A composite resin dosimeter: a new concept and design for a fibrous

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Applications of Polymer, Composite, and Coating Materials

A composite resin dosimeter: a new concept and design for a fibrous color dosimeter Kenji Kinashi, Takato Iwata, Hayato Tsuchida, Wataru Sakai, and Naoto Tsutsumi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00251 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Materials & Interfaces

A composite resin dosimeter: a new concept and design for a fibrous color dosimeter Kenji Kinashi,1* Takato Iwata,2 Hayato Tsuchida,2 Wataru Sakai,1 Naoto Tsutsumi,1

1

Faculty of Materials Science and Engineering, Kyoto Institute of Technology. Matsugasaki, Sakyo, Kyoto 606-8585, Japan 2

Master’s Program of Innovative Materials, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan.

Corresponding author: [email protected]

Abstract Polystyrene (PS)-based composite microfibers combined with a photochromic spiropyran dye, 1,3,3trimethylindolino-6ʹ-nitrobenzopyrylospiran (6-nitro BIPS), and a photostimulable phosphor, europiumdoped barium fluorochloride (BaFCl:Eu2+), were developed for the detection of X-ray exposure doses on the order of approximately 1 Gy. To produce the PS-based composite microfibers, we employed a forcespinning method that embeds a high concentration of the phosphor in the PS in a safe, inexpensive, and simple procedure. Based on optimization of the forcespinning process, fibrous color dosimeters with a high radiation dose sensitivity of 1.2−4.4 Gy were fabricated. The color of the dosimeters was found to transition from white to blue in response to X-ray exposure. The optimized fibrous color dosimeter, made from a solution having a PS/6-nitro BIPS/BaFCl:Eu2+/C2Cl4 ratio of 7.0/0.21/28.0/28.0 (wt%), and produced with a 290 mm distance between needle and collectors, a 0.34 mm 23 G needle nozzle, and a spinneret rotational rate of 3000 rpm, exhibited sensitivity to a dose as low as 1.2 Gy. In order to realize practical applications, we manufactured the optimized fibrous color dosimeter into a clothlike color dosimeter. The clothlike color dosimeter was mounted on a stuffed bear, and its coloring behavior was demonstrated upon X-ray exposure. After exposure with X-ray, a blue colored and shaped in the form of the letter “♡” clearly appeared on the surface of the clothlike color dosimeter. The proposed fibrous color dosimeters having excellent workability will be an unprecedented dosimetry and contributed to all industries utilizing radiation dosimeters. This new fibrous “composite resin dosimeter” should be able to replace traditional wearable individual radiation dose monitoring devices, such as film badges.

Keywords: radiation dosimeter, photochromism, spiropyran, scintillator, polystyrene fiber.

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Introduction There is currently a demand for personal radiation exposure monitors that are small, lightweight, flexible, battery-free, and provide a simple (visible) indication of exposure level. Radiation doses can be determined using either one of two approaches: (i) estimations based on mathematical models using reasonable assumptions or (ii) direct individual radiation dose measurements.1,2 In the latter case, there are four major types of monitoring devices in use today: the pocket dosimeter, film badge, thermoluminescent dosimeter, and optically stimulated luminescent dosimeter. The film badge is the most commonly used personnel monitoring device for those working with X-rays, γ-rays, or charged particles. In these devices, radiation exposure darkens a film inside a lightweight packet and the amount of darkening is proportional to the dose. The film holder can also contain filters composed of various materials (such as lead, tin, aluminum, or plastic), and radiation passing through the filters will produce a density distribution on the film from which the energy range and type of the radiation can be determined. The primary drawback of such systems is that they do not allow immediate results regarding the extent of exposure. Therefore, it would be beneficial to develop a new type of radiation dosimeter that has the same physical advantages as the film badge but gives immediate results. As interest in wearable devices has increased over the last decade, fiber-based technologies for use in electronic units incorporating organic solar cells have been studied.3,4 Fiber-based radiation dosimeters are also of interest, because dosimeters integrated into clothing could be utilized in all situations, such as space facilities, therefore removing the requirement for the user to carry a device. Recently, we proposed a fibrous color dosimeter consisting of a spiropyran dye, BaFCl:Eu2+ particles, and polystyrene (PS). This device is capable of indirectly detecting X-ray radiation at a dose of 5.6 Gy and provides visual feedback to the user.5 However, the sensitivity of the unit in its current state is low because the concentration of BaFCl:Eu2+ particles deposited on the PS is not yet optimized. As a result of the high photoisomerization quantum efficiency of spiropyrans and the high molar extinction coefficient of the photomerocyanine-form (PMC-form), these compounds have potential applications in optical memory and photo-optical switching in conjunction with many photochromic organic dyes.6-8 Upon exposure to light at approximately 360 nm, the spirocarbon−oxygen (C−O) bond in the colorless spiro-form (SP-form) of the spiropyran is broken and the subsequent photoisomerization leads to the deeply colored PMC-form, as shown in Scheme 1. Centrifugal spinning (or forcespinning) is a technique that allows the fabrication of materials on the micro- or nanoscale. It is also a highly efficient, low-cost, and versatile method of generating polymer micro/nanofiber assemblies, especially in comparison with the alternative technique of electrospinning.9-12 Forcespinning uses centrifugal force in association with the viscoelastic properties and the mass transfer characteristics of polymer solutions to promote the controlled thinning of solution filaments into micro/nanofibers.13 The aim of the present study was to develop a highly sensitive fibrous color dosimeter by optimizing the forcespinning parameters for the generation of PS-based microfibers containing 6-nitro BIPS as a dye and BaFCl:Eu2+ particles. The proposed fibrous color dosimeters having excellent workability will be an unprecedented dosimetry and contributed to all industries utilizing radiation dosimeters. This work also demonstrated a novel radiation color dosimeter based on a photochromic dye, which we term a “composite resin dosimeter.”


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Scheme 1 The reversible photochromic reaction between the SP- and PMC-forms.

Experimental All chemicals were commercially available and were used as-received. The photochromic spiropyran dye, 1',3',3'-trimethyl-6-nitrospiro[1(2H)-benzopyran-2,2'-indoline] (or 1,3,3-trimethylindolino-6'nitrobenzopyrylospiran, 6-nitro BIPS) was purchased from the Tokyo Kasei Co. Polystyrene (PS) and tetrachloroethylene were purchased from the Wako Co., and the BaFCl:Eu2+ phosphor powder was obtained from the Nemoto Lumi-Materials Co. This material was used because it absorbs X-rays and emits UV light. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the PS were determined by gel permeation chromatography (GPC) using Shodex GPC System-21 instrumentation (Showa Denko Co.) equipped with G5000–G3000 columns and with tetrahydrofuran (THF) as the eluent. The system was calibrated using PS standards. The Mw of the PS was found to be 252 000 g mol−1 and its polydispersion index, Mw/Mn, was 2.5. The mean particle size and associated standard deviation, σ, of the BaFCl:Eu2+ particles suspended in tetrachloroethane and in dry powder form were determined using a laser diffraction particle size analyzer (LS13 320, Beckman Coulter Co.). The optimal PS concentration for forcespinning was determined by preparing several viscous solutions of PS in tetrachloroethylene by stirring at room temperature for 12 h. The mass-based concentration of PS in these solutions was varied over 5.0−27.5 wt%, and fixed amounts of PS (7 g), 6-nitro BIPS (210 mg), and BaFCl:Eu2+ (3 g) were added to each solvent (18−133 g). Viscous solutions with higher BaFCl:Eu2+ concentrations were also prepared by adding BaFCl:Eu2+ to a solution found to be optimal for forcespinning. All solutions were homogenized using a planetary centrifugal mixer (ARE-310, Thinky Co.) operating at 2000 rpm with polystyrene balls (1/2 inch) for 10 min. The viscosity of each polymer solution was measured with a vibronic viscometer (SV-1A, and SV-100A, A&D Co.) at room temperature (25 °C). The SV-1A was used for the lower viscosity solutions (5.0, 7.5, 10.0, 12.5, 15.0, and 17.5 wt%), and the SV-100A was used for the more viscous samples (20.0, 22.5, 25.0, and 27.5 wt%). Micro/nanofibers consisting of 6-nitro BIPS/BaFCl:Eu2+/PS were prepared using a fiber making apparatus of our own design. This unit is composed of a 60 mm needle-based spinneret equipped with blunt needles rotated via an AC motor (EUROSTAR 20 high-speed digital, IKA Co.). The polymer solution loading rate is controlled by a syringe pump (KDS-100, KD Scientific Co.) operating at 50 mL h−1. The distance between the 0.34 mm needle tip (23 G) and the collectors is 290 mm. The polymer solution was fed continuously into the spinneret and was expelled by centrifugal force at a rotational rate of 3000 rpm. A schematic illustration of the system is shown in Figure 1. Fluorescence quantum yields (ΦF) were determined with a photoluminescence spectrophotometer (RF6000, Shimadzu Co.) using a 100 mm diameter Spectralon integrated sphere unit. Data were acquired under illumination with 275 nm light. Images showing the morphologies of the forcespinning fibers were obtained using scanning electron microscopy (SEM) (Hitachi S-3000, Hitachi Co.). Fiber diameters and diameter distributions (N = 100) were assessed based on the SEM images, employing image processing software (ImageJ and Minitab). An X-ray diffractometer (RINT2500, Rigaku Co.) was used to generate monochromatic 0.154 nm Cu K radiation, operating at 50 kV and 48 mA. The devices were exposed to estimated X-ray dose rates of 2.12 Gy/min that was determined using the Fricke solution. During exposure, photographic images of the associated color changes of the samples were acquired with a digital camera (TG-3 CMIIT, OLYMPUS Co.), and the chromaticity differences (∆E) were estimated using the Photoshop CS3 software package (Adobe Systems Co.). To estimate a sensitivity to the X-ray dose at ∆E = 10, we fitted the exposure dose trace of the ∆E using a Box-Lucas exponential model: ∆E = ∆ 1 − −

(1)



where ∆E is the chromaticity difference, ∆E∞ is the steady-state the chromaticity difference, r is the parameter of the dose response function, and D is the absorbed dose in the Fricke solution. A practical demonstration of X-ray exposure visualization for a clothlike sample was performed using an X-ray radiation system (HW-100W, Hitex Co.) at 60 kV, 8 mA, for 1 min (a dose corresponding to 24.5 Gy).

Figure 1 A photographic image of the forcespinning apparatus.

Results and discussion Seven viscous polymer solutions were prepared according to the procedure described above. As noted, the forcespinning system consisted of a spinneret, fiber collectors, syringe pump, supply tube and spinning motor. The viscous polymer solution was fed continuously into the spinneret and centrifugally forced through the needles to discharge either beads, beaded fibers, or fibers. The morphologies of the resulting fibers were found to be significantly affected by the concentration (and hence the viscosity) of the solution. The relationship between the solution concentration and viscosity, η, is summarized in Figure 2a. The fiber morphologies generated with an internal nozzle diameter of 0.34 mm (23 G) at a rotational rate of 3000 rpm are shown in Figures 2b−2h, and the fabrication conditions and the diameters for the fibrous composite resin dosimeters are provided in Table 1. The η of a polymer solution determined using the vibronic viscometer at a low frequency of 30 Hz can be regarded as equal to the zero-shear viscosity, η0. The η values are plotted as a function of the solution concentration divided by the critical chain overlap concentration, C* (the point at which the concentration inside a single macromolecular chain equals the solution concentration), in Figure 2a. C* is used here because it is a good indicator of the transition between dilute and semi-dilute regimes. The dimensionless product of the intrinsic viscosity [η] and the concentration of the polymer solution C, [η]C, is referred to as the Berry number. A value above unity for this parameter indicates a solution having chain entanglements.14 In this study, [η] was calculated using the Mark–Houwink–Sakurada equation: [η] = K· Mva, 15 where K and a are the Mark–Houwink constants (which are dependent on the nature of the polymer, the solvent, and the temperature) and Mv is the viscosity-averaged molecular weight. In the case that the polydispersity is close to that of a monodisperse system and a = 1, Mv can considered equal to Mw. In this study, a ≠ 1, as discussed below, and so Mv is considered equal to Mw. In this work, we used the K and a values for PS at 30 °C in dichloromethane found in the literature (K = 0.021 g cm−3 and a = 0.66).16 Based on these parameters and Mw = 252 000 g mol−1, the [η] value of PS was estimated to be 77.11 cm3 g−1. The C* value can subsequently be obtained from the relationship between 1/[η] and C*, and was estimated to be 1.25 wt%. The resulting plots of the viscosity values against C/C* can be separated into different solution regimes: dilute (C/C* < 6.0, equivalent to 5.0 and 7.5 wt%), semi-dilute unentangled (6.0 < C/C* ≤ 11.3, equivalent to 10.0 and 12.5 wt%), and semi-dilute entangled (11.3 < C/C*, equivalent


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to 15.0, 17.5, 20.0, 22.5, 25.0, and 27.5 wt%). The concentrations of the polymer solutions in the dilute regime (C/C* < 6.0) result in an insufficient degree of chain overlap, leading to the formation of polymer films (as determined by SEM images, not provided herein). Beyond the dilute regime, two distinct regimes appear, as indicated by the change in the slope of the data plot. The concentration of the solution increases to give the semi-dilute unentangled regime at 6.0 < C/C* ≤ 11.3, and the formation of polymer droplets and beaded fibers is observed, as shown in Figures 2b and 2c. Based on the SEM images obtained from the semi-dilute entangled region at C/C* > 11.3, the viscous polymer solutions in the range of 15.0–27.5 wt% tend to produce smooth fibers and/or fibers having small beads. However, crossing the C/C* boundary at 11.3 (corresponding to 15 wt%) generates fibers containing a small amount of beads. At a concentration of 27.5 wt%, a rotational speed of 3000 rpm is insufficient to produce a polymer solution jet. Consequently, in the case of a PS polymer solution containing 6-nitro BIPS and BaFCl:Eu2+, the formation of fibers requires C/C* > 14.0 (17.5 wt%), at which point the chain entanglement is sufficient to produce uniform and continuous bead-free fibers. The resulting fibers have a smooth morphology with no beads, as demonstrated in Figures 2e–2h. The fiber diameters and their distributions were found to be 4.47 ± 0.45 µm (for 17.5 wt%), 5.89 ± 0.45 µm (20.0 wt%), 9.87 ± 0.48 µm (22.5 wt%), and 10.04 ± 0.67 µm (25.0 wt%). These results show that the fiber diameter increases with increasing concentration of the polymer solution in the semi-dilute entangled regime. The energy dispersive X-ray spectroscopy (EDS) data for the fibrous composite resin dosimeters prepared using a PS concentration of 20 wt% demonstrate the presence of carbon (C) and barium (Ba) in the fibers. The corresponding EDS elemental maps are provided in Figures 2i–2k. Note that the positions of the EDS elemental maps correspond to the SEM image in Figure 2i. The mean BaFCl:Eu2+ particle size was determined to be 7.6 µm with σ = 4.1 µm in dry powder form; however, the mean particle size in tetrachloroethylene was increased to 13.7 µm with σ = 9.8 µm due to aggregation. The EDS elemental maps and the BaFCl:Eu2+ particle size data indicate that the fibrous composite resin dosimeters had BaFCl:Eu2+ particles both protruding from and embedded in the fibers. A schematic representation of the structure of a dosimeter is presented in Figure 2l.

Figure 2 (a) Correlation of viscosity, η, with C/C* for various viscous polymer solutions. SEM images and fiber diameter distributions of fibrous composite resin dosimeters prepared with viscous polymer solutions having PS concentrations of (b) 10, (c) 12.5, (d) 15.0, (e) 17.5, (f) 20.0, (g) 22.5, and (h) 25.0 wt%. Forcespinning conditions: 290 mm distance between needle and collectors, 0.34 mm 23 G needle



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nozzle, spinneret rotational rate of 3000 rpm. EDS elemental maps of the fibers prepared using a concentration of 20 wt%: (i) SEM image and EDS elemental maps for (j) C (red) and (k) Ba (green). (l) A schematic representation of the fibrous composite resin dosimeter structure. Table 1 Experimental conditions and the exposure doses required to obtain ∆E = 10 for the fibrous composite resin dosimeters. PS conc. (wt%)

1)

PS(g) 6-nitro BIPS (mg)

2+

BaFCl:Eu (g)

5.0

133

η (mPa·s) Fiber diameter (µm)

Sensitivity at ∆E = 10 (Gy)

2)

unreached

2)

34

−

7.5

86

103

63

143

− 0.33 ± 0.02

unreached

10.0 12.5

49

182

0.50 ± 0.02

unreached

40

362

1.55 ± 0.06

unreached

33

514

4.47 ± 0.45

unreached

20.0

28

937

5.89 ± 0.26

4.0

22.5

24

1917

9.87 ± 0.48

4.4

25.0

21

3850

10.04 ± 0.67

4.2

27.5

18

7180

15.0 17.5

7.0

210

3.0

1)

PS (g)/C2Cl4 (g)

2)

dilute or semi-dilute unentangled regime, no fibers produced

3)

C2Cl4 (g)

3)

−

unreached

3)

−

overly high PS conc., no polymer jet produced

The steady state fluorescence spectrum of BaFCl:Eu2+ in response to excitation with X-rays at 0.154 nm contains an intense emission peak centered at 381 nm and a much smaller peak at 360 nm. It is noteworthy that this spectrum has the same general shape as that obtained with 275 nm excitation, and that almost no peak shift is observed at room temperature. The emission peak at 381 nm is attributed to the electronic transition from the 4f65d1 (2eg) state to the 4f7 (8S7/2) ground state of Eu2+, whereas the narrow peak at 360 nm is ascribed to the intraband transition from 4f7 (6P7/2) to 4f7 (8S7/2).17 The absorptivity and internal and external fluorescence quantum yields ΦF of the BaFCl:Eu2+ at an excitation wavelength of 275 nm were 40.9%, 78.7%, and 32.2%, respectively. Prior to X-ray exposure, only the primary reflection peak assignable to the SP-form was exhibited by the fibrous composite resin dosimeters in the wavelength region of 450 nm. This reflection band results from the π–π* transitions of the chromene and indoline rings of the SP-form. Upon exposure to 0.154 nm Xrays, the dosimeters transitioned from white to blue, and a new, broad absorption band appeared, with a maximum at 604 nm.5 Dose response curves (used to establish sensitivity to the X-ray dose) for the dosimeters were obtained by plotting the chromaticity difference (∆E) against the exposure dose at an exposure rate of 2.12 Gy min−1. These plots are shown in Figure 3. The X-ray dosages in these plots were determined using the Fricke solution. The color change in response to X-ray exposure is expressed herein using the color space CIE L*a*b* parameters calculated from the spectral reflectance of each sample. The magnitude of the total color difference, ∆E, is defined by the equation ∆E = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2, where ∆L* is the lightness difference, ∆a* is the red/green difference, and ∆b* is the yellow/blue difference. All the fibrous composite resin dosimeters exhibited good response, based on the chromaticity difference with respect to the exposure dose. However, the sensitivities of the fibers prepared using PS concentrations of 5.0 to 17.5 wt% were insufficient, as all gave ∆E values below 10, and changes in color cannot be visually identified in this region. In accordance with this definition, sufficient sensitivity was obtained from the dosimeters prepared from solutions having PS concentrations of 20.0, 22.5, and 25.0 wt%, for which the doses were 4.0, 4.4, and 4.2 Gy, respectively. Consequently, the optimal BaFCl:Eu2+ concentration and fiber diameter are ≥ 20.0 wt% and ≥ 6 µm. The X-ray doses required to obtain ∆E = 10


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for the fibrous composite resin dosimeters and the associated experimental conditions are summarized in Table 1.

Figure 3 Dose response curves for the fibrous composite resin dosimeters. The broken line indicates the point at which the color change can be visually observed.

The effect of increasing the concentration of BaFCl:Eu2+ on the device sensitivity was also assessed. The BaFCl:Eu2+ concentration in the dosimeter was increased to give a BaFCl:Eu2+/PS ratio of 28.0/7.0 (w/w). In the BaFCl:Eu2+ concentration dependence, a BaFCl:Eu2+/PS ratio of 3.0/7.0 (w/w) was compared to a reference solution having PS concentrations of 20.0. The conditions used to fabricate dosimeters having varying BaFCl:Eu2+ concentration are summarized in Table 2. Figure 4 presents SEM images of the resulting specimens. The fibers (Figures 4a–4f) evidently all had a pearl-chain shape, the BaFCl:Eu2+ particles being embedded in the fiber with some distance in between the particles. These images also show that some BaFCl:Eu2+ particles aggregated together, especially upon increasing the BaFCl:Eu2+ concentration, which produced fibers containing closely packed BaFCl:Eu2+ particles. At BaFCl:Eu2+/PS ratios greater than 16.3/7.0 (w/w), so many particles were present that fibers were formed between the BaFCl:Eu2+ particles, with the fibers acting to hold the BaFCl:Eu2+ particles together.

Figure 4 SEM images and fiber diameter distributions of the fibrous composite resin dosimeters prepared with BaFCl:Eu2+/PS ratios of (a) 3.0/7.0, (b) 4.6/7.0, (c) 7.0/7.0, (d) 10.5/7.0, (e) 16.3/7.0, and (f) 28.0/7.0 w/w. Forcespinning conditions: 290 mm distance between needle and collectors, 0.34 mm 23 G needle nozzle, spinneret rotational rate of 3000 rpm.

Figure 5 shows that the fibrous composite resin dosimeters with BaFCl:Eu2+/SP ratios at or above 10.5/7.0 w/w exhibit superior chromaticity difference values, such that ∆E values of 10 were obtained upon exposure to 2.0, 1.5, and 1.2 Gy for the 10.5/7.0, 16.3/7.0, and 28.5/7.0 w/w specimens, respectively.



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The observed color changes demonstrate that the C–O bond of the initial SP-form was broken, and subsequent photoisomerization gave the colored PMC-form, having a zwitterionic structure.18 The sensitivities of the composite resin dosimeters in this study are evidently improved as compared to the performance reported by Tsuchida et al. for an unoptimized 6-nitro BIPS/BaFCl:Eu2+/PS microfiber (with a value of 5.6 Gy). Accordingly, the results also imply that the absorption efficiency (the percentage of incident radiation absorbed by the sample) of the X-ray is the most important factor. The fibrous composite resin dosimeters would poorly absorb the X-ray that enters them and be not thick enough to absorb all the X-ray photons. In other words, almost all of the X-ray energy does not contribute to the color changes. Consequently, the effect of increasing the concentration of BaFCl:Eu2+ on the device sensitivity would be particularly meaningful with respect to the device capable of realizing further high sensitivity. The X-ray doses necessary to obtain a ∆E value of 10 for dosimeters prepared with different BaFCl:Eu2+ concentrations and the associated experimental conditions are summarized in Table 2.

Figure 5 Dose response curves for the fibrous composite resin dosimeters prepared with different BaFCl:Eu2+ particle concentrations. The broken line indicates the point at which the color change can be visually observed.

Table 2 Experimental conditions and the exposure doses required to obtain ∆E = 10 for fibrous composite resin dosimeters with varying BaFCl:Eu2+ concentrations. 2+

BaFCl:Eu /PS (w/w)

PS (g) 6-nitro BIPS (mg)

1)

3.0/7.0 4.6/7.0

7.0/7.0 10.5/7.0

1)

2+

BaFCl:Eu (g) 3.0

C2Cl4 (g)

4.0

4.6 7.0

210

7.0 10.5

Sensitivity at ∆E = 10 (Gy) 3.7

28.0

3.6 2.0

16.3/7.0

16.3

1.5

28.0/7.0

28.0

1.2

corresponding to a PS conc. of 20.0 wt% in Table 1.

A preliminary experiment of X-ray exposure visualization using the fibrous composite resin dosimeters is shown in Figure 6, which presents photographic images of color changes during X-ray exposure at room temperature. Here, the leftmost images show units before X-ray exposure. The units having BaFCl:Eu2+/PS ratios at or below 10.5/7.0 w/w are seen to have been whitish blue prior to exposure, based on a combination of the colorless SP-form and small amounts of the colored PMC-form generated by the reaction with indoor light. In the case of those specimens with BaFCl:Eu2+/PS ratios at or above


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16.3/7.0 w/w, a pale pink coloration is evident, attributed to the protonated PMC-form.19 Upon X-ray exposure for 28−168 s (a dose corresponding to 1−6 Gy), a rectangular area within these dosimeters changes to blue with reflecting the results of the dose response curves as Figure 5. Finally, a practical demonstration of X-ray exposure visualization using the fibrous composite resin dosimeter with BaFCl:Eu2+/SP ratio at 28.0/7.0 w/w that was processed into a cloth is presented in Figure 7. The clothlike composite resin dosimeter was prepared from the nonwoven fabrics which was formed into a sheet and then laminated into 2−3 layers. The clothlike composite resin dosimeter was mounted on a stuffed bear, and was set in the X-ray radiation system. Upon exposure with X-ray that passed through a lead sheet cut in a “♡” shape, the blue area indicates the X-ray exposed area, and the whitish blue is the unexposed area. A blue heart-shaped X-ray image clearly appears on the clothlike composite resin dosimeter the stuffed bear is wearing, as shown in Figure 7b. It is noteworthy that the fibrous composite resin dosimeters can be processed into various shapes, e.g. clothes, cloves, and so on. Furthermore, it is possible to detect radiation doses from various angles based on “face” rather than “point”. We believe that these dosimeters show promise with regard to the evaluation of real-time X-ray exposure.

Figure 6 Photographic images of the fibrous composite resin dosimeters following various exposure doses. The leftmost photographs show the fibers prior to exposure and the exposure dose increases from left to right, up to 6 Gy.



Figure 7 Photographic images of the clothlike composite resin dosimeter (a) before, and (b) X-ray exposure through the lead sheet cut in a “♡” shape.

Conclusions The PS-based composite microfibers combined with 6-nitro BIPS, and BaFCl:Eu2+, having a sensitivities in the X-ray region and changing color by X-ray exposure doses, were successfully prepared according to a forcespinning method. A fibrous dosimeter with a maximum sensitivity, made from a solution having a PS/6-nitro BIPS/BaFCl:Eu2+/C2Cl4 ratio of 7.0/0.21/28.0/28.0 (wt%), and produced with a 290 mm distance between needle and collectors, a 0.34 mm 23 G needle nozzle, and a spinneret rotational rate of 3000 rpm, exhibited a dose as low as 1.2 Gy. We believe that this new fibrous “composite resin dosimeter” will be able to replace traditional wearable individual radiation dose monitoring devices, such as film badges, and used as an alternative to the Geiger–Muller counter, scintillation detector, and ionization chamber in a severe environment where people unable to use the hands. The concept and design of the fibrous color dosimeters will induce innovation of individual radiation dose monitoring devices and have a potential to open new markets in the not far future.

Conflicts of interest There are no conflicts of interest to declare.

Acknowledgements This work was financially supported by a JSPS Grant-in-Aid for Young Scientists (A) (no. 16H06118).

References (1) World Health Organization. Health risk assessment from the nuclear accident after the 2011 Great East Japan earthquake and tsunami, based on a preliminary dose estimation. Geneva: World Health Organization, 2013.


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(2) United Nations Scientific Committee on the Effects of Atomic Radiation. Annex J: exposures and effects of the Chernobyl accident. New York: United Nations Scientific Committee on the Effects of Atomic Radiation, 2008. (3) Yun, M. J.; Cha, S. I.; Seo, S. H.; Lee, D. Y. Highly Flexible Dye-sensitized Solar Cells Produced by Sewing Textile Electrodes on Cloth. Sci., Rep., 2014, 4, 5322. (4) Wu, S.; Liu, P.; Zhang, Y.; Zhang, H.; Qin, X. Flexible and Conductive Nanofiber-structured Single Yarn Sensor for Smart Wearable Devices. Sens. Acuators, B: Chem., 2017, 252, 697–705. (5) Tsuchida, H.; Nakamura, R.; Kinashi, K.; Sakai, W.; Tsutsumi, N.; Ozaki, M.; Okabe, T. Radiationinduced Colour Changes in a Spiropyran/BaFCl:Eu2+/Polystyrene Composite Film and Nonwoven Fabric. New J. Chem., 2016, 40, 8658–8663. (6) Raymo, F. M. Digital Processing and Communication with Molecular Switches. Adv. Mater., 2002, 14, 401–414. (7) Giordani, S.; Cejas, M. A.; Raymo, F. M. Photoinduced Proton Exchange Between Molecular Switches. Tetrahedron, 2004, 60, 10973–10981. (8) Raymo, F. M.; Tomasulo, M. Electron and Energy Transfer Modulation with Photochromic Switches. Chem. Soc. Rev., 2005, 34, 327–336. (9) Nayak, R.; Padhye, R.; Kyratzis, I. L.; Truong, Y. B.; Arnold, L. Recent Advances in Nanofibre Fabrication Techniques. Text. Res. J., 2011, 82(2), 129–147. (10) Padron, S.; Fuentes, A.; Caruntu, D.; Lozano, K. Experimental Study of Nanofiber Production through Forcespinning. J. Appl. Phys., 2013, 113(2), 024318. (11) Sarkar, K.; Gomez, C.; Zambrano, S.; Ramirez, M.; de Hoyos, E.; Vasquez, H.; Lozano, K. Electrospinning to Forcespinning™. Mater. Today, 2010, 13(10), 12–14. (12) Doan, H. N.; Tsuchida, H.; Iwata, T.; Kinashi, K.; Sakai, W.; Tsutsumi, N.; Huynh, D. P. Fabrication and Photochromic Properties of Forcespinning® Fibers Based on Spiropyran-doped Poly(methyl methacrylate). RSC Advances, 2017, 7, 33061–33067. (13) Ren, L.; Ozisik, R.; Kotha, S. P.; Underhill, P. T. Highly Efficient Fabrication of Polymer Nanofiber Assembly by Centrifugal Jet Spinning: Process and Characterization. Macromolecules, 2015, 48(8), 2593–2602. (14) Hager, B. L.; Berry, G. C. Moderately Concentrated Solutions of Polystyrene. I. Viscosity as a Function of Concentration, Temperature, and Molecular Weight. J. Polym. Sci., Polym. Phys. Ed., 1982, 20(5), 911–928. (15) Tanford, C. Physical Chemistry of Macromolecules, Wiley, New York, 1961. (16) Wagner, H. L. The Mark-Houwink-Sakurada Equation for the Viscosity of Atactic Polystyrene. J. Phys. Chem. Ref. Data, 1985, 14(4), 1101–1106. (17) Chen, W.; Kristianpoller, N.; Shmilevich, A.; Weiss, D.; Chen, R.; Su, M. X-Ray Storage Luminescence of BaFCl:Eu2+ Single Crystals. J. Phys. Chem. B, 2005, 109, 11505–11511.



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Table of Contents



Scheme 1 The reversible photochromic reaction between the SP- and PMC-forms. 20x6mm (300 x 300 DPI)


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Figure 1. A photographic image of the forcespinning apparatus. 56x40mm (300 x 300 DPI)



Figure 2 (a) Correlation of viscosity, η, with C/C* for various viscous polymer solutions. SEM images and fiber diameter distributions of fibrous composite resin dosimeters prepared with viscous polymer solutions having PS concentrations of (b) 10, (c) 12.5, (d) 15.0, (e) 17.5, (f) 20.0, (g) 22.5, and (h) 25.0 wt%. Forcespinning conditions: 290 mm distance between needle and collectors, 0.34 mm 23 G needle nozzle, spinneret rotational rate of 3000 rpm. EDS elemental maps of the fibers prepared using a concentration of 20 wt%: (i) SEM image and EDS elemental maps for (j) C (red) and (k) Ba (green). (l) A schematic representation of the fibrous composite resin dosimeter structure. 82x42mm (300 x 300 DPI)


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Figure 3 Dose response curves for the fibrous composite resin dosimeters. The broken line indicates the point at which the color change can be visually observed. 43x12mm (300 x 300 DPI)



Figure 4. SEM images and fiber diameter distributions of the fibrous composite resin dosimeters prepared with BaFCl:Eu2+/PS ratios of (a) 3.0/7.0, (b) 4.6/7.0, (c) 7.0/7.0, (d) 10.5/7.0, (e) 16.3/7.0, and (f) 28.0/7.0 w/w. Forcespinning conditions: 290 mm distance between needle and collectors, 0.34 mm 23 G needle nozzle, spinneret rotational rate of 3000 rpm. 53x24mm (300 x 300 DPI)


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Figure 5 Dose response curves for the fibrous composite resin dosimeters prepared with different BaFCl:Eu2+ particle concentrations. The broken line indicates the point at which the color change can be visually observed. 46x15mm (300 x 300 DPI)



Figure 6 Photographic images of the fibrous composite resin dosimeters following various exposure doses. The leftmost photographs show the fibers prior to exposure and the exposure dose increases from left to right, up to 6 Gy. 114x95mm (300 x 300 DPI)


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Figure 7. Photographic images of the clothlike chromic dosimeter (a) before, and (b) X-ray exposure through the lead sheet cut in a “♡” shape. 46x27mm (300 x 300 DPI)



PS conc. PS(g) (wt%)1) 5.0

6-nitro BIPS (mg)

BaFCl:Eu2+ (g)

C2Cl4 η (mPa·s) (g) 133 34

Page 22 of 24

Fiber diameter (μm) − 2) −

Sensitivity at ΔE = 10 (Gy) unreached

2)

unreached

143

0.33 ± 0.02

unreached

49

182

0.50 ± 0.02

unreached

40

362

1.55 ± 0.06

unreached

33

514

4.47 ± 0.45

unreached

20.0

28

937

5.89 ± 0.26

4.0

22.5

24

1917

9.87 ± 0.48

4.4

25.0

21

3850

10.04 ± 0.67

4.2

27.5

18

7180

7.5

86

103

10.0

63

12.5 15.0 17.5

7.0

210

3.0

1)

PS (g)/C2Cl4 (g)

2)

dilute or semi-dilute unentangled regime, no fibers produced

3)

overly high PS conc., no polymer jet produced


−

3)

−

3)

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BaFCl:Eu2+/PS (w/w) 3.0/7.0 1)

1)

PS (g)

7.0

6-nitro BIPS (mg)

BaFCl:Eu2+ (g) 3.0

210

C2Cl4 (g)

28.0

Sensitivity at ΔE = 10 (Gy) 4.0

4.6/7.0

4.6

3.7

7.0/7.0

7.0

3.6

10.5/7.0

10.5

2.0

16.3/7.0

16.3

1.5

28.0/7.0

28.0

1.2

corresponding to a PS conc. of 20.0 wt% in Table 1.



82x44mm (300 x 300 DPI)


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A composite resin dosimeter: a new concept and design for a fibrous

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