Bionanocomposites based on balata rubber

Transcription

1 Chapter 9 Bionanocomposites based on balata rubber (Swietenia macrophylla) reinforced with nanocellulose This chapter is under revision in Biomacromolecules as Green nanocomposites based on balata and natural rubber nanocomposites with cellulose nanofibre: Preparation and characterization Summary Balata latex obtained from mahogany trees (Swietenia macrophylla), an Amazonian shield rainforest species has been found to be an excellent matrix material for reinforcing nanocellulose (which is obtained by the steam explosion of banana bast fibre) to make a novel bionanocomposite. The balata composites have been compared with the NR composites. The balata composites have been found to have enhanced thermal stability, tensile strength and storage moduli compared to NR composites. The results suggest that the balata rubber and nanocellulose are good candidates for the preparation of nanocomposites for various industrial applications.

2 262 Chapter Introduction Balata latex (Manilkara bidebtata), similar to natural rubber, has not been industrially exploited as much as natural rubber mainly due to an enhanced level of crystallinity in the polymer. The crystallinity of the balata rubber results from the fact that the main component is trans-1, 4-polyisoprene which is shown in Fig. 9.1 (a). Natural rubber however has a cis structure as shown in Fig. 9.1 (b) (cis-1, 4-polyisoprene) which is amorphous. Balata rubber contains some amide functional group components and the exact structure is yet to be revealed. The trans configuration of the polymer chain leads to an increased ease of packing and therefore increased crystallinity and concomitant brittleness and reduced elongation at break. Balata sap is harvested in the Amazonian shield rainforest from the mahogany tree which is also a sought after hardwood for fine furniture. If the balata latex can be made more industrially relevant, it would provide significant reason for avoided deforestation of this hardwood, with related benefits to abatement of climate change. It would also provide a sustainable livelihood for the rainforest inhabitants such as the Makushi people of the Guyana rainforest. In this chapter, isolated nanocellulose has been utilized as functional rigid fibres in natural balata matrix and has been compared with its NR counterparts. The morphology, thermo mechanical properties, crystallinity and structure of the nanocomposite materials were compared. The nanofibres used were a homogenous nanocellulose (with a diameter ranging between 5-40 nm) dispersion. Nanocellulose prepared in this manner present a fibre like morphology, with very high aspect ratio; desirous in nanofibres used in nanocomposites [1,2]. The cellulosic unit is shown in Fig. 9.2 (a) and atomic force microscope (AFM) image of the nanocellulose in Fig. 9.2 (b).

3 Bionanocomposites based on balata rubber (Swietenia macrophylla) 263 Fig. 9.1 (a) trans-1,, 4-polyisoprene Fig. 9.1 (b) Cis-1, 4-polyisoprene Fig. 9.2 (a) Cellulose unit Fig 9.2 (b) AFM of nanocellulose Reports are there in the literature on the usage of nanocellulose as active network enhancers in rubber latex matrices with beneficial effects on the mechanical properties [3, 4] 9.2 Results and Discussion Deprotonation of cellulose and formation Zn-cellulose complex The balata rubber/cnf nanocomposites were prepated as per the procedure described in A duplicate of the nanocomposite with NR counterparts were also prepared to compare effect of the conformational change in the properties of the resultant nanocomposites. It is reported that the cellulose can be dissolved in aqueous solvent systems containing metals such as copper and zinc in Cuam and Cuen solutions [5]. The dissolution mechanism involves deprotonation of C 2 -OH and C 3 -OH during the metal complex formation. The deprotonation of cellulose is takes at alkaline medium with

4 264 Chapter 9 the presence of copper and zinc [6]. Balata latex, which is preserved in the alkaline medium has a ph of around 10, making the favorable atmosphere for the deprotonation of cellulose. The spectral characteristics of aqueous zinc chloride solutions containing cellulose were also reported [7]. The results suggest that zinc ion forms loose complexes with the C 2 or C 3 hydroxyl groups of glucopyranose. We already have reported about the formation of this Zn-cellulose three dimensional network in the crosslinked NR nanocomposites and proposed a mechanism in Fig. 6.5 and is scientifically accepted [10,11]. In this study, a similar mechanism might be assumed. The accelerators and crosslinking agents like Zinc dithiocarbamate (ZDC), zinc mercaptobenzothiozole (ZMBT) and zinc oxide (ZnO) were used in the nanocomposite preparation stage to generate active sites in the balata rubber backbone for the crosslinking of the matrix. These accelerators not only make active sites in the balata rubber backbone by breaking the C=C double bond but also have a major role in the deprotonation of the nanocellulose and make them participate in the crosslinking network. CH 3 CH 3 CH 3 C6 CH 2 OH CH 3 CH 3 CH 3 OH C1 + O O + Sulphur + Zn C3 C2 O OH Balata rubber Cellulose Zn-Cell-complex Zn-Cell-complex CH 3 CH 3 CH 3 (S)n Zn-Cell-complex (S)n CH 3 CH 3 CH 3 Zn-Cell-complex Zn-Cell-complex Zn-Cell-complex Balata rubber/cellulose nanocomposite Fig. 9.3 Proposed mechanism for interaction of the cross linked balata/cellulose nanocomposite

5 Bionanocomposites based on balata rubber (Swietenia macrophylla) 265 During the ultra-sonication process of the mixing of the filler in matrix, cellulose fibres get dispersed into nano level whose specific surface area is increased remarkably. This increased surface area favours chemical reactions with other reagents. The incorporation of fillers cause interruption in the alignment process of chains. When filler loading is increased, weak interfacial regions between filler surface and rubber matrix are formed. The resultant Zn-cellulose complex diminishes the crystalline nature of the composite and get dispersed with metal ion [11]. A reaction mechanism is suggested and is shown in Fig FTIR analysis The FTIR analyses of the nanocellulose, NR and balata nanocomposites have been done as per the procedure described in and the transmittance spectra of balata rubber nanocomposites are shown in Fig. 9.4 (a). Compared to the NR composites in Fig. 9.4 (b), there is a higher transmittance at 3400 cm -1 suggesting a higher amount of -OH presence in the natural balata latex. The higher amount of water percentage and the presence of acidamide group of a few protein components in the balata rubber make this contribution. The stretching vibration of the methylene group (2850 cm -1 ) which is attached to the isoprene backbone is more pronounced in natural rubber compared to balata. This decreased intensity of vibration of the methylene group is due to the close packing of the rubber molecules because of the trans configuration of polyisoprene units in balata. There is enough room for methylene molecule to vibrate in NR than balata due to the spacious arrangement of isoprene molecule in the former. In addition, the intensity of transmittance bands at 1247 cm -1 and 1725 cm -1 are higher in balata than natural rubber, again as expected due to the cis configuration of NR compared to the trans

6 266 Chapter 9 configuration of balata. The band at 1247 cm -1 is related to C O stretching and that at 1725 cm -1 is related to amide group stretching [12]. In addition to the characteristic transmittances of balata, those of the nanocellulose are also presented in Fig. 9.4 (a) which is recorded by pressing nanofibres in a tablet form. Transmittance at 1630 cm -1 in the nanocellulose is due to C O vibrations in the cellulose group. The vibrations of unsymmetry C O C bond and C O bond of the functional group are at 1154 cm -1 and 1045 cm -1. The transmittance at 3322 cm -1 and 3402 cm -1 are peaks attributable to the stretching vibrations of the hydroxyl group of nanocellulose and its composites respectively. In this instance, the peaks are also partially due to adsorbed water. The highest intensity of transmittance demonstrated at 3322 cm -1 is not surprising due to the three hydroxyl functional groups present in each cellulosic molecule. It should be noted that the intensity of the hydroxyl transmittance of all the composites are less than the intensity of both the neat balata matrix and the nanocellulose. This is due to the fact that when the nanocellulose fibres are added to the balata matrix, the free hydroxyls in both the balata matrix (most likely due to water present and small percentages of protein present) and the nanocellulose fibres become bound, and the total free hydroxyl absorbance is concomitantly reduced. The amount of nanocellulose is increased by increasing its percentage, the number of free hydroxyls again continue to decrease up to 0.75% nanocomposite, as all of the hydroxyl components from the balata matrix has been used up. 1% balata composites shows an increase in the intensity of the hydroxyl density. It proves that the ultimate using up of the hydroxyl density which is contributed by the balata latex and nanocellulose is in the range between 0.75% and 1% of nanocellulose content.

7 Bionanocomposites based on balata rubber (Swietenia macrophylla) 267 As presented in Fig. 9.4 (a), the transmittance spectra of balata at 2937 cm -1, 2850 cm -1 and 2830 cm -1 are attributed to the stretching vibrations of the methyl and methene in balata rubber [13]. The transmittance at 1447 cm -1 and 1370 cm -1 are attributed to the bending vibrations of methene and the vibrations of methyl groups in balata rubber. Transmittance at 879 cm -1 is due to methane vibration in the balata side chain. From the spectra of both the composites, it can be seen that there is a reduction in the transmittance peak at 1640 cm -1 of the balata composites than NR rubber composites. It contains both transmittance of acid amide I of balata rubber (1725 cm -1 ) and that of C O bond of cellulose (1638 cm -1 ). This is because of the formation of intrahydrogen bonding between balata and nanocellulose, which leads to chemical bond polarization for attending intra-hydrogen bonding. The transmittance at 1023 cm -1 is attributed to transmittance of both NH (1036 cm -1 ) in balata and primary alcohol C O (1045 cm -1 ) in nanocellulose. The formation of intrahydrogen bonding between balata and nanocellulose can be inferred from the reduction of the vibrational frequency of NH and polar bond C O [14]. It can be concluded that the hydrogen bonding has been formed between nanocellulose and balata rubber. This behaviour is not seen in natural rubber nanocomposites. The formation of hydrogen bonding between the acidamide group of a few proteins in balata and the hydroxyl group in nanocellulose enhances the interface interaction between nanocellulose and balata rubber.

8 268 Chapter Transmittance(%) Pure balata rubber 0.25% balata composite 0.5% balata composite 0.75% balata composite 1% balata composite Nanocellulose Wave number (cm -1 ) Fig. 9.4 (a) FTIR spectroscopy of balata nanocomposites Transmittance (%) Natural rubber 0.25% NR composite 0.5% NR composite 0.75% NR composite 1% NR composite Nanocellulose Wave number (cm -1 ) Fig. 9.4 (b) FTIR spectroscopy of natural rubber nanocomposites An interesting observation from the FTIR spectrum of balata rubber is the decreased intensity of the 3300 cm -1 (hydroxyl groups) peak after the reinforcement of the nanocellulose. This phenomenon is not observed in natural rubber composites as shown in Fig. 9.4 (b). Here a progressive

9 Bionanocomposites based on balata rubber (Swietenia macrophylla) 269 increase of the respective peak is observed because of the increased percentage of nanocellulose because of the reduced interaction between the nanocellulose and NR. The decreased intensity of 3400 cm -1 peak present in the nanocellulose reinforced balata composite proves that the -OH group in the starting material has been used up. These observations coupled with an increase in the intensity of the C O stretching vibration and C-H stretching vibration of the composite suggest bonding interaction between the balata matrix and the nanocellulose. The sharp decline of the 2890 cm -1 peak of nanocellulose, which is the C H stretching vibration peak characterizing the hydrogen bonds between cellulose chains, suggests that the inter-molecular hydrogen bonds break during the reinforcement of nanocellulose. The free hydroxyl groups of nanocellulose are expected to establish new hydrogen bonds with balata rubber backbone and with other nanocellulose molecules during the compounding process, leading to a more robust interfacial interaction between the two phases with a cross linked structure and improved mechanical and dynamic mechanical properties of the composites. The introduction of Zn-cellulose complex [10] by the reaction between the accelerators and the nanocellulose and the formation of a three dimensional network is proposed in Fig The existence of these strong chemical bondings between the reinforced nanocellulose and balata rubber is further proved by increased thermal stability, mechanical and dynamic mechanical properties which is discussed later.

10 270 Chapter XRD analysis The XRD analyses of the nanocellulose, NR and balata nanocomposites are done as per the procedure described in and are shown in Fig. 9.5 (a), (b) and (c). The major finding of the X-ray analysis of the balata rubber which is shown in Fig. 9.5 (a) is the presence of the characteristic peaks of its crystalline nature. It has two strong peaks at o and contrary to its cis configuration (natural rubber). The diffraction pattern recorded for a film of pure nanocellulose obtained by pressing nanofibres in a tablet form is also shown in the graph for comparison. The X-ray diffractogram for nanocellulose by this way displays typical peaks of A-type amylose allomorph. It is characterized by a strong peak at 2θ = 15.1, a very strong peak at Fig. 9.5 (a) XRD analysis of balata, natural rubber and nanocellulose

11 Bionanocomposites based on balata rubber (Swietenia macrophylla) 271 Fig. 9.5 (b) XRD analysis of balata nanocomposites % NR composite 0.5 % NR composite 0.75 % NR composite 1 % NR composite Intensity count Theta angle Fig. 9.5 (c) XRD analysis of natural rubber nanocomposites The crystalline character of balata rubber is retained in its composite structure with nanocellulose is the interesting finding of XRD analysis. It is clear from the XRD patterns of the balata composites in Fig. 9.5 (b). With the increase of the crystalline nanocellulose percentage, an increase in the intensity of the corresponding peaks leads to the conclusion that the crystallinity of the composite increases with nanocellulose percentage. The second peak of the composites resemble nanocellulose peak at 22.5 o than balata rubber at o. The crystalline nanocellulose molecules are in the Zn-cellulose complex

12 272 Chapter 9 network form as per the proposed mechanism in Fig. 9.3 and the said complex is arranged in an ordered manner which is suitably placed in between the layers of balata rubber without interrupting the crystalline nature of the later. But the broad peak of pure balata at 15 o is much more widened in the XRD of the whole composites and the peak at o is split in two when reinforced with nanocellulsoe. These findings support the formation of the ordered Zn-cellulose complex network and its orientation in the balata matrix in such a way that leads to the widening of the d-spacing in between the matrix layers. The equal splitting of the main peak at o in the whole nanocomposites suggests the ordered arrangement of two components as per the mechanism shown in Fig The interaction between the balata and the nanocellulose is proved by the additional peaks originating at 27 o in the nanocomposites XRD which is completely absent in the pure balata and the nanocellulose diffractograms. An additional ordered arrangement originates in the balata nanocomposite because of the formation of the proposed Zn-cellulose complex and the interaction between the matrix and the filler. The diffraction patterns of the various balata/nanocellulose do not exactly correspond to a simple mixing rule of the diffractograms of the two pure parent components. Indeed, the central peak (22.5 ) of the pure nanocellulose fibre is stronger than the experimental nanocellulose reinforced composite. By adding nanocellulose fibres, the peaks corresponding to A-type amylose allomorph become stronger, as expected. This shows that an increase of the cellulose content results in an increase of the global crystallinity of the composite material. This probably results from the different orientation of nanocellulose fibre within the composite of balata matrix film. The Zn-cellulose complex network distribution is most likely random in the composite structure contrary to the reference nanocellulose films, which is composed of in-plane oriented filler.

13 Bionanocomposites based on balata rubber (Swietenia macrophylla) 273 Furthermore, X-ray analysis confirmed that the processing by casting and evaporation at C did not affect the crystallinity of the nanocellulose. It is well-known that cellulose has four polymorphs, cellulose I, II, III, and IV. A detailed study is required to know whether any conformational transformation has occurred when the nanocellulose is introduced in an ordered matrix of balata rubber. The X-ray analysis of the NR and its composites is shown in Fig. 9.4 (c).the most important observation of these nanocomposites are the disappearance of the crystalline peak of the nanocellulose in the composite. The crystalline nature of the nanofibre is completely disappeared in the NR nanocomposites. The complete loss of the crystalline nature of the nanocellulose in the crosslinked composite is explained by the clear dispersion of the nanocellulose in the matrix of NR latex. The filler and the cross linking agents can be expected to enter in between the molecular chains of natural rubber making new bonds with each other. The breaking of the double bond in the natural rubber back bone makes two active sites and a cross linking network between sulfur and natural rubber. This makes the loss of crystallinity of cellulose in the composite. The amorphous nature and the cis configuration of the 1,4-polyisoprene back bone facilitate this interconnected network. Ultra-sonication causes an increase of amorphous phase of cellulose, making favourable atmosphere for the breakage of inter and intramolecular hydrogen bonds which could produce a large number of free OH groups at the surface of nanocellulose. The resultant OH groups at the cellulose surface were activated, and form complexes with Zinc (II) during the prevulcanisation of the composite. The incorporation of fillers cause interruption in the alignment process of chains. When filler loading is increased, weak interfacial regions between filler surface and rubber matrix are formed.

14 274 Chapter 9 The resultant Zn-cellulose complex diminishes the crystalline nature of the composite and dispersed with metal ion. [11]. The balata composites give a different picture because of the presence of the trans configuration of the rubber molecule. The cellulose molecules and balata chains get interacted and make new ordered arrangement with each other. This reinforcement gives two additional crystalline peaks, one at 2 theta of 18.5 o and the other at 26.4 o for the composites. The absence of these peaks at gum balata and nanocellulose reveals the introduction of new interactions between the balata and nanocellulose. The balata composites retain the crystalline peaks of the gum balata and nanocellulose in addition to the additional crystalline peaks of 2θ at 22 and at 26.4 o. These interactions strengthen the resulting composite which is evident from the mechanical and dynamic mechanical results of the prepared nanocomposites, discussed at the later part of this chapter. Hence the dispersion of the nanocellulose is somewhat ordered and the interactions between the matrix and the filler is strong in this case. The crystalline nature revealed from the XRD peaks confirm further the applicability of balata latex in situations where an ordered arrangement is needed. The ordered arrangement makes them more resistant to organic solvents in diffusion than natural rubber Mechanical properties of the composite The tensile properties of the NR and balata nanocomposites are done as per the procedure described in The importance of nanocellulose reinforced composites of polymeric materials come from the substantial improvement in the strength and modulus of the resulting composite. The increase in the strength and modulus of the materials offer a possibility of composites in practical applications. The tensile strength values of balata and natural rubber are shown in Fig. 9.6 (a). The tensile strength values of balata and natural rubber

15 Bionanocomposites based on balata rubber (Swietenia macrophylla) 275 nanocomposites at different filler loading are shown in 9.6 (b) and (c) respectively. Fig. 9.6 (a) Mechanical properties of balata and natural rubber Fig. 9.6 (a) shows that the initial modulus for balata rubber is very much higher than that of NR. This trend is observed in their nanocomposites too. Natural rubber has an elongation at break more than 600% and the elongation of the balata rubber is less than 100%. The crystallinity and the ordered arrangement of molecular chains of balata rubber are clearly evident from these results. The amorphous natural rubber molecular chains possess longer elongation and lesser tensile modulus than their trans configuration counterparts. As the balata rubber is reinforced with nanocellulose, some strong interfacial bonds are formed between the filler and the matrix (Fig.9.3) which consequently increases the tensile strength of the nanocomposites. The increment of stress level is due to the interaction between the nanocellulose and the rubber. A good interface between the nanocellulose and the rubber is very important for a material to withstand at this higher level of stress. Nanocellulose serves as an ideal candidate as a filler, thanks to their high strength. Under load, the matrix

16 276 Chapter 9 distributes the force to the nanocellulose which carries most of the applied load. From the experimental results, the addition of nanocellulose enhances the mechanical properties of balata rubber- nanocellulose composites. An increase in the filler content decreases the micro spaces between the filler and the matrix. This will strengthen the filler matrix interfacial adhesion. As a result, the values of tensile strength shows an increasing trend with increasing filler content in the composite. The presence of hydroxyl groups in the nanocellulose is responsible for its inherent hydrophilic nature. Since the matrix, balata rubber is in its liquid latex form with its highest tendency of hydrophilicity, the compatibility between the balata rubber and hydrophilic nanocellulose is much more increased at the time of mixing and reinforcing stage. Thus the hydrophobic nature of balata rubber and hydrophilic nature of nanocellulose are compromised to some extent. As a result, it becomes very easy to compound hydrophilic nanocellulose dispersion with balata rubber latex along with other crosslinking agents which are in the water solution. This results in an efficient composite with good interfacial bonding. Of the three hydroxyl groups present in a cellulose anhydroglucose unit, one is the primary hydroxyl group at C 6, while the other two are the secondary hydroxyl groups at C 2 and C 3 positions. A three dimensional network of Zn-cellulose complex is formed within the composite entity and this network is formed by reacting with these hydroxyl groups [Fig.6.5 and Fig.9.3] and Zn metal which is from the accelerators. Due to this three dimensional network between the nanocellulose fibres, the interfacial bonding between the filler and matrix is increased in the resultant composites. This in turn increases the tensile strength of the composites at all mixing ratios compared to that of balata gum rubber. The increased interfacial bonding will increases stress transfer efficiency from the matrix to the

17 Bionanocomposites based on balata rubber (Swietenia macrophylla) 277 filler with a consequent improvement in the mechanical properties of the composites. Fig. 9.6 (b) Mechanical properties of the balata rubber/nanocellulose composites Fig. 9.6 (c) Mechanical properties of the natural rubber/nanocellulose composites

18 278 Chapter 9 The addition of the filler in to the rubber matrix is expected to increase the modulus of the composites. It is evident from Fig. 9.6 (b) that an increases in the nanocellulose content in the balata matrix results in an increase in the modulus of the composites. Reinforcing crystalline cellulose in the partially crystalline matrix, imparts overall crystallinity to the composite. Furthermore incorporation of fibre into the polymer matrix reduces the matrix mobility, resulting in stiffness of the composite. As a result, Young s modulus increases with an increase in the filler content of the composites. The tensile strength of the studied composites show a maximum at 1% nanocellulose in both the balata and natural rubber nanocomposites. The tensile modulus of the balata composites increases with increasing nanocellulose content. The 0.75 % nanocomposites has the modulus higher than 1%. It means that there exists a point which shows the maximum modulus around 0.75% nanocellulose. An interesting feature is the shape of the stress vs. strain curve for the composites: the high moduli, tensile strength, tenacity and the presence of a yield point in this curve do not resemble known behaviour for vulcanized rubber but they are rather similar to the behaviour observed for semicrystalline thermoplastics. It is clear from the Fig. 9.6 (b) and (c) that the initial tensile strength and the modulus of balata rubber nanocomposites is very much higher than the respective natural rubber counter parts. This is another evidence for the effective reinforcement between balata rubber and nanocellulose. The percentage elongation and elongation at break is much higher in natural rubber and its nanocomposites than the balata and its nanocomposites. The main reason for this observation is the crystalline nature of the balata matrix. It may be concluded that the nanocellulose are homogeneously dispersed as the Zn-cellulose complex network in the balata rubber matrix during the

19 Bionanocomposites based on balata rubber (Swietenia macrophylla) 279 composite formation which resulted a substantial influence in the mechanical properties of the composite and the partial preservation of the characteristic rubber elongation Dynamic mechanical analysis of balata/nanocellulose composite The dynamic mechanical properties of the NR and balata nanocomposites are done as per the procedure described in The reinforcing effect of nanocellulose in balata rubber was confirmed by DMTA of the prepared nanocomposites. Fig. 9.7 (a) shows the storage modulus versus temperature for balata and natural rubber. The evolution of the storage modulus corresponding to the unfilled balata matrix is typical for partially crystalline high molecular weight thermoplastic polymers. Even at low temperatures, the storage modulus of the balata rubber is higher than the natural rubber. The sharp drop in the modulus of the gum balata rubber observed at around - 50 o C corresponds to the main relaxation phenomenon of the matrix, associated with its glass transition. At room temperature (20 o C), the rubbery modulus is < 500 MPa Natural rubber Balata rubber Storage Modulus (MPa) Temperature ( o C) Fig. 9.7 (a) Storage modulus of balata and natural rubber

20 280 Chapter 9 Answer: Fig. 9.7 (a) shows the plot of storage modulus verses temperature at 1 Hz for both balata and NR films prepared by evaporation. At low temperature (-100 o C), the G remains almost constant. Decreasing the temperature successively increases the values of G and the highest modulus is observed after the glass transition temperature. A sharp modulus drop appears for NR sample around -70 C and -50 o C for balata, i.e., in the glassrubber transition zone determined from DMA measurements. This modulus drop corresponds to an energy dissipation phenomenon displayed in the concomitant relaxation process where tan delta passes through a maximum. This relaxation process involves cooperative motions of long chain sequences. Above T g the modulus becomes roughly constant over a wide temperature region called the rubbery plateau region (rubbery modulus) in the case of NR. But balata rubber shows a second drop at around 50 o C. The rubbery modulus is known to depend on the degree of crystallinity of the material. The balata rubber possesses crystalline character [Fig. 9.5 (a)] and the respective regions act as physical cross-links for the polymer. In this physically cross-linked system, the crystalline domains would also act as a filler particle due to their finite size, which would increase the modulus substantially. Thus modulus of balata is always higher than the NR rubber at every range of temperature. Fig. 9.7 (a) clearly reveals that the storage modulus is different for the two curves in the temperature range between -70 and 0 C. It is obvious that the rubbery modulus is much lower for the NR than balata in this range (the difference is started from 100 MPa). This could be a clear indication of the high degree of crystallinity present in the balata film, and the drop of modulus in balata rubber around 50 C in Fig. 9.7 (a) can be attributed to the melting of the crystalline regions during the temperature scan and the

21 Bionanocomposites based on balata rubber (Swietenia macrophylla) 281 retention of this amorphous state after subsequent quenching. The two-step modulus drop observed in Fig. 9.7 (a) for the balata rubber can be ascribed to the crystallinity of the material. This type of second step modulus drop is completely absent in NR which is an amorphous polymer. In the terminal zone of all curves, the elastic modulus becomes lower and lower with temperature and the experimental setup fails to measure it due to the flow of the material. But up to 50 o C, the rubbery modulus of balata film is very much higher than that of the NR. After the degradation of crystalline regions in the balata, the rubbery moduli are the same. Storage Modulus (MPa) % Balata composite 0.5% Balata composite 0.75% Balata composite 1% Balata composite Temperature ( o C) Fig. 9.7 (b) Storage modulus of balata/nanocellulose composite Fig. 9.7 (b) and (d) shows the storage modulus values of balata and natural rubber nanocomposites respectively. In the temperature range of 0 to o C, the storage modulus increases with an increase in the percentage of the reinforced nanofiller in both cases. This could be ascribed to the fact that in this temperature range balata and natural rubber are in the glassy state and the difference between the modulus of the host matrix and the reinforcing

22 282 Chapter 9 phase is enough to generate a significant reinforcing effect. The global evolution for for both the nanocomposite films are similar to their respective neat matrices except for the drop in balata rubber at -50 o C. At low temperature, i.e., below T g, the reinforcing effect of cellulosic nano particles was low, justifying the normalization of the modulus. Indeed, the exact determination of the glassy modulus depends on the precise knowledge of the sample dimensions. At room temperature, the films were somewhat soft and it was difficult to obtain a constant and precise thickness along these samples. Above T g, a much more significant reinforcing effect of the nanoparticles was observed. This high reinforcing effect could be assigned, as already reported in the literature [16] to a mechanical percolation phenomenon of cellulose nanoparticles. Above the percolation threshold, these nanoparticles connect and form a stiff continuous network linked through hydrogen bonding [17,18]. This effect was predicted from the adaptation of the percolation concept to the classical series-parallel model. In this model and at sufficiently high temperature, i.e., when the modulus of the matrix is much lower than the one of the percolating network, the elastic modulus of the composite is simply the product of the volume fraction and modulus of the rigid percolating network [19]. We have proposed a mechanism to justify the observed results from the balata nanocomposite and is shown in Fig The critical volume fraction at the percolation threshold (long-range connectivity) of cellulose nano fibres was around 1.5 vol.%, (2.5 wt.%), as determined from their high aspect ratio. The storage modulus of the cellulose nano fibre was found to be 10 GPa. A prediction of the tensile modulus for balata composite is possible since the reinforcing nano particles display a well-defined geometrical aspect within the composite structure. It is a good indication that the stiffness of the sample and the temperature

23 Bionanocomposites based on balata rubber (Swietenia macrophylla) 283 stabilization of the composite modulus most probably result from the formation of a hydrogen bonded nanocellulose network above the percolation threshold of the nanocellulose. Fig.9. 7 (c) shows the storage modulus versus filler percentage of balata rubber nanocomposites at a frequency of 1Hz. The storage modulus (E ) values for any glassy polymer are constant around 3x10 9 Pa. Molecular motions are largely restricted to vibration and short-range rotational motions in the glassy state. Increasing the amount of nanocellulose successively increases the values of E, and the composite with 1% nanocellulose shows highest modulus. The glass transition temperature, T g, of balata rubber is around -52 o C, and the enhancement in modulus even below this temperature is a better evidence for the strong reinforcing tendency of nanocellulose in the balata matrix. The T g of natural balata rubber is o C which is increased to o C when incorporated with 1% nanocellulose. Above T g, a higher increase of the storage tensile modulus is observed when adding nanocellulose to the balata rubber matrix. The increase in the storage modulus with an increase in the filler content is in agreement with the increasing cellulose/cellulose interaction probability and density of the proposed Zn-cellulose complex network. The homogenous nature of the balata/cellulose composite which is supported by the proposed mechanism in Fig.9.3 is again proved by the single drop in the storage modulus of their nanocomposites within the experimental temperature range +100 to -100 o C. Fig. 9.7 (d) shows the storage modulus of NR/nanocellulose composites and here also a similar trent of balata composite is observed. The broad temperature range from 40 to +70 o C of the rubbery plateau is ascribed to the high molecular weight of the polymer, resulting in an entangled state of the macromolecules. But this entanglement is somewhat less than a

24 284 Chapter 9 completely amorphous polymer like natural rubber whose glass transition temperature is around -70 o C. The T g of gum NR is o C which also gets upgraded to o C with 1% nanocellulose reinforced composite. The incorporation of the crystalline cellulose particles influences the glass transition temperature of the NR matrix. Above T g, a higher increase of the storage tensile modulus is observed when adding nanocellulose to the NR matrix. The increase in the storage modulus with an increase in the filler content is in agreement with the increasing cellulose-cellulose interaction. The homogenous single phase of the NR/nanocellulose composite which is proved by their XRD is supported by the single drop in the storage modulus within the experimental temperature range +100 to -100 o C. Fig. 9.7 (c) Storage modulus vs filler percentage of balata/nanocellulose composite

25 Bionanocomposites based on balata rubber (Swietenia macrophylla) % NR composite 0.5% NR composite 0.75% NR composite 1% NR composite Storage modulus (M Pa) Temperature ( o C) Fig. 9.7 (d) Storage modulus of NR/nanocellulose composite Thus it is clear from Fig. 9.7 (b) and (d) that the highest storage modulus value of the 1% balata composite is 4850 MPa while that for NR composite is 4150 MPa. It has been found that the tensile modulus is higher for balata composites than NR composites. Because of the difference in the compatibility, the interfacial adhesion between nanocellulose and balata rubber should be higher than the interfacial adhesion between natural rubber and nanocellulose. In addition, the trans geometry and crystallinity of the balata rubber contributes to the higher modulus value. Another reason is that the extractive substances and fatty acids in the balata latex as a compatibilizer between the filler and the matrix. This effect is not promising in the case of NR composites where the fatty acid content is comparatively low. This result agrees with the lower elongation at break reported for nanocellulose reinforced balata films. At low particle content, the adhesion between the two parent materials probably dominates the mechanical behaviour and superior stiffness is observed for nanocellulose from both

26 286 Chapter 9 tensile and storage modulus measurements. At higher particle contents, the percolation effect of the improved strength has earlier been mentioned to be associated with percolation threshold, the mechanical behaviour resulting in higher mechanical performances from DMA measurements for balata composites than NR materials Thermo gravimetric analysis (TGA) of balata nanocomposites The thermal degradation properties of the nanocellulose, NR and balata nanocomposites are done as per the procedure described in The thermal degradation of the individual system nanocellulose, balata and natural rubber matrices and related composites has been investigated in terms of weight loss and weight loss rate, in order to further analyze the effects of the processing conditions. The effect of nano fillers on the thermal stability of balata and natural rubber was studied using TGA method and gives the information about the stability and thermal degradation. Fig. 9.8 (a), (b) and (c) shows the thermo gravimetric analysis of balata and NR and its nanocomposites with nanocellulose. When a rubber compound is heated at a lower temperature, the volatile components will be evaporated first and the small dip around 100 o C is due to the moisture present in the sample. On continued heating, the corresponding loss in weight arises from the degradation of the rubber, which is reflected in the curve. Thermal degradation of nanocomposites occurs as a result of chain scission, crosslink formation and crosslink breakage Fig. 9.8 (a) shows the degradation of gum balata, natural rubber and nanocellulose. It is obvious that natural rubber is more thermally stable than balata rubber but the percentage of the final char is higher in balata rubber. The decomposition temperatures for balata, NR and nanocellulose are 289.8,

27 Bionanocomposites based on balata rubber (Swietenia macrophylla) o C and o C respectively. Stereo specific structure of the cis counterpart makes it more stable to temperature than its trans configuration. Additionally, the lower molecular weight and the presence of the traces of proteins like materials in the balata rubber make it less thermally stable than natural rubber. The presence of oxygen acid amides from various proteins present in the balata rubber makes them more susceptible to thermal degradation than NR. Fig. 9.8 (a) TGA of balata, natural rubber and nanocellulose Fig. 9.8 (b) shows the degradation of the balata rubber nanocomposites. The balata composites show a higher thermal stability than the gum balata which is unexpected. Interestingly, at certain regions the decomposition temperature of the balata nanocomposites is higher than the decomposition of gum balata and nanocellulose. As discussed in the FTIR, XRD and dynamic properties, some additional chemical interactions have happened between the balata and the nanofibre during the composite formation stage.

28 288 Chapter % Balata composite 0.5% Balata composite 0.75% Balata composite 80 1% Balata composite Weight loss (%) Temperature ( o C) Fig. 9.8 (b) TGA of balata/nanocellulose nanocomposites As per the proposed mechanism in Fig. 9.3, the existence of three dimensional network of Zn-cellulose complex and the interaction between the balata and the nanocellulose is again proved by the TGA results. The said network and the interaction are formed in such way that increases the stability of the resultant nanocomposite. Thus these additional chemical interactions make the composite more stable to thermal degradation. This is evident from the thermo gravimetric analysis. The same information is given by the decreased intensity of the hydroxyl concentration in FTIR analysis and increased crystallinity by XRD and dynamic mechanical analysis of the nanocomposites. The increased percentage of nanocellulose content in the composite makes them more stable to thermal degradation. Thus the global thermal stability of the nanocomposite increases with increasing the nanocellulose percentage. The thermal degradation of 1% nanofibre reinforced balata composite has a thermal stability up to 316 o C where as the maximum thermal stability of the gum balata is 289 o C. The percentage increment in thermal stability of 1% nanocellulose reinforced balata

29 Bionanocomposites based on balata rubber (Swietenia macrophylla) 289 composite from the gum balata rubber is 8.2 and the corresponding NR rubber counterpart shows a decrease of around 6% (from to o C). An increase in the thermal stability of 8.2% with the addition of 1% nanofibre is unusual and this stability makes the balata composite suitable for application which needs higher thermal resistance. It has been shown that thermal ageing resistance of silica filled chlorinated poly (ethylene) (CPE)/natural rubber (NR) blends does not change significantly with an increase in silica loading [20]. Synthetic lattices do not exhibit the variability in natural lattices due to the control which is exercised during the preparation. Because of strain induced crystallization behaviour of NR, it possesses higher tensile strength and viscosity than synthetic lattices. However, NR/nanocellulose composites exhibit poor modulus, stability towards fillers, thermal resistance, and gas barrier properties when compared with balata /nanocellulose composites. Fig. 9.8 (c) is the TGA of the NR/nanocellulose composites and it is obvious that in the presence of nanocellulose, the decomposition temperature of the natural rubber is shifted to a low temperature range which is contrary to the balata composite decomposition. The nanocellulose shows only a single major degradation. In the case of both the rubber and its nanocomposites, the first stage degradation is due to the deterioration of polymer chain into simple products. The second stage decomposition is the volatilization of the products formed in the first step. The interaction between the NR and the nanocellulose is weak when compared to balata counterparts and their thermal stability is the sum of the thermal stability of the pure NR and the nanocellulose. Thus the thermal stability of nanocellulose filled balata samples are higher than the unfilled system. This can be explained by the well defined nanomeric level of dispersion of cellulose in the matrix in

30 290 Chapter 9 addition to the possibilities of chemical reactions between balata and nanocellulose. The polymer cellulose interaction is equivalent to or as competent as cellulose cellulose interaction. During latex stage mixing and ultrasonication, the mechanical stress applied is sufficient to reduce the molecular weight of rubber particles to create active centers for interaction with filler. Therefore, fillers are well dispersed in these systems. The homogeneous distribution and thereby the increased interaction between the nanofibre and matrix with the increasing of the filler increases the thermal stability of balata composites. 9.3 Conclusion Fig. 9.8 (c) TGA of NR/nanocellulose nanocomposites Nanocellulose isolated from banana fibre by the steam explosion process is found to be a good candidate as reinforcing filler for the preparation of green nanocomposites with a matrix of balata rubber latex. The nanofibre is very long, flexible, whereas the matrix occurs as latex globules in liquid form and semi crystalline rubber in dried solid form. Unlike natural rubber, balata

31 Bionanocomposites based on balata rubber (Swietenia macrophylla) 291 rubber shows a crystalline nature and higher modulus value because of its trans conformation. The crystalline nature gives them additional properties even in the composite entity when compared to other rubbery materials especially natural rubber. Balata-nanocellulose composites have flexibility and possibility of entanglements between the filler and matrix and the adhesion between the nanocellulose and the balata rubber is not merely physical as usual composites but chemical. The incorporation of nanocellulose in balata rubber causes an interruption in the alignment of the polymer chains by inducing additional network of a Zn-cellulose complex. When filler loading is increased, strong interfacial regions between filler surface and rubber matrix are formed and the resultant Zn-cellulose complex influences the properties of the resultant nanocomposites [10,11]. From the FTIR spectra it has been found that the intensity of the hydroxyl transmittance of all the composites is less than the intensity of both the neat balata matrix and the nanocellulose. When the nanocellulose fibres are added to the balata matrix, the free hydroxyls in both the balata matrix and the nanocellulose fibres become bound, and the total free hydroxyl absorbance is concomitantly reduced. The amount of nanocellulose is increased by increasing its percentage, the number of free hydroxyls again continue to decrease up to 0.75% nanocomposite, as all of the hydroxyl components from the balata matrix has been used up. The increased intensity of the hydroxyl density (3300 cm -1 ) of the FTIR of the balata nanocomposites [Fig.9.4 (a)] with nanocellulose percentage reveals the hydroxyl groups are free to form a three dimensional network as proposed by the mechanism. It is interesting to note that the gradual increase of the peak at 2400 cm -1 in balata nanocomposites with increasing percentage of the

32 292 Chapter 9 nanocellulose. This peak corresponding to the stretching vibrations of the C- H bond in CH 3 groups of poly isoprene units. The increase in the intensity of this peak with increase in the nanocellulose content is due to the increased distance in between the individual rubber molecules. The insertion of nanocellulose between the rubber molecules give more freedom for the stretching of the pendant -CH 3 groups of poly isoprene units. The XRD of balata composites give a different picture from the NR. The cellulose molecules and balata chains get interacted and make new ordered arrangement with each other. This reinforcement gives two additional crystalline peaks, one at 2 theta of 18.5 o and the other at 26.4 o for the composites. The absence of these peaks at gum balata and nanocellulose reveals the introduction of new interactions between the balata and nanocellulose. The balata composites retain the crystalline peaks of the gum balata and nanocellulose in addition to the additional crystalline peaks of 2θ at 22 and at 26.4 o. These interactions will strengthen the resulted composite which is evident from the mechanical and dynamic mechanical results of the prepared nanocomposites. The XRD analysis of the balata nanocomposites gives additional crystalline arrangement at 18 o and 27.5 o and proves the evidence for the three dimensional ordered arrangement of the Zn-cellulose- BR complex which is absent in natural balata rubber. These two peaks are responsible for the ordered arrangement of crystalline cellulose and Zncellulose complex form. From the mechanical analysis it may be concluded that the nanocellulose are homogeneously dispersed as the Zn-cellulose complex network in the balata rubber matrix during the composite formation. The increase of nanocellulose content in the balata matrix causes a substantial influence in the mechanical properties of the composite by increasing the Young moduli and tensile strength.

33 Bionanocomposites based on balata rubber (Swietenia macrophylla) 293 The DMA analysis show that the storage modulus is found to be systematically higher for balata composites than NR composites. Because of the difference in the compatibility, the interfacial adhesion between nanocellulose and balata rubber should be higher than the interfacial adhesion between natural rubber and nanocellulose. At higher nanocellulose content, the percolation effect of the improved strength has been associated with percolation threshold and the resulting dynamic mechanical behaviour are higher for balata composites than NR materials. The glass transition temperature, T g, of balata rubber is around -52 o C, and the enhancement in modulus even below this temperature is a better evidence for the strong reinforcing interaction of nanocellulose in the balata matrix. The T g of natural balata rubber is o C and it is increased to o C when incorporated with 1% nanocellulose. Above T g, a higher increase of the storage tensile modulus is observed when adding nanocellulose to the balata rubber matrix. The increase in the storage modulus with an increase in the filler content is in agreement with the increasing cellulose/balata rubber interaction probability and density of the proposed Zn-cellulose complex network. The proposed mechanism in Fig.9.3 is again supported by the single drop in the storage modulus of the nanocomposites within the experimental temperature range +100 to -100 o C. We can expect chemical interactions between nanocellulose and balata rubber in addition to the physical interaction. The chemical interactions include the reaction between crystalline polar nanocellulose and semi crystalline balata rubber and the Zncellulose three dimensional networks in the crosslinked balata nanocomposite which is shown in Fig The TGA analysis reveals that the balata composites show a higher thermal stability than the gum balata and the decomposition temperature of the balata