Flow distribution in shear-induced crystallisation of melt polymer: A prediction from morphological distribution of solid polymer
Graphical abstract
Introduction
Recent investigations have greatly advanced our understanding of flow-induced crystallisation (FIC) of semi-crystalline polymers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. FIC attracts much attention for its intrinsic scientific interest, as it is a typical example of a non-equilibrium and externally-controlled phase transition. FIC is also technologically important because the processing of semi-crystalline polymers provides the largest amount of commercially useful polymeric materials.
Flow can effectively introduce different levels of anisotropy into melt crystallised polymers. Moderate flow strengths result in an enhanced point-like nucleation, which speeds up the isotropic growth of polymer spherulites. High flow strengths are able to induce a coil-to-stretch transition to form shish, which directs oriented crystallisation to form a shish-kebab structure, a special chain crystalline assembly. The specific hierarchical structures formed during melting crystallisation then ultimately control the final physical, optical and mechanical properties of the moulded component.
Flow-induced melt crystallisation of polymers and corresponding oriented morphologies have been extensively studied. Nevertheless, the vast majority of the work to date has been devoted to the flow–structure relationship at a laboratory level. However, the flow history and accordingly the development of structures in industrial processes, such as injection moulding, are far from those occurring in laboratory experiments [11], [12], [13], [14], [15]. In laboratory experiments the flow and temperature conditions are constant and normally are well known. In contrast, large flow and temperature gradients are typical of many industrial processes. It is worthwhile to note that the differences between laboratory device flow and processing-like flow have been recently narrowed using a sliding plate shear cell with optical access, which allows the FIC into the processing regime [16], [17]. Importantly, both flow and thermal gradients are known to influence melt crystallisation [18], [19], [20], [21], [22], [23], [24]. The combined effects are particularly significant in the manufacture of thick materials, which are obviously not uncommon for practical interests. In injection moulded components the combined effect of flow and temperature gradients results in a highly nonuniform skin-core morphology [18], [19], [20], [21], [22], [23], [24]. Low cooling rates and strain fields lead a predominantly spherulitic structure in the core of components. Towards the surface a highly sheared zone forms which produces very anisotropic molecular arrangements. In particular, a branched shish-kebab structure [22], a special kind of polymer crystalline assembly, can form in the shear zone under certain moulding conditions in injection moulded isotactic polypropylene (iPP).
The direct measurement of flow and temperature gradients during injection moulding is extremely difficult. The polymer melt experiences shear flow only for short durations before solidifying. Sheared morphologies of solid polymers have been investigated [22], [25], [26], [27], [28], [29], [30], but the experimental results are usually correlated to the geometric positions, rather than the specific flow conditions occurring at that point. Nevertheless, the morphological distribution along the thickness of sheared polymers is directly related to the flow history of melt crystallisation. The flow distribution in complex moulded parts is often predicted by computer simulations [31], [32]. However, experimental methods to evaluate their accuracy have yet to be developed.
The final morphologies of injection moulded polymers, including thickness of crystalline lamellae and orientation, are determined by an intricate balance between the contradictory effects of flow and thermal gradients. For example, at high cooling rates the polymer segments tend to be locked or frozen “in” leading to lower levels of polymer orientation. On the other hand, sufficiently intense shear flow can induce high levels of polymer orientation. While no technique is yet available to determine the specific flow and temperature gradient of melt crystallisation, the sheared morphology is readily measured along the thickness of solid polymers. Especially, the fine X-ray beam made possible using synchrotron radiation allows the precise determination of the morphological distribution from the skin to the core [26], [27], [28]. The free energy of melt crystallisation can also be estimated from the thickness of crystalline lamellae of the solid polymers [33]. Accordingly, the possibility exists to reverse engineer the flow history imposed during melt crystallisation from detailed morphological measurements of the final solid article.
In the present work, we report on a study which aimed to predict the flow distribution of an injection-moulded iPP article from its experimentally determined morphological distribution. A low limit flow field imposed on the melt is traced out from the morphological distribution using a microrheological model [34], [35]. The effect of Cu-phthalocyanine (CuPc), a nucleating agent and pigment, on the flow distribution of iPP is also investigated. Individual CuPc molecules are a planar structure that are closely packed, as shown in Fig. 1, in parallel with each other along b-axis to form molecular stacks [36]. A flat surface full of crevices of nonuniform sizes and an anisotropic geometry has been theoretically predicted to be a good template for the heterogeneous nucleation [37], [38]. It is found that CuPc is also an efficient nucleating agent for the melt crystallisation of iPP [39], though it has been usually used as a pigment. The presence of CuPc is a typical example of seed effects on the flow-induced crystallisation of polymers.
Section snippets
Experimental section
iPP (Mw = 367,000, Mn = 74,000) and CuPc were obtained from Borealis and Polypacific Australia, respectively. CuPc was premixed with iPP in a blender at room temperature and then in a twin screw extruder at 220 °C. This is a common procedure for the mixing of additives into polymer formulations. We did not observe a large number of big particles at 220 °C using optical microscopy, although an ideal homogeneous dispersion was not expected to be achieved. The materials were labelled as iPP-0.0,
Results and discussion
Fig. 3 shows the distribution of 2D WAXS image patterns along the thickness of sheared iPP plates. As can be seen, oriented and isotropic patterns were observed, respectively, from shear to the core regions. 1D WAXS profiles were obtained from circularly integrated intensities of 2D image patterns in order to include the effect of molecular orientation on the 1D WAXS profiles. A peak-fit procedure was used to deconvolute the peaks of 1D WAXD profiles, Fig. 4. Both Gaussian and Lorentz functions
Conclusions
The flow distribution locally imposed on the melt crystallisation of iPP during injection moulding was predicted from the morphological distribution of solid state iPP using a microrheological model. The flow distribution represents a low limit flow field, including shear and elongation flows, required to generate the oriented morphology. The crystallinity and molecular orientation have been correlated to the flow history, which show the features of combined effects of flow and temperature on
Acknowledgements
This work was performed at the Australian National Beamline Facility (ANBF) with support from the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. P-W Zhu wishes to thank Dr. Mitsuyoshi Fujiyama for fruitful discussion.
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