Experimental investigations of temperature-sensitive shape memory polymer composites for 4D printing

Abstract

Shape memory polymers (SMPs) and their composites (SMPCs) have emerged as popular materials in a variety of industries due to their unique properties of shape-changing behavior in response to external stimuli. The inclusion of reinforcement may modify the SMPs to enhance their thermal and shape memory properties. Different types of bio ceramics have already been used to alter the thermal and shape memory behavior of SMPs. However, using bioactive glass (BG) as filler to modify these properties has not yet been explored. Despite the significant advantages that shape-memory polymers (SMPs) offer when combined with 3D/4D printing technology, their potential in 3D printing has been explored only to a limited extent. This work created a solvent-based 4D-printed temperature-sensitive shape memory polymer composites (SMPCs) system using polylactic acid (PLA) and bioactive glass (BG). The influence of BG on the thermal as well as shape-memory capabilities of composites was further examined. An increase in the degree of crystallinity and viscoelastic characteristics of PLA/BG composites led to improved shape memory properties, like shape fixity and shape recovery. These findings suggest the potential for using the developed SMPC printed through 4D printing technology, to develop complex shapes for self-foldable structures and smart biomedical devices in the future.

Keywords

Bioactive glass polylactic acid 4D printing shape memory polymer shape memory properties thermal characterization

Introduction

Stimuli-responsive polymers, also known as shape memory polymers (SMPs), are polymers that change shape in response to an external stimulus (i.e., temperature variation, moisture, electric/magnetic) field and then return back to their original, unaltered shape once the stimulus is removed.^1,2 SMPs are a rapidly expanding class of smart materials with many potential uses in fields as diverse as smart medical equipment, high-performance textiles, aerospace, packaging, actuators, and civil engineering due to their many advantageous properties, such as greater than 100% recoverable strain, cost-effectiveness, relatively low density, simple processability, and even potential biodegradable properties.^1,3,4 Even so, in the past, neat SMPs with no reinforcement demonstrated poor mechanical strength and recovery forces, limiting the range of their potential uses.⁵ Composite technology, the idea of adding two or more materials together to create a new material with improved qualities, has gained traction in a variety of fields.^2,6,7 Inorganic materials like hydroxyapatite particles, $S i$ -based particles (e.g., $S i O_{2}$ , $S i C$ ), carbon fibers/nanotubes, glass fibers, and graphene oxide, have been widely used as reinforcements to overcome the drawback of poor mechanical characteristics and shape memory properties.^8–14 Therefore, shape memory polymer composites (SMPCs) containing reinforcing fillers may be designed to further extend the application area of SMPs.² A further benefit of SMPs is their ease of processing, increasing the possibility of their application range. Electrospinning,¹⁵ microsphere fabrication,^10,16 and 3D or 4D printing¹⁷ are the most common processing techniques used with SMPs. Among the various fabrication techniques available for SMPs, 3D/4D printing is considered to be the most modern, with numerous advantages over traditional methods. This method can be employed to create complex shapes like controlled sequential folding, allowing for easier printing of certain structures or even making unprintable shapes available for smart biomedical implants and soft robotics.¹⁷

Depending upon the nature of SMPs, there are three popular 3D printing technologies to process SMPs, like stereolithography (SLA), fused deposition modelling (FDM), and solvent-based 3D printing (SB3DP) which is a type of direct ink writing (DIW) technique.^18,19 SLA is limited to use with only one photosensitive monomer unit at a time with slower printing speed, higher printing time and cost.^20–22 The FDM process is commonly employed in the processing of thermoplastic SMPs.¹⁷ Nevertheless, the primary disadvantage of the FDM process is its high working temperature, which has led to the thermal stresses in the materials used in this technology.²³ Moreover it is widely recognized that the SMPs used in FDM are subjected to frequent heating and cooling, which causes some undesired warpage of the printed item to occur after fabrication.²⁴ Several studies have also highlighted filament buckling as a significant challenge with FDM.²² Solvent-based 3D printing (SB3DP) is a direct ink writing method that uses a viscous ink made by dissolving a polymer in a suitable solvent. This printing method is unique because of the filament is extruded at room temperature and the rheological properties of the ink affect the solidification of the printed filament.²³ Bioinks with suspended cells and other biological fugitive substances, colloidal suspensions, high concentration polyelectrolytes solutions and thermosets (UV-curable), composites and nanocomposites sensitive to higher temperature, have all been utilized in this method to build specialized structures. Zhu et al.²⁵ used a pneumatic fluid dispenser to deposit a viscous ink to create 3D printed graphene aerogel lattice structures. Guo et al.¹⁸ utilized a solvent-cast direct-write method to create freeform 3D geometries using dissolvable thermoplastic polymers at room temperature. Kandi et al.²⁶ created a novel hybrid tracheal scaffold using a combination of synthetic and natural polymers (alginate and gelatin) using direct ink writing.

A fascinating recent development of 3D printing is 4D printing in which the printed objects or parts can be deformed in response to an external stimulus.^17,27,28 Even though 3D printing technologies have been successfully emerged for a wide variety of materials like metals, polymers as well as ceramics but the majority of materials are not suitable to be used with 4D printing because they fail to demonstrate abundant shape change in the presence of an external stimuli such as moisture, temperature variations, or an external electric/magnetic field.^27,29 Printability and “smartness,” or the ability to respond accurately to a stimulus, are prerequisites for any potential material to be used in 4D printing. Researchers are still attempting to identify smart materials to back the advancement of 4D printing technology.^17,29,30

Polylactic acid (PLA), a member of the polymer family, has gained popularity for its use in textiles, food packaging, and the biomedical field.³¹ PLA is an ideal material for use in other practical applications even in contact with the human body because of its non-petrochemical basis, high tensile strength and elastic modulus, biocompatibility, and ability to degrade into non-toxic products under physiological environment with an additional advantage of easy processability.³¹ Due to its glass transition temperature $(T_{g})$ , which serves as a transition temperature of about 60 $℃$ , this highly versatile polymer is also known to display thermo-responsive shape-memory properties.^1,32,33 Thermally activating shape memory in PLA is accomplished by heating to a switching temperature $T_{s w}$ higher than $T_{g}$ , at which these polymer chains have sufficient mobility to fully recover the original form from a temporary initial shape that was previously fixed.

Although the PLA has several advantages, certain features require customization and careful engineering to optimize its performance.³⁴ Hence diverse materials with different shapes and sizes are reported as reinforcement in PLA polymer matrix to improve its shape memory properties that can be further utilized in 4D printing to develop smart complex shapes and geometries.^35–37

Zhao et al.³⁸ suggested PLA/ ${F e}_{3} O_{4}$ as SMPC that can be recovered to its original shape in an external magnetic field. In order to create the filaments for the FDM printer, 80% PLA and 20% ${F e}_{3} O_{4}$ (magnetite) were combined to create a solid mix that was then chopped and extruded in a twin-screw extruder. They discovered that by using a magnetic field with alternating frequencies of 30 $k H z$ , shape recovery of more than 95% was accomplished in just 14–24 s. In their future work, they fabricated a smart tracheal scaffold which was capable to regain its original cylindrical shape from a temporary flat shape within 35 seconds.³⁹ Senatov et al.⁴⁰ fabricated polylactide (PLA)/15 wt% hydroxyapatite (HA) based porous scaffolds with remodeled structures with FDM. They found that the scaffolds had a shape recovery ratio of 98% and may be useful for self-fitting bio implants for small bone defect replacement.

Bio active glasses (BGs) are another important class of inorganic biomaterial within the tissue-engineering industry, due to their high bio activity and biocompatibility. The benefit of using bioactive glasses is that their composition can be specifically tailored, their capacity to reabsorb under physiological conditions can be managed, as well as physiologically relevant ionic species may be doped into these materials to trigger cellular processes in a variety of different tissues. BGs reinforcement can also result in better mechanical and biological properties of SMPCs.⁴¹ Additionally, it is anticipated that the interaction between BGs and SMPs promotes particle dispersion in SMPCs, enhancing nucleation sites, % crystallinity, and shape memory characteristics.⁴²

According to a review of the literature, it was discovered that the majority of studies focused solely on creating 4D structure with FDM process only. The current work attempts to fill this gap with the effective use of SB3DP technique for 4D printed structures with SMPC consisting of PLA matrix reinforced with BG particles. This work also examines the influence of BG reinforcement on the thermal and shape memory characteristics by analyzing the crystallization behavior, shape fixity, and shape recovery ratios of 4D printed SMPCs.

Materials and Methods

Materials

The polymer material utilized was semi-crystalline PLA manufactured by Nature Works under the commercial designation PLA 2003D and defined by the levels of d-isomer content of 4% and density 1.24 $g m / {c m}^{3}$ . Dichloromethane (DCM) was used as solvent which was procured from Fisher Scientific having the concentration of 99.5%. Bioactive glass (BG) particles (grade 45S5, nano Research Elements) created using sol-gel chemistry with average particle size of 2 $μ m$ were employed as reinforcement. Bioactive glass 45S5 was composed of 45 $w t %$ of ${S i O}_{2}$ , 24.5 $w t %$ of $C a O$ , 24.5 $w t %$ of ${N a}_{2} O$ and 6 $w t %$ of $P_{2} O_{5}$ .

Preparation of Polymer/Bioactive Glass Composite Ink for 4D Printing

Table 1 shows how several PLA/BG composite inks were created by varying the quantity of BG particles in the polymer matrix.

Table 1.

Composition of PLA/BG composite inks.

S. No.	Ink name	Composition (wt%)
S. No.	Ink name	Polylactic acid (PLA)	Bioactive glass (45S5)
1	PBG0	100	0.0
2	PBG0.5	99.5	0.5
3	PBG1.0	99.0	1.0
4	PBG1.5	98.5	1.5
5	PBG2.0	98	2.0

To produce a 2% ( $w / v$ ) polymer weight to solvent volume fraction, the PLA was first dissolved in dichloromethane (DCM) in a 0.5 $g / m l$ proportion. The mixture was mechanically stirred all night to produce a homogeneous polymer solution. To prepare the composite ink, the required quantity of BG powder was disseminated in 0.05 $g / m l$ dichloromethane (DCM) and sonicated in an ultrasonic bath for 1 $h o u r$ . The sonicated solution containing BG and PLA solution was then combined, and the resulting composite mixture was mechanically agitated for 1 $h o u r$ to improve BG particle dispersion in PLA solution. To achieve the final homogenous PLA/BG composite ink, the ink was left overnight. Following the preparation of the homogeneous ink, samples for different characterization were created using a solvent-based 3D printer with a nozzle diameter of 0.41 $m m$ and a print speed of 8 $m m / s$ . Figure 1(a) illustrates a flow chart of the ink preparation, and Figure 1(b) displays the structural setup for customized solvent-based extrusion 3D printer.

Figure 1.

(a) Flowchart for fabrication of PLA and BG composites and (b) set up for solvent-based 3D printer.

Characterization

Morphology

The morphology of surfaces was analyzed by means of scanning electron microscopy (SEM, model: Zeiss EVO 50). 10 $k V$ of accelerating voltage was used for the observations. EDX mapping in conjunction with SEM research helped in clarifying the distribution of BG powder throughout the PLA polymer matrix.

Atomic Force Microscopy

Asylum Research MFP3D-SA was utilized to photograph all samples using silicon cantilever with a resonance frequency ranging from 60 to 70 $k H z$ in tapping mode. The scan region (5 × 5 $μ m$ ) was chosen and the scans were reproducible with minimal probe tip disturbance. Average roughness value of each surface was calculated and analyzed using the Gwydion (v2.54) software. AFM was performed on three specimens of each PLA and BG mix.

X-Ray Diffraction

A Smart lab. Rigaku RU-200 diffractometer (Rigaku Co., Japan) was used to examine composites with copper $K α$ radiation (wavelength = 0.154 $n m$ ) in order to identify the phases present. For the measurement, a range of 20-80 $°$ was employed with a scanning rate of 2 $° / \min$ .

Thermal Characterization

Differential Scanning Calorimetry

The glass transition ( $T_{g}$ ), crystallization, and melting are some of the thermal transitions that may be studied with DSC. Curves between heat flow and temperature (DSC curves) were generated using a DSC-TA 250 equipment. The temperature was ramped from 20 $℃$ to 200 $℃$ (first heating cycle) and from 200 $℃$ to 20 $℃$ (cooling cycle) at 5 $℃ / \min$ in nitrogen atmosphere. For the test, samples weighing between 10 $m g$ and 15 $m g$ were used. The glass transition temperature ( $T_{g}$ ), melt temperature ( $T_{m}$ ), cold crystallization temperature ( $T_{c}$ ), endothermic enthalpy of melting ( $H_{m}$ ), and % crystallinity ( $X_{c}$ ) were all measured during thermal scanning of the samples. The percentage of crystallinity was determined using the following equation:⁴³

X_{c} = 100 \times \frac{{∆ H}_{m}}{{w \times ∆ H}_{m}^{*}}

(1)

Where

{∆ H}_{m}

is the heat of fusion for PLA/BG composites,

{∆ H}_{m}^{*}

is the heat of fusion for 100% crystalline PLA and

w

is the mass fraction of PLA in PLA/BG composites. The heat of fusion for 100% crystalline polymer was taken as

{∆ H}_{m}^{*} = 93 J g^{- 1}

.⁴⁴

Thermo-Gravimetric Analysis

A TA instrument SDT650 was used to conduct thermogravimetric analysis (TGA) at a heating rate of 10 $℃ / \min$ . For the purpose of measuring the complete thermal degradation of composites, the samples were heated from 20 $℃$ to 500 $℃$ with nitrogen flowing at a rate of 60 $m l / \min$ . Data were recorded between the change in weight of the composites or the rate of degradation with rise in temperature.

Dynamic Mechanical Properties

A dynamic mechanical analyzer, model DMA Q850 TA Instruments (New Jersey, USA), was used to test the dynamic mechanical characteristics in tensile mode. The tests were done with a fixed frequency of 1 $H z$ , temperatures from 20 $℃$ to 200 $℃$ with a heating rate of 3 $℃ / \min$ . The test specimen’s dimensions were approximately 2.0 $\times$ 5.0 $\times$ 30.0 $m m$ ( $t h i c k n e s s \times w i d t h \times l e n g t h$ ). Three samples of each composite were evaluated, and their mean value with ± SD was outlined as a result.

Shape Memory Characterization

Thermomechanical cycles were used to characterize thermally triggered shape memory at $T_{t r a n s}$ , near to the glass transition temperature ( $T_{g}$ ) of PLA, utilizing a universal tensile testing apparatus with a controlled temperature chamber. A 3D printed rectangular strip (70 $\times 10 \times 1 m m$ ) with a gauge length $50 m m$ was undergone heating and cooling cycles (Figure 2). In fact, heating the sample over the relevant $T_{t r a n s}$ and deforming the sample to the desired shape was required to undertake thermally triggered shape memory analysis. After that the material was cooled below $T_{t r a n s}$ while the deformation was held constant. Once the deformation was fixed, the external force was released. By re-heating the sample above the $T_{t r a n s}$ , the original form was recovered. The shape memory evaluation was done thrice for each composite using the steps mentioned below:

Step 1: After 10 minutes of isothermal heating above $T_{g}$ at 70 $℃$ (where the $T_{g}$ was acquired from the DSC), the sample was deformed to approx. 10 $%$ strain $(\in_{d})$ with a ramp load of 0.5 $N / \min$ .

Step 2: After reaching maximum strain $(\in_{d}$ ), the sample was allowed to cool up to $20 ℃$ at a rate of $5 ℃ / \min$ .

Step 3: Subsequently, the load was released to 0.001 N at the rate of 0.5 $N / \min$ to bring down the strain to a fixed strain $(\in_{f}$ ).

Step 4: After that, the shape memory recovery was performed when heated to $70 ℃$ again at a rate of $5 ℃ / \min$ and maintained for 60 $\min$ to record the end strain ( $\in_{r})$ .

Figure 2.

Schematic representation of shape memory cycle.

The shape fixed ratio ( $R_{f}$ ) as well as shape recovery ratio ( $R_{r}$ ) were outlined and determined as follows:

R_{f} (%) = \frac{\in_{f}}{\in_{d}} \times 100

(2)

R_{r} (%) = \frac{\in_{f} - \in_{r}}{\in_{f}} \times 100

(3)

Results and Discussion

Microstructural Characterization

All of the printed samples underwent Scanning electron microscopy (SEM) analysis to determine surface topology along with the dispersion of BG particles on top and cross-sectional surfaces. SEM images revealed good fusion between layers in top and cross-sectional surfaces as seen in Figure 3(a) and (b). SEM micrographs also revealed the presence of some cracks and pores due to the evaporation of solvent. The presence of BG particles could be observed at higher magnification. However, SEM micrographs did not provide any evidence of the dispersion of BG particles and the presence of agglomerates of BG in PLA matrix after printing. Therefore, an elemental mapping test (EDX) was run to ensure proper comprehension of BG dispersion inside the PLA matrix for all PLA/BG composites and elements like $S i$ , $C a$ , and $N a$ were observed to ensure the homogeneous dispersion of BG in the PLA matrix. Figure 3 (c) and (d) presents the elemental mapping of the top and cross-sectional surfaces of PBG2.0 composite. Elemental mapping revealed even dispersion of BG particles inside the polymer matrix. For each of the PLA/BG composites, a distinct color indicated the presence of a particular component of BG in the polymer matrix. The highlighted clustered regions corresponding to agglomeration were not seen in composites, confirming that BG particles were uniformly distributed in the polymer matrix. The effectiveness of reinforcements is increased when they are evenly dispersed in the matrix, resulting in strong interfacial adhesion.¹⁷

Figure 3.

SEM images showing the morphologies of (a) top surface, (b) cross-sectional surface along with the elemental mapping of (c) top and (d) cross sectional surface of PBG2.0.

Surface Morphology

The impact of BG reinforcement on the surface roughness of the composite’s top surfaces was clarified using AFM analysis, as shown in Figure 4. Numerous surface irregularities were noticed, which could be attributed to the fabrication process, material preparation, and solvent evaporation.⁴⁵

Figure 4.

AFM micrographs of 4D printed PLA/BG composites (A) PBG0 (B) PBG0.5, (C) PBG1.0, (D) PBG1.5, and (E) PBG2.0, respectively.

Table 2 represents the average surface roughness value of PLA/BG composites fabricated by SB3DP. The existence of BG reinforcement inside the polymer matrix was confirmed by the higher value of surface roughness seen in PLA/BG composites. However, the lower surface roughness for PBG2.0 was due to the improved deposition and settling down of BG particles at higher percentage of BG.⁴⁶

Table 2.

Average surface roughness value 4D printed composites.

Samples	Average surface roughness (nm)
PBG0	92.8 $\pm$ 7.4
PBG0.5	$145.3 \pm$ 7.8
PBG1.0	$165.5 \pm$ 8.4
PBG1.5	$314.7 \pm$ 18.8
PBG2.0	$302.2 \pm$ 10.9

XRD Analysis

XRD was used to confirm the incorporation of BG in PLA/BG composites with varying BG contents (0-2 $w t %$ ), as shown in Figure 5. Only peaks that corresponded to the crystalline PLA phase could be seen at 16.55 $°$ and 18.87 $°$ , respectively in the XRD diffractogram of pure PLA.⁴⁷ When the proportion of BG in the sample was raised from 0 to 2 $w t %$ , there was a significant improvement in the peak intensity that corresponded to crystalline PLA. Since BG was present at a smaller concentration, it was able to act as a nucleating agent, reducing the potential barrier to form PLA nuclei and increasing the crystallization degree of PLA.⁴⁸ Due to the little amount of BG in PBG0.5 and PBG1.0, it was very challenging to identify the peaks that correspond to BG. However, additional peaks were found at 23.77⁰, 26.77⁰, 29.39⁰, 33.61⁰, 48.71⁰ and 61⁰ in PLA/BG composites with higher BG contents (1.5 and 2 $w t %$ ), which were directly attributed to the crystalline BG phase that was present in the composites.^47,49 The above results demonstrated that the BG particles might be effectively reinforced into the PLA/BG composites without any chemical interaction between BG and PLA. This was confirmed by the fact that the XRD plots of PLA/BG composites only revealed the characteristic peaks of PLA and BG.

Figure 5.

XRD plot for BG, PLA, and PLA/BG composites.

Thermal Characterization

Differential Scanning Calorimetry (DSC)

The thermal characteristics of PLA after BG inclusion were further investigated using DSC analysis. Figure 6 displays the DSC thermograms for the heating and cooling whereas Table 3 provides a summary of the data obtained for the glass transition temperate ( $T_{g})$ , melting temperature ( $T_{m}$ ), crystallization temperature ( $T_{c}$ ), heat of fusion ( ${∆ H}_{m})$ and percentage crystallinity ( $X_{c}$ ) of PLA and PLA/BG composites.

Figure 6.

DSC thermograms for PLA and PLA/BG composites (a) first heating cycle and (b) first cooling cycle.

Table 3.

Thermal properties of PLA and PLA/BG composites.

Sample	$T_{g} (℃)$	$T_{c} (℃)$	$T_{m} (℃)$	${∆ H}_{m}$ ( $J / g$ )	$X_{c}$ (%)
PBG0	42.75	111.91	141.19	22.02	23.53
PBG0.5	43.67	112.32	141.75	25.27	27.13
PBG1.0	43.60	112.37	140.61	25.06	27.05
PBG1.5	43.92	112.28	143.48	26.54	28.94
PBG2.0	48.39	112.82	145.22	28.69	31.12

No significant changes in glass transition temperature were observed in PLA/BG composites up to 1.5 $w t %$ of BG. The inclusion of small BG content was expected to have little effect on $T_{g}$ because $T_{g}$ is widely recognized as an extremely intricate occurrence that depends on several variables such as chain flexibility, intermolecular interaction, and the material’s own molecular weight.^50,51 In case of PBG2.0, $T_{g}$ increased to 48.39 $℃$ , which was probably due to restricted PLA chain mobility by a high concentration of BG (2 $w t %$ ).⁵² The BG incorporation in the PLA matrix had minor effects on the crystallization temperature ( $℃)$ , which remained almost equal to 112 $℃$ for all composites. The melting temperature ( $T_{m}$ ) of all the composites ranged from 141 $℃$ to 145 $℃$ , close to PLA.

Furthermore, the % crystallinity ( $X_{c}$ ) of PLA increased for PLA/BG composites when compared to neat PLA. Ceramic filler could act as a nucleation site and promoted crystals formation.⁵³ This increased crystallinity demonstrated the effective nucleation effect of BG.⁴²

Thermo-Gravimetric Analysis

The thermal characterization of raw material as well as its composites is a very important step. This is because the way a polymer behaves when heated can suggest a processing window that stops it from breaking down when heated. Figure 7 shows weight loss curve, and Table 4 lists thermal degradation properties for PLA and PLA/BG composites.

Figure 7.

TGA and DTGA curve for PLA and PLA/BG composites.

Table 4.

Thermal degradation properties of PLA and PLA/BG composites.

Sample	$T_{1 %}$ $(℃)$	$T_{O}$ $(℃)$	$T_{D}$ $(℃)$	Residue (%) at T ≥480 $℃$
PBG0	97.13	302.3	391.4	.004
PBG0.5	95.98	260.4	373.5	1.85
PBG1.0	94.25	244.5	354.3	2.79
PBG1.5	92.92	243.2	351.6	2.53
PBG2.0	90.01	238.6	348.8	4.32

As shown by a single peak, the neat PLA and PLA/BG composites showed a similar one-step degradation pattern. As seen in Figure 7, all samples experienced an initial loss in weight at a temperature of around 100 $℃$ . The loss of mass at 100 $℃$ was caused by the evaporation of water and other volatiles contained within the material.⁵⁴ Finally, the thermal analysis curves started slowing down around 250–400 $℃$ to finish the decomposition of the PLA matrix until a constant mass was achieved. The constant mass left at the completion of each TGA experiment represented the inorganic BG particles. The presence of BG led to a higher residue content in PLA/BG composites.

When comparing the degradation properties of the composites to those of neat PLA, it was obsereved that the composite had a lower degradation onset temperature ( $T_{O}$ ) and degradation temperature ( $T_{D}$ ) than neat PLA. Therefore, the reinforcement of BG particles reduced the PLA/BG composites’s thermal stability. This behavior could be attributed to the formation of carboxylate salt by-products as a result of a reaction between both BG and PLA at higher temperatures. Surface hydroxyl groups on silica-based glasses like BG can dissociate $S i - O - S i$ bonds to ${S i O}^{-}$ and associate with ${N a}^{+}$ or ${C a}^{2 +}$ counterions in moist conditions, potentially catalysing polymer chain scission.^55,56

Dynamic Mechanical Properties

Considering the shape memory capabilities of SMPCs are heavily dependent on the polymer’s low temperature modulus and

T_{g}

, DMA study was performed.¹¹ Table 5 provides a comprehensive breakdown of the results, while Figure 8 displays storage modulus (

E^{'}

) and damping parameter (

\tan δ

) values versus temperature.

Table 5.

Dynamic mechanical properties of PLA and PLA/BG composites.

Sample	$T_{g}$ ( $℃)$	Glassy storage modulus (MPa)	Rubbery modulus (MPa)	Factor C
PBG0	40.3	1570.6 $\pm$ 21.7	202.6 $\pm$ 9.3	1
PBG0.5	47.4	1643.6 $\pm$ 41.6	296.6 $\pm$ 12.9	0.7
PBG1.0	47.8	1673.5 $\pm$ 15.5	238.4 $\pm$ 7.8	0.8
PBG1.5	48.2	1846.8 $\pm$ 15.1	321.8 $\pm$ 1.2	0.7
PBG2.0	49.9	1880.9 $\pm$ 13.8	389.6 $\pm$ 9.1	0.6

Figure 8.

Temperature dependence of (a) storage modulus $E^{'}$ and (b) damping parameter $\tan δ$ for PLA and PLA/BG composites.

When all the samples reached $T_{g}$ , both $E^{'}$ and $\tan δ$ dropped sharply. When compared to PLA, the composites had a higher storage modulus in both the glassy and rubbery regions. Higher values of modulus showed that BG particles served as a reinforcing phase in the composites.⁵⁷ Moreover, $E^{'}$ values of PLA/BG composites were plateaued at the glassy and rubbery region temperatures, highly desirable for SMPCs. Additionally, high rubbery modulus would result in a large shape recovery upon heating, while a high glassy modulus would result in a large shape fixity on cooling.^58,59

Table 5 also displays the filler efficiency (factor ) that reveals the composite reinforcing capacity. A factor $C$ value of 1 is the norm for the unfilled polymer matrix film and factor $C$ below 1 indicates the filler’s mechanical stiffening ability within the polymer matrix.^60,61 As the BG content in the composites increased, the factor $C$ values decreased, demonstrating the reinforcement of BG within the PLA matrix. PBG2.0 showed the smallest gap among $E^{'}$ values in the glassy as well as rubbery states with the lowest $C$ value (the highest effectiveness).

The values of glass transition temperature ( $T_{g}$ ) can also be characterized as the peaks of the $\tan δ$ curves, as illustrated in Figure 8(b). The $T_{g}$ of PLA/BG composites measured by DMA exhibited the same pattern as $T_{g}$ measured by DSC. The incorporation of BG particles caused a minor increase in the $T_{g}$ value of the composites as listed in Table 5. Simultaneously, a wider peak of $\tan δ$ plot (wider transition temperature) was also observed in PLA/BG composites. This indicated towards the reduction of the polymer chains mobility because of the confinement effect caused by the uniformly dispersed BG particles. The shape memory performance of SMPCs was significantly enhanced as a direct result of all of the aforementioned features.⁶²

Shape Memory Characterization

Shape memory properties like shape fixity and shape recovery ratio were calculated using the technique described in the experimental section, and their respective values are presented in the Figure 9. The result showed all samples recovered to the permanent shape at temperature above $T_{g}$ but at different shape fixity and recoverability. From 65% and 60% in neat PLA, the shape fixity and recovery values dramatically increased to 89% and 92% in PLA/BG composites. It is essential for SMPs to have both a shape fixing (netpoints) and a shape switching (reversible) part for the shape memory properties. In case of semicrystalline PLA, crystalline phase served as the fixed netpoints, while long polymer chains, played the role of the reversible soft phase.⁶ Permanent netpoints made a three-dimensional network structure that stored and remembered the permanent shape. The polymer chains worked as switching segments to fix or release the temporary shape through a change in molecular mobility caused by the formation and dissolution of reversible interactions. When the sample was deformed into the temporary shape, deformation resulted in molecular chain configuration change and displacement of netpoints followed by fixing of the temporary shape due to the entropy entrapment at the temperature below $T_{g}$ . Upon reheating above $T_{g}$ , the molecular chains transformed to a higher entropy state and the shape recovery occurred because of the release of entropic energy that had been stored in the temporary fixed shape. Thus, the entropy of the reversible soft phase and the modulus of the hard phase governed the shape memory qualities.⁶

Figure 9.

Shape-fixity ratio ( $R_{f}$ ) and Shape-recovery ratio ( $R_{r}$ ) of compositions with different wt% of BG.

In case of PLA/BG composites, reinforced micro-BG particles resulted in the increased crystallinity which served as extra net point regions, allowing the materials to fix the temporary shape and recover to their original shape with more precision. However, the incorporation of BG also resulted in increased rubbery modulus in PLA/BG composites (Table 5). This increased modulus fascinated for more entropy storage in the temporary shape leading to improved shape recovery. The results showed that the composites based on BG had a good shape memory properties with higher shape fixity and recovery ratio.^11,63

Figure 10(a) illustrates the shape memory behavior of 4D printed self-foldable box printed with PBG2.0 composite. The flat specimen was first immersed in hot water at 70 $℃$ and converted into a box (temporary shape B) followed by an immersion in cold water at 20 $℃$ to fix the temporary shape. Further, the specimen with shape B returned to permanent flat shape A within 15 seconds after being placed back into the hot water at 70 $℃$ . Figure 10(b) presents the quantitative evaluation of the shape memory thermomechanical cycle for PBG2.0 composite as mentioned in section 3.6.

Figure 10.

(a) Shape memory behavior of 4D printed self-foldable box of PBG2.0 composite in water bath and (b) quantitative evaluation of the shape memory cycle for PBG2.0 composite obtained from shape memory thermomechanical cycle.

Comparison with Prior Studies

Table 6 compares the results of the present study with a few exciting studies on various manufacturing techniques utilized with different materials. To the best of the author’s knowledge, no attempts have been made to examine the thermal and shape memory properties of PLA printed by the SB3DP technique. Furthermore, no previous study has explained the effect of BG reinforcement on the thermal and shape memory properties of PLA.

Table 6.

Comparison based on different studies on tools, techniques, methods, and applications of shape memory polymers and their composites.

Technique	Polymer matrix	Reinforced material	External/internal stimulation used	Major findings	Purposed application	Reference
Extrusion compounding with injection molding	Thermoplastic polyurethane	Magnetite (Fe₃O₄)	Temperature 280K to 380 K	The addition of Fe₃O₄ as a filler material increased the thermal diffusivity and conductivity of SMP. Shape memory properties were not evaluated	Not proposed	⁶⁴
3D printing fused deposition modeling	Polylactic acid	Bioactive glass 45S5	Not given	Mechanical and biological properties were improved by the incorporation of reinforced material. Shape memory properties were not quantified	Bone tissue scaffolding	⁶⁵
3D printing Fused deposition modeling	Polylactic acid	Zinc oxide ZnO	Water stimulus 70 $℃$	Thermal, tensile, and shape memory properties were reported	Not proposed	⁶⁶
Melt extrusion	Polyurethane	Cellulose nanocrystals (CNC)	Temperature gradient 50 $℃$ –70 $℃$	The filler material modified flexural and shape recovery properties	Biomedical applications	⁶⁷
Compression molding	Polyether ester	Carbon black CB	Electrical stimulus 70 $℃$	The addition of carbon black resulted in improved properties like mechanical, and electrical resistivity and electrically stimulated shape memory characteristics with 80% of shape recovery	Multilayer composites	⁶⁸
Casting	Poly (urethane urea)/polythiophene	Carbon black (GCB)	Electrical stimulus voltage- 40V	The study demonstrated the effects of GCB on the structure, morphology, mechanical properties, electrical conductivity, and voltage-triggered shape memory effect, and a recovery ratio of 94% was achieved at 40V	Not proposed	⁶⁹
3D printing fused deposition modeling	Blend consisting of 30, 50, and 20 wt% of SEBS, PEW, LDPE		Temperature 0 $℃$ –150 $℃$	In comparison to single constituents, the blend of SEBS-PEW-LDPE exhibits the highest degree of shape recovery	Modifiable electronics and electrical actuators	⁷⁰
3D printing fused deposition modeling	Polylactic acid	Magnetite (Fe₃O₄)	Magnetic field	Shape recovery ratio of 95% was achieved in 15s	Porous bone tissue scaffold smart tracheal scaffold	^38,39
3D printing fused deposition modeling	Polylactic acid	Hydroxyapatite (HA)	Direct heating 65 $℃$ –105 $℃$	PLA/15%HA showed shape recovery of 98%	Porous scaffold	⁴⁰
3D printing fused deposition modeling	Polylactic acid	Hydroxyapatite (HA)	Temperature $60 ℃$ –70 $℃$	The shape memory effect of the 3D printed scaffolds was around 95.77%, and they also had excellent mechanical, in vitro biocompatibility, and hydrophilic morphology	Biomedical devices	⁷¹
3D printing stereolithography	Mixture of different photopolymers i.e. 30 wt% of polyurethane acrylate, 20 wt% of epoxy acrylate, 50 wt% isobornyl acrylate and radical photoinitiator		Water immersion 85 $℃$	The observed shape fixity and recovery ratios were in the order of 96.77%±0.06% and 100.00%±0.08%, respectively	Not proposed	⁷²
3D printing direct ink writing	Polydimethylsiloxane resin	Nano silica	Temperature 70 $℃$	When heated close to and over $T_{g}$ , the lower $T_{g}$ microsphere structures showed significant compression set with full recovery. The higher $T_{g}$ microsphere structure showed no recovery	Self-bending structures	⁷³
Laminated composites	Thermosetting cyanate-based shape memory polymer and epoxy-based shape memory polymer	Toray T300-3 K plain-weave fabric with a fiber lay-up direction of±45°	Electrical heating	Development, testing, and successful application of the shape memory polymer composite based flexible deployable subsystem (SMPC-FDS)	Shape memory polymer composite based flexible deployable subsystem for Tianwen-1	⁷⁴
3D printing fused deposition modeling	Ionomeric thermoplastic	Zinc-neutralized polyethylene-co-methacrylic acid (PEMA)	Dynamic mechanical loading $20 ℃$ - 70 $℃$	Recovery ratio of 99% was achieved	Foldable structures	⁷⁵
3D printing photocuring 3D printer (anycubic photon)	Photosensitive resin		Temperature $20 ℃$ –100 $℃$	Tension- twist coupling behavior of chiral metamaterials and large deformation ability and shape memory property of the metamaterials was observed. Shape recovery ratio of 93% was achieved	Three- dimensional (3D) chiral and anti-chiral metamaterials based on face-rotating polyhedral (FRP) have been developed	⁷⁶
Solvent based 3D printing	Polylactic acid	Bioactive glass 45S5	Thermal stimulus $20 ℃$ - 70 $℃$	The effect of bioactive glass particles on morphology, thermal and shape memory properties were thoroughly explained. Highest values of shape fixity (89%) and (92%) shape recovery ratio was obtained for 2 wt% of reinforced particles	Foldable structures and smart biomedical devices	Present study

Conclusion

Polymer matrix (PLA) and reinforcing particles (BG) were used to successfully generate solvent-based 4D printed composites with outstanding shape memory characteristics in the present study. The initial steps of the composite’s characterization looked at the viability of BG particle inclusion in 4D printed PLA. Elemental mapping of the composites revealed uniform dispersion of BG particles in the PLA matrix. XRD data also confirmed the reinforcement of BG in the PLA due to the presence of characteristics peaks of BG in the XRD plots of PLA/BG composites. However, the incorporation of BG resulted in the increased surface roughness in PLA/BG composites. Additionally, the impact of BG reinforcement on thermal and shape memory characteristics was also studied. The addition of BG enhanced crystallinity by a small percentage but lowered thermal stability. Increases in BG content had a direct effect on the $T_{g}$ which in turn impacted the viscoelastic characteristics and shape-memory properties of composites. Increased levels of glass and elastic modulus were found in PLA/BG composites, leading to significant enhancements in shape memory properties. The results of this research help to enhance the shape memory performance of PLA/BG composites, which can then be used in conjunction with 3D and 4D printing to create complex shapes for uses like bio implants for soft and hard tissue engineering with the potential for use in minimally invasive surgery.

Footnotes

Acknowledgements

The authors are thankful to the central research facility (CRF) of Indian Institute of Technology Delhi, for providing research facilities for this study.

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) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Pulak Mohan Pandey

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