Catalyst-free synthesis of low-temperature thermally actuated shape memory polyurethanes with modified biobased plasticizers

Recent years have seen research into developing specific application-based materials with particular components. Bio-based polyurethanes (PUs) with self-tightening effect through shape recovery at low temperature have been designed from sesame oil-based plasticizer (HSSO). Without using a catalyst, the produced plasticizer was used to create PU samples. In contrast, orcein-based PU has been created both with and without HSSO. The prepared samples have been analyzed through instrumental as well as chemical analyses for surface chemistry, thermal stability and morphology. The gel content and water absorption capacity of HSSO based PU samples has been observed to be 99.27% and 14.94%, respectively. Shape memory study of the PU samples revealed that HSSO-based PU showed fast shape recovery at 60 °C with shape recovery rate (Rr) and shape fixing rate (Rf) of 94.44% and 5%, respectively, in 150 seconds, whereas at 36 °C the sample showed 85% Rr in 15 minutes with 93.1196 N force and 52.78% Rr without force. Low-temperature thermal actuation and high water uptake highlight the prepared samples as suitable candidates for self-tightening structures in textile and biomedical fields.


Introduction
Stimulus-responsive restoration of a polymeric material to its original shape aer specic deformation has opened a new dimension in material applications. 1 These stimuli include, but are not limited to, changes in temperature, 2 light, 3 electricity, 4 pH, 5 and other similar variables. Biomedical devices such as cardiovascular stents, 6 sutures, 7 drug-eluting stents 8 and clot removal devices, 9 and tissue engineering, 10 have made extensive use of SMPs due to their biocompatibility, 11 biodegradability, 11 and human body temperature shape recovery. [12][13][14] With their tunable transition temperatures for shape recovery and high levels of biocompatibility, shape memory polyurethanes (SMPUs) have shown great promise as responsive materials for use in biomedical devices inside the human body, [12][13][14] accompanied by coating material, packing materials and smart textile. [15][16][17] However, environmental concerns and exhausted petroleum reserves have shied the focus to SMPUs with biobased origin rather than petrochemical origin. 18 Currently, vegetable oils like soyabean oil, 19 castor oil, 20 sunower oil, 21 jatropha oil 22 and palm kernel oil, 23 are the most common source for bio-based SMPUs (bio-SMPUs), but slow curing, 24 catalytic constraint 25,26 and reduced elastic strength [27][28][29] has limited their applications in comparison to petrochemical based SMPUs. This is because the micro-phase separation of PU is negatively impacted by the lengthy dangling chains present in vegetable oil polyols (PUs). Researchers have tried to control the length of side chains either by bio-based polyester diols, 24 or by rosin-based chain extender 30 but found not ideal for biological applications due to their shape recovery below body temperature. PU based polyurethane SMPs had ability to recover its shape to maximum degree but different chain extenders and catalyst were applied to control their elastic properties 25,31,32 as shown in Table 6. Considering these limitations, ecofriendly SMPUs, with recyclable nature but high mechanical strength, have always been in dire need.
In this research, modied sesame oil-based polyols have been utilized to formulate SMPUs, without any chain extender or catalyst, but with quick shape recovery at very low thermal actuation. Two step treatment i.e. epoxidation and hydroxylation, has been performed to obtain suitable polyol as plasticizer for shape memory property.

Characterizations
Synthesized PU samples have been analyzed through FTIR-(IRSpirit) Shimadzu with diamond ATR in range 500-4000 cm −1 , TGA/DSC (STA SKZ1060A Industrial Co., Limited) from room temperature to 500°C at ramp rate of 10°C min −1 for 5 mg sample in Al crucible using oxidative environment and air as purge gas, light microscope (IRMECO GmbH & Co., IM-910). The morphology and surface roughness of synthesized PU composites have been studied by scanning electron microscope (SEM, ZEISS EVO, Carl Zeiss) and atomic force microscopy (AFM). Molar mass of treated SSO was determined by ebullioscopic method (eqn (1)) taking benzene as solvent.
where DT b = elevation in boiling point of specic solvent, K b = ebullioscopic boiling constant of specic solvent and m = molality of unknown sample. For benzene, the value of ebullioscopic boiling constant is 2.53°C kg mol −1 . 33 Iodine value (IV) was determined by reported method. 34 0.2 g sample with 10 ml chloroform was stirred with 30 ml of Hanus solution for 15 minutes. 10 ml of 15% KI and 100 ml DI water were added and titrated against 0.1 N Na 2 S 2 O 3 till yellow colour, added 2-3 drops of starch indicator and titrated again till blue colour. Calculated IV by using eqn (2).
where B and S are the volume used of Na 2 S 2 O 3 against blank solution and sample and N is the normality of Na 2 S 2 O 3 . Epoxy value (EV) was determined by HCl-acetone titration method. 35 0.25 g sample was stirred in 5 ml HCl (0.1 N) and 35 ml of acetone. 5 ml mixture, with 2-3 drops of indicator, was titrated against 0.1 N NaOH till pink color. EV value was calculated by eqn (3).
where S and B are the volume used of NaOH against sample and blank solution, and W is the weight of sample. Samples' density was determined by using mass/volume relationship. 36 Gel contents were measured by a method reported in literature. 37 Calculated amount of PU samples (HSSO-PU, Or-HSSO-PU and Or-PU) was soaked in 20 ml of dichloromethane (DCM) for 24 hours. Then samples were removed from solvent and dried at 40°C, and weighted to calculate gel contents by using eqn (4) and (5).
where W s is weight of sample and W d is weight of dried sample. Water absorption capacity was determined by a method reported in literature. 38 Weighed PU samples were dipped in deionized water for 48 hours. At regular time intervals samples were removed form water, dried and weighed to determine water absorption by eqn (6). 15 Water absorption capacityð%Þ where W t = weight of sample aer dipping, W 0 = weight of dried sample. Hemolytic activity, antioxidant activity and antibacterial activity of synthesized PU composites have been determined by reported methods 39-41 using eqn (7) and (8), respectively.
where A t is the absorbance of the test sample. A n is the absorbance of the control (saline control) A c is the absorbance of the control (Triton control).

Antioxidant activity ¼
where; A blank is the absorbance of the control reaction (containing all reagents except the extract) and A sample is the absorbance of the mixture containing the extract.

Shape memory test
Rectangular strips of PU lms having dimension 3 cm × 6 mm × 3 mm ( Fig. 1) were used to study shape memory behavior ( Fig. 1).
For shape memory study, lms were heated at 54°C, bent and cooled to maintain temporary shape. Thermal stimulus (50-60°C) was provided to lms for restoration of original shape. The displacement of shape from q max while cooling and xing is known as angle of xity (q f ). Attained position by bent lm while displacing towards original shape aer heating is known as recovered shape and the difference of angle from starting position is known as angle of recovery (q r ). Cycles of conversion original / temporary / original shape were repeated ve times. Ability of PU lms to gain temporary shape and restoration to original shape under the inuence of temperature is the shape recovery rate (R r ) and shape xity rate (R f ), calculated with eqn (9) and (10). 15 4 Results and discussions

Physico-chemical analyses
Molecular mass (MM), iodine value (IV), epoxy value (EV) and density of unmodied and modied SSO were calculated to conrm the effectiveness of modication protocol. Upsurge in Fig. 1 Shape memory bending test. MM and density of modied samples in comparison to unmodied sample, conrmed the change in molecular assembly. IV difference signposted the effective consumption of unsaturated contents during two step modication. EV counter-conrmed the effectiveness of two-step modication protocol (i.e. epoxidation and hydroxylation) ( Table 1).

FTIR analyses
SSO to ESSO modication was studied by FTIR data (Fig. 2). SSO spectra showed peaks of different functional groups (e.g. C]C, C-O-C and ]C-H) at 723 cm −1 , 1242 cm −1 and 3008 cm −1 respectively. 42,43 Aer treatment, ESSO spectra showed a reduction in C]C peak with peak area condensed thrice, whereas, absorbance peak of C-O-C became sharp and wide with peak area almost doubled. At the same time, epoxy peak with peak area 0.62 was observed at 826 cm −1 in ESSO spectral line. 42 Disappearance of ]C-H peak (3008 cm −1 ) 43 in ESSO, may be attributed to conversion of alkene into epoxy, conrmed by the appearance of epoxy peak in ESSO. The peaks present at 2347 cm −1 and 1738 cm −1 represent carbonyl functional group. 43 FTIR spectra of HSSO (Fig. 2) obtained aer treating ESSO with organic acids (acetic acid and formic acid) in ethanol and water, showed a reduction in C-O-C peak at 1242 cm −1 and also in epoxy peak at 826 cm −1 . Moreover, -OH peak appeared at 3502 cm −1 . 43 The conformation of OH induction may be done with C-O peak formation at 1037 cm −1 . 43 HSSO, as biobased diol, upon reaction with TDI resulted in HSSO-PU (Fig. 3). Peaks at 3502 cm −1 and 2245 cm −1 in HSSO FTIR spectra, represent OH in HSSO and NCO in TDI, respectively. 33 Upon reaction of HSSO with TDI, disappearance of both OH and NCO peaks in product (Fig. 3) conrmed the consumption of these functional groups along with the formation of a new functional group C-O at 1054 cm −1 . 43 Breakdown of C]N bond in TDI, resulted in new bonds, i.e. C-N with peak at 1219 cm −1 , 33 N-H with peak at 3333 cm −1 , 33 and carbonyl with peak at 1738 cm −1 . 43 Peak at 1533 cm −1 indicated the presence of cyclic alkenes. 33 Or-HSSO-PU sample along with ingredients was analyzed through FTIR (Fig. 4). Peaks of NCO and OH were found at 2245 cm −1 and 1328 cm −1 , respectively, 15,33 while HSSO hydroxyl group was observed at 3502 cm −1 . 33 Aer reaction of orcein and HSSO with TDI, both NCO and OH peaks disappeared rising new peaks in product. It is assumed that polyols OH reacted with C]N in TDI, resulting in CO bond between TDI and polyol with peak at 1054 cm −1 . 43 Similarly, N linked with H and peak appeared at 3299 cm −1 . 15 Likewise, breakup of double bond between C and N resulted in single bond le with peak at 1219 cm −1 . 15,33,43 Carbonyl peak at 1703 cm −1 , also increased in size. The peak of cyclic C]C appeared at 1533 cm −1 .
Similarly, FTIR analysis was applied on Or-PU and its reactants to observe different functional group formed (Fig. 5). Phenolic OH group was observed at 1328 cm −1 . Aer reaction between polyol and isocyanate, NCO and OH functionalities disappeared with new group formation in product, i.e. N-C, C-O and N-H. Peaks of NCO and OH were at 2254 cm −1 and 1328 cm −1 respectively. 15,33,43 Appearance of these new peaks in Or-PU conrmed the synthesis of product as shown in Fig

TGA
Thermal stability of prepared samples has been studied with TGA ( Fig. 6). Data revealed that incorporation of orcein imparts thermal instability into samples, which might be attributed to higher oxidizable contents on its surface. Thermal degradation of sample prepared from biobased material was observed least in rst two segments, which might be attributed to least oxidizable contents on its surface. In third segment (above 400°C ) thermal stability pattern reversed i.e. biobased polyol containing PU degraded quickly in comparison to samples having orcein as polyol. This reversal in degradation pattern may be attributed to fact that orcein based PU samples lost most of oxidizable components in rst two segments and became thermally stable whereas biobased polyol containing PU, at 450°C (Fig. 6), decomposed carbon chains into smaller fragments, prone to thermal degradation. 44

DSC
Prepared PU samples were analyzed through DSC (Fig. 7, Table  2). Comparative analysis, on the basis of increasing biobased polyol component, showed an augmentation trend in T g of hard segment (TDI), signposting micro-phasic separation of hard and so segments of PU samples by adding HSSO. 45 About 100°C increase in thermal melt of HSSO-PU than Or-PU is due to its well-dened microcrystalline structure which signpost increased cross linking density of rubbery so segments (polyols), moreover 10 cal. low DH m of HSSO-PU than Or-PU represents less thermal dissipation of amorphous HSSO. 46 This data evidenced a vivid micro-phasic level separation of hard and so segments in HSSO-PU.
Thermal melt enthalpy increases with increasing polyol content in PU structure. 45 Or-PU showed highest DH m of all samples indicating highest alcoholic content involvement in polymerization, which might be attributed to condensed structure of orcein, but high thermal dissipation capacity of orcein decreased thermal melt temperature of Or-PU than HSSO-PU.

Gel content & water absorption capacity
Gel content analysis was applied on prepared PU samples (Table  3). HSSO-PU was found to have the highest gel contents (99.27%) in comparison to Or-PU (98.32%) and Or-HSSO-PU    (97.66%) verifying huge quantity of constituents to be crosslinked during synthesis of PU samples. Water absorption capacity of prepared PU samples was also studied to conrm their hydrophilic nature and biodegradation behavior. Water absorption results (Table 3) indicated an upsurge with temperature rise, which links so segment's interaction with water. Swelling of material relates inversely with crosslinking density of the constituents, i.e. Or-PU composite had high swelling proportion as compared to HSSO-PU composite due to low crosslinking density.     by DSC thermal melt data, which specied well-dened microcrystalline hard segments.

SEM and AFM
SEM and AFM analyses of prepared samples (Fig. 9) at m level revealed homogeneity and smoothness in sample with biobased plasticizer (HSSO). On the contrary, orcein amalgamation imparted non-homogeneity and surface roughness (almost 40 folds to biobased plasticizer), which might be considered for the structural damage during shape memory studies.

Bioactivities
Hemolytic activity showed mild toxicity at 12 mg ml −1 concentration, which shrank with concentration drop (

Shape memory
Shape memory of PU samples (HSSO-PU, Or-HSSO-PU and Or-PU) was studied under different temperatures (Fig. 10). Or-HSSO-PU and Or-PU showed rigidity till 40°C and broke but above 40°C started melting and did not show any shape recovery behavior. This may be attributed to strong thermal dissipation behavior of orcein, which caused melting of sample at a specic temperature through structural damage. On the other side, HSSO-PU started showing exibility at 50°C. Shape memory and shape xing behaviors of HSSO-PU lms were studied in temperature range of 50-60°C (Table 5). Fast shape recovery was found with temperature rise but with more shape xing, which may be attributed to structural damages of HSSO-PU at 60°C (Fig. 11). Biodegradability and water uptake response of PU samples containing HSSO (Table 3) has widened the application scope of these materials in textile as well as biomedical not only as drug carrier but also as shape recovery materials. To recognize shape memory response of HSSO-PU at human body temperature,  shape recovery at 36°C was studied (Fig. 12). 52.78% shape recovery was found without any force whereas 85% shape recovery was observed at 93.1196 N force. Upon comparison of shape recovery data from already published works, it is being claimed that current research has neither used any types of chain extenders nor catalyst to support shape recovery rate at body temperature, furthermore, not any specially designed commercial product to impart exibility (MDI-50) has been incorporated in molecular design. This outcome i.e. synthesis without special chemical, will support HSSO-PU's expanded biomedical application domain.

Conclusion
In this research biobased polyols have been designed through an eco-friendly and facile approach. SSO has been treated to obtain hydroxylated plasticizer (HSSO), which has induced low thermal actuation shape recovery with high thermal stability in PU samples, whereas orcein has imparted rigidity and easy thermal degradation. HSSO has framed separate and well-dened hard and so segments in PU. Prepared samples have 100% thermal stability till 270°C, but shape recovery response initiated at very low thermal stimulus i.e. 36°C, which has made it a suitable candidate for self-tightening structures not only in textiles and biomedical applications like dental implants, but also in all applications that require shape recovery at very low temperature.

Conflicts of interest
There are no conicts to declare.