3.2. Crystallization Characterization of Composite Films
Crystallization behavior of the CNC/PLA melt spun fibers and films were examined in order to investigate the effect of CNC on PLA crystallization characteristics. Initial heating thermograms, seen in
Figure 2, were used to calculate the degree of crystallinity (χ) and examine the cold crystallization enthalpy (Δ
Hc) and cold crystallization temperature (
Tcc) as a function of CNC content for both the fibers and films. Equation (1) was used to calculate χ:
where χ is the degree of crystallinity, Δ
Hm is the melting enthalpy of the sample, Δ
Hc (<0) is the cold crystallization enthalpy of the sample,
is the theoretical melting enthalpy of 100% crystalline PLA,
= 93 J/g was used [
21], and wt
% is the CNC content in weight percent. The degree of crystallinity along with
|Δ
Hc| and
Tcc is seen in
Table 1.
Figure 2.
Non-isothermal initial heating thermograms of CNC/PLA (a) fibers; (b) films; (c) fiber cold crystallization peaks; and (d) film cold crystallization peaks.
Figure 2.
Non-isothermal initial heating thermograms of CNC/PLA (a) fibers; (b) films; (c) fiber cold crystallization peaks; and (d) film cold crystallization peaks.
Table 1.
Degree of crystallinity (χ), cold crystallization enthalpy (ΔHc), and cold crystallization temperature (Tcc) for cellulose nanocrystals/poly lactic acid (CNC/PLA) fibers and films.
Table 1.
Degree of crystallinity (χ), cold crystallization enthalpy (ΔHc), and cold crystallization temperature (Tcc) for cellulose nanocrystals/poly lactic acid (CNC/PLA) fibers and films.
CNC content | χ (%) | |ΔHc| (J/g) | Tcc (°C) |
---|
Fibers | Films | Fibers | Films | Fibers | Films |
---|
0 wt % CNC | 5.7 ± 2.4 | 11.5 ± 0.8 | 22.7 ± 2.6 | 16.2 ± 2.1 | 113.2 ± 0.5 | 113.0 ± 0.5 |
1 wt % CNC | 2.2 ± 2.1 | 24.1 ± 0.2 | 28.0 ± 2.9 | 7.3 ± 1.3 | 113.7 ± 0.2 | 110.2 ± 0.1 |
2 wt % CNC | 1.6 ± 1.2 | 25.3 ± 0.2 | 28.2 ± 2.8 | 5.8 ± 0.7 | 114.5 ± 1.4 | 109.9 ± 0.2 |
3 wt % CNC | 1.3 ± 1.3 | 29.7 ± 0.5 | 29.8 ± 0.7 | 1.9 ± 0.2 | 113.2 ± 1.3 | 109.3 ± 0.1 |
The fibers are highly amorphous with large |Δ
Hc| because the PLA chains do not have ample time to organize into crystalline lamella due to the rapid cooling of the fibers during spinning. However, the films’ degree of crystallinity is higher than that of the fibers and also increases with increasing CNC content due to the slow cooling rate during compression molding allowing time for crystallization coupled with CNC acting as a nucleating agent. For example, the 0 wt % CNC film has a relatively low crystallinity at approximately 11.5%, while the 3 wt % CNC film has a relatively high crystallinity at approximately 29.7%. Furthermore, both the |Δ
Hc| and
Tcc decrease with increased CNC content, seen in
Figure 2c,d and displayed in
Table 1. This increase in χ and decrease in both |Δ
Hc| and the cold crystallization peak as a function of CNC content indicates that the CNC are acting as a nucleating agent, promoting crystallization in the PLA films [
4]. As seen in
Figure 2a,b, the melting endotherms for both the fibers and films are comparable and do not change with increased CNC content. Both the fibers and films displayed double melting behavior, which is indicative of either polymorphism or melt recrystallization. Polymorphism refers to when there are multiple crystal phases present in a sample and melt recrystallization occurs when semi-melted crystals present recrystallize instead of melting further. To further investigate the crystal structure of the composite fibers and films, diffraction patterns were examined.
PLA is comprised of three primary crystal phases: α, β, and γ [
22]. The pseudo-orthorhombic, helical α phase crystal is the most stable and prevalent of the three primary crystal phases [
22,
23,
24]. The diffraction patterns of the CNC composite fibers and films are seen in
Figure 3. Both the fibers and films were comprised of primary
α phase crystals and do not display evidence of a secondary crystal phase, therefore the double melting behavior seen in
Figure 2 is due to melt recrystallization and not polymorphism. The diffraction patterns of the fibers, seen in
Figure 3a, display a broad maximum at 2θ = 16.7° corresponding to the characteristic plane of α (200 + 110) [
24] and this is in good agreement with the low crystallinity calculated using Equation (1) and displayed in
Table 1. The CNC films display characteristic peaks of crystalline PLA at 2θ = 16.7° and 19.1° which correspond to the
α phase crystalline peaks with characteristic planes of (200 + 110) [
24] and (203) [
25], respectively.
Figure 3.
Diffraction patterns obtained for the CNC/PLA (a) fibers and (b) films as a function of CNC content.
Figure 3.
Diffraction patterns obtained for the CNC/PLA (a) fibers and (b) films as a function of CNC content.
The diffraction behavior of both the composite fibers and films is dominated by the PLA phase, which is the primary phase of the composite. In order to ascertain the crystal structure of the reinforcing phase, the as-received CNC/water suspension was dried in a vacuum oven and the diffraction pattern of the resultant CNC mat is shown in
Figure 4. The CNC display primary peaks at 2θ = 15.1°, 17.5°, and 22.7° with a weak diffraction peak at 34.4° which correspond to the cellulose I crystal planes (1–10), (110), (200), and (040), respectively [
10,
26,
27]. Diffraction peaks are also seen at 2θ = 12.5° and 20.1° which is consistent with the primary peaks associated with cellulose II which are at 2θ = 12.5°, 20.1°, 22.7°, and 34.4° [
27]. The obtained crystal structure of the CNC mat is consistent with previously reported highly crystalline CNC of both cellulose I and II [
10,
26,
27]. The existence of cellulose II in the CNC is typical of CNC from Forest Products Laboratory and is likely an artifact of using dissolving pulp as the source materials [
28], but may also be attributed to the acid hydrolysis extraction method and exposure to alkali and acid treatments [
27].
Figure 4.
Diffraction pattern of a CNC mat with cellulose I and cellulose II primary diffraction peaks indicated.
Figure 4.
Diffraction pattern of a CNC mat with cellulose I and cellulose II primary diffraction peaks indicated.
To further investigate the crystal structure of the composite films as a function of CNC content, the average lamella thickness at the PLA primary diffraction peaks, 16.7° and 19.1°, was calculated using the Scherrer equation, Equation (2):
where
L is the apparent crystalline lamella thickness,
K is a dimensionless shape factor (0.9 was used as an approximate for the spherulite structure [
29]), λ is the radiation wavelength,
B is the full width at half maximum value of the diffraction peak, and θ is the Bragg angle. As seen in
Figure 5, the average lamella thickness does not significantly change for either primary diffraction peak as a function of CNC content. However, the degree of crystallinity significantly increases as a function of CNC content, as reported in
Table 1, indicating that while the crystallinity of the films is increasing the average spherulite size is constant. Therefore, as the CNC content increases, there are more spherulites of comparable size present.
Figure 5.
Average crystal lamella thickness of the two dominant diffraction peaks, 16.7° and 19.1°, for the CNC/PLA films as a function of CNC content.
Figure 5.
Average crystal lamella thickness of the two dominant diffraction peaks, 16.7° and 19.1°, for the CNC/PLA films as a function of CNC content.
3.3. Thermo-Mechanical and Mechanical Response of Composite Films
The thermo-mechanical behavior of the CNC/PLA films, both the storage modulus (
E′) and loss modulus (
E′′), was investigated as a function of CNC content. The results are seen in both
Table 2 and
Figure 6. The storage modulus increases significantly from 1.9 ± 0.3 GPa to 2.7 ± 0.0 GPa with the addition of only 1 wt % CNC. An increase in storage modulus, can be attributed to hindered polymer chain mobility due to: (1) an increase in polymer crystallinity [
30]; (2) the addition of CNC constricting polymer chain movements [
31,
32]; or (3) a combination of (1) and (2). In this case, the significant increase in storage modulus can be attributed to a combinatory effect because the degree of crystallinity steadily increases with the addition of CNC, as shown in
Table 1. Furthermore, as displayed in
Table 2, there is a slight increase in glass transition temperature (
Tg) with the addition of 3 wt % CNC. The
Tg values reported in
Table 2 are from DMA measurements, specifically the tanδ peak which is defined as
E′′/E′. It was difficult to ascertain whether there was a trend in
Tg from the initial heating thermograms; however, this may be due to the sensitivity of the measurement technique,
i.e., DSC [
33]. Additionally, the loss modulus peak shows some indication of broadening upon the addition of CNC, particularly at a higher CNC content of 3 wt %. This can be attributed to the increase in PLA matrix crystallinity coupled with the addition of CNC, which will further hinder polymer chain mobility resulting in a slight broadening the glass transition region [
17].
Figure 6.
(a) Storage modulus and (b) loss modulus of the CNC/PLA films as a function of CNC content.
Figure 6.
(a) Storage modulus and (b) loss modulus of the CNC/PLA films as a function of CNC content.
Table 2.
Thermo-mechanical behavior of CNC/PLA films as a function of CNC content
Table 2.
Thermo-mechanical behavior of CNC/PLA films as a function of CNC content
CNC content | E′ at 35 °C (GPa) | Tg, from tanδ (°C) |
---|
0 wt % CNC | 1.9 ± 0.3 | 79.3 ± 0.8 |
1 wt % CNC | 2.7 ± 0.0 | 79.5 ± 0.3 |
2 wt % CNC | 2.7 ± 0.1 | 78.8 ± 0.0 |
3 wt % CNC | 2.9 ± 0.1 | 82.2 ± 0.3 |
The elastic modulus,
E, of the composite films as a function of CNC content is shown in
Figure 7. As seen in
Figure 7, there is minimal change in elastic modulus as a function of CNC content for the CNC/PLA films. This is similar to that reported by John
et al. [
16], in which melt spun fibers having 3 wt % CNC additions had a modest 0.2 GPa increase in elastic modulus as compared to the neat PLA fibers. The previously reported elastic modulus of CNC (in the axial direction is ~60–105 GPa [
34] and 20–50 GPa [
35] in the transverse direction) is higher than that of PLA (~3 GPa), therefore, an increase in elastic modulus upon addition of CNC would be expected. However, while CNC has a higher intrinsic elastic modulus, there are several factors that may influence the modulus of the composites including: (1) the CNC/polymer interfacial interactions; (2) CNC distribution and dispersion; (3) CNC alignment; and even (4) moisture content in the composite [
7]. Poor interfacial adhesion may also lead to poor nanofiller distribution and dispersion, which has previously been reported for CN/PLA composites fabricated via melt compounding [
36]. Furthermore, because the CNC/PLA films were fabricated using direct liquid feeding of the as-received CNC/water suspension, TGA was performed on the films to investigate whether any moisture was present and representative results of the 3 wt % CNC/PLA films are seen in
Figure 8. There is no significant moisture content present; therefore excess moisture is not a dominating factor to the plateau in elastic modulus with the addition of CNC content. Furthermore, due to the introduction of water during processing, the degradation of the 3 wt % CNC film was compared to the as-received PLA pellets. As seen in
Figure 8a,b, the thermal stability of the 3 wt % CNC film is comparable to the as-received PLA pellets and does not appear to be compromised.
Figure 7.
Elastic modulus of CNC/PLA films as a function of CNC content.
Figure 7.
Elastic modulus of CNC/PLA films as a function of CNC content.
Figure 8.
Representative results of the thermogravimetric analysis (TGA) of the (a) as-received PLA pellets and (b) 3 wt % CNC/PLA film.
Figure 8.
Representative results of the thermogravimetric analysis (TGA) of the (a) as-received PLA pellets and (b) 3 wt % CNC/PLA film.