Article pubs.acs.org/Macromolecules
The Role of Soft Segment Molecular Weight on Microphase Separation and Dynamics of Bulk Polymerized Polyureas Alicia M. Castagna,† Autchara Pangon,† Taeyi Choi,† Gregory P. Dillon,‡ and James Runt*,† †
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Applied Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: The influence of poly(tetramethylene oxide) (PTMO) soft segment length on the phase-separated microstructure, state of hydrogen-bonded associations, and molecular dynamics was investigated in polyureas polymerized from the bulk. For higher PTMO molecular weights (1000 and 650 g/mol) hard segments self-assemble into ribbon-like domains, while incorporation of a 250 g/mol soft segment leads to a predominately mixed segment material. The degree of microphase separation of the hard and soft segments, however, is rather incomplete for polymers synthesized from 1000 and 650 g/mol PTMO and decreases with decreasing soft segment molecular weight. Broadband dielectric relaxation spectroscopy reveals two segmental relaxations: a soft segment rich (α) and slow segmental (α2) process. When the molecular weight is reduced from 1000 to 650 g/mol the mobility of these processes is reduced, consistent with findings from differential scanning calorimetry and dynamic mechanical analysis.
■
INTRODUCTION As a consequence of their outstanding energy absorbing capabilities polyureas have found wide ranging applications, including as coatings to prevent ballistic penetration and fracture of steel plates and laminate armor.1−3 Most recently, polyureas have been identified as candidate helmet suspension pad materials to mitigate shock wave energy and protect against traumatic brain injury.4 In recent simulations, polyureas were demonstrated to more effectively dampen blast loadings at high peak stresses than a conventional ethylene vinyl acetate foam.4 The mechanism of energy absorption in these materials, however, is not well understood. It has been proposed that a high strain rate-induced glass transition of the polymer is the dominant energy dissipation mechanism at ballistic impact time scales.5,6 Despite their widespread utility, the phase-separated microstructure−property relationships of polyureas are not welldefined. Similar to segmented polyurethane block copolymers, polyureas consist of alternating low-Tg “soft” segments and high-Tg “hard” segments, which tend to microphase separate into hard and soft phases. While this phase segregation behavior has been extensively studied in polyurethanes and urethane−ureas,7−15 fundamental studies on polyureas are relatively limited.16−18 In this paper, we explore the role of poly(tetramethylene oxide) (PTMO) soft segment molecular weight on microphase separation and the resulting dynamics of a series of bulk polymerized polyureas containing a modified methylene diphenyl diisocyanate (mMDI) hard segment. To date, investigations of the dynamics and high strain rate mechanical © 2012 American Chemical Society
properties have been evaluated on a single polyurea chemistry (PTMO = 1000 g/mol and mMDI),19−21 and it remains unclear what role hard and soft segment chemistry play in determining the microstructural properties and dynamics of this family of materials. The goal of this investigation is to elucidate the connections between molecular structure, morphology, and polymer dynamics to facilitate the future chemical design of polyureas.
■
EXPERIMENTAL SECTION
Sample Preparation. Polyurea alternating copolymers were bulk polymerized using poly(tetramethylene oxide di-p-aminobenzoate) (Versalink, Air Products) and a uritoneimine-modified diphenylmethane diisocyanate (Isonate 143L, Dow). Stoichiometry for all reactions was maintained at the conventional 95% amine to isocyanate (i.e., 5% excess isocyanate). Chemical structures for the reactants are provided in Figure 1. It is important to note that the average functionality of Isonate 143L is reported by the manufacturer to be 2.1, indicating that it consists predominately of bifunctional diphenylmethane diisocyanate and a modest amount of higher functionality isocyanate, as depicted in Figure 1. The available Versalink family from Air Products was investigated (P1000, P650, and P250), where 1000, 650, and 250 refer to the molecular weight of the PTMO repeating units. The materials herein will be referred to by their Versalink component trade name. The components were degassed for >5 h, mixed under ambient conditions, and degassed again for 1−2 min prior to coating on Teflon sheets using a film applicator to control thickness. Films (∼0.3 and 0.7 Received: August 6, 2012 Revised: September 28, 2012 Published: October 12, 2012 8438
dx.doi.org/10.1021/ma3016568 | Macromolecules 2012, 45, 8438−8444
Macromolecules
Article
intensity (the invariant, Q) using the background and absolute intensity corrected SAXS intensities
Δη2′ = cQ = c
∫0
∞
[I(q) − Ib(q)]q2 dq
(2)
where q is the scattering vector and c is a constant given by
c=
(3)
where ie is Thompson’s constant for the scattering from one electron (7.94 × 10−26 cm2) and NA is Avogadro’s number. The experimental variance is reduced from the ideal case due to diffuse phase boundaries and intersegment mixing. Therefore, the ratio of the experimental to the ideal variance (Δη2′/Δηc 2 ) yields a value between 0 and 1, ranging from complete mixing to complete phase segregation. Fourier Transform Infrared Spectroscopy−Attenuated Total Reflectance (ATR-FTIR). FTIR spectra were collected using a Nicolet 6700 FTIR spectrometer (Thermo Scientific) fitted with a diamond ATR cell. 100 scans with a resolution of 2 cm−1 were signal averaged. Dynamic Mechanical Analysis (DMA). Dynamic mechanical analysis was performed using a TA-Q800 DMA in tension at a frequency of 1 Hz, a heating rate of 3 °C/min and amplitude of 30 μm. Broadband Dielectric Relaxation Spectroscopy (DRS). It has been shown previously that dielectric spectra of polyureas are highly sensitive to water content, and drying can occur in the inert environment of the experiment.24 To minimize changes in moisture content during DRS measurements, films were dried overnight under vacuum at room temperature. A parallel plate capacitor configuration was used where samples ∼0.3−0.4 mm thick were sandwiched between brass electrodes with a top electrode diameter of 2 cm. A Novocontrol GmbH Concept 40 DRS spectrometer was used to collect isothermal relaxation spectra from 0.01 Hz to 10 MHz on heating from −120 to 240 °C under a nitrogen environment. Dielectric relaxation strength (Δε) and characteristic relaxation time (τHN) were determined for each relaxation process by fitting the dielectric loss to the appropriate form of the Havriliak−Negami (HN) function:
Figure 1. Chemical structures of Versalink diamines (where n = 13, 9, and 3 for P1000, P650, and P250, respectively) and Isonate 143L MDI. mm thick) were polymerized under ambient conditions for ∼48 h and then were transferred to a desiccant box (
