Semicrystalline block composition

Effects of Variation of Composition and Block Sequence on Properties of Copolymers Containing Semicrystalline Block(s)... 

Figure 13.5 shows the variation of melting and crystallization temperatures corresponding to each semicrystalline block within PLLA-6-PCL diblock copolymers as a function of composition [34]. For comparison purposes, solution blends of PCL and PLLA homopoljuners of equivalent molecular weights to those of the diblock copolymers were prepared and their characteristic transition temperatures were also reported in Fig. 13.5. This figure shows that the prepared PLLA and PCL blends are immiscible for the compositions examined as can be gathered by the invariance of the melting points associated with each homopolymer. Instead, the diblock copolymers exhibit signs of miscibility. In particular, the melting temperature of the PLLA block decreases as the content of PCL increases, and in the case of the L32C68 sample, the melting point depression of the PLLA block reaches 11°C.

In this study, the effects of the variations in block sequence and composition (and thus relative block length) on the material properties of two series of triblock copolymers has been investigated. One of the blocks, the hydrogenated polybutadiene (HB), is semicrystalline, and the other block, the hydrogenated polyisoprene (HI) is rubbery at room temperature. Thus in one series, the HBIB block copolymers, the end blocks are semi-... 

The dynamic mechanical behavior indicates that the glass transition of the rubbery block is basically independent of the butadiene content. Moreover, the melting temperature of the semicrystalline HB block does not show any dependence on composition or architecture of the block copolymer. The above findings combined with the observation of the linear additivity of density and heat of fusion of the block copolymers as a function of composition support the fact that there is a good phase separation of the HI and HB amorphous phases in the solid state of these block copolymers. Future investigations will focus attention on characterizing the melt state of these systems to note if homogeneity exists above Tm. 

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

Binary composite membranes constitute the chief example of membranes classified under (b) in the introductory section. They include binary polymer blends or block or graft copolymers exhibiting a distinct domain structure, filled or semicrystalline polymers and the like. 

The properties of block copolymers, on the other hand, cannot be calculated without additional information concerning the block sizes, and whether or not the different blocks aggregate into domains. The results of calculations using the methods developed in this book can be inserted as input parameters into models for the thermoelastic and transport properties of multiphase polymeric systems such as blends and block copolymers of immiscible polymers, semicrystalline polymers, and polymers containing various types of fillers. A review of the morphologies and properties of multiphase materials, and of some composite models which we have found to be useful in such applications, will be postponed to Chapter 19 and Chapter 20, where the most likely future directions for research on such materials will also be pointed out. 

The Chow equations, which constitute a large set that is too long and complex to reproduce here, are sometimes more accurate. Both of these sets of general-purpose equations (Halpin-Tsai and Chow) are applicable to many types of multiphase systems including composites, blends, immiscible block copolymers, and semicrystalline polymers. Their application to such systems requires the morphology to be described adequately and reasonable values to be available as input parameters for the relevant material properties of the individual phases. 

The semicrystalline PCL and branched amorphous PCL were also combined in a block copolymer structure giving transparent elastomeric films. The block copolymers were made by RROP of MDO or copolymerization of MDO with MMA using a semicrystalline PCL azo initiator. The polymers were degradable, transparent, and elastic, depending upon the copolymer composition and block length (Figure 2.7) [69]. 

Copolymerization provides a route for making polymers with special, desired property profiles. A statistical copolymer consisting of units A and B, for instance, has in most cases properties in between those of the homopolymers (polyA and polyB). An important deviation from this simple rule arises if either polyA or polyB is semicrystalline. The statistical copolymer (polyA-sfflt-B)) is for most compositions fully amorphous. Block and graft copolymers form in most cases a two-phase morphology and the different phases obey properties similar to those of the respective homopolymers. 

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