Switching Chirality of Hybrid Left-Right Crystalline Helicoids Built of Achiral Polymer Chains: When Right to Left Becomes Left to Right

It is widely accepted that achiral polymers crystallize in lamella helicoids of both handedness, whereas chiral polymers are known to form helicoids of only one handedness. By employing nanofocus X-ray diffraction, we show that the situation is more complex and that the lamella helicoids of achiral polymers can be in fact hybrid, i.e., containing a left- and a right-handed part. Moreover, our data demonstrate that the poly(trimethylene terephthalate) lamellae invert chirality at a certain crystallization temperature. The chirality correlates with the growth axis polarity (i.e., a versus -a) and handedness of the lamellar helicoid (L versus R) and can be conveniently expressed in terms of the chirality parameter pairs. In addition, the overall chain tilt does not affect the rate and sense of twisting, calling into question the premise of the Keith and Padden model. Instead, the inclination of the terminal segment of the crystalline stem protruding the lamellar surface is proposed to be the key factor controlling the surface stresses.