Yi Thomann

Influence of rubber particle size on mechanical properties of polypropylene–SEBS blends

Isotactic polypropylene blends with 0–20 vol % thermoplastic elastomers were prepared to study the influence of elastomer particle size on mechanical properties. Polystyrene-block-poly(ethene-co-but-1-ene)-block-polystyrene (SEBS) was used as thermoplastic elastomer. SEBS particle size, determined by means of transmission electron and atomic force microscopy, was varied by using polypropylene and SEBS of different molecular weight. With increasing polypropylene molecular weight and, consequently, melt viscosity and decreasing SEBS molecular weight, SEBS particle size decreases. Impact strength of pure polypropylene is almost independent of molecular weight, whereas impact strength of polypropylene blends increases strongly with increasing polypropylene molecular weight. The observed sharp brittle–tough transition is caused by micromechanical processes, mostly shear yielding, especially occurring below a critical interparticle distance. The interparticle distance is decreasing with decreasing SEBS particle size and increasing volume fraction. If the polypropylene matrix ligament between the SEBS particles is thinner than 0.27 μm, the blends become ductile. Stiffness and yield stress of polypropylene and polypropylene blends increase with increasing polypropylene molecular weight in the same extent, and are consequently only dependent on matrix properties and not on SEBS particle size.

Translucent acrylic nanocomposites containing anisotropic laminated nanoparticles derived from intercalated layered silicates

New acrylic nanocomposites consisting of methyl methacrylate (MMA)/n-dodecylmethacrylate (LMA) copolymers and intercalated layered silicates were prepared. The silicates were based upon bentonite which was rendered organophilic by ion exchange with N,N,N,N,-dioctadecyl dimethyl ammonium ions. Morphological, thermal, mechanical, and optical properties were examined as a function of both organophilic bentonite and LMA content. Addition of LMA improved the compatibility between the layered silicate and the acrylic matrix, thus promoting bentonite intercalation and formation of anisotropic laminated silicate nanoparticles of an average diameter of 18 nm, average length of 450 nm, and interlayer distance of 4.8 nm, as determined by WAXS, TEM, and AFM. Addition of 2–10 wt % of intercalated layered silicate accounted for improved stiffness/toughness balance, higher glass temperature, and enhanced thermal stability, with respect to the properties of the corresponding MMA/LMA copolymer. As a result of the addition of LMA, translucent acrylic nanocomposites were obtained.

Influence of stereoirregularities on the formation of the γ-phase in isotactic polypropene

Medium to high molar mass isotactic polypropenes with different amounts of stereoirregularities were characterised with respect to their crystallisation behaviour and for comparison a random copolymer of ethene and propene with 5.8 wt% ethene is used. The influence of stereoregularity and crystallisation temperature on the γ-content of the crystallised samples is studied by means of wide angle X-ray scattering, atomic force microscopy and light microscopy. The paper deals also with the temperature rising elution fractionation of an i-PP with large amounts of stereoirregularities and the influence of a nucleation agent on the γ-content. It is shown that effects which render the chainfolding in lamellae more difficult, enhance the formation of the γ-modification. The necessity of chainfolding in isotactic polypropene is discussed in terms of a model that is based on the number of chains that emerge from the lamellae surfaces of the α- and the γ-modification, respectively.

Contact‐Active Antimicrobial and Potentially Self‐Polishing Coatings Based on Cellulose

A contact-active antimicrobial coating is described that is only degraded in the presence of cellulase, which is an extracellular enzyme of numerous microbial strains. Antimicrobial DDA was grafted to a cellulose backbone via a polymeric spacer. The antimicrobial activity of the coatings, their biodegradability and their self-polishing potential were investigated. It was found that all coatings were antimicrobially active against Staphylococcus aureus. Coatings with high DS and long polymeric spacers degraded in water, while coatings with low DS and short spacers were not hydrolyzed even in the presence of cellulase. One coating was found to be selectively degradable by cellulase and recovered most of its antimicrobial activity after overloading and subsequent treatment with cellulase.

Influence of comonomer incorporation on morphology and thermal and mechanical properties of blends based upon isotactic metallocene-polypropene and random ethene/1-butene copolymers

Blends of isotactic polypropene (i-PP) with random ethene/1-butene (EB) copolymers containing 10, 24, 48, 58, 62, 82, and 90 wt % 1-butene were prepared in order to examine the influence of the EB molecular architecture on the morphology development as well as on the thermal and mechanical properties. Compatibility between i-PP and EB increased with increasing 1-butene content in EB to afford single-phase blends at a 1-butene content exceeding 82 wt %. The morphology was investigated using AFM and TEM. Improved compatibility accounted for enhanced EB dispersion and interfacial adhesion. Highly flexible as well as stiff blends with improved toughness were obtained.

New molecular and supermolecular polymer architecturesvia transition metal catalyzed alkene polymerization

Superstructure formation during crystallization has been examined as a function of isotactic poly(propene) and poly(ethene) molecular architectures, tailored by means of metallocene catalyzed propene polymerization, metallocene catalyzed ethene/alk-1-ene copolymerization, and nickel-catalyzed migratory insertion polymerization of ethene to afford methyl-branched poly(ethene) without using comonomers. The role of steric irregularities in the chain resulting from false insertion in stereoselective polymerization or from short chain branching, respectively, was investigated. Randomly distributed regio- and stereo-regularities in isotactic poly(propene) chains and variation of crystallization temperature were the key to controlled poly(propene) crystallization and predominant formation of the γ-modification. Poly(propene) melting temperature increased with increasing isotactic segment length between stereo- and regio-irregularities. Superstructures of isotactic γ-poly(propene) were analyzed by means of light and atomic force microscopy. Both types of short-chain branched poly(ethene)s, prepared by ethene/oct-1-ene copolymerization and migratory insertion homopolymerization, showed similar dependence of melting temperature on the degree of branching, calculated as the number of branching carbon atoms per 1000 carbon atoms. Phase transitions were monitored by means of wide angle X-ray scattering and pressure–volume–temperature measurements. Atomic force microscopy was applied to image both lamella- and fringed micelle-type superstructures as a function of the degree of branching.

Poly(: N -vinylimidazole)- l -poly(propylene glycol) amphiphilic conetworks and gels: Molecularly forced blends of incompatible polymers with single glass transition temperatures of unusual dependence on the composition

A series of macroscopically homogeneous poly(N-vinylimidazole)-l-poly(propylene glycol) (PVIm-l-PPG) (“l” stands for “linked by”) amphiphilic conetworks (APCNs) composed of otherwise incompatible polymers were successfully synthesized in a broad composition range (34–88 wt% PPG) by free radical copolymerization of hydrophilic N-vinylimidazole (VIm) and hydrophobic poly(propylene glycol) dimethacrylate (PPGDMA) macromolecular cross-linkers. Strikingly, while PVIm and PPGDMA homopolymers are immiscible and their blends have two distinct glass transition temperatures (Tg), the PVIm-l-PPG conetworks possess only one Tg indicating the absence of considerable phase separation in the conetworks, which was also confirmed by AFM measurements. This is in sharp contrast to the two Tgs of APCNs reported so far in the literature, on the one hand. On the other hand, the Tg values do not follow known correlations between Tg and composition, like the Fox equation or additive rule, widely applied for compatible polymers. These results indicate a strong interpolymer interaction on the molecular level between the PVIm and PPG chains in these new APCNs resulting in single Tg. Thermogravimetric analysis (TGA) shows that degradation of the conetworks occurs at high temperatures in two stages without sharp changes, but with a transition period in between. The DTG curves indicate that the components retain their chemical integrity to certain extent in these APCNs. The amphiphilic nature of the PVIm-l-PPG conetworks was confirmed by their composition dependent swelling in both polar (water, ethanol) and nonpolar (THF) solvents, that is in spite of the lack of phase separation, these new materials behave as either hydrogels or hydrophobic gels (organogels) depending on the swelling medium in a broad composition range.

Nanophasic morphologies as a function of the composition and molecular weight of the macromolecular cross-linker in poly(N-vinylimidazole)-l-poly(tetrahydrofuran) amphiphilic conetworks: bicontinuous domain structure in broad composition ranges

Macroscopically homogeneous poly(N-vinylimidazole)-linked by-poly(tetrahydrofuran) (PVIm-l-PTHF) amphiphilic conetworks (APCNs) were investigated to reveal the effect of conetwork composition in a broad composition range between 25–91 wt% PTHF content and the molecular weight of the components on phase separation and the formation of different morphological features. No macroscopic phase separation was found in these conetworks with semicrystalline PTHF phase, but the segregation of the various covalently connected phases occurs in the nanoscale. The nanophase separated APCNs possess compositionally asymmetric morphology with spherical and elongated domains together with a bicontinuous (cocontinuous) domain structure having ∼7–19 nm average domain sizes. The molecular weight of the PTHFDMA cross-linker, varying between 2170 and 10 030 g mol−1, also influences the size and distance between the phases. Moreover, morphology dependent interactions with polar and non-polar solvents, as well as amphiphilic swelling behavior were found. These nanostructured materials, due to their unique nanophasic morphology and swelling properties possess significant importance and have numerous potential applications in various fields from medicine to material science and engineering.

MORPHOLOGY OF STEREOBLOCK POLYPROPYLENE

The microstructure and morphology of stereoblock polypropylenes containing sequences of isotactic (i) and syndiotactic (s) polypropylene (PP) were studied by 13C NMR spectroscopy, wide angle x-ray scattering (WAXS), size exclusion chromatography (SEC) combined with FT-IR analysis of the fractions, and atomic force microscopy. The 13C NMR spectra show the presence of long stereoregular sequences in the respective stereoblock polypropylene which are able to form the crystal modification known from the respective stereoregular homopolymers as revealed by WAXS measurements. A bimodal molar-mass distribution is detected by SEC. All SEC fractions contain i-PP and s-PP sequences, proven by FT-IR spectroscopy. It is shown by atomic-force microscopy that the stereoblock polypropylene, synthesized by homopolymerization of propylene using iso- and syndiotactic catalysts, form a microphase-separated morphology. The microphase separation is preserved also after treatment with hot toluene. A parallel investigation was conducted on a blend of i-PP and s-PP; it exhibits macrophase separation. Treatment of the blend with hot toluene results in the dissolution of the s-PP phase.