Stem Tilt in alpha-Form Single Crystals of Isotactic Polypropylene: A Manifestation of Conformational Constraints Set by Stereochemistry and Minimized Fold Encumbrance

Two high molecular weight (MW) isotactic polypropylene (i-PP) samples, one containing a small amount of rr stereodefects denoted R1 and one conventional, commercial i-PP denoted S1, were used to grow single crystals in thin films at high temperatures (Tx = 145 °C for R1, Tx = 155 °C for S1). Elongated R2-form lathlike single crystals could be found in these two samples, indicating that the growth of this type of single crystal is a general phenomenon in i-PP and is independent of the small amount of tacticity defects. On the basis of our selected area electron diffraction (SAED) experimental results, the stems in these lathlike single crystals were tilted at an unusual 17° angle around the b-axis (i.e., a rotation of 17° of the c-axis within the ac-plane toward the a-axis direction), while in the traditional scheme of the alpha-form in i-PP, the stem axis is understood to be normal to the fold surface. This 17° stem tilt in the alpha2-form, lathlike single crystals grown at high Tx values appears to depend upon two constraints: (a) the conformational restrictions on chirality and clinicity of isotactic polyolefin stems linked by a fold, as analyzed by Petraccone et al. [Polymer 1986, 27, 1665; Eur. Polym. J. 1989, 25, 43], and (b) a minimized encumbrance at the fold surface can be achieved by those folds which start from and end on the chemical bonds normal to the fold surface with a favorable gauche conformation of i-PP chains. This is because in i-PP (and most isotactic polyolefins) with a helical stem conformation every other chemical bond is nearly parallel (in a trans conformation) or perpendicular (in a gauche conformation) to the stem axis (and thus, nearly within or normal to the fold surface). The combination of these two constraints favors a (102) fold surface in the i-PP case, as is observed in this study.