a. Mechanical characterization
The tensile strength, elongation at break, and elastic modulus of the PVA/cellulose fibers composites compared with those of unreinforced PVA are shown in Figs. 6 – 8.
Concentration of cellulose fibers [%]
 |
|
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It is apparent that the three types of fibers differently influence the mechanical properties of PVA matrix. U and H fibers lead to an improvement of mechanical properties, both the strength and modulus and elongation at break being enhanced and the type L to a decrease in strength and stiffness. The observed differences are mainly due to the different size of fibers, U and H fibers having diameters on the nanoscale while L being micron sized fibers.
It is known that an important increase in strength and stiffness of polymer matrix can be obtained at concentrations above 30% of micron sized cellulose fibers, lower concentration of 20 – 25% being effective only in the case of a good treatment of fibers or coupling agent adding. In these composites, elongation at break knows significant decreases, polymer matrix ductility being severely altered by micron sized fibers (Bledzki et al., 2003; Ganster et al., 2006). Hence, it was not expected that the low concentration of L fibers could improve PVA mechanical properties.
In contrast to conventional filled polymer systems where the increase of strength and stiffness is accompanied by an important decrease of ductility, in nanocomposites a simultaneous increase of Young’s modulus, tensile strength and elongation could appear, especially in the case of strong filler/matrix interface (Lin et al., 2009). Therefore, the incorporation of U and H fibers in PVA which provide a significant improvement of all mechanical properties (strength, stiffness and ductility) is a proof of a good interfacial adhesion in our composites.
The highest improvement of tensile strength and modulus in regard to PVA (Fig. 6 and 8) was obtained for PVA/U composites, 5 wt% U in PVA: almost 50% and 40%, respectively. The different behavior of U and H fibers as reinforcements in PVA could be a result of their different size (in the case of U fibers, the nanosized fraction were separated from the coarse fraction) and the higher tendency to form agglomerates in the case of higher aspect ratio fibers (H). The enhancement of mechanical properties of PVA/U and PVA/H composite films by comparison with neat PVA resulted from the good adhesion at the filler/matrix interface, favored by the small size of fibers and, accordingly, their high surface area. The hydrogen bonding between the OH groups of cellulose fibers and similar group of PVA matrix led to improved adhesion between phases which resulted in improved mechanical properties. Lee et al. (2009b) explained an increase of about 70% of the elastic modulus and up to 55% of the tensile strength by the intermolecular forces between cellulose fibers and PVA matrix.
b. Thermal characterization
The thermal properties of cellulose fibers reinforced PVA were determined from DSC and DTG thermograms. The main thermal transitions, glass transition temperature (Tg) and melting point (Tm), were evaluated and compared with those of the neat PVA. DSC results of PVA composites containing 5wt% of cellulose fibers, L, H and U types, are shown in Fig. 9.
PVA exhibits an endotherm close by 57°C (57.20C) corresponding to the glass transition temperature of PVA. The appearance of one Tg in the nanocomposite samples highlights the good interaction of cellulose fibers and PVA in the amorphous phase. No significant changes in Tg value are observed in the case of PVA composites containing 5 wt% U or H fibers as shown in Table 1, but a decrease with four degrees is detected in PVA filled with 5 wt% L fibers as regard to neat PVA. This behavior is caused by the decrease of the cohesive energy density in the amorphous phase of the PVA/L composites in comparison with neat PVA and can be explained by the rupture of the hydrogen bonds in PVA because of larger
size of L type fibers (tens of microns in diameter and hundred of microns in length). As a result, the rubbery properties and the increase of the mobility of the macromolecules will appear from lower temperatures. Nanometer sized fibers, H and U, will not cause massive breaks of hydrogen bonds in PVA and, the expected interactions between these cellulose fibers and PVA will not alter the supramolecular structure of the amorphous phase. Nevertheless it seemed that these new formed interactions are not strong enough to slow the chain mobility associated with glass transition since the Tg values measured on PVA/H and PVA/U composite samples are close to that of neat PVA.
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Fig. 9. DSC diagrams of neat PVA and PVA composites containing U, L and H fibers |
|
Samples |
Tg 0C |
Tm 0C |
AHm J/g |
Xc % |
|
PVA |
57.2 |
227.1 |
79.3 |
48.7 |
|
PVA/5%U |
56.8 |
226.9 |
70.5 |
45.5 |
|
PVA/5%L |
53.0 |
227.2 |
60.0 |
38.7 |
|
PVA/5%H |
57.0 |
227.3 |
78.1 |
50.4 |
 |
 |
PVA exhibits a sharp endothermic curve with a peak at 227.10C, as shown in Fig. 9, corresponding to the melting of the crystalline phase of PVA. Tm values remains roughly constant for all PVA composites whatever the type of fiber used but the heat of fusion is different depending on the filler characteristics. The degree of crystallinity (Xc) of the PVA component in the composite was obtained as follows:
where AHf and AHo are the heats of fusion for PVA and 100% crystalline PVA, respectively and w is the mass fraction of PVA in the composite. AHo was taken 163 J/g (Ramaraj et al., 2010).
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A highest degree of crystallinity is observed in PVA/H composite and the lowest in PVA/L composite. PVA filled with H type fibers show higher crystallinity than neat PVA. This could be ascribed to stronger interactions between cellulose fiber surface and adjacent PVA chains. The nucleating effect of cellulose fibers could also explain this increase of crystallinity. PVA/H composite shows a higher crystallinity as regard to PVA/U composite but the mechanical behavior of the latter is better. This is due to the fact that, besides the matrix crystallinity, other factors influence the tensile properties of the composite. The nanoscale dispersion of the filler and its orientation in the matrix are among these factors.
Thermogravimetric analysis (TGA) was used to investigate the effect of cellulose fibers on the thermal stability of the composites. In the obtained PVA thermograms (Fig. 10) three
main weight loss regions can be observed (Lee et al., 2009b; Qua et al., 2009). All the samples show an initial weight loss in the region 75 – 150 0C caused by the evaporation of water. Between 2.5 and 3.5 wt.% physically and chemically bound water was detected in neat PVA and PVA composites thermograms. Figure shows that the second degradation region is located between 220 and 300 0C and is due to the pyrolysis of cellulose fibers and to the degradation of PVA films, the weight loss being around 70% for all the samples. As reported by Qua et al. (2009), the second stage of degradation mainly involves dehydration reactions and the formation of volatile products.
The third stage weight loss occurrs above 400 0C and consists of decomposition of carbonaceous matter (Lee et al., 2009b).
It can be observed in Fig. 10 that the differences in TGA curves are negligible for neat PVA and PVA composites. However, some differences can be detected in the case of PVA/H composite. TGA’s first order derivative (Fig. 11) show a broadening of the main decomposition peak and a shift of the onset temperature for the third decomposition stage to higher temperatures. This can be due to sulfonic groups bound to the cellulose fibers by acid hydrolysis which influence the degradation process.
The onset degradation temperature (Td) and enthalpy (AHd) could be easily determinated from DTG curves (Table 2). Slightly higher onset degradation temperatures were obtained for PVA composites as regard to neat PVA, showing a marginal increase of the thermal stability caused by the cellulose fibers. Lee showed that the thermal stability of PVA composites was improved with the increase of the nanocellulose loading (Lee et al., 2009b).
|
|
|
Samples |
Td 0C |
AHd J/g |
|
PVA |
238.7 |
513.8 |
|
PVA/5%U |
242.9 |
509.2 |
|
PVA/5%L |
239.7 |
480.5 |
|
PVA/5%H |
239.8 |
525.3 |
|
Table 2. DSC data of neat PVA and PVA composites containing 5 wt.% fibers |
The lowered PVA/L decomposition enthalpy was thought to be related to the more complex structure of these fibers containing beside cellulose, hemicellulose and lignin and to the more porous fiber structure.
5. Conclusion
The application of cellulose nanofibers in polymer reinforcement is a relatively new research field. The development of fully biodegradable nanocomposites is still a challenging area.
In this chapter discussion is focused on the physical and mechanical properties of polyvinyl alcohol as a biodegradable matrix reinforced with cellulose fibers prepared by different methods. Three types of fibers with different characteristics in terms of composition, size and aspect ratio were tested as reinforcements in PVA: L fibers of micron size and high aspect ratio contain beside cellulose, hemicellulose and lignin, H fibers with a nanosize dimension and high aspect ratio and U fibers with a nanosize dimension and low aspect ratio contain only cellulose.
These cellulose fibers were obtained by mechanical treatment, acid hydrolysis and ultrasound treatment, respectively. The isolated cellulose fibers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Our work was focused on studying the changes induced in PVA characteristics by low concentration of these cellulose fibers. Similar conditions were applied in order to prepare PVA composite films, knowing the high influence of preparing and characterization conditions on PVA properties, in order to select the proper system PVA/ cellulose fibers for a target application.
The obtained composites with low concentration of cellulose fibers (1 – 5 wt.%) showed improved mechanical properties, preserving the transparency and flexibility of the original films. The highest improvement of the tensile strength and modulus in regard to PVA was obtained for PVA composite containing 5 wt% cellulose fibers prepared by ultrasound treatment: almost 50% and 40%, respectively. Favorable interfacial properties and the lack of agglomerations at low fibers concentration in PVA were supposed to explain the high values of the mechanical properties.
PVA filled with acid treated cellulose fibers showed higher crystallinity than neat PVA, as resulted from DSC analysis. Stronger interactions between cellulose fiber surface and adjacent PVA chains and the nucleating effect of cellulose fibers were proposed to explain this increase of crystallinity. Slightly higher onset degradation temperatures were obtained for PVA composites as regard to neat PVA, showing an increase of the thermal stability caused by the addition of cellulose fibers.
The biocomposites presented in this chapter are advanced materials with improved mechanical and thermal properties, high transparency and flexibility and large possibilities of application in packaging and other fields.
Future work will be focus on the study concerning the processing behavior of these materials and on achieving new preparation methods with industrial application.
