Enhancing the Gas Barrier Properties of Polylactic Acid by Means of Electrospun Ultrathin
Zein Fibers
Maria A. Busolo1,2
enhanced performance13,14
, the latest incorporation of
nanosized fibers and layered silicates, as reinforcing
materials t...
where Q is the oxygen transmission rate, L is the sample
thickness, and ∆p is partial pressure gradient across the
incorporation of the ultrathin fibers. However, the oxygen
permeability was seen to be reduced by 71% compared to
the pure...
Table 1. Thermal properties of PLA/PEG, PLA/PEG-
Sample Tc (ºC) St. dev. Tm (ºC) St. dev. ∆∆∆∆Hm (J/g)
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Enhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers


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Enhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers

  1. 1. Enhancing the Gas Barrier Properties of Polylactic Acid by Means of Electrospun Ultrathin Zein Fibers Maria A. Busolo1,2 , Sergio Torres-Giner1 and Jose M. Lagaron1 1 Novel Materials and Nanotechnology Lab., IATA-CSIC (Valencia, Spain), email: lagaron@iata.csic.es 2 Nanobiomatters, S.L. (Valencia, Spain) Abstract Ultrathin fibers of 100-500 nm in diameter were obtained by electrospinning from an alcoholic solution of zein, a prolamine from maize. The ultrathin zein fibers generated were incorporated as a reinforcement to a poly(lactic acid)/polyethylene glycol (PLA/PEG) matrix by a proprietary method to generate a novel multilayer structure consisting of a zein fiber layer laminated in a sandwich to two PLA/PEG layers. As the zein nanofibers retained the ultrathin structure inside the resulting PLA/PEG matrix, the overall composite remained transparent and colorless. The incorporation of the zein did not significantly affect the thermal behaviour or the water permeability of the original PLA/PEG matrix, but surprisingly it did reduce the oxygen permeability of the matrix by ca. 70%. This technology could thus be potentially applied to obtain highly transparent PLA films with enhanced barrier properties of interest in for instance packaging applications. Introduction The electrospinning technique was developed and patented at the beginning of the 20th century, but in recent years it has been revisited by the growing interest in nanotechnology. It is a simple, versatile and efficient method to produce high-performance polymeric fibers with diameters ranging from the micro to the nanoscale1 . The electrospinning technique requires a voltage power supply, a syringe containing the polymer solution with a needle, and a grounded collector screen. The spinning process is achieved by application of a high voltage electric field that induces charges on the solution polymer surface. The hemispherical droplet at the tip of the needle stretches and a conical jet projection of the newly created fibers emerges. As the solvent evaporates before the jet reaches the collector, solid fibers are collected in the form of non-woven mats. Some parameters rule the process, and consequently, the final properties of the fibers. Electrospinning parameters (needle-to-collector distance, flow rate, voltage); polymer solution characteristics (concentration, solvent nature, viscosity) as well as room conditions (temperature, relative humidity) define the morphology of the products, fibers or beads, and their dimensions1,2 . Because of their very large area to volume ratio, superior mechanical performance, and the versatility in the design, electrospun ultrathin fibers are being considered in different areas such as filtration, nanotube reinforcements, sensors, cosmetics, catalysis, protective clothing, biomedicine, space applications, semiconductors and micro- or nano-electronics3-6 . Zein is a natural, non-water soluble prolamine obtained from maize that has been used to improve barrier properties of bags and packagings due to its low permeability to oxygen and carbon dioxide under dry and medium relative humidity conditions. Electrospinning procedures to obtain zein nanofibers from acidic and alcoholic solutions have recently been reported2,7,8 . Zein conventional fibers have been incorporated to fabrics for work clothes reducing their permeability to liquids and gases, and increasing their mechanical resistance, wear and washing durability9 . Polylactic acid (PLA) is a linear biodegradable and biocompatible polyester produced from renewable resources with interesting physical properties, which make it an attractive alternative for polymers derived from fossil origin. In recent years, PLA has been used in many differents applications: sutures, implants and biocompounds for drug delivery systems (biomedicine); bottles and packagings for food and beverages, disposable fabrics, personal care products, etc. However, some properties such as thougness, HDT and gas barrier are not suficiently enough for its use in various applications10 . The water permeability of PLA is much lower than that of proteins and polysacharides but it is still higher than the permeability of conventional polyolefins and polyethylene terephthalate (PET)11 . By adding nanoaditives to PLA, nanocomposites with enhanced mechanical properties, thermal resistence, barrier properties, and dimensional stability can be obtained, even at low filler concentrations12 . Although many studies have reported interesting blends of PLA with other polymers of ANTEC 2009 / 2763
  2. 2. enhanced performance13,14 , the latest incorporation of nanosized fibers and layered silicates, as reinforcing materials to PLA, can potentially result in advanced materials of wider applications15-17 . The addition of polyethylene glycol (PEG), as a plastizacing agent, to PLA has also been reported to lead to increased flexibility and ductibility18 . This paper presents the optical, barrier and thermal properties of a novel PLA multilayer composite material prepared by incorporation of electrospun zein fiber mats between two PLA/PEG layers with potential interest in packaging applications. Materials A poly(lactic) acid (PLA) extrusion grade with an average molecular weigh of 150.000 g/mol was supplied by Natureworks (USA). Zein powder from maize was purchased from Sigma Aldrich Chemical and was used without further purification. Polyethyleneglycol, 850-950 g/mol (Fluka Chemika) was used as both zein compatibilizer and plastizicer for the PLA matrix. Ethanol and trichloromethane 99%, were supplied by Panreac Synthesis. Procedures PLA-PEG films preparation The PLA/PEG films were prepared by solution casting. A solution containing 4.75 wt.-% of PLA and 0.25 wt.-% of PEG in chloroform was prepared by stirring on a hot plate at 40ºC until complete pellet dissolution. The resulting solution was then cast onto Petri dishes (15 cm diameter), and 100 microns thick films were obtained after solvent evaporation at ambient conditions. The obtained PLA/PEG film was vacuum dry overnight at 70ºC, to remove residual solvent. Electrospinning process Electrospinning was performed using a plastic syringe containing the protein solutions which were disposed in horizontal lying on a digitally controlled syringe pump while the needle was directly in vertical towards the collector. Further details of the basic electrospinning setup can be found elsewhere2 . Zein ultrathin fibers were obtained from electrospinning of 33wt.-% of zein in 80% ethanol/water solutions prepared under magnetic stirring at room temperature. The governing parameters were fixed at 14 kV of power voltage, 15 cm of tip-to-collector distance and 0.33 ml/h of volumetric flow-rate. Environmental conditions were maintained stable at 24ºC and 60 %RH by having the equipment enclosed in a specific chamber with temperature and humidity control. Multilayer Assemble By using a proprietary method (patent pending) of application, the protein fiber mats were laminated to two PLA/PEG layers to obtain a multilayer system where an ultrathin layer of zein fibers was trapped between two symmetrical PLA/PEG layers. Optical microscopy The morphology of the films was investigated by optical microscopy. Observations were performed in 1x1cm film samples, using a Nikkon Eclipse 90i microscope with imaging software NIS Element BR 2.30. Scanning electronic microscopy (SEM) SEM imaging of the cryofractured composite was measured using a Hitachi S-4100 equipment operating at 8 kV. Thermal properties The thermal behaviour of PLA/PEG-zein composites was analyzed by differential scanning calorimetry (DSC) using a Perkin Elmer DSC 7. Samples of approximately 5 mg were heated in aluminium pans from 40ºC to 170ºC at a rate of 10ºC/min. Similar thermal runs were performed in empty pans in order to get a base line. Water vapour permeability (WVP) The water vapour permeability of the films was determined according to the ASTM method E96, using aluminium cells with internal and external diameters of 3.5 and 6 cm, respectively. Each sample film was sealed to a permeation cell containing liquid water, and then the permeation cells were left under controlled environmental conditions (24ºC, 40% relative humidity) and weighed regularly until steady-state was reached. The water vapour transmission rate was easily determined from the slope of the cell weight loss vs. time plot following equation 1. WVP = ( G * L ) / ( A * ∆p ) (1) where G is the slope of weigh loss vs. time straight line, L is the film thickness, and ∆p is the vapour partial pressure differential across the film. All tests were conducted in duplicate, and aluminium films were used as blanks to evaluate water vapour loss through the sealings. Oxygen permeability The oxygen transmission rate was measured using an Oxtran 100 equipment (Mocon, Minneapolis, USA), at 24 ºC and 80% relative humidity. The samples were previously purged with nitrogen during 20h before being exposed to the oxygen flow. The oxygen permeability was calculated from the oxygen transmission rate at equilibrium conditions by: PO2 = (Q * L ) / ∆p (2) 2764 / ANTEC 2009
  3. 3. where Q is the oxygen transmission rate, L is the sample thickness, and ∆p is partial pressure gradient across the sample. Results and discussion Morphology of the zein fibers SEM micrographs show that zein electrospun fibers form a non woven ultrathin structure while they are generated (Figure 1). The attained fiber diameters range between 100 to 500 nm. However, the fiber length can reach up to several centimeters. Composite characterization Figure 2 shows an optical micrograph of one of the multilayer systems prepared. From this a regular disposition of the ultrathin zein fibers is observed from within the PLA/PEG matrix. By cryofracture of the assemble, the fibers layer can be hardly distinguished within the multilayer system, indicating that a strong adhesion between the zein fiber mats and the PLA/PEG layers takes place and that the zein layer is very small, i.e. few microns, in comparison to the overall thickness. Simple optical pictures indicate that the final composite system is completely transparent and colorless similarly as the neat PLA/PEG film (Figure 3). It should be borne in mind that transparency is a key property in many applications but especially in food packaging. The results indicate that the multilayer system can be made in a way in which no apparent losses in optical properties are generated. Thermal properties of PLA/PEG-zein composites Figure 4 shows the DSC thermograms of the PLA/PEG film and of the PLA/PEG-zein composite material developed. From the DSC experiments the heat of fusion (∆Hm), melting point (Tm) and cold crystallization temperature (Tc) of the studied materials was determined and is summarized in Table 1. The results indicate that the protein fibers do not influence the thermal behavior of the PLA/PEG matrix since they are not randomly mixed with the polymer but forming a separate intermediate layer. However, it is remarkable to observe that the Tm of the PLA/PEG blend decreases compared to the neat PLA due to molecular interactions with the plasticizer13 . Barrier properties The water vapour permeability results of the films are gathered in Table 2. From this, it can be seen that the zein fibers do not significantly alter the water barrier of the PLA/PEG blend, even when zein films are about 10 fold more permeable to water than the PLA/PEG blend19,20 . Thus, while zein is a relatively hydrophobic protein it does still become strongly plasticized by water21 . The reason why water permeability does not go up in the system could be related to the fact that PLA does not uptake water to a significant extent. The hydrophobicity of the matrix may thus be constraining the fibers in the multilayer system as previously observed in other systems, such as cellulose fibers in PE, leading to very little plasticization or swelling of the zein layer. Another important contributing reason is the very small layer of the zein fibers compared to the overall thickness of the PLA/PEG film. The PLA is the higher barrier in the multilayer structure and is also the dominant phase determining the final barrier. Table 2 also indicates that the PLA films are actually 41% less permeable to water than the actual PLA/PEG blend. So, the addition of PEG to the PLA polymer does bring a negative effect on the water permeability of the blend due to the higher free volume attained and the corresponding changes in diffusion and also in solubility of water in this blend. In this context, it is also worth mentioning that the water vapour of the composite is not better than its oil-based counterpart PET, which has a WVP of 3x10-15 (Kg m / s m2 Pa)20 . As a result the resulting composite does not outperform the polyester in terms of water barrier. However, the oxygen barrier results are very significant (see Table 3). In this table zein fibers were able to reduce the oxygen permeability of the PLA/PEG matrix drastically (71%). This entails that the PLA/PEG-zein composite is 24% more barrier than the neat PLA, is equivalent to polypropylene (PP)22 and is 10 fold more permeable than PET19 . The reason for the oxygen barrier improvement is related to the fact that dry zein is a very high barrier polymer to oxygen and therefore imposes a stronger blocking effect in a multilayer system16 . Application of a simple series modeling is supporting the fact that if the permeability difference between the layers is very high, a thinner layer with higher barrier will impact more strongly the overall barrier of the multilayer system as observed with the oxygen barrier. Conclusions Electrospun zein nanofibers of 100-500 nm in diameter were obtained from alcoholic zein solutions. A new methodology alternative to melt mixing or solvent casting allowed the incorporation of electrospun ultrathin fiber in a PLA/PEG blend with no alteration of their structure and original morphology. The resulting composite film was transparent and colorless. Thermal analysis showed that melting and crystallization behavior of the PLA/PEG matrix did not change due to the incorporation of the ultrathin fibers. In the same way, water vapor permeability of PLA/PEG matrix did not significantly increase due to the ANTEC 2009 / 2765
  4. 4. incorporation of the ultrathin fibers. However, the oxygen permeability was seen to be reduced by 71% compared to the pure matrix Acknowledgements The authors would like to acknowledge the Spanish MEC project MAT2006-10261-C03 for financial support; and Dr. Eugenia Nuñez for helpful comments. References 1. Sawicka, K.; Gouma, P. J. Nanoparticle Research, 8, 769-781(2006). 2. Torres-Giner, S.; Gimenez, E.; Lagaron, J.M. Food Hydrocolloids, 22, 4, 601-614 (2008). 3. Huang, Z.; Zhang, Y.Z. Composites Science and Technology, 63, 2223-2253 (2003). 4. Lelkes, P.I.; Li, M.; Mondrinos, M.J.; Gandhi, M.R.; Ko, F.K.; Weiss, A.S. Biomaterials, 26, 5999-6008 (2005). 5. Ramakrishna, S.; Huang, Z.; Zuang, Y.; Kotaki, M.; Composites Science and Technology, 63, 2223-2253 (2003) 6. Gopal, R.; Kaur, S.; Ma, Z.; Chan, C.;Ramakrishna, S.; Matsuura, T. J. Membrane Science, 281, 581-586 (2006). 7. Okuyama, K.; Yun, K.M.; Hogan, C.J.Jr.; Matsubayashi, Y.; Kawabe, M.; Iskandar, F. Chemical Eng. Science, 62, 4751-4759 (2007). 8. Selling, G.W.; Biswas, A.; Patel, A.; Walls, D.J.; Dunlap, C.; Wei, Y. Macromolecular Chemistry and Physics, 208, 1002-1010 (2002). 9. Graham, Gogins y Schrender-Gibson, 2004; US patent 5750064 10. Singh, R.P.; Pandey, J.K.; Reddy, K.R.; Kumar, A.P. Polymer Degradation and Stability, 88, 234-250 (2005). 11. Lagaron, J.; Gimenez, E.; Sanchez-Garcia, M.D. “In environmentally compatible food packaging”. Edited by Emo Chiellini. CRC Press, 2008 12. Pluta, M.; Jeszka, J.K.; Boiteux, G. European Polymer Journa,l 43 2819-2835(2007) 13. Pereira, B.M.; Carvalho, C.A.; Rezende, E.A. Brazilian Journal of Biomedical Engineering. 19, no. 1. April 2003. Pp. 21-27. 14. Yu, L.; Dean, K.; Li, L. Prog. Pol. Science, 31, 576- 602 (2006). 15. Sinha Ray, S.; Okamoto, M. Macromolecular Rapid Communications. 24, 815-840 (2003). 16. Oksman, K.; Petersson, L. Composites Science and Technology, 66, 2187-2196 (2006). 17. Sanchez-Garcia, M.D.; Lagaron, J.M.; Gimenez, E.J.Carbohydrate Polymers, 71, 235-24 (2008). 18. Pillin, I.; Montrelay, N.; Grohens, Y. Polymer 47, 4676–4682 (2006) 19. Sanchez-Garcia, M.D.; Gimenez, E.; J.M. Lagaron. J. Plastic Film and Sheeting, 23,133(2007). 20. Parris, N.; Dickey, L.C.; Kurantz, M.J.; Moten, R.O.; Craig, J.C. J. Food Eng. 32, 766 (1997). 21. Cava, D.; Gimenez, E.; Gavara, R.; Lagaron, J.M. J. Plastic film & sheeting, 22, 265-274 (2006). 2766 / ANTEC 2009
  5. 5. Tables Table 1. Thermal properties of PLA/PEG, PLA/PEG- zein Sample Tc (ºC) St. dev. Tm (ºC) St. dev. ∆∆∆∆Hm (J/g) PLA/PEG 78.3 0.6 134.4 0.4 9.0 PLA/PEG- zein 78.9 1.4 134.4 1.1 9.9 Table 2. Water vapor permeability of PLA/PEG, PAL/PEG-zein, PLA and zein Sample WVP (Kg m/s m2 Pa) PLA/PEG (3.96 ± 1.13) x 10-14 PLA/PEG-zein (3.88 ± 0.17) x 10-14 PLA19 2.30 x 10-14 Zein20 3.36 x10-13 Table 3. Oxygen permeability of PLA/PEG, PAL/PEG-zein (24ºC, 80% RH) and other polymers Sample PO2 (m3 m/m2 sPa) PLA/PEG (5.85 ± 0.25) x 10-18 PLA/PEG + zein nanofibers (1.68 ± 1.50) x 10-18 PET19 4.26 x 10-19 PP22 8.6 x 10-18 PLA19 2.21 x 10-18 Figures Figure 1. Zein fiber mats by SEM (scale marker is 5 µm) (a) (b) Figure 2. PLA/PEG-zein composite micrographs: (a) top view by optical microscopy (scale marker is 100 µm); (b) cryofractured side view by SEM (scale marker is 10 µm). (a) (b) Figure 3. Images of nanocomposite films: (a)PLA/PEG; (b) PLA/PEG-zein ultrathin fibers Figure 4. DSC heating thermograms of the PLA/PEG blend (top) and of the PLA/PEG-zein composite (bottom) Keywords: Electrospinning, zein, poly(lactic acid), gas barrier properties. ANTEC 2009 / 2767