60th Annual Report on Research 2015

            In work supported by the ACS PRF, we have examined the properties of bottlebrush polymers in solution, in the bulk, as additives, and as thin film coatings. The work during year 3 of the project focused primarily on the use of bottlebrush additives to tailor surface properties. As shown in the schematic below, bottlebrush copolymers can segregate to films surfaces to achieve desired surface properties.

Figure 1: Schematic for the modification of polymer thin films through the addition of bottlebrush copolymers, which segregate to the film surface due to enthalpic and entropic effects.

Prior experimental and computational studies have demonstrated the surface segregation of branched polymers in blends with chemically identical linear polymers.  The migration of linear copolymer additives, where the additive contained a compatibilizing (same composition as matrix polymer) and low-surface energy component, has also been reported. In a recent study focusing on blends of bottlebrush and linear polystyrene (1:9 vol/vol), we found that bottlebrush polystyrene preferentially accumulated at air and substrate interfaces after thermal annealing. The bottlebrush side-chain composition matched that of the linear polymer matrix, and therefore segregation was driven by entropic effects. Here, we explore blends of bottlebrush copolymers that contain two different types of side chains (see Figure 1). The use of bottlebrush copolymers with two different side-chains introduces enthalpic interactions as a driving force for segregation, while simultaneously delivering a desired surface functionality.

We focus on bottlebrush polymers and copolymers with poly(dimethylsiloxane) (PDMS) side-chains. PDMS is an inorganic polymer with low surface energy (~ 20 mN/m) and low glass transition temperature (T_g ~ -120°C). We synthesized bottlebrush polymers with PDMS side-chains and bottlebrush copolymers with PDMS and poly(lactic acid) (PLA) side-chains using a ROMP grafting-through synthesis approach and their use as surface-active additives. ROMP produces branched PDMS polymers and copolymers with a higher grafting density and longer side-chains compared with previous synthetic approaches.

Scheme 1. Synthesis of bottlebrush copolymers P(XPDMS-co-YPLA)_n with mixed PDMS and PLA sidechains is carried out in a one-pot ROMP.  The resulting polymers have 75, 50, or 25 mol % PDMS sidechains.  

To determine if bottlebrush copolymers spontaneously segregate to the top of thin film blends, we first measured the water contact angle (WCA) for bulk PLA films and found a value of 65.7 ± 1°.  As a reference, bulk PDMS is extremely hydrophobic with a measured water contact angle of 107 ± 1.3°. We then measured the WCA for all blends and observed significant increases (by more than 20°) with only 1 wt % bottlebrush.  Blends with 5 wt % bottlebrush additives showed further increases in contact angle, by up to 30°. Therefore, bottlebrush copolymer additives accumulate at the free surface and introduce hydrophobic behavior, where the extent of hydrophobicity is enhanced with loading. Contact angles as a function of bottlebrush copolymer loading and side-chain composition are reported in Table 3 and Figure 4.  All films are hydrophobic (WCA > 90°), with the exception of 1 wt % P(25PDMS-co-75PLA)₄₃ which has a WCA of 85° owing to the high PLA content and the low loading of the material.  The WCA for 5 wt % blends increased by an average of 6.1° compared with the 1 wt % blends for all compositions of additives.  Similar to results from pure bottlebrush copolymer contact angles, a higher content of PDMS side-chains in the copolymer additives results in higher contact angle values for the blends.  In all cases, the water contact angle for blends was less that the value for the corresponding pure bottlebrush copolymer, indicating the presence of some PLA at the top film interface.

Figure 2. Optical microscopy images of 5 wt% blend films both before and after thermal annealing (left) and water contact angle measurements for PLA films with 1 and 5 wt% PDMS-co-PLA bottlebrush added. The content of the bottlebrush (red for PDMA and blue for PLA) is given on the bottom axis.

Blend films containing bottlebrush copolymers were analyzed by XPS to determine the composition at the top surface. XPS measurements showed an excess of PDMS at the surface, with an approximate bottlebrush content of 20 and 30 wt % bottlebrush at the top surface for 1 and 5 wt % blend films. These results combined with contact angle and optical microscopy measurements prove that bottlebrush copolymers spontaneously segregate to the polymer-air interface during spin casting without macroscopic phase separation.

This work demonstrates that low-surface energy bottlebrush copolymer additives can be used to introduce new surface properties in polymer films, such as creating a hydrophobic surface in a hydrophilic polymer film, which has potential applications in fouling reduction. We demonstrated the synthesis of PDMS bottlebrush polymers and PDMS/PLA bottlebrush copolymers by ROMP. This synthesis method enables the preparation of well-defined, highly branched PDMS bottlebrush polymers and copolymers.  These materials exhibit surfactant-like properties in thin film blends with linear PLA: Both contact angle goniometry and XPS demonstrate a spontaneous accumulation of these additives at the film surface without lateral phase segregation. Linear PDMS is found to phase separate strongly while PDMS-b-PLA diblock copolymers do not enrich the film surface as strongly as bottlebrush copolymer additives. Future work will explore the use of tailored bottlebrush copolymers to introduce different surface functionalities through entropically- and enthalpically-mediated segregation. The work described in this report has been submitted for publication and is currently under review.

This work represented a significant portion of the graduate work for Stacy Pesek and Yen-Hao Lin, both of whom defended their theses this past year.

Figure 3. Graduation photo for Verduzco laboratory members (left, Yen-Hao Lin is second from the left) and photo after Stacy Pesek thesis defense (right).