THE EFFECT OF GAMMA RADIATION ON THE SUPRAMOLECULAR STRUCTURE OF PINE WOOD CELLULOSE IN SITU REVEALED BY X-RAY DIFFRACTION

Henryk Kasprzyk, Kinga Wichłacz, Sławomir Borysiak

ABSTRACT

This study investigated the changes in the supramolecular structure of pine wood cellulose in situ treated by various of doses gamma radiation (from 20 kGy to 9000 kGy). From the dose of 120 kGy the degree of crystallinity (x_c) of wood cellulose slightly decreased, while at the dose of 500-4500 kGy dropped rapidly. At 9000 kGy occures a total degradation of cellulose, both crystalline and amorphous. Along with the increase in the gamma radiation of wood, both the crystallinity degree and the average size of the crystallites of cellulose decrease. The curve of changes in the crystallinity degree was similar to that of the changes in the average size of the crystallites of cellulose.

Key words: irradiated pine wood, cellulose crystallinity, gamma radiation, X-ray diffraction..

INTRODUCTION

The understanding of gamma irradiation of wood is important for the applications of gamma rays in the production of wood-plastic composites [1, 2], and it has been taken into consideration for pulp processing and waste water cleaning [3] or in the viscose industry [4]. Interest is placed in using gamma radiation as a mitigation method for the disinfestation of wood [5, 6] and the sterilization of paper [7].

Depending on a given dose which is used, radiation may have a positive or negative effect on the irradiated material. Gamma radiation at higher doses has a destructive influence on both physical and chemical properties of wood [8, 9, 10].

Changes in the chemical structure of wood may be presented as a sum of g-radiation of its individual components, namely cellulose, hemicellulose and lignin. However these, cellulose is more sensitive to gamma irradiation than lignin [8, 11, 12].

The cellulose polymer chain resembles a flat ribbon, with hydroxyl groups extending laterally from the edges, and hydrogen atoms oriented above and below the plane of the ribbon. This structure allows for extensive interactions between cellulose chains through hydrogen bonding between the hydroxyl groups, and Van der Waals interactions between the hydrogen atoms [3, 13]. This results in a supramolecular structure composed of crystalline regions, where the chains are arranged in highly ordered three-dimensional lattices, and amorphous regions, where there is little or no order among the chains [14].

Irradiation of wood with g-rays contributes to the degradation of its constituents and changes the crystallinity in wood cellulose. There is evidence that the change in crystallinity importantly influences the elasticity, absorptive capacity, and other industrially valuable physical properties of the fiber [3].

To characterise changes in the crystalline structure of cellulose as a result of irradiation, the degree of crystallinity (crystallinity index), has been investigated, which is the ratio between the crystalline and total (amorphous and crystalline) cellulose.

Studies on the decomposition of pure cellulose and wood cellulose caused by g-rays have been investigated by several researchers [3]. However, the detailed pathway of gamma ray decomposition of the supramolecular structure of wood cellulose in situ has not been completely clarified.

Ours study focused on the changes in the crystallite size and the degree of crystallinity of pine wood cellulose in situ treated by different doses of g-radiation of wood in the air analysed by the X-ray diffraction method.

EXPERIMENTAL

Experimental samples were selected from the sapwood part of a log and produced as prismatic beams in which the direction of grains was exactly parallel to the longest edge of each sample. The dimensions of the samples were :10(R)x 5(T) x 150(L) mm.

Samples of pine wood Pinus sylvestris L. were treated by different doses of g-radiation: 20, 60, 120, 300, 500, 1500, 4500 and 9000 kGy at room temperature. The samples were exposed to gamma radiation in the type RChM-Gamma-20 apparatus with a cobalt [⁶⁰Co] as a radiation source. The gamma radiation exposition time varied from 6 hours to 7 months.

The cellulose content was determined with Seifert method.

The supramolecular structure of pine wood cellulose in situ was analysed using wide angle X-ray scattering (WAXS) technique, using Cu-K_a radiation (1.5418 Å) provided by an X-ray generator operating at 30 kV and 25 mA with a Ni-filter. The scattering intensity was collected as a function of 2θ, i.e. the angle between the incoming and the scattered beam, as well as a function of the scattering vector with the modulus = | q |=4p/l sin(2θ/2).

The X-ray diffraction pattern was recorded in the angle range of 5-30° 2Q. The intensity was computed with the step of 0.04^o/5 s. The deconvolution of peaks was performed by the method proposed by Hindeleh and Johnson, improved and programmed by Rabiej [15]. After the separation of X-ray diffraction lines, the degree of crystallinity (x_c) was calculated according to Segal et al. [16] and Isogai and Usuda [17] from the ratio of the integral intensity of crystalline portions to the total intensity of the sample. The degree of crystallinity was calculated as average values from three measurements for each irradiation dose group. The relative value of crystallinity X/X₀ was determined from the crystallinity, where X and X₀ are the crystallinity of irradiated and unirradiated pine wood cellulose, respectively.

The average size of the crystallites of wood cellulose, measured in the orthogonal direction to the (101)-, (10)- and (002)-plane, was determined by applying the Scherrer equation

D_hkl = K l/(H cosθ)

whereas the d-spacing was calculated using the Bragg`s equation:

d = n l/2 sinθ

where D_hkl is the average crystallite size perpendicular to the plane; K is the Scherrer constant (0,9); l is the wavelength of the X-ray; H is the FWHM (the full width at half maximum) of the reflex in radians; and q is the Bragg`s angle, d is spacing of planes, n- the order of diffraction.

RESULTS AND DISCUSSION

Figure 1 shows the X-ray diffractograms of pine wood, both unirradiated (a) and irradiated (b-f) by g-rays. The peaks on the diffraction patterns shown below the data correspond to the reflections. Three reflections of the crystalline portion of cellulose were observed.

This reflex belongs to cellulose I according to Meyer and Misch [18] and Fengel et al. [19]. The reflex of the (101)- and (10)-plane of cellulose appears at 14.49⁰ and 16.53⁰ respectively; and (002)-plane at 22.38⁰ for unirradiated pine wood. For the wood irradiation dose of 4500 kGy the reflex of the (101), (10) and (002)- plane of cellulose appears at 14.30⁰, 16.26⁰ and 21.66⁰, respectively. The shifts of peaks caused by radiating are especially distinct for the (002) diffraction.

A striking change in the profile appears after a 1500 kGy treatment. The crystalline reflection almost disappeared at 9000 kGy. The pine wood irridiation with this dose gave a completely amorphous pattern (Fig. 1f). This irradiation dose leads to the total destruction of crystalline and amorphous wood cellulose - the content of cellulose in this sample is 0% (Fig. 2).

Antoine et al. [8] received the total destruction of cellulose of Picea abies by g radiation at the dose of 6.55 MJ/kg (6550 kGy), whereas gamma irradiation of oak wood with the dose of 19 MJ/kg (19000 kGy) leads to the total decrystallinity of the cellulose [20].

Figure 2 shows changes in the relative degree of crystallinity of wood cellulose in situ and cellulose contents depending on the dose of gamma radiation of Pinus sylvestris.

The degree of crystallinity (x_c) of wood cellulose slightly decreases during the initial stage of gamma radiation at the dose range (20 – 120 kGy), whereas in higher doses (300 – 4500 kGy) a decrease of the degree of crystallinity is much greater - to about 42% for 4500 kGy and is zero at the dose of 9000 kGy as compared to non-irradiated wood. Therefore, the minimum dose for the decomposition of cellulose crystals is considered to be above the dose of 120 kGy.

For oak wood Cutter et al. [20] obtained the crystallinity index of 28% for the 9500 kGy dose radiation. The difference can be considered a result of various species of wood.

Simionescu et al. [21] reported no significant increase in crystallinity during the gamma irradiation of cellulose to 1000kGy. However in our study an increase of degree of crystallinity was not observed at low doses of the irradiation. This discrepancy might be result different types of used cellulose – pure and wood cellulose in situ.

Moreover, gamma-radiation at the low doses to 1 MJ/kg (1000 kGy) does not give a visible change in the wood structure of Picea abies, Pinus sylvestris, Fagus sylvatica and Populus alba observed under electron microscopy [22, 23].

Cellulose content, as it is show on Figure 2, decreases nearly identically with the curve of the degree of crystallinity up to the dose of 500 kGy. However, when the doses of irradiation increase above the level of 500 kGy then the cellulose content falls more rapidly as compared to the degree of crystallinity. In wood samples irradiated with 4500 kGy g- rays a 0.59 relative degree of crystallinity was obtained, whereas relative cellulose content (determinted by the Seifert method) in these samples was only 0.02.

It is worth mentioning that cellulose is essentially the sole crystalline material in wood.

This discrepancy between the cellulose content and the degree of crystallinity may be explained by the mechanism of wood destruction by g-radiation.

According to the hypotesis by Goto et al. [24] gamma-rays attack the structure of cellulose fibrils. This attack produces many crystalline defects throughout the whole fibres, but they retain their original shape. Even small external forces, such as a few minutes of ultrasonication, may reveal that the fibrils are only arrays of short fragments which can be easily disordered.

Results of our studies are in agreement with this hypothesis. In our studies the degree of crystallinity (determined by the X-ray diffraction method) is the effect of the X-ray diffraction of the native cellulose with defects in its “original shape”. Therefore, it can also indicate the crystallinity of cellulose apparently present in wood. The higher dose of radiation, the bigger is the difference between the contents of “true” cellulose and “apparent” cellulose.

The temperature conditions and chemicals used in the procedure of cellulose isolation (the Seifert method) cause the decomposition of the labile structure of cellulose fibrils on short, water soluble oligomers.

The higher the dose of irradiation, the higher the degradation of cellulose and the bigger difference between the crystallinity curve and the cellulose content curve was observed (Fig. 2).

When measuring the degree of crystallinity, the effect of the reduction in weight by gamma radiation was not considered. That is why , the absolute crystallinity X_abs=X_cW, was also estimated where W is the ratio of residual weight of irradiateted wood/weight of unirradiated wood [25].Variations in the absolute value of crystallinity show a tendency similar to the variation of the degree of crystallinity (Fig. 2). These results reveal that gamma ray irradiation causes the degradation of both the crystalline and amorphous part of cellulose.

Comparable results were obtained by Goto et al. [24] - high doses of radiation have the same effect on the crystalline and amorphous structure of cellulose.

Figure 3 shows changes in the average crystallite size orthogonal to the (101)-, (10)- and the (002)- plane of pine wood cellulose as a function of doses of g-ray radiation. The average crystallite size in all directions of wood cellulose at low doses of irradiation to 500 kGy did not change significant. Above 500 kGy the average crystallite size of irradiated samples decreased rapidly similary to the degree of crystallinity.

The radiation with the dose of 4500 kGy resulted in a decrease in the crystallite size by approximately 6-8 nm in three directions: (101), (10) and (002) (Fig. 4).

Fig. 3. Changes in the average crystallite size in the (101), (10) and (002) planes of wood cellulose in situ as a function of dose level

Fig. 4. Average crystallite size in (101), (10) and (002) direction in wood cellulose in situ before g-radiation (0 - kGy) and after radiation with the doses: 20, 60, 120, 300, 500, 1500 and 4500 kGy

The changes in the average crystallite size are almost the same as the changes in the crystallinity degree.

Figure 5 shows the changes in d-spacings of the wood cellulose irradiated with different doses of g-rays. The d-spacings in wood cellulose before radiation were: d₁₀₁ = 6.046 , d₁₀ = 5.246 Å and d₀₀₂=3.972 Å. All the d-spacings steadily increased along with g-radiation of wood. At 4500 kGy , the expansion of the lattice in the lateral direction was 2.4-3.3 % of the initial value.

It is evident that irradation not only caused the decomposition of wood cellulose in situ, but also had an effect on the crystallite dimensions. Above 500-1500 kGy, the gamma rays start to break up chemical bonds of molecules leading to the destruction of the crystalline order. However, this requires some additional studies.

CONCLUSIONS

1. The degree of crystallinity was reduced with increasing doses of gamma radiation and is declined to zero at a dosage of 9000 kGy in the pine wood cellulose in situ.

2. The crystallinity changes curve is similar to cellulose content changes curve in wood up to the dose 500 kGy. Above 500 kGy the cellulose content falls more rapidly as compared to the degree of crystallinity measured by X-ray diffraction.

3. The maximum values of diffraction peaks of wood cellulose irradiated varies from 2Θ = 22.38⁰ to 2Θ = 21.66⁰ for (002)-plane.

4. The d-spacings of pine wood cellulose steadily increased along with gamma radiation of wood.

5. Irradiation of wood diminish the average crystallite size of cellulose in situ in (101)-, (10)- and (002)- direction. The crystallite size (in 002- direction) is decreased from about 3,0 nm at 20 kGy to about 2,1 nm at 4500 kGy.

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