1. Introduction Emissions of the greenhouse gas CO2 into the atmosphere contribute to global climate change [1]. As such, carbon capture and storage should be enforced to mitigate CO2 emissions [2]. CO2 is captured from large point sources, such as the flue gas in coal-fired power plants and the syngas in integrated gasification combined cycle plants [3, 4]. Therefore, CO2 separation from other gases (e.g., H2, CH4, and N2) is required. Compared with conventional separation technologies (e.g., pressure swing adsorption and chemical absorption), membrane separation technology presents smaller carbon footprint, simpler operation and maintenance, higher energy efficiency, and potentially lower costs [5]. CO2-philic membranes provide conducive permeation conditions for CO2 other than small gases, reduce gas recompression during CO2 separation from small gas molecules, and overcome the trade-off limitation between CO2 permeance and selectivity [6, 7]. Furthermore, CO2-philic membranes for practical applications should possess permeation rates larger than 100 GPU (1 GPU = 1 × 10−6 cm3 (STP)/cm2·s·cm Hg) [4]. As such, an ultra-thin selective layer (membrane active skin layer) with a thickness of less than 1μm is required [2]. Because of the pores and rough surface of the support, a gutter layer is also needed to connect the support and the selective layer. Polydimethylsiloxane (PDMS) is an excellent gutter-layer material for the fabrication of multilayer composite membranes [8-10]. Most of these composite membranes were prepared through dip-coating or interfacial polymerization [9-11]. In general, the improved interfacial adhesion between two adjacent layers can enhance the structural stability of the composite membrane. Intrinsic adhesion typically involves five adhesion mechanisms, namely, mechanical interlocking, diffusion theory, chemical bonding, electronic and adsorption theories [12]. Mechanical interlocking is weak because of the required smooth surface of PDMS gutter layer [12]. Meanwhile, the four other binding forces are limited because of the strong nonpolarity of PDMS and the polarity of most CO2-selective materials, such as poly(ethylene oxide)-based copolymers [13]. The weak interfacial adhesion between PDMS gutter layer and CO2 selective layer negatively affects the preparation of a defect-free selective layer on PDMS gutter layer and the lifetime of the multilayer composite membrane. In this case, the authors first proposed the functional surface modification of PDMS layer for CO2 separation composite membranes. Polar and active amino groups were introduced to the surface of PDMS membrane. The crucial requirement of the modification was that surface polarity should be improved without the loss of the high CO2 permeation and separation performance of PDMS layer. Previous studies have employed plasma treatment, ultraviolet (UV)/ozone treatment, laser surface modification, and surface grafting modification to realize the surface modification of PDMS membranes; however, most of these studies focused on the improvement of surface hydrophilicity and disregarded the permeation and separation performance of PDMS membrane [14-18]. Surface grafting modification is an efficient

and flexible approach to improve the surface-chemical properties of PDMS layer because of the wide variety of polymers with different functional properties [19]. Chen et al. [16, 17] proposed the attachment of poly(ethylene glycol) onto PDMS surface through grafting to improve the surface hydrophilicity of PDMS. However, this method on surface hydrophilicity is restricted because PDMS is chemically inert and lacks reactive functional groups on its surface and because a thick modified layer would be produced [19]. Moreover, plasma, UV/ozone, and laser treatments using high energy or chemical etching to introduce functional groups, such as –OH and –NH2, on the PDMS surface tend to damage PDMS and its gas permeation performance [19]. Besides, the surface modification of PDMS with 3-aminopropyltriethoxysilane (APTES) was proposed to enhance functionality of PDMS microfluidic channels in several studies [20, 21]. The modification was achieved through immersing PDMS into APTES solution [21]. In another study, to improve the pervaporation of pure organic solvents, APTES was grafted on the surface of PDMS layer through the reaction between APTES and the hydroxyls [22]. However, the stabilities of those modifications by APTES were unguaranteed. Meanwhile, the effects of modifications by APTES on gas permeability and selectivity were disregarded. In the present study, a new effective approach was proposed to realize the functional surface modification of PDMS layer and the improvement of surface polarization. The modifier containing amino groups was integrated into a PDMS membrane through the cross-linking reaction of PDMS oligomer. The polar amino groups on the surface can improve the polarity of PDMS layer. Meanwhile, the amino groups can act as the active sites for further surface initiated atom transfer radical polymerization to graft the selective layer. Hydroxy terminated PDMS oligomer and its cross-linker tetraethyl orthosilicate (TEOS) were chosen in this study. The formation of cross-linked PDMS was shown in Fig. 1 (a). Under the catalysis of dibutyltindilaurate (DBD), the hydroxys of PDMS oligomer and the ethyls of TEOS can react and strong reticular spatial structure was formed [23-26]. 3-Aminopropyltriethoxysilane (APTES) was adopted as the modifier. The hydroxys of PDMS oligomer and the ethyls of TEOS can also react under the catalysis of dibutyltindilaurate (DBD). However, the reticular spatial structure would be weakened because APTES contained only three ethyls and one amine group [27]. Their chemical structure is shown in Fig. 1. Polyacrylonitrile (PAN) hollow fiber was adopted as the support, on whose outer surface PDMS layer was prepared, as shown in Fig.1 (b).

2. Experimental

2.1. Materials 1 . 1 H yd r o x y - t e r m i n a t e d

PDMS

( v i s c o s i t y:

2550–3750 cSt) and APTES were purchased from Sigma–Aldrich and used as received. TEOS, DBD, and organic solvent (h eptane) were obtained from

Aladdin Inc. and used as received . PAN hollow fibers (outer diameter: 1.6 mm, inner diameter: 1.2 mm) with an average cut-off molecular weight of 100kDa were provided by Iuwater Co., China.

2.2. Preparation of PAN hollow fiber-supported PDMS membranes with different contents of the modifier APTES 1 . 2 T h e c r o s s - l i n k e d P D M S l a ye r w a s c o a t e d onto PAN hollow fi ber supports as follows. PDMS oligomer (0.2 g) was added in 9.4 g of heptane, and the solution was stirred for 1 min. Then, different masses of TEOS and APTES were added into the PDMS solution. The molar weight of ethoxy group (–OCH2CH3)

was

maintained

at

approximately

0.00385 mol. Five mass ratios of TEOS and APTES w e r e c h o s e n ( T a b l e 1 ) . S u b s e q u e n t l y, 0 . 1 g o f D B D was added to promote the cross -linking of PDMS. The mixed solution was stirred for 11 min and stood f o r 4 m i n . F i n a l l y, t h e P D M S c o a t i n g s o l u t i o n w a s prepared 1.3 PAN hollow fibers were pretreated with f r e e z e - d r yi n g

to

remove

the

residual

solvents

existing in the pores of hollow fibers and then cut i n t o 5 c m l o n g w i t h o n e e n d a t t a c h e d t o a n yl o n t u b e

and another end sealed with epoxy glue . PAN hollow fiber-supported PDMS membranes were fabricated by dip-coating as follows . After being pre-wetted by deionized water for 5 s, each hollow fiber was immersed into PDMS coating solution for 3 s . Then, the samples were dried for at least 72 h before characterization and gas permeation tests .

2.3. Membrane characterization and gas permeation experiments A Nicolet 5700 Fourier transform infrared (FTIR) spectrometer with a scan range of 4000–400 cm−1 and a resolution of 0.09 cm−1 was used to obtain the FTIR spectra of cross-linked PDMS with different contents of APTES. Herein, different PDMS membranes were ground with potassium bromide particles. The mass ratio of PDMS/potassium bromide was maintained at 1/50, and a fixed amount of the mixture was pressed to a flat sample for FTIR test. X-ray photoelectron spectroscopy (XPS, VG Scientific Ltd., UK) was used to measure the chemical composition of unmodified and modified surfaces of PAN hollow fiber-supported PDMS membranes. The resolution for the measurement of the binding energy was approximately 0.8 eV. Changes in water contact angle (WCA) of cross-linked PDMS surfaces with increasing mass of APTES were measured using an OCA20 optical contact angle meter (DATAPHYSI, Germany). Cross-linked PDMS was also prepared on the surface of glass slides. With the hydrostatic weighing method, a Mettler Toledo balance (ModelXS205, Switzerland) and a density determination kit were used to measure the densities of PDMS membranes with different contents of APTES. The membrane density (ρm) was calculated as follows [28]:

ρm =

MA ρ0 (1) MA  ML

where MA is the membrane weight in air; ML is the membrane weight in the auxiliary liquid; and ρ0 is the density of the auxiliary liquid. Ethanol was used as the auxiliary liquid to determine the density of PDMS. Solvent swelling measurements were carried out. After being weighed, dry PDMS membranes with different contents of APTES were immersed in pure toluene until equilibrium swelling was reached. The membranes were then pat dried and

immediately weighed. The process was performed thrice for each membrane to ensure repeatability. The swelling degree (MSD) of the membrane was calculated as follows [29]:  M wet  M dry M SD  %     M dry 

  100 (2) 

where Mwet and Mdry are the weights of the wet and dry membranes, respectively. A Tu-4 viscometer was adopted to measure the viscosity of PDMS coating solution. The measurement of viscosity was based on the standard (GB/T 1723, China) of paint viscosity measurement method. The test was conducted at room temperature twice with an error of viscosity smaller than ±3%. The kinematic viscosity of the coating solution was calculated using the following equation:

t = 0.154   11 (t < 23 s) (3) where µ stands for kinematic viscosity (mm2/s) and t refers to the retention time of the PDMS coating solution. 1 . 4 T h i c k n e s s e s o f P D M S s e l e c t i v e l a ye r s o f PAN hollow fiber-supported PDMS membranes were detected scanning

using

a

electron

Hitachi

SU -70

microscope

field-emission (FEG650,

FEI,

H o l l a n d ) o p e r a t e d a t 3 k V . P r i o r t o t h e a n a l ys i s , m e m b r a n e s w e r e c r yo g e n i c a l l y f r a c t u r e d i n l i q u i d n i t r o g e n a n d t h e n s p u t t e r e d w i t h a t h i n l a ye r o f g o l d . 1.5 <1.7**2**> The gas permeation and separation performance of PAN hollow fiber-support PDMS membranes with different contents of APTES were tested under different operating temperatures ranging from 10 °C to 50 °C. The inlet pressure (relative pressure) was controlled at 0.2 MPa , and

permeated gas flowed out at ambient pressure. Pure H2, CO2, CH4, and N2 were adopted as feed gases. The

gas

permeation

calculated

using

the

rate

and

selectivity

following

equations

were [7,

11]:

J  P / L  Q / SmΔp (4)  D  S  α A/ B  J A / J B   A  A  (5)  DB  S B 

1.6 where J (1 GPU = 1 × 10−6 cm3(STP)/cm2·scmHg) denotes the gas permeation rate, which indicates the permeability of a single gas or a component in a m i x t u r e ; L i s t h e l a ye r t h i c k n e s s ; Q r e p r e s e n t s t h e gas flow rate; Sm is the effective permeation area of the composite membrane; and Δp is the pressure difference across the membrane. Gas selectivity αA/B is the ratio of JA to JB denoting the permeation rates o f g a s e s A a n d B , r e s p e c t i v e l y.

3. Results and discussion FTIR measurements of amino adsorption at room temperature provided insights into the existence and concentration of APTES in cross-linked PDMS. As shown in Fig. 2, the appearance of IR features at 3400 cm−1corresponds to the adsorption of amino groups in APTES [6, 30, 31]. In particular, PDMS with increasing concentrations of APTES exhibits a strong bond at 3400 cm−1. The Si–O–Si stretching multicomponent peaks for PDMS are also observed between 950 and 1200 cm−1 [32]. The peaks at 1250 cm−1 correspond to –CH3 deformation vibration in PDMS [28]. Si–CH3 peak appears in the region of 785–815 cm−1, which is also observed in the FTIR spectra [30].

The atomic composition on the surface of the PAN hollow fiber-supported PDMS membranes was characterized by XPS. The change in N atomic concentration with increasing content of APTES in PDMS is shown in Fig. 3. No N atom existed in the PDMS membrane without the blending of APTES because PDMS, TEOS, and DBD did not contain N atoms. Then, N concentration increased with the increased mass ratio of APTES/PDMS oligomer. This result proved the existence of amino groups on the surface of the PDMS membrane. The concentration of N atoms on the surface of the PDMS membrane reached ~6%when the mass ratio of APTES/PDMS oligomer was increased to 4/3. This result implied the success of the functional surface modification of the PDMS membrane by blending APTES. Surface polarity of PDMS membranes can also be represented by surface hydrophilicity. In this study, the hydrophilicity of PDMS was characterized in terms of static WCA. The WCAs of cross-linked PDMS with different masses of the modifier (APTES) were measured (Fig. 4). The WCA of PDMS without APTES was measured at ~114°, which is fairly in accordance with the values reported in the current literature [15]. The WCA decreased linearly to ~87.5°when the mass ratio of APTES/PDMS oligomer was increased to 4/3. This result can be ascribed to the hydrophilic amino groups in APTES. The decrease in WCA implied the successful surface polarization of the PDMS membrane. The densities of PDMS membranes with different mass ratios of APTES/PDMS oligomer are shown in Fig. 5.In general, the density of the PDMS membrane decreased with increasing concentration of APTES. This outcome is attributed to the decreased concentration of TEOS when APTES concentration was increased (Table 1). However, relative to APTES, TEOS can combine more PDMS oligomers and cause the polymerization to form a tight network in the PDMS membrane and consequently increase the density. Therefore, the increasing content of APTES decreased the density of the PDMS membrane. Meanwhile, a decrease in the density of the PDMS membrane corresponded to an increase in fractional free volume (FFV) in the membrane [28, 33]. Fig. 6 presents the effect of APTES content on the swelling of dense PDMS membranes prepared at different APTES/PDMS oligomer mass ratios of 0, 1/3, 2/3, 3/3, and 4/3. The swelling degree of the membrane increased with decreasing content of APTES in the PDMS membrane. This behavior corresponded to the change in density of the membrane. Such observations have been ascribed to a reduced chain length of the oligomers between cross-links in other studies [28, 34]. The loose network in the PDMS membrane with

increasing content of APTES caused the increasing degree of swelling, which corresponded to the change in density. The thicknesses of the PDMS layer of PAN hollow fiber-supported membranes were measured on SEM cross-section images. Typical images of all PDMS membranes are listed in Fig. 7. Before adding the modifier (APTES), the thickness of the PDMS layer was valued at ~490 nm in Fig. 7 (a). After adding APTES, the thickness of the PDMS layer increased to ~1600 nm in Fig. 7 (c) and then decreased to ~830 nm in Fig. 7 (e). This phenomenon corresponded to the variation trend of the viscosity of PDMS coating solution (Fig. 8). Adding a certain amount of APTES into the PDMS solution can enhance the cross-linking reaction of PDMS, thereby increasing the viscosity of the solution. However, excessively low TEOS content can inhibit the cross-linking reaction of PDMS. The viscosity of the solution increased to the peak and then decreased as the mass ratio of APTES/PDMS oligomer was increased (Fig. 8). During dipcoating, more coating solutions can adhere onto the outer surface of the PAN hollow fiber when the viscosity of the coating solution was high, thereby producing a thick PDMS layer. The CO2 permeation and separation performance of hollow fiber-supported PDMS membranes with different contents of APTES were investigated under different operating temperatures. The CO2 permeation rate sharply decreased when the mass ratio of APTES/PDMS oligomer was increased to 2/3 (Fig. 9). Thereafter, a considerable rising occurred when the mass ratio of APTES/PDMS oligomer was increased to 4/3. The dramatic change in CO2 permeation rate can be attributed to the corresponding change in the thickness of the PDMS selective layer with increasing mass fraction of APTES. The gas permeation rate of PAN hollow fiber-supported PDMS membranes is mainly determined by the thickness and gas permeability of the PDMS layer when the PAN hollow fiber shows gas permeation rates (e.g., the CO2 permeation rate of the PAN hollow fiber adopted in this study was ~51000 GPU) considerably larger than that of the PDMS layer [35]. Meanwhile, the permeation resistance from the PAN hollow fiber is relatively small and negligible. Therefore, CO2 permeation rate decreased to the bottom at approximately 800 GPU when the mass ratio of APTES/PDMS oligomer was 2/3 and a 1626 nm-thick PDMS selective layer was prepared. However, a nearly linear increase in CO2 permeability was revealed with the increase in the mass fraction of APTES when the CO2 permeabilities of different PDMS selective layers were calculated on the basis of the estimated thicknesses (Fig. 10). As the cross-linking of PDMS was gradually weak with the addition of APTES, polymer chains became more flexible and more free volumes were produced in the structure of PDMS, causing the increase in CO2 permeability[34]. However, the weak cross-linking of PDMS caused the degradation of CO2 separation performance (Fig. 11). CO2/H2 selectivity, CO2/CH4 selectivity, and CO2/N2 selectivity were ~3.1, ~4.1, and ~10.3, respectively, when no APTES was added in the PDMS solution and when the operating temperature was 30 °C. This result was fairly in accordance with the values reported in other studies, indicating the successful preparation of a defect-free PDMS layer [35]. Meanwhile, CO2/H2 selectivity,

CO2/CH4 selectivity, and CO2/N2 selectivity decreased to ~2.0, 3.3, and 6.4, respectively, when the mass ratio of APTES/PDMS oligomer in the PDMS coating solution was increased to 4/3 because the weak cross-linking of PDMS worsened CO2 diffusion selectivity [34]. The CO2 permeation rate increased slightly for all PAN hollow fiber-supported PDMS membranes with different contents of APTES as the operating temperature was increased (Figs. 9 and 11). Meanwhile, CO2/H2 selectivity, CO2/CH4 selectivity, and CO2/N2 selectivity obviously decreased, exhibiting a permeability selectivity trade-off as expected. For example, the CO2 permeation rate of a PDMS membrane with 1/3 mass ratio of APTES/PDMS oligomer increased from ~1767 GPU to ~1889 GPU when the operating temperature was increased from 10 °C to 50 °C. Simultaneously, CO2/H2 selectivity, CO2/CH4 selectivity, and CO2/N2 selectivity decreased from ~3.7, ~4.3, and ~10.8 to ~2, ~3.2, and ~7.9, respectively. The mobility of CO2 molecules increased at elevated temperatures, which enhanced the driving force for diffusion. In addition, the increase in temperature caused the production of more flexible polymer chains, thereby creating more free volume cavities for molecule transport [36]. However, the decrease in CO2 selectivity should be attributed to the decreased CO2 solubility at high temperatures [37]. The comparison in gas permeation properties of PAN hollow fiber-supported PDMS membranes modified with APTES and reported PDMS membranes in literatures was shown in Table 2. The CO2 permeances of PDMS membranes in this study were medium, and CO2 permeabilities were relatively low. This was because the intrusion of PDMS coating solution in PAN hollow fibers resulted in an increase of CO2 transport resistance. Meanwhile, the resistance of PAN hollow fiber produced a negative effect on CO2 permeability. Besides, CO2/CH4 selectivity and CO2/N2 selectivity of the unmodified and modified PDMS membranes were similar to those reported in literatures. However, CO2/H2 selectivity was relatively low in this study due to the looseness of polymer chains after blending APTES.

4.

Conclusion

The modifier APTES containing an amino group was blended to PDMS for the functional surface modification of a PAN hollow fiber-supported PDMS membrane. FTIR and XPS analyses proved the existence of APTES and its active amino group. Meanwhile, the WCA of PDMS membrane decreased from ~114° to ~87.5° when the mass ratio of APTES/PDMS oligomer in PDMS coating solution was increased from 0 to 4/3. This finding indicates the surface polarization of the PDMS membrane. Moreover, the density of the PDMS membrane decreased while the swelling degree increased as the mass ratio of APTES/PDMS oligomer was increased. The behavior reflects the polymerization of PDMS to form a loose network and the increase in FFV after blending APTES. Consequently, the permeability of CO2 increased while CO2/H2 selectivity, CO2/CH4 selectivity, and CO2/N2selectivity decreased. For the CO2 permeation rate of the hollow fiber-supported PDMS membrane, the variation trend of initial decreasing and then increasing was similar to that of the viscosity of PDMS coating solution when the mass ratio of APTES/PDMS oligomer was increased.

Acknowledgements This study was supported by the National key research and development programChina (2016YFE0117900), National Natural Science Foundation-China (51676171), Zhejiang Provincial Key Research and Development Program-China (2017C04001).

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Fig. 1 Schematic diagrams of the formation of the cross-linked PDMS (a) and preparation (b) of PAN hollow fiber-supported PDMS membranes modified with amino groups.

Fig. 2 FTIR spectra of PDMS membranes with various mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane oligomer in PDMS coating solution.

Fig. 3 Concentrations of N atoms on the surface of PDMS membranes with various mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane (PDMS) oligomer in PDMS-coating solution detected through XPS.

Fig. 4 Effects of mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane (PDMS) oligomer on water contact angle of PDMS membrane.

Fig. 5 Effects of mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane (PDMS) oligomer on PDMS membrane density.

Fig. 6 Effects of mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane (PDMS) oligomer on the swelling degree of PDMS membrane.

Fig. 7 Thicknesses of polydimethylsiloxane (PDMS) selective layers on PAN hollow fibers when the mass ratios of 3-aminopropyltriethoxysilane to PDMS oligomer were 0 (a), 1/3 (b), 2/3 (c), 1/1 (d), and 4/3 (e).

Fig. 8 Effects of mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane (PDMS) oligomer on kinematic viscosity of PDMS coating solution.

Fig. 9 Effects of operating temperature on the CO2 permeation rates of PAN hollow fiber-supported PDMS membranes with various mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane oligomer.

Fig. 10 Effects of mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane (PDMS) oligomer on CO2 permeability of PAN hollow fiber-supported PDMS membranes at an operating temperature of 30 °C.

Fig. 11 Effects of operating temperatures on CO2/H2 selectivity (a), CO2/CH4 selectivity (b), and CO2/N2 selectivity (c) of PAN hollow fiber-supported PDMS

membranes at various mass ratios of 3-aminopropyltriethoxysilane to polydimethylsiloxane oligomer. Table 1 Experimental conditions of PDMS coating solution compositions. Experiments

Weights and molar quantities of TEOS and

Mass ratio of APTES/PDMS in

APTES in PDMS coating solution

PDMS coating solution

Condition 1

0.200 g (9.60 ×10−4 mol), 0 g (0 mol)

0

Condition 2

0.153 g (7.34 ×10−4 mol), 0.067 g (3.03 ×10−4 mol)

1:3

Condition 3

0.106 g (5.09 ×10−4 mol), 0.133 g (6.01 ×10−4 mol)

2:3

Condition 4

0.059 g (2.83 ×10−4 mol), 0.200 g (9.03 ×10−4 mol)

1:1

Condition 5

0.012 g (0.58 ×10−4 mol), 0.267 g (12.06 ×10−4 mol)

4:3

Table 2 Comparison in gas permeation properties of PAN hollow fiber-supported PDMS membranes modified with APTES and reported PDMS membranes in literatures Membranes

Testing

CO2

CO2

CO2/H2

CO2/CH4

conditions

permeability

permeance

selectivity

selectivity

--a

3704 GPU

--

--

3970 Barrers

--

--

4.0

--

~4.5

~3.0

3800 Barrers

--

~4.3

~3.2

--

2481 GPU

~3.8

~3.1

--

~411 GPU

3.8

--

Hollow fiber-supported

~15 psig,

PDMS

~25°C

membrane PAN flat-supported

~400 kPa,

PDMS

35°C

membrane PDMS film flat-supported PDMS membrane

206 kPa,

~3200

35°C

Barrers

15-240 psig, 35°C

PAN flat-supported

2 atm,

PDMS gutter

35°C

layer Ceramic

0.2 MPa,

flat-supported

35°C

PDMS membrane Plasma modified

--,35°C

PDMS

~950 Barrers

--

--

--

membrane PAN hollow

1166 Barrers

2370 GPU

3.2

4.1

10.3

fiber-supported

1305 Barrers

1836 GPU

2.6

3.7

9.6

PDMS

0.2 MPa,

1395 Barrers

858 GPU

2.4

3.6

8.4

membranes

30°C

1560 Barrers

1473 GPU

2.1

3.5

8.1

2.0

3.3

6.4

modified with

1644 Barrers

APTES

Note: a meant the data was unavailable. TDENDOFDOCTD

1986 GPU

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