PEO-PEE

The membrane elastic behavior of PEO-PEE giant polymersomes has been studied by a micropipette aspiration method [5], The results showed that the polymer membrane elasticity is comparable to fluid-state lipid membranes however the vesicles could sustain a greater strain before rupture, proving an enhanced polymersome toughness, which originates from membrane thickness. 

Bermudez et al. [135] studied the effect of the membrane thickness d (and thus indirectly the molecular mass MW) of PEO-PEE vesicles on the membrane bending rigidity (Kc). The scaling between Kc and d was nearly quadratic, in agreement with the established theories for lipid bilayers, hence providing a way to alter the vesicle properties by choosing the polymer blocks. 

Principally, encapsulation of proteins within the aqueous lumen of polymersomes can benefit from the extended circulation kinetics and controlled release properties of polymersomes. Neutral diblock (PEO-PDB, PEO-PEE), charged triblock (PEO-... 

The ability to encapsulate large aqueous volumes and consequently whatever is dissolved in it is one prerogative of polymersomes and represents their most promising ability. Nevertheless, while for hydrophobic and amphiphilic molecules the encapsulation process is more or less straightforward, the complex kinetics of polymersomes formations hampers the encapsulation of hydrophilic molecules. Clearly the most important parameter to consider in encapsulating water-soluble molecules is the polymersome membrane permeability. Early work carried out by Discher and co-workers [6] showed that the water permeability of PEO-PEE polymersomes was established by measuring the reduction in polymersome swelling as a function... 

Figure 17 Schematic illustration of a multicompartment micelle from (PE0)(PEE)(PFP0) miktoarm stars and binary blends of (PE0)(PEE) (PFPO) with PEO (a) hamburger micelle from the miktoarm star with a very long PEO block (0), (b) segmented wormlike micelle from the star with a short PEO block (0), and (c) hamburger micelle from blends (PEO) (PEE)(PFPO)/PEO. Reprinted from Li, Z. Hillmyer, M. A. Lodge, T. P. Macromolecules 2006, 39, 765. " °...

Amphiphilic block copolymers consisting of a hydrophobic (poly(ethyl ethylene) (PEE) and a hydrophilic polyethylene oxide)(PEO) block form monolayers at the air-water interface. The schematic molecular arrangement of this diblock is shown in Fig. 3.26. 

Fig. 3.26 Schematic of the molecular arrangement of PEE-PEO monolayer and its changes on compression. (From ref. [129])...

The starting material represents a polyblock PEE comprising PBT as the hard segments and PEO (with a molecular weight of 1000 and polydispersity of 1.3, according to GPC analysis) (Fakirov et al, 1991) as the soft segments in a ratio of 57/43 wt%. The sample was a bristle, drawn at room temperature to five times its initial length, and then annealed with fixed ends for 6 h in vacuum at a temperature of 170 °C. WAXS and microhardness measurements were performed in the same way as for the homo-PBT. 

The microhardness, degree of crystallinity and the percentage of a and p phase obtained are summarized in Table 6.3. The dependence of the microhardness H on the deformation s for drawn and annealed bristles of PEE (PBT/PEO = 57/43 wt%) is plotted in Fig. 6.5. One can see that the H variation can be split into several regimes depending on the stress applied. For the lowest deformations (e up to 25%) H is nearly constant ( 33 MPa) and thereafter in a very narrow deformation interval ( = 2-3%) H suddenly drops by 30% reaching the value of 24 MPa which is maintained in the s range between 30 and 40%. With further increases in s... 

Figure 6.5. Microhardness H vi overall relative tensile deformation e of drawn and annealed bristles of PEE (PBT/PEO = 57/43 wt%). (From Apostolov et al, 1998.)...

Similar changes can be found on the equatorial WAXS scans of drawn and annealed PEE (PBT/PEO = 57/43 wt%) taken at various tensile deformations (see Fig. 6.6). Once again, drastic changes, both with respect to the peak shape and peak angular position, can be detected at = 28.8 and 58.8%. The relative crystallinity Wc (for these drawn samples) from WAXS at various stages of sample deformation is presented in Table 6.3 since calorimetry or density measurements cannot be used for strained samples. 

In addition to the assumed microhardening of the non-crystalline phases, the crystallization of the PEO soft segments in PEE acts simultaneously in the same deformation range as can be concluded from the analysis of WAXS data (see Fig. 6.6). The scattering curves at = 28.8% and = 58.8% are different from those at lower deformations. Their shape indicates overlapping of two independent reflections from two different unit cells. The lower-angle one arises from the ... 

Figure 6.7. Model of the PEE block copolyester structure with hard (- - - -) PBT segments and soft (-0-0-0-) PEO segments in equimolar amounts. Basically four phases are possible because both the PBT and PEO are crystallizable. (From Fakirov et al., 1990.)...

After following the microhardness behaviour during the stress-induced polymorphic transition of homo-PBT and its multiblock copolymers attention is now focused on the deformation behaviour of a blend of PBT and a PEE thermoplastic elastomer, the latter being a copolymer of PBT and PEO. This system is attractive not only because the two polymers have the same crystallizable component but also because the copolymer, being an elastomer, strongly affects the mechanical properties of the blend. It should be mentioned that these blends have been well characterized by differential scanning calorimetry, SAXS, dynamic mechanical thermal analysis and static mechanical measurements (Apostolov et al, 1994). 

The same PEE was used as in the preceding section (PBT/PEO = 49/51 wt%). For the preparation of the blend, both, the homopolymer PBT and PEE were cooled in liquid N2 and finely ground. PBT was blended with PEE in the ratio PBT/PEE = 51/49 wt%. Bristles of the blend were prepared using a capillary rheometer, flushed with argon and heated to 250 °C. The melt obtained was kept in the rheometer for... 

Two emulsifier systems were used for the preparation of O/W emulsions. The first emulsifier was Synperonic PEE 127, an A-B-A block copolymer of poly(ethylene oxide), PEO (the A chains, were about 100 EO units each) and propylene oxide PPO (the B chain was about 55 PO units), supplied by UNIQEMA. The second emulsifier system was Arlatone V-100 (supplied by UNIQEMA), a nonionic emulsifier system composed of a blend of Steareth-100 (stearyl alcohol with 100 EO units), Steareth-2 (stearyl alcohol with 2 EO units), glyceryl stearate citrate, sucrose, and a mixture of two polysaccharides, namely mannan and xanthan gum (Keltrol F, supplied by Kelco). In some emulsions, xanthan gum was used as a thickener. AH emulsions contained a preservative (Nipaguard BPX). 

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