POLYMERLETTERS VOL. 2, PP. 955-958 (1964)

EQUILIBRIA IN SOLID PHASE POLYAMIDATION

In a polyamide me l t , the carboxyl and amine groups are in equilibrium with the amide groups and water; i.e.,

[-COOHI + [-NH,] = 1-CONH-] + H2O

The equilibrium constant for the reaction is

[-CONH-I X = K 1 - - B e - A ~ a / ~ ~ [-COOS] x [-NH,]

where AH, is the enthalpy change of the reaction. The formation of polymer is exothermic; and published values for -AHa (1-3) although somewhat at variance, are in the range of 6-7 kcal./mole. Thus, a de- crease in temperature shifts the equilibrium to higher molecular weights a t constant water concentration.

If s o m e fixed water vapor pressure is maintained above the melt, the water concentration is fixed at s o m e definite value characteristic of the polymer. For any given polyamide, the product (P) of the concentrations of carboxyl and amine ends at equilibrium will be proportional to the wa- ter vapor pressure. At constant water vapor pressure, a reduction in tem- perature will cause the water concentration to increase by an amount de- termined by the heat of vaporization of water from the polyamide. This effect opposes the effect of temperature on K , so that P,, is not greatly affected by temperature changes over a narrow range.

It has been known for some t i m e that the polyamide condensation reac- tion can occur quite rapidly at temperatures below the melting point of the polymer (4). In studying the polyamidation equilibrium in the solid phase, we have observed a considerable shift to higher molecular weights at a given water vapor pressure (vs. melt polymerization) as well a s a tendency to broader than normal molecular weight distributions. W e have rationalized these results by assuming that end groups are essentially excluded from the crystalline regions of the polymer and that the equilib- rium in the amorphous regions is the s a m e a s for a completely amorphous or molten polymer at the same temperature.

If f a is the fraction of amorphous polymer, the relations between the equilibrium end group concentrations for semicrystalline and amorphous or molten polymers are

[-COOHI, = f a [-COOS], (3)

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POLYMER LETTERS

P, = fa2Pa

957

From eq. 5 w e predict that we can use the experimentally observed end group products in solid phase polymerization to equilibrium to calculate f a or the percent crystallinity of the solid polymer. This has been done for 66 nylon a s well a s for 66/6 random copolymers. These results have then been compared with another method for estimating crystallinity (i.e., density) (5). The values for P a at equilibrium were determined from 66/6 random copolymers which were molten at the appropriate tem- peratures. The results are summarized in Table I.

these elevated temperatures indicate that percent crystallinity is of the order of 55-60% (5). It is possible that some premelting occurs while the polymers are held at elevated temperatures so that actual crystallin- ity is somewhat lower than that indicated by density. In any case, the calculated percent c rys t a lh i t i e s are certainly in a reasonable range; and the expected reduction in crystallinity with increasing copolymer modification is obtained. Reasonably good agreement is also obtained for results obtained at two different steam pressures.

Since homogeneous, melt-polymerized, linear polyamides are charac- terized by a fixed ratio of weight average (a,) to number average molec- ular weight (a,) and since the relative viscosity (7,) of an 8.5% solution of polymer in 90% formic acid is related closely to the weight average molecular weight, it is common to construct a standard curve relating 7, to M,. However, these curves are actually relationships between 7. and gw/2 . For a polymer with a broader than normal molecular weight dis- tribution, aw/2 deduced from 7, will be greater than a, deduced from end group measurements. This situation has been found to apply to poly- amides equilibrated under a water vapor atmosphere in the solid phase a s indicated in Table 11.

Density measurements at 2 5 O C . on 66 nylon polymer annealed at

-

TABLE I1

Molecular Weight Distribution in Polyamides Equilibrated in the Solid Phase

Polymer Temp., OC. a n gw/an pH@, a w / 2 atm. from vr from ends

66 243 1 30,400 27,700 2.20

66/6 (95/5) 243 1 25,600 23,100 2.22 66/6 (75/25) 20 5 1 25,600 24,100 2.12

66 205 1 26,600 25,600 2.08

66 245 5.4 17,400 16,350 2.14

958 POLYMER LETTERS

Our explanation for this broadening of the molecular weight distribu- tion is that it results from a microscopic nonuniformity of the solid re- agent. In considering a polymer chain which passes through crystalline and intervening amorphous regions a number of t i m e s and where the ex- tent of reaction is p in the amorphous regions and 1 in the crystalline regions, one would sti l l expect a “most probable” molecular weight dis- tribution with the number average molecular weight increased by a fac- tor l/f, a s compared with a completely amorphous polymer. However, it is not likely that each polymer chain has the s a m e amorphous length or “percent crystallinity”; and this could then lead to a broadening of the distribution. Another possible factor is the existence of a distribution of crystallite s i zes and thus of local “percent crystallinity. ”

The existence of a fairly rapid rate of solid phase polyamidation and the good correlation of the equilibrium end group product with crystallin- ity make it seem reasonable that the end groups are not present a s iso- lated defects in crystallites and that they exist in regions of relatively high mobility which we commonly think of as amorphous. This conclu- sion can also be drawn with respect to amine end groups from data on sorption of acid dyes by polyamide yarns a t pH of 3.0-4.0 (6). These show that the quantity of dye absorbed correlates well with the titrated amine end group concentration for polymer in solution.

References

(1) Heikens, D., P. H. Hermans, and G. M. Van der Want, J . Polymer

(2) Fukomoto, O., J . Polymer Sci., 263 (1956). (3) Tobolsky, A. V., and A. Eisenberg, J . Am. Chem. SOC.,

(1959). (4) P . J. Flory to Du Pont, U.S. Patent 2,172,374, September 12, 1939. (5) Starkweather, H. W., Jr., and R. E. Moynihan, J. Polymer Sci., 22,

(6) Remington, W. R., and E. K. Gladding, J . Am. Chem. SOC.,

Sci., 437 (1960).

2303

363 (1956).

2553 (1950). Joseph Zimmerman

Carothers Research Laboratory Textile Fibers Department E. I. Du Pont de Nemours and Co., Inc. Wilmington, Delaware

Received May 22, 1964 Revised July 9, 1964