# Shape Memory Effect in Micro-Sized Shape Memory Polymer Composite Chains
## 1 Introduction  
As a smart material, shape-memory polymers (SMPs) attract much interest owing to the capability to fully recover a remarkable pre-strain (on an order of hundred percent) upon applying a variety of external stimulus [1; 2], including Joule heating [3; 4], magnetism-induced heating [5; 6], moisture [7], light [8; 9], and pH change [10]. The shape memory effect (SME) could be realized on the basis of the following mechanism and the associated shaping procedure in a SMP [11]. In a glass transition based thermo-responsive SMP with the hard/soft-segment system, the function of hard segments is to act as physical crosslinks, and the SME is driven by the entropic forces which tend to shape the stretched polymer chains in the soft elements back to their original coiled configuration [12; 13]. During a typical shape memory procedure, the original shape of SMP is synthesized and solidified at below glass transition temperature (\(T_{g}\)), then it is deformed to a temporary and large-deformation shape above \(T_{g}\). Then, it is cooled well below \(T_{g}\) and the frozen soft segment lock the large-deformation configuration. When it is heated above \(T_{g}\) again, the soft segment release the hard segment, and the SMP recover toward the original shape.  
The SME has occurred in macroscale and microscale in recent years, and also demonstrated at nanoscale in SMPs [14; 15]. Hence, the prospect of SME applying in microscale or even nanoscale patterning receives great attention, including adaptive programmable materials [15], switchablemolecule-based materials [16], photoactive electroactive applications [7; 8], soft electronics [17; 18], biomedical applications (e.g., cell mechanobiology [19], biomimetic 4D printing [3], switchable dual pH-Responsiveness [10], robust microcarriers [20], soft microgrippers [21], and micropatterned containers [22]). At present, a number of SMPs are available in the market and many more are under developing [1]. Among these SMPs, some polyurethane SMPs are both thermo-responsive and moisture-responsive [9], which shows a prospective way to realize the operation and surgery in live cell at microscale and nanoscale. Some microscale polymeric machines have been developed [23; 24] for operation or surgery at cell scale, and can be actuated outside the cells. But, it is still difficult to deliver those machines into cells and the following stimulus for the artificially controlled operation. Those polymeric machines may be fabricated with SMPs to deliver and operation inside the living cells on the basis of shape memory effect.  
Considering the formation of chain structures of ferromagnetic particles of magnetorheological fluids and elastomers [25; 26], the suspended ferromagnetic particles form chains, columns, or aggregates along the direction of magnetic field. To explain the above phenomenon, several research methods have been used, including macroscopic mechanical element combination method, macroscopic continuum mechanics, microscopic dipole mechanical method, and finite element method based on multi-field coupling [26; 27; 28; 29]. Based on these methods, many models or theories have been proposed, and two typical theories among them are phase-nuclear theory and field-induced dipole moment theory [30]. Among those models, the main parameters include strength and distribution of magnetic field, properties of particles (volume fraction, size, and microstructures), viscosity of solvent, interface between particle and solvent, and time of action. However, the formation mechanism of particle chains under magnetic field is still unclear [25; 26; 27; 28; 29; 30], and the testing results are not completely consistent with the theoretical predictions [31; 32].  
In this study, we prepared nickel (Ni) particles polyurethane-SMP chains in micron and submicron scale, and the shape memory effect and associated properties are demonstrated for those vertical protrusive chains upon thermal stimulus.