticles. Also, a physicochemical explanation was disclosed, wherein a hydrophilic-hydrophobic balance between -OH and phenyl groups was proposed. In all cases, the phenyl group allows the migration of functionalized nanoparticles through the polyester matrix, enhancing the hardness and elastic modulus on the surface. Thus, the functional nanomaterial design with tunable coverage density is a powerful tool to improve the physical and superficial properties of powder coatings using low amounts of nanomaterial.

Exploring the broadening and the existence of two glass transitions due to competing interfacial effects in thin, supported polymer films

In this report, we use ellipsometry to characterize the glass transition in ultra-thin films of poly(2-vinyl pyridine) (P2VP) supported on a silicon substrate. P2VP is known to have attractive substrate interactions, which can increase the T_g of ultra-thin films compared to the bulk value. Here, we use an extended temperature range to show that the glass transition can be extremely broad, indicating that a large gradient of the dynamics exists through the film with slow dynamics near the substrate and enhanced dynamics at the free surface. To observe the effect of these two interfaces on the average thin film dynamics, cooling rate-dependent T_g (CR-T_g) measurements were used to indirectly probe the average relaxation times of the films. We demonstrate that ultra-thin films have lower fragility compared to bulk films, and, when cooled at slow cooling rates (<1 K/min), exhibit extreme broadening of the dynamics (<70 nm) and eventually complete decoupling between the free surface and substrate regions to produce films with two distinct T_g's (<16 nm). T_g,high increases with decreasing thickness in a similar manner to what has been observed in previous studies on P2VP, and T_g,low decreases with decreasing film thickness in a similar manner to what has been observed in polymer films with enhanced free surfaces and neutral substrate interactions. These observations indicate that the dynamics in thin films of P2VP can be strongly coupled over a length scale of ∼10-20 nm, resulting in two co-existing layers with two distinct glass transitions when the range of the dynamical gradients become too large to sustain (breadth of the transition > 50 K).

Unexpected thermal annealing effects on the viscosity of polymer nanocomposites

The effects of thermal annealing, 12-50 K above the glass transition temperature, on the zero-shear viscosity, η, of polymer nanocomposites (PNCs) and the corresponding host polymers were studied. For all specimens, including neat and 4 wt% dioctyl phthalate (DOP)-plasticized polystyrene (PS), neat poly(methyl methacrylate) (PMMA), and PNCs containing bare and grafted silica nanoparticles in neat and DOP-plasticized PS, the η increased with time initially, and only asymptotically approached a steady-state value after thermal annealing for ∼100 to ∼200 h. We found that this phenomenon occurred regardless of the solvent used to prepare the sample although the fractional changes in η (δη/η) are visibly bigger for tetrahydrofuran (THF). Moreover, the PNCs not plasticized by DOP showed bigger δη/η than their host polymers while the plasticized ones behave essentially the same as the neat hosts. Interestingly, some unplasticized PNCs prepared from THF exhibited smaller viscosities than the host polymer, but this anomaly disappeared on thermal annealing. By correlating the viscosity measurements with the evolution of the solvent content, average NP aggregate size and the amount of adsorbed PS on silica for samples prepared from different solvents, we infer that the temporal viscosity evolution originates from out-of-equilibrium chain conformations produced during sample preparation. Because these relaxations are limited by the rearrangement of the polymer chains adsorbed on the NP or sample substrate surface, the timescales over which η changes can be much longer than the polymer reptation time, as observed.

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.