Structure–property relationships of nanocomposites based on polylactide and MgAl layered double hydroxides

Accepted Manuscript Structure-Property Relationships of Nanocomposites Based on Polylactide and MgAl Layered Double Hydroxides Jing Leng, Purv J. Purohit, Nianjun Kang, De-Yi Wang, Jana Falkenhagen, Franziska Emmerling, Andreas F. Thünemann, Andreas Schönhals PII: DOI: Reference:

S0014-3057(15)00272-4 http://dx.doi.org/10.1016/j.eurpolymj.2015.05.008 EPJ 6909

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

3 March 2015 6 May 2015 8 May 2015

Please cite this article as: Leng, J., Purohit, P.J., Kang, N., Wang, D-Y., Falkenhagen, J., Emmerling, F., Thünemann, A.F., Schönhals, A., Structure-Property Relationships of Nanocomposites Based on Polylactide and MgAl Layered Double Hydroxides, European Polymer Journal (2015), doi: http://dx.doi.org/10.1016/j.eurpolymj.2015.05.008

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Structure-Property Relationships of Nanocomposites Based on Polylactide and MgAl Layered Double Hydroxides Jing Leng a, Purv J. Purohit a,1, Nianjun Kang b, De-Yi Wang b,*, Jana Falkenhagen a, Franziska Emmerling a, Andreas F. Thünemann a, Andreas Schönhals a,* a

BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87 12205 Berlin, Germany b

IMDEA Materials Institute, c/Eric Kandel 2, 28906 Getafe, Madrid, Spain

1

Present address: Clear Edge Filtration, Kevelaerer Straße 78, 47608 Geldern-Walbeck

(Germany)

* Corresponding Author Telephone: +49 30/8104-3384. Fax: +49 30/8104-1617. E-mail: *[email protected]; For chemistry and preparation please contact D.-Y. Wang., E-mail: *[email protected]

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ABSTRACT: Nanocomposites based on Poly(L-lactide) (PLA) and organically modified MgAl Layered Double Hydroxides (MgAl-LDH) were prepared by melt blending and investigated by a combination of Differential Scanning Calorimetry (DSC), Small- and Wide-angle X-ray scattering (SAXS, WAXS), and dielectric spectroscopy (BDS). Scanning microfocus SAXS investigations show that the MgAl-LDH is homogeneously distributed in the matrix as stacks of 6 layers and/or partly exfoliated layers. DSC and WAXS show that the degree of crystallinity decreases linearly with the content of LDH. The extrapolation of the dependencies (DSC and WAX) to zero estimates a limiting concentration of LDH CCri

of ca. 21 wt% where the

crystallization of PLA is completely suppressed by the nanofiller. The dielectric behavior of neat PLA show two relaxation regions, a β-relaxation at low temperatures related to localized fluctuations and the α-relaxation at higher temperatures due to the dynamic glass transition. The dielectric spectra of the nanocomposites show several additional relaxation processes compared to neat PLA which are discussed in detail. For the nanocomposites around 260 K (f=1 kHz) an additional process is observed which intensity increases with increasing concentration of LDH. This process is mainly attributed to the exchanged dodecylbenzene sulfonate (SDBS) molecules which are adsorbed at the LDH layers and form a mixed phase with the polymer close to the layers and stacks. An analysis of this process provides information about the molecular dynamics in the interfacial region between the LDH layers and the PLA matrix which reveal glassy dynamics in this region. In the temperature range around 310 K (f=1 kHz) a further process is observed. Its relaxation rate has an unusual saddle-like temperature dependence. It was assigned to rotational fluctuations of water molecules in a nanoporous environment provided by the LDH filler. Above the glass transition temperature a further process is observed at temperatures above. It is related to Maxwell / Wagner / Sillars polarization due to the blocking of charges at the nanofiller.

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KEYWORDS: Polymer based nanocomposites, Polylactide, Layered Double Hydroxides, Dielectric spectroscopy

3

1. Introduction Aliphatic polyesters especially polylactides (PLA) encounter nowadays various applications due to their biodegradable and/or biocompatible properties. Firstly, they are involved in the preparation of medical devices or therapies (bone surgery, suture, chemotherapy, etc.) [1,2]. Secondly, they are intensively studied as an alternative solution to partially reduce the plastic waste accumulation especially in packaging. Thirdly, there is also an increasing interest on PLAs as a promising replacement of petroleum-derived plastics, since they are extracted entirely from renewable agricultural products. Due to its biodegradation properties the polymer can enter in the natural cycle implying its return to the biomass [3,4,5]. However PLA, with a glass transition temperature around 329 K, is a relatively stiff and brittle polymer with a low deformation at break. One way to improve the properties of PLA is to incorporate nanoparticles to form polymer based nanocomposites. Among other nanoparticles like carbon nanotubes (see for instance reference [ 6 ]), layered aluminosilicates such as montmorillonite (clay), have been studied to some extent [7-15]. As a result it was found that a melt blending of the polyester with the clay leads to an intercalated or to a semi-intercalated / semi-exfoliated nanocomposite, relatively independent from the properties of the clay surface [16,17]. From a general point of view, compared with micro or macro scaled composites, polymer-based nanocomposites attract an increasing interest because of the substantial improvements in material properties such as gas and solvent barrier, toughness, mechanical strength, flame retardancy, etc [18-25]. The properties improvements can be attributed to the small size of the filler particles, its homogeneous dispersion on the nanoscale in the polymeric matrix, and thus the length scale of interaction with the polymer segments [26]. Moreover, due to the small size of the particles the

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surface to volume ratio is high, which results in a high volume fraction of an interfacial area between the polymer matrix and the nanoparticle [27]. The behavior of the polymer chains / segments in these interfacial regions like the packing density, the segmental molecular mobility or even the crystallinity can be different from those in the matrix polymer (see for instance references [28-31]). Because of its high volume fraction, the interfacial area between the matrix polymer and the nanofiller is crucial for the properties of the whole composite. A basic understanding of the interplay of the properties of the matrix and that of the interfacial area is still lacking. Recently, Layered Double Hydroxides (LDH) has attracted considerable interest as nanofiller for polymer based nanocomposites (see for instance references [29-38]. From the mineral point of view LDHs belong to the general class of anionic clay minerals and well known for their catalytic activity [39]. Due to the large amount of tightly bound water [40] and other synergistic effects they are able to enhance the flame retardancy of polymeric materials [41,42]. The most common naturally occurring LDH is hydrotalcite. Besides that, a broad range of chemical composition can be obtained due to the fact that layered double hydroxides can be synthesized. Here the structure / property relationships of nanocomposites based on poly(L-lactide) and synthetic organically modified MgAl LDH is investigated. In detail a combination of complementary methods like Differential Scanning Calorimetry (DSC), Small- and Wide-Angle X-ray Scattering (SAXS, WAXS), Size Exclusion Chromatography (SEC) and Broadband Dielectric Spectroscopy (BDS) are employed. Recently a similar combination of methods was employed to investigate nanocomposites based on polyolefins and LDH [36,37,43]. Some work on LDH filled poly(L-lactide) has be published recently [44,45].

2. Experimental Section

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2.1 Materials Layered double hydroxides (LDH) are a special class of anionic clay minerals. Like layered silicates LDH materials have a crystalline geometry. Its structure is based on brucite-like layers (Mg(OH)2) where each magnesium cation is octahedrally surrounded by hydroxyl groups. An isomorphous substitution of Mg2+ by a trivalent cation (or by a combination of other divalent or trivalent cations) occurs in the LDHs. The layers become charged and therefore anions between the layers are necessary to balance the charge. LDH can be represented by the general formula [MII1-x MIIIx (OH)2]x+ • [(An-)x/n • mH2O]x- where MII and MIII are the divalent and trivalent metal cations respectively, and A is the interlayer anion. A detailed discussion can be found in recent reviews [32,38]. The interlayer anions can be exchanged by bulky organic anionic species. This makes LDH especially suitable to prepare polymer nanocomposites because macromolecules can intercalate into the gallery of organically modified LDH and may result in the exfoliation of LDH layers. Examples of divalent cations are Mg2+, Ni2+, and Zn2+ where for the trivalent ions are commonly found Al3+, Cr3+, Fe3+, and Co3+. Here Mg2+ and Al3+ are employed as di- and trivalent cations where the LDH is fully synthetically. Sodium dodecyl benzene sulfonate (SDBS) is used as organic anion in the layer galleries of the LDH. The synthesis of organomodified MgAl-LDH (O-MgAl-LDH; O-LDH) was carried out by a onestep procedure [46]. A mixed magnesium and aluminum metal salt solution (with Mg2+:Al3+ ratio equal to 2:1 and a total metal ion concentration of 0.3M) to a SDBS solution under continuous stirring at 50° C. During the synthesis the pH value was kept constant at 10±0.2 by adding a 1M NaOH solution in the required amounts. After the addition of the mixed metal salt solution, the resulting slurry was further continuously stirred at the same temperature for 0.5 h. Finally a thermal treatment was applied at 75° C for 18 h. The final products were filtered and washed

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several times with distilled water to remove non-reacted surfactant molecules until the pH of the supernatant solution was about 7. The organomodified MgAl-LDH was then dried at 80° C till a constant weight was obtained. For the comparison, the unmodified MgAl-LDH (U-MgAl-LDH; U-LDH) was also synthesized using the same route. Although the LDH materials were carefully dried in vacuum before processing the products still might contain water in the interlayer. A detailed chemical characterization of the prepared LHD materials can be found elsewhere [46]. Polylactide was purchased as Biomer® L9000 from Biomer (Krailling, Germany). It is a predominantly L-type polylactide and was used without further purification. The melt flow index was 3.0-6.0 g/10min.

2.2 Preparation of nanocomposites The nanocomposites were prepared by melt mixing in a one step process. Before compounding, all the materials (polymer, LDH) were dried under vacuum at 323 K for 24 h. PLA contains a polar group in the main chain which can interact with the LDH for intercalation. Therefore no compatibilizer is needed like for corresponding polyolefin based nanocomposites [36,37]. The LDH was mixed in different concentrations with pure PLA to obtain the various nanocomposites. Melt

mixing

was carried

out

in a co-rotating

twin-screw microextruder (15-mL

microcompounder, DSM Xplore, Geleen, The Netherlands) at 463 K with 200 rpm screw speed for 10 min. The concentrations of O-LDH in the nanocomposites were determined based on an approximate metal hydroxide content of the filler. Details of the different samples including the sample code were given in Table 1. The extruded samples were melt pressed (above the melting temperature) to platelets (40 mm *40 mm) and thickness of ca. 0.4 mm for several minutes. After that the samples were slowly cooled down in the press (as prepared samples). 2.3 Experimental techniques

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X-ray scattering: The X-ray scattering experiments were carried out at the synchrotron micro focus beamline µSpot (BESSY II of the Helmholtz Centre Berlin for Materials and Energy). A detailed description of the beamline can be found elsewhere [47]. The beamline provides a beam diameter of 100 µm at a photon flux of 1 × 109 s-1 at a ring current of 100 mA. The divergence is less than 1 mrad (horizontally and vertically). The experiments were carried out employing a wavelength of 1.03358 Å using a double crystal monochromator (Si 111). The data were collected by a two-dimensional X-ray detector (MarMosaic, CCD 3072 × 3072 pixel with a point spread function width of about 100 µm) 820 mm behind the sample position. The obtained scattering images were converted into diagrams of scattered intensities versus scattering vector q (q is defined in terms of the scattering angle θ and the wavelength λ of the radiation, thus q = (4π/λ) sinθ) employing an algorithm of the computer program FIT2D [48]. Size Exclusion Chromatography (SEC). SEC was carried out using the following columns: “1 x PL gel mixed C, 10 µm, 300 x 8 mm” and “1 x PSS gel 1000 Å, 3µm, 300 x 8 mm” and a refractive index detector ERC 7510. As eluent Chloroform was used at a flow rate of 1.0 mL/min and a temperature of 25°C. The samples were prepared by dissolving 9 to 16 mg of the sample in 1 mL Chloroform. 100 µL of the filtered solution (0.2 µm millipore filter) was injected. Calibration was performed by polystyrene standards (PSS Mainz). Thermal analysis: Thermal analysis was carried out by differential scanning calorimeter (DSC, Seiko instruments, DSC 220C). The samples (ca. 10 mg) were measured from 248 K to 523 K with a heating and cooling rate of 10 K/min using nitrogen as protection gas. For pur PLA The cold crystallization and melting temperatures are 403 K and 440 K, whereas the corresponding enthalpies are 6 J/g and 8.2 J/g respectively (second heating run, 10 K/min). A glass transition temperature of 333 K and ∆cP of 0.51 J/g.K was also estimated from the thermogram.

8

Broadband Dielectric Spectroscopy (BDS): A high-resolution ALPHA analyzer together with an active sample holder head (Novocontrol, Montabaur, Germany) is used, to measure the complex dielectric function ε* ( f ) = ε´( f ) − iε´´( f ) (ε´-real part, ε´´-loss part and i = •-1) as function of frequency f (10-1 Hz to 106 Hz) and temperature T (160 K to 400 K). Samples were prepared in parallel plate geometry. Therefore gold electrodes with a diameter of 20 mm were evaporated on both sides of the samples. The samples were mounted between two gold-plated electrodes (20 mm) of the sample holder. Isothermal frequency scans were carried out where the temperature is controlled by a Quatro Novocontrol cryo-system with a temperature stability of 0.1 K. Details can be found elsewhere [49].

3. Results and Discussion 3.1 Characterization of the organically modified LDH In the first step the successful modification of LDH by SDBS was proven by SAXS measurements. Figure 1 compares the SAXS pattern for O-MgAl-LDH and U-MgAl-LDH. The SAXS data for unmodified LDH shows two equidistant reflections at 7.1 nm-1 and 14.11 nm-1. This is characteristic for a layered compound corresponding to a lamellar repeat distance (d = 2π/qpeak) of d = 0.88 nm. Subtracting the thickness of the hydrotalcite brucite-like LDH of 0.49 nm [50] this results in an effective interlayer distance to be 0.39 nm. By fitting two Gaussians to the data, the peak widths were determined to w = 0.69 nm-1. Form the width a mean correlation length perpendicular to the lamella normal can be calculated to lc = 2π/w = 9.23 nm. Assuming that lattice distortions can be neglected in a first approximation the crystallite thickness in the direction normal to the (001) plane can be estimated. This calculation results that the average number of layers in a stack of the unmodified LDH is approximately 10. As expected, the lamellar reflections for modified LDH shift to lower q-values. Four equidistant reflections are

9

clearly visible. The lack of reflections in the diagram of O-LDH at a q value of 7.1 nm-1 proves the absence of significant amounts of U-MgAl-LDH. A similar analysis as discussed above can be also carried for O-MgAl-LDH. After the subtraction of the thickness of the brucite sheet the effective layer distance is 2.54 nm. As expected the introduction of the SDBS provokes an essential widening of the distance between the layers. The width of the peak for the O-MgAlLDH is estimated to 0.36 nm-1. This is essentially smaller than for U-MgAl-LDH. Therefore the number of layers in a stack of the modified LDH is lower than for U-MgAl-LDH and is calculated to ca. 6. On the one side, likely the SDBS hinders the formation of large stacks and reduces the number of layers in O-MgAl-LDH. On the other side, the existence of small stacks with only few layers will lead to nanocomposites when mixed with polymers.

a)

10000

0.39 nm

Intensity [a. u.]

b) 1000

100

2.54 nm

1

-1

10

q [nm ]

Figure 1. SAXS pattern of unmodified (a) and organically modified (b) LDH.

3.2 Homogeneity of the dispersion of the nanofiller within the nanocomposites

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The homogenous, spatial distribution of the nanofiller across the sample on a macroscopic scale is one of the key parameters which determine the structure property relationships of nanocomposites. To study this in more detail space resolved experiments were employed with a microfocus using synchrotron radiation at the µSpotBeamline of BESSY [47]. The measurements were carried out at three different positions of the as prepared samples having a diameter of more than 30 mm where the spot diameter of the X-ray beam was 0.1 mm. The result is depicted in Figure 2 for PLA9 as one example. All the individual SAXS pattern nearly collapse into one chart (solid, dotted and dashed-dotted curves, positions of measurements at the sample are shown in the inset). This proves a homogeneous dispersion of O-LDH in the polymer matrix on macroscopic length scales (> 1 mm). Figure 2 gives further that in the SAXS region reflections at quite similar positions are observed for O-MgAl-LDH and for the nanocomposite. The reflections are a bit weaker and also broader this evidences that stack-like structure similar to that O-MgAl-LDH is also present in the nanocomposites. This result points to a partly exfoliated morphology with mixed nanostacks. After fitting Gaussians, the lamellar repeat unit d was found out to be similar to O-LDH. This result is similar to results obtained for previously studied nanocomposites based on polyolefines [36,37]. The stack size was determined from the widths of the peaks and the lc was calculated to be 25.6 nm which is equivalent to around 6 layers like for O-MgAl-LDH. Because of the fact that the stacks are so small even a homogenous mixing of these nanostacks in the polymer matrix gives nanocomposites.

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PLA9

Intensity [a.u.]

1000

100

O-MgAl-LDH

2

3

4

5

6

7 8 9 10

20

-1

q [nm ]

Figure 2. Overlay of three synchrotron SAXS curves from a disc sample of PLA9 with a diameter of 30 mm (solid, dotted and dashed-dotted lines). The dashed line shows the curve for organically modified LDH. The reflections in the q-range from 10 to 20 nm-1 are due to the crystal structure of the semi crystalline PLA.

3.3 Degradation stability of the nanocomposites The LDH materials contain Al and Mg ions which may cause a catalytic degradation of the PLA matrix. For a similar system an enhanced thermal degradation behavior is reported [51]. Therefore here Size Exclusion Chromatography was employed to study the quasi isothermal degradation. The investigations were carried out at samples which were stored for more than 3 years at room temperature. Figure 3 gives the elugram for neat PLA. Two elution regions having different intensities can be identified. The first one at shorter elution times corresponds to high molecular weights where the second one is due to lower molecular weights. The inset of Figure 3 gives the molecular weight distribution for the high molecular weight elution peak for the different LDH concentrations. With increasing concentration of LDH the peak maximum shifts slightly to lower molecular weight. This means with increasing concentration of LDH the molecular weight of the matrix decreases slightly due to catalytic degradation.

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Normalzied log(MW) Distribution

Refraction Index Signal [mV]

0.02

C LDH

100 80 60 40 20 0 4

0.01

5

6

log(MW [g/mol])

0.00 10

12

14

16

18

20

22

Elution Volume [ml]

Figure 3. Signal of the refractive index detector versus the elution volume for pure PLA. The inset shows the normalized molecular weight distribution versus the molecular weight: solid line – pure PLA, dashed line – PLA1, dotted line – PLA3, short dash line – PLA6, short dotted line - PLA9, short dashed dotted line – PLA12.

This becomes clearer from Figure 4 where weight averaged molecular weight MW is plotted versus CLDH. With increasing concentration of LDH MW decreases from about 190 kg/mol to about 130 kg/mol. This evidences that the nanoparticles influences slightly the degradation of the polymeric matrix likely by catalytic processes. Besides the catalytic activity also the remaining water in the intergalleries might contribute to the degradation process. The change of the molecular weight is small. This slightly decreased molecular weight will not have a significant effect on the properties studied during the course of this paper. The inset of Figure 4 gives the molecular weight distribution of the molecular weight for the release at longer elution times. Obviously the distribution is bimodal indicating two different molecular species. To compare the different samples, the distribution was normalized by the maximum value of the peak at lower molecular weights. Probably this low molecular weight compounds are degradations products. These degradation products are also found for neat PLA.

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But nevertheless the relative amount of both degradation species changes with increasing concentration of LDH. It was tried to identify the both degradation products on a molecular level by mass spectrometry. But up to now with a limited success. Therefore further experimental

Normalzied log(MW) Distribution

work is necessary to investigate this problem in more detail.

200

MW [kg/mol]

180

160

100

PLA12

80

PLA9 PLA3

60

PLA

CLDH

40 20 0 1.5

2.0

2.5

3.0

3.5

log(MW [g/mol])

140

120 0

2

4

6

8

10

12

CLDH [wt%]

Figure 4. Change of the weight averaged molecular weight MW with the concentration of LDH. The inset shows the normalized molecular weight distribution versus the molecular weight for the low molecular weight degradation products: solid line – pure PLA, dotted line – PLA3, short dotted line - PLA9, short dashed dotted line – PLA12. In the inset the other concentrations are omitted for sake of clearness.

3.4 Crystallinity of the nanocomposites investigated by DSC and WAXS Figure 5 compares the DSC curves of the as prepared samples (first heating run) for pure PLA, PLA1 and PLA12. All measurements discussed here were carried and heating rate of 10 K/min. A glass transition region as well as crystallization and melting is observed for each concentration of the nanofiller. The glass transition temperature does not change significantly with the concentration of the nanofiller. This concerns also the width of the glass transition. Interestingly compared to pure PLA for PLA1 melting enthalpies is much higher. Moreover a pronounced cold crystallization peak is observed for PLA1. This suggests that the LDH acts as nucleation agent. A similar effect was also discussed in the literature. For instance Pilla et al.

14

showed that the addition of recycled wood fibers leads to an increase of the crystallinity of PLA [52]. This will be discussed in more detail below.

PLA12

Heat Flow [a.u.]

Cold Crystalization

PLA1 Melting

exo

PLA

Glass Transition

250

300

350

400

450

500

T[K]

Figure 5. Comparison of DSC thermograms (as prepared samples, first heating, heating rate of 10 K/min) for pure PLA and selected nanocomposites: solid line - pure PLA, dashed line - PLA1, dashed dotted line – PLA12. The curves are shifted along the y-scale for sake of clearness.

Table 1 shows that for concentrations higher than 1 wt% the melting enthalpies (first heating) decrease with increasing concentration of LDH. From the melting enthalpy ∆Hm the degree of crystallization can be calculated. In a first step the enthalpy values have to be normalized to the amount of polymer. The degree of crystallization is given χ = ( 1 − CLDH ) ∆H m / ∆H m ,100% * 100 where

∆Hm,100% is the melting enthalpy of a complete crystalline PLA sample. The value of ∆Hm,100%=93 J/g is taken from references [56, 53]. In Figure 6 χ is plotted versus the concentration of LDH. χ increases for CLDH=1 wt% compared to the bulk value. This means for a small amount of LDH the degree of crystallinity increases. This gives further evidence that the nanofiller acts as nucleating agent and increases for low concentrations of LDH the crystallinity

15

compared to the bulk value. For higher values of CLDH the degree of crystallization decreases approximately linearly with increasing concentration of LDH. The extrapolation of χ to zero value results in a critical concentration of LDH CCri of ca. 21 wt%. From the DSC measurements (first heating run) it is therefore concluded that for concentrations higher than this value, the cold crystallization of PLA is completely suppressed at 21 wt% of LDH for the as prepared samples. This behavior is similar to that observed for nanocomposites based on olefins and LDH [36,37]. Absolute values of the crystallinity χ can be also calculated from the wide angle X-ray scattering pattern (WAXS) for the prepared samples. For that analysis the q range from 1.5 nm-1 to 40 nm-1 is chosen and 60 counts were subtracted from the spectra as detector background. Moreover the Kratky representation is used because q2I(q) is approximately proportional to the real number of scattered photons and more importantly to have equal condition in the whole q range. The integral Atotal =

40 nm −1

∫

1.5 nm −1

q 2 I total ( q ) dq is the total scattered intensity due to both the amorphous and

crystalline regions. From the q2Itotal(q) curves the amorphous contribution is subtracted by hand yielding to q2Icrys(q) due to the scattering of the crystalline regions including possible 40 nm−1

mesomorphic contributions [54,55]. The integral Acrys = ∫ q 2 I crys ( q ) dq is the total scattered 1.5nm−1

intensity due to the crystalline regions. The degree of crystallization is then given by χ = Acrys / Atotal .

In Figure 6 the degree of crystallization estimated from the WAXS data are also

included. The dependence of χ versus the concentration of LDH estimated from the WAXS measurements is similar to the dependence extracted from the DSC data (see Fig. 6) was also the absolute values agree in the frame of the experimental error. For pure PLA a crystallization degree below 20 % is obtained which is in agreement with literature data (see for instance [56]).

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Both data sets can be described by the same linear regression leading to a similar critical concentration of ca. 21 wt% of LDH were the cold crystallization of PLA is completely suppressed. This gives further evidence that the nanofiller first acts as crystallization agent and for higher concentration the LDH suppress the cold crystallization. The agreement of the degrees of crystallization estimated from the WAXS investigation and the DSC data for the first heating run is expected because in both cases the samples have the same thermal history (as prepared samples). 60

60 40

χ [%]

50 20

χ [%]

40 30

0 0

5

10

15

20

25

CLDH [wt%]

20

χ=0

10 0 0

5

10

15

20

25

CLDH [wt%]

Figure 6. Degree of crystallization χ vs. concentration of LDH squares – DSC measurements (first heating run), circles – WAXS data. The solid line is a linear regression using both data sets. The inset compares the concentration dependence of the degree of crystallization estimated from DSC measurements: squares – first heating run, stares – second heating run. The lines are linear regressions to the corresponding data.

The inset of Figure 6 compares the degree of crystallization estimated from the first and the second heating run of the DSC measurement versus the concentration of LDH. For the second heating the degree of crystallization measured by DSC is essentially lower than the values obtained by the first heating run. Moreover in the case of the second heating run the dependence

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of χ on the concentration is stronger than for the first one which gives rise to a lower value for the critical concentration of ca. 14 wt% where χ becomes zero (see inset Figure 6). This is different to the behavior found for polypropylene [36] and polyethylene [37] filled with LDH. Compared to polypropylene and polyethylene the crystallization rate of PLA is essentially lower. In the case of the first heating run the samples were cooled down slowly in the press. For the second heating run the samples were crystallized inside the DSC pan by cold crystallization at a heating rate of 10 K/min. Within the experimental error the different behavior observed for the different thermal histories can be understood by assuming the main influence of the nanofiller on the cold crystallization behavior is a reduction of the crystallization rate. A detailed DSC study where the cooling rate is varied in a broad range is in progress to investigate this further.

3.5 Dielectric Spectroscopy The dielectric measurements were carried out for the as prepared samples. Figure 7 displays the dielectric loss ε´´ of pure PLA versus frequency and temperature in a 3D representation. The dielectric response of a material is related to the fluctuation of dipole moments which is related to the molecular mobility of groups or segments [57]. As known in the literature bulk polylactide shows at least two relaxation processes indicated by peaks in the dielectric loss ε´´ (see for instance [55, 58 , 59 , 60 ]). The β-relaxation at low temperatures is assigned to localized fluctuations. At temperatures higher than the β-process, the α-relaxation (dynamic glass transition, segmental dynamics) takes place.

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α - relaxation

0

-1

log ε´´

β - relaxation

-2 6

350

300

T [K]

0 250

200

150

log(f

2 400

[Hz] )

4

-2

Figure 7. Dielectric loss for pure PLA versus frequency and temperature and a 3D representation.

To discuss the dielectric behavior in more detail the dielectric loss for pure PLA is plotted versus temperature at fixed frequency of 103 Hz (see Figure 8). The α- and the β-relaxations are seen as prominent peak in the dielectric loss at high and low temperatures respectively. A closer inspection of the dielectric spectra reveals that there seems to be an additional process in between the α- and the β-relaxation. The molecular origin of this relaxation process is not clear till now. Therefore a second heating run carried out. In the second heating cycle this intermediate process seems to disappear. But also the β-relaxation shifts to higher temperatures and broadens (see Figure 8). In parallel the intensity of the α-relaxation drops down and the peak shifts slightly to higher temperatures. This behavior points to an increased degree of crystallinity. To avoid such effects in the analysis of the nanocomposites here only the as prepared samples are considered.

19

The model function introduced by Havriliak/Negami [61] (HN-function) is used to analyze the dielectric measurements quantitatively. It reads

ε*HN ( ω ) − ε ∞ =

∆ε β γ

(1 + (i ω / ω0 ) )

(1)

.

ω0 is a characteristic frequency related to the frequency of maximal loss fp (relaxation rate). ∆ε denotes the dielectric strength. ε∞ describes the value of the real part ε´ for ω>> ω0. β and γ are fractional parameters (0<β≤1 and 0<βγ≤1) characterizing the shape of the relaxation time spectra. From the fit of the HN-function to the data the relaxation rate fp (maximum frequency of dielectric loss) is determined and further discussed. Conduction effects are treated in the usual ´´ s way by adding a contribution ε cond = σ 0 /[ ω ε o ] to the dielectric loss where σ0 is related to the

specific dc conductivity of the sample. The parameter s (0
0.0

-0.2

α-Relaxation

-0.5

log ε´´

-0.4 -0.6 -0.8

log ε´´

-1.0

-1.0 -2

0

β-Relaxation

2

4

6

log (f [Hz])

-1.5

-2.0

-2.5 150

200

250

300

350

400

T [K]

Figure 8. Dielectric loss versus temperature at a fixed frequency of 103 Hz for pure PLA: Circles – first heating; squares – second heating. The inset gives an example for the fitting of the HN function to the data of pure PLA:

20

circles – T=339.1 K; squares – T= 351.1 K. Lines are fits of the HN equation to the corresponding data including conductivity contribution.

Figure 9 depicts the temperature dependence of the relaxation rate of the α-relaxation for pure PLA in the relaxation map. As expected for segmental (glassy) dynamics the temperature dependence of fp,α is curved when plotted versus 1/T which can be described by the Vogel/Fulcher/Tammann (VFT-) equation [64-66]

f p ,α ( T ) = f ∞ exp(

 D T0 −A ) = f ∞ exp − T − T0  T − T0

(2)

  . 

Here A is a constant, f∞ is the pre-exponential factor and T0 is the Vogel or ideal glass transition temperature. For the dynamic glass transition for T0 a value of about 30-70 K below the thermal glass transition temperature measured by DSC etc. is found empirically. D =

A T0

* ln10 is called

fragility parameter and provides among others a useful quantity to classify glass forming systems [67,68]. Glass forming materials are called "fragile" if their fp,α(T) dependence deviates strongly from an Arrhenius-type behaviour and "strong" if fp,α(T) is close to the latter. The temperature dependence of the relaxation rate of the β-relaxation follows the Arrhenius equation

f p ,β ( T ) = f ∞ exp(

− EA ). RT

(3)

where EA is the activation energy and R the general gas constant (see inset Figure 9).

21

7 6 5

4

3 3

2 2

log(fp,β [Hz])

log (fp,α [Hz])

5

4

1 -1

1

0

1000 / T [K ]

3.5

2.60

4.0

2.65

4.5

2.70

2.75

5.0

5.5

2.80

2.85

2.90

2.95

-1

1000 / T [K ]

Figure 9. Relaxation rate of the α-relaxation (fp,α) vs. 1000/T for pure PLA. The solid line is a fit of the VFT equation to the data. The inset gives the relaxation rate of the β-relaxation (fp,α) vs. 1000/T for pure PLA. The solid line is a fit of the Arrhenius equation to the data.

Figure 10 compares the dielectric loss at a fixed frequency in the temperature domain for pure PLA and different nanocomposites. Compared to pure PLA besides the α- and the β-relaxation which were also observed for the nanocomposites three additional features are found: (1) In the temperature range around 260 K an additional peak is observed which increases in intensity with increasing concentration of LDH (relaxation region 1). (2) A further process is also observed in the temperature around 310 K (relaxation region 2). (3) At temperature above the α-relaxation a thrird process appears which increases strongly in its intensity with increasing LDH concentration (relaxation region 3). These features will be discussed in detail in the following. Relaxation region 1: Because of the fact that the intensity of relaxation region 1 increases with

increasing concentration of LDH it is obvious that this process is related to the nanoparticles. The only polar component in the system which increases with the concentration of LDH is the bulky anion dodecyl benzene sulfonate (SDBS). Therefore it is concluded that relaxation region

22

1 is related in some manner to molecular fluctuations of SDBS. There are only few investigations which consider the dynamics of the alkyl chains inside the galleries of a pure layered material [69,70]. In the case of neat montmorillonite a disordered hydrocarbon trilayer formed by the surfactant molecules is evidenced by simulation [71]. For the nanocomposites the SAXS measurements show that they have a partly exfoliated morphology. In the presence of polymer segments the alkyl tails of the surfactant (here SDBS) are desorbed further from the surface of the nanoparticles and form a phase mixed with the polymer segments. This means the polymer segments close to the layers will fluctuate together with the CH2 groups of the alkyl chains of the surfactant. Relaxation region 1 is therefore assigned to common fluctuations of polymer segments and tail groups of the surfactant or in other words fluctuations of PLA segments promoted by the presence of SDBS and the nanofiller. With increasing concentration of the nanofiler the amount of the interfacial area increases, and therefore the intensity of the relaxation region 1. Its analysis will provide information about the dynamics close to the layers.

0.0 Region 3 Region 1 Region 2

log ε´´

-0.5

-1.0

-1.5

-2.0 150

200

250

300

350

400

T [K]

Figure 10. Dielectric loss versus temperature at a frequency of 103 Hz for pure PLA and different nanocomposites: Squares – pure PLA, circle – PLA1, diamonds – PLA3.

23

Figure 11 give the temperature dependence of the relaxation rate of the relaxation region 1 in the relaxation map. For all concentrations of the nanofiller the relaxation rates for relaxation region 1 collapse into one chart. Its temperature dependence is curved when plotted versus 1/T. This result indicates that the underlying molecular fluctuations show a glassy dynamics. For all concentrations the temperature dependence can be described by a common fit of the VFT equation to the data. The inset A of Figure 11 compares the temperature dependence of relaxation of relaxation region 1 for PLA6 with the temperature dependence of a similar process found for a nanocomposite based on MgAl-LDH and polyethylene also for ca. 6 wt-% of the filler (PE6) [34]. The relaxation processes found for the two different systems show a close similarity. They are observed in the same temperature range and seem to have the same temperature dependence. To analyze the temperature dependence in more detail a derivative technique is employed [72]. This method is sensitive to the functional form of fp(T) irrespective of the prefactor. For a dependency according to the VFT-equation one gets  d log f p   dT   

[

−1 / 2

= ( A * ln( 10 ))−1 / 2 ( T − T0 ) .

(4)

]

In a plot of d log f p / d T −1/ 2 versus T a VFT-behavior shows up as a straight line. This technique is applied to the data given in the inset A of Figure 11 and plotted in inset B of the same Figure. Firstly, both data sets follow a straight line confirming the VFT temperature dependence of the relaxation rates. Secondly, the data for PE6 and PLA6 collapse into one chart. This result gives some evidence that the both processes for the different matrix polymers have a similar molecular origin. The common structural features present in both systems are the SDBS molecules. The similarities in the both processes are suggested that both relaxation modes are

24

closely related to the fluctuations of the polar SDBS surfactant molecules modified by polymer segments. These, molecules are located close to the surface of the exfoliated layers or to the surface of the nano-stacks. {d log (fp [Hz]) / dT [K] }-1/2

5

6

2

T0

1

200

5

250

300

T [K]

4 log (fp [Hz])

log (fp [Hz])

3

0 150

6

4

B

4

2

3 2 1

A

0

0

3.4

3.6

3.8

4.0

4.2

4.4

1000 / T [K-1]

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

1000 / T [K-1]

Figure 11. Relaxation rates of the relaxation region 1 versus 1/T for different nanocomposites:, circle – PLA1, diamonds – PLA3, triangles – PLA6, hexagons – PLA9, stars – PLA12. The line is a fit common of the VFT equation to all data. Inset A gives the relaxation rate for PLA6 (triangles) and PE6 (hexagons) versus 1/T. The lines are fits of the VFT equation to the corresponding data. Inset B gives d log f p / d T −1/ 2 versus T for PLA6

[

]

(triangles) and PE6 (hexagons). The line is a linear regression using both data sets.

The VFT temperature dependence of the relaxation rate of relaxation region 1 indicated glassy dynamics in the interfacial region between the nanofiller and the PLA matrix. To have a signature as a glass transition the spatial extent of these regions should be in the order of 1 to 3 nm [73-78]. Relaxation region 2: The relaxation region 3 can be observed for all concentrations of LDH. But

for CLHD=1 wt% is quite weak (see Figure 10) and for LDH concentrations higher than 3 wt% it is strongly overlaid by relaxation region 3 and conductivity effects. Therefore it can be analyzed unambiguously only for PLA3. Figure 12 gives the dielectric loss for the nanocomposite PLA3 versus frequency and temperature in a 3D representation. The different relaxation regions are evidenced by peaks in the dielectric loss and indicated by arrows. The relaxation region 2 is

25

located between relaxation region 1 and the α-relaxation. The temperature dependence of the relaxation rate of relaxation region 2 seems to be quite unusual. The temperature dependence of this relaxation process seems to have a saddle-like shape. At low temperatures the relaxation rates increases with increasing temperatures as expected. Than the relaxation rates have maximum before they seems to decrease again. A similar behavior was observed for nanocomposite systems bases on a maleic anhydride grafted polypropylene and organophilic modified montmorillonite clay [31].

Figure 12. Dielectric loss for PLA3 versus frequency and temperature and a 3D representation.

To analyze this behavior in more detail Figure 13 depicts the temperature dependence of the relaxation rate of the relaxation region 2 for PLA3 that is curved when plotted versus 1/T. At the first glance the data seems to follow a VFT temperature dependence. But however a fit of the VFT equation to the data results in a prefactor log (f∞ [Hz]) ≈ 6 that is much too small for a true

[

]

relaxation process. The inset of Figure 13 gives d log f p / d T −1/ 2 versus T for this process. The

26

data can be described neither by an Arrhenius law nor by the VFT equation. This gives evidence that the temperature dependence of relaxation region 2 have a saddle-like temperature dependence. Such a saddle-like behavior is found besides the nanocomposite system discussed above [31] also for pure LDH materials [79]. Such a saddle-like behaviour of the relaxation rate seems to be characteristic for systems which contains some amount of water in the presence of a nanoporous structure [80,81]. In a model of Rybaov et al. it is assumes that the saddle-like temperature dependence of the relaxation rate must be due to the counterbalance of two competing processes [82]. Firstly, the rotational fluctuation should have an activated temperature according to the Arrhenius equation. Secondly, the nanoporous structure will give rise to some free volume defects which will have a different temperature dependence. The combination of both effects will lead to a saddle-like temperature dependence. In the systems considered here some water is present in the intergalleries of the LDH although the filler is carefully dried in vacuum. Moreover the layered structure gives rise to given porosity. Therefore it is concluded that relaxation region 2 is related to rotational fluctuations of water molecules in the presence of defects.

27

10

{d log (fp [Hz]) / dT [K] }-1/2

7 6 5

log(fp [Hz])

4

9 8 7 6 5 4 3

3

270

280

290

300

310

320

330

T [K]

2 1

α-Relaxation Region 2 for PLA3

0 -1 2.6

2.8

3.0

3.2

3.4

3.6

3.8

-1

1000 / T [K ]

Figure 13. Relaxation rates of the relaxation region 2 versus 1/T for PLA3 (diamonds). This line is a fit of the VFT equation to the data. Relaxation rates of the α-relaxation versus 1/T for PLA and different nanocomposites obtained from the modulus representation: asterisks – neat PLA; circles – PLA1; diamonds – PLA3. The dashed line is a

[

common fit of the VFT equation to all given data for the α-relaxation. The inset gives d log f p / d T

]−1/ 2 versus T

for PLA3 (diamonds). The line is a guide to the eyes.

α-Relaxation: For the analysis of the α-relaxation the modulus representation is used where the complex electrical modulus M* is related to complex dielectric function by M*(ω)ε*( ω)=1 [83]. Figure 14 gives the modulus data for neat PLA and two nanocomposites with the lowest concentration of LDH for T=363.1 K. Like for the complex dielectric function the α-relaxation is characterized by a peak also in the modulus representation. The HN-function can be also used to analyze the data. Besides the data for relaxation region 2, Figure 13 gives also the relaxation rates for the α-relaxation. For all given concentrations the relaxation rates collapse nearly into one chart together with data for neat PLA. This indicates that this process is due to the αrelaxation of the polymeric matrix which is not influenced by the nanofiller. This is in agreement with the results obtained by DSC that Tg does not dependent on the filler concentrations (see

28

Figure 5). It is also in agreement with literature studies for other polymer based nanocomposites (see for instance [1214,84,85] but also for other nanostructured systems like ultrathin films (see for instance [86-90]). A closer inspection of the temperature dependence shows that the data for the nanocomposite seems to have some kink-like change towards higher temperatures at ca. 375 K (see also Figure 14). Probably this effect is due to the onset of the cold crystallization which is shifted to lower temperatures by the presence of the nanoparticles (see Figure 5) the lower heating rate in the dielectric compared to the DSC experiments. The only influence of the nanoparticles to the α-relaxation is an almost symmetrical broadening of the relaxation peak with increases with increasing concentration of LDH (see Figure 14). For higher concentrations of LDH the α-relaxation is strongly overlaid by process 3 and conductivity phenomena and cannot be analyzed quantitatively.

-1.5

log M´´

-2.0

-2.5

-3.0 -2

0

2

4

6

8

log(f [Hz])

Figure 14. Imaginary part of the complex dielectric modulus M´´ versus frequency at T=363.1 K: asterisk – neat PLA; circles – PLA1; diamonds – PLA3. Lines are guides for the eyes.

29

Relaxation region 3: Figure 15 gives the imaginary part of the complex electrical modulus for

the nanocomposites PLA6 and PLA9. The spectra show two pronounced peaks. At the first glance the peak at higher frequencies might be assigned to the α-relaxation in these composites because of some similarities in the frequency position. But a more careful consideration reveals that the intensity of theses peaks is much too high for the α-relaxation (compare Figure 15 and 14). Also the extracted temperature dependence of the relaxation rate of that peak is completely different from that of the α-relaxation. In parallel to the peaks observed in the imaginary part of the modulus M´´ a giant increase of the real part of the complex function ε´ with decreasing is observed (see inset of Figure 15). Such a strong increase of ε´ with decreasing frequency cannot be explained by a molecular dipole moment present in the system and is considered to be typical for interfacial polarization effects. Generally such an interfacial polarization process is caused by (partial) blocking of charge carriers at internal surfaces or interfaces of different phases having different values of the dielectric permittivity and/or conductivity at a mesoscopic length scale or at electrodes.

30

Conductivity

Interfacial Polarization due to nanofiller

-1.5

100 Step

ε´

log M´´

1000

-2.0

-2.5

10

log (f [Hz]) 1 -2

-3.0 -2

0

2

0

4

2

4

6

6

8

8

log (f [Hz])

Figure 15. Imaginary part of the complex dielectric modulus M´´ versus frequency at T=363.1 K: triangles – PLA6; hexagons – PLA9. Lines are guides for the eyes. The inset gives the real part of the complex dielectric function for PLA9 at T=361.1K.

In the considered system the Maxwell / Wager / Sillars polarization effects can have two different origins: The blocking of the charge carriers at the crystallites and/or the blocking of the charge carriers at the LDH nanofiller (LDH nanostacks and exfoliated layers). Because of the facts that the dielectric data of neat PLA does not show interfacial polarization effects and that the strongest Maxwell / Wagner / Sillars signal were observed for the highest concentration of the nanofiller the high frequency peak in the modulus spectrum is assigned to interfacial polarization effects caused by the nanofiller. In the modulus representation a conductivity contribution is transformed into a peak. Therefore the peak observed at lower frequency in the modulus representation is assigned to the conductivity (see Figure 15). From the maximum position of that peak a rate for the conductivity fCond can be extracted. Again the sum of two HN-functions can be employed to analyze the data. Figure 16 gives the temperature dependence of the rate of the conductivity for two concentrations of LDH. Generally

31

for both concentrations of LDH is quite complex and cannot be described neither by an Arrhenius nor by the VFT equation. The reason for that is unclear up to now and further investigations are needed. A possible explanation is the cold crystallization process above the glass transition temperature which changes the temperature dependence. Compared to PLA6 the rate for the conductivity is much higher for PLA12 (see Figure 16). Because of the fact that fCond is proportional to the DC conductivity that means that for PLA12 the conductivity is more than one order of magnitude higher than for PLA6. With increasing LDH also the concentration of SDBS is increased. Compared to apolar polyolefines [36,37] for the more polar PLA the SDBS molecules may partly desorb from the LDH into the polymeric matrix and cat a ionic charge carriers which enhance conductivity. This is interesting to note that for PLA12 a considerable conductivity is observed even below the thermal glass transition. As discussed above the interfacial regions between the polymeric matrix and the nanofiller have a high molecular mobility (see discussion relaxation region 1). For high concentrations of LDH the amount of this interfacial area is high which can lead to a percolating network of high mobility regions. Charge transport can take place within this percolating network even at temperature below Tg. To be complete in the inset of Figure 16 the temperature dependence of the rate for the Maxwell / Wagner / Sillars polarization fMWS is given for the same two concentrations of LDH. Due to the overlapping of the both processes the data have a considerable scatter. Also these temperature dependencies are complex and resemble similarities to that of the conductivity.

32

5

3 log(fMWS [Hz])

4

log (fCon [Hz])

2

Tg

3 2 1 2.4

2.6

2.8

3.0

3.2

3.4

-1

1000 / T [K ]

1

0 Tg -1 2.4

2.6

2.8

3.0

3.2

3.4

1000 / T [K-1]

Figure 16. Rates of the conductivity versus inverse temperature: triangles – PLA6; stars – PLA12. The temperature dependence of fcon for PLA9 is similar to that of PLA12. The inset gives the rates for the Maxwell / Wagner / Sillars polarization fMWS versus 1/T: triangles – PLA6; stars – PLA12.

Conclusions

Nanocomposites based on Poly(L-lactide) (PLA) and organically modified MgAl Layered Double Hydroxides (MgAl-LDH) were prepared by melt blending. The obtained nanocomposites were investigated by a combination of Size Exclusion Chromatography (SEC), Differential Scanning Calorimetry (DSC), Small- and Wide-angle X-ray scattering (SAXS and WAXS) and dielectric relaxation spectroscopy (BDS). The SEC measurements show that there is a small enhancement of the degradation of PLA due to the LDH nanoparticles. But the observed effects are small and will not influence the properties of the nanocomposites significantly. The SAXS measurement demonstrated the successful modification of LDH by SDBS. The distance of the layers was increased from 0.39 nm to 2.53 nm by the in cooperation of SDBS. The average stack size is 25.6 nm. Space resolved SAXS measurements at the synchrotron micro

33

focus beamline µSpot (BESSY II of the Helmholtz Centre Berlin for Materials and Energy) proofed the homogenous dispersion of O-MgAl-LDH inside the polymer matrix across the sample dimension of 30 mm were the spot size was 0.1 mm. Small angle X-ray scattering (SAXS) analysis indicates a nearly similar effective layer distance and an equal stack size of OMGAl-LDH for nanocomposites as compared to pure O-MgAl-LDH. From that it is concluded that the composites have a partly exfoliated morphology with mixed nanostacks. The thermal properties of the nanocomposites are investigated by differential scanning calorimetry. For small concentrations of LDH the melting increases. Form that it is concluded that the nanofillers act as nucleating agent. The melting enthalpies are used to calculate the degree of crystallization. For LDH concentration greater than 1 wt% the degree of crystallization decreases with increasing concentration of LDH. The extrapolation of degree of crystallization to zero leads to a limiting concentration CCri of ca. 21 wt% LDH where the crystallization should be completely suppressed by the presence of the nanoparticles. These results were confirmed by a quantitative estimation of the degree of crystallinity by WAXS measurements leading to a similar value of CCri. Moreover an extended DSC study is in preparation in broad a range of heating and cooling rates (3 orders of magnitude) including also an isothermal crystallizations process giving samples with a well-defined thermal history. Moreover the crystallization kinetics will be studied in detail. This enables in addition the estimation of the rigid amorphous phase due to both the crystallites and the nanoparticles. The dielectric relaxation behavior of neat PLA show a β-relaxation due to localized fluctuations below the thermal glass transition temperature Tg and the α-relaxation due to segmental dynamics (dynamic glass transition) for temperatures higher than Tg. Besides these relaxation processes the relaxation behavior of the nanocomposites is more complex and at least three

34

additional dielectric active processes are observed. Relaxation region 1 is related to the interfacial area between the nanofiller and the PLA matrix. It is mainly due to the fluctuations of the alkyl tails of the SDBS molecules together with PLA segments. The temperature dependence of the relaxation rate of relaxation region 2 seems to have an unusual saddle-like temperature dependence. Therefore it is related to the fluctuations of remaining water molecules in the nonporous structure of the LDH nanofillers. Relaxation region 3 is observed at temperatures higher than characteristic for the dynamic glass transition. It is assigned to interfacial polarizations effects due to the blocking of charge carriers at the nanofiller.

35

Table 1: Code and composition of the investigated nanocomposite samples. Additionally the melting temperatures and melting enthalpies for the first and second heating are given. Sample information

Melting (1. heating)

Melting (2. heating)

Sample code

amount of LDH [wt%]

Tmelt [K]

∆Hmelt [J/g]

Tmelt [K]

∆Hmelt [J/g]

PLA

0

442.6

17.4

440.3

8.2

PLA1

1

442.2

46.8

438.4

36.6

PLA3

3

442.9

35.2

433.8

26.0

PLA6

6

441.5

33.4

427.5

25.6

PLA9

9

441.4

30.8

429.1

12.0

PLA12

12

441.9

22.7

424.7

6.55

ACKNOWLEDGMENT

The authors gratefully acknowledge the assistance of Ms. S. Rolf, Ms. R. Laging and Mr. D. Neubert for their experimental help. The financial supports from the Ph.D. program of BAM (to P. J. P.) and from the China Scholarship Council (to J. L.) is highly appreciated.

Figure Captions:

36

Figure 1. SAXS pattern of unmodified (a) and organically modified (b) LDH. Figure 2. Overlay of three synchrotron SAXS curves from a disc sample of PLA9 with a

diameter of 30 mm (solid, dotted and dashed-dotted lines). The dashed line shows the curve for organically modified LDH. The reflections in the q-range from 10 to 20 nm-1 are due to the crystal structure of the semi crystalline PLA. Figure 3. Signal of the refractive index detector versus the elution volume for pure PLA. The

inset shows the normalized molecular weight distribution versus the molecular weight: solid line – pure PLA, dashed line – PLA1, dotted line – PLA3, short dash line – PLA6, short dotted line PLA9, short dashed dotted line – PLA12. Figure 4. Change of the weight averaged molecular weight MW with the concentration of LDH.

The inset shows the normalized molecular weight distribution versus the molecular weight for the low molecular weight degradation products: solid line – pure PLA, dotted line – PLA3, short dotted line - PLA9, short dashed dotted line – PLA12. In the inset the other concentrations are omitted for sake of clearness. Figure 5. Comparison of DSC thermograms (as prepared samples, first heating, heating rate of

10 K/min) for pure PLA and selected nanocomposites: solid line - pure PLA, dashed line PLA1, dashed dotted line – PLA12. The curves are shifted along the y-scale for sake of clearness. Figure 6. Degree of crystallization χ vs. concentration of LDH squares – DSC measurements

(first heating run), circles – WAXS data. The solid line is a linear regression using both data sets. The inset compares the concentration dependence of the degree of crystallization estimated from DSC measurements: squares – first heating run, stares – second heating run. The lines are linear regressions to the corresponding data. Figure 7. Dielectric loss for pure PLA versus frequency and temperature and a 3D representation.

37

Figure 8. Dielectric loss versus temperature at a fixed frequency of 103 Hz for pure PLA: Circles

– first heating; squares – second heating. The inset gives an example for the fitting of the HN function to the data of pure PLA: circles – T=339.1 K; squares – T= 351.1 K. Lines are fits of the HN equation to the corresponding data including conductivity contribution. Figure 9. Relaxation rate of the α-relaxation (fp,α) vs. 1000/T for pure PLA. The solid line is a fit

of the VFT equation to the data. The inset gives the relaxation rate of the β-relaxation (fp,α) vs. 1000/T for pure PLA. The solid line is a fit of the Arrhenius equation to the data. Figure 10. Dielectric loss versus temperature at a frequency of 103 Hz for pure PLA and

different nanocomposites: Squares – pure PLA, circle – PLA1, diamonds – PLA3. Figure 11. Relaxation rates of the relaxation region 1 versus 1/T for different nanocomposites:,

circle – PLA1, diamonds – PLA3, triangles – PLA6, hexagons – PLA9, stars – PLA12. The line is a fit common of the VFT equation to all data. Inset A gives the relaxation rate for PLA6 (triangles) and PE6 (hexagons) versus 1/T. The lines are fits of the VFT equation to the corresponding data. Inset B gives [d log f p / d T ]−1 / 2 versus T for PLA6 (triangles) and PE6 (hexagons). The line is a linear regression using both data sets. Figure 12. Dielectric loss for PLA3 versus frequency and temperature and a 3D representation. Figure 13. Relaxation rates of the relaxation region 2 versus 1/T for PLA3 (diamonds). This line

is a fit of the VFT equation to the data. Relaxation rates of the α-relaxation versus 1/T for PLA and different nanocomposites obtained from the modulus representation: asterisks – neat PLA; circles – PLA1; diamonds – PLA3. The dashed line is a common fit of the VFT equation to all given data for the α-relaxation. The inset gives

[d log f p / d T ]−1 / 2

versus T for PLA3 (diamonds).

The line is a guide to the eyes. Figure 14. Imaginary part of the complex dielectric modulus M´´ versus frequency at T=363.1

K: asterisk – neat PLA; circles – PLA1; diamonds – PLA3. Lines are guides for the eyes.

38

Figure 15. Imaginary part of the complex dielectric modulus M´´ versus frequency at T=363.1 K:

triangles – PLA6; hexagons – PLA9. Lines are guides for the eyes. The inset gives the real part of the complex dielectric function for PLA9 at T=361.1K. Figure 16. Rates of the conductivity versus inverse temperature: triangles – PLA6; stars –

PLA12. The temperature dependence of fcon for PLA9 is similar to that of PLA12. The inset gives the rates for the Maxwell / Wagner / Sillars polarization fMWS versus 1/T: triangles – PLA6; stars – PLA12.

39

Author Contributions J. L. and P. J. P. contributed equally to the dielectric measurements and all the data analysis including partly interpretation. D.-Y. W. initiated the work from the chemical point of view. N. K. and D.-Y. W. synthesized the LDH nanofiller and prepared the nanocomposites. J. F. conducted the SEC experiments including its analysis. A. T. and F. E. carried out the X-ray measurements including a partly data analysis and interpretation. A. S. initiated the physical investigations and coordinated the work, did partly data interpretation and wrote parts of the manuscript. All authors have given approval to the final version of the manuscript.

ABBREVIATIONS PLA, polylactide; LDH, Layered Doubled Hydroxide; SEC, Size Exclusion Chromatography (SEC); DSC, Differential Scanning Calorimetry; SAXS, Small Angle X-ray Scattering; WAXS, Wide Angle X-ray Scattering.

40

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For Table of Contents use only Structure-Property Relationships of Nanocomposites Based on Polylactide and Mg/Al Layered Double Hydroxides *

Jing Leng, Purv J. Purohit, Nianjun Kang, De-Yi Wang, Jana Falkenhagen, Franziska * Emmerling, Andreas F. Thünemann, Andreas Schönhals

•

Nanocomposites are investigated by an orthogonal combination of methods.

•

Space resolved SAXS investigations show that the Filler is homogeneously distributed.

•

A concentration of the filler is estimated, where crystallization is suppressed.

•

The dielectric spectra of the nanocomposites are discussed in detail.