Structure of binary mixed polymer Langmuir layers

C. Bernardini

Research output: Thesisinternal PhD, WU

Abstract

The possibility of preparing 2D stable emulsions through mixing of homopolymers in a Langmuir monolayer is the core topic of this thesis. While colloid science has achieved well established results in the study of bulk dispersed systems, accounts on properties of mixed monomolecular films are fewer, and seldom systematic. The aim of this investigation is to contribute to a deeper understanding of the subject, in order to explore opportunities to apply the acquired knowledge to the fabrication of technologically relevant materials. In particular, this study focused on a possibly applicable,  innovative strategy for the manipulation of the morphology and the patterning of mixed Langmuir monolayers: the possibility to stabilize and control a dispersion of homopolymers through the addition of a lineactant (the equivalent of a surfactant in three dimensional systems), able to adsorb preferentially at the interfacial contact line of polymer domains, thereby lowering the interfacial energy (line tension) in the system and favoring an effective dispersion of one component into the other.

The state of the art of the preparation and investigation of 2D colloids is the subject of Chapter 2, which is a comprehensive review on several systems able to yield phase–separated Langmuir monolayers, and includes a general definition of the concept of a 2D colloid, the most relevant instrumental techniques and experimental tools available, a summary of several systems suitable for preparing 2D colloid dispersions, an introduction to the concept of lineactant, and several examples, both experimental and theoretical, in which compounds acting as lineactants have been investigated. This review clearly shows that the polymer–based mixtures are a poorly explored subject, when compared to amphiphiles of natural origin, and so the rest of the thesis has been devoted to the investigation of polymer–based Langmuir monolayers.

This investigation has been carried out with two parallel approaches: classical experiments at the Langmuir trough and morphological characterization of the Langmuir monolayers with the Brewster Angle Microscope have been performed, along with Self–Consistent Field modeling of the same systems. The setup of the SCF model and comparison of SCF calculation with experimental data from the reference experiments are dealt with in Chapter 3. Surface pressure isotherms at the air/water interface were reproduced for four different polymers, poly–l–lactic acid (PLLA), poly (dimethylsiloxane) (PDMS), poly (methyl methacrylate) (PMMA), and poly (isobutylene) (PiB). The polymers are all insoluble in water, but display a different degree of amphiphilicity; therefore the four isotherms differed strongly. The polymers were described through a SCF model on a united atom level, taking the side groups on the monomer level into account. In line with experiments, the model shown that PiB spread in a monolayer which smoothly thickened at a very low surface pressure and area/monomer value. The monolayer made of PMMA had an autophobic behavior: a PMMA liquid did not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only formed after the collapse of the film at a relatively high pressure. The isotherm of PDMS had regions with extreme compressibility which were linked to a layering transition. Finally, PLLA wetted the water surface and spread homogeneously at larger areas per monomer. The classical SCF approach features only short–range, nearest–neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, a power–law van der Waals contribution was taken into account in this model. Two–gradient SCF computations were performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact was obtained, together with the associated line tension value. The SF–SCF molecularly detailed modeling of PLLA, PDMS, PMMA, and PiB monolayers, spread at the air/water surface, has proven to be consistent with experimental data: the incorporation in the model of a detailed molecular description of the monomeric features of the four compounds examined has been crucial to reproducing the features of the adsorption and pressure/area isotherms. 

In Chapter 4, the same approach was applied to the description of polymer mixtures spread at the air/water interface. The aim of this chapter was to analyze topics such as 2D phase separation and partitioning in mixed polymeric Langmuir monolayers. Two of the four polymers studied in Chapter 3 were selected in order to obtain a mixed Langmuir monolayer. A system consisting of water–insoluble, spreadable, fluid–like polymers was prepared. The polymers were polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA), combined, in some cases, with a minority of PDMS–b–PMMA copolymer. Both Langmuir trough pressure/area isotherm measurements and Brewster angle microscopy (BAM) observations were performed, and complemented with molecularly detailed self–consistent field (SCF) calculations. It was shown that PDMS undergoes a layering transition that is difficult to detect by BAM. Addition of PMMA enhanced contrast in BAM, showing a two–phase system: if this consisted of separate two–dimensional (2D) PMMA and PDMS phases, a PDMS–PMMA diblock should accumulate at the phase boundary. However, the diblock copolymer of PDMS–PMMA failed to show the expected “lineactant” behavior, i.e., failed to accumulate at the phase boundary. The calculations pointed to a non-trivial arrangement of the polymer chains at the interface: in mixtures of the two homopolymers, in a rather wide composition ratio, a vertical (with respect to the air/water interfacial plane) configuration was found, with PMMA sitting preferably at the PDMS/water interface of the thicker PDMS film, during the PDMS layering phase transition. This also explained why the diblock copolymer was not a lineactant. Both PMMA and PDMS–b–PMMA were depleted from the thin–thick PDMS film interface, and the line tension between the phases consequently increased in the binary mixtures, as well as in the ternary ones. The results shown in this chapter proved that gaining an accurate control over thin film structures at the microscopic level is a far from trivial task, and the acquisition of fundamental knowledge is necessary in order to interpret experimental data in an appropriate way.

As a consequence, in Chapter 5 an investigation based solely on SCF modeling was carried out, in order to analyze which polymer blends could have the possibility to undergo lateral phase separation in two dimensions. Specifically, the model system investigated consisted of water–supported Langmuir monolayers, obtained from binary polyalkyl methacrylate mixtures (PXMA, where X stands for any of the type of ester side groups used: M, methyl–; E, ethyl–; B, butyl–; H, hexyl–; O, octyl–; L, lauryl–methacrylate). In particular, the conditions which determined demixing and phase separation in the two–dimensional system were addressed, showing that a sufficient chain length mismatch in the ester side group moieties is able to drive the polymer demixing. When the difference in length of the alkyl chain of the ester moieties on the two types of polymers was progressively reduced, from 11 carbon atoms (PMMA/PLMA) to 4 carbons only (POMA/PLMA), the demixing tendency was also reduced; it vanished, indeed, for POMA/PLMA. In the latter case the polymer/subphase interactions affected more the distribution of the polymer coils in the blend monolayer: mixing of the two polymers was observed, but also a partial layering along the vertical direction.

Lineactancy was also considered, by selecting the mixture in which phase separation was best achieved: a third component, namely a symmetrical diblock copolymer of the type PLMA–b–PMMA, was added to a PMMA/PLMA blended monolayer. Adsorption of the diblock copolymer was observed exclusively at the contact line between the two homopolymer domains, together with a concomitant lowering of the line tension. The line tension varied with chemical potential of the diblock copolymer according to the Gibbs’ law, which demonstrated that PLMA–b–PMMA indeed acted as a lineactant (the two–dimensional analog of a surfactant) in the model system made of a binary demixed PMMA/PLMA Langmuir monolayer.

In conclusion, the requirements needed to achieve polymer blend demixing in a Langmuir monolayer are the following: spreadable, insoluble polymers, with the same amphiphilicity degree, combined to a certain chemical mismatch of the side moieties are necessary in order to cause lateral demixing at the air/water interface. The polyalkyl methacrylate example investigated in the chapter represented a suitable model system, since the methacrylate backbone guarantees that the different polymers have the same affinity towards the water subphase, while the different ester moieties drive the occurrence of lateral demixing. The dependency of the lateral demixing on the difference in length between the two ester side groups chosen was demonstrated. A rather complex interplay of forces regulates the distribution of the polymer coils in the monolayer: subtle alterations of this complex balance might favor the dewetting of the mixture in a single domain, together with the layering of the blended polymers along the direction normal to the air/water interface, as well as accumulation of one polymer at the domain edge, instead of the occurrence of the lateral phase separation. Furthermore, the possibility to control emulsification of two–dimensional demixed polymer blends was proven. This was achieved by use of a diblock copolymer, which acted as a lineactant by adsorbing at the contact line of the polymer domains. The calculations demonstrated the possibility to extend the lineactant concept, first elaborated in the context of lipid membrane investigations, to the field of study of polymer thin films.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Cohen Stuart, Martien, Promotor
  • Leermakers, Frans, Promotor
Award date27 Apr 2012
Place of PublicationS.l.
Print ISBNs9789461732149
Publication statusPublished - 2012

Keywords

  • polymers
  • colloids
  • colloidal properties
  • surface phenomena

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