Deformation-driven alignment of macromolecules or nanofibers leading to anisotropy is a challenge in functional soft materials. Here, tough cellulose hydrogels that exhibited deformation-induced anisotropy are fabricated by reacting cellulose with a small amount of epichlorohydrin (EPI) in LiOH/urea solution and subsequent treating with dilute acid. The loosely cross-linked network that was obtained via chemical cross-linking of cellulose with EPI as a large framework maintained the elasticity of hydrogels, whereas nanofibers produced by the acid treatment formed physical cross-linked networks through hydrogen bonds which could efficiently dissipated mechanical energy. Meanwhile, the nanofibers could further aggregate to form submicrobundles and participate in the formation of frameworks during the acid treatment. Under deformation, the nanofibers and submicrobundles in the physical networks synchronize easily to align with the large framework, generating the rapidly responsive birefringence behaviors with highly stable colors. Thus, the cellulose hydrogels possessing sensitively mechano-responsive behavior could be utilized as a dynamic light switch and a soft sensor to accurately detect small external force, respectively. This work opens a novel pathway to construct tough and mechanoresponsive hydrogels via a green conversion of natural polysaccharide.
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 strain−temperature two-dimensional space, three temperature regions (I, II, and III) are clearly defined with the α relaxation temperature (T α ≈ 80 °C) and the onset of melting temperature (T onset ≈ 135 °C) as boundaries, where different mechanisms dominate the nonlinear deformations after yield. In region I, microstrain in lamellar stacks ε m obtains an accelerated increase after yield and reaches a value significantly larger than corresponding macrostrain ε, 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 T α 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 T onset , melting−recrystallization dictates the nonlinear deformation. A schematic roadmap for structural evolution is constructed in strain−temperature space, which may guide the processing of microporous membranes for lithium battery separators as well as other high performance polymer fibers and films.
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 (χI–II) for the formation of the deformable crystal-cross-linked network, while the crystallinity (χf) at the frost line or for the formation of nondeformable scaffold does vary with TUR. This suggests that the former (χI–II) is mainly controlled by molecular parameters, while the latter (χf) is determined by both processing and molecular parameters of PE material.
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