Ž .Materials Science and Engineering C 18 2001 45–49www.elsevier.comrlocatermsec

Thermal stability of a ureidopyrimidinone model compound

Gordon Armstrong), Martin BuggyMaterials and Surface Science Institute, UniÕersity of Limerick, National Technological Park, Castletroy, County Limerick, Ireland

Abstract

wŽ . xA dimer of N- butylamino carbonyl -6-methylisocytosine was prepared as a model compound in order to study the thermal stabilityŽof ureidopyrimidinone supramolecular polymers during heatrcool cycles. The dimer did not self-heal i.e. disintegrate and reform

.reversibly as expected; it underwent thermal degradation in three stages once heated to its melting point of 225 8C. Infrared spectra andoptical micrographs were taken before and after these cycles, and the gases evolved at each stage of the degradation process wereidentified by gas chromatography-mass spectroscopy. A degradation mechanism is proposed whereby the dimer’s butane-1-isocyanate‘tail’ cleaves first, followed by the isocytosine ‘head’ breaking down above 244 8C. The kinetics of the degradation process were alsodetermined, from which the activation energy was calculated to be 71.5 kJrmol. In conclusion, the implications for processing relatedsupramolecular polymers are discussed. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Ureidopyrimidinone; Self-assembly; Supramolecular polymer; Thermal stability; Thermal analysis; Degradation kinetics

1. Introduction

A supramolecular polymer is an assembly of smallŽmolecules bound together by weak interactions typically

.hydrogen bonds rather than the covalent bonds found inconventional polymeric materials. Unlike conventionalpolymers, the bonds in supramolecular polymers are re-versible, and so have high dimerization constants. It hasbeen speculated that this feature may give rise to uniqueproperties: for example, Sijbesma and Meijer claimed thatthese materials should exhibit the same flow behaviour assmall molecules at elevated temperatures, yet form rugged

w xnetworks upon cooling 1 . Certain supramolecular poly-mers form liquid crystals or optically active structuresw x2–4 , which offer potential for developing novel optoelec-tronic materials.

As part of a study to determine the suitability of urei-dopyrimidinone supramolecular polymers for use as novelmaterials, a model compound was prepared to assess itsthermal stability and determine whether it exhibits self-healing, i.e. totally reversible disintegration and formation,during heatrcool cycles. This class of polymer was chosen

) Corresponding author.Ž .E-mail address: gordon.armstrong@ul.ie G. Armstrong .

because they show promise for use as coatings and hotmelts.

2. Experimental

2.1. Synthesis

wŽ . xA dimer of N- butylamino carbonyl -6-methylisocyto-Ž .sine Fig. 1 was synthesised following published proce-w x 1dures 5 . The H NMR spectrum of the product was

recorded on a Jeol 90Mhz spectrometer as ppm againstTMS and found to agree with data published by Beijer et

w xal. 5 .

2.2. DSC

Samples were placed in sealed aluminium pans andheated at 10 8Crmin in a TA Instruments Model 10 DSC;the sample cell was flushed with oxygen-free nitrogen at aflow rate of 60 mlrmin. Average sample masses were 5.5mg. For the heatrcool cycles, samples were heated to T ,m

then, as soon as the melting transition was complete, thesample was allowed cool back to room temperature so thatthe dimer could reform. This cycle was repeated up tothree times. If the dimer reformed as expected, both DSCtraces should match exactly. Each sample was recovered

0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0928-4931 01 00359-9

( )G. Armstrong, M. BuggyrMaterials Science and Engineering C 18 2001 45–4946

wŽ . xFig. 1. N- butylamino carbonyl -6-methylisocytosine.

and its IR spectrum recorded for comparison with that ofthe original material.

2.3. FTIR

ŽThe IR spectra were recorded as KBr 1% sample.wrw discs on a Perkin Elmer Spectrum 2000 FTIR spec-

trophotometer. The range 4000–500 cmy1 was scanned 16times and averaged to give the spectra shown below.

2.4. TGA

A sample of dimer was heated to 500 8C at 5 8Crmin ina TA Instruments model 951 thermobalance, calibratedusing a calcium oxalate standard according to the manufac-turer’s instructions. To determine the kinetics of degrada-tion, further samples were heated to 225, 235, 245, 255,265, 275 and 285 8C, all at 10 8Crmin. The furnace wasflushed with oxygen-free nitrogen at a flow rate of 60mlrmin during all experiments. Average sample masses of7 mg were used for TGA and EGA.

2.5. EGA

Two evolved gas analysis experiments were carried out.Both samples were heated to 500 8C at 10 8Crmin in a TA

Instruments model 2950 thermobalance, flushed with nitro-gen as before. 10 ml of samples of the exhaust gas fromthe sample chamber were passed through a stainless steelsampling tube containing tenax sorbent. The samplinginterval was centred on 250 8C for the first experiment and290 8C for the second. Afterwards, the sampling tube wasplaced in a Markes Unity thermal desorption unit coupledto a Hewlett-Packard 6890 Gas Chromatograph with aHewlett-Packard 5973 Quadrupole Mass Analyser detec-tor. Mass spectra were recorded from 45 AMU upwards toavoid spurious peaks from traces of air and moisture,which inevitably arise in mass spectrometry. These spectrawere identified by comparison with library data usingHewlett Packard’s ChemStation software.

2.6. Microscopy

Several samples were examined at 168= magnificationunder a hot-stage Olympus BX60 microscope fitted with aReichert hot stage capable of heating to 300 8C. A satu-rated solution of the dimer in chloroform was prepared anddropped onto ordinary glass microscope slides and placedin a desiccator. The solvent evaporated off slowly, leavinga powdery, even, thin film of dimer. Each slide wasmounted on the hot stage and heated slowly to the dimer’smelting point. Then the heat source was switched off andthe sample allowed return to room temperature. Pho-tographs were taken of the samples at each of these stagesfor later comparison.

3. Results and discussion

The DSC traces showed a single peak, attributed to themelting transition. An average value of 217 8C was ob-tained for T onset, 220 8C for T and 134.8 Jrg form m max

Ž .the enthalpy of fusion DH . With each extra heatrcool

Fig. 2. DSC traces after one, two and three heatrcool cycles.

( )G. Armstrong, M. BuggyrMaterials Science and Engineering C 18 2001 45–49 47

Ž . Ž .Fig. 3. FTIR spectra of the dimer before above and after below threeheatrcool cycles.

cycle, a decrease in T was observed, falling to 215 8Cm

with onset at 200 8C after three cycles. RepresentativeDSC traces are shown in Fig. 2. The enthalpies of fusionfor the second and third heating cycles were significantlylower than that of the first. On removing the dimer sam-ples from the DSC, it was noticed that the dimer hadreformed into a yellow coating covering the base of the

Ž .sample pan. Comparing the infra-red FTIR spectra of

Ždimer samples before and after the heatrcool cycles Fig.. y13 showed that the peaks around 3260 and 3150 cm

attributed to hydrogen bonding remain unchanged, evenafter three cycles. However, a new peak at 1900 cmy1 andband broadening in the 3400–2800 and 1700–1250 cmy1

regions were seen in the spectra of the heat-treated dimer.These were correlated with the presence of ammonium

w xions 6 suggesting that the dimer’s tail degraded duringthe heatrcool cycles.

Thermogravimetry showed that 89% of the initial masswas lost between 227 and 350 8C and thermal degradationoccurred in three stages, as shown in Fig. 4. Between 225and 244 8C, butane-1-isocyanate was identified as theprimary evolved gas, with some N, NY di-n-butylurea alsopresent. N, NY di-n-butylurea was the principal evolvedgas detected between 244 and 299 8C. Traces of pyridine,N-pentylidene-1-butanamine and N-butyl-acetamide werealso detected. On removal from the thermobalance, thesample pan was found to be covered with brown charcorresponding to the 12.4% residue seen in Fig. 4.

Because ammonium ions were present after the heatrcool cycles yet isocyanate groups remained, it was pro-posed that the dimer’s tail degraded first during theheatrcool cycles. If this was so, the TGA traces shouldindicate an initial mass loss of 44%, corresponding to the

Ž .loss of 100 g C H N Ormol 225 g of dimer. The5 11 2

butane-1-isocyanate detected during stage 1 of degradationcould only arise from cleavage of the dimer’s tail, and thecalculated mass loss of 41% corresponds well with theexpected result. Comparing the mass spectra of the firstand second stages showed approximately the same abun-dance of molecular ions attributable to N, NY di-n-buty-lurea throughout. N, NY di-n-butylurea was probablyformed by the reaction of two butane-1-isocyanate radicalsand, as this species was still present close to the onset ofthe third stage of degradation, it could account for theremaining 3% of the tail’s calculated mass.

Fig. 4. TGA trace of the dimer.

( )G. Armstrong, M. BuggyrMaterials Science and Engineering C 18 2001 45–4948

Table 1Kinetic data

y1 1r2Ž . Ž . Ž .Hold 8C k s t s

225 0.0012 577.5235 0.0018 385245 0.0028 247.5255 0.0035 198265 0.0021 330275 0.0062 112

Similarly, the N-pentylidene-1-butanamine and N-butyl-acetamide detected during the second degradationstep probably arose from degradation of the dimer’s tail,although there were only traces of them present. The massof the dimer’s head was calculated as 110 grmol; as thedimer’s molar mass is 225 g, this corresponds to 48.9%.46.6% of the original sample mass was lost during thesecond step; with little change to the mass spectra taken atthis point it is reasonable to attribute this loss to thedimer’s head breaking down.

Given that one species underwent degradation—thedimer—it was assumed that the process obeyed first orderkinetics. The first-order rate law is of the form

ln DrD sykt .Ž .o

The half-life of the reaction is

t1r2s ln2rk .

By repeating the procedure for isothermal holds at 108C intervals between 225 and 285 8C, it is then possible todetermine the activation energy and temperature depen-dence from the Arrhenius equation,

lnks ln AyE rRT .act

For example, 6.553 mg of dimer was heated to, andthen held at, 225 8C until the sample reached a steadymass. After 20.8 min, 5.608 mg of sample remained.Substituting into the rate law equation gave ks0.0012

Fig. 6. Thin film of dimer as seen under hot stage microscope beforeŽ . Ž .top and after bottom one heatrcool cycle.

sy1. A straight-line kinetic plot was obtained by linearregression. The correlation was 98.85% and the rate con-

Fig. 5. Arrhenius plot.

( )G. Armstrong, M. BuggyrMaterials Science and Engineering C 18 2001 45–49 49

stant, 1.2=10y3 sy1. The results for each isothermal holdare given in Table 1.

The Arrhenius plot shown in Fig. 5 was prepared fromthese k values. A straight line was fitted with a correlationof 99.43%. An activation energy of 71.5 kJrmol was

Ž .calculated from the slope E rR . Activation energiesact

determined under comparable conditions for high-densitypolyethylene range from 220 to 330 kJrmol. Those forlow-density polyethylene range from 163 to 305.3 kJrmol.w x7 .

No changes were observed under the hot-stage micro-Ž .scope Fig. 6 until the onset of melting, when the needle-

like crystals rapidly formed a bubbly-looking liquid phasewith noticeable Brownian motion. This motion abatedquickly once the heating element of the hot stage wasswitched off, causing the liquid to form pockets of solidphase dimer, which soon grew into each other. At thispoint, the dimer began to volatilise and the hot stage’sglass cover became clouded, making observation difficult.On returning to room temperature, it was noticeable thatnone of the original needle-like crystalline phase remained.The crystals had adopted a far finer morphology givingrise to a more evenly distributed film on the glass slide.

4. Conclusions

w ŽThis model-compound study of dimeric N- butyla-. xmino carbonyl 6-methylisocytosine has revealed several

new factors to be considered in developing suitable appli-cations for ureidopyrimidinone supramolecular polymers.

Though it was expected that the dimer would be capable ofrepairing itself if damaged, dimerization did not occur asexpected during the heatrcool cycles used to test thishypothesis. Rather, the dimer actually underwent thermaldegradation. Such quick degradation after melting maypose problems for the processing of ureido-pyrimidinone

w Ž . xpolymers because they use N- butylamino carbonyl -6-methylisocytosine as their end group and their degree ofpolymerisation is strongly dependent upon the mole frac-tion of end group present.

Acknowledgements

We gratefully acknowledge the financial support of theMaterials and Surface Science Institute of the Universityof Limerick.

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