Physically cross-linked semicrystalline

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... 

PE-PEP diblock were similar to each other at high PE content (50-90%). This was because the mechanical properties were determined predominantly by the behaviour of the more continuous PE phase. For lower PE contents (7-29%) there were major differences in the mechanical properties of polymers with different architectures, all of which formed a cubic-packed sphere phase. PE-PEP-PE triblocks were found to be thermoplastic elastomers, whereas PEP-PE-PEP triblocks behaved like particulate filled rubber.The difference was proposed to result from bridging of PE domains across spheres in PE-PEP-PE triblocks, which acted as physical cross-links due to anchorage of the PE blocks in the semicrystalline domains. No such arrangement is possible for the PEP-PE-PEP or PE-PEP copolymers (Mohajer et al. 1982). 

In semicrystalline polymers, the crystallites may be regarded as physical cross-links that tend to reinforce or stiffen the structure. Viewed this way, it is easy to visualize that Tg will increase with increasing degree of crystallinity. This is certainly not surprising since the cohesive energy factors operative in the amorphous and crystalline regions are the same and exercise similar influence on transitions. It has been found that the following empirical relationship exists between Tg and Tj . 

The discussion above has been limited to amorphous polymers. However, if the polymer is semicrystalline, the dotted line in Figure 1.19 is followed. Since the crystalline regions in the polymer matrix tend to behave as a filler phase and also as a type of physical cross-link between the chains, the height of the plateau (i.e., the modulus) will be governed by the degree of crystallinity]... 

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 ,. 

The thermoplastic IPNs utilize physical cross-links, rather than chemical crosslinks. Usually, these materials will flow when heated to sufficiently high temperature (hence the terminology thermoplastic), but behave as thermosets at ambient temperature, with IPN properties, often possessing dual-phase continuity. Most often, physical cross-links are based on triblock copolymers (thermoplastic elastomers being the leading material), ionomers, or semicrystalline materials. 

A physical bond that joins two or more chains together. They may arise from crystalline portions of a semicrystalline polymer, the glassy or crystalline portion of a block copolymer, or the ionic portion of an ionomer. The physical cross-link forces are affected by the temperature. 

Thermoplastic IPNs. These materials utilize physical cross-links rather than chemical cross-links. Thus, the materials may be made to flow at elevated temperatures. As such, they are hybrids between polymer blends and IPNs. Such cross-links may involve block copolymers, ionomers, and/or semicrystallinity. 

Neutralization of ethylene copolymers containing up to 5%-10% acrylic or methacryUc acid copolymer with a metal salt such as the acetate or oxide of zinc, magnesium, and barium yields products referred to as ionomers. (Commercial products may contain univalent as well as divalent metal salts.) lonomers are marked by Du Pont under the trade name Surlyn. These have interesting properties compared with the nonionized copolymer. Introduction of ions causes disordering of the semicrystalline structure, which makes the polymer transparent. lonomers act like reversibly cross-linked thermoplastics as a result of microphase separation between ionic metal carboxylate and nonpolar hydrocarbon segments. The behavior is similar to the physical cross-linking in thermoplastic elastomers (see Chapter 1 of Industrial... 

The ability of the E-plastomers to participate in the peroxide-mediated chain-extension processes can be augmented in blends with EPDM, where the mixture is homogeneously cross-linked with free radicals [5]. The use of these blends of EPDM and E-plastomers leads to improved processing and physical properties of the combination, compared to the EPDM alone, though the resulting vulcanizates are somewhat harder than the EPDM vulcanizates alone due to the presence of the semicrystalline plastomers in the vulcanized mixture. 

As pointed out by Hajji et al. [32], polymer blend nanocomposite systems can be prepared by various synthesis routes because of their ability to combine in different ways to introduce each phase. The organic component can be introduced as (1) a precursor, which can be a monomer or an oligomer (2) a preformed linear polymer (in molten, solution, or emulsion states) or (3) a polymer network, physically (eg, semicrystalline linear polymer) or chemically (eg, thermosets, elastomers) cross-linked. The mineral part can be introduced as (1) a precursor (eg, tetraethyl orthosilicate) or (2) preformed nanoparticles. Organic or inorganic polymerization generally becomes necessary if at least one of the starting moieties is a precursor. 

Characterization of Poiymer Networks by Transverse Relaxation. The NMR transverse magnetisation relaxation experiments were extensively used for quantitative analysis of the density of chemical cross-links, temporary and trapped chain entanglements and physical network junctions which are formed in fllled rubbers, semicrystalline and ionic containing elastomers, and for determination of the molecular-scale heterogeneity of polymer networks ((83), and reference therein). 

---