Cellulose Nanocrystals in Polymer Matrices - The Nanoscience and Technology of Renewable Biomaterials - page 278

CNXLs have been incorporated into many polymers, including siloxanes (Grunnert
and Winter 2000), poly(caprolactone) (Morin and Dufresne 2002; Habibi and Dufresne
2008; Habibi et al . 2008), glycerol-plasticized starch (Angles and Dufresne 2001),
styrene-butyl acrylate latex (Paillet and Dufresne 2001), poly-(styrene- co -butyl acrylate)
(poly(S- co -BuA)) (Favier et al . 1995b), cellulose acetate butyrate (Grunnert and
Winter 2002), poly(vinyl acetate) (Roohani et al . 2008; Shanmuganathan et al . 2008),
poly(vinyl alcohol)/carboxymethyl cellulose blends (He et al . 2008), epoxies (Ruiz
et al . 2000) phenol-formaldehyde (Hong et al . 2008), polypropylene (Bonini 2000),
poly(vinyl chloride) (Chazeau et al . 1999a; Chazeau et al . 1999b), and thermoplastic
starch (Orts et al . 2004). Note that most of the matrices listed above are thermoplastics,
which can compensate for the lack of CNXLs ductility with only 2% extension at break
(Marks 1967). In none of these cases were the very properties of neat CNXLs obtained
(Chazeau et al . 1999b).
CNXLs have not yet been used extensively in the common thermoplastics, e.g.
polyethylene and polypropylene, as they are thermally sensitive at the temperatures
commonly used to extrude them.

10.4 Thermal Properties

From most of the studies in this field, surprisingly, the addition of CNXLs into poly-
mers matrices seems not to affect the values of the glass-rubber transition temperature
T g , regardless of the nature of the host polymer, or the origin of the CNXLs or the
processing conditions (Azizi Samir et al . 2005a; Dufresne 2008). This observation is
unexpected if one considers the high specific area of CNXLs. There are a few cases
reported in the literature where the addition of CNXL as a filler in composite materi-
als affects the T g but only slightly. This unusual effect, which showed especially in a
moisture sensitive system (Roohani et al . 2008) was related to the plasticization effect
of water and is also linked to the strong interaction between CNXLs and the respective
matrix.
In the case of semi-crystalline polymers, it was reported that the addition of unmodi-
fied CNXLs had no influence on the melting temperature (T m ) of the nanocomposites in
plasticized starch (Angles and Dufresne 2000; Mathew and Dufresne 2002), PEO (Azizi
Samir et al . 2004a, 2005b), CAB (Grunnert and Winter 2002), and PCL-reinforced
polymers (Habibi and Dufresne 2008; Habibi et al . 2008). However, when chemi-
cally modified CNXLs were used in nanocomposites, a change of T m was observed.
Strong interactions between chemically modified CNXLs and matrices were reported
to be the origin of this T m change. Moreover, CNXLs can act as a nucleating agent
in the semi-crystalline polymers which significantly increases the crystallinity of such
nanocomposites (Ljungberg et al . 2006; Habibi and Dufresne 2008). This effect is
mainly governed by CNXL-matrix compatibility which depends on surface chemistry
considerations. Finally, transcrystallization phenomenon has also been reported in a
CNXL-filled polypropylene nanocomposite (Gray 2008).