A Review of the Current State of Research and Future Prospectives on Stimulus-Responsive Shape Memory Polymer Composite and Its Blends

Abstract

Shape-memory polymers (SMPs) possess unique properties that respond to external stimuli. The current review discusses types of SMPs, fabrication methods, and the characterization of their mechanical, thermal, and shape recovery properties. Research suggests that SMP composites, when infused with fillers, demonstrate enhanced mechanical and thermal characteristics. On the other hand, blends, particularly incorporating polylactic acid (PLA), exhibit the most efficient shape recovery. Furthermore, the crosslinking density in polymer blends impacts the shape recovery force, showcasing a correlation between energy storage capacity and shape recovery force in SMP networks. Overall, SMP blends show promising mechanical, thermal, and shape recovery features, rendering them advantageous for applications of artificial muscles, soft actuators, and biomedical devices. This review also discusses the future prospectives of SMP for robust applications.

1. Introduction

Polymers are made up of long chains of repeating monomer units [1] which can be either naturally occurring or synthetic. Common natural polymers include hemp, shellac, wool, silk, cellulose, etc. [2]; common synthetics include polyethylene, polystyrene, polyvinyl chloride, Teflon, epoxy, nylon, etc. [3,4,5]. Virgin polymers have limited application due its poor mechanical and thermal properties [6], which can be fulfilled by reinforcing suitable nanofillers in virgin polymer matrices known as polymer composites [7,8]. Shape memory polymers (SMPs) are a type of stimuli-responsive polymer that can return to their original shape when triggered by an external stimulus such as temperature change, moisture, magnetic field exposure, or electricity supply [9,10,11].

Thermo-responsive SMPs are the most studied among shape memory polymers. Figure 1a illustrates that at lower temperatures, SMPs have a stable, relaxed configuration due to the hard segments restricting the movement of the molecular chains. When heated above their glass transition temperature (T_g), the polymers become viscoelastic as the soft segments gain mobility, allowing the molecular chains to reorient and form new interactions. This transition facilitates the shape memory effect by enabling the polymer to deform and return to its original shape upon cooling temporarily. Figure 1b explains the combination of metal and polymer properties in SMPs, creating a network with gradient plasticity by controlling metal ion diffusion. Figure 1c shows 4D printing advancements, enabling precise control over SMP fabrication. Figure 1d depicts how light-responsive SMPs can be triggered by UV light, making them ideal for applications such as wearable electronics and health monitoring devices. SMPs have been used in various fields, such as sensors, actuators, and remote sensing devices, due to their superior properties, flexibility, recoverability, light weight, ease of fabrication, and environmental friendliness [12]. Although SMPs have the properties discussed above, they face challenges, such as limited recovery force, slow response time, and poor mechanical properties, which hinder their broader applicability. Therefore, nanoscale fillers can be combined into them to further improve SMP performance, making what is commonly known as shape memory polymer nanocomposites (SMPNCs). These nanocomposites could improve such materials’ mechanical properties, thermal stability, and shape recovery capability [13,14]. Thermoplastic SMPs exhibit characteristics that facilitate the ease of processing and possess the capability to regain their initial form when subjected to heat. Conversely, thermoset SMPs feature a crosslink network structure that enables them to recover their shape more effectively than their thermoplastic counterparts. However, the virgin SMP’s drawback is its low mechanical strength and high cost. To address these issues, polymer blends are created. Blends are made with two or more dissimilar polymers combined to produce new materials with superior mechanical properties than those of individual polymers. Polymer blends can offer a cost-efficient solution by employing the mixing process of two or more polymers, where one would be high in cost and the other may be low in cost, without compromising on desired functionality. Polymers such as polyethylene (PE) [15,16], polypropylene (PP) [17,18], polystyrene (PS) [19], polyvinyl chloride (PVC) [20], polylactic acid (PLA) [21], and polyethylene terephthalate (PET) [22] are among the most utilized materials as polymer blend materials. Polymer blends may be extended in the SMP category where they can provide a better shape memory effect and provide better mechanical strength. Developing SMP blends and composites has led to a new generation of high-performance polymeric materials with distinctive properties. Blends and composites of SMP allow their properties to be improved and their applications to be widened. For example, the mechanical characteristics and flexibility of SMPs can be improved by introducing elastomers. However, selecting the most appropriate blend material in SMPs for the intended application is crucial. Figure 2a shows the list of documents published in the Scopus database from 1943 to 2023 on SMPs. It was observed that there was an exponential rise in publications, and 861 documents are available on SMPs. As shown in Figure 2b, the highest number of publications on SMPs is from China, followed by the U.S., Germany, India, Japan, etc. Figure 2c shows the type of article published in the Scopus database on SMP material. It was noted that around 69% of publications on SMP material are from journals, followed by conference papers, review articles, book chapters, etc. It was observed that research on SMPs has been rapidly expanding and is attracting significant attention among researchers. The novelty of this study is to provide a thorough review of the current state of research in SMP composites and its future prospectives. Figure 3 shows the overall structure of the current review study in sequential order.

2. Classification of SMPs

SMP categorization is significant since it offers structure to the knowledge of various types and classifications of these materials and helps to select a particular SMP based on net point, composition and structure, shape memory function, and types of stimulus activation. The broad classification is shown in Figure 4.

2.1. Based on Net Points

2.1.1. Physically Crosslinked SMPs

Physically crosslinked (PCL) polymers rely on segregated domains within the polymer structure to achieve shape memory properties as shown in Figure 5a. These materials can be remolded after the physical bonds are broken, allowing for shape changes without altering the permanent shape [24].

2.1.2. Covalently Crosslinked/Chemically Crosslinked SMPs

Covalently crosslinked (CCL) SMPs, as shown in Figure 5b, are created through specific crosslinking reactions, resulting in thermoset materials. They have permanent shapes set by the crosslinks and exhibit shape memory behavior due to reversible changes in the switching segments [25].

Figure 5. (a) Physically and (b) covalently crosslinked [26].

Figure 5. (a) Physically and (b) covalently crosslinked [26].

2.2. Based on Composition and Structure

SMPs can be classified as either thermoplastic or thermoset based on their chemical structure. Thermoplastic SMPs have physically crosslinked networks, while thermoset SMPs have covalent crosslinking points. There are four classifications of SMPs based on the polymer structure and the nature of their net points, namely chemically crosslinked amorphous SMPs (class I) and (class II), physically crosslinked with amorphous SMPs (class III), and physically crosslinked with semi-crystalline SMPs (class IV). The essential characteristics of these SMPs are outlined in

Table 1. Structure-based categorization of shape-memory polymers [27].

Type	Materials	Observation	Ref.
Class I	Thermosetting PU, Styrene copolymers, Epoxy, PET-PEG copolymer, Methacrylate, PMMA-PBMA copolymers, Polynorbornen	Tg is the temperature at which a shape transition occurs, whereas vitrification fixes the secondary form.	[28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]
Class II	Acrylates, PCL-BA copolymer, PE, Poly (Propylene sebacate), PE/PP blends, Poly(ε-caprolactone), Polycyclooctene	The shape transition temperature is Tm. While the secondary shape is fixed by crystallization. The permanent shape is fixed by chemical crosslinking. Here, quick shape restoration is possible.	[45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]
Class III	PCL-b-ODX, Styrene block copolymer, PVDF/PMMA blend, PE-co-nylon 6, Polylactide-based systems oligo(ε-caprolactone), PE-co-PMCP Copolymer, PET-co-PEO, POSS telechelic, POSS-PN block	The permanent form is fixed by rigid amorphous domains, crystals, hydrogen bonds, or ionic clusters acting as physical crosslinks, but the secondary shape is fixed by soft segments with lower Tg or Tm upon cooling.	[63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80]
Class IV	Polyurethane copolymers, Copolyesters, Styrene–trans-butadiene–styrene, TBCP, PCL-based systems	The temperature of the shape transition is Tg or Tm. The permanent form is fixed by physical crosslinks (polar contact, hydrogen bonding, or crystallization with such crosslinks), whereas the secondary shape is determined by the crystallization of soft segments.	[81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97]

[27].

The molecular structure of (SMPs) consists of two main parts: switching segments and crosslinks with chemical or physical nodes (hard segments). The switching segments undergo reversible shape changes, while the memorization of the original shape is supported by crosslinked elements. It is further categorized as follows.

2.2.1. Segmented Block Copolymers

These are polymers composed of two or more chemically distinct segments. In the context of shape memory polymers (SMPs), segmented block copolymers play a crucial role in achieving shape memory properties by combining different types of polymer segments with varying properties [98].

2.2.2. Crosslinked Homopolymers

Materials made of crosslinked polymers are particularly appealing because of their numerous intriguing features. Homopolymers are polymers composed of a single type of monomeric unit. The homopolymers, when crosslinked, form a network structure that augments the mechanical strength and shape memory characteristics [99].

2.2.3. Polymer IPN/Semi-iPN

These are complex polymer structures where two or more polymers are intertwined at a molecular level. These complex structures provide improved mechanical properties and enhanced shape memory behavior [100].

2.2.4. Supramolecular Polymer Networks

These are polymers held together by non-covalent bonding, i.e., π-π stacking. The specialty here is that it offers dynamic properties that can be further exploited in SMP’s shape memory effects [101].

2.2.5. Hydrogels

Hydrogel-based SMPs are 3D networks of hydrophilic polymers, which are capable of absorbing and retaining a large amount of water. In the context of SMPs, hydrogels can be used to create stimuli-responsive materials with applications in biomedicine, drug delivery, and tissue engineering [102].

2.2.6. Polymer Composites

These composites involve incorporating fillers or reinforcements into the polymer matrix to improve specific properties. In SMP composites, the addition of fillers like carbon nanotubes or nanoparticles can enhance mechanical strength, thermal stability, and shape memory performance [103,104].

2.2.7. Polymer Blends

To obtain the desired qualities, two or more different polymers are combined with SMPs, which is known as a polymer blend. Blending can improve properties such as form memory, hardness, and mechanical strength [96]. Blends of polymers come in many varieties and possess unique characteristics. Blends can be categorized into two categories depending on the number of phases, such as miscible blends and immiscible blends, as shown in Figure 6 [104]. For instance, Polymer A blends typically feature phase domains in the range of 100 nm, exhibiting finely dispersed morphologies. Polymer B blends often have phase domains around 1 μm, presenting more pronounced phase separation. In contrast, Polymer C blends exhibit phase domains of up to 100 μm, resulting in significantly larger and distinct phase structures. These varying phase sizes influence the polymer blends’ mechanical, thermal, and optical properties, making them suitable for different applications.

Miscible Blends

Miscible blends have a single glass transition temperature (T_g) that falls between the T_g of the two components of the blend and have a homogenous morphology [104].

Immiscible Blends

Because of the extremely low entropy of mixing, most polymer blends are immiscible by nature. Since polymers are highly disordered, adding another polymer does not significantly alter the entropy of the mixture. Therefore, the mixing enthalpy must be negative for a polymer to mix spontaneously. Immiscible blends have a diverse morphology. When two immiscible polymers are combined, the resulting mixture exhibits two glass transition (T_g) and melting (T_m) temperatures, one for each polymeric component. The abrupt interphase in immiscible blends suggests no significant interactions between the two polymer components. Immiscible mixtures such as van der Waals attraction and hydrogen bonding often exhibit weak interactions. However, most valuable goods consist of immiscible mixes [104].

2.3. Based on Shape Memory Effect (SME)

SMPs are responsive materials that deform and regain their original shape under external stimuli such as heat, light, magnetic fields, and other means. This ability of these materials to memorize the original shape is known as the shape memory effect. Figure 7 illustrates three kinds of SMEs: one-way SME, two-way SME, and multi-way SME [105].

2.3.1. One-Way SME

The most common form of shape memory behavior is expressed by the one-way SME shown in Figure 7A. This technique allows the polymer to be reshaped temporarily and then return to its original geometry in response to a stimulus. The process is unidirectional, meaning the shape change only occurs once during each application cycle and stimulus removal [105]. One-way shape memory polymers are commonly used in various applications due to their simplicity and effectiveness. Dual- or triple-responsive SME occurs when different fillers in the polymer matrix respond to various stimuli, as depicted in Figure 7, showing the dual- and triple-SME behavior process.

2.3.2. Two-Way SME

The two-way SME shown in Figure 7B allows the polymer to switch from one configuration to another only by introducing and removing the stimulus. This bidirectional function is realized by programming the polymer to memorize two different shapes and hence, it cycles in response to the stimulus [105]. The two-way shape memory effect (SME) is shown in Figure 7. They can be described as reversible stimuli-responsive materials that return to their original form after being deformed and then cooled below a certain temperature. The shape memory material can remember two different shapes: the original and temporary shapes. The material can switch between these two shapes reversibly upon exposure to external stimuli, offering versatility in shape manipulation and reconfiguration.

2.3.3. Multi-Way SME

The multi-way SME shown in Figure 7C exhibits more complex behavior: the polymer changes shape in response to multiple stimuli or a sequence of stimuli. Applications of this type of SME are especially useful where complicated changes in shape and many functional states are necessary [105]. The materials exhibiting multi-shape memory can remember and recover multiple shapes sequentially, as shown in Figure 7 [105]. This property allows for complex shape changes and programmable responses in applications requiring multiple shape transformations.

Accordingly, one-way, two-way, and multi-way SMEs have a number of advantages and apply to a number of different domains. One-way, two-way, and multi-way SMEs each have distinct benefits and are appropriate for diverse applications. Continued research in this area will result in the creation of new SMPs with improved characteristics and more application possibilities.

2.4. Based on Stimulus for Activation

SMPs can be further classified depending on the type of external stimulus they respond to. This includes electroactive SMPs, ferromagnetic SMPs, and thermo-responsive SMPs, which can revert to their permanent shape based on a temperature greater than their polymer transition temperature [106,107,108,109,110,111]. The external stimuli shown in Figure 8 [112] can be applied to the SMPs, including heat, light, magnetic fields, electric and chemical (pH) changes, humidity, etc., and they play a crucial role in controlling the SME in SMPs and have applications in various fields, including medicine, aerospace, and textiles [113,114].

The SME in SMPs is typically realized based on a reversible type of thermal reaction. Thermo-initiated shape memory polymers are widely studied, which respond to heating above the glass transition temperature and activate the molecular mobility in SMPs, allowing them to recover their original shape when stimuli are removed when cooled below T_g. Indirect heating methods, such as using light, electric, or magnetic energies, have been explored as alternate methods for developing SMPs. Photo-responsive SMPs can undergo photo-reversible cycloaddition reactions when exposed to light, leading to shape recovery. Conductive materials may act as ideal heating sources for activating the SME in SMPs.

3. Fabrication Methods

The fabrication procedures significantly impact the uniformity, shape, and ultimate qualities of blends and composites. There are three main fabrication methods for blends and composites, i.e., melt blending, solution blending, and in situ polymerization [115,116,117,118,119,120,121,122,123]. The following are some of the widely used methods for the fabrication of SMP composites and blends.

3.1. Melt Blending

Melt blending is a method used to fabricate SMPs. Figure 9 shows the detailed steps of the melt blending process. As indicated in Figure 9, plastic pellets are first fed into a dehumidification device. The first step is the removal of moisture content from the pellets so that the processing conditions are improved. Dehumidified pellets are then fed into a twin-screw extrusion mechanism. They are mixed with clay minerals (nanofillers) at this point. The twin-screw extruder ensures proper blending and dispersion of the nanofillers within the polymer matrix. Afterward, there is a cooling stage combined with the pelletization of the material that has just undergone the extrusion process. This phase involves cooling the obtained extruded nanocomposite material, followed by its fragmentation into smaller pellets or granules. Such pellets exhibit enhanced manageability and are, therefore, ready for further processing [124,125,126,127].

This is the most adopted method, resulting in an intimate mix of polymers that do not share covalent connections and are commercially available. The properties of the resulting materials are aligned to meet the application requirements and also the materials cost. Melt blending offers practicality and it has been attempted over the years to improve its output. It allows for the dispersion of fillers throughout the material, although it may not achieve the same level of dispersion as solution casting.

3.2. Solution Mixing

In the solution mixing method for SMP composites and blends, the solvent acts as a medium to facilitate the blending process [125,126,127]. The detailed process is shown in Figure 10 [128]. After thorough mixing, the solvent is evaporated, leaving behind a homogeneous blend or composite material. This method is particularly useful for blending polymers that are not compatible with each other, as the solvent helps to overcome the immiscibility barrier and achieve a uniform mixture. Solution blending offers several advantages, including the ability to control the composition and morphology of the blend or composite, as well as the potential for incorporating additives or reinforcements. The fundamental limitation of this technology is that it only works with polymers that are soluble in all solvents. Solvent recovery is an issue with the widespread use of this solution mixing method [127,128].

3.3. In Situ Polymerization

The in situ polymerization process for nanocomposite fabrication is shown in Figure 11 [128]. Here, the process starts with adding nanofiller into the monomer solution, followed by polymerization. The evaporation of solvent from the polymerization solution produces polymer nanocomposite. This method permits the creation of SMP blends and composites with improved compatibility. It offers advantages such as (i) better control over the polymerization process and (ii) the potential for incorporating functional groups or additives during the polymerization process [129,130].

4. Characterization Techniques

4.1. Mechanical Characterization

The addition of fillers and the blending technique improves mechanical properties. However, selecting a suitable filler and blending material is the key challenge. This section briefly discusses important articles on the mechanical characterization of SMP composites and blends. Lin et al. [131] successfully evaluated the mechanical properties of the polypropylene/thermoplastic polyurethane (PP/TTPU) blends and stated that the study successfully combined the high strength of PP with the elasticity of TTPU, achieving a synergistic effect and fulfilling the recycling objective. Increasing TTPU content results in more cracked layers and cracks on the fractured surface. Thermoplastic polyurethane (TTPU) enhances the impact performance of PP at low temperatures due to its elasticity, which is confirmed by Figure 12. TTPU particles in PP/MA/TTPU blends act as stress concentrators, absorbing impact energy through deformation and crack formation. This rapid energy dissipation prevents immediate damage to the blends. It was concluded that the impact test results, shown in Figure 13, demonstrate that incorporating 20 wt % of TTPU (5 wt % of compatibilizer) improved the interphase issue, resulting in an impact strength (IS) of 63 J/g.

Figure 14A–E, illustrate tensile strength and Young’s modulus at the variation of MA (0 to 5 wt %), and it was observed that increasing the PP content enhances the tensile strength and Young’s modulus of blends. This is owed to the mechanical properties of the blends being influenced by the adhesion and compatibility of the two-phase materials. The addition of MA improves the tensile strength of the blends made of incompatible PP and TTPU, compensating for the high cost of TTPU. Bianchi et al. [132] analyzed the mechanical properties of SMP blends and stated that the phase inversion results in significant toughening effects, enhancing the blend’s ductility. The limited interfacial adhesion between PLA and PBAT also explains the considerable drop in stiffness and strength with limited PBAT contents, as the blend morphology changes with larger and irregularly shaped PBAT domains appearing, reducing blend stiffness and strength. Tyagi et al. [133] prepared the shape memory blend of polycarbonate (PC) and thermoplastic polyurethane (TPU) and examined the mechanical properties of PC blended with three types of TPUs. It was observed that when the TPU content in the blend was higher than 40%, TPU particles started agglomerating into bigger particles in the PC matrix. Due to this agglomeration, the mechanical properties of the blend exhibited deterioration. Lei Zhu et al. [134] evaluated the mechanical properties of PCL/POE blends in various proportions and stated that tensile yield stress and modulus decreased as POE content increased, but all blends maintained excellent toughness. Pure PCL exhibited the highest tensile yield stress and modulus among all samples, while PCL-based polymers, except PCL7POE3, displayed excellent resilience with tensile strain at break exceeding 400. The research also highlighted that the compatibility of the blends improved with increasing POE content, forming a co-continuous structure at 60 wt % POE, which positively impacted the mechanical behavior of the blends. Tekay et al. [135] determined the mechanical properties of EVA/BMA-co-iBMA blends and stated that lower tensile strength compared to compression-molded samples was due to inter-layer voids in the 3D printed parts. SEM analysis was used to examine the phase distributions of EVA/BMA-co-iBMA blends, as shown in Figure 15.

Table 2. Reported research on mechanical properties of various SMPs and their blends.

SMP	Blend	Reinforcement	Observations	Ref.
Polyurethane	-	Titaniumdiox de(TiO2) (0–5 wt %)	The tensile test data revealed that at 3 wt % TiO2/SMPU composite exhibited the highest tensile strength and largest elongation at break.	[136]
Polyurethane	-	Multiwalled carbon nanotubes (MWCNTs) (0–1.0 wt %)	For 1.0 wt % of MWCNT modulus of elasticity: increased 25%, ultimate tensile strength: increased 21%, elongation at break: 11%	[137]
Polyurethane	-	MWCNT and HNT (0–1 wt %).	At 0.1 wt % PU/MWCNT had 23.5 MPa tensile strength and 7.23% elongation, while 0.1 wt % PU/HNTs had 22.7 MPa tensile strength and 362% elongation at 1 wt %.	[138]
Copolyester thermoplastic elastomer (COPE): 50, 60, 70, 80, and 90 wt %	polycaprolactone (PCL): 10, 20, 30, 40 and 50 wt %	-	Elastic modulus was highest for COPE60 blend (49.78 MPa) and COPE90 blend (13.04 MPa), while neat COPE elastomer had 15.51 MPa.	[139]
Thermoplastic polyurethane (TPU)	polycaprolactone (PCL)	hydroxyapatite (HA) (5, 10, and 20 wt %)	Pure TPU exhibited an average modulus of 48.4 ± 0.8 MPa, while the blend 75TPU/25PCL showed 57.4 ± 0.7 MPa, and pure PCL had a modulus of 92.1 ± 4.4 MPa.	[140]
Polycaprolactone (PCL)	Polystyrene-block-Polybutadiene block-Polystyrene (SBS)	Carbon nanofibers (CNF)	As, the PCL content increases, elongation decreases.	[141]
Polylactic acid (PLA)	poly(butyleneadipate terephthalate) (PBAT)	-	Stiffness and strength decreased with increasing PBAT content.	[142]
Polylactic acid (PLA)	poly(ether ether ketone) (PEEK)	-	Tensile strength: PEEK 10%: 20.6 MPa, PEEK15: 18.9 MPa, PEEK5: 18.6 MPa, PEEK20: 16.1 MPa, Pure PLA: 15.3 MPa.	[143]

shows the reported research on the mechanical properties of various SMPs and their blends.

4.2. Thermal Characterization

SMP composites and their blends exhibit diverse thermal properties based on their composition and processing. This section presents a brief overview of the thermal characterization of SMP composites and their blends. The thermal properties of PLA/PCL blends at different concentrations of mCs were carried out by Minzimo et al. [144]. It was observed that the blend mCS as a compatibilizer acts as a toughening agent. Figure 16a–d shows different thermal properties and crystalline behavior of the PLA/PCL blends modified with mCS. The results suggested that the mCS addition causes a decrease in thermal decomposition temperature and there is an influence of mCS on thermal stability. The heating curves in the DSC test reveal that there is a change in the enthalpy of fusion (H_f) and crystallization temperature (T_c) of the blend sample with varying mCS content. At first, the H_f and T_c decreased with the addition of mCS, owing to the reduction in crystallinity. Pekdemir et al. [145] stated that Fe₃O₄ nanoparticles increased the thermal stability of the PLA/PEG nanocomposite films. The addition of Fe₃O₄ nanoparticles to the PLA/PCL blend showed enhanced thermal properties and MNP addition improved further.

Table 3. Thermal behavior of the various SMPs and their blends.

Smp	Blend	Reinforcement	Observation	Ref.
Polyurethane	PCL	PU/Graphene (1–3 wt %)	Higher graphene concentration slows crystal formation, approaching 50% crystallinity (t1/2). During cooling, the maximum Tc of PU nanocomposites shifts to lower temperatures due to graphene’s inhibition of PU soft segment crystallization.	[146]
Trans-1,4-polyisoprene (TPI)		Al2O3-GO	TGA test reveals GO (1.2 wt %) Al2O3 exhibited a 7.1 and 4.2 °C increase in the T10 values compared to the pure sample, indicating enhanced thermal stability.	[147]
PLA	PCL		The neat PLA Tc at 132 °C and Tm at 157.6 °C, while neat PCL shows a Tm at 55.2 °C. Adding GMA and NCC to PLA/PCL blends expressively affects the thermal properties.	[148]
PCL	PVC and PMMA		The peak Tm of PCL is around 57 °C. The addition 50% PMMA reduces Tm to 55 °C.	[149]

summarizes a few references related to the thermal properties of the various SMPs and their blends.

4.3. Shape Memory Behavior

Shape memory behavior refers to the capacity of a material to return to its original shape following deformation from a mechanical force or external stimuli. This behavior is caused by a structural phase transition. This section highlights the important articles that focus on the shape memory behavior of SMP. Minzimo et al. [144] analyzed the addition of modified chitosan in PLA/PCL blends, which significantly improved shape recovery rates with high persistence and memory rates, making them suitable for 4D printing applications. The modified chitosan acted as a bridge between PLA and PCL, increasing the interaction force and improving shape fixation and change during the shape memory process, as shown in Figure 17.

Shin et al. [150] study showed that increasing the TPU content in PLA/TPU blends results in a 14% increase in optimal shape setting temperature and a 12% decrease in shape recovery time. The findings highlight the blend’s potential for 4D printing applications like actuators and soft robots. The higher TPU composition caused higher recovery ratios for the non-plasticized PLA/TPU between 40 and 55 °C, and the higher content of PLA increased the shape recovery, as shown in Figure 18 [151]. The lowest molecular weight PEG (i.e., 1000 g/mole) provided the highest recovery at lower temperatures. The lowest deformation rates resulted in higher shape recovery ratios for all recovery temperatures. The 20/80 PLA/TPU blends showed higher recovery for all weights due to the strong elasticity of TPU, as shown in Figure 19 [142]. W. Nonkrathok et al. [152] reported that the PLA/PEG blend shape memory improved with a PLA-g-MA compatibilizer. PLA-g-MA enhances interfacial adhesion and chain entanglement in blends. The shape memory properties of PEG/PLA blends suffered due to the blend’s incompatibility, and to enhance the shape memory abilities of the blends, 0.45% maleic anhydride-grafted poly lactic acid (PLA-g-MA) was used as a compatibilizer; the blend with 2 wt % PLA-g-MA exhibited the best shape fixity and recovery performance. Sanaka et al. [9] reported on the addition of MXene (Ti₃C₂) in the PU matrix on shape memory properties. The test was carried out from a time frame of 10 s to 60 s for virgin PU and 0.5 wt % of MXenePU composite as shown in Figure 20 and Figure 21, respectively. It was observed that the addition of MXene in the PU matrix does not contribute to shape memory behavior.

Table 4. Shape memory behavior of SMPs their blends.

Smp	Blend	Reinforcement	Shape Memory Behavior	Ref.
PLA, Ethylene-co-vinylacetate (EVA)	Thermoplastic vulcanizates (TPVs)	-	The shape memory behavior of EVA-2.0/PLA/AD, especially the shape recovery (SR), showed significant improvement. EVA-2.0/PLA/AD TPV exhibited greater reliability and reusability in shape memory behavior compared to EVA-0/PLA/AD TPV. These enhancements can be attributed to the increased gel content, particularly in the EVA component.	[153]
PCL	Epoxy	-	Shape memory properties were assessed using visual methods and dynamic mechanical analysis (DMA). Visual shape memory testing demonstrated the recovery of the original shape when samples with temporary shapes were placed in water at 80 °C. DMA testing involved samples measuring 5 × 10 mm with a thickness of approximately 500 µm, conducted in three steps. Fixity and recovery ratios were calculated using specified equations. $\mathit{Fixity}\%=\frac{{\epsilon }_f-{\epsilon }_i}{{\epsilon }_p-{\epsilon }_i}\times 100$ $\mathit{Recovery}\%=\frac{{\epsilon }_f-{\epsilon }_r}{{\epsilon }_f-{\epsilon }_i}\times 100$	[154]
TPU	PLA	CNT	Incorporating CNT significantly enhances the shape memory efficiency of the solid composite. PLA/TPU/6 wt % CNT (70/30) exhibited a shape recovery ratio (Rr) of approximately 86% and a shape fixation ratio (Rf) close to 95%, reaching maximum values. Addition of carbon nanotubes to PLA/TPU blends improves shape memory properties by enhancing both shape recovery and fixation ratios.	[155]

shows some important articles that discusses the shape memory behavior of different SMPs and their blends.

5. Applications

SMPs and their blends find applications in various fields, as shown in Figure 22, such as aerospace engineering, sensors and actuators, automobile engineering, artificial muscles, biomedical devices, soft robotics, packages, textile engineering, and flexible electronics [156,157,158]. These SMP blends offer manipulable mechanical properties like stretchability and toughness while maintaining excellent shape memory properties, making them suitable for artificial muscles, soft actuators, and biomedical devices. Additionally, shape memory polymer composites (SMPC) have been explored for use in space-deployable structures, biomedical devices, mold manufacturing, and release devices, due to their unique shape memory effect, low density, high specific strength, biodegradability, and biocompatibility [1,2,3,4,5].

6. Conclusions

The current review discussed the overview of shape memory polymers (SMPs), their blends, and composites. The following conclusions are drawn:

• Shape memory polymers (SMPs) respond uniquely to external stimuli;
• SMP composites with fillers exhibit improved mechanical and thermal properties;
• Blends, especially those with polylactic acid (PLA), show superior shape recovery efficiency;
• SMP blends offer promising features for mechanical, thermal, and shape recovery applications.

7. Future Prospectives

Shape memory polymers (SMPs) could be enhanced for stretchability, toughness, and shape recovery by using polymer blending techniques and adding fillers [159,160,161]. Nevertheless, determining suitable fillers and polymers for blending is difficult. However, this can be made easier using artificial intelligence (AI) tools due to their ability to find the best combination and the right amount of filler [162,163,164]. They can expand untouched niches and bring about new opportunities for their usage in various fields. Below is a list of some promising prospects for the future. Future research should focus more on the shape memory characteristics of these composites. Great potential lies in using multifunctional fillers and creating hybrid composites while improving material design. Moreover, screening new mechanisms of stimuli-responsive behavior and improvements in processing techniques will remain greatly relevant for fully exploiting the potential of SMP composites. Long-term interdisciplinary collaborative efforts and advances in material science and engineering will lead to an entirely new field of responsive materials. The following are some important future applications of SMP materials.

7.1. Bioprinting

Bioprinting is an upcoming area where SMP composite materials and their blends may find their applications. SMP materials are a considerable prospect for artificial tissues/organs. For instance, materials employed in fabricating cardiac valves could dynamically change their properties in terms of the structure and performance of blood circulation pressure [165].

7.2. Self-Healing Material

Self-healing is the ability to repair itself when damage occurs, which is a potential application of SMPs. For instance, the parts used in car production or plane construction are SMPs that self-heal when exposed to hot temperatures to close previously formed cracks or damage [166]. Furthermore, it also brings benefits for materials to be safe and last longer, reducing the amount of maintenance required and increasing reliability.

7.3. Agricultural

Applying SMPs in the agricultural field may revolutionize and create plants that will be sensitive to moisture and can dynamically adjust to their surroundings [167]. SMP-based leaves may maximize transpiration at high moisture, and at dryness, they will curl to reduce water loss and maximize water use. SMP roots may also detect soil moisture and change their shape to improve the grasping capability of soil and sand [168].

7.4. Bio-Robots

Bio-robots, often known as soft robots, made with SMP, can bend with stimuli [169]. This allows robots to transform into many forms and maneuver through intricate settings, similar to living organisms. This may find use in places where conventional rigid robots cannot operate, such as medical treatments and search-and-rescue operations.

7.5. Drug Delivery

SMPs can potentially be used in targeted drug delivery [170]. These polymers can be made to release drugs when changes in pH or temperature occur. For instance, an SMP-based drug delivery system may remain dormant throughout the body but reform itself to dispense its contents where intended, as a tumor.