Investigating wettability and optical properties of PADC polymer irradiated by low energy Ar ions

    Investigating wettability and optical properties of PADC polymer irradiated by low energy Ar ions A.A. El-Saftawy, S.A. Abd El Aal, Z.M. Badawy, B.A. Soliman PII: DOI: Reference:

S0257-8972(14)00475-7 doi: 10.1016/j.surfcoat.2014.05.048 SCT 19435

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

14 January 2014 26 April 2014 26 May 2014

Please cite this article as: A.A. El-Saftawy, S.A. Abd El Aal, Z.M. Badawy, B.A. Soliman, Investigating wettability and optical properties of PADC polymer irradiated by low energy Ar ions, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.05.048

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ACCEPTED MANUSCRIPT Investigating wettability and optical properties of PADC polymer irradiated by low energy Ar ions

Accelerators & Ion Sources Dept., Nuclear Research Center, Atomic Energy Authority, P.O. 13759, Cairo-Egypt.

Central Lab. for Elemental & Isotopic Analysis, Nuclear Research Center, Atomic Energy Authority,

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A.A. El-Saftawya,*, S.A. Abd El Aalb, Z.M. Badawyc and B.A. Solimana

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P.O. 13759, Cairo-Egypt.

Experimental Nuclear Physics Dept., Nuclear Research Center, Atomic Energy Authority, P.O. 13759,

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Cairo-Egypt.

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Abstract

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*corresponding author, e-mail: [email protected] (A.A. El-Saftawy); mobile: +201142118813

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Polyallyl diglycole carbonate (PADC) polymer, commonly used in optometry, is irradiated by different fluences of low energy Ar ions to optimize its wetting behavior and optical response. Surface wettability is evaluated by contact angle method (for water and glycerol) which shows noticeable improvement. Also, adhesion and surface free energy analysis indicate that adsorption properties of PADC enhanced after irradiation. Scanning electron microscopy is applied to relate surface morphology with wettability changes and optical properties of the target polymer. UV-Vis spectra are recorded to investigate the effect of the induced defects on the optical band gap and the formed carbon clusters size. It is found that carbon domains created over the surface are responsible for band gap decrease. Photoluminescence spectral changes have been studied as well. Low energy Ar ions are suitable to improve PADC surface properties.

Keywords: Ar ions / Wettability / Photoluminescence / PADC polymer / Band gap / Carbon clusters. 1

ACCEPTED MANUSCRIPT 1. Introduction Visible light transmission and / or optical refractive index are important aspects in different applications of polymers. These include the use of polymers in all types of glazing for buildings and

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transportations and the use of solid polymers in optometry applications [1] by producing polymeric

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lenses to fix myopia, hyperopia, astigmatism and presbyopia [2,3]. Clarity, wettability, luminescence,

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UV resistance, hardness, low weight and low thickness are favorable characteristics in any lens. Corrective lenses are often made of PADC polymer which is considered as the standard lens material

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for several decades [3,4]. PADC has the previously mentioned properties besides it is the most

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economical lens material with good optical properties. Also it is lighter and thinner than other plastics or glass [1,5]. Despite the excellent properties of PADC, only few investigations about its wetting

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behavior have been reported [6,7]. Wetting properties are affected by surface layer properties [8-11].

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To alter the surface properties without affecting the bulk properties, various techniques have been

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employed such as corona [12,13], flame [14], ultraviolet [15,16], plasmas [17], electron or ion beam treatment [18-21]. Energetic-ions-polymer interactions lead to bond breaking, chain scission, crosslinking, free radicals formation, creation of unsaturated bonds and loss of volatile fragments [22]. As a result, defects appear in the polymer produce changes in its optical and structural properties [23]. Different ion species with different energies are employed worldwide to modify various physical and chemical properties of PADC. These energies ranging from few hundreds of keVs [24-27] to several MeVs [23,25,28-31]. In the present article, Ar ions with low energy (5 keV) are employed to get insight into the effect of low energy ions on wettability and optical properties of PADC polymer.

1. Experimental procedures 1.1. Samples preparation and irradiation In the present work, small samples of 1 cm x 1 cm were accurately cut from a sheet of PADC 2

ACCEPTED MANUSCRIPT polymer (of thickness 1 mm and density 1.32 g/cm3) supplied by TASTRAK, Track Analysis System Ltd., UK. PADC consists of short polyallyl chains joined by links containing carbonates and diethyleneglycole groups into a dense three dimensional network with an initiating monomer unit

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shown in fig. 1. The branching point in this net is tertiary carbon in the polyallyl chain. Out of three

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links originating from this point, two constitutes the polyallyl chain and one consists of

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diethyleneglycol dicarbonate [28,32]. High efficiency glow discharge ion source [33] with argon gas as a precursor is used to irradiate the PADC samples with fluence rate 1.13E16 cm-2s-1. The applied

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fluences range from 3.49E17 cm-2 up to 1.37E18 cm-2. For all samples, the working pressure is 1.2

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mtorr and the extracted Ar ion beam energy is 5 keV. According to SRIM code (2008 version)

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1.2. Contact angle measurement

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calculation, the projected range of Ar ions into PADC polymer is 119 Ao.

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When a liquid drop is projected over a surface it takes the shape of semi-sphere (fig. 2) due to surface energy equilibrium. From such a shape, the drop volume, height and contact area could be calculated. In the present work, the PADC samples are mounted in front of a digital CCD camera (type DSC H9 of SONY Inc.) over a horizontal optical bench. Then five liquid drops (distilled water and glycerol of 4l volume) are projected gently on different places over each sample by means of micropipette. The drops images are captured by means of the CCD camera which is connected to a computer to transfer and manipulate the captured images, fig. 2. The drops height (h) and baseline length (b) are calculated and the contact angle () is measured using the following formula [34]:





  arcsin 4bh  4h 2  b 2  

1.3. Surface morphology and roughness measurement To study the correlation between surface morphology and the induced changes in wettability 3

ACCEPTED MANUSCRIPT and optical properties of the target polymer, scanning electron microscope (SEM) type Jeol GSM 5600 LV has been used. The instrument stylus-profiler type Surtronic 3 was used to measure the average surface roughness values of the studied polymer. Four different values are taken for each sample and

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their average are considered as the roughness value. The data were measured with accuracy of ±0.3 m.

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1.4. UV-Vis spectral measurement

Using UV-Vis spectrometer type PerkinElmer UV Lambda 900, the transmittance spectrum is

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measured in the wavelength region 300-600 nm. The measurements for all samples were done with

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resolution of 10 nm at room temperature. The pristine sample is used as a reference material.

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1.5. Photoluminescence analysis technique

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SHIMADZU spectrofluorophotometer model RF-1501 is used to investigate luminescence

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spectra of pristine and irradiated PADC samples. All spectra were recorded in the reflection geometry mode at room temperature and precaution is taken to avoid stray light. The excitation source is a highpressure xenon lamp dispersed by a grating monochromator and detected by a photomultiplier through a second grating monochromator. This measuring setup made it possible to excite and detect luminescence in the wavelength range 200-900 nm without the effect of transmitted light. As a result of excitation scan for the PADC samples, excitation wavelength of 352 nm is used to excite the samples.

2. Results and discussion 2.1. PADC wetting behavior: analysis and evaluation Wettability is connected to the properties of both the probe liquid and the solid surface. The solid surface is characterized by its morphology, polarity and surface energy. These parameters control the spread of a liquid drop over the surface. Contact angle of a liquid drop projected over a surface is a 4

ACCEPTED MANUSCRIPT commonly used technique to evaluate surface wettability. Contact angle value provides an inverse measure of wettability i.e. perfect wet surface has lower contact angle than poor wet surface [26,35]. Fig. 3 shows the behavior of the liquid drops (distilled water and glycerol) projected over pristine and

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irradiated PADC samples. It is clear that the polymer having a hydrophobic nature which altered to

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hydrophilic one after irradiation. PADC samples become more hydrophilic due to Ar ion treatment and

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the contact angle decrease is related to the rate of chemical changes taking place over the surface. Also, Active sites have been created over the polymer surface due to ion bombardment which leads to bond

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cleavage. The active sites can bind to other atom or molecule in air, especially oxygen when the treated

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polymer exposed to atmosphere. Oxygen increases the surface polarity which in turn lead to liquid drop spreading over the surface due to the increase of the intermolecular interaction between them [21,37-

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40]. Due to the high polarity of water compared to glycerol, the contact angle values for water is

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higher than that of glycerol as noticed in fig. 3. The measured contact angle values were used as the

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basis for surface free energy and adhesion analysis. The adsorption characteristics of any surface are evaluated by adhesion work which in turn depends on its surface free energy (SFE). The surface tension or SFE of liquid can be measured directly using tensiometer, but it is very difficult to measure SFE of solid directly. In a situation of three phases solid-liquid-vapor interactions (as shown in fig. 2), there are exist three interfacial forces related together by Young's equation [41]:

 sv   sl   lv cos where,

 sv is

(2)

the SFE of a solid or the interfacial force between solid and vapor,  sl is the SFE

corresponding to solid-liquid interface,  lv is the SFE (surface tension) of the measuring liquid and



is the contact angle between the solid and the measuring liquid. These interfacial forces depend on the properties of the three phases. Young's equation and the measured values of the contact angle are being used as the basis for calculating the SFE of polymeric materials. In order to solve Young's equation, 5

ACCEPTED MANUSCRIPT Neumann assumed that  sl value depends on the properties of a solid and a measuring liquid. This is reflected in the so-called equation of state [9]: 0.5

 s 

2

l

(3)

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 sl   s   l  2 s  l  e   

with β=0.0001247. By combining eq. 2 and eq. 3, the result is [42-44];

1 cos  2  sv  lv  e 

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   lv  sv 

(4)

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0.5

The general equation correlating the wetting and bonding behaviors at the interface of a solid-

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liquid-vapor system is [45]:

(5)

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W a   s   l   sl

where the work of adhesion W a represents the energy of interaction between the liquid and the solid phases per unit area. By combining eq. 2 and eq. 5, the following relation results [45]:

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W a   l 1  cosΘ

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(6)

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Eq. 6 shows that the work of adhesion depends on the SFE of the measuring liquid and the contact angle which in turn depend on the type of polymer. SFE and adhesion work are estimated using eqs. 4 and 6, respectively and the results are summarized in table 1. The calculation process depends on the data presented in fig. 3 for the contact angles and the SFE values of the probe liquids [  lv for water and glycerol are 72.8 mJ/m2 and 64 mJ/m2 respectively] [46]. Table 1 shows the increase of both the SFE and the adhesion work with the ions fluence increase. This behavior is attributed to chemical changes happening over the surface and the oxidation process which increases the surface polarity. Also, the adhesion work increases with the polarity of the measuring liquid i.e. the strongest work of adhesion is found for more polar liquids (water). As seen in fig. 4, the samples’ color turned from high transparency to light brown and finally to dark brown as the ions fluence increases. Color transformation of irradiated polymer may be attributed 6

ACCEPTED MANUSCRIPT to trapped free radicals or charge species in the polymer medium [47]. Also, many of the optical transitions which result from the presence of impurities (such as free radicals) have energies in the visible part of the spectrum. As a result, the induced defects are referred to as color centers [48].

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Fig. 5 shows surface morphology of pristine and irradiated PADC samples. The noticed

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irregular protrusions on the surface increase as the ions fluence increases. The interaction of energetic

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ions with polymer surface causes the removal of low molecular contaminants such as additives, processing aids, and adsorbed species resulting in etching of the surface [49]. Also, by adsorbing ions

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energy, materials around the interaction area are heated up and evaporated. There will be debris formed

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from melted material or sputtering [49,50]. Therefore, surface morphology and roughness were changed. As the ions fluence increase, the deposited energy on the polymer surface increases which in

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turn improves roughness. The measured average surface roughness values are; 0.13 m, 0.14 m, 0.28

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m, 0.35 m and 0.36 m for 0 cm-2 , 3.49E17 cm-2, 6.75E17 cm-2, 1.01E18 cm-2, and 1.37E18 cm-2 ion

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fluences respectively. It can be seen that the roughness increases with fluence. The impact of roughness on surface wettability may be explained with the view point that the additional surface area produced by roughening the surface was regarded as effectively increases its surface energy [51-53]. It is well known that the surface roughness increases the surface area [51] which in turn improves wettability and bonding strength [54].

2.2. PADC optical properties 2.2.1. UV-Vis spectral analysis: optical band gap energy and carbonaceous clusters UV-Vis spectroscopy gives an idea about the value of optical band gap energy (Eg) and thus is an important tool for materials investigation [55]. The absorption of light by polymeric materials in the ultraviolet and visible region involves promotions of electrons in ,  and n-orbitals from the ground state to the higher energy levels [56]. The electronic transitions that are involved in ultraviolet and 7

ACCEPTED MANUSCRIPT visible regions are, *, * and n* [57]. The UV-Vis transmission spectra of pristine and irradiated PADC polymer are shown in fig. 6. All samples show a rapid increase in the transmission

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spectra with the wavelength up to a certain wavelength after which a plateau takes place. Also in the irradiated samples, knees are observed at wavelengths, 360 nm, 350 nm, 340 nm and 330 nm for ion

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fluences 3.49E17 cm-2, 6.75E17 cm-2, 1.01E18 cm-2 and 1.37E18 cm-2, respectively.

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The usual method for determining Eg from UV-Vis spectrum is by plotting (h)1/n against (h), where; is the absorption coefficient, h is the Planck’s constant and  is the frequency of the incident

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radiation, this methods (usually known as Tauc’s technique) is described in some details in literature

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[23,26,28-31,57-59]. Also n is a constant have the values of ½, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively [26]. The values of the

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absorption coefficient () for pristine and irradiated PADC samples have been determined using the

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relation [60,61]:T  exp(d ) where d is the sample thickness. In the present work, the most

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satisfactory fit for PADC polymer is obtained by plotting (h) 2 and (h) 1/2 against (h). Such plots are presented in fig. 7 for direct allowed transition (n = 1/2) and fig. 8 for indirect allowed (n = 2) transition. Indirect and direct band gap are determined by fitting the linear parts in fig. 7 and fig. 8 and make an extrapolation of the linear fit to intercept with the x-axis, where the intercept point represents the band gap energy [57]. The obtained band gaps are summarized in fig. 9. The observed decrease in the band gap energy is attributed to the decrease in PADC resistivity. This means that there is a change in the structural characteristics of this polymer upon Ar ions irradiation. It may account for the scission of the polymer chains and formation of free radicals that contains nonbonding electrons to increase conduction band electrons density [47]. As a result of ions irradiation of PADC, carbon clusters are formed due to the migration of atoms from the bulk to the surface during irradiation and the dehydrogenation processes takes place over the surface [30]. Carbonaceous clusters are generally assumed to induce conductivity and band 8

ACCEPTED MANUSCRIPT gap energy is linked to cluster size [62]. The cluster structure is like a buckminsterfullerene structure, i.e. C60 ring instead of C6 ring [63]. Accordingly, the cluster size is estimated as [64]: N (where N is the number of carbon atoms per cluster in the irradiated samples). The

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E g  34.3

cluster size depends on the energy loss during interaction and increases as the ion fluence increases

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(fig. 9). Agglomeration of clusters depends on the electronic energy loss and the ion beam fluence

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during irradiation. High electronic energy loss causes more agglomeration of clusters, which result in

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the bigger clusters on the surface [30]. Carbon enriched domains created in polymers during irradiation may be responsible for the decrease in the band gap as indicated in literature [65-67]. The band gap and

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the corresponding carbon cluster size for both direct and indirect transitions are presented in fig. 9. As the ion beam fluence increases, the number of dehydrogenation processes increase resulting in

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decrease.

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increasing the number of carbon atoms and hence the cluster size increases which causes the band gap

2.2.2. PADC photoluminescence investigation Photoluminescence (PL) spectroscopy is a common method to investigate the defects in polymeric materials. This process is initiated with excitation by external source of energy that is associated with formation of electron-hole (e-h) pairs or absorption in molecular system. Afterwards, thermalization of e-h pairs towards thermal equilibrium or transfer of energy to chromophoric sites occurs. Finally, radiative recombination of the thermalized pairs or direct emission of energy in sort of electromagnetic emission takes place [68]. Fig. 10 shows the PL spectra of PADC samples irradiated with different fluences of Ar ions. In pristine PADC, the peak c is observed at 714 nm and for ions irradiated samples, the c are observed at 712 nm, 709 nm, 707 nm and 706 nm for 3.49E17 cm-2, 6.75E17 cm-2, 1.01E18 cm-2, and 1.37E18 cm-2 ion fluences respectively. Thus noticeable changes were observed in the PL spectra of ion irradiated PADC with increased fluence due to the changes in the 9

ACCEPTED MANUSCRIPT molecular structure and the corresponding changes in the electronic properties of the polymer. The obtained PL spectra lie in the red spectral region and show a small shift towards lower wavelengths as the ion beam dose increases. This kind of shift is known as the bato chromatic effect.

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Also, it is clear that the PL intensity decreases (hypo chromatic shift) when the irradiation fluence

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increases. This behavior is caused by the induced defects and clusters formed in the polymer which

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serves as non-radiative centers [26,69]. As the doses increases, the defects concentration increases leading to the decrease in the PL intensity [23]. The emission bands in the investigated range are

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associated to the less energetic *-and-n electronic transition. This type of emission occurs in the

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unsaturated centers of the molecules, i.e. in compounds containing multiple bond or aromatic species, which is responsible for PL intensity decrease [26,70]. The difference between positions of the band

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maxima of absorption and fluorescence of the same electronic transition is known as Stokes shift.

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Stokes shift is important for practical applications of fluorescence because it allows separating strong

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excitation light from weak emitted fluorescence using appropriate optics [71]. The values of Stokes shift are indicated on fig. 10.

3. Conclusion

The presented results show that low energy argon ions are capable to induce noticeable changes in wettability and optical properties of PADC polymer. These changes are reasonable and comparable to that made by high energy ions, electrons and plasma. The results show that: 1. The wettability and SFE of the investigated polymer have been improved as indicated by the decrease of the contact angles due to the changes taking place over the surface. 2. Surface morphology changed and roughness improved which in turn enhances the surface wettability and bonding strength. 3. Color transformation of PADC may be attributed to the trapped free radicals or charge 10

ACCEPTED MANUSCRIPT species in the polymer medium. 4. The optical band gap shows a shift towards lower energies as the ion fluence increases which is attributed to the formation of carbon clusters and defects on the polymer surface.

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surface) is confirmed by the decrease in the PL intensity.

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5. The formation of clusters and defects (which serves as a non-radiative center on the polymer

Acknowledgement

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The authors would like to thank Prof. Dr. Abdelfatah I. Helal (Central Lab. for Elemental &

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Isotopic Analysis) for his valuable suggestions and discussion. Also Dr. Mustafa M. Abdel Rahman (Accelerators & Ion Sources Dept.,) is appreciated for his generous assistance and permission of using

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ACCEPTED MANUSCRIPT Table 1. SFE and adhesion work of PADC as a function of Ar ions fluence for water and glycerol. Fluence (cm-2)

Water

Glycerol

Wa (mJ/m2)

 sv (mJ/m2) Wa (mJ/m2)

0

39.25

92.92

41.13

96.14

3.49E17

43.24

100.62

42.06

97.72

6.75E17

48.32

110.08

45.51

103.43

1.01E18

56.25

123.68

50.12

110.58

1.37E18

61.41

131.58

56.25

119.1

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 sv (mJ/m2)

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Figure Captions:

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Fig. 1. Monomer unit of PADC polymer.

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Fig. 2. Some photos of the probe liquid drops image capturing system. Fig. 3. Contact angle as a function of Ar ions fluence. Fig. 4. Photographs of pristine (a) and irradiated PADC polymer samples with fluences; (b) 3.49E17 cm-2, (c) 6.75E17 cm-2, (d) 1.01E18 cm-2 and (e) 1.37E18 cm-2. Fig. 5. SEM images of pristine (a) and irradiated PADC polymer samples with fluences; (b) 3.49E17 cm-2, (c) 6.75E17 cm-2, (d) 1.01E18 cm-2 and (e) 1.37E18 cm-2. Fig. 6. UV-Vis spectra for pristine and irradiated PADC. Fig. 7. (h) 2 as a function of photon energy (h. Fig. 8. (h)1/2 as a function of photon energy (h. Fig. 9. Band gap and carbon cluster size as a function of Ar ions fluence. Fig. 10. PL spectra with related parametres.

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Fig. 1.

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

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b h



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r

80

Contact angle (degree)

70

60

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Fig. 2.

Water Glycerol

50

40

30 0.00E+000

4.00E+017

8.00E+017

1.20E+018 -2

Ion Beam Fluence (cm )

Fig. 3.

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1.60E+018

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(a)

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Fig. 4.

(b)

(c)

(d)

(e)

Fig. 5.

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90 Fluence (cm-2) = 0 3.49E17 6.75E17 1.01E18 1.37E18

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70 60

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50 40

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Transmission Percent

80

30

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20 10 350

400

450

500

550

600

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300

Wavelength (nm)

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Fig. 6.

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h)2 (cm-1eV)2

50 40 30 20

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Pristine 3.49E17 cm-2 6.75E17 cm-2 1.01E18 cm-2 1.37E18 cm-2

10 0 -10 3.6

3.8

4.0

heV)

Fig. 7.

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3.5 3.0

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2.0 1.5 1.0

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h)1/2 (cm-1eV)1/2

2.5

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Pristine 3.49E17 cm-2 6.75E17 cm-2 1.01E18 cm-2 1.37E18 cm-2

0.5

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0.0 -0.5 2.5

3.0

3.5

4.0

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heV)

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Fig. 8.

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3.8

200

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Band Gap Eg (eV)

3.6

220

3.4

180

3.2 3.0 2.8

160

Eg Direct Eg Indirect N Direct N Indirect

140

2.6

120

2.4

100

2.2 80

2.0 0.00E+000

5.00E+017

1.00E+018 -2

Ion Beam Fluence (cm )

Fig. 9.

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1.50E+018

Cluster Size N

4.0

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-2 Fluence = 0 cm Intensity = 49.16

PL intensity (au)

= 6.75E17 cm = 32.1

stokes shift = 2nm

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= 3.49E17 cm = 47.79

c= 714nm c= 712nm

-2

-2

c= 709nm

= 1.01E18 cm = 22.38

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stokes shift = 5nm

-2

c= 707nm

-2

700

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650

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= 1.37E18 cm = 6.4

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stokes shift = 7nm

Wavelength (nm)

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Fig.10.

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c= 706nm stokes shift = 8nm

750

ACCEPTED MANUSCRIPT Highlights

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1. The induced defects on PADC polymer by low energy Ar ions bombardments have been investigated. 2. Low energy ions in the range below 5 keV is not examined elsewhere. 3. The wettability behavior is improved after treatment. This property is limited in investigation for the studied polymer. 4. The optical properties changed and the optical band gap decreased. 5. The used ion source proves efficiency in improving surface properties of PADC polymer.

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