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

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.

responsive SME (after programming via stretching above its melting temperature); (c1-c4): chemo-responsive SME [after programming via stretching above its melting temperature (c1), and then immersing in acetone (c2); (c3-c4) drying in air causing the sample curling up due to rapid and uneven evaporation of acetone]; and (d) after further heating [of (c4)] in hot water to release the internal stress and subsequently to recover its original shape. 3 -Internal stress in thermo-plastic and thermoset • Thermo-plastic (without cross-linking) The internal stress in two commercial thermo-plastic items, namely polystyrene (PS) petri dish and polypropylene (PP) box, is examined by conventional photo-elasticity test [1] in Figure S2 (including direct observation via naked eyes of the petri dish). The colourful image of the photoelasticity test reveals the internal stress field produced by injection moulding. For both pieces, we used a lighter for local heating at one single point, which results in an elliptical shaped small area, indicating the change in the internal stress field. A closer-look reveals that the surface of the locally heated area becomes uneven (e.g., in Figure S2a2) due to overheating induced melting.
• Thermoset (cross-linked) Figure S3(a) is a piece of dried tough hydrogel, which is thermoset and with excellent SME [2].
Although the as-fabricated wet piece is flat [3], it becomes curly as a result of the internal stress that is built up during drying in air [4], which is similar to drying acetone wetted cross-linked PCL in Figure S1(c2-c4). We can flatten it after heating to soften it, since dry hydrogel is essentially a polymer and with the heating/water-responsive SME [5]. The photo-elasticity image of the flattened piece becomes more colourful ( Figure S3b). A droplet of water placed on the surface of the sample is able to release the internal stress underneath it, so that after drying, a circular shaped less colourful area is observed ( Figure S3c is fully dried after 1 st droplet is applied and Figure S3d is fully dried after the 2 nd droplet is applied). As a thermoset, its internal stress field can be altered, while the cross-linking prevents the polymer from flowing.

Part II
A series of additional experiments were conducted in order to identify the chemical structure of this material and subsequently to find the underlying mechanism for the observed vitrimer-like behaviour. -

Structural characterization
The structure of this material was analysed by nuclear magnetic resonance (NMR, Bruker Unity-400). Tetramethylsilane (TMS) was used as the internal standard and acetone-d6 was used as the solvent.
PBA segments form the soft domain, while MDI and BDO segments form the hard segments.
Based on the areas underneath these peaks, the ratio of soft segments to hard segments (m/n) is determined to be about 28/3. X-ray diffractometry (XRD) was further used to study the aggregation structure. DSC test was carried out by Rigaku Smartlab with a temperature variable sample cell. The scanning range was 10°−40°. The spectra were collected upon heating the material to 25 o C, 80 o C and 110 o C.
As shown in Figure S6, at 25 o C, obvious diffraction is observed at three 2θ values of 21.76°, 22.42° and 24.02°, which attributes to the crystalline PBA segments in the soft domain. Upon heating to 80 o C, the diffraction corresponding to crystalline PBA disappears, and amorphous diffraction is observed, which agrees with the DSC result in Figure S4, as the material fully melts at 80 o C.
When the material is heated to 110 o C, the intensity of amorphous diffraction decreases, which 8 means that the aggregation interaction in the hard segments decreases when the temperature is increased from 80 o C to 110 o C. As determined by the result of the above 1 H NMR test, the hard segments are formed by MDI and BOD. We may conclude that the hydrogen bonding interaction formed by N-H and C=O in urethane affects the aggregation interaction in the hard domain.  -3200 cm -1 ) and the C=O stretching (1860 cm -1 -1620 cm -1 ) regions.
As the focus of this study is on the hydrogen bonding interactions in the hard domain, two spectral regions are of our interest: the N-H stretching vibrations at 3500 cm -1 -3200 cm -1 , and the C=O stretching vibrations at 1860 cm -1 -1620 cm -1 .

• N-H stretching region
The N-H stretching region in the FTIR spectra presented in the transmittance scale and recorded upon heating is shown in Figure S8 The infrared bands at 3446 cm -1 and 3340 cm -1 are assigned to be the stretching modes of the "free" and hydrogen bonded N-H groups, respectively, at 30 o C. Upon heating from 30 o C to 120 o C, 12 while the peak position for the "free" N-H stretching band maintains almost at the same position of 3446 cm -1 , the intensity for the "free" band keeps the same as the temperature is increased from 30 o C to 120 o C. This is similar to what is reported in [6]. • C=O stretching region According to above structural analysis, the C=O stretching bands should include two components, namely, the ester C= O stretching and the urethane C=O stretching. The latter is further composed of three subcomponents corresponding to the "free", disordered hydrogen bonded, and ordered hydrogen bonded C=O stretching, respectively. The "free" C=O groups are the ones forming nonhydrogen bonds, whereas the disordered and ordered hydrogen bonded ones are the hydrogen bonded groups associated with the amorphous phase and crystalline phase, respectively. Figure S8(b) reveals the FTIR spectrum in the C=O stretching region in the transmittance scale upon heating. Only a wide band is observed at 1820 cm -1 to 1640 cm -1 , which may attribute to the overlapping of the ester C=O stretching in the range of 1780 cm -1 to 1740 cm -1 and the urethane C=O stretching in the range of 1740 cm -1 to 1660 cm -1 . Upon heating, the shift of peak position and the change in the shape of the peak are apparent. The band at 1720 cm -1 is attributed to the 13 "free" C=O groups, the band at 1700 cm -1 is the disordered hydrogen bonded C=O, and the band at 1685 cm -1 is the ordered hydrogen bonded C=O. The bands at 1700 cm -1 and 1685 cm -1 can be easily identified upon heating to 60 o C, and then both bands gradually disappear upon further heating, which means that the intensity of hydrogen bonded C=O decreases.
-Underlying mechanism for vitrimer-like behavior At this point, we may conclude that the hydrogen bonding interactions in the hard domain are kept constant upon heating to 60 o C. Upon further heating to above 80 o C, the interactions gradually decrease.
Now, the phase transition process upon heating, which is associated with the observed vitrimerlike behaviour in this material, may be schematically sketched as shown in Figure S9. At room temperature (around 25 o C), the soft segment of this material is crystalline, while the hard segment is glassy. Upon heating to around 60 o C, the soft segment melts, and the hard segment remains glassy. Hence, the material is similar to a semi-crystalline polymer [7], and the hard segment serves as the elastic part, while the soft segment functions as the transition part for the heating-responsive SME [8]. Upon further heating to over 80 o C, the hard segment gradually becomes viscous, so that the shape recovery ratio gradually decreases. At around 120 o C, the material turns out to be thermoplastic, since now it is able to flow even under the gravitational force. Upon cooling, the hard segment gradually becomes glassy, and the soft segment starts to crystallize at lower temperatures.
14 Figure S9 Schematic illustration of the phase transition process upon heating.
In addition, instead of heating, acetone is able to eliminate the bonding between urethane C=O and N-H, resulting in the material to be dissolved in acetone. Upon drying, acetone evaporates, so that the bonding between urethane C=O and N-H is re-formed. after heating for shape recovery.
In Figure S10, we schematically compare the difference between surface etching (thermo-plastic) approach and surface preheating (vitrimer) approach. While etching smooths the surface of 16 thermo-plastic ( Figure S10c1), surface preheating of vitrimer, if processed carefully according to the features of a particular material, is able to mostly keep the initial surface feature (pre-deformed) ( Figure S10d1). Therefore, the resulted surface feature of the surface etched thermo-plastic is parallel wrinkles only ( Figure S10c2), while the final surface feature of the surface preheated vitrimer is a combination of the initial surface pattern (pre-deformed) and newly formed parallel wrinkles ( Figure S10d2).