PhD: Shear induced micro- and macrostructural changes in polymer blends based on poly(vinylchloride) with poly(alkylmethacrylates) and their effect on the rheological, morphological and final properties

Aneta Vasiljevic-Shikaleska (aneta@fbe.edu.mk)
Advanced Polymer Materials, Faculty of Technology and Metallurgy, Ss. Cyril and Methodious Univeristy, Skopje, Macedonia
February, 2013
 

Abstract

The aim of this thesis was to study the structure development in polymer blends when they are forced to flow under specified stress fields and its influence on rheological and mechanical properties.

Binary polymer blends of polyvinylchloride (PVC) with polymethylmethacrylate (PMMA), polyethylmethacrylate (PEMA), polybutylmethacrylate (PBMA) and polypropylene (PP) with 90/10, 80/20, 70/30 and 30/70 were investigated. Also nanocomposite material based on PVC/PEMA 90/10 blend was prepared by incorporation of montmorillonite nanoclay.

Test samples were prepared in a molten state using a Brabender mixer and afterwards extruded at two different rotational speeds in a single screw extruder. The characterization of these materials was carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), parallel plate oscillatory rheometry, dynamic mechanical thermal analysis (DMTA), dynamometric method, differential scanning calorimetry (DSC) and X-ray analysis (XRD).

SEM micrographs revealed homogenous and uniform dispersions without visible phase boundaries. The fracture surfaces of samples processed in a brabender changed from brittle and glassy for neat PVC to ductile with high extent of plastic deformation for the nanocomposite and blends. It was also observed that additional shearing of samples in the extruder caused structural reorganization and elongated domains oriented in the flow direction were detected.

Rheological measurements performed under steady shearing conditions and dynamic oscillatory shearing were used to evaluate the shear-dependent viscosity, η(γ), the first normal stress coefficient, ψ1(γ), and the second normal stress coefficient, ψ2(γ). Analytical equations of these dependences (η(γ), ψ1(γ), ψ2(γ)) known as material functions, with the exception of ψ2(γ) which is zero, were found. Also expressions for the functions of the storage modulus, G'(ω), the loss modulus, G''(ω), and the complex viscosity η*(ω) were determined. According to these functions all samples are viscoelastic with a pseudoplastic character of the apparent viscosity and the first normal stress coefficient. Pseudoplacticity, assessed by the viscosity material function, is more expressed with samples obtained in the brabender. For example, the viscosity material function of PVC/PEMA 90/10 blend processed in the brabender is ηap = 14γ–06, while the function for the same blend additionally sheared in the extruder is ηap = 8γ–04. This noticeable change in the values of the flow index, which is closely related to the material structure, arises from the flow induced microstructures caused by the different stress field occurring in the brabender and extruder. These results are consistent with those obtained from the morphological analysis.

Good correlation between the correspondent functions from steady shear flow measurements, τ12(γ) and N1(γ), and dynamic oscillatory measurements, G''(ω), and G'(ω) is observed. This finding, being a property inherent in all viscoelastic materials, is very important because it can be used not only to test the suitability of different rheological constitutive equations and models but also may confirm a more convenient method to generate data from some polymer characteristics to others quite different or to prove the validity of data obtained from different measurements.

DMTA results show that viscoelastic properties of samples in a solid state are significantly influenced by the manner of mixing, i.e. how the components are brought together into a direct contact. Materials with noticeably higher storage modulus, from 20 to 40%, are obtained when the material is processed in the brabender instead of extruder.

Tensile properties were evaluated by the Young’s modulus, tensile strength and elongation. As expected, due to the homogenous dispersion of spherically shaped domains when blends are processed in the brabender materials with enhanced elasticity are obtained. Contrary, during extrusion the elongation of domains generates more brittle material. These results entirely agree with those obtained from SEM, DMTA and parallel plate measurements.

It has to be emphasized that experimental results confirm the assumption that during the operation of shaping of polymer systems into semifinal or final products microstructural changes do happen that undoubtedly affect strongly material end properties. Among all, the rheological properties of the studied polymer blends were the most sensitive ones to the flow induced microstructural changes.

The addition of nanoparticles improved the rheological behavior of polymer blend in a molten and solid state, without worsening its processibility. This is of particular importance because indicates that the nanocomposite material can be processed using the same operations and processing conditions as those for neat polymer blends.

Based on a thorough analysis of the obtained experimental results, observations, analytical relations of the material functions and viscoelastic properties, others very specific findings and of course the current knowledge in this field, it is shown that the rheological behaviour, being the most sensitive to flow induced structural and morphological changes, remains the main concern in polymer processing especially when polymer blends are processed. It dictates the rate and quality of final products. Therefore the rheological viscous and elastic behaviour quantitatively determined in this investigation, with incorporated molecular parameters and engineering principles can be used as a fundamental input in the mathematical formulation of the process as whole. This approach along with advanced computational tools of modelling, simulation and optimization will enable achieving products with desired specific properties.