We present investigations of the intricate crystallization and melting behavior of an alternating copolymer synthesized by photopolymerization of 2,2′-dimercaptodiethyl sulfide with di(ethylene glycol)divinyl ether. Upon increasing temperature, we observed the succession of two distinctly separated melting processes, which we related to the sequential formation and disappearance of two crystalline polymorphs. Due to their well-separated melting temperatures T m1 and T m2 , we labeled these polymorphs as LOW-T m -form and HIGH-T m -form, respectively. X-ray diffraction results confirmed differences in the parameters of the crystal unit cells. However, upon cooling from the isotropic melt, we never obtained the HIGH-T m -form and could only generate the LOW-T m -form characterized by spherulitic crystals that melted completely at T m1 . Surprisingly, simultaneously everywhere within these molten spherulites, a large number of needle-like crystals were growing as a function of the time the sample was kept (well) above T m1 . All crystals exhibited an orientation largely following the radial direction of the initial spherulites. This observation suggests that molten polymer chains remembered for some time their previous alignment within the crystalline LOW-T m -form. This memory assisted the nucleation and thus exclusively enabled the formation of the HIGH-T m -form. Only when partially crystallizing a sample aboveT m1 in the HIGH-T m -form and subsequently cooling it below T m1 allowed to achieve coexistence of both polymorphs. When heating such a sample of coexisting crystalline structures above T m1 , only the LOW-T m -form melted and the HIGH-T m -form remained. The generality of our findings has been demonstrated by similar results obtained for complementary alternating copolymers. Our study suggests that the otherwise impossible nucleation of polymorphs with a high melting temperature can be enabled by prior orientation of chains within another easily established polymorph characterized by a lower melting temperature.

We examined the formation of self-seeded platelet-like crystals from polystyrene-block-polyethylene oxide (PS-b-PEO) diblock copolymers in toluene as a function of polymer concentration (c), crystallization temperature (TC), and self-seeding temperature (TSS). We showed that the number (N) of platelet-like crystals and their mean lateral size (L) can be controlled through a self-seeding procedure. As (homogeneous) nucleation was circumvented by the self-seeding procedure, N did not depend on TC. N increased linearly with c and decayed exponentially with TSS but was not affected significantly by the time the sample was kept at TSS. The solubility limit of PS-b-PEO in toluene (c*), which was derived from the linear extrapolation of Nc→ 0 and from the total deposited mass of the platelets per area (MCc→0), depended on TC. We have also demonstrated that at low N, stacks consisting of a (large) number (η) of uniquely oriented lamellae can be achieved. At a given TC, L was controlled by N and η as well as by ∆c=c−c∗. Thus, besides being able to predict size and number of platelet-like crystals, the self-seeding procedure also allowed control of the number of stacked lamellae in these crystals.

A photocatalytic thiol‐ene aqueous emulsion polymerization under visible‐light is described to prepare linear semicrystalline latexes using 2,2’‐dimercaptodiethyl sulfide as dithiol and various dienes. The procedure involves low irradiance (3 mW cm−2), LED irradiation source, eosin‐Y disodium as organocatalyst, low catalyst loading (<0.05% mol), and short reaction time scales (<1 h). The resulting latexes have molecular weights of about 10 kg mol−1, average diameters of 100 nm, and a linear structure consisting only of thioether repeating units. Electron‐transfer reaction from a thiol to the triplet excited state of the photocatalyst is suggested as the primary step of the mechanism (type I), whereas oxidation by singlet oxygen generated by energy transfer has a negligible effect (type II). Only polymers prepared with aliphatic dienes such as diallyl adipate or di(ethylene glycol) divinyl ether exhibit a high crystallization tendency as revealed by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction. Ordering and crystallization are driven by molecular packing of poly(thioether) chains combining structural regularity, compactness, and flexibility.

Using a hydrophobic–hydrophilic poly­(styrene)-b-poly­(ethylene oxide) (PS-b-PEO) diblock copolymer, we formed sandwich-like micron-sized thin platelets via crystallization in solution. These platelets consisted of a crystalline PEO lamella confined between two glassy PS layers and were deposited on a solid substrate. Using in situ optical microscopy, we followed the temporal and spatial changes in the thickness and morphology of these sandwich-like platelets induced by exposure to humid air. As water is a good solvent for PEO but a nonsolvent for PS, we observed first the dissolution of the confined crystalline PEO layers characterized by a sharp dissolution front propagating at an almost constant velocity as expected for case II diffusion into a solid. The resulting hydrated PEO brushes absorbed water further until equilibration of the respective chemical potentials, accompanied by a change from a planar to a dome-shaped morphology that could be fully reversed by exposure to dry air. Repeated swelling–deswelling cycles demonstrated stability and reproducibility of these developments taking place at distinctly different transport rates. Here, we discuss the underlying processes of water permeation into and efflux from the PEO layers.

Using the setup of an optical microscope, we have examined the influence of illumination with white light on the crystallization behavior in molten films of poly(3-(2,5dioctylphenyl)thiophene) and poly(3-hexylthiophene). We observed a reduction in nucleation density and crystal growth rate induced by illumination. The amount of this reduction increased with the increase of light intensity. Melting of samples previously crystallized under illumination and recrystallization of the same samples in the dark showed full reversibility of the crystallization behavior, demonstrating that these changes in the crystallization behavior induced by illumination were not permanent. We tentatively suggest that absorption of photons by the polymer induced chain stiffening, possibly causing a reduction in polymer diffusivity, which, in turn, slows down the crystal growth rate and reduces the nucleation probability. We expect that many other conjugated polymers will show a similar impact of illumination on crystallization.