Elastic modulus of in - situ composites of a liquid crystalline polymer and polycarbonate

In this paper, we model the elastic modulus of in - situ composite fibers from polymer blends where a fibrous liquid crystalline polymer (LCP) phase is induced by drawing. We propose a composite model to account for the change of the elastic moduli of the reinforcing LCP phase with the draw ratio of the composite fibers. We envisage the LCP phase as a composite of a perfectly oriented chain aggregate and a randomly oriented chain aggregate which are connected in series. We then derive equations for the longitudinal and the transverse elastic moduli of the composite fibers based on the well-known Halpin-Tsai equation and the composite model of the reinforcing LCP phase. Using this approach, we are able to make a number of predictions including the transverse elastic modulus and mechanical anisotropy. Our results show that theoretical predictions of the longitudinal elastic modulus agree fairly well with experimental results for polycarbonate/Vectra composites. The proposed modulus equations will be useful in providing guidelines for fabrication and applications of this new class of polymeric materials.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38420/1/750150209_ftp.pd

Mechanical properties of in situ composites based on polycarbonate and a liquid crystalline polymer

The mechanical properties of in situ composites based on blends of polycarbonate and a liquid crystalline copoly(ester amide) (Vectra B950) have been studied as functions of the liquid crystalline polymer (LCP) concentration and the draw ratio, a processing parameter. It is shown that both the elastic modulus and the tensile strength of the in situ composites increase steadily with the LCP concentration and the draw ratio. However, the ultimate tensile strain decreases with these two parameters. A model is proposed for the longitudinal elastic modulus of the in situ composites, which is based on the Halpin-Tsai equation and Northolt's model for the LCP phase. The experimental elastic moduli of the in situ composites are found to conform fairly well with the theoretical values derived from the model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31429/1/0000347.pd

Effect of drawing on structure and properties of a liquid crystalline polymer and polycarbonate in - situ composite

Fibers (strands) with various draw ratios were spun from the liquid crystalline state of a pure aromatic liquid crystalline copoly(ester amide) and the melts of its blend with polycarbonate. Scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC) were employed to investigate the structure and properties of the resulting fibers. Mechanical properties of the fibers were also evaluated. It was found that both the crystallite size and heat of fusion of the liquid crystalline polymer (LCP) increase steadily with draw ratio. However, the crystal-nematic transition temperature of the LCP is virtually unaffected by drawing. Moreover, heat of fusion of LCP is much smaller than that of isotropic condensation polymers despite the presence of very sharp diffraction peaks in WAXS measurements. These results are ascribed to the (semi)rigid rod nature of the LCP chains and the persistence of an ordered structure in the LCP melt, i.e., entropy effect. It was further observed that tensile modulus and tensile strength along fiber axis rise with draw ratio for the composite fibers. The elastic modulus of the composite fibers were found to be as high as 19 GPa and tensile strength reached 146 MPa with draw ratios below 40 and an LCP content of 30 wt%. Compared with the thermoplastic matrix, the elastic modulus and tensile strength of the in - situ composite have increased by 7.3 times and 1.4 times, respectively, with the addition of only 30 wt% LCP. This improvement in mechanical properties is attributed to fibrillation of the LCP phase in the blend and the increasing orientation of the LCP chains along the fiber axis during drawing.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38112/1/760331303_ftp.pd

Skin-core morphology of in situ composites based on polycarbonate and a liquid crystalline polymer

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43028/1/10855_2004_Article_BF00591637.pd

High-performance polymeric materials through liquid crystallinity: In situ formed polymer composites and novel liquid crystalline epoxy thermosets.

High performance polymeric materials offer great potential as structural materials in automobile, aircraft, and space industries where weight saving is critical. The aim of my dissertation research is to develop high performance polymeric materials (both thermoplastics and thermosets) by orienting polymer chains via liquid crystalline states. In the thermoplastic system, in-situ composite fibers were prepared by extrusion and subsequent drawing of blends of a liquid crystalline copoly(ester amide) (Vectra B950) and polycarbonate, where the liquid crystalline polymer (LCP) serves as the reinforcing phase. It was found that the liquid crystalline polymer and polycarbonate are partially miscible due to the exchange reactions between the two polymers. It was also observed that both the longitudinal elastic modulus and tensile strength of the in-situ composites increase with LCP concentration and the extent of drawing. The reinforcement effect originates from the stretching of the dispersed LCP phase and the increased molecular orientation of the LCP chains during drawing. Based on this understanding, a model is proposed to correlate the elastic moduli of in-situ composites with LCP concentration and the draw ratio. Equations for the longitudinal and the transverse elastic moduli of in-situ composites are then derived based on the Halpin-Tsai equation, and a composite model or Northolt's aggregate model of the reinforcing LCP phase. Theoretical predictions of longitudinal and transverse elastic moduli are found to be in good agreement with experimental results for Vectra/polycarbonate composite fibers. In the thermoset system, the effects of curing agent, cure time, cure temperature and shearing on the phase transformations, solid state structure, and molecular orientation of a monotropic nematic liquid crystalline epoxy, diglycidyl ether of 4,4\sp\prime-dihydroxy-$\alpha$-methylstilbene (DGEDHMS) have been investigated. It was found that isothermal curing of the DGEDHMS in its isotropic phase with di- and tetra-functional amines leads to the development of liquid crystallinity. The resultant cured networks show a layered structure with the mesogenic units aligned perpendicular to the layer surface. Shearing a partially cured liquid crystalline resin orients the mesogenic core perpendicular to the shear direction.Ph.D.Applied SciencesMaterials sciencePlasticsPolymer chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129469/2/9513416.pd

Interactions of a liquid crystalline polymer with polycarbonate and poly(ethylene terephthalate)

Thermal behaviour of blends of a liquid crystalline copoly(ester amide) (Vectra B950) with two isotropic polymers has been studied by differential scanning calorimetry. One of the isotropic polymers is an amorphous polymer – polycarbonate, the other is a semi-crystalline polymer – poly(ethylene terephthalate). It was found that the glass transition temperature of polycarbonate decreases with increasing Vectra concentration in the blend, suggesting a partial miscibility between the Vectra liquid crystalline polymer (LCP) and polycarbonate. The miscibility is enhanced through heat treatment at elevated temperatures presumably due to a transesterification reaction. Moreover, the presence of the amorphous poly- carbonate hinders the crystallization of the liquid crystalline polymer in the blends. It was also observed that heat treatment of the Vectra LCP and poly(ethylene terephthalate) blends causes a loss in crystallinity and shifts in transition temperatures of poly(ethylene terephthalate), indicating that exchange reactions occur between Vectra B950 and poly(ethylene terephthalate). Based on these results, a new strategy, in situ compatibilization, is proposed to improve the interfacial adhesion between an LCP and an isotropic polymer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44740/1/10853_2004_Article_175453.pd