Apparent nucleation of a crystal‐crystal transition in poly‐1‐butene

Abstract

In a previous paper we reported on the kinetics of crystallization and a crystal-crystal transition in poly-1-butene.1 (For earlier references, refer to this paper; also, note a recent publication by Zannetti et a1.2) In the absence of stresses and high pressures the two transformations ( a ) melt + modification 2 and ( 6 ) modification 2 + modification 1 were shown under optimal conditions to be fast (minutes) and slow (days), respectively. The unstable modification 2 is characterized by a fourfold helix similar to that of poly-3-methyl-1-butene, while the stable modification 1 is characterized by a threefold helix similar to that of polypropylene.3In the course of the transformation modification 2 -f modification 1, poly-1-butene becomes more rigid and less readily permanently deformed. The acceleration of this transformation was thus of both practical and theoretical interest. In the present work we have shown that the addition of certain compounds to the polymer can bring about some acceleration of the crystal-crystal transition. From the nature of the materials causing the acceleration and from the temperature dependence of the effect, it can be concluded that an increase in nucleation rate is involved. To the best of our knowledge such an effect has not previously been reported for polymeric systems.Offset tensile yield points were used to obtain initial indication of the effectiveness of the various additives as transition accelerators (Table I). The offset tensile yield point (ASTM D638-58T) is the stress a t which the stress-strain curve departs from the straight line representing Hooke's law behavior by a specified strain, the offset. Arbitrarily, we have chosen an offset of 5% elongation.Without an additive, the control samples had an offset yield point of about 1000 psi if the determination was made 23 hr. after the molding of the test specimen. But when 2 to 5 wt.-% of certain additives was blended into the poly-1-butene, offset yield points in the range 1700-2400 psi were obtained under comparable test conditions. Crystalline polymers such as isotactie polypropylene (>95 wt.-% insolubles in boiling heptane) and high-density polyethylene (HIFAX, d = 0.96), as well as low molecular weight compounds such as biphenyl and stearic acid, were found effective. Dilatometric measurements elucidated further the effect of these additives on the crystallization rate in poly-1-butene. Two samples containing 5 wt.-% stearic acid and 5 wt.-% crystalline polypropylene were examined and compared with the parent poly-lbutene containing no additive (Table 11, Fig. 1). Table I1 shows the times required to attain standard contraction levels (0.82, 1.64, and 3.28 m1./100 g. of polymer) at various temperatures, while in Figure 1 we have illustrated one of these contractions levels to show more graphically the effect of these two additives. Both additives caused a considerable acceleration of the phase transformation a t certain temperatures, resulting in a displacement of the maximum rate toward higher temperatures. As bo...

Crystallization kinetics and polymorphic transformations in polybutene-1

“…Unlike the case of the supercooled liquid -> form 2 transformation , where evidence of heteroge neous initiation was clearly apparent, the type of initiation cannot be positively identified . Boor and l\1itchell [25] have observed th at th e addition of polypropylene 01' stearic acid accelerates the form 2 -> form 1 transformation. ThiR indicates that the process is subj ect to heterogeneous initiation.…”

Polymorphism in poly‐1‐butene: Apparent direct formation of modification I

“…Three crystalline modifications of isotactic poly-1-butene have been described. Two forms were recognized at an early date (1) and have been studied extensively (2)(3)(4)(5)(6)(7)(8). More recently, a third form has received considerable attention (3,6,9).…”

“…More recently, a third form has received considerable attention (3,6,9). Form I, T, = 13O-14O0C., is spontaneously (3,4) produced from 11; the transformation is accelerated by pressure (3), mechanical deformation (3) or, above 15OC., by nucleating agents (5). Form 11, T, = 120-126OC., crystallizes from the melt (3,4).…”

Effect of polymorphism of isotactic polybutene‐1 on peel behavior of polyethylene/polybutene‐1 peel systems

“…Research, for evaluation of the effect of the presence of a second component on the transition kinetics, employed low molecular weight compounds including biphenyl, stearic acid, dioctyl phthalate, or 1-naphtylacetamide, polymers, including isotactic polypropylene (iPP) or high-density polyethylene (HDPE), and nanofillers. 22,25,26 Blending of iPB-1 with iPP, for instance, was found to accelerate the rate of the transition from form II to form I crystals of iPB-1. 25 For pure iPB-1, at ambient temperature, full conversion of form II to form I was achieved after aging for at least 6 days, whereas in blends with 25 m% iPP, the transition was completed after 3 days.…”

“…On the other side, the kinetics of the crystal-form II to crystal-form I transformation is a function of the thermodynamic variables temperature and pressure/ deformation, 7,20 and depends additionally on the presence of additives, i.e., of a second component. 22 The phase transformation is possible between about 248 K, which is the glass transition temperature, and about 333 K. The maximum rate of transformation is observed near ambient temperature, between 293 and 313 K. 3 Quantitative calorimetric analysis of the transformation rate of a specific PB-1 with a molecular weight of 525,000 g mol 21 and a fraction of isotactic pentades of close to 83%, which initially was melt-crystallized at 323 K, revealed that the transformation at 293 K is completed after about 300 h. 3 Pressure and mechanical deformation, in general, increase the rate of phase transformation since the nucleation density within the crystals gets increased. Additives also were found to accelerate the transformation by providing surfaces which support the development of the stable form I structure of PB-1.…”