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Plasticizing and anti-plasticizing effects of polyvinyl alcohol in blend with thermoplastic starch
International Journal of Biological Macromolecules 140 (2019) 775–781
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Plasticizing and anti-plasticizing effects of polyvinyl alcohol in blend with thermoplastic starch Fatemeh Kahvand, Mohammad Fasihi ⁎ School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology, Tehran, Iran
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Article history: Received 20 July 2019 Received in revised form 11 August 2019 Accepted 21 August 2019 Available online 22 August 2019 Keywords: Thermoplastic starch Polyvinyl alcohol Phase behavior Miscibility Dynamic mechanical analysis X-ray diffractometry
a b s t r a c t Thermoplastic starch (TPS) and polyvinyl alcohol (PVA) blends were prepared by extrusion method in the presence of water/glycerol and citric acid, and their phase behavior was investigated. Results of scanning electron microscopy (SEM) and X-ray diffractometry (XRD) tests demonstrated the complete plasticization of starch during the melt mixing. PVA crystal phase was not formed in blends with less than 50 wt% PVA. Fourier-transform infrared spectroscopy (FTIR) and dynamic mechanical analysis (DMA) analysis revealed high compatibility between two phases, starch and PVA. Addition of PVA to TPS resulted in the reduction of the glass transition temperature and dynamic moduli and improvement of sample flexibility. However, at 10 wt% PVA, an anti-plasticization effect caused to increase the Tg and storage modulus of the blend. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, a wide range of petroleum-based synthetic polymers is used in various applications such as packaging, automotive, and construction industries. Almost 300 million tons of plastics consumed each year require only about 4% of the fossil resources extracted in the world to manufacture. However, if the current strong growth of plastics usage continues as expected, the plastics sector will account for 20% of total oil consumption by 2050 [1]. Rising in raw materials cost, concerning about depletion of oil resources and environmental pollution caused by the accumulation of large volumes of polymeric materials in nature, have led to increase attention to the replacement of petroleum-based by biodegradable polymers and their blends in many applications, especially packaging industry. In recent years, researchers have focused on biopolymers like proteins and carbohydrates for the production of plastics. Among biopolymers, starch has given more attention due to wide availability and low cost [2]. Starch is a semi-crystalline polymer composed of two types of alpha-glucan, amylose and amylopectin. Amylose is a relatively long linear α-glucan containing around 99% (1 → 4)-α- and (1 → 6)-α-linkages with the molecular weight of 105–106. Amylopectin is a much larger molecule with heavily branched structure [3]. Native starch cannot be considered as a thermoplastic polymer, because strong intermolecular hydrogen bonding prevents from melting [4,5]. But in the presence of plasticizers, the intermolecular bonds are weakened so that starch can ⁎ Corresponding author at: Narmak, Tehran 16846-13114, Iran. E-mail address: [email protected] (M. Fasihi).
https://doi.org/10.1016/j.ijbiomac.2019.08.185 0141-8130/© 2019 Elsevier B.V. All rights reserved.
be melted at high temperature under shear stress [4]. The addition of plasticizers to the starch leads to reduce the intermolecular force, increase the chain mobility, decrease the glass transition temperature and consequently improves the ductility and extensibility [6,7]. Changing in the mechanical properties of TPS with the variation of relative humidity, poor water resistance, and processability limited the use of it in some applications [8]. Blending is considered as an effective approach to overcome these drawbacks [8,9]. Properties of TPS blends with different polymers such as polycaprolactone [10], polylactic acid (PLA) [11], and polyvinyl alcohol (PVA) [12] have been studied. PVA is a watersoluble fully degradable polymer thermoplastic with 1,3-diol units or 1,2-diol units depends on the hydrolysis process of poly (vinyl acetate) [13]. Incorporation of PVA into starch improves the thermal and mechanical properties of the material and thus modifies the polymer structure at both molecular and morphological level [14]. The effect of using different plasticizers on properties of TPS blends has been investigated. Zhou and his coworkers found that using urea and glycerol mixture as plasticizers for TPS/PVA blend improves rheological properties by the formation of more strong and stable hydrogen bond with water and PVA/starch granules [15]. Park et al. 2005 examined glycerol, sorbitol, and citric acid as plasticizers to prepare TPS/ PVA films [16]. Most of the earlier research relates to study of the effect of the composition and plasticizers [12,17,18], crosslinking [19–21] or adding reinforcements like nanoparticles and nano and micro fibers [22–25] on the thermal, mechanical and barrier properties of the TPS/PVA blends. The phase morphology and miscibility of TPS/PVA blends by varying the PVA content was not intensely focused, yet. Some conventional
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techniques like optical microscopy and differential scanning calorimetry (DSC) are challenging to apply for phase study due to evaporation of plasticizer during these tests. This work presents a phase study on TPS/PVA blends prepared by one- step extrusion process. The plasticization effect of PVA for TPS was investigated, as well. 2. Experimental 2.1. Materials Normal corn starch (10% moisture) with about 23% amylose was obtained from Glucosan company (Qazvin, Iran). PVA 24-88 with weight average molecular weight of 120 kg/mol was obtained from Shangxin China (degree of hydrolysis: 88%). Glycerol and citric acid from Daejung Co. (Shiheung, South Korea) were used as plasticizers. 2.2. Preparation of starch/PVA blend Dry starch, plasticizers (water, glycerol and citric acid) and PVA were premixed by a high-speed mixer in room temperature and stored about 48 h to make plasticizers to penetrate to starch granules and PVA. Melt blending of pre-mixtures was carried out by a single screw extruder (L/ D: 26, KEX series, Kajaran, Iran) equipped with a 4 mm rod die the temperature profile from feeding zone to the die was 98–145–135 °C and screw speed of 31 rpm. Two TPS samples containing citric acid and without citric acid were prepared as mentioned above, to study the effect of citric acid on TPS properties. The formulations of samples are presented in Table 1. TPS included 21 wt% glycerol. Other samples included 21 wt% glycerol and 2 wt% citric acid. The remaining part of each sample (77 wt%) consisted of different ratio of PVA/starch. 2.3. Characterizations 2.3.1. Scanning electron microscopy (SEM) Morphology of starch and the cryo-fractured surface of blends were studied by using scanning electron microscope (TESCAN, Vega II) with an accelerating voltage of 30.00 kV. Before SEM, samples were coated with a thin layer of gold. 2.3.2. FTIR FTIR spectra were recorded with Perkin Elmer IR spectrum scanner at 1 cm−1 resolution over the range of 400–4000 cm−1. The samples were prepared by mixing the powder with KBr and pressing to the thin slices. 2.3.3. X-ray diffraction (XRD) X-ray diffraction patterns of samples were recorded by using an Xray diffractometer (DRON8, BOUREVESTNIK) operated in 40 kV, 20 mA in scattering angels (2θ) ranges 3.5 to 60°. Relative crystallinity (RC) was obtained by using the Eq. (1), where Ac and Aa correspond
Table 1 Samples compositions. Sample
TPS CATPS P10/S90 P20/S80 P30/S70 P40/S60 P50/S50 PVA
PVA:dry starch (wt%) 0:100 0:100 10:90 20:80 30:70 40:60 50:50 100:0
PVA + starch (wt%) 69 67 69 67 67 67 67 67
Glycerol (wt%) 21 21 21 21 21 21 21 21
Citric acid (wt%) – 2 2 2 2 2 2 2
Water (wt%) 10 10 10 10 10 10 10 10
to the area of the crystalline and amorphous phases, respectively.
RC ¼
Ac 100 Ac þ Aa
ð1Þ
Deconvolution computations performed on XRD spectra of the samples to evaluate the Ac and Aa. The deconvolution allows calculating the areas of all peaks, separately. For TPS and PVA three crystal peaks and one amorphous halo were considered. For single-phase blends, one amorphous halo was considered similar to pure materials. The complete method of deconvolution for blends was expressed by Psarski et al. [26]. 2.3.4. Dynamic mechanical analysis (DMA) Dynamic mechanical measurements of the samples with dimensions 10 × 5 mm2 and thickness of 1 mm were carried out in a NETZSCH 242c instrument, in tension mode at 1 Hz and heating rate of 5 °C/min over a temperature range of −100 to 140 °C. The specimens were prepared by hot press molding of extruded samples at 150 °C, and conditioned at room temperature before test for 4 days for equilibration. 3. Results and discussion 3.1. Characterization of thermoplastic starch During the conversion of starch to TPS, the plasticizers in starch attenuate the strong interactions between hydroxyl groups of starch molecules by forming new hydrogen bonds with starch molecules. Formation of homogenous TPS is a result of strong interactions caused by hydrogen bonds between starch and the plasticizers [25]. FTIR analysis is utilized as a suitable method for the qualitative survey of interactions between plasticizers and starch molecules. Fig. 1 presents the FTIR spectrum of starch and TPS containing 2% citric acid and without citric acid. The bands present in the range of 2800 to 3000 cm−1 are related to C\\H bond stretching, the peak at 1650 is due to angular deformation of water, the peak at 2363 cm−1 is related to the atmosphere carbon dioxide, and the band at 1460 cm−1 is due to CH2 bond. The characteristic peaks of starch molecules in the range of 994 to 1160 cm−1 is due to C\\O bond stretching which is divided into two subgroups including C\\O bond stretching of C-O-H group (in the range of 1080 to 1160) and C\\O bond stretching of C-O-C group existing in anhydroglucose ring of starch (in the range of 990 to 1030) [27]. Reduction of thermoplastic starches peak wavenumbers compared to pure starch and formation of dual peaks at 992 and 1024 cm−1 for thermoplastic starches instead of 994 cm−1 for pure starch, confirmed the new intermolecular interactions and formation of more stable hydrogen bonds with the oxygen of C-O-H group and anhydroglucose ring of starch by plasticizers. This wavenumber reduction was observed intensively in the sample containing citric acid (CATPS) which represented the higher capability of carboxyl groups of citric acid for the formation of stronger hydrogen bonds with the C\\O groups of starch. Reduction of starch wavenumber at 3436 spectrum, which represents the free intermolecular, and intramolecular bond hydroxyl groups of water and starch in TPS and CATPS samples, showed the attenuation of original hydroxyl interaction in starch by addition of plasticizers. The presence of a peak at 1737 cm−1 for CATPS was due to the formation of ester groups followed by citric acid. Partial esterification of starch results not only in the improvement of thermal resistance, but also reduces the retrogradation rate of starch crystals after converting to TPS [28]. 3.2. Microscopic analysis of TPS Fig. 2 represents SEM images of starch granules prior to plasticization and production of TPS by the extrusion process. The morphology of TPS containing citric acid was completely homogenous and without
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Fig. 1. FTIR spectra of starch and thermoplastic starches.
Fig. 2. SEM micrographs of (a) neat starch (b) CATPS (c) TPS.
un-melted starch granules. It demonstrated that the plasticization process was carried out properly in the extruder by the plasticizers. The surface structure was slightly uneven, and some un-melted
starch granules were observed in the sample lacking citric acid, which was due to the weakness of glycerol in plasticization compared to glycerol/citric acid mixture [16]. The apparent structure of TPS and CATPS
Fig. 3. Biodegradable thermoplastic starches (a) TPS, (b) CATPS.
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process condition and input feed composition. Elimination of A-type peaks in CATPS demonstrated the absence of un-melted granules in CATPS sample which was formerly confirmed by SEM images.
3.4. TPS/PVA blends characterization
Fig. 4. XRD patterns of starch and CATPS.
are presented in Fig. 3. As shown, the presence of citric acid led to more clarity and brightness of samples by improving the thermal resistance of starch during the extrusion.
Considering the results of the previous section, TPS sample containing citric acid was selected for preparation of blend with PVA. One of the significant points in the blending process is the compatibility of blend components, which is dependent on the nature and chemical structure of components. The compatibility of CATPS/PVA blends with various percentages of PVA was investigated by FTIR. Fig. 5 signifies the FTIR pattern of PVA and CATPS/PVA blend containing 30% PVA. The peaks located in 3500–3600 spectrum were due to stretching of OH group, 2800–3100 cm−1 were related to C\\H bond stretching, 1350–1400 were due to C\\H bond bending, 1031–1255 cm−1 were related to C\\O bond bending, 1650 was due to C_O bond stretching, and 1560 was related to C_C bond stretching [31,32]. The peak at 1749 cm−1 of the FTIR spectrum of PVA was due to carbonyl functional group resulted by residual acetate groups after the hydrolysis of polyvinyl acetate. Reduction of the frequency of PVA hydroxyl band from 3600 to 3400 and broadening of this peak after blending with CATPS (Fig. 5a) and the formation of dual peaks at 1031 and 995 in the blend represented new interactions between PVA and starch. In comparison with CATPS, the horizontal shift of C\\O bands in the blend (at 1071 and 1151 cm−1) also confirmed the formation of stronger hydrogen bonds between PVA and starch in blend compared to pure CATPS (Fig. 5b).
3.3. X-ray diffraction for TPS 3.5. Microscopic analysis of blends XRD pattern, which is shown in Fig. 4, was used for investigating the crystalline structure of samples. Un-plasticized corn starch showed the characteristic peaks regarding the A-type crystal structure in its XRD pattern, which can be observed in 15.2, 17.1, and 23.5 angles in Fig. 4. Unlike the un-gelatinized starch, in TPS XRD pattern, A-type crystals were substituted with V-type in 13.5 and 20.8 angles [29]. V-type crystals are caused by the rapid crystallization of amylose in single helices involving lipids or other polar molecules during extrusion or cooling [30]. The amount of these types of crystals depends on the
Fig. 6 illustrates the SEM images of the blends containing 10, 20, 30, 40, and 50% of PVA. The blend morphology similar to pure CATPS lacked the un-melted starch granules. Furthermore, the SEM images represented a homogenous morphology and probably single-phase at room temperature in samples containing less than 50% PVA. In the sample with 50% PVA, the morphology was altered, and rippling was observed, which can be related to the formation of two-phase blend in this composition.
Fig. 5. FTIR spectra of a) PVA and P30/S70 blend, and b) P30/S70 and CATPS.
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Fig. 6. SEM micrographs of CATPS/PVA blends a) P10/S90 b) P20/S80 c) P30/S70 d) P40/S60 e) P50/S50.
3.6. X-ray diffraction of blends Fig. 7 plots the XRD pattern for CATPS/PVA blends. Similar to CATPS, A-type crystal peaks were not observed in CATPS/PVA blends and they were substituted with V-type crystals in about 13.5, and 20.8 angles [29,33]. The characteristic peaks regarding PVA crystals were observed in 19.34, 23.1, and 11.54 angles [31,34]. Elimination of PVA
characteristic peaks in the crystal structure of blends, excluding the blend with 50% PVA in which minor percentage of PVA crystals were formed in 23.2°, indicated that PVA crystals were not independently formed in the blend and PVA formed a co-crystal structure with starch. This confirmed the formation of a single-phase blend of these two materials. The samples crystallinity percentages are presented in Table 2. The crystallinity percentage of CATPS/PVA blends was generally
Fig. 7. XRD patterns of CATPS/PVA blends.
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F. Kahvand, M. Fasihi / International Journal of Biological Macromolecules 140 (2019) 775–781 Table 2 Relative crystallinity of different samples. Sample
Relative crystallinity (%)
Starch CATPS P10/S90 P20/S80 P30/S70 P40/S60 P50/S50 PVA
25 21.04 23.31 23.60 20.54 21.15 20.92 31
between the crystallinity percentage of PVA and CATPS and was more close to CATPS. The crystallinity percentage of single-phase blends can be independent of blends components ratio due to the formation of new crystalline phases containing starch chains and PVA. 3.7. Dynamic mechanical analysis The results of DMA in the first heating, as well as SEM images, were used for investigation of compatibility and mechanical properties of the blends. Fig. 8 displays the changes in the storage modulus (E′) and loss
factor (tanδ) in terms of temperature for CATPS, PVA, and CATPS/PVA blends. The first weak peak of tanδ graph, which generally appeared in negative temperatures, was considered as the relaxation of plasticizer-rich phase and the sharp peak domain at high temperature (β relaxation) was considered as the glass transition temperature of the polymer. γ relaxation which was observed just in pure PVA and P50/S50 samples was resulted from the crystalline domains of PVA [35]. The variation of the Tg and E′ at 25 °C versus PVA content are given in Fig. 8c and d. The results of DMA are presented in Table 3. Increasing PVA percentage in the blends, excluding the sample containing 10% PVA, led to the reduction of the glass transition temperature and storage modulus. The presence of only one β relaxations temperature in the blends with less than 50% PVA confirmed the formation of one-phase blend, which previously received from the results of XRD test and SEM images. In fact, PVA acted as a polymeric plasticizer for starch and decreased the glass transition temperature and the dynamic modulus. However, the addition of 10% PVA to starch caused to increase the glass transition temperature up to 25 °C as well as increasing the blend modulus which can be due to the anti-plasticization effect in low PVA contents [36,37]. It might be due to the strong interaction between starch and PVA at this composition. Miscible blends with a single
Fig. 8. a) tanδ and b) storage modulus versus temperature curves for CATPS/PVA blends, and (c) Tg and (d) storage modulus of blends at 25 °C versus PVA content.
F. Kahvand, M. Fasihi / International Journal of Biological Macromolecules 140 (2019) 775–781 Table 3 Dynamic mechanical properties of TPS/PVA blends. Sample
Tα°C
Tβ1°C
TPS P10/S90 P20/S80 P30/S70 P40/S60 P50/S50 PVA
−62 −61 −60 −50 −53 −55 −65
55 80 47 36 16 14 5
Tβ2°C
Tγ°C
75 85
E′ (MPa) at 25 °C
Deviation of Tg from additive rule
183 266 137 76 86 74 32
– 30 2 −4 −19 −16 –
Tg value, usually exhibit positive or negative deviations from the Tg value determined by the additive rules. According to the study of Lu et al. [38], positive deviation indicates strong interpolymer interactions in blends, e.g., interpolymer complexes and can be attributed to a negative interaction parameter (χ). The deviation of Tg from the additive rule is reported in Table 3. The most positive deviation was observed in P10/S90 samples which could be attributed to the formation of the strong complex between PVA and starch at this composition. By further increasing the amount of PVA to 40%, the glass transition temperature was remarkably reduced and reached 16 °C. Further addition of PVA to 50%, did not change the glass transition temperature so much and caused the advent of Tβ2 temperature, which was related to PVA crystals. This demonstrates that the blend P50/S50 reached its saturation limit, and PVA phase was formed, independently. The obtained results were in consistence with the results of SEM and XRD. 4. Conclusions TPS/PVA blends were produced by extrusion method using glycerol and citric acid. The results of FTIR for TPS indicated the formation of new hydrogen bonds between starch and the plasticizer. Using citric acid as co-plasticizer resulted in the formation of stronger and more stable hydrogen bonds with starch, and consequently led to the creation of more uniform microscopic structure. Moreover, the homogenous morphology in TPS/PVA blends was observed in SEM images. The elimination of PVA crystal peaks in the XRD analysis of blends containing less than 50% PVA along with the appearance of a single peak in loss factor demonstrated high compatibility between starch and PVA. PVA acted as a polymeric plasticizer for starch in percentages higher than 10% and led to the reduction of glass transition temperature and storage modulus of the blend. While, in 10% PVA, a strong interaction caused to anti-plasticization effect, resulted in increase the Tg and storage modulus. Acknowledgments The authors would like to thank the Iran National Science Foundation (INSF) for the financial support of this study (Grant number 96010131). References [1] M. Rujnić-Sokele, A. Pilipović, Challenges and opportunities of biodegradable plastics: a mini review, Waste Manag. Res. 35 (2017) 132–140. [2] F. Versino, M.A. García, Cassava (Manihot esculenta) starch films reinforced with natural fibrous filler, Ind. Crop. Prod. 58 (2014) 305–314. [3] R.F. Tester, J. Karkalas, X. Qi, Starch—composition, fine structure and architecture, J. Cereal Sci. 39 (2004) 151–165. [4] L. Zhang, X.F. Wang, H. Liu, L. Yu, Y. Wang, G.P. Simon, J. Qian, Effect of plasticizers on microstructure, compatibility and mechanical property of hydroxypropyl methylcellulose/hydroxypropyl starch blends, Int. J. Biol. Macromol. 119 (2018) 141–148. [5] X. Qiao, Z. Tang, K. Sun, Plasticization of corn starch by polyol mixtures, Carbohydr. Polym. 83 (2011) 659–664. [6] A. Edhirej, S.M. Sapuan, M. Jawaid, N.I. Zahari, Effect of various plasticizers and concentration on the physical, thermal, mechanical, and structural properties of cassava-starch-based films, Starch-Stärke 69 (2017), 1500366. [7] R. Belhassen, F. Vilaseca, P. Mutjé, S. Boufi, Thermoplasticized starch modified by reactive blending with epoxidized soybean oil, Ind. Crop. Prod. 53 (2014) 261–267.
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Plasticizing and anti-plasticizing effects of polyvinyl alcohol in blend with thermoplastic starch Fatemeh Kahvand, Mohammad Fasihi ⁎ School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 20 July 2019 Received in revised form 11 August 2019 Accepted 21 August 2019 Available online 22 August 2019 Keywords: Thermoplastic starch Polyvinyl alcohol Phase behavior Miscibility Dynamic mechanical analysis X-ray diffractometry
a b s t r a c t Thermoplastic starch (TPS) and polyvinyl alcohol (PVA) blends were prepared by extrusion method in the presence of water/glycerol and citric acid, and their phase behavior was investigated. Results of scanning electron microscopy (SEM) and X-ray diffractometry (XRD) tests demonstrated the complete plasticization of starch during the melt mixing. PVA crystal phase was not formed in blends with less than 50 wt% PVA. Fourier-transform infrared spectroscopy (FTIR) and dynamic mechanical analysis (DMA) analysis revealed high compatibility between two phases, starch and PVA. Addition of PVA to TPS resulted in the reduction of the glass transition temperature and dynamic moduli and improvement of sample flexibility. However, at 10 wt% PVA, an anti-plasticization effect caused to increase the Tg and storage modulus of the blend. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, a wide range of petroleum-based synthetic polymers is used in various applications such as packaging, automotive, and construction industries. Almost 300 million tons of plastics consumed each year require only about 4% of the fossil resources extracted in the world to manufacture. However, if the current strong growth of plastics usage continues as expected, the plastics sector will account for 20% of total oil consumption by 2050 [1]. Rising in raw materials cost, concerning about depletion of oil resources and environmental pollution caused by the accumulation of large volumes of polymeric materials in nature, have led to increase attention to the replacement of petroleum-based by biodegradable polymers and their blends in many applications, especially packaging industry. In recent years, researchers have focused on biopolymers like proteins and carbohydrates for the production of plastics. Among biopolymers, starch has given more attention due to wide availability and low cost [2]. Starch is a semi-crystalline polymer composed of two types of alpha-glucan, amylose and amylopectin. Amylose is a relatively long linear α-glucan containing around 99% (1 → 4)-α- and (1 → 6)-α-linkages with the molecular weight of 105–106. Amylopectin is a much larger molecule with heavily branched structure [3]. Native starch cannot be considered as a thermoplastic polymer, because strong intermolecular hydrogen bonding prevents from melting [4,5]. But in the presence of plasticizers, the intermolecular bonds are weakened so that starch can ⁎ Corresponding author at: Narmak, Tehran 16846-13114, Iran. E-mail address: [email protected] (M. Fasihi).
https://doi.org/10.1016/j.ijbiomac.2019.08.185 0141-8130/© 2019 Elsevier B.V. All rights reserved.
be melted at high temperature under shear stress [4]. The addition of plasticizers to the starch leads to reduce the intermolecular force, increase the chain mobility, decrease the glass transition temperature and consequently improves the ductility and extensibility [6,7]. Changing in the mechanical properties of TPS with the variation of relative humidity, poor water resistance, and processability limited the use of it in some applications [8]. Blending is considered as an effective approach to overcome these drawbacks [8,9]. Properties of TPS blends with different polymers such as polycaprolactone [10], polylactic acid (PLA) [11], and polyvinyl alcohol (PVA) [12] have been studied. PVA is a watersoluble fully degradable polymer thermoplastic with 1,3-diol units or 1,2-diol units depends on the hydrolysis process of poly (vinyl acetate) [13]. Incorporation of PVA into starch improves the thermal and mechanical properties of the material and thus modifies the polymer structure at both molecular and morphological level [14]. The effect of using different plasticizers on properties of TPS blends has been investigated. Zhou and his coworkers found that using urea and glycerol mixture as plasticizers for TPS/PVA blend improves rheological properties by the formation of more strong and stable hydrogen bond with water and PVA/starch granules [15]. Park et al. 2005 examined glycerol, sorbitol, and citric acid as plasticizers to prepare TPS/ PVA films [16]. Most of the earlier research relates to study of the effect of the composition and plasticizers [12,17,18], crosslinking [19–21] or adding reinforcements like nanoparticles and nano and micro fibers [22–25] on the thermal, mechanical and barrier properties of the TPS/PVA blends. The phase morphology and miscibility of TPS/PVA blends by varying the PVA content was not intensely focused, yet. Some conventional
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techniques like optical microscopy and differential scanning calorimetry (DSC) are challenging to apply for phase study due to evaporation of plasticizer during these tests. This work presents a phase study on TPS/PVA blends prepared by one- step extrusion process. The plasticization effect of PVA for TPS was investigated, as well. 2. Experimental 2.1. Materials Normal corn starch (10% moisture) with about 23% amylose was obtained from Glucosan company (Qazvin, Iran). PVA 24-88 with weight average molecular weight of 120 kg/mol was obtained from Shangxin China (degree of hydrolysis: 88%). Glycerol and citric acid from Daejung Co. (Shiheung, South Korea) were used as plasticizers. 2.2. Preparation of starch/PVA blend Dry starch, plasticizers (water, glycerol and citric acid) and PVA were premixed by a high-speed mixer in room temperature and stored about 48 h to make plasticizers to penetrate to starch granules and PVA. Melt blending of pre-mixtures was carried out by a single screw extruder (L/ D: 26, KEX series, Kajaran, Iran) equipped with a 4 mm rod die the temperature profile from feeding zone to the die was 98–145–135 °C and screw speed of 31 rpm. Two TPS samples containing citric acid and without citric acid were prepared as mentioned above, to study the effect of citric acid on TPS properties. The formulations of samples are presented in Table 1. TPS included 21 wt% glycerol. Other samples included 21 wt% glycerol and 2 wt% citric acid. The remaining part of each sample (77 wt%) consisted of different ratio of PVA/starch. 2.3. Characterizations 2.3.1. Scanning electron microscopy (SEM) Morphology of starch and the cryo-fractured surface of blends were studied by using scanning electron microscope (TESCAN, Vega II) with an accelerating voltage of 30.00 kV. Before SEM, samples were coated with a thin layer of gold. 2.3.2. FTIR FTIR spectra were recorded with Perkin Elmer IR spectrum scanner at 1 cm−1 resolution over the range of 400–4000 cm−1. The samples were prepared by mixing the powder with KBr and pressing to the thin slices. 2.3.3. X-ray diffraction (XRD) X-ray diffraction patterns of samples were recorded by using an Xray diffractometer (DRON8, BOUREVESTNIK) operated in 40 kV, 20 mA in scattering angels (2θ) ranges 3.5 to 60°. Relative crystallinity (RC) was obtained by using the Eq. (1), where Ac and Aa correspond
Table 1 Samples compositions. Sample
TPS CATPS P10/S90 P20/S80 P30/S70 P40/S60 P50/S50 PVA
PVA:dry starch (wt%) 0:100 0:100 10:90 20:80 30:70 40:60 50:50 100:0
PVA + starch (wt%) 69 67 69 67 67 67 67 67
Glycerol (wt%) 21 21 21 21 21 21 21 21
Citric acid (wt%) – 2 2 2 2 2 2 2
Water (wt%) 10 10 10 10 10 10 10 10
to the area of the crystalline and amorphous phases, respectively.
RC ¼
Ac 100 Ac þ Aa
ð1Þ
Deconvolution computations performed on XRD spectra of the samples to evaluate the Ac and Aa. The deconvolution allows calculating the areas of all peaks, separately. For TPS and PVA three crystal peaks and one amorphous halo were considered. For single-phase blends, one amorphous halo was considered similar to pure materials. The complete method of deconvolution for blends was expressed by Psarski et al. [26]. 2.3.4. Dynamic mechanical analysis (DMA) Dynamic mechanical measurements of the samples with dimensions 10 × 5 mm2 and thickness of 1 mm were carried out in a NETZSCH 242c instrument, in tension mode at 1 Hz and heating rate of 5 °C/min over a temperature range of −100 to 140 °C. The specimens were prepared by hot press molding of extruded samples at 150 °C, and conditioned at room temperature before test for 4 days for equilibration. 3. Results and discussion 3.1. Characterization of thermoplastic starch During the conversion of starch to TPS, the plasticizers in starch attenuate the strong interactions between hydroxyl groups of starch molecules by forming new hydrogen bonds with starch molecules. Formation of homogenous TPS is a result of strong interactions caused by hydrogen bonds between starch and the plasticizers [25]. FTIR analysis is utilized as a suitable method for the qualitative survey of interactions between plasticizers and starch molecules. Fig. 1 presents the FTIR spectrum of starch and TPS containing 2% citric acid and without citric acid. The bands present in the range of 2800 to 3000 cm−1 are related to C\\H bond stretching, the peak at 1650 is due to angular deformation of water, the peak at 2363 cm−1 is related to the atmosphere carbon dioxide, and the band at 1460 cm−1 is due to CH2 bond. The characteristic peaks of starch molecules in the range of 994 to 1160 cm−1 is due to C\\O bond stretching which is divided into two subgroups including C\\O bond stretching of C-O-H group (in the range of 1080 to 1160) and C\\O bond stretching of C-O-C group existing in anhydroglucose ring of starch (in the range of 990 to 1030) [27]. Reduction of thermoplastic starches peak wavenumbers compared to pure starch and formation of dual peaks at 992 and 1024 cm−1 for thermoplastic starches instead of 994 cm−1 for pure starch, confirmed the new intermolecular interactions and formation of more stable hydrogen bonds with the oxygen of C-O-H group and anhydroglucose ring of starch by plasticizers. This wavenumber reduction was observed intensively in the sample containing citric acid (CATPS) which represented the higher capability of carboxyl groups of citric acid for the formation of stronger hydrogen bonds with the C\\O groups of starch. Reduction of starch wavenumber at 3436 spectrum, which represents the free intermolecular, and intramolecular bond hydroxyl groups of water and starch in TPS and CATPS samples, showed the attenuation of original hydroxyl interaction in starch by addition of plasticizers. The presence of a peak at 1737 cm−1 for CATPS was due to the formation of ester groups followed by citric acid. Partial esterification of starch results not only in the improvement of thermal resistance, but also reduces the retrogradation rate of starch crystals after converting to TPS [28]. 3.2. Microscopic analysis of TPS Fig. 2 represents SEM images of starch granules prior to plasticization and production of TPS by the extrusion process. The morphology of TPS containing citric acid was completely homogenous and without
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Fig. 1. FTIR spectra of starch and thermoplastic starches.
Fig. 2. SEM micrographs of (a) neat starch (b) CATPS (c) TPS.
un-melted starch granules. It demonstrated that the plasticization process was carried out properly in the extruder by the plasticizers. The surface structure was slightly uneven, and some un-melted
starch granules were observed in the sample lacking citric acid, which was due to the weakness of glycerol in plasticization compared to glycerol/citric acid mixture [16]. The apparent structure of TPS and CATPS
Fig. 3. Biodegradable thermoplastic starches (a) TPS, (b) CATPS.
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process condition and input feed composition. Elimination of A-type peaks in CATPS demonstrated the absence of un-melted granules in CATPS sample which was formerly confirmed by SEM images.
3.4. TPS/PVA blends characterization
Fig. 4. XRD patterns of starch and CATPS.
are presented in Fig. 3. As shown, the presence of citric acid led to more clarity and brightness of samples by improving the thermal resistance of starch during the extrusion.
Considering the results of the previous section, TPS sample containing citric acid was selected for preparation of blend with PVA. One of the significant points in the blending process is the compatibility of blend components, which is dependent on the nature and chemical structure of components. The compatibility of CATPS/PVA blends with various percentages of PVA was investigated by FTIR. Fig. 5 signifies the FTIR pattern of PVA and CATPS/PVA blend containing 30% PVA. The peaks located in 3500–3600 spectrum were due to stretching of OH group, 2800–3100 cm−1 were related to C\\H bond stretching, 1350–1400 were due to C\\H bond bending, 1031–1255 cm−1 were related to C\\O bond bending, 1650 was due to C_O bond stretching, and 1560 was related to C_C bond stretching [31,32]. The peak at 1749 cm−1 of the FTIR spectrum of PVA was due to carbonyl functional group resulted by residual acetate groups after the hydrolysis of polyvinyl acetate. Reduction of the frequency of PVA hydroxyl band from 3600 to 3400 and broadening of this peak after blending with CATPS (Fig. 5a) and the formation of dual peaks at 1031 and 995 in the blend represented new interactions between PVA and starch. In comparison with CATPS, the horizontal shift of C\\O bands in the blend (at 1071 and 1151 cm−1) also confirmed the formation of stronger hydrogen bonds between PVA and starch in blend compared to pure CATPS (Fig. 5b).
3.3. X-ray diffraction for TPS 3.5. Microscopic analysis of blends XRD pattern, which is shown in Fig. 4, was used for investigating the crystalline structure of samples. Un-plasticized corn starch showed the characteristic peaks regarding the A-type crystal structure in its XRD pattern, which can be observed in 15.2, 17.1, and 23.5 angles in Fig. 4. Unlike the un-gelatinized starch, in TPS XRD pattern, A-type crystals were substituted with V-type in 13.5 and 20.8 angles [29]. V-type crystals are caused by the rapid crystallization of amylose in single helices involving lipids or other polar molecules during extrusion or cooling [30]. The amount of these types of crystals depends on the
Fig. 6 illustrates the SEM images of the blends containing 10, 20, 30, 40, and 50% of PVA. The blend morphology similar to pure CATPS lacked the un-melted starch granules. Furthermore, the SEM images represented a homogenous morphology and probably single-phase at room temperature in samples containing less than 50% PVA. In the sample with 50% PVA, the morphology was altered, and rippling was observed, which can be related to the formation of two-phase blend in this composition.
Fig. 5. FTIR spectra of a) PVA and P30/S70 blend, and b) P30/S70 and CATPS.
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Fig. 6. SEM micrographs of CATPS/PVA blends a) P10/S90 b) P20/S80 c) P30/S70 d) P40/S60 e) P50/S50.
3.6. X-ray diffraction of blends Fig. 7 plots the XRD pattern for CATPS/PVA blends. Similar to CATPS, A-type crystal peaks were not observed in CATPS/PVA blends and they were substituted with V-type crystals in about 13.5, and 20.8 angles [29,33]. The characteristic peaks regarding PVA crystals were observed in 19.34, 23.1, and 11.54 angles [31,34]. Elimination of PVA
characteristic peaks in the crystal structure of blends, excluding the blend with 50% PVA in which minor percentage of PVA crystals were formed in 23.2°, indicated that PVA crystals were not independently formed in the blend and PVA formed a co-crystal structure with starch. This confirmed the formation of a single-phase blend of these two materials. The samples crystallinity percentages are presented in Table 2. The crystallinity percentage of CATPS/PVA blends was generally
Fig. 7. XRD patterns of CATPS/PVA blends.
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Relative crystallinity (%)
Starch CATPS P10/S90 P20/S80 P30/S70 P40/S60 P50/S50 PVA
25 21.04 23.31 23.60 20.54 21.15 20.92 31
between the crystallinity percentage of PVA and CATPS and was more close to CATPS. The crystallinity percentage of single-phase blends can be independent of blends components ratio due to the formation of new crystalline phases containing starch chains and PVA. 3.7. Dynamic mechanical analysis The results of DMA in the first heating, as well as SEM images, were used for investigation of compatibility and mechanical properties of the blends. Fig. 8 displays the changes in the storage modulus (E′) and loss
factor (tanδ) in terms of temperature for CATPS, PVA, and CATPS/PVA blends. The first weak peak of tanδ graph, which generally appeared in negative temperatures, was considered as the relaxation of plasticizer-rich phase and the sharp peak domain at high temperature (β relaxation) was considered as the glass transition temperature of the polymer. γ relaxation which was observed just in pure PVA and P50/S50 samples was resulted from the crystalline domains of PVA [35]. The variation of the Tg and E′ at 25 °C versus PVA content are given in Fig. 8c and d. The results of DMA are presented in Table 3. Increasing PVA percentage in the blends, excluding the sample containing 10% PVA, led to the reduction of the glass transition temperature and storage modulus. The presence of only one β relaxations temperature in the blends with less than 50% PVA confirmed the formation of one-phase blend, which previously received from the results of XRD test and SEM images. In fact, PVA acted as a polymeric plasticizer for starch and decreased the glass transition temperature and the dynamic modulus. However, the addition of 10% PVA to starch caused to increase the glass transition temperature up to 25 °C as well as increasing the blend modulus which can be due to the anti-plasticization effect in low PVA contents [36,37]. It might be due to the strong interaction between starch and PVA at this composition. Miscible blends with a single
Fig. 8. a) tanδ and b) storage modulus versus temperature curves for CATPS/PVA blends, and (c) Tg and (d) storage modulus of blends at 25 °C versus PVA content.
F. Kahvand, M. Fasihi / International Journal of Biological Macromolecules 140 (2019) 775–781 Table 3 Dynamic mechanical properties of TPS/PVA blends. Sample
Tα°C
Tβ1°C
TPS P10/S90 P20/S80 P30/S70 P40/S60 P50/S50 PVA
−62 −61 −60 −50 −53 −55 −65
55 80 47 36 16 14 5
Tβ2°C
Tγ°C
75 85
E′ (MPa) at 25 °C
Deviation of Tg from additive rule
183 266 137 76 86 74 32
– 30 2 −4 −19 −16 –
Tg value, usually exhibit positive or negative deviations from the Tg value determined by the additive rules. According to the study of Lu et al. [38], positive deviation indicates strong interpolymer interactions in blends, e.g., interpolymer complexes and can be attributed to a negative interaction parameter (χ). The deviation of Tg from the additive rule is reported in Table 3. The most positive deviation was observed in P10/S90 samples which could be attributed to the formation of the strong complex between PVA and starch at this composition. By further increasing the amount of PVA to 40%, the glass transition temperature was remarkably reduced and reached 16 °C. Further addition of PVA to 50%, did not change the glass transition temperature so much and caused the advent of Tβ2 temperature, which was related to PVA crystals. This demonstrates that the blend P50/S50 reached its saturation limit, and PVA phase was formed, independently. The obtained results were in consistence with the results of SEM and XRD. 4. Conclusions TPS/PVA blends were produced by extrusion method using glycerol and citric acid. The results of FTIR for TPS indicated the formation of new hydrogen bonds between starch and the plasticizer. Using citric acid as co-plasticizer resulted in the formation of stronger and more stable hydrogen bonds with starch, and consequently led to the creation of more uniform microscopic structure. Moreover, the homogenous morphology in TPS/PVA blends was observed in SEM images. The elimination of PVA crystal peaks in the XRD analysis of blends containing less than 50% PVA along with the appearance of a single peak in loss factor demonstrated high compatibility between starch and PVA. PVA acted as a polymeric plasticizer for starch in percentages higher than 10% and led to the reduction of glass transition temperature and storage modulus of the blend. While, in 10% PVA, a strong interaction caused to anti-plasticization effect, resulted in increase the Tg and storage modulus. Acknowledgments The authors would like to thank the Iran National Science Foundation (INSF) for the financial support of this study (Grant number 96010131). References [1] M. Rujnić-Sokele, A. Pilipović, Challenges and opportunities of biodegradable plastics: a mini review, Waste Manag. Res. 35 (2017) 132–140. [2] F. Versino, M.A. García, Cassava (Manihot esculenta) starch films reinforced with natural fibrous filler, Ind. Crop. Prod. 58 (2014) 305–314. [3] R.F. Tester, J. Karkalas, X. Qi, Starch—composition, fine structure and architecture, J. Cereal Sci. 39 (2004) 151–165. [4] L. Zhang, X.F. Wang, H. Liu, L. Yu, Y. Wang, G.P. Simon, J. Qian, Effect of plasticizers on microstructure, compatibility and mechanical property of hydroxypropyl methylcellulose/hydroxypropyl starch blends, Int. J. Biol. Macromol. 119 (2018) 141–148. [5] X. Qiao, Z. Tang, K. Sun, Plasticization of corn starch by polyol mixtures, Carbohydr. Polym. 83 (2011) 659–664. [6] A. Edhirej, S.M. Sapuan, M. Jawaid, N.I. Zahari, Effect of various plasticizers and concentration on the physical, thermal, mechanical, and structural properties of cassava-starch-based films, Starch-Stärke 69 (2017), 1500366. [7] R. Belhassen, F. Vilaseca, P. Mutjé, S. Boufi, Thermoplasticized starch modified by reactive blending with epoxidized soybean oil, Ind. Crop. Prod. 53 (2014) 261–267.
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