Melting temperature of semicrystalline

SC CO2 used as a carrier of drug molecules into a polymer matrix has a number of advantages such as the plasticizing ability of CO2 (based on specific interactions between CO2 and polymer moieties), which both enhances the diffusion rates of drug molecules into the polymer and facilitates solvent removal. Polymer plasticization is accompanied by the swelling of the polymer matrix, with a concomitant increase in the free volume of the polymer. Moreover, SC CO2 can reduce the melting temperature of semicrystalline polymers. These effects are crucial to the impregnation and modification of polymeric materials. 

Thermoplastics are uncrosslinked plastics up to their decomposition temperature. Flow or melting (Fig. 2) occurs above the softening point of the amorphous structure in amorphous thermoplastics and above the melting temperature of semicrystalline thermoplastics. In this thermoplastic state, the viscous liquid can be processed. Form strength is achieved by cooUng. Meltdown, solidification, and crystallization can be repeated any number of times. 

The cavity temperature influences the impression directly. The closer the cavity temperature is to the softening temperature of amorphous materials or the crystalline melting temperature of semicrystalline thermoplastics, the more accurate can the molding compound replicate the mold surface. On the other hand, the economics of the cycle time must be considered, since a high cavity temperature means very long cycle times (extending to approximately 2% per degree). 

Figure 9.14 The melting temperature of semicrystalline polyethylene as a function of the reciprocal of lamellae thickness [1, 3].

Semicrystalline partial aromatic polyamides, 139 Semicrystalline polyesters, 45 Semicrystalline polymers, melting temperatures of, 33 Semirigid foams, 203 tests for, 244 Sensitization, 246... 

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. 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

Semicrystalline polymers, 20 351, 352, 588 toughness of, 20 354 Semicrystalline resins, 19 537 Semicrystalline thermoplastics, melting temperature of, 19 538t Semiefficient (semi-EV) sulfur... 

Flow surging can occur if the temperature of the screw becomes too high in the solids-conveying section. In general, the temperature of the screw in this section needs to be less than the Tg for amorphous resins or less than the melting temperature for semicrystalline resins. Small-diameter screws will typically operate at feed... 

The dependence of the experimentally observed ODT on the ordering temperature (final quenching temperature) was shown recently by rheology measurements on SI3 and SI low molecular weight samples [91]. This dependence resembles the dependence of the melting temperature in semicrystalline homopolymers on the crystallization temperature. The existence of an equilibrium transition temperature (Todt°) was demonstrated and a procedure, analogous to the known Hofmann-Weeks plot for crystalline polymers, was proposed as a means to calculate Todt°. The phenomenon was attributed to the fact that grains when produced at different temperatures have different sizes. 

All polymers showed semicrystalline behavior, as demonstrated by WAXD measurements. DSC thermograms were presented, but LC properties were found to be difficult to determine due to the concomitant thermally-induced crosslinking of the unsaturated polymers upon melting. In some cases smectic B phases were identified by POM. The melting temperatures of the saturated analogues were lower than for the corresponding unsaturated derivatives. Optical textures of the saturated polymers showed lancets in the background, typical of a solid-like smectic phase [89]. 

The stereochemically random presence of the Cl atom limits the crystallinity. In commercial resins, it is generally further modified by copolymerization, resulting in a semicrystalline material, with a glass transition temperature about 45°C and a melt temperature of about 190°C. It is sold by Allied Corp. under the name Aclar. PCTFE can be melt processed, though not easily. Its main advantage is its extremely good water vapor barrier, which is the best of any plastic film available. The WVTR at 38°C and 90% RH is 9.8-17.7 g pm/m2 d (0.025-0.045 g mil/100 in2 24 h). 

An identical crosslink density does not necessarily lead to the same effect of cross-linking on properti. This is particularly evident in a case when crosslinks are formed between segments of polymer chains which are in the amorphous ase of semicrystalline polymer below the melting temperature of polymer crystallites. Ifcre, the effect of crosslinking on crystallinity is Icwer. 

Lactide is the cycUc dimer of lactic acid, which exists as two optical active isomers d and l. L-Lactide is the naturally occurring isomer, while OL-lactide is the synthetic blend of D-lactide and L-lactide. The polymerization of these monomers leads to either a semicrystalline polymer or an amorphous polymer. Poly(L-lactide) (PLLA), for example, is a semicrystaUine polymer with a degree of crystallinity around 37%. It has a glass transition temperature of 60-65 C and a melting temperature of approximately 175 °C. Conversely, poly(DL-lactide) (DLPLA) is an amorphous polymer with random distribution of both isomeric forms of... 

As we raise the temperature above Tg, the amorphous regions within the polymer gain increased mobility. When the temperature of UHMWPE rises above 60-90°C, the smaller crystallites in the polymer begin to melt. The melting behavior of semicrystalline polymers, including UHMWPE, is typically... 

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