Copolymers crystallization rates

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

By using diamide units, melt phasing can be avoided and at the same time the crystallization rate is rapid. Surprisingly, diese segmented copolymers widi diamide units not only crystallize quickly but also have a high fracture strain... 

According to Hoffman s crystallization theory, a drop in the heat of fusion corresponds to an exponential decrease in nucleation and crystal growth rates [63]. Implicitly, the rate of crystallization is severely retarded by the presence of 3HV comonomer [64, 69, 72]. These low crystallization rates can hamper the melt processing of these copolymers since they necessitate longer processing cycle times. 

Recrystallization of a copolymer having 15 wt % VC has been found to be nucleated by material that survives the melting process plus new nuclei (74). The maximum crystallization rate occurred at 373 K the maximum nudeation rate at 283 K. Attempts to melt all the polymer led to degradation that interfered with recrystallization. 

Physical properties are related to ester-segment structure and concentration in thermoplastic polyether-ester elastomers prepared hy melt transesterification of poly(tetra-methylene ether) glycol with various diols and aromatic diesters. Diols used were 1,4-benzenedimethanol, 1,4-cyclo-hexanedimethanol, and the linear, aliphatic a,m-diols from ethylene glycol to 1,10-decane-diol. Esters used were terephthalate, isophthalate, 4,4 -biphenyldicarboxylate, 2,6-naphthalenedicarboxylate, and m-terphenyl-4,4"-dicarboxyl-ate. Ester-segment structure was found to affect many copolymer properties including ease of synthesis, molecular weight obtained, crystallization rate, elastic recovery, and tensile and tear strengths. 

The stereoregularity—i.e., distribution of the stereosequence length in these polymers—has a marked effect on the crystallization rates and the morphology of the crystalline aggregates. These differences, in turn, influence the dynamic mechanical properties and the temperature dependence of the dynamic mechanical properties. In order to interpret any differences in the dynamic mechanical properties of polymers and copolymers of propylene oxide made with different catalysts, it was interesting to study the differences in the stereosequence length in the propylene oxide polymers made with a few representative catalysts. 

A limited number of patents concern sPS blend which are not compatible. In these cases the properties that are described are functional ones and not related to the poor toughness of sPS. For instance, blends of sPS and partially saponified ethylene-vinyl acetate copolymers exhibit improved gas barrier properties (entry 11) small amounts of sPS added to polyethylene terephthalate) (although the patent actually claims a wide range of compositions) are useful to increase the polyester crystallization rate (entry 5). 

Gausepohl et ah [31] investigated the behavior of blends between sPS and random styrene-l,l-diphenylethylene copolymers obtained by anionic synthesis. The blends were miscible for copolymer contents of 1,1-diphenylethylene lower than 15 wt% as indicated by the occurrence of a single Tg (114°C). Tm and crystallization rate were not influenced. 

The orientation of crystalline stems with respect to the interface of the microstructure in block copolymers depends on both morphology and the speed of chain diffusion, which is controlled by block copolymer molecular weight and the crystallization protocol (i.e. cooling rate). In contrast to homopolymers, where folding of chains occurs such that stems are always perpendicular to the lamellar interface, a parallel orientation was observed for block copolymers crystallized from a lamellar melt phase perpendicular folding was observed in a cylindrical microstructure. Both orientations are shown in Fig. 8. Chain orientation can be probed via combined SAXS and WAXS on specimens oriented by shear or compression. In PE, for example, the orientation of (110) and (200) WAXS reflections with respect to Bragg peaks from the microstructure in the SAXS pattern enables the unit cell orientation to be deduced. Since PE stems are known to be oriented along the c axis, the chain orientation with respect to the microstructure can be determined. 

The effects of melt temperatures of the films before quenching on the crystallization curves were compared at 80° and 100°C. The latter is higher than the glass transition temperature (Tg) of PS block, and the former is lower than the Tg. However, there were no substantial differences in the observed rate curves. It appears that melting above or below Tg of the PS block does not cause substantial change in the size of the PTHF domains. Therefore, the crystallization rate (G) of the block copolymers may be approximated by the equation for homopolymer crystallization (9) ... 

The crystallization behavior and kinetics under isothermal conditions of iPP/SBH and HDPE/SBH blends, compatibilized with PP-g-SBH and PE-g-SBH copolymers, respectively, have been investigated (71). It has been established that the LCP dispersed phase in the blends plays a nucleation role for the polyolefin matrix crystallization. This effect is more pronounced in the polypropylene matrix than in the polyethylene matrix, due to the lower crystallization rate of the former. The addition of PP-g-SBH copolymers (2.5-10 wt%) to 90/10 and 80/20 iPP/SBH blends provokes a drastic increase of the overall crystallization rate of the iPP matrix and of the degree of crystallinity. Table 17.4 collects the isothermal crystallization parameters for uncompatibilized and compatibilized iPP/SBH blends (71). On the contrary, the addition of PE-g-SBH copolymers (COP or COP 120) (2.5-8 wt%) to 80/20 HDPE/SBH blends almost does not change or only slightly decreases the PE overall crystallization rate (71). This is due to some difference in the compatibilization mechanism and efficiency of both types of graft copolymers (PP-g-SBH and PE-g-SBH). The two polyolefin-g-SBH copolymers migrate to blend interfaces and... 

The previous results have shown that the addition of PE-g-SBH copolymer compa-tibilizer to HDPE/SBH blends in a nonoriented state does not lead to an increase of the crystallization rate (71). However, the overall PE crystallization rate under... 

The thioether ketone/thioether sulfone copolymer exhibits a reduced crystallinity and a high melting point, but a glass transition temperature higher than that of the corresponding PTK. When proportion of the 4,4 -di-halobenzophenone to the 4,4 -dihalodiphenyl sulfone is selectively limited to a specific range, a copolymer moderately reduced in crystallization rate can be obtained in the form of granules. 

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