Synergistic plasticization and anti-aging effect of hyperbranched poly(1,4-butanediol citrate)/glycerol on corn starch

doi:10.21203/rs.3.rs-4312570/v1

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Synergistic plasticization and anti-aging effect of hyperbranched poly(1,4-butanediol citrate)/glycerol on corn starch

https://doi.org/10.21203/rs.3.rs-4312570/v1

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Thermoplastic starch plasticized with glycerol is very sensitive to humidity and aging. In this study, hyperbranched poly(1,4-butanediol citrate) is prepared via a one-step method, and is mixed with glycerol as the co-plasticizer of starch to prepare thermoplastic starch films. The structure of hyperbranched poly(1,4-butanediol citrate) is studied by performing Fourier-transform infrared spectroscopy and ¹H nuclear magnetic resonance analyses. The interaction between starch and co-plasticizer, and crystallinity, mechanical properties, anti-aging properties, thermal stability, transmittance, and moisture absorption of thermoplastic starch films are studied. The results illustrate that poly(1,4-butanediol citrate)/glycerol has a synergistic effect on the plasticization, anti-aging properties, transmittance, and moisture adsorption properties of thermoplastic starch films. The thermoplastic starch film with a poly(1,4-butanediol citrate)/glycerol weight ratio of 2:28 has the maximum elongation at break, the highest transmittance, the optimal ability to inhibit the long-term retrogradation of starch and the lowest moisture content at the relative humidity of 68%. Elongation at break of thermoplastic starch film with poly(1, 4-butanediol citrate)/glycerol at 2/28 and stored for 3 and 30 days are (101.1 ± 14.0)% and (91.7 ± 2.7)%, respectively, which are 1.8 and 4.4 times that of the thermoplastic starch film with only glycerol, respectively. These phenomenon not only depend on the interaction between starch and co-plasticizer, but also may be related to the compatibility between starch and co-plasticizer. Thus, the poly(1,4-butanediol citrate)/glycerol combination has potential applications in the processing of thermoplastic starch.

1 Introduction

Starch is a renewable, biodegradable, abundant, and low-cost material, which is considered to be a promising candidate for preparing packaging materials [1]. Nevertheless, native starch is not easily processed, and its products are brittle due to inter- and intra-molecular hydrogen bonds. A plasticizer is an essential additive in starch processing to improve the processing and usability of starch. The plasticizer can destroy hydrogen bonds between starch molecular chains and form new hydrogen bonds with –OH groups in starch molecules [2]. Further, the plasticizer can enhance the mobility of starch molecular chains and impart thermoplasticity to starch. Polyols, polyacids, and polyamines, especially glycerol, are the most commonly used plasticizers for starch [3-5]. Most polyols and polyacids tend to self-migrate and self-aggregate, resulting in recrystallization of starch [3, 4]. Sorbitol, despite being stable in the starch matrix, is expensive [6]. Moreover, polyamines are not suitable starch plasticizers due to their toxicity.

The ideal starch plasticizer has characteristics, such as low cost, non-toxicity, and the ability to impart thermoplasticity to starch, improve starch toughness, inhibit recrystallization of starch, and increase resistance to humidity [7, 8]. However, no single plasticizer satisfies all these requirements. Therefore, an increasing number of researchers are focusing on the use of co-plasticizers. Patel et al. [9] found that thermoplastic starch films with sorbitol/glycerol presented more compatible morphology, tensile stress, and thermal stability than those with only glycerol. Zhang et al. [10] discovered that sodium adipate/triethylene glycol at the weight ratio of 15:10 showed a synergistic effect on inhibiting short-term and long-term retrogradation of corn starch. In addition, few studies have focused on glycerol/urea co-plasticizers that can limit the amount of moisture absorbed by starch and inhibit starch recrystallization with the increase in the urea content, because of the formation of stronger hydrogen bonds between urea and starch than between glycerol and starch [2, 11-15].

In recent years, some new starch plasticizers have been identified, such as ionic liquids and non-linear polymers [16, 17]. The application of ionic liquids is limited because of the high price. Non-linear polymers include dendritic polymer, star-shaped polymer, hyperbranched polymer, etc. Our group reported that star-shaped poly(trimethylpropane-succinate-ethylene glycol esters) prepared by a multi-step reaction was superior to glycerol in improving starch toughness, inhibiting the recrystallization of starch, and increasing the resistance towards humidity [18]. Non-linear polymers, especially hyperbranched polymer, have attracted considerable attention owing to their simple preparation process, low cost, and excellent chemical properties [19-22]. Our group found that poly(glycerol citrates) synthesized by a one-step reaction was also superior to glycerol in plasticizing starch, and resisting the aging of thermoplastic starch [23]. To further reduce the cost of the plasticizer and improve the performance of thermoplastic starch, the effect of hyperbranched polyester/polyol on the structure and properties of starch is studied. First, hyperbranched poly(1,4-butanediol citrate) was prepared by a one-step method, and then, poly(1,4-butanediol citrate)/glycerol as co-plasticizers of starch were used to prepare thermoplastic starch films. The structure of hyperbranched poly(1,4-butanediol citrate) was investigated. In addition, the structure (functional groups and crystallinity) and properties (mechanical properties, anti-aging, thermal stability, transmittance, and moisture absorption characteristics) of thermoplastic starch films were studied.

2.1 Materials and chemicals

Corn starch with 25 wt% amylose and viscosity average molecular weight of 1.55 × 10⁷ was obtained from Guowei Starch Co., Ltd. (Xi'an, China). Further, analytical-grade citric acid, 1,4-butanediol, glycerol, p-toluenesulfonic acid, and sodium hydroxide were obtained from Titan Technology Co., Ltd. (Shanghai, China).

2.2 Synthesis of hyperbranched poly(1,4-butanediol citrate)

Citric acid and 1,4-butanediol at a molar ratio of 1:2 were added to the reaction vessel. The mixture was heated to 120°C for 30 min, and then, 0.5 wt% p-toluenesulfonic acid based on the absolute quality of citric acid and 1,4-butanediolas a catalyst was added. The reaction was carried out under a vacuum of 0.09 MPa for 40 min.

2.3 Structure characterization of hyperbranched poly(1,4-butanediol citrate)

The functional groups of hyperbranched poly(1,4-butanediol citrate) were recorded using a Thermo Nicolet Is50 FTIR spectroscopic scanner (USA) with the KBr tablet. The wavenumber ranged from 400 to 4000 cm^− 1, and the resolution was 2 cm^− 1. The molecular weight and distribution of hyperbranched poly(1,4-butanediol citrate) were measured by HLC-8320 gel permeation chromatography (Waters, USA). The mobile phase was tetrahydrofuran with a flow rate of 0.5 mL/min. The structure of hyperbranched poly(1,4-butanediol citrate) was determined by using an AM-500 superconducting nuclear magnetic resonance spectrometer (Bruker, Germany). The solvent used was deuterated trichloromethane. The branching coefficient (BC) of the hyperbranched poly(1,4-butanediol citrate) was calculated using the following equation [24].

, where D, T, and L represent the numbers of dendritic, terminal, and linear units on hyperbranched poly(1,4-butanediol citrate), respectively. Structural units forming the monoester, diester, and triester are known as the terminal unit (T), linear unit (L), and dendritic unit (D), respectively, which are shown in Fig. 1. The number of structural units are independent on chain length, and can be measured by the chemical shift of hydrogen atoms in ¹H NMR.

2.4 Preparation of thermoplastic starch films

Corn starch, co-plasticizers, and ultrapure water were added into a three-neck flask. Hyperbranched poly(1,4-butanediol citrate) was neutralized to pH = 7 with a sodium hydroxide solution before usage. Then, the starch dispersion was heated to 95°C and stirred at 150 rpm for 1 h. Finally, 90 mL of the resultant starch solution was poured into a flat glass dish with a size of 180 × 210 mm, and dried at 50°C for 48 h. The preparation method of neat starch film is the same as that of thermoplastic starch film. The only difference between the preparation method of neat starch film and thermoplastic starch film is that no plasticizer is added in the preparation process of neat starch film. The recipe for neat starch film and thermoplastic starch films is provided in Table 1.

Table 1

The recipe of neat starch film and thermoplastic starch films
Sample	Starch (g)	Poly(1,4-butanediol citrate) (g)	Glycerol (g)	Water (g)
CS	8.00	-	-	192.00
30G	8.00	-	2.40	189.60
2P28G	8.00	0.16	2.24	189.60
5P25G	8.00	0.40	2.00	189.60
10P20G	8.00	0.80	1.60	189.60
20P10G	8.00	1.60	0.80	189.60

2.5 Structural characterization of thermoplastic starch films

The functional groups on thermoplastic starch films and the interactions between components were determined by using a Thermo Nicolet Is50 FTIR spectroscopic scanner with attenuated total reflectance. Further, the X-ray diffraction (XRD) patterns of starch films stored at RH 68% for 3 and 30 days were recorded with a Philips X’ Pert Pro MPD system (Netherlands) in the reflection mode over a scattering angle (2θ) from 5° to 40° at 0.026°/s. The cross-section surface morphology of thermoplastic starch films was observed by using a Zeiss Sigma300 field emission scanning electron microscope (SEM, Germany). The films were placed in liquid nitrogen for 10 s, and then broken by tweezers. Their cross sections were sprayed with gold before observation.

2.6 Properties of thermoplastic starch films

Mechanical properties of thermoplastic starch films stored at RH 68% for 3 and 30 days were measured by using a YG061-1500 electronic strength tester (China). The initial length is 50 mm, and the stretching speed is 50 mm/min. The transmittance of thermoplastic starch films was recorded by using a Jena Specord S600 Ultraviolet-visible Spectrophotometer (Germany). The wavelength ranged from 200 nm to 800 nm. The thermostability of thermoplastic starch films was measured with a TG thermal analyzer (Toledo, USA). Samples were heated from 30 to 600°C at a heating rate of 10°C/min. The moisture content of thermoplastic starch films was computed according to Eq. (2). Specimens were weighed after drying at 100°C for 24 h. Then, specimens were placed in different RH (23, 43, 55, 68, and 85%) environments for 30 days and weighed.

Here M_c, m₀, and m₁ represent the moisture content of starch films, dry weight of starch films, and weight of starch films after moisture absorption for 30 days, respectively.

3.1 Structural characterization of hyperbranched poly(1,4-butanediol citrate)

The number-average molecular weight of hyperbranched poly(1,4-butanediol citrate) measured by gel permeation chromatography is 1144, and its polydispersity index is 1.50. The FT-IR spectra of monomers and hyperbranched poly(1,4-butanediol citrate) are shown in Fig. 2(a). In the FT-IR spectrum of 1,4-butanediol, the peak at 3400 cm^− 1 is assigned to –OH stretching vibration. The FT-IR spectrum of citric acid shows that the peaks at 1760 cm^− 1 and 1700 cm^− 1 are attributed to C = O stretching vibration in –COOH. In the FT-IR spectra of hyperbranched poly(1,4-butanediol citrate), a new peak appears at 1735 cm^− 1, which belongs to the C = O stretching vibration in ester groups [25]. It results from the esterification between 1,4-butanediol and citric acid.

The branching coefficient is an important parameter for hyperbranched polymers and can be determined from ¹H NMR results. The proton attribution on monomers and poly(1,4-butanediol citrate) are shown in Fig. 2(b). The branching coefficient for poly(1,4-butanediol citrate) is calculated according to the method presented in our previous report [26]. The branching coefficient of hyperbranched poly(1,4-butanediol citrate) is calculated using Eq. 1 and is 0.54.

3.2 FT-IR analysis of thermoplastic starch films

FT-IR spectra of corn starch films prepared using poly(1,4-butanediol citrate)/glycerol co-plasticizers at different proportions are shown in Fig. 3. In the FT-IR spectrum of the corn starch films, peaks at 3385, 1150, and 995 cm^-1 are the typical characteristic absorption peaks of starch and correspond to –OH, C–O in C–O–H, and C–O in C–O–C stretching vibration, respectively [2]. In contrast to the FT-IR spectrum of neat starch film, the spectra of thermoplastic starch films plasticized with poly(1,4-butanediol citrate)/glycerol display a characteristic absorption peak at 1732 cm^-1 that corresponds to the C = O stretching vibration in ester groups. All the –OH stretching vibration peaks in the FT-IR spectra of thermoplastic starch films with poly(1,4-butanediol citrate)/glycerol at the weight ratio of 0/30, 2/28, 5/25, 10/20, and 20/10, (30G, 2P28G, 5P25G, 10P20G, and 20P10G films), shift from 3385 cm^-1 to 3300 cm^-1, 3287 cm^-1, 3286 cm^-1, 3284 cm^-1, and 3280 cm^-1, respectively. This is mainly because the partial hydrogen bonds of associated hydroxyl groups in the starch film are destroyed, and –OH in starch chains forms stronger hydrogen bonds with polar groups in the plasticizer [2]. In addition, –OH stretching vibration peaks of the thermoplastic starch films shift from 3287 cm^-1 to 3280 cm^-1 with the increase in the poly(1,4-butanediol citrate) content. This indicates that hydrogen bonds formed by poly(1,4-butanediol citrate) with –OH in starch chains are stronger than those formed by glycerol.

3.3 Crystallinity of thermoplastic starch films

Starch is a type of semi-crystalline polymer. XRD patterns of the prepared thermoplastic corn starch films after storage for 3 days and 30 days are shown in Fig. 4. The peaks at 5.5, 15.0, 17.1, 19.8, 22.1, and 23.8° in the XRD patterns of the corn starch film are B-type crystals peaks of starch. The characteristic peaks in the XRD patterns of thermoplastic starch films are weaker than those of the corn starch films or even disappear. This is mainly due to the plasticizer breaking up hydrogen bonding between starch molecules. The crystallinity of 30G, 2P28G, 5P25G, 10P20G, and 20P10G films stored at RH 68% for 3 days are 18.4, 14.7, 16.3, 21.9, and 23.3%, respectively, which are calculated by Jade 6.0 (Table 2). The results show that the use of poly(1,4-butanediol citrate)/glycerol at weight ratios of 2:28 and 5:25 is more effective than the use of glycerol only in inhibiting the short-term retrogradation of starch or the recrystallization of amylose.

Table 2

Crystallinity of corn starch films plasticized by poly(1,4-butanediol citrate)/glycerol (P/G) with different proportions stored for 3 and 30 days
Sample	3 days	30 days
Sample	Crystallinity (%)
CS	24.6	33.0
30G	18.4	31.8
2P28G	14.7	19.5
5P25G	16.3	23.5
10P20G	21.9	27.5
20P10G	23.3	28.8

With the increase in the storage time, long-term retrogradation occurs due to the recrystallization of amylopectin. The characteristic crystal peak intensities of starch and thermoplastic starch films stored for 30 days are stronger than those of corresponding films stored for 3 days, as shown in Fig. 4. This is attributed to the recrystallization of amylopectin. The crystallinity of thermoplastic starch films with poly(1,4-butanediol citrate)/glycerol stored for 30 days at RH 68% are lower than that of thermoplastic starch films with glycerol. The results show that co-plasticizers are more efficient in inhibiting the long-term retrogradation of starch or recrystallization of amylopectin than glycerol alone, because of the stronger intermolecular interactions among starch, glycerol, and poly(1,4-butanediol citrate) as illustrated in Fig. 3. The crystallinity of the 2P28G film stored for 30 days is 19.5%, which is the lowest among all the thermoplastic starch films. This suggests that the poly(1,4-butanediol citrate)/glycerol weight ratio of 2:28 is optimal for inhibiting the long-term retrogradation of starch. This phenomenon not only depends on the interaction between the components but also may be related to the compatibility between the components.

3.4 Mechanical properties of thermoplastic starch films

Tensile strength and elongation at the break of the corn starch film are (24.2 ± 1.8) MPa and (2.2 ± 0.1)%, respectively. The corn starch film is brittle. Consequently, a plasticizer should be added during starch processing. The stress–strain curves of the prepared thermoplastic corn starch films after storage for 3 days are shown in Fig. 5. The thermoplastic starch films with 30 wt% glycerol or poly(1,4-butanediol citrate)/glycerol show a ductile fracture. The tensile strength and elongation at break of the 30G film stored for 3 days are 3.6 MPa and (56.0 ± 6.0)%, respectively. The elongation at break of 2P28G, 5P25G, 10P20G, and 20P10G films are (101.1 ± 14.0), (84.3 ± 10.7), (82.2 ± 2.8), and (84.6 ± 10.7)%, respectively, which are higher than that of the 30G film. In other words, the co-plasticizers, especially poly(1,4-butanediol citrate)/glycerol at a weight ratio of 2:28, have a synergistic plasticizing effect on starch. This is because the interactions between co-plasticizers and starch are stronger than those between glycerol and starch, which is evident from FT-IR analysis results for thermoplastic starch films.

The retrogradation of thermoplastic starch films occurs because of the migration of plasticizers, which results in low toughness of the thermoplastic starch. As a result, the change in the mechanical properties of thermoplastic starch with time is an important index to evaluate the plasticizer quality. The elongation at break and tensile strength of corn starch films plasticized with poly(1,4-butanediol citrate)/glycerol at different proportions and stored for 3 and 30 days are shown in Table 3. The elongation at break of the 30G film decreases by 62.9% from days 3–30, while that of 2P28G, 5P25G, 10P20G, and 20P10G films decreases by 9.3, 4.6, 11.7, and 13.9%, respectively. These results show that the co-plasticizers can significantly inhibit the retrogradation of the starch, which is consistent with the conclusion drawn from XRD analysis results. The most probable reason is the improvement in interactions between starch and plasticizers, which is evident from FT-IR analysis results for thermoplastic starch films. This may also be attributed to the improvement in the compatibility among starch, poly(1,4-butanediol citrate), and glycerol, which can be drawn from the surface morphology of starch films, as shown in Fig.S1.

Table 3

Mechanical properties of corn starch films plasticized by poly(1,4-butanediol citrate)/glycerol (P/G) with different proportions stored for 3 and 30 days
Sample	3 days		30 days
Sample	Elongation at break (%)	Tensile strength (MPa)	Elongation at break (%)	Tensile strength (MPa)
CS	2.2 ± 0.1	24.2 ± 1.8	1.9 ± 0.1	26.5 ± 2.1
30G	56.0 ± 6.0	3.6 ± 0.5	20.8 ± 1.1	5.3 ± 0.2
2P28G	101.1 ± 14.0	3.0 ± 0.2	91.7 ± 2.7	3.1 ± 0.3
5P25G	84.3 ± 10.7	2.6 ± 0.3	80.4 ± 4.1	3.9 ± 0.8
10P20G	82.2 ± 2.8	3.8 ± 0.2	72.6 ± 5.1	4.6 ± 0.2
20P10G	84.6 ± 10.7	5.1 ± 0.5	72.8 ± 1.7	5.5 ± 0.5

3.5 Transmittance of thermoplastic starch film

The light transmittance of plastic is an essential characteristic considered for packaging, optical instruments, lighting appliances, and photovoltaic materials. The transmittance of composite materials is related to not only the crystallinity of the material but also the compatibility between components. The transmittance of corn starch films plasticized with poly(1,4-butanediol citrate)/glycerol at different proportions and stored for 7 days are shown in Fig. 6. The transmittance of all the thermoplastic starch films is higher than that of the neat starch film. This is because the plasticizer destroys the crystalline structure of the starch, as can be seen from XRD analysis results. The transmittance of all the thermoplastic starch films prepared using co-plasticizers is also higher than that of the thermoplastic starch film prepared using glycerol. There might be two reasons for this. (1) Poly(1,4-butanediol citrate) increases the intermolecular interaction between starch and glycerol as shown in Fig. 3, so the crystallinity of thermoplastic starch films with co-plasticizers is lower than that of the thermoplastic starch film with glycerol (Table 2). (2) Poly(1,4-butanediol citrate) improves the compatibility between starch and glycerol. Notably, the 2P28G film has the highest transmittance, indicating that thermoplastic starch films with co-plasticizers exhibit the highest compatibility when the weight ratio of poly(1,4-butanediol citrate) to glycerol is 2:28. In conclusion, the 2P28G film has potential applications in packaging.

3.6 Thermal analysis of thermoplastic starch films

The TGA and differential thermogravimetry (DTG) curves of corn starch films plasticized with poly(1,4-butanediol citrate)/glycerol at different proportions are shown in Fig. 7. Three degradation stages exist for thermoplastic starch films. The first degradation stage below 150°C is due to the loss of free water and bound water in the starch film. The amount of lost free water and bound water in the thermoplastic starch films, except in the 30G film, is lower than that in the neat starch film. The second degradation stage below 250°C corresponds to the evaporation of glycerol in the thermoplastic starch films. The third degradation stage above 250°C is attributed to the degradation of starch and poly(1,4-butanediol citrate) in thermoplastic starch films. The temperatures corresponding to the maximum mass loss rate in thermoplastic starch films are lower than those in the case of the neat starch films. This is because thermoplastic starch films are more amorphous than neat starch films.

3.7 Moisture absorption of thermoplastic starch films

Starch tends to readily absorb moisture owing to the large number of –OH groups in starch molecules, which limits the applications of starch. The moisture content of the thermoplastic starch film with glycerol is higher than that of the neat starch film because the tendency of glycerol to absorb moisture is stronger than that of starch. The moisture absorbed at different relative humidities by corn starch films plasticized with poly(1,4-butanediol citrate)/glycerol at different proportions is shown in Fig. 8. The moisture content of thermoplastic starch films with co-plasticizers is lower than that of the thermoplastic starch film with only glycerol when these films are stored in the same environment. Co-plasticizers could form the stronger interaction with starch than glycerol, and then part of the –OH groups in starch molecules are blocked. In addition, no positive correlation is seen between the moisture content of thermoplastic starch films with co-plasticizers and the interactions among all the components. The moisture contents of 30G, 2P28G, 5P25G, 10P20G, and 20P10G films stored at RH 68% for 30 days are (11.7 ± 0.3), (9.6 ± 0.2), (11.0 ± 0.2), (10.3 ± 0.2), and (10.6 ± 0.2)%, respectively. When the relative humidity is higher than 68%, the 2P28G film has the lowest moisture content. This could be related to the better compatibility between the components [9], which will be further studied in the future.

Hyperbranched poly(1,4-butanediol citrate) is prepared, and poly(1,4-butanediol citrate)/glycerol as co-plasticizers of starch is used to prepare thermoplastic starch films. Number molecular weight and branching coefficient of hyperbranched poly(1,4-butanediol citrate) are 1144 and 0.54, respectively. Poly(1,4-butanediol citrate)/glycerol show synergistic effect on the toughness, anti-aging, transmittance and moisture adsorption of thermoplastic starch films. Poly(1,4-butanediol citrate) forms stronger hydrogen bonds with starch chains than glycerol. The optimal addition ratio of poly(1,4-butanediol citrate) to glycerol is 2/28 (2P28G). Elongation at break of 2P28G film stored for 3 and 30 days are 101.1% and 91.7%, respectively. The crystallinity of 2P28G film varies from 14.7% at 3 days to 19.5% at 30 days. Moisture content of 2P28G film storeed at RH 68% for 30 days is 9.6%. These phenomenon not only depend on the interaction between starch and co-plasticizer, but also may be related to the compatibility between starch and co-plasticizer.

The authors declare no competing financial interest.

Author Contribution

Author contributionsKZ: Methodology, Writing-original draft. DJ: Investigation, Validation. HO: Investigation, Formal analysis. YH: Investigation, Software. PC: Funding acquisition, Validation, Formal analysis. RW: Supervision, Writing-review & editing. KT: Conceptualization, Writing-review & editing. YW: Writing-review & editing, Formal analysis, Validation. FC: Methodology, Funding acquisition. PZ: Conceptualization, Writing-review & editing.

Acknowledgments

We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript. Thanks to Li Tang ([email protected]) for the experimental test support. Thanks to the Scientific Research Project of Education Department of Hunan Province (No. 21B0587) and the National Natural Science Foundation of China (No. 51603134).

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Synergistic plasticization and anti-aging effect of hyperbranched poly(1,4-butanediol citrate)/glycerol on corn starch

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Abstract

Figures

1 Introduction

2 Experimental

2.1 Materials and chemicals

2.2 Synthesis of hyperbranched poly(1,4-butanediol citrate)

2.3 Structure characterization of hyperbranched poly(1,4-butanediol citrate)

2.4 Preparation of thermoplastic starch films

2.5 Structural characterization of thermoplastic starch films

2.6 Properties of thermoplastic starch films

3 Results and discussion

3.1 Structural characterization of hyperbranched poly(1,4-butanediol citrate)

3.2 FT-IR analysis of thermoplastic starch films

3.3 Crystallinity of thermoplastic starch films

3.4 Mechanical properties of thermoplastic starch films

3.5 Transmittance of thermoplastic starch film

3.6 Thermal analysis of thermoplastic starch films

3.7 Moisture absorption of thermoplastic starch films

4 Conclusions

Declarations

Author Contribution

Acknowledgments

References

Additional Declarations

Supplementary Files

Status:

Version 1