Article pubs.acs.org/Macromolecules
Water-Triggered Modulus Changes of Cellulose Nanofiber Nanocomposites with Hydrophobic Polymer Matrices Koffi L. Dagnon,† Kadhiravan Shanmuganathan,† Christoph Weder,*,‡ and Stuart J. Rowan*,† †
Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio 44106, United States ‡ Adolphe Merkle Institute, University of Fribourg, Rte de l’Ancienne Papeterie, CH-1723 Marly 1, Switzerland S Supporting Information *
ABSTRACT: Biomimetic, stimuli-responsive nanocomposites were made using either poly(styrene-co-butadiene) (SBR) or polybutadiene (PBD) as the hydrophobic, low-modulus matrix and hydrophilic cellulose whiskers isolated from tunicates (TW) as the high-modulus filler. These materials were prepared using a template approach, which involves the formation of a percolating TW network and filling this template with either of the matrix polymers. Dynamic mechanical analysis (DMA) studies of the dry nanocomposite films reveal that the incorporation of TWs into the rubbery polymers increases the tensile storage modulus E′ significantly. The reinforcement is attributed to the formation of a three-dimensional TW network within the SBR and PBD matrices. The incorporation of the TWs did not affect the main relaxation temperature of the matrix SBR polymer, suggesting weak nanofiller− polymer interactions. Thus, the reinforcement is primarily on account of the nanofiller−nanofiller interactions, which involve hydrogen bonding. Interestingly, submersion of these hydrophobic matrix nanocomposites in water results in dramatic softening, consistent with disengagement of the TW network as a consequence of competitive hydrogen bonding with water. The kinetics of the modulus change and the amount of water uptake were shown to depend on the TW content. Given the hydrophobic nature of the matrices, it is proposed that the TWs create a percolating network of hydrophilic channels within the hydrophobic SBR and PBD matrices.
1. INTRODUCTION The development of polymer nanocomposites is of significant current interest in academia and industry.1 Such materials can combine desirable properties of the nanofiller (e.g., stiffness, thermal stability, impermeability) and the polymer (e.g., flexibility, dielectric, ductility, and processability) and cover a broad range of properties that are useful for a diverse range of applications such as sporting goods, aerospace components, and automobiles. In addition to the nanoparticle−nanoparticle interactions, the polymer−nanoparticle interface, constituted by polymer chains in the vicinity of the nanoparticles, also can play an important role in determining the polymer nanocomposite properties.2 Much work has been carried out on nanocomposites based on clay, silica, carbon nanotubes, and other inorganic-based materials.3−8 Cellulose nanocrystals (CNCs) (also referred to as whiskers or nanowhiskers) represent another attractive type of filler and are the subject of intense current investigations.9−12 CNCs are environmental friendly, biodegradable, and renewable and can be obtained from a range of renewable biosources, including tunicates, wood, cotton, and sisal.10 The dimensions of the nanofibers vary with the source of cellulose with diameters ranging from 2 to 40 nm and the length varying from 100 nm to several micrometers.10 CNCs are characterized by high modulus, high degree of crystallinity, and low density.9 These properties, combined with the fact that © 2012 American Chemical Society
their surfaces are covered with hydrogen-bonding hydroxyl groups, make CNCs interesting candidates for a broad range of applications. For example, they have shown to exhibit lyotropic liquid crystalline properties13 and have been used as templates for the self-assembly of nanoparticles14,15 and as absorbents for heavy metal ions.16 The investigation of CNCs as a reinforcing phase in polymer nanocomposites was first reported in 1995 by Favier et al.17 Since then, the interest in developing polymer nanocomposites reinforced by CNCs has grown tremendously.9,10 In this context, we have developed a new family of mechanically dynamic polymer nanocomposites18 that were inspired by the stimuli-responsive behavior of the dermis of sea cucumbers.19−23 These creatures have the ability to rapidly and reversibly alter the stiffness of their inner dermis.24,25 Studies have proposed that this dynamic mechanical behavior is achieved through a nanocomposite architecture, where a viscoelastic matrix is reinforced with rigid, high-aspect ratio collagen fibrils.25−27 The stiffness of the tissue is regulated by controlling the interactions, and thereby the stress transfer, between adjacent collagen fibrils by locally secreted proteins Received: March 6, 2012 Revised: May 9, 2012 Published: May 22, 2012 4707
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through either noncovalent28,29 or covalent30 bonds. Inspired by this natural model, we have been investigating the possibility of creating synthetic nanocomposites that exhibit a similar architecture as well as a comparable mechanical morphing capability. Our initial studies involved the use of tunicate (Styela clava) whiskers (TW, dimensions ∼20 nm × 2.2 μm and tensile modulus ∼130 GPa31) as alternative nanofillers to the collagen fibrils found in the natural model, embedded within relatively hydrophilic matrices such as poly(ethylene oxide-co-epichlorohydrin) (P(EO-co-EPI)) and poly(vinyl acetate) (PVAc).19,22 Similar studies were also conducted with CNCs isolated from cotton or microcrystalline cellulose, which have a lower aspect ratio, but are more accessible.20,32 The tensile storage modulus E′ of the dry nanocomposites was significantly enhanced compared to that of the neat polymers. The reinforcement is attributed to the formation of a threedimensional hydrogen-bonded CNC network within the polymer matrices and is well described by a percolation model.33,34 Immersion of these nanocomposites in water results in dramatic softening, consistent with disengagement of the CNC network due to competitive hydrogen bonding with water. Their stiffness is restored upon drying. Analyses using the Halpin−Kardos model35 (for the wet-state modulus) and a percolation model33 (for swollen-redried modulus) and stresstransfer studies on the basis of Raman spectroscopy36 support the conclusion that the stiffness change is, as per design, primarily a consequence of altered CNC fiber−fiber (as well as presumably fiber−matrix) interactions upon wetting/drying. The equilibrium swelling data of P(EO-co-EPI) and its TW nanocomposites in water showed modest aqueous swelling (∼30% v/v) that was not directly dependent on the whiskers content, indicating that in this system the water uptake was predominantly governed by the matrix polymer.19 By contrast, the aqueous swelling of polyurethane/cotton CNC nanocomposites was found to increase in a nonlinear fashion with increasing whisker content, showing a significant change in behavior around the percolation onset, suggesting that in this system water may be primarily transported through the cellulose whisker network.37 Thus, we sought to explore further how the polarity of the matrix might impact the water transport and switching capability of this class of nanocomposites, and as such we report herein the preparation and mechanical studies of a series of tunicate CNC or tunicate whisker (TW) nanocomposites based on hydrophobic poly(styrene-co-butadiene) (SBR) and polybutadiene (PBD) matrices. To our knowledge, no water-induced mechanical switching of cellulose nanocomposites with hydrophobic matrices has been reported.
Sulfuric Acid Hydrolysis of Tunicate Mantles. The isolation of whiskers from raw tunicates was accomplished in three steps: (1) isolation and cleaning of the mantles of tunicates, (2) bleaching of the mantles, and finally (3) hydrolysis of the fine pulp of dried bleached mantles. Tunicates were first gutted to isolate their mantles. Then, using aqueous potassium hydroxide (3 L, 5% w/w per 500 g of mantles), the mantles were heated at 80 °C for 8 h under mechanical agitation. The mantles were then scrubbed, rinsed with water, and treated twice again with the same concentration of aqueous potassium hydroxide at 80 °C for an additional 16 h under mechanical agitation. This protocol represents a minor modification of the procedure reported by Yuan et al.40 Lastly, the cleaned mantles were washed with water until the washings were at neutral pH. Bleaching was performed by adding water (3 L), acetic acid (5 mL), and sodium hypochlorite solution (>4% chlorine, 10 mL) to the cleaned mantles, and the temperature was increased to 60−70 °C. In 1 h intervals, additional same portions of acetic acid and sodium hypochlorite solution were added until the material’s color changed from pinkish to white. Finally, the bleached deproteinized mantles were washed with deionized water and stored in a refrigerator. Acid hydrolysis was performed on the dried bleached mantles. They were first disintegrated in a Waring blender (∼5 g in 500 mL of deionized water) to yield a fine cellulose pulp. Then, sulfonated tunicate whiskers were prepared by sulfuric acid hydrolysis of the fine cellulose pulp, according to the method described by Favier et al.17 with slight modifications. Sulfuric acid (98%, 500 mL) was slowly (over the course of 60−90 min) added under vigorous mechanical stirring to an ice-cooled suspension of the fine cellulose pulp in deionized water (∼4 °C). As the sulfuric acid is added, the temperature increases but was not allowed to exceed 20 °C during the course of the hydrolysis. After ∼400 mL of the acid had been added, the fine cellulose pulp reaction was removed from the ice bath and allowed to warm up to ∼40 °C during the addition of the final 100 mL of acid. After the acid addition was complete, the reaction was heated to 60 °C and maintained at this temperature for 60 min under continuous stirring. It was filtered through a small-pore fritted glass filter, and the residue was washed with deionized water until the pH reached 6. After filtration, a colloidal suspension of cellulose whiskers was obtained by adding 0.1 L of deionized water to the residue. This colloidal suspension was subsequently placed into dialysis tubing and dialyzed against deionized water for 24 h. It was then removed from the dialysis tubing, and an additional 0.2 L of deionized water was added. This resulting whisker suspension was freeze-dried using a VirTis Benchtop Shell Bath freezer (SP Industries: Warminster, PA) with an initial shelf temperature of 25 °C and condenser temperature of −100 °C. It was left in the freezer until the vacuum reached ∼5 mTorr to ensure complete drying, and the obtained tunicate whisker aerogel was stored for future use. The yield (weight ratio of whisker aerogel over dried bleached mantles) was 75%. Formation of Sulfonated Tunicate Whisker Organogels. Whisker organogels were prepared from aqueous dispersions using a solvent-exchange sol−gel process where gelation was induced through addition of acetone to the whisker dispersion. The process described is a slight modification of our previously published procedure.41 In a typical gelation process, 50 mL of aqueous cellulose whisker dispersion of concentration 4−6 mg/mL (prepared by sonicating specified amount of whiskers in deionized water for about 6−8 h) was taken in a 250 mL beaker. The dispersion was heated to a gentle boil followed by brief sonication to remove air bubbles. After cooling to room temperature, 100 mL of acetone was gently added through the walls of the beaker so as to avoid mixing and form an organic layer on top of the aqueous dispersion. Acetone gradually replaces water, and the top organic layer was exchanged with fresh acetone 1−2 times daily until the bottom portion had assembled into a mechanically coherent whisker−acetone gel (typically 5−7 days). During that process the acetone layer was gently agitated to facilitate solvent exchange. When solvent exchange was no longer visible (refractive index gradients at the sol/gel interface), the acetone gel was released from the glass beaker and rinsed twice more with dry acetone. The gel was cut into
2. EXPERIMENTAL SECTION Materials. All reagents, except acetone, were used as received. Acetone (dried with potassium carbonate), anhydrous tetrahydrofuran (THF), sulfuric acid, and potassium hydroxide were purchased from Fisher Scientific (Pittsburgh, PA). Poly(styrene-co-butadiene) (SBR1502 with 23.5% styrene, weight-average molecular weight, Mw, = 215 000 g/mol, polydispersity index, PDI, = 1.8, density = 0.94 g/cm3) and poly(butadiene) (PBD) (Mw = 200 000−300 000 g/mol, density = 1.19 g/cm3) were supplied by The Goodyear Tire and Rubber Co. (Akron, OH). Sulfonated whiskers were isolated from tunicates collected from floating docks in Point View Marina (Narragansett, RI). They were obtained through sulfuric acid hydrolysis of the tunicate mantles. The acid hydrolysis procedure described hereafter is a slight modification of our previously published procedure.21,38,39 4708
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rectangular films with length (10 mm), width (3 mm), and thickness (450−500 and 250−300 μm for the SBR and PBD nanocomposite films, respectively) were used. Tests were conducted using a temperature sweep method, a heating rate of 3 °C/min, and a fixed frequency of 1 Hz. For SBR and its nanocomposite films, the measurements were conducted from −100 to 100 °C with a strain of 0.1% and a force of 0.01 N, whereas for PBD nanocomposite films, a 25−90 °C temperature range and strain amplitude of 15 μm were used. It was difficult to conduct a temperature sweep for the neat PBD as the samples yielded too much. For investigations in the dry state, all samples were dried in vacuum (−80 kPa, 24 h, 50 °C) prior to DMA testing. In order to determine the wet modulus and the kinetics of the water-induced stiff−soft transition of the nanocomposite films, DMA experiments were conducted using the above setup with a submersible clamp, which allowed measurements while the samples were immersed in water; in this case, samples were tested at 25 °C. DMA experiments were done in triplicate. Aqueous Swelling. Specimens in the form of rectangular films with length (30 mm), width (5 mm), and thickness (450−500 and 250−300 μm thin SBR and PBD nanocomposite films, respectively) were first dried under vacuum at 60 °C for 24 h and weighed using a four-digit balance. They were then immersed in deionized water. The samples were removed every 24 h, gently blotted using filter paper, weighed, and immediately immersed in deionized water. Swelling measurements were conducted for 5 and 10 days for PBD and SBR nanocomposite samples, respectively, and were carried out in triplicate. The water uptake (WU) was determined from the relative gain in weight of the specimen as follows:
small rectangular pieces, which were stored in dry acetone in a tightly sealed bottle until use. Fabrication of SBR and PBD Nanocomposites via a Template Approach. A template approach similar to that previously reported by Capadona et al.41 was used to fabricate the nanocomposites. The polymers were dissolved in anhydrous tetrahydrofuran (THF) at room temperature and at various concentrations ranging from 30 to 125 mg/mL (for SBR) and 80 to 125 mg/mL (for PBD) by overnight stirring. The TW organogel, prepared as described above, was cut into rectangular pieces, which were weighed and placed at room temperature into SBR and PBD solutions for 16 h. Higher concentration and longer time would lead to a higher polymer content in the organogel template and afford nanocomposites of lower whisker content. The gels were removed from the polymer solution after the immersion was complete and dried at ambient temperature in a wellventilated fume hood for up to 6 h to remove most of the solvent. They were subsequently placed in a vacuum oven at 50 °C for up to 6 h to remove any residual solvent. The dried materials were compression-molded in a Carver press between Teflon sheets at room temperature at 3000 psi for 2 min and then repressed at 70−75 °C and 3000 psi for 10 min to yield 450−500 and 250−300 μm thin SBR and PBD nanocomposite films, respectively. After melt-pressing, films were allowed to cool down to room temperature. Reference samples of neat SBR and PBD were prepared using the same procedure. The volume fraction of whisker (% v/v TW) in the nanocomposites was calculated based on the weight fraction of the TWs in the polymer, using a density of 1.5 g/cm3 (density of cellulose) for the TWs and a density of 0.94 g/cm3 for SBR or 0.89 g/ cm3 for PBD. The weight fraction was determined gravimetrically from the weight (in mg) of whiskers in the wet TW organogels used to prepare the nanocomposites (this weight is obtained by drying the organogel at 40 °C for 24 h) and the weight of the final nanocomposite material. The compositions of all nanocomposites prepared and studied are compiled in Table 1.
WU =
sample code
TW (% v/v)
SBR
SBR SBR-7TW SBR-10TW SBR-13TW SBR-15TW SBR-17TW PBD PBD-10TW PBD-12TW PBD-20TW
0 7 10 13 15 17 0 10 12 20
PBD
(1)
where Mt and M0 are the mass of the sample after water immersion for a certain period of time (t) and before immersion to deionized water, respectively. The mean water uptake of each SBR nanocomposite sample was calculated for various exposure times. The ratio of mass of water absorbed at time t (Mt − M0) to the mass of water absorbed at equilibrium (M∞) can be expressed as42
Table 1. Composition of Nanocomposites Studied nanocomposite
Mt − M0 × 100 M0
Mt − M0 =1− M∞
n =∞
∑ n=0
⎡ − D(2n + 1)2 π 2t ⎤ 8 exp⎢ ⎥ 2 2 ⎣ ⎦ (2n + 1) π (2L)2 (2)
where 2L is the initial thickness of the film, D is the diffusion coefficient, and t is time in seconds. Equation 2 represents a Fickian mode of diffusion. After the equilibrium swelling, the thickness of the nanocomposite films changes only slightly, and this change is dependent on the TW content. For short immersion times, i.e., at low (Mt − M0)/M∞ values (
