fluoropolymers with very low surface energy characteristics

Fluoropolymers with very low surface energy characteristics

Paul Graham, Maureen Stone, Adrian Thorpe, Thomas G. Nevell, John Tsibouklis*

School of Pharmacy and Biomedical Sciences, University of Portsmouth, St. Michael's Building, White Swan Road, Portsmouth, PO1 2DT, UK

Abstract

The synthesis and characterisation of three homologous series of polymeric materials is reported and the surface energies of ®lm

structures prepared from these are evaluated. Dependent upon the molecular design features of these materials, the initial surface energies

of ®lm structures were within the range 6±35 mJ mÿ2 but in many cases these were found to change over time due to the penetration of the

surfaces by the liquids utilised for the contact angle measurements. # 2000 Elsevier Science S.A. All rights reserved.

Keywords: Poly(methylpropenoxy¯uoroalkylsiloxane)s; Poly(per¯uoroalkylacrylate)s; Surface energy; Contact angle goniometry

1. Introduction

The commercial implications of low-surface-energy poly-

meric materials with good ®lm-forming characteristics

become apparent when the lack of universally applicable

and environmentally friendly protection against biological

fouling is considered [1,2].

It has been suggested that low-surface-energy polymers

must possess a ¯exible linear backbone onto which side-

chains with low intermolecular interactions are attached via

suitable linking groups [3]. In this work we consider two

classes of compounds that conform to these molecular

design requirements, namely: the poly(methylpropenoxy-

¯uoroalkylsiloxane)s and the poly(per¯uoroalkylacrylate)s.

The non-¯uorinated counterparts of the former class of

materials are also considered for comparison.

2. Experimental

2.1. Materials

Divinyltetramethyldisiloxane platinum catalyst was

obtained from Fluorochem and used as received. Poly-

(methylhydrosiloxane) (Mn�2270), acrylic acid, HPLC

grade tetrahydrofuran and all alcohols were obtained from

Aldrich and used as supplied. Toluene, used in the hydro-

silylation reaction, was distilled (nitrogen atmosphere) from

benzophenone and sodium metal.

2.2. Instrumental

Solution 1H-NMR and 13C-NMR spectra were recorded

using a Jeol GSX spectrometer operating at 270.05 MHz and

67.80 MHz, respectively, using CDCl3 as solvent and TMS

as reference. IR spectra (neat; NaCl plates) were obtained

using a Perkin Elmer Paragon 1000 FT-IR spectropho-

tometer. Electron-impact mass spectra were recorded on a

Jeol JMS-DX303/DA5000 double-focusing mass spectro-

meter with Matsuda optics. The purity of all the synthesised

low-molecular-mass compounds was checked by thin layer

chromatography, TLC, (silica, 1:1 v/v n-hexane:diethyl

ether) and temperature-ramped GC using a Hewlett Packard

5890 J gas chromatograph equipped with a 25 m DB225

quartz capillary column (internal diameter 0.22 mm, ®lm

thickness 25 mm); a Jeol JMS-DX303/DA5000 mass spec-

trometric detector was employed. Experiments were per-

formed under helium (12.4 mm3 minÿ1) using an ionising

voltage of 70 eV with a current of 100 mA (temperature

pro®le: 288C, 0.5 min; 10 K minÿ1 to 80 8C, 15 min; 10

K minÿ1 to 2408C, 5 min). Molecular weight distribution

studies of the polymers were performed by gel permeation

chromatography, GPC, in tetrahydrofuran using a Waters

instrument equipped with a PHENOGEL 5 linear column

(30 cm) and a differential refractometric detector. Measure-

ments were carried out at room temperature using a solvent

¯ow rate of 1 cm3 minÿ1. A GPC calibration graph was

obtained using a series of polystyrene standards. Studies of

thermal properties were carried out using a Perkin Elmer

DSC7 differential scanning calorimeter (DSC), a Stanton

Redcroft 671 differential thermal analyser (DTA) and a

Stanton Redcroft TG-750 thermogravimetric analyser

Journal of Fluorine Chemistry 104 (2000) 29±36

* Corresponding author. Fax: �44-1705843565.

0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.

PII: S 0 0 2 2 - 1 1 3 9 ( 0 0 ) 0 0 2 2 4 - 4

(TGA). Atomic force microscopy (AFM) studies were

performed in air under ambient conditions, using a Dis-

coverer TopoMetrix TMX2000 scanning probe microscope

(SPM) which was mounted on a custom-built mass/spring

anti-vibration rig with a lateral natural frequency of 0.40 Hz

and a vertical natural frequency of 0.52 Hz.

2.3. Synthesis

All classes of materials are readily accessible; the

poly(per¯uoroacrylate)s are prepared by the bulk polymer-

isation of acrylate esters [4] whereas the synthesis of the

poly(methyl-propenoxy¯uoroalkylsiloxane)s [5] and the

poly(methylpropenoxyalkylsiloxane)s [6] involves a hydro-

silylation reaction.

2.3.1. Poly(perfluoroalkylacrylate)s, PFA

The readily obtainable monomers were polymerised, in

the bulk (no solvent; 1% w/w AIBN; 608C), to give the

corresponding poly(per¯uoroalkylacrylate)s, Fig. 1. The

polymers were puri®ed by repeated washings in methanol

and acetone and dried under reduced pressure.

2.3.2. Poly(methylpropenoxyfluoroalkylsiloxane)s, PFE

The synthesis of the poly(methylpropenoxy¯uoroalkyl-

siloxane)s, PFE, is outlined in Fig. 2.

Per¯uoroalkyl allyl ethers (CF3±(CF2)n±(CH2)2±O±CH2±

CH=CH2) were prepared from the corresponding alcohols

and allyl bromide using phase transfer catalysis [7].

For the grafting reaction [8,9], this derivative (1.0 g;

10 mol% excess against the number of Si±H groups in

the poly(methylhydrosiloxane)) was dissolved in dry,

freshly distilled toluene (100 cm3) and the appropriate

amount of poly(methylhydrosiloxane) was added. The

reaction mixture was heated to re¯ux (1108C) under

nitrogen and the divinyltetramethyldisiloxane platinum

catalyst (3% w/v solution in xylene; 100 mg equivalent

of platinum) was injected using a syringe. The reaction

mixture was re¯uxed (nitrogen atmosphere) for a further

24 h, after which time IR analysis showed that the hydro-

silylation reaction was complete. After removal of the

toluene, the polymers were puri®ed by several washes with

methanol.

2.3.3. Poly(methylpropenoxyalkylsiloxane)s, PES

The synthesis of the modi®ed polysiloxanes is outlined in

Fig. 3.

Sodium metal (1.15 g, 0.05 mol) was added to a solution

of the appropriate n-alcohol (0.06 mol) in anhydrous tetra-

hydrofuran (100 cm3) contained in a clean, dry round

bottom ¯ask (250 cm3) equipped with a double surface

condenser. After the sodium had completely dissolved

(ca. 24 h), allyl bromide (6.05 g, 0.05 mol) was added

dropwise and the mixture was allowed to re¯ux for a further

24 h. The cooled mixture was ®ltered (to remove the sodium

bromide) and the solvent was removed by rotary evapora-

tion to yield a clear yellow liquid which was further puri®ed

by column chromatography (silica, 1:1 v/v n-hexane :

diethyl ether). In each case, the pure product was obtained

as a colourless liquid. The propenoxyalkane derivative

(1.0 g; 10 mol% excess against the number of Si±H groups

in the poly(methylhydrosiloxane)) was dissolved in dry,

freshly distilled toluene (100 cm3) and the appropriate

amount of poly(methylhydrosiloxane) was added. The reac-

tion mixture was heated to re¯ux (1108C) under nitrogen

and the divinyltetramethyldisiloxane platinum catalyst

(3% w/v solution in xylene; 100 mg equivalent of platinum)

was injected with a syringe. The reaction mixture was

re¯uxed (N2 atmosphere) for a further 24 h after which

time IR analysis showed that the hydrosilylation reaction

Fig. 1. Polymer synthesis: n�3 (PFA3); n�5 (PFA5); n�7 (PFA7); n�9 (PFA9).

Fig. 2. Polymer synthesis: n�3 (PFE3); n�5 (PFE5); n�7 (PFE7); n�9

(PFE9).

30 P. Graham et al. / Journal of Fluorine Chemistry 104 (2000) 29±36

was complete. The polymers were separated and puri®ed by

several reprecipitations from tetrahydrofuran solution into

methanol.

2.4. Film formation

Films of the synthesised polymers were deposited on

glass and/or poly(methylmethacrylate) supporting sub-

strates (10 mm�10 mm�1 mm). The acrylates were depos-

ited from the melt or from CF2ClCFCl2 (0.1% w/w) solution

(dipping speed: 10 mm sÿ1) without further treatment. The,

liquid, silicone samples formed into glassy structures after

thermal crosslinking at 1058C for 16 h; it was found that a

slow cooling rate (<108C hÿ1) was important in determining

®lm quality.

2.5. Goniometry

The surface free energies of polymer samples were

determined by contact-angle goniometry using a manual

Kernco instrument or a Kruss G10 goniometer interfaced to

image capture software; both instruments were ®tted with an

enclosed thermostated cell. Relaxed ®lm structures that had

been annealed at 258C for at least 3 weeks were utilised

for the experiments. For both advancing (yA) and receding

(yR) contact angle experiments, measurements of droplets

(2±10 ml) were recorded at 25�18C using doubly distilled

water (surface tension gL�73.4 mN mÿ1 at 18.758C, litera-

ture value�73.05 mN mÿ1 at 188C [10,11]), diiodomethane

(>99%; gL�48.7 mN mÿ1 at 18.88C, literature value�50.76 mN mÿ1 at 208C [10,11]) and 1,1-ethanediol (ethy-

lene glycol, >99%; gL�47.7 mN mÿ1 at 18.88C, literature

value�48.40 mN mÿ1 at 208C [10,11]).

3. Results and discussion

3.1. Materials

The chemistry of the polyacrylates (Fig. 1) is thoroughly

documented in standard textbooks whereas that of poly(-

methylpropenoxy¯uoroalkylsiloxane)s (Fig. 2) has been

described elsewhere [5].

The preparation of the propenoxyalkanes by phase trans-

fer catalysis resulted in yields in the range 84±90% whereas

a synthetic procedure utilising the SN2 reaction of allyl

bromide with the corresponding alcohol (Williamson Synth-

esis) gave signi®cantly lower yields (61±37%). The phase

transfer reaction procedure proved to be a less complicated

route in that: (i) no potentially hazardous metallic sodium is

required, (ii) the long period of time necessary to form the

alkoxide is eliminated and (iii) in contrast to the Williamson

synthesis, the method may be employed for the preparation

of ¯uorinated ethers.

The infrared spectra of all propenoxyalkanes were char-

acterised by absorptions at 1647 cmÿ1, ethylenic n(C=C),

and at 1105 cmÿ1, n(C±O); 1H-NMR and 13C-NMR spectra

were consistent with the expected structures [6]. The purity

of all the synthesised allylic ethers was checked by TLC and

further con®rmed by GC-MS; a single component was

identi®ed in all cases. The molecular ion, [CH3(CH2)n±

O±CH2±CH=CH2]�, and characteristic fragments due to the

long hydrocarbon chain as well as those due to the allylic

system (m/z 41) were observed in all cases.

The poly(methylpropenoxyalkylsiloxane)s were synthe-

sised by the reaction of the above propenoxyalkylsiloxanes

with poly(methylhydrosiloxane) as shown in Fig. 3. The

utilisation of an excess amount of the propenoxyalkane

derivative allowed the reaction to go to completion. Any

excess propenoxyalkane was removed by repeated precipi-

tation from tetrahydrofuran solution with methanol.

The preparation of the polymers by the grafting reaction

with Karstedt catalyst requires ultra-dry conditions as

moisture causes deactivation; sodium dried toluene and a

nitrogen atmosphere are employed. An excess amount of

the allylic ether ensures the reaction goes to completion.

The reaction can be followed spectroscopically: the strong

Si±H absorption at �2165 cmÿ1 and the ±C=C stretch at

�1650 cmÿ1 are seen to progressively decrease in intensity

and eventually disappear as the reaction moves to comple-

tion. In addition, the IR spectra of the poly(methylhydro-

siloxane)s are characterised by strong absorptions in the

regions 1000±1200 cmÿ1 [n(Si±O±Si)] and 800 cmÿ1±

900 cmÿ1 [n(Si±C)]; the Si±CH3 deformation mode is at

1258 cmÿ1 (Fig. 4).

Fig. 3. Polymer synthesis: n�4 (PES4); n�6 (PES6); n�8 (PES8); n�10 (PES10); n�12 (PES12); n�14 (PES14); n�16 (PES16).

P. Graham et al. / Journal of Fluorine Chemistry 104 (2000) 29±36 31

All polymers (except for n�14 and n�16) were obtained

as viscous liquids. It was found that polymers substituted

with lower propenoxyalkanes (up to n�12) had a tendency

to trap organic solvents. Attempts to remove these by rotary

evaporation or distillation resulted in the materials cross-

linking to form clear, glassy solids. The n�14 and n�16 poly-

mers were obtained as low-melting white solids; recoveries

were 96 and 97%, respectively. In all cases, 1H-NMR spectro-

scopy was used to follow the disappearance of the Si±H bond.

GPC results (Fig. 5) demonstrated an increase in average

molar mass from 2270 to ca. 11,800 for the polymer n�16.

In accordance with expectation, this corresponds with the

addition of about 30 ether units to each poly(methylhydro-

siloxane) chain. Analogous GPC analysis results were

obtained with all other synthesised polymers.

It was found that all polymer samples could be cross-

linked on thermal annealing at 1108C. In order to examine

the effect of partial hydrosilylation on the cross-linking

reaction, a series of poly(methylpropenoxyalkylsiloxane)s

was prepared using equimolar quantities of reactants. Even

after a 24 h reaction time at 1108C, IR [n(Si±H), 2166 cmÿ1]

and 1H-NMR [d(Si±H), 4.7 ppm] spectroscopies con®rmed

the presence of unreacted Si±H moieties (1±2 sites per

average polymer chain). Film structures prepared from

partially hydrosilylated materials were found to be parti-

cularly susceptible to thermal cross-linking.

Differential scanning calorimetry showed the presence of

a melting endotherm (448C) for the higher-melting polymer

(n�16), suggesting a degree of crystallinity within the

sample; thermally cross-linked samples of the same poly-

mer did not show this endotherm. In accordance with the

NMR experiments, thermogravimetric analysis (Fig. 6),

combined with DTA (Fig. 7) and DSC experiments, con-

®rmed the presence of trapped solvent within all liquid

polymer samples. A common feature characterising the

dynamic DTA and DSC thermograms (®rst heating) of

Fig. 4. FT-IR spectrum of poly(methylpropenoxyoctylsiloxane).

Fig. 5. GPC analysis results presented as log (molecular weight) versus retention time; polystyrene provides the calibration.


the solid poly(methylpropenoxyalkylsiloxane)s (n�14 and

n�16) is the presence of a two-step exotherm. The onset of

the ®rst exotherm, for solid samples heated at 10 K minÿ1, is

at ca. 1108C. Since this is not observed in subsequent

heating cycles, it is assumed to be associated with the

cross-linking process characterising these materials.

Liquid samples exhibited a similar thermal behaviour

which, was nonetheless, complicated by the presence of an

evaporation exotherm, the position and intensity of which

were found to be a function of the nitrogen ¯ow rate as well

as the heating rate. The onset of degradation, for all poly-

mers, is at ca. 2208C.

3.2. Surface energies

Surface energies were evaluated using the surface-ten-

sion-component theory [12±15].

3.2.1. Poly(1H,1H,2H,2H-perfluoroalkyl acrylate)s

The surface energy of the poly(1H,1H,2H,2H-per¯uorodo-

decyl acrylate) ®lm structures has been evaluated as 6 mJ mÿ2

(Table 1); the material combines good hydrophobicity with

unusual incompatibility with the organic liquids used.

The large difference between the initial surface free

energies of poly(1H,1H,2H,2H-per¯uorooctyl acrylate)

(7.0�0.4 mJ mÿ2) and poly(octyl acrylate) (33.1�0.9

mJ mÿ2) demonstrate the signi®cance of ¯uorine substitution

in the molecular design requirements for such materials.

For all liquids employed, the advancing contact angles

observed for the two higher esters, poly(1H,1H,2H,2H-

per¯uorodecylacrylate) and poly(1H,1H,2H,2H-per¯uoro-

dodecyl-acrylate), were time-independent (monitored over

12 months) whereas those for the lower analogues and for

the non-¯uorinated material were found to decrease with

time. The extremely low surface energies associated with

the higher-¯uorocarbon-based materials indicate very little

af®nity for the liquids used; the time-independent behaviour

con®rms that the liquids do not become absorbed.

The length of the per¯uorocarbon chain is important in

determining the time-dependence of the wetting behaviour;

the surface free energy of poly(1H,1H,2H,2H-per¯uorooc-

tyl acrylate) increased, over time, at a much lower rate than

that of poly(1H,1H,2H,2H-per¯uorohexyl acrylate). From

observations of the dimensions of liquid drops, it appeared

that penetration of the surface by the liquids was responsible

for the observed reduction in contact angles; with the polar

protic liquids, hydrogen bonding with oxygen atoms of the

ester-linking group may be responsible for this behaviour

whereas with diiodomethane, interactions with the hydro-

carbon backbone may provide the driving force behind the

observed phenomena.The n-hexadecane contact angle is

Fig. 6. Thermogram of poly(methylpropenoxydodecylsiloxane).

Fig. 7. DTA thermogram of poly(methylpropenoxyhexadecylsiloxane).


generally accepted as the index of oleophobicity [1]. For

poly(1H,1H,2H,2H-per¯uorododecyl acrylate) this contact

angle was found to be 838. For comparison, it is worth

noting that ¯uoropolymers have been identi®ed [16,17]

with n-hexadecane values of 858, but it is the combined

effect of the hydrophobic (1258) and oleophobic nature

of poly(1H,1H,2H,2H-per¯uorododecyl acrylate) that is

responsible for the unusually low surface-energy value

associated with this material; surface energies of the same

order of magnitude (6±8 mJ mÿ2) have only been reported

for, thermodynamically unstable, Langmuir±Blodgett ®lm

structures of per¯uorocarboxylic acids [18±21].

Receding contact angles were also measured for the

two materials that exhibited time-independent behaviour.

To this end, a slightly larger drop (3±6 ml) was placed on

the surface of the sample and allowed to stand for 30 min.

Using a microcapillary pipette, a portion of liquid (1±2 ml)

was then removed, care being taken to avoid disturbing

the edge of the drop; the contact angle was measured

after ca. 30 s. Following small changes which occurred

rapidly after the drop size was reduced, receding contact

angles, yR, remained almost constant and were lower than

advancing angles, yA. For both samples, hysteresis

(H�yAÿyR, Table 1) was greatest for ethylene glycol.

The small hysteresis effects shown with water imply that

the surfaces of poly(1H,1H,2H,2H-per¯uorodecyl acrylate)

and poly(1H,1H,2H,2H-per¯uorododecyl acrylate) were

mechanically smooth and homogeneous with respect to

van der Waals' and/or hydrogen bonded interactions;

surface roughness pro®ling using AFM con®rmed the

smoothness of the surface (Ra 0.49 nm) [22,23].

3.2.2. Poly(methylpropenoxyfluoroalkylsiloxane)s

Depending on the length of the pendent ¯uorocarbon

chain, the calculated initial surface energies of the poly-

(methylpropenoxy¯uoroalkylsiloxane) ®lm structures, as

determined from advancing contact angle measurements,

range between 17 and 9 mJ mÿ2 (Table 2).

Table 1

Contact angles and hysteresisa

Sample t (minÿ1) Contact angle, (y/8); (H/8) Surface energy (mJ mÿ2)

H2O DIM EG gLWS gS

� gSÿ gS

PFDDA 0 125 (4) 112 (16) 120 (25) 5.0 0.1 1.6 5.6

30 123 (4) 112 (18) 119 (26) 5.0 0.1 2.8 6.1

PFDA 0 117 (8) 108 (12) 108 (17) 6.2 0.1 2.2 6.9

30 106 (2) 107 (15) 103 (13) 6.3 0.0 7.3 7.3

PFOA 0 114 105 108 6.9 0.0 3.7 7.0

30 89 100 89 8.7 0.2 18.0 12.1

PFHA 0 113 96 110 10.2 0.4 4.3 12.9

30 86 69 89 23.3 1.2 18.9 32.6

POA 0 96 52 70 33.0 0.0 1.5 33.1

30 54 42 63 38.6 1.3 46.5 54.2

a (H) for water, diiodomethane (DIM) and ethylene glycol (EG) on poly(1H,1H,2H,2H-perfluorododecyl acrylate) PFDDA, poly(1H,1H,2H,2H-

perfluorodecyl acrylate) PFDA, poly(1H,1H,2H,2H-perfluorooctyl acrylate) PFOA, poly(1H,1H,2H,2H-perfluorohexylacrylate) PFHA, poly(octyl acrylate)

POA and surface energies of these materials. Each contact angle value is the mean of six drops on two independently prepared polymer samples; standard

deviations were in the range 0.6±1.1.

Table 2

Contact angles for watera

Sample Time Contact angle, (yA/8) Surface energy (mJ mÿ2)

n (Ra nmÿ1)(t minÿ1)

H2O DIM EG gLWS gS

� gSÿ gS

PFE3 0.5 0 105.0 89.5 79.2 12.9 0.1 4.6 14.2

30 84.3 72.9 67.3 21.3 6.4 1.3 26.9

PFE5 1.0 0 102.4 86.9 82.6 14.1 0.8 1.7 16.4

30 89.6 82.2 80.8 16.4 9.9 0.1 17.8

PFE7 2.3 0 106.7 95.6 91.3 10.4 1.3 1.1 12.6

30 99.5 89.8 92.0 12.8 7.2 0.0 13.2

PFE9 3.1 0 109.3 94.8 94.3 10.7 1.3 0.5 12.2

30 100 94.0 94.5 11.0 7.8 0.0 11.8

PFE9u 133.3 0 117.1 102.2 102.1 7.8 0.5 0.5 8.8

30 109.5 102.7 100.9 7.7 3.1 0.2 9.4

a Diiodomethane (DIM) and ethylene glycol (EG) on poly(methylpropenoxyfluoroalkylsiloxane)s and surface energies of these materials. Each contact

angle value is the mean of six drops on two independently prepared polymer samples (standard deviations were in the range 0.6±1.9). PFE9u has not been

subjected to thermal treatment. (Ra�surface roughness as determined by AFM measurements).


All materials were found to combine good hydrophobi-

city with unusual incompatibility with the organic liquids

used. However, from observations of the dimensions of

water drops, it appeared that penetration of the surface

by this liquid was responsible for the observed reduction

in contact angles. This may be brought about by hydrogen

bonding with oxygen atoms of the ether-linking group.

Analogous behaviour was observed with diiodomethane

for all polymers except PFE9. The lower surface energy

associated with the non-crosslinked polymer (PFE9u), rela-

tive to its crosslinked counterpart (PFE9), can be readily

explained in terms of the `¯exible-backbone' molecular

design requirement [1].

Since on surfaces of PFE9 contact angles for ethylene

glycol and diiodomethane remained constant for 30 min

(Table 2), receding contact angle experiments were per-

formed in order to assess hysteresis effects. To this end, a

diiodomethane or ethylene glycol drop (8±10 ml) was placed

on the surface of the sample. The liquid was then removed in

small increments (0.5 ml) until the drop edge spontaneously

contracted to a new stationary position at which the receding

contact angle was measured. In the case of PFE9 ®lm-

structures. Hysteresis was of the order of 88 for ethylene

glycol and 128 for diiodomethane, indicating considerable

surface roughness. Surface roughness pro®ling using AFM

con®rmed these ®ndings (Ra 3.24 nm). The surface rough-

ness of the PFE ®lm-structures was found to be a function of

the length of the propenoxy¯uoroalkyl side-chain. Thus,

®lm structures prepared from the short side-chain analogue,

PFE3, exhibited hysteresis values of the order of 38; AFM

imaging further con®rmed the relative smoothness of this

surface (Ra 0.49 nm). The small hysteresis effects associated

with this material imply that the surface of PFE3 was

mechanically smooth and homogeneous with respect to

van der Waals' and/or hydrogen bonded interactions

[16,17].

3.2.3. Poly(methylpropenoxyalkylsiloxane)s

Depending on the length of the pendent hydrocarbon

chain, the calculated initial surface energies of the poly(-

methylpropenoxyalkylsiloxane) ®lm structures, as deter-

mined from advancing contact angle measurements,

range between 35 and 21 mJ mÿ2 (Table 3). It must be

noted that, because of the observed variation in surface

roughness between samples and the absence of receding

contact angle data, the reported values do not represent the

true surface energy of the materials and are presented for

reasons of comparison only. In particular, the dramatic

decrease in the calculated initial surface energy of the

n�10 polymer as compared to the n�8 analogue

(Table 3) may be a re¯ection of the large differences in

average surface roughness associated with ®lm structures

prepared using these materials whereas the corresponding

values for the n�14 and n�16 polymers are those of very

rough surfaces and, hence, of little value.

From observations of the dimensions of drops of the three

liquids under consideration, it appeared that penetration of

the surface by the liquids was responsible for the observed

reduction in contact angles and consequent time dependence

of the surface energy [14]. In the case of water, hydrogen

bonding with oxygen atoms of the ether-linking group may

provide the driving force behind the observed phenomenon

whereas with diiodomethane and ethylene glycol, enhanced

van der Waals interactions due to the hydrocarbon nature of

the side-chains may account for this effect. This hypothesis

is supported by the observation that the ethylene glycol

contact angle associated with the shortest hydrocarbon

analogue (n�4) exhibited a time-independent behaviour

Table 3

Surface roughness dataa

Sample Time Contact angle (yA/8) Surface energy (mJ mÿ2)

(n) (Ra nmÿ1) (t minÿ1) H2O DIM EG gLWS gS

� gSÿ gS

4 1.9 0 79.6 55.2 70.9 31.3 0.3 14.9 35.4

30 61.6 51.0 70.8 33.7 1.5 42.1 49.5

6 3.5 0 91.0 62.1 82.0 27.3 0.5 8.7 31.6

30 77.3 57.6 81.2 29.9 1.6 25.1 42.7

8 8.2 0 72.2 51.6 61.2 33.3 0.1 18.5 35.2

30 63.1 46.4 54.2 36.2 0.1 26.2 38.4

10 5.6 0 80.5 66.8 73.8 24.6 0.1 16.4 26.6

30 72.8 63.2 66.1 26.7 0.0 21.6 27.5

12 0.9 0 85.2 70.0 75.4 22.9 0.0 12.0 23.4

30 73.4 62.4 67.7 27.2 0.0 21.7 29.2

14 115.5 0 84.7 72.1 75.6 21.7 0.0 12.8 21.8

30 70.0 63.6 70.3 26.5 0.2 28.7 31.2

16 99.9 0 106.9 73.4 81.8 21.0 0.1 0.3 21.3

30 99.7 64.6 82.8 25.9 0.2 2.9 27.3

a (Ra value; mean of at least six samples) as determined by AFM and contact angles (mean of six drops on two independently prepared samples) for water,

diiodomethane (DIM) and ethylene glycol (EG) on poly(methylpropenoxyalkylsiloxane)s (n�4, 6, 8, 10, 12, 14, 16). The surface energies of these materials

are also given. Standard deviations were in the range 0.4±3.4.


and that for the next member of the series (n�6) was only

weakly time-dependent.

Since contact angles on the surfaces of ®lm structures

from the polymers under consideration were found to

change with time, receding contact angle experiments were

not performed. However, surface roughness pro®ling using

AFM (performed on at least six, independently prepared,

®lm samples) showed that all ®lms were relatively rough

(Table 3). The n�10 polymer was found to produce parti-

cularly rough surfaces (Ra 8.15 nm) with the greatest varia-

tion between samples whereas the n�12 analogue

reproducibly yielded the smoothest ®lm structures (Ra

0.90 nm). Structures derived from the two solid polymers,

n�14 and n�16, were very rough (Ra 115.5 and 99.9 nm,

respectively) rendering the reported surface energy values

essentially meaningless.

4. Conclusions

Three homologous series of polymers have been synthe-

sised and characterised and the surface energies of ®lm

structures prepared from these have been evaluated. Depen-

dent upon the type of backbone as well as nature and length

of the pendant hydrocarbon chain, the initial surface ener-

gies were within the range 6±35 mJ mÿ2 but in many cases

these were found to change over time due to the penetration

of the surfaces by the liquids utilised for the contact angle

measurements.

Acknowledgements

The ®nancial support of this work by EPSRC/MTD Ltd

(research studentships to MS and AT) and NERC (research

studentship to PG) is gratefully acknowledged.

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fluoropolymers with very low surface energy characteristics

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