Polymer dissolution, which is based on two main steps – solvent diffusion and chain disentanglement, is one of the important steps in numerous industries and needs process and condition optimisation for increased profits. Thus, scientists have developed certain methodologies for understanding dissolution of polymers in various solvents and under varied process conditions. Each industry can optimise the conditions as per their requirement.
Dissolution of polymers is largely governed by Gibbs free energy of mixing mentioned below:
Where, ∆G is the Gibbs free energy change on mixing, ∆H is the change in enthalpy on mixing, T is the absolute temperature and ∆S is the change in entropy on mixing
Certain factors which affects dissolution of polymers are:
- Molecular weight of polymer
- Polydispersity index of polymer
- Structure of polymer
- Polymers chemical composition
- Crystallinity of polymer
- Additives present in the polymer
- Solvent type
- Solvent to polymer ratio
- Agitation rate
- Dissolution temperature
- Dissolution time
Please comment if there are any other parameters.
Some of the application areas wherein dissolution of polymers is important are:
- Membranes
- Electronics
- Controlled drug delivery
- Tissue engineering
- Polymer recycling
- Fibre
- Adhesive
- Food
- Paint
- Sealant etc
Please comment of any other application areas, wherein, dissolution of polymers is quite important.
Stages in polymer dissolution:
When any polymer in the form of lump, granule, flake, particle or powder is added in solvent, phenomenon of dissolution starts from the surface of the solid and proceed inward. If one wants to briefly explain the steps involved, initially, polymer on the solid surface will be in un-swollen form, which on penetration of solvent starts swelling. With higher ingress of solvent, polymers start getting disentangled eventually leading to release of the polymer in the solvent media. These steps are pictorially described in the figure below.
Methodologies for understanding dissolution of polymers are:
Thus, it can be understood that polymer dissolution is very important in numerous industries and is also affected by several parameters. Thus, it becomes important to have certain methodologies developed to understand the extend of dissolution, understand the effect of parameters and thus have a repeatable process. Some of the methodologies are listed in below (they are also listed in the infographic):
- Optical microscopy
- Gravimetry
- NMR
- FT-IR
- Interferometry
- Differential refractometry
Lets see each in brief.
Optical Microscopy:
- Method involves visual observation
- Provides information on the stage of dissolution (un-swollen / swollen / disentangled / released)
- Can use dye for better / improved visualisation
- Very simple technique
Gravimetry:
- Measure weight of the film during the process of dissolution
- Requires quartz crystal micro-balance; which has a known oscillation frequency
- Addition of the film onto the quartz crystal lowers the frequency of quartz as compared
- As the polymer dissolves, frequency of the quartz starts increasing, subsequently reaching the original value, confirming complete dissolution
- Highly accurate technique
NMR:
- Measured in terms of area of proton signal resonance line
- Undissolved polymer, due to restricted mobility, will demonstrate broader proton signal resonance line
- As polymer dissolves, proton signal resonance line narrows attributed to free polymer mobility
- A convenient method
FT-IR
- Can provide real time data of polymer dissolution
- Dissolution of polymer affects the specific peaks of solvent / polymer
- A single experiment can yield huge data
Interferometery:
- One of the popular methods to understand polymer dissolution
- A beam of monochromatic light is passed through the sample.
- As the polymer dissolves, intensity of the light passing through the sample increases indicating dissolution
Differential refractometry:
- One of the oldest methodologies for understanding dissolution of polymers
- Based on the measurement of refractive index
- As polymer dissolves, refractive index of the solvent changes
- A constant final refractive index indicates complete dissolution of the polymer in the solvent
- Methods is highly dependent on polymer and solvent quantity
Please comment the name of the methodologies not listed above.
Check this interesting video; wherein, a user is trying to understand the effect of water and hydricarbon solvent on solubility of styrofoam!
Dear Readers, do go through the above literature and let me know your viewpoints in the comments section.
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References:
- Stamatialis, D. F., Sanopoulou, M., & Raptis, I. (2002). Swelling and dissolution behavior of poly (methyl methacrylate) films in methyl ethyl ketone/methyl alcohol mixtures studied by optical techniques. Journal of applied polymer science, 83(13), 2823-2834. (Link)
- Zhang, Y., Mallapragada, S. K., & Narasimhan, B. (2010). A novel high throughput method to investigate polymer dissolution. Macromolecular rapid communications, 31(4), 385-390. (Link)
- Thijs, H. M., Becer, C. R., Guerrero-Sanchez, C., Fournier, D., Hoogenboom, R., & Schubert, U. S. (2007). Water uptake of hydrophilic polymers determined by a thermal gravimetric analyzer with a controlled humidity chamber. Journal of Materials Chemistry, 17(46), 4864-4871. (Link)
- Strübing, S., Metz, H., Syrowatka, F., & Mäder, K. (2007). Monitoring of dissolution induced changes in film coat composition by 1H NMR spectroscopy and SEM. Journal of controlled release, 119(2), 190-196. (Link)
- Krasicky, P. D., Groele, R. J., & Rodriguez, F. (1987). A laser interferometer for monitoring thin film processes: Application to polymer dissolution. Chemical Engineering Communications, 54(1-6), 279-299. (Link)
- Miller-Chou, B. A., & Koenig, J. L. (2003). A review of polymer dissolution. Progress in Polymer Science, 28(8), 1223-1270. (Link)
- Video Link: https://www.youtube.com/watch?v=IYjpP_2kRLU
