Polymer Relaxation Research Articles

Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers.

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature Tg, whether there is an underlying thermodynamic transition at some finite temperature below Tg, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field.

Patches of patches: Spatial fluctuations of elastic turbulence in porous media

Polymer solutions are often injected in porous media for applications such as oil recovery and groundwater remediation. As the fluid navigates the tortuous pore space, elastic stresses build up, causing the flow to become unstable at sufficiently large flow rates—a phenomenon often known as “elastic turbulence”. However, what physical factors determine the onset of this instability, and what its spatial and temporal characteristics are, remain unknown. In this talk, I describe some of our work using direct visualization in model porous media to address this gap in knowledge. First, we show that the spacing between pores strongly influences the flow: when the pore spacing is sufficiently small, the unstable flow in the different pores exhibits a surprising bistability due to the interplay between elongation and relaxation of polymers as they are advected through the pore space (Browne et al., J. Fluid Mech. 2020). Second, using experiments in full three-dimensional porous media, we show that the onset of unstable flow in each pore is akin to a second-order phase transition, arising due to the persistence of discrete patches of instability. Thus, unstable flow is patchy across the different pores of the medium. Guided by these findings, we directly link the energy dissipated by pore-scale fluctuations to the flow resistance through the entire medium, enabling prediction of the macroscopic transport behavior (Browne and Datta, Science Advances 2021). Together, these results reveal the rich array of behaviors that can arise during the unstable flow of polymer solutions through porous media, and provide a general framework by which flow fluctuations can be predicted and controlled.

Interactions between polymers and flows in macro and micro length scales.

Polymer solution with large molecular weights show scale dependent nonlinear behavior. The dynamics of such polymer solution in a meter length scale are very different from those in a micrometer length scale. To clarify this scale dependent behavior, we have conducted several experiments that focus on interactions between polymers and flows in multiple length scales [1Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Furukawa H. Effects of the extensional rate on two-dimensional turbulence of semi-dilute polymer solution flows.Rheol. Acta. 2013; 52: 949https://doi.org/10.1007/s00397-013-0733-3Crossref Scopus (13) Google Scholar, 2Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Characteristic scales of two-dimensional turbulence in polymer solutions.AICHE J. 2014; 60: 1854https://doi.org/10.1002/aic.14364Crossref Scopus (15) Google Scholar, 3Hidema R. Murao I. Komoda Y. Suzuki H. Effects of the extensional rheological properties of polymer solutions on vortex shedding and turbulence characteristics in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2018; 254: 1https://doi.org/10.1016/j.jnnfm.2018.02.001Crossref Scopus (12) Google Scholar, 4Hidema R. Fukushima K. Yoshida R. Suzuki H. Vortex deformation and turbulent energy of polymer solution in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2020; 285104385https://doi.org/10.1016/j.jnnfm.2020.104385Crossref Scopus (3) Google Scholar, 5Fukushima K. Kishi H. Suzuki H. Hidema R. Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow.J. Fluid Mech. 2022; 933: A9https://doi.org/10.1017/jfm.2021.1058Crossref Scopus (1) Google Scholar, 6Hidema R. Hayashi S. Suzuki Hiroshi Drag force of polyethyleneglycol in flow measured by a scanning probe microscope.Phys. Rev. Fluids. 2019; 4074201https://doi.org/10.1103/PhysRevFluids.4.074201Crossref Scopus (2) Google Scholar, 7Hidema Ruri Fujito Ken-ya Suzuki Hiroshi Drag force of polyethyleneglycol in flows of polymer solutions measured by a scanning probe microscope.Soft Matter. 2022; 18: 455https://doi.org/10.1039/D1SM01305JCrossref PubMed Google Scholar].In the first part of this talk, I will highlight characteristic vortex deformation and energy transfer caused by polymers in a two-dimensional turbulent flow [1Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Furukawa H. Effects of the extensional rate on two-dimensional turbulence of semi-dilute polymer solution flows.Rheol. Acta. 2013; 52: 949https://doi.org/10.1007/s00397-013-0733-3Crossref Scopus (13) Google Scholar, 2Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Characteristic scales of two-dimensional turbulence in polymer solutions.AICHE J. 2014; 60: 1854https://doi.org/10.1002/aic.14364Crossref Scopus (15) Google Scholar, 3Hidema R. Murao I. Komoda Y. Suzuki H. Effects of the extensional rheological properties of polymer solutions on vortex shedding and turbulence characteristics in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2018; 254: 1https://doi.org/10.1016/j.jnnfm.2018.02.001Crossref Scopus (12) Google Scholar, 4Hidema R. Fukushima K. Yoshida R. Suzuki H. Vortex deformation and turbulent energy of polymer solution in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2020; 285104385https://doi.org/10.1016/j.jnnfm.2020.104385Crossref Scopus (3) Google Scholar, 5Fukushima K. Kishi H. Suzuki H. Hidema R. Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow.J. Fluid Mech. 2022; 933: A9https://doi.org/10.1017/jfm.2021.1058Crossref Scopus (1) Google Scholar]. The effects of the extensional rheological properties of polyethylene oxide solutions were focused to understand the vortex deformation in turbulent flow and turbulent statistics of the polymer solution. In the polymer solution, although the turbulent energy production was virtually zero, a characteristic peak appeared in turbulent energy. The results imply the extension and relaxation of polymers affect the energy transfer.In the second part, I will introduce a method to measure the drag force due to synthetic polymers in flowing fluids by using a scanning probe microscope [[6]Hidema R. Hayashi S. Suzuki Hiroshi Drag force of polyethyleneglycol in flow measured by a scanning probe microscope.Phys. Rev. Fluids. 2019; 4074201https://doi.org/10.1103/PhysRevFluids.4.074201Crossref Scopus (2) Google Scholar,[7]Hidema Ruri Fujito Ken-ya Suzuki Hiroshi Drag force of polyethyleneglycol in flows of polymer solutions measured by a scanning probe microscope.Soft Matter. 2022; 18: 455https://doi.org/10.1039/D1SM01305JCrossref PubMed Google Scholar]. The conformation of polymers attached to the cantilever probe was predicted, and the drag force due to the deformed polymers in a flow was calculated. The drag forces obtained by model calculations were compared to the force detected by experiments, and found to be reasonably close.

A linear route to elasto-inertial turbulence

Pipe flow of Newtonian fluids is well known to exhibit a transition from the laminar to turbulent regime usually at a Reynolds number ~ 2000, despite being linearly stable at all Reynolds numbers. In stark contrast, we show that pressure-driven pipe flow of a viscoelastic (Oldroyd-B) fluid is linearly unstable to axisymmetric perturbations. The dimensionless groups that govern stability are the Reynolds number Re = ρUmaxR/η, the elasticity number E = λη/(ρR2) and the ratio of solvent to solution viscosity β = ηs/η; here, R is the pipe radius, Umax is the maximum velocity of the base flow, ρ is the fluid density, and λ is the polymer relaxation time. The unstable mode has a phase speed close to Umax over the entire unstable region in the Re-E-β space. In parameter regimes accessible to experiments (e.g. β > 0.6 and E > 0.08), the critical Reynolds number Rec ~ 400, with the associated eigenfunctions spread out across the pipe cross section. In the asymptotic limit of E(1-β) << 1, but fixed E, the critical Reynolds number for instability and the critical wavenumber diverge as Rec ~ (E (1-β))-3/2 and kc ~ (E(1-β))-1/2 respectively. The unstable eigenfunction in this limit is localized near the centerline, implying that the unstable mode belongs to a class of viscoelastic ‘center modes'.The instability identified in this study comprehensively dispels the prevailing notion of pipe flow of viscoelastic fluids being linearly stable in the Re-W plane (W = Re E being the Weissenberg number). The prediction of a linear instability is consistent with several experimental studies on pipe flow of polymer solutions, ranging from reports of ‘early turbulence’ in the 1970's to the more recent discovery of ‘elasto-inertial turbulence’ (B. Hof and coworkers, Proc. Natl. Acad. Sci., 110, 10557–10562 (2013)), wherein transition is observed at Re much lower than 2000. An analogous center-mode instability is predicted for viscoelastic channel flows over a similar range of parameters. Thus, there is the suggestion of a universal linear mechanism that underlies the onset of turbulence in rectilinear viscoelastic shearing flows for sufficiently elastic dilute polymer solutions, marking a possible paradigm shift in our understanding of transition in such flows. We will end with a discussion of the possible transition scenarios in the Re-W-β space for viscoelastic shearing flows.

Superionic Bifunctional Polymer Electrolytes for Solid-State Energy Storage and Conversion.

Achieving superionic conductivity from solid-state polymer electrolytes is an important task in the development of future energy storage and conversion technologies. Herein, a platform for innovative electrolyte technologies based on a bifunctional polymer, poly(3-hydroxy-4-sulfonated styrene) (PS-3H4S), is presented. By incorporating OH and SO3 H functional groups at adjacent positions in the styrene repeating unit, "intra-monomer" hydrogen bonds are formed to effectively weaken the electrostatic interactions of the SO3 - moieties in the polymer matrix with embedded ions, promoting rich structural and dynamic heterogeneity in the PS-3H4S electrolyte. Upon the incorporation of an ionic liquid, interconnected rod-like ion channels, which allow the decoupling of ion relaxation from polymer relaxation, are formed in the stiff motif of the polymeric domains passivated by interfacial ionic layers. This results in accelerated proton hopping through the glassy polymer matrix, and proton hopping becomes more pronounced at cryogenic temperatures down to -35 °C. The PS-3H4S/ionic liquid composite electrolytes exhibit a high ionic conductivity of 10-3 S cm-1 and high storage modulus of ≈100MPa at 25 °C, and can be successfully applied in soft actuators and lithium-metal batteries.

Molecular basis of secondary relaxation in stiff-chain glassy polymers.

Recent progress in establishing local order in polycarbonate-like glasses using rotational echo double resonance and centerband-only detection of exchange solid-state nuclear magnetic resonance (NMR) has stimulated a renewed attempt to connect molecular motion within glassy polymers and the mechanical properties of the glass. We have in fact established a correlation between molecular motion characterized by NMR and the mechanical secondary relaxation (tan δ) for nine polycarbonate-like glasses. All of the NMR and mechanical data are for T ≪ Tg. The resulting structural insights suggest that the chains of these polymers are simultaneously both Flory random coils and Vol'kenstein bundles. The cooperative motions of groups of bundles can be described qualitatively by a variety of constrained-kinetics models of the glass. All of the models share a common trait for large-amplitude motion: an exponential increase in the time required for an inter-bundle dilation event with a linear increase in bundle group size. This dependence and a locally ordered Vol'kenstein bundle lead to an understanding of the surprising 60° (K) shift of tan δ to higher temperature for ring-fluoro-polycarbonate relative to that of polycarbonate by the apparently minor substitution of a fluorine for a hydrogen on every fourth ring.

A Hidden Relaxation Process in Poly(2-vinylpyridine) Homopolymers, Copolymers, and Nanocomposites

In broadband dielectric spectroscopy experiments, we find that introducing an air gap between the top electrode and the polymer sample reduces DC conductivity substantially, allowing the study of low-frequency relaxations, whose signal would otherwise be hidden by the DC conductivity signal. An extra process slower than the α-relaxation process is observed in poly(2-vinylpyridine) (P2VP), consistent with some earlier reports. This “slower process” was studied in two heterogeneous systems to examine the interaction between the 2VP and other species to elucidate the mechanism behind the slower process signal. In a random copolymer of styrene and 2VP, the relaxation strength of the slower process relative to the α-process increases substantially at low 2VP mole fractions. Additionally, in a composite of P2VP and octa(aminophenyl) silsesquioxane (OAPS), the presence of OAPS increases both the strength and timescale of the slower process, leaving the α-relaxation process relatively unchanged. This suggests that the slower process could be caused by cooperative polymer relaxation coupled to the transport of heterogeneous components and impurities. Further studies are needed to probe the molecular-level mechanism of this slower process and its effect on interfacial properties.

Mixed diffusion and relaxation kinetics model for hydrogels swelling

Hydrogels have a unique ability to retain moisture due to the three-dimensional (3D) polymeric network. Both swelling capacity and swelling rate of hydrogels depend on the various parameters such as solution concentration, temperature, degree of cross-linking in the gel structure, type of hydrophilic groups, pH, etc. Various kinetics models were presented for swelling rate, however, most of them only describe diffusion or polymer relaxation as the predominant process on the swelling rate. In the present work, a new mixed diffusion and relaxation kinetics model is presented by simultaneously considering both diffusion and polymer relaxation. The mixed model is able to cover the whole of swelling and provides insight into the gelling mechanism. By the presented kinetics model, it is possible to indicate the contribution of solvent molecules diffusion and polymer relaxation on the swelling rate and also determine the predominant process at different extent of swelling. It is shown that this new mixed kinetics model is a general equation and covers most of the previously reported equations. The presented mixed kinetics equation was used for modeling experimental swelling kinetics data at different conditions very well.

Transmission of sodium chloride in PDMS membrane during Pervaporation based on polymer relaxation

Polydimethylsiloxane (PDMS) composite membrane is used for treating pharmaceutical wastewater containing NaCl and solvent. In this study, the influence of feed concentrations of NaCl and isobutanol, process temperature and membrane microstructures on salt rejection are evaluated. Microstructures of PDMS membrane before and after separation are characterized by nuclear magnetic resonance (NMR), energy dispersive X-ray spectroscopy (EDS), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Positron annihilation life-time spectroscopy (PALS). The PV results show that NaCl will not spontaneously enter PDMS membrane without isobutanol. However, while NaCl feed concentration is 13 wt%, salt rejection of PDMS membrane drops from 100% to 99.09% with increasing feed concentration of isobutanol (up to 7 wt%). On the contrary, a higher temperature increases salt rejection of PDMS membrane and NaCl permeation through PDMS membrane is not through a vapor permeate process. Due to the relaxation of PDMS polymer chain, when PDMS cross-linking ratio is 0.1, the salt rejection increases from 99.87% to 100% with its thickness increasing from 10 μm to 17.5 μm. While the cross-linking ratio rises to 0.2, the salt rejection is 100% with the PDMS layer thickness of 10 μm. The relationship between relaxation of polymer chains and transport of NaCl in PDMS membrane is an excellent guidance and will be beneficial for the treatment of saline organic wastewater.

Deviatoric stress driven transient melting below the glass transition temperature in shocked polymers

The relaxation of polymers around and below their glass transition temperature is governed by a range of correlated unit processes with a wide range of timescales. The fast deformation rates of shock loading can suppress a significant fraction of these processes resulting in dynamical glass transition in rubbers. In this article, we report the inverse, a transient melting of a glassy polymer under shock loading. The large deviatoric stresses near the shock front induce fast transitions in backbone dihedral angles and a stress relaxation characteristic of polymer melts. This is followed by slower relaxation expected for glasses.

Prediction of polymer extension, drag reduction, and vortex interaction in direct numerical simulation of turbulent channel flows

Hydrodynamic and viscoelastic interactions between the turbulent fluid within a channel at Reτ=180 and a polymeric phase are investigated numerically using a multiscale hybrid approach. Direct numerical simulations are performed to predict the continuous phase and Brownian dynamics simulations using the finitely extensible nonlinear elastic dumbbell approach are carried out to model the trajectories of polymer extension vectors within the flow, using parallel computations to achieve reasonable computation timeframes on large-scale flows. Upon validating the polymeric configuration solver against theoretical predictions in equilibrium conditions, with excellent agreement observed, the distributions of velocity gradient tensor components are analyzed throughout the channel flow wall-normal regions. Impact on polymer stretching is discussed, with streamwise extension dominant close to the wall, and wall-normal extension driven by high streamwise gradients of wall-normal velocity. In this case, it is shown that chains already possessing high wall-normal extensions may attempt to orientate more in the streamwise direction, causing a curling effect. These effects are observed in instantaneous snapshots of polymer extension, and the effects of the bulk Weissenberg number show that increased WeB leads to more stretched configurations and more streamwise orientated conformities close to the wall, whereas, in the bulk flow and log-law regions, the polymers tend to trace fluid turbulence structures. Chain orientation angles are also considered, with WeB demonstrating little influence on the isotropic distributions in the log-law and bulk flow regions. Polymer–fluid coupling is implemented through a polymer contribution to the viscoelastic stress tensor. The effect of the polymer relaxation time on the turbulent drag reduction is discussed, with greater Weissenberg numbers leading to more impactful reduction. Finally, the velocity gradient tensor invariants are calculated for the drag-reduced flows, with polymers having a significant impact on the Q–R phase diagrams, with the presence of polymers narrowing the range of R values in the wall regions and causing flow structures to become more two-dimensional.

Understanding effect of fluid salinity on polymeric drag reduction in turbulent flows of slickwater fluids

Produced waters are brines drawn during the oil and gas production. These fluids are often re-injected into unconventional reservoirs along with certain additives and proppants in order to induce/sustain fractures and boost oil/gas production. The additives may transform these fluids into drag-reducing slickwater fluids that are pumped at high flowrates (turbulent conditions) to transport proppants at low viscosities. The drag-reducer polymeric additives in the slickwater fluids facilitate high flowrates by reducing drag during the flow. The present work investigated the influence of fluid salinity/hardness on polymer dynamics and its impact on the ability of the polymer to alleviate drag. The study focuses on two synthesized drag-reducing anionic acrylamide (APAM) polymers: first is a copolymer of acrylamide and acrylic acid (Base-APAM), and the second is a ter-polymer of acrylamide, acrylic acid and a functional group (Functionalized-APAM). The polymers were characterized using capillary viscometry, NMR and degree of ionicity.For evaluation of the drag reduction performance of the above polymers, a laboratory flow-loop was built to mimic the turbulent flow conditions in the wellbore. The experimental evaluation was conducted for the fluids with added polymers and range of fluid salinities to encompass plausible variations in the field. Additionally, the fluids’ viscosity performance was evaluated using conventional oil-field viscometers. The obtained experimental data was used to build a semi-empirical correlation that predict the polymer relaxation defined by Weissenberg number i.e., Wi, modelled as a function of fluid salinity and polymeric structure. The drag reduction performance may be derived based on the Re-Wi parameter space, and a correlation for the studied range of parameters has been constructed. The experimental data and models also evidently depicted that the synthesized ter-polymer provides a superior salt-tolerance compared to the co-polymer for the range of parameters studied.

In vitro‐studies of adenosine‐ β‐cyclodextrin inclusion complexes loaded into chitosan, sodium alginate and bentonite‐based nanocomposite optimized by RSM as a sustained release system

AbstractDrugs with less aqueous solubility and permeability acquire significant hurdles in designing safe and effective formulations. The purpose of this study was to prepare an adenosine (ADS): β‐cyclodextrin (β‐CD) inclusion complex that could be consistently incorporated into a nanocomposite system for topical administration. Several procedures, including kneading, microwave irradiation, and co‐precipitation, were used to develop an adenosine: β‐CD inclusion complex. ROESY, DSC, XRD, and 1H NMR spectroscopy were used to analyze the adenosine‐β‐CD inclusion complex. Then bentonite (BN) incorporated chitosan (CH) and sodium alginate (ALG) nanocomposite were synthesized by microwave irradiation method. Response Surface Methodology (RSM) design was executed to optimize the reaction parameters. The Minimum Resolution V design was implemented to find significant variables to get maximum percentage swelling. The inclusion complexes of ADS: β‐CD were directly loaded into the CH/ALG/BN nanocomposite and the adenosine release was analyzed in simulated intestinal fluids (SIF) and simulated gastric fluids (SGF). Five kinetic models were investigated to know about the drug release mechanism. After a 24‐h interval, the ADS final release was 86 ppm at pH 2, 96 ppm at pH 7, and 85 ppm at pH 7.4. Bentonite has a layered structure that allows it to introduce the drug molecule into its cavity, resulting in sustainable adenosine release. The exploratory kinetic analysis demonstrated that β‐CD influences polymer relaxation, resulting in slow release.

Unexpected stability of micrometer weakly viscoelastic jets

We study experimentally the stability of micrometer weakly viscoelastic jets produced with transonic flow focusing. Highly stable jets are formed when a low molecular weight polymer is added to water at a given low concentration, and the injected flow rate is reduced to its minimum value. In this case, the capillary instability is delayed, and the jet breakup occurs at distances from the ejector of the order of tens of thousands the jet diameter. The results indicate that the intense converging extensional flow in the ejection point builds up viscoelastic stress that does not relax in the jet even for times much longer than the polymer relaxation time. We hypothesize that the drag (shear) force exerted by the outer gas stream prevents the stress relaxation. It is also possible that partial polymer entanglement at the jet emission point contributes to this effect. We measure the jet length and the diameter at the ejector orifice and breakup point. The diameter takes values just above 2 μm at the breakup point regardless of the liquid flow rate and gas pressure.

DeePN$^2$: A Deep Learning-Based Non-Newtonian Hydrodynamic Model

A long standing problem in the modeling of non-Newtonian hydrodynamics of polymeric flows is the availability of reliable and interpretable hydrodynamic models that faithfully encode the underlying micro-scale polymer dynamics. The main complication arises from the long polymer relaxation time, the complex molecular structure and heterogeneous interaction. DeePN$^2$, a deep learning-based non-Newtonian hydrodynamic model, has been proposed and has shown some success in systematically passing the micro-scale structural mechanics information to the macro-scale hydrodynamics for suspensions with simple polymer conformation and bond potential. The model retains a multi-scaled nature by mapping the polymer configurations into a set of symmetry-preserving macro-scale features. The extended constitutive laws for these macro-scale features can be directly learned from the kinetics of their micro-scale counterparts. In this paper, we develop DeePN$^2$ using more complex micro-structural models. We show that DeePN$^2$ can faithfully capture the broadly overlooked viscoelastic differences arising from the specific molecular structural mechanics without human intervention.

Cooperative Intramolecular Dynamics Control the Chain-Length-Dependent Glass Transition in Polymers

The glass transition is a long-standing unsolved problem in materials science. For polymers, our understanding of glass-formation is particularly poor due to the added complexity of chain connectivity and flexibility; structural relaxation of polymers thus involves a complex interplay between intra- and inter-molecular cooperativity. Here we study how the glass transition temperature Tg varies with molecular weight M for different polymer chemistries and chain flexibilities. We find that Tg(M) is controlled by the average mass (or volume) per conformational degree of freedom, and that a `local' molecular relaxation (involving a few conformers) controls the larger-scale cooperative alpha relaxation responsible for Tg. We propose that dynamic facilitation where a `local' relaxation facilitates adjacent relaxations, leading to hierarchical dynamics, can explain our observations including logarithmic Tg(M) dependences. Our study provides a new understanding of molecular relaxations and the glass transition in polymers, which paves the way for predictive design of polymers based on monomer-scale metrics.

Multimodal confined water dynamics in reverse osmosis polyamide membranes

While polyamide (PA) membranes are widespread in water purification and desalination by reverse osmosis, a molecular-level understanding of the dynamics of both confined water and polymer matrix remains elusive. Despite the dense hierarchical structure of PA membranes formed by interfacial polymerization, previous studies suggest that water diffusion remains largely unchanged with respect to bulk water. Here, we employ neutron spectroscopy to investigate PA membranes under precise hydration conditions, and a series of isotopic contrasts, to elucidate water transport and polymer relaxation, spanning ps-ns timescales, and Å-nm lengthscales. We experimentally resolve, for the first time, the multimodal diffusive nature of water in PA membranes: in addition to (slowed down) translational jump-diffusion, we observe a long-range and a localized mode, whose geometry and timescales we quantify. The PA matrix is also found to exhibit rotational relaxations commensurate with the nanoscale confinement observed in water diffusion. This comprehensive ‘diffusion map’ can anchor molecular and nanoscale simulations, and enable the predictive design of PA membranes with tuneable performance.

The roles of polymer relaxation phenomena, aqueous dye solubility and the physical properties of water in the mechanism of adsorption of a disperse dye on poly(ethylene terephthalate) fibres: Part 2 dye solubility

AbstractThe marked temperature‐dependent uptake of a commercial disperse dye on poly(ethylene terephthalate) fabric achieved during an aqueous high temperature dyeing process is the likely amalgamation of at least two separate, but intrinsically inter‐related, thermally activated processes, namely, poly(ethylene terephthalate) structural relaxations, as articulated in terms of polymer chain segmental mobility and available free volume, in which glass transition adopts a principal role, as well as disperse dye aqueous solubility. Both polymer relaxation times and disperse dye solubility therefore assume crucially important roles in the mechanism of adsorption of disperse dyes on poly(ethylene terephthalate) fibres.

Melt Properties and String Model Description of Glass Formation in Graft Polymers of Different Side-Chain Lengths

Graft polymers have attracted growing interest due to their unique properties, but a fundamental understanding of their glass formation remains to be established. We perform extensive molecular dynamics simulations of a coarse-grained bead–spring polymer model to investigate the influence of side-chain length on the glass formation of general classes of graft polymers having a fixed reduced molecular mass, which are classified based on the relative rigidities of the backbone and side chains. We first examine the basic properties of the graft polymers in a melt to understand their conformational and structural properties. Our analysis indicates that the overall average size and shape of our model graft polymers exhibit a nonmonotonic dependence on the side-chain length, while the side chains undergo a smooth transition from rod-like to coiled chain conformations near a side-chain length of 10 statistical segments. We then utilize the string model to understand the structural relaxation of graft polymers in the low-temperature non-Arrhenius regime─a model that takes the average string length as a key quantity in the description of the dynamics of glass-forming liquids, along with the high-temperature activation free energy. We find the string model to be capable of quantitatively describing the structural relaxation time over the range of temperatures considered in our model graft polymers. The string model also allows us to discuss issues related to the energetic parameters of high temperature activation, and our analysis indicates that the entropic contribution to the high-temperature activation free energy is not generally negligible for polymeric glass-forming liquids, in contrast to the assumption in the entropy theory of glass formation. We expect our work to motivate further studies of the glass formation of graft polymers based on the models in which the fluid entropy and collective motion are of central importance.

Macromolecule Relaxation Directed 3D Nanofiber Architecture in Stretchable Fibrous Mats for Wearable Multifunctional Sensors.

Elastomer fiber mat sensors, which are capable of perceiving mechanical stimuli, temperature, and vapor of chemicals, are highly desirable for designing wearable electronics and human-robot interfacing devices due to good wearability, skin affinity, and durability, and so on. However, it is still challenging to fabricate multiresponsive flexible wearable sensors with three-dimensional (3D) architecture using simple material and structure design. Herein, we report an all-in-one multiresponsive thermoplastic polyurethane (TPU) nanofiber mat sensors composed of crimped elastomer fibers with deposited platinum nanoparticles (PtNPs) on the fiber surface. The 1D TPU nanofibers could be transferred to nanofibers with different 3D nanofiber architectures by controllable macromolecular chain relaxation of aligned elastomer polymers upon poor solvent annealing. The conductive networks of PtNPs on wavy TPU fibers enable the sensor susceptible to multiple stimuli like strain/pressure, humidity, and organic vapors. Besides, the 3D nanofiber architectures allow the strain sensor to detect wider tensile strain and pressure with higher sensitivity due to delicate fiber morphology and structure control. Therefore, this work provides new insights into the fabrication of multifunctional flexible sensors with 3D architecture in an easy way, advancing the establishment of a multiple signal monitoring platform for the health care and human-machine interfacing.