Microstructural reorganization in model epoxy network during cyclic hygrothermal ageing

Accepted Manuscript Microstructural reorganization in model epoxy network during cyclic hygrothermal ageing G. Bouvet, S. Cohendoz, X. Feaugas, S. Touzain, S. Mallarino PII:

S0032-3861(17)30602-X

DOI:

10.1016/j.polymer.2017.06.032

Reference:

JPOL 19765

To appear in:

Polymer

Received Date: 29 March 2017 Revised Date:

14 June 2017

Accepted Date: 15 June 2017

Please cite this article as: Bouvet G, Cohendoz S, Feaugas X, Touzain S, Mallarino S, Microstructural reorganization in model epoxy network during cyclic hygrothermal ageing, Polymer (2017), doi: 10.1016/ j.polymer.2017.06.032. 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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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Microstructural reorganization in model epoxy network during cyclic hygrothermal ageing

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G. Bouvet, S. Cohendoz, X. Feaugas, S. Touzain, S. Mallarino*

Laboratoire des Sciences de l'Ingénieur pour l'Environnement, LaSIE UMR-CNRS 7356,

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Université de La Rochelle, Avenue Michel Crépeau, 17042 La Rochelle France

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Corresponding Author: S. Mallarino (E-mail: [email protected])

ABSTRACT

The water sorption characteristics and physico-chemical properties have been

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determined for a fully cured model epoxy systems based on the DGEBA resin and DAMP amine hardener, during cyclic hygrothermal ageing (absorption-desorptionreabsorption-redesorption). Gravimetric measurements on epoxy free films were

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carried out to follow the water sorption kinetics at different temperatures (30, 40, 50 and 60°C). The water diffusion processes follow a pseudo-Fickian behaviour with two

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diffusive stages during the first sorption and a Fickian behaviour during the first desorption. During the first sorption, a plasticization phenomenon is observed leading to a decrease of rigidity and Tg but this phenomenon is reversible, showing that water reacts only with the polymeric network via low energy links. The water diffusion processes evolve during the second absorption-desorption cycle. They follow a Fickian behaviour which implies a disappearance of diffusion heterogeneity of the first sorption. This behavior was explained by a microstructure reorganization during

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ACCEPTED MANUSCRIPT cyclic ageing highlighted through the FTIR and DMA techniques: chain fractions became accessible to the water after being relaxed in the first absorption-desorption cycle. A thermodynamic approach applied to solubility and diffusion processes

phenomena in epoxy model systems. KEYWORDS

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corroborates these results. This study allows to a better understanding of diffusion

Epoxy resin, cyclic hygrothermal ageing, network microstructure, microstructural

INTRODUCTION

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1

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reorganization, water uptake, diffusion processes, thermodynamic approach.

Among the diverse methods available to protect metals against corrosion, epoxy based paints have been widely employed due to their low cost and their efficiency in corrosive environments such as seawater. However, many environmental factors

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cause degradation and thus affect the durability of the coated systems. The aminebased epoxy resins are particularly sensitive to water absorption and their physicochemical and mechanical properties are strongly affected [1, 2].

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Therefore, the characterization of the water sorption behaviour in polymeric coatings is essential for predicting the durability of such coated systems. Numerous studies

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have focused on the water sorption through epoxy resins at different temperatures. Water absorption in epoxies can be a complex process because of the involvement of different mechanisms, as indicated in reviews on the subject [3-5]. The diffusion of water molecules in an epoxy matrix has historically been described by two approaches: volumetric and interactional approaches. These two above concepts are however generally complementary and can generally occur in the same scales of time and space [6]. The network architecture such as crosslinking density [7], free volume [8, 9] and the chemical nature of the components (polar groups) [102

ACCEPTED MANUSCRIPT 12] are intrinsic factors that strongly affect water sorption behaviour in terms of both diffusion coefficients and solubility. Several water diffusion models through the polymeric systems are encountered in the literature [13-16]. However, the most widely used model is the Fick’s model [16],

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because it is the simplest and it describes fairly well the diffusive behaviour of materials [17-19]. However, studies show deviations from this model [20-24] for the long ageing times. Several interpretations of the causes of these diffusion anomalies

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have been advanced. This is often due to the complexity of the molecular interactions between water and the resin as well as the consequence of those interactions [25],

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such as the network relaxation phenomena in order to disperse for example swelling constraints due to the water absorption [26-27], the existence of two sorbed-water populations (“free” and “bound”) [4, 15, 28-30], the presence of water clusters [31], microvoid formation [8, 32-33] and heterogeneities in the amorphous phase too

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(notion of two-phase materials) [14, 34]. Chemical modifications are also proposed in literature to explain the non-Fickian diffusion behaviour such as the advancement of

[10, 36].

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crosslinking networks, oxidation [35], or hydrolysis reactions during the conditioning

So, the mechanisms of water diffusion into polymeric materials are very complex and

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still not completely understood. When the material is subjected to several water exposure cycles, the problem is even more complex. Fernandez-Garcia et al. [37] shows that the water diffusion in a particle-filled epoxy system exhibits a non-Fickian behaviour for the absorption and the reabsorption processes. In the case of bismaleimide resin, Li et al. [38] obtained the same results. They attributed the nonFickian behaviour to the formation of hydrogen bonds between the water and the resin. The diffusion coefficients in the first cycle were lower than those of the second

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ACCEPTED MANUSCRIPT cycle. For other authors, the diffusion coefficients in the first cycle were higher than those of the second cycle. They associated these decreases with the physical microdamage induced by the sorption of the first cycle [39-41]. However, few studies have been conducted on the cycle effects on the water diffusion inside the materials

analytical tool for studying water polymer interactions.

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and the absorption-desorption-reabsorption (ADR) test appear as an effective

A previous study [42-43] concerning marine paints highlighted synergies between

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various degradation factors. However, water diffusion laws inherent in these synergies were difficult to establish because the paint formulations were complex and

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involved different compounds (binder, mineral fillers, pigments, additives). In order to continue this work, a more fundamental study [44] was realized by choosing two unfilled model materials, composed solely of the binder DGEBA/TETA and DGEBA/DAMP epoxy systems, differing only in the number of polar groups. This

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study pointed out that the water/polar group interaction mechanisms govern the water pseudo-Fickian diffusion and the solubility.

The objective of the current study is to further investigate the nature of pseudo-

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Fickian diffusion behaviour via absorption-desorption-reabsorption-redesorption hygrothermal cycling. Only the DGEBA/DAMP epoxy system has been used. This

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study should contribute to a better understanding of the nature of non-Fickian diffusion behaviour in epoxy resin. 2 2.1

EXPERIMENTAL Material

The epoxy resins investigated were prepared from a Diglycidyl Ether of Bisphenol A (DGEBA) cured with methylpentanediamine (DAMP). The formulae, supplier,

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ACCEPTED MANUSCRIPT molecular weight (M) and the functionality (F) of the products are listed in Table I. All materials are used as received without further purification. M (g.mol-1)

F

DGEBA

Sigma, D.E.RTM 322

340.41

2

DAMP

Aldrich

Formulae

Table I: Structure and characteristics of the products

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Supplier

Product

116.2

4

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A stoichiometric amount of DGEBA was added to the amine hardener, mixed at room temperature and degassed under vacuum. The mixture was transferred to a

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mould, which consisted of two Teflon sheets which are separated by a spacer of about 120 µm thick.

A controlled curing protocol was used to create a homogeneous fully cured network, according to Tcharkhtchi et al. recommendation [45]. Two isotherm

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temperatures were chosen and two intermediary isotherms were used to avoid the emergence of a vitrification phenomenon. Next, a post-curing step above Tg was realized to place the material in a thermodynamic equilibrium state. So, a crosslinking

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protocol was defined as: 7 hours at 30°C, followed by 3 hours at 60°C, followed by 3 hours at 80°C followed by 3 hours at 100°C followed by 3 hours at 120°C, and

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postcured for 1 hour at 130°C.

The system was cooled to room temperature with a rate of 10°C.min-1 to avoid

physical ageing. The fully cured epoxy specimens [44] were stored in a desiccator containing silica gel desiccant to prevent moisture absorption before immersion. 2.2

Physico-chemical and mechanical characterization methods FTIR, DSC and DMTA were used to experimentally characterize the chemical

structure and the physical properties of studied system.

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ACCEPTED MANUSCRIPT 2.2.1 FTIR Fourier transform infrared (FTIR) analyses of cured materials were carried out by Thermo-Nicolet Magna IR 760 spectrometer equipped with a Smart MIRacle ATR accessory with a diamond crystal. Spectra were collected over a range 600–4000 cmwith a resolution of 4 cm−1. Each spectrum was produced by coaddition of 128

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1

scans. 2.2.2 DSC

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The glass transition temperature (Tg) measurements were performed with the TA Instruments Q100 Differential Scanning Calorimetry (DSC). The epoxy resins

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were scanned from 20 to 200°C at 10°C.min-1, under nitrogen flow. The glass transition temperature Tg is taken at the half height of the change in heat capacity (middle of transition). 2.2.3 DMTA

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Dynamic Mechanical Thermal Analyzer (DMTA) - tensile mode (Q800 TA Instruments) was used to investigate the dynamic mechanical properties of resins. 2.2.3.1 Temperature sweeps

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Samples were heated from -100°C to 180°C with a heating rate of 3°C.min-1. The first series of tests were performed with a strain amplitude of 15µm at a fixed

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frequency of 1 Hz. Considering the tan δ

curve, the temperature at the higher

maximum was recorded as the Tg, and the temperature at the lower maximum was recorded as the Tβ (β-relaxation). 2.2.3.2 Tensile tests Elastic properties were characterized by tensile tests with DMTA. After an isotherm step at 35°C during 24h, the free films were loaded with a constant stress rate of 10MPa.min-1 in order to measure the Young’s modulus.

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ACCEPTED MANUSCRIPT 2.2.3.3 Relaxation tests In order to study thermodynamic processes associated with visco-elasticity and visco-plasticity, relaxation tests were performed using DMTA–tensile mode. Using classical thermal activity theory initiated by Eyring [46] and applied for solid

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polymers [47-49], the strain rate sensitivity of the flow stress can be directly linked with molecular processes. Especially, the activation volume, Va is associated with the jump of a molecular segment over energy barrier [50]. In stress relaxation test, Va is

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expressed as follows:

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 d ln(ε&v )  Va = k BT ⋅    dσ  µs ,T with ε&v = − σ& E

(Eq.1)

(Eq.2)

where kB is the Boltzmann constant, (µs, T) are a constant microstructure and temperature,

ε&v

is the strain rate,

is the stress rate and E is the Young modulus.

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Different stress relaxation tests were carried out following an identical experimental protocol. After an isotherm step at 35°C during 24h, a constant strain was applied

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onto the sample and the stress evolution was recorded during the stress relaxation test. The evolution of strain rate can then be calculated using Eq.2. The activation

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volume Va was determined, for the applied constant strain, by using the slope of the curve ln

.

By repeating these tests for various imposed strain values (0.2%, 0.3%, 0.5%, 1%, 2%, 3%, 4% for the initial state and 0.2%, 0.3%, 0.5%, 2%, 4% for the desorbed state), several activation volume values can be determined. Their evolutions can then be followed as a function of the initial stress, resulting from the imposed strain.

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ACCEPTED MANUSCRIPT 2.3

Hygrothermal ageing The free film specimens (around 20 cm²) were immersed in deionized water at

30, 40, 50, and 60°C, respectively. After 6 weeks of immersion, the specimens were desorbed under vacuum (8mbar) at the same temperature during 2 weeks. Next, a

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second hygrothermal ageing cycle was realized reproducing these two immersiondesorption steps.

The samples were regularly removed from the solution, carefully wiped with a

χm

(%) by the specimens was calculated with the

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precision). The water content

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filter paper and mass measures were performed with a PRECISA balance (10-5 g

following expression:

χm (%) =

m(t ) − m0 ⋅100 m0

(Eq.3)

where m(t) is the mass of the wet specimen at time t, m0 is the mass of the dry

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specimen. Each point in the sorption curves represents the average of three experiments. The average standard deviation corresponds to a value <0.05% on the

Physico-chemical and mechanical characterization at wet state

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2.4

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χm (%) scale.

2.4.1 Sorption isotherms Sorption isotherms were realized on both initial dry sample and 1 cycle-aged

sample with an automatic vapour adsorption instrument (BELSORP-aqua3). These sorption isotherms were carried out à 40°C after an initial desorption step at the same temperature.

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ACCEPTED MANUSCRIPT 2.4.2 DSC The evolution of Tg during first sorption and desorption was followed by DSC thanks to stainless steel-hermetic capsule in order to avoid water evaporation. At different ageing times, a sample part was removed and a thermogram was performed

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from 20 to 200°C at 10°C.min-1 under nitrogen flow. These tests were performed for both extreme temperature of ageing, namely 30°C and 60°C. 2.4.3 DMTA

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The mechanical properties of DGEBA/DAMP at saturated state were

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characterized by DMTA using submersible clamp in tensile mode (Figure I). Mobile clamp

Fixed clamp

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Thermocouples

H2O (Controlled T°)

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Free film

DMA oven (Controlled T°)

Figure I : Diagram of submersible clamp in tensile mode

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A water-saturated sample was quickly removed from 30°C ageing tank and placed between DMTA submersible clamps (Figure I). Then, the DMTA tank was filled over sample level by 30°C deionized water, to allow total immersion. After a 30°C isotherm step, tensile tests and stress relaxation tests were performed with the same protocols than those explained in part 2.2.3.2 and 2.2.3.3. 3

RESULTS AND DISCUSSION

The first stage of this study aims to follow the water diffusion processes of the system during the first sorption-desorption cycle, and its impacts on the physico-chemical 9

ACCEPTED MANUSCRIPT properties of system. The second step focuses on the evolution of water diffusion processes during the second sorption-desorption cycle. Then, a microstructural reorganisation theory is presented. Hygrothermal ageing

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3.1

3.1.1 First sorption-desorption cycle 3.1.1.1 Diffusion phenomenon 3.1.1.1.1 First sorption

χm

within the DGEBA/DAMP networks as a function of square root of time t

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uptake

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The results of the gravimetric monitoring allow plotting the evolution of water

divided by the thickness e of the sample for the four studied temperatures (Figure II). The similar water uptake values for the three samples (for each temperature) show a good reproducibility of the measurements.

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3.5 3.0 2.5

3.0

2.5

II

30°C 40°C 50°C 60°C

III

2.0

EP

χm (%)

2.0

DGEBA/DAMP

χm (%)

1.5

1.5

I

1.0

1.0

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0.5

0.5

40°C

0.0 0

20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

t1/2/e (s1/2/cm)

0.0

0

20000 40000 60000 80000 100000 120000 140000 160000 180000

t1/2/e (s1/2/cm)

Figure II: Sorption curves of DGEBA/DAMP networks, immersed in H2O at different temperatures The sorption curves show behaviour over time which can be divided into three successive stages (independent of the microstructure and temperature):

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ACCEPTED MANUSCRIPT Step I: rapid diffusion of the water in the network involving a large increase in the mass content as a function of time; Step II: slow diffusion of water in the network involving low intake over time;

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Step III: saturation step involving no change in water content. In the case of a pure Fickian diffusion, sorption curves would show a diffusion step (Step I) followed by a saturation step (Step III). The presence of stage II in our case highlights a pseudo-Fickian behaviour, which has been related in several

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studies [13, 26, 51]. Bouvet and al. [44] explained this pseudo-Fickian behaviour by a

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Fickian diffusion with an evolution of the mean apparent diffusion coefficient during time. This evolution presents three steps: two stationary regimes surrounded by a transient regime. The authors defined two diffusion coefficients, linked to stationary regimes: D1 and D2 related to beginning and the end of sorption respectively. 3.1.1.1.2

First desorption

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Gravimetric monitoring was carried out during the desorption step to follow the DGEBA/DAMP system behaviour during this step. The desorption curves for the four

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studied temperatures are presented Figure III.

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ACCEPTED MANUSCRIPT 3.5 3.0

30°C 40°C 50°C 60°C

2.5

χm (%)

2.0

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1.5 1.0 0.5 0.0

10000 20000 30000 40000 50000 60000 70000 80000 90000 100000

t1/2/e (s1/2/cm)

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0

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Figure III: Desorption curves of DGEBA/DAMP system for the four studied temperatures For all temperatures, the desorption process consists of two steps: a step of rapid water desorption, followed by a stationary step where the water content is constant. The content at equilibrium (Table II) is zero for all temperatures, which

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indicates that the material does not have any residual water in its microstructure, i.e.

Temperature

30°C

40°C

50°C

60°C

χm∞ (%)

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that there is no water strongly linked, or irreversibly trapped.

0,00±0,02

0,00±0,02

0,01±0,04

0.01±0.03

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Table II: Equilibrium water content during the desorption A heat-activated desorption phenomenon can be noted, with a faster decrease

of the water content with increasing temperatures. The diffusion coefficients are calculated during the desorption step for the four studied temperatures, and presented in Table III. Temperature 30°C 40°C 50°C 60°C Ddesorption 8.2x10-10 13.8 x10-10 19.8 x10-10 35.3 x10-10 (cm²/s) Table III: Values of desorption diffusion coefficients of DGEBA / DAMP system 12

ACCEPTED MANUSCRIPT Contrary to the first sorption, the diffusion coefficients are constant during the desorption step at a constant temperature, showing a purely Fickian diffusion behaviour, which involves an evolution of the diffusion behaviour between sorption and desorption steps. This will be discussed later in this article. Given the nature of

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the Arrhenius diffusion, the thermodynamic parameters of the desorption are determined with the equation (Eq. 4) and compared to those of the sorption step (Figure IV and Table IV).

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 ∆G  %  ∆S   ∆H   −∆H  D = D% 0 exp  −  = D0exp   exp  −  = D0exp    RT   R   RT   RT 

(Eq. 4)

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% ≈ν ⋅Γ (ν: jump frequency, Γ : jump number), ∆ S and ∆H respectively the with D 0 entropy and enthalpy associated with the energy barrier to overcome ∆ G . -19.0 -19.5

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-20.0 -20.5

Ln(D)

-21.0 -21.5 -22.0

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-22.5

D1 Sorption D2 Sorption 1st Desorption

-23.0 -23.5

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-24.0 -24.5

3.0

3.1

1000/T (K-1)

3.2

3.3

Figure IV: Evolution of diffusion coefficients D1, D2 and Ddesorption in function of the inverse of the temperature Arrhenius parameters ∆H (kJ/mol)

D1

D2

Ddesorption

40

24

40

-

D0 (cm²/s)

-

4.6x10

4.6x10

3

3

5.7x10

-3

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ACCEPTED MANUSCRIPT Table IV: Thermodynamic parameters related to the sorption and desorption processes of DGEBA/DAMP system The enthalpy of water diffusion at the beginning of sorption and during the desorption step present the same values, meaning that the mechanisms involved during the desorption step needs the same energy than that of the beginning of

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sorption.

Regarding the pre-exponential factor D0, a slight increase is recorded in the case of desorption, which can be seen as an increase in the number of possible

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diffusion paths in relation to the beginning of sorption. This may be an indication of a microstructural evolution of the system, involving an evolution of the diffusive

3.1.1.2 Evolution of properties

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behaviour (pseudo-Fickian sorption to Fickian desorption).

The water diffusion phenomena in DGEBA/DAMP system was observed, and highlighted microstructure-property relationships. Now, the consequences of diffusion

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processes on microstructural properties of the polymer are studied. 3.1.1.2.1 Mechanical properties

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The tensile behaviour of DGEBA/DAMP system is analysed, specifically the evolution of the Young's modulus during immersion. Figure V shows three tensile

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curves from 0 to 20 MPa, for an initial (non-aged), a water saturated and a desorbed free films. The first part of the stress-strain curves allows determination of the Young's modulus, whose values are specified in Table V.

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ACCEPTED MANUSCRIPT 20 18 16 14

Initial Saturated Desorbed

10 8 6 4 2 0 0.1

0.2

0.3

0.4

0.5

0.6

ε (%)

0.7

0.8

0.9

1.0

1.1

1.2

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0.0

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σ (MPa)

12

Ageing State Initial (dry)

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Figure V: Tensile curves of DGEBA/DAMP free film in the initial, water saturated and desorbed state. Young’s modulus (MPa) 2100 ± 55

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Water saturated 1940 ± 58 Desorbed (without residual 2110 ± 58 water) Table V: Young's moduli of the DGEBA/DAMP system in different states of ageing

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Figure V shows a small decrease in the rigidity of the material in the saturated state (10% decrease). These results indicate the presence of a network plasticization

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phenomenon by diffusing water molecules. Indeed, in the most hydrophilic polymers (polyepoxide, for example), when water molecules enter a macromolecular network, they break secondary bonds between the polar groups to establish hydrogen bonds. Therefore, the breaking of these bonds leads to a decrease of the network cohesion, so an increase of the molecular mobility, which decreases the Young's modulus [52]. Moreover, these results show the same mechanical properties of the DGEBA/DAMP system at desorbed and initial states, although the presence of a drop in rigidity in the water-saturated state. This indicates a fully reversible plasticizing 15

ACCEPTED MANUSCRIPT effect of the water concerning the rigidity of the system. Indeed, at the desorbed state, the water molecules are completely removed from the network, allowing reformation of these secondary bonds, and therefore the recovery of the rigidity at its initial value.

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To complete the characterization of the desorbed state, the activation volumes of DGEBA-DAMP system were determined, both at desorbed state and at the initial state. The results are shown Figure VI.

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100

Initial Saturated Desorbed

90

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80 70

3

Va (nm )

60 50 40

20 10 0

10

σ (MPa)

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1

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30

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Figure VI: Activation volumes of DGEBA/DAMP system at the initial and desorbed state The activation volumes in the saturated and desorbed state are identical to

those of the initial dry state. Apparently, activation volumes are not affected by the water. However, at the saturated state, previous results showed a plasticization effect by water which should lead to an increase of the activation volume. Then, it can be proposed that an opposite effect exist and is related to the hindering of the macromolecular chains by the water molecules. Finally, for the desorbed state, the

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ACCEPTED MANUSCRIPT macromolecular chains have regained their initial degree of freedom during desorption. 3.1.1.2.2 Thermodynamical properties

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Numerous studies [28, 32, 53] showed that the plasticizing phenomenon of epoxy networks by water leads to a decrease of the glass transition temperature (Tg) during immersion. In order to follow this decrease, Tg measurements on free films were performed by DSC, at initial dry stage and at different immersion times [44]. The

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Tg evolution depending on the water content for the two extreme ageing

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temperatures is expressed in Figure VII for the first sorption and first desorption steps. 125

30°C_1st Sorption 30°C_1st Desorption 60°C_1st Sorption 60°C_1st Desorption Fox's law

120

110 105

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Tg (°C)

115

EP

100

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95 0.0

0.5

1.0

1.5

χm (%)

2.0

2.5

3.0

Figure VII: Tg evolution of the DGEBA/DAMP system depending on the water content at 30°C and 60°C, for the step of sorption and desorption. Figure VII shows a linear behaviour of the Tg with the mass content, independently of the ageing temperature. This evolution follows the empirical law of Fox [54-56]:

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ACCEPTED MANUSCRIPT f polymer f 1 = water + Tg Tg water Tg polymer , dry

(Eq. 5)

fwater , f polymer are respectively the water and polymer mass fractions, and Tgwater ,Tg polymer,dry are respectively the water Tg and the polymer Tg in the initial dry

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state. Several authors [57-58] proposed the existence of a water Tg, whose the value is around 134K.

Therefore, the plasticization by water depends only on the amount of the

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absorbed water, as already proposed by Maggana and Pissis [14]. The

decrease with the water content.

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phenomenological interpretation is the same as that explaining the Young's modulus

Furthermore, Figure VII confirms reversibility of the plasticization by water. Indeed, during the desorption, a re-increase in Tg was recorded with decreasing water content of the polymer, to reach the desorbed system Tg equal to the initial Tg.

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The microstructural explanation of this reversibility (reformation of secondary bonds) is then the same as that proposed for the evolution of the Young's modulus.

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3.1.1.2.3 Thermomechanical properties Although the physico-chemical and mechanical properties are clearly

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reversible, it is useful to look at the microstructural analysis of the material at initial and desorbed states. To do this, temperature sweep tests using DMA were realized. The dynamic mechanical properties (storage modulus E’ and the tan δ), measured at 1Hz, are showed in Figure VIII.

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ACCEPTED MANUSCRIPT E'

0.8 0.7 0.6

1000

E' (MPa)

0.5 0.4

Tan δ

Initial Desorbed

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0.3

100

0.2

Tan δ

0.1 0.0

10

-0.1

-80

-60

-40

-20

0

20

40

60

100 120 140 160

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Temperature (°C)

80

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Figure VIII: Evolution of storage moduli (E') and tan (δ) versus temperature for the DGEBA/DAMP system, in the initial and desorbed state Two significant relaxations can be seen, both at initial state. The main relaxation, named α-relaxation and corresponding to the glass transition temperature, occurs at 130°C±1°C (considering the maximum of the tan δ curve). This value

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obtained by DMA differs from that obtained by DSC (TgDSC= 122°C). Indeed, the DSC considers the thermodynamic system (influenced by the temperature change rate), while the DMA accesses thermomechanical quantities of the material

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(influenced by the mechanical stress frequency). Thermomechanical analysis highlights another relaxation, at a temperature of -35°C (tan δ curve), described as

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secondary relaxation or β-relaxation [59-65]. The presence of this relaxation may due to the vibrations of hydroxypropyl groups and crosslinking movements [59, 63-65]. Otherwise, no change in Tg between the initial state and the desorbed state

was observed, confirming the results of the DSC. Nevertheless, a slight change in the β-transition can be noticed. This trend is more visible in Figure IX, which shows a shrinking of the β-relaxation peak width for the desorbed system.

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0.07 0.07 0.06 0.06 0.05 0.05 0.04 0.04

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0.03 0.03

0.02

Initial Desorbed

0.02 0.01

0.01 0.00

0.00

-80

-60

-40

-20

0

Temperature (°C)

20

40

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Tan δ (Initial DGEBA/DAMP)

0.08

Tan δ (Desorbed DGEBA/DAMP)

ACCEPTED MANUSCRIPT

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Figure IX: Evolution of Tan δ as a function of temperature for the DGEBA/DAMP system in the initial and desorbed state According to Cukierman et al. [66], the β-relaxation peak width can be linked to the difficulty of movement for hydroxypropylether units. So, a narrowing of the peak width indicates an easier and more homogeneous movements of those units.

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Therefore, in the initial state, a fraction of such hydroxypropylether groups was more stressed than the others, needing more (heat) energy to relax, leading to a wider βrelaxation peak. At the desorbed state, the hygrothermal history of the material allows

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to relax these stressed network section, and finally to homogenize the microstructure.

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3.1.1.2.4 FTIR spectrum

To ensure that the chemical structure of system has not been changed by the

sorption-desorption cycle, FTIR spectroscopic analysis was carried out on initial and desorbed free films (Figure X).

20

ACCEPTED MANUSCRIPT 0.30 3403

3036

2964

1383 2828

2929

0.25

1297

1510

1249 1414 1608 1362 1183 1459 1085 1581 1107 1037

2870

0.20

829

Initial DGEBA/DAMP 0.15

0.00

2964

0.20

Desorbed DGEBA/DAMP

1085

1180

1459

1362

3378

1032 829

1107

2828

1581

0.10

3500

3000

M AN U

ATR cristal response

0.05 0.00 4000

1238

1414 1297

1510

2929

3036

2870

0.15

1383

1608

0.25

SC

Absorbance (U.A)

0.05

RI PT

ATR cristal response

0.10

2500

2000

1500

1000

Wavenumber (cm-1)

TE D

Figure X: FTIR spectra of initial and desorbed DGEBA/DAMP systems (the position at 915cm-1 is given by the dashed line) For the initial DGEBA/DAMP spectrum, the found major bands are in agreement with the results reported in literature concerning epoxy systems [67-69]. It

EP

can be noticed the absence of adsorption band around 915 cm-1 (dashed line in Figure X) assigned to the vibration of the epoxide groups. The FTIR spectrum

AC C

suggests a complete crosslink of the DGEBA/DAMP network, confirming the DSC and the DMA results. Moreover, the desorbed DGEBA/DAMP spectrum does not show the appearance or disappearance of absorbance peak, which means that there is no significant difference in the chemical structure of the desorbed material. Therefore, the system does not present any chemical degradation of its chemical structure at the end of a sorption-desorption cycle.

21

ACCEPTED MANUSCRIPT However, some peaks record a shift toward lower wavenumbers between the initial and the desorbed state. These peaks (1037, 1183, 1249 and 3403 cm-1) are attributed to the vibrations of the ether and alcohol groups constituting the hydroxypropylether group. This shift is related to the nature of the interaction forces

RI PT

of concerned links and then to the stress state of the microstructure, as shown by some studies [70]. Indeed, if stressed hydroxypropylether groups in the initial state can be found relaxed at the desorbed state (as suggested by the results of DMA

SC

above), the interaction forces of the links become weaker, and the peaks are shifted

M AN U

towards lower values of wavenumbers. 3.1.2 Second sorption-desorption cycle

The behaviour of the system in a second sorption-desorption cycle is followed to study the consequences of the microstructural reorganization on water diffusion behaviour. The sorption-desorption curves of the second cycle are presented Figure

TE D

XI:

3.0

EP

2.5

χm (%)

2.0

1.5

AC C

1.0

a)

0.5

0.0 0

20000

40000

60000

80000

100000

3.5 3.0 2.5

30°C 40°C 50°C 60°C

2.0

30°C 40°C 50°C 60°C

χm (%)

3.5

1.5 1.0

b)

0.5 0.0 120000

t1/2/e (s1/2/cm)

140000

160000

180000

0

10000 20000 30000 40000 50000 60000 70000 80000 90000 100000 1/2 1/2

t /e (s /cm)

Figure XI : Evolution of water content of DGEBA/DAMP system for four studied temperature in the second sorption (a) and second desorption (b) A pure Fickian diffusion behaviour during these two steps can be observed, regardless of the considered temperature, which indicates the disappearance of the diffusion heterogeneity present in the first sorption and is the major difference 22

ACCEPTED MANUSCRIPT between the first and the second sorption. This diffusion in the second sorption and desorption involves two steps: a fast water uptake step, followed by a stabilization step. At the end of the second desorption, the water contents are zero, indicating a total desorption of water, like the first desorption. It can then be concluded in a first

RI PT

approach that the water uptake phenomenon of system is completely reversible.

Concerning the equilibrium water content during the second sorption, a slight change is observed, as shown in Table VI.

60°C

2.91±0.04 3.07±0.03

M AN U

SC

30°C 40°C 50°C χm 1st sorption 2.80±0.04 2.81±0.03 2.85±0.01 (%) χm 2nd 2.91±0.04 2.99±0.03 3.02±0.02 sorption(%) Table VI: Equilibrium water content during first and second sorption

The equilibrium water contents are very slightly affected by temperature for both sorption. However, a slight increase of equilibrium water content is observed (+0.15%) in the second absorption. This means that the system has a slightly higher

TE D

solubility, which can be calculated (as detailed elsewhere [71]) for the first and second sorption and grouped in Table VII. Their evolutions with temperature are shown Figure XII. 30°C

40°C

50°C

60°C

AC C

EP

S1st sorption (mol.m-3.Pa) 0.434 0.253 0.154 0.097 S2nd sorption (mol.m-3.Pa) 0.446 0.269 0.163 0.103 Table VII: Water solubility of the DGEBA/DAMP network for different temperatures and for two sorption steps

23

ACCEPTED MANUSCRIPT -0.6

1st Sorption 2nd Sorption

-0.8 -1.0

-1.4 -1.6 -1.8 -2.0 -2.2 -2.4 3.1x10-3

3.2x10-3 -1

1/T (K )

3.3x10-3

SC

3.0x10-3

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Ln (S)

-1.2

M AN U

Figure XII: Evolution of the solubility vs. 1/T for two sorption steps

The solubility increases slightly in the second sorption, explaining higher saturation levels. The solubility depends on Arrhenius parameters such as:

with

(Eq. 6)

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 −H  S = S0 exp  s   RT 

Hs the enthalpy of solubility and S0 the pre-exponential factor, related to the

availability of polar group for a water molecule.

EP

Then, the enthalpy values of solubility and pre-exponential factor are calculated from

AC C

(Eq. 6) and expressed in Table VIII.

Hs (kJ.mol-

S0 (mol.m-

1

3 ) .Pa) 1st Sorption -42 2.7x10-8 2nd sorption -41 3.6x10-8 Table VIII: Thermodynamic parameters related to the process of water dissolution in the DGEBA/DAMP system during the two stages of sorption

Since the water-polymer interactions are the same as in the first sorption, the energy released by the dissolution process is identical, which explains the similar values of solubility enthalpy. The pre-exponential factor increases slightly in the

24

ACCEPTED MANUSCRIPT second sorption, revealing an increase of the number of available hydrophilic sites in the microstructure. This increase is due to the availability of polar groups near to the crosslinking nodes, initially inaccessible during the first sorption. The relaxation of some hydroxypropylether units, as revealed by FTIR and DMA results, has allowed

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the release of inaccessible polar groups, which slightly increases the material solubility.

To complete this analysis, sorption isotherms were performed for the second

SC

sorption in order to evaluate if the water diffusion law has changed. Isotherms of the first and second sorption are plotted Figure XIII, showing two similar evolutions

M AN U

between the first and second sorptions, whose characteristics have been described above in this article.

1800 1600

1st Sorption 2nd Sorption

1400

1000 800 600

EP

400

χm (%)

TE D

1200

C (mol.m-3)

2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

200 0

1000

AC C

0

2000

3000

4000

5000

6000

7000

pe (Pa)

Figure XIII: Sorption isotherms of DGEBA/DAMP system during the first and second sorption However, the slope of the isotherm of the second sorption is higher, expressing a higher solubility. The deviation from the right to the high equilibrium pressures is the same for both sorptions, revealing the same water cluster phenomenon into the material. So, this indicates an independence of this 25

ACCEPTED MANUSCRIPT phenomenon with the hygrothermal history of DGEBA/DAMP system. Although this positive deviation is identical to that of the first sorption, the limit values between the two regimes have changed between the first and the second sorption, as a result of changes in the solubility. The second regime appears to an equilibrium value of

RI PT

pressure of about 3800 Pa and an equilibrium water content about 1.22%.

After studying the system behaviour in a steady state, the kinetic aspect of diffusion in the second sorption-desorption cycle is analysed. As the first desorption,

SC

the diffusion coefficients of the second cycle are constant for the sorption and the desorption steps, showing a purely Fickian behaviour. The values of these

XIV. 30°C 11.2x10-10 9.2x10-10

40°C

50°C

60°C

16.5x10-10

29.1x10-10

44.0x10-10

13.6x10-10

21.4x10-10

38.2x10-10

AC C

EP

TE D

Temperature D 2nd Sorption (cm²/s) D 2nd Desorption (cm²/s)

M AN U

coefficients are given in Table IX and their evolutions versus 1/T are plotted Figure

26

ACCEPTED MANUSCRIPT Table IX: Diffusion coefficient values of DGEBA/DAMP system for the second ageing cycle -19.0 -19.5 -20.0

RI PT

-20.5

Ln(D)

-21.0 -21.5

D1 Sorption D2 Sorption 1st Desorption 2nd Sorption 2nd Desorption

-22.0 -22.5

SC

-23.0 -23.5 -24.0 -24.5 3.1

3.2

M AN U

3.0

1000/T (K-1)

3.3

Figure XIV: Evolution of Ln(D) of the DGEBA/DAMP system in function of the inverse of temperature for all stages of ageing An evolution of the diffusion coefficients can be noted at each ageing step: an

TE D

increase between the first sorption and first desorption, another increase for the second sorption, and a slight decrease for the second desorption to be close to the value of the first desorption. In order to characterize these evolutions, the Arrhenius

AC C

XVI.

EP

parameters of all these steps are calculated and represented Figure XV and Figure

27

ACCEPTED MANUSCRIPT

∆H (kJ/mol) 40.0

39.7

RI PT

24.41

Desorption

2nd Sorption 2nd Desorption

SC

Sorption (D1) Sorption (D2)

39.5

39.2

M AN U

Figure XV: Diffusion enthalpy of the different stages of ageing of the DGEBA/DAMP system

Do (cm²/s) 4.6E-03

6.1E-03

5.6E-03

TE D

5.7E-03

EP

4.6E-07 Desorption

2nd Sorption 2nd Désorption

AC C

Sorption (D1) Sorption (D2)

Figure XVI: Pre-exponential factor of the different stages of ageing of the DGEBA/DAMP system

In Figure XV, the enthalpy of diffusion is roughly the same for all the diffusion steps, excluding the value obtained for D2 due to its origin as explained above. Consequently, the microstructural changes that occurred during the first cycle do not impact the diffusion energy, indicating the same diffusion process.

28

ACCEPTED MANUSCRIPT In Figure XVI, a slight increase in the pre-exponential factor D0 can be seen from the desorption step. From Equation (Eq. 4), this increase involves a larger number of possible paths for diffusion of a water molecule, attributed to the

of the first sorption step. 3.2

Microstructural reorganization

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availability of some polar groups during the microstructural reorganization at the end

The overall results allow us to propose a scenario explaining the different

An initial microstructure without water is characterized by macromolecular

M AN U

•

SC

processes:

chains with polar sites. Due to internal stresses developed during the material preparation, some polar sites are stressed and inaccessible to water. •

Early in the first sorption, water molecules penetrate the microstructure and

site to polar site. •

TE D

establish hydrogen bonds with the polar sites, performing jumps from polar

At the end of the first sorption, the large number of water molecules and the

EP

energy provided by the water diffusion allows some inaccessible polar sites to relax, making them accessible to water molecules. This causes a slow

•

AC C

diffusion of water molecules until saturation. During desorption, the energy provided by the water diffusion to the outside, and low external pressure (8mbar) allows the relaxation of the inaccessible polar sites, which creates a more rapid and uniform diffusion through the availability of more hydrophilic sites (increase of diffusive paths).

•

At the 2nd sorption, the material becomes totally relaxed and homogeneous, allowing a purely Fickian diffusion and faster than the first sorption thanks to

29

ACCEPTED MANUSCRIPT more diffusive paths. The increase of accessible polar sites involves higher solubility with slightly higher equilibrium water content. •

The 2nd desorption is realized in the same way as the first desorption, the

4

RI PT

material is completely relaxed, with all its hydrophilic sites available. CONCLUSIONS

This paper was devoted to the study of the environment influence on the possible

•

SC

evolution of the microstructure and properties of DGEBA/DAMP free films:

The vacuum step of samples leads to a purely Fickian water desorption with

•

M AN U

zero content at equilibrium: water is completely desorbed from material. The water absorption by the polymer causes a plasticization by water, involving a decrease in stiffness and Tg. This phenomenon is nevertheless completely reversible with the total desorption of water. The material keeps a structural and chemical integrity after a sorption-

TE D

•

desorption cycle. Water reacts only through low-energy bonds with the DGEBA / DAMP system.

The second sorption-desorption cycle is a purely Fickian phenomenon, which

EP

•

implies a disappearance of diffusion heterogeneity of the first sorption. This

AC C

behavior is caused by a microstructural reorganization during cyclic ageing: chain fractions became accessible to water after been relaxed during the first sorption and desorption.

All these results enable the better understanding of diffusion phenomena in model epoxy system and the development of structure-property relationships.

30

ACCEPTED MANUSCRIPT ACKNOWLEDGMENT We want to thank the National Research Agency (ANR) for the financial support of this study and the PhD thesis of Geoffrey BOUVET through the project

technical support concerning the BELSORP-aqua3 tests.

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RI PT

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SC

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TE D

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TE D

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[71] D. Nguyen Dang, S. Cohendoz, S. Mallarino, X. Feaugas, S. Touzain. Effects of Curing Program on Mechanical Behavior and Water Absorption of DGEBA/TETa

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Epoxy Network, J. Appl. Polym. Sci., 129(5) (2013) 2451-2463.

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Evolution of physico-chemical properties for model epoxy systems during cyclic ageing Determination of water sorption characteristics and use of thermodynamic approach Reversible plasticization by water of DGEBA/DAMP epoxy free films Evolution of water diffusion processes: disappearance of the diffusion heterogeneity Microstructural reorganization during cyclic ageing: relaxation of chain fraction

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