Linear Rheology of a Series of Second-Generation Dendronized Wedge Polymers

Abstract

A series of second-generation dendronized wedge polymers were synthesized by ring-opening metathesis polymerization, and the linear viscoelastic response over a wide range of temperatures was investigated. From 0 to 90 °C the dynamic moduli (G′(ω) and G″(ω)) were determined, and frequency−temperature superposition was used to create master curves that showed behavior from the terminal zone to the glassy regime. An apparent extremely low rubbery plateau of ∼10 kPa was observed in both the dynamic response and in the corresponding van Gurp−Palmen plot. However, further investigation shows that the apparent rubbery plateau is related to the steady-state recoverable compliance, not the onset of entanglements. In addition, these wedge polymers exhibit an extremely low glassy modulus of ∼100 MPa at 0 °C, which is shown to increase at 1 Hz to ∼700 MPa at −80 °C for the wedge polymer 2G-EHW-311. In addition, both small-and wide-angle X-ray scattering patterns were obtained for all of the polymers investigated, and these showed that the polymer molecules adopt an extended cylinder conformation. Furthermore, based on calorimetric measurements, the polymers were found to exhibit two glass transition temperatures, with a 100 K difference between the higher (T g,hi = 26.8 ± 0.7 °C) and lower glass transition temperatures (T g,lo = −76.1 ± 1.1 °C) for the 2G-EHW-311 material. Hence, an intermediate regime extends to well below the T g,hi to T g,lo , providing an explanation for the low glassy modulus of ∼100 MPa at 0 °C and its increase to ∼700 MPa when measured at T g,hi − 100 °C and approaching the T g,lo .

Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

“…The T g of PS determined by Moynihan method is 99.0 °C. We obtained Δ c p at T g by calculating the difference between the extrapolated liquid line and glass line, which gives a value of 0.28 J g −1 K −1 and has a good agreement with the literature report value of 0.28–0.30 J g −1 K −1 …”

“…Hence, the overall contributions lead to a low Δ c p . High mass fraction of long and branched flexible side chains is commonly used in the CPS to improve the solubility issue. Meanwhile, it has been suggested for non‐CPs, introducing bulky side chains lead to high glassy‐state entropy and glassy‐state c p , while the liquid c p is not affected, which then results in a low Δ c p at T g . Same argument is presumably applicable to D–A CPs.…”

Bottlebrush polymers in the melt and polyelectrolytes in solution share common structural features

“…2, which also demonstrate a corresponding primary peak at length scales that are similar to what we expect and observe experimentally. To our knowledge, this bulk backbone-backbone correlation has, up until this point, only been observed using SAXS for short-sidechain (C18 and lower) polyolefin bottlebrushes and wedge polymers (36,37). The use of SANS also allows us to unambiguously interpret the scattering as the result of backbone-backbone correlations, whereas, in the SAXS analysis, the backbone correlations are convoluted with other density fluctuations.…”

Glass Transition Phenomenon for Conjugated Polymers

“…Besides obtaining T g from the temperature sweep measurements, it has been shown by McKenna and coworkers that the T g can also be obtained from the measurement of frequency sweep at different isotherms, in which the data in the frequency space are converted to the temperature space at a certain frequency (e.g., 0.01 rad s −1 , which corresponds to 100 s relaxation time) when the time–temperature superposition principle holds. The mass requirement for dynamic rheometry can be as low as ≈15 mg, much less in comparison to bulk DMA measurements, but still larger than the modified DMA technique mentioned above. Furthermore, since it measures the absolute value of moduli and viscosity, it is able to identify different thermal transitions, which overcomes the shortcomings in modified DMA.…”

“…In the case of traditional coil polymers, for example, poly( n ‐alkyl methacrylates), Donth reported Δ C p decreases from approximately 0.3 J (g K) −1 to 0.1 J (g K) −1 as the number of carbon in the alkyl increased from 1 to 6 . Another example is the second‐generation dendronized polymer, in which the discrete dendronized side groups are grafted onto the backbone, resulting in a stiffened backbone that allows the molecule to take an extended cylindrical configuration, a low Δ C p of 0.076 J (g K) −1 was reported by Qian et al Second, many conjugated polymers are semicrystalline, since the crystalline content does not contribute to the heat capacity change during the glass transition, Δ C p is further reduced due to the decrease of amorphous fraction . In addition, it has been commented that the regular semicrystalline polymers often exhibit a broad glass transition, which leads to the difficulty of T g determination .…”