SHORT CELLULOSE NANOFIBRIL/POLYVINYL ALCOHOL NANOCOMPOSITE FIBERS

Author

Jun Peng1,2, Craig Clemons3*, Ronald Sabo3, Tom Ellingham1,2, Lih-Sheng Turng1,2* Polymer Engineering Center, University of Wisconsin–Madison, Madison, Wisconsin 53706
Wisconsin Institute for Discovery, University of Wisconsin–Madison, Madison, Wisconsin 53715 3USDA Forest Service, Forest Products Laboratory, Madison, WI 53726 

Abstract

methylpiperidine-1-oxy radical (TEMPO)-mediated oxid- ation as a pretreatment prior to mechanical disintegration. They found the tensile strength did not improve with the addition of 1 wt% CNF [4]. A higher CNF content may have led to greater reinforcement. However, it was not possible to add more CNF due to the difficulty in dis- persing the high-aspect-ratio CNF even with intense ultra- sonication or strong mechanical stirring.
In this study, we investigated short cellulose nano- fibers (SCNF) that were mechanically isolated from enzymatically pretreated wood pulp as a reinforcement for the PVA fiber. Sufficient mechanical energy was used to prepare discrete CNF that could be used at higher weight percentages but without the need for concentrated sulfuric acid hydrolysis, as in CNC production, or the loss of hydroxyl groups for hydrogen bonding with the PVA matrix. Ultimately, we hoped to optimize the enzyme mixtures and pretreating conditions to minimize the mechanical energy needed to produce the SCNF.

Experiments
Preparation of Short Cellulose Nanofibrils (SCNF)
SCNF was prepared at the U.S. Forest Service Forest Products Laboratory (Madison, WI), according to the procedure described by Qing et al. [5]. The 1 wt% SCNF suspension was prepared to act as a reinforcing agent.

Preparation of Spinning Solution
A 99% hydrolyzed commercial-grade of PVA from Sigma-Aldrich with a weight-average molecular weight of 85,000 to 124,000 was used as the matrix polymer. A sufficient amount of PVA was dissolved in water at 90 oC with mechanical stirring for 30 min to yield 20% PVA by weight. Spinning solutions were prepared by mixing various amounts of SCNF solution, PVA solution, and water at 90oC for 60 min. Specific compositions are listed in Table 1.

Table 1. Compositions of PVA/SCNF spinning solutions and dry fiber.

Short cellulose nanofibrils (SCNF) were investigated as a reinforcement for polyvinyl alcohol (PVA) fibers. SCNF fibers were mechanically isolated from hard wood pulp after enzymatic pretreatment. Various levels of SCNF were added to PVA and gel-spun into continuous fibers. The molecular orientation of PVA was affected by a combination of wet drawing during gel spinning and post-hot-drawing at a high temperature after drying. A maximum total draw ratio of 27 was achieved with various SCNF contents investigated. The PVA crystal orientation increased when small amounts of SCNF were added, but decreased again as the SCNF content was increased above about 2 or 3%, likely due to SCNF percolation resulting in network formation that inhibited alignment. SCNF fillers were effective in improving PVA fiber tensile properties (i.e., ultimate strength and elastic modulus). Shifts in the Raman peak at ~1095 cm-1, which were associated with the C–O–C glycosidic bond of SCNF, indicated good stress transfer between the SCNF and the PVA matrix due to strong interfacial hydrogen bonding.

Introduction
Nanocellulose-based reinforcements constitute a new class of naturally-sourced reinforcements. Trees, plants, some marine creatures such as tunicates, and certain bacteria and algae form microfibrils from cellulose mole- cules. These microfibrils have a complex structural hierarchy and often act as the main reinforcing element in their respective organisms [1]. In part, this is the high reinforcing potential of native crystalline cellulose within these microfibrils that has recently led researchers to ex- tract nanocellulose from them for use in composites.

To improve the mechanical properties of PVA fibers, both CNC and CNF have been investigated as reinforce- ments. Uddin et al. [2] produced wet spun fibers from aqueous PVA solutions with more than 30% CNC, followed by hot drawing [2]. Adding 5% CNC increased the ultimate tensile strength 20% compared to neat PVA fibers. However, the tensile strength was reduced at concentrations above 5% as orientation was reduced. Peresin et al. produced PVA mats with up to 15% CNC by electrospinning and found that CNC increased the storage modulus [3]. However, properties such as fiber strength or tenacity were not measured. Endo et al. gel-spun PVA that was reinforced with CNFs prepared using 2,2,6,6-tetra-methylpiperidine-1-oxy radical (TEMPO)-mediated oxid- ation as a pretreatment prior to mechanical disintegration. They found the tensile strength did not improve with the addition of 1 wt% CNF [4]. A higher CNF content may have led to greater reinforcement. However, it was not possible to add more CNF due to the difficulty in dis- persing the high-aspect-ratio CNF even with intense ultra- sonication or strong mechanical stirring.

In this study, we investigated short cellulose nano- fibers (SCNF) that were mechanically isolated from enzymatically pretreated wood pulp as a reinforcement for the PVA fiber. Sufficient mechanical energy was used to prepare discrete CNF that could be used at higher weight percentages but without the need for concentrated sulfuric acid hydrolysis, as in CNC production, or the loss of hydroxyl groups for hydrogen bonding with the PVA matrix. Ultimately, we hoped to optimize the enzyme mixtures and pretreating conditions to minimize the mechanical energy needed to produce the SCNF.

Experiments
Preparation of Short Cellulose Nanofibrils (SCNF)
SCNF was prepared at the U.S. Forest Service Forest Products Laboratory (Madison, WI), according to the procedure described by Qing et al. [5]. The 1 wt% SCNF suspension was prepared to act as a reinforcing agent. 

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