Synthesis of thermally and mechanically improved polyurethane-urea elastomers by using novel diamines as chain extenders

2.1 Chemicals

The chemicals, reagents, and solvents used in this study were of analytical grade obtained from Sigma-Aldrich Chemie GmbH (Stienheim, Germany) and used without further purification. These include; 4,4-methylenebis(phenyl isocyanate) (MDI), poly(tetramethylene) glycol (PTMG, M_n=2000 g/mol), chlorobenzene, sulfuric acid (96% H₂SO₄, 96%_, 18 m) , nitric acid (HNO₃, 70.4%, 15.9 m), sodium sulfate (Na₂SO₄), sodium carbonate (Na₂CO₃), ethylene glycol, propanediol, sodium metal, activated carbon, ethylene glycol monomethyl ether, hydrazine monohydrate (NH₂.NH₂.H₂O, 85%), n-hexane, ferric chloride (FeCl₃), hydrochloric acid (HCl, 37.2%, 12.1 m), ammonia solution (NH₄OH, 28%, 14.5 m), 1,4-dioxane, N,N-dimethyl formamide (DMF), potassium carbonate (K₂CO₃), methanol, and ethanol.

2.2 Synthesis

2.2.1 Synthesis of chain extenders

• 1st Step: In the first step, p-nitrochlorobenzene was synthesized by adding acid mixture (nitric acid:sulfuric acid 1:1) to chlorobenzene (300 ml) keeping the temperature below 40°C. After the acid mixture addition, the temperature was slowly raised to 75–80°C and the reaction was carried out for 2 h. This reaction mixture was washed with water at room temperature and neutralized with sodium carbonate and dried over anhydrous Na₂SO₄. By cooling this solution in an ice bath, the crystals of p-nitrochlorobenzene were obtained which were then dried completely in air (Scheme 1).

• 2nd Step: In the second step sodium salts of different diols were formed by reacting diols (0.1 mol) with sodium metal (small quantity) in toluene with continuous stirring at room temperature to form disodium alkoxides (Scheme 1). When bubbles of H₂ gas were stopped, unreacted Na metal pieces were removed, leaving behind a solution of disodium alkoxides.

• 3nd Step: In the third step, p-nitrochlorobenzene was reacted with sodium salt of different diols (0.1 mol) (Scheme 1, 6a–c). Sodium salts of different diols were formed by reacting diols with sodium metal in small quantities with continuous stirring at room temperature to form 1, 2-disodium alkoxides. Subsequently, a solution of p-nitrochlorobenzene in toluene was added drop wise into 1,2-disodium alkoxides and refluxed for 4 h. Upon pouring this mixture into a beaker containing 50 ml of methanol/water mixture (1:1), brown precipitates appeared which were crystallized from n-hexane to recover the nitro group terminated ether products.

• 4th Step: In the fourth step, the nitro group terminated ethers (0.1 mol), activated carbon (5.0 g), ferric chloride (1.0 g), and ethylene glycol monomethyl ether (150 ml) were put in a flask and heated at 110°C and stirred for half an hour and then hydrazine (85%, 0.5 mole) was added and followed by reflux for 8 h. To prevent the product from separating out, the mixture was hot filtered and washed with ethylene glycol monomethyl ether. The filtrate was neutralized with hydrochloric acid (20%, 300 ml). When white precipitates appeared, ammonia was added to neutralize the excessive hydrochloric acid until a pH of 11–12 was achieved. The precipitates were washed with distilled water followed by crystallization from ethanol and a 1,4-dioxane mixture to get novel diamine as 2,2-bis(p-(p-aminophenoxy)phenyl)propane (BAPP) (6a), 1,2-di(p-aminophenoxy)ethane (BAE) (6b) and 1,2-di(p-aminophenoxy)propane (BAP) (6c). Physical properties of the synthesized diamines are given in Table 1. The yield in the case of the nitro compound was ≥80% while on reduction it decreased to ≥65% for amines.

Scheme 1:

Synthesis of different diamines as chain extender.

Table 1:

Properties of synthesized diamine as chain extenders.

Properties of chain extenders	BAPP	BAE	BAP
Physical state	Solid	Solid	Solid
Color	Yellowish brown	Golden yellow	Light brown
Yield	82%	69%	81%
λmax (nm)	325	320	310
Melting point (°C)	165	158	177
Molecular weight	320	272	286

2.2.2 Synthesis of polyurethane-urea (PUU)

4,4′-Diphenylmethane diisocyanate (MDI) (17.4 g, 0.1 mol) was taken in a 250 ml three neck round-bottom flask equipped with a mechanical stirrer, a dropping funnel, and a nitrogen inlet. The flask was then placed in a water bath at 70°C. The pre-dried PTMG (50 g, 0.05 mol) was added from a constant pressure dropping funnel into MDI while stirring for a period of 30 min. The reaction mixture was then maintained at 70–80°C for 2 h with continuous stirring under nitrogen to obtain an NCO capped polyurethane prepolymer (16). The molar ratio between –NCO and OH (NCO/OH) in the pre-polymer was kept at 2, 2.3 and 2.5 mol by taking 0.1, 0.13 and 0.15 moles of MDI, and 0.05 moles of PMTG. PUU elastomers were synthesized by the reaction of PU prepolymer with three different diamines chain extenders (6a–c). Diamines dissolved in 1,4-dioxane, added drop wise in the PU prepolymer under stirring and heating for 0.5 h. This mixture was poured into Teflon molds and kept under room temperature for 24 h to evaporate all the solvent to achieve flexible PUU films. For curing purpose, the thin films thus obtained, were kept for 3 h in a vacuum oven maintained at 60°C (Scheme 2). Similarly 9 PUU films having different types of chain extenders with varying amounts being prepared. A reference sample (PUU-10) for comparison purpose was prepared through thermal cross-linking of the urethane prepolymer cured in moisture (for the structure of the reference sample, see supporting information scheme S3). Compositions of synthesized PUU systems are given in the Table 2.

Scheme 2:

Synthesis of polyurethane-urea elastomer.

Table 2:

Composition of synthesized PUU elastomers.

Polymer	Polyol	Diisocyanate	Chain extenders	Molar ratio MDI:PTMG:CE	NCO/OH ratio of prepolymer
PUU-1	PTMG	MDI	DAPE	2:1:1	2
PUU-2	PTMG	MDI	DAPE	2.3:1:1.3	2.3
PUU-3	PTMG	MDI	DAPE	2.5:1:1.5	2.5
PUU-4	PTMG	MDI	DAPP	2:1:1	2
PUU-5	PTMG	MDI	DAPP	2.3:1:1.3	2.3
PUU-6	PTMG	MDI	DAPP	2.5:1:1.5	2.5
PUU-7	PTMG	MDI	DAPB	2:1:1	2
PUU-8	PTMG	MDI	DAPB	2.3:1:1.3	2.3
PUU-9	PTMG	MDI	DAPB	2.5:1:1.5	2.5
PUU-10	PTMG	MDI	H2O	2:1:1	2

3.1 Fourier transforms infrared attenuated total reflectance (FTIR-ATR) spectroscopy

The initial characterization of different samples was carried out by using FTIR-ATR spectroscopy. The infrared spectra were recorded on a Vertex 80v FTIR spectrophotometer (4000–400 cm^-1), resolution 2 cm^-1, 32 scans per measurement) (Bruker Optik GmbH, Ettlingen, Germany). The spectra were taken in absorbance mode by placing the samples on the ATR cell.

3.2 Thermogravimetric analysis

The thermogravimetric analysis (TGA) was carried out using a thermogravimetric analyzer (Q500, TA Instruments, USA) starting from ambient temperature to 450°C in the high-resolution mode with a heating rate of 10°C/min under nitrogen atmosphere. The accuracy of the measuring balance was 0.1% and for the temperature calibration, the well-known Curie temperature of the nickel standard sample was measured as a reference. The weight loss as a function of temperature for the reference sample and PUU was recorded.

3.3 Mechanical properties

Stress-strain behavior of the reference sample and the PUU was determined according to the ISO 527 method at a cross-head speed of 200 mm/min using a tensile testing machine from Zwick GmbH (Ulm, Germany).

3.4 Small angle X-ray scattering

Small angle X-ray scattering (SAXS) experiments were performed on the compact PU and PUU films at room temperature using a pinhole instrument designed by JJ X-rays with a Rigaku rotating anode as radiation source (Cu, K_α, λ=0.15406 nm), an Osmic multilayer optics, and a Bruker Hi-Star 2D detector. The sample-to-detector distance was about 1580 mm.

4.1 Thermogravimetric analyses (TGA)

Thermal stability of the prepared PUU elastomers was evaluated by TGA. The representative TGA thermograms and the respective differential thermal analysis (DTA) curves, for reference sample and various PUU elastomers, prepared with NCO/OH=2 are shown in Figure 3A and B, respectively. The TGA data show a maximum weight loss, i.e. ≥70% in the temperature range 390–412°C for all the samples (also see Figure 4). This weight loss has occurred due to the degradation of hard segments. Nevertheless, as evident from the DTA plots shown in Figure 3B, there is a clear shift of the peak maximum (the peak maximum corresponds to the maximum rate of weight loss with respect to temperature) towards lower temperature after employing the synthesized diamines as chain extenders. For example, the peak maximum shifts from 410°C for reference sample to 396°C for PUU with BAPP as chain extender. It is clear that the creation of the urea hard segment has a destabilizing effect on the thermal stability of the PUU samples in comparison to the reference material. This effect is more pronounced in the case of diamine having an aliphatic middle chain, i.e. BAE and BAP. At lower temperature, there are two more steps of weight loss during degradation for the diamine extended samples compared to the reference sample (Figure 3B), which are assigned to the degradation of hard segments (which resulted from the reaction of isocyanate and diamine).

Figure 3:

(A) TGA and (B) DTA data plots of reference PU (NCO/OH=2) and PUU (NCO/OH=2.5) prepared using chain extenders BAE, BAP, and BAPP, respectively.

Figure 4:

Integrated TGA data plot showing the weight loss in different temperature ranges, of reference PU (NCO/OH=2) and polyurethane-urea PUU (1-9) (NCO/OH=2.5).

Figure 4 sums up the weight loss for all the four samples discussed in Figure 3 including the reference sample. Again, the major weight loss (≥70%) is shown to be in the temperature range of 390–410°C.

4.2 Stress-strain properties

Uniaxial tensile tests were performed in order to obtain the information about the stress-strain behavior at room temperature of solution cast films of the PUU and reference sample. In each case three samples were tested and the representative stress-strain data are shown in Figure 5. A prominent feature for most of the samples (c.f. zoomed data on the right of Figure 5) is a narrow elastic region followed by a pronounced yield point appearing at about 40–50% strain. On further extension a stable neck is seen as a drop in the stress-strain curve followed by a cold drawing region followed by strain hardening at strain values (ε≥100%) as indicated by a strong rise of the stress level until the sample breaks at strain values ≥1500%. In relative terms, ductile behavior with a large degree of plastic deformation is observed for all the samples. Slightly different behavior is observed for samples crosslinked with bisphenol based diamine (BAPP) which show lower yield strength (c.f. zoomed data on right of Figure 5) and large strain at break. It is assumed that the micro-phase separated structures are weakly organized in PUU films of BAPP based samples. The samples cross-linked with BPE and BAP show higher yield strength compared to the reference sample.

Figure 5:

Representative stress-strain curves for samples containing different moles of diamines (NCO/OH=2-2.5).

The zooms on the right hand side show differences regarding the yielding behavior.

For all the tested samples the Young’s modulus data are depicted in Figure 6. The data trend may be attributed to the structure of the aliphatic/aromatic middle block of the employed diamine. Compared with BAPP there are ethylene and propylene segments are present in the structures of BPE and BAP. For example, the improved modulus and even the yield strength of the BPE and BAP based PUU samples could be due to better packing of the phase separated structures as compared with BAPP (with aromatic ring) based systems. It is also relevant to comment that the hard domains provide both physical crosslink (25) points and filler-like reinforcement to the soft segment matrix (26), (27) which leads to the enhanced modulus for BAE and BAP containing PUU as compared with the reference sample. In contrast, the rigid backbone of BAPP hinders the segregation of the hard segments leading to lower modulus values.

Figure 6:

A comparison of Young’s moduli of polyurethane elastomers cross-linked using different moles of diamines (NCO/OH=2-2.5).

4.3 Small angle X-ray scattering studies

Figure 7 shows the SAXS data for reference sample and the prepared PUU samples using various diamines as chain extenders. These measurements were carried out to study the phase separated morphology. A significant scattering peak resulting from the morphology of segmented PU and PUU systems can be observed, which corresponds to the typical hard-segment inter-domain spacing in the samples. As expected, the diamine cross-linked samples show relative weak intensity value compared to the reference material due to incorporation of soft segments. The inter-domain distance can be calculated using d₁₀₀=2π/q₁₀₀, from the peak maximum of the primary scattering peak. The estimated d₁₀₀ values differ slightly for the PUU sample from the reference material (24 nm), however, the BAPP based PUU sample shows much larger domain spacing of ~28.5 nm.

Figure 7:

Representative small angle X-ray scattering (SAXS) traces for PU (reference sample) and PUU samples.

The position of q₁₀₀ in each case is indicated by vertical lines.