Reactive Molecular Dynamics Simulations on the Disintegration of

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The Journal of Physical Chemistry

Reactive Molecular Dynamics Simulations on the Disintegration of PVDF, FP-POSS and Their Composite during Atomic Oxygen Impact †

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‡

Fanlin Zeng,*, Chao Peng, Yizhi Liu, and Jianmin Qu*, Department of Astronautic Science and Mechanics, Harbin Institute of Technology, 92 West Dazhi St., Harbin, Heilongjiang 150001, People’s Republic of China ‡ Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208, United States

†

ABSTRACT poly(vinylidene fluoride) (PVDF) is a kind of important piezoelectric polymer used in spacecraft industry. But the atomic oxygen (AO) is the most abundant element in the low earth orbit (LEO) environment. AO collision degradation is an important issue in the application of PDVF on spacecrafts. In order to investigate the erosion behaviors of PVDF during AO impacts and how to improve the stability of PVDF against AO impacts, the temperature evolution, mass loss and erosion yields of neat PVDF, neat polyhedral oligomeric silsesquioxanes compound (3, 3, 3-trifluoropropyl)8Si8O12 (FP-POSS) and the PVDF/FP-POSS composite under AO impacts, as well as some key disintegrated structures and separated chemical compositions, have been researched using the molecular dynamics (MD) simulations and the reactive ReaxFF force field. The simulation erosion yield result of PVDF is very close to the experiment results, shows our simulations are reliable. The results of the temperature evolution, mass loss and erosion yield of three materials show that the anti-erosion performance of PVDF is not outstanding. However, incorporating FP-POSS into PVDF matrix enhances the stability of PVDF against AO impact greatly and reduces the temperature rise, mass loss and the erosion yield of PVDF rapidly. A detailed analysis on the flight chemical compositions and key snapshots of the structures reveals that the erosion process on PVDF and PVDF/FP-POSS is continuous, and should be derived from the same PVDF matrix in two materials. In contrast, the erosion process on FP-POSS is stepped. The erosion will not take place until the number of AO reaches a specific value. There is a barrier for the erosion of high-energy AO because of the stable cage-like Si-O frame in FP-POSS molecules. This should be chiefly responsible for the high stability of FP-POSS and the reinforcement mechanism of FP-POSS on PVDF against AO impacts. This work is helpful for people to understand the erosion details of PVDF and POSS and provides valuable information to design effective protective structure for PVDF against AO impacts in LEO environment. Keywords: AO impacts; Erosion; Reinforcement mechanism; PVDF; POSS

1. INTRODUCTION As a kind of piezoelectric polymer, poly(vinylidene fluoride) (PVDF) is always used as piezoelectric sensors or actuators in fields where abrasion, chemical resistance, and thermal stability are required, because of its good mechanical properties, resistance to chemicals, high dielectric permittivity and unique pyroelectric and piezoelectric properties.1,2 In aerospace field, PVDF is a very good candidate to form several kinds of smart structures, for example the dust measurements,3 large inflatable antenna,4 solar-sail5 and so on, because on the one hand PVDF is very soft so it can be prepared to the thin and large structures with very light weight, and on the other hand the shape of these structures can be controlled or adjusted by applying specific electric fields, thanks to the piezoelectric properties of this polymer. PVDF is also seemed as the new concept material in aerospace during 2010-2030.6 But materials on the surface of a spacecraft are subject to the collisions with ambient atomic oxygen (AO, the energy is about ~4.5eV), the radiation of ultraviolent (UV) and the extreme temperatures, especially in low earth orbital (LEO) environment.7 AO collisions can cause the surface degradation of these materials not only because the relative speed of AOs is very high (7-8 km/s) but also because AOs are very active and strong oxidizing. In the experiment of Dargaville et al.,8 AO erosion yields were determined as 1

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2.8×10-24 cm3/atom for PVDF in LEO, and the remaining percentage weight was only 40% after 3 years. Thus the AO collision degradation is an important issue needed to be addressed for PVDF in its applications on spacecrafts. An effective approach to protect polymers in LEO is to form atomic layer deposition coatings of Al2O3 or TiO2 on the surface of materials.9 These coatings are anti-erosion but at the same time they are very hard. As for PVDF, the problem is that PVDF is always served as large flexible or inflatable structures, “soft” is primarily important in these applications. Thus the approach of depositing hard coatings is infeasible. An alternative way is to mix some compounds into PVDF matrix to form PVDF composites. In practice, many kinds of compounds, such as the clay,10 carbon black,11 graphite12 and so on, have been blended into PVDF with the objective of further improving its properties. The blending of two polymers may either result in a miscible or an immiscible system. Unfortunately, few materials are miscible when melted with PVDF. In this context, incorporation of polyhedral oligomeric silsesquioxanes (POSS) compounds into the PVDF matrix to enhance its thermal stability and anti-erosion ability is worth considering. POSS is a unique class of compound that can be depicted by the formula (RSiO1.5)n or Tn (where n is an even number and R = H, Cl or a variety of organic groups). Compared with the traditional polymers, a POSS molecule consists of a robust inorganic cage-like core with the Si-O atoms and several organic branches (R groups) bonded to the Si atoms on the corner of the cage.13,14 POSS is also called the organic-inorganic hybrid materials. On the one hand, due to the inorganic Si-O cage in POSS molecules, POSS exhibits many superior thermomechanical properties in terms of wearability, thermal stability, oxidation resistance, and relative high strength13-15 among others. On the other hand, because of the convertible organic R branches, POSS possess flexible chemical activity or properties. Fluorinated POSS are a new kind of POSS compounds with high molecular weight and density.16 These fluorinated POSS compounds are soluble in fluorinated solvents. Subsequently the miscibility of fluorinated POSS and PVDF is expected because the molecules of the former hold the CH2-CF2 groups in R branches similar in structure to the repeat unit in PVDF molecules. In theory, the incorporation of POSS into PVDF will produce PVDF/POSS nanocomposites with highly improved properties, especial the thermal stability and anti-erosion ability. Along the idea discussed above, we performed a serial of researches in the past several years. Firstly, we studied the miscibility behavior of binary mixtures of PVDF and six different kinds of POSS compounds (including four kinds of fluorinated POSS) using molecular simulations and the extended Flory–Huggins model.17 The simulation results in this work indicated that PVDF and four kinds of fluorinated POSS are fully miscible at any temperature and the reason is derived from the polar C–F bonds and the electrostatic interactions in their molecules. Whereafter, we fulfilled a molecular dynamics simulation study to investigate the effects of one kind of the fluorinated POSS (3, 3, 3-trifluoropropyl)8Si8O12 (FP-POSS) on the elastic properties of PVDF.18 In this research we found that the moduli of PVDF are improved after a small amount of FP-POSS was incorporated and the improvement effect, in general, nearly decreases with a further addition of FP-POSS. After that, we prepared different PVDF/FP-POSS nanocomposites and carried out a serial of experiments to characterize their morphological, crystallization, mechanical and piezoelectric properties, and so on.19-22 The experimental results in these researches are in good agreement with our previous simulations. We also found that a small amount of FP-POSS (wt% 2” due to the limitation of the atom numbers of the models. In order to compare the simulations results with experimental results, the AO erosion yield observations of Materials International Space Station Experiment 2 (MISSE 2)32 have been used here. In these experiments, different materials have been exposed to the LEO environment on the exterior of the International Space Station (ISS) for nearly 4 years. On the basis of these experimental results, the erosion yield of PVDF is 1.29×10-24 cm3/atom. Taking into account the density of PVDF (1.63 g/cm3), the reference value of the erosion yield of PVDF is about 2.10×10-24 g/oxygen atom. In Figure 8, it can be found that the erosion yield results for PVDF in our simulations confirms to the experimental one, especially in the relative steady stage during the AO impacts (AO 15~45). The average value of the erosion yield of PVDF is about 1.87×10-24 g/oxygen atom, which is also quite close to the experimental value. These results prove that, to a large extent, the ReaxFF force filed and our simulation models are feasible and reliable to address the erosion behaviors during AO impacts. Figure 8 also shows that the erosion yields of neat POSS and PVDF/POSS are far lower than that of neat PVDF. This result is in accordance with the mass loss results of three materials. If we compute the average erosion yields of neat POSS and PVDF/POSS, we find that they are quite similar because the average erosion yields of neat POSS and PVDF/POSS are about 0.90×10-24 g/oxygen atom and 0.96×10-24 g/oxygen atom, respectively. The erosion yield of neat POSS is slightly lower that of PVDF/POSS but the difference is not as obvious as the mass loss results show because we only consider the erosion yield in the first stage of AO impacts (NVE simulation time < 10ps). We also find the 9



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corresponding mass loss curves (time < 10ps) of neat POSS and PVDF/POSS in Figure 6b are almost overlapping. These results reveal that in the first stage of AO impacts neat POSS and PVDF/POSS have comparable ability of anti-erosion but in the latter stage (time >10ps) neat POSS possesses stronger stability against AO impacts than PVDF/POSS. If we observe the curve variations of three materials in Figure 8, we can find something interesting. The curves of neat PVDF and PVDF/POSS show similar trend but the curve of PVDF/POSS locates below that of neat PVDF. This can be explained from the structures of two materials. The same PVDF matrix in two materials should be responsible for the similar variation trend and the FP-POSS reinforcement phase in PVDF/POSS leads to a much lower erosion degree compared to neat PVDF. As for neat POSS, it can be found that the erosion yield is almost nothing before impacting 21 AOs. But the erosion yield of neat POSS rises greatly after impacting 30 AOs. It seems that there is a barrier in neat POSS. Almost nothing can be disintegrated until the barrier has been broken down. This is somewhat similar to the result of the amorphous silica in Rahnamoun and van Duin’s work.28 Taking into consideration the structure of FP-POSS molecule, we consider that at least one cage-like Si-O frame has been disintegrated when the erosion yield increases rapidly (after impacting 30~35 AOs). If so, it could arrive at the conclusion that a relative small amount of AO impacts have no erosion effect on POSS molecules but will disintegrate PVDF because we find erosion yields in neat PVDF and PVDF/POSS even the number of AO is less than 5. This should be further analyzed from the chemical compositions of the erosion yields.

Figure 8. Erosion yield results of neat PVDF, neat POSS and PVDF/POSS in the first stage of AO impacts.

The chemical compositions of the materials separating from the surface of three materials are listed in Table 1. Because of the limitation of the table size, we only list the results with an interval of 5 AOs. The chemical compositions are determined from monitoring the mass variation of each element in each output result. These simulation results show that in the first stages, atoms C, H, F and molecules FH and H2 are the first small molecules separating from the surface of neat PVDF and PVDF/POSS, and with the progress of AO collisions, oxygen atoms and oxide molecules start to be released from the surface, then with the further collision of AOs, organic compound containing carbon starts separating from the surface. Although the chemical compositions separating from two materials look similar, if we perform a comparison, we find the number of released atoms/molecules from PVDF/POSS is much lower than that of neat PVDF. The results prove once again that FP-POSS can improve the stability of PVDF against AO impact. While for neat POSS, we see there is nothing coming off the surface even after impacting 20 atomic oxygen atoms. After impacting 25 AOs, two atoms of H and F are found to separate from the surface of neat POSS. These two disintegrated atoms come from the R branch of the FP-POSS molecule, and prove that AO will primarily erode the R branches in FP-POSS molecules. By keeping AO collisions, we find silicon containing fragment separates from the surface of neat POSS after impacting 30 AOs. This result confirms to our previous analysis, the cage-like Si-O frame in a FP-POSS molecule has been indeed 10


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disintegrated in this stage, followed by an amount of small molecules containing carbon and fluorine separating from the surface during the following AO collisions. Because the carbon and fluorine atoms exist only in the R branches of FP-POSS molecules, these small molecules should come from the disintegration of the branches. This result also reveals that the R branches of FP-POSS become unstable as soon as the centric cage-like Si-O structure is broken. So the chemical compositions from neat POSS provide exact evidence of the rapid increasing of the erosion yield is derived from the breaking down of the cage-like Si-O frame in FP-POSS molecules. And these results can also explain why there is a barrier when neat POSS starts to be disintegrated.

Table 1. Chemical Compositions of the Materials Separating From the Surface of Neat PVDF, Neat POSS, and PVDF/POSS in the First Stage of AO Impacts material

number of AO added

neat PVDF

5

molecules separated from the surface 1C

neat POSS

5

PVDF/POSS

5

1F

neat PVDF

10

2H, 1FH, 1C, 1H2

neat POSS

10

PVDF/POSS

10

2H, 1H2, 1C

neat PVDF

15

3F, 2H, 1CH

neat POSS

15

1H

PVDF/POSS

15

1C

neat PVDF

20

2O, 2H, 1C, 1CO, 1CH

neat POSS

20

PVDF/POSS

20

2O, 2H, 1HO

neat PVDF

25

3H2, 2HO, 2H

neat POSS

25

1H, 1F

PVDF/POSS

25

1C, 1H

neat PVDF

30

2H, 1CH, 1HF, 1O

neat POSS

30

6H, 1SiH2, 1H2, 1HO, 1HF, 1CH

PVDF/POSS

30

4H, 1HO, 1C

neat PVDF

35

3H2O, 2O, 1CO, 1HO, 1F

neat POSS

35

2CH2, 3C, 1CH2F4, 1CH2F, 1CHF, 1CF, 1F2

PVDF/POSS

35

2O, 1HO, 1H

neat PVDF

40

4H, 2CO, 2CH, 2F, 1H2O, 1COH2, 1H2, 1CF

neat POSS

40

5H, 4F, 1C2H, 1H2

PVDF/POSS

40

2H2, 1CH2, 1HF, 1C, 1H

neat PVDF

45

3HF, 3H, 3C, 2H2, 1C2H2, 1CH4, 1CH3, 1CH2, 1HO, 1F

neat POSS

45

5H, 4HF, 1H2, 1F, 1C

PVDF/POSS

45

7H, 4H2, 3HF, 2O, 1C2H4, 1H2O, 1F, 1CHF

neat PVDF

50

7H2, 6HF, 6H, 2H2O, 2C, 1C4F6H3, 1C2H6, 1CH4, 1COH3, 1CHF, 1CH, 1F

neat POSS

50

2H2, 2F, 2HF, 1C4H3F, 1C2H2O, 1HO

PVDF/POSS

50

3HF, 2CH2F, 2CO, 2H2, 2H, 1C2HF2, 1CH2, 1CH, 1F2, 1HO, 1CH, 1C, 1F

3.4. Discussion on the disintegration mechanisms From the erosion yields and the separated chemical compositions from the surfaces, the disintegration mechanisms of AOs on different materials can be also explored. As analyzed above, the erosion yields in Figure 8 reveal that a relative small amount of AO impacts have no erosion effect on POSS molecules but will disintegrate PVDF. For neat PVDF and 11



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PVDF/POSS, the disintegration process seems to start from the very beginning of AO impact. While for neat POSS, nothing will be disintegrated until a barrier has been broken down. Naturally, one has two questions: (1) why are the erosion yields so different and (2) what are the disintegration mechanisms for different materials? Since the first one has been analyzed in section 3.3, here we present the discussion on the latter. We also pursue the answer from table 1. For neat PVDF and PVDF/POSS, when the number of AO is less than 15 the separated chemical compositions are atoms C, H, F and molecules like FH and H2, no any oxide molecules can be found. These results reveal that, at the first stage (AOs15), the effect of the oxidability of active AOs starts to appear and many oxide molecules like H2O, CO are separated from the material surfaces. From this process, it can be found that the disintegration on PVDF and PVDF/POSS should be attributed to two kinds of mechanisms, the physical impact of AOs with high kinetic energies and the chemical reaction of AOs with strong oxidability. The former takes place first and then the latter. Only when the physical impact has produced some free atoms/molecules the chemical reaction will take into effect. As for neat POSS, the disintegration mechanisms are quite similar to those in neat PVDF and PVDF/POSS but there are two distinctions should be noted. The first distinction is that the effect of the physical impact is not obvious before the number of AO reaches to a relative large value (30 AOs) because the Si-O bonds in POSS cages are stronger and more stable than C-C bonds in PVDF chains, and the former possesses a higher bond energy than the latter (110 kcal/mol vs. 83 kcal/mol). The second distinction is the effect of the chemical reaction is not as obvious as that in neat PVDF and PVDF/POSS, or in other words, the oxide molecules are not as productive as the other two cases. It seems that most AOs have been absorbed after impacting neat POSS. We suppose this should be similarly derived from the high energy Si-O bonds in FP-POSS molecules. After the continuous impacts, when the Si-O cage is broken the unsaturated silicon atoms in this cage are more attractive to AOs than other free atoms/molecules like H, F, C and so on because they prefer to form more stable Si-O bonds. In this case, most AOs trend to combine with unsaturated silicon atoms and remain in the neat POSS. This mechanism also explains why the released oxygen mass listed in Figure 6(b) is much lower in the model of neat POSS.

4. CONCLUSIONS In this article, we performed the reactive MD simulations on the disintegration of PVDF, FP-POSS, and PVDF/FP-POSS subject to the high-energy AO collisions. We investigated the temperature evolution, disintegrated structures, mass loss and erosion yields of three materials. Our simulations show that the erosion yield of PVDF is very close to the experiment results of MISSE2 and different results of three materials are consistent. This proves that our simulations are very likely to be reliable and subsequently make our other results are meaningful and valuable. The erosion process on PVDF and PVDF/FP-POSS is continuous, that is, a small number of AO can make these two materials be eroded. During the AO impacts on these two materials, the initial separated species are dominated by C, H, F, FH and H2, and then oxygen atoms and oxide molecules such as O2, H2O and CO start to be released, followed by a wide range of other PVDF degradation products. The same PVDF matrix in two materials should be responsible for this continuous erosion process. Actually, the separated chemical compositions of these two materials are the PVDF degradation products. On the contrary, the erosion process on FP-POSS is stepped, that is, the erosion will not take place until the number of AO reaches a specific value. Because of the stable cage-like Si-O frame in FP-POSS molecules, there is a barrier for the erosion of high-energy AOs on FP-POSS. During the impact procedure, no erosion yields can be found before impacting 20 AOs, after that few atoms such as H and F separate from the R branches of FP-POSS; then with the further collision of AOs, the observing of silicon containing species means the Si-O cage is broken, followed by amount of small molecules contain C, H and F atoms. During the AO impacts, The R branches in FP-POSS molecules will be unstable and be degraded rapidly as soon as the centric Si-O cage is broken. But breaking down the Si-O cage is not easy 12


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and needs continuous and amount of AO impacts. The results of the temperature evolution, mass loss and erosion yield of three materials show that the anti-erosion performance of PVDF is not outstanding. However, incorporating FP-POSS into PVDF matrix enhances the stability of PVDF against AO impact greatly and reduces the temperature rise, mass loss and the erosion yield of PVDF rapidly. This should be derived from the remarkable anti-erosion ability of FP-POSS. The results in this work are helpful in gaining insight into the erosion details of three materials during the AO impacts and in understanding the inner mechanism of the improvement of FP-POSS on the stability of PVDF against AO impacts. Some other issues such as the influences of different POSS contents and different reinforcement structures on the anti-erosion ability of PVDF are also very important and will be discussed in our future work.

AUTHOR INFORMATION Corresponding Author *E-mail [email protected], Ph +86-451-86418100 (F.Z.); [email protected], Ph 847-467-4528 (J.Q.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors would like to thank the National Natural Science Foundation of China (11102053), the Science and Technology Innovation Talents Special Fund of Harbin (Grant No. 2012RFQXG001) and the China Scholarship Council (CSC) for the financial support of this research.

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Preparation of the AO erosion model of neat PVDF. In this preparation, 32 PVDF molecular chains with polymerization degree of 50 were packed into a cubic cell and forming an equilibrium amorphous structure after a dynamic optimizing. Then the PVDF cell was put in a “well” and under collision from 50 AOs individually generated over the upper surface. The grey white atoms represent H, the grey black atoms represent C, the blue green atoms represent F and the red atom represents O. 423x122mm (120 x 120 DPI)


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Preparation of the AO erosion model of neat POSS. In this preparation, 100 FP-POSS molecules were packed into a cubic cell and forming an equilibrium amorphous structure after a dynamic optimizing. Then the FPPOSS cell was put in a “well” and under collision from 50 AOs individually generated over the upper surface. The grey white atoms represent H, the grey black atoms represent C, the blue green atoms represent F, the yellow atoms represent Si and the red atoms represent O. 397x105mm (120 x 120 DPI)



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Preparation of the AO erosion model of PVDF/POSS. In this preparation, 32 PVDF molecular chains with polymerization degree of 50 and 8 FP-POSS molecules were both packed into a cubic cell and forming an equilibrium amorphous structure after a dynamic optimizing. Then the PVDF/POSS cell was put in a “well” and under collision from 50 AOs individually generated over the upper surface. The grey white atoms represent H, the grey black atoms represent C, the blue green atoms represent F, the yellow atoms represent Si and the red atoms represent O. 422x115mm (120 x 120 DPI)


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Temperature evolution of neat PVDF, neat POSS and PVDF/POSS during AO impacts (NVE simulations). 178x129mm (300 x 300 DPI)



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Key snapshots of AO collisions on neat PVDF, neat POSS and PVDF/POSS at specific simulation steps (including the 20000 steps of NVT equilibration), the upper pictures are the snapshots after the first AO impact (step 20500) and the under pictures are the snapshots with 5000 iterations after the last AO impact (step 45500). 328x338mm (96 x 96 DPI)


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The total remaining cell mass of neat PVDF, neat POSS and PVDF/POSS during AO impacts, (a) the absolute remaining mass and (b) the normalized remaining mass. 716x265mm (120 x 120 DPI)



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Remaining and released oxygen mass in the models of neat PVDF, neat POSS and PVDF/POSS during AO impacts, (a) the remaining oxygen mass and (b) the released oxygen mass. 1159x433mm (120 x 120 DPI)


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Erosion yield results of neat PVDF, neat POSS and PVDF/POSS in the first stage of AO impacts. 227x170mm (300 x 300 DPI)


Reactive Molecular Dynamics Simulations on the Disintegration of

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