Nanocellulose: Common Strategies for Processing of Nanocomposites

Chapter 11

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Nanocellulose: Common Strategies for Processing of Nanocomposites Marcos Mariano and Alain Dufresne* Univeristy Grenoble Alpes, CNRS, Grenoble Institute of Engineering, LGP2, F-38000 Grenoble, France *E-mail: [email protected].

There has been an explosion of interest in the use of biomass as a source of renewable energy and materials. One focus of this activity has followed from the recognition that, by suitable chemical or mechanical treatments, it is possible to produce materials with dimensions in the nanometer range from many naturally occurring sources of cellulose. Cellulose-based materials are carbon neutral, sustainable, recyclable, and nontoxic; they thus have the potential to be truly green nanomaterials with many useful and unexpected properties. The potential of these nanoparticles has been proved for special functional nanomaterials but it is as a biobased reinforcing nanofiller that they have attracted significant interest. Impressive mechanical properties and reinforcing capability, abundance, low weight, and biodegradability of these nanoparticles make them ideal candidates for the processing of polymer nanocomposites. However, as for any nanoparticle, the main issue is related to their homogeneous dispersion within a polymeric matrix. This entry describes the main issues and challenges associated with the processing of nanocellulose reinforced polymer nanocomposites.

© 2017 American Chemical Society Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Introduction It is undeniable and well accepted that the development of new materials changed the world during the last century. Materials that are able to combine better conductivity and flexibility and, at the same time are light weight has allowed the growth of many technologies that are today a part of life as we know it. With a broad range of applications and characteristics, polymeric materials are probably the most remarkable class of materials developed during this period. There are no doubts that they revolutionized the way as we prepare, store and transport virtually all the products available for consumption. Despite the development of such a broad range of materials, some polymers cannot present the optimal properties for specific applications. As a way to overcome these limitations, the use of polymer as matrix for the creation of composites became widespread. The presence of a rigid filler can improve several properties of the material such as chemical, mechanical, barrier and swelling properties. However, today we are aware that it is necessary to deal with problems related to the discard of the first generation of polymers. Since they are mostly petroleum-sourced, issues of non-biodegradability and recyclability limitations became a major concern during the last decades. As a solution, the utilization of biopolymers has been offered as a way to replace traditional materials. Produced by living organisms, renewable and abundant, many of these so-called “green polymers” are readily biodegradable and their production is not petroleum-dependent. These renewable resources are replenished by the environment over relatively short periods of time. Furthermore, these polymers can also be used for the preparation of composites. In this case, we are able to create fully biobased composites by their combination with biodegradable fillers. In the last years, these green composites became more advanced. Technological and scientific research now go towards these sustainable materials as we become witnesses of nanotechnology growth. Aiming to obtain more advanced, light weight and powerful devices, the development of polysaccharide nanoparticles can be combined with biopolymers to produce eco-friendly nanocomposites. As principal candidate for the development of this kind of green nanofiller, cellulose has gained huge attention in this miniaturization process of advanced nanomaterials. Since the first publications about the subject, in the middle of 90’s, several research groups started to dedicate special attention to this topic. This interest is justifiable since cellulose is the most abundant polymer on earth and possesses very promising properties. Natural cellulose based composites have been used as engineering material for thousand of years and, still today, cellulose is largely present in our lives in textile and paper. Fortunately, the current technology provides us the tools to observe the microand nano-scale hierarchical structure of this material and find out the source of its functionality, flexibility and strength. Looking closer, the structure of a plant itself is a nanocomposite. As described by Sain and Oksman (1), the backbone of plants is composed mostly of cellulose, a polymeric carbohydrate that is, at its core, organized as nanofibers. The advancing of cellulose nanofiber technology is remarkable. In the beginning of the last decade, only 2% of world cellulose production (estimated to 204 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


1.5 x 1012 annual tons in 2005) were dedicated to producing fibers and cellulose derivatives, such as regenerated cellulose and cellulose esters. The major part of the production was destined for the paper industries (2). Since 2015, this scenario started to change in terms of commercial purposes. Some companies have commercialize nanocellulose (including nanofibrils and nanocrystals) on a large scale, and can produce tons per day (3). This is a huge advance for a material that can be considered as a feedstock for industrial applications. Today, ten years have passed since the publication of “Cellulose Nanocomposites” in ACS Symposium Series (4). During this time, research on nanocellulose has traveled a long road. New interests, perspectives and challenges can be found in literature, always supported by a growing number of publications. Here, we try to summarize some important concepts and recent breakthroughs with a focus on the processing of cellulose based nanocomposites.

Cellulose Structure and Nanocellulose Obtainment Since we are describing the use of nanocellulose or cellulose nanomaterials in nanocomposite preparation, it is important to describe some characteristics of these nanoparticles that are essential during processing. The classical representation of plant hierarchical structure is represented in Figure 1.

Figure 1. Plant hierarchical structure. Reproduced with permission from ref. (5). Copyright 2012 Elsevier.

205 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Elementary nanofibrils (CNF) are the building blocks of this structure. Subsequently with removal of lignin and other secondary materials (e.g. hemicelluloses, waxes and trace components) from pristine fiber, these fibrils can be isolated as nanoparticles. Normally this is done by the application of mechanical and enzymatic treatments, which induces the defibrillation of the microfiber structure. Several techniques are available to assist in obtaining these individualized nanofibers, including 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation as a way to facilitate the separation of the fibrils, reducing the necessary time and energy (6, 7). In turn, individual CNF are composed of amorphous and crystalline cellulosic domains. The disruption of the amorphous phase, normally performed using strong acids, allows the isolation of rod-like crystalline domains, called cellulose nanocrystals (CNC), which are extremely strong and lightweight crystalline nanoparticles. Due to their highly crystalline structure and nanometric dimensions, these particles can present Young’s modulus values in the range of 100-150 GPa combined with a high specific surface area in the range of several hundred m2g-1 (8). The isolation conditions used in obtaining these nanoparticles strongly affect their surface and bulk properties along with the cellulose source. Several publications are dedicated to describing the influence of the preparation conditions on the final surface properties, thermal stability, polymorphism and dimensions of the nanoparticles. Some of these properties will be discussed in this chapter. The possibility of obtaining particles with different properties is a quite interesting topic, which can bring valorization of residual or unexplored biomass. Different plants can present a large range of cellulose contents and nanocrystalline dimensions. While cellulose content in wood reaches between 40–50% (half in nanocrystalline and half in amorphous form), other sources such as bacterial cellulose (or even cotton) consist of more than 90% cellulose (with varied crystalline content), that makes the preparation of cellulose with high purity easy. However, several reasons (e.g. local production, specific crystalline dimensions, short treatment time, etc) can serve as motivation for the obtainment of cellulose nanoparticles from sources that are richer in lignin (e.g. sisal, soy hulls, capim dourado, etc). The production of nanocellulose and the isolation of lignin can be combined and used as a way to obtain versatile nanoparticles and aromatic chemical reagents from these materials, increasing the valorization of this biomass (9, 10). Cellulose is a polysaccharide composed of D-glucopyranose (C6H11O5) rings linked through β(1,4) bonds. With a high number of hydroxyl groups, cellulose chains are able to form hydrogen bonds with neighboring molecules. These bonds are responsible for keeping the chains packed together and for controlling the formation of the fiber microstructure. Crystalline domains are supposed to present around 7.2 -OH/nm2 on their surface (11), which, combined with their large surface area, are responsible for the properties of chemical reactivity and hydrophilicity of these nanomaterials. According to TAPPI definition (TAPPI WI3021), CNF are particles that possess diameters (d) between 5-30 nm and lengths (L) such that the aspect ratio L/d is higher than 50. Due to the presence of amorphous regions, these particles 206 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


are somewhat flexible and have high surface area. This combination of properties makes these particles perfect candidates for producing materials capable of absorbing water, and creating gels and suspensions with high viscosity. When obtained from bacteria, these nanoparticles can be easily applied in biomedicine (12). Due to their strength and stiffness, CNC are used as nanofiller in the preparation of rigid materials. Defined as particles with diameters (d) ranging from 3 to 10 nm and L/d > 5, these rod-like nanoparticles can form a very rigid 3D network of percolated nanoparticles, that is able to provide outstanding mechanical properties in different polymeric matrices. Once extracted from the original structure of the fiber, these particles can offer new features, not only based on different aspect ratios but also polymorphism, surface charge and thermal stability due to the presence of sulfate groups inserted during cellulose hydrolysis with H2SO4 (13, 14). The use of harsh or mild conditions during sulfuric acid hydrolysis also effects the level of particle sulfation and, as a consequence, the thermal resistance of the obtained nanoparticles. While some treatments, such as TEMPO oxidation, can be used as a way to increase cellulose reactivity and, in some cases, its thermal resistance, others tend to cause early degradation of the external cellulose chains of the nanoparticle (15, 16). The use of other acids seems to not induce early thermal degradation of cellulose, but cannot provide the negative charge present in the sulfate SO3- group, that results in electrostatic stability for CNC suspensions. Normally performed with strong acids, the hydrolysis of the amorphous phase can also modify the structure of isolated crystalline particles. It seems that, besides the insertion of sulfate groups, strong acids can also cause a swelling of the crystalline regions resulting in a polymorph conversion of cellulose (17–19). Under moderate conditions, these particles are normally elongated rod-like particles able to keep the original cellulose I crystalline structure. Although, some experimental conditions can cause a conversion from cellulose I to cellulose II. Some works describe this conversion when using acid (20) and mechanical (21) treatments, generating a mix of nanoparticles with both polymorphs or even a predominant distribution of cellulose II round-like particles. By choosing the adequate cellulose source and chemical/mechanical treatment, it is possible to control some of the aspects mentioned above. While CNF are broadly used in food packaging and for control of permeability (5, 22), CNC are more popularly used to improve the mechanical properties (23). Of course, these applications are not exclusive to one or another cellulosic nanomaterial, but can take advantage of its principal characteristics.


208


Table 1. Examples of nanoparticles isolated by using different methodologies Source

Form

Obtention Method

Onset Thermal Degradation (°C)

Polymorph

Ref.

Bacterial cellulose

CNF

HCl hydrolysis

-

Iα

(24)

Bacterial cellulose

CNF

Filtration

-

-

(25)

Microfibrillated Cellulose

CNF

Commercial Sample

-

I

(26, 27)

Paper pulp

CNF

Supermass colloider

-

-

(28)

Softwood

CNF

Shear Homogenization

246

I

(29)

Sonication/ Homogenization

-

-

(30)

Iα

(24)

Sugar pulp

CNF

Wood

CNF

Commercial Sample

-

Bamboo

CNC

H3PO4, HCl H2SO4, CH3COOH/HNO3

250 - 320

I

(31)

Bacterial cellulose

CNC

H2SO4 hydrolysis

-

-

(32)

Cotton

CNC

H2SO4 hydrolysis

224

I and II

(33)

Cotton

CNC

H2SO4 hydrolysis

128 – 270

I

(13)

Kenaf bast fiber

CNC

H2SO4 hydrolysis

120 - 280

I

(34)

Lyocellfibers

CNC

Ammonium persulfate

-

II

(35)

MCC*

CNC

Water/Temperature

300

I

(36)

Ramie

CNC

H2SO4 hydrolysis

-

-

(37)

Sisal

CNC

H2SO4 hydrolysis

145

I

(15)

Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Form

Obtention Method

Onset Thermal Degradation (°C)

Polymorph

Ref.

Wood

CNC

TEMPO/Sonication

-

II

(19)

Wood

CNC

H2SO4 hydrolysis

-

I and II

(20)

Wood

CNC

Ball-milling

-

I and II

(21)

Wood

CNC

N2H4/ H20

-

II

(38)

MCC means Microcrystalline Cellulose.

209


*

Source



For instance, it was reported by Rusli et al. (39) that the use of HCl or H2SO4 to prepare CNC modifies the mechanism of stress transfer between a polymeric matrix and the obtained nanoparticles. It seems that due to the presence of negative charges on the surface of the nanoparticle, H2SO4-hydrolyzed CNC can increase stress transfer. This is of importance since it is as a biobased nanofiller for mechanical reinforcement that such nanoparticles have attracted significant interest. Table 1 shows some examples of different cellulose nanomaterials that can be obtained through the application of different methodologies for nanoparticle isolation.

Preparation of Nanocomposites The preparation of nanocomposites based on cellulosic nanomaterials is a broad topic that is well explored in literature by a large number of publications. Until the middle of 2000’s, most of these publications dealt with preparation of new materials in a liquid medium, exploring film casting/evaporation. In the last 10 years, the number of publications exploring melt processing has increased significantly. Also, the use of alternative techniques such as layer-by-layer (LbL) assembly, electrospinning and resin transfer molding is now investigated (40). This is a natural consequence of the search for methodologies that can be applied at industrial scale. Since processing techniques influence the final properties of the nanocomposites (e.g. dispersion, distribution and alignment of the filler), investigation of different preparation routes became one of the major targets of research in the area (40). Influence of Aggregation The huge specific surface area of nanomaterials is one of their main assets but is also responsible for the greatest challenges during the preparation of these materials: aggregation. Nanostructures have an exponentially increasing natural tendency to aggregate, which is not different for cellulose nanoparticles. Moreover, the self-association of cellulose nanoparticles is exacerbated by the omnipresence of hydroxyl groups on their surface, which leads to particle interaction and the formation of new interparticle H bonds. To avoid it, well controlled processing conditions are very important. The distance between CNCs in a film swelled by water was calculated to be between 1.2 to 1.6 nm (e.g. 4 to 6 layers of water molecules), that shows how close these particles can be packed together in a final stage or processing (41). The polar cellulose surface is a drawback during the preparation of nanocomposites with non-polar polymers or dispersion in non-polar solvents. Attraction between the surfaces of neighboring nanofillers can be much stronger than between the filler and the non-polar medium/surface. To overcome this limitation and avoid aggregation, different physical and chemical procedures can be adopted. For example, surface modification can be used to provide new steric or electrostatic properties on particle surfaces (42). Many routes are available to perform surface modification (e.g. creation of new covalent bonds) through 210 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


chemical reaction, by inserting functional non-polar groups or polymers, or by adsorbing surfactants on the nanoparticle surface through electrostatic interactions (43). Independently of the chosen approach, the general purpose is to control (decrease) the surface energy of the nanoparticle, fitting it to a broader number of chemical environments. Aggregation decreases the surface area of the nanoparticle and reduces filler-polymer interactions, causing effects that can be harmful to nanocomposites, independent of the targeted final application of the material. For example, the presence of well-dispersed nanofiller can increase barrier properties of polymeric materials, by increasing the tortuosity of the matrix, along with the presence of crystalline particles that act as impenetrable domains for the solvent (or gas) molecules. The presence of aggregates induces the formation of voids and holes into the structure of the nanocomposite, making the diffusion of small molecules easier. In the same way, aggregation prevents the formation of a percolating network of individual particles and decreases the mechanical properties of the material (44). Also, during hot processing, the presence of aggregates increases thermal degradation of the cellulosic materials and damages the structure of the composite (45). Common Processing Methodologies As described in the last section, the most common processing methods used for the preparation of cellulose based nanocomposites include traditional film casting/evaporation and extrusion/injection molding, besides more specific or alternative techniques. Since the forces and parameters involved in these methodologies are completely different, it will deeply affect the organization and integrity of the filler, besides modifying the matrix structure. Roughly, these different processing techniques can be grouped as solvent-based and melt-processing techniques, as illustrated in Figure 2.

Figure 2. Flowchart of some popular processing methodologies applied for the preparation of cellulose nanocomposites. 211 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Solvent-Based Processing As a result of the disintegration process of cellulose macrostructure, cellulosic nanoparticles are usually obtained as an aqueous suspension. It makes sense, since water medium can help to swell cellulose pristine fibers and facilitate the destruction of its structure, saving energy, chemicals and time. At this stage, a drying step would induce irreversible aggregation of cellulose nanoparticles and lost of the nanoscale.


Casting/Evaporation As reported by Favier et al. (46–48), the first publications dealing with cellulose reinforced nanocomposites were based on casting/evaporation of an aqueous mixture of CNC and elastomeric matrix in latex form. Never-dried aqueous dispersion of CNC was therefore used. These publications attributed the outstanding mechanical reinforcement that was obtained by the addition of CNC to the formation of a percolating network of the particles. Theoretically, the obtainment of this network occurs for a critical volume content (φc) corresponding to the percolation threshold, which directly depends on the aspect ratio (L/d) of individual particles, as described by Equation I (47).

This equation shows the importance of the source from where the particles are obtained and hydrolysis conditions since long nanorods can provide the construction of the percolating network at a lower volume fraction of nanoparticles. This 3D nanoparticle network can only be obtained under specific experimental conditions, as a result of the slow evaporation of the solvent. The evaporation of the solvent, that normally takes hours, allows the progressive organization and interaction among the nanoparticles (based on the construction of new H bonds). Until now, these systems based on elastomers are still used as model systems to study the maximum reinforcement potential of particles coming from different sources. A common example is natural rubber (NR) as a matrix, used to investigate the influence of volume fraction and filler aspect ratio on the mechanical properties (49, 50). The maximum reinforcement obtained seems to mainly depend on the filler aspect ratio. In fact, it is usually reported that longer nanoparticles can provide better mechanical reinforcement, while shortest nanoparticles can present better results for barrier properties. Also, some have found that nanocomposites containing long CNC particles need a smaller volume fraction of particles to reach the percolation threshold and, for some of these systems, the casting methodology induces sedimentation of the filler resulting in a gradient of particles in the thickness of the obtained film (51). This means that one of the faces of the film has a higher concentration of nanoparticles, which can cause anomalous behavior (e.g. unexpected increase of elongation) (51, 52). 212 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Nowadays, the use of the casting/evaporation technique is not limited to a water medium and has already expanded beyond aqueous dispersion of polymers or are water-soluble polymers. This means that alternative preparation methods were adapted for these systems. Chemical modification of the nanofiller can increase filler-polymer interactions and avoid aggregation. However, some increase the particle-particle distance if large chains are attached to the nanofiller surface. This prevents the formation of a percolating network. As an alternative, the particles (normally CNC) can be transferred from water to acetone and, in sequence, to a desirable organic solvent. Some common examples are DMF (53), pyridine (54), toluene (55) and chloroform (56), increasing considerably the number of polymeric systems that can be used to produce nanocomposites by casting/evaporation.

Template Approach An interesting method, explored in a few works, was the construction of a CNC network prior to the solvent evaporation. Though laborious, Capadona et al. (57) used a slow solvent exchange process from water to a non-solvent to create a nanocellulose gel. Then, the gelled nanofiber scaffold was imbibed with a polymer (by its immersion directly in a solution of the desired polymer). Processing of nanocomposites have been traditionally limited by a maximum volume fraction of nanoparticles due to a significant increase in viscosity caused by high-aspect-ratio fillers. The advantage of the template method is the possibility of creating even neat cellulose films from the precursor gel, with unlimited nanofiber content. Annamalai et al. (58) prepared nanocomposites using this approach and compared ensuing systems to nanocomposites obtained by traditional casting/evaporation methodology. The mechanical properties of materials prepared by this template approach followed the predicted storage modulus (E′) values described by the percolation theory, what corroborates the formation of a 3D percolating structure of particles. However, the final values of E′ were higher for materials prepared by simple casting/evaporation. Recently, a similar approach was used to create tough and stretchable hydrogels (59).

LbL Assembly and Electrospinning Also using CNC suspension in a suitable liquid, the LbL technique is based on the alternating deposition of different layers, normally performed by dipping the film in different solutions. Cellulose nanomaterials can be used as a reinforcing phase, in association with other nanoparticles (60). Mechanical and barrier properties of the final material are controlled by the final number of deposited layers and their thickness. Some work has used CNC in the construction of LbL materials with the intention of controlling O2 permeability, improving electrical and mechanical properties, as well as some surface studies about material roughness and Young modulus (61–63). 213 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Also based on the use of nanocelluse suspensions, electrospinning is now a popular way to prepare materials for biomedical and pharmacological applications (64, 65). As advantages, this technique can allow the development of micro/nanofibers with very high surface area-to-volume ratio. Electrospinning is a fiber production method which uses electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in the order of some hundred nanometers. The use of nanocellulose can provide significant mechanical reinforcement to the produced fibers, besides increasing its electrical conductivity (66, 67). Melt Processing The use of melt processing for the preparation of cellulose based nanocomposites is a quite new subject with increasing number of publications in the last 5 to 10 years compared to solvent-based methods. Besides industrial interest and a short processing time (minutes, against hours in the case of casting/evaporation), this approach is also interesting since temperature is used to soften the polymer matrix and no solvent is involved, resulting in greener methodology. First, it is important to note that high temperature mixing of the rigid cellulose filler and polymeric matrix is possible because cellulose does not present classical thermal behavior of semicrystalline polymers. In other words, cellulose nanoparticles do not become soft at high temperatures since their Tg and Tm values cannot be observed for these materials. Some extrapolations indicate a possible Tg value for cellulose in a range from 220 to 250°C and, based on that, a melting temperature of 430°C (2). These values are normally above cellulose degradation temperature, especially in nanocellulose form (Table 1). With some limitations, many examples of materials produced by in situ polymerization, extrusion, injection-molding, and resin transfer molding at high temperatures can be found elsewhere (68–70). All these methods are highly appealing because they can be applied at industrial scale. However, since no solvent is present during processing, it means that nanocellulose usually needs to be solvent-free, i.e. in the dry state, even if water-assisted extrusion methods exist. In this case, simple methods based on just mixing the dried nanocellulose powder and polymer pellets have demonstrated to be inefficient due to particle aggregation in the cellulose powder, which is not reversible during processing (71). Furthermore, with solvent methods, some systems can present heterogeneities attributed to incompatibility between cellulose and most of polymeric matrices, that makes nanoparticles difficult to disperse. As discussed by Reid et al. (41), even the presence of solvent is not necessarily enough to ensure cellulose nanoparticle dispersion, and external energy may be necessary to overcome Van der Waals forces that are keeping the particles together. In the absence of solvent, other ways should be used to keep the nanoparticles apart and avoid aggregation prior to the processing. The principal approaches to preparing nanocomposites with well-dispersed filler are based on chemical modification and preparation of masterbatches. Several examples of 214 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


chemical modification of the nanocellulose surface can be found in the literature for nanocomposites prepared by extrusion, for example. As advantages, not only the improvement of particles dispersion is reported, but also a reduction in nanoparticle thermal degradation. The introduction of silane groups, surfactants and even polymerization of new groups on the particle surface have been reported (72–75). More simply, the use of a masterbatch is based on the preparation of a concentrated mixture of nanocellulose in polymer. Following this technique, a film, normally prepared by casting, can be prepared from a suspension of nanoparticles and the polymer in a suitable solvent, that allows the polymeric chains to coat the nanoparticles. This creates a mechanical barrier that avoids nanoparticle aggregation and low thermal degradation (54, 76, 77). The presence of secondary chains on the surface of the nanoparticles can be a problem when casting/evaporation preparation is used, since it prevents the formation of a percolating nertwork. This is not significant for melt processing because this percolation phenomenon of particles is generally not possible using this approach. The short preparation time requested to prepare the composites at high temperatures also limits the interactions between the particles. In addition, during melt processing, high shear forces are imposed to the sample and cause the orientation of the particles, trapped within the polymeric chains after cooling of the material (78). Recently, an attempt was made to reorganize the particles, based on their diffusion coefficient inside the polymer melt, but without success (79). As a consequence, a higher volume fraction of particles should be used to obtain similar results than for cast/evaporated materials (80). Many properties of melt processed materials are also impacted by the indirect effect of the processing conditions or by the simple presence of the nanoparticles. Some processing conditions (e.g. screw speed during extrusion, temperature, humidity content, etc) can affect the nanocomposite structure. Modification of polymer’s molecular weight and quality of dispersion are the most relevant ones (81), which are effected by the combination of the stress imposed on the sample and high temperatures. In that sense, it seems that hot-pressing is less aggressive on the sample than injection-molding or extrusion.

Crystallinity Since melt processing is performed in conditions that should cause polymer melting, cooling conditions will affect the final properties of the obtained nanocomposite. Thermal treatments of polymers are known to generate a “thermal history” which affects the organization of polymeric chains and crystal structure. Besides the thermal history of the matrix, nanocellulose can also modify the crystal structure of the polymer. Due to its small dimensions and large surface area, these particles can have a large nucleation effect on the polymer. It can increase the crystallinity of and alter the polymer structure. Some studies showed that the presence of CNC particles can increase the crystallinity of thermoplastic polymers and modify the activation energy of crystallization (79, 82–85). This effects seems to be even more significant if a reduction of polymer molecular weight occurs during extrusion (54). 215 Agarwal et al.; Nanocelluloses: Their Preparation, Properties, and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

216


Table 2. Examples of some properties obtained for nanocomposites prepared by different methods Preparation

Form

Matrix

Permeability coeficient P/Po (%)

Mechanical reinforcement E/Eo

Crystallinity variation (χ-χo)

ref

LbL

CNC

PET

0.06†

-

-

(86)

LbL

CNC

Resins

-

6.9

-

(87)

Electrospinning

CNC

PVA

-

3.7

2.0 %

(38)

Electrospinning

CNC

PCL

-

1.6

3.0 %

(88)

Electrospinning

CNF

SPI

-

7.6

-

(89)

Casting/Evaporation

CNF

PLA

1.10*

1.8

- 5.0 %

(90)

Casting/Evaporation

CNC

Gelatin

0.60*

1.3

-

(91)

Casting/Evaporation

CNC

Starch

0.74‡

25.0

-

(92)

Casting/Evaporation

CNC

PLA

1.10†

1.2

3.4 %

(93)

Casting/Evaporation

CNC

PMMA

-

3.4

-

(94)

Casting/Evaporation

CNC

PVA

-

1.3

6.4 %

(95)

Casting/Evaporation

CNC Tara Gum

0.33†

3.4

-

(96)

Casting/Evaporation

CNF

PU

-

2.4

6.7 %

(97)

Casting/Evaporation

CNC

CNF

0.33†

1.3

-

(98)

Casting/Evaporation Extrusion Roller Blade

CNC

PVAc

-

1.5 1.3 1.5

-

(81)


217


Preparation

Form

Matrix

Permeability coeficient P/Po (%)

Mechanical reinforcement E/Eo

Crystallinity variation (χ-χo)

ref

Extrusion

CNF

PLA

-

1.0

- 5.0 %

(99)

Extrusion

CNC

PA 6

-

1.5

- 1.0 %

(76)

0.76†

1

19.2 %

(100)

Extrusion

CNC PLA/PHB

Extrusion/Injection

CNC

PBAT

-

1.5

7.2 %

(101)

Melt Mixer

CNC

PLA/ Lignin

-

-

34.2 %

(102)

Where Po, Eo and χo are the initial values of the respective properties and P, E and χ the final ones. † Coefficients: Oxygen barrier ‡ water activity * Water vapour barrier Abbreviations: PA 6 (Polyamide 6); PBAT (Poly(butylene adipate-co-terephthalate); PET (Polyethylene terephthalate); PHB (Polyhydroxybutyrate ); PLA (Polylactic acid); PMMA (Polymethyl methacrylate ); PU (Polyurethane); (Polyvinyil alcohol); PVAc (Polyvinyl acetate); SPI (Soybean protein isolate).


As a consequence, the modification of matrix properties is directly related to the behavior of the nanocomposite. Permeability, mechanical properties and light transmittance are examples of properties that are strongly influenced by the amount of crystalline domains present in the matrix. Table 2 shows a compilation of some results obtained for the mechanical and barrier properties of nanocomposites preparated by different techniques.


Conclusions and Perspectives Nanomaterials can be prepared from cellulose using different strategies. They display high stiffness and are lightweight making them ideal candidates for the preparation of polymer nanocomposites. Casting-evaporation from an aqueous (or at least polar) medium is the most suited processing method, since it preserves the dispersion state of the nanoparticles in this medium and the surface hydroxyl groups lead to unexpected mechanical properties. This slow wet process gives the highest mechanical performance materials compared to other processing techniques. Indeed, during liquid evaporation strong interactions between nanoparticles can develop and promote the formation of a strong percolating network through H-bonding. This strategy is probably well adapted for niche applications. However, if industrial and large scale manufacturing of cellulose nanomaterial reinforced polymer nanocomposites is the final target, melt processing is definitively the most interesting technique, since the final product can be easily shaped by extrusion, injection-molding, blow-molding or compression-molding, but it is also the most challenging. Issues of self-aggregation and thermal degradation are usually identified as the main challenges, but different strategies have been proposed to limit these phenomena. Nevertheless, the mechanical properties of ensuing materials are always disappointing and far from the expectations if compared with percolated nanoparticles. It mainly results from the orientation of the nanoparticles but also from their inevitable coating (to avoid self-aggregation upon drying), which alters or limits access to surface hydroxyl groups, and prevents a H-bonded percolated structure. The new challenge probably consists of restoring this percolating network after shaping of the product.

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