Characterizing the Fundamental Adhesion of Polyimide Monomers on

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Characterizing the Fundamental Adhesion of Polyimide Monomers on Crystalline and Glassy Silica Surfaces: A Molecular Dynamic Study Sushmit Goyal, Hyunhang Park, Sung Hoon Lee, Elizabeth Savoy, Matthew E. Mckenzie, Aravind Raghavan Rammohan, John C. Mauro, Hyunbin Kim, Kyoungmin Min, and Eunseog Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08081 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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The Journal of Physical Chemistry

Characterizing the Fundamental Adhesion of Polyimide Monomers on Crystalline and Glassy Silica Surfaces: A Molecular Dynamic Study ‡*Sushmit Goyal1, ‡Hyun-Hang Park2, Sung Hoon Lee2, Elizabeth Savoy1, Mathew E. McKenzie1, Aravind R. Rammohan1, John C. Mauro1, Hyunbin Kim2, Kyoungmin Min3, Eunseog Cho3 1

Science and Technology Division, Corning Incorporated, Corning, New York 14831, USA 2

Corning Technology Center Korea, Corning Precision Materials Co., Ltd., Asan, Chungcheongnam-do 31454, Republic of Korea

3

Platform Technology Lab, Samsung Advanced Institute of Technology, 130 Samsung-ro, Suwon, Gyeonggi-do, 443-803, Republic of Korea

‡ These authors contributed equally to this work. * Corresponding author, email : [email protected]; Phone : 16079743634

Abstract

Understanding the interaction between polyimide and inorganic surfaces is vital in controlling interfacial adhesion behavior. Here, molecular dynamics simulations are employed to study the adhesion of polyimide on both crystalline and glassy silica surfaces,

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and the effects of hydroxylation, silica structure, and polyimide chemistry on adhesion are investigated. The results reveal that polyimide monomers have stronger adhesion on hydroxylated surfaces compared to non-hydroxylated surfaces. Also, adhesion of polyimide onto silica glass is stronger compared to the corresponding crystalline surfaces.

Finally, we explore the molecular origins of adhesion to understand why some polyimide monomers like Kapton have a stronger adhesion per unit area (adhesion density) than others like BPDA-APB. We find this occurs due to a higher density of oxygen’s in the Kapton monomer which we found to have the highest contribution to adhesion density.


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1. INTRODUCTION

Polyimides have been widely used in the coatings industry for numerous applications due to their thermal stability and chemical inertness1-4. Compared to other polymers, these qualities primarily arise due to polyimide’s − stacking which creates structures with strong intermolecular forces, stabilizing them more so than many other polymers. Polyimides also have desirable wetting properties, leading to their use in modern electronic devices like cell phones, flexible displays, and tablets5,6,7. These commercial devices typically involve an interface between polyimides and glass where the glass may be used to enable display, or at the backend as a support. For these systems, it is important to understand the adhesion between polyimide and glass to avoid structural failure8,9. The failure in these composite systems can occur at the interface between polyimide and glass known as adhesive failure, or within the polyimide layer or the underlying inorganic layer known as cohesive failure. Adhesive failure is one of the dominant failure modes in electronics 6 and hence is the focus of this paper. The degree of adhesion can be obtained from experiments and is referred to as practical adhesion

10

. Multiple direct measurement techniques can be used to obtain the practical

adhesion11-17 and the most commonly used techniques are the peel and pull-out tests. In these tests, a tape or a stud is placed on the sample and pressure is applied to calculate the force or pressure required to cause failure. While these tests provide quantitative measures of adhesion, plastic deformation in film, patterns of mechanical loading, and other factors cause inaccuracies in the measured adhesion values18,19,20. Adhesion can also be calculated by computing the thermodynamic work of adhesion required to separate the surfaces under equilibrium condition called the fundamental adhesion. This method is free of these



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limitations of practical adhesion and hence can provide more useful insight into the interactions between silica and polyimide. Hence, calculating the fundamental adhesion using computational models serve as a useful tool for this purpose5. To understand adhesion phenomena in the interfacial system comprehensively, these computational model should be capable of capturing complete atomistic details while spanning length scales of nanometers, which is the typical thickness of such interfaces. From this point of view, molecular dynamics (MD) is ideal to generate a microscopic understanding of the interaction mechanisms at the interface. Typically for simulating inorganic materials, like silicate glasses, non-bonded or partially bonded force fields, like Pedone 21,22 and BKS 23, have been used to study glass structures. Simulations with these nonbonded force fields have resulted in better understanding of the bulk properties, structure of silica and the interfacial properties with other inorganic materials like silicon24. These force fields, while allowing for dynamic bond breakage and formation as reflected by changes in local structure, cannot accurately describe the structure of organic molecules which require explicitly defined angles and dihedrals. Hence for organic molecules, bonded force fields such as PCFF25, CVFF 26 and COMPASS 27 have traditionally been used. While these bonded force fields have elucidated the bulk properties, structure, and changes in interfacial density distribution of the polyimide at the interface between polyimide and silica

28,29,30

, they have

not been used to systematically study the adhesion properties at the interface. This gap in understanding organic-glass interfaces is partially due to the absence of reliable force fields that can accurately predict their interfacial interaction. Since we are primarily interested in the interfacial interactions of an inorganic surface with a polymer, it is important to use a force field that accurately describes them. Potentials such as PCFF, CVFF, and COMPASS predict polymer properties well but are not optimized for interfacial interactions and surface properties such as surface tension and binding energy. Recently two force fields,


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ReaxFF

31,32,33

and Interface FF

34,35

, have shown promise in addressing these problems.

ReaxFF, a force field based on bond order, is able to model reactions by describing dynamic bond formation and breakage between the molecules. However, the large computational time with this force field limits the system size of the interface being studied. On the other hand, the Interface FF requires less simulation time and accurately predicts properties such as contact angle, heat of immersion35, and adsorption of peptides36 but requires key details of reaction from DFT or experiments to model reactive surfaces37,38,39. In our previous work40, we studied the adhesion of polyimide film on silica and its relationship with polyimide rigidity/coefficient of thermal expansion (CTE) using the ReaxFF.

Specifically, we show that polyimides with lower CTE tend to be more rigid,

which results in larger forces required to pull them off the silica surface but shorter distances to completely detach them. Due to the increased force to detach but shorter detachment distances, the overall work of adhesion for the three different polyimides was found to be comparable. To better understand the complex nature of these interactions, in this paper, we focus on elucidating the molecular origins of this adhesion, the effect of silica structure and hydroxylation, and the effect of polyimide structure and chemistry in greater detail. To address these questions, we present a framework to study this adhesion systematically. Specifically, we begin with careful calibration for the ability of the Interface FF to describe the bulk properties of polyimide and the surface properties of silica. We then use Interface FF to demonstrate the use of the Steered Molecular Dynamic (SMD) approach to study the effect of polyimide monomer adhesion onto crystalline and glassy silica with and without hydroxylation for four different polyimide monomers. Using the developed framework, we elucidate the role of surface structure, degree of hydroxylation, and polyimide chemistry. The rest of the paper is organized as follows: the choice of force field and the methods used to create polyimides and silica surfaces, along with the method to measure fundamental



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adhesion, are discussed in Section 2. The results are reported in Section 3, followed by discussions and the atomistic origin of various trends we observe in Section 4. Finally, in Section 5, we provide a summary and make some conclusions.

2. METHODS

The specific steps that we follow in setting up our simulations are described in this section. First, we create and validate silica structures. Next, we generate the bulk structures of polyimides to validate the ability of the force field to predict its bulk properties. We then combine polyimide monomers with silica structures to form the interfacial system, and use Steered Molecular Dynamics (SMD) to calculate the adhesion between them. We describe the details of the methods for each of these steps in the sections below.

2.1 Creating silica structures The Interface FF requires the bonds and angles between atoms to be explicitly defined, which makes generating a silica structure slightly more complex than when using a nonbonded potential. To ensure that our approach can readily be extended to other inorganic materials, we start by generating the silica glass structure using the well-established nonbonded Pedone potential

21

and a simulated annealing procedure 41. A flat surface is created

by opening the simulation box, and hydroxylating the under-coordinated oxygen atoms exposed to surface to achieve a surface density of ~ 4.7 OH/nm2. A corresponding nonhydroxylated surface is made by deleting –H and –OH pairs and introducing Si-O-Si bonds, followed by energy minimization. This method mimics the extreme condition of dehydration and provides a surface that is free of high-energy dangling bonds. The crystalline silica under


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study is the -quartz structure obtained from the model structure database shared with the ) Interface potential 35. This structure is generated by cleaving bulk -quartz along the (001 plane, which creates a perfect Q3 surface and on hydroxylation of these silica surfaces, a system with 4.7 OH/ nm2 hydroxyl density is obtained. The size of the glassy silica surface is 4.03 nm × 4.14 nm and for -quartz it is 3.33nm × 3.34nm.

2.3 Preparation of polyimide monomer and of the bulk polymer We choose four different polyimide monomers for this study. Three of them have the same 3,3’4,4’-biphenyl tetracarboxylic dianhydride (BPDA) with one of three different APB isomers : 1,3-bis(3-aminophenoxy)-benzene, 1,3-bis(4-aminophenoxy)-benzene or 1,4-bis(4aminophenoxy)-benzene. BPDA has been chosen as the benchmark data for its bulk properties and microstructure is available in the literature. The final polyimide under study is pyromellitic dianhydride and 4,4'-oxydiphenylamine (Kapton) which is one of the most commonly used polyimide. Throughout this paper, they will be abbreviated as BPDA 1,3,3APB, BPDA 1,3,4-APB, BPDA 1,4,4-APB, and Kapton, respectively. Among them, the first three BPDA series have the same chemical compositions except for the oxygen linkage connecting the benzene rings in the APB section. BPDA 1,3,4-APB has one meta linkage and two para linkages, while 1,3,3-APB and 1,4,4-APB have three meta and three para linkages, respectively. Kapton has a different chemical structure from the BPDA series (See Figure 1 for the structure in detail). By choosing these polyimide sets, we expect to see the effect of conformational variability (BPDA) and in chemical composition (BPDA vs. Kapton) on adhesion. The bulk polymer typically requires very long relaxation times due to its large number of conformational degrees of freedom. To reduce this time, many computational techniques



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have been developed. For example, the vector mapping between the atomistic structure and coarse grained structure is used by the combination of Monte Carlo (MC) and MD simulations 42. One can calculate a realistic structure by MC, which is computationally faster than MD, but it has the drawback of requiring multiple development steps before one can obtain a usable structure. On the other hand, due to increased computational power, equilibration and relaxation by MD has become tractable for fairly complex polymers. In addition, the algorithm of thermal-volume annealing can further reduce the relaxation time 43,44

. Therefore, in this study we use thermal annealing to equilibrate the BPDA, and

benchmark the structures by comparing with the result in Clancy et al.42. Briefly, the procedure for complete equilibration of the polyimide is as follows: i) the initial threedimensional structures with periodic boundary conditions are constructed based on the Configuration Biased Monte Carlo technique (Amorphous Cell module from Materials Studio 7.0 package

45

is used). The bulk structure contains 7 chains where each of them has 10

repeat-unit monomers, and the size of the simulation cell is 36 × 36 × 36 Å3 with a density of 1.37 g/cm3. ii) The annealing and relaxation procedure are performed by MD simulation using the LAMMPS simulation package46 with the Interface-CVFF force field. The structure is thermally annealed between 300−650 K for 5 cycles, relaxed at 650K for 100ps with the NVT ensemble followed by cooling to 300 K for 50 ps with the NPT ensemble using the Nosé-Hoover thermostat and barostat with a time step of 1 fs, temperature of 300 K and pressure of 1 atmosphere. Finally, the equilibration is performed at 300 K for 2 ns with the NPT ensemble. This procedure helps the polyimide to overcome the local minima in the free energy landscape, and thus drives to reach the global equilibrium state quickly.

2.4 Modeling the composite system


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We model the polyimide-silica composite to consist of the silica surface with and without hydroxylation (0% and 100%) and a polymer phase. The silica surfaces considered are the αquartz crystalline surface and the glassy silica surface. The polyimide monomers considered are BPDA 1,3,3-APB, BPDA 1,3,4-APB, BPDA 1,4,4-APB or Kapton. We use thermal annealing to equilibrate the polyimide monomers on the silica. In order for the monomer on the silica to arrive at the equilibrium conformation completely, the thermal annealing and relaxation procedures are performed by MD simulation with the Interface-CVFF force field. The composite structure is relaxed at 500K for 500ps followed by annealing to 300K for 500ps with the NVT ensembles using the Noose-Hoover thermostat. Then, the final equilibration is performed at 300K for 500ps with both NVT and NPT ensembles. This procedure helps the monomer to overcome the local minima in the free energy landscape, and thus reach the global equilibrium state quickly. The time step was 1 fs, the cutoff for van-derWaals interactions was 12 Å, and the Particle-Particle Particle-Mesh (PPPM) solver was used for the summation of long-range Coulomb interactions with the precision of 10-4.

2.5 Calculating the adhesion energy of polyimide monomers onto silica surfaces To calculate the fundamental adhesion between the silica and polyimide monomers, we use SMD which allows us to calculate the potential of mean force (PMF)47. SMD is a nonequilibrium simulation process that equates the non-equilibrium work done on the system to the equilibrium PMF using the Jarzynski inequality48. In this method, we add a fictitious particle that is connected by a virtual spring to all of the polymer atoms. This fictitious particle is displaced with a constant velocity, which applies a force on the polymer for it to detach from the silica surface. The force is calculated using the following equation, =

1 − − ∙ 2



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= −∇ where, is the spring constant of the virtual spring, is the velocity at which the fictitious particle is displaced, is the current position of the center of mass of the atoms being pulled (polyimide in this case), is the initial center of mass of the atoms and is an unit vector along the direction in which the spring is being pulled. The work done in this process is then calculated as =!

&'()

&'(*

" ∙ #

%$where + is the final position of the center of mass of the polyimide. Simulations for multiple trajectories allows us to calculate an ensemble average and use Jarzynski’s equality to relate the work done to PMF by < exp−0

>2 23452 = exp −0 3+

6

where 0 = 7 9, : is the Boltzman’s constant, and ; is the temperature which is 300K for 8

our simulations. In our simulations, we use a spring constant, , equal to 100 kcal/mol/Å2.

3. RESULTS 3.1 Validation of structures, properties, and optimization of SMD pulling velocity 3.1.1 Polyimide bulk structure and property In order to validate the bulk structure, the conformational and dynamic properties of the relaxed polymer structure, of the BPDA series, are investigated and compared to published simulation values42. (Kapton is excluded from the comparison because the selected literature only deals with BPDA series, but one can expect that the result of BPDA to be applicable to Kapton as well.) There are two basic conformational properties calculated in this work: the


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average radius of gyration squared, , and the average end-to-end distance squared, . The intermolecular pair correlation function, g(r), is also calculated to estimate the dynamic packing behavior of the bulk polymer. Table 1 compares the values of the density, , , and the ratio of to obtained by using the Amorphous Cell generator in Materials Studio with the Interface FF, to those results published by Clancy et al.42 We find that the computed density values are in good agreement with the simulated densities. The and values show some discrepancy, probably because the values in the literature were measured from the coarsegrained structure, and not a fully atomistic structure. However, we observe a consistent trend: that as the number of para-linkages in the APB section of the monomer increases, the bulk polymer packs more easily increasing the bulk density. Furthermore, Figure 2 shows the evolution of the intermolecular pair correlation function, which shows the same general trend. This confirms that the BPDA structure with more para linkages should exhibit higher packing behavior. Another metric that is useful for the validation of the structure is the ratio of to . For an ideal flexible polymer, based on the theory of freely jointed polymer chains46, this ratio should be six which is what we obtain from our calculations. From this comparison, we conclude that our bulk structure of polyimide equilibrated by classical MD shows reasonable properties as flexible polymer chains. This suggests that our method and force fields for simulating polyimides are accurate and reliable.

3.1.2 Interface force field structure and property of the silica surface Pedone potentials are known to be able to accurately reproduce bulk silica properties and structures for glassy and crystalline silica

22

. Hence, the validity of our silica surfaces and

implementation of the Interface FF are established by calculating wetting properties and comparing them with simulations and experiments from literature35. Heat of immersion is the



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enthalpy released upon immersion of silica into water, and calorimetric experiments50 give a value between 100-1300 mJ/m2, depending on ionization, cavities, and hydroxylation. We compare our calculated values to the heat of immersion and contact angle for pyrogenic silica as it has no internal cavities and surface hydroxyl density of 4.7 OH/nm2. Heat of immersion is calculated by finding the potential energy of the silica+water system and subtracting the potential energy of the silica + vacuum and the bulk water system. We obtain hydration energy of 180.3 mJ/m2 and contact angle of 0° for glassy silica and 157.4 mJ/m2 and contact angle of 14° for -quartz, which match favorably with literature values of 160 ±5 mJ/m2 and 0° respectively for cavity free pyrogenic silica.

3.1.3 Determining optimal pulling velocity for SMD simulations of the adhesion To obtain accurate evolution of the PMF with polyimide on silica, the SMD simulation should be maintained in the quasi-equilibrium state during the entire process. In this regard, checking the effect of pulling velocity is of great importance for the precise calculation of the adhesion between the polyimides and silica substrates. It is particularly true for the monomer structures which are very sensitive to any driving force toward a non-equilibrium state. Figure 3 shows the PMF curves as a function of the pulling distance for different values of pulling velocity for 1,4,4 BPDA on unhydroxylated -quartz. As the velocity is reduced, the height of the PMF curve initially decreases and converges finally at = 0.75 m/s. This implies that the calculated adhesion values for the higher pulling velocities, such as = 1.5 m/s or 15 m/s, overestimate the adhesion since the higher pulling velocity does not allow the system to access all possible microstates. Based on the result in this subsection, a fixed pulling velocity of v = 0.75 m/s is used for all subsequent SMD simulations.

3.2 Adhesion between polyimide monomer and silica surfaces


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3.2.1 Effect of hydration and silica structure We start our analysis by studying the adhesion of BPDA 1,4,4-APB on various silica structures and surface hydroxylation. First, we analyze the effect of hydration on adhesion for both the crystalline and glassy silica surfaces. We observe that hydroxylated silica have higher adhesion by 50 kJ/mol for -quartz and 100 kJ/mol for glassy silica; the PMF curve is shown in Figure 4(a). This trend is independent of silica structure as this result holds true for both crystalline silica as well as glassy silica. In addition, we observe that the adhesion for glassy silica is always higher than that for their crystalline counterparts by ~75 kJ/mol for fully hydroxlylated surfaces and ~25 kJ/mol for non-hydroxylated surfaces. To understand the detachment process, we next analyze the distance at which the monomer completely detaches from the silica surface. From Figure 4(a), we can see that a longer separation distance is required to detach the monomer from glassy silica when compared to crystalline silica, and for hydroxylated structures when compared to non-hydroxylated structures. This is illustrated in the snapshots of the systems shown in Figure 4(b) and 4(c) where due to the presence of hydroxyl layers and glassy structure, the polyimide monomer clearly has a stronger interaction with silica.

3.2.2 Effect of monomer structure and chemistry on adhesion To understand the effect of polymer structure on adhesion, we study the three isomers of BPDA: 1,4,4-APB, 1,3,4-APB, and 1,3,3-APB on hydroxylated glassy silica. Figure 5 shows the PMF for all three isomers as a function of distance between the surfaces. We find that the adhesions for all the systems are identical. Next, to understand the effect of polymer chemistry on adhesion, we study Kapton (See Figure 1 for its structure) for comparison. We first study its adhesion on various silica structures with and without hydroxylation and the results are shown in Figure 6. We find that



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the adhesion from Kapton follows the same trend as in BPDA which is that ΔABCD 5B E > ΔABCD FE GB55 2 and ΔABCD DECHIE5BG2C > ΔABCD J DECHIE5BG H . We also find that the net adhesion for BPDA is always higher than that of Kapton. This result is not surprising as the BPDA molecule is significantly larger and hence the increased adhesion could be attributed to a larger area of contact. To investigate the increased adhesion between BPDA polymorphs and Kapton, we calculate the projected area of each monomer on the silica surface and used it to normalize the adhesion for both polymers. We find that the surface area of Kapton is roughly 0.6 times the surface area of BPDA. Figure 7 shows the results for the normalized adhesion. We observe that the trend for adhesion has now reversed and Kapton adheres more strongly than BPDA for every case of silica structure.

4. Discussion In our previous work

40

we studied the correlation between the coefficient of thermal

expansion and adhesion of three different polyimides (DHBZ-BPDA, DHBZ-BTDA and BHBZ-DTDA) on silica surfaces. We found that the polyimide with lower CTE (DHBZBPDA surface hydroxyls > hydrogen in polyimide. This clearly suggests that for a given surface area of polymer, the higher the number of oxygen atoms, the higher will be the net


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adhesion with silica. On comparing BPDA to Kapton, we find that the concentration of oxygen is significantly higher in Kapton (almost 1.5 times) which leads to a higher adhesion per unit area when compared to BPDA. The higher net adhesion of BPDA is due to a longer carbon chain (34 for BPDA vs. 22 for Kapton) which is the second highest contributor to potential energy. However, the number of carbon atoms per unit area is similar between Kapton and BPDA and therefore carbon does not contribute to the overall trend for adhesion per unit area. Hence we conclude that due to the higher concentration of oxygen in Kapton, the adhesion per unit area is higher than BPDA.

5. CONCLUSIONS

We have formulated a method to create organic-inorganic composite systems and performed benchmarks for each of the components to show that the properties agree with those calculated by Emami et al.35and Clancy et al.42. On studying the adhesion between polyimide monomers and various glass surfaces, we find that hydroxylated silica surfaces have higher adhesion than non-hydroxylated surfaces. We also find that glassy silica has a higher adhesion than crystalline silica. The reason for this appears to be the additional hydrogen bonding interactions possible in the presence of surface hydroxylation and due to a rougher surface for silica glass allowing more interactions. Adhesion between the three isomeric monomers of BPDA and silica was found to be identical. When compared with the adhesion between Kapton and silica, the adhesion was found to follow the same trend as BPDA but was consistently lower. On converting the adhesion to adhesion per unit area, a more relevant metric, we find that Kapton consistently has higher adhesion per unit area than BPDA. On analyzing the interactions, we find that the contribution to adhesion is highest for oxygen followed by carbon, nitrogen, surface hydroxyls and hydrogen. As Kapton has the



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highest density of oxygen and similar densities for other atom groups, it has higher adhesion energy. This result implies that adhesion with silica for polyimides with higher oxygen density would be higher. However, polyimide packing density and chain stiffness can greatly affect adhesion by changing the ability of the film to interact with the silica surface. We have studied the adhesion of polyimide monomers with silica, and while these include many effects of chemistry and structure, it is not clear if the trends will translate to higher adhesion for polyimide films. In our previous work40, we explored the effect of polymer rigidity/CTE on adhesion. We plan to expand on the current and previous work by exploring the effect of surface attributes like roughness coupled with hydroxylation, polyimide film chemistry, and rigidity on the adhesion behavior. In the current work, while we did not observe any differences in adhesion for the isomers of BPDA on silica, we expect to see differences as surface features are modulated.

Acknowledgement We would like to acknowledge Theresa Chang, Jim Rustad, Adama Tandia and Jian Luo for fruitful and stimulating discussion on this work. The authors would also like to thank Sam Zoubi , Gautam Meda and V.P. Chen for their help and support in this work.

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H

H

O

O

H N

N

O

O

O

O

O

O

N

N O

H

O

O

O

BPDA 1,3,4-APB

BPDA 1,3,3-APB

H

O

O

O

H N O

O

O

H

N

N

O N

O

O

O

BPDA 1,4,4-APB

O

KAPTON

Figure 1. Repeat units for the Polyimide monomers for three BPDAs (left) and Kapton (right)


H

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BPDA 1,4,4-APB

BPDA 1,3,4-APB

BPDA 1,3,3-APB

Current

Literature

Current

Literature

Current

Literature

1.285

1.283

1.291

1.270

1.291

1.290

8467±657

5300±1300 6522±1818 5200±450

6983±1474 4300±520

1387±39

930±16

1099±124

890±80

1183±86

690±58

6.10

5.70

5.93

5.84

5.90

6.23

Quantities Density (g/cm3) ree2 s2

(Å2) (Å2)

ree2 s2

(Å2)

Table 1. Comparison of the conformational properties between the current study and the literature42



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Figure 2. The intermolecular pair correlation function g(r) for the bulk structure of three BPDA polyimides.


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Figure 3. Variation of the PMF curve as a function of the pulling distance for four different pulling velocities for 1,4,4 BPDA on L-quartz.



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(a)

(b)

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System

Pulling distance at detachment (Å)

c-SiO2 with 100% OH

13.9

c-SiO2 with 0% OH

13.0

g-SiO2 with 100% OH

15.1

g-SiO2 with 0% OH

13.6

(c)

Figure 4. (a) The distance at which BPDA separates from the silica surface. We observe that hydroxylated and glassy surfaces need higher separation for completely detaching. The reason for the larger separation distance is the extended hydrogen bonding which can be observed in hydroxylated surfaces as shown in Figure (b) but is absent in non-hydroxylated surfaces as shown in Figure (c)


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Figure 5. Effect of monomer structure on adhesion onto hydroxylated glassy silica. No change in adhesion among the three isomers of BPDA is observed.



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(a)

(b)

(c)

(d)

Figure 6. Comparison of adhesion of polyimide and silica for BPDA (black) and Kapton (red) for (a) crystalline silica fully hydroxylated, (b) crystalline silica with no hydroxylation, (c) glassy silica with full hydroxylation, (d) glassy silica with no hydroxylation


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(a)

(b)

(c)

(d)

Figure 7. Comparison of adhesion of polyimide and silica for BPDA (black) and Kapton (red) per projected surface area for (a) crystalline silica fully hydroxylated, (b) crystalline silica with no hydroxylation, (c) glassy silica with full hydroxylation, (d) glassy silica with no hydroxylation.



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Figure 8. Change in potential energy for each component of the silica+PI system. We observe that the majority of adhesion is from the carbon backbone of PI and surface hydroxyls on the silica surface.


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Figure 9. The adhesion per atom per unit area for each of the subgroups studied in Figure 8. Note that bulk silica has been omitted due to its minimal effect on adhesion. We can clearly see that oxygen has the largest effect on adhesion followed by carbon, nitrogen, surface hydroxyls, and finally hydrogen.



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TOC GRAPHIC


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Characterizing the Fundamental Adhesion of Polyimide Monomers on

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