Macromolecular Rapid Communications

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Bacterial Cellulose Nanopaper as Reinforcement for Polylactide Composites: Renewable Thermoplastic NanoPaPreg Thanit Montrikittiphant, Min Tang, Koon-Yang Lee,* Charlotte K. Williams, Alexander Bismarck*

Bacterial cellulose (BC) is often regarded as a prime candidate nano-reinforcement for the production of renewable nanocomposites. However, the mechanical performance of most BC nanocomposites is often inferior compared with commercially available polylactide (PLLA). Here, the manufacturing concept of paper-based laminates is used, i.e., “PaPreg,” to produce BC nanopaper reinforced PLLA, which has been called “nanoPaPreg” by the authors. It is demonstrated that high-performance nanoPaPreg (vf = 65 vol%) with a tensile modulus and strength of 6.9 ± 0.5 GPa and 125 ± 10 MPa, respectively, can be fabricated. It is also shown that the tensile properties of nanoPaPreg are predominantly governed by the mechanical performance of BC nanopaper instead of the individual BC nanofibers, due to difficulties impregnating the dense nanofibrous BC network.

1. Introduction Cellulose is the most abundant organic homo-polymer on earth. Lignocellulose fibers are often regarded as the T. Montrikittiphant, C. K. Williams Department of Chemistry, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK T. Montrikittiphant, M. Tang, A. Bismarck Polymer and Composite Engineering (PaCE) group, Department of Chemical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK Fax: +44 20 7594 5578 A. Bismarck Polymer and Composite Engineering (PaCE) group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna, Währinger Strasse 42, 1090, Vienna, Austria E-mail: [email protected] K.-Y. Lee Department of Chemical Engineering, University College London, Torrington Place, WC1E 7JE, London, UK E-mail: [email protected]

prime candidate for the production of green(er) composites.[1] This is by no means a recent concept. Indeed, one of the first successful commercial applications of cellulose fibers in modern synthetic composites dates back to the 1940s, when Henry Ford introduced a soybean fiber-reinforced phenolic resin into the body panel of a car.[2] In the late 1950s, a car manufacturer in the former German Democratic Republic, VEB Sachsenring Zwickau, further developed the concept of green(er) composites originally developed in the 1930 to produce cotton linter waste-reinforced phenolic resins from which the bodywork of the Trabant car was built. These parts, i.e., hood, roof, doors etc., fitted to a steel monocoque, were surprisingly durable, showing the potential of green composites. Currently, phenolic resin impregnated papers (under the tradename of Phenolkraft) are being produced and mainly used in the electric and electronic industry. Cellulose fibers, particularly nanocellulose fibers, have received significant attention recently for the fabrication of green(er) nanocomposites.[3–5] The major driver for utilizing nanocellulose as opposed to natural fibers

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in composite applications stems from the possibility of exploiting the high stiffness and strength of cellulose crystals,[6] estimated (theoretically) to be approximately 50–180[7–10] and 0.3–23 GPa,[11–14] respectively, depending on the assumptions made for the calculations. Nanocellulose can be obtained via two approaches: either i) topdown or ii) bottom-up. In the top–down approach, plant fibers are either treated with strong ultrasound,[15] passed through high-pressure homogenizers,[16,17] or grinders[18] to separate the micrometer technical fiber bundles into individual nanocellulose fibers. This type of nanocellulose fibers is more commonly known as nanofibrillated cellulose (NFC). Using the bottom-up approach, bacterial cellulose (BC) is produced via the fermentation of lowmolecular-weight sugars by cellulose-producing bacteria, typically from the Acetobacter species.[19] It is worth mentioning at this point that both types of nanocellulose possess very similar reinforcing ability if incorporated into polymers.[20] However, both types of nanocellulose have yet to fully deliver their potential in practical applications, particularly in terms of the mechanical performance of the resulting nanocellulose-reinforced polymer composites (Figure 1). The red dashed lines represent the properties of 100% bio-derived poly(L-lactide) (PLLA) which we have chosen as benchmark, because its mechanical performance is one of the highest among all renewable polymers and it is commercially available.[21] Approximately 80% of all reported nanocellulose-reinforced polymers performed worse than or only equally as good as PLLA. One major reason for this is the loading fraction of nanocellulose (vf ) in the composites. In order to achieve or

exceed the mechanical performance of PLLA, vf > 30 vol% were necessary. This poses a challenge in the manufacturing of nanocomposites, as conventional polymer extrusion methods cannot be used due to the tendency of nanocellulose to aggregate within the polymer matrices.[22] To produce high vf cellulose nanocomposites, the only practical solutions are to either use only water-soluble polymers[23,24] or to apply solvent exchange methods[25,26] if non-water-soluble polymers are selected or in the case of BC fibers, culture BC in the presence of the selected polymer.[23] In all cases, significant complexity and additional processing steps are required, which will result in a poor cost-performance ratio of the resulting nanocomposites and can also have deleterious effects on their properties. Therefore, a new strategy for the manufacturing of nanocellulose-reinforced polymer composites is needed. Inspiration can be drawn from the manufacturing of paper-based pre-impregnated (PaPreg) laminates, which was extensively studied in 1940s,[11,27,28] and appeared again as a manufacturing concept in 1980s.[29] PaPregs have excellent mechanical properties, with reported tensile stiffness and strength as high as 18.6 GPa and 188 MPa, respectively. In this work, we draw inspiration from the “PaPreg” concept and combine it with the current state-of-the-art nanocellulose/nanopapers to produce truly green, high-performance nanocellulosereinforced PLLA composites, overcoming the aforementioned challenges in the manufacturing of nanocellulose-reinforced polymer composites, which should result in composites with a better cost-performance ratio.

Figure 1. Tensile properties of bacterial cellulose (BC)-reinforced polymer nanocomposites reported by various authors in the literature. Reproduced with permission.[5] Copyright 2014, Wiley. The red dashed lines represent our benchmark, PLLA. vf, E, σ, PVOH, HEC, and PU denote fiber volume fraction, tensile modulus, tensile strength, polyvinyl alcohol, hydroxyethyl cellulose, and polyurethane, respectively.

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2. Experimental Section

Table 1. Tensile properties of neat PLLA, nanoPaPreg and BC nanopaper. ρ, E, σ, ε and vf, BC denote density, tensile modulus, tensile strength, strain-to-failure, and the loading fraction of BC nanopaper, respectively, of PLLA and nanoPaPreg.

2.1. Materials PLLA (L9000, MW ≥ 150 kDa, D-content ≈ 1.5%) was purchased from Biomer GmbH and used as the matrix for the nanocomposites. 1,4-Dioxane (ACS Reagent, purity ≥99%) was purchased from Sigma–Aldrich and used as solvent for PLLA. Commercialgrade BC was kindly supplied by Forschungszentrum für Medizintechnik und Biotechnologie (fzmb) GmbH in wet pellicle form containing 92 wt% water. Gluconacetobacter xylinus (wild-type strain AX5 from the stock collection of the fzmb was cultivated in glass vessels containing 3.5 L of Hestrin–Schramm medium in static culture at 30 °C for about 30 d. A bacterial suspension of 10 mL (turbidity by McFarland 3–4) was used for inoculation. The pellicles were then removed and washed twice by boiling 0.1 N aqueous NaOH for 30 min.[30]

2.2. Methods BC nanopaper with a grammage of 80 g m−2 (gsm) was manufactured by first creating a BC suspension in water containing 0.1 wt% BC. The BC was then filtered onto a 100-mesh metal mesh screen in a 0.3 × 0.3 m2 sheet former (Adirondack Machine Corp., G-FW100, New York, USA). The wet filter cake was wetpressed twice between blotting papers under a weight of 10 kg for 10 s to remove excess water, followed by a consolidation step at 120 °C under a weight of 1 t. In order to fabricate nanoPaPreg containing high vf BC, it is essential to use thin PLLA films to produce nanocomposites with the appropriate polymer content; Thin PLLA films were produced by solution casting. Briefly, PLLA pellets were dissolved in 1,4-dioxane (8 wt/vol%) at 60 °C overnight under magnetic stirring. This polymer solution was then left to cool to room temperature prior to solution casting onto a glass plate using a casting knife. 1,4-dioxane was then left to evaporate and the thin PLLA film was removed from the glass plate by immersing it into deionized water. NanoPaPregs (100 × 100 × 0.1 mm3) were fabricated by sandwiching the previously manufactured BC nanopapers between two thin PLLA films, followed by a consolidation step at 190 °C and a weight of 1 t for 5 min.

3. Results and Discussion Tensile tests performed on the BC nanopaper used in this study showed that they possessed a tensile modulus (E) and strength (σ) of 9.5 ± 0.8 GPa and 270 ± 10 MPa, respectively (Table 1). This measured tensile modulus of BC nanopaper falls in the range of values reported by various authors between 9.2[31] and 18 GPa.[32] However, it is still unclear as to why the published tensile moduli of BC nanopapers vary so significantly. The tensile strength of BC nanopaper measured in this study, on the other hand, is one of the highest reported and is in good agreement with the tensile strength obtained by Yamanaka et al.[32] The low tensile modulus and high tensile strength of our

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ρ [g cm−3]

Sample

E [GPa]

1.21 ± 0.01 3.9 ± 0.2

Neat PLLA

σ [MPa]

ε [%]

55 ± 6

1.6 ± 0.2

6.9 ± 0.5 125 ± 12

4.4 ± 0.4

BC nanopaper 1.61 ± 0.03 9.5 ± 0.8 270 ± 10

6.2 ± 0.2

a)

NanoPaPreg

f, BC = 65 ± 2 vol.%. The loading fraction was determined first by measuring the weight of BC nanopaper and the weight of the composites. Given the constituent’s bulk density, the loading fraction can be calculated.

a)v

BC nanopaper can be ascribed to the consolidation of the nanopaper during its manufacturing. It appears that applying a low consolidation pressure of only 9.81 kPa prevents damage to the nanofibers, resulting in a high tensile strength. However, this low consolidation pressure also resulted in shrinkage of the nanopapers during drying and hence affects the elasticity of the nanopaper,[33] leading to the observed lower tensile modulus. These results also corroborate the observations made by Yamanaka et al.,[32] who reported that increasing the consolidation pressure from 49 MPa to almost 2000 MPa led to a 7% increase in tensile modulus but a 65% decrease in tensile strength. The BC nanopaper also possesses a high strain-to-failure of 6.2 ± 0.2% (see Table 1). This is hypothesized to be due to the presence of fewer “crosslinking” points, i.e., hydrogen bonding sites, which formed at the physical contact points between individual BC nanofibers. This allows for the realignment of the nanofibrous network during tensile loading (see Figure S1, Supporting Information, for the stress–strain curve). The tensile properties of neat PLLA and nanoPaPreg containing 65 ± 2 vol% BC are also tabulated in Table 1. It can be seen from Table 1 that the nanoPaPregs have a tensile modulus and strength of E = 6.9 ± 0.5 GPa and σ = 125 ± 12 MPa, respectively. In order to ascertain that this improvement is due the introduction of BC nanopapers into PLLA, we then determined the crystallinity of PLLA (χc). This is because trans-crystallization of polymers is known to occur at the surface of nanocellulose.[34] The introduction of BC could result in a significant increase in χc beyond that of neat PLLA, which would affect the polymer tensile properties.[35] This would complicate the delineation of the effects of polymer crystallinity and/or the reinforcing ability of BC when analysing the measured tensile properties of nanoPaPreg. The thermal properties of PLLA including its glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures, as well as the enthalpy of crystallization (ΔHc), enthalpy of melting (ΔHm) and χc are summarized in Table S1

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(also see Table S1, Figures S2 and S3, Supporting Information, for the DSC traces of PLLA and nanoPaPreg). We found that the crystallinity of neat PLLA and the nanoPaPreg are similar, indicating that the observed improvement in the tensile properties of the nanoPaPreg can be attributed to the introduction of BC. In addition to this, the Tg of PLLA is not significantly affected by the introduction of BC nanopaper. However, this raises an important question; given the high stiffness and strength of the BC nanofibers and the high BC volume fraction vf = 65 vol% in our nanoPaPreg, it is surprising that the tensile properties of the nanocomposites only doubled when compared with neat PLLA. In order to further elucidate this, micromechanical models were used to verify the tensile stiffness and strength of our nanoPaPreg. The Cox–Krenchel model[36,37] was developed based on classical shear-lag theory with the assumptions that i) the fiber and matrix respond elastically, ii) there is a perfect fiber–matrix interface, and iii) there are no axial loads on the ends of the fibers. The Cox– Krenchel model can be expressed as: Ecomposite = n0nL v f E f + (1 − v f ) Em

(1)

where Ecomposite, η0, ηL, vf, Ef, and Em represent the predicted tensile modulus of the composite, the fiber orientation factor, the fiber volume fraction, the tensile modulus of the fiber and matrix, respectively. The limited stress transfer efficiency caused by finite fiber length, ηL, can be obtained from the “shear-lag” model: ⎛ βL⎞ tanh ⎜ ⎟ ⎝ 2 ⎠ nL = 1 − βL 2

β=

2⎡ 2 × Gm ⎤ ⎥ d⎢ ⎛ ⎞ π ⎢ E f 1n ⎥ ⎜ X ×v ⎟ ⎥ ⎢ i f ⎠ ⎝ ⎣ ⎦

Gm =

Em 2 × (1 + v )

(2)

0.5

(3)

(4)

where L is the fiber length, d is the fiber diameter, Gm is the shear modulus of the matrix, Xi is the packing of fibers in the composites, and ν is the Poisson ratio of the matrix. Xi was taken to be 3/2 as this value was used in the original publication by Cox.[36] This value is derived assuming hexagonal fiber packing within the composite with a mean center-to-center spacing of R. The input parameters for the prediction of the tensile modulus of the nanoPaPreg are η0 = 3/8 (assuming in-plane isotropic fibers in the nanocomposites), L = 0.005 mm (obtained from scanning electron micrographs), ν = 0.34[38] and d = 0.00005 mm (obtained from scanning electron micrographs),

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respectively. The model developed by Kelly and Tyson[39] can be written as: ⎡ Lv ⎛ L ⎞⎤ σ composite = ⎢ ∑ i i + v j ∑ ⎜ 1 − c ⎟ ⎥σ f + (1 − v f )σ m 2 L 2 L j ⎠ ⎥⎦ c j ⎝ ⎢⎣ i

(5)

where σcomposite is the predicted strength of the composite, σf is the fiber strength, σm is the matrix strength, and νf is the fiber volume fraction. In this equation, Lc represents the critical fiber length. νi is the fiber volume fraction of fibers of length Li, which is shorter than the critical fiber length, and νj is the fiber volume fraction of fibers of length Lj, which is longer than the critical fiber length. However, this equation was first developed for aligned discontinuous fiber-reinforced composites. In order to account for the random orientation of fibers, a virtual factor (η0,v)[40] should be introduced: ⎡ Lv ⎛ L ⎞⎤ σ composite = n0 ,v ⎢ ∑ i i + v j ∑ ⎜ 1 − c ⎟ ⎥σ f + (1 − v f )σ m 2 L 2 L j ⎠ ⎥⎦ ⎝ c j ⎢⎣ i

(6)

While the critical length of BC is not known, it may be assumed that this critical length should be similar to that of TEMPO-oxidized tunicate whiskers, which was found to be ≈1500 nm.[41] In addition to this, the percolation threshold for nanocellulose was estimated to be 1–6 vol%. Since vf = 65 vol% for the nanoPaPreg, Equation (6) can be re-written such that L → ∞:

σ composite = n0 ,v v fσ f + (1 − v f )σ m

(7)

The predicted tensile modulus and strength from these micromechanical models are summarized in Table 2. A negative deviation of the measured tensile properties of our nanoPaPreg from these predicted values can be observed. While it is right to question the assumptions made during the derivation, the simplification and applicability of these equations, it is clear from this micromechanical model analysis that the individual BC fiber stiffness and strength was not fully utilized in our nanoPaPreg. The situation can further be elucidated when the tensile fracture surfaces of BC nanopaper and its nanoPaPreg are studied (Figure 2). From this figure, it can clearly be seen that the fractured surface of the nanoPaPreg resembles that of BC nanopaper and clear layers of PLLA are observed. This structure Table 2. Predicted tensile modulus (Epredicted) and strength (σpredicted) of a PLLA/BC nanoPaPreg using Cox–Krenchel, Kelly–Tyson models and rule-of-mixture, respectively.

Model used

Epredicted [GPa]

σpredicted [MPa]

Cox–Krenchel

27.5 ± 2.8



Kelly–Tyson Rule-of-mixture

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212 ± 23

7.5 ± 0.8

192 ± 21

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modification[45] but BC modifications are rather laborious and add extra cost to the manufacturing of BC-reinforced PLLA nanocomposites. Moreover, significant bulk modification of BC will occur when it was first freeze-dried from water, followed by subsequent dispersion in the appropriate reaction medium.[46] Solvent exchanging BC-inwater through a second organic solvent into the reaction medium is required for surface-only modification of BC. However, the “green credentials” of the resulting modified BC-nanopaper-reinforced PLLA composites will be lost and potentially no longer economically/commercially viable to manufacture.

4. Conclusions

Figure 2. Scanning electron micrographs of the fracture surfaces of BC nanopaper (top) and nanoPaPreg (bottom). The scale bars represent 5 μm. The inset in the bottom figure shows an exemplary photo of the produced “nanoPaPreg” with 60 × 15 mm and a thickness of 0.1 mm.

resembles that of the Voigt model[42] used for the derivation of the rule-of-mixture (ROM) for fiber-reinforced composites under equal strain conditions. Indeed, if conventional ROM using the tensile properties of BC nanopaper instead of BC nanofibers were used to predict the tensile modulus, an even better agreement (within error) with the measured tensile modulus of nanoPaPreg is observed, particularly compared with the Cox–Krenchel model (Table 2). This fact also implies that BC nanopapers are difficult to infuse or impregnate by the rather viscous polymer melt. The tensile strength predicted by ROM is still significantly higher than that measured for the nanoPaPreg. Pull out of BC nanopaper from the PLLA matrix can also be seen, implying that the fiber–matrix interface between PLLA and BC nanopaper is still rather poor. Anyhow, even when using neat BC nanopaper reinforcements, strong and stiff nanocomposites can be produced (our data are among the top 5 reported for BC nanocomposites in the literature if our data points are inserted into Figure 1). However, there is still room for improvement. For instance, the adhesion between the nanopaper and polymer could be improved by chemical surface modification of BC[43,44] or by PLA

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We have shown in this study that high-performance nanocellulose-reinforced PLLA containing high vf of 65 vol% can be fabricated. This was achieved by laminating BC nanopapers with thin PLLA films. It was found that the nanofibrous network of BC is difficult to impregnate with PLLA. As a result, only the tensile properties of BC nanopaper can be utilized, rather than those of individual nanofibers. Nonetheless, the simplicity of the manufacturing process used in this study, i.e., papermaking, film casting, and compression molding, coupled with the performance of the resulting nanocomposites is expected to improve the costto-performance ratio of these nanoPaPregs. Comparing our results to all the BC-reinforced polymer nanocomposites in literature (Figure 1), our simple lamination method still does produce high-performance BC-reinforced PLLA nanocomposites that does not involve any wet impregnation method.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author and includes details of the PLLA and nanoPaPreg characterizations, stress-strain curves and DSC traces of the materials. Acknowledgements: The authors would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) (EP/ J013390/1, EP/H00713X/1, EP/K014676/1), Imperial Innovations POC fund (MT) and the University of Vienna for funding (KYL).nanoPaPreg (see Table S1, Figures S2 and S3, Supplementary Information for the DSC traces of PLLA and nanoPaPreg). Received: March 26, 2014; Revised: May Published online: ; DOI: 10.1002/marc.201400181

23,

2014;

Keywords: Bacterial cellulose; mechanical properties; modeling; nanocomposites; polylactide

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Bacterial cellulose nanopaper as reinforcement for polylactide composites: renewable thermoplastic NanoPaPreg.

Bacterial cellulose (BC) is often regarded as a prime candidate nano-reinforcement for the production of renewable nanocomposites. However, the mechan...
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