Heat-Induced Actuator Fibers: Starch-Containing Biopolyamide Composites for Functional Textiles

This study introduces the development of a thermally responsive shape-morphing fabric using low-melting-point polyamide shape memory actuators. To facilitate the blending of biomaterials, we report the synthesis and characterization of a biopolyamide with a relatively low melting point. Additionally, we present a straightforward and solvent-free method for the compatibilization of starch particles with the synthesized biopolyamide, aiming to enhance the sustainability of polyamide and customize the actuation temperature. Subsequently, homogeneous dispersion of up to 70 wt % compatibilized starch particles into the matrix is achieved. The resulting composites exhibit excellent mechanical properties comparable to those reported for soft and tough materials, making them well suited for textile integration. Furthermore, cyclic thermomechanical tests were conducted to evaluate the shape memory and shape recovery of both plain polyamide and composites. The results confirmed their remarkable shape recovery properties. To demonstrate the potential application of biocomposites in textiles, a heat-responsive fabric was created using thermoresponsive shape memory polymer actuators composed of a biocomposite containing 50 wt % compatibilized starch. This fabric demonstrates the ability to repeatedly undergo significant heat-induced deformations by opening and closing pores, thereby exposing hidden functionalities through heat stimulation. This innovative approach provides a convenient pathway for designing heat-responsive textiles, adding value to state-of-the-art smart textiles.


TGA/DTG of starch and OSA-g-starch
To monitor the thermal decomposition of starch before and after surface treatment, as a tool for qualitative evaluation of the grafting of OSA, TGA was conducted on starch before and after grafting (un-washed sample).The thermograms are plotted in Figure S2.[3] On the other side, OSA-g-starch presented three weight loss regions; a minor one (~ 2.7%) S-4 below 120 °C, along with the second decomposition (~12%) between 200 to 280 °C, followed by a pyrolytic decomposition (~70%) within 280 to 800 °C, with an ash content of about 15%.Significant differences were observed in the TGA/DTG thermograms of the starch and OSAg-starch.For instance, the weight loss below 200 °C, which corresponded to the evaporation of the trapped water, was considerably lower in the OSA-g-starch, indicating a more hydrophobic property of the surface-treated samples obtained by grafting of OSA molecules.Furthermore, the new stage appeared between 200 to 280 °C with the maximum decomposition temperature at ~260 °C due to the decomposition of the grafted OSA molecules. 4This decomposition temperature indicated that the surface-modified starch was amenable to the newly developed low melting-point copolyamide.Moreover, the ash content in the surface-treated sample was higher, which could be explained by the fact that the residue in the OSA-g-starch was a mixture of more ordered crystallites that required higher thermal temperatures for decomposition. 5

TGA/DTG of the matrix and biocomposites
As depicted in Figure S3, the copolyamide presented a typical polyamide decomposition profile with less than 1% residue at 700 °C. 6Although it was thermally stable below 400 °C, a rapid weight loss occurred between 400 to 500 °C owing to the decomposition of the polymeric backbone with a maximum decomposition at 460 °C.For block copolymers with different polymer blocks, double DTG peaks have been reported in the literature; nevertheless, a unimodal decomposition profile observed in the current study could suggest an alternating or random copolymer formation. 7It is worth notifying that the absence of any other decomposition peak in the copolyamide DTG curve could support the fact that no oligomers/polymers rather than polyamide were synthesized during the copolymerization reaction.On the other side, the biocomposites revealed two pronounced decomposition stages.
The first one took shape between 300 to 400 °C with a maximum decomposition temperature of ~304 °C and the second one appeared between 400 to 500 °C with a maximum S-7 decomposition of ~460 °C.In other words, the biocomposites presented the decomposition stages of both OSA-g-starch and copolyamide.For instance, PSMS30 presented approximately 22% weight loss before 400 °C due to the starch decomposition, along with about 75% weight loss between 400 to 500 °C, attributed to the polyamide degradation (Figure S3c).Obviously, the height of the first DTG peak increased upon increasing the bio-filler content, while the second one reduced, indicating an increase in the starch portion.Finally, the first decomposition stage prevailed in the biocomposite with a higher starch content, i.e., 70 wt.%.
Besides, the residue material (Table S2) increased with the increase in the starch content.
Concerning the residue of the copolyamide and OSA-g-starch at 700 °C, the residue of biocomposites had good agreement with the experimental concentration of OSA-g-starch, suggesting the effectiveness of the applied melt blending process in distributing the particles into the polymer matrix evenly.

Figure S1 .
Figure S1.The starch elements calibration curve was obtained from elemental analysis

Table S1 .
The residue of the samples at 700 °C, extracted from TGA thermograms I Extracted from TGA thermograms.II Calculated due to the portion of each component and their residue at 700 °C.S-14

Table S2 .
Shape recovery (R r ) and shape fixity (R f ) of the samples at the second and third