Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Crystallization-Induced Confinement Enhances Glassy Dynamics in Star-Shaped Polyhedral Oligomeric Polysilesquioxane−Isotactic Polystyrene (POSS−iPS) Hybrid Material Martin Tress,*,†,‡ Maximilian Vielhauer,§ Pierre J. Lutz,§,∥ Rolf Mülhaupt,§,⊥ and Friedrich Kremer‡ †
Department of Chemistry, University of Tennessee, Knoxville, 1420 Circle Dr., Knoxville, Tennessee 37996, United States Peter-Debye-Institute, University Leipzig, Linnestr. 5, D-04103 Leipzig, Germany § Freiburg Materials Research Center FMF, University of Freiburg, Stefan-Meier-Str. 21, D-79104 Freiburg, Germany ∥ Institute Charles Sadron, CNRS UPR 22, University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg, France ⊥ Institute for Macromolecular Chemistry, University of Freiburg, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany ‡
S Supporting Information *
ABSTRACT: In semicrystalline polymers, the segments around the crystallites typically relax significantly slower than in the purely amorphous phase. This results in an, on average, slower dynamics. Here we present a contrary effect in a star-shaped polymer based on a polyhedral oligomeric silesquioxane (POSS) molecule as center and isotactic polystyrene arms. Measurements by means of broadband dielectric spectroscopy reveal a reduction of the mean relaxation time by up to 1 decade. Analyzing the relaxation time distribution unravels three moieties of different dynamics beyond the crystalline fraction. These are assumed to form respective domains: a rigid amorphous fraction around crystallites, a mobile amorphous fraction, and a confined amorphous fraction of enhanced dynamics presumably located around the POSS centers. Probably, the crystallites in combination with the starlike architecture stabilize the average volume which balances the higher density of the growing crystallites by an increase in free volume in the amorphous domains.
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INTRODUCTION Crystallinity in polymers is in focus of research for a long time,1 but still certain mechanisms of this process are not yet understood.2 Nevertheless, (partially) crystalline polymers play an important role in technology and applications whereas crystallization strongly impacts the macroscopic properties.3 This is easily comprehensible considering that the chain segments in the crystallites are highly immobilized1 compared to their counterparts in the amorphous regions. In fact, it has been shown that in a semicrystalline polymer amorphous chains in close vicinity to the crystallites exhibit slower dynamics and form the so-called rigid amorphous fraction (RAF).3,4 Among other polymers,1 isotactic polystyrene (iPS) gained some interest since its relatively slow crystallization enables detailed investigations of the purely amorphous and the semicrystalline state in a wide temperature range.4−6 Thus, not only structural information obtained by X-ray scattering but also measurements of the molecular dynamics by means of broadband dielectric spectroscopy were accomplished.4−6 It was found that a degree of crystallinity of 30−40% develops which leads to a reduced dielectric relaxation strength and a broadening of the relaxation peak primarily at the low frequency side. This indicates a reduced number of mobile segments and the emergence of additional slower relaxation © XXXX American Chemical Society
modes, respectively. While the former is a direct consequence of the immobilization of segments in the crystalline domains, which do not contribute to the relaxation, the latter is attributed to the RAF in-between the crystalline domains organized in a lamellar structure and the amorphous regions of usual mobility, referred to as mobile amorphous fraction (MAF).4 From complementary X-ray measurements, the thickness of these rigid amorphous phases has been determined to 0.7 nm, which corresponds to the fact that spatial effects on glassy dynamics range on the order of ∼1 nm.7,8 The sizes of the crystalline and mobile amorphous domains are given to 2−4.5 and 6−10 nm, respectively, with the larger values applying to faster crystallization.4,6 In this regard, semicrystalline polymers share similarities with polymer nanoparticle composites, a class of materials seeing growing attention due to increasing access to fabrication and characterization on the nanoscale. In this case, nanometer sized particles are distributed in a polymer matrix to modify its properties. Attractive interactions between the nanoparticle surface and the polymer generate an interfacial layer of lower Received: October 3, 2017 Revised: December 27, 2017
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DOI: 10.1021/acs.macromol.7b02137 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules mobility9 which governs the macroscopic material properties due to the dramatically increased inner surface. Following the current picture of such composite materials, semicrystalline polymers resemble an intrinsic nanocomposite. Recently, it has been shown that the variation of chain length in polymer−nanoparticle composites can lead to qualitatively different dynamics.10 Apparently, in cases of chains that are of similar size like the interparticle distance, those chains which are in direct contact to each other but adsorbed to different adjacent particles cannot fill the space as efficient as in the bulk (or in the case of shorter chains which allow for free, nonadsorbed chains in-between). The resulting packing frustration generates increased free volume and, thus, enhanced segmental dynamics. This resembles the increased mobility observed in spatial confinement, e.g., of low molecular weight molecules in nanoporous matrices.11−13 These composite systems exemplify how chain length, or more general chain architecture, can have a severe impact on the dynamics in systems with strongly interacting nodes, in that case the particles, distributed on the nanoscale. Here, we present a POSS−polymer hybrid material which exhibits enhanced segmental mobility in the semicrystalline state. The mean relaxation rate of the segments is increased by almost 1 order of magnitude compared to the purely amorphous sample. A conversion into relaxation time distribution allows us to determine three different dynamical regions in the amorphous phase: a (bulklike) mobile amorphous fraction (MAF), a rigid amorphous fraction (RAF), and additionally a confined amorphous fraction (CAF). This unexpected dynamics resembles a spatial confinement and seems to originate from the star-shaped architecture of the polymer where a POSS particle serves as central molecule to the corners of which are 6−8 chains of isotactic polystyrene are attached.
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Consequently, a free upper interface remains even after the application of the counter electrode which allows for relaxation of mechanical stress and also structural rearrangements in the sample during phase transitions. All preparation steps from cleaning until assembly were performed in a laminar flow box. Then the sample has been annealed at a temperature of ∼150 °C (∼ Tg + 50 K) under high vacuum (10−6 mbar) for 12 h. For the actual BDS measurement, performed by an Alpha Analyzer combined with a Quattro temperature controller (both from Novocontrol), the sample was kept in a cryostat and tempered by a stream of dry nitrogen. During the measurement, different protocols of data acquisition have been employed: (i) continuous recording of spectra during particular thermal treatment to record e.g. isothermal crystallization or (ii) isothermal scanning of spectra (typical range: 380−420 K and 0.1 Hz−1 MHz) to obtain data for the activation curve. Prior to measurements of the amorphous state, the material is heated above its melting temperature Tm = 206 °C (DSC)14 to ensure it is entirely molten (210 °C for ∼1 h). To obtain semicrystalline samples, it is tempered at 460 K (187 °C) for ∼10 h to allow for isothermal crystallization. In a further experiment to monitor the evolution of the segmental dynamics during crystallization, a sample in the amorphous state was tempered at 460 K for isothermal crystallization and has been cooled down to 360 K for isothermal BDS scans several times. To analyze the dielectric data, a fit function based on an equivalent circuit model to consider the effect of the sample geometry18,19 has been developed. It includes a Havriliak−Negami function20 to describe the segmental relaxation of iPS
ε∗ (ω) = ε∞ +
(2)
with the permittivity in the high frequency limit ε∞, the dielectric relaxation strength Δε, the skewness parameters of the relaxation peaks β and γ, and the Havriliak−Negami relaxation time τHN. The latter is related to the position of the peak maximum ωc = τc−1 by20 ⎡ ⎢ sin τc = τHN⎢ ⎢ sin ⎣
MATERIALS AND METHODS
Differential Scanning Calorimetry. A detailed description of the synthesis of the POSS−iPS star polymer can be found in our previous publication.14 DSC measurements are performed with a Q2000 differential scanning calorimeter (TA Instruments) in the temperature range from 60 to 240 °C. Sample masses of 5−10 mg in standard aluminum pans were investigated by employing a heating rate of 10 K/ min. The specific heat of fusion ΔHf is extracted from the area underneath the melting peak, and the degree of crystallinity was calculated according to15
ΔHf fc = ΔHf,c
Δε (1 + (iωτHN)β )γ
( ) ⎤⎥ ⎥ ( ) ⎥⎦ πβγ 2 + 2γ
1/ β
πβ 2 + 2γ
(3)
For a detailed analysis of the relaxation times and especially their distribution, the relaxation time distribution function G(τ) is calculated according to21
⎛ ⎡ ⎛ ⎛ τ ⎞β ⎞⎤⎞ ⎜ ⎢π ⎜ ⎝ τ ⎠ + cos(πβ) ⎟⎥⎟ sin⎜γ ⎢ 2 − arctan⎜ HN sin(πβ) ⎟⎟⎥⎟ ⎜ ⎟ ⎜ ⎢ ⎝ ⎠⎥⎦⎠ ⎝ ⎣ ⎜
G(τ ) =
⎛ π ⎜1 + 2 cos(πβ) ⎝
(ττ )
(1)
HN
⎟
β
+
2β ⎞γ /2
(ττ ) HN
⎟
⎠
(4)
∞ τ=0G(τ)
d ln τ = 1, it is only a Since G(τ) obeys the normalization ∫ relative measure.20 To compare the relaxation time distribution functions of samples in the amorphous and semicrystalline state, i.e., with different absolute amount of mobile segments, the distribution functions are multiplied with the corresponding relaxation strengths. The product G(τ)·Δε is an absolute measure for the existing relaxation modes.
with the specific heat of fusion ΔHf,c = 86.5864 J/g of 100% crystalline iPS.15,16 Broadband Dielectric Spectroscopy. For the investigation of the molecular dynamics, broadband dielectric spectroscopy (BDS) is combined with a recently developed nanostructured electrode arrangement. Details about this technique can be found elsewhere;8,17,18 in brief, highly doped silicon wafers (resistivity
