Controlling the melting kinetics of polymers : a route to a new melt state

Polymers play an important role, both in nature and in the modern society. In contrast to polymers in nature, the so-called biopolymers, man-made polymers are thermally more robust and are in majority processed via the melt (plastics). In the case of thermoplastic polymers (> 70% of all synthetic polymers), the viscosity of the polymer melt poses a limit on the processability, notably for polymers possessing a high(er) molar mass M. Based on experimental evidence, the (zero-shear) viscosity of polymer melts, ? 0 , scales with Mw 3.4 (Mw is the weight-average molar mass). This implies that the melt-viscosity increases with more than a factor of 10 upon doubling the molar mass! Since the properties of polymers in the solid-state also increase with increasing molar mass, notably the strength and toughness, the processing of thermoplastic polymers, e.g. injection-moulding, extrusion, fiber spinning, is often a compromise between the ease of processing, viz. preference for lower molar mass (easy flow), and properties, with preference for high(er) molar masses. The current knowledge of polymer melts is rather well developed and based on a simple but elegant model put forward by P.G. de Gennes (Nobel prize for Physics), the so-called reptation model. In this model, the motion of a polymer chain in the molten state is hindered by its neighbors (entanglement), which generate a virtual "tube" confining the chain on a one dimensional pathway. The constraint chain dynamics gives rise to a characteristic time for a chain to diffuse its own length in the tube. Scaling as M3. The same scaling is predicted for the zero-shear viscosity. The experimentally observed discrepancy, see above ? 0~ M 3.4, from the 3.0 dependence is attributed to "contour-length fluctuations" i.e. fluctuation-driven stretching and contractions of the chain along the tube. In Chapter 2, it is shown that the zero-shear melt-viscosity of carefully prepared samples of high molar mass polyethylene (PE), possessing a narrow molecular weight distribution, indeed follow the predictions of the reptation model, viz. ? 0 scales with M3 . The advantage of high molar mass polyethylenes is that chain-end effects do not play an important role or can be ignored. As a consequence of these results, high molecular weight polyethylenes have been used as a model substance throughout the thesis, notably ultra-high-molecular-weight PE (UHMW-PE). In the solid state, entanglements can be removed effectively by dissolution of the polymer. In dilute solutions, below the so-called overlap concentration ? *, entanglements can be removed completely. In the case of crystallizable polymers, such as PE, the reduced entanglement density can be made permanent since the long chain molecules form folded-chain crystals, a well-studied phenomenon in polymer physics. A more elegant and also technologically more advanced way to generate disentangled PE crystals is via direct polymerization in the reactor. At low polymerization temperatures and low catalyst activity/concentration, individual growing chains will form their own folded chain crystals. In the limiting case where the growing chains are separated far enough from each other, monomolecular crystals can be formed. If completely disentangled PE structures can be obtained via solution-crystallization and/or via direct controlled synthesis, the intriguing question is whether this disentangled state will be preserved upon melting and what is the time scale to generate a fully entangled equilibrium polymer melt. This question is the key issue of the thesis. What happens when we start from a non-equilibrium disentangled state and cross the melting temperature into the molten state? How does the equilibrium entangled melt state get restored? In Chapter 2 it is shown that starting from the disentangled state, in this case nascent UHMW-PE powder, that it takes time to "build-up" the plateau modulus in the melt, indicative of an entanglement formation process. The entanglements formation scales as the reptation process (Mw 3). Parallel to rheology measurements, solid-sate NMR is used to monitor the change in chain mobility. The time scale to reach the equilibrium melt as probed by the NMR and Rheology experiments is very different, suggesting that restrictions in local chain mobility monitored by NMR are realized at an earlier stage than restrictions in segmental mobility inferred from rheological experiments. A peculiar phenomenon of nascent reactor powders is their high melting point, close to or equal to the so-called equilibrium melting point of PE. This phenomenon has puzzled researchers in the field for many years and various explanations have been given such as the growth of extended-chain crystals instead of folded-chain crystals or extensive reorganization during the melting process, but all these explanations were not supported by experimental data which show that nascent UHMW-PE reactor powders consist of "normal" folded-chain crystals without extensive reorganization (thickening) during the melting process. In Chapter 3 it is shown that the unusual high melting temperature of nascent UHMW-PE is related to the tight-folding (adjacent re-entry mode) in the crystals. Melting is a cooperative process over several chain stems of the same crystal in contrast with e.g. melt-crystallized samples where a chain is incorporated in various folded-chain crystals and topologically, prior to the melt, is in contact to different chains. The melting mechanism as discussed in chapter 3, can be utilized by controlling the melting process of UHMW-PE nascent reactor powders. When decreasing the heating rate, the melting process starts by detachment of single stems from the (lateral) surface of the crystals. In this process, the molten chain ends can entangle with chain ends from other partly molten crystals, whereas the core of the molecule is still in the crystal, viz. in the tight folded-chain conformation. As discussed in Chapter 4, after complete melting by this mechanism, a heterogeneous melt-state is obtained since the central part of the individual chains is prevented from taking part in the entangling process. By NMR experiments, it is observed that on decreasing the heating rate, the time required to restrict the chain conformations at the local scale increases. In rheometry it is observed that with the increasing time to restrict the chain conformations, the time needed for the modulus to buildup increases. Ultimately, it is feasible to melt the sample so slowly that the restriction in the chain conformation in part of the sample can be inhibited, maintaining the partially high local mobility. Since restrictions in the chain conformations can not be achieved, the cooperative motion needed for the translational mobility is absent. As a consequence normal chain reptation is slowed down considerably and a long-living partially disentangled melt is obtained. This new melt state shows a decreased plateau modulus and viscosity, whereas the terminal stress relaxation rates remain the same. The observations are that stress relaxation is achieved without "normal" reptation of chains in the tube. This is explained by the partial reptation of the chains since only a fraction of the whole chain is required for the stress relaxation. The consequences of a heterogeneous melt-state are discussed in Chapter 5. The observations are that the disentangled chainsegments crystallize faster than the entangled chains. This suggests that intra-molecular homogeneous nucleation occurs faster than the heterogeneous nucleation. Moreover, after crystallization from the heterogeneous melt, the solid-sate drawability is still remarkably high, indicative of a certain state of disentanglement. Thus can be drawn into a fiber in the solid state because large disentangled blocks are present in the crystal. The melting behavior of solution-crystallized UHMW-PE is studied in Chapter 6. Similar to the nascent disentangled crystals, these folded-chain crystals can be melted by consecutive detachment of chain stems from the crystal substrate. The differences in melting behavior, revealed during different heating rates, have consequences on the chain dynamics. Unlike the nascent disentangled samples, where modulus builds up with time, the solution-crystallized sample entangles immediately upon fast heating. The remarkable difference in the rate of entanglements formation can be attributed to the differences in the stacking of crystals, prior to melt. The solution-crystallized samples double their crystal size via intermixing of the regularly stacked crystals which upon melting facilitate the entanglement formation process.contrary to the nascent disentangled samples where no regular stacking occurs. An alternative route to achieve a reduction in the melt viscosity is explored in Chapter 7, by the addition of the single-walled carbon nanotubes (SWNTs). When varying the content of SWNTs, the dynamic viscosity/storage modulus shows a minimum. The decrease in viscosity is attributed to the selective adsorption of the high molar mass fraction onto the nanotube surface. The increase in viscosity upon further increasing the nanotube content is attributed to the formation of an elastic nanotube-polymer network. The concepts presented in the thesis, based on experimental validation, could have an important impact on novel processing techniques for UHMWPE, e.g. sintering of UHMW-PE into products for demanding applications such as artificial hip-and knee joints and, solventfree processing routes for UHMW-PE fibers and tapes.