Slow crystallizing polymers

Polybutylene polymers slow crystallization rates are helpful in forming hot-melt adhesives with long open times [67]. Besides the hot-melt viscosity, lap shear strength, T-peel, and open time, the shear adhesion failure temperature (SAFT) of the bonded substrate is an important property for evaluating the effectiveness of the adhesive. The polymer MFI plays an important part in determining the surface temperature at which shear adhesion failure could occur, as can be seen from Fig. 9.75. The top curve represents the SAFT test results with a 0.5-kg load, whereas the bottom curve shows test results with a 1-kg... 

Neoprene 750A. It is a medium-gel, slow-crystallizing polymer. It combines flexibility, dry tack, heat-reactivity and cohesive strength. 

In the world of molecular simulation, it would be more conventional to consider that the present model is a coarse grained model of real polymers, where the real time-scale is much longer than that of the present MD simulation time-scale. However, we did not intend to make a coarse grained model. The crystallization of polymers was shown to be rather universal. Various kinds of polymers, either fast crystallizing or slow crystallizing, were known to follow the same scheme with respect to the molecular mechanism of crystallization. So we studied this simple model expecting that the present model would also follow the same crystallization scheme and show the general molecular mechanisms of polymer crystallization. 

While, in recent years, many laboratories have demonstrated that nearly every synthetic polymer can crystallize in the form of single crystals (13) consisting of lamellae formed by regular chain folding (Figure 3), it is clear that extreme condi-tions-i.e., very slow crystallization or very dilute solutions, are required for these structures to form. Under normal conditions, such as those encountered in any industrial process, the polymer usually crystallizes in the form of less ordered, large structures, called spherulites. 

In order to record the dynamic polymer crystal growth process in-situ, two factors are significantly important. One is the use of a very high resolution technique. Such a technique can repeatedly record the same area without damage to or significant interactions with the sample. AFM has been proved to be a successful tool to fulfill this task. The other key factor is that the polymers must have an appropriate crystallization rate. It generally takes an AFM several minutes to produce an image. This requires that the polymer has a very slow crystallization rate. The crystallization rates of most semicrystalline polymers at room temperature are too fast. 

The growth of polymer spherulites involves the segregation of noncrystal-lizable material into the regions between the lamellar ribbons. The components that arc not incorporated into the crystallites include additives like oxidation stabilizers, catalyst residues, and so on. as well as comonomer units or branches. The spherulite structures and interspherulitic boundaries are held together primarily by polymer molecules which run between the twisted lamellar subunits and the spherulites themselves. Slow crystallization at low degrees of supercooling... 

C and increased both the modulus and crystallization rate three fold.234 (Slow crystallization of this polymer leads to uneconomically long cycle times in molding.) Similar techniques have used to add clays into polyethylene,235 polypropylene,236 and epoxy resins.237... 

The linear PE studied was Marlex 6015 obtained from Phillips Petroleum Co. The polymer powder was compression-molded between Teflon-coated aluminum foil at 170°C. Temperature was maintained for 15 min prior to compression for 5 min. The sample was then allowed to cool slowly to room temperature with no pressure while still in the platens of the press. Thin films were obtained which showed no macroscopic melt flow orientation and which were less than 0.002 in. thick. This sample preparation will be referred to as slow-crystallized. 

The effects of SCB and side groups are similar. They disrupt the ability of the polymer to crystallize. If the disruption is not complete, the added bulkiness will make the rate of crystallization slow down. SCB has little effect on the flow properties of a polymer, but LCB has a profound effect. We will discuss this more when we look at the differences in behavior between HDPE, LDPE, and LLDPE in Chapter 4. For now, we can illustrate the effects of branching by comparing LDPE with HDPE. The densities differ, the tensile properties differ, and the elastic character of the polymers differs greatly, even though both are made from the same monomer. 

---