Confinement Effects on the Dynamic Behavior of Poly(d,

Article pubs.acs.org/JPCB

Confinement Effects on the Dynamic Behavior of Poly(D,L‑lactic Acid) upon Incorporation in α‑Cyclodextrin M. T. Viciosa,† N. M. Alves,‡,§ T. Oliveira,‡,§ M. Dionísio,*,∥ and J. F. Mano‡,§ †

CQFM (Centro de Química-Física Molecular) and IN (Institute of Nanoscience and Nanotechnology), Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal ‡ 3Bs Research Group (Biomaterials, Biodegradables and Biomimetics), University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark, 4806-909, Taipas, Guimarães, Portugal § ICVS/3Bs PT Government Associate Laboratory, Braga, Guimarães, Portugal ∥ REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ABSTRACT: Inclusion complexes (ICs) composed of α-cyclodextrin (α-CD) and poly(D,L-lactic acid) (PDLLA), with 10/24 (IC1) and 15/46 (IC2) (% w/ w) of PDLLA incorporated/initial PDLLA weight percentage, were prepared and characterized mainly by dielectric relaxation spectroscopy (DRS). Bulk PDLLA was also analyzed for comparison. DRS was revealed to be a suitable tool to distinguish the dynamical response of the PDLLA regions constrained in between α-CD channels from the fraction incorporated inside channels. While the cooperative α-process undergoes a dramatic depletion shifting to higher temperatures (∼4.5 °C) for the PDLLA interchannels portion, it is suppressed for PDLLA chains inside pores. It was demonstrated that the broad secondary relaxation of bulk PDLLA is the Johari−Goldstein process (βJGprocess). The detection of its analogue in the ICs at higher frequencies, to a greater extent in IC1, is interpreted as a true confinement effect where the dimensions of the α-CD channels interfere with the length scale of the βJG-process. The limit predicted in the framework of the coupling model, where the α-relaxation transforms in the βJG-process, seems to be reached in the ICs. Furthermore, it was found that the length scale of the additional γ process only detected in the ICs is inferior to inter- or intrachannel dimensions.

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INTRODUCTION Poly(D,L-lactic acid) (PDLLA) is a well-known biodegradable polyester that has been widely employed in the development of biomedical devices, including drug delivery systems and supports for tissue engineering.1,2 As the glass transition occurs close to body temperature, it is highly important to understand the corresponding segmental mobility of the chains that will have crucial influence in the properties of this polymer. On the other hand, supramolecular inclusion complexes (ICs) have been widely investigated, especially using cyclodextrins (CDs) as host molecules,3−5 which are known to form stable structures with PDLLA. CDs constitute a series of cyclic oligosaccharides composed of 6, 7, and 8 D-glucose units linked by α-1,4 bonds and named α-, β-, and γ-CD, respectively. The geometry of CDs is like a hollow truncated cone forming a hydrophobic cavity, which have already been found to form ICs with both hydrophilic and hydrophobic polymers (e.g.,refs 3−5). An IC was developed in a previous work by adopting a simple and enhanced methodology.6 1H NMR and FTIR data confirmed the coalescence of PDLLA in this structure through the hydrophobic cavity interactions. XRD measurements demonstrated that through the combination of the amorphous polymer with α-cyclodextrin, a material with a well-organized arrangement was obtained adopting a crystalline channel type © 2014 American Chemical Society

organization. So, it was possible to transform amorphous PDLLA into an ordered structure, whereas most of the works regarding ICs of CDs and polymers were already prepared with semicrystalline polymers. The polymeric chains present a highly organized structure in this organization at the nanoscale level, which influences the conformational dynamics of the chains and thus its main physicochemical properties. It was shown before that the crystalline structure influences the glass transition dynamics in poly(lactic acid)-based systems;7,8 we hypothesize that polymeric chain confined within the IC supramolecular organization could undergo an even stronger confinement effect. In a previous work,6 a study of the glass transition dynamics of the IC was performed, namely by dynamic mechanical analysis (DMA). In this work, we analyze the effect of the ultraconfinement existing in the inclusion complexes on the macromolecular structure in the entire relaxational pattern accessible by dielectric relaxation spectroscopy (DRS), that includes the large scale segmental mobility derived from the glass transition dynamics and also the more local motions occurring typically at lower temperatures (or higher frequencies). DRS is able to probe dipolar fluctuations Received: May 9, 2014 Published: June 5, 2014 6972

dx.doi.org/10.1021/jp5045783 | J. Phys. Chem. B 2014, 118, 6972−6981

The Journal of Physical Chemistry B

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along a very large frequency range,9 being a very powerful tool to characterize the molecular dynamics. So, it allows detection of both subglass relaxations and the process associated with the dynamic glass transition. From the dielectric data analysis, a relaxation map could be drawn providing a picture of the dynamical behavior of the probed system in a broad range of temperature and frequency. The relaxational behavior of polylactides was already analyzed by DRS by some of us,10,11 and it was possible to detect, besides the α-relaxation, a very broad secondary relaxation, which deconvolutes in multiple processes in semicrystalline PLLA with a relatively high degree of crystallinity.12 The observation of higher glass-transition temperatures (Tg) for polymers in ICs as compared to the bulk has been reported,6,13−15 but in the present work, the focus is given to the confinement effect over the secondary relaxations for the polymer fraction incorporated into the α-CD channels.This provides a further contribution to understanding fundamental concepts such as the glass transition, under the scope of condensed matter physics. In this context, confinement studies are related to the relevance of a length scale responsible for glassy dynamics,16 offering a means to test theoretical models such as the Adam Gibbs theory17 that conceives cooperative rearranging regions with a length scale18 that increases upon temperature decrease and could be affected by the dimensions of the confining geometry. Indeed, if the dimensions of the confining host interfere with the spatial scale ξ of the molecular guest cooperative motion (in the scale of few nanometers), its molecular mobility can be dramatically changed from that of the bulk state.19−24 This could even lead to the suppression of the dynamically correlated regions,25 allowing estimation of a minimal length scale for the cooperative motion underlying the dynamic glass transition associated with the relaxation α-process, which manifests as a deviation from the typical curvature in the activation plot to Arrhenius dependence. In the limit of extreme confinement, when the pore size becomes smaller than the length scale of the α-process, it is predicted26,27 in the framework of the coupling model that the α-process is transformed into the process taken as its precursor, the Johari−Goldstein (JG) process. This extreme situation seems to be attained in the ICs as reported here. The DRS results were complemented with a differential scanning calorimetry (DSC) analysis of both hydrated and nonhydrated PDLLA and ICs, as it is known that water influences the glass transition dynamics of polylactides.11,28 Besides providing new insights on the general effect of nanogeometrical confinement on the molecular mobility of glass-forming materials, this work may be useful in the use of PDLLA in biomedical applications, namely, in the development of complex systems able to encapsulate therapeutic molecules and release them with a controlled profile.

(w/w) (IC1) and 1:1 (w/w) (IC2). After addition, the mixture was stirred for another hour at 60 °C. The heating was then stopped and the stirring was maintained for another 24 h. Finally, the obtained suspension was filtered and the white powder was washed several times with dioxane and distilled water. NMR1 showed6 that the weight percentage of PDLLA incorporated in the supramolecular structure of α-CD relative to the initial amount of PDLLA used in the composites preparation is 10%/24% and 15%/46% in, respectively, IC1 and IC2. To allow comparison, a film of bulk PDLLA was prepared by solvent casting using chloroform as solvent. Both ICs and PDLLA samples were allowed to equilibrate at environmental humidity. Techniques. Differential Scanning Calorimetry. The calorimetric experiments were carried out with a DSC Q2000 from TA Instruments Inc. (Tzero DSC technology) operating in the Heat Flow T4P option (details can be found in ref 29), coupled to an RCS cooling accessory. The calibration of DSC is of major importance to obtain reliable results from this technique.30 DSC Tzero calibration was carried out in the temperature range from 183 to 473 K. It requires two experiments: the first run with the empty cell (baseline) and the second run with equal weight sapphire disks on the sample and reference platforms (without pans). This procedure allows for cell resistance and capacitance calibration, which compensates for subtle differences in thermal resistance and capacitance between the reference and sample platforms in the DSC sensor. Enthalpy (cell constant) and temperature calibration were based on the melting peak of the indium standard (Tm = 429.75 K) supplied by TA Instruments (Lot #E10W029). Samples of less than 3 mg were used. Thermal cycling at cooling/heating rate of 10 °C.min−1 from −90 °C up to increasing final temperatures was carried out: from 80 to 140 °C for PDLLA over 6 cooling/heating scans and from 80 to 250 °C for IC2 over 7 cooling/heating scans; measurements were carried out under dry, high purity nitrogen at flow rate of 50 mL·min−1. Dielectric Relaxation Spectroscopy. For the dielectric spectroscopy measurements, PDLLA 56-μm-thick films were cut into disks of about 25 mm in diameter. For the PDLLA-αCD-ICs measurements, the powder was compressed between electrodes which were held apart by two 50-μm-thick silica spacers. The samples were placed between two stainless steel electrodes (smaller electrode with 20 mm for PDLLA and 10 mm diameter for ICs measurements) in a parallel plate capacitor, BDS 1200. The sample cell was mounted on a cryostat, BDS 1100, and exposed to a heated gas stream evaporated from liquid nitrogen in a Dewar. Temperature control was assured by the Quatro Cryosystem and performed within ±0.5 K (all modules supplied by Novocontrol). Measurements were carried out using an Alpha-N analyzer also from Novocontrol GmbH, covering a frequency range from 10−1 Hz to 1 MHz. The samples were previously held for 30 min at 120 °C to ensure water removal; therefore, the results presented here are for the anhydrous systems. After a first cooling ramp from room temperature to −120 °C, isothermal spectra were collected upon increasing different temperature steps: in the range −120 °C to −50 °C each spectrum was recorded every 5 °C, and from −48 °C to +150 °C, the spectra were recorded every 2 °C.

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EXPERIMENTAL SECTION Materials. Purac supplied the medical grade PDLLA. The molecular weights of the polymer were Mn = 31 750 and Mw = 100 000. α-Cyclodextrin (α-CD) and the organic solvents used were purchased from Aldrich and were used without further purification. For the preparation of the ICs a solution at 60 °C of PDLLA (1.2 g (IC1) and 3.2 g (IC2) in 200 mL of dioxane after complete dissolution during 3 days at 60 °C) was slowly added (dropwise) into a solution of α-CD (3.7 g in 25 mL of distilled water) also at 60 °C under continuous stirring; the final complexes have a PDLLA:α-CD proportion close to 1:3 6973



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Dielectric Data Analysis. To analyze the isothermal dielectric data, the model function introduced by Havriliak− Negami (HN) was fitted to both imaginary and real components of complex permittivity using the WinFit software (version 3.4, Novocontrol Technologies, 2011). Because multiple peaks are observed in the available frequency window, a sum of HN-functions was employed: ε*(f ) = ε∞ +

∑ j

Δεj [1 + (iωτHNj)αHNj ]βHNj

⎛ σ ⎞s − i⎜ ⎟ ⎝ ε0ω ⎠

(1)

where j is the index over which the relaxation processes are summed, Δε is the dielectric strength, τHN is the characteristic HN relaxation time, and αHN and βHN are fractional parameters (0 < αHN < 1 and 0 < αHN.βHN < 1) describing, respectively, the symmetric and asymmetric broadening of the complex dielectric function;31 the second term on the right-hand side of eq 1 takes into account the conductivity contribution that manifests mainly at high temperatures and low frequencies, where ε0 is the vacuum permittivity; σ and s are fitting parameters: σ is related to the direct current (dc) conductivity of the sample, and the parameter s (0 < s ≤ 1) reflects conductivity of ions for s = 1 and for s < 1 interfacial polarizations, including electrode polarization. From the estimated values of τHN, αHN, and βHN parameters, a model-independent relaxation time, τ = 1/(2πf max), was determined according to eq 2 (see refs 31, 32, and 33 for details): ⎡ ⎢ sin τ = τHN⎢ ⎢ sin ⎣

( (

αHNβHNπ 2 + 2βHN αHNπ 2 + 2βHN

) )

⎤1/ αHN ⎥ ⎥ ⎥ ⎦

Figure 1. Scaleup in the glass transition region of the DSC thermograms upon thermal cycling with successively higher final temperatures from 80 (black lines) to 140 °C (red line) collected for (a) PDLLA (mPDLLA = 1.28 mg) and (b) IC2 (mIC2 = 2.66 mg). For pure PDLLA, the thermogram obtained after keeping a fresh sample 30 min at 120 °C is also included (blue line in part a; it was vertically shifted to allow a better comparison).

(2)

Close to the glass transition temperature, the temperature dependence can be described by the well-known Vogel/ Fulcher/Tammann/Hesse equation34−36 which reads ⎛ B ⎞ τ(T ) = τ∞ exp⎜ ⎟ ⎝ T − T0 ⎠

the last cycle allow estimation of glass transition temperatures of Tg,onPDLLA = 51.2 °C, Tg,midPDLLA = 53.4 °C, which are close to 13 °C above that of hydrated PDLLA; from the loss of mass a water content of 7% was estimated for this sample. The estimated temperature Tg,midPDLLA = 53.4 °C will be considered as the glass transition temperature for dried PDLLA. Shifts up to 20 °C were observed in different fresh samples for which lower initial glass transition temperatures were detected, indicating different hydration levels. These results clarify the significant shift toward higher temperatures upon dehydration, confirming the plasticizing effect of water as reported for PLLA.11 Moreover, the heat flux discontinuity along the glass transition is much narrower after dehydration. Hereafter, the temperature taken in the midpoint will be taken for further discussion. The heat capacity change at the glass transition for the dried sample (after correction for the loss mass) is ΔCp = 0.42 J. K−1.g−1 = 30.5 J. K−1.mol−1 (taking the monomer mass 72.06 g·mol−1). This change in heat capacity in the glass transition region seems to be in line with the values reported for amorphous poly(lactid acid) with higher Mw and Mn of 43.8 J.K−1.mol−1 (Mw varying from 180 000 to 220 000) by Pyda et al.37 and and 0.48 J.K−1.g−1 (Mn = 116 000) by Delpouve et al.38 An additional run was carried out to obtain a dried state in a one-step experiment, consisting of maintaining the sample for 30 min at 120 °C. The thermogram taken subsequently is

(3)

τ∞ and B are constants and T0 is the so-called Vogel temperature.

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RESULTS Thermal Properties. Since polylactic acid is hydrophilic, a first thermogram was collected up to 150 °C to observe the occurrence of water evaporation, which indeed was identified by a broad endotherm (not shown). Therefore, the state of the as-prepared samples is designated as “wet”. Moreover, to observe in detail the water effect over the glass transition, a particular cycling thermal treatment was applied to fresh samples of PDLLA and to one of the ICs (IC2); the characterization of the glass transition region of the inclusion complexes is reported in ref 6. For bulk PDLLA, thermal cycling was conducted by DSC by gradually increasing the final temperature attained in each heating scan, ranging from 80 to 140 °C. A scaleup of the glass transition region is presented in Figure 1a that shows the thermograms obtained on heating at 10 K.min−1. For the wet sample, the glass transition temperature measured in the onset (Tg,on) and measured in the midpoint (Tg,mid) was determined as, respectively, 37.2 and 41.2 °C. The thermogram collected in 6974



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included in Figure 1a (blue line), being in close agreement with the one obtained for the last cycle upon the previous treatment (red line). Furthermore, no more thermal events were detected by DSC under any of the different applied conditions as expected, because PDLLA is an amorphous polymer in opposition to PLLA that can undergo crystallization depending on the applied thermal treatment.10,39,40 For the as prepared IC2, the glass transition signal is identified in the first heating run superimposed on the broad endotherm due to water evaporation (Figure 1b). In the thermograms taken upon further cycles, the glass transition is detected, although through a less intense heat flux step relative to bulk PDLLA emerging at lower temperatures relative to first cycle (see arrow in Figure 1b). It is important to note that the DSC signal is due to the incorporated polymer, since unloaded α-cyclodextrin has no thermal response besides the initial endotherm due to water release (not shown) as also reported in ref 41. In fact, single α-CD and also typically their ICs do not show detectable thermal transitions such as melting/crystallization, as pointed out by authors.42−44 Moreover, no change is observed for the temperature position of the glass transition signal for IC2 after reaching 90 °C, differently from bulk PDLLA for which the glass transition is continuously shifting until the final temperature is raised up to 120 °C. Even after heating up to 250 °C, the final temperature for thermal cycling treatment in IC2 (see Experimental section), no further changes are observed. This can be taken as an indication that water is interacting mainly with the hydrophilic moieties of cyclodextrin instead of with the PDLLA polymer itself. The glass transition temperature for PDLLA in the composite is Tg,mid‑IC2 = 57.6 ± 0.1 °C revealing an increase of 4.2 °C relative to that of bulk dried PDLLA. The relatively small heat flux change and the increase of Tg for IC2 is a consequence of the confinement effect imposed by the IC structure on the glass transition dynamics of PDLLA, reducing the mobility of the polymeric chains in the channel structure of the IC. Wide angle X-ray scattering using synchrotron radiation showed that the supramolecular channel-type crystalline structure of IC is maintained over a quite high temperature range: the supramolecular organization is maintained up to temperatures above 300 °C, then lost during the thermal degradation of the material.15 To get a further insight into the dynamical behavior of PDLLA in the inclusion complex, dielectric relaxation spectroscopy was carried out and results are described in the next section. Dielectric Properties. For the dielectric measurements the IC1, IC2, and PDLLA samples were first heated in the DRS sample holder at 120 °C (see Experimental Section). The loss curves for bulk PDLLA and the inclusion complexes are depicted at some representative temperatures in Figure 2, in the low temperature region, and in Figure 3 (IC2) in the temperature region close to the glass transition. While for pure PDLLA only a secondary process is observed in the subglass transition region (−120 to 0 °C) (Figure 2a), two secondary relaxations with higher intensity are clearly distinguished for the inclusion complex IC2, labeled γ and β in decreasing order of frequency (Figure 2b). The significant enhancement of the dielectric response in this IC is also seen in the thickness independent tan δ plot (see TOC). The two secondary processes are also observed for IC1 although less

Figure 2. Isothermal loss curves between −100 and −10 °C in steps of 10 °C of (a) bulk PDLLA and (b) IC2. (c) Comparison of IC1 (red), IC2 (blue), and PDLLA (black) loss curves at −70 °C; dashed lines are the individual HN fitting functions from which the overall fit was obtained, which is also represented as solid lines in (a), (b), and (c).

resolved. This is illustrated in Figure 2c that compares the isothermal spectra of the three systems at −70 °C. The comparison evidences the shift to higher frequencies of the ε″ spectra for the ICs, to a great extent for IC1; this will be discussed later. At higher temperatures, the relaxation process associated with the dynamic glass transition of PDLLA, labeled α-process, emerges at 60 °C. While this process is clearly detected in the bulk polymer (Figure 3a), in IC2 it comes out with lower intensity (Figure 3b). Figure 3c shows a comparison of the isothermal spectrum taken at 70 °C of bulk PDLLA and IC2. For the latter, the strong depletion (logarithmic scale is used in Figure 3c) and the shift toward lower frequencies are obvious. In the case of IC1, the spectra at these temperatures are affected by conductivity impairing the clear detection of the αprocess. However, it is possible to identify this relaxation for the two inclusion complexes through the isochronal plot of Figure 4. The comparison with the α-relaxation of bulk PDLLA 6975



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Figure 4. Isochronal plot of the imaginary part of the complex permittivity at 1 kHz for PDLLA (black circles), IC1 (red squares), and IC2 (blue triangles); the presence of α-CD depletes the segmental α-process and resolves two sub-Tg relaxations.

straining the confining geometry allowed deconvolution of another secondary relaxation, the γ-process, being detected at the highest frequencies of the tested frequency window (lower temperatures). The multimodal response of the complex dielectric permittivity of the different systems can be simulated by a sum of HN equations (eq 1). Solid lines in Figures 2 and 3 correspond to the overall fit allowing the conclusion that the spectra are well described by this method. The individual processes used to simulate the spectra at some representative temperatures, −70 °C in Figure 2c and 70 °C in Figure 3c, are illustrated as dashed lines. For IC2, ε″ spectra are well simulated by a superposition of three HN functions corresponding, respectively, to α, β, and γ relaxations; for IC1 and the bulk polymer only two HN functions were needed in the temperature range where the fit was carried out (β and γ and α and β, respectively). The shape αHN and βHN fitting parameters are presented in Table 1 for all the processes detected in PDLLA and both IC1 and IC2. For the spectral simulation a conductivity contribution was also taken into account (second term on the right-hand side of eq 1). This becomes significant at temperatures higher than that of the α-process. For the ICs, an additional Maxwell− Wagner−Sillars31,45 (MWS) process due to interfacial polarization was considered, as also found for the analogous PLLA/ montmorillonite nanocomposites.46 This can be seen by the analysis of the exponent in the conductivity contribution (s in the second term on the right-hand side of eq 1), which is usually smaller than 1 if electrode and interfacial polarization effects also manifest as mentioned above (see Experimental Section). In fact, the best fit for the dielectric loss isotherms of IC2 was achieved with s varying between 0.45 and 0.76 for 70 °C ≤ T ≤ 124 °C; for higher temperatures the increasing ionic conductivity is so high that s turns becomes close to unity as found for bulk PDLLA in the entire temperature range. Figure 3c illustrates the influence at low frequencies of the interfacial contribution due to charges blocked at IC2 interfaces in the inclusion complex as compared with that of bulk PLLA. A parameter that comes out from the fitting procedure, which allows us to gain insight into the dynamical behavior of the tested materials, is the relaxation time, τHN (see eq 1). This

Figure 3. Isothermal loss curves in the temperature region above the glass transition for 60, 64, 70, 74, and 80 °C of (a) bulk PDLLA and (b) IC2 (the inset shows the corresponding isotherms for IC1), and (c) comparison of IC2 and PDLLA loss curves at 70 °C; the low frequency tail was simulated by adding the conductivity term in eq 1: s = 1 for PDLLA corresponding to pure conductivity and s = 0.45 in IC2 indicating the influence of interfacial polarization.

confirms the attribution in the ICs to the process associated with the dynamic glass transition and the slight shift toward higher temperatures, to similar extents, in both ICs. Interestingly, in the DSC thermograms taken for IC2, the glass transition was identified by an ill-defined step in the heat flux (Figure 1b), while it is undoubtedly noticed in the isochronal plot. This indicates a higher sensitivity of DRS compared with conventional DSC. Besides the strong depletion of the main segmental α-process for the ICs, more accentuated but even noticed for IC1 and the slight shift to higher temperatures of the peak maximum (see vertical bar in Figure 4), the isochronal representation also puts into evidence the intensification of the subglass dielectric response mainly for IC2 that was already seen from the direct observation of dielectric data (remember Figure 2c). Con6976



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Table 1. HN (eq 1), VFT (eq 3), and Arrhenius Parameters for All the Processes Detected in Bulk PDLLA, IC1, and IC2a PDLLA α αHN βHN τ∞/s B/K T0/K Ea,VFT(Tg) /kJ mol−1 Ea,DMA range /kJ mol−1 Tg (DRS, τ = 100 s) m Tg(DSC)

0.69 ± 0.05 0.46 ± 0.03 (1.2 ± 0.2) × 10−13 1143 ± 58 285 ± 1 870b 532 ± 36c

IC1 β

β

IC2 γ

0.26 1 (1.6 ± 0.3) × 10−15

0.34 ± 0.02 0.80 (6.8 ± 3.5) × 10−17

0.28 ± 0.01 0.87 (8.0 ± 6.6) × 10−16

51.8 ± 0.4

52.8 ± 0.7

37.8 ± 1.1

α 0.52 ± 0.02 1 (2.6 ± 2.1) × 10−14 1202 ± 177 289 ± 4 926

β

γ

0.41 ± 0.05 1 (3.6 ± 0.8) × 10−16

0.49 ± 0.05 0.3 (1.1 ± 0.3) × 10−15

51.8 ± 0.4

37.0 ± 0.4

323K = 49.5 °C

318 K = 45.0 °C 143 326.6 K = 53.4 °C

150 330.8 K = 57.6 °C

a The glass transition temperature estimated from dielectric data for τ = 100 s is also included, along with the fragility index, m (eq 5). bActivation energy at Tg,DRS estimated from the VFT parameters (eq 3). cActivation energy estimated from the slope of the log10(τ) vs 1/T in the range of DMA measurements to allow comparison with values reported in refs 6 and 15.

states). Although the linearity of the relaxation times dependence is maintained, it is deviated to lower temperatures/lower relaxation times, to a greater extent for IC1. This can be directly seen from the ε″ isotherms compared in Figure 2c where the maxima (indicated by vertical bars) of the ICs is shifted to higher frequencies relative to the β-process in bulk PDLLA. On the other hand, the relaxation map presented in Figure 5 also includes the activation plot for the γ-process, from which slope an activation energy of 37.8 ± 1.1 and 37.0 ± 0.4 kJ mol−1 for, respectively, IC1 and IC2, was estimated. The temperature dependence of relaxation times found for the processes detected in anhydrous α-CD matrix41 are represented as dashed green lines excluding the hypothesis that the processes observed here have originated from the cyclodextrin matrix itself. Therefore, the detected γ-process seems to have genuinely originated from the ICs. In contrast to the linearity of the log of relaxation times vs 1/ T of the subglass relaxations, a curvature is exhibited for the αprocess in both the bulk and IC2 polymer, which obeys a VFTH lawsee the fit parameters in Table 1. It is important to note that the relaxation map also includes the relaxation times obtained from the isochronal plot of the dielectric loss of αIC2, which is an alternative procedure to extract the relaxation times when the isothermal loss peaks are ill-defined, as is the case here for the α-process of the inclusion complex. The agreement with the relaxation times obtained from the HN fits of the isothermal loss curves is excellent. This validates the use of isochronal data especially when the respective loss peak maxima are poorly defined in the frequency domain or out of the accessible frequency range.47−49 From the extrapolation of the VFTH equation to τ = 100 s,50,51 a glass transition temperature is estimated as 45.0 and 49.5 °C for the PDLLA and IC2, respectively. The 4.5 °C increase of the glass transition temperature for the inclusion complex as compared with the bulk is in agreement with the increase determined here by DSC (4.2 °C) and close to the value previously estimated by DMA (∼10 °C) measurements.6 So, the dielectric results also evidenced the restricted mobility of the PDLLA chains in the ICs in agreement with the previous DSC and DMA results, which revealed a Tg increase, associated with a small and broad heat flux step and a broadening of the loss factor (tan δ) DMA peaks.6,15 The difference between the

Figure 5. Relaxation map for PDLLA and ICs evidencing the linearity of the secondary relaxations and curvature of the α-process for bulk PDLLA and inclusion complexes; cross circles: τ obtained from isochronal representation. The star indicates the τ0 of the coupling model (eq 7) in good agreement with τβ,PDLLA (see text). The activation plot for the β and γ processes of anhydrous α-CD retrieved from ref 41 are included for comparison as dashed green lines. The inserted picture illustrates the intra- and interchannel PDLLA fractions, the former being in the origin of the constrained α-process observed for the ICs.

quantity is plotted against the temperature reciprocal in Figure 5 after conversion to τmax according eq 2. The linearity of the log of relaxation times vs 1/T of the subglass relaxations is clear in both IC and for the β-process in PDLLA as usually found for subglass relaxations following an Arrhenius law: τ(T) = τ∞ exp(Ea/RT), where τ∞ is the pre-exponential factor and Ea is the activation energy; R is the ideal gas constant and T the temperature. The prefactors and activation energy for the subglass processes are summarized in Table 1 for the three samples studied in this work. The estimated Ea values for this secondary process are 51.8 ± 0.4 kJ mol−1 for both PDLLA and IC2, and 52.8 ± 0.7 kJ mol−1 for IC1, being in agreement with 50 ± 3 kJ mol−1 as obtained by some of us12 for the activation energy of the secondary βrelaxation for amorphous and semicrystalline PLLA (crystallinity induced isothermally at 80 °C from both glassy and melt 6977



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τJG(T) ≈ τ0(T) = τcn[τα(T)]1 − n

Tg values estimated by DRS and determined from DSC (see Table 1) is not surprising since the relaxation time at the calorimetric glass transition temperature is not 100 s for many polymeric systems.52,53 From the VFT parameters it is possible to estimate an activation energy at a given temperature, according to Ea(T ) =

where τ0 is the primitive relaxation time of the CM, n is the coupling parameter, τc is a time characterizing the crossover from independent to cooperative fluctuations found to be close to 2 × 10−12 s for molecular glass-formers,59 τα = τKWW is the relaxation time of the Kohlrausch−Williams−Watts (KWW) function,60,61 φ(t) = exp[−(t/τKWW)βKWW] that describes the relaxation response in the time domain 0 < βKWW < 1; βKWW = 1 for a Debye response. The βKWW parameter can be estimated through the HN shape parameters of the α-process for bulk PDLLA given in Table 1, by using the empirical correlation (βKWW = (αHN.βHN)1/1.23) proposed by Alegrı ́a et al.,62 allowing one to obtain a value of βKWW = 0.39. The coupling parameter, n, is equal to 1 − βKWW, giving a value of τJG ∼ τ0 at Tg,Diel of 4.9 × 10−7 s. This value is in excellent agreement with the experimental τβ value at 318 K (Tg,Diel) as indicated by the star in the relaxation map of Figure 5, confirming the assignment of the β-process in bulk PDLLA to a βJG-process.

RB T0 2 T

(1 − )

(4)

The value of activation energy estimated at the dielectric glass transition temperature is included in Table 1. Additionally, the curvature of the log10(τ) vs 1/T plot, i.e., the degree of deviation from Arrhenius-type temperature dependence near Tg, allows one to quantitatively measure the fragility54,55 as the steepness index m according the following equation: ⎛ ⎞ ⎜ ∂(log10[τ(T )]) ⎟ m=⎜ ⎟ Tg ⎜ ⎟ ∂ T ⎝ ⎠T = T

()

g

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DISCUSSION The dielectric results reveal an evident decrease of the intensity of the segmental mobility that governs the dynamic glass transition in the inclusion complexes and a slight shift to higher temperatures, meaning that this process becomes hindered in the ICs. By contrast, in both ICs, the intensity of the β-process relative to the α-process is highly increased when compared to the bulk polymer and becomes more mobile, to a greater extent for IC1. Also, on the ICs the broad secondary process observed in PDLLA splits into two relaxation processes, which may be related to the fact that the polymer chains relax in highly restricted spatial regions as observed in semicrystalline PLLA with a crystallinity degree higher than 0.43.12 While in amorphous and semicrystalline PLLA with a low degree of crystallinity (χ ∼ 0.3) a single β-process was detected, characterized by an abnormally broad distribution of relaxation times (αHN ∼ 0.28; βHN = 0.7), in higher crystallinity specimens (0.43 ≤ χ ≤ 0.65), the broad secondary relaxation is deconvoluted in narrower multiple components. For PDLLA incorporated in the cyclodextrin matrix, the β-process is less wide (αHN ∼ 0.34 (IC1), 0.41 (IC2), βHN = 0.8 (IC1), 1 (IC2)) relative to bulk PDLLA (αHN ∼ 0.26, βHN = 1); this narrowing reveals a very local γ-process. Therefore, the incorporation into the α-CD matrix has an effect on the secondary relaxation with identical features to those imposed by the amorphous regions that persist within the crystalline regions in PLLA. For bulk PDLLA the β-process can be identified with the primitive or precursor mechanism (βJG-process) of the cooperative structural relaxation in the origin of the dynamic glass transition. If it is assumed that the attribution to a JG process also holds for the β-process detected in the ICs, the mobility enhancement (shift to higher frequencies/lower temperatures) can be seen as manifestation of a true confinement effect as also observed for the analogous relaxation observed in a pharmaceutical drug confined to a silica matrix of 8.6 and 3.6 nm pore size.63 The reason the β-process becomes more mobile in IC1 can be explained on the basis of the higher relative amount of PDLLA effectively incorporated inside the channel structure of α-CD in spite of the overall PDLLA smaller quantity used in the preparation of this IC (see Experimental Section). Consequently, PDLLA in IC1 experiences a greater confinement effect, where the dimensions of the

(5)

Fragility values typically range between m = 16 for strong systems and m ≈ 20050 for fragile systems where a marked deviation from an Arrhenius dependence occurs induced by cooperative molecular fluctuations. From the VFTH equation, m can be calculated according to m=

BTg ln(10)(Tg − T0)2

(7)

(6)

as m PDLLA = 143 and mIC2 = 150, allowing one to classify both systems as fragile glass formers.50 Interesting, the inclusion process has a fragility comparable to that of the bulk polymer. At first sight, this seems to be in contrast with the behavior found for identical ICs6,15 probed by DMA over a more limited frequency range, and for which a linear log10(f) vs 1/T plot is obtained. However, if we compare the relaxation times estimated in the present work over a time scale corresponding to DMA measurements (see indication of the equivalent range in the relaxation map in Figure 5), an activation energy of 532 ± 36 kJ mol−1 is estimated for IC2 (same designation in ref 6) in accordance with the respective value obtained previously for this system (488 ± 38 kJ mol−1)6 and for a homologous IC (580 ± 44 kJ mol−1).15 Given that PDLLA is a type A polyester having a component of the dipole moment parallel to the main chain, this α-process was previously assigned to a local segmental mode due to the transverse component of the monomeric dipoles.56 Since the PDLLA polar ester group lies in the main chain, there are no flexible side chain dipolar units, and therefore the β-process was attributed to the local twisting motions of the PDLLA chains in the glassy state (averaging angle for twisting motions of 11°).56 Therefore, this secondary process in nature is similar to the main α-process, however over a small length scale. This could indicate that the β relaxation corresponds to the so-called Johary−Goldstein (JG) process,57 βJG, or the primitive relaxation process, precursor of the α-relaxation, as postulated in the framework of the Coupling Model (CM).58 In order to test this attribution, the relaxation time of the JG-process, τβ,JG, must be compared to that of the α-process through the equation 6978



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α-CD channels interfere with the length scale of the mechanism of this secondary relaxation related with the glass transition. Hence two different effects are simultaneously manifested in the inclusion complexes: (i) a true confinement effect for the fraction that is effectively incorporated inside the α-CD channels (PDLLA intrachannels fraction) and (ii) constraining of molecular mobility for the fraction that remains outside αCD channels, even so, undergoing influence of the cyclodextrin supramolecular structure (PDLLA interchannels fraction), identical to that observed in semicrystalline specimens (see illustration in the inset of Figure 5). Therefore, the PDLLA incorporated within the cyclodextrin structure determines the dynamics of the β-process, while the polymeric fraction remaining in the IC but outside the α-CD structure originates the α-process that is relatively hindered compared to the bulk PDLLA, exhibiting characteristics similar to the α-process of the rigid amorphous phase detected in a variety of semicrystalline polymers.10,64−69 This is the reason the estimate of τ0 according eq 4 cannot be made through the parameters of the detected α-process for the ICs. For the fraction effectively incorporated inside the cyclodextrin structure, the α-relaxation was suppressed, meaning that the confining dimensions are below the length scale of the cooperative mechanism that would drive the glass transition inside the α-CD channels. This is not surprising since the inner cavity dimensions of α-CD are in the range of 4.7−5.3 Ă 70 below the nanometer dimensions at which interference occurs with the length scale of the α-process in polymeric systems.71,72 In low molecular weight materials it was found63 that the βJG and the α-process shift to lower temperatures with decreasing confining pore size; in the limit, as predicted in the framework of the coupling model, the α-process is transformed into the Johari−Goldstein (JG) process.23,24 This limit seems to be reached for PDLLA relaxing inside α-CD channels. To that extent that some large scale cooperative motion takes place, however, behind the limit of DRS detection, since the dynamics of the α and βJG relaxations are interdependent,73 the respective glass transition will be lowered relative to that of the bulk. The increase in the dielectric strength of the secondary βrelaxation relative to the bulk PDLLA means that the transverse dipolar component transfers its relaxation mechanism from the cooperative α-process to the β-process, probably undergoing a preferential alignment of the active dipoles inside α-CD channels which enhances the dielectric response. The reason the two traces of this secondary relaxation observed for IC1 and IC2 do not fall into a single chart in the relaxation map is the contribution of the respective β bulk-like process taking place between channels. This is confirmed by the higher-intensity response through the secondary relaxations in IC2, for which the fraction not incorporated in α-CD channels is the most significant. By contrast, the dynamics of the γ process once it is detected is insensitive to the kind of environment that is revealed as occurs over a length scale smaller than either intraor interchannel dimensions.

From the dielectric characterization multiple relaxation processes were identified for the inclusion complexes: two secondary processes, named γ and β in the subglass region, and the α-process associated with the dynamic glass transition, the latter only able to be characterized for IC2. At higher temperatures, MWS due to interfacial polarization owing to CD/PDLLA interfaces was detected, which is absent in the bulk polymer. For bulk PDLLA it was demonstrated that the β-process is cohesive with a Johari−Goldstein process taken as the precursor of the relaxation process responsible for the dynamical glass transition in the framework of the coupling model. This process is shifted to higher frequencies/lower temperatures in the ICs, with a greater extent in IC1. This enhancement of mobility is rationalized as a true confinement effect originating mainly over the PDLLA fraction incorporated into α-CD channels whose dimensions interfere with the length scale of the process precursor of the structural relaxation, suppressing the latter that occurs over a larger spatial scale; it is proposed that if it was possible to detect the α-process inside αCD channels, it would exhibit a lower glass transition temperature. At the same time, the nondetection of the confined α-relaxation leads to the conclusion that the limit predicted in the framework of the coupling model where this process transforms in the βJG-process is reached in the ICs. The reason the respective traces in the relaxation map do not follow in the same chart is the contribution of the β bulk-like secondary process, which is higher for IC2. Therefore, the detected α-process takes place mostly in the interchannel PDLLA fraction in a similar way to the identical relaxation observed in the rigid amorphous phase in semicrystalline PLLA; the glass transition temperature of this relatively hindered process in IC2 increases by 4.5 °C relative to bulk PDLLA as estimated from the VFT fit to the dielectric data, in good accordance with DSC measurements. Additionally, the organization imposed by the α-CD matrix allows a highly local relaxation process to be revealed, designated here as the γ-process, for which the associated length scale is below either the inter- or intrachannel dimensions. DRS is revealed to be a suitable tool to distinguish the dynamical behavior of PDLLA incorporated inside α-CD channels from the behavior of the PDLLA fraction remaining between channels. Additionally, the effect of water on the glass transition was studied in more detail by DSC for PDLLA and IC2. A significant effect was found upon dehydration for the bulk polymer whose glass transition temperature shifts ∼13 to 20 °C toward higher temperatures after water removal (depending in the initial hydration level), while no shift is observed for the inclusion complex. This is taken as an indication that water is mainly interacting with the hydrophilic moiety of α-CD. After water removal the glass transition in the IC2 inclusion complex occurs at a higher temperature relative to the bulk polymer (4.2 °C as seen by DSC) in excellent accordance with DRS observations.

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CONCLUSIONS The thermal and dielectric properties of poly(D,L-lactide acid), PDLLA, and its inclusion complexes into α-cyclodextrin (PDLLA-α-CD-IC) with 10%/24% (IC1) and 15%/46% (IC2) (w PDLLA incorporated/w PDLLA initial) were investigated by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6979



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(19) Kremer, F.; Huwe, A.; Schönhals, A.; Rózȧ ński, S. A. Molecular dynamics in Confining Space in Broadband Dielectric Spectroscopy; Schönhals, A., Kremer, F., Eds.; Springer-Verlag: Berlin, 2003; Chapter 6. (20) Ngai, K. L. In Disorder Effects on Relaxational Processes; Richert, R., Blumen, A., , Eds.; Springer: Berlin, 1993. (21) Sappelt, D.; Jäckle, J. The Cooperativity Length in Models for the Glass Transition. J. Phys. A: Math. Gen. 1993, 26, 7325−7341. (22) Fischer, E. W.; Donth, E.; Steffen, W. Temperature Dependence of Characteristic Length for Glass Transition. Phys. Rev. Lett. 1992, 68, 2344−2346. (23) Gorbatschow, W.; Arndt, M.; Stannarius, R.; Kremer, F. Dynamics of H-Bonded Liquids Confined to Nanopores. Europhys. Lett. 1996, 35, 719−724. (24) Arndt, M.; Stannarius, R.; Gorbatschow, W.; Kremer, F. Dielectric Investigations of the Dynamic Glass Transition in Nanopores. Phys. Rev. E 1996, 54, 5377−5390. (25) Koppensteiner, J.; Schranz, W.; Puica, M. R. Confinement Effects on Glass Forming Liquids Probed by Dynamic Mechanical Analysis. Phys. Rev. B 2008, 78, 054203−054214. (26) Ngai, K. L. Predicting the Changes of Relaxation Dynamics with Various Modifications of the Chemical and Physical Structures of Glass-Formers. J. Non-Cryst. Solids 2007, 353, 4237−4245. (27) Ngai, K. L. Relaxation and diffusion in complex systems; Springer: New York, 2011; Chapter 2. (28) Delpouve, N.; Stoclet, G.; Saiter, A.; Dargent, E.; Marais, S. J., Water Barrier Properties in Biaxially Drawn Poly(lactic acid) Films. Phys. Chem. B 2012, 116, 4615−4625. (29) Thermal Analysis of Polymers, Fundamentals and Applications; Menczel, J. D., Prime, R. B., Eds.; John Wiley & Sons, Inc.: Hoboken, 2009. (30) Mano, J. F.; Gómez Ribelles, J. L. Influence of the Sample Mass on the Study of the Glass Transition and the Structural Relaxation by Differential Scanning Calorimetry. J. Non-Cryst. Solids 2004, 337, 68− 77. (31) Schönhals, A.; Kremer, F. Analysis of Dielectric Spectra. In Broadband Dielectric Spectroscopy; Kremer, F., Schönhals, A., Eds.; Springer Verlag: Berlin, 2003; Chapter 3. (32) Boersema, A.; Van Turnhout, J.; Wübbenhorst, M. Dielectric Characterization of a Thermotropic Liquid Crystalline Copolyesteramide: 1. Relaxation Peak Assignment. Macromolecules 1998, 31, 7453−7460. (33) Schröter, K.; Unger, R.; Reissig, S.; Garwe, F.; Kahle, S.; Beiner, M.; Donth, E. Dielectric Spectroscopy in the αβ Splitting Region of Glass Transition in Poly(ethyl methacrylate) and Poly(n-butyl methacrylate): Different Evaluation Methods and Experimental Conditions. Macromolecules 1998, 31, 8966−8972. (34) Vogel, H. The Temperature Dependence Law of the Viscosity of Fluids. Phys. Zeit. 1921, 22, 645−646. (35) Fulcher, G. S. Analysis of Recent Measurements of the Viscosity of Glasses. J. Am. Ceram. Soc. 1925, 8, 339−355. (36) Tammann, G.; Hesse, W. The Dependency of Viscosity on Temperature in Hypothermic Liquids. Z. Anorg. Allg. Chem. 1926, 156, 245−257. (37) Pyda, M.; Bopp, R. C.; Wunderlich, B. Heat Capacity of Poly(lactic Acid). J. Chem. Thermodyn. 2004, 36, 731−742. (38) N. Delpouve, A.; Saiter, J. P.; Dargent, E. Cooperativity Length Evolution During Crystallization of Poly(lactic acid). Eur. Polym. J. 2011, 47, 2414−2423. (39) Hernández Sánchez, F.; Molina Mateo, J.; Romero Colomer, F. J.; Salmerón Sánchez, M.; Gómez Ribelles, J. L.; Mano, J. F. Influence of Low-Temperature Nucleation on the Crystallization Process of Poly(L-lactide). Biomacromolecules 2005, 6, 3283−3290. (40) Wang, Y. M.; Mano, J. F. Influence of Melting Conditions on the Thermal Behavior of Poly(L-lactic Acid). Eur. Polym. J. 2005, 41, 2335−2342. (41) Kaminski, K.; Adrjanowicz, K.; Kaminska, E.; Grzybowska, K.; Hawelek, L.; Paluch, M.; Tarnacka, M.; Gruszka, I.; Kasprzycka, A. Impact of Water on Molecular Dynamics of Amorphous α-, β-, and γ

ACKNOWLEDGMENTS Portuguese Foundation for Science and Technology (FCT) for financial support through the PTDC/FIS/115048/2009 project. M. T. Viciosa also thanks FCT for the postdoctoral grant SFRH/BPD/39691/2007.

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