@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}

}

@article{Kim2024,

title = {3D printable modular soft elastomers from physically cross-linked homogeneous associative polymers},

author = {Myoeum Kim and Shifeng Nian and Daniel A. Rau and Baiqiang Huang and Jinchang Zhu and Guillaume Freychet and Mikhail Zhernenkov and Li-Heng Cai},

doi = {10.1021/acspolymersau.3c00021},

year  = {2024},

date = {2024-01-01},

urldate = {2024-01-01},

journal = {ACS Polymers Au},

volume = {4},

number = {2},

pages = {98–108},

abstract = {Three-dimensional (3D) printing of elastomers enables the fabrication of many technologically important structures and devices. However, there remains a critical need for the development of reprocessable, solvent-free, soft elastomers that can be printed without the need for post-treatment. Herein, we report modular soft elastomers suitable for direct ink writing (DIW) printing by physically cross-linking associative polymers with a high fraction of reversible bonds. We designed and synthesized linear-associative-linear (LAL) triblock copolymers; the middle block is an associative polymer carrying amide groups that form double hydrogen bonding, and the end blocks aggregate to hard glassy domains that effectively act as physical cross-links. The amide groups do not aggregate to nanoscale clusters and only slow down polymer dynamics without changing the shape of the linear viscoelastic spectra; this enables molecular control over energy dissipation by varying the fraction of the associative groups. Increasing the volume fraction of the end linear blocks increases the network stiffness by more than 100 times without significantly compromising the extensibility. We created elastomers with Young's moduli ranging from 8 kPa to 8 MPa while maintaining the tensile breaking strain around 150%. Using a high-temperature DIW printing platform, we transformed our elastomers to complex, highly deformable 3D structures without involving any solvent or post-print processing. Our elastomers represent the softest melt reprocessable materials for DIW printing. The developed LAL polymers synergize emerging homogeneous associative polymers with a high fraction of reversible bonds and classical block copolymer self-assembly to form a dual-cross-linked network, providing a versatile platform for the modular design and development of soft melt reprocessable elastomeric materials for practical applications.},

keywords = {Advanced (Bio)Manufacturing, Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@article{Nianb,

title = {Self-assembly of flexible linear-semiflexible bottlebrush-flexible linear triblock copolymers},

author = {Shifeng Nian and Zhouhao Fan and Guillaume Freychet and Mikhail Zhernenkov and Stefanie Redemann and Li-Heng Cai},

doi = {10.1021/acs.macromol.1c01911},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Macromolecules},

volume = {54},

number = {20},

pages = {9361–9371},

abstract = {Block copolymer (BCP) self-assembly is a fundamental process in which incompatible blocks spontaneously form organized microstructures with broad practical applications. Classical understanding is that the domain spacing is limited by the contour length of the polymer backbone. Here, using a combination of molecular design, chemical synthesis, small-/wide-angle X-ray scattering, transmission electron microscopy, and electron tomography, we discover that this molecular picture does not hold for architecturally semiflexible BCPs. For strongly segregated linear-semiflexible bottlebrush-linear triblock copolymers, the size of the bottlebrush domain can be twice the bottlebrush backbone contour length. The mechanism of such anomalous self-assembly is likely that the interfacial repulsion between the incompatible blocks is large enough to pull a part of the linear end blocks into the bottlebrush domain. This effectively increases the bottlebrush domain size. Moreover, the semiflexible bottlebrush widens the regime for the cylinder morphology that is associated with the volume fraction of the end blocks fCSFB (0.10, >0.41). This window is much wider than that for flexible linear BCPs, fCF (0.14, 0.35), and that predicted by the recent self-consistent field theory for linear-bottlebrush BCPs of the same chemistry and molecular architecture. Our experimental findings reveal previously unrecognized mechanisms for the self-assembly of architecturally complex BCPs.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Nian2021,

title = {Three-dimensional printable, extremely soft, stretchable, and reversible elastomers from molecular architecture-directed assembly},

author = {Shifeng Nian and Jinchang Zhu and Haozhe Zhang and Zihao Gong and Guillaume Freychet and Mikhail Zhernenkov and Baoxing Xu and Li-Heng Cai},

doi = {10.1021/acs.chemmater.0c04659},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Chemistry of Materials},

volume = {33},

number = {7},

pages = {2436–2445},

abstract = {3D printing elastomers enables the fabrication of many technologically important structures and devices such as tissue scaffolds, sensors, actuators, and soft robots. However, conventional 3D printable elastomers are intrinsically stiff; moreover, the process of printing often requires external mechanical support and/or post-treatment. Here, we exploit the self-assembly of a responsive linear-bottlebrush-linear triblock copolymer to create stimuli-reversible, extremely soft, and stretchable elastomers and demonstrate their applicability as inks for in situ direct-write printing 3D structures without the aid of external mechanical support or post-treatment. By developing a procedure for controlled synthesis of such architecturally designed block copolymers, we create elastomers with extensibility up to 600% and Young's moduli down to ∼102 Pa, 106 times softer than plastics and more than 102 times softer than all existing 3D printable elastomers. Moreover, the elastomers are thermostable and remain to be solid up to 180 °C, yet they are 100% solvent-reprocessable. Their extreme softness, stretchability, thermostability, and solvent-reprocessability bode well for future applications.},

keywords = {Advanced (Bio)Manufacturing, Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Zhang2020,

title = {Capillary transfer of soft films},

author = {Yue Zhang and Mengtian Yin and Yongmin Baek and Kyusang Lee and Giovanni Zangari and Li-Heng Cai and Baoxing Xu},

doi = {10.1073/pnas.2000340117},

year  = {2020},

date = {2020-03-01},

urldate = {2020-03-01},

journal = {Proceedings of the National Academy of Sciences},

volume = {117},

number = {10},

pages = {5210–5216},

publisher = {National Academy of Sciences},

abstract = {Existing transfer technologies in the construction of film-based electronics and devices are deeply established in the framework of native solid substrates. Here, we report a capillary approach that enables a fast, robust, and reliable transfer of soft films from liquid in a defect-free manner. This capillary transfer is underpinned by the transfer front of dynamic contact among receiver substrate, liquid, and film, and can be well controlled by a selectable motion direction of receiver substrates at a high speed. We demonstrate in extensive experiments, together with theoretical models and computational analysis, the robust capabilities of the capillary transfer using a versatile set of soft films with a broad material diversity of both film and liquid, surface-wetting properties, and complex geometric patterns of soft films onto various solid substrates in a deterministic manner.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2020a,

title = {The rheology property of organogels based on 3D helical nanofilament bnetworks self-assembled by bent-core liquid crystals},

author = {Xing Zheng Wang and Chen Jing Yang and Li-Heng Cai and Dong Chen},

doi = {10.7498/aps.69.20200332},

year  = {2020},

date = {2020-01-01},

urldate = {2020-01-01},

journal = {Acta Physica Sinica},

volume = {69},

number = {8},

pages = {86102},

publisher = {物理学报},

abstract = {In the B4 phase of bent-core liquid crystals, smectic layers of tilted achiral bent-core molecules are chiral and polar, which, driven by intra-layer structural mismatch, eventually twist into helical nanofilaments. We design a NOBOW/hexadecane organogel system, which is different from traditional organogel system, and the studied organogels show reversible gel-liquid transitions under temperature cycles. At high temperature, the NOBOW molecules dissolve in hexadecane and the storage modulus and viscous modulus show typical liquid characteristics. At low temperature, the mobility of NOBOW molecules decreases and the storage modulus of the organogels increases as the temperature decreases. We conduct a rheology experiment to systematically investigate the viscoelasticity of the organogel to understand the property of the organogel and develop the application in soft matter. The viscoelastic studies of the organogels reveal that the helical nanofilaments are internally strained and their 3D networks are relatively stiff, which provides an in-depth insight into the properties of the organogels and paves the way for their applications in soft matter.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@article{Haney2019,

title = {Millimeter-size Pickering emulsions stabilized with Janus microparticles},

author = {Bobby Haney and Dong Chen and Li-Heng Cai and David Weitz and Subramanian Ramakrishnan},

doi = {10.1021/acs.langmuir.9b00058},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {Langmuir},

volume = {35},

number = {13},

pages = {4693–4701},

publisher = {American Chemical Society},

abstract = {The ability to make stable water-in-oil and oil-in-water millimeter-size Pickering emulsions is demonstrated using Janus particles - particles with distinct surface chemistries. The use of a highly cross-linked hydrophobic polymer network and the excellent water-wetting nature of a hydrogel as the hydrophobic and hydrophilic sides, respectively, permit distinct wettability on the Janus particle. Glass capillary microfluidics allows the synthesis of Janus particles with controlled sizes between 128 and 440 μm and control over the hydrophilic-to-hydrophobic domain volume ratio of the particle from 0.36 to 12.77 for a given size. It is shown that the Janus particle size controls the size of the emulsion drops, thus providing the ability to tune the structure and stability of the resulting emulsions. Stability investigations using centrifugation reveal that particles with the smallest size and a balanced hydrophilic-to-hydrophobic volume ratio (Janus ratio) form emulsions with the greatest stability against coalescence. Particles eventually jam at the interface to form nonspherical droplets. This effect is more pronounced as the hydrogel volume is increased. The large Janus particles permit facile visualization of particle-stabilized emulsions, which result in a better understanding of particle stabilization mechanisms of formed emulsions.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Nian2019,

title = {Molecular architecture directs linear-bottlebrush-linear triblock copolymers to self-assemble to soft reprocessable elastomers},

author = {Shifeng Nian and Huada Lian and Zihao Gong and Mikhail Zhernenkov and Jian Qin and Li-Heng Cai},

doi = {10.1021/acsmacrolett.9b00721},

year  = {2019},

date = {2019-01-01},

urldate = {2019-01-01},

journal = {ACS Macro Letters},

volume = {8},

number = {11},

pages = {1528–1534},

publisher = {American Chemical Society},

abstract = {Linear-bottlebrush-linear (LBBL) triblock copolymers represent an emerging system for creating multifunctional nanostructures. Their self-assembly depends on molecular architecture but remains poorly explored. We synthesize polystyrene-block-bottlebrush polydimethylsiloxane-block-polystyrene triblock copolymers with controlled molecular architecture and use them as a model system to study the self-assembly of LBBL polymers. Unlike classical stiff rod-flexible linear block copolymers that are prone to form highly ordered nanostructures such as lamellae, at small weight fractions of the linear blocks, LBBL polymers self-assemble to a disordered sphere phase, regardless of the bottlebrush stiffness. Microscopically, characteristic lengths increase with the bottlebrush stiffness by a power of 2/3, which is captured by a scaling analysis. Macroscopically, the formed nanostructures are ultrasoft, reprocessable elastomers with shear moduli of about 1 kPa, two orders of magnitude lower than that of conventional polydimethylsiloxane elastomers. Our results provide insights on exploiting the self-assembly of LBBL polymers to create soft functional nanostructures.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Wu2017,

title = {Tough self-healing elastomers by molecular enforced integration of covalent and reversible networks},

author = {Jinrong Wu and Li-Heng Cai and David A. Weitz},

doi = {10.1002/adma.201702616},

year  = {2017},

date = {2017-10-01},

urldate = {2017-10-01},

journal = {Advanced Materials},

volume = {29},

number = {38},

pages = {1702616},

publisher = {John Wiley & Sons, Ltd},

abstract = {Self-healing polymers crosslinked by solely reversible bonds are intrinsically weaker than common covalently crosslinked networks. Introducing covalent crosslinks into a reversible network would improve mechanical strength. It is challenging, however, to apply this concept to “dry” elastomers, largely because reversible crosslinks such as hydrogen bonds are often polar motifs, whereas covalent crosslinks are nonpolar motifs. These two types of bonds are intrinsically immiscible without cosolvents. Here, we design and fabricate a hybrid polymer network by crosslinking randomly branched polymers carrying motifs that can form both reversible hydrogen bonds and permanent covalent crosslinks. The randomly branched polymer links such two types of bonds and forces them to mix on the molecular level without cosolvents. This enables a hybrid “dry” elastomer that is very tough with fracture energy 13500 Jm−2 comparable to that of natural rubber. Moreover, the elastomer can self-heal at room temperature with a recovered tensile strength 4 MPa, which is 30% of its original value, yet comparable to the pristine strength of existing self-healing polymers. The concept of forcing covalent and reversible bonds to mix at molecular scale to create a homogenous network is quite general and should enable development of tough, self-healing polymers of practical usage.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2017,

title = {Biocompatible amphiphilic hydrogel-solid dimer particles as colloidal surfactants},

author = {Dong Chen and Esther Amstad and Chun Xia Zhao and Li-Heng Cai and Jing Fan and Qiushui Chen and Mingtan Hai and Stephan Koehler and Huidan Zhang and Fuxin Liang and Zhenzhong Yang and David A. Weitz},

doi = {10.1021/acsnano.7b03110},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {ACS Nano},

volume = {11},

number = {12},

pages = {11978–11985},

abstract = {Emulsions of two immiscible liquids can slowly coalesce over time when stabilized by surfactant molecules. Pickering emulsions stabilized by colloidal particles can be much more stable. Here, we fabricate biocompatible amphiphilic dimer particles using a hydrogel, a strongly hydrophilic material, and achieve large contrast in the wetting properties of the two bulbs, resulting in enhanced stabilization of emulsions. We generate monodisperse single emulsions of alginate and shellac solution in oil using a flow-focusing microfluidics device. Shellac precipitates from water and forms a solid bulb at the periphery of the droplet when the emulsion is exposed to acid. Molecular interactions result in amphiphilic dimer particles that consist of two joined bulbs: one hydrogel bulb of alginate in water and the other hydrophobic bulb of shellac. Alginate in the hydrogel compartment can be cross-linked using calcium cations to obtain stable particles. Analogous to surfactant molecules at the interface, the resultant amphiphilic particles stand at the water/oil interface with the hydrogel bulb submerged in water and the hydrophobic bulb in oil and are thus able to stabilize both water-in-oil and oil-in-water emulsions, making these amphiphilic hydrogel-solid particles ideal colloidal surfactants for various applications.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Xing2017,

title = {Graphene oxide induced crosslinking and reinforcement of elastomers},

author = {Wang Xing and Hengyi Li and Guangsu Huang and Li-Heng Cai and Jinrong Wu},

doi = {10.1016/j.compscitech.2017.03.006},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {Composites Science and Technology},

volume = {144},

pages = {223–229},

abstract = {Conventional elastomer processing requires crosslinking elastomer using specific chemical reagents and reinforcing it using filler particles. Here we report a method to simultaneously crosslink and reinforce styrene-butadiene rubber (SBR) using graphene oxide (GO). We find that GO not only acts as an effective reinforcing filler, but also is capable of generating free radicals upon heating, enabling covalent crosslinking of SBR. Moreover, the interaction between GO surface and SBR polymers results in an interfacial layer in which the density of crosslinks increases towards to the GO surface, thus interfacial layer shows much slower relaxation dynamics than the bulk rubber. The unique role of GO allows GO/SBR nanocomposites to have better mechanical properties than SBR crosslinked with conventional sulfur or dicumyl peroxide. The concept of using GO as both a filler and crosslinking agent may enable the discovery of polymeric nanocomposites with exceeding mechanical properties.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@article{Qin2017,

title = {Ultrafast nanofiltration through large-area single-layered graphene membranes},

author = {Yanzhe Qin and Yongyou Hu and Stephan Koehler and Li-Heng Cai and Junjie Wen and Xiaojun Tan and Weiwei L. Xu and Qian Sheng and Xu Hou and Jianming Xue and Miao Yu and David Weitz},

doi = {10.1021/acsami.7b00504},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {ACS Applied Materials and Interfaces},

volume = {9},

number = {11},

pages = {9239–9244},

abstract = {Perforated single-layered graphene has demonstrated selectivity and flux that is orders of magnitude greater than state-of-the-art polymer membranes. However, only individual graphene sheets with sizes up to tens of micrometers have been successfully fabricated for pressurized permeation studies. Scaling-up and reinforcement of these atomic membranes with minimum cracks and pinholes remains a major hurdle for practical applications. We develop a large-area in situ, phase-inversion casting technique to create 63 cm2 high-quality single-layered perforated graphene membranes for ultrafast nanofiltration that can operate at pressures up to 50 bar. This result demonstrates the feasibility of our technique for creating robust large-area, high quality, single-layered graphene and its potential use as a pressurized nanofiltration membrane.},

keywords = {Polymers and Soft Matter},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@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}

}