Supporting information for

Transcription

1 Supporting information for High-performance and moisture-stable cellulosestarch nanocomposites based on bioinspired coreshell nanofibers Kasinee Prakobna, 1, 2 Sylvain Galland, 1, 2 and Lars A. Berglund * 1, 2 1Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE Stockholm, Sweden 2Wallenberg Wood Science Centre, KTH Royal Institute of Technology, SE Stockholm, Sweden blund@kth.se 1

2 Schematic illustration of CNF and AP Figure S1. Schematic illustration of structures of cellulose nanofibers (CNF) and amylopectin (AP) Cellulose nanofibers (CNF) have been widely used as reinforcement materials in nanocomposites. CNF consists of highly aligned cellulose molecules, in the form of glucan polymers of D-glucopyranose units linked together by β-(1-4)-glucosidic bonds. Amylopectin (AP) is one of the components of starch. AP molecules are highly branched polysaccharide polymers. The backbone of AP is a glucan polymer of D-glucopyranose units linked together by α-(1-4)-glucosidic bonds, while the branching take place with α-(1-6) bonds. Molecular weight analysis of AP Table S1. Properties of the native amylopectin (AP) and its branched constituent analyzed by SEC-MALLS combined with enzymatic debranching. Native AP Debranched AP 2

3 Amylose fraction (wt%) Mw (g/mol) 1.86 x x 10 3 Mn (g/mol) 6.77 x x 10 3 PDI DP 1.03 x Amylopectin (AP) is a highly branched polysaccharide composed of (1 4)-α-glucosidic segments linked through (1 6)-α-glucosidic bonds at branching points. In the present study, commercial amylopectin from maize was purchased from Sigma Aldrich (GmbH, Germany). It is believed that the difference in fine structure such as molecular weight of AP may dictate to largely distinctive mechanical properties in the final material. In order to gain information on our matrix component, the structural analysis of native AP and its branched constituent was carried out using SEC-MALLS combined with enzymatic debranching. The analytical method was described by Vilaplana et al (2010). As the SEC-MALLS system was performed in DMSO, there is no concern for the aggregation of AP during the measurement. The result obtained from SEC- MALLS analysis is shown in Table S1. The weight- and number average molecular weight (Mw and Mn) of native AP are 1.86 x 10 8 and 6.77 x 10 7 g/mol, respectively. A fraction of amylose (7.4 wt%) was also separated from the native AP and appearing in the corresponding chromatogram. It is possible to characterize the branch segment of AP via selective cleavage of the branching position. The debranched chains show Mw and Mn of 6.73 x 10 3 and 3.56 x 10 3, respectively. There is also a large difference of DP in the native AP (1.03 x 10 6 ) and debranched 3

4 AP (37.4). The result suggests that the native AP is a hyperbranched polysaccharide consisting of enormous short branch chains. Kinetic association of AP on CNF Figure S2. Effect of mixing time and initial concentration of AP on their adsorption on CNF. The binding capacity of AP on CNF in an aqueous suspension was studied. The experiments were conducted with the mixture of CNF/AP suspension at 10 wt% and 50 wt% AP. At low AP content, all of the introduced AP was successfully adsorbed on CNF as indicated in Figure S2. The term adsorption may in the present case refer to a looser form of association than in classical adsorption. The effect of initial AP concentration on binding capability was observed and it shows concentration dependence characteristic. It was observed that the adsorbed amount of AP at two different concentrations was abruptly increased at the initial stage of mixing and remains constant after 6 hr. We conducted the kinetic study to ensure that the equilibrium adsorption was reached within 24 hr, which is an applied condition for our study. XRD measurement of CNF, AP and Core-shell CNF/AP nanocomposite films 4

5 Figure S3. XRD diffractograms of CNF, AP, and Core-shell CNF/AP 23 wt% nanocomposite. Table S2. Crystallite size and degree of crystallinity compared between Neat CNF film and Core-shell CNF/AP nanocomposite. Sample Nanocomposite 110 C C2 200 C3 Degree of (nm) (nm) (nm) crystallinity Neat CNF CNF/AP 23 wt% Core-shell X-ray diffraction measurements were carried out to characterize the crystallinity of CNF, AP and Core-shell CNF/AP nanocomposite films. The diffraction patterns of those films were measured from rotating specimen. The X-ray diffractograms of CNF and CNF/AP film exhibit a typical pattern of cellulose I (see Figure S3). The presence of adsorbed AP in the Core/shell CNF/AP nanocomposite film results in decreased total crystallinity, since the cellulose content is decreased and AP is amorphous (see Figure S3). The decrease in crystallinity from 0.58 to

6 is consistent with the AP content (23 wt%) in the nanocomposite film. Also, as expected, there is no significant change in crystallite size of the cellulose (see Table S2). Density and porosity measurements The density of all films was determined by measuring their air-dried weight and dividing by their volume. The volume of all samples was obtained by measuring their dimensions using a digital caliper and micrometer pressure gauge. Three measurements were made of each dimension, and an average was calculated for one specimen. At least five specimens were used for density measurement of each sample, and the reported data was an average value of all specimens. Porosity of CNF and CNF/AP fillms was estimated from the density of material according to the formula: Porosity = 1 - Where is the density of CNF cellulose and taken as 1500 kg.m -3. 6

7 Observation of fracture surface Figure S4. FE-SEM cross-section images of tensile fracture samples of: (a) Neat CNF, (b) Coreshell CNF/AP 23 wt%, and (b) ``Mixed CNF/AP 25 wt%. Figure S5. TEM positive and negative images of RuO4 stained ultra-thin sections of: (a, b) Coreshell CNF/AP 23 wt%, and (c, d) ``Mixed CNF/AP 25 wt%, respectively. 7

8 Figure S6. Proposed schematic model of the nanocomposite prepared from Core-shell CNF/AP nanofibers. The tensile fracture samples of CNF and CNF/AP nanocomposite films were investigated by fractography. The features of the CNF lamellae show that they are important units and that the pull-out lenghts are larger for Neat CNF than for the Core-shell CNF/AP composite. This indicates that adhesion between CNF lamellae are improved as AP is added. In Mixed CNF/AP, lamellae features are less distinct. There are also signs of AP-rich regions in the Mixed CNF/AP. Figure S4 is in agreement with the hypothesis that the structure in Core-shell CNF/AP is homogeneous at a finer scale than in Mixed CNF/AP. This apparently translates to better tensile properties under humid conditions. TEM positive and negative images of Coreshell CNF/AP and Mixed CNF/AP after being exposed to RuO 4 staining and cryogenic ultramicrotome cutting are shown in Figure S5. At microscale, it can be clearly observed that the structure of Core-shell CNF/AP is much more homogeneous than that of Mixed CNF/AP. The proposed schematic model of nanostructure in the Core-shell CNF/AP nanocomposite is 8

9 presented in Figure S6. Individual CNF/AP nanofibers form larger bundles in the Core-shell CNF/AP nanocomposite. Moisture sorption of Core-shell CNF/AP and ``Mixed CNF/AP nanocomposites Table S3. Moisture sorption of CNF/AP nanocomposite films at various RH%. Difference between the experimental data and calculated values based on rule of mixture are compared. RH% Core-shell CNF/AP 15 wt% Simply mixed CNF/AP 25 wt% Measured value Calculated value Measured value Calculated value Moisture sorption behavior of CNF/AP nanocomposite films were studied using dynamic vapor sorption testing within the relative humidity range of RH%. Core-shell CNF/AP 15 wt% and ``Mixed CNF/AP 25 wt% were selected for this study. The experimental data of moisture sorption is compared with the calculated moisture content for the CNF/AP composite (M CNF/AP ) based on the rule of mixtures; M CNF/AP = M CNF W CNF + M AP W AP where M is measured moisture content for the components, W is weight fraction and the subscripts CNF and AP refer 9

10 to the components. For the Core-shell nanocompsite, it is interesting to observe that the measured value is lower than the calculated value, especially at high RH%. On the other hand, the ``Mixed nanocomposite shows higher moisture sorption compared to the expected value based on rule of mixture. It suggests that the molecular interaction between adsorbed AP and CNF is favorable. As a consequence, water molecule has less accessibility to the material, which resulted in lower moisture sorption on the Core-shell nanocomposite. Mechanical properties in tension Table S4. Tensile properties of CNF, AP, and CNF/AP nanocomposite films at 50 RH% (values in parentheses are standard deviations). Note that yield strength is determined based on intersection of the slope in elastic and plastic regions. Mixed CNF/AP Core-shell CNF/AP Tensile properties Neat CNF AP nanocomposites nanocomposites AP weight fraction (%) AP weight fraction (%) Density (kg/m 3 ) Porosity (%) 8.5 NA Modulus (GPa) (0.66) (0.24) (0.81) (0.74) (0.52) (0.11) (0.34) (0.22) 10

11 Tensile strength (MPa) (26.2) (4.2) (26.2) (20.3) (12.0) (17.9) (23.2) (17.1) Yield strength (MPa) (1.8) (2.0) (7.0) (7.8) (8.5) (5.8) (4.3) (1.8) Elongation (%) (1.5) (1.8) (2.1) (1.3) (1.1) (1.3) (1.7) (1.5) Work of fracture (MJ/m 3 ) (3.8) (0.7) (3.7) (2.3) (1.0) (3.2) (3.7) (3.1) Table S5. Tensile properties of CNF, AP, and CNF/AP nanocomposite films at 85 RH% (values in parentheses are standard deviations). Note that yield strength is determined based on intersection of the slope in the elastic and plastic regions. ``Mixed CNF/AP Core-shell CNF/AP Tensile properties Neat CNF AP nanocomposites nanocomposites AP weight fraction (%) AP weight fraction (%) Modulus (GPa)

12 (0.73) (0.34) (1.05) (0.44) (0.38) (0.49) (0.54) (0.46) Tensile strength (MPa) (26.2) (2.7) (20.0) (12.1) (2.9) (7.8) (12.0) (12.4) Yield strength (MPa) (2.0) (1.5) (7.0) (3.3) (1.0) (3.0) (4.1) (1.8) Elongation (%) (1.5) (0.6) (1.7) (1.7) (0.3) (0.5) (0.7) (0.6) Work of fracture (MJ/m 3 ) (3.8) (0.2) (2.1) (1.8) (0.2) (1.0) (1.3) (1.3) Dynamic mechanical analysis (DMA) with humidity scanning The dynamic mechanical testing was carried out using a Perkin-Elmer DMA 7. The experiment was performed with humidity scanning in the range of 5-90 RH%. The testing was carried out in tension mode at 30 C. Constant amplitude of 2 µm was set. The static load was adjusted to be equal to 120 % of the dynamic load. A frequency of 1 Hz was applied. To release residual stresses from the material, all specimens were first subjected to a fast humidity scanning (5-90 RH%) with a ramp rate of 60 RH%/hr. Consecutively, the specimens were conditioned at initial humidity of 5 RH% for 3 hr. After conditioning, a slow humidity scanning (5-90 RH%) with a ramp rate of 5 RH%/hr was set. The storage modulus of all samples was recorded. During the slow scanning, the result was calculated and reported as relative storage modulus. 12

13 Figure S7. Relative storage modulus as a function of relative humidity for Neat CNF, Core-shell CNF/AP 23 wt%, and ``Mixed CNF/AP 26 wt% Relative storage modulus as a function of relative humidity is presented in Figure S7 for Neat CNF, Core-shell CNF/AP 23 wt% and ``Mixed CNF/AP 36 wt%. The relative storage modulus of Neat CNF decreased with increasing relative humidity. The softening behavior of Core-shell CNF/AP 23 wt% is fairly similar as compared to the Neat CNF. However, the Core-shell CNF/AP nanocompostie exhibited higher relative storage modulus throughout the entire range of relative humidities. The ``Mixed CNF/AP 36 wt% shows distinctive softening behavior, which is different from the other two materials. Initially, the Mixed CNF/AP shows an unexpected increase in modulus with increased humidity. Possibly, the reason is film shrinkage as the AP is plasticized by moisture. The non-adsorbed polymer may increase its entropy by a change in conformation towards a state more similar to a random coil. Most likely, the non-adsorbed AP in bulk state is highly sensitive to moisture. As this phase is plasticized, its modulus decreases 13

14 substantially. The stress transfer between CNF nanofibers is less efficient and the modulus decreases more rapidly with relative humidity. Thermomechanical properties Figure S8. (a) Storage modulus and (b) Tan δ of films for CNF, AP, Core-shell CNF/AP and ``Mixed CNF/AP nanocomposites The films of CNF, AP, Core-shell CNF/AP and ``Mixed CNF/AP nanocomposites were characterized by dynamic mechanical analysis (DMA). In Figure S8(a), there seems to be no difference in E between CNF and all CNF/AP nanocomposites. Their modulus was almost constant over a wide range of temperature (-100 to 250 C). It is apparent that AP has lower storage modulus than CNF-based films. Thermal degradation of AP was observed around 280 C. For this reason, it is difficult to distinguish differences in thermal relaxation behavior between the two nanocomposites. Their expected glass transition temperature (T g ) is in the same range as the thermal degradation process, see Figure 8(b). 14