bioadesão de polímeros líquidos por ângulo de contato

8/7/2019 bioadeso de polmeros lquidos por ngulo de contato

1/15

J. Adhesion Sci. Technol., Vol. 21, No. 34, pp. 227241 (2007) VSP 2007.Also available online - www.brill.nl/jast

The effect of polymer surface on the wetting and adhesion

of liquid systems

P. F. RIOS 1,2, H. DODIUK 1, S. KENIG 1,, S. McCARTHY 2 and A. DOTAN 1

1

Department of Plastics Engineering, Shenkar College of Engineering and Design,12 Anna Frank St, Ramat-Gan 52526, Israel

2 Department of Plastics Engineering, University of Massachusetts at Lowell,883 Broadway Street, Lowell, MA 01854-5130, USA

Received in final form 28 December 2006

AbstractYoungs equation describes the wetting phenomenon in terms of the contact angle between

a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid

solid interactions, an additional parameter related to the adhesion between the liquid drop and the

solid surface is also of importance in cases where liquid sliding is involved. It is postulated that

wetting which is related to the contact angle, and interfacial adhesion, which is related to the slidingangle, are interdependent phenomena and have to be considered simultaneously. A variety of models

that relate the sliding angle to the forces developed along the contact periphery between a liquid drop

and a solid surface have been proposed in the literature. Here, a modified model is proposed that

quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across

the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter

can be defined depending on the mass of the drop, the contact angle and the sliding angle. To verify

the proposed approach the adhesion strength parameter has been calculated, based on experimental

results, for a number of polymer surfaces and has been correlated with their composition and structure.

The interaction strength parameter can be calculated for any smooth surface from measurements of

the contact and the sliding angles.

Keywords: Molecular bonding; molecular structure; wetting; contact angle; sliding angle; interfacial

adhesion; polymer surfaces.

1. INTRODUCTION

The energetics of solid surfaces and their effect on the interaction with liquids

play an important role in a variety of applications such as adhesive bonding,

polymer coatings, printing, etc., where a high degree of wetting is desired. In other

To whom correspondence should be addressed. Tel.: (972-3) 613-0111; Fax: (972-3) 613-0019;

e-mail: [email protected]


2/15

228 P. F. Rios et al.

applications such as water and ice repellency and anti-sticking, easy-cleaning or

self-cleaning surfaces, wetting is undesirable.

The thermodynamics between a liquid and a solid were first described by Young

in 1805 [1]. The so-called Youngs equation relates the surface tensions of a liquid,

a solid and a gas surrounding them to the contact angle formed between the liquid

and the solid substrate. Adamson [2] defined wetting as the case where the contact

angle between the liquid drop and the solid surface approaches zero. Non-wetting

was defined as the case where the contact angle is greater than 90. Generally, when

the water contact angle is less than 90 the surface is considered hydrophilic. When

the water contact angle is greater than 90 the surface is considered hydrophobic.

However, the contact angle is not the only parameter that defines liquidsolid

interactions, an additional parameter related to the adhesion between the liquid drop

and the solid surface is of importance in cases where liquid sliding is involved.

Adhesion can be determined by the critical angle at which the drop is detached

from the solid surface upon tilting the solid plane. Hence, the sliding angle is

defined as the tilting angle at which a liquid drop, with a certain weight, begins

to slide down the tilted solid plane. Murase and co-workers [3, 4] showed that

a high contact angle does not necessarily relate to a low sliding angle. For

instance, a fluoropolymer with a water contact angle of 117 possesses a higher

sliding angle than poly(dimethylsiloxane) with a contact angle of 96. Previous

studies [37] correlated the sliding angle to the forces that developed at the contact

periphery between the liquid drop and the solid surface. Several other studies[814] correlated the drop retention and sliding angle to hysteresis phenomenon

manifested by different advancing and receding contact angles, which are also

a consequence of forces at the liquidsolid contact periphery. Recently, it was

proposed that the wetting phenomenon that is related to the contact angle and

the adhesion phenomenon that is related to the sliding angle are interdependent.

Furthermore, in this new approach, the adhesion forces that develop between the

liquid and the solid can be related to the contact area in addition to the contact

perimeter, as described by Rios and co-workers [15].

2. ANALYSIS AND MODELING

The radius R that a liquid drop makes with a smooth surface can be calculated

from its density , mass m and the contact angle of the liquid with the solid

(Fig. 1). Assuming that the drop is a perfect sphere, the radius of the drop, R, can

be expressed according to equation (1):

R =

3m

(2 3cos + cos3 )

1/3. (1)


3/15

The effect of polymer surface on the wetting and adhesion of liquid systems 229

Figure 1. Schematic representation of a water drop on a smooth solid surface.

The radius r , of the contact area between the drop and the solid, is given by

equation (2):r = R sin . (2)

Equations (1) and (2) are valid for hydrophobic surfaces ( > 90) as in Fig. 1,

as well as for hydrophilic surfaces ( < 90). When the horizontal smooth plane is

tilted, the contact area is assumed to remain circular with a radius r , though tilting

deforms the drop. At a certain sliding angle the drop is detached from the surface

and slides. Equations (1) and (2) assume that there are no moments and rotational

forces acting on the drop, as depicted in Fig. 2. The driving force for the drop

sliding is the gravitational force (mg sin ) and the adhesion force opposing this

movement is FA. At the onset of drop motion, the forces acting on the drop will be

at equilibrium as described by equation (3):

FA = mg sin . (3)

The correlations between the contact angle, sliding angle and the interaction

energies between a liquid and a smooth or rough surface have been postulated

previously. These previous models [3, 4, 6, 7] are based on the assumption that

the adhesion of the liquid drop to the solid is the result of the forces acting at the

contact periphery between the drop and the solid as given by equation (4):

FA = KA2 r, (4)

where KA is a constant with units of surface tension (N/m) or surface energy (J/m2).

KA represents the energy of adhesion between the two phases. Combining equations

(3) and (4) the Von Buzgh and Wolfram equation (5) is obtained [5]:

sin =KA2 r

mg. (5)

Furthermore, substituting r in equation (5) from equations (1) and (2) the

following expression is obtained:

sin =KA2

g

3

(2 3cos + cos3 )

1/3sin m2/3. (6)


4/15


Figure 2. Schematic representation of a water drop on a tilted solid surface.

Hence, for a given liquid and solid (i.e., and are constant) sin depends onthe mass of the drop to the 2/3 power. KA is the interaction energy (invariant for

a given surface chemistry) and is expressed by equation (7), which was obtained in

a very similar form by Miwa and co-workers [7]:

KA =

(2 3cos + cos3 )

3

1/3g

2

m

2/3sin

sin . (7)

Other models [8 14] are derived from the thermodynamic work of adhesion WA,

which is the work required to separate a liquid from a solid phase as expressed by

the YoungDupre equation (8) [16]:

WA e = LV(1 + cos ), (8)

where LV is the liquid surface tension and the contact angle. The spreading

pressure e is small when is finite and becomes significant when approaches

zero. Thus, in most polymer surfaces, e can be neglected. Therefore, the Young

Dupres equation becomes:

WA = LV(1 + cos ). (9)

Furmidge [8] derived equation (10) from equation (9), by taking into account theadvancing (a) and receding (r) contact angles:

mg sin = wLV(cos r cos a), (10)

where w is the maximum width of the contact area of the drop with the solid surface,

perpendicular to the sliding direction.

In all hysteresis-based-type models [914], Furmidges equation (10) is the basis

for the calculation of interaction energies of liquid drops on tilted solid surfaces.

All these models discussed previously are based on the assumption that the

adhesion of the liquid drop to the solid surface is the result of the forces developed

along the periphery of the liquid drop which is in contact with the solid surface.

However, when considering the sliding angle other forces should also be taken into

account. The adhesion of a liquid drop to a solid is the result of intermolecular


5/15


forces at the interfacial area. These interfacial forces attract the liquid molecules to

the solid molecules across the whole contact area of the drop with the solid and not

only along the periphery. Consequently, the proposed model hypothesizes that the

force of adhesion is proportional to the contact area between the drop and the solid,

thus:

FA = KR r2, (11)

where KR is a constant representing the adhesion strength between the liquid and

the solid, with units of force/area (N/m2 = Pa). Combination of equations (3) and

(11) leads to equation (12):

sin =KR r

2

mg

. (12)

Furthermore, substituting r in equation (12) from equations (1) and (2) the

following expression is obtained:

sin =KR

g

3

(2 3cos + cos3 )

2/3sin2 m1/3. (13)

Hence, for a given liquid and solid (i.e., and are constant), sin depends on

the mass of the drop to the 1/3 power. KR is the interfacial adhesion strength

(invariant for a given surface chemistry) and is expressed by:

KR =

(2 3cos + cos3 )

3

2/3

g

m

1/3 sin

sin2 . (14)

3. EXPERIMENTAL

To verify the proposed analysis and the derived models, five chemically different but

smooth polymer surfaces were evaluated. Three non-polar, hydrophobic polymers

( > 90); poly(tetrafluoroethylene) (PTFE Virgin, Lanza Nuova, Italy), Silicone

rubber (Moldmax 30, Smooth-on, USA) and Polypropylene (R50PP homopoly-mer, Carmel Olefins, Israel), and two polar, hydrophilic polymers ( < 90); poly-

carbonate (PC, Makrolon 3108, Bayer, Germany) and poly(methyl methcrylate)

(PMMA, Diakon CLH 952T, Lucite International, USA) were examined. The PP,

PC and PMMA samples were produced by injection molding and the silicone sam-

ple was produced by casting and curing the silicone compound in a mold. All sam-

ples were prepared using highly polished molds, thus rendering smooth surfaces.

The PTFE sheet possessed a smooth surface without further surface treatment. Be-

fore measurements, all samples were cleaned with iso-propanol and dried.

The static contact angle was measured according to the sessile drop method

using a contact angle analyzer (OCA 20, Dataphysics Instruments, Germany).

Two liquids were used for the measurements: deionized and ultra-filtered water

(0.2 m filter) and di-iodomethane (DIM, Merck, Germany). Water has a high


6/15


surface energy (72.2 mJ/m2) and is a highly polar liquid that can form hydrogen

bonds. In contrast, DIM is mostly non-polar, having relatively high surface energy

(50.8 mJ/m2) [17]. These two liquids are commonly used as the polar and non-

polar liquids for determining solid surface energies [18]. In a preliminary step, it

was found that there were no significant changes in the contact angle as a function of

liquid drop volume in the range of 130 l. The sliding angle was measured using

a tilting unit (TBU90E, Dataphysics Instruments, Germany), which is part of the

contact-angle analyzer. A drop was first deposited on the horizontal substrate and

after equilibrium it was tilted at a rate of 100/min until the onset of drop motion

(sliding). The sliding angle was found to depend on drop volume. The contact

angles and sliding angles were measured using video-based software (SCA 20,

Dataphysics Instruments, Germany). The solids surface energies were calculated

using the two-liquid method, as described by Owens, Wendt and Kaelble [19, 20].Accordingly, the interfacial energies between the solids and liquids are a function of

the geometric mean of the polar and dispersion components of the surface energies

of the liquid and the solid. Using Youngs equation for the contact angle of each

liquid (in this case water and DIM) the solids surface energies were derived.

4. RESULTS AND DISCUSSION

4.1. Contact angles

Figure 3 shows the contact angles for 5 l water drops on PMMA, PC, PP, silicone

and PTFE, respectively. The increase in contact angle from PMMA to PTFE is

evident.

Table 1 summarizes the results for static contact angles of water and DIM and

the calculated surface energies for the different substrates. All measurements were

made in various sample locations and for different drop volumes. As pointed out

earlier, contact angles do not depend significantly on drop volume. The results given

in Table 1 represent the averages and standard deviations for all measurements and

volumes up to 30 l.It must be noted that the surface energies obtained for silicone and PTFE are

somewhat higher than those currently accepted (around 20 mJ/m2). One- or two-

liquid methods for surface energy calculation (like the one used in this work) are

known to be less accurate than other more sophisticated multi-liquid methods, but

are used because of their simplicity.

Based on thermodynamic considerations a liquid will completely wet a solid

surface, provided that the surface energy of the solid is greater than that of the liquid

[21]. Since both, water and DIM, have higher surface energies than all the solid

surfaces studied, both liquids wet these solids only partially. The higher the

difference between the surface energies of the liquid and the solid, less wetting

is expected. Due to the higher surface energy of water compared to DIM, water

contact angles on all polymer surfaces studied are higher than those of DIM, as


7/15


Figure 3. Sessile drops for static water contact angle measurements: (a) PMMA, (b) PC, (c) PP,

(d) silicone, (e) PTFE.

Table 1.

Static contact angles on smooth polymer surfaces

Parameter PMMA PC PP Silicone PTFE

water (deg) 72.5 3.3 81.3 0.7 96.7 2.5 103.1 2.5 111.9 3.0

DIM (deg) 33.7 1.3 28.8 3.3 52.8 1.6 65.0 1.7 59.8 3.5

Surface energy (mJ/m2) 44.1 44.7 33.0 26.0 31.5

seen in Table 1. Moreover, the low-surface-energy polymers (PP, silicone and

PTFE) exhibit higher contact angles with both liquids than the higher-surface-energy polymers (PMMA and PC). PP has a higher surface energy than silicone and

PTFE but a lower surface energy than PC and PMMA. Thus, water and DIM both

wet PP more than they wet silicone and PTFE and wet it less than they wet PC and

PMMA. It can be seen that, although PTFE and silicone have close surface energy

values, PTFE exhibits a higher water contact angle but a lower DIM contact angle

than silicone. This may be attributed to the fact that water possesses both polar and

dispersion surface energy components, while DIM has mostly the dispersion surface

energy component [17] and PTFE is non-polar while the silicone rubber may have

some polarity due to the siloxane backbone and/or additives in the formulation.

This phenomenon is also observed for PC and PMMA. The surface energies of

these polymers are close; however, PMMA is more polar than PC, hence water wets

PMMA more than it wets PC and DIM wets PC more than it wets PMMA.


8/15


4.2. Sliding angles

Figure 4 depicts the averaged water sliding angles as a function of drop volume

(350 l) for the studied polymer substrates. For all polymer substrates, small dropsdo not slide even for vertical (90) solid position. As the drop volume increases the

drops starts to slide. The sliding angle decreases with drop volume. This general

behavior is in agreement with previous studies [3, 4, 7, 15].

Equations (6) and (13) describe the relations between the sine of the sliding angle

and m, the mass of the liquid drop for the perimeter based models and for the contact

area based model, respectively. For the perimeter models, sin depends on m (or

volume V) to the 2/3 power, while for the contact area model sin depends on

m (or V) to the 1/3 power. In Fig. 5 log(sin ) is plotted as a function of logV.

It can be seen that for all studied polymers, except PP, the experimental data fit acurve that starts with a high negative slope and becomes more moderate as the drop

Figure 4. Water sliding angles for smooth polymer surfaces as a function of water drop volume from

3 l to 50 l.

Figure 5. log(sin ) as a function of log V for water drop volumes from 3 l to 50 l.


9/15


volume increases. Figures 6 and 7 describe the behavior of low volume drops and of

high volume drops, respectively (for all polymers except PP). In the case of PTFE,

for volumes less than 20 l, water drops will not slide even when the solid surface

is vertical. For 2035 l drops the experimental data follow a straight line with a

slope of0.878. Finally, for higher drop volumes up to 50 l the slope is reduced

to0.415. Similar behavior is observed for PMMA. Water drops with volumes less

than 10 l will not slide, drop volumes between 10 and 25 l show a slope of0.676

and for higher volume drops the slope is 0.422. For PC, drops do not slide below

5 l, for drop volumes between 5 and 20 l the slope is 0.9513 and for higher drop

volumes the slope is 0.272. Finally, for silicone, all water drops in the range of

3 l to 50 l show a sliding angle lower than 90. For drop volumes from 320 l

the slope is 0.627 and for higher drop volumes the slope is reduced to 0.500. In

the case of the above-mentioned four polymers, the slopes for small drop volumes

Figure 6. log(sin ) as a function of log V for small water drops.

Figure 7. log(sin ) as a function of log V for large water drops.


10/15


average 0.783 and the slopes for the high drop volumes average 0.402. This

analysis is consistent with the concept that for small volume drops, sin behaves

according to m (or V) to the 2/3 power, i.e., consistent with the perimeter models.

For higher drop volumes sin follows m (or V) to the 1/3 power, i.e., consistent

with the contact area model. Rios and co-workers [15] have previously suggested

that the ratio of the contact perimeter to the contact area is proportional to 1/r; thus,

for small drops the contact perimeter is dominant over the contact area and for larger

drops the contact area is dominant over the contact perimeter.

In a recent paper, Gao and McCarthy [22] consider a liquid droplet on a horizontal

surface that rolls from one point to another (movement caused by vibration, or

wind). This movement is described as a tank-tread fashion i.e., the water

molecules that move are only those that wet the new surface, as well as the ones

that dewet the wet surface. These molecules are the ones that are situated at thethree-phase contact line, while the molecules at the two-phase solidliquid contact

area remain stationary. Gao and McCarthys model describes the movement of a

rolling drop on a flat surface, where a no-slip condition is assumed. Furthermore,

the model leads to the conclusion that the drop adhesion will be controlled mainly

by the contact perimeter. However, in the other extreme case, the drop may slide

rather than roll on a tilted surface, where a slip condition may prevail. In this case

the adhesion will be controlled by the whole contact area. It can be concluded

that both contributions, those of the peripherial forces and those of the areal forces,

should be considered for the real case of combined rolling and sliding. The relativecontributions of the perimeter and area forces depend on the size of the drop

and the substrate chemical composition as demonstrated by the data presented in

Figs 46. An integrated model that takes into account both forces, i.e., at the

perimeter (a 3-phase boundary) and at the contact area (a 2-phase boundary) should

be considered.

Table 2 summarizes the sliding angles for 15 mg and 50 mg DIM drops on the

different polymers.

The results shown in Table 1, Table 2 and Fig. 4 show that PTFE has a higher water

contact angle than silicone, and also a higher water sliding angle. Furthermore,PTFE has the highest water contact angle and the highest water sliding angle. If

hydrophobicity is characterized by a combination of high contact angle and low

sliding angle, then PTFE cannot be described as being more hydrophobic than the

other polymers. For both liquids PTFE has the largest sliding angle and silicone

possesses the lowest one. PMMA has a higher water sliding angle but a lower

Table 2.

DIM sliding angles (degrees) on smooth solid polymer surfaces (15 and 50 mg drops)

Drop weight PMMA PC PP Silicone PTFE

(mg)

15 13 1 23 3 20 3 10 1 44 16

50 4 2 8 2 7 1 3 1 22 3


11/15


DIM sliding angle than PP and PC. For small liquid drops, PP has a higher water

sliding angle than PC but for high drop volumes the water sliding angles adopt

similar values (slightly higher for PC). The DIM sliding angle is also slightly higher

for PC but very close to that of PP. The analysis of the surface interactions based

on these complex and even contradictory phenomena of the contact angle and the

sliding angle is difficult. However, some of these difficulties may be resolved by

evaluating the interaction parameters calculated from equations (7) and/or (14),

once the contact angle and the sliding angle are measured.

4.3. Adhesion strength

Table 3 summarizes the interaction energy KA calculated from equation (7) and

the adhesion strength KR calculated from equation (14). As stated above, theperimeter model fits the small drop case, and the area model is more appropriate for

high volume drops. Consequently, the experimental sliding angles of small drops

(15 mg drop for all cases except, 20 mg drop for water on PTFE) were used for the

calculation of KA and the sliding angles for large drops (50 mg) were used for the

calculation of KR. The respective liquids densities used were 1.00 g/cm3 for water

and 3.32 g/cm3 for DIM.

It is expected that the interaction energy or adhesion strength for water (a polar

liquid) on a polar polymer should be higher than those for DIM (non-polar liquid).

Conversely, the interaction energy or adhesion strength for DIM on a non-polarpolymer should be higher than those for water. As can be observed in Table 3

these principles are followed by KR, but not for KA. Hence, these results indicate

that KR may be used as a single parameter for the description of the liquidsolid

interactions.

Figure 8 showns the values of KR for the different polymers. PMMA and PC

(two polar polymers) show both higher KR for water than for DIM. Since PMMA is

more polar than PC it shows a higher KR for water than PC. Conversely, PC shows

a higher KR for DIM than PMMA. PP and PTFE (two non-polar polymers) show

higher KR for DIM than for water. In the case of silicone KR for water is slightlyhigher than that for DIM. As discussed previously (Section 4.1), this may be due

to the siloxane backbone and/or additives in the formulation of the silicone rubber.

Table 3.

Interaction energy, KA (mJ/m2) and adhesion strength, KR (Pa), for water and DIM

Polymer KA (mJ/m2) KA (mJ/m

2) KR (Pa) KR (Pa)

Water DIM Water DIM

PMMA 7.9 2.6 8.7 1.7

PC 3.3 4.3 4.4 2.9

Silicone 4.7 2.7 3.0 2.2

PP 9.0 4.8 3.0 4.2

PTFE 18.0 10.2 10.0 14.3


12/15


Figure 8. Adhesion strength parameter KR for water and DIM on different polymers.

This polarity contribution explains also the observation that KR for DIM on silicone

is lower than KR for DIM on PP.

PTFE has the highest contact angle of all studied polymers. Thus, PTFE is

traditionally classified as a highly hydrophobic polymer. According to this study

PTFE possesses the highest adhesion strength for both liquids. Consequently, based

on the adhesion parameter, PTFE is the least liquidphobic. Silicone has a lower

contact angle than PTFE, as well as much lower adhesion values. Hence, based onthe adhesion strength, silicone is highly liquidphobic.

4.4. Molecular interactions

The relationships of the contact and sliding angles with entropy and enthalpy

have been previously suggested by Murase and co-workers [3, 4]. An alternative

approach is presently proposed based on the intermolecular forces that develop

between the liquid and the solid. Upon deposition of a liquid drop on a solid surface,

interactions are formed between the polymer and the liquid molecules across the

contact area. From the inception of contact until equilibrium is achieved, the drop

adopts its static contact angle. These interactions induce the surface molecules to

rearrange their conformation to create a new equilibrated interface.

As the solid plane is tilted, the gravitational force will cause detachment of the

drop from the surface. As pointed out earlier KR is a measure of the forces needed

to detach a liquid drop from the solid. It can be assumed that KR is composed of

two terms: KB related to the forces needed to break the interfacial bonds between

the solid and the liquid and can be attributed to the chemical nature and the types of

interactions between them (polar, non-polar, hydrogen bond, etc.) and KS related

to the stress induced upon wetting by the formation of a new interfacial molecularorder in the solidliquid interface. Equation (15) expresses these two contributions:

KR = KB +KS. (15)


13/15


Figure 9 describes the interaction between water and silicone and compares it with

water and PTFE. In the case of silicone a bond is formed between the electropositive

hydrogen (in the methyl group) and the electronegative oxygen (in the water

molecule). In the case of PTFE, a bond is formed between the electronegative

fluorine atom and the electropositive hydrogen (in the water molecule). The latter

interaction is stronger than the former one as a result of the higher electronegativity

of the fluorine atom. Figure 10 describes the interaction between DIM and silicone

and compares it with DIM and PTFE. In this case the interaction between the

electropositive hydrogen in the methyl group and the electronegative iodine of the

DIM molecule is weaker than the interaction between the more electronegative

fluorine atom and the more electropositive hydrogen of the DIM molecule. Thus, for

both liquids, KB for PTFE is expected to be higher than that for silicone. Moreover,

because of the flexible siloxane main chain and the pendant methyl groups, siliconeis expected to have some surface mobility, while the high electronegativity of the

(a) (b)

Figure 9. Interaction between (a) water molecule and silicone surface and (b) water molecule and

PTFE surface (continuous line, primary covalent bond; dashed line, secondary bond).

(a) (b)

Figure 10. Interaction between (a) DIM molecule and silicone surface, (b) DIM molecule and PTFE

surface (continuous line, primary covalent bond; dashed line, secondary bond).


14/15


(a) (b)

Figure 11. Interaction between (a) DIM molecule and silicone surface, (b) DIM molecule and PP

surface (continuous line, primary covalent bond; dashed line, secondary bond).

fluorine atoms in PTFE results in a stiff structure [23]. Thus, PTFE will induce a

higher level of stress upon wetting by the liquid. Hence, regardless of the liquid

type, KS for PTFE will be significant higher than that for silicone. Integration of

these effects leads to the conclusion that the interaction strength KR of PTFE will be

significantly higher than that of silicone. This conforms to the experimental findings

and model calculations.

Silicone and PP interact with water and DIM in a similar manner through themethyl groups; thus, KB for both polymers should be similar. However, the carbon

main chain in PP is stiffer than the siloxane main chain in silicone, therefore KS for

PP should be slightly higher than KS for silicone leading to a higher KR for PP with

respect to silicone, consistent with the observations. The interaction between DIM

and silicone compared with DIM and PP is described in Fig. 11.

5. CONCLUSIONS

Experimental results indicate that the contact angle along with the sliding angledescribe the surface energetics of liquidsolid systems. A model is proposed based

on the interfacial forces that develop between a liquid drop and a solid surface. The

model assumes that the interfacial adhesion between the drop and the solid across

the whole contact area, and not only along the perimeter, should be considered

when determining the interface interactions. This approach is applicable especially

for larger drops. The model leads to the definition of an adhesion strength parameter

that can be calculated using measured contact and sliding angles and which can be

used as a single parameter for the description of the liquidsolid interactions. The

resulting model was verified using two liquids, water and di-iodomethane, and fivechemically different, smooth polymer surfaces: two hydrophilic, PC and PMMA,

and three hydrophobic materials, silicone, PP and PTFE. Although PTFE has the

highest contact angle of all studied materials, it also has the highest sliding angle


15/15


leading to the highest adhesion strength with both liquids. Consequently, based on

the adhesion strength, PTFE is less liquidphobic than silicone and PP and even less

liquidphobic than PMMA and PC.

The interaction strength parameter was postulated to be composed of two compo-

nents. The first depends on the interactions between the liquid and the solid surface

atoms, depending on the chemical composition of both the liquid and the solid. The

second depends on the stresses needed to induce the equilibrium conformation in

the liquid molecules. The latter is expected to relate to the surface structure and

flexibility of the polymer molecules. The calculated interaction strength values, ac-

cording to the proposed analysis and model, are found to be in good agreement with

the experimental results.

REFERENCES

1. T. Young, Phil. Trans. Roy. Soc. London, 95 65 (1805).

2. A. W. Adamson, Physical Chemistry of Surfaces, 5th edn. Wiley, New York, NY (1990).

3. H. Murase, K. Nanishi, H. Kogure, T. Fujibayashi, K. Tamura and H. Haruta, J. Appl. Polym.

Sci. 54, 2051 (1994).

4. H. Murase and T. Fujibayashi, Prog. Org. Coatings 31, 97 (1997).

5. A. Von Buzgh and E. Wolfram, Kolloid Z. 149, 125 (1956).

6. E. Wolfram and R. Faust, in: Wetting, Spreading and Adhesion, J. F. Padday (Ed.), pp. 213222.

Academic Press, London (1978).

7. M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto and T. Watanabe, Langmuir 16, 5754(2000).

8. C. G. L. Furmidge, J. Colloid Sci. 17, 309 (1962).

9. E. B. Dussan V. and R. T. P. Chow, J. Fluid Mech. 137, 1 (1983)

10. E. B. Dussan V., J. Fluid Mech. 151, 1 (1985).

11. B. J. Briscoe and K. P. Galvin, Colloids Surfaces 52, 219 (1991).

12. C. W. Extrand and Y. Kumagai, J. Colloid Interface Sci. 170, 515 (1995).

13. A. Carre and M. E. R. Shanahan, J. Adhesion 49, 177 (1995).

14. O. Albenge, C. Lacabanne, J. D. Beguin, A. Konen and C. Evo, Langmuir18, 8929 (2002).

15. P. F. Rios, H. Dodiuk, S. Kenig, S. McCarthy and A. Dotan, J. Adhesion Sci. Technol. 20, 563

(2006).

16. A. J. Kinloch, in: Adhesion and Adhesives, p. 83. Chapman and Hall, New York, NY (1987).17. A. J. Kinloch, in: Adhesion and Adhesives, p. 30. Chapman and Hall, New York, NY (1987).

18. F. Garbassi and M. Morra, in: Polymer Surfaces: From Physics to Technology, pp. 169200.

Wiley, New York, NY (1998).

19. D. K. Owens and R. C. Wendt, J. Appl. Polym. Sci. 13, 1741 (1969).

20. D. H. Kaelble, J. Adhesion 2, 66 (1970).

21. R. A. Malloy, in: Plastic Part Design for Injection Molding. An Introduction, pp. 439440.

Hanser, Munich (1994).

22. L. Gao and T. J. McCarthy, Langmuir22, 6234 (2006).

23. D. Deanin, in: Polymer Structure, Properties and Applications, pp. 434437. Cahners, Boston,

MA (1972).

Date post:	08-Apr-2018
Category:	Documents
Upload:	liliane-pedreiro
View:	218 times
Download:	0 times

bioadesão de polímeros líquidos por ângulo de contato

Documents