Polymer phase nucleation

As suggested by Barrett (2), it is assumed that following the particle nucleation stage, the polymerization proceeds in the particle (monomer/polymer) phase with no mass transfer limitation. Therefore, the dispersion polymerization is similar to a mass or suspension polymerization, and kj can not be assumed to be constant even at isothermal conditions, since kp and even kp are dependent on the degree of polymerization because of a gel effect. (2., ,D However, since the application of the model is for a finishing step, with polymer molecular weight and viscosity fairly well established, further changes in kp and kp should be minimal. 

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

A rep < 1, Des < 1, the nucleation dynamics is stochastic in nature as a critical fluctuation in one, or more, order parameters is required for the development of a nucleus. For DeYep > 1, Des < 1 the chains become more uniformly oriented in the flow direction but the conformation remains unaffected. Hence a thermally activated fluctuation in the conformation can be sufficient for the development of a nucleus. For a number of polymers, for example PET and PEEK, the Kuhn length is larger than the distance between two entanglements. For this class of polymers, the nucleation dynamics is very similar to the phase transition observed in liquid crystalline polymers under quiescent [8], and flow conditions [21]. In fast flows, Derep > 1, Des > 1, A > A (T), one reaches the condition where the chains are fully oriented and the chain conformation becomes similar to that of the crystalline state. Critical fluctuations in the orientation and conformation of the chain are therefore no longer needed, as these requirements are fulfilled, in a more deterministic manner, by the applied flow field. Hence, an increase of the parameters Deiep, Des and A results into a shift of the nucleation dynamics from a stochastic to a more deterministic process, resulting into an increase of the nucleation rate. 

M/SC on many, in particular, polymer substrates the desorption of adatoms should be taken into consideration. In this case the probability of M/SC phase nucleation at PVD, usually designated as condensation coefficient C, is defined by a competition between desorption of adatoms and their irreversible fixing on a substrate surface. Irreversibility of M/SC atom fixing is provided either by its aggregation with other atoms or by immobilization on surface defect which can be surface steps, outputs of dislocations, vacancies, etc. [42],... 

Wettability and Surface Morphology. Surface chemical studies on crystallizable polymers have ignored, in general, the nature of the nucleating phase—i.e., vapor, solid, or liquid—and the details of formation of the polymer melt-nucleating phase interface which on solidification by cooling results in a polymer solid-nucleating phase interface (22). 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

On the contrary, in the second binodal region (b) in Fig. 20.1-7, where the local mixing point finds the polymer-rich solution as the continuous phase, dispersed spharaids of nesrly polymer-free fluid are nucleated. These do meins then coalesce to p red nee a foam structure whose walls are composed of the solidified dispersed polymer phase. To obtain an open-cell foam with low resists ace to flow, defects clearly must occur in the waits of (be cells.39 Soch a structure is shown in Fig. 20.1-66. Tbe dense film on the surface can be promoted by a brief exposure of the cast or apan nascent membrane to air to obtain a more concentrated region at the surfhee prior to immersion in the nonsolvent precipitation hath, which then sets the dease layer in place and proceeds to nucleate the subetracture as described above, This evaporation step, however, is not required in all cases to produce acceptuble skins.56,65,66... 

Some binary systems do not show any depression at all, indicating that T and T do not depend on blend composition. This is found when the second dispersed phase does not influence the normal crystallization behavior of the matrix polymer no nucleating activity, no influence on sphemUte growth rate, etc. 

The rapid chain transfer to monomer in VC polymerization, resulting in high rates of radical desorption and readsoiption, may enable a radical to nucleate more than one of the miniemulsion droplets. It therefore seems reasonable to expect that in miniemulsion polymerization of VC, formation of the polymer phase may involve a somewhat similar sequence of events as those proposed for bulk and suspension polymerization. However, it should noted that the miniemulsions prepared by the methods described have a broad droplet size distribution, implying that for the smallest droplets, stage 3 may be the final step. 

Two types of phase equilibria are superimposed phase separation equilibria in amorphous and ciystalhne phases. The thermodynamic equilibrium of the system presented in Figure 6.2 corresponds to polymer crystallization. The amorphous phase is metastable. It can exist fi om the beginning of crystalline phase nucleation through subsequent crystallization imtil the equilibrium state for the crystalline polymer is reached. A system can degenerate into many simple systems for polymers that erystallize readily in dilute solutions where polymer segregates into separate crystallites. Polymers, capable of partial crystallization at the ejq)ense of the formation of local regular blocks of chains (e.g., PVC) can exist as a metastable amorphous-erystalhne system. In such cases, one phase repre-... 

Fig. 10.23 Illustration of three basic situations of crystal nucleation. From left to right are primary nucleation in the bulk polymer phase, secondary nucleation on the smooth growth front, and tertiary nucleation at the terrace of the growth front...

---