Noncrystalline block copolymers

Kinetics of Aggregation and Dimensions of Supramolecular Structure in Noncrystalline Block Copolymers... 

Noncrystalline aromatic polycarbonates (qv) and polyesters (polyarylates) and alloys of polycarbonate with other thermoplastics are considered elsewhere, as are aHphatic polyesters derived from natural or biological sources such as poly(3-hydroxybutyrate), poly(glycoHde), or poly(lactide) these, too, are separately covered (see Polymers, environmentally degradable Sutures). Thermoplastic elastomers derived from poly(ester—ether) block copolymers such as PBT/PTMEG-T [82662-36-0] and known by commercial names such as Hytrel and Riteflex are included here in the section on poly(butylene terephthalate). Specific polymers are dealt with largely in order of volume, which puts PET first by virtue of its enormous market volume in bottie resin. 

A new class of elastomers uses block copolymers, which contain segments of different monomers. These form crystalline and noncrystalline regions in the polymer, and these polymers have elastomeric properties, as well as high fracture toughness. 

Polymer chain segments of pure PP and pure PE placed one after the other form block copolymers that have an increased degree of crystallinity. Depending on the manufacturing process, copolymers with an ethylene fraction of up to 30 % can also be noncrystalline, thus forming an ethylenepropylene elastomer. 

One means of this estimation depends on the ability to establish a gradient of magnetization, m r, t) between different types of domains, and subsequently monitor the resultant approach to equilibrium. Examples include crystalline and noncrystalline domains within a homopolymer, and block copolymers where dimensions of separated phases are controlled by block length and polymer blends. The usual diffusion equation describes the behaviour of the magnetization as it approaches equilibrium from an initial nonequilibrium state,... 

Linear low-density polyethylene (LLDPE) and PS resins were the same as described previously (Chapter 21). The various block copolymers that were used as compatibilizers have also been described (I). A series of crystalline copolymers (Q series) was prepared by hydrogenation of diblock and triblock copolymers of styrene and butadiene [styrene-hydrogenated butadiene (SEB) and styrene-hydrogenated butadiene—styrene (SEBS)J (1). Triblock copolymers of styrene and butadiene [styrene-butadiene-styrene (SBS)] and a noncrystalline hydrogenated block copolymer (SEBS) (Kraton) were supplied by Shell Chemical Co. Diblock copolymers of styrene and butadiene [styrene-butadiene (SB) (Vector)] were obtained from Dexco Polymers. The characteristics of the resins are given in Table I. 

Crystallization Kinetics. Typical isothermal crystallization curves, which were measured using cast films of both types of copolymers, are presented in Figures 9 and 10. At the early stage of crystallization, the effect of the noncrystalline PS block on the rate curves was only shifts of the degree of crystallinity [1 — A(t)] vs. time curve along the time axis. However, the extent of shift does not correspond to PS content in the block copolymers. The change in crystallization temperature also causes the crystallization curves to shift. At the initial stage of crystallization, these rate curves could be superimposed on the rate curve of the homo-PTHF (Mn = 6 X 104), and an Avrami exponent... 

A morphological term used in noncrystalline systems, such as block copolymers, in which the chemically different sections of the chain separate, generating amorphous phases. 

Consider now a block copolymer composed of two chemically dissimilar blocks each of which is noncrystalline. The same factors that are involved in homopolymer mixing will still be operative so that phase separation would be a priori expected. However, since the sequences in the block copolymer are covalently linked, macrophase separation characteristic of binary blends is prevented. Instead, microphase separation and the formation of separate domains will occur. The linkages at the A-B junction points further reduce the mixing entropy. There has to be a boundary between the two species and the junction point has to be placed in this interphase. The interphase itself will not be sharp and will be composed of both A and B units. Mixing of the sequences, and homogeneity of the melt, will be favored as the temperature is increased. There is then a transition temperature between the heterogeneous and homogeneous melt, known as the order-disorder transition. 

Experimental evidence has shown that the levels of crystallinity, based on the crystallizable blocks, are usually well below unity and are comparable to the values found with the corresponding homopolymers. The chain units of the crystallizable and noncrystallizable blocks mix in the noncrystalline phase. For sufficiently long chain lengths the observed melting temperatures of the crystalline components in block copolymers are comparable to and approach those of the corresponding homopolymers. These widespread observations are not consistent with nor can they be explained by regularly folded chain structures.(252)... 

For copolymers, as a general rule, the more irregular and random the repeat unit arrangements, the greater is the tendency for the development of noncrystallinity. For alternating and block copolymers there is some likelihood of crystallization. However, random and graft copolymers are normally amorphous. 

In semicrystalline block copolymers, the presence of a noncrystalline block enables modification of the mechanical and structural properties compared to a crystalline homopolymer, through introduction of a rubbery or glassy component. Crystallization in homopolymers leads to an extended conformation, or to kinetically controlled chain folding. In block copolymers, on the other hand, equilibrium chain folding can occur, the equilibrium number of folds being controlled by the size of the second, noncrystallizable block. The structure of block copolymers following crystallization has been reviewed [1,145]. 

Figure 1 Schematic outline of microphase separation in an ABA block copolymer in which both components are noncrystalline (see text)...

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