Time and Temperature as Key Parameters Controlling Dynamics and

Jun 26, 2017 - The annealing behavior of poly(propylene glycol) (PPG, Mw = 4000 g/mol) incorporated into the nanoporous aluminum oxide (AAO) membranes...
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Time and Temperature as Key Parameters Controlling Dynamics and Properties of Spatially Restricted Polymers Magdalena Tarnacka,*,†,‡ Olga Madejczyk,†,‡ Kamil Kaminski,*,†,‡ and Marian Paluch†,‡ †

Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland Silesian Center of Education and Interdisciplinary Research, University of Silesia, ul. 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland

‡

ABSTRACT: The annealing behavior of poly(propylene glycol) (PPG, Mw = 4000 g/mol) incorporated into the nanoporous aluminum oxide (AAO) membranes of different pore diameter was investigated by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). The presence of two subsets of molecules of different dynamics (interfacial and core) allows to produce conditions, where only one fraction (interfacial) is vitrified. Upon these conditions, annealed samples undergo continuous changes reflected in a shift of dielectric processes (segmental and normal mode). This unusual behavior was observed to be a reversible effect strictly governed by chain diffusion, indicated by the coupling between the equilibration constant (τanneal) and the relaxation times of normal mode (τNM). Despite the similarity in macroscopic changes observed upon the annealing of 2D and 1D materials [Napolitano, S.; et al. Nat. Commun. 2011, 2, 260], the examined behavior occurs accordingly to a totally different mechanism than the one reported for thin polymer films. Nevertheless, on the other hand, it reveals the possibility of exploring aging effects up to now not accessible for bulk materials.

I. INTRODUCTION Nanomaterials are currently one of the most explored class of materials. This enormous interest results from their unusual properties often different than the one observed in the case of bulk materials. In fact, the spatial nanometric restriction becomes a novel tool that provides outstanding opportunity of producing the materials with unique physical properties and morphologies satisfying the industrial requirements (i.e., solar batteries,1 fuel cells,2,3 or drug carriers4,5). One can add that there are also many important problems related to these materials yet to be addressed, i.e., response of the soft matter to different kind of confinements or the physics underlying unusual properties of spatially restricted materials. As discussed, the change in molecular dynamics as well as shift in the phase transition temperatures is a result of many factors, i.e., the size and the degree of confinement, the host−guest interaction, geometry of the sample, etc. However, recent works revealed that the physical properties of material confined at nanoscale result mostly from the interfacial interaction. As reported, the polymer chains adsorb and interact with the substrate surface. As a consequence, layer of reduced mobility is formed. The equilibration of such system at time scale much longer than reptation time, τrep, often identified as segmental relaxation times, leads to the increase in the number of irreversible adsorbed chains.6−9 As a consequence, the dynamical properties of the confined materials become a function of the interfacial layer thickness10,11 due to the perturbation of polymer density at the substrate surface upon the annealing experiments.12 One can assume that if the density perturbation of the interfacial layer (and the formation of irreversible adsorbed © XXXX American Chemical Society

chains or molecules) is responsible for the tuning of nanomaterials properties as thin films (1-dimensional confinement), the density effect should be significantly magnified in the case of nanoporous materials (2-dimensional confinement), where surface-to-volume ratio is much greater. In this context, the incorporation of liquids to pores of varying pore diameter seems to be the best test and verification whether there is any relationship between behavior of soft matter under various kinds of spatial restriction. Herein, we examined in detail the annealing effect on the molecular dynamics of poly(propylene glycol) (PPG, Mw = 4000 g/mol) incorporated into the nanoporous aluminum oxide (AAO) membranes of different pore diameter (d = 18 and 150 nm), monitored by means of broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). (Note that the molecular dynamics study on the behavior of PPG within AAO temples was a subject of a previous paper.13 Thus, herein, we focused only on the annealing experiments.) Poly(propylene glycols) are unique glass-forming polymers with a net dipole moment parallel to the main chains (type A polymer14), which fluctuation leads to a well visible relaxation process, slower than the segmental one, so-called normal mode. Thus, it is possible to explore and estimate the impact of the annealing on both the segmental dynamics (responsible for liquid−glass transition) and the global chain dynamics (reflected as a normal mode process). For this purpose, samples were quenched deep below Tg of Received: March 23, 2017 Revised: June 15, 2017

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DOI: 10.1021/acs.macromol.7b00616 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Time evolution of the loss spectra measured for PPG confined in 18 nm (a) and 150 nm (b) nanochannels at indicated temperatures. As insets, the shapes of structural relaxation of indicated samples before and after the annealing are compared. Additionally, the chemical structure of PPG with n = 68 (Mw = 4000 g/mol) is presented. K. AAO membranes filled with PPG were also placed in a similar capacitor (diameter: 10 mm; membrane: 0.005 mm).23,24 Nevertheless, the confined samples are a heterogeneous dielectric consisting of a 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*Polymer and ε*AAO. Thus, the measured total impedance is related to the individual values through 1/Z*c = 1/Z*Polymer + 1/Z*AAO. The BDS annealing measurements were performed as follow. Samples were quenched deep below Tg of bulk material (Tg,bulk = 202 K), heated to the given annealing temperatures, Tanneal, and measured as a function of time at different temperatures (213−225 K). 3.2. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (Mettler-Toledo) was carried out on the crushed membranes filled with PPG. The samples were contained in sealed crucibles and scanned with a heating rate of 10 K/min over a temperature range from 160 to 300 K. Each measurement was repeated thrice. For annealing experiments, samples were quenched deep below Tg of bulk material (Tg,bulk = 202 K), heated to the given annealing temperature (Tanneal = 213 K), and then measured as a function of time. Immediately after the annealing, samples were cooled and scanned with a heating rate of 10 K/min over a temperature range from 160 to 300 K. This second scan was followed by third scan, where samples were again cooled and scanned again with a heating rate of 10 K/min up to 300 K.

bulk material, heated to the given annealing temperatures, Tanneal, and then measured as a function of time at different temperatures (213−225 K). Values of Tanneal were chosen to correspond to conditions, where only one subset of molecules (interfacial described by higher Tg2) are vitrified, while others (core fraction with lower Tg1) are not (Tg1 < Tanneal < Tg2).13,15−21 At these conditions, we were able to monitor changes within core molecules manifested by a shift in dielectric loss peak as a function of time, which, we believe, is caused by the reorientation of interfacial molecules, undergoing physical aging process and affecting the motions of the core subset.

II. EXPERIMENTAL SECTION 1. Material. Poly(propylene glycol) (PPG, terminated by OH) with purity higher than 98% was supplied by Sigma-Aldrich. The chemical structure is presented as an inset in Figure 1a. The nanoporous aluminum oxide membranes used in this study were supplied from Synkera Co. Details concerning porosity, pore distribution, etc., can be found at the Web page of the producer.22 2. Samples Preparation. PPG was transferred into the flask together with the AAO membrane. (Prior to filling, AAO membranes were dried in an oven at 423 K under vacuum to remove any volatile impurities from the nanochannels.) For all experiments the required amount of AIBN as a catalyst was 0.5%. Then, the whole system was maintained at T = 293 K in a vacuum (10−2 bar) for 24 h to the compound flow into the nanocavities. After completing the infiltration process, the surface of AAO membrane was dried, and the excess sample on the surface was removed by use of a paper towel. In the experiment, we used membranes with a different pore diameters: 150 and 18 nm. 3. Methods. 3.1. Broadband Dielectric Spectroscopy (BDS). Isobaric measurements of the complex dielectric permittivity ε*(ω) = ε′(ω) − iε″(ω) were carried out using the Novocontrol Alpha dielectric spectrometer over the frequency range from 10−2 to 106 Hz at ambient pressure. The temperature stability controlled by the Quatro Cryosystem using a nitrogen gas cryostat was better than 0.1

III. RESULTS AND DISCUSSION The time evolution of dielectric spectra recorded upon the annealing of PPG incorporated into the pore diameter of 18 and 150 nm are presented in Figures 1a and 1b, respectively. As the experiment proceeds, both processes move by more than half a decade toward lower frequencies for all samples. Consequently, the retardation in the molecular dynamics can be seen. Since this effect is detectable for all pore sizes, one can assume it is clearly confinement independent. Note that a similar effect was also reported for low molecular weight glassB

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Figure 2. (a, b) Time dependence of relaxation times of τα (scaled to the initial value) of PPG within 18 and 150 nm, respectively. Red solid lines represent the best stretch exponential fits. As an inset in panel a, the time evolution of τNM upon the experiment is shown for d = 18 nm. (c) Evolution of τanneal determined for segmental process and normal mode plotted versus 1000/Tanneal. Red solid lines represent the guide for eye. (d) Plot of log(τanneal) versus log(τα(NM)). Red lines represent the best linear fits.

technique. The relaxation times of segmental, τα, and normal mode, τNM, processes were determined from the fitting analysis of obtained spectra to the superposition of two Havriliak− Negami (HN) functions.27 Using the well-known protocol, τα and τNM were estimated from τHN,28 then scaled vs initial value, and plotted as a function of time as presented in Figures 2a and 2b. As illustrated, the system undergoes continuous changes until it fully equilibrates, as observed for all examined samples. For quantifying this effect, rescaled relaxation times were further analyzed by the stretch exponential function, usually utilized for aging experiments,29,30 to determine the equilibration constant, τanneal (τ = A exp(−[t/τanneal]β) + τ∞, where A, β, and τ∞ are constant).31 Estimated values of τanneal plotted versus 1000/Tanneal are presented in Figure 2c. As shown, τanneal reveals different temperature sensitivity depending on the pore diameter. For the smallest pore size (d = 18 nm), τanneal

former, as triphenyl phosphite (TPP),21 making this kind of behavior rather universal. Additionally, by comparing the spectra before and after the annealing (loss peaks were arbitrarily shifted vertically to superpose at maximum), narrowing of the distribution of relaxation times of both processes has been detected, suggesting that the dynamics in the annealed samples becomes more homogeneous (see insets in Figure 1a,b).13,21,25 It should be also mentioned that the chain mode has lower sensitivity to the sample equilibration since both processes (normal and segmental mode) tend to merge with time.26 In order to explore equilibration of the confined PPG more deeply, a series of annealing experiments at various temperatures and pores of varying diameter were carried out. Each sample was quenched from the room temperature deeply below Tg,bulk, heated to indicated Tanneal, and measured by the BDS C

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Figure 3. (a) DSC curves obtained for PPG within templates of d = 150 nm measured before (solid), immediately after the annealing at 213 K (dotted), and as a third scan (dashed). As an inset, the pore diameter dependence of ζ determined from the DSC measurements is presented. Solid points were taken from ref 13. (b) Values of Tg1 and Tg2 determined from performed DSC measurements.

by Napolitano et al.,6 where the shift of the segmental relaxation process was discussed in terms of the density perturbation leading to the thickening of “the layer of chains irreversibly adsorbed onto the substrate” with time.6,10−12 Nevertheless, it should be pointed out that although the macroscopic change related to the slowing down of the dynamics of annealed sample seems to be the same in both thin films and porous AAO membranes, the molecular mechanism responsible for that can be much different. In this context, one can recall that materials under 2D restriction revealed the presence of two glass transition temperatures, which is not observed in the case of thin films, where no evidence of vitrification of interfacial layer is detected. Moreover, one has to remember that the thickness of interfacial layer within thin polymer films is usually reported to be around 2−3 nm for various substrates. On the other hand, the length scale of surface molecules in AAO nanoporous materials is in some cases around 10 nm.17,21 Additionally, it is worthwhile to stress that herein we examined PPG of low molecular weight (Mw = 4000 g/mol), which may have a significant impact on the observed effects. Just to add that generally polymers of high molecular weight are deposited on substrates due to high stability and quality of produced materials. Therefore, we can expect that the mechanism of adsorption of low and high molecular weight polymers can be much different. Consequently, for the latter systems, irreversible effects are observed, while in the case of the former one, a scenario similar to the low molecular weight simple liquids might be seen, indicating a change in mechanism of adsorption depending on the polymer molecular weight.34,35 Finally, one can recall that no change in dynamics of segmental and normal mode is recorded for the experiments carried out at higher temperatures (above Tg1 of interfacial molecules). That indicates that vitrification of the polymers attached to the surface is crucial for this scenario to occur in context of 2D materials.21

increases with a decreasing temperature, while for d = 150 nm, τanneal changes only slightly at examined T conditions. One can assume that the higher the degree of confinement, the longer the equilibration. Interestingly, τanneal determined from the time dependence of segmental and chain modes are comparable (see circles in Figure 2c). This observation seems to be rather unexpected taking into account the differences in the origin and relaxation times of both processes. One can assume that as the surface coverage increases, the new coming molecules more loosely interact with the surface and are adsorbed by fewer segments forming loops, tails, and trains;6,32 note that the number of those structures depends on the chains rigidity and, for some cases, the interfacial dynamics.33 The time evolution of polymer chain conformation of already adsorbed and newly attached molecules is correlated with interaction between both small segment and whole chains, which change upon the annealing. In this context, it is expected that both examined herein processes (segmental and normal modes) have comparable sensitive to occurring fluctuation, as observed in the presented data. For a deeper understanding of the correlation between τanneal, τα, and τNM, we plotted log(τanneal) versus log(τα) and log(τNM) and fitted to the linear function (see Figure 2d). Interestingly, a significant decoupling in the case of the segmental relaxation process can be observed, where the decoupling parameter (slope of linear fitting function), s, varies from s = 0.04 to s = 0.6 for d = 150 nm and d = 18 nm, respectively. As observed, the value of s strongly depends on the pore size. On the other hand, a totally different scenario can be seen in the case of normal mode, where s oscillates around 1 for all samples, suggesting coupling between the normal mode and the constant of annealing (see Figure 2d). Thus, one can consider the observed equilibration of the confined system to be directly related to the chain dynamics/diffusion. Interestingly, a similar observation upon the annealing experiments of materials under 1D confinement was reported D

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To verify whether behavior discussed herein might have the same features and origin as reported by Napolitano et al. for thin polymer films,6 additional DSC measurements according to the BDS protocol have been performed. As it can be seen in Figure 3a, the annealing at 213 K leads to the shift of both Tg’s and decrease in the distance between them. Nevertheless, in contrast to the thin films studies, we observed a reduction of the interfacial layer, ζ, due to the annealing, which is more pronounced in the case of d = 150 nm. Note that parameter ζ was calculated from DSC data, as proposed by Park and McKenna (ζ = d[1 − (1 − ΔCp2/ΔCtotal)1/2]/2, where ΔCp,total = ΔCp,1 + ΔCp,2; ΔCp,1 and ΔCp,2 are the changes of the heat capacity at Tg,1 and Tg,2).15 It is very important finding suggesting totally different scenario than the one reported in the case of thin films. Namely, the decrease of ζ (due to equilibration of frozen polymers at the interface) indicates a clear exchange transport process between interfacial and core molecules, leading to densification of the latter fraction of polymers. As a consequence, the dynamics of segmental and normal mode slows down. Interestingly, it agrees well with our previous finding that the equilibration constant (τanneal) scales well with the chain diffusion (τNM). Therefore, the whole process is governed by the vitrified interfacial layer and chain diffusion. It is also worthwhile to mention that after the scan performed immediately after the annealing, we carried out the additional measurement (third). As observed, the examined annealing effect leads to reversible changes in the system (see Figure 3b) due to the different mechanism of adsorption.34,35 Additionally, one can recall recent studies on confined materials showing that below T g1 the system enters isochoric conditions.36−39 Therefore, we can suppose that due to the annealing, systems move from one isochoric condition to other, characterized by higher density and slower dynamics. Consequently, the origin of the slowing down of the dynamics in porous materials seems to be as expected, much different than the one reported by Napolitano et al. for thin films,6,10−12 taking into account all the differences between them as i.e. the geometry, thickness, and properties.

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AUTHOR INFORMATION

Corresponding Authors

*(M.T.) E-mail [email protected]. *(K.K.) E-mail [email protected]. ORCID

Magdalena Tarnacka: 0000-0002-9444-3114 Olga Madejczyk: 0000-0001-8129-9224 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.K. and O.M. are thankful for financial support from the Polish National Science Centre within OPUS project (Dec. no 2015/17/B/ST3/01195).

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IV. CONCLUSION In conclusion, the observed equilibration of nanoporous materials which arose from the vitrified surface molecules is governed by the chain diffusion and results in the reduction of interfacial layer and densification of core fraction, indicating a mass transport from the nanochannels walls to their center governed by the chain diffusion. As presented, it is a reversible effect allowing to achieve and tune the desired physical properties of nanomaterials just by time and temperature conditions. Additionally, one can assume that the investigated density perturbation makes this system a unique model for examining the density effects, giving the remarkable chance of exploring more deeply the behavior of the soft matter. In particular, one can recall that the observed effect mimics, to some extent, the physical aging process, where glasses, evolving toward equilibrium with time, change their physical properties and densities.29,30,40,41 Since we can easily follow changes in the dynamics upon the spontaneous densification of the sample within the experimental window, it can open new and exciting ways to explore aging effects up to now not accessible for the bulky materials. E

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