Copolymers glass transition behavior

Polynadimides. The exact structure of the network crosslink is somewhat controversial but it could resemble the structure shown in Fig. 10.6. These polymers cumulate all the problems encountered in other polymers Is it really pertinent to consider that we are in the presence of hexa-functional crosslinks In this case, how do we take into account their copolymer effect In fact, if the black junctions in Fig. 10.6 connect one crosslink directly to another, we are in the presence of crosslink lines rather than dispersed individual crosslinks. Does this feature modify the whole glass transition behavior There is, to our knowledge, no satisfactory answer to these questions, and the research field remains largely open in this domain. 

The glass transition behavior of copolymers can also be analyzed using the free volume concept. Block and graft copolymers usually have multiple glass transition temperatures that are near to the values of Tg for each constituent homopolymers. In contrast, random or statistical copolymers usually have a single Tg between that of the corresponding homopolymers. If a series of such copolymers are produced from two monomers by varying their relative... 

Lee, S.-G., Lee, J. H., Choi, K.-Y., and Rhee, J. M., Glass transition behavior of polypropylene/polystyrene/styrene-ethylene-propylene block copolymer blends, Polym. Bull, 40, 765-771 (1998). 

Historically, polymers have resulted from at least three stages of development (1) many new polymers were formed from a large supply of monomers, (2) various chemical techniques were utilized to copolymerize different monomers in order to gain various chemical and physical properties than were unavailable from the monomers alone, and (3) the plastics industry has ventured into blending two or more polymers to yield new plastics with properties superior to those of the blend s individual polymeric components. Here we describe the glass transition behavior of polymer blends and copolymers, but cannot present an exhaustive review. For that purpose the reader is directed to several excellent reviews (Noshay and McGrath 1977 Paul and Barlow 1982 MacKnight and Karasz 1989 Bates and Fredrickson 1990 Folkes and Hope 1993 Hale and Bair 1997). 

Dual glass transition behavior is observed in immiscible binary polymer blends or block copolymers which have undergtme microphase separation to create discrete domains of each type of polymer in the rrratrix. The itaconate structures considered here are comb-branch polymers with relatively short side chains which plasticize the polymer efficiently when the chain lengdis are Cj to Cfy but then undergo a subtle change in bdiavior at long chain lengths. While there is no apparent... 

Bowmer and Tonelli [161] have also observed that the magnitude of the glass transition (ACp) increases with the ethylene content of the copolymer, goes through a maximum at about 30 mol%, and then continually decreases until no glass transition is observed at more than 80 mol% of ethylene. This may constitute further evidence in favor of the explanations put forward by Naqvi for the thermal stability behavior of similar copolymers reported by Braun et al. [159]. Initially, with increasing content of nonpolar ethylene units in the co-... 

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. 

Park et al. [20] reported on the synthesis of poly-(chloroprene-co-isobutyl methacrylate) and its compati-bilizing effect in immiscible polychloroprene-poly(iso-butyl methacrylate) blends. A copolymer of chloroprene rubber (CR) and isobutyl methacrylate (iBMA) poly[CP-Co-(BMA)] and a graft copolymer of iBMA and poly-chloroprene [poly(CR-g-iBMA)] were prepared for comparison. Blends of CR and PiBMA are prepared by the solution casting technique using THF as the solvent. The morphology and glass-transition temperature behavior indicated that the blend is an immiscible one. It was found that both the copolymers can improve the miscibility, but the efficiency is higher in poly(CR-Co-iBMA) than in poly(CR-g-iBMA),... 

The nature of the hard domains differs for the various block copolymers. The amorphous polystyrene blocks in the ABA block copolymers are hard because the glass transition temperature (100°C) is considerably above ambient temperature, i.e., the polystyrene blocks are in the glassy state. However, there is some controversy about the nature of the hard domains in the various multiblock copolymers. The polyurethane blocks in the polyester-polyurethane and polyether-polyurethane copolymers have a glass transition temperature above ambient temperature but also derive their hard behavior from hydrogen-bonding and low levels of crystallinity. The aromatic polyester (usually terephthalate) blocks in the polyether-polyester multiblock copolymer appear to derive their hardness entirely from crystallinity. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

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. 

In the case of statistic copolymers of two monomers (binary copolymers) the glass transition temperature steadily changes with the molar amounts of the two monomers. In many cases, a similar behavior is observed with some mechanical properties (tensile strength, impact strength, stiffness, and hardness) (see Chap. 1). Deviations can occur in copolymers, which contain only a few percent of one comonomer. 

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