Cellulose-based nanocomposites prepared via mini-emulsion polymerization: Understanding the chemistry of the nanocellulose/matrix interface

doi:10.1016/j.colsurfa.2014.01.077

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 448, 20 April 2014, Pages 1-8

https://doi.org/10.1016/j.colsurfa.2014.01.077 Get rights and content

Highlights

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Nanocomposite dispersions were prepared via miniemulsion polymerization.
•
Cellulose nanocrystals (CNCs) was incorporated as reinforcing nanofiller.
•
Silane coupling agent was used to promote the adhesion between the CNCs and the polymer particles.
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NMR was used as a tool to investigate the mechanism of interaction.

Abstract

The miniemelusion polymerization of acrylic monomers in the presence of cellulose nanocrystals (CNCs) and methacryloxypropyl trimethoxysilane (MPMS) as a coupling agent is proven to be an efficient approach for the one-pot synthesis of a ready-for-use stable nancomposite dispersion. In order to gain further insight on the location of the coupling agent within the polymer particles and understand how the silane coupling agent affects the degree of binding between the CNC and the polymer matrix, liquid and solid state NMR (¹³C and ²⁹Si) investigation were carried out on the nanocomposite. It was shown that the sol fraction contained only the styrene/ethylehexylacrylate (ST/EHA) copolymer, while the CNCs and the MPMS moiety were accumulated on the gel fraction. Based on the analysis, it was concluded that the MPMS is mostly located at the interface of the CNCs and the copolymer matrix.

Graphical abstract

Introduction

Nanocellulose, encompassing, cellulose nanocrystals, nanofibrillated cellulose and bacterial cellulose have emerged as one of the most promising biobased nanofiller with outstanding reinforcing potential when incorporated within a polymer matrix [1], [2]. More specifically, if CNC are considered, then such huge reinforcing potential is the consequence of their high aspect ratio, nanosized scale, high Young modulus amounting 130 GPa, as well as their aptitude to set up percolated networks [1]. The high degree of transparency in nanocomposites based on cellulose nanofillers is another feature that deserves emphasis [3]. Actually, due to the cross section of the cellulose nanofiller being less than one-tenth of the visible light wavelength, the optical properties of the resulting nanocomposite is expected to remain unchanged as long as good dispersion within the host matrix is ensured.

CNC consists of aggregate of monocrystalline cellulose domains that have unfolded molecular chains parallel to the microfibril axis. Their typical size ranges from 5 to 15 nm in width and from 100 to 500 nm in length [4]. The basic procedure for the isolation of CNCs is an acid-catalyzed hydrolysis of amorphous and paracrystalline cellulose macromolecules. Calling on this approach, CNC from a wide variety of cellulose sources [5], such as bacterial cellulose [6], algal cellulose (valonia), tunicin, plants (woody and non-woody) [7], [8], [9], and cotton, were prepared and their morphology was characterized.

Currently, two major techniques can be adopted to synthesize cellulosic nanocomposites films [10]. The first technique is solvent casting via water or organic solvent evaporation. In the second approach, the matrix was extruded in the presence of the desired amount of freeze-dried cellulose nanoparticles. In our previous work [11], [12], [13], [14], a one-step approach for preparing stable aqueous nanocomposite dispersions, based on CNCs and an acrylic copolymer, were successfully implemented adopting a mini-emulsion polymerization approach.

The polymerization reaction was conducted in the presence of a MPMS, which was used as a silane coupling agent to functionalize the polymer particles. The reinforcing efficiency and the melt rheological behavior of these nanocomposite films were shown to be largely affected by the amount of the coupling agent [13], [14]. To account for this effect, it was hypothesized that the silane was located mainly on the surface of the particle which caused the organic methacrylic functionalities to be oriented toward the interior of the micelles and the silanol groups to protrude into the water phase. This arrangement enables the cellulose CNC to attach more easily to the polymer particles during the film-formation process. The resulting connections not only contributed to the uniform distribution of the nanofiller within the entire polymer matrix, but also reduced the risk agglomeration during the film-formation process.

In the present work, an NMR method was used to gain further insight on the locus of the silane coupling agent (MPMS) within the polymer particles; these results would allow us to further explore the effects of the silane on the degree of binding between the CNC and the polymer matrix after film-formation. For this purpose, a solvent extraction was carried out on the nanocomposite films that were obtained after coalescence and film-formation at room temperature of the nanocomposite dispersion, and the sol and gel fraction was characterized by using liquid- and solid-state NMR analyses.

Section snippets

Materials

Styrene (Sty) and 2-ethyl hexylacrylate (EHA) were purified via distillation under reduced pressure and kept in the refrigerator until used. The reagents, 2,2-azobis(isobutyronitrile) (AIBN), sodium dioctylsulfano succinate (DOSS), γ-methacryloxypropyl trimethoxysilane (MPMS) and hexadecane (HD), which are commercial products supplied by Aldrich, were used without further purification. Deionized water was used during all polymerizations and treatment processes.

CNCs preparation

An aqueous colloidal suspension of

Sample preparation

Three samples were prepared and characterized, i.e.: (i) nanocomposite film, (ii) gel part and (ii) sol fractions. The nanocomposite films were extracted with refluxing THF for 24 h to isolate the gel and sol fractions. The “gel portion” is the solid resulting after the extraction of the “sol fraction” and subsequent drying under vacuum to eliminate the solvent.

On the one hand, ²⁹Si NMR experiments were performed to check the presence of MPMS and establish the state of silicon atoms (T⁰, T¹,…),

Results and discussion

In this research work, the interactions between CNCs/silane coupling agent/copolymer have been studied with liquid- and solid-state NMR. For this purpose, the nanocomposite films were extracted in tetrahydrofuran (THF) for 24 h to eliminate the soluble polymer (extractable). The sediment fraction, which was designated gel, was isolated and dried under vacuum until the solvent was eliminated. The supernatant fraction designated sol, was separated and the soluble polymer isolated after the

Conclusion

The NMR techniques utilized in this study have confirmed the static distribution of styrene and 2-ethylhexyl acrylate units within the macromolecular chains. Additionally, these experiments have facilitated the understanding of the different types of interactions between the CNCs/silane/matrix. Using liquid- and solid-state ¹³C and ²⁹Si NMR, it has been shown that neither the CNCs nor the MPMS are likely to affect the copolymer composition during the in situ mini emulsion polymerization of

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Cellulose-based nanocomposites prepared via mini-emulsion polymerization: Understanding the chemistry of the nanocellulose/matrix interface

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

CNCs preparation

Sample preparation

Results and discussion

Conclusion

J. Colloid Interface Sci.

Colloids Surf., A

Nanocomposites materials from latex and cellulose whiskers

Polym. Adv. Technol.

Nanofibrillated cellulose: surface modification and potential applications

Colloid Polym. Sci.

Surface modification of bacterial cellulose nanofibers for property enhancement of optically transparent composites: dependence on acetyl-group DS

Biomacromolecules

Review cellulosic bionanocomposites: a review of preparation, properties and applications

Polymers

Optimization of the isolation of nanocrystals from microcrystalline cellulose by acid hydrolysis

Cellulose

Phase separation behavior in aqueous suspensions of bacterial cellulose nanocrystals prepared by sulfuric acid treatment

Langmuir

Effect of conditions on the properties behavior of wood cellulose nanocrystals suspensions

Biomacromolecule