Browsing by Subject "Liquid crystal"

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Liquid Crystals for Mobility Control
(1986-05) Sommer, Francis Stephen; Schechter, Robert S.
Novel Liquid Crystal Resins for Stereolithography: Mechanical and Physical Properties
(1997) Schultz, J. W.; Ullett, J. S.; Chartoff, R. P.; Pogue, R.T.
This paper considers photocurable liquid crystal (LC) monomers which are a new class of stereolithography resins. These resins form polymers with high upper-use temperatures. The rodlike molecules can be aligned by an external force. When cured in an aligned state, the aligned structure is "locked in" resulting in materials with anisotropic physical and mechanical properties. FTIR spectroscopy, thermo-mechanical analysis (TMA), dynamic mechanical analysis (DMA), and large strain mechanical tests were applied to liquid crystal photo-polymers, both in the green state and after postcure. These measurements showed that the photo-polymerization reaction locked in the molecular order. Elastic modulus in the glassy state, revealed approximately a factor of two difference between the directions parallel and perpendicular to the alignment. Thermal expansion measurements showed an anisotropic linear expansion that was very small, and sometimes negative in the alignment direction. Finally, these resins demonstrated high glass transition temperatures which could be advanced to as high as 150°C by postcuring.
Thermal-Expansion and Fracture Toughness Properties ofParts made from Liquid Crystal Stereolithography Resins
(1998) Ulett, J.S.; Benson-Tolle, T.; Schultz, J.W.; Chartoff, R. P.
Liquid crystal (LC) resins are a new kind ofstereolithography material that can produce parts with structured or ordered morphologies instead ofthe amorphous morphologies that result from standard resins. The LC molecules can be aligned before cure resulting in an anisotropic crosslinked network when the laser induced polymerization "locks-in" the alignment. Previous papers have explored liquid crystal orientation dynamics [1], the effects of orientation on viscoelastic and mechanical properties [2,3], and the processing ofLC resins by stereolithography [4]. This paper considers the effects ofmorphology on fracture toughness and thermal-expansion properties. Both toughness and thermal-stability continue to be important issues for stereolithography parts. The use ofLC resins may provide a way to significantly improve performance in both ofthese areas, and in addition result in parts with high upper use . temperatures.