**3. Results and discussion**

#### **3.1. Prepolymer characterization by high field NMR.**

The substrate polymer (HTPB) is the key component that affects the elastomeric properties of PUs. Knowledge on the polymer structure and composition is essential for synthesis of PUs with required properties and understanding the various advantages, the polymer can offer. We have used the high field 1D and 2D NMR techniques for characterization of HTPB prepolymers. Analysis of microstructure and sequence distribution of monomer units can be discerned from the analysis of quantitative 1H/13C NMR spectra. Although 13C NMR spectroscopy is good in terms of a wider range of chemical shifts and thus offering less possibility of overlapping peaks, problems associated with questionable assignments occasionally arise from steric-sensitive environments in the carbon skeleton. Additionally, the Nuclear Overhauser Enhancement (NOE) of different types of carbon is usually not equal and the wide spin-lattice relaxation time (t1) range makes quantitative measurements of carbon signals difficult. A combination of NMR techniques such as 1H, 13C{1H}, 13C{1H}-DEPT (Distortionless Enhancement By Polarization Transfer) and 1H/13C-HMQC (Hetero-nuclear Multiple Quantum Coherence) permit assignments of all 1H and 13C resonance peaks. To our knowledge, hitherto the actual physical characteristics of the HTPB are not precisely known, particularly its absolute number-average molecular weight (ܯഥ). In the current work, we have examined the 1H and 13C NMR spectra of HTPB in order to precisely determine its number-average degree of polymerization (ܦܲതതതത ), and thus, ܯഥ of the polymer. A typical 13C{1H} NMR spectrum (200MHz, CDCl3) of free radical HTPB prepolymer is shown in Fig.1. For convenience, resonances in the spectrum can be divided in to three distinct regions, *i.e.* (a) an olefinic region: δ 113-144, (b) a carbon bearing hydroxyl end group region: δ 56 – 65, and (c) an aliphatic region: δ 24-44. However, due to complex nature of the prepolymer, a complete assignment of all signals was not possible. The, methine and methylene carbons were distinguished by using the DEPT technique. The 13C{1H}-DEPT spectrum of the polymer, recorded in CDCl3, is depicted in Fig.2.

232 Polyurethane

Polyurethane

PU-II HTPB/TDI/

PU-IIp HTPB/TDI/

**Table 1.** The basic composition of the one step PUs.

*2.2.2. Filled PUs (propellant): PU-IIp* 

**3. Results and discussion** 

\*Hard segment content= ሼሾݓ்ூ ݓைା்ெሿΤ ሽ ݓ௧௧ ൈ ͳͲͲǡ ݓ ൌ ݓ݄݁݅݃ݐǤ

cast in to the Teflon coated mould and cured at 60 °C for 5 days.

**3.1. Prepolymer characterization by high field NMR.** 

system Binder component Fillers (%) Hard segment content

The PUs were prepared in bulk by one step procedure. Mixing was carried out in a pilot mixer with facility for circulation of hot/cold water around the mixer jacket. The HTPB and BDO-TMP mixture were taken in the pilot mixer and stirred for 10 minutes. The calculated amount of TDI was added to the mixer, and the contents were stirred for 20 minutes at 40 ± 1 °C. The binder slurry was cast in to a Teflon coated mould and cured at 60 °C for 3 days.

The basic propellant composition that uses 68% ammonium perchlorate (AP) and 18% aluminum (Al) powder was taken up for study. AP (with purity > 99%) was used in bimodal distribution (3:1) having average particle size 280 m and 49 m, respectively. Particle size of AP and Al powder (mean diameter = 33.51 μm) were measured by a CILAS Particle Size Analyzer-1180 model. Dioctyl adipate (saponification value = 300 mg KOH/g) was used as a plasticizer. The mixing was carried out in two phases. In the first phase, all the ingredients, except the curing agent, were premixed thoroughly for about 3 h at 38 2 °C. Hot water was circulated through the jacket of the mixer bowl to keep a constant temperature throughout the mixing cycle. A homogeneous test of the slurry was carried out after completion of the premix to confirm the uniform dispersion of AP and Al powder. In the second phase of mixing, a calculated amount of curing agent, *i.e.* toluene diisocyanate (TDI) was added to the premixed slurry, and further mixed for 40 minutes at 40 1 °C. The propellant slurry was

The substrate polymer (HTPB) is the key component that affects the elastomeric properties of PUs. Knowledge on the polymer structure and composition is essential for synthesis of PUs with required properties and understanding the various advantages, the polymer can offer. We have used the high field 1D and 2D NMR techniques for characterization of HTPB prepolymers. Analysis of microstructure and sequence distribution of monomer units can be discerned from the analysis of quantitative 1H/13C

(BDO +TMP) --- 7.25/7.34/7.43/7.52/7.61

(BDO +TMP) AP- 68, Al -18, 7.25/7.34/7.43/7.52/7.61

PU-I HTPB/TDI --- 4.34

(% w/w)\*

**Figure 1.** 13C{1H} NMR (CDCl3, 200 MHz) spectrum of free radical HTPB prepolymer.

HTPB-Polyurethane: A Versatile Fuel Binder for Composite Solid Propellant 235

calculated observed

**Figure 3.** Expanded 13C{1H} NMR spectrum of δ 127-132 region of free radical HTPB prepolymer.

Signal Sequence# Chemical shift (δ values)

*vinyl*-1, 2-CH2-\*CH=CH-CH2- *trans*-1, 4-unit.

1 v-T\*-v 131.01 131.6 2 c-T\*-v , t-T\*-v 130.55 131.2 3 v-C\*-v 129.87 130.5 3 v-T\*-c ,v-T\*-t 129.76 130.4 4 c-T\*-c, c-T\*-t 129.30 130.1 5 t-T\*-c, t-T\*-t 129.30 129.9 6 t-\*T-v, c-\*T-v 129.11 129.8 7 c-\*C-c, t-\*C-c 128.91 129.4 8 c-\*C-t, t-\*C-t 128.91 129.3 9 c-\*C-v, t-\*C-v 128.60 128.8 10 v-\*T-t 127.64 128.4 11 v-\*T-v 127.45 128.3 12 v-\*C-c 127.31 128.0 13 v-\*C-t 127.31 127.8 # c = *cis*-1, 4-unit; t = *trans*-1, 4-unit; v = *vinyl*-1, 2-unit; v-T\*-v = *vinyl*-1, 2-CH2-CH=CH\*-CH2- *vinyl*-1, 2-unit; and v-\*C-t =

**Table 2.** 13C{1H} Assignment of triad sequence of free radical HTPB prepolymer (δ 127-132 region).

In the aliphatic region (δ 24-44), the DEPT spectrum showed six sharp negative resonances at δ 38.6, 34.4, 32.8, 30.4, 27.4, and 24.9. A positive signal at δ 43.4, was

**Figure 2.** 13C{1H}DEPT-135 spectrum of free radical HTPB prepolymer.

The delay in the DEPT sequence was chosen in such a way that methine carbons appeared as positive peak, whereas both methyl and methylene carbons appeared as negative peak. In the olefinic region, the DEPT spectrum showed a set of positive signals in the range of δ 142-144, that corresponds to methine (-*C*H=) carbons, whereas a set of negative signal at δ 113-115, corresponds to the methylenic (=CH2) carbons of *vinyl*-1,2- unit. The fine splitting of the signals is due to the tacticity of the monomer units. A set of positive signals in the range of δ 125 – 134 was ascribed to the compositional splitting of the two olefinic carbons (-CH=CH-) in central *cis*-1,4- or *trans*-1,4- unit, present in different combination of homotriads, heterotriads, and symmetric and non-symmetric isolated triads (Frankland et al., 1991). A total of thirteen signals were observed in the olefinic double bond region, *i.e.*δ 127-132. (Fig.3). Each of the resonance line has been assigned to the methine carbon of 1,4 unit in the possible set of three consecutive monomer units (*cis*-1,4-; *trans*-1,4-; and *vinyl*-1,2-unit). When surrounded by 1,2-units, the methine carbon of 1,4-unit would have different chemical shift due to their different distance from the *vinyl*-1,2-side group. The chemical shifts of methine carbon signals in various possible triad sequences were calculated by a known method and then, compared with that of observed one to assign the signals. Besides, the assignment of the triad resonances was made based on the values reported in literature for polybutadiene (Elgert et al., 1975). The results, thus, obtained are summarized in Table 2.

**Figure 2.** 13C{1H}DEPT-135 spectrum of free radical HTPB prepolymer.

summarized in Table 2.

The delay in the DEPT sequence was chosen in such a way that methine carbons appeared as positive peak, whereas both methyl and methylene carbons appeared as negative peak. In the olefinic region, the DEPT spectrum showed a set of positive signals in the range of δ 142-144, that corresponds to methine (-*C*H=) carbons, whereas a set of negative signal at δ 113-115, corresponds to the methylenic (=CH2) carbons of *vinyl*-1,2- unit. The fine splitting of the signals is due to the tacticity of the monomer units. A set of positive signals in the range of δ 125 – 134 was ascribed to the compositional splitting of the two olefinic carbons (-CH=CH-) in central *cis*-1,4- or *trans*-1,4- unit, present in different combination of homotriads, heterotriads, and symmetric and non-symmetric isolated triads (Frankland et al., 1991). A total of thirteen signals were observed in the olefinic double bond region, *i.e.*δ 127-132. (Fig.3). Each of the resonance line has been assigned to the methine carbon of 1,4 unit in the possible set of three consecutive monomer units (*cis*-1,4-; *trans*-1,4-; and *vinyl*-1,2-unit). When surrounded by 1,2-units, the methine carbon of 1,4-unit would have different chemical shift due to their different distance from the *vinyl*-1,2-side group. The chemical shifts of methine carbon signals in various possible triad sequences were calculated by a known method and then, compared with that of observed one to assign the signals. Besides, the assignment of the triad resonances was made based on the values reported in literature for polybutadiene (Elgert et al., 1975). The results, thus, obtained are

**Figure 3.** Expanded 13C{1H} NMR spectrum of δ 127-132 region of free radical HTPB prepolymer.

