Significant Improvement in Thermal and UV Resistances of UHMWPE

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Significant improvement in thermal and UV resistances of UHMWPE fabric through in situ formation of polysiloxane-TiO2 hybrid layers Jiangtao Hu, Qianhong Gao, Lu Xu, Mingxing Zhang, Zhe Xing, Xiaojing Guo, Kuo Zhang, and Guozhong Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04914 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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ACS Applied Materials & Interfaces

Significant improvement in thermal and UV resistances of UHMWPE fabric through in situ formation of polysiloxane-TiO2 hybrid layers Jiangtao Hu1, Qianhong Gao1, 2, Lu Xu1, Mingxing Zhang1, 2, Zhe Xing1, Xiaojing Guo1, Kuo Zhang1, Guozhong Wu1* 1: CAS Center for Excellence on TMSR Energy System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019 Jialuo Rd., Jiading Dist., Shanghai, 201800, China. 2: University of Chinese Academy of Sciences, Beijing 100049, P. R. China. Dr. Jiangtao Hu, Mr. Qianhong Gao made the same contribution to this work. Abstract: Anatase nanocrystalline titanium dioxide coatings were produced on ultra-high molecular weight polyethylene (UHMWPE) fabric by radiation-induced graft polymerization of γ-methacryloxypropyl trimethoxy silane (MAPS), subsequent co-hydrolysis of the graft chains (PMAPS) with tetrabutyl titanate, followed by boiling water treatment for 180 min. The resulting material was coded as UHMWPE-g-PMAPS/TiO2 and characterized by attenuated total reflection infrared spectrometry, differential scanning calorimetry, X-ray diffraction, thermal gravimetry, ultraviolet absorption spectroscopy, etc. The predominant form of TiO2 in the thin film was anatase. The coating layer was composed of two sublayers: an inner part consisting of an organic-inorganic hybrid layer to prevent photocatalytic degradation of the matrix by TiO2 film, and an outer part consisting of anatase nanocrystalline TiO2 capable of UV absorption. This UHMWPE-g-PMAPS/TiO2 composite exhibited much better thermal resistance than conventional UHMWPE fabric, as reflected by the higher melting point, decreased maximum degradation rate, and higher char yield at 700 oC. Compared with UHMWPE fabric, UHMWPE-g-PMAPS/TiO2 exhibited significantly enhanced UV absorption and excellent duration of UV illumination. Specifically, the UV absorption intensity was 2.4-fold higher than that of UHMWPE fabric; the retention of the break strength of UHMWPE-g-PMAPS/TiO2 reached 92.3 % after UV-irradiation. This work provides an approach for addressing the issue

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of self-degradation of TiO2-coated polymeric materials due to the inherent photoactivity of TiO2. Keywords: radiation-induced graft polymerization; sol-gel process; titanium oxide; UHMWPE fabric; UV resistance 1. INTRODUCTION Fabrication of functional textile fabrics which exhibit very unique properties including conductive, self-cleaning, flame retardancy, hydrophobic, antibacterial, antistatic, and other properties without negative impact on the inherent properties of the pristine textile fabrics has become a growing research interest in recent years1-5. Ultra-high molecular weight polyethylene (UHMWPE) fiber, a third-generation high-performance fiber, exhibits very excellent physical and mechanical properties such as low specific weight, high strength, high modulus, high impact resistance, excellent chemical resistance, a low dielectric constant, self-lubricating properties and so on6. Despite the many excellent properties, UHMWPE fibers also have limitations such as chemical inertness and a lack of functional groups on the surface; UHMWPE fibers are also not readily bound to most materials and exhibit poor UV resistance, which is a major drawback to the application of UHMWPE fibers7. Overcoming these problems remains a formidable challenge to date. Many techniques have been adopted to address each aspect of these two shortcomings of UHMWPE fiber. For example, some strategies such as chemical modification8, chemical grafting9, corona discharging10, oxygen plasma11-13, high energy laser14, UV15, and gamma irradiation16 have been employed to improve the surface activity of UHMWPE fiber, and some of them have been industrialized. However, few of these studies simultaneously paid attention to the UV resistance of the fibers and the related literature is also very limited. Based on extensive research, it has been found that the introduction of UV absorbers into materials is one effective approach for circumventing UV damage. Among them TiO2 was selected as a suitable UV-absorber due to its high UV-absorption ability and excellent physicochemical properties17. Deposition of TiO2 nanoparticles on the surface of UHMWPE fiber may improve its UV resistance and expand its usage as reinforcements, self-cleaning


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fabrics, or other fascinating applications. Titanium dioxide (TiO2), an excellent UV absorber, has attracted great attention worldwide owing to its environmentally-friendly nature, excellent photocatalytic property, corrosion resistance and high heat resistance18-19. Current research mainly takes advantage of photocatalytic properties of TiO2 to develop self-cleaning textiles and photocatalysts or its high UV absorption capability to protect materials from UV damage20-22. However, the excellent photoactivity of TiO2 may result in the photodegradation of organic supports of TiO2 films, such as plastics, textiles, and resins, which considerably limits the practical application of TiO2 film as photocatalytic and UV absorbing agents23. To address the aforementioned issues while retaining the excellent properties of TiO2, inert shells that are generally composed of SiO2, MgO, Al2O3, ZrO2, CeO2, and their mixtures or a variety of polymers, have been coated onto TiO2 cores 21, 24-26. These inert species are most commonly deposited on the surface of TiO2 to offer a desired coat formation. For commercialization of such products, the preparation of TiO2-SiO2 and other core-shell particles must be optimized and the limitations addressed through further research. First, there is no covalent bonds exist between these nanoparticles and the organic supports, the efficiency of binding between the nanoparticles and organic supports must be improved. Secondly, the synthesized core-shell particles are relatively large because TiO2 powder generally undergoes agglomeration27. Lastly, the core-shell particles exhibit relatively poor UV-absorption ability because the active sites of the TiO2 film are generally completely or partly shielded by the inert shell28. Alternative approaches have been developed to resolve the above-mentioned problems, such as in situ coating of the film on textile fibers through sol-gel technology, improving the surface properties by surface modification techniques (e.g. plasma treatment), and using adhesives, resins, and organosilanes as agents to promote cross-linking between the TiO2 particles and fabrics29-31. Such methods can improve the ability of TiO2 films to adhere to the supports and prevent agglomeration of the TiO2 particles, but cannot completely avert photodegradation of the organic supports of the TiO2 film; such approaches may also negatively affect the efficiency of UV absorption by TiO2.



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Based on the previously reported works, the mentioned approaches are only capable of solving one aspect of aforementioned problems. To the best of our knowledge, there is no report on improvement of the adhesion of the TiO2 film to the support while simultaneously avoiding agglomeration and photodegradation of the organic supports of the TiO2 film; thus, further research is needed. In this study, a new kind of UHMWPE fabric-based composite was synthesized by radiation-induced graft polymerization of γ-methacryloxypropyl trimethoxy silane (MAPS) with subsequent co-hydrolysis of the graft chains (PMAPS) using tetrabutyl titanate. This composite is not susceptible to photodegradation of the organic support of the TiO2 film and also exhibits excellent heat resistance, UV shielding performance and higher retention of the break strength after UV-irradiation. The influences of molecular structure and the mechanisms underlying the structure-activity relationship were evaluated. 2. EXPERIMENTAL 2.1 Raw materials UHMWPE fabrics were purchased from Beijing Tongyizhong Specialty fiber Technology

&

Development

Co.

Ltd.,

plain

weave,

206.85

g/m2.

γ-methacryloxypropyl trimethoxy silane (CR), methanol (AR), ethanol (AR), tetrabutyl titanate (TBT, AR), acetic acid (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd (China), and used as received. 2.2 Preparation of UHMWPE-g-PMAPS The simultaneous γ-radiation-induced graft polymerization is normally initiated by free-radical reactions. Generally, the grafting process follows two steps: 1) create active sites for graft polymerization and 2) the introduction of the compound which will participate in the process of graft polymerization. In detail, the grafting process consists of two mechanisms: namely “grafting from” and “grafting to”. In the “grafting from” process, the active sites are created on the matrix, so when the matrix contacts with the monomer solution, the monomer can polymerize and grow from the bottom-up. In the “grafting to” process, the active sites are tethered to the oligomer which can be connected to the matrix by a radical coupling reaction to form a covalent bond. During the process of γ-radiation, the free radicals generated in


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UHMWPE fiber at room temperature were mainly alkyl radicals32. The chemical reactions occurring in the simultaneous radiation-induced graft polymerization are illustrated in Scheme 1. Before graft polymerization, the UHMWPE fabric was rinsed with deionized water, and then extracted with boiling acetone in a Soxhlet apparatus over 24 h to remove the impurities attaching on the surface of UHMWPE fabric. Simultaneous radiation-induced graft polymerization: typically, UHMWPE fabric was placed into irradiation tubes and a methanolic solution with a defined MAPS monomer concentration was added to keep the fabric immersed in the solution. The solution was bubbled with nitrogen for 15 min to eliminate oxygen. The irradiation tubes were then sealed and irradiated with a 60Co γ-ray source for 17 h at room temperature; the total absorbed dose was 20 kGy. After graft polymerization, the fabric was extracted with boiling menthol in a Soxhlet apparatus over 12 h to remove the residual monomer and homopolymer. Finally, the grafted UHMWPE fabric was dried under vacuum at 60 oC to constant weight. The degree of grafting (DG) of the grafted UHMWPE fabric was determined according to Eq. 1: DG (%) = (W1-W0)×100/W0

(1)

where W0 and W1 are the weights of the samples before and after grafting polymerization, respectively. In this work, the DG of UHMWPE-g-PMAPS was 50.1%. A mixture of ethanol (20 ml), acetic acid (1.5 ml), and deionized water (1 ml) was added dropwise to an ethanolic solution (20 ml) of tetrabutyl titanate (20 ml) with vigorous stirring. After stirring the mixture for 5 min, UHMWPE-g-PMAPS (0.3-0.5 g) was added, followed by continuous stirring for 10 h at 50 oC. Ethanol solution (10 ml) containing deionized water (1 ml) was then added dropwise over 20 min, followed by continuous stirring for 5 h at 60 oC; white TiO2 particles were thus produced. The product was then hydrothermally treated in a boiling water bath for 3 h and finally ultrasonic cleaned at room temperature to remove the TiO2 nanoparticles that were not chemically bound to the UHMWPE fabric. The weight of the coated



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TiO2 nano-particles was determined as the weight increase of UHMWPE-g-PMAPS, and was also calculated according to Eq. 1. The weight of TiO2 nanoparticles in the UHMWPE-g-PMAPS/TiO2 composite was 4.7%. 2.3 UV-Radiation Treatment The pristine and modified fibers obtained from the corresponding fabrics were irradiated following the accelerated photo-ageing procedures33 (UV lamp: 10 W, 260-340 nm, Hualun, Shanghai; Filter, Shanghai Seagull Colored Optical Glass). The distance between the fibers and mercury light was 20 cm. 2.4 Measurements Attenuated total reflection infrared spectrometry (ATR-IR) was recorded in the region of 4000 to 600 cm-1 on a Bruker Tensor 207 FT-IR spectrometer attached to an attenuated total reflection (ATR) apparatus; the resolution of the wave number was 4 cm-1. X-ray diffraction (XRD) spectrum was measured on a RIGAKU D/Max2200 XRD instrument equipped with Cu-Kα radiation (λ = 1.54Å). Thermogravimetric analysis (TGA) was performed on a TG 209 F3 Tarsus (NETZSCH, Germany) instrument from 50 to 700 °C under nitrogen atmosphere with a heating rate of 10 °C/min. The temperature at which the weight loss is 5 wt% is defined as the initial degradation temperature (Tdi). A scanning electron microscope (JEOL JSM-6700F, Japan) was employed to observe the morphologies of the pristine UHMWPE fabric and functionalized UHMWPE fabrics. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI-5702 electron spectrometer using an Al Kα line excitation source with the C 1s at 285.0 eV as a reference. DSC measurements were carried out by using a PerkinElmer DSC-822e analyzer; 5-10 mg samples were sealed in aluminum crucibles for measurements. The measurements were performed from 20 to 180 oC with a heating rate of 10 oC/min. All the performances were carried out in a nitrogen atmosphere with a gas flow of 20 mL/min.


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The

monofilaments

of

the

pristine,

UHMWPE-g-PMAPS

and

UHMWPE-g-PMAPS/TiO2 fibers were measured with an electronic single fiber tensile strength tester (LLY-06E, Laizhou Electron Instrument Co. Ltd.) according to the Chinese Standard GB/T14337-2008. The fibers were clamped by pneumatic clamps, and the distance between two fixtures was 20 mm. All tests were carried out at 25 oC with a crosshead speed of 10 cm/min. In each test, at least 50 monofilaments were tested, and the average value of these tests was reported. UV-Vis spectra were conducted using a Ruili 1100 spectrophotometer (Cary 50, America) from 200 to 800 nm, and integrating sphere was used as an affiliated fitting in solid UV test. 3 RESULTS AND DISCUSSION 3.1 Design and characterization of UHMWPE-g-PMAPS/TiO2 fabric As described above, it is difficult to avoid photodegradation of the organic supports in the TiO2 nanoparticle composites while retaining the excellent UV-absorption

property

of

the

supported

TiO2.

Therefore,

we

designed

organic-inorganic hybridized coating between TiO2 nanoparticles and its organic supports to address the aforementioned problem. A commercially available silane-based acrylate monomer, γ-methacryloxypropyl trimethoxy silane, was grafted onto UHMWPE fabric by γ-ray radiation induced graft polymerization and the graft chains were then co-hydrolyzed with tetrabutyl titanate for in situ formation of the TiO2 thin film. The product was then treated in boiling water according to the method described in the literature34. The preparation process is illustrated in Scheme 2. The newly developed process has the following advantages: first, co-hydrolysis of the graft chains with tetrabutyl titanate leads to the formation of Si-O-Ti chemical bonds between the graft layer and TiO2 layer, which improve the strength of adhesion between the TiO2 nanoparticles and UHMWPE fabric. Second, in-situ generation of the TiO2 nanoparticles through the sol-gel process not only prevents agglomeration of the TiO2 nanoparticles, but also results in the formation of TiO2 coatings with a uniform thickness. Third, there is an interface between the UHMWPE fabric and TiO2 inorganic layer comprising Si-O-Ti and Si-O-Si chemical bonds, which prevents photodegradation of the organic species supporting the TiO2 nanoparticles. PMAPS was selected as an interface layer because the energy of the Si-O-Si bond is up to 460



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kJ/mo1, which is considerably higher than that of UV irradiation energy (333-406 kJ/mol). The amount of TiO2 attached to the UHMWPE fabric is important because it determines the most properties of UHMWPE-g-PMAPS/TiO2. Effect of tetrabutyl titanate (TBT) concentration on the amount of loaded TiO2 nanoparticles on the surface of UHMWPE fabric is shown in Fig.1. The loading amount of TiO2 increases with the increasing TBT concentration, as long as the TBT concentration is lower than 30%. When the TBT concentration is higher than 30%, the loading amount of TiO2 changes slightly. These results indicate that when the attached TiO2 is up to a certain amount, the PMAPS grafting chains are mostly covered by TiO2 film, and they cannot continue to anchor the precursor of TiO2 to the surface of UHMWPE-g-PMAPS. Furthermore, higher TBT concentration is beneficial to the formation of TiO2 nanoparticles in the solution which also hindered the diffusion of TBT to the interface of UHMWPE-g-PMAPS. The presence of PMAPS graft chains in the UHMWPE-g-PMAPS samples was confirmed by FT-IR spectroscopy (Fig. 2), where the new absorption peaks at 1082 cm-1 and 820 cm-1 can be ascribed to Si-O-C and Si-C stretching vibrations35. The absorption peak at 1725 cm−1 is due to the antisymmetric stretching vibration of the ester carbonyl in the carboxyl groups; the peak at 1160 cm-1 is a characteristic C(=O)-O-C stretching vibration36. The data suggest that MAPS was successfully grafted to the surface of the UHMWPE fabric. The broad peak around 2500-3700 cm-1 could be ascribed to the stretching vibration of the hydroxyl groups of Ti-OH, and the peak at 630 cm-1 is assigned to the Ti-O-Ti vibration37. The above results further indicate that the TiO2 nanoparticles were successfully attached to the surface of the UHMWPE fabric. In order to determine the surface composition and chemical states of the UHMWPE-g-PMAPS/TiO2 composite, the samples were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3, only peaks corresponding to C were detected for the pristine UHMWPE fabric (Fig. 3a). In contrast, additional characteristic Si2s, Si2p, Ti2s, Ti2p, Ti3s, and Ti3p peaks were observed for the


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UHMWPE-g-PMAPS/TiO2 composite (Fig. 3b). This observation indicates the existence of Ti and Si elements on the surface of UHMWPE-g-PMAPS/TiO2. However, the intensity of the peaks of atomic Si is very weak, probably owing to coverage of the PMAPS molecular chains by TiO2 nanoparticles. To further confirm formation of Ti-O-Si bonds in the UHMWPE-g-PMAPS/TiO2 composite, the Si2p peak was resolved into three peaks at 104.6, 101.8, and 102.7 eV (Fig. 4). The sub-peaks at 104.6, 101.8, and 102.7 eV were assigned to Si-O-Si38, Si-C39, and Si-O-Ti40, respectively. Fig. 5 presents representative SEM images of the UHMWPE fabric (a, a’), UHMWPE-g-PMAPS (b, b’), and UHMWPE-g-PMAPS/TiO2 (c, c’, c’’) at magnifications of ×100 (a, b, c), ×5000 (a’, b’, c’) and ×30000 (c’’). The microstructure of the UHMWPE fabric is clearly the same before and after radiation graft polymerization (Fig. 5 a, b). Comparison of the surface morphology of a single fiber shows that the surface of the pristine and grafted UHMWPE fibers is smooth, although there are many cracks with dimensions of several nanometers for UHMWPE-g-PMAPS, caused by accumulation of the grafting polymer chains. After sol-gel coating with TiO2 sol, the xerogel of TiO2 was formed on the fiber surface, making the surface rougher. The UHMWPE fabric was clearly covered with a uniform and dense film of TiO2 (Fig. 5 c, c’ and c’’). The external layer of the UHMWPE fabric was covered by TiO2 nanoparticles that impart unique features to the UHMWPE fabric. 3.2 Analysis of crystal structure The crystalline structure and particle size of the resultant TiO2 nanoparticles formed on the UHMWPE fabric were characterized by XRD (Fig. 6). In order to analyze the crystal structure of the prepared nanoparticles, the observed peaks were assigned by indexing the observed peaks to the standard peaks of anatase phase TiO2; impurity peaks were not detected. A series of peaks of the obtained nanoparticles at 26° (101), 38° (004), 48° (200), 54° (105), 63° (204), 70° (220), and 75° (215) are observed in the spectrum of TiO2 (Fig. 6a), which are consistent with the data list for JCPDS Card No. 21-127241. The diffraction peaks associated with the anatase phase



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at 26°, 38°, and 48°42 were high intensity peaks, as shown in Fig. 6a, indicating predominance

of

the

anatase

microstructure

of

the

film.

This

UHMWPE-g-PMAPS/TiO2 composite exhibited good UV-absorption ability due to the presence of anatase TiO2 on the surface. To determine the crystallite size of the TiO2 particles, the full width at half-maximum (FWHM) of the (101), (004), and (200) reflections was measured according to Scherrer’s equation43. The average calculated crystallite size was 5.3 nm. XRD

patterns

of

the

UHMWPE

fabric,

UHMWPE-g-PMAPS,

and

UHMWPE-g-PMAPS/TiO2 are also shown in Fig. 6b. In detail, two distinct diffraction peaks at 21.6o and 24.1o, corresponding to the (110) and (200) planes of the orthorhombic crystal of polyethylene44, were observed for the UHMWPE fabric and modified UHMWPE fabrics, and there was no change in the XRD peaks for the UHMWPE fabric treated with PMAPS and PMAPS/TiO2. A minor diffraction peak at 19.7o, corresponding to the (010) plane of the monoclinic crystal45, was also found in the XRD patterns of the UHMWPE fabric and UHMWPE-g-PMAPS, but the intensity of this peak was too low to be detected after coating the fabric with the TiO2 nanoparticles. After the graft polymerization process, a broad peak appeared at 2θ = 16-20°, which is believed to be due to the amorphous graft chains of PMAPS. However, after coating the fabric with TiO2 nanoparticles, this peak disappeared; simultaneously, the intensity of the diffraction peaks due to the monoclinic crystals also

decreased

compared

with

those

of

the

UHMWPE

fabric

and

UHMWPE-g-PMAPS. This phenomenon may be explained as follows: first, in order to obtain anatase TiO2, the sample was boiled in hot water (90-100 oC) for 3 h. The monoclinic phase of UHMWPE is unstable and may be transformed to the orthorhombic form after heat treatment; thus, the intensity of the diffraction peaks of the monoclinic crystal decreased after coating the fabric with TiO2 nanoparticles46. Second, during formation of the anatase TiO2 nanoparticles, the graft chain and UHMWPE chain in the amorphous zone underwent crystallization on the surface of the TiO2 nanoparticles because nano-TiO2 acts as a nucleating agent47. Thus, the peak of the amorphous graft chain species disappeared after coating the fabric with TiO2


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nanoparticles. However, the crystallinity of the new crystalline regions formed by the graft chain and UHMWPE chain in the amorphous zone is low and there are many crystal flaws which reduce the thermal stability of the crystalline regions, as reflected in the DSC curve of UHMWPE-g-PMAPS/TiO2 which showed an endothermic peak at about 88.4 oC, caused by melting of the crystalline regions mentioned above. 3.3 Thermal Properties The thermal properties of polymeric materials are generally characterized based on the melting point (Tm) and thermogravimetric behavior. The former indicates the mobility of the molecular chains with increasing temperature, and the latter denotes the thermal stability of the molecular chains48. The DSC profile is illustrated in Fig. 7, and the important parameters, such as the first melting peak (Tm1), the second melting peak (Tm2), and the endothermic heat (∆H) obtained from these curves are listed in Table 1. During the DSC measurement, once the UHMWPE fabric melts, it cannot re-solidify to its original state at a high cooling rate, and the extent of the amorphous region increases. Thus, only the values from the first heating thermograms are presented herein. In order to clearly distinguish both melting peaks and calculate the endothermic heat, the spectra of pristine UHMWPE fabric and UHMWPE-g-PMAPS were fitted by Gaussian function49. Each curve can be divided into two peaks and the corresponding fitting curves are inserted in Fig. 7 (a’, b’). The pristine UHMWPE fabric exhibited two endotherms (Fig. 7a), i.e., a main peak at 144.9 oC and a shoulder peak at 152.5 oC. Similar results were previously reported by other researchers50-51. The main endothermic peak is considered to be due to melting of the orthorhombic crystals and the transition of orthorhombic to pseudo-hexagonal phase. The shoulder peak reflects the melting of pseudo-hexagonal crystals. From the spectrum of UHMWPE-g-PMAPS (Fig. 7b), it can be seen that Tm1 of the orthorhombic crystals remained almost unchanged, whereas the Tm2 peak of the pseudo-hexagonal phase became indiscernible after radiation graft polymerization; moreover, the value of ∆H1 increased, while that of ∆H2 decreased. The above



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phenomenon may be ascribed to the phase transition from pseudo-hexagonal to orthorhombic due to the introduction of the graft chains. The DSC spectrum of UHMWPE-g-PMAPS/TiO2 is different from that of UHMWPE fabric and UHMWPE-g-PMAPS. The new endothermic peak at 88.4 oC is assigned to the crystals of the grafted chain and UHMWPE chain in the amorphous form. Moreover, the temperature at which the main endothermic peak is observed increases compared with that of UHMWPE fabric and UHMWPE-g-PMAPS. In the melting process of the UHMWPE fiber, the defect-crystal interface is believed to melt first. Considering the impact of graft polymerization on the fibrous structure, the increase of the melting point is ascribed to restricted chain relaxation near the interfaces and the increased crystallinity. The restricted chain relaxation near the interfaces is ascribed to the increased crosslinking density caused by radiation crosslinking in the amorphous region and the crosslinked graft chains. It is believed that crosslinking and chain scission occurred simultaneously in the irradiation process52. Radiation scission reduces the degree of chain entanglement and is responsible for the formation of new crystal during the hydrothermal treatment, and induced crystallization of titanium dioxide47, 53. This occurs not only on the surfaces of the TiO2 nanoparticles, but also in the non-crystalline region of UHMWPE fabric. Consequently, the melting points and melting enthalpy of UHMWPE-g-PMAPS/TiO2 increased compared with those of UHMWPE-g-PMAPS. Thermogravimetric analysis provided data such as the temperature of the maximum degradation rate (Tmax), the initial decomposition temperature (Tdi), and char yield (Yc) at 700 oC, as summarized in Table 2. Fig. 8 shows the TG and DTG curves of the UHMWPE fabric and treated fabric; these samples have different Tdi and Tmax values, indicating that they undergo different thermal degradation mechanisms. The Tdi of the treated fabric was reduced after surface modification with PMAPS and TiO2, ascribed to the poor heat resistance of the graft chain. In contrast, the intensities of the DTG peaks of the treated fabric were lower than that of UHMWPE fabric, suggesting that thermal decomposition is delayed because of the presence of PMAPS and TiO2. In addition, other typical parameters of the composite, such as Tmax and Yc,


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were higher than that of UHMWPE fabric, also indicating that the incorporation of PMAPS and TiO2 was beneficial for improving the thermal stability of the fabric. The underlying cause of this phenomenon is that a large amount of PMAPS/TiO2 will generate more inert silica and titania layers that can act as a thermal barrier to inhibit further degradation of the inner part of the matrix. More attractively, the experimental Yc of UHMWPE-g-PMAPS/TiO2 is much larger than its theoretical value calculated by the ‘Mixture Rule’

54

(Table 2), demonstrating that there is a synergistic effect in

the UHMWPE-g-PMAPS/TiO2 composite. The reason behind this phenomenon may be ascribed to the higher photooxidation ability of Si-modified TiO255, compared with the usual TiO2 which may accelerate the dehydrogenation reaction of molecular chains of UHMWPE fiber and lead to the increment of Yc. 3.4 UV resistance of the UHMWPE and modified UHMWPE fabrics The

UV

resistance

of

UHMWPE

fabric,

UHMWPE-g-PMAPS

and

UHMWPE-g-PMAPS/TiO2 was evaluated from the absorption spectra presented in Fig. 9. All of them showed strong absorptions in the range of 200-380 nm. The maximum absorbance of UHMWPE fabric reached 0.56, indicating that UHMWPE fabric readily absorbs UV light and then undergoes degradation. Compared with original UHMWPE fabric, UHMWPE-g-PMAPS exhibits improved UV absorbance under 261 nm indicating the graft chain enhanced the UV absorption capacity of the material as the introduction of silicon atoms. Similar result was also reported in literature 54, in which the fiber coated with hyperbranched polysiloxane can improve the UV absorption capacity and the UV resistance of the material. With regard to UHMWPE-g-PMAPS/TiO2, it has a broader absorption range and higher absorbance than UHMWPE-g-PMAPS, showing better UV resistance; specifically, the maximum absorption intensity of UHMWPE-g-PMAPS/TiO2 is 1.35, which is about 2.4 and 1.3 times that of UHMWPE fabric and UHMWPE-g-PMAPS respectively. This superior absorption capacity is ascribed to the following reasons. First, TiO2 possessing excellent UV absorption capacity can absorb UV light to produce a pair of electron and hole. When these electron and hole recombination, the energy partly converts to harmless heat. Second, the silicon atom of PMAPS entering into the lattice of TiO2



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suppressed the grain growth. The grain refinement leads to greater quantum size effect. Hence the Si-modified TiO2 should possess high photocatalytic properties including photooxidation and photoreduction55. Moreover, it also decreases recombination rate of holes and electrons, and facilitates charge transfer or excitation with lower energy. Third, the polysiloxane-TiO2 hybrid coating is multi-functional with UV reflective, refractive, and absorptive properties, and also decreases the transmittance of fabric. 3.5 Mechanical Properties In the practical application, the mechanical behavior is a quite important property of the material; especially these used as functional material in the natural environment. The mechanical property of the pristine UHMWPE, UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS/TiO2 fibers before and after the UV-irradiation is shown in Fig. 10; moreover, all tested fibers are obtained from the corresponding fabrics. For the pristine UHMWPE fiber, the average break strength was determined to be 64.5 cN. For the UHMWPE-g-PMAPS fibers, the break strength decreased to 61.5 cN, the decrement mainly ascribed to chain scission caused by γ-irradiation degradation. UHMWPE-g-PMAPS/TiO2 fiber exhibited considerably higher break strength (66.3 cN) than pristine UHMWPE and UHMWPE-g-PMAPS fibers, demonstrating that TiO2 coating not only can offset the harmfulness caused by the γ-irradiation damage, but also provides additional protection to avoid breaking down of fibers. The reason is mainly due to the formation of crosslinked network on the surface of UHMWPE fiber composed of Si-O-Si, Si-O-Ti and Ti-O-Ti chemical bonds. The cross linked network intensifies the molecular force between chains, and therefore hampers the slippage of molecular chains and prevents fiber from fracture during stretching. The UV-irradiation of various fibers was done under an accelerated photoageing procedure according to a standard method by ISO 4892 (part 3)33, the instruments and device parameters are listed in section 2.3. The experiments were carried out continuously (24 h/day) at room temperature. The mechanical properties of samples were tested regularly at predetermined UV exposure times. The break strength of various fibers after a 96 h irradiation is also shown in Fig. 10. With regard to the irradiated fibers, the retentions of break strength are about 72.2% to 92.3% of those of


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fibers without irradiation, the magnitude of which follows the order: UHMWPE fiber < UHMWPE-g-PMAPS fiber < UHMWPE-g-PMAPS/TiO2 fiber. The rapidly decreased break strength of UHMWPE fiber is due to the main-chain scission and dehydrogenation after a long time irradiation. UHMWPE-g-PMAPS has slightly better

mechanical

properties

compared

with

UHMWPE

fiber.

Although

UHMWPE-g-PMAPS has a higher UV absorption capacity compared with UHMWPE fiber, but the graft chains without hydrolysis to form a compact Si-O-Si crosslinked network which, to some extent, can prevent the degradation of the UHMWPE fiber54. With regard to UHMWPE-g-PMAPS/TiO2, it shows the highest retention of break strength. In addition to the advantages of UV shielding of TiO2 coating discussed in section 3.4, the dense TiO2 coating can prevent the oxygen from penetrating into the interior of the fibers, and slow down the process of photo-oxidation of UHMWPE fibers.

Conclusion Nanocrystalline titania films were generated in situ on UHMWPE fabrics by radiation-induced graft polymerization and a low-temperature sol-gel process. A uniform film composed of anatase phase TiO2 with a grain size of about 5.3 nm was obtained. The radiation grafting and sol-gel process did not damage the orthorhombic crystalline phase of UHMWPE fabric, but caused disordering of the monoclinic crystalline phase and the amorphous phase. After modification, the thermal properties and UV resistance of the composite were enhanced relative to that of UHMWPE fabric. The UHMWPE-g-PMAPS/TiO2 showed excellent duration of UV illumination, and the retention of the break strength reached 92.3 % after 96 h UV-irradiation. Coating of TiO2 nanoparticles on textiles such as UHMWPE fabric is a feasible approach for the protection of materials from UV damage, which can be applied in the textile industry.

Acknowledgements We greatly appreciate supports from the National Natural Science Foundation of China (11305243, 11275252, 11405249, 21306220) and the “Knowledge Innovation



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Program” of Chinese Academy of Sciences.

Corresponding Author Prof. G.Z. Wu [email protected]

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Captions of Tables, Figures and Schemes Table 1. Typical data from DSC analyses of UHMWPE fabric, UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS /TiO2 Table

2.

Characteristic

data

from

TG

analyses

of

UHMWPE

fabric,

UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS/TiO2 Scheme 1. The structure of UHMWPE-g-PMAPS Scheme 2. Process of coating titania layer on the surface of UHMWPE fiber, and chemical structure of the UHMWPE-g-PMAPS/TiO2. Fig. 1 Amount of loaded TiO2 nanoparticles on the surface of UHMWPE fabric as a function of TBT concentration. Fig. 2 Fourier transform infrared spectra of UHMWPE (a), UHMWPE-g-PMAPS (b) and UHMWPE-g-PMAPS/TiO2 (c). Fig. 3 XPS wide scan spectra of UHMWPE fabric (a) and UHMWPE-g-PMAPS/TiO2 (b). Fig. 4 High-resolution Si2p spectrum of UHMWPE-g-PMAPS/TiO2. Fig. 5 SEM images of UHMWPE fabric (a, a’), UHMWPE-g-PMAPS (b, b’), UHMWPE-g-PMAPS/TiO2 (c, c’ and c’’); magnification: a, b, c ×100; a’, b’, c’ ×5000; c’’×30000. Fig. 6 (a) Normalized XRD spectra of UHMWPE, UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS/TiO2; (b) magnification of box region in part (a) Fig. 7 DSC spectra of UHMWPE fabric (a), UHMWPE-g-PMAPS (b), UHMWPE-gPMAPS/TiO2 (c) and spectra fitted by Gaussian function: UHMWPE fabric (a’), UHMWPE-g-PMAPS (b’). Fig. 8 TG and DTG curves of UHMWPE fabric, UHMWPE-g-PMAPS, UHMWPE-g-PMAPS/TiO2. Fig. 9 UV-Vis absorption spectra of UHMWPE fabric, UHMWPE-g-PMAPS and UHMWPE-g-PMAPS/TiO2. Fig.

10

Mechanical

properties

of

UHMWPE,

UHMWPE-g-PMAPS

UHMWPE-g-PMAPS/TiO2 fibers before and after UV-irradiation.


and

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Table 1. Typical data from DSC analyses of UHMWPE fabric, UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS /TiO2 Sample

Tm1 (oC)

Tm2 (oC)

∆H1 (J/g)

∆H2 (J/g)

∆H (J/g)

UHMWPE fabric

144.9

152.5

65.7

48.6

114.3

UHMWPE-g-PMAPS

146.7

152.2

95.3

18.8

114.1

UHMWPE-g-PMAPS /TiO2

88.4

154.7

109.7

188.1

297.8



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Table 2 Characteristic data from TG analyses of UHMWPE fabric, UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS/TiO2 Sample

o

Tdi ( C)

o

Tmax ( C)

Yc at 700oC (%) Intensity Experimental

Calculated

UHMWPE fabric

443

479

3.33

1.8

----

UHMWPE-g-PMAPS

351

481

1.8

9.9

----

UHMWPE-g-PMAPS/TiO2

337

488

1.61

18.1

14.6


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TOC: Significant improvement in thermal and UV resistances of UHMWPE fabric through in situ formation of polysiloxane-TiO2 hybrid layers Jiangtao Hu, Qianhong Gao, Lu Xu, Mingxing Zhang, Zhe Xing, Xiaojing Guo, Kuo Zhang, Guozhong Wu

A new kind of UHMWPE fabric-based composite was synthesized by radiation-induced graft polymerization of γ-methacryloxypropyl trimethoxy silane (MAPS) with subsequent co-hydrolysis of the graft chains (PMAPS) using tetrabutyl titanate. This composite is not susceptible to photodegradation of the organic support of the TiO2 film and also exhibits excellent heat resistance, UV shielding performance and higher retention of the break strength after UV-irradiation.



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Scheme 1. The structure of UHMWPE-g-PMAPS 158x109mm (300 x 300 DPI)


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Scheme 2. Process of coating titania layer on the surface of UHMWPE fiber, and chemical structure of the UHMWPE-g-PMAPS/TiO2. 146x60mm (300 x 300 DPI)



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Fig. 1 Amount of loaded TiO2 nanoparticles on the surface of UHMWPE fabric as a function of TBT concentration. 242x189mm (300 x 300 DPI)


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Fig. 2 Fourier transform infrared spectra of UHMWPE (a), UHMWPE-g-PMAPS (b) and UHMWPE-gPMAPS/TiO2 (c). 114x91mm (300 x 300 DPI)



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Fig. 3 XPS wide scan spectra of UHMWPE fabric (a) and UHMWPE-g-PMAPS/TiO2 (b). 80x63mm (300 x 300 DPI)


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Fig. 4 High-resolution Si2p spectrum of UHMWPE-g-PMAPS/TiO2. 80x62mm (300 x 300 DPI)



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Fig. 5 SEM images of UHMWPE fabric (a, a’), UHMWPE-g-PMAPS (b, b’), UHMWPE-g-PMAPS/TiO2 (c, c’ and c’’); magnification: a, b, c ×100; a’, b’, c’ ×5000; c’’×30000. 109x83mm (300 x 300 DPI)


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Fig. 6 (a) Normalized XRD spectra of UHMWPE, UHMWPE-g-PMAPS, and UHMWPE-g-PMAPS/TiO2; (b) magnification of box region in part (a) 124x49mm (300 x 300 DPI)



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Fig. 7 DSC spectra of UHMWPE fabric (a), UHMWPE-g-PMAPS (b), UHMWPE-g- PMAPS/TiO2 (c) and spectra fitted by Gaussian function: UHMWPE fabric (a’), UHMWPE-g-PMAPS (b’). 140x111mm (300 x 300 DPI)


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Fig. 8 TG and DTG curves of UHMWPE fabric, UHMWPE-g-PMAPS, UHMWPE-g-PMAPS/TiO2. 150x104mm (300 x 300 DPI)



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Fig. 9 UV-Vis absorption spectra of UHMWPE fabric, UHMWPE-g-PMAPS and UHMWPE-g-PMAPS/TiO2. 124x94mm (300 x 300 DPI)


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Fig. 10 Mechanical properties of UHMWPE, UHMWPE-g-PMAPS and UHMWPE-g-PMAPS/TiO2 fibers before and after UV-irradiation. 119x92mm (300 x 300 DPI)


Significant Improvement in Thermal and UV Resistances of UHMWPE

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