Strain-induced Crystallization Research Articles

Additive manufactured thermoplastic elastomers for low-stress driven elastocaloric cooling

Exploiting the strain-induced crystallization of soft elastomeric polymers elastocaloric cooling has recently been explored as an environmental-friendly alternative to conventional refrigeration. Elastomers require a much lower applied stress to induce the elastocaloric effect compared to shape memory alloys. Several prototype coolers employing these soft polymers have been demonstrated to achieve a moderate temperature span under lower stresses. Here, we investigate the elastocaloric properties and potential cooling performance of five thermoplastic elastomers that can be 3D printed, both in the form of filaments and as Additive Manufactured (AM) parts. The materials were first characterized as filaments to screen for the elastomers with the highest elastocaloric effects. A large adiabatic temperature change of 17.8 K was obtained in the Ultimaker98A filament. AM parameters were optimized to achieve parts with satisfactory functional stability while maintaining their elastocaloric effect. As a printed part, NinjaFlex achieved a high material coefficient of performance (COPmat) of 3.2 with 1.74 J/g input work at ∼0.1 Hz, driven by a stress of 5.7 MPa. Implementing AM elastocaloric elastomers creates opportunities for the development of full-scale low-activation-stress regenerative elastocaloric cooling components that enable optimizing flow structures and enhanced heat-transfer performance.

Structure–Property Relationships of Epoxidized Natural Rubber Vulcanizates

The effects of epoxide content on the microstructure and characteristics of epoxidized natural rubber (ENR) were examined. Self-crosslinking between epoxide groups possibly occurred during the vulcanization, with C–O–C linkages in the ENR. Depending on the epoxide level, the self-crosslinking in ENR improved the maximum torque, overall crosslink density (CD) and stability at elevated temperature of the rubber. With increasing epoxide content, the moduli and tensile strengthes of the ENR also increased. The highest tensile strength was achieved at 25 mol% epoxide, exceeding that of crosslinked NR by 46%. The self-crosslinking also facilitated strain-induced crystallization (SIC), reducing the onset strain of crystallization and increasing the degree of crystallinity. Even though increased epoxide content in the NR made the crosslink network distribution less homogeneous, the CD increased so much that the thermomechanical and mechanical characteristics of ENR were improved. The C–O–C linkages from ENR self-crosslinking were also sufficient to boost the thermal stability, complementing the conventional sulfide links.

Crystal Growth of 3D Poly(ε-caprolactone) Based Bone Scaffolds and Its Effects on the Physical Properties and Cellular Interactions.

Extrusion additive manufacturing is widely used to fabricate polymer-based 3D bone scaffolds. However, the insight views of crystal growths, scaffold features and eventually cell-scaffold interactions are still unknown. In this work, melt and solvent extrusion additive manufacturing techniques are used to produce scaffolds considering highly analogous printing conditions. Results show that the scaffolds produced by these two techniques present distinct physiochemical properties, with melt-printed scaffolds showing stronger mechanical properties and solvent-printed scaffolds showing rougher surface, higher degradation rate, and faster stress relaxation. These differences are attributed to the two different crystal growth kinetics, temperature-induced crystallization (TIC) and strain-induced crystallization (SIC), forming large/integrated spherulite-like and a small/fragmented lamella-like crystal regions respectively. The stiffer substrate of melt-printed scaffolds contributes to higher ratio of nuclear Yes-associated protein (YAP) allocation, favoring cell proliferation and differentiation. Faster relaxation and degradation of solvent-printed scaffolds result in dynamic surface, contributing to an early-stage faster osteogenesis differentiation.

Dynamic Monte Carlo simulations of strain-induced crystallization in multiblock copolymers: Effects of microphase separation

Semicrystalline multiblock copolymers holding alternatingly crystallizable and noncrystallizable blocks are good thermoplastic elastomers that are commonly processed through strain-induced crystallization. The crystalline microdomains play the role of physical crosslinks in the network of noncrystallizable blocks for high elasticity, and meanwhile their size diversity determines high toughness. We scheduled three integrated steps to study three physical effects of the coexisting noncrystallizable components, i.e. dilution, microphase separation and asymmetric block rigidity, on the size diversity of crystalline microdomains yielded by strain-induced crystallization in multiblock copolymers. We firstly considered the dilution effects in the extremely concentrated and diluted cases presumably generated by concentration fluctuations of crystallizable blocks, as responsible for the large-end and small-end crystalline microdomains, respectively. Our dynamic Monte Carlo simulations of strain-induced crystallization of the extremely concentrated and diluted crystallizable blocks in diblock and tetrablock copolymers demonstrated the dilution effects that the concentrated cases exhibit crystallization similar with bulk homopolymers, but the diluted cases make smaller crystalline microdomains than the cases without stretching, thus strain-induced crystallization in these two cases enhances the size diversity of crystalline microdomains (DOI: 10.1039/D2SM00193D). Hereby in the second step, we further compared the concentrated and diluted cases of strain-induced crystallization with and without microphase separation, and observed that microphase separation influences little to the concentrated cases, but makes more rather than smaller crystalline microdomains in the diluted cases, thus enhances their size diversity in a way different from the dilution effects. Higher extents of microphase separation will however weaken this enhancement. Our observations paved the way towards a better understanding of strain-induced crystallization in thermoplastic elastomers.

Competition between Strain-Induced Crystallization and Cavitation at the Crack Tip of Unfilled and Carbon Black-Filled Natural Rubber

Competition between Strain-Induced Crystallization and Cavitation at the Crack Tip of Unfilled and Carbon Black-Filled Natural Rubber

Vulcanization kinetics of natural rubber and strain softening behaviors of gum vulcanizates tailored by deep eutectic solvents

Rubber processing industries call for ecofriendly, green activators for reducing zinc oxide (ZnO) activator usages and volatile organic compounds emissions. Herein low cost deep eutectic solvents (DESs) are used for regulating vulcanization kinetics of natural rubber (NR) gums and for tailoring mechanical behaviors of the gum vulcanizates. A highly active DES based on choline chloride/thiourea (1/3) pair (CCT) is identified. Its combination with ZnO at lowered amounts allows preparation of high-strength vulcanizates at lowered vulcanization temperature. The 120 °C-cured vulcanizate containing 1 phr (parts per hundred parts of rubber) ZnO and 0.2 phr CCT demonstrates crosslinking density 105 mol/m3 and tensile strength 21 MPa, being higher than those (94 mol/m3 and 16 MPa) of the 140 °C-cured vulcanizate containing 2 phr ZnO activator. Normalization of deformational stress with respect to theoretical elastic modulus of ideal rubbery network reveals that stress (deformation energy) and softening at low strains are closely related to crosslinking density and strain-induced crystallization behavior. Crosslinking density could be independently regulated by introduction of DESs and by variation of vulcanization temperature, paving the way for tailoring elasticity and dissipation of vulcanizates manufactured in an energy-saving efficient vulcanization approach.

Elastocaloric effect: Impact of heat transfer on strain-induced crystallization kinetics of natural rubber

When natural rubber is rapidly loaded and unloaded there occurs a variation of its temperature called the elastocaloric effect. The objective of this work was to determine whether this temperature variation could be responsible for a delay in strain-induced crystallization (SIC) kinetics and stress relaxation. To this end, the SIC kinetics of cross-linked natural rubber samples with different thicknesses was studied at constant elongation after adiabatic stretching using thermal, mechanical and in-situ WAXS characterizations. The coupling of these techniques revealed that, at a fixed elongation λ (between 5 and 6), the crystallization kinetics and stress relaxation were faster in the thinner sample (with a short heat exchange time constant), thus providing evidence that heat transfer affected the SIC kinetics and should be considered when interpreting experimental results.

Development of order during strain induced crystallization of polymers, case of PET

This study aims at enlightening the kinetics and the nature of the precursor for crystallization in strain induced crystallization (SIC) of PET, combining ex situ and in situ X-ray diffraction on well controlled tensile experiments. Attention is paid too to annealing to assess “stability” of the phases that are generated. It is concluded that SIC in PET consists in the appearance of ordered entities the periodicities of which are those of crystal. Strain hardening corresponds to perfectioning without leading to actual crystal under tension. The kinetics of formation can be directly related to the stress, whatever the set on tensile conditions (temperature, strain rate). However, microstructure of PET can evolve after loading in case of annealing.

The effect of electron beam irradiation and fillers on natural rubber prepared by latex mixing: A Small and Wide Angle X-ray scattering study

The natural rubber (NR) cured by electron beam (EB) irradiation prepared by latex mixing was studied via small angle and wide angle x-ray scattering (SAXS and WAXS) techniques for the first time. In this study, the effect of EB irradiation doses (100–250 kGy) and fillers (graphene nanoplatelets (GE), mixture of silica and graphene nanoplatelets (TSi/20%GE) on the mean distance of crosslink and strain-induced crystallization were investigated. The SAXS experiments revealed that the maximum position of the plot (qmax) was increased from 1.76 nm to 1 to 1.99 nm-1, indicating that the mean distance between crosslinks of NR decreased due to the occurrence of crosslink networks after EB irradiation processes. It was found that the mean distance between crosslinks of all EB-irradiated NR was insignificantly different (qmax ∼ 1.94–1.99 nm−1) although the higher crosslink density was found in NR with the higher EB dose. Likewise, the addition of fillers did not also affect the mean distance between crosslinks of NR. The discussion led to the conclusion that the mean distance of NR depended on the type of crosslink networks. The WAXS study revealed that the intensities of crystallinity reflection peaks increased in the function of EB irradiation dose. It can be seen in all NR types that molecular chains of NR were still in amorphous regions although the high stretching was applied. Additionally, the crystallinity percentage of EB-irradiated NR in stretching mode was always lower than that in retraction mode, which the highest crystallinity percentage was found in the case of NR250 (41.89% crystallinity at 500% strain). Based on WAXS results, the conclusions were in agreement with NR cured by other vulcanization systems. Finally, it was found that addition of TSi/20%GE in NR pulled down the level of strain – induced crystallization, which the crystallinity percentage was 27.75% at 500% strain, while the crystallinity percentage of NR added GE was 36.19% at the same strain.

Block Poly(carbonate-ester) Ionomers as High-Performance and Recyclable Thermoplastic Elastomers.

Thermoplastic elastomers based on polyesters/carbonates have the potential to maximize recyclability, degradability and renewable resource use. However, they often underperform and suffer from the familiar trade-off between strength and extensibility. Herein, we report well-defined reprocessable poly(ester-b-carbonate-b-ester) elastomers with impressive tensile strengths (60 MPa), elasticity (>800 %) and recovery (95 %). Plus, the ester/carbonate linkages are fully degradable and enable chemical recycling. The superior performances are attributed to three features: (1) Highly entangled soft segments; (2) Fully reversible strain-induced crystallization and (3) Precisely placed ZnII -carboxylates dynamically crosslinking the hard domains. The one-pot synthesis couples controlled cyclic monomer ring-opening polymerization and alternating epoxide/anhydride ring-opening copolymerization. Efficient convresion to ionomers is achieved by reacting vinyl-epoxides to install ZnII -carboxylates.

Prolonging rubber fatigue life using hysteresis of strain-induced crystallization of natural rubber

Recently, soft robots have attracted considerable attention; however, they often undergo large deformations. Thus, their lifespans are critical issues. Therefore, this study proposes a novel method for extending the lives of rubber materials. We extend the fatigue life of rubber materials by focusing on the hysteresis of the strain-induced crystallization characteristics of rubber. Strain-induced crystallization is a phenomenon wherein rubber molecules exhibit crystal structures by elongation. These crystal structures are generated by elongation and disappear with relaxation. However, hysteresis occurs in both the generation and disappearance processes. Therefore, if the rubber is not completely relaxed, the disappearance of the crystal layers is slowed by hysteresis, and the formation of cracks is hindered. Herein, we select two types of rubbers: those exhibiting strain-induced crystallization characteristics (natural rubber (NR)) and those that do not (styrene-butadiene rubber (SBR)). First, wide-angle X-ray diffraction is used to confirm the formation of crystal layers under stretching for both types of rubbers. Fatigue life test is conducted using rectangular test pieces made of NR and SBR by repeated tensile tests. Subsequently, a fatigue life test is conducted for a straight-fiber-type pneumatic artificial muscle prepared by NR and SBR. The results reveal that the fatigue life is approximately 100 times longer than normal, thereby confirming the effects of prolonging the fatigue life using the hysteresis of strain-induced crystallization.

VISCOELASTICITY AND DYNAMIC FATIGUE CRACK GROWTH BEHAVIOR OF NATURAL RUBBER/CIS-POLYBUTADIENE RUBBER COMPOSITES

ABSTRACT Because cis-polybutadiene rubber (BR) can improve the fatigue crack growth (FCG) resistance of natural rubber (NR) in the low-tearing-energy (G) range, a blending system of NR/BR is often used in tire materials. In this study, based on relationship of (FCG rate (dc/dN) − G) established from the perspective of fracture mechanics and then inspired by Persson's theory and starting from the perspective of viscoelasticity, the FCG behavior and mechanism of NR/BR were investigated. NR/BR with different blending ratios were prepared, and dc/dN under different G inputs (500/1500 J/m2) was measured. According to the viscoelastic parameters (storage modulus E′, loss factor tan δ, and loss compliance modulus J″) recorded in situ, energy dissipation distribution at the crack tip (energy consumption for FCG inside crack tip: G0 and energy loss in linear viscoelastic zone near crack tip: G0f [v, T]) was determined, and the relationship of (viscoelasticity − dc/dN) was finally set up. When G = 500 J/m2, blending BR can reduce dc/dN as compared with pure NR. On one hand, with a higher BR fraction, an increased cross-linking density and enhanced filler network provided greater rigidity, which increased E′; on the other hand, a low glass transition temperature and flexible chain of BR reduced hysteresis, which decreased tan δ. The joint action led to a decrease in J″, which caused more G0f (v, T) and less G0, resulting in the ultimate reduction of dc/dN. In contrast, for G = 1500 J/m2, when the BR content was >50 phr, dc/dN showed a significant increase. Although more BR evidently decreased J″ and then led to a large amount of G0f (v, T), due to absence of strain-induced crystallization, the chain orientation of BR was hard to resist FCG when G increased. Finally, the morphology of the crack tip propagation path was captured to corroborate the different orientation characteristics of NR and BR and their effects on FCG behavior.

Fatigue Characteristics of Poly(ether-b-amide) Elastomers during Cyclic Dynamic Tests and the Underlying Microstructural Evolution

The dynamic fatigue behavior of the poly(ether-b-amide) (PEBA) elastomers was characterized by a combination of step increase load test (SILT), step increase strain test (SIST), and single load dynamic creep (SLDC). PEBA had crystalline hard segments of polyamide 12 (PA12) and soft segments of poly(tetramethylene oxide) (PTMO). The crystalline morphologies before and after the tests were studied by ex situ wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Fast stress relaxation can be seen under the applied strain well below than the elongation at break. Remarkable dynamic creep can be observed under the applied load triggering yielding of the specimen, which is substantially less than the ultimate tensile strength. Although the presence of high content of amorphous PTMO renders high entropic elasticity, the disintegration of the crystalline hard domains of PA12 is expected to determine the relaxation rate, creep rate, and modulus, as strain-induced crystallization in soft domains did not occur and no trace was found by WAXD. SAXS patterns showed that the orientation of the PA12 lamellae at lower stresses or strains can largely be restored. However, the fractured and highly aligned PA12 lamellae resulted in classic four-point or two-lobe SAXS patterns, which implied the underlying microstructural changes after the cyclic dynamic tests.

Effect of dispersibility of graphene nanoplatelets on the properties of natural rubber latex composites using sodium dodecyl sulfate

Abstract Natural rubber latex/graphene nanoplatelet (NRL/GNP) composites containing GNP-pristine and GNP–SDS were prepared by a simple mechanical mixing method. The main objective was to study the effect of dispersibility of GNP on the properties in NRL. X-ray diffraction confirmed the adsorption of sodium sulfate dodecyl (SDS) on the GNP surface. The results showed that high filler loading diminished the physical and mechanical properties of the composites but successfully endured to satisfy electrical conductivity to the NRL/GNP composites. Besides, the SDS surfactant-filled system demonstrated better physical, tensile, electrical, and thermal stability properties than the GNP-pristine. The intercalated and dispersed GNP–SDS increased the number of routes for stress and heat transfer to occur and facilitated the formation of conductive pathways as well, leading to the improvement of the properties as compared to NRL/GNP-pristine composites. However, as the GNP–SDS loading exceeded 5 phr, the GNP–SDS localized in the interstitial layer of NRL, restricted the formation of crosslinking, and interfered with the strain-induced crystallization ability of the composites.

Comparative Structure-Property Relationship between Nanoclay and Cellulose Nanofiber Reinforced Natural Rubber Nanocomposites.

Natural rubber (NR) nanocomposites reinforced with five parts per hundred rubber (phr) of two different nano-fillers, i.e., nanoclay (abbrev. NC) and cellulose nanofiber (abbrev. CNF), were prepared by using latex mixing approach, followed by mill-compounding and molding. The morphology, stress–strain behavior, strain-induced crystallization, and bound rubber of the NR nanocomposites were systematically compared through TEM, tensile test, WAXS, DMA, and bound rubber measurement. The aggregated CNFs were observed in the NR matrix, while the dispersed nanosized clay tactoids were detected across the NR phase. The reinforcement effects of NC and CNF were clearly distinct in the NR nanocomposites. At the same nano-filler content, the addition of NC and CNF effectively accelerated strain-induced crystallization of NR. The high tensile strength obtained in the NC-filled NR nanocomposite was attributed to strain-induced crystallization of NR accelerated by well-dispersed NC. However, the larger tensile modulus and low strain for the CNF-filled NR were related to the formation of immobilized NR at the interface between CNF aggregate and NR. The immobilization effect of NR at the CNF surface offered by a mutual entanglement of CNF aggregate and NR chain led to local stress concentration and accelerated strain-induced crystallization of CNF/NR nanocomposite. From the present study, the NR nanocomposites combined with 5 phr CNF shows high-tensile modulus and acceptable breaking tensile stress and strain, suggesting the application of CNF/NR based nanocomposite in automotive and stretchable sensors for next-generation electronic devices.

Enhanced Phase Stability of Compressive Strain-Induced Perovskite Crystals.

Control of strain in perovskite crystals has been considered as an effective strategy to ensure the phase stability of perovskite films where a compressive strain is particularly preferred over a tensile strain due to a lowered Gibbs free energy by the unit cell contraction effect. Here we adapt the strategy of strain control into perovskite solar cells in which the compressive strain is applied by utilizing a thermal expansion difference between the perovskite film and an adjacent layer. Poly(4-butylphenyldiphenylamine), with a higher thermal expansion coefficient compared to that of perovskite, is employed as a substrate for perovskite crystal growth at 100 °C, followed by cooling to room temperature. The applied compressive strain at the interface, as a result of a greater contraction of the polymer compared to the perovskite film, is confirmed by grazing incidence X-ray diffraction showing a red peak shift with increasing secondary angle. The compressive strain-induced perovskite film shows relatively constant absorbance spectra as a function of time. In the meantime, the absorbance spectra of a film without strain control exhibit a gradual decay with developing an Urbach tail. Importantly, the effect of strain engineering is remarkably prominent in the long-term photovoltaic performance. The photocurrent drops by 41% over 911 h without controlling strain, which is significantly improved by employing compressive strain, showing only a 6% drop in photocurrent from a shelf-stability test without encapsulation. It is also noted that an S-shaped kink appears in the current-voltage curves since 579-h-long storage for the device without strain control, leading to unreliable and overestimated fill factor and conversion efficiency. On the other hand, a 16% increase in fill factor with a stable performance is derived over 911 h from the compressive strain-induced device.

Inelastic response of thermo-oxidatively aged carbon black filled polychloroprene rubber. Part I: Viscoelasticity

The first part of a two parts study focuses on the inelastic response of carbon black-filled polychloroprene rubber undergoing thermo-oxidative aging. More precisely, the prominence of the present work is laid on the attempt to provide a better understanding of the network modification due to thermo-oxidative aging and its consequences on the macroscopic mechanical responses including viscoelasticity (in Part I) and Mullins effect (in Part II). For this purpose, the polychloroprene rubber specimens are exposed to a temperature of 120 °C for a duration of 2, 3, and 4 days to induce three different aging levels before undergoing various tests. Results indicated that during thermo-oxidative aging, crosslinking dominates chain scission, and the strain-induced crystallization becomes more pronounced. Furthermore, it is observed that the inelastic responses such as stress relaxation and strain-induced crystallization are strongly affected by this thermo-oxidative aging-induced network alteration.

Manipulating the mechanical properties of cis-polyisoprene nanocomposites via molecular dynamics simulation

Attributed to the reinforcing effect of the NPs, filled natural rubber (NR) exhibits more excellent mechanical properties compared to pure natural rubber and has been attracting considerable scientific and technological attention. However, only a few studies have shown a systematical understanding of the structure-mechanics relation of filled natural rubber. Herein, a common strategy used to study the effects of the critical structural factors on strain-induced crystallization (SIC) and mechanical properties at the molecular level is to adopt molecular dynamic simulation (MD). Increasing the cross-linkage of the polymer matrix improves the entanglement degree, and the mechanical properties are reinforced by the high orientation of the polymer chains due to the high entanglement degree and the dissipation of energy due to the unentangling. Surprisingly, we find that the stress-strain behaviour of filled natural rubber is significantly manipulated by nanoparticle (NP) content, size and strength of interfacial interaction, evidenced by the increase of the chain orientation, SIC, and energy dissipated by NPs and polymer matrix, compared with the case of pure natural rubber. The underlying mechanism is the adsorption of the polymer by NPs, which leads to the orientation and crystallization of the polymer chains induced by NPs during the deformation process and energy dissipation through the slip of NPs and polymer chains. In general, this work demonstrates a detailed molecular-level structure-mechanics relation of filled natural rubber and provides some rational guidelines for experimentally designing and fabricating high-performance elastomer nanocomposites.

Modeling strain-induced dual-phase transformation in semicrystalline polylactide

Polylactic acid or polylactide (PLA) can form amorphous, semicrystalline, or highly crystalline structure. In this paper, we provide a model for semicrystalline PLA under strain-induced dual-phase transformation, between mesomorphization and crystallization. In the proposed constitutive model, we use an internal state variable approach by considering initial microstructure, strain-induced dual-phase transformation, intermolecular features, and preferred network orientation-induced anisotropy. The effective contribution of the newly created phases is considered in both the elastic-viscoplastic intermolecular resistance and the viscohyperelastic network resistance using a micro–macro transition approach. We find a good agreement between the model and experimental data for the semicrystalline PLA in terms of progressive evolution of the mesomorphic/crystalline phases along with stress–strain behavior up to very large strains and over a wide range of crystallinities. The relationship between initial crystallinity, strain-induced molecular ordering, newly created phases, and mechanical response is discussed.

First evidence and characterization of strain-induced crystallization heterogeneity in natural rubber under homogeneous strain states

This paper provides the first evidence of strain-induced crystallization heterogeneities at the macroscopic scale, accompanied with strong memory effects during uniaxial cyclic tensile loadings in unfilled natural rubber. Specimens with two different sulphur amounts, 1.2 phr and 1.6 phr, and two different geometries, small and large widths, have been considered. The crystallinity has first been evaluated in the middle of each specimen thanks to a quantitative surface calorimetry approach based on infrared thermography. The calorimetric approach was then used to simultaneously access the crystallinity at different points of the tested specimens and to investigate spatial variations in the crystallinity along a profile at the middle of each specimen. SIC was found to occur heterogeneously along the profile, i.e. at the macroscopic scale, whatever the geometry and the sulphur amount are. Moreover, crystallization and melting initiate in the same zones during load-unload cycles. Several families in terms of heat sources due to SIC have been identified.