EFFECT OF UNIAXIAL DEFORMATION, ANNEALING AND CARBON NANOTUBES ON THE MORPHOLOGY AND MECHANICAL PROPERTIES OF POLY (BUTYLENE TEREPHTHALATE) AND PBT NANOCOMPOSITES

ABSTRACT The goal of this investigation is to elucidate the interrelations between the strain-induced crystallization behavior, morphology and mechanical properties of poly (butylene terephthalate) PBT and its nanocomposites with multi-walled carbon nanotubes (MWNTs). The mechanical properties of semicrystalline polymers such as PBT depend upon the processing conditions, which affect the crystallization behavior and the resulting crystal morphology developed within the processed sample. PBT is observed to undergo strain-induced crystallization during uniaxial deformation, with concomitant changes in the polymer crystal as a function of the applied strain history. In the current work polymer morphology was investigated with wide angle XRD, differential scanning calorimetry (DSC) and polarized light microscopy (PLM). DSC results indicate an increase in crystallinity due to strain-induced crystallization during uniaxial cold-stretching, which was further confirmed with XRD analysis of the samples. Analyses of the samples under polarized light pre-and post-stretching clearly show that there is a transformation of the spherulitic crystals of the pre-stretch morphology into elongated oblong crystals, as the imposed strain exceeds a critical value. Annealing of PBT was done under different conditions to probe the effects of changes in the crystallinity obtained upon thermal treatment on polymer morphology and mechanical properties. The annealed samples were found to have high crystallinity, high Young's modulus, and low yield stress values as compared to unannealed samples processed under similar conditions. To investigate the effects of nanoparticle loadings on PBT crystal morphology and mechanical properties, pure PBT was melt mixed with different concentrations of multi-walled carbon nanotubes (MWNTs). Due to the increased nucleation rate effect associated with the incorporation of MWNTs, the PBT crystallization temperature was increased and the crystal size decreased with the increasing concentration of MWNTs. Tensile tests performed on PBT and their nanocomposite samples revealed decreases in the elongation at break values. Research is ongoing to understand the relationship between the MWNT loading levels and mechanical properties along with study of orientation of MWNTs under tensile load and its effect on strain-induced crystallization

Parallel-Disk Viscometry of a Viscoplastic Hydrogel: Yield Stress and Other Parameters of Shear Viscosity and Wall Slip

The rheology, i.e., the flow and deformation properties, of hydrogels is generally a very important consideration for their functionality. However, the accurate characterization of their rheological material functions is handicapped by their ubiquitous viscoplasticity and associated wall slip behavior. Here a parallel-disk viscometer was used to characterize the shear viscosity and wall slip behavior of a crosslinked poly(acrylic acid) (PAA) carbomer hydrogel (specifically Carbopol® at 0.12% by weight in water). It was demonstrated that parallel-disk viscometry, i.e., the steady torsional flow in between two parallel disks, can be used to unambiguously determine the yield stress and other parameters of viscoplastic constitutive equations and wall slip behavior. It was specifically shown that torque versus rotational speed information, obtained from parallel-disk viscometry, was sufficient to determine the yield stress of a viscoplastic hydrogel. Additional gap-dependent data from parallel-disk viscometry could then be used to characterize the other parameters of the shear viscosity and wall slip behavior of the hydrogel. To investigate the accuracy of the parameters of shear viscosity and apparent wall slip that were determined, the data were used to calculate the torque values and the velocity distributions (using the lubrication assumption and parallel plate analogy) under different flow conditions. The calculated torques and velocity distributions of the hydrogel agreed very well with experimental data collected by Medina-Bañuelos et al., 2021, suggesting that the methodologies demonstrated here provide the means necessary to understand in detail the steady flow and deformation behavior of hydrogels. Such a detailed understanding of the viscoplastic nature and wall slip behavior of hydrogels can then be used to design and develop novel hydrogels with a wider range of applications in the medical and other industrial areas, and for finding optimum conditions for their processing and manufacturing