Vitrimer-Like Shape Memory Polymers: Characterization and Applications in Reshaping and Manufacturing

Article Vitrimer-Like Shape Memory Polymers: Characterization and Applications in Reshaping and Manufacturing

1 2, 3, 3 4 Tao Xi Wang , Hong Mei Chen *, Abhijit Vijay Salvekar †, Junyi Lim , Yahui Chen , Rui Xiao 5 and Wei Min Huang 3,*

1 College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China; [email protected] 2 College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China 3 School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore; [email protected] (A.V.S.); [email protected] (J.L.) 4 School of Physical Science and Technology, Soochow University, Suzhou 215006, China; [email protected] 5 Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, China; [email protected] * Correspondence: [email protected] (H.M.C.); [email protected] (W.M.H.) Current Address: Polymerize Pte. Ltd., Singapore 048580, Singapore. † Received: 8 June 2020; Accepted: 8 October 2020; Published: 12 October 2020

Abstract: The shape memory effect (SME) refers to the ability of a material to recover its original shape, but only in the presence of a right stimulus. Most polymers, either thermo-plastic or thermoset, can have the SME, although the actual shape memory performance varies according to the exact material and how the material is processed. Vitrimer, which is between thermoset and thermo-plastic, is featured by the reversible cross-linking. Vitrimer-like shape memory polymers (SMPs) combine the vitrimer-like behavior (associated with dissociative covalent adaptable networks) and SME, and can be utilized to achieve many novel functions that are difficult to be realized by conventional polymers. In the first part of this paper, a commercial polymer is used to demonstrate how to characterize the vitrimer-like behavior based on the heating-responsive SME. In the second part, a series of cases are presented to reveal the potential applications of vitrimer-like SMPs and their composites. It is concluded that the vitrimer-like feature not only enables many new ways in reshaping polymers, but also can bring forward new approaches in manufacturing, such as, rapid 3D printing in solid state on space/air/sea missions.

Keywords: vitrimer; shape memory; cross-linking; reversible; 3D printing; reshaping

1. Introduction

As a typical thermo-plastic, above its melting temperature (Tm), polycaprolactone (PCL) is able to flow and can be reshaped into another shape. This new shape remains after solidification [1]. We can repeat this reshaping process again and again. However, after cross-linking, the PCL strip (refer to Figure1) pre-stretched above its melting temperature is able to recover its original shape upon either heating or immersing in acetone. After pre-deformation to fix a temporary shape, a process called programming (e.g., Figure1b to Figure1c), a material with the ability to recover its original shape, but only at the presence of a right stimulus, is technically called shape memory material (SMM), and the phenomenon associated with this shape recovery process (e.g., Figure1c to Figure1d, which is heat-induced, or Figure1c to Figure1e, which is chemically induced) is known as the shape memory

Polymers 2020, 12, 2330; doi:10.3390/polym12102330 www.mdpi.com/journal/polymers Polymers 2020, 12, 2330 2 of 28

Polymers 2020, 12, x FOR PEER REVIEW 2 of 29 effect (SME) [2–5]. Similar to drying of wet hydrogel membranes in air [6], during drying of acetone wettedshape PCL, memory uneven effect evaporation (SME) [2–5 of]. Similar acetone to may drying cause of thewet PCLhydrogel to curl membrane (Figure1se in to air Figure [6], d1uringf) and andrying internal of stress acetone field wetted is resulted PCL, uneven inside ofevaporation the material. of acetone Subsequent may cause heating the may PCL etoffectively curl (Figure flatten 1e it (Figureto Figure1f to Figure1f) and1d) an and internal eliminate stress the field internal is resulted stress inside as well. of Referthe material to Figure. Subsequent S1 in the Supplementary heating may Materialseffectively (Part flatten I) for it a ( snapshotFigure 1f ofto theFigure actual 1d) experiment.and eliminate After the internal cross-linking, stress as PCL well becomes. Refer to thermoset, Figure andS1 therefore in the Supplementary its permanent Materials shape (Part is fixed I) for [7 a– snapshot9]. It has of both the actual the heating-responsive experiment. After cross SME-linking, and the chemoPCL (acetone)-responsivebecomes thermoset, and SME. therefore Furthermore, its permanent as revealed shape in is S2 fixed (thermo-plastic [7–9]. It has polystyrene, both the heating PS and- responsive SME and the chemo (acetone)-responsive SME. Furthermore, as revealed in Figures S2 thermo-plastic polypropylene, PP) and S3 (dry thermoset hydrogel) in the Supplementary Materials (thermo-plastic polystyrene, PS and thermo-plastic polypropylene, PP) and S3 (dry thermoset (Part I), the local internal stress in a thermoset can be eliminated or minimized via the SME, while for hydrogel) in the Supplementary Materials (Part I), the local internal stress in a thermoset can be thermo-plastics, not only the local internal stress is difficult to remove, but also melting induced eliminated or minimized via the SME, while for thermo-plastics, not only the local internal stress is distortion may occur. difficult to remove, but also melting induced distortion may occur.

FigureFigure 1. 1.Shape Shape memory memory effect eﬀect in inthermoset thermoset polycaprolactone polycaprolactone (PCL). (PCL). (a) Original (a) Original strip shaped strip sample; shaped sample;(b) stretching (b) stretching at high temperatures at high temperatures when the material when the is soft; material (c) temporary is soft; shape (c) temporary after programming; shape after programming;(d) recovered( dshape) recovered after heating; shape (e after) recovered heating; shape (e) recoveredafter immersi shapeng in after acetone immersing (slightly sw ino acetonellen); (slightly(f) after swollen); taking out (f) afterof acetone, taking sample out of curls acetone, because sample of uneven curls because evaporation of uneven of acetone. evaporation [(b)–(c): of acetone.Programming [(b)–(c): to Programming fix the temporary to ﬁx shape; the temporary (c)–(d): recovery shape; via (c)–( heatd):- recoveryresponsive via SME; heat-responsive (c)–(e): recovery SME; via chemo-responsive SME; (f)–(d) heating to eliminate the deformation induced during acetone (c)–(e): recovery via chemo-responsive SME; (f)–(d) heating to eliminate the deformation induced evaporation]. during acetone evaporation].

AlthoughAlthough thermoset thermoset isis well well-known-known for for better better thermal/shape thermal/shape stability stability and higher and higherstrength, strength, being reprocess-able via injection/extrusion etc., at high temperatures (above Tm) is the remarkable being reprocess-able via injection/extrusion etc., at high temperatures (above Tm) is the remarkable advantage of thermo-plastic. Because of the increasing demand for recycling, being reprocess-able advantage of thermo-plastic. Because of the increasing demand for recycling, being reprocess-able becomes more and more important at present [10,11]. becomes more and more important at present [10,11]. Both the glass transition and melting/crystallization may be utilized for the SME in shape Both the glass transition and melting/crystallization may be utilized for the SME in shape memory memory polymers (SMPs). For the melting/crystallization-based SME in a SMP, cross-linking, either polymersphysical (SMPs). or chemical, For the which melting determines/crystallization-based the permanent SME shape in aof SMP, the SMP cross-linking,, is required either [12,13 physical]. There or chemical,are different which ways determines to describe the the permanent underlying shape mechanism of the SMP,s for the is required SME in SMPs [12,13. From]. There the aremechanics diﬀerent waysof materials to describe point the of underlying view, there mechanisms are at least two for theparts SME in a in SMP, SMPs. one From is the the elastic mechanics part, which of materials stores pointthe ofelastic view, energy there after are atprogramming least two parts and releases in a SMP, the one elastic is theenergy elastic to drive part, shape which recovery, stores the and elastic the energyother after is the programming transition part, and which releases change the elastics its stiffness energy according to drive shape to whether recovery, theand right the stimulus other is is the transitionpresented part, and which hold changesthe deformed its sti ﬀtemporaryness according shape toafter whether programming. the right Depending stimulus is on presented the actual and holdworking the deformed mechanism, temporary the elastic shape part after might programming. be the cross Depending-linked network on the actual only working, or together mechanism, with additional contribution from a portion of yet softened transition part during programming [14–16].

Polymers 2020, 12, 2330 3 of 28 the elastic part might be the cross-linked network only, or together with additional contribution from a portion of yet softened transition part during programming [14–16]. It is apparent that a reversible cross-linking system/network, which appears and disappears on demand in a well-controlled manner via, e.g., heating/cooling, is able to combine the advantages of both thermoset and thermo-plastic, and in the meantime, largely avoid their own problems. The potential application of this kind of material is enormous. Vitrimer is a newly coined term for this type of polymers with a reversible cross-linking system/network upon thermal cycling [17–19]. Although strictly speaking, according to [20], vitrimer belongs to the group of associative covalent adaptable networks (CANs) (i.e., the original cross-link is only broken when a new covalent bond to another position has been formed [20]), the group of dissociative CANs (i.e., the chemical bonds are ﬁrst broken and then formed again at another place, for instance the reversible Diels–Alder reaction between furans and maleimides in organic polymer networks [20]) is seemingly more interesting and useful. Interested readers may refer to above mentioned references for more details. In recent years, many new types of vitrimers have been invented via diﬀerent approaches for enhanced performance and/or for some special features (such as, the novel SME or self-folding/unfolding [21–25], i.e., reshaping), malleability and reprocessability by hot-pressing (i.e., associated with recycling as thermo-plastics) and heat-assisted healing, etc., [21,25–35]. Current methods to characterize vitrimers are mostly based on the Arrhenius law [17,33,35]. Some dissociative CAN vitrimers have been available in the market for some years. We used a commercial vitrimer polyurethane (PU) in the course of this study. Upon thermal cycling within a relatively lower temperature range, if the network (cross-linking) in a vitrimer is strong enough to serve as the elastic part to store elastic energy, similar to those semi-crystalline SMPs, the heating-responsive SME can be achieved [12]. Since the cross-linking in vitrimers is reversible during thermal cycling to higher temperatures, we may examine the shape memory performance of a vitrimer to systematically characterize its vitrimer-like behavior (dissociative CAN) as a function of temperature. From the engineering application point of view, such a kind of information is essential in order to use the material in real practice. The purpose of this paper is two-fold. Section2 presents a heating-responsive SME based approach to systematically characterize the vitrimer-like behavior of a commercial vitrimer-like polymer. Section3 presents a range of ways to permanently or temporarily reshape/manufacture vitrimers and their composites, including rapid 3D printing on space/air/sea missions. Main conclusions are summarized in Section4. Unless otherwise stated, in all experiments mentioned in Section3, the vitrimer-like polymer characterized in Section2 is used.

2. Characterization of Vitrimer-Like Behavior via SME

2.1. Material and Sample Preparation The polymer (TPU 262A) used in the course of this study is from Taiwan PU Corporation (TPUCO, Taiwan). This material is claimed to be thermo-plastic by the manufacturer. As-received material is in pellet form. Diﬀerential scanning calorimetry (DSC) test was carried out on a piece of small original pellet for two continuous thermal cycles between 50 C and 200 C at a temperature ramping rate of 10 C/min − ◦ ◦ ◦ using a Q200 DSC machine (TA Instrument, New Castle, DE, USA). According to the result of the second cycle (red line in Figure2), the melting temperature (T m) of this material is about 50 ◦C, and at around 60 ◦C it fully melts. Upon cooling, its crystallization temperature (Tc) is found to be about 0 ◦C. However, after 15 min at room temperature (about 23 ◦C), which according to Figure2, is slightly above the crystallization starting temperature of this material, it is able to fully crystallize. An additional Polymers 2020, 12, 2330 4 of 28

DSC test carried out between 100 C and 200 C (Figure S4 in the Supplementary Materials, Part I) − ◦ ◦ reveals that the glass transition temperature range of this material is between 50 ◦C and 30 ◦C. Polymers 2020, 12, x FOR PEER REVIEW − −4 of 29

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FigureFigure 2. Di 2.ff Differentialerential scanning scanning calorimetry calorimetry ( (DSC)DSC) result result of of as as-received-received pellet pellet for fortwo twocontinuous continuous thermalthermal cycles. cycles. Figure 2. Differential scanning calorimetry (DSC) result of as-received pellet for two continuous FlatFlatthermal pieces pieces cycles. with with a thicknessa thickness of of 11 mm or or 0.3 0.3 mm mm were were prepared prepared via hot via-compressing hot-compressing at 130 °C at. 130It ◦C. was observed that at 110 °C, this polymer is already very easy to flow (refer to Figure 5). Hence, It was observed that at 110 ◦C, this polymer is already very easy to flow (refer to Figure 5). Hence, Flat pieces with a thickness of 1 mm or 0.3 mm were prepared via hot-compressing at 130 °C. It unlessunless otherwise otherwise stated, stated, the the applied applied pressurepressure was always always very very small small in inall allhot hot-compression-compression tests tests reportedwas observed here. thatSamples at 11 were0 °C, cutthis from polymer the hot is -alreadycompressed very pieceseasy to into flow the (r eferrequired to Figure size for5). testing.Hence, reported here. Samples were cut from the hot-compressed pieces into the required size for testing. Polytetrafluoroethyleneunless otherwise stated, (PTFE) the applied thin film pressure was used was asalways the interfacial very small layer in betweenall hot-compression the polymer testsand Polytetrafluoroethylenethereported metallic here mold. Samples to en (PTFE)sure were easy cut thin separationfrom film the washot after-compressed used hot-compression. as the pieces interfacial intoConsequently, the layerrequired between the size textile for the testing.pattern polymer and theofPolytetrafluoroethylene the metallic PTFE film mold was torecorded ensure(PTFE) on thin easy the film separationsurface was ofused the afteras hot the-compressed hot-compression. interfacial layersamples between as Consequently, shown the polymerin Figure the and 3a, textile patternwhichthe ofmetallic theis a strip PTFE mold-shaped filmto en wassure sample easy recorded cut separation from on the the after hot surface- compressedhot-compression. of the piece. hot-compressed Consequently, A few dots wer samplesthee marked textile aspattern on shown the in Figuresampleof3 thea, whichPTFE surface film is for awas strip-shapedthe recorded purpose on of samplethe manual surface cut measurement of fromthe hot the-compressed hot-compressedof the real samples deformation piece.as shown during A in few Figure thermo dots 3a,- were markedmechanicalwhich on is the a strip sampletesting.-shaped Figure surface sample 3b for reveals thecut from purpose the 3 theD surface hot of- manualcompressed over an measurement area piece. of 1 mmA few × of 1 dotsmm the realscannedwere deformation marked by Talyscan on the during thermo-mechanicalsample150 (Ta ylorsurface Hobson, for testing. the Warrenville, purpose Figure3 ofb IL, revealsmanual USA), the measurementin which 3D surface the fluctuation of over the anrealarea in deformation height of 1 mmof the during surface1 mm thermo scannedof the- by × Talyscanhotmechanical-compressed 150 (Taylor testing. sample Hobson, Figure is 3around Warrenville,b reveals 20 theμm. 3 IL, DSamples surface USA), were over in which ancut area from the of the fluctuation1 mm hot ×-compressed 1 mm in scanned height pieces by of Talyscan the into surface the of 150 (Taylor Hobson, Warrenville, IL, USA), in which the fluctuation in height of the surface of the the hot-compressedrequired size for testing. sample is around 20 µm. Samples were cut from the hot-compressed pieces into hot-compressed sample is around 20 μm. Samples were cut from the hot-compressed pieces into the the required size for testing. required size for testing.

(a) (b)

Figure 3. Typical(a) strip-shaped 1-mm thick sample prepared by hot-(compressionb) (a) (standard rule is included for reference) and 3D surface morphology (b) (scanned area: 1 mm × 1 mm, the unit for the FigurescaleFigure 3. barTypical 3. isTypical μm). strip-shaped strip-shaped 1-mm 1-mm thick thick samplesample prepared prepared by by hot hot-compression-compression (a) (standard (a) (standard rule is rule is included for reference) and 3D surface morphology (b) (scanned area: 1 mm 1 mm, the unit for the included for reference) and 3D surface morphology (b) (scanned area: 1 mm × 1 ×mm, the unit for the scalescaleSome bar isbar pelletsµ m).is μm). were dissolved in acetone (99% purity) at a concentration of 20 g/100 mL and then kept on stirring for 24 h. Thereafter, the solution was poured into a Petri dish without covering for Some pellets were dissolved in acetone (99% purity) at a concentration of 20 g/100 mL and then kept on stirring for 24 h. Thereafter, the solution was poured into a Petri dish without covering for

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Some pellets were dissolved in acetone (99% purity) at a concentration of 20 g/100 mL and then kept on stirring for 24 h. Thereafter, the solution was poured into a Petri dish without covering for 24 h for complete evaporation of acetone. Flat pieces of about 1 mm in thickness were obtained in this way.

Herein,Polymers the 20 samples20, 12, x FOR cut PEER from REVIEW this kind of flat pieces are named as acetone-treated sample,5 while of 29 the samples prepared by hot-compression are named without acetone-treated sample. 24 h for complete evaporation of acetone. Flat pieces of about 1 mm in thickness were obtained in this 2.2. Characterizationway. Herein, the samples cut from this kind of flat pieces are named as acetone-treated sample, while the samples prepared by hot-compression are named without acetone-treated sample. A series of experiments, including nuclear magnetic resonance (Figure S5), X-ray diffractometry (XRD)2.2. (Figure Characterization S6), and Fourier-transform infrared spectroscopy (FTIR) (Figures S7 and S8) were conducted to identify the chemical structure of this material. Refer to Part II of Supplementary A series of experiments, including nuclear magnetic resonance (Figure S5), X-ray diffractometry Materials for details. (XRD) (Figure S6), and Fourier-transform infrared spectroscopy (FTIR) (Figures S7 and S8) were According to the 1H NMR spectrum in Figure S5 (Part II in the Supplementary Materials), conducted to identify the chemical structure of this material. Refer to Part II of Supplementary this TPUMaterials is composed for details. of poly(1,4-butylene adipate glycol) (PBA), diphenyl-methane-diisocyanate (MDI), andAccording 1,4-butylene to the glycol1H NMR (BDO). spectrum PBA in segmentsFigure S5 (Part form II the in the soft Supplementary domain (crystalline), Materials) while, this MDI and BDOTPU is segments composed form of poly(1,4 the hard-butylene segments adipate (glassy). glycol) Refer(PBA), to diphenyl Figure- S9methane (Part- IIdiisocyanate in the Supplementary (MDI), Materials),and 1,4- thebutylene soft/ hardglycol segment (BDO). PBA structure segments enables form the the soft heating-responsive domain (crystalline), SMEwhile inMDI this and material BDO at relativelysegments lower form temperatures the hard segments (below (glassy) 80 ◦C),. Refer in whichto Figure the S9 soft(Part segments II in the Supplementary serve as the Materials), transition part, whilethe the soft/hard hard segments segmentwork structure as the enables elastic the part. heating-responsive SME in this material at relatively lowUnlesser temperatures otherwise stated,(below 80 herein °C), in the which stress the andsoft seg strainments are serve meant as the for transition the engineering part, while stress the and hard segments work as the elastic part. engineering strain, respectively. Unless otherwise stated, herein the stress and strain are meant for the engineering stress and A Q800 DMA machine from TA Instruments (New Castle, DE, USA) was used for all dynamic engineering strain, respectively. mechanicalA Q800 analysis DMA (DMA) machine tests from (in TA filmInstruments tension (New mode Castle, at a rampingDE, USA) ratewas used of 1 ◦forC/ allmin dynamic from room temperaturemechanical to 115analysis◦C). The(DMA) applied tests frequency(in film tension and oscillationmode at a ramping strain were rate 1 of Hz 1 and°C/min 0.07%, from respectively. room The samplestemperature used to here115 are°C).20 The mm applied5 mm frequency1 mm and strips. oscillation The DMA strain result were presented 1 Hz and in 0.07% Figure, 4 is × × for therespectively sample without. The samples acetone used treatment, here are 20 which mm × confirms5 mm × 1 mm that strips. the melting The DMA transition result presented of this material in endsFigure at around 4 is for 60 ◦theC. sample In Figure without5, we plotacetone the treatment, strain versus which heating confirms temperature that the melting relationship transition using of the samethis set material of experimental ends at around data for 60 Figure °C. In4 .Figure Four zoom-in5, we plot views the strain of four versus selected heating temperature temperature ranges are alsorelationship included. using Although the same we set cannot of experimental see any significantdata for Figure feature 4. Four in bothzoom Figure-in views2 (DSC) of four andselected Figure 4 temperature ranges are also included. Although we cannot see any significant feature in both Figure (DMA) at around 100 C upon heating, Figure5 [in particular, inset (d)] clearly shows that from 107 C 2 (DSC) and Figure◦ 4 (DMA) at around 100 °C upon heating, Figure 5 [in particular, inset (d)] clearly ◦ onward the material becomes more and more non-elastic and continuously extends during cyclic shows that from 107 °C onward the material becomes more and more non-elastic and continuously stretching.extends Hence, during cyclic the material stretching. gradually Hence, the turns material to be gradually less viscous, turns to i.e., bewith less viscous, less friction. i.e., with At less around 110 ◦C,friction. the material At around appears 110 °C not, thebeing material able appears to maintain not being its shapeable to anymore,maintain its so shape that theanymore, material so that becomes thermo-plasticallythe material becomes reprocess-able. thermo-plastically reprocess-able.

FigureFigure 4. Typical 4. Typical dynamic dynamic mechanical mechanical analysisanalysis ( (DMA)DMA) re resultsult of of sample sample without without acetone acetone treatment. treatment.

Polymers 2020, 12, 2330 6 of 28 Polymers 2020, 12, x FOR PEER REVIEW 6 of 29

Polymers 2020, 12, x FOR PEER REVIEW 6 of 29

Figure 5. Strain versus heating temperature relationship in DMA test (same set of experimental data Figure 5. Strain versus heating temperature relationship in DMA test (same set of experimental data for Figure4). ( a): Zoom-in view from 44 ◦C to 46 ◦C; (b) zoom-in view from 89 ◦C to 91 ◦C; (c) zoom-in for FigureFigure 45.). Strain(a): Zoom versus-in heating view from temperature 44 °C to relationship 46 °C; (b) inzoom DMA-in test view (same from set 89 of °Cexperimental to 91 °C; ( datac) zoom - view from 99 ◦C to 101 ◦C; (d) zoom-in view from 107 ◦C to 109 ◦C. in viewfor Figurefrom 994). °C(a): to Zoom 101 -°Cin ;view (d) zoom from -44in °C view to 46 from °C; (107b) zoom °C to-in 109 view °C from. 89 °C to 91 °C; (c) zoom- Thein heating-responsive view from 99 °C to 101 shape °C; (d memory) zoom-in performanceview from 107 °C of to the 109 samples °C. with/without acetone treatment was characterizedThe heating- viaresponsive DMA. In eachshape test, memory the 1-mm performance thick sample of was the heated samples to a prescribedwith/without temperature. acetone treatmentThe was heating characterized-responsive via shapeDMA. memory In each test,performance the 1-mm of thick the samplesamples was with/without heated to aacetone prescr ibed Aftertreatment three minutes was characterized of temperature via DMA. stabilization, In each test, the the sample1-mm thick was sample stretched was heated to about to a25% prescr strainibed at a temperature. After three minutes of temperature stabilization, the sample was stretched to about 25% straintemperature rate about. After 0.2%/ threes. After minutes cooling of temperature back to room stabilization, temperature, the sample the sample was stretched was unloaded. to about This 25% ends strain at a strain rate about 0.2%/s. After cooling back to room temperature, the sample was unloaded. the programmingstrain at a strain process, rate about which 0.2%/s is. After the ﬁrst cooling part back of a to full room shape temperature, memory the cycle. sample Technically was unloaded. speaking, This ends the programming process, which is the first part of a full shape memory cycle. Technically this heatingThis ends temperature the programming is called process, the programming which is the first temperature part of a full (T shaped) and memory the maximum cycle. Technically deformation speaking, this heating temperature is called the programming temperature (Td) and the maximum strainspeaking, applied this here heating is called temperature the maximum is called programming the programming strain ( εtemperaturem)[36]. In the(Td) next and partthe maximum of a full shape memorydeformationdeformation cycle, strain whichstrain applied applied is the here recoveryhere is is called called process, the the maximummaximum the programmed programming programming sample strain strain was (εm ()ε heated[m36) ][.36 In] .the toIn next the previousnextpart part of a offull a fullshape shape memory memory cycle, cycle, which which is is the the recoveryrecovery process,process, the the programmed programmed sample sample was was heated heated to to programming temperature (Td) for shape recovery. Two typical DMA results, in which Td was 80 ◦C the previous programming temperature (Tdd) for shape recovery. Two typical DMA results, in which inthe both previous tests, programming but one sample temperature was acetone (T ) treatedfor shape and recovery. the other Two was typical without DMA acetone results, treatment, in which Td was 80 °C in both tests, but one sample was acetone treated and the other was without acetone areTd was presented 80 °C in Figureboth tests,6. Since but theone horizontal sample was axis acetone is time, treated the stress, and strain,the other and was temperature without acetone during treatment, are presented in Figure 6. Since the horizontal axis is time, the stress, strain, and eachtreatment, shape memoryare presented cycle arein presentedFigure 6. asSince a function the horizontal of time. axis is time, the stress, strain, and temperaturetemperature during during each each shape shape memory memory cycle cycle are presented as as a afunction function of time.of time.

(a) (b) Figure 6. Typical DMA(a) results for characterization of shape memory performance(b) (T : 80 C; ε : Figure 6. Typical DMA results for characterization of shape memory performance (Td: 80d °C; ɛ◦m: m ~25%).∼25%). (a) Acetone-treated(a) Acetone-treated sample; sample; (b (b)) without without acetoneacetone treatment. treatment. Figure 6. Typical DMA results for characterization of shape memory performance (Td: 80 °C; ɛm: Two∼25%).Two most (a )most Acetone important important-treated parameters parameters sample; (b to) to without evaluateevaluate acetone the shape shape treatment. memory memory performance performance of a of polymer a polymer are are the shapethe shape ﬁxity fixity ratio ratio (R f()R andf) and shape shaperecovery recovery ratioratio ( (RRrr),), which, which, for for polymers polymers without without apparent apparent Two most important parameters to evaluate the shape memory performance of a polymer are the shape fixity ratio (Rf) and shape recovery ratio (Rr), which, for polymers without apparent

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Polymers 2020, 12, x FOR PEER REVIEW 7 of 29 creep/relaxation at room temperature after programming (as the material used in this study is; creep/relaxation at room temperature after programming (as the material used in this study is; refer refer to [37] for the experimental results of this material in our previous investigation) are defined to [37] for the experimental results of this material in our previous investigation) are defined by [36], by [36], ε1 R f == ((1)1) εm1 𝑓𝑓 ɛ 𝑅𝑅 ε1 𝑚𝑚ε2 Rr == −ɛ ((2)2) ε1 ɛ1 − ɛ2 where ε is the strain after programming and 𝑅𝑅ε𝑟𝑟 is the residual strain after heating for shape recovery. where ɛ11 is the strain after programming and ɛ22 is theɛ residual1 strain after heating for shape recovery. AsAs mentioned mentioned in [in12 ],[12 both], both the shape the shape fixity ratio fixity and ratio shape and recovery shape ratio recovery of a polymer ratio of are a programming polymer are temperatureprogramming and temperature maximum programmingand maximum strain programming dependent. strain dependent. R R InIn Figure Figure7 ,7, Rf fand and Rr r(upon (upon heating heating to to T Td,d, which which is is the the programming programming temperature, temperature, for for shape shape recovery)recovery) are are plotted plotted as as a a function function of of T Tdd.. AsAs thethe materialmaterial aroundaround thethe clamping clamping areas areas might might be be pulled pulled outout of of the the clampers clampers during during stretching, stretching, which which is is di difficultfficult to to recover recover in thein the subsequent subsequent heating heating process process in ain DMA a DMA test, test, the the shape shape recovery recovery ratios ratios of of the the samples samples without without acetone acetone treatment treatment obtained obtained based based on on thethe displacement displacement manually manually measured measured using using the the pre-marked pre-marked dots dots in in the the middle middle area area of of these these samples samples (refer(refer toto FigureFigure3 3a)a) areare alsoalso included in in Figure Figure 7.. It It appears appears that that Rf inR allf in samples all samples is about is about 100%, 100%, while whileRr decreasesRr decreases with with the theincrease increase in inTd T. dBoth. Both acetone acetone-treated-treated and and without without acetone-treatedacetone-treated samplessamples essentiallyessentially shareshare the the same same trend trend in in the theR Rr rversus versus T Tddrelationship relationship. .The The manuallymanually measuredmeasured resultsresults ofof withoutwithout acetone-treatedacetone-treated samplessamples indicate that for for T Tdd upup to to 80 80 °C◦C,, upon upon heating heating to tothe the previous previous Td, Tthed, thecorresponding corresponding Rr isR raboutis about 100%. 100%. When When Td approaches Td approaches 110 110°C, the◦C, corresponding the corresponding Rr dropsRr drops to only to onlyabout about 10% 10%or les ors less in inall allexperiments. experiments. The The results results of of RRr robtainedobtained from from DMA test andand manualmanual measurementmeasurement are are only only slightly slightly di different.fferent.

Figure 7. Shape ﬁxity ratio (Rf) and shape recovery ratio (Rr) as a function of programming temperature f r (TFigured) for samples7. Shape with fixity/without ratio acetone(R ) and treatment. shape recovery ratio (R ) as a function of programming temperature (Td) for samples with/without acetone treatment. Additional tests were manually carried out to further investigate the inﬂuence of thermal history on Rr Additionalusing 1-mm tests thick were samples. manually Manual carried measurement out to further based investigate on pre-marked the influence dots (refer of thermal to Figure history3a) wason R usedr using in the1-mm calculation. thick samples. All these Manual tests measurement were carried outbased according on pre-marked to the following dots (refer sequence. to Figure 3a) was used in the calculation. All these tests were carried out according to the following sequence. (1) Heat the sample to a prescribed pre-heating temperature (Tp). Three pre-heating temperatures (1) Heat the sample to a prescribed pre-heating temperature (Tp). Three pre-heating temperatures (namely, 75 ◦C, 90 ◦C, and 120 ◦C) were applied in the course of this study. (namely, 75 °C, 90 °C, and 120 °C) were applied in the course of this study. (2) Cool the sample to a predetermined programming temperature, Td ( Tp), and then gently and (2) Cool the sample to a predetermined programming temperature, Td ≤(≤Tp), and then gently and slowly stretch it by hands so that the strain between the two marked dots reaches around 100%. slowly stretch it by hands so that the strain between the two marked dots reaches around 100%. Three programming temperatures (namely, 50 °C, 75 °C, and 90 °C) were applied. According to

Polymers 2020, 12, 2330 8 of 28 Polymers 2020, 12, x FOR PEER REVIEW 8 of 29

ThreeFigure programming 2, these three temperatures temperatures (namely, are all 50well◦C, above 75 ◦C, the and crystallization 90 ◦C) were applied. temperature According of this to Figurematerial.2, these three temperatures are all well above the crystallization temperature of this material. (3)(3) Further cool the sample backback toto roomroom temperaturetemperature forfor fullfull crystallization.crystallization. (4)(4) Heat the sample inin aa step-by-stepstep-by-step mannermanner toto fivefive reheatingreheating temperaturestemperatures (T(Trr)) fromfrom 8080 ◦°C,C, 9090 ◦°C,C, 100100 ◦°C, 110 ◦°C, to finallyfinally 120 ◦°C. The corresponding shapeshape recoveryrecovery ratiosratios afterafter eacheach heatingheating areare plotted inin FigureFigure8 8.. Legend Legend in in Figure Figure8 8is is in in (T (Tpp:T:Tdd) format to indicate thethe actualactual thermalthermal historyhistory before finalfinal step-by-stepstep-by-step heatingheating ofof aa sample.sample.

Figure 8. Relationship between shape recovery ratio (Rr) and reheating temperature (Tr) for pre-heated (toFigure Tp) and 8. Relationship then deformed between (upon coolingshape torecovery Td) samples. ratio ( LegendRr) and is reheating in (Tp:Td )temperature format, where (T Tr)p forand pre Td- indicateheated ( theto T pre-heatingp) and then deformed and programming (upon cooling temperatures, to Td) samples. respectively, Legend of is a sample.in (Tp:Td) format, where Tp and Td indicate the pre-heating and programming temperatures, respectively, of a sample.

Hence, Sample (75:75) was pre-heated to 75 ◦C and then programmed at 75 ◦C. Same as that in Hence, Sample (75:75) was pre-heated to 75 °C and then programmed at 75 °C. Same as that in Figure7, it is able to mostly recover its original shape upon heating to 80 ◦C. However, upon further Figure 7, it is able to mostly recover its original shape upon heating to 80 °C. However, upon further heating to 110 ◦C and above, Rr is slightly over 100%, which should be due to the easy to ﬂow nature heating to 110 °C and above, Rr is slightly over 100%, which should be due to the easy to flow nature of this material at such high temperatures (refer to Figure5d). For Sample (90:90), its Rr is about 82% of this material at such high temperatures (refer to Figure 5d). For Sample (90:90), its Rr is about 82% upon heating to 80 ◦C. Further heating to its programming temperature of 90 ◦C, Rr improves slightly. upon heating to 80 °C. Further heating to its programming temperature of 90 °C, Rr improves slightly. With further increase in heating temperature, Rr gradually and continuously increases until around With further increase in heating temperature, Rr gradually and continuously increases until around 110 ◦C, where the material becomes dimensionally unstable. For the other three samples, they were 110 °C, where the material becomes dimensionally unstable. For the other three samples, they were pre-heated to 120 ◦C and then programmed at lower temperatures. Apparently, Sample (120:90) has pre-heated to 120 °C and then programmed at lower temperatures. Apparently, Sample (120:90) has very much limited capability for shape recovery (less than 10%), while Sample (120:75) is slightly better very much limited capability for shape recovery (less than 10%), while Sample (120:75) is slightly (20% or less). The shape recovery ratio of Sample (120:50) is over 50%, which is much better than Sample better (20% or less). The shape recovery ratio of Sample (120:50) is over 50%, which is much better (120:75) and Sample (120:90). According to [12], a higher shape recovery ratio implies more/better than Sample (120:75) and Sample (120:90). According to [12], a higher shape recovery ratio implies elastic part, which releases the elastic energy stored during programming to drive shape recovery. more/better elastic part, which releases the elastic energy stored during programming to drive shape 2.3.recovery. Vitrimer-Like Behavior

2.3. VitrimerAccording-Like toBehavior [12], the underlying mechanism for the SME in polymeric materials is a two-part/component system, in which one (elastic part) is always elastic to store elastic energy after programmingAccording and to then [12] the, the stored underlying elastic energymechanism serves for as thethe drivingSME in force polymeric for later materials on shape is recovery, a two- whilepart/component the other (transition system, in part) which is able one to(ela changestic part) its sti isﬀ alwaysness to functionelastic to as store the switcherelastic energy in a shape after memoryprogramming cycle. and then the stored elastic energy serves as the driving force for later on shape recovery, while the other (transition part) is able to change its stiffness to function as the switcher in a shape memory cycle.

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According to Figure S4 (Part II of the Supplementary Materials), the soft segment is PBA, and the hard segment is MDI-BDO. Figure S9 in Part II in the Supplementary Materials schematically illustrates the phase transition process upon heating. While below 80 ◦C, the SME of this material is the result of the soft/hard segment system, upon further heating to over 80 ◦C, the glassy hard segments gradually become viscous. Correspondingly, the network gradually disappears. Consequently, the shape recovery ratio of the polymer gradually decreases. After combining the results in Figures7 and8 together, we may conclude that below 80 ◦C, the material is similar to a semi-crystalline polymer, i.e., thermoset. The chemically or physically crosslinked network is essentially the elastic part. However, upon further heating the network starts to gradually weaken, which results in continuous decrease in the shape recovery ratio. Above 110 ◦C, the material becomes easy to flow, which indicates that the network is either weakened to the level that is not sufficient to keep the shape or fully removed (refer to the DMA result in Figure5d). The reason for less recovery upon heating to Td and then more recovery upon further heating [e.g., Sample (90:90)] is because the network is weakened (evidenced by higher strain and more fluctuation in Figure5b,c). Therefore, further softening at higher temperatures (refer to DMA result in Figure4) is required for more recovery. After heating to eliminate all the network, in the subsequent cooling process, the network starts to form gradually, but not so significant even upon cooling to 75 ◦C, as the corresponding shape recovery ratio is still less than 20%. Upon further cooling to 50 ◦C, the shape recovery ratio is dramatically increased to over 50%. It can be concluded that the reversible cross-linking in this material is dissociative CAN. In Figure9, we schematically plot the relationship between cross-linking (in %) versus temperature in a thermal cycle. For the TPU investigated in this study, according to Figure S9 (Part II in the Supplementary Materials), the reversible network (cross-linking) is the hard segments (MDI-BDD) (Figure S5 in Part II of the Supplementary Materials), which gradually disappear upon gradual heating from 80 ◦C to about 110 ◦C. In the subsequent cooling process, the network slowly reappears. V In the heating process, the network starts to disappear upon reaching around TDs, which means that below this temperature, the material is 100% cross-linked and we may consider it as a kind of semicrystalline thermoset. Therefore, it can have excellent SME, i.e., being able to almost 100% recover its original shape, if the applied programming temperature and strain are right [12]. With the increase in temperature, the network gradually disappears, and hence the corresponding shape recovery is V reduced accordingly. At about TD f , the network is fully eliminated and consequently, the material becomes thermo-plastic and is easy to flow (i.e., with high melting flow index [37]). In the cooling V V process, cross-linking starts at around TCs and at about TC f , the network reaches 100%. Based on Figure9, it is expected that part of the network can be removed upon heating to between V V V TDs and TD f . After subsequent cooling to TC f , the new cross-linking (100%) includes two parts, one is the remaining of the previous network, and the other is newly formed. By selecting the actual heating V V temperature between TDs and TD f , the fraction of the newly formed network can be tailored. If we heat V V the material to a temperature between TDs and TD f , and then stretch it (programming), after cooling, elastic energy is stored in the remaining of the previous network, while the newly formed network is mostly stress free. The slight difference observed in Figure7 between samples with /without acetone treatment implies the density of the reversible network is roughly the same before and after treatment (thermal cycling or dissolving in acetone). In the heating process, dramatic decrease in shape recovery is observed, when the remaining network is not able to serve as a strong continuous elastic spring, which should follow the percolation theory, to effectively store elastic energy. Polymers 2020, 12, 2330 10 of 28 Polymers 2020, 12, x FOR PEER REVIEW 10 of 29

FigureFigure 9. 9.Schematic Schematic sketchsketch ofof cross-linkingcross-linking (in (in %) %) versus versus temperature temperature relationship relationship upon upon heating heating (network eliminating) and cooling (gradual cross-linking). (network eliminating) and cooling (gradual cross-linking).

Apparently,Based on Figure the programming 9, it is expected strain that also part aﬀ ofects the the network continuity can be of theremoved elastic upon spring heating (network). to Forbetween instance, over and stretching. After maysubsequent cause onecooling functionally to , the continuous new cross- elasticlinkingspring (100%)to includes split into two two parts, one is𝑉𝑉 the remaining𝑉𝑉 of the previous network, and𝑉𝑉 the other is newly formed. By selecting the or more smaller𝑇𝑇𝐷𝐷𝐷𝐷 springs,𝑇𝑇𝐷𝐷𝐷𝐷 which results in permanent𝑇𝑇𝐶𝐶𝐶𝐶 plastic deformation and hence the reduced actual heating temperature between and , the fraction of the newly formed network can be shape recovery capability of the material. As schematically illustrated in Figure 10a, if a material is tailored. If we heat the material to 𝑉𝑉a temperature𝑉𝑉 between and , and then stretch it slightly stretched at high temperatures,𝑇𝑇𝐷𝐷𝐷𝐷 the network𝑇𝑇𝐷𝐷𝐷𝐷 is able to elastically deform together with the (programming), after cooling, elastic energy is stored in the remaining𝑉𝑉 of the previous𝑉𝑉 network, while matrix (Figure 10b). However, if it is over-stretched, dislocation,𝑇𝑇𝐷𝐷𝐷𝐷 another𝑇𝑇𝐷𝐷 type𝐷𝐷 of permanent plastic the newly formed network is mostly stress free. deformation, at the end of the network occurs (Figure 10c), which reduces the shape recovery ratio The slight difference observed in Figure 7 between samples with/without acetone treatment accordingly. In the cooling process, the percolation theory for the relationship between the network and implies the density of the reversible network is roughly the same before and after treatment (thermal shape recovery capability is also applicable. Such a kind of relationship in both heating and cooling cycling or dissolving in acetone). processesIn the provides heating the process, possibility dramatic to simultaneously decrease in shape reset recovery both the is permanentobserved, when shape the and remaining temporary Polymersshapenetwork of 20 a20 is vitrimer, 1 not2, x FORablein PEER to one serve REVIEW processing as a strong step. continuous elastic spring, which should follow the percolation11 of 29 theory, to effectively store elastic energy. Apparently, the programming strain also affects the continuity of the elastic spring (network). For instance, over stretching may cause one functionally continuous elastic spring to split into two or more smaller springs, which results in permanent plastic deformation and hence the reduced shape recovery capability of the material. As schematically illustrated in Figure 10a, if a material is slightly stretched at high temperatures, the network is able to elastically deform together with the matrix (Figure 10b). However, if it is over-stretched, dislocation, another type of permanent plastic deformation, at the end of the network occurs (Figure 10c), which reduces the shape recovery ratio accordingly. In the cooling process, the percolation theory for the relationship between the network and shape recovery capability is also applicable. Such a kind of relationship in both heating and cooling processes provides the possibility to simultaneously reset both the permanent shape and temporary shape of a vitrimer in one processing step.

Figure 10. SchematicSchematic illustration illustration of ofdislocation dislocation at atthe the end end of ofa network a network due due to over to over-stretching.-stretching. (a) (Originala) Original (without (without stretching); stretching); (b) ( bstretched) stretched within within elastic elastic limit; limit; (c ()c )over over-stretched,-stretched, dislocation dislocation which is permanent, occurs at the end of thethe networknetwork (the(the networknetwork isis lessless extended).extended).

For simplicity, in above mentioned experimental investigation, we only take the heating/cooling temperature as the parameter without considering the influence of heating/cooling time/speed. Since this material is able to fully crystallize at room temperature, which is slightly above its crystallization starting temperature (Figure 2), it is logical to expect that heating/cooling time/speed should be important factors as well. It is clear that this polymer is indeed a vitrimer-like polymer (dissociative CAN), in which there is a network that can be gradually eliminated upon heating to over 80 °C that is above its melting finish temperature (according to Figures 2 and 4), and upon cooling, the network gradually reappears. The shape recovery ratio (Rr) essentially reveals the evolution of the network structure of a polymer, and may be utilized to systematically characterize the vitrimer-like behavior of a polymer upon thermal cycling. Such a kind of information is essential in order to apply the material in real engineering applications. It can be concluded that the Rr versus programming temperature relationship of a vitrimer-like SMP (e.g., Figure 7) is programming strain and heating/cooling time/speed dependent. The relationship between Rr and cross-linking (in %) is nonlinear and sophisticated.

3. Applications of Vitrimer-Like SMPs and Their Composites A combination of the vitrimer-like behavior and SME enables many new ways to manipulate polymers, far beyond folding/unfolding, reprocessing and heating assisted healing, which have been focused on in most of the studies reported so far [21,25,26]. In this section, in addition to these well- known applications, we explore some more possibilities in reshaping (inside/outside, microscopically/macroscopically, permanent/temporary) and manufacturing using vitrimer-like SMPs and their composites. Above characterized commercial vitrimer-like SMP is used to demonstrate all applications reported in Sections 3.1–3.4.

Polymers 2020, 12, 2330 11 of 28

For simplicity, in above mentioned experimental investigation, we only take the heating/cooling temperature as the parameter without considering the inﬂuence of heating/cooling time/speed. Since this material is able to fully crystallize at room temperature, which is slightly above its crystallization starting temperature (Figure2), it is logical to expect that heating /cooling time/speed should be important factors as well. It is clear that this polymer is indeed a vitrimer-like polymer (dissociative CAN), in which there is a network that can be gradually eliminated upon heating to over 80 ◦C that is above its melting ﬁnish temperature (according to Figures2 and4), and upon cooling, the network gradually reappears. The shape recovery ratio (Rr) essentially reveals the evolution of the network structure of a polymer, and may be utilized to systematically characterize the vitrimer-like behavior of a polymer upon thermal cycling. Such a kind of information is essential in order to apply the material in real engineering applications. It can be concluded that the Rr versus programming temperature relationship of a vitrimer-like SMP (e.g., Figure7) is programming strain and heating /cooling time/speed dependent. The relationship between Rr and cross-linking (in %) is nonlinear and sophisticated.

3. Applications of Vitrimer-Like SMPs and Their Composites A combination of the vitrimer-like behavior and SME enables many new ways to manipulate polymers, far beyond folding/unfolding, reprocessing and heating assisted healing, which have been focused on in most of the studies reported so far [21,25,26]. In this section, in addition to these well-known applications, we explore some more possibilities in reshaping (inside/outside, microscopically/macroscopically, permanent/temporary) and manufacturing using vitrimer-like SMPs and their composites. Above characterized commercial vitrimer-like SMP is used to demonstrate all applications reported in Section 3.1, Section 3.2, Section 3.3, Section 3.4.

3.1. Superimposing of Permanent and Temporary Shapes Although some ductile polymers may be programmed at low temperatures, for the heating-responsive SME, in most cases, it is ideal to carry out programming at high temperatures when the polymer is soft and ductile [14]. The transition that might be applied to soften a polymer includes the glass transition and melting transition. While the glass transition temperature ranges in the heating process and cooling process of a polymer are about the same for most polymers, the melting transition temperature range is normally well above the crystallization temperature range (refer to Figure2 for an example). Programming a polymer based on the melting transition can be done either in the melting temperature range (or above) upon heating or in the crystallization temperature range (or above) upon cooling (after pre-heating). Hence, in the case of, for instance, comfort fitting in direct contact with human body [38,39], we are able to programme the polymer investigated in Section2 at body temperature or even room temperature without worrying of being either too hot or short of time in programming (fitting), which are problems in those polymers in which the glass transition is utilized in programming. In Figure 11A, a piece of hot-compressed sample (without acetone treatment material) is pre-heated to its melting temperature (about 70 ◦C) and then hand-stretched when it is cooled to room temperature (about 23 ◦C) for a while. Since the middle part of the sample is stretched at lower temperatures, similar to the stress induced crystallization in PCL [1], i.e., the crystalline component increases remarkably after stretching, which results in improved transparency on account of minimized density differences between crystalline and amorphous regions, and hence reduced refractive index fluctuations [40], this area becomes transparent. After heating to the melting temperature again, it fully recovers its original shape (including the original surface pattern). Figure 11B reveals the typical surface morphology after stretching and the resulted coloring effect (light interference) due to the surface pattern in the middle transparent part, when it is placed in front of a LED computer monitor. Polymers 2020, 12, x FOR PEER REVIEW 12 of 29

body temperature or even room temperature without worrying of being either too hot or short of time in programming (fitting), which are problems in those polymers in which the glass transition is utilized in programming. In Figure 11A, a piece of hot-compressed sample (without acetone treatment material) is preheated to its melting temperature (about 70 °C) and then hand-stretched when it is cooled to room temperature (about 23 °C) for a while. Since the middle part of the sample is stretched at lower temperatures, similar to the stress induced crystallization in PCL [1], i.e., the crystalline component increases remarkably after stretching, which results in improved transparency on account of minimized density differences between crystalline and amorphous regions, and hence reduced refractive index fluctuations [40], this area becomes transparent. After heating to the melting temperature again, it fully recovers its original shape (including the original surface pattern). Figure 11B reveals the typical surface morphology after stretching and the resulted coloring effect (light Polymersinterference)2020, 12, due 2330 to the surface pattern in the middle transparent part, when it is placed in front12 of of 28 a LED computer monitor.

(A) (B)

FigureFigure 11. 11. (A) Evolution of of surface surface morphology morphology in in hot hot-compressed-compressed sample sample (1- (1-mmmm thick) thick) used used in this in thisstudy study (refer (refer to Section to Section 2) 2in) inone one shape shape memory memory cycle. cycle. (a) ( Originala) Original (opaque); (opaque); (b) (bafter) after stretching stretching the themiddle middle part part (transparent) (transparent) of the of thepreheated preheated sample sample at room at room temperature temperature;; (c) after (c) afterheating heating for shape for shaperecovery recovery and cooling and cooling for crystallization for crystallization at room at room temperature temperature (opaque). (opaque). (B) Coloring (B) Coloring effect eﬀ(lightect (lightinterference) interference) in the in middle the middle-stretched-stretched area area(transparent). (transparent). (a) After (a) After stretching stretching at room at room temperature; temperature; (b1) (b1and,b2 ():b2 sample): sample with with two two di ﬀdifferenterent angles angles before before LED LED computer computer monitor. monitor.

ToTo reconfigure reconfigure a polymer a polymer into anotherinto another permanent permanent shape withoutshape without complete complete removal ofremoval the originally of the definedoriginally permanent defined permanent shape is apparently shape is apparently an advantage an advantage of vitrimer of [22 vitrimer,41,42]. [ According22,41,42]. According to Figure9 to, reshapingFigure 9, intoreshaping another into permanent another shapepermanent can actually shape becan done actually during be eitherdone heatingduring oreither cooling heating when or thecooling network when is partiallythe network or fully is eliminated.partially or Stretchingfully eliminated. under di Stretchingfferent conditions under different (depending conditions on the thermal(depending history on the and thermal the way history of deformation) and the way may of deformation) result in diff erentmay result colors in (opaque differen ort colors transparent) (opaque withinor transparent) the deformed within area. the deformed area. AnAn example example to to superimpose superimpose a a new new permanent permanent surface surface pattern pattern (using(using aa coin) coin) atop atop the the existing existing oneone (produced (produced during during sample sample preparation preparation via hot-compressing via hot-compressing as shown as in Figureshown3) isin demonstrated Figure 3) is indemonstrated Figure 12, in in which Figur thee 12 first, in impressionwhich the first is done impression upon heating is done to upon around heating 100 ◦ Cto (Figure around 12 100(a1)), °C and(Figure the second12(a1)), impression and the second is done impression after the pre-heatedis done after sample the pre (to-heated less than sample 80 ◦C) (to is less just than cooled 80 back°C) is tojust room cooled temperature back to room (Figure temperature 12(b1)). ( WhileFigure the12(b1 first)). While impression the first (the impressio mirror imagen (the mirror of one image side of of aone coin) side is superimposedof a coin) is superimposed on the original on permanentthe original surface permanent pattern surface produced pattern during produced hot-compression during hot- (Figure 12a), the second impression (the mirror image of the other side of the coin) is temporary (Figure 12b), and can be fully removed after heating again (Figure 12c). Note that the maximum depth of the surface feature of a standard bank coin is about 0.1 mm [43,44], which is a lot more than that of the surface pattern of the hot-compressed sample (about 20 µm, refer to Figure3b). In Figure 12, in order to record the results after each step, the sample was heated twice to superimpose two surface patterns, one is permanent and the other is temporary, one by one. In fact, both the permanent pattern and temporary pattern can be superimposed within one heating-cooling process, i.e., the second surface pattern can be impressed onto the sample when the network is fully established in the cooling process. Furthermore, according to Figures8 and9, we can actually superimpose a few permanent and temporary shapes during one heating-cooling process. Polymers 2020, 12, x FOR PEER REVIEW 13 of 29

compression (Figure 12a), the second impression (the mirror image of the other side of the coin) is temporary (Figure 12b), and can be fully removed after heating again (Figure 12c). Note that the Polymersmaximum2020 ,depth12, 2330 of the surface feature of a standard bank coin is about 0.1 mm [43,44], which is13 a of lot 28 more than that of the surface pattern of the hot-compressed sample (about 20 μm, refer to Figure 3b).

Figure 12. SuperimposingSuperimposing new new feature feature atop atop e existingxisting surface surface pattern pattern.. (a) ( aAfter) After heating heating the the sample sample (1

(1mm mm thick) thick) placed placed atop atop a coin a coin to around to around 100 100°C [as◦C shown [as shown in (a1 in), (samplea1), sample is fully is fullytransparent transparent at high at hightemperatures] temperatures] to superimpose to superimpose the surface the surface pattern pattern of the of coin; the coin;(b) after (b) afterpre-heated pre-heated sample sample (to less (to than less

than80 °C 80) cooled◦C) cooled to room to room temperature temperature and and then then placing placing the the other other side side of of the the coin coin atop atop for second impression [as[as shownshown inin ((b1)];)]; ((cc)) afterafter heatingheating forfor shapeshape recovery.recovery.

In solidFigure state, 12, in vitrimer-like order to record SMP the has results the advantage after each to step, reconﬁgure the sample into anotherwas heated shape twice either to permanentlysuperimpose ortwo temporarily surface patterns, on demand. one is permanent and the other is temporary, one by one. In fact, both the permanent pattern and temporary pattern can be superimposed within one heating-cooling 3.2.process, Heat-Assisted i.e., the second Healing surface without pattern Altering can Surface be impressed Feature onto the sample when the network is fully establishedHealing, in inthe particular cooling heat-assistedprocess. Furthermore, healing, has according been extensively to Figures investigated 8 and 9, we in can recent actually years (e.g.,superimpose in [45–50 a]). few In permanent most applications, and tempora both shapery shapes recovery during and one strength heating recovery-cooling areprocess. simultaneously required.In solid Ideally, state, healingvitrimer should-like SMP be repeatablehas the advantage and the healingto reconfigure process into should another take shape as less either time aspermanently possible. or temporarily on demand. For a thermo-plastic, unless heating is restricted within a very small local area, heating to or over3.2. Heat its- meltingAssisted temperatureHealing without tends Altering to fail Surface the material Feature to maintain the original shape/dimension, in particular,Healing, surfacein partic patternular heat (refer-assisted to Figure healing, S2(a2) has in Partbeen I ofextensively the Supplementary investigated Materials). in recent On years the other(e.g., in hand, [45– the50]). broken In most network applications, in a thermoset both shape ismostly recovery permanent. and strength However, recovery vitrimer are simultaneously (in particular forrequired. dissociative Ideally, CAN healing type) isshould able to be keep repeatable the original and shapethe healing and reconstruct process sho a networkuld take via as heat-assisted less time as healingpossible. for strength recovery in a repeatable and rapid manner. BasedFor a thermo on the-plastic, characterized unless heating properties is restricted of the vitrimer-like within a very SMP small reported local area, in Sectionheating2 to, we or over can selectits melting an optimized temperature temperature tends to tofail heat thethe material whole to piece maintain of a sample the original for healing shape/dimension, within a short in periodparticular, of time, surface while pattern the sample (refer to keeps Figure the S2 original(a2) in shapePart I/ dimension.of the Supplementary Two 1-mm Mate thickrials examples). On the as reportedother hand, in Sectionthe broken2 are network demonstrated in a thermoset in Figure is 13 mostly, in which permanent. Figure 13However,A is healing vitrimer of surface (in particular cutting producedfor dissociative by a sharp CAN blade type) via is immersingable to keep into the 90 original◦C hot water, shape and and Figure reconstruct 13B is healing a network of throughout via heat- thicknessassisted healing cutting for produced strength by recovery a pen-knife in a viarepeatable heating and using rapid a hairdryer. manner. According to Figures2 and4, heatingBased to 80 on◦C the triggers characterized the heating-responsive properties of SME,the vitrimer which helps-like SMP to close reported the cutting in Section (shape 2, recovery). we can Uponselect furtheran optimized heating temperature to below its to easy heat to the ﬂow whole temperature piece of anda sample then coolingfor healing back within enables a short the network period toof time, be re-established, while the sample while keeps the originalthe original surface shape/dimension. pattern largely Two remains. 1-mm thick The reversible examples networkas reported in vitrimer-likein Section 2 are SMPs demonstrated provides ain temperatureFigure 13, in which winder Figure (between 13A is about healing 90 ◦ofC surface to about cutting 100 ◦ producedC for the materialby a sharp investigated blade via inimmersing Section2) tointo simultaneously 90 °C hot water, achieve and shape Figure recovery 13B is andhealing strength of throughout recovery withthickness minimum cutting impact produced on the by original a pen-knife shape via/dimension heating using of the a sample.hairdryer. According to Figures 2 and 4, heatingIn Figure to 8013, °C it appears triggers that the shape heating recovery-responsive is not 100%, SME, in which particular helps in to the close sample the with cutting throughout (shape cutting.recovery). In Upon order further to reveal heating the exactto below reason its easy behind to flow this, temperature further experiments and then cooling were carried back enables out on 1-mmthe network thick samples.to be re-established, Figure 14 presents while the the original results ofsurface three pattern SME tests, largely in which remain thes. The programming reversible temperaturesnetwork in vitrimer are room-like temperature,SMPs provides 65 a ◦temperatureC and 95 ◦C, winder respectively, (between while about the 90applied °C to about maximum 100 °C programming strain is approximately the same. Since at room temperature the material is hard, stretching is done by a tensile machine. Programming of other two samples at higher temperatures is Polymers 2020, 12, 2330 14 of 28

conducted by hands (in a similar way to investigate the inﬂuence of thermal history on Rr in Section 2.2). As we can see, full shape recovery is only observed in the sample stretched at 65 ◦C, while the other two samples stretched at room temperature and 95 ◦C are not able to fully recover. As explained above, incomplete recovery in the sample programmed at 95 ◦C is due to the reason of signiﬁcant reduction in cross-linking. On the other hand, for the sample programmed at room temperature, incomplete shape recovery after heating should be the result of fractured cross-linking when the sample is stretched at low temperatures, which is a phenomenon that has been reported [12,51]. Thus, it may be concluded Polymers 2020, 12, x FOR PEER REVIEW 14 of 29 that incomplete healing (shape recovery) observed in Figure 13 is most likely associated with the reducedfor the material shape recovery investigated capability in Section of the polymer2) to simultaneously after low temperature achieve programming,shape recovery since and cuttingstrength is donerecovery at room with temperature.minimum impact on the original shape/dimension of the sample.

(A)

(B)

Figure 13. HeatHeat-assisted-assisted healing. ( A) Shallow Shallow surface cutting using sharp blade. ( a) Original; (b) after

cutting; ((cc)) afterafter heating heating in in 90 90◦ C°C hot hot water. water. (B ()B Throughout) Throughout thickness thickness cutting cutting using using pen-knife. pen-knife. Insets Insets are zoom-inare zoom view-in view of the of cut.the cut. (a) After (a) After cutting; cutting; (b) after (b) after heating heating using using hairdryer. hairdryer. (1) Front (1) Front side; side; (2) back (2) back side. side.

In Figure 13, it appears that shape recovery is not 100%, in particular in the sample with throughout cutting. In order to reveal the exact reason behind this, further experiments were carried out on 1-mm thick samples. Figure 14 presents the results of three SME tests, in which the programming temperatures are room temperature, 65 °C and 95 °C, respectively, while the applied maximum programming strain is approximately the same. Since at room temperature the material is hard, stretching is done by a tensile machine. Programming of other two samples at higher temperatures is conducted by hands (in a similar way to investigate the influence of thermal history on Rr in Section 2.2). As we can see, full shape recovery is only observed in the sample stretched at 65 °C, while the other two samples stretched at room temperature and 95 °C are not able to fully recover. As explained above, incomplete recovery in the sample programmed at 95 °C is due to the reason of significant reduction in cross-linking. On the other hand, for the sample programmed at room temperature, incomplete shape recovery after heating should be the result of fractured cross-linking when the sample is stretched at low temperatures, which is a phenomenon that has been reported [12,51]. Thus, it may be concluded that incomplete healing (shape recovery) observed in Figure 13 is

Polymers 2020, 12, x FOR PEER REVIEW 15 of 29 Polymers 2020, 12, 2330 15 of 28 most likely associated with the reduced shape recovery capability of the polymer after low

temperaturePolymers 2020, 1 programming,2, x FOR PEER REVIEW since cutting is done at room temperature. 15 of 29

most likely associated with the reduced shape recovery capability of the polymer after low temperature programming, since cutting is done at room temperature.

(A) (B) (C)

Figure 14. Typical SME tests programmed at three different temperatures. (A) Room temperature Figure 14. Typical SME tests programmed at three diﬀerent temperatures. (A) Room temperature programming. (a) Original shape (opaque); (b) after stretching at room temperature (the middle part a b programming.is transparent); ( ) Original(A) (c) after shape heating (opaque); to 80 (°C) (opaque); after(B) stretching (d) afterat heating room to temperature 100 °C (C(opaque).) (the middle(B) part is transparent); (c) after heating to 80 C (opaque); (d) after heating to 100 C (opaque). (B) Programmed ProgrammedFigure 14. Typical at 65 °C SME. (a) testsOriginal programmed◦ shape (opaque); at three (b )different after programming temperatures. at ◦65 (A °C) Room (opaque); temperature (c) upon at 65 ◦C.heating (programming.a) Original to 65 °C ( shapea )for Original shape (opaque); shaperecovery (opaque); ((transparent);b) after (b) after programming ( stretchingd) after fully at room crystallized at 65temperature◦C (opaque);at room (the temperature middle (c) uponpart heating (opaque). (C) Programmed at 95 °C. (a) Original shape; (b) after programming at 95 °C (opaque); (c) to 65 ◦C foris transparent); shape recovery (c) after (transparent); heating to 80 (°Cd) after(opaque); fully (d) crystallized after heating atto room100 °C temperature (opaque). (B) (opaque). uponProgrammed heating atto 6595 °C°C. (fora) Originalshape recovery shape (opaque); (transparent); (b) after (d) programming after cooling backat 65 to°C room(opaque); temperature (c) upon (C) Programmed at 95 ◦C. (a) Original shape; (b) after programming at 95 ◦C (opaque); (c) upon forheating crystallization to 65 °C for(opaque). shape recovery (transparent); (d) after fully crystallized at room temperature heating to 95 C for shape recovery (transparent); (d) after cooling back to room temperature for (opaque).◦ (C) Programmed at 95 °C. (a) Original shape; (b) after programming at 95 °C (opaque); (c) crystallization3.3. Fromupon 2D heating (opaque).-Shape to to 95 Surface °C for Wrinklingshape recovery/3D-Shape (transp arent); (d) after cooling back to room temperature Accordingfor crystallization to Figure (opaque).s 7 and 8, the reversible network in this vitrimer-like SMP is gradually 3.3. From 2D-Shape to Surface Wrinkling 3D-Shape removed upon heating to above 80 °C/ till about 110 °C, where the material starts to flow (Figure 5). 3.3. From 2D-Shape to Surface Wrinkling/3D-Shape AccordingOn the toother Figures hand, 7in and the 8subsequent, the reversible cooling networkprocess, the in network this vitrimer-like gradually reinstalls. SMP is This gradually feature removed can be applied via gradient heating (i.e., heating only selected area) to achieve surface upon heating toAccording above 80to FigureC tills about7 and 8 110, the C,reversible where network the material in thisstarts vitrimer to-like ﬂow SMP (Figure is gradually5). On the other wrinkling/patterning,removed upon heating◦ 2D to- shapeabove to80 3D °C- shapetill◦ about switching, 110 °C, whereand tran thesition material from starts uniform to flow shape (Figure to non 5).- hand, in theuniformOn subsequentthe other shape, hand, etc., cooling inas theschematically subsequent process, illustrated cooling the network process, in Figure graduallythe 15 network using reinstalls.agradually piece of pre reinstalls. This-stretched feature This vitrimer feature can- be applied via gradientlikecan heating SMPbe appliedstrip. (i.e., via heating gradient only heating selected (i.e., area) heating to achieve only selected surface area) wrinkling to achieve/patterning, surface 2D-shape to 3D-shapewrinkling/patterning, switching, and transition2D-shape to from 3D-shape uniform switching, shape and to tran non-uniformsition from uniform shape, shape etc., to as non schematically- uniform shape, etc., as schematically illustrated in Figure 15 using a piece of pre-stretched vitrimer- illustrated in Figure 15 using a piece of pre-stretched vitrimer-like SMP strip. like SMP strip.

Figure 15. Illustration of typical approaches applicable for reshaping of vitrimer-like SMP strip (side view for all sketches) via gradient preheating and then heating for shape recovery. (a) Original shape;

Figure 15. Illustration of typical approaches applicable for reshaping of vitrimer-like SMP strip (side Figure 15. Illustration of typical approaches applicable for reshaping of vitrimer-like SMP strip (side view view for all sketches) via gradient preheating and then heating for shape recovery. (a) Original shape; a b for all sketches) via gradient preheating and then heating for shape recovery. ( ) Original shape; ( ) after stretching; (c) after gradient heating; (d) after heating for shape recovery. (i) Wrinkling; (ii) bending;

(iii) folding (local non-through thickness heating); (iii0) surface patterning (local surface heating); (iv) transition from uniform thickness to non-uniform thickness (local through thickness heating).

As presented in Figure 15, in general, there are three steps in all approaches. First (pre-deformation), the original strip (a) is programmed via stretching (b) preferably at high temperatures for better shape ﬁxity ratio and higher shape recovery ratio (refer to Figure 14). Polymers 2020, 12, x FOR PEER REVIEW 16 of 29

(b) after stretching; (c) after gradient heating; (d) after heating for shape recovery. (i) Wrinkling; (ii) bending; (iii) folding (local non-through thickness heating); (iii′) surface patterning (local surface heating); (iv) transition from uniform thickness to non-uniform thickness (local through thickness Polymers 2020heating)., 12, 2330 16 of 28

As presented in Figure 15, in general, there are three steps in all approaches. AlthoughFirst illustrated (pre-deformation), here is the the case original of uniaxial strip (a) stretching, is programmed programming via stretching may (b) be preferably carried out at high in other modes,temperatures such as biaxial for better stretching, shape compression,fixity ratio and and higher twisting, shape etc., recovery or a combination ratio (refer to of Figure them. 14). InAlthough the next illustrated step (c) (preheating), here is the case local of uniaxial/gradient stretching, heating pr isogramming applied to may eliminate be carried the out network in other in one modes, such as biaxial stretching, compression, and twisting, etc., or a combination of them. (Figure 15i,ii) or more (Figure 15iii,iii ,iv) prescribed areas. After cooling back to room temperature, In the next step (c) (preheating),0 local/gradient heating is applied to eliminate the network in one new network is formed without any elastic energy stored inside of it. In Figure 15(ci), the surface of (Figure 15i,ii) or more (Figure 15iii,iii′,iv) prescribed areas. After cooling back to room temperature, the samplenew network is preheated, is formed while without in Figure any elastic 15(cii), energy the preheatedstored inside area of it. is In much Figure deeper. 15(ci), the surface of Subsequently,the sample is preheated, upon heating while in forFigure shape 15(cii), recovery, the preheated which area is is the much third deeper. step (shape recovery), surface wrinklesSubsequently, are observed upon heating in Figure for shape 15(di), recovery, while which in Figure is the 15 third(dii), step the (shape sample recovery), bends towardsurface the un-preheatedwrinkles side.are observed in Figure 15(di), while in Figure 15(dii), the sample bends toward the un- Figurepreheated 16 side.presents typical wrinkles produced on a 1-mm thick sample following approach (i) in FigureFigure 15. 16 The presents resulted typical parallel wrinkles wrinkles produced look on a similar 1-mm thick to what sample is following reported approach in [52], in(i) whichin pre-stretchedFigure 15 acrylonitrile. The resulted butadiene parallel wrinkles styrene look (ABS) similar that to is what thermo-plastic is reported wasin [52 dipped], in which in acetone pre- to stretched acrylonitrile butadiene styrene (ABS) that is thermo-plastic was dipped in acetone to slightly etch its surface. After surface drying and then heating for shape recovery, parallel wrinkles slightly etch its surface. After surface drying and then heating for shape recovery, parallel wrinkles were formed on the surface of ABS sample. In Figure S10 (Part III of Supplementary Materials), were formed on the surface of ABS sample. In Figure S10 (Part III of Supplementary Materials), we we schematicallyschematically comparecompare the didifferenceﬀerence between between surface surface et etchingching approach approach and and surface surface preheating preheating (vitrimer)(vitrimer) approach. approach. While While surface surface preheating preheating (vitrimer)(vitrimer) approach approach is isable able to tomaintain maintain the theoriginal original surfacesurface pattern pattern (as in (as Figure in Figure 16), surface 16), surface etching etching approach approach can hardly can hardly keep thekeep original the original surface surface structure, in particularstructure, if in the particul surfacear featureif the surface is small feature in size. is small in size.

Figure 16. Surface wrinkles produced following approach (i) in Figure 15. The actual surface feature is a combinationFigure 16. Surface of initial wrinkles surface produced pattern following (refer to approach Figure3 (i)) and in Figure wrinkles. 15. The In actual the preparation surface feature of the sample,is a acombination cotton swab of wasinitial soaked surface inpattern acetone (refer and to Figure then applied 3) and wrinkles. to slightly In the etch preparation the surface of the of 50% sample, a cotton swab was soaked in acetone and then applied to slightly etch the surface of 50% pre- pre-stretched 1-mm thick hot compressed sample. Subsequently, the sample was heated in 75 ◦C water for shapestretched recovery. 1-mm thick hot compressed sample. Subsequently, the sample was heated in 75 °C water for shape recovery. In Figure 15(ciii), instead of the whole surface layer, some local areas are preheated. Upon heating In Figure 15(ciii), instead of the whole surface layer, some local areas are preheated. Upon for shape recovery, the sample folds (Figure 15(diii)). Figure 17 is an example of such 2D to 3D folding heating for shape recovery, the sample folds (Figure 15(diii)). Figure 17 is an example of such 2D to after heating.3D folding The after 1 mmheating. thick The sample 1 mm wasthick pre-stretched sample was pre by-stretched 30%. Local by heating30%. Local (two heating straight (two lines) was done using a 3D printing pen (from eSUN, PR China) to slowly tough the sample surface during writing (without ﬁlament). The pen head was adjusted to be about 100 ◦C. After heating for shape recovery, the initial surface pattern produced during hot-compressing is still clearly visible. If the depth of the preheated area is relatively small, as illustrated in Figure 15(ciii0), the result might be a patterned surface as shown in Figure 15(diii0). Of course, preheating might be throughout the whole thickness of the sample (Figure 15(civ)), so that after overall-heating of the whole sample for shape recovery, some parts of the sample shrinks, while some other parts fully or partially maintain the shape (Figure 15(div)). It should be pointed out that it might be necessary to restrain the sample in step (c) of Figure 15 from undesired distortion during the last heating process. Polymers 2020, 12, x FOR PEER REVIEW 17 of 29

straight lines) was done using a 3D printing pen (from eSUN, PR China) to slowly tough the sample surface during writing (without filament). The pen head was adjusted to be about 100 °C. After heating for shape recovery, the initial surface pattern produced during hot-compressing is still clearly visible. If the depth of the preheated area is relatively small, as illustrated in Figure 15(ciii′), the result might be a patterned surface as shown in Figure 15(diii′). Of course, preheating might be throughout the whole thickness of the sample (Figure 15(civ)), so that after overall-heating of the whole sample Polymers 2020, 12, 2330for shape recovery, some parts of the sample shrinks, while some other parts fully or partially 17 of 28 maintain the shape (Figure 15(div)). It should be pointed out that it might be necessary to restrain the sample in step (c) of Figure 15 from undesired distortion during the last heating process.

Figure 17. Folding following approach (iii) in Figure 15 (two different angles of view). Figure 17. Folding following approach (iii) in Figure 15 (two diﬀerent angles of view). A combination of the preheating methods mentioned above can result in different ways of A combinationfolding. of Figure the preheating18 presents twomethods examples of mentioned2D to 3D folding. above In Figure can 18 resultA, after inpre- distretchingﬀerent to ways of folding. 20%, the right part of the top sample surface of a 1 mm thick sample is preheated to high temperatures Figure 18 presentsusing the two same examples 3D printing ofpen 2Dmentioned to 3D in above folding.. Thus, after In Figure 2nd heating 18 forA, shape after recovery, pre-stretching the to 20%, the right part ofright the part top of the sample sample bends surface down ( ofFigure a 1 18 mm(Aa)). thickFigure 18 sample(Ab) is 3D isscanned preheated cross-section to of high the temperatures line marked as A-A in Figure 18(Ac) (optical image, top view). We can see the original surface pattern using the samelargely 3D printingremains. Figure pen 18B mentioned is a combination in of above. surface preheating Thus, afterand deep 2nd-line heatingpreheating (again for shape recovery, the right part ofusing the the sample 3D printing bends pen) of down a piece (Figureof pre-stretched 18(Aa)). 1-mm thick Figure sample. 18(Ab) The resulted is 3D 3D scanned shape after cross-section of the line markedheating as A-A for shape in Figure recovery can18(Ac) be pre- (opticaldesigned. image, top view). We can see the original surface pattern largely remains. Figure 18B is a combination of surface preheating and deep-line preheating (again using the 3D printing pen) of a piece of pre-stretched 1-mm thick sample. The resulted 3D shape after heating for shape recovery can be pre-designed. Polymers 2020, 12, x FOR PEER REVIEW 18 of 29

(A)

(B)

Figure 18. Controlled folding. (A) Partial-bending. (a) Photo; (b) 3D line scanning result (top surface) Figure 18. Controlledof A-A folding.section; (c) top (A view,) Partial-bending. in which A-A section (isa )marked. Photo; (B)( 3Db) folding. 3D line (a) Photo scanning of resulted result (top surface) of A-A section; (cshape) top (red view, mark pen in colored which areas A-A are meant section to guide is for marked. local heating) (B; )(b 3D) 3D scanning folding. result (a (unit) Photo of resulted shape (red mark penis in mm colored for all). areas are meant to guide for local heating); (b) 3D scanning result (unit is in mm for all). If a vitrimer is transparent, sophisticated patterns, e.g., integrated with wrinkles and/or other particular surface patterns, may be used as optical lens for anti-counterfeit applications [44]. Local heating to selectively release internal elastic stress in pre-deformed vitrimer to achieve so called digital coding has been reported in [53]. For simplicity, a strip is used for illustration in Figure 15. There are many other possible shapes for different applications.

Polymers 2020, 12, 2330 18 of 28

If a vitrimer is transparent, sophisticated patterns, e.g., integrated with wrinkles and/or other particular surface patterns, may be used as optical lens for anti-counterfeit applications [44]. Local heating to selectively release internal elastic stress in pre-deformed vitrimer to achieve so called digital coding has been reported in [53]. For simplicity, a strip is used for illustration in Figure 15. There are many other possible shapes for diﬀerent applications. Since upon heating to above its melting temperature but less than 80 ◦C, this vitrimer-like SMP is transparent and has excellent SME (Figure 14), local preheating may be carried out by laser engraving method in a 3D pixel manner, when the material is transparent.

3.4. VitrimerPolymers Composites 2020, 12, x FOR PEER REVIEW 19 of 29

While mostSince previous upon heating works to onabove vitrimer its melting composites temperature are but focused less than on 80 their°C, this capability vitrimer-like of SMP recycling and self-healingis [transparent54–57], rapid and reshapinghas excellent toSME fix ( aFigure new 14 permanent), local preheating/temporary may be shape carried (including out by laser to reshape the distributionengraving of inclusions)method in a 3D is pixel of great manner, potential when the in material many is engineering transparent. applications. Since the topic of composite is rather wide, in this section, we will only discuss some special 3.4. Vitrimer Composites functions that vitrimer is able to offer based on its reversible network. While most previous works on vitrimer composites are focused on their capability of recycling 3.4.1. Formationand self and-healing Realignment [54–57], rapid/Healing reshaping of to Embedded fix a new permanent/temporary Magnetic Particle shape Chains (including to reshape the distribution of inclusions) is of great potential in many engineering applications. Since the topic of composite is rather wide, in this section, we will only discuss some special As illustrated in Figure 19a, we can load magnetic particles (such as, Fe3O4 powder) into vitrimer functions that vitrimer is able to offer based on its reversible network. via melting mixing or with the help of a solvent (e.g., acetone for the vitrimer-like SMP in Section2). The randomly3.4.1. distributed Formation and particles Realignment form/Healing regular of Embedded chains, if Magnetic a magnetic Particle field Chains is applied when the material is softened (withAs itsillustrated shape /indimension Figure 19a, we maintained) can load magnetic at high particles temperatures (such as, Fe (Figure3O4 powder) 19b). into Of vitrimer course, if local heating is applied,via melting chains mixing are or with only the formed help of withina solvent the (e.g., heated acetone area, for the which vitrimer results-like SMP in tailorablein Section 2). embedded patterns. IfThe conductive-magnetic randomly distributed particles particles form (e.g., regular nickel, chains, Ni, if nano a magnetic/micro field powder) is applied are used,when the the resulted compositematerial becomes is softened electrically (with moreits shape/dimension conductive maintained) along the directionat high temperatures of the particle (Figure chains,19b). Of and thus course, if local heating is applied, chains are only formed within the heated area, which results in may be heatedtailorable via embedded joule heating patterns. for If shape conductive recovery.-magnetic On particles the one (e.g., hand, nickel these, Ni, nano/micro magnetic powder) chains are able to switch theirare used, direction, the resulted if a composite different becomes magnetic electrically field is more applied conductive on the along softened the direction material of the (while the shape maintains)particle chains, (Figure and 19 thusc). may Furthermore, be heated via joule the brokenheating for magnetic shape recovery. chains, On e.g.,the one after hand, shape these memory cycling, canmagnetic be healed chains in are the able same to switch way (Figuretheir direction, 19d). Accordingif a different tomagnetic Figure field9, the is applied magnetic on the field can be softened material (while the shape maintains) (Figure 19c). Furthermore, the broken magnetic chains, applied aftere.g., preheatingafter shape memory or pre-local-heating cycling, can be healed at in lower the same temperatures way (Figure 19 tod). form According chains, to Figure which 9, provide great flexibilitythe magnetic in processing. field can be applied Hence, after the preheating magnetic or particlespre-local-heating can be at lower manipulated temperatures for to reinforcement form along a particularchains, which direction provide globally great orflexibility locally. in Refer processing. to Figure Hence, S11 (Partthe magnetic III of Supplementary particles can beMaterials) for an experimentalmanipulated demonstrationfor reinforcement along of the a directionparticular direction switching globally of theor locally. micro Refer sized to NiFigure powder S11 chains (Part III of Supplementary Materials) for an experimental demonstration of the direction switching embeddedof inside the micro this sized vitrimer-like Ni powder chains TPU. embedded inside this vitrimer-like TPU.

Figure 19. Formation and realignment/healing of magnetic particle chains. (a) Randomly distributed Figure 19. Formation and realignment/healing of magnetic particle chains. (a) Randomly distributed magnetic particles; (b) chains formed after heating and applying a magnetic field; (c) switching of magnetic particles;magnetic particle (b) chains chains upon formed heating after and heating applying and a different applying magnetic a magnetic field; (d) broken field; magnetic (c) switching of magnetic particlechains due chains to, e.g.,upon shape memory heating cycling. and applying a different magnetic field; (d) broken magnetic chains due to, e.g., shape memory cycling. 3.4.2. Rapid Permanent/Temporary Reshaping with Reinforcement Layer in the Middle Glass/carbon fiber-reinforced polymeric composites have been extensively used in many engineering applications [54,55,58,59]. Vitrimer with a fabric layer of glass/carbon fiber in the middle for reinforcement has the advantage of permanent and/or temporary reshaping. Carbon fabric provides the additional convenience in Joule heating of such composites. In Figure 20a, two pieces of

Polymers 2020, 12, 2330 19 of 28

3.4.2. Rapid Permanent/Temporary Reshaping with Reinforcement Layer in the Middle Glass/carbon fiber-reinforced polymeric composites have been extensively used in many engineering applications [54,55,58,59]. Vitrimer with a fabric layer of glass/carbon fiber in the middle for reinforcement has the advantage of permanent and/or temporary reshaping. Carbon fabric provides thePolymers additional 2020, 12 convenience, x FOR PEER REVIEW in Joule heating of such composites. In Figure 20a, two pieces20 of 29 the vitrimer-like SMP strips (thickness of each is 0.3 mm) reported in Section2 are hot-compressed at 110the◦C vitrimer with a- layerlike SMP of commercial strips (thickness glass of fabric each is in 0.3 between mm) reported to form in a Section single piece.2 are hot Subsequently,-compressed at it is wrapped110 °C with around a layer a shaft of commercial and then heated glass fabric in hot in water between of twoto form diff erenta single temperatures, piece. Subsequently, one is around it is wrapped around a shaft and then heated in hot water of two different temperatures, one is around 70 ◦C (Figure 20b) and the other is 90 ◦C (Figure 20(d1)). After cooling back to room temperature, the70 free-standing °C (Figure 20b) shapes and the of other both is of 90 them °C (Figure are about 20(d1 the)). After same cooling (Figure back 20(b,d2)). to room Aftertemperature, reheating the in free-standing shapes of both of them are about the same (Figure 20(b,d2)). After reheating in 80 °C 80 ◦C water, one recovers its original flat shape (Figure 20c), while the other appears slightly twisted water, one recovers its original flat shape (Figure 20c), while the other appears slightly twisted (Figure (Figure 20e). After programming to flatten the twisted piece at about 70 ◦C (Figure 20f), the sample 20e). After programming to flatten the twisted piece at about 70 °C (Figure 20f), the sample returns returns to its twisted shape upon reheating in 80 C water (Figure 20g), which confirms that this new to its twisted shape upon reheating in 80 °C water◦ (Figure 20g), which confirms that this new shape shape is permanent. According to Figure7, we can increase the temperature in Figure 20(d1) to achieve is permanent. According to Figure 7, we can increase the temperature in Figure 20(d1) to achieve better shape fixity. This experiment also demonstrates the feasibility to achieve simultaneous permeant better shape fixity. This experiment also demonstrates the feasibility to achieve simultaneous andpermeant temporary and reshaping temporary based reshaping on the based cross-linking on the cross versus-linking temperature versus temperature relationship relationship (e.g., Figures (e.g.,7– 9 forFigure this particulars 7–9 for this polymer). particular polymer).

FigureFigure 20. 20.Vitrimer Vitrimer reinforced reinforced with with glass glass fabric fabric in the in middle the middle (a standard (a standard ruler is ruler included is included as reference). as reference). (a) Original shape; (b) after wrapping around a shaft and heated in about 70 °C water; (c) (a) Original shape; (b) after wrapping around a shaft and heated in about 70 ◦C water; (c) after heating after heating in 80 °C water; (d1) after wrapping around a shaft and heated in 90 °C water; (d2) free- in 80 ◦C water; (d1) after wrapping around a shaft and heated in 90 ◦C water; (d2) free-standing sample standing sample after shaft is removed; (e) after heating in 80 °C water; (f) after programming after shaft is removed; (e) after heating in 80 ◦C water; (f) after programming (ﬂattening) at about 70 ◦C; (flattening) at about 70 °C; (g) after heating in 80 °C water for shape recovery. (g) after heating in 80 ◦C water for shape recovery. 3.4.3.3.4.3. Comfort Comfort Fitting Fitting of of Wearable Wearable Items Items aroundaround Body-TemperatureBody-Temperature TheThe heating-responsive heating-responsive shape shapememory memory phenomenonphenomenon in in polymers polymers is is mostly mostly based based on on either either the the glassglass transition transition or or melting melting/crystallization./crystallization. As brieﬂybriefly mentioned above, above, the the temperature temperature ranges ranges for for thethe glass glass transition transition of of a a polymer polymer in in the the heatingheating andand cooling processes processes are are about about the the same, same, while while the the meltingmelting temperature temperature range range of of a a polymer polymer isis normallynormally much higher higher than than that that for for crystallization. crystallization. Thus, Thus, we can soften a polymer upon heating to above its melting temperature, and then program it at lower temperatures during the cooling process. If a polymer can be deformed around body temperature or room temperature, while its melting temperature is much higher than the room temperature, this kind of material can be used in comfort fitting around human body temperature. PCL is a good example of such, and has been used extensively in different splints.

Polymers 2020, 12, 2330 20 of 28 we can soften a polymer upon heating to above its melting temperature, and then program it at lower temperatures during the cooling process. If a polymer can be deformed around body temperature or room temperature, while its melting temperature is much higher than the room temperature, this kind of material can be used in comfort fitting around human body temperature. PCL is a good example of such,Polymers and 20 has20, 1 been2, x FOR used PEER extensively REVIEW in different splints. 21 of 29 Comfort fitting is required in many wearable items [38]. For those items in direct contact with humanComfort body, such fitting as splint,is required the meaningin many ofwearable comfort items fitting [38 is]. two-fold.For those items Oneis in comfort direct contact during with fitting andhuman the other body, is such comfort as splint, after fitting.the meaning It is ideal of comfort that fitting fitting is is carried two-fold. out atOne room is comfort temperature during or fitting around humanand the body other temperature. is comfort Apartafter fitting. fromthe It is requirement ideal that fitting on the is fitting carried temperature out at room range, temperature we may or need longaround enough human time body for fitting temperature. as well. Apart PCL isfrom a good the requirement material for splints,on the fitting but lacks temperature flexibility range, [1]. Hence, we aftermay fitting, need long PCL enough splints aretime rigid. for fitting as well. PCL is a good material for splints, but lacks flexibility [1].In Hence, many after wearable fitting, items, PCL splints such asare two rigid. examples presented in Figure 21, one is elbow band and In many wearable items, such as two examples presented in Figure 21, one is elbow band and the other is toe sock-shoes, after modification (either coated with above mentioned vitrimer-like SMP the other is toe sock-shoes, after modification (either coated with above mentioned vitrimer-like SMP or soaked in its acetone solution and then dried in air), a combination of comfort during and after or soaked in its acetone solution and then dried in air), a combination of comfort during and after fitting is achieved. There are more than five minutes for fitting at around body temperature or at room fitting is achieved. There are more than five minutes for fitting at around body temperature or at temperature, while the wearable items are still reasonably soft after fitting. Hence, later on they are room temperature, while the wearable items are still reasonably soft after fitting. Hence, later on they flexibleare flexible and canand becan elastically be elastically taken taken off offwithout without much much di difficulty.fficulty. The programmed programmed shapes shapes are are well well maintainedmaintained even even in in hot hot days, days, unless unless they they are are heated heated to to around around 60 60◦ C°C to to recover recover their theiroriginal original shapeshape for nextfor roundnext round of fitting. of fitting. The original The original shapes shapes of both of examples both examples in Figure in 21 Figure are actually 21 are notactually permanent, not andpermanent, can always and be can reshaped always upon be reshaped heating upon to 110 heating◦C to fixto 110 a new °C permanentto fix a new shape,permanent since shape, the SMP since used herethe is SMP vitrimer. used here is vitrimer.

(A) (B)

FigureFigure 21. 21.Typical Typical instant instant comfort comfort fitting fitting items items (modified (modified with vitrimer-likewith vitrimer SMP).-like SMP). Both were Both produced were byproduced soaking inby the soaking acetone in solutionthe acetone ofthis solution vitrimer-like of this vitrimer TPU and-like then TPU drying and then in air. drying (A) Flexible in air. elbow(A) band.Flexible (a) Original elbow band. (after ( modification);a) Original (after (b1 ,modification);b2) after fitting (b1 (with,b2) flexibility);after fitting ( c1(with,c2) afterflexibility); being elastically(c1,c2) takenafter o beingff, the elastically programmed taken shape off, the maintains. programmed (B) shape Toe sock-shoes. maintains. ( Top:B) Toe original sock-shoes. socks; Top: bottom: original after modificationsocks; bottom: and after fitting modification (the programmed and fitting shape (the programmed remains after shape being remains taken o ffafter). being taken off).

Nowadays,Nowadays, elastic elastic textilestextiles (e.g.,(e.g., spandex) that that are are highly highly stretchable stretchable in inone one or ortwo two inin-planeplane directionsdirections are are widely widely usedused inin sportssports clothes. In In Figure Figure 22 22a, a,the the stress stress versus versus strain strain relationship relationship of a of a typicaltypical commercialcommercial spandex in in uniaxial uniaxial stretching stretching along along both both in in-plane-plane directions directions is plotted. is plotted. We We can can use such a kind of elastic textile to cover one or both sides of a piece of vitrimer-like SMP and then use such a kind of elastic textile to cover one or both sides of a piece of vitrimer-like SMP and then hot-compress into one piece. If the applied temperature is so high that the network is fully eliminated, hot-compress into one piece. If the applied temperature is so high that the network is fully eliminated, vitrimer is able to penetrate into the small gaps within textile, which results in strong bonding vitrimer is able to penetrate into the small gaps within textile, which results in strong bonding between between textile and vitrimer. The stress versus strain relationships of above investigated vitrimer- textile and vitrimer. The stress versus strain relationships of above investigated vitrimer-like SMP and like SMP and its composite (0.3 mm thick with both surfaces covered by spandex and hot compressed its composite (0.3 mm thick with both surfaces covered by spandex and hot compressed at 110 C) under at 110 °C) under cyclic stretching at room temperature are presented in Figure 22b,c, respectively.◦ cyclicThe stretchingtotal thickness at room of the temperature composite is are measured presented and in used Figure in the22b,c, calculation respectively. of the The stress total in thicknessFigure 22c. According to Figure 22a, unless an extremely high pressure is applied during hot compression, which causes significant stretching in the elastic textile, the contribution of the elastic textile on the mechanical strength of the resulted composite is practically negligible. Hence, it is not a surprise that the strength of the composite appears to be reduced. In addition, the elastic textile not only smooths

Polymers 2020, 12, 2330 21 of 28

of the composite is measured and used in the calculation of the stress in Figure 22c. According to Figure 22a, unless an extremely high pressure is applied during hot compression, which causes signiﬁcant stretching in the elastic textile, the contribution of the elastic textile on the mechanical strength of the resulted composite is practically negligible. Hence, it is not a surprise that the strength Polymers 2020, 12, x FOR PEER REVIEW 22 of 29 of the composite appears to be reduced. In addition, the elastic textile not only smooths the stress theversus stress strain versus curve strain (i.e., curve high yielding(i.e., high peaks yielding are removed),peaks are butremoved), also changes but also apparent changes propagation apparent propagationfront movement front in movement the stress in plateau the stress range plateau into gradual range into strain gradual hardening. strain hardening.

(a)

(b) (c)

FigureFigure 22 22.. (a(a) )Uniaxial Uniaxial stretching stretching of of a apiece piece of of typical typical commercial commercial spandex spandex in in two two in in-plane-plane directions. directions. (b(b) )Original Original vitrimer vitrimer-like-like SMP SMP (0.3 (0.3 mm mm thick) thick) in in cyclic cyclic stretching stretching to to 60% 60% maximum maximum strain strain with with an an incrementincrement of of 10% 10% in in each each cycle. cycle. Apparent Apparent necking necking-propagation-propagation phenomenon phenomenon is is observed. observed. (c (c) )Cyclic Cyclic stretchingstretching of of vtrimer vtrimer-like-like SMP SMP (0.3 (0.3 mm mm thick) thick) with with spandex spandex [as [as in in (a (a)])] hot hot-compressed-compressed on on both both sides. sides. StrainStrain rate: rate: 10 10-2-2/s/ s(in (in all all tests tests reported reported here). here).

ThisThis kind kind of of composite composite can can be be either either reshaped reshaped into into another another permanent permanent shape shape after after heating heating to 110 to °C110 (Figure◦C (Figure 23A )23 orA) programmed or programmed for comfort for comfort fitting ﬁtting at body at body temperature temperature or b orelow below after after preheating preheating to lessto less than than 80 °C 80 ◦(FigureC (Figure 23 B23).B). Even Even the the associated associated strain strain is ishigh high inin the the process process of of reshaping, reshaping, the the elastic elastic textiletextile (spandex) (spandex) is is soft soft enough enough to to ensure ensure that that its its influence inﬂuence is is always always minimum. minimum. On On the the other other hand, hand, after reshaping, either permanent or temporary, for thin plastic composites as mentioned here, they have good flexibility and elasticity for comfort wearing after fitting and can be easily removed in a quasi-elastic manner.

Polymers 2020, 12, 2330 22 of 28 after reshaping, either permanent or temporary, for thin plastic composites as mentioned here, they have good ﬂexibility and elasticity for comfort wearing after ﬁtting and can be easily removed in aPolymers quasi-elastic 2020, 12 manner., x FOR PEER REVIEW 23 of 29

(A) (B)

FigureFigure 23. 23.Flexible Flexible composites covered covered with with dual dual-directionally-directionally stretchable stretchable elastic elastic textile textile (spandex, (spandex, in intwo two different different colors colors in in I Iand and II, II, respectively) respectively) on on both both sides. sides. ( (AA)) Rapid Rapid reshaping reshaping into into another another new new permanentpermanent shape. shape. (a(a)) As-fabricated As-fabricated flatflat piece;piece; ((bb––dd)) didifferentfferent newnew permanentpermanent shapes. ( (BB)) Fitting Fitting at at bodybody temperature temperature (temporary (temporary shape). shape). Top: Top: on on wrist wrist for for fitting; fitting; bottom: bottom: two two di differentfferent angles angles of of view view of theof programmedthe programmed free-standing free-standing piece piece after after fitting. fitting.

3.4.4.3.4.4. Controlled Controlled Unfolding Unfolding to to Minimize Minimize ImpactImpact CurvedCurved elastic elastic tapes tapes (similar(similar toto metallicmetallic measuringmeasuring tape) have been used as elastic hinges hinges for for unfoldingunfolding of of deployabledeployable structures, structures, such such as, as, solar solar array array panels, panels, in space in space applications. applications. However, However, huge hugeimpact impact is produced is produced during during unfolding, unfolding, in particular in particular at atthe the end end ofof the the deployment, deployment, which which results results in in continuouscontinuous vibration vibration and and/or/or turbulence turbulence of of the the whole whole structure structure that that requires requires additional additional power power and timeand fortime stabilization for stabilization and re-positioning. and re-positioning. AA very very interesting interesting feature feature of of this this vitrimer-like vitrimer-likeSMP SMP isis thatthat afterafter beingbeing heated to over 80 °C◦C,, it it has has apparentapparent shear-thickening shear-thickening eeffectffect [60[60],], i.e.,i.e., withwith thethe increaseincrease inin shearshear rate,rate, its viscosity dramatically increasesincreases as as well. well. ThisThis eeffect,ffect, which is is far far more more effective effective than than normal normal damping damping when when a polymer a polymer is in is inthe the viscous viscous state, state, helps helps to to minimize minimize the the impact impact in in the the final final deploymentdeployment stage. As As schematically illustratedillustrated in in Figure Figure 24 24a,a, the the middle middle partpart ofof aa piecepiece ofof curvedcurved elasticelastic tape (represented by by a a piece piece of of largelarge sized sized metallic metallic measuring measuring tape) tape) is coveredis covered by aby vitrimer-like a vitrimer-like SMP SMP layer layer (0.3 mm(0.3 mm thick) thick) on its on inner its sideinner [61 side]. After [61] heating. After heating to soften to thesoften vitrimer the vitrimer layer, the layer, tape the is benttape is by bent 90◦. by Subsequent 90°. Subsequent cooling cooling results inresults hardening in hardening of the vitrimer of the layer, vitrimer which layer, is designed which is to designed be thick enoughto be thick to have enough enough to have stiffness enough after coolingstiffness/hardening after cooling/hardening to prevent the elastic to prevent tape from the recovering elastic tape (Figure from 24recb,overing bottom ( piece).Figure Upon24b, bottom heating (e.g.,piece). via Upon Joule heating heating) (e.g., of the via vitrimerJoule heating) layer, of its the restraint vitrimer on layer, the elastic its restraint tape is on gradually the elastic removed. tape is Hence,gradually recovery removed. (deployment) Hence, recovery occurs, but(deployment) the unfolding occurs, speed but is the slow unfolding because ofspeed the shear-thickeningis slow because effofect the of shear this vitrimer-thickening layer effect at highof this temperatures. vitrimer layer Thick at high elastic temperatures. fabric (in pinkThick color elastic in fabric Figure (in 24 pinkb) is usedcolor to in cover Figure the 24 bentb) is areaused forto cover good the thermal bent area insulation for good to savethermal energy insulation in the to case save of energy Joule heating in the forcase activation. of Joule heating for activation.

Polymers 2020, 12, 2330 23 of 28 Polymers 2020, 12, x FOR PEER REVIEW 24 of 29

FigureFigure 24.24. Controlled unfolding upon heating to minimize impact during deployment of elastic hinge. ((aa)) IllustrationIllustration ofof hingehinge withwith vitrimervitrimer layerlayer only;only; ((bb)) actualactual hingeshinges (pink(pink colorcolor fabricfabric isis usedused toto covercover bothboth sidessides ofof thethe bentbent areaarea forfor thermalthermal insulation).insulation).

3.5.3.5. Additional Permanent Cross-LinkingCross-Linking 3.5.1. Reconfigurable Two-Way Actuation 3.5.1. Reconfigurable Two-Way Actuation Two-way (or called reversible) actuation has been achieved in some polymeric materials based Two-way (or called reversible) actuation has been achieved in some polymeric materials based on melting/crystallization transition [62–65]. These materials are able to switch between two shapes, on melting/crystallization transition [62–65]. These materials are able to switch between two shapes, one corresponding to the high temperature shape and the other to the low temperature shape. Same as one corresponding to the high temperature shape and the other to the low temperature shape. Same shape memory alloy-based actuators [66], an elastic stress field, either inside of the polymer or as shape memory alloy-based actuators [66], an elastic stress field, either inside of the polymer or outside (in the form of elastic spring, which also includes elastic structures and constant/variable outside (in the form of elastic spring, which also includes elastic structures and constant/variable load), is required for automatic re-programming during thermal cycling. The former (with internal load), is required for automatic re-programming during thermal cycling. The former (with internal elastic stress field) is called material two-way actuation, and the latter is called mechanical two-way elastic stress field) is called material two-way actuation, and the latter is called mechanical two-way actuation [67]. actuation [67]. According to Figure9, we may heat a pre-deformed sample to partially remove the network, According to Figure 9, we may heat a pre-deformed sample to partially remove the network, and then cool it to form new network. Consequently, there are two networks in the material, one is the and then cool it to form new network. Consequently, there are two networks in the material, one is remaining of the previous network, which is pre-strained, and the other is newly formed, which is the remaining of the previous network, which is pre-strained, and the other is newly formed, which strain free. Hence, an internal elastic stress field is introduced into the material. Same as the material is strain free. Hence, an internal elastic stress field is introduced into the material. Same as the material two-way SME in shape memory alloys [67], such a kind of internal stress field is required for two-way two-way SME in shape memory alloys [67], such a kind of internal stress field is required for two- (reversible) actuation of polymers upon thermal cycling without applying any external loading [68]. way (reversible) actuation of polymers upon thermal cycling without applying any external loading However, practically it might be difficult to precise control the actual fraction of the new network, [68]. However, practically it might be difficult to precise control the actual fraction of the new since there are many processing parameters involved. network, since there are many processing parameters involved. So far, the most applicable approach to realize material two-way actuation in polymers is to So far, the most applicable approach to realize material two-way actuation in polymers is to introduce two networks into a semi-crystalline polymer in two steps during curing [69]. The second introduce two networks into a semi-crystalline polymer in two steps during curing [69]. The second network is introduced into the polymer after the material is deformed, so that the first network is network is introduced into the polymer after the material is deformed, so that the first network is pre-strained, while the 2nd network is strain free. The resulted polymer is able to switch between two pre-strained, while the 2nd network is strain free. The resulted polymer is able to switch between two shapes upon thermal cycling. But both high temperature and low temperature shapes are permanent. shapes upon thermal cycling. But both high temperature and low temperature shapes are permanent. Without modifying the chain structure, this vitrimer-like SMP investigated in Section2 can be Without modifying the chain structure, this vitrimer-like SMP investigated in Section 2 can be cross-linked with dicumyl peroxide (DCP) to form interpenetrating polymer network (IPN) [70]. cross-linked with dicumyl peroxide (DCP) to form interpenetrating polymer network (IPN) [70]. The The resulted polymer is thermoset and has excellent heating-responsive SME even being programmed resulted polymer is thermoset and has excellent heating-responsive SME even being programmed at at 100 C. Since now there are two networks inside of the polymer, one is permanent (IPN) and the 100 °C◦. Since now there are two networks inside of the polymer, one is permanent (IPN) and the other other is reversible (vitrimer), we can reprogram the network associated with the vitrimer-like behavior. is reversible (vitrimer), we can reprogram the network associated with the vitrimer-like behavior. Hence, the two-way actuation of the resulted polymer is re-configurable. Hence, the two-way actuation of the resulted polymer is re-configurable. 3.5.2. Rapid Additive Manufacturing in Solid State 3.5.2. Rapid Additive Manufacturing in Solid State Current technologies for additive manufacturing (also known as 3D printing) are mostly developed Current technologies for additive manufacturing (also known as 3D printing) are mostly for on-earth environment, which not only relies on the gravitational force, but also requires minimum developed for on-earth environment, which not only relies on the gravitational force, but also vibration/disturbance. However, in many situations, e.g., on space missions where gravity is close to requires minimum vibration/disturbance. However, in many situations, e.g., on space missions zero [71,72], while on air/sea missions (on-board of airplanes/ships) where severe random vibration where gravity is close to zero [71,72], while on air/sea missions (on-board of airplanes/ships) where severe random vibration is unavoidable, it is very hard to use liquid/powder form of raw materials in 3D printing. Even for volumetric additive manufacturing via tomographic reconstruction [73],

Polymers 2020, 12, 2330 24 of 28 is unavoidable, it is very hard to use liquid/powder form of raw materials in 3D printing. Even for volumetric additive manufacturing via tomographic reconstruction [73], which is one of the recently developed approaches for rapid 3D printing, good accuracy in cross-linking of polymeric liquid is a challenge, if the printing platform is unstable (such as, on air/sea missions) that normal vibration isolation tables cannot handle. Vitrimer-like SMPs appear to be the right material to realize rapid 3D printing in solid state for above mentioned application scenarios [74], i.e., instead of cross-linking liquid polymers as in current volumetric additive manufacturing via tomographic reconstruction [73], vitrimer-like SMPs can be cross-linked at high temperatures, while they are still in the solid state and transparent. Cross-linking might be UV activated or laser-induced heat activated on space missions or air/sea missions. After cross-linking, the printed model is thermoset, while the uncross-linked part is still vitrimer-like, which can be removed upon heating to the easy to ﬂow temperature (e.g., 120 ◦C for this vitrimer TPU) or washed away by a special solvent (e.g., acetone for this vitrimer TPU). Any deformation in the model induced during the process to remove the uncross-linked part can be eliminated via activating the SME. Refer to Figure S12 in Part III of Supplementary Materials for a schematic illustration of the major steps in 3D printing.

4. Conclusions Vitrimer-like shape memory polymers (SMPs) combine the vitrimer-like behavior (associated with dissociative covalent adaptable networks) and shape memory phenomenon. This kind of polymers can be utilized to achieve many novel functions that are diﬃcult to be realized by conventional polymers. In this paper, we used a commercial polymer to demonstrate how to characterize the vitrimer-like behavior based on the heating-responsive SME. The relationship between the shape memory performance, which is associated with the reversible cross-linking, and pre-process (in particular, thermal history) was obtained for this polymer. Such a kind of information provides the foundation for a series of examples presented here to reveal the potential applications of vitrimer-like SMPs and their composites. It can be concluded that apart from conventional applications, such as, re-processability and heat-assisted self-healing, the vitrimer-like feature not only enables many new ways in reshaping polymers (inside/outside, temporarily/permanently, at macroscopic/microscopic scale), but also can bring forward new approaches in manufacturing, such as, rapid 3D printing in solid state.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/10/2330/s1, Figure S1: Shape memory effect in thermoset polycaprolactone (PCL) [cross-linked with 10 wt. % of benzoyl peroxide (BPO)], Figure S2: Internal stress fields in commercial thermo-plastic PS petri dish (a) and PP box (b), Figure S3: Photo-elasticity images of tough hydrogel (thermoset), Figure S4: Differential scanning calorimetry (DSC) result between 100 C and 200 C at temperature ramping rate of 10 C/min using Netzsch DSC 214 − ◦ ◦ ◦ machine (NETZSCH Group, Germany), Figure S5: 1H NMR spectrum in acetone-d6, Figure S6: XRD spectra recorded at 25 ◦C, 80 ◦C and 110 ◦C upon heating, Figure S7: FTIR spectrum recorded at room temperature, Figure S8: FTIR spectra upon heating from 30 ◦C to 120 ◦C (a) in the N-H stretching region and (b) in the C=O stretching region, Figure S9: Schematic illustration of the phase transition process upon heating, Figure S10: Schematic comparison of surface wrinkles formed by surface etching of thermo-plastic (c) and surface preheating of vitrimer (d), Figure S11: Switching of Ni micro powder chains inside vitrimer thin film (about 0.1 mm, produced from acetone solution of this vitrimer-like PU mixed with Ni powder), Figure S12: Rapid 3D printing in solid state on space/sea missions. Author Contributions: T.X.W., H.M.C., A.V.S., J.L., Y.C. and Y.K. conducted some of the experiments. T.X.W., J.L. and R.X. helped in data analysis and discussions. T.X.W., H.M.C. and W.M.H. prepared the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This project is partially supported by the National Natural Science Foundation of China (Grant No. 11828201). Conflicts of Interest: The authors declare no conflict of interest, and manuscript is approved by all authors for publication. Polymers 2020, 12, 2330 25 of 28

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