Polymers coalesced from their cyclodextrin inclusion complexes: What can they tell us about the morphology of melt‐crystallized polymers?

Abstract

Cyclodextrins (CDs) are cyclic polysaccharides with nano-size, largely hydrophobic cavities, and exteriors covered with hydrophilic hydroxyl groups, making them water soluble. Threading and filling their cavities with polymer chains produces noncovalently bonded crystalline inclusion compounds (ICs). In this study, we formed fully covered, stoichiometric ICs between guest poly(L-lactic acid), poly(e-caprolactone), and nylon-6 chains and host a-CD. Coalesced samples of all three polymers were obtained after appropriately removing the stacked a-CD host channels from their ICs. Distinct differential scanning calorimetriy (DSC) thermograms were observed for as-received and coalesced samples, with the coalesced samples crystallizing faster at higher temperatures from their melts, and this distinction was maintained even after extensive, long-time melt-annealing (hours, days, and weeks). We believe this is due to the largely unentangled chains with extended conformations that are more densely packed in the initially coalesced samples. When small amounts ($2 wt %) of the coalesced polymers are used as self-nucleating agents for their as-received samples, the resulting self-nucleated samples show DSC thermograms similar to those of the neat coalesced polymers, including their long-time stability to melt-annealing. Coalesced polymers, whether neat or in samples they self-nucleate, may conserve their organization in the melt (largely extended and unentangled chains) for long periods, because the process of entangling the many chains influenced by a single initially extended unentangled coalesced chain, after it randomly coils, is extremely sluggish. By contrast, in melt-crystallized or solution-cast samples, polymer chains generally become fully randomly coiled, interpenetrate, and entangle after being heated and held in their melts for comparatively much shorter times. For example, we have recently observed (DSC) that ultra high molecular weight, gel-spun spectra polyethylene (PE) fibers V R did not conserve or retain any memory of their as-spun and highly drawn semicrystalline morphology even after spending as little as 2 min in the melt. As a consequence of the comparison to the behavior of coalesced polymer melts, we believe that polyethylene chains in Spectra fibers V R must be at least intimately dispersed within their crystalline regions, and likely partially coiled and entangled in their noncrystalline regions, thereby facilitating their rapid transformation into a full entanglement network of randomly coiling chains in the melt.

Self-Nucleation of Crystalline Phases Within Homopolymers, Polymer Blends, Copolymers, and Nanocomposites

“…Gurarslan et al [33] developed a method to produce highly oriented polymer melts, employing cyclodextrins (CDs). These water-soluble polysaccharides are nanometric in size and have internal hydrophobic cavities and external hydrophilic groups.…”

“…The process for production of a homogeneous random melt for these structures takes much longer times than for a conventional crystalline sample. According to Gurarslan et al [33], the proximity of the crystals to the amorphous zone, in an as-received sample, facilitates chain mobility and allows the formation of a homogeneous melt. However, the coalesced sample exhibits uniform order throughout, making the creation of random coiled chains in the melt more difficult.…”

“…Gurarslan et al [33] employed 2 wt% of the coalesced sample as self-nucleating agent in the as-received PCL sample and found that the crystallization temperature of the self-nucleated sample was the same as that of the coalesced sample. This experiment demonstrates that a small quantity of pre-order structure is able to selfnucleate a polymeric sample and that there is no need for crystalline fragments.…”

“…Once PP is divided into many small and isolated droplets, the heterogeneities responsible for PP nucleation are also divided among the different droplets, [33]. Copyright (2012) John Wiley and Sons whereas some heterogeneities can migrate to the LLDPE matrix component during processing.…”

Coalesced Poly(ε-caprolactone) Fibers Are Stronger

“…9 The coalesced polymer chains are more extended, less entangled, and more aligned in nematic-like coalesced regions 16 and have higher noncrystalline densities. 13 However, since melt pressed films are isotropic, we believe that they do not reflect the real mechanical strength of coalesced fibers.…”