Conselho Nacional de Desenvolvimento Científico e Tecnológico / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Tese (Doutorado) / Abstract: Since this thesis presents two independent studies on cellulose nanocrystals
(CNCs), the abstract was divided in two sections referring to chapters II and III,
respectively. Comprehensive morphological and structural investigation of cellulose I and II
nanocrystals prepared by sulfuric acid hydrolysis Cellulose has several polymorphs. These polymorphs differ by crystal packing (i.e. unit cell parameters), polarity of the constituting chains and hydrogen bond patterns
established between them. Most of cellulose polymorphs result from chemical treatments
of the native polymorph, the so-called cellulose I (Cel-I) (Wada et al., 2008). In Cel-I, the
chains are parallel and can be packed into two allomorphs, namely Iα and Iβ. Among the
cellulose polymorphs, cellulose II (Cel-II), in which the chains are antiparallel, can be
prepared from Cel-I by two distinct processes: Mercerization or Regeneration.
Mercerization is an essentially solid-state process during which cellulose fibers are
swollen in concentrated alkali media and recrystallized into cellulose II upon washing
and drying (removal of the swelling agent). Unlike the mercerization process, in process
known as regeneration, cellulose is first dissolved in an appropriated solvent and
subsequent reprecipitated by adding a non-solvent, leading the chains to recrystallize into
into Cel-II polymorph. The Cel-I to Cel-II transition is irreversible, which suggests that
Cel-II is thermodynamically more stable (Habibi et al., 2010).
Cell-II is the second most extensively studied polymorph due to its technical
relevance. Nevertheless, so far, most of investigations involving Cel-II have focused on
fibers and only a few recent studies have been carried out on CNCs. Cel-II nanocrystals
have been prepared either by acid hydrolysis of mercerized fibers (Hirota et al., 2012;
Kim et al., 2006; Yue et al., 2012), mercerization of Cel-I CNCs (Jin et al., 2016), or after
recrystallization of fractions of short cellulose chains in solution (Dhar et al., 2015; Hirota
et al., 2012; Hu et al., 2014; Sèbe et al., 2012). However, while these studies have
generally combined the data from several imaging, diffraction and spectroscopic
techniques, a complete structural picture of the nanocrystals has not been reported so far.
In this context, the purpose of the research work presented in chapter II was to
produce, characterize and compare CNCs obtained from eucalyptus wood pulp using
three different methods: i) classical sulfuric acid hydrolysis (CN-I), ii) acid hydrolysis of
cellulose previously mercerized by alkaline treatment (MCN-II), and iii) solubilization of cellulose in sulfuric acid and subsequent recrystallization in water (RCN-II). The
morphology, crystal structure, crystallinity index, surface charge and degree of
polymerization of these nanocrystals were characterized by complementary techniques,
namely elemental analysis, zetametry, viscometry, transmission electron microscopy
(TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier-transform
infrared and solid-state nuclear magnetic resonance spectroscopies (FTIR and NMR,
respectively).
The three types of prepared CNC exhibit different morphologies and crystalline
structures. When the acid hydrolysis conditions are set-up in such a way that the
crystalline domains in the initial wood pulp and mercerized cellulose (WP and MWP,
respectively) are preserved (60 wt% H2SO4, 45°C, 50 min), the resulting nanocrystals
retain the fibrillar nature of the parent fibers (i.e., the chain axis is parallel to the long axis
of the acicular particles) and their initial allomorphic type (I for WP and II for the MWP).
In both cases, the particles are mostly composed of a few laterally-bound elementary
crystallites, in agreement with what was shown for cotton CNCs by Elazzouzi-Hafraoui
et al. (2008). The unit nanocrystals in CNCs from mercerized cellulose (MCN-II) are
shorter but broader than those prepared from cellulose I fibers (CN-I). If harsher
conditions are used (64 wt% H2SO4, 40°C, 20 min), resulting in the depolymerisation and
dissolution of native cellulose, the short chains (with degree of polymerization DP ≈ 17)
recrystallize into Cel-II ribbons upon regeneration in water at room temperature. In these
somewhat tortuous ribbons, the chain axis would lie perpendicular to the long axis of the
nanocrystal and parallel to its basal plane. In addition, these nanoribbons are very similar
in shape and molecular orientation to mannan II nanocrystals prepared by
recrystallization of mannan (Heux et al., 2005), a linear polymer of β-(1,4)-D-mannosyl
residues, suggesting that this mode of crystallization may be a feature of short-chain linear
β-(1,4)-linked polysaccharides.
Although similar ribbons of recrystallized cellulose II have been reported by other
authors, to our knowledge, it is the first time that a detailed morphological and structural
description is proposed in terms of particle morphology, crystal structure and chain
orientation. By comparison with the fibrillar nanocrystals prepared by acid hydrolysis of
native or mercerized cellulose fibers, the unique molecular and crystal structure of the
nanoribbons imply that a higher number of reducing chain ends are located at the particle
surface, which may be important for subsequent chemical modification and specific
potential applications such as biosensing and bioimaging agents. Therefore this study offers scope to a better understanding of crystalline structure and morphology of CNC
obtained by regeneration process with sulfuric acid. Mechanical properties of natural rubber nanocomposites reinforced with high
aspect ratio cellulose nanocrystals isolated from soy hulls
At present, the most promising application of CNCs is as reinforcement material
in the field of polymer nanocomposites.The incorporation of CNCs in polymer matrices
generally leads to polymer-based nanocomposite materials with higher mechanical and
barrier properties than the neat polymer or conventional composites. Among various
factors that influence the efficiency of the reinforcing effect of CNCs, their intrinsic
characteristics, including crystallinity and aspect ratio, play a key role (Dufresne, 2012;
Favier et al., 1995; Mariano et al., 2014). It is also well-known that these characteristics
depend on the source of the original cellulose, on the extraction method and its conditions
(including pretreatment). However, it is widely accepted that the raw starting material is
the most important factor (Beck-Candanedo et al., 2005; Dufresne, 2012; Elazzouzi-
Hafraoui et al., 2008). The reinforcement capability of CNCs is therefore directly linked
to the source of cellulose as well as its biosynthesis. Thus, the optimization of the
extraction procedure and further characterization of CNCs from different sources of
cellulose are crucial for an efficient exploitation of these sources, allowing the selection
of the appropriate source (i.e. with targeted morphology) to suit specific end user
applications (Brinchi et al., 2013).
Natural rubber (NR) is a perfect polymer matrix to be used as a model system to
study the effect of filler reinforcement, owing to its high flexibility and low stiffness. Its
properties can be tailored by the addition of reinforcing fillers of various surface
chemistries and aggregate size/aspect ratios to suit the targeted application. CNCs
extracted from different sources have already been studied as nanoreinforcement in NRbased
nanocomposites, including CNCs isolated from capim dourado (Siqueira et al.,
2010), rachis of palm date tree (Bendahou et al., 2009), sugarcane bagasse (Pasquini et
al., 2010; Bras et al., 2010), sisal (Siqueira et al., 2011), and bamboo (Visakh et al., 2012).
So far, little results have been reported in the literature on the isolation of CNCs
from soy hulls or their use in nanocomposites (Flauzino Neto et al., 2013, Silvério et al.,
2014). In this study, CNCs were isolated from soy hulls by sulfuric acid hydrolysis
treatment. The resulting CNCs, referred to as CNCSH in the following, were characterized
using transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray
diffraction (XRD), wide-angle X-ray scattering (WAXS). These CNCSH were used as a
reinforcing phase in a NR matrix to prepare nanocomposite films by casting/evaporation
at 1, 2.5 and 5 wt% (dry basis) loading levels. The effect of CNCSH on the structure, as well as thermal and mechanical properties of NR, was investigated by means of scanning
electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), dynamic
mechanical analysis (DMA), tensile tests and thermogravimetric analysis (TGA).
For the acid hydrolysis treatment, were chose milder conditions compared to those
described in Flauzino Neto et al. (2013) in order to avoid as much as possible the
hydrolysis of crystalline cellulose domains. The CNCSH was found to have a type I crystal
structure, high crystallinity (crystallinity index ≈ 80%), large specific surface area
(estimated to be 747 m2.g-1 from geometrical considerations) and high aspect ratio
(around 100). This aspect ratio is the largest ever reported in the literature for a plant
cellulose source. Futhermore, from microscopic observations it is clearly seen that CNCSH
does not consist of partially hydrolyzed microfibril since it displays the classical rod-like
morphology of CNC. Thus, soy hull was found to be an interesting source of raw material
for the production of CNC, due to the characteristics of the obtained nanocrystals
associated with low lignin content and wide availability of this agro-industrial residue. In
the meantime, the reuse of this agro-industrial residue goes towards sustainable
development and environment-friendly materials. To tailor the dimensions of CNC and
take full advantage of this source, special care needs to be paid to the extraction process
and its conditions. A milder acid hydrolysis is preferable to improve the extraction yield,
preserve the crystallinity of native cellulose and obtain high aspect ratio CNC.
As expected, a high reinforcing effect is observed even at low filler contents when
using this nanofiller (CNCSH) to prepare nanocomposites with a natural rubber (NR)
matrix by casting/evaporation. For instance, by adding only 2.5 wt% CNC, the storage
tensile modulus at 25°C of the nanocomposite was about 21 times higher than that of the
unfilled NR matrix. This reinforcing effect was higher than the one observed for CNCs
extracted from other sources. It may be assigned not only to the high aspect ratio of these
CNCs but also to the stiffness of the percolating nanoparticle network formed within the
polymer matrix. Moreover, the sedimentation of CNCs during the film processing by
casting/evaporation was found to take place and play a crucial role on the mechanical
properties. Thus, both the high aspect ratio of the CNC and sedimentation due to the
processing technique are involved in the good mechanical results obtained. Indeed, if
sedimentation occurs, then a multilayered film results and the CNC content in the lowest
layers is higher than the average CNC content. It means that CNC mechanical percolation
can occur in the lowest layers for an average CNC content which is lower than the
percolation threshold. Hence, the system can be considered as constituted of parallel layers in the direction of the mechanical solicitation (tensile mode), and the CNC-rich
layers can support a higher stress leading to a higher modulus value. Moreover, if high
aspect ratio CNC is used, then percolation can occur in the lowest layers for lower average
CNC contents. An important contribution of this work is to highlight the importance of
the sedimentation of CNC during the evaporation step on the mechanical properties of
the nanocomposites which is rarely mentioned in the literature.
| Identifer | oai:union.ndltd.org:IBICT/urn:repox.ist.utl.pt:RI_UFU:oai:repositorio.ufu.br:123456789/18105 |
| Date | 01 February 2017 |
| Creators | Flauzino Neto, Wilson Pires |
| Contributors | Otaguro, Harumi, Cerqueira, Daniel Alves, Lucas, Alessandra de Almeida, Morais, Luis Carlos de, Schmidt, Vivian Consuelo Reolon, Dufresne, Alain |
| Publisher | Universidade Federal de Uberlândia, Programa de Pós-graduação em Química, Brasil |
| Source Sets | IBICT Brazilian ETDs |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Source | reponame:Repositório Institucional da UFU, instname:Universidade Federal de Uberlândia, instacron:UFU |
| Rights | info:eu-repo/semantics/openAccess |
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