Poly(vinyl

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Ind. Eng. Chem. Res. 2010, 49, 2176–2185

Preparation and Characterization of Cross-Linked Starch/Poly(vinyl alcohol) Green Films with Low Moisture Absorption Kunal Das,† Dipa Ray,*,† N. R. Bandyopadhyay,‡ Anirudhha Gupta,† Suparna Sengupta,§ Saswata Sahoo,| Amar Mohanty,| and Manjusri Misra⊥

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Department of Polymer Science and Technology, UniVersity College of Science and Technology, UniVersity of Calcutta, 92 A.P. C Road, Kolkata 700009, India, School of Materials Science and Engineering, Bengal Engineering and Science UniVersity, Shibpur, Howrah 711103, India, Indian Association for the CultiVation of Science, 2 A & B Raja S. C. Mallick Road, Calcutta 700032, India, and Bioproducts DiscoVery and DeVelopment Centre, Department of Plant Agriculture, Crop Science Building, and School of Engineering, Thornbrough Building, UniVersity of Guelph, Guelph, ON, Canada N1G 2W1

The effects of incorporating different cross-linking agents into starch/poly(vinyl alcohol) (PVA) blend were examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), mechanical characterization, dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), moisture absorption tests, and scanning electron microscopy (SEM). Different cross-linkers such as borax, formaldehyde, epichlorohydrin, and ZnO were used to cross-link the films at the same weight percent. The films were prepared by gelatinization followed by a solution casting method. SEM microstructures revealed that the destructuring of starch granules was significantly influenced by the presence of cross-linkers during the gelatinization process. The extent of cross-linking was measured through the determination of the normalized gel mass and the normalized swelling degree of the films in dimethyl sulfoxide (DMSO) solvent. Tensile strength and tensile modulus were highest in borax cross-linked film, while the highest flexibility was achieved with the epichlorohydrin cross-linked film. Damping was remarkably increased, and moisture absorption was considerably decreased in the cross-linked films. The onset of thermal degradation was lowered, but the char yield was slightly increased in the cross-linked films. 1. Introduction There has been a growing interest in the use of biodegradable polymers in order to reduce the environmental pollution caused by plastic wastes. So there is an urgent need to develop renewable resource-based, environmentally benign materials, which will reduce the problem of plastic wastes and will be a solution to the uncertainty of the petroleum supply. Among different kinds of biodegradable polymers, starch is most promising with immense potential and low price. But starch is brittle and has poor mechanical properties and high moisture sensitivity. To overcome these drawbacks and to make starchbased products commercially acceptable, it is suitably modified or blended with other materials.1-9 Poly(vinyl alcohol) (PVA), which is a biodegradable and water-processable synthetic polymer, is often blended with starch to modify its properties. But starch/PVA blends also suffer from various shortcomings. Some researchers have worked on starch/PVA blends to modify their properties. Preparation of starch/PVA/citric acid films by casting method and investigation on the effect of citric acid on the mechanical properties and water resistance of the films were reported by some researchers.10,11 Some researchers studied the performance and biodegradation behavior of starch/PVA blend films.12 They used hexamethoxy methylmelamine as the crosslinking agent in the presence of a catalytic amount of citric acid. They reported that the cross-linked films showed little or no disintegration when exposed to aqueous environment, and cross* To whom correspondence should be addressed. Tel.: +91-0332350 1397. Fax: +91-033-2351 9755. E-mail: roy.dipa@ gmail.com. † University of Calcutta. ‡ Bengal Engineering and Science University. § Indian Association for the Cultivation of Science. | Department of Plant Agriculture, University of Guelph. ⊥ School of Engineering, University of Guelph.

linking did not affect the biodegradability of the films. Starch/ PVA hydrogels were prepared by some workers using ultraviolet radiation.13,14 Some researchers15 prepared cross-linked starch/ PVA blends using various plasticizers such as poly(ethylene glycol) and glycerol. The cross-linking of starch/PVA blends was carried out with epichlorohydrin. The results showed a decrease in intensity of the -OH band due to cross-linking. They also studied thermal, mechanical, and surface properties of borax cross-linked starch/PVA.16 They reported that the mechanical properties and thermal stability of starch/PVA blends treated with borax were higher than the pure blends. An optimization of water absorption behavior of the starch/PVA blend was reported by some scientists.17 They used borax as cross-linker in the presence of citric acid and observed that water absorption was increased with the increase in starch content. They reported that the cross-linking with borax improved the mechanical and thermal properties of starch/PVA blends. The physicomechanical, thermal, and swelling properties of cross-linked starch/PVA composite films were reported by Ramaraj.18 They observed that the tensile modulus and solubility resistance improved in the composite films, but there was a decrease in tensile strength, tensile elongation, and burst strength. They cross-linked PVA with glutaraldehyde and blended with starch. The cross-linked films showed significant improvement in tensile strength, tensile modulus, tear strength, and burst strength over the un-crosslinked films. Extruded corn starch/glycerol/PVA blends were prepared by Mao et al.19 Significant improvement of both tensile strength and elongation break was observed by them. They reported in their dynamic mechanical analysis that the decreasing trend in glass transition temperatures (Tg values) was proportional to the glycerol content. Jiang et al.20 prepared thermoplastic blends by melt blending of acetylated starch (TPAS) and poly(ethylene-co-vinyl alcohol) (EVOH). They observed a

10.1021/ie901092n  2010 American Chemical Society Published on Web 02/02/2010

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Table 1. Composition of Plasticized Starch-Based Films sample ID

starch (wt %)

PVA (wt %)

glycerol (wt %)

zinc oxide (wt %)

formaldehyde (wt %)

borax (wt %)

epichlorohydrin (wt %)

U Z F B E

50 50 50 50 50

50 50 50 50 50

30 30 30 30 30

0 5 0 0 0

0 0 5 0 0

0 0 0 5 0

0 0 0 0 5

significant improvement in the mechanical properties. Good molecular interaction and miscibility between the components were clearly observed in their dynamic mechanical analysis. Li et al.21 reported the detailed study of dynamic mechanical analysis of starch modified with carboxymethylcellulose (CMC), and they observed a very high storage modulus value in the case of modified starch. Tough thermoplastic starch modified with polyurethane microparticles were successfully prepared by Wu et al.22 in an intensive mixer. Han et al.23 elaborately reported the strong interaction of starch and PVA through X-ray diffraction (XRD) analysis. Film blowing of thermoplastic starch was done by Thunwall et al.24 from natural, oxidized, and hydroxyl propylated grades of the native material. They reported that storage modulus values increased in the case of hydroxy propylated starch compared to the native one. Thus, various attempts are being made by researchers to overcome the inherent drawbacks of starch in various ways. Some work has also been done on cross-linking of starch/PVA blend, but extensive research is yet to be done to establish their suitability as low-cost packaging material. In this work, we prepared starch/PVA films cross-linked with different crosslinkers by a solution casting process. Four different types of cross-linking agents were used, such as epichlorohydrin, borax, formaldehyde, and ZnO at the same concentration (5 wt % with respect to the dry weight of the starch and PVA). There are a few reports available on the use of epichlorohydrin and borax as cross-linking agents for starch/PVA blends. Formaldehyde and ZnO were chosen as the two other cross-linkers on the basis of their chemical reactivity and applications. The films were characterized by XRD, Fourier transform infrared spectroscopy (FTIR), scanning electron microcopy (SEM), mechanical testing, moisture absorption, dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). An attempt has been made to establish their structure-property correlation.

and pH 10 was maintained by the dropwise addition of 0.5 N NaOH solution. Then gelatinization as well as cross-linking reaction was carried out at 80 °C for 2 h. Glycerol was added

2. Experimental Section 2.1. Materials. Soluble starch (Merck) was used as the matrix material. The poly(vinyl alcohol) used was a CDH Co. product with a molecular weight of approximately 14000 and was soluble in hot water. Glycerol, a SRL product, was used as the plasticizer. Epichlorohydrin (CDH product; MW, 92.53), zinc oxide (Merck product; MW, 81.71), borax (SRL product; MW, 201.22), and formaldehyde (CDH product; 37-41% (w/v)) were used as cross-linkers in different sets. 2.2. Gelatinization of Starch and Preparation of Film. A calculated amount of starch and PVA were added to distilled water, and the mixture of starch was gelatinized at 80 °C for 120 min. Glycerol was added to the hot solution (30 wt % based on the total dry weight of starch and PVA). The solution was then poured onto a polypropylene dish and was allowed to dry by evaporation at room temperature (U). Four sets of crosslinked films were prepared with four different cross-linkers such as epichlorohydrin (E), zinc oxide (Z), formaldehyde (F), and borax (B). In each case, 5 wt % of the cross-linking agent was used. For preparing epichlorohydrin, zinc oxide, and formaldehyde cross-linked films, the cross-linking agent was added to the calculated mixture of starch and PVA in distilled water

Figure 1. XRD graph of (a) dry starch granules, gelatinized starch with glycerol, and PVA film with glycerol and (b) un-cross-linked and crosslinked films (U, Z, E, B, and F). Table 2. Percent Crystallinity and Crystallite Size of the Cross-Linked Starch/PVA Films peak at 2θ ) 19.5° sample ID

% crystallinity

crystallite size (nm)

U Z F B E

22.7 21.9 50.5 47.2 58.4

8.8 7.5 10.2 8.8 6.7

peak at 2θ ) 17.1° % crystallinity

crystallite size (nm)

30.2 27.0 46.7 38.4 29.7

16.1 21.3 10.2 12.8 12.1

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to the hot solution at a 30 wt % ratio, based on the total dry weight of starch and PVA. The solution was then poured onto a polypropylene dish and was allowed to dry by evaporation at room temperature. For the borax cross-linked film (B), the crosslinker was added at the last stage when gelatinization was over and was cast immediately. The chemical compositions of the starch/PVA blends are given in Table 1. 2.3. Testing. 2.3.1. X-ray Diffraction. X-ray diffraction studies of the samples were carried out by high-resolution X-ray diffractometer (Expert PRO) with a scanning rate of 2°/min with Cu KR radiation operating at 45 kV and 40 mA within a scan range of 2θ ) 2-60°. 2.3.2. Swelling Degree and Gel Mass Determination. The degree of cross-linking was characterized by measuring the gel mass (GM) and swelling degree (SD) of the film. The determination of GM and SD was carried out following the method described in selected literature.25,26 The un-cross-linked as well as the cross-linked films were immersed in DMSO solvent in which the starch/PVA films are completely soluble. After 2 h, the swollen film was taken out, filtered out, and weighed. The weight of the insoluble, swollen part was referred to as ms. Afterward, the insoluble part was first rinsed in water and then in ethanol in remove DMSO. The insoluble part was dried at 80 °C for 6 h and weighed. The dried weight was referred to as md. The surface area normalized gel mass (NGM) and swelling degree (NSD) were calculated by the following formulas.

NGM ) md /A

(1)

NSD ) ms - md /(mdA)

(2)

where A is the surface area of the samples immersed in the solvent. 2.3.3. SEM Study. The surface morphology of the film samples was investigated with HTACHI S 3400N, using a voltage of 15 kV by coating with Au. 2.3.4. FTIR Analysis. FTIR analysis was done in the ATR mode with an FTIR-8300 SHIMADZU. 2.3.5. Mechanical Testing. Tensile tests were done as per ASTM D 882-97 using Instron 5500R. Ten samples were tested for each set, and the mean value was reported. 2.3.6. Dynamic Mechanical Analysis. DMA was done in the temperature range of -50 to 120 °C with DMA Q800 V20.9 Build 27, maintaining a frequency of 1 Hz and amplitude of 15 µm in tensile mode. 2.3.7. Moisture Uptake Test. The moisture uptake test was carried out by taking films of dimensions of approximately 20 × 15 × 0.28 mm3. The films were dried, weighed, and conditioned at 27 °C in a desiccator containing KNO3 saturated solution to ensure 93% RH. The samples were weighed at desired intervals until an equilibrium weight was reached. The moisture uptake (%MU) of the samples was calculated as follows:

Figure 2. Schematic diagram showing the probable cross-linking reactions in the films.


MU/% ) {(Mt - M0)/M0} × 100

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(3)

where Mt and M0 are the weight of the sample at time “t” in 93% RH and the initial weight of the sample before exposure to 93% RH, respectively. At short times, the mass of water (Mt s M0) sorbed at time t can be expressed as27 Mt - M 0 2 ) M∞ L

ΠD √t

(4)

where M∞ is the mass sorbed at equilibrium, 2L is the thickness of the polymer film, and D is the diffusion coefficient. Diffusion coefficients of the biocomposite films are calculated according to eq 4. 2.3.8. Thermogravimetric Analysis. Thermogravimetric analysis of the film samples was done with TGA-Model Pyris Diamond TGA/DTA (Perkin-Elmer; DTA, differential thermal analysis) with a heating rate of 5 °C/min in a nitrogen environment. 3. Results and Discussion Starch and the PVA molecules, having a large number of hydroxyl groups in their structures, remain associated with one another by inter- and intramolecular H-bonding in the blend. The plasticizer molecules enter between the starch and the PVA molecules, reduce the intermolecular force of attraction, and also take part in H-bonding with them. The cross-linking agents react with the -OH groups present in starch and PVA and make ether linkages with the available hydroxyl groups. This helps to increase the mechanical properties of the films as well as reduce their water absorption behavior. 3.1. XRD Study. The XRD graphs of the individual components (starch granules, gelatinized starch plasticized with glycerol, and PVA film plasticized with glycerol) are shown in Figure 1a. The characteristic diffraction peaks were observed at 17.8, 21.8, and 24.1° for starch granules, at 19.8, 22.6, and 40.6° for PVA film, and at 16.9 and 22° for the gelatinized starch sample. The un-cross-linked starch/PVA blend film (U) and the cross-linked films (E, Z, F, and B) were subjected to X-ray diffraction studies, and the XRD graphs are shown in Figure 1b. The sharp peak at 19.6° in E, F, and B could be assigned to the ordered arrangement of PVA molecules in the films. The small hump at 40° in E was also characteristic for PVA. A small peak appeared at 17.6°, which corresponded to the plasticized starch in the film. These observations reveal the fact that the crystallinity of E, F, and B was mainly due to PVA. The high physical interactions between the PVA molecules due to H-bonding were prevalent, which might be due to their lower participation in cross-linking. The starch molecules lost their crystallinity to a significant extent, which could be due to their greater participation in the cross-linking reaction. In U, the 22° peak was sharp, which could be due to contribution from both starch and PVA. The 19.6° peak appeared as a small hump. The percent crystallinity of the film samples and the crystallite sizes were calculated as per standard methods with respect to the 19.5 (PVA) and 17.1° (starch) peaks28 (Table 2). It was observed that crystallinity was lower in U and Z compared to that of E, F, and B. Thus, preferential cross-linking of one component over another might have occurred in the presence of different cross-linkers. The extent of cross-linking and molecular rearrangement controlled their mechanical properties, dynamic mechanical, moisture absorption, and thermal degradation behavior.

Figure 3. Normalized gel mass (a) and normalized swelling degree (NSD) of the cross-linked films in DMSO.

3.2. Swelling Degree and Gel Mass. The surface area normalized gel mass (NGM) is directly related to the mass of thermoplastic starch and PVA molecules involved in crosslinked network formation. The normalized swelling degree (NSD) is related to the cross-link density of the network formed. The higher the cross-link density, the lower is the NSD and the higher is the NGM. The probable cross-linking reactions with different cross-linkers have been shown schematically in Figure 2. The NGM and NSD of the films are shown in Figure 3a,b respectively. In E, incorporation of flexible epichlorohydrin molecules might have increased the interchain distance and NSD was high. In F, Z, and U, the solvent molecules could not penetrate the closely packed chains, resulting in a low NSD. The NGM was very high in F, which can be attributed to the high cross-link density of the sample. The high NGM value in U was due to compact packing of the starch and PVA molecules and strong H-bonding between them. The moderately high NGM value in E also indicated a significant amount of cross-linking in them. In Z, the presence of ZnO particles might have hindered the solvent molecules to enter between the chains, reduced their swelling, and increased their gel mass content. In B, three -OH groups attached to the B atom took part in cross-linking with the -OH groups of the starch and PVA molecules. This increased the rigidity of the sample. This resulted in low NSD and very high NGM values. 3.3. Morphology. The surface morphology of the films was examined under SEM. It was clearly observed from Figure 4a,b that the starch granules were not fully collapsed during

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Figure 4. (a, b) Partially collapsed starch granules dispersed in PVA matrix in the un-cross-linked film (U): (a) lower magnification; (b) higher magnification. (c, d) Partially collapsed starch granules and fine particles of ZnO dispersed in PVA matrix (Z): (c) lower magnification; (d) higher magnification. (e) Gelatinized starch granules, embedded within the PVA matrix (E). (f) Good blending of starch and PVA in presence of borax (B). (g) Uniform blending of starch granules with PVA matrix in presence of formaldehyde showing a longitudinal pattern.

gelatinization in U and were present as partially destructured granules in a continuous matrix of PVA. Similar morphology was observed in Z (Figure 4c) along with the fine ZnO particles which remained dispersed uniformly in the PVA matrix (Figure 4d). In E (Figure 4e), the starch granules were collapsed to a much smaller size and were embedded within the PVA matrix. The starch/PVA interface was highly diffused, indicating strong bonding through epichlorohydrin. Figure 4f shows the surface features of B. Here, the starch granules were completely collapsed during gelatinization, and there was a uniform blending between starch and PVA resulting in the single-phase morphology. In formaldehyde cross-linked film (F), a smooth and uniform surface morphology was observed with some longitudinal patterns on them (Figure 4g). Here also, the starch

molecules were collapsed completely, participated in crosslinking, and showed a homogeneous morphology. It was thus observed that under the same processing condition, the destructuring of the starch granules were significantly influenced by their reaction with the cross-linking agents. The SEM observations fully corroborated with the XRD findings. The collapse of starch granules observed from SEM micrographs in E, F, and B clearly supported the findings of XRD observations. 3.4. FTIR Study. FTIR spectra of all the films (U, Z, E, F, and B) are presented in Figure 5a-c. Figure 5a shows the spectra of the films in the range of 750-1200 cm-1. The peaks appeared at 856 and 920 cm-1 due to -C-OsC-ring vibration in granular starch29 and at 993 cm-1 due to -C-H bending of


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Figure 5. FTIR spectra of the starch/PVA blend in un-cross-linked (U) and cross-linked states (Z, E, B, and F) in the ranges of (a) 750-1200, (b) 1200-1800, and (c) 1800-4000 cm-1.

vinyl groups. The peaks at 1076 and 1145 cm-1 were assigned to the -C-O-C- stretching of the ether groups present in the structures. In Figure 5b, all the films show absorption bands at 1240, 1336, and 1415 cm-1 due to deformation vibration of -CH2 in -CH2OH. In E, a sharp, broad peak appeared at 1328 cm-1 instead of 1336 cm-1. In F, this peak was very sharp and narrow and it shifted to 1344 cm-1. This clearly indicates the incorporation of additional -CH2 groups in the structures of E and F, and the shift of the peak position was due to the difference in their mode of vibrations depending on their cross-linked structures. The characteristic -C-H bending peaks for -CH2 groups at 1458 cm-1 appeared as small humps in U and Z. In B this peak appeared at 1458 cm-1 with a higher intensity, which could be due to decreased hydrogen bonding between starch and PVA molecules. In E and F, this peak became very sharp and shifted to1462 and 1460 cm-1, respectively. The peaks at 1650 cm-1 could be assigned to the -C-H stretching of the

monosubstituted vinyl group. In F and B, some sharp peaks appeared in the region of 1500-1800 cm-1, which was difficult to assign (Figure 5b, inset). Figure 5c showed a broad absorption peak in the range of 3000-3650 cm-1 due to O-H stretching vibrations. This band was significantly lowered in B because the -OH groups took part in complex formation with the borate ions resulting in a highly rigid structure.16 A typical methylene C-H stretch appears in all the films at 2931 cm-1. In comparison to the spectrum of U, Z, E, and F, this peak was lowered in B. The reason is that no additional -CH2- group was incorporated in B due to cross-linking. But in E and F, cross-linking incorporated additional -CH2 groups apart from those already present in the structures. The weakness, disappearance, and shift of the characteristic absorption bands might result from various interactions between the different components.

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Figure 6. (a) Tensile strength, (b) tensile modulus, and (c) % strain of the un-cross-linked (U) and the cross-linked starch/PVA blend films (E, B, Z, and F).

3.5. Mechanical Property. The tensile properties of the films are shown in Figure 6a-c. From Figure 6a, it was observed that the tensile strength was 160% higher in B compared to U. While in Z, E, and F, the tensile strength decreased by 12.2, 20.2, and 14.7%, respectively, in comparison to U. A similar trend was observed for the tensile modulus also. The tensile modulus was increased by 390% in B and decreased by 22, 33, and 12% in Z, E, and F, respectively, compared to U (Figure 6b). This significant increase in mechanical properties in B could be due to the rigid network formation between the three -OH groups of borax and the -OH groups of starch and PVA, while, in E, F, and Z, the cross-linkers lowered the compact packing of the starch and PVA molecules, which lowered their mechanical properties. A significant increase in strain was observed for E (80%), while it was lowered by 21.6 and 71% in F and B, respectively, with respect to U (Figure 6c). Incorporation of covalently bonded flexible epichlorohydrin molecules between the starch and PVA molecules lead to such a high strain value. In F and B, cross-linking reduced the mobility of the chains considerably and lowered the strain values. For Z, the strain was similar to that of U. Thus, cross-linking changed the mechanical properties of the films to a significant extent. 3.6. Dynamic Mechanical Properties. Parts a-c of Figure 7 show the storage modulus (E′), loss modulus (E′′), and damping parameter of the films in the temperature range of -50

to +100 °C. It is observed that the storage modulus of all the cross-linked films were much higher than the un-cross-linked one at -50 °C (Figure 7a). It suggests that incorporation of cross-links between the starch and PVA molecules helped to store a larger amount of elastic energy during dynamic loading even at a very low temperature. The ZnO dispersed films showed the highest E′ as ZnO might have played the role of reinforcing filler. This was followed by E, where epichlorohydrin, being a flexible molecule, took part effectively in elastic energy absorption. Figure 7b shows the variation of loss modulus of the films as a function of temperature. The loss modulus value was highest for E, but it decreased steeply as the temperature was raised. Here due to the flexible nature of the epichlorohydrin molecules, the interchain distance was increased and the viscous dissipation was lowered. But in Z and F, the drop in loss modulus value was much less with the rise in temperature, because, with the incorporation of formaldehyde cross-links and intermittently dispersed ZnO particles, a larger amount of applied mechanical energy was expended in viscous dissipation. In U, the loss modulus value was much less because the molecules were very closely packed by strong H-bonds, not allowing much viscous deformation. And in B, a rigid network was formed which reduced the viscous dissipation. The damping behavior of the films as a function of temperature has been shown in Figure 7c. Damping was very high at -50 °C in E and F. In B and Z,


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Figure 7. (a) Storage modulus, (b) loss modulus, and (c) damping parameter of the un-cross-linked (U) and the cross-linked starch/PVA blend films (E, B, Z, and F).

damping was less than that of U at -50 °C. Damping was very high in E, which showed two relaxation peaks at -24 and +28 °C, respectively. Similar two relaxation peaks were evident in F and Z also. The lower one around -24 °C might be due to the secondary transition of starch, in the presence of glycerol,19 and the peak at the higher temperature 28-38 °C might be due to the primary transition of starch molecules.19 In B, this peak shifted to 50 °C, which could be attributed to the rigid network formation of starch molecules. A similar nature was shown by U, but the damping was much less than that of B due to the strong H-bonding between starch molecules which acted like physical cross-links and reduced the mobility of the chains. PVA exhibited very weak relaxation peaks around 73 °C in U, Z, E, and F (Figure 7c). This fully corroborates with our XRD observations reported in section 3.1. 3.5. Moisture Uptake at 93% RH. The effect of crosslinking on the moisture absorption behavior of the films is shown in Figure 8a. Moisture absorption depends on the availability

of free hydroxyl groups as well as on the ease of diffusion of the water molecules across the thickness of the film sample. The moisture absorption of the un-cross-linked film (U) was found to be 52%, while there was a significant decrease in that of the cross-linked films. Set Z, B, E, and F showed moisture absorption of 30, 38, 24, and 29%, respectively, at 93% RH condition, indicating that availability of hydroxyl groups decreased considerably due to cross-linking. However, the extent of decrease varied depending on the type of cross-linkers used. The diffusion coefficient (D) of the films were calculated using eq 4 and are shown graphically in Figure 8b. The D value was high for U (12 × 10-.15 m2/s) but was lowered in all the crosslinked films (6 × 10-15 m2/s for Z, 9 × 10-15 m2/s for B, 3.9 × 10-15 m2/s for E, and 3.7 × 10-15 m2/s for F. The D value of B is higher compared to those of Z, E, and F. A complex formation takes place between the -OH groups of starch, PVA, and glycerol with the borate ion, where three -OH groups form bonds with boron. This might have increased the interchain

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Figure 8. (a) Percent moisture absorption and (b) the diffusion coefficients of un-cross-linked (U) and the cross-linked starch/PVA blend films (E, B, Z, and F) when kept in 93% RH environment.

distances, which facilitated the passage of water molecules through the film sample and increased the moisture absorption. In Z, ZnO remained as filler but effectively lowered the availability of -OH groups for absorbing moisture. 3.6. Thermogravimetric Analysis. The thermal degradation behavior of individual components such as gelatinized starch and PVA film has been shown in our previous work.30 The thermograms of the un-cross-linked as well as the crosslinked films are shown in Figure 9a,b. There was a remarkable difference in the thermal degradation behavior of the crosslinked films compared to that of U. The TGA graphs exhibited a three-step degradation process. A small peak appeared around 215 °C in all the cross-linked films, which might be due to breakage of cross-links during the first stage of decomposition. Similar observation has been reported by other authors.16 This was followed by the major degradation process, which began from 232 °C for the cross-linked films (Z, B, E, and F) and from 258 °C for the un-cross-linked one (U). The end-set temperature for degradation varied significantly in all films, 386 °C for U, 319 °C for Z, 361 °C for F, 354 °C for B, and 340 °C for E. This reflected generation of various intercomponent bonds in the presence of different cross-linkers which gave rise to different threedimensional networks and led to a complex mode of degradation. In U, a third degradation step occurred at 415 °C which could be attributed to the degradation of PVA. But

Figure 9. (a) TGA and (b) DTGA thermograms of the un-cross-linked (U) and cross-linked starch/PVA blend films (E, B, Z, and F) with the rise in temperature.

in cross-linked films, this degradation peak was significantly diminished. The residue left at 500 °C was slightly higher for B (22%) and Z (19%) than other films (13-15% for U, E, and F). 4. Conclusion Starch/PVA films plasticized with glycerol and cross-linked with the same concentration of different cross-linkers such as borax (B), formaldehyde (F), epichlorohydrin (E), and ZnO (Z) were prepared and characterized for different properties. X-ray diffraction patterns of the films revealed preferential crosslinking of one component over another. SEM revealed that the starch granules were not fully collapsed during gelatinization in the case of U and Z. From the resemblance observed XRD and SEM findings, it was concluded that ZnO particles were present only as fillers and did not take part in the chemical crosslinking reaction. In E, F, and B, the morphology was significantly different from that of U, indicating chemical cross-linking between the components. The borax cross-linked films (B) exhibited the highest mechanical properties. The tensile strength and tensile modulus of B increased by 160 and 390%, respectively, compared to that of U. The complex formation between three -OH groups of the borate ion and those of starch and PVA resulted in a highly rigid three-dimensional network structure. Such a structure might have facilitated the diffusion of moisture across the film thickness, and B showed the highest


diffusion coefficient value among all the cross-linked films. The moisture absorption was lowest for E (24%), while that of U was 52%. Damping was significantly increased in E, F, and B compared to that of U. The thermal degradation process was influenced by the generation of new bonds with the cross-linkers. There was an increase in char residue at 500 °C in the case of B and Z. Thus, the performance of starch/PVA films can be enhanced considerably by cross-linking reaction, which can open new avenues for their commercial exploitation as a low-cost packaging material. Acknowledgment D.R. is thankful to AICTE (All India Council for Technical Education), Government of India, for granting her a project. S.S.G. is grateful to DST, Government of India, for awarding her a Research Associateship. M.M. and A.M. are thankful to the University of GuelphsThe Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) 2008 and 2009 BioeconomyIndustrial Uses Research Program for partial financial support. Literature Cited (1) Avella, M.; Errico, M. E.; Rimedio, R.; Sadocco, P. Preparation of biodegradable polyesters/high-amylose-starch composites by reactive blending and their characterization. J. Appl. Polym. Sci. 2002, 83, 1432. (2) Bhattacharia, M. Stress relaxation of starch/synthetic polymer blends. J. Mater. Sci. 1998, 33, 4131. (3) Choi, E. J.; Kim, C. H.; Park, J. K. Structure-property relationship in PCL/starch blend compatibilized with starch-g-PCL copolymer. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2430. (4) Carvalho, A. J. F.; Job, A. E.; Alves, N.; Curvelo, A. A. S.; Gandini, A. Thermoplastic starch/natural rubber blends. Carbohydr. Polym. 2003, 53, 99. (5) Dubois, P.; Krishnan, M.; Narayan, R. Starch-polyvinyl alcohol crosslinked filmsPerformance and biodegradation. Polymer 1999, 40, 3091. (6) Kiatkamjornwong, S.; Sonsuk, M.; Wittayapichet, S.; Prasassarakich, P. Cellulose microfibrils from potato tuber cells: Processing and characterization of starch-cellulose microfibril composites. Polym. Degrad. Stab. 1999, 66, 323. (7) Petersen, K.; Nielsen, P. V.; Olsen, M. B. Physical and mechanical properties of biobased materials starch, Polylactate and polyhydroxybutyrate. Starch/Staerke 2001, 53, 356. (8) Van, S. J. J. C.; De, W. D.; Vliegenthart, J. F. C. Mechanical properties of thermoplastic waxy maize starch. J. Appl. Polym. Sci. 1996, 61, 1927. (9) Follain, N.; Joly, C.; Dole, P. Properties of starch based blends. Part 2. Influence of poly vinyl alcohol addition and photocrosslinking on starch based materials mechanical properties. Carbohydr. Polym. 2005, 60, 185. (10) Park, H. R.; Chough, S. H.; Yun, Y. H.; Yoon, S. D. Biodegradability of chemically modified starch (RS4)/PVA blend films: Part 2. J. Polym. EnViron. 2005, 13, 375. (11) Yoon, S. D.; Chough, S. H.; Park, H. R. J. Appl. Polym. Sci. 2006, 100, 2554. (12) Chen, L.; Imam, S. H.; Gordon, S. H.; Greene, R. V. Starchpolyvinyl alcohol crosslinked filmsPerformance and biodegradation. Polym. Degrad. 1997, 5, 111.

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ReceiVed for reView July 9, 2009 ReVised manuscript receiVed December 29, 2009 Accepted January 1, 2010 IE901092N

Poly(vinyl

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