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Cellulose whiskers reinforced polyvinyl alcohol copolymers nanocomposites
European Polymer Journal 44 (2008) 2489–2498
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Cellulose whiskers reinforced polyvinyl alcohol copolymers nanocomposites Mehdi Roohani a,b, Youssef Habibi a, Naceur M. Belgacem a, Ghanbar Ebrahim b, Ali Naghi Karimi b, Alain Dufresne a,*
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 14 May 2008 Accepted 26 May 2008 Available online 2 June 2008
Keywords: Nanocomposite Cellulose Polyvinyl alcholol Whiskers
a b s t r a c t Nanocomposite materials were prepared from copolymers of polyvinyl alcohol and polyvinyl acetate and a colloidal aqueous suspension of cellulose whiskers prepared from cotton linter. The degree of hydrolysis of the matrix was varied in order to vary the hydrophilic character of the polymer matrix and then the degree of interaction between the filler and the matrix. Nanocomposite films were conditioned at various moisture contents, and the dynamic mechanical and thermal properties were characterized using dynamic mechanical analysis and differential scanning calorimetry, respectively. Tensile tests were performed at room temperature to estimate mechanical properties of the films in the non linear range. All the results show that stronger filler/matrix interactions occur for fully hydrolyzed PVA compared to partially hydrolyzed samples. For moist samples, a water accumulation at the interface was evidenced. The reinforcing effect was found to be all the higher as the degree of hydrolysis of the matrix was high. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Poly(vinyl alcohol) (PVA) is the largest synthetic watersoluble polymer produced in the world [1]. It is prepared by the hydrolysis of polyvinyl acetate. The degree of solubility, and the biodegradability as well as other physical properties can be controlled by varying the molecular weight (MW) and the degree of hydrolysis (saponification) of the polymer [2]. Indeed, the chemical characteristics of these polymers, e.g. the reactivity of the numerous hydroxyl groups depends strongly on the residual acetyl group content or the degree of hydrolysis. In practice, the partially hydrolyzed grades can be considered as copolymers of vinyl alcohol and vinyl acetate. In the range from about 97 to 100 mol% (or fully hydrolyzed grades) hydrolysis the
* Corresponding author. Tel.: +33 4 76 82 69 95; fax: +33 4 76 82 69 33. E-mail address: [email protected] (A. Dufresne). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.05.024
relationship between the degree of hydrolysis and properties of the ensuing polymers produces very clear differences in the property profiles17 opening the way of using them in a broad field of applications, of among which matrix for biodegradable nanocomposites. In recent years, nanocomposites have attracted significant scientific attention because of their phenomenal electrical, barrier and mechanical properties. They are defined as composite materials for which one of the phases has at least one dimension in the nanometer range (1–100 nm). A large variety of nanocomposites have been prepared using PVA as a matrix and nanoreinforcement like layered silicate [4–8], silica [9–11], cadmium sulfide nanoparticles [12] and carbon nanotubes [13–15]. The preparation methods are usually solution casting or in situ polymerization. As most of the present-day nanofillers used to prepare nanocomposites are inorganic, their processability, biocompatibility and biodegradability are much more limited than those of naturally organic ones.
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a Ecole Française de Papeterie et des Industries Graphiques, Institut National Polytechnique de Grenoble (EFPG-INPG), BP65, 38402 Saint-Martin d’Hères Cedex, France b Department of Wood and Paper Technology, Faculty of Natural Resources, The University of Tehran, Karaj, P.O. Box 31585-4314, Iran
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In nature, a large number of animals and plants synthesize extracellular high-performance skeletal biocomposites that consist of a matrix reinforced by fibrous biopolymers. In fact, during the last decade there has been a growing interest in using cellulose nanocrystals or whiskers [16– 18]. Rånby and Ribi [19,20] were the first to produce stable suspensions of colloidal-sized cellulose crystals by sulfuric acid hydrolysis of wood and cotton cellulose. Using acid hydrolysis, native cellulose suspensions have been prepared from a variety of sources, including bacterial cellulose [21,22], microcrystalline cellulose [23], sugar beet primary cell wall cellulose [24], cotton [25], tunicate cellulose (or tunicin) [26] and softwood pulp (mostly black spruce) [27]. The investigation for preparing cellulose nanocomposites started around 1994. Nanocomposites were prepared by solution casting of various matrices and nanoreinforcements in aqueous medium [26,28]. Favier et al. [26] were the first to use cellulose whiskers from the tunic of tunicate Microcosmus fulcatus (i.e. tunicin whiskers) as reinforcing nanofillers in a copolymer of styrene and butyl acrylate. Polyvinyl acetate is well suited for blends with cellulose whiskers since it is highly polar and can be dispersed in water solutions. Sriupayo et al. [29] prepared and characterized achitin whisker reinforced PVA nanocomposite films with or without heat treatment. Improved thermal stability, tensile strength and water resistance were observed. The presence of the a-chitin whiskers was found to decrease the elongation at break and they did not have any effect on the crystallinity of the PVA matrix. Kvien and Oksman [30] prepared unidirectional reinforced nanocomposite by orientation of cellulose whiskers in PVA. Cellulose nanocomposites based on cellulose nanocrystals from sisal whiskers and polyvinyl acetate (PVAc) prepared by solution casting was also reported [31]. A stabilization of the polymeric matrix towards water absorption was observed and it was suggested that this stabilization could enhance the behavior of polar polymers under humid conditions and widens their applicability. The preparation and characterization of pea starch nanocrystals reinforced PVA nanocomposites were also reported [32]. Improved physical properties were observed. In the present study, we investigated the thermal and mechanical properties of nanocomposite films obtained from cotton cellulose whiskers and different grades of PVA. Variation in the degree of hydrolysis changes the residual ester content and amount of hydroxyl groups of copolymers. Because of the mutual affinity of PVA and cellulose whiskers with hydroxyl-rich surfaces, it is anticipated that strong interactions may form between cellulose whiskers and PVA copolymers that can be tailored by adjusting the degree of hydrolysis. The resulting hydrogen bonds should strengthen the interface significantly with a positive impact on the mechanical properties of the composite material.
sis were used as matrix. Two of them were purchased from Aldrich and four of them were kindly provided by Clariant GmbH (Muttenz, Switzerland) with trade name MowiolÒ (Table 1).
2. Experimental section
PVA grade
Codification
Mw (103 g mol1)
Degree of hydrolysis (%)
2.1. Materials
Aldrich Aldrich Mowiol Mowiol Mowiol Mowiol
1 2 3 4 5 6
146–186 124–186 100 150 175 145
99 89 81.5 87.5 92.4 99.4
2.1.1. Polymer matrices Six different grades of PVA copolymers with different molecular weights (Mw) and different degrees of hydroly-
2.1.2. Cellulose whiskers Cellulose nanocrystals or whiskers were extracted from cotton linter. Colloidal suspensions of whiskers in water were prepared as described elsewhere [33–35]. The cotton linter was milled with a laboratory milling device to obtain fine particulate substance. The cotton fibers were extracted in a 2 wt.% aqueous NaOH solution for 12 h at room temperature under mechanical stirring and then filtered and rinsed with distilled water after this treatment step. Acid hydrolysis was done in 65 wt.% aqueous sulfuric acid solution (11 wt.% cotton fibers). The resulting suspension was held at 45 °C under mechanical stirring for 45 min to allow fibers hydrolysis. The yield of the cellulose whiskers, with respect to the initial amount of cotton linters, was 64%. The ensuing suspension was diluted with cold distilled water followed by centrifugation at 10,000 rpm and 10 °C for 20 min. This process was repeated until a pH = 4 was reached (5 times). Next, dialysis against distilled water was performed to remove free acid in the suspension, as detected by the neutrality of the dialysis effluent. The dispersion of whiskers was completed by an ultrasonic treatment using a Branson sonifier and the suspension was filtered over a N°1 fritted glass filter in order to remove residual aggregates. The resulting suspension was subsequently stored in the refrigerator after adding several drops of chloroform in order to avoid bacterial growth until used. The whisker content was determined by weighting aliquots of the solution before and after drying. 2.1.3. Nanocomposites processing Polyvinyl acetate copolymers aqueous solutions were prepared. Depending on the proportion of cotton whiskers in PVA matrix, a given amount of PVA (5 g) was dissolved in 80 g distilled water at 90 °C for 60 min under mechanical stirring. Then, the solutions were kept under stirring to reach room temperature. To obtain films with different compositions, the solutions were mixed with a specific amount of the aqueous dispersion of cotton whiskers and mixed for 30 min. The resulting mixture was cast in a Teflon mold and placed in a 35 °C oven to evaporate water. Cotton whisker content in the final composites were 0, 3, 6, 9 and 12 wt.% for PVA copolymers from Aldrich and 0, 3 and 12 wt.% for MowiolÒ matrix. The obtained films around 200–350 lm thick were demolded after approximately 72 h. Table 1 Codification and properties of PVA grades used as matrix
15–79 23–88 30–92 28–99
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2.1.4. Film conditioning As PVA films absorb moisture to different extents, depending on the water vapor partial pressure, precise conditioning must be ensured in any determination of mechanical and thermal properties [3]. In order to evaluate the effect of moisture content on the composite structure, the moisture content of the nanocomposite films was achieved by conditioning the samples at room temperature in desiccators at controlled humidity containing saturated salt solutions. Four relative humidity (RH) conditions were used, namely 0, 35, 75 and 98%. The saturated salt solutions were P2O5, CaCl2.6H2O, NaCl and CuSO4.5H2O, respectively. The temperature range was within 20–25 °C. Conditioning was achieved for at least two weeks to ensure the equilibration of the water content in the films with that of the atmosphere and controlled by the stabilization of the sample weight.
3. Results and discussion
2.2. Experimental methods
3.2. Morphological characterization of nanocomposite films
Scanning electron micrographs of cellulose whiskers were taken with a ZEISS-ULTRA55 scanning electron microscope with field emission gun (SEM–FEG) with an acceleration voltage of 15 kV. Whiskers were deposited from a droplet of a dilute suspension on a microgrid (200 mesh, Electron Microscopy Sciences, Hatfield, PA, USA). Scanning electron microscopy (SEM) was performed to investigate the morphology of the nanocomposite films with a JEOL JSM-6100 instrument. The specimens were frozen under liquid nitrogen, then fractured, mounted, coated with gold/palladium on a JEOL JFC-1100E ion sputter coater, and observed. SEM micrographs were obtained using 7 kV secondary electrons. Differential Scanning Calorimetry (DSC) was carried out with a DSC Q100 (TA Instruments, New Castle, DE, USA) equipped with a manual liquid nitrogen cooling system. Conditioned samples were placed in hermetically closed DSC crucibles and tested in the range of 50– 250 °C at a heating rate of 10 °C min1 under a nitrogen atmosphere. Sample weight was between 10 and 15 mg. The glass transition temperature (Tg) was taken as the inflection point of the specific heat increment at the glass–rubber transition while the melting temperature Tm was taken as the peak temperature of the melting endotherm. Three samples were used to characterize each material. Dynamic mechanical properties of the nanocomposite films were measured in tensile mode using a RSA3 DMA fitted with a LN2 cooling system. The measurements were carried out at a constant frequency of 1 Hz, a strain rate of 0.008%, previously established as the linear viscoelasticity domain of the material. A temperature range of 60– 250 °C, a heating rate of 5 °C min1 and distance between jaws of 5 mm were used. The samples were prepared by cutting rectangular strips from the films with a width of 5 mm. Tensile testing was carried out using a RSA3 DMA with a cross head speed of 0.05 mm s1 and an initial distance between jaws of 10 mm. The samples were prepared by cutting rectangular strips from the films with a width of 2–3 mm. Three samples were used to characterize each material.
The morphology of the nanocomposite films was characterized by SEM. Fig. 2 shows the fractured surface of the unfilled partially hydrolyzed PVA matrix (PVA 4, panel a) and related nanocomposite film reinforced with 12 wt.% cellulose whiskers (panel b), and unfilled fully hydrolyzed PVA matrix (PVA 6, panel c) and related nanocomposite film filled with 12 wt.% cotton whiskers (panel d), just after fracture. The freshly fractured surface of the unfilled films either partially or fully hydrolyzed PVA (Fig. 2A and C) is smooth and uniform. For nanocomposite films (Fig. 2B and D) no clear evidence of the presence of the cellulosic nanoparticles is observed. This is due to the scale of the reinforcing phase, but it also indicates that no aggregates are present in the nanocomposite films and that the dispersion of the filler within the polymeric matrix is rather good. However, the aspect of the fractured surface is different and appears to be more chaotic, mainly for the filled fully hydrolyzed nanocomposite (Fig. 2D). It could be ascribed to a higher degree of crystallinity of the material and/or strong interactions between the filler and the matrix leading to a fracture path through PVAcoated nanoparticles.
3.1. Whiskers characterization
Fig. 1. Scanning electron micrograph (SEM–FEG) of a dilute suspension of cotton whiskers.
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Fig. 1 shows an electron micrograph of the cotton whiskers. They occurred as rod-like nanoparticles which dimensions are known to depend on the origin of the cellulose [16]. The average diameter and length were calculated using digital image analysis (Axone). At least 500 measurements were carried out for both diameter and length. The average diameter and length of the cotton whiskers were approximately 14.6 ± 3.9 and 171.6 ± 48.2 nm, respectively. The presented errors are the standard deviations of the distributions. The average aspect ratio (L/d, L being the length and d the diameter) of these whiskers is therefore around 11–12. It is close to the values reported for cotton whisker in the literature [34].
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Fig. 2. Scanning electron micrographs from the fractured surfaces of PVA 4-based nanocomposite films reinforced with 0. (A) and 12 wt.%, (B) cotton whiskers, and PVA 6-based nanocomposite films reinforced with 0, (C) and 12 wt.% and (D) cotton whiskers.
3.3. Thermal analysis of nanocomposite films The thermal properties of cotton whiskers reinforced PVA copolymers were determined from DSC thermograms. Thermal characteristics are collected in Tables 2 and 3. The glass transition temperature of unfilled polymeric matrices and nanocomposite films reinforced with cotton whiskers up to 12 wt.% are reported in Table 2. Tg values are given for samples conditioned in different atmospheres. Regardless the composition, Tg decreases as the humidity content increases owing to the well-known plasticizing effect of water molecules for PVA chains. The two fully hydrolyzed PVAs (PVA 1 and 6) display the lowest Tg values in the dry state (0% RH conditioned samples). Tg values corresponding to the six different matrices conditioned at 0% RH are plotted in Fig. 3 as a function of the whisker content. For these dry samples, Tg of partially hydrolyzed samples remains roughly constant when adding cellulose whiskers, whereas those corresponding to fully hydrolyzed samples (PVA 1 and 6, Fig. 3) slightly increased. It could be ascribed to stronger interactions between the filler and the fully hydrolyzed matrices. In moist atmosphere, Tg is found to significantly increases when adding cotton whiskers regardless the degree of hydrolysis of the matrix (Table 2). It is most probably ascribed to the competitive interactions between PVA, water and cellulose whiskers surface. Both PVA/cellulose and water/cellulose interactions tend to shift Tg towards higher
Table 2 Glass transition temperature for MowiolÒ copolymer films reinforced with cellulose whisker as a function of whisker content and relative humidity Sample
Whiskers content (wt.%)
RH (%) 0
35
75
98
1
0 3 6 9 12
40.4 44.3 45.0 44.6 45.6
45.5 46.7 45.9 46.2 46.6
0.9 2.1 2.3 2.6 2.7
21.7 18.1 11.9 8.2 4.7
2
0 3 6 9 12
66.6 66.0 65.3 65.6 66.1
45.7 45.9 46.1 45.5 45.8
9.1 8.2 6.4 10.5 5.5
18.3 16.8 8.2 6.2 3.1
3
0 3 12
58.8 60.1 58.5
46.7 47.1 48.4
0.7 9.1 6.2
31.8 28.2 6.0
4
0 3 12
70.1 69.6 68.7
47.9 47.4 48.9
7.2 12.5 10.9
27.4 19.3 9.6
5
0 3 12
70.6 69.8 67.3
47.4 48.3 48.7
4.5 5.5 9.5
39.4 19.4 14.2
6
0 3 12
47.6 48.8 49.9
50.8 48.0 48.4
0.5 3.1 6.9
35.2 23.4 2.3
Table 3 Temperatures of the calorimetric transitions of cotton whiskers filled PVA conditioned at 0% RH and using data obtained from the DSC curves, melting temperature Tm, associated heat of fusion DHm and degree of crystallinity (Xc and Xp) Sample
Whiskers content (wt.%)
Tm (°C)
DHm (J g1)
Xc
Xp
1
0 3 6 9 12
226.6 225.6 223.1 222.4 222.0
47.5 44.1 44.0 42.8 39.8
0.294 0.272 0.272 0.264 0.246
0.294 0.281 0.289 0.290 0.279
2
0 3 6 9 12
190.9 189.9 190.3 188.0 190.0
23.5 23.0 19.8 19.4 17.4
0.145 0.138 0.123 0.120 0.107
0.145 0.142 0.130 0.132 0.122
3
0 3 12
185.9 182.9 184.1
20.4 18.7 14.0
0.126 0.116 0.086
0.126 0.119 0.098
4
0 3 12
191.1 191.1 190.8
27.0 25.8 21.4
0.167 0.159 0.132
0.167 0.164 0.150
5
0 3 12
203.6 203.1 198.9
37.2 30.8 24.9
0.230 0.190 0.154
0.230 0.196 0.175
6
0 3 12
229.7 228.1 224.9
62.2 54.3 48.6
0.385 0.336 0.300
0.385 0.346 0.341
Xc = DHm/DHm° and Xp = Xc/w; where DHm° = 161.6 J g1 is the heat of fusion for 100% crystalline PVA [from 190.3ATHAS Data Bank] and w is the weight fraction of polymeric matrix material in the composite.
80
Tg (ºC)
70 60 50
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Data Bank and was considered to be independent on the degree of hydrolysis. Two values of the degree of crystallinity were reported. The first one was calculated from the sample weight (Xc) and the other one (Xp) took into account the amount of matrix material in the composite. Both the melting point and the degree of crystallinity increased with increasing the degree of hydrolysis, as shown in Fig. 4 that displays the evolution of Tm (Fig. 4A) and Xc (Fig. 4B) as a function of the whiskers content for the different degrees of hydrolysis. This is ascribed to strong interactions between adjacent PVA chains by hydrogen bonding forces. As a consequence, highly crystalline polymers and bigger crystalline domains could be formed. The inverse trend was observed when dealing with PVAc, which is a totally amorphous polymer. Thus, the increase of the PVAc content decreases the extent of such interactions. Both the melting point and the degree of crystallinity of the matrix material tend to remain roughly constant or slightly decrease as the whiskers content increases. Tm values remains roughly constant for partially hydrolyzed materials when increasing the whiskers content whereas it decreases for fully hydrolyzed PVA matrices. Again, this is ascribed to stronger interactions between the cellulosic surface and polymeric matrix for the latter. These interactions most probably restrict the capability of the matrix chains to form big crystalline domains. A similar decrease of Tm was reported by Azizi Samir et al. [35], for tunicin whiskers reinforced poly(ethylene oxide) (PEO). The decrease of Tm was ascribed to two effects. The first one was a morphological phenomenon related to a decrease of the PEO crystallites size due to the formation of a close cellulose network within the matrix, mainly for high filler content. The second effect originated from the expected strong interactions between ether oxygen groups of PEO and hydroxyl groups of cellulose whiskers. 3.4. Dynamic mechanical properties of nanocomposite films
40 30
0
5
10
15
Whiskers Content (wt%) Fig. 3. Glass transition temperature of PVA/cotton whisker nanocomposites conditioned at 0% RH as a function of the whiskers content, PVA 1 (d), PVA 2 (s), PVA 3 (N), PVA 4 (D), PVA 5 (j) and PVA 6 (h). The solid lines serve to guide the eye.
temperatures. The former phenomenon is due to a lowering of the molecular mobility of PVA chains in the interfacial zone and the latter can result from a redistribution of water molecules within the matrix, diffusing toward the cellulosic surface. Such a relocalization effect can decrease the plasticizing effect of water as already reported for tunicin whiskers reinforced starch [36]. The values of the melting temperature Tm and corresponding heat of fusion, DHm, for nanocomposite films conditioned at 0% RH and reinforced with cotton whiskers are collected in Table 3. The degree of crystallinity of the material was calculated from DHm values. The value corresponding to a 100% crystalline PVA was taken from ATHAS
Favier et al. [26] reported the first demonstration of the reinforcing effect of cellulose whiskers. The authors measured by DMA in the shear mode a spectacular improvement in the modulus after adding tunicin whiskers into a thermoplastic matrix. These outstanding properties were ascribed to a mechanical percolation phenomenon [26,37], yielded by cellulosic nanoparticles interactions through hydrogen bond forces. Fig. 5 shows the evolution of the logarithm of the storage tensile modulus for nanocomposite films prepared using PVA 1 and conditioned at different relative humidity levels as a function of temperature. For low temperatures, it was difficult to observe any change in the storage modulus neither with moisture variation nor with filler content. As it is well known, the exact determination of the glassy modulus depends on the precise knowledge of the samples dimensions. In this case, and especially for highly moist samples, the films were soft and it was very difficult to obtain a constant and precise thickness along these samples. In order to minimize this effect, the glassy elastic tensile modulus was normalized at the observed average value, regardless the composition or moisture level.
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230
40 30
210
Xp (%)
Tm (º C)
220
200
10
190 180
20
0
5 10 Whiskers Content (wt%)
15
0
0
5
10
15
Whiskers Content (wt%)
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Fig. 4. (A) Melting temperature and (B) degree of crystallinity of PVA/cotton whisker nanocomposites conditioned at 0% RH as a function of the whiskers content, PVA 1 (d), PVA 2 (s), PVA 3 (N), PVA 4 (D), PVA 5 (j) and PVA 6 (h). The solid lines serve to guide the eye.
Fig. 5. Evolution of the logarithm of the storage tensile modulus as a function of temperature for PVA 1-based nanocomposite films reinforced with 0 (d), 3 (s), 6 (N), 9 (D) and 12 wt.% (j) of cotton whiskers and conditioned at (A) 0, (B) 35, (C) 75 and (D) 98% RH.
The unfilled PVA-based matrices display a typical behavior of semicrystalline polymer with four distinct zones. In the glassy state, the tensile storage modulus E’ slightly decreases with temperature but remains roughly constant. Then, the modulus drops at a temperature that depends on the moisture conditions. This relaxation phenomenon is associated to the glass–rubber transition of the PVA copolymer amorphous phase. The magnitude of this modulus drop is relatively weak because of the semi-
crystalline state of the material. Then, amorphous rubbery domains coexist with crystalline domains, which play the role of both filler particles and physical cross-links. At higher temperatures the modulus drops sharply and irremediably due to the melting of the crystalline zones of the PVA copolymer. Regardless the moisture content, the addition of cotton whiskers increases the rubbery modulus of the PVA matrix. It results in a lower modulus drop at Tg. Similar effects
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Ec ¼
ð1 2w þ wvR ÞES ER þ ð1 vR ÞwE2R ð1 vR ÞER þ ðvR wÞES
ð1Þ
where the subscripts S and R refer to the soft and rigid phase, respectively, i.e. polymeric matrix and filler. vR is the volume fraction of filler, and w and ER correspond to the volume fraction and modulus of the stiff percolating network, respectively. w can be written as
w ¼ 0 for vR < vRc b R vRc w ¼ vR v1v for vR vRc Rc
ð2Þ
where vRc and b correspond to the critical volume fraction at the percolation threshold and the corresponding critical exponent, respectively. It means that all the stiffness of the material results from the formation of infinite aggregates of cellulose whiskers. It is now of interest to compare the performances of cotton whiskers/PVA nanocomposites to the data predicted from this model based on the percolation concept. The critical volume fraction at the percolation threshold vRC can be calculated from the aspect ratio of the rod-like nanoparticles and b = 0.4 for the critical exponent in a three-dimensional problem according to the percolation theory [16,17]. With an aspect ratio of 11–12 it comes vRC = 6.4–5.8. A value of six was taken in the prediction. The densities of crystalline cellulose and PVA copolymers were taken as 1.5 and 1.3 g cm3, respectively. For the tensile modulus, ER, of a film of cellulose whiskers literature reports values ranging between 5 and 15 GPa depending on the source of cellulose [16,26,35,39]. An average value of 10 GPa was chosen for the calculation. Both experimental and predicted moduli are reported in Table 4 for PVA 1-based nanocomposite films reinforced with 12 wt.% cotton whiskers. The order of magnitude of the predicted data agrees with the experimental ones. However, the experimental values are systematically lower than predicted ones. It is a good indication that the cellulose whiskers percolation phenomenon is affected by filler/PVA interactions. These interac-
Table 4 Experimental and predicted rubbery modulus of PVA 1-based nanocomposite films reinforced with 12 wt.% cotton whiskers RH (%)
Experimental modulus (MPa)
Predicted modulus (MPa)
0 35 75 98
250a 292a 244b 93c
379 422 390 328
a b c
Estimated at 150 °C. Estimated at 100 °C. Estimated at 75 °C.
tions can partially hinder the formation of the expected percolating cellulosic network. However, these experimental data could also be affected by the low temperature storage modulus normalization. Also, it is found that for the moister nanocomposite sample (conditioned at 98% RH) the gap between experimental and predicted data is particularly significant. The high water content most probably leads to strong water/cellulose interactions that hinder more significantly the inter-whiskers interactions. Fig. 6 shows the evolution of the tangent of the loss angle (tan d) as a function of temperature for PVA 4-based nanocomposites conditioned at 0 (Fig. 6A) and 98% RH (Fig. 6B). The observed relaxation phenomenon is associated with the glass transition of the polymeric matrix. It is shifted to lower temperatures as the moisture content in the sample increases in agreement with DSC experiments. However, no shift of the peak position is observed for the moist samples upon whiskers addition, regardless the moisture content. This observation seems to disagree with DSC data reported in Table 2 for PVA 4 conditioned at 98% RH that showed an increase of Tg as the cotton whiskers increased. It is most probably ascribed to the wellknow mechanical coupling effect that tends to decrease the relaxation temperature as the filler content increases as a result of the decrease of the magnitude of the modulus drop at Tg [40]. These two effects probably counterbalance and cancel mutually. Also, it is found in Fig. 6 that the magnitude of the relaxation process strongly decreased when increasing the cotton whiskers content for the dried materials (Fig. 6A) whereas it remained roughly constant for the moister ones (Fig. 6B). It is generally observed that the magnitude of the main relaxation process is reduced in presence of the filler. It is ascribed to a decrease of the matrix material amount, responsible for damping properties, i.e. a decrease in the number of mobile units participating to the relaxation phenomenon. However, new damping mechanisms can be introduced by the filler particles [40]. Possible new damping mechanisms include: (i) particle–particle slippage or friction, (ii) particle–polymer motion at the filler interface and (iii) change in the properties of the polymer by adsorption onto the filler particle. It is generally accepted that if significant specific interactions between polymer and filler occur, this will tend to create a layer of polymer surrounding each filler particle. The resulting polymeric layer has different properties compared with those of bulk polymer. Assuming the dispersed phase particles to be rigid, this leads to an immobilized polymer layer contributing to the effective filler volume fraction in the compound. In
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were observed for each matrix regardless its hydrolysis state and the moisture content. Because the degree of crystallinity of the polymeric matrices was found from DSC measurements to remain roughly constant or even slightly decrease as the whiskers content increases, this increase of the relaxed modulus is most probably ascribed to a mechanical reinforcing effect of the filler. For highly moist samples, the rubbery modulus was found to increase with temperature because of the temperature scan and concomitant loss of water content during DMA test. Previous studies [16,17,26,37] have shown that the high reinforcing effect of cellulose whiskers reinforced nanocomposites can be well predicted following the adaptation of the percolation concept to the classical phenomenological series-parallel model of Takayanagi et al. [38]. The main advantage of this approach was to account for interactions between the fillers and for the hydrogen-bonding forces that hold the percolating cellulose whiskers network. Details of the calculation can be found elsewhere [16,26,39,40]. In this approach, the elastic tensile modulus Ec of the composite is given by the following equation
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0.4
tan δ
0.3
tan δ
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 50
0.2 0.1 0
60
70
80
-0.1 -40
90 100 110 120 130
Temperature (º C)
-20
0
20
40
60
Temperature (º C)
Fig. 6. Evolution of the tangent of the loss angle (tan d) as a function of temperature for PVA 4-based nanocomposite films reinforced with 0 (d), 3 (s) and 12 wt.% (N) of cotton whiskers and conditioned at (A) 0 and (B) 98% RH.
the light of these remarks, it is clear that filler/matrix interactions are much lower for the nanocomposite films conditioned at 98% RH compared to those conditioned at 0% RH. It could be ascribed to the formation of an aqueous or at least highly moist interface due to a redistribution of water molecules as already suggested from DSC measurements.
2
1 3.5. Tensile properties of nanocomposite films
0
75 0
3
Whiskers
6
9
Content
%
0 RH
Tensile Modulus (GPa)
12
(wt%)
Tensile Strength (MPa)
3 2.5 2 1.5 1
0
10 5 Whiskers Content (wt%)
15
120 100 80 60
Tensile tests were performed at room temperature for all nanocomposite films. All samples display an initial zone of elastic deformation. The stress–strain curves for dry samples exhibited a peak after a period of elastic deformation (yield) and the stress dropped and leveled. For wet samples it was followed by a nearly monotonically increase of the stress during the plastic deformation, up to
Strain at Break (%)
Tensile Modulus (GPa)
Fig. 7. Evolution of the Young’s modulus as a function of both the whisker content and relative humidity for nanocomposites PVA 1-based nanocomposite films reinforced with cotton whiskers.
Yield Stress (MPa)
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3
0
5
10
Whiskers Content (wt%)
15
140 120 100 80
0
10 5 Whiskers Content (wt%)
15
50 40 30 20 10 0
0
5
10
15
Whiskers Content (wt%)
Fig. 8. Evolution of the tensile mechanical properties for PVA 2-based nanocomposite films reinforced with cotton whiskers and conditioned at 0% RH, (A) tensile modulus, (B) tensile strength, (C) yield stress and (D) strain at break vs. cotton whiskers content. The solid lines serve to guide the eye.
2.5
2
1.5
1 80
85 90 95 100 Degree of Hydrolysis (%)
Fig. 9. Evolution of the relative tensile modulus of 12 wt.% cotton whiskers reinforced PVA nanocomposite films conditioned at 0% RH as a function of the degree of hydrolysis of the matrix. The solid lines serve to guide the eye.
the failure. These behaviors are classical for semicrystalline polymers mechanically characterized below and above their glass transition temperature, respectively. Fig. 7 shows the evolution of the Young’s modulus, as a function of whisker content and relative humidity for nanocomposite films prepared from PVA 1. The tensile modulus of nanocomposite films increased when increasing the whisker content for each relative humidity condition. For a given filler content, increasing the humidity resulted in a clear and significant decrease of the tensile modulus because of the shift of the glass transition temperature towards lower values, below the room temperature. Fig. 8 represents the evolution of the mechanical properties of PVA 2-based nanocomposite films conditioned at 0% RH. Compared to the unfilled PVA matrix, the addition of cotton whiskers induces an increase of the tensile modulus, tensile strength and yield stress. At the same time, the strain at break decreases from 29.5% for the unfilled matrix down to 9.1% for the nanocomposite film reinforced with 12 wt.% of cotton whiskers. Fig. 9 shows the evolution of the relative tensile modulus of PVA-based nanocomposite films reinforced with 12 wt.% of cotton whiskers and conditioned at 0% RH as a function of the degree of hydrolysis of the matrix. The relative tensile modulus is defined as the ratio of the modulus of the composite to the one of the unfilled matrix. It is clear that this reinforcing effect can be affected by factors such as the degree of crystallinity and Tg of the matrix. However, the relative tensile modulus tends globally to increase as the degree of hydrolysis of the PVA matrix increases. It could be ascribed to increased interactions between PVA chains and cellulose whiskers via hydrogen bonding. When increasing the degree of hydrolysis of the polymeric matrix, the number of hydroxyl groups increases and consequently, the number of hydrogen bonds increases. Such interactions lead to more rigid materials as the whiskers content increases. 4. Conclusion The goal of this work was to compare the mechanical and thermal properties of biopolymer based nanocomposites prepared from cotton whiskers and copolymers of polyvinyl alcohol and polyvinyl acetate. The cotton whis-
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kers, prepared by acid hydrolysis of cotton linter, consisted of slender rod-like nanoparticles. The average length and width of these whiskers were about 172 and 15 nm, respectively, leading to an average aspect ratio around 11–12. After blending the suspension of cotton whiskers with the solution of PVA nanocomposite films were prepared by a casting/evaporation technique. The samples were conditioned at different relative humidities (RH). All the results show that stronger filler/matrix interactions occur for fully hydrolyzed PVA compared to partially hydrolyzed samples. These stronger interactions result in an increase of the glass temperature, and a decrease of both the melting point and degree of crystallinity of the polymeric matrix in dry atmosphere. For moist samples, Tg increases significantly upon whiskers addition regardless the degree of hydrolysis of the matrix, because of the formation of a water layer at the interface, the PVA matrix becoming less plasticized by water. This water accumulation was also evidenced from DMA tests. A reinforcing effect of cotton whiskers was observed from both DMA and tensile tests. However, this reinforcing effect remains systematically lower than the prediction from a percolation approach. This is a good indication that the mechanical percolation of cellulose whiskers is affected by cellulose/ PVA interactions. The reinforcing effect was found to be all the higher as the degree of hydrolysis of the matrix was high.
Acknowledgments The authors thank Mrs. Bertine Khelifi, M. Raphaël Passas (EFPG-INPG) and Mrs. Francine Roussel-Dherbey (CMTC) for their help with the SEM–FEG imaging of the cotton whiskers and Michel Muguet (Air Liquide, France) for supplying MowiolÒ samples. Mehdi Roohani also thanks the Iranian Ministry of Science for its financial support through a Fellowship.
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[30] Kvien I, Oksman K. Appl Phys A 2007;87(4):641–3. [31] Garcia de Rodriguez NL, Thielemans W, Dufresne A. Cellulose 2006;13:261–70. [32] Chen Y, Cao X, Chang PR, Huneault MA. Carbohydr Polym 2008;73(1):8–17. [33] Dong XM, Kimura T, Revol JF, Gray DG. Langmuir 1996;12(8): 2076–82. [34] De Souza Lima MM, Wong JT, Paillet M, Borsali R, Pecora R. Langmuir 2003;19(1):24–9. [35] Azizi Samir MAS, Alloin F, Sanchez J-Y, Dufresne A. Polymer 2004;45(12):4033–41. [36] Anglès NM, Dufresne A. Macromolecules 2000;33(22): 8344–53. [37] Favier V, Chanzy H, Cavaillé JY. Macromolecules 1995;28(18): 6365–7. [38] Takayanagi M, Uemura S, Minami S. J Polym Sci C 1964;5: 113–22. [39] Dufresne A, Helbert W, Cavaillé JY. Polym Compos 1997;18(2): 198–210. [40] Dufresne A. Compos Interface 2000;7(1):53–67.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Cellulose whiskers reinforced polyvinyl alcohol copolymers nanocomposites Mehdi Roohani a,b, Youssef Habibi a, Naceur M. Belgacem a, Ghanbar Ebrahim b, Ali Naghi Karimi b, Alain Dufresne a,*
a r t i c l e
i n f o
Article history: Received 15 February 2008 Received in revised form 14 May 2008 Accepted 26 May 2008 Available online 2 June 2008
Keywords: Nanocomposite Cellulose Polyvinyl alcholol Whiskers
a b s t r a c t Nanocomposite materials were prepared from copolymers of polyvinyl alcohol and polyvinyl acetate and a colloidal aqueous suspension of cellulose whiskers prepared from cotton linter. The degree of hydrolysis of the matrix was varied in order to vary the hydrophilic character of the polymer matrix and then the degree of interaction between the filler and the matrix. Nanocomposite films were conditioned at various moisture contents, and the dynamic mechanical and thermal properties were characterized using dynamic mechanical analysis and differential scanning calorimetry, respectively. Tensile tests were performed at room temperature to estimate mechanical properties of the films in the non linear range. All the results show that stronger filler/matrix interactions occur for fully hydrolyzed PVA compared to partially hydrolyzed samples. For moist samples, a water accumulation at the interface was evidenced. The reinforcing effect was found to be all the higher as the degree of hydrolysis of the matrix was high. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Poly(vinyl alcohol) (PVA) is the largest synthetic watersoluble polymer produced in the world [1]. It is prepared by the hydrolysis of polyvinyl acetate. The degree of solubility, and the biodegradability as well as other physical properties can be controlled by varying the molecular weight (MW) and the degree of hydrolysis (saponification) of the polymer [2]. Indeed, the chemical characteristics of these polymers, e.g. the reactivity of the numerous hydroxyl groups depends strongly on the residual acetyl group content or the degree of hydrolysis. In practice, the partially hydrolyzed grades can be considered as copolymers of vinyl alcohol and vinyl acetate. In the range from about 97 to 100 mol% (or fully hydrolyzed grades) hydrolysis the
* Corresponding author. Tel.: +33 4 76 82 69 95; fax: +33 4 76 82 69 33. E-mail address: [email protected] (A. Dufresne). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.05.024
relationship between the degree of hydrolysis and properties of the ensuing polymers produces very clear differences in the property profiles17 opening the way of using them in a broad field of applications, of among which matrix for biodegradable nanocomposites. In recent years, nanocomposites have attracted significant scientific attention because of their phenomenal electrical, barrier and mechanical properties. They are defined as composite materials for which one of the phases has at least one dimension in the nanometer range (1–100 nm). A large variety of nanocomposites have been prepared using PVA as a matrix and nanoreinforcement like layered silicate [4–8], silica [9–11], cadmium sulfide nanoparticles [12] and carbon nanotubes [13–15]. The preparation methods are usually solution casting or in situ polymerization. As most of the present-day nanofillers used to prepare nanocomposites are inorganic, their processability, biocompatibility and biodegradability are much more limited than those of naturally organic ones.
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a Ecole Française de Papeterie et des Industries Graphiques, Institut National Polytechnique de Grenoble (EFPG-INPG), BP65, 38402 Saint-Martin d’Hères Cedex, France b Department of Wood and Paper Technology, Faculty of Natural Resources, The University of Tehran, Karaj, P.O. Box 31585-4314, Iran
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In nature, a large number of animals and plants synthesize extracellular high-performance skeletal biocomposites that consist of a matrix reinforced by fibrous biopolymers. In fact, during the last decade there has been a growing interest in using cellulose nanocrystals or whiskers [16– 18]. Rånby and Ribi [19,20] were the first to produce stable suspensions of colloidal-sized cellulose crystals by sulfuric acid hydrolysis of wood and cotton cellulose. Using acid hydrolysis, native cellulose suspensions have been prepared from a variety of sources, including bacterial cellulose [21,22], microcrystalline cellulose [23], sugar beet primary cell wall cellulose [24], cotton [25], tunicate cellulose (or tunicin) [26] and softwood pulp (mostly black spruce) [27]. The investigation for preparing cellulose nanocomposites started around 1994. Nanocomposites were prepared by solution casting of various matrices and nanoreinforcements in aqueous medium [26,28]. Favier et al. [26] were the first to use cellulose whiskers from the tunic of tunicate Microcosmus fulcatus (i.e. tunicin whiskers) as reinforcing nanofillers in a copolymer of styrene and butyl acrylate. Polyvinyl acetate is well suited for blends with cellulose whiskers since it is highly polar and can be dispersed in water solutions. Sriupayo et al. [29] prepared and characterized achitin whisker reinforced PVA nanocomposite films with or without heat treatment. Improved thermal stability, tensile strength and water resistance were observed. The presence of the a-chitin whiskers was found to decrease the elongation at break and they did not have any effect on the crystallinity of the PVA matrix. Kvien and Oksman [30] prepared unidirectional reinforced nanocomposite by orientation of cellulose whiskers in PVA. Cellulose nanocomposites based on cellulose nanocrystals from sisal whiskers and polyvinyl acetate (PVAc) prepared by solution casting was also reported [31]. A stabilization of the polymeric matrix towards water absorption was observed and it was suggested that this stabilization could enhance the behavior of polar polymers under humid conditions and widens their applicability. The preparation and characterization of pea starch nanocrystals reinforced PVA nanocomposites were also reported [32]. Improved physical properties were observed. In the present study, we investigated the thermal and mechanical properties of nanocomposite films obtained from cotton cellulose whiskers and different grades of PVA. Variation in the degree of hydrolysis changes the residual ester content and amount of hydroxyl groups of copolymers. Because of the mutual affinity of PVA and cellulose whiskers with hydroxyl-rich surfaces, it is anticipated that strong interactions may form between cellulose whiskers and PVA copolymers that can be tailored by adjusting the degree of hydrolysis. The resulting hydrogen bonds should strengthen the interface significantly with a positive impact on the mechanical properties of the composite material.
sis were used as matrix. Two of them were purchased from Aldrich and four of them were kindly provided by Clariant GmbH (Muttenz, Switzerland) with trade name MowiolÒ (Table 1).
2. Experimental section
PVA grade
Codification
Mw (103 g mol1)
Degree of hydrolysis (%)
2.1. Materials
Aldrich Aldrich Mowiol Mowiol Mowiol Mowiol
1 2 3 4 5 6
146–186 124–186 100 150 175 145
99 89 81.5 87.5 92.4 99.4
2.1.1. Polymer matrices Six different grades of PVA copolymers with different molecular weights (Mw) and different degrees of hydroly-
2.1.2. Cellulose whiskers Cellulose nanocrystals or whiskers were extracted from cotton linter. Colloidal suspensions of whiskers in water were prepared as described elsewhere [33–35]. The cotton linter was milled with a laboratory milling device to obtain fine particulate substance. The cotton fibers were extracted in a 2 wt.% aqueous NaOH solution for 12 h at room temperature under mechanical stirring and then filtered and rinsed with distilled water after this treatment step. Acid hydrolysis was done in 65 wt.% aqueous sulfuric acid solution (11 wt.% cotton fibers). The resulting suspension was held at 45 °C under mechanical stirring for 45 min to allow fibers hydrolysis. The yield of the cellulose whiskers, with respect to the initial amount of cotton linters, was 64%. The ensuing suspension was diluted with cold distilled water followed by centrifugation at 10,000 rpm and 10 °C for 20 min. This process was repeated until a pH = 4 was reached (5 times). Next, dialysis against distilled water was performed to remove free acid in the suspension, as detected by the neutrality of the dialysis effluent. The dispersion of whiskers was completed by an ultrasonic treatment using a Branson sonifier and the suspension was filtered over a N°1 fritted glass filter in order to remove residual aggregates. The resulting suspension was subsequently stored in the refrigerator after adding several drops of chloroform in order to avoid bacterial growth until used. The whisker content was determined by weighting aliquots of the solution before and after drying. 2.1.3. Nanocomposites processing Polyvinyl acetate copolymers aqueous solutions were prepared. Depending on the proportion of cotton whiskers in PVA matrix, a given amount of PVA (5 g) was dissolved in 80 g distilled water at 90 °C for 60 min under mechanical stirring. Then, the solutions were kept under stirring to reach room temperature. To obtain films with different compositions, the solutions were mixed with a specific amount of the aqueous dispersion of cotton whiskers and mixed for 30 min. The resulting mixture was cast in a Teflon mold and placed in a 35 °C oven to evaporate water. Cotton whisker content in the final composites were 0, 3, 6, 9 and 12 wt.% for PVA copolymers from Aldrich and 0, 3 and 12 wt.% for MowiolÒ matrix. The obtained films around 200–350 lm thick were demolded after approximately 72 h. Table 1 Codification and properties of PVA grades used as matrix
15–79 23–88 30–92 28–99
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2.1.4. Film conditioning As PVA films absorb moisture to different extents, depending on the water vapor partial pressure, precise conditioning must be ensured in any determination of mechanical and thermal properties [3]. In order to evaluate the effect of moisture content on the composite structure, the moisture content of the nanocomposite films was achieved by conditioning the samples at room temperature in desiccators at controlled humidity containing saturated salt solutions. Four relative humidity (RH) conditions were used, namely 0, 35, 75 and 98%. The saturated salt solutions were P2O5, CaCl2.6H2O, NaCl and CuSO4.5H2O, respectively. The temperature range was within 20–25 °C. Conditioning was achieved for at least two weeks to ensure the equilibration of the water content in the films with that of the atmosphere and controlled by the stabilization of the sample weight.
3. Results and discussion
2.2. Experimental methods
3.2. Morphological characterization of nanocomposite films
Scanning electron micrographs of cellulose whiskers were taken with a ZEISS-ULTRA55 scanning electron microscope with field emission gun (SEM–FEG) with an acceleration voltage of 15 kV. Whiskers were deposited from a droplet of a dilute suspension on a microgrid (200 mesh, Electron Microscopy Sciences, Hatfield, PA, USA). Scanning electron microscopy (SEM) was performed to investigate the morphology of the nanocomposite films with a JEOL JSM-6100 instrument. The specimens were frozen under liquid nitrogen, then fractured, mounted, coated with gold/palladium on a JEOL JFC-1100E ion sputter coater, and observed. SEM micrographs were obtained using 7 kV secondary electrons. Differential Scanning Calorimetry (DSC) was carried out with a DSC Q100 (TA Instruments, New Castle, DE, USA) equipped with a manual liquid nitrogen cooling system. Conditioned samples were placed in hermetically closed DSC crucibles and tested in the range of 50– 250 °C at a heating rate of 10 °C min1 under a nitrogen atmosphere. Sample weight was between 10 and 15 mg. The glass transition temperature (Tg) was taken as the inflection point of the specific heat increment at the glass–rubber transition while the melting temperature Tm was taken as the peak temperature of the melting endotherm. Three samples were used to characterize each material. Dynamic mechanical properties of the nanocomposite films were measured in tensile mode using a RSA3 DMA fitted with a LN2 cooling system. The measurements were carried out at a constant frequency of 1 Hz, a strain rate of 0.008%, previously established as the linear viscoelasticity domain of the material. A temperature range of 60– 250 °C, a heating rate of 5 °C min1 and distance between jaws of 5 mm were used. The samples were prepared by cutting rectangular strips from the films with a width of 5 mm. Tensile testing was carried out using a RSA3 DMA with a cross head speed of 0.05 mm s1 and an initial distance between jaws of 10 mm. The samples were prepared by cutting rectangular strips from the films with a width of 2–3 mm. Three samples were used to characterize each material.
The morphology of the nanocomposite films was characterized by SEM. Fig. 2 shows the fractured surface of the unfilled partially hydrolyzed PVA matrix (PVA 4, panel a) and related nanocomposite film reinforced with 12 wt.% cellulose whiskers (panel b), and unfilled fully hydrolyzed PVA matrix (PVA 6, panel c) and related nanocomposite film filled with 12 wt.% cotton whiskers (panel d), just after fracture. The freshly fractured surface of the unfilled films either partially or fully hydrolyzed PVA (Fig. 2A and C) is smooth and uniform. For nanocomposite films (Fig. 2B and D) no clear evidence of the presence of the cellulosic nanoparticles is observed. This is due to the scale of the reinforcing phase, but it also indicates that no aggregates are present in the nanocomposite films and that the dispersion of the filler within the polymeric matrix is rather good. However, the aspect of the fractured surface is different and appears to be more chaotic, mainly for the filled fully hydrolyzed nanocomposite (Fig. 2D). It could be ascribed to a higher degree of crystallinity of the material and/or strong interactions between the filler and the matrix leading to a fracture path through PVAcoated nanoparticles.
3.1. Whiskers characterization
Fig. 1. Scanning electron micrograph (SEM–FEG) of a dilute suspension of cotton whiskers.
MACROMOLECULAR NANOTECHNOLOGY
Fig. 1 shows an electron micrograph of the cotton whiskers. They occurred as rod-like nanoparticles which dimensions are known to depend on the origin of the cellulose [16]. The average diameter and length were calculated using digital image analysis (Axone). At least 500 measurements were carried out for both diameter and length. The average diameter and length of the cotton whiskers were approximately 14.6 ± 3.9 and 171.6 ± 48.2 nm, respectively. The presented errors are the standard deviations of the distributions. The average aspect ratio (L/d, L being the length and d the diameter) of these whiskers is therefore around 11–12. It is close to the values reported for cotton whisker in the literature [34].
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Fig. 2. Scanning electron micrographs from the fractured surfaces of PVA 4-based nanocomposite films reinforced with 0. (A) and 12 wt.%, (B) cotton whiskers, and PVA 6-based nanocomposite films reinforced with 0, (C) and 12 wt.% and (D) cotton whiskers.
3.3. Thermal analysis of nanocomposite films The thermal properties of cotton whiskers reinforced PVA copolymers were determined from DSC thermograms. Thermal characteristics are collected in Tables 2 and 3. The glass transition temperature of unfilled polymeric matrices and nanocomposite films reinforced with cotton whiskers up to 12 wt.% are reported in Table 2. Tg values are given for samples conditioned in different atmospheres. Regardless the composition, Tg decreases as the humidity content increases owing to the well-known plasticizing effect of water molecules for PVA chains. The two fully hydrolyzed PVAs (PVA 1 and 6) display the lowest Tg values in the dry state (0% RH conditioned samples). Tg values corresponding to the six different matrices conditioned at 0% RH are plotted in Fig. 3 as a function of the whisker content. For these dry samples, Tg of partially hydrolyzed samples remains roughly constant when adding cellulose whiskers, whereas those corresponding to fully hydrolyzed samples (PVA 1 and 6, Fig. 3) slightly increased. It could be ascribed to stronger interactions between the filler and the fully hydrolyzed matrices. In moist atmosphere, Tg is found to significantly increases when adding cotton whiskers regardless the degree of hydrolysis of the matrix (Table 2). It is most probably ascribed to the competitive interactions between PVA, water and cellulose whiskers surface. Both PVA/cellulose and water/cellulose interactions tend to shift Tg towards higher
Table 2 Glass transition temperature for MowiolÒ copolymer films reinforced with cellulose whisker as a function of whisker content and relative humidity Sample
Whiskers content (wt.%)
RH (%) 0
35
75
98
1
0 3 6 9 12
40.4 44.3 45.0 44.6 45.6
45.5 46.7 45.9 46.2 46.6
0.9 2.1 2.3 2.6 2.7
21.7 18.1 11.9 8.2 4.7
2
0 3 6 9 12
66.6 66.0 65.3 65.6 66.1
45.7 45.9 46.1 45.5 45.8
9.1 8.2 6.4 10.5 5.5
18.3 16.8 8.2 6.2 3.1
3
0 3 12
58.8 60.1 58.5
46.7 47.1 48.4
0.7 9.1 6.2
31.8 28.2 6.0
4
0 3 12
70.1 69.6 68.7
47.9 47.4 48.9
7.2 12.5 10.9
27.4 19.3 9.6
5
0 3 12
70.6 69.8 67.3
47.4 48.3 48.7
4.5 5.5 9.5
39.4 19.4 14.2
6
0 3 12
47.6 48.8 49.9
50.8 48.0 48.4
0.5 3.1 6.9
35.2 23.4 2.3
Table 3 Temperatures of the calorimetric transitions of cotton whiskers filled PVA conditioned at 0% RH and using data obtained from the DSC curves, melting temperature Tm, associated heat of fusion DHm and degree of crystallinity (Xc and Xp) Sample
Whiskers content (wt.%)
Tm (°C)
DHm (J g1)
Xc
Xp
1
0 3 6 9 12
226.6 225.6 223.1 222.4 222.0
47.5 44.1 44.0 42.8 39.8
0.294 0.272 0.272 0.264 0.246
0.294 0.281 0.289 0.290 0.279
2
0 3 6 9 12
190.9 189.9 190.3 188.0 190.0
23.5 23.0 19.8 19.4 17.4
0.145 0.138 0.123 0.120 0.107
0.145 0.142 0.130 0.132 0.122
3
0 3 12
185.9 182.9 184.1
20.4 18.7 14.0
0.126 0.116 0.086
0.126 0.119 0.098
4
0 3 12
191.1 191.1 190.8
27.0 25.8 21.4
0.167 0.159 0.132
0.167 0.164 0.150
5
0 3 12
203.6 203.1 198.9
37.2 30.8 24.9
0.230 0.190 0.154
0.230 0.196 0.175
6
0 3 12
229.7 228.1 224.9
62.2 54.3 48.6
0.385 0.336 0.300
0.385 0.346 0.341
Xc = DHm/DHm° and Xp = Xc/w; where DHm° = 161.6 J g1 is the heat of fusion for 100% crystalline PVA [from 190.3ATHAS Data Bank] and w is the weight fraction of polymeric matrix material in the composite.
80
Tg (ºC)
70 60 50
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Data Bank and was considered to be independent on the degree of hydrolysis. Two values of the degree of crystallinity were reported. The first one was calculated from the sample weight (Xc) and the other one (Xp) took into account the amount of matrix material in the composite. Both the melting point and the degree of crystallinity increased with increasing the degree of hydrolysis, as shown in Fig. 4 that displays the evolution of Tm (Fig. 4A) and Xc (Fig. 4B) as a function of the whiskers content for the different degrees of hydrolysis. This is ascribed to strong interactions between adjacent PVA chains by hydrogen bonding forces. As a consequence, highly crystalline polymers and bigger crystalline domains could be formed. The inverse trend was observed when dealing with PVAc, which is a totally amorphous polymer. Thus, the increase of the PVAc content decreases the extent of such interactions. Both the melting point and the degree of crystallinity of the matrix material tend to remain roughly constant or slightly decrease as the whiskers content increases. Tm values remains roughly constant for partially hydrolyzed materials when increasing the whiskers content whereas it decreases for fully hydrolyzed PVA matrices. Again, this is ascribed to stronger interactions between the cellulosic surface and polymeric matrix for the latter. These interactions most probably restrict the capability of the matrix chains to form big crystalline domains. A similar decrease of Tm was reported by Azizi Samir et al. [35], for tunicin whiskers reinforced poly(ethylene oxide) (PEO). The decrease of Tm was ascribed to two effects. The first one was a morphological phenomenon related to a decrease of the PEO crystallites size due to the formation of a close cellulose network within the matrix, mainly for high filler content. The second effect originated from the expected strong interactions between ether oxygen groups of PEO and hydroxyl groups of cellulose whiskers. 3.4. Dynamic mechanical properties of nanocomposite films
40 30
0
5
10
15
Whiskers Content (wt%) Fig. 3. Glass transition temperature of PVA/cotton whisker nanocomposites conditioned at 0% RH as a function of the whiskers content, PVA 1 (d), PVA 2 (s), PVA 3 (N), PVA 4 (D), PVA 5 (j) and PVA 6 (h). The solid lines serve to guide the eye.
temperatures. The former phenomenon is due to a lowering of the molecular mobility of PVA chains in the interfacial zone and the latter can result from a redistribution of water molecules within the matrix, diffusing toward the cellulosic surface. Such a relocalization effect can decrease the plasticizing effect of water as already reported for tunicin whiskers reinforced starch [36]. The values of the melting temperature Tm and corresponding heat of fusion, DHm, for nanocomposite films conditioned at 0% RH and reinforced with cotton whiskers are collected in Table 3. The degree of crystallinity of the material was calculated from DHm values. The value corresponding to a 100% crystalline PVA was taken from ATHAS
Favier et al. [26] reported the first demonstration of the reinforcing effect of cellulose whiskers. The authors measured by DMA in the shear mode a spectacular improvement in the modulus after adding tunicin whiskers into a thermoplastic matrix. These outstanding properties were ascribed to a mechanical percolation phenomenon [26,37], yielded by cellulosic nanoparticles interactions through hydrogen bond forces. Fig. 5 shows the evolution of the logarithm of the storage tensile modulus for nanocomposite films prepared using PVA 1 and conditioned at different relative humidity levels as a function of temperature. For low temperatures, it was difficult to observe any change in the storage modulus neither with moisture variation nor with filler content. As it is well known, the exact determination of the glassy modulus depends on the precise knowledge of the samples dimensions. In this case, and especially for highly moist samples, the films were soft and it was very difficult to obtain a constant and precise thickness along these samples. In order to minimize this effect, the glassy elastic tensile modulus was normalized at the observed average value, regardless the composition or moisture level.
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40 30
210
Xp (%)
Tm (º C)
220
200
10
190 180
20
0
5 10 Whiskers Content (wt%)
15
0
0
5
10
15
Whiskers Content (wt%)
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Fig. 4. (A) Melting temperature and (B) degree of crystallinity of PVA/cotton whisker nanocomposites conditioned at 0% RH as a function of the whiskers content, PVA 1 (d), PVA 2 (s), PVA 3 (N), PVA 4 (D), PVA 5 (j) and PVA 6 (h). The solid lines serve to guide the eye.
Fig. 5. Evolution of the logarithm of the storage tensile modulus as a function of temperature for PVA 1-based nanocomposite films reinforced with 0 (d), 3 (s), 6 (N), 9 (D) and 12 wt.% (j) of cotton whiskers and conditioned at (A) 0, (B) 35, (C) 75 and (D) 98% RH.
The unfilled PVA-based matrices display a typical behavior of semicrystalline polymer with four distinct zones. In the glassy state, the tensile storage modulus E’ slightly decreases with temperature but remains roughly constant. Then, the modulus drops at a temperature that depends on the moisture conditions. This relaxation phenomenon is associated to the glass–rubber transition of the PVA copolymer amorphous phase. The magnitude of this modulus drop is relatively weak because of the semi-
crystalline state of the material. Then, amorphous rubbery domains coexist with crystalline domains, which play the role of both filler particles and physical cross-links. At higher temperatures the modulus drops sharply and irremediably due to the melting of the crystalline zones of the PVA copolymer. Regardless the moisture content, the addition of cotton whiskers increases the rubbery modulus of the PVA matrix. It results in a lower modulus drop at Tg. Similar effects
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Ec ¼
ð1 2w þ wvR ÞES ER þ ð1 vR ÞwE2R ð1 vR ÞER þ ðvR wÞES
ð1Þ
where the subscripts S and R refer to the soft and rigid phase, respectively, i.e. polymeric matrix and filler. vR is the volume fraction of filler, and w and ER correspond to the volume fraction and modulus of the stiff percolating network, respectively. w can be written as
w ¼ 0 for vR < vRc b R vRc w ¼ vR v1v for vR vRc Rc
ð2Þ
where vRc and b correspond to the critical volume fraction at the percolation threshold and the corresponding critical exponent, respectively. It means that all the stiffness of the material results from the formation of infinite aggregates of cellulose whiskers. It is now of interest to compare the performances of cotton whiskers/PVA nanocomposites to the data predicted from this model based on the percolation concept. The critical volume fraction at the percolation threshold vRC can be calculated from the aspect ratio of the rod-like nanoparticles and b = 0.4 for the critical exponent in a three-dimensional problem according to the percolation theory [16,17]. With an aspect ratio of 11–12 it comes vRC = 6.4–5.8. A value of six was taken in the prediction. The densities of crystalline cellulose and PVA copolymers were taken as 1.5 and 1.3 g cm3, respectively. For the tensile modulus, ER, of a film of cellulose whiskers literature reports values ranging between 5 and 15 GPa depending on the source of cellulose [16,26,35,39]. An average value of 10 GPa was chosen for the calculation. Both experimental and predicted moduli are reported in Table 4 for PVA 1-based nanocomposite films reinforced with 12 wt.% cotton whiskers. The order of magnitude of the predicted data agrees with the experimental ones. However, the experimental values are systematically lower than predicted ones. It is a good indication that the cellulose whiskers percolation phenomenon is affected by filler/PVA interactions. These interac-
Table 4 Experimental and predicted rubbery modulus of PVA 1-based nanocomposite films reinforced with 12 wt.% cotton whiskers RH (%)
Experimental modulus (MPa)
Predicted modulus (MPa)
0 35 75 98
250a 292a 244b 93c
379 422 390 328
a b c
Estimated at 150 °C. Estimated at 100 °C. Estimated at 75 °C.
tions can partially hinder the formation of the expected percolating cellulosic network. However, these experimental data could also be affected by the low temperature storage modulus normalization. Also, it is found that for the moister nanocomposite sample (conditioned at 98% RH) the gap between experimental and predicted data is particularly significant. The high water content most probably leads to strong water/cellulose interactions that hinder more significantly the inter-whiskers interactions. Fig. 6 shows the evolution of the tangent of the loss angle (tan d) as a function of temperature for PVA 4-based nanocomposites conditioned at 0 (Fig. 6A) and 98% RH (Fig. 6B). The observed relaxation phenomenon is associated with the glass transition of the polymeric matrix. It is shifted to lower temperatures as the moisture content in the sample increases in agreement with DSC experiments. However, no shift of the peak position is observed for the moist samples upon whiskers addition, regardless the moisture content. This observation seems to disagree with DSC data reported in Table 2 for PVA 4 conditioned at 98% RH that showed an increase of Tg as the cotton whiskers increased. It is most probably ascribed to the wellknow mechanical coupling effect that tends to decrease the relaxation temperature as the filler content increases as a result of the decrease of the magnitude of the modulus drop at Tg [40]. These two effects probably counterbalance and cancel mutually. Also, it is found in Fig. 6 that the magnitude of the relaxation process strongly decreased when increasing the cotton whiskers content for the dried materials (Fig. 6A) whereas it remained roughly constant for the moister ones (Fig. 6B). It is generally observed that the magnitude of the main relaxation process is reduced in presence of the filler. It is ascribed to a decrease of the matrix material amount, responsible for damping properties, i.e. a decrease in the number of mobile units participating to the relaxation phenomenon. However, new damping mechanisms can be introduced by the filler particles [40]. Possible new damping mechanisms include: (i) particle–particle slippage or friction, (ii) particle–polymer motion at the filler interface and (iii) change in the properties of the polymer by adsorption onto the filler particle. It is generally accepted that if significant specific interactions between polymer and filler occur, this will tend to create a layer of polymer surrounding each filler particle. The resulting polymeric layer has different properties compared with those of bulk polymer. Assuming the dispersed phase particles to be rigid, this leads to an immobilized polymer layer contributing to the effective filler volume fraction in the compound. In
MACROMOLECULAR NANOTECHNOLOGY
were observed for each matrix regardless its hydrolysis state and the moisture content. Because the degree of crystallinity of the polymeric matrices was found from DSC measurements to remain roughly constant or even slightly decrease as the whiskers content increases, this increase of the relaxed modulus is most probably ascribed to a mechanical reinforcing effect of the filler. For highly moist samples, the rubbery modulus was found to increase with temperature because of the temperature scan and concomitant loss of water content during DMA test. Previous studies [16,17,26,37] have shown that the high reinforcing effect of cellulose whiskers reinforced nanocomposites can be well predicted following the adaptation of the percolation concept to the classical phenomenological series-parallel model of Takayanagi et al. [38]. The main advantage of this approach was to account for interactions between the fillers and for the hydrogen-bonding forces that hold the percolating cellulose whiskers network. Details of the calculation can be found elsewhere [16,26,39,40]. In this approach, the elastic tensile modulus Ec of the composite is given by the following equation
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0.4
tan δ
0.3
tan δ
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 50
0.2 0.1 0
60
70
80
-0.1 -40
90 100 110 120 130
Temperature (º C)
-20
0
20
40
60
Temperature (º C)
Fig. 6. Evolution of the tangent of the loss angle (tan d) as a function of temperature for PVA 4-based nanocomposite films reinforced with 0 (d), 3 (s) and 12 wt.% (N) of cotton whiskers and conditioned at (A) 0 and (B) 98% RH.
the light of these remarks, it is clear that filler/matrix interactions are much lower for the nanocomposite films conditioned at 98% RH compared to those conditioned at 0% RH. It could be ascribed to the formation of an aqueous or at least highly moist interface due to a redistribution of water molecules as already suggested from DSC measurements.
2
1 3.5. Tensile properties of nanocomposite films
0
75 0
3
Whiskers
6
9
Content
%
0 RH
Tensile Modulus (GPa)
12
(wt%)
Tensile Strength (MPa)
3 2.5 2 1.5 1
0
10 5 Whiskers Content (wt%)
15
120 100 80 60
Tensile tests were performed at room temperature for all nanocomposite films. All samples display an initial zone of elastic deformation. The stress–strain curves for dry samples exhibited a peak after a period of elastic deformation (yield) and the stress dropped and leveled. For wet samples it was followed by a nearly monotonically increase of the stress during the plastic deformation, up to
Strain at Break (%)
Tensile Modulus (GPa)
Fig. 7. Evolution of the Young’s modulus as a function of both the whisker content and relative humidity for nanocomposites PVA 1-based nanocomposite films reinforced with cotton whiskers.
Yield Stress (MPa)
MACROMOLECULAR NANOTECHNOLOGY
3
0
5
10
Whiskers Content (wt%)
15
140 120 100 80
0
10 5 Whiskers Content (wt%)
15
50 40 30 20 10 0
0
5
10
15
Whiskers Content (wt%)
Fig. 8. Evolution of the tensile mechanical properties for PVA 2-based nanocomposite films reinforced with cotton whiskers and conditioned at 0% RH, (A) tensile modulus, (B) tensile strength, (C) yield stress and (D) strain at break vs. cotton whiskers content. The solid lines serve to guide the eye.
2.5
2
1.5
1 80
85 90 95 100 Degree of Hydrolysis (%)
Fig. 9. Evolution of the relative tensile modulus of 12 wt.% cotton whiskers reinforced PVA nanocomposite films conditioned at 0% RH as a function of the degree of hydrolysis of the matrix. The solid lines serve to guide the eye.
the failure. These behaviors are classical for semicrystalline polymers mechanically characterized below and above their glass transition temperature, respectively. Fig. 7 shows the evolution of the Young’s modulus, as a function of whisker content and relative humidity for nanocomposite films prepared from PVA 1. The tensile modulus of nanocomposite films increased when increasing the whisker content for each relative humidity condition. For a given filler content, increasing the humidity resulted in a clear and significant decrease of the tensile modulus because of the shift of the glass transition temperature towards lower values, below the room temperature. Fig. 8 represents the evolution of the mechanical properties of PVA 2-based nanocomposite films conditioned at 0% RH. Compared to the unfilled PVA matrix, the addition of cotton whiskers induces an increase of the tensile modulus, tensile strength and yield stress. At the same time, the strain at break decreases from 29.5% for the unfilled matrix down to 9.1% for the nanocomposite film reinforced with 12 wt.% of cotton whiskers. Fig. 9 shows the evolution of the relative tensile modulus of PVA-based nanocomposite films reinforced with 12 wt.% of cotton whiskers and conditioned at 0% RH as a function of the degree of hydrolysis of the matrix. The relative tensile modulus is defined as the ratio of the modulus of the composite to the one of the unfilled matrix. It is clear that this reinforcing effect can be affected by factors such as the degree of crystallinity and Tg of the matrix. However, the relative tensile modulus tends globally to increase as the degree of hydrolysis of the PVA matrix increases. It could be ascribed to increased interactions between PVA chains and cellulose whiskers via hydrogen bonding. When increasing the degree of hydrolysis of the polymeric matrix, the number of hydroxyl groups increases and consequently, the number of hydrogen bonds increases. Such interactions lead to more rigid materials as the whiskers content increases. 4. Conclusion The goal of this work was to compare the mechanical and thermal properties of biopolymer based nanocomposites prepared from cotton whiskers and copolymers of polyvinyl alcohol and polyvinyl acetate. The cotton whis-
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kers, prepared by acid hydrolysis of cotton linter, consisted of slender rod-like nanoparticles. The average length and width of these whiskers were about 172 and 15 nm, respectively, leading to an average aspect ratio around 11–12. After blending the suspension of cotton whiskers with the solution of PVA nanocomposite films were prepared by a casting/evaporation technique. The samples were conditioned at different relative humidities (RH). All the results show that stronger filler/matrix interactions occur for fully hydrolyzed PVA compared to partially hydrolyzed samples. These stronger interactions result in an increase of the glass temperature, and a decrease of both the melting point and degree of crystallinity of the polymeric matrix in dry atmosphere. For moist samples, Tg increases significantly upon whiskers addition regardless the degree of hydrolysis of the matrix, because of the formation of a water layer at the interface, the PVA matrix becoming less plasticized by water. This water accumulation was also evidenced from DMA tests. A reinforcing effect of cotton whiskers was observed from both DMA and tensile tests. However, this reinforcing effect remains systematically lower than the prediction from a percolation approach. This is a good indication that the mechanical percolation of cellulose whiskers is affected by cellulose/ PVA interactions. The reinforcing effect was found to be all the higher as the degree of hydrolysis of the matrix was high.
Acknowledgments The authors thank Mrs. Bertine Khelifi, M. Raphaël Passas (EFPG-INPG) and Mrs. Francine Roussel-Dherbey (CMTC) for their help with the SEM–FEG imaging of the cotton whiskers and Michel Muguet (Air Liquide, France) for supplying MowiolÒ samples. Mehdi Roohani also thanks the Iranian Ministry of Science for its financial support through a Fellowship.
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