Nanostructural Effects in High Cellulose Content Thermoplastic Nanocomposites with a Covalently Grafted Cellulose-Poly(methyl methacrylate) Interface.

A critical aspect in materials design of polymer nanocomposites is the nature of the nanoparticle/polymer interface. The present study investigates the effect of manipulation of the interface between cellulose nanofibrils (CNF) and poly(methyl methacrylate) (PMMA) on the optical, thermal, and mechanical properties of the corresponding nanocomposites. The CNF/PMMA interface is altered with a minimum of changes in material composition so that interface effects can be analyzed. The hydroxyl-rich surface of CNF fibrils is exploited to modify the CNF surface via an epoxide-hydroxyl reaction. CNF/PMMA nanocomposites are then prepared with high CNF content (∼38 wt %) using an approach where a porous CNF mat is impregnated with monomer or polymer. The nanocomposite interface is controlled by either providing PMMA grafts from the modified CNF surface or by solvent-assisted diffusion of PMMA into a CNF network (native and modified). The high content of CNF fibrils of ∼6 nm diameter leads to a strong interface and polymer matrix distribution effects. Moisture uptake and mechanical properties are measured at different relative humidity conditions. The nanocomposites with PMMA molecules grafted to cellulose exhibited much higher optical transparency, thermal stability, and hygro-mechanical properties than the control samples. The present modification and preparation strategies are versatile and may be used for cellulose nanocomposites of other compositions, architectures, properties, and functionalities.

Polycaprolactone Nanocomposites Reinforced with Cellulose Nanocrystals Surface-Modified via Covalent Grafting or Physisorption: A Comparative Study.

In the present work, cellulose nanocrystals (CNCs) have been surface-modified either via covalent grafting or through physisorption of poly(n-butyl methacrylate) (PBMA) and employed as reinforcement in PCL. Covalent grafting was achieved by surface-initiated atom transfer radical polymerization (SI-ATRP). Two approaches were utilized for the physisorption: using either micelles of poly(dimethyl aminoethyl methacrylate)-block-poly(n-butyl methacrylate) (PDMAEMA-b-PBMA) or latex nanoparticles of poly(dimethyl aminoethyl methacrylate-co-methacrylic acid)-block-poly(n-butyl methacrylate) (P(DMAEMA-co-MAA)-b-PBMA). Block copolymers (PDMAEMA-b-PBMA)s were obtained by ATRP and subsequently micellized. Latex nanoparticles were produced via reversible addition-fragmentation chain-transfer (RAFT) mediated surfactant-free emulsion polymerization, employing polymer-induced self-assembly (PISA) for the particle formation. For a reliable comparison, the amounts of micelles/latex particles adsorbed and the amount of polymer grafted onto the CNCs were kept similar. Two different chain lengths of PBMA were targeted, below and above the critical molecular weight for chain entanglement of PBMA (Mn,c ∼ 56 000 g mol-1). Poly(ε-caprolactone) (PCL) nanocomposites reinforced with unmodified and modified CNCs in different weight percentages (0.5, 1, and 3 wt %) were prepared via melt extrusion. The resulting composites were evaluated by UV-vis, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and tensile testing. All materials resulted in higher transparency, greater thermal stability, and stronger mechanical properties than unfilled PCL and nanocomposites containing unmodified CNCs. The degradation temperature of PCL reinforced with grafted CNCs was higher than that of micelle-modified CNCs, and the latter was higher than that of latex-adsorbed CNCs with a long PBMA chain length. The results clearly indicate that covalent grafting is superior to physisorption with regard to thermal and mechanical properties of the final nanocomposite. This unique study is of great value for the future design of CNC-based nanocomposites with tailored properties.

Preparation and characterization of functionalized cellulose nanocrystals.

In this work, a series of functional nanocrystals (F-CNCs) was successfully produced by an efficient preparation method, combining acid hydrolysis and Fischer esterification with various organic acids. Functionalities such as ATRP initiators, double bonds, triple bonds, and thiols could be incorporated on CNCs. Surface modification was confirmed by FT-IR, XPS, and elemental analysis. Physical properties of F-CNCs were assessed by AFM, XRD and TGA. Moreover, ATRP initiator functionalized CNCs were utilized to graft poly(methyl methacrylate) via ATRP, thiol functionalized CNCs were reacted with Ellman's reagent to determine the thiol content and dye disperse red 13 was attached to alkyne functionalized CNCs to estimate the propiolate content. The herein presented method is a highly versatile and straightforward procedure for the preparation of F-CNCs which is believed to be a better alternative for the commonly utilized, extensive, multistep, and time consuming post functionalization methods.

Facile preparation route for nanostructured composites: surface-initiated ring-opening polymerization of ε-caprolactone from high-surface-area nanopaper.

In this work, highly porous nanopaper, i.e., sheets of papers made from non-aggregated nanofibrillated cellulose (NFC), have been surface-grafted with poly(ε-caprolactone) (PCL) by surface-initiated ring-opening polymerization (SI-ROP). The nanopaper has exceptionally high surface area (∼300 m(2)/g). The "grafting from" of the nanopapers was compared to "grafting from" of cellulose in the form of filter paper, and in both cases either titanium n-butoxide (Ti(On-Bu)4) or tin octoate (Sn(Oct)2) was utilized as a catalyst. It was found that a high surface area leads to significantly higher amount of grafted PCL in the substrates when Sn(Oct)2 was utilized as a catalyst. Up to 79 wt % PCL was successfully grafted onto the nanopapers as compared to filter paper where only 2-3 wt % PCL was grafted. However, utilizing Ti(On-Bu)4 this effect was not seen and the grafted amount was essentially similar, irrespectively of surface area. The mechanical properties of the grafted nanopaper proved to be superior to those of pure PCL films, especially at elevated temperatures. The present bottom-up preparation route of NFC-based composites allows high NFC content and provides excellent nanostructural control. This is an important advantage compared with some existing preparation routes where dispersion of the filler in the matrix is challenging.