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Preparation of PAA-g-PEG/PANI polymer gel electrolyte and its application in quasi solid state dye-sensitized solar cells.

INTRODUCTION

Since the first report by O'Regan and Gratzel in 1991 [1], dye-sensitized solar cell (DSSC) has attracted widespread scientific interests due to its low cost, high conversion efficiency, and simple preparation procedure [2, 3], A typical DSSC based on liquid electrolyte has been achieved a photoelectric conversion efficiency of 12% [4]. However, the use of liquid electrolyte displays some practical problems such as leakage and volatilization of liquid. These problems are considered as some of the critical reasons restricted the long-term performance and practical use of the DSSC [5]. Therefore, it is necessary to seek for new electrolyte with high efficiency, good stability, and easy preparation. Recently, many works have been done to replace the liquid electrolyte with solid state [6-10] or quasi solid state (QS) type charge transport materials [5, 11-15], Especially, the QS gel electrolyte attracts more attention due to its high ionic conductivity, good interfacial filling properties, and relatively high long-term stability.

Superabsorbent polymers have a network structure with a suitable degree of crosslinking. Such a structure makes it able to absorb a large amount of water to form a stable hydrogel, and the absorbed water is hard to be removed even under some pressure [16, 17], This unique property makes it could be used in QS gel electrolyte. On the other hand, conducting polymer is a good candidate material in DSSC due to its low cost, high conductivity, high specific capacitances, good stabilities, and catalytic activity for [I.sup.-]/[I.sub.3.sup.-] reaction [18, 19].

In this article, a microporous superabsorbent of poly(acrylic acid)-g-poly(ethylene glycol)/polyaniline (PAA-g-PEG/PANI) was synthesized. The porous PAA-g-PEG was synthesized by aqueous solution graft copolymerization, then aniline solution mixed with HC1 was absorbed into PAA-g-PEG network and in situ polymerized in the net work; thus, the PAA-g-PEG/PANI hybrid polymer was obtained. Using the hybrid polymer as host, a polymer gel electrolyte with the conductivity of 11.50 mS [cm.sup.-1] was prepared. The gel electrolyte was used to assemble QS DSSC, which achieves a high efficiency of 6.38%.

EXPERIMENTAL

Materials

Acrylic acid and aniline monomer were distilled under reduced pressure before use. Polyethylene glycol (molecular weight of 20,000, PEG-20000) and potassium peroxydisulfate (KPS), as a radical initiator for the synthesis of PAA-g-PEG hybrid, were used as received. AA-methylene bisacrylamide (NMBA), as a crosslinker for preparing the hybrid, was purified by recrystallization from 66 wt% ethanol/water solution. Organometallic sensitized dye N-719, [[RuL.sub.2][(NCS).sub.2], L = 4,4-dicarboxylate-2,2-bipyridine], was purchased from Solaronix SA. Titanium (IV) isopropoxide, OP emulsification agent (Triton X-100), and other reagents were obtained from Shanghai Chemical Agent, China and used as received. All the chemicals were of analytical reagent grade.

Preparation of PAA-g-PEG Superabsorbent Polymer

PAA-g-PEG superabsorbent polymer was prepared by modifying the procedure from Refs. 10] and [20]-22: 1.0 g PEG and 10 g acrylic acid (AA) were dispersed in 15 mL distilled water. Initiator KPS (weight ratio of KPS to AA was 0.8%) and NMBA (weight ratio of NMBA to AA was 0.05%) were added to the mixed solution system. Then, a graft copolymerization reaction took place under stirring in nitrogen atmosphere at 80[degrees]C. After completion of the copolymerization reaction, the system was cooled to room temperature. The resultant product was filtered and then immersed in excess distilled water to remove any impurities. Then, the product was vacuum dried at 80[degrees]C for more than 12 h to a constant weight.

Synthesis of PAA-g-PEGIPANI Hybrid Polymer and Gel Electrolyte

PAA-g-PEG/PANI hybrid polymer was prepared by the following procedures [20, 21]: 0.2 g of PAA-g-PEG was immersed in a predetermined amount of aniline aqueous solution mixed with HCl (pH = 2.5) at 4[degrees]C for more than 48 h, which resulted in the absorption of ANI into the network of PAA-g-PEG polymer. Such a swollen PAA-g-PEG gel was dispersed in aqueous solution containing a suitable amount of KPS (mass ratio of KPS to aniline was 1.0) at 4[degrees]C for more than 48 h in dark, which resulted in an in situ polymerization of ANI monomers inside the 3D network of PAA-g-PEG to form PAA-g-PEG/ PANI. The PAA-g-PEG/PANI polymer was washed and dried like the preparation of PAA-g-PEG, thus the PAA-g-PEG/PANI hybrid superabsorbent was obtained.

The gel electrolyte was prepared by soaking 0.2 g of PAA-g-PEG/PANI sample in liquid electrolyte. The liquid electrolyte consisted of 0.1 M tetrabutylammonium iodide, 0.1 M tetramethylammonium iodide, 0.1 M LiI, 0.1 M KI, 0.1 M Nal, 0.06 M [I.sub.2] in mixed organic solvent of A-methyl-2-pyrrolidone (NMP) and acetonitrile (AC) (NMP/AC = 2/8). After soaking adequately, a black color PAA-g-PEG/PANI gel electrolyte was obtained.

Fabrication of DSSC

Microporous Ti[O.sub.2] film (10 [micro]m thick) was prepared as described previously [23], The Ti[O.sub.2] colloid was dropped on a FTO glass by a doctor scraping technique, and sintered at 450[degrees]C for 30 min in air, the process was repeated twice. The Ti[O.sub.2] film was immersed in 50 mM Ti[Cl.sub.4] (aqueous) at 70[degrees]C for 30 min and then sintered at 450[degrees]C for 30 min. The resultant Ti[O.sub.2] film was immersed in a dye N-719 ethanol solution (2.5 X [10.sup.-4] M) for 24 h. Thus, a dye-sensitized Ti[O.sub.2] electrode was obtained. The QS-DSSC was assembled by injecting the gel electrolyte based on PAA-g-PEG or PAA-g-PEG/PANI into the aperture between the Ti[O.sub.2] film electrode (anode electrode) and the Pt electrode (counter electrode). The two electrodes were clipped together and wrapped with thermoplastic hot-melt Surlyn.

Characterizations and Measurements

The morphology of the PAA-g-PEG and PAA-g-PEG/PANI were observed by using a scanning electron microscope (SEM, S-5200, Hitachi, Tokyo, Japan). The powdered samples were identified by Fourier transform infrared (FTIR) spectroscopy on a Nicolet Impact 410 FTIR spectrophotometer (Inspiratech 2000, Warwickshire, UK) using KBr pellets.

The swelling ratio (SR, g/g) of the sample was measured according to the equation below [11]:

Swelling ratio (SR) = [[W.sub.2] - [W.sub.1]]/[W.sub.1] (1)

where [W.sub.1] is the mass of dried sample (g) and [W.sub.2] is the mass of swollen gel electrolyte (g). The ionic conductivity of gel electrolyte was measured by using model DDSJ-308 digitized conductivity meter (Shanghai Reici Instrument Factory, China). The instrument was calibrated with 0.01 M KC1 aqueous solution before experiments [24].

Photovoltaic Test

The photovoltaic test of DSSC was carried out by measuring photocurrent-photovoltage (J-V) characteristic curves under simulated solar illumination at 100 m W x [cm.sup.-2] (AM 1.5) from a 100-W xenon arc lamp (XQ-500W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere. The fill factor (FF) and overall light to electrical energy conversion efficiency ([eta]) of the solar cell were calculated according to the following equations [25]:

FF = [V.sub.max] x [J.sub.max]/[V.sub.OC] x [J.sub.SC] (2)

[eta](%) = [[V.sub.max] x [J.sub.max]]/[P.sub.in] x 100% = [[V.sub.OC] x [J.sub.SC]] x FF/[P.sub.in] (3)

where [J.sub.SC] is the short-circuit current density (mA x [cm.sup.-2]), [V.sub.OC] the open-circuit voltage (V), [P.sub.in] is the incident light power, [J.sub.max] (mA x [cm.sup.-2]), and [V.sub.max] (V) are the current density and voltage at the point of maximum power output in the J-V curves, respectively.

RESULTS AND DISCUSSION

Appearance of the Polymer Gel Electrolytes

The SEM images of the PAA-g-PEG and PAA-g-PEG/PANI gel electrolytes are shown in Fig. 1. It can be seen that the hydrogel displays an obvious porous network structure (Fig. 1a). Such a three-dimensional (3D) network structure in conjunction with the hydrophilic groups of PAA-g-PEG enable these gel electrolytes to absorb a large amount of organic solvent, and the absorbed solvent is hardly released even under some pressure or heat [16, 17]. Furthermore, this porous network structure prompt aniline molecules to be absorbed into PAA-g-PEG and in situ polymerize in the network. This can be verified by SEM image of PAA-g-PEG/PANI (Fig. 1b), which presents a rough surface with sheath-like coverings. According to the report [20, 21], the PAA-g-PEG network with many oxygen-containing groups (-COOH, -OH) are inclined to integrate with conductive PAIN chains (containing -N[H.sub.2] groups) via physical or chemical interaction, which may make a positive influence to the conductivity of PAA-g-PEG/PANI gel electrolytes.

FTIR Spectra of Samples

Figure 2 shows the FTIR spectroscopy of PAA-g-PEG (a), PANI (b), and PAA-g-PEG/PANI (c). For PAA-g-PEG in curve (a), the peaks related to O-H vibration appear at 3300-3100 [cm.sup.-1] (stretching vibration), 1250 [cm.sup.-1] (wagging vibration), and 918 [cm.sup.-1] (bending vibration), whereas another two absorption bands at 1730 and 1170 [cm.sup.-1] are attributed to the C=O bending and C-O-C stretching vibration, respectively. These results demonstrate the existence of a large number of oxygen-containing groups, including -OH and -COOH. Moreover, other visible peaks are assigned to -C[H.sub.3] stretching (2960 [cm.sup.-1]), --C[H.sub.3] bending (1450 [cm.sup.-1]), -C[H.sub.2] scissoring (1400 [cm.sup.-1]), C-H out-of-plane bending (808 [cm.sup.-1]), and C-H out-of-plane deformation vibration [11, 12, 24, 26].

For PANI in curve (b), the peaks at 1560 and 1490 [cm.sup.-1] are ascribe to the stretching of quinoid and benzenoid rings, respectively. The peak around 1290 [cm.sup.-1] arises from the C-N stretching of a secondary aromatic amine. The peak around 1240 [cm.sup.-1] can be interpreted as C-[N.sup.+] stretching vibration in the polaron structure. The band centers at 1140, 798, and 598 [cm.sup.-1] arise from C-H in-plane deformation vibration, C-H out-of-plane bending vibration, and C-H out-of-plane deformation vibration. These characteristic peaks are coinciding with the previous reports [20-24].

For PAA-g-PEG/PANI, curve (c) shows a similar FTIR spectrum as curve (a) except for some peaks shift. The peaks at 1560 and 1500 [cm.sup.-1] responsible for C-C stretching mode of the quinoid and benzenoid rings in PANI [21]. The C-H out-of-plane bending vibration at 804 [cm.sup.-1] and C-H out-of-plane deformation vibration at 619 [cm.sup.-1] show a little red shift, which may caused by the hydrogen bond interaction formed in the hybrid polymeric components after the introduction of PANI [13]. According to the FTIR analysis, it can be confirmed that the PANI was formed inside of PAA-g-PEG polymer network, which is significant for the conductivity improvement of PAA-g-PEG/PANI gel electrolyte.

Influence of Aniline Concentration on the Conductivity of Gel Electrolyte PAA-g-PEG/PANI

Figure 3 shows the influence of aniline concentration on the conductivity of gel electrolyte PAA-g-PEG/PANI. From Fig. 3, it can be seen that a maximum value reaches 11.50 mS/cm at an aniline concentration of 0.66 wt%, the increasing of conductivity encounters bottleneck and reverse as the aniline concentration of more than 0.66 wt%. According to the previous report [20], a lower aniline concentration will leads to a slower velocity for the polymerization reaction of PANI, resulting in lower PANI yield and electrical conductivity. Furthermore, only part aniline monomers can be absorbed into the PAA-g-PEG network due to its osmosis character, when the aniline concentration exceeds 0.66%, mostly aniline monomers and oligomer distribute outside of PAA-g-PEG network, and will be washed out in the preparation process.

Liquid Absorbency and Conductivity of PAA-g-PEG and PAA-g-PEGIPANI Polymers

Table 1 is the liquid electrolyte absorbency capability and conductivity properties of PAA-g-PEG (a) and PAA-g-PEG/ PANI (b). From Table 1, it can be seen that the SR (g/g) of PAA-g-PEG is slightly higher than PAA-g-PEG/PANI. It might be due to the introduction of PANI inside of the network, which increases the crosslinking density of the polymer and leads to the decrease of liquid absorbency [27]. Even PAA-g-PEG can absorbs a little more liquid electrolyte than PAA-g-PEG/PANI, its conductivity (9.28 mS [cm.sup.-1]) is less than that of PAA-g-PEG/PANI (11.5 mS [cm.sup.-1]) due to the introduction of the conductivity PANI chains inside of the network. As former mentioned, once the second chain of PANI is formed, the whole system becomes an interpenetrated network structure and an electrical conductive channel is formed with PANI chain [20]. Benefited from the excellent conductivity of PAA-g-PEG/PANI, the light-to-electric energy conversion efficiency of DSSC will improved significantly [28-30].

Influence of Temperature on the Conductivity of Gel Electrolytes

Figure 4 shows the conductivity-temperature ([sigma]-T) relationship of PAA-g-PEG and PAA-g-PEG/PANI gel electrolytes. The conductivities of these samples increase linearly as the temperature rises, which coincides with our previous results [16, 17, 31]. The conductivity-temperature ([sigma]-T) behaviors of gel electrolyte can be described by Arrhenius equation 4 shown as follows:

1n [sigma] = 1n A = 1n A - Ea/RT (4)

where [E.sub.a] is the activation energy, R is the molar gas constant, A is a meaningless constant, and T is absolute temperature. According to the experimental data, the activation energies ([E.sub.a]) and A of PAA-g-PEG/PANI gel electrolyte are calculated as 9.06 kJ [mol.sup.-1] and 445, the [E.sub.a] and A for the gel electrolyte based on PAA-g-PEG are calculated as 9.53 kJ [mol.sup.-1] and 489, respectively. The similar activation energies indicate the similar ionic conduction mechanism and the free-volume for PAA-g-PEG/PAN1 and PAA-g-PEG systems [5]. However, because of the introduction of PAN1 into the polymer network, PAA-g-PEG/PAN1 shows a higher conductivity (11.50 mS [cm.sup.-1]) than PAA-g-PEG (9.28 mS [cm.sup.-1]).

Photovoltaic Performance of the DSSCs

Figure 5 shows the photocurrent versus photovoltage curves of the DSSCs based on gel electrolyte PAA-g-PEG (a) and PAA-g-PEG/PANI (b). Under illumination with a simulated solar light of 100 mW [cm.sup.-2] (AM 1.5), the photovoltaic parameters of the DSSCs including short circuit photocurrent density ([J.sub.SC]), open circuit voltage ([V.sub.OC]), FF, and the light-to-electric energy conversion efficiency ([eta]) are listed in Table 2. It can be seen that all the photovoltaic parameters of DSSC with PAA-g-PEG/PANI gel electrolyte are better than that with PAA-g-PEG system.

The light to electric energy conversion efficiencies ([eta]) for the QS DSSCs with PAA-g-PEG gel electrolyte and PAA-g-PEG/PANI gel electrolyte are 5.5% and 6.38%, respectively, and the [eta] (6.38%) is higher than the previously published reports [32, 33]. It could be attributed to the introduction of PANI in the PAA-g-PEG polymer. As mentioned previously that the introduction of PANI increases the conductivity of gel electrolyte, which accelerates the mobility of charge carriers in the electrolyte. It means the [I.sub.3.sup.-] can be transported to the Pt-coated counter electrode rapidly and efficiently reduced at the counter electrode, instead of the recombination with Ti[O.sub.2] anode film [5], Besides, PANI has a good electrocatalytic activity for the [I.sub.3.sup.-]/[I.sup.-] redox reaction (which causes a lower energy lose from [I.sub.3.sup.-] to [I.sup.-]), which decreases the overpotential and enhances the open circuit voltage [27]. In other word, PANI chains on PAA-g-PEG polymer host leads to a decreased dark current and a higher light-current, which are beneficial for the increase of FF value.

The insets are the photographs of PAA-g-PEG (a) and PAA-g-PEG/PANI (b) gel electrolytes. From the insets, it can be seen that PAA-g-PEG presents red-brown color, whereas PAA-g-PEG/PANI is completely black, which is a direct evidence for the insertion of PANI.

CONCLUSIONS

A novel PAA-g-PEG/PANI copolymer was synthesized by a two steps solution polymerization method. Using the polymer as host, a polymer gel electrolyte with high conductivity of 11.5 mS [cm.sup.-1] was prepared. Based on the electrolyte, a QS-DSSC was assembled. Because of the high conductivity of PANI and its catalytic function for [I.sup.-]/[I.sub.3.sup.-] system in PAA-g-PEG/PANI gel electrolyte, the QS-DSSC with photocurrent density of 13.7 mA [cm.sup.-2], open circuit potential of 694 mV, FF of 0.670, and efficiency of 6.38% was obtained. The results indicate that the addition of PANI has a positive effect on the gel electrolyte and the photovoltaic performance of the DSSC.

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Qin Liu, Jihuai Wu, Zhang Lan, Min Zheng, Gentian Yue, Jianming Lin, Miaoliang Huang

Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China

Correspondence to: Jihuai Wu; e-mail: jhwu@hqu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: U1205112; 21301060; 50842027; 61306077.

DOI10.1002/pen.23900

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Liquid electrolyte absorbency
and conductivity ([sigma]) of the polymers.

                                   [sigma] (mS
                                 [cm.sup.-1] at
Samples               SR (g/g)   20 [degrees]C)

PAA-g-PEG (a)           8.32           9.28
PAA-s-PEG/PANI (b)      8.00          11.50

TABLE 2. Photovoltaic performances of the DSSCs
with electrolyte PAA-g-PEG and PAA-g-PEG/PANI.

Gel                     [J.sub.sc]      [V.sub.oc]
electrolyte          (mA [cm.sup.-2])      (mV)        FF    [eta](%)

PAA-g-PEG (a)              13.0             647      0.652     5.50
PAA-g-PEG/PANI (b)         13.7             694      0.670     6.38
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Author:Liu, Qin; Wu, Jihuai; Lan, Zhang; Zheng, Min; Yue, Gentian; Lin, Jianming; Huang, Miaoliang
Publication:Polymer Engineering and Science
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Date:Feb 1, 2015
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