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Advancing thermal field-flow fractionation for industrial polymer characterization
Toney, Michael David
Toney, Michael David
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2023
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Abstract
Field-flow fractionation (FFF) is a family of techniques known for its open channel design and ability to separate polymers and colloids. The applied field in an FFF technique determines the physiochemical properties by which the separation occurs. A well-established theory relates retention time to a retention parameter that is defined by the interaction between the analyte and the applied field as well as the analyte’s diffusion coefficient (D). This imparts the ability for FFF techniques to be used as both a separation and characterization tool. In the case of flow FFF, the retention parameter is dependent solely on D which subsequently relates to hydrodynamic size. For thermal FFF (ThFFF), the retention parameter is dependent on the Soret coefficient (ST), a ratio of the thermal diffusion coefficient (DT) and D. Existing studies of the thermophoresis of polymers in dilute solutions have revealed trends that may be utilized to solve challenges in polymer analysis. Specifically, DT has been observed to be molar mass independent (above 103 to 104 Da), polymer-solvent dependent, and recently – dependent on architecture. The polymer-solvent dependence of DT has been leveraged to characterize the composition of di- and tri-block polymer systems. This polymer-solvent dependence, while useful in driving separations, has proven to be challenging when analyzing new polymer chemistries because of incomplete understanding of thermophoresis and the resulting trial-and-error approach to selecting a carrier liquid that imparts sufficient polymer retention.
This thesis aims to increase the adoption of ThFFF by expanding the scope of polymer chemistries and architectures studied. The work presented here leveraged a leading polymer thermophoresis model to identify a suitable carrier liquid for characterizing the architecture of bottlebrush polymers. The ST values were calculated from measured ThFFF retention times and their relationships to the degree of polymerization of the brush backbone and sidechains were established. Information about bottlebrush architecture was then obtained using the recently introduced Soret contraction factor (g”), defined as the ratio of the Soret coefficient of a branched polymer (ST,br) to that of linear polymer (ST,lin) with the same molar mass. Linear analogs were not available for these polyacrylates-containing bottlebrush polymers and thus ST,lin values were approximated using models for thermal and translational diffusion. A plot of log g” versus the number of chain-ends of a bottlebrush showed the expected decrease in log g” when the number of chain ends increased from 120 to 400. Differences in log g” were also noticeable between 30% and 100% grafting densities. This work demonstrated the feasibility of estimating ST,lin and opens the door for architecture characterization in the absence of a linear polymer analog.
The g” approach described above has been successfully utilized for model polymer systems derived from well controlled synthesis and orthogonal characterization. Ultimately, the question is whether g” can be used to characterize polymers in complex formulations of industrial importance. A polydimethylsiloxane (PDMS) containing formulation was targeted because of its relative ‘greenness’ when compared to petrochemical derived polymers. This PDMS system proved to be challenging on multiple fronts. First, DT calculations did not yield trends useful to solvent selection due to the observed double Hansen Solubility sphere. This yielded two distinct values for DT for each solvent system, both of which were inaccurate (>100% difference). Second, the industrial formulation contained a low (< 10%) amount of crosslinked PDMS amidst a large amount (> 90%) of PDMS diluent as well as gels and microgels. This low level of crosslinked PDMS and the broad size polydispersity required development of a sample preparation procedure. Different sample preparation methods were evaluated using ThFFF-MALS and the molar mass profiles indicated that centrifugation followed by filtration was the most suitable. Next, PDMS samples with different levels of crosslinking were analyzed and the log g” distribution for the more crosslinked sample was observed to extend to lower values (more contraction). This is as expected and showed that the g” approach can be used for a new polymer chemistry, PDMS, and a complex sample mixture. To date, there has been no additional verification of the Soret contraction approach. PDMS offered a unique opportunity to address this gap because this polymer can be depolymerized and GC-MS used to determine products indicative of branching. GC-MS results confirmed the degree of branching trend indicated by g” values and is an important step forward.
The final project presented in this thesis explores the link between DT and the glass transition temperature (Tg). This work presents a comprehensive compilation of polymer thermal diffusion data and the first comparison of experimental DT and Tg. Across multiple solvent systems, a strong positive correlation was observed. To understand if there is a physical connection between these two properties, the results of a first principles Tg model were compared to experimental DT values. This work suggests that entropic forces may contribute to DT, a factor which was explicitly ignored in the derivation of the current leading predictive model. In addition, Tg may be an alternative metric for carrier liquid selection for ThFFF analyses.
In summary, ThFFF has become an increasingly powerful tool for the characterization of complex polymer systems. This thesis presents the first reported estimation of ST for a linear polymer utilizing leading models of translational and thermal diffusion, which has expanded the accessibility of g” analysis beyond systems with available linear analogs. The scope of architecture studied by g” now includes bottlebrushes and crosslinked networks along with the previously reported branched systems. Analyses of crosslinked PDMS also presents the first application of g” to an industrial polymer system. These results were also the first to be verified by alternative means. In addition, a potential link between DT and Tg was observed that could lead to improvements in predictive models. This relationship may also serve as a simple metric for approximating the relative magnitude of DT for polymers.
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