International Journal of Biological Macromolecules 67 (2014) 147–153
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Preparation and characterization of potato starch nanocrystal reinforced natural rubber nanocomposites K.R. Rajisha a , H.J. Maria b , L.A. Pothan c , Zakiah Ahmad d , S. Thomas b,e,∗ a
Department of Chemistry, CMS College, Kottayam, Kerala, India School of Chemical Sciences, Mahatma Gandhi University, Priyadarsini Hills, Kottayam, Kerala, India Department of Chemistry, Bishop Moore College, Mavelikara, Kerala 690101, India d UiTM, Mara Malaysia, 40450 Shah Alam, Selangor, Malaysia e Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarsini Hills, Kottayam, Kerala, India b c
a r t i c l e
i n f o
Article history: Received 16 December 2013 Received in revised form 5 March 2014 Accepted 7 March 2014 Available online 19 March 2014 Keywords: Bionanocomposite Starch Nanocrystal
a b s t r a c t Potato starch nanocrystals were found to serve as an effective reinforcing agent for natural rubber (NR). Starch nanocrystals were obtained by the sulfuric acid hydrolysis of potato starch granules. After mixing the latex and the starch nanocrystals, the resulting aqueous suspension was cast into film by solvent evaporation method. The composite samples were successfully prepared by varying filler loadings, using a colloidal suspension of starch nanocrystals and NR latex. The morphology of the nanocomposite prepared was analyzed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM analysis revealed the size and shape of the crystal and their homogeneous dispersion in the composites. The crystallinity of the nanocomposites was studied using XRD analysis which indicated an overall increase in crystallinity with filler content. The mechanical properties of the nanocomposites such as stress–strain behavior, tensile strength, tensile modulus and elongation at break were measured according to ASTM standards. The tensile strength and modulus of the composites were found to improve tremendously with increasing nanocrystal content. This dramatic increase observed can be attributed to the formation of starch nanocrystal network. This network immobilizes the polymer chains leading to an increase in the modulus and other mechanical properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite materials find considerable interest recently among researchers and industrialists. Synthetic nanomaterials lack easy processability, biocompatibility, and biodegradability and hence have limitations compared to natural nanomaterials. Biodegradable nanocomposites which have superior thermal, barrier and mechanical properties can be synthesized from biopolymer and nanosized reinforcements. They are formed by the combination of natural polymers and inorganic solids which possess at least one dimension in the nanometer scale. Similar to conventional nanocomposites, which involve synthetic polymers, these biohybrid materials exhibit improved structural and functional properties of great interest for different applications. The
∗ Corresponding author. Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarsini Hills, Kottayam–686560, Kerala. Tel.: +9447223452/+481 2730003. E-mail addresses: [email protected], [email protected], [email protected] (S. Thomas). http://dx.doi.org/10.1016/j.ijbiomac.2014.03.013 0141-8130/© 2014 Elsevier B.V. All rights reserved.
properties inherent to the biopolymers, biocompatibility and biodegradability, open new prospects for these hybrid materials with special incidence in regenerative medicine and environment friendly materials (green nanocomposites). Research on bionanocomposites can be regarded as a new interdisciplinary field closely related to significant topics such as biomineralization processes, bioinspired materials, and biomimetic systems. The upcoming development of novel bionanocomposites introducing multifunctionality represents a promising research topic that takes advantage of the synergistic effect of biopolymers with inorganic nanometer-sized solids. Raw materials often used for these new nanostructured composites are natural polymers; clay and nanowhiskers of chitin and cellulose. A possible source of inspiration for the design of new high performance bionanomaterials is due to the fact that they offer excellent reinforcement [1]. Starch is an abundant biopolymer, which is totally biodegradable. Starch nanocrystals obtained from starch have been used as fillers in polymeric matrices leading to desired reinforcing effect [2–4]. During the last decade, nano-material derived from natural polymers have been used as reinforcement in polymers and have been named as “green” bionanocomposites [5]. Many research
148
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
works published in the area of starch based bionanocomposites show the relevance on this topic in the current scenario of polymer research [6–9]. Chitin, cellulose and starch are the crystalline residue that can be obtained from different natural polysaccharides with a uniform structure after acidic or alkaline hydrolysis. These nanoparticles from different sources have different geometrical characteristics. For example, the nanocrystals obtained from cellulose [10] and chitin [11] have a rod shaped structure while that of starch have a platelet shaped structure [12]. However, nanocrystals from polysaccharides have the limitation of being hydrophilic and hence are incompatible with hydrophobic polymeric matrices. Therefore, the nanocrystal–matrix interface is usually the weakest point in a biocomposite, which results in the poor performance of the final composite. In order to improve the compatibility between the reinforcing phase and the polymer matrix, the nanoparticles can be physically or chemically modified [12]. An effective method for chemical modification is to graft polymer chains from the matrix directly onto the surface of reinforcing nanoparticles before composite preparation [13]. The formation of a continuous interphase between polysaccharide phase and the polymer matrix phase can improve interfacial adhesion. In addition, entanglements between grafted and ungrafted polymer chains are expected to occur if the molar weight is high enough. By this approach, nanocomposite materials with high content of reinforcing nanoparticles can be prepared. The main polysaccharides of interest as materials for nanocomposite preparation are starch and cellulose, but an increasing attention is also given to the application of the other biopolymers such as chitin and chitosan [14]. Starch is a well-known polymer naturally produced by plants in the form of granules which can be obtained mainly from maize, potatoes, corn, and rice. The properties of these starch granules vary from plant to plant, but are generally composed of, amylose (in most cases about 20% of the granule), and amylopectin. Amylose that builds up to 15–35% of the granules in most plants is a primarily linear polysaccharide with ␣-(1–4)-linked d-glucose units. Amylopectin is a highly branched molecule, with ␣ (1–4) linked d-glucose backbones and exhibits about 5% of ␣ (1–6)-linked branches, which have a profound effect on the physical and biological properties. Amylose is semi crystalline and soluble in hot water while amylopectin is insoluble in hot water. During its biodegradation, starch undergo enzyme-catalysed acetal hydrolysis where the ␣-1,4 link in amylopectin is attacked by glucosidases.[15]. The structural differences between these two polymers contribute to significant differences in starch properties and functionality The focus of this work is to process nanocomposite materials consisting of natural rubber filled with potato starch nanocrystals which are completely biodegradable and thereby replacing the conventionally used carbon black (manufactured by burning oil or natural gas in controlled conditions). In the past decades, research works were focused on the development of other reinforcing agents to replace carbon black in rubber compounds. Recently, Novamont (Novara, Italy), working in partnership with Goodyear Tire and Rubber, has developed tires using nanoparticles derived from corn starch, partially replacing the conventional carbon black and silica used in making tires. This patented innovation, called Biotred, not only presents environmental advantages but also reduces the rolling resistance of tires [16]. Many attempts have been reported to blend polysaccharide nanocrystals with polymeric matrices [17–20]. The resulting nanocomposite materials display outstanding mechanical properties and thermal stability. For example, starch nanocrystals consist of crystalline nanoplatelets about 6–8 nm thick with a length of 20–40 nm and a width of 15–30 nm [20]. They have been used as a new kind of fillers, showing interesting reinforcing and barrier properties in natural rubber [21–23].
In this approach, potato starch nanocrystals constitute another possible filler for natural rubber which can have an admirable contribution in developing new environmental friendly strong composites [5]. Starch nanocrystals, the nanoscale biofiller derived from native starch granules, have been compounded with many different kinds of polymer matrices. The intrinsic rigidity of starch nanocrystals, special platelet-like morphology, strong interfacial interactions, and the percolation network organized by nanocrystals, contribute to the mechanical performance, thermal properties, solvent absorption, and barrier properties of the composites [24]. In the present work, a new nanocomposite based on natural rubber filled with potato starch nanocrystals was prepared. The nanocrystals were characterized by TEM and XRD. By varying the weight percentage of starch nanocrystals in the NR matrix the surface morphology, swelling behavior, and crystallinity of the various composites were investigated. The mechanical properties of the composites were also analyzed. 2. Experimental 2.1. Materials Natural rubber latex was kindly supplied by Rubber Research Institute of India (RRI, Kottayam, India). It contained spherical particles of natural rubber with an average diameter around 1 m, and the dry rubber content determined was about 60 wt %. The density of dry NR, was 1.4 g cm−3 , and it contained ∼98% of cis-1,4-polyisoprene. Potato starch powder was purchased from Luba chemicals, Mumbai. Other chemicals like 36 N H2 SO4 , BaCl2 , sodium azoture (protectant against microorganisms) etc. needed for the preparation of starch nanocrystals were obtained from local sources. 2.2. Preparation of potato starch nanocrystals Potato starch nanocrystals were prepared by sulfuric acid hydrolysis of native potato starch powder. About 36 g of potato starch granules were mixed with 250 ml of 36 N H2 SO4 for 5 days at 40 ◦ C, with a stirring speed of 100 rpm. The aqueous suspension was washed by successive centrifugation with distilled water until neutrality (confirmed by litmus paper testing). At this stage, starch suspension can be believed to be broken into nanocrystals, which changes the refractive index of the solution which is evident from the opaque nature of the suspension. The dispersion was completed by a further 10 min ultrasonic treatment in a B12 Branson sonifier. The resultant aqueous suspension constituted of starch fragments with a homogenous distribution in size. The solid fraction of this aqueous suspension had a weight concentration of about 3.4 wt %. 2.3. Preparation of NR latex/starch nanocrystal nanocomposite films The aqueous suspension of potato starch nanocrystal and the NR latex were mixed in various proportions at ambient temperature using a mechanical stirrer (IKA-RW 28). The mixing was carried out for 15 min to ensure uniform dispersion. Then the mixture was kept for 1 h in order to ensure homogenization and also for the sedimentation of impurities. The mixture was stored under vacuum and stirred on a rotavapor for 10 min in order to degass the mixture and to avoid the formation of irreversible bubbles during water evaporation. The films with uniform thickness were obtained by casting on a glass mold and was evaporated at 40 ◦ C in a ventilated oven for 4–6 h and then heated at 60 ◦ C under vacuum for 2 h. The resulting films were conditioned at room temperature
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
149
Fig. 1. Flow chart showing the preparation of potato starch nanocomposites.
in desiccators containing P2 O5 Fig. 1 shows the flow chart of the preparation method adopted. The weight fractions of the sample and their compositions are listed in Table 1. 3. Characterization techniques 3.1. Transmission electron microscopy (TEM) The morphology of the nanocrystals was investigated by means of TEM (JEM-2100HRTEM). The micrographs were obtained in point to point resolution 0.194 nm, operating at an accelerating voltage of 200 kV. The suspension was placed on a 300 mesh Cu grid (35 mm diameter). 3.2. Field emission scanning electron microscopy (FESEM) Morphology of the composites was studied using field emission scanning electron microscopy. A FESEM, CARC-ZEISS, SUPRA40 UP was used to observe the morphology of starch nanocrystal and different starch nanocomposite. The films were sputter coated with platinum to ensure conduction observed with an accelerating voltage of 5 kV. 3.3. X-ray diffraction (XRD)
The kinetics of water absorption was determined for all compositions. The specimens were films with dimensions around 10 × 10 × 0.2 mm3 . The films were thin enough to ensure unidirectional diffusion. After immersion in water, samples were removed at different times, wiped with filter paper to remove surface water and weighed with an analytical balance with 0.1 mg resolution. Water induced dimensional changes were measured with a micrometer having an accuracy of 0.1 mm. The water uptake (WU) was calculated using Eq. (1) WU% =
mt − m0 × 100 m0
(1)
mo and mt are the weights of the sample before and after a time t of immersion, respectively Water diffusion coefficient (D) of the nanocomposites was calculated using Eq. (2) D=
h 2 4m∞
(2)
where, D = water diffusion coefficient; h = thickness of each sample; = slope of the linear portion of the curve; m∞ = weight of the sample at equilibrium 4. Results and discussion
X-ray diffraction patterns were taken by using Ni- filtered Cu-K␣ radiation ( = 0.154 nm) by Bruker D8 Advanced x-ray diffractometer. The latex nanocomposite samples were scanned in step mode by 1.5% min scan rate in the range of 2 < 12◦ . The specimens of 1 × 1 cm films were used for the analysis. 3.4. Mechanical measurement Tensile tests: The non linear mechanical properties of the samples were studied using Universal Testing Machine in accordance with ASTM D 412- 2002. Dumbbell shaped specimens 5 mm wide, 8 mm long and about 1 mm thick were used for tensile measurements. The tensile curves obtained were analyzed for strain at break and modulus. The tests were conducted at a crosshead speed of 500 mm/min at ambient temperature. The mechanical properties of nano filled latex composites were studied at ambient temperature. Table 1 Composition of potato starch nanocrystal/NR latex and nanocomposite films. Sample
Natural rubber latex (dry wt%)a
Starch content (dry wt%)
NR100 NR95 NR90 NR85 NR 80
100 95 90 85 80
0 5 10 15 20
a
3.5. Water uptake
Dry rubber content of 10 mg NR latex found to be 6 gm.
4.1. Characterization of starch nanocrystals The morphological analysis of starch nanocrystal was done using x-ray diffraction analysis and transmission electron microscopy. The XRD pattern displayed two weak peaks at 2 = 11.0◦ , double peak at 2 = 14.16◦ and 2 = 19.5◦ and a strong peak at 2 = 22.19о and 2 = 25.88◦ (Fig. 2). From the XRD data, it was observed that the diffraction patterns recorded for the lyophilized sample of pure potato starch nanocrystal displayed a typical A-type amylose with x-ray diffraction patterns at 2 with the first peak around 15◦ , the second peak near 18◦ , and the third main reflection around 23◦ . A comparative study of the XRD peaks of native starch with prepared starch nanocrystals were also conducted. From the diffractograms, both the native potato starch and hydrolysed potato starch show the characteristic peak at 18◦ and 23◦ . But these two peaks are more prominent in the diffractogram of potato starch nanocrystal and this indicates the very high extent of crystallinity in starch nanocrystals. The TEM analysis of potato starch nanocrystals lead to the image shown in Fig. 3a. The presence of nanocrystal particles is evidenced by the small dark spots (a few spots are encircled to highlight them). From the TEM image, the average diameter of the nanocrystals is found to be 3 nm which can be confirmed from the particle size analysis (Fig. 3b). The samples were found to be free of aggregation and impurities as per the TEM observation. Thus the method used
150
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
Fig. 2. X-ray diffraction patterns of starch nanocrystals and potato starch powder.
to synthesize nanocrystals does not create impurities. Nanocrystals of the same of nanometers range (9–20 nm) have been reported by Dufresne et al. [25]. 4.2. Morphology of the nanocomposite films The morphology of the NR/starch nanocrystal material was determined from field emission scanning electron microscopy and XRD analysis. The distribution level of the filler within the matrix was evaluated by observing the FESEM image of the surface fractured films. Fig. 4a and b show the nanocomposite films of NR nanocomposite with 5 and 10 wt % of nanocrystals respectively. In the FE-SEM micrograph a homogenous dispersion of starch nanocrystals which appears like white dots can be observed. A uniform distribution of the nanocrystal in the matrix is clearly seen for the compositions. Such an even and uniform distribution of the filler in the matrix is essential for obtaining optimum properties. As the filler loading is increased the nanocrystals show a tendency to agglomerate which can be observed in Fig. 4(b) image. The particle size of the nanocrystal was observed to be in the range 12 nm and it is in agreement with some earlier work in this area [25].
Fig. 4. FESEM micrograph of (a) NR95 (b) NR90.
4.3. X-ray diffraction: structural analysis; Influence of starch content The nanocomposite films of NR/starch nanocrystals were characterized by x-ray diffraction. X-ray patterns were collected for different compositions and are displayed in Fig. 5. The diffractograms of unfilled natural rubber and pure potato starch nanocrystal films have also been shown for comparison.
Fig. 3. (a) TEM micrograph of starch suspension. (b) Particle size distribution of (∼50 counts) starch nanocrystals from the TEM data.
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
Fig. 5. X-ray diffraction patterns of starch nanocrystals/NR nanocomposite films.
The diffraction pattern recorded for a film of pure potato starch nanocrystals obtained by pressing freeze-dried nanocrystals displays typical peaks of A-type amylose allomorph. It is characterized by two weak peaks at 2 = 10.1◦ and, a strong peak at 18.2◦ , and a strong peak at 23.5◦ . The natural rubber film (NR100) displays a typical behavior of a fully amorphous polymer. It is characterized by a broad hump located around 2 = 18◦ . By adding starch nanocrystals, the peaks corresponding to A-type amylose allomorph become stronger and stronger. This shows that an increase of the starch content results in an increase of the global crystallinity of the composite material. Furthermore, x-ray analysis confirmed that the processing by casting and evaporation at 40 ◦ C did not affect the crystallinity of the starch nanocrystals.
151
Fig. 6. Stress–strain curves of starch nanocrystal/NR nanocomposites.
4.4. Mechanical properties The starch nanocrystal obtained from sulfuric acid hydrolysis was very promising reinforcing material for polymer nanocomposites because of their high stiffness and strength studies on the reinforcing effect of starch nanocrystals with other fillers like clays, organoclays, carbon black, fly ash, and chitin whiskers in natural rubber matrix reports that starch nanocrystal is a good substitute for carbon black, as it can induce a reinforcing effect in terms of stiffness, strength and elastic behavior. These nanocomposites from renewable resources have the added advantages such as, low cost, easy availability, good biocompatibility, and ease of modification chemically and mechanically.
Fig. 7. (a) Tensile strength, (b), tensile modulus and (c) elongation at break, for starch nanocrystals/NR nanocomposite.
152
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
Table 2 Water diffusion coefficient (D) of NR/nanocrystal composites. Nanocomposites samples (vol%)
Thickness of sample (h) (mm)
Angle of linear ()
Weight of sample at infinity (Mω) (g)
Diffusion coefficient (D) (cm2 s−1 )
NR film NR 95 NR 90 NR 85 NR 80
1.35 1.65 1.42 1.12 1.81
0.38 0.40 0.56 0.61 0.63
1.23 1.31 1.38 1.48 1.50
0.08 0.95 1.01 3.24 3.97
The tensile behavior of the potato starch nanocrystal reinforced with NR latex nanocomposite films was analyzed at room temperature. Fig. 6 shows the stress–strain curves of starch–natural rubber composite with increasing starch content. It is found that stress increases regularly with strain till the sample breaks. As the filler loading is increased the tensile strength as well the modulus increases remarkably. The sharp increase in the tensile strength of the NR nanocomposites with addition of the filler shows the reinforcing effect of starch nanocrystal. The elongation at break of the nanocomposites was found to be decreased from a value of 1351 for the neat rubber to 536 when 20 wt% of the starch nanocrystals were added. Tensile modulus of the nanocomposites also was improved significantly with filler loading. In Fig. 7, the tensile modulus of the NR/potato starch nanocrystal composites is plotted as a function of starch content. From the graph it is clear that tensile modulus also increases in almost linear fashion. The reinforcing effect of the starch nanocrystals strongly depended on their ability to form a rigid three-dimensional network, resulting in strong interactions such as hydrogen bonds between the filler. Tensile test experiments give clear evidence for the presence of a three-dimensional network within the nanocomposites samples. In higher concentration, there is more number of possible polymer–filler interactions which enhances the reinforcement. The schematic representation given in Fig. 8 shows the possible presence of a three-dimensional network within the nanocomposites. Even though an agglomerated morphology was observed at 10 wt% nanocrystal loading the property of the nanocomposites were not affected adversely. Thus, a modification of dispersion method used can possibly give a better dispersion that can result in a further improvement of properties and will be a scope of future studies.
For all the compositions the water uptake increases tremendously at its first zone (below 20 h) and then reaches a maximum. After reaching the maximum point, there is a decrease for the next 5 h. On further soaking there is a slight decrease with respect to time and reaches at equilibrium. The diminution of water uptake may be due to the partial release or leaching of starch nano particles in water. The further increase in water uptake with time may be due to the migration of starch nanocrystals towards the aqueous phase during swelling. During the exposure of this starch nanocrystals to water, starch domains gets swollen, and the interface between filler and NR matrix weakens which can create free volumes and thereby inducing an increase in swelling rate by the phenomenon known as overshooting effect. This has already been reported for polystyrene-co-butyl acrylate filled with potato starch micro crystal [26]. Diffusion coefficient (D) of NR composites calculated using Eq. (2) is given in Table 2. It was observed that the water diffusion coefficient (D) increases continually as the wt% of starch nanocrystals is increased. It may be due to the formation of a continuous polar network of starch nanocrystal within the NR matrix which seems to favor the swelling of the films by water. The increase in the water diffusion coefficient can be explained as, due to the pathways created by the networks and also due to hydrogen bonding. Increased starch content results in more absorption of water. This has been reported by Duferense at al [12]. The polar network of starch nanocrystals within the NR matrix is due to the interaction between the starch nanocrystals results in increase rate of water absorption. From the water swelling behavior given in Fig. 9 it can be observed that the unfilled NR matrix displays the lowest swelling and on addition of nanocrystals induces an increase in water diffusivity. Diffusion coefficient increases roughly and linearly. This can
4.5. Swelling behavior The swelling behavior of the composite samples after immersion in distilled water were plotted as a function of time. Swelling studies of samples with a filler loading from 5 to 20 wt % were carried out. The diffusion of water molecules was found to be strongly influenced by the microstructure of the nanocomposites and a clear trend is observed with respect to the starch content.
Fig. 8. Schematic representation of possible network formation of starch nanocrystals in the natural rubber matrix.
Fig. 9. Water uptake of NR/starch nanocrystal composites.
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
be explained based on the formation of a continuous polar network of starch nanocrystals within the NR matrix. The network seems to favor the swelling by water. The polar network of starch nanocrystals within the NR matrix due to the interaction between the starch nanocrystals and polymeric chains is represented (Fig. 8). The improvement in mechanical properties on adding starch nanocrystals also support this. 5. Conclusion Potato starch nanocrystals appeared to be an effective reinforcing agent for natural rubber. Starch nanocrystals were obtained after sulfuric acid hydrolysis of potato starch granules (filler). TEM observations confirmed that the crystals extracted from potato starch granules via acid hydrolysis are in the nanometer dimensions. Nanocomposite material were obtained by casting and evaporating a mixture of NR latex and aqueous suspension of potato starch nanocrystal. Four different compositions were prepared with different wt% of filler for the matrix. FESEM observations confirmed that the filler is evenly distributed in the matrix. The homogenous dispersion of the filler into the polymer matrix is one of the key parameters for obtaining excellent mechanical properties. Wide-angle x-ray analysis showed that the processing by casting and evaporation at 40◦ C did not affect the crystallinity of starch nanocrystal. The diffractograms revealed that, by adding starch nanocrystals into NR resulted in an increase of the global crystallinity of the composite material. The mechanical properties of the prepared composite materials were studied through tensile testing. The reinforcing effect of the prepared nanocrystals thus appears to be an excellent nanofiller for NR latex and have the potential for replacing conventional polymer composite and nanocomposite which are a real threat to the environment. The overall improvement in property was expected to be due to the possible formation of a three-dimensional network of starch nanocrystals within the nanocomposites network.
153
References [1] Y. Habibi, A. Dufresne, Biomacromolecules 9 (2008) 1974–1980. [2] A. Dufresne, Can. J. Chem. 86 (2008) 484–494. [3] A. Helene, P. Jean-Luc, M. Sonia, D. Danielle, D. Alain, Macromol. Symp. 221 (2005) 95–104. [4] A. Helene, M. Sonia, D. Danielle, A. Dufresne, Macromol. Symp. 233 (2006) 132–136. [5] M. Darder, P. Aranda, E. Ruiz-Hitzky, Adv. Mater. 19 (2007) 1309–1319. [6] H.Y. Kim, D.J. Park, J.Y. Kim, S.T. Lim, Carbohyd. Polym. 98 (2013) 295–301. [7] D. LeCorre, J. Brass, A. Dufresne, Carbohyd. Polym. 87 (2012) 658–666. [8] D. Chen, D. Lawtona, M.R. Thompson, Q. Liub, Carbohyd. Polym. 90 (2012) 709–716. [9] H.-Y. Kim, J.A. Hanb, D.K. Kweonc, J.D. Parkd, S.T. Lima, Carbohyd. Polym. 93 (2013) 582–588. [10] O. van den Berg, J.R. Capadona, C. Weder, Biomacromolecules 8 (2007) 1353–1357. [11] Y. Lu, L. Weng, L. Zhang, Biomacromolecules 5 (2004) 1046–1051. [12] H. Angellier, L. Choisnard, S. Molina-Biosseau, P. Ozil, A. Dufresne, Biomacromolecules 5 (2004) 1545–1551. [13] W. Thielemans, M.N. Belgacem, A. Dufresne, Langmuir 22 (2006) 4804–4810. [14] P.J. Jenkins, A.M. Donald, Starch-Stärke 49 (1997) 262–267. [15] T. Materne, F. Corvasce, and P. Leitz, European patent no. EP0995775A1, 2000. [16] H. Angellier, S. Molina-Boisseau, L. Lebrun, A. Dufresne, Macromolecules 38 (2005) 3783–3792. [17] L. Goetz, A. Mathew, K. Oksman, P. Gatenholm, P. Ragauskas, Carbohyd. Polym. 75 (2009) 85–89. [18] E. Kristo, C.G. Biliaderis, Carbohyd. Polym. 68 (2007) 146–158. [19] S.A. Paralikar, J. Simonsen, J. Lombardi, J. Membr. Sci. 320 (2008) 248–258. [20] O. van den Berg, M. Schroeter, J.R. Capadona, C. Weder, J. Mater. Chem. 17 (2007) 2746–2753. [21] M. Labet, W. Thielemans, A. Dufresne, Biomacromolecules 8 (2007) 2916–2927. [22] H. Angellier, S. Molina-Boisseau, L. Lebrun, A. Dufresne, Macromolecules 38 (2005) 3783–3792. [23] H. Angellier, S. Molina-Boisseau, A. Dufresne, Macromolecules 38 (2005) 9161–9170. [24] N. Lin, J. Huang, P.R. Chang, D.P. Anderson, J. Yu, J. Nano Mater. (2011), http://dx.doi.org/10.1155/2011/573687. [25] A. Dufresne, Natural rubber green nanocomposites, in: S. Ranimol, S. Thomas (Eds.), Rubber Nanocomposites: Preparation, Properties, and Applications., John Wiley & Sons, Singapore, 2010, pp. 113–145. [26] A. Dufresne, J.Y. Cavaille, J. Polym. Sci., Part B: Polym. Phys. 36 (1998) 2211–2224.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Preparation and characterization of potato starch nanocrystal reinforced natural rubber nanocomposites K.R. Rajisha a , H.J. Maria b , L.A. Pothan c , Zakiah Ahmad d , S. Thomas b,e,∗ a
Department of Chemistry, CMS College, Kottayam, Kerala, India School of Chemical Sciences, Mahatma Gandhi University, Priyadarsini Hills, Kottayam, Kerala, India Department of Chemistry, Bishop Moore College, Mavelikara, Kerala 690101, India d UiTM, Mara Malaysia, 40450 Shah Alam, Selangor, Malaysia e Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarsini Hills, Kottayam, Kerala, India b c
a r t i c l e
i n f o
Article history: Received 16 December 2013 Received in revised form 5 March 2014 Accepted 7 March 2014 Available online 19 March 2014 Keywords: Bionanocomposite Starch Nanocrystal
a b s t r a c t Potato starch nanocrystals were found to serve as an effective reinforcing agent for natural rubber (NR). Starch nanocrystals were obtained by the sulfuric acid hydrolysis of potato starch granules. After mixing the latex and the starch nanocrystals, the resulting aqueous suspension was cast into film by solvent evaporation method. The composite samples were successfully prepared by varying filler loadings, using a colloidal suspension of starch nanocrystals and NR latex. The morphology of the nanocomposite prepared was analyzed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM analysis revealed the size and shape of the crystal and their homogeneous dispersion in the composites. The crystallinity of the nanocomposites was studied using XRD analysis which indicated an overall increase in crystallinity with filler content. The mechanical properties of the nanocomposites such as stress–strain behavior, tensile strength, tensile modulus and elongation at break were measured according to ASTM standards. The tensile strength and modulus of the composites were found to improve tremendously with increasing nanocrystal content. This dramatic increase observed can be attributed to the formation of starch nanocrystal network. This network immobilizes the polymer chains leading to an increase in the modulus and other mechanical properties. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite materials find considerable interest recently among researchers and industrialists. Synthetic nanomaterials lack easy processability, biocompatibility, and biodegradability and hence have limitations compared to natural nanomaterials. Biodegradable nanocomposites which have superior thermal, barrier and mechanical properties can be synthesized from biopolymer and nanosized reinforcements. They are formed by the combination of natural polymers and inorganic solids which possess at least one dimension in the nanometer scale. Similar to conventional nanocomposites, which involve synthetic polymers, these biohybrid materials exhibit improved structural and functional properties of great interest for different applications. The
∗ Corresponding author. Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarsini Hills, Kottayam–686560, Kerala. Tel.: +9447223452/+481 2730003. E-mail addresses: [email protected], [email protected], [email protected] (S. Thomas). http://dx.doi.org/10.1016/j.ijbiomac.2014.03.013 0141-8130/© 2014 Elsevier B.V. All rights reserved.
properties inherent to the biopolymers, biocompatibility and biodegradability, open new prospects for these hybrid materials with special incidence in regenerative medicine and environment friendly materials (green nanocomposites). Research on bionanocomposites can be regarded as a new interdisciplinary field closely related to significant topics such as biomineralization processes, bioinspired materials, and biomimetic systems. The upcoming development of novel bionanocomposites introducing multifunctionality represents a promising research topic that takes advantage of the synergistic effect of biopolymers with inorganic nanometer-sized solids. Raw materials often used for these new nanostructured composites are natural polymers; clay and nanowhiskers of chitin and cellulose. A possible source of inspiration for the design of new high performance bionanomaterials is due to the fact that they offer excellent reinforcement [1]. Starch is an abundant biopolymer, which is totally biodegradable. Starch nanocrystals obtained from starch have been used as fillers in polymeric matrices leading to desired reinforcing effect [2–4]. During the last decade, nano-material derived from natural polymers have been used as reinforcement in polymers and have been named as “green” bionanocomposites [5]. Many research
148
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
works published in the area of starch based bionanocomposites show the relevance on this topic in the current scenario of polymer research [6–9]. Chitin, cellulose and starch are the crystalline residue that can be obtained from different natural polysaccharides with a uniform structure after acidic or alkaline hydrolysis. These nanoparticles from different sources have different geometrical characteristics. For example, the nanocrystals obtained from cellulose [10] and chitin [11] have a rod shaped structure while that of starch have a platelet shaped structure [12]. However, nanocrystals from polysaccharides have the limitation of being hydrophilic and hence are incompatible with hydrophobic polymeric matrices. Therefore, the nanocrystal–matrix interface is usually the weakest point in a biocomposite, which results in the poor performance of the final composite. In order to improve the compatibility between the reinforcing phase and the polymer matrix, the nanoparticles can be physically or chemically modified [12]. An effective method for chemical modification is to graft polymer chains from the matrix directly onto the surface of reinforcing nanoparticles before composite preparation [13]. The formation of a continuous interphase between polysaccharide phase and the polymer matrix phase can improve interfacial adhesion. In addition, entanglements between grafted and ungrafted polymer chains are expected to occur if the molar weight is high enough. By this approach, nanocomposite materials with high content of reinforcing nanoparticles can be prepared. The main polysaccharides of interest as materials for nanocomposite preparation are starch and cellulose, but an increasing attention is also given to the application of the other biopolymers such as chitin and chitosan [14]. Starch is a well-known polymer naturally produced by plants in the form of granules which can be obtained mainly from maize, potatoes, corn, and rice. The properties of these starch granules vary from plant to plant, but are generally composed of, amylose (in most cases about 20% of the granule), and amylopectin. Amylose that builds up to 15–35% of the granules in most plants is a primarily linear polysaccharide with ␣-(1–4)-linked d-glucose units. Amylopectin is a highly branched molecule, with ␣ (1–4) linked d-glucose backbones and exhibits about 5% of ␣ (1–6)-linked branches, which have a profound effect on the physical and biological properties. Amylose is semi crystalline and soluble in hot water while amylopectin is insoluble in hot water. During its biodegradation, starch undergo enzyme-catalysed acetal hydrolysis where the ␣-1,4 link in amylopectin is attacked by glucosidases.[15]. The structural differences between these two polymers contribute to significant differences in starch properties and functionality The focus of this work is to process nanocomposite materials consisting of natural rubber filled with potato starch nanocrystals which are completely biodegradable and thereby replacing the conventionally used carbon black (manufactured by burning oil or natural gas in controlled conditions). In the past decades, research works were focused on the development of other reinforcing agents to replace carbon black in rubber compounds. Recently, Novamont (Novara, Italy), working in partnership with Goodyear Tire and Rubber, has developed tires using nanoparticles derived from corn starch, partially replacing the conventional carbon black and silica used in making tires. This patented innovation, called Biotred, not only presents environmental advantages but also reduces the rolling resistance of tires [16]. Many attempts have been reported to blend polysaccharide nanocrystals with polymeric matrices [17–20]. The resulting nanocomposite materials display outstanding mechanical properties and thermal stability. For example, starch nanocrystals consist of crystalline nanoplatelets about 6–8 nm thick with a length of 20–40 nm and a width of 15–30 nm [20]. They have been used as a new kind of fillers, showing interesting reinforcing and barrier properties in natural rubber [21–23].
In this approach, potato starch nanocrystals constitute another possible filler for natural rubber which can have an admirable contribution in developing new environmental friendly strong composites [5]. Starch nanocrystals, the nanoscale biofiller derived from native starch granules, have been compounded with many different kinds of polymer matrices. The intrinsic rigidity of starch nanocrystals, special platelet-like morphology, strong interfacial interactions, and the percolation network organized by nanocrystals, contribute to the mechanical performance, thermal properties, solvent absorption, and barrier properties of the composites [24]. In the present work, a new nanocomposite based on natural rubber filled with potato starch nanocrystals was prepared. The nanocrystals were characterized by TEM and XRD. By varying the weight percentage of starch nanocrystals in the NR matrix the surface morphology, swelling behavior, and crystallinity of the various composites were investigated. The mechanical properties of the composites were also analyzed. 2. Experimental 2.1. Materials Natural rubber latex was kindly supplied by Rubber Research Institute of India (RRI, Kottayam, India). It contained spherical particles of natural rubber with an average diameter around 1 m, and the dry rubber content determined was about 60 wt %. The density of dry NR, was 1.4 g cm−3 , and it contained ∼98% of cis-1,4-polyisoprene. Potato starch powder was purchased from Luba chemicals, Mumbai. Other chemicals like 36 N H2 SO4 , BaCl2 , sodium azoture (protectant against microorganisms) etc. needed for the preparation of starch nanocrystals were obtained from local sources. 2.2. Preparation of potato starch nanocrystals Potato starch nanocrystals were prepared by sulfuric acid hydrolysis of native potato starch powder. About 36 g of potato starch granules were mixed with 250 ml of 36 N H2 SO4 for 5 days at 40 ◦ C, with a stirring speed of 100 rpm. The aqueous suspension was washed by successive centrifugation with distilled water until neutrality (confirmed by litmus paper testing). At this stage, starch suspension can be believed to be broken into nanocrystals, which changes the refractive index of the solution which is evident from the opaque nature of the suspension. The dispersion was completed by a further 10 min ultrasonic treatment in a B12 Branson sonifier. The resultant aqueous suspension constituted of starch fragments with a homogenous distribution in size. The solid fraction of this aqueous suspension had a weight concentration of about 3.4 wt %. 2.3. Preparation of NR latex/starch nanocrystal nanocomposite films The aqueous suspension of potato starch nanocrystal and the NR latex were mixed in various proportions at ambient temperature using a mechanical stirrer (IKA-RW 28). The mixing was carried out for 15 min to ensure uniform dispersion. Then the mixture was kept for 1 h in order to ensure homogenization and also for the sedimentation of impurities. The mixture was stored under vacuum and stirred on a rotavapor for 10 min in order to degass the mixture and to avoid the formation of irreversible bubbles during water evaporation. The films with uniform thickness were obtained by casting on a glass mold and was evaporated at 40 ◦ C in a ventilated oven for 4–6 h and then heated at 60 ◦ C under vacuum for 2 h. The resulting films were conditioned at room temperature
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
149
Fig. 1. Flow chart showing the preparation of potato starch nanocomposites.
in desiccators containing P2 O5 Fig. 1 shows the flow chart of the preparation method adopted. The weight fractions of the sample and their compositions are listed in Table 1. 3. Characterization techniques 3.1. Transmission electron microscopy (TEM) The morphology of the nanocrystals was investigated by means of TEM (JEM-2100HRTEM). The micrographs were obtained in point to point resolution 0.194 nm, operating at an accelerating voltage of 200 kV. The suspension was placed on a 300 mesh Cu grid (35 mm diameter). 3.2. Field emission scanning electron microscopy (FESEM) Morphology of the composites was studied using field emission scanning electron microscopy. A FESEM, CARC-ZEISS, SUPRA40 UP was used to observe the morphology of starch nanocrystal and different starch nanocomposite. The films were sputter coated with platinum to ensure conduction observed with an accelerating voltage of 5 kV. 3.3. X-ray diffraction (XRD)
The kinetics of water absorption was determined for all compositions. The specimens were films with dimensions around 10 × 10 × 0.2 mm3 . The films were thin enough to ensure unidirectional diffusion. After immersion in water, samples were removed at different times, wiped with filter paper to remove surface water and weighed with an analytical balance with 0.1 mg resolution. Water induced dimensional changes were measured with a micrometer having an accuracy of 0.1 mm. The water uptake (WU) was calculated using Eq. (1) WU% =
mt − m0 × 100 m0
(1)
mo and mt are the weights of the sample before and after a time t of immersion, respectively Water diffusion coefficient (D) of the nanocomposites was calculated using Eq. (2) D=
h 2 4m∞
(2)
where, D = water diffusion coefficient; h = thickness of each sample; = slope of the linear portion of the curve; m∞ = weight of the sample at equilibrium 4. Results and discussion
X-ray diffraction patterns were taken by using Ni- filtered Cu-K␣ radiation ( = 0.154 nm) by Bruker D8 Advanced x-ray diffractometer. The latex nanocomposite samples were scanned in step mode by 1.5% min scan rate in the range of 2 < 12◦ . The specimens of 1 × 1 cm films were used for the analysis. 3.4. Mechanical measurement Tensile tests: The non linear mechanical properties of the samples were studied using Universal Testing Machine in accordance with ASTM D 412- 2002. Dumbbell shaped specimens 5 mm wide, 8 mm long and about 1 mm thick were used for tensile measurements. The tensile curves obtained were analyzed for strain at break and modulus. The tests were conducted at a crosshead speed of 500 mm/min at ambient temperature. The mechanical properties of nano filled latex composites were studied at ambient temperature. Table 1 Composition of potato starch nanocrystal/NR latex and nanocomposite films. Sample
Natural rubber latex (dry wt%)a
Starch content (dry wt%)
NR100 NR95 NR90 NR85 NR 80
100 95 90 85 80
0 5 10 15 20
a
3.5. Water uptake
Dry rubber content of 10 mg NR latex found to be 6 gm.
4.1. Characterization of starch nanocrystals The morphological analysis of starch nanocrystal was done using x-ray diffraction analysis and transmission electron microscopy. The XRD pattern displayed two weak peaks at 2 = 11.0◦ , double peak at 2 = 14.16◦ and 2 = 19.5◦ and a strong peak at 2 = 22.19о and 2 = 25.88◦ (Fig. 2). From the XRD data, it was observed that the diffraction patterns recorded for the lyophilized sample of pure potato starch nanocrystal displayed a typical A-type amylose with x-ray diffraction patterns at 2 with the first peak around 15◦ , the second peak near 18◦ , and the third main reflection around 23◦ . A comparative study of the XRD peaks of native starch with prepared starch nanocrystals were also conducted. From the diffractograms, both the native potato starch and hydrolysed potato starch show the characteristic peak at 18◦ and 23◦ . But these two peaks are more prominent in the diffractogram of potato starch nanocrystal and this indicates the very high extent of crystallinity in starch nanocrystals. The TEM analysis of potato starch nanocrystals lead to the image shown in Fig. 3a. The presence of nanocrystal particles is evidenced by the small dark spots (a few spots are encircled to highlight them). From the TEM image, the average diameter of the nanocrystals is found to be 3 nm which can be confirmed from the particle size analysis (Fig. 3b). The samples were found to be free of aggregation and impurities as per the TEM observation. Thus the method used
150
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
Fig. 2. X-ray diffraction patterns of starch nanocrystals and potato starch powder.
to synthesize nanocrystals does not create impurities. Nanocrystals of the same of nanometers range (9–20 nm) have been reported by Dufresne et al. [25]. 4.2. Morphology of the nanocomposite films The morphology of the NR/starch nanocrystal material was determined from field emission scanning electron microscopy and XRD analysis. The distribution level of the filler within the matrix was evaluated by observing the FESEM image of the surface fractured films. Fig. 4a and b show the nanocomposite films of NR nanocomposite with 5 and 10 wt % of nanocrystals respectively. In the FE-SEM micrograph a homogenous dispersion of starch nanocrystals which appears like white dots can be observed. A uniform distribution of the nanocrystal in the matrix is clearly seen for the compositions. Such an even and uniform distribution of the filler in the matrix is essential for obtaining optimum properties. As the filler loading is increased the nanocrystals show a tendency to agglomerate which can be observed in Fig. 4(b) image. The particle size of the nanocrystal was observed to be in the range 12 nm and it is in agreement with some earlier work in this area [25].
Fig. 4. FESEM micrograph of (a) NR95 (b) NR90.
4.3. X-ray diffraction: structural analysis; Influence of starch content The nanocomposite films of NR/starch nanocrystals were characterized by x-ray diffraction. X-ray patterns were collected for different compositions and are displayed in Fig. 5. The diffractograms of unfilled natural rubber and pure potato starch nanocrystal films have also been shown for comparison.
Fig. 3. (a) TEM micrograph of starch suspension. (b) Particle size distribution of (∼50 counts) starch nanocrystals from the TEM data.
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
Fig. 5. X-ray diffraction patterns of starch nanocrystals/NR nanocomposite films.
The diffraction pattern recorded for a film of pure potato starch nanocrystals obtained by pressing freeze-dried nanocrystals displays typical peaks of A-type amylose allomorph. It is characterized by two weak peaks at 2 = 10.1◦ and, a strong peak at 18.2◦ , and a strong peak at 23.5◦ . The natural rubber film (NR100) displays a typical behavior of a fully amorphous polymer. It is characterized by a broad hump located around 2 = 18◦ . By adding starch nanocrystals, the peaks corresponding to A-type amylose allomorph become stronger and stronger. This shows that an increase of the starch content results in an increase of the global crystallinity of the composite material. Furthermore, x-ray analysis confirmed that the processing by casting and evaporation at 40 ◦ C did not affect the crystallinity of the starch nanocrystals.
151
Fig. 6. Stress–strain curves of starch nanocrystal/NR nanocomposites.
4.4. Mechanical properties The starch nanocrystal obtained from sulfuric acid hydrolysis was very promising reinforcing material for polymer nanocomposites because of their high stiffness and strength studies on the reinforcing effect of starch nanocrystals with other fillers like clays, organoclays, carbon black, fly ash, and chitin whiskers in natural rubber matrix reports that starch nanocrystal is a good substitute for carbon black, as it can induce a reinforcing effect in terms of stiffness, strength and elastic behavior. These nanocomposites from renewable resources have the added advantages such as, low cost, easy availability, good biocompatibility, and ease of modification chemically and mechanically.
Fig. 7. (a) Tensile strength, (b), tensile modulus and (c) elongation at break, for starch nanocrystals/NR nanocomposite.
152
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
Table 2 Water diffusion coefficient (D) of NR/nanocrystal composites. Nanocomposites samples (vol%)
Thickness of sample (h) (mm)
Angle of linear ()
Weight of sample at infinity (Mω) (g)
Diffusion coefficient (D) (cm2 s−1 )
NR film NR 95 NR 90 NR 85 NR 80
1.35 1.65 1.42 1.12 1.81
0.38 0.40 0.56 0.61 0.63
1.23 1.31 1.38 1.48 1.50
0.08 0.95 1.01 3.24 3.97
The tensile behavior of the potato starch nanocrystal reinforced with NR latex nanocomposite films was analyzed at room temperature. Fig. 6 shows the stress–strain curves of starch–natural rubber composite with increasing starch content. It is found that stress increases regularly with strain till the sample breaks. As the filler loading is increased the tensile strength as well the modulus increases remarkably. The sharp increase in the tensile strength of the NR nanocomposites with addition of the filler shows the reinforcing effect of starch nanocrystal. The elongation at break of the nanocomposites was found to be decreased from a value of 1351 for the neat rubber to 536 when 20 wt% of the starch nanocrystals were added. Tensile modulus of the nanocomposites also was improved significantly with filler loading. In Fig. 7, the tensile modulus of the NR/potato starch nanocrystal composites is plotted as a function of starch content. From the graph it is clear that tensile modulus also increases in almost linear fashion. The reinforcing effect of the starch nanocrystals strongly depended on their ability to form a rigid three-dimensional network, resulting in strong interactions such as hydrogen bonds between the filler. Tensile test experiments give clear evidence for the presence of a three-dimensional network within the nanocomposites samples. In higher concentration, there is more number of possible polymer–filler interactions which enhances the reinforcement. The schematic representation given in Fig. 8 shows the possible presence of a three-dimensional network within the nanocomposites. Even though an agglomerated morphology was observed at 10 wt% nanocrystal loading the property of the nanocomposites were not affected adversely. Thus, a modification of dispersion method used can possibly give a better dispersion that can result in a further improvement of properties and will be a scope of future studies.
For all the compositions the water uptake increases tremendously at its first zone (below 20 h) and then reaches a maximum. After reaching the maximum point, there is a decrease for the next 5 h. On further soaking there is a slight decrease with respect to time and reaches at equilibrium. The diminution of water uptake may be due to the partial release or leaching of starch nano particles in water. The further increase in water uptake with time may be due to the migration of starch nanocrystals towards the aqueous phase during swelling. During the exposure of this starch nanocrystals to water, starch domains gets swollen, and the interface between filler and NR matrix weakens which can create free volumes and thereby inducing an increase in swelling rate by the phenomenon known as overshooting effect. This has already been reported for polystyrene-co-butyl acrylate filled with potato starch micro crystal [26]. Diffusion coefficient (D) of NR composites calculated using Eq. (2) is given in Table 2. It was observed that the water diffusion coefficient (D) increases continually as the wt% of starch nanocrystals is increased. It may be due to the formation of a continuous polar network of starch nanocrystal within the NR matrix which seems to favor the swelling of the films by water. The increase in the water diffusion coefficient can be explained as, due to the pathways created by the networks and also due to hydrogen bonding. Increased starch content results in more absorption of water. This has been reported by Duferense at al [12]. The polar network of starch nanocrystals within the NR matrix is due to the interaction between the starch nanocrystals results in increase rate of water absorption. From the water swelling behavior given in Fig. 9 it can be observed that the unfilled NR matrix displays the lowest swelling and on addition of nanocrystals induces an increase in water diffusivity. Diffusion coefficient increases roughly and linearly. This can
4.5. Swelling behavior The swelling behavior of the composite samples after immersion in distilled water were plotted as a function of time. Swelling studies of samples with a filler loading from 5 to 20 wt % were carried out. The diffusion of water molecules was found to be strongly influenced by the microstructure of the nanocomposites and a clear trend is observed with respect to the starch content.
Fig. 8. Schematic representation of possible network formation of starch nanocrystals in the natural rubber matrix.
Fig. 9. Water uptake of NR/starch nanocrystal composites.
K.R. Rajisha et al. / International Journal of Biological Macromolecules 67 (2014) 147–153
be explained based on the formation of a continuous polar network of starch nanocrystals within the NR matrix. The network seems to favor the swelling by water. The polar network of starch nanocrystals within the NR matrix due to the interaction between the starch nanocrystals and polymeric chains is represented (Fig. 8). The improvement in mechanical properties on adding starch nanocrystals also support this. 5. Conclusion Potato starch nanocrystals appeared to be an effective reinforcing agent for natural rubber. Starch nanocrystals were obtained after sulfuric acid hydrolysis of potato starch granules (filler). TEM observations confirmed that the crystals extracted from potato starch granules via acid hydrolysis are in the nanometer dimensions. Nanocomposite material were obtained by casting and evaporating a mixture of NR latex and aqueous suspension of potato starch nanocrystal. Four different compositions were prepared with different wt% of filler for the matrix. FESEM observations confirmed that the filler is evenly distributed in the matrix. The homogenous dispersion of the filler into the polymer matrix is one of the key parameters for obtaining excellent mechanical properties. Wide-angle x-ray analysis showed that the processing by casting and evaporation at 40◦ C did not affect the crystallinity of starch nanocrystal. The diffractograms revealed that, by adding starch nanocrystals into NR resulted in an increase of the global crystallinity of the composite material. The mechanical properties of the prepared composite materials were studied through tensile testing. The reinforcing effect of the prepared nanocrystals thus appears to be an excellent nanofiller for NR latex and have the potential for replacing conventional polymer composite and nanocomposite which are a real threat to the environment. The overall improvement in property was expected to be due to the possible formation of a three-dimensional network of starch nanocrystals within the nanocomposites network.
153
References [1] Y. Habibi, A. Dufresne, Biomacromolecules 9 (2008) 1974–1980. [2] A. Dufresne, Can. J. Chem. 86 (2008) 484–494. [3] A. Helene, P. Jean-Luc, M. Sonia, D. Danielle, D. Alain, Macromol. Symp. 221 (2005) 95–104. [4] A. Helene, M. Sonia, D. Danielle, A. Dufresne, Macromol. Symp. 233 (2006) 132–136. [5] M. Darder, P. Aranda, E. Ruiz-Hitzky, Adv. Mater. 19 (2007) 1309–1319. [6] H.Y. Kim, D.J. Park, J.Y. Kim, S.T. Lim, Carbohyd. Polym. 98 (2013) 295–301. [7] D. LeCorre, J. Brass, A. Dufresne, Carbohyd. Polym. 87 (2012) 658–666. [8] D. Chen, D. Lawtona, M.R. Thompson, Q. Liub, Carbohyd. Polym. 90 (2012) 709–716. [9] H.-Y. Kim, J.A. Hanb, D.K. Kweonc, J.D. Parkd, S.T. Lima, Carbohyd. Polym. 93 (2013) 582–588. [10] O. van den Berg, J.R. Capadona, C. Weder, Biomacromolecules 8 (2007) 1353–1357. [11] Y. Lu, L. Weng, L. Zhang, Biomacromolecules 5 (2004) 1046–1051. [12] H. Angellier, L. Choisnard, S. Molina-Biosseau, P. Ozil, A. Dufresne, Biomacromolecules 5 (2004) 1545–1551. [13] W. Thielemans, M.N. Belgacem, A. Dufresne, Langmuir 22 (2006) 4804–4810. [14] P.J. Jenkins, A.M. Donald, Starch-Stärke 49 (1997) 262–267. [15] T. Materne, F. Corvasce, and P. Leitz, European patent no. EP0995775A1, 2000. [16] H. Angellier, S. Molina-Boisseau, L. Lebrun, A. Dufresne, Macromolecules 38 (2005) 3783–3792. [17] L. Goetz, A. Mathew, K. Oksman, P. Gatenholm, P. Ragauskas, Carbohyd. Polym. 75 (2009) 85–89. [18] E. Kristo, C.G. Biliaderis, Carbohyd. Polym. 68 (2007) 146–158. [19] S.A. Paralikar, J. Simonsen, J. Lombardi, J. Membr. Sci. 320 (2008) 248–258. [20] O. van den Berg, M. Schroeter, J.R. Capadona, C. Weder, J. Mater. Chem. 17 (2007) 2746–2753. [21] M. Labet, W. Thielemans, A. Dufresne, Biomacromolecules 8 (2007) 2916–2927. [22] H. Angellier, S. Molina-Boisseau, L. Lebrun, A. Dufresne, Macromolecules 38 (2005) 3783–3792. [23] H. Angellier, S. Molina-Boisseau, A. Dufresne, Macromolecules 38 (2005) 9161–9170. [24] N. Lin, J. Huang, P.R. Chang, D.P. Anderson, J. Yu, J. Nano Mater. (2011), http://dx.doi.org/10.1155/2011/573687. [25] A. Dufresne, Natural rubber green nanocomposites, in: S. Ranimol, S. Thomas (Eds.), Rubber Nanocomposites: Preparation, Properties, and Applications., John Wiley & Sons, Singapore, 2010, pp. 113–145. [26] A. Dufresne, J.Y. Cavaille, J. Polym. Sci., Part B: Polym. Phys. 36 (1998) 2211–2224.
