Terminal Regime Research Articles

Volumetric Measurements Of Particle-Wake Interactions

Research on freely falling particles has primarily focused on wake dynamics and vortex shedding of individual particles in quiescent flow. When these particles fall collectively, the wakes of surrounding particles alter the flow fields. Hence, we conducted volumetric experiments to investigate how the settling and wake dynamics of particles are affected by the wakes of other settling particles. Negatively buoyant 12 mm particles of (sphere, flat cuboid, circular and square cylinders) are first released individually into quiescent water. Then, the particles are released individually into the bulk wakes of 20 mono-dispersed particles. Using four high-speed cameras and LEDs, we simultaneously capture both 3D particle and fluid motions in the terminal velocity regime. The imaging domain measures 90 mm x 90 mm x 40 mm. Our results show that all trailing particles settling through the bulk wakes gain additional downward momentum from the turbulent wakes, falling faster than in the quiescent flow. Upstream of the particle, the vortices in the bulk wake interact with the developing shear layer along the particle. The wake downstream of the trailing particle also appears more chaotic than that in a quiescent flow.

Polymer architecture effect on rheology and segmental dynamics in poly (methyl methacrylate)-silica nanocomposite melts.

Architecturally different polymer chains lead to fundamentally different rheological responses and internal dynamics, which can be utilized to rationalize advanced thermoplastic nanocomposites with tunable mechanical behavior. In this work, three model poly (methyl methacrylate) (PMMA) polymers with linear, bottlebrush, and star architectures with the same total molar mass were investigated in their neat form, and nanocomposites with well-dispersed silica nanoparticles using rheology and broadband dielectric spectroscopy (BDS). The master curves of the dynamic moduli obtained by time-temperature superposition (TTS) over the entire range from the Rouse regime to the terminal flow and a sequence of significantly different relaxation modes were observed for the samples with linear and branch chains. While linear chains form an entangled polymer network, the branched bottlebrush, and star chains show a viscoelastic response with no sign of rubbery entanglement plateau and a weak arm relaxation regime between Rouse and terminal flow, akin to other branched polymers. Moreover, branched chains showed a higher fragility index (m = 3.46 for the bottlebrush and 5.36 for the star) compared to linear chains (m = 3.29) due to dynamical heterogeneities induced by arm relaxation. The addition of nanoparticles affects only the terminal relaxation regime, where the whole chain motion is hindered by the attractive nanoparticles. The dynamics of the polymer segment were investigated by performing broadband dielectric spectroscopy (BDS) at a frequency range from 10-2 Hz to 107 Hz. The results revealed more than 10 times slower segmental relaxation for the star homopolymers and a slowdown in the α-relaxation process for all three architectures in their composite form. The dynamical slowdown in the composites is temperature dependent and more pronounced at low temperatures (leading to approximately equal to 80 times slower dynamics for nanocomposite with bottlebrush PMMA at 150 °C) due to prolonged relaxation of the interfacial polymer compared to the matrix chains. The results from this study have practical applications in fields such as gas separation and polymeric electrolyte membranes, where simultaneous improvement of segmental mobility and mechanical moduli is highly desired.

POSS increased the toughness and mechanical modulus of acrylic copolymers by increasing the entanglement density

Polyhedral oligomeric silsesquioxane copolymerized with butyl acrylate (BA), methyl methacrylate (MMA), and acrylic acid (AA) induced ca. twofold increase of fracture energy Γ and of Young's modulus E of cast films at only 1 wt% content. The copolymer of composition BA:MMA:AA:POSS 56:41:2:1 mol% was synthesized in situ by seeded semi-batch emulsion polymerization. Films cast from the corresponding emulsions were optically transparent and transmission electron microscopy (TEM) showed that POSS was predominantly dispersed at nearly single unit in the acrylic matrix. Shear rheology and time-temperature superposition (TTS) analysis demonstrated an entangled state with the absence of terminal regime, and POSS increased the segmental relaxation time τe and the rubbery modulus Ge, hence the free volume and packing length p decreased, relative to the neat copolymer. The efficient dispersion of POSS and its size smaller than the virtual tube diameter suggests that POSS is intercalating the molecular mesh. Therefore, the entanglement density increased and induced an increase of tensile mechanical modulus. The mesh intercalation by POSS means that it did not act as stress concentrator and consequently the critical fracture strain ε* and the fracture energy Γ also increased. These results evidenced the topological constraints imposed by the bulky POSS to hinder chains past each other thus inducing an increase of entanglement density and this manifested in simultaneous mechanical reinforcement and resistance to fracture.

Determination of the molecular weight distribution of ultrahigh molecular weight polyethylene from solution rheology

We investigate the linear rheology of ultrahigh molecular weight polyethylene (UHMWPE) solutions with the aim of determining the molecular weight distribution of the polymer. The UHMWPE is dissolved in oligo-ethylene in order to avoid issues related to unfavorable interactions with the solvent. To prepare the solutions, UHMWPE, solvent, and a fixed amount of antioxidants are mixed by means of a corotating twin-screw microcompounder. All prepared solutions are within the concentrated regime, as confirmed by the scaling laws of the main rheological parameters (plateau modulus, relaxation time, and zero-shear viscosity) with concentration. Based on the viscoelastic response of the solutions, we adopt a heuristic approach to extrapolate the linear viscoelastic behavior of the melt, according to a time-concentration superposition principle. Such a technique allows us to span many decades of angular frequency, eventually attaining the terminal relaxation regime. The latter is difficult to achieve by direct measurements in the molten state because of experimental issues such as extremely long experimental times and thermal limits. The viscoelastic spectrum of the melt is used to obtain the molecular weight distribution (MWD) according to the time-dependent diffusion/double reptation model. The MWD of UHMWPE evaluated by using this approach agrees well with data obtained from gel permeation chromatography.

Predictive Mesoscale Simulation of Flow-Induced Blend Morphology, Interfacial Relaxation, and Linear Viscoelasticity of Polymer–Elastomer Blends

Mesoscale simulations of two (cross-linking vs non-cross-linking) representative polymer–elastomer (Nylon 6/ACM) blends are performed without introducing freely adjustable parameters. The simulation allows for a direct investigation into the interplay between flow history (modeled herein by oscillatory shear flow), blend morphology, and viscoelasticity of the polymer composite. Detailed spectral analysis is utilized to resolve the stress relaxations in the bulk and interface regions, respectively, in an effort to gain insight into the origins of blend elasticity. The results reveal a notable impact of flow history on the blend morphology, and the substantially altered storage modulus response in the terminal regime agrees well with early experimental observations on a broad variety of immiscible polymer blends. The interfacial relaxation, in particular, is found to formally commence when the stress relaxations of individual polymer components in the bulk phase are nearly complete, helping to release the remaining stress that was previously dictated by repulsion-driven tube dilations─instead of chain reptation─in the interface region. Importantly, the last feature suggests that the bulk and interfacial relaxations of a polymer blend cannot be fully decoupled. Cross-linking between the elastomers (ACM) adds further complexity to the features of storage modulus response while partly offsetting the flow-induced phase separation. The generality of the simulation protocols described herein along with the affordable computational expense is expected to greatly facilitate future processing designs on polymer–polymer and polymer–elastomer blends.

Viscoelastic properties of comb-shaped ring polystyrenes

In this study, we examined the viscoelastic properties of a series of comb-shaped ring (RC) polystyrene samples with different branch chain lengths, i.e., the molecular weights of the ring backbone Mbb (≃ 4Me where Me is the entanglement molecular weight) and branch chains Mbr (≃ Me, 2Me, and 4Me). Even for the RC sample with the shortest branch chains, a plateau region was observed for the dynamic modulus G*(ω) in the middle angular frequency ω region, suggesting that intermolecular branch chain entanglement occurred. In the ω region between the plateau and terminal regions, G*(ω) was observed with a weaker ω dependence than the terminal relaxation. This behavior was more pronounced for RC samples with shorter branch chains and for the corresponding linear comb (LC) samples than for the RC ones. The molecular weight dependence of the zero-shear viscosity η0 and the steady-state recoverable compliance Jeo of the RC and LC samples was evaluated, and the effects of different backbone molecular structures (i.e., ring or linear) on the terminal relaxation behavior was discussed. Moreover, the G*(ω) data were analyzed with two models: the comb-Rouse model, in which the structures of the RC/LC molecules are taken into account by graph theory, and the Milner-McLeish model for entangled star-shaped polymers. The former model qualitatively described the terminal relaxation behavior of G*(ω) at low ω but failed to reproduce the plateau in the middle ω range. Conversely, the latter model described the entanglement plateau in the middle ω range, but the difference in the terminal relaxation regime for the RC/LC samples seen in the data and the comb-Rouse model disappeared.

Modelling the effect of hydrogen bonding on elongational flow of supramolecular polymer melts

Hydrogen bonding is the most common noncovalent reversible interaction leading to supramolecular polymeric assemblies. Shabbir et al. (Macromolecules 48:5988–5996, 2015) reported both linear and nonlinear rheological data for a model system consisting of pure poly(n-butyl acrylate) (PnBA) homopolymer and three PnBA-poly(acrylic acid) (PnBA-PAA) copolymers with different numbers of acrylic acid (AA) side groups. Hydrogen bonds between the AA groups not only cause the storage and loss modulus to shift in the direction of a power law scaling of 0.5 in the terminal relaxation regime, but also the elongational viscosity shows increasing strain hardening with a strongly nonlinear dependence on the number of hydrogen bonding groups. Based on the “Sticky Rouse” model and a constitutive equation of the Doi-Edwards type with consideration of chain stretch, we model the effect of hydrogen bonding on the elongational viscosity of the PnBA-AA copolymers. We show that the elongational viscosity data are consistent with a Sticky Rouse relaxation modulus of the AA associations characterized by a constant modulus G_{A} and a constant sticker life time tau_{A}, while the complexity of the hydrogen assemblies as quantified by the Sticky Rouse time increases with the concentration of AA groups from the order of seconds (3% AA) to hours (6%AA) and to 1 day (13%AA), and leads to extreme strain hardening. The elongational stress shows a steady state at large strains and the stretch reaches a limiting value independent of strain rate. At the highest concentration of AA groups investigated (38%AA), the PnBA-AA copolymer is a weak gel fracturing at a critical strain, and the sticker life time loses its significance. The effect of the Sticky Rouse time on self-healing is discussed.

Universal Scaling for the Exit Dynamics of Block Copolymers from Micelles at Short and Long Time Scales.

The correlation function for the exit of poloxamer copolymers from equilibrated micelles is found to show up to four regimes depending on the chain flexibility: an initial fast reorganization, a logarithmic intermediate regime, followed by an exponential intermediate regime, and a final exponential decay. The logarithmic intermediate regime has been observed experimentally and attributed to the polydispersity of the polymer samples. However, we present dynamic single-chain mean-field theory simulations with chains of variable flexibility which show the same logarithmic relaxation but with strictly monodisperse systems. In agreement with our previous studies, we propose that this logarithmic response arises from a degeneracy of energy states of the hydrophobic block in the micelle core. For this to occur, a sufficiently large number of degenerate conformational states are required, which depend on the polymer flexibility and therefore should not be present for rigid polymers. Experimental results for monodisperse polymeric samples claiming the absence of such a logarithmic response may also lack a sufficient number of hydrophobic blocks for the required number of configurational states for this type of response to be seen. The insight gained from analyzing the simulation results allows us to propose a modified Eyring equation capable of reproducing the observed dynamic behavior. On scaling experimental results from different sources and systems according to this equation, we find a unique master curve showing a universal nature of the intermediate regimes: the logarithmic regime together with the secondary exponential decay. The terminal exponential regime at long times proposed by the standard Halperin and Alexander model is beyond the range of the data analyzed in this article. The universality observed suggests an entropic origin of the short-time dynamic response of this class of systems rather than the polydispersity.

Acrylic‐styrene/montmorillonite nanocomposites. Melt viscoelasticity and chain dynamics in nanoconfinement

AbstractThe chain dynamics of co(acrylic‐styrene)/montmorillonite (MMT) nanoclay nanocomposites in the melt was studied by shear rheometry. Copolymers containing 60% butyl acrylate (BA), 38% styrene (sty), and 2% methacrylic acid (MAA) were synthesized in the presence of MMT nanoclay, that is, in‐situ, and the MMT content ranged from cMMT = 5–25 wt%. Recently, it was shown that nanoclay was exfoliated when cMMT ≤ 5 wt% and intercalated at higher concentrations, and the tensile modulus increased over an order of magnitude with MMT content [Romo‐Uribe, Polym Adv Technol, 2021]. Here, the chain dynamics was investigated using shearrheology. The results revealed that nanoclay slowed down the chain dynamics in the terminal regime, the terminal relaxation time τt increased up to an order of magnitude. Furthermore, nanoclay induced 10‐fold increase of rubbery modulus Ge with the consequent reduction of molecular weight between entanglements Me and reduction of packing length p. Furthermore, the nanoconfinement imposed on the polymer chains reduced the energy dissipation in the transition regime (as indicated by the damping tan δ) thus correlating with the increase of the glass transition temperature, Tg. Therefore, understanding and controlling nanoconfinement and its influence on chain dynamics would enable tuning the physical properties of polymer‐clay nanocomposites.

Study of the effect of inter-pass temperature on weld overlap start-stop defects and mitigation by application of laser defocusing

Laser keyhole initiation and termination-related defects, such as cracking and keyhole cavities due to keyhole collapse, are a well-known issue in laser keyhole welding of thick section steels. In longitudinal welding, run-on and run-off plates are used to avoid this problem. However, such an approach is not applicable in circumferential welding where start/stop defects remain within the workpiece. These issues can hinder industry from applying laser keyhole welding for circumferential welding applications. In this paper, the effect of inter-pass temperature on laser keyhole initiation and termination at the weld overlap start-stop region was investigated. This study has identified that defects occurring within this region were due to laser termination rather than laser initiation because of keyhole instabilities regardless of the thermal cycle. The laser termination defects were mitigated by applying a laser defocusing termination regime to reduce the keyhole depth gradually and control the closure of the keyhole.

Shear‐induced textures and viscoelasticity in the nematic and isotropic phase of semiflexible thermotropic polymers. A rheo‐optical study

The striking similarities in response to shear flow of nematic semi‐flexible thermotropic liquid crystalline polymers (LCPs) is reported. Insights into their complex flow was gained by in‐situ polarized optical microscopy (POM) whereas spatial correlations below optical resolution were characterized by small‐angle light scattering (SALS). The LCPs contained mesogenic units separated by ten (10) alkyl spacers, the polyether TPB10 of = 17 kDa and the polyester PSHQ10 of = 53 kDa. The quiescent nematic phase consisted of microdomains randomly oriented, that is, polydomain texture. Shear stretched and aligned the μm‐sized domains. After cessation of shear the texture sprang back and relaxed, PSHQ10 produced banded texture, whereas TPB10 only produced a disordered texture. Strikingly, high shear rate induced monodomains. Upon shear cessation the monodomain in PSHQ10 slowly relaxed to a polydomain, whereas TPB10 maintained a defect free texture. Shear reversal induced crisscross structure in both LCPs and SALS showed asymmetric “butterfly” patterns revealing a tilted microstructure. Shear rheometry in the nematic phase revealed that: (i) PSHQ10 displayed a terminal and rubbery regime, indicating the existence of entanglements; (ii) TPB10 exhibited only a liquid‐like behavior where G″ ≫ G′. The rubbery modulus Ge of PSHQ10 was ca. five times higher in the isotropic than in the nematic phase indicating that a change of conformation is involved in the nematic‐to‐isotropic transition. This study has resulted in correlations between rheological properties and shear‐induced microstructure thus developing new insights into their underlying deformation mechanisms.

Rheological behavior of molecular vs network chalcogenide supercooled liquids.

The viscoelastic behavior of supercooled glass-forming liquids along the binary join As4S3-GeS2 with As4S3 contents varying from 81.25 to 9 mol. % and correspondingly with structures varying from predominantly molecular to a three-dimensional tetrahedral network is studied by small-amplitude oscillatory shear parallel plate rheometry. The storage shear modulus G' shows a scaling behavior of G'(ω) ∼ ωn in the terminal (low-frequency) regime, where n varies between 1 and 2 and shows an increasingly anomalous departure from the expected value of 2 (Maxwell scaling) with increasing molecule content. A concomitant departure from the Maxwell scaling is also observed for the loss modulus G″ at frequencies above the G'-G″ crossover. On the other hand, the variation in the phase angle δ with the complex modulus indicates that the molecular liquid does not display a purely viscous response even at the lowest frequencies. These results, combined with an analysis of the relaxation spectra of these liquids, suggest that the anomalous behavior of molecular liquids may be linked to their rather broad relaxation spectrum and the presence of slow relaxation processes associated with molecular clusters. Additionally, these liquids are also characterized by a wide high-frequency plateau in the relaxation spectral density that can be linked to the rotational dynamics of the constituent molecules. Such fundamental differences between the rheological behavior of molecular and network liquids may explain the significantly higher fragility of the former.

Universality of steady shear flow of Rouse melts

The data set of steady and transient shear data reported by Santangelo and Roland Journal of Rheology 45: 583–594, (2001) in the nonlinear range of shear rates of an unentangled polystyrene melt PS13K with a molar mass of 13.7 kDa is analysed by using the single integral constitutive equation approach developed by Narimissa and Wagner Journal of Rheology 64:129–140, (2020) for elongational and shear flow of Rouse melts. We compare model predictions with the steady-state, stress growth, and stress relaxation data after start-up shear flows. In characterising the linear-viscoelastic relaxation behaviour, we consider that in the vicinity of the glass transition temperature, Rouse modes and glassy modes are inseparable, and we model the terminal regime of PS13K by effective Rouse modes. Excellent agreement is achieved between model predictions and shear viscosity data, and good agreement with first normal stress coefficient data. In particular, the shear viscosity data of PS13K as well as of two polystyrene melts with M = 10.5 kDa and M = 9.8 kDa investigated by Stratton Macromolecules 5 (3): 304–310, (1972) agree quantitatively with the universal mastercurve predicted by Narimissa and Wagner for unentangled melts, and approach a scaling of Wi−1/2at sufficiently high Weissenberg numbers Wi. Some deviations between model predictions and data are seen for stress growth and stress relaxation of shear stress and first normal stress difference, which may be attributed to limitations of the experimental data, and may also indicate limitations of the model due to the complex interactions of Rouse modes and glassy modes in the vicinity of the glass transition temperature.Graphical abstract

Effect of pressure of pulsating water jet moving along stair trajectory on erosion depth, surface morphology and microhardness

The objective of the study is to investigate the erosion effects of periodic water clusters interacting with flat AISI 304 surface to evaluate erosion regimes, surface morphology and variation in the subsurface microhardness below the disintegrated grooves. Stairs trajectory within the standoff distance 5–101 mm with step height 2 mm and length 20 mm was used. The pulsating water jet was changed by varying the supply pressure of the liquid from 40 MPa–100 MPa, at a traverse speed of 5 mm/s which is equivalent to constant impingement distribution of 4000 impingements per mm. Morphology of the PWJ defines different erosion regimes based on specific value of supply pressure and standoff distance. Increasing of supply pressure shifts prevalence of both impact pressure and stagnation pressure at higher standoff distances leading to the delay in formation of incubation, acceleration, culmination, depletion, and termination regime. Impinged surface features such as micro-pits, craters, upheaved surface and isolated voids near the periphery of the trace were observed along with PWJ footprint. The values of the micro-hardness showed strengthening of material subsurface at selected standoff distances. At certain standoff distance effect of supply pressure is inverse to micro-hardness values. Determination of studied technological parameters can be used for effective surface peening operation at v > 5 mm and surface activation of the metals. Results opens a new avenue for determination of erosion resistance of material.

Self-Disinfecting Copper Beds Sustain Terminal Cleaning and Disinfection Effects throughout Patient Care.

Microbial burden associated with near-patient touch surfaces results in a greater risk of health care-associated infections (HAIs). Acute care beds may be a critical fomite, as traditional plastic surfaces harbor the highest concentrations of bacteria associated with high-touch surfaces in a hospital room's patient zone. Five high-touch intensive care unit (ICU) bed surfaces encountered by patients, health care workers, and visitors were monitored by routine culture to assess the effect U.S. Environmental Protection Agency (U.S. EPA)-registered antimicrobial copper materials have on the microbial burden. Despite both daily and discharge cleaning and disinfection, each control bed's plastic surfaces exceeded bacterial concentrations recommended subsequent to terminal cleaning and disinfection (TC&D) of 2.5 aerobic CFU/cm2 Beds with self-disinfecting (copper) surfaces harbored significantly fewer bacteria throughout the patient stay than control beds, at levels below those considered to increase the likelihood of HAIs. With adherence to routine daily and terminal cleaning regimes throughout the study, the copper alloy surfaces neither tarnished nor required additional cleaning or special maintenance. Beds encapsulated with U.S. EPA-registered antimicrobial copper materials were found to sustain the microbial burden below the TC&D risk threshold levels throughout the patient stay, suggesting that outfitting acute care beds with such materials may be an important supplement to controlling the concentration of infectious agents and thereby potentially reducing the overall HAI risk.IMPORTANCE Despite cleaning efforts of environmental service teams and substantial compliance with hand hygiene best practices, the microbial burden in patient care settings often exceeds concentrations at which transfer to patients represents a substantial acquisition risk for health care-associated infections (HAIs). Approaches to limit HAI risk have relied on designing health care equipment and furnishings that are easier to clean and/or the use of no-touch disinfection interventions such as germicidal UV irradiation or vapor deposition of hydrogen peroxide. In a clinical trial evaluating the largest fomite in the patient care setting, the bed, a bed was encapsulated with continuously disinfecting antimicrobial copper surfaces, which reduced the bacteria on surfaces by 94% and sustained the microbial burden below the terminal cleaning and disinfection risk threshold throughout the patient's stay. Such an intervention, which continuously limits microbes on high-touch surfaces, should be studied in a broader range of health care settings to determine its potential long-range efficacy for reducing HAI.

Nonlinear rheological behavior of gelatin gels: In situ gels and individual gel layers filled with hard particles

HypothesisPreviously, we examined the impact of two preparation procedures on the mechanical properties of native gelatin gels. In this work, we extend our research by considering hard spherical particles as fillers. We expect that the presence of these fillers significantly affects the viscoelastic response. ExperimentsWe prepared fresh in situ gels and individual gel layers filled with glass micro-beads up to 1% w/w and aged for 24 and 48 h. The hydrogels were made of 3 and 5% w/w gelatin at 5 °C, and we performed large amplitude oscillatory shear (LAOS) tests. We analyzed the intracycle linear and nonlinear response using normalized Lissajous-Bowditch curves. Utilizing the MITlaos software, we decomposed the total intracycle stress into elastic and viscous contributions and calculated the Chebyshev harmonics coefficients. FindingsThe fresh in situ gels exhibit severe progressive stiffening during the strain sweeps and a subsequent sharp decrease of both moduli. The filled layers show smoother yielding than the in situ gels. The fillers increase the dynamic moduli, affect the terminal LAOS regime, and enhance the intracycle nonlinearities at higher concentrations. The Lissajous-Bowditch curves of the aged layers indicate elastoplastic behavior, which is more pronounced for the 48 h filled gel layer than the native counterpart.

Microrheological study of PVA/borax physical gels: Effect of chain length and elastic reinforcement by sodium hydroxide addition

In the present work, we have applied DWS-based microrheology to study poly (vinyl alcohol) (PVA) + borax physically cross-linked gels which are typically “living” networks. At observation times longer than the lifetime of the network connection (non-covalent reversible links based on hydrogen bonds) the system flows, but exhibits elastic character at shorter time scales. We determined gel points Cb* in borax concentration at ~27 °C using viscoelastic loss tangent uniqueness method for two molecular masses of PVA (89,000 and 205,000 g mol−1) at polymer concentration CP = 4.4% below Ce. We verified that loss tangent independency of frequency is equivalent to the Winter-Chambon (W-C) law in the critical regime for elastic and viscous moduli: G′ ∝ G″~ωn. The continuity between the terminal flow regime ω <100 rd/s and the high frequency W-C one is established allowing to cover [~1–105 rad/s]. The proportionality found between Cb* and the molecular mass needs to be verified on other chain lengths. We then studied the effect of sodium hydroxide addition to PVA/borax systems since borax dissociation in water gives borate ions (responsible for establishing “elastic active” reversible reticulations between PVA chains) and boric acid which doesn't contribute to elasticity building. The results show that NaOH addition (i.e. pH increase) transforms almost all boric acid, already attached to PVA chains by monodiol complexation, into borate ions leading to didiol complexation. This, therefore, induces gelation for systems initially in sol state or elastically reinforces physical gels. Lifetime measurements of physical reticulations with and without NaOH reveal a change in borax dependency, possibly due to the increase of the association rate compared to the dissociation one i.e. an enhancement in the reforming-breaking ratio rates.

Effect of intermolecular interactions on the viscoelastic behavior of unentangled polyamide melts

We have studied the effect of ionic interactions on the dynamics of polyamides in the molten state, both in the terminal relaxation regime and in the vicinity of the glass transition, using amorphous and end-blocked PA 6I polyamides. The density of interactions was varied by substituting different amounts of 5-sulfoisophthalic acid lithium salt. First, the effect of molecular weight distribution for PA 6I was studied in detail. As the molecular weight increased, a clear rubbery plateau appeared and the longest relaxation time was shifted to lower frequencies. The reference temperature used to construct the master curves was chosen to be Tg so that all master curves overlap in the glassy region. The main effect of introducing ionic groups was the increase in Tg by 10–40 °C. For comparable molecular weights, master curves of PA 6I and substituted polyamides overlapped in the complete range of frequencies when using an appropriate reference temperature. The reference temperature, at least up to 15 mol. %, seems to be correlated to the width of the glass transition that increases with increasing ionic content. The Rouse model was used to fit the viscoelastic response of all unentangled polyamides, showing that hydrogen bonds and/or ionic interactions did not play a role in the viscoelasticity of these polyamides in the molten state. The dynamic fragility indices determined from William–Landel–Ferry parameters decrease with increasing ionic content. Small-angle x-ray scattering measurements show that no segregation of ionic domains occurs in unentangled ionic copolyamides, while entangled ionic copolyamides show only weak segregation.We have studied the effect of ionic interactions on the dynamics of polyamides in the molten state, both in the terminal relaxation regime and in the vicinity of the glass transition, using amorphous and end-blocked PA 6I polyamides. The density of interactions was varied by substituting different amounts of 5-sulfoisophthalic acid lithium salt. First, the effect of molecular weight distribution for PA 6I was studied in detail. As the molecular weight increased, a clear rubbery plateau appeared and the longest relaxation time was shifted to lower frequencies. The reference temperature used to construct the master curves was chosen to be Tg so that all master curves overlap in the glassy region. The main effect of introducing ionic groups was the increase in Tg by 10–40 °C. For comparable molecular weights, master curves of PA 6I and substituted polyamides overlapped in the complete range of frequencies when using an appropriate reference temperature. The reference temperature, at least up to 15 mol. %, se...

Economic Opportunity and Strategic Dilemma in Colonial Development: Britain, Japan and Malaya’s Iron Ore, 1920s to 1950s

The importance of Malaya’s iron ore has been downgraded from both an economic and geo-strategic perspective. Under colonial administration, iron-ore mines owned and operated by Japanese enterprises and individuals, and the resultant exports to Japan, were central to economic development in the east-coast states of the peninsula. But this also made British Malaya a site of international contestation as Anglo–Japanese relations became estranged in the run up to World War Two in Asia and the Pacific. British administrators had to balance economic opportunity with strategic cost, and securing iron ore was a key factor in Japan’s occupation of Malaya after December 1941. Pre-war dilemmas resurfaced after the Pacific War. Nevertheless, exports to Japan both resumed and expanded. The combination of Japanese regeneration and Southeast Asian development in the context of the Cold War once again placed Malaya’s iron ore at the centre of geo-strategic and economic priorities for what was fast becoming the terminal colonial regime. This analysis is supported by the wide range of primary sources consulted from the records of the Malayan state administrations to British intelligence assessments, as well as the evaluation of quantitative data.

POSS driven chain disentanglements, decreased the melt viscosity and reduced O2 transmission in polyethylene

Untethered octaisobutyl-polyhedral oligomeric silsesquioxane (POSS) molecularly dispersed into low-density polyethylene (LDPE) induced entanglement dilation and dynamics acceleration, reduced the melt viscosity and acted as nano-plugs thus reducing O2 transmission. POSS nanoparticles were melt mixed with LDPE using a twin-screw extruder and adding up to 10 wt% POSS. High-resolution transmission electron microscopy (HRTEM) demonstrated excellent dispersion of POSS into LDPE, at a single POSS unit at low concentrations. The dynamics was probed by shear rheometry constructing master curves at Tref = 190 °C using the time-temperature superposition (TTS) principle. In the low concentration range (cPOSS<5 wt%) the terminal (liquid-like) regime showed that POSS nanoparticles induced accelerated dynamics, decreasing the terminal relaxation time τt. This translated into an anomalous, non-Einstein, reduction of viscosity. Increasing concentration to c∼10 wt% POSS formed clusters of ca. 10 units, the terminal relaxation time increased and the melt exhibited higher viscosity than the neat LDPE, as predicted for particulate filled suspensions. Therefore, at low concentrations the size of POSS (ca. 1.5 nm), smaller than entanglement mesh size of polyethylene (tube diameter dt = 3.28 nm), would drive chain disentanglement and be responsible for the dilution effect. At higher concentrations POSS aggregated and therefore increased the viscosity of the nanocomposite, a behavior akin to Einstein's prediction. Strikingly, the intercalation of POSS into the entangled molecular mesh reduced oxygen transmission, suggesting that POSS acted as nano-plugs by filling the voids in the entangled mesh.