Inkjet printing of polymer solutions is a key area of interest for depositing high-throughput, precise patterns of functional polymer. The polymer patterns are used in different applications such as textiles, organic circuits and OLEDs.
The viscosity and concentration of the polymer solution, level of branching in the polymer, molecular weight (MW) and molecular weight distribution (MWD) of the polymer can negatively impact the efficacy of this printed pattern.
Therefore, it is important to understand the effect of inkjet printing on polymer degradation. In this article, the MW and MWD of various polymers are demonstrated as a function of time during inkjet printing. The data shown in this article is taken from references [1-2].
To characterize the polymers after various durations of inkjet printing, two different Malvern Panalytical Viscotek Size Exclusion Chromatography (SEC) systems were used. A Malvern Panalytical Viscotek GPCmax, which includes a low pulsation isocratic pump, autosampler, and an online eluent degasser, was the front end of both systems.
One system was comprised of a single Malvern Panalytical Viscotek refractive index (RI) detector. It ran THF as the mobile phase, was calibrated with a series of poly(methyl methacrylate) (PMMA) standards, and then used to analyze the polymer MWD and MW with inkjet printing time.
Commonly known as a Conventional Calibration system, this system will only report MWs and MWDs relative to the chemistry of the standard (which is PMMA in this case).
The other system was integrated with a Malvern Panalytical Viscotek TDA305 system with light scattering detectors, RI, and viscometer. It ran MEK as the mobile phase, was calibrated with a single PMMA standard, and then used for characterizing the degree of branching in the polymers and verifying the MW’s measured with the first system.
Commonly known as a Triple Detection system, this system reports the MW and MWD as well as the intrinsic viscosity (IV) by incorporating the data from the viscometer detector. Using the light scattering detector, the calculated MW and MWD are regarded as “absolute” MW, in that they are independent of the structure and chemistry of the standard. Branching information can be determined by combining the information from the light scattering detectors and the viscometer.
In this study, six polymers were used. Table 1 shows a summary of those polymers. HB, LB, and MB refer to the relative level of branching in the samples: low branching, medium branching, and high branching.
Table 1. Summary of polymers analyzed after inkjet printing [1-2] .
|PMMA 90 kDa
|PMMA 310 kDa
|PMMA 468 kDa
Effect of MW on Degradation
The effect of inkjet printing time on the weight average MW (Mw) for PMMA 90 kDa, PMMA 310 kDa, and PMMA 468 kDa samples is shown in Figure 1.
Figure 1. Molecular weight of samples PMMA 90 kDa , PMMA 310 kDa , and PMMA 468 kDa as a function of time during inkjet printing 
It is very clear that sample PMMA 468 kDa distinctly degrades, whereas sample PMMA 90 kDa does not degrade much, if at all. Samples PMMA 468 kDa and PMMA 310 kDa appear to converge in MW at longer times in the experiment.
From this, it can be determined that high molecular weight polymers will degrade faster compared to low molecular weight polymers. This is further analyzed in Figure 2, which shows how the MWD changes with inkjet printing time. Initially, the largest chains appear to degrade, followed by smaller chains, until the distribution seems to stabilize above ~ 200 hours of printing.
Figure 2. Molecular weight distribution of sample PMMA 468 kDa when printed for a period of 0 (solid line), 150 (---), 200 (.-.), and 250 h (..-..) 
Degree of Branching from SEC
The scientists also studied the branching effect on the MW degradation with inkjet printing time. As a baseline for these tests, they sought to understand the differences in the degrees of branching between the samples PMMA 468 kDa, HB, LB, and MB.
Using Triple Detection GPC, these differences in branching can be visualized and quantified by mapping the data acquired on a Mark-Houwink plot (example shown in Figure 3). The Mark-Houwink plot shows the IV as determined by the viscometer detector as a function of the MW as determined by the light scattering detector.
Figure 3. An example Mark-Houwink plot showing IV as a function of MW for a branched sample (red) and a linear reference (black). Quantitative branching information can be calculated from the difference between the sample and the reference.
Qualitatively, samples that are more highly branched will be closer to the bottom right of the graph while more linear samples will be towards the upper left. The number of branches, g' (ratio of branched and linear IV’s), and branching frequency can be quantified by comparing the curves to a known linear reference (either run on the GPC or from known Mark-Houwink parameters).
Triple detector GPC was used to determine MWD and g' of these samples, as shown in Figures 4a and 4b, respectively.
Figure 4. a) Molecular weight distributions of samples LB (...), MB (---), and HB (solid line); b) g' for linear sample PMMA 468 kDa (...), branched samples LB and MB (---), and branched sample HB (solid line) 
As expected, the linear sample displayed a g’ near 1 while the branched samples had increasingly lower values with an growing level of branching. As shown in Figure 4a, the wide and multi-modal MWD is more typically seen with highly branched samples.
Scientists established how inkjet printing degrades polymers by analyzing the polymer MW after inkjet printing for various amounts of time. The scientists of the two papers referenced in this article explored how the initial MW of the polymer and the degree of branching influenced the rate of degradation.
For comparison purpose, traditional calibration GPC can rapidly and effectively examine the MW and MWD of similar polymer samples. This works very well for analyzing the degradation of polymers over time.
Triple detection GPC is capable of analyzing a wide range of polymers (similar or not) and achieving significant amounts of information including, but not limited to, IV, MW, MWD, and degree of branching. This works well for comparing polymers with widely different structures or various chemistries.
1. Wheeler, Joseph S.R., Stuart Reynolds, Steven Lancaster, Veronica Sanchez Romanguera, and Stephen Yeates. "Polymer degradation during continuous inkjet printing." Polymer Degradation and Stability. 105. 2014. pp. 116-121.
2. Wheeler, Joseph S.R., Amelie Longpre, Daniel Sells, Daryl McManus, Steven Lancaster, Stuart Reynolds, and Stephen Yeates. "Effect of branching on degradation during inkjet printing." Polymer Degradation and Stability. 128. 2016. pp. 1-7.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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