Polymer-CD-ICs

Fig. 2 Schematic representation of polymer-CD IC formation, the coalescence process, and the coalesced polymer...

Warm water washing of polymer-CD-ICs containing polymer guests insoluble in water or treatment with amylase enzymes serves to remove the host CDs and results in the coalescence of the guest polymers into solid samples. X-ray diffraction, DSC, TGA, and FUR and NMR spectroscopies are typically used to characterize the coalesced polymer samples. 

PCL, PLLA, or both were included. The a-CD-IC resulting from the suspension of PLLA-a-CD-IC in the PCL solution was found to contain only PCL guest chains, while the a-CD-IC resulting from the suspension of PCL-a-CD-IC in the PLLA solution contained only a very small amount of PLLA guest chains. The above results were interpreted to signify the importance of both guest polymer hydrophobicity and guest/host steric compatibility in the formation of polymer-CD-ICs. 

Nonstoichiometric Polymer-CD-ICs as Nucleating Agents for the Crystallization of Polymers [103-106]... 

Formation of polymer-CD-ICs with amounts of host CDs insufficient to completely thread and confine the guest polymers, results in polymer-CD-IC crystals with portions of the guest polymer chains emerging from the host CD crystalline surfaces [102], When a nonstoichiometric (n-s) polymer-CD-IC, with only partially included chains, is added in small quantities to a bulk sample of the same polymer, which can crystallize and has a Tm below the decomposition temperature of CD-ICs... 

C), it has been observed that its crystallization from the melt is enhanced [103-106], Melt crystallized polymers nucleated with n-s polymer-CD-ICs crystallize more rapidly, evidence greater levels of crystallinity, higher melt crystallization temperatures, and semicrystalline morphologies characterized by crystals which are smaller and more uniformly distributed than in un-nucleated pure bulk samples. 

The effectiveness/efficiency of nucleation with (n-s) polymer-CD-ICs was observed to be at least comparable or superior to that produced by more traditional nucleation agents, such as talc, and for that matter pure CDs. The alteration of crystalline morphology achievable by using n-s polymer-CD-ICs as nucleants in melt-crystallization is demonstrated in Fig. 19. It is very clear that the morphology of PCL melt-crystallized in the presence of a small amount of (n-s) a-PCL-C-IC is very distinct from the pure melt-crystallized PCL. Crystal sizes are drastically reduced and more homogeneously distributed for the nucleated PCL. 

The distinct changes in morphology achieved by melt-crystallization of polymers in the presence of small amounts of their (n-s) polymer-CD-ICs results in changes in other physical properties as well. For example, in Table 2 a comparison of the properties of poly(3-hydroxy butyrate) (PHB) melt-spun with and without the presence of (n-s) PHB-a-CD-IC are compared [104], The mechanical properties of the PHB/( -s) PHB-a-CD-IC fibers are superior. In fact, their lower elongation at break likely contributes to the removal of the stickiness normally observed between melt-spun PBH fibers [104],... 

There is an important additional advantage of using (n-s) polymer-CD-ICs to nucleate the melt-crystallization of polymers. Because CDs are nontoxic, biocompatible, and biodegradable, they may be safely utilized in (n-s) polymer-CD-IC nucleants to fabricate both permanent and biodegradable/bioabsorbable implants that are also nontoxic. 

It should also be mentioned that block copolymers, such as PCL-PPG-PCL, may be utilized to form polymer-CD-ICs [102-104] that are effective as nucleating agents. For example, if P-CD is used to form an IC with PCL-PPG-PCL, only the central PPG blocks will be included [101], and the unincluded terminal PCL blocks will extend from the surfaces of the b-PPG-P-CD-IC crystals. By altering the PCL block lengths we may be able to tailor their ability to control the melt-crystallization of PCL. In comparison to (n-s) PCL-a-CD-ICs, or (n-s) polymer-CD-ICs in general, block copolymer-CD-ICs with some blocks able to be included, while other blocks are excluded, may potentially be superior nucleating agents. 

The formation of and coalescence from polymer-CD-ICs are briefly described along with the methods used to characterize both the ICs and the coalesced guest polymers. Here the focus is on the structural organization of solid polymer samples coalesced from their CD-ICs, which are found to be quite distinct from solid polymer samples formed from their randomly coiling and entangled solutions and melts. The alteration and control of properties displayed by solid polymers coalesced from their CD-ICs are also discussed. 

Formation of polymer-CD-ICs (see Fig. 11.If and 11.2) begins with either the dissolution or suspension of CD cage-structure crystals (see Fig. 11.1c) or their conversion to chaimel- or columnar-stmcture crystals (see Fig. 11.le) containing only water of hydration as a guest. " The formation of crystalline polymer-CD-ICs from solutions requires the combination of solutions... 

As noted in Fig. 11.2, guest polymers may be recovered from their CD-ICs in the form of bulk solid samples by treating the polymer-CD-IC crystals with warm water or another CD solvent, which is a non-solvent for the guest... 

The dipolar interactions between polar host CDs and included guests with permanent dipole moments are considered to affect at least the conformations and structures of soluble small-molecule guest/host CD-ICs.Our observations of the preference for inclusion of poly (e-caprolactone) (PCL) over poly(L-lactic acid) (PLLA) chains in both dissolved and crystalline suspended a-CD (a-CDcs), as well as the displacement of PLLA chains by PCL from PLLA-a-CD-ICs when suspended in PCL solution, indicate that dipole-dipole electrostatic interactions may not be critical to the formation of polymer-CD-ICs. If they were, then we might expect PLLA to be preferentially included in a-CD compared to PCL, because two PLLA repeat units and two ester group dipoles occupy each a-CD, while only single PCL... 

Once again the preference of PCL over PLLA inclusion and the fact that all hydrocarbon polyolefins can form CD-lCs" imply that hydrogen-bonding between included guest polymers and host CDs is not likely crucial in the formation of polymer-CD-ICs. 

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