Microphotography of the surface was investigated by the secondary electron emission method using a scanning electron microscope - microcomputer assembly ...
MICROBIOLOGICAL
MODIFICATION OF ORGANIC FIBRES
E. V. Pisanova, V. S. Detsuk, M. B. Vainshtein, E. V. Voevoda, V. G. Grishchenkov, and V. V. Meshkov
UDC 539.216.1:539.25:678.019.38
Modification0f the surface of fibres by various methods (-y-radiation, chemical or plasma treatment) makes it possible, in many cases, to raise the level of interfacial interaction in reinforced composites and thereby considerably improve their properties [1]. A basic difficulty thereupon is the need to preserve the original structure and properties of the fibre in bulk only a thin layer of the fibre should be modified. In this respect, modification of fibres by use of microorganisms is promising [2-4]. At present the biodegradation of synthetic polymers is being studied intensively; however the action of bacteria or microscopic fungi on man-made fibres has been studied comparatively little [2-7]. Microbiological corrosion of block or film polymer specimens leads, as a rule, to changes in the bulk of the material, and, as a result, to a loss in strength, reduction in density, and other undesirable effects [8]. At the same time, the features of the supermolecular structure of highly oriented synthetic fibres permit one to hope for an occurrence of biochemical processes only on the surface, without breakdown in the mechanical properties of the fibres. The object of the work described in the present article was to study the structure and properties of man-made fibres which had been subjected to the action of microorganisms. As objects of study we chose the organic fibres Arimid S (technical specification 6-06-31-580-87), Oksalon (technical specification 6-06-23-38-4), and SVM (technical specification 6-06-454-79), which are among the promising fillers for polymeric composites. The following microorganisms were used: 1) Caulobacter bacteroides, strain 2; 2)Bacillus cereus, strain "Bactisubtil"; 3) Pseudomonas putida, strain BS 394-b; and 4) Pseudomonas, strains 640-x. Bacteria of the Caulobacter family are known as typical representatives of overgrowth microflora; the Pseudomonas strains are characterized by a high degradative activity with respect to synthetic organic substances [9]. Bacteria of the Bacillus family are able to change the chemical structure of the surface of aramide fibres [4]. Biochemical treatment of the fibres was carried out by placing them ina growth medium with the bacteria for 14 days under normal conditions and with moderate aeration. Control specimens were placed in sterile growth media for the same time and under the same conditions. At the end of treatment, the fibres were sterilized, repeatedly washed with hot distilled water, and dried. Study of the surface of monofilaments was carried out by use of a JSM-50A scanning electron microscope. Microphotography of the surface was investigated by the secondary electron emission method using a scanning electron microscope - microcomputer assembly as described in [10]. ESR spectra were recorded on an RÉ-1301 radiospectrometer at 373 K. The paramagnetic probe was used; nitroxyl radicals were introduced into the studied specimens from the vapor over an 8-h period. The correlation time of the nitroxyl radical was calculated by the known procedure of [11]. To evaluate wetting and adhesion in the monofilament-binder systems we used the following disperse polymers: polysulfone (PS), polyearbonate (PC), and epoxy resin ÉD-8, having a particle size of about 30/~m. The wetting angle was determined from the contour of small drops of melt formed on heat-treatment of solid particles of polymer present on the fibre over the course of 60 min, at an assigned temperature [12]. The adhesive strength of unions of monofilaments with the thermoplastic binder was determined by the three-fibre method of [13], which is based on measuring the force needed to withdraw the fibre from a thin layer of polymer. The strength of elementary filaments and adhesive joints was measured on an FO-1C breaking machine (Germany) at a loading rate of 0.8 g/sec and a fibre length of 20 mm.
Institute of Mechanics of Metallopolymeric Systems, Belorussian Academy of Sciences of the Academy of Sciences Gomel'. Institute of Bioehemistry and Physiology of Microorganisms, Russian Academy of Sciences, Pushchino, Moscow Oblast'. Translated from Khimicheskie Volokna, No. 3, pp. 18-21, May-June, 1993. 174
0015-0541/93/2503-0174512.50
©1993 Plenum Publishing Corporation
TABLE 1. Relative Activity of Various Microorganisms in Modifying the Surface of Fibres* 1
Strain of microorganism
Fibre
/~_1
Arlmid $VM Oxalone
+ M M
]
~.2 + M _
I
~3 + + q-
I
.w.*4 M -+
*Monitoring was carried out visually using an electron microscope; plus indicates a high degree of change in the surface; m indicates medium; and minus indicates a lack of change.
Fig. 1. Microphotographs of the surface of arimid (a, b, d), oksalon (c, e), and SVM (f) fibres, modified by strains No. 1 (b), 2 (d), 3 (a, f), and 4 (c, e). Magnification: 1000 x (ac) and 5000 x (d-f). 175
TABLE 2. Characteristics of Microrelief of Fibre Surface at an Unevenness Size of 0.07-9.00/xm
Fibre
~ype of IAngular distribution parameters ~reatment SSA IN,Bm-2 IO,deg. ,I x I V
SVM
--
1,052
1,5
17,6
0,60
0,48
Arimid
--
1,032 1,412
8,9 0,5
14,l 42,6
O,Ol --0,17 --0,66 --0,24
1,066
1,4
19,5
--0,15
--0,85
8,% n
b
a
2
d
c
1
2
54
5
1
25
#
5
Fig. 2. Histograms of proportion of surface area of SVM (a, b) and arimid (c, d) fibres occupied by unevennesses of various size (in/xm): 1) 0.07-0.14; 2) 0.14-0.28; 3) 0.280.56; 4) 0.56-1.12; 5) 1.12-2.25; a, c) starting fibres; b, d) modified fibres. Analysis of the limited number of publications devoted to the action of microorganisms on man-made fibres shows that the results of this acfion differ considerably, depending on the microorganisms used, the medium, and the duration of the treatment. The direction of action of the very same microorganism can be different, even for fibres of closely similar chemical nature, for example, Kevlar and polycaproamide [5], terlon, fenilon, and SVM [4]. In Table 1 we give the results of visual monitoring of the surface of treated fibres. Strain No. 3 proved active with respect to all three organic fibres, while the action of the remaining microorganisms was selective. Oksalon fibre proved resistant to strain No. 2; SVM, to strain No. 4. In the remaining cases a more or less well-expressed change in the surface was observed, while the control specimens were unchanged. As is evident from the electron-microscope photographs (Fig. 1), characteristic elements of the surface of modified fibres are numerous excrescences, of various forms. The reason for their generation may be either adsorpfion of metabolism products of the microorganisms on the fibre [2-4, 6], or biochemical degradafion of macromolecule in the surface layer accompanied by a change in supermolecular structure of this layer [5, 7,
8]. A more detailed study of the fibre surfaces was carried out by the secondary electron emission method [ 10]. Thereupon we calculated such statisfical characterisfics as the specific surface area (SSA), defined as the ratio of the real surface to the nominal; the density of the extremes N, the mean angle of inclination of roughnesses 0, and also the coefficients of asymmetry, ×, and of excess % in angular distribution. The use of a probe having a diameter from 0.07 to 9.00/xm in diameter made it possible to determine the distribufion parameters of unevennesses, both integrally in this range, and also selecfively in six subranges. Integral characteristics of the surface of the starting and modified SVM and arimid fibres are shown in Table 2. .As is evident from the data in Table 2, the mean angle of inclinafion of the unevennesses and the SSA are reduced for both types of fibre after treatment with the microorganisms, the change being more significant for arimid. At the same fime, 176
TABLE 3. Properties of Organic Monofilaments with Various Surface Treatments
Fibre
[---"~Wetting ang[I.e [ • ~- I l l with. melts of IAdhenszve ~orm o~ i,~ I ~ lindzcated sub-|strength treatment I ~ I~ ~ ö f union I =~ ~ I .~ _ I P S a t ~ED-8 ~ i t h P C ,~, MPa
~
_ 15,7 0,78 S t r a i n No. i 15,7 0,72 Oksalon 8,8 0,65 StrainNo. 4 8,8 0,64 SVM -13,5 3,90 Strain No. 3 13,5 3,67 Arimid
-
-
38 25 42 30 41 26
26 13 25 18 27 19
39 51 -
-
-46 57
*Area of union for arimid fibres, 2.5 x 10 .3 mm2; for SVM, 3.8 x 10 -3 m m 2. It was not possible to test oksalon by this method because of its low strength and small diameter (breakage of the fibre takes place; not shift of it relative to the matrix). the density of the extremes rises markedly in both cases, that is, the unevennesses are smoothed out, but they become larger. The changes in the coefficients of asymmetry and excess also indicate significant changes in the microtopography of the surface. The excess coefficient, which characterizes the deviation of the angular distribution of unevennesses from Gaussian, changes in the direction of negative values for both fibres. However, the asymmetry in both cases approaches zero, which indicates a greater isotropicity of the surface of the treated fibres. Thus, as a result of the life-acüvity of the microorganisms the coarse projections are destroyed; however, depressions (pores, microcracks) are probably filled with metabolism products, so that the characteristic size of the unevennesses as a whole is reduced. A selective analysis of the microrelief made it possible to determine the gain in SSA and density of the extremes which correspond to various sizes of the unevermesses. An important characteristic of the relief is the proportion of the surface 6, which is occupied by unevennesses of a definite size. This may be defined (in %) as the product of the characteristic area of the unevenesses by the number of them (density of the maxima for the corresponding size d): 7td 2
6 = --T- N. 100.
The dependence of 6 on the size of the unevennesses is shown in Fig. 2. It is evident that the change in surface relief takes place differently for SVM and arimid fibres, although there are common features. In both cäses the total area of the coarse unevenesses rises (1-2/xm), which are also discernible on the microphotographs (Fig. 1). Apparently these structural formations are products from the metabolism of microorganisms which are rather strongly bonded to the surface. Moreover, for both types of fibre the number of fine (0.1-0.3/xm) structural elements increases, although the spectrum of unevennesses is different for SVM and arimid. In the case of arimid, a uniform increase in the proportion of unevelmesses which are 1-3 ixm in size is observed (Fig. 2), while for SVM a maximum increase in the proportion of very fine ( - 0.1/xm) unevennesses is characteristic. For these fibres there is a certain mean size of unevennesses (0.3-0.6/xm) which are essentially not affected by microprobe treatment, and in the case of arimid the proportion of formations from 0.6-1.2/xm in size even decreases. Thus, as a result of treating fibres with microorganisms a smoothing out of fine unevennesses takes place, with simultaneous increase in the amount of them. Moreover, new structural elements 1-2/xm in size are formed; this is apparently caused by nonuniformity in the microbial treatment. An additional evaluation of structural changes in the surface layers of fibres was carried out by the ESR method. The correlation time of the nitroxyl radical on the surface of the starting arimid fibre is 9.1 x 10 -10 sec. Treatment of this fiber with strain No. 2 reduces the correction time to 7.5 × 10 -1° sec; and with strain No. 3, to 2.4 × 10 -1° sec. The decrease in correlation time of the radical indicates an increase in mobility of the segments of the polymer chains, which is brought about by a loosening of structure in the presence of very fine unevennesses. This agrees with the increase in proportion of unevennesses which are 0.07-0.14/xm in size (Fig. 2). Moreover, apparently the proportion of still finer structural elements (down to those comparable in size with the size of segments) also increases. 177
As is evident from Table 3, the change in microrelief and chemical structure of the surface of fibres as a result of treating them with microorganisms leads to an increase in degree of interaction at the fibre-polymeric matrix boundary. For all types of fibres, an improvement in wettability of their surface is observed, both with a thermoplastic and also with an epoxy binder. The adhesion of fibres (SVM and arimid) to a thermoplastic matrix also rises. It is to be noted that the fibre strength changes only slighfly as a result of biochemical treatment. Apparenüy microbiological degradation affects only a thin surface layer of the fibre. This permits orte to consider the action of microorganisms on man-made fibres to be on the efficient methods of modifying them, which makes it possible to improve the fibre-polymeric binder combinations without reducing its strength.
CONCLUSIONS Treatment of organic fibres with microorganisms makes it possible to change the surface microrelief without reducing fibre strength. The action of the microorganisms is selective: its intensity depends both on the nature of the fibre and also on the variety of microbe. As a result of the life-activity of the microorganisms, the following occur as a rule; smoothing out of coarse projections and filling in of depressions on the fibre surface, while the number of fine (0.1-0.3 /xm) structural elements increases significantly. Simultaneously smooth excrescences of size up to a few micrometers appear. Modification of the surface of fibres using microorganisms leads to an improvement in their wetting with melts of polymeric binders, and also to an increase in adhesion at the interface in organoplastics.
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178
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