ELSEVIER

Polymer Degradation and Stability 55 (1997) Y5- IO2 0 1996 Elsevier Science Limited

Printed in Northern Ireland. All rights reserved

PII: SO141.3910(96)00130-9 0141-3910/97/$17.W

Effect of polyol structure and molecular weight on the thermal stability of segmented

poly(urethaneureas)

Tzong-Liu Department of Chemical Engineering, National

Wang* & Tar-Hwa Hsieh Kaohsiung Institute of Technology, Kaohsiung, Taiwan 80782, ROC

(Received 15 March 1996: accepted 26 April 1996)

The thermal stabilities of two series of segmented poly(urethaneureas) have been correlated with their soft-segment molecular weights and structure. Thermal stability was measured by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) in air and in nitrogen. It was shown that polyurethanes based on polytetramethylene glycol (PTMG) are generally more stable for both series in air and in nitrogen than polyurethanes based on polyethene glycol (PEG) and polypropylene glycol (PPG). In contrast, the lower stability for PPG-based polyurethanes may be attributed to the lower extent of interurethane hydrogen bonding arising from the incomplete phase separation between the soft and hard segments and, an inferior mutual stabilization effect in this polyurethane. The TGA curves in air also revealed that higher soft-segment molecular weights increase the thermal stability of polyurethanes based on PTMG or PEG. It is suggested that the crystallization effect in both kinds of polyurethanes enhances the phase separation and increases the degrees of interurethane hydrogen bonding in both samples. In a nitrogen atmosphere, the importance of interurethane hydrogen bonding decreases due to the dissociation of hydrogen bonding at elevated temperature. Hence, the B series polyurethanes show higher decomposition temperatures due to the higher soft segment concentrations in these materials. 0 1996 Elsevier Science Limited

1 INTRODUCTION

Segmented polyurethanes are an important subclass of the family of thermoplastic elasto- mers. However, these materials are generally not very thermally stable, especially above their softening temperatures.

role in the decomposition of ether-based TPUs. Thus, in general, the ester-based TPUs normally exhibit better thermal and oxidative stabilities than do the ether-based TPUs.

Several studies have reported the results of the thermal degradation of ester- and ether-based thermoplastic polyurethanes (TPUs) which have been performed under vacuum, air and nitrogen.‘-5 It was found that the polyester based polyurethanes exhibit equally rapid degradation in air and in nitrogen, indicating that a nonoxidative mechanism was involved. In con- trast, the significantly improved thermal stability of ether-based TPUs under vacuum and nitrogen indicates that the oxidative process plays a major

By varying the concentration of the soft segment, it was also observed that degradation starts in the hard segment while the apparent weight loss is correlated with the soft segment.6 It is also claimed that polyurethanes based on PTMG degrade easily in air by oxidation on the carbon atom (a carbon) next to the oxygen’ while breakage of the C-O bond and subsequent unzipping was proposed as the mechanism in an inert atmosphere.’ Moreover, it has been proven previously that the polyether soft segment and the hard segment are more stable when mixed in a urethane copolymer, indicating that there is a mutual stabilization effect in segmented polyurethanes.6

* To whom correspondence should be addressed. This paper concerns the investigation of the 95

96 Tzong-Liu Wang, Tar-Hwa Hsieh

thermal stability of polyether-based poly- urethanes. The polyurethanes were synthesized from 4, 4’-diphenylmethane diisocyanate (MDI), three kinds of polyol, and ethylene diamine (EDA) chain extender. In this study, three different kinds of polyether diols (polyethene glycol, polypropylene glycol and polytetra- methylene glycol) and molecular weights of 1000 and 2000 were used to compare the effect of the diol structure and molecular weight on the thermal stability of segmented polyurethanes.

2 EXPERIMENTAL

2.1 Materials

4,4’-Methylene bis(4-phenylisocyanate) (Du Pont, Inc.) was distilled under reduced pressure. Polyethylene glycol (Hayashi Chemicals), poly- propylene glycol (Wako Chemicals) and poly- tetramethylene glycol (Terathane, Du Pont, Inc.) with MW = 1000 and 2000, were degassed under vacuum at 55°C at 4.5 mm Hg for 3 h to remove any absorbed water, then stored over type 48, molecular sieves. Dimethyl sulfoxide (DMSO) (Nacalai Tesque, Inc.), methyl isobutyl ketone (MIBK, Hayashi Chemicals) and ethylene di- amine (EDA, Tokyo Chemicals) were distilled under reduced pressure. N,N’-dimethylacetamide (DMAc, Aldrich Chemicals) was used as received.

2.2 Poiymerization

Two series of segmented polyetherurethanes were synthesized by a prepolymer technique. A typical procedure is described below.

In a flame-dried 250ml two-necked round- bottomed flask equipped with a stirrer, a condenser fitted with a drying tube and a thermometer, 0.02 mol of polyol (PEG, PPG or PTMG) with MW = 1000 and 0.04 mol of MD1 in 70ml of a 50/50 (v/v) mixture of DMSO and MIBK were added. This flask was flushed with a slow stream of nitrogen during the addition. The reaction was then heated and stirred at 85°C for 3 h. Then it was cooled to room temperature, and 0.02 mol of ethylenediamine in 30 ml of DMSO was added to the rapidly stirred solution. The reaction was stirred for an additional hour at room temperature, then the viscous solution was poured into water to isolate the polymer.

The block copolymers based on 2000 MW polyol (PEG, PPG, or PTMG) were prepared in a similar manner.

2.3 Characterization

Simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) experi- ments (SDT) on the urethane copolymers were carried out on films placed in a platinum sample pan using a Du Pont SDT-2960 analyzer. Sample films ranging from 3 to 4 mg were cut into small pieces, loaded into the platinum pan and sealed in the sample chamber. The samples were heated from 50 to 600°C in a nitrogen atmosphere and static air at the rate of lO”C/min. During the heating period, the weight loss and temperature difference were recorded as a function of temperature.

Infrared spectra of the thin polymer films were obtained using a Bio-Rad FTS 165 Fourier transform infrared spectrometer. The spectra were obtained over the frequency range of 4000-400cm-’ at a resolution of 2 cm-‘. For comparison, infrared spectra after the samples had been heated at 270 and 300°C for 1 h were also taken.

Differential scanning calorimetry (DSC) ther- mograms from - 100 - 200°C were obtained using a Perkin-Elmer DSC 7 analyzer at a heating rate of lOC/min under a dry nitrogen purge.

For convenience, designation of the two series of samples is given in Table 1.

3 RESULTS AND DISCUSSION

3.1 The effect of polyol structure

The effect of polyol structure on the stability of both series of polyurethanes in a nitrogen atmosphere is illustrated in Figs 1 and 2. As shown in these figures, in a nitrogen atmosphere all TGA curves display a slower initial and then a more rapid degradation process, suggesting a two-step mechanism for the degradation. From the TGA curves, it was found that degradation was comparatively slow in the solid polymers, indicating that they are reasonably stable up to their melting points. In contrast, PEG-PU and PTMG-PU of both series have similar stability and are more stable than PPG-PU. Weight loss

Efect of structure and molecular weight on thermal stability

Table 1. Designation of poly(urethaneureas)

97

Polyol PU type Series A Series B Degradation in Degradation in

type (MW = 1000) (MW = 2000) air nitrogen

PEG PEG-PU PEG-PU-1 PEG-PU-2 PEG-PU-1 A PEG-PU-IN PEG-PU-2A PEG-PU-2N

PPG PPG-PU PPG-PU-1 PPG-PU-2 PPG-PU-1 A PPG-PU-1N PPG-PU-2A PPG-PU-2N

PTMG PTMG-PU PTMG-PU-1 PTMG-PU-2 PTMG-PU-1 A PTMG-PU-1N PTMG-PU-2A PTMG-PU-2N

started at lower temperatures and was more rapid in PPG-PU. If the criteria for stability are taken as the temperature at which 10% weight loss occurred, the same conclusion can be reached. The variations of these temperatures with the polyol type are given in Table 2.

In comparison with the curves in a nitrogen stream, the TGA curves for both series of polyurethanes in air show a similar trend. The temperature stability is increasing in the order: PPG-PU < PEG-PU < PTMG-PU, as is evident from Figs 3 and 4.

It is postulated that the degree of phase separation plays a major role in the decomposi- tion of polyurethane copolymers. Since it has been indicated that the initial degradation occurs in the hard segments, we can derive some important observations from infrared spectra recorded after the samples were degraded at elevated temperature (300”(Z), as shown in Figs 5 and 6.

The apparent decrease in the absorbance region ranging from 1630 - 1730 cm-’ (based on the C=C aromatic ring breathing at ca. 1600 cm-’ as internal standard) indicates that strong

120

- PEG-PU-IN

100

2

5 80 m ._

2 60 c_-

I .z 40

s

20

\ I I I I I I A-., -____

0

0 100 200 300 400 500 600

Temperature CC)

Fig. 1. SDT curves of A series polyurethanes in nitrogen.

degradations have occurred in the urethane and urea groups, with the degradation rate of urethane groups being more rapid than that of urea groups. As seen from endothermic peaks of DTA curves in Figs 1 and 2, it is possible that the degradation at the initial stages is merely a reflection of the volatilization of diol and diamine produced by decomposition of the urethane and urea groups.

Since free and hydrogen bonded urethane carbonyl groups are assigned at ca. 1725 - 1730 and 1705 - 1710 cm-‘, respectively,Y it appears that the initial degradation of polyurethane copolymers depends strongly on the extent of interurethane hydrogen bonding. Hence, the thermal stability of both series of polyurethane copolymers in air can be ranked in the order: PPG-PU < PEG-PU < PTMG-PU according to the degrees of interurethane hydrogen bondings of these polyurethanes (Figs 7 and 8).

The TGA curves for both series of pol- yurethanes in a nitrogen stream show a similar trend, with thermal stabilities generally following degrees of interurethane hydrogen bonding. Nevertheless, T,,,, of PEG-PU-2N is a little

120

100

s z 80

z

2 60

- PEG-PU-LN

- - - PPG-PU-2N

1

0 100 200 300 400 500 600

Temperature (“C)

Fig. 2. SDT curves of B series polyurethanes in nitrogen.

98 Tzong-Liu Wang, Tar-Hwa Hsieh

Sample

PEG-PU-1N 284.7 318.0 317.2 316.7 PPG-PU-IN 281.0 310.4 337.5 340.5 PTMG-PU-1N 306.8 326.8 346.5 340.5 PEG-PU-2N 301.5 334.1 329.7 331 .o PPG-PU-2N 289.1 320.9 300.2 333.7 PTMG-PU-2N 310.4 331.8 353.8 334.7 PEG-PU-1A 282.5 314.3 322.3 342.9 PPG-PU-IA 274.2 307.6 316.0 340.5 PTMG-PU-1 A 288.1 318.9 338.5 364.3 PEG-PU-2A 280.2 318.9 317.2 333.3 PPG-PU-2A 260.7 297.6 309.3 309.5 PTMG-PU-2A 285.9 325.7 309.5 228.6

Table 2. Characteristic temperatures on SDT curves

T!Jl

Ton (“0 T,,,.,, (“C) First weight loss (onset temp.) (10% weight loss) maximum T,,(“C)

DTA peak (“C)

T dZ -_____

409.5 388.1 418.4 413.2 380.8 418.4 376.2 361.9 395.2 345.2 333.3 342.9

higher than that of PTMG-PU-2N. This may be attributed to the fact that all interurethane hydrogen bonds for PEG-PU-2N and PTMG-PU- 2N dissociated at higher decomposition tempera- tures (T,,,,) and more phase mixing in PEG-PU-

120

- PEG-PU-IA

. mMG_PU_IA

80 -

60 -

40 -

20 -

0 100 200 300 400 500 600

Temperature (“C)

- PEG-PU-IA

___-_ PPG-PU-IA

..------. mMG_p”_I*

0 100 200 300 400 500 600

Temperature (“C)

Fig. 3. SDT curves of A series polyurethanes in air: (a) TGA curves: (b) DTA curves.

2N had a protection function on the hard segment.

According to Antipova et al.‘” urethane groups can behave as antioxidants, and it has been stated”.” that urea and amine groups may have a

120 - PEG-PC2A

--___ PPG-PUZA

_________ mMG_PU_ZA

100 200 300 400 500 600

Temperature (“C)

,

- PEG-PU-2A

_-_-- PPG-PU-2A

.._. PmG_PU_2A J.-h' ;:., :: ,J, :

___, _' -.

..__, ,... . .

_' . . _._._.__.__.__.._-~-~~-----'~ . ..__

0 100 200 300 400 500 600

Temperature (“C)

Fig. 4. SDT curves of B series polyurethanes in air: (a) TGA curves: (b) DTA curves.

Effect of structure and molecular weight on thermal stability 99

a: PEG-PU- 1 b: PFG-PU- 1 c: PTh4G-PU- 1

4ooo 3500 3ooo 2500 2C!QO 1500 loo0 500

Wavenumber (cm-‘)

Fig. 5. The infrared spectra of A series polyurethanes heated at 300°C for 1 h.

stabilizing effect on the soft segment. Therefore, the presence of hard segments can increase the stability of the soft segment, while the soft segment may have a protection function on the hard segment and hence increase the stability of segmented polyurethanes. The above facts can be interpreted as the mutual stabilization effect of segmented polyurethanes.

Combining the above results, it is generally concluded that the degree of phase separation has a tremendous effect on the thermal stability of polyurethane copolymers and there is a mutual stabilization effect between the soft and hard segments. When polymer samples degrade in air, lower decomposition temperatures manifest the importance of the extent of interurethane hydrogen bonding. Consequently, stronger inter- urethane hydrogen bondings arising from sharper

il: PEG-PU-2 b: PPG-PU-2 c: PTMG-PU-2

4000 3500 3000 2500 2000 1500 loo0 500 4cKKl 3500 3000 2500 2000 1500 loo0 500

Wavenumber (cm-‘) Wavenumber (cm”) Fig. 6. The infrared spectra of B series polyurethanes

heated at 3oWC for I h. Fig. 8. The infrared spectra of B series polyurethanes at

room temperature.

4000 3500 3000 2500 2000 1500 loo0 500

Wavenumber (cm-‘)

Fig. 7. The infrared spectra of A series polyurethanes at room temperature.

domain/matrix interfaces reduce degradation rates. It means that a higher thermal stability correlates with a higher degree of phase separation in this case. However, in the case of degradation in a nitrogen atmosphere, more phase mixing favors the thermal stability of polyurethanes after the dissociation of inter- urethane hydrogen bonding at a higher decom- position temperature.

In the discussion of degradation mechanism, the TGA curves in the air are rather more complex as compared to the curves in a nitrogen stream. For both series, Figs 3 and 4 show more than two stages of degradation. In contrast, the temperatures at 10% weight loss for both series are lower than those of the same samples degraded in a nitrogen atmosphere. Since the exothermic peaks of DTA curves did not occur

a: PEG-PU-2 b: PPG-PlJ-2 c: PTMG-PU-2 1

100 Tzong-Liu Wang, Tar-Hwa Hsieh

when DTA was carried out under Nz, it is apparent that the lower stability in air and the multistage process must be related to oxidation reactions in the soft and hard segments.

Subsequently, as compared to Figs 5 and 6, the IR results of the samples degraded at 270°C (Figs 9 and 10) clearly show the disappearance of characteristic absorbance regions at ca. 2600- 3000, 930 - 950, 1100 - 1105 and 1445 - 1450cm-’ arising from both series of polyether polyols, indicating the degradation of soft segments. It is probable that the first exothermic peak is caused by the oxidation of the soft segment. Additionally, as also shown in Figs 9 and 10, the absorbance peaks assigned to the carbonyl group (at ca. 1723 - 1728 cm-‘) rose during degradation for PTMG-PU which may be attributed to the initial oxidative attack on the carbon atom cy to the oxygen, resulting in chain scission in the C-O bond with the formation of oxidation products containing the carbonyl group. The mechanism has been proposed before.’ No obvious increase in the carbonyl absorbance region for PEG-PU and PPG-PU may indicate that oxidation mechanisms of PEG and PPG are somewhat different from that of PTMG. The mechanisms of the degradation reactions are beyond the scope of this paper. On the other hand, it is found that the char residues over 500°C for both series are higher in air than those in nitrogen. Since it has been stated” that oxidation reactions of the hard segment occur in a later stage than those of the soft segment, it is felt that the higher char residues are caused by

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-‘) Fig. 9. The infrared spectra of A series polyurethanes

heated at 270°C for 1 h.

c: PTMG-PU-2

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-‘) Fig. 10. The infrared spectra of B series polyurethanes

heated at 270°C for 1 h.

the oxidation reactions of the hard segment and hence increase the weight residues of samples.

3.2 The effect of polyol molecular weight

Figures 11 and 12 depict the effect of polyol molecular weight on the thermal stability of segmented polyurethanes in air. As Fig. 11 shows, PPG-PU-1A has a higher onset tempera- ture and 10% weight loss temperature than PPG-PU-2A. It seems that the longer soft segment is detrimental to the phase separation due to the presence of the methyl group side chain and hence reduces the extent of inter- urethane hydrogen bonding of PPG-PU-2A, as is evident from Figs 7 and 8. In contrast, the temperature at 10% weight loss for PEG-PU-1A is lower than that of PEG-PU-2A, indicating that the longer soft segment favors the phase separation and enhances the extent of inter- urethane hydrogen bonding of this type of segmented poly(urethaneureas). The TGA curves for PTMG-PU materials show a similar trend, as also seen in Fig. 11. The higher microphase separation in both segmented poly(urethaneureas) may be due to the crystalli- zation effect in the PEG and PTMG soft segments, which has been conformed by the melting peaks in DSC curves (Fig. 13).

The TGA curves for both series in a nitrogen stream have a similar trend in stability to those in air. The picture presented in Fig. 12 clearly shows that PEG-PU-2N is more stable in the initial stage and degrades rapidly in the later. It appears that PEG-PU-2N has a higher mutual stabilization

Effect of structure and molecular weight on thermal stability 101

x a 80

a .- $ EC 60

PEG-PU-IA _________ PEG_PU_2A

PPG-PU- 1 A __ _ __ _ _ _ _ PPG_PUJA

100 200 300 400 500 600100 200 300 400 500 600100 200 300 400 500 600

Temperature (“Cl Temperature (“C) Temperature (“C)

Fig. 11. Effect of polyol molecular weight on the thermal stability of segmented polyurethanes by thermal analysis in air.

effect than does PEG-PU-1N due to the hard segment being protected by a higher soft segment concentration in this polymer and hence increas- ing its degradation temperature (T,, and T,,,,). However, the later stage caused mainly by the soft segment decomposition and oxidation process is more rapid in PEG-PU-2N. This result can also be attributed to the higher soft segment

concentration in this sample. A similar trend was observed for the TGA curves of PTMG-PU and PPG-PU materials. The difference in the stability of PPG-PU in air and in nitrogen may be attributed to the dissociation of interurethane hydrogen bonding at a higher temperature in nitrogen. Consequently, the decomposition tem- perature of PPG-PU-1A is higher than that of

ILU

PEG-PU-IN _________ PEG_PU_2N

t

PPC-PU-IN ____-____ PPG_PU_2N

PTMG-PN-IN _________ mMG_PU_zN

loo 200 300 400 500 600100 200 300 400 500 600100 200 300 400 500 600 Temperature (‘C) Temperature (“C) Temperature (“C)

Fig. 12. Effect of polyol molecular weight on the thermal stability of segmented polyurethanes by thermal analysis in nitrogen.

Tzong-Liu Wang, Tar-Hwa Hsieh

- PEG-P&2 . mG_P”_2

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Madorsky, S. L. & Straus, S., J. Polym. Sci., 36 (1959) 183.

-100 -50 0 50 100 150 200

Temperature (“C)

Fig. 13. DSC curves of PEG-PU-2 and PI’MG-PU-2.

PPG-PU-2A as mentioned above, while in a nitrogen atmosphere the temperature stability of PPG-PU-2N is higher than that of PPG-PU-1N due to a higher soft segment concentration.

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