How strong can a thermoset shape memory polymer push?


Mr. Jizhou Fan (a current Ph.D. student) and Dr. Guoqiang Li (professor) published in Nature Communications

“Jizhou Fan and Guoqiang Li. High enthalpy storage thermoset network with giant stress and energy output in rubbery state. Nature Communications, Volume 9, Article number 642, (February, 2018).”

Can you imagine that a cylindrical polymer rod with a diameter of 4.6 cm and programmed height of 11 cm can jack up a car by simple heating? This can be done by our new shape memory polymers (SMPs) which is designed by a new energy storage mechanism.

Thermoset shape memory polymers have been a topic of intensive research for years. In addition to shape memory, which means a deformed temporary shape can restore its original permanent shape upon stimulation, such as heat, light, moisture, pH, etc., SMPs can also release stress if free shape recovery is not allowed. The fact that SMPs can memorize both shape and stress has rendered them with many potential applications such as actuators, self-healing, sealants, morphing structures, stent, suture, soft robot, smart textile, etc.

However, low output in stress and energy in the rubbery state has been a bottleneck for wide-spread applications of thermoset SMPs. In the literatures, the stable recovery stress is from tenths to several MPa and the energy output is several tenths MJ/m3. This suggests that the energy released by a 1cm3 sample can only lift 4 bottles of water (500g/bottle) by 1cm, a capability similar to a baby.

They believe that if more energy can be stored during the deformation process, or programming, more energy output can be achieved. Currently, entropy reduction is the acknowledged mechanism for energy storage in thermoset polymers. In addition to entropy reduction, they believe that enthalpy increase due to chemical bond length change is an option. Conventional polymer networks cannot store energy through enthalpy increase because the coiled segments in the rubbery state can endure giant deformation or ordering or entropy reduction without stretching the chemical bonds, or if the bond is stretched, it causes fracture. Hence, enriching the steric hindrance of the polymer network is a solution. Following this idea, a commercially available epoxy (EPON 826) is reacted with a rigid diamine named isophorone diamine (IPD), which can provide a large steric hindrance. A stable recovery stress of 17 MPa and energy output of 2.12 MJ/m3 in rubbery state and in bulk form are obtained and largely maintained.

More details can be found from the link below: