No abstract

ber of hydrogen bonds. In the case of PLLA 7 % water content, all are implicated in at least one hydrogen bond and a number of them in more than one.In summary, it can be stated that a major reason for the faster degradation of PGA, in comparison to PLLA, is the considerably faster swelling which is most likely caused by a much higher solubility of water in PGA than in PLLA. The major structural reason for this behaviour seems to be the presence of an extra nonpolar methyl group in PLLA that leads to a reduced density of polar interaction energy. Another finding is that the water molecules in the simulated less swollen states, on average, move in pairs, while at higher water content (PLLA with 7 % water content in weight) even clusters of three molecules are observed.In the future, the investigations will be extended to copolymers and quantum chemical investigations.Shape-memory polymers (SMP) are stimuli-sensitive materials with the ability to change their shape when heated to a temperature higher than a certain switching temperature COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2006, 8, No. 5

Shape-memory polymers (SMP) are dual shape materials, which can fix a temporary shape besides their original shape. The process of deformation under application of an external stress above a transition temperature (T trans ) and fixation by cooling below T trans is called programming. The original, permanent shape can be recovered by application of heat. [1][2][3][4][5] Heating could be realized directly by an increase in environmental temperature or indirectly by exposing to alternating magnetic fields [6,7] or illuminating with IR-light. [8] Shapememory polymers have a high innovation potential as biomaterial, especially for minimally invasive surgery. One basic concept in the development of biodegradable polymer systems with shape-memory properties is based on covalently crosslinked polymer networks, [9][10][11][12] containing switching segments from semi-crystalline poly(e-caprolactone) [10,11,13] or poly[(e-caprolactone)-co-glycolide]. [12] The latter ones enable an accelerated hydrolytic degradation because of the presence of easily hydrolizable ester bonds in the form of glycolate ester bonds. [12,14] Another approach is the development of AB copolymer photoset networks, which are obtained from poly(e-caprolactone)dimethacrylate with n-butylacrylate or cyclohexylmethacrylate by UV-crosslinking. [10,11] These polymer networks showed excellent shape-memory properties and as tested so far good tissue-compatibility, in cell culture and in vivo tests. [15][16][17][18][19] Amorphous polymer networks were developed based on poly[(L-lactide)-ran-glycolide)] chain segments. [20,21] This material is transparent and has a shape-memory capability, but its mechanical properties have to be further improved. Polymer networks from poly[(L-lactide)-ran-glycolide)]dimethacrylates (DM) tend to be brittle below the glass transition temperature T g of 55°C and are difficult to handle in the course of the network synthesis and extraction without destroying the samples. The incorporation of a second amorphous phase with a low glass transition temperature (T g,l ) into these materials, which keeps the material elastic, is a potential strategy to effectively improve the elastic properties of poly[(L-lactide)-ran-glycolide]-segment based networks. Formation of an amorphous mixed phase is supposed to allow an adjustment of the switching temperature for the shape-memory effect to a temperature range between roomand body temperature.In a first approach, to obtain dilactide-based amorphous polymer networks with increased toughness and elasticity at room temperature, multi-phase polymer networks were synthesized from amorphous and degradable ABA triblock macrodimethacrylates based on poly(rac-lactide)-b-poly(propylene oxide)-b-poly(rac-lactide). Atactic poly(propylene oxide) (PPG) with a number average molecular weight M n of 4000 g mol -1 and a low T g of -73°C formed the B-block while the molecular weight of A-blocks from poly(rac-lactide) was varied. Variation in this block length resulted in networks with a T g varying between 11°C ...

For polymer-based degradable implants, mechanical performance and degradation behavior need to be precisely controlled. Based on a rational design, this work comprehensively describes the properties of photo-crosslinked polymer networks prepared from poly(rac-lactide)-block-poly(propylene glycol)-block-poly(rac-lactide) dimethacrylate precursors during degradation. By varying the length of poly(rac-lactide) blocks connected to a central 4 kDa polyether block, microphase separated networks with adjustable crosslinking density, hydrophilicity/hydrophobicity ratio, thermal, and mechanical properties are obtained. The materials are characterized by a low water uptake, controlled mass loss, and slowly decreasing wet-state E moduli in the kPa range.

Structured dispersion particles suitable for pressure sensitive adhesives (PSA) were synthesized via swelling polymerization technique (EP 359562). Particles consisting of poly(n-butyl acrylate) copolymerized with different types of carboxylic acids were used as seeds. The final particles were synthesized by swelling polymerization process, using 6 wt % styrene or 6 wt % methyl methacylate. The resulting particle morphology was analyzed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). From previous works (Coll Surf A 2001, 183-185, 725-737; J Appl Polym Sci 2004, 91, 2610 -2623 where two-step emulsion polymerization was used on similar particles, it is expected that the particle morphology is affected by the polarity of the monomer used for swelling polymerization because of the phase compatibility (thermodynamic parameter). In this work, the seed particles used were always of a glass transition temperature (T g ) below polymerization temperature. The diffusion of the growing polymer chains from the swelling polymerization is therefore mainly affected by their own T g and the influence of the carboxy groups on the chain length of the entering radicals (kinetic parameter). The different morphologies of the single particles are discussed qualitatively. The effects of reaction parameters are compared with the results given in the previous work. The structure of the corresponding dispersion films was characterized using AFM. Correlations to macroscopic properties such as the cohesive strength and peel adhesion to different substrates are discussed. The results are also compared with the application properties of the corresponding unmodified particles, statistical copolymers, and to blends with small sized PMMA or PS particles.