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This report describes work toward the establishment of design criteria for producing polymeric materials having a negative Poisson ratio. This unusual mechanical behavior is also known as auxetic behavior and is characterized by a material getting fatter when stretched. Our approach deals with a molecular-scale concept by which a nematic liquid crystalline field orients rigid rods in a main chain polymer - some of which are attached to the chain at lateral positions along the rod as opposed to attachment at the ends of the rods (terminal attachment). The nematic field-induced parallel orientation of all rods is subsequently overcome by a mechanical field when stretched. The side-attached rods will then orient roughly normal to the stretch direction resulting in an increase in the interchain separation and to an auxetic response. We have made model compounds to examine this concept; a series of nematic polymers having end- and side-attached rods; and crosslinked nematic polymers having end- and side-attached rods. X-ray data is consistent with our postulated effect and mechanical testing has shown a strong sensitivity to the molecular structure.

During the first year of the grant period, the key personnel were brought on board-Dr. Bertrand Guichard from Paris and Puwei Lin a new Phd student at USM. The target monomers were selected and synthetic work was begun. Two general synthetic plans were followed: one to prepare in "inverted honeycomb/reentrant cell" macrocycle and the other to prepare monomers for a main chain liquid crystalline polymer. The first of these, the macrocycle, was sought to examine the idea that in-plane stretching deformation of such a structure could lead to a negative Poisson ratio material. In turn a network of such reentrant cells could produce-by a flexing or hinging motion- a polymeric negative Poisson ratio material. Synthetic work along these lines, although interesting in its own right, did not produce the desired macrocyle. Instead, there was premature macrocycle formation early in the synthetic route which- although producing intriguing new macrocycles- did not advance this area of the project. After numerous attempts at this target, it was decided to concentrate exclusively on the second approach- the main chain liquid crystalline polymer approach., DEPSCOR-93.

The First European Tri-Service Advanced Materials Workshops: Compressive Strength in Rigid-Rod Polymers is held at Pembroke College, University of Cambridge, September 3-5, 1992. The Workshop brings together a small and select group of experts in the Physics, Engineering, Chemistry, and Materials aspects of compressive problems in these polymers. The abstracts which follow reflect the lectures by the distinguished scientists at the Workshop. It is the intent that as a result of intensive discussion concerning origins and manifestations of compressive failure in rigid-rod polymer systems, rational approaches for solving these problems may arise.

UV-VIS spectral comparison with model compounds and isolation of the major product from preparative photolysis of one of the models has been used to show that the predominant photochemical reactions in the solid state of an aryl cinnamate polymer are 2+2 cycloaddition and photo-Fries rearrangement., This article is from 'Multifunctional Materials Held in Boston, Massachusetts on November 29-December 1 1989. Materials Research Society Symposium Proceedings. Volume 175,' AD-A253 089, p35-40.

A series of nitroimine, Siamese-twin dimers joined by an aliphatic chain with n = 2 to 10 were examined by DSC, microscopy, and x-ray diffraction. All dimers except the one having n = 3 undergo transitions to a nematic mesophase without exhibiting distinct relationships. The N-I transitions, in contrast, exhibit a very pronounced alternation with even numbered spacers having much higher clearing temperatures. Magnetically aligned monodomains of the nematic mesophase were examined by x-ray diffraction. In addition to the broad equatorial arcs (yielding interchain separations 1.5 to 5.7A), up to eight aperiodically spaced meridional reflections were recorded. By positing two possible 'coupling' modes in which either one or both phenyl rings in neighboring dimers overlap, a linear array of coupled nitroimine dimers having two repeating distances along its length is produced. Using such a copolymer- like model, the spacings of the aperiodic meridional reflections and their relative intensities could be reproduced for a random sequence of the two repeat distances and an equal (50:50) probability of single or double overlapped phenyl rings. Keywords: Liquid crystals; Copolymers.