Reduced Chain Mobility Research Articles

RAD51 regulates eukaryotic chromatin motions in the absence of DNA damage.

In yeasts and higher eukaryotes, chromatin motions may be tuned to genomic functions, with transcriptional activation and the DNA damage response both leading to profound changes in chromatin dynamics. The RAD51 recombinase is a key mediator of chromatin mobility following DNA damage. As functions of RAD51 beyond DNA repair are being discovered, we asked whether RAD51 modulates chromatin dynamics in the absence of DNA damage and found that inhibition or depletion of RAD51 alters chromatin motions in undamaged cells. Inhibition of RAD51 increased nucleosome clustering. Predictions from polymer models are that chromatin clusters reduce chain mobility and, indeed, we measured reduced motion of individual chromatin loci in cells treated with a RAD51 inhibitor. This effect was conserved in mammalian cells, yeasts, and plant cells. In contrast, RAD51 depletion or inhibition increased global chromatin motions at the microscale. The results uncover a role for RAD51 in regulating local and global chromatin dynamics independently from DNA damage and highlight the importance of considering different physical scales when studying chromatin dynamics.

Sequence programmable nucleic acid coacervates.

Nature uses bottom-up self-assembly to build structures with remarkable complexity and functionality. Understanding how molecular-scale interactions translate to macroscopic properties remains a major challenge and requires systems that effectively bridge these two scales. Here, we generate DNA and RNA liquids with exquisite programmability in their material properties. Nucleic acids are negatively charged, and in the presence of polycations, they may condense to a liquid-like state. Within these liquids, DNA and RNA retain sequence-specific hybridization abilities. We show that intermolecular hybridization in the condensed phase cross-links molecules and slows down chain dynamics. This reduced chain mobility is mirrored in the macroscopic properties of the condensates. Molecular diffusivity and material viscosity scale with the intermolecular hybridization energy, enabling precise sequence-based modulation of condensate properties over orders of magnitude. Our work offers a robust platform to create self-assembling programmable fluids and may help advance our understanding of liquid-like compartments in cells.

Modulating Perfluorinated Ionomer Functionality via Sidechain Chemistry

AbstractIonomers are important to electrode function in energy conversion devices, such as fuel cells and electrolyzers, as the catalyst‐binding nanometer‐thick films under confinement, which compromises mass transport of species. Mitigating confinement effects is necessary for improved performance of polymer electrolyte fuel cells. Studies on perfluorosulfonic acid (PFSA) ionomers provided insights into origins of transport resistances, but limited improvements by current strategies necessitate a need for alternative chemistries to enhance film function. This work investigates new ionomers based on sidechain modifications of PFSA to tune local acidity and structure. Their application‐relevant properties (hydration, thermal‐transition) and nanomorphology are systematically characterized using environmental ellipsometry and grazing‐incidence X‐ray scattering, respectively, to develop chemistry‐structure‐property relationships in thin‐film motif (≤100 nm). Introducing acid sites to sidechains improves hydration and nano‐phase‐separation but reduce chain mobility. Replacing the hydrophilic end‐groups with hydrophobic perfluoro‐groups enhances mobility but disrupts phase‐separation. Although modifying sidechain functional groups alters ionomer‐substrate interactions (Pt/C), chemistry has greater influence on structure‐properties. Proton conductivity correlates with hydration, regardless of chemistry. This work improves the fundamental understanding of chemistry‐function relationships of ionomers by providing a novel means to independently control their side‐chain vs. end‐group and observe resulting effects on interfacial properties to modulate electrode function in electrochemical technologies.

Self-Healing Sulfonated Poly(ether ether ketone)-Based Polymer Electrolyte Membrane for Direct Methanol Fuel Cells: Effect of Solvent Content.

Proton exchange membranes (PEMs) with superior characteristics are needed to advance fuel cell technology. Nafion, the most used PEM in direct methanol fuel cells (DMFCs), has excellent proton conductivity but suffers from high methanol permeability and long-term performance degradation. Thus, this study aimed to create a healable PEM with improved durability and methanol barrier properties by combining sulfonated poly(ether ether ketone) (SPEEK) and poly-vinyl alcohol (PVA). The effect of changing the N,N-dimethylacetamide (DMAc) solvent concentration during membrane casting was investigated. Lower DMAc concentrations improved water absorption and, thus, membrane proton conductivity, but methanol permeability increased correspondingly. For the best trade-off between these two characteristics, the blend membrane with a 10 wt% DMAc solvent (SP10) exhibited the highest selectivity. SP10 also showed a remarkable self-healing capacity by regaining 88% of its pre-damage methanol-blocking efficiency. The ability to self-heal decreased with the increasing solvent concentration because of the increased crosslinking density and structure compactness, which reduced chain mobility. Optimizing the solvent concentration during membrane preparation is therefore an important factor in improving membrane performance in DMFCs. With its exceptional methanol barrier and self-healing characteristics, the pioneering SPEEK/PVA blend membrane may contribute to efficient and durable fuel cell systems.

Enhanced interfacial thermal conductance in functionalized boron nitride/polylactic acid nanocomposites: A molecular dynamics study

The relatively low thermal conductivity of biodegradable polylactic acid (PLA) has limited its applications in various fields. To address this issue, the incorporation of nanofillers, such as boron nitride nanosheets (BNNSs), has emerged as an effective method to enhance PLA's thermal properties. However, the thermal conduction of polymer-based nanocomposites is strongly influenced by interfacial thermal resistance. In this study, we investigate the impact of pristine and surface-treated BNNSs on the thermal behavior of PLA using molecular dynamics simulations. To enhance interfacial interactions and reduce chain mobility during heat transfer, we chemically modify the surface of BNNSs by introducing three different functional groups (NH2, OH, and COOH) with varying polarities. Our findings suggest that oxygen-containing groups, namely –OH and –COOH, exhibit stronger interfacial interactions compared to the other cases. We also systematically apply different percentages of these functional groups (i.e., 2.5, 5, and 7.5 %) and observe that a higher number of functional groups leads to a greater improvement in interfacial thermal transport, attributed to the enhanced phonon coupling effect. To complete the discussion, we thoroughly study the influence of random and agglomerated patterns of functional groups distribution.

The effect of coupling agent on the mechanical properties of injection molded polypropylene/wheat straw composites

The objective of this work is to evaluate the effect of maleic anhydride-grafted polypropylene (MAPP) as coupling agent in polypropylene (PP)-based composites filled with ground wheat straw (WS) particles. The WS and the MAPP content in the composites was 10 wt% and 2 wt%, respectively. The samples were fabricated through melt compounding using a twin-screw extruder and then formed into dumbbell-shaped specimens by injection molding. The mechanical properties of neat PP and the composites with and without coupling were evaluated based on tensile and flexural tests and dynamic mechanical analyses (DMA). The experimental results showed that incorporating WS into the PP reduces its tensile strength by ~3 MPa, while improving its Young’s modulus by 0.14 GPa. The addition of MAPP compensated for the loss in tensile strength without affecting the modulus. Similar observations were made during the flexural tests as well, in which case, however, there was no loss revealed in the strength in the presence of WS due to the different types of load. The results of DMA analyses indicated an improved stiffness of WS-containing samples throughout the whole analyzed temperature range of 20-120 °C as a consequence of reduced chain mobility of PP caused by the stiff straw particles.

Enhanced mechanical performance of mSLA-printed biopolymer nanocomposites due to phase functionalization

Functionalized phases can effectively increase the mechanical properties of nanocomposites through interfacial bonding. This work demonstrates masked stereolithography (mSLA) of biopolymer-based nanocomposites and the improvement of their mechanical properties by the functionalization of both polymer matrix and nanoparticles with methacrylate groups. 3D printable nanocomposite inks were prepared from plant-derived acrylated epoxidized soybean oil (AESO), polyethylene glycol diacrylate (PEGDA), and nano-hydroxyapatite (nHA). Both AESO and nHA were further functionalized with additional methacrylate groups. We hypothesized that the additional functionalization of AESO and surface functionalization of nHA would improve the tensile strength and fracture toughness of these nanocomposites by increasing the degree of crosslinking and the strength of the interface between the matrix and nanoparticles. Curing efficiency, rheology, and print-fidelity of the nanocomposites were evaluated. Mechanical test specimens were prepared by mSLA-based 3D printing. Tensile mechanical properties, Poisson's ratio, and Mode-I fracture toughness were measured by following ASTM standards. Fracture surfaces of the tested specimens were studied using scanning electron microscopy. Thermomechanical behavior, especially glass transition temperature (Tg), was studied using dynamic mechanical analysis (DMA). Functionalized AESO (mAESO) improved rheological, tensile, and fracture mechanical properties. For instance, by replacing AESO with mAESO, tensile strength, Young's modulus, fracture toughness (K1c), and Tg increased by 33%, 53%, 40%, and 38% respectively. In addition, the combination of both functionalized nHA and mAESO improved the fracture toughness of the 10% volume fraction nHA nanocomposites but made them less extensible presumably due to reduced chain mobility due to greater crosslinking.

Dynamic electrical conductivity induced by isothermal crystallization in aluminum hydroxide filled silicone rubber

In this paper, taking aluminum hydroxide (ATH) filled silicone rubber as a model composite, we report the “n-shape” dynamic electrical conductivity during isothermal crystallization in nonconductive particles filled insulating polymers. During the initial crystallization, rapid growth of dominant lamellae blocks the transport path of impurity ions, contributing to the remarkable reduction of conductivity. Addition of ATH fillers could reduce the silicone rubber content and randomness of crystals formation, lessening the amplitude of decreased conductivity. Afterward, the silicone rubber composites further shrink due to the development of subsidiary lamellae. It results in low activation energy of ion migration and thereby enhances conductivity, whose amplitude is positively related to the crystalline phase. Finally, continuous growth of rigid amorphous fraction with reduced chain mobility impedes the ions transport, decreasing the conductivity again. The growth of rigid amorphous fraction is accelerated in silicone rubber with more ATH fillers because of more constraint of ATH on molecular chains. Our findings afford a simple means to manipulate insulation performance of nonconductive particles filled semi-crystalline polymers and provide a guideline for their use in real operation.

Functionalization of multi-walled carbon nanotube and its effect on shape memory behavior of nanocomposite based on thermoplastic polyurethane/polyvinyl chloride/multi-walled carbon nanotube (TPU/PVC/MWCNT)

Nanocomposites based on thermoplastic polyurethane/polyvinyl chloride/multi-walled carbon nanotubes were prepared by a solution method and their shape memory properties were studied. The blend ratios were 70/30, 60/40, and 50/50 (w/w) of thermoplastic elastomer polyurethane/polyvinyl chloride. To improve the dispersion of multi-walled carbon nanotubes, at 0.5% and 1% (wt), in polymer matrices, three-step chemical modifications were performed, including oxidation, chlorination and grafting of poly (ɛ-caprolactone) diol. The occurrence of surface modifications in MWCNTs was monitored using Fourier transform infrared spectroscopy, elemental analysis, scanning electron microscopy and thermal gravimetric analysis techniques. SEM micrographs revealed a good dispersion of MWCNTs in the polymer matrix due to better matrix-filler interactions compared to filler–filler interactions. The results showed that the main indices of shape memory behavior (shape fixity and shape recovery) of nanocomposite samples were affected by the polymer blend ratio, nanoparticle concentration and surface modification. Increasing the content of polyvinyl chloride led to reduced shape fixity and shape recovery, which was attributed to the lack of rigid structural nature of polyvinyl chloride. The presence of nanoparticles contributed to enhanced shape fixity and weaker shape recovery. This can be explained by lower polymer chain mobility in the presence of multi-walled carbon nanotubes. The reduction in chain mobility in nanocomposites reinforced by surface-modified multi-walled carbon nanotubes was more significant due to higher distribution efficiency of nano-scale fillers in the polymer matrix.

Size-dependent Creep Master Curve of Individual Electrospun Polymer Nanofibers

While there is great interest in polymer nanofibers due to their high strength, methods to measure their time-temperature superposition (TTS) curves are lacking. The objective of this work is to demonstrate one such method and thereby to predict room temperature creep compliance over many decades of time. A complimentary measurement is also presented to estimate the associated activation energy. The experimental method is demonstrated for polyacrylonitrile (PAN) nanofibers using an on-chip surface micromachined stepper motor actuator. It is first shown that the method yields good agreement with previous room temperature measurements. Subsequently, data up to 95 °C, near the glass transition temperature, is presented. Time-temperature superposition master curves are then constructed, and activation energies for two narrow diameter ranges (221 ± 27 nm and 150 ± 9 nm) are determined. The activation energy of the 221 nm fiber agrees well with the bulk PAN value, while the 150 nm fiber is 50% larger, indicating the importance of higher chain packing and reduced chain mobility in thinner fibers. TTS curves spanning 7 (221 nm) and 9 (150 nm) decades are presented. Over this time span the creep compliance increases by a factor of approximately 10 for each. This work demonstrates a viable method to measure polymer nanofiber TTS curves from which quantitative activation energy can be determined, and from which creep compliance values over time can be predicted.

The effect of the GNP-SDS loadings on the properties of the NRL/GNP-SDS composites

Stretchable conductive polymer composites (CPC) are fabricated by incorporating the conductive particles into the polymer matrix. In this paper, CPC was fabricated by incorporating the sodium dodecyl sulfate (SDS) modified graphene nanoplatelet (GNP) into natural rubber latex (NRL) by varying loading from 0phr to 9phr using a simple mechanical stirring method. The effect of the GNP-SDS loadings on the properties of the composites were study by investigated the crosslink density, tensile properties, morphology of the tensile fracture surface and electrical conductivity. The crosslink density of the composites shows a decreased trend. Then, due to the well dispersed GNP-SDS, the tensile strength increased but decreased at high filler loading caused by the agglomeration issue. The tensile modulus also increased with increasing filler loading due to the intrinsic high modulus of GNP and the reduction of chain mobility. However, the electrical properties of the composites improved as GNP-SDS loading increased and achieved a percolation threshold at 7phr.

Reversing and non-reversing effects of PEEK-HA composites on tuning cooling rate during crystallization

The development of bio-compatible materials for bone implants bone healing is one of the key research areas in biomaterials. In this paper, composites of polyether-ether-ketone (PEEK) (a bio-compatible polymer) and hydroxyapatite (HA) were synthesized using thermal process, The qualities of the composites were analyzed with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) for characterizing the nature of stress and bonding. The morphological study of the PEEK-HA composites were studied through Scanning electron microscopy. The compressive nature of the composites as well as raw materials was observed based on XRD data with internal lattice-strain analysis. The temperature-modulated differential scanning calorimetry (TM-DSC) and conventional differential scanning calorimetry (DSC) were performed for analyzing the crystallization process. The TM-DSC showed that both reversing and non-reversing processes were active during the crystallization between 160 °C–330 °C. However, a higher residual time of PEEK at elevated temperatures tends to facilitate polymer degradation and spread the crystallization process over a broader temperature range. Addition of HA particles provided heterogeneous nucleation sites for crystallization but also reduced the mobility of polymer chains. The combination of enhanced nucleation rate, reduced chain mobility and polymer degradation processes together lead to a wide range of crystallinity in PEEK-HA composites.

Synthesis, structure and gas separation properties of ethanol-soluble, amphiphilic POM-PBHP comb copolymers

We report the synthesis of a series of alcohol-soluble comb copolymers, specifically poly(oxyethylene methacrylate)-co-poly(2-(4-benzoyl-3-hydroxy phenoxy) ethyl acrylate) (POM-PBHP), with various compositions, as well as their use in CO2 capture membranes. The POM-PBHP comb copolymers were synthesized through a low-cost free-radical polymerization method and analyzed based on Fourier transform infrared spectroscopy and nuclear magnetic resonance. The interactions and structures of the comb copolymers were characterized via X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. The comb copolymers showed good solubility in volatile nontoxic ethanol, allowing them to be coated onto microporous polymer supports to form thin-film composite membranes. While both neat POM and PBHP membranes showed very poor separation properties, the POM-PBHP comb copolymers were effective at separating CO2 from N2. This is because the POM chains with ether oxygen groups have a strong affinity for CO2, leading to high CO2 permeance, whereas the rigid PBHP chains with aromatic rings hinder N2 permeance based on reduced chain mobility. All the full-atomistic molecular dynamics simulations based on density functional theory were also used to characterize the molecular structure and binding energy for CO2, which is consistent with the experimental results. The performance of the membranes was greatest when the content of PBHP in the copolymer was 15 wt%. The CO2/N2 selectivity and CO2 permeability reached 46.6 and 111 Barrer, respectively, which represents one of the highest values for alcohol-soluble, additive-free polymeric membranes and is similar to that of commercial PEBAX membrane.

Docosane-Organosilica Microcapsules for Structural Composites with Thermal Energy Storage/Release Capability.

Organic phase change materials (PCMs) represent an effective solution to manage intermittent energy sources as the solar thermal energy. This work aims at encapsulating docosane in organosilica shells and at dispersing the produced capsules in epoxy/carbon laminates to manufacture multifunctional structural composites for thermal energy storage (TES). Microcapsules of different sizes were prepared by hydrolysis-condensation of methyltriethoxysilane (MTES) in an oil-in-water emulsion. X-ray diffraction (XRD) highlighted the difference in the crystalline structure of pristine and microencapsulated docosane, and 13C solid-state nuclear magnetic resonance (NMR) evidenced the influence of microcapsules size on the shifts of the representative docosane signals, as a consequence of confinement effects, i.e., reduced chain mobility and interaction with the inner shell walls. A phase change enthalpy up to 143 J/g was determined via differential scanning calorimetry (DSC) on microcapsules, and tests at low scanning speed emphasized the differences in the crystallization behavior and allowed the calculation of the phase change activation energy of docosane, which increased upon encapsulation. Then, the possibility of embedding the microcapsules in an epoxy resin and in an epoxy/carbon laminate to produce a structural TES composite was investigated. The presence of microcapsules agglomerates and the poor capsule-epoxy adhesion, both evidenced by scanning electron microscopy (SEM), led to a decrease in the mechanical properties, as confirmed by three-point bending tests. Dynamic mechanical analysis (DMA) highlighted that the storage modulus decreased by 15% after docosane melting and that the glass transition temperature of the epoxy resin was not influenced by the PCM. The heat storage/release properties of the obtained laminates were proved through DSC and thermal camera imaging tests.

Confinement Effects on the Crystallization of Poly(3-hydroxybutyrate)

Poly(3-hydroxybutyrate) (PHB) is taken as an example to explore (i) whether the confined crystallization occurring in anodized aluminum oxide (AAO) nanopores is the same as that in ultrathin films, and (ii) whether the interfacial effect of curve surface is the same as flat surface. The crystallization behavior of PHB in AAO and thin films (sandwiched between two plates) has been compared. A curvature-dependent crystallization behavior of PHB is identified. Stable intermediate structures of PHB confined in narrow pores (diameter <100 nm), which have never been observed in bulk, are obtained for the first time. In larger pores (pore diameter >100 nm), crystallization occurs both in the center and interfacial regions in contact with AAO inner wall. This implies that the strength of interfacial layer weakens with decreased curvature and has been further proved by the crystallization behavior of its sandwiched ultrathin film. It is found that the highly restricted interface layer incapable of crystallization is about 30 nm for film, which is much thinner than that for nanorods (approximately 100 nm). We have also identified two different relaxations of PHB nanorods corresponding to the interfacial effect and spatial confinement, respectively. While the relaxation correlated to the interfacial effect with a slower relaxation time strengthens, the relaxation corresponding to spatial confinement with a faster relaxation time (bulklike α-relaxation) weakens with decreased pore size. This is completely different from that of sandwiched thin film, where only a thickness-independent α-relaxation same as bulk is observed 1 ( Macromolecules 2006, 39, 5967). Therefore, the reduced crystallization kinetics of thin film is attributed to the reduction of long-range chain mobility. By contrast, the inhibited crystallization of PHB nanorods in AAO is attributed to the reduced segmental dynamics and the reduced chain mobility of PHB layer at interface.

Viscoelasticity and Dynamics of Intercalated Polymer/Bentonite Nanocomposites

AbstractThis research investigates confinement‐induced dynamics retardation behavior in intercalated acrylic/clay nanocomposites. Nanostructured waterborne latex contains up to 3 wt% nanoclay, and transmission electron microscopy shows well‐dispersed intercalated morphology. Dynamics in the melt is analyzed using the time‐temperature superposition principle. Confinement induces dynamics retardation, and the entanglement relaxation time τe increases over an order of magnitude. The cooperative motion retardation also reduces viscous dissipation, as measured by the damping tanδ. Moreover, the rubbery modulus increases &gt;10‐fold. The increase of rubbery modulus suggests that confinement hinders chain mobility. The system is then naturally segregated, with intercalated chains having slow dynamics. Hence, thermal analysis of freshly cast films shows one broad glass transition with temperature T g,i for all the nanocomposites, and upon annealing the nanocomposites exhibits two T g's. The T g,high suggests regions of reduced chain mobility (i.e., intercalated chains); the T g,low is associated to the bulk material with regions of higher mobility. The breadth T g,high – T g,low increases with clay content indicating that the regions of fast and slow dynamics become more decoupled as confinement increases.

Thermomechanical characteristics of ODF-silica Nafion® nanocomposite for PEMFCs application

Previous work on Nafion® nanocomposite membranes used oxadiazole oligomer (ODF) to functionalize silica nanoparticle through silanation, the product of which then used as a filler. A simple solvent casting was used as a fabrication method to produce polymer film with different filler contents. In this work, thermomechanical properties of the polymer electrolyte membranes were studied by dynamic mechanic thermal analysis (DMTA). The composite membranes showed the β relaxation peak at 60-80 °C. As ODF-SiO2 filler was dispersed in the matrix, the physical crosslinking brought about by the nanoparticles reduced chain mobility, increased the temperature of the β relaxation. Starting at 90 °C, the shoulder of α relaxation was observed and the α peak significantly broadened, as the amount of the filler increased. This could be explained by an extra facilitating factor, which is probably the gradual desorption of water trapped in the ODF-silica particles. The water further plasticized the matrix, replenishing the evaporating water clusters, and prolonging increased ion-hopping into higher temperature regions. Additionally, the morphology of the composites was investigated. SEM images showed that the membranes with 5 wt% filler had a good dispersion while agglomeration started to be observed at 10 wt% loading. Finally, the fuel cell with 5wt% loading composite was tested for polarization, which showed that the highest power density was 0.97 W cm-2, an acceptable value for fuel cell application.

Graphene oxide reduces the hydrolytic degradation in polyamide-11

Graphene oxide (GO) was incorporated into polyamide-11 (PA11) via in-situ polymerization. The GO-PA11 nano-composite had elevated resistance to hydrolytic degradation. At a loading of 1 mg/g, GO to PA11, the accelerated aging equilibrium molecular weight of GO-PA11 was higher (33 and 34 kg/mol at 100 and 120 °C, respectively) compared to neat PA11 (23 and 24 kg/mol at 100 and 120 °C, respectively). Neat PA11 had hydrolysis rate constants (kH) of 2.8 and 12 ( × 10−2 day−1) when aged at 100 and 120 °C, respectively, and re-polymerization rate constants (kP) of 5.0 and 23 ( × 10−5 day−1), respectively. The higher equilibrium molecular weight for GO-PA11 loaded at 1 mg/g was the result of a decreased kH, 1.8 and 4.5 ( × 10−2 day−1), and an increased kP, 10 and 17 ( × 10−5 day−1) compared with neat PA11 at 100 and 120 °C, respectively. The decreased rate of degradation and resulting 40% increased equilibrium molecular weight of GO-PA11 was attributed to the highly asymmetric planar GO nano-sheets that inhibited the molecular mobility of water and the polymer chain. The crystallinity of the polymer matrix was similarly affected by a reduction in chain mobility during annealing due to the GO nanoparticles' chemistry and highly asymmetric nano-planar sheet structure.

Polyimide-SiO2-TiO2 nanocomposite structural study probing free volume, physical properties, and gas transport

Polyimide-SiO2-TiO2 nanocomposite membrane permeability, diffusivity, and selectivity were evaluated using polyimide block copolymers designed from 6F and [6F-DABA], and [SiO2-TiO2] using He, H2, CO2, O2, N2, and CH4. Nanocomposite membranes were synthesized using an in-situ sol-gel method to improve the integration of inorganic and organic phases. Metal alkoxide type and polymer functional groups were key factors affecting physical properties and gas transport. [6F-DABA]-[SiO2-TiO2] nanocomposite fractional free volume (FFV) increased 29% as compared to the unmodified [6F-DABA] polyimide. Dynamic Mechanical Thermal Analysis revealed that [SiO2-TiO2] and [TiO2] within functionalized multi-block reduced chain mobility. This led to a Tg increase from 334°C to 359°C, and tanδ to decrease from 1.9 to 0.35. In general, inorganic concentration and composition retarded polymer segmental motions, inhibited polymer chain packing, and increase predicted FFV. [6F-DABA-50]-[SiO2-TiO2] membrane's exhibited a simultaneous gas selectivity and permeability improvement. He permeability improved from 58 (unmodified polyimide) to 83 Barrers, and He/CH4 gas selectivity increased from 221 (unmodified) to 334. Membrane gas separation performance of several organic-inorganic materials exceeded Robeson's “upper bound.”

Polylactide/cellulose nanocrystal composites: a comparative study on cold and melt crystallization

Polylactide (PLA) composites with pristine cellulose nanocrystals (CNC) and acetylated one (aCNC) were prepared for the crystallization study. The roles of CNC and aCNC in cold and melt crystallization of PLA were explored. Both CNC and aCNC have good nucleation activity during cold crystallization of PLA, but also highly impede transport of adjacent chain segments to the growing surface, acting as the role of physical barrier in the glassy bulk. Within the experimental temperature range, growth dominates the overall kinetics, rather than nucleation. Therefore, barrier role overwhelms nucleation agent one and as a result, the cold crystallization rates of composites decrease as compared with neat PLA, accompanied by decreased degrees of crystallinity. During melt crystallization, although the presence of CNC and aCNC leads to sharply increased system viscosities, reducing chain mobility, nucleation is the dominant role as the systems crystallize from the melts. Thus, the presence of CNC and aCNC promotes melt crystallization of PLA, and the composites show far higher crystallization rates and degrees of crystallinity than neat PLA. Besides, the surface acetylation of CNC improves its nucleation ability during melt crystallization of PLA, and as a result, the composite with aCNC has denser fold surfaces relative to the one with CNC. But the presence of these two kinds of particles has no evident influence on the lamellar structure of PLA whether in the cold or in melt crystallization. This work can provide useful information on the crystallization control of PLA using CNC.