Crystallization of Isotactic Poly(methylmethacrylate) in Monolayers and Thin Films

Langmuir-Blodgett monolayers of isotactic PMMA exhibit a pressure-induced transition upon compression, that can be described in terms of a two-dimensional crystallization process, analogous to a normal melt crystallization. These water surface crystallized monolayers can be used to prepare highly crystalline thin films of isotactic PMMA with tailor-made orientational characteristics.

Oxidation of Monolayers of Partly Converted Dimethoxy-Substituted Poly(p-phenylenevinylene) Precursor Polymers at the Air-Water Interface

We observed that the poly(p-phenylenevinylene) units in Langmuir monolayers of partly converted dimethoxy-substituted poly(p-phenylenevinylene) precursor polymers oxidize at the air-water interface. This reaction even happened in the dark and therefore can not be attributed to a photooxygenation reaction with singlet oxygen. We assume that ground-state triplet oxygen is polarized at the air-water interface and forms a weakly bound complex with the double bond to give a reactive intermediate state, which lowers the activation energy of the oxidation reaction. The air-water interface thus works as a catalyst in this reaction.

Secondary Crystallization of Isotactic Polystyrene

When isotactic polystyrene (i-PS) is crystallized from the melt or from the glassy state at rather large supercooling an additional melting peak appears on the curve during scanning in a differential calorimeter. The overall rate of crystallization deduced from the total peak areas as a function of crystallization time did not fit the Avrami equation well. When we omit the area of the additional melting peak in the kinetic analysis a much better fit is obtained. We also observed that no lamellar thickening occurs during isothermal crystallization. In view of the low degree of crystallinity of i-PS these results lead to the idea that a secondary crystallization process takes place within the amorphous parts of the spherulites resulting in this additional melting peak on the DSC curve. The large supercooling needed and the increase in peak area with increasing molecular weight make us suppose that intercrystalline links are probably responsible for the additional melting peak of bulk-crystallized i-PS. Electron microscopic studies of surface replicas of i-PS support this view.

Thin-Film Behavior of Poly(methyl methacrylates). 1. Monolayers at the Air-Water Interface

The monolayer behavior of PMMA of varying tacticities at the air-water interface was studied. A difference in lateral cohesive energy is argued to be responsible for the fact that the pressure area isotherms of isotactic PMMA deviate strongly from those of syndiotactic PMMA. At low surface pressures the isotactic PMMA monolayer can be characterized as an expanded type monolayer, whereas syndiotactic PMMA forms a condensed type monolayer. At areas of about 20 Å2/monomeric unit a transition can be observed in the pressure-area isotherm of isotactic PMMA. This transition was studied as a function of molecular weight, temperature, and compression speed. From the results it is deduced that the transition corresponds to a two-dimensional pseudocrystallization process in which the isotactic PMMA assumes a double-helix conformation in the monolayer, similar to the three-dimensional crystal structure. From an Avrami analysis it is inferred that the kinetics of this crystallization can be described by an activated nucleation, followed by a one-dimensional growth.

Rheological Behavior of Precursor PPV Monolayers

The rheological behavior of different precursor poly(p-phenylene vinylene) (prec-PPV) monolayers at the air-water interface was investigated using an interfacial stress rheometer (ISR). This device nicely reveals a transition of the precursor poly(2,5-dimethoxy-1,4 phenylene vinylene) (prec-DMePPV) monolayer from Newtonian to elastic behavior with increasing surface pressure. The transition is accompanied by an increase in the modulus. This behavior coincides with the coagulation of different 2D condensed domains as revealed by Brewster angle microscopy (BAM). However, partly converted prec-DMePPV monolayers show elastic behavior even at low surface pressures, although a sudden increase of the moduli does occur. This phenomenon is attributed to enhanced hydrophobic interactions between the conjugated moieties in the partly converted polymers. The latter also explains the stretching behavior of the partly converted prec-DMePPV upon transfer in Langmuir-Blodgett-type vertical dipping. The increase of the moduli which is observed is much more gradual in the precursor poly(2,5-dibutoxy-1,4-phenylene vinylene), prec-DBuPPV, a monolayer which is in agreement with the expected expanded state of the latter monolayer.

Development of cisgenic apples with durable resistance to apple scab

Most of the apple (Malus × domestica) growers are facing serious disease problems with apple scab which is caused by the fungus Venturia inaequalis. Developing a resistant variety in apple through classical breeding is very slow and inefficient. So, we aim at improving existing apple varieties through a new concept called “cisgenesis” which saves time and effort compared to classical breeding. Malus floribunda proved to be a good source of natural scab resistance genes. The genes HcrVf1 and HcrVf2, consisting of promoter, coding and terminator sequences in their natural configuration, were isolated from Malus floribunda and cloned into the binary vector pMF1. Apple cv. ‘Gala’ was transformed with pMF1 containing HcrVf1 and HcrVf2, individually or in combination. pMF1 can be used to obtain marker-free plants by recombinase-based excision of a fragment carrying undesired gene sequences, such as antibiotic-selection marker genes, leaving behind only the gene(s)-of-interest and one recombination site. Using this vector it is therefore possible to stack several genes by retransformation using the same selection procedure. In order to obtain durable resistance, we have the intention to combine different resistance genes from Malus either by stacking them one by one or by introducing them all together in one T-DNA. Performance of all different types of cisgenic plants will be evaluated by monitoring scab resistance levels phenotypically and by determining gene expression profiles through quantitative RT-PC

Design, synthesis and properties of a degradable polyurethane scaffold for meniscus regeneration

Longitudinal lesions in menisci are among the most frequent orthopedic problems of the knee. Repair by simple techniques is only limited to the vascular part of the meniscus. For repair of the avascular part of the meniscus a scaffold, which will assist the body in the formation of new meniscus cell tissue, might be applicable. In this study a biomedical segmented polyurethane with poly(ε-caprolactone) as soft segment and 1,4-butanediisocyanate and 1,4-butanediol as uniform hard segments has been synthesised. The material has a micro phase separated morphology and excellent mechanical properties. A porous scaffold was prepared via a combination of liquid–liquid phase separation and salt leaching. The foams prepared combined a very high interconnectivity and porosity with the desired compression modulus. After six months of implantation in the knees of beagles full ingrowth with cells was obtained and it was found that meniscus like tissue had been formed in the scaffold. Moreover, compression behaviour appeared to be comparable to native meniscus tissue