EUROPEAN

POLYMERJOURNAL

European Polymer Journal 41 (2005) 1234–1242

www.elsevier.com/locate/europolj

Surface characterization of macroporous glycidylmethacrylate based copolymers by inverse gas chromatography

A.B. Nastasovic a,*, A.E. Onjia b, S.K. Milonjic b, S.M. Jovanovic c

a Institute for Chemistry, Technology and Metallurgy, Center for Chemistry, Studentski trg 12–16, 11001 Belgrade,

Serbia and Montenegrob Vinca Institute of Nuclear Sciences, Chemical Dynamics Laboratory, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro

c Faculty of Technology and Metallurgy, Karnegijeva 4, 11001 Belgrade, Serbia and Montenegro

Received 21 October 2004; received in revised form 21 December 2004; accepted 22 December 2004

Available online 26 January 2005

Abstract

Two samples of macroporous crosslinked poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate), PGME,

with different porosity parameters were synthesized by suspension copolymerization and modified by ring-opening reac-

tion of the pendant epoxy groups with ethylene diamine, EDA. Inverse gas chromatography at infinite dilution was

used for the determination of adsorption properties of PGME, and copolymer modified with ethylene diamine,

PGME-en. Thermodynamic parameters of adsorption, dispersive components of the surface free energies, and the

acid/base constants for the copolymer samples were calculated. The calculated dispersive surface energy values, cdS,for PGME and PGME-en are comparable with the literature data for nonconductive polymers.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Thermodynamic parameters; Adsorption; Inverse gas chromatography; Glycidyl methacrylate based copolymers

1. Introduction

Inverse gas chromatography (IGC) has been used for

investigations of the structure of polymers, interactions

of various liquids and gases with polymeric materials,

and polymer–polymer miscibility [1–17]. The method

being simple, relatively rapid, with good accuracy, low

cost and available equipment provides valuable ther-

modynamic information for physico-chemical character-

ization of polymers. The analysis of the adsorption

0014-3057/$ - see front matter � 2005 Elsevier Ltd. All rights reserv

doi:10.1016/j.eurpolymj.2004.12.020

* Corresponding author. Tel./fax: +38 111 635 839.

E-mail address: anastaso@helix.chem.bg.ac.yu (A.B.

Nastasovic).

properties of macroporous crosslinked poly(glycidyl

methacrylate-co-ethylene glycol dimethacrylate), [abbre-

viated PGME] and copolymer with attached ethylene

diamine, [abbreviated PGME-en] is particularly interest-

ing, bearing in mind their use in biotechnological and

biomedical applications, heavy and platinum metal

sorption, catalysis, etc. [18–23]. Yet, only a few data

regarding surface properties of PGME were published

[24]. However, no data have been found in the literature

on PGME-en surface properties, in terms of the Lewis

acid/base properties and surface free energy. This paper

reports investigations of the surface properties of

PGME and PGME-en by IGC at infinite dilution.

Retentions of ten organic compounds of different chem-

ical nature and polarity (non-polar, donor or acceptor)

ed.

A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242 1235

were measured in the temperature range 333–363 K. The

thermodynamic parameters of adsorption (enthalpy and

entropy), the dispersive and specific interactions of var-

ious organics, as well as the KD and KA values which

describe the ability of a polymer surface to act as elec-

tron acceptor or donor, were calculated for the initial

and the modified macroporous copolymer samples and

compared.

2. Experimental

Two macroporous PGME samples with different

porosity parameters were synthesized by a radical sus-

pension copolymerization [25]. The monomer phase

(80.9 g) containing a monomer mixture (24.2 g GMA

and 10.3 g EGDMA), azobisisobytironitrile (AIBN) as

an initiator (0.8 g), and 45.6 g of inert component (cyclo-

hexanol and tetradecanol) was suspended in the aqueous

phase consisting of 237.6 g of water and 2.4 g of poly(N-

vinyl pyrrolidone), Kollidone 90. The copolymerization

was carried out at 70 �C for 2 h and at 80 �C for 6 h at a

stirring rate of 200 rpm. After completion of the reac-

tion, the copolymer particles were washed with water

and ethanol, kept in ethanol for 12 h and dried in vac-

uum at 40 �C. The synthesized samples were purified

by the Soxhlet extraction with ethanol. The samples pre-

pared in this way were labeled as 1 (10 wt% of dodecanol

in the inert component) and 2 (20 wt% of dodecanol in

the inert component). For further investigations, a frac-

tion with particle size of 150–500 lm was used.

Synthesized copolymer samples were modified with

tenfold excess of ethylene diamine in toluene at 70 �Cfor 7 h. Modified samples were filtered, washed with eth-

anol, dried, and labeled as 1a and 2a.

Specific surface areas of obtained copolymers were

determined by the BET method from the low-tempera-

ture nitrogen adsorption isotherms obtained at 77 K

using a high-vacuum volumetric apparatus. The samples

were degassed at 373 K and 1 mPa for 2 h. The values

found for PGME samples 1 and 2 were 46.7 and

27.6 m2/g, respectively. The specific surface areas of

PGME-en samples 1a and 2a were 57.6 and 48.6 m2/g,

respectively.

Adsorption properties of the prepared copolymer

samples were examined using a Hewlett Packard 5890

Series II gas chromatograph, with a flame ionization

detector (FID) maintained at 250 �C. The detector oper-ated in its maximum sensitivity mode and Varian Star

4.5 software was employed to collect the data. To ensure

flash vaporization of the adsorbate vapor, the injector

was set at 220 �C. Dry nitrogen at a flow rate in the

range from 24 to 28 cm3/min was used as the carrier

gas, while methane as an unretained compound, was

used to determine the dead volume. The column flow

was controlled by a soap-bubble flowmeter and the out-

let pressure, assumed to be atmospheric, measured by a

precision barometer. Appropriate corrections to the

pressure drop through the column were made on the

basis of the pressure at the inlet and outlet ports.

A stainless steel column, 50 cm long and 2.2 mm i.d.,

was cleaned with both polar (C2H5OH) and nonpolar

(C6H14) solvents prior to packing. After each packing,

the column was conditioned overnight in a stream of

nitrogen (flow rate 5 cm3/min) at 363 K.

The organic adsorbates (obtained from various com-

mercial sources) were of analytical grade and injected

into the chromatograph by means of a 1 ll Hamilton

microsyringe. Each adsorbate was injected at least three

times and retention times were averaged. These mean

retention times were used in further calculations of the

net retention volumes. Retention times for each adsor-

bate with all copolymer samples were measured in the

temperature range from 333 to 363 K, at 10 K interval.

To ensure a valid approximation of zero coverage, min-

imum detectable amounts of adsorbate vapor were in-

jected. Relevant IGC results from the retention data

collected were calculated by a program developed for

the purpose [26].

3. Results and discussion

The sorption properties of macroporous copolymer

samples were investigated at infinite dilution conditions

(Henry�s law region), where lateral interactions between

the molecules adsorbed at the surface can be neglected.

Well-defined Gaussian peaks for the same retention

volumes of repeated injections of various small quanti-

ties of adsorbate vapor indicate that the measurements

were performed at the linear part of the adsorbate

isotherms.

The theory relating IGC data to the surface proper-

ties of solids is given in details elsewhere [27,28]. In

IGC at infinite dilution, the thermodynamic functions

of adsorption of organic compounds onto an adsorbent,

packed into a chromatographic column, may be calcu-

lated from the adsorbate net retention volume, VN, as

follows:

V N ¼ ðtR � t0ÞF f

TT f

� �p0 � pW

p0

� �j; ð1Þ

where tR is the adsorbate retention time, t0 is the reten-

tion time of an unretained compound (in our case meth-

ane), Ff, is the flow rate measured with a soap bubble

flowmeter at temperature Tf, T is the column tempera-

ture, p0 is the column outlet pressure (taken as baromet-

ric), pW is the vapor pressure of water at Tf, and j is the

James–Martin gas compressibility correction factor.

The surface partition coefficient, KS, defined as the

net retention volume per unit of the adsorbent surface

area, was calculated as:

1236 A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242

KS ¼V N

mSa

; ð2Þ

where Sa, is the specific surface area of polymer and m is

the mass of polymer in the column.

Table 1 presents the surface partition coefficients, KS,

of selected adsorbates for the copolymer samples at all

selected temperatures.

As expected, the log of partition coefficients of the n-

alkane homologue series linearly increases with increas-

Table 1

Partition coefficients, KS (cm3/m), for test adsorbates on PGME and

Adsorbate T (K) KS (cm3/m2)

1

n-Pentane 333 0.753

343 0.533

353 0.413

363 0.294

n-Hexane 333 1.181

343 0.867

353 0.761

363 0.594

n-Heptane 333 3.373

343 2.203

353 1.606

363 1.151

n-Octane 333 8.355

343 4.852

353 3.601

363 3.110

Chloroform 333 8.254

343 4.691

353 2.804

363 1.788

Ethyl acetate 333 33.19

343 16.65

353 8.703

363 4.928

Diethyl ether 333 4.221

343 2.264

353 1.485

363 0.900

Tetrahydrofuran 333 16.16

343 9.260

353 5.387

363 3.324

Cyclohexane 333 0.676

343 0.490

353 0.388

363 0.331

Benzene 333 25.88

343 14.46

353 8.405

363 5.015

ing chain length. This trend is valid for all temperatures

and all polymer samples.

The shortening of retention of all adsorbates was ob-

served for modified samples (1a and 2a). Chemical mod-

ification of PGME with ethylene diamine leads to

different chromatographic behavior of the modified

and the initial copolymer samples. Namely, in addition

to ester groups present in all samples, PGME-en samples

contain ANH, ANH2 and AOH groups which are a

PGME-en samples at indicated temperatures

2 1a 2a

0.457 0.082 0.441

0.430 0.053 0.348

0.415 0.041 0.312

0.398 0.024 0.274

1.521 0.237 1.020

1.365 0.153 0.764

1.272 0.103 0.607

1.167 0.067 0.515

5.828 0.648 2.679

5.237 0.407 1.860

4.770 0.259 1.348

4.358 0.172 1.059

20.10 1.832 7.129

14.68 1.088 4.687

11.42 0.648 3.140

8.927 0.416 2.315

24.26 2.954 17.32

14.59 2.43 15.78

8.854 1.647 11.09

5.563 1.176 8.822

96.28 8.666 59.91

51.98 5.605 33.54

28.93 3.052 18.96

16.28 1.736 11.05

9.643 0.257 1.290

6.525 0.206 1.255

4.33 0.173 1.229

2.993 0.139 1.211

43.38 1.281 7.398

27.95 1.093 7.340

17.21 0.875 7.139

10.68 0.743 6.961

1.618 0.216 0.964

1.224 0.137 0.764

0.972 0.103 0.563

0.861 0.061 0.484

72.34 1.729 6.987

44.90 1.115 6.403

27.57 0.805 6.310

16.34 0.635 6.053

2

3

4

1 n-C5

n-C6

n-C7

n-C8

A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242 1237

result of modification with EDA. Moreover, for macro-

porous copolymers, an additional explanation was sug-

gested in the literature. Shorten n-alkane retention

times for macroporous PGME modified with ethanol

amine compared with those for the initial copolymer,

Lukas et al. [29] explain by the change in internal struc-

ture, i.e. pore distribution, caused by chemical modifica-

tion. The fact that our values of specific surface area for

modified samples are higher than those for the corre-

sponding initial ones, substantiates our assumption that

internal structure actually changes due to modification

with EDA. Different chromatographic behavior of

poly(methyl methacrylate-co-ethylene glycol dimethac-

rylate), PMME, and PGME was observed by Onjia et

al. [24]. The higher KS values for PMME for all adsor-

bates, with the exception of methanol and phenol, they

explained by the presence of epoxy groups in PGME,

which made it more polar than PMME.

Longer retention of benzene in comparison with that

of n-hexane, observed for all samples, can be ascribed to

the contribution of three p-bonds in the benzene ring,

which are capable of interacting specifically with the

polymer surfaces.

In gas/solid partition chromatography (GSC), lower

KS values for c-C6H12 than for the corresponding n-al-

kane (n-C6H14) are commonly observed [26]. For geo-

metric reasons, the six C-atoms can simultaneously

approach the plane in the case of n-C6H14, whereas only

three or four of them in the case of c-C6H12. This is a

consequence of the existence of both ‘‘boat’’ and ‘‘chair’’

isomers of cyclohexane causing a steric effect on

adsorption.

2.75 2.80 2.85 2.90 2.95 3.00 3.05

-1

0

1lnK

s

T-1 103, K-1

CHCl3

EtAcDEETHFc-C

6

Benzene

2.75 2.80 2.85 2.90 2.95 3.00 3.05-4

-3

-2

-1

0

1

2

31a

lnK

s

T-1 10

3, K

-1

n-C5

n-C6

n-C7

n-C8

CHCl3

EtAcDEETHFc-C6

Benzene

(a)

(b)

Fig. 1. Plots of lnKS vs. 103/T for indicated adsorbates with

samples 1 and 1a.

3.1. Enthalpy and entropy of adsorption

The thermodynamic data describing adsorption can

be derived from the partition coefficients and their tem-

perature dependence. The standard free energy change

of adsorption, DG0a, was calculated from the relation

[30,31]:

DG0a ¼ �RT ln

KSpS;gPS

� �ð3Þ

or

DG0a ¼ �RT ln V N þ C; ð4Þ

where R is the gas constant, T is the column tempera-

ture, pS,g is the adsorbate standard state vapor pressure,

and PS is the two-dimensional standard-state (surface)

spreading pressure of the adsorbed gas. The standard

reference state is taken as pS,g = 101 kN/m2 (101 kPa)

and PS = 0.338 mN/m [32]. The constant C takes into

account the mass and specific area of the adsorbent in

the column and the standard states of adsorbates in

the mobile and adsorbed states [33].

The differential heats of adsorption of the adsorbates,

qd, at zero coverage, i.e. the standard-state enthalpies of

adsorption, �DH 0a, were obtained from the temperature

dependence of KS, viz.

dðlnKSÞdð1=T Þ ¼ �DH 0

a

R: ð5Þ

Plots of lnKS vs. 1/T of the used compounds with

copolymer samples are shown in Figs. 1 and 2. The val-

ues of �DH 0a, calculated from the least-squares determi-

nation of the corresponding slopes, according to Eq. (5),

are compiled in Table 2. The heats of vaporization,

�Hvap, for the investigated adsorbates (taken from

Ref. [34]) are also presented for comparison. For all

copolymer samples �DH 0a of n-alkanes increases linearly

with the number of carbon atoms. The more negative

�DH 0a, the greater the interaction between the adsorbate

and adsorbent. The slope of the linear function �DH 0a

vs. number of carbon atoms (for n-alkanes) may be ta-

ken to represent an increment in the enthalpy of adsorp-

tion contributed by each CH2 group.

The corresponding standard-state entropy changes of

adsorption, �DS0a, were calculated from:

DS0a ¼

DH 0a � DG0

a

T: ð6Þ

Table 2

DH 0a (kJ/mol) and DHvap (kJ/mol) values for listed adsorbates

with PGME and PGME-en samples

Adsorbate �DH0a (kJ/mol) �DHvap

(kJ/mol)

1 2 1a 2a

n-Pentane 30.96 4.50 39.79 15.71 26.75

n-Hexane 22.50 8.56 42.12 22.93 31.73

n-Heptane 35.97 9.49 44.56 30.94 36.66

n-Octane 34.64 26.64 49.95 37.41 41.53

Chloroform 51.56 48.30 30.59 22.17 31.40

Ethyl acetate 64.23 58.12 53.47 55.38 35.69

Diethyl ether 51.40 38.38 20.39 2.07 27.37

Tetrahydrofuran 53.15 45.68 18.43 1.90 32.16

Cyclohexane 24.54 21.46 40.87 23.35 29.97

Benzene 54.96 48.21 34.31 4.62 30.72

2.75 2.80 2.85 2.90 2.95 3.00 3.05-1

0

1

2

3

4 2

lnK

s

T-1 103, K-1

n-C5

n-C6

n-C7

n-C8

CHCl3

EtAcDEETHFc-C6

Benzene

2.75 2.80 2.85 2.90 2.95 3.00 3.05

-1

0

1

2

3

4

5

2a

lnK

s

T-1 103, K-1

n-C5

n-C6

n-C7

n-C8

CHCl3

EtAcDEETHFc-C6

Benzene

Fig. 2. Plots of lnKS vs. 103/T for indicated adsorbates with

samples 2 and 2a.

1238 A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242

The averaged entropy values, hDS0ai, with stan-

dard deviation (n = 4) of investigated adsorbates in

the temperature range 333–363 K are shown in

Table 3.

3.2. Dispersive component of the surface free energy

The dispersive surface energy, cdS, is a very important

parameter that describes the material surface to establish

non-polar interactions with other substances. The sur-

face characteristics significantly influence its adhesive

properties, wettability, coating ability, permeability, cor-

rosive properties and biocompatibility [35]. Intermolecu-

lar interactions in the adsorbate/adsorbent system may

be dispersive and specific, which corresponds to the dis-

persive, cdS, and the specific, csS, component of free sur-

face energy, cS, of the adsorbent [36]:

cS ¼ cdS þ csS: ð7Þ

For a given adsorbate, the standard free energy

change of adsorption (DG0a) is the sum of energies of

adsorption attributed to dispersive and specific interac-

tions. Adsorption of non-polar adsorbates, such as n-

alkanes, is caused by dispersive interactions, whereas

for polar adsorbates both London and acid-base interac-

tions contribute to DG0a:

DG0a ¼ DGd

a þ DGsa; ð8Þ

where DGda and DGs

a are the dispersive and the specific

component of the standard free energy change of

adsorption, respectively.

For n-alkanes DG0a ¼ DGd

a and changes with the num-

ber of carbon atoms in the molecules. The increment of

adsorption energy corresponding to the methylene

group may be calculated from:

DGCH2¼ �RT ln

V N;n

V N;nþ1

� �; ð9Þ

where VN,n and VN,n+1 are the net retention volumes of

two consecutive n-alkanes having n and (n + 1) carbon

atoms, respectively. This parameter is independent of

the chosen state of the adsorbed molecule.

The dependence of DG0a on the number of carbon

atoms of n-alkanes at 333 and 363 K for samples 1

and 1a is shown in Fig. 3. Similar plots were obtained

for other temperatures, but for the sake of brevity these

diagrams are not presented in this paper.

The slopes of linear functions given in Fig. 3 repre-

sent the increment in DGCH2. Table 4. reports

the DG0CH2

values for the n-alkane series adsorbed on

investigated copolymer samples, respectively, at various

temperatures. The �DG0CH2

values increase with increas-

ing number of carbon atoms of n-alkanes, and de-

creasing temperature. Also, calculated �DG0CH2

values

for PGME and PGME-en vary in the range 2.1–

3.5 mJ/m2 and are comparable with published data. On-

jia et al. [24] found �DG0CH2

values of 2.76–3.05 mJ/m2

and 2.85–2.94 mJ/m2, in the temperature range 338–

353 K for PMME and PMME, respectively. The re-

ported �G0CH2

value for PMMA is 2.68 mJ/m2 (at

298 K) [37].

Dorris and Gray used the incremental amount of free

energy of adsorption corresponding to the adsorption of

one methylene group to determine the dispersive compo-

nent of surface free energy [36]:

Table 3

Averaged entropy values (in kJ/mol) with standard deviation (n = 4) of investigated adsorbates in the temperature range 333–363 K

Adsorbate 1 2 1a 2a

h�DS0ai SD h�DS0ai SD h�DS0ai SD h�DS0ai SD

n-Pentane 47.9 0.18 27.3 0.05 92.8 0.49 6.81 0.24

n-Hexane 18.9 0.29 25.1 0.06 91.0 0.12 21.5 0.20

n-Heptane 50.6 0.17 33.5 0.04 90.0 0.03 37.5 0.19

n-Octane 39.4 0.81 7.8 0.11 97.6 0.08 48.8 0.19

Chloroform 90.0 0.12 71.2 0.19 35.1 0.47 4.8 0.56

Ethyl acetate 116.4 0.11 89.3 0.24 94.9 0.48 85.0 0.23

Diethyl ether 95.2 0.31 49.1 0.19 25.1 0.10 43.3 0.02

Tetrahydrofuran 89.1 0.05 58.3 0.32 5.81 0.13 58.4 0.06

Cyclohexane 29.7 0.31 13.3 0.28 88.0 0.43 23.1 0.26

Benzene 90.4 0.01 61.8 0.33 51.3 0.36 49.6 0.13

5 6 7 85

10

15

20

25

Sample 1333 K363 K

Sample 1a333 K363 K-∆

Ga0 ,k

J/m

ol

nC

Fig. 3. Variation of DG0a vs. number of carbon atoms of n-

alkanes at 333 and 363 K on samples 1 and 1a.

A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242 1239

cdS ¼DG2

CH2

4cCH2N 2

AaCH2

; ð10Þ

where cCH2is the surface free energy of a solid contain-

ing only methylene groups such as polyethylene

(cCH2= 36.8 � 0.058T (�C) mJ/m2), NA is Avogadro�s

number, and aCH2is the cross-sectional area of an ad-

sorbed CH2 group (0.06 nm2).

The calculated cdS values are reported in Table 4. For

all samples, the cdS values gradually decrease with

increasing temperature, which is consistent with the

literature data [35]. The calculated dispersive surface

energy values (with exception of sample 2) are compara-

ble to those found in literature for non-conductive

Table 4

The DGCH2(kJ/mol) and cdS (mJ�1) values for investigated copolymer

T (K) 1 2

�DGCH2cdS �DGCH2

cdS

333 2.29 30.14 2.86 46.91

343 2.15 27.19 2.87 48.11

353 2.12 26.91 2.71 43.88

363 2.34 28.12 2.87 42.17

polymers. For example, the cdS values of analogous

poly(dimethacrylate)s differing by the presence and the

type of the heteroatom in the ester group are relatively

low and lie in the range 24.8–39.8 mJ/m2 at 323 K, while

for PMMA, the cdS value at 333 K is 35.9 mJ/m2 [35].

Surprisingly, we obtained considerably higher values in

the whole temperature range for PGME sample 2. At

333 K, the cdS value for sample 2 is 71.03 mJ/m2, which

is comparable to that of 80 mJ/m2 reported by Papirer

et al. [38] for silica, or values for conducting polypyr-

roles in the range 30–60 mJ/m2 at 323 K, found by

Chehimi et al. [39]. Generally, IGC method provides

cdS values for polymers, fillers and fibers over a wide

range of temperatures, which is advantageous over, for

example, contact angle measurements. Nevertheless, in

some cases, IGC yields the dispersive surface energy val-

ues higher than other techniques, like for heterogeneous

high-energy surfaces, microporous and lamellar materi-

als, in the presence of impurities, etc. Bearing in mind

that sample 2 has the lowest SS value of all samples,

the overestimated cdS value might be attributed to differ-

ent internal structure and possible microporosity.

The dispersive surface energy differs even for the

polymers of similar chemical composition. Onjia et al.

[24] reported cdS values of 50.1 mJ/m2 for PGME and

53.9 mJ/m2 for PMME at 338 K. The lower cdS value

found for PGME was explained by the enhanced polar-

ity, caused by the epoxy groups, which were not present

in PMME. It is consistent with the literature data, i.e.

samples

1a 2a

�DGCH2cdS �D GCH2

cdS

3.51 71.03 2.58 38.24

3.40 67.86 2.48 35.99

3.34 65.14 2.27 30.60

3.21 62.70 2.15 28.10

300 320 340 360 380 400

15

20

25

Tb, K

-∆G

a0 ,kJm

ol-1

1an-AlkanesCHCl

3

EtAcDEETHFBenzene

Fig. 4. Plots of �DGsa versus Tb for sample 1a.

0 10 20 30 40

0

5

10

15

20

25

30

351

2a

1a

2

THF

DEE

EtAc

CHCl3

-∆HaS /A

N*

DN/AN*

Fig. 5. Plots of �DH sa=AN

� versus DN/AN* for PGME and

PGME-en samples.

Table 5

Acid/base constants PGME and PGME-en samples

Copolymer sample KD KA KD/KA

1 1.25 0.61 2.04

2 1.41 0.68 2.06

1a 2.67 0.30 8.95

2a 1.07 0.07 14.58

1240 A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242

that the increase in the polar functional groups reduces

the values of cdS [35]. However, based on our results,

an unambiguous conclusion regarding the influence of

the chemical modification of PGME with ethylenedia-

mine on the dispersive surface energy cannot be drawn.

However, if we rule out data for sample 2, chemical

modification of sample 1 leads to the increase of cdS val-

ues in the sample 1a. Voelkel [35] also observed higher cdSvalue for the polymer with a nitrogen atom in the ester

group, poly(N-methyl diethanolamine dimethacrylate),

PNDM (38.9 mJ/m2) compared with analogous poly-

(dimethacrylate)s.

3.3. Acid/base interactions

A comprehensive insight in the Lewis acid/base sur-

face interactions provides better understanding of the

influence of the physical and mechanical properties of

the polymers, as well as their ability to change via chem-

ical modifications, which is of great importance for their

applications [36]. The presence of acid and base sites on

the polymer surface enhances the possibility of specific

intermolecular interactions with solvents, plasticizers,

other polymers or fillers, which is of significance to the

composite or blend performances [39]. According to

Brookman and Sawyer, the specific standard free energy

change of adsorption, DGsa, corresponding to the acid-

base surface interactions, may be described as the differ-

ence between the point of the specific polar adsorbate

and the reference n-alkane having the same boiling

point, Tb [40]. Consequently, VN data are related to

DGsa by the following equation:

DGsa ¼ RT ln

V N

V refN

� �: ð11Þ

For the sake of clarity, �DGsa vs. Tb plots for only

one copolymer sample are presented (Fig. 4).

The DGsa values for the polar probes located above

the n-alkane line, indicate that the surfaces have both

donor and acceptor character resulting from their spe-

cific interactions with both the strong base (THF) and

the strong acid (CHCl3).

Following the Saint Flour and Papirer approach, the

base constant, KD, and the acid constant, KA, of the

copolymer may be obtained as a slope and intercept

from the relationship [41]:

�DH sa

AN � ¼ DNAN � KA þ KD; ð12Þ

where AN* and DN are Gutmann�s electron-acceptor

and electron-donor numbers of the adsorbates [42].

Fig. 5 shows the dependencies of �DH sa=AN

� on DN/

AN*, plotted for all copolymer samples. The calculated

KD and KA constants are summarized in Table 5.

The overall acid/base character of PGME and

PGME-en can be evaluated from the KD/KA ratio.

Namely, for KD/KA > 1, the surface is considered to be

basic, and for KD/KA > 1 acidic. Consequently, as seen

from the data in Table 4, the surface of PGME (samples

1 and 2) exhibits a basic character. Furthermore, the

resulting KD/KA values for PGME-en (samples 1a and

2a) imply that copolymer modified with ethylene dia-

mine has a much more pronounced basic character.

The obtained results are in agreement with the literature

data, according to which the introduction of amino,

ether or sulfo groups into the polymer leads to a sig-

nificant increase in surface affinity for nucleophilic inter-

actions [6]. Dieckmann et al. [43] also reported

exceptionally high KD/KA values of 20.7 and 17.2 for

A.B. Nastasovic et al. / European Polymer Journal 41 (2005) 1234–1242 1241

polyester and polyether with pyridine groups, respec-

tively. Also, our results agree very well with expectation

from the analysis of the repeating units in the PGME

and PGME-en. The basic sites identified in PGME are

ester functionality in the side chain and epoxy group.

Weak Lewis acidic sites are localized on the ACH3

groups and on H atoms in the AOACH2 groups. On

the other hand, the basic sites in PGME-en are ester

functionality in the side chain, O atom in the hydroxyl

group as well as N atom in the primary and secondary

group. The weak Lewis acidic sites in PGME-en are also

localized on the ACH3 groups and on H atoms in the

AOACH2 groups. Interestingly, though 1a and 2a have

the same chemical structure, the KD/KA values for

PGME-en samples differ noticeably. Namely, for the

assessment of the acid/base character of the polymer

surface, besides KD and KA values for polymer and

adsorbate, numerous factors must be taken into consid-

eration, such as accessibility of the acidic and basic sites

in the polymer, due to steric hindrance, molecular con-

formation and morphology [17]. For example, Hamieh

et al. [17] showed an important effect of the PMMA tac-

ticity on the acid/base properties, i.e. basic character of

PMMA decreased in the order atactic > syndiotac-

tic > isotactic. A considerable change in surface area is

noticed for sample 2a (27.6 m2/g), compared with the

initial one, and sample 2 (48.6 m2/g), suggesting a

change of the internal structure during modification.

Therefore, in the case of crosslinked macroporous

copolymers, porosity parameters, i.e., specific surface

area and pore diameter also influence the degree of mod-

ification reactions, accessibility of the acidic and basic

sites, and consequently, the Lewis acid/base properties

of the copolymer.

IGC at infinite dilution was widely studied to get a

better understanding of the influence of the Lewis acid-

ity-basicity on the adhesion, rheological properties, as

well as the mechanical properties of polymers [14].

The importance of the method is that it allows the

examination of different porous materials, which surface

properties can hardly be determined by other methods,

like contact angle measurements [44]. Also, informations

gathered by IGC, concerning the Lewis acid/base prop-

erties and dispersive surface energy, cdS, can be useful to

follow the changes in surface properties occurring as a

result of modification of poly(glycidyl methacrylate-co-

ethylene glycol dimethacrylate), PGME, with EDA.

Among other possibilities of applications, macropo-

rous crosslinked poly(glycidyl methacrylate-co-ethylene

glycol dimethacrylate), PGME, and copolymer modified

with ethylene diamine, PGME-en, proved very useful as

column packings in gas chromatography (GC) [29]. As

the separation ability of PGME and PGME-en depends

on the porous properties of the copolymer and its polar-

ity, the knowledge of surface properties in terms of the

copolymer ability to undergo dispersive interactions (de-

scribed by the dispersive component of surface free en-

ergy, cdS) and to act as electron donor or acceptor

(described by parameters KD and KA) gathered by

IGC, are of great importance for elucidation of the rela-

tionship between surface properties and chromato-

graphic performances of the initial and the modified

copolymer.

4. Conclusions

Adsorption properties of macroporous poly(glycidyl

methacrylate-co-ethylene glycol dimethacrylate), PGME,

and copolymer modified with ethylene diamine, PGME-

en, have been investigated by means of inverse gas chro-

matography at infinite dilution, in the temperature range

from 333 to 363 K. Chemical modification of PGME

with ethylene diamine leads to different chromato-

graphic behavior of the modified and the initial copoly-

mer samples, i.e. to the shortening of retention of all

adsorbates for modified samples. The calculated values

for the entropy changes of adsorption of the investigated

adsorbates are of the same order as those obtained

experimentally for all copolymer samples. Differences

observed between the experimental and the predicted en-

tropy changes, i.e. the additional entropy loss found

experimentally, may result from the restricted rotational

and vibrational freedom of adsorbates on the copolymer

surfaces. The calculated dispersive surface energy values,

cdS, for PGME and PGME-en are comparable with the

literature data for nonconductive polymers. According

to KD/KA ratio, both the initial and modified copolymers

have basic character, which is much more pronounced in

the case of PGME-en. Porosity parameters of cross-

linked macroporous copolymers influence the accessibil-

ity of the acidic and basic sites, and consequently, the

Lewis acid/base properties of the copolymer.

Acknowledgements

This work was supported by the Serbian Ministry of

Science and Environmental Protection (Projects No.

1948 and 1978). The authors are grateful to Novartis

Pharma AG (Basel, Switzerland) for the donation of

the HP5890II gas chromatograph used in this work.

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