Disentangled polyethylene with sharp molar mass distribution : implications for sintering

S. Talebi

    Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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    Abstract

    Synthetic polymers, often referred to as plastics, have surpassed steel two decades ago in production volume. At present, over 200 million tones of plastics are produced annually. There are various reasons for the almost exponential growth of plastics since World War II such as cheap production routes (based on oil) , ease of processing complexly shaped parts e.g. via injection-molding and a large range of properties ranging from soft rubbers to fibers stronger than steel. Synthetic polymers are long chain molecules and the majority is processed via the molten state, the so-called thermoplastics. The properties of polymeric products are not solely determined by the chemistry and chemical structure of the polymer molecules but equally well by the processing step, notably the orientation of the long chain molecules. A prime example in this respect is the simplest polymer: polyethylene which is used to make flexible containers but is also the base material for the super-strong Dyneema® fiber of DSM. Thermoplastic polymers, viz. synthetic polymers which are processed via the molten state (melt) possess a high melt-viscosity which is strongly increasing with increasing molar mass. In fact, the so-called zero-shear viscosity, ¿o, scales with Mw3.4, viz upon doubling the molar mass, the melt-viscosity increases more than 10 times! Properties such as toughness and strength also increase with increasing molar mass and hence polymer processing in practice is a balance between ease (speed) of processing, which requires a low(er) molar mass and properties of the end-product where a high(er) molar mass is required. The reason for the strong dependence of the melt-viscosity on molar mass is related to existence of intermolecular topological interactions which is referred to as entanglements, the long chains are inter-hooked like in cooked spaghetti. The number of entanglements per molecule is dependent on the chemical structure and is expressed in the so-called , the average molecular weight in between two neighboring entanglements. If we accept the current models of polymer rheology that chain entanglements play a dominant role in the melt-viscosity of polymer melts then the question arises whether the entanglement density can be changed in favor of a low(er) melt-viscosity and hence easier processability? An effective way to remove chain entanglements is via dissolution. In solutions the chains are relatively separated from each other and in the case of dilute solutions, below the so-called overlap concentration, the chains are completely separated physically and hence all entanglements have been removed. Upon removal of the solvent, the chains re-entangle again but for crystallizable polymers the chain disentanglement can be made permanent in the solid state. A well-studied system in this respect is ultra-high molecular weight polyethylene, UHMW-PE, which is considered to be an intractable polymer, viz. can not be processed via the melt in view of an excessively high melt-viscosity related to its molar mass, well above 106 g/mol. UHMW-PE can be dissolved in solvents such as decalin at elevated temperatures and upon cooling lamellar UHMW-PE crystals are formed. After crystallization the solvent can be removed, e.g. by extraction, and solid UHMW-PE is obtained which is free of entanglements. In the solid state this so-called disentangled UHMW-PE is ductile and easy to deform into oriented tapes with a high degree of chain alignment. However, upon heating into the melt the favorable chain topology is lost and re-entangling is a very fast, Entropy-driven process and, consequently, in terms of rheology and processing no beneficial effect could be obtained, the melt-viscosity is again prohibitive high. The use of solvents to generate dis-entangled UHMW-PE is rather cumbersome but used in practice for the production of superstrong polyethylene fibers, e.g. Dyneema® by DSM. A novel and much more elegant route to generate disentangled polymer crystals is via direct synthesis in a reactor as described in Chapter 2. Upon polymerization at low temperatures and by using a single site homogeneous catalyst, the growing chains experience a "cold" environment and crystallize individually into folded-chain crystals, most probably monomolecular crystals, viz. one long chain forms one crystal. In fact, this is an easy and direct approach towards complete dis-entangling. The first aim of this work, as described in Chapter 2, is to produce disentangled UHMWPE with different molar mass and narrow polydispersity directly in a polymerization reactor. A single site catalyst, a so called FI catalyst, was used for this purpose. A linear dependency between the polymerization time and the molar mass in the initial stage of the polymerization was observed, indicative of living, or controlled, polymerization. Furthermore, the influence of polymerization parameters such as temperature, co-catalyst to catalyst ratio and type of solvent on molar mass of synthesized polymer have been investigated. By controlling polymerization parameters, polyethylenes with different molar masses up to 9x106 g/mol and narrow molecular weight distributions in the disentangled state were produced. However, determination of the molar mass and polydispersity becomes a challenge via conventional gel permeation chromatography (GPC). In Chapter 3, a melt rheometry technique based on the "modulus model" was utilized to measure molar mass and PDI and make a comparison with the GPC data. The method works by converting the relaxation spectrum from the time domain to the molecular weight domain, then using a regularized integral inversion to recover the molecular weight distribution curve. These findings are of relevance in determining very high molar masses that cannot be obtained conclusively with the existing chromatography techniques. It is to be noted that in calculating molar mass from relaxation modulus the ill-posed nature of the inverse problem is dealt. Therefore, the possible error in the data obtained might be to check it by other means, such as GPC for molar masses up to 3x106 g/mol. From the results obtained it was concluded that UHMWPE with different molar masses and narrow distributions has been synthesized successfully. These disentangled materials, so-called nascent or virgin reactor powders, have been studied regarding their rheological behavior, especially addressing the re-entangling process upon heating into the melt. In Chapter 4, an investigation was performed using an oscillatory rheometry on the formation of entanglements and chain dynamics in a disentangled polymer melt. It is demonstrated that the modulus build-up with time at a fixed frequency and at constant strain and temperature shows two distinct regions, defined by two distinct slopes in the modulus build up. Region I, corresponding to the steep slope, arises due to mixing of disentangled chains, whereas Region II shows a slow increase in the modulus build up with time – following the reptation dynamics in the melt. Region I shows a strong dependence on the rate of entropy gain by the crystals on going from the solid to the liquid state. The rate of entropy gain was varied either by heating rate, on annealing of the samples in the vicinity of the equilibrium melting temperature. Molar mass dependence is exhibited during entangling process of the chains, which also shows the influence on the build-up time from the disentangled to the entangled melt state. Further experiments were performed on a disentangled sample to investigate the influence of applied frequency and strain on the entangling process. It was shown that the build-up time to reach a plateau of 2MPa increased considerably at the low frequency of 1rad/s. This difference in the build-up time at the low and the high frequencies suggest differences in the chain dynamics of the different chain regions of the disentangled polymer melt. These observations suggest that the total build-up time will be different for the different chain segments. From the studies of the strain effect, it was observed that the entangled state influences the border of the linear viscoelastic regime and the non-linear region. Disentangled melts show a non-linear regime behavior at much lower strain amplitude, indicative of easier disengagement of chains. These findings suggest a possibility of influencing the entangling process by controlling the mixing of chain segments at different regions – such as slow melting via annealing. The annealing experiment performed on the disentangled polymer in the rheometer showed that the slope of region I for fast heated sample was much higher than the slope of the annealed sample, clearly indicating the influence on the rate of entropy gain during melting of the crystals and its effect on mixing of the initially disentangled chains. The melting of disentangled crystals and the influence of entanglements on polymer crystallization is discussed in Chapter 5. The high melting temperature of nascent UHMWPE has been a matter of debate for a long time. By measuring the melting point of two different molar mass UHMWPEs having narrow polydispersities with the help of DSC the non-linear heating rate dependence of the melting temperature of the nascent polyethylenes was observed suggesting the presence of an activation barrier in melting of the crystals. Furthermore, the effect of annealing below and above the melting temperature was investigated for the nascent polymer. The subsequent heating run on the annealed samples below melting temperature showed two distinct melting peaks suggesting the existence of two populations of crystallites. The subsequent cooling on the annealed samples well above the equilibrium melting temperature resulted in lowering the onset of crystallization. The influence of annealing in the vicinity of melting temperature on the onset of crystallization temperature was also investigated. Chapter 6 addresses some of the technological aspects of homogenous polymerization and the disentangled polymer obtained from such a polymerization.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • Chemical Engineering and Chemistry
    Supervisors/Advisors
    • Rastogi, Sanjay, Promotor
    • Lemstra, Piet, Promotor
    • Duchateau, Robbert, Copromotor
    Award date3 Dec 2008
    Place of PublicationEindhoven
    Publisher
    Print ISBNs978-90-386-1477-9
    DOIs
    Publication statusPublished - 2008

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