Polyethylene terephthalate (PET) is one of the largest commercial thermoplastics due to its strength, thermal stability, chemical resistance, transparency and barrier properties.
However, its crystallinity, low wettability and poor adhesion properties make mixing with cellulose a challenge.
Therefore, decorating cellulose surfaces with hydrophobic groups is a key solution.
While the literature on the production of natural cellulose-PET composites is highly relevant and discussed at length at this point, the intersection of the answer sets between the terms "PET" and "cellulose" in Chemical Abstracts gives a large number of shocks that cannot be fully addressed here.
For convenience, we have produced a graphical overview of selected works and related literature to illustrate the function of trifluoroacetic acid as an effective dispersion medium for cellulose nanocrystals in the supplementary data.
Despite advances in the production of CNCs, their suspension in common organic media remains a challenge in order to generate composite materials.
Ultimately, for industrial-scale applications, the goal is to achieve the dispersion of natural CNCs in organic solvents without modifying the surface.
Experimental part
1.1 Production of cellulose nanocrystalline suspension
CNC was prepared using the gaseous hydrogen chloride method described by Kontturi and colleagues.
With a CNC load of 20 mg mL-1, the resulting fine white powder was found to form an opaque suspension immediately after mixing with anhydrous trifluoroacetic acid (TFA).
CNCs were suspended in TFA for 1 min, 30 min, 1 h, 6 h, 12 h, 24 h, 48 h and 72 h, and then quenched in water to prepare analytical samples.
The resulting precipitate is filtered with deionized water and washed with ether.
Prior to analysis, solids are dried overnight under vacuum.
Quality recovery rates ranged from 90-100% for all cases.
1.2 Transmission electron microscopy (TEM) and atomic force microscopy (AFM) imaging
TEM imaging at 100 kV accelerating voltage on JEOL 1230 transmission electron microscope.
2.5 μL of nc-TFA suspension collected at different time points, diluted with fresh TFA to 0.0005 wt%, deposited on a luminescent carbon-coated TEM grid, negatively stained with 2% uranyl acetate solution for 5 min.
AFM imaging was performed on Asylum Research atomic force microscopy using a silicon probe (f=70 KHz), tap mode, and a scan rate of 0.8 Hz/s.
Dilute the CNC-TFA suspension (2.5 μL) to 0.0005 wt% with fresh TFA deposited on a freshly sheared mica matrix.
1.3 Thermogravimetric analysis (TGA)
TGA data was measured using the Shimadzu TGA-50 thermogravimetric analyzer.
CNC-TFA suspensions collected at different time points were characterized at a heating rate of 10 °C min-1 at room temperature to 450 °C nitrogen.
1.4 Spectral analysis
Fourier transform infrared (FT-IR) spectroscopy was measured on a Nicolet 6700 FTIR spectrophotometer (Thermoelectronics) with a resolution of 4cm-1.
CNC-TFA suspensions collected at different time points were analyzed in KBr wafers.
A Bruker AVANCE 500 MHz spectrometer and an 11.7 Tesla wide-bore magnet were used for solid-state 13C cross-polarization-magic-angle rotational nuclear magnetic resonance spectroscopy (CP-MAS NMR) studies.
All measurements were made using a 1.0 g CNC-TFA suspension sample.
1.5 Powder X-ray diffraction (XRD)
XRD measurements were performed on the Bruker D8 ADVANCE powder diffractometer.
The diffraction intensity of Cu radiation (40 kV and 25 mA) was measured in a 2θ range from 10◦ to 80◦.
1.6 Preparation of composite film
PET is taken from commercial water bottles, washed with ethanol and air-dried.
The tissue (10 ml) and processing center (100 mg) were added and mixed and stirred for 24, 48 years or 72 h.
The resulting homogeneous suspension is poured into a glass dish in a fume hood and the film is organized in the form of evaporation in approximately 30 min.
Add deionized water to wash off the remaining tissue and the resulting white film harvest and dry under several layers of paper under a 2 h 5 kg load, then air dry overnight in a fume hood.
The same PET is stirred in TFA for the same time and dried under the same conditions to obtain a control film containing only PET.
The second group is cellulose nanocrystals stirred in TFA for 24, 48 or 72 h, followed by the addition of PET flakes in the last 5 min.
1.7 Scanning electron microscopy (SEM) and Brunol-Emmett-Taylor (BET) adsorption isothermal analysis
Hitachi S-4100T scanning electron microscopy was used to perform scanning electron microscopy analysis of CNC-PET composites.
1:10 nc-PET aged for 72 h in 10 mL TFA and control PET aged in 10 mL TFA for 72 h, coated with gold and imaged.
BET data was collected using the Micromeritics ASAP2020 absorption analyzer and vacuum degassed at RT for 18 h prior to analysis.
Results and discussion
2.1 Production of CNC suspension
Over the course of 72 hours, the initially free-flowing CNCs and TFA suspensions become increasingly viscous and gradually clarify.
The suspension at early time points was initially homogeneous but proved to be metastable and exhibited very slow sedimentation behavior, which can be delayed by a warming of 60 °C, after which the mixture remains completely homogeneous.
Tyndall effect measurements after the CNC-controlled suspension was stirred in TFA for 24 h showed the presence of particles > 50 nm.
Over the course of the second 24 hours, a gradual weakening of the effect can be seen and almost disappears after 5 days.
2.2TEM and AFM analysis
In TEM images, bundled CNCs are mainly seen at t=0 and early time points.
During aging, needle-like nanocrystals (length 145±43 nm) are formed.
The results of AFM imaging were broadly consistent with TEM, showing both aggregates and needle-like CNCs (thickness 10.8±4 nm) with identical aggregates.
2.3 Thermogravimetric analysis
For TGA measurements (Figure 4), analyze as a control a freshly prepared CNC sample, which is different from the t = 1 min sample, which is suspended in TFA, quenched with deionized water after 1 min and separated.
Thermal degradation is evaluated from the first derivative (midpoint) and tangent (start, end) of the weightlessness curve.
For native CNC and t=1 min samples, initial slight weight loss up to 100◦C can be attributed to volatilization of the adsorbed water.
The two samples are then decomposed at about 340°C at the midpoint of the curve, consistent with the decomposition of pure cellulose.
Samples precipitated from TFA for 0.5 h to 72 h have an increasingly pronounced mass loss between room temperature and the onset of decomposition.
Samples from 0.5 h to 24 h underwent two-stage thermal decomposition, while samples with t = 48 h and 72 h again underwent single-stage thermal decomposition, but in a wider range, with the midpoint about 50°C lower than the native CNC and t = 1 min samples.
While the stage decomposition of the second t = 0.5 h, 1 h, 6 h and 12 h samples is roughly related to the control and t = 1 min samples, the first stage begins to decompose between about 240-250 °C.
This supports infrared and NMR data (Section 3.4), indicating that CNCs are modified over time by increasing acylation.
The intermediate sample of T = 0.5, 1, 6, and 12 h appears to contain CNCs from both populations; One is very similar to natural cellulose and follows the thermal decomposition path of the control sample, while the other roughly follows the thermal decomposition path of the t = 48 h and 72 h samples.
2.4 Spectral analysis
Stacked FT-IR spectra of CNCs in TFA at different time points.
In all samples, the characteristic peaks of cellulose OH stretch, CH stretch, and OH bending were located at 3433 cm-1, 2898 cm-1, and 1643 cm-1, respectively.
The fingerprint region peak of the control CNC sample also almost overlaps the spectrum of microcrystalline cellulose in the literature.
The new peak at 1785 cm-1 is only visible in the sample with t = 6 h and clearly visible at 72 h, which is characteristic of the C=O function of trifluoroacetate.
The stacked CP-MAS 13C-NMR plots of the control sample and the t=72h sample also show a new peak of the trifluoroacetate C=O group at 160.1 ppm and the CF3 quartet at 115.3 ppm, as well as hydrogen-free glucose-free carbon consistent with the literature NMR data for cellulose.
2.5XRD analysis
XRD was used to analyze the change of crystallinity of nanosilicon carbide in TFA with increasing suspension time.
The CNC features a 2θ peak at 22.5◦, corresponding to the crystal aircraft.
It was observed that this peak decreased over time, with a peak at 20.40 °C, reminiscent of the characteristic peak of cellulose trifluoroacetate.
The position of this peak indicates an increase in plane separation of the crystal, and its widening indicates an inhomogeneous and/or disordered composition.
2.6 SEM and BET analysis of CNC-PET composites
When the CNC-PET composite film was prepared by the above method, both the composite film and the PET control cast by TFA showed a porous structure.
This makes it difficult to measure the contact angle because the water droplets are gradually absorbed into the dry film.
However, measuring the evolution of the droplets over the same time period allowed us to determine that the hydrophilicity of cnc-containing films was significantly increased compared to control PET.
Cellulose-PET composite membranes were also observed to absorb more water than porous PET membranes.
Porosity analysis showed that the surface area of the cellulose PET film and the control PET film were the same in the experimental error range, 11.9 and 12.5 m2/g, respectively.
According to the pore size distribution, it can be seen that the composite of PET and cellulose has no significant effect on the overall porosity.
As expected, the TGA curve of the composite showed the thermal behavior of both materials.
conclusion
The data provided in this paper shows that native CNCs can be physically dispersed in TFA and can be re-separated as if precipitated out in a short period of time.
Applications requiring CNC suspension in organic media can take advantage of TFA's excellent solvent properties and CNC suspension stabilization potential, as well as its high volatility and water miscibility, so it can be easily removed from samples by evaporation or washing.
As CNC-tfa suspensions age, trifluoroacetyl groups gradually increase on the surface of the CNC with consequent changes in rheology and transparency, which may be useful in applications of interest in hydrophobic CNCs.
This study only solves the problem of CNC-PET nanocomposites, which is a simple problem of mixing a CNC suspension with a PET solution in TFA and then drying and washing the resulting film, but the method should be suitable for combining any organic-soluble polymer with non-functionalized natural cellulose.
bibliography
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