In addition to basic research for the development of new methods and strategies of synthesis, the focus of the Polymer Chemistry and Technology (PCT) Group is on practice-oriented synthetic chemistry. The cornerstones of our activities are the synthesis and characterisation of products that are industrially and technologically exploitable and marketable as well as the development of technical manufacturing processes. Our research is devoted to the synthesis and modification of synthetic polymers and renewable materials, whereas there is a strong focus on fundamentals of photopolymerization, applied photopolymers, biomaterials, polymers and materials with defined architecture and polymer characterization.
In tissue engineering (TE), artificial biomaterials have emerged over the past decade as an alternative to auto-transplants. The main characteristics of these materials include good biocompatibility as well as biodegradability. Up to now, most materials used in TE are based on polyesters, which are limited in their scope of application, especially as scaffolds, due to their undesirably slow degradation behavior and the formation of acidic degradation products which can lead to tissue inflammation or even necrosis. Therefore, alternatives to ester functionalities, which show enhanced degradability, are of interest. Furthermore, advanced polymer processing methods are requested, such as Additive Manufacturing or Electrospinning
Photopolymers exhibiting biocompatibility and biodegradability are promising materials for the application in the field of Tissue Engineering. The possibility of structuring compounds via Additive Manufacturing Technologies (AMT) such as Microstereolithography, Digital Light Processing (DLP) or Two-photon Induced Photopolymerization (TPIP) enables the fabrication of constructs with complex geometries and high resolutions of about 10µm - or even 200nm in the case of TPIP - in order to mimic the cellular structures of natural materials such as bone.
State of the art compounds for these applications are (meth)acrylate-based. However, methacrylates exhibit low photoreactivity and acrylates are prone to side-reactions with proteins inside the human body giving explanation for the high irritation and sometimes toxicity. Furthermore, the formation of poly(acrylic acid) upon hydrolytic degradation provokes a local decrease of the pH value and can cause the formation of non-excretable precipitate in the presence of calcium ions. Hence, these materials are less suitable for biomedical applications.
Therefore we have established the photopolymerization chemistry with vinylesters, vinylcarbonates and other enes that show significantly lower toxicity.
Diseases of the cardiovascular system are principal causes for morbidity and mortality in all western countries. The total direct and indirect costs of cardiovascular diseases (CVD) and stroke for 2010 were estimated at $503.2 billion for the United States alone. Electrospinning (ES) has attracted the interest of biomedical research as it is possible to manufacture seamless, non-woven, nanofibrous tubes with high (>20 m2/g) surface areas which mimic the extracellular matrix (ECM) and are potentially suitable as vascular grafts. Unfortunately, currently used materials show low compliancy (ePTFE) and/or are non-degradable (Thermoplastic polyurethane elastomers (TPUs)). Therefore degradable TPUs are of high interest.
The number of patients with minor and major accidents coming into accident and emergency care is steadily rising due to increasing life spans and the aging of our society. The fixation and adhesion between tissues, implants or scaffolds have to be refined, but the number and versatility of biomimetic and biocompatible adhesives that can be used for such purpose is limited. Up to now, adhesives based on cyanoacrylates, polyurethanes, epoxy resins or poly(methyl methacrylates) still bear several drawbacks ranging from possible allergic response, lack of mechanical strength to toxic side products. Therefore, new biomimetic glues for bonding of tissue-tissue and tissue-implant interfaces are urgently needed.
The extracellular matrix (ECM) forms the mechanical framework for living tissue and controls many functions including tissue morphogenesis, regulation of cell differentiation and proliferation, and processes for regeneration after injuries and is therefore a central consideration in the field of tissue engineering and regenerative medicine. To replace the damaged tissue artificial hydrogels are required with low cytotoxicity. Multiphoton Polymerization is a suitable tool to structure such hydrogels including the placement of signals to stimulate and guide cell differentiation.
As far as the aspect of sustainability is concerned, renewable raw materials play a key role in polymer technology. The use of recoverable raw materials is especially efficient if benefits in synthesis provided by nature are fully exploited, i.e. if only few physical, chemical or biotechnological modification steps are necessary to obtain industrially utilizable materials and products. Research topics at the institute are e.g. the synthesis of reactive carbohydrate and protein derivatives, usable as components for water-borne UV-curable resins , and the synthesis of so-called glycopolymers from saccharide-based (glucose, sucrose, glucamine..) monomers. Application of poly-functional carbohydrate monomers or addition of commercially available cross-linkers yields hydrophilic cross-linked polymers , which are tested as polymer supports in e.g. biomedicine. Printability and wettability of polypropylene can be significantly improved by photochemical immobilization of carbohydrates on the polymer surface. Other main fields of research are the functionalization of polysaccharides such as cellulose and starch and we are also engaged in the modification of wood and lignin.
Topics: Fundamentals of Photopolymerization
Advantages of photopolymerizable formulations can for sure been found in the solvent free processing and the high curing speed that allows the conversion of typically (meth)acrylate-based monomers within the fraction of a second. Furthermore, a large variety of monomers is commercially available so that the mechanical properties and other polymer characteristics can be easily tuned. One of the key components is the photoinitiator that absorbs light and transfers it to chemical energy by starting the radical or cationic polymerization process.
During the last 25 years research of the PCT group has focused on radical photoinititiators. Besides water-soluble and covalently linked photoinitiators, new chromophores were aimed. With Ivocerin(c) a milestone for dental composites has been created. Besides this Ge-based compounds also Si and Sn-based long wavelength initiators have been explored. As migration of uncunsumed copounds is always an issue, self-initiating monomers and non-aromatic photoinitiators have been developed.
Oxygen inhibition in radical photopolymerization is a major drawback as it leads to tacky surfaces and frequently requires expensive nitrogen inertization. Beside such physical methods there are a large variety of chemical methods available that try to circumvent these disadvantages. However, a recent study that compares most of the strategies known from literature shows that there is up to now only a moderate improvement possible and no general strategy available.
Cationic photopolymerization offers not only wide range of polymerizeable monomers (e.g. epoxides, oxetanes, vinyl ether) but also a clear advantage compared to the classical free radical photopolymerization, which is the independence of the process from oxygen. The state of the art cationic photoinitiators (so called photo acids) are usually iodonium or sulfonium salts of fluorometallate anions (e.g. BF4-; PF6-; AsF6-; SbF6-) or [BAr4F]--anions. These anions suffer from different drawbacks like toxicity, hydrolysis or tough synthesis.
Very recently we have developed a new class of cationic photoinitiators (sulphonium as well as iodonium salt) based on a fluorinated alkoxyaluminate with a low toxicity, high stability and outstanding reactivity. Excellent activity has been demonstrated for epoxides and ring opening monomers like oxazolines, cyclic carbonates, cyclic orthoesters etc. Based on that success the cation and anion has been widely changes to give new high performance photoacids
Generally, in Multi Photon Induced Photopolymerization (MPIP, also called Two Photon induced Polymerization) a resin, that contains mainly a multifunctional monomer (typically acrylate-based) and a photoinitiator, is cured inside the focal point of a fs pulsed near infrared laser beam to produce a desired 3D shape with a resolution down to 100 nm. As usual single photon initiators have only limited two photon absorption properties specialized molecules have to be desinged containing a long planar π-system and strong electron donor and or acceptor groups are required. Initiators known from literature have the disadvantages of deactivation of the excited singlet state by fluoreszenz and cis-trans isomerization of the double bonds.
Designed Polymer Architecture
Classical photopolymers are based on the (meth)acrylate chemistry. Typical chain growth polymerization of these monomers leads to a fast gelation already at relatively low conversion. As polymerization continous shrinkage stress occurs and a inhomogenous network with brittle behaviour is formed. Strategies that circumvent this drawback include addition fragmentation chain transfer agents and classical thiol ene polymerization
Photopolymers suffer from their polymer architecture which is typically formed by a radical chain growth reaction. The high number of crosslinks along the polymer backbone (branches every second carbon atom) leads to brittle materials with low impact strength. Furthermore, during photocuring, the gel point is reached already after ~ 20% conversion which leads to high shrinkage stress of the material. Although thiol-ene polymerization has demonstrated in the last decade that it is able to circumvent most of these drawbacks, new problems arise such as bad odor or poor storage stability.
Thiol-Ene polymerization is known since the 50’s of the last century and has gained tremendously increasing interest during the last decade, thanks to the rediscovery by Hoyle and recent efforts by Bowman. Advantages such as low oxygen inhibition and shrinkage and the formation of uniform networks in a click chemistry style are accompanied by up to now unsolved disadvantages such as unpleasant odour, poor storage stability and low modulus materials.
We have demonstrated that this chemistry could speed up low toxic vinylester and vinylcarbonate polymerization and also the suitability in the formation of hydrogels in the field of tissue engineering.
Azide grafting chemistry is another field of high importance which is now applied in the high resolution multiphoton chemistry for site selective surface modification of hydrogels
UV curing of photopolymerizable formulations has been used for more than a half century for protective and decorative coatings of paper, wood, metals or plastics. Beside classical coating, this technique is currently also considered for other more advanced applications such as dental filling materials, optical materials, biocompatible and biodegradable polymers or additive manufacturing technologies.
Frontal polymerization is a method of macromolecular synthesis, characterized by a steady state polymerization front that constantly propagates through the reaction vessel.
The process can be triggered by a one time energy input via an external heat source or the heat of a photopolymerization reaction. Due to the exothermic behavior of polymerization reactions, the local reaction can initiate further polymerization in the surrounding monomer by cleavage of a thermal initiator. At low heat loss, it is possible to generate a self-sustaining, thermal front which propagates step by step through a theoretically unlimited monomer volume.
We have demonstrated that frontal polymerization of classical acrylate systems is also possible in water. Furthermore, a new system for radical induced cationic polymerization of epoxides has been established that allows the curing of composite materials
In cooperation with the Institute of Materials Sciences and Technology from the TU Wien and different companies new materials are developed for Light based Additive Manufacturing including Ceramic 3D Printing (Lithoz), the Hot Lithography technology (Cubicure) and Multiphoton Polymerization (UpNano). Toughening of Photopolymers, Biomaterials or High Resolution Structuring are the principle aims.
Since more than 20 years there is a continous industry cooperation with a dental company. The main aims are to develop new efficient photoinitiators the have good solubility in aqueous dental primer formulations and especialy long wavelength absorption for excellent cure depth of highly filled composites. In the later case, a milestone in dental photoinitiators has been brought to market, Ivocerin(c) . Another ongoing topic is shrinkage stress of dental filling composites. Here the Addition Fragementation Chain Transfer concept has been introduced to the market due to its low retardation, excellent shrinkage reduction and surprsing improvement of toughness. Furthermore, cyclopolymerizable monomers and vinylcyclopropane were used to counteract shrinkage.
Photopolymerizable glues and cements that offer debonding on demand (DoD) through an external stimulus are of great interest for the fields of recycling and repair. State-of-the-art DoD solutions often require a high-energy impulse (e.g., >200 °C, strong force), which is due to the typical glassy nature of such photopolymer networks. We have developed some DoD solutions for methacrylate-based monomers in the field of dentistry.
Polymer characterization is an essential tool to follow polymer formation and their final properties. Besides classical method, their is a set of unique equipment available, especially due to the strong focus on photopolymer sciences.
Polymer characterization includes the determination of molecular weight and molecular weight distribution, molecular structure, conformation of the polymer in solution, thermal properties and mechanical properties. The monomer and oligomer characterization according to type, purity and additives is also regularly carried out in our laboratory.
RT-NIR/MIR-Photorheometry is a world wide unique equipment in our group. It allows to follow photocuring reaction under simulateous evaluation of the rheological and mechanical properties, chemical information on the monomer conversion and shrinkage behaviour.