CC BY
12
UDC 678.046.39:678.7-1:678.743.21
G. V. Kozlov, G. E. Zaikov, A. K. Mikitaev, Kh. S. Abzaldinov
THE AGGREGATION PROCESS AS LARGE-SCAL DISORDER CAUSE IN NANOCOMPOSITES POLYMER/ORGANOCLAY
Keywords: nanocomposite, organoclay, aggregation, tactoid, reinforcement degree, anisotropy.
It has been shown that organoclay platelets aggregation in ''packets " (tactoids) results in large-scale disorder intensification, that reduces nanofiller anisotropy degree. In its turn, this factor decreases essentially nanocomposites reinforcement degree. The interfacial adhesion role in anisotropy level definition has been shown.
Ключевые слова: нанокомпозит, глина, агрегация, тактоид, степень усиления, анизотропия.
Показано, что агрегация пластин органоглины в "пачки" (тактоиды) приводит к усилению крупномасштабных беспорядков, что снижает степень анизотропии нанонаполнителя. В свою очередь, этот фактор существенно уменьшает степень усиления нанокомпозитов. Показана роль межфазной адгезии в определении уровня анизотропии.
Introduction
Organoclay belongs to anisotropic nanofillers, for which their anisotropy degree, i.e. ratio a of organoclay platelets (aggregates of platelets) length to thickness, has large significance [1]. The reinforcement degree En/Em of nanocomposites polymer/organoclay can be estimated according to the equation [1]:
E = 1 + 2aCa<pn , (1)
/77
where En and Em are elasticity moduli of nanocomposite and matrix polymer, respectively, Ca is an orientation factor, which is equal for organoclay to about 0.5 [1], pn is organoclay volume content.
In case of organoclay platelets aggregation, i.e. their "packets" (tactoids) formation [2] such "packets" thickness increasing takes place in comparison with a separate platelet, that results in length/thickness ratio a reduction at platelet constant length and, as consequence, nanocomposites reinforcement degree decreasing is realized according to the equation (1). The present work purpose is the analytical study of organoclay aggregation, i.e. large-scale disorder, influence on reinforcement degree on the example of nanocomposites plasticat of poly(vinyl chloride) -organomodified montmorillonite.
Experimental
The plasticat of poly(vinyl chloride) (PVC) of mark U30-13A, prescription 8/2 GOST 5960-72 was used as a matrix polymer. The modification product of montmorillonite (MMT) of deposit Gerpegezh (KBR, Russian Federation), modified by urea with content of 10 mass % with cation-changing caposity of 95 mg-eq/100 g of clay was applied as a nanofiller. Organoclay contents were varied within the limits of 1-10 mass %.
The nanocomposites PVC-MMT preparation was performed as follows. The components were mixed in a two-speed blender R 600/HC 2500 of firm Diosna, the design of which ensures intensive intermixing in turbulent regime with blends high homogenization and blowing by hot air. After components intensive intermixing the composition was cooled up to temperature 313 K and processed on a twin screw
extruder Thermo Haake, model Reomex RTW 25/42, production of German Federal Republic, at temperature 418-438 K and screw rotation speed of 48 rpm.
Sheet nanocomposite was obtained by a hot rolling method at temperature (433+10) K during 5-15 min. The samples in the shape of a two-sided spade with sizes according to GOST 112 62-80 were cut out by punch. Uniaxial tension mechanical tests have been performed on the universal testing apparatus Gotech Testing Machine CT-TCS 2000, production of German Federal Republic, at temperature 293 K and strain rate of ~ 2x10-3 s-1.
Results and Discussion
The nanofiller initial particles aggregation is the main process, enhancing large-scale disorder level in polymer nanocomposites. For each nanofiller type this process has its specific character, but in case of anisotropic nanofillers (organoclay, carbon nanotubes) application this process always decreases their anisotropy degree, i.e. aspect ratio a, that reduces nanocomposites reinforcement degree according to the equation (1). Let us consider the physical bases of the value a reduction at organoclay content increasing in the considered nanocomposites. As it is known [3], organoclay platelets number Npl in "packet" (tactoid) can be determined as follows:
Np= 24 - 5.7ba , (2)
where ba is the dimensionless parameter, characterizing the level of interfacial adhesion polymeric matrix-nanofiller, which is determined with the aid of the following percolation relationship [2]:
E = 1+ 11Pa)1■7, (3)
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where c is constant coefficient, which is equal to 1.955 for intercalated organoclay and 2.90 - for exfoliated one.
In its turn, the value pn can be determined according to the well-known formula [2]:
• (4)
Pn
where Wn is nanofiller mass content; pn is its density, which for nanoparticles is determined as follows [2]:
pn= 188(z?p)1/3,kg/m3, (5)
where Dp is the initial nanoparticle diameter, which is given in nm.
In case of organoclay parameter Dp is determined as mean arithmetical of its three basic sizes: length, width and thickness, which are equal to 100, 35 and 0.65 nm, respectively [2].
An alternative method of the value Npl estimation gives the following equation [2]:
Np,dp,
(6)
((pi - 1)001 ^upl where % is relative volume content of montmorillonite in tactoid ("effective particle" [4]); dpl is thickness of organoclay separate platelet; d001 is interlayer spacing, i.e. the distance between organoclay platelets in tactoid, which can be estimated according to the following formula [3]:
dom = 1.21 ba, nm. (7)
In its turn, parameter % is determined as follows [3]:
% = -, (8)
<Pn + <Pif
where is a relative fraction of interfacial regions in nanocomposite, estimated with the aid of the following percolation relationship [2]:
E = 1 + 11((pn + <) ^ m
(9)
The comparison of Npl value calculations according to the equations (2) and (6) showed their close correspondence.
In Fig. 1 the dependence Npl(<n) for nanocomposites PVC/MMT is adduced.
Fig. 1 - The dependence of organoclay platelets number per one tactoid Npl on nanofiller volume content <n for nanocomposites PVC-MMT
As one can see, at quite enough small values <n<0.05 fast growth of Npl occurs, i.e. strong aggregation of organoclay initial platelets, and at <„>0.05 the value Npl achieves the asymptotic branch: Npi «22. As it was noted above, the reduction of nanofiller anisotropy degree, characterized by parameter a, was defined by its aggregation, the level of which could be characterized by parameter Npl. In Fig. 2 the
dependence a( N2,) for the considered nanocomposites,
pi
where quadratic shape of dependence was chosen with the purpose of its linearization.
600 JT2 lvpl
Fig. 2 - The dependence of organoclay anisotropy degree a on its platelets number per one tactoid Npl for nanocomposites PVC-MMT
As was to be expected, the organoclay anisotropy degree, characterized by the parameter a, reduction is observed at its platelets aggregation, characterized by the value Np,, enhancement, which is expressed analytically by the following equation:
a = 10.5 - 0.018Np,, (10)
where the value a was estimated according to the equation (1).
Theoretical method of parameter a (a7) estimation can be obtained as follows. Organoclay aggregates (tactoids) anisotropy degree can be determined according to the equation:
(11)
T Lpl a = -
org
t,
where Lpl is organoclay platelet length, which is equal to ~ 100 nm [2], torg is its tactoid thickness.
In its turn, the value torg is determined as
follows:
(12)
lorg
^001^/ +1.
Besides, it should be borne in mind, that experimental value a in the equation (1) is determined on the basis of reinforcement degree E„/Em, i.e. on the basis of mechanical tests results. This means, that value a depends on conditions of stress transfer on interfacial boundary polymeric matrix-organoclay, i.e. on the parameter ba value. Then parameter aT can be determined finally as follows:
a =—1— (13)
121b aNp+ 1
In Fig. 3 the comparison of experimental a and calculated according to the equation (13) a values of organoclay tactoids aspect ratio, characterizing large-scale disorder level for the considered nanocomposites, is adduced. As one can see, good enough correspondence of experiment and theory is obtained (average discrepancy of a and aT makes up ~ 9 %).
The equations (10) and (13) allow to predict reinforcement degree En/Em on organoclay known structural characteristics.
In Fig. 4 the comparison of theoretical curves E„/Em(<„), calculated according to the equation (1), where parameter aT was determined according to the formulas (10) and (13), and corresponding the
experimental data is adduced. As one can see, a good both qualitative (the theoretical curves are reflected experimental dependence maximum without existence of maximums for parameters Npi and d0M) and quantitative correspondence of theory and experiment (their average discrepancy makes up less 2.5 %).
dependences En/Em(pn) for nanocomposites PVC/MMT are adduced. As it follows from this comparison, both lower values of reinforcement degree and its decay at Wn >7 mass % are due precisely by organoclay platelets aggregation in "packets" (tactoids).
Ed Em
0 4
Fig. 3 - The comparison of experimental a and calculated according to the equation (13) aT organoclay anisotropy degree for nanocomposites PVC-MMT
Fig. 4 - The comparison of calculated according to the equation (1) with usage of the formulas (10) (1) and (13) (2) for parameter aT determination and experimental (3) dependences of reinforcement degree En/Em on nanofiller volume content ф„ for nanocomposites PVC-ММТ
Let consider in conclusion the influence of organoclay platelets aggregation or large-scale disorder on the considered nanocomposites reinforcement degree. In Fig. 5 the experimental and calculated according to the equation (1) at organoclay aggregation minimum level (Np/=9.70, ba=2.51, a=8.6), corresponding to organoclay content Wn=1 mass %,
© G. V. Kozlov - Senior Researcher, Kh.M. Berbekov Kabardino-Balkarian State University, Nal'chik, Russia, [email protected]; G. E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, A. K. Mikitaev - Doctor of Chemistry, Full Professor, Head of Organic Chemistry and Macromolecular Compounds Department, Kh.M. Berbekov Kabardino-Balkarian State University, Nal'chik, Russia; Kh. S. Abzaldinov - Ph.D., Associate Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia.
© Г. В. Козлов - ст. науч. сотр., Кабардино-Балкарский госуд. ун-тет им. Х.М. Бербекова, Нальчик, Россия, [email protected]; Г. Е. Заиков - д-р хим. наук, проф. каф. технологии пластических масс КНИТУ, Казань, Россия, А. К. Микитаев - д-р хим. наук, проф., зав. каф. органической химии и высокомолекулярных соединений, Кабардино-Балкарский госуд. ун-тет им. Х.М. Бербекова, Нальчик, Россия; Х. С. Абзальдинов - канд. хим. наук, доц. каф. технологии пластических масс КНИТУ.
Fig. 5 - The comparison of calculated according to the equation (1) at the condition of organoclay minimum aggregation (1) and experimental (2) dependences of reinforcement degree En/Em on nanofiller volume content for nanocomposites PVC-ММТ
Conclusions
Thus, the present work results have demonstrated that organoclay platelets aggregation in "packets" (tactoids) results in large-scale disorder enhancement, that reduces nanofiller anisotropy degree. In its turn, this factor decreases essentially reinforcement degree (or elasticity modulus) of nanocomposites polymer-organoclay, moreover at large enough organoclay contents (>7 mass %) reinforcement degree reduction at nanofiller content growth is observed. The important role of interfacial adhesion in anisotropy level determination has been shown.
References
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