The unusual interactions between polymer grafted cellulose nanocrystal aggregates

Orlando J Rojas,Arcot R Lokanathan,Janne Laine,Henry Bock,Eero Kontturi

doi:10.1039/c3sm51494c

Orlando J Rojas, Arcot R Lokanathan + Show 3 more

Open Access

https://doi.org/10.1039/c3sm51494c

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Abstract

Using computer simulations we study how a corona of polymer molecules grafted to cellulose nanocrystal aggregate (CNA) particles influences the interaction between pairs of parallel CNAs. The resulting distance and orientation (face-to-face versus edge-to-edge) dependence is very rich and counterintuitive. Although the unperturbed polymer corona assumes cylindrical symmetry relatively quickly as the degree of polymerisation increases, the polymer mediated interactions between the grafted particles are strongly orientation dependent. Rather unexpectedly we find that the forces in the face-to-face orientation are much larger than in the edge-to-edge configuration although in the latter case the distance between the particle surfaces is much smaller. The reason for this effect is that overall the face-to-face orientation leads to larger chain confinement. Interestingly, we find that the deviations of the polymer mediated interactions from cylindrical symmetry are larger in the case of longer grafted molecules compared to shorter ones. When the distance between the CNAs becomes larger and the overlap of the polymer coronas becomes small, the orientation dependence of the mediated interaction vanishes and the particles behave as cylindrical rods. However, this is only a crossover point where the behaviour of the system inverts to slightly larger forces in the edge-to-edge compared to the face-to-face configuration. Thus, even though the polymer density around the CNAs is nearly perfectly cylindrically symmetric the polymer mediated interactions are strongly orientation dependent, revealing the polygon character of the CNA cross-section.

Highlights

The physical and geometrical properties of cellulose nanocrystals (CNC) have attracted recent interest of the research communities as new applications are discovered for these naturally occurring nanoparticles.[1,2] It is well known that CNCs have high aspect ratios, L/w spanning a range that varies with the CNC source, between 10 for cotton and ca. 70 for tunicate.[1,3,4] The subject to debate, is the exact geometry of CNC cross-sections.CNCs are extracted from native cellulose micro brils via acid hydrolysis, which breaks the disordered regions and leaves the crystalline regions intact
This intense debate inspires the question: are the exact shape and twist of cellulose nanocrystal aggregate (CNA) important for their technical application? In this paper we focus on the importance of edges, i.e. deviations of the particle shape from cylindrical symmetry
Since the exact shape of CNAs is quite elusive and varies between different cellulose-producing species, we focus on fundamental aspects, in particular the existence of edges along the particle surface, while foregoing other details such as the speci c surface chemistry

Summary

Introduction

The physical and geometrical properties of cellulose nanocrystals (CNC) have attracted recent interest of the research communities as new applications are discovered for these naturally occurring nanoparticles.[1,2] It is well known that CNCs have high aspect ratios (length-to-width), L/w spanning a range that varies with the CNC source, between 10 for cotton and ca. 70 for tunicate.[1,3,4] The subject to debate, is the exact geometry of CNC cross-sections. This results in slightly larger intermolecular distances and stretching of the cellulose chains, which the crystal accommodates by twisting It is unknown, if and how this twist is inherited by the CNAs from its underlying CNCs. If the origin of the chiral nematic phases is the twist of the constituting particles, the CNAs should be twisted too, because they are the ones most o en used in experiments reporting chiral phases While this study is inspired by cellulose nanocrystal aggregates, rod shaped nanocrystals with nm-sized and polygon-shaped cross-sections have been produced from various inorganic materials.[18,19] It is likely that the cross-section of the closely related natural chitin nanocrystals[20,21] could be polygonal For this discussion, it is important to consider that CNCs and CNAs are almost never used in their native state, because they are essentially insoluble in all relevant solvents. This contribution intends for the rst time to reveal such aspects that are otherwise elusive to experimentation

Model and simulation

Shape of the grafted polymer shell

Grafted polymer-mediated interactions between the CNAs

Findings

Conclusions