Cross-linking elastic recovery

Polyethylene blends having toughness and elastic recovery comparable to those of plasticized PVC comprised >50 wt% of a copolymer of ethylene and either butene or hexene [LLDPE, p = 880-915 kg MI < 1 dg min , long-chain branching =0.5-1.5 long chalns/l.OOOC, M > 200 kg mol ] >5 wt% of a copolymer of ethylene and either vinyl acetate or ethyl acrylate, EVAc or EEA and 5-30 wt% liquid hydrocarbon oil. The blends showed essentially no yield point and behavior similar to that of cross-linked materials, although they were not cross-linked (strain recovery). They were found competitive with plasticized PVC in terms of both physical properties and economics... 

Fig. 3. Effect of cross-link density where A represents tear strength, fatigue life, and toughness B, elastic recovery and stiffness C, strength and D,...

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

The effect on resilience (approximate rate of recovery from deformation) of reducing is more complex. At relatively low degrees of cross linking, the system exhibits rubbery elasticity. As decreases due to further cross linking, T increases and as it approaches the test temperature, a point of maximum damping is achieved. Here the resilience is at a minimum. Further decrease in Me increases resiliency until the sample become an elastic solid. 

The properties of block copolymers differ from those of a blend of the correponding homopolymers or a random copolymer (Chapter 7) with the same overall composition. An important practical example is the ABA-type styrene/butadiene/styrene triblock copolymer. These behave as thermoplastic elastomers. Ordinary elastomers are cross-linked by covalent bonds, e.g., vulcanization (see Chapter 2) to impart elastic recovery property, as without this there will be permanent deformation. Such cross-linked rubbers are therraosets and so cannot be softened and reshaped by molding. However, solid thermoplastic styrene/butadiene/styrene triblock elastomers can be resoftened and remolded. This can be explained as follows. At room temperature, the triblock elastomers consist of glassy, rigid, polystyrene domains... 

Thermoplastic elastomers contain sequences of hard and soft repeating units in the polymer chain. Elastic recovery occurs when the hard segments act to pull back the more soft and rubbery segments. Cross-linking is not required. The six generic classes of TPEs are, in order of increasing cost and performance, styrene block copolymers, polyolefin blends, elastomeric alloys, thermoplastic urethanes, thermoplastic copolyesters, and thermoplastic polyamides. 

The cross-links formed in the molecular structure are key to the superior heat resistance of XLA. As the temperature increases, crystallites gradually disappear and cross-links take over keeping the network structure retention. After cooling down, crystallites will reform. This makes XLA very different from conventional melt-spun fibers, which rely on crystallites for both recovery and heat resistance. Figure 3.2 shows the percent tenacity retention of chlorine-treated XLA elastic fibers and elas-tane fibers evaluated under accelerated conditions. XLA elastic fiber retains more than 80% of the mechanical tenacity up to 40 hours of treatment. However, the mechanical property of elastane quickly deteriorates to 45% of its starting tenacity within only 10 hours. 

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 ,. 

Their results were explained on the basis of elastic recovery (it means cross-linking density) and crystallinity. EVA increased the amorphous fraction of the blends and enhanced its radiation cross-linking. Heat shrinkability increased with the increase in crystallinity and decreased with the increase in gel fraction. DMPTMA level, EVA content, and radiation dose decreased the heat shrinkability. The amnesia rating also decreased with increasing radiation dose and EVA content. 

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