Effects of molecular weight on poly(ω-pentadecalactone) mechanical and thermal properties

Abstract

A series of poly(ω-pentadecalactone) (PPDL) samples, synthesized by lipase catalysis, were prepared by systematic variation of reaction time and water content. These samples possessed weight-average molecular weights (M_w), determined by multi-angle laser light scattering (MALLS), from 2.5 × 10⁴ to 48.1 × 10⁴. Cold-drawing tensile tests at room temperature of PPDL samples with M_w between 4.5 × 10⁴ and 8.1 × 10⁴ showed a brittle-to-ductile transition. For PPDL with M_w of 8.1 × 10⁴, inter-fibrillar slippage dominates during deformation until fracture. Increasing M_w above 18.9 × 10⁴ resulted in enhanced entanglement network strength and strain-hardening. The high M_w samples also exhibited tough properties with elongation at break about 650% and tensile strength about 60.8 MPa, comparable to linear high density polyethylene (HDPE). Relationships among molecular weight, Young's modulus, stress, strain at yield, melting and crystallization enthalpy (by differential scanning calorimetry, DSC) and crystallinity (from wide-angle X-ray diffraction, WAXD) were correlated for PPDL samples. Similarities and differences of linear HDPE and PPDL molecular weight dependence on their mechanical and thermal properties were also compared.

Introduction

Polyethylene is the most widely used commodity polymer. It is found in many consumer products, such as milk jugs, detergent bottles, margarine tubs, garbage containers, water pipes, just to name a few. Poly(ω-pentadecalactone) (PPDL) is a new type of thermoplastic that can be synthesized by lipase catalysis [1], [2], [3]. The chemical structure of PPDL, with 14 methylene groups and an in-chain ester linkage in each repeating unit, is very similar to that of linear high density polyethylene (HDPE) (Scheme 1). Polyethylene (PE) cannot be easily decomposed into small molecules after usage. To achieve extensive degradation of the PE carbon backbone, treatment of PE with strong oxidized agents such as nitric acid [4], ozone [5] and permanganic acid [6] or pyrolysis at high reaction temperatures (above 370 °C) [7], [8] is required. Therefore, “white pollution” [9], [10] from un-recycled PE plastics is a mounting problem that mankind must confront. An important advantage of PPDL over PE is that the former has ester groups in the backbone that are susceptive to chain breakage. Consequently, gentle enzymatic hydrolysis can, in principle, be used to decompose PPDL back into monomer building blocks. Currently, on-going studies are in progress to find a suitable enzyme for PPDL biological recycling both in our laboratory and elsewhere [11].

Chemical catalysts such as potassium alkoxides [12], diethylzinc [13], [14] and yttrium isopropoxide [15] can be used for conversion of macrolactones to polyesters. For ω-pentadecalactone (PDL), the use of immobilized lipase catalysis has been proven to be superior to chemical catalyzed routes for making polymers, resulting in more rapid polymerization kinetics as well as yielding polyesters of relatively higher molecular weight. Immobilized lipase-catalyzed polymerization of macrolactones was first published by Uyama et al. [16]. Our laboratory demonstrated that, using Novozym 435 that consists of Candida antarctica lipase B (CALB) physically immobilized on a macroporous support, the polymerization process of PDL gives PPDL with number-average molecular weight (M_n) up to 8.6 × 10⁴ in yields exceeding 90% (route is illustrated in Scheme 1) [17]. Thus, enzyme-catalyzed preparation of PPDL with high molecular weight has provided suitable materials for evaluation of their physico-mechanical properties. For example, mechanical properties of PPDL with M_n 6.5 × 10⁴ (polydispersity, M_w/M_n, 2.0) [18] and thiol-functionalized PPDL telechelics [19] have been studied.

Previous crystallographic work [20] indicated that the a and b parameters of PPDL unit cells are slightly larger than those of PE's and the until cell parameter along the fiber axis is much larger than that of PE's. DMA tests showed that PPDL with M_n 6.5 × 10⁴ had a glass transition at −27 °C. Also, the observed high storage modulus was attributed to high crystallinity as determined by DSC and WAXS [18]. PPDL-based copolymers with other monomers including trimethylene carbonate [2], p-dioxanone [3] and ω-caprolactone [21] were synthesized and studied by thermal and X-ray analysis. Copolymers were found to be highly crystalline random copolymers over the entire composition range. This behavior was attributed to co-crystallization of comonomer units in a common lattice or isomorphic substitution of comonomer units.

The current study aimed to investigate the effect of PPDL molecular weight on its mechanical, thermal and rheological properties. The chosen synthetic methods enabled the preparation of PPDL with M_w values up to 48.1 × 10⁴ and PDI values close to 2.0. Films were prepared by press-molding at 130 °C and tensile testing was performed on these samples. Based on the shape of stress–strain curves, a brittle-to-ductile transition along with maximum elongation at break was observed. Since the chemical structure of PPDL is similar to polyethylene (PE), its thermal and mechanical properties were compared with those of a commercially obtained PE sample [22]. In addition, PPDL films with different molecular weights were analyzed by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA) to reveal the molecular weight dependence of mechanical, thermal, crystalline and rheological properties. It was found that Young's modulus and stress at break exhibit distinct crystallinity dependence; in addition, differences, of elongation at break and true stress at break, between PPDL and linear PE, are discussed.