Surface and wetting characteristics of textured bisphenol-A based polycarbonate surfaces: Acetone-induced crystallization texturing methods Ahmed Owais, 1 Mazen M. Khaled, 2 Bekir S. Yilbas, 3 Numan Abu-Dheir, 3 Kripa K. Varanasi, 4 Kamal Y. Toumi 4 1 Renewable Energy Science and Engineering Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni- Suef University, Beni-Suef 62511, Egypt 2 Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 3 Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 4 Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge Massachusetts 02139-4307 Correspondence to: M. M. Khaled (E - mail: [email protected]) ABSTRACT: Polycarbonate (PC) sheet is a promising material for facile patterning to induce hydrophobic self-cleaning and dust repel- ling properties for photovoltaic panels’ protection. An investigation to texture PC sheet surfaces to develop a self-cleaning structure using solvent induced-crystallization is carried out using acetone. Acetone is applied in both liquid and vapor states to generate a hierarchically structured surface that would improve its contacts angle and therefore improve hydrophobicity. The surface texture is investigated and characterized using atomic force microscopy, contact angle technique (Goniometer), optical microscopy, ultraviolet- visible spectroscopy (UV–vis) and Fourier transform infrared spectroscopy. The findings revealed that the liquid acetone-induced crystallization of PC surface leads to a hierarchal and hydrophobic surface with an average contact angle of 1358 and average trans- mittance <2%. However, the acetone vapor induced-crystallization results in a slightly hydrophilic hierarchal textured surface with high transmittance; in which case, average contact angle of 898 and average transmittance of 69% are achieved. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43074. KEYWORDS: crystallization; hydrophobicity; polycarbonate sheet; solar cells; surface texturing Received 29 April 2015; accepted 21 October 2015 DOI: 10.1002/app.43074 INTRODUCTION In the last decade, several studies have been performed to develop the different techniques to design and produce hydro- phobic surfaces by controlling the surface topography and chemistry. 1,2 Accumulation of dust, 3 as well as snow, 4 over the polycarbonate (PC) sheet surface becomes problematic and adversely affects the photovoltaic (PV) efficiency owing to low- ering the transmittance of the solar radiation reaching the active area of the PV panel. 5 Lotus leaf has gained the main the focus of the scientists who are interested in that field of research because its surface is rough and hydrophobic, contact angle >1508 and sliding angle <108. 6 Several crop plants also have the same characteristics like the Lotus leave, for example, Bras- sica, Alchemellia, and Lupinus. 7 Mimicking the nature and gen- erating surface hydrophobicity improves the performance of the PV devices through minimizing the dust accummulation at the surface. One of the promising materials to be modified to develop hydrophobicity is the PC sheet. PC sheet is one of the protective covers for PV panels due to its high mechanical flexi- bility and low density. It was reported that surface texturing of PC sheet at micro/nano scales resulted in a hydrophobic tex- ture. 8 Texturing the hydrophobic surfaces enhances the non- wetting properties by increasing the trapped air between the surface texture posts. This, in turn, leads to a superhydrophobic behavior of the textured surface, since liquid droplets lay on the air pockets. Surface texturing and modification of polymers toward achieving surface hydrophobicity and characterization were central interest by the researchers. 9–12 This is mainly because of easiness of surface modification and processing of polymers to achieve hydrophobic characteristics. Additional Supporting Information may be found in the online version of this article. V C 2015 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.43074 43074 (1 of 10)
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Surface and wetting characteristics of textured bisphenol-A basedpolycarbonate surfaces: Acetone-induced crystallization texturingmethods
Ahmed Owais,1 Mazen M. Khaled,2 Bekir S. Yilbas,3 Numan Abu-Dheir,3 Kripa K. Varanasi,4
Kamal Y. Toumi4
1Renewable Energy Science and Engineering Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef 62511, Egypt2Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia3Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia4Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge Massachusetts 02139-4307Correspondence to: M. M. Khaled (E - mail: [email protected])
ABSTRACT: Polycarbonate (PC) sheet is a promising material for facile patterning to induce hydrophobic self-cleaning and dust repel-
ling properties for photovoltaic panels’ protection. An investigation to texture PC sheet surfaces to develop a self-cleaning structure
using solvent induced-crystallization is carried out using acetone. Acetone is applied in both liquid and vapor states to generate a
hierarchically structured surface that would improve its contacts angle and therefore improve hydrophobicity. The surface texture is
investigated and characterized using atomic force microscopy, contact angle technique (Goniometer), optical microscopy, ultraviolet-
visible spectroscopy (UV–vis) and Fourier transform infrared spectroscopy. The findings revealed that the liquid acetone-induced
crystallization of PC surface leads to a hierarchal and hydrophobic surface with an average contact angle of 1358 and average trans-
mittance <2%. However, the acetone vapor induced-crystallization results in a slightly hydrophilic hierarchal textured surface with
high transmittance; in which case, average contact angle of 898 and average transmittance of 69% are achieved. VC 2015 Wiley Periodicals,
Inc. J. Appl. Polym. Sci. 2016, 133, 43074.
KEYWORDS: crystallization; hydrophobicity; polycarbonate sheet; solar cells; surface texturing
Received 29 April 2015; accepted 21 October 2015DOI: 10.1002/app.43074
INTRODUCTION
In the last decade, several studies have been performed to
develop the different techniques to design and produce hydro-
phobic surfaces by controlling the surface topography and
chemistry.1,2 Accumulation of dust,3 as well as snow,4 over the
polycarbonate (PC) sheet surface becomes problematic and
adversely affects the photovoltaic (PV) efficiency owing to low-
ering the transmittance of the solar radiation reaching the active
area of the PV panel.5 Lotus leaf has gained the main the focus
of the scientists who are interested in that field of research
because its surface is rough and hydrophobic, contact angle
>1508 and sliding angle <108.6 Several crop plants also have
the same characteristics like the Lotus leave, for example, Bras-
sica, Alchemellia, and Lupinus.7 Mimicking the nature and gen-
erating surface hydrophobicity improves the performance of the
PV devices through minimizing the dust accummulation at the
surface. One of the promising materials to be modified to
develop hydrophobicity is the PC sheet. PC sheet is one of the
protective covers for PV panels due to its high mechanical flexi-
bility and low density. It was reported that surface texturing of
PC sheet at micro/nano scales resulted in a hydrophobic tex-
ture.8 Texturing the hydrophobic surfaces enhances the non-
wetting properties by increasing the trapped air between the
surface texture posts. This, in turn, leads to a superhydrophobic
behavior of the textured surface, since liquid droplets lay on the
air pockets. Surface texturing and modification of polymers
toward achieving surface hydrophobicity and characterization
were central interest by the researchers.9–12 This is mainly
because of easiness of surface modification and processing of
polymers to achieve hydrophobic characteristics.
Additional Supporting Information may be found in the online version of this article.
VC 2015 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.4307443074 (1 of 10)
roughness. This can be related to the hydrostatic pressure that is
applied on the immersed PC surface in the liquid acetone. How-
ever, the textured PC sheet surfaces due to vapor-induced crystalli-
zation have small crystals owing to the high nucleation density.
Despite of the enlargement of the crystals grain-sizes due to increas-
ing the exposure time of the PC sheet surface to the acetone vapor,
however, reaching large crystals with perfectness stills arduous.33
The size of the generated spherules and the polymer surface
degree of crystallinity is directly proportional to the depth to
which acetone diffuses. Furthermore, pores formation takes
place if the layer depth is larger than the width of the spherule;
however, incomplete spherule coverage results in case of less
layer depth than the spherule dimensions.
For the solid-vapor method of polymer crystallization, the ace-
tone vapor condenses over the PC surface. Therefore, the PC
becomes in contact with, only, 1 mL of acetone on average. As
a consequence, the mass transfer in this case becomes consider-
ably lower than that in case of immersing the surface in the liq-
uid acetone. Mass transfer, which is a driving-force dependent,
affects the diffusion process significantly. The mass transfer
equation between two phases is34:
N A5k a DCA (1)
where NA: the mass transfer rate of component A, k: the mass
transfer coefficient, a: the transfer area, DCA: the concentration
driving force. The driving force is the result of the difference
between the concentration of the liquid in the bulk and the
concentration of the liquid in the formed boundary-film inter-
face. Therefore, the concentration of the liquid in the bulk is
larger, in case of the immersion in liquid acetone, than that in
case of exposing the polymer surface to acetone vapor.
Figure 4. (A) 40 lm scale and (B) 5 lm scale line profiles of the AFM micrographs for a textured PC surface by immersion in pure liquid acetone for
10 min. (C) 40 lm scale and (D) 5 lm scale line profiles of the AFM micrographs for a textured PC surface by exposure to pure acetone vapor for
24 h. The four profiles prove the formation of the hierarchical structure after the solvent-induced crystallization process. Line profiles A and C (at 40
lm) show different textured surfaces in micro (for A) and nano (for C) scales due to surface crystallization by treatment with acetone liquid and vapor,
respectively. Line profiles B and D show different textured surfaces with dense (for B) and sprinkled (for D) pillars due to surface crystallization by treat-
ment with acetone liquid and vapor, respectively.
ARTICLE WILEYONLINELIBRARY.COM/APP
WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.4307443074 (5 of 10)