Advanced
stretchable electronic sensors with a complex structure place higher
requirements on the mechanical properties and manufacturing process
of the stretchable substrate materials. Herein, three kinds of polyurethane
acrylate oligomers were synthesized successfully and mixed with a
commercial acrylate monomer (isobornyl acrylate) to prepare photocurable
resins with a low viscosity for a digital light processing three-dimensional
(3D) printer without custom equipment. Results showed that the resin
containing poly(tetrahydrofuran) units (PPTMGA-40) exhibited optimal
mechanical properties and shape recoverability. The tensile strength
and elongation at break of PPTMGA-40 were 15.7 MPa and 414.3%, respectively.
The unprecedented fatigue resistance of PPTMGA-40 allowed it to withstand
100 compression cycles at 80% strain without fracture. The transmittance
of PPTMGA-40 reached 89.4% at 550 nm, showing high transparency. An
ionic hydrogel was coated on the surface of 3D-printed structures
to fabricate stretchable sensors, and their conductivity, transparency,
and mechanical performance were characterized. A robust piezoresistive
strain sensor with a high strength (∼6 MPa) and a wearable
finger guard sensor were fabricated, demonstrating that this hydrogel-elastomer
system can meet the requirements of applications for advanced stretchable
electronic sensors and expand the usage scope.
Shape memory polymer
(SMP)-based 4D printing combines the advantages
of SMP and 3D printing to form active materials with delicate structure.
Nowadays, studies of SMP-based 4D printing materials mainly focus
on cross-linked (meth)acrylate of which the permanent shape cannot
be changed for their covalent linkage, limiting the usage of 4D printing
materials. In this paper, a novel (meth)acrylate monomer with an aldehyde
group (2-(methacryloyloxy)ethyl 4-formylbenzoate, MEFB) and hyperbranched
cross-linker (HPASi) are synthesized to build (meth)acrylate systems
(IEMSis) with dynamic imine bonds for 4D printing. The flexible chain
structure of HPASi significantly enhances the toughness of IEMSis,
which is 33–97-fold higher than that of the one without HPASi
(IEM). The addition of HPASi also endows IEMSis good shape memory
properties, and the shape fixity and shape recovery ratios of them
are 97.5–97.6 and 91.4–93.7%, respectively. At the same
time, IEMSis can undergo a stress relaxation process by dynamic exchanges
of imine bonds under relatively mild conditions without a catalyst
to acquire an ability of permanent shape reconfiguration. The shape
retention ratio of IEMSi3 is 84.3%. In addition, the 4D-printed structures
displayed here indicate that these 4D printing systems have a myriad
of potential applications including aerospace structures, soft robotic
grippers, smart electron switches, and intelligent packaging, while
the reconfigurability shown by IEMSi3 will expand the scope of application
fields of 4D printing materials.
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