Structure and Dynamics of Hyperbranched Polymer/Layered Silicate Nanocomposites

Dynamics of Polymer Chains in Poly(ethylene oxide)/Silica Nanocomposites via a Combined Computational and Experimental Approach

The dynamics of polymer chains in poly(ethylene oxide)/silica (PEO/SiO₂) nanoparticle nanohybrids have been investigated via a combined computational and experimental approach involving atomistic molecular dynamics simulations and dielectric relaxation spectroscopy (DRS) measurements. The complementarity of the approaches allows us to study systems with different polymer molecular weights, nanoparticle radii, and compositions across a broad range of temperatures. We study the effects of spatial confinement, which is induced by the nanoparticles, and chain adsorption on the polymer's structure and dynamics. The investigation of the static properties of the nanocomposites via detailed atomistic simulations revealed a heterogeneous polymer density layer at the vicinity of the PEO/SiO₂ interface that exhibited an intense maximum close to the inorganic surface, whereas the bulk density was reached for distances ∼1-1.2 nm away from the nanoparticle. For small volume fractions of nanoparticles, the polymer dynamics, probed by the atomistic simulations of low-molecular-weight chains at high temperatures, are consistent with the presence of a thin adsorbed layer that exhibits slow dynamics, with the dynamics far away from the nanoparticle being similar to those in the bulk. However, for high volume fractions of nanoparticles (strong confinement), the dynamics of all polymer chains were predicted slower than that in the bulk. On the other hand, similar dynamics were found experimentally for both the local β-process and the segmental dynamics for high-molecular-weight systems measured at temperatures below the melting temperature of the polymer, which were probed by DRS. These differences can be attributed to various parameters, including systems of different molecular weights and nanoparticle states of dispersion, the different temperature range studied by the different methods, the potential presence of a reduced-mobility PEO/SiO₂ interfacial layer that does not contribute to the dielectric spectrum, and the presence of amorphous-crystalline interfaces in the experimental samples that may lead to a different dynamical behaviors of the PEO chains.

Static and Dynamic Behavior of Polymer/Graphite Oxide Nanocomposites before and after Thermal Reduction

Nanocomposites of hyperbranched polymers with graphitic materials are investigated with respect to their structure and thermal properties as well as the dynamics of the polymer probing the effect of the different intercalated or exfoliated structure. Three generations of hyperbranched polyester polyols are mixed with graphite oxide (GO) and the favorable interactions between the polymers and the solid surfaces lead to intercalated structure. The thermal transitions of the confined chains are suppressed, whereas their dynamics show similarities and differences with the dynamics of the neat polymers. The three relaxation processes observed for the neat polymers are observed in the nanohybrids as well, but with different temperature dependencies. Thermal reduction of the graphite oxide in the presence of the polymer to produce reduced graphite oxide (rGO) reveals an increase in the reduction temperature, which is accompanied by decreased thermal stability of the polymer. The de-oxygenation of the graphite oxide leads to the destruction of the intercalated structure and to the dispersion of the rGO layers within the polymeric matrix because of the modification of the interactions between the polymer chains and the surfaces. A significant increase in the conductivity of the resulting nanocomposites, in comparison to both the polymers and the intercalated nanohybrids, indicates the formation of a percolated rGO network.

Interface and Interphase in Polymer Nanocomposites with Bare and Core-Shell Gold Nanoparticles

Metal nanoparticles are used to modify/enhance the properties of a polymer matrix for a broad range of applications in bio-nanotechnology. Here, we study the properties of polymer/gold nanoparticle (NP) nanocomposites through atomistic molecular dynamics, MD, simulations. We probe the structural, conformational and dynamical properties of polymer chains at the vicinity of a gold (Au) NP and a functionalized (core/shell) Au NP, and compare them against the behavior of bulk polyethylene (PE). The bare Au NPs were constructed via a systematic methodology starting from ab-initio calculations and an atomistic Wulff construction algorithm resulting in the crystal shape with the minimum surface energy. For the functionalized NPs the interactions between gold atoms and chemically adsorbed functional groups change their shape. As a model polymer matrix we consider polyethylene of different molecular lengths, from the oligomer to unentangled Rouse like systems. The PE/Au interaction is parametrized via DFT calculations. By computing the different properties the concept of the interface, and the interphase as well, in polymer nanocomposites with metal NPs are critically examined. Results concerning polymer density profiles, bond order parameter, segmental and terminal dynamics show clearly that the size of the interface/interphase, depends on the actual property under study. In addition, the anchored polymeric chains change the behavior/properties, and especially the chain density profile and the dynamics, of the polymer chain at the vicinity of the Au NP.

Molecular Dynamics in Nanocomposites Based on Renewable Poly(butylene 2,5-furan-dicarboxylate) In Situ Reinforced by Montmorillonite Nanoclays: Effects of Clay Modification, Crystallization, and Hydration

This study deals with poly(butylene 2,5-furan-dicarboxylate), PBF, a renewable bio-based polyester expected to replace non-eco-friendly fossil-based homologues. PBF exhibits excellent gas barrier properties, which makes it promising for packaging applications; however, its rather low and slow crystallinity affects good mechanical performance. The crystallization of this relatively new polymer is enhanced here via reinforcement by introduction in situ of 1 wt % montmorillonite, MMT, nanoclays of three types (functionalizations). We study PBF and its nanocomposites (PNCs) also from the basic research point of view, molecular dynamics. For this work, we employ the widely used combination of techniques, differential scanning calorimetry (DSC) with broad-band dielectric relaxation spectroscopy (BDS), supplemented by polarized light microscopy (PLM) and thermogravimetric analysis (TGA). In the PNCs, the crystalline rate and fraction, CF, were found to be strongly enhanced as these fillers act as additional crystallization nuclei. The improvements in crystallization here correlate quite well with those on the mechanical performance recorded recently; moreover, they occur in the same filler order, in particular, with increasing MMT interlayer distance (from ∼1 to ∼3 nm). In the amorphous fraction of the polymer, the chain diffusion (calorimetric T_g and dynamic α process) is easier in the PNCs due to their slightly smaller length, while in the semicrystalline state, it decelerates by crystal-induced constraints. The local polymer dynamics (β process, below T_g) was found to be independent of the PNC composition, however, sensitive to structural changes of the matrix. Finally, a filler-induced dynamics was additionally recorded in the PNCs (α* process), arising possibly from the polymer located at the MMT surfaces. α* follows the changes in polymer chain length and decelerates with crystallization, whereas its activation energy decreases with mild hydration. The combined results on α* with the DSC and TGA findings, provide proof for weak MMT-PBF interactions. Overall, our results, along with data from the literature, suggest that such furan-based polyesters reinforced with properly chosen nanofillers could potentially serve well as tailor-made PNCs for targeted applications.

Universal localization transition accompanying glass formation: insights from efficient molecular dynamics simulations of diverse supercooled liquids

The origin of the precipitous dynamic arrest known as the glass transition is a grand open question of soft condensed matter physics. It has long been suspected that this transition is driven by an onset of particle localization and associated emergence of a glassy modulus. However, progress towards an accepted understanding of glass formation has been impeded by an inability to obtain data sufficient in chemical diversity, relaxation timescales, and spatial and temporal resolution to validate or falsify proposed theories for its physics. Here we first describe a strategy enabling facile high-throughput simulation of glass-forming liquids to nearly unprecedented relaxation times. We then perform simulations of 51 glass-forming liquids, spanning polymers, small organic molecules, inorganics, and metallic glass-formers, with longest relaxation times exceeding one microsecond. Results identify a universal particle-localization transition accompanying glass formation across all classes of glass-forming liquid. The onset temperature of non-Arrhenius dynamics is found to serve as a normalizing condition leading to a master collapse of localization data. This transition exhibits a non-universal relationship with dynamic arrest, suggesting that the nonuniversality of supercooled liquid dynamics enters via the dependence of relaxation times on local cage scale. These results suggest that a universal particle-localization transition may underpin the glass transition, and they emphasize the potential for recent theoretical developments connecting relaxation to localization and emergent elasticity to finally explain the origin of this phenomenon. More broadly, the capacity for high-throughput prediction of glass formation behavior may open the door to computational inverse design of glass-forming materials.

Structure and Dynamics of Biobased Polyester Nanocomposites

The structure and the dynamics of two bio-based polyester polyols are investigated in the bulk and close to surfaces in polymer/layered silicate nanocomposites. The morphology of the neat polymers as well as the structure of the nanohybrids are investigated with X-ray diffraction and their thermal properties are studied by differential scanning calorimetry. One of the investigated polyesters is amorphous, whereas the second one is a semicrystalline polymer with intriguing thermal behavior. Hybrids have been synthesized over a broad range of compositions and intercalated structures are always obtained. The thermal transitions in the nanocomposites are observed only when the polymers are in excess outside the completely filled galleries. The glass transition, whenever it can be resolved, appears insensitive to the presence of the inorganic material, whereas the way the crystallization takes place depends on the composition of the nanohybrid. Dielectric relaxation spectroscopy was utilized to study the polymer dynamics. It revealed multiple relaxation processes for the neat polymers both below and above their glass transition temperatures, whereas in the nanocomposites, similarities and differences are observed depending on the specific mode of the dynamic process.

Segmental Dynamics and Cooperativity Length of PMMA/SAN Miscible Blend Intercalated in Organically Modified Nanoclay

The effect of nanoconfinement on the segmental dynamics of a poly(methyl methacrylate) (PMMA)/poly(styrene- ran-acrylonitrile) (SAN) miscible blend, intercalated into the interlayer spacing of the organically modified nanoclay (OMNC), was investigated using dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) methods. We reported an unusual phenomenon in which the weak interfacial interactions between the polymer chains and OMNCs was responsible for increase in segmental mobility at the glass-transition temperature ( T_g). Remarkably, we found a positive correlation between dynamic fragility and thermodynamic fragility, in which both fragilities decreased under nanoconfinement. The cooperative length of segmental motions, or length of cooperatively rearranging regions, ξ_CRR, decreased from 2.64 nm for the PMMA/SAN blend to 1.86 nm for the PMMA/SAN/OMNC nanocomposite. The segmental mobility of the PMMA/SAN/OMNC model was also studied using the molecular dynamics simulations. The simulation results showed the increased segmental mobility of the PMMA/SAN chains in the presence of OMNCs, which is in agreement with the DMA and DSC results.

An investigation of the impact of an amino-ended hyperbranched polymer as a new type of modifier on the compatibility of PLA/PBAT blends

Poly(lactic acid) (PLA)/poly(butylene adipate-co-terephthalate) (PBAT) blends using amino-ended hyperbranched polymers (HBP) as modifiers were prepared by melt-mixing through a double-roller mill and injection molding. It was found that when the content of HBP was 2.5 phr, the elongation at break and the impact strength of PLA/PBAT blends both reached peak values. Moreover, by addition of HBP, the ΔTg of the blends was smaller. These results, together with Scanning electron microscope (SEM) images on the fractured morphology of the blends, indicate that the compatibility between PLA and PBAT is improved upon addition of HBP. The mechanism of the impact of HBP on the improvement of the compatibility between PLA and PBAT is proposed based upon Fourier transform infrared (FTIR) spectra.

Polymer Conformation under Confinement

The conformation of polymer chains under confinement is investigated in intercalated polymer/layered silicate nanocomposites. Hydrophilic poly(ethylene oxide)/sodium montmorillonite, PEO/Na⁺-MMT, hybrids were prepared utilizing melt intercalation with compositions where the polymer chains are mostly within the ~1 nm galleries of the inorganic material. The polymer chains are completely amorphous in all compositions even at temperatures where the bulk polymer is highly crystalline. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) is utilized to investigate the conformation of the polymer chains over a broad range of temperatures from below to much higher than the bulk polymer melting temperature. A systematic increase of the gauche conformation relatively to the trans is found with decreasing polymer content both for the C⁻C and the C⁻O bonds that exist along the PEO backbone indicating that the severe confinement and the proximity to the inorganic surfaces results in a more disordered state of the polymer.

Dynamics of Hyperbranched Polymers under Confinement: A Dielectric Relaxation Study

The effect of severe confinement on the dynamics of three different generations of hyperbranched polyesters of the Boltorn family is investigated by dielectric relaxation spectroscopy (DRS). The polymer chains are intercalated within the galleries of natural montmorillonite (Na+-MMT), thus forming 1 nm polymer films confined between solid walls. The structure of the nanocomposites is studied with X-ray diffraction and the thermal behavior of the polymers in bulk and under confinement is determined by differential scanning calorimetry. The glass transition temperatures of the polymers show a clear dependence on the generation whereas the transition is completely suppressed when all the polymer chains are intercalated. The dynamic investigation of the bulk polymers reveals two sub-Tg processes, with similar behavior for the three polymers with the segmental relaxation observed above the Tg of each polymer. For the nanocomposites, where all the polymer chains are severely confined, the dynamics show significant differences compared to that of the bulk polymers. The sub-Tg processes are similar for the three generations but significantly faster and with weaker temperature dependence than those in the bulk. The segmental process appears at temperatures below the bulk polymer Tg, it exhibits an Arrhenius temperature dependence and shows differences for the three generations. A slow process that appears at higher temperatures is due to interfacial polarization.

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.

Hyperbranched polymers: advances from synthesis to applications

This review summarizes the advances in hyperbranched polymers from the viewpoint of structure, click synthesis and functionalization towards their applications in the last decade.

A comparison of homopolymer and block copolymer structure in 6FDA-based polyimides

Two homopolyimides and the corresponding block copolyimide, all based on the 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA), have been synthesized and fully atomistic models have been studied using molecular dynamics (MD) simulation. The respective diamines were 1,3-phenylenediamine (mPDA) and 2,3,5,6-tetramethyl-1,4-phenylenediamine (durene). These polyimides are potential candidates for gas separation applications. The synthesized polymers were processed as dense flat membranes. The effects of diamine structure were investigated at the molecular level and an attempt to compare the structural features of homo- and block copolyimides was made. Amorphous models were generated using a hybrid pivot Monte Carlo-MD sampling preparation technique. Average model densities were validated against experimental measurements on the dense films. Cohesive energies, Hildebrand solubility parameters, conformational characteristics, intermolecular interactions and available void spaces were analysed for each system. The durene diamine was found to hinder stacking and increase the available space. This is associated with the steric effect of the methyl substituents. In general, 6FDA-mPDA/durene exhibits an intermediate behaviour with respect to its base polyimides. For most of the examined properties, the differences between different size simulated systems were minor with the exception of the free volume distribution.