Studies on the Temperature and Time Induced Variation in the

Aug 19, 2016 - (46-49) In contrast to PI, the segmental dynamics of PPG was enhanced significantly in comparison to the bulk macromolecules. However, ...
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Studies on the Temperature and Time Induced Variation in the Segmental and Chain Dynamics in Poly(propylene glycol) Confined at the Nanoscale Magdalena Tarnacka,*,†,‡ Kamil Kaminski,† Emmanuel U. Mapesa,∥ Ewa Kaminska,§ and Marian Paluch†,‡ †

Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland Silesian Center for Education and Interdisciplinary Research, University of Silesia, 75 Pulku Piechoty 1A, 41-500 Chorzow, 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 ∥ Institute of Experimental Physics I, University of Leipzig, Linnéstraße 5, 04103 Leipzig, Germany ‡

ABSTRACT: The effect of 2D confinement on the dynamics of the normal mode (chain mobility) and segmental relaxation in poly(propylene glycol) (PPG) has been studied with the use of broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). It is shown that both processes become faster with increasing degree of confinement. Interestingly, the crossover from VFT to the Arrhenius-like behavior of chain and segmental dynamics, observed in the examined system, is strictly related to the vitrification of the adsorbed polymers. We also report that the mean relaxation times of the normal, τNM, and segmental modes, τα, depend on the thermal history of confined PPG and can be significantly modified using different thermal treatments. It is demonstrated that annealing of the samples below the crossover temperature, Tc, leads to a systematic shift of the segmental relaxation and normal mode toward lower frequencies, resulting in an increase in the glass transition temperature of the spatially restricted PPG. Taking into account recent studies, we allude this new experimental observation to the density equilibration: after annealing, a system with higher density characterized by more homogeneous dynamics can be obtained. It is therefore possible to modify and control the properties of the confined material by using different thermal treatment protocols. Our results offer a better understanding of the behavior of the spatially restricted soft matter and the interplay between mobilities at two completely different length scales. dependently on the used experimental techniques.28−32 However, some explanation of this finding has been proposed recently: it was claimed that most likely segmental fluctuations in thermal equilibrium decouple from the nonequilibrium dynamics in the glass, which can be understood in view of the free volume hole diffusion (FVHD) model developed by Cangialosi et al.33,34 and furthermore discussed by Kremer et al.35 On the other hand, the properties of soft matter incorporated in pores, so-called 2D geometrical restriction, are always considered as due to interplay between surface and finite size effects. There are many experimental reports showing depression of the melting or glass transition temperatures with reduced pore size indicating the former factor could be more dominant.18,36 However, studies by Richert et al.37,38 shed a new light on this issue: in the case of hard confinement, for the liquid close to the hard interface, slowing down of the

I. INTRODUCTION Because of the increasing number of the applications of nanomaterials in different kinds of industries, there is a strong emphasis to understand and control the behavior of soft matter under spatial confinement.1−3 Specifically, great attention is paid to the field of the glass transition phenomenon, a major unsolved problem in condensed matter physics.4−6 There are basically two methods of studying properties and dynamics of confined materials. The first one is related to the deposition of the sample on a hard substrate (1D), while the other is based on the incorporation of liquids into pores (2D).7−16 In the literature, there are numerous investigations on the polymers confined in nanometric supported layers or free-standing films with the main aim being to better understand interactions between the solid substrate and used materials.17−27 Interestingly, over the years a lot of controversy arose due to the contradictory results published by many research groups. The best illustration of that are data presented for poly(styrene) (PS) supported on different substrates. In this particular case, authors reported the glass transition temperature, Tg, of this polymer to vary by more than 80 K © XXXX American Chemical Society

Received: June 9, 2016 Revised: July 27, 2016

A

DOI: 10.1021/acs.macromol.6b01237 Macromolecules XXXX, XXX, XXX−XXX

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various thermal treatments of the sample. This experimental observation indicates mutual interaction between different fractions of the molecules and points to the possibility of density equilibration among them.

dynamics is noted. In comparison, in the case of soft confinement (microemulsion placed within fluid medium), significant acceleration of mobility of molecules is observed. These conclusions were previously confirmed by Bodiguel et al.39,40 and Wang and McKenna41−43 for thin films supported on hot glycerol. As discussed above, dynamics of macromolecules subjected to 1D and 2D confinement has been a subject of intensive studies in the past. However, there is a special group of macromolecules classified as a type A, which attracted special attention. For those polymers, information about segmental and chain dynamics can be obtained directly from dielectric measurements. The observation of the latter mobility, which is manifested as a separate relaxation process (usually referred to as normal mode44) at low frequency, is possible because the net dipole moment is aligned parallel to the main backbone of macromolecule. Interestingly, there are few studies showing the impact of 1D and 2D confinement on the chain and segmental dynamics in the literature. Mapesa et al.45 studied segmental and chain mobility of poly(cis-1,4-isoprene) (PI) thin films supported on highly conductive silicon wafers. They found the segmental dynamics to be completely unaffected by the film thickness for a series of polymers of varying molecular weights. On the other hand, significant enhancement of normal mode for the higher molecular weight macromolecules (reptation dynamics regime) was observed. Additionally, the authors showed that the conformation of chains and their mobility are also related to the concentration of the solution from which thin films were prepared. Recently dynamics of PI has been also investigated under 2D confinement in uniaxial pores made of anodic aluminum oxide (AAO).13 It was shown that the Tg and mean relaxation times of the segmental process and normal mode were unaffected by the applied geometrical constraint. Nevertheless, the distribution of relaxation times of both processes broadened in pores. Consequently, the authors pointed out that chain dynamics is retarded with respect to the bulk PI. In addition, the results were discussed in term of confinement and adsorption effects. However, the former was observed to be negligible for the studied system while the latter was reported to control the motion and conformation of polymer chains. There have been also studies of molecular dynamics of poly(propylene glycol) (PPG), another type A polymer, confined in various nanoporous templates.46−49 In contrast to PI, the segmental dynamics of PPG was enhanced significantly in comparison to the bulk macromolecules. However, below a certain pore size (d < 3 nm), the segmental dynamics was noted to slow down, suggesting strong interplay between confinement (leading to the enhancement of dynamics) and adsorption (contributing to the observed slowing down) effects.49 In contrast, the relaxation rate of the normal mode was shown to be slowed down under spatial restriction. In this paper, we examine the molecular dynamics and glass transition temperature, T g, of PPG confined in AAO membranes as a function of pore size. Our results reveal that the dynamics of both processes (the α-relaxation and the most intense chain (normal) modes) differs from their bulk counterpart in the close vicinity of the glass transition temperature. A combination of DSC and BDS techniques allowed us to detect and determine the presence of the double glass transition (Tgs), connected with viftrification of interfacial and core molecules. It was found that the lower Tg increases (ΔTg = 5−6 K), whereas higher one slightly decreases due to

II. EXPERIMENTAL SECTION 1. Materials. Poly(propylene glycol) (PPG Mw = 4000 g/mol). with purity higher than 98% was supplied by Sigma-Aldrich. The chemical structure is presented as an inset in Figure 1b. For clarity,

Figure 1. DSC thermograms of bulk and confined samples. As insets in (a) and (b), the length scale of interfacial layer plotted as a function of pore diameter and the chemical structure of poly(propylene glycol) (PPG) with n = 68 (Mw = 4000 g/mol) are presented. PPG of Mw = 4000 g/mol has the degree of polymerization (n) equal to 68. The nanoporous alumina oxide membranes used in this study (supplied from Synkera Co.) are composed of uniaxial channels (open from both sides) with well-defined pore diameter. Details concerning porosity, pore distribution, etc., can be found in Table 1 and at the Web site of the producer.50 2. Samples Preparation. Prior to filling, AAO membranes were dried in an oven at 423 K under vacuum to remove any volatile impurities from the nanochannels. After cooling, they were placed in PPG. Then, the whole system was maintained at T = 343 K under vacuum (10−2 bar) for 24 h to let polymer 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 different pore diameters: 150, 73, 35, and 18 nm. The filling ratio of the pores was determined from weighing empty and B

DOI: 10.1021/acs.macromol.6b01237 Macromolecules XXXX, XXX, XXX−XXX

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Consequently, the glass transition temperature Tg of adsorbed polymers is much higher compared to the bulk. As presented in Figure 1, a significant shift of the primary (Tg1) and secondary (Tg2) transition to lower and higher temperatures, respectively, can be observed with an increase in degree of confinement. The difference between the two Tgs for the smallest studied pores (18 nm) reaches a value of 46 K (ΔT = Tg2 − Tg1). A comparable difference between the two glass transition temperatures has been observed for poly(methyl methacrylate) (PMMA) confined in AAO membranes (ΔT = 45 K).26 Taking advantage of calorimetric measurements, the length scale of interfacial layer, ζ, can be calculated from the following equation19

Table 1. Details Concerning Porosity, Pore Diameter, and Distribution of AAO Membranes parameter pore diameter [nm] pore density [cm−2] pore period [nm] estimated porosity [%] pore fullness after the inflitration [%]

membranes 18 6 × 1010 44 15 67

35 1 × 1010 94 13 79

73 2 × 109 243 10 80

150 9 × 108 367 15 69

filled membranes. It was calculated taking into account the porosity of the nanoporous material and the density of the material with the assumption that it is the same as in the case of bulk PPG. A filling of around 70−80% was achieved (see Table 1). 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 Quatro Cryosystem using nitrogen gas cryostat was better than 0.1 K. Dielectric measurements of bulk PPG were performed in a parallelplate cell (diameter: 10 mm; gap: 0.1 mm). AAO membranes filled with PPG were also placed in a similar capacitor (diameter: 10 mm; membrane: 0.005 mm).51,52 It should be added that dielectric measurements on empty membranes were carried out to evaluate the contribution of AAO, which turned out to be negligible to the recorded data. Dielectric measurements were performed in the temperature range 173−221 K for bulk and confined systems. Two temperature protocols were employed in the measurements of confined PPG: slow cooling and heating following fast quenching (rate ∼ 50 K/min). 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, with a heating rate of 10 K/min over a temperature range from 160 to 300 K. Each measurement was repeated thrice.

⎡ ⎛ ⎞1/2 ⎤ ΔCp ,1 d⎢ ⎜ ⎟ ⎥ ζ = ⎢1 − ⎜ ΔCp ,1 + ΔCp ,2 ⎟⎠ ⎥ 2 ⎝ ⎣ ⎦

(1)

where d is the pore diameter and ΔCp,1 and ΔCp,2 are the changes of the heat capacity at lower (Tg,1) and higher (Tg,2) glass transition temperatures, respectively. It should be noted, however, that this equation has some limitations: when calculating ζ, one assumes that (i) the volume of the material in the surface layer is proportional to the step change of its heat capacity, (ii) the density of the material inside does not change along the pore radius, and finally (iii) the shape of the pore is cylindrical. Values of ΔCp,1 and ΔCp,2 are presented in Table 2, Table 2. Values of Tgs, Tc, and ΔCp Obtained from Calorimetric and Dielectric Measurements DSC measurements sample bulk 150 nm 73 nm 35 nm 18 nm

III. RESULTS AND DISCUSSION In Figure 1, representative thermograms measured for bulk PPG and material confined in membranes of different pore sizes are depicted. Two endothermic processes located above and below Tg of the bulk macromolecules (Figure 1) for all confined samples are observed. They are a manifestation of the presence of a double glass transition phenomenon, reported for confined supercooled liquids.18−20,53−55 The origin of this finding can be explained taking into account a two-layer model, proposed by Park and McKenna.19 According to this approach, there are two subsets of molecules: (i) a “core” set, which comprises molecules at the center of the pore, and (ii) those at the interface which interact with the surface of pore walls. Because of a significant difference in mobility, they vitrify at completely different temperatures as shown in Figure 1. Another approach suggested in the literature considers an additional layer of molecules between the interfacial and core ones, characterized by moderate dynamics and density.56 This approach is more realistic since it takes into account a plausible continuous change in mobility and density of the confined material as the distance from the pore walls increases. Nevertheless, DSC measurements enabled us to distinguish just two fractions of molecules: core molecules (in the middle of the pores) characterized by faster dynamics and lower Tg, with respect to the bulk PPG, which is attributed to the loose packing of the polymer chains due to larger free volume;19 and interfacial molecules, attached to the pore walls, and slowed down due to creation of H-bonds with the pore walls.

Tg1 [K]

Tg2 [K]

202.3 195.3 218.8 190.4 225.3 188.1 230.3 185.9 231.9

BDS measurements

ΔCp,1 [J/kg]

ΔCp,2 [J/kg]

Tc [K]

Tg [K] for 100 s

0.67 0.50 0.40 0.31 0.25

0.57 0.48 0.35 0.30

219 224 229 231

199.4 192.5 189.7 184.2 179.6

while the thickness of interfacial layer plotted as a function of the pore diameter is shown as an inset in Figure 1a. As can be observed, the ζ parameter is significantly confinement-dependent and decreases with the lowering pore size. It is noteworthy that the estimated values are significantly higher than those reported for low molecular weight glass-forming liquids confined in nanoporous material.19,57 Hence, very comparable results were reported for PMMA26 confined within AAO temples; here, ζ was about 14 nm for the sample confined in 80 nm pores, which corresponds quite well to the estimated interfacial layer of PPG molecules, i.e., 12 nm for the sample infiltrated into 73 nm nanochannels. Additionally, our results are also comparable to those determined for salol25 confined within AAO membranes and poly(vinyl alcohol) (PVA)/silica nanocomposites.58 In Figure 2, dielectric loss spectra obtained for PPG (bulk and confined in AAO membrane with a pore size of 18 nm) are presented. As can be seen for the bulk sample (Figure 2a), spectra display three clearly visible dielectric processes: (i) the dc conductivity connected to the charge transport and located at the lowest frequencies, (ii) the most intense modes connected to the global chain dynamics and related to the fluctuations of the end-to-end vector of the chain (so-called C

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Figure 2. (a, b) Dielectric loss spectra collected for bulk PPG and PPG infiltrated into 18 nm pores, respectively. (c) Relaxation map of bulk and confined samples. (d) Dielectric spectra collected above the Tc. As the inset in (d), the real part of permittivity measured for PPG confined in 18 nm pores is presented. (e) Shape of the segmental relaxation measured for bulk PPG and the sample incorporated in 18 nm pores at 217 K.

normal modes), and (iii) the α-relaxation at the highest frequencies assigned to the cooperative motions of segments, which is considered to be responsible for the viscous flow and the liquid-to-glass transition. In the case of the PPG incorporated in AAO membranes, apart from the relaxation processes mentioned above, an additional one can be observed at very low frequencies (Figure 2b). Although this mode is hardly visible in the as-measured loss spectra, it can be seen in the real part of permittivity as an additional step (see inset to Figure 2d). According to previous investigations, this process is an interfacial one and is often discussed in terms of reorientational motions of molecules adsorbed at the pore walls.37,38,59−61 As presented in Figure 2d, the α-loss peak of confined samples is shifted toward higher frequencies with confinement, indicating the enhancement of the molecular dynamics of the studied systems.62 Comparing the shape of segmental relaxation (α-loss peaks were arbitrarily shifted vertically to superpose at maximum), it is found that the distribution of relaxation times becomes systematically broader with the reduction pore size (Figure 2e). Interestingly, the same observation is made for the normal mode. This reflects increasing heterogeneity of the soft matter confined in porous materials.25,63,64 Similar broadening of the chain and segmental loss peaks were also reported in the case of another type A polymer poly(cis-1,4-isoprene) (PI) confined in self-ordered AAO membranes.13 It was observed that the low frequency slope of chain and segmental processes of confined PI was less than 1. Interestingly, in the case of the former mode, such a phenomenon has been explained by considering that a true terminal relaxation rate has not been reached under confinement. Consequently, although the mean relaxation time of this

process seemed to be unaffected by spatial restriction, its broadening was interpreted as a clear sign of retarded chain dynamics.65 Nevertheless, it should be underscored that although we observed the broadening of the chain and segmental relaxation processes for the PPG incorporated in AAO pores, the dynamics of both modes was clearly enhanced in the vicinity of the glass transition (see Figure 2d). From the fitting analysis of obtained spectra by three superposed Havriliak−Negami (HN) functions with an additional term related to the dc conductivity, we determined the mean relaxation times of the normal and segmental modes, τNM and τα, respectively. To estimate precisely relaxation times of the interfacial process, the real part of permittivity has been analyzed with the use of the well-known Kramers−Krönig derivative method.66 As can be seen (Figure 2c), the temperature dependence of the normal mode and segmental relaxation times remains the same, within experimental uncertainty, for all examined samples at high temperatures. However, as temperature decreases, a pronounced crossover from VFT to Arrhenius-like behavior of relaxation times of both processes can be observed for the spatially restricted PPG. Note that this change of behavior of relaxation times occurs at some temperature, which we labeled as the crossover temperature, Tc. As presented in Figure 2c, this effect is confinement dependent and Tc increases with reduction of the pore size (see dotted lines in Figure 2c).25,62 Therefore, the most significant difference in the relaxation times can be observed between the bulk sample and material confined within the membrane with the smallest pore diameter (18 nm). Evidently, the dynamics becomes faster and the glass transition temperature shifts to lower temperatures with the decrease in pore size.25,59,62 D

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Figure 3. (a) Relaxation map of segmental relaxation of bulk PPG and sample confined in 18 nm pores measured after different thermal treatments. As the inset in (a), the relaxation map of normal modes is presented. (b) Dielectric loss spectra obtained for PPG incorporated in 18 nm pores at 225 K after various thermal treatments of the sample. (c, d) Time evolution of the loss spectra measured for PPG confined in 18 nm nanochannels at indicated temperatures. As the inset in (c), time dependence of relaxation times of segmental relaxation and normal modes scaled to the initial value. (e) Shape of structural relaxation before and after annealing performed at T = 218 K. (f) DSC thermograms obtained before and after annealing of PPG within AAO membrane of pore diameter 18 nm at 223 K.

polymer via H bonds. However, silanization (which eliminates the possibility of H-bonding) of the applied nanochannels pores reduces this effect, and an enhancement of the chain dynamics was noted, similar to our observations and like in case of PPG removed from silica composites.71 Temperature dependences of segmental and chain modes relaxation times shown in Figure 2c were fitted to the VFT equation:72−74

On the other hand, we noted that the observed deviation in the relaxation times is less pronounced in the case of the chain modes. It becomes noticeable only in the close vicinity of the glass transition temperature. This observation is rather unique in view of recent studies, which show that the same friction factor governs the T-dependence of segmental and chain dynamics. We make reference here to recent studies by Adrjanowicz et al.67 and Tarnacka et al.,68 who considered the role of density fluctuations quantified by the Ev/Hp ratio (where Ev and Hp are activation barriers calculated for the structural process at isochoric and isobaric conditions) in controlling dynamics of confined liquids.69 They showed that for the processes characterized by low or high Ev/Hp (molecular dynamics governed by either density or temperature), respectively, significant or minor deviation in relaxation times can be noted due to density fluctuations induced by vitrification of adsorbed molecules. Noteworthy, too, are high-pressure studies on PPG,70 which showed that the normal and segmental relaxation processes tend to merge at higher compression, indicating higher sensitivity of the latter process to density variation. Consequently, change in the temperature dependence of segmental relaxation times under confinement is much sharper and noticeable with respect to the normal mode. It is also worth mentioning that significant slowing down of the normal modes relaxation rate with decreasing pore size has been reported for PPG confined in porous glasses (pore size