It is shown that the application of a chip calorimeter, allowing very fast cooling and heating rates up to 10,000 K/s. can be successfully applied to study reorganization phenomena in crystallizable polymers. In this research both bulk Polyamide 6 (PA6) as well as an immiscible (polystyrene/styrene-maleic anhydride copolyrner)/Polyamide 6 blend with dispersed Polyamide 6 droplets of sub-micrometer size have been studied. The blends with sub-micrometer PA6 droplets have been shown to crystallize at low temperature via a homogeneous nucleation mechanism, due to a lack of heterogeneities in the small droplets. Upon fast cooling with more than 500 K/s crystallization of PA6 could be totally prevented. No cold crystallization takes place upon subsequent heating with 500 K/s. Upon fast heating of 2000 K/s after isothermal crystallization, the 'real', initial melting of the crystallites formed at very low temperature could be obtained, which was not possible with standard DSC apparatus or HPer DSC. However, even heating, with 5000 K/s was not fast enough to completely avoid reorganization. Annealing experiments in the melt-range clearly show the fast reorganization of PA6 crystallites, resulting in improved stability within a timescale in the order of 0.01-0.1 s. Most probably reorganization takes place mainly in the crystalline state, and only to a lesser extent via a melting-recrystallization-remelting process. The reorganization process is not hindered by the confined dimensions of the Polyamide 6 droplets, because both bulk PA6 as well as the confined (PS/SMA2)/PA6 blend give rise to identical reorganization phenomena. (c) 2006 Elsevier Ltd. All rights reserved.

The genesis and stability of different PA6 crystalline polymorphs, dispersed as micro- and submicrometer sized droplets inside an amorphous polymer matrix, are discussed over a very broad temperature range. Different PA6 droplet sizes lead to different PA6 crystallization events in a 100°C wide temperature window that extends down to 85°C. Static WAXD and DSC experiments on micrometer sized PA6 droplets indicate the formation of a stable γ-crystal phase in the region between 175 and 130°C. Sub-micrometer sized PA6 droplets only crystallize at 85°C in the β-phase. Upon heating above the PA6 glass transition, these crystals progressively increase their perfection and ultimately transform into the α-phase around 170°C.

In this paper the relation between the blend phase morphology and the fractionated crystallization behavior of PA6 in reactively compatibilized immiscible PS/PA6 and (PPE/PS)/PA6 immiscible blends is studied. Reactive compatibilization is used as an effective tool for controlling the blend phase morphology, and to reduce the PA6 dispersed droplet size. As reactive compatibilizers, SMA2 and SMA17 are used, which differ in their level of miscibility with the amorphous PS and (PPE/PS) components. With SMA2 a strong shift of PA6 crystallization to much higher supercoolings than before is found after compatibilization resulting in crystallization at temperatures as low as 85 °C. This is ascribed to the strong decrease of the droplet sizes down to 100–150 nm. Nucleation experiments show that heterogeneous bulk nucleation can be reintroduced in the submicron-sized PA6 droplets by adding enough nucleating agents of sufficient small size. The degree of fractionated crystallization is found to depend on the interface between PA6 droplets and surrounding medium, as it is influenced by vitrification of the matrix polymer and by the location of the compatibilizers SMA2 and SMA17. The method used for mixing the reactive compatibilizer with the blend components also affects the fractionated crystallization process.

Crystallization kinetics and crystallinity development of PA6 droplets having sizes from 0.1 to 20μm dispersed in immiscible uncompatibilized PS/PA6 and reactively compatibilized (PS/Styrene-maleic anhydride copolymer=SMA2)/PA6 blends are reported. These blend systems show fractionated crystallization, leading to several separate crystallization events at different lowered temperatures. Isothermal DSC experiments show that micrometer-sized PA6 droplets crystallizing in an intermediate temperature range (Tc∼175°C) below the bulk crystallization show a different dependency on cooling rate compared to bulk crystallization, and an athermal crystallization mechanism is suggested for PA6 in this crystallization temperature region. The crystallinity in these blends decreases with PA6 droplet size. Random nucleation, characteristic for a homogeneous nucleation process, is found for sub-micrometer sized PA6 droplets crystallizing between Tc 85 and 110°C using isothermal DSC experiments. However, crystallization in the PA6 droplets is most likely initiated at the PA6-PS interface due to vitrification of the PS matrix during crystallization. Very imperfect PA6 crystals are formed in this low temperature crystallization region, leading to a strongly reduced crystallinity. These crystals show strong reorganization effects upon heating.

(PPE/PS)/PA6 and PS/PA6 blends were prepared by means of melt-extrusion. They were compatibilized using the reactive styrene-maleic anhydride copolymer with 2wt% maleic anhydride (SMA2). The effect of compatibilization on the phase inversion and the stability of the resulting co-continuous blend structures were investigated using scanning electron microscopy, dissolution and extraction experiments. The onset of co-continuity shifted towards lower PA6 concentrations according to the change in blend viscosity ratio. The melting order of the components inside the extruder could result in a change in the observed co-continuity interval in slowly developing phase morphologies. The unmodified co-continuous blends were not stable and did break-up into a droplet/matrix type of morphology upon annealing in the melt depending on the blend composition. Although the stability of the threads during annealing improved upon compatibilization because of the lower resulting interfacial tension, the decreased possibility for recombination and coalescence during flow reduced the co-continuous region for the compatibilized blends. It is proposed that a dynamic equilibrium between break-up and recombination phenomena after the initial network formation is necessary to maintain the network structure.

In this paper the crystallization behavior of PA6, dispersed as droplets in various immiscible amorphous polymer matrices, is reported. PA6 was melt-mixed at various compositions with PS, (PPE/PS 50/50 wt/wt) and PPE using twin-screw extrusion. The phase morphologies of the obtained blends were analysed using SEM, etching experiments and image analysis. The crystallization behavior of PA6 was investigated by dynamic and isothermal DSC experiments. In case PA6 is dispersed as droplets, fractionated crystallization behavior occurs, characterized by several crystallization events at different, lowered crystallization temperatures. It is found to depend on the blend morphology (size of the droplets) and the thermal history of the samples (heterogeneous nucleation density). The PA6 droplet size distribution is shown to strongly influence the crystallization behavior of the droplets. Vitrification of the matrix appears to cause nucleation in the droplets at the interface. Decreasing the PA6 droplet size results in slower overall crystallization rates.