3D printing of dynamic covalent polymer network with on-demand geometric and mechanical reprogrammability.

Delicate geometries and suitable mechanical properties are essential for device applications of polymer materials. 3D printing offers unprecedented versatility, but the geometries and mechanical properties are typically fixed after printing. Here, we report a 3D photo-printable dynamic covalent network that can undergo two independently controllable bond exchange reactions, allowing reprogramming the geometry and mechanical properties after printing. Specifically, the network is designed to contain hindered urea bonds and pendant hydroxyl groups. The homolytic exchange between hindered urea bonds allows reconfiguring the printed shape without affecting the network topology and mechanical properties. Under different conditions, the hindered urea bonds are transformed into urethane bonds via exchange reactions with hydroxyl groups, which permits tailoring of the mechanical properties. The freedom to reprogram the shape and properties in an on-demand fashion offers the opportunity to produce multiple 3D printed products from one single printing step.

High-accuracy optical extensometer realized by two parallel cameras and two-dimensional digital image correlation.

A conventional optical extensometer realized by a single common camera and two-dimensional digital image correlation (2D-DIC) often provides unsatisfactory strain results owing to the out-of-plane motion of the specimen. In this work, we propose an improved optical extensometer based on two parallel cameras and 2D-DIC. In the proposed extensometer, the gauge points are selected at the image centers of two cameras, which are negligibly affected by the out-of-plane translation and rotation, leading to higher accuracy of strain measurement as compared with the conventional optical extensometer. A rigid out-of-plane translation experiment and four repeated uniaxial tensile tests were conducted to verify the feasibility, reliability, and accuracy of the proposed method. Experimental results indicate that the proposed method has a strong ability to resist the effect of out-of-plane motion and experimental vibrations. Moreover, the strain measurement results obtained with the proposed method were found to be in excellent agreement with those obtained with a strain gauge, and the strain errors between them were only a few microstrains. Given that no compensation method is required, the proposed method is easy to implement with 2D-DIC and can be used for specimens of different sizes by adjusting the distance between the two cameras.