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.

Incorporating small fractions (typically <10 mol %) of ionic groups into polymer chains has been shown to have profound effects on thermal, mechanical, and charge transport properties. 1,2 These ionomers are used in a range of applications including performance coatings, packaging, membranes, and actuators. Ionic species tend to self-assemble into microphase-separated domains due to Coulombic interactions that serve as thermoreversible cross-links.Although ion aggregation and properties of ionomers have been studied extensively using a number of different techniques (including dynamic mechanical analysis (DMA), 3À5 rheology, 6 X-ray and neutron scattering, 2,7À11 and scanning transmission electron microscopy (STEM) 2,11À13 ) little is known about the effect of these ionic clusters on the molecular dynamics of this class of materials. Dielectric relaxation spectroscopy has been used to investigate the dynamics of some ionomers, predominantly on poly(styrene-co-methacrylic acid) and poly(ethyleneco-methacrylic acid) ionomers, 1,14,15 but very little has been done on sulfonated polystyrene. 16,17 We intend to elucidate the connection between aggregate morphology and dynamics by applying an integrated and systematic approach using sulfonated polystyrene (SPS) as a model ionomer.In our previous paper, we demonstrated that sulfonation has an important effect on the structure and dynamics of sulfonated polystyrene acid copolymers (having 3.5, 6.7, and 9.5 mol % sulfonation), the precursors to the ionomers investigated herein. 18 In this publication we will focus on the effect of neutralizing these materials with zinc. In our investigation, morphology was characterized using X-ray scattering and scanning transmission electron microscopy (STEM) while the dynamics were investigated using dielectric relaxation spectroscopy (DRS).' EXPERIMENTAL SECTION Sample Preparation. Atactic polystyrene, purchased from Pressure Chemical (M w = 123 kg/mol, PDI = 1.06), was sulfonated according a previously published procedure. 18 SPS acid copolymers were neutralized by dissolving in a 90/10 v/v mixture of toluene/ methanol. A stoichiometric amount of Zn acetate was dissolved in a 50/50 v/v mixture of toluene/methanol and added slowly into the gently agitated SPS solution to achieve different degrees of neutralization. The reaction was held at 50°C for 2 h. In this publication the ionomers are designated as SPSx-yZn, where x is the mole fraction of sulfonation (3.5, 6.7, or 9.5) and y is the percent neutralization (0, 25, 50, 75, and 100).The neutralized ionomers were solvent cast at ambient conditions, air-dried for 1À2 days, and then dried under vacuum at 120°C for at least 24 h. Ionomers with T g around or above 120°C (SPS6.7-yM and SPS9.5-yM) were further annealed at ∼ T g þ 20°C for another day. The dried ionomer films were then hot pressed at 160°C and used for X-ray scattering, STEM imaging, and dielectric spectroscopy. All materials ABSTRACT: Scanning transmission electron microscopy (STEM), X-ray scattering, and dielectric relaxa...

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The structure and dynamics of sulfonated polystyrene (SPS) acid copolymers were investigated using X-ray scattering and broadband dielectric relaxation spectroscopy (DRS). Evidence of acid group aggregation was found at 3.5, 6.7, and 9.5 mol % sulfonation. An increase in the segmental relaxation time and dielectric relaxation strength was observed with increasing acid content. The average state of dipolar interactions was quantified as a function of temperature using the Onsager equation and the Kirkwood−Fröhlich correlation factor. Two high-temperature relaxation processes at frequencies 4−7 decades below the segmental relaxation were identified and are proposed to originate from Maxwell−Wagner−Sillars interfacial polarization and association dynamics of various assemblies of acid groups.

Ionomers, polymers containing ionic functionality, have been a significant research focus over the past 5 decades. 1 Their enhanced properties, such as toughness and moduli, over their non-ionic counterparts make these materials attractive for a breadth of applications including performance coatings, films, adhesives, and tough thermoplastic parts. 1 This property enhancement arises from the microphase separation of ionic species into aggregates due to strong electrostatic interactions, which behave as thermoreversible cross-links. 1 Extensive investigation of the morphology of these ionic rich domains and resulting mechanical and dynamic mechanical properties of these materials has been performed. 1À11 However, the fundamental connection between aggregate structure and molecular dynamics is not well-defined. Dielectric relaxation spectroscopy (DRS) is a powerful tool for probing molecular dynamics and can reveal information about segmental chain motion, ion motion and polarization processes.In an effort to elucidate the correlation between ionic aggregate structure and dynamics, we systematically investigated a model ionomer, sulfonated polystyrene (SPS), with varying degrees of sulfonation and neutralization as well as neutralizing ion types, utilizing X-ray scattering, scanning transmission electron microscopy, and DRS. In two previous publications, we discussed the effect of sulfonation on the structure and dynamics of SPS acid copolymers and the influence of neutralizing these copolymers with zinc. 12,13 In the current paper we examine the role of neutralizing ion type (Na, Cs, and Zn) and discuss the effects of sulfonation and neutralization on structure and dynamics of Cs-and Na-neutralized SPS.' EXPERIMENTAL SECTION Sample Preparation. Atactic polystyrene, purchased from Pressure Chemical (M w = 123 kg/mol, PDI = 1.06), was sulfonated according to a previously published procedure. 12 SPS acid copolymers were neutralized by dissolving in a 90/10 v/v mixture of toluene/methanol. A stoichiometric amount of metal acetate (Na, Cs, and Zn) was dissolved in a 50/50 v/v mixture of toluene/methanol and added slowly into the gently agitated SPS solution to achieve different percentages of neutralization. The reaction was held at 50°C for 2 h. In this publication, the ionomers are designated as SPSxÀyM, where x is the mole percent of sulfonation (3.5, 6.7, or 9.5), y is the percent neutralization (0, 25, 50, 75, and 100), and M is the neutralizing ion (Na, Cs, or Zn). In total, 23 polymers were studied and are listed in Table 1.The neutralized ionomers were solvent cast at ambient conditions, air-dried for 1À2 days, and then dried under vacuum at 120°C for at least 24 h. Ionomers with T g around or above 120°C (SPS6.7ÀyM and SPS9.5ÀyM) were further annealed at ∼T g þ 20°C for another day. The dried ionomer films were then hot pressed at 160°C and used for X-ray scattering, STEM imaging, and dielectric spectroscopy. All materials were stored in vacuum desiccators prior to characterization. Selected samples were sent ...