## 3 Results and Discussion
### Mechanical Properties of BSMs-PDADMAC Films  
The results discussed in the previous section provide evidence that the weight fraction of PDADMAC W has the marked influence on the composition and structural features of resulting PDADMAC-BSM systems. The formulation with W = 0 is represented as a stable aqueous suspension of negatively charged BSMs. The formulation with W = 0.036 corresponds to a stable aqueous suspension of saturated positively charged PDADMAC  
Figure 1: Electrophometric mobility of BSM–PDADMAC complex particles vs. Z. [BSM] = 4 \(\times\) 10\({}^{-5}\) M; 0.01 M phosphate buffer with pH 7. BSM complex particles. At \(0.036<W<1\), the formulation is considered as the mixture of the saturated complex particles and unbound PDADMAC. At \(W=1\), the formulation is an aqueous solution of the virgin PDADMAC. To study the influence of the composition of PDADMAC-BSM samples on their mechanical properties, the films prepared from the binary PDADMAC-BSM mixtures were tested. Note that BSMs are characterized by a well-pronounced ability for film formation, whereas PDADMAC is not prone to do it. For this reason, a PDADMAC content in the binary mixtures W was varied from 0 to 0.5. Figure 2 shows stress-strain diagrams for several typical films prepared from BSMs with W = 0 (curve 1) and PDADMAC-BSM mixtures with different W (curves 4-8). For all samples studied (nine in total), mechanical characteristics are listed in Table 1 (columns 3-5). The numbers at the curves in Figure 2 correspond to the sample numbers in Table 1.  
As follows from the data in Table 1, the growth in W or increasing PDADMAC content in the mixture results in noticeable changes in the mechanical characteristics of BSMs-PDADMAC films--a 20-fold increase in elastic modulus, 2-fold increase in the strength, and 2-fold decrease in the ultimate strain of the samples. Thus, the addition of the inelastic hydrophilic PDADMAC to the elastic BSMs is accompanied by the growth in rigidity of the films and decrease in their elasticity. In other words, the variation of PDADMAC content in a BSMs-PDADMAC formulation allows one to control the rigidity-elasticity balance of the final material. For a more detailed analysis of this effect, let us consider the dependences of the mechanical properties of the films on PDADMAC content W.  
Figure 3 shows the dependences of elastic modulus E\({}_{0}\) and ultimate strain on W. Note that both curves are characterized by well-pronounced inflection at PDADMAC content \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**W**} & \multicolumn{4}{c}{**PDADMAC-BSM Films**} & \multicolumn{2}{c}{**Strength of**} \\ \cline{3-6}  & & **Elastic Modulus (\(\varepsilon_{0}\)), mPa** & **Difuent (\(\varepsilon_{\prime}\)), \%** & **Polymer-Sand Crust P\({}_{f}\), MPa** \\ \cline{2-6}
**1** & **2** & **3** & **4** & **5** & **6** \\ \hline
1 & 0 & \(4.5\pm 0.4\) & \(3.5\pm 0.3\) & \(580\pm 60\) & 0.8 \\
2 & 0.036 & \(5.0\pm 0.4\) & \(3.8\pm 0.3\) & \(570\pm 55\) & 1.1 \\
3 & 0.059 & \(5.5\pm 0.45\) & \(4.0\pm 0.3\) & \(560\pm 55\) & 1.9 \\
4 & 0.077 & \(9.0\pm 0.65\) & \(5.05\pm 0.25\) & \(550\pm 55\) & 2.7 \\
5 & 0.111 & \(11.0\pm 0.8\) & \(5.5\pm 0.15\) & \(480\pm 50\) & 3.5 \\
6 & 0.2 & \(15.0\pm 1.0\) & \(5.7\pm 0.2\) & \(330\pm 45\) & 8.0 \\
7 & 0.33 & \(17.0\pm 2.0\) & \(6.6\pm 0.25\) & \(300\pm 40\) & 13.0 \\
8 & 0.5 & \(90.0\pm 8.0\) & \(7.2\pm 0.4\) & \(270\pm 20\) & 18.0 \\
9 & 1 & - & - & - & 45.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of BSMs–PDADMAC films and polymer-sand crusts with different PDADMAC content W.  
Figure 2: Stress–strain diagrams for the films prepared from BSL (W = 0) (1) and BSMs–PDADMAC mixtures with W = 0.077 (4), 0.111 (5), 0.2 (6), 0.333 (7) and 0.5 (8). Numbers at the curves correspond to the sample numbers in Table 1.  
\(\mathrm{W_{in}}\) in the range from 0.04 to 0.06.