Microstructure and Surface Properties of Fibrous and Ground

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Microstructure and Surface Properties of Fibrous and Ground Cellulosic Substrates Emília Csisz
ar*,† and Erika Fekete‡ †

Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, H-1521 Budapest, P.O. Box 91, Hungary ‡ Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri
ut 59-67, Hungary ABSTRACT: Cotton and linen fibers were ground in a ball-mill, and the effect of grinding on the microstructure and surface properties of the fibers was determined by combining a couple of simple tests with powerful techniques of surface and structure analysis. Results clearly proved that the effect of grinding on cotton fiber was much less severe than on linen. For both fibers, the degree of polymerization reduced (by 14.5% and 30.5% for cotton and linen, respectively) with a simultaneous increase in copper number. The increased water sorption capacity of the ground substrates was in good agreement with the X-ray results, which proved a less perfect crystalline structure in the ground samples. Data from XPS and SEM-EDS methods revealed that the concentration of oxygen atoms (bonded especially in acetal and/or carbonyl groups) on the ground surfaces increased significantly, resulting in an increase in oxygen/carbon atomic ratio (XPS data: from 0.11 to 0.14 and from 0.16 to 0.29 for cotton and linen, respectively). Although grinding created new surfaces rich in O atoms, the probable higher energy of the surface could not be measured by IGC, most likely due to the limited adsorption of the n-alkane probes on the less perfect crystalline surfaces.

’ INTRODUCTION The interest in cellulose-based materials increased considerably in recent years both in academic settings and in industry. Cellulosic materials are renewable, environmentally friendly, and sustainable resources, and they are applied extensively in various forms and in a wide range of different sectors. The number of papers published in this field increases exponentially, and a large amount of information has been accumulated concerning the chemical, physical, and morphological behavior of the cellulosic materials. The most important cellulosic fibers are cotton and linen; they are viewed as environmentally safe and sustainable. Both fibers are traditionally used in the textile area for household textiles, as well as for apparel. Recently, linen and other cellulosic fibers like hemp are increasingly used for the preparation of composites with synthetic and biopolymers, where the matrix is reinforced with the natural fibers. Since the application of cellulosic substrates has broadened continuously, surface properties are becoming increasingly important. Several papers have published on the properties of these fibers, and the information collected in this field is increasing significantly.1 Since these materials are usually used in powder form in numerous new applications, such as natural fiber reinforced composites, the effect of grinding on the surface properties of the fibers has a great significance. Grinding is employed in order to achieve both size reduction and an increase in surface area of the substance being treated. r 2011 American Chemical Society

When grinding is applied to polymeric substances such as cellulosic fibers, however, the size reduction is accompanied by physical and chemical alteration of the polymer. These alterations may occur in the crystal structure and degree of polymerization. The influence of grinding on the structure of cotton cellulose was investigated by Earland and Raven.2 Physicochemical properties of the substrate were tested as a function of grinding time, and varied in a wide range from 0.5 to 20 h. Results proved that during ball milling the degree of polymerization reduced to less than 200 with a very great increase in copper number, but little increase in methylene blue absorption. The authors underlined that the large values of copper number arose not only from the hydrolytic fission between glucose residues, but from the oxidative fission of glucose rings between C2 and C3 to produce aldehyde groups. Flax fibers were prepared by cutting, milling, and freezegrinding for differential thermogravimetric analysis, and the effect of preparation on particle size, surface characteristics, fiber ends of the samples, and pyrolysis properties were investigated by Sharma and his co-workers.3 Scanning electron microscopy revealed that fiber ends prepared by the three methods were distinctly different from each other. While in cut fibers the scissor Received: March 19, 2011 Revised: May 11, 2011 Published: June 09, 2011 8444

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Langmuir marks were visible, the samples prepared by milling had rounded fiber ends caused by abrasion and the heat generated. In addition, the mean length of the fibers was lowest in cut samples. The results also confirmed that particle size of the samples can affect the pyrolysis pattern. Some 40
50 years ago, other workers examined the effect of grinding on the crystallinity and molecular size. They showed that grinding rapidly destroyed the fiber structure resulting in profound changes in the crystallinity. Grinding in a ball mill produced pronounced increase in moisture sorption along with loss of fiber structure. X-ray diffraction proved that the increase in moisture sorption was accompanied by a decrease in lateral order.4
7 Despite the increasing interest in cellulose-based fibers in powder form, only a very few and relatively old publications focused on changes in their structural and surface properties, affected by grinding. Thus, it is necessary to get sufficiently detailed information about the substrate surface composition and morphology that can affect their performance in the different applications. In this article, we will be concerned mainly with the microstructure and surface properties of the cotton and linen fibers when ground. These measurements are not only of theoretical interest, but also of considerable technological importance. By combining a couple of simple, widely applied tests, like water sorption capacity, degree of polymerization (DP), and copper number, to powerful techniques of surface and structure analysis, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and inverse gas chromatography (IGC), novel information on the surface properties and morphological structure of linen and cotton fibers, and on the changes in properties that occurred during grinding of the fibrous materials, can be obtained.

’ EXPERIMENTAL SECTION Materials. Raw cotton and linen roving originating from middle eastern Asia were used for the experiments. The analytical grade chemicals for classical tests and the chromatography grade probes used for IGC were purchased from Sigma-Aldrich Chemicals. Grinding. Cotton and linen fibers were cut into lengths of 1
3 mm and ground in a Mixer Mill MM400 (Retsch GmBH, Germany) at a frequency of 30 1/s, with 11 stainless steel balls for 2.5 min. The mass ground was kept constant at 1.5 g for both fibers. The fraction studied in this work was composed of particles with diameters enclosed between 200 and 315 μm, collected by sieving. Classical Analytical Methods. Moisture regain of the samples was determined by exposing the triplicate samples, previously dried over P2O5 for 5 days, to an atmosphere of 65% rh at 25 °C for 5 days. The average viscometric degree of polymerization was measured according to the ASTM standard D4243-99.8 The test method measures the specific viscosity of a solution of the cellulosic sample in cupri-ethylenediamine. From this measurement the intrinsic viscosity of the solution is deduced, and from this the degree of polymerization is easily calculated. Copper number was determined by Braidy’s method.9 Copper number is defined as the weight of copper reduced by 100 g of a sample of cellulose fiber, from cupric to the cuprous state. It is determined by a quantitative modification of Fehling’s test. The reduction is due to the activity of the aldehyde (
CHO) group present in oxycellulose and hydrocellulose. The method serves to detect cellulose damage and to estimate the quantity of reducing groups. X-ray Diffraction. X-ray diffraction patterns were obtained in a Philips model PW 3710 based PW 1050 Bragg
Brentano parafocusing

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goniometer using Cu KR radiation (λ = 0.154 18 nm), graphite monochromator, and proportional counter. The XRD scans were digitally recorded with a step size of 0.04° and evaluated with profile fitting methods. X-ray Photoelectron Spectroscopy. XPS spectra were collected by a VG Microtech instrument consisting of a XR3E2 X-ray source, a twin anode (Mg KR and Al KR), and a CLAM2 hemispherical analyzer using Mg KR radiation. Detailed scans were recorded with 50 and 20 eV pass energy.

Scanning Electron Microscopy and Electron Probe Microanalysis. Surface morphological investigations were made by a JEOL 5500 LV electron microscope in low vacuum (30 Pa) mode with a back scattered electron detector. The accelerating voltage was 20 kV and the working distance 20
21 mm. The samples were fastened to the copper sample holder by adhesive carbon tape and measured without any coating. Atomic composition of the surface was investigated by energy dispersive spectroscopy in low vacuum (30 Pa) mode with a Si(Li) detector. Inverse Gas Chromatography (IGC). In IGC the solid substrate with unknown surface is packed into the column and the known materials are the vapor probes that are injected into the column packed with the solid sample. The fibrous and powder cellulosic fibers were packed into the stainless steel column having an inner diameter of 5 mm and a length of 50 cm. Approximately 1.5 g portions of the samples were filled into the column. IGC measurements were carried out using a Perkin-Elmer Autosystem XL apparatus. Vapor samples of 5
20 μL were injected into the column, and retention peaks were recorded by a FID detector. Each reported value is the result of at least three parallel measurements. Highpurity nitrogen was used as carrier gas, and its flow rate changed between 5 and 20 mL/min depending on measurement temperature and on the type of adsorbent. IGC measurements were carried out after conditioning the samples at 105 °C for 16 h under a constant flow of nitrogen, in the temperature range between 30 and 70 °C and increased by 10 °C increments. A new column was prepared for each set of experiments. By using IGC, the surface characteristics can be derived from retention times or volumes. Besides numerous parameters, the dispersion component of the surface tension of the adsorbent (γsd) can be determined with this technique. The determination based on the fact that the free enthalpy of adsorption (ΔGA) is related to the net retention volume (V),10 i.e. ΔGA ¼
RT ln V þ C

ð1Þ

where C is a constant depending on the reference state selected, R is the universal gas constant, and T is the absolute temperature of the column. The commonly used approach to determine γsd was proposed by Dorris and Gray.11 If the value of RT ln V, which is derived from the retention volumes measured with n-alkanes of different chain lengths, is plotted against the number of carbon atoms in their chain, we obtain a straight line, the slope of which is given by eq 2
RT ln

Vn ¼ 2Nðγds Þ1=2 aCH2 ðγCH2 Þ1=2 Vnþ1

ð2Þ

where Vn and Vnþ1 are the retention volumes of n-alkanes with n and nþ1 carbon atoms, respectively, aCH2 is the surface area occupied by a
CH2
group, and γCH2 is the surface tension of polyethylene (Figure 1).

’ RESULTS AND DISCUSSION Microstructure of the Ground and Fibrous Cotton and Linen Substrates. Cotton and linen fibers were ground in a ball

mill at a frequency of 30 1/s for 2.5 min and sieved to collect particles with diameter between 200 and 315 μm. Grinding the long fibers to particles, cotton and linen substrates undergo 8445

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Figure 1. Diagram illustrating the determination of γsd from the retention volumes (V) of n-alkanes of different chain lengths, proposed by Dorris and Gray.11

complex changes, but to a varying extent. As scanning electron micrographs in Figures 2 and 3 illustrate, under identical grinding conditions linen shows a greater change in external form compared to cotton. The mechanical action in the grinding process ruptures the cotton fibers to small particles (Figure 2) by reducing the lengths but leaving the lateral dimensions practically unaffected. The ground particles still show the main characteristics of the original fiber, such as convolutions and ribbonlike form. Cotton consists of a single biological cell. This compact fiber structure offers a high degree of resistance to further disruption namely to the breakdown to fibrils. Obviously, the applied grinding is not drastic enough to disintegrate the fiber to fibrils or further to elementary fibrils. The effect of grinding on cotton is much less severe than on linen. While the nature of the breakdown of cotton fiber induced by grinding is particulate, rather than fibrillar, a quite different effect can be found when linen is subjected to similar treatment (Figure 3). Linen fiber is a multicellular fiber and consists of a bundle of cells fused together. The cells are cemented together laterally by the middle lamella, containing pectin and other noncellulosic materials. During the applied grinding, linen fibers are degraded in length and disintegrated into elementary fibers before being destroyed into units with sheetlike structure. The rough surface of these elongated tabular particle sheets contains small irregularities. The units of the ground sample do not possess any clearly recognizable structure and do not show similarity to the external form of the original linen fiber at all. The size reduction by grinding is accompanied by further physical and chemical changes in the cellulosic fibers. As the measured data in Table 1 show, the short grinding process results in significant differences in the bulk properties of the substrates. For both fibers, the degree of polymerization reduced with an increase in copper number and water sorption capacity. Obviously, the applied mechanical action is intensive enough to reduce the molecular mass. The rupture of cellulose chain molecules is accompanied by a slight increase in carbonyl groups expressed by copper number. Furthermore, the significant increase

Figure 2. Scanning electron micrographs of fibrous (a) and ground (b) cotton.

in water sorption for both fibers reveals12 that the accessible surface of the substrates was enlarged by grinding. Comparing the DP values of the linen and cotton substrates, we can see that the percent loss suffered by the linen during grinding is roughly 2 times (i.e., 30.5%) that suffered by the cotton (i.e., 14.5%). The higher differences between the data of fibrous and ground linen listed in Table 1 demonstrate again that linen is more sensitive to grinding than cotton. Degradation of the fibers that occurred during grinding induces observable changes in the crystal structure, which can be detected by X-ray diffraction. For both fibers, a significant difference can be seen between the spectra of fibrous and ground samples. X-ray diagrams of linen in Figure 4 show the broadening of the cellulose I reflections at 2θ 14.7, 16.5, and 22.7, as well as their reduction in intensity. These results clearly prove that the applied grinding process slightly destroys the crystalline structure of the fibers, producing less perfect cellulose crystals. For both fibers, the crystallites have many more defects in the ground samples than in the fibers. Furthermore, the crystallites in the ground samples are reduced in size (cotton, from 9.1 to 5.8 nm; linen, from 5.8 to 4.6 nm) and contain more imperfections. It is clear that there is a limited change in the crystal structure of the ground fibers, which, nevertheless, can be characterized by the same crystalline/amorphous ratio. 8446

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Figure 4. X-ray spectra of fibrous and ground linen.

Figure 3. Scanning electron micrographs of fibrous (a) and ground (b) linen.

Table 1. Characteristics of Ground and Fibrous Cellulosic Substrates water sorption

degree of

substrate

capacity (%)

polymerization

copper number

cotton ground cotton fibrous

7.77 ( 0.19 7.29 ( 0.02

1880 ( 50 2200 ( 100

1.14 ( 0.05 1.07 ( 0.03

linen ground

10.06 ( 0.03

1730 ( 120

1.77 ( 0.03

linen fibrous

9.32 ( 0.01

2490 ( 60

1.68 ( 0.05

X-ray results are in a good agreement with the water sorption capacity of the samples (Table 1). Moisture regain can indicate the compact crystal structure of the substrate. By grinding, the water sorption is slightly increased, which reveals the slight expansion of the accessible internal surface in the conditioned fibers. Chemical Composition of the Surface. Cotton and linen fibers are mainly composed of cellulose. The noncellulosic substances present in raw cotton and linen constitute approximately 5% and 30% of the total fiber weight, respectively. In cotton, the noncellulosic incrusting matters, such as waxes, pectin, proteins, and minerals, are mainly found in the cuticle and

the primary wall, surrounding the fiber cellulose core. In linen, hemicelluloses, pectin, lignin, and some fats and waxes were analyzed mainly in the middle lamella and primary wall. For both fibers, the waxy surface layers protect the fibers against the environment during growth and affect their surface properties and processing behavior as well. Furthermore, the noncellulosic matters present in the bast linen are also responsible for cementing of the large numbers of ultimate cells both longitudinally and transversely.13 XPS is an analytical technique which can be successfully used for surface characterization of cellulosic substrates in a few nanometer depth. XPS provides quantitative information not only on the chemical composition of the surface, but also on the differently bonded carbon atoms. For cellulosic fibers, the surfaces consist mainly of carbon and oxygen, but the O/C ratio for the components of the surface (i.e., cellulose, waxes, lignin) is different. Thus, the amount of noncellulosic surface constituents can be estimated. Furthermore, XPS can make a difference in the oxidative states of the carbon atoms. Chemical shifts for carbon (C1s) in pulp fibers can be classified into four categories: unoxidized carbon (C—C), carbon with one oxygen bond (C—O), carbon with two oxygen bonds (O—C—O or CdO), and carbon with three oxygen bonds (OdC—O). In XPS spectra of pure cellulose, only two peaks appear which can be attributed to C—O (alcohols and ethers) and O—C—O (acetal) moieties. An additional C1 peak in the spectra of unpurified cellulosic samples corresponds to nonoxidized alkane-type carbon atoms (C—C) and originates from noncellulosic constituents like lignin, extractives, and waxy materials. The amount of alkyl carbons (C—C) decreases in the following order: extractives > lignin > carbohydrates. Thus, from the O/C atomic ratios and the relative amounts of the different carbons, quantitative data on the surface chemical composition can be obtained.14
17 The C1 XPS spectra of fibrous and ground substrates are shown in Figure 5. All the XPS data for the two fibers before and after grinding are collected in Table 2. The results show that very low O/C values (cotton, 0.11; linen, 0.16), and very high relative concentrations of the unoxidized (C
C) carbons (cotton, 87.8%; linen, 89.2%) can be detected in the original fibers, which indicate the presence of extractives in the fiber surfaces. The measured O/C ratio of cotton fiber agrees well with the theoretical O/C value for model (fatty acids and resin acids) extractives (0.11),14 and confirms that the surface of cotton fiber is perfectly covered by waxes rich in hydrocarbons. Results in Table 2 also reveal that, besides the predominant nonoxidized 8447

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Figure 5. XPS spectrum of C1s peak of cotton and linen in fibrous and ground forms.

Table 2. Effect of Grinding on the O/C Atomic Ratio and the Relative Amounts of Different Carbons Determined from the C1s Peak in the XPS Spectrum of Cotton and Linen substrate

O/C

C1 (%) C—C

C2 (%) C—O

C3 (%) CdO, O—C—O

C4 (%) O—CdO

cotton ground

0.14

83.5

6.3

10.2

0.0

cotton fibrous

0.11

87.8

5.5

6.6

0.0

linen ground

0.29

76.2

9.0

14.8

0.0

linen fibrous

0.16

89.2

3.7

7.2

0.0

alkane-type carbon atoms (C1, 87.8%), only a negligible amount of carbon is present as C2 (C—O, alcohols or ethers, 5.5%) and C3 (O—C—O, CdO, acetal or carbonyl, 6.6%) moieties. Buchert and her co-workers18 found slightly higher O/C ratio (0.14) for cotton knitted fabric manufactured from raw cotton fiber, and the relative amount of the C1 and C2 moieties was significantly different (77.0% and 16.0%, respectively). Furthermore, they found C4 peak which was attributed to carbon atoms in carboxyl groups (O—CdO, 2.4%). The O/C value for linen (Table 2) is somewhat higher (0.16) than the theoretical value for model extractives (0.11),14 implying that the surface of linen is less homogeneous in composition than that of cotton, which almost exclusively consists of waxy matters.

The higher O/C ratio agrees with the higher noncellulosic content of linen. Furthermore, these results indicate that, besides the extractives in the primary wall of the multicellular linen, the XPS technique can also detect a small amount of cementing materials derived from the binding layers between the elementary fibers. The relative amounts of different carbons indicate the predominance of nonoxidized carbons (C1, 89.2%). In the same study of the former research group,18 woven linen fabric from field retted flax fibers was characterized. A higher O/C ratio (0.24) as well as lower C1 (C
C, 66.2%) and significantly higher C2 (C
O, 25.9%) concentrations were detected. Furthermore, carboxyl groups were also detected in linen and attributed to galacturonic acids in pectin constituent. Additionally, a large part of the noncarbohydrate material was expected to be lignin. The O/C atomic ratio was also higher for both of the tested fibers, namely mechanically extracted (green) and dew-retted flax fibers (0.22 and 0.25, respectively).19 C1, C2, and C3 peaks were detected, and the relative concentration for C1 (green, 52.6%; dew-retted, 40.8%) was lower; for C2 and C3, it was higher. The authors underlined that there is no lignin present on the fiber surface, but it is enriched with waxy materials. Grinding changes the chemical composition of the surfaces. For both fibers, the concentration of oxygen on the surface increases, resulting in an increase in oxygen/carbon atomic ratio. For cotton, the changes are less significant; the O/C ratio increases only from 0.11 to 0.14. In the case of linen, however, the applied grinding increases the O/C ratio from 0.16 to 0.29. Results prove that grinding only partially modifies the perfect hydrophobic waxy coverage of the cotton fibers (illustrated by Figure 2b), but destroys considerably the multicellular structure of linen (illustrated by Figure 3b) producing new and less waxy surfaces. For both fibers, none of the C1, C2, and C3 moieties remain unaltered. A decrease in C
C carbons (cotton, 87.8 f 83.5%; linen, 89.2 f 76.2%) and an increase in C3 carbons (cotton, 6.6 f 10.2%; linen, 7.2 f 14.8%) attributed to carbonyl and/or acetal moieties can be observed. These data are in a good correlation with the DP and copper number data (Table 1), which also demonstrated the formation of the carbonyl end groups by the polymer chain degrading effect of the applied grinding process. XPS data also confirm that linen shows a more powerful response to ball milling than cotton. While XPS can characterize approximately a 5
10 nm thin surface layer,20 the thickness of the surface layer analyzed by EDS is significantly higher. That can be the reason why the O/C ratios for cotton and linen measured by EDS are significantly higher (0.28 and 0.32, respectively, for the fibrous materials, Table 3) than those obtained by XPS (0.11 and 0.16, respectively, for the fibrous materials, Table 2). XPS probably analyzes only the cuticle, a complex mixture of fatty acids, high molecular weight alcohols and esters, which has been estimated to have a thickness of 12 nm for cotton.21 EDS is a particle-beam technique. A beam of accelerated electrons is focused on the surface of a specimen, producing characteristic X-rays within a small volume (typically 1
9 cubic micrometers) of the specimen.22 Consequently, this method is able to analyze not only the very thin waxy cuticle, but the primary wall, which has a thickness of 0.1
0.2 μm for cotton,23 and the secondary wall in a certain depth. Furthermore, a certain amount of cementing materials can also be detected in linen. Primary wall consists mainly of less ordered cellulose accompanied by pectin, hemicelluloses, and proteins. In the secondary 8448

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Table 3. Effect of Grinding on the Carbon and Oxygen Atomic Composition (in Atomic %) of Cotton and Linen Surfaces Determined by EDS substrate

O

C

O/C

cotton ground

30 ( 5

70 ( 5

0.43

cotton fibrous

22 ( 5

78 ( 5

0.28

linen ground

30 ( 3

70 ( 4

0.43

linen fibrous

24 ( 3

76 ( 3

0.32

Figure 7. Effect of temperature of measurement on the dispersion component of surface tension (γsd) of fibrous (9) and ground (b) linen samples.

Figure 6. Effect of temperature of measurement on the dispersion component of surface tension (γsd) of fibrous (9) and ground (b) cotton samples.

wall, only crystalline and highly ordered cellulose is present. Cementing materials in linen present in the middle lamella consist of pectin and other noncellulosic materials, like hemicelluloses and lignin. Since O/C ratios between 0.31 and 0.40 are reported for lignin, and a value of 0.83 can be calculated for cellulose,17 the higher O/C ratios measured by EDS can be attributed to the cellulose, hemicelluloses, pectin, and lignin constituents of the deeper layers in the fibers. Despite the different thickness of the surface layers analyzed by the two methods, the data reveal similar changes by grinding: a significant increase in O/C ratio for both fibers. Surface Energetics. Inverse gas chromatography at infinite dilution is an objective method for evaluating the changes in surface properties of the fibers more precisely,24 and to obtain quantitative information on the new surfaces produced by grinding of the fibers. As we shall see later, the dispersive component of the surface free energy (γsd) determined by the method illustrated in Figure 1 characterizes accurately the surface energetics of the fibrous and ground substrates. Results presented in Figures 6 and 7 show that the dispersion component of surface tension determined at 30 °C is around 40
45 mJ/m2 for all the four samples; thus, the surface energy of the substrates is quite similar. In a previous study25 a slightly lower γsd value was measured for the fibrous cotton (39.0 ( 0.9 mJ/m2 at 30 °C); however, in that case the sample in the column was not preconditioned. Without conditioning, we could measure a lower energy of the surface covered partially by water. Furthermore, we found that the γsd values decrease continuously with increasing measurement temperature in the range 30
70 °C (Figures 6 and 7). The slight decrease in the γsd values

as a function of increasing temperature, measured without any further changes in the fiber surface, can be explained by the thermal expansion of the samples, as well as the decreasing adsorption ability of the probes with increasing temperature. Basically all authors in the literature agree with the negative temperature coefficient within this temperature range. Interestingly, the dependence of the dispersion component of surface energy of ground cotton on temperature is slightly higher than that of the fibrous cotton and the two linen samples. The lower γsd values of ground cotton measured at higher temperatures (60 and 70 °C, Figure 6) can probably be explained by different factors. As the results in the previous chapters proved, there are significant differences in all of the properties of the four samples investigated. As a consequence, further investigations are needed to understand the factors (e.g., rate of thermal expansion, adsorption ability of the adsorbates, etc.) contributing to the greater effect of the temperature on the dispersion component of surface energy of the ground cotton. Comparing the γsd values of the fibrous cotton and linen to those of the ground samples, we can see that, except for the ground and unground cotton fibers at 30 and 40 °C which have almost the same γsd values, the surface energy of the fibrous cotton and linen substrates in the range 30
70 °C is slightly higher than the surface energy of the ground fibers. These results were surprising since our starting assumption was that the new surfaces created by grinding of the fibrous samples can be characterized by increased surface energy. Furthermore, the comparative values of the fibrous and ground samples in Figures 6 and 7 are inconsistent with the O/C ratios measured by XPS or EDS. These unexpected values of the surface energy may be explained by the XRD data that reveal the slightly destroyed crystalline structure with lattice defects and the less perfect cellulose crystals in the ground samples. The relation between the γsd values and the crystallinity of the cellulosic substrates was demonstrated recently.26,27 The relatively high γsd value measured on crystalline sample (41.3 mJ/m2, microcrystalline cellulose) and the relatively low γsd values for amorphous substrate (27.4 mJ/m2, starch) proved that, on the flat cellulose surface in a highly crystalline substrate, the alkane probe can adopt a particular conformation which contributes to an optimal interaction between the probe and the surface, and 8449

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Langmuir consequently to a longer retention time. On a molecular scale rough surface, the retention time will be shorter due to the hindered access. Although grinding creates new surfaces rich in O atoms, the probable higher energy of the surface cannot be measured by IGC, since the adsorption of the probes is less effective on the less perfect crystalline surface of the ground samples. The parameters measured by IGC depend largely on the adsorption of the probes on the solid surface. This interaction is strongly influenced by the sample preparation and the measurement conditions. Thus, the determined surface characteristics are not material constants.24 Under appropriate standard conditions, for example, the surface modifying effect of different treatments on a certain sample can be determined reliably.25 However, for comparative characterization of samples differing deeply in morphology, texture, size, etc., the method cannot be applied.

’ CONCLUSION Cotton and linen fibers were ground in a ball-mill, and a fraction of the particles with a diameter 200
315 μm was analyzed to determine the effect of grinding on the microstructure and surface properties of the fibers. By classical analytical methods, degree of polymerization, copper number, and water sorption capacity were determined and compared with the results derived from powerful techniques of surface and structure analysis, such as X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and inverse gas chromatography. By grinding the long fibers to particles, cotton and linen undergo complex changes, but to a varying extent. Almost all of the results proved that the effect of grinding on cotton was much less severe than on linen. For both fibers, the degree of polymerization reduced with a simultaneous increase in copper number and water sorption capacity. The increased water sorption capacity indicated that the enlargement of the accessible surface was in good agreement with the X-ray results. The new surfaces created by grinding exhibited different chemical characters. As the results from XPS and SEMEDS analysis revealed, the concentration of oxygen on the surface increased, resulting in an increase in oxygen/carbon atomic ratio. A decrease in C
C carbons and an increase in C3 carbons (carbonyl and/or acetal) measured by XPS were in a good correlation with the DP and copper number data, which also proved the increase in carbonyl end groups in the ground substrates. In spite of grinding creating new surfaces rich in O atoms, the probable higher energy of the surface could not be measured by IGC, since the adsorption of the probes is less effective on the less perfect crystalline surface of the ground samples. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Research was partially financed by grants from the Hungarian National Science Foundation (Grants OTKA K82044 and K67936). This work is connected to the scientific program of the “Development of quality-oriented and harmonized RþDþI strategy and functional model at BME” project supported by the New Hungary Development Plan (Project ID: TAMOP-4.2.1/ B-09/1/KMR-2010-0002).

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The authors are grateful to Dr. B
ela Koczka, Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, for the SEM-EDS analysis; to Istv
an Saj
o, Chemical Research Center, Hungarian Academy of Sciences, for the XRD work; and to Dr. Katalin Josepovits, Department of Atomic Physics, Budapest University of Technology and Economics, for the XPS analysis. The skilful technical assistance of technician
va Bandi, M
aria Kiszel
ak, Ramona Bende, as well as students E and Zsuzsanna Ol
ah, is gratefully acknowledged.

’ REFERENCES (1) Kim, J.; Yun, S.; Ounaies, Z. Macromolecules 2006, 39, 4202– 4206. (2) Earland, C.; Raven, D. J. J. Text. Inst. 1977, 2, 69–73. (3) Sharma, H. S. S.; Faughey, G.; McCall, D. J. Text. Inst. 1996, 87, 249–257. (4) Hermans, P. H. Text. Res. J. 1946, 16, 545–555. (5) Nelson, M. L.; Conrad, C. M. Text. Res. J. 1948, 18, 149–154. (6) Nelson, M. L.; Conrad, C. M. Text. Res. J. 1948, 18, 155–164. (7) Howsmon, J. A. In Cellulose and Cellulose Derivatives; Ott, E., Spurlin, H. M., Grafflin, M. W., Eds.; Interscience Publishers, Inc.: New York, 1954; Part I, Chapter 4, p 424. (8) ASTM D4243 - 99(2009) Standard Test Method for Measurement of Average Viscometric Degree of Polymerization of New and Aged Electrical Papers and Boards (9) Nevell, T. P. In Methods in Carbohydrate Chemistry; Whistler, R. L., Ed.; Academic Press: New York, 1963; Vol. III, p 44. (10) Conder, J. R.; Young, C. L. Physicochemical Measurement by Gas Chromatography; John Wiley & Sons: Chichester, U.K., 1979; p 146. (11) Dorris, G. M.; Gray, G. J. Colloid Interface Sci. 1980, 77, 353– 362. (12) Kr€assig, H. A. Cellulose, Structure, Accessibility and Reactivity, 1st ed.; Gordon and Breach Science Publishers: Amsterdam, 1993; p 167. (13) Focet, B; Marzetti, A.; Sharma, H. S. S. In The Biology and Processing of FlaxTail; Sharma, H. S. S., Van Sumere, C. F., Eds.; M Publications: Belfast, 1992; p 329. (14) Laine, J.; Stenius, P.; Carlsson, G.; Str€om, G. Cellulose 1994, 1, 145–160. (15) Belgacem, M. N.; Czeremuszkin, G.; Sapieha, S.; Gardini, A. Cellulose 1995, 2, 145–157. (16) Buchert, J.; Carlsson, G.; Viikari, L.; Str€om, G. Holzforschung 1996, 50, 69–74. (17) Toth, A.; Faix, O.; Rachor, G.; Bertoti, I.; Szekely, T. Appl. Surf. Sci. 1993, 72, 209–213. (18) Buchert, J.; Pere, J.; Johansson, L.-S.; Campbell, J. M. Text. Res. J. 2001, 71, 626–629. (19) Zafeiropoulos, N. E.; Vickers, P. E.; Baillie, C. A.; Watts, J. F. J. Mater. Sci. 2003, 38, 3903–3914. (20) Briggs, D.; Seah, M. P. Practical Surface Analysis; John Wiley & Sons: New York, 1992; Vol. 1, Auger and X-ray Photoelectron Spectroscopy. (21) Ryser, U.; Holloway, P. J. Planta 1985, 163, 151–163. (22) Electron Microprobe Analysis Information, Electron Microprobe Laboratory, University of Minnesota. http://probelab.geo.umn. edu/electron_microprobe.html. (23) Peters, R. H. Textile Chemistry, The Chemistry of Fibres; Elsevier Publishing Company Ltd.: Amsterdam, 1963; Vol. I, Chapter 6. (24) Fekete, E.; M
ocz
o, J.; Puk
anszky, B. J. Colloid Sci. 2004, 269, 143–152. (25) Csisz
ar, E.; Fekete, E. Text. Res. J. 2010, 80, 1307–1316. (26) Papirer, E.; Brendle, E.; Balard, H.; Vergelati, C. J. Adhes. Sci. Technol. 2000, 14, 321–337. (27) Belgacem, M. N.; Blayo, A.; Gardini, A. J. Colloid Interface Sci. 1996, 182, 431–436.

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