The efficacy of electrospun polyurethane fibers with TiO 2 in a real time weathering condition

Abstract

This study focuses on the behavior of electrospun polyurethane fibers filled with nano, micro spheres and nanotube TiO₂, thus nTiO₂, mTiO₂ or TiNT, under real time ultraviolet (UV) radiation. Analyses were conducted using scanning electron microscopy (SEM), attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analysis and differential scanning calorimetry (DSC) techniques. Significant changes of thermoplastic polyurethane (TPU) + 0.5% nTiO₂ and TPU + 0.5% mTiO₂ fibers morphology, that is, agglomerated TiO₂ particles, were observed on material surfaces due to TPU fiber degradation, also supported by FTIR spectra given as reduction in the peaks intensities after UV radiation, especially in the case of TPU + 0.5% nTiO₂. Further, after UV radiation, the initial degradation temperature reduction of the hard segment for all materials was up to 40℃. A significant reduction in the final degradation temperature (from 397.8℃ to 274.1℃) was noticed for the mTiO₂ filled fibers, suggesting their photocatalytic activity, while T_g was increased up to 20℃ for almost all fillers added to TPU, thus indicating lower molecular chain mobility upon UV radiation.

Keywords

electrospinning polyurethane scaffolds ultraviolet degradation

Titanium dioxide (TiO₂) has been known as an almost ideal photocatalyst due to its cost-effectiveness, non-toxicity, chemical stability, biocompatibility and its high oxidizing photogenerated holes.^1,2 The photocatalytic mechanism of the TiO₂ involves the generation of electron/hole pairs (e⁻ CB/ h⁺ VB) under light with appropriate wavelengths. Due to excitation, electrons move to the conduction band leaving matching holes in the valance band, thus being good reducing and highly oxidizing agents, respectively.³ Reactive oxidizing compounds and electron/hole pairs formed during photocatalysis can then act as decomposing agents of organic and inorganic impurities, as well as biological species.⁴ The decomposing property has been clearly shown for the TiO₂ when used in self-cleaning applications.⁵ Other applications include antibacterial finishes,⁶ ultraviolet (UV)-protective coatings,⁷ hydrophilic coatings,⁸ waste water treatment,⁹ the generation of hydrogen as a source of energy,¹⁰ etc. Efforts have been made to overcome the TiO₂ drawback concerning its activation only in the UV light region due to its wide band gap. Thus, to be active in the visible light region pure TiO₂ needs to be modified by dyes,¹¹ doped by metals or non-metals,¹² etc.

Electrospinning is one of the most significant nanofiber production techniques of the last century. It is based on the principle of fine fiber formation with the help of an electric field. The electrostatic forces stretch a polymer solution (or melt), which undergoes linear stretching and bending instability with simultaneous solvent evaporation as it travels to the opposite polar collector and finally results in a dried nanofibrous nonwoven material formation. Due to the uniqueness of the nanofibers and electrospun materials properties, such as nanoscaled diameter, high surface to volume ratio, porosity and interconnections between pores, high specific surface area, versatility in polymers and fillers selected, etc.; these materials have found a number of applications.¹³ The most significant application areas include biomedicine,¹⁴ drug delivery,¹⁵ environmental protection,¹⁶ mechanical to electrical energy conversion,¹⁷ passive thermal energy storage,¹⁸ etc.

In electrospun materials, TiO₂ has been explored in the shape of nanoparticles directly electrospun from polymer solution¹⁹ or chemically bonded to the polymer. The photocatalytic activity of the nanoparticles is greater than the bulk material itself.²⁰ Another aspect is the production of TiO₂ fibers by electrospinning of the precursors followed by heat treatment.²¹ The materials have been developed for the photodegradation of toxic dyes,^13,22 solar to electrical energy conversion devices^13,23 and hydrogen generation.^13,24

In our previous study,²⁵ the behavior of polycaprolactone (PCL) + TiO₂ electrospun fibers was investigated under UV irradiation (utilizing UV lamp), while in this study we were interested in the behavior of the electrospun fibers of thermoplastic polyurethane (TPU) + TiO₂ after real time UV radiation. Electrospun TPU materials containing TiO₂ are to be used as temporary scaffolds for cell cultures, and thus tissue thin layer repairs. The addition of the TiO₂ particles can help in the protection from bacterial contamination during tissue layer growth. Since after complete tissue layers growth the electrospun scaffolds are disposed of, an important step in their environment disposal is their rate of degradability. In this respect, the TiO₂ nanoparticles can help in the degradability enhancement. For this reason, the electrospun TPU particles with TiO₂ were exposed to natural weather, thus natural UV light, to help determine how the materials will behave in the real condition. The study further investigates material behavior in regard to TiO₂ shapes, thus not only nano spheres (nTiO₂), but also micro spheres (mTiO₂) and nanotubes (TiNT) were added to the polymer solutions prior to electrospinning. From this point of view, materials suggested for fabrication and characterization will have bi-functionality, that is, the antibacterial function and after usage, the polymer photodegradation enhancement function.

Experimental details

The materials used in this work were commercial TPU, DESMOPAN 588E, purchased from Bayer, Germany, titanium dioxide (TiO₂) (nano-, micro-particles), N,N dimethylformamide (DMF) and tetrahydrofuran (THF), all purchased from Sigma Aldrich. The nano (nTiO₂) and micro (mTiO₂) particles mean diameters and densities were 21 nm, 4.26 gcm^–3 and 0.1 µm to several µm, 3.9 gcm^–3, respectively. In addition, titanium nanotubes (TiNT) were synthesized by the hydrothermal method as described previously.²⁵ TPU single polymer and polymer + filler solutions were prepared by dissolution in DMF and THF with mass ratios of 1:1, polymer concentration of 14% and filler concentrations of 0.5 and 1.0 wt%. The solutions were homogenized by continuous stirring and additional ultrasonification.

Electrospinning and UV radiation

TPU and TPU with TiO₂ electrospun materials were prepared on a standard needle electrospinning device, NT-ESS-300, NTSEE Co. Ltd, South Korea. The processing parameters were electrical voltage of 15 kV, flow rate of 1 mlh^–1 and needle tip to collector distance of 18 cm. As-prepared electrospun materials were exposed to UV radiation in the real time weathering condition during the summer period for a minimum of 30 days. The samples were removed from sun exposure during random rainy days only.

Characterization techniques

To examine material behavior before and after UV radiation, all the samples were evaluated conducting scanning electron microscopy (SEM), attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. Fiber morphology was observed under a FEG SEM QUANTA 250 from FEI with no metal coating. Changes in the chemical structure were analyzed with a FTIR Spectrum One, Perkin Elmer with ATR chamber. DSC analysis was carried using a Mettler Toledo DSC 822e with samples of ∼10 mg under nitrogen. The samples were first heated from 25℃ to 180℃ and held for 10 minutes to erase the thermal history, and afterwards they were cooled to –100℃ and again heated back to 180℃, all at the rate of 10℃min^–1. The thermal stability was determined using TGA analyzer Q500 with samples of ∼10 mg, in the temperature range of 25–600℃ and rate of 10℃min^–1 in nitrogen atmosphere.

This paper discusses the most significant results obtained from the characterization techniques of the as-prepared TPU with TiO₂ electrospun materials.

Results and discussion

Fiber morphology before and after UV radiation

Figure 1 shows SEM images of the electrospun materials before and after UV radiation for the neat TPU (Figures 1(a) and (b)) and the composite materials: TPU + 0.5% nTiO₂ (Figures 1(c) and (d)), TPU + 0.5% mTiO₂ (Figures 1(e) and (f)) and TPU + 0.5% TiNT (Figures 1(g) and (h)). These images show the most significant changes in regard to fiber appearance. The neat TPU fibers generally had a cylindrical shape, and uniformity along the length before UV radiation (Figure 1(a)). After UV radiation these fibers did not show a drastic change in their morphology. The main change observed was in the shape of the pores, thus changing from rectangular to more oval ones (Figure 1(b)). This is also a result of the increase in the fiber junctions and agglutination after UV radiation. The fillers added, especially the nano and micro TiO₂ particles, were observed inside or on the near surface along the fiber length, mostly as agglomerations (Figures 1(c) and (e)). The TiNT could not be observed clearly under the SEM (Figure 1(g)). After UV radiation, the most appealing changes were the fiber agglutinations, thus pore clogging and a high level of fiber degradation (Figures 1(d), (f) and (h)) due to the photocatalytic activity of the TiO₂ particles.

Figure 1.

Scanning electron microscopy (SEM) images of the electrospun materials: thermoplastic polyurethane (TPU) (a), TPU + 0.5% nTiO₂ (c), TPU + 0.5% mTiO₂ (e), TPU + 0.5% TiNT (g) before and after 30 days of ultraviolet radiation (b), (d), (f), (h).

The photocatalytic activity was suspected to be due to the TiO₂ agglomerations observed on the surface, resulting in fibers disappearance. Similar observations were reported in our previous study when PCL with TiO₂ electrospun fibers were irradiated for 5 and 10 days under an UV lamp. The results suggested no photocatalytic activity in the case of the TiNT fillers.²⁵

Thermal stability before and after UV radiation

Figures 2(a)–(f) present the TGA and differential thermal gravimetric (DTG) analysis curves of the electrospun materials before and after UV radiation. Their characteristic temperatures during thermal degradation are given in Table 1. The degradation of the neat electrospun TPU involves two steps. The first step has an initial temperature at 283.1℃ and is related to the degradation of the TPU hard segments. The second step has an initial temperature at 323.1℃ and is related to the degradation of the TPU soft segments. The maximum degradation rates were at temperatures of 320.9℃ and 361.2℃, respectively, Figures 2(b), (d) and (f). At 0.5% of all TiO₂ fillers, there was a decrease in the initial degradation temperature (T_5%). This suggests decreased thermal stability or weak places in the fibers due to the agglomeration of the TiO₂ fillers. Sometimes the fillers can slow down the degradation due to the interaction with the polymer chains or polymer free radicals and will inhibit the propagation. Thus, they will not have an influence on the polymer thermal stability.²⁵ The degradation of the soft segments results at higher initial temperatures (T² _int ) for the nano and micro filled fibers, thus improving thermal stability of the soft segment. The final degradation temperature (T_fin) increases from 388℃ up to 420.5℃, for all filler types, suggesting that the TiO₂ particles enhance the temperature interval of degradation for the electrospun TPU with TiO₂ fibers. In regard to maximum degradation rates, the peak temperatures (T¹_max) of the hard segments shift to lower values after the addition of the 0.5% of all filler types, while the peak temperatures (T²_max) of the soft segments shift to higher values. This suggests that the filler addition results in faster and slower degradation rates, respectively.

Figure 2.

Thermogravimetric and differential thermal gravimetric analysis curves of the electrospun fibers: thermoplastic polyurethane (TPU) + mTiO₂ (a), (b), TPU + nTiO₂ (c), (d) and TPU + TiNT (e), (f), before and after ultraviolet (UV) radiation.

Table 1.

Characteristic temperatures during thermal degradation before and after ultraviolet (UV) radiation

Electrospun fibers	T_5% (℃)	T² _int (℃)	T_fin (℃)	Δm₁ (%)	Δm₂ (%)	Residue 700℃ (%)	T¹_max (℃)	T²_max (℃)
Before UV radiation
TPU	283.1	323.1	388.0	28.5	35.0	3.5	320.0	361.2
TPU+0.5% mTiO₂	280.0	338.3	397.8	33.7	44.7	21.3	315.2	375.1
TPU+0.5% nTiO₂	258.8	333.9	394.4	29.7	45.9	23.8	293.7	375.0
TPU+0.5% TiNT	260.2	314.9	420.5	38.7	50.4	8.2	290.4	384.2
After 30 days of UV radiation
TPU	275.1	341.3	399.2	38.4	32.0	7.9	342.4	375.5
TPU+0.5% mTiO₂	248.1	344.6	274.1	37.7	33.1	30.0	344.2	379.0
TPU+1% nTiO₂	236.0	340.3	404.5	39.9	38.6	21.1	339.4	376.7
TPU+0.5% TiNT	224.1	311.6	388.5	34.0	48.0	16.1	312.1	353.1

TPU: thermoplastic polyurethane.

The UV radiation reduced the initial degradation temperature for all electrospun materials, up to almost 40℃. This suggests lower thermal stability due to the free radical formation. However, T² _int increased after UV radiation for the nano and micro filled fibers, while T_fin reduced for all filler added materials except for the TPU + 0.5% nTiO₂ and the neat TPU. Drastic reduction in the final degradation temperature (from 397.8℃ to 274.1℃) confirms the photocatalytic activity of the mTiO₂ filler. The percentage of the residue after UV radiation was higher due to crosslinking structure formation in TPU.

Melting and crystallization behavior before and after UV radiation

To understand the behavior of electrospun TPU with TiO₂ fibers under UV radiation, during melting and crystallization, the materials were heated and cooled in the range from –100℃ to 180℃. Figures 3(a)–(f) show the DSC curves of the electrospun neat TPU and TPU with TiO₂ fibers before and after 30 days of UV radiation. The transition temperatures are given in Table 2, indicating the glass transition temperature (T_g), the melting temperatures (T_m₁, T_m₂) and the crystallization temperatures (T_c₁, T_c₂). Index 1 stands for low and index 2 for high-ordered domains in the hard segment of the semicrystalline TPU. The T_g of the soft segment in the neat electrospun TPU was at –38.8℃. Temperatures of melting T_m₁, T_m₂ were at 62.1℃ and 122.5℃, while temperatures of crystallization T_c₁, T_c₂ were at 2.9℃ and 102℃, respectively. Compared to the T_g of TPU prepared by injection molding/compression (T_g = –13.7℃),²⁶ the electrospun TPU has showed significantly lower T_g, which might be explained by the structural changes of the polymer solution during electrospinning. Similar has been discussed by others.²⁷ The addition of the 0.5% nano and micro TiO₂ resulted in the T_g increase, while the opposite effect was observed for the TiNT filler. The shift of the T_g to lower values is due to the larger separation between the soft and the hard segment in the TPU, while the shift of the T_g to higher values suggests that the interaction between TiO₂ and TPU restricted the mobility of the molecular chains in the TPU soft segment. The variations in both effects might be due to agglomerations and nonuniform distribution of the fillers along the fibers. The melting temperature for all TPU with TiO₂ electrospun fibers has shown small increase of ∼3℃ (T_m₁) or negligible change (both increase and decrease) (T_m₂) compared to the neat TPU. The increase of the melting temperatures suggests a higher ordered structure, while the small decrease is due to the reduction in the structural order of the hard segment. The crystallization temperatures T_c₁ of the low-ordered domains in the hard segment showed a significant decrease of 20–30℃ in case of the 0.5% nTiO₂ and TiNT. The T_c₂ for all filler types added in the fibers resulted in an increase for almost 10℃, with a new peak around 150℃. The increase in the crystallization temperature suggests faster crystallization compared to the electrospun neat TPU.

Figure 3.

Differential scanning calorimetry curves during heating and cooling of the electrospun fibers: thermoplastic polyurethane (TPU) + mTiO₂ (a), (b), TPU + nTiO₂ (c), (d) and TPU + TiNT (e), (f), before and after ultraviolet (UV) radiation, respectively.

Table 2.

Characteristic transitional temperatures of electrospun fibers before and after ultraviolet (UV) radiation

Electrospun fibers	T_g (℃)	T_m₁ (℃)	T_m₂ (℃)	T_c₁ (℃)	T_c₂ (℃)
Before UV radiation
TPU	−38.8	62.1	122.5	52.9	102.0 (–)
TPU+0.5% mTiO₂	−40.0	65.1	119.1	55.3	109.0 (151.5)
TPU+0.5% nTiO₂	−42.3	65.7	123.3	29.2	107.9 (158.4)
TPU+1% nTiO₂	−38.1	65.1	122.3	52.5	109.7 (154.0)
TPU+0.5% TiNT	−35.5	65.1	121.5	21.3	103.7
After 30 days of UV radiation
TPU	−31.5	65.3	121.9	58.2	106.8
TPU+0.5% mTiO₂	−20.3	65.8	123.7	58.3	112.9
TPU+1% nTiO₂	−23.5	65.4	122.7	58.6	111.1
TPU+0.5% TiNT	−21.6	64.4	120.3	58.8	110.8

TPU: thermoplastic polyurethane.

After 30 days of UV radiation, the T_g of the neat TPU increased for ∼7℃ with a smaller increase in the T_m and T_c. The increase in the melting temperature suggests a higher ordered crystalline structure in the hard segment after UV radiation. The filler addition increased the T_g for up to 20℃. The increase in the T_g indicated the reduction of the molecular chain mobility after UV radiation due to the formation of the crosslinking structure in TPU. The crystallization temperatures of the hard segments were increased for up to 37.5℃ (T_c₁) for the TPU + 0.5% TiNT. The peaks above 150℃ disappeared after UV radiation.

ATR-FTIR analysis before and after UV radiation

Figures 4 –7 present the ATR-FTIR spectra of the electrospun TPU without and with the TiO₂ fillers, before and after 30 days of UV radiation. Typical peaks of the electrospun TPU (according to supplier, this is an ether/ester type of polyurethane) are marked in Figure 4. The wavelength at 3334 cm^–1 confirms the N-H stretching, while the C-H bond stretching is at 2969 cm^–1. The C = O (carbonyl group) stretching is at 1726 and 1702 cm^–1. At 1595 cm^–1 is NH bending and aromatic C–C stretching, following 1530 cm^–1 for C = N stretching and N–H bending, 1418 cm^–1 for C-C stretching of the aromatic ring, 1311 cm^–1 for C = N stretching, NH, 1225 cm^–1 for C = N stretching, NH, and 1169 and 1078 cm^–1 for C–O–C stretching. The difference between the ester and the ether groups is in the frequency of the C-O-C bond stretching, which corresponds to the wavelength of 1100 cm^–1 for the asymmetric ether and urethane stretching, or 1250 and 1150 cm^–1 for the ester groups.²⁸ The Ti-O-Ti bond in TiO₂ is identified with peaks below 700 cm^–1,²⁹ but due to the low filler quantity the peaks were weak. After UV radiation there was a change in the color of the samples, from white to yellow, which suggests the formation of the quinoid structure due to photo-oxidative degradation.³⁰ The most significant changes after UV radiation were noticed for the electrospun TPU, TPU + 0.5% nTiO₂ and TPU + 0.5% TiNT, showing a reduction in the intensity peaks for the urethane, aromatic and alkane groups. The changes in the wavelengths (Figure 6) were noticed as follows: 2959, 1707, 1413, 1220, 1174 and 1073 cm^–1. The results suggest TPU degradation due to the filler photocatalytic activity, which is less obvious in the case of the neat TPU after UV radiation (Figure 4). Less change in the intensities was observed for the mTiO₂ filled TPU, which could be due to the nonuniform filler distribution, agglomeration or even less efficient capsulation during electrospinning.

Figure 4.

Attenuated total reflection-Fourier transform infrared spectroscopy spectra of the neat electrospun thermoplastic polyurethane (TPU), before and after 30 days of ultraviolet (UV) radiation.

Figure 5.

Attenuated total reflection-Fourier transform infrared spectroscopy spectra of the electrospun thermoplastic polyurethane (TPU) and TPU + 0.5% nTiO₂, before and after 30 days of ultraviolet (UV) radiation.

Figure 6.

Attenuated total reflection-Fourier transform infrared spectroscopy spectra of the electrospun thermoplastic polyurethane (TPU) + 0.5% nTiO₂, before and after 30 days of ultraviolet (UV) radiation.

Figure 7.

Attenuated total reflection-Fourier transform infrared spectroscopy spectra of the electrospun thermoplastic polyurethane (TPU) + 0.5% TiNT, before and after 30 days of ultraviolet (UV) radiation.

Conclusion

This study considers the effect of TiO₂ nano, micro particles and nanotubes on electrospun TPU material properties under 30 days of UV radiation in real time weathering condition. The materials are to be used as temporary scaffolds for cell cultures, and thus tissue thin layer repairs. The addition of the TiO₂ has a bi-functional role, which on one hand helps in antibacterial protection during cell cultures and growth, and on the other hand after disposal should help in easier and faster TPU degradation. Thus, the study helps one to understand material behavior when exposed to the natural weathering condition. It was revealed that a significant change in fiber morphology was observed in the case of TPU + 0.5% nTiO₂ and TPU + 0.5% mTiO₂, that is, agglomerated TiO₂ particles were observed on material surfaces due to accelerated degradation of the TPU fibers. The FTIR spectra supported this physical observation by the reduction in the peak intensities after UV radiation, especially in the case of TPU + 0.5% nTiO₂ fibers. In regard to thermal degradation, after UV radiation initial degradation temperature reduction of the hard segment, up to almost 40℃, was observed for all materials. Significant reduction in the final degradation temperature (from 397.8℃ to 274.1℃) was noticed for the mTiO₂ filled fibers, confirming their photocatalytic activity. The DSC analysis showed that T_g was increased for up to 20℃ for almost all filler added TPU, suggesting reduction of the molecular chain mobility after UV radiation.

Footnotes

Declaration of conflicting interests

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work has been fully supported by Croatian Science Foundation under the project IP-2016-06-6878, Custom Tailored Fibrous Scaffold Prototype for Tissue Cells Culture via Combined Electrospinning, COMBOELECTROSPUN.

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