Temperature dependence of the dynamics and interfacial width in nanoconfined polymers via atomistic simulations.

We present a detailed computational study on the temperature effect of the dynamics and the interfacial width of unentangled cis-1,4 polybutadiene linear chains confined between strongly attractive alumina layers via long, several µs, atomistic molecular dynamics simulations for a wide range of temperatures (143-473 K). We examine the spatial gradient of the translational segmental dynamics and of an effective local glass temperature (TgL). The latter is found to be much higher than the bulk Tg for the adsorbed layer. It gradually reduces to the bulk Tg at about 2 nm away from the substrate. For distant regions (more than ≈1.2nm), a bulk-like behavior is observed; relaxation times follow a typical Vogel-Fulcher-Tammann dependence for temperatures higher than Tg and an Arrhenius dependence for temperatures below the bulk Tg. On the contrary, the polymer chains at the vicinity of the substrate follow piecewise Arrhenius processes. For temperatures below about the adsorbed layer's TgL, the translational dynamics follows a bulk-like (same activation energy) Arrhenius process. At higher temperatures, there is a low activation energy Arrhenius process, caused by high interfacial friction forces. Finally, we compute the interfacial width, based on both structural and dynamical definitions, as a function of temperature. The absolute value of the interfacial width depends on the actual definition, but, regardless, the qualitative behavior is consistent. The interfacial width peaks around the bulk Tg and contracts for lower and higher temperatures. At bulk Tg, the estimated length of the interfacial width, computed via the various definitions, ranges between 1.0 and 2.7 nm.

Capillary filling of star polymer melts in nanopores.

The topology of a polymer profoundly influences its behavior. However, its effect on imbibition dynamics remains poorly understood. In the present work, capillary filling (during imbibition and following full imbibition) of star polymer melts was investigated by molecular dynamics simulations with a coarse-grained model. The reversal of imbibition dynamics observed for linear-chain systems was also present for star polymers. Star polymers with short arms penetrate slower than the prediction of the Lucas-Washburn equation, while systems with long arms penetrate faster. The radius of gyration increases during confined flow, indicating the orientation and disentanglement of arms. In addition, the higher the functionality of the star polymer, the more entanglement points are retained. Besides, a stiff region near the core segments of the stars is observed, which increases in size with functionality. The proportion of different configurations of the arms (e.g., loops, trains, tails) changes dramatically with the arm length and degree of confinement but is only influenced by the functionality when the arms are short. Following full imbibition, the different decay rates of the self-correlation function of the core-to-end vector illustrate that arms take a longer time to reach the equilibrium state as the functionality, arm length, and degree of confinement increase, in agreement with recent experimental findings. Furthermore, the star topology induces a stronger effect of adsorption and friction, which becomes more pronounced with increasing functionality.

Topology sorting: Separating linear/star polymer blend components by imbibition in nanopores.

We report the imbibition and adsorption kinetics of a series of symmetric linear/star cis-1,4-polyisoprene blends within the long channels of self-ordered nanoporous anodic aluminum oxide (abbreviated: AAO). Using in situ nanodielectric spectroscopy, we followed the evolution of the longest chain modes in the blends with a judicious selection of molar masses for the constituent components. We demonstrated differences in the imbibition kinetics of linear and star components based on the relative viscosities (e.g., polymers with lower zero-shear viscosity penetrated first the nanopores). Following the complete imbibition of the pores, the adsorption time, τads, of each component was evaluated from the reduction in the dielectric strength of the respective chain modes. In the majority of blends, both components exhibited slower adsorption kinetics with respect to the homopolymers. The only exception was the case of entangled stars mixed with shorter linear chains, the latter acting as a diluent for the star component. This gives rise to what is known as topology sorting, e.g., the separation of linear/star blend components in the absence of solvent. Moreover, a simple relation (τads ∼ 10 × tpeak; tpeak is the time needed for the complete filling of pores) was found for linear polymers and stars. This suggested that the characteristic timescale of imbibition (tpeak) governs the adsorption process of polymers. It further implied the possibility of predicting the adsorption times of high molar mass polymers of various architectures by the shorter imbibition times.

Nopadiene: A Pinene-Derived Cyclic Diene as a Styrene Substitute for Fully Biobased Thermoplastic Elastomers.

The bicyclic 1,2-substituted, 1,3-diene monomer nopadiene (1R,5S)-2-ethenyl-6,6-dimethylbicyclo[3.1.1]hept-2-ene was successfully polymerized by anionic and catalytic polymerization. Nopadiene is produced either through a facile one-step synthesis from myrtenal via Wittig-olefination or via a scalable two-step reaction from nopol (10-hydroxymethylene-2-pinene). Both terpenoids originate from the renewable ß-pinene. The living anionic polymerization of nopadiene in apolar and polar solvents at 25 °C using organolithium initiators resulted in homopolymers with well-controlled molar masses in the range of 5.6-103.4 kg·mol-1 (SEC, PS calibration) and low dispersities (D) between 1.06 and 1.18. By means of catalytic polymerization with Me4CpSi(Me)2NtBuTiCl2 and (Flu)(Pyr)CH2Lu(CH2TMS)2(THF), the 1,4 and 3,4- microstructures of nopadiene are accessible in excellent selectivity. In pronounced contrast to other 1,3-dienes, the rigid polymers of the sterically demanding nopadiene showed an elevated glass temperature, Tg,∞ = 160 °C (in the limit of very high molar mass, Mn). ABA triblock copolymers with a central polymyrcene block and myrcene content of 60-75 mol %, with molar masses of 100-200 kg/mol were prepared by living anionic polymerization of the pinene-derivable monomers nopadiene and myrcene. This diene copolymerization resulted in thermoplastic elastomers displaying nanophase separation at different molar ratios (DSC, SAXS) and an upper service temperature about 30 K higher than that for traditional petroleum-derived styrenic thermoplastic elastomers due to the high glass temperature of polynopadiene. The materials showed good thermal stability at elevated temperatures under nitrogen (TGA), promising tensile strength and ultimate elongation of up to 1600%.

A Terrylene-Anthraquinone Dyad as a Chromophore for Photothermal Therapy in the NIR-II Window.

A terrylenedicarboximide-anthraquinone dyad, FTQ, with absorption in the second near-infrared region (NIR-II) is obtained as a high-performance chromophore for photothermal therapy (PTT). The synthetic route proceeds by C-N coupling of amino-substituted terrylenedicarboximide (TMI) and 1,4-dichloroanthraquinone followed by alkaline-promoted dehydrocyclization. FTQ with extended π-conjugation exhibits an optical absorption band peaking at 1140 nm and extending into the 1500 nm range. Moreover, as determined by dielectric spectroscopy in dilute solutions, FTQ achieves an ultrastrong dipole moment of 14.4 ± 0.4 Debye due to intense intramolecular charge transfer. After encapsulation in a biodegradable polyethylene glycol (DSPE-mPEG2000), FTQ nanoparticles (NPs) deliver a high photothermal conversion efficiency of 49% under 1064 nm laser irradiation combined with excellent biocompatibility, photostability, and photoacoustic imaging capability. In vitro and in vivo studies reveal the great potential of FTQ NPs in photoacoustic-imaging-guided photothermal therapy for orthotopic liver cancer treatment in the NIR-II window.

Dynamically and structurally heterogeneous 1-propanol/water mixtures.

1-propanol/water mixtures over the whole composition range (0 < XV ≤ 1; XV is the 1-propanol volume fraction) are shown to be structurally and dynamically heterogeneous. By combining structural (x-ray diffraction), thermodynamic (differential scanning calorimetry) and dynamical probes (dielectric spectroscopy) we construct the pertinent phase diagram. It consists of liquid 1-propanol, liquid water, hexagonal ice and different hydrates, the latter sharing the same lattice. The phase diagram can be discussed in terms of four regimes, all having in common a droplet arrangement of the minority component. When water droplets are strongly confined by 1-propanol (regime I, 0.92 < XV ≤ 1; "soft" confinement), water is unable to crystallize. It has dynamics reminiscent to the ultra-viscous water phase known as high-density liquid (HDL). When water droplets are moderately confined (regime II, 0.75 < XV ≤ 0.92) water can crystallize via homogeneous nucleation. Strikingly, the homogeneous nucleation temperature is at 205 K, well within "no-man's land." The result is in line with earlier reports that soft confinement is the key to enter into the "no-man's land". When 1-propanol is the minority component (regimes III and IV), the structure and the dynamics are dominated by the 1-propanol/water interface with the formation of hydrates. The corresponding dynamical features suggest a link between hydrate formation and the two metastable phases of ultra-viscous water, HDL and low-density liquid.

Conductivity of Ionic Liquids In the Bulk and during Infiltration in Nanopores.

The conductivity of ionic liquids (ILs) in nanopores is essential when considering their application as materials for energy. However, no consensus has been reached about the influence of confinement on the mobility of the ions. A series of ILs bearing the same cation, 1-butyl-3-methylimidazolium ([BMIM]+), and six different anions ([Cl]-, [Br]-, [I]-, [BF4]-, [PF6]-, and [TFSI]-) with radii from 0.168 to 0.326 nm were investigated with respect to their self-assembly, the thermodynamics, and the ionic conductivity in the bulk, during flow and under confinement in cylindrical nanopores with sizes in the range from 400 to 25 nm. In the bulk, the [BMIM]+[X]- exhibits weak ordering as a result of cation-anion correlations (charge alteration peak), and nanophase separation of polar/apolar groups. Liquid-to-glass temperatures were found to differ by ∼50 K, their viscosities by a factor of ∼270, and their conductivities by a factor of 24 (all at a temperature of 303 K). Electrostatic interactions were largely responsible for variations in the glass temperature, the viscosity, and the conductivity. Confined ILs behave differently from the bulk. The majority of ILs in the bulk were prone to crystallization during heating but were unable to crystallize in the smaller pores. Changes in dc-conductivity were used as markers of the phase state. This allowed the construction of the effective phase diagrams under confinement. The ILs penetrate the pores with an effective viscosity of the order of their viscosity in their bulk state. However, within the pores the dc-conductivity was reduced relative to bulk, indicating the immobilization of ions at the pore walls. Hydrophobization of the pore walls by hexamethyldisilazane could partially restore the conductivity. ILs are model systems where the phase state and ion mobility can be controlled by confinement.

When crystals flow.

Semicrystalline polymers are solids that are supposed to flow only above their melting temperature. By using confinement within nanoscopic cylindrical pores, we show that a semicrystalline polymer can flow at temperatures below the melting point with a viscosity intermediate to the melt and crystal states. During this process, the capillary force is strong and drags the polymer chains in the pores without melting the crystal. The unexpected enhancement in flow, while preserving the polymer crystallites, is of importance in the design of polymer processing conditions applicable at low temperatures, e.g., cold drawn polymers such as polytetrafluoroethylene, self-healing, and in nanoconfined donor/acceptor polymers used in organic electronics.

Non-equilibrium Effects of Polymer Dynamics under Nanometer Confinement: Effects of Architecture and Molar Mass.

The non-equilibrium dynamics of linear and star-shaped cis-1,4 polyisoprenes confined within nanoporous alumina is explored as a function of pore size, d, molar mass, and functionality (f = 2, 6, and 64). Two thermal protocols are tested: one resembling a quasi-static process (I) and another involving fast cooling followed by annealing (II). Although both protocols give identical equilibrium times, it is through protocol I that it is easier to extract the equilibrium times, teq, by the linear relationships of the characteristic peak frequencies with time and rate, respectively, as log(fmax) = C1 - k log(t) and log(fmax) = C2 + λ log(ß). Both thermal protocols establish the existence of a critical temperature (at Tc, where k → 0 and λ → 0) below which non-equilibrium effects set-in. The critical temperature depends on the degree of confinement, 2Rg/d, and on molecular architecture. Strikingly, establishing equilibrium dynamics at all temperatures above the bulk, Tg, requires 2Rg/d ∼ 0.02, i.e., pore diameters that are much larger than the chain dimensions. This reflects non-equilibrium configurations of the adsorbed layer that extent away from the pore walls. The equilibrium times depend strongly on temperature, pore size, and functionality. In general, star-shaped polymers require longer times to reach equilibrium because of the higher tendency for adsorption. Both thermal protocols produced an increasing dielectric strength for the segmental and chain modes. The increase was beyond any densification, suggesting enhanced orientation correlations of subchain dipoles.

Ordering kinetics of a tapered copolymer based on isoprene and styrene.

High molar mass copolymers with a tapered interface are mechanically tough materials with an accessible order-to-disorder transition temperature and hence processability. We report the first ordering kinetics for a tapered tetrablock copolymer in comparison to a conventional diblock copolymer made sequentially. We show that tapered copolymers belong to the Brazovskii "universality class," where fluctuations play a dominant role. Consequently, the order-to-disorder transition has a very weak, fluctuation-induced first-order character. The ordering kinetics of the lamellar phase from the supercooled disordered melt revealed several distinct differences associated with the range of metastability (increased), the timescales (bimodal), and the exact mechanism of ordering. The results are discussed in terms of the reduced interaction parameter and the introduction of structural defects within the lamellar grains.

Effect of confinement on the dynamics of 1-propanol and other monohydroxy alcohols.

We report the effect of confinement on the dynamics of three monohydroxy alcohols (1-propanol, 2-ethyl-1-hexanol, and 4-methyl-3-heptanol) differing in their chemical structure and, consequently, in the dielectric strength of the "Debye" process. Density functional theory calculations in bulk 1-propanol identified both linear and ring-like associations composed of up to five repeat units. The simulation results revealed that the ring structures, with a low dipole moment (∼2 D), are energetically preferred over the linear assemblies with a dipole moment of 2.18 D per repeat unit. Under confinement in nanoporous alumina (in templates with pore diameters ranging from 400 to 20 nm), all dynamic processes were found to speed up irrespective of the molecular architecture. The characteristic freezing temperatures of the α and the Debye-like processes followed the pore size dependence: Ta,D=Ta,D bulk-A/d1/2, where d is the pore diameter. The characteristic "freezing" temperatures for the Debye-like (the slow process for confined 1-propanol is non-Debye) and the α-processes decrease, respectively, by 6.5 and 13 K in confined 1-propanol, by 9.5 and 19 K in confined 2-ethyl-1-hexanol, and by 9 and 23 K in confined 4-methyl-3-heptanol within the same 25 nm pores. In 2-ethyl-1-hexanol, confinement reduced the number of linearly associated repeats from approximately heptamers in the bulk to dimers within 25 pores. In addition, the slower process in bulk 2-ethyl-1-hexanol and 4-methyl-3-heptanol, where the signal is dominated by ring-like supramolecular assemblies, is clearly non-Debye. The results suggest that the effect of confinement is dominant in the latter assemblies.

Layers of Distinct Mobility in Densely Grafted Dendrimer Arborescent Polymer Hybrids.

Melts of multiarm stars of 1,4-polybutadiene (dendrimer arborescent hybrids) with very high branching functionality (f) and small arm molar mass behave as jammed colloids and show distinct layers of segmental mobility. Three mobility layers were identified, comprising outer, intermediate, and near-core segments, all displaying a Vogel-Fulcher-Tammann temperature dependence. The respective glass temperatures increase as f^{1/2}. Our findings pave the way for further progress in this field by reconsidering previous theoretical treatments based on a single friction coefficient in hybrid nanoparticles such as densely grafted stars.

Designing cryo-enzymatic reactions in subzero liquid water by lipidic mesophase nanoconfinement.

Cryo-enzymology provides the possibility to develop unconventional biological reactions and detect intermediates in ultrafast enzymatic catalysis processes, but also illuminates the understanding of life principles in extremely cold environments. The scarcity of biological or biomimetic host systems that provide liquid water at subzero temperatures inhibits the prosperity of cryo-enzymology. Here we introduce cryo-enzymatic reactions in subzero water nanoconfined within lipid mesophases formed by conventional lipids. We show that the enzymatic reactions that ensue outperform the homologue catalytic processes run at standard temperatures. We use phytantriol-based lipidic mesophases (LMPs), within which water remains in the liquid state down to -120 °C, and combine crystallization and dynamic studies of the confined water to provide a fundamental understanding of the physical status of water at subzero temperatures, which sets the stage for cryo-enzymatic reactions in these environments. In the model horseradish peroxidase oxidization, the cation free-radical product is stabilized in LMPs at -20 °C, in contrast to the fast-consuming reactions at temperatures above 0 °C. Furthermore, the LMP system also supports the cascade reaction and lipase reaction at subzero temperatures, at which enzymatic reactions with both hydrophilic and hydrophobic substrates are successfully carried out. Our designed LMP system opens access to the nature of confined water in the biomimetic environment and provides a platform for low-temperature biomacromolecule reconstitution and the cryogenic control of enzymatic reactions in bionanotechnology.

Polarity Matters: Dielectric Relaxation in All-cis-Multifluorinated Cycloalkanes.

The polarity of all-cis-multifluorinated cyclohexanes can be fine-tuned by the number and relative orientation of fluoro substituents, giving rise to a series of compounds with strong dipole moments. Simulations provided the energetics, the dipole moments, and the respective molecular polarizabilities, while dielectric spectroscopy gave information on the dielectric permittivities and the molecular dynamics. In special cases, dipole moments in excess of 6 D and dielectric permittivities of over 300 were obtained by simulation and experiment. Melting temperatures within a given family of multifluorinated cyclohexanes were found to scale with the molecular volume. The less-symmetric all-cis-octafluorotetrahydronaphthalene did not readily crystallize, permitting an investigation of the molecular dynamics in an energetically unfavorable yet rigid and facially polarized isomer. The resulting dynamics above the glass temperature conform to the structural α-relaxation and to the celebrated Johari-Goldstein ß-relaxation.

Nanometer Confinement Induces Nematic Order in 1-Dodecanol.

The phase state and molecular dynamics of 1-dodecanol are studied in the bulk and under nanometer confinement within self-ordered nanoporous alumina templates. A rotator phase in the bulk is absent under confinement. A nematic liquid crystalline phase is formed instead in pores with diameters from 400 down to 25 nm. Results are based on the changes in temperature-dependence of dielectric permittivity and X-ray diffraction. The phase diagram under confinement is explored, and the limits of the nematic-to-isotropic and crystalline-to-nematic phase transitions are identified. The phase diagram allows for a direct transition from the liquid to the low-temperature crystalline phase in pores with a diameter below 20 nm. Furthermore, we report on the dielectric fingerprint of the rotator phase and the molecular dynamics in bulk 1-dodecanol.

Interfacial Interactions During In Situ Polymer Imbibition in Nanopores.

Using in situ nanodielectric spectroscopy we demonstrate that the imbibition kinetics of cis-1,4-polyisoprene in native alumina nanopores proceeds in two time regimes both with higher effective viscosity than bulk. This finding is discussed by a microscopic picture that considers the competition from an increasing number of chains entering the pores and a decreasing number of fluctuating chain ends. The latter is a direct manifestation of increasing adsorption sites during flow. At the same time, the longest normal mode is somewhat longer than in bulk. This could reflect an increasing density of topological constraints of chains entering the pores with the longer loops formed by other chains.

Spatially and Temporally Resolved Heterogeneities in a Miscible Polymer Blend.

Mapping the spatial and temporal heterogeneities in miscible polymer blends is critical for understanding and further improving their material properties. However, a complete picture on the heterogeneous dynamics is often obscured in ensemble measurements. Herein, the spatial and temporal heterogeneities in fully miscible polystyrene/oligostyrene blend films are investigated by monitoring the rotational diffusion of embedded individual probe molecules using defocused wide-field fluorescence microscopy. In the same blend film, three significantly different types of dynamical behaviors (referred to as modes) of the probe molecules can be observed at the same time, namely, immobile, continuously rotating, and intermittently rotating probe molecules. This reveals a prominent spatial heterogeneity in local dynamics at the nanometer scale. In addition to that, temporal heterogeneity is uncovered by the nonexponential characteristic of the rotational autocorrelation functions of single-molecule probes. Moreover, the occurrence probabilities of these different modes strongly depend on the polystyrene: oligostyrene ratios in the blend films. Remarkably, some probe molecules switch between the continuous and intermittent rotational modes at elevated temperature, indicating a possible alteration in local dynamics that is triggered by the dynamic heterogeneity in the blends. Although some of these findings can be discussed by the self-concentration model and the results provided by ensemble averaging techniques (e.g., dielectric spectroscopy), there are implications that go beyond current models of blend dynamics.

A "Rigid-Flexible" Approach for Processable Perylene Diimide-Based Polymers: Influence of the Specific Architecture on the Morphological, Dielectric, Optical, and Electronic Properties.

Conjugation-break flexible spacers in-between π-conjugated segments were utilized herein toward processable perylene diimide (PDI)-based polymers. Aromatic-aliphatic PDI-based polymers were developed via the two-phase polyetherification of a phenol-difunctional PDI monomer and aliphatic dibromides. These polyethers showed excellent solubility and film-forming ability and deep lowest unoccupied molecular orbital (LUMO) levels (-4.0 to -3.85 eV), indicating the preservation of good electron-accepting character or characteristics, despite the non-conjugated segments. Their thermodynamic properties, local dynamics, and ionic conductivity demonstrate the suppression of PDI's inherent tendency for aggregation and crystallization, suggesting PDI-polyethers as versatile candidates for organic electronic applications. Their dynamics investigation using dielectric spectroscopy revealed weak dipole moments arising from the distortion of the planar perylene cores. Blends of the PDI-polyethers (as electron acceptors) with P3HT (as a potential electron donor component) showed UV-vis absorbances from 350 to 650 nm and a tendency of the PDI-polyethers to intertwine with rr-P3HT and restrain its high crystallization tendency.

H-Shaped Copolymer of Polyethylene and Poly(ethylene oxide) under Severe Confinement: Phase State and Dynamics.

The self-assembly and the dynamics of an H-shaped copolymer composed of a polyethylene midblock and four poly(ethylene oxide) arms (PE-b-4PEO) are investigated in the bulk and under severe confinement into nanometer-spaced LAPONITE clay particles by means of small- and wide-angle X-ray diffraction (SAXS, WAXS), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), rheology, and dielectric spectroscopy (DS). Because of the H-shaped architecture, the PE midblock is topologically frustrated and thus unable to crystallize. The superstructure formation in the bulk is dictated solely by the PEO arms as inferred by the crystallization/melting temperature relative to the PEO homopolymer. Confinement produced remarkable changes in the interlayer distance and PEO crystallinity but left the local segmental dynamics unaltered. To reconcile all structural, thermodynamic, and dynamic effects, a novel morphological picture is proposed with interest in emulsions. Key parameters that stabilize the final morphology are the severe chain confinement with the associated entropy loss and the presence of interactions (hydrophobic/hydrophilic) between the LAPONITE and the PEO/PE blocks.

Afterglow Effects as a Tool to Screen Emissive Nongeminate Charge Recombination Processes in Organic Photovoltaic Composites.

Disentangling temporally overlapping charge carrier recombination events in organic bulk heterojunctions by optical spectroscopy is challenging. Here, a new methodology for employing delayed luminescence spectroscopy is presented. The proposed method is capable of distinguishing between recombination of spatially separated charge carriers and trap-assisted charge recombination simply by monitoring the delayed luminescence (afterglow) of bulk heterojunctions with a quasi time-integrated detection scheme. Applied on the model composite of the donor poly(6,12-dihydro-6,6,12,12-tetraoctyl-indeno[1,2-b]fluorene-alt-benzothiadiazole) (PIF8BT) polymer and the acceptor ethyl-propyl perylene diimide (PDI) derivative, that is, PIF8BT:PDI, the luminescence of charge-transfer (CT) states created by nongeminate charge recombination on the ns to µs timescale is observed. Fluence-dependent, quasi time-integrated detection of the CT luminescence monitors exclusively emissive charge recombination events, while rejecting the contribution of other early-time emissive processes. Trap-assisted and bimolecular charge recombination channels are identified based on their distinct dependence on fluence. The importance of the two recombination channels is correlated with the layer's order and electrical properties of the corresponding devices. Four different microstructures of the PIF8BT:PDI composite obtained by thermal annealing are investigated. Thermal annealing of PIF8BT:PDI shrinks the PDI domains in parallel with the growth of the PIF8BT domains in the blend. Common to all states studied, the delayed CT luminescence signal is dominated by trap-assisted recombination. Yet, the minor fraction of fully separated charge recombination in the overall CT emission increases as the difference in the size of the donor and acceptor domains in the PIF8BT:PDI blend becomes larger. Electric field-induced quenching measurements on complete PIF8BT:PDI devices confirm quantitatively the dominance of emissive trap-limited charge recombination and demonstrates that only 40% of the PIF8BT/PDI CT luminescence comes from the recombination of fully-separated charges, taking place within 200 ns after photoexcitation. The method is applicable to other nonfullerene acceptor blends beyond the system discussed here, if their CT state luminescence can be monitored.