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Quantitative description of polychloroprene and piperylene-styrene blend films surface morphology.

INTRODUCTION

Polychloroprene adhesive is based on the synthetic chloroprene rubber that possesses similar characteristics to natural rubber and has higher polarity [1]. Polychloroprene rubber is suitable for broad range of substrates for assembly, which shows high peel and green strengths, high resistance to moisture, chemicals and oils, excellent ageing properties, and has high temperature resistance [2-4].

The properties of adhesive depend on chemical nature and molecular weight of polychloroprene. They can be altered by the modifications of conditions and experimental variables during polymerization [1, 5]. A high amount of 1,4-trans units in the polymer leads to polychloroprene that has a high crystallization degree [1].

On the other hand, great advantages may be offered by heterogeneous blends with varying phase-separated structures and superior physical properties [6-9]. A practical way to eliminate or reduce some unfavorable surface properties of polychloroprene, connected to wetting and adhesion, is addition of various polymeric additives, which change adhesive performance [10-12]. Extensive trials have shown that piperylene-styrene copolymer could be used as an adhesion promoter for polychloroprene adhesive to make it suitable for thermoplastic rubber bonding [10, 13]. However, polychloroprene/piperylene-styrene copolymer blend components are immiscible and have segregated structure with domains predominantly formed from the individual homopolymers [12]. The changes in the proportion of the components vary the structure and surface morphology of the blend film. It was determined that the component with lower surface free energy (polychloroprene) is enriched at the surface to minimize polymer-air surface tension [12, 13]. The changes of the blend composition and surface morphology lead to the changes of surface roughness [14]. Herein the careful quantitative analysis of atomic force microscopy (AFM) topographical images, which was simultaneously recorded with lateral force microscopy (LFM) data, was applied to measure the morphological changes of the CR/PSC film surfaces. Root-mean-square roughness [R.sub.q] was used because of the most universal reported measurement of surface roughness and also because of the ease, with which it may be determined and calculated.

Generally, surface roughness has a great influence on many physical properties, such as optical [15], tribological [16, 17], wetting [18], and adhesion [19-22]. Therefore, an accurate description of the roughness is needed for quality control and for finding the correlation with other properties.

Characterization of surface roughness involves two steps: instrumental measurement and quantification. The best known parameters--the root-mean-square roughness and arithmetic mean roughness--are rather inadequate to provide a complete description. The limitation can be overcome using correlation or fractal analyses, which provide valuable information not only on the height deviation of the roughness profile, but also on its lateral distribution. These analyses could be based on the various techniques used for the characterization surface roughness, such as stylus prifilometry, light scattering topography, X-ray scattering, or atomic force measurements. AFM seems to be more useful technique to accesses the roughness evolution [23-25]. AFM topographical imaging has been widely used to measure the surface roughness of polymer blend films for the high surface sensitivity and the simplicity of samples preparation [26-30].

However, only few studies related to quantitative analysis of polymer blends surface features have been found [31-34]. Gemeinhardt and Moore [31] have used small-angle X-ray scattering to investigate the phase separated morphology of polyester/polyamide blend employing a two-parameter correlation function. Ghosh et al. [32] have analyzed AFM images including surface plot, section, roughness, and power spectral density analysis to study the surface morphology of silicone rubber and tetrafluorethy-lene/propylene/vinylidene fluoride terpolymer blends. It was found that the surface morphology of the blends is governed primary by the silicone rubber in the blend. Mail-hot et al. [33] have used AFM to investigate the chemical evolution of poly(vinyl methyl either) and polystyrene blends in photooxidation process. The diameter and height average of the hill-like structures and the root-mean-square of surface roughness were analyzed in this study. Surface enrichment and phase separation of polystyrene and poly (methylmethacrylate) blends were investigated using roughness parameters by Prosycevas et al. [34].

The aim of this study was to find the correlation between the surface roughness parameters and morphology of polychloroprene and piperylene-styrene copolymer blend films using correlation and fractal analyses of AFM topographical images for the roughness description.

EXPERIMENTAL

Materials

Polychloroprene (CR) is produced by free radical emulsion polymerization of 2-chloro-1,3-butadiene monomer. Polychloroprene, named Baypren 330-1, was manufactured by Bayer AG, Germany. It is a chloroprene rubber with high rate of crystallization, which gives solution of high viscosity. Baypren 330-1 is particularly suitable for the use in the formulation of contact adhesive, which yields very strong bonds after the short setting time.

Piperylene-styrene copolymer (PSC) was obtained by radical polymerization mechanism [14]. The infrared spectrum of copolymer films shows that a double bond is found to be in position 2,3 of the piperylene chains. However, in some chains it is possible to find double bond in positions 1,2 or 3,4. The nature of reactants, their stoichiometry, and copolymerization conditions determined PSC structure and its ultimate properties. The ratio of piperylene and styrene mainly influences the copolymer adhesion properties. It was found [10] that copolymer with piperylene/styrene weight ratio of 30/70 wt% possesses good balance of adhesion and cohesion properties. Such PSC is noted for light colour, colour stability, and resistance to acid. Characteristics of polymers used are given in Table 1 [12].

Preparation of Polymer Films

Polymer blends were prepared by dissolving of each polymer in a 2:1 mixture of ethyl acetate and n-hexane in a laboratory mixer (150 rpm for 4 h) by mixing homopolymers solutions in the derived proportions. The solid content of the polychloroprene solution was 20 wt%. The homogeneous polymer mixture was obtained by magnetic stirring at (20 [+ or -] 2)[degrees]C temperature for 5 min.

Polymer films were prepared by solution casting on a Teflon panel and was dried for more than 72 h. The residual solvent from the polymer films was evaporated under mechanically created vacuum at 50[degrees]C for 4 h according to standard procedure. The thickness of dry films, measured by thickness gauge, was (300-400) [+ or -] 10 [micro]m.

AFM Measurements

AFM experiments were carried out at room temperature using atomic force microscope NT-206 (Microtestma-chines) and SPM processing software SurfaceView. The topographical images were collected using a V shaped silicon cantilever (force constant of 0.35 N/m, tip curvature radius < 10.0 nm, cone angle 20[degrees]) operating in the contact image mode with 12 x 16 [micro][m.sup.2] field-of-view.

The image roughness calculation, obtained from the in-build software, is based on finding a median surface level for the image and then evaluating the standard deviation within N x N range. For this three-dimensional N x N image of data heights z(x,y) discrete approximations to root-mean-square (rms) roughness, [R.sub.q] is given by:

[R.sub.q] = [square root of ([1/[N.sup.2]][N.summation over (i=1)] [N.summation over (j=1)] ([z.sub.ij] - [z.sub.av])[.sup.2])] (1)

where i and j are pixel locations on the AFM image, [z.sub.ij], [z.sub.av] are the height values at i and j locations, average height value within the given, respectively. N is the number of data points in the image.

[FIGURE 1 OMITTED]

RESULTS AND DISCUSSION

As it was shown earlier [10, 12, 13], the incorporation of PSC in CR significantly alters its adhesion properties. The improvement of adhesion properties of CR/PSC blend films, in conformity with surface energy investigation of the different blend film sides, were attributed to the increase of copolymer concentration at substrate/blend interface. This presumption was further confirmed by LFM images observations [14]. The lateral force images were quantitatively analyzed using lateral force-image height correlation function. It was found that lateral parameter values of different film sides of CR/PSC 60/40 wt% blend markedly differ and are close to that reported for homopolymers (upside, to CR; underside, to PSC). Thus, it was shown that correlation analysis of LFM images leads to differentiate blend components [14].

AFM topographical images of CR and PSC films are shown in Fig. 1a and b, respectively. PSC surface topography shows a smooth continuous film with [R.sub.q] roughness of 21.0 nm. In contrast, CR surface contains a large number of hills and valleys and [R.sub.q] value is found to be 144.9 nm.

The different surface morphology was observed in the case of various CR and PSC blend compositions. The blend containing 15 wt% of PSC shows structure with the sea-island morphology and roughness of [R.sub.q] = 92.3 nm (Fig. 2a). It may be assumed that bright areas, the continuous phase, is CR-rich phase and dark areas, the disperse phase, is PSC-rich phase. Thus, the CR/PSC blend of 85/15 wt% has two-phase structure, when PSC dispersive phase is continuously distributed in the CR matrix.

Both the lateral size and height of the surface hills become larger, when CR/PSC blend ratio of 75/25 wt% is applied (Fig. 2b). For this surface topography the roughness of [R.sub.q] = 172.5 nm is characteristic.

[FIGURE 2 OMITTED]

AFM images reveal the decrease in the size of PSC-rich phase with the increase of the copolymer content up to 40 wt%. Disperse phase (dark colour) is replaced by the continuous matrix (bright background) in this blend composition (Fig. 2c). Besides, the similar roughness values are observed for both CR and CR/PSC blend of 60/40 wt% films. [R.sub.q] roughness of this blend is found to be 144.8 nm. As was previously discussed in Refs. 13 and 14, it may be attributed to the enrichment of the component with lower surface free energy at the film surface, which took place in the CR/PSC blends at higher PSC content.

However, it is required to perform the detailed quantitative analysis for accurate description of specific roughness properties of CR/PSC blend, because [R.sub.q] values themselves are often difficult to interpret, since roughness is dependent on scan size, lateral and vertical resolution, and sampling density. To make complete understanding about the surface morphology of the blend, it is useful to analyze the fractal dimension of the films surface roughness. Fractal dimension is the defining characteristic of a fractal model and measures the roughness of a fractal curve.

Using AFM recorded topographical images of the blends, the fractal parameter of the surface was calculated from the correlation function. In this case the correlation function was evaluated directly from the AFM topographical images by taking six cross-sections of 2D images and merging together. The data of profilograms were discretizated and their empirical characteristics were found. Discretization was carried out by Mathcad 2001 Professional software (MathSoft, USA). The empirical data of profilograms were grouped and presented as height histograms. It is evident that the solid curves shown in Fig. 3 are Gaussian fits for the height distribution. Gaussian behavior was assessed by fitting the data to the normal distribution function:

P(z) = 1/[square root of 2[pi][sigma]]exp(-(1/2) [(z - [mu])/[sigma]][.sup.2]), (2)

where P(z) is the probability density of realizing a particular height, z is the magnitude of the height of location along the profile length, and [mu] and [sigma] are the mean and standard deviation of all the height values in the profile, respectively.

Kolmogorov-Smirnov test was applied for a normal distribution. It was found that the data fail Kolmogorov-Smirnov tests with significance level [alpha] of 0.05. Consequently, the results showed that data have normal distribution.

For the same sample the height histograms, which were measured at different image positions, are close to each other. This means that for the same sample statistically the height distribution does not change from place to place.

The histogram, displayed in Fig. 3a, shows that PSC film surface roughness height distribution curve is narrow with mean size of 100 nm, implying that the hills on the surface do not have significant size variation. CR film polydispersity is significantly higher. In this case roughness heights vary in the interval 257.0-754.5 nm and mean size of roughness height is found to be 499.3 nm.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The surface height distribution curves of different CR/PSC blends have unimodal character and distinct maximums, the position of which changes in the dependence on the copolymer content. Addition of 15-25 wt% of PSC reduces the mean size of roughness height of the blend film down to 185.4-380.1 nm and curves maximum shifts to the value closer to PSC (Fig. 3b and c). On the other hand, PSC content of 40 wt% broadens the surface height distribution curves and both maximum roughness height and the median height increase up to 917.9 and 668.2 nm, respectively (Fig. 3d).

For the characterization of CR/PSC blend films surface the correlation function K(x) of AFM topographical image profilograms was defined as:

K(x) = <[z(x) - z(0)][.sup.2]>, (3)

where z(x) and z(0) are the image height at the coordinate x = [([x.sub.1], [y.sub.1])] and the reference position (0,0), respectively. The angle bracket <...> in this definition indicates a statistical average. The reference position can be chosen at any point in the height profile. Figure 4 shows the surface height-height correlation function K(x) obtained from AFM measurements of CR, PSC, and their blends of different compositions by fitting the experimental results with:

K(x) = [[sigma].sup.2] exp(-(x/[xi])[.sup.2H]) (4)

where x is scanning length, [xi] is lateral parameter, H is Hurst's (or roughness) exponent. Hurst's exponent is the measure of "jaggedness" of roughness profile. This correlation function treats the surface as self-affine fractal. The parameter H is constant and ranges between 0 and 1. The exponent H is related to the fractal dimension D by D = 2 - H. The fractal dimension is related to surface roughness, an almost smooth surface has a low (slightly greater than one) fractal dimension, while an extremely rough surface has a fractal dimension that approaches to two.

Roughness (standard deviation) [sigma] and lateral parameter [xi] represent the vertical and lateral sizes of the hills or valleys of the investigated surface, respectively. To provide a more quantitative comparison of the relevant parameters from analysis of the blend composition, values for [xi], [sigma], H, and D are summarized in Table 2.

As it can be seen, value of lateral parameter [xi] of CR is higher than that of PSC. [xi] parameter for CR/PSC blends has intermediate values comparing with those of homopolymers. However, it is close to that reported for CR in the case of CR/PSC 60/40 wt% blend. This effect may be associated with CR enrichment at the CR/PSC blend film surface.

Roughness [sigma] values, calculated from the asymptotic value of K(x), were found to be in good agreement with roughness [R.sub.q] values obtained from the in-build software of the AFM instrument. It is interesting to note that [sigma] values for profilogram obtained only for single cross-section are also similar to those of merged profilograms. As in the case of parameter [xi], [sigma] increases with the increase of PSC content and in the case of CR/PSC 60/40 wt% is quite close to that of CR film. Thus, the correlation function parameters more clearly characterize the blends topography evolution than that of roughness [R.sub.q].

The resulting scattering curves of the CR, PSC, and their blends are shown in Fig. 4. The solid lines represent the exponential fits to the scattering data and show the excellent agreement between the theoretical and experimental curves. The scattering curve for CR is above PSC one and indicates the higher dimensions of the hills of CR film surface. The curves of CR/PSC blend films shift down to PSC curve with the increase of copolymer content up to 25 wt%, indicating the increase of the hills size. The plot obtained from CR/PSC 60/40 wt% film surface is consistent with plot obtained from the CR film. Thus, the surface roughness for this blend film becomes similar to CR homopolymer film.

The fractal dimension value D was obtained as an average over 128 cross sections. In the Table 2 fractal dimension is presented as a function of blend composition. The fractal dimension range between 1.09 for CR and 1.03 for PSC. Addition of 15 wt% of PSC decreases the fractal dimension of CR down to 1.04. Further increase of PSC content increases D value and in the case of CR/PCP 60/40 wt% it becomes the same as CR, i.e., D = 1.09. This confirms the assumption that at the higher copolymer content the blend surface is enriched by CR. In this way, the study of the fractal dimension of AFM topographical images is also valuable to follow the evolution process of surface morphology of CR/PSC films.

Thus, the excellent agreement of data demonstrates the possibilities of AFM topographical imaging for describing the blend morphology evolution.

CONCLUSIONS

CR/PSC films obtained by solution casting have been studied using AFM topographical imaging. Changes in surface morphology and roughness have been observed depending on the blend composition. Enrichment of CR at the blend film surface, when content of PCP reaches 40 wt%, was proved by root-mean-square roughness values of AFM topographical images.

For the description of the roughness evolution of CR/PSC blend films the fractal and correlation analyses, derived from the AFM topographical images, were used. The surface roughness was described using parameters, such as the roughness, lateral parameter, and fractal dimension. The results obtained from the analysis of blend surfaces exhibit similar behavior, all parameters obtained for CR/PSC blend of 60/40 wt%, are close to those reported for CR film. The analysis suggests that the proposed statistical description of AFM topographical images roughness is useful tool for the estimation of evolution process of blend surface morphology.

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Virginija Jankauskaite, (1) Kristina Zukiene, (2) Stase Petraitiene (3)

(1) Department of Clothing and Polymer Products Technology, Kaunas University of Technology, LT-51424 Kaunas, Lithuania

(2) Research Laboratory of Polymer Products, Kaunas University of Technology, LT-51424 Kaunas, Lithuania

(3) Department of Mathematical Research in Systems, Kaunas University of Technology, LT-51368 Kaunas, Lithuania

Correspondence to: Virginija Jankauskaite: e-mail: virginija.jankauskaite@ktu.lt
TABLE 1. Characteristics of polymers.

 Glass transition
 Molecular temperature [T.sub.g]
Polymer weight [M.sub.w] ([10.sup.3]) ([degrees]C)

CR 300 -45
PSC 35 55

TABLE 2. Parameters extracted from AFM topographical images
profilograms.

 CR/PSC composition (wt%)
Parameters 100/0 85/15 75/25 60/40 0/100

Lateral parameter, [xi] (nm) 1.11 0.95 1.03 1.08 0.79
Roughness, [sigma] (nm) 117.2 85.0 70.8 117.2 16.2
Hurst's exponent, H 0.91 0.96 0.93 0.91 0.97
Fractal dimension, D 1.09 1.04 1.07 1.09 1.03
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Author:Jankauskaite, Virginija; Zukiene, Kristina; Petraitiene, Stase
Publication:Polymer Engineering and Science
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Date:Jun 1, 2007
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