Mechanism for “remembering” original shape and transient shape.
Common mechanisms: Entanglements, Crosslinks and hard-domains.
Transient shape fixed usually with crystalline phase or the glassy state.
Revert back to original shape by heating.
Heating above Tm (if the crystalline phase is used to fix the transient shape)
Heating above Tg (if the glassy phase is used to fix the transient shape)
How Shape Memory Polymers Work Original: Chemical Cross-Links Temporary: Glassy Phase Lendlein et al. Original: Crystalline Hard domains (Physical cross-links) Temporary: Crystallites Original: Chemical Cross-Links Temporary: Crystallites
Shape Memory Alloys (SMA)
- Extensive work has been carried out in the last 10 years.
- Constitutive equations and modeling fairly well developed.
Shape Memory Polymers (SMP)
- SM effect can be seen for large deformation
- Manufacturing methods are conventional and cheap
Shape Memory Polymers Representative Application Time series photographs that show the recovery of a shape-memory tube. (a)- (f) Start to finish of the process takes a total of 10 s at 50°C (Marc Behl et al 2007).
Shape Memory Polymers
SMP fibers for comfort wear
MEMS devices, temperature sensors
Intravenous needles and implantable
Films and fibers used in insulation applications
Rewritable digital storage devices
Morphing Aircraft Wings
Shape Memory Mechanism in CSMP’s Deform Cool Unload Heat Amorphous polymer Cross-link Crystallite Legend Melting Crystallization T > T r T < T r State 1 State 4 State 2 State 3 Stretch Nominal Stress 1 2 3 4
Shape Memory Mechanism in GSMP’s Deform Cool Unload Heat Amorphous polymer Cross-link Glassy polymer Legend Glass Transition T > T r T < T r State 1 State 4 State 2 State 3 Stretch Nominal Stress 1 2 3 4
Modeling (Salient Features)
Constitutive Modeling – Mathematical description of how a material responds to deformations.
It is a relation between two physical quantities (often described by tensors).
Modeling of polymers– Write equations for stress tensor in terms of deformation gradient.
Change in Entropy and internal energy is macroscopic manifestation of changes in microstructure.
Modeling (Salient Features)
Above Tr the material behavior is rubber like
Hard domains act as cross-links in thermoplastic SMP’s
Chemical cross-links in the case of thermoset SMP’s
Cooling in deformed shape causes partial
crystallization / glass transition
Crystallization – drop in stress
Glass-Transition- stress remains constant or increases
Semi-crystalline polymer is anisotropic
Unloading the the specimen below Tr, a small recovery strain observed.
Heating above Tr, return to original shape
Need to account for the influence of each phase
Amorphous rubbery phase above the recovery temperature.
Semi-crystalline polymers: amorphous and crystalline phases
Glassy polymers: amorphous and glassy phases (mixture region)
Each phase can have its own stress free state
Modeling - Natural Configurations
In most traditional approaches the response of the material is assumed to be known from a single configuration.
Well known that a body can be stress-free in more than one configuration
Solid which can exist in two different phases (e.g. Austenite and Martensite) with different symmetries.
Polymers, which can exist in the amorphous and crystalline phase
Modeling - Natural Configurations Natural configurations associate with a viscoelastic melt