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Surface forces and friction between non-polar surfaces coated by temperature-responsive methylcellulose
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2014 (English)In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, ISSN 0927-7757, E-ISSN 1873-4359, Vol. 441, 701-708 p.Article in journal (Refereed) Published
Abstract [en]

Methylcellulose is a heterogeneous polymer that exposes both methyl groups and -OH-groups to the solution, and the solvent quality of water for methylcellulose deceases with increasing temperature. In bulk solution this leads to aggregation into fibrils at high temperatures. In this report we address how temperature affects adsorbed layers of methylcellulose on hydrophobized silica surfaces in contact with an aqueous methylcellulose solution. The layers were imaged using PeakForce tapping mode atomic force microscopy, in order to determine how the additional adsorption that occurs with increasing temperature affects the layer structure. Surface force and friction measurements were carried out using the AFM colloidal probe method. The data demonstrate that the normal surface forces were rather insensitive to temperature, whereas the friction forces changed significantly with increasing temperature. At low loads the friction increases with increasing temperature, whereas at high loads the reverse is observed. These findings are discussed in terms of how the worsening of the solvent condition affects the aggregation state in the adsorbed layer, and the polymer-surface affinity.

Place, publisher, year, edition, pages
2014. Vol. 441, 701-708 p.
Keyword [en]
Temperature-responsive polymer, Cellulose ethers, Methylcellulose friction, Load bearing capacity, Atomic force microscopy, Surface forces
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-95377DOI: 10.1016/j.colsurfa.2013.10.038ISI: 000329260800090Scopus ID: 2-s2.0-84887548022OAI: oai:DiVA.org:kth-95377DiVA: diva2:527990
Funder
VinnovaSwedish Foundation for Strategic Research
Note

QC 20140131. Updated from manuscript to article in journal.

Available from: 2012-05-23 Created: 2012-05-23 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Bulk and interfacial properties of cellulose ethers
Open this publication in new window or tab >>Bulk and interfacial properties of cellulose ethers
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work summarizes several studies that all concern cellulose ethers of the types methylcellulose (MC) hydroxypropylmethylcellulose (HPMC) and ethyl(hydroxyethyl)cellulose (EHEC). They share the feature of negative temperature response, as they are soluble in water at room temperature but phase separate and sometimes form gels at high temperatures. The different types of viscosity transitions occurring in these three cellulose ethers are well-known. However, earlier studies have not solved the problem of why both HPMC and EHEC, as the temperature increases, exhibit a viscosity decrease just before the viscosity increases, whereas MC only has one transition temperature where the viscosity increases. With our investigations we have aimed to compare the effect of temperature on bulk solutions and on adsorbed layers of the different polymers using a range of techniques.

Light scattering and cryo transmission electron microscopy (cryo-TEM) was employed to study aggregation of MC, HPMC and EHEC in solution. The solvent quality of water is reduced for all three polymers in solution as the temperature increases, and this infers an onset of aggregation at a certain temperature. The aggregation rate follows the order EHEC > HPMC > MC. Cryo-TEM pictures of solutions frozen from high temperatures showed closely packed fibrils forming dense networks in MC solution. Some fibrils were also found in HPMC solution above the transition temperature, but they did not interconnect readily. This is explained by the bulky and hydrophilic hydroxypropyl groups attached to HPMC. EHEC has similar substituents, while MC only has short and hydrophobic methyl groups attached to the main chain.

An amphiphilic liquid, diethyleneglycolmonobutylether (BDG) was used as an additive to change the properties of MC solutions in water. With 10 wt% BDG added, the effect was similar in viscosity and light scattering measurements as well as cryo-TEM pictures, inducing a temperature response resembling that of HPMC in pure water. 5 wt% of BDG was enough to change the aggregation type and induce a transition temperature with viscosity decrease. The effect of the additive is rationalized by BDG acting as a hydrophobic and bulky substituent in MC, similar to the large substituents in HPMC and EHEC.

Two instruments, a quartz crystal microbalance with dissipation (QCM-D) and an ellipsometer, were used in parallel to determine the changes with temperature on an adsorbed layer of MC and HPMC on silica kept in water and in polymer solution. The silica needed to be hydrophobized for significant adsorption to take place. Adsorption was similar for both polymers at low temperatures, whereas a sharp transition in several layer properties occurred for HPMC, but not for MC, close to the solution viscosity transition temperature. Atomic force microscopy (AFM) was used to measure attractive and repulsive forces and also friction forces between MC layers in polymer solution. The small changes in normal forces with temperature infer that the hydrophobic groups in MC are mostly depleted from the surface. The surface–polymer interactions increase with increasing temperature and the layer becomes more cohesive, which induces a higher load bearing capacity and lower friction when measured at high loads. AFM imaging was employed to obtain the height distribution in MC adsorbed layers. These images indicate that fibril-like structures were formed at a lower temperature in the surface layer than in bulk solution.

The different preferences for adsorption and for aggregation in MC and HPMC above the solution transition temperatures are explained by the fibril formation in MC shielding hydrophobic parts of the polymer from the solution, and thus counteracting adsorption, but also fast aggregation. The viscosity decrease in HPMC and EHEC is conferred to intra-chain contraction and aggregation into less extended structures.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 41 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:22
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-95379 (URN)978-91-7501-347-3 (ISBN)
Public defence
2012-06-01, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20120523

Available from: 2012-05-23 Created: 2012-05-23 Last updated: 2012-09-25Bibliographically approved

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