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Studies on poly(urethane–urea)s based on zinc salt
of mono[hydroxyethoxyethyl]phthalate
R. Jayakumar a,b,*, S. Radhakrishnan c^ , S. Nanjundan b
a (^) Division of Environmental and Chemical Engineering, The Research Institute of Industrial Technology, Engineering Research Institute, Chonbuk National University, Chonju 561-756, South Korea b (^) Department of Chemistry, Anna University, Chennai 600 025, India c (^) Department of Chemistry, Merit International Institute of Technology, Ootacamund, The Nilgiries 643 002, India
Received 28 February 2003; received in revised form 24 June 2003; accepted 21 July 2003
Abstract
Zinc containing poly(urethane–urea)s having ionic links in the main chain were synthesized by the reaction of hexamethylene diisocyanate or tolylene-2,4-diisocyanate with 1:1 mixture of zinc salt of mono(hydroxyethoxyethyl) phthalate and each of the bisureas such as hexamethylene-bis(x,N-hydroxyethylurea), tolylene-2,4,-bis(x,N-hydroxyethyl- urea), hexamethylene-bis(x,N-hydroxypropylurea) and tolylene–2,4-bis(x,N-hydroxypropylurea) using di-n-butyltin dilaurate as catalyst. These polymers were characterized by FT-IR, 1 H NMR and 13 C NMR spectroscopy, elemental analysis, solubility test, viscosity measurement and X-ray diffraction analysis. Thermal properties of the polymers were determined by differential scanning calorimetry and thermogravimetric analysis. Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Zinc salt of mono(hydroxyethoxyethyl)phthalate; Ionic monomer; Bisureas; Poly(urethane–urea)s; Thermal studies
- Introduction
Urethane-based materials are of commercial interest in many applications owing to their abrasion resistance, low temperature flexibility, high strength and aging and chemical resistance. There are various ways of combining polyols and diisocyanates in order to produce tailor-made
polyurethanes [1,2]. The proper combination of these and other reagents results in versatile poly- mers, which have wide range of applications as foams, elastomers, coatings and elastomeric fibers [3–6]. Poly(urethane–urea)s are a class of very impor- tant copolymers. They are composed of a class of elastomers exhibiting superior extensibility, tough- ness and extensively used ranging from textile fibers to medical prosthesis [7,8]. Introduction of urea group into the polymer backbone is expected to improve the solubility of the polymer without de- creasing the thermal stability significantly. High
Reactive & Functional Polymers 57 (2003) 23– www.elsevier.com/locate/react
REACTIVE
FUNCTIONAL
POLYMERS
- (^) Corresponding author. Fax: +82-063-270-2306. E-mail address: [email protected] (R. Jayakumar).
1381-5148/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2003.07.
modulus polyurethane–urea elastomer has many practical applications; the most important are in the automotive industry [9,10]. Incorporation of metal and functional groups into the polymers have emerged and find wide applications as aqueous thickeners, impregnates, coatings, textile seizers, adhesives [11,12], additives [13], resins [14,15], catalysts [16] and in the bio- medical field [17,18]. Ionic diols containing ionic linkages between COO^ and M 2 þ^ are of our in- terest. They are very important starting materials for the synthesis of ionic polymers in which the metal is incorporated into the backbone of the polymer. Metal containing polymers with ionic links formed between COO^ and M 2 þ^ in the backbone have already shown by us [19–25]. The present investigation is aimed at the syn- thesis and characterization of hexamethylene diis- ocyanate [HMDI] and tolylene-2,4-diisocyanate [TDI]-based poly(urethane–urea)s from zinc salt of mono(hydroxyethoxyethyl)phthalate [Zn[HEEP] 2 ], hexamethylene bis(x,N-hydroxyethylurea) [HBHEU], tolylene-2,4,-bis(x,N-hydroxyethylurea) [TBHEU], hexamethylene bis(x,N-hydroxypropyl urea) [HB HPU] and tolylene-2,4-bis(x,N-hydroxypropylu- rea) [TBHPU].
- Experimental
2.1. Materials
Phthalic anhydride (BDH), diethylene glycol (Fluka), di-n-butyltin dilaurate (DBTDL) (Fluka), ethanolamine (Aldrich), propanolamine (Aldrich) and zinc acetate of extra pure grades were used without any purification. HMDI and TDI (Fluka) were also used without prior purification. The solvents such as acetone, methanol, dimethyl
formamide (DMF), dimethyl sulfoxide (DMSO), dimethyl acetamide (DMAc), benzene, toluene, m- cresol and chloroform were purified by standard procedures. Zn[HEEP] 2 was synthesized as re- ported in our previous paper [23]. The structure of Zn[HEEP] 2 is shown in Fig. 1. Bisureas such as HBHEU, TBHEU, HBHPU and TBHPU were synthesized according to the reported method [20,21].
2.2. Synthesis of poly(urethane–urea)s
Zn[HEEP] 2 (0.005 mol) and any one of the bisureas, HBHEU, HBHPU, TBHEU or TBHPU (0.005 mol) dissolved in 200 ml of DMSO were added to a three necked flask fitted with a nitrogen inlet, a condenser and a dropping funnel. To this 2–3 drops of DBTDL was also added as catalyst. Then 0.01 mol of HMDI or TDI dissolved in 25 ml of DMSO was slowly added with constant stirring under nitrogen atmosphere for 25–45 min at 95 °C. After the addition the mixture was kept under stirring at the same temperature for 4 h. Then 50 ml of DMSO was added to the mixture and the contents were filtered. The filtrate was poured into a large excess of vigorously stirred chloroform to precipitate the product. The product was further washed with acetone several times and dried at 60 °C in vacuum. In the similar method, using Zn[HEEP] 2 and four different bisureas, eight zinc containing poly(urethane–urea)s were synthesized based on HMDI or TDI. The polymers were en- coded as Zn[HEEP] 2 -HMDI-HBHEU (I), Zn[HE EP] 2 -HMDI-HBHPU (II), Zn[HEEP] 2 -HMDI-TB HEU (III), Zn[HEEP] 2 -HMDI-TBHPU (IV), Zn [HEEP] 2 -TDI-HBHEU (V), Zn[HEEP] 2 -TDI-HB HPU (VI), Zn[HEEP] 2 -TDI-TBHEU (VII) and Zn [HEEP] 2 -TDI-TBHPU (VIII).
2.3. Measurements
The FT-IR spectra of the polymers were re- corded on a Testscan Shimadzu FT-IR 8000 series spectrophotometer at room temperature using KBr pellets. The 1 H- and 13 C NMR and DEPT spectra of the polymers were recorded on a JEOL GSX-400 MHz spectrometer using DMSO-d 6 as
HO(CH 2 ) 2 O(CH 2 ) 2 OOC COOZn OOC^ COO(CH 2 ) 2 O(CH 2 ) 2 OH
Zn[HEEP] (^2)
Fig. 1. Structure of Zn[HEEP] 2.
observed between 2931 and 2857 cm^1. The car- bonyl stretching of the urethane, urea and ester groups show a peak at 1687–1685 cm^1. The car- boxylate ion of the zinc salt gives two broad peaks between 1624 and 1472 cm^1. This confirms the presence of ionic linkage in the polymer. These bands were not found in metal-free analogues of these polymers. The C–H out of plane bending vi-
bration of aromatic system shows a peak at 772– cm^1. The FT-IR spectra of TDI-based polymers show a peak at 3306–3290 cm^1 due to –NH stretching. The C–H asymmetrical and symmetri- cal stretchings due to the methyl and methylene groups are observed between 2921 and 2852 cm^1. The peak at 1691–1684 cm^1 is attributed to the
Table 2 Elemental analysis data of the polymers
Polymer (^) Analytical data found (calculated) C (%) H (%) Zn (%) I 52.43 (52.11) 6.83 (6.39) 4.98 (5.45) II 53.24 (52.88) 6.99 (6.58) 4.80 (5.33) III 53.31 (52.85) 6.13 (5.86) 4.93 (5.43) IV 54.02 (53.60) 6.34 (6.05) 4.67 (5.30) V 53.89 (53.58) 5.59 (5.33) 4.89 (5.40) VI 54.62 (54.31) 5.82 (5.54) 4.60 (5.28) VII 54.73 (54.31) 5.15 (4.81) 4.62 (5.37) VIII 54.36 (55.02) 5.29 (5.02) 4.41 (5.25)
Scheme 1. Synthesis of zinc containing poly(urethane–urea)s.
carbonyl stretching of urethane, urea and ester groups. The carboxylate ion shows two peaks in the range from 1643 to 1461 cm^1. The C–H out of plane bending vibration of aromatic system shows a peak at 752–746 cm^1.
3.2.2. 1 H NMR spectra The 1 H NMR spectra of HMDI-based polymers show signals for the –NH protons of the urethane and urea groups which are shifted to down field due to the inter- and intramolecular hydrogen bonding between –NH group and C@O group and with the S@O group of the solvent (DMSO-d 6 ). The –NH protons appear between 8.25 and 8.15 ppm. The
aromatic protons show signals between 7.42 and 7.13 ppm. The urethane peak is observed between 6.64 and 6.53 ppm. The urea group appears be- tween 5.84 and 5.62 ppm. The methylene group attached to ArCOO– and NHCOO– group shows signal between 4.23 and 4.08 ppm and between 3. and 3.85 ppm, respectively. The signal in the region of 3.51–3.43 ppm is attributed to the –CH 2 –O– CH 2 – group. The signals at 3.21–3.09 ppm are due to the methylene group adjacent to –NHCOO and –NHCONH groups. The other methylene protons in the poly(urethane–urea)s show broad peaks be- tween 1.51 and 1.18 ppm. Fig. 2 shows the 1 H NMR spectrum of polymers II (a) and IV (b).
Fig. 2. 1 H spectrum of (a) Zn[HEEP] 2 -HMDI-HBHPU, (b) Zn[HEEP] 2 -HMDI-TBHPU, (c) Zn[HEEP] 2 -TDI-TBHPU.
respectively. The urea carbonyl carbon shows resonance signal at 166.08–166.02 and 155.17– 155.14 ppm. The urethane carbonyl carbon signals are observed at 153.78–153.73 and 152.13–152. ppm. The aromatic carbons to which the carbox- ylate and ester carbonyl carbon groups are at- tached show resonance signals at 135.55–135. and 129.46–129.42 ppm, respectively. The aromatic carbons gave signals between 136.17 and 121.33 ppm. The methylene carbons in the –OCH 2 CH 2 OCH 2 CH 2 O– group show signal at 72.03–68.71 ppm. The peak at 17.35–17.32 ppm is due to the methyl carbon attached to aromatic ring. The methylene group attached to –COON- HAr and –NHCONHAr group shows signal at 63.58–63.52 and 38.12–38.10 ppm, respectively.
3.2.4. Inherent viscosity The inherent viscosities of the polymers in DMSO were found to be very low (Table 1). This
may be due to fact that in a more polar solvents like DMSO the ionic links present in the polymer chain dissociate resulting in reduction of the mo- lecular weight of the polymer when the concen- tration is low [20,24,25]. The HMDI-based polymers show higher inherent viscosities than TDI-based polymers. This is due to the higher hydrodynamic volume of HMDI-based polymers will be more than TDI-based polymers.
3.3. Thermal analysis
The TGA data are given in Table 3. All the metal containing polymers are structurally similar, but they show different thermal behavior. The initial decomposition temperature (IDT) values are found between 233 and 272 °C. It is observed that TDI-based polymers show higher IDT than HMDI-based polymers. This may be due to the presence of stiff phenylene rings in the main chain.
Fig. 4. 13 C NMR spectra of Zn[HEEP] 2 -TDI-HBHPU.
It was found that 50% weight loss occurred be- tween 328 and 334 °C for HMDI-based polymers. In the case of TDI-based polymers 50% weight loss occurred between 335 and 352 °C. In general, all the poly(urethane–urea)s show slightly higher stability than the metal containing polyurethanes [23], this may be due to the presence of a higher degree of hydrogen bondings and lower amount of metal ion in the poly(urethane–urea)s. In all the cases, the residual weight at 800 °C corresponds to roughly the amount of ZnO formed. All the polymers show two stage decompositions. The glass transition temperatures (Tg ) of the polymers are shown in Table 3. All the polymers show a single Tg value. This reveals that the ab- sence of formation of mixture of homopolymers or a block copolymer. The difference in the Tg values between TDI- and HMDI-based poly(urethane– urea)s and the same of bisureas is high. Though HMDI units have flexible chain, due to increase in the crystallinity of HMDI-based polymers (re- vealed by X-ray studies), the Tg values increase and they become lower than that of TDI-based poly(urethane–urea)s.
3.4. X-ray diffraction
X-ray diffraction patterns of the TDI-based polymers VII and VIII do not show sharp peaks. Thus, they can be considered as amorphous in nature. HMDI-based poly(urethane–urea)s are partially crystalline in nature and they show some sharp peaks with a broad peak at 2h ¼ 20–25. Among the polymers HMDI-based poly(ure- thane–urea)s I and II have the maximum crystal-
linity due to the presence of hexamethylene units in both bisurea and the diisocyanate units which helps in the folding of the chain to form crystalline regions. The HMDI-based polymers III and IV have low crystallinity because in these polymers the bisurea unit contains a tolylene ring and not a hexamethylene unit. The low crystallinity of the polymers V and VI is due to the presence of hexamethylene groups only in the bisurea units and not in diisocyanate units. The crystallinity of the polymers follow the order II > I > IV > III
VI > V.
- Conclusions
Zinc containing poly(urethane–urea)s were prepared by the reaction of HMDI or TDI with Zn[HEEP] 2 and bisureas. The presence of ionic linkage in the polymers was confirmed by FT-IR spectra. The polymers were soluble in DMF, DMSO and DMAc and insoluble in most of the common organic solvents. Thermal properties of the polymers showed that the TDI-based polymers are more stable than the respective HMDI-based polymers. From the analytical methods it was found that the metal content in poly(urethane– urea)s are lower than the calculated values indi- cating that the reactivity of bisureas is higher than that of metal containing ionic diols relative to the diisocyanates. The inherent viscosities of the polymers were very low due to the occurrence of chain dissociation in DMSO solvent. HMDI- based polymers have higher inherent viscosities than the TDI-based ones. X-ray diffraction pat-
Table 3 DSC and TGA data of the polymers
Polymer Tg (°C) IDT (°C) (^) Temperature at wt. loss (°C) Wt. loss at 10% 25% 50% 75%^800 °C (%) I 9.3 236 293 322 329 383 93. II 8.8 233 298 325 331 386 93. III 22.7 241 287 318 328 375 93. IV 22.1 238 299 323 334 391 93. V 10.4 244 310 338 351 407 93. VI 9.8 242 298 334 338 410 93. VII 23.9 272 288 328 335 415 93. VIII 23.1 264 312 340 352 389 95.
