Vassilios Galiatsatos

Amphiphilic networks. XI. Mechanical properties and morphology

The bulk properties of two types of amphiphilic networks, poly(2-hydroxyethyl methacrylate)-l-polyisobutylene (PHEMA-l-PIB, H-network) and poly(N,N-dimethylacrylamide)-l-polyisobutylene (PDMAAm-l-PIB, A-network), have been investigated. Tensile strengths decreased considerably by swelling, and the decrease was more severe by swelling in water than in n-heptane. Elongations increased by swelling in water; however, the change was not consistent upon swelling in n-heptane. The hardness of dry networks decreased with increasing PIB content, while for wet networks it was similar to dry networks containing 85 wt % PIB. Small-angle X-ray scattering showed that average interdomain spacings decreased with increasing PIB content. According to dynamic mechanical thermal analysis (DMTA) the glass transition temperatures (T g ) of the respective hydrophobic and hydrophilic components shift toward each other with increasing PIB content. A liquid-liquid transition (T u ) above the T g of the hydrophilic component was apparent by DMTA, but could not be found by differential scanning calorimetry (DSC).

Designing heterogeneity into bimodal elastomeric PDMS networks

Bimodal poly(dimethylsiloxane) (PDMS) networks prepared using a dual-step process have been studied. It is shown that these networks exhibit a significant increase in equilibrium modulus. This reinforcement can be obtained by adjusting a process parameter, the heterogeneity time, during network preparation. The increase in modulus is attributed to the clustering of the short chains that is effected through proper use of heterogeneity time.

Quantitative prediction of molecular optical polarizability anisotropy: benzene, substituted benzene and rigid-rod oligomers

Quantitative predictions of optical polarizability and its anisotropy are effected through a method that combines intrinsic molecular optical polarizability (as this is determined via semi-empirical quantum calculations) with inter- and intramolecular contributions to molecular optical polarizability (as those are manifested in depolarized Rayleigh scattering experiments). The approach is based on that proposed earlier by Dewar by which one can predict mean optical polarizabilities. Dewars methodology is extended to predict not only the mean but also the anisotropy of the optical polarizability. It is shown that the technique is applicable to unsubstituted benzene and a number of substituted benzenes by quantitatively reproducing experimental values of mean optical polarizability and optical anisotropy simultaneously. The technique is further applied to model oligomers of three rigid-rod polymers, cis- and trans-poly(p-phenylene benzobisoxazole) as well as trans-poly(p-phenylene benzobisthiazole), to show excellent agreement with previously published experimental and theoretical results.

Computer simulation of sulfur vulcanization

Recent advances in polymer network modelling techniques have enabled the development of coarse-grained computer algorithms which predict polymer gelation while maintaining chemical reactivity. These computer simulations have proven to be a powerful tool in the study of network microstructure, otherwise not obtainable. Previous applications of the algorithms to irradiation and end-linking have given insight into the interconnectivity of chains and the formation of network defects. We propose modifications to this already existing and tested algorithm in order to model sulfur vulcanization. The modifications include the introduction of reactive crosslinkers off the constituent polymer backbones. The validation of the sulfur vulcanization simulation was performed with high-(Z)-poly(buta-1,4-diene) systems with TMTD employed as the crosslinker in order to assure mostly monosulfidic crosslinks. The predicted values of moduli from simulation are particularly encouraging. Upon comparison, the moduli determined from simulation varied from 1–15% for the experimental systems examined. This is accepted as very good agreement in the light of the absence of accounting for chain entanglements. The paper also reports results for network sub-structures such as dangling chain ends and intramolecular loops.

Depolarized Rayleigh scattering from simple molecular liquids: Investigation of transitions in the melt

Depolarized Rayleigh scattering photometry has been employed to study the temperature dependence of the optical anisotropy for benzene, hexafluorobenzene, and carbon tetrachloride. The depolarized Rayleigh intensity from carbon tetrachloride is entirely due to scattering by pairs of density fluctuations and increases monotonically with temperature. The total depolarized Rayleigh ratio RVH for benzene and hexafluorobenzene decreases smoothly and monotonically with temperature, in contradiction to a recent report [J. Chem. Phys. 95, 1223 (1991)]. The temperature dependence of the total depolarized Rayleigh ratio can be analyzed in terms of an increasing contribution due to scattering by pairs of density fluctuations and a decreasing contribution due to orientational fluctuations of optically anisotropic molecules. No anomalous behavior is observed anywhere in the liquid range for these three fluids.

Depolarized Rayleigh scattering of polymeric materials and their oligomers: Instrument design, calibration, and data analysis issues

The importance of obtaining dependable values of molecular optical polarizability anisotropy for polymers and their oligomers lies in their subsequent use in the rotational isomeric state theory and in calculations involving the components of the optical polarizability tensor. This work describes a depolarized Rayleigh scattering photometer able to determine optical polarizability anisotropy of oligomers, melts, and polymer compounds in solution with a combination of exceptional accuracy and high sensitivity. Two separate calibration techniques are reported that ensure the accuracy and preciseness of the results obtained from the measurements. Finally, data analysis procedures are reported that show the sensitivity of the results to the internal field correction.

Strain birefringence in real elastomeric networks: modifications of theory and comparison to experimental data

A theory of strain birefringence in real elastomeric networks was previously proposed by one of us. The theory was based upon the modified constrained chain theory of rubberlike elasticity developed by Erman and Monnerie. Recent modifications have been made to the constrained chain theory which necessitate alterations in the affected portions of the theory of strain birefringence. These alterations and their effect on predicted birefringence are examined. The modified predictions of the strain birefringence theory will be examined and compared to both experimental data and the original predictions of the theory. The agreement with experiment is found to be satisfactory