New testing and calculation method for determination viscoelasticity of optical glass.

Viscoelastic properties of glass within molding temperatures, such as shear relaxation modulus and bulk relaxation modulus, are key factors to build successful numerical model, predict forming process, and determine optimal process parameters for precision glass molding. However, traditional uniaxial compression creep tests with large strains are very limited in obtaining high-accuracy viscoelastic data of glass, due to the declining compressive stress caused by the increasing cross-sectional area of specimen in testing process. Besides, existing calculation method has limitation in transforming creep data to viscoelasticity data, especially when Poisson's ratio is unknown at molding temperature, which further induces a block to characterize viscoelastic parameter. This study proposes a systematic acquisition method for high-precision viscoelastic data, including creep testing, viscoelasticity calculation, and finite element verification. A minimal uniaxial creep testing (MUCT) method based on thermo-mechanical analysis (TMA) instrument is first built to obtain ideal and accurate creep data, by keeping compressive stress as a constant. A new calculation method on viscoelasticity determination is then proposed to derive shear relaxation modulus without the need of knowing bulk modulus or Poisson's ratio, which, compared with traditional method, extends the application range of viscoelasticity calculation. After determination, the obtained viscoelastic data are further incorporated into a numerical simulation model of MUCT to verify the accuracy of the determined viscoelasticity. Base on the great consistence between simulated and measured results (uniaxial creep displacement), the proposed systematic acquisition method can be used as a high accuracy viscoelasticity determination method.

Chitosan derivative-based self-healable hydrogels with enhanced mechanical properties by high-density dynamic ionic interactions.

Chitosan derivative-based self-healable hydrogels with enhanced mechanical properties are reported, which were prepared by polymerization of acrylic acid (AAc) in 2-hydroxypropyltrimethyl ammonium chloride chitosan (HACC) solution. The PAAc/HACC hydrogels exhibit tensile fracture stress as high as 3.31 MPa and a Young's modulus of 2.53 MPa. They can maintain their original shape after 30 repeated compression cycles under various strain conditions, with a compression stress of more than 60 MPa at 99% strain. The damaged PAAc/HACC hydrogels can heal together in the presence of a NaCl salt solution with a self-healing efficiency of up to 61%. In addition, the PAAc/HACC hydrogels have high ionic conductivity and can serve as electrolytes for supercapacitors. The analysis suggests that all these good properties of the PAAc/HACC hydrogels mainly result from their high-density dynamic ionic interactions structure.

Superior hybrid hydrogels of polyacrylamide enhanced by bacterial cellulose nanofiber clusters.

Hybrid polyacrylamide/bacterial cellulose nanofiber clusters (PAM/BC) hydrogels with high strength, toughness and recoverability were synthesized by in situ polymerization of acrylamide monomer in BC nanofiber clusters suspension. The hybrid gels exhibited an extremely large elongation at break of 2200%, and a high fracture stress of 1.35MPa. Additionally, the original length of hydrogels could be recovered after releasing the tensile force. Compressive results showed that the PAM/BC hybrid gels could reach a strain of about 99% without break, and was able to completely recover its original shape immediately after releasing the compression force. The compressive stress at 99% reached as high as 30MPa. Nearly no hysteresis in cyclic compressive tests was observed with these hybrid gels. The FT-IR, XRD and TGA analysis showed that hydrogen bonds between the PAM chains and BC nanofiber clusters mainly contributed to the superior mechanical properties of hybrid hydrogels. The cell viability results suggested that PAM/BC hybrid hydrogel was benign for biomedical application. These PAM/BC hydrogels offer a great promise as biomaterials such as bone and cartilage repair materials.

Dual Physically Cross-Linked Double Network Hydrogels with High Mechanical Strength, Fatigue Resistance, Notch-Insensitivity, and Self-Healing Properties.

Double-network (DN) hydrogels with high strength and toughness have been developed as promising materials. Herein, we explored a dual physically cross-linked polyacrylamide/xanthan gum (PAM/XG) DN hydrogel. The nonchemically cross-linked PAM/XG DN hydrogels exhibited fracture stresses as high as 3.64 MPa (13 times higher than the pure PAM single network hydrogel) and compressive stresses at 99% strain of more than 50 MPa. The hydrogels could restore their original shapes after continuously loading-unloading tensile and compressive cyclic tests. In addition, the PAM/XG DN hydrogels demonstrated excellent fatigue resistance, notch-insensitivity, high stability in different harsh environments, and remarkable self-healing properties, which might result from their distinctive physical-cross-linking structures. The attenuated total reflectance infrared spectroscopy (ATR-IR) and dynamic thermogravimetric analysis (TGA) results indicated that there were no chemical bonds (only hydrogen bonds) between the XG and PAM networks. The PAM/XG DN hydrogel synthesis offers a new avenue for the design and construction of DN systems, broadening current research and applications of hydrogels with excellent mechanical properties.