Usage: Polycaprolactone (PCL) has received a lot of interest as a biodegradable polymer in a variety of biomedical applications. The degradation of PCL via hydrolysis of its ester linkages under physiological conditions has earned this polymer the FDA’s approval for use in the human body as surgical sutures, controlled release drug delivery devices, and scaffold for tissue repair. PCL’s rheological and viscoelastic properties may be manipulated to tailor its degradation kinetics to suit specific anatomical site, which makes it of special interest for the design and manufacture of long term implantable devices. Triple detection GPC is an ideal method for the analysis of biodegradable polymers and to overcome the low dn/dc of PCL in THF associated with a poor light scattering response, solvent enhanced light scattering technique (SELS) was employed.
Manufacturing: PCL is derived from the chemical synthesis of non-renewable crude oil. Two main pathways to produce polycaprolactone are the polycondensation of a hydroxycarboxylic acid and the ring-opening polymerisation (ROP) of monomeric units of ε -caprolactone shown in Figure 1.
Chemical Formula: (C6H10O2)n
Chemical Structure: PCL consists of a sequence of methylene units, between which ester groups are formed.
Figure 1. Synthesis of PCL by ring-opening polymerization (ROP) of ε-caprolactone
Three low molecular weight PCL samples were dissolved in tetrahydrofuran (THF) at a concentration of 5 mg/mL. Analysis was completed on a Viscotek Triple Detector Array (TDA) 302 equipped with a Refractive Index (RI) detector, Viscometer, Right Angle (90o) Light Scattering (RALS) and Low Angle (7o) Light Scattering (LALS). Separation was performed using 3 Viscogel I-MBLMW-3078 columns in series. Samples were injected at a volume of 200 µL and eluted through the system at flow rate of 1 mL/min in pure acetone. A temperature of 30oC was maintained during separation and detection.
PCL samples were prepared in THF and injected into a system in pure acetone because PCL has a larger dn/dc in acetone than THF. While PCL is soluble in acetone and THF is miscible with acetone, the mobile phase does not have to dissolve the sample; only maintain its solubility during the analysis. The LS detector sensitivities are thus enhanced since the response signal is dependent solely on the dn/dc of the sample in the mobile phase. An overlay of the RI signal for different PCL injections can be seen in Figure 2. Each injection has a large signal to noise response as well as a symmetric mono-modal distribution. Using the data in Figure 2, along with the RALS, LALS and Viscometer data, the Mn, Mw and Mw/Mn were calculated. The data analysis results can be seen in Table 1.
Figure 2. Overlay of Refractive Index (RI) signals versus retention volume of different PCL sample injections.
Table 1. Results of Triple Detection analysis of PCL samples.
Determination of PCL’s molecular weight and polydispersity is essential to predict the final product’s performance. SELS provides increased detector response, even for low molecular weight polymers, which increases the accuracy of enhanced GPC techniques. Analyzing the polymeric component of biomaterials will allow for control of properties such as drug release rate, toxicity or stability.