Polycaprolactone
Polycaprolactone (PCL) is a synthetic, biodegradable, and biocompatible polyester produced through the ring-opening polymerization of ε-caprolactone, characterized by its semicrystalline structure, hydrophobicity, and slow degradation rate via hydrolysis, making it suitable for long-term biomedical applications.[1][2] PCL exhibits a low glass transition temperature of approximately -60°C and a melting point ranging from 59°C to 64°C, which contribute to its flexibility, viscoelasticity, and ease of processing into various forms such as films, fibers, and scaffolds.[2] Its mechanical properties, including adjustable strength and elasticity, can be tailored by varying molecular weight and blending with other materials, enhancing its utility in load-bearing applications.[3] Biocompatibility is well-established, with PCL meeting ISO 10993 standards for interaction with human tissues and fluids, as its degradation products—primarily carbon dioxide and water—are non-toxic and fully eliminated from the body.[2] The polymer's degradation occurs in two phases: initial hydrolysis of ester bonds followed by enzymatic breakdown, with the rate depending on factors like molecular weight (typically 1-4 years for biomedical uses) and environmental conditions, allowing controlled release in drug delivery systems.[2][4] PCL finds extensive use in tissue engineering for creating 3D scaffolds that support cell growth in bone, skin, and nerve regeneration, as well as in sutures like Monocryl™ and drug-eluting implants like Capronor™ for contraception.[2][3] In esthetic medicine, PCL-based fillers such as Ellansé® stimulate collagen production for facial and hand rejuvenation, leveraging its biocompatibility and gradual resorption.[2] Emerging applications include wound dressings and artificial blood vessels, driven by ongoing research into nanocomposites for improved mechanical and biological performance.[5]Overview and Properties
Chemical Structure
Polycaprolactone (PCL) is a biodegradable aliphatic polyester synthesized via ring-opening polymerization of the cyclic monomer ε-caprolactone, which has the molecular formula C₆H₁₀O₂.[6] The resulting polymer features a repeating unit of –[O–(CH₂)₅–CO]–, forming a linear chain with ester linkages that connect the oxygen and carbonyl groups, giving the overall chemical formula (C₆H₁₀O₂)ₙ, where n represents the degree of polymerization.[7] This structure consists of five methylene (–CH₂–) groups flanked by an ester (–COO–) linkage, contributing to the polymer's flexibility and hydrophobicity at the molecular level.[8] Commercial grades of PCL typically exhibit number-average molecular weights (Mₙ) ranging from 3,000 to 80,000 g/mol, with the degree of polymerization influencing chain length and structural regularity.[6] Higher molecular weights generally promote greater chain entanglement and semi-crystalline ordering due to the ability of the linear aliphatic chains to fold and pack into crystalline domains, while lower weights result in more amorphous characteristics.[9] PCL can be incorporated into copolymers, such as multiblock or graft structures with poly(lactic acid) (PLA), where PCL segments are covalently linked to PLA chains to form hybrid repeating units that maintain the core ester-based backbone.[10] These structural integrations allow for tailored chain architectures without altering the fundamental PCL repeating motif.[11] The ε-caprolactone monomer lacks chiral centers, resulting in PCL chains with uniform structural regularity along the backbone, facilitating linear extension and contributing to the polymer's semi-crystalline nature through van der Waals interactions between methylene sequences. This uniformity ensures consistent chain conformation in standard syntheses, unlike stereoregular polymers with chiral units.[12][6]Physical and Mechanical Properties
Polycaprolactone (PCL) is a semicrystalline polymer characterized by distinct thermal properties that influence its processability and performance. Its melting point typically ranges from 58°C to 60°C, allowing for low-temperature processing, while the glass transition temperature is approximately -60°C, resulting in a rubbery state at room temperature. The degree of crystallinity is around 56%, which is influenced by the linear aliphatic structure of the polymer chains that enables ordered packing but limits complete crystallization.[13] PCL exhibits a density of approximately 1.14 g/cm³, contributing to its lightweight nature in bulk applications. It demonstrates good solubility in organic solvents such as chloroform and dichloromethane, facilitating solution-based processing, but remains insoluble in water due to its hydrophobic polyester backbone.[14][15] In terms of mechanical properties, PCL displays moderate tensile strength of 10.5–16.1 MPa and a Young's modulus of about 0.35 GPa for solid forms, indicating flexibility rather than rigidity. Its elongation at break can reach up to 800%, highlighting exceptional ductility that arises from the soft, semicrystalline matrix.[13] Rheologically, PCL melt exhibits low viscosity due to its low melting point, making it suitable for extrusion and molding processes; it also shows shear-thinning behavior with a Newtonian plateau at low shear rates, enhancing flow under processing conditions.[16] Electrically, PCL is an insulator with a dielectric constant of approximately 3.2 across microwave frequencies and low inherent conductivity, suitable for non-conductive applications.[17] In certain formulations, PCL demonstrates shape memory effects, where temporary shapes are fixed below the glass transition or crystallization temperature and recover to the original form upon heating to around 60°C, with recovery rates exceeding 90% in optimized systems.[18]| Property | Value | Notes |
|---|---|---|
| Melting Point | 58–60°C | Enables low-temperature melt processing[13] |
| Glass Transition Temperature | -60°C | Contributes to rubbery behavior at ambient conditions[13] |
| Crystallinity | ~56% | Semicrystalline nature affects mechanical ductility[13] |
| Density | 1.14 g/cm³ | Typical for bulk PCL[14] |
| Tensile Strength | 10.5–16.1 MPa | For solid PCL[13] |
| Young's Modulus | 0.35 GPa | Indicates flexible response[13] |
| Elongation at Break | Up to 800% | High ductility[13] |
| Dielectric Constant | ~3.2 | At 0.5–3.5 GHz[17] |