A Rheological Investigation of Entanglement in Side-Chain Liquid-Crystalline Azobenzene Polymethacrylates

Abstract

The shear rheological behavior was investigated in a series of high molar mass liquid-crystalline polymers (PMA4 homopolymer and copolymer samples) carrying an azobenzene mesogenic chromophore group in the side chains. The focus was on studying the entanglement effects and testing selected reptation models to ascertain their ability to reproduce the complex shear modulus of the copolymers. We found that ordinary dynamic models worked for the nematic PMA4 copolymers, nicely reproducing the rheological response of the materials. We were able to obtain microscopic information on the materials, such as Rouse time and entanglement molar mass, in a consistent way, as well as to get insight on the macroscopic effects of tube dilatation induced by the nematic order on the master curves of the entangled polymers. Model improvements, accounting for the different nature of the co-units, were also proposed in this work that singled out the friction coefficients of the co-units MMA and MA4. The monomeric friction coefficient ζ0MA4 was found to be constant throughout the series, (5±2)×10–9 kg s–1. Likewise, ζ0MMA had the same value throughout the series, very similar to literature data for PMMA homopolymers, (2.0±0.6)×10−8 kg s−1. Finally, in the framework of the packing length model, constant packing lengths of 3.5 Å and 13 Å throughout the series were found for MMA and MA4 co-units, respectively. Also, it resulted that the viscoelastic behavior of any PMA4 random copolymer could be predicted, provided that the response of the extreme homopolymers of the series has been characterized

Heterogeneity and Dynamics in Azobenzene Methacrylate Random and Block Copolymers: A Nanometer–Nanosecond Study by Electron Spin Resonance Spectroscopy

“…Standard samples of polystyrene (molar mass 1-500 kg mol -1 ) were used for calibration. Based on previous measurements, 26 we estimate that the absolute values of the molar masses in the distribution of these copolymer samples may differ by 10% from the respective SEC values. Differential scanning calorimetry (DSC) measurements of random copolymers were carried out with a Perkin-Elmer DSC7 calorimeter, recording thermograms on heating at 10 K min -1 after cooling at 40 K min -1 from about 383 K. The DSC curves of block copolymers were recorded with a Mettler Toledo DSC30 calorimeter on heating from about 225 K at 10 K min −1 after cooling at 20 K min −1 from about 473 K.…”

“…40,41 No discontinuity was observed in the investigated temperature range for any sample, not even across Tni or Todt. 9,26,32 9/36 40,41 (Table 3) is also shown.…”

“…Moreover, the shear moduli in dynamic experiments carried out in the linear response regime did not show any heterogeneous behavior across the characteristic temperatures of the different samples. 26,32 However, the different chemical and structural arrangement of the block copolymers led to the failure of the time-temperature superposition 42 principle (TTS) in B20 and B10. Therefore, viscosity was only evaluated by creep-recovery experiments for these copolymers.…”

“…The best fits are superimposed to the experimental curves in Figure 3. Values of Tb and T0 are given in Table 3, where values 26 of η∞ are also reported for the sake of completeness. In Table 3, the fitting parameters according to a VF dependence are also reported for the shift factor 45 a(T) of G' in B20 and B10.…”

Onset of entanglement and reptation in melts of linear homopolymers: consistent rheological simulations of experiments from oligomers to high polymers

“…In this work, the complex shear modulus of poly(ethyl acrylate) (PEA) melts is predicted according to four models based on reptation. Since the molar mass of the samples ranges from oligomers to high polymers with about 15M e , indication could be inferred on the mass where the entanglement is expected to onset and its influence for a correct calculation of the experimental master curves (Andreozzi et al 2013;Lin and Juang 1999). Indeed, special care has been devoted to simulate in a consistent way the rheological response at the different masses.…”