Research Article www.acsami.org
Influence of Copolyester Composition on Adhesion to Soda-Lime Glass via Molecular Dynamics Simulations Ben Hanson,† John Hofmann,‡ and Melissa A. Pasquinelli*,† †
Fiber and Polymer Science Program, North Carolina State University, Raleigh, North Carolina 27695, United States Eastman Chemical Company, Kingsport, Tennessee 37662, United States
‡
ABSTRACT: Copolyesters are a subset of polymers that have the desirable properties of strength and clarity while retaining chemical resistance, and are thus potential candidates for enhancing the impact resistance of soda-lime glass. Adhesion between the polymer and the glass relates to the impact performance of the system, as well as the longevity of the bond between the polymer and the glass under various conditions. Modifying the types of diols and diacids present in the copolyester provides a method for fine-tuning the physical properties of the polymer. In this study, we used molecular dynamics (MD) simulations to examine the influence of the chemical composition of the polymers on adhesion of polymer film laminates to two soda-lime glass surfaces, one tin-rich and one oxygen-rich. By calculating properties such as adhesion energies and contact angles, these results provide insights into how the polymer−glass interaction is impacted by the polymer composition, temperature, and other factors such as the presence of free volume or pi stacking. These results can be used to optimize the adhesion of copolyester films to glass surfaces. KEYWORDS: adhesion, temperature effects, copolyester, soda-lime glass, molecular dynamics
1. INTRODUCTION Violent storms such as hurricanes and tornadoes have the potential to cause billions of dollars in property damage. Hurricane Sandy alone caused an estimated $65 billion in property damage due to its impact on heavily populated areas.1 A study of a hurricane in the 1980s that hit Houston found that a large amount of property damage was caused by roofing gravel that was carried by the high wind that was breaking the windows of the largely glass building edifices. Windows in these types of structures are typically some variation of soda-lime glass.2 In such circumstances, the shards of broken glass propelled by high wind speeds have the potential to cause significant bodily harm as well as additional property damage. This damage may have been reduced by preventing the broken glass from becoming airborn. Soda-lime silica glass, an easily synthesized and homogeneous glass, is probably the most important and certainly the most prevalent oxide glass. It is used for a wide variety of applications, including structural glass, containers, insulating materials, bioactive surfaces,3,4 and as a key support for modern electronic devices. Key properties include high resistance to crystallization, the ability to shape stable structures due to its high viscosity at liquid temperatures, potential use as a fiber, and control of index of refraction through changes in the material composition.5 A disadvantage of this glass is its low impact resistance, shattering with even moderate impacts.6 To make glass more impact resistant, meaning that it will not shatter into small dangerous pieces when it breaks, there are two approaches that utilize polymer films.7 For existing © XXXX American Chemical Society
windows, the most economical choice is to apply an external polymer film, which eliminates the need for costly and time intensive retrofitting, but the efficacy is largely dependent on the quality of the film and how well it adheres and keeps the glass within the window frame after impact.8 During the manufacturing of new windows a thin polymer film can also be laminated to a sheet of glass. This approach tends to be much more effective since the polymer is directly applied to the glass and then heated to a melt. Polymer from a melt state can achieve stronger adhesion.9 Additionally, applying the polymer and then bringing it to a melt allows for a larger range of polymer types to be bonded to the glass. When selecting the polymer to be used when creating laminated windows, a number of properties are important. First the optical properties of the polymer need to have a negligible effect on the window transparency. Second, the strength of the polymer needs to be such that it holds together during impact, yet still allows for the natural flexing and movement that a window undergoes with heating and cooling and with impact from winds.9 Third, the polymer must be stable under prolonged exposure to sunlight and not experience yellowing, clouding, shrinking, and cracking that can occur as polymers age.10 Finally, the adhesion between the polymer and glass needs to be strong enough to avoid delamination without causing brittleness and cracking.11 Received: February 18, 2016 Accepted: May 5, 2016
A
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Copolyesters have a surface energy that enables them to be adhesively bonded to the types of glass commonly used in laminated structures. One of the most common and versatile is poly(ethylene terephthalate) (PET).12 The strength, durability, and ease of manufacture of PET materials have made them ideal for a range of applications, from packaging and as a substrate to consumer goods, such as water bottles and as textiles.13 One method for expanding the usefulness of PET is by combining it with another monomeric unit as a copolyester. In the case of PET, the addition of an ethylene glycol (EG) repeat unit can reduce crystallinity, which lowers the glass transition temperature (Tg) of the resulting polymer melt. This combination allows for a greater range of processing temperatures and provides greater flexibility while retaining some of the key physical properties that make PET so desirable. Additionally, it prevents the formation of large crystal structures caused by crystallization at high temperatures which can result in PET opacity and brittleness.14,15 Copolyesters have already been used in a wide range of consumer products. Their desirable processing properties allow them to be molded into a number of shapes, while retaining its strength and chemical resistance. In addition to these copolyesters being used in a range of consumer goods, such as reusable watter bottles and containers, these polymers have also been used to reinforce other polymers, fabrics, fibers (glass and plant based), and other materials.16 In this study, we used molecular dynamics (MD) simulations to examine the effect of copolyester composition on the polymer adhesion with soda lime glass surfaces, one of which is tin-rich and the other is oxygen-rich. Changes in composition also can influence other properties, such as the free volume present. Other factors that impact adhesion, such as temperature, were also investigated.
Table 1. Composition of Molecular Model of Soda-Lime Glass SiO2 Na2O CaO MgO SnO
oxygen-rich glass
tin-rich glass
73.73% 11.74% 8.66% 5.87%
71.27% 11.27% 8.31% 5.61% 3.54%
To facilitate the transition from the crystalline structure to an amorphous structure, a number of Si−O bonds were selected at random and deleted, and each system was equilibrated with MD simulations employing the isothermal−isobaric (NPT) ensemble at 1 atm and at an elevated temperature (∼2000 K). Nonbonded van der Waals forces were truncated at 10 Å with long-range electrostatics solved using the particle−particle particle-mesh solver with a convergence accuracy of 1.0 × 10−5. The velocity Verlet algorithm was used to integrate through time with a 0.5 fs time step for approximately 2 ns. Each glass model was then stepwise cooled to room temperature with descending intervals of 50 K, where the system was allowed to “rest” for 100 ps at each interval. If the desired density was not achieved, additional connectivity was eliminated and the annealing procedure repeated. It should be noted that the resulting system consisted of small silicon oxide groups of varying sizes similar to the starting configuration used in other soda-lime glass studies.23 The surface that will be in contact with the polymer was formed for each system by slicing a section from the center of the glass simulation cell. We made several sections with a thickness of approximately 1 Å and chose the most electrostatically neutral section for further studies. This section was used as the surface of the glass slab with the overall thickness of the section sliced being approximately 20 Å. If the cut was made at exactly 20 Å, the slab would have an overall charge due to an imbalance of positive and negative ions present. Minor adjustments were made to the thickness, slowly increasing the thickness beyond the 20 Å cut until the glass slab was electrostatically neutral. The final dimensions of each glass surface is 7.5 nm × 7.5 nm × 2.0 nm and have a molecular surface roughness (Connolly surface area/flat surface area) of 2.4. The resulting molecular models of each glass surface are depicted in Figure 1. 2.2. Molecular Models of Water Droplet. We performed wettability experiments on our glass surface by placing a droplet of water on the surface of the glass and allowing the water molecules to slowly spread to form a semihemisphere on the surface of the glass. The water droplet was created by randomly placing 525 water molecules in a simulation box. Fully atomistic MD simulations were performed to equilibrate the system under NPT conditions at 1 atm and 298 K using the TIP3P force field.24 Nonbonded van der Waals forces were truncated at 10 Å with the long-range electrostatics solved using the particle−particle particle-mesh solver with a convergence accuracy of 1.0 × 10−5. Additionally, the system used the velocity Verlet algorithm to integrate with a time step of 1.0 fs. The system was considered equilibrated when the total energy and density fluctuated around a constant value for approximately 2 ns. As a test of proper equilibration, the density of the water was calculated and was approximately 1.04 g/cm3. The droplet was cooled to approximately 250 K and placed in contact with the glass surface. To prevent evaporation, the droplet was slowly warmed to 298 K at a rate of 24 K/ns. The droplet was considered stable when there was not significant spreading for approximately 2 ns. The contact angle was measured by first neglecting the more structured atoms closest to the surface and beginning the angle measurement a small distance from the surface.25 Measurements were taken at different points along the edge of the droplet and averaged to determine the final value. The contact angle of water on the glass surface was also measured experimentally. Measurements were taken according to the Sessile drop method in which a drop of ultrapure water is deposited on the glass surface and photographed using contact angle measurement
2. METHODOLOGY We performed fully atomistic molecular dynamics (MD) simulations to examine the role of copolyester composition on its adhesion to tinrich and oxygen-rich soda-lime glass surfaces. All MD simulations were done with the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)17 simulation software on the henry2 cluster at the North Carolina State University High-Performance Computing center. Due to the difficulty of properly modeling amorphous silica glass, a number of force fields were investigated for this study. Two of the most prominent force fields are the van Beest, Kramer, and van Santen (BKS) force field18 and the potential created by Behnam Vessal.18 The BKS force field uses a potential similar to the Buckingham potential, which employs an exponential to model the repulsive portion of the nonbonded force. Vessal parameterized a three body potential that has been used to reproduce amorphous silica structure. Unfortunately, even though these force fields do an excellent job for modeling silica,19 they have not been parameterized for all of the ions present in sodalime glass, particularly MgO. The CVFF_aug20 force field was chosen for the glass surface because the force field has been parameterized for metallic oxide, zeolite, and glass systems. The use of a CVFF-based21 force field for both the glass and polymer will prevent any errors that can occur when mixing force fields. 2.1. Molecular Models of Glass Surfaces. The desired compositions of the oxygen-rich and tin-rich glass surface are given in Table 1. These values were provided by Eastman Chemical and were adapted from a thesis on tin in silicate glasses.22 Accelrys’ Materials Studio v. 5.526 was employed to generate a bulk model of a perfect cristobalite (SiO2) crystal. To obtain the desired composition, Si and O atoms were randomly deleted along with their respective connectivity and replaced with the desired free ions. B
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Snapshot of the final molecular models of the (a) oxygen-rich and (b) tin-rich glass surfaces, with the corresponding color scheme for the atoms given on the right. equipment (AST VCA Optima XE). This procedure was repeated a minimum of 5 times for both glass surfaces. For each case, software was subsequently utilized to measure the contact angle between the glass surface and the water droplet and the average value reported. 2.3. Molecular Models of Copolyesters. The polymers used in this study were five copolyesters with a 50/50 mixture of terephthalic and isophthalic acid and varying percentages of ethylene glycol (EG) and cyclohexanedimethanol (CHDM). The repeat unit structure is given in Figure 2. Each monomer was built in Accelrys’ Materials
Figure 3. Graph of the specific volume as a function of temperature for the 100% EG system, with lines indicated of how the glass transition temperature is calculated. the free volume present within the polymer matrix. Scienomics MAPS software platform27 was used to calculate the free volume of the polymer. The program uses a similar methodology as the calculation of the Connolly surface in that a probe of a specific diameter was used to measure the void areas present in the polymer. For the purpose of this calculation, a probe with a radius the size of a water molecule (1.4 Å) was employed. The free volume was calculated for systems equilibrated at 298 K. 2.4. Molecular Model of Copolyester−Glass System. Each polymer was combined with each glass surface by first unwrapping the copolyester chains across periodic boundaries perpendicular to the plane of the glass surface, and then placing the copolyester layer on top of the glass surface such that the closest polymer atom was within 2 Å of the glass surface. The systems were simulated in the canonical ensemble with periodic boundaries in the dimensions parallel to the glass surface and using a time step of 0.5 fs. Simulating the system periodic in only two directions creates an infinite slab of the glasspolymer system. Parallel tempering was employed to accelerate the formation of a good interaction between the polymer layer and the glass surface. Care was given to use a temperature profile that ensured that the parallel tempering systems exchanged at the optimal rate of 0.25.28 The parallel tempering procedure was considered complete once the system that began at the lowest temperature (298 K) had fully explored the entire range of temperatures and returned to the original temperature a minimum of three times. A snapshot of the final configuration of the combined system is found in Figure 4 2.5. Simulation Analysis. Once each polymer-glass system was fully equilibrated, the systems were simulated under NVT conditions at room temperature (298 K) for a minimum of 3 ns. The interaction energy between the polymer in the glass was calculated by taking the overall potential energy of the system and subtracting the sum of the internal energy of the polymer and the internal energy of the glass, given as
Figure 2. Molecular structures of the copolyester repeat units, (a) CHDM and (b) EG, with the isophthalic form (left) and terephthalic form (right). Studio version 5.526 and the polymer builder module was used to create a random block copolymer. The percentages of each monomer present in the final chain was fixed with the type of diol present varying with the following mixtures, 100% CHDM, 75% CHDM 25% EG, 50% CHDM 50% EG, 25% CHDM 75% EG, and 100% EG. The amorphous cell module of Accelrys’ Materials Studio was used to create the copolyester layers. For each chemical composition, 10 chains with a molecular weight of approximately 30 kDa were built at a very low system density. To obtain experimentally relevant densities, each system was simulated in the NPT ensemble at 1 atm and at an elevated temperature (750 K). Nonbonded van der Waals forces were truncated at 10 Å with the long-range electrostatics solved using the particle−particle particle-mesh solver with a convergence accuracy of 1.0 × 10−5. The velocity Verlet algorithm was used to integrate with a time step of 0.5 fs. Once the system had run for approximately 5 ns, it was cooled to 298 K at a rate of 50 K/ns. The glass transition temperature (Tg) of the copolyester was investigated to validate our models and to examine the changes in the physical properties of the polymer caused by the changing composition. This transition is marked by a shift in the rate of change in the specific volume as a function of temperature. To calculate the Tg of each copolyester composition, each layer was simulated at a range of temperatures around the hypothesized Tg at 1 atm using the NPT ensemble, and the specific volume of the systems were plotted against the system temperature. Figure 3 presents such a plot for the 100% EG system. A linear fit was applied to each linear region, and where the two lines converge was defined as the Tg. The influence of copolyester composition on the internal organization of the polymer was investigated through examination of
E inter = Etotal − (Epolymer + Eglass)
(1)
Where Einter is the overall interaction energy, Etotal is the total potential energy of the cell, Epolymer is the internal energy for the polymer, and C
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
planarity of each ring was determined by first calculating two vectors between carbon atoms in the ring. The cross product of these vectors was used to calculate a vector normal to the plane of the aromatic ring. If the magnitude of the cross product between these normal vectors is close to zero, then the rings are considered parallel.
3. RESULTS AND DISCUSSION We performed MD simulations in order to study the influence of copolyester composition and temperature on adhesion with soda-lime glass to investigate the efficacy of using copolyesters for use in safety glass. Copolyesters have the desired strength and optical properties necessary to prevent the shattering of glass after impact while retaining clarity. We studied a range of copolyester compositions (Figure 2) that contain a 50/50 mixture of terephthalic and isophthalic acid as the diacid and the diol ranging from 100% CHDM to 100% EG. A large portion of glass used in commercial windows is often called float glass. This name is due to a portion of the industrial process that involves the fluid glass floating on a bed of molten tin.30 During this process, a small amount of tin diffuses into the glass, causing the resulting product to have two chemically distinct sides.31,32 To account for this effect, we created two types of amorphous molecular models of the soda-lime glass surfaces: one that is oxygen-rich and one that is tin-rich. The glass and polymer systems were combined and simulated at a range of temperatures to investigate the role temperature has on adhesion. 3.1. Characterization of Glass Model. To validate the appropriateness of the molecular model of each surface, we calculated the density and water contact angles. The interaction of a surface with a compound can be measured experimentally by calculating the wettability of the surface. This measurement is performed by placing a droplet of liquid on the surface and measuring the apparent contact angle (θ). This value is defined as the angle created between the solid/liquid interface and the line tangent to the vapor/liquid interface. If the liquid is water and θ is smaller than 90°, the surface has high wettability and is deemed to be hydrophilic; larger than 90° it is considered to have low wettability and is hydrophobic. The experimental water contact angle results found in Table 2 indicate that the different surfaces have similar water contact
Figure 4. Snapshot of the polymer−glass interface from the final step in the MD trajectory of the 50% CHDM copolyester (top) interacting with the oxygen-rich glass surface (bottom). (Eglass) is the internal energy of the glass. The internal energies of the individual components were calculated by summing the energy due to internal connectivity (bonds, bends, dihedrals, and impropers) along with the van der Waals and electrostatic energies calculated only between atoms of the same group. The overall adhesion energy (Eadh) between the two components is then calculated by dividing −Einter by surface area of the glass surface (AvdW), defined as Eadh = − E inter /A vdW
(2)
The AvdW term was calculated via the solvent-excluded surface, or the Connolly surface,29 which is determined by rolling a ball of a particular radius (which was set to the radius of a water molecule, 1.4 Å) along the surface of the glass. The area calculated is considered the area of the surface with which the polymer is able to interact. To quantify the internal structure and coordination between the polymer and glass, the radial distribution function (RDF) (g(r)) was calculated, which is a numerical method for measuring the correlation between particles during an MD simulation given as g (r ) =
1 Nρ
N
Table 2. Properties of the Oxygen-Rich and Tin-Rich Surfaces of Soda-Lime Glass
N
∑ ∑ δ(r − rij) i=1 j=1
density (g/cm3)
contact angle
(3)
experiment simulation
where N is the number of selected atoms present, ρ is the number density of the atoms and rij is the distance between atoms of type i and j. The g(r) values quantify the average probability of two selected particles being separated by the specified distance (r) relative to the bulk number density. This calculation is performed at regular intervals throughout the simulation trajectory, then producing information about the internal structure of the system averaged over time. Additional analysis was performed on the internal structure of the copolyester when in contact with the glass surface. We examined the way in which polymer self-organized, specifically the π stacking of the aromatic rings within the polymer chain. The procedure for quantifying the amount of π stacking begins with examining the distribution of aromatic rings present within the polymer melt. The RDF between the center of mass of the rings is calculated to determine the first coordination shell. This value (6.8 Å) was used as a cut off when examining which rings should be further examined. The
oxygen-rich glass
tin-rich glass
oxygen-rich glass
tin-rich glass
23.8° 21.1°
18.2° 15.6°
2.5 2.21
2.5 2.23
angles, with the tin-rich side having a slightly lower contact angle (18.2°) suggesting it is slightly more hydrophilic, but not enough to be statistically significant. Comparable water contact angles were calculated from the MD simulation and thus we conclude that the model of the glass surface is able to reproduce the contact angle of water reasonably well. Note that the experimental contact angle calculated for the tin-rich side is approximately 30° less than those reported by Gladushko.33 This difference can possibly be attributed to the use of a different surface preparation and a change in tin concentration at the air interface. D
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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and may contribute to a decrease in the Tg of the polymer system. Comparing the free volume as a percentage of the total volume of the system, the trend is consistent with that of the total free volume, monotonically decreasing with increasing amounts of EG present on the copolyester. 3.3. Characterization of Copolyester−Glass System. Figure 5 provides the calculated Eadh (from eq 2) as a function of polymer composition at two different temperatures (298 and 400 K). For the oxygen-rich surface, Eadh with the glass is lowest where the diol present is 100% CHDM. Eadh monotonically increases as the percentage of EG diol rises to a maximum for the 100% EG system. The curve for the tin-rich surface has a similar trend with the minimum Eadh occurring for the 100% CHDM system and a maximum value for the 100% EG system. However, the values calculated using the tin-rich surface were consistently less than those for the oxygen-rich surface. We analyzed the molecular level details of each system to determine potential underlying causes of these trends. The first potential cause examined was the orientation of the polymer chains and their interaction with the atoms of the glass surface, which was quantified by calculating g(r) between different components of the glass and the surface. The g(r) plot between the polymer carbonyl oxygen and the silicon atoms has a small peak at approximately 2 Å as observed in Figure 6, which
As mentioned in the Methodology section, the density of the glass was used to determine when the simulation cell had reached an amorphous state and no longer needed to be modified. The density values found in Table 2 are within 12% of the experimentally calculated values. This correlation was used as an additional test that the glass model was properly replicating real world conditions. 3.2. Characterization of Copolyester Layers. The copolyesters started with a 50/50 mixture of terephthalic and isophthalic acid combined with different percentages of ethylene glycol (EG) and cyclohexanedimethanol (CHDM). As found in Table 3, the Tg values monotonically increase as the percentage of CHDM present in the copolyester Table 3. Properties of the Copolyester as a Function of Composition
Tg (K) free volume (Å3) free volume (% of total volume)
100% CHDM
75% CHDM
50% CHDM
25% CHDM
100% EG
387.2 5977.7 1.48
389.2 2040.9 0.56
391.5 1698.6 0.47
395.4 838.1 0.30
399.7 541.8 0.22
composition decreases, reaching a maximum at 100% EG. This rise in Tg is indicative of greater intermolecular forces within the copolyester systems as the amount of CHDM is decreased. The resulting Tg values are higher than those found experimentally, but this discrepancy may be attributed to simulations being unable to achieve cooling rates equitable to those generated experimentally. For comparison, the polymer systems were cooled at a rate of 12.5 K/ns and a typical experimental cooling rate is 20 K/min. Additionally, the Tg value calculated for the 100% EG system is similar to the previously reported PET Tg that was also calculated from simulation.34 However, the resulting trend highlights the changing internal energy with change in copolyester composition. The free volume calculated for the copolyester systems also follows a monotonic trend with the greatest free volume present at the 100% CHDM system and decreasing to a minimum at 100% EG system. The increased free volume in the polymer is linked to increased mobility35 within the system
Figure 6. g(r) values calculated between Si and carbonyl O molecules at all copolyester compositions. Values were calculated for oxygen-rich glass surface.
Figure 5. Adhesion energies calculated as a function of copolyester composition at 298 K (red) and 400 K (blue) for the (left) oxygen-rich and (right) tin-rich glass surfaces. Error bars indicate the standard deviation. The initial peak height from the RDF is plotted as dotted lines, with the peak values represented on the right axis. E
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 7. (a) Percent of aromatic rings participating in π stacking calculated as a function of copolyester composition. Error bars represent the standard deviation. (b) Snapshot of π stacking rings in a 100% CHDM copolyester system.
corresponds to a close contact occurring between the surface of the polymer interacting with the top layer of the glass. By examining the height of this first peak, an interesting trend was observed. Figure 5 contains a plot of the heights of this initial peak at the different copolyester compositions, where the resulting curves mirror those of the adhesion values. This correlation indicates that the strength of adhesion is strongly tied to the close contact between the carbonyl oxygen located on the polymer molecule and the silicon atoms on the surface of the glass. Additionally, the role of the polymer orientation was examined. The diacids used to form the copolyester chain contain aromatic rings as shown in Figure 2. These rings have a propensity to align in a parallel manner due to π stacking that occurs when the electron clouds of the aromatic rings favorably interact with the electron clouds of another aromatic ring. This stacking provides strong, stabilizing interactions between polymer chains. In order to quantify the amount of π stacking present, the instantaneous percentage of rings that participate in π stacking is averaged over the entire simulation run and plotted as a function of polymer composition in Figure 7. The amount of π stacking present within the polymer increases monotonically as the amount of diol present transitions from 100% CHDM to 100% EG. This change suggests that there are stronger interactions between the individual polymer chains. An examination of a snapshot of rings participating in π stacking found that the majority of these interactions occur in the parallel displaced geometry. In this geometry, the rings do not perfectly align, but are slightly offset. In order to examine the effect of temperature on adhesion, the previously generated systems were heated to 325, 350, 375, 400, and 425 K. The above procedure was repeated and the Eadh of that was calculated for the various polymer compositions and at each temperature. The resulting values were plotted as a function of temperature in Figure 8. It was found that the Eadh monotonically decrease with increasing temperature. The influence of temperature on adhesion has been observed experimentally for a number of polymeric materials.36 It was found that adhesion is low at temperatures where the mobility of the polymer chains is limited, eventually reaching a maximum, and then adhesion rapidly decreases with rising temperature until the polymer no longer adheres. This result may be attributed to both entropic effects of increased mobility of the polymer chains that occurs at an elevated temperature, and due to the enthalpic effects becoming less prominent at
Figure 8. Adhesion energy with the oxygen-rich glass surface as a function of temperature for all copolyester compositions. Error bars represent the standard deviation.
high temperatures. Additionally, the density of the polymer melt decreases as the temperature rises, lowering the probability of the important polymer carbonyl oxygen group coming near to the surface silicon atoms and thus reducing the total amount of interactions present. It should be noted that glass surfaces are often hydrated depending on processing conditions and exposure to air, and the polymer film can be hydrated as well; we have also studied the effects of hydration on Eadh, which is the subject of a future manuscript.
4. CONCLUSIONS A series of copolyester polymers at varying compositions were simulated in contact with oxygen-rich and tin-rich soda-lime glass surfaces using MD simulations. The RDF plots between the carbonyl oxygen atoms and silicon atoms indicate that the polymer composition increases to have more EG content, there is an increase in the amount of close contacts between these atom groups, which mirrors the trends occurring in the total system Eadh. Additionally, as the composition of the polymer transitioned from CHDM to EG, the Tg monotonically increased. This trend was mirrored by the rise in the amount of π stacking occurring within the copolyester system. These results provide insight into the underlying physics of copolyester adhesion with soda-lime glass and how modifying the copolyester composition may be used to achieve a desired level of adhesion with a glass surface. The ability to fine-tune the adhesion level, along with the polymer durability and F
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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(19) Pedone, A. Properties Calculations of Silica-Based Glasses by Atomistic Simulations Techniques: A Review. J. Phys. Chem. C 2009, 113, 20773−20784. (20) Hill, J. R.; Freeman, C. M.; Subramaniam, L. Reviews in Computational Chemistry. In Use of Force Fields in Materials Modeling; Lipkowitz, K. B., Boyd, D. B., Eds.; John Wiley & Sons, Inc.: New York, 2007; Vol. 16; pp 141−216 10.1002/9780470125939.ch3. (21) Dauber-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T. Structure and Energetics of Ligand Binding to Proteins: Escherichia coli Dihydrofolate ReductaseTrimethoprim, A Drug-Receptor System. Proteins: Struct., Funct., Genet. 1988, 4, 31−47. (22) Karim, M. M. A. A Study of Tin Oxides in Silicate Based Glasses. Ph.D. Thesis, University of Warwick, 1995. (23) Yuan, X.; Cormack, A. N. Local Structures of MD-Modeled Vitreous Silica and Sodium Silicate Glasses. J. Non-Cryst. Solids 2001, 283, 69−87. (24) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926−935. (25) Chen, J.; Hanson, B. J.; Pasquinelli, M. A. Molecular Dynamics Simulations for Predicting Surface Wetting. AIMS Mater. Sci. 2014, 1, 121−131. (26) Accelrys Materials Studio, Release 5.5.2; Accelrys Software Inc.: San Diego, CA, 2004. (27) Scienomics, S. MAPS, version 3.4. 2015. (28) Earl, D. J.; Deem, M. W. Parallel Tempering: Theory, Applications, and New Perspectives. Phys. Chem. Chem. Phys. 2005, 7, 3910−3916. (29) Connolly, M. Solvent-Accessible Surfaces of Proteins and Nucleic Acids. Science 1983, 221, 709−713. (30) Pilkington, L. A. B. Review Lecture. The Float Glass Process. Proc. R. Soc. London, Ser. A 1969, 314, 1−25. (31) Sieger, J. Chemical Characteristics of Float Glass Surfaces. Glass Surfaces: Proceedings of the Fourth Rolla Ceramic Materials Conference on Glass Surfaces, St. Louis, Missouri, USA, 15−19 June, 1975; NorthHolland: Oxford, 1975; pp 213−220. (32) Townsend, P.; Can, N.; Chandler, P.; Farmery, B.; LopezHeredero, R.; Peto, A.; Salvin, L.; Underdown, D.; Yang, B. Comparisons of Tin Depth Profile Analyses in Float Glass. J. NonCryst. Solids 1998, 223, 73−85. (33) Gladushko, O.; Chesnokov, A. Identification of Float Glass Surfaces. Glass Ceram. 2005, 62, 308−309. (34) Boyd, S. U.; Boyd, R. H. Chain Dynamics and Relaxation in Amorphous Poly(ethylene terephthalate): A Molecular Dynamics Simulation Study. Macromolecules 2001, 34, 7219−7229. (35) Roth, C. B.; Dutcher, J. R. Glass Transition and Chain Mobility in Thin Polymer Films. J. Electroanal. Chem. 2005, 584, 13−22. (36) Kinloch, A. Adhesion and Adhesives: Science and Technology; Springer Science & Business Media, 2012.
optical properties, indicate that copolyesters may be commercially viable as a laminate material in safety glass.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Eastman Chemical Company for financial support of this work through the Eastman Chemical Center for Excellence at NC State. The authors also want to thank Damon Billodeaux at the Eastman Chemical Center for Excellence at NC State for all of his help acting as an interface between the two groups.
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REFERENCES
(1) Baade, R. A.; Baumann, R.; Matheson, V. Estimating the Economic Impact of Natural and Social Disasters, with an Application to Hurricane Katrina. Urban Studies 2007, 44, 2061−2076. (2) Savage, R. P. Hurricane Alicia, Galveston and Houston, Texas, August 17−18, 1983; National Academies, 1984. (3) O’Donnell, M. D.; Watts, S. J.; Hill, R. G.; Law, R. V. The Effect of Phosphate Content on the Bioactivity of Soda-Lime-Phosphosilicate Glasses. J. Mater. Sci.: Mater. Med. 2009, 20, 1611−1618. (4) Wallace, K. E.; Hill, R. G.; Pembroke, J. T.; Brown, C. J.; Hatton, P. V. Influence of Sodium Oxide Content on Bioactive Glass Properties. J. Mater. Sci.: Mater. Med. 1999, 10, 697−701. (5) Bagley, B. G.; Vogel, E. M.; French, W. G.; Pasteur, G. A.; Gan, J. N.; Tauc, J. The Optical Properties of a Soda-Lime-Silica Glass in the Region from 0.006 to 22 eV. J. Non-Cryst. Solids 1976, 22, 423−436. (6) Wiederhorn, S. M. Fracture Surface Energy of Glass. J. Am. Ceram. Soc. 1969, 52, 99−105. (7) Norville, H.; King, K.; Swofford, J. Behavior and Strength of Laminated Glass. J. Eng. Mech. 1998, 124, 46−53. (8) Memari, A.; Behr, R.; Kremer, P. Dynamic Racking Crescendo Tests on Architectural Glass Fitted with Anchored Pet Film. J. Arch. Eng. 2004, 10, 5−14. (9) Morison, C. The Resistance of Laminated Glass to Blast Pressure Loading and the Coefficients for Single Degree of Freedom Analysis of Laminated Glass. Ph.D. Thesis, Cranfield University, 2010. (10) Schuler, C.; Bucak, Ö .; Albrecht, G.; Sackmann, V.; Gräf, H. Time and Temperature Dependent Mechanical Behaviour and Durability of Laminated Safety Glass. Structural Engineering International 2004, 14, 80−83. (11) El-Din, N. M. S.; Sabaa, M. W. Thermal Degradation of Poly(vinyl butyral) Laminated Safety Glass. Polym. Degrad. Stab. 1995, 47, 283−288. (12) Norville, H. S.; Conrath, E. J. Design Procedure for BlastResistant Laminated Glass. ASTM Spec. Technol. Publ. 2002, 1434, 159−170. (13) Polyethylene Terephthalate (PET or PETE); Encyclopædia Britannica, Ed.; Encyclopædia Britannica; 2015. (14) Hamonic, F.; Prevosto, D.; Dargent, E.; Saiter, A. Contribution of Chain Alignment and Crystallization in the Evolution of Cooperativity in Drawn Polymers. Polymer 2014, 55, 2882−2889. (15) Demirel, B.; Yaras, A.; Elcicek, H. Crystallization Behavior of PET Materials. Journal of BAÜ Fen Bil. Enst. Dergisi 2011, 13, 26−35. (16) Josef, H.; Dominik, M.; Gottfried, M.; Winfried, Z. Copolyesters, Their Production and Uses. US Patent US3558557 A, 1971. (17) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1−19. (18) van Beest, B. W. H.; Kramer, G. J.; van Santen, R. A. Force Fields for Silicas and Aluminophosphates Based on ab initio Calculations. Phys. Rev. Lett. 1990, 64, 1955−1958. G
DOI: 10.1021/acsami.6b01851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
