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J Polym Environ DOI 10.1007/s10924-008-0112-9
ORIGINAL PAPER
Biodegradable Films Based on Blends of Gelatin and Poly (Vinyl Alcohol): Effect of PVA Type or Concentration on Some Physical Properties of Films G. G. D. Silva Æ P. J. A. Sobral Æ R. A. Carvalho Æ P. V. A. Bergo Æ O. Mendieta-Taboada Æ A. M. Q. B. Habitante
Ó Springer Science+Business Media, LLC 2008
Abstract The aim of this work was to develop biodegradable films based on blends of gelatin and poly (vinyl alcohol) (PVA), without a plasticizer. Firstly, the effect of five types of PVA with different degree of hydrolysis (DH) on the physical properties of films elaborated with blends containing 23.1% PVA was studied. One PVA type was then chosen for the study of the effect of the PVA concentration on the mechanical properties, color, opacity, gloss, and water solubility of the films. The five types of PVA studied allowed for films with different characteristics, but with no direct relationship with the DH of the PVA. Therefore, the PVA CelvolÒ418 with a DH = 91.8% was chosen for the second part, because they produced films with greater tensile strength. The PVA concentration affected all studied properties of films. These results could be explained by the results of the DSC and FTIR analyses, which showed that some interactions between the gelatin and the PVA occurred depending on the PVA concentration, affecting the crystallinity of the films. Keywords Biopolymer Protein Mechanical properties Color Solubility
G. G. D. Silva P. J. A. Sobral (&) R. A. Carvalho P. V. A. Bergo A. M. Q. B. Habitante Food Engineering Department, FZEA, University of Sa˜o Paulo, P.O. Box 23, 13635-900 Pirassununga, SP, Brazil e-mail:
[email protected] O. Mendieta-Taboada Universidad Nacional de San Martı´n, Tarapoto, Peru
Introduction The question of environmental concern caused by synthetic packaging has favored, in a general way, an increase in research on biodegradable materials, which are elaborated from renewable raw materials. Polysaccharides [1] and proteins [2–4] are biopolymers capable to produce thin and flexible films with biodegradable character. Gelatin was one of the first macromolecules employed in the production of biomaterials [2]. This biopolymer still attracts the attention of researchers because it is produced abundantly practically worldwide, has a relatively low cost and possesses excellent functional and filmogenic properties [2, 4]. For this reason gelatin has been studied in film technology both alone [5–10] and in blends with other biopolymers [11]. In a general way, gelatin based films present good mechanical resistance, despite their reduced water vapor barrier [6]. On the other hand, these films present high susceptibility to room temperature and relative humidity conditions due to the hydrophilic nature of gelatin. Thus, an increasing of the room temperature and/or of the relative humidity may reduce the mechanical resistance and increase the extensibility of this material, for instance. This behavior leads to difficulty in food packaging applications. Several alternatives have been studied to minimize this problem, such as chemical or enzymatic modifications of the gelatin [7, 12], the use of plasticizer blends [8], or different plasticizers with different hydrophilicity [9], and the incorporation of lipids [13], amongst others. However, in a general way, these results have not necessarily been satisfactory. A possible alternative to improve the mechanical properties of these materials could be the mixture of these biopolymers with synthetic polymers. Low density
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polyethylene is the most studied synthetic polymer blended with biopolymers [14]. However, this polymer needs chemical treatments to become biopolymer compatible [15]. Another synthetic polymer, poly(vinyl alcohol) (PVA) has also attracted increasing attention for the implementation of the mechanical properties of films based on polysaccharides [16–19] and proteins [20–24]. PVA is a non-toxic, water-soluble, synthetic polymer, which has been employed in biomaterial technology for the pharmaceutical and biomedical areas due to its excellent film forming, emulsifying, and adhesion properties [25]. Despite its synthetic character, this polymer was recognized recently as biodegradable [26]. The PVA is obtained by hydrolysis of poly(vinyl acetate), then, different types of PVA can be produced depending on the degree of hydrolysis involved. And the degree of hydrolysis of PVA can affect the physical properties of this polymer and of its films [25]. Chiellini et al. [21, 22] elaborated biodegradable films based on PVA and waste gelatin, with sugar cane bagasse as the filler, and Maria et al. [23] and Bergo et al. [24] developed the same types of film but without the filler. However, more data on the physical properties of films based on blends of PVA and gelatin, produced by casting, are necessary. Thus the objective of this work was the development of biodegradable films based on blends of gelatin and poly(vinyl alcohol) (PVA) with no plasticizer, and the characterization of their mechanical properties, color, opacity, moisture and water solubility. These properties are of interest for food packaging applications. More specifically, this work involved two steps: in the first step, the effect of the degree of hydrolysis of five types of PVA on the physical properties of films elaborated with blends containing 23.1% de PVA was studied; and, in the second part, the PVA CelvolÒ418 was chosen to verify the effect of the PVA concentration on the physical properties of the films prepared with blends of gelatin and this PVA.
Experimental Material A commercial pigskin gelatin (A type), donated by the industry Gelita do Brasil Ltda (Sa˜o Paulo, Brazil), and five types of poly(vinyl alcohol) (PVA), with different degree of hydrolysis (DH): CelvolÒ504 (DH = 88.0%), CelvolÒ418 (DH = 91.8%), CelvolÒ425 (DH = 95.7%); CelvolÒ350 (DH = 98.0–98.8%) and CelvolÒ125 (GH = 99.7%), kindly furnished by the Industry Celanese Ltd. (Baytown, USA), were used in this work.
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Film Preparation The films were produced from film forming solutions (FFS) with a concentration of 2 g of macromolecules (gelatin ? PVA)/100 g FFS, being that gelatin and PVA were previously prepared separately. The gelatin (solution A) was initially hydrated with water at room temperature in a beaker, and subsequently dissolved at 55 °C [6], in a water bath with digital temperature control (±0.5 °C). And, the PVA (solution B) was dissolved in water at 95 °C [27], with magnetic stirring for 30 min. To obtain the desired FFS, solutions A and B were mixed in the pre-determined proportion, and homogenized with magnetic stirring at room temperature for 15 min, to obtain the following conditions: 23.1 g PVA/ 100 g of macromolecules for the first part of this work, and 9.1; 16.7; 23.1; 28.6 and 33.3 g PVA/100 g of macromolecules for the second part. An analytical balance was used for all weightings (±0.0001 g) (Scientech, SA210). Additionally, in the second part of this work, some films based on pure gelatin (0 g PVA/100 g of macromolecules) or pure PVA (100 g PVA/100 g of macromolecules) were produced using FFS containing in all cases 2 g macromolecule/100 g FFS, for the purpose of comparison with the films prepared with blends of gelatin and PVA. The films were obtained by casting the FFS on Plexiglas plates (12 9 12 cm2), always with the same dry matter density to guarantee constant thickness, and drying at 30 °C and relative humidity of (55–65%) for 24–28 h in an oven with air renewal and circulation (Marconi, MA037) and PID control (±0.5 °C) of temperature [6]. Film Characterization In the first part of this work, the films prepared with blends of gelatin and PVA were characterized for their mechanical properties, color, opacity, humidity and water solubility. In the second part, in addition to these characterizations, the films prepared with blends of gelatin and PVA were characterized to determine their gloss and Fourier-transform infrared spectra. Also, the films based on both pure polymers as well as both pure polymers in water, were analyzed by differential scanning calorimetry. The films were conditioned in desiccators containing saturated solutions of NaBr in distilled water (relative humidity of 58%) at 22 ± 3 °C for 7 days for posterior characterization, performed in a climatized room, at 22 ± 3 °C and relative humidity between 55% and 65%.
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Film Thickness
Film Solubility and Humidity
The thickness of the films was measured averaging ten different positions, using a digital micrometer (Mitutoyo, ±0.001 mm) with a 6.4 mm diameter probe.
The solubility of the films (*2 cm) was determined after 24 h of immersion in distilled water (50 mL) with shaking in a shaker (Marconi, MA141) for 24 h at room temperature [29]. After this period, the samples were dried in order to determine their final dry mass. Considering that the initial dry mass of the samples was known, the solubility of the films was calculated in terms of the solubilized dry mass. The sample humidity was determined by drying in an oven at 105 °C for 24 h.
Mechanical Properties The mechanical properties were determined using tensile and puncture tests with the help of a texturometer mod. TA.XT2i (Stable Micro Systems) and the software Texture Expert 1.15 (SMS).
Gloss Puncture Tests In the puncture tests, the films were fixed in a 52.6 mm diameter cell and perforated by a 3 mm diameter probe, moving at 1 mm/s. The puncture force and the displacement of the probe at break were determined directly from the force 9 probe displacement curves. Thus, the puncture deformation could be calculated considering that the stress was perfectly distributed along the film at breaking point [6]. Tensile Tests Tensile tests were run using rectangular 100 mm 9 15 mm samples, initial (lo) grips separation of 80 mm and crosshead speed of 0.9 mm/s [8]. The tensile strength (force/initial cross-sectional area) and elongation at break (Dl/lo) were determined directly from the stress 9 strain curves, and the elastic modulus was calculated as the inclination of the linear initial portion of this curve (with deformation in %). Color and Opacity Film color was determined with a colorimeter (HunterLab, model Miniscan XE) with D65 (day light) and a measurement cell with an opening of 30 mm, using the CIELab color parameters [28]. The color of the films was expressed as the difference of color {DE* = [(DL*)2 ? (Da*)2 ? (Db*)2]0.5} knowing DL*, Da* and Db*, the differentials between the color parameter of the samples and the color parameter of the white standard used as the film support [8, 9]. The opacity of the films was determined according to the Hunterlab method [8], with the same equipment used for the color measurements, also operating in the reflectance mode. The opacity (Y) of the samples was calculated as the relationship between the opacity of each sample on the black standard (Yb) and the opacity of each sample on the white standard (Yw). This calculation (Y = Yb/Yw) was automatically carried out by the Universal Software 3.2 (Hunterlab Associates Laboratory).
The gloss of the films was measured at 208 and 608 angles from the normal to the drying surface using a glossimeter (Rhopoint NGL 20/60) according to Villalobos et al. [30]. FTIR Spectroscopy FTIR spectra were recorded using a Perkin Elmer spectrometer Spectrum One (Perkin-Elmer, USA) equipped with a universal attenuated total reflectance (UATR) accessory, according to Vicentini et al. [31]. The spectra were recorded between 4,000 and 400 cm-1 with a 4 cm-1 spectral resolution. For each spectrum, 20 scans were coadded. The data were analyzed using the program FTIR Spectrum Software (Perkin Elmer). Differential Scanning Calorimetry Samples were analyzed by differential scanning calorimetry using a DSC TA 2010 controlled by a TA5000 module (TA Instruments, New Castle, DE, USA) and with a quench-cooling accessory. Aliquots of the order of 10 mg, weighed (±0.01 mg) on a high precision balance (Ohaus, Analytical Plus), were conditioned in hermetic aluminum pans, and heated at 5 °C min-1, in an inert atmosphere (45 mL/min of N2) [32]. The reference was an empty pan. The equipment was calibrated with an indium sample (Tm = 156.6 °C, DHm = 28.71 J g-1) (TA Instruments). The glass transition temperature (Tg) was calculated as the inflexion point of the base line, caused by the discontinuity of the specific heat of the sample, and the melting temperature (Tm) was considered as the peak temperature of the endotherms. All these properties were calculated with the help of the software Universal Analysis V1.7F (TA Instruments). Two types of analyses were performed: (a) the films based on both pure macromolecules were pre-conditioned at 22–25 °C and 55–65% of relative humidity for two weeks, and heated between -50 and 200 °C in double, or even, in triple runs, always after quench cooling with liquid
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23.0 ± 1.8c 15.5 ± 1.4d 15.6 ± 1.4a 15.5 ± 1.4a Different letters in the same column indicate significant difference (p \ 0.05) between means obtained in Duncan test using the software SAS
0.8 ± 0.08d 1.1 ± 0.16c 3.1 ± 0.09c 3.2 ± 0.13c 2.6 ± 0.11b 2.7 ± 0.13c -1.0 ± 0.01a -1.0 ± 0.02a 91.7 ± 0.07ab 91.6 ± 0.11b 27.7 ± 1.2a 28.0 ± 1.7a 5.1 ± 0.5a 5.0 ± 0.5a 80.4 ± 4.6ab 73.9 ± 6.2c 32.3 ± 2.6a 30.8 ± 2.9a CelvolÒ350 CelvolÒ125
1.5 ± 0.1a 1.4 ± 0.2a
1.1 ± 0.24 3.2 ± 0.10 2.8 ± 0.07 -1.0 ± 0.02 91.7 ± 0.09 24.4 ± 1.1 5.1 ± 0.5 75.2 ± 3.9 1.5 ± 0.2 31.5 ± 4.0 Celvol 425
28.1 ± 3.2b
35.3 ± 2.4a
28.3 ± 1.9b 14.9 ± 1.0
a c
14.7 ± 1.3 2.0 ± 0.31 3.6 ± 0.15
bc b
3.1 ± 0.12 -1.0 ± 0.03
a b
91.4 ± 0.11 27.4 ± 2.3
c a c
5.2 ± 1.0 82.3 ± 4.5 1.2 ± 0.1
a a
26.5 ± 5.3 Celvol 418
Ò
ab
13.5 ± 0.7b 2.4 ± 0.59a
b a
3.3 ± 0.20b 3.1 ± 0.14a
a a
-1.0 ± 0.03a 91.6 ± 0.20b
c a
26.5 ± 1.5b 5.0 ± 0.7a
a a
78.4 ± 7.3b 1.4 ± 0.1a
a b
26.8 ± 1.7b CelvolÒ504
Ò
S (%) X (%) Y DE* b* a* L* EM (MPa/%)
All the films produced in this work were visibly homogenous and transparent, with an excellent appearance, typical of gelatin-based films, independent of the PVA type tested. The results obtained in the puncture (puncture force and deformation) and tensile (tensile strength, elongation at break and elastic modulus) tests, and the color parameters (luminosity, a*, b*, total difference of color), opacity, humidity and water solubility determinations of the films based on blends of gelatin with different types of PVA, are presented in Table 1. The control of film thickness by controlling the relation between the dry weight of the film forming solution and the support area was efficient. In general, the thickness of the films varied between 0.079 and 0.082 mm. In relation to puncture force, it was observed that the films made with PVA with a high degree of hydrolysis (CelvolÒ425, CelvolÒ350 and CelvolÒ125) were more (p \ 0.05) resistant to puncture than the films produced using PVA with a low degree of hydrolysis (CelvolÒ504 and CelvolÒ418). However, the degree of hydrolysis did not affect the puncture deformation of these films, which remained near 1.4% (Table 1). Considering the tensile tests, the PVA CelvolÒ418 produced films (p \ 0.05) with higher tensile strength (82.3 MPa), although with similar rigidity (p \ 0.05) to the films produced using PVA CelvolÒ125 and CelvolÒ350, in which the elastic modulus was 27–28 MPa/% (Table 1). Less resistant films (p \ 0.05) were produced with PVA CelvolÒ125 and CelvolÒ425. On the other hand, no effect of PVA type was observed on the values for elongation at break of the films prepared with blends of gelatin and PVA, as observed in the puncture tests.
E (%)
Effect of Different PVA Types on the Physical Properties of the Films Based on Blends of Gelatin and PVA
T (MPa)
Results and Discussions
D (%)
Duncan’s multiple range test was applied to compare the means for the film properties, with a level of significance of a = 0.05, using the SAS program (Version 9.1, SAS Institute Inc., Cary, NC, USA). The linear regressions were performed with the software Excel2000.
F (N)
Statistical Analyses
PVA types
nitrogen; (b) and the pure gelatin or the pure PVA were mixed with equal quantities of distilled water in the DSC pans, and maintained at rest for 30 min for hydration. The samples were then heated between 0 and 150 °C in a single run.
Table 1 Puncture force (F), puncture deformation (D), tensile strength (T), elongation at break (E), elastic modulus (EM), luminosity (L*), a*, b*, difference of color (DE*), opacity (Y), humidity (X) and water solubility (S) of films based on blends of gelatin and different types of poly(vinyl alcohol) (PVA)
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Although the PVA type affected some of the mechanical properties, as can be seen above, it was not possible to establish a relationship between the degree of hydrolysis of the PVA and the mechanical properties of those films. This behavior may be due to the complexity of the structural forces involved in the formation of the polymeric matrix, as a function of the characteristics of the macromolecule system. The gelatin and PVA films produced in this work were more resistant to puncture than some films produced from gelatin and plasticized with sorbitol and/or glycerol [6, 8, 9], and also more resistant to tensile stress than films based on pure pigskin gelatin plasticized with glycerol (21 MPa) or sorbitol (47 MPa) [8], and on bovine hide gelatin with glycerol (15 MPa) [7], amongst others. On the other hand, the elongation at break and the puncture deformation values of the films produced in this work were lower than those cited above, due to the presence of plasticizer in that films. Variations in the parameters of color and opacity, statistically significant in some cases, were observed with no logical behavior in relation to the PVA type. However, despite this, the films produced in this work could be considered to have little color (DE* \ 4) and low opacity (Y \ 2.5) (Table 1). In fact, the films obtained in the present work presented, in a general way, color similar to the films produced with pure pigskin gelatin and plasticized with various polyols (DE* = *3) [9], but were less colored than films based on the myofibrillar protein of Nile Tilapia (DE* = 7–8) [33], and more colored than films based on ovoalbumins (DE* = 1.7–2.3) [28]. In the case of opacity, the films produced in this work showed values slightly higher than films based on pure pigskin gelatin (Y = 0.2–0.8) [9], but comparable to that of films produced with the muscle proteins of Nile Tilapia (Y = 2–7) [33]. The PVA type practically did not affect the humidity of the films prepared with blends of gelatin and PVA. Different film humidity (p \ 0.05) was observed for films produced with PVA CelvolÒ504 (Table 1). The least
water-soluble film was that produced with the PVA CelvolÒ125, which presented the lower (p \ 0.05) degree of hydrolysis. Carvalho and Grosso [7] determined the solubility of films based on bovine gelatin to be around 31%, which is comparable to some values determined in this work (Table 1). Thus, a reduction of the water solubility of gelatin-based films requires the use of PVA with low degree of hydrolysis. Considering the results presented in Table 1, and the fact that there is no logical and generic relationship between the degree of hydrolysis of the different PVA types and film properties studied, the PVA CelvolÒ418 was chosen for the second part of this work because it produced films with greater mechanical resistance to tensile stress. Effects of the PVA Concentration on the Properties of Films Prepared with Blends of Gelatin and PVA CelvolÒ418 Color and Opacity The parameters color difference (DE*), a*, b*, L* and opacity of the films based on gelatin and PVA blends with different PVA concentrations are presented in Table 2. Once again, although there was some variation in the parameters of color observed, in some cases statistically significant, all the films could be considered as very low colored (DE* \ 4) and with very low opacity (Y \ 3), despite the PVA concentration effect. However, it was remarkable that the films based on the pure polymers presented the lowest values for opacity (p \ 0.05), being, therefore, more transparent than their mixtures. Gloss Contrary to color, gloss is a surface property, related to the superficial texture, or in other words, to its degree of polish. The results for gloss of the drying surface are presented in Table 3. This property has an arbitrary scale and is
Table 2 Luminosity (L*), a*, b*, difference of color (DE*) and opacity (Y) of films based on blends of gelatin and PVA CelvolÒ418 L*
0
91.7 ± 0.10b
-1.0 ± 0.04b
2.6 ± 0.18c
3.1 ± 0.15c
0.4 ± 0.15c
c
c
b
b
1.0 ± 0.42b
b
9.1
a*
DE*
CPVA
91.5 ± 0.19
b*
-1.0 ± 0.02
2.9 ± 0.10
3.3 ± 0.18
16.7 23.1
c
91.6 ± 0.07 91.4 ± 0.07c
c
-1.0 ± 0.03 -1.0 ± 0.02c
3.0 ± 0.13 3.2 ± 0.15a
3.4 ± 0.11 3.6 ± 0.11b
1.4 ± 0.03b 2.6 ± 0.64a
28.6
91.2 ± 0.13d
-0.9 ± 0.01b
3.1 ± 0.27a
3.7 ± 0.20a
2.8 ± 0.69a
33.3
91.3 ± 0.23d
-0.9 ± 0.04b
2.8 ± 0.28bc
3.5 ± 0.31b
2.3 ± 0.56a
a
a
d
0.4 ± 0.09c
100
92.1 ± 0.13
-0.9 ± 0.01
ab
Opacity
d
1.9 ± 0.02
2.6 ± 0.13
CPVA = concentration of PVA (g/100 g of macromolecules); different letters in the same column indicate significant difference (p \ 0.05) between means obtained in Duncan test using the software SAS
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J Polym Environ Table 3 Gloss values measured at 20° and 60° of films based on blends of gelatin and PVA CelvolÒ418 CPVA
208
0
234.6 ± 13.3a
180.2 ± 5.2a
d
171.5 ± 6.6c
b
184.4 ± 1.9b
bc
183.9 ± 1.6b
bc
175.3 ± 1.8c
c
9.1 16.7 23.1 28.6
608
131.3 ± 14.4 208.4 ± 12.5 192.8 ± 17.8 199.1 ± 10.9
33.3
189.2 ± 17.0
185.0 ± 2.1b
100
202.0 ± 11.1bc
197.9 ± 5.8a
CPVA = concentration of PVA (g/100 g of macromolecules); different letters in the same column indicate significant difference (p \ 0.05) between means obtained in Duncan test using the software SAS
dimensionless. At lower angles (e.g., 208), high gloss samples are better differentiated, whereas higher angles (e.g., 608) are better for low gloss surface measurements [30]. Thus, considering that the films presented high gloss, the results obtained at 208 were more considered. It can be observed in Table 3 that the PVA concentration affected the gloss of the samples. The pure gelatin and pure PVA films presented the highest and lowest (p \ 0.05) gloss values at 208, respectively. The films produced in the present work were more glossy than those based on whey protein isolate (gloss = 87–96 at 458) [34] and on hydroxypropyl methylcellulose (gloss \ 100 at 20, 60 and 858) [30]. Mechanical Properties Increasing PVA concentrations, from 0 to 33.3 g PVA/ 100 g macromolecules in the blend, caused a linear reduction (R2 = 0.85) in the resistance to puncture of the films, attaining values lower than that of the pure PVA films (Fig. 1). On the other hand, increasing the PVA concentration in the blend did not affect (p [ 0.05) the puncture deformation of the films (Fig. 1), which deformed less (p \ 0.05) than the pure PVA films (12.6%). According to the results of the tensile tests, the tensile resistance (Fig. 2) and rigidity (Fig. 3) of the films prepared with blends of gelatin and PVA decreased linearly (R2 C 0.96) with increasing PVA concentration in the blend, tending to the value of the pure PVA films, contrary to that observed in the puncture tests (Fig. 1). This tensile behavior is typical of homogeneous and thermodynamically miscible systems, and agrees with the results of Ke and Sun [35], who observed an increase (practically linear) in the tensile strength of films based on blends of cornstarch and poly(lactic acid) with different PVA concentrations. However, Chiellini et al. [21], working with films plasticized with glycerol, observed a minimum
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Fig. 1 Puncture force and puncture deformation of films based on blends of gelatin and PVA CelvolÒ418 as a function of the concentration of PVA (CPVA) in blends
resistance at 50:50 gelatin:PVA, similar to that observed in the puncture tests (Fig. 1). As in the puncture tests, no significant differences (p [ 0.05) in elongation at break of the films were observed, which practically remained the same as that of the pure gelatin based films. The elongation at break of pure PVA films (134%) was higher than that of the films prepared with blends of gelatin and PVA (Fig. 2). This behavior of elongation at break was similar to that observed by Chiellini et al. [16]. The films based on pure PVA produced in this work presented tensile resistance equivalent to those produced by Chiellini et al. [21] (35 MPa) and Chiellini et al. [16] (42 MPa), although being less deformable, the latter two presenting values of 200% and 225%, respectively, for films that could be considered as thick (0.3 mm).
Fig. 2 Tensile strength and elongation at break of films based on blends of gelatin and PVA CelvolÒ418 as a function of the concentration of PVA (CPVA) in blends
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Fig. 3 Elastic modulus of films based on blends of gelatin and PVA CelvolÒ418 as a function of the concentration of PVA (CPVA) in blends
In an attempt to explain these results, some DSC curves of the pure gelatin and pure PVA films were obtained (Fig. 4). These samples presented moisture values of 13.2% and 6.3%, respectively. It can be observed that both films presented partially crystalline structures. The DSC curves obtained in the first scan presented a glass transition followed by a fusion of the crystalline portion. The glass transitions occurred at (Tg) 57.5 and 26.3 °C for the gelatin and PVA films, respectively, and the initial melting temperatures (Tm) were 89.0 and 110.7 °C, respectively.
The DSC curve obtained in the second scan for the gelatin films was typical of amorphous materials. Only one glass transition, shifted to a lower temperature (55.5 °C) than that determined in the first scan, could be observed. This occurred because the cryogenic cooling before the second scan did not allow the formation of new microcrystalline junctions in the macromolecules [6]. This Tg value was similar to that observed by Sobral and Habitante [32], working with pigskin gelatin samples. In the case of pure PVA films, the disappearance of the endothermic peak was only observed in the third scan, i.e., suggesting that some crystallites were not melted during the first heating, so the sample was still partially crystalline after the first scan (Fig. 4). Moreover, the glass transition shifted to a higher temperature, at 70.3 °C. This value was practically identical to that found (70.9 °C) by Sreedhar et al. [36], also working with pure PVA. This inverse behavior, with the value of Tg of the second and third scan being higher than that observed in the first scan, which is apparently strange, allows for the suggestion that the sample of PVA might be constituted of several fractions of different molecular weight. This could also explain the broadening of the endothermic peak observed in the DSC curves of the first scan of the PVA films, or, in other words, of the great distance between Tg and Tm. These results of the differential scanning calorimetry are of interest to explain the behavior of the mechanical properties of the films based on pure macromolecules [5]. According to the results for Tg, determined in the first scan, the mechanical resistances (Figs. 1, 2) of the pure polymer films were higher for the material with a higher value for Tg, which was gelatin. This relation between Tg and mechanical resistance was also observed by Vanin et al. [9]. Water Solubility and Film Humidity
Fig. 4 DSC curves of films of pure gelatin (high) and pure PVA CelvolÒ418 (botton): numbers indicate scanning
Despite the lower hygroscopicity of the pure PVA film (8%), the humidity of the films prepared with blends of gelatin and PVA was not affected by the PVA concentration up to 28.6% of PVA, remaining near 14.5% (Fig. 5). These results suggest that higher PVA concentrations would be necessary to reduce the effect of the higher hygroscopicity of the gelatin. On the other hand, increasing the PVA concentration in the blends caused a linear increase (R2 = 0.98) in the film water solubility (Fig. 5). These results are in agreement with Chiellini et al. [16, 22], who observed that increasing the PVA concentration increased the solubility of films based on PVA blended with polysaccharides or gelatin, respectively. In the present study, the pure gelatin films presented the lowest (p \ 0.05) solubility (14.2%), which was lower than that determined for gelatin films with plasticizers [7].
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Fig. 5 Humidity and solubility in water of films based on blends of gelatin and PVA CelvolÒ418 as a function of the concentration of PVA (CPVA) in blends
The sensitivity of PVA to ambient relative humidity, and therefore the humidity of the PVA film after conditioning at a specific relative humidity, is directly related to its degree of hydrolysis and/or molecular weight. Increasing the degree of hydrolysis (DH) causes an increase in the resistance to water. According to Jang and Lee [37], PVA can be classified as completely hydrolyzed with a DH = 97.5–99.8%, and partially hydrolyzed with a DH = 87–89%. Thus the low hygroscopicity of the PVA CelvolÒ418 can be explained by its intermediary DH (91.8%). Also, according to Finch [25], the water solubility of PVA depends on its crystallinity and on the structure of the amorphous region of the polymer. In an attempt to explain the behavior of the water solubility of the films, samples of pure gelatin and pure PVA, with no previous treatment, were mixed with equal parts of distilled water and analyzed by DSC. Figure 6 shows that the gelatin presented a single, highly visible endothermal peak, with initial and peak temperatures of 28.3 and 32.3 °C, respectively, but that the PVA presented two peaks. The first peak was very broad, with an initial temperature of 46.2 °C and a peak temperature of 72.3 °C (DT = 26.1 °C), and the second peak was very sharp (DT = 3.4 °C), with initial and peak temperatures of 98.8 and 102.2 °C, respectively. To a certain extent, these results are of interest to explain the crystallinity of the PVA films (Fig. 4), because the heat treatment of solution B (see Material and Methods) was done at 95 °C, not sufficient to dissolve the crystallites relative to the second endothermal peak (102 °C). On the other hand, the results of the film solubility tests could be explained by the respective glass transition temperatures (Fig. 4). The PVA film had a Tg very close to room temperature (26.3 °C), which could have favored its
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Fig. 6 DSC curves of PVA CelvolÒ418 (a) and gelatin (b) with water (1:1)
complete solubilization, while the gelatin film had a higher Tg (57.5 °C). Evidently this is only a conjecture, and more studies are necessary to explain the results of film solubility. The DSC curves of the pure gelatin samples were similar to the curves obtained by Sobral and Habitante [32], working with bovine hide gelatin gels. Similar DSC curves for pure PVA can be observed in the work of Jang and Lee [37], who analyzed blends with equal parts of completely hydrolyzed (DH = 99.0–99.8%) and partially hydrolyzed (DH = 85.0–88.5%) PVA. The results of these authors corroborate with the suggestion that the PVA CelvolÒ418 was not constituted of a highly homogeneous mixture of PVA fractions. FTIR Spectra Despite being a qualitative analysis, the positions of the peaks obtained on the spectrum in FTIR spectroscopy are sensitive to conformations at the macromolecular or molecular levels [27]. The spectra of the films prepared with blends of gelatin and PVA were almost similar to that of pure gelatin films, indicating there were no major changes in the functional groups of the gelatin, induced by interactions between the PVA and the gelatin (Fig. 7). There are three characteristic signals in the FTIR spectra of proteins: amides I, II and III [38, 39]. The amide I band arises from the stretching of the C=O bond of the proteins; while the amide II band is due to vibrations of the N–H bond and vibrations due to stretching of the C–N bond, and amide III corresponds to vibrations in the planes of the
J Polym Environ Fig. 7 FTIR spectra of films based on blends of gelatin and PVA CelvolÒ418 with 0 (a), 9.1 (b), 16.7 (c), 23.1 (d), 28.6 (e), 33.3 (f) and (g) 100 g of PVA/ 100 g of macromolecules in blends
C–N and N–H of the amide bonds, or vibrations of the CH2 groups of glycine [40], an amino acid abundant in gelatin [2, 4]. The amide II and amide III peaks are less sensitive to the secondary structure of proteins like gelatin [39, 41]. An analysis of the amide I band of gelatin films (Fig. 7a) showed two peaks at 1,641 (appearing as a shoulder) and 1,633 cm-1. The peak at 1,633 cm-1, characteristic of the coiled structure of gelatin [39], could also be associated with CO and CN stretching [41], and was insensitive to the PVA content. On the other hand, the shoulder appearing at 1,641 cm-1 became a true peak at 1,645 cm-1 in the films with 9.1% and 16.7% of PVA (Fig. 7b, c), and also appearing as a shoulder at 1,641 cm-1 for films containing 23.1, 28.6 and 33.3% of PVA (Fig. 7d, e, f). These bands were not visible in the spectrum of the pure PVA films (Fig. 7g). In the amide II band, three peaks appeared at 1,520, 1,532 and 1,538 cm-1 in the films based on pure gelatin, constituting the top of a big peak. However, in the films prepared with blends of gelatin and PVA, these three peaks turned into two peaks, which appeared at 1,539 and 1,544– 1,548 cm-1. In the domain of amide III, a highly visible peak appeared at 1,234 cm-1, and a less visible peak at 1,204 cm-1 for films of pure gelatin. This last peak was insensitive to the concentration of PVA, but the peak at 1,234 cm-1 shifted to 1,238 for films with 9.1–28.6% of PVA, and then to 1,241 cm-1 when the PVA concentration was 33.3% (Fig. 7f). A characteristic peak was observed at 1,048 cm-1 for the film of pure PVA (Fig. 7g) which is sensitive to crystallization [27]. This behavior may indicate some interaction between the gelatin and the PVA,
affecting the crystallinity of the films [27]. Maria et al. [23] analyzed films based on blends of gelatin and PVA plasticized by glycerol, by X-ray diffraction, and observed patterns typical of partially crystalline materials. The spectrum of the pure gelatin films also revealed a peak at 3,297 cm-1, normally associated with NH stretching [41]. However, the spectra of the films prepared with blends of gelatin and PVA and the pure PVA film, showed a peak at 3,288 cm-1, which is normally associated with OH stretching vibration.
Conclusions The different degree of hydrolysis of the PVA affected the physical properties of the films produced with blends of PVA and gelatin, but with no logical and direct relationship between the values for the physical properties and the degree of hydrolysis of the PVA. Thus the PVA that resulted in films with greater mechanical resistance to tensile stress was chosen for the second part of this work. It was verified that the PVA concentration affected the properties of the films based on blends of gelatin and PVA. The mechanical resistance, rigidity and water solubility of the films prepared with blends of gelatin and PVA varied between the values shown for these properties by the pure gelatin and pure PVA films. The resistance to puncture presented a minimum at 33.3% PVA, but neither puncture deformation nor elongation at break were affected by the PVA concentration, which also failed to affect the humidity of the films up to a concentration of 28.6% of PVA in the
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blends, despite this being less hygroscopic than the gelatin. According to the FTIR analysis, some interactions between the gelatin and the PVA occurred depending on the PVA concentration, affecting the crystallinity of the films, as also observed by the DSC analyses. Acknowledgements To FAPESP for the research Grant (05/577818), for the IC fellowship (05/54418-0) of GGDS, and for the Postdoctoral fellowship (05/54688-7) of PVAB; and to CNPq for the Postdoctoral fellowship of OMT, and PQI fellowship of PJAS.
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