Polymer: December 2007

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The Glass Transition Temperature in
Homologous Series of Linear Polymers
B. M. GRIEVESON*
(Polymer, Volume 1, 1960, Pages 499-512)

Members of three homologous series of linear aliphatic polyesters were synthesized and their glass transition temperatures are reported. The theory of glass transition temperatures in random copolymers is applied to polymers in homologous series by treating them ag copolymers of the first member of the series with polymethylene. It is shown that each methylene group added to a polymer to Jorm the next member of the series makes a constant specific contribution to the glass transition temperature lust as does the addition of each homopotymer unit in a random copolymer. There is a discrepancy between the observed and predicted values of the contribution made by each methylene group.

INTRODUCTION

A KNOWLEDGE of the relationship between the chemical structure and glass transition temperature (Tg) of polymers is an important aid in the search for materials with specific physical properties in a given temperature range. The effect of changing chemical structure in homologous series of polymers has been studied by many workers. Interest has been mainly concentrated on locating glass transition temperatures, although some quantitative measurements of specific volume-temperature relationships in the region of Tg have been made (1, 2). The results of these investigations have shown that the insertion of new chemical units into a polymer chain leads to a change in the glass transition temperature. The direction and amount of the change can be predicted qualitatively from a knowledge of the steric nature of the inserted unit and its effect on the configuration of the polymer and the interaction between polymer chains. From studies of glass transition temperatures in random copolyrners, Gordon and Taylor (3), Mandelkern and co-workers (1), and Wood (4) have shown that each homopolymer makes a specific partial contribution to the glass transition temperature in proportion to its own Tg and its weight fraction in the copolymer. If the quantitative relationship developed for random copolymers is applied to the insertion of a new chemical unit into a homopolymer, it should be possible to predict the resultant change in glass transition temperature.

Three homologous series of linear aliphatic polyesters have been prepared together with some random and block copolyesters and polyester melt blends. The glass transition temperatures of these polymers have been measured, and these results together with those for homologous series studied by other workers are examined in the light of the theory of glass transition temperatures in random copolymers.

EXPERIMENTAL

Preparation of polyesters

The polyesters were prepared by condensation of dibasic acids with equimolar proportions of a glycol at 200°C in a stream of oxygen-free nitrogen. In order to obtain polymers of the requisite number-average molecular weight (greater than 10,000), in a reasonable time, it was necessary to reduce the pressure in the reactor to 1 mmHg when the polymer molecular weight reached 5,000. Condensation was continued until the number-average molecular weight reached at least 10,000.

The oxalate, malonate and n-alkylmalonate polyesters were prepared from the diethyl esters of the corresponding acids since the acids themselves are unstable at the reaction temperature. The diethyl esters were heated at 150°C with equimolar proportions of the glycol until the theoretical amount of ethyl alcohol was almost completely distilled out of the reaction mixture. The temperature was then raised to 200°C and the polycondensation completed under reduced pressure. Polymers of the desired molecular weight were obtained with relative ease in some systems, but when necessary 0.01 per cent of anhydrous zinc acetate was added as a condensation catalyst.

The diethylene succinate-sebaeate random copolymers were prepared from a mixture of succinic acid and sebacic acid in the required proportions, together with an equimolar amount of diethylene glycol, by the same condensation technique as described above.

The block copolymers were prepared from poly(diethylene succinate) and poly(diethylene sebacate), both of approximately 2,000 number-average molecular weight. These low molecular weight polyesters were mixed inthe required proportions with an equimolar amount of hexamethylene di-isocyanate (H.M.D.I.) and allowed to react at 60°C to give a block copolymer having a molecular weight of approximately 20,000. Although this method of block copolymer formation involves the introduction of urethane linkages into the polyester chain, it has the advantage that it canbe performed at relatively low temperatures. At elevated temperatures, ester interchange occurs easily and would result in a disordering of the units of the two blocks, thus producing a random copolymer. The introduction of urethane linkages into the polyesters will result in changes from the Tg of the pure polymers. H.M.D.I. was chosen as the linking agent because, although it is less reactive than the aromatic di-isocyanates, its structure is much more similar to that of the linear aliphatic polyesters. It should, therefore, cause smaller deviations from the true Tg of the polyesters. For purposes of comparison, high molecular weight homopolymers of diethylene succinate and diethylene sebacate were also prepared by chain-extending polyesters of molecular weight 2,000 with H.M.D.I.

Molecular weight measurements

Number-average molecular weights were estimated by end-group analysis. The hydroxyl end-groups were measured by acetylation with pyridine/acetie anhydride reagent and the carboxyl end-groups were titrated with 0.1 N alcoholic potash. The reproducibility of the molecular weight measurements (about +/- 5 per cent at mol. wt. 2,000) became poorer at higher molecular weights, but the accuracy was sufficient to allow a molecular weight versus glass transition temperature relationship to be established for poly(diethylene adipate), Figure 1. After this relationship was obtained, molecular weight

measurements were only used to follow the polyesterification and to check that all the polyesters prepared had a molecular weight above 10,000.

Measurements of glass transition temperature

A penetrometer technique similar to that described by Edgar and Ellery (5) was used to make a preliminary approximate estimate of Tg for each polyester. A fiat-ended needle was fixed in the bottom of a vertical brass spindle which was mounted in a rigid frame so as to move freely in a vertical direction. The spindle was loaded to give a pressure of 400 g/mm^2 at the needle point. A dial gauge (reading to 0.0005 in.) was mounted in the frame to measure the vertical movement of the spindle. The polymer sample in a small tray 1 in. in diameter and 0.3 in. deep was gently heated to about 30°C above its melting point and then damped in the penetrometer frame below the needle point. The polymer and the lower part of the apparatus were immersed in n-hexane which had been cooled to -100°C and the penetrometer needle was lowered onto the polymer surface. The temperature of the hexane hath was raised by about 1°C per minute and readings of the dial gauge were taken at 2 min intervals. Figure 2 shows a typical penetrometer curve. The intersection of the tangents to the two arms of the curve gave a reproducible temperature Tb (the penetrometric brittle poin0 which generally lay about 5°C above the dilatometric glass transition temperature Tg (see Table 2). With all of the polyesters studied, smooth penetrometer curves like Figure 2 were obtained. This finding differs from that of Edgar (6) who found that with poly(ethylene terephthalate) an abrupt change occurred in the slope of the penetrometer curve at the temperature at which the dilatometric Tg was observed.

REFERENCES

1 MANDELKERN, L., MARTIN, G. M. and QUINN, F. A., J. Res. nat. Bur. Stand., 1957, 58, 137

2 ROGERS, S. S. and MANDELKERN, L., J. Phys. Chem., 1957, 61, 985

3 GORDON, M. and TAYLOR, J. S., J. Appl. Chem., 1952, 2, 493

4 WOOD, L. A., J. Polym. Sci., 1958, 28, 319

5 EDGAR, O. and ELLERY, E., J. Chem. Soc., 1952, p. 2633

6 EDGAR, O., J. Chem. Soc., 1952, p. 2638

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