The polymerization mechanism of methylol-functional benzoxazine
monomers is reported using a series of monofunctional benzoxazine
monomers synthesized via a condensation reaction of ortho-, meta-, or para-methylol–phenol,
aniline, and paraformaldehyde following the traditional route of benzoxazine
synthesis. A phenol/aniline-type monofunctional benzoxazine monomer
has been synthesized as a control. The structures of the synthesized
monomers have been confirmed by 1H NMR and FT-IR. The polymerization
behavior of methylol monomers is studied by DSC and shows an exothermic
peak associated with condensation reaction of methylol groups and
ring-opening polymerization of benzoxazine at a lower temperature
range than the control monomer. The presence of methylol group accelerates
the ring-opening polymerization to give the ascending order of para-, meta-, and ortho-positions in comparison to the unfunctionalized monomer. Furthermore,
rheological measurements show that the position of methylol group
relative to benzoxazine structure plays a significant role in accelerating
the polymerization.
Co-extruded films with up to 65 layers of two rheologically mismatched polymer systems – polystyrene/poly(methylmethacrylate) (PS/PMMA) and hard/soft thermoplastic polyurethanes (TPUs) – were successfully produced using a combination of a 9-layer feedblock, low-pressure drop multiplier dies, and external lubricants. Formation of viscoelastic instabilities was studied using a custom visualization and by finite element method (FEM) simulations of a standard multiplier. The results showed that the flow inside the standard multiplier die is highly non-uniform, with severe gradients in shear and normal stresses and viscous encapsulation occurring mainly in the initial multiplication stages where there is enough material available in the low-viscosity layers to proceed with the encapsulation. To mitigate layer degradation the standard 2- or 3-layer feedblock was replaced with a 9-layer one, thereby decreasing the thickness of each layer at the end of the feedblock. Also, subsequent layering was performed using a low flow resistance die. This new multiplier die yields a more uniform flow profile and imparts a more homogeneous thermo-mechanical history on the melt which results in an improved layer stability. Simulations showed that in the standard die the second normal-stress difference (N2) responsible for elastic instabilities at the edges of the die are very high. These can be reduced by inducing slip at the wall resulting in be much improved layer uniformity and stability. This was accomplished experimentally via the use of external lubricants, and the resulting layered structure was indeed much better than was possible to achieve with the conventional multiplier dies.
In this paper, covalently linked graphene oxide–poly(ethylene glycol) methyl ether methacrylate–reversible addition‐fragmentation chain transfer (GO–PEGMEMA–RAFT) and physically mixed GO–PEGMEMA hydrogel nanocomposites are synthesized. Spectroscopic and imaging techniques such as UV–vis, Fourier transform infrared, Raman spectroscopy, and transmission electron microscopy show that the PEGMEMA is successfully grafted on GO sheets. The rheology of the nanocomposites is studied by small angle oscillatory shear, which shows a competition between reinforcement and lubrication behavior of GO. In the case where lubrication effect dominates reinforcement, the covalently linked GO–PEGMEMA–RAFT has higher G′ compared to the physically mixed GO‐PEGMEMA. Hence, in the covalently linked system, the grafted polymer chains appear to minimize the lubrication effect.
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