**1. Introduction**

Cellulose nanocrystals (CNCs) are rod-shaped nanoparticles with a thickness of 3–10 nm and a length of few hundreds of nanometers [1]. They can be extracted from pulp fibers, microcrystalline cellulose, and wood using a variety of reagents including strong acids, bases, oxidizing agents, and ionic liquids [2–4]. CNCs have shown great potential in a wide range of applications due to their advantageous mechanical properties, high surface area, biocompatibility, and biodegradability [5,6]. Furthermore, the possibility to modify their surfaces has led to CNCs with a wide range of surface properties and functionalities [7,8]. Due to their mechanical strength, CNCs have often been used to reinforce synthetic and bio-based polymers [9–11]. One main obstacle is the poor interfacial

adhesion between CNCs and their matrices, which results in weak interfaces and reduced mechanical reinforcement [12,13].

Most of the commonly used polymers such as polyolefins are hydrophobic, while CNCs are hydrophilic. Two main approaches have been used to overcome this issue. One approach relies on the use of compatibilizers or coupling agents such as maleic anhydride-grafted-polyethylene (MAPE) for nanocellulose/polyethylene composites [14,15]. The other approach reduces the hydrophilicity of CNCs by reacting the surface hydroxyls with a variety of hydrophobic chemicals through relatively simple reactions such as acetylation and carbanilation or through grafting bulkier chemicals and polymers [16–18]. In a previous report by the authors, the surface properties of CNCs were tailored by grafting alcohols of different chain lengths onto their whole surface (almost all available surface hydroxyls, i.e., a degree of surface modification of ca. 100%) using 2,4-toluene diisocyanate (TDI) as a linker (Figure 1) [19]. Four alcohols were explored: ethanol, 1-butanol, 1-hexanol, and 1-octanol. As a result of this surface modification, CNCs with a tailored water contact angle of up to 120◦ were prepared, which was a result of growing a hydrophobic shell around the CNCs. These modified CNCs were used to reinforce poly(butylene succinate) (PBS), a hydrophobic polymer. The resultant composite showed improved interfacial adhesion, which was dependent on the alcohol chain length as confirmed by microscopic and thermomechanical investigations. This approach was proved to be simple for tailoring the surface properties of CNCs by only varying the alcohol chain length. It is also expected to improve the interfacial adhesion between CNCs and other hydrophobic matrices.

**Figure 1.** Tailoring the hydrophilicity of cellulose nanocrystals (CNCs) by grafting alcohols of different chain lengths on the CNC surface using toluene diisocyanate (TDI) as a linker.

PBS, which is produced by the polycondensation of succinic acid and butanediol, has shown great potential in many applications including automotive and packaging industries due to its similar properties to polyolefins and its advantageous biodegradability [20–23]. It still suffered some drawbacks in terms of its mechanical and gas barrier properties and its slow crystallization [24]. In order to modulate its properties, PBS has been processed with a variety of polymers, fibers, microparticles, and nanoparticles including CNCs, starch nanocrystals, silicon nitride, carbon nanotubes, calcium carbonate, etc. [25–27]. This resulted in significant changes in its mechanical, morphological, optical, and thermal properties [28–32], which were strongly influenced by the size and distribution of the generated spherulites upon PBS crystallization [33]. Therefore, the impact of many of these "modifiers" on the crystallization kinetics of PBS has been isothermally and non-isothermally investigated using

Avrami and Ozawa models and their combination [34,35]. In general, the modifiers or nanofillers acted as nucleating agents, and their nucleation activity was dependent on their amount, size, and morphology [36–38]. For instance, Filizgok et al. studied the influence of the shape of different carbon-based nanoparticles on the crystallization kinetics of PBS and showed that carbon nanotubes have a stronger nucleation activity than carbon black and fullerene [39].

This paper studies the crystallization kinetics of PBS upon the addition of CNCs of different surface properties to shed more light on the impact of interfacial adhesion on the nucleation activity of CNCs in PBS composites.
