Poly(urethane–urea) based on functionalized polystyrene with HMDI: Synthesis and characterization

2.1 Materials

1,6-Hexamethylenediamine (HMDA), 1,6-Hexamethylenediisocyanate (HMDI), 1,2-Ethylenediamine (EDA), 1,4-Butanediamine (BDA), Deuterated dimethysulfoxide (d₆ DMSO), Phenol and 1,1,2,2-tetrachloroethane are purchased from Aldrich Chemical Co. Tetrahydrofuran, Toluene, Methanol, and Dimethyl acetamide are purchased from S.D fine Chemical Co. and used as such without further purification. The functionalized hydroxy terminated polystyrene (OH–PSt–OH) is synthesized using the known route (Arun et al., 2013).

2.2 General procedure for the synthesis of block copolymer

Segmented block copolymers are synthesised with various mole ratios of soft to hard segments (Table 1) with three different chain extenders. The synthetic route for the synthesis of copolymer is presented in Scheme 1. The preparation of the copolymer sample having code of 3a is here given as an example. Functionalized polystyrene having M_w of 2000 g/mol (6 g, 0.003 mol) is placed in a 100 ml three-necked round-bottom flask, fitted with mechanical stirrer, nitrogen inlet and heated in an oil bath. When the temperature reached 120 °C, a small excess of HMDI (1.1, 0.006 mol) is added with stirring for 3 h. After 3 h, the temperature is raised to 150 °C and then 0.35 g (0.003 mol) of HMDA is added with constant stirring for a period of 2 h. Thus the polymer is formed. The solid polymer is transparent, while being semicrystalline. The materials have a slightly cream in hue. Yield: 6 g (80%). ¹H NMR (δ, ppm): 6.5–7.3 (b, aromatic proton), 4.8 (b, >NH), 3.2 (b, –CH₂– connected to urea group), 3.0 (b, –CH₂– connected to urethane group) 1.3–1.8 (b, aliphatic proton).

Close

Scheme 1 The reaction pathway for the synthesis of semi-crystalline copolymer.

Export to PPT

2.3 Characterization

Infrared spectra are recorded using Alpha Bruker FT-IR connected with alpha-T accessory with a resolution of 4 cm⁻¹ recorded between 30 °C and 120 °C. Samples are prepared by adding a droplet of a polymer solution (HFIP (1 g/L)) on a pressed KBr pellet and the measurements are taken at room temperature (Wolinska-Grabczyk, 2004).The degree of crystallinity of the rigid segments in the polymers could be estimated with the following equations:

The heights (h) of the amorphous and crystalline urethane peaks are related by the factor “C” with a value of 2.4 (Arun et al., 2009).

The inherent viscosity of the copolymers at a concentration of 0.1 dL/g in a 1:1 (molar ratio) mixture of phenol/ 1,1,2,2-tetrachloroethane is determined at 25 °C using capillary Ubbelohde.

2.4 Swelling ratio of polymer

The equilibrium swelling ratio is measured on pieces of injection-moulded polymer bars. The samples are placed in desiccators with a layer of demineralised water for four weeks at room temperature. The swelling ratio is defined as the weight gain of the polymer according to Eq. (3):

2.5 Solvent resistivity

Injection moulded sample of dimension 10 × 10 × 2 mm are used for solvent resistance measurements. Previously weighed sample is dipped in 50 ml of organic solvents taken in a flask and shaken for 60 min. After that, the solvent is decanted and the flask is dried for 24 h at 70 °C. It is weighed again and from the weight loss, the amount of polymer sample dissolved in that solvent is calculated by using Eq. (4).

3.1 Synthesis of polyurethane–urea

PUU is synthesized by two step poly addition reaction. In the first step, a prepolymer is synthesized in the melt using one mole of PSt with two moles of diisocyanate followed by the reaction with different chain extenders (EDA, BDA and HMDA) leads to high molecular weight poly(urethane–urea). Reaction of hydroxyl groups with the isocyanate group’s results in urethane group further reaction of diamine with an isocyanate leads to urea groups in the polymer chain. Polyurethanes with diamine extenders have high melting temperatures, sometimes too high to melt synthesis and melt process without degradation. The Chemical structure of synthesized PUU is shown in Scheme 1.

3.2 Molecular weight measurements

The hydroxyl terminated polystyrene shows the inherent viscosity of 0.1 dL/g. The PUU copolymer shows high inherent viscosities (0.36–2.0 dL/g), suggesting that the polymer is of high molecular weight. The PUU based on HMDA has inherent viscosities ranged between 0.5 and 2.0 dL/g whereas the inherent viscosity EDA and BDA based PUU ranged between 0.36–1.1 and 0.45–1.6 dL/g respectively. The data are shown in Table 1. In all the three series, the inherent viscosity is linearly increasing with increasing the molecular weight as shown in Fig. 1. It shows that as the length of the soft segment increases, the inherent viscosity increases accordingly.

Close

Figure 1 Effect of soft segment length on the molecular weight of the polymer.

Export to PPT

3.3 FT-IR spectra

Two different carbonyl groups that can form hydrogen bonds are urethane and urea groups (Weber and Heckmann, 1998). Urea groups can form either bidentate or monodentate hydrogen bonds. The FT-IR spectra of 1a, 2a, and 3a series at room temperature are shown in Fig. 2. IR spectrum shows characteristic peak at 3303–3318 cm⁻¹ for crystalline N—H groups. The amorphous and crystalline urethane carbonyls appear around 1740 and 1730 cm⁻¹ respectively. The monodentate crystalline urea carbonyl group appears around 1686–1669 cm⁻¹ group. In addition, absorption bands arising from asymmetric —CH₂ stretching centred at 2917 cm⁻¹ and symmetric —CH₂ stretching at 2852 cm⁻¹ is also observed.

Close

Figure 2 FT-IR spectrum of three copolymers at room temperature: ●, 1a; ▴, 2a; ♦, 3a.

Export to PPT

The temperature dependent FTIR spectra of 1a, 2a and 3a in the region of 1500–1800 cm⁻¹ are presented in Fig. 3. In the case of 1a series, some of the H-bonded carbonyl urethane in PUU breaks up during heating to form non-H-bonded free carbonyl. The hydrogen bonded urethane carbonyl peak will appear at lower wave number around 1735 cm⁻¹ and highest intensity, which is attributed to a reduction in the electron density of the carbonyl groups due to hydrogen bonds (Yilgor et al., 2006). The peak around 1745 cm⁻¹ is assigned to stretching frequency of non-bonded urethane carbonyl groups. The peak around 1710 cm⁻¹ due to amorphous urea carbonyl and the H-bonded urea carbonyl groups is centred around 1660–1690 cm⁻¹ corresponds to bidendate and monodendate assemblies (Table 2). The effect of temperature on the crystallinity of the hard segment is given in Fig 4. In EDA based PUU, upon heating, the peak intensity of H-bonded urethane C⚌O groups and urea groups decreases. Similarly, on cooling from 120 to 30 °C, the H-bonded urethane C⚌O peak intensity increases and the peak for free urethane C⚌O group decreases.

Close

Figure 3 Temperature dependent FTIR spectrum of three copolymer at different temperatures: ●, 1a; ▴, 2a; ♦, 3a.

Export to PPT

Close

Figure 4 Crystallinity studies of three polymers: (a), 1a; (b), 2a; (c), 3a.

Export to PPT

Remarkably, the crystallinity of the EDA based PUU is slightly decreased up to 120 °C. Whereas, the crystallinity of the BDA and HMDA based PUU decreases drastically with increasing the temperature clearly suggesting that the former is formed much stable hydrogen bonding than the later two polymers. It is concluded that the physical crosslinks decrease as the length of the carbon chain increases i.e., close packing between hard and soft segment decreases, so that crystallinity decreases in 3a and 2a with respect to 1a series.

3.4 WAXS

The WAXS graph of the 2a sample is presented in Fig. 5 and the graph shows three prominent peaks observed at 1.4, 10.69 and 20.32 two thetas corresponding to three different planes and the length of the crystalline planes is 10, 2.5 and 1.5 A⁰ respectively. Three peaks at 2 theta values of 1.4, 10.69 and 20.32 deg corresponding to (0 0 1), (1 0 0) and (1 1 1) planes of PUU are observed. These data are shown in Tables 4 and 5 (Theivasanthi Alagar, 2010). Also, the WAXS pattern of the polymer shows sharp peaks which clearly suggesting that the hard segments are in crystalline form. However, the presence of broad peaks at the bottom of the sharp peak proves that the materials are not completely crystalline; in fact the crystalline hard segments are embedded in the amorphous soft matrix.

Close

Figure 5 WAXS at room temperature: (a), 1a; (b), 3a.

Export to PPT

In contrary to 1a sample, 2a and 3a show no characteristic peaks (in fact a shallow peak is observed for 3a sample and presented in Fig. 5) in WAXS measurement. This highly crystalline nature of the EDA based polymer is also supported by the IR and DSC data which are presented in Tables 2 and 3. However, the presence of broad peaks at the bottom of the sharp peak proves that the materials are not completely crystalline; in fact the crystalline hard segments are embedded in the amorphous soft matrix.

3.5 DSC

The DSC thermograms of the second heating and the first cooling curve of the polymers 1a, 2a and 3a are given in Fig. 6. Heating curve shows two peaks corresponds to the Tg of soft phase and melting endotherm of hard phase. The cooling curve of 1a and 2a series shows crystallization exotherm and this is not observed in 3a series and the results are presented in Table 3. Table 3 shows that the melting endotherm depending upon the hard segment content and for the second series from 1a to 1d was found to be between 266 and 261 °C. The Tg of SS for the polymers 1a to 1d was started at 89 °C and ends at 86 °C. The effect of hard segment content on the Tg of the polymer shows that as the concentration of HS increases, the Tg of the SS increases due to some of the HS mixed with the SS matrix thereby increasing the soft segment Tg. Also, Table 3 shows that the melting of hard segment is depending on the size of the chain extender used in the preparation PUU and for the samples 1a, 2a and 3a they are 266, 217 and 157 °C respectively. This value shows that the hard segment based EDA crystallizes in a better way than the BDA and HMDA counterparts. This explanation also supported by the shift in the Tg value of the polymer to a higher temperature upon changing the chain extender from EDA to HMDA. HMDA based polymer has higher Tg value with low Tm due to the lower crystallinity. Similar trend is found out in IR and WAXS measurements. The crystallinity of the hard segment is decreased in the order of HMDA < BDA < EDA.

Close

Figure 6 DSC heating curve and cooling curve of three polymers: ●, 1a; ▴, 2a; ♦, 3a.

Export to PPT

3.6 TGA

TGA curve of the 1a, 2a, and 3a polymer samples is given in Fig. 7 and the data are presented in Table 3. Generally the thermal stability of a polymer will be determined by the strength of its weakest bond. For the polymers in series 1, 2 and 3, they are structurally similar but chain length is varied, so it shows different thermal behaviour. The decomposition temperature values of the polymer are found between 413 and 415 °C. All the materials show the single stage decomposing temperature centred around 413 °C. Above that temperature, PUU decomposed completely. From Table 3, one observes that, as the HS content increases, the thermal stability increases slightly, which is due to the increase in toughness of the system.

Close

Figure 7 TGA thermograms of the copolymer: □, PSOH; ●, 1a; ▴, 2a; ♦, 3a.

Export to PPT

3.7 Swelling ratio of polymer

The swelling property of the polymer is measured and presented in Table 3. PUU is semi-crystalline and therefore physical crosslinks between the chains are possible. The density of physical crosslinking depends on the hard segment content. The physical crosslinking point opposes the water molecule to penetrate through the soft matrix. Therefore on increasing the hard segment, the swelling ratio decreases (Yildiz and Hazer, 2000). The swelling ratio of PUU–EDA is 0.5–6 wt.%, PUU–BDA is 0.8–8 wt.% and PUU–HMDA is 2–9 wt.%. The swelling ratio decreases in the order of 1a < 2a < 3a due to close packing between soft and hard segments. The effect of hard segment content on the swelling ratio is presented in Table 3 and reveals that the swelling ratio increases with decrease in the hard segment content on the copolymer.

3.8 Solvent resistivity

Solvent resistance behaviour of multi-block copolymers was studied by suspending injection moulded bars in chloroform at room temperature for 1 h. The PSt dissolves completely in chloroform with in 1 h as shown in Table 3. The multiblock copolymer has dramatic lower weight losses in chloroform. Although solubility decreases with increasing hard segment content, the absorbed high values of solubility are due to the highly polar nature of the urethane–urea linkage present in the polymer chain. The weight losses after 1 h in chloroform increase strongly with decreasing HS concentration. The Solvent resistivity of PUU–EDA is 56–86 wt.%, PUU–BDA is 67–96 wt.% and PUU–HMDIA is 75–100 wt.%. The solubility increases in the order of HMDA > BDA > EDA due to chain flexibility and or low crystalline of the HS. The effect of hard segment content on the solubility of the copolymer is presented in Fig. 8 and the graph shows that the solubility increases with decrease in hard segment content due to decrease in the physical crosslinking points.

Close

Figure 8 Solvent resistivity of the copolymer with respect to HS concentration: ●, 1a; ▴, 2a; ♦, 3a.

Export to PPT