Journal Pre-proof NIR induced self-healing polyurethane/polypyrrole nanocomposites Haohao Wu, Dekun Sheng, Xiangdong Liu, Yan Zhou, Li Dong, Fance Ji, Shaobin Xu, Yuming Yang PII:
S0032-3861(20)30026-4
DOI:
https://doi.org/10.1016/j.polymer.2020.122181
Reference:
JPOL 122181
To appear in:
Polymer
Received Date: 9 October 2019 Revised Date:
8 January 2020
Accepted Date: 13 January 2020
Please cite this article as: Wu H, Sheng D, Liu X, Zhou Y, Dong L, Ji F, Xu S, Yang Y, NIR induced self-healing polyurethane/polypyrrole nanocomposites, Polymer (2020), doi: https://doi.org/10.1016/ j.polymer.2020.122181. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Graphical Abstract: Polypyrrole (PPy) can well disperse in polyurethane, which significantly improve the mechanical properties and thermal stability of composites. Meanwhile, the material can be repaired quickly under near-infrared light (NIR).
NIR induced self-healing polyurethane/polypyrrole nanocomposites Haohao Wu a,b, Dekun Sheng a , Xiangdong Liu a*, Yan Zhou a,b, Li Dong a,b, Fance Ji a,b, Shaobin Xu a,b, Yuming Yanga,b* a
CAS Key Laboratory of High-Performance Synthetic Rubber and its Composite
Materials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b
University of Science and Technology of China, Hefei 230026, China
Abstract Polypyrrole nanoparticles (PPy) are widely used in cancer treatment because of its excellent photo-thermal effect. To our best knowledge, the application of such property in self-healing polymers have rarely been reported. Here, thermoplastic polyurethanes (TPU) were prepared by two-step method, and then TPU/PPy nanocomposites were prepared by solution blending. The stress and strain of TPU were increased from 8.20 MPa and 1540% to 13.50 MPa and 1650% with 0.25wt% PPy respectively. Meanwhile, the thermal stability was also improved significantly. After the composite film was cut and spliced together, the fracture was irradiated with NIR, the mechanical strength of the composite material could be restored to over 80% in only 30 s. The strengthening effect of PPy in polyurethane and the rapid repair performance of composite materials can well solve the contradiction that self-repairing materials cannot improve the repair efficiency and mechanical properties simultaneously, and at the same time broaden the application area of PPy.
Keywords: Polyurethane; Polypyrrole nanoparticles; NIR laser; Photo-thermal effect; self-healing
Introduction Inspired by nature's ability to repair wounds spontaneously,scientists introduced the concept of self-healing into polymer materials and enabled polymer materials with self-healing capacity under certain conditions (such as heat [1-3], light [4-7], electric [8], electromagnetic wave [9]) after damage. That not only extends the lifetime of polymer materials, but also improves the safety of materials and saves maintenance costs. Since 2001, when White [10] first introduced self-healing materials, a large amount of literature on self-healing materials has been reported so far. According to the repair mechanism, self-healing materials can be divided into two categories: external self-healing materials (ESHM) and intrinsic self-healing materials (ISHM) [11]. ESHM containing healing agents embedded in a "container", such as microcapsules and microvascular network [12,13], have been developed relatively early. When polymer materials are damaged, the healing agents are released from the container at the damaged areas to heal the cracks by chemical reaction. Different from the ESHM, the ISHM could repair without healing agents, but through the reversible property of dynamic chemical bonds in polymer chains. ISHM can also be divided into dynamic covalent and dynamic non-covalent self-healing materials [14,15]. Among them, reversible dynamic covalent bond includes Diels-Alder bond [3,6,16,17], disulfide bond [18-20]
and acylhydrazone bond [21] etc, while reversible dynamic non-covalent bond contains metal coordination bond [22,23], host and guest bond [24], hydrogen bond [1,25] and ionic bond [26] etc. Recently, Kim et al [19] reported a polyurethane containing aromatic disulfide bond that can be repaired at room temperature. Kang et al [25] used hydrogen bond as reversible bond to prepare polyurethane, which can be healed at room temperature or under water. Although the introduction of reversible bonds into polyurethane make it can be repaired under mild condition, the mechanical properties of materials are reduced to some extent. So it is still a huge challenge to make materials have better self-healing function without reducing mechanical properties. Photo-triggered self-healing materials have many advantages such as spatial control and remote activation [27]. In particular, near-infrared (NIR) light can harmlessly penetrate into biological tissue. Therefore, photo-triggered self-healing materials have great potential applications. As we know, many photo-thermal conversion agents have been applied to self-healing polymer materials, such as carbon-based materials (carbon nanotubes and graphene [28,29] etc) and metal-based materials (silver nanowires and gold nanoparticles [4,5,30] etc). However, most of them in nanocomposites have lower photo-thermal conversion effect and need surface modification to promote dispersion. So, to find a photo-thermal conversion agent that has both good photo-thermal effect and compatibility with polymers remains a challenge[31]. Polypyrrole (PPy) is widely used in the field of bioelectronics and biomedicine
because of its inherent characteristics such as good stability, high conductivity, high photo-thermal conversion efficiency and better biocompatibility [32,33]. At the same time, the presence of a large amount of N-H in the structure of PPy [34] may enable it to better interact with the urethane bond and ester bond in polyurethane. That may result in the formation of physical crosslink in the polyurethane, thus improving the mechanical property and thermal stability of polyurethane. However, the application of PPy to photo-triggered self-repairing polymers have been rarely reported. In this paper, thermoplastic polyurethanes (TPU) were synthesized by two-step
method,
in
which
polycaprolactone
diol
(PCL)
and
polytetrahydrofuran (PTMG) was used as soft segment, isophorone diisocyanate (IPDI) was used as hard segment and 1,4-butanediol (BDO) was used as chain extender. TPU/PPy nanocomposites used TPU as matrix blended with PPy in solution. And then the influence of PPy on the mechanical properties and thermal properties of TPU were studied. Besides, NIR photo-triggered self-healing properties of TPU/PPy nanocomposites were also been studied. The result shows that the addition of PPy could not only enhance the mechanical properties and thermal stability of TPU, but also endows composites with highly efficient self-repair performance under NIR irradiation (repair efficiency can reach over 80% after 30 seconds of illumination). The enhancement effect of PPy in polyurethane system and the quick repair performance of composite material have solved the contradiction problem that
self-healing material cannot improve the repair efficiency and mechanical property at the same time, which have broadened the application field of PPy to some extent. 1. Experimental 1.1 Experimental Materials Isophorone diisocyanate (IPDI , 99%), polytetrahydrofuran (PTMG, Mn=1000, 100 ºC vacuum for 2 h), Pyrrole (Py), dibutyltin dilaurate (DBTDL, 95%), 1,4-butanediol (BDO, ≥ 99.5%) and Iron chloride hexahydrate (FeCl3⋅6H2O, 99%) were all purchased from Aladdin. Polyvinyl alcohol (PVA, Mn=1750±50, ≥99.0%) was purchased from Sinopharm Chemical Reagent limited corporation. Polycaprolactone diol (PCL, Mn=2000, 100 ºC vacuum for 2 h) was purchased from Jining Huakai Resin limited corporation. Dimethylformamide (DMF, dehydrated before use) was purchased from Beijing Chemical Works. 1.2 Preparation of PPy The PPy was synthesized following the literature (the details process was given in the supporting information) [35]. The average particle size of synthesized PPy is around 75 nm, which could be seen from Fig. S1. 1.3 Preparation of TPU TPU was prepared by two-step method, which is displayed in Fig.1. Excessive IPDI were put into a dried glass flask which equipped with a magnetic, stirring in oil bath at 100 oC . PTMG (5.00 g),PCL (40.34 g) and
DBTDL (0.01wt% of TPU) were added into the flask and stirred for 2 h under a N2 atmosphere at 100 oC. After the synthesis of the pre-polymer, BDO was dissolved in DMF and added to the reactor. When the reaction was over, the product was dried at 60 ºC in vacuum oven for further characterization.
Fig.1. The synthetic route of TPU
1.4 Preparation of TPU/PPy nanocomposites A certain amount of PPy nanoparticles and TPU was dispersed in DMF, respectively. After the TPU is completely dissolved, mixed the two dispersions. Then the mixture was ultrasonic dispersed for 60 min to make the PPy and TPU fully mixed. Finally, The mixture was poured into the poly tetra fluoroethylene (PTFE) mould and kept at 65 ºC for two days to obtain TPU/PPy films. TPU /PPy nanocomposites with different contents of PPy were also prepared by the same procedure. According to the addition quality of PPy, the TPU/PPy nanocomposites were named TPU/PPy-x, where x is the quality score of PPy. Five different nanocomposites were prepared by using 0wt%,0.1
wt%, 0.25 wt%, 0.5 wt% and 1 wt% PPy in the TPU (named as TPU/PPy-0, TPU/PPy-0.1, TPU/PPy-0.25, TPU/PPy-0.5, TPU/PPy-1,respectively). 1.5 Characterization Vertex 70 (Bruker, Germany) was used to recorded the FTIR spectra in the range of 500-4000 cm-1. Using thermal analyzer (TA Instruments, Q20, USA) test the melting temperature of nanocomposites with Differential scanning calorimetry (DSC). And the test was conducted in a N2 atmosphere from 0 oC to 60 oC with a heating rate of 2 oC min-1. Thermogravimetric analysis (TGA, Mettler Toledo, Switzerland) was used to assess the thermal stability of nanocomposites. The test was carried out under a nitrogen stream at a temperature range of 25 oC to 800 oC and a heating rate of 10 oC min-1. Using scanning electron microscopy (SEM, XL-30 ESEM FEG, FEI Company) observed the morphologies of fractured surfaces of films. The dispersion state of PPy in the TPU matrix were obtain by using transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, USA). A UV3600 spectrometer (Shimadzu, Japan) was used to record the ultraviolet-visable-near Infrared (UV-vis-NIR) absorption spectrum of all nanocomposites. NIR light with wavelength of 808 nm was generated by the laser system (LSR808H-5W-FA), which was used for NIR light-responsive tests. Universal testing machine (Instron 1121) was used to test mechanical properties of all samples at a strain rate of 20 mm/min. The dimensions of the tensile test and self-healing test samples are the same, and the dimensions is 50mm×4mm×0.5mm. The
average of the results of five individual tensile tests was recorded for each sample. 1.6 Characterization of self-healing property Repairing properties was evaluated by comparison the tensile strength of samples before and after repair. Each samples were cut into two segments from the middle, and the two cut surfaces were brought back together. Then the fracture was placed under an NIR laser light. After Near infrared radiation, the sample was placed in room temperature and air for two days, next tensile tests were carried out. The fracture stress (σ) of the healed samples were recorded. The self-healing efficiencies was defined as Eq(1): Healing Efficiency % =
σ of healing sample × 100% σ of original sample
2. Results and Discussions 2.1 FT-IR study of TPU/PPy-x FTIR spectroscopy was used to test whether the reaction is terminated. As shown in Fig.2, it's obvious that the -NCO characteristic peak near 2270 cm-1 can't be observed. Besides, the characteristic peak of -NH- can be observed near 3382 cm-1. That all means the successful prepartion of TPU/PPy-0[36]. Infrared spectra of other composite materials can also be observed in Fig.2. However, Compared to TPU/PPy-0, the characteristic peaks of PPy (the FTIR spectroscopy of PPy can be seen in Fig.S2) are not obvious in TPU/PPy nanocomposites and there is no significant change in characteristic peaks of TPU. This may be due to there are a large number of carbonyl groups in the
polyurethane that we synthesized by using PCL as soft segment, while the maximum amount of PPy added is only 1%, so it is difficult to use FTIR to directly detect the interaction between them.
Fig. 2. FT-IR spectra of TPU/PPy nanocomposites
2.2 Thermal properties and Mechanical properties of TPU/PPy-x The thermal stability of composite materials plays a decisive role in practical applications. So we used TGA to test the thermal stability of samples. The results were shown in Table 1 and Fig.3a. Besides, the TGA curves of PPy was given in Fig.S3. The TGA curves of TPU and TPU/PPy nanocomposites all exhibit two stages of degradation. Previous studies have demonstrated that TPU undergoes two-stage degradation [37,38]. The first stage is ascribed to the decomposition of urethane bond and ester bond in the TPU, the second stage belong to the degradation of aliphatic chain of TPU. Compared to TPU/PPy-0, as the content of PPy increases, the thermal stability of the composite gradually
becomes better. The cause of this phenomenon may be the interaction between PPy and urethane bond and ester bond in TPU, where a large amount of N-H on the surface of PPy (The XPS of PPy was shown in Fig.S4) can form hydrogen bonds with urethane bond and ester bond [39-41]. Therefore, the first degradation stage of hard segments has been strengthened. Compared to the first stage, the second stage thermal decomposition temperature has hardly changed. This is because the second stage is decomposition of the aliphatic chain of TPU, which has little interaction with PPy.
Fig. 3. (a) TGA curves of TPU/PPy-x (b) The stress-strain curves of the TPU/PPy-x
The mechanical properties affect directly whether the material can be used. Therefore we used universal testing machine to measure the mechanical properties of all samples. The results illustrated in Fig.3b and Table 1. From Fig.3b, it's obvious that the incorporation of PPy improve the mechanical properties of TPU well. By comparsion, it was find that the amount of PPy added was 0.25wt%, the nanocomposites has the best mechanical strength. The tensile strength and elongation at break can reach to 13.50 MPa and 1650%, respectively. These results are mainly attributed to the strong interaction between PPy and polymer matrix, which can act as numerous physical
cross-linking points inside the linear polyurethane, resulting in a significant improvement in the mechanical properties of the nanocomposites. And when the amount of PPy exceeds 0.25wt%, the tensile strength is getting lower, but still higher than that of pure sample. This may be that excessive PPy aggregates in the composite, which leads to the decrease of mechanical strength. Table 1 TGA and Stretch data of TPU/PPy-x thermal properties
Sample
mechanical properties
temperature
temperature at maximum
Ultimate
Elongation
Young's
at 5%
weight loss rate(ºC)
tensile
at
modulus
Strength
Break [%]
[MPa]
weight Peak1(ºC) Peak2(ºC) loss(ºC)
[MPa]
TPU/PPy-0
291.49
347.23
409.67
8.2±1.5
1540±130
25.7±4.1
TPU/PPy-0.1
299.24
363.80
409.97
10.0±0.9
1474±85
37.9±3.5
TPU/PPy-0.25
313.13
367.85
409.62
13.5±0.5
1650±54
57.2±2.5
TPU/PPy-0.5
321.81
-
411.90
12.3±0.4
1600±70
67.5±8.6
TPU/PPy-1
325.33
-
412.79
12.0±0.5
1540±89
63.9±5.2
3.3 Morphology and Struture of TPU/PPy-x Microscopic morphology of TPU/PPy nanocomposites were characterized by SEM and the fracture surface of samples were given in Fig.4. Irregular wrinkled stripes presented on the surface of TPU/PPy-0, due to the crystallization of PCL segments (Fig.4a). However, with the increase of PPy content in TPU, the fracture surface of samples become rougher (Fig.4b,4c,4e). As show in Fig.4d and Fig.4f , which are enlarged view of Fig.4c and Fig.4e respectively, it's obvious that PPy agglomerates on the polymer surface when
the content of PPy exceeds 0.50wt%. But PPy has no obvious aggregation on the polymer surface when the content of PPy below 0.25wt% (Fig.4b).
Fig. 4. SEM images of (a) TPU/PPy-0, (b) TPU/PPy-0.25,(c) (d)TPU/PPy-0. 5,(e) (f)TPU/PPy-1 (the scale bar of a,b,c,e:10 µm and the scale bar of d, f : 5 µm)
In order to further observe the distribution of PPy in TPU, the sample was observed by TEM. As shown in Fig.5a, when the content of PPy is 0.25wt%, PPy can be well dispered in TPU. But in Fig.5b, when the content of PPy is 1wt%, it is clearly that PPy aggregate in TPU matrix. This result could explain the tensile properties very well. When the content of PPy is below 0.25wt%, PPy disperses uniformly in polyurethane, which indicates that the PPy can have better interaction with TPU, thereby improving the tensile strength of TPU/PPy nanocomposites. When the content of PPy exceeds 0.50wt%, excessive PPy does not disperse well in polyurethane and then agglomerates. The excessive PPy disturb the orientations of polymer chain at high loadings, which lead to decrease of the mechanical properties.
Fig. 5. TEM images of (a) TPU/PPy-0.25, (b) TPU/PPy-1
DSC measurements were carried out to monitor the crystallization properties of TPU/PPy nanocomposites. The DSC curves of the TPU/PPy nanocomposites are shown in Fig.6. First heating scans of all the TPUs show two peaks of melting temperature (Tm). The peaks at lower and higher temperatures can be ascribed to the melting of less ordered structures and better ordered structures, respectively [42,43]. The pure TPU has two distinct melting peaks, at 32.13oC and 37.63oC, respectively. With the addition of PPy,the first Tm almost disappeared, but the second Tm raised to around 39℃. This means that with the addition of PPy, the less ordered structures gradually reduce, and the better ordered structures gradually increase. This phenomenon may be due to the well-dispersed PPy facilitates the crystallization of TPU, act as heterogeneous nucleation agent. Besides, the surface of PPy has a large amount of N-H, which can form hydrogen bonds with urethane bond and ester bond, thereby promoting the aggregation of polyurethane on the surface of PPy and promoting to form better ordered structures. This result is consistent with changes in the
thermal stability and mechanical properties of the TPU/PPy nanocomposite.
Fig. 6. DSC curves of TPU and TPU/PPy-x
3.4 Photo-thermal effect of TPU/PPy-x Nanocomposites have better photo-thermal conversion performance and reach higher temperatures in a shorter period of time, which can achieve rapid repair. Firstly, the absorbance intensity of TPU/PPy-x in UV-vis-NIR was studied by UV 3600 spectrometer, results are given in Fig.S4. In which, the concentration of TPU/PPy-x was 0.1 g/mL. The spectra reveal that pristine TPU/PPy-0 hardly possess photo-absorbing properties, while the TPU/PPy nanocomposites show excellent photo-absorption properties between 800 and 1000 nm. Specifically, with the increasing content of PPy, the intensity of photo-absorption of TPU/PPy was enhanced. Next, temperature changes of samples with irradiation time were measured. The sample were placed under NIR emitter, at the same time, a thermocouple was used to record the temperature of the nanocomposites in real time and the probe of thermocouple was placed in the film (the thickness of all film is about
1mm, as show in Fig.7a), the result was shown in Fig.7b. Nanocomposites material in this experiment can basically reach the equilibrium temperature within two minutes. Attributed to the excellent photo-thermal effect of PPy, the heating rate of nanocomposites are significantly faster than that of pure TPU, and the equilibrium temperature reached by the nanocomposites are significantly higher than that of pure TPU. It's obvious that the temperature of the TPU/PPy-0.25 increased from room temperature to 150 oC within 120 seconds, and the temperature of the TPU/PPy-1 could reach to 150 oC in just 80 seconds. Besides, the heating rate of samples are high at the beginning and then declines gradually, this because of the fast heat dissipation at higher temperature[6,44]. Compared with other photo-thermal conversion agents, such as
carbon
nanotubes
and
graphene[6,28,29],
metal
nanowires
and
nanoparticles[4,5,30],PPy show higher photo-thermal conversion efficiency, which can reach high temperature at low content.
Fig. 7. (a) Schematic illustration of experimental setup for light-thermal conversion and (b) Variations of temperature of the samples with increasing illumination time (Light intensity:1.0 W cm-2 , distance from the sample surface to laser: 9 cm)
To further examine the healing performance under NIR light,
TPU/PPy-0.25 was chose as the research object. The process and mechanism of self-healing under NIR light show in Fig.8. Firstly, samples were cut into dumbbells, and then the dumbbell shaped samples were completely cut off. Next the two cut surfaces were brought back together and placed under an NIR laser light,then self-healing was processed by irradiating. After NIR light irradiation, the sample was placed in air at room temperature for two days. At last, tensile tests were carried out to assess the repair efficiency. The self-healing efficiency was determined by the ratios of the tensile strength of healed sample to origin sample (as shown by Eq(1)). Fig.9b presents the mechanical healing efficiencies of TPU/PPy-0.25 with different irradiation time of NIR, and the TPU/PPy-0 (Fig.9a) was used as a comparison. TPU/PPy-0 show low healing efficiency, whose healing efficiency was only 50% under NIR in 30 seconds. As the irradiating time increases, the healing efficiency of TPU/PPy-0 gradually increase. When the irradiating time exceeds 2 minutes, the healing efficiency of TPU/PPy-0 can reach to 100%. This all because that TPU/PPy-0 can be heated under NIR (as show in Fig.7b), whose temperature can be raised to 54.8℃ in two minutes, which is higher than the melting temperature of TPU/PPy-0. So polymer chains can move to bring the cut surface closer together, and rearranging and twining at the incision. When the temperature is lowered, polymer chains crystallize on the incision, and repairing it.
Fig. 8. The process (a) and mechanism (b) of NIR light-induced self-healing
Compared to TPU/PPy-0, TPU/PPy-0.25 shows better repair performance under NIR. The healing efficiency of TPU/PPy-0.25 can reach to 84% only need 30 seconds. From Fig.7b, we know that TPU/PPy-0.25 can be raised to 150 oC within 2 minutes under NIR, which is much higher than the melt temperature of TPU/PPy-0.25. So polymer chain of TPU/PPy-0.25 can move more easily than TPU/PPy-0, causing the repair to proceed faster. By comparison, we know that PPy plays a very important role in the repair process of composite materials: absorbing light and then transforming it into heat. As light is turned on, the temperature of the exposed area increases rapidly, which favors polymer chain inter-diffusion on the fracture surfaces so that the cracked surface can be healed after irradiation (as show in Fig.8b). But when the irradiating time exceeds 90 seconds, composite material deforms gradually.
This may be because overexposure generate excessive heat, which may cause deformation of the nanocomposites material.
Fig. 9. The healing efficiency of (a)TPU/PPy-0 and (b)TPU/PPy-0.25
4. Conclusions In summary, the NIR light-induced self-healing TPU/PPy nanocomposites were successfully prepared by solution blending. The introduction of the PPy to the polymer matrix improved the thermal stability and the mechanical strength of TPU, and endowed the TPU/PPy nanocomposites with excellent NIR light-responsive property. Meanwhile, the influence of PPy content on the mechanical strength and photo-thermal effect of the nanocomposites were also investigated. When the content of PPy was 0.25wt%, the nanocomposites have best mechanical strength and better photo-thermal effect. After repair experiments, the introduction of PPy can significantly accelerate the repair
efficiency of composite materials under near-infrared light. That can well solve the contradiction that self-repairing materials cannot simultaneously improve the repair efficiency and mechanical properties, and apply PPy into nanocomposites.
Author information *Corresponding authors. Tel: +86 43185262080 E-mail:
[email protected] (Y. Yang)
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Highlights: 1.
Polypyrrole nanoparticles (PPy) have excellent photo-thermal effect, which are used in near-infrared (NIR) light-induced self-healing materials.
2.
The introduction of PPy significantly enhances mechanical strength and thermal stability of the composite.
3. PPy can promote the crystallization of polyurethane(TPU) to increase its melting temperature. 4. TPU/PPy nanocomposites can be repaired quickly under NIR.
The authors declare no conflict of interest.