Poly methyl methacrylate (PMMA) has many uses, including as plexiglass, high-speed optic cable, teeth filling material and bone cement. It is generally an amorphous material, lacking strength and features of semi-crystalline polymers. However the mixture of stereoregular PMMA (i.e. mixture of isotactic and syndiotactic PMMA), forms a supramolecular structure. This structure is called a "stereocomplex". The syndiotactic PMMA wraps around the isotactic chain, forming a 9/1 double stranded helix, having an asymmetric unit consisting of 1 isotactic unit and 2 syndiotactic units. Thus formed physical gels has "physical" crosslinks that improve the mechanical properties of PMMA. We study the properties of this gel, and are developing the gel spinning technology to make these stereocomplex fibres with much better mechanical properties. The goal is to then use these fibers in PMMA matrix to form self-reinforced composites.

The stereocomplex forms only in a certain solvents. In these solvents, a viscosity drop is observed when dilute solutions of isotactic and syndiotactic PMMA are mixed. This suggests a reduction in the number of chains present in the solution. When such solutions are evaporated, the PMMA residue has crystalline regions with a rather high melting point (180 C). X-ray data as well as NMR and IR data in the literature suggest that a helical structure is responsible for this behavior. We are interested in the rheology of such physical gels, as well in understanding how and why stereocomplex forms. We mix the isotactic and syndiotactic PMMA in a stereocomplex forming solvent (DMF). If the concentration of the polymer is high enough, a physical gel forms. We have used this dope to make PMMA fibers by the gel-spinning method.
The gel spinning set-up. A: heated syringe with the stereoregular PMMA solution in DMF. B:methanol cooling bath at -20C . C: pick-up reel. D: heating zone. E: stretching reel.
 
The crucial feature of this method is gel formation in the polymer solution right as it flows out of the spinneret. This is achieved by heating the solution in the syringe above its gel-temperature (A). Once the polymer solution exits the spinneret, it cools down and forms a gel. To further promote gel formation, the resulting fiber is dragged through a cooling bath (B) and picked up at a set pick-up rate (C). Furthermore, the fiber is heated (D) and stretched (E). We are currently investigating the rheology of the dope solutions and of gels formed and the mechanical and thermal properties of the fibers formed.

The resulting high-strength semi-crystalline PMMA fibers will be used to make self-reinforced composites. In a regular fiber-reinforced composite, high-strength fibers are embedded in a polymeric matrix. The key feature that dictates the resistance to stress fracture failure is the fiber-matrix interface. This interface is governed by adhesion forces, which are in turn dependent on material similarity. The more chemically similar the two materials, the better the adhesion and therefore tighter fiber-matrix interface. In a self-reinforced composite, the fibrous and non-fibrous materials are chemically identical, thus providing a very good fiber-matrix interfacial interaction.

 
Idealized view of the fiber-matrix interface in self-reinforced composites. It should be noted, that the fibers are not as perfectly aligned in a realistic composite. The colors reflect the similarity of the materials used and black lines denote phase boundaries. Note that there is no phase boundary between the fiber and the matrix in a self-reinforced composite.
 
We also investigate the thermal properties of these composites and observe the melting point of stereocomplex fiber is around 180 C. This is very high compared to the melting point of either of the stereoregular PMMA and is correlated to the degree of crystallinity and the method of stereocomplex preparation. The mechanical properties of the fibers are being investigated. Young's modulus of the fibers needs to be higher than the regular PMMA in order for the fiber to have any reinforcing effect on the composite. Furthermore, the mechanical properties of the composite as a whole will be investigated as related to Young's modulus and fracture toughness,and being a self reinforced composites we expect remarkable improvements in obtainable mechanical properties.