Shape-memory hydrogels (SMHs) are potential candidate materials for biomedical applications as they can mimic the elastic properties of soft tissue and exhibit shape transformations at body temperature. Here we explored, whether architectured SMHs can be designed by incorporating oligo(ε-caprolactone) (OCL, ${\overline M _n}$ = 4500 g·mol-1, T m = 54 °C) side chains as switching segment into hydrophilic polymer networks based on N-vinylpyrrolidone as backbone forming component and oligo(ethylene glycol)divinylether (OEGDVE, ${\overline M _n}$ = 250 g·mol-1) as crosslinker. By utilizing NaCl and NaHCO3 as porogene during thermal crosslinking architectured hydrogels having pore diameters between 30 and 500 µm and wall thicknesses ranging from 10 to 190 µm in the swollen state were synthesized. According to the porous microstructure, a macroscopic form stability was obtained when the polymer networks were swollen until equilibrium in water. Material properties were investigated as function of the OCL content, which was varied between 20 and 40 wt%. In compression experiments the architectured hydrogels exhibited strain fixity and strain recovery ratios above 80%. These architectured SMHs might enable biomaterial applications as smart implants with the recovery of bulky structures from compact shapes.

Crosslinked poly[ethylene-co-(vinyl acetate)] (cPEVA) has been recently introduced as a polymer material, which can be functionalized with various shape-memory effects by solely altering the thermomechanical treatment called programming.

In this study two series of cPEVAs with different vinyl acetate contents of 18 wt% (cPEVA18) and 28 wt% (cPEVA28) comprising different crosslink densities were investigated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) in the temperature range of -130 °C to 120 °C. DMTA tests were performed in torsion mode, because such movements are highly relevant in the context of complex shape changes in shape-memory polymer based devices. Finally, the obtained DMTA results were compared with DMTA conducted in tension mode. Swelling experiments revealed a gel content in the range from 81% to 90% for cPEVA18 samples while for cPEVA28s a complete conversion was observed. The degree of swelling was found to decrease substantially with increasing crosslink density for both cPEVA series.

The influence of VA content and extent of crosslinking on the appearance of the respective melting (T m) and glass transition (T g) as well as the thermomechanical properties of cPEVA systems could be demonstrated by discussing both DSC and DMTA results. The temperature range of mechanical stability correlates with the VA content and is determined by decreasing T m values. The cross links do barely alter the stiffness of a PEVA up to the T m rang, but lead to constant mechanical rigidity in the rubbery range above T m.

Multifunctional polymer-based biomaterials, which combine degradability and shapememory capability, are promising candidate materials for biomedical implants. An example is a degradable multiblock copolymer (PDC), composed of poly(p-dioxanone) (PPDO) as hard and poly(ε-caprolactone) (PCL) as switching segments. PDC exhibits a unique linear mass loss during hydrolytic degradation, which can be tailored by the PPDO to PCL weight ratio, as well as an excellent thermally induced dual-shape effect. PDC can be synthesized by co-condensation of two oligomeric macrodiols (PCL-diol and PPDO-diol) using aliphatic diisocyanates as coupling agent. Here, we investigated whether different morphologies could be obtained for PDCs synthesized from identical oligomeric macrodiols (PCL-diol with M n = 2000 g·mol-1 and PPDO-diol with M n = 5300-5500 g·mol-1) with 2, 2(4), 4-trimethyl-hexamethylene diisocyanate (TMDI) and 1, 6-hexamethylene diisocyanate (HDI), respectively. More specifically, atomic force microscopy (AFM) was utilized for an investigation of the surface morphologies in solution casted PDC thin films in the temperature range from 20 °C to 60 °C. The results obtained in differential scanning calorimetry (DSC) and AFM demonstrated that different morphologies were obtained when TMDI (PDC-TMDI) or HDI (PDC-HDI) were used as linker. PCL related crystals in PDC-HDI were more heterogeneous and less ordered than those in PDCTMDI, while HDI resulted in a larger degree of crystallinity than TMDI. This research provides some new suggestions for choosing a suitable coupling agent to tailor the required morphologies and properties of SMPs with crystallizable switching segments.

Phase-segregated multiblock copolymers (MBC) as well as covalently crosslinked multiphase polymer networks, which are composed of crystallizable oligo(ε-caprolactone) (OCL) and oligo(ω-pentadecalactone) (OPDL) segments have been recently introduced as degradable polymer systems exhibiting various memory effects. Both types of copolyesterurethane networks can be synthesized via co-condensation of the respective hydroxytelechelic oligomers and 2,2(4),4-trimethyl-hexamethylene diisocyanate (TMDI) as aliphatic linker.

In this work the dual-shape properties as well as the temperature-memory capability of thermoplastics and covalently crosslinked copolyesterurethanes containing OCL and OPDL domains are explored. Both copolyesterurethane networks exhibited excellent dual-shape properties with high shape fixity ratios R f ≥ 93% and shape recovery ratios in the range of 92% to 100% determined in the 2nd and 3rd test cycle, whereby the dual-shape performance was substantially improved when covalent crosslinks are present in the copolymer.

A pronounced temperature-memory effect was achieved for thermoplastic as well as crosslinked copolyesterurethanes. Hereby, the switching temperature T sw could be adjusted via the variation of the applied deformation temperature T deform in the range from 32 °C to 53 °C for MBC and in the range from 29 °C to 78 °C for multiphase polymer networks.