Confined crystallization phenomena in immiscible polymer blends with dispersed micro- and nanometer sized PA6 droplets, part 3: crystallization kinetics and crystallinity of micro- and nanometer sized PA6 droplets crystallizing at high supercoolings
Introduction
When a crystallizable polymer is isolated in a confining volume, its crystallization behavior can be drastically changed compared to its bulk crystallization. In immiscible blends, where the confinement is brought about by the fine dispersion of crystallizable droplets inside a polymer matrix, several crystallization events at different, lowered crystallization temperatures can be observed [1], [2], [3], [4], [5], [6], [7]. The main reason for this so-called ‘fractionated crystallization’ phenomenon was found to be the lack of active heterogeneities in the isolated droplets. The spectrum of supercoolings obtained upon cooling reflects the difference in nucleating activity of various heterogeneities in the melt, present in the dispersed droplets [1]. This crystallization phenomenon can in fact be viewed as a quite general crystallization mechanism for crystallization in dispersions, because basically similar observations have been made for liquid emulsions [8], [9], [10] and metals [11], [12]. It is observed in various polymer systems, when the polymer is present in a confined, isolated state, i.e. copolymers with nanometer sized crystallizable domains obtained via micro-phase separation and dispersions of polymers using a solvent or oil. [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29].
When the available heterogeneities are confined to a small portion of the droplets, the remaining heterogeneity-free droplets are forced to nucleate at rates governed by the molecular characteristics of the sample. Therefore, in the extreme, crystallization will occur via homogeneous nucleation, which will generally require the highest supercooling for crystallization because of the higher activation energy needed for chain association in the absence of a nucleating substrate. This is the reason why in a number of studies on dispersed droplet systems, the lowest crystallization temperature obtained is usually connected to homogeneous nucleation [2], [4], [14], [20], [30]. Obviously, this is necessary but not sufficient to prove homogeneous nucleation. The mechanism of homogeneous nucleation is expected to proceed totally differently from a heterogeneous one. Instead of nucleation predetermined by the present heterogeneous nucleation sites, homogeneous nucleation is a random process of associations of chains (segments).
The interest in the crystallization kinetics of confined, supercooled polymers is steadily increasing [16], [17], [18], [19], [22], [23], [31]. Random nucleation, characteristic for homogeneous nucleation, was found in various confined systems using optical microscopy [27], [29], dilatometry [13], DSC [18], [23] or real-time SAXS/WAXD experiments [16], [17], [19]. In a number of cases a strong temperature dependence was found for the confined, supercooled polymer [16], [17], [18], [32]. Reiter et al. [22] were able to directly visualize independent, random crystallization of nano-compartments of PEO, by using real-time AFM measurements. For immiscible polymer blends with small crystallizable droplets, however, a detailed kinetic study has never been performed.
Fractionated crystallization and homogeneous nucleation can be expected to have important consequences for the final semicrystalline structure of the confined polymer. Several authors report a drop in the degree of crystallinity upon fractionated crystallization, both in immiscible blends [2], [5], [30], [32], [33], [34] as well as for the confined crystallization of micro-phase separated domains in crystallizable block copolymers [13], [14], [15]. In most cases it is assumed that the decrease of crystallinity can be attributed to the formation of thinner and less perfect crystalline structures at higher degrees of supercooling. Everaert et al. [34] have recently obtained interesting results using time-resolved SAXS/WAXD experiments on a blend system where POM was finely dispersed in a PPE/PS matrix. From the evolution of the lamellar crystal thickness with crystallinity, it was found, in addition, that also the lateral dimensions of the crystallites were found to be affected, which in the case of small crystallizing droplets were stated to be limited by the size of the droplets. These observations indicate the important effects of confinement on the resulting semicrystalline structure, with respect to the dimensions of the crystallites and their low crystallization temperatures.
In two earlier publications [5], [6] we reported on the fractionated crystallization phenomena when PA6 was dispersed as (sub)-micrometer sized domains in immiscible blends with PS or PPE/PS as matrix. In these reports, it has been shown that a broad range of crystallization temperatures could be obtained for PA6, mainly via adjusting the blend morphology. For uncompatibilized immiscible blends with micrometer-sized PA6 droplets, crystallization mainly took place at intermediate temperatures [5]. For the same immiscible blends, reactively compatibilized with styrene-maleic anhydride (SMA) copolymers, PA6 droplet sizes in the sub-micrometer range were generated and very low crystallization temperatures were obtained [6].
In this paper, uncompatibilized PS/PA6 blends as well as blends reactively compatibilized with SMA2 (SMA with 2 wt% anhydride functionalities) will be investigated. The phase morphology and rheology of these blend systems were described in detail in a previous publication [35]. These blend systems cover a broad range of both PA6 droplet sizes (between 0.1 and 20 μm) as well as PA6 crystallization temperatures (between 80 and 185 °C). As such, these blends form interesting model systems to study crystallization kinetics and crystallinity changes over a very broad temperature range, using normal thermal histories, e.g. without the need to use high cooling rates. Thus it is possible to do so at both intermediate as well as low crystallization temperatures by performing isothermal DSC experiments. In addition, the crystallinity of the PA6 in the sub-micrometer sized droplets crystallizing at different temperatures is investigated via DSC experiments.
Section snippets
Materials, preparation and phase morphology characterization
Polyamide-6 (PA Akulon K123) was provided by DSM Research, Geleen, The Netherlands. Atactic polystyrene (PS Styron E680) was supplied by DOW Benelux, Terneuzen, The Netherlands. The miscible polyphenylene-ether/polystyrene (PPE/PS) 50/50 wt/wt mixture was prepared by mixing polyphenylene-ether (PPE) (supplied by GE Plastics, Bergen op Zoom, The Netherlands) and PS (supplied by DOW) in a Haake Rheocord 90 twin-screw extruder [36]. Styrene-maleic anhydride copolymer SMA2 (SEA 0579) was provided by
Crystallization of PA6 at intermediate supercooling
In a previous paper [5] it was found that dispersing of PA6 to droplet sizes of about 1–10 μm in a PS matrix, resulted in fractionated crystallization, causing a second PA6 crystallization peak around 170 °C, about 15 °C lower than the PA6 bulk crystallization temperature. It was also observed that the number of crystallization peaks was independent of the cooling rate, with the exception of very high cooling rates leading to an overlap of peaks. Interestingly, however, the intermediate PA6
Conclusions
It has been shown that the crystallization kinetics of PA6 in immiscible blends of PS/PA6 and (PS/SMA2)/PA6 blends can be studied over a very broad temperature range, without the need of using high cooling rates. For immiscible PS/PA6 blends with PA6 droplets of micrometer size, exhibiting only a moderate decrease of crystallization temperature compared to the PA6 bulk crystallization, an athermal nucleation mechanism is suggested based on a nucleation process which takes place in a very small
Acknowledgements
The authors are indebted to the Research Fund of the KULeuven (GOA 98/06), and to the Fund for Scientific Research-Flanders, Belgium for the financial support given to the MSC-laboratory.
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