Self-propelling colloids with finite state dynamics.

Endowing materials with the ability to sense, adapt, and respond to stimuli holds the key to a progress leap in autonomous systems. In spite of the growing success of macroscopic soft robotic devices, transferring these concepts to the microscale presents several challenges connected to the lack of suitable fabrication and design techniques and of internal response schemes that connect the materials' properties to the function of the active units. Here, we realize self-propelling colloidal clusters which possess a finite number of internal states, which define their motility and which are connected by reversible transitions. We produce these units via capillary assembly combining hard polystyrene colloids with two different types of thermoresponsive microgels. The clusters, actuated by spatially uniform AC electric fields, adapt their shape and dielectric properties, and consequently their propulsion, via reversible temperature-induced transitions controlled by light. The different transition temperatures for the two microgels enable three distinct dynamical states corresponding to three illumination intensity levels. The sequential reconfiguration of the microgels affects the velocity and shape of the active trajectories according to a pathway defined by tailoring the clusters' geometry during assembly. The demonstration of these simple systems indicates an exciting route toward building more complex units with broader reconfiguration schemes and multiple responses as a step forward in the pursuit of adaptive autonomous systems at the colloidal scale.

Fabrication and nanoscale properties of PEDOT:PSS conducting polymer nanospheres.

Electrically conducting nanospheres of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with tailored size were prepared using a solvent displacement technique. To fabricate these nanostructures, dried PEDOT:PSS was dissolved in ethylene glycol (EG) and the solution was precipitated in deionized water. The proposed fabrication route allowed obtaining a water-based dispersion of PEDOT:PSS nanospheres with good optical properties. To determine the physical properties of the nanospheres, we followed a nanoscale approach, using atomic force microscopy. Our nanoscale mechanical and electrical investigations showed that the nanospheres retained good physical and conductivity properties, compared to the commercial product. Moreover, the local studies indicated that the reprecipitation process and the spherical shape lead to a different arrangement of the PSS and PEDOT phases.

Gold nanoparticles endowed with low-temperature colloidal stability by cyclic polyethylene glycol in ethanol.

The colloidal stability of metal nanoparticles is tremendously dependent on the thermal behavior of polymer brushes. Neat polyethylene glycol (PEG) presents an unconventional upper critical solution temperature in ethanol, where phase segregation and crystallization coexist. This thermal behavior translated to a PEG brush has serious consequences on the colloidal stability in ethanol of gold nanoparticles (AuNPs) modified with PEG brushes upon cooling. We observed that AuNPs (13 nm diameter) stabilized with conventional linear PEG brushes (Mn = 6 and 11 kg mol-1) in ethanol suffer from reversible phase separation upon a temperature drop over the course of a few hours. However, the use of a polymer brush with cyclic topology as a stabilizer prevents sedimentation, ensuring the colloidal stability in ethanol at -25 °C for, at least, four months. We postulate that temperature-driven collapse of chain brushes promotes the interpenetration of linear chains, causing progressive AuNP sedimentation, a process that is unfavorable for cyclic polymer brushes whose topology prevents chain interpenetration. This study reinforces the notion about the importance of polymer topology on the colloidal stability of AuNPs.

Water dynamics and self-assembly of single-chain nanoparticles in concentrated solutions.

Single-chain polymer nanoparticles (SCNPs) are soft nano-objects consisting of uni-macromolecular chains collapsed to a certain degree by intramolecular crosslinking. The similarities between the behaviour of SCNPs and that of intrinsically disordered proteins suggest that SCNPs in concentrated solutions can be used as models to design artificial micro-environments, which mimic many of the general aspects of cellular environments. In this work, the self-assembly into SCNPs of an amphiphilic random copolymer, composed by oligo(ethylene glycol)methyl ether methacrylate (OEGMA) and 2-acetoacetoxy ethyl methacrylate (AEMA), has been investigated by means of the dielectric relaxation of water. Direct evidence of segregation of the AEMA repeating units is provided by comparison with the dielectric relaxation of water in similar solutions of the linear hydrophilic polymer, P(OEGMA). Furthermore, the results of comparative studies with similar water solutions of an amphiphilic block copolymer forming multi-chain micelles support the single-chain character of the self-assembly of the random copolymer. The overall obtained results highlight the suitability of the dielectric spectroscopy to confirm the self-assembly of the amphiphilic random copolymers into globular like core-shell single-chain nanoparticles at a concentration well above the overlap concentration.

Effect of environmental humidity on the ionic transport of poly(ethylene oxide) thin films, investigated by local dielectric spectroscopy.

The effect of humidity on the ionic transport in the amorphous phase of poly(ethylene oxide) thin films has been studied by local dielectric spectroscopy. We explored a controlled humidity range between 15% RH and 50% RH. AFM-based local dielectric imaging allowed the thin film topography and the corresponding dielectric contrast maps to be obtained simultaneously. No humidity effect on the film topography was observed whereas large variation of the dielectric signal could be detected. In addition, we observed a clear dielectric contrast in different locations on the thin film surface. At selected regions with high contrast in the dielectric maps, we performed nanoDielectric Spectroscopy (nDS) measurements covering the frequency range from 5 Hz to 100 kHz. By modeling these spectroscopy results, we quantified the conductivity of the amorphous phase of the semicrystalline poly(ethylene oxide) films. The crystalline fraction of the PEO thin films was extracted and found to be about 36%, independently of humidity. However, the average conductivity increased by a factor of 25 from 2 × 10-10 to 5 × 10-9 S cm-1, by changing environmental humidity in the explored % RH range.

Direct Observation of Dynamic Tube Dilation in Entangled Polymer Blends: A Combination of Neutron Scattering and Dielectric Techniques.

We report a microscopic observation of the time-dependent dynamic tube dilation process on isofrictional bidisperse melts. By applying neutron spin echo (NSE) and dielectric techniques on blends of long polyisoprene (PI) chains with short PI additives with different topology, we access the dynamics of the tube dilation process on a molecular scale. The time-dependent tube dilation is directly revealed by NSE as an additional time dependence of the dynamic structure factor in the local reptation regime. We identify the characteristic time of tube dilation as the terminal time of the additive.

How Confinement Affects the Nucleation, Crystallization, and Dielectric Relaxation of Poly(butylene succinate) and Poly(butylene adipate) Infiltrated within Nanoporous Alumina Templates.

This work describes the successful melt infiltration of poly(butylene succinate) (PBS) and poly(butylene adipate) (PBA) within 70 nm diameter anodic aluminum oxide (AAO) templates. The infiltrated samples were characterized by SEM, Raman, and FTIR spectroscopy. The crystallization behaviors and crystalline structures of both polymers, bulk and confined, were analyzed by differential scanning calorimetry (DSC) and grazing incidence wide angle X-ray scattering (GIWAXS). DSC revealed that a change in the nucleation process occurred from heterogeneous nucleation for bulk samples to homogeneous nucleation for infiltrated PBA and to surface-induced nucleation for infiltrated PBS. GIWAXS results indicate that PBS nanofibers crystallize in the α-phase, as well as their bulk samples. However, PBA nanofibers crystallize just in the ß-phase, whereas PBA bulk samples crystallize in a mixture of α- and ß-phases. The crystal orientation within the pores was determined, and differences between PBS and PBA were also found. Finally, broadband dielectric spectroscopy was applied to study the segmental dynamics for bulk and infiltrated samples. The glass temperature was found to significantly decrease in the PBS case upon infiltration, while that of PBA remained unchanged. These differences were correlated with the higher affinity of PBS to the AAO walls than PBA, in accordance with their nucleation behavior (surface-induced versus homogeneous nucleation, respectively).

Broadband dielectric spectroscopy to validate architectural features in Type-A polymers: Revisiting the poly(glycidyl phenyl ether) case.

Broadband dielectric spectroscopy (BDS) is a powerful technique that allows studying the molecular dynamics of materials containing polar entities. Among a vast set of different applications, BDS can be used as a complementary tool in polymer synthesis. In this work, we will show how BDS can be used to validate architectural features in Type-A polymers, those having a net dipole moment component along the chain contour. Specifically, we will focus on the evaluation of the dielectric relaxation of poly(glycidyl phenyl ether) (PGPE) samples designed and synthesized with a variety of topologies and regio-orders: linear regio-regular chains synthesized from monofunctional and bifunctional initiators, macrocyclic regio-regular chains, and linear and macrocyclic regio-irregular chains. Our study highlights the impact of using BDS as a complementary characterization technique for providing topological details of polymers, which are otherwise not possible with many traditional techniques (e.g., NMR and mass spectrometry).

Facile Access to Completely Deuterated Single-Chain Nanoparticles Enabled by Intramolecular Azide Photodecomposition.

Access to completely deuterated single-chain nanoparticles (dSCNPs) has remained an unresolved issue. Herein, the first facile and efficient procedure to produce dSCNPs is reported, which comprises: i) the use of commercially available perdeuterated cyclic ether monomers as starting reagents, ii) a ring-opening copolymerization process performed in bulk to produce a neat dSCNP precursor, iii) a standard azidation reaction to decorate this precursor with azide moieties, and iv) a facile intramolecular azide photodecomposition step carried out under UV irradiation at high dilution providing with highly valuable, completely deuterated soft nano-objects from the precursor. dSCNPs are used to investigate by means of neutron-scattering measurements the form factor (radius of gyration, scaling exponent) of polyethylene oxide (PEO) chains in nanocomposites with different amounts of dSCNPs. Moreover, to illustrate the possibilities offered by the synthetic route disclosed in this communication for potential applications, the significant reduction in viscosity observed in a pure melt of polyether-based single-chain nanoparticles when compared to a melt of the corresponding linear polymer chains is shown.

Ionic transport in the amorphous phase of semicrystalline polyethylene oxide thin films.

We present a detailed study on the ionic transport properties of polyethylene oxide (PEO) thin films prepared under different conditions. Using a state-of-the-art Atomic Force Microscopy (AFM) methodology, we simultaneously acquired the nanostructured topography of these semicrystalline polymer films as well as the corresponding dielectric function; in the latter case by probing the frequency-dependent tip-sample electrical interactions. By means of this AFM protocol, we studied the ionic conductivity in the PEO amorphous phase and its dependence on film preparation conditions. In general, for any preparation method, we found a distribution of conductivities ranging from 10-14 to 10-6 S cm-1. Specifically, PEO thin films crystallized from the melt presented relatively high conductivity values, which decreased in the PEO films prepared from solutions at room temperature depending on solvent polarity. We discuss our results by considering the molecular arrangement of the polymer segments in the complex amorphous phase, which is strongly influenced by the PEO crystallization route.

Kinetic differences in the intercalation of linear and cyclic penta(ethylene oxide)s into graphite oxide leading to separation by topology.

In the present study, the structural differences between linear and cyclic oligo(ethylene oxide)s are demonstrated to cause large kinetic differences in the intercalation of both topologies into the interlayer space of graphite oxide (GO). This study is performed with room-temperature liquid oligomers: penta(ethylene glycol), penta(ethylene glycol) dimethyl ether, 15-crown-5 and 2-hydroxymethyl-15-crown-5. Cyclic compounds exhibit 100 to 1000 times slower intercalation rates than the linear oligomers and exhibit induction periods of several hours prior to intercalation. This enormous difference in the intercalation rate resulted in the selective exclusion of cyclic molecules from the linear ones in experiments performed with their blends, as evidenced by the evolution of glass transition and crystallization. An increase in the concentration of the cyclic molecules in the non-intercalated liquid from 70 wt% to values as high as 99 wt% was accomplished. This study reveals the potential use of selective intercalation mediated by GO as a tool for separation of cyclic and linear oligo(ethylene oxide)s.

Reaching the ideal glass transition by aging polymer films.

Searching for the ideal glass transition, we exploit the ability of glassy polymer films to explore low energy states in remarkably short time scales. We use 30 nm thick polystyrene (PS) films, which in the supercooled state basically display the bulk polymer equilibrium thermodynamics and dynamics. We show that in the glassy state, this system exhibits two mechanisms of equilibrium recovery. The faster one, active well below the kinetic glass transition temperature (Tg), allows massive enthalpy recovery. This implies that the 'fictive' temperature (Tf) reaches values as low as the predicted Kauzmann temperature (TK) for PS. Once the thermodynamic state corresponding to Tf = TK is reached, no further decrease of enthalpy is observed. This is interpreted as a signature of the ideal glass transition.

Complex nonequilibrium dynamics of stacked polystyrene films deep in the glassy state.

We investigate the kinetics of enthalpy recovery in stacked glassy polystyrene (PS) films with thickness from 30 to 95 nm over a wide temperature range below the glass transition temperature (Tg). We show that the time evolution toward equilibrium exhibits two mechanisms of recovery, in ways analogous to bulk PS. The fast mechanism, allowing partial enthalpy recovery toward equilibrium, displays Arrhenius temperature dependence with low activation energy, whereas the slow mechanism follows pronounced super-Arrhenius temperature dependence. In comparison to bulk PS, the time scales of the two mechanisms of recovery are considerably shorter and decreasing with the film thickness. Scaling of the equilibration times at various thicknesses indicates that the fast mechanism of recovery is compatible with the free volume holes diffusion model. Conversely, the slow mechanism of recovery appears to be accelerated with decreasing thickness more than predicted by the model and, therefore, its description requires additional ingredients. The implications, from both a fundamental and technological viewpoint, of the ability of thin polymer films to densify in relatively short time scales are discussed.

Dielectric relaxation of polymers: segmental dynamics under structural constraints.

In this article we review the recent polymer literature where dielectric spectroscopy has been used to investigate the segmental dynamics of polymers under the constraints produced by self-structuring. Specifically, we consider three cases: (i) semicrystalline polymers, (ii) segregated block-copolymers, and (iii) asymmetric miscible polymer blends. In these three situations the characteristics of the dielectric relaxation associated with the polymer segmental dynamics are markedly affected by the constraints imposed by the corresponding structural features. After reviewing in detail each of the polymer systems, the most common aspects are discussed in the context of the use of dielectric relaxation as a sensitive tool for analyzing structural features in nanostructured polymer systems.

Role of Temperature and Pressure on the Multisensitive Multiferroic Dicyanamide Framework [TPrA][Mn(dca)3] with Perovskite-like Structure.

A multistimuli response to temperature and pressure is found in the hybrid inorganic-organic perovskite-like [TPrA][Mn(dca)3] compound, which is related to a first-order structural phase transition near room temperature, Tt ≈ 330 K. This phase transition involves a transformation from room temperature polymorph I, with the noncentrosymmetric space group P4̅21c, to the high temperature polymorph II, with the centrosymmetric space group I4/mcm, and it implies ionic displacements, order-disorder phenomena, and a large and anisotropic thermal expansion (specially along the c-axis). As a consequence, [TPrA][Mn(dca)3] exhibits a dielectric anomaly, associated with the change from a cooperative to a noncooperative electric behavior (antiferroelectric (AFE)-paraelectric (PE) transition). The former implies an AFE distribution of electric dipoles in polymorph I, related to the described off-shift of the apolar TPrA cations and the order-disorder of the polar dca ligands mechanisms, that are different from those reported, up to now, for others perovskite-type hybrid compounds. Such cooperative electric order, below Tt ≈ 330 K, coexisting with long-range antiferromagnetic ordering below T = 2.1 K render the [TPrA][Mn(dca)3] a new type-I multiferroic material. In addition, the obtained experimental results reveal that this compound is also a multistimuli-responsive material, with a very large sensitivity toward temperature and applied external pressure, δTt/δP ≈ 24 K kbar(-1), even for small values of pressure (P < 2 kbar). Therefore, this material opens up a potential interest for future technological applications, such as temperature/pressure sensing.

A high-temperature dielectric process as a probe of large-scale silica filler structure in simplified industrial nanocomposites.

The existence of two independent filler-dependent high-temperature Maxwell-Wagner-Sillars (MWS) dielectric processes is demonstrated and characterized in detail in silica-filled styrene-butadiene (SB) industrial nanocomposites of simplified composition using Broadband Dielectric Spectroscopy (BDS). The uncrosslinked samples are made with 140 kg mol(-1) SB-chains, half of which carry a single graftable end-function (50% D3), and Zeosil 1165 MP silica incorporated by solid-phase mixing. While one high-temperature process is known to exist in other systems, the dielectric properties of a new silica-related process - strength, relaxation time, and activation energy - have been evidenced and described as a function of silica volume fraction and temperature. In particular, it is shown that its strength follows a percolation behavior as observed with the ionic conductivity and rheology. Moreover, activation energies show the role of polymer layers separating aggregates even when they are percolated. Apart from simultaneous characterization over a broad frequency range up to local polymer and silanol dynamics, it is believed that such high-temperature BDS-measurements can thus be used to detect reorganizations in structurally-complex silica nanocomposites. Moreover, they should contribute to a better identification of dynamical processes via the described sensitivity to structure in such systems.

Impact of next generation sequencing techniques in food microbiology.

Understanding the Maxam-Gilbert and Sanger sequencing as the first generation, in recent years there has been an explosion of newly-developed sequencing strategies, which are usually referred to as next generation sequencing (NGS) techniques. NGS techniques have high-throughputs and produce thousands or even millions of sequences at the same time. These sequences allow for the accurate identification of microbial taxa, including uncultivable organisms and those present in small numbers. In specific applications, NGS provides a complete inventory of all microbial operons and genes present or being expressed under different study conditions. NGS techniques are revolutionizing the field of microbial ecology and have recently been used to examine several food ecosystems. After a short introduction to the most common NGS systems and platforms, this review addresses how NGS techniques have been employed in the study of food microbiota and food fermentations, and discusses their limits and perspectives. The most important findings are reviewed, including those made in the study of the microbiota of milk, fermented dairy products, and plant-, meat- and fish-derived fermented foods. The knowledge that can be gained on microbial diversity, population structure and population dynamics via the use of these technologies could be vital in improving the monitoring and manipulation of foods and fermented food products. They should also improve their safety.

Segmental and Chain Dynamics of Polyisoprene-Based Model Vitrimers.

Polymer vitrimers are a new class of materials that combine the advantages of thermoplastics and thermosets. This is due to the dynamic nature of the chemical bonds linking different chains. However, how this property affects the polymer dynamics at different length scales is still an open question. Here, we investigate the dynamics of model vitrimers based on well-defined polyisoprene (PI) chains using broadband dielectric spectroscopy. In this way, we study the polymer dynamics from the segmental to the whole chain scale, taking advantage of the fact that PI belongs to the class of molecules that exhibit a net dipole moment associated with the end-to-end vector. Three distinct relaxation phenomena are identified. The fastest relaxation is attributed to the segmental PI dynamics with a small influence of the cross-linking. An intermediate relaxation attributed to the dipolar character of the cross-linker is also observed. The slower identified relaxation component, corresponding to limited fluctuations of the end-to-end PI chains, is found to be determined by the dynamics of the clusters formed by the cross-linkers with an average time scale orders of magnitude faster than that of the terminal relaxation as inferred from the viscous flow.

Direct evidence of two equilibration mechanisms in glassy polymers.

We investigated the kinetics of enthalpy recovery of several glass-forming polymers at temperatures significantly below the glass transition temperature (Tg) and for aging times up to one year. We find a double-step recovery at relatively low aging temperatures for the longest investigated aging times. The enthalpy recovered after the two-step decay approximately equals that expected by extrapolation from the melt. The two-step enthalpy recovery indicates the presence of two time scales for glass equilibration. The equilibration time of the first recovery step exhibits relatively weak temperature dependence, whereas that of the second step possesses pronounced temperature dependence, compatible with the Vogel-Fulcher-Tammann behavior. These results, while leaving open the question of the divergence of the relaxation time and that of a thermodynamic singularity at a finite temperature, reveal a complex scenario of glassy dynamics.

Diversity of thermophilic bacteria in raw, pasteurized and selectively-cultured milk, as assessed by culturing, PCR-DGGE and pyrosequencing.

Thermophilic lactic acid bacteria (LAB) species, such as Streptococcus thermophilus, Lactobacillus delbrueckii and Lactobacillus helveticus, enjoy worldwide economic importance as dairy starters. To assess the diversity of thermophilic bacteria in milk, milk samples were enriched in thermophilic organisms through a stepwise procedure which included pasteurization of milk at 63 °C for 30 min (PM samples) and pasteurization followed by incubation at 42 °C for 24 h (IPM samples). The microbial composition of these samples was analyzed by culture-dependent (at 42 °C) and culture-independent (PCR-DGGE and pyrosequencing of 16S rRNA gene amplicons) microbial techniques. The results were then compared to those obtained for their corresponding starting raw milk counterparts (RM samples). Twenty different species were scored by culturing among 352 isolates purified from the counting plates and identified by molecular methods. Mesophilic LAB species (Lactococcus lactis, Lactococcus garvieae) were dominant (87% of the isolates) among the RM samples. However, S. thermophilus and Lb. delbrueckii were found to be the dominant recoverable organisms in both PM and IPM samples. The DGGE profiles of RM and PM samples were found to be very similar; the most prominent bands belonging to Lactococcus, Leuconostoc and Streptococcus species. In contrast, just three DGGE bands were obtained for IPM samples, two of which were assigned to S. thermophilus. The pyrosequencing results scored 95 operational taxonomic units (OTUs) at 3% sequence divergence in an RM sample, while only 13 were encountered in two IPM samples. This technique identified Leuconostoc citreum as the dominant microorganism in the RM sample, while S. thermophilus constituted more than 98% of the reads in the IPM samples. The procedure followed in this study allowed to estimate the bacterial diversity in milk and afford a suitable strategy for the isolation of new thermophilic LAB strains, among which adequate starters might be selected.