Preparation and characterization of electrospun shape memory polyurethane/graphene quantum dot nanocomposite scaffolds for tissue engineering

Shape memory polyurethanes (SMPUs) are typically synthesized using polyols of low molecular weight (MW~2,000 g/mol) as it is believed that the high density of cross-links in these low molecular weight polyols are essential for high mechanical strength and good shape memory effect. In this study, polyethylene glycol (PEG-6000) with MW ~6000 g/mol as the soft segment and diisocyanate as the hard segment were used to synthesize SMPUs, and the results were compared with the SMPUs with polycaprolactone PCL-2000. The study revealed that although the PEG-6000-based SMPUs have lower maximum elongations at break (425%) and recovery stresses than those of PCL-based SMPUs, they have much better recovery ratios (up to 98%) and shape fixity (up to 95%), hence better shape memory effect. Furthermore, PEG-based SMPUs showed a much shorter actuation time of < 10 s for up to 90% shape recovery compared to typical actuation times of tens of seconds to a few minutes for common SMPUs, demonstrated their great potential for applications in microsystems and other engineering components.

Shape memory polyurethane (SMPU) with its outstanding characteristics is categorized as smart materials and has been utilized in a wide range of applications. In this study, a series of palm kernel oil polyol (PKOp) - based SMPU with the combination of polycaprolactone (PCL) and polyethylene glycol (PEG) as soft segment was synthesized and characterized for the first time. The synthesized SMPUs were examined via several techniques such as Fourier transform infrared, x-ray diffraction, thermal analysis, tensile and shape memory test. The combination of PCL and PEG in PKOp—based SMPU has overcome the drawbacks of PKOp—based PU with only PCL or PEG as soft segment. PU-PCL4PEG4 exhibited tensile strength, Young’s modulus, and shape fixity of 5.7 MPa, 53.9 MPa, and 97%, respectively. For the variation of PEG molar ratio, PU-0.6PEG demonstrated good modulus (151.3 MPa) and shape fixity (99%) but its tensile strength and tensile strain at break were compromised as compared to other samples.

Shape memory polyurethanes (SMPUs) are typically synthesized using polyols of low molecular weight, Mw, and high hydroxyl number as it is believed that high density of cross-links in these polyols are essential for high performance shape memory polymers. In this study, polyethylene glycol (PEG-6000) with Mw ~ 6000 g/mol and low hydroxyl number (OH ~ 18 mg K OH/g) as the soft segment and diisocyanate as the hard segment were used to synthesize SMPUs. It revealed that although the PEG-6000 based SMPUs have lower maximum elongation at break (425%) and recovery stress than those of PCL-2000 polyol based SMPUs, they have much better shape recovery ratio (98%) and shape fixity (95%). Furthermore, these SMPUs showed a much shorter actuation time of <10sec for up to 85% shape recovery, much shorter than those low Mw SMPUs, clearly demonstrated their great potential for applications.

In this study, different shape memory polyurethane (SMPU)-based electrospun nanofibers containing Fe3O4 magnetic nanoparticles (MNPs) were prepared. The shape-memory behavior of these scaffolds enables minimally invasive applications within biological systems, while their magnetic properties enhance the growth, proliferation, and differentiation of the bone cells. SMPU was synthesized via a two-step pre-polymerization method. The effects of MNPs on hydrogen bonding, crystallinity, thermal properties, hydrophilicity, water absorption, and mechanical and shape memory properties of SMPU were investigated. The results revealed that MNPs restricted the hydrogen bonds formation and promoted microphase separation in SMPU hard and soft segments. Moreover, a reduction in the degree of crystallinity of oft segments in the SMPU nanocomposites was observed by the addition of MNPs. Scanning electron microscopy was employed to determine the average diameters and size distributions of the SMPU nanofibers. In addition, the results showed that the prepared electrospun nanofibrous mats have adequate mechanical and shape memory properties for practical biomedical applications. Bioactivity studies indicated that the presence of MNPs could enhance the In-vitro cell cultivation of MG63 bone cells on the nanofibers. These results indicated that the prepared electrospun nanofibers could be utilized as a potential candidate for shape memory-assisted smart wound healing applications.

By using the nanostructure and nanocomposites of shape memory polyurethane (PU), various stimulus-responsive polymer materials which can be actuated by heating, electric power, and water were prepared. For the electroactive shape memory polyurethane nanocomposites, poly(e-caprolactone) as soft segment and the functionalized carbon nanotubes(CNTs) were employed through chemical modification, grafting, surface coating, and cross-linking. The dominant enhancement in mechanical, thermal and shape memory properties were achieved in the sample of cross-linked nanocomposites of PU and CNTs. The water-triggered shape memory effect was obtained from the POSS-PU synthesized from poly(ethylene oxide) and POSS as hydrophilic and hydrophobic components in soft and hard segments, respectively. The actuation of these different stimulus-responsive polymer materials is demonstrated in this report.

Shape memory polyurethanes are usually fabricated with low-molecular weight polyols through a two-step copolymerization, which often results in difficulty attaining both desired shape memory switch temperature and optimal thermomechanical properties. Here we present a series of shape memory polyurethane copolymers having urethane chains as soft segments. The structure and shape memory properties of copolymers were investigated with differential scanning calorimetry, dynamic mechanical analysis, small angle x-ray scattering, and thermomechanical tests. Increasing the length of the urethane soft segments enhanced phase separation, while it brought little change to the glass transition temperature (T g). Based on the urethane soft segments, some rigid chain extenders could be readily introduced into the backbone of copolymers, resulting in better phase separation. All polyurethane copolymers exhibited more than 90% of shape recovery. The shape recovery of the materials was proved to be inversely proportional to the fraction of hard phase and directly proportional to the stability of hard domains. The copolymers containing longer soft and hard segments and rigid chain extenders exhibited higher deformation stress and thus larger recovery stress. The copolymerization employing urethane chains as soft segments can greatly expand flexibility for molecular design and favor the optimization of shape memory properties.

Multi-level self-healing ability of shape memory polyurethane coating with microcapsules by induction heating

Shape memory polyurethane (SMPU) is a very versatile material that has a broad array of applications. The selection of soft segments and hard segments play critical roles in determining the structure-property behaviors of SMPU. This research was conducted to evaluate the role of distinct types of diisocyanate on the final properties of polyurethane (PU). Palm kernel oil polyol (PKO) based PU were produced by using two-step bulk polymerization method with variations of diisocyanates. Isophorone diisocyanate (IPDI), 4,4-methylenebis (cyclohexyl isocyanate) (HMDI) and hexamethylene diisocyanate (HDI) were used in the preparation of PU and the soft segment crystallinity, thermal and shape memory properties of the PU were evaluated. Based on the analyses, it was found that different types of diisocyanate and combination of diisocyanates had huge impact on the properties of the synthesized PU. The Fourier transformation infrared (FTIR) analysis revealed that IPDI based PU achieved the highest hydrogen bonding index value which promoted the phase separation. This is in accordance with differential scanning calorimetric (DSC) and x-ray diffraction (XRD) analysis which showed that IPDI based PU exhibited crystalline soft phase, hence resulted in an excellent shape fixity behavior. On the other hand, HDI and HMDI based polyurethane prepared showed absence of crystalline soft phase based on the DSC thermogram and XRD diffractogram. These results suggest the phase mixing phenomenon between soft and hard segments which contributed to low shape memory behavior of the resulting polyurethane.

The dependence of electrical resistivity on specimen temperature and imposed tensile strains was determined for shape memory polyurethane (SMPU) composites of carbon nanofiber (CNF), oxidized carbon nanofiber (ox-CNF), and carbon black (CB). The SMPU composites with crystalline soft segments were synthesized from diphenylmethane diisocyanate, 1,4-butanediol, and poly(caprolactone)diol in a low-shear chaotic mixer and in an internal mixer. The materials synthesized in the chaotic mixer showed higher soft segment crystallinity and lower electrical percolation thresholds. The soft segment crystallinity reduced in the presence of CNF and ox-CNF; although the reduction was lower in the case of ox-CNF. The composites of CB showed pronounced positive temperature coefficient (PTC) effects which in turn showed a close relationship with non-linear thermal expansion behavior. The composites of CNF and ox-CNF did not exhibit PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CNF and ox-CNF showed weak dependence on strain, while that of composites of CB increased by several orders of magnitude with imposed tensile strain. A corollary of this study was that a high level of crystallinity may cause a PTC effect and prevent any actuation through resistive heating. However, a carefully tailored compound which has reduced crystallinity and which requires minimum amount of filler may prevent PTC phenomenon and could supply necessary electrical conductivity over the operating temperature range, while offering enough soft segment crystallinity and rubberlike properties for excellent shape memory function.

Shape-memory polyurethane (SMPU) having segmented structure, with 4,4-diphenylmethane diisocyanate as hard segment, poly caprolactone diol 4000 as soft segment, and 1,4-butane diol as chain extender was synthesized using two-step polymerization technique. This pure SMPU was blended with unmodified and modified multiwalled nanotubes (MWNTs), using twin screw extruder and melt spinning technique. X-Ray diffraction, differential scanning calorimetry, morphological analysis, and shape-memory behavior results for the SMPUs are presented in this study. An improvement in the thermal stability and degradation temperature of the SMPU was observed especially in the soft segments, upon 0.1% loading of MWNT. Slight decrease in crystallinity was recorded for MWNT/SMPU as compared to virgin SMPU. The recovery ratio of SMPU fiber increased to 90% with 0.1 wt% loading of MWNT, but the spinnability decreased due to reduced crystallinity and viscosity with an addition of nanotubes to the polyurethane matrix.

Mechanical robustness and flexibility of shape memory polyurethane (SMPU) make them a prominent candidate in various field. However, the shape memory characteristics are hampered due to the lower breaking stress and strain originating from the slippage of hard segments during deformation and entropic elasticity of the segments. Herein, SMPU is synthesised by modification of Polycaprolactone diol (PCL) based soft segment by introducing a linear chain diisocyante, that is hexamethylene diisocyanate (HDI) as the mixing segment and rigid MDI (4,4′-methylene bis-phenyl diisocyanate) as the hard segment. The HDI based soft segment is expected to improve the chain flexibility, and MDI will retain the strength factor. The SMPU is characterised by chemical, structural and thermal analysis. The stress relaxation behaviour of the film was analysed w.r.t time and correlated with recovery studies using the Maxwell model. The thermomechanical conditions are optimised to attain higher shape fixity (SF) and shape recovery (SR) and the SMPU shows maximum SF (60.8%) and SR (97%) at 70°C temperature and 50% strain condition. Also, SMPU shows the tensile strength of 23.4 MPa with elongation at break of nearly 1270%. Thus, the combination of both diisocyanate and soft segments imparts strength and ductility to the SMPU.

Water vapor permeability of cotton fabrics coated with shape memory polyurethane

Shape memory polyurethanes (SMPU) are one of the advanced materials that have potential applications in the field of biomedical particularly vascular stent. This paper studies the effect of incorporating palm oil polyol (POP) up to 40% molar ratio in place of petroleum‐based polyol in the preparation of SMPU due to environmental concern. Polycaprolactone diol was utilized as the soft segment while 4,4′‐diphenylmethane diisocyanate and 1,4‐butanediol as the hard segments. The SMPU was prepared using two‐step prepolymer method, and the fabricated samples were characterized to study the effect of POP on the thermal properties, tensile, and shape memory behavior of polyurethane. The results obtained have shown that SMPU with incorporation of POP showed good shape fixity (100%) and elongation at break (245%) up to 20% molar ratio of POP. The presence of dangling chains of fatty acid in POP was believed to enhance the flexibility of SMPU molecular chains by acting as a plasticizer. On the other hand, the shape recovery of SMPU remains high even at 40% molar ratio of POP, and the thermal stability of SMPU increased with the addition of POP. It is proposed that the synthesized POP‐based SMPU is a suitable candidate for cardiovascular stent as they possessed desired thermal, mechanical, and shape memory properties.

The surface-modified nanoparticle, monmorillonite (MMT), was chemically bonded to shape memory polyurethane (SMPU) to improve shape memory and mechanical properties compared to the conventional composite prepared by melt-mixing process. Cloisite 30B was selected as MMT, because the functional group on its surface, methyl tallow bis-2-hydroxyethyl ammonium group, could be used for chemical bonding with SMPU and the reduced surface hydrophilicity rendered MMT disperse better in SMPU matrix during chemical reaction compared to bare MMT. Two types of SMPU, differing according to their soft segment (PTMG) and MMT content, were compared in mechanical and shape memory properties. Maximum stress went up as high as 57 MPa, and strain remained above 1000% for all of SMPUs. Shape recovery improved up to 97%, and did not decrease after four repetitive cyclic tests. Here, results demonstrating the advantages of chemical bonding of MMT-linked SMPU, compared to MMT and PU composite made by melt-mixing method, are discussed.

ABSTRACTSelf‐healing shape memory polyurethanes (SHSMPUs) integrate shape recovery and damage restoration, offering enhanced reliability and adaptability for biological applications. In this study, PCL‐SS SHSMPUs are synthesized using polycaprolactone (PCL) as the soft segment and hexamethylene diisocyanate (HDI) and 2‐hydroxyethyl disulfide (HEDS) as the hard segment. Increasing hard segment content enhances tensile strength but reduces elongation at break, with PCL‐SS3 showing the highest tensile strength (14.9 MPa) and PCL‐SS1 achieving the greatest elongation at break (1120.7%). All polyurethanes exhibit a shape fixation rate above 90%, and PCL‐SS2 achieves the highest shape recovery rate of 74%. The self‐healing properties are studied under varying time, temperature, and hard segment content, revealing that PCL‐SS1 achieves the highest healing efficiency of 97.6% after 12 h of self‐healing at 90°C. Furthermore, multi‐walled carbon nanotubes (MWCNTs) are introduced to develop polyurethane composites with near‐infrared (NIR) light‐induced self‐healing functionality. The polyurethane composites with 5% MWCNTs achieve the highest healing efficiency of 85.5% within 1 min of NIR irradiation. However, excessive MWCNTs content reduces mechanical properties due to poor dispersion. This study offers a basis for optimizing and designing self‐healing shape memory polyurethanes for applications in biomedical devices and smart materials.