Article pubs.acs.org/Macromolecules
Effect of Thermal History on the Microstructure of a Poly(tetramethylene oxide)-Based Polyurea Alicia M. Castagna,†,§ Autchara Pangon,† 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 S Supporting Information *
ABSTRACT: The role of thermal history on the nanoscale segregated structure of a bulk polymerized polyurea containing oligomeric poly(tetramethylene oxide) soft segments is investigated in the present study. Temperature-dependent unlike segment demixing was explored in two series of experiments: at constant heating (and cooling) rate and on annealing at selected elevated temperatures. Tapping mode atomic force microscopy on the as-polymerized polymer demonstrates that the polyurea hard segments self-assemble into a ribbon-like morphology that is generally preserved on annealing, although ribbon coarsening was observed at the highest annealing temperature. The results from the constant heating rate synchrotron X-ray scattering experiments demonstrate that the nanoscale structure begins to reorganize at temperatures as low as ∼70 °C, and the very significant changes in mean interdomain spacing observed at much higher temperatures are largely retained on returning to ambient conditions. Although there was surprisingly no detectable difference in the degree of hard/soft segment segregation in the longer time annealing experiments, changes in interdomain spacing were detected at the lowest annealing temperature (120 °C) used in this study. In combination with the findings from the synchrotron X-ray experiments, this demonstrates that domain reorganization is clearly both time and temperature dependent. The results from X-ray scattering and AFM experiments are also supported by those from FTIR spectroscopy and thermal analysis. directly correlating morphology using findings from small-angle X-ray scattering (SAXS) and DSC experiments have revealed links to changes in microphase-separated structure in polyurethanes with temperature.25,26 In a recent investigation, we reported on the nanoscale morphology and molecular dynamics of a series of polyureas based on a modified methylene diphenyl diisocyanate (mMDI) with varying molecular weight poly(tetramethylene oxide) (PTMO) soft segments.27 This particular polyurea chemistry is of principal interest in shock and blast management applications.28−30 DSC revealed three endothermic peaks at ∼70, 140, and 165 °C, the origin of which was speculated to arise from changes in morphology, possibly due to trapped non-equilibrium structures due to the kinetic constraints arising from bulk polymerization. In the present work, we investigate the role of various annealing conditions on the microstructure and phase separation of a bulk-polymerized mMDI−PTMO polyurea.
1. INTRODUCTION Polyureas, generally containing alternating soft and hard segments, are a class of elastomeric polymers that are broadly similar to segmented polyurethane copolymers. Thermodynamic incompatibility between hard and soft segments gives rise to a nanoscale-segregated morphology that plays a critical role in determining the physical and mechanical properties of these materials. Relatively recently, polyureas have been investigated as coatings for shock absorption applications to prevent ballistic penetration and fracture of steel plates and armor1−3 and are also of interest for protection against traumatic brain injury.4 Although there are a number reports that focus on high strain rate and ballistic energy absorption of these materials,5−7 a fundamental understanding of the structure−property relationships, and their control through processing and thermal treatments, remains in the early stages. The microphase separation behavior has been extensively studied for polyurethanes and poly(urethaneurea)s,8−16 and their thermal properties have been revealed to be rather complex. Differential scanning calorimetry (DSC) thermograms of polyurethanes commonly exhibit a variety of endothermic transitions at high temperatures.17 Initial assignment of the origin of these transitions included hydrogen bond dissociation;18,19 however, later annealing studies revealed that these transitions are thermal history dependent.20−24 Studies © 2013 American Chemical Society
Received: April 25, 2013 Revised: July 3, 2013 Published: August 16, 2013 6520
dx.doi.org/10.1021/ma400856w | Macromolecules 2013, 46, 6520−6527
Macromolecules
Article
phase separation were quantified from the SAXS profiles using the ratio of experimental to theoretical electron density variance (Δη2 ′/Δηc 2 ), according to the methodology of Bonart and Müller.12 The theoretical electron density variance for complete phase separation can be expressed as
2. EXPERIMENTAL SECTION 2.1. Materials. The polyureas were prepared via bulk polymerization of polytetramethylene oxide-di-p-aminobenzoate (Versalink P1000, Air Products) and a uritoneimine-modified diphenylmethane diisocyanate (Isonate 143L, Dow) as reported elsewhere.27 To ensure completeness of the reaction, the polymerization was performed at 5% excess isocyanate. The chemical structures of the reactants are displayed in Figure 1. It is important to note that Isonate 143L consists
Δηc 2 = ϕ1ϕ2(η1 − η2)2
(1)
where ϕ1 and ϕ2 are volume fractions, and η1 and η2 are electron densities, of the hard and soft phases, respectively. For the purposes of the present study, MDI, the urea linkage and benzene ring of the diamine were considered as the hard segment while PTMO and the two carbonyl groups of the diamine were considered as the soft segment. The experimental electron density variance was determined from total integrated scattering intensity (Q) of the scattered intensity, after background correction.
Δη2 ′ = cQ = c
∫0
∞
{I(q) − Ib(q)}q2 dq
(2)
where c is a constant defined by
c=
1 = 1.76 × 10−24 mol2/cm 2 2π 2ieNA 2
and ie is Thompson’s constant for the scattering from one electron (7.94 × 10−26 cm2) and NA is Avogadro’s number (6.02 × 1023 mol−1). Temperature-dependent synchrotron SAXS and WAXD were performed at beamline X27C at the National Synchrotron Light Source, Brookhaven National Laboratory, using an X-ray wavelength 1.371 Å. The sample-to-detector (Mar CCD detector) distances for SAXS and WAXD were 1.877 and 0.1605 m, respectively. Samples were heated and subsequently cooled in the temperature range 30− 240 °C, and the temperature ramp was stopped for 1 min at 10 °C intervals. The duration of each 10 °C change in temperature was ∼15 s. SAXS and WAXD patterns were collected at each T interval for 20 s. The isotropic 2D data obtained were azimuthally averaged to yield 1D patterns and were corrected for background and sample transmission. The relative invariant (in arbitrary units) was calculated from the SAXS profiles to investigate the temperature dependence of the phase separation behavior. For an ideal two-phase system with sharp interfaces, Q is defined by
Figure 1. Chemical structures of Versalink P1000 diamine and Isonate 143L mMDI. of a diisocyanate and triisocyanate with an average NCO functionality provided by manufacturer of 2.1. Based on the reported functionality, the fractions of di- and triisocynate components are estimated to be 88.5% and 11.5%, respectively. From Flory’s theory of network formation, assuming that all functional groups have the same reactivity and are independent of the status of other groups in the same molecule,31 we determine that network formation is likely for this system (the probability that each branch unit is connected to another branch unit through chain segments = 0.77). In brief, the components were degassed for more than 5 h, mixed under ambient condition, and degassed again for 1−2 min prior to casting on Teflon sheets by using a film applicator to control thickness. Films obtained were cured under ambient conditions for ∼48 h and were subsequently maintained under
