Effect of Ceramic Nanoparticles on the Rheological Properties of

Teresa Mikołajczyk, Maciej Boguń Technical University of Łódź Department of Man-Made Fibres ul. Żeromskiego 116, 90-543 Łódź, Poland

Effect of Ceramic Nanoparticles on the Rheological Properties of Spinning Solutions of Polyacrylonitrile in Dimethylformamide Abstract The effects of nanoparticles of silica, hydroxyapatite and montmorillonite on the rheological properties of polyacrylonitrile spinning solutions in dimethylformamide, including rheological parameters n and K have been examined. The mechanism of the rheological behaviour of solutions containing nanoparticles is explained. These solutions are designed for the preparation of new-generation PAN fibres as precursor fibres to be carbonised and then used in medical applications. Key words: rheological properties, ceramic nanoparticles, PAN.

in contrast to fibres from regenerated cellulose. The incorporation of suitable nanoparticles containing Ca, Si and P into precursor fibres will allow carbon fibres to be prepared which containing these required elements in their structure. According to reports in the literature, it is possible to use compounds such as hydroxyapatite, montmorillonite and silica for medical purposes [1-4]. If carbon fibres are to be used for implants, they should have good strength properties and at the same time increased porosity. These properties are dependent on those of precursor fibres, and the formation of the latter by the wet process from a solution will make it possible to control the fibre spinning operations so as to achieve required properties [5-8].

n Introduction Present-day medicine is constantly searching for new materials designed for implants that could stimulate and support the process of bone reconstruction. This is an incentive to develop new types of carbon fibres which can form the basis for constructing implants which contain elements such as Ca, Si or P in their structure. The technology of carbon fibre manufacture requires the use of a suitable precursor. As is well known, the use of polyacrylonitrile fibre as a precursor of carbon fibre is most beneficial due to the high yield of the carbonisation process,

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It is also known that the porosity of carbon fibres is a resultant feature of precursor porosity (including, first of all, the content of pores with the largest diameter) as well as of the porosity created by the gaseous products emitted during carbonisation [9]. The aim of the present study is to assess the effect of the quantity of incorporated ceramic nanoparticles on the rheological properties of polyacrylonitrile spinning solutions in DMF, and to determine the stability of rheological parameters after

prolonged storage at a temperature of 20°C. An investigation directed in this way will allow an appropriate content of ceramic nanoparticles in the spinning solutions to be selected for preparing a new type of precursor PAN fibres, which could be carbonised and used for implants that could stimulate and support the reconstruction of bones and joints. Considering the destination of the PAN fibres investigated, it would be advantageous to include as great amounts as possible of suitable nano-additives in the fibre. However, as has been determined earlier (e.g. in [10] for another fibre-grade polymer) an amount of 5% of SiO2 nanoparticles can cause not only a decrease in the fibre's strength value, but can also lead to worse spinnability. The results of the studies on the development of fibre formation conditions and the properties of such fibres will be the subject of subsequent papers.

n Object of Study The fibre-forming polymer used in this study was Mavilon (produced by Zoltek), a PAN terpolymer, with an intrinsic viscosity η of 1.29 as determined by the viscosimetric method in DMF at 20°C, and having the following composition:

Table 1. Characteristics of the ceramic nanoparticles used in the study. Size of grain, nm

Size of lamellae, nm

Distance between packages, nm

SiO2

50 - 150

-

-

Hydroxyapatite

30 -120

-

-

Montmorillonite

-

800 x 500

2.3

Type of additive

FIBRES & TEXTILES in Eastern Europe January / March 2005, Vol. 13, No. 1 (49)

curves within the range of slightly higher shearing rates.

§ 93-94% by wt. of acrylonitrile units, § 5-6% by wt. of methyl acrylate units, § about 1% by wt. of sodium allylsulphonate. The following ceramic nanoparticles were added to the spinning solutions: § hydroxyapatite, § montmorillonite, § silica. Their characteristics are given in Table 1. For rheological measurements, we used 22% PAN solutions in DMF containing a given additive in a quantity of 1%, 2% or 3% in relation to the polymer weight.

The ceramic nanoparticles were characterised on the basis of their sizes as determined by the electron microscopy method. In the case of montmorillonite, its inter-layer distances were determined on the basis of the first peak position in the WAXS X-ray diffractograms. If the reflexes determined occur above 2Θ angle of 2°, they can be recorded by the WAXS, as well as the SAXS method. The lower limit of recording for high-quality diffractometers ranges within 2°. Many WAXS diffractograms for polymer were made (montmorillonite composites are presented in [11]), on the basis of which the structure periods and the inter-layer distances were elaborated using Bragg's law. The measurements with X-ray radiographic examination were carried out with a first-class quality Zeiffert diffractometer. The measurement results are presented in Table 1.

n Measuring Methods The rheological properties of spinning solutions were measured by means of a Rheotest RV rotary rheometer at a temperature of 20°C and a shearing rate of 146.8 l/s, using an H cylinder. The rheological parameters n and K were established from the flow curves in a logarithmic system, neglecting the shearing stress range below 10 graduations of reading, as it is known that for a polymer with a linear, quite rigid chain structure, the orienting effect of this shearing can be negligibly small at very low shearing rates. The system structure is totally determined by Brownian movement, which causes a chaotic distribution of macromolecules. The internal friction of such a system will be constant and maximum. Furthermore, the power model of Ostwald de Vaele

The measurements with electron microscopy were carried out at the Institute of Fibre Physics, Technical University of Łódź, while the X-ray radiographic examinations were carried out at the Department of Textile Engineering, University of Bielsko-Biała.

n Discussion of Results Based on the flow curves obtained (Figure 1) of 22% PAN spinning solutions containing 1%, 2% or 3% of montmorillonite, one can state that these solutions are non-Newtonian fluids, rarefied by shearing, without a flow limit. A similar character of flow curves is observed in the case of the PAN spinning solutions containing SiO2 (Figure 3) and hydroxyapatite (Figure 5). For all solutions, the tangential stress increases less

τ = k Yxn where: n, k - the rheological parameters of the model, τ - the shearing stress, Yx - the shearing rate, which is an approximate model, considerably better approximates the flow

Table 2. Rheological properties of 22% PAN spinning solutions in DMF containing various contents of ceramic nanoparticles. Parameters n and K after storage for 24 h

Parameters n and K after storage for 48 h

Type of nanoparticles in spinning solution

Content of nanoparticles

n

K

n

K

-

-

0.964

27.5

0.936

32.3

1

0.959

25.1

0.896

28.2

2

0.959

26.0

0.977

26.1

3

0.953

29.9

0.978

30.1

1

0.964

26.3

0.947

30.6

2

0.963

27.0

0.978

29.1

3

0.955

33.3

0.931

45.8

1

0.982

25.4

0.966

27.0

2

0.981

26.5

0.971

28.0

3

0.955

32.4

0.938

36.0

Montmorillonite

SiO2

Hydroxyapatite


than proportionally with the increase in shearing rate, while the curves pass through the origin of the coordinates. On the other hand, the apparent dynamic viscosity decreases with the increase in the shearing rate, which is typical for polymeric fluids, and the increased content of nanoparticles does not change the character of this relationship. The flow curves of spinning solutions containing 1% and 2% of nanoparticles (SiO2, hydroxyapatite or montmorillonite) practically overlap (Figures 1, 3 and 5). This would indicate that such a low content of nanoparticles does not significantly change the rheological properties of spinning solutions, while the increase in the content of any of the nanoparticles up to 3% brings about a change in rheological parameter n, and makes the non-Newtonian character of spinning solutions more evident (Figure 7), in comparison with the PAN solution which has no nanoparticles. It has been found that with the increase in any of the nanoparticles used, the nonNewtonian character of fluids is more and more visible, as indicated by the decreased value of rheological parameter n and the simultaneously increased parameter K (Table 2). The smallest effect on the values of rheological parameters n and K, in relation to those of 22% PAN solutions without nanoparticles, is shown by the solutions containing montmorillonite. Its content of 1-2% practically does not change the rheological parameter n, which may testify to a progressing intercalation of montmorillonite in the spinning solution. Similarly, the spinning solutions containing 1-2% of SiO2 show negligible changes in the rheological parameter n, which may be connected with the low quantities of silica nanoparticles. The solution containing 3% of SiO2 shows its non-Newtonian character to a greater extent. On the other hand, the solutions containing hydroxyapatite behave in a completely different manner. A low addition of 1% or 2% brings about an increase in parameter n to a level of 0.98, which causes the character of this spinning solution to become more similar to that of a Newtonian fluid. However, the increase in the hydroxyapatite content to 3% decreases this parameter to 0.95, which is a lower value than that of the PAN solution with no additive. Storing spinning solutions for a prolonged period of time at a temperature

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35.0

1800 1600

dynamic viscosity, Pa s

30.0

shear stress, Pa

1400 1200 1000

3% montmorillonite 2% montmorillonite 1% montmorillonite

800 600

20.0 15.0

5.0

200

0

10

20

30

40

50

60

70

80

0.0

90

shearing rate, 1/s

Figure 1. Dependence of shearing stress on the shearing rate for 22% PAN solutions in DMF containing montmorillonite.

20

30

40

50

shearing rate, 1/s

60

70

80

90


35.0

1400

shear stress, Pa

10

40.0

1600

1200 1000 3% silica 1% silica 2% silica

800 600

30.0 25.0 20.0

3% silica

15.0

2% silica

10.0

400

1% silica

5.0

200

0.0

0

10

20

30

40

50

shearing rate, 1/s

60

70

80

90

of 20°C causes an increase in the rheological parameter K with a simultaneous decrease in parameter n (Table 2), which indicates that these solutions are subject to a slow ‘aging’ process. The use of the maximum content (3%) of ceramic nanoparticles results from previous findings concerning another fibreforming polymer [10]. Contents higher than 3% cause the spinning process to proceed in a less stable manner. The incorporation of ceramic nanoparticles into spinning solutions makes the non-Newtonian character of the fluid more distinct, and increases the effect of rarefaction by shearing. In accordance with the commonly accepted explanation of rarefaction by shearing [12], the macromolecules of polyacrylonitrile are considerably entangled in a stationary fluid. Their effective dimensions with an immobilised continuous phase are considerable. The presence of silica nanoparticles, layers of intercalated montmorillonite or hydroxyapatite inside and among

0

10

20

30

40

50

60

70

80

90

shearing rate, 1/s

Figure 3. Dependence of shearing stress on the shearing rate for 22% PAN solutions in DMF containing SiO2.

30

0

Figure 2. Dependence of dynamic viscosity on the shearing rate for 22% PAN solutions in DMF containing montmorillonite.

1800

0

3% montmorillonite 2% montmorillonite 1% montmorillonite

10.0

400

0

25.0

Figure 4. Dependence of dynamic viscosity on the shearing rate for 22% PAN solutions in DMF containing SiO2.

these tangles may intensify this effect. An additional factor that increases this effect is the tendency of nanoparticles to agglomerate, and in the case of montmorillonite, the circumstance that only a partial process of montmorillonite intercalation takes place. During shearing the polymer chains are straightened and disentangled as the shearing rate increases. This results in a gradual decrease in the internal friction of the system, due to the smaller dimensions of such particles and the weaker interactions between them. These interactions also weaken due to the presence of silica or hydroxyapatite nanoparticles or montmorillonite layers with dimensions of micrometers between polymer macromolecules. These layers are apt to be arranged in parallel, according to the direction of shearing stresses. On the other hand, this effect can be weakened by the ongoing process of montmorillonite intercalation, which, due to the decrease in the ‘thickness’ of its packages and their proximity, leads to a decreased distance between macromolecules. We have found similar ef-

fects of SiO2 and montmorillonite on the mechanism of rheological behaviour of spinning solutions in the case of another fibre-forming polymer [13]. The presence of nanoparticles in the system is also connected with the course of solvation phenomena, which fact explains the mechanism of rarefaction by shearing. The presence of silica nanoparticles facilitates a gradual tearing off of the solvation sheath with an increase in the shearing rate, followed by a decrease in the internal friction of the system. This effect may be weaker in the case of montmorillonite, while the presence of its layers between PAN macromolecules undoubtedly causes the polymer-solvent interaction to weaken. In the case of hydroxyapatite, these interactions are affected by the presence of nanoparticles which, due to their hydrophilic character, show no compatibility with solvent. This fact remains in relation to the considerably lower stability of spinning solutions containing hydroxyapatite, and the tendency of nanoparticles to agglomerate


35.0

1200

30.0


1400

shear stress, Pa

1000 800

3% hydroxyapatite

600

2% hydroxyapatite 400

20.0

1% hydroxyapatite

15.0

2% hydroxyapatite 3% hydroxyapatite

10.0

1% hydroxyapatite

200 0

25.0

5.0

0

10

20

30

40

50

0.0

60

0

10

20

shearing rate, 1/s

Figure 5. Dependence of shearing stress on the shearing rate for 22% PAN solutions in DMF containing hydroxyapatite.

1600

34.0


36.0

shear stress, Pa

1400 1200 1000 800 3% silica 3% montmorillonite

200 0

20

30

shearing rate, 1/s

40

50

Figure 7. Dependence of shearing stress on the shearing rate for 22% PAN solutions in DMF containing nanoparticles.

after 48 h and to precipitate from the solution in the state of rest. On the other hand, the solutions containing silica nanoparticles and intercalated montmorillonite show a considerably higher stability of rheological parameters over a long period of time. The different effects of the ceramic nanoparticles on the rheological behaviour of the spinning solutions under investigation, and the different values of the rheological parameters n and K, can be accounted for by the different characters of the nanoparticles (organophilic or hydrophilic) and their different physical structures (granular or laminar, as in the case of montmorillonite).

n Conclusions § The PAN spinning solutions in DMF containing nanoparticles of silica, hydroxyapatite or montmorillonite are non-Newtonian fluids rarefied by shearing, without any flow limit.

3% silica 3% hydroxyapatite 3% montmorillonite

32.0

without nanoparticles

30.0 28.0 26.0

20.0

10

60

22.0

without nanoparticles

0

50

24.0

3% hydroxyapatite

400

40

Figure 6. Dependence of dynamic viscosity on the shearing rate for 22% PAN solutions in DMF containing hydroxyapatite.

1800

600

30

shearing rate, 1/s

0

10

30

shearing rate, 1/s

40

50

Figure 8. Dependence of dynamic viscosity on the shearing rate for 22% PAN solutions in DMF containing nanoparticles.

§ The incorporation of nanoparticles slightly intensifies the non-Newtonian behaviour of fluids, and brings about a significant increase in the rheological parameter K. This effect is greater in the case of solutions containing 3% of silica or hydroxyapatite nanoparticles than in the case of solutions with intercalated montmorillonite. § The values of rheological parameters n and K depend on the chemical composition of nanoparticles, their compatibility in the polymer-solvent system and physical form.

Acknowledgements The authors thank Prof. K. Haberko and Prof. S. Błażewicz (AGH Cracow) for providing the nanoparticles.

References 1. T. Peltola, M. Jokinen, B. Vettola, H. Rahiala, A. Yli-Urpo, Biomateriale 22, (2001), 589-598. 2. L. Borum, O.C. Wilson, Biomateriale 24, (2003), 3681-3688.


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3. J. Heikkilä, J.A. Aho, A. Yli-Urpo, O.H. Andersson, H.J. Aho, R.P. Happonen, Acta Orthop Scand, Dec. (1993), 678-82. 4.J. Deszczyński, G. Szczęsny, Orthopaedics, Rehabilitation and Rheumatology, No 2 (2000) (in Polish). 5. I. Smitzis, Colloid Polym. Sci., 255, (1977), 948. 6. K.E. Perepelkin, Chemickée Vlákna 23 (4-5), (1974), 71. 7. S.P. Papkow, Chim. Volok. 4, (1981), 13. 8. T. Mikołajczyk, I. Krucińska, K. KameckaJędrzejczak, Textile Res. J., 59, (1989), 557. 9. T. Mikołajczyk, I. Krucińska, Fibres & Textiles in Eastern Europe 3 (3), (1995), 44. 10. T. Mikołajczyk, D. Wołowska-Czapnik, M. Boguń, Fibres & Textiles in Eastern Europe, 12(3), 19-23 (2004). 11. S. Ray, M. Okamoto, Polymers, 28 (2003), p. 1539-1641. 12. J. Ferguson, Z. Kembłowski, ‘Applied Rheology of Fluids’, Łódź (1995). 13. T. Mikołajczyk, M. Szczapińska, ‘Rheological Properties of Spinning Solutions of Modified Polyimidoamide Containing Ceramic Nanoparticles’, Fibres & Textiles in Eastern Europe, 13(1), 24-27 (2005). Received 28.04.2004

Reviewed 08.10.2004

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