OVERVIEW: Processing and fabrication are vital operations for the transformation of polymers from a material commodity into useful articles. Over the last seven years, we at MMI have been pursuing the optimization of polymer processing in two key areas: (1) vacuum assisted resin transfer molding (VARTM) of epoxies for high performance composites and (2) injection molding of engineering thermoplastics, particularly thermotropic liquid crystalline polymers (TLCPs).

TLCPs combine the virtues of superior tensile properties with the ability to injection mold with very easy flow through the spontaneous ordering of molecules. The critical processing issue is the development of high anisotropy during TLCP processing. Although the concurrence of high tensile properties and high directional orientation is of great benefit in fiber spinning, severe anisotropy can be a plague in obtaining balanced properties in net-shape injection-molded parts. The rigid nature of the mesogenic segments in TLCP molecules usually leads directly to a high orientational bias favoring the direction of flow with injection molding and other directional processing of thermotropes. Upon recrystallization, the high molecular orientation often leads to very favorable properties in the direction of orientation and lower physical properties in the transverse direction.

The high specific strength of TLCPs makes them excellent potential candidates for the manufacture of strong lightweight net-shape parts intended for use in transportation applications. The singular challenge in affecting their use in this manner is gaining control over the severe anisotropy that can often result during melt processing. Towards this end, our research in collaboration with Northwestern University was targeted at: (1) improved understanding of how processing conditions influence orientation and physical properties; and (2) using the results to develop new modeling strategies for injection-molding to predict orientation formation based upon a so-called 'polydomain' description of orientation which facilitates a direct connection to experimental measurements of orientation. An important part of this research effort is an extensive 3-D mapping of orientation in fabricated parts processed under defined conditions with model TLCPs.

Molding a plaque or other part with a broad aspect ratio will incur great flow complexity and, therefore, complex states of orientation. Shear flow dominates near the surface while transverse stretching dominates near the mid-plane resulting in bimodal cross-ply orientation, as schematically shown in the figure of a cross section. Depending upon thickness, a “skin/core” structure results with a high molecular alignment in the ‘skin’ that may be maximized in a direction different from that in the “core”. Shear asserts increasing dominance with decreasing sample thickness. Research efforts were focused on Celanese Vectra® (HBA/HNA) copolyesters, as well as a copolyester based upon dihydroxy-alpha-methyl stilbene as a molecular mesogen.

A type of soft X-ray spectroscopy called Near-Edge X-ray Absorption Fine Structure (NEXAFS) performed at the national Synchrotron Light Source (NSLS) was used to determine the molecular orientation on the surface (2 nm deep) across injection molded plaques.  A novel 6-axis sample manipulator was utilized to facilitate the physical translation, and the incident and azimuthal angle rotations of the samples.

The NEXAFS results were compared with surface orientation determinations made by using Attenuated Total Reflectance Fourier Transform Infrared (FTIR-ATR) dichroism. The results from NEXAFS and FTIR-ATR were then comprehensively compared to define the state of surface orientation in the plaques. These results along with extensive orientation determinations made by 2-dimensional Wide-Angle X-ray Scattering (2D-WAXS) performed at the Advanced Photon Source at Argonne National Laboratory were used to guide the successful development of improved computer modeling of the processing of TLCPs within the framework of the Moldflow® software package.