This work takes a simple phenomenological approach to
the questions of when, how, and why a brittle polymer glass turns
ductile and vice versa. Perceiving a polymer glass as a hybrid, we
recognize that both the primary structure formed by van der Waals
forces (network 1) and chain network (i.e., the vitrified entanglement
network) (network 2) must be accounted for in any discussion of the
mechanical responses. To show the benefit of this viewpoint, we first
carried out well-defined melt-stretching experiments on four common
polymer glasses (PS, PMMA, SAN, and PC) in a systematic way either
at a fixed Hencky strain rate to a given degree of stretching at several
temperatures or at a given temperature to different levels of stretching
using the same Hencky rate. Then we attempted to preserve the effect
of melt-stretching on the chain network structure by rapid thermal
quenching. Subsequent room-temperature tensile extension of these
melt-stretched amorphous polymers reveals something universal: (a)
along the direction of the melt-stretching, the brittle glasses (PS,
PMMA, and SAN) all become completely ductile; (b) perpendicular to
the melt-stretching direction, the ductile glass (PC) becomes brittle
at room temperature. We suggest that the transformations (from brittle
to ductile or ductile to brittle) arise from either geometric condensation
or dilation of load bearing strands in the chain network due to the
melt-stretching. Regarding a polymer glass as a structural hybrid,
we also explored two other cases where the ductile PC becomes brittle
at room temperature: (1) upon aging near the glass transition temperature;
(2) when blended with PC of sufficiently low molecular weight. These
results indicate that (i) the strengthening of the primary structure
by aging can raise the failure stress σ* to a level too high
for the chain network to sustain and (ii) the PC blend becomes brittle
upon weakening the chain network by dilution with short chains.
A cytocompatible porous scaffold mimicking the properties of extracellular matrices (ECMs) has great potential in promoting cellular attachment and proliferation for tissue regeneration. A biomimetic scaffold was prepared using silk fibroin (SF)/sodium alginate (SA) in which regular and uniform pore morphology can be formed through a facile freeze-dried method. The scanning electron microscopy (SEM) studies showed the presence of interconnected pores, mostly spread over the entire scaffold with pore diameter around 54~532 μm and porosity 66~94%. With significantly better water stability and high swelling ratios, the blend scaffolds crosslinked by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) provided sufficient time for the formation of neo-tissue and ECMs during tissue regeneration. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) results confirmed random coil structure and silk I conformation were maintained in the blend scaffolds. What’s more, FI-TR spectra demonstrated crosslinking reactions occurred actually among EDC, SF and SA macromolecules, which kept integrity of the scaffolds under physiological environment. The suitable pore structure and improved equilibrium swelling capacity of this scaffold could imitate biochemical cues of natural skin ECMs for guiding spatial organization and proliferation of cells in vitro, indicating its potential candidate material for soft tissue engineering.
Modulating nitrogen‐rich nitrides with favorable electron structure to enhance hydrogen production activity is challenging due to thermodynamically unfavorable characteristics. Herein, an ultrathin heterojunction of metallic Co and nitrogen‐rich nitride (Co‐Mo5N6) is prepared through the ammonia annealing process as a robust electrocatalyst for hydrogen evolution reaction (HER). Density functional theory simulations and experiments reveal that the obtained Co‐Mo5N6 enables electron redistribution between the nitrogen‐rich phase and Co for more negative H2O adsorption energy, decreasing the subsequent energetic barrier of dissociation (0.05 eV) and optimizing H* absorption (ΔGH* = 0.1 eV). The structure is connected by nanosheet (≈1.2 nm) building blocks with abundant interstitial spaces, open and connective channels, and strong capillary forces, which accelerate mass transfer and electrical conductivity. The Co‐Mo5N6 exhibits excellent HER activity with an extremely low overpotential of 19 mV at 10 mA cm−2 and Tafel slope of 29.0 mV dec−1. Notably, the required overpotential is only 280 mV to achieve a high current density of 1000 mA cm−2 which is better than commercial Pt/C. This work not only improves the understanding of the catalytic activity and the electron redistribution of nitrogen‐rich nitrides, but also presents a new strategy to design other nitrogen‐rich metal nitrides (such as W2N3, Ta5N6).
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