THERMOMECHANICAL PROPERTIES OF COPOLYMERS BASED ON POLYSTYRENE

R. M. KESSENIKH, A. V. PETROV, T. K. ZHITKOVA

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

In the present work we investigated the thermomechanical properties and softening temperatures of copolymers of styrene with 9-ethylcarbazol-3,6-diallyl ether of dicarboxylic acid (d.e.d.k.). The ether concentration was 5, 10, and 30 wt.%.

The investigations were carried out on Kargin scales. When a load was applied, the stress in the specimen, $\sigma$, was $0.6 \div 4.5\ \text{kg}/\text{cm}^2$. The time of action of the load was varied within the range from $1\ \text{sec}$ to $30\ \text{min}$. The holding time of the specimen at constant temperature was $20\ \text{min}$.

Fig. 1. Thermomechanical curves for the copolymer 95% styrene + 5% d.e.d.k.; $\sigma = 0.6\ \text{kg}/\text{cm}^2$: 1 — $\tau = 30\ \text{min}$; 2 — $\tau = 1\ \text{min}$; 3 — $\tau = 10\ \text{sec}$

Figure 1 shows thermomechanical curves (TMC) for the copolymer of styrene with 10% d.e.d.k. under the same load but for different times of its action. The curves consist only of two sections: a gently sloping part and a steep branch. From such curves the softening temperature $T'_c$ was determined from the point of intersection of the extensions of the linear sections located on different sides of the bend [1].

An increase in $\sigma$ causes a shift of the TMC toward lower temperatures (Fig. 2); consequently, in this case $T'_c$ depends both on the magnitude of the load and on the time of its action. At the same time, the relaxation time $\tau$ changes according to the law [3]

\[ \tau = \tau_0 \cdot e^{\frac{U_0-\alpha\sigma}{RT}}, \tag{1} \]

where $U_0$ is the activation energy at $\sigma = 0$.

$\alpha$ is a constant quantity for the given copolymer.

When a constant load is applied, deformation in polymers in the region of $T'_c$ begins to manifest itself most noticeably when the time of action of the load is comparable with the relaxation time $\tau$ of the system.

From the thermomechanical curves in Fig. 1, values of \(T_c'\) were found for various relaxation times, and the dependence \(\lg \tau = \varphi \left( \dfrac{1}{T} \right)\) was plotted (Fig. 3). With increasing \(\sigma\), the straight lines \(\lg \tau = \varphi \left( \dfrac{1}{T} \right)\) shift toward lower temperatures and form a kind of bundle converging at \(T = T_p\), where \(T_p\) is the pole temperature.

In this case, at \(\sigma = \mathrm{const}\), equation (1) will have the form

\[ \lg \tau = \lg \tau_0 + \frac{U_0 - \alpha \sigma}{2.3 \cdot R} \cdot \left( \frac{1}{T} - \frac{1}{T_p} \right). \tag{2} \]

Fig. 2. Thermomechanical curves for the copolymer 95% styrene + 5% D.E.D.K., \(\tau = 10\) sec:
1 — \(\sigma = 4.5\ \mathrm{kg/cm^2}\); 2 — \(\sigma = 3.0\ \mathrm{kg/cm^2}\); 3 — \(\sigma = 0.9\ \mathrm{kg/cm^2}\); 3 — \(\sigma = 0.6\ \mathrm{kg/cm^2}\)

Fig. 3. Dependence \(\lg \tau = \varphi \left( \dfrac{1}{T} \right)\) for the copolymer 95% styrene + 5% D.E.D.K.:
1 — \(\sigma = 0.6\ \mathrm{kg/cm^2}\); 2 — \(\sigma = 4.5\ \mathrm{kg/cm^2}\)

The tangent of the angle of inclination of this dependence to the abscissa axis characterizes the activation energy of the deformation process at the given value of \(\sigma\)

\[ U = \frac{\lg \tau_1 - \lg \tau_2}{\dfrac{1}{T_1} - \dfrac{1}{T_2}} \cdot 2.3 \cdot R. \tag{3} \]

The dependence \(U = \varphi(\sigma)\) for the copolymer with 10% D.E.D.K. is shown in Fig. 4, from which it is seen that, with increasing load on the copolymer, a decrease in the energy of the deformation process is observed.

The value \(U_0\) at \(\sigma = 0\) was determined from the equation

\[ U = U_0 - \alpha \sigma \tag{4} \]

and is equal for the indicated material to \(86\ \mathrm{kcal/mol}\).

The values of the constant coefficients \(\alpha\) for the polymers studied were determined from the slope of the dependence \(U = \varphi(\sigma)\) and lie within the limits \(2.8 \div 3\ \dfrac{\mathrm{kcal \cdot cm^2}}{\mathrm{mol \cdot kg}}\).

Figure 5 presents TMC for copolymers with different concentrations of D.E.D.K.; from these it is seen that an increase in D.E.D.K. leads to a lowering of \(T_c'\).

Owing to the fact that \(T_c'\) depends on the magnitude of the applied load, the glass-transition temperatures of the copolymers studied were determined by extrapolating the dependence \(T_c' = \varphi(\sigma)\) at \(\sigma = 0\) [2]. The experimental results are given in Table 1.

The decrease in the glass-transition temperature \(T_c\) with increasing D.E.D.K. concentration indicates that the plasticization effect predominates over the “crosslinking” effect. In the presence in polystyrene

Fig. 4. Dependence \(U=\varphi(\sigma)\) for a copolymer of 95% styrene + 5% D.E.D.K.

of chemical bonds between macromolecules, its \(T_c\) increases [4]; in the present case, such bonds apparently do not form, while mainly D.E.D.K. molecules become crosslinked with one another, hindering the growth of polystyrene chains, reducing its packing density and \(T_c\).

Table 1

Fig. 5. Thermomechanical curves for copolymers of styrene with D.E.D.K. \(\tau = 10\) sec; \(\sigma = 0.6\ \mathrm{kg/cm^2}\).

1 — 70% styrene, 30% D.E.D.K.;
2 — 90% styrene, 10% D.E.D.K.;
3 — 95% styrene, 5% D.E.D.K.

Thus, increasing the concentration of D.E.D.K. leads to a decrease in the copolymer \(T_c\), which is apparently due to loosening of the copolymer structure.

REFERENCES

1. A. V. Sidorovich, E. V. Kuvshinskii. VMS, 1698, 1961.
2. A. V. Sidorovich, E. V. Kuvshinskii. VMS, 2, 778, 1960.
3. S. B. Ratner, M. D. Frenkel, Yu. I. Bryukvin. Plastic Masses: 8, 1967.
4. S. K. Kuvshinskii. VMS, 7, 1554, 1965.