Physical properties of poly(ɛ-caprolactone) coalesced from its α-cyclodextrin inclusion compound

Poly(ɛ-caprolactone) (PCL) is a biodegradable/bioabsorbable polyester used in such biomedical applications as drug delivery and suture manufacturing. PCL has relatively poor physical properties, however, limiting its load-bearing applications. In this work, PCL was processed with α-cyclodextrin (α-CD) to form an inclusion complex (IC). The host α-CD was then stripped away to yield bulk PCL with largely extended, un-entangled polymer chains, a process referred to as coalescence. The changes in thermal, physical, and solid-state rheological properties resulting from this coalescence process were examined. It was found that elongating and un-entangling the PCL chains in this manner resulted in substantial increases in melt-crystallization temperatures, Tcs, up to 25 °C, depending on the cooling rate from the melt. Coalescence also increased the elastic storage modulus, decreased tan δ, increased the average hardness and Young's modulus by 33 and 53%, respectively, and produced a closer packing of chains in the non-crystalline sample regions, without affecting the overall PCL crystallinity. Interestingly, the reorganized PCL chains in the non-crystalline regions of coalesced samples did not revert to the normal randomly-coiled entangled melt even after heating well above Tm (90 °C) for a month. The addition of small amounts (a few wt%) of coalesced PCL was also found to effectively nucleate the melt-crystallization of as-received PCL. Thus, the semi-crystalline morphology of PCL may be controlled by melt-processing with coalesced PCL added as a nucleant, that is not only necessarily non-toxic and biodegradable/bioabsorbable, but is also chemically compatible.

Melt-crystallization temperature as a function of cooling rate for as-received and coalesced PCLs.