Luminescence properties and mechanical response of CNC/PVA co-assembly membranes
Stable dispersion of cellulose nanocrystals (CNCs) in suspension is necessary for the uniaxial assembly of CNC. Table 1 shows the zeta-potential of CNC in the suspension with poly (vinyl alcohol) (PVA) with different ratios, which indicated that neat CNC suspension owned an extremely high zeta potential of -72.2 mV. As the PVA percentage increased, the zeta potential decreased but remained at a high level of > 30 mV, which indicated the suspensions were still highly stable (Kiprono et al. 2018). In order to eliminate the chiral structure, the zeta potential of the CNC dispersion should not be too high to achieve the uniaxial assembly with higher orientation. Thus, the CNC/PVA system should have better assembly potential than neat CNC.
Table 1
Zeta potential values of CNC/PVA suspension.
Samples
|
CNC-100
|
CNC-80
|
CNC-60
|
CNC-50
|
CNC-40
|
CNC-20
|
Zeta Potential (mV)
|
-72.2
|
-46.5
|
-44.3
|
-43.5
|
-42.2
|
-38.9
|
Figure 1a shows the absorption spectra of CNC/PVA co-assembled membranes. Since both cellulose and PVA had no absorption in UV and visible range, the absorption peaks around 368 nm must come from the structural color of CNC assembly. The photoluminescence (PL) spectra Fig. 1b show excitation peaks at similar wavelengths and the emission wavelengths (Em) were around 368 nm. Those results suggested that CNCs could uniaxially assemble with PVA and emit blue light under UV light.
Figure 2a and 2b proved the uniaxial assemble structure still existed in the CNC/PVA composites, but PVA took up a large volume, and it wrapped CNC around it. As seen in Fig. 2c and 2d, for the fracture surfaces of CNC-50 and CNC-40 displayed similar results, there were not apparent chiral structures, and they were both smooth. However, in Fig. 2e, CNC-100 showed a different format under the larger observation size than CNC-40 and CNC-50. It could be the large scale of chiral nematic structure though it had a high orientation in small size less than 1 µm. This was also the reason for the EQE of the CNC/PVA uniaxial co-assembly composites were much higher than that of the neat CNC membrane.
The percentage of CNC also affects the mechanical and tensile-response properties of the CNC/PVA membranes. CNC-80 and CNC-60 showed a low elongation at break, which was 4.2% and 7.0%, respectively, when that of neat CNC membrane was less than 0.1%. Those membranes displayed no tensile responsiveness. By contrast, the elongation at the break of CNC-50, CNC-40, and CNC-20 could be more than 90%. Furthermore, CNC-50 and CNC-40 exhibited shifts in excitation wavelengths (Ex) during stretching, while CNC-20 showed little responsiveness. In detail, the Ex of CNC-50 showed an apparent blue shift when the elongation reached 40%, and it continued to decrease to 346 nm at the break, compared with the initial Ex wavelength of 368 nm, as shown in Fig. 3. The Ex of CNC-40 sample displayed similar results, and the maximal decrement of Ex wavelengths was 22 nm and 17 nm for CNC-50 and CNC-40, respectively. Emission wavelengths (Em) of CNC-50 and CNC-40 also showed a slight blue shift overall when the elongation from 0 to 90% or 94%.
Besides, the luminesce intensity also increased during the stretching. As shown in Table 2, the emission quantum-efficiencies (EQEs) of CNC-50 and CNC-40 were 48.30% and 60.60%, respectively, both of which were much higher than neat CNC uniaxial assembled membranes (13.90%) (Gan, Feng, Liu, Zheng, Li and Huang 2019). During stretching, the EQEs of CNC-50 and CNC-40 could increase to 61.10% and 63.52%, respectively, and their photoluminescent lifetime also increased slightly. Figure 4 also illustrates that the luminous intensity of CNC/PVA membranes was higher after tensile treatment.
Table 2
EQE of CNC-50 and CNC-40 before and after stretching.
Samples
|
lifetime (ns)
|
EQE (%)
|
CNC-40
|
1.26
|
48.30
|
CNC-40 at 80% elongation
|
1.27
|
61.11
|
CNC-50
|
1.18
|
60.60
|
CNC-50 at 80% elongation
|
1.22
|
63.52
|
Responsiveness mechanism
The mechanisms of tensile response and photoluminescent enhancement were studied with the uniaxial assembly structure of CNC as the beginnings shown in Scheme 1. CNC assembly was regarded as a dense hexagonal arrangement (DHA) in the PVA matrix (Wang et al. 2015), and the Bragg-diffraction wavelength (λ) of the uniaxially assembled CNC could be expressed by Eq. (1) (Zhu et al. 2016):
$$m?=2\text{D}\sqrt{{{n}_{\text{e}\text{f}\text{f}}}^{2}-{sin}^{2}?}$$
1
where m is the diffraction order. For CNCs, the m should be 1 (Gan, Feng, Liu, Zheng, Li and Huang 2019). neff is the effective refractive index, D is the vertical periodicity, and θ is the incident angle.
In this work, the incident angle was 90°, so the value of sin2θ was 1. The refractive index of CNC and PVA were 1.534 (Shopsowitz et al. 2010) and 1.520–1.550 (Natarajan et al. 2017), respectively. Since the λ was 368 nm, the D should be 158.2 nm, which was slightly larger than the average length of CNC. The difference should be ascribed to the introduction of PVA between CNC.
After stretching, the Ex of both CNC-40 and CNC-50 firstly decreased a little at the elongation of 20%, and then showed a substantial decrement as the elongation increased from 20–40%, then steeply decrease until the break. The decrease of Ex wavelength might be from the change in D and neff. However, we found the change was mainly attributed to the neff. The neff decreased with an increasing strain due to the vacuum, which owned a lower refractive index. In byte DHA of CNC in PVA, we regarded CNC as a cylinder with a radius of r (4.5 nm), a length of L (144 nm), and a minimum vertical distance between CNC particles of e. The edge of hexagonal was coded as R. According to the thermogravimetry analysis (TGA) results in Table 3, the CNC component of CNC-50 and CNC-40 was calculated to be 64.5 wt% and 43.7 wt%, respectively, with Eq. (2) (Liu et al. 2019):
$$C=\frac{{W}_{\text{C}\text{N}\text{C}/\text{P}\text{V}\text{A}}-{W}_{\text{P}\text{V}\text{A}}}{{W}_{\text{C}\text{N}\text{C}}-{W}_{\text{P}\text{V}\text{A}}}\times 100\%$$
2
where WCNC/PVA, WCNC, and WPVA were the residual weight of CNC/PVA, CNC, and PVA at 800 °C, respectively. With the densities of CNC and PVA (1.56 g cm− 3 (Giese et al. 2015) and 1.19 g cm− 3 (Wang and Walther 2015), respectively), the volume fraction (VCNC) of CNC-50 and CNC-40 should be 58.1% and 37.2%, so R could be calculated by Eq. (3):
$$R=\sqrt{\frac{\pi {\text{r}}^{2}\text{L}}{2\sqrt{3}D\times {V}_{\text{C}\text{N}\text{C}}}}$$
3
The R of CNC-50 and CNC-40 were 5.36 nm and 6.70 nm, respectively, which indicated CNCs might be tightly arranged in the PVA matrix. We first assumed the D changed, which meant it must decrease during stretching to keep consistency with the decrease in λ. However, the decreasing D implied the material was shrinking, which was clearly contradicted to the tensile test conditions.
Then, we assumed the D was constant and the neff decreased during stretching due to the vacuum generation, so we could calculate the neff of stretched membranes with Eq. (4) (Zhao et al. 2012):
n eff 2 = n12f1 + n22f2 (4)
where n1 and n2 are the effective refractive index of PVA/CNC and vacuum, the value of n2 is 1, and f2 is the fraction of vacuum and air. Apparently, f2 = 1 - f1.
The results of f1 were shown in Table 4. The Δf1 displayed the same tendency of the change by stretching the CNC-50 and CNC-40 membranes. Those results suggested that the membrane did not gain much vacuum or air at first, so the λ nearly did not change when the elongation was smaller than 20%. By contrast, when the elongation increased further, the plastic strain of PVA led to the introduction of vacuum and air. Then the f2 increased rapidly, and thus the Ex decreased obviously. Subsequently, with stationary stretching, the vacuum generation of CNC-50 and CNC-40 showed consistent results of Δf1 with the change of elongation. Finally, the materials fractured and resulted in a massive change in neff, which led to a large decrease in Ex wavelength again.
Table 3
Residual weight of different CNC/PVA membranes at 800 ℃.
Samples
|
CNC-100
|
CNC-50
|
CNC-40
|
CNC-20
|
CNC-0
|
Residual weight (wt%)
|
26.02
|
17.13
|
11.92
|
4.76
|
1.03
|
CNC content (wt%)
|
100.0
|
64.5
|
43.7
|
15.0
|
0.0
|
Table 4
λ, neff, f1, and Δf1 at different elongation for CNC-50 and CNC-40.
CNC-50
|
CNC-40
|
Elongation
|
λ (nm)
|
neff
|
f1
|
Δf1
|
Elongation
|
λ (nm)
|
neff
|
f1
|
Δf1
|
20%
|
366
|
1.529
|
0.989
|
-
|
20%
|
367
|
1.531
|
0.994
|
-
|
40%
|
354
|
1.500
|
0.926
|
0.063
|
40%
|
357
|
1.508
|
0.941
|
0.053
|
60%
|
352
|
1.496
|
0.915
|
0.011
|
60%
|
355
|
1.503
|
0.931
|
0.010
|
80%
|
350
|
1.491
|
0.909
|
0.006
|
80%
|
354
|
1.500
|
0.926
|
0.005
|
90%
|
346
|
1.482
|
0.884
|
0.023
|
94%
|
351
|
1.494
|
0.910
|
0.016
|
The results were also coincident with SEM and XRD analysis. Compared with the unscratched membrane in Fig. 2c, the stretched membrane in Fig. 6a seemed to own better orientation, and the traces of vacuum during stretch could be found. Meanwhile, the CNC-40 fracture surfaces displayed similar results, but the orientation was not noticeable. The XRD results in Fig. 7a and 7b displayed a typical diffraction peaks at 2θ values of around 22.0° in all samples, which is coincident of diffraction planes of (200) in the cellulose I crystal (French 2013). The diffraction peaks at 2θ values of around 20° belong to the PVA (Kim et al. 2020). The crystallinity of CNCs can be represented by Segal crystallinity index (CrI) (French et al. 2013). CrI of CNC-40 and CNC-50 decreased from 73.73–58.86% and from 79.25–64.77% respectively after stretching, which results from the decrease of relative density of CNCs under the same scan area. On the other hand, in the stretching process, although D is constant, the parallel distance between CNCs might be smaller, because the stretching produced a shrinking perpendicular to the stretching direction (Yu et al. 2018), so the EQE showed a lift after stretching. Besides, the R of CNC-40 was larger. Thus the EQE increase was greater than that of CNC-50. In addition, although CNC-20 was also tough, the CNC content was too low (15.0 wt%). Thus, the formation of the vacuum could be hard, and it showed little response to mechanical stimulation. These results also confirm our assumption that the decrease of the Ex wavelength and the luminescence enhancement of CNC/PVA membrane was due to the neff change should be correct.