Cellulose nanostructure-based biodegradable nanocomposite foams: A brief overview on the recent advancements and perspectives

The second step is the biological assimilation of polymer fragments by microorganisms and their mineralization [3,6,29]. Contact angle measurements showed a decrease in the affinity of the modified CNFs for polar solvents, which means that the CNFs became hydrophobic [43]. In addition, cell size decreases in the presence of nucleating agents due to increased melt viscosity and strength, which reduces bubble expansion.

Finally, cell growth and stabilization is achieved by gas expansion from the polymer into nucleated bubbles. The final step here is to cool the foamed product either in a solvent or in air. It was observed that increasing the foaming pressure from 12 to 14 MPa resulted in changes in the cellular morphology of the foams.

With the specific design of the screw configurations, the PBAs can be transferred directly into the polymer melt. Based on the results obtained, the authors preferred that the foaming of PVOH be carried out at a die temperature below 160◦C due to the obtained small cell size and high cell density. In addition, the concentration of the CBA also determines the type of morphology that develops in the foams, as indicated by Lin et al.

Figure 2. (a) Publication history on biodegradable polymers; (b) publication history on biodegradable polymer foams from 2008 to 2018 (information obtained from Scopus) (Keywords: biodegradable polymers and biodegradable polymer foams)

Influence of Cellulose Nanostructures on Crystallisation and Morphological Properties of Foam

Above this strain, a decrease in cell density and an increase in cell size were observed, which was attributed to overloading the MFC relative to its spreading capacity. In this study, although the crystallinity of the material was high in the presence of CNF, the foaming of PHBV was prevented, possibly due to the decreased solubility of CO2 in the crystalline regions. Another possible reason is that the melting strength of the material was too high in the presence of CNF to allow cell nucleation.

The cell morphologies of the foamed nanocomposites with varying CNC concentrations at a fixed amount of AC (5%) are depicted in Figure 7. Figure 8 shows that the increase in CNC content resulted in the increase in cell density, possibly due to the increased number of nucleation sites, while the cell size decreased with the increase in CNC content. A high Tc contributes significantly to increasing the solidification of the material to avoid overgrowth of the cells.

In that study, although the crystallinity of the material was high in the presence of CNF, the foaming of PHBV was inhibited, possibly due to the reduced solubility of CO2 in the crystalline regions. Cell morphologies of foamed nanocomposites with different concentrations of CNC at a fixed amount of AC (5%) are shown in Figure 7. Due to the larger number of nucleation sites created by CNC nanoparticles, the number of pores and, consequently , cell density increased, while pore size decreased compared to pure PLA foam.

In all these investigations, the cell morphology was shown to depend on the CN concentration, regardless of the processing method used. In addition, the cell size decreased with the increase in CN content, which may be caused by the increased melt strength and viscosity, which limited the overgrowth of the cells. This was related to the high degree of dispersion of the modified CNC, which improved the interaction with PLA.

Their large aspect ratio, which leads to increased filler-matrix interaction sites, increases the cell density due to the increased number of nucleated cells. Furthermore, the cell size decreases due to the limitation of cell overgrowth and improved melt strength through the incorporation of the nanoparticles.

Figure 7. SEM images of the cross-section of the foamed composites: (a) PBS/AC (5%); (b) PBS/CNC (3%)/AC (5%); (c) PBS/CNC (5%)/AC (5%); (d) PBS/CNC (10%)/AC (5%) [100]

Properties of Cellulose-Nanostructured Nanocomposite Foams

Incorporation of nanoparticles increases melt viscosity of nanocomposites, which results in reduced cell expansion rate. Cell density also increased with increasing CNF concentration, possibly due to the increased number of nucleation sites with the addition of filler. This was explained in terms of the interplay between the nucleation effect of the CNF surface and the rheological effect on cell growth.

A dramatic decrease in pore diameter could also be the result of the improved viscoelastic properties of PCL. The clustering of CNC particles at higher loads, as revealed in SEM images (not included), could be a possible reason for the microphase separation, which could lead to inferior mechanical properties of the foam. The Young's modulus of the nanocomposite foams decreased with the addition of CNC particles because, in the presence of CNC particles, the cell density improved, indicating a high volume ratio of pores, hence the observed decrease in modulus.

Furthermore, the flexibility and toughness of the foam were found to depend on the fraction of open cells. Dynamic mechanical analysis (DMA) is an effective technique for investigating material response. With this technique, the morphology and viscoelastic properties of the semi-crystalline polymer can be determined.

Furthermore, E0 decreases with the increase in temperature as a result of the increase in the mobility of polymer chains. In the draw state, the E0 of the foam increased approx. 2.2 times with the increase in CNC concentration at 30◦C, which was also related to the crystallinity of the foam. However, the storage modulus of foamed PCL samples was much smaller than solid samples due to the presence of voids in the foamed samples.

In addition, the cellular structure of polymer foams has also been shown to affect the degradation of the foam material. This decrease was associated with sulfate concentration during CNC preparation, which reduced the thermal stability of the foam. The FR materials of renewable materials gained interest due to the formation of insulating-char layer on the surface of the burning sample.

The cone calorimetric properties of the two materials were also examined under a heat flux of 35 kW/m2.

Figure 10. The rheological results of PCL and PCL/CNC nanocomposites: (a) storage modulus and (b) complex viscosity as a function of angular frequency [104]

Application of CN-Biopolymer Foams

The authors observed a reduction in PHRR of PLA/CNC containing 20 wt% CNC while 20 wt% MCC showed little or no significant influence on PHHR of PLA. Most importantly, the fire growth rate index (FIGRA) which is a parameter used to evaluate the fire characteristics of building products was also determined. The thermal insulating properties of the prepared foams were also determined, and the thermal conductivity measurements indicated a slight decrease from 40.7 mW/(mK) for neat CNF foam to mW/(mK) for CNF-HAP composite foam.

This reduction was associated with the unique morphology of the composite as pure HAP has higher thermal conductivity than cellulose fibers. 132] also investigated the thermal conductivity of CNC/PVOH composite foams prepared in the presence of 1,2,3,4-butane tetracarboxylic acid (BTCA) as a cross-linking agent. The thermal conductivities of the regular CNC and CNC/PVOH composite foams were in the range of Wm−1K−1.

However, with the incorporation of 25 wt.% BTCA, a 35% decrease in the thermal conductivity of PVOH/CNC foam containing 10 wt.% PVOH was observed compared to pure CNC foam. At this concentration of BTCA, the foam sample exhibited very small cell sizes, which resulted in the lowest thermal conductivity. The nanocomposite foam containing 10 wt.% GO and 10 wt.% SEP showed a reduction in thermal conductivity in the radial orientation from 18 mW−1K−1 (CNF foam) to 15 mW−1K−1 (composite foam).

However, the inferior flammability properties of cellulose-based foams can be adapted by incorporating FR substances or chemical modifications of CN with FR materials. Wettability determines the level of hydrophobicity/hydrophilicity of the foam material, which gives an indication of the material's surface energies. With the addition of CNF particles, a decrease in contact angles was observed; is indicative of increased wettability of the material, which is beneficial for cell adhesion.

CNC showed mostly viable cells, which is an indication of good compatibility of PCL/CNC foam material with cells. However, in this case, the performance of the foamed material was also shown to be dependent on the CNC concentration.

Biodegradation of CN-Based Biopolymer Foams

Very few studies have evaluated the wettability of CN biopolymer foams, which is a characteristic property in biomedical applications. Highly hydrophilic materials are known to possess very high surface energy, which is desired for cell adhesion in tissue engineering. In the other study [104], the performance of PCL/CNC foam for tissue engineering scaffold application was investigated.

Conclusions

Structure and properties of surface acetylated cellulose nanocrystals/poly(butylene adipate-co-terephthalate) composites.Polym. Production and characterization of biodegradable PLA_PBS blends reinforced with cellulose nanocrystals extracted from hemp fibers. Ind. Investigation of the effect of graft length on the interfacial toughness of a cellulose/poly (ε-caprolactone) two-layer laminate.Compos.

Reinforcing effect of poly(butylene succinate) (PBS)-grafted cellulose nanocrystal on cured PBS/polylactic acid blends. Carbon hydr. Effect of cellulose nanocrystals (CNC) on rheological and mechanical properties and crystallization behavior of PLA/CNC nanocomposites. Carbohydrate. Effect of cellulose nanocrystal surface modification on non-isothermal crystallization of poly(β-hydroxybutyrate) composites. Carbon hydr.

Surface modification of cellulose nanowhiskers and their use as nanoreinforcers in polylactide: A sustainably integrated approach. Comparative study of the influence of chitosan as coating of thermoplastic starch foam from potato, cassava and corn starch.Prog. Foaming of poly-(3-hydroxybutyrate-co-3-hydroxyvalerate)/organo-clay nano-biocomposites by a continuous supercritical CO2-assisted extrusion process.Eur.

The role of cellulose nanofibers in supercritical foaming of polylactic acid and their effect on foam morphology. Soft fabric. Effect of pressure drop rate on cell nucleation in continuous processing of microcellular polymers. Polym. In Proceedings of the 16th European Meeting on Supercritical Fluids, Lisbon, Portugal, 25-28 April 2017; Available online: http://eventos.fct.unl.pt/emsf2017/home (accessed 1 May 2018).

Microcellular foaming of poly(lactic acid)/silica nanocomposites in compressed CO2: Critical effect of crystallite size on cell morphology and foam expansion. Ind. Influence of cellulose nanofibers on the morphology and physical properties of poly(lactic acid) foaming by supercritical carbon dioxide.Macromol. Mechanical properties of cellulose nanofiber (CNF)-reinforced polylactic acid (PLA) prepared by twin-screw extrusion. Composition.

Kinetic evaluation of the size-dependent dissolution performance of solvent-free microcellular polylactic acid foams.Ken.