EFFECT OF PLASTICIZATION ON THE THERMAL EXPANSION OF POLYMERS

A. V. PETROV, R. M. KESSENIKH

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

The wide use of polymers as electrical insulation has at present led to intensive study of their physicomechanical properties. The most important technical characteristics of these materials are the relatively simple processing technology, the permissible operating temperature, and the magnitude of mechanical strength. One of the accessible methods for regulating the structure of polymers and their physicomechanical properties is plasticization and modification, which to a considerable extent affect the glass-transition temperature \((T_c)\), its dielectric and mechanical properties \([1, 2]\).

The introduction into a polymer of a low-molecular-weight plasticizer or modifier leads to an increase in the flexibility of the macromolecule and a decrease in the relaxation time of its structural elements. The effectiveness of their action depends first of all on the ability of these systems to combine, and is due to structural or molecular plasticization. At present there are a number of works \([2, 3, 4, 5, 6]\) devoted to the study of this problem.

A decrease in \(T_c\), and consequently an increase in the mobility of the segments of the macromolecule, can be explained from the energetic point of view, when the molecules of the plasticizer, penetrating between the chains of the polymer, thereby reduce the potential barriers that hinder molecular rearrangements.

On the other hand, the introduction of plasticizers should promote an increase in the free volume in polymers, which also leads to a decrease in \(T_c\).

Fox and Flory \([7]\) showed that the relative free volume \((f_c)\) at the glass-transition temperature should be constant for all polymers. Ferry et al. showed that for most polymer systems \(f_c = 0.025 \pm 0.003\).

On the basis of this concept, Simha and Boyer \([8]\) put forward the assumption according to which the glass-transition process is associated with the attainment of a constant value of the free volume. In doing so they established that

\[ \Delta \beta \cdot T_c = 0.12, \]

\[ \beta_b \cdot T_c = 0.2. \tag{1} \]

Here \(\Delta \beta\) is the difference between the coefficients of volume expansion at temperatures lying above \((\beta_b)\) and below \((\beta_c)\) the glass-transition temperature.

The purpose of the present work is to verify the applicability of the indicated theories to plasticized and modified thermoplastic polymers.

Methods and Objects of Investigation

Polyvinyl chloride (PVC) of grade PF-4 was used as the object of investigation. Dibutyl sebacate (DBS) served as the plasticizer. The amount of introduced plasticizer was varied from 2 to 100 weight percent relative to the weight of PVC (Table 1).

Table 1

Characteristics of the thermal expansion of the materials studied

To study the influence of modification on the polymer \(T_c\), a copolymer of styrene with 9-ethylcarbazole-3,6-diallyl ester of dicarboxylic acid (PS + D.E.D.K.), developed in the problem laboratory of TPI, was used. Its structural formula has the form

\[ \left[ -\mathrm{CH_2}-\mathrm{CH}-\mathrm{CH_2}-\mathrm{O}-\mathrm{C}(=\mathrm{O})- \begin{matrix} \text{9-ethylcarbazole fragment} \end{matrix} -\mathrm{C}-\mathrm{O}-\mathrm{CH_2}- \right. \]

\[ \left. -\mathrm{CH}-\mathrm{CH_2}-\mathrm{CH}-\mathrm{CH_2}- \right]_n \]

From the indicated materials, specimens in the form of tablets, 3 mm thick and 10 mm in diameter, were prepared by pressing followed by annealing.

The coefficients of volumetric expansion were measured using a quartz dilatometer. A Linnik IZK-50 interferometer [10] served as the indicator of specimen elongation. The structural design...

The design of this apparatus is shown in Fig. 1. To ensure tighter contact between the surface of the specimen (1) and the dilatometer table (2), they were ground in. In this case, in contrast to the design proposed in [10], the upper platform (3) of the dilatometer was made separately from the movable rod (4) and was ground to the specimen. This made it possible to eliminate the error caused by loose contact between the surfaces of the specimen and the platforms of the quartz dilatometer. The movable tube of the dilatometer (4) rests with its lower end on the upper quartz platform (3) located on the specimen. This design provides easier adjustment of the dilatometer and is convenient when installing the specimen.

Temperature was measured using a thermocouple and a type PP DC potentiometer; the sensor was a chromel–copel thermocouple. The rate of temperature rise was kept constant and was 0.7 deg/min.

Fig. 1. Schematic diagram of a quartz dilatometer

Experimental Part

Figure 2 shows the dependence of the coefficient of volume expansion of PVC on temperature, from which it follows that at \(T > T_c\) the rate of increase in specific volume increases.

Fig. 2. Dependence of the volume expansion of polyvinyl chloride on temperature

A similar character is exhibited by this dependence for all the systems we investigated.

Proceeding from the free-volume concept of Fox and Flory, the increase in the volume of a substance with increasing temperature at \(T < T_c\) is due only to anharmonic vibrations of atoms and small atomic groupings of the polymer. In this case the coefficient of volume expansion will be determined mainly by the magnitude of intra- and intermolecular potential barriers [6].

As follows from Table 1, the introduction of DBS leads to a monotonic decrease in \(T_c\) of polyvinyl chloride and simultaneously to an increase in \(\beta_c\). This fact indicates that the plasticizer molecules reduce the energy barriers of interaction, thereby increasing the number of possible conformations and the rate of transition from some conformations to others.

At \(T \gg T_c\), the free volume reaches such a value that it becomes sufficient for the realization of segmental motion. The increase in free volume at \(T > T_c\) is associated not only with anharmonic-

... with vibrations of atoms, but also with intense local Brownian motion.

The experimental results presented for plasticized PVC indicate that the empirical relation (1) is satisfied satisfactorily for them. Thus, by applying this rule to plasticized polymers, their coefficients of volume expansion can be found from the known value of \(T_c\).

However, the fact is noteworthy that at high plasticizer concentrations the value of \(\beta_c\) in absolute magnitude approaches \(\beta_b\), while the constants of equation (1) have somewhat underestimated values. This phenomenon is associated with the limited compatibility of the plasticizer with the polymer at high concentrations. Thus, this method can provide an additional qualitative estimate of the compatibility limit of the polymer–plasticizer system.

Application of the free-volume method to modified polymers showed that introduction into polystyrene of the modifier—D.E.D.K.—lowers the \(T_c\) of this copolymer. In this case the constants of equation (1) agree satisfactorily with the experimental data. The decrease in \(T_c\) in this case is due mainly to a decrease in the rigidity of the molecular chain, i.e., to a reduction in intramolecular energy barriers.

For rigid macromolecular chains, whose mobility is to a considerable extent limited by strong intramolecular interaction, the realization of segmental motion requires a significantly larger free volume than follows from equation (1). Therefore such polymers, as a rule, do not obey the indicated condition [11].

Conclusions

On the basis of the experimental data presented above and their discussion, the following conclusions may be drawn:

1. In thermoplastic polymers with sufficiently flexible macromolecular chains, the introduction of a plasticizer or modifier does not affect the magnitude of the free volume at \(T = T_c\), and the behavior of such systems obeys the empirical rule of Simha and Boyer.

2. Use of the free-volume principle makes it possible to determine the maximum permissible concentrations of a plasticizer that has limited compatibility with the polymer.

3. For rigid-chain polymers the free volume at \(T = T_c\) is somewhat larger than follows from the Simha–Boyer rule.

REFERENCES

1. V. A. Kargin, T. L. Slonimskii. Brief Essays on the Physical Chemistry of Polymers. Moscow, “Khimiya,” 1967.

2. A. A. Tager. Physical Chemistry of Polymers. Leningrad, “Khimiya,” 1968.

3. G. P. Andrianova, N. F. Bakeev, P. V. Kozlov. High-Molecular Compounds, 13A, 266, 1971.

4. A. A. Tager, V. E. Dreval’ et al. High-Molecular Compounds, 14A, 1381, 1972.

5. K. Tinius. Plasticizers. Leningrad, “Khimiya,” 1964.

6. Yu. S. Lipatov. Physical Chemistry of Filled Polymers. Kiev, “Naukova dumka,” 232, 1967.

7. T. Fox, P. Flory. J. Appl. Phys., 21, 581, 1950.

8. R. Simha, R. Boyer. J. Chem. Phys., 37, 1003, 1962.

9. R. Boyer. Transitions and Relaxation Phenomena in Polymers. Edited by A. Ya. Malkin. Moscow, “Mir,” 305, 1968.

10. V. A. Presnov. Linnick interferometer for measuring coefficients of thermal expansion. Izv. vuzov SSSR, “Fizika,” 1969, No. 4.

11. A. V. Petrov. Candidate’s dissertation. Tomsk, 1969.