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.

No abstract

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.

No abstract

In this work the effect of strain hardening on poly-vinylidene fluoride (PVDF) extrusion blown film is investigated. Controlled long chain branching is introduced via a multi-functional initiator to produce two PVDF samples with different molecular weight and chain architecture. The branched samples are compared to two reference resins having identical molecular weight and no chain branching. All samples are characterized by size exclusion chromatography (SEC) coupled with triple detection system comprising a Differential Refractive Index (DRI) detector, Intrinsic Viscosity (IV) detector, and Multi-Angle Laser Light Scattering (MALLS) detector, to determine their molecular weights and their distribution as well as to detect chain branching via measuring the coil size in dilute solution. The rheological properties are determined using oscillatory measurement and melt strength at 230°C while extensional viscosity measurements are conducted at 180°C to determine strain hardening at different extension rates. The resins are evaluated using a small scale extrusion blown film set-up to determine the blow-up ratio and the minimum thickness achievable. The characterization results show that the control samples are different in molecular weight and almost identical in the polydispersity index (Mw/Mn). The branched samples, however, have higher molecular weight and a slightly broader molecular weight distribution. Light scattering data together with inherent viscosity data show that the branched samples have a lower radius of gyration (RG) and inherent viscosity (IV) over the entire molecular weight distribution confirming chain branching. The rheological properties in oscillatory measurements show that the branched samples exhibit almost identical viscosities as the control samples. However, a broader transition from the Newtonian to the non-Newtonian region for the branched sample is observed confirming the SEC—MALLS results. This is corroborated using extensional viscosity and melt strength measurements, which show a significant strain hardening and an increase in melt strength, respectively. Blown film experiments show that the samples containing chain branching could be processed under similar process conditions as the control samples, and with a higher blow up ratio thereby achieving 5 μm film thickness with high clarity.

The solution properties of polymethylmethacrylate in dimethylsulfoxide were studied using multi-angle light scattering and an online viscometer coupled to size exclusion chromatography. The data demonstrate that DMSO is a suitable eluent for the determination of the molecular weight of PMMA by SEC. The Mark-Houwink coefficients K and a were determined as a function of temperature. At 35 C, DMSO is a theta solvent for PMMA, and it becomes a good solvent as the temperature increases. The unperturbed dimension K h was calculated using three different procedures. The data do not show evidence of a conformational transition controlled by the temperature such as the one reported in benzene and confirm the existence of a general relationship between the ratio K=K h and a. In the case of PMMA, the comparison of the dipole moment of the side group to that of the solvent can be used to predict the existence of a conformational transition.

Detailed knowledge on structural information is required to derive the rheological properties of branched polymers. Size-exclusion chromatography with triple-detection (TD-SEC), comprising a concentration, a light scattering, and a viscosity detector, is a powerful tool to analyze the degree of branching of polymers as a function of their molar masses. However, TD-SEC alone is incapable of fully deconvoluting complex polymer systems. In this study we discuss a more sophisticated approach that includes coupling of TD to our recently described novel online two-dimensional liquid chromatography method (2D-LC), based on solvent gradient at near-critical conditions in the first dimension. Thus, a contour plot of the branching ratio is presented, and unique detailed information on the degree of branching can be derived for branched polycarbonate (PC) sample. Furthermore, the molar mass distributions of separated linear and branched PC chains as well as their fractions in the polymer are quantified. The corresponding data are correlated to Monte Carlo simulations of the polycondensation process of a branched PC, and both methods show a high level of agreement in the determined molar mass distributions of the linear and branched PC chains as well as their fractions. Finally, the influence of chemical structure on rheological properties is demonstrated.