Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Maintaining the Hydrophilic−Hydrophobic Balance of Polyesters with Adjustable Crystallinity for Tailor-Made Nanoparticles Damiano Bandelli,†,‡ Christian Helbing,‡,§ Christine Weber,†,‡ Michael Seifert,§ Irina Muljajew,†,‡ Klaus D. Jandt,*,‡,§ and Ulrich S. Schubert*,†,‡
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Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany ‡ Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany § Chair of Materials Science (CMS), Department of Materials Science and Technology, Otto Schott Institute of Materials Research, Faculty of Physics and Astronomy, Friedrich Schiller University Jena, Löbdergraben 32, 07743 Jena, Germany S Supporting Information *
ABSTRACT: To explore the relationship between thermal properties of a polymer and the biological performance of the resulting nanoparticle, all other parameters, including the hydrophobicity, should be kept constant. For this purpose, a gradient and a block copolyester were tailor-made via the triazabicyclodecene catalyzed ring-opening copolymerization of δ-valerolactone (δVL) and δ-decalactone (δDL) to match the hydrophobicity of poly(εcaprolactone) (PεCL). The degree of crystallinity of the semicrystalline materials was significantly reduced due to the incorporation of amorphous PδDL segments, as confirmed by dynamic scanning calorimetry. Atomic force microscopy revealed short and randomly oriented crystals in the gradient copolymer but longer and parallel aligned crystals for the block copolymer and PεCL. The stiffness of nanoparticles (Dh ≈ 170 nm) prepared from the polyesters correlated to the bulk crystallinity. The set of nanoparticles with constant hydrophobicity and size will facilitate direct access to the influence of the nanoparticle crystallinity on biological processes such as enzymatic degradation, drug release, and cellular uptake.
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INTRODUCTION Polymeric nanoparticles represent highly promising materials for the targeted delivery of actives. They are often composed of a biodegradable polymer core serving as a reservoir for pharmaceutically active compounds, while stealth polymers1 or targeting ligands2 can be attached to its shell.3 The interdisciplinary field and the modularity of the concept offer a vast parameter landscape, rendering strict systematic investigations extremely complex. However, the latter are required to understand nanoparticle mediated drug delivery, which is one key factor for the development of a truly personalized medicine. Although the physicochemical characterization of nanoparticle carrier systems alone has been established,4 investigations of structure−property relationships with a predicting character regarding, e.g., release profiles of actives are still missing. The vast majority of degradable nanocarriers is composed of polyesters such as poly(ε-caprolactone) (PεCL) or polylactide (PLA).5 Encapsulated actives are released by enzymatic degradation.6 Besides other factors such as the hydrophobicity, the molar mass, or the chemical composition of the polymers, the crystallinity of polyesters influences the enzymatic degradation rate and, hence, the release from polyester-based nanoparticles.7−9 However, a clear statement can only be made © XXXX American Chemical Society
if only one parameter is varied, but all other parameters are kept constant. Although such investigations exist regarding the influence of the degree of crystallinity for PLA stereocomplexes in thin films,10 the issue is more complex for aqueous nanoparticle suspensions and has, to the best of our knowledge, not been clarified yet. Whereas, e.g., the size of a polymer nanoparticle can be easily varied by using the identical polymer material,11 a variation of the polyester crystallinity is often accompanied by a variation of the chemical composition. Unfortunately, a constant hydrophilic−hydrophobic balance (HHB) of the materials is difficult to maintain because polyesters with elongated alkyl spacers are more crystalline but also more hydrophobic than polyesters with shorter alkyl spacers.12,13 We therefore selected PεCL as a well-known semicrystalline reference material and approached the issue by developing polyesters that would feature a different degree of crystallinity but the same hydrophobicity, i.e., the same fraction of ester moieties per polymer chain. The copolymerization concept relies on δ-lactones as monomers (Scheme 1). Lacking one Received: April 30, 2018 Revised: June 19, 2018
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DOI: 10.1021/acs.macromol.8b00925 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Schematic Representation of the Ring-Opening Polymerization of ε-Caprolactone (εCL), δ-Valerolactone (δVL), and δ-Decalactone (δDL) Yielding the Homo- and Copolyesters P1 to P10
Size exclusion chromatography (SEC) measurements were performed on a Shimadzu system equipped with a CBM-20A system controller, a LC-10AD VP pump, a RID-10A refractive index detector, a SPD-10AD VP UV detector, and a SDV linear S column from PSS (Polymer Standards Service GmbH, Mainz, Germany) at 40 °C using chloroform:triethylamine:2-propanol (94:4:2) as eluent at a flow rate of 1 mL min−1. The system was calibrated against PMMA standards (410−88 000 g mol−1), which were purchased from PSS. For the measurements of the matrix-assisted laser desorption/ ionization time-of-flight (MALDI-ToF) mass spectra, an Ultraflex III ToF/ToF instrument (Bruker Daltonics, Bremen, Germany) was used. The instrument is equipped with a Nd:YAG laser and a collision cell. All spectra were measured in the positive reflector mode using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] (DCTB) as matrix and sodium iodide (NaI) as doping salt. The instrument was calibrated prior to each measurement with an external PMMA standard (2500 g mol−1) from PSS. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere on a Netzsch TG 209 F1 Iris from room temperature to 600 °C at a heating rate of 10 K min−1. Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 204 F1 Phoenix under a nitrogen atmosphere from −150 to 210 °C. Three cycles were recorded for each sample using a cooling rate of 20 K min−1 between the heating runs. The first and the second heating run were conducted at a heating rate of 20 K min−1. For the third heating run, a heating rate of 10 K min−1 was applied. The glass transition temperature (Tg, inflection value reported) and the melting temperature (Tm) values are reported from the second heating run. Dynamic light scattering (DLS) and ζ-potential measurements were performed on a Zetasizer Nano ZS (Malvern Instruments, Herrenberg, Germany) at 25 °C (λ = 633 nm) at an angle of 173°. Each measurement was performed five times. The mean particle size was approximated as the effective (Z-average) diameter and the width of the distribution as the dispersity index (PDI) of the particles obtained by the cumulants method assuming a spherical shape. A Leica DM 2700 equipped with a linkam heating stage was used to prepare polymeric spherulites. For this purpose, a small amount of the polymer was placed on a clean glass slide and heat-treated with the same temperature profile as described for the DSC measurements. The formation of spherulites was clarified by light microscopy with crossed polarizers. Shape and dimensions of the nanoparticles were investigated by scanning electron microscopy (SEM) with an AURIGA 60 CrossBeam workstation (Carl Zeiss AG, Oberkochen, Germany). Additionally, atomic force microscopy (AFM) measurements were performed with a Dimension 3100 and Catalyst (both from Bruker, Vecco, Santa Barbara, CA) equipped with a nanoscope IV and VIII controller, respectively, to determine the nanoparticle shape and stiffness. Measurements were performed at room temperature by using standard tapping mode silicon cantilevers from Bruker (model RTESP, Vecco, Santa Barbara, CA) with a resonance frequency in the
methylene unit compared to PεCL, poly(δ-valerolactone) (PδVL) represents a semicrystalline polyester with a similar melting temperature as the reference material. Substituents at the six-membered monomer ring are known to significantly decrease the crystallinity of the corresponding polyesters, which in fact are often amorphous.13 We hence selected δdecalactone (δDL), comprising four additional methylene moieties compared to εCL, as a second monomer to compensate for the “missing” methylene moiety of δVL. A copolymer consisting of 80 mol % of δVL and 20 mol % of δDL would hence feature the same fraction of ester moieties as PεCL. δ-Lactones can be polymerized via ring-opening polymerization (ROP) using cationic initiators,14 the standard catalyst tin(II) octoate (Sn(Oct)2),15 and organic base catalysts.16−20 The negative free enthalpy and entropy of the ROP of δlactones make the resulting polyesters polymers featuring a classical ceiling temperature.13 In view of this fact, we relied on the highly active catalyst triazabicyclodecene (TBD), which has already been successfully applied for the homopolymerization of δVL21 and δDL22,23 at room temperature. The synthetic development of the tailor-made copolyesters we describe herein includes detailed kinetic studies to elucidate the microstructure and is complemented by an extensive characterization of the thermal and mechanical properties of the materials. Dynamic scanning calorimetry and polarized light microscopy were applied as integrating methods24 to bulk samples. Atomic force microscopy25,26 was applied to correlate the bulk properties with the mechanical properties of nanoparticles prepared from the materials.
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EXPERIMENTAL SECTION
Materials. δ-Valerolactone (δVL, 98%) and δ-decalactone (δDL, 97%) were purchased from TCI. ε-Caprolactone (εCL, 97%) was purchased from Sigma-Aldrich and dried over calcium hydride. Benzyl alcohol (BnOH, 99.8%, water content
