Shape-Memory Polymer Actuator

Material

Actuation

Chamber temperature 20°C

Heating rate 8°C/s

Programmed bend angle 100°

Live State

Temperature

20.0°C

Recovery ratio

Storage modulus E(T)GLASSY

2000 MPa

How it works

A shape-memory polymer (SMP) is a covalent or physical network with two families of segments: fixed net-points that remember one permanent shape, and flexible switching segments whose mobility is frozen below the glass-transition temperature T_g.

Programming — deform the network above T_g into a temporary shape, then cool it under strain. Below T_g the switching segments vitrify and lock in that shape (strain fixity ≈ 100%) even with the load removed.

Recovery — reheating past T_g restores segmental mobility. Entropic elasticity stored in the stretched net-points then pulls the material back toward its permanent (undeformed) shape. The fixed-strain fraction f decays with a temperature-gated first-order rate law:

df/dt = −k(T)·f
k(T) = k_max / (1 + exp(−(T − Tg)/w))
Recovery ratio Rr(t) = 1 − f(t)

The storage modulus collapses by two-to-three orders of magnitude across the same transition — glassy (≈1–2 GPa) to rubbery-plateau (≈1–10 MPa) — modeled here with the matching sigmoid:

E(T) = E_r + (E_g − E_r) / (1 + exp((T − Tg)/w))

Chamber temperature — the target temperature the actuator is driven toward.
Heating rate — how fast the actual temperature ramps toward that target (thermal lag).
Programmed bend angle — total curvature locked in the temporary shape.
Material — sets T_g and the transition width w, which controls whether recovery snaps sharply or unrolls gradually.

Real-world relevance: this is the same working principle behind SMP self-deploying stents, foldable space structures, and thermally triggered soft-robotic grippers.

Glassy (fixed) Rubbery (recovering)

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