Abstract
Hydroxyazetidinium salts were used to surface-modify cellulose nanocrystals (CNC) by grafting the salts onto the sulphate ester groups on the CNC surfaces. The grafting was confirmed by ζ-potential measurements and by the thermal degradation behaviour of the modified CNC. The thermal stability (onset of degradation) of the CNC was improved by the surface modification (almost 100 °C). Composites containing surface-modified or unmodified CNC (0.1, 1.0 and 10 wt%) with an ethylene-based copolymer as matrix were produced by compression moulding. The thermal stability of the composites was not, however, markedly improved by the surface grafting onto the CNC. It is suggested that this is due to a degrafting mechanism, associated with the alkaline character of the system, taking place at high temperatures. Model experiments indicated, however, that this did not occur at the conditions under which the composites were produced. Furthermore, in the case of a reference based on pH-neutralised polymeric system and modified CNC, an upward shift in the onset of thermal degradation of the composite was observed. The addition of the CNC to the polymer matrix had a strong influence of the mechanical performance. For example, the tensile modulus increased approximately three times for some systems when adding 10 wt% CNC. The surface grafting of the hydroxyazetidinium salts appeared mainly to affect, in a positive sense, the yield behaviour and ductility of the composites. The results of the mechanical testing are discussed in terms of interactions between the grafted units and the matrix material and between the grafted groups.
Highlights
There is a growing demand for a reduction in the use of fossil-based materials and for the development of stronger lightweight materials that could reduce the amount of material needed and provide lighter structures
Hydroxyazetidinium salts were used to surfacemodify cellulose nanocrystals (CNC) particles, and the grafting of the salts onto the sulphate ester groups of the CNC was confirmed by f-potential measurements and by the thermal degradation behaviour of the modified nanocrystals
In agreement with earlier studies on similar systems [23], the grafting improved the thermal stability of the CNC, i.e. the temperature for the onset of thermal degradation was raised by almost 100 °C
Summary
There is a growing demand for a reduction in the use of fossil-based materials and for the development of stronger lightweight materials that could reduce the amount of material needed and provide lighter structures. The drawbacks associated with such fibres in thermoplastic matrices are well known and include, for instance, limited adhesion between the hydrophilic fibres and, in most cases, the hydrophobic nature of the polymer matrix, the poor dispersion of the fibres in the matrix, and the moisture uptake and degradation of the fibres at the temperatures used for thermoplastic forming. Some of these drawbacks can be reduced by chemical modification of the fibres and/or by using compatibilisers/coupling agents Some of these drawbacks can be reduced by chemical modification of the fibres and/or by using compatibilisers/coupling agents (see e.g. [7,8,9,10]), but improvements are still desired
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