Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Process-Oriented Structure Tuning of PBT/PTHF Thermoplastic Elastomers Matthias Neb́ ouy,† Andre ́ de Almeida,† Soleǹ e Brottet,‡ and Guilhem P. Baeza*,† †
Univ Lyon, INSA-Lyon, CNRS, MATEIS, UMR 5510, F-69621 Villeurbanne, France Univ Lyon, INSA-Lyon, CNRS, Institut des Nanotechnologies de Lyon, UMR 5270, F-69621 Villeurbanne, France
‡
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ABSTRACT: The multiscale structure of thermoplastic elastomers (TPEs) made of poly(butylene terephthalate)/ polytetrahydrofuran (PBT/PTHF) segmented block copolymers is investigated in a systematic way with the aim to characterize the impact of the processing route on it. By means of atomic force microscopy and small-angle scattering techniques, we first evidence the presence of thick (ca. 11 nm) and well-aligned ribbons in hot-pressed samples, whereas thinner and branched filaments (ca. 5 nm) are observed when solvent casting is used. Besides, differential scanning calorimetry suggests that all the so-formed structures can “remember” their initial state after being melted, a property that we assign to the persistent phase separation of the constituents in the viscoelastic liquid. At a smaller length scale, combining transmission and reflection wide-angle X-ray scattering experiments further reveals the influence of the processing method on the molecular order within the crystallites. In particular, it makes emerge the pronounced orthotropic character of the whole samples set, being to our knowledge totally ignored in the TPE’s literature. In light of this extensive characterization, we end the article with a discussion on the relationship between the crystalline network’s structure and the corresponding chains topology.
1. INTRODUCTION Thermoplastic elastomers (TPEs) based on segmented block copolymers cover a broad class of industrial applications ranging from daily consumer goods, dashboard elements, bitumen modifiers, adhesives, or electrical insulators.1−4 Their development started in the 1960s with the synthesis of the (still) popular styrene−butadiene−styrene triblock copolymer,5−7 offering outstanding mechanical properties coupled with an exceptional ease of processing thanks to its thermoreversible nature. Since then, a great variety of TPEs based on glassy8,9 or crystalline10 hard segments (HSs) assemblies combined with different soft segments (SSs)11,12 have been designed, all based on the same philosophy, i.e., the phase separation of their constituents. As a matter of fact, while at low temperature the association of the HSs leads to the formation of a rigid network conferring the material its elasticity, their dissociation at higher temperatures engenders a solid−liquid transition allowing the material to be (re)shaped easily. Beyond their reversible character, TPEs (and more generally block copolymers) present the great asset to be highly sensitive to the processing methods,13−16 i.e., tunable in terms of structure and properties, another important advantage with respect to their vulcanized counterparts. Among them, poly(ether−ester)s composed of poly(butylene terephthalate) (PBT) and polytetrahydrofuran (PTHF) units are synthesized industrially for more than 40 years because of their versatile © XXXX American Chemical Society
mechanical behavior, chemical resistance, thermal stability, and rapid crystallization.17−25 Nevertheless, in spite of an intense research activity aiming at understanding their structure− properties relationship, the impact of the processing conditions on the morphology of these semicrystalline copolymers still remains elusive, albeit highly relevant for the production at an industrial scale. Once synthesized, commercial TPEs are most often stored as extruded granules that one may then use as raw materials for the design of final products through injection, hot-pressing, or solvent-based methods. While the process impact seems overall well-established, it appears however that only a few academic studies have tackled this major issue in a systematic way.18,26 Very often, TPEs characterizations are actually limited to a dual study containing, on one hand, popular thermomechanical characterizations such as dynamic temperature sweeps or tensile tests and, on the other hand, microscopy, scattering, and calorimetry databarely correlated to the properties. Many articles propose, for example, to study the impact of the HSs fraction (i.e., length or number) along the copolymer chains, leading routinely to higher crystalline content, melting point, and plateau modulus.10,27−31 Received: June 15, 2018 Revised: July 26, 2018
A
DOI: 10.1021/acs.macromol.8b01279 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) Chains microstructure of HS30, HS40, and HS65. (b, c) Chemical formulas of the corresponding hard (PBT) and soft (PTHF) units.
experiments performed on TPEs processed through various methods in section 3. On the basis of these experimental evidence, we finally discuss the connections between the hardphase morphology and the resulting chains topology in section 4.
Besides, it is worth noting that this class of materials is particularly well-adapted to the use of atomic force microscopy (AFM), whereas electronic techniques (TEM and SEM) are preferred for nanocomposites because of their enhanced electronic contrast.32 (The latter techniques are however of a little use for TPEs,18,27 requiring staining procedures to improve the images quality.33) In fact, AFM performed on TPEs takes advantage of the mechanical properties contrast at the mesoscale, i.e., between the hard and soft domains, resulting most often in sharp “phase” images obtained through tapping mode.10 However, while it is true that many AFM micrographs realized from various TPEs can be found in the literature, the lack of a systematic characterization of the processing impact still limits our fundamental understanding of the network formation. For instance, we note the extensive use of spin-coating and drop- or solvent-casting31,34−38 that we attribute to their capacity to create clear-cut crystallites observable easily through AFM. On the contrary, hot-pressed samples being undoubtedly the most relevant for industrial applications are only scarcely investigated.39 Both kinds of processes are in consequence too rarely confronted for a given chemical composition.18,26 Following the idea of a systematic structural characterization, we also observed that X-ray scattering experiments (WAXS-SAXS) as well as microscopy investigations always were performed on the top view of film-shaped samples.27,39,40 Strikingly, while both solvent methods and melt-processing routes undoubtedly generate anisotropic stress fields, we realized that probing the TPEs from different directions, and in particular along the side view of the films, was completely ignored in the academic literature. The latter effects, leading likely to orthotropic morphologies, are yet well-known for nanocomposites,41,42 requiring therefore a deeper work for the description of the structure−properties relationship in TPEs. To address the qualitative impact of the processing methods and the anisotropic morphologies they may induce in film-like samples, we propose in this work a systematic study of the TPEs structure based on a complementary set of experimental techniques. By means of DSC, AFM, SAXS, and coupled WAXS-R/WAXS-T (reflection/transmission) investigations, we give new insights into both the hard-phase structure and the resulting copolymers conformations. The article is organized as follows: After having presented the TPEs chemical microstructure, we describe the processing routes and the experimental methods in section 2. We present then the results from DSC, AFM, WAXS-T, and WAXS-R
2. MATERIALS AND METHODS Materials. The segmented copolymers denoted HS30, HS40, and HS65 in the article were synthesized via polycondensation and provided by DSM, The Netherlands. They contain respectively 30, 40, and 65 wt % poly(butylene terephthalate) (PBT). The connection between the PBT segments is made through 1 kg mol−1 (HS65) or 2 kg mol−1 (HS30 and HS40) PTHF soft segments. The TPEs’ total average molecular weight and polymolecularity index have been measured through SEC being respectively Mn = 25 kg mol−1 and Ip = 2. Further details such as the average molecular weight of the PBT hard segments MHS and their average number in the chains ⟨N⟩ can be found in Figure 1 and Table 1.
Table 1. TPEs Microstructure sample
HSa [wt %]
Mna [kg mol−1]
Ipa
MSSa [kg mol−1]
MHSb [kg mol−1]
⟨N⟩b
HS30 HS40 HS65
30 40 65
25.0 25.0 25.0
2 2 2
2.0 2.0 1.0
1.0 1.5 2.1
7.7 6.5 7.7
a
Data provided by DSM. bAverage values calculated from ref 1.
These materials are produced via melt transesterification and subsequently polycondensation of dimethyl terephthalate (DMT), 1,4-butanediol (BDO), and poly(tetramethylene oxide) (“PTMO” or “PTHF”) of various molecular weights (1 and 2 kg mol−1 in the present work). Tetrabutyl titanate (TBT) is used as a catalyst, and the polyether is protected against oxidation during polymerization by 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene. The polycondensation is performed at a temperature of 250 °C under vacuum, with a typical polymerization time of 200 min. Some details on the chemical route can be found in ref 20. Subsequent to the polymerization, the material is extruded as strands and pelletized. The so-formed granules are subsequently dried at 100 and 140 °C for 2 and 5 h, respectively, and used as such in the following study. Samples Preparation. TPEs films were prepared through four different processes that we describe below (see also Figure 2). Drop-casting (DC) was performed by preparing a solution of TPEs in hexafluoroisopropanol (HFIP) at low polymer concentration (1 wt %). A small quantity of this mixture was then dropped onto a mica substrate resulting, after a few minutes evaporation at room temperature, in a very thin film (thickness
