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Effect of cobalt fillers on polyurethane segmentations investigated by synchrotron small angle X-ray scattering.

1. Introduction

The versatility of segmented polyurethane (PU) is based on the combination of a rubbery soft segment (SS) and a semicrystalline hard segment (HS). Their properties are varied with the ratio of hard/soft segments and their segmentation. In addition to their conventional uses in forms of PU thermoplastics and foams, the incorporation of particulate fillers to obtain PU composites enhances the applications as smart materials. The magnetic fillers in polymeric matrix give rise to magnetic hysteresis, magnetoviscoelasticity, magnetorheology, and magnetoelectricity [1]. Cobalt (Co) is a sot ferromagnetic material with high magnetocrystalline anisotropy and saturation magnetization. It follows that Co is often found in functional magnetic compounds and composites. In our previous work [2], the incorporation of 20%-60wt.% Co powders in PU gave rise to magnetic permeability useful for magnetic devices and components. Moreover, the change in the dielectric properties of PU by the inclusion of Co can be utilized in antistatic propose. However, the configuration of HS and SS in PU/Co composites which dictates the mechanical and chemical properties was not investigated. Only differential thermal calorimetry (DSC) spectra imply that PU chains were disrupted.

Small-angle X-ray scattering (SAXS) is a nondestructive technique in morphological investigation of nanostructures [3]. The morphology changes can also be monitored in real time by the dynamic analysis without a need for special sample preparation process. The measured data are commonly presented as the plot between the intensity of scattered X-ray (7) and the scattering vector (q) which is given by [3]

q = 4[pi] sin [theta]/[lambda], (1)

where [lambda] is the wavelength of the X-ray and [theta] is half the scattering angle with respect to the direction of the incident X-ray. These SAXS proiles are related to the contrast in the electron density ([DELTA][rho]) of different microphases.

In the case of PU and PU composites, SAXS proiles can be employed to complement DSC in the investigation of segmentation [4-10]. The arrangement of HS and SS can be deduced by fitting the characteristic SAXS peak with the equations from theoretical models [4]. With this knowledge, this work employed the synchrotron SAXS to study the effect of Co fillers of up to 60 wt.% on the segmentation between HS and SS of PU. The configuration can be concluded from the agreement between the experimental SAXS profiles and the theoretical model.

2. Models

The segmentation in PU can occur according to a variety of models, for example, lamellar model, Zernike-Prins model, Percus-Yevick model, and Teubner-Strey model [4]. The nanostructural arrangement can be concluded from the agreement between experimental results and the appropriate scattering model.

2.1. Lamellar Model. For the lamellar model illustrated in Figure 1(a), HS and SS are separated as parallel layers called lamellae. If the edge effect is neglected, the scattering intensity perpendicular to a stack containing a considerable number of infinite lamellae, [I.sub.inf](q), can be given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (2)

where the subscripts H and S denote the parameters associated with HS and SS, respectively. [phi], d, and [sigma] are the volume fraction, thickness, and standard deviation of each segment. Assuming that the lamellar thickness follows the Gaussian distribution with [psi] = q[d.sub.H], the term 1 - exp(-[q.sup.2] [[sigma].sub.H]/2) exp(-iq[d.sub.H]) = [[phi].sub.H] exp(-i[psi]) is a Fourier transform of the lamellar thickness distribution for HS. Likewise, the expression for SS can be derived using [chi] = q[d.sub.S]. Equation (2) can be reduced to

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (3)

In the case of isotropic assembly of lamellar stacks, the scattering intensity becomes

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (4)

where [I.sub.0] is a scaling factor which depends on several parameters in the measurement including the scattering contrast, surface area per unit volume, and intensity of incident X-ray.

2.2. Zernike-Prins Model. Originally proposed for a liquid, the Zernike-Prins model has been applied to segmented copolymers. To explain the scattering from dense disorder systems, the crystal imperfection and separated minor phase are treated like the scattering bodies in arbitrary direction for one-dimensional random statistical lattice as illustrated by Figure 1(b). The observed scattering intensity from globular domains of HS on a distorted lattice in SS matrix is composed of the form factor, P(q), and the structure factor, S(q)

I(q) = [I.sub.0]P(q)S(q). (5)

By omitting the interference effect, the term P(q) representing the scattering from spherical bodies with radius, R, and volume, V, can be written as

P(q) = 9[([DELTA][rho]).sup.2][V.sup.2][{[sin(qR) - qRcos(qR)]/[(qR).sup.3]}.sup.2]. (6)

On the other hand, S(q) corresponds to the interference by the scattering from neighboring bodies aligning in one dimension which is given by

S(q) = [1 - [[phi].sup.2.sub.H]]/[1 - 2[[phi].sup.2.sub.H]cos(qd) + [[phi].sup.2.sub.H]], (7)

where d is the mean distance between the center of randomly distributed neighboring bodies. The parameter [phi] = exp (-[q.sup.2][sigma]/2) is the volume fraction of each segment. By substituting (6) and (7) into (5), the scattering model can be expressed as

I(q) = [I.sub.0]9[([DELTA][rho]).sup.2][V.sup.2][{[sin(qR) - qRcos(qR)]/[(qR).sup.3]}.sup.2] x {[1 - [[phi].sup.2.sub.H]]/[1 - 2[[phi].sup.2.sub.H]cos(qd) + [[phi].sup.2.sub.H]]}. (8)

2.3. Percus-Yevick Model. The scattering equation from globular domains in a liquid-like dispersion is based on statistical-mechanical treatments by Percus and Yevick. In Figure 1(c) the structure factor of matrix containing spheres of radius, R with the thickness of the boundary layer th is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (9)

The parameter x = 2q[R.sub.H] where [R.sub.H] = R + th. Three other parameters ([alpha], [beta], [gamma]) in (9) are defined in terms of the volume fraction occupied by the spheres, [phi]:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (10)

By substituting the form factor from (6) and the structure factor from (9), the scattering intensity in (5) becomes

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (11)

2.4. Teubner-Strey Model. The Teubner-Strey model illustrated in Figure 1(d) is commonly used for explaining the scattering from microemulsion systems. This random two-phase model assumes the spatial dependence of a pair correlation function, [gamma](r), of the form

[gamma](r) = [d/2[pi]r] exp(-r/[xi]) sin(2[pi]r/d). (12)

Te sine term indicates the periodicity in the correlation function with the domain size characterized by d. The exponential term expresses the loss of long range order with the correlation length [xi], The scattering intensity is written as

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. (13)

3. Experimental

PU composites incorporating 0, 20, 40, and 60 wt.% Co powders (99.98%, [phi] [approximately equal to] 2 [micro]m) are, respectively, referred to as PU, PU20, PU40, and PU60. Polyether polyol (SS) and diphenylmethane-4,4 '-diisocyanate (HS) were used as starting materials in the conventional prepolymer method. Firstly, polyol (20 g) was mixed with Co dispersed in silicone oil, and the mixture was heated to 75[degrees]C. Under constant stirring without creating air bubble, diisocyanate (2.1 g) was then added. After the homogeneous mixture became viscous by the addition of the catalyst, PU/Co composites were tape-casted to obtain approximately 300 [micro]m thick samples. They were dried and received thermal treatment at 80[degrees]C for 180 min.

The characterizations of PU/Co composites by microscopy, DSC, vibrating sample magnetometry, complex permittivity, and permeability spectroscopy were presented elsewhere [2]. The SAXS experiment was carried out at BL 2.2, Siam Photon Laboratory, Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand [11]. The X-ray energy used was 8keV. A CCD (Mar SX165) was used to record the 2D SAXS patterns. The sample-detector distance was calibrated using scattering of silver behenate powder. Te patterns were then circularly averaged to obtain 1D SAXS proiles. The peak of SAXS proiles was curve-itted according to the lamellar (see (4)), Zernike-Prins (see (8)), Percus-Yevick (see (12)), and Teubner-Strey (see (16)) models using the least square method in MatLab.

4. Results and Discussion

Te SAXS profiles of PU and PU/Co composites shown in Figure 2 exhibit the single peak around q = 0.8-0.9 [nm.sup.-1]. Such peak is commonly observed in PU. The intensity of the peak is decreased with the increase in the Co loading. This implies that HS and SS are increasingly mixed with the reduction in clear scattering boundaries [6, 7]. In addition to the reduction in intensity, the peak position corresponding to HS distance calculated by using Bragg's law is slightly shifted by the inclusion of Co fillers. It can be inferred that the length of the HS has been reduced by the inclusion of Co. The densely packed Co aggregates may also shield some X-ray but should not affect the scattering because their sizes are much larger than the characteristic size measurable by the measurement setup.

The curve fitting with four different models is shown in Figures 3, 4, 5, and 6, and the obtained parameters are listed in Table 1. The best it of the unfilled PU peak is obtained in the case of Zernike-Prins model. The Percus-Yevick model also agrees well with experimental SAXS profiles in a narrower q range around the peak. These results indicate that the PU segmentation resembles the configurations of globular domains of HS in Figures 4 and 5. However, the values of R obtained from these two models are greatly different. Instead, the d from the Zernike-Prins model is comparable to the length scale in the lamellar and Teubner-Strey models.

The effect of Co fillers on PU segmentations is evident from the comparison of SAXS profiles. The profile in the case of 20 wt.% Co is still consistent with the Zernike-Prins model but the shape of the peak is better described by the Percus-Yevick model. The higher loadings of 40-60 wt.% Co lead to larger discrepancy between experimental results and theoretical models. It is inferred that each segment is increasingly disrupted and both segments become mixture. Only the Percus-Yevick model still fits well with the SAXS profiles around the peak. From this model, the R in PU/Co composites is slightly less than that of unfilled PU, and the minimum R with the thinnest shell occurs in the case of 40 wt.% Co. Interestingly, the lengthscale is monotonically decreased with the increase in Co loading when the lamellar model is applied. The fluctuation in the R from both Zernike-Prins and Percus-Yevick models with the loading is likely to originate from the large size distribution of the scattering bodies.

5. Conclusions

Te peaks of SAXS profiles analyzed by fitting with the theoretical models led to the conclusion about arrangements of HS and SS in PU filled with 0-60 wt.% Co. The experimental result in unilled PU had a better agreement with the scattering models from globular HS domains (i.e., Zernike-Prins and Percus-Yevick models) than the lamellar and Teubner-Strey models. The introduction of 20 wt.% Co fillers into PU gave rise to the configuration best described by globular HS domains in a liquid-like dispersion according to Percus and Yevick. The high loading levels of 40-60 wt.% severely affected the segmentation of PU. The SS was increasingly mixed with the HS in these highly loaded PU composites, and only Percus-Yevick model can adequately fit the experimental results. The peak position corresponding to the length of hard segment was slightly shifted by the increase in Co fillers.

http://dx.doi.org/10.1155/2013/493867

Acknowledgment

Te irst author would like to thank Tailand Toray Science Foundation for funding his Ph.D. scholarship.

References

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Krit Koyvanich, (1) Chitnarong Sirisathitkul, (1) and Supagorn Rugmai (2)

(1) Magnet Laboratory, Division of Physics, School of Science, Walailak University, Nakhon Si Tammarat 80161, Thailand

(2) Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima 30000, Thailand

Correspondence should be addressed to Chitnarong Sirisathitkul; schitnar@wu.ac.th

Received 25 October 2012; Revised 22 December 2012; Accepted 26 December 2012

Academic Editor: Philip Harrison

Table 1: Parameters obtained from the fitting of SAXS peaks of
PU composites with four theoretical models.

Co loading   Lamellar                   Zernike-Prins
(wt.%)

                [d.sub.H] (nm)             R (nm)

0            7.156 [+ or -] 0.022   2.029 [+ or -] 0.007
20           6.479 [+ or -] 0.014   3.175 [+ or -] 0.009
40           6.193 [+ or -] 0.014   2.962 [+ or -] 0.008
60           6.166 [+ or -] 0.028   3.582 [+ or -] 0.007

Co loading        Zernike-Prins          Percus-Yavick
(wt.%)

                     d (nm)                 R (nm)

0            7.365 [+ or -] 0.006   3.100 [+ or -] 0.0120
20           7.278 [+ or -] 0.013   3.047 [+ or -] 0.0290
40           6.654 [+ or -] 0.017   2.925 [+ or -] 0.0450
60           6.893 [+ or -] 0.012   2.970 [+ or -] 0.0640

Co loading       Percus-Yavick             Teubner-Strey
(wt.%)

                    th (nm)                [zeta] (nm)

0            0.620 [+ or -] 0.0136     5.8074 [+ or -] 0.0664
20           0.675 [+ or -] 0.0186     3.8435 [+ or -] 0.1750
40            0.25 [+ or -] 0.0530   2.0970 [+ or -] 6.87e - 19
60           0.450 [+ or -] 0.0324     2.2108 [+ or -] 0.128

Co loading   Teubner-Strey
(wt.%)

                    d (nm)

0            7.475 [+ or -] 0.019
20           7.700 [+ or -] 0.035
40           7.550 [+ or -] 0.026
60           7.774 [+ or -] 0.037
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Title Annotation:Research Article
Author:Koyvanich, Krit; Sirisathitkul, Chitnarong; Rugmai, Supagorn
Publication:Advances in Materials Science and Engineering
Date:Jan 1, 2013
Words:2587
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