Anchor segment

Diblock copolymers consist of one sequence of anchor segments and a second sequence of backbone segments. The relative lengths of the two sequences can be controlled to provide a wide variety of adsorption and barrier characteristics. Typical commercial dispersants may use alkane... 

Yamashita et al.2,99) also investigated the copolymerization of PMMA macromonomers both with a mixture of HEMA and perfluoroalkyl acrylate and with a MMA-methacrylic acid mixture here again, the PMMA grafts originating from the macromonomer play the role of anchoring segments, and surface accumulation of the functional backbone segments is well established. 

Another important cluster for microsomal anchoring frequently consists of a group of prolines (Pro-Pro-X-Pro) that form a hinge between the N-terminal hydro-phobic membrane anchoring segment (highly variable) and the globular part of the protein. 

Another example of surfactant adsorption quantification of XPS was described by Coombes et al. [38]. They introduced the concept of using PEG as anchoring segments as part of PEG-dextran (PEG-DEX) conjugates. The CIs XPS data in Fig. I0 shows that the incorporation of PEG-DEX could be observed... 

Covalent links or acid-base interactions are alternatives to physical adsorption of the anchor segments of the dispersants. Such procedures are valuable in special cases, where other factors compensate for added complication of the polymerization process. 

The adsorption of block and graft copolymers is more complex, as the intimate structure of the chain will determine the extent of adsorption [37]. Random copolymers adsorb in an intermediate fashion compared to that of the corresponding homopolymers. Block copolymers retain the adsorption preference of the individual blocks. The hydrophilic block (e.g., PEO the buoy) extends away from the particle surface into the bulk solution, while the hydrophobic anchor block (e.g., PS or PPO) provides a firm attachment to the surface. Figure 6.14 shows the theoretical prediction of diblock copolymer adsorption according to SF theory. In this case, the surface density cr was plotted versus the fraction of anchor segments v, and adsorption was shown to depend on the anchor/buoy composition. 

Figure 6.15 Adsorbed amount (mgm versus fraction of anchor segment for an A-B-A triblock copolymer (PEO-PPO-PEO).

Figure 6.18 shows the hydrodynamic thickness versus fraction of anchor segment for an ABA block copolymer of PEO-PPO-PEO [37]. The theoretical (SF) predictions of adsorbed amount and layer thickness versus fraction of anchor segment are shown in the inserts of Figure 6.18. In the presence of two buoy blocks and a central anchor block (as in the above example), the A-B-A block showed a similar...

Figure 6.18 Hydrodynamic thickness versus fraction of anchor segment for PEO-PPO-PEO block copolymer onto polystyrene latex. The inset shows the mean field calculation of thickness versus anchor fraction, using SF theory.

The sialyltransferases are membrane-bound proteins located in the endoplasmic reticulum (ER) and in the Golgi apparatus. Information about their sequence homology is limited, but they do appear to share a common topography [35]. A catalytic domain resides at the C-terminus followed by an N-terminal segment that anchors the enzyme into the ER or Golgi membrane. Soluble, catalytically active sialyltransferases that lack the anchor segment have been isolated from milk, serum, and other body fluids, suggesting that this N-terminal anchor is not necessary for the enzyme to retain catalytic activity. However, the ability to obtain from natural sources quantities of most sialyltransferases that would be needed for synthesis applications is hampered by low tissue concentrations and difficult purifications. 

Effective surfactants in CO2 are amphipatic macromolecules containing an anchoring segment (the so-called lipophilic anchor) that became adsorbed on the surface of the polymer phase by grafting and/or physical adsorption, and a stabilizing portion that is soluble in the continuous medium. The stabilizing moiety is usually constituted by a fluorinated or silicone chain. 

Fig. 27.5. The major portion of the sucrase-isomaltase complex, containing the catalytic sites, protrudes from the absorptive cells into the lumen of the intestine. Other domains of the protein form a connecting segment (stalk), and an anchoring segment that extends through the membrane into the cell. The complex is synthesized as a single polypeptide chain that is split into its two enzyme subunits extracellularly. Each subunit is a domain with a catalytic site (sucrase-maltase) and isomaltase-maltase sites. In spite of their maltase activity, these catalytic sites are often called just sucrase and isomaltase.

Nmr relaxation measurements allow the molecular motions of the segments of a polymer molecule to be investigated. The spectral line width, which is proportional to I/T2 (i.e. the reciprocal of the spin-spin relaxation time of proton nmr) is a direct measure of segment mobility. It would be anticipated that the anchor segments in the trains attached to the surface of the adsorbent particle would display low mobility. On the other hand, those segments in loops and tails that project into the continuous phase should possess much higher mobility. Two distinct linewidths would therefore be expected for the two different types of segments. 

The 1,1-dihydroperfluorooctylacrylate side chain is C02-philic and the acrylate backbone forms thfe anchor segment. A dispersion polymerization of MMA in supox ritical CO2 using PFOA was effective in producing monodisperse PMMA microspheres with diameters around 2 pm. 

As I have shown, the origin for the finite amorphous fraction formed by the (prevaiUng) loops is due to the balance between the entropic spring force created by the finite separation of the anchoring segments on the one hand side and the effort to remove the loop segments from the thermodynamically preferred crystalline phase on the other side. 

Figure 7 Predictions of Scheutjens and Fleer theory for the adsorption of diblock copolymers surface density o versus fraction of anchor segment v. (Reproduced with permission from Ref. 16.)... 

Figure 9 Adsorbed amount versus fraction of anchor segment for poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) ABA triblock copolymers adsorbed onto poly(styrene latex). Inset shows the mean-field calculations of surface coverage versus fraction of anchor segment Vx obtained using Scheutjens and Fleer theory. (J. A. Shar, T. Obey, and T. Cosgrove Colloids and Surfaces. In press.)... 

Numerous examples of these systems have been studied experimentally. Tirrell et ah measured the adsorption of AB copolymers of polystyrene and polyvinylpyridine on mica. A wide range of molecular weights was tested, and it was found that, for most samples, about 200 ng/cm of polymer adsorbed to the surface, and that the buoy segments easily formed polymer combs , unless the size of the anchoring segment was too large. The maximum surface density was found with small... 

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