No abstract

The crystallization behavior of poly(1-butene) (P1b) was investigated by polarized light microscopy (PLM), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), dilatometry, and also by time-and temperature-resolved small-angle X-ray scattering experiments (SAXS). Observations in the PLM indicate a temperature-dependent change in the mechanism of crystallization. When crossing a certain critical crystallization temperature, the morphology changes from spherulites to quadratic, platelike single crystals. Investigations of samples with different molar mass show that the transition temperature is molar mass-dependent; on decreasing the molar mass the transition shifts to lower temperatures. As proved by WAXS, both the spherulites and the single crystals are of the metastable form II. The morphological change is also observed in AFM images obtained after a rapid cooling of the samples to room temperature; the difference in the morphological appearance is preserved through the transformation from form II to form I. According to dilatometric measurements, the change in the crystallization mechanism leads to variations in the temperature dependence of the crystallization rate and also to a steplike increase in the crystallinity. The results of SAXS experiments show that the formation of P1b crystallites is governed by the same general laws as for other polymers studied before. Both the crystallization temperature, T c, and the melting temperature, Tf, are linearly dependent on the reciprocal crystalline layer thickness, dc -1 , but with different slopes and different limiting temperatures for dc -1 f 0. The observations are again indicative for a crystal development in two steps: First an initial form appears which then transforms into the final lamellar crystallites. As a new feature, in direct correspondence to the two different crystallization mechanisms observed microscopically, two different crystallization lines (dc -1 vs Tc) show up, indicating the occurrence of two different initial states. On the other hand, only one common melting line (Tf vs dc -1 ) is found, which means that the two crystallization mechanisms produce crystallites with similar surface free energies. We discuss the peculiar crystallization properties of P1b by comparing the radius of gyration Rg of the chains in the melt with the crystal thickness dc and propose that the change in the crystallization mechanism could be due to a change from foldedchain to chain-extended crystallization, taking place when dc gets larger than Rg.

The morphology of a liquid-crystalline ABA triblock copolymer with polystyrene (PS) blocks (12 vol %) and a side chain nematic liquid crystalline B block has been studied by TEM and temperature dependent SAXS measurements. Above the clearing temperature (T c = 122 °C) the morphology is characterized by a body-centered cubic (bcc) lattice of polystyrene spheres. The transition to the nematic phase induces a reversible transition to a morphology of hexagonally packed cylinders by coalescence of the PS spheres along the [1,1,1] direction of the bcc lattice.

SAXS and TEM measurements are employed in order to study the morphology of poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene) (SEBS) with 29 wt % styrene (Kraton G1652) in bulk and at interfaces to different polymers. Temperature-dependent SAXS measurements of the SEBS bulk sample reveal that PS cylinders are hexagonally packed in the EB matrix. The lattice constant increases during cooling from 200 to 120 °C from 27.5 to 29.5 nm and simultaneously an increase of the cylinder radius occurs from 7 to 7.5 nm. The lattice constant obtained by SAXS is in agreement with TEM measurements on ultrathin sections of the bulk sample. TEM tilting experiments confirm the existence of cylindrical microdomains in the SEBS bulk phase. Furthermore, the behavior of SEBS at the interface with various polymers is studied by TEM and peel tests. TEM measurements show that SEBS forms one lamella at the interface PS homopolymer having a molecular weight much larger than the PS blocks. This results only in a very weak adhesion between SEBS and PS homopolymer measured by a peel test using bilayer specimens. In contrast there is a strong adhesion at the interface of SEBS with poly(3,5-dimethylphenylene ether) (PPE) or isotactic polypropylene (i-PP) after thermal annealing. This can be explained by the miscibility of the PS blocks with PPE and of the EB matrix of the block copolymer with i-PP, respectively. This leads to interfacial phase transitions, to cooperative orientation processes of PS cylinders, and finally to diffusion processes of disordered micelles as verified by TEM micrographs.

No abstract

Chemical fixation of the greenhouse gas carbon dioxide with diepoxides followed by melt-phase polyaddition of the resulting difunctional cyclic carbonates with 1,12diaminododecane (DDA) yields semicrystalline polyhydroxurethane (PHU) thermoplastics. Also, 100% biobased semicrystalline PHU thermoplastics are feasible. Opposite to conventional polyurethane syntheses, neither isocyanates nor phosgene are required as intermediates. Preferably, melt-phase polyaddition is performed in a twin-screw compounder in the absence of catalysts, which also catalyze side-reactions. Calorimetric measurements and small-angle X-ray scattering reveal the fundamental structure−property relationships governing PHU crystallization. The PHU melting temperatures vary between 40 and 115 °C, and PHU Young's moduli range from 220 to 1430 MPa. Moreover, non-isocyanate PHU thermoplastic elastomers (TPHE) are readily tailored via melt-phase polyaddition of diamine-terminated flexible PHU prepolymers serving as soft segments combined with semicrystalline PHU as hard segments. As verified by means of thermal analysis (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (SAXS), and microscopy (AFM), the careful balance between soft and semicrystalline hard-segment incorporation accounts for nanophaseseparation, which in the past has failed as a result of phase intermixing resulting from strong hydrogen bonding between soft and hard segments. For the first time, tailored PHU thermoplastics are employed in extrusion-based additive manufacturing by means of fused deposition modeling (FDM) or fused filament fabrication (FFF). Clearly, the presence of hydroxyl groups and their hydrogen bonding improves filament fusion and adhesion essential for achieving mechanical properties similar to PHU melt extrusion without encountering warpage.

Dilatometric and X-ray scattering experiments of the crystallization kinetics of a sample of poly(ethylene-co-octene) show pronounced melt memory effects, i.e., the shapes of isotherms and characteristic times vary systematically with the temperature of the melt prior to cooling to the crystallization temperature. The temperature range of the effect is limited; crystallization kinetics remains constant below a melt temperature T(m)l and above a melt temperature T(m)h and varies only in-between. Analysis shows that the melt memory effect is caused by a variation of the characteristic time of a first order crystallization process. The process can be assigned to the in-filling of crystallites into objects of a previously generated precursor structure.

No abstract