The past decade has witnessed remarkable advances in the performance and capabilities of shape memory polymers (SMP), which are stimuli-responsive materials that change its shape upon exposure to the appropriate external stimulus, such as temperature. This responsive nature of SMP brings many potential applications, such as actuators, biomedical devices, and clinical products among others. One promising new class of SMPs is ionomers and their composites due to their ease in processability and great mechanical properties. In this work, ionomer-based SMPs and their composites were successfully applied in films and 3D printing to enable the investigations of the shape memory behavior from the nanoscale to macroscale.
Physiological SMP with a transition temperature at 37°C, which is human body temperature, applies to many potential applications in clinical or skin-contacting products. The physiological SMP was assembled by mixing zinc salts of sulfonated poly (ethylene-co-propylene-co-ethylidene norbornene) (Zn-SEPDM) ionomer with lauric acid (LA), and the SMP composite shows a shape memory transition temperature at ~37°C. The physiological SMP solution was cast into film on roll-to-roll, which provides a fast, stable, and continuous processing of the material. The shape memory behavior of the film product was studied. The fixity was constant at ~80% for 5 shape memory cycles. The first cycle recovery (R=58%) at 37°C was much lower than subsequent cycles recovery (R>82%). It was also found that a lower storage temperature at 6°C for programmed film sample could improve both the fixity and recovery. For the programmed film after one week, sample with storage temperature at 19°C was F=85%, R=65%, while the sample with storage temperature at 6°C was F=90%, R=75%.
Microscopic and nanoscopic scale textures and patterns on surfaces provide a handle to modulate properties, such as surface wetting, optical properties, and dry adhesion. Creating surface textures and patterns in a SMP film enables switchable surface properties due to its surface pattern changing ability. Continuing with Zn-SEPDM based composite solution cast SMP film, shape memory effects for micropattern and nanopattern are demonstrated for SMP composed of mixtures of the Zn-SEPDM ionomer and one of three different low molar mass fatty acids (FAs): lauric acid (LA), stearic acid (SA), and zinc stearate (ZnSt). This work demonstrates the ability to tailor the surface pattern switching temperature (Tc) by simply varying the selected FA. Surface pattern memory and recovery are shown for materials with 20 wt% LA, SA, or ZnSt, where Tc = 50, 80, and 100 °C, respectively. Recovery efficiencies for micropatterns are greater than 92% for all three materials and 73% for a nanopattern for the ionomer/ZnSt material.
In addition to study of ionomer based SMP material, the near net shape production of SMP material at macroscopic scale was demonstrated using fused deposition modeling (FDM)-based 3D printing. The extrusion based FDM 3D printing technique has many advantages for polymer processing, such as flexible of product design, low tooling cost, and minimal material waste. However, little work has been done on applying SMP materials in 3D printing processing using FDM. The shape memory behavior is likely dependent on the processing details. To understand the ability of FDM to directly produce arbitrary SMP-based objects, a commercially available ionomer SMP material, Surlyn 9520, which is zinc-neutralized poly (ethylene-co-methacrylic acid), was studied with FDM printer. The shape memory behavior was investigated for the 3D printed samples, and the results were compared with compression molded samples. The initial fixity for 3D printed and compression-molded samples was similar, but the initial recovery was much lower for the 3D printed sample (R = 58%) than that for the compression-molded sample (R = 83%). The poor recovery in the first cycle is attributed to polyethylene crystals formed during programming that act to resist the permanent network recovery. This effect is magnified in the 3D printed part due to the higher strain (lower modulus in the 3D printed part) at a fixed programming stress. The fixity and recovery in subsequent shape memory cycles are greater for the 3D printed part than for the compression-molded part. Moreover, the programmed strain can be systematically modulated by inclusion of porosity in the printed part without adversely impacting the fixity or recovery.
In conclusion, SMP based on ionomers and their composites were successfully applied to roll-to-roll film fabrication, patterning surface, and 3D printing technique. The shape memory behavior of the ionomer based SMP was studied in both microscale and macroscale, which can promote the development of ionomer based SMP in fabrication and potential application.