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Spatially Resolved Characterization of Cellulose Nanocrystal– Polypropylene Composite by Confocal Raman Microscopy

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Spatially Resolved Characterization of Cellulose Nanocrystal– Polypropylene Composite by Confocal Raman Microscopy Spatially Resolved Characterization of Cellulose Nanocrystal– Polypropylene Composite by Confocal Raman Microscopy Umesh P. Agarwal,* Ronald Sabo, Richard S. Reiner, Craig M. Clemons, Alan W. Rudie USDA FS, Forest Products Laboratory, Madison, WI 53726 USA Raman spectroscopy was used to analyze cellulose nanocrystal (CNC) The CNCs can be incorporated as the load-bearing –polypropylene (PP) composites and to investigate the spatial distribution component in composites to enhance the properties of synthetic of CNCs in extruded composite filaments. Three composites were made and other polymer matrices. This will give rise to composites from two forms of nanocellulose (CNCs from wood pulp and the nano- with improved strength. Obtaining information on composition scale fraction of microcrystalline cellulose) and two of the three and structure of the CNC composite is essential for correlating composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cmÀ1, structure to mechanical properties. Research investigations are respectively, obtained at 1 lm spatial resolution showed that the CNCs needed to understand the physicochemical aspects of the were aggregated to various degrees in the PP matrix. Of the three CNCs, the polymer matrix, and their interactions. This is composites analyzed, two showed clear existence of phase-separated extremely valuable because such findings will allow new regions: Raman images with strong PP and absent/weak cellulose or vice experimental strategies for optimizing composite processing versa. For the third composite, the situation was slightly improved but a and production. In the present investigation, composites of PP clear transition interface between the PP-abundant and CNC-abundant containing 2 wt. % CNC were investigated. A low concentra- regions was observed, indicating that the CNC remained poorly dispersed. tion of CNC was considered desirable from the view points of The spectroscopic approach to investigating spatial distribution of the efficient reinforcement at low cost. composite components was helpful in evaluating CNC dispersion in the Although a number of techniques were used in the CNC-PP composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs. composite characterization, this report focuses on analysis using Raman spectroscopy. The technique requires no sample Index Headings: Raman mapping; Confocal Raman microscopy; Nano- preparation and provides structural information that is sensitive crystalline cellulose; Polypropylene; Composite. to intra- and inter-molecular interactions. Confocal Raman microscopy15 was selected for detailed analysis at the microscopic level because it provides chemically specific information at high spatial resolution. Compared to electron INTRODUCTION microscopy, which has much higher resolution but poor There is growing interest in making composites that are contrast between CNCs and PP, Raman microscopy easily based on renewable materials.1,2 Because such composites are discriminates between these. Raman mapping cannot image biodegradable and environmentally friendly, they are desirable single CNCs and is expected to generate information on the replacements for conventional composites based on petroleum distribution of the CNCs in the CNC-PP composite. Among feedstock. One such category of materials composed of wood numerous other applications, the mapping technique has been fiber/flour and plastics is wood plastic composites.3–5 In these used to investigate the distribution of cellulose and lignin in the materials, wood fiber is used as an inexpensive component cell walls of woody tissues.16,17 along with a thermoplastics matrix such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). Another EXPERIMENTAL approach to making green composites has been to use cellulose The four samples analyzed by Raman techniques (Fourier from a variety of sources: higher plants, algae, bacteria, and 6–8 transform (FT) Raman and confocal micro-Raman) included a tunicates. In particular, bacterial cellulose has been shown to PP control and three cellulose/PP composites as listed in Table be capable of mechanical reinforcement of both thermoplastic I. The methods used in their preparation were as follows. and thermoset resins for making composites.9,10 Cellulose Cellulose Nanocrystal Preparation. CNC was prepared by nanocrystals (CNCs) are such a material that is mechanically sulfuric acid hydrolysis of wood pulp using 64% sulfuric acid strong and nano-dimensional and have the advantage of being (acid : cellulose = 8 : 1 (v : w)) by a procedure similar to that of conveniently produced from naturally abundant cellulose Dong et al.18 PP (Certene PHM-20) and maleic anhydride fibers. CNCs are the rod-shaped nano-size crystalline regions grafted polypropylene (MAPP) (Eastman Chemical; Epolene that are contained within cellulose microfibrils and are isolated G-3015) were obtained commercially. Microcrystalline cellu- when native cellulose is subjected to strong acid hydroly- lose (MCC) labeled as Avicel 591f was obtained from FMC sis.11,12 The dimensions of the CNCs vary depending upon the Biopolymer. This MCC contained carboxymethylcellulose as a source of cellulose and range from 3 to 20 nm in diameter and dispersant. 100 to 1000 nm in length.8,12,13 The Young’s modulus of CNC The CTAB (cetyl trimethyl ammonium bromide) form of has been reported to range from 138 to 167 GPa.8,13,14 CNC was prepared by neutralizing an aliquot of CNC with solid sodium bicarbonate. A stoichiometric amount of CTAB Received 15 December 2011; accepted 22 March 2012. was dissolved in a small amount of warm water (1/10 of CNC * Author to whom correspondence should be sent. E-mail: uagarwal@fs. fed.us. aliquot). (The CNC preparation typically produces CNC DOI: 10.1366/11-06563 containing about 1wt. % sulfur (as surface sulfate esters); the 0003-7028/12/6607-0750$2.00/0 750 Volume 66, Number 7, 2012Ó 2012 Society for Applied Spectroscopy APPLIED SPECTROSCOPY TABLE I. Samples analyzed by Raman mapping. CNC particles were deposited on a TEM grid (400 mesh Formvar/carbon filmed grid prepared with blow discharge) by Sample ID Descriptiona floating the grid on CNC suspension (;0.05 wt% concentra- PP-CON PP, control tions) droplets for ;2 minutes then rinsing through 2% AC_CNC-PP Acetylated CNC:PP = 2:98 aqueous uranyl acetate stained and blotted dry. FRACT_CNC-PP NanoF-MCC:MAPP:PP = 2:2:96 Raman Analysis and Mapping. The FT-Raman spectra MAPP_CNC-PP CNC:MAPP:PP = 2:2:96 were obtained using a Bruker RFS-100 spectrometer (Bruker a PP = Polypropylene; CNC = cellulose nanocrystal; MAPP = maleated Optics Inc.) equipped with a 1 W, 1064 nm Nd : YAG diode- polypropylene; nanoF-MCC = nano-crystalline fraction of microcrystalline pumped laser. The PP and composite samples were directly cellulose. analyzed whereas CNC and acetylated CNC were sampled in the ‘‘aluminum well’’ sampling accessory. The laser power used for sample excitation was 600 mW, and 1024 scans were amount of CTAB was calculated as a 1 : 1 molar stoichiom- accumulated with the use of a double-sided forward–backward etry.) The CTAB solution was added quickly to the stirred CNC aliquot. This suspension was centrifuged and the clear scanning mode. The samples were analyzed in 1808 geometry. liquid decanted. The CNC was washed by suspending in water Bruker’s OPUS software program was used to find peak and centrifuging three times. A solvent exchange was positions and process the spectral data. performed on the CNCs by suspending them in a series of For Raman mapping, sample cross-sections, 30 lm thick, of solvents three times each and centrifuging. The solvents used extruded fibers were analyzed using a 633 nm based Raman were 95% ethanol followed by acetone, diethyl ether, and then microprobe spectrometer, LabRam HR 800 (Horiba Jobin xylenes (Sigma-Aldrich, 98.5%). Yvon). This spectrometer is equipped with a confocal CNC-PP Composite Preparation (MAPP_CNC-PP). In a microscope (Olympus BX40), a piezoelectric x, y stage, and round-bottom flask, 43 g of 4.65 wt. % CTAB-CNCs in a charge-coupled device (CCD) detector. The instrument xylenes were added to 150 mL of xylenes, 2 g of MAPP, and provides ultra-high spectroscopic resolution and unique 30 mL of pyridine. These were refluxed under nitrogen with polarized 633 nm laser (20 mW) excitation. Spectra of samples stirring for 16 hours. PP (96 g) was dissolved in 800 mL of PP-CON, AC_CNC-PP, FRACT_CNC-PP, and MAPP_CNC- xylenes at boiling. The CNC/MAPP reaction mixture was PP from various sample regions were obtained by using the added to the dissolved polypropylene and mixed for 10 serial (raster) mapping technique. Each sample was placed on minutes. The composite was allowed to cool, transferred onto an automated piezoelectric x, y mapping stage and Raman aluminum foil, and the xylenes allowed to evaporate (solution spectra were obtained at different points on the surface by casting method). The composite was then placed in a vacuum moving it equivalently under the microscopic objective. For oven at 125 8C overnight. mapping in the x, y plane at a chosen stepping resolution (2, 1, Acetylated CNC-PP Composite (AC_CNC-PP). Acetylat- or 0.5 lm), the specimen was moved in the two spatial ed CNCs were prepared according to similar reaction dimensions (x and y) and a spectrum was recorded at each (x, procedures described above where acetic anhydride was y) position. Lower stepping resolution
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