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Industrialized food production is in urgent search for alternative packaging materials, which can serve the requirements of a globalized world in terms of longer product shelf lives, reduced freight weight to decrease transport costs, and better barrier functionality to preserve its freshness. Polymer materials containing organically modified nano clay particles as additives are one example for a new generation of packaging materials with specific barrier functionality to actually hit the market. Clay types used for these applications are aluminosilicates, which belong to the mineral group of phyllosilicates. These consist of nano-scaled thin platelets, which are organically modified with quaternary ammonium compounds acting as spacers between the different clay layers, thereby increasing the hydrophobicity of the mineral additive. A variety of different organically modified clays are already available, and the use as additive for food packaging materials is one important application. To ensure valid risk assessments of emerging nano composite polymers used in the food packaging industry, exact analytical characterization of the organically modified clay within the polymer matrix is of paramount importance. Time-of-flight SIMS in combination with multivariate statistical analysis was used to differentiate modified clay reference materials from another. Time-of-flight SIMS spectra of a reference polymer plate, which contained one specific nano clay composite, were acquired. For each modified clay additive, a set of characteristic diagnostic ions could be identified, which then was used to successfully assign unknown clay additives to the corresponding reference material. Thus, the described methodology could be used to define and characterize nano clay within polymer matrices. Copyright (c) 2014 John Wiley & Sons, Ltd.
Proteins are natural polypeptides produced by cells; they can be found in both animals and plants, and possess a variety of functions. One of these functions is to provide structural support to the surrounding cells and tissues. For example, collagen (which is found in skin, cartilage, tendons and bones) and keratin (which is found in hair and nails) are structural proteins. When a tissue is damaged, however, the supporting matrix formed by structural proteins cannot always spontaneously regenerate. Tailor-made synthetic polypeptides can be used to help heal and restore tissue formation.
Synthetic polypeptides are typically synthesized by the so-called ring opening polymerization (ROP) of α-amino acid N-carboxyanhydrides (NCA). Such synthetic polypeptides are generally non-sequence-controlled and thus less complex than proteins. As such, synthetic polypeptides are rarely as efficient as proteins in their ability to self-assemble and form hierarchical or structural supramolecular assemblies in water, and thus, often require rational designing. In this doctoral work, two types of amino acids, γ-benzyl-L/D-glutamate (BLG / BDG) and allylglycine (AG), were selected to synthesize a series of (co)polypeptides of different compositions and molar masses.
A new and versatile synthetic route to prepare polypeptides was developed, and its mechanism and kinetics were investigated. The polypeptide properties were thoroughly studied and new materials were developed from them. In particular, these polypeptides were able to aggregate (or self-assemble) in solution into microscopic fibres, very similar to those formed by collagen. By doing so, they formed robust physical networks and organogels which could be processed into high water-content, pH-responsive hydrogels. Particles with highly regular and chiral spiral morphologies were also obtained by emulsifying these polypeptides. Such polypeptides and the materials derived from them are, therefore, promising candidates for biomedical applications.
Synthesis, assembly and thermo-responsivity of polymer-functionalized magnetic cobalt nanoparticles
(2018)
This thesis mainly covers the synthesis, surface modification, magnetic-field-induced assembly and thermo-responsive functionalization of superparamagnetic Co NPs initially stabilized by hydrophobic small molecules oleic acid (OA) and trioctylphosphine oxide (TOPO), as well as the synthesis of both superparamagnetic and ferromagnetic Co NPs by using end-functionalized-polystyrene as stabilizer.
Co NPs, due to their excellent magnetic and catalytic properties, have great potential application in various fields, such as ferrofluids, catalysis, and magnetic resonance imaging (MRI). Superparamagnetic Co NPs are especially interesting, since they exhibit zero coercivity. They get magnetized in an external magnetic field and reach their saturation magnetization rapidly, but no magnetic moment remains after removal of the applied magnetic field. Therefore, they do not agglomerate in the body when they are used in biomedical applications. Normally, decomposition of metallic precursors at high temperature is one of the most important methods in preparation of monodisperse magnetic NPs, providing tunability in size and shape. Hydrophobic ligands like OA, TOPO and oleylamine are often used to both control the growth of NPs and protect them from agglomeration. The as-prepared magnetic NPs can be used in biological applications as long as they are transferred into water. Moreover, their supercrystal assemblies have the potential for high density data storage and electronic devices. In addition to small molecules, polymers can also be used as surfactants for the synthesis of ferromagnetic and superparamagnetic NPs by changing the reaction conditions. Therefore, chapter 2 gives an overview on the basic concept of synthesis, surface modification and self-assembly of magnetic nanoparticles. Various examples were used to illustrate the recent work.
The hydrophobic Co NPs synthesized with small molecules as surfactants limit their biological applications, which require a hydrophilic or aqueous environment. Surface modification (e.g., ligand exchange) is a general idea for either phase transition or surface-functionalization. Therefore, in chapter 3, a ligand exchange process was conducted to functionalize the surface of Co NPs. PNIPAM is one of the most popular smart polymers and its lower critical solution temperature (LCST) is around 32 °C, with a reversible change in the conformation structure between hydrophobic and hydrophilic. The novel nanocomposites of superparamagnetic Co NPs and thermo-responsive PNIPAM are of great interest. Thus, well-defined superparamagnetic Co NPs were firstly synthesized through the thermolysis of cobalt carbonyl by using OA and TOPO as surfactants. A functional ATRP initiator, containing an amine (as anchoring group) and a 2-bromopropionate group (SI-ATRP initiator), was used to replace the original ligands. This process is rapid and facial for efficient surface functionalization and afterwards the Co NPs can be dispersed into polar solvent DMF without aggregation. FT-IR spectroscopy showed that the TOPO was completely replaced, but a small amount of OA remained on the surface. A TGA measurement allowed the calculation of the grafting density of the initiator as around 3.2 initiator/nm2. Then, the surface-initiated ATRP was conducted for the polymerization of NIPAM on the surface of Co NPs and rendered the nanocomposites water-dispersible. A temperature-dependent dynamic light scattering study showed the aggregation behavior of PNIPAM-coated Co NPs upon heating and this process was proven to be reversible. The combination of superparamagnetic and thermo-responsive properties in these hybrid nanoparticles is promising for future applications e.g. in biomedicine.
In chapter 4, the magnetic-field-induced assembly of superparamagnetic cobalt nanoparticles both on solid substrates and at liquid-air interface was investigated. OA- and TOPO-coated Co NPs were synthesized via the thermolysis of cobalt carbonyl and dispersed into either hexane or toluene. The Co NP dispersion was dropped onto substrates (e.g., TEM grid, silicon wafer) and at liquid-air (water-air or ethylene glycol-air) interface. Due to the attractive dipolar interaction, 1-D chains formed in the presence of an external magnetic field. It is known that the concentration and the strength of the magnetic field can affect the assembly behavior of superparamagnetic Co NPs. Therefore, the influence of these two parameters on the morphology of the assemblies was studied. The formed 1-D chains were shorter and flexible at either lower concentration of the Co NP dispersion or lower strength of the external magnetic field due to thermal fluctuation. However, by increasing either the concentration of the NP dispersion or the strength of the applied magnetic field, these chains became longer, thicker and straighter. The reason could be that a high concentration led to a high fraction of short dipolar chains, and their interaction resulted in longer and thicker chains under applied magnetic field. On the other hand, when the magnetic field increased, the induced moments of the magnetic nanoparticles became larger, which dominated over the thermal fluctuation. Thus, the formed short chains connected to each other and grew in length. Thicker chains were also observed through chain-chain interaction. Furthermore, the induced moments of the NPs tended to direct into one direction with increased magnetic field, thus the chains were straighter. In comparison between the assembly on substrates, at water-air interface and at ethylene glycol-air interface, the assembly of Co NPs in hexane dispersion at ethylene glycol-air interface showed the most regular and homogeneous chain structures due to the better spreading of the dispersion on ethylene glycol subphase than on water subphase and substrates. The magnetic-field-induced assembly of superparamagnetic nanoparticles could provide a powerful approach for applications in data storage and electronic devices.
Chapter 5 presented the synthesis of superparamagnetic and ferromagnetic cobalt nanoparticles through a dual-stage thermolysis of cobalt carbonyl (Co2(CO)8) by using polystyrene as surfactant. The amine end-functionalized polystyrene surfactants with different molecular weight were prepared via atom transfer radical polymerization technique. The molecular weight determination of polystyrene was conducted by gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry techniques. The results showed that, when the molecular weight distribution is low (Mw/Mn < 1.2), the measurement by GPC and MALDI-ToF MS provided nearly similar results. For example, the molecular weight of 10600 Da was obtained by MALDI-ToF MS, while GPC gave 10500 g/mol (Mw/Mn = 1.17). However, if the polymer is poly distributed, MALDI-ToF MS cannot provide an accurate value. This was exemplified for a polymer with a molecular weight of 3130 Da measured by MALDI-TOF MS, while GPC showed 2300 g/mol (Mw/Mn = 1.38). The size, size distribution and magnetic properties of the hybrid particles were different by changing either the molecular weight or concentration of the polymer surfactants. The analysis from TEM characterization showed that the size of cobalt nanoparticles stabilized with polystyrene of lower molecular weight (Mn = 2300 g/mol) varied from 12–22 nm, while the size with middle (Mn = 4500 g/mol) and higher molecular weight (Mn = 10500 g/mol) of polystyrene-coated cobalt nanoparticles showed little change. Magnetic measurements exhibited that the small cobalt particles (12 nm) were superparamagnetic, while larger particles (21 nm) were ferromagnetic and assembled into 1-D chains. The grafting density calculated from thermogravimetric analysis showed that a higher grafting density of polystyrene was obtained with lower molecular weight (Mn = 2300 g/mol) than those with higher molecular weight (Mn = 10500 g/mol). Due to the larger steric hindrance, polystyrene with higher molecular weight cannot form a dense shell on the surface of the nanoparticles, which resulted in a lower grafting density. Wide angle X-ray scattering measurements revealed the epsilon cobalt crystalline phases of both superparamagnetic Co NPs coated with polystyrene (Mn = 2300 g/mol) and ferromagnetic Co NPs coated with polystyrene (Mn = 10500 g/mol). Furthermore, a stability study showed that PS-Co NPs prepared with higher polymer concentration and polymer molecular weight exhibited a better stability.
Due to its bioavailability and (bio)degradability, poly(lactide) (PLA) is an interesting polymer that is already being used as packaging material, surgical seam, and drug delivery system. Dependent on various parameters such as polymer composition, amphiphilicity, sample preparation, and the enantiomeric purity of lactide, PLA in an amphiphilic block copolymer can affect the self-assembly behavior dramatically. However, sizes and shapes of aggregates have a critical effect on the interactions between biological and drug delivery systems, where the general understanding of these polymers and their ability to influence self-assembly is of significant interest in science.
The first part of this thesis describes the synthesis and study of a series of linear poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA)-based amphiphilic block copolymers with varying PLA (hydrophobic), and poly(ethylene glycol) (PEG) (hydrophilic) chain lengths and different block copolymer sequences (PEG-PLA and PLA-PEG). The PEG-PLA block copolymers were synthesized by ring-opening polymerization of lactide initiated by a PEG-OH macroinitiator. In contrast, the PLA-PEG block copolymers were produced by a Steglich-esterification of modified PLA with PEG-OH.
The aqueous self-assembly at room temperature of the enantiomerically pure PLLA-based block copolymers and their stereocomplexed mixtures was investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). Spherical micelles and worm-like structures were produced, whereby the obtained self-assembled morphologies were affected by the lactide weight fraction in the block copolymer and self-assembly time. The formation of worm-like structures increases with decreasing PLA-chain length and arises from spherical micelles, which become colloidally unstable and undergo an epitaxial fusion with other micelles. As shown by DSC experiments, the crystallinity of the corresponding PLA blocks increases within the self-assembly time. However, the stereocomplexed self-assembled structures behave differently from the parent polymers and result in irregular-shaped clusters of spherical micelles. Additionally, time-dependent self-assembly experiments showed a transformation, from already self-assembled morphologies of different shapes to more compact micelles upon stereocomplexation.
In the second part of this thesis, with the objective to influence the self-assembly of PLA-based block copolymers and its stereocomplexes, poly(methyl phosphonate) (PMeP) and poly(isopropyl phosphonate) (PiPrP) were produced by ring-opening polymerization to implement an alternative to the hydrophilic block PEG. Although, the 1,8 diazabicyclo[5.4.0]unde 7 ene (DBU) or 1,5,7 triazabicyclo[4.4.0]dec-5-ene (TBD) mediated synthesis of the corresponding poly(alkyl phosphonate)s was successful, however, not so the polymerization of copolymers with PLA-based precursors (PLA-homo polymers, and PEG-PLA block copolymers). Transesterification, obtained by 1H-NMR spectroscopy, between the poly(phosphonate)- and PLA block caused a high-field shifted peak split of the methine proton in the PLA polymer chain, with split intensities depending on the used catalyst (DBU for PMeP, and TBD for PiPrP polymerization). An additional prepared block copolymer PiPrP-PLLA that wasn’t affected in its polymer sequence was finally used for self-assembly experiments with PLA-PEG and PEG-PLA mixing.
This work provides a comprehensive study of the self-assembly behavior of PLA-based block copolymers influenced by various parameters such as polymer block lengths, self-assembly time, and stereocomplexation of block copolymer mixtures.
This paper presents shape-memory foams that can be temporarily fixed in their compressed state and be expanded on demand. Highly porous, nanocomposite foams were prepared from a solution of polyetherurethane with suspended nanoparticles (mean aggregate size 90 nm) which have an iron(III) oxide core with a silica shell. The polymer solution with suspended nanoparticles was cooled down to -20 degrees C in a two-stage process, which was followed by freeze-drying. The average pore size increases with decreasing concentration of nanoparticles from 158 mu m to 230 mu m while the foam porosity remained constant. After fixation of a temporary form of the nanocomposite foams, shape recovery can be triggered either by heat or by exposure to an alternating magnetic field. Compressed foams showed a recovery rate of up to 76 +/- 4% in a thermochamber at 80 degrees C, and a slightly lower recovery rate of up to 65 +/- 4% in a magnetic field.
Boronic ester bonds can be reversibly formed between phenylboronic acid (PBA) and triol moieties. Here, we aim at a glucose-induced shape-memory effect by implementing such bonds as temporary netpoints, which are cleavable by glucose and by minimizing the volume change upon stimulation by a porous cryogel structure. The polymer system consisted of a semi-interpenetrating network (semi-IPN) architecture, in which the triol moieties were part of the permanent network and the PBA moieties were located in the linear polymer diffused into the semi-IPN. In an alkaline medium (pH = 10), the swelling ratio was approximately 35, independent of C-glu varied between 0 and 300 mg/dL. In bending experiments, shape fixity R-f approximate to 80% and shape recovery R-r approximate to 100% from five programming/recovery cycles could be determined. R-r was a function of C-glu in the range from 0 to 300 mg/dL, which accords with the fluctuation range of C-glu in human blood. In this way, the shape-memory hydrogels could play a role in future diabetes treatment options.
Polyester-based shape-memory polymer actuators are multifunctional materials providing reversible macroscopic shape shifts as well as hydrolytic degradability. Here, the function-function interdependencies (between shape shifts and degradation behaviour) will determine actuation performance and its life time. In this work, glycolide units were incorporated in poly(epsilon-caprolactone) based actuator materials in order to achieve an accelerated hydrolytic degradation and to explore the function-function relationship. Three different oligo[(epsilon-caprolactone)-co-glycolide] copolymers (OCGs) with similar molecular weights (10.5 +/- 0.5 kg center dot mol(-1)) including a glycolide content of 8, 16, and 26 mol% (ratio 1:1:1 wt%) terminated with methacrylated moieties were crosslinked. The obtained actuators provided a broad melting transition in the range from 27 to 44 degrees C. The hydrolytic degradation of programmed OCG actuators (200% of elongation) resulted in a reduction of sample mass to 51 wt% within 21 days at pH = 7.4 and 40 degrees C. Degradation results in a decrease of T-m associated to the actuating units and increasing T-m associated to the skeleton forming units. The actuation capability decreased almost linear as function of time. After 11 days of hydrolytic degradation the shape-memory functionality was lost. Accordingly, a fast degradation behaviour as required, e.g., for actuator materials intended as implant material can be realized.
The macroscale function of multicomponent polymeric materials is dependent on their phase-morphology. Here, we investigate the morphological structure of a multiblock copolymer consisting of poly(L-lactide) and poly(epsilon-caprolactone) segments (PLLA-PCL), physically cross-linked by stereocomplexation with a low molecular weight poly(D-lactide) oligomer (PDLA). The effects of blend composition and PLLA-PCL molecular structure on the morphology are elucidated by AFM, TEM and SAXS. We identify the formation of a lattice pattern, composed of PLA domains within a PCL matrix, with an average domain spacing d0 = 12 - 19 nm. The size of the PLA domains were found to be proportional to the block length of the PCL segment of the copolymer and inversely proportional to the PDLA content of the blend. Changing the PLLA-PCL / PDLA ratio caused a shift in the melt transition Tm attributed to the PLA stereocomplex crystallites, indicating partial amorphous phase dilution of the PLA and PCL components within the semicrystalline material. By elucidating the phase structure and thermal character of multifunctional PLLA-PCL / PDLA blends, we illustrate how composition affects the internal structure and thermal properties of multicomponent polymeric materials. This study should facilitate the more effective incorporation of a variety of polymeric structural units capable of stimuli responsive phase transitions, where an understanding the phase-morphology of each component will enable the production of multifunctional soft-actuators with enhanced performance.
The chemical nature, the number length of integrated building blocks, as well as their sequence structure impact the phase morphology of multiblock copolymers (MBC) consisting of two non-miscible block types. It is hypothesized that a strictly alternating sequence should impact phase segregation. A library of well-defined MBC obtained by coupling oligo(epsilon-caprolactone) (OCL) of different molecular weights (2, 4, and 8 kDa) with oligotetrahydrofuran (OTHF, 2.9 kDa) via Steglich esterification results in strictly alternating (MBCalt) or random (MBCran) MBC. The three different series has a weight average molecular weight (M-w) of 65 000, 165 000, and 168 000 g mol(-1) for MBCalt and 80 500, 100 000, and 147 600 g mol(-1) for MBCran. When the chain length of OCL building blocks is increased, the tendency for phase segregation is facilitated, which is attributed to the decrease in chain mobility within the MBC. Furthermore, it is found that the phase segregation disturbs the crystallization by causing heterogeneities in the semi-crystalline alignment, which is attributed to an increase of the disorder of the OCL semi-crystalline alignment.