The 920−960 cm-1 region of the infrared spectrum of the δ phase syndiotactic polystyrene/ethylbenzene predominantly involves bands arising from helical structures and has been studied here as
a function of heating temperature. Curve fitting and deconvolution showed the necessity, for annealing
temperatures above 120 °C, of adding a third component at 940 cm-1 to peaks at 934 and 943 cm-1. The
934 and 943 cm-1 peaks behave as a doublet, primarily due to the δ phase, with the splitting increasing
on annealing. The 940 cm-1 peak is solely due to the γ phase helix. Importantly, these assignments provide
the opportunity for using these bands for further studies of complexation/decomplexation in such systems.
The reduction in absorbance of 934 and 943 cm-1 peaks occurs at a lower temperature than the rise in
the 940 cm-1 absorbance, and this is attributed to disordering of the δ phase helices prior to reformation
as γ phase helices. Similar measurements using deuterated syndiotactic polystyrene also showed helix
and zigzag bands, but without a predominantly δ phase helix peak. Solvent peaks in the deuterated
syndiotactic polystyrene/ethylbenzene system were used to monitor the δ to γ phase transition. In
combination with DSC and TGA data, the melting temperature of the α phase was found to be depressed
by 7 °C for the deuterated polymer, while the δ to γ phase transition temperature was shown to be around
24 °C higher.
Poly(vinylidene fluoride) (PVDF) experimental samples containing sparsely distributed chain branching were compared to commercial reference samples. The results showed a lower onset of shear thinning for the branched samples over the reference counterparts. The storage modulus of the branched samples at low frequency shows a significant increase for the low-molecular weight sample while the higher-molecular weight sample showed a moderate increase suggesting a strong contribution of chain branching. The branched PVDF samples exhibited a lower radius of gyration (R G ) and intrinsic viscosity than the commercial samples over the entire molecular weight range. NMR revealed the presence of tertiary carbons, suggesting that the branches are covalently bonded and not of a physical nature. Extensional viscosity data showed that the branched samples display a significant degree of strain hardening while the reference samples exhibit a small degree of strain hardening.
Branching is a molecular metric that strongly influences the application properties of polymers. Consequently, detailed information on the microstructure is required to gain a deeper understanding of structure-property relationships. In the present case, we employ high-performance liquid chromatography to characterize the branching in a poly(bisphenol A carbonate) (PC). To this end, a method was developed based on a mobile phase gradient in a very narrow range (±1.4 vol %) around the point of adsorption (98.9/1.1 vol % chloroform/methyl tert-butyl ether), which we refer to as solvent gradient at near-critical conditions. Application of such gentle gradient enabled separation of PC according to end-groups. The separation mechanism was confirmed by collecting fractions of a separated sample and subsequently analyzing these by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Hyphenating the developed gradient method with size-exclusion chromatography as the second dimension (2D-LC) enabled separation of linear and branched PC chains and determination of the molar mass distribution of the fractions. A reversed elution order was observed for branched species in 2D-LC, meaning that low molar mass chains exhibited higher elution volumes in the first dimension than higher molar masses. This finding was explained by influences of end-groups as well as the architecture of the branched polymer chains.
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