Wetting transition in a binary polymer blend

Abstract

We have used composition depth profiling, based on nuclear-reaction analysis and secondary ion mass spectroscopy, to study segregation at the free surface of a partly miscible binary mixture consisting of random olefinic copolymers. The equilibrium surface excess data, analysed within a mean-field Cahn approach, point to a wetting transition. The surface phase diagram obtained was confirmed by the observed dynamics of the segregation from a coexistence composition: the monotonic and halted growth of the surface layer was observed at temperatures above and below the predicted wetting point, respectively.

Full text

Europhys. Lett.,50 (1), pp. 35–40 (2000)
EUROPHYSICS LETTERS 1 April 2000
Wetting transition in a binary polymer blend
J. Rysz 1, A. Bud kowski 1(∗),A.Bernasik
2,J.Klein
3,K.Kowalski
2,
J. Jedli´
nski2and L. J. Fetters4
1Smoluchowski Institute of Physics, Jagellonian University
Reymonta 4, 30-059 Krak´ow, Poland
2Surface Spectroscopy Laboratory, University of Mining and Metallurgy and
JCCASR Jagellonian University - Reymonta 23, 30-059 Krak´ow, Poland
3Department of Materials and Interfaces, Weizmann Institute of Science
76100 Rehovot, Israel
4Exxon Research and Engineering Company - Annandale, NJ 08801, USA
(received 15 November 1999; accepted 18 January 2000)
PAC S. 36.20.-r – Macromolecules and polymer molecules.
PAC S. 68.10.-m – Fluid surfaces and fluid-fluid interfaces.
PAC S. 68.45.Gd – Wetting.
Abstract. – We have used composition depth profiling, based on nuclear-reaction analysis
and secondary ion mass spectroscopy, to study segregation at the free surface of a partly
miscible binary mixture consisting of random olefinic copolymers. The equilibrium surface
excess data, analysed within a mean-field Cahn approach, point to a wetting transition. The
surface phase diagram obtained was confirmed by the observed dynamics of the segregation
from a coexistence composition: the monotonic and halted growth of the surface layer was
observed at temperatures above and below the predicted wetting point, respectively.
Wetting phenomena are classified primarily [1] in terms of the contact angle Θ, describing
the geometrical arrangement of two coexisting phases φ1and φ2at the surface. An alternative
approach (due to Cahn [2]) is provided by the picture of surface segregation described by
the profile φ(z) of blend composition φvs. distance zfrom the surface [2–5] (fig. 1a)): A
surface composition φs<φ
2decaying smoothly to its bulk value φ1characterises partial
wetting. In the case of complete wetting a macroscopically thick layer of the second phase φ2
(say with thickness larger than the double width of the φ1/φ2interface 2w) resides at the
surface and excludes the bulk phase φ1from the surface. In a generic situation, complete
wetting occurs close to the critical temperature TC(critical-point wetting) and the transition
to partial wetting is observed [6,7] at TWfor a larger |TC−T|value as predicted by the Cahn
model assuming short-ranged, temperature-independent surface potential fs[2,3, 5]. Different
scenarios, such as reversal wetting transition [8] or the sequence of two transitions [3, 9],
were revealed in recent experiments. The discontinuous (first-order) [6, 9] and continuous
(second-order or critical) [7–9] (generic) wetting transitions have been reported for small
(∗) E-mail: ufbudkow@cyf-kr.edu.pl
c
EDP Sciences

36 EUROPHYSICS LETTERS
molecule systems, where observations could be convoluted by the presence of gravitational and
convective fields. These effects are negligible for thin films composed of polymer blends [10,11].
In addition, macromolecular length scales are large enough to facilitate the determination of
the composition profiles φ(z) with modern profiling techniques [12, 13]: the complete wetting
behaviour [10, 11, 14] and the reversal wetting transition [8] have been observed for various
polymer mixtures, mostly in spinodal demixing experiments [8,14]. The wetting transition in a
polymer blend, despite its relevance to modern technological applications [15], huge theoretical
attention [5] and very intensive research [10,11], so far eluded experimental observation.
In this letter we present results of two types of studies on the surface segregation, free
of the spinodal decomposition effects, indicating the wetting transition at the free surface of
a model binary polymer blend. First, the equilibriu m properties of the segregation from the
one-phase region of phase diagram are analysed within the Cahn approach [2], pointing to the
transition [16]. This is confirmed by experiments focused on the dynamics of the segregation
Fig. 1 – Segregation and wetting characteristics of the d75/h66 mixture. Different symbols (data
points) and line types (calculated from a mean-field model [2, 5]) correspond to various temperatures:
◦T=59◦C; ♦and — T=64◦C; •T=79◦C; and---T=92◦C. a) Composition-depth profiles
φ(z) corresponding to the free surface (z= 0) partially (T=64
◦C, w=30.1 nm) and completely
(T=92
◦C, w=59.8 nm) wetted from the blend at coexistence. b) Temperature variation of
coexistence compositions φ1and φ2denoted by binodal [17] (dotted line). Bulk compositions φbulk
are marked for which the segregation from the one-phase region and from the coexistence composition
was studied. Solid bars represent the evaluated uncertainty of TC,TW,φ1and φ2. c) Variation of
the equilibrium surface excess z∗as a function of φbulk/φ1. The inset shows a typical NRA profile
with z∗marked by the shaded area. d) The Cahn construction [2] predicting TW=67±5◦C: The
trajectories −2κ(dφ/dz)vs. φ, marked by thick lines, correspond to the profiles of fig. 1a). The
“bare” surface energy derivative (−dfs/dφ), derived from the z∗data (panels c) and b)), determines
the starting points (at the surface composition φs) of the trajectories.

J. Rysz et al.:Wetting transition in a binary polymer blend 37
from the coexistence composition.
We used the mixture composed of two random poly(ethylene-ethylethylene) copolymers of
mean microstructure (C4H8)1−x(C2H3(C2H5))x. Such blends, where the two components have
different ethylethylene fractions x1and x2, create an attractive model system, as bulk [17–19]
and surface [10, 11, 18, 20–22] interactions may be tailored by a suitable choice of x-values.
The molecular characteristics of the pair used in this study, d75 (x=0.75), which is partially
deuterated, and h66 (x=0.66), are as follows: for d75: degree of polymerisation N=
1625; statistical segment length a=0.64 nm; degree of deuteration = 0.40; glass transition
temperature Tg=−46 ◦C; for h66: N= 2030; a=0.68 nm; Tg=−54 ◦C. Both polymers
had polydispersity index <1.08. The d75/h66 mixture displays a phase equilibrium with
TC= 101 ±4◦C and the binodal (see fig. 1b)) described within the Flory-Huggins model by
the interaction parameter χ=(0.371/T −2.7×10−5)(1+0.212φ), dependent on the d75
volume fraction φ[17].
The segregation equilibrium studies were performed for monolayer samples of various over-
all d75 concentration (and thickness Dca. 500 nm), prepared by spin coating from a toluene
solution onto polished silicon wafers. The segregation dynamics observations were made for
bilayers of a pure h66 film (D= 220 ±50 nm) on top of a pure d75 layer (D= 170–800 nm):
the h66 film was spin coated onto freshly cleaved mica and then float-mounted onto the d75
layer. The samples were annealed in a vacuum oven (≤10−2Torr) at temperatures T(stable
to ±1◦C) for different times t, and stored at T<T
guntil required for the experiments. The
profiles φ(z) of the deuterated d75 blend component normal to the sample surface were deter-
mined either by nuclear reaction analysis (NRA) [12] or by secondary ion mass spectroscopy
(SIMS) [13] with a depth resolution in the range of 9 to 20 nm, as described earlier [12,13].
The segregation from the one-phase region of the phase diagram is described in terms of
the (integrated) surface excess z∗of d75, represented in the inset to fig. 1c) by the shaded
area. z∗was measured from the profiles of the monolayer samples annealed at T=64and
92 ◦Cfort>1 day. The times used were sufficient to reach the equilibrium, due to a large
molecular mobility of the polyolefines [19]. In contrast to the situation at the free surface, no
segregation was observed at the blend interface with the substrate [20]. The equilibrium z∗
values were determined for bulk compositions φbulk marked by diamonds in the phase diagram
of fig. 1b). Figure 1c) shows the corresponding segregation isotherms with z∗plotted as a
function of the normalised bulk composition φbulk/φ1.
To analyse the segregation data of fig. 1c) we follow the standard Cahn procedure [2],
reviewed recently [5] and used previously [18,20–22]. The excess free energy functional F[φ]
is represented as a sum of the bulk and “bare” surface fsfree energies, dependent on φ(z)and
its surface value φs, resp ectively [2, 23]:
F[φ]
kBT=∞
0
dz[∆f(φ)+κ(dφ/dz)2]+fs(φs).(1)
Here κis the φ-dependent coefficient [24] and ∆f(φ) is the energy needed to create a unit
volume with composition φfrom a bulk region with φbulk [24].
To minimise F[φ] we analyse the Cahn construction (fig. 1d)), that is, the φ-dependence
of two quantities: the trajectory −2κ(dφ/dz) (equal to 2(κ∆f)1/2and specified entirely by
bulk parameters [24]) and the derivative (−dfs)/dφ) driving the segregation. While each
equilibrium profile φ(z) [25] is determined by its own trajectory, the starting point of the
latter (at φs) is given by the intersection of both, −2κ(dφ/dz) and (−dfs/dφ), relations. The
reversal procedure [18, 20–22] allows us to determine the (−dfs/dφ) relation, whenever the
profiles φ(z) with the φsvalues, or equivalently the segregation data, are known. Diamonds

38 EUROPHYSICS LETTERS
Fig. 2 – Dynamics of surface enrichment from a coexistence phase φ1:◦T=59
◦C<T
W(w=
28.3 nm); •T=79
◦C>T
W(w= 38.6 nm). SIMS comp osition-depth profiles φ(z) of the surface
layer (of thickness L) growing from the bulk phase φ1(of width d), a) after 12.6 days at 59 ◦C, b) after
73.2 days at 59 ◦C(z∗=19.2±1.5 nm), c) after 3.4 days at 79 ◦C corresponding [19] to 18.8 days at
59 ◦C. An additional film, rich in d75 (at z>200–300nm), acted as a material reservoir. d) Variation
of Lwith the parameter (t/d) reduced to 59 ◦C [11]. Dashed lines are a guide to eye.
in fig. 1d) represent the (−dfs/dφ)vs. φrelation derived from the experimental input of
fig. 1c). An additional point (◦) corresponds to the final stage of the segregation dynamics
detected at 59 ◦C (fig. 2b)), where the equilibrium surface excess is attained for the bulk
composition φ1. A non-linear shape of the (−dfs/dφ) function was observed and examined
previously [18, 20–22] for the other dx1/hx2mixtures. Figure 1d) shows that the (−dfs/dφ)
vs. φrelation is, for the values <4×10−3nm, hardly temperature dependent. This result
enables us to evaluate the wetting point TW=67±5◦C. Typical computed trajectories
(fig. 1d)) and profiles (fig. 1a)) correspond to partial (solid lines) and complete (dashed lines)
wetting at T=64and92
◦C, respectively. The Cahn analysis suggests the critical wetting
transition. This model also predicts a phenomenon prerequisite to the continuous transition
(an enrichment-depletion duality), which was observed recently in the d52/h66 blend [22].
To study the dynamics of the segregation from the coexistence composition we followed
the idea of Steiner et al. [10, 11] and measured the bilayer samples annealed at T=59
and 79 ◦C for times from 5 h to 2.4 months. Such long annealing times (10 times longer
than used previously [10, 11]) were necessary to distinguish the partial from the complete
wetting behaviour. Typical profiles corresponding to two temperatures are shown in figs. 2a)-
b) and c), respectively. They reflect the structure of the sample after the transient initial
stage, characterised by interdiffusion leading to coexisting compositions, is completed. The
profiles φ(z) may be divided into three regions: i) the surface layer of thickness L; ii) the bulk
phase φ1of width d[26]; iii) the d75-rich layer acting as a material reservoir. At the late stage
of the annealing a different shape of the surface layer was observed at both temperatures:

J. Rysz et al.:Wetting transition in a binary polymer blend 39
While the profile characteristic of complete wetting was measured at T=79
◦C (fig. 2c)),
it was never observed at T=59
◦C for equivalent and even much longer annealing times.
Instead the profile typical of partial wetting was monitored persistently (figs. 2a) and b)).
The profiles resembling fig. 2c) were observed previously [10, 11] for the d88/h78 and
d66/h52 mixtures: It was established that the surface layer thickness Lgrows at the expense
of the material reservoir, while the thickness dof the bulk phase φ1remains unchanged. This
process is limited by the diffusion of the dx1blend component across the bulk phase and
therefore Ldepends on the parameter (t/d) rather than time t. The surface layer thickness L,
growing with (t/d), attained macroscopic dimensions (L/(2w)≥1.2 [10]) indicating a complete
wetting regime.
To examine the segregation dynamics data of this study we represent them in fig. 2d) as a
plot of Lvs. the parameter (t/d) reduced to 59 ◦C [11]. The data set corresponding to 79 ◦C(•)
reproduces the monotonic evolution of the surface layer to a macroscopic surface phase φ2(here
with L/(2w)=1.3) reported previously for critical-point wetting [10,11]. On the contrary,
the data points of T=59
◦C(◦) after an initial increase level off (compare figs. 2a) and b))
at the value L=38±2 nm, smaller than the double interfacial width (2w=56.6 nm). This
observation clearly indicates a completed build-up of the surface enriched layer corresponding
to a partial wetting regime.
There are two conclusions. First, we have demonstrated for the first time the wetting
transition for a polymer blend: the dynamics of the segregation from the coexistence com-
position characteristic of complete and partial wetting was observed in the d75/h66 mixture
at T=79and59
◦C, respectively (fig. 2d)). Second, this observation is in accord with the
prediction, TW=67±5◦C, of the Cahn approach based on the surface excess data (fig. 1).
Previously this model has been used to describe other wetting-related phenomena, such as
the enrichment-depletion duality [22] or the extended critical-point wetting regime [21]. The
present results show that for polymer mixtures the Cahn theory can provide reasonable pre-
dictions of the surface phase diagram. Further experimental studies are needed to explain
how this diagram is altered by the long-ranged surface forces [3, 4, 7, 9].
∗∗∗
We thank Profs. K. Binder and U. Steiner for useful discussions. Partial support from
the Reserve for Individual Research of the Rector of Jagellonian University, the German-Israel
Foundation (GIF) and the Ministry of Science and Arts (Israel) is gratefully acknowledged.
REFERENCES
[1] Young T.,Philos. Trans. R. Soc. London,95 (1805) 65.
[2] Cahn J. W.,J. Chem. Phys.,66 (1977) 3667; Schmidt I. and Binder K.,J. Phys. (Paris),
46 (1985) 1631.
[3] de Gennes P.-G.,Rev. Mod. Phys.,57 (1985) 827.
[4] Dietrich S., in Phase Transitions and Critical Phenomena, edited by C. Domb and J. L.
Lebowitz, Vol . II (Academic, New York) 1988, pp. 1-218.
[5] Binder K.,Acta Polym.,46 (1995) 204; Adv. Polym. Sci.,138 (1999) 1 and references therein.
[6] Moldover M. R. and Schmidt J. W.,J. Chem. Phys.,79 (1983) 379; Bonn D., Kellay H.
and Wegdam G. H.,Phys. Rev. Lett.,69 (1992) 1975.
[7] Ragil K. et al.,Phys. Rev. Lett.,77 (1996) 1532.
[8] Genzer J. and Kramer E. J.,Phys. Rev. Lett.,78 (1997) 4946; Europhys. Lett.,44 (1998)
180.

40 EUROPHYSICS LETTERS
[9] Shahidzadeh N. et al.,Phys. Rev. Lett.,80 (1998) 3992.
[10] Steiner U., Klein J., Eiser E., Budkowski A. and Fetters L. J.,Science,258 (1992)
1126.
[11] Steiner U. and Klein J.,Phys. Rev. Lett.,77 (1996) 2526; Mater. Res. Soc. Symp. Proc.,464
(1997) 121.
[12] Kerle T. et al.,Acta Polymer.,48 (1997) 548.
[13] Bernasik A. et al., in ECASIA ’97, edited by I. Olefjord, L. Nyborg and D. Bryggs (John
Wiley & Sons, Chichester) 1997, pp. 775-778; Schwarz S. A. et al.,Mol. Phys.,76 (1992) 937.
[14] Bruder F. and Brenn R.,Phys. Rev. Lett.,69 (1992) 624; Krausch G. et al.,Macromolecules,
26 (1993) 5566; Straub W. et al.,Europhys. Lett.,29 (1995) 353.
[15] Mayes A. M. and Kumar S. K.,MRS Bulletin, January issue (1997) 43; Service R. F.,
Science,278 (1997) 383; B¨
oltau M. et al.,Nature,391 (1998) 877.
[16] Such a conclusion, valid for the short-ranged surface potential fs, might not be true if the long-
range interfacial interactions are also present [11]. To check this conclusion we have performed
the dynamics experiments.
[17] Scheffold F. et al.,J. Chem. Phys.,104 (1996) 8786.
[18] Budkowski A.,Adv. Polym. Sci.,148 (1999) 1; Budkowski A. et al.,J. Polym. Sci. Polym.
Phys. Ed.,36 (1998) 2691.
[19] Losch A. et al.,J. Polym. Sci. Polym. Phys. Ed.,33 (1995) 1821.
[20] Scheffold F. et al.,J. Chem. Phys.,104 (1996) 8795.
[21] Budkowski A., Scheffold F., Klein J. and Fetters L. J.,J. Chem. Phys.,106 (1997) 719.
[22] Budkowski A., Rysz J., Scheffold F. and Klein J.,Europhys. Lett.,43 (1998) 404; Bud-
kowski A. et al.,Vacuum,54 (1999) 273.
[23] Equation (1) corresponds to the long-wavelength limit: R<1, where R=a(N/6)1/2(dφ/dz)max .
R=0.1atT=64◦C for φbulk =φ1,N=Nh66 and a=ah66.
[24] κ= ((1 −φ)a2
d75 +φa2
h66)/(36φ(1 −φ)), ∆f(φ)=f(φ)−f(φbulk )−(φ−φbulk)(∂f /∂φ)φbulk,
f(φ)=φln φ/Nd75 +(1−φ)ln(1−φ)/Nh66 +χφ(1 −φ).
[25] The replacement of the short- [18, 20–22] by long- [11] ranged fsterm in eq. (1) leads to no
experimentally detectable changes in the equilibrium profile φ(z) corresponding to the one-
phase region of the phase diagram; JonesR.A.L.,Phys. Rev. E,47 (1993) 1437; Genzer J.,
Faldi A., Oslanec R. and Composto R. J.,Macromolecules,20 (1996) 5438.
[26] A depletion region of the bulk phase φ1, adjacent to the surface layer, with local concentration
φd≈φ1was also detected [11].