Variation in the Molecular Dynamics of DGEBA Confined Within AAO

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Variation in the Molecular Dynamics of DGEBA Confined Within AAO Templates Above and Below the Glass Transition Temperature Magdalena Tarnacka, Mateusz Dulski, Monika Geppert-Rybczy#ska, Agnieszka Talik, Ewa Kaminska, Kamil Kaminski, and Marian Paluch J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07522 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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The Journal of Physical Chemistry

Variation in the Molecular Dynamics of DGEBA Confined within AAO Templates Above and Below the Glass Transition Temperature Magdalena Tarnacka †‡*, Mateusz Dulski $‡, Monika Geppert-Rybczyńska §, Agnieszka Talik †‡, Ewa Kamińska #, Kamil Kamiński†‡*, Marian Paluch†‡ †Institute of Physics, University of Silesia, 75 Pulku Piechoty 1, 41-500 Chorzow, Poland ‡Silesian Center of Education and Interdisciplinary Research, University of Silesia, 75 Pulku Piechoty 1A, 41500 Chorzow, Poland $ Institute of Materials Science, University of Silesia, 75 Pulk Piechoty 1, 41-500 Chorzow, Poland § Institute of Chemistry, University of Silesia, Szkolna 9, 40 -006 Katowice, Poland # Department of Pharmacognosy and Phytochemistry, Medical University of Silesia in Katowice, School of Pharmacy with the Division of Laboratory Medicine in Sosnowiec, Jagiellonska 4, 41-200 Sosnowiec, Poland * Corresponding author: (MT) [email protected] (KK) [email protected]; [email protected]

Abstract In this paper, we have investigated the molecular dynamics above and below the glass transition temperature of bisphenol-A diglycidyl ether (known as DGEBA, Mn = 340 g/mol) infiltrated in nanoporous alumina (AAO) templates of various pore size by means of Broadband dielectric and Raman spectroscopies. It was found that the temperature dependence of the structural relaxation times is different under confinement with respect to the bulk sample even in the high temperature regime. Interestingly below the glass transition temperature, the slow secondary process (β) was not detected in dielectric loss spectra of confined DGEBA; while the relaxation times of the faster secondary process (γ) were unaffected by the pore size. In order to explain this phenomenon, two different scenarios, considering either suppression of the motions related to this mobility or enhancement of its dynamics, were taken into account. Additional annealing experiments, that lead to density perturbation, enabled us to recover bulk-like temperature dependence of structural relaxation

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times for all confined systems. This finding was discussed in view of the outcome of Raman and Contact Angle measurements, that have shown rather weak interactions between DGEBA and template. It is also worthwhile to add that except for the clear broadening of the fast secondary relaxation peak, the relaxation times of this process varied within experimental uncertainties due to annealing. I.

Introduction

In the glassy state (below the glass transition temperature, Tg) of numerous compounds, secondary relaxation processes related to some local and non-correlated molecular motions can be detected. Accordingly to their molecular origin, one can classify them into two types: (i) those having intermolecular character (so-called Johari-Goldstein (JG) relaxations) and (ii) those of intra-molecular characteristic (caused by the motion of only a part of the molecule, non-JG)1. However, the former ones are the most important, since they are considered to be directly related to the structural relaxation (responsible for the liquid-to-glass transition). In this context, it should be mentioned that secondary relaxations, that are genuine JG type, satisfy the Coupling Model (CM) prediction2,3,4 as well as in most cases, they are sensitive to the sample densification. The close correlation between the JG and the structural process has been tested and verified in the majority of glass formers of varying interactions, in binary mixtures, and at different thermodynamic conditions4,5,6. However, not much has been done to probe the connection between the structural and secondary relaxation processes under nanometric spatial confinement, which seems to be a powerful strategy to explore more deeply the dynamics of the soft matter. One can recall that the properties of the confined materials often differ notably from the bulk. As widely reported in the literature, the molecular dynamics above Tg is often enhanced in the vicinity of the glass transition temperature for the liquids incorporated into nanoporous templates or deposited as thin films on various substrates. Although, recent 2 ACS Paragon Plus Environment

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studies have indicated that the change in the dynamics of confined materials might be strongly dependent on the thermal history, annealing of the sample etc.

7,8.

As reported for the spin-

coated polymer thin films (1D confinement), the prolonged annealing time (longer than reptation time, τrep) results in the recovery of the bulk-like properties due to the perturbation of polymer density at the substrate surface and the increase in the number of irreversibly adsorbed chains9,10. Note that this effect was observed for various polymers deposited on aluminum substrates. Similar behavior was also noted for the polymers as well as low molecular

glass

formers

incorporated

into

nanoporous

alumina

templates

(2D

confinement)11,12. However, in this case, the density perturbation resulting in the variation of the structural or segmental relaxation times and Tg was observed only at some specific temperatures, located below the temperature of vitrification of the polymers/molecules attached to the pore walls11,12. At these conditions, aging of the interfacial layer affects the motions of the molecules located at the center of the nanochannels13. Consequently, bulk-like molecular dynamics of the sample infiltrated into the porous matrix might be recovered11,12,13. Aside from studying the dynamics of structural/segmental relaxation processes under various kinds of nanometric spatial restriction, the behavior of secondary relaxation is far from being established at these specific conditions. One can recall studies on systems incorporated within nanoporous silica based materials (SBA-15 and MCM-41), i.e., ibuprofen14,15,16, where enhancement of the dynamics of the  relaxation process was observed. Completely different situation was noted in poly(isobutyl vinylether)14, where secondary relaxation was almost unaffected by the confinement and annealing; while at the same time segmental relaxation times changed by more than few decades. It is also worth mentioning poly(methyl methacrylate) (PMMA) confined within alumina (AAO) templates, where one well-resolved β-loss peak can be seen both above and below Tg, independently on the applied pore diameter17. Nevertheless, it should be added that the behavior of PMMA 3 ACS Paragon Plus Environment


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under spatial restriction strongly depends on its molecular weight, Mn. Interestingly, no difference in τβ between both bulk and confined polymer can be observed for the polymer of Mn = 6 kg/mol,; while for PMMA of Mn= 120 kg/mol, a significant enhancement of β-process was noted and discussed in terms of some conformational changes occurring within macromolecules incorporated in nanoporous template18. Except of these papers, there is lack of comprehensive studies exploring the effect of density perturbation on the dynamics of local processes observed below Tg under confinement. This issue might be related to the fact that usually secondary processes are either of small amplitude or they are hardly detectable in the loss spectra measured for the confined low-molecular-weight glass formers, probably due to: (i) a small amount of the sample, (ii) additional interactions with the host material, which affect the geometry and conformation of the molecules, (iii) variation in the angle of dipole moment librations etc. Note that recent studies on vapor-deposited ultrastable glasses have revealed a significant suppression of intensity of β-process (about ~ 70%) originating probably from the improved packing efficiency and elimination of the large cone angles librations due to the confinement19. The main purpose of this paper is to explore more deeply the effect of 2D confinement on the molecular dynamics of epoxy resin monomer, bisphenol-A diglycidyl ether (known as DGEBA, Mn = 340 g/mol), characterized by various dielectric relaxation processes (including two prominent secondary relaxations: JG and non-JG that can be detected below Tg)4,20,21,22. Epoxy resins are one of the most important thermosetting polymers used in various fields of industry, for instance in paints and coatings, adhesives, industrial tooling (i.e., to produce molds, master models, laminates), electrics and electronics as well as composite materials. Herein, we examined how the variation of density and surface interactions affects the dynamics of local motions responsible for the secondary relaxation processes in DGEBA. We believe that the obtained results will give us crucial information about the physics of DGEBA 4 ACS Paragon Plus Environment

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and confined systems useful for further development in the fields of chemistry materials science, chemical engineering and/or chemistry.

I.

Experimental section

2.1. Materials Bisphenol-A diglycidyl ether (known as DGEBA of Mn = 340 g/mol) of purity higher than 99% was supplied from Sigma Aldrich and used as received. The nanoporous aluminum oxide membranes used in this study were supplied from Synkera Co. The chemical structure of investigated compound and the scheme of applied membranes are presented in Figure 1. 2.2. Sample preparation/Infiltration procedure Prior to filling, AAO membranes were dried in an oven at T = 423 K under vacuum to remove any volatile impurities from the nanochannels. After cooling, they were placed in DGEBA. Then, the whole system was maintained at T = 303 K in a vacuum (10-2 bar) for t = 24 h to let the compound flow into the nanocavities. After completing the infiltration process, the surface of AAO membranes was dried and the excess sample on the AAO surface was removed by use of a metal blade and paper towel. 2.3. BDS measurements Isobaric measurements of the complex dielectric permittivity ε*(ω) = ε’(ω)–iε”(ω) were carried out using a Novocontrol Alpha dielectric spectrometer over the frequency range from 10-2 to 106 Hz at ambient pressure. The temperature was maintained with a QuatroCryosystem using a nitrogen gas cryostat; control was better than 0.1 K. Dielectric measurements of bulk DGEBA were performed in a parallel-plate cell (diameter: 10 mm, gap: 0.1 mm) immediately after preparation of the amorphous sample. AAO membranes filled with DGEBA were also placed in a similar capacitor (diameter: 10 mm, membrane: 0.005 mm)23,24. Nevertheless, one has to remember that the confined samples are heterogeneous dielectrics composed of a



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matrix and an investigated compound. Because the applied electric field is parallel to the long pore axes, the equivalent circuit consists of two capacitors in parallel, composed of ε*DGEBA and ε*AAO. Thus, the measured total impedance is related to the individual values through 1/Z*c=1/Z*DGEBA +1/Z*AAO, where the contribution of the matrix is marginal. The measured dielectric spectra were corrected according to the method presented in Ref. [25]. Timedependent dielectric measurements were made in the temperature range T = 259 – 263 K. 2.4. DSC measurements Calorimetric measurements were carried out by Mettler-Toledo DSC apparatus equipped with a liquid nitrogen cooling accessory and an HSS8 ceramic sensor (heat flux sensor with 120 thermocouples). Temperature and enthalpy calibrations were performed by using indium and zinc standards. The sample was prepared in an open aluminum crucible (40μL) outside the DSC apparatus. Samples were scanned at various temperatures at a constant heating rate of 10 K/min over the temperature range T = 230 – 290 K. 2.5. Raman measurements Raman measurements were carried out using a WITec alpha 300 M system equipped with a He– Ne laser (λ = 633 nm at 30 mW of power) and a high sensitivity back-illuminated CCD camera. The excitation laser radiation was coupled into a microscope through a single-mode optical fiber with a 50 mm diameter. An air Olympus MPLAN (50×/0.50NA) objective was used whereas Raman scattered light was focused onto a multi-mode fiber (100 mm diameter) and monochromator grating with a 600 line/mm. Temperature and time-dependent spectra were collected in the 780 - 3200 cm-1 range, each accumulated by 20 scans, the integration time of 10 s and a resolution of 3 cm−1. Isothermal time-dependent measurements for DGEBA confined in aluminum oxide with 150 nm pore diameter were performed at T = 261 K using a THMS600 Linkam stage with a temperature stabilization of ±0.5 °C within intervals of 210 s. The spectrometer's monochromator was calibrated using the Raman scattering line of a silicon plate 6 ACS Paragon Plus Environment

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(520.7 cm−1). All data were manipulated by performing a baseline correction, cosmic ray removal, and peak fitting analysis using GRAMS 9.2 software package.

II.

Results and discussion

Molecular dynamics. The molecular dynamics of the bulk DGEBA is characterized by four various relaxation processes detected either above or below the glass transition temperature, Tg. In supercooled state (above Tg), one can distinguish: (i) the dc conductivity, connected to the charge transport and located at the lowest frequencies, and (ii) structural (alpha) relaxation, reflecting the cooperative motions of the whole molecules and responsible for the liquid-to-glass transition, see Figure 2(a). In the glassy state (below Tg), two other processes, so-called secondary relaxations (labeled herein as beta and gamma) can be detected, see Figure 2(b). Accordingly to the literature, the slower process (β) is an intermolecular JohariGoldstein (JG) relaxation4,20,21,22; while, the faster one (γ-process) originates probably from the libration of an epoxide (oxirane) ring26 in samples of low molecular weight (such as studied herein). Note that in the case of polymerized epoxy resin γ-process is usually related to the short motion of the DGEBA backbone and the libration of ether linkage

27,28.

As

observed, all processes move toward lower frequencies with decreasing temperature; however, as expected the secondary relaxations are characterized by lower sensitivity to the temperature changes than the structural mode. The dielectric spectra of DGEBA incorporated into AAO membranes of various pore diameter collected both below and above Tg are presented in Figure 2(c-f). As illustrated above Tg, all systems reveal the presence of two relaxation processes (dc conductivity and α-process, see Figure 2(a,c,e)). It is visible that the shape of structural relaxation peak becomes broader with decreasing pore size in the vicinity of the glass transition temperature, Tg, as widely reported for the confined systems

11,12,29,30

(see Figure 2 (g), where the α-loss peaks were shifted vertically to superpose at the maxima). 7 ACS Paragon Plus Environment


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One can recall that this effect is usually discussed in the context of increased heterogeneity31,32, induced by additional interactions between host and guest materials. A different behavior can be observed below Tg, where the lack of the one of the secondary relaxation (in this case β-process) can be easily detected in dielectric loss spectra of confined systems, independently on the pore diameter (see Figure 2(b,d,f)). It is a quite intriguing finding since secondary processes are considered to be local motions of molecules and they should be less sensitive to the spatial restrictions than the structural process. We assume that the observed behavior might occur due to two most likely scenarios: (1) the beta relaxation is suppressed/or of reduced intensity under confinement and/or (2) this process is enhanced (its relaxation times are shifted to higher frequencies). First scenario can be supported by recent work by Yu et al.19 on vapor-deposited toluene, n-propanol and 2picoline, where a significant suppression of the β-process in a highly stable glasses was reported (about ~ 70% when compared to the ordinary glass). This effect was explained considering the improved packing density as well as the reduction of the cone angle reorientational motions19. However, one can mention that in contrast to the vapor-deposited glasses, materials incorporated into porous templates are characterized by the lower density than the bulk ones33. Therefore, one can expect an increased intensity of the beta process. Nevertheless, it should be noticed that, in fact, the higher free volume of nanoporous materials indicates a lower number of molecules per volume, which also might result in the suppressed intensity of the secondary relaxation processes. Thus, the observed herein suppression of the β-process might be connected to the changes in density and potentially to the additional intermolecular interactions with the pore walls, i.e., formation of hydrogen bonds between the hydroxyl groups attached to walls of AAO templates and oxygen from the epoxy rings of DGEBA, which furthermore affects the mobility of this moiety as well as spatial molecular rearrangements of the investigated system. Note that the last hypothesis is in agreement with 8 ACS Paragon Plus Environment

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recent Raman experiments on triphenyl phosphite (TPP) confined within AAO templates34. In that case, it was shown that there is no liquid-to-liquid transition in TPP due to the spatial reorganization of the molecules under confinement34. On the other hand, for the second scenario, one has to remember that the β-process of DGEBA is a genuine JG relaxation (βJGprocess)4,20,21,22. Accordingly to the CM model2,3,4, the dynamics of this process should mimic the behavior of the α-process. Thus, one can expect that βJG might be faster in the glassy state since generally, the structural process gets faster for the liquids incorporated into the nanoporous matrix when compared to the bulk. As observed, structural relaxation times determined for DGEBA confined within AAO template of d = 18 nm are shifted around 1.5-2 decades to the higher frequencies in the vicinity of the Tg when compared to the bulk material, see Figure 3. Note that similar scenario has been also reported for ibuprofen (IBU) confined within nanoporous silica zeolites (SBA-15 and MCM-41). In these systems, the enhancement of both α and βJG processes was observed15,16. Considering the faster secondary relaxation process (γ), one can find that it is easily detectable in the loss spectra measured for DGEBA infiltrated into the porous matrix. In Figure 2(h), the dielectric spectra of bulk and confined DGEBA collected at T = 163 K are presented. It is worthwhile to stress that the presented γ-loss peaks were again shifted vertically to superpose at the maxima. At the first sight, it seems that relaxation times of this process are very similar to those measured for the bulk material. However, a clear broadening of the γ-relaxation peak with increasing confinement is observed. It is quite unexpected result taking into account that local motions of some pendant oxirane ring are responsible for this process. However, to explain that one should mention that accordingly to the very simple twolayer model, proposed by Park and McKenna, there are two fractions of molecules: (i) located at the center of the nanochannels (so-called a “core” set), and (ii) the ones close to the interface (interfacial set), interacting with the surface of the walls of alumina pores. Both of 9 ACS Paragon Plus Environment


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them are characterized by significantly different mobility and Tg35. Therefore, one can note that in the glassy state, we measured the average signal coming from the local libration of the oxirane ring in both the interfacial layer and core molecules. Since there are most likely Hbonds formed between a host and guest molecules, the distribution of the γ-relaxation times gets broader under confinement. Alternatively, one can consider that this broad loss peak might, in fact, be composed of two relaxations (γ and βJG), where the βJG-process is both (i) suppressed and (ii) its relaxation times are shifted towards higher frequencies. It is quite intuitive hypothesis since generally, the enhancement of the structural dynamics should induce a shift of the βJG relaxation process towards higher frequencies in material incorporated into nanoporous membrane. Above issue will be discussed in details in the further part of the manuscript. In order to analyze the temperature behavior of the observed processes (and next to determine the values of the glass transition temperature, Tg), the collected dielectric data were analyzed with the Havriliak-Negami (HN) function with an additional term describing the dc conductivity36:

  "

 i  dc   0 1  i HNi 



HNi



 HNi

(1)

where α and β are the shape parameters representing the symmetric and asymmetric broadening of given relaxation peaks, Δε is the dielectric relaxation strength, τHN is the HN relaxation time, ε0 is the vacuum permittivity and  is an angular frequency ( =2πf). Note that the relaxation times of structural, τα, and secondary, τβ and τγ, processes were estimated from τHN accordingly to the equation given in Ref. [14]. The temperature dependences of structural and secondary relaxations times are presented in Figure 3(a) and Figure 3(b), respectively. As illustrated in Figure 3(a), τα of DGEBA incorporated in the larger pores (d = 150 nm and d = 100 nm) reveals bulk-like behavior at high temperatures (no confinement 10 ACS Paragon Plus Environment

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effect); while at some specific temperatures, a clear change in the slope in this dependence is observed (Figure 3(a)). Surprisingly for the smallest pore size, a slightly different scenario was detected. The structural dynamics of DGEBA within AAO templates of d = 18 nm was slowed down at high temperatures; however on cooling, again a change in the temperature dependence of α- relaxation times (a crossover from VFT to the Arrhenius-like behavior) was noted It should be stressed that the retardation of the structural process under confinement is usually assigned to the reduced molecular mobility at the interface due to the strong surface interaction (including, i.e., the formation of the hydrogen bonding). Nevertheless, it is worthwhile to stress that herein, we followed the dynamics of the core molecules that do not interact with the pore walls. Therefore, this interpretation does not explain our experimental data. To explain a change in slope of τα(T)-dependence of DGEBA incorporated in nanoporous of varying diameter, one can refer to Ref. [33,37], where a similar crossover effect was also reported for the liquids confined within the nanoporous silica membranes. As discussed in the literature, this change in dynamics of the confined system is a manifestation of the vitrification of the molecules attached to the interface (interfacial subset)35. Thus, the temperature, at which the change in the τα(T)-dependences occurs, can be assigned as the glass transition temperature of the interfacial layer (Tc). The values of Tc obtained for different pore diameters are listed in Table 1. As observed, Tc reveals pronounced pore size dependence and moves to higher temperature with increasing degree of confinement11,12,29,30. Due to the observed deviation in the slope of τα(T)-dependences of confined systems at Tc, the obtained relaxation times were fitted by a combination of both Vogel- FulcherTammann (VFT) and Arrhenius equations. Above Tc, the VFT equation was applied:  DT T0  T  T0

     exp

  , 

(2) 11

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here τ∞ is the relaxation time at finite temperature, T0 is the value of temperature, where τ goes to infinity, DT is the fragility parameter; while below Tc, the Arrhenius equation was used:  Ea   ,  k BT 

     exp

(3)

where kB is the Boltzmann constant and Ea is the activation energy. Note that in the case of confined systems, the VFT equation was used only for an accurate determination of a point (temperature), at which the slope changes; while, Tgs of confined DGEBA were estimated from the extrapolation of Arrhenius fits. Obtained values of the glass transition temperature (where τα (Tg) = 1 s) of bulk and confined DGEBA were added to Table 1. As shown, an increasing degree of confinement results in a significant reduction of Tg for all studied nanomaterials. Note that for the smallest pore size (d = 18 nm), the difference between Tg,bulk and Tg,18nm reaches ΔTg = -10 K (where ΔTg = Tg,18nm-Tg,bulk). As a complementary to the BDS studies, calorimetric measurements have been carried out. DSC thermograms revealed the presence of two endothermic events, assigned to the two different glass transitions, see Figure 4. As illustrated, the detected Tgs are located above and below the glass transition temperature of bulk DGEBA, where the lower one (Tg,l) is connected with the vitrification of the core molecules; while the higher one (Tg,h) - to the vitrification of interfacial set. All estimated values of both Tgs are listed in Table 1. It should be stressed that the obtained values are in good agreement with those determined from dielectric data and agrees well with the two-layer model predictions11,12,29,35. In addition using values of the heat capacity change at both Tgs, we estimated also the length scale of interfacial layer (accordingly to the approach proposed by Park and McKenna35) and plotted versus pore diameter, please see Supplementary Information (Figure S1). One can add that evolution of this parameter was comparable to these determined for other liquids/polymers incorporated into nanoporous matrices. 12 ACS Paragon Plus Environment

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To explain a drop in the Tg of the materials confined in alumina nanoporous templates, Alexandris et al.25 considered the role of the interfacial energy between host and guest materials. They found that a drop in the glass transition temperature of the confined polymers correlates with an increasing interfacial energy, γSL. Surprisingly, they obtained quite unintuitive results suggesting that the stronger interactions at the interface, the lower Tg. Interestingly, further studies on the modified poly(propylene glycols) (PPG) in nanoporous materials indicated also an important role of specific interactions between polymer and AAO membrane in the variation of the glass transition temperature38. Inspired by the work of Alexandris et al.25, we have measured the surface tension of a liquid, γL, and contact angle, θ, of DGEBA on aluminum oxide, what allows the calculation of its interfacial energy, γSL (for details please see Supplementary Information, Table S1). As illustrated, DGEBA has a relatively high value of contact angle (θ = 35.1°) and surface tension (γL = 47.4 mN m-1), where γSL ~ 20 mN m-1 (see Table 2). Therefore, it seems that DGEBA molecules are rather weakly attached to the surface; nevertheless, the ΔTg plotted versus γSL remains in a general trend reported in the literature25. The temperature dependences of the secondary relaxation times are presented in Figure 3(b). As discussed above, it was impossible to extract τβ from dielectric spectra collected for DGEBA incorporated into the nanoporous. On the other hand in the case of τγ, there is no significant difference (within experimental uncertainty) between bulk and confined systems. One can recall that a similar behavior was also observed for poly(methyl methacrylate) (PMMA of Mn = 6 kg/mol) confined within AAO templates, where τβ of both bulk and confined PMMA were comparable, indicating no confinement effect on the βrelaxation17. Although, it should be noted that this process in PMMA is a genuine JG relaxation. Nevertheless, for PMMA of Mn= 120 kg/mol, a totally different scenario (the enhancement and broadening of the β-process under confinement due to changes in local 13 ACS Paragon Plus Environment


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conformation of the polymer backbone) were reported 18. Moreover, it should be stressed that the activation energy, Ea, of the γ-relaxation (estimated from the Arrhenius equation) decreases with confinement, where Ea estimated for this process in bulk DGEBA (Ea,bulk) is equal to ~ 30 kJ/mol; while for the confined systems, Ea,confined ~ 20 kJ/mol. In this context, one can recall recent studies by McKenzie et al.39, where the local dynamics of phenyl ring motions (γ-process) in polystyrene (PS) thin films were explored by β-detected NMR. As reported, the Ea of γ-process was lower near free interface when compared to the bulk, indicating that there are changes in dynamics in the close proximity of the free surface that decrease activation barriers for the local molecular rearrangements39. Importantly, the authors found that the dynamics at free and buried surface is enhanced (however in a different way) for single and bilayer thin films, respectively, returning to the bulk-like behavior over a length scale of ~ 10 nm39. One can add that a similar decrease in Ea of βJG and γ-processes has been also reported for IBU confined within nanoporous silica zeolites 15,16. In order to examine the effect of time and density perturbation on the molecular dynamics above and below Tg (especially the behavior of the βJG relaxation), we carried out series of annealing experiments. Note that the dynamics of JG secondary relaxations depends on the thermal history of the glass, including the cooling/heating rate and/or the aging time40,41. We expected that the applied protocols enabled us to influence the dynamics of the βJG process and visualize its presence in the loss spectra of DGEBA confined in nanoporous alumina. For that purpose, samples were quenched from the room temperature deeply below Tg,bulk (down to T = 173 K), then heated to the indicated Tanneal (where Tc< Tanneal< Tg,confined) and measured as a function of time at different temperatures (Tanneal = 259 – 263 K) by means of BDS technique. It should be highlighted that at the studied range of temperatures, the examined system is highly heterogeneous (the interfacial fraction of DGEBA molecules is vitrified and the core set is not). Thus, it is possible to follow the aging process of the 14 ACS Paragon Plus Environment

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interfacial layer by monitoring the dynamics of the structural process of the core molecules. One can recall that recent work on the confined TPP and PPG indicated that such annealing is accompanied by a shift in dielectric loss peak11,12,13. However, it should be stressed that no changes in the molecular dynamics can be observed at higher temperatures (above Tc of the interfacial molecules), indicating that the vitrification of the molecules at the surface is crucial for the observed scenario (the shift of the structural relaxation) to occur12. Herein, except for the monitoring the shift of the structural relaxation process, we would also like to see, whether the annealing has any impact on the local dynamics of the confined DGEBA below Tg. As illustrated for DGEBA confined within nanochannels of d = 100 nm (see the inset of Figure 5(a)), the structural relaxation peak moves towards lower frequencies with the time upon annealing at T = 263 K, indicating the slowing down of its dynamics. It should be noted that the same effect was also observed for the samples confined in membranes of lower and higher pore diameters (both d = 150 nm and d = 18 nm). The change in the position of the maximum of the peak is marked by the arrows. One can add that a similar behavior was observed in TPP and PPG. Although, the shift of the structural process was much more significant in those two materials11,12. To study the effect of the annealing on the structural and local dynamics of DGEBA confined in porous AAO template of d = 18 nm and d = 100 nm, immediately after the annealing process was finished, samples were quenched (down to T = 163 K) and measured on heating. As shown in Figure 5(a), the obtained structural relaxation times significantly differ from those determined before the time-dependent measurements. First, surprisingly, there is no crossover temperature and change in slope in τα(T)-dependences, resulting in a bulk-like behavior of structural relaxation times of confined DGEBA in the whole temperature range (within experimental uncertainty) independently on the pore diameter. Consequently, the difference between the glass transition temperature of the bulk and 15 ACS Paragon Plus Environment


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confined DGEBA decreases significantly (Tg,confined ~ Tg,bulk, where ΔT = 2-3 K). Second, for the sample annealed in nanochannels of the smallest diameter d = 18 nm, the effect of retardation of the structural process at high temperatures vanishes almost completely (see Figure 5(a) and 5(c)). It should be highlighted that to the best of our knowledge, it is a first report demonstrating such a scenario for the liquids incorporated in porous materials. One can recall that similar recovery of the bulk Tg (within experimental error) due to prolonged annealing time was also reported for the macromolecules deposited as thin films7,8,9,10,42, which was discussed as a result of the removal of the free surface 42. One can suppose that the recovery of the bulk dynamics of DGEBA confined in alumina nanoporous might be due to the ongoing density perturbation at the interface or alternatively to dewetting process of the interfacial layer proceeding at low temperatures. In this context, it should be mentioned that the contact angle between DGEBA and alumina is relatively high at room temperature, suggesting rather low contact of this materials with AAO. Moreover, taking into account the reports indicating that the contact angle increases with decreasing temperature for the similar systems43, one may speculate that surface interactions between host and guest materials decreases with a reduction of temperature and progress of annealing. Consequently, bulk-like dynamics of the structural process can be recovered. It is also worthwhile to add that along with the described above effect, the narrowing of the structural relaxation loss peak during the annealing was also observed (see inset of Figure 5(c)). On the other hand below Tg, the relaxation times of the γ- process do not change (or change within experimental uncertainty) after annealing of DGEBA at different temperatures in pores of varying diameters, see Figure 5(b) and Figure 5(d). However, in contrast to the structural relaxation, this process becomes significantly broader after annealing. The change in the width is clearly visible on the left side of the γ-loss peak, see the inset in Figure 5(b). To explain this finding, one might assume that the JG relaxation starts to separate from the γ16 ACS Paragon Plus Environment

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process. However, it cannot be fully recovered due to most likely suppression of the libration of the dipole moments related to this kind of mobility. It can be stressed that such scenario is in the line with the behavior of the structural relaxation process, which shifts to lower frequencies upon annealing. Since both structural and JG relaxation processes are directly connected to each other, one can also expect a slowing down of the latter mobility during time dependent measurements. In order to explain the behavior of DGEBA under confinement (including, i.e., retardation of the structural relaxation process for DGEBA infiltrated in the smallest pore diameter d = 18 nm and the behavior of secondary relaxation), we explored more deeply the annealing process for DGEBA infiltrated into nanoporous templates by means of Raman spectroscopy. As a first, we analyzed the bulk spectrum (as a reference) in comparison to the literature data44, see Figure 6(a). The bulk spectrum might be divided into three characteristic regions: (1) 3200 - 2800 cm-1, (2) 1620 - 1300 cm-1 and (3) 1300 - 800 cm-1. The region (1) is dominated by the stretching vibration of the bonds in the CHx (where x = 1, 2, 3) group, where ν(CH) modes of the aromatic ring overlap with the symmetric and asymmetric ν(CH3) modes of methyl units, occurring in the 3071 - 3055 cm-1 region; while, the vibration of ν(CH2) at the epoxy ring refers to bands at 3006 and 2760 cm-1, as well as modes of ether ν(O-CH2) located at 2929, 2873, 2836 cm-1 44. The region (2) is mainly associated with the bands linked to the stretching ν(C-C) vibration within the aromatic ring (1608, 1581 cm-1) and the deformational mode of methine and methyl groups 44. In turn, the main bands in the region (3) centered at 1249, 1231, 1186, 1112 and 830 cm-1 correspond to the ν(C-O) stretching and deformational δ(CH2) vibration within the epoxy ring ν(CCO), in-plane deformation of the gem-dimethyl δ(C-CH3) group, the C-H out-of-plane bending modes of the para-disubstituted phenyl group and in-plane vibration of methylene δ(CH2) groups at the epoxy ring, respectively. Other bands with lower intensity are usually ascribed to the deformation modes 17 ACS Paragon Plus Environment


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of the CHx groups as well as skeletal modes of C-C within different fragments of DGEBA backbone 44,45,46,47,48,49. Raman spectra of confined DGEBA are presented in Figure 6(b,c), respectively. Note that for better understanding the ongoing spectral changes, we carried out also the theoretical calculations (for details please see Supplementary Information). As observed, the bands from the region (1) of confined system in general, are slightly shifted (~5 cm-1) towards lower wavenumbers with respect to the spectrum of the bulk sample. The red shift is, however, within the experimental error connected to the spectrometers’ resolution. More interesting observation appeared when we compare the bands’ intensity, which decreases significantly in the case of DGEBA confined within alumina pores with respect to the bulk system. The lower intensity of ν(CHx) bands, might be due to geometry restriction or interactions between DGEBA and alumina template, see Figure 6(b,c). Similar observation can be made for the region (2), where in the case of bands linked to the vibration within the aromatic ring, only slight variation is observed when compared to bulk DGEBA. As a result, the bands originated from the deformation vibration of methine and methylene groups, centered at 1459, 1427, and 1410 cm-1 are shifted about 2 - 3 cm-1 for all samples incorporated into nanoporous with respect to the bulk. It is probably due to the intramolecular reorganization enforced by geometry restriction imposed by the alumina nanochannels. One can also add that except for few bands of lower intensity, generally other bands behave like in the bulk material. The lower intensity bands might originate from the appearance of some molecular interactions between DGEBA molecules with the walls of the AAO membrane as suggested by theoretical models I and II (see Figure 7). A similar trend of behavior is observed for the bands from region (3). However, the appearance of bands around 1042 cm-1 in confined systems might correlate to the methylene group vibration within an epoxy fragment of the resin due to the possible interaction of this moiety with the hydroxyl group of the pore wall. Accordingly to


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model II, the relatively high intensity of the band at 1249 cm-1, and the presence of band at 1042 cm-1, not observed in the bulk DGEBA44, are linked to the rotation of the molecular fragment (Ph-O-CH2-Ep), as a result of the interaction of resin molecule with the hydroxyl unit anchored to the aluminum oxide wall (Figure 7). It is worthwhile to add that a small shift of bands assigned to the vibration of main DGEBA backbone and only relatively small modification of the bands’ intensity correspond to the small impact of the restriction geometry on the structural reorganization of the investigated molecules. For details considering the impact of temperature on Raman spectra of the confined systems please see Supplementary Information (Figure S2). Next, we carried out time-dependent isothermal measurements for the samples prepared using two different thermal protocols (see Figure 8(a)). First approach corresponds to DGEBA slowly cooled from room temperature down to T = 261 K (RI, red spectra shown in Figure 8(b)), while the second one refers to DGEBA rapidly quenched down to T = 173 K and then slowly heated to T = 261 K (RII, blue spectra shown in Figure 8(c)). Presented spectra were collected for DGEBA confined within pores of d = 150 nm; however, it should be highlighted that comparable scenarios have been detected also in nanoporous of smaller diameter. Surprisingly, the initial Raman spectra of both systems differ from each other depending on the sample preparation, mostly in the bands’ intensity centered at high wavenumbers of the region (1) and in a smaller extent in the regions (2) and (3). In the case of the RI approach, the initial spectrum seems to be comparable to the one obtained at room temperature and from theoretical calculations (t = 0 min in Figure 8(b)). Only a slight bands’ shift towards higher wavenumber (blue shift), probably due to a slight molecular reorganization enforced by the mutual interacting resin molecules and lowering temperature, was observed. In this case, no special modification of molecular interactions between DGEBA molecules and aluminum oxide walls occurs. Upon annealing, only small changes in



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Raman spectra were noticeable. We found a red shift (within experimental uncertainty) of the bands towards the position characteristic for the molecular geometry with respect to the spectrum collected at room temperature. It may result from the slight molecular reorganization heading towards more energetically stable configuration (t = 300 min in Figure 8(b)). On the other hand, more pronounced modifications can be seen in the case of the system prepared by RII. Here, one can find the reduced intensity of the high wavenumbers band at region (1) and increased intensity of some bands of regions (2) and (3), see blue spectrum collected after t = 0 min in Figure 8(c). This change can be easily explained taking into account two interacting DGEBA molecules (as presented in model III in Figure 7). Such molecular configuration strongly modifies the character of methine vibration located at the aromatic ring, leading to the comparable intensities of bands. What is more, this configuration causes that the intensity of the band at 1429 cm-1 (linked to the C-CH3 moieties) raises significantly, probably due to the appearance of the resonance enforced by the mutual interaction of symmetrically arranged methyl groups originating from two molecules. A similar increase in the intensity can be seen for the bands at 1247 and 1236 cm-1, ascribed to the in-plane vibration of C-H group at the aromatic ring as well as the deformational mode of CHx (x = 1, 2) at the epoxy ring (initial spectrum in Figure 8(c)). In this context, one can assume that due to quenching of the system, part of the molecules was frozen in a metastable configuration, probably in form of bimolecular subclusters (see model III); while the other part of resin molecules probably was immobilized in the vicinity of the AAO channel wall, as suggested by models I and II. After t = 300 min, a gradual increase of bands’ intensity ascribed to the ν(CH) vibration and a decrease in the intensity of other bands in regions (2) and (3) was detected, see spectrum collected after t = 300 min in Figure 8(c). Note that the band’s intensity of RII after t = 180 min becomes comparable to the one observed for the sample prepared by RI, due to the progressive molecular reorganization leading to the


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disruption of the bimolecular subcluster formed via rapid freezing of the system temporarily immobilizing DGEBA molecules as well as to weaker interactions between host and guest material (Figure 8(c)). One can add that a similar procedure was also applied for other confined systems (d = 18 nm and d = 100 nm), and interestingly, a comparable scenarios were detected. As indicated above, the application of nanoporous membranes forces some molecular/density packing of specific (different than in the bulk) configuration, which might be responsible for, i.e., the clearly retarded molecular dynamics of DGEBA confined within d = 18 nm, see Figure 3. One can add that the systems incorporated into AAO membranes of d = 18 nm are characterized by the highest surface-to-volume ratio; therefore, the impact of interfacial interaction will be the highest, what might lead to more pronounced deviations in the dynamics, when compared to the other pore sizes. Note that Raman studies confirm the “confined-induced” conformation changes of DGEBA molecules (including the formation of subclusters between the molecules), see Figure 6. Furthermore, it can be observed that quenching of confined systems enabled us to freeze those states of specific configuration, which seem to undergo continuous changes upon annealing leading to the recovery of the bulk-like behavior, as observed by variation of both dielectric and Raman spectra, see Figure 5 and Figure 8. This behavior seems to originate from the density equilibration of the interfacial molecules, resulting in the configuration changes leading to more or less bulk characteristic. Moreover, one can recall that DGEBA is characterized by rather high value of contact angle (θ = 35°, see Table 2), indicating that the wetting of DGEBA is relatively low, i.e., when compared with TPP or PPG (see Table 2). Consequently, interactions between DGEBA molecules and the alumina template are weak. Thus, one can assume that the annealing of quenched DGEBA confined within AAO templates might, in fact, be connected with some absorption-desorption processes occurring at the interface, resulting in the recovery 21 ACS Paragon Plus Environment


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of bulk-like properties due to weakening of interfacial interactions. Note that the contact angle for the similar systems tends to increase with lowering temperature resulting in decreasing strength of surface interactions43; therefore, one might assume that due to fast quenching some interfacial layer of DGEBA might be frozen on the interface. Thus, the aging of the interfacial molecules might be probably accompanied by dewetting processes as well. Consequently, bulk-like dynamics of the structural process for DGEBA incorporated into nanoporous is recovered. This supposition seems to be confirmed by independent dielectric and Raman measurements. One can mention that Raman spectra of the samples obtained by RII evolve with time reaching the band arrangement similar to RI and bulk sample, just like in the case of dielectric data. As a result, the molecular dynamics of confined DGEBA after the annealing follows the bulk dependence. Moreover, one can assume that “confined-induced” changes within DGEBA molecules might not only affect the structural relaxation but also be responsible for the behavior of the secondary relaxation, having its own contribution, to the suppression of β- process, decreasing Ea and/or broadening of the γ-loss peaks. Note that below the Tg dielectric spectroscopy measures the average signal coming from both fractions (interfacial and core). Thus, it is possible that some dipole moments of DGEBA might be strongly suppressed due to some configuration changes, making them difficult to monitor. Nevertheless, one can add that recent studies on nanoporous materials revealed that below Tc (vitrification of interfacial molecules) the system reaches isochoric conditions, governed by the negative pressure conditions, what can also contribute to the observed results30,37,50,51. III. Conclusions The molecular dynamics above and below the glass transition temperature of DGEBA infiltrated in nanoporous alumina was investigated by means of BDS and Raman spectroscopy techniques. Although, we noticed all characteristic features of materials incorporated into well-defined cylindrical pores of varying diameter, including two Tgs and the deviation from the VFT to Arrhenius-like behavior of τα(T)-dependences, the structural 22 ACS Paragon Plus Environment

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dynamics of DGEBA within d = 18 nm was surprisingly retarded at high temperatures, where usually bulk-like behavior is observed. On the other hand below the Tg, only the faster secondary relaxation (γ) was clearly visible. Interestingly, this process was only slightly affected by the confinement with respect to the bulk. It is worthwhile to mention that the activation energy of - process was lower with respect to the bulk sample. We attributed this behavior to the conformational and density changes occurring at the interface. Above assumption was further confirmed by both dielectric and Raman time-dependent measurements, highlighting the impact of both these effects (conformation and density) on the overall behavior of the soft matter under 2D confinement. As shown, DGEBA undergoes conformational and density changes upon confinement and annealing, what contributes greatly to the behavior of the examined epoxy resin. Additionally, we found that the prolonged annealing time allows to fully recover the bulk-like structural dynamics above Tg, independently on the applied pore size. As a consequence, interactions between DGEBA and alumina membranes become weaker. This phenomenon was discussed in view of density perturbation at the interface or desorption of the interfacial layer. Simultaneously, the pronounced broadening of the fast secondary relaxation below the Tg was observed. We interpreted this finding as due to increasing separation of the βJG process, that was not visible in the loss spectra of confined DGEBA. Nevertheless, we did not observe a well-resolved bulk-like β-process, despite the recovery of the bulk-like structural dynamics above the Tg.

Supporting Information. Details concerning both (i) the theoretical calculations performed to support Raman measurements and (i) surface Tension and Contact Angle measurements. Note that data for water and ethylene glycol were taken from Ref. [52]. Analysis of the pore diameter dependence of the length scale of interfacial layer estimated form calorimetric data and the temperature dependence of Raman spectra obtained for confined systems in the reference to bulk.



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AUTHOR INFORMATION Corresponding Author * (MT) e-mail: [email protected] (KK) e-mail: [email protected]; [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT K.K. is thankful for a financial support from the Polish National Science Centre within OPUS project (Dec. no 2015/17/B/ST3/01195). M.D. is thankful for a financial support from the Polish National Science Centre within SONATA project (Dec. no 2017/26/D/ST8/01117 References 1

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(b)

(a)

-

liberation of epoxide rings

O O O

O

Bisphenol-A diglycidyl ether (DGEBA)


Page 29 of 35

Figure 1. The chemical structure of the investigated compound (a) and the applied constrain medium (b).

(a) bulk

(b) bulk

dc conductivity -process

10

-process

-process

-1

"

"

101

100

10-2

T = 253 K T = 293 K T = 5 K

10-1

10-1

100

TTg

101

102

103

104

105

106

10-1

100

101

Freq. / Hz

(c) (c) d = 100 nm

103

104

(d) d = 100 nm

T = 253 K T = 293 K T = 4 K

105

106

T = 253 K T = 173 K T = 10 K

10-3

"

"

10-2

102

T = 253 K T = 173 K T = 10 K

Freq. / Hz

10-3

TTg 100

101

102

103

104

105

106

100

101

102

Freq. / Hz

105

T = 173 K T = 253 K T = 10 K

2x10-4

"

"

10

-3

104

(f) d = 18 nm

T = 253 K T = 293 K T = 4 K

(e) d = 18 nm

103

Freq. / Hz

10-4

10-4

T>Tg 10-1

100

T

Variation in the Molecular Dynamics of DGEBA Confined Within AAO

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