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|Title: ||The role of nucleating agents on flow induced crystallisation of polymers|
|Authors: ||Invigorito, Carmine|
|Keywords: ||Polymer Crystallisation|
Flow induced crystallisation
|Issue Date: ||2012|
|Publisher: ||© Carmine Invigorito|
|Abstract: ||Isotactic polypropylene (iPP) is one of the widely used commercial thermoplastics. Physical properties of iPP can be tailored to the requirements with respect to structure, microstructure and processing, thus research continues in the development and modification of the polymer. With the advancement of chemistry, as our understanding in tailoring of the molecular structure has enhanced, iPP has become more of a generic name.
With the developments in polymer technology, including polymer processing, it is apparent that to achieve dimension stability one requires fast crystallisation of the semi-crystalline polymer. For an example, in the injection molding of the material, with increasing efficiency of the molding machines to shorten the processing time, production of mould products is faster than crystallisation rate of the polymer. Relatively slow crystallisation rate, imposes limitation in production cycle of the material. Considering that crystallisation is nucleation and growth process, where nucleation is normally controlled by the presence of heterogeneity, a way to suppress the nucleation barrier is inclusion of an additive that can promote nucleation. Such a class of additives are called nucleating agents, where in simplicity their efficiency is defined by shift in the crystallization temperature to higher values at fast cooling rates.
Thus the subject of polymer crystallisation in the presence of nucleating agents has attracted a great interest. In fact, the possibility to gain shorter induction time of polymer crystallisation by controlling nucleation, is the desired goal by many researchers. Furthermore, control of crystallisation from polymer melt is also an important method for modification of the subsequent solid-state properties, allowing design of materials to new applications.
It is also to be realized that shaping of the polymer products requires handling of the viscoelastic melt under flow conditions. The inclusion of flow plays a prominent role in stretching of molecules, where disengagement of chains occurs from the entangled network. Thus the presence of additive and its influence in crystallisation of the polymer during flow demands fundamental in-depth understanding.
The open helical structure of iPP provides several possibilities of tailoring nucleating agents that can suppress the nucleation barrier by epitaxy matching and enhance the overall crystallisation rate. One of the successful nucleating agents for iPP is the derivative of sugar, sorbitol. Considering its origin, the additive is well accepted for packaging purposes and is often known as clarifier, because it enhances nucleation rate to an extent that the crystal aggregate dimensions reduces below the wavelength of visible light, leading to development of the transparent semi-crystalline products having higher modulus and strength compared to the amorphous materials having glass transition below room temperature.
The primary objective of this thesis is to study crystallization of polymer molecules during flow in the presence of nucleating agents having different chemical structures and properties. For this purpose, isotactic polypropylene (iPP) having molar mass 365K g/mol and molar mass distribution 5.4, obtained from a commercial source is utilised. The morphology of semicrystalline polymer in the quiescent and flow conditions is revealed using time resolved optical microscopy and X-ray scattering (SAXS/WAXS) techniques. Rheological aspects of polymer melt in the presence of nucleating agents are manifested. Thermal changes are followed by DSC.
The effect of a commercial nucleating agent such as NA11 has been studied on the crystallisation kinetics of polymer with and without application of shear. In the absence of shear, crystallisation of the iPP from quiescent melt with and without the nucleating agent is investigated. The probable epitaxial matching between the NA11 molecules and the polymer promotes remarkable enhancement in the overall crystallisation rate of the polymer.
In the presence of shear, different shear rates and shear times are varied and applied to the polymer melt. The observations are that the resulting polymer morphology show strong dependence on the shear rate compared to shear time. In particular, also for the neat polymer, when the applied shear rate is beyond a critical threshold the resulting polymer crystalline structure is influenced and a diverse degree of orientation can be retained. This orientation is higher when polymer crystallisation occurs in the presence of NA11.
Influence of an innovative nucleating agent, commercially known as Irgaclear xt386, is examined on the crystallisation behaviour of isotactic polypropylene. Unlike NA11 the nucleating agent Irgaclear xt 386 shows miscibility in polymer melt. Thus a monotectic phase diagram of the nucleating agent and iPP is examined and proposed. At high temperature the nucleating agent dissolves in the iPP melt and forms homogeneous liquid. While on cooling, with crystallisation of the nucleating agent, liquid to solid phase separation occurs. This crystallisation is observed above the melting point of iPP. The crystallised nucleating agent, spread over the polymer melt, provides heterogeneous nucleation site suppressing the nucleation barrier for polymer crystallisation thus shifting the crystallisation temperature to higher values.
On the application of constant shear, by varying shear rate and shear time, influence of nucleating agents on iPP prior and after its crystallisation is followed in time by optical microscopy and time resolved SAXS. The studies are further extended to different shear temperatures with the view to influence relaxation process of polymer chains.|
|Description: ||This thesis is Restricted Access until 15th October 2017. A Doctoral Thesis submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Version: ||Closed access|
|Appears in Collections:||Closed Access PhD Theses (Materials)|
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