Synthesis and Dynamic Properties of Ultra-Small-Branched Star Poly(ε-caprolactone)s and Their Application to Alternative Plasticizers

Abstract

In this study, we develop ultra-small-branched star poly(ɛ-caprolactone)s (USB-SPCLs) as nontoxic alternative plasticizers for flexible poly(vinyl chloride) (PVC) and investigate the interplay of dynamic properties with plasticization. We successfully synthesize the three- and six-branched SPCLs with extremely small branched segments using a facile pseudo-one-pot process in a pilot scale and investigate the effect of ultra-small branches on their crystallization behaviors. The number of branched segments and the individual branched segment lengths for USB-SPCLs are precisely controlled via manipulating monomer-to-core ratio, adjusting monomer-to-polymer conversion, end-capping the terminal hydroxyl groups, and vacuum purification, which results in USB-SPCLs having the branched segments below five degree of polymerization with a high yield exceeding 93%. The molecular weights obtained from 1H NMR spectroscopy are consistent with that obtained from MALDI-TOF-MS and the molecular weight distributions are narrow with Mw/Mn ≤ 1.2, indicating that USB-SPCLs have mono-dispersed branches. USB-SPCLs have low melting temperatures and broad double-melting peaks attributed to their extremely small branches, and the crystallization behaviors for USB-SPCLs depend on the end group concentration. On the other hand, the glass transitions for USB-SPCLs depend on the total molecular weights, regardless of the number and length of branched segments. The extremely small branched effects on molecular dynamics are investigated using USB-SPCLs. USB-SPCLs interestingly show total-molecular-weight-dependent glass transitions regardless of the molecular architecture parameters, such as the number and length of branches, whereas typical star polymers with polymeric large branches show the end-group-concentration-dependent glass transitions. The viscoelasticity of USB-SPCLs does not depend exponentially on the individual branched molecular weight, as observed in typical star polymers, and instead follows the Mark–Houwink power law and the Bueche-modified Rouse model for unentangled linear polymers. The flow activation energy and the longest Rouse relaxation time of USB-SPCLs show that the individual branches of USB-SPCL are dynamically equivalent and that a whole USB-SPCL molecule moves with a simple uni-motion. These results suggest that a whole USB-SPCL molecule presumably acts as a dynamically-equivalent single coarse-grain unit because of the extremely small branches on the scale of 20‒40 atoms. USB-SPCL is used as a nontoxic plasticizer for the production of phthalate-free flexible PVC. USB-SPCL is a transparent liquid at room temperature and exhibits unentangled Newtonian behavior due to its extremely short branched segments. USB-SPCL is biologically safe without producing an acute toxicity response. Torque analysis measurements reveals that USB-SPCL offers a faster fusion rate and a higher miscibility with PVC compared to a typical plasticizer, DEHP. The solid-state 1H NMR spectrum reveals that PVC and USB-SPCL are miscible with an average domain size of less than 8 nm. The flexibility and transparency of the PVC/USB-SPCL mixture are comparable to the corresponding properties of the PVC/DEHP mixture, and the stretchability and fracture toughness of PVC/USB-SPCL are superior to the corresponding properties of the PVC/DEHP system. Most of all, PVC/USB-SPCL shows excellent migration resistance with a weight loss of less than 0.6% in a liquid phase, whereas DEHP migrated out of PVC/DEHP into a liquid phase with a weight loss of about 10%. The dynamic effects of unentangled star-shaped polymers with extremely small branches on the plasticization of miscible polymer blends are investigated using USB-SPCLs, PVC, and their blends. Photon correlation dynamics of USB-SPCLs supports our previous suggestion that a whole USB-SPCL molecule acts as a single coarse-grain unit with dynamically-equivalent branches because of the extremely small branches, resulting in the total-molecular-weight-dependent Rouse dynamic behaviors of USB-SPCLs, regardless of the molecular architectures. The dynamic light scattering intensity autocorrelation curves of miscible PVC/USB-SPCL blends reveal that strong intermolecular interactions between PVC and USB-SPCL molecules determine the dynamic homogeneous behaviors of the blends despite their significantly different mobilities. The molecular motions of the blends depend on the total-molecular-weight-dependent Rouse dynamic behaviors of USB-SPCLs. These dynamic results clearly show the plasticization of the entangled neat linear PVC matrix by distinctive and rapid molecular mobility of USB-SPCLs. Free-volume-dependent dynamic behaviors of the pyrene-labeled and -doped PVC blends with different amount of USB-SPCL are analyzed using temperature-dependent FS techniques, to investigate the correlation between the dynamic behaviors of individual polymer chains and the controlled free-volume changes. The individual PVC component in the PVC/USB-SPCL blends interestingly exhibited broad thermal glass transition range from the glass transition of the whole blend system to the glass transition of original PVC, differ from a typical glass transition dynamics of miscible polymer blend with single glass transition temperature, indicating the heterogeneous glass transition dynamic behaviors of the miscible PVC/USB-SPCL blend system. These results suggest that the motion of individual PVC component in PVC/USB-SPCL blends depends on both the enlarged free volume by fast-moved USB-SPCL molecules and the dynamic constraint by entangled PVC chains while general glass transition dynamics is sufficiently described by free-volume of the whole blend system.