Positron annihilation lifetime spectroscopy was used to study the free volume parameters in polypropylene oxide-based tri-isocyanate terminated prepolymer/polycaprolactone diol bi-soft urethane/urea membranes (PU/PCL) with PCL content from 5 up to 25 wt%. Positron annihilation lifetime spectroscopy measurements carried out in the temperature range 298-324 K mirrored the phase separation of the various soft and hard segments present in the membranes. The size and amount of free volume holes determined by positron annihilation lifetime spectroscopy appear to be correlated with the CO 2 gas permeability through the membranes

A P De Lima

D P Queiroz

M F Ferreira Marques

M Norberta De Pinho

P M Gordo

P Major

Zs Kajcsos

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Vol. 113 (2008) ACTA PHYSICA POLONICA A No. 5
Proceedings of the 37th Polish Seminar on Positron Annihilation, La̧dek-Zdrój 2007
Free-Volume Studies in
Polycaprolactone/Poly(propylene oxide)
Urethane/Urea Membranes
by Positron Lifetime Spectroscopy
M.F. Ferreira Marquesa,b,∗, P.M. Gordob, A.P. de Limab,
D.P. Queirozc, M. Norberta de Pinhoc, P. Majord
and Zs. Kajcsosd
aDepartment of Chemical Engineering, Instituto Superior de Engenharia
3031-199 Coimbra, Portugal
bICEMS, Department of Physics, University of Coimbra
3004-516 Coimbra, Portugal
cDepartment of Chemical Engineering, Instituto Superior Técnico
1049-001 Lisboa, Portugal
dKFKI Research Institute for Particle and Nuclear Physics
P.O.B. 49, 1525 Budapest, Hungary
Positron annihilation lifetime spectroscopy was used to study the
free volume parameters in polypropylene oxide-based tri-isocyanate termi-
nated prepolymer/polycaprolactone diol bi-soft urethane/urea membranes
(PU/PCL) with PCL content from 5 up to 25 wt%. Positron annihilation
lifetime spectroscopy measurements carried out in the temperature range
298–324 K mirrored the phase separation of the various soft and hard seg-
ments present in the membranes. The size and amount of free volume holes
determined by positron annihilation lifetime spectroscopy appear to be cor-
related with the CO2 gas permeability through the membranes.
PACS numbers: 61.41.+e, 78.70.Bj, 82.30.Gg
1. Introduction
Segmented polyurethanes are a class of synthetic polymers widely used in
the medical field, due to their high biocompatibility especially as biomaterials in
contact with blood [1]. They usually exhibit a two-phase microstructure, which is
caused by the physical differences between the soft and the hard segments.
∗corresponding author; e-mail: fatima@lipc.fis.uc.pt
(1359)
1360 M.F. Ferreira Marques et al.
In recent years, positron annihilation lifetime spectroscopy (PALS) has
gained growing interest as a useful tool to probe the microscopic properties
of polymeric materials, predominantly to investigate free-volume distributions
in various crystalline and amorphous solids [2–7]. In polymers, the positron
may form and annihilate from a bound state called positronium (Ps) that will
be localised in the free-volume holes. The primary annihilation mechanism of
the long-lived triplet state of the positronium (o-Ps) is a pick-off annihilation
with electrons of the host medium. The o-Ps lifetime is sensitive to both the
electron density in the neighbourhood of the Ps and to the free-volume hole
size [2–7]. For an o-Ps confined in an infinitely deep potential well the semi-
empirical Tao–Eldrup model and its extensions [8–12] establish a correlation be-
tween the free–volume hole radius, R, and the o-Ps pick-off lifetime, τo−Ps, giv-
ing τo−Ps = 0.5[1 − R/R0 + (1/2π) sin(2πR/R0)]−1, where R0 = R + ∆R and
∆R = 1.656 Å. The average radius R of the free-volume holes in polymers and the
average volume of the holes, Vf = (4/3)πR3, can be calculated from the measured
o-Ps lifetime. The fractional free volume, FFV, is a product of the free volume
Vf and positronium intensity I, as FFV = CVfI, where C is a constant which is
empirically estimated from conventional free-volume theory. It must be under-
lined here that this model has principal limitations (e.g. [13]) as this apparent free
volume integrates only the sites seen by the o-Ps. However, this model has been
successfully applied to many systems, including membranes [14, 15].
The present work is an extension of studies of polyurethanes/urea mem-
branes with two soft segments [14, 16, 17]. Our aim was to study how the free-
volume properties are influenced by phase separation and how the free volume
correlates with gas permeability.
2. Experimental
Cross-linked polyurethane membranes with two soft segments were pre-
pared by extending a polypropylene oxide-based tri-isocyanate terminated pre-
polymer (PU), supplied by Hoechst Portugal, with polycaprolactone diol (PCL)
from Aldrich. Dibutyltin dilauryate (DBTDL), supplied by Aldrich, was used as
a catalyst with toluene solvent (Merck).
Our fast-fast coincidence PALS setup with Pilot-U scintillators and XP
2020Q photomultipliers [17] had a time resolution of 270 ps. Sample–source sand-
wiches with 22Na of ca. 0.5 MBq activity closed between Ni foils were employed.
All measurements were carried out in the temperature range of 298–324 K, well
above the Tg of the present membranes (223 < Tg < 233 K). The lifetime spectra
(LS) had an integral number of ca. 3.0× 106 counts and were evaluated applying
the LT (version 9) program [18].
3. Results and discussion
The LS data were deconvoluted into three and four lifetime components with
lifetimes τi and intensities Ii. The four component analysis proved to be more
Free-Volume Studies . . . 1361
consistent along the concentration range. Subscripts i = 1 to 4 are ascribed to sin-
glet states of positronium (p-Ps), free positrons, and two long-lived triplet states
(o-Ps) associated with pick-off annihilation in free-volume holes of two different
sizes, respectively. The latter components could indicate the coexistence of two
phases, corresponding to separated domains of the two soft segments (polypropy-
lene oxide and polycaprolactone) present in the membranes. Such results were also
observed in other two polyurethanes/urea membranes obtained through the intro-
duction of two soft segments — polypropylene oxide/polybutadiene and polypropy-
lene oxide/polydimetylsiloxane [16, 17].
In Fig. 1a, lifetime τ4 decreases slightly with increasing PCL content and
increases with temperature, resulting in a similar dependence for the R4 radius.
Increasing the temperature in the samples brings about a systematic increase in
the average volume of the holes, because, as expected, the chain movements above
Tg are no longer frozen. By increasing the temperature, the thermal movements
also increase. The lifetime τ3 also decreases below 15 wt% PCL, remaining al-
most constant for higher values. However, the temperature affects differently the
intensities I3 and I4, with increasing values of I4 for higher temperatures and the
opposite tendency for I3. The smaller free volumes, R3, may correspond to holes
in the crystalline regions connected with the aggregates of urethane urea groups,
whereas the larger ones, R4, might be ascribed to holes in amorphous regions. As
compared to previous results in [16, 17], R3 has about the same value as the other
polyurethane membranes for the same content of the soft segment. The decrease
in τ4 with increasing PCL may be ascribed to an increase in cross-linking, resulting
in a decrease in hole sizes. However, the rising tendency of I4 with increasing PCL
content means an increase in the hole density, so that an increase in porosity can
be expected.
The temperature dependences of R4 and R3 are shown in Fig. 1b. The
effect of temperature on R4 is essentially the same for all PCL contents. For
higher PCL content (15 to 25 wt%) the R4 radii are practically the same at each
temperature, which might indicate a sort of saturation of the hole sizes due to
a higher degree of cross-linking. As for R3, it is apparent that the temperature
dependence is attenuated for higher PCL concentrations, with stronger variations
occurring for 5 and 10 wt% PCL contents only. For these lower concentrations, the
smaller holes become even smaller with increasing temperature, suggesting that
increasing temperature enhances the degree of cross-linking of the soft segments,
a mechanism which seems to freeze for higher PCL content.
The CO2 permeability and the intensities I3 and I4 vs. PCL content at 298
K are shown in Fig. 1c. The permeability exhibits a tendency to increase with
increasing PCL content. Both I4 and I3 increase with increasing PCL content,
but in a different manner. For lower PCL concentration I4 and the permeability
go parallel, but at higher concentration I3 follows the behaviour of the permeabil-
ity. The FFV, combination of parameters Vf and I, was calculated for the smaller
1362 M.F. Ferreira Marques et al.
Fig. 1. (a) PALS parameters and the calculated radii of the Ps trapping sites as a
function of PCL content for three different temperatures. (b) Temperature dependence
of the free volume holes radii for different PCL content. (c) Relative intensities of the
long lifetime components (◦) and the CO2 permeability ( ) observed at 298 K as a
function of PCL content.
and the larger holes. A very weak dependence of the total free volume fraction,
related to both kinds of holes, with CO2 permeability was observed, suggesting
that, apart from the unoccupied space, other features are also important for the
gas permeation (thus, involving actual transfer of matter), e.g., the connectivity
of the voids. This observation is in agreement with results from amorphous poly-
mers [13]. The permeability observed in these membranes is probably defined by
an interplay of larger and smaller holes, the latter kind of “ingredients” probably
increasing the interconnectivity. These membranes are phase separated with hard
and soft segments. The expected changes of domain sizes and crystallinity may
also contribute to the increase in permeability with PCL content. The observed
enhancement of o-Ps intensity in the membranes with increasing PCL content is
consistent with a reduction in the crystalline part of the membrane and a subse-
quent rise of the amorphous part. This effect may ease the gas diffusion path in
the membranes, compensating even for an overall reduction in free-volume sizes,
thus explaining the observed improvement in permeability with increasing PCL
content.
For the same content of PU, these membranes have lower CO2 permeability
when compared to the membranes with soft segments PBDO [16] or PDMS [17].
The types of soft segments can also affect the permeation behaviour, because
they could change the degree of microphase separation and the crystallinity of the
Free-Volume Studies . . . 1363
membranes, questions which can still not be answered here but will be addressed
later.
4. Conclusions
In bi-soft segment urethane/urea membranes studied by PALS, two types of
free-volume holes of different sizes have been identified, corresponding probably to
two separate domains of the soft and hard segments. In principle, the overall free
volume in polymer membranes definitely influences the gas permeation properties.
CO2 permeability was observed to be correlated to the density and size of both
the larger and the smaller holes, the latter probably contributing to increased in-
terconnectivity of the voids. The fractional free volume seen by the o-Ps may (and
probably does) differ from the free volume involved in gas permeation. Positrons
just experience an average static structural pattern, and do not feel forces due to
concentration gradients that drive gas molecules across pathways through wobbled
interconnections among the holes. The changes observed in the free-volume pa-
rameters obtained by PALS are smaller than those found in permeability. This is
not fully surprising, as, e.g., the fractional free volume seen by o-Ps must exhibit
a saturation limit when full trapping is reached, which is probably the case here.
Studies on gas permeation [19] clearly indicate the influence of many factors, e. g.
chain flexibility, chemistry at the free-volume walls, diffusion barriers, etc. The in-
terconnectivity must play a crucial role in the gas permeation and definitely needs
further systematic studies, by PALS and other techniques.
Acknowledgments
The authors gratefully acknowledge support received through the bilat-
eral Hungarian-Portuguese Scientific exchange projects P-23/03 and P-23/05;
M.F.F.M. thanks ISEC, too.
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Free-Volume Studies in Polycaprolactone/Poly(propylene oxide) Urethane/Urea Membranes by Positron Lifetime Spectroscopy

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Free-Volume Studies in Polycaprolactone/Poly(propylene oxide) Urethane/Urea Membranes by Positron Lifetime Spectroscopy

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