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Molecular Dynamic Evaluation of starch-PLA blends nanocomposite with organoclay by proton NMR relaxometry
Polymer Testing 32 (2013) 1181–1185
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Analysis method
Molecular Dynamic Evaluation of starch-PLA blends nanocomposite with organoclay by proton NMR relaxometry Luciana M. Brito a, Fabián Vaca Chávez b, Maria Inês Bruno Tavares a, *, Pedro J.O. Sebastião b, c a
Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco J, Cidade Universitária, Ilha do Fundão, CP 68525, Rio de Janeiro, RJ 21945-970, Brazil b Condensed Matter Physics Centre, University of Lisbon, Lisbon, Portugal c Departamento de Física, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2013 Accepted 6 July 2013
Starch and PLA were used alone and in blends to prepare nanostructured materials using both hydrophilic and organophilic clays, and PVA. All nanostructured materials were obtained by the solution intercalation method using water and chloroform as solvents. These systems were characterized by using conventional X-ray diffraction (XRD), conventional NMR and the non-conventional fast field cycling (FFC) NMR technique. The spin-lattice relaxation times were measured as a function of the Larmor frequency. The FFC results showed that the starch has only one relaxation time related to the amorphous region. PLA hybrids presented two distinct spin-lattice relaxation times. The blends of the two polymers also showed two relaxation times. The renormalized Rouse formalism was applied to describe the polymer molecular dynamics behavior in the studied systems containing starch. By adding clay or PVA, differences could be observed in relaxation time corresponding to the more amorphous region, indicating that, when adding clay and PVA, the effect that each has on the dynamics of the mixture is cancelled out. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Proton spin-lattice relaxation time Fast-field cycling Starch PLA Nanocomposites
1. Introduction Polymers from renewable resources have attracted increasing attention over the last two decades for two main reasons: first, environmental concerns, and second the realization that our petroleum resources are finite [1–3]. Among the main disadvantages of biodegradable polymers obtained from renewable sources are their dominant hydrophilic character, fast degradation rate and, in some cases, unsatisfactory mechanical properties, particularly in wet environments. In principle, the properties of natural polymers can be significantly improved by blending with synthetic polymers [2].
* Corresponding author. E-mail address: [email protected] (M.I. Bruno Tavares). 0142-9418/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2013.07.002
Polymer blending is a widely used technique whenever modification of properties is required, because it uses conventional technology at low cost. The usual objective of preparing a novel blend of two or more polymers is not to change the properties of the components drastically, but to capitalize on the maximum possible performance of the blend. Unfortunately, PLA and starch are thermodynamically immiscible since PLA is hydrophobic and starch is hydrophilic. This leads to poor adhesion between the two components and, consequently, to poor and irreproducible performance [3]. Studies recently published have shown different ways to increase the interaction of these two polymers with different preparation techniques and with the addition of different compatibilizing agents, by applying various techniques to analyze these materials [3–6]. Solid-state NMR enables analyzing samples using different techniques that possess distinct pulse sequences.
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L.M. Brito et al. / Polymer Testing 32 (2013) 1181–1185
Chemical shift displacements, signal intensity and line width, as well as the relaxation times can be used to characterize the molecular order and dynamics in the studied systems [7–10]. In particular, the measurement of proton spin-lattice relaxation time, T1, and proton spin– spin relaxation time, T2.with low-field NMR spectrometers has been shown to be a useful method to characterize the molecular motions with different characteristic times [11– 15]. Both relaxation times characterize the system behavior at the molecular level, because they are sensitive to molecular dynamics at different time scales. Hence, changes in molecular mobility are normally detected and can be accompanied by T1 and T2 measurements [13,16,17]. In order to study the molecular dynamics in different time scales (e.g. different frequencies) using T1, it is necessary to use both high and low-field NMR. The measurement of relaxation parameters using standard lowfield NMR techniques is very much conditioned by signalto-noise ratio limitations at low frequencies. The use of low-field NMR fast field cycling (FFC) techniques permits the measurement of the spin-lattice relaxation times at low frequencies with good NMR signals [18,19]. The main applications of field cycling NMR relaxometry to be considered are: surface related relaxation processes of fluids in porous materials; polymer dynamics, biopolymers and biological tissue, liquid crystals and lipid bilayers. The field-cycling technique has been also applied to melts, solutions and networks of numerous polymer species [20– 23]. Some parameters can be varied in the experiments such as temperature, the molecular weight, the concentration and the cross-link density. The objectives of this study were to obtain starch-PLA blends and their nanostructured materials generated by the addition of hydrophilic and organophilic clays and PVA through solution mode and to characterize all of them by FFC NMR, through the effect of different quantity of clays. 2. Materials and methods 2.1. Samples The NatureWorksÒ PLA Polymer 2002D sample was supplied by NatureWorks LLC. The starch used was a commercial product obtained from potatoes and supplied by Yoki. PVA was supplied by Bentec. The organophylic clay Viscogel B8 was supplied by Bentec and the homoionic clay NT25 by Bentonit União Nord. Ind. e Com. Ltda. 2.2. Preparation of starch-PLA nanocomposite blends The starch-PLA blends (with 0.5% of PVA and without PVA) were prepared by solution casting, employing chloroform and distilled water as the solvents. First, the PLA solutions were prepared separately and were stirred at room temperature for 24 h, to completely dissolve the PLA, using a magnetic stirrer. This condition was previously determined for both polymers, since the boiling point of chloroform had to be considered, and an increase of temperature would favor the evaporation of the chloroform before the polymer
was dissolved. After this time, the gelled starch solution was added to PLA solution. This new solution was stirred at room temperature for 48 h. Then, suspensions of organophylic (Viscogel B8) and homoionic clay (NT25) containing 1, 3 and 5% clay were added. After blending the samples, the new solutions were stirred again with the magnetic stirrer at room temperature for 24 h. This solution was put into Teflon plate and the solvent was evaporated, using an oven with forced air circulation for five days, to complete eliminate the solvent. After the solvent was completely removed, the film was taken out for further analyses. 2.3. X-ray analyses X-ray analyses were carried out in a Rigaku D/Max 2400 diffractometer, with nickel-filtered CuKa radiation of wavelength 1.54 Å, at room temperature. The 2q scanning range was varied from 2 to 40 , with 0.02 steps, operated at 40 KV and 30 mA. 2.4. NMR measurements The 1H NMR spin-lattice relaxation time was measured as a function of the Larmor frequency nL ¼ uL/(2p) in the frequency range 10 kHz-300 MHz, using three NMR spectrometers. All measurements were made at a temperature of 25 C. The low-frequency measurements between 10 kHz and 9 MHz were performed using a home-developed fast-fieldcycling NMR relaxometer [18]. This experimental technique allows for the measurement of T1 at frequencies usually inaccessible by standard NMR spectrometers. This is possible because the magnetic field in the equipment used is switched up and down between two values: a fixed high value corresponding to the signal acquisition radiofrequency of 8.9 MHz and a variable low value at which the relaxation time is to be measured [18–20]. In FFC experiments, 20 scans and 30 points were recorded and repeated twice for each frequency. A variable field electromagnet and a 7T superconducting magnet were used in conjunction with a Bruker Avance II NMR console for measurements at Larmor frequencies between 15100 MHz and at 300 MHz, respectively. The T1 measurements were done using the inversion-recovery pulse sequence with phase cycling to compensate for any dc-bias components in the free induction decay (FID) signal. 3. Results and discussion Fig. 1 shows the data obtained from the X-ray diffraction analyses of the starch-PLA blends, with and without addition of PVA and Viscogel B8 organoclay. First, the diffraction filler peaks of interest, to characterize the nanostructure materials molecular organization and nanoparticle dispersion, are those that appear in the range of 2 to 5 in the 2q scale. They give an indication of the clay dispersion mode in the polymeric matrix. These peaks are related to the basal spacing of the clay’s d001 plane. Viscogel B8 is characterized by a single diffraction peak at 2q ¼ 3.52 , due to the organophilic group’s incorporation between the clay lamellae.
L.M. Brito et al. / Polymer Testing 32 (2013) 1181–1185
1183
Starch-PLA-PVA-B8 1%
Relative intensity
Starch-PLA-PVA Starch-PLA-B8 1%
Starch-PLA
Starch film 0
10
20
30
40
2θ (°) Fig. 1. X-ray diffraction patterns of Starch, Starch-PLA and the mixtures obtained with PVA and organoclay Viscogel B8 at 1%.
In relation to the dispersion, the 1% of Viscogel B8 caused the disappearance of the clay peaks or they are shifted to lower angles. According to these data, this clay is shown to have a good interaction with the polymer blend matrix. In addition, changes occurred in the crystallinity of the mixture when Viscogel B8 clay and PVA were added. Both the addition of PVA and B8 at a quantity of 1% were able to make the polymer blend (starch-PLA) less crystalline, confirmed by the disappearance or decrease of the peak at about 2q ¼ 20 relative to PLA (Brito et al., 2012). However, with the addition of both PVA and clay to the polymer blend, there was reversal of the effects caused by the addition of each agent separately. These observations are corroborated by the results obtained in the NMR-FFC analysis. Figs. 2–6 show the dispersion profiles for starch, PLA, starch PLA-blend and their nanostructured materials. The data obtained suggests the presence of two types of 1H population in the samples studied, corresponding to formation of distinct spin-lattice relaxation domains, except for the starch sample. For this sample, only one type of proton population and one T1 domain was observed. Fig. 2 shows the proton spin-lattice relaxation time, T1H, of the starch, PLA and starch-PLA blend measured as a function of frequency. It can be observed that the T1H of the starch-PLA blend is larger than that of pure starch, therefore a restriction of starch molecular motion caused by the PLA action at lower frequencies can be assumed, due to the predominant PLA molecular dynamics. The molecular motions of starch could only be observed at higher frequencies (1 MHz-20 MHz) as the T1H of the blend is smaller than that of pure PLA. Due to the method used to obtain this blend, as described in Section 2.2, there was encapsulation of the starch by the PLA matrix. This encapsulation occurred because of the different evaporation times of the solvents. In this case, PLA was dissolved in chloroform, and water was used to gelatinize the starch. In the mixture, chloroform was evaporated first, encapsulating the starch matrix, and the water subsequently evaporated, forming a material with a porous structure.
Fig. 2. Proton spin-lattice relaxation time T1 of polymers starch, PLA and starch-PLA blend measured as a function of the frequency.
Figs. 3 and 4 show the relaxation time, T1H, versus frequency for the composites of starch with homoionic clay (NT25) and organoclay (Viscogel B8). The unique structure of the nanocomposites is determined by the occurrence of two polymer phases in the system, the bulk phase and the confined phase of polymer intercalated into clay galleries. Molecular dynamics of polymer melts in the bulk phase is generally described using the Rouse model or the renormalized Rouse model, whereas the reptation model is applied to describe the molecular dynamics of confined polymers [20–22]. Renormalized Rouse dynamics for bulk polymer with molecular weight above the critical mass Mw > Mc is expressed by different power laws, corresponding to different motional regimes, such as high and low-mode and inter-segment dipolar interaction limits. This model has been successfully applied in experimental studies of semicrystalline or amorphous polymers [23].
Fig. 3. Proton spin-lattice relaxation time T1H of pure starch and starch composites with organoclay (Viscogel B8) measured as a function of frequency. The dotted and dashed lines are the best fits of the renormalized Rouse model. The fitting parameters of the power law dependence of T1 as a function of the Larmor frequency are explained in the text as shown above.
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L.M. Brito et al. / Polymer Testing 32 (2013) 1181–1185
Fig. 4. Proton spin-lattice relaxation time T1H of pure starch and starch composites with homoionic clay (NT25) measured as a function of frequency. The dotted and dashed lines are the best fits of the renormalized Rouse model. The fitting parameters of the power law dependence of T1as a function of the Larmor frequency are explained in the text as shown above.
Fig. 6. Proton spin-lattice relaxation time T1H of starch-PLA blend with organoclay (Viscogel B8) and PVA measured as a function of frequency. The dotted line is the best fit of the renormalized Rouse model. The fitting parameters of the power law dependence of T1as a function of the Larmor frequency are shown in the graphs.
The results in Figs. 3 and 4 indicate the presence of only one type of proton population. When applying the renormalized Rouse model for samples of starch and its composites, we obtained power laws similar to those found in the literature [20,23]. The composite with clay NT25 (Fig. 3) showed power laws with exponents w1 and w0.5. According to previously published studies [20], these exponents are related to freer movements of the macromolecules (isotropic phase). The formation of exfoliated materials can thus be suggested, due to higher affinity of the starch matrix (hydrophilic) with the homoionic clay. Composites of starch with the organophilic clay Viscogel B8 at 1% showed a 0.5 power law (red dashed line), indicating the formation of an exfoliated material, and a 0.4
power law (blue dotted line) for composites with Viscogel B8 at 5%. This exponent is related to slower movements and a more ordered sample (mesophase), indicating the formation of an intercalated material. Fig. 5 shows the relaxation times of PLA with homoionic clay (NT25 at 1% and 5%) and organically modified montmorillonite clay (Viscogel B8 at 1%). The B8 results obtained with 5% are not shown due to large variability. The results show the presence of two types of 1H population. The first type of proton population is represented by empty symbols and is associated with the longer T1H, while the second proton population, with shorter values of T1H, is represented by filled symbols. In this case, the renormalized Rouse model does not apply, probably because the material is semi-crystalline and the assignment of the two relaxation times to the crystallinity of different regions of the polymer is not clear. There was an increase in the values of T1H with the addition of clay NT25 at both concentrations used, so these values of T1H could be measured at frequencies below 15 MHz. In contrast, with organoclay B8, these values remained similar to those of the pure polymer. The effect of Viscogel B8 on the T1H values of the nanocomposites is derived from the dispersion mode of the clay in the polymeric matrix, due to its better affinity with the polymer matrix, since this clay has an organic interlayer, facilitating the interaction between the polymer chain and filler lamellae. These findings are in agreement with those already published by Brito et al. (2012), where both clays used to obtain the polymeric nanocomposites showed good dispersion in the polymeric matrix. An increase of the T1H values for clay NT25 might be correlated with possible formation of an intercalated material. Fig. 6 shows the relaxation times for the mixtures of starch and PLA with the addition of polyvinyl alcohol (PVA) (surfactant agent, 0.5% w/w) and adding the organoclay Viscogel B8 at 1%. The results of the 5% concentration are not presented due to large variation of the data.
Fig. 5. Proton spin-lattice relaxation time T1 of pure PLA and PLA composites with organoclay (Viscogel B8) and homoionic clay (NT25) measured as a function of frequency. The dotted lines are the best fits of the renormalized Rouse model. The fitting parameters of the power law dependence of T1 as a function of the Larmor frequency are shown in the graphs.
L.M. Brito et al. / Polymer Testing 32 (2013) 1181–1185
Two types of proton populations can be identified according to the T1H values associated with the individual phases of starch-PLA blends. They depend on the type of clay and the presence of PVA. Shorter values of T1H showed differences in molecular dynamics with the addition of clay and PVA. However, when comparing the values of T1H obtained for the starch-PVA-PLA-B8 1% blend with the others, it can be seen that values of T1H (filled triangles) are related to the values of the relaxation time of starch with Viscogel B8 at 1% (Fig. 6). Probably, the addition of PVA was able to separate the mixture. The lower values of T1H in the starch-PVA-PLA blend indicate a similar trend to the starch-PLA-B8 1% blend. We believe the effect of compatibility of the PVA and Viscogel B8 at 1% is similar. However, in the starch-PVA-PLA-B8 1% blend, the lower values of T1H are similar to the T1H values of the pure starch. In this case, the effects of the PVA and Viscogel B8 in the mixture appear to cancel each other. 4. Conclusions According to the results obtained, the measurement of the relaxation time can elucidate the molecular mobility changes in the starch-PLA matrix caused by the addition of nanoparticles and PVA. FFC was successfully used to determine the molecular motions, in addition to providing important information about the miscibility of the starchPLA blends. The obtained results concerning the molecular mobility might also be important in further studies of other physical properties of these systems. Acknowledgments We are grateful to CNPq and CAPES for the financial support of this work.
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References [1] L. Yua, K. Dean, L. Li, Progress Polymer Science 31 (2006) 576– 602. [2] M. Avella, J.J. De Vlieger, M.E. Errico, S. Fischer, P. Vacca, M.G. Volpe, Food Chemistry 93 (2005) 467–474. [3] N. Le Bolay, A. Lamure, N.G. Leis, A. Subhani, Chemical Engineering and Processing 56 (2012) 1–9. [4] F. Yua, K. Prashanthaa, J. Soulestina, M.F. Lacrampea, P. Krawczak, Carbohydrate Polymers 91 (2013) 253–261. [5] R. Ouhib, B. Renault, H. Mouaziz, C. Nouvel, E. Dellacherie, J. Six, Carbohydrate Polymers 77 (2009) 32–40. [6] Q. Gong, L.Q. Wang, K. Tu, Carbohydrate Polymers 64 (2006) 501– 509. [7] J. Grandejean, Clay Minerals 41 (2006) 567–586. [8] D.L. Vanderhart, A. Asano, J.W. Gilmanof, Chemistry of Material 13 (2001) 3781–3795. [9] D.L. Vanderhart, G.B. McFadden, Solid State Nuclear Magnetic 7 (1996) 45–66. [10] D.A. Costa, E.P. Silva, C.M.F. Oliveira, M.I.B. Tavares, Journal of Applied Polymer Science 64 (1998) 1635. [11] N.M. Silva, M.I.B. Tavares, E.O. Stejskal, Macromolecules 33 (2000) 115. [12] M. Preto, M.I.B. Tavares, E.P. da Silva, Polymer Testing 26 (2007) 501–504. [13] A.A. Passos, M.I.B. Tavares, R.C.P. Neto, L.A. Moreira, A.G. Ferreira, Polímeros Ciência e Tecnologia 21 (2011) 1–5. [14] D.K. Resende, C.B. Dornelas, M.I.B. Tavares, Polímeros 20 (2010) 231–235. [15] T. Paragkumar, D. Edith, J. Six, Applied Surface Science 253 (2006) 2758–2764. [16] L.M. Brito, M.I.B. Tavares, Journal of Nanoscience and Nanotechnology 12 (2012) 4508–4513. [17] G.C.V. Iulianelli, P.M.C. Maciel, M.I.B. Tavares, Macromolecular Symposia 229–300 (2011) 227–233. [18] D.M. Sousa, G.D. Marques, J.M. Cascais, P.J. Sebastiaeo, Solid State Nuclear Magnetic Resonance 38 (2010) 36–43. [19] F. Noack, Progress in nuclear magnetic, Resonance Spectroscopy 18 (1986) 171–276. [20] R. Kimmich, E. Anoardo, Progress in Nuclear Magnetic Resonance Spectroscopy 44 (2004) 257–320. [21] K.M. Nampoothiri, N.R. Nair, R.P. John, Bioresource Technology 101 (2010) 8493–8501. [22] N. Fatkullin, R. Kimmich, E. Fischer, C. Mattea, U. Beginn, M. Kroutieva, New Journal of Physics 6 (2004) 1. [23] M.S.S.B. Monteiro, F. Chávez, P.J. Sebastião, M.I.B. Tavares, Polymer Testing 3 (2013) 553–566.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Analysis method
Molecular Dynamic Evaluation of starch-PLA blends nanocomposite with organoclay by proton NMR relaxometry Luciana M. Brito a, Fabián Vaca Chávez b, Maria Inês Bruno Tavares a, *, Pedro J.O. Sebastião b, c a
Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco J, Cidade Universitária, Ilha do Fundão, CP 68525, Rio de Janeiro, RJ 21945-970, Brazil b Condensed Matter Physics Centre, University of Lisbon, Lisbon, Portugal c Departamento de Física, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2013 Accepted 6 July 2013
Starch and PLA were used alone and in blends to prepare nanostructured materials using both hydrophilic and organophilic clays, and PVA. All nanostructured materials were obtained by the solution intercalation method using water and chloroform as solvents. These systems were characterized by using conventional X-ray diffraction (XRD), conventional NMR and the non-conventional fast field cycling (FFC) NMR technique. The spin-lattice relaxation times were measured as a function of the Larmor frequency. The FFC results showed that the starch has only one relaxation time related to the amorphous region. PLA hybrids presented two distinct spin-lattice relaxation times. The blends of the two polymers also showed two relaxation times. The renormalized Rouse formalism was applied to describe the polymer molecular dynamics behavior in the studied systems containing starch. By adding clay or PVA, differences could be observed in relaxation time corresponding to the more amorphous region, indicating that, when adding clay and PVA, the effect that each has on the dynamics of the mixture is cancelled out. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Proton spin-lattice relaxation time Fast-field cycling Starch PLA Nanocomposites
1. Introduction Polymers from renewable resources have attracted increasing attention over the last two decades for two main reasons: first, environmental concerns, and second the realization that our petroleum resources are finite [1–3]. Among the main disadvantages of biodegradable polymers obtained from renewable sources are their dominant hydrophilic character, fast degradation rate and, in some cases, unsatisfactory mechanical properties, particularly in wet environments. In principle, the properties of natural polymers can be significantly improved by blending with synthetic polymers [2].
* Corresponding author. E-mail address: [email protected] (M.I. Bruno Tavares). 0142-9418/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2013.07.002
Polymer blending is a widely used technique whenever modification of properties is required, because it uses conventional technology at low cost. The usual objective of preparing a novel blend of two or more polymers is not to change the properties of the components drastically, but to capitalize on the maximum possible performance of the blend. Unfortunately, PLA and starch are thermodynamically immiscible since PLA is hydrophobic and starch is hydrophilic. This leads to poor adhesion between the two components and, consequently, to poor and irreproducible performance [3]. Studies recently published have shown different ways to increase the interaction of these two polymers with different preparation techniques and with the addition of different compatibilizing agents, by applying various techniques to analyze these materials [3–6]. Solid-state NMR enables analyzing samples using different techniques that possess distinct pulse sequences.
1182
L.M. Brito et al. / Polymer Testing 32 (2013) 1181–1185
Chemical shift displacements, signal intensity and line width, as well as the relaxation times can be used to characterize the molecular order and dynamics in the studied systems [7–10]. In particular, the measurement of proton spin-lattice relaxation time, T1, and proton spin– spin relaxation time, T2.with low-field NMR spectrometers has been shown to be a useful method to characterize the molecular motions with different characteristic times [11– 15]. Both relaxation times characterize the system behavior at the molecular level, because they are sensitive to molecular dynamics at different time scales. Hence, changes in molecular mobility are normally detected and can be accompanied by T1 and T2 measurements [13,16,17]. In order to study the molecular dynamics in different time scales (e.g. different frequencies) using T1, it is necessary to use both high and low-field NMR. The measurement of relaxation parameters using standard lowfield NMR techniques is very much conditioned by signalto-noise ratio limitations at low frequencies. The use of low-field NMR fast field cycling (FFC) techniques permits the measurement of the spin-lattice relaxation times at low frequencies with good NMR signals [18,19]. The main applications of field cycling NMR relaxometry to be considered are: surface related relaxation processes of fluids in porous materials; polymer dynamics, biopolymers and biological tissue, liquid crystals and lipid bilayers. The field-cycling technique has been also applied to melts, solutions and networks of numerous polymer species [20– 23]. Some parameters can be varied in the experiments such as temperature, the molecular weight, the concentration and the cross-link density. The objectives of this study were to obtain starch-PLA blends and their nanostructured materials generated by the addition of hydrophilic and organophilic clays and PVA through solution mode and to characterize all of them by FFC NMR, through the effect of different quantity of clays. 2. Materials and methods 2.1. Samples The NatureWorksÒ PLA Polymer 2002D sample was supplied by NatureWorks LLC. The starch used was a commercial product obtained from potatoes and supplied by Yoki. PVA was supplied by Bentec. The organophylic clay Viscogel B8 was supplied by Bentec and the homoionic clay NT25 by Bentonit União Nord. Ind. e Com. Ltda. 2.2. Preparation of starch-PLA nanocomposite blends The starch-PLA blends (with 0.5% of PVA and without PVA) were prepared by solution casting, employing chloroform and distilled water as the solvents. First, the PLA solutions were prepared separately and were stirred at room temperature for 24 h, to completely dissolve the PLA, using a magnetic stirrer. This condition was previously determined for both polymers, since the boiling point of chloroform had to be considered, and an increase of temperature would favor the evaporation of the chloroform before the polymer
was dissolved. After this time, the gelled starch solution was added to PLA solution. This new solution was stirred at room temperature for 48 h. Then, suspensions of organophylic (Viscogel B8) and homoionic clay (NT25) containing 1, 3 and 5% clay were added. After blending the samples, the new solutions were stirred again with the magnetic stirrer at room temperature for 24 h. This solution was put into Teflon plate and the solvent was evaporated, using an oven with forced air circulation for five days, to complete eliminate the solvent. After the solvent was completely removed, the film was taken out for further analyses. 2.3. X-ray analyses X-ray analyses were carried out in a Rigaku D/Max 2400 diffractometer, with nickel-filtered CuKa radiation of wavelength 1.54 Å, at room temperature. The 2q scanning range was varied from 2 to 40 , with 0.02 steps, operated at 40 KV and 30 mA. 2.4. NMR measurements The 1H NMR spin-lattice relaxation time was measured as a function of the Larmor frequency nL ¼ uL/(2p) in the frequency range 10 kHz-300 MHz, using three NMR spectrometers. All measurements were made at a temperature of 25 C. The low-frequency measurements between 10 kHz and 9 MHz were performed using a home-developed fast-fieldcycling NMR relaxometer [18]. This experimental technique allows for the measurement of T1 at frequencies usually inaccessible by standard NMR spectrometers. This is possible because the magnetic field in the equipment used is switched up and down between two values: a fixed high value corresponding to the signal acquisition radiofrequency of 8.9 MHz and a variable low value at which the relaxation time is to be measured [18–20]. In FFC experiments, 20 scans and 30 points were recorded and repeated twice for each frequency. A variable field electromagnet and a 7T superconducting magnet were used in conjunction with a Bruker Avance II NMR console for measurements at Larmor frequencies between 15100 MHz and at 300 MHz, respectively. The T1 measurements were done using the inversion-recovery pulse sequence with phase cycling to compensate for any dc-bias components in the free induction decay (FID) signal. 3. Results and discussion Fig. 1 shows the data obtained from the X-ray diffraction analyses of the starch-PLA blends, with and without addition of PVA and Viscogel B8 organoclay. First, the diffraction filler peaks of interest, to characterize the nanostructure materials molecular organization and nanoparticle dispersion, are those that appear in the range of 2 to 5 in the 2q scale. They give an indication of the clay dispersion mode in the polymeric matrix. These peaks are related to the basal spacing of the clay’s d001 plane. Viscogel B8 is characterized by a single diffraction peak at 2q ¼ 3.52 , due to the organophilic group’s incorporation between the clay lamellae.
L.M. Brito et al. / Polymer Testing 32 (2013) 1181–1185
1183
Starch-PLA-PVA-B8 1%
Relative intensity
Starch-PLA-PVA Starch-PLA-B8 1%
Starch-PLA
Starch film 0
10
20
30
40
2θ (°) Fig. 1. X-ray diffraction patterns of Starch, Starch-PLA and the mixtures obtained with PVA and organoclay Viscogel B8 at 1%.
In relation to the dispersion, the 1% of Viscogel B8 caused the disappearance of the clay peaks or they are shifted to lower angles. According to these data, this clay is shown to have a good interaction with the polymer blend matrix. In addition, changes occurred in the crystallinity of the mixture when Viscogel B8 clay and PVA were added. Both the addition of PVA and B8 at a quantity of 1% were able to make the polymer blend (starch-PLA) less crystalline, confirmed by the disappearance or decrease of the peak at about 2q ¼ 20 relative to PLA (Brito et al., 2012). However, with the addition of both PVA and clay to the polymer blend, there was reversal of the effects caused by the addition of each agent separately. These observations are corroborated by the results obtained in the NMR-FFC analysis. Figs. 2–6 show the dispersion profiles for starch, PLA, starch PLA-blend and their nanostructured materials. The data obtained suggests the presence of two types of 1H population in the samples studied, corresponding to formation of distinct spin-lattice relaxation domains, except for the starch sample. For this sample, only one type of proton population and one T1 domain was observed. Fig. 2 shows the proton spin-lattice relaxation time, T1H, of the starch, PLA and starch-PLA blend measured as a function of frequency. It can be observed that the T1H of the starch-PLA blend is larger than that of pure starch, therefore a restriction of starch molecular motion caused by the PLA action at lower frequencies can be assumed, due to the predominant PLA molecular dynamics. The molecular motions of starch could only be observed at higher frequencies (1 MHz-20 MHz) as the T1H of the blend is smaller than that of pure PLA. Due to the method used to obtain this blend, as described in Section 2.2, there was encapsulation of the starch by the PLA matrix. This encapsulation occurred because of the different evaporation times of the solvents. In this case, PLA was dissolved in chloroform, and water was used to gelatinize the starch. In the mixture, chloroform was evaporated first, encapsulating the starch matrix, and the water subsequently evaporated, forming a material with a porous structure.
Fig. 2. Proton spin-lattice relaxation time T1 of polymers starch, PLA and starch-PLA blend measured as a function of the frequency.
Figs. 3 and 4 show the relaxation time, T1H, versus frequency for the composites of starch with homoionic clay (NT25) and organoclay (Viscogel B8). The unique structure of the nanocomposites is determined by the occurrence of two polymer phases in the system, the bulk phase and the confined phase of polymer intercalated into clay galleries. Molecular dynamics of polymer melts in the bulk phase is generally described using the Rouse model or the renormalized Rouse model, whereas the reptation model is applied to describe the molecular dynamics of confined polymers [20–22]. Renormalized Rouse dynamics for bulk polymer with molecular weight above the critical mass Mw > Mc is expressed by different power laws, corresponding to different motional regimes, such as high and low-mode and inter-segment dipolar interaction limits. This model has been successfully applied in experimental studies of semicrystalline or amorphous polymers [23].
Fig. 3. Proton spin-lattice relaxation time T1H of pure starch and starch composites with organoclay (Viscogel B8) measured as a function of frequency. The dotted and dashed lines are the best fits of the renormalized Rouse model. The fitting parameters of the power law dependence of T1 as a function of the Larmor frequency are explained in the text as shown above.
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Fig. 4. Proton spin-lattice relaxation time T1H of pure starch and starch composites with homoionic clay (NT25) measured as a function of frequency. The dotted and dashed lines are the best fits of the renormalized Rouse model. The fitting parameters of the power law dependence of T1as a function of the Larmor frequency are explained in the text as shown above.
Fig. 6. Proton spin-lattice relaxation time T1H of starch-PLA blend with organoclay (Viscogel B8) and PVA measured as a function of frequency. The dotted line is the best fit of the renormalized Rouse model. The fitting parameters of the power law dependence of T1as a function of the Larmor frequency are shown in the graphs.
The results in Figs. 3 and 4 indicate the presence of only one type of proton population. When applying the renormalized Rouse model for samples of starch and its composites, we obtained power laws similar to those found in the literature [20,23]. The composite with clay NT25 (Fig. 3) showed power laws with exponents w1 and w0.5. According to previously published studies [20], these exponents are related to freer movements of the macromolecules (isotropic phase). The formation of exfoliated materials can thus be suggested, due to higher affinity of the starch matrix (hydrophilic) with the homoionic clay. Composites of starch with the organophilic clay Viscogel B8 at 1% showed a 0.5 power law (red dashed line), indicating the formation of an exfoliated material, and a 0.4
power law (blue dotted line) for composites with Viscogel B8 at 5%. This exponent is related to slower movements and a more ordered sample (mesophase), indicating the formation of an intercalated material. Fig. 5 shows the relaxation times of PLA with homoionic clay (NT25 at 1% and 5%) and organically modified montmorillonite clay (Viscogel B8 at 1%). The B8 results obtained with 5% are not shown due to large variability. The results show the presence of two types of 1H population. The first type of proton population is represented by empty symbols and is associated with the longer T1H, while the second proton population, with shorter values of T1H, is represented by filled symbols. In this case, the renormalized Rouse model does not apply, probably because the material is semi-crystalline and the assignment of the two relaxation times to the crystallinity of different regions of the polymer is not clear. There was an increase in the values of T1H with the addition of clay NT25 at both concentrations used, so these values of T1H could be measured at frequencies below 15 MHz. In contrast, with organoclay B8, these values remained similar to those of the pure polymer. The effect of Viscogel B8 on the T1H values of the nanocomposites is derived from the dispersion mode of the clay in the polymeric matrix, due to its better affinity with the polymer matrix, since this clay has an organic interlayer, facilitating the interaction between the polymer chain and filler lamellae. These findings are in agreement with those already published by Brito et al. (2012), where both clays used to obtain the polymeric nanocomposites showed good dispersion in the polymeric matrix. An increase of the T1H values for clay NT25 might be correlated with possible formation of an intercalated material. Fig. 6 shows the relaxation times for the mixtures of starch and PLA with the addition of polyvinyl alcohol (PVA) (surfactant agent, 0.5% w/w) and adding the organoclay Viscogel B8 at 1%. The results of the 5% concentration are not presented due to large variation of the data.
Fig. 5. Proton spin-lattice relaxation time T1 of pure PLA and PLA composites with organoclay (Viscogel B8) and homoionic clay (NT25) measured as a function of frequency. The dotted lines are the best fits of the renormalized Rouse model. The fitting parameters of the power law dependence of T1 as a function of the Larmor frequency are shown in the graphs.
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Two types of proton populations can be identified according to the T1H values associated with the individual phases of starch-PLA blends. They depend on the type of clay and the presence of PVA. Shorter values of T1H showed differences in molecular dynamics with the addition of clay and PVA. However, when comparing the values of T1H obtained for the starch-PVA-PLA-B8 1% blend with the others, it can be seen that values of T1H (filled triangles) are related to the values of the relaxation time of starch with Viscogel B8 at 1% (Fig. 6). Probably, the addition of PVA was able to separate the mixture. The lower values of T1H in the starch-PVA-PLA blend indicate a similar trend to the starch-PLA-B8 1% blend. We believe the effect of compatibility of the PVA and Viscogel B8 at 1% is similar. However, in the starch-PVA-PLA-B8 1% blend, the lower values of T1H are similar to the T1H values of the pure starch. In this case, the effects of the PVA and Viscogel B8 in the mixture appear to cancel each other. 4. Conclusions According to the results obtained, the measurement of the relaxation time can elucidate the molecular mobility changes in the starch-PLA matrix caused by the addition of nanoparticles and PVA. FFC was successfully used to determine the molecular motions, in addition to providing important information about the miscibility of the starchPLA blends. The obtained results concerning the molecular mobility might also be important in further studies of other physical properties of these systems. Acknowledgments We are grateful to CNPq and CAPES for the financial support of this work.
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