Thermal properties enhancement of poly(lactic acid) by corn cob cellulose nanocrystals

Abstract

Cellulose nanocrystals were extracted from agricultural waste corn cob using acid hydrolysis followed by freeze drying. Poly(lactic acid)/corn cob cellulose nanocrystals (PLA/CCNC) composites were prepared using solvent casting. The properties of CCNC were characterized using transmission electron microscope (TEM), zeta potential analyzer, and thermogravimetric analyzer (TGA). The effects of CCNC on the thermal properties of PLA were examined using differential scanning calorimetry (DSC) and TGA. From the SEM and TEM results, the irregular shaped and micron-sized corn cob powder was transformed to needle-like shaped nanocellulose (aspect ratio approximately 30.80) after the acid hydrolysis process. TGA results show that the thermal stability of CCNC is higher than that of corn cob powder. The zeta potential of CCNC is −24.6 mV, which indicates there is a repulsion force between the individual CCNC and making them disperse uniformly and stable in aqueous media. DSC and TGA results show that the crystallinity and thermal stability of PLA were increased by the incorporation of CCNC. This demonstrates that the CCNC is a potential bio-nanofiller with good thermal stability and nucleating-ability for PLA.

Keywords

Corn cob nanocellulose agricultural waste poly(lactic acid)thermal properties

Introduction

The biopolymer is used to replace conventional petroleum-based plastics, attributed to its sustainability, biodegradability, and biocompatibility. Poly(lactic acid) (PLA) is a commercial available polyester-based biopolymer synthesized from renewable resources. PLA exhibits high modulus and strength, biocompatibility, high transparency, low toxicity, and good processability.^1,2 However, the disadvantages of PLA should be mentioned, for example, low ductility, low toughness, and low crystallization rates.³ PLA/thermoplastic blending, PLA/elastomer blending, and PLA/nanofiller processing are some of the feasible methods used to modify their selected properties.^4,5

Various types of nanofillers (e.g., nanoclay, graphene oxide, halloysite nanotube, nanocellulose) have been used to enhance the thermal stability and crystallinity of the PLA. In order to achieve the biodegradability and sustainability, the bio-based natural filler is always the first option, due to the fact that some inorganic nanofiller might sacrifice the biodegradability and “less” environmentally friendly.^6,7 Nanocellulose reinforced PLA is one kind of green nanocomposites thanks to the biodegradable and renewable behavior of both PLA and nanocellulose.

Nanocellulose derived from cellulose-based material has attracted attention from researchers due to its lightweight, high strength, and stiffness, as well as the availability of renewable resources.^8,9 CNC is extracted from agriculture waste and biomass, such as sugarcane bagasse, banana peel, rice straw, wheat straw, cotton waste, corn cob and etc. Nanocellulose can be classified into three main groups depending on their size, extraction, and production methods, i.e. cellulose nanocrystals (CNC), nanofibrillated cellulose, and bacterial nanocellulose.^10

-13 CNC are rod-like cellulose crystals with 10–120 nm in width and several hundred nanometers in length. CNC is highly crystalline attributed to the removal of the amorphous part through acid hydrolysis treatment, which producing high purity cellulose crystals.

Nanocellulose had been used as a filler to increase the mechanical, thermal, and gas barrier properties of polymeric materials.¹⁴ The CNC can be chemically treated to increase its dispersibility and compatibility with various polymer.¹⁵ Some of the documented research work on CNC reinforced biopolymer composites includes poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/rice straws CNC nanocomposites,¹⁶ PLA/coffee silverskin CNC nanocomposites¹⁷ and carboxymethyl cellulose/grain straws CNC nanocomposites.¹⁸

Corn is one of the important agricultural crops in the world, and thus the left-over corn residue/waste should be concerned in the context of sustainable development. It has been reported that the left-over corn residue is about 1 kg/kg of corn grain or 4 tonnes/acre.¹⁹ Thus, in line with sustainable development, we should make some efforts to utilize the corn-cob and make it as a value-added product, for example, bio-ethanol and nanocellulose.²⁰

There are some approaches to improve the thermal properties of PLA by the incorporation of CNC. However, some studies found that adding CNC increases the crystallinity but at the cost of a reduction in the thermal stability of PLA.^21,22 Arrieta et al.²¹ has found that the crystallinity of PLA was increased significantly by CNC. However, the maximum degradation temperature of the PLA was shifted to a lower temperature. In this work, CNC was extracted from corn cob using acid hydrolysis followed by freeze-drying. The PLA/corn cob cellulose nanocrystals (CCNC) composites were prepared using solvent casting. It is our aim to improve the thermal properties of PLA (e.g., thermal stability, degree of crystallinity) by using CCNC. The novelty of this study is to investigate the feasibility of CCNC to fulfill some of the requirement for the PLA properties enhancement, i.e. (1) to make fully bio-based PLA green nanocomposites; (2) to act as nucleating agent and increase the crystallinity of PLA; (3) to increase the thermal stability of PLA. Accordingly, the effects of CCNC on the thermal properties of PLA were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

Materials and methods

Materials

Poly(lactic acid) (PLA) used in this study was purchased from NatureWorks® (Ingeo™ 3250 D). The molecular weight, melting temperature (T_m ), and density of the PLA is 120 kDa, 153°C, and 1.25 g cm⁻³, respectively. The corn cobs were obtained from corn farms at Sik, Kedah, Malaysia. Sodium hydroxide, chloroform, and acetic acid (glacial) were supplied by Merck (Germany). Sodium chlorite (Technical Grade, 80.00%) and sulfuric acid (ACS reagent, 95.0–98.0 wt%) was supplied by Sigma-Aldrich (USA).

Extraction of corn cob nanocellulose

The extraction procedure of corn cob nanocellulose was modified from the methods described by Khoo et al.⁵ and García-García et al.²³ The corn cobs (CC) were washed and dried before grounded using a miniature grinder (Mill Powder Tech Solutions, model: RT-34, Taiwan). Further, lignin and hemicellulose were removed from the corn cob powder (CCP) with sodium hydroxide aqueous solution for 4 h at 100°C under mechanical stirring and then washed several times with distilled water until the alkali was completely removed, and finally dried at 40°C for 24 h in an air-circulating oven. The CCP was then bleached with a solution made up of equal parts of acetate buffer (27 g NaOH and 75 mL glacial acetic acid, diluted to 1 L of distilled water) and aqueous chlorite (1.7 wt% NaClO₂ in water) at 80°C for 6 h. Further, the CCP was washed repeatedly in distilled water until the pH of the solution became neutral and subsequently dried again at 40°C for 24 h in an air-circulating oven. The treated CCP was then acid hydrolyzed for 1 h using sulfuric acid (H₂SO₄) at 49% (v/v) under vigorous and constant stirring. Next, 150 mL distilled water were added to quench the process and hydrolysis reaction was stopped after 60 min. The hydrolyzed cellulose was then centrifuged at 3500 rpm for at least five times wash until the solution turns turbid. The precipitate was dialyzed with water to remove non-reactive sulfate groups, salts and soluble sugars, until neutral pH was reached. The obtained corn cob nanocellulose (CCNC) was stored in a refrigerator.

Freeze-drying of CCNC

First, the CCNC was frozen at −70°C overnight. Then, the frozen sample was transferred to the freeze dryer equipment (Labconco Freeze Dry Systems, USA). The freeze-drying was carried out at a temperature of −50°C and a pressure of 0.09 mbar for 24 h.

Microstructure analysis of CCP

The morphology of CCP was characterized using a field emission electron microscope (FESEM, Supra 35VP, Carl Zeiss, Germany). The sample surfaces were sputtered with gold before the FESEM examination.

Nanostructure morphology assessment

The morphology of CNCC was characterized with a transmission electron microscope (Carl Zeiss, model: Zeiss Libra 120 Plus, Germany) operating at an accelerating voltage of 120 kV. A drop of 0.1% (w/v) nanocellulose suspensions was deposited on the surface of Cu grid covered with a thin carbon film. The grid was negatively stained with uranyl acetate in order to increase the contrast during the TEM measurement.

Zeta potential of CCNC

The zeta-potential of the CCNC was measured using a Zetasizer NanoZS instrument (Malvern, UK), under the following conditions: dispersant water, material refractive index 1.47, dispersion refractive index 1.33, dispersant dielectric constant 78.5 and viscosity 0.8872 cP. The zeta potential experiments were performed in a cuvette consisting of 4 mL 0.1% (w/v) CCNC suspension at temperature of 25°C.

Preparation of PLA/CCNC composites

The PLA/CCNC (blending ratio = 95:5) was prepared using solvent casting method. Based on our previous study, the optimum loading of CNC was achieved at 5 wt%. The CCNC agglomeration often occurs when it’s loading exceed 5 wt%, and this subsequently influences the overall performance of the PLA/CCNC films. The predetermined amount of freeze-dried CCNC was redispersed in chloroform. The PLA was dissolved by using chloroform in another beaker. Both PLA and CCNC solution was then mixed and stirred for 40 min until PLA is fully dissolved and CNCC redispersed homogenously in the suspension before they were cast onto the glass Petri dish. The PLA/CCNC solution was poured onto Petri dish and dried for 24 h under room temperature.

Thermal properties characterization

Thermogravimetric Analyzer (Pyris 6 TGA, PerkinElmer, USA) was used to evaluate the thermal decomposition of CCNC and PLA/CCNC nanocomposites. The specimens were heated from 30°C to 600°C at a heating rate of 10°C/min under nitrogen atmosphere. The onset decomposition temperature (T_onset ), temperature at 50% weight loss (T₅₀ ), decomposition temperature (T_d ) and T_max was determined from the TGA test.

Differential scanning calorimeter (Pyris 6 DSC, PerkinElmer, USA) was used to evaluate the thermal behavior of PLA/CCNC nanocomposites. The specimens were scanned from 30°C to 190°C at a heating rate of 10°C/min. The glass transition temperature (T_g ), melting temperature (T_m ) and cold-crystallization temperature (T_cc ) were determined. The degree of crystanility (χ_c ) of PLA/CCNC composites was calculated using Equation (1).

χ_{c} = [(Δ H_{m} - Δ H_{c c}) / (Δ H_{f} \times W_{P L A})] \times 100 %

where ΔH_m is the heat of fusion of the sample; ΔH_cc corresponds to the cold crystallization enthalpy; ΔH_f corresponds to the heat of fusion of 100% crystalline material, and W_PLA is the net weight fraction of the PLA. The ΔH_f of 100% crystalline PLA is approximately 93.6 J/g.²⁴

Results and Discussion

Morphology of CCP and CCNC

Figure 1 shows the SEM micrograph of the corn cob powder (CCP) prior to acid hydrolysis. Most of the CCP are irregular shaped particles. From Figure 2, it can be seen that the CCNC exhibited needle-like structure, with approximately 520.5 nm in length and 16.9 nm in width (aspect ratio = 30.80). The amorphous cellulose of CNC was removed by mineral acid and thus the isolated cellulose is normally in rod-like or needle-like shape.^25,26 Similar finding was observed for the nanocellulose synthesized from soy hull,²⁷ sugarcane bagasse,²⁸ and rice husk²⁹ by acid hydrolysis. Table 1 lists the aspect ratio of CCNC and others nanocellulose. The aspect ratio of the nanocellulose is depending on the agricultural resources, extraction technique and parameter. It can be seen that the aspect ratio of the CCNC is lower than the banana peel¹² and soy hull.²⁷ Nevertheless, the aspect ratio of the CCNC (synthesized from corn cob in our study) is comparable to the sugarcane bagasse-nanocellulose,²⁸ while higher than the rice straw-nanocellulose¹³ and rice husk-nanocellulose.²⁹

Figure 1.

FESEM micrograph taken from corn cob powder (CCP).

Figure 2.

TEM micrograph taken from CCNC.

Table 1.

Aspect ratio of nanocellulose extracted from different agricultural resources.

Agricultural resource	Aspect ratio	Reference
Banana peel	∼41.7	Tibolla et al.¹²
Corn cob	∼30.8	This study
Rice husk	∼15.0	Johar et al.²⁹
Rice straw	∼10.5	Lu and Hsieh¹³
Soy hull	∼44.0	Flauzino Neto et al.²⁷
Sugarcane bagasse	∼30.0	Kumar et al.²⁸

Thermal properties of CCP and CCNC

Figure 3(a) and 3(b) show the TGA and derivative thermogravimetric (DTG) curves of CCP and CCNC. The thermal decomposition characteristics of the CCP and CCNC were summarized in Table 2. The T_onset and T₅₀ are referred to the onset decomposition temperature and temperature recorded as 50% weight loss, respectively. The T_d1 and T_d2 referred to first- and second-stage decomposition temperature, respectively. The T_max is the maximum temperature taken from DTG curve (maximum peak). Note that the CCP shows two steps of decomposition (T_d1 and T_d2 ) while CCNC exhibits only one decomposition step. According to Carrier et al.,³⁰ the decomposition in the range of 190–240°C is associated to the thermal degradation temperature of hemicelluloses whereas the decomposition in the range of about 240–370°C was corresponding to thermal degradation temperature of cellulose. This indicates that the hemicellulose of CCNC has been fully removed after the chemical treatment process. The char residue of the CCP could be due to the ash-forming elements from alkali metals.³¹ It is worth to mention that the T_onset (245.2°C) and T₅₀ (295.5°C) of CCNC is higher than that of CCP (T_onset = 190.2°C; T_max = 228.0°C). This suggested that the thermal stability of CCNC is higher than that of CCP. In addition, the char residue of CCNC is higher than CCP. This is attributed to the higher char formation by the sulfate group which may act as flame retardant during the pyrolysis process.³² The increase in char formation for cellulose nanocrystals is due to the sulfate group which can act as dehydration catalyst and facilitates the char residue formation.³³

Figure 3.

(a)TGA and(b)DTG curves of CCP and CCNC.

Table 2.

TGA results of CCP and CCNC.

Materials designation	Decomposition temperature					Char residue (%)
Materials designation	T_onset	T₅₀	T_d1	T_d2	T_max	Char residue (%)
CCP	190.2	311.0	238.1	353.6	228.0	11.9
CCNC	245.2	322.8	—	363.2	295.5	15.4

Zeta potential of CCNC

Figure 4 shows that the zeta potential of the CCNC is –24.6 mV. The zeta potential plays an important role in determining the colloidal stability of nanoparticles in aqueous solutions. Nanoparticle with zeta potential of negative value often suggested that the particle would repulse each other and disperse well in water.³⁴ Usually, nanocellulose suspensions with absolute value of zeta potentials approximately 25 mV are quite stable.³⁵ Whereas, nanoparticles with a zeta potential in the range of –15 mV to 15 mV have been reported cause flocculation in the suspension.³⁶ Accordingly, higher electrostatic repulsion between the sulfuric acid treated CCNC could provide sufficient surface charges to stabilize the suspension.³⁷ Controlling the zeta potential of nanocellulose is essential since it would influence the film formation and roughness.³⁸

Figure 4.

Zeta potential of CCNC.

Thermal properties of PLA/CCNC composites

Thermogravimetric analysis (TGA)

Figure 5(a) and 5(b) show the TGA and DTG curves of PLA and PLA/CCNC (95/5) composites. The decomposition temperatures, T_d and char residue of the PLA/CNCC composites are summarized in Table 3. Note that T _onset records the onset decomposition temperature; T_d refers to the end decomposition temperature; while T_max is the maximum temperature taken from the DTG curves (maximum peak). In Figure 5(a), the weight loss from 30°C to 100°C is due to the moisture content of the PLA and PLA/CCNC, while the following weight loss until 351°C is associated to the main decomposition of the PLA molecular chains. The early decomposition of PLA and PLA/CNCC is caused by considerable amounts of tightly bound water whereas the main thermal degradation of PLA is caused by hydroxyl end-initiated ester interchange mechanism and chain homolysis.³⁹ Note that there is a peak at approximately 380°C (c.f. Figure 5(b)), which is preceded by a shoulder, could be associated to the decomposition of CCNC.

Figure 5.

(a) TGA and (b) DTG curves of PLA and PLA/CCNC (95/5) composites.

Table 3.

TGA characteristics of PLA and PLA/CCNC (95/5) composites.

Materials designation	Decomposition temperature (°C)				Char residue at 550°C
Materials designation	T_onset	T₅₀	T_d	T_max	Char residue at 550°C
PLA	301.5	348.1	370.4	351.9	0
PLA/CCNC (95/5)	330.9	363.0	396.1	365.0	1.10

Interesting to note that the T_onset , T_d and T_max of PLA/CCNC is higher than that of PLA, this indicates that the CCNC can increase the thermal stability of PLA. The enhancement in thermal stability is associated to the interaction between PLA and nanocellulose.⁴⁰ The thermal stability of polymer is often influenced by the intermolecular bonding between nanofiller and polymer matrix. Strong intermolecular bonding could increase the bond dissociation energy needed for chain cleavage of macromolecules.⁴¹ Hence, interfacial adhesion between nanocellulose and polymer matrix are required for the improvement of the thermal stability. The incorporation of nanocellulose greatly improved the thermal stability of nanocomposites by enhancing the filler–polymer matrix interaction through hydrogen bonding.⁴² According to Wang et al.,⁴³ the thermal decomposition of PLA composites was increased from 339.26°C to 350.69°C by the incorporation of 5 wt% nanocellulose. As shown in Table 3, the char residue of PLA/CCNC is approximately 1.10 wt%, while there is no char remaining for PLA. The char residue could be associated to the ash-forming elements from alkali metals which can be often found in the agricultural waste (e.g., corn cob). Subsequently, the char formation delay the thermal decomposition.⁴⁴

Differential scanning calorimetry (DSC)

Figure 6 shows the DSC curves of PLA and PLA/CCNC (95/5) composites. The DSC thermal characteristics are summarized in Table 4. There is not much difference of the T_g and T_m for the unfilled and filled PLA. The T_cc of the PLA (103.7°C) was shifted to 96.2°C by the incorporation of CNCC. The decrease in cold crystallization temperature indicates that the CCNC is acting as a nucleating agent, and promoting crystallization in the PLA. The nucleus density of PLA crystallites increased compared to pure PLA with the addition of CNC.⁴⁵ It is known that CNC exhibits high crystallinity due to the removal of the amorphous part of cellulose. The crystallinity index of CNC extracted by sulphuric acid hydrolysis is in the range of 73–77%.^46,47 Adding CCNC into PLA will make more crystals to grow and nucleate. Hence, CCNC acts as heterogeneous nucleus and promote PLA chains crystallization at lower temperature.

Figure 6.

DSC curves of PLA/CCNC (95/5) composites.

From Table 4, it can be seen that the X_c of PLA is 9.8%, this indicates that the PLA prepared using solvent casting is highly amorphous. According to Luzi et al.,⁴⁸ the PLA prepared using chloroform solvent casting is about 8.9 ± 3.4%. In this study, adding CCNC into PLA increase the X_c significantly (from 9.8% to 31.0%). This suggests that the CCNC can act as nucleating agent for the PLA.⁴⁹ The slow evaporation of solvent during the casting process may allow more time for CNC to promote crystallization of PLA in the ordered regions.⁴⁵ According to Yu et al.,⁴⁵ the crystallinity of PLA/nanocellulose (24.3%) is higher than that of unfilled PLA (X_c = 16.4%) prepared by solvent casting. Table 5 lists the crystallinity of the PLA/CCNC and other PLA/nanocellulose nanocomposites. It can be seen that the crystallinity of the PLA/CNCC (95/5) (synthesized from corn cob in our study) is comparable to the PLA/coffee skin-CNC¹⁷ and PLA/bamboo pulp-CNC.⁴⁰ As we further analyze the crystallinity of the samples, i.e. comparison of the X_c of PLA vs X_c of PLA/CNC for each formulation in Table 5, it was found that the percentage improvement of crystallinity of PLA/coffee skin-CNC and PLA/bamboo pulp-CNC is 193% and 119%, respectively. It is worth to mention that for the PLA/CCNC the percentage improvement of crystallinity is approximately 316%. The dramatic improvement of crystallinity of PLA clearly shows the great nucleation-ability of CCNC. This is attributed to the better dispersion and improved interfacial interaction between CCNC and PLA.⁵⁰ It is known that the nucleating efficiency of nanofillers is depends on the dispersibility, interfacial interaction and surface properties.⁵¹ Better dispersion of CCNC leads to more nucleating sites and more surface to grow the PLA crystals, and thus give higher crystallinity.

Table 4.

DSC characteristics of PLA and PLA/CCNC (95/5) composites.

Materials designation	T_g (°C)	T_cc (°C)	T_m (°C)	X_c (%)
PLA	62.1	103.7	167.4	9.8
PLA/CCNC (95/5)	61.2	96.2	166.6	31.0

Table 5.

Crystallinity of the PLA/CCNC and other PLA/nanocellulose nanocomposites.

Nanocellulose source	Formulation	Crystallinity, χ_c (%)	References
Corn Cob	Unfilled PLA	9.8	In this study
	PLA/CCNC (95/5)	31.0
Coffee skin	Unfilled PLA	12.7	Sung et al.¹⁷
	PLA/CNC (95/5)	24.5
Bamboo pulp	Unfilled PLA	28.1	Dhar et al.⁴⁰
	PLA/CNC (97/3)	33.5

Conclusions

In this study, cellulose nanocrystal was successfully extracted from renewable resources, i.e. agriculture waste corn cob using acid hydrolysis and freeze-drying method. The CCNC exhibited needle-like morphology (evidenced from TEM) with an aspect ratio of approximately 30.80. The CCNC also demonstrated high thermal stability (T_onset = 245.2°C; T_max = 295.5°C) and good colloidal stability (zeta potential = −24.6 mV). Furthermore, CCNC can act as a good nucleating agent for PLA by increasing the crystallinity of PLA significantly (from 9.8% to 31.0%). In addition, CCNC shifted the onset decomposition temperature of PLA to a higher temperature (from 301.5°C to 330.9°C), which indicates that the PLA/CCNC composites are stable in higher processing/service temperature. This study shows that CCNC is a good bio-nanofiller to improve the crystallization and thermal stability of PLA simultaneously.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors gratefully acknowledge the financial support of the Universiti Sains Malaysia Research University (RU) Grants (grant account number: 8014024).

ORCID iDs

Wei Keat Ng

Wen Shyang Chow

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