fuel, we studied low concentrations (<1wt%) of polymer in hydrocarbon solvents.
Neutron scattering experiments require the use of deuterated solvents to achieve contrast and since deuterated Jet-A is not available, we use a range of deuterated single component solvents (cyclohexane, dodecane, and toluene) that are constituents of Jet- A[15]. Studies were also conducted in tetrahydrofuran which interferes with hydrogen bonds and reduces the association strength, easing the comparison between very strongly associating end-groups.
We provide neutron scattering evidence of large supramolecular structures formed by complementary associating polymers in several hydrocarbon solvents. It is determined that there is a significant difference between the aggregation in aliphatic and aromatic solvents. Using the sizes determined by light and neutron scattering we obtain accurate estimates for the overlap concentrations in a variety of solvents, which is lower than was initially estimated. Finally, a method is developed to estimate the aggregation number of the supramolecules even above the overlap concentration. The data in this study proves that SANS can be used to study very dilute solutions (<0.1wt%) of telechelic polymers in hydrocarbon solvents and paves the way for the systematic development of a relationship between the chemical architecture and supramolecular structure of pairwise complementary associating telechelic polymers.
5.2 Experimental
Laboratories. Cylindrical quartz “banjo” cells used in scattering experiments were purchased from Hellma Analytics.
Polymers used for the experiments were end-functionalized model polybutadienes (Figure 5.3).
Figure 5.3: Structure of Telechelic Polymers. The upper box shows the polymer backbone with dendritic groups (middle) attached at both ends. For a given polymer, all of the Y groups on the dendritic end groups consist of one type of associating group from the bottom box.
The synthetic details are described in the PhD thesis of Jeremy Wei[19]. The polymers have dendritic end-group structure, allowing for different numbers of functionalities (1, 2, 4 or 8). The active end groups were either carboxylic acid donors or tertiary amine acceptors with a protected ester form of the carboxylic acid serving as the non-associative control. The amine and acid end-groups are able to form charge assisted hydrogen bonding pairs[16] with their opposite members (Figure 5.4).
Figure 5.4: Schematic of hydrogen bonding interaction between a Tetra Acid end-group and a Tetra Amine end-group. This consists of four charge-assisted hydrogen bonds.
A series of polymers with a range of molecular weights and end-group types (Table 5.1) were used in our experiments.
Table 5.1: Characteristics of associating polymers from MALLS/GPC.
The naming of the polymers consists of describing the end-group type followed by the molecular weight of the polybutadiene backbone rounded to the nearest
100kg/mol. For example diTetraAcid-100kPB has four carboxylic acid functionalities at each end of the polymer and an approximately 100kg/mol molecular weight of the backbone.
5.2.2 MALLS/GPC
Polymer absolute molecular weight[32] and polydispersity (PDI) were obtained using Gel Permeation Chromatography with a Wyatt DAWN EOS multi angle laser light scattering detector in series with a Waters 410 differential refractometer. Separation was achieved using four Agilent PLgel columns connected in series (pore sizes 103, 104, 105, and 106 Å) with degassed THF at 0.9mL/min as the mobile phase. Samples were prepared at 5mg/mL concentration in THF and filtered through a 0.45μm PTFE membrane syringe filter immediately before injection. Data was analyzed using Wyatt Astra Software (v5.3.4) with the Berry method[33, 34] used to obtain Mw and Rg.
5.2.3 Making Sample Solutions
Sample solutions for SANS were made using perdeuterated solvents (dodecane- d26, cyclohexane-d12, tetrahydrofuran-d8, and toluene-d8). Solutions of ester and base polymers were prepared by weighing out polymer on a Mettler precision balance (±0.01mg) into new glass scintillation vials with PTFE lined caps and adding the appropriate amount of solvent using a precision syringe (±1%). These were placed on a wrist action shaker at room temperature overnight. Sample solutions of acid polymers in Tetrahydrofuran (THF) were prepared in an identical manner. For solutions of acid polymers in dodecane, toluene, and cyclohexane, the polymer was weighed out into a new scintillation vial with septum cap and a stirbar was added. The vial was degassed by
pumping down to ~100mTorr and refilling with argon three times. Previously degassed solvent was then added to the vial using syringe transfer. The vial was stirred overnight at 60℃, immersing the vial in an oil bath on a magnetic stir/heat plate (IKA RCT). The oxygen-free conditions prevent degradation of the polymer at elevated temperature.
Mixed solutions of acid and base polymers were prepared by combining equal volumes of base polymer solutions and acid polymer solution in a new vial and subsequently placing the mixture on a wrist action shaker for at least 1 hour.
5.2.4 Small Angle Neutron Scattering at NIST and ORNL
Small Angle Neutron Scattering experiments were conducted at the National Institute of Standards and Technology (NIST) on beamline NG-3[35] and at Oak Ridge National Laboratory (ORNL) on beamline CG-2[36, 37] at the High Flux Isotope Reactor (HFIR). Samples were placed in Hellma quartz cylindrical cells with 2mm path length used at NIST (120-QS-2) and 5m path length used at ORNL (120-QS-5). Temperature was controlled by a recirculating water bath at NIST and by Peltier at ORNL. All scattering experiments were conducted at 25℃. 2D scattering patterns were taken for each sample using three detector distances (1.3 – 13m at NIST and 0.3 – 18.5m at ORNL) and associated configurations (see sections 3.2.4 and 3.2.5) with refractive lenses[38] used at NIST to extend the q-range. The overall scattering vector ranges were (0.0015 < q(Å-1) < 0.4) at NIST and (0.002 < q(Å-1) < 0.8) at ORNL, with the effective limits for a given sample determined by the signal to noise ratio.
The 2D scattering patterns were corrected for electronic noise, neutron background, detector sensitivity, and empty cell scattering. They were then normalized by the incident neutron flux and radially averaged resulting in absolute intensity I(q).
Patterns for the three detector distances were combined using Igor PRO macros developed by Steve Kline[39] that were also used for subsequent processing and analysis.
The solvent background scattering was subtracted from the data as described in section 3.2.4.
Data was analyzed using two scattering functions I(q): the polymer excluded volume function[40] used for polymers with non-gaussian fractal scaling and the Beaucage function[41, 42] used for generalized fractal objects like particles and aggregates. The polymer excluded volume function has the form
𝐼(𝑞) = 1
𝜈U1�2𝜈𝛾 �1
2𝜈, U� − 1
𝜈U1 𝜈� 𝛾 �1
𝜈, U� (5.1)
where γ(x,U) is the incomplete gamma function and U is given by
𝑈 = 1
6 𝑞
2𝑅
𝑔2(2𝜈 + 1)(2𝜈 + 2)
(5.2)and the parameters Rg and ν are the radius of gyration and the mass fractal scaling of the polymer coil.
The Beaucage function is an empirical function combining Guinier and Porod scattering[43] with the form
𝐼(𝑞) =𝐵𝑘𝑔𝑑+ 𝐺exp�−𝑞2𝑅𝑔2
3 �+𝐵 �𝑒𝑟𝑓 �𝑞𝑅𝑔
√6��3𝑃
𝑞𝑃 (5.3)
where Rg is the radius of gyration of the scattering feature and P is the scattering exponent in the power law region with G and B being empirical scaling parameters.