Improved dispersibility of nanofibrillated cellulose via simple microwave-assisted esterification

Figures & data

Figure 1. Schematic diagram of NFC esterification using simple microwave-assisted process.

Table 1. Reaction conditions and parameters to modify NFC using simple microwave heating.

Samples	Molar ratio (NFC:LA)	Amount of HCl (wt%)	MW power (Watts)	MW time (min)	MW energy^a (J)
Effect of reagent and catalyst
5LA-5HCl	1:5	5	800	1	48,000
10LA-5HCl	1:10	5	800	1	48,000
15LA-5HCl	1:15	5	800	1	48,000
10LA-3HCl	1:10	3	800	1	48,000
10LA-7HCl	1:10	7	800	1	48,000
Effect of microwave power and time
180P-5m	1:10	5	180	5	54,000
180P-10m	1:10	5	180	10	108,000
300P-5m	1:10	5	300	5	90,000
300P-10m	1:10	5	300	10	180,000
600P-1m	1:10	5	600	1	36,000
600P-3m	1:10	5	600	3	108,000
800P-1m	1:10	5	800	1	48,000
800P-3m	1:10	5	800	3	144,000

^a Energy (J) = Power (Watt) × Time (sec).

Table 2. Effects of the NFC:LA molar ratio and amount of HCl on the degree of substitution (DS) of mNFC.

Samples	NFC:LA ratio	Amount of HCl (wt%)	DS
5LA-5HCl	1:5	5	0.52 ± 0.06^b
10LA-5HCl	1:10	5	0.66 ± 0.05^a
15LA-5HCl	1:15	5	0.35 ± 0.10^d
10LA-3HCl	1:10	3	0.14 ± 0.06^e
10LA-7HCl	1:10	7	0.45 ± 0.08^c

Statistically significant variations in DS values at the 95% confidence interval are indicated by letters a-e (p < 0.05).

Figure 2. The Relationship between microwave energy and final reaction’s temperature (a) and weight reduction of mNFC suspension after microwave heating (b); Low weight reduction (less than 10%) denotes the loss of solely water molecules, but medium to high weight reduction (>10%) indicates the loss of water molecules as well as solid mNFC ejected from the reaction bottle due to steam explosion.

Figure 3. Chemical structure analysis: NFC and mNFC with different DS levels (mNFC-Low DS, mNFC-Med DS, and mNFC-High DS) via; (a) FTIR, (b) ¹H liquid-state NMR, and (c-d) XPS spectra.

Table 3. Comparing the DS values of mNFC assessed from three different techniques.

Samples	Degree of substitution (DS)
	DS^1 (Titration)	DS^2 (NMR)	DS^3 (XPS)
mNFC-Low DS	0.22	0.37	0.82
mNFC-Med DS	0.43	0.66	1.45
mNFC-High DS	0.66	1.01	1.82

DS¹ determined by the titration method.

DS² determined by the ¹H NMR spectra.

DS³ determined by the XPS spectra.

Figure 4. (a) X-ray diffraction patterns and crystallinity index (embedded in top right corner) of NFC and mNFC; (b) TEM images of NFC and mNFC-High DS.

Figure 5. Thermograms of NFC and mNFC with Low DS, Med DS, and High DS; (a) DSC, (b) TGA, and (c) DTG.

Figure 6. Dispersion of NFC and mNFC with different DS levels in solvents ranging in the order of decreasing polarity.

Figure 7. Photographs of PLA solution (pure PLA) and PLA/NFC solution and PLA/mNFC-High DS solution (a) and the subsequent casted films of the aforementioned solutions with thickness of 30–50 µm (b).

Figure 8. SEM images of (a) film surface and (b) fractured surface morphology of pure PLA film and nanocomposite films of PLA/NFC and PLA/mNFC-High DS.