This paper deals with the characterization of a broad range of linear polyethylenes (PE) by means of thermorheology. It is demonstrated, in which way the thermorheological behavior can be related to the comonomer type and content of copolymers. In the first part of this paper, well-established analytical and rheological methods are applied to distinguish linear and branched samples. The resolution limit of these methods is demonstrated by the investigation of a blend from a linear-low density PE (LLDPE) and a branched low density PE (LDPE). Special attention is paid to nuclear-magnetic resonance (NMR) spectroscopy in order to reliably determine the comonomer type and content of the samples chosen. The main focus of this paper lies on thermorheological investigations and correlations with the molecular structure. The comparatively high sensitivity of such investigations is highlighted: even small amounts of long-chain branches (LCB) are revealed. The activation energy (E
a) of linear samples increases with growing comonomer length and content, respectively. As such, thermorheology is demonstrated to be an interesting rheological “tool” to get an insight into branching structures. Moreover, the experimental effort is relatively small as solely usual dynamic-mechanical experiments at different temperatures are required.
This study reports a reactive extrusion process leading to very high levels of anhydride grafting (2.5-3 wt %) along polypropylene backbone without recovering grafted PP waxes at the die exit. Such high graftings are attainable without excessive degradation of the PP chain by using a brominated reagent. Simultaneously, this brominated reagent allows the tuning of the grafted PP crystallinity via epimerization of the PP backbone. Indeed, the synthesis of a mainly isotactic/atactic stereoblock polymer containing high levels of grafted succinic anhydride moieties is demonstrated by NMR and melting enthalpies recorded by DSC are definitely observed depressed and broadened. Grafting levels of around 3 wt % have been achieved and ascertained by both chemical titration and NMR spectroscopy. In addition, FTIR spectroscopy reveals an unusual observation: for the first time, only one single pair of symmetric and asymmetric carbonyl stretching bands are observed on those grafted PP, while, in other processes of anhydride grafting, those symmetric and asymmetric bands were both split in at least two bands. This suggests, for the here reported process, the absence of interacting grafted anhydride rings, i.e., absence of closely grafted anhydride moieties and absence of poly(maleic anhydride). All those observations support that this ''bromine route'' brings a really new grafting process for PP.
The synthesis and the separation of the four stereoisomers of 2,4,6,8,10-pentamethylundecane (PMU) are described together with their characterization by Raman spectroscopy. In parallel, theoretical calculations of the Raman and vibrational Raman optical activity (VROA) spectra are reported and analyzed in relation with the recorded spectra. A very good agreement is found between the experimental and theoretical spectra. The Raman spectra are also shown to be less affected by the change of configuration than the VROA spectra. Nevertheless, by studying the overlap between the theoretical Raman spectra, we show clear relationships between the spectral fingerprints and the structures displaying a mixture of the TGTGTGTG conformation of the (4R,6s,8S)-PMU (isotactic compound) with the TTTTTTTT conformation of the (4R,6r,8S)-PMU (syndiotactic compound). Then, the fingerprints of the VROA spectra of the five conformers of the (4R,8R)-PMU have been related to the fingerprints of the regular (TG)(N) isotactic compound as a function of the torsion angles. Since the (TT)(N) syndiotactic compound has no VROA signatures, the VROA spectroscopy is very sensitive to the helical structures, as demonstrated here.
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