The microstructural study of swift heavy 75MeV Oxygen-ion and 100MeV Ag-ion irradiated PVDF thin films by atomic force microscopy.

Rana, Dinesh Singh ; Chaturvedi, D.K. ; Quamara, J.K. 等

Introduction

Polyvinylidene fluoride (PVDF) has been traditionally acknowledged as a material of immense practical utility owing the ability to maintain its excellent physical, chemical, electrical and mechanical properties over a wide range of temperature and frequency. It is a semi-crystalline polymer with --CH2-CF2-as repeating unit and exhibits unique piezoelectric, pyroelectric, ferroelectric, and nonlinear optical properties, which promote their use in many technological applications including sensors (biomedical cardiopulmonary sensors, ultrasonic transducer, radiation detector) [1] actuators (hydrophone, microphone, headphone, generator) [2], nonlinear optical component and fiber optics[3] and ferroelectric memory [4].

PVDF is also one of the rare polymer that exhibit diverse crystalline forms, having at least five phases known as [alpha], [beta], [gamma], [delta] and [epsilon] [5-7]. Earlier reports have shown that the [alpha]- phase comprises helical structure with chain conformation- trans-gauche-transgauche' (TGTG') is inactive with respect to piezo- and pyroelectric properties, while [beta]- phase posses all-trans planar zigzag conformation exhibits the most activity, and hence become the focus for various transducers applications.

In addition to its high piezoelectric coefficient, the advantages such as flexibility, bio-compatibility, lightness, and low acoustic and mechanical impedance make PVDF a favorable material for implantable medical devices, micro actuators [8] and MEMS (microelectromechanical systems) applications.

The swift heavy ion (SHI) irradiation is a relatively new technique for tailoring the surface structure of polymeric material for specific technological applications including nuclear and space. The energetic ion when traverse through the material medium it losses its energy either in displacing atoms (of the sample) by elastic collisions or ionizing the atoms by inelastic collision. The former is the dominant process at low energies whereas the inelastic collisions dominate at high energies where the displacements of atoms due to elastic collisions are insignificant. This interaction of high energetic ion with medium may change surface structural properties of PVDF films. It has been recognized that the surface structural properties of PVDF polymer influence its many other important properties such as adhesion, friction, biocompability, crystallinity, wettability etc.

The behavior of PVDF exposed to different kinds of radiation [9-21] has been studied before. These studies reveal the enhancement in electrical conductivity and change in crystallinity of PVDF [11, 13-15, 18-19 and 21]. The decrease in crystallinity has been reported under electron and low-energy ion implantation [13-15] whereas an increase in crystallinity has been reported under electron, X-ray and [gamma]-ray irradiations [11, 15, 18-19 and 21]. The crystallinity plays a crucial role in determining piezoelectric, mechanical, optical, electrical and even thermal properties of polymers [13]. The SHI irradiation effect on the surface properties of various polymers have been widely studied with scanning tunneling microscope (STM) [22] and near surface study tools such as atomic force microscope (AFM) and transmission electron microscope (TEM) but the surface properties of PVDF thin film is not yet fully explore.

The aim of the present work is to investigate the surface microstructual and other properties, on scales from few micron to nanometers, of PVDF thin films of different thicknesses before and after irradiation with 100 MeV Ag-ion, and 75 MeV Oxygenion beams at different fluences by using Atomic Force Microscopy (AFM).

Experimental Details

The poly-vinylidene fluoride used in the present study was procured form DuPont (U.K.) in flat film forms of thicknesses 9 [micro]m, 12 [micro]m and 20 [micro]m. The samples of size 1 sq. cm were irradiated with 100 MeV Ag-ion beam at fluence 1.875X[10.sup.11] ions/[cm.sup.2] and with 75 MeV oxygen-ion beam at fluences 5.625X[10.sup.11] ions/[cm.sup.2] and 5.675X[10.sup.12] ions/[cm.sup.2] using the PELLETRON facility at Inter University Accelerator Centre, New Delhi. The pristine and SHI exposed PVDF films of different thicknesses were later investigated by AFM techniques. The scanning probe microscope Solver PRO 47 (NT-MDT, Russia) operating in the semi contact mode (Trapping mode) was used. Images were acquired using 'Golden' silicon probes (NTMDT, Russia) with resonance frequencies of 260 kHz (tip radius of 10 nm). All measurements were performed with the instrument mounted in a vibration isolation system. The scanning probe microscope is also used to estimate the surface roughness. A series of shots with equal dimensions were taken from different parts of surface (dimensions of the shots were chosen as 1.6 x 1.6 [micro]m to 2 x 2 [micro]m).

AFM Investigations

Atomic force microscopy (AFM), or scanning probe microscopy (SPM), has been turned out to be indispensable tool for investigation of surface morphology, microstructure, mechanical and other properties of polymeric material. An AFM can be used in different modes for producing the topographic image of the sample surface. AFM in tapping mode is of particular interest in determining topography and phase morphology in polymer films. In tapping mode, the silicon probe tip oscillates at its resonance frequency as it rasters across the sample surface, experiencing only intermittent contact with the surface. The surface topography is represented by the height image in trapping mode atomic force microscopy (TMAFM). Following the surface morphology, constant oscillation amplitude is used as the feed back signal via the z displacement of the piezo- ceramics. The amplitude image is obtained by recording the variation of the root mean square (RMS) of the amplitude before the feedback loop. In this mode the lateral resolution is around one nanometer.

Result and Discussions

Pristine and SHI irradiated PVDF

The AFM micrographs of pristine and 100 MeV Ag-ion irradiated PVDF films of different thicknesses (9 [micro]m, 12 [micro]m and 20 [micro]m) are shown in following figures (Fig. 1 to 6) while the AFM micrographs of 20 [micro]m PVDF film irradiated with 75 MeV Oxygen-ion at different fluences (5.625 X[10.sup.11] ions/[cm.sup.2] and 5.675X[10.sup.12] ions/[cm.sup.2]) are presented in figures 7 and 8. For all figures, the images on the left represents a two dimensional image and the one on the right the three dimensional image. Analysis of a topographic AFM images allows us to obtain the size histogram of the grains, grain density h, their mean side d and their size dispersion. The atomic force microscopy is used to estimate the surface roughness. The roughness and other structural parameters measurement results of the analyzed AFM micrographs of pristine and irradiated samples are reported in Table 1.

The roughness parameters are determined on each image obtained in tapping mode (height image) and are defined as: (i) [S.sub.q] (Root Mean Square roughness parameter) is the standard deviation of the Z values within a given area and calculated by the equation:

[S.sub.q] = [[summation over (i)] [(Zi - Zav).sup.2]/N].sup.1/2] (1)

where Zav is the average of the Z values within the given area, Zi is the current Z value, and N the number of points within the given area.

(ii) [S.sub.a] is the mean roughness. This is the mean value of the surface relative to the centre plane and is

calculated using following relation;

Ly Lx [S.sub.a] = 1 / Lx .Ly [integral][integral]|f(x,y) | dx dy 00 (2)

where f(x, y) is the surface relative to the centre plane and Lx and Ly are the dimensions of the surface.

It is observed from the AFM micrograph (Fig. 1, 3 and 5) of pristine PVDF films of different thicknesses (9 [micro]m, 12 [micro]m and 20 [micro]m) that the surface of samples has different morphological patterns which is in line of our earlier FTIR and XRD analysis (Rana etal, 2009) of the pristine samples that the pristine samples are the mixture of [alpha]-, [beta]-, and [gamma]- phases. At least two different morphological patterns easily identified in which one is white and the other is dark in every pristine sample (see Fig. 1, 3 and 5). If one looks into smaller scale, one sees totally different structures of surface morphology in every pristine sample. Thus all pristine PVDF samples show granular microstructure and this microstructure has a matrix and granular grains inlaid on this matrix. The three dimensional AFM images of pristine samples show uniform mountain features with sharp deeps at the bottom.

Figures 2, 4 and 6 demonstrated the surface morphological changes after 100 MeV Ag-ion irradiation of PVDF samples of different thicknesses. The irradiated samples also show granular microstructure with the formation of different kind of grains. The Ag-ion irradiated PVDF samples of different thicknesses show decrease in average surface roughness. The average surface roughness decreases drastically in case of 20 [micro]m PVDF sample irradiated with Ag-ion which suggests the smoothening of surface and this relative smoothness is probably due to the sputtering effects. The Ag-ion irradiated PVDF samples also show some small craters-hillocks at the edge of the films. The formation of hillocks in the present case has been attributed to nuclear energy loss induced collision cascades which take place near the surface and are responsible for displaced atoms forming clusters.

The decrease in the average surface roughness is also observed in 20 [micro]m PVDF samples irradiated with 75 MeV Oxygen-ion beam at different fluences and this decrease in the average surface roughness is depend on the fluence. No hillocks were observed for the sample irradiated with 75 MeV Oxygen-ion beam at fluences 5.625 X[10.sup.11] ions/[cm.sup.2], though the formation of small craters at some places has been observed. It also observed at lower fluence 5.625 X[10.sup.11] ions/[cm.sup.2] that 3D, AFM topographic image (Fig. 7) shows greater degree of crystallinity along with uniform distribution of nano crystalline particles. Whereas, feeble craters and hillocks are observed on the edge of the film at the higher fluence 5.675 X[10.sup.12] ions/[cm.sup.2], with decrease in the degree of crystallinity.

The AFM study also show that the average grain size decreases upon SHI irradiation and further decrease in grain size is observed at lower fluence. It is well know that the grain size of the material influences its hardness properties. Classically, one would expect an increase in hardness (H) for the decrease in grain size according to the Hall-Petch equation given below;

H = [H.sub.0] + K [D.sup.-1/2] (3)

where [H.sub.0] is the lattice friction stress in the absence of grain boundaries, K is constant and D is the grain size. Interestingly we observed the decrease in hardness with decrease in grain size of PVDF thin films upon ion irradiation i.e., the reverse Hall-Petch effect. Since, the Hall-Petch relation is found to be effective for materials with grain sizes ranging from 1 millimeter to 1 micrometre and this relation is no longer valid for the material having grain size smaller than 100nm[23-24] and hence it could not be applicable to our samples as the grain size of our samples are less than 100nm range. The decrease in hardness can be resolved on the fact that dislocation pile-up process prohibited and never results in grain boundary diffusion when the size of dislocations begins to approach the size of the grains. The lattice resolves the applied stress by grain boundary sliding, resulting in a decrease in the material's yield strength (hardness). This reverse Hall-Petch effect may likely be the result of unrecognized pores in samples. It is important to mention that the AFM imaging does not make it possible to determine the exact pore size or crater size, since AFM analysis is restricted to the surface. Therefore, the term "mean pore size" refers to the spaces between the PVDF grains on the surface.

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Conclusion

AFM investigations of surface morphology and local structure of pristine and SHI irradiated PVDF thin films of different thicknesses have been performed. The roughness values such as average roughness, [S.sub.a], root mean square roughness, [S.sub.q], and other physical parameters such as crystallinity, grains size, grain density, surface skewness, Ssk, hardness and entropy of pristine and irradiated samples have been estimated. It has been shown that the average surface roughness and grain size decreases upon SHI irradiation and the decrease in surface roughness are dependent on the type of swift heavy ion beam and its energy and fluences. There is no strong evidence for the formation of craters-hillocks after the Oxygen-ion beam irradiation. Both pristine and SHI irradiated films demonstrate amorphously nanocrystalline composition. The surface morphology of films deteriorated at the higher fluence. This study shows that the PVDF thin films become softer as grain size is reduces upon irradiations.

References

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Dinesh Singh Rana (1), D.K. Chaturvedi (2) and J K Quamara (3)

(1,2) Institute of Instrumentation Engineering, Kurukshetra University, Kurukshetra-India

(3) Department of Physics, National institute of Technology, Kurukshetra-India Email: [email protected], [email protected]

Table 1: Roughness and other structural parameters measurement results
of pristine and and SHI irradiated PVD samples of different thicknesses
(9 [micro]m, 12[micro]m and 20 [micro]m)

Samples/
Properties                  Pristine PVDF

Thickness       9 [micro]m   12 [micro]m   20 [micro]m
of samples

Peak-to-        134.32       56.44         320.66
peak, Sy        nm           nm            nm

Ten point       66.66 nm     28.18nm       161.85
height, Sz

Average         70.71 nm     28.47nm       220.36

Average         16.07 nm     4.17nm        26.58 nm
Roughnes,
Sa

Second          73.63        29.02         223.24
moment

Root Mean       20.54nm      5.62nm        35.82nm
Square, Sq

Surface         -0.321       0.159         -0.859
skewness,
Ssk

Coefficient     0.225133     1.37175       2.45335
of kurtosis,
Ska

Entropy         10.36        8.48          11.05

Redundancy      -0.467       -0.464        -0.327

Crystallinity   48.3%        52.56%        51.53%

Samples/        100 Mev Ag-ion irradiated
Properties      PVDF samples (fluence
                1.875X [10.sup.11] ion/[cm.sup.2]

Thickness       9 [micro]m   12 [micro]m   20 [micro]m
of samples

Peak-to-        134.26       70.52 nm      50.65 nm
peak, Sy        nm

Ten point       65.57 nm     30.07 nm      25.11 nm
height, Sz                   nm
Average         76.75nm      22.24 nm      28.161nm
                             nm
Average         7.99nm       3.98 nm       3.99 nm
Roughnes,
Sa

Second          77.48        22.84         28.68
moment

Root Mean       10.57 nm     5.19 nm       5.41nm
Square, Sq

Surface         -0.314       0.107         -0.25
skewness,
Ssk

Coefficient     1.66482      1.19005       1.51
of kurtosis,
Ska

Entropy         9.39         8.37          8.42

Redundancy      -0.331       -0.367        -0.5

Crystallinity   44.53%      50.28%         48.5%

                75 Mev              75 Mev
Samples/        oxygen -            oxygen-
Properties      ion                 ion
                irradiated          irradiated
                (fluence;           (fluence;
                5.675X[10.sup.11]   5.675X[10.sup.11]
                ion/[cm.sup.2])     ion/[cm.sup.2])

Thickness       20 [micro]m         20 [micro]m
of samples

Peak-to-        156.87 nm           179.92 nm
peak, Sy

Ten point       78.53 nm            89.88 nm
height, Sz
Average         83.48 nm            97.93nm

Average         16.95 nm            12.53 nm
Roughnes,
Sa

Second          86.12               99.43
moment

Root Mean       21.17 nm            17.18 nm
Square, Sq

Surface         0.056               -0.423
skewness,
Ssk

Coefficient     -0.163705           2.45448
of kurtosis,
Ska

Entropy         10.39               10.02

Redundancy      -0.427              -0.655

Crystallinity   53.69%              47.08%