Enthalpy Relaxation of Polyamide 11 of Different Morphology Far Below the Glass Transition Temperature

Abstract

Polyamide 11 (PA 11) samples of different supermolecular structure, including the crystal-free glass and semi-crystalline PA 11 of largely different semi-crystalline morphology, were prepared by fast scanning chip calorimetry (FSC). These samples were then annealed at different temperatures well below the glass transition temperature Tg. The main purpose of the low-temperature annealing experiments was the calorimetric detection of mobility of chain segments at temperatures as low as −40 °C (≈Tg − 80 K) where still excellent impact resistance is predicted. It was found that annealing PA 11 at such low temperature, regardless the thermal history and supermolecular structure including crystallinity as well as crystal shape and size, permits distinct enthalpy relaxation at rather short time scale with the structural changes reverting on subsequent heating as detected with pronounced sub-Tg-enthalpy-recovery peaks. The main glass transition, associated to large-amplitude segmental mobility, as well as relaxations at temperatures only slightly below Tg are even more distinctly sensitive to the crystal morphology. In contrast to spherulitically grown lamellar crystals, presence of high-specific-surface area nanometer-sized ordered domains causes a shift of the glass transition temperature of the amorphous phase to higher temperature, proving stronger coupling of ordered and amorphous phases than in case of lamellae. In addition, the increased coupling of the crystalline and amorphous phases slows down the cooperative rearrangements on annealing the glass slightly below Tg. The performed study contributes to further understanding of the spectrum of structural relaxations in PA 11 including the effect of presence of crystals. Enthalpy relaxation and consequently the reduction of entropy at temperatures slightly below Tg strongly depends on the semi-crystalline morphology, while an only minor effect is seen on low-temperature annealing at Tg − 80 K, possibly indicating different molecular mechanisms for the processes occurring in both temperature ranges. The low-temperature process even seems proceeding in the crystalline fraction of the material.

Reaching the Ideal Glass in Polymer Spheres: Thermodynamics and Vibrational Density of States

“…Physical aging results in the development of a pronounced endothermic overshoot, increasing in magnitude with aging time, whose onset is located ≈100 K below T g and with breadth encompassing a wide range of temperatures up to T g . This feature is well documented in glasses of different nature aged substantially below T g [12,60,[65][66][67][68][69][70][71], and is explained taking into account a fast mechanism of equilibrium recovery with relatively low activation energy and a time scale much smaller than that of the main α relaxation [59,72]. These results point toward the universality of a fast evolution to a relative minimum in the free energy, theoretically identified with the concept of marginal glass [73].…”

Physical Aging Behavior of a Glassy Polyether

“…Our isochoric experiments (see Figure 4 ) are in line with those previous reports, as they show that, even at C, mild conditions of aging, in the case of Figure 4 , s, result in a significant drop of . In calorimetry, this effect is generally associated with a low temperature endothermic overshoot [ 27 , 30 , 57 , 58 ], in ways analogous to what it is shown in Figure 3 . The latter underlines a devitrification step taking place at temperatures at which the relaxation, increasing to extremely large time scales at temperatures not far below , is of no relevance.…”

Kinetics of homogeneous crystal nucleation of polyamide 11 near the glass transition temperature

“…Information about possible nuclei formation on cooling faster than 1000 K/s is not available. However, a cold-crystallization peak is typically observed when heating the glassy and crystal-free sample to above T g , 43,44 indicative of nuclei formation during prior cooling and/or annealing the glass, which is not quantified yet. Such thermal profiles, that is, fast cooling of the melt followed by annealing the glass slightly below T g is often observed in industrial processing of this material, for example injection molding.…”

“…Here, depending on the component geometry/wall thickness and molding parameters, cooling rates up to several 100 K/s may be observed, [45][46][47] perhaps suppressing crystallization but permitting formation of nuclei which might grow to crystals over long time, causing, for example, aging/ change of properties. 48,49 Classical enthalpy relaxation of the glass has been treated in a separate study 44 lubricant, and is designed for processing by extrusion. 50 The melt-flow volume index, mass-average molar mass, and polydispersity are 1 cm 3 /10 min (235 C, 2.16 kg), 50 17.2 kg/mol, and 2, 51 respectively.…”

“…Here, depending on the component geometry/wall thickness and molding parameters, cooling rates up to several 100 K/s may be observed, 45–47 perhaps suppressing crystallization but permitting formation of nuclei which might grow to crystals over long time, causing, for example, aging/change of properties 48,49 . Classical enthalpy relaxation of the glass has been treated in a separate study 44 and crystal formation below T g has been proven for another representative of the polyamide family, namely PA 6 36 …”