Cross-link physical

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

This unusual behavior results from unsolvated crystalline regions in the PVC that act as physical cross-links. These allow the PVC to accept large amounts of solvent (plasticizers) in the amorphous regions, lowering its T to well below room temperature, thus making it mbbery. PVC was, as a result, the first thermoplastic elastomer (TPE). This mbber-like material has stable properties over a wide temperature range (32,138—140). 

The multiple emulsion technique includes three steps 1) preparation of a primary oil-in-water emulsion in which the oil dispersed phase is constituted of CH2CI2 and the aqueous continuous phase is a mixture of 2% v/v acetic acid solution methanol (4/1, v/v) containing chitosan (1.6%) and Tween (1.6, w/v) 2) multiple emulsion formation with mineral oil (oily outer phase) containing Span 20 (2%, w/v) 3) evaporation of aqueous solvents under reduced pressure. Details can be found in various publications [208,209]. Chemical cross-linking is an option of this method enzymatic cross-linking can also be performed [210]. Physical cross-linking may take place to a certain extent if chitosan is exposed to high temperature. 

When plastics act as a physical cross-link and strength properties are indirectly related to the modulus of hard phase and morphology of the blend, the filler effect is analyzed by the following equation ... 

Thomas V, Kumari TV, and Jayabalan M. In vitro studies on the effect of physical cross-linking on the biological performance of aliphatic polyfurethane urea) for blood contact applications. Biomacromolecules, 2001, 2, 588-596. 

B. Type 3 physical cross-linked gels, agarose... 

The structure of these gel-like systems of micelles is very different from that of conventional electrophoresis media made from chemically and physically cross-linked polymers of polyacrylamide and agarose [75], The absence of chemical or physical cross-links in the Pluronic gel-like phases may allow a larger degree of freedom for macromolecular transport around the obstacles that make up the medium than occurs in conventional electrophoresis media. 

Elasticity measurements can serve as a measure of the degree of interconnection in gels. Covalently cross-linked networks can be distinguished from physically cross-linked networks by the use of a technique termed mechanical spectroscopy [333]. Compression of gels has also been used to assess the physical structure [28,168,303]. 

Pure PHEMA gel is sufficiently physically cross-linked by entanglements that it swells in water without dissolving, even without covalent cross-links. Its water sorption kinetics are Fickian over a broad temperature range. As the temperature increases, the diffusion coefficient of the sorption process rises from a value of 3.2 X 10 8 cm2/s at 4°C to 5.6 x 10 7 cm2/s at 88°C according to an Arrhenius rate law with an activation energy of 6.1 kcal/mol. At 5°C, the sample becomes completely rubbery at 60% of the equilibrium solvent uptake (q = 1.67). This transition drops steadily as Tg is approached ( 90°C), so that at 88°C the sample becomes entirely rubbery with less than 30% of the equilibrium uptake (q = 1.51) (data cited here are from Ref. 138). 

These physical cross-links are, in general, not permanent and may disappear on swelling or increase in temperature. The corresponding networks are referred to as "physical" or "thermoreversible" and are not considered in this chapter. The reader may refer to Burchard and Ross-Murphy [17-19] for further information on such materials. 

The Material of the Example. Poly(ether ester) (PEE) materials are thermoplastic elastomers. Fibers made from this class of multiblock copolymers are commercially available as Sympatex . Axle sleeves for automotive applications or gaskets are traded as Arnitel or Hytrel . Polyether blocks form the soft phase (matrix). The polyester forms the hard domains which provide physical cross-linking of the chains. This nanostructure is the reason for the rubbery nature of the material. 

Formation of physical cross-links by the anchorage of chain ends in semicrystalline domains and production of permanent entanglements is shown in the HBIB block copolymers. No such arrangement exists for the inverted polymer HIBI. (No attempt has been made to show possible chain folding, or superstructure development of their... 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

Hydrophobic domain (physical cross linking point)... 

Turner and coworkers111 118 119 used specially purified rubber samples and irradiated them in the absence of air. They found the yield of the physical cross-linking to decrease with the dose the initial yield is G = 3.5. However, the yield of chemical cross-linking was found to be 1.3. 

PVA can be prepared by chemical or physical cross-linking general methods for chemical cross-linking are the use of chemical cross-linkers or the use of electron beams or y-... 

---