Crystallization, melting and morphology of poly(hexamethylene sulfide)

Abstract

Poly(hexamethy1ene sulfide) fractions, ranging in molecular weight from 4600 to 20000, were isothermally crystallized from the melt state in the temperature range 50-70°C. The dependence of the melting temperature on the crystallization temperature was analyzed in experiments involving low and high levels of crystallinity. The equilibrium melting temperature corresponds to 366 K and the enthalpy of fusion was found to be 40,5 cal/g ( = 170 J/g). The isothermal overall crystallization rates were analyzed using dilatometric and calorimetric techniques. The Avrami exponent is an integral value, n = 3, and the crystallinity varies with increasing molecular weight from 65% to 55%. The morphology is spherulitic and it was found that the growth rate is linear in the whole range of crystallization temperatures. The temperature coefficient of the crystallization was analyzed using nucleation theory, and comparison between dilatometric and growth data revealed that the overall rate and growth rate-temperature coefficients are approximately equal.

Test of the reptation concept: crystal growth rate as a function of molecular weight in polyethylene crystallized from the melt

“…The average undercooling predicted with eq 39 and 40 at the regime I -* II transition is thus µ = 16.22 ± 0.17 °C, which is to be compared to the experimental value _ = 16.46 ± 0.39 °C exhibited in Table I. It may be concluded that eq 39 and 40 provide an accurate and meaningful representation of the growth rate data inasmuch as they reproduce the absolute growth rates in both regimes I and II as a function of molecular weight and temperature within experimental error and at the same time yield the correct mean undercooling at the regime I -*-II transition.…”

“…( 71)ß-5736/ 7'ß-°.955 1 5/7,( 77The nature of the determination of s from eq 28 and 29 is shown schematically in Figure 3. Selecting a fixed AT, f (40) say 14.96 °C in regime I, which is exactly 1.5 °C above -. = 16.46 °C, one then plots log G¡ vs log nz for the 11 specimens.…”

“…The regenerated growth rate data were reanalyzed in the form of eq 26a and 27a assuming Kg(í) = 2Ag{n) on the basis Tp°= 418.7 K = 145.5 °C and Qd* = 5736 cal mol'1 and with eq 35 to account for the reduction in Tm caused by finite molecular weight. The results are expressed in eq 39 and 40 to follow. The value of Kgm = 1.91 X 10s deg2 " Calculated from Flory-Vrij expression, eq 35.…”

Analysis of the isothermal crystallization of isotactic polypropylene nucleated with sorbitol derivatives

“…One of the most used extrapolation methods52–57 for the determination of the T m in conditions of thermodynamic equilibrium is based on the implicit relationship between the crystal size and the T m proposed by Gibbs58 and Thomson:59 where T is the equilibrium melting temperature, σ ec is the interfacial free energy associated with the basal face of the crystallite and ξ is the crystal size. In this expression, it is assumed that the lateral dimensions of the lamellar crystallite are much greater than its thickness and do not depend on the nature of the interfacial structure 60.…”

“…If the family of crystals that gave rise to T m I were formed exclusively due to crystalline segregation during the isothermal crystallization process, the crystallites formed from the recrystallization phenomenon after the melting of the family associated with T m II would give rise to a third melting peak, above that of T m I because they would have been developed at a higher T c . One of the most used extrapolation methods [52][53][54][55][56][57] for the determination of the T m in conditions of thermodynamic equilibrium is based on the implicit relationship between the crystal size and the T m proposed by Gibbs 58 and Thomson: 59…”