Amination of Poly(ether imide) Membranes Using Di- and Multivalent Amines

Amination of Poly(ether imide) Membranes Using

Di- and Multivalent Amines

This paper is dedicated to Prof. Dieter Paul to honor his life-long contribution to membrane research

Wolfgang Albrecht,*1 Barbara Seifert,1 Thomas Weigel,1 Michael Schossig,1 Andreas Hollander,2 Thomas Groth,1

Roland Hilke1

1GKSS Research Center Geesthacht GmbH, Institute of Chemistry, Kantstr. 55, D-14513 Teltow and Max-Planck-Str.1,D-21502 Geesthacht, GermanyFax: þ49-3328 352452; E-mail: [email protected]

2Fraunhofer Institute of Applied Polymer Research, Geiselbergstr. 69, D-14476 Golm, Germany

Keywords: diamines; functionalization of polymers; membranes; polyimides; polyimines

Introduction

Polyimides (PI) are a group of polymers that are attractive

for membrane formation because of their excellent film-

forming and mechanical properties as well as good thermal

and moderate chemical resistance. Membranes of PIs are

commercially used for gas separation[1–6] and as support

membrane for composite membranes,[7–10] particularly

coatedwith hydrophobic layers such as silicone rubbers. An

application as support membrane for coating with hydro-

philic layers and as ultrafiltration membrane is desirable.

However, the hydrophobic character of PI polymers, similar

to many other membrane-forming polymers, causes an

insufficient wettability inducing incomplete coating and a

high adsorption of solutes onto the membrane in aqueous

systems. The former effect is related to the formation of

pinholes in the coating, the latter with a decline in flux as

consequence of fouling. Addition of hydrophilic modifiers

such as polyvinylpyrrolidone[11–13] or polybenzimida-

zole[14] to the membrane-forming polymer solution is one

way to generate a higherwettability. Another way is surface

modification, a frequently applied technique to make

membrane surface more hydrophilic or to generate cova-

lently bound chemical groups. At this technique, the

membrane surface is e.g. treated with plasma,[15–18] ion

beam etching,[19] UV laser ablation,[20] or graft poly-

Full Paper: Covalently aminated polyimidemembranes areprepared by wet chemistry using different amines as modi-fiers. During this synthesis process carbonyl groups of theimide ring react with amine groups forming amide groupsmaintaining the macromolecular structure and an additionalamide group bearing free amine groups. The reaction se-quence was verified by FTIR-ATR and XPS measurements.Using poly(ethylene imine)s, highly aminated polyimidemembranes with amine contents higher than 500 nmol/cm2

membrane can be produced which can be used as basicfunctions for further modification. As a result of amination,the hydrophobic polyimide membranes were strongly hydro-philized. Depending on the process parameters for functio-nalization and the applied modifier, a symmetric or anasymmetric distribution of amine functions across the mem-brane was observed. The distributions of amine functionswere determined by SEM microscopy (EDX mode) and byAR-XPS (take-off angle resolvedXPS). Calculations suggestthat in the case of an asymmetric distribution the pores of thesurface layer are almost completely filled with a swollennetwork of themodifier, which allows the covalent binding ofsecond modifiers in order to obtain new types of compositemembranes.

510 Macromol. Chem. Phys. 2003, 204, 510–521

Macromol. Chem. Phys. 2003, 204, No. 3 � WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003 1022-1352/2003/0302–510$17.50þ.50/0

merization[21–24] to generate radicals onto the surface

(pretreatment step) or to introduce chemical functions

(functionalization step), which are chemically reactive.

Such types of functionalization are also applied for PIs in

order to increase the wettability,[16] to covalently immobi-

lize poly(ethylene imine)s,[17] and, what is very important

for special applications of PIs, to increase the adhesion

between PI and metals for high performance packages of

high-density wiring in a low dielectric matrix.[15,24,25]

Another strategy of heterogeneous functionalization

involves surface treatment with chemical solutions initiat-

ing surface-selective reactions. This technique ensures

simplicity, low cost operation, and a good implementation

into continuous production cycles. A prerequisite is the

presence of reactive chemical groups. Besides electrophilic

substitution of the aromatic rings, e.g. by sulfonation,[26]

PIs offer the possibility of another reaction type: It has been

known for a long time that imide groups react with bases to

open the imide ring system.[27] This reaction type is

frequently applied using bases such as NaOH, KOH, and

NR4OH to improve metal/PI adhesion.[28,29] In this reac-

tion, the imide ring is opened and an amide and a

carboxylate salt are formed.[30–34] In generalization of this

reaction type, the electrophilic imide group of PIs reacts

with a nucleophilic agent. The same behavior can be expec-

ted if amines are used as nucleophilic agents. However,

choosing a suitable modifier can additively generate

chemical functions. Using this reaction and various amines,

PIs were modified to improve the lamination between PIs

and epoxy resin layers,[35] for cross-linking of PI mem-

branes by heterogeneous phase reactions,[36,37] for increas-

ing polymer solution viscosity,[38] or for compatibilization

of organic/organic and organic/inorganic composites[39,40]

using homogeneous reaction conditions. However, a com-

prehensive discussion of chemical reaction behavior and

possibilities to influence the properties during functionali-

zation was still not investigated.

The subject of this work is the surface functionalization

of poly(ether imide) (PEI) membranes by reaction of imide

groups with amines using wet-chemical technique. In this

investigation, diamines were chosen to study reaction

chemistry characterized by amine content, XPS and FTIR-

ATR spectra. Multivalent amines from ethylene imine type

are applied to create membranes with a high content of

reactive amine functions onto themembrane surface or over

their cross section. Modified membranes are characterized

with respect to the amine functionality, contact angle, and

distribution of amine functions across the membrane.

Experimental Part

Materials

Asymmetric PEI flat membranes with ultrafiltration propertiesprepared on a non-woven support (GMT,Germany) are used as

plainmembrane for amination in heterogeneous phase.Difunc-tional amines like hexamethylene diamines (HDA, code: 1),N,N0-dimethylethylene diamine (code: 2), or N,N,N0,N0-tetramethylethylene diamines (code: 3) and polyvalent amineslike poly(ethylene imine)s (Pei) with different molecularweight (Mw¼ 750 kg/mol, code: 4;Mw¼ 0.8 kg/mol, code: 5)and diethylene triamine (DETA, code: 6) as well as AcidOrange II are purchased by Sigma-Aldrich, Germany; 1-propanol fromMerck-Schuchardt, Germany. All chemicals areapplied as received.

Modification of Membranes

The dry flat membrane was mounted onto a stainless steelcylinder (diameter: 130 mm) and fixed with clamping rings.This device was immersed into a thermostat bath, containingthe modifying solution – the amines solved in a 1:1 mixture of1-propanol and water. Usually, the concentration of modifyingamine was 2 wt.-%. Reactions were carried out at 70 8C and astirring rate of 40 rpm for the time indicated. After quenchingin cool water the membrane was demounted, carefully washedwithwater to remove adsorbedmodifier, and stored inwet stateat 4 8C until use. If drying was necessary (characterizationtechniques) the wet membrane was dried at room temperature.According to the planed experiments the PEI membrane wascontacted with the modifying solution on the active layermembrane side (code:A) or on the support layer side (code: B).

Methods

Determination of Amine Content

An Acid Orange II assay was used for determining content ofamine functions according to Uchida et al.[41] but with vari-ation in the experimental conditions. Briefly, membranesamples of defined size (1.33 cm2) are immersed in a solutionof 500mmol/LAcidOrange II in distilledwater at pH3 (dilutedHCl) at which a protonation of amines is given. After shakingfor 24 h at room temperature the samples are intensely washedwith water at pH 3. The amount of bound dye was quantifiedafter detachment in distilled water at pH 12 (diluted NaOH).The detachment was achieved after 15 min shaking at roomtemperature. After that, the optical density of the solution wasmeasured spectrometrically at a wavelength of 492 nm usingSpectra Fluor Plus spectrometer (Tecan, Germany). Theconcentration of amine groups was quantified with help of acalibration curve and the assumption that one amine group iscomplexed with one equivalent value of Acid Orange II. Lastassumption is only met at low amine content where the aminegroup is freely accessible for the dye molecule. Thereforeusing this technique the amine content can be underdeterminedin comparison to the real amount because of limited acces-sibility at higher amine content.

Contact Angle Measurements

Contact angles were measured to characterize changes in thewettability of the membranes. The captive bubble method wasusedwhere an air bubble is injectedwith a stainless steel needlefrom a syringe onto the inverted membrane surface. Bubbles

Amination of Poly(ether imide) Membranes Using Di- and Multivalent Amines 511

were always larger than 3 mm in diameter. Advancing andreceding contact angle measurements were performed with agoniometer (Carl Zeiss, Germany) by stepwise withdrawing/adding of air from/to the captured bubble. At least tenmeasurements of different bubbles on at least three differentlocations of the membrane were averaged to yield the contactangles and their standard derivation. Generally, the contactangle was measured at the active layer of the membraneindependent on the modification side of the membrane.

FTIR-ATR Measurement

Fourier-transformed infrared spectra of dry membranes in asingle-beammodusweremeasuredwithMagna IRTM Spectro-meter 550 (Nicolet, USA) in the attenuated total reflectionmodus to detect the covalent functionalization of membraneswith amines with respect to the transition of imide into amidegroups and structure elements of the modifier. For datainterpretation, the spectra were normalized to the absorptionintensity at a wave number of 1 230 cm�1 (C–O valence ofether bridge), which should not be influenced by thefunctionalization reaction.

Scanning Electron Microscopy (SEM) Investigations

Morphological investigation of cross section was made using afield-emission scanning electronmicroscope, type JSM6400 F(Joel, Japan) at an acceleration voltage of 15 kV in secondary(SE) and back scattering electrons (BSE) modus. Line scansare carried out to detect copper distribution across themembrane (EDX).To prepare the investigated samples the amine-functiona-

lized membranes were stained with copper[42,43] by dippingthem into an 1 wt.-% solution of CuCl2 for one hour followedby intense rinsing in deionized water and drying at roomtemperature. For cross section investigations, samples of 3mmin diameter were stamped from the stained membrane,mounted to a holder, and cut with the cryo-ultramicrotomeUltracut UCTwith FC-S cryostage unit (Leica, Germany). Thesample sections were mounted on a specimen holder andcoated with carbon using coater EPA 101 (Gatan, Germany).For energy dispersive X-ray analysis back scattered electronswere detected using EDAX analyzer type DX 4 (EDAX, USA)combined with Digital Image System (Point electronic,Germany). The scan was carried out along the membranecross section. Signals were detected for copper and carbon.Ratio of the copper Ka- and the carbon signal intensity wasused to evaluate the copper and therefore the amine distributionacross the functionalized polymer membrane. Because ofstrong variations of the ratio in the practically polymer-freeregion of the scanning area with very low intensity levels thedata were averaged manually to the baseline.

X-Ray Photoelectron Spectroscopy (XPS)

The elemental analysis of the membrane surface and thebinding states were determined by AR-XPS (take-off angleresolved XPS) measurements using a Kratos Axis 165instrument (Kratos Analytical, UK). A monochromatic Al Ka

beam source (1 486.6 eV) was applied. A magnetic lens andelectrostatic lenses were used for focusing emitted electrons

into the analyzer. The signal was averaged over a spot size of0.3 mm� 0.7 mm. The emission was analyzed at differenttake-off angles in order to vary the sampling depth. The surfacecomposition and the binding states (908 relative to thehorizontal, sampling depth: about 7 nm) as well as chemicalstructure of functionalized material at different samplingdepths (take-off angle dependency, sampling depth: less than7 nm) were determined. Generally, the active layer side wasinvestigated by XPS measurement independent from themodification side of the membrane. The N1s peak was fittedwith four peaks of Gaussian shape, which can be attributed toamine (N1s(1) – 399.4 eV), amide (N1s(2) – 399.9 eV), imide(N1s(3) – 400.4 eV), and ammonium (N1s(4) – 401.8 eV)structures. The component positions were calibrated using theimide peak of an unmodified PEI membrane. A semi-quan-titative evaluation was carried out based on the fractional peakareas of total amine (free amine and ammonium) and imide.

Results

Reaction of Amines with PI

As shortly described in the introduction, the electrophilic

imide group of PIs should be reactive towards nucleophilic

substitution at the activated carbonyl group, which offers a

new opportunity for a covalent functionalization of PIs. The

expected general reaction sequence is depicted in Figure 1

for the case of the reaction of PEI with bifunctional

modifying agents bearing a monofunctional amine group.

Amine-containing modifiers are preferred because of their

high nucleophilicity. The nucleophilic agents consists of an

amine group, a second functional group X of different

chemical nature, and a spacer between them. The spacer is

Figure 1. Expected reaction sequence for the functionalizationof poly(ether imide) using bifunctional modifiers bearing amonofunctional amine group.

512 W. Albrecht et al.

defined as any chemical group. However, the whole chemi-

cal structure has to support the nucleophilic character of the

amine group in the reacting modifier. In the investigation

described here, the functional group X was an amine or

imine group. According to this expected reaction type, a

carbonyl group of the imide ring should react with the

amine group of the modifier. One amide group is formed

maintaining the macromolecular structure and a second

amide group arises additionally bearing the functional

group X. According to this reaction sequence a poly-

[(amidic amide)-co-imide] or a poly(amidic amide) is

formed after the conversion of the imide group depending

on the degree of conversion. Principally, the expected

reaction should be implemented if the amine containing the

modifying agents has a primary or a secondary amine group

for amide formation. Furthermore, since the applied amines

havemore than one amine group, all can react with the large

number of imide functions. This multi-reaction can be

connected with cross-linking.

Solubility experiments with PI membranes modified

with amines of ethylene imine type (code 4–6)were carried

out in order to confirm or to exclude the crosslinking

reaction. The membranes were treated 2 h at 80 8C under

weak shaking with N,N-dimethylacetamide (DMA), which

is a typical solvent for PI polymers. It appeared that the

treatment of PI membranes with a high molecular weight

Pei (code 4) resulted in the insoluble membranes in DMA

(which plausibly means the crosslinked ones) whereas the

PI membranes modified with a low molecular weight Pei

(code 5) or DETA (code 6) were in DMA completely

soluble. It can be assumed that the concentration of amine

inside the PI matrix is a dominant factor influencing the

crosslinking. A low concentration of the amine preferably

induces the crosslinking while its high concentration

induces only the functionalization. The PI membrane used

in the experiments has a rejection for the high molecular

weight Pei (code 4) higher than 90% and, therefore, during

the coating with Pei (code 4), only a very small amount of

Pei (code 4) can get into the pores of the membrane (PI

matrix). On the other hand, the molecules of both other

(small) amines can enter the pores of this membrane virtu-

ally without any limitation. In the former case, practically

all amine groups per molecule (which have access to the

pores) react with the imide groups whereas in the latter case

virtually only one amine group per molecule can react

because of a high ratio of amine to imide groups. Heter-

ogeneous functionalizations were used here because of

their simplicity and further investigations on the crosslink-

ing of the PI membrane matrix are under work.

In a first approach, different symmetrically structured

low molecular weight diamines were used for the PI mem-

brane functionalization in order to verify the actual reaction

mechanism. The active layer of the membrane was brought

into contact with the modifying solution (A) and the amine

groups on the membrane surface were determined. The

results depicted in Figure 2 showed that the reactivity of

the diamine was highest if a symmetrically structured

secondary diamine was used for functionalization. Their

higher nucleophilicity in comparison to an analogous

primary diamine seems to be the reason. As expected, the

tertiary diamine does not react with the activated carbonyl

group of PIs. In the latter case, a low and relatively constant

amine content was found which can be discussed as

mechanical anchoring but not as a chemical binding.

In the reaction with amine modifiers, the imide group is

transformed into an amide group and a second amide group

is formed in addition which links to one or more free amine

groups. This alteration of the chemical composition should

be qualitatively detectable in the infrared (IR) spectrum. A

strong reduction of absorption intensity in the carbonyl

stretching bands of imide group, the formation of new

amide carbonyl stretching and of new amine group absorp-

tion bands are expected as well as an increase of absorption

intensity ofC–H-stretching vibration depending on the kind

of amine. However, PIs itself have rich absorption spectra.

Therefore, the investigation was focused on prominent

adsorption peaks, which are expected to change during

modification. HDAwas used as amino modifier because of

its simple spectroscopic absorption bands. Characteristic

IR bands of HDA are located at wave numbers of

1 480 cm�1 (CH2 deformation), 1 595 cm�1 (NH deforma-

tion), 2 865 cm�1, and 2 925 cm�1 (CH2 valence) as well as

3 345 cm�1 (NH valence).

The results of IR investigations are displayed in Figure 3

for two different wave number ranges containing parts of

absorption bands from non-modified (top) and modified

membranes (bottom). The FTIR-ATR spectra of non-

modified PEI showed prominent absorption peaks at

3 068/3 038 cm�1 (C–H valence of the ring system),

Figure 2. Dependency of the amine group content of poly(etherimide) membranes on the time of functionalization using differentsymmetric diamines. A – active layer side functionalization; (1) –hexamethylene diamine; (2) – N,N0-dimethylethylene diamine;(3) – N,N,N0,N0-tetramethylethylene diamine.


2 968 cm�1 (C–H valence of CH3 groups in the backbone),

1 777 and 1 720 cm�1 (carbonyl C O stretching in imide

ring system, imide I-band), 1 356 cm�1 (C–N stretch, imide

II band) and 1 234 cm�1 (C–O valence of ether bridge). In

the spectra of the HDA-modified PEI membranes main

changes are found: In the higher wave number region

around 3 300 cm�1 the vibrations of theN–Hvalence of free

amine groups were found. In addition, a strong increase in

absorption intensity at around 2 925 cm�1 and 2 865 cm�1

from the C–H valence vibration of the HDA spacer part was

generated by functionalization. In thewave numbers region

between 2 000 to 1 200 cm�1 a decrease of the imide bands’

intensity (1 777 and 1 720 cm�1, imide band I) as well as

1 356 cm�1 (imide band II) was observed, in particular. The

deformation vibrations of the CONH group formed by

functionalization arises between 1 660 cm�1 (C O stretch

of CONH group, amide I band) and 1 550 cm�1 (N–H band

of CONH group, amide II band) but their intensity is not

as strong as the decrease in intensity of imide groups.

However, these data verify the reaction of PIs with amino

modifiers resulting in a covalent binding of the amino

modifier to the PI and the formation of amine functionalities.

XPS investigationswere carried out to illustrate the reac-

tion sequence of PIs with amines. The elemental composi-

tions of the surfaces are summarized in Table 1. The

chemical functionalities were analyzed only qualitatively

because with XPS we cannot distinguish between carbon

atoms in amide and in imide groups in C1s spectrum and

only slight shifts were observed in N1s spectra. The

nitrogen atoms in imide groups of an unmodified sample

were used for calibrating the binding energy of this

component (Figure 4). For these investigations, PEI

membranes modified with polyvalent amines were applied

because of their higher amine content (cf. next chapter).

Table 1 shows the measured composition of modified

samples as well as calculated data for unmodified PEI and

for the modifier Pei for comparison. As can be seen, the

chemical composition data for PEI, calculated and mea-

sured, differ only slightly. Significant differences were

found between the unmodified and the modified samples in

the content of carbon and nitrogen but not as dominate as

expected for oxygen. The differences in C and N content

depend on the functionalization conditions. The highest

amine functionalization degree in conjunction with the

highest N-content and lowest C-content was realized using

high molecular weight Pei (4) and support side functiona-

lization (B) that is also in agreement with amine content

determination (cf. next chapter).

For the interpretation of XPSmeasurements with respect

to the binding states, the N (1s) peak was fitted with four

peaks of a Gaussian shape, corresponding to nitrogen atoms

in imide, amide, amine, and ammonium groups. Fitted

spectra are depicted in Figure 4.

The evaluated data of Figure 4 are summarized in Table 2

on basis of area fractions of the components. They should be

evaluated only qualitatively. Adding the total amine group

concentration (free amine and ammonium), the functiona-

lization degree was increased in series of A5<A4<B4

Figure 3. FTIR-ATR spectra of the plain (top in the figures) anda hexamethylene diamine functionalized poly(ether imide)membrane (bottom in the figures).

Table 1. Chemical surface composition of different aminated poly(ether imide) samples.

Sample C O N C/N Impurities

a.-% a.-% a.-% a.-%

PEIunmodified, calculated 78.2 16.9 4.9 15.9 –PEIunmodified, measured 80.8 14.5 4.7 17.2 0PEI, active layer side, low MW Pei (A5) 77.6 14.8 7.1 10.9 0.5PEI, active layer side, highMWPei (A4) 77.4 14.7 7.9 9.8 0PEI, support side, high MW Pei (B4) 76.8 11.4 11.2 6.9 0.6Peicalculated 63.2 0 36.8 1.7 –


which agree with dye adsorption measurements (cf. next

chapter). However, the amide concentration and the reduc-

tion of the imide group content documenting the covalent

bindings were found in the series A5>B4>A4. Both

expected rankings were in agreement with the modifier’s

accessibility to the pore system of the membrane or – in

other words – to the pore area that can be functionalized.

The low molecular weight Pei (5) has the best accessibility

and transforms the highest amount of imide groups into

amide groups (practically complete accessibility). In con-

trast, lowestbinding imide/amidetransformationwasobserv-

ed with lowest accessible pore area, which is realized with

high-molecular Pei modifier (4) and active layer functiona-

lization (A). The ranking with respect to the number of

amine functionalities was influenced by molecular weight

of modifier. With a high molecular weight modifier one

imide/amide transformation multiplies the amine content

of the membrane in contrast to a low molecular weight

modifier where according to the molecular weight only few

amine groups are introduced. Using high molecular weight

modifier, a low imide/amide transformationrate is related toa

high content of amine groups and vice versa.

In summary, the XPS data demonstrate that the modifier

was covalently bound to the PI according to the expected

reaction sequence.

Surface Modification of PEI Membranes UsingPolyvalent Amines of Ethylene Imine Type

According to the discussion in the previous chapter, a high

amine content after a short functionalization time and

minimal chemical alterations of the PI backbone structure

can be expected if polyvalent amines are applied. There-

fore, amines of the ethylene imine type with different

molecular weight were used for the modification. Second-

ary amine groups in these modifiers are responsible for an

efficient functionalization.

The results of amine concentration determination in

dependence on the functionalization time are summarized

in Figure 5. Themembranes were exposed to the modifying

solution on the active layer side (A) or on the support layer

side (B). The results illustrate that membranes with

different but high contents of amine groups can be prepared.

The highest amine concentration was obtained using a high

molecular weight Pei (4) followed by the low molecular

weight Pei (5) and DETA (6). Especially with (4) a relative

constant level of amine groups was reached after a func-

tionalization time as short as 60 s. Since the whole

accessible surface is probably covered with the amine

agents, a further attachment of molecules is hindered by

Figure 4. Gaussian fits of the N1s spectra (the componentsrepresent the following four types of nitrogen atoms: amine, amide,imide, and ammonium) for different aminated poly(ether imide)membranes. Take-off angle: 908; PEI – unmodifiedmembrane;A– active layer side functionalization; B – support sidefunctionalization; (4) – high molecular weight poly(ethyleneimine); (5) – low molecular weight poly(ethylene imine).

Table 2. Area fractions of N1s-components for different aminated poly(ether imide) membranes.

Sample Area of N1s peaks

N1s(1) amine N1s(2) amide N1s(3) imide N1s(4) ammonium N1s(1)þ (4) aminetotal

a.-% a.-% a.-% a.-% a.-%

PEIunmodified 0 0 100 0 0PEI, active side, low MW Pei (A5) 28 19.9 44.1 8.1 36.1PEI, active side, high MW Pei (A4) 16 2 58.8 23.2 39.2PEI, support side, highMWPei (B4) 45.7 14.3 35.4 4.6 50.3


electrostatic repulsion. In the range of this plateau, a content

of amine groups of 500 to 600 nmol/cm2 membrane was

detected relatively independent on the direction of

membrane functionalization. The same independence of

functionalization degree on the direction of membrane

functionalization was observed using DETA as modifying

agents. But a continuous increase of amine group concen-

tration was measured with increasing of the functionaliza-

tion time. Using modifier (5), a difference between the

active layer side (A) and support layer side (B) functiona-

lization was observed although the membrane has no

rejection ability with regard to the modifying agents at a

convective permeation process. Probably, the active layer

hinders the free diffusion of the molecules into the mem-

brane. Therefore, the accessibility of modifier molecules to

the whole membrane pore system will be retarded in

particular to the membrane support layer pores.

The fact that highest functionalization degree was

observed for modifier (4) by support side functionalization

(B) is surprising, since themembrane has a high rejection to

the modifier (larger than 90% at active layer side streaming

mode). Probably, under the applied conditions for support

side functionalization the modifier can diffuse near to or

into the active layer of the asymmetric structured mem-

brane. Due to the asymmetric characteristic of the mem-

brane the fractional specific surface area increases from the

support side to the active layer side. Therefore, a larger

pore area is accessible for modifier at functionalization of

the support side since the active layer is only a small part of

membrane cross section. As a consequence, a higher

amount of amine can be bound and will be detected with

XPS measurements (Table 1) in the case of support side

functionalization (B) in comparison to the active side

functionalization (A). This finding will be discussed further

from the viewpoint of the contact angle measurements

and the distribution of amine groups along the modified

membrane cross section (cf. this and the following chapter).

The contact angle data in dependence on the functiona-

lization time are plotted in Figure 6 for three different

amines that are identical with the modifying agents at the

Figure 5. Content of amine groups at aminated poly(etherimide) membranes in dependence on the functionalization time.A – active layer side functionalization; B – support sidefunctionalization; (4) – high molecular weight poly(ethyleneimine); (5) – low molecular weight poly(ethylene imine); (6) –diethylene triamine.

Figure 6. Contact angle of aminated poly(ether imide) membranes in dependence on thefunctionalization time. A – active layer side functionalization; (4), (5), (6) according toFigure 5.


amine group determination. Top curves are related to the

advancing, bottom curves to the receding contact angle.

The results manifest a considerable decrease of the

contact angle compared with the unmodified sample. As

expected, the functionalization results in more hydrophilic

surface properties. In particular, at short functionalization

times the contact angle decreases strongly to a relative

constant level at longer functionalization times. The stron-

gest decrease was observed for the Pei with the highest

molecular weight (4) from 75 to 388 (advancing angle) andfrom 55 to 228 (receding angle), which is in agreement with

the estimated amine content. Both other imines (5) and (6)

show a comparable behavior, but contact angles after longer

functionalization (plateau level) are higher in comparison

to modifier (4). In general, the amination results in a better

wettability of PEI membranes. Surprising for the high

molecular weight imine (4), the contact angle values are the

same if the membrane was exposed to the modifier solution

on the support side (B) (Figure 7), which is in agreement

with the XPS analysis (Table 1). Since a complete acces-

sibility of modifier molecules (coiled molecules) in the

active layer can be excluded (high rejection), nonbonding

parts of the coupled flexible macromolecular modifier

molecules are probably disentangled during binding to the

membrane. Then their size is reduced and the pore system

of the active layer is more accessible for modification. This

interpretation is supported by the contact angle measure-

ments since the hydrophilic amine groups have the ten-

dency to orientate in the aqueous phase. Consequently,

similar contact angles were found no matter which surface

was exposed to the solution during functionalization.

Distribution of Amine FunctionsAcross the Membrane

The functionalization of a dense film generally results in an

asymmetric distribution of functionalities. In contrast, the

functionalization of a porous membrane can result in a

symmetric or an asymmetric distribution of functional

groups across the membrane. For the verification of these

effects, we investigated in a first approach different modi-

fied and copper stainedmembranes by SEM using SEmode

for morphological investigation, BSE-mode for element

contrast images, and EDX line scans for the detection of

the copper distribution across the membrane. Besides the

direction of functionalization, the molecular weight of the

Pei used as modifying agents was varied in the limits from

practical complete permeation (rejection smaller than 5%)

to a practical complete rejection (rejection larger than

90%). Because of its intensity with respect to the back-

ground (see the X-ray spectrum in Figure 8) the Cu Ka line

was used for data evaluation.

SEM micrographs of the investigated membranes are

shown in Figure 9 (left side). The lines in these figures

represent the scan routes of element distribution measure-

ment. Micrographs demonstrate the structural asymmetry

of membrane morphology. Data evaluation with respect to

the relative copper content is depicted in Figure 9 (right

side) in dependence on the kind of amine (A4, A5) and

functionalization direction (A4, B4). Obviously, a practical

symmetric distribution of copper and, therefore, of the

amine groups was generated if the modifying agents can

easily access to the pore system of the membrane (A5).

Probably, the marginally asymmetric characteristic found

was caused by the structural asymmetry of membrane

morphology according to the above discussion of the pore

area available for functionalization. As a consequence,

asymmetric membrane morphology of the plain PEI mem-

brane induces the minor asymmetric character of copper

distribution. A completely symmetric copper distribution

can be expected only if a membrane with a symmetric

structure is used. In spite of using an asymmetrically

structuredmembrane, a significant differencewas observed

if a high molecular weight modifying agent was used

Figure 7. Contact angle of functionalized poly(ether imide)membranes in dependence on the functionalization time. B –support layer side functionalization using high molecular weightpoly(ethylene imine) (4).

Figure 8. X-ray spectrum of a copper-stained aminated poly-(ether imide) membrane.


(A4, B4). Irrespective of the functionalization direction, the

maximum of relative copper content was found at the

position of the active layer. The intensity of copper signal

for the samples A4 or B4 (both high molecular weight Pei)

wasmuch higher than for sample A5 (lowmolecular weight

modifier) on the one hand. On the other hand, the copper

signal was much higher for support side functionalization

(B4) compared with active side functionalization (A4).

According to the above discussion, the higher modification

degree was caused by a larger pore area accessible for

functionalization.

In addition to SEM mapping, the distribution of amine

functions was evaluated in a second approach using AR-

XPS measurements at different sampling depths (variation

of the take-off angle). Compositional data of the analyzed

membrane surface are plotted in Figure 10, here presented

as ratio of C/N in dependence on the sampling depth. In this

figure, the upper limit of the C/N ratio represents the data

calculated for unmodified PEI. The lower limit is related to

Pei assuming a linear structural configuration and a

molecular weight for the monomer unit of 43 D. Between

both limits the data of the investigated samples are located

with different dependencies of C/N ratio on the sampling

depth. For sample A5, the C/N ratio was found to be nearly

10 and independent of the sampling depth within the

precision of measurement. This result indicates a sym-

Figure 9. SEM micrographs of different aminated poly(ether imide) membranes (left side) and EDX linescan of the same samples in dependence on the position in the membrane cross section (codes according toFigure 5).


metric distribution of amine function in the analyzed

region, which is in agreement with the SEM mapping.

Furthermore, the significant decrease of the C/N ratio for

sample A4 and B4 with smaller sampling depth illustrates

an asymmetric functionalization of the amine groups across

the membrane cross section. The highest content of amine

groups is found in or near to the membrane’s active layer,

which is also in agreement with SEM mapping data.

Moreover, the average C/N ratio represents a parameter for

the content of amine groups in the analyzed membrane

layer: The higher the ratio the lower is the amine content.

The following ranking was found for amine content:

A5<A4<B4. This ranking is in complete agreement with

data of SEM mapping.

The N1s components were evaluated for a symmetrically

(A5) and an asymmetrically functionalized sample (B4) in

dependence on the sampling depth. The spectra and fitted

Gaussian functions for the components are plotted in

Figure 11 as a function of the take-off angle. Besides the

general decrease of intensity with increased take-off angle,

the intensity of the amine peak (N1s(1)) remains relative

constant at symmetric functionalization whereas a strong

increase was observed at asymmetric functionalization.

Both tendencies are come with a marginal or a larger de-

crease of the intensities of the imide peak (Ns1(3)),

respectively. AR-XPS data also prove the expected state

of amine distribution across the membrane according to

SEM investigations as discussed before.

Using the sampling depth dependent N-data of AR-XPS

measurement and the calculated N-data of PEI and Pei, an

average Pei content in the analyzed layer can be estimated

roughly. The results of the calculations are shown in

Figure 12. The data document a concentration of about 9 to

10 wt.-% Pei at a symmetrical functionalization (A5)

independent on the sampling depth, as expected. In the case

of an asymmetrical functionalization, a Pei concentration of

12 to 20 wt.-%was found at a sampling depth of 7 nm.With

a decrease in the sampling depth to 2 nm the content of Pei

Figure 10. Carbon/nitrogen intensity ratios of different ami-nated poly(ether imide) membranes in dependence on thesampling depth in the AR-XPS measurement. PEI – plainpoly(ether imide) membrane; Pei – poly(ethylene imine); A –active layer side functionalization; B – support side functionaliza-tion; (4) – high molecular weight poly(ethylene imine); (5) – lowmolecular weight poly(ethylene imine).

Figure 11. N1s spectra and Gaussian function fits for different aminated poly(etherimide) membranes in dependence on the take-off angle for a symmetric (A5) and anasymmetric functionalized poly(ether imide) membrane. A – active layer sidefunctionalization; B – support side functionalization; (4) – high molecular weightpoly(ethylene imine); (5) – low molecular weight poly(ethylene imine).


increases to20–40wt.-%dependingon the functionalization

conditions. According to ideas on the surface porosity in the

active layer of membranes with ultrafiltration properties,

these data suggest that the pore system in the 2 nm-layer is

completely filled with covalently bound Pei, on the one

hand, and, on the other hand, that the degree of pore filling is

much lower at a higher sampling depth. The filling of the

membrane pore system, therefore, proceeds in a highly

asymmetric manner along the membrane cross section.

According to the presented data it will be expected that

amination of PI membranes by di- or polyvalent amines is

connected with changes in the separation properties

depending on the conditions of functionalization. Data will

not be reported here in detail because a further publication

on the separation properties of modified membranes is

under preparation. However, for readers information, some

changes in separation properties will be briefly discussed

using results of water permeability. The different modifiers

of ethylene imine type cause the following effects in the

separation profile related to modification conditions: First,

amination using a modifier like DETA is connected with a

pore opening effect by polymer degradation of membrane

active layer. Water permeability will be strongly increased.

Second, amination using lowmolecular weight Pei causes a

preferable functionalization effect. Water permeability will

be relatively constant independent on functionalization

time. Third, amination using high molecular weight Pei is

connected with the described pore filling effect. Water

permeability is strongly reduced.

Conclusions

In wet-chemical functionalization of PIs using di- and

polyamine as modifiers the modifier will be bound cova-

lently to the PI. According to this reaction, the carbonyl

group of the imide ring reacts with the nucleophilic amine

group of the modifier forming one amide group, maintain-

ing the macromolecular structure and generating an

additional second amide group bearing the amine group

or amine groups. Our investigations verified this reaction

mechanism by chemical techniques, by FTIR-ATR mea-

surements as well as by XPS data.

A degree of amination higher than 500 nmol/cm2

membrane can be obtained using high molecular weight

modifiers. The modification results in a strong reduction of

the contact angle of the modified membrane not depending

on the functionalization direction (active layer side (A) or

support layer side (B)) of the membrane. This fact should

allow further functionalization steps to bind other chemical

functions covalently. According to the conditions of func-

tionalization, the distribution of amine functions across the

membrane can be varied from a nearly symmetric to a

strongly asymmetric scheme as shown by SEMmicroscopy

(EDX-mode) and by AR-XPS. The asymmetric functiona-

lization can permit a complete filling of the pores in the very

thin layer.

In conclusion of all, asymmetrically functionalized

membranes with modifier-filled pores, where the modifier

is covalently bound to themembrane support, canbe applied

e.g. as basic membrane for new kinds of composite mem-

branes. Since the covalent binding of the composite layer to

the support their mechanical properties and stability should

be significantly improved which should allow the applica-

tion of new type composite membrane under harsh condi-

tions. Since their separating layer is very thin the separation

performance should be very high. PI membranes

with preferable functionalization effect should allow the

development of modified surfaces, e.g. with increased

biocompatibility in a simple process as is shown in a

first investigation recently.[44] At last, pore opening

simultaneous to functionalization can be the starting point

for a simple new route to develop functionalized mem-

branes with amicrofilter separation profile from ultrafilters.

Acknowledgement: Financial founding of this research waspartially supported by the German Government (BMBF project03N4026B). The authors thankDoris Micheli,Heike Schmidt, andMarion Aderhold for their excellent assistance.

Received: May 22, 2002Revised: October 9, 2002

Accepted: November 12, 2002

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Figure 12. Content of poly(ethylene imine) of different ami-nated poly(ether imide) membranes in dependence on thesampling depth. A – active layer side functionalization; B –support side functionalization; (4) – high molecular weightpoly(ethylene imine); (5) – low molecular weight poly(ethyleneimine).


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Amination of Poly(ether imide) Membranes Using Di- and Multivalent Amines