1H-13C CPMAS andT2 Relaxation Solid-State NMR Measurements of Melamine-Based Polycondensed Chemical...

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2204

1H-13C CPMAS and T2 Relaxation Solid-StateNMR Measurements of Melamine-BasedPolycondensed Chemical Gels

Dedicated to Prof. H. W. Spiess on the occasion of his 65th birthday.

Chrystelle C. Egger,* Volker Schadler, Jerome Hirschinger, Jesus Raya,Burkhard Bechinger

Gel networks prepared from aqueous melamine-formaldehyde resins via a sol–gel route werecharacterised by 13C CPMAS (where MAS stands for magic angle spinning)-NMR and T2

relaxation. We show that within the aqueous networks, the type and amount of functionalgroups on the melamine ring is strongly influencedby the initial melamine/formaldehyde ratio, the solconcentration and the amount of catalyst. Residualmethylol groups can be etherified by postalkoxyla-tion using primary alcohols resulting in chemicalgels with different bridging units and residual etherlinkages as also confirmed by T2 measurements. Inaddition, in the case of initially alkoxylated gels thealcoholysis process can be followed by varying theamount of catalyst used.

Introduction

Hydrogels represent a special type of soft matter which

can be considered as ‘solidified’ water entrapped by a

chemical network. The unique thermodynamic, rheologi-

cal and release properties of hydrogels are already of great

use in a couple of large-scale applications, e.g. in the food,

hygiene and drug formulation business. Although the

C. C. EggerKeele University, Lennard Jones Laboratories, Keele ST5 5BG, UKE-mail: [email protected]. SchadlerISIS-BASF Group, 8 Allee Gaspard Monge, 67083 Strasbourg,FranceJ. Hirschinger, J. Raya, B. BechingerInstitut Le Bel, 4 Rue Blaise Pascal, 67000 Strasbourg, France

Macromol. Chem. Phys. 2007, 208, 2204–2214

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

chemical nature of hydrogels can be very diverse ranging

from inorganic to organic networks that are obtained

either by chain growth or step growth crosslinking mech-

anisms the physical properties of any hydrogel can be

narrowed down to a crosslinking density (distribution)

and a solution parameter describing the degree of swelling

or phase separation of the polymer from the solvent. When

the gel is obtained from a ‘sol’, i.e. a molecular solution, by

a chemically triggered polycondensation or polyaddition

this so-called sol–gel process has been shown to give rise

to a multitude of well-known porous structures, e.g. for

silica.[1] The chemical connectivity of silica hydrogels and

its derivatives leading to micro- or mesoporous materials

has been studied in detail using, e.g. solid-state (29Si) NMR

spectroscopy.[2] While these studies have enabled the

design of functional porous inorganic materials from

silica-based hydrogels, the somewhat analogous gels

DOI: 10.1002/macp.200700255

1H-13C CPMAS and T2 Relaxation Solid-State NMR Measurements . . .

obtained via chemical polycondensation of organic pre-

cursors is much less understood. Organic hydrogels syn-

thesised by reacting phenolic and amino resins in water

have been largely used to produce aerogels and xerogels;[3]

particular phenol-based hydrogels and their corresponding

aerogels currently enjoy a renaissance as such aerogels can

be further carbonised to produce nanoporous carbon (e.g.

to be used as electrodes).[4] In contrast to the vast number

of potential applications of such organogels and their pre-

cursors, there is a striking lack of understanding concern-

ing their structure and dynamics. Here, we present a first

comprehensive solid-state NMR study which aims at char-

acterising the structure and dynamics of model melamine-

formaldehyde chemically formed hydrogels. First of all,

the study of melamine hydrogels helps to better under-

stand the mechanism of the organic sol–gel process but

also shows the potential for further gel modifications

that lead to improved properties. Chemical modification of

Figure 1. Schematic reactivity of melamine towards formaldehyde addacid treatment either from the alcohol-derivatised melamine orpolycondensed form: secondary or tertiary amines, hydroxyl groups, amelamine precursors.



the hydrogels may, for instance, lead to porous materials

showing a specific functionality on the pore walls; it may

also influence the drying process itself. Speciation through13C liquid-state NMR spectroscopy has been carried out

over the last decade on melamine or melamine–urea

systems,[5–9] however, mainly on the liquid-state (on small

physically polycondensed aggregates formed with tem-

perature or time) where application relevant issues such as

storage stability play an important role. Even when the

detailed formation of methylene bridges resulting from

the physical polycondensation is investigated, it is in the

liquid-state.[10] Recent liquid-state 13C NMR work has

focussed on the influence of the temperature in the storage

stability of various melamine-based resins and although

the work has been carried out on physical gels of

melamine-formaldehyde only it does emphasise the

various factors influencing the polycondensation pro-

cess.[11,12] The interest has been very recently extended to

ition followed by alkoxylation. Polycondensation can take place uponfrom its ether form. Note the various functions present in thelkyl ether bridges between two amino functions from two adjacent

www.mcp-journal.de 2205

https://www.researchgate.net/publication/239207575_The_storage_stability_of_melamine_formaldehyde_resin_solutions_III_Storage_at_elevated_temperatures?el=1_x_8&enrichId=rgreq-9c6dde1b-994e-45f5-b215-805f3dd2efdc&enrichSource=Y292ZXJQYWdlOzIyOTY5MjA0NTtBUzoxODcyNzYzMDY5NTIxOTJAMTQyMTY2MTU1OTA3OA==

C. C. Egger, V. Schadler, J. Hirschinger, J. Raya, B. Bechinger

2206

colloidal melamine–urea-formaldehyde resins.[13–16] Solid-

state multinuclear NMR (1H, 15N and 13C)[17,18] or relaxation

experiments[19] have also been carried out on physically

precipitated phases but not on macroscopic chemical

hydrogels. So far, very little is reported about the chemical

connectivity and functionalities in the gel state.[20] Here

we employ 13C magic angle spinning (MAS) NMR spectros-

copy to characterise different gels as obtained chemically

from melamine-formaldehyde resins. In 13C NMR spectros-

copy under MAS, the line broadening of the NMR signal as

typically observed for strong gels is reduced by averaging

the anisotropy of the dipolar coupling. Using this tech-

nique, species of distinct chemical shift within the gel can

be distinguished as synthesised under well-defined poly-

merisation conditions.[21] Other NMR works can sometime

be carried out, such as diffusion measurements coupled

to relaxation experiments, when the rate of release of a

component in a complex aqueous structure such as cement

is targeted.[22]

In Figure 1, the formation of melamine-formaldehyde

resins via the addition of formaldehyde under basic

conditions is shown. The resins can be further alkoxylated

via acid catalysis either (i) by adding alcohols or (ii) by

condensing the methylol groups to form crosslinks

between the melamine rings. More details about the

chemistry of such melamine resins are described in the

literature.[23–25]

From the mechanistic scheme presented in Figure 1,

it becomes easy to realise that the degree of catalysed

polycondensation will be greatly influenced by the ratio

melamine/formaldehyde, but also the ratio melamine-

formaldehyde/catalyst (catalyst¼ acid) and the degree of

hydrolysis of the ether functions into alcohol. Although the

degree of polycondensation can be measured indirectly

(e.g. gel stiffness), the solid-state NMR technique is a very

Table 1. Experimental parameters for the 11 MFGs studied, prepaformaldehyde are given in molar ratio, melamine-formaldehyde to a

MFG1a MFG1b MFG

MF no. 1 1

M:F (moles) 1:1.51 1:1.51 1

[MF] wt.-% 16.8 18.9 1

Initial MF/Ac (weight) 2 2

Post-treatment – – 5 wt.-%

MFG3a MFG3

MF no. 3 3

M:F:1-butanol (moles) 1:5:5 1:5:5

[MF] wt.-% 25 17.5

Initial MF/Ac (weight) 18 18



suited technique as it not only gives the amount of

crosslinking, but it also gives information about the type

and amount of functions left in the gel for further reactions

or curing. In addition, results from spin–spin relaxation

measurements give good estimation of the water mobility

within the gels and add onto the 1H-13C CPMAS experi-

ments.

In this work, we study model melamine-formaldehyde

gels (MFG) with 1H-13C CPMAS and T2 relaxation measure-

ments. We investigate the influence of the formaldehyde

content, the catalyst content, the alkoxylation step and

also the influence of the post-treatment step, on both the

structure and the dynamics of various MFGs.

Experimental Part

Synthesis of the Gels

Three different types of melamine-formaldehyde resins have been

used in this work: (i) MF1 with a ratio melamine-to-formaldehyde

equal to 1:1.51 and (ii) MF2 with ratio 1:3.15 and (iii) MF3 the

methylol functions of which being fully etherified by 1-butanol.

All resins were provided by BASF Aktiengesellschaft and used

without further purification. MF1 and MF2 have been dissolved

in water using a homogeniser of the Ultra-Turrax type. When

starting with MF1 or MF2 resins, gels have been prepared by

adding a given amount of formic acid to a propylene vial

containing a melamine-formaldehyde solution. The vial is then

placed in a water bath at 60 8C for 3 h and kept at room

temperature thereafter until further NMR measurements are

carried out. Instead, MF3 is dissolved into methyl ethyl ketone

(MEK) and hydrochloric acid is used as the catalyst. Post-

treatments on gels prepared from MF1 and MF2 consist of

dipping the gels into a 5 wt.-% solution of 1-butanol with pH

stabilised to about 2–3 using formic acid, at 80 8C for 2 h. The

synthesis details are summarised in Table 1.

red from 3 different melamine-formaldehyde resins (melamine/cid is a mass ratio).

1a-PT MFG2a MFG2b MFG2a-PT

1 2 2 2

:1.51 1:3.15 1:3.15 1:3.15

6.8 17.0 17.0 17.0

2 2 1.5 2

1-butanol – – 5 wt.-% 1-butanol

b MFG3c MFG3d MFG3e

3 3 3

1:5:5 1:5:5 1:5:5

25 25 25

5 10 15

DOI: 10.1002/macp.200700255


Solid-State NMR Measurements

The spectra showed below have been recorded on a Bruker Avance

300 spectrometer operating at 300.133 MHz (7.05 T), equipped

with a 4 mm double resonance MAS probe (1H-X, X ranging from15N to 31P). A Bruker Cooling Unit was used to keep the

temperature at 300 K (0.1 K error). Each sample was introduced

into a 4 mm HRMAS ZrO2 rotor fitted with a 50 mL cylindrical

insert. For all MAS NMR experiments, samples were spun at 6 kHz.

The Hartmann-Hahn conditions for all 13C CP/MAS spectra were as

follows: n1(13C)¼50 kHz, n1(1H)¼56 kHz, n1(1H decoupling)¼80 kHz, contact time¼ 2 ms, recycle time¼ 5 s. Although the signal

intensities observed in 1H-13C CPMAS NMR spectra are not quanti-

tative due to the dependence of magnetisation transfer on the

dynamics, environment and functional groups involved, it is here,

however, possible to compare spectra quantitatively against one

another since acquisition parameters have been kept constant

amongst all experiments. 13C spectral deconvolutions and curve

fittings were carried out using TopSpin 2.0 (Bruker) using a

Simplex algorithm with a line shape composition of 50% Gaussian

and Lorentzian contributions, respectively. A typical 13C CPMAS

Figure 2. A typical 13C CPMAS NMR spectrum for a nonalkoxylated MFcontributions: (i) the aromatic carbon nuclei from the triazine ring (150methylol/methoxy bridges (60–80 ppm) or the methylene bridges (40these two regions are also given for MFG2a.



spectrum along with the deconvolution result is displayed in

Figure 2.

Peak assignments are essentially based on previous liquid-state

work,[5–12] but also occasionally on solid-state work available in

the literature.[17,18,20] As indicated in Figure 2, the peaks in the

150–175 ppm region correspond to the aromatic carbons of the

triazine ring. The 40–80 ppm region divides into two parts: (i)

60–80 ppm corresponding to carbon atoms present in methoxy

functions and (ii) 40–60 ppm corresponding to carbon atoms in

methylene units. Within each region, due to the broadness of the

peaks, peak assignment cannot be categorical and various func-

tions are sometimes possible. The amplitude of the 13C signals

stemming from the triazine ring (150–175 ppm) have been used to

normalise the spectra such that a quantitative comparison of the

signal intensities in the 40–80 ppm region becomes possible

(the acquisition parameters are the same for all 1H-13C CP/MAS

experiments). However, due to the fact that only amplitudes and

not signal areas are used to normalise the spectra, spectra

comparison still remains approximate. The amplitude factors are

given in Table 2 under the gel names and are relative to the gel

which has its amplitude set to 1, along with the deconvolution

G (here MFG2a), along with the assignments of the two main signal–175 ppm), (ii) the aliphatic carbon nuclei (40–80 ppm) with either the–60 ppm). Result of the 13C spectral deconvolution and curve fitting of




Table2.

Expe

rimen

tal13

Cch

emic

alsh

iftsf

orM

FG1a

ndM

FG2-

type

gels

alon

gw

ithth

ein

tegr

alfo

reac

hde

conv

olut

edpe

akgi

ven

perr

egio

n(15

0–17

0pp

m,4

0–80

ppm

).As

sign

men

tsar

eal

sogi

ven

and

refe

rto

the

liter

atur

e.

Carbontype

13Cch

emicalsh

ifts

(deconvolution%)

ppm

MFG

1a

1

MFG

1b

T1.3665a)

MFG

1a-PT

T1.5665

MFG

2a

T0.10811

MFG

2b

T6.2621

MFG

2a-PT

T53.5854

Tri

azi

ne

rin

gb

)

ArC

–N

H2

16

7.8

(22

)1

67

.9(1

9)

17

1.5

(6)

16

9.2

(13

)1

66

.9(1

3)

16

9.9

(35

)

ArC

–N

H(C

H2O

–R

)1

65

.5(5

0)

16

5.4

(52

)1

67

.6(6

2)

16

7.6

(65

)1

65

.6(5

3)

16

7.9

(49

)

ArC

–N

(CH

2O

–R

) 21

57

.9(2

7)

15

7.9

(28

)1

60

.4(3

1)

15

9.1

(22

)1

57

.3(3

3)

16

1.5

(16

)

–C

H2O

–

–N

(R1)C

H2O

CH

2–

N(R

2)–

––

–7

3.6

(33

)–

–

–N

(R)–

CH

2O

Ho

r–

N(R

)–C

H2O

CH

2–

NH

––

––

71

.8(3

8)

–

–N

H–

CH

2O

Ho

r–

N(C

H2O

H) 2

or

–N

H–

CH

2O

–C

H2O

H–

––

66

.8(3

)6

5.3

(2)

–

–C

H2–

–N

(R1)–

CH

2–

N(R

2)–

––

58

.9(3

7)

––

–N

H–

CH

2–

N(C

H2)–

[5]

53

.9(5

9)

55

.8(3

2)

55

.2(1

9.5

)5

6.1

(63

)5

4.6

(60

)5

6.0

(80

)

–N

H–

CH

2–

N(C

H2)–

[10

]–

51

.6(2

4)

50

.3(4

4)

––

50

.5(2

0)

–N

H–

CH

2–

NH

–4

7.5

(41

)4

7.5

(44

)–

––

–

Sha

rp

1-B

uta

no

l–

––

––

64

.4

36

.4

21

.1

15

.8

a) M

FG1

bT

1.3

66

5:

am

pli

tud

efa

cto

rfr

om

MFG

1b

toM

FG1

a;

b) A

rCst

an

ds

for

aro

ma

tic

carb

on

sfr

om

the

tria

zin

eri

ng

.

2208Macromol. Chem. Phys. 2007, 208, 2204–2214

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/macp.200700255


results for each gel, distinct for each region of the spectrum (the

triazine region between 155 and 175 ppm and the functions

carried by the triazine ring between 40 and 80 ppm). In the case of

nonalkoxylated materials, the chemical shifts of the aromatic

carbons follow the rule that the more substituted the amine

function it carries, the smaller the chemical shift (from 168 to

164 ppm).[6] In the case of the MF3-based material, both the solvent

and the pH are different and consequently affect the 13C chemical

shifts. The solvent and pH effects are discussed in the text.

T2 relaxation measurements have been performed at 300 K

using a Hahn echo pulse sequence with echo times varying

from 5� 10�6 to 0.1 s. Fits for relaxations curves followed an

exponential decay and also used a Simplex algorithm. The T2 value

of bulk water measured under the same conditions is 250 ms.

Results and Discussion

1H-13C: CPMAS

Formaldehyde acts as a crosslinker between melamine

units (see Figure 1) as its addition to the melamine ring

creates alcohol (methylol) functions which can then

Figure 3. 1H-13C CPMAS spectra of gels prepared with nonalkylatedresins (MF1 or MF2). Top: Influence of the formaldehyde content,MFG1a and MFG2a; middle: Influence of the acid content, MFG2aand MFG2b; bottom: Influence of the resin concentration, MFG1aand MFG1b. Compared spectra have been normalised to thestrongest peak of MFG1a or MFG2a.



undergo polycondensation, the degree of which will

impact the density and robustness of the final gels. In

order to study the influence of the formaldehyde content

on the chemical structures of the resulting products, two

different gels (MFG1a and MFG2a) have been synthesised

using nonalkoxylated resins which have a ratio of

melamine-to-formaldehyde differing by a factor of 2 while

keeping constant the relative amount of formic acid used

to catalyse the reaction (see details in Table 1). The

resulting 1H-13C CPMAS spectra of the two gels are shown

in Figure 3 (top). Apart from the small chemical shift

between the two spectra (�1.4 ppm) which could be

attributed to slight pH variation, there are two main

differences between the two spectra: (i) the 56.1 ppm peak

for MFG2a is stronger than that at 53.9 ppm for MFG1a

and (ii) two extra peaks at 73.6 and 66.8 ppm appear on the

MFG2a spectrum. From the results gathered in Table 2, it

appears that after normalisation to the strongest peak in

the MFG1a spectrum, the peak at 56.1 ppm for MFG2a is

�7 times stronger than that of MFG1a (this factor seems

smaller if one looks at the spectra, which is due to the fact

that only amplitudes and not areas have been used to

normalise the spectra), which suggests more methylene

bridges [–NH–CH2–N(CH2)–] in the case of higher formal-

dehyde content in the resin (MFG2a). Furthermore, there is

less nonreacted ArC–NH2 in MFG2a than in MFG1a (13.0

and 22.2%, respectively) and there is no detectable –NH–

CH2–NH– signal in MFG2a (no peak at 47 ppm) suggesting

a relatively high degree of substitution on the triazine

ring, which is in complete agreement with the presence of

more crosslinkers in MFG2a than in MFG1a. More

importantly, the two extra peaks in MFG2a are assigned

to –N(R1)CH2OCH2–N(R2)– (73.6 ppm) and to various

possible methoxyamine groups (66.8 ppm) clearly show-

ing the presence of methoxy bridges or N-methylol

functions in the case of the high-formaldehyde content

gel which do not exist in MFG1a. This result obtained in

the solid-state of chemical hydrogels is in perfect agree-

ment with that previously found in the liquid-state with

physical gels.[17]

Similarly, the influence of acid content has been studied

on both resin types. The resulting 1H-13C CPMAS are given

in Figure 3 (middle) for MF2-based materials and the same

conclusions can be derived from MF1-based materials

(data not shown). A slight variation in acidity (MF/Ac¼ 2

compared to¼ 1.5) greatly affects the intensity of the

peaks at 73.6–71.8 and 66.8–65.3 ppm (amplitude differ-

ence varying around 5–7 following a normalisation based

on signal intensity). The triazine region differs between

both spectra, thus matching of intensity for normalisation

is not sufficient: indeed it seems that the area for the signal

correlated to the triazine ring for MFG2a is larger than

that of MFG2b. Consequently, any quantification on the

40–80 ppm region is risky and cannot be attempted.



Figure 5. 1H-13C CPMAS spectra of gels prepared with alkylatedresins (MF3). Top: Influence of the resin concentration, MFG3a andMFG3b; bottom: Influence of the acid content, MFG3c, MFG3d andMFG3e. Compared spectra have been normalised to the strongestpeak of MFG3a or MFG3c. For MFG3c, conclusive assignment ofthe sharp peaks is not possible as they are very weak.

Figure 4. 1H-13C CPMAS spectra of gels prepared with nonalkylatedresins (MF1 or MF2) before and after post-treatment with a 5 wt.-%1-butanol solution. Top: on a low-formaldehyde content resinMFG1a and MFG1a-PT; bottom: on a high-formaldehyde contentresin MFG2a and MFG2a-PT. Compared spectra have been normal-ised to the strongest peak of MFG1a or MFG2a.

2210

The influence of the resin concentration on the final gel

has also been investigated and the spectra of two gels

MFG1a and MFG1b prepared using MF1 at 16.8 and

18.9 wt.-%, respectively, (MF/Ac¼ 2) are displayed in

Figure 3 (bottom). The relative amount of each type of

carbon within the triazine ring is very similar in both cases

(Table 1) and so are the relative amounts of the different

methylene units [compare 58.7 and 41.2% against

(32.1þ 24.3) and 43.6]. However, when compared to the

triazine intensities there are 1.3 times more –NH–CH2–

N(CH2)– units in MFG1b than in MFG1a, clearly showing

that an increase in the resin concentration favours

cocondensation between melamine units.

Two samples MFG1a and MFG2a have been post-treated

chemically with 1-butanol (see the Experimental Part for

detail) in order to tackle possibilities of gel postgrafting.

The resulting 1H-13C CPMAS spectra are shown in Figure 4,

and the first noticeable feature is that both the aromatic

and the aliphatic part of the MFG1a spectrum remain

almost unchanged after treatment with 1-butanol, whereas

in the case of MFG2a sharp peaks appear in the 0–70 ppm

region. These signals are indicative of more mobiles

species when compared to the rest of gel. As the spectrum

was obtained by crosspolarisation the latter cannot be

from sites exhibiting isotropic motions and therefore is

part of the gel phase rather than unconnected freely

moving molecules. The peaks at 64.4/36.4/21.1/15.8 ppm

are a clear signature of 1-butanol whereas the signal

emerging at 33.8 ppm is from the background (the noisy



background is unfortunately due to a lack of material for

this particular experiment). The width at half height of

the sharp peaks varies approximately from 40 to 70 Hz,

suggesting rather mobile species. According to these

results, it seems that the presence of either N-methylol

functions or methoxy bridges within MFG2a have allowed

the n-1-butanol to react with the polymeric network,

which was not possible with MFG1a.

The 1H-13C CPMAS spectra of various gels prepared with

MF3 (melamine-formaldehyde resin almost completely

etherified with 1-butanol) under different conditions (acid

content or resin concentration) are presented in Figure 5. It

is interesting to note that the spectrum of the post-treated

gel is very similar to that prepared with MF3 [clearly

seen in Figure 5 (top)], which corroborates the grafting of

1-butanol molecules inside the gel MFG2a. Extra sharp

peaks at 7.5/28.8/36.3/207.8 ppm are also present, char-

acteristic of MEK, utilised here as the solvent to dissolve

the MF3 resin. Hence, that implies that solvent molecules,

in equilibrium with their enol-tautomer have reacted with

the melamine-based gel, suggesting a higher reactivity of

melamine units towards MEK than themselves. The MF3

resin is initially almost entirely capped with 1-butanol

and probably stays so since the acid content is rather

DOI: 10.1002/macp.200700255


Table3.

Expe

rimen

tal13

Cch

emic

alsh

iftsf

orM

FG3-

type

gels

alon

gw

ithth

ein

tegr

alfo

reac

hde

conv

olut

edpe

akgi

ven

perr

egio

n(15

0–17

0pp

m,4

0–80

ppm

).As

sign

men

tsfo

rbot

hth

ebr

oad

and

shar

ppe

aks

are

also

give

nan

dre

fer

toth

elit

erat

ure.

Carbontype

13Cch

emicalsh

ifts

(deconvolution%)

ppm

MFG

3a

1a)

MFG

3b

T0.8638

MFG

3c

1

MFG

3d

T0.4237

MFG

3e

0.5201

Tri

azi

ne

rin

gb

)

ArC

–N

H2

–1

66

.6(1

0)

––

–

ArC

–N

H(C

H2O

–R

)1

64

.6(4

4)

16

4.0

(31

)1

60

.8(2

3)

16

3.8

(32

)1

64

.0(3

8)

ArC

–N

(CH

2O

–R

) 21

55

.7(5

6)

15

5.8

(58

)1

53

.0(7

7)

15

6.0

(68

)1

56

.2(6

2)

–C

H2O

–

–N

–( C

H2O

C4H

9) 2

or

–N

(R1)–

CH

2O

C4H

97

5.9

(26

)7

5.9

(28

)7

3.7

sbc)

77

.1sb

c)7

5.8

(33

)

–N

H–

CH

2O

C4H

9o

r–

N(R

)–C

H2O

CH

2–

NH

–7

1.5

(13

)7

1.6

(14

)–

71

.5(4

4)

71

.9(9

)

–O

CH

2C

3H

7o

r–

NH

–C

H2O

Ho

r–

N(C

H2O

H2) 2

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Figure 6. Exponential decay of MFG1a magnetisation when sub-ject to a Hahn echo sequence. Inset: static 1H NMR spectrum.

2212

small in this case (MF/Ac¼ 18) and the water necessary to

hydrolyse the ether linkages only comes from the source of

acid (HCl solution) before polycondensation can take place.

It is likely that there is very little hydrolysed species

available for self-condensation whereas an excess of MEK

favours reaction of the free methoxyamine with the enol-

form of the ketone. When the acid content has been set to

MF/Ac¼ 18 the 1H-13C CPMAS spectra remain unchanged

upon variation of the initial MF3 concentration between

17.5 and 18.9 wt.-% [Figure 5 (top)]. Again, this observation

is very likely related to the low amount of acid present

in these two systems, which first need to be hydrolysed

before they can polycondense. However, when the acid

content is raised from MF/Ac¼ 15 to MF/Ac¼ 5, a clear

change can be seen as shown in Figure 5 (bottom).

Although the spectrum for MF/Ac¼ 15 looks very similar

to the previous ones, it is strongly modified when MF/Ac

decreases as both the 1-butanol and MEK 13C signatures

disappear.

On the basis of literature data, the assignment of the

carbons in the triazine ring is reverse for the alkoxylated

and nonalkoxylated resins. For the nonalkoxylated resins,

the higher the chemical shift, the less substituted the

amine on which the carbon is attached.[6,20] However, if it

were the case, there would be more than 50% of primary

amines attached to the triazine ring for the MF3-based

gels, which is extremely unlikely considering that there are

5 mol of 1-butanol for 5–5.5 mol of formaldehyde for 1

melamine ring (all the methylol groups are capped with

1-butanol). There seems to be a solvent effect from MEK

modulating the 13C chemical shifts which renders the

various aromatic carbons more unshielded as the sub-

stitution on the amino group decreases. Although it is

logical with the findings presented in Table 3 (more than

50% of tertiary amines) it goes against what has been

described elsewhere[20b] where the hexa(methoxymethyl)-

melamine resin in dissolved in iso-butanol). The assign-

ment given here has not yet been verified specifically.

Static 1H T2 Relaxation NMR

In order to compare the structural data gathered via13C CPMAS with dynamical data, 1H T2 relaxation measure-

ments were performed. Note that T2 relaxation of highly

Table 4. Observed T2 relaxation values in for the 11 gels studied (the

MFG1a MFG1b

Observed T2 (T10S3 s) 12 15

MFG3a MFG3b

T2 (T10S3 s) 30 32



swollen hydrogels reflects the proton exchange process,

thus the degree of hydration of the network rather than

the crosslinking density (as attributed in solvent-free

polymer networks).[26,27] For the present system contain-

ing many exchangeable protons with H2O, T2 is expected

to be shorter the stronger the hydration. In Table 4, T2

values are listed for the various hydrogels studied. In all

cases, the decay resulting from the relaxation phenom-

enon is monoexponential, suggesting that there is a single

predominant contribution to the relaxation process (see

Figure 6). In the case of MFG1 and MFG2, the relaxation

process can been seen as a proton exchange between the

polymer network and the water, whereas it is more

complicated for the MFG3-type since MEK and water

are both present in the reaction media. Consequently, only

the main findings about MGF1 and MFG2 will be discussed

in detail below: (i) When the resin concentration is in-

creased from 16.9 wt.-% (MFG1a) to 18.3% (MFG1b) a

slightly higher T2 value is found for MFG1b (15 ms

compared to 12 ms) indicating a decrease in gel-associated

water. This is consistent with the deconvoluted 13C data

above showing a slightly increased crosslink density thus a

reduced amount of hydrophilic groups. (ii) When comparing

MFG1a with MFG2a again an increase in T2 is observed

which is obviously due to a decrease in free NH2 groups on

the melamine (cf. 13C data Table 2), thus a smaller number

T2 value for bulk water is measured at 250 ms).

MFG1a-PT MFG2a MFG2b MFG2a-PT

20 15 11 23

MFG3c MFG3d MFG3e

25 32 29

DOI: 10.1002/macp.200700255


of exchangeable protons. (iii) The influence of catalyst is

tested by comparing the T2 values of 15 and 11 ms for a

resin/acid ratio of 2 and 1.5 for MFG2a and MFG2b, re-

spectively. This suggests that at higher amounts of catalyst

(MFG2b) a slightly lower crosslink density is achieved,

reflected by an increase in the amount of gel-associated

water (hydrophilic swelling) in agreement with mechan-

ical observations. (iv) Finally, when T2 of the butoxylated

gels MFG1a-PT and MFG2a-PT is compared with T2 of the

respective samples without post- treatment a slightly

higher water mobility is found for the ‘hydrophobised’

samples (20–23 ms against 11–15 ms). Again, this can be

attributed to a reduced amount of proton-exchangeable

groups as a result of the etherification. T2 values of the

MFG3 specimens are all very similar (around 25–32 ms)

and slightly larger than that from MFG1 and MFG2 gels.

Semiquantitative estimations of T2 values of 1H from

water associated with the polymer backbone is possible if

one considers the protons from water in exchange with

that of the polymer (the exchangeable protons), in addition

to the water molecules H-bonded to the network (the

interfacial water) and finally the bulk water not affected

by the network.[26] Per melamine-formaldehyde motif the

number of exchangeable protons can be estimated to be

about 0–3 (exchangeable protons left on the amino groups

after polycondensation) and interfacial protons are about

30–36 in number (considering the number of donor and

acceptor centres in the motif, and also that two water

molecules have been shown to reorient anisotropically per

hydroxyl group of a carbohydrate[27]). Consequently, the

proportion of gel-associated water protons would roughly

be about 30–35% for MFG1 or MFG2. Then, the contribu-

tion of both the H-bonded and exchangeable protons to the

observed T2 value can be estimated using a similar ap-

proach to ref.[26] [Equation (6)], knowing that the bulk

water relaxes in 250 ms under the same conditions.

The estimates are of a few milliseconds only. These

results show that the observed relaxation rate is largely

dominated by the gel-associated water and that small

changes in the composition of the chemical gels will affect

the observed relaxation process.

Conclusion

This work gives a comprehensive study of a model organic

chemical hydrogel based on melamine-formaldehyde

chemistry. It aims at gaining insights about the chem-

istry of the gels formed by acid catalysis of melamine-

formaldehyde resins and is based on 13C CPMAS and T2

relaxation NMR experiments. When varying the formal-

dehyde or acid content, the nature of the resin is modified,

and despite the complexity of the systems studied, we find



interesting features. Indeed, we show that (i) the content of

formaldehyde influences the type of bridging units present

in the gel, (ii) the amount of acid affects the gel crosslink-

ing density and degree of hydration, (iii) the initial type of

resin used impacts on the nature of the final material. We

also demonstrate that the type of bridges present in the

MFG specimens is of great importance when carrying

out postmodification reactions. Our study corroborates

the previous literature on physical gels of melamine-

formaldehyde resins but beyond that it lays out a real

potential in chemically designing hydrogels with tailored

properties (e.g. polarity, possibility of further postgrafting,

stability of the network upon drying), opening for instance

a new route towards organic hierarchical porous materials.

Acknowledgements: Dr. Marc Fricke and Dr. Vijay Raman arewarmly thanked for useful discussions and for their help inpreparing and modifying the hydrogels.

Received: May 8, 2007; Revised: July 20, 2007; Accepted: July 24,2007; DOI: 10.1002/macp.200700255

Keywords: hydrogels; melamine; solid-state NMR

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DOI: 10.1002/macp.200700255

1H-13C CPMAS andT2 Relaxation Solid-State NMR Measurements of Melamine-Based Polycondensed Chemical...

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