Hydrolytic degradation conditions

Thus, an increase in polymer crystallinity should generally decrease polymer degradation rates, as extensively reported for PLA matrices under alkaline hydrolytic degradation conditions.Other researchers reported accelerated hydrolysis in neutral media with increasing polymer crystallinity for PLLA and PLLA/PDLA blends. A possible explanation to this behaviour has been related to the consideration that, upon crystallization of PLLA, hydrophilic terminal groups (-OH and -COOH) can be condensed in the amorphous area between the crystalline regions (see... 

By varying the material parameters, the hydrolytic degradation mechanism, behavior, and rate can be controlled. In other words, these parameters are carefully manipulated when PLA-based materials are biomedically and environmentally applied by using the hydrolyzability function. In the following section, the hydrolytic degradation conditions are temperature of 37°C and pH of about 7, unless otherwise specified. 

Hammer and coworkers prepared PEG-h-PCL polymersomes entrapping DXR (Fig. 11a). The release of DXR from the polymersomes was in a sustained manner over 14 days at 37 °C in PBS via drug permeation through the PCL membrane, and hydrolytic degradation of the PCL membrane [228]. The release rate of encapsulated molecules from polymersomes can be tuned by blending with another type of block copolymer [229]. Indeed, the release rate of encapsulated DXR from polymersomes prepared from mixtures of PEG- -PLA with PEG- -PBD copolymers increased linearly with the molar ratio of PEG- -PLA in acidic media (Fig. lib). Under acidic conditions, the PLA first underwent hydrolysis and, hours later, pores formed in the membrane followed by final membrane... 

Too little has been published about the flow properties of PET as a criterion for processing. The results of melt flow index (MFI) testing conditions do not correlate with the processing behavior in the case of PET. This may be caused by the discrepancy between the shear rates in testing and processing. MFI is defined as the amount of polymer melt (in g) extruded within 10 min through an orifice of specified diameter at a standard load and temperature. In the case of PET, this method was not very popular until recently due to the sensitivity of this material to hydrolytic degradation. 

As an alternative, stable high-coverage nonpolar RPC sorbents phases have been prepared by cross-linking hydrophobic polymers at the silica surface, either via free radical 143 or condensation 101 polymerization chemistry. In this case, the underlying silica becomes partly protected from hydrolytic degradation due to the presence of the hydrophobic polymer film coating that effectively shields the support material. Similar procedures have been employed to chemically modify the surface of other support materials, such as porous zirconia, titania, or alumina, to further impart resistance to degradation when alkaline mobile-phase conditions are employed. Porous polystyrene-divinylbenzene sorbents, be-... 

Once a protein has reached its correct location and has acquired its proper function, it usually has a limited lifetime, which may average only a few hours or a few days. The protein is then hydrolytically degraded back to its constituent amino acids.128 Defective and damaged proteins are usually degraded much more rapidly than are intact proteins.129 130 Under conditions of starvation, proteins are broken down more rapidly than usual to supply the cell with energy. 

Starch-free BSG was subjected to reaction with water (autohydrolysis) in a 2-L stainless steel Parr reactor model 4532 (Moline, IL), to cause the hydrolytic degradation of hemicelluloses, operating under optimized conditions (liquid-to-solid ratio of 8 1 [w/w], standard heating temperature profile up to 190°C, isothermal reaction at 190°C for 2.5 min) (2). After the reactor was cooled down, the oligosaccharide-containing liquor (OCL) was separated from the residual solid by filtration (Whatman no. 1 filter paper). 

To demonstrate polymerase activity in a model cell, Chakrabarti et al. [79] encapsulated polynucleotide phosphorylase in vesicles composed of dimyris-toylphosphatidylcholine (DMPC). This enzyme can produce RNA from nucleoside diphosphates such as adenosine diphosphate (ADP) and does not require a template, so it has proven useful for initial studies of encapsulated polymerase activity (Fig. 10a). Furthermore, DMPC liposomes are sufficiently permeable so that 5-10 ADP molecules per second enter each vesicle. Under these conditions, measurable amounts of RNA in the form of polyadenylic acid were synthesized and accumulated in the vesicles after several days incubation. The enzyme-catalyzed reaction could be carried out in the presence of a protease external to the membrane, demonstrating that the vesicle membrane protected the encapsulated enzyme from hydrolytic degradation. Similar behavior has been observed with monocarboxylic acid vesicles [80], and it follows that complex phospholipids are not required for an encapsulated polymerase system to function. 

Ni(II) in 74] that contribute to the polarization of the substrate molecule. Those interactions may also modulate the piCa of the urea upon coordination to avoid its premature deprotonation While O coordinated urea has almost the same piCa as the free molecule ( 14) (89, 90), N coordination was shown to dramatically increase urea acidity (down to p fa = 2-6 in aqueous solution cf. Section 111.A) (87). This result should be even more pronounced in the pi 3-N,0 mode. Even if one considers the relatively lower polarizing power of Ni(II) compared to the metals used in those studies, deprotonation is still likely to occur upon bidentate substrate under conditions optimal for urease (pH 4-8) if modulating interactions are not present. In that sense, model systems currently available are not mature yet. In view of these considerations, it may not be surprising that none of the model complexes mediates the direct hydrolytic degradation of parent urea. 

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