Chain Relaxation Research Articles

Structural Characteristics of Fibrillar Crystals in Uniaxially Stretched Isotactic Polypropylene Dominated by Temperature and Strain

AbstractThe spherulitic morphology of isotactic polypropylene can be transformed into the oriented fibrillar morphology through hot stretching processes with varying temperature (Ts) or altering strain (εt). The effects of Ts and εt on the structural characteristics of fibrillar crystals are comprehensively investigated with respect to crystal orientation, long periodic spacing, lamellar thickness (Lc), crystallinity (Xc), melting point, and chain relaxation behavior. Small‐angle X‐ray scattering patterns illustrate that the fibrillar crystals consist of alternated stacks of crystalline lamellae and amorphous layers. High Ts leads to a low orientation degree of lamellae, whereas large εt facilitates a high orientation level. The Xc and mean Lc are improved continuously with the increasing of Ts or εt, indicating a stretching‐enhanced crystallization behavior driven by the two factors. The endothermic profiles reveal that new chain‐folded lamellae with relatively thinner thickness form during the hot stretching process. The formation of thinner lamellae is dominated by the melting–recrystallization mechanism. This work would provide guidance for optimizing process conditions to manipulate the microstructure of hot‐stretched semicrystalline polymers.

Interplay between Chain Relaxation Time and Melt Crystallization Time in Microinjection Molding of Polyoxymethylene

Interplay between Chain Relaxation Time and Melt Crystallization Time in Microinjection Molding of Polyoxymethylene

Polysaccharide Hydro-Gel from Arid Plant Seed

There is a limited information on the structure and gelation process in mucilagepolysaccharides because of the complex conformation and non-homogeneity of the constituent polysaccharides that typically show large molecular weights in excess of 1 MDa. Only a few works have been done in this area of characterizing mucilage structure and rheological behaviour for different gel solutions. Exploring this area would surely open the new horizons for drug delivery, soil conditioning and pharmaceutical industry. In this study, we aimed to extract polysaccharides found in P.ovata Seed (Isabgol) via mucilage extraction controlling molecular weight of polymers in hot water extraction stage and tried to formulate gel formation dissolving different conc. of extracted freeze-dried material along with the characterization of gel network through Rheological studies and Dynamic Light Scattering (DLS) studies in different solvents. The study confirmed the relationship between molecular associations, chain relaxation time and entanglement behaviour of P. ovata hot water extracted (HW) solutions using polydispersity index and shear rheology. Moreover, two power law regions obtained for different concentration solution are of great importance because of their wide applications in the industrial prospective.

Solid polymer electrolytes with fragment-separated microphase for advanced Li ion transport and highly stable lithium metal batteries

Solid polymer electrolytes (SPEs) with high ionic conductivity and good interfacial compatibility are one of the main factors constraining the performance of lithium metal batteries. However, the most common PEO (Polyethylene oxide)-based SPEs always lack of considerable relaxation of PEO segments, and therefore fail to apply into practical situations. Herein a novel method to prepare fragment-separated epoxy polymer electrolytes (FSEEs) characterized with fragment-separated microphase patterns via a rigid-and flexible monomers cross-link reaction is presented to enhance the overall SPE performance. The fragment-separated microphase can separate the PEO chains in framework and improve the relaxation of PEO chains, optimize the Li+ transport, where the ionic conductivity can be up to 0.56 mS cm−1 at 25 °C. The fragment-separated Li+ transport mechanism in FSEEs also provide better interfacial compatibility than that of PEO due to the even Li+ distribution at interface. The utilization of FSEE-S in Li/Li symmetric cells demonstrated a stable voltage plateau after cycling for 2000 h. Additionally, Li/LFP batteries exhibit favorable 0.5 C performance under 45 °C, lasting over 600 h and exceeding 150 cycles. This study offers a promising approach to SPE design, which may catalyze further exploration of the role of microphase in SPE performance.

Macromolecular Crowding Effect on Chitosan-Hyaluronic Acid Complexation and the Activity of Encapsulated Catalase.

The associative phase separation of charged biomacromolecules plays a key role in many biophysical events that take place in crowded intracellular environments. Such natural polyelectrolyte complexation and phase separation often occur at nonstoichiometric charge ratios with the incorporation of bioactive proteins, which is not studied as extensively as those complexations at stoichiometric ratios. In this work, we investigated how the addition of a crowding agent (polyethylene glycol, PEG) affected the complexation between chitosan (CS) and hyaluronic acid (HA), especially at nonstoichiometric ratios, and the encapsulation of enzyme (catalase, CAT) by the colloidal complexes. The crowded environment promoted colloidal phase separation at low charge ratios, forming complexes with increased colloidal and dissolution stability, which resulted in a smaller size and polydispersity (PDI). The binding isotherms revealed that the addition of PEG greatly enhanced the ion-pairing strength (with increased ion-pairing equilibrium constant Ka from 4.92 × 104 without PEG to 1.08 × 106 with 200 g/L PEG) and switched the coacervation from endothermic to exothermic, which explained the promoted complexation and phase separation. At the stoichiometric charge ratio, the enhanced CS-HA interaction in crowded media generated a more solid-like coacervate phase with a denser network, slower chain relaxation, and higher modulus. Moreover, both crowding and complex encapsulation enhanced the activity and catalytic efficiency of CAT, represented by a 2-fold increase in catalytic efficiency (Kcat/Km) under 100 g/L PEG crowding and CS-HA complex encapsulation. This is likely due to the lower polarity in the microenvironment surrounding the enzyme molecules. By a systematic investigation of both nonstoichiometric and stoichiometric charge ratios under macromolecular crowding, this work provided new insights into the complexation between natural polyelectrolytes in a scenario closer to an intracellular environment.

Stearate‐Modified Montmorillonite and Polylactic Acid Synergistically Enhanced the Microcellular of Thermoplastic Polyurethane Elastomer Foam and Improved Its Dimensional Stability

Thermoplastic polyurethane (TPU) elastomer is a commonly used material for foam preparation. However, its application is hindered by significant shrinkage behavior. To reduce the foam shrinkage, polylactic acid (PLA) and calcium stearate‐modified montmorillonite (MMT‐St) are incorporated into the polyurethane matrix. The TPU/PLA/MMT‐St composite foams are formed using an intermittent supercritical carbon‐dioxide‐foaming process. The addition of PLA inhibits the relaxation of TPU chain segments, while the introduction of calcium stearate (GaSt2) improves the dispersion of MMT within the matrix, increases the nucleation sites of cell growth, and improves cell structure. The cell diameter decreases from 10 to 4 μm, while the cell density increased from 4.1 × 1012 to 15.8 × 1012 cell cm−3. In addition, the cell distribution range narrows from 4–19 to 2–6 μm. Incorporating MMT‐St increases the crystallinity and thermal stability of TPU/PLA, mitigates chain relaxation at room temperature, and enhances foam dimensional stability. As a result, this leads to a decrease in shrinkage rate from 27.5% to 5%. As both matrices of the material are thermoplastic, this composite foam possesses recyclability with enhanced material utilization.

Impact of Sacrificial Hydrogen Bonds on the Structure and Properties of Rubber Materials: Insights from All-Atom Molecular Dynamics Simulations.

The inclusion of sacrificial hydrogen bonds is crucial for advancing high-performance rubber materials. However, the molecular mechanisms governing the impact of these bonds on material properties remain unclear, hindering progress in advanced rubber material research. This study employed all-atom molecular dynamics simulations to thoroughly investigate how hydrogen bonds affect the structure, dynamics, mechanics, and linear viscoelasticity of rubber materials. As the modified repeating unit ratio (β) increased, both interchain and intrachain hydrogen bond content rose, with interchain bonds playing a predominant role. This physical cross-linking network formed through interchain hydrogen bonds restricts molecular chain movement and relaxation and raises the glass transition temperature of rubber. Within a certain content of hydrogen bonds, the mechanical strength increases with increasing β. However, further increasing β leads to a subsequent decrease in the mechanical performance. Optimal mechanical properties were observed at β = 6%. On the other hand, a higher β value yields an elevated stress relaxation modulus and an extended stress relaxation plateau, signifying a more complex hydrogen-bond cross-linking network. Additionally, higher β increases the storage modulus, loss modulus, and complex viscosity while reducing the loss factor. In summary, this study successfully established the relationship between the structure and properties of natural rubber containing hydrogen bonds, providing a scientific foundation for the design of high-performance rubber materials.

Structuring restricted amorphous molecular chains in the reinforced cellulose film by uniaxial stretching

The construction of the preferred orientation structure by stretching is an efficient strategy to fabricate high-performance cellulose film and it is still an open issue whether crystalline structure or amorphous molecular chain is the key factor in determining the enhanced mechanical performance. Herein, uniaxial stretching with constant width followed by drying in a stretching state was carried out to cellulose hydrogels with physical and chemical double cross-linking networks, achieving high-performance regenerated cellulose films (RCFs) with an impressive tensile strength of 154.5 MPa and an elastic modulus of 5.4 GPa. The hierarchical structure of RCFs during uniaxial stretching and drying was systematically characterized from micro- to nanoscale, including microscopic morphology, crystalline structure as well as relaxation behavior at a molecular level. The two-dimensional correlation spectra of dynamic mechanical analysis and Havriliak-Negami fitting results verified that the enhanced mechanical properties of RCFs were mainly attributed to the stretch-induced tight packing and restricted relaxation of amorphous molecular chains. The new insight concerning the contribution of molecular chains in the amorphous region to the enhancement of mechanical performance for RCFs is expected to provide valuable guidance for designing and fabricating high-performance eco-friendly cellulose-based films.

Multiscale simulations of viscoelastic fluids in complex geometries using a finitely extensible nonlinear elastic transient network model

This paper presents a novel implementation of a numerical scheme for predicting complex flows of viscoelastic fluids using a finitely extensible nonlinear elastic (FENE) transient network model. This model extends the FENE model by incorporating chain interactions and accounting for the way in which the maximum chain length, drag, and relaxation time are influenced by entanglement and disentanglement processes. Three different initial networks are considered (disentanglement, entanglement, and aleatory), and the influence of variables such as the kinetic rate constants, elasticity, and chain length on the microstate concentration, stresses, and drag force is investigated. It is shown that although the concentrations of the microstates are independent of the Weissenberg number and the maximum extension length, the stresses and hence the drag are influenced by them.

Investigation on the Correlation between Biaxial Stretching Process and Macroscopic Properties of BOPA6 Film.

Biaxially oriented polyamide 6 (BOPA6) films were prepared by extrusion casting and biaxial stretching with polyamide 6. The effects of different biaxially oriented on the macroscopic properties of BOPA6 were investigated by characterizing the rheological, crystallization, optical, barrier and mechanical properties. The results show that the increase of stretching temperature leads to the diffusion and regular stacking rate of BOPA6 chain segments towards crystal nuclei increases, the relative crystallinity increases, reaching 27.87% at 180 °C, and the mechanical strength and optical performance decrease. Heat-induced crystallization promotes the transformation of β-crystals to α-crystals in BOPA6, resulting in a more perfect crystalline structure and enhancing oxygen barrier properties. BOPA6 chains are oriented, and strain-induced crystallization (SIC) occurs during the biaxial stretching. Further increasing the stretch ratio, the relative crystallinity increased to 30.34%. The machine direction (MD) and transverse direction (TD) tensile strength of BOPA6 (B-33) are nearly two times higher than the unstretched film, reaching 134.33 MPa and 155.28 MPa, respectively. In addition, the permeation decreases to 57.61 cc·mil/(m2 day), and the oxygen barrier performance has improved by nearly 30% compared to the sample B-22. BOPA6 has a high storage modulus at a high stretching rate (300%/s). Rapid chain relaxation would promote the molecular chain disorientation, destroy the entangled network of the molecular chain, and lead to a decrease in tensile strength, reducing to about 110 MPa.

Pullulan-Based Spray-Dried Mucoadhesive Microparticles for Sustained Oromucosal Drug Delivery.

Mucoadhesive microparticles for oromucosal drug delivery offer several advantages, including intimate contact with the mucosa, delivery to less accessible regions, extended residence time, sustained drug release, reduced irritation, and improved patient compliance. In this study, pullulan was used to prepare mucoadhesive spray-dried microparticles for delivering benzydamine hydrochloride (BZH) to oral mucosa. The BZH-pullulan spray-dried microparticles had a mean size of <25 μm with an angle of repose values between 25.8-36.6°. Pullulan markedly extended drug-release time to >180 min, ~9 times greater than the duration (i.e., 20 min) reportedly achieved by chitosan. Kinetic analysis showed the drug-release rate was concentration dependent and jointly controlled by drug diffusion and polymer chain relaxation. Further, pullulan was mucoadhesive and was able to retain up to 78.8% w/w of microencapsulated gold nanoparticle probes at the mucosal membrane. These data strongly suggest that BZH-pullulan microparticles have great potential for oromucosal drug delivery, by providing elongated residence time in situ and sustained drug release for the treatment of local diseases.

Dynamical relaxation of a long-range Kitaev chain

Dynamical relaxation of a long-range Kitaev chain

Chitosan hydrogels enriched with bioactive phloroglucinol for controlled drug diffusion and potential wound healing

We report a facile strategy to prepare chitosan (CS) hydrogels that eliminates the need for chemical crosslinking for advanced biomedical therapies. This approach gives controlled properties to the hydrogels by incorporating a natural bioactive phenolic compound, phloroglucinol (PG), into their microstructure. The adsorption of PG onto CS chains enhanced the hydrogels' antioxidant activity by up to 25 % and resulted in a denser, more entangled structure, reducing the pore size by 59 μm while maintaining porosity above 94 %. This allowed us to finely adjust pore size and swelling capacity. These structural properties make these hydrogels well-suited for wound healing dressings, promoting fibroblast proliferation and exhibiting excellent hemocompatibility. Furthermore, to ensure the versatility of these hydrogels, herein, we demonstrate their potential as drug delivery systems, particularly for dermal infections. The drug release can be controlled by a combination of drug diffusion through the swollen hydrogel and relaxation of the CS chains. In summary, our hydrogels leverage the synergistic effects of CS's antibacterial and antifungal properties with PG's antimicrobial and anti-inflammatory attributes, positioning them as promising candidates for biomedical and pharmaceutical applications, more specifically in advanced wound healing therapies with local drug delivery.

Effect of short-chain branches in high-molecular-weight component on tensile properties of polyethylene solids

The effects of short-chain branches (SCBs) in the high-molecular-weight (MW) chains on the solid-state tensile properties of polyethylene (PE) have been investigated using blend samples of linear PE with a polydisperse MW distribution and PE with very narrow MW distribution and SCBs with four carbons. The introduction of SCBs in the high-MW chains had almost no effect on the crystalline structure, but relaxation of the interlamellar amorphous chains was only activated by blending the high-MW chains with a large fraction of SCBs (2.0 wt%). The blends of PE with 0.4 wt% SCBs resulted in significant increases in the strain-hardening modulus and strength without greatly decreasing the strain at break. The improvement in the strain-hardening modulus and strength by adding a small amount of SCBs (0.4 wt%) was significant compared with the improvement by adding linear PE or branched PE with a large amount of SCBs (2.0 wt%). For a small amount of SCBs, the SCBs will be incorporated into the crystalline structure, which enhances the fragmentation and subsequent orientation of the crystalline structure under yielding deformation. Consequently, blending the branched PE with a small amount of SCBs in the high-MW chains leads to significant improvement of the tensile properties.

Biomimetic shape-morphing actuators with controlled bending deformation and photo-mediated 3D shape programming for flexible smart devices

Biomimetic soft actuators with programmable morphing behaviors and remote actuations have drawn significant attention in recent years. In this work, we present the network architecture of a four-arm crystalline shape memory polymer actuator via efficient thiol-acrylate coupling of polycaprolactone precursor bearing crosslinkable methacrylate end-groups with pentaerythritol, followed by post-approach surface chemistry of polydopamine (PDA) nanospheres coating. The excellent photothermal transfer efficiency of the coated PDA can enable rapid temperature increase over the transition temperature of polymeric matrix and optically drive the shape morphings. Combined with built-in internal stress, anisotropic polymer chain relaxation of the strained thin actuator film would generate asymmetric contractions and thus causes controlled bending deformations via surface directing of an 532 nm green laser. Beyond this, the dynamic nature of transesterification reaction also provides the actuator film with reprogrammability. Remarkably, the morphing behaviors of a flexible plane film into various soft robots including 3D airplane and chiral helix were achieved under a 300 mW/cm2 simulated light irradiation. The results presented in this study clearly provide a new method for designing actuators with spatiotemporally programmable morphing behaviors and will pave way for the construction of flexible smart devices.

New Carriers for Bioadhesive Gastroretentive Drug Delivery Systems Based on Eudragit® EPO/Eudragit® L100 Interpolyelectrolyte Complexes

The aim of this study was the analysis of interpolyelectrolyte complexes (IPECs) based on Eudragit® EPO and Eudragit® L100 as prospective carriers for gastroretentive drug delivery systems (GRDDS) using two model drugs: metronidazole (MZ) and acyclovir (ACR). Eudragit® EPO/L100 IPECs with different pH concentrations were characterized by different degrees of swelling in mimicking fasted stomach medium (0.1 M HCl) and saved their shape for 6 h. The microenvironmental changes in IPEC structures in acidic medium were investigated using FT-IR spectroscopy, thermal and elemental analysis. IPEC samples showed bioadhesive properties that were not significantly different from the positive control (Carbopol) in the test with the mucin compacts. The release rate of metronidazole (class I BCS) from IPEC matrices increased with the increasing degree of swelling. IPEC 1 provided 49.62 ± 6.20% and IPEC 2 reached 87.69 ± 5.15% of metronidazole release after 6 h in mimicking fasted stomach medium (0.1 M HCl). The total amount of released acyclovir (class III BCS) from IPEC 1 was 25.76 ± 5.67% and from IPEC 2 was 21.48 ± 5.00%. Release of both drugs was controlled by relaxation of polymeric chains in matrices according to the Peppas–Sahlin model. According to the received results, investigated interpolymer complexes are prospects for further evaluation as carriers for gastroretentive bioadhesive systems.

Functionally responsive hydrogels with salt-alkali sensitivity effectively target soil amelioration

The long-standing crisis of soil salinization and alkalization poses a significant challenge to global agricultural development. High soil salinity-alkalinity, water dispersion, and nutrient loss present major hurdles to soil improvement. Novel environmentally friendly gels have demonstrated excellent water retention and slow-release capabilities in agricultural enhancement. However, their application for improving saline-alkali soil is both scarce and competitive. This study proposes a new strategy for regulating saline-alkali soil using gel-coated controlled-release soil modifiers (CWR-SRMs), where radical-polymerized gels are embedded on the surface of composite gel beads through spray coating. Characterization and performance analysis reveal that the three-dimensional spatial network structure rich in hydrophilic groups exhibits good thermal stability (first-stage weight loss temperature of 257.7 °C in thermogravimetric analysis) and encapsulation efficiency for fulvic acid‑potassium (FA-K), which can enhance soil quality in saline-alkali environments. The molecular chain relaxation under saline-alkali conditions promotes a synergistic effect of swelling and slow release, endowing it with qualifications as a water reservoir, Ca2+ source unit, and slow-release body. The results of a 6 weeks incubation experiment on 0–20 cm saline-alkaline soil with different application gradients showed that the gradient content had a significant effect on the soil improvement effect. Specifically, the T2 (the dosage accounted for 1 % of soil mass) treatment significantly increases water retention (30 % ~ 90 %), and nutrient levels (30 % ~ 50 %), while significantly decreasing soil sodium colloid content (30 % ~ 60 %) and soil pH (10 % ~ 15 %). Furthermore, PCA analysis indicates that the addition of 1 % CWR-SRMs as amendments can significantly adjust the negative aspects of soil salinity and alkalinity. This highlights the excellent applicability of CWR-SRMs in improving saline-alkali agricultural ecosystems, demonstrating the potential value of novel environmentally friendly gels as an alternative solution for soil challenges persistently affected by adverse salinity and alkalinity.

Biodegradable photo-crosslinked polycaprolactone/polydopamine elastomers with excellent light driven programmable shape memory and chemical degradation properties

Fabrication of biodegradable shape memory polymer with remotely controllable shape actuation is of great significance in the biomedical field but remains challenging. Herein, we present a simple strategy to fabricate a monolayer-based stretchable and mechanically robust polycaprolactone/polydopamine elastomer via efficient thiol-ene click chemistry. The resultant elastomers exhibit desirable photothermal transfer efficiency and can enable rapid temperature increase over the melting temperature of polymeric matrix, and quantitative results demonstrate that the crosslinked film exhibited excellent shape memory properties with shape fixity (Rf) and shape recovery ratios (Rr) approaching 92.3 % and 95.6 %, respectively. Combined with photo stimuli, anisotropic polymer chain relaxation of the prestretched film can generate asymmetric contractions and eventually give rise to ut out-of-plane bending actuations upon photo stimulation, meanwhile, numerical simulation reveals the interaction mechanism of light with film. Beyond this, we further demonstrate that the bending angle is correlated with the parameters of prestretch strain, film thickness as well as irradiation time, and the maximum value can reach 158° with prestretch strain of 200 % and film thickness of 0.3 mm. In particular, the bent structures could be reversibly deformed into plane state via photo-directed corresponding opposite surfaces. Remarkably, the in vitro degradation properties of the elastomers on PBS-T buffer solutions demonstrated that the degradation was composed of induction stage and acceleration stage. This work will pave way for designing biodegradable light-induced shape memory materials toward biomedical device fields and so on.

Multiple relaxation dynamics under electric field enables tunable viscoelastic response of poly(methyl methacrylate) above glass transition temperature.

Optimizing mechanical performance is crucial for the practical utilization of stimuli-responsive polymers, while complex viscous and elastic behavior hinders a deep understanding of functional polymers under external field excitation. Here, we demonstrate the in situ dynamic and static mechanical responses under electric stimuli of poly(methyl methacrylate) (PMMA) above glass transition temperature (Tg) by applying a direct current electric field vertically to the mechanical loading. The results show that the electro-mechanical response of PMMA is directly correlated to chain relaxation modes with different length scales: for local segments, polarization provides resistance for molecular motion, manifested by enhanced moduli, increased transient viscosity, and a wider linear viscoelastic range, whereas in a larger spatial range, polarization-induced conformation change causes faster relaxation, reduced elastic modulus, and a lowered modulus plateau. Moreover, flow viscosity is reduced because of weaker friction between chain segments under polarization. Our results suggest effective strategies for precisely tuning the viscoelastic behavior of polymers above Tg through electric stimuli.

One-pot preparing EVA elastomer auxetic foam through stress relaxation of molecular chains by heat annealing

It is a challenge to prepare closed-cell auxetic foam by a simple method without destroying the properties of the foam. In this paper, auxetic foams were prepared directly from ethylene-vinyl acetate copolymer (EVA) resin in one step by annealing immediately after sc-CO2 foaming. The effects of annealing time and foaming temperature on the cell structure and Poisson's ratio were investigated using scanning electron microscopy (SEM) and Poisson's ratio testing. The reasons for the formation of EVA foams with negative Poisson's ratio were analyzed by dynamic mechanical thermal analysis (DMA) and differential scanning calorimetry (DSC). It was found that annealing treatment maintained the high movement ability of molecular chains and provided sufficient time for the molecular chains to relax, so that the stress generated by the foaming process can relax quickly and reach a new stable state, thus weakening the elastic shrinking of EVA foam. Under the action of the pressure difference between the inside and outside of the cell formed by the different diffusion rates of air and CO2, the cell concaves inward to produce a reentrant structure, and then the foam shape is fixed with the continuous improvement of crystallization. The obtained EVA auxetic foam has excellent resilience. Finally, it is proved that the method has good universality for the preparation of auxetic foams with most elastomers, so this study has important implications for the preparation of closed-cell elastomer polymer auxetic foams.