Sodium polyacrylate has many usages such as latex and food thickener, inorganic pigment dispersant, fabric sizing agent etc. Having carboxylic groups, a weak ionic functional group, sodium polyacrylate behaves as an ionic polymer. Often interactions other than size exclusion effect are observed for the SEC analysis of ionic polymers. When such interactions are present, the elution of the polymer may not be solely based on its molecular size. Therefore, it may interfere with an accurate molecular weight determination calculation of the polymer. Sodium polyacrylate of three different molecular weights were analyzed using Asahipak GF-7M HQ (an Aqueous/Organic (multi-solvent use) SEC column).
The effects of mobile phase pH on the analysis of sodium polyacrylate are summarized.

Water ---- Polymer exists as dissociated form (-COO-). Since the packing material of GF-7M HQ, polyvinyl alcohol, is slightly negatively charged, an ionic interaction (ion exclusion) between the polymer and the packing material occurs. This resulted in the elution of polymer as front.
Mobile phase (pH 3) ---- polymer dominates in none dissociated form (-COOH). With this form, the low polarity of the polymer experiences hydrophobic interaction with the packing material. The polymer is adsorbed completely and will not elute from the column.
Mobile phase (pH 5) ---- Polymer dissociates in some level, and it exhibits as a mixture of -COO- and -COOH forms. The polarity of the polymer is higher than that at pH 3, however added salt in the mobile phase helps to suppress the ion exclusion effect. Although the presence of -COOH form causes hydrophobic interaction in addition to the size exclusion effect; this results in lowering of recovery rate and delay of elution. These phenomena are more substantial for the larger MW polymers.
Mobile phase (pH 7) ---- Polymer dissociates and -COO- form dominates. The higher polarity inhibits the hydrophobic interaction; less adsorption, appears as lesser delay in elution, is observed. Addition of ~10% acetonitrile can suppress the remaining hydrophobic interaction.
Mobile phase (pH 9) ---- No adsorption and delay in elution is observed since hydrophobic interaction is well suppressed as polymer is in fully dissociated form.


The pH of mobile phase is a key factor for the SEC analysis of ionic polymers as shown in this experiment. It is important to optimize the conditions for each ionic polymer since polarity and ionic strengths differ between polymers.

Sample :50 μL
Sodium polyacrylate 0.1 % each PSS-USA（Polymer Standards Service-USA）

 

	Mp	Mn	Mw	Mw/Mn
1.	272,900	222,100	335,400	1.51
2.	90,500	7,8400	131,200	1.67
3.	7,500	6,200	8,300	1.34

Column       : Shodex Asahipak GF-7M HQ (7.5 mm I.D. x 300 mm)
Eluent       : H2O
               50 mM Sodium phosphate buffer
               50 mM Sodium phosphate buffer (pH7.0)/CH3CN=90/10
Flow rate    : 0.6 mL/min
Detector     : UV (210 nm)
Column temp. : 30 °C