1

Polyurethane urea membranes for membrane blood

oxygenators: synthesis and gas permeation properties

Tiago Mendonça Eusébio

Abstract Nonporous symmetric (PU) and integral asymmetric (PEU) poly(ester urethane urea) membranes were

synthetized and characterized in terms of: i) structure by Scanning Electron Microscopy (SEM), and ii) gas permeation properties in a custom-made set-up constructed and optimized for the measurement of the N2, CO2 and O2 permeation fluxes at constant temperature.

The membranes were synthesized by a modified version of the phase inversion technique where polyurethane (PUR) and polycaprolactone-diol (PCL-diol) prepolymers react in a solvent mixture of dimethyl formamide (DMF) and diethyl ether (DEE), during the casting solutions preparation step. Total polymer to solvent weight

ratio, solvent evaporation time and PCL quantity were varied. SEM micrographs showed that the integral asymmetric poly(ester urethane urea) membranes have a characteristic cross section structure with no visible dense layer but instead three distinct porous regions.

The CO2 permeabilities obtained for the nonporous symmetric membranes with 0, 5 and 15% wt. ratio of PCL were 163 Barrer, 94 Barrer and 218 Barrer, respectively. The average permeances obtained for the integral asymmetric poly(ester urethane urea) membranes prepared

with, total polymer/total solvent ratio 1/1, 5 minutes solvent evaporation time and PCL content between 0-15

wt.% were 0.13 ± 0.01 × 10−5 cm3cm−2s−1cmHg−1 for CO2, 0.011 ± 0.003 × 10−5 cm3cm−2s−1cmHg−1 for O2 and

0.004 ± 0.001 × 10−5 cm3cm−2s−1cmHg−1 for N2.

1 – Introduction

Extracorporeal membrane oxygenation (ECMO) is a medical technique

of providing prolonged artificial breathing and heart support to patients

whose cardiovascular and or pulmonary systems are not functioning

normally. [1] Nowadays, membrane blood oxygenators MBOs are the only

type of oxygenators used.

The ideal MBO has to fulfil a two-fold goal: i) promote efficient gas

exchange and ii) be blood compatible or hemocompatible. [2] The average

membrane surface area of commercial oxygenators is approximately 2 m2.

Considering the referred volumetric fluxes and a feed pressure of 76 cmHg,

the membrane should exhibit CO2 and O2 permeances of approximately

0.22×10−5 cmSTP3 cm−2s−1cmHg−1 and 0.27×10−5 cmSTP

3 cm−2s−1cmHg−1,

respectively. [3]

Polyurethanes, having extensive structure and property diversity are

one of the most bio- and blood compatible material and have played a

major role in the development of many medical devices. Polyurethanes are

characterized by durability, elasticity, elastomer-like character, fatigue

resistance, compliance, and acceptance or tolerance by the body [4].

Works by Boretos et. al. [5] and Marzec et. al. [6] describe biomedical

applications of polyurethanes.

Bi-soft segment polyurethanes contain not one but two different types

of SS‘s and are formed when the second SS is used to extend an

isocyanate terminated prepolymer containing the first SS.

Studies by the de Pinho’s research group show that the gas permeation

properties of bi-soft segment poly(urethane urea) membranes were

influenced by the type and content of the second soft segment. The CO2

permeability increases with the type of second SS in the order PCL, PBDO,

and PDMS; For PUR/PBDO membranes CO2 permeability increased with

the decrease of PBDO; For PUR/PDMS membranes increased with the

increase of PDMS; For PUR/PCL membranes CO2 permeability was very

low for membranes containing 0-15% of PCL. [7], [8], [9]

The blood compatibility properties of the PUR/PBDO and PUR/PDMS

bi-soft segment membranes were studied by de Queiroz et. al. [8] and Zhou

[10]. Results showed that the hemocompatibility of these types of

membranes was limited particularly in terms of thrombogenicity for the

PUR/PBDO membranes, and of platelet adhesion for the PUR/PDMS

membranes. The hemocompatibility of bi-soft segment PUR/PCL

membranes with PCL-diol content ranging from 0% to 15% (w/w), was also

studied by the de Pinho’s research group ([11], [12]) and it was concluded

that these presented enhanced hemocompatibility properties when

compared to the PUR/PBDO and PUR/PDMS membranes.

Despite the very promising results in terms of hemocompatibility, the

gas permeability of the dense homogeneous PUR/PCL membranes was

very low. One method to increase the gas permeabilities and

simultaneously preserve the enhanced hemocompatibility of the PUR/PCL

membranes is to synthesize them as integral asymmetric PUR/PCL

membranes [3]. These membranes are composed of a very thin top dense

layer, where all of the resistance to the gas transport phenomena is

present, and by a bottom thicker porous support layer which offers little or

no resistance to gas transport.

2 – Mass transport phenomena in homogeneous membranes

The mass transport in non-porous homogeneous polymer membranes

is assumed to occur by a solution-diffusion-desorption mechanism. The

steady state diffusive flux in the y-direction, JAy, is described by the Fick's

First Law (Eq.1):

JAy = −DAm

dcAm

dy

(1)

where JAy is the flux of species A in terms of moles per unit of time and

unit of membrane surface area. The gradient of concentration is dcA

dy, with

cAm being expressed as the molar concentration of solute A in the polymer.

The quantity DAm is the diffusion coefficient and can be regarded as a

proportionality between the flux and the concentration gradient . [13]

The integration of the First Fick’s Law over the total membrane

thickness, δ, with the boundary conditions: i) On the feed side, y = 0, the

penetrant concentration in the polymer is cA0(m); ii) On the permeate side,

y = δ, the penetrant concentration is cAmδ, results in Eq.2.

JA =DAm

δ(𝑐Am0 − 𝑐Amδ) (2)

For ideal systems, where the solubility is independent of concentration,

the concentration inside the polymer is proportional to the applied pressure.

This behavior is normally observed with gases in elastomers. Since the

solubility of the gases in elastomeric polymers is very low, Henry's law can

be applied. The equilibrium at the membrane/gas interfaces is therefore

described by the relationship of the concentration inside the polymer with

the external pressure, given by the sorption coefficient, SA, (Eq.3).

𝑆𝐴 =𝑐𝐴𝑚0

𝑝𝑓

=𝑐𝐴𝑚𝛿

𝑝𝑝

(3)

In the considered system, cAm0 and cAmδ are not known, unlike pf and

pp that are known. Applying Henry’s Law (Eq. 3), to Eq.2, and considering

the pressure pf on the feed side and pp on the permeate side, it is obtained

Eq. 4:

Supervisors: Prof. Eduardo Jorge Morilla Filipe;

Dr. Mónica Cristina Faria Besteiro. Thesis to obtain the Master of Science Degree in: Chemical Engineering

November 2017

Keywords:

gas permeability

bi-soft segment polyurethanes

integral asymmetric membranes

Membrane Blood Oxygenators

2

JA =SADA

δ(pf − pp) (4)

Considering the Solution-diffusion model, Permeability (P𝐴) is a function

of Diffusivity (D𝐴) and Solubility (S𝐴):

P𝐴 = D𝐴 . S𝐴 (5)

Considering Eq.5, Eq. 4turns in Eq.6:

JA =PA

δ(pf − pp)

(6)

Eq. 6 shows that the flux of a component through a membrane is

proportional to the pressure difference across the membrane and

inversely proportional to the membrane thickness. [14]

Usually, Permeability is represented in Barrer units:

Barrer = 10−10 (cm3cm

cm2 s cmHg)

Substituting the flux given by Fick’s first law in the mass balance of

penetrant gas A through a nonporous symmetric membrane it is obtained

the Fick’s second Law, expressed by Eq.7.

∂cAm

∂t= DA

∂2cAm

∂y2

(7)

For the initial and boundary conditions of a system where the

concentration of gas in one side of an initially gas-free membrane is raised

to cA0(m), at time t = 0, while the other side is maintained at zero, Rogers

et al. [15] suggests a solution for Fick’s second Law admitting that the

relation between the concentration of gas at the interface of the polymer

and the external gas pressure is given by Eq.8:

KA =nAm0

pf

(8)

where nAm0 is the quantity of gas, as the pressure-volume product,

dividing by the volume of polymer. Despite the sorption coefficient, KA, be

defined without units, can be understood as equivalent units of (pressure-

volume of dissolved gas/volume of polymer)/external pressure. Taking

into consideration the units of the sorption coefficient, it is possible to

obtain the permeability coefficient, kA, by Eq.9.

kA = DA𝐾𝐴 (9)

The integration of the solution of the 2nd Fick’s Law, in the form of

Fourier series assigning coefficients to meet the boundary and initial

requirements, is represented by Eq.10: [15]

pp =AD𝐴𝐾𝐴

Vδpf [t −

δ2

6D𝐴

+2

π2D𝐴

∑(−1)m

m2exp (−

D𝐴m2π2t

δ2)

∞

m=1

] (10)

where V is the receiving chamber volume in which the diffused gas

accumulates, A is the membrane surface area, When t → ∞ , exponential

terms are elapsed, becoming negligibly small and pressure becomes

linear with time (Eq.11).

pp = (AD𝐴𝐾𝐴

Vδ) pf (t −

δ2

6D𝐴

) (11)

The intercept on the time axis, tc, can be obtained by solving the Eq.

11 for 𝑡 when pp = 0, resulting in Eq.12, allowing the calculation of the

diffusion coefficient, 𝐷𝐴.

𝑡𝑐 =𝛿2

6𝐷𝐴

(12)

Analogously, the intercept on the pressure axis, pc affords a simple

calculation of the solubility, 𝐾𝐴 , from Eq.13. [15]

𝐾𝐴 = −6V

Aδ

pc

pf

(13)

Rogers et al. [15] suggested an alternative method, known as the early

approximation, specially used for permeation experiments where the time

required to establish the linear relation between the pressure and the time

is very long. It is based on a treatment of the exact solution in Fourier

series in which a transformation formula is applied to the right hand

leading to another solution to Fick’s second law (Eq.14).

dpp

dt=

2A

V𝐾𝐴pf√

DA

πt∑ e

[−(δ2

4DAt)(2m+1)2]

∞

m=0

(14)

Because of the inverted placement of t in the exponentials, this series

converges most rapidly for very small values of t rather than for large

values. For times sufficiently short, Eq. 14 may be approximated by

neglecting all but the leading term in the series of exponentials. Then it is

convenient to multiply by √t, and take the logarithms on both sides. Thus,

Eq. 15 is obtained.

ln (√tdpp

dt) = ln (

2A

V𝐾𝐴pf

2√DA

π) −

δ2

4DAt (15)

Plotting ln (√tdpp

dt) against

1

t a straight line is determined. From the slope

of this line, DA can be obtained according to the relation expressed by

Eq. 16.

slope = −δ2

4DA

(16)

After determined DA, the solubility can be obtained by solving Eq. 17, for

KA.

𝐾𝐴 = √π

DA

V

2A

1

pf

(√tdp

dt) exp (

δ2

4DAt) (17)

3 – Experimental

3.1 – Materials

The PUR prepolymer, supplied by Fabrires-Produtos Químicos, SA, had a molecular weight of 3500 Da, approximately, and prepolymer PCL-

diol, provided by Sigma-Aldrich, had a molecular weight of about 530 Da. The solvents used in membranes’ synthesis were dimethylformamide (DMF) (w / w% grade, 99.8%) and diethyl ether (DEE) (w / w% grade,

99.7%) provided by Panreac. Tin-Octoate (C16H30O4Sn) (wt.%, 95%), also of the Aldrich brand, was used as the catalyst.

Permeation tests were performed using nitrogen (purity ≥ 99.999%),

carbon dioxide (purity ≥ 99.98%) and industrial oxygen (purity ≥ 99.5%)

provided by Air Liquide. All gases were used as received.

3.2 – Synthesis of poly(ester urethane urea) membranes

Integral asymmetric poly(urethane urea) membranes, were

synthesized by a modified version of the phase inversion technique where

the prepolymers PUR and PCL-diol reacted in a solvent system of DMF

and DEE in presence of the Tin-Octoate catalyst. Upon this preparation,

the casting solution was left in agitation for about 2 hours. In a second

step, the casting solution was spread on a glass plate using a 250 μm

casting knife. After a solvent evaporation step, the glass plate was

introduced into the coagulation bath (distilled water) where it was left for

about 12 hours. When removed from the bath, the membranes were dried

in an oven (35 °C) for at least 36 hours. The PEU membranes, were

synthesized using different total polymer to total solvent weight ratio, the

PUU membranes were prepared varying solvent evaporation times and

the PEUU membranes, were synthesized with solvent evaporation time of

1 or 5 minutes, varying the PCL content.

3

The nonporous symmetric poly(urethane urea) membranes,

designated PU membranes, were synthesized by the solvent evaporation

method, varying the PCL content.

Table 1 shows the chemical composition, solvent time evaporation

and total polymer to total solvent weight ratio of the PEU, and PU

membranes.

3.3 – Pressure method

An in house built set-up was used for gas permeation measurements

by the pressure method at constant temperature. A temperature controlled

unit was built and calibrated to perform gas permeation tests, where

pressure variation is recorded online, at very short intervals of time with

high precision. Figure 1 represents a diagram of the set-up where the

permeation flux measurements of the PUU, PEUU and PU membranes

were carried out. The unit was composed of a feed pressure sensor (PfT)

(Setra, Model 205, Massachusetts, USA), a permeation cell, a cylindrical

buffer of 12.6 cm3 and a pressure transmitter (PpT), (Intelligent

Transmitter Paroscientific, Series 6000, model 6100A-CE Inc.

Washington, USA), attached to a Paroscientific model 710 display unit,

connected to a computer. The pressure values were recorded in the

software Digiquartz Assistant® version 1.0 (Paroscientific Inc,

Washington, USA). The receiving chamber volume was 27.7 ± 0.1 cm3. In

each measurement, a sample of membrane was placed in the permeation

cell, after which, a single pure gas (N2, CO2 or O2) was fed at constant

pressure, and all the outlet valves were closed. The pressure at the

permeate side was measured as a function of time by the PpT sensor. The

tubbing system of the set-up consisted mainly of stainless steel 316 tube

with 1/8 inch internal diameter, provided by Hook®. Several types of tube

fitting, of different materials (stainless steel, titanium and brass)

GYROLOK®, and needle valves 3700 Series, also provided by Hook®,

were used.

4 – Results and Discussion

4.1 – PEU membranes prepared with different total polymer to total

solvent weight ratio

The PEU1, PEU2 and PEU3 integral asymmetric membranes,

containing 0, 5 and 10% wt. of PCL, respectively, were synthesized with

total polymer to total solvent weight ratio of 2/3, DMF to DEE weight ratio

of 3, and solvent evaporation time of 30 seconds.

Upon visual inspection, it was verified that the membranes did not

present a cohesive and continuous structure but instead net or web-like

morphology. The PEU1 membrane showed the least homogeneous

structure of varying thickness and with visible holes; the PEU2 membrane

also presented a fragile net-like structure; and the PEU3 membrane

presented a more homogeneous structure, continuous, cohesive and with

no visible holes. These results indicate that the increase of the amount of

PCL in the PEU membranes improves the structural characteristics.

Furthermore, the structure of the PEU membranes suggests that the batch

of the prepolymer PUR used in this study was manufactured with higher

amounts of solvent, than the one used in previous studies ([3], [9]),

supplied by another producer. Taking into account this assumption, other

membrane casting solutions and conditions were tested in order to obtain

the optimal film forming conditions, such as: variation of total polymer/total

solvent ratio and variation of the solvent evaporation time.

The PEU1, PEU4 and PEU5 membranes, containing only PUR and

no PCL, were synthesized with polymer/solvent weight ratio of 2/3, 1/1 and

3/2, respectively, while maintaining the mass proportions among DMF /

DEE solvents equal to 3, and the solvent time evaporation of 30 seconds.

When the membranes were visually compared in terms of physical

appearance, it was found that the PEU1 membrane presented a web like

incomplete film structure, with several holes and zones of limited

thickness; that the PEU5 membrane was slightly more rigid, malleable and

sticky, which difficulted its manipulation, than the PEU1 membrane. On

the other hand, the PEU4 membrane was more homogeneous and

consistent, malleable, elastic and with a uniform, non-web like structure. It

was observed that the greater the total polymer to total solvent ratio, the

easier it is to obtain a membrane with a complete film. However, an excess

of polymer may result in less elastic and very sticky membranes. This

indicates that to obtain membranes with optimal structural characteristics,

there must be a balance between the amount of polymer and the solvent

system. It was determined that, for the materials used, the most suitable

total polymer to total solvent ratio is 1/1.

Figure 2 shows S.E.M. images of the top surface, bottom surface and

cross-section of the PEU3, PEU4 and PEU5 membranes. Comparing the

SEM images of the top surface of the PEU3, PEU4 and PEU5 membranes

((a), (b) and (c)), it is observed that the PEU3 membrane presents an

active surface fully constituted of concavities and pores. Unlike the PEU3

membrane, it is observed in the PEU4 membrane active surface a

complete film structure, with pores of two different sizes spread along the

surface. Like the PEU3 membrane, it is observed that the PEU5

membrane top surface is fully constituted of pores, apparently larger and

deeper like the PEU3 membrane pores.

Relatively to the bottom surface SEM images ((d), (e), (f)), it is

observed that PEU3 membrane has a bottom surface full of pores and

concavities, like its top surface. The PEU4 membrane shows a bottom

surface characterized by two pore regimes of two different sizes, spread

Figure 1 - Scheme of the set-up used in permeation flux measurements by constant volume method.

Table 1 – Chemical composition, solvent evaporation time and polymer to solvent ratio of the PEU and PU membranes.

Membrane

PUR/PCL

diol

(wt.%)

HS

Type

SS

Type

Solvent

evaporation

time (min)

total

polymer/total

solvent ratio

(wt. %)

PEU1 100/0 I PPO 0.5 2/3

PEU2 95/5 I and

II

PPO &

PCL 0.5 2/3

PEU3 90/10 I and

II PPO & PCL

0.5 2/3

PEU4 100/0 I PPO 0.5 1/1

PEU5 100/0 I PPO 0.5 3/2

PEU-1-100 100/0 I PPO 1 1/1

PEU-5-100 100/0 I PPO 5 1/1

PEU-10-100 100/0 I PPO 10 1/1

PEU-15-100 100/0 I PPO 15 1/1

PEU-1-95 95/5 I and

II PPO & PCL

1 1/1

PEU-1-90 90/10 I and

II

PPO &

PCL 1 1/1

PEU-1-85 85/15 I and

II PPO & PCL

1 1/1

PEU-5-95 95/5 I PPO 5 1/1

PEU-5-90 90/10 I and

II PPO & PCL

5 1/1

PEU-5-85 85/15 I and

II PPO & PCL

5 1/1

PU100 100/0 I PPO - 1/1

PU95 95/5 I and

II PPO & PCL

- 1/1

PU85 85/5 I and

II PPO & PCL

- 1/1

4

along the film surface. The PEU5 membrane bottom surface has pores,

apparently in a lower number than its top surface, spread along the

surface.

Observing the SEM images of the cross sections ((g), (h) and (i)), the

PEU3, PEU4 and PEU5 membranes appears to have a distinct cross

section structure with no visible dense layer but instead two porous

regions: for the PEU3 and PEU5 membranes, close to the upper and

bottom surface small pores in large number are observed, while on the

most inner part of the membrane a thicker phase with larger pores and

less numerous can be noted; The same is observed for the PEU4

membrane, except the fact that near bottom surface are observed large

concavities, correspondent to the large pores observed in the bottom

surface image (e). The cross-section of the PEU5 membrane has a torn

aspect. When the samples were prepared and cut for the SEM, regard its

elastomeric and sticky nature, it was not achieved a clean cut of the

membrane pieces. Note that the cross-section structures look drained by

the knife, contrary to the PEU4 membrane cross-section. Due this, the two

pores region became harder to identify and to limit.

4.2 – PEU membranes prepared with different solvent evaporation

time

In order to study the effect of the solvent evaporation time on the

membrane morphology, PEU membranes, containing only PUR, were

prepared with a polymer/solvent ratio of 1/1, a DMF/DEE weight ratio

equal to 3 and solvent evaporation times of 1, 5, 10 and 15 minutes to

render PEU-1-100, PEU-5-100, PEU-10-100 and PEU-15-100

membranes, respectively. It was observed that all of these membranes

presented structural sustainability, were homogeneous and elastic.

Figure 3 shows SEM images of the top surface, bottom surface and

cross-section of the PEU-1-100, PEU-5-100 and PEU-10-100

membranes.

The top surface SEM images ((a), (b) and (c)) show that the porous

definition decreases in the order of the PEU1, PEU2 and PEU3

membranes.

Comparing the SEM images of the bottom surface ((d), (e) and (f)) for

the PEU-1-100 membrane is observed two types of pores, large

concavities and smaller ones between them; for PEU-5-100 and PEU-10-

100 membrane it is observed that pores are fully spread along the surface,

being smaller and in large number in the PEU-5-100 membrane and larger

and in small number for the PEU-10-100 membrane. Actually, the top and

the bottom surfaces of the PEU-10-100 membrane are very similar.

The cross-section of the PEU-1-100, PEU-5-100 and PEU-10-100

membranes represented in ((g), (h) and (i)), appears to have a distinct

cross section structure with no visible dense layer but instead two porous

regions: close to the upper and bottom surface small pores in large

number are observed, while in the most inner part of the membrane is

observed a thicker phase with larger pores and less numerous. Note that

the upper layer seems to be the less porous region (upper denser layer),

resembling the dense layer of the integrally skinned membranes. It is

observed that the upper denser layer becomes more distinct in the order

of the PEU-1-100, PEU-5-100 and PEU-10-100 membranes. In the same

order, it is verified an apparent increase of the membrane thickness. A

possible explanation is the increase of the solvent evaporation time

promotes the formation of a dense layer in the upper surface but has no

effect on the pore regime of the inner part.

4.3 – PEU membranes prepared with different PUR to PCL weight

ratios

With the objective of studying the effect of the PCL content on the

membrane morphology, the PEU membranes were prepared with varying

percentages of PCL and with constant polymer/solvent weight ratio 1/1,

constant DMF/DEE weight ratio equal to 3. PEU-1-100, PEU-1-95, PEU-

1-90 and PEU-1-85 membranes were synthesized with 0, 5, 10 and 15%

of PCL, respectively, and solvent evaporation time of 1 minute. The PEU-

5-100, PEU-5-95, PEU-5-90 and PEU-5-85 membranes were prepared

with 0, 5, 10 and 15%, of PCL respectively, and solvent evaporation time

of 5 minutes. It was observed that every PEU membrane presented

structural sustainability, were homogeneous and elastic. It was also noted

that membranes were whiter, less yellow and also stickier with the

increase of PCL amount used in its preparation.

Figure 4 shows SEM images of top surface, bottom surface and cross-

section of the PEU-1-95, PEU-1-90 and PEU-1-85 membranes prepared

with solvent evaporation time of 1 minute and PCL wt.% of 5, 10 and 15%,

respectively. It can be observed that the number of pores and its diameter

decreases in the order of the PEU-1-95, PEU-1-90 and PEU-1-85

membrane, being almost completely invisible in the PEU-1-85 membrane

top surface.

Observing the SEM images of the bottom surface of the PEU-1-95,

PEU-1-90 and PEU-1-85 membranes ((d), (e) and (f)) it is observed that

all of the three have similar aspect, with pores in the same number and of

the same size.

In the SEM images of the cross sections ((g), (h) and (i)), one can see

that the PEU-1-95, PEU-1-90 and PEU-1-85 membranes appears to have

a distinct cross section structure with no visible dense layer but instead

two porous regions: close to the upper and bottom surface small pores in

large number are observed, while in the most inner part of the membrane

is observed a thicker phase with larger pores and less numerous. Note

that the upper layer seems to be the less porous region (upper denser

layer), resembling the dense layer of the integrally skinned membranes. It

is observed that the upper denser layer becomes more distinct and the

number and the size of the pores in the inner region decreases, in the

order of the PEU-1-95, PEU-1-90 and PEU-1-85 membrane.

The cross-section of the PEU-1-90 and PEU-1-85 membranes have a

torn aspect. When the samples were prepared and cut for the SEM, regard

its elastomeric and sticky nature, it was not achieved a clean cut of the

pieces of the PEU-1-90 and PEU-1-85 membranes. Note that the cross-

section structures look drained by the knife, contrary to the PEU-1-95

cross-section. Due this, the two pores region became harder to identify

and to limit.

Figure 5 shows SEM images of top surface, bottom surface and cross-

section of the PEU-5-95, PEU-5-90 and PEU-5-85 membranes prepared

with a solvent evaporation time of 5 minutes and PCL wt.% of 5, 10 and

15%, respectively.

It can be observed that the number of pores and its diameter

decreases in the order of the PEU-5-95, PEU-5-90 and PEU-5-85

membrane, being almost completely invisible in the PEU-5-90 and PEU-

5-85 membranes top surface.

Observing the SEM images of the bottom surface of the PEU-5-95,

PEU-5-90 and PEU-5-85 membranes ((d), (e) and (f)) it is observed that

the PEU-5-95, PEU-5-90 membranes have similar aspect, with pores in

the same number and of the same size. The PEU-5-85 membrane shows

a bottom surface with a greater number of pores of the same size of the

other membranes.

In the SEM images of the cross sections ((g), (h) and (i)), one can see

that the PEU-5-95, PEU-5-90 and PEU-5-85 membranes appears to have

a distinct cross section structure with no visible dense layer but instead

two porous regions: close to the upper and bottom surface small pores in

large number are observed, while in the most inner part of the membrane

is observed a thicker phase with larger pores and less numerous. Note

that the upper layer seems to be the less porous region (upper denser

layer), resembling the dense layer of the integrally skinned membranes. It

is observed that the upper denser layer becomes more distinct, and the

number of pores in the inner region decreases, in the order of the PEU-5-

95, PEU-5-90 and PEU-5-85 membrane.

4.4 – Nonporous symmetric PU membranes

The nonporous symmetric PU membranes prepared were translucent,

glassy and very sticky. Figure 6 shows the SEM images of the top surface

and cross-section of the PU1 membrane. The PU1 membrane is

completely dense with no visible pores. The same was observed for the

PU2 and PU3 membranes.

The thickness of the nonporous symmetric PU100 membranes was

obtained from 5 measurements using the ImageJ software. The total

thickness of the PU95 and PU85 membranes was obtained from 5

measurements using a digital caliper.

The symmetric dense PU100, PU95 and PU85 membranes have total

membrane thickness of 107 Barrer, 126 Barrer and 114 Barrer,

respectively.

5

PEU3 PEU4 PEU5

Top

surface

(a) (b) (c)

Bottom

surface

(d) (e) (f)

Cross-

section

(g) (h) (i)

Figure 2 – SEM images of samples of PEU3: (a) top, (b) cross-section, (c) bottom; PEU4: (d) top, (e) cross-section, (f) bottom; PEU5: (g) top, (h) cross-section, (i) bottom.

PEU-1-100 PEU-5-100 PEU-10-100

Top surface

(a) (b) (c)

Bottom

surface

(d) (e) (f)

Cross-section

(g) (h) (i)

Figure 3 – SEM images of samples of PEU-1-100: (a) top, (b) cross-section, (c) bottom; PEU-5-100: (d) top, (e) cross-section, (f) bottom; PEU-10-100: (g)

top, (h) cross-section, (i) bottom.

6

PEU-1-95 PEU-1-90 PEU-1-85

Top surface

(a) (b) (c)

Bottom

surface

(d) (e) (f)

Cross-section

(g) (h) (i)

Figure 4 – SEM images of samples of PEU-1-95: (a) top, (b) cross-section, (c) bottom; PEU-1-90: (d) top, (e) cross-section, (f) bottom; PEU-1-85: (g) top,

(h) cross-section, (i) bottom.

PEU-5-95 PEU-5-95 PEU-5-95

Top surface

(a) (b) (c)

Bottom

surface

(d) (e) (f)

Cross-section

(g) (h) (i)

Figure 5 – SEM images of samples of PEU-5-95: (a) top, (b) cross-section, (c) bottom; PEU-5-90: (d) top, (e) cross-section, (f) bottom; PEU-5-85: (g) top,

(h) cross-section, (i) bottom

7

4.5 – Gas Permeation by the Pressure Method

In the pressure method, the permeate pressure, 𝑝𝑝, (mbar) is

measured directly as function of time (s) in experimental setup, resulting

in plots similar to the one showed in Figure 7.Figure 7 - CO2 permeate

pressure (mbar) measurement as a function of time (s) for the PEU-5-85

membrane (CO2 feed pressure of 3.4 bar).

At the beginning of each experiment (t = 0s) the relative permeate

pressure was 0 mbar and all of the experiments were performed at feed

pressures ranging between 0.5 e 4.0 bar absolute.

The N2, CO2 and O2 permeation parameters of the PEU and PU

membranes were determined by the pressure method.

The permeation flux of the membranes was determined by the slope

of the steady state zone of the permeate pressure vs time graph (Figure

7). dV

dt=

dn

dt

RTSTP

PSTP

(18)

Where dn

dt is the molar flow of gas passing to the permeate side,

obtained from Eq. 8 considering the bath temperature (T) at the time of

the experiments, and the volume of the permeate compartment in the

experimental setup (Vinst).

dn

dt=

dpdt⁄ Vinst

RT

(19)

Substituting Eq. 7 in Eq. 8 one obtains Eq. 9.

dV

dt=

dpdt⁄ Vinst

RT

RTSTP

PSTP

(20)

The volumetric flux (J), defined by Eq. 10, is obtained by dividing the

volumetric flow (Eq. 9) by the effective surface area of the permeation cell.

J =dV

dt⁄

a

(21)

The slopes of the straight lines of the permeate pressure vs time plots,

were used to obtain the volumetric flux (J) at each feed pressure.

Considering that at the beginning of the measurement, the receiving

chamber has 1 atm of air, one can define TMP as the difference between

the feed pressure and atmospheric pressure (𝑇𝑀𝑃 = 𝑝𝑓 − 𝑝𝑎𝑡𝑚). J vs

TMP curves for the four PEU-5-100 membrane samples are presented in

Figure 8. The permeance (Perm) of each sample of the PEU-5-100

membrane was obtained by the Eq. 22. Results show that the permeance

for each of the four PEU-5-100 membranes samples was 0.10, 0.12, 0.12,

and 0.17 (10−5 cmSTP3 cm−2s−1cmHg−1).

4.6 – CO2 permeation properties of the PU nonporous symmetric

membranes

The PU nonporous symmetric membranes were tested with CO2 by

the pressure method. Figure 9 shows the average volumetric fluxes

J(10−5 cm3cm−2s−1) of membranes PU100, PU95 and PU85 as a function

of the transmembrane pressure (cmHg) and shows the average total

membrane thickness (δ), the average permeance (Perm) and permeability

coefficient (PCO2).

The permeability coefficient (P), in units of Barrer, was obtained by

multiplying the mean permeance (Perm) by the dense layer thickness (Eq.

23).

The only difference between the three PU membranes was the PCL

content: 0, 5 and 15 %wt. for the PU100, PU95 and PU85 membranes,

respectively. The average CO2 permeance of the PU100, PU95 and PU85

membranes is 0.15× 10−5cmSTP3 cm−2s−1cmHg−1, 0.07 ×

10−5 cmSTP3 cm−2s−1cmHg−1, 0.19 ×10−5 cmSTP

3 cm−2s−1cmHg−1,

respectively. The values of CO2 Perm have the same order of magnitude

and increase in the order of the PU95, PU100, and PU85 membranes.

Perm =dJ

d(TMP) [

cmSTP3

cm2. s. cmHg] (22)

P = Perm×δ [cmSTP

3 . cm

cm2. s. cmHg10−10]

(23)

Figure 7 - CO2 permeate pressure (mbar) measurement as a function of time (s) for the PEU-5-85 membrane (CO2 feed pressure of 3.4

bar).

0

5

10

15

20

25

30

35

0 50 100 150

pp

(mb

ar)

t(s)

Figure 8 – CO2 volumetric fluxes (10−5 cm3cm−2s−1) as function of TMP (cmHg), obtained for samples i) (green); ii) (blue); iii) (black)

and iv) (grey) of PEU-5-100 membrane.

0

5

10

15

20

25

30

35

0 50 100 150 200

J(1

0^-

5 c

m^3

/cm

^2/s

)

TMP(cmHg)

Top surface Cross Section

(a) (b)

Figure 6 - SEM images of samples of PU100: (a) top surface, (b) cross-section.

Figure 9 – Average CO2 volumetric fluxes J(10−5 cm3cm−2s−1)

as function of TMP (cmHg) for the PU100 (blue), PU95(orange)

and PU85(grey) membranes.

0

20

40

60

80

0 100 200 300 400

J(1

0^-

5 c

m^3

cm

^-2

s^-

1)

TMP(cmHg)

8

The CO2 permeability obtained for the PU membranes was 163, 94, and

218 Barrer for the PU100, PU95 and PU85 membranes, respectively.

Thus, it can be concluded that there is no direct correlation between the

increasing amounts of PCL polymer in these membranes and the CO2

permeability properties.

4.7 – CO2 permeation properties of the PEU integral asymmetric

membranes prepared with different solvent evaporation time

Figure 10 (a) shows the average volumetric fluxes

J(10−5 cm3cm−2s−1) of the PEU-5-100, PEU-10-100 and PEU-15-100

membranes as a function of TMP (cmHg) and Table 2 shows the values

average permeance obtained for the PEU membranes. The only

difference between the four PEU membranes was the solvent evaporation

time used for the synthesis: 1, 5, 10 and 15 minutes for the PEU-1-100,

PEU-5-100, PEU-10-100 and PEU-15-100 membranes, respectively.

Results show that the average permeance of the PEU-1-100 membrane

(10 ± 8×10−5 cmSTP3 cm−2s−1cmHg−1) is several orders of magnitude

greater than the other PEU membranes.

The average CO2 permeance of the PEU-5-100, PEU-10-100 and

PEU-15-100 membranes is 0.14 ± 0.03 × 10−5cmSTP3 cm−2s−1cmHg−1,

0.11±0.01 × 10−5 cmSTP3 cm−2s−1cmHg−1, 0.14±0.01 ×

10−5 cmSTP3 cm−2s−1cmHg−1, respectively, and a ANOVA-test performed

for these results found that there is no significant difference for a

confidence interval of 99.5%. Results show that there is no correlation

between the solvent evaporation time and the CO2 permeance. In fact, the

CO2 permeance value obtained for the nonporous symmetric PU

membranes is the same order of magnitude as the PEU-5-100, PEU-10-

100 and PEU-15-100 asymmetric membranes.

For a solvent evaporation time equal to and greater than 5 mins the

solvent evaporation time seems to have no effect on the CO2 permeance,

but for a solvent evaporation of 1 minutes a permeance approximately 100

times higher is observed. A possible explanation for this phenomenon can

be that a solvent evaporation time of 1 min is insufficient for the formation

of an upper denser layer, near the surface of the membranes, which in

turn offers a higher resistance to gas permeation when compared to

microporous membranes. In order to obtain a thin denser layer at the

surface of the membrane and increase the resistance to gas transport,

there needs to be enough time to allow the evaporation of the more volatile

solvent, DEE, before the polymer membrane be quenched in the water

bath [14]. The thickness of this denser layer is expected to be greater for

increasing solvent evaporation times. This is confirmed by the fact that the

membranes synthesized with solvent evaporation times larger than 1 min

showed much higher resistance to CO2 permeation. Taking into account

these results one can conclude that the PEU-1-100 membrane has a

structure and behavior that resembles more a microporous membrane

rather than an integral asymmetric membrane

4.8 – CO2, N2, and O2 permeation properties of the PEU membranes

with different PU/PCL weight ratios.

4.8.1 – CO2 permeation of integral asymmetric PEU Membranes

Figure 11 shows the average volumetric fluxes J(10−5 cm3cm−2s−1) vs

TMP (cmHg) and Table 2 shows the average permeation (Perm).

These four PEU membranes were prepared under the same

conditions, with a solvent evaporation time of 5 mins being the only

difference between them the PCL content which is 0, 5, 10 and 15 wt.%

for the PEU-5-100, PEU-5-95, PEU-5-90 and PEU-5-85 membranes,

respectively.

Results show that the CO2 permeance measured for the PEU

membranes were 0.14±0.03×10−5 cmSTP3 cm−2s−1cmHg−1, 0.11 ± 0.01×

10−5 cmSTP3 cm−2s−1cmHg−1, 0.12 ± 0.02×10−5 cmSTP

3 cm−2s−1cmHg−1 and

0.13 ± 0.03×10−5 cmSTP3 cm−2s−1cmHg−1 for the PEU-5-100, PEU-5-95,

PEU-5-90 and PEU-5-85 membranes, respectively, and a ANOVA-test

performed for these results found that there is no significant difference for

a confidence interval of 99.5%. The values of CO2 Perm have the same

order of magnitude and increase in the order of the PEU-5-95, PEU-5-90

and PEU-5-85 and PEU-5-100 membranes. The CO2 permeability

obtained for the PEU membranes was 18, 26, 15 and 22 Barrer for the

PEU-5-100, PEU-5-95, PEU-5-90 and PEU-5-85 membranes,

respectively. Thus, it can be concluded that there is no direct correlation

between the amount of PCL polymer in these membranes and the

permeability properties.

4.8.2 – N2 permeation of integral asymmetric PEU Membranes

Figure 12 shows the average volumetric fluxes of N2 as function of the TMP for the PEU-5-100, PEU-5-95, PEU-5-90 and PEU-5-85 membranes and Table 2 shows the average N2 permeation (Perm). These four PEU

membranes were prepared under the same conditions, with a solvent evaporation time of 5 mins being the only difference between them the PCL content which is 0, 5, 10 and 15 wt.% for the PEU-5-100, PEU-5-95,

PEU-5-90 and PEU-5-85 membranes, respectively. Results show that the N2 permeance measured for the PEU

membranes was 0.003±0.001×10−5 cmSTP3 cm−2s−1cmHg−1, 0.006 ±

0.003×10−5 cmSTP3 cm−2s−1cmHg−1, 0.004 ± 0.001×

10−5 cmSTP3 cm−2s−1cmHg−1 and 0.004 ± 0.001×

10−5 cmSTP3 cm−2s−1cmHg−1 for the PEU-5-100, PEU-5-95, PEU-5-90 and

PEU-5-85 membranes, respectively. The values of N2 Perm have the same order of magnitude and increase in the order of the PEU-5-100,

PEU-5-90, PEU-5-85 and PEU-5-95 membranes.

Figure 10 - Average CO2 volumetric fluxes 𝐽(10−5 𝑐𝑚3𝑐𝑚−2𝑠−1) as function of the transmembrane pressure (cmHg) for the PEU-5-100 (blue), PEU-10-100 (orange) and PEU-15-100 (grey) membranes

prepared with solvent evaporation times of 5, 10 and 15 mins.

0

10

20

30

40

50

60

0 100 200 300 400

J(1

0^5

cm

^3/c

m^2

/s)

TMP(cmHg)

Figure 11 - Average CO2 volumetric fluxes 𝐽(10−5 𝑐𝑚3𝑐𝑚−2𝑠−1) as function of TMP (cmHg) of PEU-5-100 (blue), PEU-5-95 (orange),

PEU-5-90 (grey) and PEU-5-85 (yellow) membranes.

0

10

20

30

40

50

60

0 100 200 300 400

J(1

0^-

5 c

m^3

/cm

^2/s

)

TMP(cmHg)

Figure 12 - Average N2 volumetric fluxes 𝐽(10−5 𝑐𝑚3𝑐𝑚−2𝑠−1) as function of TMP (cmHg) of the PEU-5-100 (blue), PEU-5-95

(orange), PEU-5-90 (grey), PEU-5-85 (yellow).

0

0.5

1

1.5

2

2.5

0 100 200 300 400

J(1

0^-

5

cm^3

/cm

^/s/

cmH

g)

TMP(cmHg)

9

Results show that the PEU-5-100, PEU-5-95, PEU-5-90 and PEU-5-

85 average N2 permeances are several orders of magnitude lower than N2

average permeances obtained for the PEU-1-100, PEU-1-95, PEU-1-90 and PEU-1-85 membranes, prepared with 1 min of solvent evaporation

time. This indicates that the resistance to transport is highly dependent of the solvent evaporation time and there are no evidences of a direct correlation between the PCL content of the PEU membranes and the N2

permeances.

4.8.3 – O2 permeation of integral asymmetric PEU membranes

Figure 13 shows the average volumetric fluxes of O2 for the PEU-5-100, PEU-5-95, PEU-5-90 and PEU-5-85 membranes as function of TMP. Table 2 shows the O2 permeance (Perm) for the PEU-5-100, PEU-5-95, PEU-5-90 and PEU-5-85 membranes.

Results show that the O2 permeance measured for the PEU

membranes was 0.011×10−5 cmSTP3 cm−2s−1cmHg−1, 0.008×

10−5 cmSTP3 cm−2s−1cmHg−1, 0.010×10−5 cmSTP

3 cm−2s−1cmHg−1 and

0.015×10−5 cmSTP3 cm−2s−1cmHg−1 for the PEU-5-100, PEU-5-95, PEU-5-

90 and PEU-5-85 membranes, respectively. The values of O2 Perm have the same order of magnitude and increase in the order of the PEU-5-95, PEU-5-90, PEU-5-100 and PEU-5-85 membranes. Thus, it can be

concluded that there is no direct correlation between the amount of PCL polymer in these membranes and the permeability properties.

5 – Determination of CO2 diffusion and solubility coefficients

Analysis of this section of the graphs together with Equations

pertaining to the time-lag/late approximation method and the early

approximation method were carried out, in order to obtain CO2 diffusion

and solubility coefficients for the PU95 non-porous symmetric membrane.

A point of divergence between theory and application in this work, was

the operational conditions and the initial and boundary conditions. Both,

time-lag and early approximations are solutions of the Fick’s second Law

under the conditions that initially the membrane is gas free and that the

pressure in the receiving chamber is maintained near zero, which means

that measurements must be initialized with vacuum in both chambers.

However, during this work, pressure on the receiving chamber was

maintained near atmospheric pressure during measurements, and at

initial instants the membrane samples were under gas feed pressure.

Thereafter, in the calculations where equation related to the time-lag and

the early approximation were necessary, instead of the feed pressure, pf,

it was used the pressure difference between permeate and feed side of

the membrane, TMP.

Because of this, values obtained and discussed in this section, must

be considered preliminary results, and a motivation for future work.

5.1 – Time-lag method

In order to determine the DCO2and KCO2

coefficients by the time-lag

method, the graphical extrapolation of the straight line in the pressure vs time graph Figure 14 which corresponds to the steady state, was

performed to the pressure vs time curve for the PU95 membrane (Figure

14) to obtain the values of the characteristic time, tc, and the

characteristic pressure, pc. Eq. 12 and Eq. 13 were used to determine DCO2

and KCO2, respectively, and the permeability coefficient, 𝑘CO2

, was

obtained from Eq.9.

Results show that the diffusion coefficient were 5.6×10−7cm2s−1 and

the solubility coefficients were 124.7×10−2cmHg 𝑐𝑚𝐶𝑂2

3 cmHg−1𝑐𝑚𝑚𝑒𝑚𝑏3 .

The permeability coefficients obtained were 6.9 ×10−7𝑐𝑚2𝑠−1, that corresponds to 106 Barrer.

5.2 – Early approximation method

According to the early approximation method, Eq.16, Eq. 17 and Eq.

9 were used to determine DCO2, KCO2

and kCO2, respectively, using the

slope and y-intercept. Figure 15 shows the lower times of the ln (√tdp

dt)

vs 1

t plot obtained from the CO2 permeate pressure measurement as a

function of time for the PU95 membrane. The plot points correspond to the early times of Figure 14, for a maximum time of 12 seconds, which is assuredly in the range of the transient section. Results show that the

diffusion coefficient were 96.9×10−7cm2s−1 and the solubility coefficients

were 5.7×10−2cmHg cmCO2

3 cmHg−1cmmemb3 . The permeability coefficients

obtained were 5.6 ×10−7cm2s−1, that corresponds to 85 Barrer.

6 – Conclusions

A novel experimental set-up capable of recording the evolution of

pressure online, in intervals of 1.4 seconds, with milibar precision, at

constant temperature were built and validated. The set-up allows to

perform measurements, at chosen constant temperature, with a duration

up to 90 minutes.

Figure 13 - Average O2 volumetric fluxes 𝐽(10−5 𝑐𝑚3𝑐𝑚−2𝑠−1) as function of TMP (cmHg) of PEU-5-100 (blue), PEU-5-95 (orange),

PEU-5-90 (grey), PEU-5-85 (yellow).

0

1

2

3

4

5

6

7

0 100 200 300 400

J(1

0^-

5 c

m^3

/cm

^2/s

)

TMP(cmHg)

Figure 14 – CO2 permeate pressure (mbar) measurement as a

function of time (s) for the PU95 membrane (CO2 feed pressure of

1.9 bar).

-0.1

-0.05

0

0.05

0.1

0.15

0 25 50 75 100 125 150p

p(c

mH

g)

t(s)

Figure 16 - Average CO2 volumetric fluxes 𝐽(10−5 𝑐𝑚3𝑐𝑚−2𝑠−1) as function of TMP (cmHg) of the PEU-5-100 (blue), PEU-5-95

(orange), PEU-5-90 (grey), PEU-5-85 (yellow).

Table 2 – Average permeance of N2, CO2 and O2, obtained for PEU

membranes.

Membrane 1𝐏𝐞𝐫𝐦 𝐍𝟐 1𝐏𝐞𝐫𝐦 𝐂𝐎𝟐 1𝐏𝐞𝐫𝐦 𝐎𝟐

PEU-5-100 0.003 0.14 0.011

PEU-5-95 0.006 0.11 0.008

PEU-5-90 0.004 0.12 0.010

PEU-5-85 0.004 0.13 0.015

1Perm have units of (10−5 cm3cm−2s−1cmHg−1)

Figure 15 – ln (√tdp

dt) vs

1

t plot obtained from the CO2 permeate

pressure (mbar) measurement as a function of time (s) for the

PU95 membrane (CO2 feed pressure of 1.9 bar).

-6.20

-6.00

-5.80

-5.60

0.05 0.10 0.15 0.20

Ln(d

pp

/dt.

t^0

.5)

1/t (s^-1)

10

For membranes with a range of permeances between 0.003×

10−5𝑐𝑚3𝑐𝑚−2𝑠−1𝑐𝑚𝐻𝑔−1 and 3.6×10−5𝑐𝑚3𝑐𝑚−2𝑠−1𝑐𝑚𝐻𝑔−1, it was

possible to obtain reproducible results, being the measurements

uncertainty lower than the variability associated to the membranes

synthesis.

Poly(ester urethane urea) membranes prepared by a modified version

of the phase inversion technique, by reacting a polyurethane (PUR)

prepolymer with a polycaprolactone (PCL) prepolymer, displayed

asymmetric cross-sectional structures that were tailored upon the

variations of the casting solutions and conditions. The increase of the total

polymer to total solvent weight ratio resulted in more uniform, completely

formed membranes, however, an excess of polymer resulted in less

elastic, very sticky membranes which are difficult to manage. It is

concluded that, to be able to obtain membranes with optimal structural

characteristics there must be a certain balance between the total amount

of polymer and solvent and, for the particular case of the prepolymers

used in this work, the most suitable composition was total polymer/total

solvent weight ratio 1/1.

The integral asymmetric poly(urethane urea) membranes appear to

have a characteristic cross section structure where the expected very thin

active layer and much thicker porous bottom layer are not easily identified.

Instead three distinct regions can be seen: two regions that boarder the

upper (upper denser layer) and bottom surface composed of very small

pores in large number and a thicker region located at the center part of the

membrane composed of larger pores and less numerous.

The increase of the amount of PCL in the PEU membranes improves

the structural characteristics and membranes become whiter, less yellow

and stickier. It was also verified that as the PCL content increases, the

distinction between the smaller-pore regions and the internal large-pore

area become more noticeable and, apparently, the number of pores in the

inner region decreases even more.

The nonporous symmetric membranes, PU100, PU95 and PU85,

prepared with a PCL content of 0, 5 and 15%, respectively, were dense,

translucent, glassy and very sticky. The total membrane thickness of these

membranes was between 107 and 126 μm.

The experimental measurements of the permeation fluxes obtained by

the pressure method were coherent with the ones obtained with the

volumetric method. Furthermore, in the built set-up of pressure method,

the fact of the measurements are made online allows a more efficient and

a quickest obtainment of the results and it is capable to detect and record

very low permeation fluxes, which is a major advantage of the pressure

method over the volumetric method.

The CO2 permeance values obtained for the nonporous symmetric

membranes with 0, 5 and 15% weight ratio of PCL were 0.15×10−5,

0.07×10−5 and 0.19×10−5 cm3cm−2s−1cmHg−1, respectively.

The highest CO2 permeance value (10.4×10−5 cm3cm−2s−1cmHg−1)

was measured for the asymmetric membranes prepared with the lowest

evaporation time (1 minute). The permeance values obtained for the

asymmetric membranes prepared with higher solvent evaporation time

were two orders of magnitude lower: 0.14×10−5, 0.11×10−5 and

0.14×10−5cm3cm−2s−1cmHg−1, for 5, 10 and 15 minutes of solvent

evaporation time, respectively, and these are of the same order of

magnitude as for the nonporous symmetric membranes. Thus, the solvent

evaporation time, above 5 minutes, does not affect the permeability

properties of the membranes.

The CO2, N2 and O2. permeances of the asymmetric membranes

prepared with a solvent evaporation time of 5 minutes and varying content

of PCL, were determined and results showed that: i) the CO2 permeances

were 0.14 × 10−5, 0.11 × 10−5, 0.12 × 10−5, 0.13 × 10−5

cm3cm−2s−1cmHg−1 for the membranes containing 0, 5, 10, 15% wt. of

PCL, respectively; ii) the N2 permeances were 0.003 × 10−5, 0.006 × 10−5,

0.004 × 10−5, 0.004 × 10−5 cm3cm−2s−1cmHg−1 for the membranes with 0,

5, 10, 15% wt. of PCL, respectively; iii) the measured O2 permeances were

0.011 × 10−5, 0.008 × 10−5, 0.010 × 10−5, 0.015 × 10−5

cm3cm−2s−1cmHg−1 for the membranes with 0, 5, 10, 15% wt. of PCL,

respectively. It is therefore concluded that the amount of PCL in these

membranes is not directly correlated to the gas permeability properties.

Results show that the membranes with the higher permeance are the

PEU-5-90 and the PEU-5-85, which were prepared with polymer to solvent

weight ratio of 1/1, a solvent evaporation time of 5 minutes and PCL

content of 10 and 15 wt.%, respectively. Considering the enhanced

hemocompatibility of the membranes containing 15 wt.% of PCL, found in

previous studies ([11], [12]), it is concluded the PEU-5-85 membrane,

prepared with 1/1 of total polymer to total solvent weight ratio, 5 minutes

of solvent time evaporation and 85/15 of PUR/PCL wt. %, is the most

promising membrane to be applied in future MBOs.

In order to be considered for future MBOs, and functioning at a TMP

of 76 cmHg, the PEU-5-85 membrane, must have a surface area of 3.4 m2

to ensure adequate CO2 fluxes. One method of achieving higher gas

fluxes is by tailoring of the PEU-5-85 membrane with a thinner upper

denser layer.

The CO2 permeability obtained for the PU95 membrane by the time-

lag method and by the early approximation method was 105 Barrer and

85 Barrer, respectively, and are coherent with the experimental CO2

permeability obtained from the permeation experiments (94 Barrer).

7 – References

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bypass,” Contin. Educ. Anaesthesia, Crit. Care Pain, vol. 6, no. 5, pp. 176–181, 2006.

[2] D. F. Stamatialis et al., “Medical applications of membranes:

Drug delivery, artificial organs and tissue engineering,” J. Memb. Sci., vol. 308, no. 1–2, pp. 1–34, 2008.

[3] M. Faria, M. Rajagopalan, and M. N. De Pinho, “Tailoring bi-soft

segment poly (ester urethane urea) integral asymmetric membranes for CO2 and O2 permeation,” J. Memb. Sci., vol. 387–388, no. 1, pp. 66–75, 2012.

[4] R. J. Zdrahala and I. J. Zdrahala, “Biomedical Applications of Polyurethanes: A Review of Past Promises, Present Realities, and a Vibrant Future,” J. Biomater. Appl., vol. 14, no. 1, pp. 67–

90, 1999. [5] J. W. Boretos and W. S. Pierce, “Segmented Polyurethane: A

New Elastomer for Biomedical Applications,” Science (80-. )., vol. 158, no. 3807, pp. 1481–1482, 1967.

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