Christof Hübner

Modular ambient temperature functionalization of carbon nanotubes with stimuli-responsive polymer strands

Cyclopentadienyl end-capped poly(N-isopropylacrylamide) (PNIPAM-Cp, Mn = 5400 g mol−1, PDI = 1.13) was synthesized via a combination of RAFT (Reversible Addition–Fragmentation Chain Transfer) polymerization and modular conjugation (characterized via Nuclear Magnetic Resonance (NMR) as well as Electrospray Ionization-Mass Spectrometry (ESI-MS)), and reacted with untreated Single Walled Carbon Nanotubes (SWCNTs) as dienophiles in a Diels–Alder reaction with PNIPAM-Cp (diene) at ambient temperature in the absence of any catalyst. The obtained stimuli-responsive hybrid materials display thermo-responsive behaviour evidenced via UV-VIS-spectroscopy and Dynamic Light Scattering (DLS). The grafting density of the PNIPAM chains at the surface of the SWCNTs was determined via Thermogravimetric Analysis (TGA), Elemental Analysis (EA) and X-ray Photoelectron Spectroscopy (XPS), to be close to 0.0288 chains per nm2.

Thermoplastic Nanocomposites with Carbon Nanotubes

The potential impact of light-weight structures with multifunctional properties for engineering applications drives significant research and development activities on nanocomposites. Polymer nanocomposites especially those with carbon nanotubes (CNTs) are very attractive for conductive composites with good structural characteristics. The biggest challenge facing the commercial success of CNT based composites is the intrinsic strength of their agglomerates which prevents good filler dispersion in the matrix, a key attribute for any reinforcement. This chapter is a review of the CNT incorporated thermoplastic composites processed mainly containing economic multi-walled carbon nanotubes (MWCNTs). We present an overview of CNTs (their structure, production process, properties, surface modification, applications etc.) and the mechanism of their dispersion in thermoplastic matrices. The processing of thermoplastic matrices via the conventional twin-screw compounding approach is discussed along with the influence of process parameters on MWCNT dispersion. The role of secondary processing operation on composite properties is also highlighted. The mechanical, electrical and thermal properties of the thermoplastic-CNT composites are reviewed highlighting the key findings and shortcomings. We conclude throwing some light on the developments on multi-scale reinforcements in polymeric matrices.

Hetero Diels–Alder Chemistry for the Functionalization of Single‐Walled Carbon Nanotubes with Cyclopentadienyl End‐Capped Polymer Strands

Single-walled carbon nanotubes (SWCNTs) are pre-functionalized with a pyridinyl-based dithioester to undergo a hetero Diels-Alder (HDA) reaction with cyclopentadienyl end-capped poly(methyl)methacrylate (Mn = 2700 g mol(-1) , PDI = 1.14). Fourier transform infrared spectroscopy, thermogravimetric analysis, elemental analysis (EA), and X-ray photoelectron spectroscopy (XPS) evidence the success of the grafting process. The estimated resulting grafting density (from XPS and EA) via the HDA reaction increases by a factor of more than two (0.0774 chains·nm(-2) via XPS) compared with typical values obtained via a direct cyclopentadiene driven Diels-Alder conjugation onto non-functional SWCNTs under similar conditions.

Twin-screw extrusion of multi walled carbon nanotubes reinforced polycarbonate composites: Investigation of electrical and mechanical properties

1, 3 and 5 wt.% multi walled carbon nanotubes (MWCNT) reinforced polycarbonate (PC) composites were processed in a twin-screw extruder (L/D=52) with two different screw speeds, throughputs and screw configurations. Extruded strands were characterized for dispersion and measurement of electrical resistivities while the pelletized extrudates were injection molded to produce samples for mechanical and further electrical property measurements. The absolute resistance of the melt was recorded with an online melt resistance setup developed by our group. The volume resistivity of pure PC (1017 Ω.m) was lowered to 104 − 105 Ω.m on an injection molded PC-1 wt. % MWCNT composite. 3 wt.% MWCNT incorporated composites showed volume resistivity less than 1 Ω.m independent of process conditions. At lower filler contents the volume resistivity of injection molded samples were higher than those observed on the extruded strands and this effect diminished with increasing MWCNT loadings; owing to the loss of CNT network contacts due to shear induced filler orientation and core-skin effects. The quality of dispersion was exceptional for all filler concentrations at any process condition owing to the affinity of MWCNT towards PC due to the lower interfacial energy difference between the reactants and high polarity of PC. The modulus and strength of the composites increased with filler addition, however at 5 wt.% filler loading the strength of the composites processed with lower SMEs was less than that observed on the 1 wt.% MWCNT reinforced PC composite. The elongation of the composites at maximum tensile strength were comparable to that of neat PC except for composites with 5 wt.% MWCNT loading processed with lower SMEs. Composites with identical filler loadings which were processed with higher SMEs showed higher notched impact strength values principally because of the ability of very well dispersed filler fractions to inhibit crack propagation. The significance of the results obtained in this work stems from the fact that we were able to produce composites with substantial improvements in electrical properties with improvements/without significant loss in mechanical properties. This is one of the few exceptions to results achieved with commercially available MWCNT reinforced polymer composites processed on a large scale.

Multiscale dispersion-state characterization of nanocomposites using optical coherence tomography

Nanocomposite materials represent a success story of nanotechnology. However, development of nanomaterial fabrication still suffers from the lack of adequate analysis tools. In particular, achieving and maintaining well-dispersed particle distributions is a key challenge, both in material development and industrial production. Conventional methods like optical or electron microscopy need laborious, costly sample preparation and do not permit fast extraction of nanoscale structural information from statistically relevant sample volumes. Here we show that optical coherence tomography (OCT) represents a versatile tool for nanomaterial characterization, both in a laboratory and in a production environment. The technique does not require sample preparation and is applicable to a wide range of solid and liquid material systems. Large particle agglomerates can be directly found by OCT imaging, whereas dispersed nanoparticles are detected by model-based analysis of depth-dependent backscattering. Using a model system of polystyrene nanoparticles, we demonstrate nanoparticle sizing with high accuracy. We further prove the viability of the approach by characterizing highly relevant material systems based on nanoclays or carbon nanotubes. The technique is perfectly suited for in-line metrology in a production environment, which is demonstrated using a state-of-the-art compounding extruder. These experiments represent the first demonstration of multiscale nanomaterial characterization using OCT.

Opportunities and (in Particular) Risks of Use (Utilization Phase) of Plastic Structural Components

The previous chapters have described the opportunities and risks of plastics as materials, whereas in this chapter and the chapters, “Opportunities and Risks Involved in Designing Structural Components Made of Polymers,” “Plastics and Structural Components – The Environment and Recycling,” they will be discussed in terms of the structural components made of plastics. The opportunities often predominate due to the advantages of plastics over other materials, for example metals, ceramics, and wood. To avoid repetition, this chapter will deal primarily with the risks of the utilization phase of plastic products. The specific applications used to illustrate this are listed in the keywords.

Opportunities and Risks Involved in Designing Structural Components Made of Polymers

In the introduction to this book, and to this chapter in particular, the polymer engineering process is presented from a holistic point of view, i.e., with all of its material sources and ramifications. In the sections that follow, this point of view will be demonstrated on the basis of a number of examples of structural component development from the recent past; some of these projects extend into the future as well. Following an introductory text, each example of component development will be supplemented by a qualitative evaluation table with explanations as required. These tables are also qualitative checklists for avoiding future errors based on the example of the specific application considered in each case.