Joerg Christian Tiller

PVDF and PEtOx were cross-linked to obtain a low T _g material with inhibited crystallisation due to a high T _g barrier surrounding the seed crystals. The resulting material is self-enforcing upon strain by strain-induced crystallisation.

Double‐network hydrogels (DNHs) have outstanding mechanical properties, making them suitable for biomedical applications, such as artificial cartilage. One example of a biocompatible DNH with a mechanical behavior that exactly matches that of natural cartilage is based on poly(2‐methyl‐2‐oxazoline) (PMOx) as primary and poly(acrylic acid) (PAA) as a secondary network, which is stabilized by hydrogen bonds formed between the protonated PAA and the PMOx carbonyl groups. This DNH is shifting the deprotonation of PAA to a pH of 7.6, which makes it applicable in natural tissue. However, implanted hydrogels often encounter varying pH conditions within different tissues or in response to inflammation and infection. Thus, a further shift of the pKa toward more basic conditions is necessary. In this study, the secondary network in the PMOx‐based DNHs was systematically varied by copolymerizing acrylic acid (AA) and acryl amide (AAm) and the mechanical properties were investigated in PBS buffer with varying pH. A ratio of 10 wt.% AAm in the secondary network resulted in a PMOx/P(AA ₉₀ ‐co‐AAm ₁₀ ) DNH that is not deprotonated even at pH 9 and does not change its dimensions or its cartilage‐like mechanical properties up to this pH.

Shape memory polymers typically utilize the glass transition or melting temperature to switch from one shape to another. The goal of this work is to check if the α‐relaxation, which is attributed to the migration of chains through lamellar crystals, can be utilized as novel switch for any crosslinkable semi‐crystalline thermoplastic. Using cross‐linked low‐density polyethylene (x‐LDPE) as an example, it is shown that the α‐relaxation is indeed suitable as switch, when the polymer is crystallized under strain to an intermediate shape. It is demonstrated that post‐stretching at a temperature between the α‐relaxation and the melting temperature, followed by subsequent cooling to room‐temperature switches x‐LDPE from the intermediate to a temporary shape and that reheating to this temperature initiates the retraction back to the intermediate shape. Switching between intermediate and temporary shapes is shown to be accompanied by a reversible change in morphology.

Rendering established antibiotics by derivatization or formulation is a promising way to quickly obtain higher active antibiotics and to address antibiotic resistant bacterial strains. The antibiotic ciprofloxacin (CIP) conjugated with amphiphilic blockcopoly(2-oxazoline)s (POx) shows greatly enhanced antimicrobial activity, reduces resistance formation, and is active against CIP-resistant bacterial cells with overexpressed efflux pumps. In order to design CIP conjugates with comparable performance, but higher biocompatibility, the hydrophobic POx-block was substituted by a modified α-Tocopherol (VitE), which is predominantly referred to as vitamin E. Modification of VitE with 4-bromomethylbenzoyl bromide leads to a highly active initiator for POx synthesis. Using this initiator for the living cationic polymerization of 2-methyl-2-oxazoline leads to fully end-group functionalized PMOx. Conjugation of these polymers with CIP results in non-cytotoxic conjugates with improved CIP activity. These VitE-PMOx-EDA-xCIP conjugates enter E. coli cells via their efflux pumps. In case of E. coli with overexpressed efflux pumps (typical for resistant bacteria), the molar activity of the novel CIP conjugates is up to 100 times higher than free CIP, indicating potential of polymer conjugation with antibiotics to overcome bacterial resistances. [ ]

Smart materials react to external triggers by changing size, color, mechanical properties, or permeability. The next generation of smart materials will be able to not only recognize and react to external triggers but also to their dynamics. The only existing example of such a material is heating rate‐sensitive polymorphous cross‐linked syndiotactic polypropylene. This study presents a new principle of a heating rate‐sensitive material on the example of cross‐linked and fully amorphous quenchable semi‐crystalline polyethylene terephthalate (x‐PET). The x‐PET is stretched to high elongation above its melting temperature and constrained quenched to a fully amorphous state. Then the polymer is heated to 120–170 °C with different heating rates. Due to its heating‐rate sensitivity, x‐PET shrinks to different stabilized lengths dependent on the heating rate. The new length can be used to read out the heating rate and to specifically answer to this by mechanically switching a process. Detailed analytics of this process reveal that amorphous stretched x‐PET is starting the retraction above T_g and simultaneously stopping it by crystallization. The different rates of these processes result in the heating rate sensitivity of x‐PET.

Cross‐linking of polymers is an efficient method to tailor the end‐use properties of polymer materials. Cross‐linking using a chemical agent, e.g., dicumyl peroxide (DCP), allows for a spatially uniform network formation in the melt state. In addition, it is also associated with side reactions which influence the final properties of the plastic part. This work investigates the influence of DCP concentration on the tribological properties of a cross‐linked low‐density polyethylene (LDPE) grade. In particular, high DCP concentrations up to 20 phr are chosen in order to explore the effect of a high degree of cross‐linking. The viscoelastic properties below and above the melting temperature are studied in detail to support the interpretation of the tribological results. Rheological investigations allow one to monitor the cross‐linking of the long‐chain branched LDPE. The data and the subsequent optical analysis show that wear already is significantly reduced at a low DCP concentration of 1 phr because of the covalent bonds caused by cross‐linking. A high DCP concentration of 20 phr yields an increase of coefficient of friction which can be explained by the low stiffness and the resulting high contact area in the case of highly cross‐linked LDPE.