4D printing light-/thermo-responsive shape memory composites based on thermoplastic polyurethane/polylactic acid/polyaniline blends

Abstract

In this work, a series of polylactic acid/thermoplastic polyurethane/polyaniline (PLA/TPU/PANI) blends with different weight ratios were prepared by Fused deposition molding First, six groups of PLA/TPU (U9A1/U8A2/U7A3/U6A4/U5A5/U4A6) and three groups of PLA/TPU/PANI (821/823/825) with different ratios were fabricated by melt blending. Then, the effects of different filament forming and printing process parameters on print resolution and quality were investigated. Next, printed samples were characterized by Fourier transform infrared (FTIR), Thermogravimetric analysis (TGA), Scanning electron microscopy (SEM) and mechanical experiments. The results of FTIR and TGA showed no chemical reaction between different components, and uniform distribution of the material was observed in the SEM. The tensile and compressive curves of the samples showed an inverted U-shape. Finally, the shape-memory property was evaluated by differential scanning calorimetry. For PLA/TPU blends, U8A2 had the best shape memory capability ( $R_{r}$ = 80.8% and $R_{f}$ = 100%). Based on the excellent shape memory performance of PLA/TPU, the addition of PANI can introduce a light-actuated mechanism to form a binary-driven shape memory material. The composite materials prepared in this work can be applied to tissue engineering scaffolds, medical devices, soft robots and so on.

Graphical Abstract

Keywords

4D printing light/thermo-responsive shape memory composites polylactic acid/thermoplastic polyurethane/polyaniline blends

Introduction

Intelligent materials, especially shape memory polymer composites (SMPC), can recover the state from programmed conditions to the initial situation under a specific stimulus. According to the shape memory actuation method, SMPC can be categorized into thermal responsive,¹ light-actuated,² electrical-driving,³ magnetized-stimulating,⁴ PH-incentive⁵ and water-triggered,⁶ and so on. The thermal responsive SMPC is the most familiar due to the easy manipulation, such as constructing the warm environment via direct heating (warm water⁷) or indirect heat transfer (Joule heat⁸ and light irradiation warming²). Joule thermal triggering usually requires that the material be electrically conductive and equipped with the appropriate power supply and wires, which is a tedious process. The photothermal SMPC should have the photothermal conversion ability and a photo source that emits specific wavelengths of light.² In addition, photo stimulation has the advantages of fixed-point transformation and remotely controllable deformation.⁹ Therefore, this paper will focus on photothermal-driven SMPC.

Additionally, structure is essential for the optimum use of photosensitive materials. The commonly fabricating methods include injection molding,¹⁰ compression,¹¹ electrospinning¹² and 3D printing,¹³ etc. Among these, 3D printing can manufacture structures with complex shapes, it has high molding accuracy and fast printing speed. Fused deposition molding (FDM) is a widely used 3D printing technology, which has the advantages of low equipment price, easy operation, and a wide range of material sources.^1,7,14 Such as polylactic acid (PLA) is the most common material and generally originates from renewable plant starch, cassava, etc. Besides, as a shape memory material, PLA has weak shape memory properties ( $R_{r}$ about 10%), high driving temperature (>100°C) and high brittleness.¹⁵ Meanwhile, other thermal shape-memory materials (such as PCL and thermoplastic polyurethane (TPU)) generally cannot be printed by the FDM, because of their low strength. If material is to the printability of FDM, the shape memory properties will be affected accordingly. To solve this contradiction, the optimum approach is to prepare new SMPC by blending materials. Previously, our group prepared two categories of SMPCs, PCL/PLA blends and PCL/TPU blends. Among them, PCL/PLA blends have excellent shape memory properties, but the driving temperature is still too high (>90°C).¹ The PCL/TPU blends have medium shape memory properties with driving temperature below 60°C, but the mechanical strength is relatively low.²

Therefore, the fabrication of PLA and TPU composites is a solution. So far, scholars have investigated the shape memory behavior of PLA/TPU blends under different pre-deformation temperatures¹⁶ and annealing effects¹⁷ by varied preparing processes.^18,19 However, both PLA and TPU do not have the photothermal conversion ability. Thus, the photothermal conversion agents need to be added to the matrix to impart them with the photo-driving shape memory properties. Based on the previous report,² photothermal conversion materials include carbon-based materials (i.e. graphene, graphene oxide (GO), carbon nanotube (CNT), and carbon black), metal nanoparticles, and noble metals (gold and silver), polydopamine (PDA), polypyrrole (PPy), polyaniline (PANI), dyes, rare earth, etc. And researchers have added CNT,²⁰ carbon nanofiber,²¹ montmorillonite,²² PPy,²³ and GO²⁴ into PLLA/TPU blends and explored their shape memory behavior. However, as fillers, they are either expensive or complicated to fabricate. Polyaniline is a conductive material with infrared thermal conversion properties, good biocompatibility, and low price. Wibowo et al.²⁵ studied 3D printed PCL/PANI electroactive scaffolds for bone tissue engineering, Min-Su Heo et al.²⁶ explored the near-infrared light-responsive shape memory polymer properties from maleated polyolefin elastomer and PANI. But few studies were about the dual-drive shape properties and FDM technology based on PLA/TPU/PANI blending (or 4D printing²⁷). Commonly, 4D printing is based on the traditional 3D printer and intelligent materials (such as shape memory materials) to fabricate structures which can change form or function after giving specific stimulus.

Therefore, this work focuses on the properties of the PLA/TPU/PANI composites under different mass ratios. The main research contents include filament-forming parameters, 3D printing process parameters, thermal properties (thermogravimetric analysis (TGA), fourier transform infrared (FTIR), differential scanning calorimetry (DSC)), mechanical properties, microscopic phase distribution and dual-driving shape memory effects.

Experimental section

Materials

Thermoplastic polyurethane powder (Texalan 385A, Mw = 50,000 g/mol, particle size less than 45 μm) was purchased from Austin Novel Materials Technology (China), and PLA powder (Mw = 50,000 g/mol, particle size less than 35 μm) was purchased from Dongguan xingwang Plastic Raw material Co., Ltd. (China), and PANI powder (Mw = 60,000 g/mol, particle size less than 30 μm) was purchased from Anhui Kuer Biological Engineering (China). All materials are used directly without any other treatment.

Filament preparation of TPU/PLA and TPU/PLA/PANI blends

Thermoplastic polyurethane/polylactic acid composites were prepared by mixing different ratios of PLA with TPU as the base material. Here, six TPU/PLA blends were prepared and named UxAy for simplicity, where U represents TPU, A represents PLA, x and y is the weight ratio of the two materials (i.e. U9A1 means the weight ratio of TPU and PLA is 9:1). The PLA and TPU were dried in a vacuum oven for 48 h before mechanical mixing, and then the composite filaments were prepared by an extruder (Figure 1(a)). Table 1 shows the forming parameters. Afterward, three TPU/PLA/PANI composite filaments (i.e. 821, 823, and 825, Figures 1(a)–7) were fabricated by selecting the best formulation (U8A2) as the base material. The numbers of the blends represent the weight content of PANI in the TPU/PLA/PANI blends of 1%, 3% and 5%, respectively, and the detailed information is in “Results and Discussion” (Table 1).

Figure 1.

(a) Prepared composites filament. (b) 3D printing process with masking tapes. (c) 3D printed samples.

Table 1.

Parameters of the filament forming and 3D printing of the TPU, TPU/PLA and TPU/PLA/PANI composites.

Label	U	U9A1	U8A2	U7A3	U6A4	U5A5	U4A6	821	823	825
Extruding Speed (rpm/s)	15	15	20	30	30	30	30	20	20	20
Extrusion temperature (°C)	78	82	85	87	90	95	95	102	102	102
Nozzle temperature (°C)	175	175	180	180	180	180	180	180	180	180
Printing speed (mm/s)	10	20	30	50	50	50	50	20	20	20
Hotbed temperature (°C)	Room temperature	Room temperature	Room temperature	35	35	40	45	Room temperature	Room temperature	Room temperature

Figure 2.

Thermogravimetric analysis analysis results: (a) The pure thermoplastic polyurethane and neat polylactic acid. (b) thermoplastic polyurethane/polylactic acid composites.

Figure 3.

Fourier transform infrared results of the pure thermoplastic polyurethane, neat polylactic acid and thermoplastic polyurethane/polylactic acid composites.

Figure 4.

Mechanical properties of the pure thermoplastic polyurethane and thermoplastic polyurethane/polylactic acid blends: (a) The tensile curves of the pure thermoplastic polyurethane, thermoplastic polyurethane/polylactic acid blends; (b) Flexural curves of the pure thermoplastic polyurethane, thermoplastic polyurethane/polylactic acid blends.

Figure 5.

Scanning electron microscopy images of the cross-section view of the 3D printed rectangular of the thermoplastic polyurethane/polylactic acid composites: (a-1). U4A6, (b-1). U5A5, (c-1). U6A4, (d-1). U7A3, (e-1). U8A2, (f-1). U9A1, and the (a-2), (b-2), (c-2), (d-2), (e-2), (f-2) are their enlarged graph, respectively.

Figure 6.

(a) Scanning electron microscopy curve of the pure thermoplastic polyurethane, pure polylactic acid and thermoplastic polyurethane/polylactic acid composites. (b) The shape memory recovery ratio and shape memory fixed rate of pure thermoplastic polyurethane and thermoplastic polyurethane/polylactic acid blends with different compositions.

Figure 7.

(a) Thermogravimetric analysis analysis. (b) fused deposition molding spectra. (c) Stress-strain curves of 3D printed samples. (d) Mechanical results of flexural tests.

Table 2.

TGA results of PLA/TPU composite filaments with different weight ratio.

Label	U	A	U9A1	U8A2	U7A3	U6A4	U5A5	U4A6
$T_{1}$ (°C)	337.5	352.6	338.2	339.3	342.6	346.1	347.4	349.3
$T_{2}$ (°C)	552.7	452.9	547.9	543.2	525.1	521.6	517.0	515.5
Residual content (%)	4.09	0	2.6	1.9	1.55	0.77	0.75	0.48

3D printing

Samples were prepared by the Creativity CR2020 printer (Shenzhen Chuangxiang 3D Technology Co. Ltd., China). As shown in Figure 1(b), when printing TPU, U9A1, U8A2 and TPU/PLA/PANI (821, 823, 825) composites, considering the high viscosity of the materials, masking tape was applied to the hotbed to facilitate sample removal. The fill rate was maintained at 100% throughout the 3D printing process, and the detailed printing parameters for both the TPU and PLA/TPU blends are shown in Table 1. Figure 1(c) shows the 3D printed samples used for mechanical, SEM and shape memory tests.

Shape memory testing

Five rectangles (80 × 10 × 0.8 mm³, Figure 1(c)) were printed for each group of materials to measure and average to evaluate the shape memory performance, which contains two parameters, shape memory fixation rate ( $R_{f}$ ) and shape memory recovery rate ( $R_{r}$ ). Here, the measurement process consists of four steps, heating to programming, cooling to fixation, demolding, and heating again to shape recovery. In addition, we used a medical infrared lamp (Philips E27, 50W) to measure the photosensitive shape memory properties. Meanwhile, a handheld infrared thermometer was chosen for testing the sample temperature during the illumination.

Characterizations

TGA testing

Thermal stability and decomposition temperature of composite filaments were tested in nitrogen atmosphere with a TGA analyzer (STA 449F3TGA, Mettler-Toledo, Germany Netsch). The starting temperature was room temperature and the ending temperature was 800°C, with an increased temperature rate of 10°C/min.

FTIR analysis

The functional groups of the components in the composites were observed by FTIR (Satellite FTIR spectrometer, Mattson) to analyze whether chemical reactions or new substances were formed between the composites (PLA, TPU and PANI). The resolution was set to 4 cm⁻¹ and the scanning range was from 400 cm⁻¹ to 4000 cm⁻¹.

Mechanical measurement

The tensile and bending properties of the samples were tested on an electronic universal testing machine (Wance, ETM504C, Shenzhen, China) with a loading speed of 100 mm/min and 2 mm/min, respectively. Regarding the measurement results, the experimental apparatus directly provides the function of tensile stress-strain curves, compressive load and displacement and a summary of the mechanical properties of the measured sample. In addition, five samples of each material were 3D printed for testing and take the average. Detailed information is provided in “Results and Discussion 3”.

SEM observation

The microtopography of the composites was performed by SEM (HITACHI’s TM4000PLUS) on cross sections of 3D printed rectangular specimens that were prepared via liquid nitrogen fracture and then gold-coated (Hitachi MC1000).

DSC testing

The composites were tested by DSC (DSC214, Germany Netzsch) in the nitrogen atmosphere, 5 mg of each material was placed in an aluminum pan and gas purging at a rate of 40 mL/min. The DSC measurement process consists of three temperature changes, i.e. heating-cooling-heating. The first heating is to eliminate the thermal history, and the second heating curve is used as the final result. The temperature change was set to be room temperature −240°C–−50°C–240°C with a ramp rate of 10.0°C/min. The test results can provide the glass transition temperature ( $T_{g}$ ) and the melting point ( $T_{m}$ ), which help to obtain the extrusion temperature for filament formation and to obtain the shape memory deformation temperature of the material.

Results and discussion

PLA/TPU composite filaments forming and 3D printing procedure

Considering the characteristics of the FDM printing process, the fabricated composite filaments should be strong enough to withstand the forces exerted by the feeding gears to ensure printing smoothly. Commonly, the dimension of the fabricated filament depends on the combination of extrusion temperature, extrusion speed, pulling speed, preload force and cooling method. The extruding temperature generally depends on the melting point of the component materials. An extrusion temperature usually above about 10.0°C than the melting point of all components is usually desirable because high temperature will affect the thermal characteristics of the material, such as yellowing (Figures 1(a) and 2). If the temperature is too low, some of the components are not melted, resulting in a rough surface of the filament (Figures 1(a)–3). At the same time, excessive preload will flatten the filament (Figures 1(a)–4). Besides, insufficient coordination between hauling speed, extrusion velocity, preload force and cooling method can affect the dimensional accuracy of filament (Figures 1(a)–5). In addition, due to the high viscosity of TPU, the filament was not been shaped thoroughly before packaging and was prone to bonding. Therefore, a combination of water cooling and air cooling was chosen to fix the filament shape. Finally, the optimal filament forming parameters of each composite filament were obtained through experimental verification (Table 1), and Figures 1(a) and 6 shows the prepared filaments for subsequent 3D printing research.

Meantime, Table 1 revealed the optimum printing parameters of each material based on the fabricated composite filaments. Among all PLA/TPU composites, due to TPU, U9A1 and U8A2 having high viscosity, the printed samples were attached to the hotbed closely, thus difficult to remove. Therefore, as Figure 1(b) shows, the masking tape was attached to the hot bed before printing TPU, U9A1 and U8A2 composites. Meanwhile, set the hotbed temperature at room temperature to ensure the smooth removal of the sample. Similar to our previous study,⁷ when the nozzle temperature is too low during 3D printing, the filament cannot be melted completely, which affects the overall printing fluency. Higher temperatures can impair the quality of the printed structures, such as yellowing, structural collapse, and dimensional inaccuracy. As for the fill rate, printing consumables and printing time are directly proportional to the fill rate, and the lower the fill rate, the faster the heat transfer rate and the shorter the shape memory recovery time. In this work, the fill rate was set to 100% in order to better measure the performance of the 3D printed samples. A low print speed was chosen to improve the print quality, where the previous layer was solidified before depositing the new layer. Based on the optimal printing parameters (Table 1), samples were printed for micromorphological observations, mechanical experiments and shape memory tests.

TGA analysis

The thermal stability and thermal decomposition of TPU/PLA composites were obtained by TGA analysis. The initial decomposition temperature was set to

T_{1}

, and the mass loss of the sample at this point was 5%. The final temperature at

T_{2}

and no further weight change occurred here. As revealed in Figure 2(a), we also tested the matrix TPU and PLA as a reference. Combining Figure 2(a) with Table 2, we can find that the

T_{1}

T_{2}

of TPU and PLA were 337.5°C/552.7°C and 352.6°C/452.9°C, respectively. With the PLA weight ratio in the TPU/PLA blends gradually increased, their curves all fell in the pure TPU and pure PLA (Figure 2(b)). That is to say, the

T_{1}

of the composites were 338.2°C (U9A1), 339.3°C (U8A2), 342.6°C (U7A3), 346.1°C (U6A4), 347.4°C (U5A5) and 349.3°C (U4A6), respectively. Moreover, when the temperature reached 547.9°C (U9A1), 543.2°C (U8A2), 525.1°C (U7A3), 521.6°C (U6A4), 517.0°C (U5A5) and 515.5 (U4A6), the thermal decomposition was completed with the residual weight less than 5%. The reason is that there was no chemical reaction between the two components. The properties of both extrusion temperature (<110°C) and 3D printing temperature (<200°C) were lower than the initial decomposition temperature of the prepared composites (>300°C). Even after the treatment of hot extrusion (filament-forming procedure) and 3D printing, the prepared TPU/PLA composites remained stable and did not show thermal decomposition.

Table 3.

Summary of tensile and flexural testing results.

Mechanical properties	Label	U	U9/A1	U8A2	U7/A3	U6/A4	U5/A5	U4A6
Elongation at break (%)	Mean	1586.867	23.360	19.419	16.169	11.931	7.893	6.553
	SD	20.280	1.011	0.553	0.367	0.216	0.114	0.285
	Confidence interval (95%)	[1560.01, 1606.38]	[22.6, 23.89]	[18.34, 20.07]	[15.25, 16.73]	[11.83, 12.11]	[7.39, 8.25]	[5.86, 6.95]
Ultimate tensile stress (MPa)	Mean	9.318	11.691	12.093	11.912	11.833	10.642	9.544
	SD	0.081	0.328	0.112	0.055	0.141	0.262	0.093
	Confidence interval (95%)	[8.99, 9.50]	[10.88, 12.16]	[11.82, 12.24]	[11.14, 12.33]	[11.12, 11.96]	[9.80, 11.28]	[9.22, 9.96]
Tensile modulus of Elasticity (MPa)	Mean	82.573	80.190	85.082	97.078	112.821	141.540	149.884
	SD	0.972	0.152	0.421	0.461	0.274	0.387	0.412
	Confidence interval (95%)	[81.42, 83.32]	[79.07, 80.92]	[84.60, 85.64]	[95.72,.98.24]	[112.31, 113.31]	[140.33, 142.11]	[149.16, 150.40]
Flexural strength (MPa)	Mean	13.389	12.353	11.273	10.986	9.209	7.396	7.293
	SD	0.917	0.970	1.002	0.690	0.357	0.386	0.368
	Confidence interval (95%)	[11.51, 14.21]	[11.37, 12.74]	[11.22, 12.45]	[10.25, 11.95]	[8.61, 9.99]	[6.26, 8.04]	[6.33, 8.11]
Bending modulus of elasticity (MPa)	Mean	45.328	63.032	79.033	91.208	104.157	104.233	120.468
	SD	0.588	0.965	0.970	0.838	0.890	0.866	0.661
	Confidence interval (95%)	[43.73, 47.30]	[62.28, 65.06]	[78.27, 81.07]	[90.01, 93.50]	[102.03, 107.44]	[103.23, 109.51]	[117.22, 123.58]
Maximum load(N)	Mean	39.846	40.163	40.713	41.859	36.162	28.346	27.195
	SD	0.530	0.664	0.752	0.687	0.953	0.568	1.342
	Confidence interval (95%)	[38.40, 40.15]	[39.51, 40.97]	[39.75, 41.83]	[38.64, 44.94]	[35.82, 37.47]	[27.18, 29.77]	[26.61, 28.90]

FTIR measurement

The FTIR analysis spectrums were elucidated in Figure 3 to characterize the chemical structure of samples. For TPU, hydrogen bonding was distributed in the range of 3000–3500 cm⁻¹. The asymmetric and symmetric methylene stretching peaks fell in 3000 ∼ 2800 cm⁻¹, respectively. 1730 ∼ 1700 cm⁻¹ was the hydrogen-bonded urethane groups (amide I), the peak at 1072 cm⁻¹ belonged to C–O–C stretching.^28,29 For PLA, 2996 cm⁻¹ and 2946 cm⁻¹ were assigned to the stretching modes of asymmetric and symmetrical C–H bonds, respectively. The stretching vibration of the C = O bond peak located at 1755 cm⁻¹, and 1182 cm⁻¹ corresponds to the stretching of the ether bond.³⁰ Even though the preparation process experienced high temperatures, no new peaks appeared in FTIR, which indicated that the mixing of TPU and PLA did not produce a chemical reaction, but only a physical mixing, and the results were also consistent with the TGA test.

Mechanical properties measurement

Figure 4 and Table 3 manifested the mechanical properties of pure TPU and TPU/PLA composites. The tensile stress-strain curves in Figure 4(a) show the typical elastomeric behaviour of the matrix material TPU. With the addition of PLA, the ultimate tensile stress of TPU/PLA composites presented an inverted U-shape and reached the maximum value when PLA content was 20%. Meanwhile, with the increase of PLA content, the elongation at break reduced and the tensile modulus of elasticity increased accordingly, and the flexural strength showed a decreasing trend, while the flexural modulus of elasticity also increased accordingly. The maximum load first increased and then decreased as the PLA increased. This indicated that when the content of PLA was ≤20%, the TPU/PLA blends still had some toughness attributed to the TPU. Whereafter, due to the high strength and brittleness of PLA and its increasing dominance in composite, the properties of other prepared TPU/PLA composites gradually approached the pure PL. Additionally, to verify the mechanical properties stable of the prepared composite materials, Table 3 also presented the standard deviation and 95% confidence interval. According to the value, we can find that the mechanical properties of the composites are stable. In conclusion, the trend of mechanical properties with the varied weight ratio of each component also showed that no new substances were produced between TPU and PLA after blending, which was also constant with the results of TGA and FTIR testing.

Microstructure of the composites

The SEM of the 3D printed rectangular cross-section of the TPU/PLA composites are exhibited in Figure 5. For the composites, TPU was used as the matrix material and PLA was mixed with it as filler in different weight ratios. When TPU was in the dominant state, such as the U9A1, U8A2 and U7A3, TPU tended to agglomerate, while less PLA is evenly dispersed in it. With the PLA content continuing to increase, the co-continuous phase morphology diminished and the components were is uniformly distributed in the composites. That is because PLA and TPU were physically mixed without chemical reaction, which was also consistent with the previous TGA, DSC and FTIR analyses.

DSC analysis

As displayed in Figure 6(a), the DSC analysis results provided the glass transition temperature ( $T_{g}$ ) and melting point ( $T_{m}$ ) of the tested specimens. Therein, TPU as the matrix material, its strength was improved to satisfied the requirements of FDM. Figure 6(a) showed the presence of two peaks for TPU, 45.3°C and 54.2°C, respectively. Namely, the increased strength of the substrate material TPU leaded to the appearance of a significant melting peak. As for PLA, the melting temperature is about 110.8°C. With the increase of PLA content in TPU/PLA composites, two peaks appeared in the blends, which belong to the melting point of TPU and PLA, respectively. They were also the $T_{g}$ and $T_{m}$ of the TPU/PLA blends. The addition of PLA to the TPU resulted in a slight decrease in the $T_{g}$ of the composites and correspondingly slight increase in the melting temperature. It also clarified that no new peaks were generated and no chemical reaction occurred between the components of the composites. Moreover, when TPU/PLA composites are used as a class of thermal shape memory materials, the TPU acts as the soft part and the PLA as the hard part. Furthermore, $T_{g}$ and $T_{m}$ of the material can assist in offering the approximate range of filament extrusion temperature, printing temperature and shape memory driving temperature of the material.

Shape memory properties measurements

In general, the shape-memory fixed ratio and shape memory recovery rate are chosen to evaluate the shape memory properties of TPU/PLA blending composites. Herein, the matrix material TPU was the soft part of the thermal responsive shape memory composites, and PLA was the hard part. From the DSC test results (Figure 6(a)), the approximate region of the driving temperature of the shape memory materials can be obtained. Usually, they are above the glass transition temperature (about 1°C–20°C). Therefore, it is a prerequisite that the sample can maintain its shape and possess definite strength and resilience, Table 4 displays the programming temperature for each composite. Moreover, because the melting temperature of PLA was above 100°C, thus we did not measure it.

Table 4.

Programming temperature of each composite.

Label	U	A	U9A1	U8A2	U7A3	U6A4	U5A5	U4A6	821	823	825
Temperature (°C)	47.5	>100	51	51.5	53	54	56	58	53	53.5	54

Same as previous studies,⁷ five sets of samples were printed for testing and averaged, and Figure 6(b) showed the shape memory testing results of pure TPU and TPU/PLA blends. Herein, we can find that the $R_{f}$ of TPU was about 94.2%, and $R_{r}$ was only 51%, which was attributed to its increased strength to meet the printability of FDM processing. To improve the shape memory performances of the TPU, PLA was blended with TPU to measure the shape memory effects of the TPU/PLA blends with varied weight ratios. According to Figure 6(b), it can be concluded that the shape memory fixation rate of the all six composites reached 100%. In particular, the shape memory recovery rate of U9A1 and U8A2 was significantly higher, reaching 80%. In both, there was a two-phase phenomenon. PLA, as the hard segment providing structural support, and elastomer TPU, as the soft segment (or the active phase), can flow well after the temperature is above the melting point of TPU, realizing the shape memory recovery function of the composites. However, with the gradual increase of PLA content, the energy stored in TPU was not enough to drive the shape memory recovery of the structure due to its high stiffness, leading to a gradual weakening of shape memory recovery effects. Furthermore, the shape memory properties of the composites in this work were better compared with other studies.³¹

In summary, the U8A2 composite had moderate mechanical properties and the highest shape memory recovery rate (80.8%) among all prepared composites. Therefore, it was chosen as the substrate material and blended different proportions of PANI to measure the photosensitive shape memory characteristics. The mass content of PANI in the TPU/PLA/PANI composites was set as 1%, 3% and 5%, and labeled as 821,823 and 825, respectively. Before mixing, TPU, PLA and PANI were dried in a vacuum drying oven at 45°C for 48 h. Subsequently, referring to the preparation method of TPU/PLA composite filaments, the three components were blended in different mass ratios and prepared into filaments with a diameter of 1.75 mm±0.5 mm by an extrusion mechanism. Table 1 exhibited the filament-forming and 3D printing parameters for 821,823 and 825. Besides, the temperature of the hotbed was set to room temperature with masking tape in order to remove the samples more easily.

The effect of PANI on the U8A2 composite properties

The effect of PANI on U8A2 was analyzed by TGA, FTIR and mechanical properties. According to the TGA analysis (Figure 7(a)) it can be found that the addition of PANI does not significantly change the thermal decomposition trend of U8A2, and the residual mass increases with the increase of PANI content. This indicates that PANI does not decompose completely at 800°C. The FTIR curves (Figure 6(b)) characterized the functional groups and chemical bonds of PANI and U8A2 composites. Typical peaks of PANI were found in the TPU/PLA/PANI blends (821,823 and 825) according to the spectra. For example, at 526 cm⁻¹ was the ring deformation vibration of PANI. 826 cm⁻¹ and 913 cm⁻¹ were mono-substituted benzene and C-H out-of-plane deformation, respectively. The absorption peak at 1570 cm⁻¹ corresponded to the quinoid ring structure in PANI. The vibration at 1592 cm⁻¹ was due to the characteristic C = C-C stretch of aromatic nuclei of PANI. 3062 cm⁻¹ was the OH stretching vibration of water molecules physically absorbed on the PANI backbone in association with aromatic C-H stretching vibration.³² A band around 3327 cm⁻¹ was responsible for the N-H stretching vibration of PANI.^33,34 Therefore, the newly prepared TPU/PLA/PANI composites also did not generate any other new functional groups. The composites of the three components were simply blended. In addition, Figure 7(c) and (d) and Table 5 disclosed the change in mechanical properties. The mechanical strength increased slightly with the increase of the proportion of PANI. Here, the composite stiffness increased with increasing PANI content, with 825 exhibiting more brittleness than 821. It was also found that the elongation at break decreased with increasing PANI loading. Meanwhile, based on the SD and 95% confidence interval, the mechanical properties of the composites are stable.

Table 5.

Summary of tensile and flexural testing results.

Mechanical properties	Label	821	823	825
Elongation at break (%)	Mean	19.963	16.706	16.169
	SD	0.365	0.154	0.292
	Confidence interval (95%)	[18.72, 20.74]	[15.43, 17.17]	[15.65, 16.30]
Ultimate tensile stress (MPa)	Mean	12.951	12.810	11.912
	SD	0.073	0.289	0.066
	Confidence interval (95%)	[12.02, 13.31]	[12.11, 13.33]	[11.74, 12.17]
Tensile modulus of Elasticity (MPa)	Mean	93.102	95.793	97.078
	SD	0.334	0.376	0.863
	Confidence interval (95%)	[92.00, 94.20]	[94.87, 96.47]	[95.13, 99.52]
Flexural strength (MPa)	Mean	11.733	10.314	10.616
	SD	0.672	0.512	0.505
	Confidence interval (95%)	[9.77, 12.73]	[9.18, 11.52]	[9.50, 11.41]
Bending modulus of elasticity (MPa)	Mean	85.095	85.571	85.965
	SD	1.520	0.561	0.401
	Confidence interval (95%)	[81.35, 88.77]	[82.60, 88.53]	[83.22, 89.33]
Maximum load(N)	Mean	43.403	44.579	44.915
	SD	0.763	0.563	0.475
	Confidence interval (95%)	[42.79, 44.46]	[43.54, 45.38]	[44.51, 46.21]

Differential scanning calorimetry analysis was carried out to investigate the effect of PANI on the driving temperature. As shown in Figure 8(a), for TPU/PLA/PANI composites, the addition of PANI amplified the peak of $T_{g}$ (TPU), and the melting temperatures of TPU and PLA were also slightly increased. In other words, the actuating temperature of TPU/PLA/PANI composites was increased with the increase of PANI, and Table 4 shows detailed information. Figure 8(b) exhibited the shape memory performances of TPU/PLA/PANI blends. The shape memory fixation rate remained at 100% and the shape memory recovery rate fluctuated around 80%. This indicates that a small amount of PANI had no significant effect on the shape memory characteristics of U8A2.

Figure 8.

(a) Fused deposition molding analysis of U8A2 and thermoplastic polyurethane/polylactic acid/polyaniline composites. (b) Shape memory properties of U8A2 and thermoplastic polyurethane/polylactic acid/polyaniline blends.

Subsequently, the photosensitive shape memory performance of the composites (821, 823 and 825) was tested in a process similar to the temperature triggering. Firstly, the 3D printed samples were deformed around the programmed temperature under illumination, and then cooled and shaped in room temperature water. The shape memory properties were evaluated by placing the samples under an infrared light (the distance between the light and the product was about 20 cm). Compared with the thermal actuation, the infrared light triggered $R_{f}$ for the three composites (821, 823 and 825) was also 100%, while $R_{r}$ fluctuated around 80%. The increasing PANI content in TPU/PLA/PANI composites enhanced the photothermal conversion ability. Thus, the recovery time of shape memory was also shortened. In addition, the recovery time of shape memory triggered by warm water was shorter than that stimulated by infrared light. Because the heat transfer was faster in hot water, the temperature of the sample can rise to the glass transition temperature in a short time to achieve shape recovery.

Following, uncovered box, rectangular bar and reseau structure were printed using 825 and U8A2 to verify the shape memory effects, and Figure 9 shows the shape-changing under different stimuli. As the temperature of 825 continued to rise under IR illumination until the temperature exceeded the melting point of PLA, the box opened completely. Namely, the combination of the movement of the hard segment (PLA) and gravity effects leaded to the flat situation (shape memory recovery), the detailed information is in the supplemental video.

Figure 9.

Dual actuating shape memory processing: The shape recovery procedure of U8A2 and 825 blend under thermal actuating (54°C), light-actuating.

Conclusion and outlook

In this work, six types of TPU/PLA composites were fabricated at first. Then, the filament forming and FDM printing parameters were explored and the optimum values were selected. Whereafter, the TGA, FTIR, mechanical properties, SEM and DSC analyses were executed, and based on these results, it can be concluded that TPU and PLA were only physically mixed. No new substance was generated and the composite material was characterized as a superposition of the PLA and TPU. Thermoplastic polyurethane/polylactic acid composites had thermally responsive shape memory properties. According to the shape memory test, U8A2 had the best shape memory recovery ratio (around 80.8%). Based on the excellent shape memory performance of U8A2, the addition of PANI can introduce a light-actuated mechanism to form a binary-driven shape memory material.

Combining the above characterization with 3D printing parameters allow easy, fast and low-cost fabrication of the dual-drive functional structure. For potential applications, subsequent research can favor the preparation of soft robots with remotely controlled deformation based on photosensitive shape memory properties. Considering the biocompatibility of the matrix material (TPU, PLA and PANI), the materials can also be applied to tissue engineering, such as tissue engineering scaffolds with photothermal therapy effects, tracheal scaffolds and pathological intestinal scaffolds with remotely controlled deformation.

Supplemental Material

Footnotes

Acknowledgements

We would like to thank the Analytical and Testing Center of Northwestern Polytechnical University for DSC, SEM and TGA tests.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project was sponsored by the National Natural Science Foundation of China (Grant No. 51905438), The National Key Research and Development Program of China (Grant No. 2019QY(Y)0502), the Fundamental Research Funds for the Central Universities (Grant No. 31020210506006), the Key Research and Development Program of Shanxi Province (Grant No. 2022GY-228), and the Fund of Foshan Science and Technology Research (2018AB002711).

Data availability

The raw data required to reproduce these findings are available to download.

ORCID iD

Qinghua Wei

Supplemental Material

Supplemental material for this article is available online.

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