Wheat straw was purchased from Home Depot, USA. Polyvinyl alcohol (PVA, 98-99 % hydrolyzed,. Mw 31000-50000), hydrogen peroxide (30 wt.% in. H2O) ...
Thermal and Mechanical Properties of Cellulose Nanofibers Reinforced Polyvinyl Alcohol Composite Films Md. Nuruddin1, Raju Gupta2, Alfred Tcherbi-Narteh1, Mahesh Hosur1, Shaik Jeelani1
ABSTRACT: This research was aimed to use ball milling method to extract cellulose nanofibers (CNFs) from a bio-waste (i.e. wheat straw), and also to use the extracted cellulose nanofibers as reinforcing materials in polyvinyl alcohol (PVA) thin film. To study the effect of cellulose nanofibers (CNFs) on mechanical and thermal properties of polyvinyl alcohol (PVA) nano-composite films, thin film nano-composites were loaded with different loading of cellulose nanofibers (CNFs) by weight percent (i.e. 1,3,5 and 7% loading). As a result of the research, we found that the tensile and thermal properties of PVA thin composite increased up to 5% loading of cellulose nanofibers (CNFs). In contrast, the tensile as well as thermal properties of PVA nano-composite film degraded because of poor dispersion and agglomeration of CNFs. KEYWORDS: Extract, hydrolysis, nanofibers, nano-composite, agglomeration
1. 1INTRODUCTION Cellulose is a natural polymer consisting of linear homo polysachharide β-(1,4)-D-glucose units linked together by β-1-4-linkages [1]. It is used for various applications since its most abundantly found in natural resources, environment friendly and biocompatible in nature. Cellulose nanofibers (CNFs) are extracted from cellulose using different methods like high pressure homogenizer [2,3], enzyme assisted hydrolysis [4,5], and acid hydrolysis treatment process [6-8]. Cellulose nanofibers (CNFs) provides excellent mechanical properties [9], large specific area, low coefficient of thermal expansion, low cost and availability [10], better biodegradability, high aspect ratio (L/D), biocompatibility and renewability [11]. Polyvinyl alcohol (PVA) is a water soluble synthetic polymer, widely used as a matrix for fabrication of biodegradable polymer composites due to its biodegradability, biocompatibility, high tensile strength, excellent resistance and adhesive properties [12]. In this study, wheat straw was used to extract cellulose nanofibers (CNFs) by organic acid treatment followed by ballmilling process to use it as reinforcing materials. PVA nanocomposite thin films 1 2
Department of Materials Science & Engineering Department of Aerospace Engineering
were prepared by reinforcing CNFs at various loading and these different CNFs loaded thin composite films were evaluated and compared against their mechanical, morphological and thermal properties.
2. EXPERIMENTAL 2.1 MATERIALS Wheat straw was purchased from Home Depot, USA. Polyvinyl alcohol (PVA, 98-99 % hydrolyzed, Mw 31000-50000), hydrogen peroxide (30 wt.% in H2O), ethanol (≥ 99.5%), formic acid (≥ 95%), sodium hydroxide pellets and glycerol (≥ 99%) were purchased from Sigma–Aldrich (St. Louis, MO, USA). 2.2 ISOLATION NANOFIBERS (CNFS)
OF
CELLULOSE
Pretreatment of wheat straw was done using formic acid treatment followed by hydrogen peroxide bleaching [13-14]. CNFs were obtained from pretreated wheat straw by ball milling treatment process according to Nuruddin et.al. [15]. For this process, a mixture of approximately 10 gm of bleached cellulose and 10 ml of 80% ethanol was prepared and was allowed to mill for 120 minutes in a Mixer/Mill 8000DTM (SPEX Sample Prep, USA) using zirconia ceramic grinding vial and ball of diameter 0.5 inch. The mixture was then washed
repeatedly with distilled water and centrifuged to bring the pH value of cellulose between 6 and 7.
2.3
PREPARATION OF POLYVINYL ALCOHOL NANOCOMPOSITE FILMS PVA granules and distilled water was mixed along with 30% glycerol to prepare 10 wt.% PVA solution, which was then heated and magnetically stirred to completely dissolve the polymer. The desired CNFs suspension (0.5%) was then added and sonicated for uniform dispersion of CNFs in PVA solution. The mixture was then poured in petri dishes to allow water to evaporate then the film was demolded and stored.
3. CHARACTERIZATION 3.1 SCANNING ELECTRON MICROSCOPE
Morphological study was done by observing fracture surface of neat PVA and CNFs modified PVA nanocomposites using Scanning Electron Microscope (SEM) to analyze the effect of CNFs incorporation into the polymer matrix. Figure 1 shows the SEM image of tensile specimen of nanocomposite films which shows smooth and uniform fracture surface of neat PVA while comparatively rougher fracture surface of the specimen that were incorporated with CNFs and well dispersed. The rough surface is caused by the restriction of propagation of crack by the presence of better interaction (hydrogen bonding) between CNFs and PVA polymer matrix. So, roughest surface can be observed in SEM image at 5% loading of CNFs showing the maximum restriction by CNFs. In contrast, SEM image of fractured surface of 7% loaded CNFs shows agglomeration of CNFs.
(SEM) JEOL JSM-6400 scanning electron microscope (SEM) was used at 20kV accelerating voltage to take scanning electron micrograpth of wheat straw, cellulose, and CNFs while morphological study of fracture surface of PVA and CNFs was done at 10 kV accelerating voltage. 3.2 DIFFERENTIAL CALORIMETER (DSC)
1 [ ∆𝐻 ] (1−𝑚𝑓) ∆𝐻0
%
c
d
SCANNING
DSC analysis was performed at a heating rate of 10ºC/min from -10 to 240 ºC with two heating and one cooling scan. The crystallinity degree was calculated by: 𝐶𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑛𝑖𝑡𝑦 = 1
b
a
×100%
e
(1)
Where ΔH, is the enthalpy of melting, ΔHo is enthalpy of melting for 100% crystalline PVA and (1 – mf) is the weight fraction of PVA in the sample. 3.3 TENSILE TEST MTS 809 Axial/Torsional Test system was used to perform tensile test of neat PVA and CNFs reinforced PVA films according to ASTM D882. Five samples for each nanocomposites each with size of 5mm x 80mm rectangular strips and thickness of 200-300 μm were tested at 30 mm grip separation, 500 mm/min crosshead speed and 100 N load cell.
4. RESULT AND DISCUSSIONS 4.1 MORPHOLOGICAL CHARACTERIZATION OF CNFS AND NANOCOMPOSITE FILMS
Figure 1: SEM images of fracture surfaces of (a) neat PVA, (b) PVA/1% B-CNFs, (c) PVA/3% B-CNFs, (d) PVA/5% B-CNFs and (e) PVA/7% B-CNFs
4.2 THERMAL STABILITY NANOCOMPOSITE FILMS
OF
Differential scanning calorimetry (DSC) analysis was performed to study the crystallization and melting phenomena of neat PVA and CNFs modified PVA in order to evaluate the thermal properties, which is shown in Figure 2.
FIGURE 3: Stress-strain curves of neat PVA and ball milled cellulose nanofibers (B-CNFs) reinforced PVA composite films.
1.5
Heat Flow, W/g
1.0
Neat PVA PVA/1% B-CNFs PVA/3% B-CNFs PVA/5% B-CNFs PVA/7% B-CNFs
Cooling Scan
5. CONCLUSION
0.5
0
-0.5
Heating Scan
-1.0
-1.5 120
140
160
180
200
220
Temperature, C
FIGURE 2: DSC thermograms of neat PVA and B-CNFs reinforced PVA composite films
DSC thermograms from figure 2 shows that from cooling scan addition of CNFs increases the crystallinity and crystallization temperature of PVA till 5% loading of CNF while both crystallinity and crystallization temperature decreases for 7% loading. Heating scan shows similar trend for melting temperature, heat of fusion and crystallinity. 4.3 MECHANICAL PERFORMANCE NANOCOMPOSITE FILMS
The aim of this study was to prepare CNFs reinforced PVA nanocomposite and see analyze the effect on mechanical and thermal properties. Reinforcement of CNFs increases the hydrogen bonding between the fibers and polymers which resulted in better improvement in mechanical and thermal properties of the PVA nanocomposite thin films as observed from tensile test and DSC analysis. At the same time, loading of CNFs more than 5% (i.e. 7% CNFs loading) into PVA matrix system causes the degradation in mechanical and thermal properties due to the formation of agglomeration. Ball milling causes better interaction of CNFs with PVA polymer matrix by exposing more hydroxyl groups on the surface.
ACKNOWLEDGEMENT The authors are grateful to the NSF-CREST (grant no. 1137681) and NSF-EPSCoR (grant no. 1158862) for the financing support to carry out this research.
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REFERENCES
Tensile test was performed to analyze the mechanical performance of the nanocomposite films. The data for tensile strength, tensile modulus and elongation at break was obtained from the tensile test for both PVA thin film and CNFs reinforced PVA thin films and were compared as shown in figure 3. A significant improvement in tensile strength and modulus was observed with increased CNFs loading, which was found highest for 5% loading of CNFs where improvement in tensile strength was about 41-49 % and elastic modulus was about 258-267 %.
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40 Neat PVA PVA/1% B-CNFs PVA/3%-B-CNFs PVA/5% B-CNFs PVA/7% B-CNFs
Stress, MPa
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