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Polymeric Biomaterials
Published in Joyce Y. Wong, Joseph D. Bronzino, Biomaterials, 2007
Hai Bang Lee, Gilson Khang, Jin Ho Lee
Polymers have very long chain molecules which are formed by covalent bonding along the backbone chain. The long chains are held together either by secondary bonding forces such as van der Waals and hydrogen bonds or primary covalent bonding forces through crosslinks between chains. The long chains are very flexible and can be tangled easily. In addition, each chain can have side groups, branches and copolymeric chains or blocks which can also interfere with the long-range ordering of chains. For example, paraffin wax has the same chemical formula as polyethylene (PE) [(CH2CH2)n], but will crystallize almost completely because of its much shorter chain lengths. However, when the chains become extremely long {from 40 to 50 repeating units [–CH2CH2–] to several thousands as in linear PE} they cannot be crystallized completely (up to 80 to 90% crystallization is possible). Also, branched PE in which side chains are attached to the main backbone chain at positions normally occupied by a hydrogen atom, will not crystallize easily due to the steric hindrance of side chains resulting in a more noncrystalline structure. The partially crystallized structure is called semicrystalline which is the most commonly occurring structure for linear polymers. The semicrystalline structure is represented by disordered noncrystalline (amorphous) regions and ordered crystalline regions which may contain folded chains as shown in Figure 3.1.
Polyanhydride Microspheres As Drug Delivery Systems
Published in Max Donbrow, Microcapsules and Nanoparticles in Medicine and Pharmacy, 2020
Edith Mathiowitz, Robert Langer
The “hot-melt” method can be used for encapsulation of both drugs and dyes. The organic solvents used (silicon and olive oils) are effective, because of the low solubility of various drugs in them. The microencapsulation procedure is reproducible with respect to yield and size distribution: the disadvantage of this approach is the moderate temperatures to which the drug must be exposed. One way to overcome this problem is to synthesize polymers with lower melting points. This is possible to achieve by changes in the backbone of the polymer.
Polymer Technologies
Published in Ghenadii Korotcenkov, Handbook of Humidity Measurement, 2020
The grafting is other approach used for polymers functioning (Uchida and Ikada 1996; Ranby 1999; Kang and Zhang 2000; Zhao and Brittain 2000). Among the surface-modification techniques developed to date, surface grafting has emerged as a simple, useful, and versatile approach to improve surface properties of polymers for many applications. According to Gopal et al. (2007), the grafting has the following advantages: (1) the ability to modify the polymer surface to have distinct properties through the choice of different monomers; (2) the controllable introduction of graft chains with a high density and exact localization to the surface, without affecting the bulk properties; and (3) long-term chemical stability, which is assured by covalent attachment of graft chains (Gopal et al. 2007). The latter factor contrasts with physically coated polymer chains that can in principle be removed rather easily. The experiment showed that surface grafting provides versatile techniques for introducing functional groups such as amine, imine, hydroxyl, carboxylic acid, sulfonate, and epoxide onto a broad range of conventional polymeric substrates, most of which have a nonpolar, less reactive surface (see Table 19.6). As is known, hydroxyl, amine, carboxyl, and sulfone groups are hydrophilic functional groups (Van der Bruggen 2009). Usually functional groups are localized on the side chains. A typical polymer consists of a backbone, typically made up of a main chain of long strands of monomer units—from ten to millions—and side chains. A side chain is simply a relatively short branch of the polymer molecule, usually several atoms or groups of atoms, that are connected to the polymer backbone. There may be a few or many of them. Sometimes even the branches (side chains) have branches (side chains). The presence of these side chains can affect the physical properties of a polymer. For example, high-density polyethylene, with its near-absence of side chains, is harder, more abrasion resistant, and will withstand higher temperatures, compared to low-density polyethylene that has numerous molecular branches or side chains. The functional groups introduced with help of side chains can be utilized to further reaction with small or large molecules through covalent or noncovalent linkage. Functionalization is achieved by either direct grafting of functional monomer or postderivatization of graft chains.
Preparation and performance analysis of novel liquid crystal alignment films based on macromolecular photosensitiser and photosensitive polyimide
Published in Liquid Crystals, 2021
Chengyun Yuan, Gang Zhu, Fei Wang, Xinjian Gong, Pengfei Geng, Zhenwei Gao, Yinghan Wang
The solubility test of BB-CA and BF-CA is carried out and the results are shown in Table 1. The results show that BB-OH and BF-OH have excellent solubility in polar solvents (such as DMF, NMP). According to the principle of similar compatibility, large polar hydroxyl groups increase the polarity of the polymer backbone, resulting in poor solubility of BB-OH and BF-OH in non-polar solvents (such as CH2Cl2). BB-CA and BF-CA are synthesised through the esterification of hydroxyl groups with CL, their solubility in non-polar solvents has been greatly improved. This is because the consumption of hydroxyl groups greatly reduces the polarity of the polymer backbone, resulting in increased solubility in non-polar solvents. Compared with BF-CA, the main chain of BB-CA is more rigid resulting in slightly poorer solubility in CH2Cl2. The solubility test shows that BB-CA and BF-CA have similar dissolution behaviour in NMP, which can be used to their common solvent.