Block copolymer physical cross-links

Thermoplastic elastomers, as they are called, are constructed according to a similar principle. They are block copolymers in which a flexible block Tq < application temperature) occurs between two rigid blocks (Tq > application temperature). The different blocks are mutually incompatible. The rigid blocks form physical cross-links. At the application temperature, the material behaves as a cross-linked elastomer. At higher temperatures, the rigid blocks are also above the glass-transition temperature, the physical cross-links are lost, and the material can be deformed like a thermoplast. 

To ascertain control of the molecular weight, structure, and composition, block copolymers are usually synthesized in anionic polymerization. The block copolymers of conunercial interest are specifically prepared from monomers that upon polymerizatiOTi yield immiscible macromolecular blocks, a smaller one rigid and the other flexible. The rigid blocks form physical cross-links that upon heating above the transition point make the copolymer flow. Thus, these materials belong to the growing family of thermoplastic elastomers. 

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... 

Today, new copolymers are making use of the hard-soft block strategy where the hard segment is a block portion as PE that readily crystallizes forming a physical cross-link. The soft segment consists of blocks formed from alpha-olefins, such as 1-butene, 1-hexene, and 1-octane, where the substituted alkane-arm discourages crystallization. 

The major difference between block and graft copolymers is the position of the second kind of unit. Thus, information that applies to block copolymers can often be applied to graft copolymers. So, domains where physical cross-linking occurs via crystallization can occur in either block components or within graft copolymers where the necessary symmetry occurs. 

One of the most important discoveries relating to synthesis and physical behavior was made by Dr. Milkovich while at the Shell Development Co. He and his colleagues showed that triblock copolymers containing polystyrene-polydiene-polystyrene blocks in appropriate sizes could behave as a physically cross-linked but linear thermoplastic elastomer. Thus Dr. Milkovich was involved with two very crucial discoveries in this field. Interestingly, he received his M. S. degree at Syracuse with Professor Szwarc and his Ph.D. at Akron with Professor Morton. I was pleased that Dr. Milkovich accepted my invitation to be a plenary speaker at the symposium, along with Professors Szwarc and Morton. 

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). 

The situation is quite different with block copolymers. As an example we again take a copolymer of styrene and butadiene, but now as a three-block copolymer, SBS. The incompatibility of polystyrene and polybutadiene now results in a phase separation, which is enabled by the circumstance that the blocks can live their own life . The polystyrene chain ends clog together into PS domains, which lie embedded in a polybutadiene matrix. These glassy domains act as physical cross-links, so that the polymer has the nature of a thermoplastic rubber. The glass-rubber transitions of PS and BR both remain present in between these two temperatures the polymer is in a, somewhat stiffened, rubbery condition (see Figure 3.8). This behaviour is dealt... 

PEO-PPO-PEO triblock copolymers (Pluronics or Poloxamers) form reversible physically cross-linked hydrogels under certain concentration range and temperature. The use of this system in tissue engineering is scarce because of its inability to degrade. Di- or tri-block copolymers of PEG with PLA have been developed to overcome this problem. Multiple blocks of PEG and PLA, synthesized by condensation reaction of L-lactic acid in the presence of succinic acid. 

Polymersomes generated from simple AB block copolymers display inter-digitated membrane. The robust entanglement within the hydrophobic layer can be considered as a physical cross-link able to enhance the mechanical properties of those aggregates compared to liposomes [5]. Tri-block (BAB) hydrophobic-hydrophilic-hydrophobic copolymers are similar to diblock copolymers since there... 

In contrast, thermoplastic elastomers vulcanize by a physical cross-linking, that is, by formation of hard domains in a soft matrix. Here, hard and soft refer to glass transition temperatures relative to application temperatures. The properties of these thermoplastic elastomers follow directly from their structures. All thermoplastic elastomers (TPEs, plastomers) are copolymers with long sequences of hard and soft blocks. They can be block polymers, segment polymers, or graft polymers. 

Block copolymers often phase segregate into an A-rich phase and a B-rich phase. If one repeat unit (or phase) is a soft phase and the other is a hard glassy or crystalline phase, the result can be a thermoplastic elastomer. The crystalline or hard glassy phase acts as a physical cross-link. The... 

Block copolymers of the type (sty) -(bu) -(sty) form thermoplastic elastomers, i.e., physically, and therefore reversibly, cross-linked products (see Section 5.5.4). They are produced by anionic polymerization, since the physical cross-linking properties can only be achieved by long, molecularly homogeneous blocks, and thus only with specific ratios of m/n. 

Thermoplastic IPN Polymer alloy, containing two or more polymers in a co-continuous network form, each physically cross-linked. The cross-linking originates in crystallinity, ion cluster formation, presence of hard blocks in copolymers, etc. 

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. 

A process in which melted plastic is injected into a mold cavity, where it cools and takes the shape of the cavity. Bosses, screw threads, ribs, and other details can be integrated, which allows the molding operation to be accomplished in one step. The finished part usually does not require additional work before assembling. Any IPN in which the individual polymers are thermoplastic. The polymers may contain physical cross-links as in ionomers where ionic clusters join two or more chains together. Nowadays, phase-separated polymeric systems, e.g., block and graft copolymers or thermoplastic polyurethanes, are frequently considered thermoplastic IPNs. 

Block copolymers are useful in many applications where a number of different polymers are connected together to yield a material with hybrid properties. For example, thermoplastic elastomers are block copolymers containing a rubbery matrix (polybutadiene or polyisoprene) containing glassy hard domains (often polystyrene). The block copolymer, a kind of polymer alloy, behaves as a rubber at ambient conditions, but can be molded at high temperatures because of the presence of the glassy domains that act as physical cross-links. In solution, attachment of a water-soluble polymer to an insoluble polymer leads to the formation of micelles in amphiphilic block copolymers. The presence of micelles leads to structural and flow characteristics of the polymer in solution, that differ from either parent polymer. 

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