Physically cross-linked polymers composites

Although attempts have been made to investigate the morphological stracture of T -SMPUs, by smdying phase composition and microdomain sizes (the latter defining the physically cross-linked polymer netwoifcs), the structure has not yet been demonstrated systematically (Takahashi et al, 1996 Lin and Chen, 1998a,b Lee et al., 2001). 

Albertsson and coworkers [240-244] carried out extensive research to develop polymers in which the polymer properties are altered for different applications. The predominant procedure is ring-opening polymerization which provides a way to achieve pure and well defined structures. They have utilized cyclic monomers such as lactones, anhydrides, carbonates, ether-lactones. The work involved the synthesis of monomers not commercially available, studies of polymerization to form homopolymers, random and block copolymers, development of cross-linked polymers and polymer blends, surface modification in some cases, and characterization of the materials formed. The characterization is carried out with respect to the chemical composition and both chemical and physical structures, the degradation behavior in vitro and in vivo, and in some cases the ability to release drug components from microspheres prepared from the polymers. 

Physical immobilization methods do not involve covalent bond formation with the enzyme, so that the native composition of the enzyme remains unaltered. Physical immobilization methods are subclassified as adsorption, entrapment, and encapsulation methods. Adsorption of proteins to the surface of a carrier is, in principle, reversible, but careful selection of the carrier material and the immobilization conditions can render desorption negligible. Entrapment of enzymes in a cross-linked polymer is accomplished by carrying out the polymerization reaction in the presence of enzyme the enzyme becomes trapped in interstitial spaces in the polymer matrix. Encapsulation of enzymes results in regions of high enzyme concentration being separated from the bulk solvent system by a semipermeable membrane, through which substrate, but not enzyme, may diffuse. Physical immobilization methods are represented in Figure 4.1 (c-e). 

Moreover, such a hnear relationship tends to demonstrate that the actual cross-linking degree of the APCNs is only dependent on the initial molar fraction in PCLDMA crosshnkers introduced in the polymerization medium and therefore attests for the absence of additional (physical) cross-links that could be due to polymer chain entanglements. Therefore and owing to the control over the molecular parameters of the APCNs, a correlation between gel stracture and swelling behavior could be predicted from the Flory-Rehner relationship in the particular case of this study with a constant composition in PCL of about 30%. 

Figure 7.10. The effect of light on the dynamic properties of physical cross-links between azobenzene-modified poly(acrylate) and poly(cyclodextrin). The relaxation modulus in stress relaxation experiments is plotted after application at time zero of a fixed strain to a 0.7% polymer solution in water (polymer structure, cf. Fig. 7.1, with n = 11 and x=3) with polycyclodextrin at 0.25% (lower curves) or 0.5% (higher moduli). The sample was either incubated for 24h in the dark (dark-adapted, closed symbols) or continuously exposed to UV before and after loading in the rheometer (open symbols). Details on the samples composition are given in Pouliquen et al. (2007).

SMP based on miscible blends of semicrystalline polymer/amorphous polymer was reported by the Mather research group, which included semicrystalline polymer/amorphous polymer such as polylactide (PLA)/poly vinylacetate (PVAc) blend [21,22], poly(vinylidene fluoride) (PVDF)/PVAc blend [23], and PVDF/polymethyl methacrylate (PMMA) blend [23]. These polymer blends are completely miscible at all compositions with a single, sharp glass transition temperature, while crystallization of PLA or PVDF is partially maintained and the degree of crystallinity, which controls the rubbery stiffness and the elasticity, can be tuned by the blend ratios. Tg of the blends are the critical temperatures for triggering shape recovery, while the crystalline phase of the semicrystalline PLA and PVDF serves well as a physical cross-linking site for elastic deformation above Tg, while still below T ,. 

If stereocomplex formation were to occur only between I-PMMA and S-PMMA, then the mixing of isotactic-atactic-isotactic (lAtl)- and syndiotactic-atactic-syndiotactic (SAtS)-PMMA stereoblock polymers in complexing solvents should result in stereocomplexes imbedded in an atactic matrix (Figure 2). Here the stereocomplexes would act as physical cross-links providing a network structure from a single chemical composition. From an application viewpoint, such a material should possess interesting properties compared to conventional PMMA. 

For other water-based formulation such as tile adhesives, cements and grouts, a little more scope exists for selecting polymers in other physical forms. In these end uses, the composition should be easy to apply or spread on such as the rear surface of a tile, whilst having sufficient viscosity to resist the force of gravity on the tile placed on a vertical surface, until drying is complete. This requirement is well met by the extreme shear thinning profile of cross-linked polymers. Consequently, precipitation polymers or alkali-activated polymers are often selected. Inverse emulsions and LDPs less well suited as the small amount of hydrocarbon oil that would be present in a formulation based on this polymer type can be undesirable in the end use. 

Polyvinyl alcohol (PVA) is one of the most popular polymers used for electrospun composite fiber production due to its thermostability, excellent biocompatibility, and high mechanical performance [53-56], However, its applications are limited by its high hydrophilieity, through which it dissolves immediately on contact with water. Therefore, PVA nanofibers have usually been modified by either chemical or physical cross-linking to improve their water resistance and mechanical properties [57],... 

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