Stretch-induced Crystallization Research Articles

Chain-Level Analysis of Reinforced Polyethylene through Stretch-Induced Crystallization.

Herein, we propose a large-scale simulation approach to perform the stretch-induced crystallization of entangled polyethylene (PE) melts. Sufficiently long (1000 ns) united-atom molecular dynamics (UAMD) simulations for 16000 chains of 1000 consecutive CH2 united-atom particles under periodic boundary conditions were performed to achieve the crystallinity observed in experiments. Before the isothermal crystallization process, we applied uniaxial stretching as pre-elongation to the embedded strain memory on the entangled PE melts. We confirmed significant differences in the morphologies of crystal domains and scattering patterns for pre-elongation ratios of 400% and 800%. The obtained scattering patterns were consistent with the experimental results. Uniaxial stretching MD simulations revealed that the elastic modulus at 800% pre-elongation was stronger than that at 400% pre-elongation. From this observation, we can derive the structure-property relationship, wherein the magnitude of the pre-elongation governs the crystal domain structures and mechanical properties.

Molecular dynamics simulation of stretch-induced crystallization of star polymers as compared to their linear counterparts

Abstract The linear and star polyethylene during static crystallization and stretch-induced crystallization has been investigated by molecular dynamics simulations. The findings demonstrate that the branching point of the star polymer system does not participate in crystallization, and the crystallization ability of the segments near the branching point and at the end of the chains is inferior. Due to the existence of branching points, the mobility and conformational extension of chain segments are weak, and the entanglement degree is higher than that of linear systems. For stretch-induced crystallization, stretching promotes the extension of molecular chains and arranges them along the stretching direction. The crystal nucleation and growth in linear and star polymer systems are significantly faster than in static crystallization. The mobility of the chain segments close to the branching point is partially enhanced by stretching, while the branching point still substantially affects the chain conformation and segment orientation. It is worth mentioning that we have verified some crucial results that cannot be observed in the experiments at the microscopic scale.

Phantom Chain Simulations for the Fracture of Energy-Minimized Tetra- and Tri-Branched Networks

Effects of branch functionality on mechanical properties of polymer networks are yet to be fully elucidated, although multi-functional approaches have been mainly attempted. For instance, Fujiyabu et al. [Sci. Adv. 2022, 8, abk0010] recently reported that polymer networks made from tri-branch prepolymers exhibit superior mechanical properties to tetra-branch analogues. Although they attribute the difference to stretch-induced crystallization observed in tri-branch poly (ethylene glycol) networks, the mechanism still needs to be clarified. In this study, we performed coarse-grained molecular simulations to extract the effect of branch functionality. The prepolymers were replaced by bead-spring phantom chains, and gelation was simulated by a Brownian dynamics scheme. We subjected the resultant networks to energy minimization and uniaxial stretch by introducing breakage for elongated segments. In the stress–strain relation thus obtained, stress and strain at the break were larger for tri-branch networks than that for tetra-branch analogues, consistent with the experiment. The superiority of tri-branch networks is observed in a wide range of the conversion ratio in gelation, molecular weights of prepolymers, and polymer concentrations. The result implies that the mechanical superiority of tri-branch networks to tetra-branch ones is due to a fundamental structural difference generated during gelation.

Ductile poly(lactic acid)-based blends derived from poly(butylene succinate-co-butylene 2,5-thiophenedicarboxylate): Structures and properties

Because of superior tensile strength, biodegradability, and biocompatibility, poly(lactic acid) (PLA) has emerged as one among the growth-oriented biodegradable materials. But it has been limited to some extent in practical applications due to poor ductility. Consequently, in order to improve the drawback of poor ductility of PLA, ductile blends were obtained by melt-blending of poly(butylene succinate-co-butylene 2,5-thiophenedicarboxylate) (PBSTF25) with PLA. PBSTF25 has a good improvement on the ductility of PLA due to its excellent toughness. Differential scanning calorimetry (DSC) showed that PBSTF25 promoted the cold crystallization of PLA. Wide-angle X-ray diffraction (XRD) results revealed that PBSTF25 experienced stretch-induced crystallization throughout the stretching procedure. Scanning electron microscopy (SEM) showed neat PLA had a smooth fracture surface, but the blends had rough fracture surface. PBSTF25 can improve the ductility and processing properties of PLA. When the addition of PBSTF25 reached 20 wt%, tensile strength was 42.5 MPa and elongation at break increased to 156.6 %, approximately 19 times as much as PLA. The toughening effect of PBSTF25 was better than that of poly(butylene succinate).

Enhancing Tensile Strength and Toughness via Moderate Orientation of Amorphous Molecular Chains in Biobased Biodegradable Poly(lactic acid)

The preparation of high-toughness and high-strength poly(lactic acid) (PLA) is of great significance for promoting its application in packing, construction, and medical fields. Compared to blending with polymeric elastomers, hot stretching is an effective method to improve the toughness of PLA without sacrificing its strength. However, it is still unclear which part of the structural change after hot stretching is responsible for the increased toughness of PLA. In this work, the semicrystalline structure and mechanical properties of the PLA samples stretched at different temperatures were studied. First, the semicrystalline structure of the PLA samples was studied by combining differential scanning calorimetry and wide-angle X-ray diffraction analyses. When the stretching temperature was relatively low (e.g., 65 °C), the mesophase, an intermediate phase between crystal and amorphous, was formed. Furthermore, significant stretch-induced crystallization was observed due to the enhanced chain mobility when the stretching temperature was 65–75 °C. With further increasing the stretching temperature, the crystallinity of the PLA samples decreased sharply to nearly 0 due to the rapid chain relaxation. Intriguingly, the PLA samples stretched at 85 °C showed the highest elongation at break 238.4% and high strength 75.8 MPa, which was attributed to its moderate molecular chain orientation not only hindering physical aging but also retaining the entanglement network of PLA molecular chains. The findings in this work provide an insightful understanding of the improved toughness of PLA, which will facilitate the large-scale practical application of PLA.

Structural evolution of UHMWPE gel fibers as high degree plasticized system during stretching: An in-situ wide and small angle X-ray scattering study

Ultra-high molecular weight polyethylene (UHMWPE) gel fibers filled with high content of plasticization deformed at room temperature were characterized by using in-situ wide-angle X-ray diffraction (WAXD)/small-angle X-ray scattering (SAXS), and ex-situ differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The structural evolution dependence on the plasticization content and stretching speed was stressed. The gel fibers with 96 wt% plasticization exhibited higher orientation than the fibers with 88 wt% plasticization stretched at the same stretching speed. Both gel fibers presented the stretch-induced crystallization as quantified by an increase in crystallinity index. Partial crystallites were found to be induced in the manner of fragmentation-recrystallization accompanied by fibrillization. However, when stretched beyond a threshold strain, the orientation factor, crystallinity index, and long period of both gel fibers changed slowly, which corresponded to the transition from the inability of the structure to continue evolving smoothly. A new mechanism for the structural evolution of high-plasticized UHMWPE gel fibers was proposed to summarize the above results. • The structural evolution in high content plasticized UHMWPE systems was studied. • Effect of degree of plasticization on the structural evolution was compared. • Effect of stretching speed on the structural evolution was compared. • In-situ WAXD/SAXS technique was used for the study.

Stretch-Induced Crystallization of Cellulose Spun from Ionic Liquid Solution.

With the emergence of efficient green solvents, structural regulation of regenerated cellulose is highly desired in the solution process from an industrial perspective. Cellulose fiber and films are viewed as a "composite" comprising amorphous and crystalline fractions. The regulation of the crystalline structure is of great importance for the properties of cellulose materials. In this study, we found stretch-induced crystallization behavior during the transition from solution to gel via coagulation. The crystallinity index of the hydrogel fiber increases with the stretch ratio (SR). X-ray diffraction revealed that the cellulose II hydrate formed in the stretched hydrogel fibers. The mechanical properties and thermal stability of the dry fibers greatly improved against the SR. This crystallization behavior depends on the concentration of the solution and the type of ionic liquid. This stretch-induced crystallization provides an efficient method for structural regulation in cellulose solution processing.

Facile fabrication of high nanofiller-content natural rubber nanocomposites for reversible plasticity shape memory polymers

Reversible plasticity shape memory (RPSM) polymers that undergo a large deformation at room temperature and subsequently recover to their original shape upon heating is a new type of shape memory material just discovered in recent years. Herein, the RPSM polymers based on crosslinked natural rubber (NR) composites were fabricated by a simple ultrasonic mixing of zwitterionic chitin slurry, the precursor of chitin nanofibers (ChNFs), and sulfur-prevulcanized natural rubber latex (NRL) in aqueous media. The results showed that with the increase of the nanofiller content, the elastomer deformation of ChNFs/NR composites turned to plasticity deformation under uniaxially stretching. Meanwhile, the room-temperature plasticity deformation of ChNFs/NR composites with high nanofiller loading (>20 wt%) could be rapidly recovered in hot water (90 °C). Interestingly, it was also found that the stretch-induced crystallization of NR occurred at low strain (λ = 1) for the plasticity deformed ChNFs/NR composites, which is far lower than that of neat NR (λ > 4). Moreover, the stretch-induced crystals even persisted in the stretched high-content ChNFs/NR composites when the stress was unloaded. The significantly advanced stretch-induced crystallization of NR indicated the presence of strong interfacial interaction between ChNFs and NRL. To explain the RPSM behavior of as-prepared ChNFs/NR composites, the formation of ChNFs network and its water-induced softening mechanism were proposed to resist or give way to the recovery force of chemical crosslinking network in NR. This work provides a new insight into the fabrication of RPSM composites for promising applications.

Studying the effects of carbon nanotube contents on stretch-induced crystallization behavior of polyethylene/carbon nanotube nanocomposites using molecular dynamics simulations.

In the present work, we used molecular dynamics simulations to study the effects of carbon nanotube (CNT) contents on stretch-induced crystallization behavior in CNT filled polyethylene systems. During high-temperature stretching, the stretching is responsible for the orientation of CNTs, which then facilitates segment orientation and conformational transition from the gauche-conformation into the trans-conformation in interfacial regions. The systems with higher CNT contents have a higher degree of orientation and higher contents of trans-conformation during stretching, resulting in the formation of more precursors. During subsequent crystallization, the initial crystallization rate increases with the increase of the CNT content due to the increase in precursor contents in interfacial regions. However, after the CNT content exceeds a certain value, a filler network would be formed by CNTs, which can restrict chain movements and then lead to a decrease in the overall crystallization rate in the systems with high CNT contents.

Strain Rate Dependence of Stretch-Induced Crystallization and Crystal Transition of Poly(dimethylsiloxane)

Strain Rate Dependence of Stretch-Induced Crystallization and Crystal Transition of Poly(dimethylsiloxane)

The formation of crystal cross-linked network in sequential biaxial stretching of poly(ethylene terephthalate): The essential role of MD pre-stretch

Combining a homemade uniaxial stretching device and the simultaneous in-situ synchrotron radiation WAXS measurement, the structural evolution during transverse direction (TD) stretching of machine direction (MD) pre-stretched Poly(ethylene terephthalate) (PET) film has been investigated. Current work reveals that increasing the MD pre-stretch before the occurrence of stretch induced crystallization (SIC) can enhance the crystallization and orientation in TD stretching. The concept of parallel chains structures (PCS) cross-linked network is proposed to interpret the rubber-like mechanical behavior. An appropriate stretch ratio window in stretching above the glass transition temperature (Tg) is existed, where the PCS cross-linked network before crystallization can be induced by stretching above Tg. With the help of the PCS cross-linked network, the chain orientation is retained more effectively under stretching, promoting the formation of subsequent crystallization and crystal cross-linking network. The strain hardening in different pre-stretched samples occurs accompanied by the same crystallinity and various chains orientation. This demonstrates that the strain hardening is determined by the crystal cross-linked network rather than the orientation. Current work supplies a guidance for getting the improved film products of PET with enhanced comprehensive mechanical properties, process stability and higher chain orientation.

Toughening Polylactic Acid by a Biobased Poly(Butylene 2,5-Furandicarboxylate)-b-Poly(Ethylene Glycol) Copolymer: Balanced Mechanical Properties and Potential Biodegradability.

Polylactic acid (PLA) is a biodegradable thermoplastic polyester produced from natural resources. Because of its brittleness, many tougheners have been developed. However, traditional toughening methods cause either the loss of modulus and strength or the lack of degradability. In this work, we synthesized a biobased and potentially biodegradable poly(butylene 2,5-furandicarboxylate)-b-poly(ethylene glycol) (PBFEG50) copolymer to toughen PLA, with the purpose of both keeping mechanical strength and enhancing the toughness. The blend containing 5 wt % PBFEG50 exhibited about 28.5 times increase in elongation at break (5.5% vs 156.5%). At the same time, the tensile modulus even strikingly increased by 21.6% while the tensile strength was seldom deteriorated. Such a phenomenon could be explained by the stretch-induced crystallization of the BF segment and the interconnected morphology of PBFEG50 domains in PLA5. The Raman spectrum was used to identify the phase dispersion of PLA and PBFEG50 phases. As the PBFEG50 content increased, the interconnected PBFEG50 domains start to separate, but their size increases. Interestingly, tensile-induced cavitation could be clearly identified in scanning electron microscopy images, which meant that the miscibility between PLA and PBFEG50 was limited. The crystallization of PLA/PBFEG50 blends was examined by differential scanning calorimetry, and the plasticizer effect of the EG segment on the PLA matrix could be confirmed. The rheological experiment revealed decreased viscosity of PLA/PBFEG50 blends, implying the possible greener processing. Finally, potential biodegradability of these blends was proved.

Stretch-Induced Intermediate Structures and Crystallization of Poly(dimethylsiloxane): The Effect of Filler Content

Stretch-induced crystallization (SIC) and structural transitions of poly(dimethylsiloxane) (PDMS) filled with different contents of nanosilica were studied with an in situ synchrotron radiation wid...

Stretchable and fatigue-resistant materials

In developing a material for a load-bearing application, attention inevitably falls on the resistance of the material to the growth of a crack, characterized by toughness under monotonic load, and by threshold under cyclic load. Many methods have been discovered to enhance toughness, but they do not enhance threshold. For example, stretch-induced crystallization and inorganic fillers have made the toughness of natural rubber well above 10000 J/m 2 , but have left the threshold of natural rubber around 50 J/m 2 . Here we describe a principle of stretchable and fatigue-resistant materials. To illustrate the principle, we embed unidirectional fibers of a soft and stretchable material in a matrix of a much softer and much more stretchable material, and adhere the fibers and the matrix by sparse and covalent interlinks. When the composite is cut with a crack and subject to a load, the soft matrix shears readily and delocalizes the high stretch of a fiber over a long segment. A threshold of 1290 J/m 2 is reached, below which the composite does not suffer any mode of failure (fiber break, kink crack, or matrix fracture). The principle of stretchable and fatigue-resistant materials is applicable to various materials, layouts, and methods of fabrication, opening an enormous design space for general applications.

Molecular Dynamics Simulation of Stretch-Induced Crystallization in Polyethylene: Emergence of Fiber Structure and Molecular Network

Molecular processes of fiber formation and network genesis in polyethylene (PE) are studied by molecular dynamics simulation of crystallization from highly stretched melt. We consider a system comprising 150 molecules of 500 united atoms, which is much larger than that of our previous work on fiber structures and their deformation [Polymer 2013, 54, 3086], aiming to study crystal nucleation and growth separately. Rapid elongation of the isotropic melt gives rise to incipient crystal clusters or embryos much smaller than critical nuclei. Subsequent isothermal crystallization of the stretched melt leads to three distinct stages of structure formation. The initial stage is a period waiting for critical nucleation. In the second stage that follows, we observe independent growth of isolated lamellae and resulting polynomial increases in crystallinity. In the last stage the crystals come to collide with each other to give well-aligned stacked lamellae. By dividing the system into mesh cells (pixels) and using a...

The effect of water absorption on stretch-induced crystallization of poly(ethylene terephthalate): An in-situ synchrotron radiation wide angle X-ray scattering study

The structural evolution of poly (ethylene terephthalate) (PET) with distinct water absorption during uniaxial deformation slightly above glass transition temperature (Tg) were investigated by in-situ synchrotron radiation wide angle X-ray scattering (WAXS). Results show that the locally ordered structures are not conducive to stretch-induced crystallization (SIC) although the corresponding entropy is more close to crystals compared to random chains, which is due to the limited chain mobility therein. The “frozen orders” should be destroyed to achieve the critical chain mobility required by SIC even with the presence of water as plasticizer. Unexpectedly, absorption of water does not enhance but suppresses SIC though it does promote the chain mobility, originating from lower stress level with higher water contents. Stress and chain mobility directly correlate with chain stretch and orientation in positive and negative manners, respectively, demonstrating that chain stretch and orientation also play important role in SIC around Tg.

Stretch-Induced Crystallization and Phase Transitions of Poly(dimethylsiloxane) at Low Temperatures: An in Situ Synchrotron Radiation Wide-Angle X-ray Scattering Study

Stretch-induced crystallization (SIC) and phase transitions of poly(dimethylsiloxane) (PDMS) have been studied with the in situ synchrotron radiation wide-angle X-ray scattering technique (WAXS) du...

Thermodynamics and mechanics of stretch-induced crystallization in rubbers.

The aim of the present paper is to provide a quantitative prediction of the stretch-induced crystallization in natural rubber, the exclusive reason for its history-dependent thermomechanical features. A constitutive model based on a micromechanism inspired molecular chain approach is formulated within the context of the thermodynamic framework. The molecular configuration of the partially crystallized single chain is analyzed and calculated by means of some statistical mechanical methods. The random thermal oscillation of the crystal orientation, considered as a continuous random variable, is treated by means of a representative angle. The physical expression of the chain free energy is derived according to a two-step strategy by separating crystallization and stretching. This strategy ensures that the stretch-induced part of the thermodynamic crystallization force is null at the initial instant and allows, without any additional constraint, the formulation of a simple linear relationship for the crystallinity evolution law. The model contains very few physically interpretable material constants to simulate the complex mechanism: two chain-scale constants, one crystallinity kinetics constant, three thermodynamic constants related to the newly formed crystallites, and a function controlling the crystal orientation with respect to the chain. The model is used to discuss some important aspects of the micromechanism and the macroresponse under the equilibrium state and the nonequilibrium state involved during stretching and recovery, and continuous relaxation.

Filler-induced heterogeneous distribution of stretch-induced crystallization in natural rubber: An in-situ synchrotron-radiation micro-focused scanning X-ray diffraction study

Crystallinity distribution of natural rubber around filled micrometer-size glass beads under different strains are studied with in-situ micro-focused scanning X-ray diffraction (μ-SXRD), where the glass beads are taken to serve as a model of filler. The experimental results suggest, due to the gradient stress field around the glass bead, the oscillatory distribution of crystallinities around the glass bead tends to form soft-hard double network with multi-scale hierarchical structures and spontaneously responds to external strains. The double network spreads stress in larger region and avoids stress concentration, in which the hard domains with high crystallinities effectively sustain the external stress while the soft domains with low crystallinity store elastic energy. According to the equal strain and equal stress conditions, the quantitative calculation on enhancement factor indicates that the soft-hard double network structure can increase the fracture energy to about three orders of magnitude as comparing to that of structure with homogeneous distribution of crystallinity.

Biaxial stretch-induced crystallization of poly(ethylene terephthalate) above glass transition temperature: The necessary of chain mobility

Abstract Strain-induced crystallization (SIC) of poly(ethylene terephthalate) (PET) slightly above glass transition temperature (Tg) is investigated by adopting a sequential biaxial stretching mode in this work. Small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) are employed to study variation of orientation parameter and crystallinity of stretch films in machine and transverse draw ratio space. Our results reveal that improving chain mobility by stretching is necessary before crystallization occurs slightly above Tg, during which the total external work of sequential biaxial stretching is not dissipated but “stored” in polymer chains with high mobility. The total crystallinity is precisely correlated with the total external work, which does not relate to stretch direction and demonstrate that enhanced chain mobility have no directionality. Considering the structures that restrict the polymer chains in the initial sample, a molecular model is proposed to interpret the structure changes during SIC of PET above Tg.