19 research outputs found
Mesoscale bicontinuous networks in self-healing hydrogels delay fatigue fracture
Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-ealing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rate G(0) above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transition G(tran) far above G(0). In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, and G(tran) corresponds to the rupture of the hard phase network
Preparation of Polyethylene and Ethylene/Methacrylic Acid Copolymer Blend Films with Tunable Surface Properties through Manipulating Processing Parameters during Film Blowing
Polymer films based on polyethylene (PE) and ionomer ethylene/methacrylic acid (EMAA) copolymer blend were prepared by film blowing, whose surface properties were tuned by varying processing parameters, i.e., take up ratio (TUR). Blends of PE/EMAA copolymer were firstly prepared by the melt-mixing method, before being further blown to films. The wettability of the film was investigated by measuring the contact angle/water-film encounter time, and optical properties, i.e., the haze and transmittance. The wettability was found to be enhanced with the increase of TUR. So too was the haze, while the transmittance was found to be almost independent of TUR. The XPS and AFM results directly show the increasing polar functional groups (–COO−) on the surface and roughness with increasing TUR. Further analysis of the 2D SAXS and WAXS unveiled the origin of the invariant transmittance, which resulted from the minor change of the crystallinity and the monotonic increase of the haze, with TUR resulting from the evolution of crystal orientation. In addition to other post-modification methods, the current study provides an alternative route to prepare large-scale PE films as the template for the advanced potential applications, i.e., covering in the layer of roof, the privacy of protective windows, and multitudes of packaging
Counterion-Induced Nanosheet-to-Nanofilament Transition of Lyotropic Bent-Core Liquid Crystals
The smart flexibility of phase transitions in liquid crystals (LCs) makes them suitable for various applications and is an important research field in contemporary science, engineering, and technology. Unlike most reports focused on bent-core LCs in the thermotropic situation, in our present study, we designed and synthesized a fully rigid bent-core molecule with the sulfonic acid group replacing conventional flexible chains. A rich variety of counterion-induced supramolecular LC phase behaviors have been systematically investigated. It was found that the smectic phase with nanosheets tends to transform to the hexagonal phase with nanofilaments when the protons of the sulfonic acid group are partially replaced by alkali metal ions. The experimental results show that the nanoaggregate and phase transition are controlled by the displacing ratio of alkali metal ions rather than the molecular concentration. Another interesting feature is that the achiral bent-core molecules self-assemble into columns by helical stacking and present macroscopic chirality, indicating that spontaneous chiral symmetry breaking occurs in the columnar phase. The fully rigid bent-core molecules reveal surprisingly hierarchical molecular self-assemblies with the smectic-to-hexagonal phase transition, which was not previously observed in supramolecular complexes. The findings will provide new possibilities for applications in LC-based photonic devices, biosystem switches, and supramolecular actuators
Stretch-induced crystal-crystal transition of polybutene-1: An in situ synchrotron radiation wide-angle X-ray scattering study
Deformation induced crystal-crystal transition of polybutene-1 (PB-1) from forms II to I at different temperatures is studied with in situ synchrotron radiation wide-angle X-ray scattering (WAXS). Analyses on the evolution of crystallinity and orientations of forms II and I during tensile deformation show that stretch accelerates the transformation from forms II to I, which is interpreted based on either a direct crystal-crystal transition or an indirect approach via an intermediate state of melt, namely a melting recrystallization process. A three-stage mechanical deformation including linear deformation, stress plateau, and strain hardening is observed in the engineering stress-strain curves, which corresponds to a process of incubation, nucleation, and gelation of form I crystals. It establishes a nice correlation between phase transition and mechanical behavior in this study
Structural Evolution of Hard-Elastic Isotactic Polypropylene Film during Uniaxial Tensile Deformation: The Effect of Temperature
The effects of temperature on the
nonlinear mechanical behaviors
of hard-elastic isotactic polypropylene films are systematically studied
with in-situ ultrafast synchrotron radiation small- and wide-angle
X-ray scattering techniques (SAXS/WAXS) during uniaxial tensile deformation
at temperatures from 30 to 160 °C. Based on the mechanical behaviors
and structural evolutions in strainâtemperature two-dimensional
space, three temperature regions (I, II, and III) are clearly defined
with the α relaxation temperature (<i>T</i><sub>α</sub> â 80 °C) and the onset of melting temperature (<i>T</i><sub>onset</sub> â 135 °C) as boundaries, where
different mechanisms dominate the nonlinear deformations after yield.
In region I, microstrain in lamellar stacks Δ<sub>m</sub> obtains
an accelerated increase after yield and reaches a value significantly
larger than corresponding macrostrain Δ, during which neither
slipping, melting, nor cavitation occurs. We propose stress-induced
microphase separation of interlamellar amorphous to be responsible
to the hyperelastic behavior in region I. Above <i>T</i><sub>α</sub> in region II, due to reduced cohesive strength
and enhanced chain mobility, the irreversible reduction of crystallinity
and the formation of slender cavities suggest that crystal slipping
overwhelms microphase separation and plays the major role in nonlinear
deformation, during which chains in lamellar crystals are pulled out
and recrystallize into nanofibrillar bridges. In region III above <i>T</i><sub>onset</sub>, meltingârecrystallization dictates
the nonlinear deformation. A schematic roadmap for structural evolution
is constructed in strainâtemperature space, which may guide
the processing of microporous membranes for lithium battery separators
as well as other high performance polymer fibers and films
Coupling of Multiscale Orderings during Flow-Induced Crystallization of Isotactic Polypropylene
The
sequence and coupling of intra- and interchain orderings in flow-induced
crystallization (FIC) of partially cross-linked isotactic polypropylene
(iPP) is studied with <i>in situ</i> Fourier transform infrared
spectroscopy (FTIR) and synchrotron radiation X-ray scattering techniques,
which reveal that multiscale structural intermediates emerge prior
to the onset of crystallization. Upon imposing flow, intrachain conformational
ordering or coilâhelix transition (CHT) occurs first, which
is directly correlated with external stress. As helical content is
built up at large strain, density fluctuation happens, and sufficient
long helices may result in orientation ordering before FIC. The results
demonstrate that stress induced intrachain CHT is the essential structural
intermediate in FIC, which can be further coupled with interchain
orientation and density providing either helical content or length
meets the criterions for the phase transitions. We propose that coupling
among external stress, intrachain conformational, and interchain orientation
and density orderings to be the molecular mechanism for FIC of polymer
forming helical structures
From Molecular Entanglement Network to Crystal-Cross-Linked Network and Crystal Scaffold during Film Blowing of Polyethylene: An in Situ Synchrotron Radiation Small- and Wide-Angle Xâray Scattering Study
Combining a homemade film blowing
machine and an in situ synchrotron
radiation source with small- and wide-angle X-ray scattering (SAXS
and WAXS) capability, an investigation of film blowing of polyethylene
(PE) has been studied. From the die exit to the positions above the
frost line, four zones defined with different structural features
are observed with SAXS and WAXS measurements. In zone I, precursor
and crystal structures emerge from the polymer entanglement network
during cooling and extension, which lead to the formation of a deformable
crystal-cross-linked network at the boundary between zones I and II.
The occurrence of the crystal-cross-linked network enhances the effective
chain stretching during further deformation in zone II. Crystallization
is largely accelerated, which generates crystals with high orientation.
Further increasing the crystallinity results in the deformable crystal-cross-linked
network transforming into a nondeformable crystal scaffold at the
frost line (the boundary between zones II and III), which stabilizes
the bubble and prevents further deformation. In zones III and IV,
the scaffold and the entire sample are gradually filled up by crystals,
respectively. Interestingly, increasing the take-up ratio (TUR) does
not influence the critical crystallinity (Ï<sub>IâII</sub>) for the formation of the deformable crystal-cross-linked network,
while the crystallinity (Ï<sub>f</sub>) at the frost line or
for the formation of nondeformable scaffold does vary with TUR. This
suggests that the former (Ï<sub>IâII</sub>) is mainly
controlled by molecular parameters, while the latter (Ï<sub>f</sub>) is determined by both processing and molecular parameters
of PE material
Extension-Induced Crystallization of Poly(ethylene oxide) Bidisperse Blends: An Entanglement Network Perspective
The role of long chains in extension
flow-induced crystallization
was studied with a combination of extension rheological and <i>in situ</i> small-angle X-ray scattering (SAXS) measurements
at 52 °C. To elucidate the effects of long chains, bidisperse
blends of polyÂ(ethylene oxide) (PEO) with the long-chain concentration
above the overlap concentration were prepared, constructing long-chain
entanglement network in short-chain matrix. Rheological data of step
extension on PEO melt are divided into two regions with fracture strain
of pure short-chain sample as a boundary. Distinctly different features
of crystallization kinetics and crystal morphologies are observed
in these two regions, exactly corresponding to rheological behavior.
A new mechanism based on entanglement network perspective is proposed,
in which the second entanglement network constructed by long chains
has three effects: (i) helping flow to change the free energy of polymer
melt more effectively; (ii) ensuring the specific work can impose
on the system; (iii) favoring the formation of precursors. This mechanism
captures both rheological observation and crystallization behavior
successfully and offers a new viewpoint for FIC study