Our publications are classified into four areas: (1) Polymers and Soft Matter, (2) Advanced (Bio)Manufacturing, (3) Biomaterials, and (4) Bioengineering. Some of them are theoretical works or experimental works including theoretical component. Please click tags to sort papers of each category and Google Scholar for citations.
2024
11.
Jinchang Zhu, Yi He, Yong Wang, Li-Heng Cai
Voxelated bioprinting of modular double-network bio-ink droplets Journal Article
In: Nature Communications, vol. 15, pp. 5902, 2024.
Abstract | Links | Tags: Advanced (Bio)Manufacturing, Biomaterials, Theory
@article{Zhu2024,
title = {Voxelated bioprinting of modular double-network bio-ink droplets},
author = {Jinchang Zhu and Yi He and Yong Wang and Li-Heng Cai},
url = {https://doi.org/10.1038/s41467-024-49705-z},
doi = {10.1038/s41467-024-49705-z},
year = {2024},
date = {2024-01-01},
urldate = {2024-01-01},
journal = {Nature Communications},
volume = {15},
pages = {5902},
abstract = {Analogous of pixels to two-dimensional pictures, voxels—in the form of either small cubes or spheres—are the basic building blocks of three-dimensional objects. However, precise manipulation of viscoelastic bio-ink voxels in three-dimensional space represents a grand challenge in both soft matter science and biomanufacturing. Here, we present a voxelated bioprinting technology that enables the digital assembly of interpenetrating double-network hydrogel droplets made of polyacrylamide/alginate-based or hyaluronic acid/alginate-based polymers. The hydrogels are crosslinked via additive-free and biofriendly click reaction between a pair of stoichiometrically matched polymers carrying norbornene and tetrazine groups, respectively. We develop theoretical frameworks to describe the crosslinking kinetics and stiffness of the hydrogels, and construct a diagram-of-state to delineate their mechanical properties. Multi-channel print nozzles are developed to allow on-demand mixing of highly viscoelastic bio-inks without significantly impairing cell viability. Further, we showcase the distinctive capability of voxelated bioprinting by creating highly complex three-dimensional structures such as a hollow sphere composed of interconnected yet distinguishable hydrogel particles. Finally, we validate the cytocompatibility and in vivo stability of the printed double-network scaffolds through cell encapsulation and animal transplantation.},
keywords = {Advanced (Bio)Manufacturing, Biomaterials, Theory},
pubstate = {published},
tppubtype = {article}
}
Analogous of pixels to two-dimensional pictures, voxels—in the form of either small cubes or spheres—are the basic building blocks of three-dimensional objects. However, precise manipulation of viscoelastic bio-ink voxels in three-dimensional space represents a grand challenge in both soft matter science and biomanufacturing. Here, we present a voxelated bioprinting technology that enables the digital assembly of interpenetrating double-network hydrogel droplets made of polyacrylamide/alginate-based or hyaluronic acid/alginate-based polymers. The hydrogels are crosslinked via additive-free and biofriendly click reaction between a pair of stoichiometrically matched polymers carrying norbornene and tetrazine groups, respectively. We develop theoretical frameworks to describe the crosslinking kinetics and stiffness of the hydrogels, and construct a diagram-of-state to delineate their mechanical properties. Multi-channel print nozzles are developed to allow on-demand mixing of highly viscoelastic bio-inks without significantly impairing cell viability. Further, we showcase the distinctive capability of voxelated bioprinting by creating highly complex three-dimensional structures such as a hollow sphere composed of interconnected yet distinguishable hydrogel particles. Finally, we validate the cytocompatibility and in vivo stability of the printed double-network scaffolds through cell encapsulation and animal transplantation.
10.
Baiqiang Huang, Shifeng Nian, Li-Heng Cai
A universal strategy for decoupling stiffness and extensibility polymer networks Journal Article
In: Science Advances, vol. 10, pp. eadq3080, 2024.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Huang2024,
title = {A universal strategy for decoupling stiffness and extensibility polymer networks},
author = {Baiqiang Huang and Shifeng Nian and Li-Heng Cai},
doi = {10.1126/sciadv.adq3080},
year = {2024},
date = {2024-01-01},
urldate = {2024-01-01},
journal = {Science Advances},
volume = {10},
pages = {eadq3080},
abstract = {Since the invention of polymer networks in the 19th century (e.g., crosslinked natural rubber by Goodyear), it has been a dogma that stiffer networks are less stretchable, a trade-off inherent to the molecular nature of polymer network strands. Here, we report a universal strategy for decoupling the stiffness and extensibility of single-network elastomers. Instead of using linear polymers as network strands, we use foldable bottlebrush polymers, which feature a collapsed backbone grafted with many linear side chains. Upon elongation, the collapsed backbone unfolds to release stored length, enabling remarkable extensibility. By contrast, the network elastic modulus is inversely proportional to the network strand mass and is determined by the side chains. We validate this concept by creating a series of unentangled single-network elastomers with nearly constant Young's modulus (30 kPa) while increasing tensile breaking strain by 40-fold, from 20% to 800%. We show that this strategy applies to networks of different polymer species and topologies. Our discovery opens an avenue for developing polymer networks with extraordinary mechanical properties.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
Since the invention of polymer networks in the 19th century (e.g., crosslinked natural rubber by Goodyear), it has been a dogma that stiffer networks are less stretchable, a trade-off inherent to the molecular nature of polymer network strands. Here, we report a universal strategy for decoupling the stiffness and extensibility of single-network elastomers. Instead of using linear polymers as network strands, we use foldable bottlebrush polymers, which feature a collapsed backbone grafted with many linear side chains. Upon elongation, the collapsed backbone unfolds to release stored length, enabling remarkable extensibility. By contrast, the network elastic modulus is inversely proportional to the network strand mass and is determined by the side chains. We validate this concept by creating a series of unentangled single-network elastomers with nearly constant Young's modulus (30 kPa) while increasing tensile breaking strain by 40-fold, from 20% to 800%. We show that this strategy applies to networks of different polymer species and topologies. Our discovery opens an avenue for developing polymer networks with extraordinary mechanical properties.
2023
9.
Shifeng Nian, Baiqiang Huang, Guillaume Freychet, Mikhail Zhernenkov, Li-Heng Cai
Unexpected folding of bottlebrush polymers in melts Journal Article
In: Macromolecules, vol. 56, no. 6, pp. 2551–2559, 2023.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Nian2023a,
title = {Unexpected folding of bottlebrush polymers in melts},
author = {Shifeng Nian and Baiqiang Huang and Guillaume Freychet and Mikhail Zhernenkov and Li-Heng Cai},
doi = {10.1021/acs.macromol.2c02053},
year = {2023},
date = {2023-01-01},
urldate = {2023-01-01},
journal = {Macromolecules},
volume = {56},
number = {6},
pages = {2551–2559},
abstract = {Bottlebrush molecules are branched polymers with a long linear backbone densely grafted by many relatively short linear side chains. Such a unique molecular architecture enables bottlebrush polymers with properties and functions inaccessible by their linear counterparts. The existing understanding is that, in melts of bottlebrush polymers, the interbackbone distance decreases as the grafting density of side chains becomes smaller. Here, we experimentally discover a behavior opposite to all existing works: the interbackbone distance increases monotonically as the grafting density decreases. To explain these remarkable experimental findings, we develop a theory by accounting for the incompatibility between the backbone and side chains within a bottlebrush molecule. The backbone polymer folds into a cylindrical core with all grafting sites on its surface to reduce interfacial free energy. As the grafting density decreases, the backbone collapses; this process not only increases the diameter of the cylindrical core but also reduces the distance between grafting sites in space, such that the extension of side chains is not alleviated. Our discovery presents a paradigm-shifting understanding of the molecular structure of bottlebrush polymers.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
Bottlebrush molecules are branched polymers with a long linear backbone densely grafted by many relatively short linear side chains. Such a unique molecular architecture enables bottlebrush polymers with properties and functions inaccessible by their linear counterparts. The existing understanding is that, in melts of bottlebrush polymers, the interbackbone distance decreases as the grafting density of side chains becomes smaller. Here, we experimentally discover a behavior opposite to all existing works: the interbackbone distance increases monotonically as the grafting density decreases. To explain these remarkable experimental findings, we develop a theory by accounting for the incompatibility between the backbone and side chains within a bottlebrush molecule. The backbone polymer folds into a cylindrical core with all grafting sites on its surface to reduce interfacial free energy. As the grafting density decreases, the backbone collapses; this process not only increases the diameter of the cylindrical core but also reduces the distance between grafting sites in space, such that the extension of side chains is not alleviated. Our discovery presents a paradigm-shifting understanding of the molecular structure of bottlebrush polymers.
8.
Jinchang Zhu, Li-Heng Cai
All-aqueous printing of viscoelastic droplets in yield-stress fluids Journal Article
In: Acta Biomaterialia, vol. 165, no. 2023, pp. 60–71, 2023.
Abstract | Links | Tags: Advanced (Bio)Manufacturing, Bioengineering, Theory
@article{Zhu2023,
title = {All-aqueous printing of viscoelastic droplets in yield-stress fluids},
author = {Jinchang Zhu and Li-Heng Cai},
url = {https://doi.org/10.1016/j.actbio.2022.09.031},
doi = {10.1016/j.actbio.2022.09.031},
year = {2023},
date = {2023-01-01},
urldate = {2023-01-01},
journal = {Acta Biomaterialia},
volume = {165},
number = {2023},
pages = {60–71},
publisher = {Elsevier Ltd},
abstract = {All-aqueous printing of viscoelastic droplets (aaPVD) in yield-stress fluids is the core of an emerging voxelated bioprinting technology that enables the digital assembly of spherical bio-ink particles (DASP) to create functional tissue mimics. However, the mechanism of aaPVD is largely unknown. Here, by quantifying the dynamics of the whole printing process in real-time, we identify two parameters critical to aaPVD: (1) acceleration of print nozzle, and (2) droplet/nozzle diameter ratio. Moreover, we distinguish three stages associated with aaPVD: droplet generation, detachment, and relaxation. To generate a droplet of good roundness, the ink should be a highly viscous shear-thinning fluid. Using particle image velocimetry and scaling theory, we establish a universal description for the droplet displacements at various printing conditions. Along the direction of nozzle movement, the droplet displacement is determined by the detachment number, a dimensionless parameter defined as the ratio between the dragging force from the nozzle and the confinement force from the supporting matrix. Perpendicular to the direction of nozzle movement, the droplet displacement is determined by the Oldroyd number, a dimensionless parameter that describes the yielded area of the supporting matrix near the print nozzle. For a relaxed droplet, the droplet tail length is independent of droplet/nozzle diameter ratio but determined by the nozzle acceleration. We conclude that printing droplets of good fidelity requires a relatively large droplet/nozzle diameter ratio and intermediate nozzle accelerations. These ensure that the droplet is more solid-like to not flow with the nozzle to form a tadpole-like morphology and that the confinement force from the yield-stress fluid is large enough to prevent large droplet displacement. Our results provide the knowledge and tools for in situ generating and depositing highly viscoelastic droplets of good roundness at prescribed locations in 3D space, which help establish the foundational science for voxelated bioprinting. Statement of significance: Analogues of pixels to two-dimensional (2D) pictures, voxels – in the form of small cubes or spheres – are the basic units of three-dimensional (3D) objects. All-aqueous printing of viscoelastic droplets (aaPVD) is the core of voxelated bioprinting, an emerging technology that uses spherical bio-ink voxels as building blocks to create 3D tissue mimics. Unlike existing technologies relying on the classic Rayleigh-Plateau instability to generate droplets, aaPVD exploits previously unexplored nonlinear fluid dynamics of complex fluids to precisely manipulate viscoelastic droplets in 3D space. The developed knowledge and tools not only help advance biomanufacturing but also stimulate new research directions in soft matter and complex fluids.},
keywords = {Advanced (Bio)Manufacturing, Bioengineering, Theory},
pubstate = {published},
tppubtype = {article}
}
All-aqueous printing of viscoelastic droplets (aaPVD) in yield-stress fluids is the core of an emerging voxelated bioprinting technology that enables the digital assembly of spherical bio-ink particles (DASP) to create functional tissue mimics. However, the mechanism of aaPVD is largely unknown. Here, by quantifying the dynamics of the whole printing process in real-time, we identify two parameters critical to aaPVD: (1) acceleration of print nozzle, and (2) droplet/nozzle diameter ratio. Moreover, we distinguish three stages associated with aaPVD: droplet generation, detachment, and relaxation. To generate a droplet of good roundness, the ink should be a highly viscous shear-thinning fluid. Using particle image velocimetry and scaling theory, we establish a universal description for the droplet displacements at various printing conditions. Along the direction of nozzle movement, the droplet displacement is determined by the detachment number, a dimensionless parameter defined as the ratio between the dragging force from the nozzle and the confinement force from the supporting matrix. Perpendicular to the direction of nozzle movement, the droplet displacement is determined by the Oldroyd number, a dimensionless parameter that describes the yielded area of the supporting matrix near the print nozzle. For a relaxed droplet, the droplet tail length is independent of droplet/nozzle diameter ratio but determined by the nozzle acceleration. We conclude that printing droplets of good fidelity requires a relatively large droplet/nozzle diameter ratio and intermediate nozzle accelerations. These ensure that the droplet is more solid-like to not flow with the nozzle to form a tadpole-like morphology and that the confinement force from the yield-stress fluid is large enough to prevent large droplet displacement. Our results provide the knowledge and tools for in situ generating and depositing highly viscoelastic droplets of good roundness at prescribed locations in 3D space, which help establish the foundational science for voxelated bioprinting. Statement of significance: Analogues of pixels to two-dimensional (2D) pictures, voxels – in the form of small cubes or spheres – are the basic units of three-dimensional (3D) objects. All-aqueous printing of viscoelastic droplets (aaPVD) is the core of voxelated bioprinting, an emerging technology that uses spherical bio-ink voxels as building blocks to create 3D tissue mimics. Unlike existing technologies relying on the classic Rayleigh-Plateau instability to generate droplets, aaPVD exploits previously unexplored nonlinear fluid dynamics of complex fluids to precisely manipulate viscoelastic droplets in 3D space. The developed knowledge and tools not only help advance biomanufacturing but also stimulate new research directions in soft matter and complex fluids.
7.
Shifeng Nian, Shalin Patil, Siteng Zhang, Myoeum Kim, Quan Chen, Mikhail Zhernenkov, Ting Ge, Shiwang Cheng, Li-Heng Cai
Dynamics of associative polymers with high density of reversible bonds Journal Article
In: Physical Review Letters, vol. 130, no. 22, pp. 228101, 2023.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Nian2023,
title = {Dynamics of associative polymers with high density of reversible bonds},
author = {Shifeng Nian and Shalin Patil and Siteng Zhang and Myoeum Kim and Quan Chen and Mikhail Zhernenkov and Ting Ge and Shiwang Cheng and Li-Heng Cai},
doi = {10.1103/PhysRevLett.130.228101},
year = {2023},
date = {2023-01-01},
urldate = {2023-01-01},
journal = {Physical Review Letters},
volume = {130},
number = {22},
pages = {228101},
publisher = {American Physical Society},
abstract = {An associative polymer carries many stickers that can form reversible associations. For more than 30 years, the understanding has been that reversible associations change the shape of linear viscoelastic spectra by adding a rubbery plateau in the intermediate frequency range, at which associations have not yet relaxed and thus effectively act as crosslinks. Here, we design and synthesize new classes of unentangled associative polymers carrying unprecedentedly high fractions of stickers, up to eight per Kuhn segment, that can form strong pairwise hydrogen bonding of ∼20kBT without microphase separation. We experimentally show that reversible bonds significantly slow down the polymer dynamics but nearly do not change the shape of linear viscoelastic spectra. This behavior can be explained by a renormalized Rouse model that highlights an unexpected influence of reversible bonds on the structural relaxation of associative polymers.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
An associative polymer carries many stickers that can form reversible associations. For more than 30 years, the understanding has been that reversible associations change the shape of linear viscoelastic spectra by adding a rubbery plateau in the intermediate frequency range, at which associations have not yet relaxed and thus effectively act as crosslinks. Here, we design and synthesize new classes of unentangled associative polymers carrying unprecedentedly high fractions of stickers, up to eight per Kuhn segment, that can form strong pairwise hydrogen bonding of ∼20kBT without microphase separation. We experimentally show that reversible bonds significantly slow down the polymer dynamics but nearly do not change the shape of linear viscoelastic spectra. This behavior can be explained by a renormalized Rouse model that highlights an unexpected influence of reversible bonds on the structural relaxation of associative polymers.
2022
6.
Shifeng Nian, Li-Heng Cai
Dynamic mechanical properties of self-assembled bottlebrush polymer networks Journal Article
In: Macromolecules, vol. 55, no. 18, pp. 8058–8066, 2022.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Nian,
title = {Dynamic mechanical properties of self-assembled bottlebrush polymer networks},
author = {Shifeng Nian and Li-Heng Cai},
url = {https://pubs.acs.org/doi/10.1021/acs.macromol.2c01204},
year = {2022},
date = {2022-01-01},
urldate = {2022-01-01},
journal = {Macromolecules},
volume = {55},
number = {18},
pages = {8058–8066},
abstract = {We systematically investigate the effects of composition on the dynamic mechanical properties of bottlebrush polymer networks self-assembled by linear–bottlebrush–linear triblock copolymers. We fix the molecular architecture of the bottlebrush, which consists of 51 poly(dimethyl siloxane) (PDMS) side chains of 5 kg/mol and has a molecular weight of 255 kg/mol, and increase only the volume fraction f of the linear poly(benzyl methacrylate) (PBnMA) blocks. As f increases from 0.05 to 0.41, the network shear modulus G at room temperature increases from ∼4 to ∼100 kPa. Yet, depending on the network morphology, the relation between G and f exhibits two regimes. (i) For sphere morphology, G is nearly a constant; yet, because of a large fraction of loops, the absolute value of G is about 40% of the stiffness Gm of the PDMS bottlebrush matrix. (ii) For cylinder morphology, G increases slowly with f but remains nearly 4 orders of magnitude lower than 109 Pa for the glassy cylinders formed by the end PBnMA blocks. We explain this remarkable behavior by modeling the polymer as a polycrystalline material consisting of randomly oriented grains, and each grain is a fiber-reinforced composite. We propose a modified Halpin–Tsai model to describe the shear modulus of such a polycrystalline material: G = Gm(1+ζf)/(1–f), in which ζ is an adjustable parameter that describes the grain size relative to the fiber diameter. Above the glass-transition temperature of end blocks, the reinforcement to network modulus from the glassy fibers diminishes, such that G becomes a constant of the matrix stiffness. Our results not only reveal previously unexplored molecule–structure–property relations of self-assembled bottlebrush polymer networks but also provide a new class of soft, solvent-free, and reprocessable polymeric materials with a wide range of controllable stiffness.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
We systematically investigate the effects of composition on the dynamic mechanical properties of bottlebrush polymer networks self-assembled by linear–bottlebrush–linear triblock copolymers. We fix the molecular architecture of the bottlebrush, which consists of 51 poly(dimethyl siloxane) (PDMS) side chains of 5 kg/mol and has a molecular weight of 255 kg/mol, and increase only the volume fraction f of the linear poly(benzyl methacrylate) (PBnMA) blocks. As f increases from 0.05 to 0.41, the network shear modulus G at room temperature increases from ∼4 to ∼100 kPa. Yet, depending on the network morphology, the relation between G and f exhibits two regimes. (i) For sphere morphology, G is nearly a constant; yet, because of a large fraction of loops, the absolute value of G is about 40% of the stiffness Gm of the PDMS bottlebrush matrix. (ii) For cylinder morphology, G increases slowly with f but remains nearly 4 orders of magnitude lower than 109 Pa for the glassy cylinders formed by the end PBnMA blocks. We explain this remarkable behavior by modeling the polymer as a polycrystalline material consisting of randomly oriented grains, and each grain is a fiber-reinforced composite. We propose a modified Halpin–Tsai model to describe the shear modulus of such a polycrystalline material: G = Gm(1+ζf)/(1–f), in which ζ is an adjustable parameter that describes the grain size relative to the fiber diameter. Above the glass-transition temperature of end blocks, the reinforcement to network modulus from the glassy fibers diminishes, such that G becomes a constant of the matrix stiffness. Our results not only reveal previously unexplored molecule–structure–property relations of self-assembled bottlebrush polymer networks but also provide a new class of soft, solvent-free, and reprocessable polymeric materials with a wide range of controllable stiffness.
2020
5.
Li Heng Cai
Molecular understanding for large deformations of soft bottlebrush polymer networks Journal Article
In: Soft Matter, vol. 16, no. 27, pp. 6259–6264, 2020.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Cai2020,
title = {Molecular understanding for large deformations of soft bottlebrush polymer networks},
author = {Li Heng Cai},
doi = {10.1039/d0sm00759e},
year = {2020},
date = {2020-01-01},
urldate = {2020-01-01},
journal = {Soft Matter},
volume = {16},
number = {27},
pages = {6259–6264},
publisher = {Royal Society of Chemistry},
abstract = {Networks formed by crosslinking bottlebrush polymers are a class of soft materials with stiffnesses matching that of 'watery' hydrogels and biological tissues but contain no solvents. Because of their extreme softness, bottlebrush polymer networks are often subject to large deformations. However, it is poorly understood how molecular architecture determines the extensibility of the networks. Using a combination of experimental and theoretical approaches, we discover that the yield strain γy of the network equals the ratio of the contour length Lmax to the end-to-end distance R of the bottlebrush between two neighboring crosslinks: γy = Lmax/R - 1. This relation suggests two regimes: (1) for stiff bottlebrush polymers, γy is inversely proportional to the network shear modulus G, γy ∼ G-1, which represents a previously unrecognized regime; (2) for flexible bottlebrush polymers, γy ∼ G-1/2, which recovers the behavior of conventional polymer networks. Our findings provide a new molecular understanding of the nonlinear mechanics for soft bottlebrush polymer networks. This journal is},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
Networks formed by crosslinking bottlebrush polymers are a class of soft materials with stiffnesses matching that of 'watery' hydrogels and biological tissues but contain no solvents. Because of their extreme softness, bottlebrush polymer networks are often subject to large deformations. However, it is poorly understood how molecular architecture determines the extensibility of the networks. Using a combination of experimental and theoretical approaches, we discover that the yield strain γy of the network equals the ratio of the contour length Lmax to the end-to-end distance R of the bottlebrush between two neighboring crosslinks: γy = Lmax/R – 1. This relation suggests two regimes: (1) for stiff bottlebrush polymers, γy is inversely proportional to the network shear modulus G, γy ∼ G-1, which represents a previously unrecognized regime; (2) for flexible bottlebrush polymers, γy ∼ G-1/2, which recovers the behavior of conventional polymer networks. Our findings provide a new molecular understanding of the nonlinear mechanics for soft bottlebrush polymer networks. This journal is
2015
4.
Li-Heng Cai, Thomas E. Kodger, Rodrigo E. Guerra, Adrian F. Pegoraro, Michael Rubinstein, David A. Weitz
Soft Poly(dimethylsiloxane) elastomers from architecture-driven entanglement free design Journal Article
In: Advanced Materials, vol. 27, no. 35, pp. 5132–5140, 2015.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Cai2015a,
title = {Soft Poly(dimethylsiloxane) elastomers from architecture-driven entanglement free design},
author = {Li-Heng Cai and Thomas E. Kodger and Rodrigo E. Guerra and Adrian F. Pegoraro and Michael Rubinstein and David A. Weitz},
doi = {10.1002/adma.201502771},
year = {2015},
date = {2015-09-01},
urldate = {2015-09-01},
journal = {Advanced Materials},
volume = {27},
number = {35},
pages = {5132–5140},
abstract = {Soft, solvent-free poly(dimethylsiloxane) elastomers are fabricated by a one-step process via crosslinking bottlebrush polymers. The bottlebrush architecture prevents the formation of entanglements, resulting in elastomers with precisely controllable low moduli from 1 to 100 kPa, below the lower limit of traditional elastomers; moreover, the solvent-free nature enables their negligible adhesiveness compared to commercially available silicone products of similar stiffness.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
Soft, solvent-free poly(dimethylsiloxane) elastomers are fabricated by a one-step process via crosslinking bottlebrush polymers. The bottlebrush architecture prevents the formation of entanglements, resulting in elastomers with precisely controllable low moduli from 1 to 100 kPa, below the lower limit of traditional elastomers; moreover, the solvent-free nature enables their negligible adhesiveness compared to commercially available silicone products of similar stiffness.
3.
Li-Heng Cai, Sergey Panyukov, Michael Rubinstein
Hopping diffusion of nanoparticles in polymer matrices Journal Article
In: Macromolecules, vol. 48, no. 3, pp. 847–862, 2015.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Cai2015,
title = {Hopping diffusion of nanoparticles in polymer matrices},
author = {Li-Heng Cai and Sergey Panyukov and Michael Rubinstein},
doi = {10.1021/ma501608x},
year = {2015},
date = {2015-01-01},
urldate = {2015-01-01},
journal = {Macromolecules},
volume = {48},
number = {3},
pages = {847–862},
publisher = {American Chemical Society},
abstract = {We propose a hopping mechanism for diffusion of large nonsticky nanoparticles subjected to topological constraints in both unentangled and entangled polymer solids (networks and gels) and entangled polymer liquids (melts and solutions). Probe particles with size larger than the mesh size ax of unentangled polymer networks or tube diameter ae of entangled polymer liquids are trapped by the network or entanglement cells. At long time scales, however, these particles can diffuse by overcoming free energy barrier between neighboring confinement cells. The terminal particle diffusion coefficient dominated by this hopping diffusion is appreciable for particles with size moderately larger than the network mesh size ax or tube diameter ae. Much larger particles in polymer solids will be permanently trapped by local network cells, whereas they can still move in polymer liquids by waiting for entanglement cells to rearrange on the relaxation time scales of these liquids. Hopping diffusion in entangled polymer liquids and networks has a weaker dependence on particle size than that in unentangled networks as entanglements can slide along chains under polymer deformation. The proposed novel hopping model enables understanding the motion of large nanoparticles in polymeric nanocomposites and the transport of nano drug carriers in complex biological gels such as mucus.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
We propose a hopping mechanism for diffusion of large nonsticky nanoparticles subjected to topological constraints in both unentangled and entangled polymer solids (networks and gels) and entangled polymer liquids (melts and solutions). Probe particles with size larger than the mesh size ax of unentangled polymer networks or tube diameter ae of entangled polymer liquids are trapped by the network or entanglement cells. At long time scales, however, these particles can diffuse by overcoming free energy barrier between neighboring confinement cells. The terminal particle diffusion coefficient dominated by this hopping diffusion is appreciable for particles with size moderately larger than the network mesh size ax or tube diameter ae. Much larger particles in polymer solids will be permanently trapped by local network cells, whereas they can still move in polymer liquids by waiting for entanglement cells to rearrange on the relaxation time scales of these liquids. Hopping diffusion in entangled polymer liquids and networks has a weaker dependence on particle size than that in unentangled networks as entanglements can slide along chains under polymer deformation. The proposed novel hopping model enables understanding the motion of large nanoparticles in polymeric nanocomposites and the transport of nano drug carriers in complex biological gels such as mucus.
2013
2.
Evgeny B. Stukalin, Li-Heng Cai, N. Arun Kumar, Ludwik Leibler, Michael Rubinstein
Self-healing of unentangled polymer networks with reversible bonds Journal Article
In: Macromolecules, vol. 46, no. 18, pp. 7525–7541, 2013.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Stukalin2013,
title = {Self-healing of unentangled polymer networks with reversible bonds},
author = {Evgeny B. Stukalin and Li-Heng Cai and N. Arun Kumar and Ludwik Leibler and Michael Rubinstein},
doi = {10.1021/ma401111n},
year = {2013},
date = {2013-09-01},
urldate = {2013-09-01},
journal = {Macromolecules},
volume = {46},
number = {18},
pages = {7525–7541},
publisher = {American Chemical Society},
abstract = {Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory we study a simple model of autonomic self-healing of unentangled polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. We study the reaction kinetics of reversible bonds in this simple model and analyze the different stages in the self-repair process. The formation of bridges and the recovery of the material strength across the fractured interface during the healing period occur appreciably faster after shorter waiting time, during which the fractured surfaces are kept apart. We observe the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium (very low) density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess nonequilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface (formation of bridges) occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk. © 2013 American Chemical Society.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory we study a simple model of autonomic self-healing of unentangled polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. We study the reaction kinetics of reversible bonds in this simple model and analyze the different stages in the self-repair process. The formation of bridges and the recovery of the material strength across the fractured interface during the healing period occur appreciably faster after shorter waiting time, during which the fractured surfaces are kept apart. We observe the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium (very low) density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess nonequilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface (formation of bridges) occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk. © 2013 American Chemical Society.
2011
1.
Li-Heng Cai, Sergey Panyukov, Michael Rubinstein
Mobility of nonsticky nanoparticles in polymer liquids Journal Article
In: Macromolecules, vol. 44, no. 19, pp. 7853–7863, 2011.
Abstract | Links | Tags: Polymers and Soft Matter, Theory
@article{Cai2011,
title = {Mobility of nonsticky nanoparticles in polymer liquids},
author = {Li-Heng Cai and Sergey Panyukov and Michael Rubinstein},
doi = {10.1021/ma201583q},
year = {2011},
date = {2011-01-01},
urldate = {2011-01-01},
journal = {Macromolecules},
volume = {44},
number = {19},
pages = {7853–7863},
publisher = {American Chemical Society},
abstract = {We use scaling theory to derive the time dependence of the mean-square displacement 〈Δr2〉 of a probe nanoparticle of size d experiencing thermal motion in polymer solutions and melts. Particles with size smaller than solution correlation length undergo ordinary diffusion (〈Δr2(t)〉 ∼ t) with diffusion coefficient similar to that in pure solvent. The motion of particles of intermediate size (< d < a), where a is the tube diameter for entangled polymer liquids, is subdiffusive (〈Δr2(t)〉 ∼ t1/2) at short time scales since their motion is affected by subsections of polymer chains. At long time scales the motion of these particles is diffusive, and their diffusion coefficient is determined by the effective viscosity of a polymer liquid with chains of size comparable to the particle diameter d. The motion of particles larger than the tube diameter a at time scales shorter than the relaxation time τe of an entanglement strand is similar to the motion of particles of intermediate size. At longer time scales (t > τe) large particles (d > a) are trapped by entanglement mesh, and to move further they have to wait for the surrounding polymer chains to relax at the reptation time scale τrep. At longer times t > τrep, the motion of such large particles (d > a) is diffusive with diffusion coefficient determined by the bulk viscosity of the entangled polymer liquids. Our predictions are in agreement with the results of experiments and computer simulations. © 2011 American Chemical Society.},
keywords = {Polymers and Soft Matter, Theory},
pubstate = {published},
tppubtype = {article}
}
We use scaling theory to derive the time dependence of the mean-square displacement 〈Δr2〉 of a probe nanoparticle of size d experiencing thermal motion in polymer solutions and melts. Particles with size smaller than solution correlation length undergo ordinary diffusion (〈Δr2(t)〉 ∼ t) with diffusion coefficient similar to that in pure solvent. The motion of particles of intermediate size (< d < a), where a is the tube diameter for entangled polymer liquids, is subdiffusive (〈Δr2(t)〉 ∼ t1/2) at short time scales since their motion is affected by subsections of polymer chains. At long time scales the motion of these particles is diffusive, and their diffusion coefficient is determined by the effective viscosity of a polymer liquid with chains of size comparable to the particle diameter d. The motion of particles larger than the tube diameter a at time scales shorter than the relaxation time τe of an entanglement strand is similar to the motion of particles of intermediate size. At longer time scales (t > τe) large particles (d > a) are trapped by entanglement mesh, and to move further they have to wait for the surrounding polymer chains to relax at the reptation time scale τrep. At longer times t > τrep, the motion of such large particles (d > a) is diffusive with diffusion coefficient determined by the bulk viscosity of the entangled polymer liquids. Our predictions are in agreement with the results of experiments and computer simulations. © 2011 American Chemical Society.