1875-3892 © 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Chinese Heat Treatment Society doi: 10.1016/j.phpro.2013.11.052

Physics Procedia 50 ( 2013 ) 328 – 336

Available online at www.sciencedirect.com

ScienceDirect

International Federation for Heat Treatment and Surface Engineering 20th CongressBeijing,China,23-25 October 2012

Study on the Surface Structure and Properties of PDMS/PMMAAntifouling CoatingsX.M.Zhang , L.Li, Y.Zhang

College of Material Science and Chemistry Engineering, Harbin Engineering University, Harbin 150001, China

Abstract

Polydimethylsiloxane/ Poly (methyl methacrylates) copolymers (PDMS/PMMA) were synthesized by macromolecule initiationpolymerization in the PDMS content (molar fraction) of 27.5%, 61.7%, 72.8% and 78%. The surface coatings of thesecopolymers were then prepared by spinning-coating technology and examined with AFM. The morphologies of the copolymercoatings demonstrated that all of the copolymers had island shape micro-phase separation structure. Both the size of theseparation and the RMS value increased with the PDMS content increasing from 27.5 % to 72.8% and then decreased when thePDMS content reached 78%. The less obvious phase separation for as prepared coatings were strengthened after annealing. Inaddition, the hydrophobic surface coating of PDMS/PMMA copolymer had a good resistance to protein and diatom adsorption,which is demonstrated by the water contact angle testing and adsorption experiment of BSA protein, nitzschia closteriumminutissima. And the resistances of copolymer surfaces to protein and diatom adsorption increased after annealing treatment.© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Chinese Heat Treatment Society.

Keywords: PDMS/PMMA copolymer; Antifouling Coating; Surface Structure; Protein Adsorption; Diatom Adsorption

1. Introducture

Block copolymers with micro-phase separation structure have attracted much attention in recent years, due to itssuperiorities in antifouling, anticoagulant, optics and microelectronics, etc. (Yebra et al., 2004), (Xie et al., 2011),(Ni et al., 2012), (Iimura et al., 1998), (Gavrilov et al., 2011), (Kaczmarek and Chaberska, 2009). The micro-phaseseparation structures tend to be formed in block copolymers(Sharma et al., 2006), (Mani et al., 1998), owing to theincompatibility and bonding force between different chain segments. It has been proved that the biological

* Corresponding author. Tel.: +86-451-8251-8173; fax: +86-451-8251-8173E-mail address: jollyer@163.com.

© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Chinese Heat Treatment Society

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

329 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

antifouling performance and anticoagulant properties of polymer coating surfaces are related to the surface micro-phase structure and size. The structure can be multilayer, columnar and sphere according to the components of thecopolymers. But for coatings, copolymer can form different morphologies (Templin and Franck, 2007).

Considerable interests have been aroused for polysiloxane (PDMS) due to the excellent properties, such as highflexibility, good oxidative stability, high heat resistance, low energy, biocompatibility and environmental friendlycharacteristics. Copolymers with PDMS segments are widely used in thermoplastic elastomer, surfactant, lubricant,waterproof agent and antifoamer. However, the insuperable disadvantages, such as high-cost and low cohesivestrength, limit their further application in many fields (Deng et al., 2007), (Sun etal., 2004), (Liu et al., 2012), (Rathet al., 2009). In recent years, a variety of methods have been tried to synthesize PDMS copolymers with multiphasestructures, looking forward to improve their service performances. It has been recognized thatpolymethylmethacrylate (PMMA) has better light transmission, ultraviolet radiation resistance and stability tohydrolysis (Deng et al., 2007), (Richmond et al., 2004), (Varma et al., 2003), (Shi et al., 2010). Therefore,copolymers made from PMMA and PDMS should fuse together the advantages of these two components, and havemany unique properties (Chang et al., 1999). In addition, the incompatibility between PDMS and PMMA willcertainly endow their copolymers the micro-phase separation structures. This further gives the copolymers somesignificantly better performances.

Over the past few years, many studies have been given to copolymers of PDMS/PMMA, from development ofnew synthetic strategies to application. All of the findings show that PDMS/PMMA copolymers will have vastprospect due to their excellent performances (Tan et al., 2010), (Tumer et al., 2004), (Bes et al., 2003), (Brown et al.,2001). The purpose of this paper is to prepare copolymer coatings with certain surface morphology, and study theinfluence of copolymer compositions on the microphase separation structure, as well as the relationship betweenphase structure and properties of protein and diatom adsorption resistance, in the hope of controlling the antifoulingperformance by adjusting the contents of copolymers. Several PDMS/PMMA copolymers were synthesized bymacromolecule initiation polymerization method under mild condition. After a brief description of the samplespreparation and characterization, the influence of the composition of PDMS/PMMA on their morphology will bediscussed. Then the effect of micro-phase separation structure on protein and diatom adsorption will be described indetail. The obtained results will be helpful to researchers in antifouling field.

2. Experiment Section

PDMS/PMMA copolymers with different PDMS molar contents were synthesized through two steps. Firstly,macromolecules azo initiator (MAI) was synthesized by estrification and condensation reaction of polydimethylsiloxane double blocked by hydroxypropyl (HO-PDMS-OH) (Mw=2,000) and 4, 4’ Azobis (4-cyanopentanoic acid)(ACPA) at room temperature, with self-made N, N-dimethylpyridin-4-amine -4-methylbenzenesulfonic acidmonohydrate (DMAP-PTSA ) as catalyst and N, N’-dicyclohexyl carbodiimide DCC as activator at roomtemperature. Afterward, PDMS/PMMA copolymers were synthesized by macromolecule initiation polymerizationusing MAI as initiator and methyl methacrylates (MMA) as monomer in toluene solvent, under nitrogen protectioncircumstances at 75 for 12 hours. And, the products were precipitated in mixture of hexane and absolute ethylalcohol. After vacuum drying at 40 for 12 hours, the white copolymer powders were obtained. And the PDMSmolar contents were respectively 27.5%, 61.7%, 72.8% and 78% accordingly.

The PDMS/PMMA copolymer coatings were prepared on polished silicon wafers by spin-coating technique usingtoluene as coating solvent. Before using, silicon wafers were washed in Piranha solution and distilled water forseveral times then vacuum dried at room temperature. After the coatings were coated, solvent was evaporatedquickly and PMMA/PS copolymer coatings were obtained.

Thermal analysis of PDMS/PS copolymers was carried out through differential scanning calorimetry (DSC)method, with heating speed of 10 per minute, and argon served as a sweep gas when testing. The surfacemorphologies of PDMS/PMMA copolymer coatings were observed by atomic force microscope (AFM) in tappingmode. To study the wettability of the PMMA/PMMA copolymer coatings, water contact angles were investigated bywater contact angle analyzer. In protein adsorption experiments, PDMS/PMMA copolymer coating samples wereimmersed in phosphate buffer solution (PBS) of BSA at 0.5 mg/ml for 3 hours. The coating samples weredisinfected with ethanol solution and PBS before using. After removing the sample from the solution, the protein

330 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

concentration in PBS solution was assay by UV-Vis spectrophotometer. Simultaneously, in diatom adsorptionexperiment, the coating samples were immersed in artificial seawater with nitzschia closterium minutissima for 45days. All of the samples, solutions and utensils were sterilized by steam sterilization before using. Then the diatomon the coating surfaces was washed away by a certain quantity of deionized water and the concentration of diatomwater solution were tested by UV-Vis spectrophotometer.

3. Results and Discussion

3.1. Thermal Analysis of PDMS/PMMA Copolymers

To make clear that PDMS/PMMA copolymer should have micro-phase separation structure, thermal analysis wascarried out. Fig.1 showed the DSC curves of PMMA, HO-PDMS-OH and PDMS/PMMA copolymers with PMMAmolar contents were 27.5% and 61.7%. It illustrated that there were two glass transition points in PDMS/PMMAcopolymers. They located at about -125 and 110 when PMMA molar content was 25.7% and about -127 and94 when PMMA molar content was 61.7%. Glass transition points of both PMMA and PDMS existed in onePDMS/PMMA copolymer. This proved that PDMS/PMMA copolymer should have micro-phase separationstructure.

Temperature ( )

a— pure PMMA

b— pure HO-PDMS-OH

c— PDMS27.5PMMA

d— PDMS61.7PMMA

Hea

tflo

wen

doth

erm

ic

Fig. 1. DSC curves of PMMA, HO-PDMS-OH and PDMS/PMMA copolymers

3.2. Phase Morphology of PDMS/PMMA Copolymer Coatings

Microscopy is an extremely useful tool for studying the modified surfaces as it provides real-space coatingmorphology and nanostructure. It also provides detailed topographical information about surface features in terms ofroughness values of the interfaces (Rasmont et al., 2000). Therefore, various PDMS/PMMA copolymer coatingsurfaces created in this investigation were characterized using tapping mode AFM. Fig. 2 showed 2D and 3D AFMimages of the surface morphology for the spin-coated PDMS/PMMA copolymer coatings with different PDMScontents. Atomic force microscopy analysis results can illustrate the changes in surface topography clearly. Thecoatings surfaces were rough, showing islands of varying heights. In the case of light tapping mode, the interactionlargely depends on a meniscus force on the polymer surface, i.e., hydrophilicity and wettability. Due to thehydrophobic patches appear as low spots and the hydrophilic areas are as high spots, the phase data for thehydrophobic patches are dark and the hydrophilic areas are light in AFM topographical data obtained under lighttapping mode (Lee et al., 2008). It is well known that PMMA is relatively hydrophilic compared with PDMS. Onthe other hand, toluene is good for PDMS and poor for PMMA, as a result PMMA was apt to form raised phase, andPDMS formed pits. In the images, the high-resolved phase formed by micro-phase separation are clearly seen withthe dark parts correspond to domains formed by PDMS blocks, and the light parts formed by PMMA. From Fig. 2

331 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

we can see that phase separation can be observed on the surfaces of all PDMS/PMMA copolymer coatings. BothPDMS and PMMA phase regions in copolymer PDMS27.5PMMA (Fig.2a) performed by islands with small size. Bycontrast, the proportion of PDMS phase increased evidently for the morphology of PDMS61.7PMMA (Fig.2b).Moreover, there was an apparent change in the morphology of PDMS78PMMA (Fig.2d) that the PMMA phases hadconnected together. The phase region size of PDMS/PMMA copolymers with 27.5%, 61.7%, 72% and 78% PDMScontents through cross-sectional were respectively 45nm, 100nm, 130nm and 175nm, increased with the increasingof PDMS contents in PDMS/PMMA copolymers.

a1 a2

b1 b2

c1c2

d2d1

Fig. 2. AFM images of surface topology of PDMS/PMMA copolymer coating with PDMS molar contents were (a1) (a2)27.5%; (b1)(b2)61.7%; (c1) (c2) 72%; (d1) (d2) 78%

332 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

The coatings mentioned above were prepared by spinning-coated method, therefore the surface morphologies ofPDMS/PMMA copolymer coatings were acquired mainly through quickly evaporating the solvent. Thus it might notachieve completely thermodynamic equilibrium for the surface structures of copolymers. From the AFM images(Fig.2c and d) we know that large area of PMMA rich regions even emerged in surfaces of copolymer coatings with72.8% and 78% PDMS. As a result, it was necessary to anneal the copolymer coatings to put the surface structuresin state of thermodynamic equilibrium. Fig. 3 showed the AFM images of PDMS/PMMA copolymer coatings with72.8% and 78% PDMS annealed at 120 under vacuum condition for 16 hours.

a1 a2

b1b2

Fig. 3. AFM images of surface topology of PDMS/PMMA copolymer coating after annealing with PDMS molar contents were (a1) (a2) 72%;(b1) (b2) 78%

From Fig. 3 we can see that phase regions were evenly distributed. In general, there will be violent thermalmotion of molecular chains in copolymer above room temperature. This made the surface structure of coatingsachieve thermodynamic equilibrium state. The glass transition temperatures of PDMS and PMMA in PDMS/PMMAcopolymer were respectively -120 and 110 . As annealing temperature of 120 is warmer than either of the two,thermal motion could happen in either PDMS or PMMA section. At the same time, the incompatibility betweenPDMS and PMMA section is larger at higher temperature. Those made the interface boundary of two sectionsobvious.

Root meant square (RMS) roughness values Rrms and maximum roughness values Rmax for various surfaces aregiven in Fig. 4. Before annealing, surface roughness Rrms and Rmax increased with increasing PDMS contentswhen molar content of PDMS is less than 72.8%. On the other side, surface roughness decreased to a minimumvalue when PDMS content is 78%. The Rrms 3.134nm and Rmax 13.4nm for PDMS contents of 72.8% was themaximum value. However, the extent of change was different in different regions. The growth was the maximumwhen PDMS content was from 61.7% to 72.8%. This was illustrated that the best ranges of composition for micro-phase separation was about 61.7~72.8% PDMS molar content. It is important to mention here that RMS roughnessis a statistical parameter and therefore, gives an average of the roughness for the entire scan (Nathawat et al., 2009).It was well known that many reasons could lead to the morphologies of PDMS/PMMA copolymer varied accordingPDMS content. For instance, PDMS segment with low surface free energy tended to rich on coating surface,solubilities of PDMS and PMMA in toluene were different and there were acting forces among silicon wafer, PDMSand PMMA Contrast surface roughness, we can see that for the surface roughness after annealing, Rmax was largerbut Rrms was less than before annealing.

333 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

Fig. 4. Rrms and Rmax values for PDMS/PMMA copolymer coatings with different PDMS content

3.3. Protein Resistance of PDMS/PMMA Copolymers

The marine befouling process can be broadly into four stages: firstly, the accumulative total attaching of organicmacromolecules, such as protein, form conditional film. Then, the metabolic products of bacteria and seaweed, etc.adhere to the conditional film, form microbial film. Thirdly, primary multicellular organisms, herbivores anddecomposing organisms adhere on the microbial film. Finally, the depositing of large marine invertebrate organismforms fouling organism layer. Every stage of fouling process provides conditions for the next stage. In consequence,the forming of conditional film is the foundation of fouling process, and it is important to investigate the proteinadsorption performance of antifouling coatings. It was suggested that the maximum absorbance of protein occurredin wavelength of 278nm (Fig. 5a). The absorbance of protein solution at that wavelength was in proportion to thesolution density (Li et al., 2009). Therefore, the absorbance in wavelength of 278nm could be used to quantitativeassay of protein adsorption performance. Fig. 5b showed the standard curve obtained through the absorbance testingof proteins solution with different concentration at wavelength of 278nm. In Fig. 5b, the points were measuredvalues and the line was absorbance (A) to concentration (C) fitted curve with correlation coefficients of 0.9996expressed as formula 1.

A = 0.5479C+0.1733 (1)

In this paper, we immersed PDMS/PMMA copolymer coating samples with dimension of 1cm by 1cm in 3mlprotein solution, respectively. The protein adsorbed amounts of each sample were obtained by calculation and theresults were shown in Fig.5c.

Protein absorbed amount of coating is related to its surface condition, such as the morphology. Compared withthe primary BSA amount, the adsorption on surfaces of all the PDMS/PMMA copolymer coatings we studied wasfairly small. This indicated that PDMS/PMMA copolymer had good property in inhibiting BSA adsorption. Thismade the formation of conditional coatings difficult. The protein adsorption of PDMS61.7PMMA copolymer withmedial RRMS value of 0.957nm was smaller than that of others. This was in line with the idea that proteinadsorption could be inhibited only when the phase area of copolymer has suitable size. After annealing, the proteinadsorption in copolymer surfaces decreased, due to the change of surface morphology.

334 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

a

240 260 280 300 320 34-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Abso

rban

ce

Wavenumber ( nm)

b

c

1.5

2.0

2.5

3.0

3.5

4.0

4.5

BSA

adso

rbed

amou

nt (m

g/cm

2 )

Molar content of PDMS ( %)

before annealing after annealing

27.5 61.7 72.8 78

Fig. 5. (a)UV spectra of BSA solution; (b) Standard absorbance curve of BSA absorbance to concentration at wavelength of 278nm; (c) BSAadsorbed amount on the surface of PDMS/PMMA copolymer coatings with different contents

3.4. Diatom Adsorption Performance of PDMS/PMMA Copolymer Coatings

To investigate the antifouling of PDMS/PMMA copolymer coatings, nitzschia closterium minutissima adsorptionexperiment was carried out. And UV-Vis spectra of diatom solution were tested. It was suggested that the maximumabsorption occurred in wavelength of 682nm (Fig. 6a). The absorbance of diatom solution near that wavelength is inproportion to the solution density. Therefore, the absorbance in wavelength of 682nm can be used to quantitativeassay of the adsorption of diatom. Fig. 6b showed the standard curve obtained through testing the absorbance ofdiatom solution with different concentration at wavelength of 682nm. On the graph the points were measured valuesand the line was absorbency (A) to concentration (C) fitted curve with correlation coefficients of 1 expressed asformula 2.

C=9.1326A-0.5631 104Cell/ml (2)

In this paper, we immersed PDMS/PMMA copolymer coating samples with diameter of 5.08cm in 50ml diatomsolution, respectively. The concentration of diatom washed from surfaces of coating samples were obtained bycalculation and the results were shown in Fig.6c.

Diatom absorbed amount of coating is also concerned with its surface morphology. Compared with theconcentration of nitzschia closterium minutissima growth in artificial seawater, the adsorption on surfaces of all thePDMS/PMMA copolymer coatings we studied was quite small. In addition, nitzschia closterium minutissima barelyexisted on the surface of copolymer coatings after washed. This indicated that PDMS/PMMA copolymer had goodproperty in inhibiting diatom adsorption. The diatom adsorption of PDMS72.8PMMA copolymer with hasmaximum RRMS value was smaller than any other surfaces. After annealing, the diatom adsorption in copolymersurfaces decreased, due to the change of surface morphology.

335 X.M. Zhang et al. / Physics Procedia 50 ( 2013 ) 328 – 336

4. Conclusions

(1) A series of PDMS/PMMA copolymers with 27.5%, 61.7%, 72.8% and 78% molar content of PDMS weresynthesized respectively by macromolecule initiation polymerization, and their films were prepared by spin-coatingmethod.

(2) All of the copolymers had micro-phase separation structure with island shape. Furthermore, with the increasein the PDMS content, the surface pattern changed in shape, size and RMS value, which were embodied in the bothincreasing of phase region size and RMS value when the molar conent of PDMS is less than 72.8%. However,surface segregation was not apparent for copolymers with 72.8% and 78% PDMS, and surface segregation inPDMS/PMMA copolymer coatings was effectively improved by annealing treatment.

(3) BSA protein adsorption experiment results identified that all the studied PDMS/PMMA copolymer coatingscan inhibit protein adsorption. The protein resistance of copolymer coatings was improved through annealingtreatment.

(4) Diatom adsorption experiment results illustrated that PDMS/PMMA copolymer had good property ininhibiting diatom adsorption. The adsorption of nitzschia closterium minutissima on surfaces of all thePDMS/PMMA copolymer coatings was quite small. In addition, nitzschia closterium minutissima barely existed onthe surface of copolymer coatings after washed. After annealing, the adsorption of diatom on copolymer surfacesdecreased.

c

12

13

14

15

16

17

18

Conc

entra

tion

(104 C

ell/m

l)

Molar content of PDMS ( %)

before annealing after annealing

27.5 61.7 72.8 78

a

600 650 700 750 8000.16

0.17

0.18

0.19

0.20

0.21

0.22

0.23

0.24

0.25

0.26

Abs

orba

nce

Wavenumber (nm)

b

0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Con

cent

ratio

n (

104 C

ells

/ml)

Absorbance

Fig. 6. (a)UV spectra of nitzschia closterium minutissima solution; (b) Standard absorbance curve of nitzschia closterium minutissimaconcentration to absorbance at wavelength of 682nm; (c) Concentration of nitzschia closterium minutissima washed from surfaces of

PDMS/PMMA copolymer coatings with different contents

Acknowledgements

This work is supported by the Natural Science Foundation of Heilongjiang Province of China (No. E201105),and the National Nature Science Foundation of China (Grant No. 51309067).

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