A better understanding of structure-property relations is necessary to design novel materials. In this study, we investigate the morphology and chemical structure of five commercial grades of propylene-based polymers in relation to the change in yield- stress as a function of strain-rate. Substantial emphasis has been laid on understanding the chain microstructure in the relation to chain dynamics in the amorphous phase. Heterogeneous Ziegler–Natta catalysis was used to prepare the samples with differing ratios of propylene and ethylene units. Various analytical techniques such as WAXS, SAXS, solution- and solid-state NMR were employed to characterize their structure. The results indicate a reduction in crystallinity, melting temperature, long-period and crystal thickness with increasing ethylene content. Solid-state NMR data reveal the presence of four components in these samples, which is an extension of the traditional three phase model found in most semi-crystalline polymers. The additional fourth phase is attributed to a rubber-like component that is primarily composed of chain segments rich in ethylene units and shows an increase in chain dynamics with increasing ethylene content in the samples. Mechanical experiments show that yield stress decreases with increase in the amount ethylene which can be correlated to the observed increase in chain dynamics in the amorphous phase.
Polymers derived from biomass form a fascinating field of research, offering novel materials with differentiated properties. Many of the renewable monomers can be polymd. through step-growth or, more specifically, polycondensation chem. However, several challenges are met when performing such syntheses, which may be due to a variety of reasons. For example, sugar-based monomers may be thermally less stable and/or less reactive than conventional monomers, complicating bulk polymns. in the melt. In addn., polarity differences between vegetable oil derivs. and, e.g., starch- or cellulose-based starting materials can cause compatibility issues during copolymn. reactions as well as during their subsequent application. Apart from these monomer-related challenges, new and more sustainable synthetic routes such as isocyanate-free polyurethane synthesis also require significant adaptations of the way we approach step-growth polymn. In this lecture, the mentioned aspects are discussed and possible solns. to these synthetic challenges are discussed by means of examples studied in our labs.
Performance polymers derived from biomass represent a fascinating and increasingly important field of research, as such macromolecules offer differentiated material properties as compared to conventional polymers from fossil feedstock.1,2 The aim of our research is to understand the chemistry of bio-based building blocks and the structure-property relations of the resulting novel polymers. For thermosetting polymer systems, some of the main challenges include enhancing the polymer functionality, mechanical performance and thermal stability. A range of fully aliphatic, 1,4:3,6-dianhydrohexitol (DAH) based polyesters and polycarbonates are presented, designed for thermosetting coating applications. In this paper, we focus on the reactivity of the DAH isomers under different reaction conditions as well as on methods to improve the polymer functionality.
First- and second-generation Newkome-type dendronized norbomene macromonomers were synthesized and polymerized by ring-opening metathesis polymerization (ROMP). In the case of the second-generation macromonomer, the rate of polymerization was highly dependent on the initial concentration of the macromonomer; quasi-quantitative polymerization was only achieved when the concentration was higher than 50 mM. Adding a linker between the polymerizable group and the dendron increased the rate of polymerization and it was possible to reach quantitative conversions at lower concentration. Doubling the length of the linker further improved the polymerization to a rate comparable with the polymerization of the first-generation dendronized macromonomer. The dendronized polymers presented herein consist of a poly(amide)-based dendron attached to the poly(norbomene) backbone. Because of the properties of these components such as biocompatibility, we foresee these polymers having possible applications in therapeutics.
A set of 303 R X bond dissociation free energies (BDFEs) at 298.15 K in acetonitrile, along with corresponding values of polar, steric and radical stability or resonance descriptors for each R-group and X-group, has been calculated at the G3(MP2)-RAD level of theory in conjunction with CPCM solvation energies. The R-groups were chosen to cover the broad spectrum of steric, polar and radical stability properties of propagating polymeric radicals, while the X-groups included a variety of nitroxides, dithioester fragments (*SC(Z)=S) and halogens, chosen to be representative of control agents used in nitroxide mediated polymerization (NMP), reversible addition fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP). The data have been used to design, parametrize and test a linear free energy relationship that can predict the BDFEs of any R and X combination based on the polar, steric and radical stability or resonance properties of the separate R and X groups. The final equation is BDFE[R-X] = -20.8 theta[R] - 9.73 IP[R] - 1.10 RSE[R] + 192 theta[X] + 57.4 EA[X] - 62.0 Resonance [X] - 250, where the steric descriptors theta[R] and theta[X] are measured as Tolman's cone angle of Cl-Rand CH3-X respectively, the polar descriptors IP[R] and EA[X] are the (gas-phase) ionization energy of R* and electron affinity of X* respectively, and the radical stability or resonance descriptors RSE[R] and Resonance[X] are measured as the standard radical stabilization energy for R* and the inverse HOMO-LUMO energy gap for X*. This general model was also fitted to the individual cases of ATRP, RAFT, and NMP and was used to analyze similarities and differences in structure-reactivity trends among the different types of polymerization process. We show how the equation can be used to select appropriate initial leaving groups for a given polymerization, or predict the correct sequence of monomer addition in block copolymer synthesis.
