montmorillonite nanocomposites obtained from

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opening polymerization or by condensation polymerization [5]. Polymer ... Improvement of biodegradable polymers' properties by the addition of nanofillers has.
Evaluation of nanoparticles’ influence on poly(lactic acid) nanocomposites obtained by the solution intercalation method Luciana Macedo Brito, Maria Inês Bruno Tavares Instituto de Macromoléculas Professora Eloisa Mano da Universidade Federal do Rio de Janeiro (IMA/UFRJ), Centro de Tecnologia, Bloco J, Ilha do Fundão, Rio de Janeiro, RJ, Brazil, CP: 68525, CEP: 21945-970

Abstract

PLA nanocomposites were prepared by adding organically modified montmorillonite clay (Viscogel B8) and a homoionic clay (NT25) and unmodified silica (A200) and modified organo silica (R972) were obtained by the solution intercalation method. The materials were characterized by wide angle X-ray analysis, FT-IR and proton spin-lattice relaxation time, through low-field nuclear magnetic resonance relaxometry (LF-NMR). All clays and silicas showed good dispersion level with the polymeric matrix and the FTIR analysis showed the efficiency of the solvent evaporation. The relaxation time was different for each type of nanoparticle. The nanocomposite with homoionic clay NT25 became more rigid, indicating the formation of an intercalated nanocomposite, unlike the organoclay Viscogel B8. With the addition of the silicas, there was an increase in the relaxation time of the polymer matrix, but the interaction of R972 was better than that of the A200, indicating good interaction after the incorporation of nanoparticles in the PLA matrix.

1. Introduction Biodegradable and biocompatible polymers have attracted significant attention in the last decade. They have become the polymer matrix of choice for many researchers in the field of nanocomposites because they are environmentally friendly, since they degrade under the action of physicochemical and biological factors in a humid environment, and they can be employed in many biomedical and pharmaceutical applications such as sutures and biologically active controlled-release devices [1]. The most important advantage of biodegradable polymers is the disappearance of implanted foreign materials from the body as a result of their biodegradation. Important biodegradable polymers used in biomedical applications are poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(e-caprolactone) (PCL), poly(3-hydroxybutyrate) (PHB) and copolymers made of polyglycolide, chitosan and soy protein [2-4]. In this respect, poly(lactic acid) is rapidly gaining recognition as one of the most popular biopolymers. PLA is a biodegradable polyester synthesized from l- and d-lactic acid, the last of which is produced from the fermentation of sugar and polysaccharides such as sugar feedstocks and corn, wheat and other starch sources, either by ringopening polymerization or by condensation polymerization [5]. Polymer nanocomposites are a class of materials that have gained substantial popularity among researchers both in academia and industry due to their new properties, which are superior to those of virgin polymers and conventional composites [3]. Improvement of biodegradable polymers’ properties by the addition of nanofillers has good potential for designing eco-friendly materials for several applications, since the efficiency of nanofillers can be significant thanks to their high aspect ratio, even at a low loading (1–5 wt%). Various nanocomposites have been developed in recent years. The extensively studied ones are based on layered silicates, due to their availability and low cost [6-8]. In the case of biodegradable polymers, various authors [9-11] have recently reported the preparation and characterization of PLA nanocomposites with modified and unmodified montmorillonites. Polymer/layered-silicate nanocomposites can be prepared by solution casting, in situ polymerization and melt intercalation. In the first case, the inorganic material is thoroughly mixed and dispersed in a polymer solution, resulting in the insertion of polymer chains into the galleries of the clay, which swells. In situ polymerization involves monomer intrusion into the clay galleries followed by polymerization. Finally, in the melt intercalation process the molten

polymer is blended with the clay, optimizing the polymer/layered silicate interactions and facilitating the insertion of the polymer chains [12-14]. Many techniques can be used together in complement to investigate the properties and characteristics of nanocomposites, such as wide angle X-ray analysis (WAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and nuclear magnetic resonance (NMR). Solid-state NMR enables analyzing samples in many different ways using solutions. The responses come from chemical shift differences, peak intensity and line width, as well as from relaxation data [15-19]. In particular, low-field NMR spectroscopy has been shown to be a useful methodology to measure relaxation times [20-25]. Its spectroscopy can determine the values of proton spin-lattice relaxation time, with a time constant T 1, and proton spin–spin relaxation time, with time constant T2. Both relaxation times allow evaluating the sample behavior at the molecular level, because both time constants are sensitive to molecular motions at the MHz scale. Hence, changes in molecular mobility are normally detected and can be accompanied by T1 and T2 measurements [21-28]. The objective of this study was to prepare PLA/montmorillonite nanocomposites employing solution intercalation and to characterize them by proton spin-lattice relaxation time, determined through LF-NMR relaxometry.

2. Material and Methods 2.1 Samples NatureWorks®PLA Polymer 2002D sample was supplied by NatureWorks LLC. The organophylic clay Viscogel B8 was supplied by Bentec and the sodic clay NT25 by Bentonit União Nord. Ind. e Com. Ltda.

PLA nanocomposites were prepared by solution intercalation. First, a PLA solution was prepared and stirred vigorously at room temperature for 24 h. Then, a suspension of montmorillonite was added in varying proportions of 1, 3 and 5% and the solution was again stirred vigorously at room temperature for 24 h. This solution was spread onto a glass plate and the solvent was evaporated. Solvent elimination was confirmed by its band disappearing using infrared spectroscopy. After the solvent was completely removed, the film was taken out for further analyses.

2.2 NMR measurements The relaxation measurements were performed with a Maran low-field NMR spectrometer (Resonance Instruments, Oxford, UK), operating at 23MHz (hydrogen nucleus). Proton spin-lattice relaxation times were determined directly by the traditional inversion-recovery pulse sequence (recycle time - 180º–τ–90º - acquisition) using 40 data points, with 4 scans for each and a range of τ varying from 0.1 to 27 ms, with 10 s of recycle delay and 90º pulse of 4.5 ms, calibrated automatically by the instrument’s software. The T1 values and relative intensities were obtained by fitting the exponential data with the aid of the WINFIT program. Distributed exponential fittings as a plot of relaxation amplitude versus relaxation time were performed by using the WINDXP software.

2.3 X-ray analyses X-ray analyses were carried out with a Rigaku Miniflex diffractometer, operating at the Cu Ka wavelength (1.542 Å). The scanning range was varied from 2θ = 2º to 40º.

2.4 Fourier transform infrared measurements The infrared spectra were recorded with a Varian 3100 FT-IR spectrometer at room temperature. A zinc selenide (ZnSe) internal reflection element (IRE) with a fixed angle of incidence of 45º was used for attenuated total reflection (ATR) measurements.

3. Results and Discussion 3.1 FTIR Since the PLA and their nanocomposite films were obtained by the solution intercalation method, the first investigation after film formation regarded the solvent elimination, which was accompanied by the FTIR technique through solvent band disappearance. ATR-FTIR is generally used for surface characterization, but the intensity of the observed bands depends on the wavelength of the ATR-FTIR spectrum. For this reason, comparison between ATR-FTIR and FTIR-transmission spectra is not generally recommended [24]. Figures 1, 2 and 3 show the FTIR spectra of the PLA and their nanocomposites. The peaks located at about 1756 cm−1 and 1180 cm−1, which belong to the CO stretching and C–O–C stretching of PLA, respectively [25], are visible in all the IR spectra. This shows that non-chemical structural modifications occurred in

the polymer matrix after the nanoparticles were incorporated. The solvent was not completely eliminated, since a peak located at 750 cm-1 was detected in all the samples. According to these results, more time was necessary to eliminate the residual solvent in the films. After three days, no more solvent was detected in the FTIR. Figures 4, 5 and 6 show the FTIR spectra of the nanocomposite films after the complete residual solvent elimination. CHCl3

Transmittance (%)

240

PLA 3% NT25 5%

160

NT25 3% NT25 1% 80

3900 3600 3300 3000 2700 2400 2100 1800 1500 1200

900

600

-1

Wavenumber (cm )

Figure 1 - FTIR spectra of PLA and PLA/NT25 nanocomposites.

CHCl3

Transmittance (%)

300

PLA 3% 200

B8 5% B8 3% 100

B8 1%

0 3900 3600 3300 3000 2700 2400 2100 1800 1500 1200 -1

Wavenumber (cm )

Figure 2 - FTIR spectra of PLA and PLA/B8 nanocomposites

900

600

Transmitence (%)

CHCl3

PLA 3%

R972 0,1% A200 0,1%

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 3 - FTIR spectra of PLA and PLA/silica nanocomposites

CHCl3

Intensity (u.a.)

Transmitance (%)

NT25

PLA POWDER PLA FILM 3% NT25 5% NT25 3% NT25 1%

4000

3500

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2500

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500

-1

Wave number (cm )

Figure 4 - FTIR spectra of PLA and PLA/NT25 nanocomposites

CHCl3

(%) Transmitance Intensity (u.a.)

B8

PLA POWDER PLA FILM 3% B8 5% B8 3% B8 1% 4000

3500

3000

2500

2000

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-1

Wave number (cm )

Figure 5 - FTIR spectra of PLA and PLA/B8 nanocomposites

500

CHCl3

Intensity (u.a.) (%) Transmitance

PLA POWDER PLA FILM 3% R972 R972 0,1% A200 A200 0,1%

4000

3500

3000

2500

2000

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1000

500

-1

wave number (cm )

Figure 6 - FTIR spectra of PLA and PLA/silica nanocomposites

3.2 X-ray analyses

The characterization of the PLA, PLA/MMT and PLA/silica samples was accomplished using the conventional X-ray diffraction measurement, using a theta range from 2 to 40º. NMR relaxometry measured by low-field NMR was also used. Figures 7 and 8 show the XRD patterns obtained for the PLA, clay and their nanocomposite samples. The pattern of the PLA is characterized by a broad peak with maximum at approximately 2θ = 17°, indicating its completely amorphous molecular organization. The XRD of the polymer matrix was not influenced by the presence of the filler with respect to the crystallinity or molecular organization induction. Therefore, a displacement of the amorphous line to low angles was seen for all nanocomposites, due to the filler dispersion. The diffraction peaks of interest in characterizing nanocomposites’ molecular organization and nanoparticle dispersion are those which appear in the range of 2º to 5º, in 2θ scale, which gives an indication of the clay dispersion mode in the polymeric matrix. These peaks are related to the basal spacing, peaks of the clay’s d001 plane. NT25 is characterized by a single diffraction peak at 2θ = 5.0°, corresponding to the basal reflection (d001). On the other hand, Viscogel B8 is characterized by a single diffraction peak at 2θ = 3.52°, due to the organophilic group’s incorporation between the clay lamellae. Both clays appeared to have good interaction with the polymeric matrix, especially the NT25 (Fig. 8). Indeed, a shift of the clay diffraction peak to lower angles

was observed, corresponding to an increase of the interlayer distance for NT25 and B8. Moreover, in the case of NT25 nanocomposites, a significant decrease of the peak intensity was observed, which may account for the formation of a disordered structure.

Figure 7. X-ray diffraction patterns of organoclay (Viscogel B8) and PLA/MMT nanocomposites.

There was disappearance of a shift of the clay peaks to lower angles in all the nanocomposites formed with the homoionic clay (NT25). These results probably indicate the formation of an intercalated/exfoliated mixed nanocomposite structure. 4000

NT25

Relative intensity

3000

PLA 3% 2000

NT25 5%

1000

NT25 3% NT25 1%

0 10

20

30

40

2°)

Figure 8. X-ray diffraction patterns of clay (NT25) and PLA/NT25 nanocomposites.

PLA 3% 3000

Relative Intensity

R972

R972 0,1%

2000

A200

1000

A200 0,1%

0 10

20

30

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2

Figure 9. X-ray diffraction patterns of both silica (A200 and R972) and PLA/silica nanocomposites.

3.2 NMR measurements The nuclear relaxation times are presented in Tables 1 and 2. It can be observed that there was an increase in relaxation times of the PLA with the addition of the sodic clay (NT25). The increase in relaxation time of the polymer, indicating lower molecular mobility, means greater restriction of motion of the chains because they are more constricted. This indicates the formation of a rigid material, which is normally associated with the formation of an intercalated nanocomposite. Table 1. Proton spin-lattice relaxation times of PLA and PLA/NT25 nanocomposites determined by low-field NMR, employing 1 exponential value _______________________________________________________ PLA/NT25

T1H (ms) 1 EXP

_______________________________________________________ PLA FILME 3%

607

PLA + NT25 1%

620

PLA + NT25 3%

625

PLA + NT25 5%

533

____________________________________________________

The decrease in T1H values with the increase of the Viscogel B8 proportion indicates lower polymer-polymer interaction and the generation of a polymer interface interaction with the nanoparticles, indicating the formation of a system with good degree of exfoliation. The predominant effect on T1H for nanocomposites containing Viscogel B8 is derived from the dispersion mode of clay in the polymeric matrix. The values of T1H parameter decreases, as a consequence of the degree exfoliation increases, since the polymer chains are around the clay lamellae, they are under influence of paramagnetic metals (which are present in the clay structure) and cause an influence in the relaxation process, accelerating it, as a consequence the hydrogen’s of the polymeric matrix relaxing rapid comparing to the PLA itself. These data are in agreement with those already published by Vanderhart (2001). Table 2. Proton spin-lattice relaxation times of PLA and PLA/B8 nanocomposites, determined by low-field NMR, employing 1 exponential value _______________________________________________________ PLA/VISCOGEL B8

T1H (ms) 1 EXP

_______________________________________________________ PLA FILME 3%

604

PLA + B8 1%

593

PLA + B8 3%

578

PLA + B8 5%

519

____________________________________________________

Table 3 shows the relaxation times of the PLA nanocomposites with unmodified and modified silica. There was an increase in the polymer relaxation time with the increase in the concentration of nanoparticles in the polymer nanocomposite formed with silica R972 because it presented a higher relaxation time than those formed with silica A200. This indicates there was a decrease in molecular mobility due to the good interaction after the incorporation of the nanoparticles in the PLA matrix.

Table 3. Proton spin-lattice relaxation times of PLA and PLA/silica nanocomposites, determined by low-field NMR, employing 1 exponential value _______________________________________________________ PLA/silica

T1H (ms) 1 EXP

_______________________________________________________ PLA FILM 3%

607

PLA + A200 0.1%

627

PLA + R972 0.1%

639

______________________________________________________

From the relaxation time measurements allow plotting the domain curves of the relaxation time values according to the changes in the molecular structural after the incorporation of nanoparticules in the polymeric matrix.

Figures 10 and 11 show the domain curves for the PLA/NT25 and PLA/Viscogel B8 nanocomposites.

400

Relative intensity

PLA 3%

PLA+NT25 5%

200

PLA+NT25 3% 0

PLA+NT25 1% 10

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1000

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1000000

1E7

Time (s)

Figure 10. Distribution curves obtained by low-field NMR of PLA and PLA/NT25 nanocomposites.

400

Relative intensity

PLA 3%

PLA+ B8 3%

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PLA+ B8 3%

0

PLA+ B8 1%

10

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1000

10000

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1000000

1E7

Time (s)

Figure 11. Distribution curves obtained by low-field NMR of PLA and PLA/B8 nanocomposites.

The domain curves shown in Figure 10 and 11 indicate that no significant changes in domains occurred in the nanocomposites formed with sodic clay NT25. On the other hand, the domains of the nanocomposites made with Viscogel B8 at concentrations of 1 and 3% were wider. This result indicates that a more organized and possibly more intercalated nanocomposite was formed. At a concentration of 3%, the field was narrower than in the pure material. This narrowing indicates the formation of an amorphous material with a more exfoliated structure, in turn indicating a reduction of the rigidity at the molecular level [26]. Figure 12 shows the domain curves for the nanocomposites containing 0.1% PLA/A200 and 1% PLA/R972. The distribution of the domain curves provides information on the molecular mobility, interaction of the polymer matrix with the nanoparticles and structural organization. For this system, the silica introduction changed the molecular organization, due to the base broadening of the nanocomposites. This corroborates the fact that the insertion of nanoparticles caused an increase in the chain disorder, generating different forms of molecular organization. A small deviation of the domain curves to higher values is a result of the increase in the relaxation values shown in the table, confirming the decrease in the molecular mobility of the hydrogen nuclei. Finally, the appearance of only one domain for the nanocomposites in relation to the polymer matrix, indicating the same basic molecular structure of the PLA for both unmodified (A200) and modified silica (R972).

300

Relative intensity

PLA 3% 200

A200 0,1% 100

R972 0,1%

0

10

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1000000

1E7

Time (s)

Figure 12. Distribution curves obtained by low-field NMR of PLA and PLA/silica nanocomposites

4. Conclusions According to the results obtained, the use of the relaxation time can elucidate the molecular mobility changes in the PLA matrix caused by the addition of clays. Lowfield 1H NMR relaxometry was particularly useful to study the non-exfoliated and exfoliated PLA/clay nanocomposite materials.

Acknowledgment We are grateful to CNPq for the financial support of this work.

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