Applications of FT Raman Spectroscopy and Micro Spectroscopy Characterizing Cellulose and Cellulosic Biomaterials

FT Raman spectroscopy and micro spectroscopy were used for the investigation of cellulose, cellulose derivatives and cellulosic plant fibres. Lattice structures of cellulose, polymorphic modifications I and II, as well as amorphous structure, were clearly identified by means of FT Raman vibrational spectra. Chemometric models were developed utilizing univariate calibration as well as methods of multivariate data analyses of FT Raman spectral data for the fast prediction of cellulose properties. Cellulose properties like the degree of crystallinity Xc_Raman, the degree of substitution DS_CMC, DS_AC and cellulose reactivity were determined. In situ/ in vivo FT Raman micro spectroscopy was used for the characterization of cellulose structures of flax and hemp fibres. Orientational and stress dependent FT Raman experiments were carried out.     

NIR FT Raman Spectroscopy–a Rapid Analytical Tool for Detecting the Transformation of Cellulose Polymorphs

This is a report of the combined use of NIR FT Raman spectroscopy and X-ray diffraction measurements (WAXS) to investigate the polymorphic transformation of cellulose I into cellulose II. For this reason samples of cellulose I were swelled in different concentrations of NaOH and dissolved in different molten inorganic salts hydrates (LiCl·2ZnCl₂·6H₂O, LiClO₄·3H₂O, LiSCN·2,5H₂O and ZnCl₂·4H₂O). NIR FT Raman spectra of the alkali treated samples were recorded. They characterize the pure modifications cellulose I and II as well as mixtures of the two polymorphic phases. The results of the Raman measurements were confirmed by X-ray scattering. The paper demonstrates that FT Raman vibrational spectroscopy is a powerful, rapid analytical method which may be used to follow the polymorphic transformation of cellulose I into cellulose II.     

Effect of alkaline treatment on cellulose supramolecular structure studied with combined confocal Raman spectroscopy and atomic force microscopy

The aim of this work was to investigate the morphological and structural changes associated with mercerization of cellulose fibres with combined confocal Raman and atomic force microscopy (AFM). During mercerization the alkali induces a change in polymorphic lattice from cellulose I to II. This was observed by confocal Raman spectroscopy from cellulose samples treated with 10, 15 and 25% aqueous sodium hydroxide solution. AFM images from the same samples illustrated that microfibrils were swollen and more granular in cellulose II than in cellulose I. Raman spectral images in plane and depth directions showed that the polymorphous cellulose structure was uniform throughout the cell wall, whereas the microfibril orientation varied between fibre cell wall layers. The changes in microfibril orientation on the sample surfaces were confirmed by AFM images measured from the same sample position.     

Examination of cellulose textile fibres in historical objects by micro-Raman spectroscopy

The investigation and characterisation of historical objects can be an exacting piece of work because of the small quantity of material that can be investigated and the degradation of the material and its value, which sometimes demands only non-destructive methods. In this study, as one such method, Raman spectroscopy was used to investigate the cellulose fibres of painting canvases and linings. Historical samples of fabrics were taken from different paintings and their linings from different locations in Slovenia. Raman spectra were recorded on the fibres of these historical samples. Additionally, a database of the Raman spectra of modern cellulose fibres was created and compared with the literature data. Differences in the Raman spectra of different cellulose fibres were observed, and on this basis fibres of different types were discriminated. The recorded Raman spectra of historical samples were compared with the database spectra of modern cellulose fibres. Strong luminescence effects because of the changes caused by ageing, degradation products and surface contamination caused difficulties in interpreting the Raman spectra of historical fibres. The luminescence effects were partly overcome by prolonged exposition times and previous "signal quenching" with the laser. The Raman spectra of historical cotton showed no luminescence effects, and only slight differences to the reference spectra of modern cotton fibres appeared, whereas the Raman spectra of historical flax fibres were overwhelmed with luminescence and showed changes in spectra through degradation. The research showed that by using Raman spectroscopy the identification and differentiation of different cellulose fibres and materials that accompany cellulose in the fibres are possible and that degraded and aged material can be differentiated.     

Mechanical Properties of Regenerated Cellulose Fibres for Composites

Summary: A broad variety of regenerated cellulose fibres was subjected to single fibre tensile tests in order to determine the modulus of elasticity, tensile strength, and failure strain. The results were compared to glass fibres and flax fibres, which are considered the most important technical and natural fibres, respectively. With regard to their modulus of elasticity and tensile strength, regenerated cellulose fibres showed clearly lower values than glass fibres, even when their low density was taken into account. The average modulus of elasticity and tensile strength of regenerated cellulose fibres was also lower than the values measured for flax fibres, but when variability was considered, both fibres performed similarly. In terms of interfacial shear strength with polypropylene, lyocell fibres performed significantly less well than sized glass fibre and ramie fibre. The most important difference between regenerated cellulose fibres and both glass and flax fibres is their high failure strain and thus high work to fracture. The high work to fracture of regenerated cellulose fibres makes them particularly useful for composite applications where high fracture toughness is required.     

Crystallinity changes in lyocell and viscose-type fibres by caustic treatment

One of the most important treatments performed on cellulosic fibres to improve properties such as dimensional stability, tensile strength and lustre, is mercerisation. The aim of this work was to study the crystallinity, accessibility and unit cell structure changes occurring in three types of regenerated cellulose fibres (lyocell, modal and viscose) that were mercerised with caustic soda solutions of different concentrations. Differences were observed between the behaviour of the viscose type fibres (viscose and modal) and that of the lyocell fibres. For the viscose type fibres, the proportion of crystalline regions increased at low alkali concentrations, while for lyocell fibres a decrease in crystallinity was observed. In all three fibres there was a transformation from cellulose II to amorphous cellulose. While for lyocell the transformation was partial, the modal and in particular the viscose fibres showed a complete transformation, and the swelling agent caused the fibre to dissolve at high caustic concentrations.     

Orientation Dependent FT Raman Microspectroscopy on Hemp Fibers

Summary: FT Raman microspectroscopy was used for polarization experiments on strained hemp fibre cells. The cellulosic plant fibers were macerated with alkaline and enzymatic solutions. Those cleaned and refined single fiber cells were subjected to micro tensile tests as well as to polarization measurements under the FT Raman microscope. Mechanical parameters of the fiber cells (e.g. E-modulus) were determined and changes in orientation of the  (C O C) structure units of the cellulose were considered with respect to fiber stress and molecular fiber structures. Intensity ratios R₁ and R₂ calculated on the polarized micro FT Raman spectra of the strained fibers describe the order parameter 〈P₂〉 and 〈P₄〉 allowing the quantitative determination of the orientation of the structure units  (C O C) of fiber cellulose with respect to the fiber cell axis.     

Studies of dyestuffs in fibres by Fourier Transform Raman spectroscopy

FT Raman spectroscopy is shown to be a powerful tool for the direct observation of low levels of dyestuffs in acrylic fibres. In the systems studied here, it is also found to yield more specific information than the complementary infrared technique. In contrast to conventional Raman spectroscopy with visible excitation, the near infrared laser used here does not induce fluorescence in the dye. Acrylic polymer bands can be efficiently subtracted to yield superbly detailed spectra of the dye in its physiochemical state in the fibre. The subtraction spectra provide valuable information on the nature of the dye and can be compared with spectra of pure dyes. Caution should be taken when subtracting spectra since small shifts in the frequency of the excitation laser line passing through the interferometer can generate artefacts in the final subtraction spectrum.     

Thermogravimetric measurement of amorphous cellulose content in flax fibre and flax pulp

Usually the raw material for flax pulp production is a blend which contains fibres and shives. In order to better understanding the structure of these materials and the effects of flax pulping, X-ray diffraction and thermogravimetry analysis under air atmosphere have been used. There was a significant effect of the fibre size on the composition, crystallinity, and thermal behaviour of the flax pulps. On the other hand, data obtained from thermogravimetric analysis have been modelled on the basis of two cellulose types characterized by different crystallinity levels, using kinetics equations based on the nucleation concept. As a result of these simulations, composition of the samples, pulp crystallinity and the proportion of amorphous cellulose are calculated.     

Influence of the Absorbed Water on the Tensile Strength of Flax Fibers

The complex structure of flax fibres involves many chemical biomolecules located in an amorphous matrix in which cellulose micrifibrils are embbeded. The drying of flax fibres influences significantly their tensile strength. This result can be explained by the creation of damages within the fibre and by the modification of the chemical composition of the matrix components. This loss of water involves a modification of the adhesion between the cellulose microfibrils and the matrix. This modification is due to the evolution of the components ensuring the transfer of load between the microfibrils and thus conditioning the strength of the cellular wall.     

Electron beam irradiated textile cellulose fibres.: ESR studies and derivatisation with glycidyl methacrylate (GMA)

Powdered samples of cotton, flax and viscose from textile fibres were subjected to electron beam irradiation (20-400 KGy). ESR signals were observed after irradiation. The intensity of the signals depended on the administered dose and exhibited an exponential decay with time. The ESR spectra, whose pattern depended drastically on the crystallinity of the samples, indicated the simultaneous presence of at least three carbon-centred radical species. When the whole irradiated textile fibres were quenched in a solution of glycidyl methacrylate (GMA) a significant increase of the weight was observed, the increase being strongly dependent on the administered dose and on the time interval elapsed between irradiation and quenching. Spectroscopic characterisation evidenced that GMA acted as cellulose radicals scavenger on the unsubstituted carbon of the double bond and was covalently linked to the fibre through a new C-C bond while maintaining unaltered the ester and the epoxide groups, available for further derivatisations. Because irradiation of the whole textile fibres in the range 20-200 KGy does not result in significant degradation of the material, GMA quenching of fibres irradiated in this dose range may represent an attractive route to their functionalisation.     

Thermal and crystallization studies of short flax fibre reinforced polypropylene matrix composites: Effect of treatments

The effect of fibre treatments on thermal stability of flax fibre and crystallization of flax fibre/polypropylene composites was investigated. For thermal stability study, flax fibres have been treated using maleic anhydride, maleic anhydride polypropylene copolymer, vinyltrimethoxy silane and alkalization. In order to compare thermal stability of flax fibres thermogravimetry (TG) analysis has been used. Kinetic parameters have been determined by Kissinger method. Results showed that all treatments improved thermal stability of flax fibres. For crystallinity analysis, three different techniques have been used, differential scanning calorimetry analysis (DSC), pressure–volume–temperature (PVT) measurements for analysis of volume shrinkage and polarized optical microscopy (POM). All techniques results showed that addition of flax fibre increased crystallization rate. Besides, depending on fibre surface treatment and crystallization temperature, flax fibre/PP composites can show transcrystallinity.     

Elastic coils: deformation micromechanics of coir and celery fibres

Natural fibres, such as flax and hemp, are typically chosen as reinforcing elements in composites to replace traditional glass fibres due to their high stiffness, strength and low strain to failure. Some plant fibres such as coir and celery however possess high strains to failure, which could be utilised in a composite to enhance toughness. This paper reports on the use of Raman spectroscopy to follow the molecular deformation of single fibres of coir and celery. The technique is also used to characterise the orientation of the cellulose structure within the fibres. It is shown by mechanical testing of fibres that both celery and coir possess a non-linear stress–strain curve. Coir fibres however exhibit high strain to failure, whereas celery fibres are shown to have a much lower value of this parameter, despite having a similar coiled fibrillar structure. It is shown by using polarised Raman spectroscopy, and rotating the specimens with respect to the polarisation axis of the laser and measuring the intensity of the 1095 cm⁻¹ Raman band, that both celery and coir fibres combine both axial and transverse orientation, due to their coiled structures. This is also confirmed by birefringence measurements. By following the shift in the central position of this Raman band as a function of cyclic deformation of the fibres, it is shown that the coir fibres recover their molecular deformation, whereas the celery does not show the same level of recovery. This difference between the fibres is postulated to be due to the fact that coir possesses an interlaced fibrillar structure, which remains intact, whereas the celery sub-fibrils unravel and orient towards the fibre axis during deformation.     

Influence of plasma treatment on the adhesion between a polymeric matrix and natural fibres

The aim of this work is to study the influence of low-pressure plasma treatment on cellulose fibres to improve the adhesion between a polymeric matrix and natural fibres used as reinforcement. To evaluate fibre wettability, contact angle measurements were carried out on flax fibres after treatment with plasma under several conditions. Similarly, contact angle measurements were performed without plasma treatment. A comparison between all the samples led to the definition of the optimal plasma treatment conditions. Once the latter were determined, composite materials were prepared with treated and untreated flax fibres and a low-density polyethylene matrix. Composites, with different fibre contents (5 and 40%) and different fibre lengths (1 and 10 mm), were manufactured using a mixer and a hot plate press. The tensile strengths of the composites were assessed to determine optimal fibre content and length, and the plasma treatment effect was also quantified. It was found that the higher the fibre content, the higher the tensile strength, and the higher the Young’s modulus; however, fibre length did not affect tensile strength. Regarding plasma treatment, composites with treated fibres exhibited a considerably improved tensile strength and Young’s modulus. Plasma treatment effects were also studied by X-ray photoelectron spectroscopy and by differential scanning calorimetric. Finally, an analysis of the fibre surface and an interaction study between the matrix and the fibres was conducted with scanning electron microscopy.     

Thermal properties of biocomposites

Thermal properties of new biocomposites prepared from modified starch matrix reinforced with natural vegetable fibres were studied. DSC and TG methods were applied to study thermal behaviour of biocomposites. Biocomposites were obtained in the laboratory twin-screw extruder. Two kinds of natural fibres were used, i.e. flax and cellulose in the amount of 0–40 mass%. DSC curves of biocomposites reveal glass transition temperature, attributed to the amorphous nature of the plasticized starch matrix. In general, incorporating natural fibres into modified starch matrix leads to an increase in glass transition temperature. Thermal degradation of modified starch matrix and cellulose reinforced biocomposites proceeds in three steps, whereas the degradation process of flax reinforced biocomposites occurs in two steps. For unreinforced matrix as well as for all biocomposites, regardless of type and amount of reinforcement, the major mass loss is observed at the temperature above 300°C. The increase in thermal stability with introduction of natural fibre is observed for both flax and cellulose reinforced biocomposites.     

The Young's modulus of a microcrystalline cellulose

This research is concerned with an investigation into the determination of the micromechanical properties of particulate form of cellulose; namel microcr stalline cellulose. Using the technique of Raman spectroscop the shift in the 1095cm^–1 Raman band, characteristic of cellulose, with strain is monitored and compared to the deformation of natural cellulose fibres (flax and hemp). From the values of the shift rate of the 1095cm^–1 band for flax and hemp and the experimentally-determined value for microcrystalline cellulose the value for the Young's modulus of microcrystalline cellulose was estimated to be 25±4GPa. It has been shown that this value is consistent with the measured degree of crystallinity of microcrystalline cellulose. Theoretical modelling has also enabled the Young's modulus for compacted microcrystalline cellulose to be determined for fibres in either 2-D in-plane and 3-D arrangements. These values have been show to be consistent with recent direct measurements of the modulus of compacted material.     

Vibrational study and spectra-structure correlations in ammonium saccharinate: comparison with the alkali saccharinates

The FT IR spectra, at temperatures from liquid-nitrogen boiling (LNT) up to room temperature (RT), as well as the RT Raman solid-state spectra of protonated and deuterated ammonium saccharinate and of a series of alkali (Na, K, Rb, Cs) saccharinates are studied. The spectral assignments are aided with ab initio calculations on the free saccharinato anion at the HF/6-31 + + G(d,p) level. Attention is paid to the ND, CO and SO2 stretching regions. Correlation splitting is believed to be responsible for the presence of a v(CO) doublet. The averaged v(CO) frequency in (purely ionic) ammonium saccharinate is found to be the lowest in the so far studied saccharinates, along with the assumptions that the v(CO) frequency (or the corresponding averaged value) can have predictive value for the type of the metal-to-saccharinato ligand/ion bonding. The appreciably higher contribution of the dominating internal coordinate in the corresponding normal vibration in case of v(as)(SO2) than in v(s)(SO2) makes it suitable for spectra-structure correlations. Contrary to RT, even though no phase transitions were observed in the studied temperature range, some polycentered character is prescribed to the hydrogen bonds in which the ammonium ions of effective symmetry C8 participate at LNT. Certain structural predictions about the saccharinates of K, Rb and Cs are made.     

A New Kinetic Model for Water Sorption Isotherms of Cellulosic Materials

Summary: Some natural fibres like flax, hemp and others show excellent mechanical properties which make them a promising choice for the reinforcement of polymers. For natural fibre reinforced composites, hydrophilicity is a problem with respect to dimensional changes, fibre to matrix adhesion and long term stability. The interaction of differently prepared and modified fibres with water vapour has been investigated by dynamic vapour sorption. It has been found that the sorption and desorption kinetics of cellulosic fibres can be excellently fitted by assuming two parallel, independent first order processes. This empirical model, defined here as the “Parallel Exponential Kinetics” model (PEK-model), reveals two distinct mechanisms with slow and fast exchange of water vapour respectively, related to different sorption sites. The specific sorption mechanisms are represented by individual sorption-desorption isotherms as components of the total isotherms. The results suggest a relation to the differing types of amorphous regions in the fibres and/or to the different states of “bound” or “free” water, discussed for hydrophilic materials. The PEK-model proved to be consistently applicable for sorption and desorption over the whole humidity range, and also for all tested cellulose fibres. It is especially useful for a clearer distinction of different fibre types or modifications and can be successfully used for an extended fibre characterization.     

Swelling of carboxymethylated cellulose fibres

Swelling of cotton cellulose fibres having different proportions of carboxyl groups in the H-form was studied. The carboxyl groups were introduced by carboxymethylation under different reaction conditions. By studying the swelling of modified cellulose samples (water retention value of non-dried fibre) it was shown that the concentration of sodium hydroxide was the dominant factor among the investigated reaction parameters. The number of acidic groups was found to play a significant but not determinative role in the level of improvement in swelling caused by carboxymethylation. A linear correlation was observed between swelling and iodine sorption capacity. The degree of collapse of the highly accessible structure of cellulose during drying (hornification) was larger in the case of more accessible carboxymethylated fibres than for the alkali treated sample. The degree of hornification increased with growing swellability and with growing number of carboxyl groups in the investigated interval (40–120 mmol carboxyl/mol cellulose). This type of modified cellulosic fibre could be used for enhanced entrapping and release of chemicals. This revised version was published online in August 2006 with corrections to the Cover Date.