Nucleation-controlled dual semicrystalline morphology of polyamide 11

Crystallization with Nodular Aggregation near the Glass Transition Temperature for Syndiotactic Polypropylene

The isothermal crystallization from the melt state of syndiotactic polypropylene (sPP) has been studied by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and optical microscopy. The WAXD and SAXS results show the crystallization mechanism near the glass transition temperature in which the crystalline and mesomorphic nodules cover the entire sample with the formation of aggregation regions. For the SAXS analysis, the scattering function for the three-component system has been suggested. Furthermore, to analyze the growth kinetics of the aggregation region for sPP, the time-dependent structure factor combined with the homogeneous and inhomogeneous nucleation-and-growth kinetics has been suggested. The analysis shows that the growth kinetics of the aggregation region for sPP is the homogeneous nucleation-and-growth. The growth velocity of the aggregation region is a natural extrapolation of that of spherulite to the high supercooling region. These results might indicate that the crystallization with the nodular aggregation is a fundamental crystallization process near the glass transition temperature for polymers.

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

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 T_g. 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 (≈T_g − 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-T_g-enthalpy-recovery peaks. The main glass transition, associated to large-amplitude segmental mobility, as well as relaxations at temperatures only slightly below T_g 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 T_g. 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 T_g strongly depends on the semi-crystalline morphology, while an only minor effect is seen on low-temperature annealing at T_g − 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.