Negative thermal expansion in oriented crystalline polymers

Choy, C. L.; Chen, F. C.; Young, K.

doi:10.1002/pol.1981.180190213

Cited by 128 publications

(88 citation statements)

References 23 publications

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“…Experimental studies have shown that thermal expansion in oriented polymer fibers can be highly anisotropic [23,27]. Typically, oriented polymers show small expansions and sometimes contractions in the length direction on heating along with diameter direction expansion of significantly larger magnitude [27].…”

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

“…Typically, oriented polymers show small expansions and sometimes contractions in the length direction on heating along with diameter direction expansion of significantly larger magnitude [27]. Thermal contraction in the fiber length direction has been attributed to contributions from both crystalline and non-crystalline regions in the oriented fiber [27].…”

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

“…The thermal expansion due to the crystalline regions is negative in the chain direction for polymers like polyethylene and nylon [28]. However, this contribution is small and the major contribution to the negative thermal expansion in the fiber direction comes from the rubber-like elasticity of the extended amorphous tie molecules in the inter-crystalline regions [27]. These chains remain highly extended after drawing as a result of the formation of the crystalline bridges.…”

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

“…When heated, the modulus of the amorphous tie molecules increases due to the thermodynamics of rubber elasticity [29] and thereby further compresses the crystalline bridges since the modulus of the crystalline bridges does not change appreciably on heating. The net result is a thermal contraction in the draw direction (up to -40x10 -5 K -1 ) that is much larger than the change in dimensions of the crystal lattice [27]. This mechanism operates only at temperatures above the T g of the polymer.…”

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

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Stretchable artificial muscles from coiled polymer fibers

Spinks

2016

J. Mater. Res.

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Soft robots are being developed to mimic the movement of biological organisms and as wearable garments to assist human movement in rehabilitation, training, and tasks encountered in functional daily living. Stretchable artificial muscles are well suited as the active mechanical element in soft wearable robotics, and here the performance of highly stretchable and compliant polymer coil muscles are described and analyzed. The force and displacements generated by a given stimulus are shown to be determined by the external loading conditions and the main material properties of free stroke and stiffness. Spring mechanics and a model based on a single helix are used to evaluate both the coil stiffness and the mechanism of coil actuation. The latter is directly coupled to a torsional actuation in the twisted fiber that forms the coil. The single helix model illustrates how fiber volume changes generate a partial fiber untwist, and spring mechanics shows how this fiber untwist generates large tensile strokes and high gravimetric work outputs in the polymer coil muscles. These analyses highlight possible as yet unexplored means for further enhancing the performance of these systems. Disciplines Engineering | Science and Technology Studies AbstractSoft robots are being developed to mimic the movement of biological organisms and as wearable garments to assist human movement in rehabilitation, training, and tasks encountered in functional daily living. Stretchable artificial muscles are well suited as the active mechanical element in soft wearable robotics and here the performance of highly stretchable and compliant polymer coil muscles are described and analysed. The force and displacements generated by a given stimulus are shown to be determined by the external loading conditions and the main material properties of free stroke and stiffness. Spring mechanics and a model based on a single helix are used to evaluate both the coil stiffness and the mechanism of coil actuation. The latter is directly coupled to a torsional actuation in the twisted fiber that forms the coil. The single helix model illustrates how fiber volume changes generate a partial fiber untwist and spring mechanics shows how this fiber untwist generates large tensile strokes and high gravimetric work outputs in the polymer coil muscles. These analyses highlight possible as yet unexplored means for further enhancing the performance of these systems.

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Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

Section: Origin Of Anisotropic Volume Expansion In Oriented Polymer Fmentioning

confidence: 99%

See 2 more Smart Citations

Stretchable artificial muscles from coiled polymer fibers

Spinks

2016

J. Mater. Res.

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“…However, in addition to them, the other factors, iPP crystal orientation 9) and the tie molecules pulled out from the iPP crystalline region 26,27) will also be deeply connected with this anisotropic CLTE phenomenon. We will now carry out the analysis of the CLTE anisotropy by introducing these factors together with the TEMT results obtained in this study.…”

Section: Correlation Of Clte Anisotropy With Irregularities In the Eomentioning

confidence: 99%

Study on Three-dimensional Structures in Injection-molded iPP/Poly(ethylene-co-octene) by Transmission Electron Microtomography

et al. 2006

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One of the conventional solutions for such issue is adding inorganic fillers (talc, glass fiber and mica) with high aspect ratios, and suppressing the volume expansion by simple mechanical restraints [4][5][6] . However, such filler-PP composites have caused poor appearance and weight increase in molded articles fabricated using them. Recently, new techniques were proposed to reduce the CLTE of the injection-molded iPP-based parts in which thermoplastic elastomers (TPE) having well-controlled structures are used as an additive instead of fillers to tune the CLTE [7][8][9][10] . In these papers, the elastomers inclusions were highly deformed both along the FD and transverseto-flow (TD) directions and co-continuously dispersed with the iPP. Abstract A phase-separated structure of the injection-molded isotactic polypropylene (iPP)/poly(ethylene-co-octene) (EOR) binary blend was studied in three-dimension (3D) by transmission electron microtomography (TEMT). Highly oriented EOR domains along both flow-(FD) and transverse-to-flow (TD) directions resulting in stacking lamella-sheet like structures to normal direction (ND) were confirmed. Some irregularities in morphology and intervals between the EOR sheets, and thickness heterogeneity of the sheets, were observed more frequently in the TD rather than in the FD. Using the 3D information obtained by the TEMT, we have tried to elucidate massive anisotropy in linear thermal expansion coefficient (CLTE) along the injection directions in this blend. We found that the CLTE anisotropy was well correlated with the lamella-like sheets arrays and their irregularities.

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Bioinspired Structures for Soft Actuators

Wei

Ghosh

2022

Adv Materials Technologies

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biological energy into mechanical output. In addition to the actively controlled actuation behavior, plants also produce passive motions driven by the swelling and drying of cellulose materials on the cell walls under the ambient moisture change. [3] These cell-level actuators are shaped, assembled, and structured with other components hierarchically, determining their macroscopic actuation performance. From the mechanical point of view, the actuation behaviors are governed by the stress and strain distribution at various levels of the structure composed of well-arranged active components and supportive matrices. The opening of a pinecone best manifests this principle to release seeds at low humidity conditions. [4] Consisting of two macroscopic layers of tissues with different coefficients of hygroscopic swelling, the scales of pinecone bend outward as the outer layer shrinks more significantly under decreasing humidity. Interestingly, the swelling ability of the two layers is governed at the cellular level, as the special cellulose fibril arrangements on the cell walls effectively modify the stiffness of the cells. While bioinspiration aims to reproduce a functional outcome by drawing on ideas from nature or understanding the principles that underlie natural processes, biomimicry seeks to replicate the mechanism underlying the specific functional behavior found in nature. Notwithstanding the subtle distinction, understanding the fundamentals of biological functions allows us to appreciate why cells and organisms have the structures they do and provides clues to create synthetic systems. Structural design solutions in nature have always been a source of inspiration for scientists and engineers to advance their fields, such as robust yet resilient mechanical structures, [5] biomimetic functional textiles, [6] dry adhesives, [7] drag reduction surfaces, [8] etc. Given that actuation is essential for almost all manmade machines to accomplish their designed functions, scientists are continually looking at every natural environment to find inspiration and ideas to create the new generation of automatons soft robots that can stretch, flex, and morph with high degrees of freedom (DOF). Compared to the conventional rigid actuators such as electric motors, the soft actuators have mechanical properties and actuation behaviors closer to the natural actuators. Therefore, the lessons from nature, particularly the intimate relationship between the structures of biological actuators and their behavior from cellular processes to whole organism functions, provide tremendous insights into designing soft actuators.Biological organisms present marvelous morphing behaviors from the quiescent blooming of flowers to the energetic wing-flapping of birds that have always inspired humans to design better-engineered products. The diversity of natural motion is attributed primarily to the intricate and hierarchical structure of actuators that are self-assembled from nanoscale structures to superstructures. Compared to the biological actuat...

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Negative thermal expansion in oriented crystalline polymers

Cited by 128 publications

References 23 publications

Stretchable artificial muscles from coiled polymer fibers

Stretchable artificial muscles from coiled polymer fibers

Study on Three-dimensional Structures in Injection-molded iPP/Poly(ethylene-co-octene) by Transmission Electron Microtomography

Bioinspired Structures for Soft Actuators

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