Shaw Ling Hsu

Normal vibrational analysis of a rigid rod polymer: poly(p-phenylene terephthalamide)

The authors have analysed the vibrational spectra of poly(p-phenylene terephthalalamide). The structure used is identical to that obtained from X-ray diffraction and all the internal coordinates have been defined. A specific set of non-redundant symmetry coordinates was used in the normal vibrational calculation. Satisfactory assignments were made in the region above 800 cm−1. The authors have estimated the ultimate tensile modulus using diagonal force constants and the dispersion curves associated with PPTA. The modulus value was obtained from the slope of the longitudinal acoustic vibration near the zone centre. A value of 241 GPa was obtained.

Structure–property relationships of polyimides: a molecular simulation approach

Abstract Chain flexibility and mechanical properties of four polyimides with different chemical structures are simulated by molecular dynamics and molecular mechanics techniques to establish some structure–property relationships. The oxygen linkage in the diamine moiety of a polyimide gives the highest flexibility whereas the sulfonyl linkage imparts the lowest flexibility to the polymer chain. A more flexible polyimide has smaller characteristic ratio, lower solubility parameter, lower elastic modulus, and larger yield strain. These simulated values show good agreement with experimental data.

Vibrational spectroscopic studies of linear and cyclic alkanes CnH2n+2, CnH2n with 24⩽n⩽288: Chain folding, chain packing and conformations

Evidence of chain folding in long chain linear alkanes CnH2n+2n⩾168 was obtained by Raman and FTi.r. spectroscopy. Investigation of cyclic alkanes CnH2n with 24⩽n⩽288 using vibrational spectroscopy allows for identification of specific bands due to folding as well as determination of conformational sequences in the fold region. The cyclic alkanes with n=48, 72 and 96 were crystallized from solution with monoclinic structures, whereas the alkanes with n=144 and 288 were obtained as orthorhombic crystals. It has been found that the cyclic alkanes with n=72 and 96 crystallize from the melt as crystallites with a high degree of orthorhombic character.

Spectroscopic study of the electric field induced microstructural changes in poly(vinylidene fluoride)

Abstract Infra-red spectra (4000-400 cm−1) have been obtained for poly(vinylidene fluoride) films in the presence of high electric fields up to 2.6 MV cm−1 in strength. Both band intensity and polarization studies have revealed that dipolar re-orientation can be correlated with the macroscopic electric effect. The frequency shift as a function of field strength for the β phase bands can be attributed to the non-equivalent reorientation behaviour of the bands in the unit cell. However, the change in the frequency of the α bands shifted irreversibly with the applied electric field. These changes cannot be attributed to reorientation behaviour alone.

An infra-red spectroscopic study of structural reorganization of a uniaxially drawn VDF/TrFE copolymer in an electric field

A spectroscopic study has been conducted for the analysis of electric field induced microstructural changes in a uniaxially drawn 7525 vinylidene fluoride/trifluoroethylene copolymer. Based on the relative intensities of the infra-red active bands measured, changes in the average orientation of the CH2 dipoles after poling could be determined. Upon applying an electric field of 1.0 MV cm−1 to the sample, the mean-squared value of cos α for the CH2 dipoles in the crystalline domain with respect to the film plane was found to change irreversibly from 0.46 to 0.34. Additionally, an increase in the degree of crystallinity of approximately 8% was observed after poling. Factor analysis was utilized in our spectroscopic analysis. Infra-red spectra for both the crystalline and amorphous regions have been constructed. These spectra are identical to those previously measured for isotropic samples. The conformational distribution for samples after poling contains a larger fraction of trans segments than unpoled samples.

Raman characterization of orientation in poly(lactic acid) films

Poly(lactic acid) is a new biopolymer material which is marketed by Cargill Dow Polymers under the tradename Nature Works*. One major application for this material is biaxially oriented films for food packaging because it possesses excellent barrier for flavor constituents, deadfold and heat sealability. Shrinkage must be minimized when the film is heat sealed for these applications and, therefore, characterization of the orientation of the amorphous phase of PLA films is necessary. Raman spectroscopy methodology has been developed to quantify orientation in PLA films. Bands were assigned to crystalline and amorphous phases of PLA such that orientation in both phases could be monitored. Raman depolarization ratios were used to characterize uniaxial systems but were insufficient for most biaxial draws. A new phenomenon for oriented films involving Raman band shifts was observed in these systems, and was shown to be capable of determining orientation, even for symmetrical biaxially drawn films. The origin of these shifts, as well as their use for the quantification of orientation will be discussed. Further, since the line widths of the bands could be used to quantify crystallinity, both crystallinity and orientation could be determined with one measurement.

Deformation studies of polymers by time resolved Fourier transform infra-red spectroscopy: 1. Development of the technique

Abstract We have constructed a miniature closed-loop servo-controlled hydraulic tensile tester interfaced to our fast scanning Fourier transform infra-red spectrometer. Macroscopic information in the form of stress-strain curves can be collected along with the microstructural changes in the form of infra-red data. This technique in conjunction with our newly developed time resolved Fourier transform spectroscopy package has been used to follow structural changes in macromolecules.

Spectroscopic characterization of mesogen order in discotic liquid crystal

Infrared and Raman spectra of the discotic liquid crystal molecules benzene–hexa‐hexanoate (BH6), benzene–hexa‐heptanoate (BH7), benzene–hexa‐octanoate (BH8), and benzene–hexa‐nananoate (BH9), and of mixtures of BH7 with BH8, are reported as a function of temperature. The 1615 cm−1 infrared band is strong in the Raman spectrum of BH7 and is taken as a characteristic of and diagnostic for central core disorder in all these molecules and mixtures. The aliphatic side chains are shown to disorder at much lower temperatures than the central core, and order in the central core is shown to remain throughout the liquid crystalline region.

Vibrational spectroscopic characterization of rigid rod polymers: 3. Microstructural changes in stressed polymers

Abstract Highly oriented poly(p-phenylene benzobisthiazole) films were stressed in a miniature hydraulic tensile tester interfaced to a fast scanning Fourier transform infra-red spectrometer. The changes in relative intensity, polarization, frequency and shape of infrared bands have been useful in the elucidation of microstructures. With the short time resolution achievable, vibrational bandshape and frequency have been shown to change at different rates when an external stress was applied.

Deformation Studies of Polymers by Time-Resolved Infrared Spectroscopy IV: A Modified Approach

Recent studies have demonstrated how effective the time-resolved Fourier transform spectroscopic technique is in obtaining dynamic information about microstructural change in the millisecond time scale. In this current study, the time efficiency of the method has been improved by the development of a scheme implementing the use of a separate microprocessor to maintain synchronization between the interferometer movement and the external event. This method eliminates the need for accurate external timing circuits and their interface to the FT-IR. Therefore, data collection software of the FT-IR system need not be modified as in previous cases.