Solvent-Free One-Shot Synthesis of Thermoplastic Polyurethane Based on Bio-Poly(1,3-propylene succinate) Glycol with Temperature-Sensitive Shape Memory Behavior.

In this work, a new family of fully biobased thermoplastic polyurethanes (TPUs) with thermo-induced shape memory is developed. First, a series of TPUs were successfully synthesized by the one-shot solvent-free bulk polymerization of bio-poly(1,3-propylene succinate) glycol (PPS) with various molecular weights (M n = 1000, 2000, 3000, and 4000), 1,4-butanediol (BDO), and 4,4'-methylene diphenyl diisocyanate (MDI). These polyurethanes (PUs) are denoted as PPS-x-TPUs (x = 1000, 2000, 3000, and 4000), where x represents the M n of PPS in the polymers. To determine the effect of the molecular weight of the soft segment of PU, all PPS-TPUs were formed with the same hard segment content (32.5 wt %). The soft segment with high molecular weight in PPS-4000-TPU caused a high degree of soft segment entanglement and formed many secondary bonds. PPS-4000-TPU exhibited better mechanical (tensile strength: 64.13 MPa and hardness: 90A) and thermomechanical properties (maximum loading: 2.95 MPa and maximum strain: 144%) than PPS-1000-TPU. At an appropriate shape memory programming temperature, all synthesized PPS-x-TPUs exhibited excellent shape memory behaviors with a fixed shape rate of >99% and a shape recovery rate of >86% in the first round and 95% in the following rounds. Therefore, these bio-TPUs with shape memory have potential for use in smart fabrics.

Introduction

Nowadays, significant attention is being paid to the study and development of “smart” materials or stimulus-responsive shape memory polymers (SMPs). SMPs reversibly change shape from a deformed shape to their original shape in response to an externally imposed stimulus, which may be stress, temperature, light, humidity, solvent, a change in pH, or an electric or magnetic field.[1−5] SMPs have many technical advantages over other shape memory materials and have an extensive range of applications in high-performance textiles, biomedical devices, actuators, packing films, heat-shrinkable tubes, and many others.[6−10] Polymers with shape memory behavior must have cross-links at net points that determine their original shape and switching segments with a phase transition temperature (Ttrans) that fixes their makeshift shape. The rubber elasticity of the cross-links that are created by chemical bonds or physical interactions strongly affects the properties of SMPs and is the key driver of polymer shape recovery.[11,12] Hence, the shape memory behaviors depend mainly on the microphase separation of the hard and soft segments, which is determined by molecular properties (the composition of the hard and soft segments and the molecular weight).

Polyurethanes (PUs) have been developed as the perfect material for use in shape memory applications owing to their biocompatibility, biodegradability (polyester-based), and easy tuning of their structural properties and Ttrans by manipulating their segmental composition.[13,14] Specifically, shape memory PUs (SMPUs) that are formed from renewable monomers have attracted a great deal of interest because of their technological advantages over petrochemical-based PUs with regard to sustainability, biocompatibility, and environmental friendliness.[15,16] Recently, most bio-SMPUs have included natural oil-based polyols as their soft segment, and many research groups have synthesized some plant oil-based bio-SMPUs.[17] Remarkably, biopolyols that are derived from biobased 1,3-propanediol (PDO) monomers seem to be more attractive because of their affordability because PDO is manufactured from renewable resources.[18] Notably, all renewably sourced poly(1,3-propylene succinate) glycols (PPS polyols) that are derived from cornstalk-based PDO and succinic acid have been commercialized by Tai Chin Chemical Industry Co., Ltd.

PPS polyols are entirely biobased polyester-based (polyethylene terephthalate-based) polyols and have well-known advantages of biodegradability, biocompatibility, and transparency.[19−21] The development or application of conventional organic solvent-based SMPUs involves the acute emission of volatile organic compounds, so ecofriendly and biobased SMPUs that are not associated with the emission of volatile organic compounds have been developed in response to an increasing awareness of conservational degradation and the need for environmental protection. The solvent-free synthesis of thermoplastic PUs (TPUs) has been less studied than the synthesis of solvent-based PUs and waterborne PUs.[22] In particular, the synthesis of thermal-responsive shape memory solvent-free bio-PPS-based TPUs by one-shot polymerization has never been discussed. Prominently, one-shot polymerization is commonly used as an industrial application. Solvent-free and one-shot polymerization synthesis of TPUs has substantial theoretical and practical value. TPU is a class of PU that is synthesized using polyols as the soft segment, chain extenders, and diisocyanate as the hard segments.[23,24] TPUs have various applications, including those in biomedical devices, drug carriers, tissue engineering scaffolds, anti-biofouling, and insulation. Nonetheless, the high flammability of TPUs and its ability to create large quantities of toxic gases and smoke during its combustion restrict its widespread application in some aforementioned fields. However, the aromatic backbone improves the thermal stability and char formation of TPUs [TPUs based on 4,4′-methylene diphenyl diisocyanate (MDI)] and reduces smoke production compared to aliphatic TPUs [TPUs based on hexamethylene diisocyanate (HDI) and pentamethylene diisocyanate (PDMI)]. This is because HDI and PDMI are more volatile and can cause lung injury. Rather, MDI is more stable than both owing to its aromatic structure.[25] In addition, several studies have been published on the development of biodegradable TPUs with diverse properties despite the fact that SMPUs based on renewable bio-PPS polyols are rare.

For the abovementioned reasons, this study focuses on the solvent-free one-shot synthesis of thermally activated shape memory TPUs (SMTPUs), PPS-x-TPUs, from 100% renewably resourced PPS polyols (Mn = 1000, 2000, 3000, and 4000), 1,4-butanediol (BDO), and MDI. The hard segment content was fixed to observe the effect of the soft segment content on shape memory behavior. A hard segment of over 50 wt % causes the soft segment to disperse in the hard segment, affecting the shape memory properties and resulting in permanent deformation to a degree that increases with its weight percentage.[26] To solve this problem, the hard segment content of all samples was fixed at 32.5 wt %. Increasing the mole proportion of the hard segment increased the degree of entanglement of the soft segment and the strength of the secondary bonds in a manner that depended on the molecular weight of the soft segment. The effect of the molecular weight of the soft segment on the mechanical, thermomechanical, and shape memory properties of these PPS-x-TPUs was investigated in detail. This study opens up the novel prospect of the development and extensive use of truly ecologically friendly shape memory TPUs.

Experimental Section

Materials

Poly(1,3-propylene succinate) glycol (PPS) (Mn = 1000, 2000, 3000, and 4000 g/mol) was kindly provided by Tai Chin Chemical Industry Co., Ltd., Taiwan. MDI and BDO were kindly provided by Lidye Chemical Co., Ltd., Taiwan. Dibutyltin dilaurate (DBTDL or T12) was purchased from Alfa Aesar. Dimethylacetamide (DMAC, 99.8%) and N,N-dimethylformamide (DMF, 99.8%) were purchased from Sigma-Aldrich.

Synthesis of PPS-x-TPUs

PPS-x-TPUs were synthesized using a solvent-free one-shot polymerization procedure. To remove water, PPS polyols and the chain extender BDO were heated in a vacuum oven at 80 °C for 4 h. MDI was melted in the oven at 60 °C for 2 h. In a typical synthesis of PPS-1000-TPU, PPS polyols (Mn = 1000) and the chain extender BDO were thoroughly mixed using a mechanical stirrer in polypropylene beakers, which were then placed in the oven to maintain the temperature at 100 °C. Next, MDI and catalyst T12 were added to the reaction mixture and mechanically stirred at room temperature for 30 s. The reaction mixture was poured into a Teflon-coated pan and cured in an oven at 120 °C for 3 h. The final product was dissolved in DMAC and poured into a mold and evaporated at 80 °C for 24 h to obtain the film. PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU were synthesized using the same procedure as for PPS-1000-TPU, mentioned above. Table presents the procedure in detail.

Table 1

Composites of PPS-x-TPUs

PPS-x-TPUs	[A]/[B]/[C]a	HSb wt %
PPS-1000-TPU	1.60:1.00:0.60	32.5
PPS-2000-TPU	2.94:1.00:1.94	32.5
PPS-3000-TPU	4.40:1.00:3.40	32.5
PPS-4000-TPU	5.97:1.00:4.97	32.5

[A] = [MDI hard segment], [B] = [PPS-polyol soft segment], and [C] = [BDO chain extender hard segment].

Hard segment.

Instruments

Fourier Transform Infrared Spectrometry

The chemical structure of the synthesized PPS-x-TPUs was characterized by 16 scans using a PerkinElmer Spectrum One FT-IR spectrometer (ATR testing model) in the transmission mode in the range 4000–650 cm–1 with a resolution of 4 cm–1.

1H Nuclear Magnetic Resonance Spectroscopic Analysis

1H nuclear magnetic resonance (NMR) spectra of the samples were recorded with a Bruker AVANCE-III 400 MHz spectrometer (Billerica, MA, USA). The samples were dissolved in dimethyl sulfoxide solvent, in 5 mm NMR tubes, at room temperature.

Gel Permeation Chromatography

Molecular characteristics [number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI)] were measured by gel permeation chromatography (GPC) using a Viscotek GPCmax VE-2001. DMF was used as a mobile phase solvent at a flow rate of 1 mL min–1 at 60 °C. The column was 300 × 810 mm.

Melting Flow Index

Melting flow index (MI) experiments were performed using a Gotech GT-7100-MI instrument. The temperature of the MI heater was set to 200 °C, and the samples were loaded at 2.16 kg. The testing method was consistent with the JIS-K7210 standard.

Shore Hardness Test

A Shore-A-type hardness tester (TECLOCK GT-7100-MI) was used to test the hardness of PPS-x-TPUs. The bulk size of the standard samples of PPS-TPUs was 30 × 30 × 3 mm.

Differential Scanning Calorimetry

The melting temperature (Tm) and glass transition temperature (Tg) were determined by differential scanning calorimetry (DSC) using a HITACHI SIINT SII X-DSC7000 instrument. The samples (8–10 mg) were sealed in an aluminum pan, maintained at −80 °C for 10 min, and then heated to 220 °C at a ramping rate of 10 °C min–1. After 3 min, a cooling scan from 220 to −80 °C was performed at a cooling rate of −10 °C min–1. Throughout the process, the samples were in a purging stream of nitrogen gas. Tm and Tg were measured from the second scan.

Thermogravimetric Analysis

The thermal stability of PPS-x-TPUs (samples ranging from 5 to 10 mg) was measured by thermogravimetric analysis (TGA) using a NETZSCH TG 209 F3 instrument. TGA began at 35 °C and proceeded with heating to 600 °C at a ramping rate of 10 °C min–1. The variation of weight with temperature was recorded.

Tensile Testing Machine

Tensile tests of dumbbell PPS-x-TPU samples were carried out using a Cometech A2 universal tensile testing machine following the ASTM D412-C standard. Test specimens were strained at a crosshead speed of 500 mm/min at room temperature until they broke. The test comprised three specimens, whose tensile strength and elongation at breakage were recorded.

Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) was conducted using a HITACHI DMS 6100 instrument in the tension mode with a dynamic stretching of 0.5%. The temperature was increased ranging from −100 to 100 °C at a rate of 5 °C min–1, and the frequency of the instrument was 1 Hz. The sample size was 40 mm long × 10 mm wide × 0.25 mm thick.

Shape Memory Property Testing

A shape memory property test was conducted using a HITACHI DMS 6100 in the SS-control mode. The PPS-x-TPU film sample was 40 mm long × 10 mm wide × 0.25 mm thick. The temperature range for testing shape memory was based on an analysis of E′ (storage modulus), which yielded the temperatures associated with the glassy state (fixing temperature) and rubbery state (recovery temperature) of PPS-x-TPUs. First, the sample was held at the deformation temperature (recovery temperature) for 5 min; it was then strained to its maximum strain (below the yield strain) at a strain rate of 300 mN min–1 at the deformation temperature. When the sample strain exceeded the maximum strain, nonreversible deformation occurred and the sample did not recover. Second, the sample was cooled to the fixing temperature at a cooling rate of 3 °C min–1 under a strain and the fixing temperature was then maintained for 5 min. Third, the force was released at a rate of −300 mN min–1 at the fixed temporary shape temperature. Finally, the sample was heated to the recovery temperature at a heating rate of 3 °C min–1 and this temperature was maintained for 5 min. This testing cycle was repeated three times for each sample to determine the reproducibility of the results concerning shape memory properties. The shape memory is evaluated using the strain fixity (Rf) and strain recovery (Rr) ratios, which are defined as follows.[27−29]where N is the number of cycles; εl is the maximum strain of the loaded sample at the fixing temperature; εu is the residual strain of the unloaded sample; and εf is the final strain when the stress on the sample is zero at the recovery temperature.

Results and Discussion

Chemical Structure of PPS Polyols and PPS-x-TPUs

PPS-x-TPUs (x = 1000, 2000, 3000, and 4000) were synthesized by solvent-free one-shot bulk polymerization using PPS (Mn = 1000, 2000, 3000, and 4000 g/mol), MDI, and BDO. Figure shows the chemical structures of the PPS polyol and PPS-x-TPUs, which were confirmed by Fourier transform infrared (FT-IR) analyses. Figures and S1 also show the FT-IR spectrum of the PPS polyols, which includes a bimodal absorption band at 3750–3250 cm–1, which corresponds to the stretching vibration of the −OH group. The spectrum also includes bands at 2950 and 2850 cm–1 due to the symmetric and asymmetric stretching vibrations of the −CH2– group, respectively. The absorption bands at 1750 and 1235 cm–1 were ascribed to the C=O and C–O stretching vibrations of the ester unit, respectively.[30,31]

Figure 1

Chemical structure of (a) bio-PPS polyols and (b) bio-PPS-x-TPU.

Figure 2

FT-IR spectra of biobased PPS polyols.

Figures and S1 display the FT-IR spectra of the synthesized PPS-x-TPUs. These spectra are very similar because of the same chemical structure; they exhibit stretching and bending vibrational −NH absorption bands at 3330 and 1530 cm–1, which are characteristic of urethane groups, and the vibration band at 2730 cm–1, which is characteristic of N=C=O, disappears and becomes that of the urethane group, indicating the successful reaction of the −OH group with the −NCO group. The appearance of new absorption peaks at 1596 and 1460 cm–1 reveals the presence of a benzene group. The spectrum also includes absorption bands at 1410 cm–1 due to the stretching vibrations of the C–C aromatic benzene ring group. All of the characteristic absorbance peaks of PPS and PPS-x-TPUs are clearly present. Besides, the chemical structure of the PPS polyols and PPS-x-TPUs was further scrutinized by 1H NMR analysis (Figures S2 and S3). These results indicate that all MDI reacted with PPS polyols and the chain extender.

Figure 3

FT-IR spectra of biobased PPS-x-TPUs.

Analysis of MI

The MI of PPS-x-TPUs was obtained according to the JIS-K7210 standard. The melted PPS-x-TPUs passed through a die (diameter = 2.095 mm) in 10 min at a set temperature of 200 °C with a sample loading of 2.16 kg. Table presents the MI of the PPS-x-TPUs. The MI of the PPS-x-TPUs decreased strongly as the molecular weight of the PPS polyols increased. PPS-1000-TPU had the lowest molecular weight and PPS-4000-TPU had the highest molecular weight of all of the polymers. The MI and molecular weight were positively correlated. PPS-x-TPUs with a high molecular weight were formed, and they were expected to exhibit high degrees of molecular chain entanglement. Hence, the MI of PPS-x-TPUs decreased as the molecular weight increased. Herein, the hard segment weight percentage of every PPS-x-TPU was fixed at 32.5%. However, as the molecular weight of PPS polyols increased from Mn = 1000 to 4000, the proportional amount of the hard segments ([A] and [C]) increased (from 1.60 to 5.97 and 0.60 to 4.97 M ratio). A larger proportional amount of the hard segment is responsible for a more exothermic one-shot bulk polymerization reaction, explaining why the molecular weight of PPS-x-TPUs increases with the molecular weight of the PPS polyols. Accordingly, the MI of the PPS-4000-TPU is 0.5 g 10 min–1, which limits the polymer processing. A larger hard segment mole ratio is associated with more secondary bonds (such as interhydrogen bonds, benzene π–π stacking, and van der Waals’ bonds) and more entanglements, disfavoring flow of the molten PPS-4000-TPU and resulting in a lower MI.

Table 2

MI, Number Average Molecular Weight (Mn), Weight Average Molecular Weight (Mw), and Polydispersity Index (PDI) of Biobased PPS-x-TPUs

PPS-x-TPUs	MI (g 10 min–1 at 200 °C)	Mn (g/mol)	Mw (g/mol)	PDI
PPS-1000-TPU	7.5	94,930	132,546	1.396
PPS-2000-TPU	3.5	147,327	222,710	1.512
PPS-3000-TPU	4	124,563	185,757	1.491
PPS-4000-TPU	0.5	147,247	265,507	1.803

Gel Permeation Chromatography

The Mn, Mw, and PDI of the PPS-x-TPUs were determined by GPC analysis (Figure S4), and the relevant data are summarized in Table . The Mn and Mw of PPS-x-TPUs were in the ranges 94,930–147,247 and 132,546–265,507, respectively. PPS-4000-TPU has a higher molecular weight than PPS-1000-TPU because it had a higher hard segment mole ratio ([A] is 5.97 and [C] is 4.97). The PDI values of PPS-x-TPUs were in the range 1.396–1.803, which are highly consistent with the molecular weights. However, all of the PPS-x-TPUs had a PDI below 2.0, which is nevertheless high enough for polymer processing (injection and extrusion) and various applications.

Mechanical Properties

Figure and Table present the mechanical properties and typical stress–strain curves of the PPS-x-TPUs. From Table , the tensile strength of the PPS-x-TPU samples gradually increases with the molecular weight of PPS polyol, from 25.88 ± 0.03 for PPS-1000-TPU to 64.13 ± 4.35 for PPS-4000-TPU, probably owing to many secondary bonds and the high molecular chain entanglement. However, elongation at breakage declines from 862.95 ± 18.65 for PPS-1000-TPUs to 783.21 ± 16.14 for PPS-4000-TPU. The decrease in elongation at breakage with the increasing molecular weight of PPS polyol is attributable to the many entanglements of PPS polyol soft segments.[32] Additionally, as the TPU’s hard segment mole rate increases, its E100 properties also increase. When PPS-x-TPUs are tensed, the molecular chains become ordered and crystallization (stretching-induced crystallization) appears. PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU have a higher hard segment mole ratio than PPS-1000-TPU, and the hard segments have more strong secondary bonds to interact with each molecular chain. Therefore, a higher hard segment mole ratio increases the E100 (modulus at 100% deformation) values of PPS-x-TPUs. Interestingly, the higher hard segment mole ration of PPS-4000-TPU exhibits the microphase separation; therefore, the hard segments will be like fillers in the PPS-4000-TPU matrix. Therefore, the tensile strength of PPS-4000-TPU is higher than that of other PPS-x-TPUs. The results of the tensile testing show that the mechanical properties of the PPS-x-TPUs can be tuned over a wide range and that PPS-4000-TPUs have the best mechanical properties of the PPS-x-TPUs herein. Shore-A hardness values (Table ) varied similar to the tensile strength with increasing molecular weight of PPS polyol. Intriguingly, all of the PPS-x-TPUs had the same hard segment values (32.5%), but they exhibited different values of hardness probably because the molecular weight of the polyols and the hard segment mole ratio increased (A = 1.60–5.97 and C = 0.60–4.97).

Figure 4

Tensile stress–strain curves of PPS-x-TPUs.

Table 3

Mechanical Properties of the PPS-x-TPUs

PPS-x-TPUs	modulus at 100% deformation (MPa)	tensile strength (MPa)	elongation at break (%)	Shore-A hardness
PPS-1000-TPU	3.64 ± 0.05	25.88 ± 0.03	862.95 ± 18.65	75
PPS-2000-TPU	5.00 ± 0.35	27.20 ± 2.10	1046.99 ± 56.42	82
PPS-3000-TPU	5.22 ± 0.02	25.85 ± 1.02	907.01 ± 14.23	87
PPS-4000-TPU	6.10 ± 0.23	64.13 ± 4.35	783.21 ± 16.14	90

Thermal Properties

Differential Scanning Calorimetry

Figure a plots the typical second heating DSC curves of PPS polyols. In Figure a, no obvious melting peaks are witnessed because of the amorphous nature of PPS polyols. Because the chemical structural units of PPS-polyols are odd carbons, the chemical structure is not easy to crystallize. Therefore, no melting peaks are observed. Figure b plots the typical second heating DSC curves of PPS-x-TPUs. The glass transition temperatures (Tg) of PPS-1000-TPU, PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU are 3, −3, −4, and −4 °C, respectively. Clearly, the Tg of the PPS-x-TPUs decreased as the molecular weight of the soft segments (PPS polyols) increased possibly because the free volume increased and because of the conformation of main chains (−CH2–CH2−). The Tg values of PPS-3000-TPUs and PPS-4000-TPUs did not exhibit such a huge drop because the higher crystallinity of the hard segment in PPS-4000-TPU constrains the mobility of the molecular chain of the PPS soft segment in the amorphous region and so inhibits the decrease in Tg values.[33] Therefore, the Tg of PPS-4000-TPU is similar to that of PPS-3000-TPU. PPS-1000-TPU did not yield any Tm peak because of the lower mole ratio of the hard segment. The structure was not sufficiently dense to be crystalline. Nevertheless, PPS-2000-TPU and PPS-3000-TPU exhibited broad Tm peaks at 124.2 and 122.7 °C, whereas PPS-4000-TPU exhibited a strong Tm peak at 143.4 °C because the PPS polyol with the higher mole ratio of hard segments created more ordered domains and a larger fraction of ordered crystalline segments. The increase in the hard segment mole ratio implies a higher molar amount of isocynate, chain extender (chain extender), and long hard segment units, thus easing microphase separation and formation of ordered hard domains. PPS-4000-TPU has the hardest segment mole ratio than other PPS-x-TPUs. Therefore, the ordered hard domains are easily crystallized, and as a result, the PPS-4000-TPU shows the Tm peak.

Figure 5

Second heating DSC curves of PPS polyols (a) and PPS-x-TPUs (b).

Thermaogravimetric Analysis

The thermal decomposition of PPS-x-TPUs was studied by making TGA measurements. Figure plots the corresponding TGA and derivative thermogravimetric analysis (DTG) curves. The TGA curves (Figure a) show that decomposition occurred in two steps from 300 to 500 °C. As reported in the literature,[34,35] the two-stage thermal decomposition of PPS-x-TPUs involves the degradation of the hard segments [urethane linkages in the first stage (Td1)] and soft segments [PPS polyol in the second stage (Td2)]. Table S1 shows that Td1, Td2, and the 5% weight loss (Td,5%) temperatures hardly varied among the PPS-x-TPUs, revealing that the molecular weight of the PPS polyol had only a tiny effect on the thermal decomposition of PPS-x-TPUs owing to their similar chemical structures of PPS polyols with different molecular weights and the same hard segment weight percentage. Notably, the Td,5% of all PPS-x-TPUs exceeds 310 °C, indicating that these polymers have excellent thermal stability and proving that they can all undergo injection and extrusion processing.

Figure 6

TGA (a) and DTG (b) traces of PPS-x-TPUs.

Shape Memory Properties

Programming Deformation/Recovery and Fixed Temperature

The thermal transitions of PPS-x-TPUs were examined using DMA. Figure shows the variations of the loss factor (tan δ) and storage modulus (E′) with temperature and composition for the synthesized PPS-x-TPUs. Table presents the Tg values that were obtained from tan δ. From Figure , the Tg of PPS-1000-TPUs that contain the PPS-1000 soft segment was −2.17 °C, whereas that of PPS-4000-TPUs (with the PPS-4000 soft segment) was −18.25 °C. Tg decreased as the molecular weight of PPS polyols increased. The Tg values that were obtained from Figure were similar to those obtained from the DSC experiments. Tg declined and the tan δ peak became broader as the molecular weight of the polyols increased. These results suggest that the mobility of the soft segment increased (free volume increased) with the molecular weight of polyol. The loss modulus quantifies energy dissipation: a higher loss modulus corresponds to a greater damping capacity.[30]

Figure 7

Storage modulus and tan δ curves of PPS-x-TPUs.

Table 4

Tg Analysis of All PPS-x-TPUs

PPS-x-TPUs	Tg (°C)
PPS-1000-TPU	–2.17
PPS-2000-TPU	–12.45
PPS-3000-TPU	–19.87
PPS-4000-TPU	–18.25

PPS-1000-TPU has the highest loss modulus, which relates to the highest tan δ peak intensity compared to other PPS-x-TPUs as shown in Figure and the lowest hardness as shown in Table . The maximum peak strength in the tan δ curves is not only related with the volume fraction of the relaxation phase during the Tg process but also associated with the amorphous phase. The low hard segment mole ratio of PPS-1000-TPU had no improvement in the crystallinity of the PPS polyol and therefore causes the amorphous phase. Besides, damping capacity is one of the reasons for this phenomenon because of more viscous dissipation of the potential energy. The decrease in the tan δ peak intensity upon the addition of PPS polyol (Mn = 2000, 3000, and 4000) is related to effect of the increase in the hard segment mole ratio on Tg. A lower peak intensity in the tan δ curves represents a higher storage modulus, a lower damping capacity, and a greater hardness as a result of entanglements of the soft segments. However, soft segment entanglements can also affect the thermal ability of PPS-1000-TPUs from 55 to 90 °C (Figure ), consistent with PPS-1000-TPUs lower than PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU hard segment mole ratio. The higher hard segment mole ratio of PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU exhibits a smooth plateau at 55–90 °C, reflecting excellent thermal ability because of soft segment entanglements. PPS-1000-TPU has the highest tan δ peak intensity and lowest E′ (Figure ) above 50 °C, reflecting the α relaxation phenomenon of the hard segments.[36,37] The synthesized PPS-x-TPUs can be easily deformed from their temporary shape into a rubbery state, reducing the entropy of their molecular chains, as follows. First, freeze the molecular chain and release the stress to cause the PPS-x-TPUs to enter the glassy state; subsequently, increase the temperature so they enter the rubbery state to initiate recovery of their original shape because the entropy of elastomers is greater in the rubbery state. The shape memory test parameters are set based on all of the abovementioned DMA results considerations.[26,27,38]

The glassy state and rubbery state, which are associated with the E′ values, determine the fixed and recovery temperatures in the shape memory tests. Figure shows that PPS-1000-TPU is in the glassy state below −10 °C and in the rubbery state between 25 and 50 °C. PPS-2000-TPU is in the glassy state below −25 °C and both PPS-3000-TPU and PPS-4000-TPU are in this state below −30 °C; they are all similar in the rubbery state between 50 and 90 °C. The rubber plateaus of PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU have wider temperature ranges than that of PPS-1000-TPU, so they can easily adopt a temporary shape. The rubber plateau has an important role in shape memory. Table S2 presents the deformation/recovery and fixed temperatures.

Shape Memory Testing

Shape memory testing debatably has two stages, which are (i) shape fixing and (ii) shape recovery. In this work, the shape memory properties of synthesized PPS-x-TPUs are determined by making DMA measurements. Figure presents the three rounds of cyclic DMA testing of the PPS-x-TPUs, and Table S3 presents the shape fixity and shape recovery ratios of PPS-x-TPUs. All of the PPS-x-TPUs have outstanding fixity ratios (>99.60%) in the glassy state (PPS-1000-TPU at −25 °C and PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU at −45 °C) because the molecular chains of PPS-x-TPUs have no mobility in this state, so their temporary shapes are effectively fixed.[26] Some of the molecular chains in PPS-4000-TPU are relaxed and the associated mobility causes shape recovery, although the temporary shape of PPS-4000-TPU in the glass transition state is not well-fixed, as observed in Figure . Therefore, the temporary shapes of PPS-x-TPUs can be fixed well in the glassy state.

Figure 8

Stress–strain–temperature diagram for three consecutive shape memory rounds for (a) PPS-1000-TPU, (b) PPS-2000-TPU, (c) PPS-3000-TPU, and (d) PPS-4000-TPU.

The maximum stress of PPS-x-TPUs at which the strain is reversible is determined. From Figure d and Table S3, the sample PPS-4000-TPU has very good thermomechanical properties (maximum stress: 2.95 MPa and maximum strain: 144%). The soft entanglements and secondary bonds in PPS-4000-TPU form a forced framework that maintains the temporary shape under massive loading. As shown in Figure , the rubbery plateau is extended as the number of entanglements and secondary bonds increase. Shape memory testing of PPS-1000-TPU failed (Table S3) possibly because the deformation temperature exceeded the hard segment α relaxation temperature. Therefore, the permanent shape of PPS-1000-TPU was disrupted and the strain exceeded the instrument limit in the first step of the test, so the test was stopped. Table S3 reveals that all PPS-x-TPUs had an impressive shape recovery ratio (>95.00%) in the second (Rr (2)) and third (Rr (3)) rounds. In the rubbery state, PPS-TPUs exhibited physical cross-links as secondary bonds and soft segment entanglements, so the deformation was reversed, increasing entropy. Nevertheless, in the first round (Rr (1)), all PPS-x-TPUs permanently deformed, revealing that their recovery ratios in this round were lower than those in the following two rounds. PPS-x-TPU molecular chains undergo reorganization, including orientation and the decoupling of soft entanglements, forming a nearly ideal elastic network after the first round.[15,26] Consequently, all PPS-x-TPUs exhibited similar, outstanding recovery ratios in the following rounds (Rr (2)) and third (Rr (3)). Herein, hard domains have an important role in shape recovery.[27] The shape recovery ratios of all PPS-x-TPUs were similar because they all had the same wt % (32.5%) of hard segments.

The shape recovery ratio of PPS-4000-TPU in the glass transition state was slightly lower than those of the other PPS-x-TPUs in the series. The lower mobility of the molecular chain yielded a lower recovery ratio and therefore a longer recovery time to the original shape. PPS-x-TPUs have great potential to form compounds with PCL (Tm is 60 °C), increasing the fixed temperature to the ambient temperature and easily fixing the temporary shape at room temperature. This technique is well-known and widely used.[38−41]

Conclusions

In this work, a series of thermoresponsive shape memory PUs (PPS-x-TPUs) from bio-PPS polyols (Mn = 1000, 2000, 3000, and 4000) were successfully synthesized by solvent-free one-shot bulk polymerization. All synthesized PPS-x-TPUs underwent two-step thermal decomposition, exhibiting good thermal stability with a Td,5% above 310 °C. Tg was measured using both DSC and DMA. Both methods revealed that Tg decreased, as the molecular weight of PPS polyols increased. The bio-based PPS-4000-TPU exhibits the impressive mechanical properties (hardness: 90A and tensile strength: 64.13 ± 4.35 MPa) owing to soft segment entanglements and relatively many secondary bonds. Soft segment entanglements between 50 and 90 °C affect the thermal mechanical properties of PPS-x-TPUs (PPS-2000-TPU, PPS-3000-TPU, and PPS-4000-TPU) but they did not reduce the modulus. Finally, the shape memory performance of the synthesized PPS-x-TPUs was assessed. All PPS-x-TPUs exhibited a perfect shape fixity ratio (>99.60%) and an excellent recovery ratio (>95.00%) in the glass state and the rubbery state, respectively, reflecting good shape memory behavior. In conclusion, the polymers that were synthesized in this work are suitable for coating clothes. Their recovery temperatures allow them to be dried in a clothes dryer (45–68 °C) to help them recover from deformation. Because when using a temperature treatment to recover the wrinkles of PPS-coated textiles, PPS-x-TPU coating will recover the shape and also help the wrinkled textile recover the shape. We can use the recovering force of the PPS-x-TPUs at recover temperature (50–75 °C) to make the textile straight and smooth.