Morphological transitions in langmuir films of a diblock copolymer from polyethylene glycol and poly-L-lactide

Lipopolymers which are composed of lipids covalently linked to polyethylene glycol (PEG)  head groups, have been used to improve the stability of bilayer membranes which is a necessary  property for the preparation of drug carriers. Furthermore, the presence of PEG increases the  circulation time of the pharmaceuticals. PEGylated vesicles can also be prepared from  amphiphilic diblock copolymers and a direct incorporation of the agent into such vesicles becomes possible. It has been shown that an increased therapeutic effect of the drug can be  achieved through conjugation with PEG over a relatively broad range of chain length and admixed portions of PEG. However, the exact mechanisms responsible for this PEG induced  effects are not yet known. On the other hand, a certain risk of an immune defence reaction  (immunogenicity) against PEGylated drugs has also become apparent, which makes it  necessary to identify the optimum amount of PEG moieties on surfaces. PEG molecules may  reside on a surface and take different conformations depending on grafting density, length of  the PEG chains and their interactions with surrounding water molecules. Due to these multiple  parameters, such a system is complex and difficult to control. One possible approach is to  reduce its complexity and to design experiments that allow for a precise adjustment of certain  parameters. A simplified and easily controllable system is a Langmuir film which can be  prepared in different phases through a precisely adjustable surface density. Accordingly,  experiments on Langmuir films can provide useful information on the topic of the correlation  of surface density and interactions between molecular units and the resulting physical  properties.

In the here presented work, we investigated morphological transitions upon compression of  Langmuir films of a diblock copolymer composed of two short blocks of methyl-terminated  PEG (mPEG) and poly-L-lactide acid (PLLA). These two blocks are balanced in terms of  molecular weight and therefore also in terms of hydrophilicity and lipophilicity (HLB value).  Due to this well-balanced HLB value, one may expect the formation of regularly arranged  structures as for example micelles. At rather high surface densities, the morphologies observed  in the Langmuir films showed a transition from curved fibers arranged in regularly spaced  circles to straightened fibers that were preferentially aligned parallel to each other. This  transition passed through an intermediate state of randomly oriented short fiber-like structures. We also investigated the influence of changes of compression rate and temperature on the  behavior of the diblock copolymer in Langmuir films. Surprisingly, only the transition from  circular to parallel arranged fibers showed a dependence on the rate of compression while the  other regions of the isotherm were not affected. Upon increasing the temperature, this transition  was shifted towards a higher surface pressure and a smaller area per molecule. The temperature  dependence of the interactions between PEG and water was considered to be responsible for  this observation. By studying the miscibility of the homopolymers mPEG and PLLA in  Langmuir films, we have clearly identified repulsive interactions between these polymers, allowing for the conclusion that, in Langmuir films of the block copolymer, PLLA and PEG  phase-separated and arranged above and below the air/water interface, respectively.  Accordingly, the mPEG chains formed a layer in the sub-phase region, which, due to the  covalent links to the PLLA chains, was forced to stay close to this interface. With increasing  surface density, the PLLA chains ordered at the interface with an interspacing governed by the  hydrodynamic volume taken by the PEG blocks. The mPEG chains underwent conformational  transitions in water and stretched into the sub-phase at the highest compression, which we  interpreted as a transition from a PEG quasi-brush regime to a PEG brush induced by  compression. Compression-expansion experiments showed that this transition was reversible as demonstrated by the re-appearance of well assembled circular arrangements of fibers upon  expansion of a film that was previously compressed into the brush state. Finally, we have  studied the influence of the molecular weight of PLLA on structure formation of the diblock  copolymer, concluding that curved fibers were present for all mPEG2000-PLLA diblock  copolymers used in this study. The curvature of the fibers was attributed to the increased surface  stress which was reinforced by a mismatch in size of the two polymer blocks. The regular array  of well arranged circular fibers, however, was only observed for the block copolymer with  balanced molecular weights.