Gelatin-Based Blends and Composites. Morphological and Thermal

Jul 10, 2001 - Cast films presented thermal and mechanical properties, which make them good candidates as biodegradable self-fertilizing mulching film...
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Biomacromolecules 2001, 2, 806-811

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Gelatin-Based Blends and Composites. Morphological and Thermal Mechanical Characterization Emo Chiellini,*,† Patrizia Cinelli,† Elizabeth Grillo Fernandes,† El-Refaie S. Kenawy,‡,§ and Andrea Lazzeri⊥,| Department of Chemistry & Industrial Chemistry, University of Pisa, Via Risorgimento 35, 56126 Pisa, Italy; Department of Chemistry-Faculty of Science, Tanta University, Tanta, Egypt; and Department of Chemical Engineering, Industrial Chemistry & Material Science, University of Pisa, Via Diotisalvi 2, 56126 Pisa, Italy Received February 9, 2001; Revised Manuscript Received May 16, 2001

Gelatin, a natural macromolecular proteic material from renewable resources, is widely used in many industrial applications. In this study gelatin scraps, deriving from pharmaceutical capsule productions, were turned into films by casting water solutions or suspensions producing flexible and consistent films. Gelatin was blended with poly(vinyl alcohol), a biodegradable synthetic polymer, in order to improve mechanical properties in the films. Gelatin was blended also with sugar cane bagasse, a lignin cellulosic waste from sugar cane processing. These blends showed good interface adhesion between gelatin and sugar cane fibers and mechanical properties of practical interest for up to 20% of sugar cane content by weight. Glutaraldehyde was used in different amounts as a cross-linking agent increasing elongation at break especially for amount above 1%. Morphology, thermal, thermomechanical, and mechanical properties of the samples were investigated by scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and tensile tests, respectively. Cast films presented thermal and mechanical properties, which make them good candidates as biodegradable self-fertilizing mulching films. Introduction Plastic wastes contribute to a significant part of the overall waste volume in municipal landfills1 and represent a serious environmental concern for the resistance of most synthetic plastics to microbial attack. Plastics represent, however, an integral part of contemporary life, and their share in both commodities and high-tech applications is expected to increase in the future. This paradox has stimulated research activities on different fronts of plastics waste production and management with particular reference to recovery of their free energy content via mechanical recycling, biorecycling, photobiodegradation, and controlled combustion.2,3 The use of biobased polymeric materials as one of the components in the production of plastic items presents several advantages connected with their origin and amenability to biodegradation.4 Among biopolymers, proteins have shown to be versatile materials that combine many characteristics relevant for technical applications such as good processability both in the melt and in solution and good film-forming * To whom all the correspondence should be addressed. Telephone: +3950-918299. Fax: +39-50-28438. E-mail: [email protected] † Department of Chemistry & Industrial Chemistry, University of Pisa. ‡ International Center for Science and High Technology. § ICS-UNIDO fellow on leave from the Department of Chemistry, Polymer Research Group, Faculty of Science, University of Tanta, Tanta, Egypt. ⊥ Department of Chemical Engineering, Industrial Chemistry & Material Science, University of Pisa. | Current address: Massachusetts Institute of Technology, Cambridge, MA 02139.

properties.5 Proteins are often used for the formulation of products such as coatings, capsules in pharmaceutical and food industries, adhesives, surfactants, and plastic items. However, the higher price of proteins and proteinaceous materials as compared to some other biopolymers, especially starch and cellulosics, has limited research on their technical applications. As a part of our continuing research program aimed at the preparation and evaluation of biobased and petrobased, environmentally degradable, polymeric materials for agroindustrial and packaging applications, studies on the applicability of scraps of animal gelatin from pharmaceutical industry (waste gelatin, WG) in the formulation of blends with poly(vinyl alcohol) (PVA) and composites with sugar cane bagasse (SCB) as biobased filler were undertaken.6 It was previously observed that the increase of PVA in WG/PVA blends lowered the mineralization rate in soil burial tests.6 Accordingly, a proper balance of WG/PVA blend composition can be expected to modulate the degradation time of the mulching films. Gelatin based films can display a selffertilizing character as consequence of their ultimate bio assimilation by the soil microflora, thanks to the presence of the proteic nitrogen. Gelatin represents a typical renewable material from natural resources of animal origin. It is made up of macromolecules consisting of amino acids residues in variable relative proportions and distributions along the macromolecular backbone.7,8 Gelatin scraps generated in

10.1021/bm015519h CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001

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different manufacturing processes comprising pharmaceutical, cosmetics, tannery, and food segments may often constitute a concern for the environment connected to their disposal.9 They strongly swell in water and have high carbon and nitrogen content that may lead to high oxygen demand once they reach the sewage drainage system, wastewater treatment plants, and eventually freshwater streams. A labor intensive and expensive treatment is required for correct management in their disposal. Value-added solutions as aimed at defraying their disposal cost are then sought. Hardening of gelatin with low molecular weight aldehydes (formaldehyde, glutaraldehyde) is well documented in the literature. Cross-linking is predominantly due to Schiff’s base formation by condensation of the formyl group and the -amino groups present in lysine and hydroxylysine residues. This reaction can take place in high yield even at room temperature.10-12 Sugar cane bagasse (SCB) is an agro-industrial byproduct from sugar juice extraction from canes and represents a renewable, low-cost, nonabrasive, high-moisture-absorbing and -retaining, biodegradable filler.13 The principal components in bagasse are fiber (ca. 45 wt %), water insolubles (ca. 2-3 wt %), water solubles (ca. 2-3 wt %), and water (ca. 50 wt %). SCB is not a homogeneous material with regard to fiber sizes that can range from 1 to 25 mm in length depending on the workup applied in cane processing.14 Poly(vinyl alcohol) (PVA) is a hydrolysis product of poly(vinyl acetate) (PVAc) and is a polar, water-soluble, synthetic polymer. Besides, it is recognized as one of the few synthetic polymers truly biodegradable under both aerobic and anaerobic conditions.15 In the present paper, we report on thermal and mechanical properties of blends and composites based on waste gelatin (WG), poly(vinyl alcohol) (PVA), and sugar cane bagasse (SCB) as attained by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and tensile testing. Materials and Methods Materials. Poly(vinyl alcohol) (PVA) was a Hoechst product, Mowiol 08/88, with an average molecular weight of 67 kDa and 88% hydrolysis degree. Waste gelatin (WG) consisting of net shape rubbery scraps as derived from the punching process applied in the pharmaceutical soft capsules manufacturing, was kindly supplied by Rp Scherer Co., Alexandria, Egypt. These scraps were used as received and contained several additives from original formulation as pigments and glycerol. Sugar cane bagasse (SCB) was kindly supplied by the Brazilian company Copersucar. SCB fibers were dried in an oven at 50 °C for 24 h and then pulverized with a blade grinder. The ground bagasse was sieved and the fraction passing through a mesh sieve of 0.212 mm was collected. Elemental analysis on SCB and WG are reported in Table 1. Glutaraldehyde (GA) was Aldrich product commercialized as 50 wt % aqueous solution and was used as cross-linking agent in various weight proportions without any further purification treatment. Sample Preparation. Waste gelatin (WG) was suspended in water at 50 °C under stirring for 30 min. For blends

Table 1. Elemental Analysis (wt %) of Sugar Cane Bagasse (SCB) and Waste Gelatin (WG) sample SCB WG

C 48.72 42.43

H

N

protein

fat

fibers

ash

9.10

2.60

42.62

6.05

6.32

0.34 14.77

Table 2. Composition (wt %) of the Tested Blends and Composites Based on Waste Gelatina un-cross-linked specimen

cross-linked specimen

sample

WG

PVA

sample

WG

PVA

GA

PVA WG WGP10 WGP20 WGP30 WGP50 WGP70 WGP80 WGP90 WGSCB20b

0 100 90 80 70 50 30 20 10 80

100 0 10 20 30 50 70 80 90 0

WGX1 WGX2 WGX3 WGP80X WGSCB20Xb

99.75 99 97.5 20 79.75

0 0 0 79.75 0

0.25 1.00 2.50 0.25 0.25

b

a WG: waste gelatin. PVA: poly(vinyl alcohol). GA: glutaraldehyde. Containing 20 wt % of sugar cane bagasse (SCB).

preparation a 10 wt % PVA/water solution was introduced in a 10 wt % WG/water suspension and the resulting homogeneous mixture was stirred at 70 °C for 20 min. Both WG suspensions as well as the WG scraps were stored at 4 °C to avoid spontaneous contamination by fungi. WG/SCB composites were prepared by adding the required amount of bagasse in a WG/water suspension. The mixture was stirred for 30 min at 50 °C. For cross-linked samples the desired amount of glutaraldehyde/water solution was introduced in a WG/water suspension and the mixture was stirred for 5 min at room temperature. Films were obtained by casting of the water suspension in Teflonated aluminum trays followed by water evaporation. Composition of the tested blends and composites is reported in Table 2. Scanning Electron Microscopy (SEM). SEM inspection was carried out on a JEOL T300 scanning electron microscope. The film samples were prepared by critical point drying followed by sputtering with gold and observation at 10 keV. TGA. A Mettler TA4000 System consisting of a TG50 furnace, a M3 microbalance and a TA72 Graph Ware was used for thermal degradation measurements. Samples of ca. 10 mg were heated in the temperature range of 25-600 °C at a scanning rate of 10 °C‚min-1 under nitrogen atmosphere with a flow rate of 200 mL‚min-1. DSC. A Mettler TA4000 System consisting of a DSC-30 cell and a TA72 Graph Ware was used for thermodynamic transitions evaluation. Samples of about 10 mg were heated from -100 to +220 °C at a scanning rate of 10 °C‚min-1 under a nitrogen flow of ca. 80 mL‚min-1. All samples were analyzed in duplicate. DMTA. Materials properties from dynamic-mechanical measurements were obtained from a Perkin-Elmer DMA-7. Three point bending method was performed by using a staticto-dynamic stress ratio of 110% at 1 Hz frequency and a heating rate of 5 °C‚min-1. Samples were maintained for 10 days in a air-conditioned room (50% relative humidity at 23 °C) before testing.

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Figure 1. SEM photomicrographs of WG/SCB composite film containing 20 wt % of SCB (WGSCB20): (a) upper surface (500×); (b) freeze-fractured transversal section (200×).

Tensile Tests. The tensile properties of the prepared films were determined by using an Instron Universal tensile testing machine (Model-4300). Dog-bone-shaped specimens were cut and then maintained in an air-conditioned room (50% relative humidity at 23 °C) for 10 days before testing. Specimens, type IV (ASTM-D638) had a rectangular cross section of 6 mm and a gauge length of 40 mm with a thickness of 0.3 mm. Experiments were run out at a crosshead speed of 20 mm‚min-1. Values were averaged on at least five specimens. Results and Discussion WG cast films appeared red, cohesive, and flexible. Blending with sugar cane conferred a dark color to the films. WG/SCB composites resulted harder than the flexible WG films. Increasing of SCB content produced more fragile films. When blends were prepared by mixing of WG water suspension and PVA water solution a homogeneous water suspension was produced, thus showing compatibility of the two components in the solvent. Whereas cast films appeared homogeneous only for a limited amount (20%) of one component into the other. Thus, in blends with the same amount of PVA and gelatin, phase separation and opacity were evident. SEM. WG/SCB composites resulted harder than the flexible WG films. Figure 1 shows photomicrographs of a

Chiellini et al.

Figure 2. SEM photomicrographs of freeze fractured transversal sections of WG/PVA blend film containing 50 wt % of PVA (WGP50) (750×): (a) as prepared; (b) after tensile test.

characteristic WG/SCB composite film obtained from solution casting. This composite revealed coarse surfaces (Figure 1a) with SCB fibers cemented and covered by gelatin. The transversal section (Figure 1b) showed empty spaces with random distribution of SCB fibers (diameter between 10 and 50 µm) tightly embedded within the continuous gelatin matrix. In response of a good adhesion between the two components. In Figure 2, SEM micrographs of a blend based on the same amount of WG and PVA, (WGP50) are reported. Clear indications of phase segregation of the two components were obtained from the SEM photomicrographs. Thus, the figure shows the freeze fractured transversal sections of WGP50 blend film as prepared and after submission to tensile test (Figure 2b). In this last case the phase separation is particularly evident with one of the two components (round structure) separating from the other constituting the continuous matrix. TGA. Integral decomposition traces of WG/PVA blends cast films and their parent polymers show up to three overlapped steps leaving about 15 wt % of residue at 600 °C, in nitrogen atmosphere (Figure 3). The first weight loss was attributed to water and/or volatile components evaporation corresponding to 6-8 wt %. The second and third steps represent the sequence of pyrolytic reactions. PVA sample showed to be about 40 °C more stable than WG with Ton

Mulch from Waste Gelatin

Figure 3. Thermogravimetric traces of waste gelatin (WG) and poly(vinyl alcohol) (PVA) based blends: (- -) PVA; (--) WGP80; (- -) WGP50; (- - -) WGP20; (s) WG.

Figure 4. DSC traces of WG/PVA blends and their parent conditioned at 50% of relative humidity (RH) at room temperature: (‚‚‚) PVA; (- ) WGP80; (- - -) WGP50; (- -) WGP20; (s) WG.

(onset temperature corresponding to the crossover of tangents drawn on both sides of the decomposition trace) at 276 °C. The decomposition of WG/PVA blends takes place at a temperature value comprised between those corresponding to the parent components. Their stability decreases while increasing the content of the less stable component. In fact, Ton changes from 250 °C for WGP20 to 272 °C for WGP80. Onset temperature of SCB was 239 °C that is equivalent to that of WG. However, Ton of WG/SCB composite with 20 wt % of SCB (WGSCB20) decreased slightly to 223 °C. Formulations based on WG presented above showed that its pyrolysis can be delayed by the presence of the more stable second component. These results can be a consequence of the degree of WG dispersion in the blend. Thus, the more stable component should play a kind of shielding role appreciably refraining the decomposition of less thermally stable component. DSC. Figure 4 shows DSC traces for WG/PVA blends and their parent components after conditioning at 50% of relative humidity (RH) at room temperature. Adsorbed water plays just a strong influence on the polymer properties of hydrophilic polymers such as PVA and gelatin. In particular, the glass transition temperature (Tg) can be lowered substantially due to the increase in the mobility of individual polymer chains.16 Thus, PVA and WG/PVA blends presented a broad endothermic peak around 100 °C due to water

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evaporation. Although this process may interfere with the detection of other thermal transitions, the specimens were not dried. High dehydration can indeed induce irreversible cross-linking in gelatin samples17 leading thus to a change in the properties of obtained materials. The change of the baseline associated with glass transition of plasticized PVA (ca. 7 wt % of water) was observed at about 22 °C. This value slightly moves toward lower temperatures with increasing WG content in the blends. For example, WGP80 (20 wt % WG) showed a Tg at about 15 °C and WGP50 at 7 °C. For high WG concentration (WGP20) Tg was not any longer detectable. This decrease of PVA Tg can be attributed to the additional plasticizer effect of glycerol present on WG. In WG-cast film the baseline change at -32 °C can be attributed to the gelatin-glycerol system as observed by Fraga et al.18 WG itself displays one specific heat capacity change with endothermic relaxation at 67 °C in accordance with a previous study.19 The temperature of this transition for WG increased with increasing of PVA content in the blends and became less evident until to vanish in the blend containing 80 wt % PVA. This trend was attributed to a partitioning of glycerol between WG and PVA. Consequently, WG resulted less plasticized in the blend. Fraga18 observed a Tg at 120 °C and a broad less pronounced Tg at 190 °C. Moisture absorption shifted these values and at 20 wt % of moisture content Tgs were located at -30 and +80 °C, respectively. The first glass transition was associated with gelatin soft blocks composed of sequences in which are present mainly R-amino acids, including glycine at every third position. The second glass transition was associated with gelatin rigid blocks composed of sequences mainly made up of the imino acids proline and hydroxyproline including glycine at every third position. A second endotherm on WG was observed at about 130 °C and attributed to the overlaying of different thermal events related to gelatin rigid blocks and water evaporation. This second endotherm was no more present when PVA was added to WG/PVA blends. It can be supposed that the more energetic water evaporation is hiding the WG endotherm. The melting temperature (Tm) of PVA decreases of about 6 °C in going from PVA cast films (191 °C) to WGP50 blend (185 °C) for example, in accordance with previous studies.19 Besides, the melting peak became less sharp due to the lower level of order in the crystalline phase until it is no longer detectable for 80 wt % WG (WGP20). DMTA. The curves of storage modulus (E′) and tan δ of WG/PVA blends as a function of temperature and composition are shown in Figure 5. Storage modulus of blends decreased with increasing of WG content. In the glass state the values of E′ were about 30, 6, and 4 GPa for respectively WGP90, WGP70, and WGP50. In the rubbery state the values of E′ showed a decrement of about 100 times in comparison with starting values. For temperatures above 70 °C, WG began to flow. The same behavior was observed for the maximum of the tan δ detected at 30.8, 29.1, and 23.9 °C, respectively. The broad dispersion observed for the blend containing 10 wt % of WG (WGP90) can be

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Figure 5. Dependence of storage modulus on temperature and composition of blends based on waste gelatin (WG) and poly(vinyl alcohol) (PVA): (s) WGP90; (--) WGP70; (- -) WGP50.

Figure 7. Dependence of Young’s modulus, tensile strength and elongation at break on gutaraldehyde content in cross-linked waste gelatin (WG).

Figure 6. Dependence of Young’s modulus, tensile strength and elongation at break on composition of the waste gelatin (WG)/poly(vinyl alcohol) (PVA) blends.

interpreted as a plasticizer effect played by the glycerol on PVA phase. Tensile Test. Plasticized PVA and WG cast films (after conditioning at 50% RH) presented elongation at break of 211% and 116%, Young’s modulus of 387 and 78 MPa, and tensile strength of 35 and 11 MPa, respectively (Figure 6). With addition of up to 20 wt % of WG to PVA (WGP90, WGP80) the elongation at break of blends increased up to ca. 260%. This behavior can be attributed to the additional plasticization of PVA by the glycerol present in WG. In accordance to this plasticizer effect played by the glycerol, tensile strength and Young’s modulus decreased. However, for blends with higher WG content these three mechanical properties dropped down to the values typical of WG. Thus, WG/PVA blends with compositions up to 20 wt % of WG appeared to be more flexible than those with higher amounts

of WG (WGP50, WGP70) whose films became harder and started to turn opaque. Introduction of SCB in WG sharply reduced both elongation at break and tensile strength while increasing Young’s modulus. WG/SCB20 composite (20 wt % of SCB) showed an elongation at break of 14% that was about 10 times lower than that of WG. Young’s modulus increased almost three times the value of the parent polymer (228 MPa). However, any significant changes on the tensile strength were not observed for this amount of natural filler. When increasing the glutaraldehyde concentration from 1 to 2.5 wt %, cross-linked WG showed a lowering in the Young’s modulus and at the same time a significant increase on elongation at break (Figure 7). Thus, Young’s modulus was 78 MPa for WG, 33 MPa for WGX1 (GA ) 0.25 wt %) and WGX2 (GA ) 1 wt %), and 10 MPa for WGX3 (GA ) 2.5 wt %). Elongation at break of WG (116%) increased to 140% for both WGX1 and WGX2 and to 300% for WGX3. By the other way, tensile strength did not significantly changed resulting placed at about 11 MPa. Cross-linking of gelatin with glutaraldehyde (GA) give rise to formation of short aliphatic segments between gelatin chains. When a warm solution of gelatin is cooled, not only chemical cross-linking will be present but also a physical one too (recovered collagen triple-helix structure).7 Consequently, it can be supposed that the cross-linked gelatin at ambient temperature will be the result of these two processes. With increasing of the GA concentration the probability of triple-helix recovering will be reduced and the structure of the cross-linked gelatin will be like that of a random coil polymer. This consideration can explain why, when GA concentration is increased from 1 to 2.5 wt % there is a

Mulch from Waste Gelatin

significant increase of the elongation at break and a concomitant decrease of the Young’s modulus associated with only a slight increase of tensile strength. A similar behavior was observed in water sensitivity and mineralization rates.6 So, it can be supposed that the balance of the chemical and physical cross-linking plays an important role on materials based on gelatin. WG blends containing PVA (WGP80X) or SCB (WGB20X) and cross-linked with 0.25 wt % GA did not show any significant differences in mechanical properties when compared with non-cross-linked parent specimen. Conclusion Blends and composites based on pharmaceutical grade gelatin scraps (WG) were prepared with poly(vinyl alcohol) (PVA) and sugar cane bagasse (SCB), respectively. Thermal degradation of WG/SCB composites was similar to that of the parent WG. WG/PVA blends on the contrary appeared more stable than WG as a result of a kind of shielding effect promoted by the more stable component PVA. WG/PVA blends showed two glass transition temperatures corresponding to each plasticized component thus suggesting the formation of immiscible blends within the composition range studied. This result was confirmed by SEM that clearly showed the establishment of a fairly distinct two-phase system. The cross-linking of WG with GA increased elongation at break of the corresponding cast films when GA concentration was higher than 1 wt %. This last result interestingly indicated that a guided balance of both physical and chemical cross-linkings of WG could allow for the attainment of films with modulated properties of mechanical strength and propensity to biodegradation. Indeed tensile strength decreased and elongation at break increased by increasing the WG content in the blends. For WG/SCB composites, a decrease of both mechanical properties as a consequence of the increase of the rigidity of the composite caused by the SCB addition was observed. WG/SCB composites showed a dark color that interestingly might result a good structural attribute for the potential application of derived films in soil solarization and weed control. The formulations containing up to 20 wt % of SCB display in fact mechanical properties applicable in agricultural mulching practices. The investigated WG/PVA blends and WG/SCB composites displayed even at high filler contents good thermal stability.

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Acknowledgment. The financial support by MURST through the PRIN’98 “Synthesis of Polymeric Materials for Unconventional Applications” is gratefully acknowledged. Grateful recognition has to be given also to the ICS-UNIDO for having acted as proactive mandator through his action on Environmentally Degradable Polymers & Plastics and the financial support of a fellowship to E.-R.S.K. Mrs. Irene Anguillesi is acknowledged for her assistance in performing the mechanical measurements. References and Notes (1) Doane, W. M. J. Polym. Mater. 1994, 11, 229-237. (2) IUPAC 38th Microsymposium on Recycling of Polymers, Prague, Czech Republic, July 14-17, 1997. Kahovec, J., Ed. Macromol. Symp. 1998, 135, 373 pp. (3) IUPAC International Symposium on Recycling of Polymers: Science and Technology, Marbella, Spain, September 18-20, 1991. Heitz, W., Eds.; Makromol. Chem., Macromol. Symp. 1992, 57, 395 pp. (4) Feil, H. Macromol. Symp. 1998, 127, 7-11. (5) de Graaf, L. A.; Kolster, P. Macromol. Symp. 1998, 127, 51-58. (6) Kenawy, E.-R.; Cinelli, P.; Corti, A.; Miertus, S.; Chiellini, E. Macromol. Symp. 1999, 144, 351-364. (7) Rose, P. I. Gelatin. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G. Eds.; John Wiley & Sons: New York, 1987; Vol. 7, pp 488513. (8) Yannas, I. V. J. Macomol. Sci. ReV. Macromol. Chem. 1972, C7, 49-104. (9) Environmental Bio-Process (M) Sdn. Bhd., GEL-OUTsGelatin Eradication System. http://www.malaysia-web.com/cyberdir/environ/ gelout.htm. (10) Digenis, G. A.; Gold, T. B.; Shah, V. P. J. Pharm. Sci. 1994, 83, 915-921. (11) Davis, P; Tabor, B. E. J. Polym. Sci.: Part A 1963, 1, 799-815 (12) Akin, H.; Hasirci, N. J. Appl. Polym. Sci. 1995, 58, 95-100. (13) Rowell, R. M.; Sanadi, A. R.; Caulfield, D. F.; Jacobson, R. E. Utilization of Natural Fibers in Plastic Composites: Problems and Opportunities. In LignocellulosicsPlastics Composites; Lea˜o, A. L., Carvalho, F. X., Frollini, E., Eds.; USP & UNESP: Sa˜o Paulo, Brazil, 1997; pp 23-59. (14) Gomez, A.; Ga´lvez, L.; De La Osa, O. Sugar Cane Bagasse. Utilization for Production of Composites. State of the Art in Cuba. In LignocellulosicsPlastics Composites; Lea˜o, A. L., Carvalho, F. X., Frollini, E., Eds.; USP & UNESP: Sa˜o Paulo, Brazil, 1997; pp 281-324. (15) Matsumura, S.; Tomizawa, N.; Toki, A.; Nishikawa, K.; Toshima, K. Macromolecules 1999, 32, 7753-7761. (16) Chartoff, R. P. Thermoplastic Polymers. In Thermal Characterization of Polymeric Materials, 2nd ed.; Turi, E. A., Ed.; Academic Press: San Diego, CA, 1997; V1, Chapter 3, pp 483-743. (17) Yannas, I. V. J. Polym. Sci.: Part. A-2 1968, 6, 687-694. (18) Fraga, A. N.; Wiliams, R. J. J. Polymer 1985, 26, 113-118. (19) Fernandes, E. G.; Kenawy, E.-R.; Miertus, S.; Chiellini, E. J. Appl. Polym. Sci., in press.

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