amine crosslinked networks

Accepted Manuscript The effect of pendant alkyl chain length on the barrier properties of epoxy/amine crosslinked networks J.H. Vergara, J.J. La Scala, C.K. Henry, J.M. Sadler, S.K. Yadav, G.R. Palmese PII:

S0032-3861(17)31008-X

DOI:

10.1016/j.polymer.2017.10.042

Reference:

JPOL 20084

To appear in:

Polymer

Received Date: 2 August 2017 Revised Date:

13 October 2017

Accepted Date: 18 October 2017

Please cite this article as: Vergara JH, La Scala JJ, Henry CK, Sadler JM, Yadav SK, Palmese GR, The effect of pendant alkyl chain length on the barrier properties of epoxy/amine crosslinked networks, Polymer (2017), doi: 10.1016/j.polymer.2017.10.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis and characterization of model amidoamines reveal a dependence of inter-polymer oxygen concentration on water transport within epoxy/amine networks.

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The effect of pendant alkyl chain length on the barrier properties of epoxy/amine crosslinked networks a

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J.H. Vergara, J.J. La Scala, C.K. Henry, J.M. Sadler, S.K. Yadav and G.R. Palmese

Introduction

temperature (Tg) and thermal stability, while aliphatic amines tend to have faster curing kinetics allowing them to cure epoxy resins at room temperature. Amines can be modified to impart desirable properties to the resulting epoxy/amine matrix. A good example of a modified amine, which is commercially available, for coating applications is Ancamide 507 (Figure 1).

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It has been estimated that the annual cost due to corrosion and corrosion prevention is approximately $2.5 trillion globally 1, per year, which is 3.4% of the global gross domestic product. 2 Not counting for just the economic costs, corrosion can lead to structural failures with dire consequences such as corrosion failures automobiles, bridges, buildings, naval craft, aircraft, 3-12 and gas pipelines. The performance and durability of a coating depends on many factors. The coating substrate, pretreatment of the substrate, thickness of the coating, adhesion as well as external factors including acidity, temperature, and humidity are some of the factors at play in 13-15 the performance of an anti-corrosive coating. Epoxy resins have thus found wide use in adhesives, composites, and surface coating applications in the automotive and aerospace industries owing to their good adhesive properties as well as having high modulus and 16-19 thermal stability. Although a large variety of epoxy resins are commercially available, the most commonly used epoxy resins are based on diglycidyl ether of bisphenol A (DGEBA) of varied molecular weights, which account for about 80% of the 20-22 epoxy resin market. The curing agent has a profound influence on the curing reaction mechanisms, curing conditions, process feasibility, pot life, and most importantly, the cured epoxy network structure, which affects thermomechanical properties, hygrothermal behavior, and 23-26 thermal stability. Among the many types of curing agents, amines are of the most important due to their versatile 20 properties, processability, and ease of use. Amine curing agents can generally be grouped into two categories: aromatic and aliphatic amines. Aromatic amines tend to have better high temperature properties, such as high glass transition

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Aliphatic amido amine crosslinkers have been commercially used in epoxy coatings to enhance water barrier properties for corrosion prevention. However, there is only an empirical understanding of how these aliphatic polyamine crosslinkers affect properties of thermoset polymers, which limits our ability to design newer and more environmentally friendly coating systems. In this work, a series of model aliphatic amido amine curing agents are synthesized and used to create model epoxy/amine networks found in commercial coating formulations. These model systems are interrogated to understand the effects of pendant alkyl chain length material properties including modulus, glass transition temperature, solubility, diffusivity, and permeability of water. This work shows that increasing the pendant aliphatic chain reduces the solubility of water while simultaneously increasing the diffusivity to water in the polymer network.

a.

Drexel University, Department of Chemical and Biological Engineering, Philadelphia, PA 19104 b. U.S. Army Research Lab, 4600 Deer Creek Loop, Aberdeen Proving Grounds, MD 21005

Figure 1. A representative structure for Ancamide 507, which contains reaction products of TOFA with TEPA and reaction products of epoxidized oleic acid with TEPA.

In this case, tetraethylenepentamine (TEPA) has been modified with tall oil fatty acids (TOFA) to impart hydrophobicity to the amine curing agent; when cured with an epoxy, the resulting epoxy/amine thermoset has improved water barrier properties when compared to its unmodified 27 analog. It is important to note that both TEPA and TOFA are mixtures of various compounds, making fundamental studies investigating the effect of pendant aliphatic chains on material properties of epoxy/amine thermosets difficult. The water barrier properties of anticorrosive play a critical 28 role in protecting metals from corrosion. Once water penetrates through to the polymer/metal interphase (i.e., region near the interface that is molecularly or morphologically affected by the nearness of the interface), it can adsorb on the oxide surface and substitute the electrostatic interactions between the coating and the metal thereby reducing the adhesion between the polymer and the 29, 30 substrate. In addition to reducing adhesion, water

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Figure 2 shows the synthetic scheme for the production of model mono amidoamines. In this scheme, Diethylenetriamine (0.467 mol, 48.26 g) was heated to 90 °C and mixed in a two neck round bottom flask for 30 min. A fatty methyl ester with desired alkyl chain length was then added dropwise to the hot DETA at a 1 methyl ester : 10 DETA equivalent ratio and allowed to react for 12 hrs. The reaction products were diluted in 150 mL of chloroform and washed with 100 mL of water twice to remove unreacted DETA. The chloroform phase was dried with MgSO4 and evaporated under vacuum at 70 °C using a rotary evaporator to isolate the desired monoamidoamines shown in Figure 2. The mono-amidoamine structures 1 were characterized by H-NMR and were collected via a Varian Unity Inova NMR apparatus (500 Mhz) with a spectral window of ±2000 Hz and 32 scans per sample.

Figure 2. Synthetic scheme for the production of AMAMn crosslinkers

2.3 Polymer Preparation

DGEBA was individually cured with DETA and each of the four AMAMn (n = 4, 6, 8, and 10) crosslinkers to produce crosslinked polymer systems. DGEBA and crosslinker were mixed at stoichiometric ratio (one epoxy ring to one amine hydrogen) and drawn down with a 4 mil bird applicator on a PVF wrapped steel panel. The drawn film was then cured at a temperature of 80 °C for 3 hrs and subsequently post cured at 160 °C for 1.5 hrs. The resulting films were on average 100 μm in thickness and were easily peeled off the PVF-wrapped panels and stored in a desiccator to prevent moisture uptake. Polymer bars were made using a homemade rectangular silicon molds where the desired polymer composition was cured at a temerature of 80 °C for 3 hrs and subsequently post-cured at 160 °C for 1.5 hrs. These samples were then polished down to dimensions of 17.5 mm in length, approximately 11.9 mm in width, and approximatley 3.5 mm in thickness.

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infiltration into the coating matrix can lead to swelling and plasticization of the matrix, which reduces the modulus and 31-34 Tg. Swelling of the coating matrix can lead to microcrack formation along the matrix/filler interface and exacerbate corrosion by increasing the permeation of water and oxygen to 34, 35 the coating/metal interface. Microcrack formation tends to occur in the interphase due to poor modulus and surface energy matching; this occurs when there is a different 36, 37 composition of epoxy/amine at the interface. An ideal barrier coating for corrosion control would minimize swelling of the coating matrix and bar both water and oxygen from reaching the polymer/metal interface thus depriving the essential reactants for corrosion to occur. It stands to reason that creating model building blocks for epoxy/amine coatings is the first step in elucidating the key contributions of epoxy/amine monomer structures to corrosion performance. Recent work suggests that the polarity of an epoxy/amine 38-40 network has a profound effect on their barrier properties. This work aims to systematically evaluate the role of pendant alkyl chains of model amido amine cured epoxy networks on the material properties, specifically water barrier properties and thermomechanical properties, of epoxy/amine thermosets for coating applications. To this end, model amido amine crosslinkers (AMAMn) with varying alkyl chain length were synthesized and cured with DGEBA to obtain homogenous epoxy/amine systems with varying pendant alkyl chains. By increasing the alkyl chain length of these AMAMn crosslinkers, the polarity of an epoxy/amine network can be changed with limited sensitivity to thermal properties (Tg). These systems were used to elucidate the effect of increasing alkyl pendant chains on water transport and material properties of conventional epoxy networks with model amido amine curing agents to help identify optimum coatings formulations for corrosion control applications. This work has found no governing relationship between the crosslinking density, molecular weight between crosslinks, and the free volume estimated from group contribution theory to the solubility and diffusivity of water in epoxy/amine polymers. However, this work will show that the solubility and diffusivity of water are strongly correlated to enthalpic interactions manifested by the chemical composition of the epoxy/amine polymer itself, i.e. the inter-polymer oxygen concentration.

Experimental 2.1 Materials

Diglycidyl ether of bisphenol A (DGEBA) was purchased from Miller Stevenson Inc. and was used as received. Diethylenetriamine (DETA) ACS Reagent grade (99.5%), methyl hexanoate, methyl octanoate, methyl decanoate, methyl dodecanoate, and chloroform were purchased from Sigma Aldrich and was used as received. Magnesium Sulfate was purchased from BDH and was used as received. Polyvinyl fluoride, PVF (Tedlar) was used as release film for making thin epoxy/amine films and was acquired from DuPont. 2.2 Synthesis of AMAMn Crosslinkers

2.4 Dynamic Mechanical Analysis (DMA) Thermomechanical properties were measured using a Thermal Analysis Q800 DMA apparatus. Properties of interest include storage and loss modulus, as well as the Tg of DGEBA cured with modified and unmodified amines, crosslinking density (ν), and molecular weight between crosslinks (Mc). Each sample was tested at a frequency of 1 Hz and amplitude of 10 μm in single cantilever mode. Each sample was shaped into rectangular slab geometry with dimensions (length, width, and thickness) of 17.5 mm, 11.9 mm, and 3.5 mm to keep sample 41 stiffness at an acceptable range for rubbery modulus. Each sample was relaxed in the apparatus by ramping the temperature by 2 °C/min from room temperature to 140 °C and allowing them to cool to room temperature to erase the thermal history of each sample. Samples cured with model amido amines were then equilibrated at –120 °C for 10 min and tested using a temperature ramp of 2 °C/min from –120 °C to 140 °C, Samples cured with DETA were tested using a

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(1) (2)

In these equations ρ, R, T, and E are the density of the polymer, universal gas constant, absolute temperature and 41 storage modulus in the rubbery regime (T = Tg + 40 °C). 2.5 Density Measurements

2.6 Fourier Transform Infrared Spectroscopy (FTIR)

For the purpose of examining the effect of alkyl chain length of the AMAMn crosslinkers on physical properties, it is important to ensure that all reactive moieties have been consumed so that the effect that unreacted species may have on material properties such as Tg, Mc, ν, ρ, solubility (S), and diffusivity (D) of water through a resulting polymer can be eliminated. Figure 3 shows the FTIR spectra of post-cured resin samples. Epoxy, primary amine, and secondary amine peaks are not present indicating that the samples are fully cured within the limits of FTIR sensitivity.

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The densities for all samples were measure via a density gradient column as described in ASTM D1505. All specimens were thin films with an average thickness of 100 μm; these o films were immersed in DI water at 23 C for about 10 min prior to exposure to the density gradient to ensure no air bubbles would form. The density of each composition was taken to be the average of at least three representative samples from a film of desired composition.

3.2 Curing

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methyl ester ratios (< 10:1) did not yield sufficiently pure mono amides as more methyl ester present in the reaction increases the concentration of diamide. Diamide formation is highly unfavorable as curatives because diamides synthesized from DETA produce monomers with only one labile hydrogen, which does not allow any crosslinking. The purity of AMAMn structures was found to be about 98–99%. AMAM0 and AMAM2 could not be successfully synthesized due to cyclization of the monoamide to an imidazoline. The AMAMn crosslinkers that were successfully synthesized (n = 4, 6, 8, 10) were used in curing.

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temperature ramp of 2 °C/min from –120 °C to 180 °C. The Tg was taken to be the peak maximum of the loss modulus (E’’) curve. Based on rubber elasticity theory, the molecular weight in between crosslinks (Mc) and crosslinking density (ν) were obtained by using Eqs. 1 and 2, respectively.

2.7 Dynamic Vapor Sorption (DVS)

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FTIR in the near infrared region (nIR) was used to assess the extent of cure in the thermosets presented here. A Nicolet 6700 with a CaF2 beam splitter was employed using a -1 resolution of 4 cm in transmission. The near infrared region is an ideal spectroscopic tool to assess the extent of cure due to the fact that all of the reactive species can be monitored. The peaks of interest are located at 4530 cm-1, 4925 cm-1, and 6510 cm-1, which are the epoxy, primary, and combined 42, 43 primary and secondary amine peaks, respectively.

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Water barrier properties of model amido amines cured with DGEBA were measured using a TA model Q5000SA Dynamic Vapor Sorption device. Each sample was dried in situ at 85 °C and at 0% RH before exposure to humidity to ensure fully dry samples before testing. Each composition was exposed to 80% RH at 24, 36, 48, and 60 °C.

Figure 3. nIR spectra of postcured samples of DGEBA cured with AMAM6, AMAM8, AMAM10, as well as an uncuredAMAM6 sample.

AMAM4,

2.8 Wide Angel X-Ray Scattering (WAXs)

WAXs was used to determine whether or not nanophase separation is present in the specimens studied here. A detailed experimental protocol and analysis can be found in the supplementary information. All samples were found to be free micro and nano phase separation.

Results and Discussion 3.1 AMAMn Characterization Characterization of each amido-amine can be found in the supplemental information. Reactions with lower DETA to

3.3 Thermomechanical Properties Figure 4 shows the storage (E’) and loss moduli (E’’) as a function of temperature for all samples studied. Additionally, Table 1 contains a summary of thermomechanical data for all samples tested. This includes room temperature modulus, rubbery modulus (T = Tg + 40 °C), and the Tg obtained as the peak position of the E’’ curve. Figure 4 shows that the sub-Tg molecular motions are altered significantly by the addition of the amido group in all modelamido amines. Fully crosslinked epoxy/amines exhibit a broad Beta-relaxation peak beginning at –100 °C at a frequency of 1

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Amines

E'RT

ρ

E'R 3

v

Mc 3

Tg, E''

(g/cm )

(MPa)

(mol/m )

(g/mol)

(°C)

1.82 2.43

1.2041 1.1700

36 22

3200 2200

380 530

137 99

n=4

2.28

1.1665

10

1000

1140

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Table 1. Summary of thermomechanical properties of DETA and AMAMn cured DGEBA polymers.

The peak area of the loss moduli vs. temperature in the sub-ambient regime for the model amido amines also decreased as the aliphatic chain length decreased and plateaued from AMAM8 and AMAM10. The implication here is that less energy is needed for these motions to be fully activated. In other words, AMAMn samples possess intrinsically higher polymer dynamics, more motions below the glass transition, than DGEBA-DETA. It is important to note that the free volume is not static, or in other words the hole-free volume can be distributed throughout the polymer without a change in energy. These motions may facilitate the transport of water into the matrix by virtue of dispersing the free volume in the network. Based on the results shown in Table 1, the storage modulus at room temperature (ERT) is enhanced for all AMAMn samples, compared to DETA. ERT decreased linearly with the length of the aliphatic chain length likely because the density decreased accordingly, as glassy modulus is generally linked to 47 sample density. The modulus values for the AMAMn polymers studies are also within the range of a typical 48 pigment-free binder for anticorrosive coatings. Molecular simulations based on DGEBA cured with AMAMn crosslinkers concluded that the methylene units on the pendant alkyl chain can be treated as a solute such that an ideal mixture is formed 49 between the polymer backbone and the methylene units. Further, a linear relationship between volume fraction of alkyl pendant chains with both the density and the coefficient of thermal expansion of the polymers in question was 49 established. By taking the methylene units to be a “solute” in a polymer backbone “solvent”, we can make a similar linear relationship (Eq. 3) between the ERT and pendant alkyl chain length where ERT,0 is the modulus of free alkyl chain polymer, Em,RT is the modulus of the methylene chain, and Фm is the methylene volume fraction.

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Hz. As the crosslink density is reduced, the onset of the transition remains unchanged while the peak position, breadth, and height are diminished. Here, we see that the highly crosslinked DGEBA-DETA system requires more energy to fully activate all sub-zero relaxation mechanisms when compared to the model amido amine systems. This may be due to an increased crosslinking density which consequently causes an increased phenyl ring concentration in the DGEBADETA polymer. Rotation of phenyl rings about the parapositioned carbons at sub-ambient conditions is considered to be one of the most energy-intensive fundamental motions 44-46 during the β-transition.

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The depression in Tg by ~70 Tg °C observed in the AMAMn samples relative to DETA samples was not solely due to the addition of low Tg alkyl chains but also due to the loss of functionality and crosslinking capacity in AMAMn crosslinkers when compared to DETA. The Tg of all AMAMn polymers are well within the range of standard anticorrosive coatings and as 48 such the Tg is appropriate for coating applications. With regard to the latter, the AMAMn loses a primary amine group replaced by a non-reactive amide group. Amine crosslinkers with lower concentrations of labile hydrogens yield a lower crosslinked polymer, which generally yields polymers with lower Tgs. The Tgs of the AMAMn samples are insensitive to the increase in aliphatic chain length. The insensitivity of Tg to increasing pendant alkyl chain length is evidence that the reduction in Tg is mostly due to the loss of labile hydrogens. Previous work on fatty acid modified DETA showed a similar trend where the Tg was fairly constant for two similar 51 amidoamines with different alkyl chain lengths. Further, molecular simulations attribute a lack of increased segmental flexibility in the face of increasing alkyl chain length to the 49 absence of intermonomer methylene correlation. Thus, the modulus, and as will be shown, S and D of water in epoxy/amine polymers with AMAMn type crosslinkers can be tuned without changing the Tg of the system. From a thermomechanical perspective, epoxy/amine polymers made from AMAMn crosslinkers show similar properties to commercially used amido amines crosslinkers (i.e. A507), thus they are expected to perform adequately from a thermomechanical perspective. The Mc increased as the aliphatic chain length in the AMAMn crosslinkers increased; and conversely, the v decreased with an increasing aliphatic chain length. The increase in Mc and decrease in v was expected, as increasing the molecular weight of the curing agent increases the molecular weight of the unit monomer and hence should increase Mc.

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Figure 5 shows ERT as a function of Фm with the modulus data being regressed to Eq. 3. From the regression one can see that indeed the modulus of the AMAMn polymer systems is governed by the ideal mixing rules between the alkyl chains (solute) the alkyl chain-free polymer (solvent). The ERT value associated with a methylene volume fraction equal to unity is approximately 0.22 GPa, which is in the range of error with the 50 modulus of polyethylene. This is another confirmation of the alkyl chains existing as solutes in the polymer solution (ideal solution). It is important to note that all samples studied have no separation. The absence of a second alpha-transition in the sub-zero regime of the E’’ thermograms indicate that no microphase separation occurs in the samples studied. Moreover, SEM and SAX data support this conclusion.

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For one-dimensional penetrant diffusion through a slab with

3.4 Water Barrier Properties

thickness L and constant diffusion coefficient or diffusivity, D, Fick’s second law is given in Eq. 3 where C is the concentration of penetrant and t is time. (3) The absorption of water into glassy polymers generally exhibits Fickian behaviour when the transport of water is completely controlled by diffusion. Non-Fickian diffustion may occur when the polymer itself undergoes degradation as a result of the penetrant, or when the polymer relaxation time 32, 52 scale is similar to the diffusion time scale. Assuming a

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Figure 6 shows the Arrhenius plots of both diffusivity and solubility of water. The Ea and ΔH can be calculated by taking the product of the universal gas constant by that of the slope of each plot, respectively. Table 2 shows a summary of all DVS results including S, D, P, Ea, Do, ΔH, and the solvation preexponential factor So. It is clear from Figure 6 that the diffusivity of the AMAMn systems increased as the length of the pendant aliphatic chain is increased. It is also clear that the solubility of water of the AMAMn systems decreased as the pendant alkyl chain length increased.

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A similar relationship should apply for S, where So is a constant pre-exponential factor, Po is another constant preexponential factor given by the product of Do and So, ΔH is the enthalpy for sorption (heat of solution), R is the universal gas constant, and T is absolute temperature.

Fickian diffusion is characterized by: an initially linear plot of 1/2 M(t) vs. t , and that the absorption curve smoothly 53 approaches Meq. However, in some cases it is beneficial to weigh the initial sorption data more heavily than the late time water uptake data to obtain better fits for D. In these cases, the value D can be calculated from the initial slope of M(t) as a 1/2 function of t /L using the Eq. 5 where Meq is the ultimate solubility of a sample at a particular temperature and external 54 relative humidity. ( )

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Eq. 6 where Do is the pre-exponential factor, Ea is the activation energy for diffusion, R is the universal gas constant, and T is absolute temperature.

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This equation was fit to gravimetric water uptake data to obtain the diffusivity of water in the polymers studied. The permeability of each sample was taken to be the product of the S and D. As for Fickian behaviour in a non-infinite slab, all of the water sorption experiments resulted in a plateau in absorbed mass as a function of time, demonstrating the solubility limit of the sample for water, S. Further, D often exhibits an Arrhenius relationship with temperature given by

Table 2. Summary of dynamic vapor sorption results for DGEBA cured with DETA and all AMAMn.

T (°C)

(g/100g)

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2.61 2.65

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2.99

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AMAM10

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1.15E-08

1.31E-08 2.22E-08

3.42E-08 5.88E-08

4.51E-08

1.35E-07

2.25 2.29

4.28E-09 1.00E-08

9.63E-09 2.30E-08

2.36 2.37

2.12E-08 5.39E-08

5.00E-08 1.28E-07

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3.92E-08 9.99E-08

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2.27

1.05E-07

2.38E-07

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1.82 1.83

8.56E-09 2.45E-08

1.56E-08 4.48E-08

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1.87 1.91

6.61E-08 1.43E-07

1.24E-07 2.73E-07

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1.27E-08

2.27E-08

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1.86

2.84E-08

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(kJ/mol)

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4.85

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48 1.92 7.66E-08 1.47E-07 60 2.01 2.02E-07 4.06E-07 However, a trend is not seen for the enthalpy of solvation the polymer network. or for the pre-exponential factor for solvation. This may be due Figure 7 shows the solubility and diffusivity of water in all to the combined effect of insensitivity of S on temperature and AMAMn samples studied as a function of the methylene unit the fact the solubility limit in these polymers are very low. It is volume fraction, Φm. This plot emphasizes the effect of also likely that the insensitivity of S to temperature may be increasing the length of the pendant aliphatic chains on water due to the fact that all water uptake measurements were barrier properties. It is clear that both S and D are well taken below the Tg of each sample. The activation energy for correlated to the volume fraction of methylene chains in the diffusion appears to increase as pendant aliphatic chain length polymers. This corroborates that the AMAMn samples follow increased. A higher Ea signifies an increased response to ideal mixing rules. That is, at a given temperature the variations in temperature. In other words, the Eas indicate that diffusivity and the solubility can be expressed by Eqs. 8 and 9, the diffusion coefficient varies more with temperature. where Dw,0 is the diffusivity of water in an AMAM0 cured with The Do is seen to increase dramatically as the length of the DGEBA polymer, Фm is the volume fraction of methylene pendant aliphatic chain increased. This may be due to the chains, Dw,m is the diffusivity of water through the methylene enhanced polymer dynamics below Tg. The increased motion chains, Sw,0 is the solubility of water of a AMAM0 cured with of the polymer below Tg may be enhancing the diffusivity of DGEBA polymer, and Sw,m is the solubility of water in the water by effectively increasing the dispersion of free volume in methylene chains.

Figure 7. Solubility and Diffusivity in AMAMn samples as a function of Φm

Figure 8. Dependence of solubility of water at 24 °C to (a) [aliphatic] and (b) [O].

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From the linear regression we can obtain a value of 7.01E-8 2 cm /s for Dw,m and 1.47 g/100g for Sw,o. Both of these values agree with measurements of water diffusivity and solubility in 56 low density polyethylene. Yet, in examining the DGEBA/AMAMn polymers alone, we cannot gain a full understanding of the water transport in epoxy-amine polymers. There is evidence in the literature that the free volume and the polarity of an epoxy/amine resin are linked through nanovoid structures concentrated around the crosslink junctures (i.e., 57 the tertiary amines and hydroxyls). The literature suggests that the steric hindrance, caused by these crosslink junctures and their polar groups (i.e., tertiary amines and hydroxyls) is the main suspect for why the distribution of free volume in these systems are concentrated around the crosslinking junctures. In other words, the distribution of the nanovoids, and hence the free volume, is governed by the location and concentration of these polar groups. One might be so inclined to look for correlations between crosslinking density and the transport properties, S and D. Upon doing so, it was found that, for both crosslinking density and free volume estimated from group contribution theory in the epoxy/amine polymers studied here, no correlation was found for S or D. This analysis

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the AMAMn samples. Figure 8b indicates that dilution of [O] can also be brought about by increasing the phenyl ring concentration in the epoxy amine polymer network. For the case of PACM and MDA cured DGEBA samples, the solubility of water also depends on [O]. Thus, solubility appears to be governed quite well by [O] for AMAM polymers with different fatty acid adduct lengths, DETA, and other amine crosslinkers with significantly different chemical makeup. Figure 9 shows the diffusivity as a function of oxygen concentration in the polymer. Diffusivity is inversely proportional to oxygen concentration in the AMAMn samples; it follows that the diffusivity is also proportional to the aliphatic content, which is proportional to the volume fraction of methylene chains in the AMAMn samples. It can be seen that [O] is very well correlated with the diffusivity of all samples studied, but the general trend of [O] does not fit the DGEBA/DETA polymer. The β-transition for all the AMAMn samples was greatly subdued compared to that of DETA; the increased local polymer mobility exhibited by the DGEBA-DETA system may be the reason D does not follow the trend observed for the AMAMn samples. Yet, DGEBA/PACM and DGEBA/MDA also had large β-transitions, while the diffusivity of water in these polymers did follow the general trend so the β-transition is not the likely cause for this effect. It is also possible that the minimum diffusivity for water in these epoxy2 amine systems is ~4E-9 cm /s. To further investigate the role of inter-polymer oxygen concentration on the solubility of water in these systems, the Flory-Huggins model was used to correlate the ultimate solubility of water in the samples tested here to the polymerpenetrant interaction parameter, χ12, commonly referred to as the Flory-Huggins interaction parameter. In a binary penetrant-polymer mixture, the activity of the penetrant (water, phase 1) in and the polymer (DGEBA cured with DETA, AMAM4, AMAM6, AMAM8, AMAM10, or PACM and MDA, phase 2) can be represented by Eq. 8, where r is the ratio of polymer molar volume to penetrant molar volume, a1 and =% are the water activity and water volume fraction, respectively, and χ12 is the Flory-Huggins interaction parameter for the binary mixture. The Flor-Huggins parameter, χ12, can also be estimated using solubility parameters via Equation 9 where δ1 and δ2 are the solubility parameters for water and glassy polymer, respectively, v1 is the molar volume of water, T is the temperature of the system and R is the ideal gas constant.

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can be found in the supplementary information. It is clear that there is something else is at work here and that free volume is certainly not the governing parameter for the solubility and diffusivity of water. There are generally three suspects that have been reported for trends in water absorption and diffusion: free volume of the polymer, chemistry and polarity of the polymer, and for thermosets, crosslink density. For polymers with the same chemistry that are cured to different extents, samples with higher crosslinking 58 have lower water absorption. In fact, for water swelling polymers, this is an accepted method for calculation of 59 crosslink density. Analysis of Tables 1 and 2 show that crosslink density does not govern the water transport of the polymers studied in this work. For example, Mc was relatively constant as n increased in the AMAMn polymers, yet the diffusivity increased and solubility decreased. Although ν can be predicted by water swelling experiments in water, these results show that is not applicable for poor solvents for the polymer, such as water for these epoxy-amines, or is not applicable as the chemistry of the material is changed rather than simply the degree of cure. Figure 8a shows the dependence of the solubility of water on aliphatic concentration. The solubility of water decreased as the aliphatic concentration increased for samples with similar chemistry (i.e., DETA and the AMAMn samples). We prepared similar epoxy-amine polymers using 4,4’-methylene dianiline and PACM (4,4’-Methylenebiscyclohexanamine) instead of the AMAMn amidoamines to determine the effect of chemistry. It is important to note here that this trend in aliphatic content does not hold for samples with dissimilar chemistries and that the solubility does not appear to be a function of aliphatic content for all amines studied. Figure 8b shows the dependence of the solubility of water at 24 °C on oxygen concentration. Note that oxygen concentration refers to the concentration of oxygen within the

polymer network including the oxygens in ethers and hydroxyls. These results indicate that addition of alkyl pendant chains dilute the concentration of oxygen present in the polymer network thus reducing the solubility of water for

Figure 10. χ12 as a function of [O] in DGEBA cured with DETA, AMAM4, AMAM6, AMAM8, AMAM10, PACM and, MDA.

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Using these equations and the solubility obtained from sorption experiments, it is possible to regress the FloryHuggins interaction parameter χ12. Figure 10 shows the χ12 as a function of the alkyl chain length in the AMAMn crosslinkers cured with DGEBA where n = 0 is taken to be DGEBA/DETA. The open symbols represent χ12 from group contribution theory, the closed symbols represent χ12 from experiments. Note that as n increases, the aliphatic content of the polymer increases and the oxygen concentration decreases. Decreasing the oxygen concentration by increasing the alkyl chain length in the AMAMn effectively increases χ12 of the polymers studied. Thus, the polarity of an

epoxy/amine greatly affects its barrier properties, as also 17, 38, 39, 57, 60, 61 shown by others. Although the oxygen content in the polymer strongly governs the water transport, the FloryHuggins parameter analysis shows that the basis for the water transport effects are thermodynamic in nature. Specifically, the A useful empirical relationship between oxygen concentration and the Flory-Huggins interaction parameter may be derived from experimental data to predict solubility and diffusivity. However, not enough samples were studied here to present an accurate picture of all epoxy/amine compositions. So now we apparently have four parameters that potentially affect water transport in epoxy-amine polymers: oxygen content in the polymer chains, crosslink density, nanovoid content (free volume), and polymer dynamics. Of these, oxygen content in the polymer, including its effects on free volume, appears to be the dominant factor that affects water solubility and diffusion in epoxy-amine polymers.

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Figure 11. Schematic representation of an epoxy/amine network with a higher concentration of oxygen (top) and an epoxy/amine network with a lower concentration of oxygen (bottom). The green circles represent crosslinking junctures that contain polar groups, the black and grey lines represent polymer chains, and the blue circles with arrows represent water molecules moving through the network. The arrows indicate the direction and “speed” the water molecule is diffusing, longer arrows indicate faster diffusion.

It is useful to point out again that the literature supports the notion that free volume and its distribution throughout an epoxy/amine polymer network is governed by the distribution and concentration of the polar groups present in the final network by virtue of their connection to crosslink junctures. Figure 11 is a schematic representation of water diffusing through an epoxy/amine network with high [O] and low [O]. Water molecules can only reside in the free volume of the polymer network. Note that the distribution of free volume in an epoxy/amine network is believed to be concentrated around the crosslink junctures and the dispersal of the total free volume depends on the local motions of these junctures at a given temperature. In other words, these diffusion

pathways are not constant, but rather fluctuating as the local conformations of the network change due to the thermal energy of the polymer. The increased concentration of crosslinking junctures necessitates an increase in [O]. Moreover, it has been shown here that D and S are both functions of [O] rather than purely functions of free volume. Increasing the [O] in epoxy/amine polymers effectively increases the solubility of water and decreases the diffusivity of water. Furthermore the oxygen content of an epoxy/amine can effectively be controlled by the chemistry of the amine curing agent. In the case of DGEBA cured with the AMAMn curing agents, the oxygen content is effectively controlled by increasing n. For DGEBA cured with PACM and MDA the

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other epoxy-amine polymers. In fact, an increase in oxygen concentration in the polymer backbone directly increased the solubility of water and decreased its diffusivity. Thus, the model that describes water transport in the epoxy-amine polymers is that the water aggregates near the oxygen groups in the epoxy-amine and diffuses slowly around the oxygen where the oxygen acts as “pot holes” that retard the diffusion of water, while water molecules away from the oxygen groups diffuse relatively fast until they come in contact with water. Yet, a critical concentration of aliphatic content appears to be found where addition of more methylene groups to the pendant alkyl chain no longer improves the water barrier properties because the solubility decreases to a lesser extent than diffusivity increases beyond for pendant alkyl chains with more than four carbon atoms. The thermo-mechanical properties of the resulting amido-amine polymers are very similar to that of commercial epoxy/amido-amines and have reduced glass transition temperature and higher modulus than DGEBA/DETA. These changes in properties make the resulting DGEBA-AMAMn polymers highly applicable for corrosion control coatings and the added knowledge of water transport indicates that only short (four carbons) alkyl modifiers to the amines are necessary to minimize water transport. Future studies to examine oxygen diffusion need to be completed to identify optimum corrosion control coatings formulations.

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oxygen content is controlled by virtue of their chemical structure; that is, PACM has two cycloaliphatic rings while MDA has two aromatic rings, both of which effectively dilute [O] when cured with DGEBA at a stoichiometric ratio. Each hydroxyl, oxygen, and tertiary amine available in the free volume of the epoxy/amine network may interact with a water molecule effectively “slowing” down its diffusion n process. This has been shown through the results presented thus far, and is most marked representation is the effect of [O] on D and χ12. The parameter that is indicative of barrier performance is the permeability. Table 2 shows that the permeability of model epoxy amine systems follows this order for all temperatures AMAM4 < DETA < AMAM6 < AMAM8 < AMAM10. The diffusivity of all samples studied generally follow this order for all temperatures AMAM4 < DETA < AMAM6 < AMAM8 < AMAM10. In other words, the epoxy/amine system with the best barrier performance to water was the AMAM4 system. An optimum occurs with two competing results, diffusivity increases with aliphatic content while solubility decreases. Increasing hydrophobicity of an epoxy/amine polymer network should decrease the permeability with respect to water. What this work shows is that in a homogenous network of DGEBA and a mono-amido amine that the solubility does not decrease as much as the diffusivity increases as the pendant amine chain length is increases. Thus, there is a critical pendant chain length. Although the water cannot reach as high of a saturation content in the polymer, the water is more mobile and thus the flux of water into the polymer becomes greater as aliphatic content is increased beyond that of C4 addition. Previous work on fatty acid modified DETA showed that addition of aliphatic content via modification of DETA improved the permeability of an epoxy/amine 51 thermoset. Also, the water uptake studies were done with liquid water, whereas all water uptake presented here has been done with humid nitrogen. Chain lengths greater than C8 were also not explored. The order of magnitude for the diffusion of water in polyethylene at room temperature is on 2 the order of 10E-7 cm /s, which is much faster than that in epoxy amine thermosets, which are in the order of about 10E2 9 cm /s. Thus if one considers the AMAMn polymers to be an ideal mixture of pendant alkyl chains in a “solution” of alkyl chain free AMAMn polymer, the pendant alkyl chain appear to behave as dissolved PE.

Amidoamine (AMAMn) crosslinkers with pendant alkyl chains of four different lengths (n = 4, 6, 8, and 10) were synthesized and cured with DGEBA to generate novel crosslinked polymers. While increasing the alkyl chain length in AMAMn crosslinkers reduces the solubility of water in the polymer networks, the diffusivity of water increases. The solubility and diffusivity of water in these polymers were directly related to the concentration of oxygen in the polymer network. Although a model of free volume governing water transport were valid for the DGEBA/AMAMn formulations, it was not when considering

Acknowledgments

We acknowledge the financial support of the US Army Research Laboratory (ARL) under the Center for Sustainable Corrosion Prevention (CSCP) under cooperative agreement W911NF-13-2-0046. The authors would also like to thank the Louis Stokes Alliance for Minority Participation Bridget to the Doctorate Fellowship (LSAMP-BTD) for financial support. We 1 thank Drexel University’s Department of Chemistry for the HNMR facilities. We also thank DuPont for providing Polyvinyl fluoride, PVF (Tedlar).

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Highlights

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1. A structure-property relationship for the addition of pendant alkyl chain segments in epoxy/amine polymers is discussed. 2. A governing relationship between intermonomer oxygen concentration ([O]) and solubility of water in epoxy/amine thermosets is presented here.