Strain-induced Crystallization Research Articles

Building Ultrastrong, Tough and Biodegradable Thermoplastic Elastomers from Multiblock Copolyesters Via a "Reserve-Release" Crystallization Strategy.

Simultaneously attaining high strength and toughness has been a significant challenge in designing thermoplastic elastomers, especially biodegradable ones. In this context, we present a class of biodegradable elastomers based on multiblock copolyesters that afford extraordinary strength, toughness, and low-strain resilience despite expedient chemical synthesis and sample processing. With the incorporation of the semi-crystalline soft block and the judicious selection of block periodicity, the thermoplastic materials feature low quiescent crystallinity ("reserve") albeit with vast potential for strain-induced crystallization ("release"), resulting in their significantly enhanced ultimate strength and energy-dissipating capabilities. Moreover, a breadth of mechanical responses of the materials - from reinforced elastomers to shape-memory materials to toughened thermoplastics - can be achieved by orthogonal variation of segment lengths and ratios. This work and the "reserve-release" crystallization strategy herein highlight the double crystalline multiblock chain architecture as a potential avenue towards reconciling the strength-toughness trade-off in thermoplastic elastomers and can possibly be extended to other biodegradable building blocks to deliver functional materials with diverse mechanical performances.

A bio-based, self-healable, conductive rubber film with oxidized cellulose nanofiber segregated network

Rubber composites are indispensable in all areas of our daily lives. However, the formation of permanent crosslinked networks in rubber materials makes it difficult to recycle, resulting in a non-negligible waste of resources. In this paper, a vulcanization-free, fully bio-sourced rubber composite was prepared by using oxidized natural rubber (oNR) and oxidized cellulose nanofibers (TOCFs). TOCFs are selectively dispersed between the latex particles to form a segregated network. Meanwhile, the formation of hydrogen-bonding between oxygenated polar groups of oNR and abundant hydroxyl and carboxyl groups of TOCFs improves their interfacial interactions. This special structure promotes strain-induced crystallization (SIC) behavior of oNR matrix, giving its tensile strength up to 14.7 MPa. Furthermore, the oNR/TOCFs film shows excellent self-healing efficiency (96 %) at 40 °C for 5 h. The hygroscopicity of the TOCFs segregated network can turn the oNR/TOCFs film to be a conductive film by regulating the absorbed water content. The film has high conductivity (0.05 S/m) at a water content of 8.99 wt%, and the resistance change (RV/R0) can be varied between 1–5.9 × 10−6 at a water content range of 0–8.99 wt%, which makes it have potential for a wide range of humidity monitoring applications.

Scalable Layered Heterogeneous Hydrogel Fibers with Strain-Induced Crystallization for Tough, Resilient, and Highly Conductive Soft Bioelectronics.

The advancement of soft bioelectronics hinges critically on the electromechanical properties of hydrogels. Despite ongoing research into diverse material and structural strategies to enhance these properties, producing hydrogels that are simultaneously tough, resilient, and highly conductive for long-term, dynamic physiological monitoring remains a formidable challenge. Here, a strategy utilizing scalable layered heterogeneous hydrogel fibers (LHHFs) is introduced that enables synergistic electromechanical modulation of hydrogels. High toughness (1.4MJ m-3) and resilience (over 92% recovery from 200% strain) of LHHFs are achieved through a damage-free toughening mechanism that involves dense long-chain entanglements and reversible strain-induced crystallization of sodium polyacrylate. The unique symmetrical layered structure of LHHFs, featuring distinct electrical and mechanical functional layers, facilitates the mixing of multi-walled carbon nanotubes to significantly enhance electrical conductivity (192.7 S m-1) without compromising toughness and resilience. Furthermore, high-performance LHHF capacitive iontronic strain/pressure sensors and epidermal electrodes are developed, capable of accurately and stably capturing biomechanical and bioelectrical signals from the human body under long-term, dynamic conditions. The LHHF offers a promising route for developing hydrogels with uniquely integrated electromechanical attributes, advancing practical wearable healthcare applications.

Crack tip stress intensification in strain-induced crystallized elastomer

Natural rubber (NR) exhibits strain-induced crystallization (SIC), enhancing tearing strength and crack resistance. However, the reinforcement mechanism along with nonuniform strain around a crack tip remains unclear. We reveal the nonuniform stress field around a crack tip using the DIC-based deformation field data and a hyperelasticity approach. A hyperelastic strain energy density function (W) is derived to be able to replicate stress-strain data across various deformations, encompassing equal and unequal biaxial, uniaxial, and pure shear stretching. These data cover the full range and magnitude of deformations around the crack tip. SIC significantly impacts the singular behaviors of strain and stress near the crack tip, causing a pronounced stress increase and strain decrease within the SIC zone that extends up to approximately 100 μm away from the crack tip. This results in a distinct crossover in singularity power-law index between the SIC zone and the fully amorphous zone. With increasing crack opening, the stress upturn intensifies, and the crossover shifts away from the crack tip due to SIC zone enlargement and local crystallinity increase. These findings deepen our understanding of the physics of SIC near crack tips and its reinforcement mechanism in strain-induced crystallizable soft solid materials.

Graphene Nanoplatelet Induced Microphase Separation in Poly(ether-block-amide)s

The inclusion of graphene nanoplatelets (GNP) in segmented block copolymers offers a route to manipulate microphase separation for tailoring the mechanical properties of thermoplastic elastomers. GNP loading, lateral size, surface chemistry, interactions with the copolymer hard (HS) and soft (SS) segments and the relative ratio of HS:SS determine the mechanical properties achievable. To test this hypothesis, two different GNPs with similar surface chemistry but which differed in lateral dimensions by one order of magnitude (GNP1, ∼2μm and GNP2, ∼20μm) were melt mixed with three different poly(ether-block-amide)s (PE-b-A)s with variable HS and SS content from high to low. The inclusion of the larger lateral sized GNP2 had a more pronounced effect on PE-b-A morphology as it was more effective at hindering SS chain mobility resulting in microphase separation and suppression of the glass transition temperature (Tg) of the PE-b-A with the largest SS content. At low loadings GNP2 preferentially locates to the HS region, inducing reorganisation of this phase resulting in increased microphase separation. Strain induced crystallisation (SIC) phenomena were also observed for the lowest HS content PE-b-A, behaviour not evident for the PE-b-A with the largest HS content as the SS are not long enough to allow SIC. Inclusion of GNP2 to the PE-b-A with the largest HS content resulted in the largest increase in Young’s modulus (E) of 46%, tensile strength (σ) of 37% and elongation at break (ε) of 53% relative to the unfilled polymer.

Strain softening of carbon black/isoprene rubber/nitrile rubber ternary vulcanizate nanocomposites

The influence of filler selective distribution on the strain softening behaviors (Payne and Mullins effects) has been seldom studied for ternary rubber nanocomposites. Herein selected distribution of carbon black (CB) in the CB/isoprene rubber (IR)/nitrile rubber (NBR) nanocomposites is regulated simply by varying compounding sequences. The location of CB does not significantly influence the Payne effects and slightly influences the Mullins effect in the first cyclic tension run. However, it influences the Mullins effect of the demullinized nanocomposites (in the third deformation runs) markedly, and the CB concentration in the crystallizable IR phase tends to improve damping ratio at high strains. The influence of CB selective localization on the Mullins effect involves the strain-induced crystallization of the IR phase. For tensile performances, the CB concentration in the IR and NBR phases tends to improve strain hardening at high strains and stress at low strains, respectively. This work provides a way to tailor the strain softening and tensile behaviors of the ternary rubber nanocomposites by control of the filler selective dispersion.

Control by the curing time of the strain induced crystallization and elastocaloric properties in natural rubber and natural/waste rubber blends

This study provides a systematic investigation of the influence of the curing time on strain-induced crystallization (SIC) and elastocaloric (eC) effect in peroxide-cured natural rubber (NR) and NR/ground tyre rubber (GTR) blends. To do so, materials were prepared with three distinct curing times (t10, t50, and t90). Results reveal that extended curing times correlate with increased network hardness and elastic modulus, promoting the number of elastically active chains. GTR fillers accelerate vulcanization but reduce final network chain density due to their partially devulcanized state and their contribution to the vulcanization of the blends. Both SIC and eC effects improve with curing time, with properties at t50 closely matching those at t90, indicating an optimal crosslink density at such times. The temperature span during mechanical cycling increases linearly with curing time, largely unaffected by GTR presence. The material's coefficient of performance (COP) peaks at t50, suggesting this curing time offers the best balance between mechanical energy dissipation and temperature change, highlighting its potential for eco-friendly heating/cooling applications.

Tough Polymer Gels Reinforced by Strain-Induced Crystallization

Tough Polymer Gels Reinforced by Strain-Induced Crystallization

Critical parameters governing elastocaloric effect in polyisoprene rubbers for solid-state cooling

The aim of this work is to study the critical parameters governing the eCe (elastocaloric effect) of poly-isoprene rubber (NR and IR) in use conditions of a cooling device, i.e. under partial cyclic loading. The effect of mechanical cyclic loading parameters (pre-extension ratio, waveform and frequency) on eCe was first studied. It shows that the eCe increases when the mimimum pre-extension ratio is increased and frequency is lowered from 1Hz to 0.001Hz, as it promotes strain-induced crystallization. However, it also leads to a decrease in potential cooling power, from 7 to 0.01 MW/m3 (i.e. 8 kW/kg to 0.01 kW/kg). At intermediate frequency (f ≈ 0.1 Hz), the comparison of square and triangular waveforms demonstrated that the former enables greater temperature variation. This is due to its holding step (at maximum extension ratio), which maximizes crystallization but also promotes stress relaxation, resulting in increased mechanical losses. Consequently, the square and triangular loadings have a COPmat of 12 and 25, with a temperature variation of 4.2K and 3.6K, respectively. Regarding the formulation, rubbers that do not have a great tendency to crystallize such as synthetic polyisoprene rubber seem to have the best COPmat, while to maximize ΔT, the best of our formulations are NR crosslinked with sulfur and having a crosslink density close to 1.5 × 10−4 mol/cm3, which combine large strain induced crystallization and entropic elasticity. This material has a COPmat of about 27 and allows a ΔT≈4K, i.e.performance comparable to that of the best Shape Memory Alloys (at equivalent COPmat).

Temperature effects on the lifetime reinforcement due to strain-induced crystallization in carbon black filled natural rubber under non-relaxing torsion: comparison with non-relaxing tensile loadings

This paper investigates the lifetime reinforcement at different temperatures of carbon black filled natural rubber under non-relaxing torsion. Torsion fatigue tests with different angle ratios have been carried out with axisymmetric-shaped specimens. Finite element analysis was used to predict the local mechanical state by taking into account temperature effects and stress softening heterogeneities. It has been shown that when the temperature of the test is increased, the mechanical response does no longer stabilize, contrary to what is observed when the test is carried out at ambient temperature. A strategy has been proposed to evaluate an equivalent loading ratio, in order to compute the minimum loading as the Cauchy stress normal to the crack plane for the minimum torsion angle applied and to represent the results in the Haigh diagram. It was found that the lifetime reinforcement under torsion loading decreases when the temperature is increased to 90 °C, but is still observed. The lifetime reinforcement under torsion at 90 °C was of the same amplitude as the one obtained with the same material under uniaxial tension at the same temperature. The study has been completed by post-mortem analysis at the macro- and the microscale in order to characterize damage mechanisms at 90 °C.

The poker-chip experiments of synthetic elastomers explained

In a recent study, Kumar and Lopez-Pamies (J. Mech. Phys. Solids 150: 104359, 2021) have provided a complete quantitative explanation of the famed poker-chip experiments of Gent and Lindley (Proc. R. Soc. Lond. Ser. A 249: 195–205, 1959) on natural rubber. In a nutshell, making use of the fracture theory of Kumar, Francfort, and Lopez-Pamies (J. Mech. Phys. Solids 112: 523–551, 2018), they have shown that the nucleation of cracks in poker-chip experiments in natural rubber is governed by the strength — in particular, the hydrostatic strength — of the rubber, while the propagation of the nucleated cracks is governed by the Griffith competition between the bulk elastic energy of the rubber and its intrinsic fracture energy. The main objective of this paper is to extend the theoretical study of the poker-chip experiment by Kumar and Lopez-Pamies to synthetic elastomers that, as opposed to natural rubber: (i) may feature a hydrostatic strength that is larger than their uniaxial and biaxial tensile strengths and (ii) do not typically exhibit strain-induced crystallization. A parametric study, together with direct comparisons with recent poker-chip experiments on a silicone elastomer, show that these two different material characteristics have a profound impact on where and when cracks nucleate, as well as on where and when they propagate. In conjunction with the results put forth earlier for natural rubber, the results presented in this paper provide a complete description and explanation of the poker-chip experiments of elastomers at large. As a second objective, this paper also introduces a new fully explicit constitutive prescription for the driving force that describes the material strength in the fracture theory of Kumar, Francfort, and Lopez-Pamies.

Effect of Stereoregularity on Strain-Induced Crystallization and Mechanical Properties of Natural Rubber

Effect of Stereoregularity on Strain-Induced Crystallization and Mechanical Properties of Natural Rubber

Comparative Investigation of Nano-Sized Silica and Micrometer-Sized Calcium Carbonate on Structure and Properties of Natural Rubber Composites.

Fillers have been widely used in natural rubber (NR) products. They are introduced to serve as a strategy for modifying the final properties of NR vulcanizates. Silica and calcium carbonate (CaCO3) are among the fillers of choice when the color of the products is concerned. In this case, a special focus was to compare the vulcanizing efficiency of NR filled with two different filler types, namely nano-sized silica and micrometer-sized CaCO3. This study focused on the effects of the loading level (10-50 parts per hundred parts of rubber, phr) on the final properties and structural changes of NR composites. The results indicated that increased filler loading led to higher curing torques and stiffness of the rubber composites irrespective of the type of filler used. The better filler dispersion was achieved in composites filled with CaCO3 which is responsible for less polarity of CaCO3 compared to silica. Good filler distribution enhanced filler-matrix interactions, improving swelling resistance and total crosslink density, and delaying stress relaxation. The modulus and tensile strength of both composites also improved over the content of fillers. The CaCO3-filled composites reached their maximum tensile strength at 40 phr, exceeding, by roughly 88%, the strength of an unfilled sample. Conversely, the maximum tensile strength of silica-filled NR was at 20 phr and was only slightly higher than that of its unfilled counterpart. This discrepancy was ascribed to the stronger rubber-filler interactions in cases with CaCO3 filler. Effective rubber-filler interactions improved strain-induced crystallization, increasing crystallinity during stretching and reducing the strain at which crystallization begins. In contrast, large silica aggregates with poor dispersion reduced the overall crosslink density, and degraded the thermomechanical properties, tensile properties, and strain-induced crystallization ability of the NR. The results clearly indicate that CaCO3 should be favored over silica as a filler in the production of some rubber products where high performance was not the main characteristic.

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.

Biaxially strain-induced polymer crystallization studied by dynamic Monte Carlo simulations

Polymer crystallization under biaxial stretching is a basic process for rubber-tire strain-hardening performance as well as for plastic film formation. Its specific microscopic mechanism different from that under uniaxial stretching has not yet been systematically investigated. We conducted dynamic Monte Carlo simulations of biaxially strain-induced crystallization in half-half cross-stretched bulk polymers. We found small thermodynamic but significant kinetic differences between biaxially and uniaxially strain-induced polymer crystallizations under the same conditions of strains, temperatures and strain rates. Furthermore, the cross-stretching among neighboring polymers raises a constraint to lamellar thickening in biaxially strain-induced crystallization, which results in low crystallinity and small crystal sizes in agreement with experimental observations. Our simulations brought new insights into a better understanding of biaxially strain-induced polymer crystallization.

Structure – property relationships of different natural rubber grades

The effects of various natural rubber (NR) grades, namely RSS 3, STR 5L, STR 10, STR 20 and STR 20CV, towards their final properties were investigated. The molecular characteristics of these NR grades were assessed by gel permeation chromatography and by determining protein contents. The highest molecular weight was found for RSS 3, while STR 20CV had the lowest molecular weight. No relationship between molecular weight and protein content was seen in this study. A higher molecular weight rubber provided greater maximum torque and torque difference, and a better resistance to degradation during processing. However, no significant difference was found among molecular weights higher than 1.25 × 106 g/mol. The final torque during processing and the Mooney viscosity of rubber increased with molecular weight due to tie chain entanglements, as later confirmed by a dynamic mechanical analyzer. Thermo-stability and stress relaxation of the various NR grades were also largely governed by their molecular weights. Stability of rubber at an elevated temperature improved with molecular weight. Also, the tensile properties and the strain-induced crystallization varied with the molecular weight. The tensile strength of various NR grades varied from 15 to 18 MPa while the elongation at break was 600%–800%. The molecular weight of NR was found to have a great effect on the processing, thermomechanical, dynamic, and mechanical properties of the vulcanizates. In addition, a narrow molecular weight distribution also affected the mechanical properties. The crystallinities of the NRs during stretching were closely related to their tensile behavior, and the alignment of crystal structures during stretching was almost parallel to the stretching direction in all NR samples. The molecular weight of NR appears to be a key factor associated with the mechanical properties and reinforcement behavior of vulcanizates.

Novel polybutadiene rubber with long cis-1,4 and syndiotactic vinyl segments (CVBR) for high performance sidewall of all-steel giant off-the-road tire

The novel polybutadiene rubbers (CVBR) carrying 88–97% of long cis-1,4 polybutadiene segments (cis-PB, cis-1,4 content >98.3%) and 3–12% of long syndiotactic1,2-polybutadiene segments (sPB, 1,2 content >87.0%) could be successfully synthesized via in-situ coordination polymerizations of butadiene in hexanes in our lab and also be scaled up via continuous polymerization process in pilot (scale: 200 t/a). The natural rubber (NR)-based compound formulations containing different contents of CVBR, and carbon black (CB) fillers were developed and the optimized NR/CVBR/CB vulcanizates with 40% of CVBR were manufactured for the sidewall of all-steel giant off-the-road tire. The storage modulus of NR/CVBR/CB compounds and the elastic modulus, tensile strength and tear strength of NR/CVBR/CB vulcanizates could be remarkably increased by increasing CVBR content. The crack generation and growth could be effectively restricted due to both the fast relaxation, the rigid polymer-filler interface and synergistic effect of high cis-polybutadiene and crystalline 1,2-polybutadiene particles on the strain-induced crystallization behaviors of rubber matrix. The heat build-up of NR/CVBR/CB vulcanizates could be greatly reduced by introduction of cis-PB segments in CVBR for the easy mobility and low hysteresis of macromolecular chains. The NR/CVBR/CB vulcanizates with high strength and elastic modulus, good flex fatigue crack resistance and low heat build-up would meet the requirement for high performance sidewall in manufacturing giant all-steel off-the-road tires.

Graphene-Based Hybrid Fillers for Rubber Composites.

Graphene and its derivatives have been confirmed to be among the best fillers for rubber due to their excellent properties, such as high mechanical strength, improved interface interaction, and strain-induced crystallization capabilities. Graphene rubber materials can be widely used in tires, shoes, high-barrier conductive seals, electromagnetic shielding seals, shock absorbers, etc. In order to reduce the graphene loading and endow more desirable functions to rubber materials, graphene-based hybrid fillers are extensively employed, which can effectively enhance the performance of rubber composites. This review briefly summarizes the recent research on rubber composites with graphene-based hybrid fillers consisting of carbon black, silica, carbon nanotubes, metal oxide, and one-dimensional nanowires. The preparation methods, performance improvements, and applications of different graphene-based hybrid fillers/rubber composites have been investigated. This study also focuses on methods that can ensure the effectiveness of graphene hybrid fillers in reinforcing rubber composites. Furthermore, the enhanced mechanism of graphene- and graphene derivative-based hybrid fillers in rubber composites is investigated to provide a foundation for future studies.

Strain-Induced Crystallization in Tetra-Branched Poly(ethylene glycol) Hydrogels with a Common Network Structure

Strain-Induced Crystallization in Tetra-Branched Poly(ethylene glycol) Hydrogels with a Common Network Structure

Molecular chain orientation and mechanical properties of hydrogenated nitrile-butadiene rubbers with different acrylonitrile content and crosslink density

A study of the thermal and mechanical properties, as well as the molecular chain orientation of sulfur-vulcanized hydrogenated nitrile-butadiene rubber (HNBR) with acrylonitrile (ACN) content of 34 and 36 wt% and different crosslink density is presented. The objective is to establish the relationships between structural changes during stretching and the tensile and elastic behavior of HNBR having different chain microstructure. The (vulcanized) HNBR samples are amorphous at room temperature with glass transition temperatures that increase with increasing ACN content and crosslink density. The samples do not exhibit strain-induced crystallization. Stretching at room temperature only induces orientation of the amorphous segments along the stretching direction, which contributes to improved tensile strength, strain-hardening, and toughness, while preserving excellent elastic recovery of the rubbers. Precise correlations are established between the degree of orientation of the amorphous chain and the deformation in the framework of the affine deformation model of rubber elasticity.