Effect of citric acid on thermal, phase morphological, and mechanical properties of poly(l-lactide)-b-poly(ethylene glycol)-b-poly(l-lactide)/thermoplastic starch blends

Yaowalak Srisuwan; Prasong Srihanam; Theeraphol Phromsopha; Yodthong Baimark

doi:10.1515/epoly-2023-0057

Article Open Access

Effect of citric acid on thermal, phase morphological, and mechanical properties of poly(l-lactide)-b-poly(ethylene glycol)-b-poly(l-lactide)/thermoplastic starch blends

Yaowalak Srisuwan , Prasong Srihanam , Theeraphol Phromsopha and Yodthong Baimark

Published/Copyright: October 24, 2023

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From the journal e-Polymers Volume 23 Issue 1

Abstract

This work investigated the thermal, morphological, and tensile properties of poly(l-lactide)-b-poly(ethylene glycol)-b-poly(l-lactide) (PLLA-PEG-PLLA)/thermoplastic starch (TPS) blends with 3 wt% citric acid (CA) treatment of TPS. The blends with PLLA-PEG-PLLA/CA-TPS ratios of 100/0, 90/10, 80/20, and 60/40 (w/w) were investigated and compared with PLLA-PEG-PLLA/CA-free TPS blends. Crystallizability of the blends decreased and thermal stability increased as the TPS content increased. The thermal stability of the blends was found to improve after CA treatment of TPS. The PLLA-PEG-PLLA/CA-TPS blends showed better phase compatibility than those of the PLLA-PEG-PLLA/CA-free TPS blends. The tensile properties of the blends were improved by CA treatment of TPS. In conclusion, improvement in thermal stability, phase compatibility, and tensile properties of the PLLA-PEG-PLLA/TPS blends was obtained by CA treatment of TPS. The resulting PLLA-PEG-PLLA/CA-TPS blends could potentially be used to prepare biodegradable and flexible bioplastics.

Keywords: polylactide; block copolymer; thermoplastic starch; citric acid; phase compatibility

1 Introduction

An important synthetic bioplastic is poly(l-lactic acid) or poly(l-lactide) (PLLA) because of its bio-renewability, biocompatibility, and biodegradability (1,2,3,4,5,6). The utilization of PLLA in plastic applications is considered to solve the problem of non-biodegradable petroleum-based plastic waste (7). However, low flexibility and high production cost of PLLA are the main drawbacks compared with traditional petroleum-based plastics which restrict its range of applications (8,9,10,11,12).

Poly(l-lactide)-b-poly(ethylene glycol)-b-poly(l-lactide) (PLLA-PEG-PLLA) block copolymers are more flexible and more hydrophilic than PLLA due to the characteristics of PEG blocks (13,14,15). PLLA-PEG-PLLA reacts with a chain extender by post melt blending (14,15) and by in situ ring-opening polymerization (16) to control its melt flow ability for conventional melt processing. PLLA-PEG-PLLA is blended with low-cost thermoplastic starch (TPS) to reduce its production cost and to increase its biodegradation rate (17,18). The results showed that the PLLA-PEG-PLLA/TPS blends exhibited good phase compatibility compared with the PLLA/TPS blends. However, blending of TPS decreased the flexibility of PLLA-PEG-PLLA due to poor mechanical properties of TPS (12,19,20). Thus, the improvement in flexibility of PLLA-PEG-PLLA/TPS blends emerged as a challenge in this work.

Wang et al. (21,22) prepared PLLA/cornstarch blends in the presence of glycerol and citric acid (CA) by one-step extrusion processing. They found that the starch was degraded by CA treatment to improve the dispersion and compatibility in the blends. Kahar et al. (23) systematically investigated the effect of CA on the blend morphology and tensile properties of polyethylene/natural rubber/TPS blends. They found that the phase compatibility improved and the tensile properties increased after CA modification of TPS. Ibrahim et al. (24) treated TPS with CA to improve compatibility in PLLA/TPS blends. The results demonstrated that tensile stress, strain at break, and Young’s modulus of the PLLA/TPS blends increased when the TPS was treated with CA. The tensile stress and Young’s modulus of the blends increased with the CA content until CA content was 3 wt%. However, PLLA-PEG-PLLA/TPS treated with CA blends have not been reported so far. Thus, we hypothesized that CA treatment of TPS could improve phase compatibility and tensile properties of the PLLA-PEG-PLLA/TPS blends.

The objective of this work was to improve phase compatibility and tensile properties of the PLLA-PEG-PLLA/TPS blends by CA treatment. For this purpose, TPS was treated with CA before melt blending with PLLA-PEG-PLLA. The effects of CA treatment and blend ratio on the thermal, morphological, and tensile properties of the PLLA-PEG-PLLA/TPS blends were determined and discussed. The PLLA-PEG-PLLA/CA-free TPS blends were also prepared for comparison.

2 Materials and methods

2.1 Materials

A chain-extended PLLA-PEG-PLLA with melt flow index of 26 g·10 min⁻¹ (determined at 190°C under 2.16 kg load force) was synthesized in our research unit, as described in our previous work (16). TPS, (Tapioplast^TM) prepared from tapioca starch (79% amylopectin) was supplied by SMS Corporation Co., Ltd. (Pathum Thani, Thailand). CA was obtained from Merck.

2.2 CA treatment of TPS

TPS and CA were dried at 50°C under vacuum overnight before melt blending using a HAAKE PolyLab OS Rheomix batch-mixer (Thermo Scientific, USA) at 150°C for 10 min with a rotor speed of 100 rpm. TPS with and without 3 wt% CA treatment was prepared and designed as 3% CA-TPS and CA-free TPS, respectively.

2.3 Preparation of PLLA-PEG-PLLA/TPS blends

PLLA-PEG-PLLA and 3% CA-TPS were dried at 50°C under vacuum overnight before melt blending in a HAAKE PolyLab OS Rheomix batch-mixer (Thermo Scientific, USA) at 180°C for 5 min with a rotor speed of 100 rpm. PLLA-PEG-PLLA/3% CA-TPS blends with blend ratios of 100/0, 90/10, 80/20, and 60/40 (w/w) were investigated. PLLA-PEG-PLLA/CA-free TPS blends were also prepared by the same method for comparison.

2.4 Characterization of treated TPS and PLLA-PEG-PLLA/TPS blends

The reaction between TPS and CA was characterized with a Fourier-transform infrared (FTIR) spectrometer (Invenio-S, Bruker) equipped with an attenuated total reflectance accessory in the range of 4,000−500 cm⁻¹ with a 4 cm⁻¹ resolution and 32 accumulated scans.

The thermal transition properties of the blends were determined with a Pyris Diamond differential scanning calorimeter (DSC, Perkin Elmer) over the temperature range 0–200°C with a heating rate of 10°C·min⁻¹ under a nitrogen gas flow. Thermal history of the blends was erased by melting at 200°C for 3 min followed with fast quenching to 0°C with cooling rate of 100°C·min⁻¹ before the DSC heating scan. The degree of crystallinity (X _c) of the blend was calculated using Eq. 1 (9). Each DSC result was the average from three measurements.

(1) X c ( % ) = [ ( D H m − D H cc ) / ( 93.6 ´ W PLLA ) ] × 100

where ∆H _m and ∆H _cc are enthalpies of melting and cold crystallization, respectively. The ΔH _m of 100% X _c PLLA is 93.6 J·g⁻¹ (9,25). W _PLLA is the PLLA weight fraction.

The thermal decomposition properties of the blends were determined with a SDT Q600 thermogravimetric analyzer (TGA, TA-Instrument) over the temperature range of 50–1,000°C at a heating rate of 20°C·min⁻¹ under a nitrogen gas flow.

The phase morphology of cryofractured surfaces for the blend films were examined with a JSM-6460LV scanning electron microscope (SEM, JEOL) at an acceleration potential of 15 kV. Cryofracture surfaces were prepared by fracturing under liquid nitrogen. The samples were sputter-coated with gold prior to scan.

The tensile properties of the blend films (100 mm × 10 mm) were measured with a LY-1066B universal mechanical tester (Dongguan Liyi Environmental Technology Co., Ltd) at 25°C with a 100 kg load cell, a 50 mm·min⁻¹ crosshead speed, and a 50 mm gauge length. Each tensile property was averaged from at least five samples.

3 Results and discussion

3.1 FTIR analysis

The FTIR spectra of the CA-free TPS and 3% CA-TPS are shown in Figure 1. They showed broad peaks in the range of 3,700−3,000 cm⁻¹ referred to the stretching hydroxyl groups, the peaks at 2,925 cm⁻¹ attributed to the C–H stretching groups, and the peaks at 1,640 cm⁻¹ related to the bending hydroxyl groups of starch (17,18,26). For 3% CA-TPS in Figure 1(b), an additional peak at 1,742 cm⁻¹ assigned to the ester carbonyl groups of the TPS treated with CA was detected (23,27). This confirmed that an acetylation reaction between the hydroxyl groups of starch and carboxylic acid groups of CA had occurred (23,24).

Figure 1

FTIR spectra of (a) CA-free TPS and (b) 3% CA-TPS.

Figure 2 shows expanded FTIR spectra of TPS in the region of C–O stretching vibration. The peaks at 1,150 cm⁻¹ referred to C–O stretching vibration of C–O–H groups of starch (23). The height of this peak for 3% CA-TPS in Figure 2 (red line) was higher than that of the CA-free TPS in Figure 2 (black line) using the peak at 758 cm⁻¹ (C–O–C ring vibration in starch) as the reference peak. This indicates the content of C–O–H groups of 3% CA-TPS was more than the CA-free TPS. The results implied that glycosidic bonds of starch were hydrolyzed with CA to form new hydroxyl groups (23). The molar mass of starch then decreased by acid-catalyzed hydrolysis with CA (28).

Figure 2

Expanded FTIR spectra in C–O stretching region (700−1,200 cm⁻¹) of CA-free TPS and 3% CA-TPS.

3.2 Thermal transition properties

The thermal transition properties of the blends with and without 3% CA treatment including glass transition temperature (T _g), cold crystallization temperature (T _cc), melting temperature (T _m), and degree of crystallinity (X _c) were investigated from DSC heating curves as shown in Figure 3. The DSC results are summarized in Table 1. All the blends had similar T _g values in the range of 30−33°C. The T _cc and T _m values of pure PLLA-PEG-PLLA were 71.3°C and 159.6°C, respectively. The T _cc peaks shifted to higher temperatures (73.8−78.0°C) and the T _m peaks shifted to lower temperatures (156.4−157.8°C) when the TPS was incorporated. This shifting of T _cc and T _m peaks suggested that the cold crystallization of PLLA end-blocks was suppressed by the addition of TPS and induced formation of imperfect PLLA crystallites (17). The T _cc peaks slightly shifted to higher temperature as the TPS content increased. However, the TPS content did not significantly affect shifting of the T _m peaks.

Figure 3

DSC heating curves of (top) PLLA-PEG-PLLA/CA-free TPS and (bottom) PLLA-PEG-PLLA/3% CA-TPS blends for blend ratios of (a) 100/0, (b) 90/10, (c) 80/20, and (d) 60/40 (w/w).

Table 1

Thermal transition properties of PLLA-PEG-PLLA/TPS blends with and without CA treatment

PLLA-PEG-PLLA/TPS (w/w)	T _g (°C)	T _cc (°C)	T _m (°C)	X _c (%)
Pure PLLA-PEG-PLLA	30.0 ± 0.6	71.3 ± 0.3	159.6 ± 0.2	25.2 ± 0.4
PLLA-PEG-PLLA/CA-free TPS
90/10	31.4 ± 0.2	73.8 ± 0.2	157.3 ± 0.1	22.4 ± 0.5
80/20	32.2 ± 0.2	74.1 ± 0.3	156.6 ± 0.2	19.8 ± 0.4
60/40	33.0 ± 0.3	76.4 ± 0.1	156.4 ± 0.3	17.6 ± 0.7
PLLA-PEG-PLLA/3% CA-TPS
90/10	32.5 ± 0.4	76.2 ± 0.2	157.8 ± 0.4	19.4 ± 0.6
80/20	32.3 ± 0.2	77.3 ± 0.3	157.3 ± 0.2	18.2 ± 0.5
60/40	30.4 ± 0.7	78.0 ± 0.1	157.4 ± 0.1	13.9 ± 0.4

All the blends had X _c values (13.9−22.4%) lower than the pure PLLA-PEG-PLLA (25.2%). The X _c values steadily decreased as the TPS content increased for both the blend series. It should be noted that the X _c values of the PLLA-PEG-PLLA/3% CA-TPS blends were lower than the PLLA-PEG-PLLA/CA-free TPS blends for the same TPS content as shown in Figure 4. This indicates that the shorter CA-treated TPS chains showed more suppression of the crystallization of PLLA end-blocks in the blends than the longer CA-free TPS chains.

Figure 4

Effect of CA treatment on X _c of PLLA-PEG-PLLA/TPS blends.

3.3 Thermal decomposition properties

The thermal decomposition properties of the blends were studied from thermogravimetric (TG) and derivative TG (DTG) curves as shown in Figures 5 and 6, respectively. The TG and DTG results including residue weight at 1,000°C, temperature at maximum decomposition rate of PLLA blocks (PLLA-T _d,max), and temperature at maximum decomposition rate of PEG blocks (PEG-T _d,max), are summarized in Table 2. The TG curve of pure PLLA-PEG-PLLA exhibited two weight loss steps in the ranges of 250−350°C and 350−450°C due to thermal decompositions of PLLA end-blocks and PEG middle-blocks, respectively (14,15,16,17). The pure PLLA-PEG-PLLA showed complete decomposition at a temperature around 450°C. From Table 2, the PLLA-T _d,max and PEG-T _d,max peaks of the pure PLLA-PEG-PLLA are seen to have been 294°C and 416°C, respectively.

Figure 5

TG curves of (top) PLLA-PEG-PLLA/CA-free TPS and (bottom) PLLA-PEG-PLLA/3% CA-TPS blends for various blend ratios.

Figure 6

DTG curves of (top) PLLA-PEG-PLLA/CA-free TPS and (bottom) PLLA-PEG-PLLA/3% CA-TPS blends for various blend ratios (T _d,max peaks as shown).

Table 2

Thermal decomposition properties of PLLA-PEG-PLLA/TPS blends with and without CA treatment

PLLA-PEG-PLLA/TPS (w/w)	Residue weight at 1,000°C (%)^a	PLLA-T_d,max (°C)^b	PEG-T_d,max (°C)^b
Pure PLLA-PEG-PLLA	0.3	294	416
PLLA-PEG-PLLA/CA-free TPS
90/10	1.6	342	414
80/20	3.4	355	415
60/40	7.0	359	415
PLLA-PEG-PLLA/3% CA-TPS
90/10	1.9	346	417
80/20	3.1	361	416
60/40	6.6	367	418

^aObtained from TG curves.

^bObtained from DTG curves.

From TG curves, it was found that all the blends also exhibited two weight loss steps in the ranges 100−380°C and 380−450°C. The temperature ranges for main thermal decompositions of PLLA end-blocks in the blends significantly shifted to higher temperature for both the blend series. From DTG curves, the PLLA-T _d,max peaks of both the blend series dramatically shifted to higher temperature as the TPS content increased, indicating that the thermal stability of PLLA end-blocks in the blends increased with the increase in the TPS content. Hydrogen bonding could occur between the oxygen atoms of PEG middle-blocks and hydroxyl groups of starch to improve thermal stability of the PLLA end-blocks in PLLA-PEG-PLLA/starch blends (17). In addition, products from pyrolysis of starch induced a shielding effect that enhanced thermal stability of the compatible polymer/starch blends (17,19,30). However, the PEG-T _d,max did not change significantly when the TPS was blended.

This indicates that the addition of TPS with and without CA treatment improved thermal stability of the PLLA end-blocks. However, the weight losses in the temperature range 100−300°C of the blends increased with the TPS content. This was due to lower thermal stability of the TPS than PLLA-PEG-PLLA (17,29). As would be expected, residue weight at 1,000°C of the blends increased as the TPS content increased because of increase in the TPS ashes.

From Table 2, it can be seen that the PLLA-T _d,max peaks of PLLA-PEG-PLLA/3% CA-TPS blends were at higher temperature than the PLLA-PEG-PLLA/CA-free TPS blends for the same TPS content. This may be explained by the content of hydroxyl groups in 3% CA-TPS being more than the CA-free TPS as described in FTIR analysis reported above. Therefore, higher content of hydrogen bonds in the PLLA-PEG-PLLA/3% CA-TPS blends enhanced the higher thermal stability of the blends.

3.4 Phase morphology

The phase morphology of the blends was investigated from SEM images of the film cryo-fractured surfaces as shown in Figure 7. The cryo-fractured surface of pure PLLA-PEG-PLLA film in Figure 7(a) was homogeneous with rough surfaces suggesting it had a ductile character (13,14). All the blend films in Figure 7(b)−(g) showed that many voids which occurred from TPS phases fell out during film fractionation indicating phase separation between PLLA-PEG-PLLA matrices and dispersed TPS. The number and size of these voids increased as the TPS content increased.

$Figure 7 Cryo-fractured surfaces of (a) pure PLLA-PEG-PLLA and PLLA-PEG-PLLA/CA-free TPS blends for blend ratios of (b) 90/10, (c) 80/20, and (d) 60/40 (w/w) as well as PLLA-PEG-PLLA/3% CA-TPS blends for blend ratios of (e) 90/10, (f) 80/20, and (g) 60/40 (w/w) (all bar scales = 10 µm).$

Figure 7

Cryo-fractured surfaces of (a) pure PLLA-PEG-PLLA and PLLA-PEG-PLLA/CA-free TPS blends for blend ratios of (b) 90/10, (c) 80/20, and (d) 60/40 (w/w) as well as PLLA-PEG-PLLA/3% CA-TPS blends for blend ratios of (e) 90/10, (f) 80/20, and (g) 60/40 (w/w) (all bar scales = 10 µm).

It is clearly seen that the void sizes in the blends that contained 3% CA-TPS (Figure 7(e)−(g)) were smaller than in the blends containing CA-free TPS (Figure 7(b)−(d)) which suggested that the phase compatibility in the blends containing 3% CA-TPS was better than that in CA-free TPS (17,31,32). It has been reported that modification of TPS with CA improved the dispersion and phase compatibility of TPS in the polyethylene (33) and PLLA (24). This may be explained by acidic hydrolysis of starch with CA through melt processing leading to decreased molar mass of starch (23,28,34) and development of hydroxyl groups (23) to improve the phase compatibility between PLLA-PEG-PLLA and TPS. In addition, the unreacted CA could develop an in situ compatibilization effect by reacting with both the PLLA end-chains and TPS during melt blending (35) and could create hydrogen bonding (36) to improve the interfacial interaction between PLLA and TPS.

3.5 Tensile properties

The tensile properties of the blend films were determined from stress–strain curves as shown in Figure 8 and the averaged tensile properties are summarized in Table 3. The ultimate tensile strength, elongation at break, and Young’s modulus of the pure PLLA-PEG-PLLA films were 17.4 MPa, 258%, and 243 MPa, respectively. These tensile properties decreased when both the CA-free TPS and 3% CA-TPS were blended and the TPS content was increased. This is due to the poor mechanical properties of the TPS (12,19,20). However, the tensile properties of the blend films containing 3% CA-TPS were higher than the blend films that contained CA-free TPS for the same TPS content as shown in Figure 9. This may be due to the good phase compatibility in the blend films that contained 3% CA-TPS as described above in the SEM analysis. The CA treatment can increase the tensile properties of the PLLA/TPS blends by improving its phase compatibility (24). The tensile results support a conclusion that the tensile properties of the PLLA-PEG-PLLA/TPS blends were improved by CA treatment of TPS.

Figure 8

Tensile curves of (top) PLLA-PEG-PLLA/CA-free TPS and (bottom) PLLA-PEG-PLLA/3% CA-TPS blends for various blend ratios.

Table 3

Tensile properties of PLLA-PEG-PLLA/TPS blends with and without CA treatment

Sample	Ultimate tensile strength (MPa)	Elongation at break (%)	Young’s modulus (MPa)
Pure PLLA-PEG-PLLA	17.4 ± 1.1	258 ± 24	243 ± 31
PLLA-PEG-PLLA/CA-free TPS
90/10	12.9 ± 2.3	156 ± 21	232 ± 28
80/20	11.5 ± 1.4	86 ± 14	192 ± 24
60/40	6.1 ± 0.7	24 ± 6	84 ± 15
PLLA-PEG-PLLA/3% CA-TPS
90/10	13.1 ± 2.4	184 ± 27	234 ± 26
80/20	12.4 ± 1.5	142 ± 18	205 ± 27
60/40	7.5 ± 1.2	73 ± 7	103 ± 14

Figure 9

Effect of CA treatment on tensile properties of PLLA-PEG-PLLA/TPS blends.

4 Conclusion

In this study, TPS was treated with 3 wt% CA (3% CA) before melt blending with the PLLA-PEG-PLLA for PLLA-PEG-PLLA/TPS blend ratios of 90/10, 80/20, and 60/40 (w/w). The FTIR analysis indicated that both the acetylation and acidic hydrolysis reactions of TPS with CA had occurred. For PLLA-PEG-PLLA/TPS blends, the crystallization ability of PLLA-PEG-PLLA matrices decreased and the thermal stability increased as the TPS content increased, as determined from DSC and TGA analyses, respectively. The addition of 3% CA-TPS showed more suppression in crystallization of the PLLA end-blocks and more improvement in thermal stability of the PLLA end-blocks for the PLLA-PEG-PLLA/TPS blends than with the addition of CA-free TPS. The blends containing 3% CA-TPS showed better phase compatibility and higher tensile properties than those of the blends containing CA-free TPS for same blend ratio as investigated from SEM analysis and tensile test, respectively. PLLA-PEG-PLLA/TPS treated with CA blends can be used as fully biodegradable and flexible bioplastics.

Acknowledgements

The authors like to thank the Centre of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education, Thailand for providing the tensile test.

Funding information: This research project is financially supported by Thailand Science Research and Innovation (TSRI).
Author contributions: Yaowalak Srisuwan: conceptualization, methodology, investigation, writing – original draft, and funding acquisition; Prasong Srihanam: validation, investigation, writing – review and editing; Theeraphol Phromsopha: validation, investigation, and writing – review and editing; Yodthong Baimark: conceptualization, methodology, investigation, writing – original draft, writing – review and editing, supervision, project administration, and funding acquisition.
Conflict of interest: The authors have no conflict of interest.

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Received: 2023-05-27

Revised: 2023-09-01

Accepted: 2023-09-09

Published Online: 2023-10-24

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/epoly-2023-0057

Keywords for this article

polylactide; block copolymer; thermoplastic starch; citric acid; phase compatibility

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